Nanoengineering: The Skills and Tools Making Technology Invisible 1788018672, 9781788018678

Table of contents :
Cover
Nanoengineering: The Skills and Tools Making Technology Invisible
Preface
Contents
Part 1 - The Flatlands of the Nanoverse
Chapter 1 - The World of Graphene
1.1 Introduction
1.2 Graphene Transfer Using Off-the-shelf Office Equipment
1.3 Engineering High-tech Composite Materials
1.4 ‘Stitching’ Together Ultrastrong Nanosheets
1.5 Graphene Aerogel Walls
1.6 Whispers About Graphene's Electrical Properties
1.7 Defect-free Graphene Could Solve the Dendrite Problem in Batteries
1.8 Printing Electronics with Highly Conductive Inks
1.9 Self-healing Electronic Tattoos
1.10 Stick-on Skin Biosensors
1.11 Implant for Deep Brain Imaging
1.12 Graphene Bubbles Enhance Photoacoustic Imaging
1.13 Real-time Monitoring of Insulin
1.14 Self-powered Biosensor Contact Lens
1.15 Solar-driven Water Purification with Multifunctional Papers
1.16 Optoelectronics on Regular Paper
1.17 Graphene Origami Folded with ‘Colors’
1.18 Transparent Electrodes for Liquid Crystal Displays
1.19 Ultrathin Encapsulation for Organic Light-emitting Diodes
1.20 How to Directly Measure the Surface Energy of Graphene
1.21 A 2D Electron Microscope
1.22 Water Slippage in Graphene Nanochannels
1.23 Light-induced Active Ion Transport Through Graphene Membranes
1.24 Membranes for Large-scale Energy Storage Systems
1.25 Protecting Against Electrostatic Discharging Failures
1.26 Inspiration from Spider Webs
1.27 Graphene Quantum Dots Made from Agricultural Waste
1.28 Outstanding Thermal Conductivity of Graphene Composites
Chapter 2 - The Growing Landscape of Two-dimensional Materials
2.1 Introduction
2.2 High-performance Synthesis of 2D Metal Oxides and Hydroxides
2.3 Flexible, Low-power, High-frequency Nanoelectronics
2.4 Large-yield Synthesis of 2D Antimonene Nanocrystals
2.5 Freestanding Borophene Synthesized for the First Time
2.6 2D Spacer Materials for Surface Plasmon Coupled Emission Sensing
2.7 2D Oxides Juice-up Sodium-ion Batteries
2.8 Holey 2D Nanosheets for Efficient Energy Storage
2.9 Atomristor: Memristor Effect in Atomically Thin Nanomaterials
2.10 Photostriction of Molecular 2D Nanosheets
2.11 A Nano Squeegee to Clean Nanosheets
2.12 Studying Strain Effects in 2D Materials Using Kelvin Probe Microscopy
2.13 Optothermoplasmonic Patterning of 2D Materials
2.14 Let's Do the Twist: Rotation-tunable 2D Electronics
Chapter 3 - Not Found in Nature: Metamaterials and Metasurfaces
3.1 Introduction
3.2 Bottom-up
Assembled Chiral Metamolecules
3.3 Ultrathin Plasmonic Chiral Metamaterials
3.4 Large-area
Tunable Metasurfaces
3.5 How to Realize Metasurfaces in Novel Plasmonic Materials
3.6 Full-color
3D Metaholography
3.7 Van der Waals Heterostructures with Tunable Interfacial Coupling
3.8 A Rewritable Metacanvas for Photonic Applications
3.9 Dynamic Plasmonic Pixels
3.10 Improving Terahertz Detection with Metasurfaces
Part 2 - Nanotechnology Unleashed
Chapter 4 - Plasmonics
4.1 Introduction
4.2 Naked-eye Plasmonic Colorimetry
4.3 Monitoring UV Exposure with a Tunable Adhesive Patch
4.4 Nanosensor Gels Detect Therapeutic Levels of Radiation
4.5 Plasmonics in the Clouds
4.6 Reversible Assembly of Plasmonic Nanoparticles
4.7 Biofoam Beats Conventional Plasmonic Surfaces
4.8 Black Gold Maximizes the Light Absorption of Nanomaterials
4.9 Nanopatterning Holograms onto Commercial Contact Lenses
4.10 Multiple Electromagnetic Responses from Accordion-like Plasmonic Nanorods
Chapter 5 - Nanobiotechnology
5.1 Introduction
5.2 An Alternative to Antibiotics: Weakening the Grip of Superbugs
5.3 Cell Sex Impacts the Biological Uptake of Nanoparticles
5.4 Early Cancer Detection with Protein Corona ‘Fingerprints’
5.5 Micromotors Deliver Drug Payloads in the Gastrointestinal Tract
5.6 Titanium Implant Material with Multifunctional Nano–Bio Interface
5.7 All-natural Nanobiotechnology as an Alternativeto Synthetic Agrochemicals
5.8 Advanced Protein Design Drives Complex Nano-assemblies
5.9 Towards Self-powered, Brain-linkede-Vision
5.10 ‘Cyborg’ Microfilter Actively Cleans Contaminated Water
5.11 Growing Bone and Cartilage Tissue from Nanosilicates
5.12 Fabricating Tissue Engineering Scaffolds via Controlled Ice Crystallization
5.13 Augmenting Nerve Regeneration
Chapter 6 - Nanomedicine
6.1 Introduction
6.2 Bacteria-produced Nanoparticles Kill Cancer Cells
6.3 Light-triggered Local Anesthesia
6.4 Repairing the Cancer Cell Suicide Mechanism
6.5 Drug-loaded Nanobullets Fired from Microcannons
6.6 Iron Oxide Nanoparticles Inhibit Tumor Growth
6.7 Replacing Animal Models with Biomimetic Blood–Brain Barrier Models
6.8 Controlled Release of Hydrogel from Nanotubes
6.9 Piezoelectric Platform Inhibits Cancer Cell Proliferation
6.10 Combating Opioid Drug Abuse
6.11 Defining the Immunological Effects of Nucleic Acid Nanoparticles
6.12 Testing Nanomedicine in Space
6.13 The Impact of Nanoparticle Design on Parkinson's Disease Therapies
6.14 Metal–Organic Frameworks Enhance Sonodynamic Cancer Therapy
6.15 Magnetically Propelled MOFBOTs Perform Microrobotic Drug Delivery
6.16 MOF-encapsulated DNA for Gene Therapy
Chapter 7 - Characterization
7.1 Introduction
7.2 Using ‘Big Data’ to Shed Light on the Complexity of Nanomaterials
7.3 Helical Nanopropellers Measure Local Viscosity
7.4 New Phenomena in Ultrasonic Scattering in Nanofluids
7.5 Surface Area as a Highly Relevant Dose Metric for Nanotoxicity Assessments
7.6 AFM Imaging and Characterization of Nematodes in Their Natural Environment
7.7 Accurate Simulations at the Nanoscale Depend on Appropriate Interatomic Potentials
7.8 How to Detect Biological Contamination of Nanoparticles
7.9 Nanophotonic AFM Probe Provides Ultrafast and Ultralow Noise Detection
Part 3 - Engineering at the Nanoscale
Chapter 8 - Fabrication Moves to the Printer
8.1 Introduction
8.2 Highly Conductive Nanocomposites for the 3D Printer
8.2.1 Nanotribological Printing
8.3 3D Printing of Living Responsive Devices
8.4 Customized Design of Anticounterfeiting Marks
8.5 Next-generation
Inkjet Color Printing
8.6 3D Printing with Nanoengineered Bioinks
8.7 Why Burn Bagasse When You Can 3D Print Its Nanocellulose
8.8 Nanocellulose Dramatically Improves the 3D Printability of Carbon Nanotubes
8.9 Miniature 3D Printed High-performance Heaters
8.10 Molecular Printing with Light-actuated Pens
8.11 Scanning Probe + Microbeads = Low-cost, High-resolution Optical Lithography
8.12 Printed Decal Electronics for the Internet-of-Things
8.13 Taking Ice Lithography to the Next Level
8.14 Nanotechnology in a Bubble
8.15 Writing Nanotubes with a Nano Fountain Pen
Chapter 9 - (Mostly Wearable) Electronics
9.1 Introduction
9.2 Skin-inspired Haptic Memory Device
9.3 Self-powered Analog Smart Skin
9.4 Moisture-powered Electronics
9.5 Reversibly Controlling the Learning Properties of Memristors
9.6 Measuring Sunlight Exposure with Stick-on Epidermal Electronic Tattoos
9.7 Batch Assembly of Reconfigurable, Multimodal 3D Electronics
9.8 Green and Flexible Protein-based Electronics
9.9 Stretchable Nanogenerators for Wearable Health Monitors
9.10 Power Up Your T-shirt with Print Designs
9.11 Mission Impossible: Remote Destruction Capability of Silicon Electronics
9.12 A Solid-state Nanopore Platform for Digital Data Storage
9.13 Eminently Wearable Laser-induced Graphene Composites
Chapter 10 - Sensors and Diagnostics
10.1 Introduction
10.2 Keeping a Food Allergen Detector on Your Key- chain
10.3 Disposable Biosensors Made from Newspaper
10.4 Nanocurve-based Sensor Reads Facial Expressions
10.5 En Route to Artificial Retinas
10.6 Voltage-activated Carbon Monoxide Sensor
10.7 Nature-inspired Skin-mimicking Sensors
10.8 Multiplexed Planar-array Analysis from Withina Living Cell
10.9 Real-time Detection of Individual Viruses in Complex Solutions
10.10 Nanotube Chip Captures and Analyzes Circulating Tumor Cells in Blood
10.11 Flexible, Implantable Nutrient Sensors Based on Metal–Organic Frameworks
10.12 Using Household Items to Make Multisensory ‘Paper Skin’
10.13 DNA Walkers Amplify Molecular Fluorescent Signals
10.14 Optical Voltage Nanosensors Created with DNA Origami
10.15 Self-propelled Swimming Nanodiamonds
10.16 Iontronic Sensing Paper: A New Touch for Pressure Sensors
10.17 Brain-on-a-chip Engineered on Nanowire Scaffolds
Chapter 11 - Energy Harvesting and Storage
11.1 Introduction
11.2 ‘Artificial Leaf’ Uses Visible Light to Accelerate Chemical Reactions
11.3 Scavenging Wind and Solar Energy in Cities
11.4 Thermoelectric Paper Utilizes Waste Heat to Power Electronics and Sensors
11.5 Reclaiming Energy from Thermal Waste
11.6 Generating Electricity from Water Evaporation
11.7 Harvesting Water Energy with a Wearable Triboelectric Generator
11.8 Powering Piezoelectric Nanogenerators with Onion Skin Biowaste
11.9 Piezoelectric Nanogenerators Made from Spider Silk
11.10 Battery-free Electronic Toys
11.11 Self-healing, Highly Stretchable Supercapacitor
11.12 Phosphorene Anchoring Material for Lithium–Sulfur Batteries
11.13 Dredging the Polysulfide ‘Flooding’ in Li–S Batteries
11.14 Nanostructured Conductive Polymer GelsImprove Li-ion Batteries
11.15 The Stressful Life of High-capacity Electrodes
11.16 Nanostructured Phosphides for Efficient Water Splitting
11.17 Fully Stretchable Sodium-ion Batteries
Part 4
Chapter 12 - For Nanoengineers, Anything Goes
12.1 Introduction
12.2 Sculpting Liquids
12.3 DNA Molds Template Nanoparticles into 2D Patterns
12.4 Kirigami Nanofluidic Devices
12.5 Towards Carbon Nanotube-based Quantum Devices
12.6 Manipulating Colloids with Mobile Nanotweezers
12.7 Large-scale Synthesis of Metal Nanoclusters with Thermal Shocks
12.8 Replicating Nacre with Nanomimetics
12.9 Turning Hair into a Biomedical Nanomaterial
12.10 Three-terminal Nanoelectro mechanical Switch Breaks Performance Records
12.11 ‘Whispering Gallery’ Modes Control Artificial Atoms for Quantum Computing
12.12 Exploring Applications of Quasicrystals at Small Scales
12.13 Nanoengineered Surfaces Prevent Frost Formation
12.14 Spatially Controlling the Formation of Ice
12.15 A True Random Number Generator Based on Carbon Nanotubes
12.16 Nanocoating Wiggles Surfaces Clean
12.17 Wood Nanotechnology for Selective Oil/Water Separation
12.18 Highly Tunable Adhesives with Kirigami-inspired Structures
12.19 A Second Skin with Switchable Wettability
12.20 Nanotechnology Coming to a Hair Salon Near You
12.21 Smart Droplets Clean and Repair Surfaces
12.22 How to Transform a Greenhouse Gas into a Nanomaterial
12.23 Designing Safer Metal Oxide Nanoparticles
12.24 Driving a Single Microtubule Just with Light
12.25 Energy-saving Windows Made from Common Glass and Cheap Nanocrystals
12.26 How Cool Is That: Cooling in Color
Subject Index

Citation preview

Nanoengineering The Skills and Tools Making Technology Invisible

     

Nanoengineering The Skills and Tools Making Technology Invisible

By

Michael Berger

Nanowerk LLC, Germany Email: [email protected]

Print ISBN: 978-­1-­78801-­867-­8 PDF ISBN: 978-­1-­83916-­036-­3 EPUB ISBN: 978-­1-­83916-­041-­7 A catalogue record for this book is available from the British Library © Michael Berger 2020 All rights reserved Apart from fair dealing for the purposes of research for non-­commercial purposes or for private study, criticism or review, as permitted under the Copyright, Designs and Patents Act 1988 and the Copyright and Related Rights Regulations 2003, this publication may not be reproduced, stored or transmitted, in any form or by any means, without the prior permission in writing of The Royal Society of Chemistry or the copyright owner, or in the case of reproduction in accordance with the terms of licences issued by the Copyright Licensing Agency in the UK, or in accordance with the terms of the licences issued by the appropriate Reproduction Rights Organization outside the UK. Enquiries concerning reproduction outside the terms stated here should be sent to The Royal Society of Chemistry at the address printed on this page. Whilst this material has been produced with all due care, The Royal Society of Chemistry cannot be held responsible or liable for its accuracy and completeness, nor for any consequences arising from any errors or the use of the information contained in this publication. The publication of advertisements does not constitute any endorsement by The Royal Society of Chemistry or Authors of any products advertised. The views and opinions advanced by contributors do not necessarily reflect those of The Royal Society of Chemistry, which shall not be liable for any resulting loss or damage arising as a result of reliance upon this material. The Royal Society of Chemistry is a charity, registered in England and Wales, Number 207890, and a company incorporated in England by Royal Charter (Registered No. RC000524), registered office: Burlington House, Piccadilly, London W1J 0BA, UK, Telephone: +44 (0) 20 7437 8656. Visit our website at www.rsc.org/books Printed in the United Kingdom by CPI Group (UK) Ltd, Croydon, CR0 4YY, UK

Preface Nanoengineering is a branch of engineering that exploits the unique properties of nanomaterials—their size and quantum effects—and the interaction between these materials, in order to design and manufacture novel structures and devices that possess entirely new functionality and capabilities, which are not obtainable by macroscale engineering. Nanoengineering is not exactly a new science, but rather an enabling platform technology with applications in most industries. While the term nanoengineering is often used synonymously with the general term nanotechnology, the former technically focuses more closely on the engineering aspects of the field, as opposed to the broader science and general technology aspects that are encompassed by the latter. Other closely related terms used in this context are nanofabrication and nanomanufacturing. One possible approach to distinguish between them is by using the criterion of economic viability. The connotations of industrial scale and profitability associated with the word manufacturing imply that nanomanufacturing is an economic activity with industrial production facilities with more or less fully automated assembly lines (think microchip production). By contrast, nanofabrication is more of a research activity in a laboratory environment, based on developing new materials and processes—it's more a domain of skilled craftsmen and not of mass production. One of the most fascinating aspects of nanoengineering is the incredibly small scale at which it takes place. Consider this example: the first working transistor, built by Bell Labs' John Bardeen, Walter Brattain, and William Shockley in 1947, measured roughly 1 centimeter across. Today, logic transistor density has surpassed a staggering 100 million transistors per square

  Nanoengineering: The Skills and Tools Making Technology Invisible By Michael Berger © Michael Berger 2020 Published by the Royal Society of Chemistry, www.rsc.org

v

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Preface

millimeter. That means that the same surface area of Bell Labs' original transistor can now contain more than 10 billion transistors! This book is a collection of essays about researchers involved in nanoengineering and many other facets of nanotechnologies. This research involves truly multidisciplinary and international efforts, covering a wide range of scientific disciplines such as medicine, materials sciences, chemistry, toxicology, biology and biotechnology, physics, and electronics. A total of 176 very specific research projects are showcased and you will meet the scientists who develop the theories, conduct the experiments, and build the new materials and devices that will make nanoengineering a core technology platform for many future products and applications. By catching a glimpse of the wide-­ranging, painstaking, and intricate work that is taking place in these and thousands of other laboratories around the world, you will begin to appreciate that nanotechnology advances are developed not in big leaps but one tiny step at a time. Among many other astonishing inventions and developments, you will read about self-­powered biosensor contact lenses; electronics, including the batteries that power them, invisibly woven into garments; cyborg microfilters that clean contaminated water; nanogenerators that convert breathing, heart beats, or blood flow into electricity; and tiny sensors that can analyze the inside of a living cell. A common aspect of many of the nanoengineered elements underlying these feats is their scale: they are not visible to the naked eye. For all intents and purposes, this technology, powerful as it may be, has become invisible to us. It is also quite amazing how much of nanotechnology-­related research is inspired by nature's designs. As a matter of fact, nature is full of examples of sophisticated nanoscopic architectural feats. Whether it is structural colors; adhesion; porous strength; or bacterial navigation and locomotion—they underpin the essential functions of a variety of lifeforms, from bacteria to berries, wasps to whales. Each of the following stories from the wide field of nanoengineering is based on a scientific paper published in a peer-­reviewed journal. Although each chapter revolves around scientists who were interviewed for this book, many, if not most, of the scientific accomplishments covered here are the result of collaborative efforts by several scientists and research groups, often from different organizations and different countries. These stories take you on a journey of scientific discovery into a world so small that it is not open to our direct experience. While our five senses are doing a reasonably good job at representing the world around us on a macroscale, we have no existing intuitive representation of the nanoworld, ruled by laws entirely foreign to our experience. This is where molecules mingle to create proteins; where you wouldn't recognize water as a liquid; and where minute morphological changes would reveal how much ‘solid’ things such as the ground or houses are constantly vibrating and moving.

Preface

vii

You will catch a glimpse of how diverse, wide-­ranging, and intriguing this research field is and what kind of amazing and exciting materials and applications nanoengineers have in store for us. Some stories are more like an introduction to nanotechnology, some are about understanding current developments, and some are advanced technical discussions of leading edge research. Reading this book will shatter the monolithic terms nanotechnology and nanoengineering into the myriad of facets that they really are. Michael Berger

     

Contents Part 1 : The Flatlands of the Nanoverse Chapter 1 The World of Graphene 

3



3



1.1 Introduction  1.2 Graphene Transfer Using Off-­the-­shelf Office Equipment  1.3 Engineering High-­tech Composite Materials  1.4 ‘Stitching’ Together Ultrastrong Nanosheets  1.5 Graphene Aerogel Walls  1.6 Whispers About Graphene's Electrical Properties  1.7 Defect-­free Graphene Could Solve the Dendrite Problem in Batteries  1.8 Printing Electronics with Highly Conductive Inks  1.9 Self-­healing Electronic Tattoos  1.10 Stick-­on Skin Biosensors  1.11 Implant for Deep Brain Imaging  1.12 Graphene Bubbles Enhance Photoacoustic Imaging  1.13 Real-­time Monitoring of Insulin  1.14 Self-­powered Biosensor Contact Lens  1.15 Solar-­driven Water Purification with Multifunctional Papers  1.16 Optoelectronics on Regular Paper  1.17 Graphene Origami Folded with ‘Colors’  1.18 Transparent Electrodes for Liquid Crystal Displays 

  Nanoengineering: The Skills and Tools Making Technology Invisible By Michael Berger © Michael Berger 2020 Published by the Royal Society of Chemistry, www.rsc.org

ix

4 6 8 11 12 13 16 18 20 21 24 26 28 30 33 35 37

Contents

x



1.19 Ultrathin Encapsulation for Organic Light-­emitting Diodes  1.20 How to Directly Measure the Surface Energy of Graphene  1.21 A 2D Electron Microscope  1.22 Water Slippage in Graphene Nanochannels  1.23 Light-­induced Active Ion Transport Through Graphene Membranes  1.24 Membranes for Large-­scale Energy Storage Systems  1.25 Protecting Against Electrostatic Discharging Failures  1.26 Inspiration from Spider Webs  1.27 Graphene Quantum Dots Made from Agricultural Waste  1.28 Outstanding Thermal Conductivity of Graphene Composites  Chapter 2 The Growing Landscape of Two-­dimensional Materials 



2.1 Introduction  2.2 High-­performance Synthesis of 2D Metal Oxides and Hydroxides  2.3 Flexible, Low-­power, High-­frequency Nanoelectronics  2.4 Large-­yield Synthesis of 2D Antimonene Nanocrystals  2.5 Freestanding Borophene Synthesized for the First Time  2.6 2D Spacer Materials for Surface Plasmon Coupled Emission Sensing  2.7 2D Oxides Juice-­up Sodium-­ion Batteries  2.8 Holey 2D Nanosheets for Efficient Energy Storage  2.9 Atomristor: Memristor Effect in Atomically Thin Nanomaterials  2.10 Photostriction of Molecular 2D Nanosheets  2.11 A Nano Squeegee to Clean Nanosheets  2.12 Studying Strain Effects in 2D Materials Using Kelvin Probe Microscopy  2.13 Optothermoplasmonic Patterning of 2D Materials  2.14 Let's Do the Twist: Rotation-­tunable 2D Electronics 

38 40 43 45 47 50 52 54 57 58 61 61 61 63 64 66 68 69 71 74 76 78 80 82 83

Contents

xi

Chapter 3 Not Found in Nature: Metamaterials and Metasurfaces 

86



86 88 90 92



3.1 Introduction  3.2 Bottom-­up Assembled Chiral Metamolecules  3.3 Ultrathin Plasmonic Chiral Metamaterials  3.4 Large-­area Tunable Metasurfaces  3.5 How to Realize Metasurfaces in Novel Plasmonic Materials  3.6 Full-­color 3D Metaholography  3.7 Van der Waals Heterostructures with Tunable Interfacial Coupling  3.8 A Rewritable Metacanvas for Photonic Applications  3.9 Dynamic Plasmonic Pixels  3.10 Improving Terahertz Detection with Metasurfaces 

95 96 98 100 102 104

Part 2 : Nanotechnology Unleashed Chapter 4 Plasmonics 

109



109 110



4.1 Introduction  4.2 Naked-­eye Plasmonic Colorimetry  4.3 Monitoring UV Exposure with a Tunable Adhesive Patch  4.4 Nanosensor Gels Detect Therapeutic Levels of Radiation  4.5 Plasmonics in the Clouds  4.6 Reversible Assembly of Plasmonic Nanoparticles  4.7 Biofoam Beats Conventional Plasmonic Surfaces  4.8 Black Gold Maximizes the Light Absorption of Nanomaterials  4.9 Nanopatterning Holograms onto Commercial Contact Lenses  4.10 Multiple Electromagnetic Responses from Accordion-­like Plasmonic Nanorods 

112 114 116 117 119 121 123 125

Chapter 5 Nanobiotechnology 

128



128



5.1 Introduction  5.2 An Alternative to Antibiotics: Weakening the Grip of Superbugs  5.3 Cell Sex Impacts the Biological Uptake of Nanoparticles 

129 131

Contents

xii



5.4 Early Cancer Detection with Protein Corona ‘Fingerprints’  5.5 Micromotors Deliver Drug Payloads in the Gastrointestinal Tract  5.6 Titanium Implant Material with Multifunctional Nano–Bio Interface  5.7 All-­natural Nanobiotechnology as an Alternative to Synthetic Agrochemicals  5.8 Advanced Protein Design Drives Complex Nano-­assemblies  5.9 Towards Self-­powered, Brain-­linked e-­Vision  5.10 ‘Cyborg’ Microfilter Actively Cleans Contaminated Water  5.11 Growing Bone and Cartilage Tissue from Nanosilicates  5.12 Fabricating Tissue Engineering Scaffolds via Controlled Ice Crystallization  5.13 Augmenting Nerve Regeneration 

132 134 136 138 140 144 146 149 152 154

Chapter 6 Nanomedicine 

156



156



6.1 Introduction  6.2 Bacteria-­produced Nanoparticles Kill Cancer Cells  6.3 Light-­triggered Local Anesthesia  6.4 Repairing the Cancer Cell Suicide Mechanism  6.5 Drug-­loaded Nanobullets Fired from Microcannons  6.6 Iron Oxide Nanoparticles Inhibit Tumor Growth  6.7 Replacing Animal Models with Biomimetic Blood–Brain Barrier Models  6.8 Controlled Release of Hydrogel from Nanotubes  6.9 Piezoelectric Platform Inhibits Cancer Cell Proliferation  6.10 Combating Opioid Drug Abuse  6.11 Defining the Immunological Effects of Nucleic Acid Nanoparticles  6.12 Testing Nanomedicine in Space  6.13 The Impact of Nanoparticle Design on Parkinson's Disease Therapies  6.14 Metal–Organic Frameworks Enhance Sonodynamic Cancer Therapy  6.15 Magnetically Propelled MOFBOTs Perform Microrobotic Drug Delivery  6.16 MOF-­encapsulated DNA for Gene Therapy 

157 158 160 161 163 164 167 169 172 175 177 179 181 182 184

Contents

xiii

Chapter 7 Characterization 

187



187



7.1 Introduction  7.2 Using ‘Big Data’ to Shed Light on the Complexity of Nanomaterials  7.3 Helical Nanopropellers Measure Local Viscosity  7.4 New Phenomena in Ultrasonic Scattering in Nanofluids  7.5 Surface Area as a Highly Relevant Dose Metric for Nanotoxicity Assessments  7.6 AFM Imaging and Characterization of Nematodes in Their Natural Environment  7.7 Accurate Simulations at the Nanoscale Depend on Appropriate Interatomic Potentials  7.8 How to Detect Biological Contamination of Nanoparticles  7.9 Nanophotonic AFM Probe Provides Ultrafast and Ultralow Noise Detection 

187 190 192 196 198 200 202 204

Part 3 : Engineering at the Nanoscale Chapter 8 Fabrication Moves to the Printer 

209



210



8.1 Introduction  8.2 Highly Conductive Nanocomposites for the 3D Printer  8.2.1 Nanotribological Printing  8.3 3D Printing of Living Responsive Devices  8.4 Customized Design of Anticounterfeiting Marks  8.5 Next-­generation Inkjet Color Printing  8.6 3D Printing with Nanoengineered Bioinks  8.7 Why Burn Bagasse When You Can 3D Print Its Nanocellulose?  8.8 Nanocellulose Dramatically Improves the 3D Printability of Carbon Nanotubes  8.9 Miniature 3D Printed High-­performance Heaters  8.10 Molecular Printing with Light-­actuated Pens  8.11 Scanning Probe + Microbeads = Low-­cost, High-­resolution Optical Lithography  8.12 Printed Decal Electronics for the Internet-­of-­Things  8.13 Taking Ice Lithography to the Next Level  8.14 Nanotechnology in a Bubble  8.15 Writing Nanotubes with a Nano Fountain Pen 

210 212 214 217 219 220 223 224 226 228 230 233 236 239 240

Contents

xiv

Chapter 9 (Mostly Wearable) Electronics 

243



243 243 246 248



9.1 Introduction  9.2 Skin-­inspired Haptic Memory Device  9.3 Self-­powered Analog Smart Skin  9.4 Moisture-­powered Electronics  9.5 Reversibly Controlling the Learning Properties of Memristors  9.6 Measuring Sunlight Exposure with Stick-­on Epidermal Electronic Tattoos  9.7 Batch Assembly of Reconfigurable, Multimodal 3D Electronics  9.8 Green and Flexible Protein-­based Electronics  9.9 Stretchable Nanogenerators for Wearable Health Monitors  9.10 Power Up Your T-­shirt with Print Designs  9.11 Mission Impossible: Remote Destruction Capability of Silicon Electronics  9.12 A Solid-­state Nanopore Platform for Digital Data Storage  9.13 Eminently Wearable Laser-­induced Graphene Composites 

249 253 255 257 259 261 263 265 268

Chapter 10 Sensors and Diagnostics 

271



271



10.1 Introduction  10.2 Keeping a Food Allergen Detector on Your Key-­chain  10.3 Disposable Biosensors Made from Newspaper  10.4 Nanocurve-­based Sensor Reads Facial Expressions  10.5 En Route to Artificial Retinas  10.6 Voltage-­activated Carbon Monoxide Sensor  10.7 Nature-­inspired Skin-­mimicking Sensors  10.8 Multiplexed Planar-­array Analysis from Within a Living Cell  10.9 Real-­time Detection of Individual Viruses in Complex Solutions  10.10 Nanotube Chip Captures and Analyzes Circulating Tumor Cells in Blood  10.11 Flexible, Implantable Nutrient Sensors Based on Metal–Organic Frameworks  10.12 Using Household Items to Make Multisensory ‘Paper Skin’  10.13 DNA Walkers Amplify Molecular Fluorescent Signals 

271 274 275 277 279 281 283 286 288 291 292 295

Contents



xv

10.14 Optical Voltage Nanosensors Created with DNA Origami  10.15 Self-­propelled Swimming Nanodiamonds  10.16 Iontronic Sensing Paper: A New Touch for Pressure Sensors  10.17 Brain-­on-­a-­chip Engineered on Nanowire Scaffolds 

297 299 301 302

Chapter 11 Energy Harvesting and Storage 

305



305



11.1 Introduction  11.2 ‘Artificial Leaf’ Uses Visible Light to Accelerate Chemical Reactions  11.3 Scavenging Wind and Solar Energy in Cities  11.4 Thermoelectric Paper Utilizes Waste Heat to Power Electronics and Sensors  11.5 Reclaiming Energy from Thermal Waste  11.6 Generating Electricity from Water Evaporation  11.7 Harvesting Water Energy with a Wearable Triboelectric Generator  11.8 Powering Piezoelectric Nanogenerators with Onion Skin Biowaste  11.9 Piezoelectric Nanogenerators Made from Spider Silk  11.10 Battery-­free Electronic Toys  11.11 Self-­healing, Highly Stretchable Supercapacitor  11.12 Phosphorene Anchoring Material for Lithium–Sulfur Batteries  11.13 Dredging the Polysulfide ‘Flooding’ in Li–S Batteries  11.14 Nanostructured Conductive Polymer Gels Improve Li-­ion Batteries  11.15 The Stressful Life of High-­capacity Electrodes  11.16 Nanostructured Phosphides for Efficient Water Splitting  11.17 Fully Stretchable Sodium-­ion Batteries 

306 308 309 311 312 314 316 317 319 321 323 324 327 329 331 333

Part 4 Chapter 12 For Nanoengineers, Anything Goes 

337



337 337



12.1 Introduction  12.2 Sculpting Liquids  12.3 DNA Molds Template Nanoparticles into 2D Patterns  12.4 Kirigami Nanofluidic Devices 

339 342

Contents

xvi



12.5 Towards Carbon Nanotube-­based Quantum Devices  12.6 Manipulating Colloids with Mobile Nanotweezers  12.7 Large-­scale Synthesis of Metal Nanoclusters with Thermal Shocks  12.8 Replicating Nacre with Nanomimetics  12.9 Turning Hair into a Biomedical Nanomaterial  12.10 Three-­terminal Nanoelectromechanical Switch Breaks Performance Records  12.11 ‘Whispering Gallery’ Modes Control Artificial Atoms for Quantum Computing  12.12 Exploring Applications of Quasicrystals at Small Scales  12.13 Nanoengineered Surfaces Prevent Frost Formation  12.14 Spatially Controlling the Formation of Ice  12.15 A True Random Number Generator Based on Carbon Nanotubes  12.16 Nanocoating Wiggles Surfaces Clean  12.17 Wood Nanotechnology for Selective Oil/Water Separation  12.18 Highly Tunable Adhesives with Kirigami-­inspired Structures  12.19 A Second Skin with Switchable Wettability  12.20 Nanotechnology Coming to a Hair Salon Near You  12.21 Smart Droplets Clean and Repair Surfaces  12.22 How to Transform a Greenhouse Gas into a Nanomaterial  12.23 Designing Safer Metal Oxide Nanoparticles  12.24 Driving a Single Microtubule Just With Light  12.25 Energy-­saving Windows Made from Common Glass and Cheap Nanocrystals  12.26 How Cool Is That: Cooling in Color 

Subject Index 

344 346 347 349 351 353 354 356 359 361 362 364 366 367 369 371 373 375 376 378 381 383 386

Part 1 The Flatlands of the Nanoverse

           

Chapter 1

The World of Graphene 1.1 Introduction Since graphene was isolated for the first time in 2004, more than 200 000 scholarly articles have been published and over 30 000 patents have been filed with regard to this material, a testament to the enormous interest from both scientific and commercial communities. Graphene research has evolved into a vast field, with huge scientific and financial efforts (the European Graphene Flagship project alone has a 10-­year budget of €1 billion) going into the study of its synthesis and fundamental properties, as well as the development of commercial applications. In the past few years, it has become clear that graphene is not at all the only two-­dimensional (2D) supermaterial (they are called 2D because they extend in only two dimensions: length and width; as the material is only one-­ atom thick, the third dimension, height, is considered to be zero). More than 2500 other layered, atomically thin, materials have already been identified, ready to be studied and exploited. While these materials cover an amazing range of electrical, chemical, optical, and mechanical properties, perhaps the most astounding discovery is that they can be combined freely to create altogether new materials, so-­called van der Waals heterostructures. By stacking together any number of atomically thin layers, it becomes possible to create novel metamaterials and devices otherwise not achievable by traditional three-­dimensional (3D) bulk materials. Since all atoms and molecules attract each other by the ubiquitous van der Waals forces, there are virtually no limitations to how all these new, superthin materials can be assembled into stacks—akin to atomically thin LEGO™ blocks.

  Nanoengineering: The Skills and Tools Making Technology Invisible By Michael Berger © Michael Berger 2020 Published by the Royal Society of Chemistry, www.rsc.org

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Graphene is a flat honeycomb lattice made of a single layer of carbon atoms, which are held together by a backbone of overlapping sp2 hybrid bonds. This nanocrystal is a basic building block for other graphitic materials (nanotubes, fullerene, graphite). Since a typical carbon atom has a diameter of about 0.33 nm, there are about 3 million layers of graphene in 1 mm of graphite. Graphene's striking physical, electronic, and chemical properties originate from the 2D electron confinement within this one-­atom-­thick layer. Graphene has emerged as one of the most promising nanomaterials because of its unique combination of superb properties: it is not only one of the thinnest but also one of the strongest materials; it conducts heat better than all other materials; it is a superb conductor of electricity; it is optically transparent, yet so dense that it is impermeable to gases—not even helium and hydrogen, the smallest gas atoms, can pass through it. Although it is counterintuitive to our idea of ‘perfection equals best performance’, defects in graphene can provide an advantage for certain applications. Removing a carbon atom from where it is supposed to be and/or adding other atoms to the graphene structure can open numerous possibilities to improve the properties of graphene—a process called tuning. For instance, poking holes into graphene to create holey graphene can change the microscopic distribution of electrons and thereby increase the quantum capacitance of graphene by at least fourfold. These amazing properties, and its multifunctionality, make graphene  suitable for a wide spectrum of applications, ranging from electronics to optics, sensors, membranes, coatings, and biodevices.

1.2 Graphene Transfer Using Off-­the-­shelf Office Equipment One of the most common ways of producing high-­quality graphene in sizeable quantities is by chemical vapor deposition (CVD) of methane gas onto a metallic substrate, usually copper. In order for the graphene to then be used in its intended application, it needs to be transferred from the growth substrate to a target substrate—a challenging but extremely important process step. This is typically done by spin-­coating a supporting polymer layer and then chemically dissolving away the copper to release the graphene film from the substrate. Although CVD is an industrially scalable process, the transferred graphene produced in this way is prone to ripping and tearing, as well as contamination from the chemical agents used to remove the growth substrate. Electrochemical and dry delamination of CVD-­grown graphene has been demonstrated, but the material still suffers from some processing-­ related contamination.

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“After the growth process, the graphene is firmly attached to a copper foil and while it may well be the world's strongest material, it requires almost nothing to tear or contaminate the graphene,” says Peter Bøggild, a professor at Technical University of Denmark (DTU). “So, it's hard! If you do an image search on ‘graphene transfer’ in Google, it is obvious that researchers have tried hundreds of complicated recipes to solve this problem.” “Imagine moving a piece of kitchen film from one surface to another, without it becoming curled or dirty,” he continues. “It's almost impossible. Then, imagine doing the same with graphene, which is 25 000 times thinner (kitchen film about 10 µm, graphene 0.0003 µm). In 2009, the enthusiasm for discovering how easily the graphene could be grown on copper foil was quickly replaced by the question: how do we move it to where we are going to use it?” Bøggild and a team of researchers from Denmark, China, and Korea, developed a simple method for transferring A4-­size sheets of CVD graphene from copper foils onto a target substrate using a commercially available polyvinyl alcohol (PVA) polymer foil as a carrier substrate and an off-­the-­shelf office laminator (Figure 1.1) This do-­it-­yourself approach requires few tools and low-­cost materials—it is safe and easy enough to be carried out in school physics classes. There are no chemicals involved besides water, no spinner, and no dangerous etchants.

Figure 1.1 (a) Schematic illustration of the PVA lamination transfer process of

graphene: (1) copper foils with as-­grown graphene are oxidized in de-­ ionized water at room temperature for more than 8 hours (overnight); (2) the oxidized foils are subsequently laminated with a commercially available PVA film using a commercial hot-­roll laminator. (3) After a 30-­s bake on a hot plate at 110 °C, the PVA film is mechanically delaminated from the copper surface, taking the graphene film along with it (4). (b) Graphene transferred from a 10 cm × 10 cm copper foil; the dashed lines serve as a visual guide marking the edges of the graphene sheet on PVA film. (c) PVA–graphene film (from panel (b)) compared to a four-­ inch SiO2 on Si wafer. Reproduced from http://dx.doi.org/10.1021/acs. chemmater.8b04196 with permission from American Chemical Society, Copyright 2019.

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“PVA is widely used in industry already. It's non-­toxic, biodegradable and can be washed away using only water,” says Tim Booth, an associate professor at DTU who led the work. “It's an ideal material to base an industrial process on.” “We can transfer monolayer graphene from copper foils using a cheap office laminator in just a few minutes,” explains Abhay Shivayogimath, the paper's first author. “It is fast and easy. The best part is that we show that this method beats the two most common existing transfer methods by lengths. It is a process that anyone can learn in 20 minutes, with an equipment cost of no more than a few hundred dollars.” The team's invention could be quite exciting for graphene producers. Now, they can supply graphene in bulk on water-­soluble backing foil that customers can laminate onto their desired substrate. Customers don't need any special experience, equipment, or chemicals to complete the transfer. The researchers note that this process can also be used to transfer other 2D materials, not just graphene, from metal surfaces. But how about the quality? According to the team, this do-­it-­yourself method beats the other two dominant methods: chemical etching and bubbling transfer. Because only water is used in the whole process, there is virtually no contamination left on the graphene. “We have shown that graphene transferred via PVA lamination has lower and more homogeneous residual doping, higher charge carrier mobility, and fewer transfer-­induced defects as compared to standard chemical etching or electrochemical delamination transfers,” Shivayogimath concludes. “By avoiding any complex processing, the need for catalyst etching and recovery, and the use of organic solvents; and using only non-­toxic and biodegradable materials, we have demonstrated a process which is greener, lower cost, highly scalable and more convenient for graphene growers and end-­users than existing techniques, and has considerable potential for further development and use in research and industry.” Featured scientists: Abhay Shivayogimath; Professors Peter Bøggild (https://bit.ly/2FE6xuZ) and Tim Booth Organization: Technical University of Denmark (DTU), Lyngby (Denmark) Relevant publication: A. Shivayogimath, P. Whelan, D. Mackenzie, B. Luo, D. Huang, D. Luo, et al., Do-­It-­Yourself Transfer of Large-­Area Graphene Using an Office Laminator and Water, Chem. Mater., 2019, 31(7), 2328–2336.

1.3 Engineering High-­tech Composite Materials Carbon fiber-­reinforced plastic (CFRP) composite materials, with their excellent strength and low specific weight properties, are suitable for all kinds of engineering applications, and they have been increasingly replacing conventional materials such as metal sheets. CFRP can be

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found in cars, trains, airplanes, sports equipment, windmill blades, and in many other applications where ‘lightweighting’ is an important aspect of engineering. Of particular interest in this field are graphene-­based composites. Research groups worldwide are working on the development of industrially manufacturable graphene sheets that have high strength and toughness in all sheet directions. High-­performance graphene films have many promising applications as lightweight structural materials for a variety of uses, from aircraft and automobile bodies to windmill blades and sports equipment; as flexible conductive materials for increasingly popular portable and wearable electronics; and as electromagnetic interference (EMI)-­shielding materials, for eliminating the EM pollution of diverse electronic devices. “However, improvements in mechanical and electrical properties are needed before inexpensive graphene-­based composites can be used for high-­performance applications,” says Qunfeng Cheng, a professor at Beihang University in Beijing. “Today, the common engineering solution is based on cross-­plied carbon fibers in a polymer resin requiring high-­temperature cure. In contrast, we have demonstrated cross-­linked graphene sheets that are manufacturable from graphene platelets, which are resin-­free, processable at low temperature, and contain less than 10 wt% additives.” “The strength of these graphene sheets in all in-­plane directions matches that of plied carbon fiber composites, and they can absorb much higher mechanical energy before failing than carbon fiber composites that are currently used in diverse commercial products,” Cheng points out. “Today's carbon fiber composites are expensive in part because the carbon fibers are produced at extremely high temperatures, which can exceed 2500 °C. In contrast, our process can use graphite that is cheaply dug from the ground and processed at temperatures below 45 °C.” This advance provides a universal strategy for converting inexpensive graphene platelets into high-­performance polymer-­free graphene sheets for the development of next-­generation lightweight multifunctional materials. These graphene sheets could potentially replace the expensive CFRP fiber composites used in industrial engineering, as well as support new applications in increasingly popular flexible electronics. Unlike the conventional approach for making graphene composites, where graphene platelets are uniformly layered in a host polymer, the researchers fabricated cross-­linked graphene sheets from successive use of π–π and covalent cross-­linking agents. The interplatelet bonding provides bridges between adjacent graphene layers so that no host polymer resin is needed, resulting in polymer-­free nanocomposites. In their experiments, the team demonstrated simultaneous enhancement of tensile strength, durability, electrical conductivity, and electromagnetic interference shielding efficiency, thereby opening the path to the design of

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layered nanocomposite materials using multiple types of quantitatively engineered chemical bonds between nanoscale building blocks. This approach eliminates the need for a polymer matrix. “We were intrigued by natural nacre, also known as mother-­of-­pearl, which gives some seashells their high fracture toughness,” Cheng explains the inspiration for this work. “Nacre is composed of parallel platelets that are bound together by thin layers of organic material, similar to the way bricks in a wall are held together by mortar. Inspired by nacre, we have developed various interfacial bridging strategies for designing layered nanocomposites using multiple types of quantitatively engineered chemical bonds between nanoscale building blocks.” He adds that graphene oxide platelets, having oxygen-­containing groups and sp2-­conjugated areas, provide abundant bonding sites for covalent and π–π bonding agents. Next steps for the team will be to develop high-­performance graphene sheets using different cross-­linking agents, which will meet the requirements for specific applications, such as structural and electronic applications. Another goal is to develop a fabrication process for large-­scale production for commercial applications. “Searching for manufacturable sheets having high strength and toughness in all sheet directions for diverse applications is a continuing challenge,” concludes Cheng. “In addition, multifunctional nanocomposites that exhibit high electromagnetic interference shielding efficiency, smart separation, are mechanical-­adaptive or self-­healing, etc., will be in strong demand. Right now, the main challenge is to find a way to produce high-­ performance graphene nanocomposites on a large scale for commercial applications.” Featured scientists: Professor Qunfeng Cheng (https://bit.ly/2HteuXh); Professor Ray Baughman Organizations: Beihang University, Beijing (China); University of Texas at Dallas, Dallas, TX (USA) Relevant publication: S. Wan, Y. Li, J. Mu, A. Aliev, S. Fang, N. Kotov,  et al., Sequentially bridged graphene sheets with high strength, toughness,  and electrical conductivity, Proc. Natl. Acad. Sci. U. S. A., 2018, 115(21), 5359–5364.

1.4 ‘Stitching’ Together Ultrastrong Nanosheets Introducing graphene sheets into a polymer matrix through solution mixing, melt mixing, and in situ polymerization are common processes used to make graphene nanocomposites. However, these methods usually result in nanocomposite films with poor mechanical and electrical properties due to

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Figure 1.2 Left: photograph of π-­bridged rGO film displaying its flexibility. Middle: high-­resolution scanning electron microscope image of the fracture surface of the film. Right: illustration of the long-­chain π-­bridging. Reproduced from http://dx.doi.org/10.1016/j.matt.2019.04.006, with permission from Elsevier, Copyright 2019.

the low content of graphene, poor dispersion, and weak interfaces between graphene and the polymer matrix. These disadvantages severely limit the practical applications of these nanocomposites. Another approach to assemble graphene into macroscopic graphene nanocomposites has also been developed by the team at Cheng's laboratory at Beihang University. They followed up on their previous research (reported above) with a novel strategy to ‘stitch’ reduced graphene oxide (rGO) nanosheets into ultrastrong, super-­tough, and highly conductive graphene films using only small amounts of cross-­linker. They show that bridging of long-­chain π–π bonding agents between neighboring rGO nanosheets can provide substantial improvements in multiple properties, including tensile strength, toughness, electrical conductivity, EMI shielding capability, and resistance to mechanical damage (Figure 1.2). “Our graphene films not only demonstrate a record tensile strength of almost 1.1 GPa, but exceptional abilities to absorb mechanical energy, transport charge, and shield electromagnetic interference that are comparable to, or even superior to, graphene films annealed at much higher temperatures,” notes Cheng. “Our process uses abundant natural graphite as a raw material at room temperature. This novel strategy can provide an inspiration for converting low-­priced graphite powders into much higher performance macroscopic graphene films for diverse commercial uses in the future.” Cheng notes that numerous high-­performance graphene-­based nanocomposites have been demonstrated through various interface design strategies in the past. “However, these graphene-­based nanocomposites usually show only moderately enhanced mechanical properties at the expense of decreased electrical conductivity due to the addition of insulating cross-­linkers, impeding their practical applications.” In their work, the team demonstrates that long-­chain π-­bridging can simultaneously enhance the tensile strength, toughness, and electrical

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conductivity of graphene films. They found that less than 7 wt% content of π–π bonding agent (polymerized BPDD) delivered the best performance results. The resulting films have record values compared with the previously reported graphene-­based nanocomposites bridged by various interface interactions under ambient conditions. Cheng points out that, compared with other interface design strategies, long-­chain π-­bridging can not only provide highly efficient stress transfer and large slippage space for graphene nanosheets, but also induce alignment of graphene nanosheets, resulting in remarkable electrical conductivity comparable to that of high-­temperature annealed graphene films. The team's strategy of assembling ultrastrong graphene films through long-­ chain π-­bridging could also inspire an avenue for the assembly of graphene nanosheets and other nanoscale building blocks into high-­performance macroscopic nanocomposites using various cross-­linkers. “In addition,” says Cheng, “the strengthening and toughening mechanisms can also give us impetus to overcome the conflict of attaining strength and toughness, two properties that are often mutually exclusive, in structural materials and traditional composites.” There are two key scientific problems to be solved to further improve the performance of graphene films. First, the researchers will endeavor to develop a large-­scale manufacturing technique to allow the integration of this bridging strategy into commercial applications. Second, they will continue to develop higher performance graphene films incorporating interface bridging and structure optimization for uses in extreme environments, such as ultrahigh or ultralow temperature. The properties of the team's graphene films are still lower than those of single-­layer graphene nanosheets, so they will continue their search for solutions to further enhance the tensile strength, toughness, and electrical conductivity of macroscale graphene films for many applications. In addition, although these graphene films show high performance at room temperature, their performance in extreme environments, such as outer space with intense radiation, and frequent variations of high and low temperatures, needs to be investigated. “One main challenge is to precisely design interface interactions between adjacent microscale graphene nanosheets to achieve a tensile strength higher than current carbon fiber-­reinforced nanocomposites,” Cheng concludes.

Featured scientist: Professor Qunfeng Cheng (https://bit.ly/2HteuXh) Organization: Beihang University, Beijing (China) Relevant publication: S. Wan, Y. Chen, Y. Wang, G. Li, G. Wang, L. Liu,  et al., Ultrastrong Graphene Films via Long-­Chain π-­Bridging, Matter,  2019, 1(2), 389–401.

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1.5 Graphene Aerogel Walls When graphene sheets are neatly stacked on top of each other and formed into a three-­dimensional shape, it becomes graphite, commonly known as the lead in pencils. Because graphite is simply packed-­together graphene, it has fairly poor mechanical properties. But if the graphene sheets are separated with air-­ filled pores, the three-­dimensional structure can maintain its properties. This porous graphene monolith structure is called a graphene aerogel. If the gas phase is replaced by a liquid, without the graphene network losing its stability, then we speak of a hydrogel. Highly compressible graphene aerogels possess extraordinary properties that exceed the performance of natural materials—superior compressive elasticity; ultrahigh porosity; outstanding tolerance for harsh environment; large specific surface area; high electrical and thermal conductivity. These aerogels have shown great potential as multifunctional frameworks to be applied in such areas as high-­performance damping, large strain sensors, fast oil–water separation, flame retardation, thermal insulation, electromagnetic interference shielding, and sound absorption. However, very low mechanical strength and limited macroscale have hampered the practical applications of graphene aerogels. A team of researchers from Beijing Institute of Technology and Tsinghua University have developed a novel sol–gel method by introducing air bubbles and ice crystals as dual templates to prepare graphene hydrogels that can be air-­dried without shrinkage. The air bubbles are introduced in the graphene oxide solutions as the soft template, to produce the isotropic large pores in the reduced graphene hydrogels. The ice crystals impale the graphene walls to introduce many interconnected channels among the isotropic large pores. The team employs a facile wet-­press assembly strategy to fabricate ultrastrong, superelastic graphene aerogels with infinite macroscale. After the graphene hydrogel is prepared, it is compressed in order to squeeze out the air bubbles and solution through the interconnected channels in the gel. The remaining structure is then dried in air to enhance the stiffness of the 3D framework. “The dried aerogel doesn't show any obvious volume shrinkage, a result that we attribute to the framework stiffness of the gels, which could resist the solvent evaporation capillary pressure,” notes Professor Liangti Qu from the Beijing Institute of Technology, who led this work. “An unlimited number of these graphene aerogel ‘bricks’ can be assembled together during the pressing process, much like building a wall with LEGO™ bricks.” The researchers demonstrated that the combined graphene aerogels form a highly oriented, dense, multiple-­arch microstructure that possesses arbitrary macroscale, outstanding compressive strength (47 MPa, over ten times higher than the best ever reported), superelasticity (>97% strain), and high conductivity (378 S m−1).

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Qu notes that this ultrastrong, superplastic graphene aerogel could find applications as advanced multifunctional structural materials for buildings, spacecraft, and vehicles. “Our next steps will include investigations with regard to extending their fabrication strategy to other graphene-­based hybrid and composite aerogels and improving and expanding the functionality of the aerogels,” he says. “We also would like to develop more facile and energy-­ efficient fabrication methods in order to make this process attractive for commercialization.” Featured scientist: Professor Liangti Qu (https://bit.ly/2GZMloi) Organizations: Beijing Institute of Technology; Tsinghua University,  Beijing (China) Relevant publication: H. Yang, Z. Li, G. Sun, X. Jin, B. Lu, P. Zhang, et al., Superplastic Air-­Dryable Graphene Hydrogels for Wet-­Press Assembly of Ultrastrong Superelastic Aerogels with Infinite Macroscale, Adv. Funct. Mater., 2019, 29(26), 1901917.

1.6 Whispers About Graphene's Electrical Properties A whispering gallery is an acoustic phenomenon that architects have used for centuries when building cathedrals. Sound waves travelling around the curved walls of a dome, such as the one in St. Paul's Cathedral in London, allow a person standing near one part of the wall to easily hear a faint sound originating at any other part of the wall. Similarly, but much more recently, researchers have built whispering galleries for light, which are nowadays found in applications ranging from sensing, spectroscopy, and communications to the generation of laser frequency combs or to boost the performance of solar cells. Yet another—but nanoscale—electronic analog of this classical wave effect has been created in graphene where it acts as a circular wall of mirrors for electrons. These whispering galleries for graphene electrons open the way to building devices that focus and amplify electrons, just as lenses focus light and resonators (like the body of a guitar) amplify sound. A research team from the O.Ya.Usikov Institute for Radiophysics and Electronics, Kharkiv, Ukraine and the Institute of Bioelectronics (ICS-­8), Forschungszentrum Jülich, Germany, reports the experimental results of studying the response characteristics in a millimeter waveband, whispering gallery mode sapphire resonator to single-­layer graphene at different distances of graphene from the resonator. Instead of whispering words (vibrations with a frequency range of 200–600 Hz), the scientists used this millimeter-­sized whispering gallery to send out electromagnetic waves of rather large frequencies (about 40 GHz) and excite the whispering gallery modes (WGM) in sapphire disks.

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“Interestingly, we found that the evanescent field of the resonance system is extremely sensitive to the environment, surrounding materials and disk coatings,” says Dr Dmitry Kireev, one of the authors of a report on this research. “A series of experiments showed that such WGM resonance can be used to detect the conductivity and thickness of the materials deposited on top, and/or can even be used for advanced biosensing.” In order to test the level of detection, the scientists decided to explore single-­layer graphene material using the WGM technique. They found that the resonator system is indeed responsive to the presence of graphene. Moreover, resonance dumping is more pronounced when the graphene is in direct contact with the system, and less pronounced when the graphene is slightly apart from the resonator. “This confirms that part of the resonance energy is absorbed by graphene,” explains Kireev. “A series of experiments confirmed that the contactless WGM resonant technique allows detecting the electrical conductivity of graphene with precision comparable to a standard DC conductivity measured through steady-­state electrical measurements.” This means that the resonator technique can be used to reveal the conductivity/sheet resistance of graphene in the millimeter waveband in a contactless and non-­invasive way. Moreover, the demonstrated WGM resonator technique paves the way for contactless studies of other one-­atom-­thick materials in a wide range of wavelengths, including sub-­millimeter waves. Featured scientist: Dr Dmitry Kireev (https://bit.ly/2CoSSr2) Organizations: O.Ya.Usikov Institute for Radiophysics and Electronics, Kharkiv (Ukraine); Institute of Bioelectronics (ICS-­8), Forschungszentrum Jülich, Jülich (Germany) Relevant publication: A. Barannik, N. Cherpak, I. Protsenko, A. Gubin, D. Kireev and S. Vitusevich, Contactless exploration of graphene properties using millimeter wave response of WGM resonator, Appl. Phys. Lett., 2018, 113(9), 094102.

1.7 Defect-­free Graphene Could Solve the Dendrite Problem in Batteries Rechargeable lithium-­ion (Li-­ion) batteries are the dominant technology not only for portable electronics but also as the battery of choice for electric vehicles and electric grid energy storage applications. In a Li-­ion battery, the cathode (positive electrode) is a lithium metal oxide, while the anode (negative electrode) is graphite. Researchers are looking for ways to replace graphite with lithium metal as the anode to boost the battery's energy density.

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Lithium metal-­based batteries such as Li–sulfur and Li–air batteries have received considerable attention because the packing density of lithium atoms is highest in its metallic form and Li metal can store ten times more energy than graphite. However, there are safety and performance concerns for these types of batteries that arise from the formation of dendrites on the metal electrodes; an issue that has been known about and investigated since the 1960s. These dendrites form when metal ions accumulate on the surface of the battery's electrode, as it degrades during the charging process. Dendrites are often responsible for the highly publicized violent battery failures that are reported in the news. When these branch-­like filament deposits elongate sufficiently to penetrate the barrier between the two halves of the battery, they can cause electrical shorts, overheating, and fires. They also cause significant cycling efficiency losses. To avoid dendrites, researchers are experimenting with new battery electrolyte chemistries, new separator technologies, and new physical hosts for the lithium metal. Carbon hosts, in particular, are very promising, since they may be added to the anode with little additional cost and minimal modification of the manufacturing process, and they are becoming an important way to stabilize Li metal anodes. However, there are seemingly contradictory findings reported in hundreds of prior publications on the subject. The hosts, which are predominantly made from various nanostructured carbons such as graphene, are, in some cases, very effective in eliminating dendrites. In other cases, they don't work at all, or actually make the dendrite problem worse. The design of such host systems has been largely Edisonian: researchers use a trial-­and-­error approach to find an architecture/structure that works better than the rest. An international research team from Sichuan University in China and Clarkson University in the USA has discovered a key design rule for Li metal batteries: if you want to suppress dendrites, you have to use a defect-­free host. More generally, carbon defects catalyze dendrite growth in metal anodes. These findings address the major scientific problem of explaining how the structure and chemistry of the carbon per se affects dendrite growth (Figure 1.3). “We discovered a critical and unexpected relationship between the host (graphene) chemical/structural defectiveness and its ability to suppress dendrites,” explains Professor David Mitlin, who led the work. “To do this, we created what may be the world's most pure and ordered graphene and compared it to a standard graphene based on reduced graphene oxide. Using such opposite materials provided unique and fundamental insight into the way lithium dendrites form and grow.”

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Figure 1.3 Left half: defect-­free graphene protecting the lithium metal anode from the electrolyte. Right half: defective graphene catalyzing dendrite growth. (Image: Mitlin Research Group, Clarkson University).

“The key finding, which will rationally guide future lithium battery design efforts, is that the carbon defects are themselves catalytic for dendrite growth,” points out Professor Wei Liu from Sichuan University's Institute of New Energy and Low Carbon Technology, who collaborated with Mitlin in this research. “Much of the ‘damage’ to the anode ultimately responsible for the dendrites occurs even before the battery is fully charged for the first time. Defects in the carbon host corrode the electrolyte at low voltages, leading to early dendrite formation.” The team hypothesized that it was the host structure/chemistry that mattered, but needed to create ideal model systems to test the hypothesis. Professor Wei Liu's unique flow assisted sonication approach allowed them to create nearly defect-­free graphene. Literally, such oxygen-­free and structural defect-­free graphene has never been synthesized before by a wet chemistry method. This graphene is 1–3 atomic layers thick and with only about 1.5% oxygen. This is much purer than the typical 8% or more oxygen found in most graphene materials. “It served as a perfect test bed to explore our hypothesis,” says Liu. “Without such a pristine structure, it would not have been possible to obtain the conclusive answers to the dendrite growth problem.” He emphasizes that this, in itself, is a transformative accomplishment for the carbon and energy communities, since previously only vapor deposition could obtain such ideal defect-­free structures. The team then compared their defect-­free graphene to a standard, highly defective Hummers graphene baseline found in the literature. “A direct one-­to-­one comparison allowed us to obtain unique insight into the role of carbon defects on Li dendrite growth,” says Mitlin. “A

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critical new finding is that solid electrolyte interphase (SEI) formation occurring before metal plating actually dictates dendrites. The fate of the Li metal anode is in effect sealed once the carbon host forms SEI at the initial charge!” Going forward, the researchers plan to commercialize defect-­free graphene host materials for next-­generation lithium batteries. They also want to further understand the complex relationship between carbon defects and metal dendrites by examining carbons with tuned structure/chemistry for lithium, sodium, and potassium batteries. Featured scientists: Professors David Mitlin (https://bit.ly/2VT7RjK) and Wei Liu Organizations: Clarkson University, Potsdam, NY (USA); Sichuan University, Chengdu (China) Relevant publication: W. Liu, Y. Xia, W. Wang, Y. Wang, J. Jin, Y. Chen,  et al., Pristine or Highly Defective? Understanding the Role of Graphene Structure for Stable Lithium Metal Plating, Adv. Energy Mater., 2018, 9(3), 1802918.

1.8 Printing Electronics with Highly Conductive Inks Numerous research efforts have already demonstrated the feasibility of fabricating graphene-­based electronics through high-­throughput ink printing. However, the electrical conductivity of the printed ink patterns is far from satisfactory. In inkjet printing of graphene, however, the diameter of the printing nozzle regularly defaults to be tens of micrometers; thus, increased graphene sheet sizes beyond the safe range of the nozzle could restrict the printing fluency and cause clogging. “Generally, achieving satisfactory results it is a trade-­off between the sizes of the graphene sheets and the chosen printing strategies,” says Yanlin Song, a professor at the Institute of Chemistry, Chinese Academy of Sciences, and Director of Key Laboratory of Green Printing. “Direct-­ink writing offers an attractive way to break the routine and meet the printability with demanding sheet sizes. The nozzles' diameters range from sub-­micrometer to millimeter scale to accommodate the inks. More importantly, the extrusion-­based procedure plays a crucial role in directing the orientation of graphene sheets to pass through the nozzle during printing.” The team fabricated graphene patterns with ordered microstructure and excellent conductivity. To do this, they realized the assembly of giant graphene oxide (GGO) sheets during the printing process. The synergetic

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effect of printing-­induced orientation and evaporation-­induced interfacial assembly facilitated the formation of laminar-­structured GGO patterns. The resulting reduced graphene oxide (rGO), after chemical reduction, showed remarkable electrical conductivity in printed graphene electronics (up to 4.51 × 104 S m−1—which is among the highest reported in printed graphene electronics). The researchers attribute the high conductivity to the introduction of GGO as the printing ink, as well as the ordered structure from the process of assembly. Song points out that the size effect is also important for the enhancement of the conductivity. “A greater aspect ratio of the building blocks and their better alignment can give rise to a higher electrical conductivity of the printed patterns due to the closer stacking and lower intersheet contact resistance.” A direct-­write printing assembly of GGO could be particularly attractive for application in soft actuators, where higher electrical conductivity contributes to better sensitivity of the devices; consequently, it enables actuation with lower driving voltage. Because of their high conductivity, mechanical flexibility, and advantage in pattern design, printed graphene circuits/electrodes are promising for applications in electronic and optoelectronic devices; sensors and actuators; energy storage and electrochemical devices; and biological applications. “Large-­sized graphene sheets as building blocks exhibit outstanding behavior in vacuum-­filtered graphene films and wet-­spun graphene fibers, but rarely are they adopted in the printing process,” notes Song. “Inspired by the high-­performance results from macroscopic assembled graphene, we anticipate the properties of printed graphene can be tremendously enhanced by combined this material with versatile assembling strategies.” The team also demonstrated the use of their flexible graphene electrodes toward application in electrodriven actuators. The actuators could perform controllable shape change under electric signals. “By tailoring the ink composition and printing parameters, we could easily control the width and thickness of the electrodes,” says Song. “That allowed us to fabricate the actuators with arbitrary geometries, enabling them to achieve complex deformations for application in soft actuators, bionic robots, artificial muscles, smart sensors, etc.” Going forward, the scientists are planning to investigate the assembly of various forms, and the resulting functionality, of the printed graphene-­based materials. Direct-­ink writing is a versatile printing strategy to build up structures of both planar and three-­dimensional shapes, as well to construct patterns for micro devices and large-­area applications.

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Featured scientists: Professor Yanlin Song's research group (https://bit. ly/2OeQWG6) Organization: Key Laboratory of Green Printing, Institute of Chemistry, Chinese Academy of Sciences, Beijing (China) Relevant publication: W. Li, F. Li, H. Li, M. Su, M. Gao, Y. Li, et al., Flexible Circuits and Soft Actuators by Printing Assembly of Graphene, ACS Appl. Mater. Interfaces, 2016, 8(19), 12369–12376.

1.9 Self-­healing Electronic Tattoos Electronic tattoos (e-­tattoos)—or epidermal electronics—are an extremely thin form of wearable electronics. They are lightweight and soft, which allows them to be intimately mounted on human skin for non-­invasive, high-­ fidelity sensing. During the operation of e-­tattoos, they are constantly exposed to external mechanical inputs, such as bending, twisting, pressing, and cutting, which may cause mechanical damage and lead to malfunction. Researchers in China have demonstrated a self-­healing, silk e-­t attoo that shows high sensitivity to multiple stimuli, including strain, humidity, and temperature, based on a unique graphene/silk fibroin combination (Figure 1.4). “We developed our self-­healing, biocompatible, and multifunctional e-­tattoo by incorporating graphene with silk fibroin/Ca2+ films,” explains Yingying Zhang, Associate Professor in the Department of Chemistry and the Center for Nano and Micro Mechanics at Tsinghua University. “We show that custom-­designed and highly flexible e-­tattoos can be facilely prepared through screen printing or direct writing of a graphene/silk fibroin/Ca2+ suspension.” The graphene flakes distributed in the matrix form an electrically conductive path that is responsive to environmental changes, such as strain, humidity, and temperature variations, endowing the e-­tattoo with high sensitivity to multiple stimuli. Remarkably, the e-­tattoo can be healed with an efficiency of 100%, even after being fully fractured, within 0.3 seconds, simply by wetting it with a droplet of water because of the reformation of hydrogen and coordination bonds at the fractured interface. “Based on the superior capabilities of our e-­tattoos, we believe that such skin-­like devices hold great promise for manufacturing cost-­effective artificial skins and wearable electronics,” notes Zhang. The silk fibroin used by the team is a natural protein material produced by silkworms. It has been identified as particularly suitable for wearable electronic applications because of its mechanical durability, tunable secondary structure, all-­aqueous processing, and good biocompatibility.

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Figure 1.4 Schematic illustration showing the fabrication process of the Gr/

SF/Ca2+ e-­tattoo and its stable adhesion on human skin. Reproduced from http://dx.doi.org/10.1002/adfm.201808695 with permission from John Wiley and Sons, © 2019 WILEY-­VCH Verlag GmbH & Co. KGaA, Weinheim.

Despite the large potential of silk fibroin for use as wearable electronics and even e-­tattoos, its applications in wearable electronics are largely limited to its use as a substrate, rather than use as the active functional material. Graphene shows great promise for e-­tattoo applications because it is mechanically robust, electrochemically stable, and biocompatible. Moreover, graphene materials can be functionalized through various physical and chemical approaches, thereby providing plenty of room for tailoring the properties and performance of graphene-­based sensors. “The integration of graphene materials and silk has been explored previously, but the majority of studies focus on mechanical-­enhanced materials,” notes Zhang. “We found that silk/graphene composite materials are highly promising for emerging epidermal electronics.” Her team explored the performance of their e-­tattoo as a strain, humidity, and temperature sensor and found that it can monitor a variety of daily life sensations, including heart rhythm and electrical activity, breathing, and temperature, with high sensitivity, fast response, and long-­term stability. Rather than having to rely on various instruments to monitor different parameters of human health, this opens the possibility of an effective, inexpensive, and simple method for multifunctional monitoring over extended periods of time.

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“Our research group focuses on the scientific, physical, and chemical properties of the preparation of nanocarbon and silk materials, and focuses on the development of new materials for flexible wearable systems,” Zhang concludes. “At present, we mainly focus on carbon nanotubes, graphene, and natural silk protein materials, explore their preparation methods, structure– performance correlation, and develop their applications in the fields of wearable electronics.” “In my view, there are still many challenges to be overcome towards ideal wearable sensors and energy equipment,” she adds. “The ultimate direction in this field is the integration of various wearable electronics into smart systems, which will contribute to the development of medical applications, health care, and artificial intelligence.” Featured scientists: Professor Yingying Zhang's research group (https:// bit.ly/2FmA8dF) Organization: Department of Chemistry and the Center for Nano and Micro Mechanics, Tsinghua University, Beijing (China) Relevant publication: Q. Wang, S. Ling, X. Liang, H. Wang, H. Lu and Y. Zhang, Self-­Healable Multifunctional Electronic Tattoos Based on Silk and Graphene, Adv. Funct. Mater., 2019, 29(16), 1808695.

1.10 Stick-­on Skin Biosensors Graphene is the thinnest electrically conductive material and is optically transparent, mechanically robust, electrochemically stable, and biocompatible—ideal prerequisites for fabricating epidermal electronics or tattoo-­t ype biosensors. In these and similar applications, the total thicknesses of the graphene-­ based sensors are tens or hundreds of micrometers, which is too large to fully conform to the skin, like a real tattoo, and act as electronic skin. Moreover, while the aforementioned graphene biosensors were successfully patterned by photolithography, the process can be costly and time consuming. A team, led by Professors Deji Akinwande and Nanshu Lu, has developed a stretchable and transparent graphene-­based electronic tattoo (GET) sensor that is only hundreds of nanometers thick but demonstrates high electrical and mechanical performance. “We demonstrate that a GET can be fabricated through a simple wet-­ transfer/dry-­patterning process directly on tattoo paper, allowing it to be transferred on human skin exactly like a temporary tattoo, except this sensor is transparent,” says Lu. “Due to its ultrathinness, a GET can fully conform to the microscopic morphology of human skin via just van der Waals interactions, and can follow arbitrary skin deformation without mechanical failure or delamination for an extended period of time.” (See Figure 1.5).

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Figure 1.5 Graphene-­based electronic tattoo mounted on skin. GET on skin compressed (middle) and stretched by 25% (right), respectively. Reproduced from http://dx.doi.org/10.1021/acsnano.7b02182 with permission from American Chemical Society, Copyright 2017.

As dry electrodes, the graphene–skin interface impedance is found to be as low as that between gel electrodes and skin, which guarantees a high signal-­ to-­noise ratio. However, the wet-­transfer/dry-­patterning that the researchers developed can ensure minimal chemical contamination on the graphene surface, which enables a clean graphene–skin contact. In addition to electrophysiological measurements (electroencephalogram, electrocardiogram, electromyogram), the team's fabricated GET is also able to measure skin temperature and hydration. This type of ultrathin graphene electronic tattoo has potential uses in mobile health and fitness monitoring, neonatal monitoring, rehabilitation, athletic training, human–machine interface, etc. Going forward, the researchers plan to make their e-­tattoo go active, i.e., build active electronics such as diodes and transistors using 2D materials on the e-­tattoo. “The ultimate goal is to build all functionalities including amplification, wireless data, and power transmission into ultrathin, ultrasoft e-­tattoos to obtain a complete monitoring system,” concludes Lu. Featured scientists: Professors Deji Akinwande (https://bit.ly/2Ob7EXb) and Nanshu Lu (https://bit.ly/2HxnKK6) Organization: The University of Texas at Austin, Austin, TX (USA) Relevant publication: S. Kabiri Ameri, R. Ho, H. Jang, L. Tao, Y. Wang, L. Wang, et al., Graphene Electronic Tattoo Sensors, ACS Nano, 2017, 11(8), 7634–7641.

1.11 Implant for Deep Brain Imaging Notwithstanding the progress neuroscientists have made in understanding the microscale function of single neurons and the macroscale activity of the human brain—a comprehensive understanding of the brain still remains an elusive goal. Over the past several years, nanoscale analysis tools and the design and synthesis of nanomaterials have generated optical, electrical, and chemical methods that can readily be adapted for use in neuroscience and brain activity mapping.

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“Although advances in optical technologies such as multiphoton microscopy and optogenetics have revolutionized our ability to record and manipulate neuronal activity, integration of optical modalities with electrical recordings is challenging due to generation of light-­induced artifacts,” Duygu Kuzum, an assistant professor of Electrical and Computer Engineering at the University of California, San Diego, points out. Scientists from the laboratories of Professor Kuzum and Professor Anna Devor, developed a transparent, graphene microelectrode, neural implant that eliminates light-­induced artifacts to enable cross-­talk-­free integration of two-­photon microscopy, optogenetic stimulation, and cortical recordings in the same in vivo experiment. This new class of transparent brain implant is based on monolayer graphene. It offers a practical pathway to investigate neuronal activity over multiple spatial scales, extending from single neurons to large neuronal populations. Conventional metal-­based microelectrodes cannot be used for simultaneous measurements of multiple optical and electrical parameters, which are essential for comprehensive investigation of brain function across spatiotemporal scales. Since they are opaque, they block the field of view of the microscopes and generate optical shadows that impede imaging. More importantly, they cause light-­induced artifacts in electrical recordings, which can significantly interfere with neural signals. Transparent graphene electrode technology addresses these problems and allows seamless and cross-­talk-­free integration of optical and electrical sensing and manipulation technologies. In their work, the researchers demonstrate that by careful design of key steps in the fabrication process for transparent graphene electrodes, the light-­induced artifact problem can be mitigated and virtually artifact-­ free local field potential recordings can be achieved within operating light intensities. “Optical transparency of graphene enables seamless integration of imaging, optogenetic stimulation, and electrical recording of brain activity in the same experiment with animal models,” Kuzum explains. “Different from conventional implants based on metal electrodes, graphene-­based electrodes do not generate any electrical artifacts upon interacting with light used for imaging or optogenetics. That enables cross-­t alk-­free integration of three modalities: imaging, stimulation, and recording to investigate brain activity over multiple spatial scales extending from single neurons to large populations of neurons in the same experiment.” The team's fabrication process avoids any crack formation in the transfer process, resulting in a 95–100% yield for the electrode arrays. This fabrication quality is important for expanding this technology to high-­density, large-­area transparent arrays to monitor brain-­scale cortical activity in large animal models or humans. “High optical transmittance of these graphene arrays supports simultaneous two-­photon imaging down to about 1 mm directly beneath the

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transparent microelectrodes,” notes Kuzum. “For the first time, we show that transparent graphene electrodes can be employed for cross-­talk-­free integration of three different modalities, two-­photon imaging, optogenetics, and electrical recordings of cortical potentials at the same time in the same in vivo experiment.” Transparent graphene electrode technology could be directly applied to investigate numerous different questions related to basic neuroscience research and neurological disorders. This technology can also provide a viable complementary alternative to invasive micro-­needle arrays (sharp penetrating electrodes) for multimodal measurements/manipulations within the penetration depth of multiphoton microscopy. In cases where depth-­resolved electrical recordings are not required, optically transparent graphene surface arrays allow seamless integration with depth-­resolved optical imaging modalities, while circumventing the need to insert invasive probes into brain tissue. According to Kuzum, this technology is also well suited for neurovascular and neurometabolic studies, providing a ‘gold standard’ neuronal correlate for optical measurements of vascular, hemodynamic, and metabolic activity. “It will find application in multiple areas, advancing our understanding of how microscopic neural activity at the cellular scale translates into macroscopic activity of large neuron populations,” she says. “Combining optical techniques with electrical recordings using graphene electrodes will allow us to connect the large body of neuroscience knowledge obtained from animal models to human studies mainly relying on electrophysiological recordings of brain-­scale activity,” she adds. Next steps for the team involve employing this technology to investigate coupling and information transfer between different brain regions. Advancements in measurement technology play a critical role in neuroscience, enabling scientific inquiry and powering scientific discovery. This is also the goal of the ongoing BRAIN Initiative. Close collaborations and interactions between engineers and neuroscientists are very important to come up with innovative technologies, such as transparent graphene electrodes, to overcome fundamental limitations of neuroscience research.

Featured scientists: Professors Duygu Kuzum (https://bit.ly/2KkSVuj) and Anna Devor (https://bit.ly/2KiCnTX) Organization: University of California, San Diego, CA (USA) Relevant publication: M. Thunemann, Y. Lu, X. Liu, K. Kılıç, M. Desjardins, M. Vandenberghe, et al., Deep 2-­photon imaging and artifact-­free optogenetics through transparent graphene microelectrode arrays, Nat. Commun., 2018, 9(1), 2035.

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1.12 Graphene Bubbles Enhance Photoacoustic Imaging Researchers have demonstrated that the coupling of pristine graphene sheets on practically any polymer surface can be accomplished in mild reaction conditions and in aqueous medium. The method leaves the 2D planar structure of graphene intact, preserving its original features. This novel hybrid construct enables in vivo photoacoustic signal enhancement and is a very promising step forward for the implementation of photoacoustic imaging (PAI), a powerful preclinical diagnostic tool. Imaging and drug delivery based on miniaturized devices are key for the future of personalized medicine. One of the main issues is the ability to detect diseases in their earliest stages. This increases the chance of success of any therapy. PAI is among the imaging methods with the highest resolution and this allows a less invasive way to detect tumors at very early stages. The targeting is key, both for a localized diagnostic and to bring a drug focally to the diseased tissue. The photoacoustic effect, discovered and studied by Alexander Graham Bell more than 135 years ago, occurs when light hits an absorber and the locally accumulated thermal energy is converted and dissipated as mechanical energy by the emission of ultrasound waves that are detected by a transducer. The light wavelength used in biomedical diagnostics is in the near-­infrared (NIR) spectral window, where light is less attenuated by tissue (and water). Endogenous metabolites, such as the haemoglobin in red blood cells, behave in this way. “This effect is used in photoacoustic imaging and it can be enhanced by exogenous devices as, for example, our hybrid assembly made by the stable coupling of pristine graphene with microbubbles,” explains Gaio Paradossi, a professor in the Dipartimento di Scienze e Tecnologie Chimiche at Università di Roma Tor Vergata. “The efficiency in the enhancement of the photoacoustic signal makes such a device an unprecedented multimodal contrast agent for ultrasound and PAI.” Paradossi and his team reported a proof-­of-­concept technique, tested in vivo, where they fabricated a hybrid injectable device for use as an efficient and versatile photoacoustic contrast agent. The researchers' technique couples pristine graphene with polymer-­ shelled microbubbles. The design is based on PVA polymer microbubbles, which are coupled to pristine graphene sheets through surfactant moieties covalently bound to the available functional groups on the surface of the microbubbles. “At the center of our work is pristine graphene, the intact form of graphene,” Paradossi points out. “Most of the applications reported in the literature highlight the use of graphene oxide (GO) or reduced graphene oxide (rGO). These forms of graphene, not directly obtained by graphite exfoliation,

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derive from chemical modifications of the 2D structure of graphene in very harsh conditions, which introduce kinks and irregularities in the carbonic material.” “Such modifications make GO and rGO more reactive and more processable than pristine graphene, but jeopardize the electrical, optical, and mechanical properties of this material,” he adds. In their work, the researchers present a technique to couple pristine graphene with polymer-­shelled microbubbles. “Why are polymer-­shelled microbubbles such an exotic support for pristine graphene? Microbubbles are the best contrast agents for enhancing ultrasound and it is a natural choice if ultrasound or photoacoustic imaging are the goals,” says Paradossi. “Another important issue pointed out in our paper is the exceptional stability of the coupling to the polymer surface of the microbubbles is an asset for the biocompatibility of graphene.” This work contains several novel elements:     ⁃ The use of pristine graphene leaves its relevant properties unchanged. ⁃ A general strategy for attaching pristine graphene to a large number of hydrophilic polymer surfaces in a stable way using mild conditions and aqueous media. ⁃ The assembly of a truly hybrid system where a hydrophobic moiety, i.e., graphene, is coupled with a hydrophilic moiety, i.e., the PVA-­shelled microbubble, to obtain a novel multifunctional device implementing the potentialities of photoacoustic imaging. ⁃ These results have been a by-­product of the work presently carried out within the frame of the European project TheraGlio (“Developing theranostics for Gliomas”), where the goal is to develop a multimodal imaging system for theranostics (therapy + diagnosis) for patients bearing malignant glioma, the most common primary brain tumor.     Going forward, the team will address the biocompatibility of their graphene microbubbles; the ability to target pathological cells tissues; and ultrasound-­ assisted drug delivery. As for biocompatibility, graphene is anchored to the surface of the PVA-­ shelled microbubble in a stable way and loss of graphenic material was not monitored in physiological media. PVA is already known as a biocompatible polymer and it has been used for the fabrication of echogenic microbubbles with long shelf-­life and good chemical versatility. The microbubbles can also be converted to drug-­delivery systems by loading drugs directly onto the surface by physisorption. Ultrasound can be used to excite the microbubbles—to ‘shake’ them—and release the drug molecules. “More sophisticated methods are under study in our lab, consisting in tethering liposomes on the shell containing oligonucleotides cargo which can be transfected upon ultrasound irradiation,” Paradossi notes.

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In conclusion, anchoring graphene on PVA microbubble surfaces opens a path to a high-­resolution clinical diagnostic tool, by combining the appealing features of both the PVA microbubble (as an efficient ultrasound scatterer) and graphene (as a strong NIR absorber with high thermal conductivity). Featured scientist: Gaio Paradossi Organization: Dipartimento di Scienze e Tecnologie Chimiche at Università di Roma Tor Vergata, Rome (Italy) (https://bit.ly/2FVmX2a) Relevant publications: Y. Toumia, F. Domenici, S. Orlanducci, F. Mura, D. Grishenkov, P. Trochet, et al., Graphene Meets Microbubbles: A Superior Contrast Agent for Photoacoustic Imaging, ACS Appl. Mater. Interfaces, 2016, 8(25), 16465–16475. Y. Toumia, S. Orlanducci, F. Basoli, S. Licoccia and G. Paradossi, “Soft” Confinement of Graphene in Hydrogel Matrixes, J. Phys. Chem. B, 2015, 119(5), 2051–2061.

1.13 Real-­time Monitoring of Insulin Insulin is a rather small molecule (molecular weight 5.8 kDa) and weakly charged in solution (isoelectric point 5.8) and, therefore, low concentrations (90%) and flexibility. The team systematically benchmarked their results against the performance of existing state-­of-­the-­art commercial multistacked barriers and industrially viable ALD aluminum oxide films, and demonstrated that the nanolaminate films can be effectively integrated in organic light-­ emitting diodes (OLEDs), enabling half-­life times of 880 hours in ambient air. They also demonstrated the usability of their nanolaminates as a potential material for inclusion in standard multistacked barrier layers, to enhance the performance of existing ALD aluminum oxide and produce next-­generation moisture barriers. “Our results highlight the potential of such heterogeneous material integration and the use of such nanolaminates as building block to engineer new functionalities and form factors for flexible and wearable technology,” concludes Sagade.

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Featured scientist: Dr Abhay Sagade (https://bit.ly/2TW0w24) Organization: Department of Engineering, University of Cambridge,  Cambridge (UK) Relevant publication: A. Sagade, A. Aria, S. Edge, P. Melgari, B. Gieseking, B. Bayer, et al., Graphene-­based nanolaminates as ultra-­high permeation barriers, npj 2D Mater. Appl., 2017, 1(1), 35.

1.20 How to Directly Measure the Surface Energy of Graphene Surface energy is defined as the energy cost to increase a material's surface per unit area. Therefore, it indicates how likely molecules are to adsorb (or desorb) onto the surface, or how strongly it forms (non-­covalent) bonds with other materials. Access to accurate surface energy values for graphene is thus not only of fundamental interest, but also provides a useful reference for anyone involved in research into graphene properties, (surface) modifications, and the implementation of graphene in devices. Scientists in the UK demonstrated the successful application of the graphene surface force balance (g-­SFB), a technique they had developed previously. They used it to directly measure the surface energy of pure graphene (Figure 1.10). While a limited number of studies have reported surface energies for graphene, they all relied only on estimates based on indirect measurements: for instance, by pushing down a graphene beam onto an underlying graphite substrate; by studying the intercalation of noble gas atoms between the graphene and a graphite substrate; by contact-­angle measurement; or by gas chromatography measurements. The drawback of these approaches is that the measurements may be influenced by the adsorption of airborne contaminants and, in the case of contact-­angle measurements, by both the method of measurement and the underlying substrate. “Our work is of fundamental interest to a broad community and will aid the advancement of fundamental measurements of 2D and other nanomaterials,” says Nicole Grobert, a professor in the Department of Materials at the University of Oxford. She led this work, together with Susan Perkin, an associate professor of Physical Chemistry at the University of Oxford. “The fact that we were able to measure two pristine graphenes also demonstrates that the set-­up of the g-­SFB is robust and that the graphene surfaces are contamination free,” explains Christian van Engers, who works on synthesis of 2D nanomaterials and the development of g-­SFB. “Such measurements will pave the way to further the development of tailored nanomaterials device design for industrial applications.”

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Figure 1.10 Direct measurement of the surface energy of graphene. Crossed cyl-

inder geometry (left): the use of crossed cylinders (equal to sphere-­ on-­flat) allows for facile conversion of measured values to that of parallel plates. Radius (R) (top right): the radius of curvature can be measured in situ though the analysis of secondary (not shown) and primary interference fringes resulting from the reflections between the semi-­transparent gold mirrors. Distance (D) (bottom right): can be directly measured by studying the number of fringes between spectral reference lines (not shown). FN; FL: normal and lateral forces can be directly measured. (Image: Perkin Group, University of Oxford).

Previous measurements generally used graphite rather than graphene, or relied on the use of complicated models. A large fraction of previous work by Grobert's group, which lead to the successful creation of the graphene lenses for surface forces measurements, was devoted to development of the production of high-­quality large-­area graphene and the transfer of these large-­area, atomically flat graphenes. Both the controlled production and the transfer of high-­quality large-­area graphene are still major bottle-­necks in the application of graphene in general. Equally important, in this work, was the careful design and accurate description of the contact mechanics for reliable interpretation of the measured data. “Having achieved all of the above, we are establishing the g-­SFB as a robust research tool,” says Grobert. “For example, a large portion of energy storage technology relies on the use of porous carbon as anode material. The graphene surfaces used in the g-­SFB can serve as a model of a two-­ dimensional carbon pore allowing detailed studies of pore–electrolyte interactions and various other processes that occur within these technologies and have yet to be understood. Ultimately, the g-­SFB could lead to tailored design of highly efficient storage devices.”

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The SFB-­technique relies on the use of atomically smooth surfaces and the use of white light interferometry. Traditionally, this has been done using muscovite mica—an insulator. As the researchers point out, the strengths of the SFB lie in its ability to measure very small displacements and forces between surfaces, across liquids or air. For example, the short-­range layering of liquid molecules near surfaces, but also forces extending over a much longer range, such as those resulting from electrostatic repulsion between electric double layers on the surfaces, have been measured recently. However, so far it has not been possible to perform studies with control of the potential of both surfaces. The g-­SFB is unique in that it allows the detailed study of structure, dynamics, and physical properties of electrolytes at electrified interfaces. “We wanted to develop a SFB in which we could apply a potential to both surfaces,” Perkin notes. “In 2014, we showed that graphene could be used to meet all criteria to replace mica and described the g-­SFB for the first time.” “The creation of the g-­SFB was only possible through the long-­term collaboration—and the access to blue-­sky research funds—between internationally leading experts in theory and experiment of surface force balances and in nanomaterials synthesis, processing, and application,” she adds. The team hopes that their work will improve understanding of chemical and physical processes near graphitic interfaces. This has started with their direct measurement of the surface energy of graphene in dry nitrogen and liquids, which impacts the scientists' fundamental understanding and may impact the design of devices incorporating graphene. Now, they are applying the g-­SFB to study processes in energy storage devices, such as double-­layer capacitors. “We expect to be able to perform a lot of exciting studies, starting with the study of the potential dependent behavior of ionic liquids and other electrolytes in confinement,” Perkin explains the team's next steps. “For example, we know that when an electric field is applied between electrodes, ions migrate to the electrode surface to screen the electric field. However, our knowledge of the exact structure of the ions near the surface, the dynamics of this process, and how this affects the physical properties of the layer is limited. The g-­SFB would be uniquely suited to study these phenomena.” One challenge is the stability of the graphene surface. When the surfaces are brought into, or moved out of, contact, the process is often not reversible, indicating that the graphene becomes damaged. The researchers believe that this is due to strong adhesion between the graphenes, in combination with defects in the lattice. “We are now trying to improve this, for example by using graphene with millimeter-­sized grains that have also been developed within our team, to avoid the influence of any grain boundaries in our area of contact,” concludes Grobert. “With the wide range of 2D materials that are emerging, other 2D systems are potential candidates to serve as model systems.”

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Featured scientists: Professors Nicole Grobert (https://bit.ly/2OeUSXx) and Susan Perkin (https://bit.ly/2K7E5HF) Organization: Department of Materials, University of Oxford, Oxford (UK) Relevant publications: C. van Engers, N. Cousens, V. Babenko, J. Britton, B. Zappone, N. Grobert, et al., Direct Measurement of the Surface Energy of Graphene, Nano Lett., 2017, 17(6), 3815–3821. L. Griffin, K. Browning, S. Clarke, A. Smith, S. Perkin, M. Skoda, et al., Direct measurements of ionic liquid layering at a single mica–liquid interface and in nano-­films between two mica–liquid interfaces, Phys. Chem. Chem. Phys., 2017, 19(1), 297–304. M. Gebbie, A. Smith, H. Dobbs, A. Lee, G. Warr, X. Banquy, et al., Long range electrostatic forces in ionic liquids, Chem. Commun., 2017, 53(7), 1214–1224.

1.21 A 2D Electron Microscope Ever since Hans Busch invented the first electron lens in 1926, the electron microscope has been humanity's most important magnifying glass. The much smaller wavelength of high-­energy electrons compared with that of light means that a whole new world was now “open for inspection”—the micro-­ and nanoscale. Modern-­day electronics or communication technology would not exist without electron microscopy, and the same can be said for many other types of technology. Since the early days, electron microscopes have evolved into a multitude of powerful techniques that allow even individual atoms to be imaged. Beyond imaging, the focused beam of electrons is also used for analysis of the chemical composition of materials, their crystal structure, and many other useful things. With the discovery of graphene, electron microscopy has a chance to enter “flatland”. Electrons moving in solids take on different behaviors depending on the properties of the solid—they become quasiparticles. The quasiparticle in graphene is called a Dirac fermion, and resembles a photon more than it does an electron. For instance, Dirac fermions move at a constant speed regardless of their energy, at about 300 times slower than the speed of light. One consequence of these “light-­like” properties is the suppression of scattering. In graphene, electrons can move more than a micrometer between collisions at room temperature, and, when cooled down, they can move up to 30 µm and maybe even further. Graphene is like a “vacuum” for electrons.

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Researchers at the Technical University of Denmark and RWTH Aachen University in Germany have proposed using graphene as a two-­dimensional vacuum chamber, and building a two-­dimensional electron microscope, where the electrons fly from the electron gun to the target without ever leaving the graphene sheet. Such an instrument could be used to study the scattering behavior of electrons in graphene devices, the edges of graphene, and the interaction between graphene and other objects (particles, quantum dots, antennas) deposited on the surface—which are commonly used to augment and tune the behavior of graphene devices. This is not as crazy as it sounds. Electron optics in graphene was suggested for the first time 10 years ago, when it was pointed out that electrons that move from an electron-­rich region (n-­doped) to an electron-­deprived region (p-­doped), where the carriers are called “holes” (missing electrons), in a process that is called Klein tunneling. The peculiar rules of graphene optics with p–n junctions can be summarized as follows:     1. When the electron hits the barrier at a 90-­degree angle, it will be transmitted with 100% probability. 2. When the angle is non-­perpendicular, the electron will either be reflected OR transmitted across the junction, in accordance with Snell's law—as in light optics—but, sensationally, with a negative refractive index!     This makes a single p–n junction act as a focusing lens, or as other powerful optical components. In the past few years, leading research groups in Korea, Europe and the USA have demonstrated that electron optics do work, in a series of elegant and beautiful experiments. The DTU group, led by Peter Bøggild, carried out calculations both with semi-­classical (raytracing) and quantum mechanical simulations, to prove the basic concept and explore how far the analogy between two-­dimensional and three-­dimensional electron microscopes can be pushed. Using a point contact and an aperture to mimic the electron source, p–n junctions to build focusing and collimating lenses, and large electrodes to pick up electrons after hitting the target, all that is left is a weak magnetic field to gently deflect the electron beam and allow it to scan across the target. The simulations show that the high-­performance graphene devices and existing electron optics components are sufficient to make the system work in practice, and that the focused electron beam can indeed be used to probe not just the shape, but also the electronic properties of edges, as well as objects deposited on top of the graphene. One object of particular interest for the team was a circular p–n junction. Since electrons can penetrate the barrier by Klein tunneling, electronic states can be excited inside the cavity (think of standing sound waves in a closed room). This leads to strange current emission patterns, as the rules of electrons escaping from the circular potential are governed by the strong angular dependence of the Klein tunneling.

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Peter Bøggild got the idea for the microscope while preparing a lecture on electron microscopy and also writing a research proposal on ballistic transport in graphene: “Many of the components in electron microscopy happen to have a counterpart in graphene electronics, so we started to explore the 2D electron microscope on a conceptual level. Now we will try to build one.” Bøggild does not know yet how well this will work: “We shall see whether this particular apparatus can tell us anything important, but there are so many wonderful things you could do with focused beams of electrons in graphene. We are having a great time coming up with devices that rely on focused spin-­polarized electron beams, coherent electron beams, or perhaps time-­resolved measurements. Short bursts of focused electron beams could be detected picoseconds later with femtosecond time resolution, after having passed through a graphene device. Since the carriers all move at the same velocity, the temporal response could tell us what the electrons did while passing through device—think, for instance, of echoes.” Christoph Stampfer from RWTH Aachen elaborates: “It is very exciting to think that fundamental studies of ballistic electron transport are potentially leading to a new experimental tool—the 2D electron microscope—offering new possibilities to investigate nature.” Another area considered by the team is the use of electron optics to improve making room-­temperature ballistic electronics, to provide faster switching and more complex functionality compared with present-­day semiconductor-­ based components. Featured scientists: Professors Peter Bøggild (https://bit.ly/2G47LS0) and  Christoph Stampfer (https://bit.ly/30GqlYB) Organizations: Technical University of Denmark, Lyngby (Denmark); RWTH, Aachen (Germany) Video: https://youtu.be/MBubNlpIahI Relevant publication: P. Bøggild, J. Caridad, C. Stampfer, G. Calogero, N. Papior and M. Brandbyge, A two-­dimensional Dirac fermion microscope, Nat. Commun., 2017, 8, 15783.

1.22 Water Slippage in Graphene Nanochannels For most viscous liquids, the solid surface of the surrounding channel walls poses friction resistance to the flowing liquid, causing a, sometimes complete, loss of velocity at the liquid–solid interface. As a result, the energy required for nanofluidic applications with regular surfaces is enormous, because of the tremendous hydraulic resistance inside nanoscale conduits (the hydraulic resistance is inversely proportional to the fourth power of the conduit dimension). Simply put, the smaller the channel,

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the more pressure you need to drive the liquid. Imagine drinking a beverage with a straw: the thinner the straw is, the more difficult this becomes and the harder you have to suck. Graphitic nanoconduits, for example, carbon nanotubes (CNTs) and graphene nanochannels and their membrane forms, may provide a solution to this problem. Researchers discovered that superfast water transport occurs in these structures and this has inspired great interest for numerous nanofluidic applications. Further development and applications of these graphitic nanoconduits requires accurate measurement and understanding of water flow enhancement and surface slippage in single nanoconduits. Although great efforts have been made to explore water transport in single CNTs, water transport in single graphene nanochannels has yet to be unambiguously studied. “There are a number of challenges for water transport studies in single graphene nanochannels, which to some extent are more difficult to overcome than those in single carbon nanotubes,” says Chuanhua Duan, an assistant professor in the Department of Mechanical Engineering at Boston University. “First, single graphene nanochannels with well-­controlled dimensions and atomically smooth graphene surfaces need to be fabricated on the target substrate. Second, ultralow flow rate due to nanoconfinement and the corresponding pressure difference across the single graphene nanochannel need to be precisely measured. So far, there have been only limited efforts to resolve these two challenges.” Most previous studies have focused on flow-­rate measurement across carbon membrane structures, which consist of numerous individual conduits. The feature size and quantities of conduits on the membrane structures are based entirely on statistical estimation. This leads to inaccuracies, as the flow behavior could be strongly affected by the dimensions of individual conduits. This motivated Duan and his collaborators to understand the flow behavior inside single nanoscale conduits. Duan and an international team of researchers address these two issues and have developed a technique to accurately measure the hydraulic resistance inside graphene nanofluidic channels. With their technique, the team experimentally quantified the water flow enhancement inside graphene-­coated nanofluidic channels. This observation makes increasing the efficiencies of most nanofluidic applications possible. “In this work, we employ our recently developed method to fabricate graphene nanochannels with three-­side graphene coverage to overcome the channel fabrication challenge,” comments Duan. “For the measurement challenge, we use capillary flow to optically measure the flow rate in individual graphene nanochannels, while employing a hybrid nanochannel design to avoid inaccurate estimation of the driving pressure.” In their experiments, the scientists connected another nanoscale conduit to their graphene-­coated conduit and measured how fast the water can flow

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inside this hybrid system. This method is akin to weighing something with known weights. This way, the hydraulic resistance ratio of the two conduits is determined. Since the resistance of one conduit is known, it is possible to calculate the hydraulic resistance of the other (graphene-­coated) conduit. This then allows a calculation of how fast the water can flow and how slippery the graphene surface is. This is the first time it has been possible to experimentally quantify water slippage in a single graphene nanofluidic channel. According to the team's results, coating the surface with graphene will greatly enhance the flow inside nanofluidic conduits and can lead to exceptional performance of nanofluidic applications in areas such as seawater desalination, energy harvesting, nanofiltration, and lab-­on-­a-­chip technology. “Before applying this technique to industry, we need to have a better control of the graphene qualities and properties,” cautions Duan. “Also, further investigations of the flow enhancement dependence on surface charge density, roughness, stress, and defects of graphene would be very beneficial.” Featured scientist: Professor Chuanhua Duan (https://bit.ly/2OSVySO) Organization: Department of Mechanical Engineering, Boston University, Boston, MA (USA) Relevant publications: Q. Xie, M. Alibakhshi, S. Jiao, Z. Xu, M. Hempel, J. Kong, et al., Fast water transport in graphene nanofluidic channels, Nat. Nanotechnol., 2018, 13(3), 238–245. Q. Xie, F. Xin, H. Park and C. Duan, Ion transport in graphene nanofluidic channels, Nanoscale, 2016, 8(47), 19527–19535.

1.23 Light-­induced Active Ion Transport Through Graphene Membranes Nanofluidic channels feature unique unipolar ionic transport when properly designed and constructed. Researchers active in nanofluidics have begun to adopt reconstructed layered two-­dimensional sheets—such as graphene oxide or clay—as a promising material platform for nanofluidics. These membranes contain a high volume fraction of interconnected 2D nanochannels. Compared with other materials used for nanofluidic devices—such as anodized aluminum oxide membrane, block copolymer membrane, and

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nanofluidic crystals—a unique feature of layered membranes is that the channels are horizontally aligned and the channel height (i.e., the spacing between the layers), which is responsible for confinement of the electrolyte, remains uniform throughout the entire thin film. “However, mass and charge transport in existing membrane materials follows their concentration gradient,” Wei Guo, a professor at the Technical Institute of Physics and Chemistry, Chinese Academy of Sciences, Beijing, points out. “Attaining anti-­gradient transport as effective as natural counterparts remains a great challenge in fully abiotic nanosystems.” A team of researchers, led by Guo, demonstrated a coupled photon–electron ion transport phenomenon through graphene oxide membranes. It shows a straightforward way in which to power the transport in 2D-­layered materials using light energy. “Using the energy of light, cations are able to move thermodynamically uphill over a broad range of concentrations, at rates orders of magnitude faster than that via simple diffusion,” Guo explains. “Based on this mechanism, we developed photonic ion switches, photonic ion diodes, and photonic ion transistors as the fundamental elements for active ion sieving and artificial photosynthesis on synthetic nanofluidic circuits.” This is the first discovery of photo-­induced active (anti-­gradient) ion transport in 2D-­layered materials with extraordinarily high pumping rates. It provides a completely new way for remote, non-­invasive, and active control of the transport behaviors in synthetic membrane materials. “Using light to control the mass and charge transportation in fully synthetic membranes is the dream of a materials scientist, like me,” says Guo. “As far as I know, many research groups, currently, are engaged in this field. However, their findings are restricted to use light as a gate, allowing or prohibiting the transport. In contrast, we use light as a motive force to realize active transport.” Upon asymmetric light illumination, a net cationic flow through the layered graphene oxide membrane is generated, from the non-­illuminated region to the illuminated region. This is the first time this phenomenon has been reported (Figure 1.11). Against a concentration gradient, the pumping rates for cations can be five orders of magnitude higher than via simple diffusion. The team established a theoretical model and performed molecular dynamics simulations to unveil the mechanism. Light irradiation reduces the local electric potential on the graphene oxide membrane, following a carrier diffusion mechanism. When the illumination is applied to an off-­center position, an electric potential difference is built up across the GO strip, which can drive the transport of ionic species. Superior to existing molecular transport systems, the light-­induced active ion transport reported in this work does not rely on lipid or liquid membranes, which significantly improves its robustness and compatibility. In addition, it does not hinge on specific ion-­binding shuttle molecules to achieve the transmembrane ion transport. Thus, its transport range can be at the scale of centimeters.

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Figure 1.11 Schematic illustration of the generation of net ionic flow through the graphene oxide membrane upon asymmetric light illumination. Reproduced from https://doi.org/10.1038/s41467-­019-­09178-­x under the terms of the CC BY 4.0 license, http://creativecommons.org/ licenses/by/4.0/.

This work provides a new route for remote, non-­invasive, and active control of the transport behaviors in synthetic membrane materials. It demonstrates a way to fabricate innovative membrane materials for active ionic sieving, artificial photosynthesis, and modular computation on integrated nanofluidic circuits. Following the mechanism proposed in this work the researchers constructed photonic ion switches, photonic ion diodes, and photonic ion transistors as the fundamental elements for light-­controlled nanofluidic circuits. “So far in our lab, the photo-­induced active ion transport systems has been developed to the third generation,” notes Guo. “The photo-­induced active ion transport phenomenon can be also found in almost the whole family of 2D semiconductors. There is tremendous room to further exploit their unique photoresponsiveness in liquid-­processable colloidal 2D materials. The present work opens up exciting new possibilities.” “Now, we are trying to amplify the generation of photocurrent and voltage, and scale up the membrane materials with, for example, printing techniques,” he concludes. “Also, we intend to further extend the scope of the materials with which the active transport behaviors can take place.” Featured scientist: Professor Wei Guo Organization: Technical Institute of Physics and Chemistry, Chinese Academy of Sciences, Beijing (China) Relevant publication: J. Yang, X. Hu, X. Kong, P. Jia, D. Ji, D. Quan, et al., Photo-­induced ultrafast active ion transport through graphene oxide membranes, Nat. Commun., 2019, 10(1), 1171.

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1.24 Membranes for Large-­scale Energy Storage Systems As a promising large-­scale energy storage technology, redox flow batteries (RFBs) are attracting increasing research attention. RFBs are distinct from conventional solid-­state, rechargeable batteries in that the active materials involved in the electrode reactions are soluble. Although much progress has been made in identifying redox species for potential large-­scale RFBs, one key limitation for their further development lies in finding low-­cost and high-­performance separators/ membranes. “For RFB separators, the essential requirement is achieving high ionic conductivity with minimal cross-­over at low cost,” points out Guihua Yu, a professor in Materials Science and Engineering, Mechanical Engineering, at the Texas Materials Institute, University of Texas at Austin. “Commercial membranes (Nafion) have high proton conductivity, but the production process is fairly laborious and expensive, accounting for almost 40% of the stack cost in RFBs, and the cross-­over problem is sometimes inevitable. Therefore, a new class of alternative membranes with facile production processes and competitive ionic conductivity are critically needed.” In a study, Yu and his team demonstrated a proof-­of-­concept graphene oxide (GO) membrane as a separator for large-­scale energy storage RFBs. Their work shows that the two-­dimensional nanochannel structure and low frictional water flow inside micrometer-­thick GO laminates make this material an attractive candidate membrane for large-­scale energy storage systems (Figure 1.12). GO laminate membranes offer tunable interlayer spacing and can act as molecular or ionic sieves to prevent the cross-­over of large-­sized redox species. Exploring GO membranes as RFB separators, the team found that they achieve high rejection of large molecules or ions and high ionic conductivity at the same time. Moreover, changing the degree of oxidation, or using bacterial cellulose as an additional filling component, can further adjust the microstructure, mechanical stability, and ion transport behavior. “By designing large-­size differences between charge carriers and redox species, GO membranes can achieve high rejection and high ionic conductivity in various redox flow battery systems,” explains Yu. “Furthermore, tailoring the degree of oxidation or using bacterial cellulose as the hybrid component demonstrates the flexibility to tune the microstructure and ionic transport of GO-­based membranes. Finally, RFBs with GO-­based separators show a similar charge/discharge profile to commercial Nafion 212 and achieve a stable cycling performance with a high coulombic efficiency of about 98%.”

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Figure 1.12 Schematic illustration of the working mechanism of graphene oxide

membranes for redox flow batteries. Reproduced from http://dx.doi. org/10.1016/j.chempr.2018.02.003 with permission from Elsevier, Copyright 2018.

Yu notes that the production of GO nanosheets can be low cost and large scale. “Although the stability and performance of GO membranes in the flow mode still need to be further improved, our proof-­of-­concept demonstration of using GO membranes with tunable interlayer space, versatile chemical modification, and rational composite design, provides useful guidelines for future development of next-­generation functional separators for potentially large-­scale energy storage systems.” The key for making GO-­based membranes work for RFBs is to utilize the 2D nanofluidic structure from the stacking of GO sheets to conduct charge carriers. “This is a fundamentally new type of membrane system compared to the polymer-­based membranes for vanadium ions,” Yu points out. “A similar membrane structure based on GO sheets has already been explored and studied in ionic and molecular sieving and other environmental  applications and shows great potential to achieve the selection of different molecules.”

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The scientists are planning to extend the concept of 2D nanochannel structures to other materials in order to design more robust membranes, since the mechanical strength of GO membranes, especially in flowing liquid, is not exceptionally high. They are also considering other new types of membranes for RFBs based on bioderived materials with hydrophilic groups, which could potentially lead to low-­cost and high-­performance membranes with good mechanical properties. “A deeper understanding of ionic transport mechanism and blocking of redox species in novel types of membranes with specific nanostructures will be an interesting research direction,” Yu concludes. “Apart from functionality, it is also important to examine the scalability and cost implications of new-­t ype membranes in order to study their usefulness in practical applications.” Featured scientist: Professor Guihua Yu (https://bit.ly/2Y6hQVh) Organization: Texas Materials Institute, University of Texas at Austin, Austin, TX (USA) Relevant publication: L. Zhang, Y. Ding, C. Zhang, Y. Zhou, X. Zhou, Z. Liu, et al., Enabling Graphene-­Oxide-­Based Membranes for Large-­Scale Energy Storage by Controlling Hydrophilic Microstructures, Chem, 2018, 4(5), 1035–1046.

1.25 Protecting Against Electrostatic Discharging Failures One of the most pervasive reliability problems facing the computer chip industry is electrostatic discharging (ESD) failure caused by the rapid, spontaneous transfer of electrostatic charge induced by a high electrostatic field. In order to protect integrated circuits (IC) from being damaged by ESD, chip manufacturers place dedicated ESD protection structures at input/output (I/O) and power lines on the chip. For decades, conventional on-­chip ESD protection structures for ICs have relied on in-­Si p–n junction-­based device structures (e.g., diodes, bipolar junction transistors, metal–oxide–semiconductor field-­effect transistors (MOSFETs), and silicon-­controlled rectifiers), which have many inherent disadvantages, such as substantial parasitic capacitance, p–n junction leakage current, and large Si area consumption, that are unsuitable for ICs at nanoscale nodes. That is why researchers have been working on novel ESD protection designs for use in future ICs. However, latent ESD failure mechanisms, accurate ESD device modeling, and predictive ESD computer-­aided design verification are some of the challenges that need to be overcome.

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A research team led by Professors Albert Wang (University of California at Riverside) and Ya-­Hong Xie (University of California at Los Angeles) has devised a novel above-­IC graphene-­based nanoelectromechanical system (NEMS) switch structure (gNEMS) for on-­chip ESD protection utilizing the unique properties of graphene. The gNEMS ESD switch is a two-­terminal device with a vacuum gap between a conducting substrate (Si or metal serving as the anode) at the bottom and a suspended graphene membrane on top serving as the discharging path. During normal IC operation, the gNEMS switch is at the OFF state and the graphene ribbon is suspended over the trench. When an ESD surge occurs at I/O, the induced electrostatic force pulls down the suspended graphene membrane to touch the bottom conducting layer, forming a discharging path from graphene to ground. “Our novel gNEMS ESD switch can be fabricated in the back end of line fabrication of ICs (where the individual devices such as transistors, capacitors, and resistors get interconnected with wiring on the wafer) through 3D heterogeneous integration, and therefore can eliminate many design problems inherent to traditional in-­Si ESD protection structures,” says Dr Qi Chen. Chen is first author of a paper that provides practical design guidelines for using the gNEMS switch as on-­chip ESD protection for ICs. He points out that, compared with conventional active ESD devices based on p–n junctions, this passive gNEMS switch has several advantages: theoretically, it has zero leakage and minimum parasitic capacitance; it also shows dual-­polarity ESD protection features while the conventional counterparts can only work for single-­polarity protection, which can largely reduce the ESD device area consumptions. Furthermore, graphene shows superior current and heat handling capability. “Most importantly” he notes, “due to the graphene ESD switch being a passive device, it can be fabricated at the CMOS (complementary metal–oxide– semiconductor) back end through 3D heterogeneous integration, instead of taking up large chip areas with core IC designs.” The research team developed a CMOS-­compatible process to fabricate the graphene NEMS ESD switches. During the fabrication process, graphene is synthesized by the chemical vapor deposition method instead of by micro-­exfoliation. According to the team, the CVD method also provides a promising route for graphene ESD switch mass production. “We also conducted systematic transient characterization of our gNEMS ESD switches by transmission line pulse (TLP) measurement for human body model ESD protection, revealing transient ESD discharging behaviors related to device dimensions and TLP pulse shapes,” says Chen. “The results provide practical design guidelines for using graphene ESD switches as on-­ chip ESD protection for ICs.”

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Going forward, the team will try to fabricate the graphene ESD switch by 3D heterogeneous integration at the IC backed with foundry-­processed chips for real applications. Furthermore, they are planning to improve ESD performance of the switches by designing novel NEMS structures and improving the quality of the graphene material used. “While graphene's unique properties make it an ideal material for ESD protection, the main challenge is the quality of the graphene material,” Chen concludes. “Currently, due to graphene quality issues during CVD synthesis, reliability and uniformity is a major challenge for the mass production and real application of graphene for ESD applications.” Featured scientists: Professors Albert Wang and Ya-­Hong Xie; Dr Qi Chen Organizations: University of California at Riverside, CA; University of  California at Los Angeles, CA (USA) Relevant publication: Q. Chen, J. Ng, C. Li, F. Lu, C. Wang, F. Zhang, et al., Systematic transient characterisation of graphene NEMS switch for ESD protection, Micro Nano Lett., 2017, 12(11), 875–880.

1.26 Inspiration from Spider Webs Highly flexible and sensitive strain sensors are essential components of wearable electronic devices. The piezoresistive effect of graphene, combined with its other properties such as ultra-­translucency, superior mechanical flexibility and stability, high restorability, and carrier mobility, make it a very promising material for fabricating high-­sensitivity strain sensors. Potential application areas for these sensors could be found in flexible display technology, robotics, smart clothing, electronic skin, body monitoring, human–machine interfaces, in vitro diagnostics, and implantable devices. Already, researchers have demonstrated examples of highly sensitive graphene strain sensors, for instance as dual-­function sensors and switches. Inspired by the high level of flexibility exhibited by spider webs, scientists in Hong Kong have developed a novel design for highly flexible and sensitive piezoresistive sensors based on an elastomer-­filled graphene woven fabric (E-­GWF) structure. This technique mimics the distinct core–shell structure of spider webs. This fabrication method could also be extended to other 1D and 2D materials for many emerging practical applications. A team from The Hong Kong University of Science and Technology (HKUST) reports that, in addition to excellent sensing capabilities, the

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Figure 1.13 Schematic flowchart of the fabrication procedure of an E-­GWF. Reproduced from http://dx.doi.org/10.1021/acsami.8b18312 with permission from American Chemical Society, Copyright 2018.

E-­GWF also shows unusual switching behavior at large strains, owing to reversible cracking and reconnection of interconnected graphene tubes (Figure 1.13). “The combination of high sensitivity, flexibility, and stretchability makes the spider-­web-­like wearable strain sensor suitable for mounting on human skin,” says Qingbin Zheng, a research assistant professor of Mechanical and Aerospace Engineering. “In addition, our semi-­transparent strain sensor can improve user experience without any significant impact on daily activities.” Furthermore, this strain sensor can strengthen interactions between humans and smart systems, especially in shape-­conforming systems such as electronic skin, elastic displays, epidermal sensors, personalized health monitoring, and human–machine interfaces. These sensors should be able to maintain good functionality when conformably in contact with human skin/muscle with curvilinear surfaces.

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Researchers and material scientists have long been fascinated by spider silk—ultrastrong and extensible self-­assembling biopolymers that out-­ perform the mechanical characteristics of many human-­made materials, including steel. The HKUST team built a spider web with silk whose structure consists of a bundle of silk fibrils, known as spidroin, which are encapsulated by about 100–200-­nm thick outer layers made up of lipids, glycoproteins, and skin. The core is 2–3-­µm thick and dominates the tensile strength and elasticity. The molecular structure of the silk consists of regions of protein crystals separated by less-­organized protein chains. The primary structural modules give rise to diverse secondary structures that, in turn, determine the functions of different silks. Inspired by this unique architecture, the researchers developed their design for a highly flexible and sensitive piezoresistive sensor, which mimics the geometric and functional characteristics of a spider web. “It was proposed previously that there are three critical components that need to be considered when designing new functional materials, including the chemical composition, nano-­ and microstructure, and architecture,” Zheng explains. “The design of our E-­GWF involves using graphene/PDMS as the main constituent to ensure high flexibility; nanoscale graphene/PDMS microfibers as the nano-­ and microstructural framework to provide high sensitivity; and woven fabric as the assembled architecture to guarantee structural integrity.” “These three components make E-­GWFs an ideal bioinspired hierarchical material for strain sensors arising from the multiscale structures spanning from woven fabric, PDMS microfiber to nanoscale graphene,” he adds. Although the working range of the sensor has been increased up to 20% durable strain, much higher stretchability is required for monitoring a full range of human motion, such as in the joints of the human body. The team expects that additional structural engineering, such as prestraining, wrinkling, and patterning, will enable them to increase the stretchability of the sensors significantly.

Featured scientist: Qingbin Zheng (https://bit.ly/2WRdj7J) Organization: The Hong Kong University of Science and Technology, Hong Kong SAR (China) Relevant publication: X. Liu, D. Liu, J. Lee, Q. Zheng, X. Du, X. Zhang, et al., Spider-­Web-­Inspired Stretchable Graphene Woven Fabric for Highly Sensitive, Transparent, Wearable Strain Sensors, ACS Appl. Mater. Interfaces, 2018, 11(2), 2282–2294.

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1.27 Graphene Quantum Dots Made from Agricultural Waste Rice husks—the outer, protective covering of rice kernels, which makes up more than 20 wt% of the entire kernel—are the by-­product of rice milling. Given the sheer volume of this agricultural waste, roughly 120 million tons a year, researchers have been exploring ways to utilize this silica-­rich biomass for the synthesis of valuable materials. “Due to the high concentration of silica in rice husks, most current research focuses on the preparation of silicon-­based materials, which exhibit broad applications in the fields of adsorption, catalysis, energy storage, etc.,” explains Dr Luyi Sun, an associate professor in the Institute of Materials Science at the University of Connecticut. “It is worth pointing out that there is also a large amount of organic components (ca. 72–85 wt%) in rice husks, which is typically wasted in the preparation of these silica materials.” In their work, Sun and co-­workers developed an advanced method for the comprehensive use of rice husks. They fabricated high-­quality graphene quantum dots (GQDs) from the organic components of rice husks, and simultaneously obtained mesoporous silica nanoparticles with a high surface area from the inorganic content. This work was a collaborative effort with Dr Xin Zhang at Guangdong University of Technology, Dr Bin Liu's group and Dr Yuhua Wang's group at Lanzhou University, Dr Weixing Wang at South China University of Technology, as well as Dr Steven L. Suib's group at the University of Connecticut. As Sun notes, the preparation of graphene quantum dots usually adopts either a bottom-­up or a top-­down method. In their work, the team developed a new approach that combines the advantages of both of these two methods. “This is the key to utilizing rice husks as the raw material to obtain high-­ quality GQDs,” says Sun. “In addition to fabricating GQDs with a high yield, we simultaneously synthesized valuable mesoporous silica nanoparticles from the inorganic component of rice husks. So, this work realizes a truly comprehensive utilization of rice husk biomass.” The team's as-­prepared GQDs from rice husks can emit bright blue light when irradiated by an ultraviolet lamp. They also show some unique photoluminescent behaviors. “We investigated the photoluminescent properties of our rice husk-­ derived GQDs (RH-­GQDs) to explore their further use,” says Dr Zhaofeng Wang, the first author of this work. “We found that the emissions of the GQDs are strongly dependent on the surrounding temperature, excitation wavelength, as well as the lateral size. These unique features provide us with many opportunities to further adjust the emissions of RH-­GQDs for various uses.”

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He adds that the team has confirmed the bioactivities of the RH-­GQDs. Therefore, the researchers expect that the RH-­GQDs will be very suitable for biomedical applications, such as bioimaging and bioprobes. Featured scientists: Professor Luyi Sun's research group (https://bit. ly/2X6JIaV) Organization: Institute of Materials Science, University of Connecticut, Storrs, CT (USA) Relevant publication: Z. Wang, J. Yu, X. Zhang, N. Li, B. Liu, Y. Li, et al., Large-­Scale and Controllable Synthesis of Graphene Quantum Dots from Rice Husk Biomass: A Comprehensive Utilization Strategy, ACS Appl. Mater. Interfaces, 2016, 8(2), 1434–1439.

1.28 Outstanding Thermal Conductivity of Graphene Composites The discovery of the unique heat conduction properties of graphene in 2008 motivated numerous, practically oriented studies of the use of single-­layer and few-­layer graphene in various composites and coatings. The general idea of such studies is incorporation of graphene fillers into the base, i.e., matrix, material in order to improve the resulting thermal conductivity of the composite. Certain types of such composites, termed thermal interface materials (TIMs), can be used for improving heat conduction from computer chips, light-­emitting diodes, microwave sources, or other devices, to heat sinks. There is an increasing demand for better TIMs for heat removal in electronics and energy conversion applications. Commercially available TIMs with a thermal conductivity in the range 0.5–5 W mK−1 no longer meet the industry requirements. A research team led by Dr Alexander Balandin, Distinguished Professor at the Department of Electrical and Computer Engineering, University of California, Riverside (UCR), has demonstrated that the epoxy-­based composites with a high loading fraction—up to 45 vol%—of the randomly oriented graphene fillers can deliver thermal conductivity values above 12 W mK−1. This value exceeds the industry's target, which is 10 W mK−1, according to the Semiconductor Research Corporation. The UCR team has also determined that graphene composites reach a distinctive thermal percolation threshold at a loading fraction above 20 vol%. Thermal percolation is a term used to describe formation of the continuous interconnecting network of fillers, allowing heat to travel mostly via these thermal conductive passes rather than through the matrix (Figure 1.14).

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Figure 1.14 Top left: schematic of the composite with the volume fraction of fill-

ers. Bottom left: schematic showing the heat propagation from one filler to the other. Right: scanning electron microscopy image of the epoxy composite with 45 vol% of graphene fillers. The microscopy image of the high loading composites clearly shows overlapping of the graphene fillers inside the epoxy matrix. The overlapping fillers confirm the formation of the percolation network at this high loading fraction of graphene. Reproduced from http://dx.doi.org/10.1021/ acsami.8b16616 with permission from American Chemical Society, Copyright 2018.

The team established that graphene fillers outperformed boron nitride fillers (h-­BN)—another highly thermally conductive material—for thermal conductivity enhancement. The reported study clarified the debated mechanism of the thermal percolation, and is expected to facilitate development of the next generation of efficient TIMs. “The unexpected finding of this study was that the thermal properties of composites with a high loading of graphene are strongly influenced not only by the in-­plane thermal conductivity of few-­layer graphene fillers, but also by their cross-­plane thermal conductivity,” Balandin explains. “In such composites, heat mostly travels via the thermally conductive pathways of few-­ layer graphene. As a result, heat transfer from one graphene filler to another graphene filler—across the atomic planes and interfaces—becomes the bottle-­neck for thermal transport.” The first studies of graphene composites found that even small loading fractions of randomly oriented graphene fillers—up to 10 vol%—can increase the thermal conductivity of epoxy composites to about 5 W mK−1, which is a 25-­fold enhancement compared with the thermal conductivity of epoxy itself. Most of the studies of thermal composites with graphene were limited to relatively low loading fractions below 10 vol%. This was due to difficulties in preparing high loading fraction composites with a uniform dispersion of graphene fillers.

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Changes in viscosity and graphene filler agglomeration complicated synthesis of a consistent set of samples with the loading substantially above 10 vol%. The thermal conductivity achieved in these graphene composites was not sufficient for replacing TIMs with conventional fillers such as metals. The UCR team developed a new approach to composite preparation, which allowed it to increase the loading of graphene, and achieve substantially higher values of thermal conductivity, meeting the industry's demands. “This is an important development for practical applications of graphene,” says Balandin. “High values of thermal conductivity, above 10 W mK−1, can be achieved with inexpensive liquid phase exfoliated graphene. The graphene loading required for it is less than that in commercial TIMs, which have thermal conductivity below 5 W mK−1. Graphene TIMs can be much better and less expensive than the conventional ones.” The composites with a high loading of graphene fillers have the potential to deliver high thermal conductivity and low thermal contact resistance. Recent technological developments have demonstrated that liquid phase exfoliated graphene can be produced inexpensively and in large quantities. Various methods of reduction of graphene oxide have also been reported. The progress in graphene synthesis makes few-­layer graphene fillers practical, even for composites with a high loading. “We are now focusing on achieving the minimum thermal resistance of graphene TIMs with the surfaces of interest,” adds Balandin. “This metric includes the thermal contact resistance of the composite with the surfaces and materials of interest. Other important goals are further optimization of the filler sizes and thicknesses.” Featured scientists: Professor Alexander Balandin’s research group (https://bit.ly/2YKRR5a) Organization: University of California, Riverside, CA (USA) Relevant publication: F. Kargar, Z. Barani, R. Salgado, B. Debnath, J. Lewis, E. Aytan, et al., Thermal Percolation Threshold and Thermal Properties of Composites with High Loading of Graphene and Boron Nitride Fillers, ACS Appl. Mater. Interfaces, 2018, 10(43), 37555–37565.

Chapter 2

The Growing Landscape of Two-­ dimensional Materials 2.1  Introduction Inspired by the unique optical and electronic properties of graphene, two-­ dimensional (2D) layered materials—as well as their hybrids—have been intensively investigated in recent years, driven by their potential applications in future electronic devices, nanocomposites, medical devices, photovoltaics, and thermoelectrics. The broad spectrum of atomic-­layered crystals includes transition metal dichalcogenides, semiconducting dichalcogenides, monoatomic buckled crystals such as black phosphorous (BP or phosphorene), and diatomic hexagonal boron nitride (h-­BN). This class of materials can be obtained by exfoliation of bulk materials to small scales, or by epitaxial growth and chemical vapor deposition (CVD) for large areas. These atomically thin, single-­or few-­layer crystals feature strong intralayer covalent bonding and weak interlayer van der Waals bonding, resulting in superior electrical, optical, and mechanical properties.

2.2  H  igh-­performance Synthesis of 2D Metal Oxides and Hydroxides As two-­dimensional materials gain more and more importance—thanks to their exotic electronic properties and abundant active sites—the development of high-­yield, efficient, fast, and low-­cost synthesis methods to advance these materials from the laboratory to industry has become an urgent issue.

  Nanoengineering: The Skills and Tools Making Technology Invisible By Michael Berger © Michael Berger 2020 Published by the Royal Society of Chemistry, www.rsc.org

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“Previous research has shown that ions always play a key role in the synthesis of 2D materials,” says Jun Zhou, a professor at Huazhong University of Science and Technology, Wuhan National Laboratory for Optoelectronics. “However, it should be noted that when the synthesis process occurs in solution, desolvation is a necessary step because ions are in the solvated state in solution. Unfortunately, though, the energy consumption for desolvation increases the overall activation energy, thus limiting the reaction rate.” A team led by Zhou has developed a general and rapid molten salts method—widely studied for the synthesis of nanomaterials, such as graphene and perovskite—that can synthesize various ion-­intercalated 2D metal oxides and hydroxides, such as cation-­intercalated manganese oxides, cation-­ intercalated tungsten oxides, and anion-­intercalated metal hydroxides. The most significant result of this work is the ability to obtain high-­quality 2D materials within just 1 minute by using very cheap and common source materials. This is a big step towards the commercialization of 2D materials. “The key feature of our method is the direct use of naked ionized ions without hydration in the molten state salt to quickly induce the growth of 2D metal oxides and hydroxides,” explains Zhou. “In our technique, by adding precursors into the low-­cost molten salts for only 1 minute, we could obtain high-­ yield 2D materials simply by washing the salts. Even without centrifugation or sedimentation, we do not observe particles or nanowires in the final products.” To demonstrate the potential applications of their 2D ion-­intercalated metal oxides and hydroxides in energy storage, the team printed a flexible solid-­state supercapacitor based on carbon nanotube (CNT)-­coated A4 paper (as a current collector), which they coated with a 2D dispersion of Na2W4O13. According to the researchers, these supercapacitors show good electrochemical performance with an excellent rate capability, demonstrating the potential applications of these 2D ion-­intercalated metal oxides and hydroxides in energy storage and beyond. “Although we only reported eight 2D ion-­intercalated metal oxides and hydroxides, the versatility of this molten salt synthesis process gives us confidence that reasonably tuning the precursors and molten salts will allow various 2D oxides and hydroxides to be synthesized and the scope of the accessible oxides and hydroxides to expand to other 2D materials with attractive properties,” concludes Zhou. Featured scientist: Professor Jun Zhou Organization: Wuhan National Laboratory for Optoelectronics, Huazhong University of Science and Technology, Wuhan (China) Video: https://youtu.be/fSfAxnh5Na4 Relevant publication: Z. Hu, X. Xiao, H. Jin, T. Li, M. Chen, Z. Liang et al., Rapid mass production of two-­dimensional metal oxides and hydroxides via the molten salts method, Nat. Commun., 2017, 8:15630.

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2.3  F  lexible, Low-­power, High-­frequency Nanoelectronics Molybdenum disulfide (MoS2), an abundant mineral that is often used in steel alloys or as an additive in lubricants, is increasingly being studied for use in nanoelectronics. This semiconducting material exhibits unique physical, optical, and electrical properties correlated with its single-­atomic-­ layer structure. Importantly for electronics applications, and in contrast to graphene, MoS2 has a band gap. In the past, the performance of synthesized MoS2 has been poor, especially when integrated onto flexible substrates. A new study has now resulted in the highest performance for CVD-­grown monolayer MoS2 device properties on flexible substrates to date. A team from The University of Texas at Austin has demonstrated, for the first time, highly flexible and robust CVD monolayer MoS2 -­based radio-­ frequency (RF) transistors operating at GHz range (Figure 2.1). “Our advance has been made possible by improvements in MoS2 synthesis, transfer, and device integration onto plastics,” explains Maruthi Nagavalli Yogeesh, a PhD student in Professor Deji Akinwande's research group. “Particularly important is that we discovered that the electrons in MoS2 can travel at high velocities sufficient to afford GHz frequencies.” The team's discovery is important because it demonstrates that MoS2 is a higher performance material compared with conventional thin-­film transistors made from organic semiconductors, metal oxides, or amorphous silicon devices. The demonstration of large-­area (millimeter-­scale) monolayer MoS2-­based flexible RF transistors operating at GHz performance is very promising for the design of low-­power and high-­frequency flexible RF nanoelectronics systems for the emerging field of wearable systems and the ‘internet-­of-­things’. For their monolayer MoS2 devices, the team employed a CVD process on SiO2/Si. They then used a poly(methyl methacrylate) (PMMA)-­supported wet-­transfer process to transfer the resulting monolayer MoS2 onto various substrates.

Figure 2.1  Schematic  of the CVD-­grown MoS2 FET device. (Image: Jo Wozniak/Deji Akinwande, The University of Texas at Austin).

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Subsequently they fabricated flexible MoS2 -­based RF transistors, which showed a high intrinsic cutoff frequency of 5.6 GHz and power gain of 3.3 GHz. “These numbers imply that flexible MoS2 transistors can be fabricated over large plastic sheets and offer wireless connectivity at common cellular and bluetooth bands,” Akinwande points out. “To this end, we have successfully demonstrated the circuit building blocks for mobile communication systems: amplifier, mixer, and wireless AM receiver. In addition, our transistors are mechanically robust enduring 10 000 bending cycles.” The team's immediate next goal is to demonstrate novel flexible wireless radio systems based on their MoS2 field-­effect transistors (FETs). “There are many challenges ahead, like how to successfully integrate different building blocks of RF systems such as antennas, amplifiers, and mixers,” says Yogeesh. “We are also exploring how to successfully fabricate passive (inductors, capacitors, and resistors) and active components (transistors) simultaneously on the flexible platform.” Featured scientists: Professor Deji Akinwande's Nano research group (https://bit.ly/2Ob7EXb) Organization: The University of Texas at Austin, Austin, TX (USA) Relevant publication: H. Chang, M. Yogeesh, R. Ghosh, A. Rai, A. Sanne, S. Yang et al., Large-­Area Monolayer MoS2 for Flexible Low-­Power RF Nanoelectronics in the GHz Regime, Adv. Mater., 2015, 28(9):1818–1823.

2.4  L  arge-­yield Synthesis of 2D Antimonene Nanocrystals Density functional theory computations have shown that monolayered arsenene and antimonene—which are indirect wide band gap semiconductors—become direct band gap semiconductors under strain. Such dramatic transitions of electronic properties could open a new door for nanoscale transistors with high on/off ratio, blue/UV optoelectronic devices, and nanomechanical sensors based on new ultrathin semiconductors. Following up on these theoretical predictions, researchers demonstrated two high-­yield methods for fabricating antimonenes: a nanosheet antimonene solution by liquid exfoliation and high-­quality single crystals of few-­ layer antimonene by CVD growth. “Our findings advance the field of nanocrystal inks in two aspects,” says Professor Haibo Zeng, Director of the Institute of Optoelectronics & Nanomaterials at Nanjing University of Science and Technology. “Firstly, through simply heating a mixture of metal–organic precursors we were able to synthesize a wide range of transparent conducting oxide (TCO) nanocrystal inks, which can be assembled into high-­performance electrodes.”

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This one-­pot process can be used for various oxide nanocrystal inks with yields as high as 10 g. The nanocrystals formed are of high crystallinity, uniform morphology, monodispersity, and high ink stability and feature effective doping. “Secondly,” adds Zeng, “the inks can be readily assembled into films with a surface roughness of 1.6 nm. Typically, a sheet resistance of 110 Ω sq−1 can be achieved with a transmittance of 88%, which is the best performance for TCO nanocrystal ink-­based electrodes described to date.” These electrodes can thus drive a polymer light-­emitting diode with a luminance of 2200 cd m−2 at 100 mA cm−2. They could therefore find applications in various solution-­based, even flexible, optoelectronics. In contrast to previous TCO nanocrystal synthesis methods, Zeng's team report a facile and universal one-­pot method for the synthesis of a wide range of TCO nanocrystal inks and the corresponding electrodes for all-­solution-­ processed devices with good performances. The proposed approach is generic for various TCOs as well as other oxide (e.g., CoO, MnO, Fe3O4, CdO) nanocrystal inks. “Our TCO nanocrystals are highly crystalline, with a uniform morphology and a narrow size distribution, as well as an effective doping control and a high colloidal stability over one year, making them suitable to be used as inks to print smooth, crack-­free, highly transparent, and conductive films,” notes Jianping Ji, a postgraduate researcher in Zeng's laboratory and first author of a paper on this work. TCO nanocrystal inks are compatible with flexible and stretchable substrates, which can be simply prepared by solution-­based techniques, such as spin-­coating, inkjet printing, spraying, and roll-­to-­roll production. The resulting high-­performance transparent electrodes have a significant potential in low-­cost and solution-­based fields, such as organic light-­emitting diodes (OLEDs), solar cells, photodetectors, wearable electronics, and smart electrochromic windows. In recent years, the hot-­injection method has become very popular for the synthesis of monodisperse colloidal nanocrystals, which have been used to prepare indium tin oxide (ITO) and gallium-­doped zinc oxide nanocrystal inks. “Nevertheless, such a process involves a chemical reaction between the injected sources and the mother solution and suffers from two disadvantages,” explains Zeng. “First, a small dose of injection is usually chosen to achieve high-­quality doping, but this injection mode leads to a low throughput and is difficult to scale up. Second, the doping quality is very sensitive to the dopant element and the injection parameters. All of the parameters have to be changed to prepare oxide nanocrystal inks with different dopants. Thus, our motivation has been to develop a one-­pot and universal strategy for the fabrication of a wide range of TCO nanocrystal inks.” In order to be used as transparent conductive electrodes for flexible and stretchable optoelectronics in commercial applications, the main hurdle for TCO nanocrystal inks is a lack of compatibility with current industrial solution-­based process, such as roll-­to-­roll or blade-­coating production. The researchers say that this could be solved by optimizing the viscosity and surface tension of the inks.

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Aside from the problem of assembling films, the electrical properties of TCO nanocrystal electrodes need to be further improved by short ligand exchanges or optimal posttreatments, which can meet the requirements for practical application. Featured scientists: Professor Haibo Zeng's research group (https://bit. ly/2W21a38) Organization: Nanjing University of Science and Technology, Nanjing (China) Relevant publications: C. Huo, X. Sun, Z. Yan, X. Song, S. Zhang, Z. Xie et al., Retraction of “Few-­Layer Antimonene: Large Yield Synthesis, Exact Atomical Structure, and Outstanding Optical Limiting”, J. Am. Chem. Soc., 2017, 139(9):3568–3568. J. Ji, X. Song, J. Liu, Z. Yan, C. Huo, S. Zhang et al., Two-­dimensional antimonene single crystals grown by van der Waals epitaxy, Nat. Commun., 2016, 7(1), 13352.

2.5  F  reestanding Borophene Synthesized for the First Time Borophene, the atomically flat form of boron, differs from graphene and other two-­dimensional materials in an important way: it cannot be reduced from a larger natural form because bulk boron is not naturally layered. While graphite is composed of stacks of atomically thin sheets that can be peeled off one at a time to make graphene, there is no such analogous process for making 2D boron. Or so researchers had thought. Previous research proposed that 2D sheets of boron cannot form without the support of a substrate, which was believed to have a predominant role in the crystallization of borophene. The theory was that the crystalline order of the substrate controls the crystalline order of the thus-­synthesized borophene. The experimental realization of borophene had been achieved only via atomic-­layer deposition and molecular beam epitaxy on a silver substrate under ultrahigh vacuum conditions. This previous work also suggested that there are primarily two planar phases of borophene, namely, β12 and X3, of which X3 is metastable whereas β12 is a higher temperature phase. Now, however, an international team of researchers has synthesized freestanding borophene—β12, X3 and intermediate phases—for the first time, and in a scalable manner. The team discovered a facile and scalable liquid-­phase synthesis method for freestanding borophene sheets via sonochemical exfoliation.

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“We have synthesized freestanding borophene with the energetically favorable structure via sonochemical exfoliation of boron powder (∼20 µm) in various solvents such as dimethylformamide (DMF), acetone, isopropyl alcohol, water, and ethylene glycol,” says Dr Prashant Kumar, Department of Physics, Indian Institute of Technology, Patna. “Acetone and ethylene glycol as solvents have yielded monolayers within 12 and 20 hours, respectively, while other solvents did not yield monolayers. In contrast, water and isopropyl alcohol display fairly good exfoliation down to few monolayers, but the observed sheet sizes are small. The use of DMF, in particular, has resulted in multilayered sheets having ordered crystal structure.” Because borophene in its 2D form is metallic, it holds promise for possible applications in energy storage, catalysis, photovoltaics, and various sensing applications. “Another intriguing feature of borophene is its atomic configuration,” Professor Ajayan Vinu, Director Global Innovative Center for Advanced Nanomaterials (GICAN) at The University of Newcastle in Australia, points out. “Unlike graphene, which is structurally isotropic in nature, the β12 phase of borophene is anisotropic and that in turn is expected to exhibit higher carrier density and mechanical stiffness in preferred direction.” In anisotropic materials, their mechanical or electronic properties—like their electrical conductivity—vary with different crystallographic orientations. In contrast, the properties of an isotropic material like graphene are the same in all directions. In their work, Vinu, Prashant, and their collaborators explored freestanding borophene sheets for their potential applications, including gas sensing, photosensing, molecular sensing, and strain sensing. In addition, the team synthesized hybrids of freestanding borophene with other 2D materials (graphene and MoS2). “We need to follow up our findings with further detailed exploration of the role of solvents in the exfoliation process and the resulting structural phases,” Vinu concludes. “Also, filtering out borophene from the reaction solution is a challenge and we need to overcome that.”

Featured scientists: Dr Prashant Kumar; Professor Ajayan Vinu Organizations: Indian Institute of Technology, Patna (India); Global Innovative Center for Advanced Nanomaterials (GICAN) (https://bit.ly/30ibaob), The University of Newcastle, Newcastle (Australia) Relevant publication: P. Ranjan, T. Sahu, R. Bhushan, S. Yamijala, D. Late, P. Kumar et al., Freestanding Borophene and Its Hybrids, Adv. Mater., 2019, 31, 1900353.

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2.6  2  D Spacer Materials for Surface Plasmon Coupled Emission Sensing Inexpensive and rapid handheld sensors for biomarkers could revolutionize health care, especially in the resource-­limited settings that are widespread in low-­ and middle-­income countries. The challenge in fabricating these biosensors is to detect weak emission signals when the biomarkers are at very low concentrations, as in the early stages of a disease. Nanomaterials like graphene and fullerenes provide an excellent platform to enhance weak signals from biomarkers. While graphene and fullerenes perform very well for detecting isolated biomarkers, their ability to amplify emission of biomarkers in a real physiological setting is limited due to their strong interactions with other biomolecules, such as proteins and lipids. A team of researchers from Clemson University and Sri Sathya Sai Institute of Higher Learning collaboratively developed new sensing platforms that use two-­dimensional materials beyond graphene. Similar to the fullerene-­based sensors, these inexpensive biosensors are fabricated by coating thin films of silver with an overcoat of two-­dimensional spacer layers, including MoS2, WS2, and boron nitride. In these sensors, the isotropic fluorescence from dye-­stained biomarkers couples with plasmons from the silver film through the overcoated spacer layers (Figure 2.2). “Metallic thin films are double-­edged swords that both simultaneously enhance and quench the biomarker emission,” says Pradyumna Mulpur, a graduate student at Sri Sathya Sai Institute of Higher Learning and first author of this paper. “We previously found that nanocarbons are good spacer layers in addition to protecting the silver film from oxidation.”

Figure 2.2  Silver  thin films overcoated with MoS2 can enhance the emission from

dyes, such as rhodamine, by 17 times and thus improve their sensitivity at low concentrations. (Image: Pradyumna Mulpur).

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He notes that, despite their great potential, facile synthesis, and high stability, nanocarbons interact very strongly with a variety of biomolecules through a delocalized π-­electron cloud. This limits their ability to enhance weak emissions from the desired biomarkers in real blood samples, which contain thousands of proteins and lipids. The emergence of 2D materials beyond graphene presents new opportunities to overcome challenges posed by nanocarbons, by selectively interacting with desired biomarkers. “Following the realization of graphene, we were able to chemically exfoliate and isolate thin layers of other materials such as MoS2 and WS2,” recounts Ramakrishna Podila, an assistant professor in the Department of Physics and Astronomy at Clemson University and principal investigator of this study. “Unlike graphene, MoS2 and WS2 do not exhibit strong π-­interactions with biomolecules. We used this to our advantage to show that emission of dyes coupled to biomarkers as in the traditional enzyme-­linked immunosorbent assay (ELISA) tests can be enhanced by ∼20 times, even with MoS2 and WS2.” The researchers expect to extend this sensor design to real-­time diagnostics in point-­of-­care settings in the near future. However, some limitations, such as realizing continuous and uniform MoS2 and WS2 films without affecting the underlying silver film, still persist. The team hopes to build on the advances in bottom-­up synthesis methods to resolve these challenges. Featured scientists: Pradyumna Mulpur; Professor Ramakrishna Podila Organizations: Nano-­Bio Lab (https://bit.ly/2UFZpYS), Clemson University, Clemson, SC (USA); Sri Sathya Sai Institute of Higher Learning (https:// bit.ly/2QgeNqe), Anantapur (India) Relevant publication: P. Mulpur, S. Yadavilli, A. Rao, V. Kamisetti and R. Podila, MoS2/WS2/BN-­Silver Thin-­Film Hybrid Architectures Displaying Enhanced Fluorescence via Surface Plasmon Coupled Emission for Sensing Applications, ACS Sens., 2016, 1(6):826–833.

2.7  2D Oxides Juice-­up Sodium-­ion Batteries Sodium-­ion batteries are a promising alternative to lithium-­ion batteries, particularly for home-­based and grid-­level storage solutions. Despite its lower energy density, sodium is more abundant and more evenly distributed geographically than lithium—and it is cheaper. In some countries, such as Saudi Arabia, it is abundant as a natural by-­product of the water desalination process. Tin monoxide (SnO) has been demonstrated to have excellent physical and chemical properties and has a large theoretical capacity as a battery anode, for instance, for sodium-­ion batteries. Unfortunately, though, it

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Figure 2.3  Schematic  illustration of the crystal structure of two-­dimensional SnO. (Image: Professor Husam Alshareef, KAUST).

also exhibits large volume change during the sodiation and lithiation process (>300%), which makes it unsuitable as a high-­performing anode material. Researchers have attempted various ways to mitigate the volume change in this important material. For instance, SnO material has often been mixed with different forms of carbon to mitigate its volume change. A new strategy from the laboratory of Professor Husam Alshareef at King Abdullah University of Science & Technology (KAUST) is different (Figure 2.3). “Recognizing that SnO actually has a two-­dimensional structure, we set out to find a chemical process to control the degree of exfoliation of this oxide during chemical synthesis,” says Alshareef. “By carefully choosing the reactants, solvents, and reaction conditions, we found a way to precisely control the number of atomic layers in each SnO sheet. In fact, we could easily go from just a couple of atomic layers to 20 atomic layers in each SnO sheet.” This study represents a different approach for designing SnO anodes and mitigating their volume change. The KAUST team demonstrated that, when the number of SnO atomic layers in one SnO nanosheet is less than five, the sodium-­ion batteries could last for thousands of cycles. “The sodiation density was 452 mA h g−1 after 1000 cycles and showed no sign of instability, which is one of the best reported performances for this anode material,” says Fan Zhang, PhD student and lead author of the study.

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“With our process, the volume change is largely mitigated due to the atomic-­ scale thickness of the SnO sheets.” “We have been focusing on tin-­based anodes in our group,” adds Alshareef. “The results in this present work are one of the best results we have obtained for this oxide in sodium-­ion batteries.” “The natural next step is to make full-­cell batteries with the appropriate cathode materials,” he concludes. “The combined effects of cathode material type, electrolyte, and separator used should be evaluated since they also may affect the overall performance of the battery. The challenge is to optimize the other components of the battery to leverage the full potential of the SnO anodes. Once the full cells are optimized, larger batteries and even battery packs could be constructed and tested.” Featured scientist: Professor Husam Alshareef Organization: King Abdullah University of Science & Technology, Thuwal (Saudi Arabia) Relevant publication: F. Zhang, J. Zhu, D. Zhang, U. Schwingenschlögl and H. Alshareef, Two-­Dimensional SnO Anodes with a Tunable Number of Atomic Layers for Sodium Ion Batteries, Nano Lett., 2017, 17(2):1302–1311.

2.8  H  oley 2D Nanosheets for Efficient Energy Storage Two-­dimensional nanocrystals offer exciting opportunities for both fundamental studies and many technological applications, due to their unique and fascinating properties. Transition metal oxide (TMO) nanomaterials have been widely studied as electrodes for alkali-­ion storage, because they generally exhibit improved capacity and rate capability compared with their bulk counterparts due to the abundant active sites and the shortened ion diffusion distance. However, it is critically challenging to synthesize TMO nanosheets with confined thickness, as they are intrinsically non-­layered materials that cannot be mechanically or chemically exfoliated to form 2D nanosheets, as conventional layered materials do. In the literature, TMOs are mostly studied in the form of zero-­ dimension nanoparticles, one-­dimensional (1D) nanotubes and nanowires, and 3D nanoclusters or microclusters. There are very few reports studying 2D nanostructured TMOs, not to mention those with confined thickness. A research team from The University of Texas at Austin and Argonne National Laboratory has developed a general and facile bottom-up

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synthesis strategy by employing graphene oxide as a sacrificial template to prepare various 2D holey TMO nanosheets, including mixed metal oxides and simple metal oxides. “The most exciting results of our research are that we developed a general sacrificial template strategy for controlled synthesis of holey TMO nanosheets with tunable pore sizes for improved alkali-­ion storage properties,” explains Guihua Yu, a professor in Materials Science and Engineering, Mechanical Engineering, at the Texas Materials Institute. “This approach is universal for the synthesis of various 2D holey TMO nanosheets, including mixed transition metal oxides and simple oxides. This unique holey structure can minimize the restacking of 2D nanosheets and provide more active sites for alkali-­ion storage.” The researchers show that 2D holey TMO nanosheets composed of chemically interconnected metal oxide nanoparticles inherit the strong mechanical properties from graphene oxide, maintaining the holey morphology and displaying minimal structural changes during the lithiation processes (Figure 2.4). The team's results show that the 2D holey nanostructured TMO materials are a promising material platform for improving electrochemical performance because of the synergistic effects of the structure-­derived excellent chemical/mechanical stability and the enhanced charge transport properties.

Figure 2.4  Schematic  of the general sacrificial template strategy to synthesize 2D

holey TMO nanosheets. Two transition metal (TM) cations are mixed with graphene oxide (GO) and then anchored on surfaces of reduced graphene oxide (rGO) templates during solution-­phase reaction. 2D holey mixed transition metal oxide (MTMO) nanosheets composed of interconnected MTMO nanocrystals are formed after removing rGO templates during postcalcination. Reproduced from https://doi. org/10.1038/ncomms15139 under the terms of the CC BY 4.0 license, http://creativecommons.org/licenses/by/4.0/.

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The general strategy reported in this work provides guidance for the synthesis of other 2D holey nanosheets materials as a unique material platform for advanced energy storage and conversion devices, and can be extended beyond oxides to phosphides, sulfides, and selenides. The researchers demonstrated that the hole size of 2D holey oxide nanosheets can be adjusted by tuning the thermal annealing temperature and the precursor ratio. Compared with other nanostructure forms and bulk particles, these nanosheets exhibit significantly enhanced charge-­storage and transport properties. “Our general method represents a promising bottom-­up strategy to synthesize 2D TMO nanosheets materials from those with intrinsically non-­layered structures,” Yu points out. “In addition, advanced in situ TEM (transmission electron microscopy) characterization shows that the 2D holey ZMO nanosheets inherit strong mechanical properties from graphene oxides, displaying minimal structural changes during the lithiation/delithiation processes and even under mechanical pressing states.” “Moreover,” he continues, “Operando XRD (X-­ray diffraction) and XAS (X-­ray absorption spectroscopy) results show that holey ZMO nanosheets deliver high capacity due to the formation of ZnLi alloy, as well as the reversible transformation between Mn2+ and Mn3+. This also provides valuable guidance for the fundamental understanding of electrochemical reaction of these oxide nanomaterials in batteries.” As the team explains, the implications of this work are two-­fold. First, a general synthetic strategy is developed by employing graphene oxide as a sacrificial template to prepare various 2D holey TMO nanosheets, and it can be extended to other materials beyond oxides. Second, given the potential merits of 2D nanostructures, including tunable porosity and inherently strong mechanical stability, these 2D holey nanosheets could be promising candidates for emerging applications in energy storage, energy conversion systems, and electrocatalysis. “We think that future directions will be focused on extending the general method to synthesize holey nanosheets in other material systems, such as phosphides, sulfides, selenides, maybe even phosphates and sulfates,” says Yu. “One critical challenge is the controlled deposition of metal ion precursors on the graphene oxide template and the following thermal annealing treatment, as the deposition of precursors on graphene oxide for different materials is certainly variable. Substantial efforts need to be devoted to exploring the optimal material systems, reaction precursors, and the thermal treatment conditions.” “But,” he concludes, “we believe this proposed general strategy may open up a promising avenue for designing 2D nanostructures for versatile materials, especially those with intrinsically non-­layered structures, for broad applications in nanoscience and nanotechnology.”

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Featured scientists: Professor Guihua Yu's research group (https://bit. ly/2Y6hQVh) Organization: Materials Science and Engineering, Mechanical Engineering, Texas Materials Institute, The University of Texas at Austin, Austin, TX (USA) Relevant publication: L. Peng, P. Xiong, L. Ma, Y. Yuan, Y. Zhu, D. Chen et al., Holey two-­dimensional transition metal oxide nanosheets for efficient energy storage, Nat. Commun., 2017, 8:15139.

2.9  A  tomristor: Memristor Effect in Atomically Thin Nanomaterials In trying to bring brain-­like (neuromorphic) computing closer to reality, researchers have been working on the development of memory resistors, or memristors, which are resistors in a circuit that ‘remember’ their state even if power is lost. Today, most computers use random access memory (RAM), which moves very quickly as a user works but does not retain unsaved data if power is lost. Flash drives, on the other hand, store information when they are not powered, but they work much slower. Memristors could provide a memory that is the best of both worlds: fast and reliable. “Previously there was work by Mark Hersam's group on memristor effect in monolayer MoS2 in a planar configuration,” says Deji Akinwande, an associate professor at The University of Texas at Austin. “This prior work has inspired us to consider vertical sandwich structures for memristors because of the massively higher density and scalability. However, everyone we consulted doubted such an effect could be possible in a vertical configuration, because the active layer spacing between metal electrodes is less than 1 nm, so leakage current will kill the device.” Akinwande and his team discovered a non-­volatile memory effect in atomically thin 2D materials such as MoS2. This effect is similar to memristors or RRAM in metal oxide materials. This experimental work is supported by preliminary calculations. These devices can be collectively labeled atomristor, in essence, memristor effects in atomically thin nanomaterials or atomic sheets. Because MoS2 and related materials are crystalline and have a good electronic barrier to prevent current flowing, it is possible to realize working atomristors (Figure 2.5). “It had been a conventional belief that you could not scale the memory layer below ∼5 nm,” says Akinwande. “Our work overturns this thinking and demonstrates that materials as thin as 0.7 nm can afford memory effects.”

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Figure 2.5  Illustration  of electric field-­induced memristor effect in single-­layer MoS2. (Image: Professor Deji Akinwande, University of Texas-­Austin).

The team's work features the thinnest memory devices and it appears to be a universal effect available in all semiconducting 2D monolayers. The scientists explain that the unexpected discovery of non-­volatile resistance switching (NVRS) in monolayer transitional metal dichalcogenides (MoS2, MoSe2, WS2, WSe2) is likely due to the inherent layered crystalline nature that produces sharp interfaces and clean tunnel barriers. This prevents excessive leakage and affords stable phenomena so that NVRS can be used for existing memory and computing applications. “Our work opens up a new field of research in exploiting defects at the atomic scale and can advance existing applications such as future generation high-­density storage, and 3D cross-­bar networks for neuromorphic memory computing,” notes Akinwande. “We also discovered a completely new application, which is non-­volatile switching for radio-­frequency communication systems. This is a rapidly emerging field because of the massive growth in wireless technologies and the need for very low-­power switches. Our devices consume no static power, an important feature for battery life in mobile communication systems.” Notwithstanding the team's discovery, there remain fundamental questions regarding mechanisms of NVRS in single-­layer atomic sheets. Hence, they are working with theorists and materials scientists to elucidate the detailed physics at the atomic level. “From a scientific view, the biggest challenge is a detailed understanding of the physics,” Akinwande concludes. “From an application point of view, the biggest challenge for maximum performance is to improve the endurance from ∼100 cycles today to more than a million cycles.”

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Featured scientists: Professor Deji Akinwande's Nano research group (https://bit.ly/2Ob7EXb) Organization: The University of Texas at Austin, Austin, TX (USA) Relevant publication: R. Ge, X. Wu, M. Kim, J. Shi, S. Sonde, L. Tao et al., Atomristor: Nonvolatile Resistance Switching in Atomic Sheets of Transition Metal Dichalcogenides, Nano Lett., 2017, 18(1):434–441.

2.10  Photostriction of Molecular 2D Nanosheets Photostriction is the property of certain materials to undergo a change in internal strain—and therefore shape—upon exposure to light. This means that the energy from light is converted into mechanical motion of the material. Photostriction induced by light–matter interaction is of significant interest due to its rich photophysics and broad applications in wireless photoactuators, adaptive optics, and artificial muscle technologies. Molecular charge-­transfer crystals show promise as a novel class of photostrictive materials. In contrast to inorganic materials with high strength covalent or ionic bonds, organic molecular solids have weak intermolecular interactions, such as hydrogen bonds, π–π stacking, van der Waals, and charge transfer-­induced electrostatic interactions, which result in distinct lattice deformations under photoexcitation due to the large intermolecular spacing. In addition, molecular charge-­transfer crystals display long-­range stacked electron donor and acceptor components. This stacking is driven by unique molecular arrangements and leads to dipole formation and additional spontaneous polarization. Furthermore, the superior optoelectronic properties of charge-­transfer solids can lead to changes in the surface electric field and elastic strain due to the converse piezoelectric effect, which leads to deformation of the sample. These clues suggest the possibility of molecular charge-­transfer systems with useful photostrictive properties. Employing the photostrictive effect, researchers have fabricated a flexible two-­dimensional charge-­transfer molecular (sub-­nanometer) nanosheet and observed a sizeable photostrictive effect of 5.7% with a fast, sub-­millisecond response. This photostrictive effect arises from excess charge carrier-­induced lattice dilation and conformational change, which is higher than that of some conventional ferroelectronics and polar semiconductors. “The photostrictive mechanism presented here is very new, which extends the photostrictive research field in the research community,” remarks Shenqiang Ren, a professor in the Mechanical and Aerospace Engineering, and

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the Research and Education in Energy, Environment & Water (RENEW) Institute at the University at Buffalo, The State University of New York. “In addition, our findings yield a new platform for 2D molecular charge-­transfer nanosheets with potential applications in flexible 2D photo-­driven microsensors and actuators.” To fabricate their ultrathin charge-­transfer network nanosheets, the team used dibenzotetrathiafulvalene (DBTTF) as the electron donor and C60 fullerene as the electron acceptor. “The excited, cationic state of DBTTF is found to be flat, and this flattening upon photoexcitation increases the length of the unit cell, contributing to the distinct photostriction phenomenon observed in the film,” explains Ren. “The fullerene C60 is known to be an acceptor possessing a number of characteristic features, namely, high-­level electronic structure, lower symmetry, and intrinsic polarizability. Due to the high intermolecular interactions between DBTTF and C60 molecules, a long-­range ordered packing arrangement of molecular charge-­transfer nanosheets can be formed.” Due to the fast, sub-­millisecond response time of the flexible 2D charge-­ transfer molecular nanosheet described in this work, it could serve as a direct convertor between light and mechanical energy and can play a role in a vast array of technological applications, including light-­controlled gas storage, microactuation, microsensing, wireless photoactuators, adaptive optics, and artificial muscle technologies. Next, the researchers will actively investigate other organic charge-­transfer systems to elucidate if they display similar photostrictive properties. Secondly, they are planning to investigate the potential flexible photostrictive applications of charge-­transfer DBTTF/C60 molecular nanosheets, like wearable photoactuators. “According to the mechanism presented in this work, we propose that there should be some other kinds of less-­complicated 2D charge-­transfer materials possessing high photostrictive effect and fast response time,” Ren concludes. “In addition, for the next generation of flexible electronics, the flexibility and stretchability also need to be taken into consideration.”

Featured scientists: Professor Shenqiang Ren's research group (https://bit. ly/2wqT59y) Organization: University at Buffalo, The State University of New York, Buffalo, NY (USA) Relevant publication: Z. Zhang, R. Remsing, H. Chakraborty, W. Gao, G. Yuan, M. Klein et al., Light-­induced dilation in nanosheets of charge-­ transfer complexes, Proc. Natl. Acad. Sci. U. S. A., 2018, 115(15):3776–3781.

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2.11  A Nano Squeegee to Clean Nanosheets Two-­dimensional materials represent the ultimate scaling down of materials because 2D materials are only a single molecular layer thick. One type of material that nanotechnology researchers are particularly interested in are called van der Waals heterostructures—assemblies of artificially stacked 2D layers of different materials. By precisely stacking various 2D materials on top of each other in a predetermined sequence, researchers can create materials that, due to their unique interlayer coupling, have special optoelectronic properties; this makes them of considerable interest for next-­generation nanoelectronics and other unique devices. Unfortunately, the study of 2D monolayers is plagued by trapped contaminants in between the 2D sheets, as well as between the 2D sheets and the underlying substrate. These contaminants make it difficult to obtain precise and reproducible experimental observations. This means that, when conducting investigations, scientists do not know whether they are measuring the intrinsic material behavior or the interaction of the material with the contaminants. And that, of course, impacts the investigations of the fundamental physics of these materials. Clean and homogeneous interfaces are critical for van der Waals heterostructures and any trapped contaminants between 2D layers lead to poor interface quality. A clean interface reduces the interlayer spacing (which increases interlayer coupling) and leads to a reduction in externally induced heterogeneity caused by trapped contaminants between layers. Findings by a team of scientists from the US Naval Research Laboratory (NRL) present a simple technique for removing these contaminants in a process similar to a squeegee, which enables unambiguous investigation of the intrinsic properties of monolayer 2D materials. Figure 2.6 illustrates the operating concept of the atomic force microscopy (AFM) flattening technique that the team developed, in which the tip of AFM squeezes out contaminants trapped between the 2D layer and the substrate. “Clean and homogeneous interfaces are essential for understanding and leveraging the unique physics of 2D materials,” points out Matthew Rosenberger, a National Research Council Postdoctoral Fellow at NRL. “This has been well known for many years, but it remains challenging to actually obtain clean interfaces in practice. Our work advances the field of 2D materials because we have developed a simple technique for achieving such clean and homogeneous 2D material interfaces, which will aid other researchers in understanding the intrinsic behaviors of 2D materials.” He adds that a necessary condition for successful AFM flattening is that the adhesion between the 2D material and the substrate must be sufficient to prevent contaminants from migrating back between the 2D layer and the substrate after removal of the AFM tip.

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Figure 2.6  (a,  b) Cartoon illustrating the general concept of AFM flattening. After

mechanical transfer, there are contaminants trapped between the 2D layer and the substrate, which are often a combination of a uniform thickness layer and bubbles, as shown in (a). Applying a normal load with the AFM tip pushes the contaminants out from beneath the tip and induces direct contact between the 2D layer and the substrate. Scanning the tip across the surface, while applying normal load, causes the contaminants to collect into a pocket, leaving behind a region without contaminants, as shown in (b). (c) AFM topography of mechanically transferred CVD WSe2 on h-­BN. The arrows show the general raster pattern used to flatten an area. Starting from the top left corner, the AFM tip scans to the right (blue arrows) and the left (white arrows), and then the tip moves down between each scan line (black arrows). (d) AFM topography after flattening a 6 µm × 6 µm area of the WSe2 sample. Reproduced from http://dx.doi.org/10.1021/acsami.8b01224 with permission from American Chemical Society, Copyright 2018.

In their experiments, the team found that this is not always the case. It appears that substrate surface roughness plays an important role in the adhesion between a 2D layer and the substrate. These findings should be of interest for other research teams because they demonstrate a simple technique for creating clean 2D material interfaces using only an atomic force microscope, which is a common instrument in the field of 2D materials. Although this technique cannot facilitate the large throughput that is required for industrial-­scale processes, it is a new approach for gaining a better understanding of the behavior of 2D materials; an essential prerequisite for developing real-­world technologies. “The primary next stage in our investigations will be to utilize the squeegee technique on many types of 2D material samples in order to

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gain insight into the fundamental physics of these materials,” Rosenberger concludes. “Understanding these fundamental physics will help us to design better technologies that are relevant to the Navy, such as wearable and stretchable electronics and cost-­effective chemical and biological sensors.” Featured scientist: Dr Matthew Rosenberger Organization: US Naval Research Laboratory, Annapolis, MD (USA) Relevant publication: M. Rosenberger, H. Chuang, K. McCreary, A. Hanbicki, S. Sivaram and B. Jonker, Nano-­“Squeegee” for the Creation of Clean 2D Material Interfaces, ACS Appl. Mater. Interfaces, 10(12):10379–10387.

2.12  S  tudying Strain Effects in 2D Materials Using Kelvin Probe Microscopy At the nanoscale, materials become exceedingly sensitive to external perturbations. This is a relevant factor for wearable and flexible applications, where materials would always bear a certain degree of variable strain/stress. “Effects of strain can be significant, for example some materials can transform from being semiconductors to metallic with strain,” says Syed Ghazi Sarwat, a PhD candidate in Harish Bhaskaran's group at the University of Oxford. “This can be quite frustrating as a device could completely lose its functionality. However, strain can also be exploited, for instance in catalysis or in making field-­effect strain sensors.” However, so far it has been very difficult to characterize strain effects in 2D materials, which is crucial to understanding strain–matter interactions. Conventionally, strain in materials is studied using optical spectroscopy. While being a robust approach, this technique is generally limited to the microscale and to materials that have direct band gaps. That means that graphene, which has zero band gap, and many other materials that have indirect band gaps, cannot be studied for strain effects. Furthermore, flexible substrates on which 2D materials reside can mask the strain effects. In their work, the Bhaskaran group demonstrates that this limitation can be overcome by using a technique based on Kelvin Probe Microcopy (Figure 2.7). In this work, the team reveals strain-­induced modulation in the heterostructures of ultrathin 2D materials. These materials are being intensively pursued for next-­generation electronic and optoelectronic devices. “We show that strain can strongly modulate the electronic energy landscape (energy band diagrams) of the heterostructures, which can surprisingly also result in the materials to behave differently than would be expected in their isolated forms,” explains Sarwat. “We also find that the junctions that form at the interfaces between the materials get modulated from strain as well.”

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Figure 2.7  A  schematic illustration of the strain characterization set-­up. Top right inset: a schematic of the two-­point bending apparatus. Bottom left inset: illustrating the heterostructure consisting of graphene/WS2 monolayer/WS2 multilayer. Reproduced from http://dx.doi.org/10.1021/ acs.nanolett.8b00036 with permission from American Chemical Society, Copyright 2018.

As an example, he notes that if such a heterostructure were to be used as an LED, strain alone could cause fluctuations in the illumination intensity, and perhaps even blank it out. Sarwat emphasizes that the team's results can aid any study that requires realization of strain modulation in individual components with a nanoscale resolution in a multicomponent system, and in the study of spatial strain distribution in a homogeneous system. They are also important for the practical aspects of device engineering. Next in their investigations, the researchers aim to use atomic scale, as well as finite element modeling, to verify the nature of band alignment and strain transfer, and possibly even harness strain modulation in devices such as gas and chemical sensors. Featured scientists: Syed Ghazi Sarwat and Harish Bhaskaran research groups (https://bit.ly/1RIC6lS) Organization: Univeristy of Oxford, Oxford (UK) Relevant publication: S. Sarwat, M. Tweedie, B. Porter, Y. Zhou, Y. Sheng, J. Mol J et al., Revealing Strain-­Induced Effects in Ultrathin Heterostructures at the Nanoscale, Nano Lett., 2018, 18(4):2467–2474.

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2.13  O  ptothermoplasmonic Patterning of 2D Materials Traditional top-­down patterning techniques—including photolithography, electron-­beam lithography, and ion-­beam lithography—have been employed to fabricate high-­quality micro-­and nanopatterns of diverse two-­dimensional materials. However, these fabrication techniques typically require complex and expensive instruments and multistep processing. An alternative approach is laser processing of 2D materials, which is a remotely controlled, one-­step, maskless, and low-­cost fabrication technique. The downside to this process is the requirement for high optical power and the fact that the fabrication of sub-­micrometer patterns is still challenging. To overcome these problems, researchers have developed an all-­optical lithographic technique called optothermoplasmonic nanolithography (OTNL) to achieve high-­throughput, versatile, and maskless patterning of different atomic layers. “Our motivation to develop this novel optical tool has been to achieve simple, low-­power, high-­throughput, and precise patterning of 2D materials with all-­optical control,” says Dr Linhan Lin, first author of a paper on this work. Taking graphene and molybdenum disulfide (MoS2) monolayers as examples, the team at The University of Texas at Austin, led by Professors Yuebing Zheng and Deji Akinwande, show that both thermal oxidation and sublimation in the light-­directed temperature field can lead to direct etching of the atomic layers. They demonstrate the low-­power (∼5 mW µm−2) and high-­resolution (down to 300 nm) patterning of both graphene and MoS2 monolayers through exploiting thermal oxidation and sublimation at the highly localized thermoplasmonic hotspots (which are created by plasmon-­enhanced optical heating). The researchers also discovered that gold (Au) nanoparticles can reduce the formation energy (∼0.6 eV) of carbon (C) monovacancies through bonding between undercoordinated C and Au, leading to a significant Au-­ catalyzed graphene oxidation and reducing the required operational optical power. “The concept of optothermoplasmonic patterning can be widely applied in the laser processing of atomic-­thin materials at different scales (10−9 to 10−3 m), which can be further explored for all-­optical and precise morphologic control at quantum regions,” says Jingang Li, the co-­first author of the paper. In this work, the team further demonstrates programmable patterning of 2D materials into complex and large-­scale nanostructures by steering the laser beam.

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There are several specific potential applications of the OTNL approach.    ⁃ Plasmonic biosensors: patterning of periodic structures on graphene with tunable plasmonic resonances can be used for the sensing of different biomolecules. ⁃ Field-­effect transistors: all-­optical fabrication of transistors based on different 2D materials. ⁃ Photon detectors: precise control of 2D materials for the use of photodetector.    “The OTNL technique can be widely applied to fabricate different electronic and photonic devices based on 2D materials,” Zheng concludes. “Compared to conventional lithographical methods, OTNL will provide a new way for simple, low-­cost, and large-­scale fabrication of diverse patterns with arbitrary configurations. We foresee various applications of OTNL in fabrication of ultrathin devices for photon detection, field-­effect transistors, light-­emitting diodes, biosensors, and so on.” Future research in optical patterning of 2D materials will focus on the improvement of patterning resolution to enable light–matter coupling at nanoscale. Potential challenges in this area include how to maintain the quality of 2D materials and how to dynamically control the nanoheaters optically and precisely. Featured scientists: Dr Linhan Lin (https://bit.ly/2Gl9wdo); Research groups of Yuebing Zheng and Deji Akinwande Organization: The University of Texas at Austin, Austin,TX (USA) Video: https://youtu.be/OEZyGTWOSUg Relevant publication: L. Lin, J. Li, W. Li, M. Yogeesh, J. Shi, X. Peng et al., Optothermoplasmonic Nanolithography for On-­Demand Patterning of 2D Materials, Adv. Funct. Mater., 2018, 28(41):1803990.

2.14  L  et's Do the Twist: Rotation-­tunable 2D Electronics Ever since the isolation of graphene in 2004, the rapidly growing family of two-­dimensional materials (more than 2500 other layered, atomically thin materials have been identified so far) cover an amazing range of electrical, chemical, optical, and mechanical properties. Perhaps the most astounding discovery is that they can be combined freely to create altogether new materials, so-­called van der Waals heterostructures. By stacking together any number of atomically thin layers, it becomes

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possible to create novel metamaterials and devices otherwise not achievable by traditional three-­dimensional materials. Researchers commonly observe a relative rotation between individual layers of 2D materials. Importantly, these interlayer rotation angles, i.e., the angle at which two individual layers are oriented towards each other, influence the electronic properties of the resulting material system. Scientists have not yet been able to explain why, during CVD growth of 2D layers, there is such a wide range of rotation angles of nucleated domains, leading to significant grain-­boundary formation. However, control over the rotation of the constituent 2D flakes is required in order to precisely tailor a material's electronic properties. This rotation-­ tunable design parameter is at the core of the emerging field of nanoscale twistronics. Now, researchers have revealed a general moiré-­driven mechanism that governs the interlayer rotation. “We have discovered that for a small flake of crystalline material that is weakly bonded to a crystalline substrate, it is possible to induce rotation of the flake by applying strain to the substrate,” says Harley T. Johnson, a professor at the University of Illinois at Urbana-­Champaign. “At the core of our findings is the concept of the van der Waals dislocation, which is the term we use to describe the commensurability/incommensurability defect in bilayer crystalline materials.” “Realizing that relative strain and rotation between a crystalline bilayer can be understood in terms of moiré patterns, which consist of van der Waals dislocations, it gives us a clear way to explain the energetics that control rotation of 2D material flakes,” he adds. “We are starting to understand the mechanics of these weakly interacting bilayer systems in new ways, and as we do, we realize that many of the interesting experimental observations that have been made in the past few years can be explained using the new framework that we have developed.” The fact that moiré patterns are dislocations implies that the energetics of the dislocations will have interesting consequences for certain material systems at certain length scales—and researchers are seeing those implications in nanoscale 2D material systems. As a result of this new understanding it will become possible for materials scientists to ‘dial in’ the desired rotation between 2D layers by applying the appropriate amount of strain to the substrate. For example, this new observation may make it possible to achieve the particular relative rotation between flake and substrate required for the recently discovered ‘magic’ angle effect in bilayer graphene, at which unconventional superconductivity can be found. The next stage for Johnson's team is to explore the precise strain–rotation relationships for a variety of materials and flake sizes, so that one can use this observation for the rational design of next-­generation microelectronic and optoelectronic devices.

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“We are continuing to explore the implications of the delicate energy balance associated with commensurability and incommensurability in 2D material systems, and the possible effects this energy balance will have for electronic properties of devices,” he concludes. “Obviously there are challenges in achieving synthesis and fabrication of device geometries, and in working with certain 2D materials, but the variety of materials that are now available is also tremendously interesting—this is the promise of van der Waals heterostructures.” Featured scientist: Professor Harley T. Johnson Organization: University of Illinois at Urbana-­Champaign, Urbana and Champaign, IL (USA) Relevant publication: P. Pochet, B. McGuigan, J. Coraux and H. Johnson, Toward Moiré engineering in 2D materials via dislocation theory, Appl. Mater. Today, 2017, 9:240–250.

Chapter 3

Not Found in Nature: Metamaterials and Metasurfaces 3.1  Introduction The prefix meta indicates that the characteristics of the material are beyond what we see in nature. Metamaterials are artificially crafted composite materials that derive their properties from internal microstructure, rather than chemical composition as found in natural materials. Human-­made, optical metamaterials have been touted for more than a decade for their ability to manipulate light in extraordinary ways. In theory, satellite imaging and interstellar telescopes could be dramatically smaller with metamaterial lenses that are one hundred times thinner than conventional ones. The core concept of metamaterials is to craft materials by using artificially designed and fabricated structural units to achieve the desired properties and functionalities. These structural units—the constituent artificial ‘atoms’ and ‘molecules’ of the metamaterial—can be tailored in shape and size, the lattice constant and interatomic interaction can be artificially tuned, and ‘defects’ can be designed and placed at desired locations. By engineering the arrangement of these nanoscale unit cells into a desired architecture or geometry, one can tune the refractive index of the metamaterial to positive, near-­zero, or negative values. Thus, metamaterials can be endowed with properties and functionalities unattainable in natural materials.

  Nanoengineering: The Skills and Tools Making Technology Invisible By Michael Berger © Michael Berger 2020 Published by the Royal Society of Chemistry, www.rsc.org

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For instance, in order for invisibility cloak technology to obscure an object or, conversely, for a ‘perfect lens’ to inhibit refraction and allow direct observation of an individual protein in a light microscope, the material must be able to precisely control the path of light in a similar manner. Metamaterials offer this potential. Although metamaterials have already revolutionized optics, their performance has been limited by their inability to function over broad bandwidths of light. Designing a metamaterial that works across the entire visible spectrum remains a considerable challenge. One hitch is that any such material needs to interact with both the electric and the magnetic fields of light. Most natural materials are blind to the magnetic field of light at visible and infrared wavelengths. Previous metamaterial efforts have created artificial atoms composed of two constituents—one that interacts with the electric field, and one with the magnetic field. A drawback to this combination approach is that the individual constituents interact with different colors of light, and it is typically difficult to make them overlap over a broad range of wavelengths. Another difficulty for engineers is that the ‘atoms’ and ‘molecules’ of metamaterials can be designed in countless variations. A certain shape, collectively, might bend light. Change that shape, the size, the spacing, or the material and that might amplify the effect, or diminish it, or cause something entirely different to happen, like twist the light one direction or another, or change its intensity or color. Among the most sought after properties of metamaterials is the negative index of refraction of light and other radiation. Negative refraction is based on the equations developed in 1861 by Scottish physicist James Maxwell. All known natural materials have a positive refractive index so that light that crosses from one medium to another gets slightly bent in the direction of propagation. For example, air at standard conditions has the lowest refractive index in nature, hovering just above 1. The refractive index of water is 1.33. That of diamond is about 2.4. The higher a material's refractive index, the more it distorts light from its original path. In some metamaterials, however, negative refraction occurs so that light and other radiation gets bent backwards as it enters the structure. With negative refraction materials, many applications become possible in electronics manufacturing, lithography, biomedicine, insulating coatings, heat transfer, space applications, and, perhaps, new approaches to optical computing and energy harvesting. The fascinating functionalities of metamaterials typically require multiple stacks of material layers, which not only lead to extensive losses but also bring a lot of challenges in nanofabrication. Many metamaterials consist of complex metallic wires and other structures that require sophisticated fabrication technology and are difficult to assemble. The unusual optical effects do not necessarily imply the use of volumetric (3D) metamaterials. It is also possible to manipulate light with the help of two-­dimensional structures—so-­called metasurfaces (or flat optics).

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3.2  Bottom-­up Assembled Chiral Metamolecules The application of chiral nanoparticles in chemistry, biology, and medicine is of great importance for the development of new molecular nanosystems. In chemistry, chirality usually refers to molecules and is a ubiquitous phenomenon in the natural world. A chiral molecule is a type of molecule that lacks an internal plane of symmetry and has a non-­superimposable mirror image; examples of this are amino acids and sugars. Two mirror images of a chiral molecule are called enantiomers or optical isomers. Pairs of enantiomers are often designated as right-­handed and left-­handed. Inspired by chiral molecular structures, scientists are developing strategies to build artificial chiral materials by mimicking natural molecular structures using functional materials. Specifically, metal nanomaterials exhibit tailorable optical properties upon excitation of surface plasmons and become one of the most promising components to realize chiral optical metamaterials. Understanding the optical chirality of organic molecules at the atomic scale is extremely challenging due to the difficulty in observing and controlling the atomic configuration at sub-­nanometer level. To that end, researchers have developed a macroscopic model to understand the origin of chirality, because the structures of the chiral metamolecules are observable under a microscope and also the optical chirality is several orders of magnitude stronger than the intrinsic chirality of organic molecules. In this research, scientists from The University of Texas at Austin and The City University of New York, mimic the organic chiral molecules found in nature by using optothermoelectric nanotweezers to assemble different metallic and dielectric meta-­atoms into chiral metamolecules using a laser (Figure 3.1). These metamolecules provide a macroscopic model to understand the origin of chirality at the atomic scale, while the approach provides a versatile platform to fabricate active chiral devices in nanophotonics and optofluidics. There are also other existing techniques to assemble colloids into chiral metamolecules. For example, DNA origami technology provides a programmable template for a precise organization of colloids into a variety of two-­dimensional or three-­dimensional chiral architectures. However, the tunability is still limited to a few pre-­established discrete states and on-­ demand reconfigurability is still elusive. This motivated the team to develop an on-­demand assembly technology, which can deal with colloidal particles in a wide range of materials and sizes, with versatile control of the molecular geometry and reconfigurability. “We use a laser beam to build chiral metamolecules ‘atom-­by-­atom’ mimicking the organic chiral molecules in nature, with their optical chirality analyzed in situ,” says Dr Linhan Lin, first author of a paper reporting

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Figure 3.1  A  color scanning electron microscopy (SEM) image of an assembled chiral metamolecule. (Image: Zheng Group, The University of Texas at Austin).

this research. “Specifically, the molecular structure is tunable and reconfigurable through steering the laser beam to obtain their enantiomers and diastereomers.” Using colloids (meta-­atoms) to mimic the atoms for bottom-­up assembly of chiral molecules is conceptually simple, while technically challenging, Lin explains. The technical bottle-­neck arises from the difficulty in picking up diverse meta-­atoms and organizing them into a specific geometry, which is beyond the capability of a traditional self-­assembly approach. To address this challenge, the team led by Professors Andrea Alù, Brian A. Korgel and Yuebing Zheng, propose using light as a tool to control the meta-­ atoms and to bond these meta-­atoms into molecular structures. This novel approach to build and detect chiral metamolecules with light could benefit applications in chiral sensing of drug molecules (the capability to switch the chiral metamolecules between their enantiomers enables them to identify and differentiate drug molecules upon application) and the fabrication of chiral photonic devices (the capability to selectively pick colloids with different sizes and materials enables the fabrication of active nanophotonic devices). In the next stages of their research, the team will investigate the interaction between the chiral metamolecules and organic chiral molecules in microfluidic devices for chiral sensing, i.e., identify the handedness of the chiral molecules. They will also add fluorescence colloids into the chiral metamolecules to investigate the interaction between the emitted photon and the chiral metamolecules to achieve colloidal chiral light sources. “With the development of fabrication and assembly technology in nanomaterials, we expect that the assembly of chiral metamolecules will become

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more rational and intelligent,” Zheng concludes. “We anticipate that in the near future, the assembly of chiral materials based on the required optical performance will become possible. Beyond the assembly technology itself, the capability to identify the meta-­atoms smartly and to design the geometry of chiral metamolecules based on the performance requirement will be the major challenges.” Featured scientists: Dr Linhan Lin (https://bit.ly/2Gl9wdo); Professors Andrea Alù, Brian A. Korgel, and Yuebing Zheng Organizations: The University of Texas at Austin, Austin, TX; The City University of New York, New York, NY (USA) Video: https://youtu.be/rkqpgL6DqiE Relevant publication: L. Lin, S. Lepeshov, A. Krasnok, T. Jiang, X. Peng, B. Korgel, et al., All-­optical reconfigurable chiral meta-­molecules, Mater. Today, 2019, 25, 10–20.

3.3  Ultrathin Plasmonic Chiral Metamaterials As we have seen in the previous sections, chiral metamaterials with strong chiroptical properties are an interesting new platform for optical signal modulation. Although plasmonic superchiral fields have been successfully applied to detect the chiral structures of proteins, it has remained challenging to detect the structural handedness of drug molecules due to their small size and the thinner film adsorbed onto the surface of metamaterials. Researchers have achieved these materials by stacking arrays of crosses, split rings, and rods with an interlayer twist. “These previously developed chiral metamaterials rely on site-­specific symmetry breaking, where the nanostructures in the top layer have a site-­specific twist with respect to the nanostructures in the bottom layer,” explains Yuebing Zheng, an assistant professor in the Department of Mechanical Engineering at The University of Texas at Austin. “It has remained unknown whether the twist between lattice directions of two periodic arrays can introduce strong optical chirality without the need to control the rotation of each unit.” His group's findings show that lattice-­dependent twist can also induce strong chiroptical properties, which show higher potential for practical applications due to the cost-­effective and high-­throughput fabrication, and their high tunability. The team reports a new type of plasmonic chiral metamaterial—called moiré chiral metamaterial (MCM)—by stacking two layers of identical achiral gold nanohole arrays into moiré patterns (Figure 3.2). “The chiroptical responses of the moiré chiral metamaterials can be precisely tuned by the in-­plane rotation between the two layers of nanohole

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Figure 3.2  Structural  property of MCMs. Schematic illustration and corresponding tilted-­view SEM images of MCMs with opposite handedness. (Image: Zheng Group, The University of Texas at Austin).

arrays,” says Zilong Wu, a PhD candidate and graduate research assistant in Zheng's group, and first author of a paper on this research. “We also demonstrated the label-­free enantiodiscrimination of chiral proteins and chiral drug molecules at the picogram level. With their ultrathin thickness (∼70 nm, which is only ∼1/10 of the operation wavelength), strong chirality, and high tunability, moiré chiral metamaterials will advance a variety of photonic and optoelectronic applications.” He points out that the existing chiral metamaterials on substrates are either inherently chiral plasmonic nanostructures or anisotropic achiral plasmonic nanostructures stacked into chiral structures with site-­specific twists. These prerequisites often lead to the use of sophisticated lithographic techniques such as e-­beam lithography and focused ion-­beam lithography that limit the practical applications. In addition, there are still problems in current techniques to achieve high tunability during the fabrication of chiral metamaterials. The researchers' new MCMs significantly lower the fabrication cost and complexity by combining conventional nanosphere lithography and a common wet-­transfer process.

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“Furthermore, we have demonstrated the label-­free enantiodiscrimination of chiral proteins using MCMs”, notes Wu. “The MCMs are also applied to distinguish R-­thalidomide (a drug molecule for pain killing) from its ‘evil enantiomer’ (S-­thalidomide, which causes deformity of newly born babies) at picogram level.” “Moiré chiral metamaterials can pave the way for commercially applicable chiral metamaterials due to the potential for high-­throughput fabrication,” says Zheng. “In addition, our enantiodiscriminative sensing results have shown the high sensitivity in detection of structural chirality of small chiral drug molecules, which is of crucial importance to the medical industry because the enantiomers of many chiral drugs have harmful effects on the human body.” In addition to enantiodiscriminative sensors that are significant to medical, food, health care, and the environmental industry, the strong optical chirality in MCMs can also be applied for broadband polarizers and polarization-­dependent photodetectors. Going forward, the scientists aim to improve the large-­scale uniformity of MCMs and the precision in controlling the relative in-­plane rotation angle between the two gold nanohole layers. Such improvement will pave the way for chiroptical devices including broadband polarizers and polarization-­ sensitive photodetectors. Featured scientists: Zilong Wu, Zheng research group (https://bit. ly/2WcDDN8) Organization: Department of Mechanical Engineering, The University of Texas at Austin, Austin, TX (USA) Relevant publication: Z. Wu and Y. Zheng, Moiré Chiral Metamaterials, Adv. Opt. Mater., 2017, 5(16), 1700034.

3.4  Large-­area Tunable Metasurfaces Graphene has shown extraordinary optical properties due to strong surface plasmon polaritons supported by the graphene nanostructure. Graphene metasurfaces show plasmonic resonance bands that can be tuned from mid-­ infrared (MIR) to terahertz (THz) regimes. These plasmonic devices can be used for biosensing, spectroscopy, light modulation, and communication applications. Commonly fabricated graphene plasmonic metasurfaces are usually single band. Plasmonic metasurfaces with multiband resonance peaks have proven to be extremely effective in molecular detection, with ultrahigh sensitivity and accuracy. Multiband devices at variable electromagnetic spectra are also highly desired to meet the strong demand for ever-­increasing accuracy and high speed in surveillance and communication systems.

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Researchers at The University of Texas at Austin, led by Professors Yuebing Zheng and Deji Akinwande, demonstrated for the first time an effective method to pattern large-­area graphene—which is grown by chemical vapor deposition (CVD)—into moiré metasurfaces with gradient nanostructures having multiband resonance peaks in the mid-­infrared range. “In our work, the CVD graphene is patterned into moiré metasurfaces via combination of moiré nanosphere lithography (MNSL) with oxygen reactive ion etching (RIE),” notes Zilong Wu, researcher at University of Texas at Austin, and the paper's first author. “In brief, colloidal polystyrene (PS) nanospheres self-­assemble into a monolayer on substrates with graphene. A second monolayer of PS nanospheres is then deposited on top of the first PS monolayer via a similar process.” Wu explains that the relative rotation angle between the first and second PS monolayer can be controlled to obtain various moiré patterns. The following RIE with O2 creates voids between closely packed nanospheres and etches away graphene that is exposed to O2 plasma by voids. After removal of the nanosphere residue, graphene sheets with moiré patterns are then left on the substrates (Figure 3.3).

Figure 3.3  (a–c)  Schematics of the fabrication processes of graphene moiré meta-

surfaces on Si substrates (blue). θ denotes the relative rotation angle between the bottom (green) and top (red) monolayers of nanospheres. (d) SEM image of a representative graphene moiré metasurface. Reproduced from http://dx.doi.org/10.1002/adom.201600242 with permission from John Wiley and Sons, © 2016 WILEY-­VCH Verlag GmbH & Co. KGaA, Weinheim.

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“By varying the relative rotational angle between top and bottom monolayers of PS nanosphere during MNSL, the size and shape of the graphene nanostructures in the metasurfaces change significantly,” Wu adds. Wei Li, the paper's joint first author, points out that “our results demonstrate that graphene moiré metasurfaces with gradient and complex moiré patterns can be controllably fabricated in cost-­effective and scalable way.” Maruthi Yogeesh, a graduate student in Professor Akinwande's research group, and a co-­author of the paper working on Fourier transform IR characterization of these graphene metasurfaces, notes that these devices support surface plasmon resonance with a wavelength that is highly dependent on the shape and size of the nanostructures. “Under light illumination, the surface plasmon resonances can be excited. Due to the large gradient and the high complexity of graphene moiré metasurfaces, they are promising candidates for tunable multiband plasmonic devices,” he says. “We prove this by measuring the transmission extinction of graphene moiré metasurfaces with different patterns. The number and the wavelength of resonance peaks of the graphene metasurfaces are highly dependent on the patterns.” “We have demonstrated large-­area graphene moiré metasurfaces by MNSL,” Professor Zheng concludes. “Our studies reveal that the extinction spectra of the graphene metasurfaces can be controlled by the size and shape of the graphene nanostructures. We also demonstrate for the first time multiband graphene metasurfaces in mid-­infrared range by the complex and gradient nanostructures in moiré patterns.” “These tunable and multiple plasmon resonance modes in the MIR and THz range of electromagnetic spectra make graphene moiré metasurfaces very promising candidates for ultrathin light modulators, biosensors, flexible optoelectronics, and photodetectors,” says Professor Akinwande (who, in 2016, received the prestigious presidential early career award (PECASE) for his group's outstanding work on 2D material-­ based electronics). Featured scientists: Professors Yuebing Zheng's (https://bit.ly/2WcDDN8) and Deji Akinwande's (https://bit.ly/2Ob7EXb) research groups Organization: The University of Texas at Austin, Austin, TX (USA) Relevant publication: Z. Wu, W. Li, M. Yogeesh, S. Jung, A. Lee, K. McNicholas, et al., Tunable Graphene Metasurfaces with Gradient Features by Self-­ Assembly-­Based Moiré Nanosphere Lithography, Adv. Opt. Mater., 2016, 4(12), 2035–2043.

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3.5  H  ow to Realize Metasurfaces in Novel Plasmonic Materials Compact optical components are crucial to realize miniaturized optical systems and integrated optoelectronic devices. Plasmonic metasurfaces—structured materials in 2D with rationally designed, sub-­ wavelength-­scale building blocks—have drawn great interest because they can control light based on sub-­wavelength structures. These planar devices are attractive for applications ranging from high-­resolution imaging to 3D holography. Recent work has further shown that metasurfaces with a thickness below 200 nm can achieve efficient polarization control and ultrasensitive biosensing. Integrated optoelectronic devices based on metasurfaces, however, are challenging to achieve, because traditional plasmonic materials, including silver and gold, are not compatible with current semiconducting processing. Recently, titanium nitride (TiN) has received attention as an unconventional plasmonic material because of its potential CMOS (complementary metal–oxide–semiconductor) compatibility as well as exceptional mechanical strength and high-­temperature stability required for operation in extreme conditions. Despite tremendous interest, experimental demonstration of TiN metasurfaces has not been reported. One major challenge is that traditional metasurface designs require stringent accuracy in fabrication, while patterning TiN nanostructures precisely is extremely difficult. Work by the Odom group at Northwestern University describes the design and prototyping of single-­crystalline TiN plasmonic metasurfaces based on sub-­wavelength hole arrays. “We developed an evolutionary algorithm to design plasmonic metasurfaces and an associated prototyping method for single-­crystalline TiN films,” says Teri W. Odom, Charles E. and Emma H. Morrison Professor of Chemistry and Professor of Materials Science and Engineering at Northwestern University. “Implemented under the object-­oriented paradigm, the versatile, black-­box design program was highly customizable and user-­friendly.” The algorithm employed a novel objective function that can achieve multispot focusing, where both the optical patterns and focal intensity can be tuned accurately. The sizes and shapes of nanoholes can be tailored to achieve multiphase TiN metasurfaces. Previously, Odom's team has reported a lattice evolution algorithm as an efficient approach to design nanohole metasurfaces in a gold film. In that work, the researchers developed a highly efficient, universal algorithmic method based on evolutionary principles for the design of ultrathin achromatic lenses.

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The team's new work features an object-­oriented lattice evolution algorithm and a parallel patterning process, to prototype designs. Using anisotropic nanohole shapes, the team demonstrated dynamic tuning of the optical responses by changing the polarization state of incident light. “The optimize-­and-­prototype platform we established here can contribute to expanding metasurface designs to a wide range of applications requiring unconventional materials systems,” notes Odom. “This might be of great interest to the computational materials science community interested in developing efficient design methods for structured metamaterials.” This platform could also be used in fast, versatile patterning techniques for prototyping nanostructures on unconventional materials. Finally, the nanophotonics community interested in achieving compact, micro-­optics systems for high-­temperature operations could put this new prototyping technique to good use. Featured scientists: Teri W. Odom's research group (https://bit.ly/2xCVsY9) Organization: Northwestern University, Chicago, IL (USA) Relevant publication: J. Hu, X. Ren, A. Reed, T. Reese, D. Rhee, B. Howe, et al., Evolutionary Design and Prototyping of Single Crystalline Titanium Nitride Lattice Optics, ACS Photonics, 2017, 4(3), 606–612.

3.6  Full-­color 3D Metaholography Holography is an important branch of optics, which designs and reconstructs electromagnetic waves with the whole information of light, i.e., phase and amplitude. This technique enables a light field—the product of laser light scattered off objects—to be recorded and later reconstructed when the original light field is no longer present, due to the absence of the original objects. Holography can be thought of as somewhat similar to sound recording, whereby a sound field created by vibrating matter, such as musical instruments or vocal cords, is encoded in such a way that it can be reproduced later, without the presence of the original vibrating matter. Holograms are already a ubiquitous part of our lives. They are in our wallets—protecting credit cards, bank notes, and passports from fraud— in grocery store scanners, and in biomedical devices. However, the realization of a matrix-­free, full-­color 3D display for the naked eye is still some way off. The principle of holography has greatly expanded visualization possibilities, in particular through the widespread use of computer-­generated holograms. The large pixel size and the limited phase modulation ability of

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Figure 3.4  An  illustration of full-­color metaholography. (Image: State Key Laboratory of Optical Technologies on Nanofabrication and Microengineering, Chinese Academy of Science).

traditional computer-­generated holograms, however, usually restricts the performance of holography applications, e.g., the limited viewing angle and the 3D full-­color imaging ability. A research team, led by Professor Xiangang Luo from the State Key Laboratory of Optical Technologies on Nanofabrication and Microengineering (SKLOTNM), Institute of Optics and Electronics, Chinese Academy of Sciences, and Professor Minghui Hong's team from the Department of Electrical and Computer Engineering, National University of Singapore, have demonstrated that full-­color 3D metaholography imaging with extended viewing angles can be realized by a single layer of a nanostructured metallic surface (Figure 3.4). “We have demonstrated full-­color 3D metaholography imaging by a metahologram which is constructed using electron-­beam lithography (EBL) to fabricate nanoslit antenna arrays, i.e., nanorectangular holes, in a 75 nm-­ thick chromium film,” says Xiong Li, a researcher at SKLOTNM and the first author of a paper on this work. He notes that the phase modulation within the entire visible color range is fully controlled by spatially adjusting the orientation angle of the nanorectangular holes, due to the intrinsic achromatic feature of the structure. The minimum pixel size of the hologram is only 200 × 200 nm2, which is much smaller than the wavelength of visible light. So the viewing angle of the holographic projection can be larger than ± 90°, i.e., imaging in whole half-­space. “In order to overcome the cross-­talk among different colors that normally exists in current metasurface holography, we introduced an off-­axis illumination method to shift the holographic image in different colors and successfully reconstructed all visible colors in the imaging area,” explains Li. Taking advantages of the achromatic feature of the structure, the team also demonstrated full-­color holography based on seven primary colors and 3D holographic imaging.

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The scientists conclude that their novel approach offers new routes for many exciting applications, such as the angle-­tuned dynamic metahologram and matrix-­free 3D display for the naked eye. Various fields, including data storage, security, and authentication, can also benefit from this technique. Featured scientists: Professors Xiangang Luo's (https://bit.ly/2DfpF22) and Minghui Hong's (https://nus.edu/2Uez6o5) research groups Organizations: Institute of Optics and Electronics, Chinese Academy of Sciences, Chengdu (China); National University of Singapore, Singapore (Singapore) Video: https://youtu.be/8h549Wp73g0 Relevant publication: X. Li, L. Chen, Y. Li, X. Zhang, M. Pu, Z. Zhao, et al., Multicolor 3D meta-­holography by broadband plasmonic modulation, Sci. Adv., 2016, 2(11), e1601102.

3.7  V  an der Waals Heterostructures with Tunable Interfacial Coupling Ever since the first demonstration of 2D materials, the scientific community has pursued the possibility of building materials by artificial stacking of different ultrathin materials one on top of the other. These materials are called van der Waals heterostructures (vdWHs). Due to their unique interlayer coupling and optoelectronic properties, these materials are of considerable interest for the next generation of nanoelectronics. Conventional 2D heterostructures are usually composed of two layers of oppositely charged carrier types using inorganic materials. One of the challenges when creating 2D heterostructures is the painstaking stacking of the individual components on top of each other. Researchers have now found, for the first time, that there can also be charge transfer (CT)-­induced interfacial coupling between two different pairs of organic CT layers. Moreover, external stimuli can be harnessed to tune the interfacial coupling to control the physical properties of organic van der Waals heterostructures. “The interface of heterostructures can provide many possibilities to generate new phenomena,” says Shenqiang Ren, an associate professor in Mechanical Engineering at Temple University. “Our group has years of experience of investigating organic CT complexes. As the interface between two layers of organic CT complex is different from the single phase, such as intralayer and interlayer CT interactions, it is exciting to investigate the interfacial coupling within organic van der Waals heterostructures.”

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The most significant finding in this work is that the researchers found strong and anisotropic interfacial charge-­transfer coupling in two-­ dimensional organic van der Waals heterostructures. As the team (a collaboration between Ren's group and Professor Michael L. Klein's High-­Performance Computing group at Temple University) points out, such interfacial CT coupling enables external stimuli-­controlled physical properties in organic vdWHs. To prepare 2D vdWHs, researchers use a conventional CVD approach. However, the weak interlayer interaction between the monolayer 2D film and the substrate leads to island growth rather than continuous monolayers. The approach demonstrated by the Temple University team allows for large-­scale (mm2 range) assembly of freestanding 2D CT heterostructures with controlled orientation and unique physicochemical properties. As they note, the interfacial coupling can be tuned over a large range by external fields (ferroelectric and magnetic) with strong enhancement of current and capacitance, due to the coupling across two CT pairs along the vertical and horizontal orientations. “As our vdWHs show multifunctional external stimuli dependent response (such as piezoresistance, electroresistive, and magnetoconductance), they can be applied for multifunctional sensors,” says Ren. “For example, the vdWHs exhibit excellent pressure-­dependent sensitivity with a high piezoresistance coefficient of −4.4 × 10−6 Pa−1.” “The organic vdW heterostructures formed by the combination of two pairs of organic CT complex with tunable interfacial coupling show overwhelming advantages over single components,” he adds. “They exhibit unique physicochemical properties covering a vast majority of opto-­electrical-­magnetic-­ mechanical topics.” The researchers' next steps will be focused on the synthesis of wafer-­scale two-­dimensional vdWHs towards artificial intelligence applications. Since the organic vdWHs demonstrate a fast response to external stimuli with change in conductivity, this opens up practical applications for multistimuli sensors.

Featured scientists: Professor Shenqiang Ren's Renewable and Emerging Nanomaterial (REN) Laboratory (https://bit.ly/2Z8wZpE) Organization: Department of Mechanical Engineering, Temple University, Philadelphia, PA (USA) Relevant publication: B. Xu, H. Chakraborty, V. Yadav, Z. Zhang, M. Klein and S. Ren, Tunable two-­dimensional interfacial coupling in molecular heterostructures, Nat. Commun., 2017, 8(1), 312.

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3.8  A  Rewritable Metacanvas for Photonic Applications Tunable materials can change their innate optical properties on demand, rather than relying on mechanical components to focus an object such as a camera lens or telescope eyepiece. Substances like vanadium dioxide (VO2), for instance, can transition between opaque and transparent under a particular set of conditions, like a certain temperature, making them difficult to incorporate into useful devices. Previous tunable photonic devices based on VO2 are mostly lithographically made, and switchable but not fully reconfigurable. This is because they have permanent patterns and fixed functionalities. Note that reconfigurable devices allow changes of patterns and functionalities from X to Y to Z, etc., while the patterns in switchable devices are fixed and can only be switched between X-­on and X-­off. “In stark contrast, our metacanvas is lithography-­free and fully reconfigurable,” says Junqiao Wu, a professor in the Department of Materials Science and Engineering at the University of California, Berkeley. “Both the patterns and the functionalities of the metacanvas can be arbitrarily reconfigured, which leads to many more degrees of freedom in metasurface design and functionalities.” He adds that one piece of metacanvas is able to function as different optical components—hologram, phase array, polarizer, modulator, etc.—at different times and on command, which has never been achieved in any of the previous VO2 devices. The metacanvas is a completely new generation of technology compared to all previous works. The metacanvas is based on the phase transition of vanadium dioxide, whose conductivity, color, and structure all change with temperature. Specifically, VO2 is in an insulating phase at room temperature, and changes into a metallic phase at temperatures above 67 °C. Raising the temperature of its environment to 67 °C, VO2 still stays in the insulating phase due to a slight delay (so-­called hysteresis) in the phase transition. Then, a writing laser beam is focused onto the VO2 film, and locally heats up the material into the metallic phase. By moving the laser beam under the control of a computer, almost arbitrary metallic patterns can be written onto the insulating film as background. These patterns can be erased simply by cooling the surrounding temperature, so that the entire VO2 film goes back to the insulating phase. The metallic pattern on the insulating background is the metamaterial, and would deliver the photonic functions. “Analogous to the revolution from pre-­printed transparency slides to the current digital projectors enabled by the modern digital light processing technology, our approach enables full dynamic control and true reconfigurability in metamaterials,” notes Assistant Professor Jie Yao, who, together with Wu, led this work.

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The motivation behind this research lies in the principal and experimental limitations of conventional optics by their fixed functionalities. To construct an optical system, one has to purchase a large number of optics and arrange them in a complex layout, for example, lenses with different focusing lengths, linear polarizers working at different wavelengths, etc. Researchers sometimes feel frustrated when a specific optics component is missing, and have to order and wait for its arrival. The reconfigurable photonics enabled by the metacanvas would fill this technology gap. Now, researchers have successfully achieved a rewritable photonic platform on which nearly arbitrary metamaterials can be rapidly and repeatedly written and erased—just like an Etch-­A-­Sketch™ drawing toy to write metamaterials. “Previously, the functionality and/or technical specification of an optical device is fixed after fabrication, leading to fundamental limitations,” says Kaichen Dong, the first author on a paper about this work. “With the metacanvas, one can realize in situ reconfiguration of optical/photonic devices, and even fully reconfigurable photonic circuitry/system. Moreover, the real-­time manipulation of light enabled by metacanvases allows scientists a peek into temporal evolution of photonic phenomena. Similar examples exist in other fields, such as the field-­programmable gate arrays (FPGA) in electrical engineering, which greatly improved embedded systems.” Such a metacanvas could benefit applications in the field of photonic metamaterials and could function as a fully reconfigurable optical/photonic device, which can change its functionality or technical specifications in real time. For example, a single device can serve as a linear polarizer in the beginning, but transform into a beam steer at a later time when needed. Immediate application might be a reconfigurable phase array, which would greatly boost the performance of radar, communication, biomedical sciences, holography, optical tweezers, etc. “Furthermore, the metacanvas could enable dynamic transition in optics without physically replacing the optical components,” notes Wu. “Hence, multiple metacanvases could work together to construct a dynamic optical system without moving parts, where photonic elements can be field-­programmed to deliver complex, system-­level functionalities.” This work also helps scientists to investigate dynamic optical phenomena by experimentally making smooth transitions between static stages, without interrupting the optics. “Moreover, since different microstructures can be rapidly and repeatedly patterned on the metacanvas, it could help engineers test their structural design of optics in a fast and economical way—just write their designs on the metacanvas and see whether they work as expected,” adds Dong. “We call this a ‘physical simulator’.”

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Finally, the idea of a metacanvas itself is inspiring, and could motivate researchers to look into other materials with phase transition for similar and new applications. Going forward, the team is seeking opportunities to unleash the full potential of the metacanvas, such as in three dimensions. Aided with more advanced facilities, one can envision further application of the metacanvas technology to more advanced industrial and scientific applications. “Reconfigurability will still be intensively pursued in the photonic field, because it is able to significantly improve the performance of optical systems, and lead to new findings/inventions which are difficult, or even impossible, to be experimentally achieved otherwise,” says Wu. “New mechanisms to fulfill the reconfigurability will be found, and existing approaches will be technically improved—we could eventually have an ‘optical FPGA’ in the future.” “One of the largest challenges is the cost of reconfigurable devices,” Wu concludes. “Reconfigurable photonic devices have to be more cost effective before they could be practically utilized.” Featured scientists: Professor Junqiao Wu's research group (https://bit. ly/2DdWM6i) Organization: Department of Materials Science and Engineering, University of California, Berkeley (USA) Video: https://youtu.be/v9coab4JWJ8 Relevant publication: K. Dong, S. Hong, Y. Deng, H. Ma, J. Li, X. Wang, et al., A Lithography-­Free and Field-­Programmable Photonic Metacanvas, Adv. Mater., 2017, 30(5), 1703878.

3.9  Dynamic Plasmonic Pixels Plasmonic metamaterials are human-­made media that acquire unusual optical properties due to nanostructuring. They operate by harnessing properties of resonant surface plasmons, which are collective oscillations of free electrons on the surface of metallic nanostructures. For more than 20 years, researchers have designed a variety of optical metamaterial-­based devices, from those hiding objects for certain colors of light to those sensitive to tiny concentrations of substances. Many of these demonstrations so far present structures that are static in time. In other words, once they are fabricated, their properties are fixed. “These technologies become much more powerful if they are able to react to external stimuli—such as electric fields—to reconfigure themselves in order to offer a different set of properties,” says Jake Fontana, PhD, a research physicist at the US Naval Research Laboratory in Washington, DC. “This motivated us to build upon established work in plasmonic color generation to craft plasmonic devices that could be dynamically tuned.”

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Fontana and his collaborators from the US Naval Research Laboratory and the Air Force Research Laboratory, demonstrate a display pixel that can switch on and off at least 1000 times faster than pixels that use conventional liquid-­crystal materials (Figure 3.5). “We show that plasmonic gold nanorods—which interact very strongly with light—can be aligned using electric fields, and we use this alignment to control the amount of light that can pass through the pixel,” explains Fontana. “By engineering the dimensions and material structure of the nanorods, we design pixels that work with different colors of light, both visible and infrared.” The researchers take advantage of two key properties of metal nanorods in order to achieve functional pixels. First, the nanorods couple strongly to external electric fields, enabling alignment of individual nanorods without the need for near–neighbor interactions, as required for liquid-­crystal materials. Second, the absence of the near–neighbor interactions enables the nanorods to respond very quickly to the application and removal of the electric fields. By combining these two effects, it becomes possible to use an electric field to rotate the nanorods within a pixel to rapidly modulate its optical properties. In their work, the team details how the attributes of the nanorods can be engineered to optimize the pixel's operation. They illustrate, first, by shortening or lengthening the nanorod dimensions, and second, by growing a thin silver shell over the gold nanorod core, that the nanorods can be tailored to match desired light wavelengths across the visible and infrared spectra.

Figure 3.5  Schematic  rendering of the plasmonic pixel structure. (Image: Jake Fontana and Robert Gates, US Naval Research Laboratory).

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They characterize the performance of the plasmonic pixels through chromaticity and luminance metrics, and they showcase the spatial, spectral, and temporal control of light by fabricating a seven-­segment numerical indicator. The team points out that the fast switching time of their plasmonic nanorod pixels could significantly reduce motion blur and other temporal artifacts in displays, in comparison with conventional LCDs. In addition to the visible light display applications, they envision the dynamic plasmonic nanorod pixel as a generic spatial light modulation platform for arbitrarily controlling the phase, amplitude, and polarization of light on a fast time scale. This may enable integration with optical information processing and communication technologies. “Our approach could enable technologies that have been challenging to implement with traditional liquid-­crystal materials, which have limited switching speeds,” says Fontana. “One example is field-­sequential color displays, where the red, green, and blue components that make up a color image are displayed in rapid succession in time—so quickly that the eye perceives them as a single full-­color image.” “Presenting the color-­component images rapidly enough requires a material with a fast response time, such as the plasmonic nanorods in our study,” he adds. “This contrasts with traditional LCDs that use a white light illuminant with individual color subpixels that must be patterned across the display. By eliminating the need for color subpixels, a field-­ sequential color display can pack more pixels per area, improving display resolution.” One important step of the team's ongoing investigations will be the integration of the plasmonic nanorod materials into arrays with larger quantities of pixels, while scaling down the size of individual pixels. Featured scientist: Dr Jake Fontana Organization: US Naval Research Laboratory and the Air Force Research Laboratory, Washington, DC (USA) Relevant publication: N. Greybush, K. Charipar, J. Geldmeier, S. Bauman, P. Johns, J. Naciri, et al., Dynamic Plasmonic Pixels, ACS Nano, 2019, 13(4), 3875–3883.

3.10  I mproving Terahertz Detection with Metasurfaces Terahertz (THz) radiation is a very safe type of radiation due to its low-­energy photons, with more than a hundred times less energy than that of photons in the visible light range. This means that THz imaging could, in some

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applications, replace X-­rays, which are much more damaging to materials and organic tissue. But precisely because it has low energy, THz radiation is challenging to detect. Already THz technology is becoming a key element in security applications (such as airport scanners), wireless data communication, medical diagnostics, astronomy, and quality control. Terahertz frequencies, which occupy a middle ground between microwaves and infrared light, are seen as the future of wireless communications because they offer a higher bandwidth capacity for data transmission than in the currently used microwave radiation. A key technology for existing photonic devices operating in the THz frequency range is an ultrafast photoconductive (PC) switch, an optoelectronic element that changes electrical conductivity under illumination between a highly resistive (OFF) and highly conductive (ON) state within a sub-­ picosecond time period. “The PC switch performance relies on the following three essential requirements: (1) a sub-­picosecond recombination time for photoexcited charge carriers; (2) a high contrast in conductivity for the ON and OFF states; and (3) an efficient conversion of photons to charge carriers,” Dr Oleg Mitrofanov from University College London (UCL), explains. “To produce an efficient PC switch, all three requirements must be met simultaneously.” Addressing these challenges, Mitrofanov and his collaborators from UCL and Sandia National Laboratories have developed a perfectly absorbing photoconductive metasurface that can be switched from an electrically insulating state to a highly conductive state with light of specified wavelength. This perfect absorption without the incorporation of any metallic structures or back reflectors is enabled by nanostructures supporting two resonant modes of opposite symmetry—odd and even with respect to the metasurface plane—provided that the two modes are degenerate and critically coupled. This metasurface—a very thin layer of carefully nanostructured material— is practically invisible when it is placed onto glass. The perfect absorption within this metasurface allows one to make the active region of THz wave detectors significantly thinner in comparison with conventional detectors. Apart from reducing the size of THz detectors, it also improves their efficiency (Figure 3.6). To demonstrate the practical applicability of their design, the researchers integrated this metasurface into photoconductive THz detectors and achieved THz pulse detection with a signal-­to-­noise ratio of more than six orders of magnitude using an unprecedentedly low level of ultrafast laser excitation of 100 µW. “Our work has direct implications for THz photoconductive detectors, which can be made smaller and would require optical power about ten times less than the conventional detectors,” Mitrofanov points out. “We also

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Figure 3.6  Conceptual  illustration of the perfectly absorbing photoconductive

metasurface comprising a network of resonators with broken symmetry and integrated into a THz detector. The inset at the bottom left illustrates schematically two resonant modes (magnetic dipoles Hx and Hz) supported by the resonators. Reproduced from http://dx.doi. org/10.1021/acs.nanolett.8b05118 with permission from American Chemical Society, Copyright 2019.

anticipate that functionality achieved with this photoconductive network can be applied to other applications, for example efficient modulators and THz emitters.” The scientists are interested in integrating these photoconductive metasurfaces into practical THz devices, for example in THz near-­field microscopy probes, which would enable THz imaging and spectroscopy with a spatial resolution much better than the diffraction limit. Mitrofanov adds that the concept of a photoconductive metasurface, where not only optical, but also electronic properties are engineered, introduces another dimension for metasurface research, which he anticipates will find several practical applications. Featured scientist: Dr Oleg Mitrofanov (https://bit.ly/30O3476) Organization: University College London, London (UK) Relevant publication: T. Siday, P. Vabishchevich, L. Hale, C. Harris, T. Luk, J. Reno, et al., Terahertz Detection with Perfectly-­Absorbing Photoconductive Metasurface, Nano Lett., 2019, 19(5), 2888–2896.

Part 2 Nanotechnology Unleashed

           

Chapter 4

Plasmonics The fascination with nanotechnology stems from the unique quantum and surface phenomena that matter exhibits at the nanoscale, making possible novel materials and revolutionary applications. The collection of stories in this book is barely scratching the surface of the vast and rapidly growing body of nanotechnology-­related research across a broad range of disciplines. Nanotechnology is not an industry, nor is it a single technology or a single field of research. What we call nanotechnology consists of sets of enabling technologies applicable to many traditional industries. Therefore it is more appropriate to speak of nanotechnologies in the plural. Chapters 4–7 contain a selection of intriguing developments in four key areas—plasmonics, nanobiotechnology, nanomedicine, characterization— to give the interested reader an idea of the incredibly diverse aspects that make up current nanotechnology research and development.

4.1 Introduction Nanoplasmonics research focuses on the optical phenomena in the nanoscale vicinity of noble-­metal surfaces, which serve as nanoscale analogs of radio antennas and are typically designed by using antenna theory concepts. Surface plasmons are ‘ripples’ of electrons that resonate across the surface of a metal nanoparticle when triggered by light (by converting free photons into localized charge-­density oscillations). The light they receive at one wavelength (i.e., the color) is radiated at the same wavelength, and that can inform scientists about the particle and its environment, for instance, the presence of chemicals. Plasmons also enable photochemistry and selectively catalyzed chemical reactions.

  Nanoengineering: The Skills and Tools Making Technology Invisible By Michael Berger © Michael Berger 2020 Published by the Royal Society of Chemistry, www.rsc.org

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Plasmonics allows breaking the diffraction limit of light into sub-­ wavelength dimensions, enabling strong-­field enhancements. This plasmonic field enhancement is key to numerous applications, ranging from surface-­ enhanced spectroscopy, sensing, non-­linear optics, and light-­activated cancer treatments to the enhancement of light absorption in photovoltaics and photocatalysis.

4.2 Naked-­eye Plasmonic Colorimetry Point-­of-­care diagnostics, food safety screening, and environmental monitoring will massively benefit from the label-­free, inexpensive, rapid, handheld sensor devices that are currently under development. During the past few years, research efforts have been devoted to maximizing surface-­enhanced Raman spectroscopy (SERS) signals from molecules located inside or near nanoscale gaps between plasmonically active metallic nanostructures on the SERS substrates. SERS relies upon a fundamental phenomenon in physics called the Raman effect—the change in the frequency of monochromatic light (such as a laser) when it passes through a substance. Properly harnessed, Raman scattering can identify specific molecules by detecting their characteristic spectral fingerprints. In SERS, the Raman effect is greatly enhanced when it is close to a rough metal surface consisting of gold or silver nanoparticles, due to surface plasmon resonance. In recent years it has been demonstrated that single-­ molecule detection with SERS is possible. Developing these impressive research results into large-­area plasmonic sensing platforms has been a major challenge, though. “To date, there has been a lot of work reported on either SERS or plasmonic sensing but very few have reported sensing with the same device for both SERS and plasmonics, let alone plasmonic colorimetry naked-­eye sensing,” says University of Illinois alumnus Zhida Xu. For the first time ever, researchers at the University of Illinois' Micro and Nanotechnology Laboratory, led by Logan Liu, have reported the combination of naked-­eye plasmonic colorimetry and high-­enhancement and high-­ uniformity SERS in one sensor. They call this dual-­mode sensor FlexBrite (Figure 4.1). “What has motivated us is the desire for a label-­free photonic sensor, which can take both qualitative and quantitative measurement,” recalls Xu, who was first author on a paper on this research. “SERS is good at qualitative but bad at quantitative analysis, while plasmonic sensing has the opposite characteristics. So we designed a structure with excellence in both functions.” The revolutionary method of label-­free molecule detection developed by the team can easily answer what substances, and how much of each, are in a liquid. “At first glance, FlexBrite is a thin, bendable, plastic-­based wafer that shines purple in the light,” says Xu. “At the nanoscale, however, it's criss-­ crossed with tiny bumps. We call these structures ‘nano-­mushrooms’, which bend the light reflected off them and account for FlexBrite's color-­changing properties, allowing researchers to analyze liquids much more efficiently.”

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Figure 4.1 Overview of FlexBrite substrate. (a) Photograph of one piece of FlexBrite. (b) Scanning electron microscopy image of the surface of FlexBrite. Reproduced from http://dx.doi.org/10.1039/C5NR08357E with permission from the Royal Society of Chemistry.

“We conducted a lot of research and simulations to design this ‘nano-­ mushroom’ structure in order to obtain its unique properties,” explains Xu. “The plasmonic color shift of FlexBrite induced by refractive index of liquid is so drastic that it can be detected by the naked eye, making the testing equipment very simple and low cost. Even a cell phone camera can be used to analyze the color shift.” As a result, FlexBrite can be used to tell what is in a liquid and how much of each substance there is, even down to trace amounts if a handheld spectrometer is used. Detection of chemicals in liquid can be quickly done and the concentration of the chemicals in the liquid can be quickly measured. Applications include finding pollutants in water; detecting trace amounts of the narcotic methamphetamine in a drink; or determining the amount of ethanol in gasoline; all within 1 minute and at very low cost. The researchers say that their current fabrication process for FlexBrite results in a unit cost of about US$ 2 per square centimeter. This compares very favorably to the $40–$120 cost per chip of SERS sensors on the market today. “With mass production processes the cost for FlexBrite could be brought even further down,” says Xu. Featured scientists: Professor Logan Liu's Nanobionics research group (https://bit.ly/2DbLsYA) Organization: Nanobionics Group, University of Illinois, Urbana, IL (USA) Video: https://youtu.be/2iHsW_8oSxQ Relevant publication: Z. Xu, J. Jiang, X. Wang, K. Han, A. Ameen, I. Khan et al., Large-­area, uniform and low-­cost dual-­mode plasmonic naked-­ eye colorimetry and SERS sensor with handheld Raman spectrometer, Nanoscale, 2016, 8(11):6162–6172.

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4.3 Monitoring UV Exposure with a Tunable Adhesive Patch Moderate exposure to sunlight has significant health benefits—vitamin D production, beneficial modulation of blood pressure, and psychological effects of wellbeing. However, exposure to ultraviolet (UV) radiation is also a major risk factor for most skin cancers. That means that, while moderate exposure to sunlight is recommended, there is a fine line to walk between beneficial and harmful amounts of UV exposure. To take the guesswork out of assessing the exposure to damaging UV rays, several wearable consumer UV sensors have already hit the market. Researchers in Mexico have proposed a simple and low-­cost stick-­on nanoplasmonic patch made of optically active silver nanoparticles (AgNPs) embedded in a film of nanopaper. The patch changes color once it has been exposed to a certain amount of UV light (Figure 4.2). “Before Sun exposure, the patch embeds silver nanoparticles with an average diameter,” explains Dr Eden Morales-­Narváez, a research scientist in the Biophotonic Nanosensors Laboratory at the Centro de Investigaciones en Óptica A.C. in Mexico. “Upon UV radiation, the original size of the particles

Figure 4.2 Operational mechanism of the wearable nanoplasmonic patch.

Before Sun exposure (A), the patch embeds silver nanoparticles with an average diameter, a. The original size of the particles embedded in the patch undergoes a modulation caused by a photolytic phenomenon upon Sun/UV radiation (B), resulting in a new average particle diameter, b. This modulation of the nanoplasmonic properties of the patch is responsible for a change in color that is visually observable. Reproduced from http://dx.doi.org/10.1021/acs.analchem.7b04066 with permission from American Chemical Society, Copyright 2017.

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embedded in the patch undergoes a modulation, resulting in a new average particle diameter. This modulates the nanoplasmonic properties of the patch, resulting in a visually observable color change.” The main scaffold of this UV-­responsive patch is made of bacterial cellulose nanopaper (average thickness of 16 µm), which is a flexible, lightweight, optically transparent, and biocompatible material. Other approaches are based on polymer technology and require extra filters to be tuned according to the skin type of the user, which increases the complexity and cost of the resulting device. According to the team, their entire device is manufactured following a simple and green route, is scalable, and costs about US$ 0.05 for each patch (at laboratory scale). Also, it can be tuned—in terms of nanoparticle density—according to the skin type of the user without the need for extra filters. The World Health Organization classifies skin types into six groups (I– VI) ranging from pale or freckled skin (type I) to naturally brown and black (types V and VI). The level of Sun/UV exposure that these groups can bear in a safe way varies significantly. Accordingly, a meaningful UV sensor needs to be tuned to take these differences into account. The operating principle of this nanoplasmonic patch is based on the phenomenon that silver nanoparticles change size upon exposure to UV light. To make sure that this effect happens within minutes and not hours, the team ran numerous experiments with a solar simulator in order to find the AgNP-­ decorated nanopaper with the fastest reaction time. “Importantly, as we have previously demonstrated, due to the nanostructure of bacterial cellulose, the spatial distribution of nanoparticles deposited within nanopaper is completely different when compared with the disposition of nanoparticles embedded within other conventional paper substrates such as nitro/cellulose,” Morales-­Narváez notes. “Moreover, it is worth mentioning that nanopaper and its optical transparency facilitates that the plasmonic properties of the embedded nanoparticles can be virtually completely exposed.” He cautions that commercialization of this device is not going to happen quickly as the team needs to do more testing, find investors, and deal with regulatory requirements.

Featured scientist: Dr Eden Morales-­Narváez Organization: Biophotonic Nanosensors Laboratory, Centro de Investigaciones en Óptica A.C., Léon (Mexico) Relevant publication: J. Barajas-­Carmona, L. Francisco-­Aldana and E. Morales-­Narváez, Wearable Nanoplasmonic Patch Detecting Sun/UV Exposure, Anal. Chem., 2017, 89(24):13589–13595.

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4.4 Nanosensor Gels Detect Therapeutic Levels of Radiation Ionizing radiation (e.g., X-­rays, γ-­rays) is widely used in the treatment of cancer, but can cause significant damage to healthy cells. The overarching goal of radiotherapy is to safely, accurately, and efficiently deliver ionizing radiation in order to treat diseases, typically cancer. However, radiation-­induced toxicity and accidental overexposure are serious concerns that can adversely affect the health of a patient. That is why radiation protection for patients has emerged as an important facet of radiotherapy. Medical physicists and oncologists need to expose the diseased tissue to maximal doses while sparing surrounding tissues and organs. A novel sensor technology can help medical physicists and oncologists to effectively plan fractionated radiotherapy in the clinic, reduce accidental overexposure, and reduce radiation-­induced toxicity. Existing sensors used for detecting clinical doses of radiation are cumbersome, expensive, and need specialized equipment and/or a highly skilled specialist for their operation. Many of these sensors cannot be molded into various shapes and size to match the contours of the body exposed to radiation. With these factors in mind, a team led by Kaushal Rege, a professor in chemical engineering at Arizona State University, in collaboration with Banner-­M.D. Anderson Cancer Center, has developed a novel gel-­based device as a colorimetric sensor that can rapidly detect clinical doses of ionizing radiation. “The burden of complex fabrication procedures, high costs, cumbersome operation procedures, need for specialized equipment, need for operators with specialized training, and/or stringent environmental control (e.g., oxygen-­free environments) motivated us to develop a simple colorimetric sensor to detect ionizing radiation that can be translated to clinical radiotherapy applications,” explains Rege. “The unique advantage of our sensor lies in the fact that the mode of detection is a visible change in color—typically, colorless to maroon—which makes it among the most simple detection methods available,” adds Karthik Pushpavanam, the paper's first author. “This change in color is visible only 15 minutes after exposure to radiation. Increasing the amount of radiation delivered to the hydrogel increases the intensity of color developed in the gel. This change in intensity can be used for dose quantification by using a simple spectrophotometric technique.” The hydrogel can be molded into various shapes depending on the user requirement. The efficacy, simplicity of operation and detection, robustness, and relative low cost make this current gel-­based colorimetric sensor device a powerful technology for detecting ionizing radiation for applications, including clinical radiotherapy. These nanosensor gels can be used to report the dose of radiation delivered to different locations in the exposed area. Furthermore, this technology

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will help effectively plan radiotherapy treatment, which is typically administered in fractions of 2 Gy dose per day leading to a cumulative dose of 20–70 Gy over the course of the treatment (1 Gy is 1 joule of energy absorbed by a mass of 1 kg). Monitoring the dose delivered during each fraction can allow for adjustments in dose as required. This, in turn, can reduce accidental overexposure and reduce radiation-­induced toxicity, ultimately leading to improved patient outcomes. The team's gel-­based sensor device contains gold ions in the form of a gold salt prior to ionizing radiation exposure. Ionizing radiation leads to splitting of water (hydrolysis) in the device, which results in the formation of free radicals that generate electrons. These electrons reduce gold ions to gold atoms, which further nucleate and grow into gold nanoparticles. The plasmonic properties of gold nanoparticles impart color to the originally translucent and colorless hydrogel. The development of this color is rapid and facilitates visual detection of radiation, unlike existing sensors. The intensity of the color depends on the extent of nanoparticles formed in the gel, which, in turn, is reflective of the dose delivered to the gel. This intensity can be used to calibrate the gel, leading to quantitative determination of the dose delivered. The skin is at risk during all radiotherapy treatments as it is the first point of contact during radiation. Determining the absolute radiation dose entering and leaving the skin during treatment is a priority to ensure the safety of the patient. In this study, the researchers employed their hydrogel to detect and determine radiation dose delivered to the skin as well as to the surface of the thorax using anthropomorphic phantoms. They were able to quantify the radiation dose with an error rate of less than 10%. This sensor response was comparable to existing and commercially used sensors indicating its readiness for use in clinical applications. “We will be evaluating these gel-­based detectors in animal models in order to evaluate their preclinical efficacy and biocompatibility,” Rege describes the team's next steps. “These studies will lead to a critical evaluation of this technology before translation to actual clinical applications.” Featured scientists: Professor Kaushal Rege's Bioengineering laboratory (https://bit.ly/30TKjir) Organization: Department of Chemical Engineering, Arizona State University, Tempe, AZ (USA) Relevant publication: K. Pushpavanam, S. Inamdar, J. Chang, T. Bista, S. Sapareto and K. Rege, Detection of Therapeutic Levels of Ionizing Radiation Using Plasmonic Nanosensor Gels, Adv. Funct. Mater., 2017, 27(21):1606724.

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4.5 Plasmonics in the Clouds Optical materials composed of plasmonic nanoparticles have revolutionized the ability to control light—yet these plasmonic materials are typically limited to only a few phases of matter, either as two-­dimensional (2D) solids or as dilute liquids. Researchers at the US Naval Research Laboratory have experimentally realized a plasmonic aerosol by efficiently transitioning liquid suspensions of gold nanorods into the gas phase and simultaneously measuring their optical spectra. They demonstrate that these aerosols are optically homogeneous, thermodynamically stable, with wide wavelength tunability (by controlling the aspect ratio of the nanorods), and have extremely large sensitivities to their environment (Figure 4.3). This novel plasmonic material could potentially open the door to many interesting applications, ranging from geoengineering, vacuum microelectronics, molecular diagnostics, and nanomedicines, to nanojet printing and non-­linear optics. “The aerosols are very sensitive to their surrounding environment, which may be used in the laboratory to aid in probing the nanoscale mechanisms governing macroscale atmospheric processes,” explains Jake Fontana, PhD, a research physicist at the US Naval Research Laboratory in Washington, DC. Today, one of the largest uncertainties in accurately predicting climate and extreme weather events is the relationship between aerosol particles and cloud systems, which is a poorly understood non-­linear process (the aerosol particles serve as nucleation sites for water molecules to condense into droplets that can then form into clouds). For instance, as the researchers write in their paper, recent work posited that aerosol particles from the exhaust of ships enhanced the intensity and electrification of storms, showing that the density of lightning strikes doubled over shipping lanes. Moreover, ultrafine aerosol particles with diameters

Figure 4.3 Conceptual rendering of a plasmonic aerosol. (Image: Jake Fontana and Robert Gates).

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below 50 nm, once thought to be too small to influence cloud formation, have recently been shown to significantly intensify the convective strength of cloud systems. “Fundamentally, we solved a decades-­old problem of simultaneously aerosolizing and measuring plasmonic nanoparticles in the gas phase, thereby merging the fields of plasmonics and aerosols,” Fontana points out. “Globally, we anticipate plasmonic aerosols will open up broad and innovative approaches to understanding the underlying physics of inaccessible climatology, astronomy, petroleum, and medical environments.” While studies on micrometer-­sized aerosols have been carried out for decades, this is the first demonstration of efficiently aerosolizing plasmonic nanoparticles from the liquid to the gas phase, while simultaneously measuring them optically. The aim of this work was to develop an approach to understand the relationship between aerosol nanoparticles and cloud systems in the laboratory to potentially help in addressing current geoengineering challenges. “To that end, we are interested in experimentally measuring the response of the aerosols in different gas environments as well as refining our bench-­ top aerosolization and detection apparatus,” concludes Fontana. Featured scientist: Dr Jake Fontana Organization: US Naval Research Laboratory, Washington, DC (USA) Relevant publication: J. Geldmeier, P. Johns, N. Greybush, J. Naciri and J. Fontana, Plasmonic aerosols, Phys. Rev. B, 2019, 99, 08112(R).

4.6 Reversible Assembly of Plasmonic Nanoparticles The optical manipulation of plasmonic nanoparticles—metal nanoparticles that are highly efficient at absorbing and scattering light—has advantages for applications such as nanofabrication, drug delivery, and biosensing. To that end, researchers have been developing techniques for the reversible assembly of plasmonic nanoparticles that can be used to modulate their structural, electrical, and optical properties. The latest such technique is a low-­power assembly that is enabled by thermophoretic migration of nanoparticles due to the plasmon-­enhanced photothermal effect and the associated enhanced local electric field over a plasmonic substrate. An international research team, led by Yuebing Zheng, Assistant Professor of Mechanical Engineering and Materials Science & Engineering at The University of Texas at Austin, has developed a new optical assembly technique known as plasmon-­enhanced thermophoresis to assemble plasmonic nanoparticles reversibly by optically controlling a temperature field.

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This plasmon-­enhanced thermophoresis can be exploited to confine plasmonic nanoparticles in a higher temperature regime under a thermoelectric field. “It is well known that plasmonic nanoparticles and their assemblies can manipulate light at the sub-­wavelength scale where the intense localized electromagnetic field strongly couples with nanoscale objects, leading to various light–matter interactions and applications,” Dr Linhan Lin, the paper's first author, explains. “However, the manipulation and assembly of nanoparticles using traditional optical tweezers requires high optical power and strict optical alignment. The technique we developed here overcomes the high power requirement; with the operation power at least three orders of magnitude lower than that used in optical tweezers.” Moreover, as he points out, the team achieved dynamical and parallel manipulation of the assembly arrays and applied them to improve SERS of molecules in native liquid environments. “The capability to assemble plasmonic nanoparticles at a low optical power paves the way towards biomolecular analyses in their native environment, smart drug delivery with plasmonic nanoparticles, and bottom-­up assembly of metamaterials,” notes Lin. Previous works, which exploit thermophoresis for particle trapping, could not achieve the stable confinement of nanoparticles in the hot region where the laser beam is. The migration of nanoparticles towards the cold region limits the trapping stability and versatile manipulation of the nanoparticles. To overcome this limitation, the team exploits a cationic surfactant to modify the surface charge of the nanoparticles and design the thermoelectric field on the particles to trap it in the hot region, which enables the light-­driven versatile manipulation of the nanoparticles. This work proposes a general strategy to modify the surface charge of plasmonic nanoparticles and achieve trapping and assembly of the nanoparticles, which will advance the multiple fields of optical manipulations, nanofabrication, and plasmofluidics. The novel method can solve current challenges in assembling various nanoparticles with different size, shape, and components for applications. Due to the generality of the method, it can be extended to trap and manipulate dielectric nanoparticles, quantum dots, and biological cells, paving the way towards bottom-­up assembly of functional devices and point-­of-­care medical research and diagnostics. For example, this manipulation of plasmonic nanoparticles will open up a new window of opportunity in drug screening and cellular biology. The plasmonic nanoparticle assembly can be widely applied to biological sensing of live cells by enhancing the Raman signal of molecules on the cellular membrane, as well as drug delivery in live cells. Going forward, the team aims to, (1) establish prediction models for the optical assembly of nanoparticles and test the ultimate limits, and (2) apply the plasmonic nanoparticle assemblies to cellular biology and disease diagnostics.

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“Our work at the interface of thermophoresis, colloid and surface chemistry, and plasmonics will advance multiple fields of materials science and engineering,” concludes Zheng. “Our future work will be focused on developing prediction models for various types of nanoparticles, testing the ultimate limits of plasmon-­enhanced opto-­thermo-­fluidics in nanoparticle manipulation, and applying assemblies of nanoparticles as functional structures and metamaterials to life sciences, point-­of-­care diagnosis, and national security.” The challenges for the scientists will be to achieve three-­dimensional control of nanoparticle assemblies together with functional molecules at the single-­nanoparticle and single-­molecule level, and to do this without the use of plasmonic substrates. Featured scientists: Professor Yuebing Zheng's research group (https://bit. ly/2WcDDN8) Organization: The University of Texas at Austin, Austin, TX (USA) Video: https://youtu.be/PT3I7sJ-­x-­M Relevant publication: L. Lin, X. Peng, M. Wang, L. Scarabelli, Z. Mao, L. Liz-­Marzán et al., Light-­Directed Reversible Assembly of Plasmonic Nanoparticles Using Plasmon-­Enhanced Thermophoresis, ACS Nano, 2016, 10(10):9659–9668.

4.7 Biofoam Beats Conventional Plasmonic Surfaces So far, most of the applications of plasmonic nanostructures rely on solid two-­dimensional (2D) substrates such as silicon, glass, plastic, or paper. These substrates offer rather limited accessible surface area, thus severely limiting the volumetric density of the nanostructures. “To address this issue, we have introduced a novel optically active 3D material with a high density of plasmonic nanostructures that can be treated as a frozen solution of metal nanostructures in which the nanostructures are completely accessible,” says Dr Rajesh R. Naik of the Air Force Research Laboratory (AFRL). “We believe that this 3D plasmonic biofoam opens up lots of opportunities in using the unique optical properties of plasmonic nanostructures.” Naik, who is Chief Scientist, 711th Human Performance Wing, AFRL at Wright–Patterson Air Force Base in Dayton, Ohio, together with Professor Srikanth Singamaneni from the Washington University in St. Louis, and their teams have demonstrated that the SERS and photothermal performance of this novel 3D material is superior compared to that of conventional 2D plasmonic surfaces. This novel, optically active, multifunctional material platform is based on biomaterial foam, which can be created from various biopolymers, in this case bacterial nanocellulose and regenerated silk fibroin. The nanocellulose

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Figure 4.4 Schematic illustration showing the fabrication of plasmonic biofoams. Reproduced from http://dx.doi.prg/10.1021/acs.nanolett.5b04320 with permission from American Chemical Society Copyright 2017.

material is processed into freeze-­dried aerogel and then uniformly coated with a high density of plasmonic nanostructures (in this case gold nanorods) (Figure 4.4). “The highly open porous structure of the biomaterial foam combined with high volumetric density of nanostructures makes plasmonic foam a solid-­ state analog of highly concentrated solution of plasmonic nanostructures,” notes Singamaneni. This approach opens the door for using biocompatible materials as bioplasmonic foams that can be used in a variety of applications, mainly in biomedical and energy-­harvesting investigations. Naik and his collaborators see three main application areas for the new material platform.     1. Plasmonic biofoam, owing to the high density of plasmonic nanostructures, is ideally suited for chemical and biological sensing based on SERS. 2. The large and tunable absorption of plasmonic nanostructures combined with high photothermal efficiency of these nanostructures make plasmonic biofoam an excellent candidate for solar energy harvesting and photothermal steam generation. 3. The ability of loading and releasing cargo from these materials with external triggers, such as light, makes them excellent carriers for catalysts and drugs, for example, optically triggered release of entrapped reagents in wound dressings.     In this present work, the team has employed plasmonic nanostructures to achieve optically active biofoams. However, the functionality of the

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biofoams can be broadened with the incorporation of other functional nanomaterials, such as graphene, carbon nanotubes, and catalytically active nanomaterials. “We would like to explore the use of other nanomaterials, making the aerogels more robust and creating other 3D hybrid functional materials using this approach,” says Naik. Also, in this study, the thickness of the plasmonic foam is higher than the light penetration depth. To minimize the amount of nanostructures employed, in a next step the researchers will limit the nanostructures to the top portion of the aerogel, while the bottom portion will serve as a support layer. “We have employed gold nanorods as plasmonic materials in this study; considering that gold is an expensive material, it would be ideal to replace gold with other materials that are relatively inexpensive, while preserving the unique optical properties of these functional aerogels,” concludes Singamaneni. “In recent years, a lot of efforts have been devoted to developing inexpensive plasmonic materials. These developments would certainly broaden the applications of plasmonic materials and plasmonic biofoams.” Featured scientists: Dr Rajesh R. Naik (https://bit.ly/2XcR7oN); Professor Srikanth Singamaneni (https://bit.ly/2UVGb1f) Organizations: AFRL at Wright–Patterson Air Force Base, Dayton, OH (USA); Washington University in St. Louis, St. Louis, MO (USA) Relevant publication: L. Tian, J. Luan, K. Liu, Q. Jiang, S. Tadepalli, M. Gupta et al., Plasmonic Biofoam: A Versatile Optically Active Material, Nano Lett., 2015, 16(1):609–616.

4.8 Black Gold Maximizes the Light Absorption of Nanomaterials Maximizing light absorption of nanomaterials has been an emerging research field in recent years due to its attractiveness in a wide range of applications that involve conversion or utilization of solar energy. However, most of the concepts reported are based on multilayered architecture inspired by optical impedance matching concepts that require complicated non-­scalable fabrication processes such as electron-­beam lithography. Efforts to maximize light absorption via nanostructuring remain scarce. A group of researchers in Australia is now one of the first to report such a material—a nanolayer of black gold nanotubes.

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“Maximizing light absorption of plasmonic nanostructures for photochemistry systems has always been the core of our research interest,” says Dr Charlene Ng, a postdoctoral fellow at CSIRO. “The motivation to conduct this work comes from the search for an appropriate fabrication strategy that is simple, cheap, and robust to create plasmonic nanostructures that exhibit strong light absorption in the visible range.” “The most exciting result in our work is the ability to create gold surfaces that are nanostructured and appear black to the human eye due to its broad band, high absorption of the visible light,” adds Associate Professor Daniel E. Gómez from RMIT University. “It can be fabricated over large surface areas in a robust and cost-­efficient manner. Furthermore, it also exhibits the flexibility to adhere to arbitrary surfaces that makes it attractive for a wide range of photo-­related applications.” “Most importantly,” he adds, “the fabrication process is not limited to only gold; we have also demonstrated the fabrication of black nickel using a similar method.” The researchers strongly believe that this study could provide a new paradigm for the use of highly absorbing metal nanostructures to effectively harvest the entire visible spectrum for photo-­related applications, such as solar fuel production, photodetection and photovoltaics. In this present work, the team demonstrates a black gold film of merely 400 nm in thickness acting as a broad band super-­absorber that is capable of absorbing >92% of the incident light energy, up to a wavelength of 600 nm. The intriguing light absorption capability of this black gold film is related to the high aspect ratio and closely packed gold nanotubes with a tapered wall thickness and high exposed surface area. The majority of nanomaterials that exhibit high light absorption involve the complexity of employing multiple materials, high-­cost fabrication processes (e.g., lithography methods), and planar geometry that limits the exposed surface area for photochemical reactions to occur. On the other hand, the black gold film is fabricated by a simple, cheap, and scalable template-­assisted physical vapor technique, which makes it highly attractive and versatile for advancing the field. “We also present experimental evidence that shows how this material can drive photochemical transformations under visible light, demonstrating its attractiveness as a novel material for a wide range of photochemical applications,” Ng notes. The next stage in the team's investigations is to integrate this novel black gold material into plasmon-­based devices, such as plasmonic solar cells or photodetectors. Other applications could be plasmon-­derived photocatalytic systems, surface-­enhanced Raman spectroscopy, or solar water purification. “Future directions in this research field will move towards more cost-­ efficient fabrication strategies such as bottom–up processes and large-­scale

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deposition of thin films, in order to integrate these nanomaterials into practical devices,” concludes Ng. “However, the challenge still lies in the availability of fabrication strategies that can provide devices at macroscopic scales.” Featured scientists: Professor Daniel E. Gómez and Dr Charlene Ng Organization: RMIT University, Melbourne (Australia) Relevant publication: C. Ng, L. Yap, A. Roberts, W. Cheng and D. Gómez, Black Gold: Broadband, High Absorption of Visible Light for Photochemical Systems, Adv. Funct. Mater., 2016, 27(2), 1604080.

4.9 Nanopatterning Holograms onto Commercial Contact Lenses The humble contact lens, widely used to correct common vision problems, is on its way to becoming a ‘smart’ diagnostic tool that brings self-­powered wearable electronics right to your eyeball. Tears contain diagnostic information regarding ocular diseases, and they can be used as a surrogate medium for analyzing blood chemistry. One recent example is a graphene-­based contact lens sensor for diagnosing glaucoma, another demonstration has been a glucose-­sensing contact lens for people with diabetes to monitor their blood sugar levels. Researchers have also developed a method to produce optical nanostructures onto contact lenses. This aids in functionalizing and nanotexturing an off-­the-­shelf, commercial contact lens (Figure 4.5). “Integrating nanoscale features into commercial contact lenses for application in low-­cost biosensors has been a challenge so far,” says Dr Haider Butt, Senior Lecturer of Micro Engineering and Nanotechnology at the School of Engineering, University of Birmingham. “With our new method we have been able to print holograms on contact lenses in a single step by using lasers. The method is very simple, inexpensive, does not require clean room environment and can be used for printing nanostructures (surface holograms) on any commercially available contact lens. In effect we are making wearable holograms.” “Already, various nanostructures have been fabricated within hydrogels that act as optical transducers,” he adds. “However, integrating nanoscale features into hydrogel structures represents a fundamental challenge for producing functional contact lenses. Most nanofabrication approaches to form optical structures in polymers are costly and time consuming.” Previously used methods, such as nanoimprinting (via UV exposure), involve multiple process steps and are not compatible with all commercial

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Figure 4.5 Different designs of holographic nanostructures fabricated on contact lenses. Each image demonstrates the diffraction colors observed at various angles (scale bar = 5 mm). Reproduced from https://doi. org/10.1021/acsnano.8b00222 under the terms of the CC BY 4.0 license, https://creativecommons.org/licenses/by/4.0/.

silicon-­hydrogel contact lenses. In contrast, the method of producing nanostructures on a soft, fragile, and thin hydrogel-­based surface demonstrated by Butt and his team is fast and requires only a single step. “Other groups doing similar research use in-­house produced contact lenses, with materials and geometries different from the ones commercially sold,” notes Butt. “In our work, we used commercial contact lenses and functionalized their surface to convert them into a sensing device.” The researchers developed a holographic laser ablation method to produce optical nanostructures. They used a black dye on the contact lens to facilitate the interaction between the interfering laser beams and the lens material. “With our method, we can rapidly create low-­cost optical nanostructures by direct laser interference patterning (DLIP) in holographic Denisyuk reflection mode to create ablative interference fringes on the contact lens surfaces,” explains Butt. “The holographic DLIP system enables us to produce different types of nanopatterns on the soft and fragile surfaces of the lens.”

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He notes that the contact lens material (silicon-­hydrogel), which is not purely solid at ambient humidity conditions, withstood up to three laser pulses without significant damage to its relatively thin matrix. The team fabricated the optical nanostructures at the edge of the contact lens in order to prevent any sight obstruction or interference of vision. The nanopatterns can be designed in various shapes and could be used for sensing as well as cosmetic purposes. However, the primary use for this kind of nanotextured lense will be in sensing and monitoring eye-­related disorders, like glaucoma and dry eyes. Tears contain a lot of biomarkers that provide information regarding the workings of the body and various eye-­related disorders could also be monitored. The team is also working towards using the holograms for monitoring eye pressure for glaucoma patients. Going forward, the scientists are interested in producing more sophisticated patterns on the contact lenses. At the moment, they create two-­ dimensional nanopatterns, which display a rainbow-­like hologram effect. In future, they want to pattern 3D nanostructures for more sophisticated functionalities. Featured scientist: Dr Haider Butt (https://bit.ly/2ZbxyPA) Organization: School of Engineering, University of Birmingham,  Birmingham (UK) Relevant publication: B. Al Qattan, A. Yetisen, H. Butt, Direct Laser Writing of Nanophotonic Structures on Contact Lenses, ACS Nano, 2018, 12(6):5130–5140.

4.10 Multiple Electromagnetic Responses from Accordion-­like Plasmonic Nanorods Well-­defined, complex nanostructures for metamaterials with unique optical properties—such as negative refractive index, strong artificial optical activity, and perfect absorption—are usually prepared by top-­down approaches, including direct laser writing, multiple e-­beam lithography, and membrane-­ projection lithography. However, these methods have two critical drawbacks: (1) large-­scale device fabrication is practically impossible due to the time-­ and energy-­consuming process; and (2) the operating window is limited (not working in the visible or UV regime) due to the large feature size of the structures. In a recent breakthrough, scientists in Korea have combined block copolymer (BCP) self-­assembly and an anodized aluminum oxide template to fabricate unique complex nanostructures over a large area. The team was led by Jin Kon Kim, a professor in the Department of Chemical Engineering at Pohang University of Science and Technology (POSTECH),

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and Director of National Creativity Research Initiative Program for Smart Block Copolymers, and Junsuk Rho, a professor in the Department of Chemical Engineering at POSTECH. In this work, the scientists introduce a novel method to fabricate a high-­ density array of plasmonic nanorods over a large area (2.5 cm × 2.5 cm), exhibiting multiple electromagnetic responses. They demonstrate the fabrication of sophisticated nanostructures that are difficult to realize through a top-­down process in a large optical device by taking advantage of BCP self-­assembly. “Though block copolymers self-­assemble into various nanodomains such as lamellae, gyroids, cylinders, and spheres, these nanostructures cannot provide complex plasmonic metamaterials with unique optical properties,” explains Kim. “To address this issue, we confined the lamellae-­forming polystyrene-­block-­poly (methyl methacrylate) copolymer (PS-­b-­PMMA) inside the cylindrical pores of an AAO template grafted with thin neutral brush layers to form stacked lamellar rods.” After the AAO template was removed, a 5 nm-­thick layer of silver was thermally deposited on only the polystyrene nanodomains, generating ‘accordion-­like’ silver nanorods with one hemispherical cover and five side stripes over a large area (Figure 4.6). Due to the unique geometry of the nanostructures, the array has two magnetic responses and one electric response from visible to near-­infrared (NIR) wavelengths. Through finite-­difference time-­domain simulations, the scientists determined the transmittance dip at 600 nm to correspond to a magnetic response, while the other dips at 800 and 1200 nm correspond to electric responses. These multiple resonances are specifically applicable for multi-­analyte detection. Furthermore, magnetic response is known to be an essential optical property for realizing metamaterials with negative refractive index, which makes this work a contribution to the commercialization of practical metamaterials working in the visible or NIR region.

Figure 4.6 Multiple resonances exhibited by ‘accordion-­like’ silver nanorods array. (Image: Professor Jin Kon Kim, POSTECH).

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“Our fabrication method is well suited to make multi-­analyte sensors, anti-­ transmittance films, and metamaterials with unique optical properties, such as negative refractive index and perfect absorption,” notes Kim. “Other extraordinary optical properties, such as circular dichroism, could be obtained by introducing chirality on the BCP nanostructures,” he adds. “Although we fabricated complex plasmonic nanostructures, the generation of a single or double helix will remain a challenge in BCP self-­assembly.” Featured scientists: Professors Jin Kon Kim (https://bit.ly/2W8wwBp) and Junsuk Rho Organization: Department of Chemical Engineering, Pohang University of Science and Technology (POSTECH), Pohang (South Korea) Relevant publication: M. Kim, J. Mun, D. Bae, G. Jeon, M. Go, J. Rho et al., Accordion-­like plasmonic silver nanorod array exhibiting multiple electromagnetic responses, NPG Asia Mater., 2018, 10(4):190–196.

Chapter 5

Nanobiotechnology 5.1  Introduction Nanobiotechnology is the application of nanotechnologies in biological fields. Chemists, physicists, and biologists each view nanotechnology as a branch of their own subject and collaborations in which they each contribute equally are common. One result is the hybrid field of nanobiotechnology that uses biological starting materials, biological design principles, or has biological or medical applications. While biotechnology deals with metabolic and other physiological processes of biological subjects, including microorganisms, in combination with nanotechnology, nanobiotechnology can play a vital role in developing and implementing many useful tools for the study of life at the smallest scales. Already, the integration of nanomaterials with biology has led to the development of diagnostic devices, contrast agents, analytical tools, therapy, and drug-­delivery vehicles. One particular area at the intersection of nanomedicine and nanobiotechnology is tissue engineering—a difficult task where living cells must be organized into tissues with structural and physiological features resembling natural structures in the body. The ultimate goal of tissue engineering as a medical treatment concept is to replace or restore the anatomic structure and function of damaged, injured, or missing tissue—ultimately providing doctors with the ability to replace entire organs. At the core of tissue engineering is the construction of three-­dimensional scaffolds out of biomaterials to provide mechanical support and guide cell growth into new tissues or organs.

  Nanoengineering: The Skills and Tools Making Technology Invisible By Michael Berger © Michael Berger 2020 Published by the Royal Society of Chemistry, www.rsc.org

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5.2  A  n Alternative to Antibiotics: Weakening the Grip of Superbugs There is a growing medical emergency in our society caused by the constant growth in the number of people affected by untreatable bacterial infections. The discovery of antibiotics in the early 20th century gave us decades of relative safety, a period that is coming to an end—the antibiotics that saved millions of lives in the last century are increasingly powerless against a growing number of antibiotic-­resistant bacteria. According to the US Centers for Disease Control (CDC), more than 23 000 Americans die each year from infections caused by germs that are resistant to antibiotics. While this number in itself is alarming, the scary thing is that unusually resistant germs—bacteria that are resistant to all or most antibiotics tested, and are uncommon or carry special resistance genes—are constantly developing and spreading. The scientists at the CDC, who are not prone to panic easily, call this type of bug nightmare bacteria. Laboratory tests in the USA uncovered unusual resistance more than 200 times in 2017 in these nightmare bacteria alone. This problem is not new. Already in 2012, the then Director General of the World Health Organization (WHO), Dr Margaret Chan, warned vividly that the growing threat of antibiotic-­resistant bacterial strains may pose grave risks for society: “A postantibiotic era means, in effect, an end to modern medicine as we know it. Things as common as strep throat or a child's scratched knee could once again kill.” Chan pointed out that there is a global crisis in antibiotics caused by rapidly evolving resistance among microbes responsible for common infections, which threatens to turn them into untreatable diseases. Every antibiotic ever developed is at risk of becoming useless. Recent research may offer some hope. Using a combination of laboratory experiments and supercomputer simulations, researchers discovered why Staphylococcus bacteria—the leading cause of health care-­related infections—can be so tough to beat. This work could point the way to new treatments for now-­invincible bacterial foes, not by developing a new antibiotic that would kill these bacteria, but by making them weaker so that they are more easily attacked by our immune system. Staph infections are caused by Staphylococcus bacteria, types of germs commonly found on the skin of healthy individuals. Most of the time, these bacteria cause no problems or result in relatively minor skin infections. The research team showed that methicillin-­resistant Staphylococcus aureus adhere to their hosts—us humans—with exceptional mechanical resilience. That is what makes pathogenic bacteria so persistent. “Understanding the physical mechanisms that underlie this persistent stickiness at the molecular level is instrumental to combat these invaders,” says Rafael Bernardi, a research scientist in the Theoretical and Computational Biophysics group at the Beckman Institute.

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Combining experiments and sophisticated computer simulations, Bernardi and the late Klaus Schulten from the Beckman Institute teamed up with Lukas Milles and Hermann Gaub from the Physics Department at Ludwig-­Maximilian-­University Munich, to decipher the mechanism responsible for staph adhesion. Using an atomic force microscope (AFM), the University of Munich team was able to measure the forces that govern the interaction between an individual adhesin (a staph protein) and its human target molecule. Independently, the Illinois team investigated the same protein complex by performing computationally intensive steered molecular dynamics (SMD) simulations, carried out using the National Center for Supercomputing Applications Blue Waters supercomputer, deconstructing the mechanism of the interaction between staph adhesion factors and human proteins. The scientists were surprised by the shear forces that were necessary to rupture the interaction between the bacterial and human proteins: they are much stronger than any other non-­covalent interaction known. They discovered that the mechanism that makes staph bacteria cling so tightly to its human host is a series of hydrogen bonds arranged in a corkscrew shape that works like superglue to clamp bacteria protein molecules to human ones. “It's easy to break one hydrogen bond,” says Bernardi. “What makes this attachment so strong is that you have to break all the bonds at once in order to detach the protein molecules.” “The unbinding force of a single adhesin–human protein complex measured was exceptional, about an order of magnitude stronger than any other protein–protein interaction known,” he continues. “The rupture forces reached over two nanonewtons, a regime generally associated with the strength of covalent bonds, and nearly an order of magnitude stronger than most other known protein–protein interactions.” The combination of innovative simulation methods and experimental confirmations showed that the extreme physical strength of the staph adhesion is largely independent of protein sequence and biochemical properties, but is rather a built-­in physical property—an invasive advantage for these staphylococci. The whole description of this unexpected mechanism is new and shows how bacteria evolved to take advantage of simple hydrogen bonds in a remarkable way. These findings expand our understanding of why pathogen adhesion is so resilient and may open new ways to inhibit staphylococcal invasion. Understanding the mechanism of staph infection at the molecular, and now the atomic, level may open new avenues for an intelligent design of antimicrobial therapies. The development of anti-­adhesion therapy could promote the detachment of staph bacteria, facilitating bacterial clearance by our own immune system. “The main challenge now is to design a drug that targets this bacterial adhesion mechanism and either blocks or at least weakens it,” Bernardi concludes. “As a more general aspect, the combined use of AFM experiments and

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SMD simulations should greatly contribute to the identification of new binding mechanisms in bacterial adhesins, thus helping to show how they regulate biofilm formation. In diagnosis and therapy, this combined approach could represent a powerful platform for the treatment of microbial infections.” Featured scientist: Rafael Bernardi Organization: Theoretical and Computational Biophysics Group (https:// bit.ly/2XdYFaJ), Beckman Institute, Urbana, IL (USA) Video: https://youtu.be/ji63Sw3iM0M Relevant publication: L. Milles, K. Schulten, H. Gaub and R. Bernardi, Molecular mechanism of extreme mechanostability in a pathogen adhesin, Science, 2018, 359(6383), 1527–1533.

5.3  C  ell Sex Impacts the Biological Uptake of Nanoparticles One of the key issues in nanomedicine is the question of how to effectively transport therapeutic nanoparticles and associated drugs to, and into, cells. Researchers have been studying the underlying mechanisms that so far have prevented successful clinical translation of a majority of medical nanotechnology applications. It turns out that when nanoparticles enter a biological environment, such as blood, they immediately become exposed to a range of factors that can impact their effectiveness and efficiency. It has become quite clear over time that the cellular response to nanomaterials depends on the physiological environment. For instance, biological responses to nanoparticles are temperature dependent. Researchers have also discovered the crucial role of biomolecular coronas for nanoparticle–cell interactions: the biological responses to nanoparticles are strongly dependent on the type and amount of associated proteins in the composition of the biomolecular corona. Scientists have now identified yet another, so far overlooked, factor that impacts nanoparticle uptake. An international team of researchers discovered that cell sex is an important overlooked factor at the nano–bio interfaces. More specifically, depending on their sex, cells respond differently to the same type of nanoparticles. According to Morteza Mahmoudi, an assistant professor at Brigham and Women's Hospital, Harvard Medical School, these findings have the capacity to optimize clinical translation of nanoparticles and also to help researchers to better design and produce safe and efficient therapeutic sex-­specific nanoparticles. It has been established that the differences between female and male embryos originate at a very early developmental stage, prior to the initiation of hormonal changes; therefore, genetically and structurally driven sexual dimorphisms are expected at this early stage.

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For their study, the team selected human amniotic stem cells (hAMSCs), which represent one of the earliest sources of somatic stem cells, to uncover the potential effect of sex differences in nanoparticle uptake. As these cells are extracted from the amniotic layer of the placenta, they are in their early passages, which can provide more accurate nanoparticle update outcomes compared with other cell types. They also tested whether cell sex affects the uptake nanoparticles in different cell types. For that they repeated the nanoparticle uptake experiments with somatic cells derived from the salivary gland of adult male and female patients. Here, as well, the results show different outcomes. As a nanoparticle model system the researchers selected monodispersed commercially available quantum dots. “In this work, our team tested the hypothesis that the observed significant difference in the quantum dot uptake between male and female cells could be because of, but not limited to, (1) variations in secreted biomolecules (i.e., paracrine factors) and/or (2) sex-­based variation of cell functions and structures (e.g., various membrane composition and intracellular pathways and, also, cell stiffness, i.e., structural differences in their cytoskeleton),” explains Mahmoudi. “We were able to demonstrate the importance of cell sex in uptake of nanoparticles. Consequently, we suggest that cell sex is an overlooked factor in research relevant to the nano–bio interface.” “Considering that our study revealed differences between male and female cells based on analysis of a limited number of factors and also that the effect of cell sex varied between cell types (fibroblasts and hAMSCs), it is likely that there are other undiscovered differences that could influence nanoparticle uptake,” he concludes. “Further research will be required on a greater variety of cell types.” Featured scientists: Professor Morteza Mahmoudi's NanoBio Interactions Laboratory (https://bit.ly/2FfhSlM) Organization: Brigham and Women's Hospital, Harvard Medical School, Boston, MA (USA) Relevant publication: V. Serpooshan, S. Sheibani, P. Pushparaj, M. Wojcik, A. Jang, M. Santoso, et al., Effect of Cell Sex on Uptake of Nanoparticles: The Overlooked Factor at the Nanobio Interface, ACS Nano, 2018, 12(3), 2253–2266.

5.4  E  arly Cancer Detection with Protein Corona ‘Fingerprints’ When nanoparticles enter the blood stream, or other biological fluid, they are quickly surrounded by a layer of proteins. This so-­called protein corona on the nanoparticles hinders interactions between the targeting ligands on the nanoparticles and their binding partners on the cell surface.

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Previous research findings in 2014, also reported by Mahmoudi's laboratory (see Section 5.3), have indicated that different individuals may have a personalized protein corona owing to their distinct type, severity and period of disease, heterogeneity, individual genetic variations, and environmental factors. This means that the type of disease has a crucial role in the protein composition of the nanoparticle corona and that exactly the same nanoparticles may have different therapeutic and/or toxic impacts in different individuals. By combining the concepts of disease-­specific protein corona and sensor-­ array technology, researchers have now created a label-­free platform for the early detection and identification of diseases. “The possibility to measure panels of specific and selective biomarker proteins has the potential to revolutionize cancer screening, detection, and monitoring,” says Morteza Mahmoudi. “Our protein corona sensor array, rather than detecting a specific biomarker, provides pattern recognition of the corona protein composition adsorbed on the liposomes. By selecting disease-­specific corona profiles of different nanoparticles, using supervised classifiers, we created a unique protein pattern, which was the ‘fingerprint’ of each type of cancer.” The results obtained by Mahmoudi and an international collaboration of researchers revealed that, although no single protein corona composition from a single nanoparticle provides this ‘fingerprint’ feature, the pattern of corona composition derived from the nanoparticle sensor array provides a unique ‘fingerprint’ for each type of cancer. As a feasibility study, the team focused on five distinct human cancers (lung cancer, glioblastoma, meningioma, myeloma, and pancreatic cancer) as a model disease ex vivo. The sensor array is composed of three different cross-­reactive liposomes with various lipid compositions: anionic liposomes, cationic liposomes, and zwitterionic liposomes. The researchers characterized the protein corona profiles at the surface of these liposomes by nano liquid chromatography tandem mass spectrometry after exposure to the plasma of patients diagnosed with each of the five cancers. It is obvious that by increasing the sensor-­array elements (i.e., the nanoparticle type and the physicochemical characteristics), the disease detection accuracy of the protein corona sensor-­ array technology will be enhanced. “To probe the capacity of this platform for very early detection of cancers, we used cohort plasma obtained from healthy people who were later diagnosed with lung, pancreas, and brain cancers several years after plasma collection and the outcomes revealed that the approach could identify and discriminate the cancers,” Mahmoudi explains. “Our results suggest that the disease-­specific protein corona sensor array will not only be instrumental in the screening, detection, and identification of diseases, but may also help identify novel protein pattern markers whose role in disease development and/or disease biology has not been appreciated so far,” he concludes.

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Featured scientists: Professor Morteza Mahmoudi's NanoBio Interactions Laboratory (https://bit.ly/2FfhSlM) Organization: Brigham and Women's Hospital, Harvard Medical School, Boston, MA (USA) Relevant publication: G. Caracciolo, R. Safavi-­Sohi, R. Malekzadeh, H. Poustchi, M. Vasighi, R. Zenezini Chiozzi, et al., Disease-­specific protein corona sensor arrays may have disease detection capacity, Nanoscale Horiz., 2019, 4, 1063–1076.

5.5  M  icromotors Deliver Drug Payloads in the Gastrointestinal Tract In 2015, researchers at the University of California, San Diego demonstrated a micromotor fueled by stomach acid that can take a bubble-­powered ride inside a mouse. These tiny motors, each about one-­fifth the width of a human hair, may someday offer a safer and more efficient way to deliver drugs or to diagnose tumors. Continuing this pioneering effort toward enhanced motor-­based site-­ specific delivery, the research team, led by Professors Joseph Wang and Liangfang Zhang at the Nanoengineering Department at University of California, San Diego (UCSD), has developed an enteric micromotor consisting of a magnesium (Mg)-­based motor body with an enteric polymer coating (Figure 5.1). “The new motors, aimed at controlling and enhancing site-­specific delivery in the gastrointestinal (GI) tract, consist of water-­powered magnesium-­ based tubular micromotors coated with an enteric polymer layer,” Wang explains. “The microscale robot can deliver payload to a particular location via dissolution of their enteric polymeric coating to activate their propulsion at the target site towards localized tissue penetration and retention.” The enteric coating can shield the motors from an acidic gastric fluid environment (pH 1–3) but dissolves in intestinal fluid (pH 6–7) to expose the motors to their fuel and start the movement. Wang points out that, by simply tuning the thickness of the pH-­sensitive coating it is feasible to selectively activate the propulsion, at desired regions of the GI tract, and thus to control localized tissue penetration and retention. The team evaluated the properties and functions of the synthesized enteric magnesium micromotors in a mouse model. The in vivo results demonstrate that these motors can safely pass through the stomach and accurately activate in the GI tract without causing noticeable acute toxicity. “Our micromotor-­based GI transporter system offers innovative combination of accurate positioning and active propulsion towards effective localized GI delivery and improved retention in the GI tract,” notes Zhang. “These developments are of particular importance for the emerging microbiome research.” The use of advanced pH-­sensitive materials for precise local manipulation of the microrobot for site-­specific active delivery—compared to conventional

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Figure 5.1  Top  row: top view of a Mg micromotor with scanning electron micros-

copy (SEM) characterization and energy-­dispersive X-­ray (EDX) images of the Mg and Au in the micromotor. Scale bar: 1 µm. Bottom row: side view of an enteric magnesium micromotor with SEM characterization and EDX images of the Mg and Au in the micromotor. Scale bar: 5 µm. Reproduced from http://dx.doi.org/10.1021/acsnano.6b04795 with permission from American Chemical Society, Copyright 2016.

passive diffusion-­driven delivery vehicles—is expected to pioneer novel delivery approaches and advance the emerging field of medical nano/micromotors and nanorobotics. Future studies by the UCSD team aim at evaluating the delivery efficiency and enhanced therapeutic efficacy associated with the operation and propulsion of their nanomotor carriers. Similar nanobiotechnology nanomachines may find a variety of applications, including diagnostics, drug delivery, or nanosurgery. Featured scientists: Professors Joseph Wang (https://bit.ly/2Ocb5Na) and Liangfang Zhang (https://bit.ly/2Xk0io1) Organization: Nanoengineering Department, University of California, San Diego, CA (USA) Video: https://youtu.be/8H2xnTu7ANw Relevant publication: J. Li, S. Thamphiwatana, W. Liu, B. Esteban-­Fernández de Ávila, P. Angsantikul, E. Sandraz, et al., Enteric Micromotor Can Selectively Position and Spontaneously Propel in the Gastrointestinal Tract, ACS Nano, 2016, 10(10), 9536–9542.

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5.6  T  itanium Implant Material with Multifunctional Nano–Bio Interface The implantation of orthopaedic devices is associated with a high risk of postoperative complications that increases substantially with each revision surgery. Revision surgeries are required primarily for two reasons: (1) implants frequently do not integrate successfully, leading to loosening, and (2) bacterial infections that result in biofilm formation. In the last 10 years, there were 410 767 revision hip replacements and 480 440 knee replacements in Australia alone; and the number of these replacements conducted annually continues to grow. Although the cumulative rate of revisions over a 10-­year period is less than 5% for all patients, it is unacceptably high, at 16%, for patients suffering from bone-­related conditions, such as osteoporosis, osteoarthritis, and ostomalacia, or those having poor bone structure. The situation is similar with knee replacement surgeries, with up to 10.5% of knee implants requiring revision. The significant increase in the number of patients requiring revision surgery is driven by our aging population, which more frequently has poor bone quality and worse implant integration. An international research team from Japan and Australia, led by Professors Wojciech Chrzanowski and Seiji Yamaguchi, propose a two-­pronged strategy to address this outstanding clinical problem by combating infections and providing bioactivity for titanium implants. “Our nanostructured surfaces simultaneously are highly antimicrobial as well as bioactive,” says Chrzanowski, an assistant professor at the Australian Institute for Nanoscale Science and Technology. “The goal of combining both functions without inducing cytotoxicity has thus far proved elusive. Unlike other approaches that use highly toxic antimicrobial compounds and induce undesired cytotoxicity, our approach not only exhibits outstanding antimicrobial activity but it also promotes the formation of bone-­like structures.” Yamaguchi and Chrzanowski and their collaborators have designed novel gallium-­containing nanostructured interfaces that are capable of sustainably releasing gallium ions (Figure 5.2). “In this way, we will achieve highly desired antimicrobial activity without compromising the ability of the implant to bind to bone,” Seiji Yamaguchi, an assistant professor in the Department of Biomedical Sciences at Chubu University, points out. “In fact, gallium ions are likely to further improve bone bonding ability as they show no cytotoxicity and strikingly enhance mineralization of the matrix directly on the surface, thus supporting bone tissue formation.”

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Figure 5.2  Bioactive  titanium slowly releases Ga3+ ions to inhibit adhesion and prolif-

eration of bacteria while at the same time stimulating new bone formation. The metal also releases Ca2+ ions to form bone-­like apatite on its surface and directly bond to bone in a living body. (Image: Chubu University).

The team's innovative technology is very simple, applicable to any shape and size of the implant, and inexpensive. Furthermore, it does not require any sophisticated chemistries or instrumentation, all of which is a significant advantage from a commercialization point of view. The major novelty of the work is its two-­pronged strategy: co-­presentation of ions and structure that regulate bioactivity and which provides high antimicrobial activity. Target applications for this work are titanium implants such as bone plates, intramedullary nails, bone screws, and spinal implants. The 3D printing of this kind of implant has become commonplace and the geometries of the implants has become relatively complex. These complexities make traditional approaches of surface modification, such as PVD (physical vapor deposition) or CVD, not applicable. The Australian team's method is fully compatible with any size and geometry and all currently used titanium alloys. “These features make the technology particularly attractive for industry,” notes Yamaguchi. “We are currently fast-­tracking our technology towards large animal studies and commercialization with a Japanese-­based implant producer.” Going forward, the team is pursuing work in three areas. First, the design of the surface modification methodology, which resulted in (a) desired nanostructures, and (b) the incorporation of gallium, which is slowly but sustainably released in simulated body conditions. Second, conformation of the antimicrobial activity using the most challenging type of multidrug-­resistant bacteria. And, lastly, the conformation of bioactivity using ISO standard tests.

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Featured scientists: Professors Wojciech Chrzanowski (https://bit. ly/2HGsc8y) and Seiji Yamaguchi (https://bit.ly/2HQ49Uw) Organizations: Australian Institute for Nanoscale Science and Technology, Sydney (Australia); Chubu University, Kasugai (Japan) Relevant publication: S. Yamaguchi, S. Nath, Y. Sugawara, K. Divakarla, T. Das, J. Manos, et al., Two-­in-­One Biointerfaces—Antimicrobial and Bioactive Nanoporous Gallium Titanate Layers for Titanium Implants, Nanomaterials, 2017, 7(8), 229.

5.7  A  ll-­natural Nanobiotechnology as an Alternative to Synthetic Agrochemicals Widespread use of synthetic agrochemicals in crop protection has led to serious concerns over environmental contamination and increased resistance in plant-­based pathogenic microbes. In an effort to develop biobased and non-­synthetic alternatives, nanobiotechnology researchers are looking to plants that possess natural antimicrobial properties. Thymol, an essential oil component of thyme, is known for its antimicrobial activity. However, it has low water solubility, which reduces its biological activity and limits its application through aqueous medium. In addition, thymol is physically and chemically unstable in the presence of oxygen, light, and heat, which drastically reduces its effectiveness. Commercially available thymol formulations have a high concentration of synthetic surfactants. These surfactants are of no use in the target application and make the formulation synthetic in nature. Sometimes this impairs the activity of active compounds and can lead to toxic effects when they enter the food chain. Scientists in India have overcome these obstacles by preparing thymol nanoemulsions where thymol is converted into nanoscale droplets using a plant-­based surfactant known as saponin (a glycoside of the Quillaja tree). Due to this encapsulation, thymol becomes physically and chemically stable in the aqueous medium (the researchers' emulsion remained stable for three months) (Figure 5.3). In their work, the team shows that the antibacterial and antifungal properties of nanoscale thymol not only prevent plant disease, but also enhance plant growth. “Normally, emulsions are prepared with synthetic surfactants where the amount of surfactant used is 2–3 times that of the water-­immiscible active compound,” according to Dr Vinod Saharan from the Nano Research Facility Laboratory, Department of Molecular Biology and Biotechnology, at

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Figure 5.3  Model  of thymol nanoemulsion. (Image: Dr Vinod Saharan, Maharana Pratap University of Agriculture and Technology).

Maharana Pratap University of Agriculture and Technology. “We have developed our thymol nanoemulsion by using a much lower concentration of saponin as a surfactant.” “It is exciting how nanoscale thymol is more active,” says Saharan, who led this work, in collaboration with Washington University in St. Louis and Haryana Agricultural University, Hisar. “We found that nanoscale droplets of thymol can easily pass through the surfaces of bacteria, fungi, and plants and exhibit much faster and strong activity. In addition, nanodroplets of thymol have a larger surface area, i.e., more molecules on the surface, so thymol becomes more active at the target sites.” The thymol nanoemulsion developed by the team is an eco-­friendly, biodegradable formulation that is safe for ecosystems and can be a potential alternative for synthetic agrochemicals (antibacterial, fungicide, plant growth regulator, etc.) for food crops. The team's next steps will be to investigate how other micronutrients and bioactive compounds can be blended with the nanoscale thymol emulsions. “Thymol nanoemulsion has application in the food and agriculture sector as an environment safe antibacterial and antifungal agent,” Saharan concludes. “Beside this, it also acts as a plant growth regulator and further can be exploited as food preservative and used to prevent postharvest losses in agriculture.”

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Featured scientist: Dr Vinod Saharan Organization: Nano Research Facility Laboratory, Department of Molecular Biology and Biotechnology, Maharana Pratap University of Agriculture and Technology, Udaipur (India) Relevant publication: S. Kumari, R. Kumaraswamy, R. Choudhary, S. Sharma, A. Pal, R. Raliya, et al., Thymol nanoemulsion exhibits potential antibacterial activity against bacterial pustule disease and growth promotory effect on soybean, Sci. Rep., 2018, 8(1), 6650.

5.8  A  dvanced Protein Design Drives Complex Nano-­ assemblies The manufacture of nanoparticles has reached a very high level of control of their shape, size, and chemical nature. However, assembling nanoparticles in a controlled manner and with clearly defined functionalities in three-­ dimensional space remains quite a challenge. Dr Erik Dujardin, Research Director, Groupe Nanosciences, PicoLab at CEMES in France explains what the challenges are:    ●● Controlling order at the desired length scale, up to macroscopic dimensions; ●● Controlling the relative arrangement of the building blocks when these are of different chemical nature, hence bearing different physical properties (this is a unique aspect of self-­assembly: nanoparticles could be mixed in various combinations and their respective properties could therefore lead to unforeseen synergies); ●● Controlling the spatial organization of nanoparticles in three dimensions; and ●● Introducing dynamics, i.e., the possibility of reconfiguring the assemblies at will. This would open the way to have nanomaterials with a portfolio of properties and one would be able to go from one to the other by switching from one form of the assembly to another rather than by refabricating a new one.    “All these challenges require work on the ‘cement’ that brings and holds the nanoparticles together,” says Dujardin. “Forces—e.g., electrostatic, hydrophobic, hydrogen bonding—between nanoparticles and molecular ligands tethered to the nanoparticle surface are what scientists have been designing to create smart ‘cement’ over the past decades. Supramolecular chemistry has provided many such examples but nature is using this everywhere, too. Take for example DNA strands that interact smartly with each other and with proteins; or proteins that form assemblies that are complex, specific, and reconfigurable.”

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Scientists have used DNA to create architectures—origami—of predetermined geometries at the one-­nanometer scale, and this has been a tremendously active research field led by the pioneering work of Nadrian Seeman at New York University. “Yet the chemistry of DNA makes it excellent as a structuring cement, but much less so as a functional material: DNA is a perfectly designable scaffold—the shape of folded DNA strand can be predicted with a household computer—but it has very poor intrinsic properties,” Dujardin points out. “In contrast, proteins combine the ability to create scaffolds and to foster complex chemistry with organic molecules but, more importantly, with inorganic materials such as, for example, tooth enamel or bones.” He notes that the strongest biomolecule pair is the streptavidin/biotin couple, which has long been used as a strong cement to assemble nanoparticles (this was the pioneering work of Stephen Mann at the University of Bristol). Unfortunately proteins are extremely difficult to engineer. The most common approach consists of isolating a natural protein and modifying it in painstakingly small steps, one or a few amino acids at a time, and then seeing whether this has an effect on its properties. An alternative is to consider simpler polypeptides. Although easier to work with and quite efficient, as demonstrated early on by Derek Woolfson (also at the University of Bristol), these lack the potential for the robust 3D structure that DNA and proteins have. “This is where Philippe Minard's work at Université Paris-­Sud blew my mind when I discovered it almost a decade ago,” says Dujardin. “He could design fully artificial proteins (alpha-­repeat proteins). Not one at a time, but 1 billion together; all of them sharing a same robust 3D scaffold. Each of them different from the neighbor for a small (∼15%) fraction of the total sequence. Their size, about 5 nm, was close to the one of nanoparticles with known interesting properties (for example, gold or silver nanoparticles with plasmonic properties, fluorescent semiconducting quantum dots, magnetic particles used in MRI (magnetic resonance imaging), other metal nanoparticle with catalytic properties, etc.).” With such a population of similar yet different proteins, you can apply Darwin's evolution principle, as was demonstrated by Stanley Brown in the early 1990s whereby you expose this population of proteins to a target (for example, one chosen protein) and you fish out of the 1 billion individuals the few that adhere to the target. They are then multiplied by a standard phage display method. By repeating this selection test three times, it is possible to identify the few artificial proteins that exhibit super-­affinity for the target. And now you have created a pair of proteins with very high affinity for each other. Minard's team can produce as many pairs of proteins as they want: they just need to change the target and start over again. “This is when Philippe and I started working together because I wanted to drive the assembly of particles using the protein pair formation,” notes Dujardin. “To do so we attach one protein to one nanoparticle

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type and the other protein to another nanoparticle. This is done using standard coupling chemistry. We also made sure that the proteins would not be perturbed by the nanoparticle surface and remain active for pair formation.” Specifically, the researchers wanted to have several pairs, so that in the end they could chose to assemble A with B or C with D, for example. While this is straightforward with DNA, it has been out of reach with proteins. However, in 2013, Minard and his team succeeded in creating proteins. Dujardin and Minard have now taken a first step towards the goal of protein-­driven assembly of nanoparticles. In this ground-­breaking work, they show that gold nanoparticles with a diameter of 10 nm can be assembled using two different protein pairs (Figure 5.4). The novelty lies in several aspects, since the researchers:    ●● work with fully folded proteins, not peptides or DNA; ●● do not serendipitously modify existing proteins by one or two amino acids, but rather create fully artificial proteins, the size of which can be varied and the chemistry of which (at least on one out of six sides) can be randomly modified; ●● select the most suitable proteins to address a particular objective (e.g., direct the strong self-­assembly of nanoparticles); and ●● work at the interface between the biomolecule (used as a smart cement) and nanomaterials (which bring physical properties), which is still virgin territory for researchers.    They further demonstrate that the strength of the nanoparticle assembly is the same as the pure protein pair formation, which implies that the proteins do the job and are not denatured or distorted by their attachment onto the nanoparticle surface. Finally, the scientists found that they can disassemble the nanoparticles again by using a trick: injecting a large quantity of free proteins unattached to a particle results in unpairing of the nanoparticle aggregates. “An interesting observation we made is that the more proteins you attach on the nanoparticle surface, the larger the super-­assembly gets,” says Dujardin. “We observed the formation of large (many tens of micrometers and possibly up to millimeter size) freestanding films made of a single layer of particles. When only few (1, 2, or 3) proteins are attached, then you produce clusters of only a few particles (∼20).” While this work has shown that protein design leads to assemblies with very different topology (film vs. clusters), the team now wants to demonstrate that the strength of the protein–protein interaction can be transferred to the assembly of functional nanomaterials. Specifically, they want to show that they can assemble nanoparticles of different types. For example, combine plasmonic gold nanoparticles with

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Figure 5.4  Schematic  flowchart of the Au NP self-­assembly driven by the α-­Rep protein pair formation. Step I: anionic thiopeptide surface capping of citrate-­stabilized Au nanoparticles. Step II: protein functionalization by ligand exchange. Step III: nanoparticle self-­assembly by protein pair recognition. Inset: TEM image of a massive nanoparticle film formed between A3-­ and α17-­functionalized Au nanoparticles. Scale bar, 2 µm. Reproduced from http://dx.doi.org/10.1021/acsnano.5b04531 with permission from American Chemical Society, Copyright 2016.

fluorescent quantum dots to benefit from the enhanced light field near the gold nanoparticle to promote more intense fluorescence. It also would be feasible to bring together proteins with enzymatic properties and catalytic nanoparticles for ‘green chemistry’ applications. As Dujardin points out, “what we have demonstrated with our simple system of designable cement based on artificial proteins could now be made as complex as any materials scientist could wish in order to combine nanoparticle properties.”

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Besides multiple-­component assemblies, another important step going forward will be the ability to control the organization of nanoparticles in three dimensions. “For this, the rigid scaffold of the alpha-­repeat proteins will be very useful,” says Dujardin. “Then, we can not only decide to put nanoscale building block A (proteins, inorganic nanoparticles, organic nanoparticles, etc.) close to B, but also A at the center, B on the left of A, C on the top of A, D below A, and so on.” The ability to create hybrid organic–inorganic interfaces with a rigid protein on a solid surface would open a whole new realm in designing the metal interface, which is so important in catalysis, colloidal synthesis, and optical properties of nanoparticles, but also in bioimaging, hyperthermia, therapeutics, etc. Virtually any application that depends on positioning a nanoparticle in a specific location could benefit from this work. “Material design and sculpting at the nanoscale and below, down to atomic scale, is a really hard-­to-­crack nut and I think that proteins can play a role if we can purpose-­design them,” concludes Dujardin. “Proteins contribute to life and life is, in part, a nanoscale phenomenon. So let's use these molecules at the nanoscale—since they have been optimized for that—but for non-­biological objectives.” Featured scientist: Dr Erik Dujardin (https://bit.ly/2KmBvMu) Organization: Groupe Nanosciences, PicoLab, CEMES, Toulouse (France) (https://bit.ly/2TTkBtX) Relevant publications: T. Nayak, H. Andersen, V. Makam, C. Khaw, S. Bae, X. Xu, et al., Graphene for Controlled and Accelerated Osteogenic Differentiation of Human Mesenchymal Stem Cells, ACS Nano, 2011, 5(6), 4670–4678. K. Gurunatha, A. Fournier, A. Urvoas, M. Valerio-­Lepiniec, V. Marchi, P. Minard, et al., Nanoparticles Self-­Assembly Driven by High Affinity Repeat Protein Pairing, ACS Nano, 2016, 10(3), 3176–3185. P. Jain, A. Soshee, S. Narayanan, J. Sharma, C. Girard, E. Dujardin, et al., Selection of Arginine-­Rich Anti-­Gold Antibodies Engineered for Plasmonic Colloid Self-­Assembly, J. Phys. Chem. C, 2014, 118(26), 14502–14510.

5.9  Towards Self-­powered, Brain-­linked e-­Vision Sensory substitution with flexible electronics is one of the intriguing fields of research that takes place in nanotechnology laboratories around the world. Scientists already fabricate electronic devices that can replicate, to some degree, some of the human senses, such as touch (electronic skin—e-­skin), smell (e-­nose), and taste (e-­tongue).

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Figure 5.5  Optical  images of the vision e-­skin. Reproduced from http://dx.doi. org/10.1002/adfm.201800275 with permission from John Wiley and Sons, © 2018 WILEY-­VCH Verlag GmbH & Co. KGaA, Weinheim.

In line with this focus on human senses, in the future, artificial retinas integrated with the human body may not only repair damaged vision, but also expand it to see a wider range of wavelengths (e.g., ultraviolet light). The human retina is a film-­shaped tissue behind the vitreous body in the eye. Photosensitive cells in the retina convert incoming light energy into bioelectric signals that are carried to the brain by the optic nerve. Researchers in China have demonstrated a new self-­powered, brain-­linked vision e-­skin for mimicking the human retina. “The general idea of our device design of brain-­linked vision electronic skin is constructing an integrated flexible system including photodetector array, information analyzer, signal transmitter, and electricity power unit,” says Xinyu Xue, a professor at the College of Sciences at Northeastern University, Shenyang. “While various research groups already have reported flexible photodetecting electronics, a battery-­free, flexible, and efficient power-­supply unit remains an important bottle-­neck of the flexible vision e-­skin. Another problem is to input the photo detecting signal into the brain for participating in the vision perception and relevant behavior intervention.” “Our self-­powered vision e-­skin is different from traditional complex integrated systems and combines the electricity-­generating, photodetecting, and neurobionics of signal transmission into one single chemical/physical process,” explains Xue. “In this process, the photodetecting units in the e-­skin harvest human-­motion energy and output triboelectric signals containing the photodetecting information, acting as both the power source and the photodetecting signal for mimicking vision.” The team's novel fabrication process employs, in successive order: standard photolithography; printed circuit board technique; polydimethysiloxane (PDMS) soft-­template method; electron-­beam evaporation process; and electrochemical polymerization. The resulting film device is a polypyrrole/PDMS triboelectric photodetecting, pixel-­addressable matrix (Figure 5.5).

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“Our new technique lowers the fabrication cost of traditional complex sensory-­substitution systems and can be easily extended to various brain– machine interaction applications,” Xue points out. To demonstrate the workings of their self-­powered vision e-­skin, the scientists attached it to the corner of a person's eye. The motion of blinking eyes generated enough output power of the triboelectric generator to power the device and, in this test setting, detect UV illumination. “The e-­skin can map single-­point and multipoint illumination stimuli (visual-­image recognition) via the multichannel data acquisition method,” says Xue. “In the next stages of our work, we will investigate self-­powered multiperception e-­skin, including tactility, gustation, olfaction, and audition. And we will also try to further investigate the brain–device interaction for practical purposes.” Featured scientist: Professor Xinyu Xue Organization: College of Sciences, Northeastern University, Shenyang (China) Relevant publication: Y. Dai, Y. Fu, H. Zeng, L. Xing, Y. Zhang, Y. Zhan, et al., A Self-­Powered Brain-­Linked Vision Electronic-­Skin Based on Triboelectric-­Photodetecting Pixel-­Addressable Matrix for Visual-­Image Recognition and Behavior Intervention, Adv. Funct. Mater., 2018, 28(20), 1800275.

5.10  ‘Cyborg’ Microfilter Actively Cleans Contaminated Water Researchers often use living systems as inspiration for the design and engineering of micro-­and nanoscale propulsion systems, actuators, sensors, and robots. With regard to propulsion, a lot of attention has been devoted to self-­ propelled, chemically powered micro/nanoscale motors, such as catalytic nanowires, microtube engines, or spherical Janus microparticles. One application area for micromotors is water decontamination, for instance, the active degradation of organic pollutants in solution. “Although microrobots have recently proved successful for remediating decontaminated water at the laboratory scale, the major challenge in the field is to scale up these applications to actual environmental settings,” notes Professor Joseph Wang, Chair of Nanoengineering and Director, Center of Wearable Sensors at the University California, San Diego. “In order to do this, we need to overcome the toxicity of their chemical fuels, the short time span of biocompatible magnesium-­based micromotors, and the small domain operation of externally actuated microrobots.” In their work on self-­propelled biohybrid microrobots, Wang and his team were inspired by recent developments of biohybrid cyborgs that integrate self-­propelling bacteria with functionalized synthetic nanostructures to transport materials.

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“These tiny cyborgs are incredibly efficient for transporting materials, but the limitation that we observed is that they do not provide large-­scale fluid mixing,” says Wang. “We wanted to combine the best properties of both worlds. So, we searched for the best candidate to create a more robust biohybrid for mixing and we decided on using rotifers (Brachionus) as the engine of the cyborg.” These marine microorganisms, which measure between 100 and 300 micrometers, are amazing creatures as they already possess sensing ability, energetic autonomy, and provide large-­scale fluid-­mixing capability. They are also are very resilient and can survive in very harsh environments and are even one of the few organisms that have survived via asexual reproduction. “Taking inspiration from the science fiction concept of a cybernetic organism, or cyborg—where an organism has enhanced abilities due to the integration of some artificial component—we developed a self-­propelled biohybrid microrobot, that we named rotibot, employing rotifers as their engine,” says Fernando Soto, first author of a paper on this work. This is the first demonstration of a biohybrid cyborg used for the removal and degradation of pollutants from solution. The technical breakthrough that allowed the team to achieve this task is based on a novel fabrication mechanism exploiting the selective accumulation of functionalized microbeads in the mouth of the microorganism: the rotifer serves not only as a transport vessel for active material or cargo, but also acts as a powerful biological pump, as it creates fluid flows directed towards its mouth to feed. The specific active sites of the functionalized microbeads on the rotifer's mouth are subjected to high flow rate, generated by the coordinated strokes of cilia bands of the rotifers, which induce a flow field towards the mouth (Figure 5.6). The researchers use this flow, without an external force, to get the pollutants towards their ‘cyborg microfilter’. The team exploited the negative charge on the cilia surface of the rotifers to confine positively charged functional particles—plastic microbeads with enzymes that are used to degrade or capture pollutants in water. “For example, we demonstrated the accelerated decontamination of bacteria (Escherichia coli), nerve agents (methyl paraoxon), and heavy metal ions (Cd and Pb) from aqueous solutions turning the cyborg into a kind of micro-­ Roomba,” says Soto. The team is looking to push the limits of what their new platform can do. Initially, they aim at making it fully degradable by replacing the plastic (latex and polystyrene) microbeads utilized in this proof-­of-­concept study with biodegradable functional microparticles. They also hope that this method can be expanded to other decontamination applications by changing the functionalization of the attached microbeads. Forming swarms of microcyborgs by inducing collective behavior, taking advantage of their group response to specific stimuli, is another item on their list.

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Figure 5.6  Mechanism  for the formation of the rotibot: (A) scheme, (B) micros-

copy images, and (C) scanning electron microscopy images, illustrating the steps toward rotifer (blue) uptake of the functionalized microbeads (yellow). (i) Microbeads approaching the rotifer's mouth due to a strong directional flow, (ii) upon contact with the cilia the beads adhere in their tips, and (iii) the cilia within the inner lips of the rotifer accumulate the particles in that location and free up the cilia for subsequent adhesion. Scale bars: B: 50 μm; C: 5 μm. Reproduced from http://dx. doi.org/10.1002/adfm.201900658 with permission from John Wiley and Sons, © 2019 WILEY-­VCH Verlag GmbH & Co. KGaA, Weinheim.

“A main challenge for us is the creation of a universal platform that can be translated into diverse applications,” Wang concludes. “This will require the fine-­tuning of the cyborg cleaners' operation and the synchronization between the microorganism engine and the desired task.” Featured scientists: Professor Joseph Wang's research group (https://bit. ly/2Ocb5Na) Organization: University of California, San Diego, CA (USA) Video: https://youtu.be/2twgoEJp97k Relevant publication: F. Soto, M. Lopez-­Ramirez, I. Jeerapan, B. Esteban-­ Fernandez de Ávila, R. Mishra, X. Lu, et al., Rotibot: Use of Rotifers as Self-­Propelling Biohybrid Microcleaners, Adv. Funct. Mater., 2019, 29(22), 1900658.

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5.11  G  rowing Bone and Cartilage Tissue from Nanosilicates Human stem cells have shown potential in medicine as they can transform into various specialized cell types such as bone and cartilage cells. The current approach to obtaining such specialized cells is to subject stem cells to specific instructive protein molecules known as growth factors. However, the use of growth factors in the human body can generate harmful side effects, including unwanted tissue growth (e.g., tumors). Researchers at Texas A&M University have demonstrated that a specific type of nanoparticle, nanosilicates, can grow bone and cartilage tissue from stem cells in the absence of growth factors. These nanoparticles are similar in shape to a coin, but 10 billion times smaller. Nanosilicates consist of minerals such as sodium, silicate, magnesium, and lithium, which are already present in the body. The capability of investigators to accurately and sensitively discern changes in cell behavior after nanomaterial treatment requires them to pick biomolecule targets, like genes or proteins of interest, ahead of time. While research is driven by scientifically based hypotheses, sometimes cell outcomes can be difficult to predict, especially for unexplored nanomaterials. The process of RNA-­sequencing provides researchers with a means to capture a broad view of the cell function of a cell population or even a single cell. Using emerging bioinformatics techniques, they can uncover signaling pathways or genes that were not previously considered. Two-­dimensional nanomaterials have emerged as a new generation of materials due to their unique properties relative to macroscale counterparts. However, little is known about their interactions with human cells following exposure to these nanomaterials. The Texas A&M team investigated the interactions of 2D nanosilicates, a layered clay, with human mesenchymal stem cells (hMSCs) at the whole-­transcriptome level by high-­ throughput sequencing (RNA-­seq; also called whole-­transcriptome shotgun sequencing). RNA-­seq uses next-­generation sequencing to reveal the presence and quantity of RNA in a biological sample at a given moment. RNA-­seq is used to analyze the continuously changing cellular transcriptome. Cells use genetic code (DNA) to produce different genetic messages to direct certain cellular functions such as to form new bone, blood vessels, skin, etc. These messages are called messenger RNA (mRNA, or transcriptome) and perform a variety of functions, one of which is to instruct cells on how to behave. So, the transcriptome is the set of all RNA molecules in one cell or a population of cells. The term “whole-­transcriptome” means that scientists are looking at all of these messages (mRNA) sent within the cell at a single point in time. “We demonstrated that nanosilicates exhibit some very interesting capabilities when applied to adult human stem cells,” says Akhilesh K. Gaharwar,

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Figure 5.7  Image  showing how nanosilicates direct differentiation of stem cells towards bone and cartilage lineages. (Image: Inspired Nanomaterials and Tissue Engineering (iNanoTE) Lab, Texas A&M University).

an assistant professor at Texas A&M University. “When these stem cells are treated with nanosilicates, they start showing behavior that is typically observed during regeneration of bone and cartilage. This indicates strong biomedical potential for these nanoparticles to be used as a possible therapy for osteoarthritis or osteoporosis.” Two-­dimensional nanosilicates are highly bioactive and have been shown to direct the change or differentiation of human stem cells towards bone and cartilage tissue (Figure 5.7). The cellular response to nanosilicates is believed to originate from the unique physical and chemical composition of the nanoparticles. This premise of mineral-­based particles affecting cell behavior has opened the door to the development of a new class of therapies, specifically for osteoporosis or arthritis/osteoarthritis, as described here.    1. Osteoporosis causes bones to become weak and brittle—so brittle that a fall or even mild stresses, such as bending over, can cause a fracture. The 2D nanosilicates can act as a unique additive that helps to stimulate bone growth. If these nanosilicates can be delivered to bone cells, then they can stimulate bone cells to deposit bone matrix that can strengthen the surrounding weak bone. 2. Osteoarthritis is characterized by progressive degradation of cartilage and is often localized at joints. Due to the lack of vascularity within intra-­articular spaces, systemic delivery of morphogens and growth factors supporting cartilage matrix synthesis often results in limited improvement. The 2D nanosilicates can be delivered to the intra-­ articular space to stimulate chondrocytes to synthesize new cartilage matrix.    RNA-­seq, which the research team employed here, is a powerful tool for an accurate quantification of expressed transcripts that largely overcomes limitations and biases of microarrays. For example, cell–nanoparticle

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interactions can result in significant changes in cellular behavior that can be observed using this technique. In layman's terms: RNA-­seq takes a snapshot of gene activity of the cell at any given moment. This is similar to taking a high-­resolution photograph during a football game and identifying the reaction of every fan during the touchdown. “RNA-­seq is suitably sensitive to investigate the interaction of a wide variety of nanomaterials with cells,” explains Dr Irtisha Singh, a co-­investigator of the study. “With the combination of nanotechnology and computational biology, we can better understand how a material's chemistry, shape, and size can contribute to cell functions. Understanding cellular responses following treatment with nanomaterials will aid in evaluating their application for a range of biomedical and biotechnology applications.” In this present work, the scientists investigated the interactions of 2D nanosilicates with hMSCs by employing transcriptome dynamics to uncover triggered biophysical and biochemical cellular pathways. In doing so, they observed widespread changes in the gene expression profile (>4000 genes) following nanosilicate exposure. In addition, transcriptomic dynamics of nanosilicate-­treated hMSCs identifies key genes and enriched gene ontology pathways and categories related to stem-­cell differentiation, specifically toward osteochondral lineages. “We validated the RNA-­seq findings using in vitro studies, which support the ability of nanosilicates to direct hMSC differentiation toward bone and cartilage lineages,” Gaharwar notes. He points out that this study also investigated surface-­mediated kinase signaling triggered by 2D nanosilicates. “This work enables further development of nanomaterial-­based therapeutics for regenerative medicine. More generally, transcriptomic analysis by next-­generation sequencing provides a comprehensive and objective snapshot of cellular behavior following exposure to nanomaterials.” “Furthermore, this study demonstrates the utility of next-­generation sequencing for the study of cellular interactions on nanoengineered substrates and the role this approach is likely to play in this rapidly expanding field of regenerative medicine.” The study was performed using human cells in in vitro to show that nanosilicates can be used to grow bone and cartilage tissue. The team's next step will be to demonstrate the formation of bone and cartilage in vivo. Specifically, they are looking to develop implantable scaffolds containing these nanosilicates to improve local tissue growth for both bone and cartilage. “Moreover, we will use computational biology and genomics to design next-­ generation bioactive materials for regenerative medicine,” says Gaharwar. The ability to customize a therapy to a specific tissue simply by changing the mineral content within the nanoparticles presents great potential within the field of regenerative engineering.

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“We believe this new field of mineralomics can provide a viable alternative to current treatments existing today,” conclude James Carrow and Lauren Cross, graduate students in Gaharwar's laboratory and co-­first authors of the paper. “From a technological perspective, there is a greater push to incorporate ‘big data’ into experimental biology and bioengineering through improved modeling and analytics. This coupling of computer science with biology has the potential to accelerate the discovery of new and beneficial regenerative therapies. The public availability of data from these gene expression studies provides a great service for those looking to find new trends in cell behavior.” Featured scientist: Professor Akhilesh K. Gaharwar's Inspired Nanomaterials and Tissue Engineering (iNanoTE) Laboratory (https://bit. ly/2HRIiuL) Organization: Texas A&M University, College Station, TX (USA) Relevant publication: J. Carrow, L. Cross, R. Reese, M. Jaiswal, C. Gregory, R. Kaunas, et al., Widespread changes in transcriptome profile of human mesenchymal stem cells induced by two-­dimensional nanosilicates, Proc. Natl. Acad. Sci. U. S. A., 2018, 115(17), E3905–E3913.

5.12  F  abricating Tissue Engineering Scaffolds via Controlled Ice Crystallization Each year, a large number of people around the world suffer from chronic skin wounds. Often, chronic wounds, such as skin ulcers, are seen in older people suffering from circulation disorders and diabetic patients whose skin tissue has a poor capacity of regeneration. Currently, many treatment approaches focus primarily on managing the wounds. Researchers have taken a nanotechnology-­based tissue engineering approach to accelerate the regeneration and repair of damaged tissues at the wound site by directing cells and tissues to grow towards the target site. Their hope is that this leads to the development of affordable and functional biodegradable wound dressings for accelerated healing of chronic skin wounds, by promoting regeneration of local tissues. “We show that 3D scaffolds with both aligned nanofibers and aligned interconnected macrochannels can be created with various biomacromolecules, including silk fibroin, using a facile guided ice-­crystal growth and nanofiber assembly strategy,” explains Dr Linpeng Fan. In this work, a research team at the Institute for Frontier Materials at Deakin University, Australia, developed a facile assembly strategy based on the growth of ice crystals to produce biomimetic 3D-­active nanofibrous

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scaffolds. They show that this cell-­ and tissue-­inducing scaffold can capture and direct peripheral healthy cells and tissues to grow towards the target site and thus accelerate regeneration of damaged tissues. “So far, it has been an enormous challenge to couple aligned nanofibers with aligned macrochannels, which always go in the opposite direction,” Dr Fan points out. “We found that this can be achieved by manipulating ice crystallization in such a way that it guides the assembly of oriented nanofibers.” In this process, the oriented nanofibers guide the oriented growth of large ice crystals along the fiber direction into the scaffold, which in turn leads to further assembly of the nanofibers to form aligned macrochannels in their long-­axis direction. Once the aligned ice crystals in the scaffold were removed, the resulting biomimetic anisotropic 3D scaffold had both radially co-­aligned nanofibers and interconnected macrochannels. “More importantly” adds Dr Fan, “the scaffold can direct cell filtration and migration, deposition of collagen proteins of cells, as well as vascularization and 3D neurite growth.” The strategy could inspire the design and development of multifunctional 3D-­active nanofibrous scaffolds for engineering different tissues. For example, these 3D scaffolds could be used for accelerated wound healing by recruiting and directing peripheral healthy cells to grow inwards radially. This work could also inspire the development of novel multifunctional scaffolds for engineering complex tissues such as blood vessels. The team's preliminary in vivo results demonstrate that this kind of 3D-­active scaffold can significantly accelerate tissue regeneration compared to traditional 3D scaffolds. “Based on this, we are developing a kind of novel wound dressing for the healing of chronic wounds,” Dr Fan concludes. “We hope we can produce affordable and functional dressings to help people in need, especially diabetics for healing of their chronic ulcers by promoting regeneration of local tissues.”

Featured scientist: Dr Linpeng Fan Organization: Institute for Frontier Materials (https://bit.ly/2WmNIHZ), Deakin University, Geelong (Australia) Relevant publication: L. Fan, J. Li, Z. Cai and X. Wang, Creating Biomimetic Anisotropic Architectures with Co-­Aligned Nanofibers and Macrochannels by Manipulating Ice Crystallization, ACS Nano, 2018, 12(6), 5780–5790.

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5.13  Augmenting Nerve Regeneration Microtubules are architectural struts in nerves. They have dynamic parts that constantly assemble and disassemble, but also stable parts that remain assembled. In recent years, researchers working in neurobiology have been intrigued by the idea of microtubule-­stabilizing drugs as a therapy to augment nerve regeneration. “Previously it had been proposed that damaged nerves could be made to regenerate by using drugs that make the microtubules in the nerve more stable,” says Peter Baas, a professor in the Department of Neurobiology & Anatomy at Drexel University College of Medicine. “We show that a better idea is to increase the amount of the dynamic parts of the microtubules. We do this by reducing the levels of fidgetin, a protein that normally exists in nerves to keep the dynamic parts of microtubules from elongating too much.” To reduce fidgetin levels, Baas and his collaborators use a nanotechnology approach. The two key elements in this approach are the novel idea of reducing fidgetin to increase dynamic parts of microtubules—as opposed to microtubule-­stabilizing drugs—and the use of a small interfering RNA (siRNA)-­encapsulated nanoparticle delivery platform. The team uses RNA interference, which stops new fidgetin from being synthesized, delivered via nanoparticles. The nanoparticles readily enter cells, tissues, and organs in a manner that is potentially superior to other methods of delivery, such as viruses. “Based on a hydrogel/sugar glass composite, our delivery platform is a hybrid nanoparticle capable of encapsulating and controllably releasing a broad range of therapeutically relevant materials ranging from gaseous nitric oxide to larger macromolecules such as chemotherapeutic agents and phosphodiesterase inhibitors,” Baas notes. He points out that the composition of the nanoparticles can be varied so that they release their load at different rates, which means that treatment regimens can be refined and adjusted. According to the team, fidgetin reduction can be used to augment nerve regeneration in a variety of injury situations, such as spinal cord injury, traumatic brain injury, and peripheral nerve injury. While this present work only deals with a cell culture model (adult primary DRG neurons), the researchers are planning to take the method into animal models (Figure 5.8). “The big challenge is not only to get the nerves to grow better but to enable them to find their appropriate target tissues to enable functional recovery,” concludes Baas. “Given that the dynamic parts of microtubules are known to

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Figure 5.8  Microtubule  mass is increased in the axons of adult DRG neurons as

a result of fidgetin knockdown. (A) Representative images of neuronal β–III–tubulin immunofluorescence (IF) staining show more extensive axonal outgrowth after fidgetin knockdown compared to (A′) control, and also show denser microtubule mass within axons, as indicated by more intense IF-­staining. Shown are inverted images with black and white reversed for enhanced clarity. Reproduced from http://dx. doi.org/10.1038/s41598-­017-­10250-­z under the terms of the CC BY 4.0 license, http://creativecommons.org/licenses/by/4.0/.

be very important for this, we think fidgetin reduction has a great chance of helping where to date nothing else has.” Featured scientist: Professor Peter Baas (https://bit.ly/2xwW4gp) Organization: Department of Neurobiology & Anatomy at Drexel University College of Medicine, Philadelphia, PA (USA) Relevant publication: T. O. Austin, A. J. Matamoros, J. M. Friedman, A. J. Friedman, P. Nacharaju, W. Yu, et al., Nanoparticle Delivery of Fidgetin siRNA as a Microtubule-­based Therapy to Augment Nerve Regeneration, Sci. Rep., 2017, 7(1), 9675.

Chapter 6

Nanomedicine 6.1  Introduction Nanotechnology is becoming a crucial driving force behind innovation in medicine and health care, with a range of advances including nanoscale therapeutics, biosensors, implantable devices, drug-­delivery systems, and imaging technologies. Universities have also begun to offer dedicated nanomedicine degree programs such as the MSc program in Nanotechnology for Medicine and Health Care at the University of Oxford. A nanotechnology-­based system, for instance, to eradicate cancer, needs four elements.    1. Molecular imaging at the cellular level so that even the slightest overexpressions can be monitored. 2. Effective molecular targeting after identifying specific surface or nucleic acid markers. 3. A technique to kill the cells that are identified as cancerous, based on molecular imaging, simultaneously by photodynamic therapy or drug delivery. 4. A postmolecular imaging technique to monitor the therapeutic efficacy.    Various nanotechnological approaches for effective drug delivery have been developed and some of them have already been successfully commercialized. Most prominent nanodrug delivery systems that are in the marketplace are oncology related and based on liposomal, solid nanoparticle-­based, protein–polymer conjugates, and polymer–drug conjugate-­based delivery platforms.

  Nanoengineering: The Skills and Tools Making Technology Invisible By Michael Berger © Michael Berger 2020 Published by the Royal Society of Chemistry, www.rsc.org

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6.2  B  acteria-­produced Nanoparticles Kill Cancer Cells Photothermal therapy (PTT) is a form of cancer treatment where a therapeutic agent absorbs energy from photons, usually from near-­infrared light, and dissipates it partially in the form of heat. When the therapeutic agents, for instance, nanoparticles, are located in close vicinity to the tumor site, the temperature increase can lead to cell damage, i.e., it kills the cancer cell. For PTT to work effectively, it is most important to design photothermal conversion agents with low toxicity and high therapeutic efficiency. In order to kill cancer cells, a team of Chinese scientists has used naturally occurring bacterial magnetic nanoparticles (BMPs)—magnetosome extracted from magnetotactic bacteria—to substitute synthetic nanoparticles for photothermal cancer therapy. Magnetotactic bacteria are organisms that produce chains of organelles (compartments located inside the bacterial cell wall) called magnetosomes that contain magnetic iron nanoparticles. Magnetosomes allow bacteria to orient themselves along the Earth's magnetic field lines in order to migrate to more favorable environments. “Compared with engineered magnetic nanoparticles, BMPs have specific features such as large-­scale production, monodispersity, good biocompatibility, high crystallinity, and close-­to-­bulk magnetization, besides being covered with a lipid bilayer,” explains Professor Linlin Li from the Beijing Institute of Nanoenergy and Nanosystems, Chinese Academy of Sciences. “In previous reports, BMPs were considered as good materials for the cancer therapy in hyperthermia treatments. With high saturation magnetization, they are also used for MRI in vivo. Up to now, no attention has been paid to photothermal therapy based on bacterial magnetic nanoparticles.” BMPs not only act as photothermal conversion agents, explains Li, but also enhance the T2-­weighted MRI at the tumor location for guiding therapy and monitoring the nanoparticle biodistribution. “Our in vitro and in vivo results all prove that the BMPs have good biocompatibility, high photothermal conversion efficiency, efficient photothermal killing effect on tumors, and T2-­weighted MRI,” says, Dr Chuanfang Chen from the Institute of Electrical Engineering, Chinese Academy of Sciences. “A single dose of therapy could completely remove tumor tissues without recurrence on liver cancer-­bearing mice.” The layer of biomembrane that covers the BMPs is particularly useful as it removes the need for a postsynthetic surface modification step for escaping destruction by the body's immune system. Using the BMPs as both photothermal conversion agent and MRI contrast agent opens a new window for highly targeted cancer therapy and cancer theranostics. By integrating active targeting ligands onto the surface of BMPs the team hopes to achieve an even more efficient tumor therapy.

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Featured scientists: Professor Linlin Li (https://bit.ly/2W9ekHu); Dr Chuanfang Chen Organization: Beijing Institute of Nanoenergy and Nanosystems, Institute of Electrical Engineering, Beijing (China) Relevant publication: C. Chen, S. Wang, L. Li, P. Wang, C. Chen, Z. Sun, et al., Bacterial magnetic nanoparticles for photothermal therapy of cancer under the guidance of MRI, Biomaterials, 2016, 104, 352–360.

6.3  Light-­triggered Local Anesthesia Systemic pain medicine—which acts on the whole nervous system, rather than a specific area, to lessen pain—can have side effects such as nausea, drowsiness, or lack of concentration. Besides pain relief, numerous serious medical conditions require medications that cannot be taken orally, but must be dosed intermittently, on an as-­needed basis, and over a long period of time. Controlled and long-­term drug release has been recognized as one of the most promising biomedical technologies for certain types of chronic diseases. In order to avoid the inconvenience of repeated injections, remotely triggerable drug-­delivery systems have seen a lot of research interest. In these applications, a depot of drug is administered once, then repeatedly actuated via a safe external trigger, such as an electrical impulse, a magnetic field, or near-­infrared (NIR) light. Researchers have now also demonstrated a system that provides phototriggered release of local anesthetics in a manner that could be adjusted by varying the dose and duration of irradiation. “We showed that gold nanoparticle-­modified liposomes could be used to provide adjustable, on-­demand infiltration anesthesia,” says Changyou Zhan, a researcher in the laboratory of Daniel Kohane at Harvard Medical School. “After the initial numbness after injection wore off, irradiation of the site of injection with near-­infrared light led to the return of local anesthesia, once daily over five days.” “From a clinical point of view, this is important in that it demonstrates a method by which patients would be able to take control of relatively local pain, being able to deliver local analgesia on demand, for the duration and with the intensity desired,” Kohane elaborates. “In the postoperative setting, technologies like these could make pain management easier for patients, and minimize the extent to which opioids and other systemic pain killers would have to be taken—and all their side effects.” In order to achieve repeated on-­demand local anesthesia, the researchers chemically tethered gold nanorods—with their ability to convert NIR light

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Figure 6.1  Transmission  electron microscopy (TEM) image of liposomes conjugated with gold nanorods (yellow arrows in the right panel). (Image: Changyou Zhan, Harvard Medical School).

into heat—to liposomes containing tetrodotoxin and dexmedetomidine (Figure 6.1). The gold nanorods would raise the temperature of the adjacent liposomal lipid bilayer above its transition temperature, so that it would change from an ordered gel phase to a disordered liquid-­crystalline phase, and release analgesic compounds. The team tested the ability to provide repeated sensory blockade triggered by remote NIR irradiation in vivo in rats. “In theory, this sort of technology could be applicable to a range of excitable tissues—nerve, muscle, brain, heart, spinal cord,” Kohane points out. “From the point of view of nanotechnology, it is an unusually clear demonstration of the ability of nanoscientifically based triggering methods to induce local effects.” Now, the challenge for the team is to make the formulations last longer and be triggerable at lower energies. The latter is important for increasing the depth in the body at which the particles can be triggered, and reducing the probability of tissue injury (burns), which could be caused by high irradiances and/or prolonged irradiation times. The scientists note that such devices could also be adapted to use for other excitable tissues, e.g., in the brain to prevent or treat seizures.

Featured scientist: Professor Daniel Kohane (https://bit.ly/2UIkUot) Organization: Harvard Medical School, Boston, MA (USA) Relevant publication: C. Zhan, W. Wang, J. McAlvin, S. Guo, B. Timko, C. Santamaria, et al., Phototriggered Local Anesthesia, Nano Lett., 2015, 16(1), 177–181.

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6.4  Repairing the Cancer Cell Suicide Mechanism Cancer is a very complex disease and the exact cause is not clearly understood yet. Extensive biomedical research suggests a combination of genetic and environmental factors. Treatment for cancer can take many forms and primarily depends on the type of the disease, its progression, and other factors. Under normal circumstances, apoptosis is a highly regulated and controlled biochemical process that leads to cell death. In other words, this is a natural process by which cells ‘commit suicide’. Cancer cells can outsmart and evade this normal cellular mechanism, leading to uncontrolled cellular growth. Mitochondria are the primary controllers of cellular suicide. Mitochondria are known as the ‘powerhouse of the cell’ and function as a battery to supply energy for the manufacture of ATP (the energy storage molecule adenosine triphosphate). “In the 1950s, Otto Warburg (Nobel Prize Winner, 1931) proposed a linkage between cancer and functional damage to mitochondria. Although this theory has long been debated, emerging studies have instigated further validation that mitochondrial metabolism theoretically is a plausible target for cancer therapy,” says Dipanjan Pan, an assistant professor in the Department of Bioengineering at the Beckman Institute of Advanced Science and Technology, University of Illinois at Urbana-­Champaign. “To trigger the suicide switch back on, our work uses a highly selective nanotechnology-­based approach to deliver a widely available small molecule commonly found as a by-­product of a water chlorination process.” In a preclinical model, Pan and his team demonstrated remarkable success of these agents. “With further development, this approach could offer new hope for cancer patients with much desired improvements in their treatment regimen with affordable treatment options,” notes Pan. In this work, as part of the quest by nanotechnology researchers to cure cancer, the researchers have demonstrated, in a laboratory setting, that therapeutic molecules can specifically be delivered to cancer cells in mice by selectively modulating the mitochondrial metabolic function. The target is a ‘gatekeeper’ enzyme in mitochondria known as pyruvate dehydrogenase kinase (PDK). PDK is activated in a wide range of cancers, which results in selective inhibition of pyruvate dehydrogenase to suppress mitochondrial apoptosis in cancer cells. Dichloroacetic acid (DCA) has been used in the past for cancer treatment with conflicting literature reports demonstrating its success. This agent is a common by-­product of chlorination of water. “DCA is easily absorbed by the body and can even penetrate the blood– brain barrier. Due to this rather easy access to the brain, the molecule can pose severe neurological and other toxicity effects,” explains Pan. “Our unique approach developed a ‘camouflaged’ form of DCA, which is not activated until it reaches the intended target; i.e., the cancer cell.”

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“Furthermore,” he adds, “the agent forms tiny nanoparticulates, which are selectively taken in by the tumors. We further tested similar compounds to show that not only DCA but other similar halogenated molecules can be used with comparable efficacy.” The team is now in the process of conducting safety studies with the agent. Once the safety is established, the first human studies can be initiated. Given the fact that these agents are derived from widely available chemicals, with a long history of human use (nearly 40 years), the scientists envision a rapid translation to eventual use in humans. These therapeutic agents will be administered in a ‘protected’ (prodrug) form, which precludes the release of the drug during systemic circulation. By virtue of the tiny size of the nanoparticles derived from these prodrugs, they will be specifically taken up by the ‘leaky’ tumor vasculature. The camouflaged form of the drug then becomes activated once it enters the cancer cell. The approach permits use of lower drug dosages, reducing overall toxicity while increasing overall efficacy. “Lack of selectivity is a key issue in developing drugs for oncology applications,” Pan points out. “An average drug discovery process from identification to regulatory approval could easily take more than ten years. It is therefore of utmost interest to the cancer research community to propose potential therapeutic approaches that simultaneously tackle selectivity issues and in parallel quickly translate into an affordable, widely available drug.” Featured scientist: Professor Dipanjan Pan (https://bit.ly/316bPcz) Organization: Department of Bioengineering at the Beckman Institute of Advanced Science and Technology, University of Illinois at Urbana-­ Champaign, Urbana and Champaign, IL (USA) Relevant publication: S. Misra, M. Ye, F. Ostadhossein and D. Pan, Pro-­ haloacetate Nanoparticles for Efficient Cancer Therapy via Pyruvate Dehydrogenase Kinase Modulation, Sci. Rep., 2016, 6(1), 28196.

6.5  D  rug-­loaded Nanobullets Fired from Microcannons The goal of a vast amount of nanomedicine research is the perfect drug carrier: it is injected into the body and transports itself to the correct target, such as a tumor, and delivers the required therapeutic drug dose at this target. This idealized concept was first proposed by German physician Paul Ehrlich at the beginning of the 20th century and was nicknamed the ‘magic bullet’ concept. Taking this ‘bullet’ concept literally, Joseph Wang and Sadik Esener, professors of nanoengineering at UC San Diego, and their teams have developed acoustically triggered microcannons, capable of versatile loading and effective firing of nanobullets, as novel tools toward advancing microscale tissue penetration of therapeutic payloads.

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“Designing the right tool for efficient tissue penetration at small scales is important for diverse biomedical applications in drug delivery and microsurgery applications,” notes Wang. “Our new nanobullets could eventually be used to directly drive drugs deep into diseased tissues.” “Ultrasound-­triggered vaporization of perfluorocarbon microemulsion is an extremely attractive candidate for externally triggering the actuation of micro-­ and nanoscale ballistic tools,” says Wang. “This technique is biocompatible and has been previously used to enhance the permeation and delivery of therapeutics into blood vessels and tissue. We expect that our new ultrasound-­based ballistic approach could lead to efficient delivery devices capable of delivering a wide range of payloads deep into an identified target.” To construct their device, the team first fabricated the hollowed cone structure of the microcannon using template electrodeposition of electrochemically reduced graphene oxide and gold on the walls of micropores in a polycarbonate membrane (Figure 6.2). They then infiltrated a liquid gel matrix by gravitational and capillarity forces into the hollow microcannons anchored in the template membrane. Entrapped in this gel were 1-­µm silica nanobullets; it also contained a perfluorocarbon (PFC) emulsion. “Upon application of a focused ultrasound pulse by a piezoelectric transducer, the nanobullets are ejected rapidly from the microcannon,” explains Wang. “The mechanism responsible for the propulsion thrust of the nanobullets relies on the momentum associated with the ultrasound-­induced spontaneous vaporization of the PFC emulsion droplet into a rapidly expanding microbubble.” Firing of a large amount of nanobullets has been modeled theoretically by the team and demonstrated experimentally. The experimental data support the theory that the ejection of nanobullets from the microcannon occurs due to vaporization of perfluorocarbon fluid, presenting remarkably high power, fast displacement speed, and a large tissue-­penetration depth.

Figure 6.2  Ultrasound  (US)-­triggered nanobullet firing based on vaporization of PFC droplets as a propulsion system. (Image: Department of Nanoengineering, UC San Diego).

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Creation of a functioning microscale cannon will allow efficient loading and firing of nanoscale therapeutic and imaging cargoes as nanoprojectiles, deep into biological tissues. The team plans to develop this microbullet platform technology for delivering a wide range of therapeutic payloads—including co-­delivery of multiple drugs—and expanding the practical utility of the acoustic microcannons, for use as single or array devices for transporting drug cocktails and vaccines, respectively. Featured scientists: Professors Joseph Wang (https://bit.ly/2Ocb5Na) and Sadik Esener (https://bit.ly/2TUpKlh) Organization: University of California, San Diego, CA (USA) Relevant publication: F. Soto, A. Martin, S. Ibsen, M. Vaidyanathan, V. Garcia-­Gradilla, Y. Levin, et al., Acoustic Microcannons: Toward Advanced Microballistics, ACS Nano, 2015, 10(1), 1522–1528.

6.6  Iron Oxide Nanoparticles Inhibit Tumor Growth The intravenous iron-­replacement product ferumoxytol and other iron oxide nanoparticles are being used for treating iron deficiency, as contrast agents for MRI, and as drug carriers. For the first time, researchers have shown an intrinsic therapeutic effect of ferumoxytol on the growth of early mammary cancers and lung cancer metastases in liver and lungs. They showed that ferumoxytol can activate the immune system to attack cancer cells. “This is the first description of an intrinsic therapeutic effect of iron oxide nanoparticles against cancer,” says Saeid Zanganeh, PhD, a postdoctoral scholar in the Department of Radiology, Molecular Imaging Program at Stanford University. “Our data have broad implications for diagnostic and therapeutic nanoparticle applications,” he adds. “Since ferumoxytol is FDA-­approved for intravenous treatment of iron deficiency, it could be applied ‘off label’ to protect the liver in patients from metastatic seeds and potentiate tumor-­associated macrophages modulating cancer immunotherapies.” Previous studies have described the use of iron oxide nanoparticles for targeted delivery of therapeutics. In these studies, the nanoparticles served as carriers of therapeutic drugs to the tumor microenvironment. In this new study, the scientists observed an intrinsic therapeutic effect of iron oxide nanoparticles themselves. “Our proposed approach to suppress liver metastases by repetitive systemic ferumoxytol administrations should be achievable without significant impact on iron load or liver toxicity,” notes Zanganeh. He adds that, due to the large surface area and chemically defined surface structure, iron oxide nanoparticles, such as ferumoxytol, can be conjugated

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or functionalized to numerous ancillary molecules to further amplify the desired immune-­modulating properties. The scientist explains that ferumoxytol could be locally delivered to unresectable tumors via interventional procedures, to micrometastases in confined spaces (e.g., peritoneal seeds), or to tumor resection margins at the end of a tumor surgery, in order to induce a proinflammatory reaction against cancer cells and suppress tumor growth in the interval between tumor surgery and initiation of postsurgical chemotherapy or irradiation. On the other hand, patients with primary tumors that typically metastasize to the liver could receive protective ferumoxytol medications to prevent early metastatic seeds. In the next stage of their study, the researchers will be more focused on the evaluation of nanoparticle–drug combinations. Potential clinical applications of ferumoxytol-­mediated proinflammatory immune responses could entail potentiating the efficacy of other M1-­activating cancer immunotherapies, such as anti-­CD47 monoclonal antibodies or antibodies blocking interleukin IL-­4 or IL-­13 signaling. Featured scientist: Dr Saeid Zanganeh Organization: Department of Radiology, Molecular Imaging Program at Stanford University, Stanford, CA (USA) Relevant publication: S. Zanganeh, G. Hutter, R. Spitler, O. Lenkov, M. Mahmoudi, A. Shaw, et al., Iron oxide nanoparticles inhibit tumour growth by inducing pro-­inflammatory macrophage polarization in tumour tissues, Nat. Nanotechnol., 2016, 11(11), 986–994.

6.7  R  eplacing Animal Models with Biomimetic Blood–Brain Barrier Models The blood–brain barrier (BBB) has the fundamental function to protect the brain from neurotoxic compounds, pathogens, and circulating blood cells. In order for nanomedicine applications to deliver a therapeutic compound from the blood system into the brain, this selective biological barrier has to be overcome. Crossing the BBB is the object of intensive research in nanotechnology and medicine for developing new therapies against brain cancer and for the treatment of neurodegenerative diseases. For this reason, it is extremely important to develop realistic models of the BBB, which mimic as accurately as possible the in vivo environment. In the past, different BBB models have been proposed, from static 2D models, to 2D microfluidic systems, and, more recently, 3D systems. However, until now, the technological limitations of the available fabrication techniques did not allow reproduction of the brain's microcapillaries.

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The development of high-­resolution 3D-­printing technologies has now enabled researchers to develop a realistic 3D biohybrid microfluidic model of the BBB inspired by the in vivo neurovasculature. Researchers in Italy have, for the first time, faithfully reproduced the microcapillaries of the neurovascular system on a 1 : 1 scale. Moreover, they have developed an accurate mathematical model that was fundamental both for the design of the biohybrid device and for the characterization of its fluid-­ dynamic properties. This model will continue to be useful for evolving and refining the prototype design. “The biohybrid BBB developed in our laboratory allows us to carry out high-­throughput screening of different drugs/compounds/nanovectors, and to evaluate their ability to cross the BBB,” says Gianni Ciofani, an associate professor at Polytechnic University of Torino and the principal investigator of the Smart Bio-­Interfaces group at the Italian Institute of Technology (IIT). “Furthermore, our biohybrid platform facilitates a rigorous study of the BBB crossing by avoiding the use of animal models, thus overcoming the issues related to the scarce accessibility of the brain and limiting important ethical concerns.” In this model, the brain's microcapillaries are characterized by both an artificial and a biological component, and are therefore defined as biohybrid. Specifically, the artificial structure consists of 3D porous tubes with the typical diameter size of brain microcapillaries (about 10 µm) and it is fabricated by using a high-­resolution 3D-­printing technology: two-­photon lithography. The printed porous microtubes provide a scaffold for the growth of endothelial cells, which externally envelop the tubular structures, thus developing a biological barrier. This barrier, similarly to the BBB, separates a luminal compartment (inside the artificial blood vessel) from the external abluminal region (represented by the brain) (Figure 6.3).

Figure 6.3  Scanning  electron microscopy image of the biohybrid BBB model. (Image: Gianni Ciofani, IIT).

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Finally, in order to further mimic the in vivo conditions, the flow rate of the solution inside the luminal biohybrid microvessels is set to a value comparable to that of brain microcapillaries (1 mm per second average speed). “The novelty of our work mainly consists in the fabrication of a reliable platform to carry out high-­throughput quantitative investigations of drug delivery to the brain,” says Ciofani. “The in vitro model provides a closed system where the different variables such as drug concentration, blood flow speed, pH, and temperature can be easily tuned and monitored, thus providing precious and detailed information about the BBB crossing in real time and at cellular/subcellular level.” Although several recently developed drug compounds show great potential for the treatment of brain diseases, most of them cannot be used in a clinical environment because they are unable to cross the BBB. One particular problem consists of the identification of the different biochemical mechanisms involved in the BBB crossing of specific molecules. Some of these pathways have already been discovered, but faithful and reliable BBB models will allow researchers to further investigate these mechanisms at subcellular resolution and in real time. The ultimate goal of this work is to modify, i.e., functionalize, anticancer nanovectors in such a way that they can cross the BBB through the blood system and target diseased tissues in the brain. The development of innovative treatments to treat brain diseases represents an important market for pharmaceutical companies and a huge scientific/technological challenge involving both public and private research centers. Since the accessibility of the central nervous system is extremely limited, the possibility to exploit a reliable platform to test the BBB crossing of new drug compounds opens up new frontiers for targeted brain therapy. “Our platform can be adopted to test drug delivery to the brain for the treatment of many diseases characterized by an impairment of the neurovasculature, for example, cancer, stroke, Alzheimer's, and Parkinson's disease,” Ciofani points out. “For these specific applications, the architecture of brain microvasculature in physiologic and pathologic condition will be reconstructed by using a reverse engineering approach and different drug treatments can then be tested.” Going forward, the team will dedicate future studies to test the BBB crossing of a variety of different nanoparticles and anticancer agents. Moreover, they will functionalize these nanovectors with different ligands in order to promote their delivery through the BBB and their selective targeting to specific cell types, such as cancer cells. “We strongly believe that nanotechnology-­based solutions such as nanocarriers and nanovectors for theranostic applications show enormous potential for the treatment of brain pathologies,” Ciofani notes. “However, in order to achieve a realistic implementation of nanomaterials into the clinical practice, it is extremely important to address safety issues. Since most nanomaterials also accumulate in peripheral body regions—e.g.,

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spleen, liver, and kidney—it is of extreme importance that such nanovectors release the drug compounds only in the brain, and, specifically, in the region of the diseased tissue.” “In this context, the new generation of remotely triggered smart nanomaterials represents, in our opinion, the future for the treatment of brain diseases. The BBB model we developed represents a powerful testing tool to achieve these goals,” he concludes. Featured scientists: Professor Gianni Ciofani's Smart Bio-­Interfaces group (https://bit.ly/2Gn8dtq) Organization: Italian Institute of Technology, Genova (Italy) Relevant publication: A. Marino, O. Tricinci, M. Battaglini, C. Filippeschi, V. Mattoli, E. Sinibaldi, et al., A 3D Real-­Scale, Biomimetic, and Biohybrid Model of the Blood–Brain Barrier Fabricated through Two-­Photon Lithography, Small, 2017, 14(6), 1702959.

6.8  Controlled Release of Hydrogel from Nanotubes Since the early days of nanotechnology in medicine, nanocarriers—nanostructures that are used to transport therapeutic compounds inside living organisms—have been attracting the interest of researchers because of their great potential in targeted drug delivery, due to a wide range of possibilities for surface modifications and compatibilities. Another attractive issue of these nanoscale cargo systems is their ability to release active molecules on demand. Popular structures for the development of nanodelivery systems are hollow tubular nanoparticles. Researchers from Kazan Federal University and the University of Palermo show that a hydrogel can be confined within the cavity of halloysite nanotubes (HNTs) by means of an easy strategy (Figure 6.4). “The alginate network inside the HNTs cavity can be triggered by chemical stimuli (by calcium chelators) altering the kinetics, which results in the release of the cargo,” says Giuseppe Lazzara, professor, Department of Physics and Chemistry at the University of Palermo. “We have demonstrated that halloysite with tunable hydrophilic/hydrophobic interfaces can act as nanotemplate for the synthesis of drug-­delivery systems based on biopolymer hydrogels.” Current strategies regarding the use of hollow tubular nanostructures for drug delivery involve smart end-­stoppers at the edges of these tubes that are broken under particular conditions (such as a certain pH value) or triggered by external stimuli (such as light) in order to release the cargo. The fabrication of theses nanocarriers usually requires two steps—first the cargo is loaded and then the caps are attached to the tube openings in order to seal them.

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Figure 6.4  Dark-­  field and fluorescence microscopy image of the distribution of modified halloysite nanotubes filled with the nanohydrogel in human cells. (Image: Prof. Rawil F. Fakhrullin, Kazan Federal University).

The novelty of the researchers' fabrication strategy is that it requires only one step. It is based on reverse inorganic micelles formed by halloysite nanotubes and their dispersions in oil phase. This approach enforces the confinement of alginate hydrogel within the halloysite, thereby generating a nanomaterial with promising properties for sustained delivery of active molecules in general. The team selected doxycycline, a broad-­spectrum antibiotic, as a model drug for loading and releasing from the confined hydrogel. The possibility of triggering the drug release from the hydrogel was exploited by adding ethylenediaminetetraacetic acid (EDTA)—which is a Ca2+ chelator—which can control the hydrogel rupturing. The confinement of the hydrogel within the halloysite cavity was imaged with advanced state-­of-­art methodologies (TEM micrographs and EDX elemental mapping, fluorescence experiments on marked polymers). Biological tests on primary rat skin fibroblasts and human prostate cancer cells (PC3) demonstrated the biocompatibility of the hybrid hydrogel and the carrier distribution within the cells. “There are two main implications of our work,” Rawil F. Fakhrullin, professor, Bionanotechnology Group, Institute of Fundamental Medicine and Biology, Kazan Federal University, notes. “First, a new, easy strategy to control nanoarchitectures in hybrid inorganic/organic gel materials and, second, an alternative to end-­stopper formation on nanotubular carriers for a further control of sustained/triggered release of the cargo.” Next, the researchers plan a deeper investigation of the suitability of the hybrid nanocarrier for drug delivery by in vitro and in vivo experiments. Future investigations will also focus on the calcium and alginate release in

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cells and their effects, in addition to the possibility of triggering the nanohydrogel disruption by chemical stimuli (pH or calcium-­ion complexing agents). The team's approach might open up new avenues for the fabrication of a wide range of functional hybrid clay–organic materials. “Although our strategy is very promising in nanomedicine, we think that the features explored for such a system are promising for other applications as well,” Fakhrullin concludes. “For instance, we are exploring the use of this methodology to develop smart protections for artwork that are relevant for the preservation of cultural heritage. Furthermore, the confinement of alginate hydrogel within the halloysite lumen might be a promising approach for the controlled cleaning of solid substrates. Another relevant application could be functional packaging obtained by the filling of polymeric matrices with the modified nanotubes. The nanocomposites might be suitable for long-­term protection, as sensors, and responsive materials to specific external stimuli.” Featured scientists: Professors Giuseppe Lazzara (https://bit.ly/2Dlromq) and Rawil F. Fakhrullin (https://bit.ly/2UHCMnG) Organizations: Department of Physics and Chemistry, University of Palermo, Palermo (Italy); Bionanotechnology Group, Institute of Fundamental Medicine and Biology, Kazan Federal University, Kazan (Russia) Relevant publication: G. Cavallaro, G. Lazzara, S. Milioto, F. Parisi, V. Evtugyn, E. Rozhina, et al., Nanohydrogel Formation within the Halloysite Lumen for Triggered and Sustained Release, ACS Appl. Mater. Interfaces, 2018, 10(9), 8265–8273.

6.9  P  iezoelectric Platform Inhibits Cancer Cell Proliferation Acquired resistance against chemotherapy drugs is as old and as widespread as the use of these agents and is a major limitation for the successful treatment of cancer. Different physical stimulations (e.g., electrical, thermal, and radiation) can be exploited in order to induce the drug sensitization and to promote chemotherapeutic effects. Here, nanoparticles can act as nanotransducers, able to actively respond to external stimuli and convert different forms of energy. There is a large body of work that demonstrates that inorganic nanomaterials are extremely powerful tools in biomedical fields and they have been widely investigated as diagnostic and therapeutic agents for cancer imaging and treatment. The efficacy of nanomaterials in cancer therapy is associated with different strategies allowing for their site-­specific targeting (which is key for reducing the adverse side effects of chemotherapy in healthy tissues).

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Clinical studies have demonstrated that low-­intensity alternating electric fields are able to affect the proliferation of cancer cells and potentiate the cytotoxic effects of chemotherapeutic drugs. Key is the ability to deliver the electric stimulation to cancer cells without affecting the function of the surrounding healthy cells. Piezoelectric nanomaterials have been shown to be a very promising tool for a wireless, non-­invasive, and targeted electric stimulation of cells and tissues. When these nanomaterials undergo mechanical deformation, for instance by being activated through ultrasound, they are able to generate electric potentials on their surface thanks to the direct piezoelectric effect. Researchers in Italy have reported an innovative nanotechnological approach for inhibiting the proliferation of breast cancer cells. In this work, for the first time, a wireless treatment based on piezoelectric nanoparticles has been exploited to remotely deliver electric stimulations to breast cancer cells. “We have shown that chronic electric stimulations mediated by piezoelectric nanoparticles result in the ability to significantly reduce the breast cancer cell proliferation by affecting the ion homeostasis and the organization of the mitotic spindles during cell division,” explains Attilio Marino, a postdoctoral fellow at the IIT. “In order to specifically target the electric stimulation to a particularly aggressive type of breast cancer cells (HER2-­positive), we modified the surface of piezoelectric nanoparticles with antibodies that recognize and bind to these cells. This ‘wireless’ stimulation approach, in principle, would allow perturbing and reducing cancer cell proliferation without affecting the viability of healthy tissues.” The team's nanoparticle platform consists of piezoelectric barium titanate nanoparticles coated with antibodies against HER2, a receptor highly expressed on the membranes of HER2-­positive cancer cells. This allows the targeted electric therapy to be extremely localized: it facilitates delivering antiproliferative electric stimuli in the proximity of and inside cancer cells (Figure 6.5). The team's approach, even if specific for HER2-­positive breast cancer cells, demonstrates the potential and versatility for treatment of other types of cancer. Key here is the fact that the physical nature of the electric stimulation leads to bypassing and reducing the acquired drug resistance of the cancer cells. This phenomenon could be exploited to develop efficient anticancer therapies by synergistically combining chronic electrical stimulations and treatments with chemotherapeutic agents. “Our approach has been developed and tested on breast cancer cell cultures; next, the targeting ability and the anticancer efficiency of this nanotechnological platform need to be validated also in the 3D environment of the breast cancer,” notes Gianni Ciofani of the IIT's Smart Bio-­Interfaces group, who led this work. “In particular, we will intensively investigate the accumulation/retention of the nanomaterials at the tumor site. Moreover,

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Figure 6.5  Scheme  of the functionalization of barium titanate nanoparticles (BTNPs) with anti-­HER2 antibody through DSPE–PEG–biotin coating. Reproduced from https://doi.org/10.1038/s41598-­018-­24697-­1 under the terms of the CC BY 4.0 license, http://creativecommons.org/ licenses/by/4.0/.

we are planning to include the piezoelectric nanoparticles in more complex nanosystems to not only deliver remote electric stimulations to cancer cells, but also to locally release anticancer drugs.” Ciofani is a pioneer in the field of piezoelectric nanomaterials for biointerface applications. Previous research by his group has demonstrated the benefits of remote cell stimulation mediated by piezoelectric nanoparticles. Out of this body of work grew the idea to apply this innovative stimulation method as anticancer treatment to improve the promising results of electric field treatments and molecularly targeted therapy. While this present work was specific to HER2-­positive breast cancer cells, the scientists also are testing the efficacy of their nanoplatform in counteracting the growth of other types of aggressive cancer cells, such as, for example, glioblastoma multiforme. In addition to piezoelectric nanoparticles, the group is adopting other types of nanoparticles, which are able to convert alternating magnetic field energy or light energy into heat. These nanomaterials can be exploited for magnetothermal or photothermal therapy, respectively. All these nanosystems offer alternative solutions that have the potential to be used in clinical environments in the near future. “Although the main application of this work is to inhibit the cancer cell growth, it is worth highlighting that the proposed nanotechnology-­based solutions can be adopted not only for the treatment of breast cancer, but also for many different high-­impact diseases, especially at central nervous system level,” Marino points out. “Indeed, the possibility to remotely deliver electrical stimulations into the brain without the need of implanting electrodes can be extremely useful for the treatment of Parkinson's and Alzheimer's diseases.”

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Featured scientists: Dr Attilio Marino; Professor Gianni Ciofani (https:// bit.ly/2Gn8dtq) Organization: Smart Bio-­Interfaces laboratory (https://bit.ly/2Gn8dtq), Italian Institute of Technology, Genova (Italy) Relevant publication: A. Marino, M. Battaglini, D. De Pasquale, A. Degl' Innocenti, G. Ciofani, Ultrasound-­Activated Piezoelectric Nano­particles Inhibit Proliferation of Breast Cancer Cells, Sci. Rep., 2018, 8(1), 6257.

6.10  Combating Opioid Drug Abuse Opium has been used and abused for centuries, but recent opioid abuse statistics paint a scary picture. In the USA alone, the latest figures (which are likely to be underreported) from October 2017 show 68 400 annual overdose deaths. The three most important opiates, legally prescribed as pain killers, are morphine, codeine (which has been used as a cough remedy), and thebaine, which is further refined by chemical processes to create higher value therapeutics such as oxycodone and hydrocodone, better known by brand names such as OxyContin and Vicodin, respectively. “Drug abuse and dependence/addiction are complex disorders that are regulated by a wide range of interacting networks of genes and pathways that control a variety of phenotypes,” says Morteza Mahmoudi, an assistant professor in the Department of Anesthesiology at Brigham and Women's Hospital and Harvard Medical School. “Therefore, both identification of the at-­risk population and treatment of the addiction disorders are strongly reliant on the development of new and innovative approaches for understanding the mechanisms underlying drug dependency and addiction.” Whereas nanotechnologies are widely researched for medical applications, their potential application in drug abuse—diagnostic, drug detoxification, and therapeutic application—is poorly investigated. “When searching the literature, there are fewer than expected reports on developing nanotechnologies for the treatment of drug abuse, detecting drugs in human urine and saliva, reducing the toxic concentrations of drugs in blood, and also for probing the effect of drug abuse on biochemical or structural variations of brain tissue,” Mahmoudi points out. He notes that the development of new and innovative approaches for understanding the mechanisms underlying addiction are of crucial importance in the field of opioid drug abuse. Novel nanotechnology approaches may lead to: (1) identification of at-­risk individuals in the population; (2) new therapeutic targets; and (3) personalized selection of appropriate treatments (Figure 6.6). Mahmoudi and his collaborators draw attention to the potential role of nanotechnologies in several areas.

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Figure 6.6  Scheme  showing differences in the biomolecular corona structures after

interaction of nanoparticles with plasma of healthy individuals and people with a risk of drug dependency and patients with drug addiction disorders. The changes in the biomolecular corona structures can be driven by variations of plasma protein and metabolomic compositions of healthy, at-­risk, and drug addiction disorder individuals. Reproduced from http://dx.doi.org/10.1021/acschemneuro.8b00127 with permission from American Chemical Society, Copyright 2018.

Drug addiction therapy. Nanotechnologies can be designed and engineered to regulate brain signaling pathways that are associated with drug addiction. This particular approach needs development of multifunctional nanoparticles that have a unique capacity to transmigrate across the blood–brain barrier and target the desired site of the brain. A better understanding of brain anatomical changes. Opioid drugs demonstrate a great capacity for altering important brain areas. These anatomic functional variations can alter the biochemical balance of the brain and affect many of the brain's critical functions, including problem solving, decision making, and the reward circuit system. Real-­time monitoring of particular neurotransmitters and brain anatomical changes using highly sensitive approaches is of great importance to shed more light on the mechanistic role of drug abuse on brain functions. Enhanced sensitivity of drug detection approaches. The use of engineered nanoparticles can substantially enhance identification and discrimination capacity of healthy, at-­risk populations and addiction patients in a short period of time, with reliable and reproducible outcomes. Although nanotechnologies show great potential in overcoming the issues of conventional drug monitoring approaches, their usage for this purpose is poorly investigated.

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Microbiome–gut–brain axis. Exploring and understanding the relationship between gut bacteria and drug addiction is still in its infancy, and in-­depth studies are required to better understand and define the mechanisms involved in opioid addiction and dependency. It appears, though, that changes in the gut microbiota can affect opioid tolerance in the cell bodies of extrinsic sensory neurons. The microbiome can also play a role in the development of opioid tolerance and alter important synaptic transcripts in the brain's reward circuitry. Therefore, it is legitimate to hypothesize that nanotechnology-­facilitated manipulation of gut microbiota profiles may have the capacity to minimize adverse effects of drug abuse. Monitoring blood plasma variations. Proteomic characterization of blood plasma is of central importance for possible biomarker discovery studies. However, advances in the fields of proteomics and genomics have not yet translated into a workable platform for either the development of suitable drugs to identify at-­risk populations or diminishing the risk of addictive relapse. This is largely due to the fact that no individual plasma protein is specific and sensitive enough to identify the source of its corresponding genetic variation and gene–gene interactions, and appropriate technology platforms for accurate and sensitive multivariate plasma analysis relevant to opioid drug addiction have remained elusive. “One of the central reasons for the limited use of nanotechnologies in drug-­abuse applications is the lack of investment from government and foundations in the field of drug-­abuse nanotechnology” notes Mahmoudi. “Therefore, we believe that specific funding for drug-­abuse nanotechnology should be established and substantially increased to empower the use of nanotechnologies in the field, which may pave a way for emerging of breakthrough discoveries for diagnosis and therapeutic applications in drug abuse.” “With the help of funding agencies, together with establishment of collaborations between nanomedicine and drug-­abuse experts, we believe that nanotechnologies will provide a unique capacity for both predictive and therapeutic approaches in opioid dependency and addiction in the foreseeable future,” he concludes. Featured scientist: Professor Morteza Mahmoudi (https://bit.ly/2YImeJN) Organization: Department of Anesthesiology at Brigham and Women's Hospital and Harvard Medical School, Boston, MA (USA) Relevant publication: M. Mahmoudi, S. Pakpour and G. Perry, Drug-­Abuse Nanotechnology: Opportunities and Challenges, ACS Chem. Neurosci., 2018, 9(10), 2288–2298.

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6.11  D  efining the Immunological Effects of Nucleic Acid Nanoparticles Therapeutic nucleic acids (TNAs) are increasingly being considered for the treatment of a wide variety of disorders, such as cancer, metabolic disorders, viral infections, and cardiovascular and inflammatory diseases. However, over the past decade only three TNA formulations have resulted in successful clinical translation. The primary barrier to clinical use of TNAs has been their rapid recognition by the body's immune system, often resulting in uncontrollable, severe inflammatory responses. However, attempts to deliver TNAs using nanocarriers (e.g., liposomes and polyplexes) have also been unsuccessful, as demonstrated by several recent examples of biotechnology companies dropping nanoparticle-­formulated TNAs after experiencing toxicity and even patient death in clinical trials. “Programmable nucleic acid nanoparticle (NANP) technology is a relatively new field that has already given rise to a host of self-­assembling nucleic acid nanoparticles that are increasingly viewed as promising biological materials for medical applications,” says Kirill A. Afonin, an assistant professor in the Department of Chemistry at the University of North Carolina at Charlotte. “Programmable self-­assembling NANPs are amenable to chemical modifications, control over functionalization, and consistent batch-­to-­batch formulation.” “The Drug Information Association, in close collaboration with the US FDA and industry, formed the Oligonucleotide Safety Working Group, which spent several years before releasing a justification that therapeutic nucleic acids deserve to be a separate category of drug products because they resemble small molecules and biologics, and yet have their own properties,” says Marina A. Dobrovolskaia from the Frederick National Laboratory for Cancer Research sponsored by the National Cancer Institute, Nanotechnology Characterization Laboratory. However, clinical translation of these materials is still in its infancy—no NANPs have reached clinical trials yet. The reason for this is that the immunotoxicity and immunomodulatory effects of NANPs are mostly unknown and must be defined to permit successful translation of this technology into the clinic. “Despite early attempts by us and others to link NANPs' physicochemical properties to their immune recognition, the results of these studies are either incomplete or inconclusive,” says Afonin. “Immunostimulation—cytokines and chemokines induction, lymphocyte proliferation, and upregulation of surface markers—is an essential step before anyone attempts to understand the immunogenicity.” Since no such study has ever been done for NANPs, a team led by Afonin and Dobrovolskaia conducted the first detailed and systematic study that

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involved 25 representative NANPs, originally designed by different groups, screened for the induction of 29 different cytokines. The study was done in human blood collected from more than 100 different donors. The authors show that immunological recognition of NANPs depends on multiple physicochemical parameters, including particle size, 3D structure, composition, and connectivity. Their major findings include the following:    ⁃ Traditional TNAs (depending on sequence, backbone, and modifications) stimulate activation of the immune cells. Unlike these TNAs, NANPs require delivery into the cells, otherwise they are invisible to the immune cells. This finding has never been described before. ⁃ Traditional TNAs stimulate a type I interferon (IFN) response. The researchers experimentally confirmed that type I IFN can be used as a biomarker of NANPs' immune recognition. In addition, they also showed that NANPs induce type III IFN, which has not been investigated in detail by scientists working with traditional TNAs. Both experimental confirmations of type I IFN induction by NANPs and a role for type III IFN are novel findings, which have not been published before in the context of the physicochemical properties of NANPs. ⁃ Traditional TNAs are recognized by various immune cells. However, plasmacytoid dendritic cells (pDCs) are the primary producers of type I IFN. Here, the team experimentally confirmed that pDCs are also responsible for both type I and type III IFN induction by NANPs. This study has never been done before. ⁃ It has been shown before that traditional TNAs are recognized by endosomal Toll-­like receptors (TLR). It has also been shown that TLR specificity to traditional nucleic acids is as follows: TLR3, double stranded RNA; TLR7 and TLR8, single-­stranded RNA; and TLR9, DNA. Here for first time, the team confirmed that NANPs are taken up by the endolysosomal pathway and induce IFN response through TLRs. However, the study demonstrated that TLR7 (not TLR3) is the receptor involved in the recognition of RNA cubes. It also showed that TLR9 is not the receptor recognizing DNA cubes.    Overall, these findings highlight the key parameters that inform the way NANPs interact with the immune system. These new insights improve the current understanding of the immunostimulatory properties of NANPs and pave the way to the development of a new auxiliary molecular language that can be expressed through the script of rationally designed NANPs. Such communication with the immune system will become instrumental, for example, in either achieving desirable activation of immune cells that are beneficial for vaccines and cancer immunotherapies, or reducing undesirable immunostimulation that commonly limits the translation of systemically delivered traditional nucleic acid-­based therapies.

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“We hope that our study helps facilitate bridging the already narrowing gap between basic research on NANPs and advanced pharmaceuticals containing these novel materials,” Afonin concludes. “This paves the way to the rational design and production of NANPs not only based on their functionality, but also based on their immunotoxicity, thus allowing us to create efficient delivery platforms with non-­immunogenic NANPs and novel adjuvants and immunomodulators with others.” Featured scientists: Professor Kirill A. Afonin (https://bit.ly/2LVv0CI); Dr Marina A. Dobrovolskaia (https://bit.ly/2KQqJj9) Organizations: Department of Chemistry, University of North Carolina at Charlotte, Charlotte, NC (USA); Frederick National Laboratory for Cancer Research, Frederick, MD (USA) Relevant publication: E. Hong, J. Halman, A. Shah, E. Khisamutdinov, M. Dobrovolskaia and K. Afonin, Structure and Composition Define Immunorecognition of Nucleic Acid Nanoparticles, Nano Lett., 2018, 18(7), 4309–4321.

6.12  Testing Nanomedicine in Space In chemistry, free radicals are atomic or molecular species with unpaired electrons on an otherwise open shell configuration. These unpaired electrons are usually highly reactive, so radicals are likely to take part in chemical reactions. Free radicals can damage components of a cell's membranes, proteins, or genetic material by oxidizing them—the same chemical reaction that causes iron to rust. This is called oxidative stress (OS). Many forms of cancer are thought to be the result of reactions between free radicals and DNA, resulting in mutations that can adversely affect the cell cycle and potentially lead to malignancy. Oxidative stress is also believed to play a role in neurodegenerative diseases such as Alzheimer's and Parkinson's. In order to counteract intracellular damage by free radicals, cells have developed a so-­called intracellular antioxidant system. This process transforms free electrons into a non-­reactive form by proteins (enzymes). In addition, the external supply of antioxidant agents is a common strategy for OS prevention and treatment. Here, nanotechnology has provided dramatic improvements in controlling or eliminating biological oxidation reactions. This may provide a new basis for pharmacological treatment of diseases related to oxidative stress. Three of the most-­studied nanoparticle redox reagents at the cellular level are fullerenes, carbon nanotubes, and rare earth oxide nanoparticles (particularly cerium, i.e., nanoceria). “Conditions that decrease gravity load for several days or weeks, such as prolonged bed rest or long space flights, cause the deterioration and

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weakening of the postural muscles, and this phenomenon has been shown to be tightly correlated with oxidative stress,” says Gianni Ciofani, an associate professor at Polytechnic University of Torino and the principal investigator of the Smart Bio-­Interfaces group at the Italian Institute of Technology. “This inspired us to investigate the potentially protective role of nanoceria against oxidative stress associated with microgravity and cosmic radiations in space.” Ciofani and his team tested nanoceria as antioxidant in skeletal muscle cells during their maturation process (differentiation) under different gravity levels, obtained by culture on Earth and aboard the International Space Station (ISS) during the Italian Space Agency Biomission Vita (NANOROS project, 2016-­7-­U.0). “Our findings support the application of antioxidant nanomaterials to skeletal muscle tissue culture for protection from the noxious effects of microgravity and cosmic radiation, which result in muscle mass and force loss and limit human operations and permanence in space,” says Giada Genchi, a postdoctoral fellow in Ciofani's group. “On Earth, these deteriorations are usually associated with aging or pathologies and are exhibited over longer time intervals compared to those occurring in space. This makes our experiment predictive of the possible efficacy of antioxidant nanomaterials based on nanoceria in the treatment of several musculodegenerative conditions.” Most importantly, nanoceria promotes the transcription of genes (Lmna and H2afx) associated with DNA protection from oxidative stress. These genes are also involved in a number of common metabolic processes, such as aging or adipogenesis. Their alteration is known to cause degenerative diseases. By endowing nanoparticles with self-­regenerative biomimetic antioxidant effects, the scientists' goal is to achieve a reduction of OS, thereby minimizing its negative impact on skeletal muscle building blocks (contractile proteins, DNA, etc.) both on Earth and in space. “Our work encourages further studies on nanomaterials in microgravity for the promotion of extended human space travel, as well as for the identification of suitable biomedical protocols for the treatment of conditions typically associated with aging and disease on Earth,” Genchi points out. “Besides shedding light on the protective role of nanoceria against microgravity-­and space radiation-­induced OS, it should also motivate the scientific community to investigate in-­depth nanomaterial interaction with biological matter in microgravity.” The team's present study will be complemented by another study on skeletal muscle cell proliferation under the same conditions, aiming at elucidating the role of these antioxidants in different stages of life of the skeletal muscle cell. Another experiment with skeletal muscle cells and nanoceria will be performed aboard the ISS in 2019, and transcriptomic analyses will be validated by accurate investigations on gene promoters, as well as on gene products. According to the team, a number of future studies should address: (1) the targeting of nanoceria to specific anatomical sites; (2) the administration

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routes of antioxidant nanoparticles (food supplement, systemic, etc.); (3) long-­term fate and effect of the nanoparticles in the intact organism level; and (4) the molecular underpinnings of the therapeutic effects of nanoceria. One of the main challenges in this research field is the uncertain future of the ISS after 2020. This highlights the need for a stronger international cooperation to access new real microgravity platforms, as well as simulated microgravity facilities on Earth able to reproduce highly energetic cosmic radiation. Featured scientists: Professor Gianni Ciofani's Smart Bio-­Interfaces laboratory (https://bit.ly/2Gn8dtq) Organization: Italian Institute of Technology, Genova (Italy) Relevant publication: G. Genchi, A. Degl'Innocenti, A. Salgarella, I. Pezzini, A. Marino, A. Menciassi, et al., Modulation of gene expression in rat muscle cells following treatment with nanoceria in different gravity regimes, Nanomedicine, 2018, 13(22), 2821–2833.

6.13  T  he Impact of Nanoparticle Design on Parkinson's Disease Therapies α-­Synuclein is a protein whose function in the healthy brain is currently unknown. It is of great interest to Parkinson's researchers because it is a major constituent of Lewy bodies, protein clumps that are the pathological hallmark of Parkinson's disease (PD). Scientists believe that the self-­assembly of α-­synuclein into oligomers and fibrils is linked to the progress and pathogenesis of the disease. As PD predominantly afflicts the elderly population, it is rational to assume that a slight delay in α-­synuclein fibrillation may prevent or postpone PD pathogenesis. In recent years, researchers have found that some nanoparticles can inhibit α-­synuclein fibrillation and also lead to the destruction of preformed fibrils. So far, however, it has not been clearly explained how nanoparticles interfere with the fibrillation reaction network—primary and secondary nucleation and elongation steps—that makes up the consequent fibrils. Matters are complicated by the fact that the available data on the therapeutic or toxic impacts of nanoparticles in amyloid-­related diseases are contradictory: depending on their physicochemical properties and concentration, nanoparticles show different acceleratory or inhibitory effects on the fibrillation process. “The detailed mechanism by which fibril formation is inhibited is not well understood. Our recent study revealed how nanoparticles affect α-­synuclein fibrillation,” says Morteza Mahmoudi, an assistant professor in the Department of Anesthesiology at Brigham and Women's Hospital and Harvard

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Medical School. “We showed that electrostatic forces play a dominant role in the α-­synuclein fibrillation process and hence, charged nano-­objects are the best choice to block charged residues mediating self-­assembly of α-­ synuclein. They can inhibit the primary and secondary nucleation and elongation phases.” The study's results suggest that important characteristics such as surface charge and surface functional group should be considered in the development of nanotechnology-­based therapeutic approaches. “Understanding the detailed mechanism of interactions between α-­ synuclein and nano-­objects with different surface properties can pave a new way to screen or design effective nanoparticles against α-­synuclein fibrillation/fibrils,” Mahmoudi points out. The team found that the physicochemical properties of nano-­objects play a crucial role in dictating their interactions with α-­synuclein. The charged nano-­objects exhibit a strong affinity to charge residues of α-­synuclein. The electrostatic force first triggers initial attachment of nanoparticles to α-­ synuclein and then van der Waals forces stabilize this interaction. “We show that charged nanoparticles inhibited the fibrillation process and dissociated preformed fibrils in a safe manner,” Mahmoudi notes. “We also showed that they induce the formation of off-­pathway oligomers—relevant because a growing body of evidence supports the critical role of oligomeric species of α-­synuclein in the pathogenesis and progress of PD.” In a next step, the researchers will study the in vivo therapeutic effects of nano-­objects by probing how their physicochemical properties direct their biological fates and how they may dictate their interactions with α-­synuclein in animal models of PD. “The passage of nanoparticles across the blood–brain barrier (BBB) is one of the most challenging issues associated with approaches developed for diagnosis and treatment of neurodegenerative diseases,” Mahmoudi concludes. “We are planning to develop small-­sized nanoparticles, functionalized with ligands facilitating nanoparticle transport across the BBB, to improve nanoparticle delivery to the brain and study how physicochemical properties of nanoparticles govern their therapeutic/toxic impact in vivo.” Featured scientist: Professor Morteza Mahmoudi (https://bit.ly/2ULFJ2o) Organization: Department of Anesthesiology at Brigham and Women's Hospital and Harvard Medical School, Boston, MA (USA) Relevant publication: H. Mohammad-­Beigi, A. Hosseini, M. Adeli, M. Ejtehadi, G. Christiansen, C. Sahin, et al., Mechanistic Understanding of the Interactions between Nano-­Objects with Different Surface Properties and α-­Synuclein, ACS Nano, 2019, 13(3), 3243–3256.

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6.14  M  etal–Organic Frameworks Enhance Sonodynamic Cancer Therapy Sonodynamic therapy (SDT) has attracted wide attention as a novel treatment strategy for deep-­seated tumors due to its safety, tissue-­penetration depth, and low cost. In SDT, ultrasound can cause irreversible and fatal damage to tumor cells via heat, cavitation, or sonosensitizers. However, traditional organic sonosensitizers (such as porphyrins and their derivatives) tend to suffer from low water solubility, fast metabolism, and elimination from the blood circulation, resulting in a low concentration in the desired treatment locations and thereby significantly reducing SDT efficacy. Although the conjugation of nanoparticles with organic sonosensitizers improves the sensitization effect of SDT, premature leakage, instability, complicated preparation, and the high cost of these conjugations hinder their further clinical application. This situation has motivated researchers to explore the development of novel inorganic sonosensitizers with great stability and high reactive oxygen species (ROS) yield. Researchers in China have reported the excellent potential of metal– organic framework (MOF)-­derived mesoporous carbon nanostructures containing porphyrin-­like metal centers (PMCS) in sonodynamic therapy augmentation. The high specific surface area and porous structure of PMCS result in excellent gas adsorption properties, providing more nucleation sites for reducing the cavitation threshold intensity (Figure 6.7).

Figure 6.7  Schematic  illustration of the sonosensitization process of PMCS for cancer therapy. Reproduced from http://dx.doi.org/10.1002/ adma.201800180 with permission from John Wiley and Sons, © 2018 WILEY-­VCH Verlag GmbH & Co. KGaA, Weinheim.

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The scientists demonstrated, through theoretical calculations as well as experiments, that these MOF-­containing porphyrin-­like structures can generate effective ROS and microjets to destroy cancer cells. “We have discovered a novel structure-­dependent sonosensitizer that could potentially be applied for sonodynamic cancer therapy and explored its possible antitumor mechanism with regard to reactive oxygen species and cavitation effect,” says Huiyu Liu, a professor at the College of Life Science and Technology, Beijing University of Chemical Technology. Liu and her team believe that this PMCS can play an important role in further exploring the SDT mechanism associated with MOF-­derived carbon nanostructures and designing inorganic sonosensitizers with high ROS yield and excellent stability. “The porphyrin-­like porous structure in PMCS enables it to produce reactive oxygen species efficiently to kill tumor cells,” explains Xueting Pan, first author of a paper on this work. “In order to explore the still unexplained particle-­mediated cavitation in SDT, we used a high-­speed camera to capture the cavitation bubbles growth and collapse and generation of microjets with PMCS under ultrasound. These phenomena can help us to better understand the mechanisms involved in SDT.” In the next stage of their investigations, the researchers will look for the spectral range of sonoluminescence and provide a theoretical basis for exploring and designing novel sonosensitizers. “Future directions of our work may include the development of novel sonosensitizers and exploration of SDT mechanisms,” Liu concludes. “In the face of a complex tumor microenvironment, how to effectively exert the therapeutic efficacy of novel sonosensitizers and achieve clinical translation are main challenges.” Featured scientist: Professor Huiyu Liu (https://bit.ly/2GleEgy) Organization: College of Life Science and Technology, Beijing University of Chemical Technology, Beijing (China) Video: https://youtu.be/9mjALByt1sY Relevant publication: X. Pan, L. Bai, H. Wang, Q. Wu, H. Wang, S. Liu, et al., Metal-­Organic-­Framework-­Derived Carbon Nanostructure Augmented Sonodynamic Cancer Therapy, Adv. Mater., 2018, 30(23), 1800180.

6.15  M  agnetically Propelled MOFBOTs Perform Microrobotic Drug Delivery Metal–organic frameworks (MOFs) are porous materials that consist of regular arrays of metal atoms surrounded by organic ‘linker’ molecules to form unique cage-­like structures. Due to this hollow structure, MOFs have an extraordinarily large internal surface area.

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This high surface-­to-­volume ratio makes them ideal materials for storing gases or other payloads, such as biomolecules of various sizes. One potential application of MOFs is as microcarriers for biomedical payloads. “While initial efforts have been made to produce mobile MOF-­based small-­scale machines, the locomotion features of most of these systems lack the level of sophistication of current state-­of-­the-­art micro-­and nanoswimmers,” says Dr Josep Puigmartí-­Luis from ETH Zurich. “For example, the controlled directionality of chemically propelled MOF crystals has yet to be addressed.” By applying concepts developed in micro-­ and nanorobotics, Puigmartí-­ Luis and an international team of collaborators demonstrate the controlled motion and delivery of cargo payloads embedded in MOFs. The researchers' helical MOF-­based micromachine, propelled by artificial bacterial flagella (ABF), can swim and follow complex trajectories in three dimensions under the control of weak rotational magnetic fields. “We focused our research on ZIF-­8 MOFs due to their excellent biocompatibility and degradation characteristics in relatively mild acidic conditions,” explains Puigmartí-­Luis. “We also show that our highly integrated multifunctional micromachine can successfully release drugs to a designated location, where the pH values are around 6, which corresponds to the similar acidic conditions found in tumor microenvironments.” To fabricate their MOFBOTs, the researchers 3D-­printed helical frameworks and coated them with nickel and then titanium to make them magnetic and biocompatible. After functionalization with polydopamine (PDA), a coating of ZIF-­8 MOF crystals was grown on the ABF surface. The thickness and compactness of the ZIF-­8 coatings can be tailored by variation of the reactant concentration and reaction time (Figure 6.8). “To illustrate the potential of our integrated micromachine, we have shown that these swimmers are tumor-­responsive and can act as selectively automated and targeted drug-­delivery platforms,” Puigmartí-­Luis points out. “We

Figure 6.8  SEM  image of multiple ZIF-­8@ABFs. Reproduced from http://dx.doi.

org/10.1002/adma.201901592 with permission from John Wiley and Sons, © 2019 WILEY-­VCH Verlag GmbH & Co. KGaA, Weinheim.

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have also demonstrated that the motion of our MOFBOTS can be precisely controlled and that they can follow complex trajectories within microfluidic channels to deliver drugs in a regioselective manner.” The team is already working on the fabrication of fully degradable MOFBOTs for biomedical applications. “In the future, we plan to design new MOFBOTs that will be loaded with real drugs, and, ideally, we will investigate them in vivo,” Puigmartí-­Luis concludes. “A big challenge will be to chemically define the MOF that will enable the loading of specific drugs and at the same time avoids the interaction with undesired healthy tissues.” Featured scientist: Dr Josep PuigmartÍ-­Luis (https://bit.ly/2WegVnG) Organization: ETH Zurich (Switzerland) Video: https://youtu.be/Az_wNXOtxUg Relevant publication: X. Wang, X. Chen, C. Alcântara, S. Sevim, M. Hoop, A. Terzopoulou, et al., MOFBOTS: Metal–Organic-­Framework-­Based Biomedical Microrobots, Adv. Mater., 2019, 31(27), 1901592.

6.16  MOF-­encapsulated DNA for Gene Therapy Gene therapy is the process of adding or replacing missing or defective genes in a patient's cells in order to treat gene-­based diseases, such as certain types of cancer, hemophilia, muscular dystrophy, and immune deficiencies. A critical challenge in gene therapy is the safe and effective delivery of genetic materials across cell membranes into target cells. So far, most approaches have relied on modified viruses, external electrical fields, or harsh chemicals. These methods can be costly, inefficient, or cause undesirable stress and toxicity to cells. Although viral systems have been the most effective method for delivering genetic matter into cells, they pose significant safety problems. This has led researchers to explore nanoscale organic (e.g., liposomes, polymers, peptides) and non-­organic (e.g., various metal nanoparticles, nanodiamonds, and carbon nanotubes) non-­viral delivery systems. “Despite the progresses and improvements made in the design and desired properties of the non-­viral vectors, there are still many challenges ahead, including the toxicity of residual catalysts used in the synthesis of organic vectors, the tedious synthesis process of inorganic vectors for nucleic acids loading, and the limited understanding of their efficient gene expression,”

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explains Professor Zhiyong Tang from the National Center for Nanoscience and Technology, CAS Key Laboratory of Nanosystem and Hierarchical Fabrication, Beijing. “Therefore, it is necessary to develop novel non-­viral vectors with facile synthetic preparation, better biocompatibility and biodegradability, and higher expression efficacy.” Demonstrating an example for such a novel delivery vector, Tang and his team of collaborators have developed a facile one-­pot strategy to encapsulate plasmid DNA into nanoscale metal–organic frameworks (MOFs) and a MOF– polymer system via the biomimetic mineralization and co-­precipitation method, respectively. “The use of MOFs as potential non-­viral vectors for intracellular delivery of proteins, DNA, or RNA is still in the preliminary stage,” says Tang. “Moreover, the focus of intracellular investigations is centered on utilizing low molecule weight oligonucleotide combined with MOFs or MOF–polymer systems, which is not applicable for nucleic acids with high molecule weight to be delivered safely and efficiently.” Plasmid DNA (pDNA), with its large rope-­like loop structure and high molecule weight (above 4000 bp) has been widely investigated to evaluate the gene transfer capacity of varied vectors based on its gene expression. However, researchers have encountered numerous challenges in large DNA molecule delivery, including the high cost and toxicity in the synthesis of organic vectors and the tedious synthesis process, especially for inorganic vectors modified with polymer-­capping agents. Demonstrating a promising alternative synthesis method, the team led by Tang developed a rapid and economical one-­pot method to encapsulate large pDNA molecules—plasmid DNA expressing enhanced green fluorescent protein (pEGFP-­C1)—into ZIF-­8 and ZIF-­8–polymer vectors. ZIF-­8 is a member of the MOF family and is popular with biomedical researchers due to its excellent biocompatibility and degradation characteristics in relatively mild acidic conditions. The researchers' experiments showed that pDNA molecules could be well distributed throughout the MOF nanostructures and benefited from effective protection against the enzymatic degradation. Tang says that the pDNA could be well loaded, released, and protected in both the MOF and MOF–polymer systems. “Impressively, the protected pDNA exhibits the high efficacy of intracellular gene expression, particularly within the ZIF-­8-­polymer vector.” This work not only offers a rapid and convenient way to load gene molecules into nanostructures for effective intracellular transportation and expression, but also opens a new avenue to develop MOF-­based non-­viral vectors for various gene therapies, such as RNA silencing, DNA vaccines, and immunotoxin treatment.

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Featured scientist: Professor Zhiyong Tang Organization: National Center for Nanoscience and Technology, Chinese Academy of Sciences Key Laboratory of Nanosystem and Hierarchical Fabrication, Beijing (China) Relevant publication: Y. Li, K. Zhang, P. Liu, M. Chen, Y. Zhong, Q. Ye, et al., Encapsulation of Plasmid DNA by Nanoscale Metal–Organic Frameworks for Efficient Gene Transportation and Expression, Adv. Mater., 2019, 31(29), 1901570.

Chapter 7

Characterization 7.1  Introduction Characterization of materials is a wide-­ranging field that encompasses the use of a variety of highly specialized (and often expensive) instruments to perform tasks like atomic-­resolution imaging, nanoscale composition analysis, surface analysis, electron microscopy, and traditional chemical characterization. It also includes toxicity testing, which is a highly relevant aspect for all nanoparticles and nanomaterials that are used in nanomedicine or could be released into the environment.

7.2  U  sing ‘Big Data’ to Shed Light on the Complexity of Nanomaterials For decades, computational physicists, chemists, and nanoscientists have been assuming that the lowest energy structures are the most representative. Since very few physical systems are ever in the ground state, this assumption biases the results towards an idealized minority. Using multivariate data analytics scientists can, for the first time, identify the truly quintessential nanostructures that actually are representative. “Using these structures—which are rarely the low energy ones—we can bias predictions toward the majority of the sample, and obtain better agreement with experiment,” says Dr Amanda S. Barnard, who leads the CSIRO Virtual Nanoscience Laboratory in Canberra, Australia.

  Nanoengineering: The Skills and Tools Making Technology Invisible By Michael Berger © Michael Berger 2020 Published by the Royal Society of Chemistry, www.rsc.org

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Together with Dr Michael Fernandez, Barnard has developed a methodology to identify nanoparticles with unique combinations of features and, in general, a feasible way of in silico characterization of intractable nanomaterial spaces. “To the best of our knowledge this is the first statistical characterization of nanostructure ensembles and the identification of their representative members,” the two scientists point out. “This is a demonstration of one of the many ways that data science can impact the study of complex nanoscale materials. It is not enough to have big data; we have to make sure it is useful. Unless we can clearly identify which data points (nanostructures) are actually important, big data will provide us with nothing but noise.” Ever since research and manufacture of nanomaterials has moved into the mainstream, scientists have grappled with a considerable characterization challenge. Health and environmental considerations, for instance, need to deal with the complexities that affect the biological interactions of the nanomaterial; or take the nanoelectronics and nanosensor areas, where even minute changes in a nanoscale material or structure could massively change performance. The enormous complexity arises from the sheer vastness of potential combinatorial variations that can be developed by choosing different nanomaterial size (including agglomeration and aggregation), solubility/dispersibility, chemical form, chemical reactivity, surface chemistry, shape, and porosity. “The number of possible materials is increasing exponentially, along with their intrinsic structural complexity, making even the application of efficient density functional theory unfeasible,” says Barnard. “Nanomaterials are more complicated, because their properties are intrinsically linked to their size, shape, and types of surfaces. Unexpected variability in shape can have detrimental effects in the nanoparticle behavior and their functional properties. This represents a tremendous challenge because the selection of experimentally significant samples becomes increasingly difficult and requires knowledge of the relevant sizes, shapes, and structural complexity a priori.” The age of big data has arrived, and scientists are discovering more useful structure/property correlations all the time. Correlations show how structures and properties are related, but not why. “To understand the mechanisms that underpin structure/property relationships we need to undertake detailed studies of a limited set of ‘representative’ nanostructures,” Barnard points out. “With our approach, the truly representative and ‘pure type’ nanostructures can now be definitely identified, taking the guess work and assumption out of choosing the appropriate model systems for these types of studies.” With the resulting data, detailed studies of structure/property relationships can be made based on the structures that accurately represent the complexity and diversity of real samples. Since any hypothetical nanostructure can be described by a linear combination of archetype and prototypes,

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Figure 7.1  Not  all nanoparticles are created equal. Identifying the archetypes and prototypes from among the myriad of possibilities is the first step in reducing the complexity of real, polydispersed ensembles. (Image: Dr Barnard, CSIRO).

researchers can easily incorporate polydispersivity and heterogeneity into structure/property predictions. These predictions will be one step closer to reality (Figure 7.1). In a paper on this work, Barnard and Fernandez apply multivariate statistical techniques to the study of nanocarbons, using virtual nanodiamonds ranging from 437 to 6107 carbon atoms, and graphene nanoflakes ranging from 16 to 2176 carbon atoms. “For the first time, we use k-­means clustering, archetypal analysis, and principal component analysis to explore the diversity of 182 (pure) nanodiamonds, 117 buckydiamonds, 311 unpassivated (radical) graphene nanoflakes, and 311 structurally equivalent hydrogen passivated nanoflakes,” Barnard explains their work. “These analyses are based on structural features characterizing geometry, interatomic distances, bond angle, surface-­ to-­volume ratio, carbon-­to-­hydrogen ratio, and hybridization fraction; many of which can be preselected without undertaking expensive electronic structure simulations.” “As we have shown, these methods are a powerful way of finding the truly representative nanostructures, which can be used in linear combination,” she adds. “One of the great advantages of using big data sets is that we can encode the predictions with distributions that accurately reproduce real-­ world observations. Now that we know we can identify prototypical and archetypal nanostructures, we plan to investigate how the selection process is impacted by different statistical distributions of the data.” However, no matter how effective and efficient synthesis and processing strategies are, samples of nanoparticles will always exhibit some persistent polydispersivity. It is simply not economically feasible to ensure that every particle is atomically identical. “This means that polydispersivity must be included when we predict structure/property relationships, but this is extremely challenging for the

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computational community who need to reduce the sample to a subset of representative structures,” cautions Barnard. “However, until now, there has been no way of knowing if we have been choosing the right ones.” Big data and the application of multivariate data analytics to the complex world of nanomaterials is a new direction in nanoscience and nanotechnology, paved with enormous opportunity. Although a range of well-­established and reliable statistical methods exist, there are conceptual barriers that are proving challenging to overcome. As the two scientists conclude, “it will be hard for a community accustomed to systematically seeking ‘one perfect result’ to embrace the uncertainty and messiness that is characteristic of big data; harder still to accept that creating data is not the same as creating value from data. It may be some time before data-­driven research is accepted as another branch of nanoscience, but it is worth the effort as there are certain problems that cannot be solved any other way.” Featured scientists: Drs Amanda S. Barnard (https://bit.ly/2w6iXHO) and Michael Fernandez Organization: CSIRO Virtual Nanoscience Laboratory, Canberra, ACT (Australia) Relevant publication: M. Fernandez and A. Barnard, Identification of Nanoparticle Prototypes and Archetypes, ACS Nano, 2015, 9(12), 11980–11992.

7.3  Helical Nanopropellers Measure Local Viscosity In medical applications, artificial micro/nanorobots have gained a lot of attention over the last decade due to their promise to behave as ingestible vehicles of drug delivery to deep tissue regions. This can have a significant impact in the field of cancer therapeutics, for example, where the drug can be delivered to the exact location of the diseased tissue with very high efficiency. Researchers find that magnetic helical nanomachines that mimic the swimming characteristics of Escherichia coli bacteria are particularly promising because of their extremely small size (0.5 to 3 µm) and their capability of navigating in various biological fluids, such as human blood, and even in the peritoneal cavity of live mice. The field of nanomachines has developed rapidly over the last few years, with several groups exploring new methods of navigation, and demonstrating their potential benefits as therapeutic tools. In the future, these nanomachines could be used in a clinical environment, where they are injected close to a specific diseased site and are navigated to a deep location in a tissue in a completely untethered and safe manner. The nanomachines can then perform tasks like sensing or therapy at the particular site, without affecting the functionality of adjoining cells and tissues.

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Figure 7.2  Experimental  set-­up and determination of precession angle. Schematic

and experimental photographs of a precessing nanomachine. Scale bar: 2 µm. Reproduced from http://dx.doi.org/10.1002/adfm.201705687 with permission from John Wiley and Sons, © 2018 WILEY-­VCH Verlag GmbH & Co. KGaA, Weinheim.

Work by a team from the Indian Institute of Science (IIS) extends the possibility of using helical nanomachines as a tool to measure the localized mechanical properties of a heterogeneous environment that is ubiquitous in biological systems (Figure 7.2). This technique can be useful to gain valuable insights into the physiological changes of a cell in response to a disease or a drug, leading to better therapeutics. “Helical nanomachines that have been so far utilized as carriers of therapeutics to specific cells, can now use mechanical sensing for potential disease diagnostic purposes as well,” notes Dr Arijit Ghosh. In this work, Ghosh and his collaborators have shown for the first time that artificial helical nanomachines can be used to measure the local viscosity of a heterogeneous media with a speed and accuracy that have never been achieved with existing methods of viscosity measurement. The helical nanomachines can give almost real-­time measurements of the local viscosity as they are navigated across fluid boundaries. The capability of untethered navigation using magnetic fields and viscosity measurement empowers the nanomachines to create a map of the viscosity of any complex environment, such as the inside of a living cell. “This technique of creating a viscosity map can provide significant insight into microfluidic environments like mixing or co-­flowing fluids, polymerization, and gelation,” explains Ghosh. “It can also bring out significant information of the changes in the physicochemical properties of the inside of a cell under the influence of a drug or various other physiological conditions. Lastly, the technique shows the potential of using nanomachines for in vivo detection of diseases using mechanical sensing as the measurement technique.” The Optics, Nanostructures & Quantum Fluids Laboratory of Professor Ambarish Ghosh at IIS is a pioneer in the field of helical magnetic nanomachines, which are currently being pursued by several research groups worldwide and holds a lot of promise for biomedicine.

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“We have found that the swimming dynamics of the nanomachines that replicates the swimming of bacteria, can be precisely controlled and manipulated using external magnetic fields showing ‘run and tumble’ kind of motion as found in bacterial systems,” notes Ghosh. “We have performed the characterization of this dynamic behavior using both theory and experiments which brought out the effect of changing viscosity on the dynamics of the nanomachine.” The team pursued their present work with the goal of demonstrating that viscosity measurement can indeed be performed with these nanomachines in an environment that has spatial heterogeneities. The scientists show that the measurements can be performed almost in real time and a spatial map of viscosity could be created. This work also extends to different kinds of fluids, called shear-­thinning fluids, that are similar to blood. The IIS researchers collaborated with Technion University in Israel to understand the dynamics of the nanomachines in shear-­thinning fluids, which could also be successfully characterized. In addition, the nanomachines were found to be sensitive enough to measure small variations in viscosity due to temperature changes. As a next step, the scientists are planning to use the helical nanomachines to perform viscosity measurements inside live cells under different physicochemical cues. Potentially, the technique could one day be utilized for disease diagnosis and therapy of a localized tissue or cell in a deep in vivo environment that is impossible to reach otherwise in a non-­invasive manner. “The main challenges for the future of autonomous therapeutic nanomachines are imaging and tracking of these micro/nanoscale machines in in vivo locations, successful navigation through the treacherous paths in a tissue using external forces, without losing the payloads, and biodegradability to eliminate the potential risks if they are lost in the body,” concludes Ghosh. Featured scientists: Dr Arijit Ghosh; Professor Ambarish Ghosh Organization: Optics, Nanostructures & Quantum Fluids Laboratory (https://bit.ly/2Eo2H9M), Indian Institute of Science, Bangalore (India) Relevant publication: A. Ghosh, D. Dasgupta, M. Pal, K. Morozov, A. Leshansky and A. Ghosh, Helical Nanomachines as Mobile Viscometers, Adv. Funct. Mater., 2018, 28(25), 1705687.

7.4  N  ew Phenomena in Ultrasonic Scattering in Nanofluids Ultrasonics is a promising, non-­invasive characterization technique for fluids. The scattering of ultrasound through colloidal suspensions allows determination of accurate particle size distribution, density, and concentration. Controlling these properties enables accurate characterization of

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nanomedicine drugs and understanding of nanoparticles present in biological systems. Ultrasound is particularly helpful in analyzing optically opaque samples, which have to be heavily diluted so they can be analyzed with optical methods. Suspension of nanoparticles has many uses in foods, cosmetics, health care products, agrochemicals, and drug-­delivery systems. Being able to measure their properties is a crucial element of the production process and of great importance in a number of industries. Work by scientists at Loughborough University in the UK and Université du Havre in France, demonstrates, for the first time, new phenomena in ultrasonic scattering in nanofluids. The team shows that shear-­mode effects cannot be neglected as one approaches the nanoscale and their complex dependencies upon particle size, concentration, applied frequency, and density that are contrasted between the suspended particles and the suspending medium are carefully investigated. “In order to understand the measurements we make, we need to use a model, a set of equations, and calculations, which tell us how the properties of the particles and fluid affect the loss of amplitude and speed of the wave,” explains Dr Valerie Pinfield, group leader at Loughborough. “The model we use has two parts: a multiple scattering theory, and a model for the scattering from a single particle.” In the past, the model used by researchers has been limited because it made some approximations about the shear waves, which are produced at the particles; it assumed that those waves die away in a very short distance, and do not have any effect on the particles nearby. Although they do die away in a very short distance, they can affect the neighboring particles when the suspension is very concentrated (i.e., there are a lot of particles in a small space) (Figure 7.3). “The shear waves themselves can be scattered by particles nearby and may be partly converted back into a compressional wave (an ultrasonic wave),” Pinfield notes. “This means we did not lose as much of the energy from the compressional wave as we thought. The process of wave conversion and reconversion is referred to as multimode scattering and, for many years, its effect has been ignored because we did not have a suitable model to calculate it—until now.” The outcome of the team's work is a model that can be used in online ultrasonic instrumentation. This will enable ultrasonics to be used with confidence as a process-­monitoring technique in a wide range of industrial contexts. It becomes possible to obtain knowledge of the physical properties of a nanofluid based upon its attenuation spectra. These models are of interest in the mathematical physics community for homogenization of advanced material properties (composite materials/ metamaterials), as well in the research field of non-­destructive evaluation. In addition, they have relevance for the use of ultrasonics as a research technique to study novel suspended materials such as nanoparticles, micellar materials, and some drug-­delivery systems.

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Figure 7.3  As  the size of the solid particles in a suspension becomes less than a

few micrometers and concentration levels increase, complicated shear-­ wave reconversion phenomena happen around the particles—leading to reduced attenuation of an ultrasonic beam. (Image: Loughborough University).

Dr Michael Forrester, first author of a paper on this work, states that, “We use two different ultrasonic spectrometers and compare the result to our new multiple scattering model, which is very computationally efficient, making it ideal for implementation in particle characterization and sizing systems—especially for suspensions that are optically opaque and impossible to analyze using traditional light-­scattering techniques. Indeed, our model is very successful even at high concentrations where other models break down.” He notes that, without inclusion of shear-­wave reconversion effects, the attenuation is found to be much higher than experimental observation. The shear-­wave phenomena occur in concentrated suspensions and become dominant as the system approaches the nanoscale. These processes take energy away from the ultrasonic wave, which causes a reduction in its amplitude. By measuring the attenuation (loss in amplitude) and the wave speed for an ultrasonic wave travelling through the suspension, it is possible to find out the concentration of particles, how big they are, or about their properties, e.g., their density. Whereas multiple scattering models of the past, such as the Lloyd–Berry model, neglected shear-­wave effects, Forrester and his collaborators show without doubt that they cannot be neglected in the analysis of nanofluids with ultrasonics. The team's model matches experimental results almost exactly in the frequency range investigated (1–20 MHz), which is the range valid for medical and biological analyses. “In this frequency regime, previous multiple scattering models have failed at comparatively low concentrations and at the lower range of the frequency

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scale, whereas we find that the new model very accurately matches the experiments,” says Forrester. “We examine in detail silica particles of 100 nm, 214 nm, 430 nm, and 1000 nm in water suspensions and measure the attenuation over the broad band of frequencies and concentrations.” This work is essential to help fulfill the potential of ultrasonic testing in nanofluids, showing the regimes where shear modes have the greatest effect; to that end, the scientists have identified peaks occurring in the attenuation at specific particle sizes and concentrations. In this present work, the researchers have investigated the frequency regime with great success up to 20 MHz. Going forward, they aim to study higher frequencies and find the limitations of the modeling. Online monitoring of suspended particulate systems is an extremely widespread need across many industries, for example, food, health care, chemical, agrochemical, petrochemical, nuclear, etc. In particular, there is a challenge with monitoring of highly concentrated suspensions, for which many existing process-­analytical technologies are unsuited. Ultrasonics as a technique lends itself to online monitoring, since it does not rely on sample dilution as do other techniques. However, the limitations of existing models for data interpretation preclude the use of ultrasonics in many online monitoring applications, particularly those involving highly concentrated suspensions (for example, in nuclear waste). “Therefore the impact of the development of a reliable model such as ours for ultrasonic monitoring is potentially highly significant, since it would enable its implementation in a wide range of industrial applications,” says Forrester. “Improved monitoring leads to better process control, resulting in cost reductions, lower energy use, and potentially improved or novel products.” He adds that ultrasonics also has the potential for significant impact in process monitoring of novel processing techniques, such as those currently in design for nanoparticle production in the health care and pharmaceutical fields. “This developing field brings with it new challenges for online process-­ analytical technologies to provide the necessary process control and optimization. Ultrasonic monitoring is well placed to satisfy these requirements, provided that suitable models are in place to permit usable information to be extracted from measured data,” he concludes. “Delivering a workable model which can be implemented into instrumentation is a critical step in achieving this potential impact.” Featured scientist: Dr Valerie Pinfield (https://bit.ly/2DsfXtz) Organization: Chemical Loughborough (UK)

Engineering,

Loughborough

University,

Relevant publication: D. Forrester, J. Huang, V. Pinfield and F. Luppé, Experimental verification of nanofluid shear-­wave reconversion in ultrasonic fields, Nanoscale, 2016, 8(10), 5497–5506.

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7.5  S  urface Area as a Highly Relevant Dose Metric for Nanotoxicity Assessments In the fields of toxicology and ecotoxicology, doses are commonly expressed in weight concentration for non-­soluble compounds because this is very convenient experimentally. However, when it comes to nanoparticles, the weight of the nanomaterial is not a relevant parameter, especially when it is necessary to compare different kinds of nanoparticles—such as carbon or metals, for example—because their density is very different. “The two other ways of expressing the dose are the number of particles and the total surface they express,” says Dr Emmanuel Flahaut, CNRS Research Director at the Centre Interuniversitaire de Recherche et d'Ingénierie des Matériaux (CIRIMAT), Université Paul Sabatier in Toulouse. “Both parameters are, a priori, much more relevant in terms of biological impact. However, our results show that surface area leads to the best correlation when it comes to the comparison of different nanoparticles. We advise to use this metric for future work in the field, which will allow a much more relevant comparison of the results from one group to another.” Flahaut and his collaborators show that that the usual approach based on mass concentrations fails to compare the toxicities of different engineered carbon nanoparticles (C-­NPs). The most significant result of this study is that the researchers demonstrate that the surface area can be used as a relevant metric to compare the toxicity of different kinds of carbon nanoparticles, but, more importantly, for the first time it can also be used a priori as a criterion for predicting the toxicity—at least with the team's experimental conditions—of carbon nanoparticles, ranging from zero-­dimensional (0D) to two-­dimensional (2D). For this study, the scientists used four different types of C-­NPs: few-­layer graphene (2D); nanodiamonds (0D); double-­walled carbon nanotubes (1D); and multiwalled carbon nanotubes (1D). They monitored and compared the inhibition of Xenopus laevis larvae growth after in vivo exposure to all of these different carbon nanoparticles for 12 days using different dose metrics. Even with different C-­NP physicochemical characteristics (i.e., structure and morphology), the team found that growth inhibition mostly depends on the surface area. “A full characterization of engineered nanoparticles must be provided in every ecotoxicological study in order to allow the toxicity comparison of C-­NPs, which should be done on the basis of surface area instead of weight concentration as is usually the case,” says Flahaut. “The use of this metric would help in the definition of a more realistic risk assessment strategy for carbon-­based nanoparticles in the aquatic environment.” The idea to investigate different metrics to express the dose–response results was not new in the field. However, this is the first time a clear demonstration has been given with comparison of different carbon nanoparticles with different geometries, evidencing that this result can be generalized.

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“The motivation for this study came from our earlier work dealing with the environmental impact of different kinds of carbon nanotubes when we realized that it was impossible to really compare our own results based on weight concentration,” Flahaut notes. These results may also be useful for researchers working in the field of nanocomposite materials, for example, because interfaces between a nanofiller and the matrix always play a crucial role. Taking into account the surface of interaction between the matrix and the load, instead of only thinking in terms of weight or volume ratio, may lead to interesting approaches for data analysis. This approach may help to re-­analyze earlier work and gives indications for further research in the field. Flahaut's team, in collaboration with ECOLAB, for instance, is currently investigating the influence of surface functionalization on ecotoxicity, because modifying the interface with water should also have very important consequences. The interaction between nanoparticles and cells or tissues on a larger scale is largely controlled by what happens at the interface, i.e., at the surfaces. However, the exact nature of this surface is rather difficult to assess because many biomolecules, such as proteins, but also salts present in water, tend to accumulate there very quickly and often in a rather dynamic way. “We are now working on the influence of surface functionalization on ecotoxicity in order to improve our understanding of what is taking place at the interface and to check if our conclusions are truly general or if some limitations apply,” Flahaut describes the team's current work.

Featured scientist: Dr Emmanuel Flahaut Organization: Centre Interuniversitaire de Recherche et d'Ingénierie des Matériaux (CIRIMAT) (https://bit.ly/2IVziqD), Université Paul Sabatier, Toulouse (France) Relevant publications: A. Mottier, F. Mouchet, C. Laplanche, S. Cadarsi, L. Lagier, J. Arnault, et al., Surface Area of Carbon Nanoparticles: A Dose Metric for a More Realistic Ecotoxicological Assessment, Nano Lett., 2016, 16(6), 3514–3518. L. Muzi, F. Mouchet, S. Cadarsi, I. Janowska, J. Russier, C. Ménard-­Moyon, et al., Examining the impact of multi-­layer graphene using cellular and amphibian models, 2D Mater., 2016, 3(2), 025009. F. Mouchet, C. Gancet, E. Flahaut, E. Pinelli, J. Boutonnet and L. Gauthier, International standardized procedures for in vivo evaluation of multi-­ walled carbon nanotube toxicity in water, Toxicol. Environ. Chem., 2016, 98(8), 829–847.

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7.6  A  FM Imaging and Characterization of Nematodes in Their Natural Environment Caenorhabditis elegans (C. elegans), a free-­living soil nematode, has become an important experimental model for biomedical research. This nematode has been successfully employed in genetics, aging research, behavioral assays, drug screening, and (nano)toxicology. C. elegans nematodes are tiny round worms and their cuticle (their outer covering) is a very important indicator of the animals' wellbeing. As long as the C. elegans nematodes are extensively used in biomedical research— i.e., for drug screening, toxicity testing—quantitative imaging and evaluation of mechanical properties will help to diagnose the effects of drugs on nematodes. “We obtained high-­resolution atomic force microscopy (AFM) scans both from large areas (i.e., head or tail regions) and from selected cuticle regions clearly demonstrating the periodicity of annular rings typical for C. elegans nematodes,” says Rawil F. Fakhrullin, Professor, Bionanotechnology Group, Institute of Fundamental Medicine and Biology, Kazan Federal University. “Our results indicate that the epicuticle nanostructure may play an important role in nematodes locomotion and body pressure maintenance. Importantly, we found that the younger larval nematodes exhibit a much rougher cuticle surface, apparently to provide better adhesion to wet substrates.” The team used the PeakForce Tapping non-­resonance imaging mode based on the direct measurement of surface topography with simultaneous force curve capturing at each pixel of the scan. They found that the PeakForce Tapping mode allows imaging C. elegans nematodes both in air and in their native liquid environment (Figure 7.4). This method might find applications in the ‘worm community’ as a powerful tool to elucidate the effects of drugs and toxicants on nematodes by imaging and mechanical mapping of the cuticle. Nanoscale imaging of the nematodes is challenging because the conventional methods based on electron microscopy do not allow imaging the animals in their natural aqueous environments. The nematode cuticle has been previously studied using scanning electron microscopy, which requires that the samples are dehydrated and sputter-­coated with a thin metal layer, thus severely compromising the original surface structure. The team's novel approach lifts this limitation. Fakhrullin notes that, until 2015, there were no reports on direct AFM imaging of C. elegans whole animals. “A limited use of soft X-­ray contact microscopy allowed fabrication of polymer replicas further analyzed with an atomic force microscope in air in contact mode, yielding topography images of cuticle and internal organs, i.e., pharynx, hypodermal, and neuronal cell nuclei.”

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Figure 7.4  A  scheme demonstrating the immobilization of C. elegans nema-

todes on a polyelectrolyte-­coated (PAH/PSS/PAH) glass slide (not drawn to scale, red polymer network represents cationic poly(allylamine hydrochloride) (PAH), blue represents anionic polystyrenesulfonic acid (PSS), LbL (layer by layer). Reproduced from http://dx.doi. org/10.1016/j.nano.2016.10.003 with permission from Elsevier, Copyright 2016.

Very recently, another paper reported the successful use of tapping mode AFM to visualize whole-­animal mounts of C. elegans microworms in air. “However,” says Fakhrullin, “no quantitative information on the cuticle was reported due to the limitations of tapping mode AFM imaging. Also, the scans of the whole animals reported demonstrate the typical shrunk and collapsed cuticle surfaces characteristic of SEM imaging, which is likely to be caused by dehydration and air imaging.” The team plans to perform similar AFM measurements with live nematodes. This will require new approaches to immobilize living worms on various substrates and to image the live animals in PeakForce Tapping mode. “Also, we expect to visualize and map the mechanical properties and the surface features of live worms at different ages and after treatment with several toxicants,” adds Fakhrullin. This method can be employed in screening for anti-­nematode chemicals— many nematode species are parasitic, which stimulates the active search for new ways of killing them. The scientists' future work will focus on imaging and quantitative nanomechanical characterization of live nematodes. They anticipate that PeakForce Tapping mode AFM applied to investigate the live nematodes will provide the scientists in this field with invaluable insights on cuticle structure and the effects of various toxicants on the nematodes.

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Featured scientist: Professor Rawil F. Fakhrullin Organization: Institute of Fundamental Medicine and Biology, Kazan Federal University, Kazan (Russia) Relevant publications: G. Fakhrullina, F. Akhatova, M. Kibardina, D. Fokin and R. Fakhrullin, Nanoscale imaging and characterization of Caenorhabditis elegans epicuticle using atomic force microscopy, Nanomedicine, 2017, 13(2), 483–491. M. Allen, R. Kanteti, J. Riehm, E. El-­Hashani and R. Salgia, Whole-­animal mounts of Caenorhabditis elegans for 3D imaging using atomic force microscopy, Nanomedicine, 2015, 11(8), 1971–1974.

7.7  A  ccurate Simulations at the Nanoscale Depend on Appropriate Interatomic Potentials Over the past 20+ years, many science and engineering fields have seen a growing interest in the use of atomistic computer simulations, such as molecular dynamics and Monte Carlo methods. This is especially true for nanoscience and nanotechnology, where more or less all advances require a detailed understanding of the manipulation of matter on an atomic, molecular, or supramolecular scale. Nevertheless, there have been reservations over the accuracy of simulation results, notwithstanding the use of correct simulation procedure and conditions. Improper choice of force field—known as interatomic potential—is one of the main concerns. The problem arises from the fact that a multitude of these interatomic potentials exist and it is very difficult to develop a universal interatomic potential that works appropriately for all applications. “However, the robustness, accuracy, and validity of atomistic simulations hinge on the appropriate choice of force fields,” says Seyed Moein Rassoulinejad-­Mousavi, a doctoral student at the University of Missouri's College of Engineering. “Force fields are key for modeling the interaction between atoms of a matter under study, and the challenge is to have an accurate force field working for any specific material at any desired temperature.” To serve this objective and make a benchmark as well as a shortcut for users to find their best force fields, Rassoulinejad-­Mousavi, together with Dr Yijin Mao and Professor Yuwen Zhang from the Multiscale Thermal Transport Laboratory, have examined a number of force fields for materials that are popular in micro-­and nanotechnologies. For their studies, the team used a repository of interatomic potentials from the National Institute of Standards and Technology (NIST IPR) and the Sandia National Laboratories (LAMMPS) databases to study the accuracy of the force fields for frequently used materials at practical temperatures for real-­world applications (Figure 7.5).

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Figure 7.5  The  figure shows uniaxial tensile strain and mechanical strength of a copper cubic nanostructure with length of 20 nm in 240 ps (240 × 10−12 seconds). (Image: University of Missouri).

“Based on our calculations, we concluded that inadequate choice of force field strongly affects the simulation results and gives rise to some inconveniences for calculations,” says Rassoulinejad-­Mousavi. “Some of the interatomic potentials seem to be useful and accurate for predicting one or two of the elastic constants or elastic modulus, not all of them.” “We also found that the elemental potentials that have been generated for a specific alloy or compound are not expected to necessarily work for all of the species present in the compound,” he continues. The novelty of this work is that the team showed for the first time which, among the many, available models are accurate for any specific range of temperature or application. The results presented are useful for interested researchers in the field of atomistic study of mechanical properties of materials, and increase the assurance of the users to see which interatomic potential fits well to their specific problem. The ultimate goal is to investigate all single elements of the periodic table, binary (two elements) and trinary (three elements), and higher order (four or more elements) compounds, to eventually prepare a handbook of interatomic potentials for users and researchers in nanoscale simulations.

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Featured scientists: Professor Yuwen Zhang's Multiscale Thermal Transport Laboratory (https://bit.ly/2vie2Dj) Organization: University of Missouri, Columbia, MO (USA) Relevant publication: S. Rassoulinejad-­Mousavi, Y. Mao and Y. Zhang, Evaluation of copper, aluminum, and nickel interatomic potentials on predicting the elastic properties, J. Appl. Phys., 2016, 119(24), 244304.

7.8  H  ow to Detect Biological Contamination of Nanoparticles Given the growing impact of nanotechnology on health care, pharmacy, and medicine, there is an increasing and urgent need for the development of reliable methods to detect the biological contamination of nanoparticles. Engineered nanomaterials such as carbon nanotubes, fullerenes, graphene nanosheets, quantum dots, dielectric, and metal nanoparticles hold high promise for a variety of biomedical applications. The most well known are diagnostic and therapeutic applications. For example, semiconductor quantum dots and metal (silver and gold) nanoparticles can both detect and destroy pathogenic microbes. Similarly, nanohybrids made of metals and semiconductors can also be used in fighting tumors (the concept of photodynamic therapy). Another very promising way to kill tumor cells is hyperthermia treatment mediated by magnetic nanoparticles. In addition, magnetic nanomaterials are indispensable for improving the sensitivity and contrast of magnetic resonance imaging. Nanomaterials can also be used as nanovehicles for targeted drug delivery and in tissue engineering. When it comes to diagnostic and therapeutic applications of nanomaterials, assessment of their cytotoxicity is a standard and required procedure. Typically, engineered nanomaterials are not sterile and can be contaminated with biological species, such as endotoxins. Endotoxins (or bacterial lipopolysaccharides) are large and heat-­stable molecules that are the major components of the cell wall of Gram-­negative bacteria. An exposure to endotoxins can cause fever, septic shock, and even death. “It should be noted that the contamination of engineered nanomaterials with endotoxin can easily happen at any stage of their production and handling,” points out Yuriy Garbovskiy, PhD, a researcher at the UCCS BioFrontiers Center & Department of Physics, University of Colorado. “This biological contamination of nanoparticles, if not detected, can alter the results of the cytotoxicity test routinely used to characterize these nanomaterials. As

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a result, the cytotoxicity of 100% pure and biocontaminated nanoparticles can differ very significantly.” That is why existing protocols and good laboratory practice require testing of engineered nanomaterials for sterility and pyrogenicity—the capacity to cause fever—to assess their possible contamination with endotoxins. Available commercial chromogenic assays for the detection and quantification of biocontaminants such as endotoxin can interfere with nanoparticles, thus leading to unreliable data. Consequently, there is a strong need for the development of new ways to assess the level of biological contamination of engineered nanomaterials. Garbovskiy has published findings that offer a solution to the aforementioned problems of nanoparticle interference and correct quantification of biocontaminants such as endotoxins. His proposed method is based on optical absorbance measurements. “The basic idea is very simple,” says Garbovskiy. “Once contaminated nanoparticles are dispersed in the contaminant-­free dispersion medium, some fraction of the biocontaminants adhering to the nanoparticle surface becomes detached from the surface and freely dispersed in the medium. As a result, the measured optical absorbance of the system under study (contaminated nanoparticles dispersed in the dispersion medium) can be written as a sum of two components”. “The first component accounts for the absorbance of light by freely dispersed biocontaminants,” he continues. “This component can be used to assess the biological contamination of nanoparticles. The second component originates from the combined absorbance of light by nanoparticles and the biocontaminants attached to them.” Garbovskiy provides an analysis of how the level of the biological contamination of nanoparticles can be assessed by analyzing the dependence of the optical absorbance of contaminated nanoparticles on the concentration of nanoparticles and on the constant characterizing the binding of biocontaminants to the surface of nanoparticles. Garbovskiy's proposed model should be of great interest to researchers trying to apply optical techniques for the detection and quantification of the biological contamination of nanomaterials. Featured scientist: Yuriy Garbovskiy (https://bit.ly/2IMrTJX) Organization: UCCS BioFrontiers Center & Department of Physics, University of Colorado, Boulder, CO (USA) Relevant publication: Y. Garbovskiy, Biological Contamination of Nanoparticles and Its Manifestation in Optical Absorbance Measurements, Anal. Chem., 2017, 89(14), 7282–7285.

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7.9  N  anophotonic AFM Probe Provides Ultrafast and Ultralow Noise Detection Photothermal induced resonance (PTIR) has found application in the characterization of materials in fields ranging from photovoltaics, plasmonics, and polymer science, to biology and geology. PTIR combines the spatial resolution of AFM with the specificity of absorption spectroscopy, enabling mapping of composition and electronic band gap, material identification, and biomolecule conformational analysis with nanoscale spatial resolution. For instance, PTIR can provide information on a solar cell's composition and defects with nanoscale precision, by measuring how much light the sample absorbs over a broad range of wavelengths, from visible light to the mid-­infrared. PTIR signal transduction relies on the thermal expansion of the sample, which generally is a small quantity, especially for very thin samples. Furthermore, PTIR is a relatively slow technique. Scientists at the Center for Nanoscale Science and Technology at National Institute of Standards and Technology have implemented an integrated near-­field cavity-­optomechanics readout concept to realize fully functional nanoscale AFM probes capable of ultralow detection noise, within an extremely wide measurement bandwidth (>25 MHz) in ambient conditions, surpassing all previous AFM probes. “The sensitive transducers used in our work allow measuring samples as thin as a molecular monolayer with high signal-­to-­noise ratio and improve the measurement throughput,” says Dr Andrea Centrone, who, together with Dr Vladimir A. Aksyuk, led this work. “A couple of years ago, we developed a technique named scanning thermal infrared microscopy (STIRM) that relies on temperature-­sensitive AFM probes to provide both chemical composition and thermal conductivity information at the nanoscale,” recounts Centrone. “However, STIRM probes trade measurement sensitivity for time resolution and were of limited use. For such measurements to be successful, it was clear that a new approach capable of overcoming the trade-­off between measurement sensitivity and time resolution was necessary.” The transducers resulting from this new work break the trade-­off between AFM measurement precision and ability to capture transient events. For PTIR, this capability improves the time resolution, signal-­to-­noise ratio, and throughput by a few orders of magnitude each. As a first practical application the scientists leveraged these characteristics to measure the intrinsic thermal conductivity of individual metal–organic framework (MOF) microcrystals, a property not measurable by conventional techniques. MOFs are a class of nanoporous materials that are promising for catalysis, gas storage, sensing, and thermoelectric applications, where accurate knowledge of thermal conductivity is critically important.

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“Capturing the sample thermalization dynamics with high precision enables measuring the thermal conductivity with low uncertainty (