Bio-Inspired Nanomaterials and Nanotechnology [1 ed.] 9781617614552, 9781608761050

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Copyright © 2009. Nova Science Publishers, Incorporated. All rights reserved. Zhou, Yong. Bio-Inspired Nanomaterials and Nanotechnology, Nova Science Publishers, Incorporated, 2009. ProQuest Ebook Central,

Copyright © 2009. Nova Science Publishers, Incorporated. All rights reserved. Zhou, Yong. Bio-Inspired Nanomaterials and Nanotechnology, Nova Science Publishers, Incorporated, 2009. ProQuest Ebook Central,

NANOTECHNOLOGY SCIENCE AND TECHNOLOGY SERIES

BIO-INSPIRED NANOMATERIALS AND NANOTECHNOLOGY

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NANOTECHNOLOGY SCIENCE AND TECHNOLOGY SERIES Safe Nanotechnology Arthur J. Cornwelle 2009. ISBN: 978-1-60692-662-8 National Nanotechnology Initiative: Assessment and Recommendations Jerrod W. Kleike (Editor) 2009. ISBN 978-1-60692-727-4 Nanotechnology Research Collection - 2009/2010. DVD edition James N. Ling (Editor) 2009. ISBN 978-1-60741-293-9 Nanotechnology Research Collection - 2009/2010. PDF edition James N. Ling (Editor) 2009. ISBN 978-1-60741-292-2

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Safe Nanotechnology in the Workplace Nathan I. Bialor (Editor) 2009. ISBN 978-1-60692-679-6 Strategic Plan for NIOSH Nanotechnology Research and Guidance Martin W. Lang (Author) 2009. ISBN: 978-1-60692-678-9 Nanotechnology in the USA: Developments, Policies and Issues Carl H. Jennings (Editor) 2009. ISBN: 978-1-60692-800-4 New Nanotechnology Developments Armando Barrañón (Editor) 2009. ISBN: 978-1-60741-028-7 Electrospun Nanofibers and Nanotubes Research Advances A. K. Haghi (Editor) 2009. ISBN: 978-1-60741-220-5 Nanostructured Materials for Electrochemical Biosensors Umasankar Yogeswaran, S. Ashok Kuma and Shen-Ming Chen 2009. ISBN: 978-1-60741-706-4

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Magnetic Properties and Applications of Ferromagnetic Microwires with Amorpheous and Nanocrystalline Structure Arcady Zhukov and Valentina Zhukova 2009. ISBN 978-1-60741-770-5 Electrospun Nanofibers Research: Recent Developments A.K. Haghi (Editor) 2009. ISBN 978-1-60741-834-4 Nanotechnology: Environmental Health and Safety Aspects Phillip S. Terrazas (Editor) 2009. ISBN: 978-1-60692-808-0 Nanofibers: Fabrication, Performance, and Applications W. N. Chang (Editor) 2009. ISBN: 978-1-60741-947-1 Carbon Nanotubes: A New Alternative for Electrochemical Sensors Gustavo A. Rivas, María D. Rubianes, María L. Pedano, Nancy F. Ferreyra, Guillermina Luque and Silvia A. Miscoria 2009. ISBN: 978-1-60741-314-1

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Polymer Nanocomposites: Advances in Filler Surface Modification Techniques Vikas Mittal (Editor) 2009. ISBN: 978-1-60876-125-8 Bio-Inspired Nanomaterials and Nanotechnology Yong Zhou (Editor) 2009. ISBN: 978-1-60876-105-0

Zhou, Yong. Bio-Inspired Nanomaterials and Nanotechnology, Nova Science Publishers, Incorporated, 2009. ProQuest Ebook Central,

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NANOTECHNOLOGY SCIENCE AND TECHNOLOGY SERIES

BIO-INSPIRED NANOMATERIALS AND NANOTECHNOLOGY

YONG ZHOU

Copyright © 2009. Nova Science Publishers, Incorporated. All rights reserved.

EDITOR

Nova Biomedical Books New York

Zhou, Yong. Bio-Inspired Nanomaterials and Nanotechnology, Nova Science Publishers, Incorporated, 2009. ProQuest Ebook Central,

Copyright © 2010 by Nova Science Publishers, Inc. All rights reserved. No part of this book may be reproduced, stored in a retrieval system or transmitted in any form or by any means: electronic, electrostatic, magnetic, tape, mechanical photocopying, recording or otherwise without the written permission of the Publisher. For permission to use material from this book please contact us: Telephone 631-231-7269; Fax 631-231-8175 Web Site: http://www.novapublishers.com NOTICE TO THE READER The Publisher has taken reasonable care in the preparation of this book, but makes no expressed or implied warranty of any kind and assumes no responsibility for any errors or omissions. No liability is assumed for incidental or consequential damages in connection with or arising out of information contained in this book. The Publisher shall not be liable for any special, consequential, or exemplary damages resulting, in whole or in part, from the readers‘ use of, or reliance upon, this material. Any parts of this book based on government reports are so indicated and copyright is claimed for those parts to the extent applicable to compilations of such works.

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Independent verification should be sought for any data, advice or recommendations contained in this book. In addition, no responsibility is assumed by the publisher for any injury and/or damage to persons or property arising from any methods, products, instructions, ideas or otherwise contained in this publication. This publication is designed to provide accurate and authoritative information with regard to the subject matter covered herein. It is sold with the clear understanding that the Publisher is not engaged in rendering legal or any other professional services. If legal or any other expert assistance is required, the services of a competent person should be sought. FROM A DECLARATION OF PARTICIPANTS JOINTLY ADOPTED BY A COMMITTEE OF THE AMERICAN BAR ASSOCIATION AND A COMMITTEE OF PUBLISHERS. LIBRARY OF CONGRESS CATALOGING-IN-PUBLICATION DATA Available upon request ISBN 978-1-61761-455-2 (ebook)

Published by Nova Science Publishers, Inc.  New York

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CONTENTS

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Preface

ix

Chapter I

Biomimetic Mineralization and Mesocrystals An-Wu Xu

Chapter II

Artificial Fossilization Process: A Shortcut to Nanostructured Materials from Natural Substances Jianguo Huang

Chapter III

Nano-Fabricated Structures for Biomolecule Analysis Noritada Kaji, Yukihiro Okamoto, Manabu Tokeshi and Yoshinobu Baba

Chapter IV

Bionic Superhydrophobic Surfaces Based on Colloidal Crystal Technique Yue Li, Weiping Cai, Guotao Duan, Sung Oh Cho

Chapter V

Chapter VI

Biologically Targeted Nanoparticles as Cancer Therapeutics Andrew Z. Wang, Aleksandar F. Radovic-Moreno and Omid C. Farokhzad Nanomaterials: Biopolymer-assisted Green Synthesis Shuyan Gao

1

31 41

59

97

119

Chapter VII Lithographically-Structured, Biologically-Inspired, Gripping Devices Charles Chin, Timothy G. Leong and David H. Gracias

167

Chapter VIII Protein Engineering Tools for Interfacing Proteins and Solid Supports with Exquisite Chemical Control Julio A. Camarero

183

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Chapter IX

Chapter X

Chapter XI

Contents

Bacilli, Green Algae, Diatoms and Red Blood Cells – How Nanobiotechnological Research Inspires Architecture I. C. Gebeshuber, M. Aumayr, O. Hekele, R. Sommer, C. G. Goesselsberger, C. Gruenberger, P. Gruber, E. Borowan, A. Rosic and F. Aumayr

207

Preparation and Application of Bio-inspired Colloidal Systems Aimin Yu, Ian R. Gentle and Gao Qing Lu

245

Biopolyelectrolyte Multilayer Microshells: Assembly, Property and Application Xia Tao and Yan-Zhen Zheng

263

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Index

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PREFACE Nanoscience is a rapidly evolving field of studying and working with matter on an ultrasmall scale, which was invented late in the twentieth century. Learning from bio-systems in Nature, scientists began to design experiments to specifically couple biology with nanofabricated materials, devices and tools. Such bio-inspired nanoscience utilizes biological processes, shape, chemical and physical functionality of biomolecules for atom-levelly controllable fabrication of advanced materials with extreme precision. This book presents our current knowledge and understanding of bio-inspired nanoscience by a collection of eleven impressive chapters, covering several remarkable aspects. Chapter I by Anwu Xu summarizes the developments of biomimetic synthesis strategies of materials with specific size, shape, orientation, composition, and hierarchical organization. Jianguo Huang in Chapter II describes a so-called ―artificial fossilization process‖ developed to achieve inorganic replicas of the biological species which possess the corresponding finest structure details and morphological hierarchies all the way down to nanometer scale. Chapter III by Noritada Kaji and coworkers surveys fundamental fabrication techniques for microand nanostructures on silicon and glass substrates, various approaches for biomolecule separation based on different separation mechanisms, and practical applications such as DNA separation from the aspect of ―nanomaterials and nanotechnology for bio-analytical chemistry‖. Chapter IV by Yue Li et al. reviews the bionic self-cleaning effect based on the superhydrophobic surfaces of hierarchical micro/nano structures fabricated with colloidal crystal techniques. Omid C. Farokhzad and coworkers in Chapter V discuss the rationale for molecularly targeting nanoparticles, the various classes of targeting ligands, the formulation of targeted nanoparticles, and highlight interesting examples from the preclinical data on targeted nanoparticle therapeutics. In Chapter VI Shuyan Gao focuses attention on the biopolymer-assisted green synthesis and properties of structured metal, semiconductor, and magnetic nanomaterials. David H. Gracias and coworkers in Chapter VII state the recent development of engineering technologies in an effort to mimic one of nature‘s simplest machines: a grasping appendage which opens or closes on-demand. Chapter VIII by Julio A. Camarero reviews the progress of new chemical and biological technologies for the sitespecific immobilization of proteins onto inorganic materials and their potential applications to the fields of micro and nanotechnology. In Chapter IX by Gebeshuber et al., a variety of biological systems are introduced: Bacillus subtilis, the green alga Euglena gracilis, diatoms

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Yong Zhou

and red blood cells. Subsequently results of bionanotechnological research performed (by physicists) on these systems are presented. In the next step, the systems and the results are discussed with an architect, resulting in a multitude of ideas, possible approaches, experiments and projects. The layer-by-layer (LbL) self-assembly method, which is based on the stepwise electrostatic self-assembly of oppositely charged species, has emerged as a promising and versatile approach to the fabrication of functional core-shell particles with well-defined shell structures. In Chapter X, Aimin Yu et al. highlight their recent work on the preparation of biocompatible colloidal systems via biofunctionalization of mesoporous silica particles with nano-structured shell composition via the LbL self-assembly technique, and their applications in ultra-sensitive immunoassay and biomolecule encapsulation. Xia Tao and coworkers in Chapter XI describe the fabrication of hollow biopolyelectrolyte microshells with the LBL method, and key properties of these microshells including their mechanical stability and potential applications in drug loading/release and pollutant remediation. In working on this book, the editor had great pleasure interacting with the authors. He is grateful to all of them for their friendly and competent co-operation. Thanks are due to the publisher for expedient support. We sincerely hope that this book will provide researchers in these fields with newest developments in this rapidly evolving field for advancing research. We also wish to stimulate the next generation of breakthroughs of the bio-inspired nanosciences, which will further enrich human life.

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Yong Zhou Nanjing, China September, 2009

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In: Bio-Inspired Nanomaterials and Nanotechnology Editor: Yong Zhou

ISBN: 978-1-60876-105-0 © 2010 Nova Science Publishers, Inc.

Chapter I

BIOMIMETIC MINERALIZATION AND MESOCRYSTALS An-Wu Xu Division of Nanomaterials and Chemistry, Hefei National Laboratory for Physical Sciences at Microscale, the Structure Research Laboratory of CAS, the School of Chemistry and Materials, University of Science and Technology of China, Hefei, China

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ABSTRACT Self-assembly into highly ordered superstructures and control over the shape and the size of inorganic materials is an important character of natural growth phenomena. In biological systems, the results of long-term evolutionary optimization processes are intimately related to specific functions. Bio-inspired materials synthesis is a powerful strategy for the synthesis of advanced materials with complex shape, hierarchical organization and controlled size, structure and polymorph in aqueous environments under ambient conditions. Increased understanding of biomineralization mechanisms has greatly enhanced the possibilities of biomimetic mineralization and template synthesis approaches. Bio-inspired materials with complex structures and advanced functions always attract attention because of their unique properties, which have paved the way to many potential applications. Organic templates such as biopolymers and synthetic amphiphilic polymers can be employed to understand the interaction of the organic matrix with the developing inorganic crystals at a molecular level and to address the question which factors lead to the remarkable crystallographic orientation of the crystalline phase, crystal growth and nanoparticle assembly, which is often observed in biomineralization. Clear evidence has shown that crystallization does not necessarily proceed along the classical crystallization process, which is the attachment of ions/molecules to a primary nanoparticle forming a single crystal, instead, crystallization can also proceed along a particle mediated self-assembly pathway. Mesocrystal is a Corresponding author. Tel:+86-551-3602346; Fax:+86-551-3600724. E-mail address: [email protected] (A. Xu) Zhou, Yong. Bio-Inspired Nanomaterials and Nanotechnology, Nova Science Publishers, Incorporated, 2009. ProQuest Ebook Central,

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An-Wu Xu quasi-single crystal consisting of ordered assemblies of small, anisotropic, and vectorially aligned nanoparticles, thus forming an entirely new class of porous metamaterials through mesoscopic transformations and nanoparticle precursors. Such composite crystals are of considerable interest to a broad range of disciplines including materials chemistry and life science as well as crystallization-related fields in general. It has been found that nanoclustered crystal growth, mediated by organic templates, is a basic characteristic of biomineralization that enables the formation of composite materials with elaborate morphologies and structures. During the past decade, exploration as well as application of these bio-inspired synthesis strategies has led to novel materials with specific size, shape, orientation, composition, hierarchical organization and assembled superstructures. This overview tries to capture the concepts and recent progress in this rapidly developing field, and prospects for the future in this field has been discussed.

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1.1. INTRODUCTION Learning from nature is a constant principle for there are numerous mysterious features in nature, which have developed over millions of years of evolution and will inspire us to develop new functional materials. Bio-inspired and biomimetic concepts borrowed from Nature have been developed for synthesizing novel functional materials such as bio-inorganic materials, bio-inspired, multiscale structured materials, bio-nanomaterials, hybrid organic/inorganic implant materials, and smart biomaterials [1]. These bio-inspired, smart materials are attracting much interest because of their unique properties, which have paved the way to many potential applications. Interfaces between biomolecules and inorganic materials have been the focus of research in various fields such as biochemistry, materials chemistry, biomedicine and bionanomaterials. Materials with complex shapes and interface functions always attract attention and fascination. In view of the huge time frame Nature had to optimize and perfect functional materials, it is obvious that scientists are highly interested to develop synthetic strategies that mimic these natural processes. Especially promising materials in this respect are biominerals, which combine complex morphology over different length scales with superior materials properties and environmentally friendly synthesis and biocompatibility. This makes them very attractive archetypes for materials chemists. To mimic the synthesis of these materials, the main purpose is not to simply emulate a particular biological architecture or system, but to abstract the guiding principles and ideas and use such knowledge for the preparation of new synthetic materials and devices. Based on these concepts a rapidly developing research field has evolved, which can be summarized as bioinspired or biomimetic materials chemistry [2]. The creation of superstructures resembling naturally existing biominerals with their unusual shapes and complexity, is meanwhile an important branch in the broad area of biomimetics [3]. During the past decade, exploration as well as application of these bioinspired synthesis strategies has resulted in materials with specific size, shape, orientation, composition, and hierarchical organization [4]. This chapter will summarize these developments and give some examples what can already be achieved by applying natures‘ strategies for biomineral synthesis. This review can not be comprehensive, therefore we have

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Biomimetic Mineralization and Mesocrystals

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selected some topics, those are closely mimicking biomineralization strategies. We consider biomimetic mineralization as mineralization in aqueous solutions under ambient or nearly ambient conditions borrowing strategies from biomineralization processes. This chapter is organized into five sections. We have structured the paper in a way that first the main concepts underlying biomineralization are introduced and then, the specific examples for biomimetic mineralization are given in the following. The main contents of this review involve strategies for biomineralization, biomimetic mineralization and mechanisms, Non classical crystallization and mesocrystals, and bio-inspired functional nanomaterials. Finally, summary and outlook is discussed.

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1.2. BIOMINERALIZATION Biomineralization is the process by which living organisms secrete inorganic minerals in form of skeletons, shells, bone, teeth, magnetic iron minerals in bacteria, etc. (Figure 1) [5]. It is already a rather old process in the development of life, which was adapted by living beings probably at the end of the Precambrium more than 500 million years ago [6]. Materials found in nature combine many inspiring properties such as sophistication, miniaturization, hierarchical organizations, hybridation, resistance and adaptability [7]. Elucidating the basic components and building principles selected by evolution to propose more reliable, efficient and environment respecting materials requires a multidisciplinary research. Biominerals are highly organized from the molecular to the nano- and macroscales, often in a hierarchical manner, with intricate nanostructures those ultimately make up a lot of different functional soft and hard tissues (Figure 1). Under genetic control, biological tissues are produced in aqueous environments under mild physiological conditions by using biomolecules, primarily proteins but also carbohydrates and lipids. Biomolecules both collect and transport raw materials, and consistently and uniformly self- and co-assemble subunits into short- and long-range-ordered nuclei and substrates [8]. For example, magnetotactic bacteria, are able to form nano-sized, membrane-bound magnetic iron minerals, magnetite (Fe3O4) or greigite (Fe3S4), by a mineralization process with precise biological control over iron accumulation and mineral deposition [9]. The unexpected and unusual features of these biogenic magnetite crystals are not only a narrow size distribution, but above all, a diameter range of 40±120 nm, which thus allocates them the highest magnetic moment. This diameter range corresponds to magnetite crystals with a single magnetic domain [10] (Figure 1f). More examples of biominerals can be addressed as the caption in Figure 1. Whether in controlling biomineral formation, biological functions or physical performance, bimolecules are an indispensable part of biological structures and systems. A simple conclusion is that nextgeneration biomimetic systems should include biomolecules in synthesis, assembly or function [11].

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a

b

c

d

e

f

Figure 1. Representative examples of biologically synthesized complex biominerals. (a) scanning electron microscope (SEM) image of mouse enamel. It is a hard, wear-resistant material with highly ordered micro/nano superstructure comprised of hydroxyapatite crystallites that assemble into woven rod architecture (inset: schematic cross-section of a human tooth) [12]. (b) SEM image of a growth edge of abalone (Haliotis rufescens) displaying aragonite platelets (blue) separated by organic film (in orange) that finally becomes nacre (mother-of-pearl). This is a layered, tough, and high-strength biocomposite (inset: transmission electron microscope (TEM) image [13]. (c) sponge spicule (with a cross-shaped apex shown in inset) of Rosella has layered silica with excellent optical and mechanical properties, a biological optical fiber (SEM image) [14]. (d) SEM image of siliceous skeletal structures in diatomaceous earth. Actinopoda and diatoms, single-celled organisms, create amorphous siliceous units that are resting spores with highly intricate and symmetrical geometrical shapes [15]. (e) SEM of the peripheral layer of a dorsal arm plate (DAP) from Ophiocoma wendtii with the microlenses structures in brittlestars. Skeletal elements of echinoderms are each composed of a single crystal of oriented calcite shaped into a unique, three-dimensional mesh. Ophiocoma wendtii is a highly photosensitive species, and it changes color markedly, from homogeneous dark brown during the day to banded grey and black at night [16]. (f) magnetite nanoparticles formed by magnetotactic bacterium (Aquaspirillum magnetotacticum, inset: TEM image) are single-crystalline, single-domained and crystallographically aligned [9].

It is commonly assumed in the biomineralization field, that the remarkable biomaterials morphologies are fabricated under total control of specific biomolecules so that biomineralization is eventually a genetically controlled process, which transforms the genetically engineered organic scaffolds into soft and hard matter. On the other hand, the recent nacre retrosynthesis example [17] has already indicated that the natural biopolymers can be replaced by synthetic polymer analogues so that some of the biomineralization mechanisms are much related to physicochemical principles such as nucleation inhibition rather than specific biopolymer structures and functions. Although a protein may exhibit a complex structure, its actual function may be simple, for example, serving as a polyelectrolyte in a biomineralization process. Therefore, it makes sense to investigate, on how far physicochemical principles play a role in biomineralization. An excellent example

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for this is the pattern formation in diatoms, which can be explained by a phase separation model of amphiphilic polyamines (Figure 2) [18].

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Figure 2. Schematic drawing of the templating mechanism by the phase-separation model (a–d) and comparison with the stages of the developing cell wall of C. wailesii (e–h). (a) The monolayer of polyamine-containing droplets in close-packed arrangement within the silica-deposition vesicle guides silica deposition. (b, c) Consecutive segregations of smaller (about 300 nm) droplets open new routes for silica precipitation. (d) Dispersion of 300 nm droplets into 50 nm droplets guides the final stage of silica deposition. Silica precipitation only occurs within the water phase (white areas). The repeated phase separations produce a hierarchy of self-similar patterns. (e–h) SEM images of valves in statue nascendi at the corresponding stages of development. Reprinted from Ref. [18] with permission of the American Association for the Advancement of Science.

In this model, amphiphilic polyamines phase separate and form an emulsion of closely packed microdroplets in a hexagonally arranged monolayer within the flat silica deposition vesicle (SDV, Figure 2a). Phase separation is induced by coordination of positively charged polyamines with phosphate, and also incorporated in the polyamine itself in form of phosphoserins [19], which acts as a cross linking agent [20]. The silica precursors in form of a polyamine stabilized sol are located at the aqueous interfaces between the microdroplets. As a consequence, upon silica formation, a honeycomb-like hexagonal framework is produced (Figure 2e). After partial consumption of the polyamines by inclusion into silica, a further segregation of the initial microdroplets into smaller droplets is assumed (Figure 2b). The newly created interfaces again serve as the template for silica deposition (Figure 2f) and a further fraction of the polyamines is consumed. This leads to a further phase separation of the polyamines into 300 nm sized droplets (Figure 2c) and silica deposition at the aqueous interface between the droplets (Figure 2g). A final phase separation of the nanodroplets into only 50 nm sized droplets (Figure 2d) with subsequent mineralization of the aqueous interface between the droplets results in the observed hierarchically hexagonal self-similar patterns. This pattern formation mechanism, fully based on a series of consecutive phase separation steps, results in a pattern, which matches that of the diatom valve of Coscinodiscus [18].

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Above result indicates that such physicochemical principles can be transferred to biomimetic mineralization for the generation of advanced materials as summarized for the polyamine case in Ref. [21]. It will be interesting to reveal further physicochemical mechanisms in bio- and biomimetic mineralization such as the minimization of interface energies by the formation of an amorphous surface layer and growth inhibition by foreign additives/impurities [22]. These principles will make us to a deeper understanding of biomineralization processes and thus extend the toolbox of biomimetic mineralization by a transfer of biomineralization principles to the synthetic materials field.

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1.3. BIOMIMETIC MINERALIZATION Biomimetic mineralization based on the biomineralization principle of templating of inorganic structures by soft organic templates has already been transferred to materials synthetic science. Organic templates can be employed to understand the interaction of the organic matrix with the developing inorganic crystals at a molecular level and to address the question which factor result in the remarkable crystallographic orientation of the crystalline phase, which is often found in biomineralization. The templates are thus used as mimics of an oriented structural matrix in biomineralization. Importance in this respect is Langmuir monolayers as a template, because they are available with different head groups and can be compressed so that a range of regular template structures can be adjusted. There have been numerous reports on the selective nucleation of certain crystal faces under Langmuir monolayers as summarized in Refs [23], and initially, epitaxy or stereochemical resp. geometrical match between the arrangement of the charged groups of the monolayer and the ion arrangement on the nucleated crystal surface was discussed. More recently, Cavalli et al. demonstrated that flexible self-organizing -sheet lipopeptide monolayers led to a new growth habit of calcite and thus the formation of indented calcite crystals. This study confirmed the importance of flexibility of the template in crystal oriented nucleation [24]. In addition to Langmuir monolayers, self assembled monolayers (SAM‘s) can also be used to investigate the influence of the functional groups and other parameters on the mineral deposition and orientation. The advantage is that the SAM‘s are chemically fixed to the substrate and that they can be patterned by PDMS stamping or surface lithography. The most remarkable result with patterned SAM‘s was reported by Aizenberg et al. for the direct fabrication of large micropatterned calcite single crystals [25], which can be considered to be a rough mimic of the oriented single crystal calcite microlens arrays in brittlestars [16]. First, photoresist micropatterns were formed on a glass surface by photolithography. Then, the surface was coated with Au or Ag. As a very important step, a localized nanoregion of a polar alkanethiol was deposited on the surface with an AFM tip serving as a single nucleation center for calcite with a known orientation. Then, the remainder of the metal surface was coated with alkanethiols with varying length and functionality, which created a disordered surface and therefore favored the formation of amorphous calcium carbonate (ACC). First, a mesh of metastable ACC filled the interstices of the framework followed by site specific nucleation of a calcite single crystal at the deposited nucleation spot. This single crystal grew by transformation of the surrounding ACC finally leading to a micropatterned single crystal

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as imaged by polarization microscopy. The micropatterned surface was found to serve for the release of stresses, water and impurities during the formation of the final crystal. As a soft template used in biomimetic synthesis, double hydrophilic block copolymers (DHBCs) are a new class of amphiphilic macromolecules of rapidly increasing interest. They are water-soluble polymers in which amphiphilicity can be induced through the presence of a substrate or by temperature and pH changes. Their chemical structure can be tuned for a wide range of applications such as colloid stabilization, crystal growth modification, induced micelle formation. In particular, mineralization processes can be controlled by using DHBCs as inspired by biology, which have a molecular head group reacting with the metal ions and a central non-reactive part similar to proteins containing hydrophilic and mineralophilic sites [26]. Such polymers help to control the size, mineral forms, structure and assemblies of inorganic crystals. Indeed, original superstructures have been prepared, as well as aligned hydroxyapatite whiskers or mineral crystals having complex morphologies [2, 26]. More recently, a DHBC polymer with high molecular weight has been applied for the crystallization process of BaCO3 crystals [27]. At the critical point between aggregation towards long fibres and spheres, the short nanofibers at starting pH 5.5 self-organized towards most striking dynamic ring structure patterns on the large scale. Figure 3 shows typical SEM images of the obtained quasiperiodic wave patterns grown in solution for 1 day. Light microscopy images indicate that those patterns were already formed in the aqueous solution. This periodic wave pattern has multiple centres, from which concentric rings with even spacing radiate outwards (Figure 3a), reminiscent of the target (concentric) waves in the spatially extended Belousov-Zhabotinsky (BZ) reaction. A set of coupled chemical reactions necessary for the establishment of a reaction-diffusion system could be formulated including an autocatalytic formation of a Ba-polymer complex. On the substrate, many groups of concentric rings grow at the same time and stop when merging with each other. The enlarged SEM image shows that each ring (band) is composed of short nanorods standing on the substrate instead of lying, and tending to form bundles on the substrate, which are organized into a circular pattern around the center (Figure 3b, c). It is noteworthy that the experimental window is narrow for the formation of this concentric circle pattern [27]. The periodic pattern formation in this reaction system belongs to a self-organization process, in which competition between autocatalytic particle growth and educt diffusion occurs. This concentric circle pattern is a vivid and ubiquitous phenomenon in the reactiondiffusion system. In the meantime, numerical simulations using a modified Oregonator model for the reaction-diffusion equations qualitatively agree with the experimental observations. Figure 4d shows the simulation result for the BZ reaction, similar to the experimental ring patterns. It is important to note that similar patterns can also be found in natural minerals; spiral patterns of nacre (aragonite CaCO3) have been found on the growing inner surface of nacre [28], and in that case, screw dislocations were believed to be responsible for spiral growth of nacre. Calcite crystals with exposed (001) faces have recently been obtained in the presence of poly(sodium 4-styrene sulfonate-co-N-isopropylacrylamide) (PSS-co-PNIPAAM) (Figure 4a, b) [29]. Usually, calcite is not able to expose the (001) faces because these faces are composed of only CO32 or Ca2+ ions in a hexagonal orientation, respectively, and therefore are highly charged faces. Such highly charged faces would exhibit high surface energies and

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cannot exist in the absence of growth modifiers. The fact that this face now becomes dominant can be ascribed to multiple Coulomb binding of the negatively charged polymer molecules to the positively charged (001) plane. This leads to surface stabilization and inhibition of growth along this direction. The oriented self-assembly of subunits toward larger, single-crystalline superstructures results in mesocrystal formation based on nonclassical crystallization processes [30]. A Cerius2-colour model of the double truncated trigonal calcite structure viewed along the 001 face is shown in Figure 4c, in agreement with our experimental observations. This assignment of faces and orientation can be well revealed by polarized optical microscopy images of calcite mesocrystals which show that the exposed faces are not birefringent, confirming that the new exposed face is (001). The (001) direction, due to its symmetry, is the only axis in calcite which is not birefringent. In addition, the mesocrystals are not symmetrical along the [001] axis. While one side of the mesocrystals exhibits the truncated (001) face as shown in Figure 4b, the opposite side of the mesocrystal has the shape of a pyramid tip. This anisotropic shape supports the model of mesocrystal assembly from nanoparticles by dipole fields suggested in the previous work [31], and leading to an assembly with opposite charge of the opposite sides along the [001] axis with the truncated (001) face being the positive face, the pyramid tip negatively charged.

Figure 3. SEM images (a-c) of the obtained concentric circle pattern of BaCO3 crystals grown for 1 day. [polymer] = 1 g L 1, [BaCl2] = 10 mM, starting pH = 5.5. (d) showing a simulated pattern using a modified Oregonator model for the reaction-diffusion equations. Reproduced with permission from Angew. Chem. Int. Ed., 2006, 45, 4451. Copyright © 2006, Wiley-VCH.

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e

Figure 4. SEM images (a, b) of the obtained calcite superstructure with (001) faces exposed grown for 6 days, polymer: 1 g L−1, [CaCl2] = 10 mM. (c) Cerius2-colour model of double truncated trigonal calcite structure viewed upon the 001 face (yellow), red = (104) faces. (d) SEM image of the obtained vaterite hexagonal plates grown for 6 days at a middle polymer and CaCl2 concentration. Polymer: 0.5 g L 1, [CaCl2] = 5 mM. (e) SEM image of the aragonite superstructure obtained at the lower polymer and CaCl2 concentrations. Polymer, 0.1 g L 1, [CaCl2] = 1.25 mM, 6 days. Reproduced with permission from Wiley-VCH.

When polymer and CaCl2 with an intermediate concentration (polymer: 0.5 g L 1, [CaCl2] = 5 mM) was used for crystallization, the vaterite phase was obtained after the same reaction time (6 d). Figure 4d presents a typical SEM image of the obtained sample showing predominant vaterite hexagonal plates, slightly contaminated with minor traces of aragonite. Obviously, these particles with the diameter ranging from about 15 to 20 m are uniform. Moreover, a high resolution SEM image (Figure 4d) shows that an individual hexagonal plate is composed of hundreds of primary hexagonal small plates. Thin primary platelet-like crystals are aligned to a multilayered stack, and build up the complete mesocrystal. Although on the nanometer scale the superstructure looks random, the whole crystal is a quite regular hexagon with sharp facets and edges. It has to be noted that the results clearly indicate that kinetic rather than thermodynamic factors control the formation of the vaterite phase. The interactions between anionic moieties in the polymer and calcium cations in solution or at mineral surfaces are believed to be responsible for initiating and stabilizing non-equilibrium crystal polymorphs and superstructures. The aragonite phase was selectively produced when crystallization was carried out at the lower polymer and Ca2+ concentrations. The morphology and structure of as-synthesized sample produced at a polymer concentration of 0.1 g L 1 and 1.25 mM CaCl2 (reaction time:

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6 days) is shown in Figure 4e, displaying “sheaf bundle” crystals as the predominant morphology. This structure was also observed in a previous study using surface adsorbed triblock copolymer microgels as additive [32]. In this work, successful realization of polymorph switching of calcium carbonate could prove useful for a deeper understanding of the related biomineralization processes of CaCO3. In addition, polymorph switching in general is of great technical importance. Different kinds of polymers can be employed for mineral crystallization. The presence of polymers clearly improves the quality of oriented assembly, as the nanoparticle surfaces are obviously ‗code‘ by the selective polymer adsorption for subsequent oriented attachment. An impressive example is the formation of BaSO4 or BaCrO4 fibers of about 30 nm in diameter, which are, however, defect free up to hundreds of micrometers in length. These primary fibers further assemble to hierarchical fiber bundles and cones (Figure 5). The sodium salt of polyacrylic acid serves as a very simple structure-directing agent for the room temperature synthesis of highly ordered cone-like crystals [33] or very long, extended nanofibers of BaCrO4 or BaSO4 with hierarchical and repetitive growth patterns [34]. Temperature and concentration variation allow the control the finer details of the architecture, namely length, axial ratio, opening angle, and mutual packing [35]. The observed [210] growth axis implies that the polyanion adsorbs onto all parallel faces to this axis on the nucleated nanoparticles, just leaving the negatively charged (210) faces free for direct interaction. This makes them the highest energy faces, which fuse together by oriented attachment to form the fibers.

Figure 5. Complex forms of BaSO4 bundles and superstructures produced in the presence of 0.11 mM sodium polyacrylate (Mn = 5100), at room temperature, [BaSO4] = 2 mM, pH = 5.3, 4 days. (a) highly ordered funnel-like superstructure with multiple cones aligned orderly; (b) zoomed image of the detailed superstructures with repetitive patterns; (c) zoomed image of the aligned bundles; (d) magnified surface structures of the nanofiber bundles. Reprinted with permission from Nano Lett., 2003, 3, 379. Copyright © 2003, American Chemical Society.

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At least three different formation mechanisms of the superstructures shown in Figure 5 can be identified, which are all based on oriented attachment of nanparticles. The chosen crystallization path was determined by the availability of nanoparticles in solution available for further oriented attachment and polymer complexes were identified as the earliest species, subsequently transforming to amorphous nanoparticles, which themselves are the precursors of the crystalline nanoparticles for oriented attachment. The complexity of this biomimetic crystallization system is remarkable and shows the significant analytical challenges associated with an already seemingly simple two components biomimetic mineralization system. These results make it very clear that the nanoparticles alignment has many similarities to a controlled ‗polymerization‘ process, where the defined nanoparticles take the role of the organic monomers. Controlled nucleation or initiation in a short period of time sets the basis of a process that is terminated by the depletion of a material or external stimuli, such as electric fields, or curvature or stress fields. In this way, monodisperse aggregates and superstructures can be created. This similarity between controlled assembly and controlled polymerization has also been demonstrated by other reports. Specific biopolymers can exert a strong influence on both crystal nucleation and growth rates in addition to temperature, pH, ionic strength and composition. Recently, investigation of biomineralization of inorganic nano- and microstructures and their relevance to biogenic templates has emerged as an active research field between biomimetics and nanotechnology. Biopolymers, existing in living organisms due to their role in biomineralization, are often a natural soluble additive choice for morphogenesis of complex superstructures. It has been found that chiral copolymers of phosphorylated serine and aspartic acid with molar masses between 15000 – 20000 g/mol were very efficient additives for the generation of helical calcite superstructures consisting of elongated 70 nm wide, uniform and highly aligned calcite nanoparticles where the helix turn corresponding to the copolymer enantiomer [36]. The helical structures formed when a high degree of phosphorylated Ser (75 mol%) and 25 mol% Asp in the copolymer were adopted in combination with the ten-fold Ca2+ concentration with respect to the monomer, which is similar to the conditions where a shellfish forms a shell. The formation mechanism of the chiral crystalline superstructure is still unclear at present and needs to be investigated in the future.

1.4. NON CLASSICAL CRYSTALLIZATION AND MESOCRYSTALS Besides the classical crystallization in templates or confined reaction environments, biomimetic mineralization can also follow non classical particle mediated assembly pathways. Some evidences were found that crystallization does not necessarily proceed along the classical crystallization process, which is the attachment of ions/molecules to a primary particle forming a single crystal. Instead, crystallization can also proceed along particle based reaction routes [37]. Particle mediated crystallization pathways were identified, which produces crystals in the process of a so-called mesoscopic transformation including selfassembly or transformation of metastable or amorphous precursor particles [38]. Such

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crystallization pathways especially apply to systems far from equilibrium, for which the classical thermodynamic considerations are not valid anymore to predict the morphology or size of the crystals. But even for systems, which were so far considered to crystallize via the classical pathway, indications were found that nanoparticles are involved in the crystallization process. This nanoparticle mediated crystallization pathway involving mesoscopic transformation is called ―Non-Classical Crystallization‖ and involves multiple nucleation events of nanoparticles, which form a nanoparticle superstructure in contrast to a single nucleation event to form a single crystal. Non classical crystallization involves selforganization of pre-formed nanoparticles to an ordered superstructure, which then can fuse to a single crystal. Overall crystallization pathways are depicted in Figure 6 and discussed below in more detail under the respective headings.

d) Amorphous particles Nucleation clusters

?

or Liquid droplets

Crystal growth Temporary Stabilization

Primary nanoparticles > 3 nm

a)

b)

Crystal with complex shape

Mesoscale assembly

c)

Mesocrystal

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Amplification Mesoscale assembly Single crystal

Oriented attachment

Fusion

Fusion

Figure 6. Schematic representation of classical and non-classical crystallization process. Pathway a) represents the classical crystallization pathway where nucleation clusters form and grow until they reach the size of the critical crystal nucleus growing to a primary nanoparticle, which is amplified to a single crystal (path a). The primary nanoparticles can also arrange to form an iso-oriented crystal, where the nanocrystalline building units can crystallographically lock in and fuse to form a single crystal (Oriented Attachment, path b). If the primary nanoparticles get covered by a polymer or other additive before they undergo a mesoscale assembly, they can form a mesocrystal (path c). Note: Mesocrystals can even form from pure nanoparticles. There is also the possibility that amorphous particles are formed, which can transform before or after their assembly to complicated morphologies (path d). Reproduced with permission from Wiley-VCH.

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One of the so far most investigated synthetic mesocrystals is the hexagonal prismatic seed crystal of fluoroapatite, formed in a gelatin gel, which further grows to spherical particles via dumbbell intermediates. In addition, structural defects attributed to a collagen triple helix strand as well as self-similar nano-subunits nucleated by gelatin were detected, which agree with the mesocrystal scheme presented in Fig. 6c but for hexagonal building units. Although the above system is very well investigated, the formation process of the mesocrystal is still not fully revealed. Precursor nanoparticles were already experimentally found and dipole fields suggested being responsible for their almost perfect alignment, but the system proves to be highly complex on several hierarchy levels. The full growth mechanism was so far only reported for two mesocrystal examples. One of them is a copper oxalate mesocrystal. Here, nanoparticles were found to arrange almost perfectly to a mesocrystal, which could be influenced in terms of morphology by hydroxymethylpropylcellulose. The polymer influences nucleation, nanocrystal growth and aggregation by face selective interaction. Upon aggregation of the nanocrystals, a mesocrystal is formed as intermediate but is apparently not stable due to the low repulsive electrostatic and steric forces. Later, the “brick by brick” selfassembly mechanism could be experimentally revealed in a time dependent study. The face selective PAA adsorption onto orthorhombic K2SO4 crystals led to the formation of tilted unit crystals, which were assembled in a diffusion limited condition resulting in various complex morphologies such as helices or zig-zag assembly of twinned crystals. A conclusive explanation of the various possibilities of particle growth in an anisotropic diffusion field was proposed. It is remarkable that the K2SO4-PAA system has six hierarchical levels from the nm scale to that of several hundreds of microns, which is a typical feature of biominerals and was so far only rarely reported for a synthetic material [39]. The superstructure designed at each level was controlled by changing the polymer concentration and the observed hierarchy was attributed to the interaction between crystals and polymers and the diffusion-controlled conditions. A similar hierarchical system has recently been reported for potassium hydrogen phthalate and PAA [40]. Again, plate-like units are composed of aligned crystalline nanocrystals, therefore, the well facetted plates on the micron scale can be considered as mesocrystals although the authors of the original paper also discuss mineral bridges between the subunits as a possibility for the explanation of their mutual crystallographic alignment. As the concept of particle assembly in diffusion fields coupled with face selective polymer adsorption was demonstrated for both – inorganic and organic crystals, it seems to be much more versatile than known so far. Uniform NH4TiOF3 mesocrystals comprising orientationally ordered primary crystallites have been recently prepared by a simple, room-temperature surfactant-mediated route [41]. Figure 7a-c show typical SEM images of the obtained particles, which have regular square morphology. High-magnification cross-sectional SEM image of a particle (Figure 7c) shows that this particle consists of small nanoparticles. TEM measurements provide more details of the structure of the as-prepared NH4TiOF3 mesocrystals (Figure 7d-f). The regions of low contrast between individual crystallites indicate interstices with diameters of about 5–10 nm exist within these particles (Figure 7e). The interstitial walls are formed by nanocrystals whose lengths can extend to 25 nm or more, which is consistent with the 26 nm in the [100]/[010] directions from the XRD results. SAED of an area ca. 250 nm in diameter (Figure 7d) shows single-crystal diffraction with minor distortions, indicating that the whole

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assembly of particles behaves as a single crystal. The distortions come from the mismatch between boundaries of the small particles, which are typical for a mesocrystal. Importantly, when the sample was moved in the microscope in diffraction mode, the pattern remained the same, except for some minor brightness changes, as clearly shown in Figure 7f. CaCO3 (vaterite) mesocrystals with a hexagonal morphology and uniform size were successfully prepared in the presence of an N-trimethylammonium derivative of hydroxyethyl cellulose via aggregation-mediated crystallization using a simple gas-diffusion method (Figure 8) [42]. Uniform hexagonal particles show sharp facets and edges, and assembled by the aggregation of spherical nanoparticle subunits (Figure 8b, [polymer]: 1 g L 1, [CaCl2] = 10 mM) or hexagonal nanosheet subunits (Figure 8d, [polymer]: 0.5 g L 1, [CaCl2] = 10 mM), depending on the adapted polymer concentrations. ED pattern and HRTEM data confirmed that these hexagonal micrometer-sized plates display 3D highly oriented superstructures. The selective adsorption of polymer molecules on specific faces of crystals plays a key role in this mesoscale transformation. This mesoscale transformation involving cooperative reorganization of coupled inorganic and organic components can be relevant for the model of matrix-mediated nucleation in biomineralization. An understanding of the 3Doriented aggregation will be helpful in controlling the aggregation-driven formation of complex, higher-order structured materials, and further provide new insights into biomineralization mechanisms.

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a

d

f

b

e

c

Figure 7. SEM (a-c) and TEM (d-f) images of the obtained uniform hexagonal NH4TiOF3 mesocrystals in the presence of 23.1 wt% Brij 58 at 35 oC for 20 h. (a) top view of an as-obtained particle, (b) crosssectional view of a particle, (c) high-magnification cross-sectional image of a particle. (d-f) TEM, HRTEM and ED images of an as-prepared particle. Reproduced with permission from Royal Society of Chemistry.

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Figure 8. SEM images of the obtained uniform hexagonal plates of vaterite mesocrystals grown in the presence of polymer for 1 day, a full view of vaterite CaCO3 particles (a), and high resolution SEM image (b) showing each hexagonal plate consists of nanoparticles. [polymer]: 1 g L 1. [CaCl2] = 10 mM. (c) SEM image and (d) high resolution SEM image of vaterite mesocrystals under [polymer]: 0.5 g L 1. [CaCl2] = 10 mM. The edges of primary hexagonal discs are perfectly parallel to each other, indicating each large particle is a single crystalline aggregate with a preferred c-axis orientation. Reproduced from Wiley-VCH with permission.

A typical example for a non-classical crystallization pathway of mesocrystal formation and morphology evolution of calcite CaCO3 crystals in the presence of a polystyrenesulfonate (PSS) was also demonstrated [43]. Variation of the concentration of calcium chloride and PSS solutions by a CO2 gas diffusion technique can result in the formation of unusual CaCO3 superstructures, which transformed from the typical calcite rhombohedra, to rounded edges, to truncated triangles, and finally to concavely bended lens-like superstructures (Figure 9). The strong binding effect of PSS to free calcium ions will shift the mechanism from traditional ionic growth to mesoscale assembly. In addition, PSS can also bind selectively to the otherwise non exposed (001) calcite face, resulting in mesocrystals composed of truncated triangular units instead of the typical rhombohedra. Usually, calcite is not able to expose the (001) faces because these are composed of only CO32 or Ca2+ ions in a hexagonal orientation, respectively. The fact that this face becomes dominant can be ascribed to multiple Coulomb binding of the negatively charged polymer molecules to the positively charged [001] plane, leading to surface stabilization and inhibition of growth along this direction. The oriented self-assembly of subunits toward larger, single-crystalline superstructures is an example for mesocrystal formation, which is currently identified to be relevant in a wide range of crystallization processes. Those crystals are porous and composed of almost perfectly 3D-aligned calcite nanocrystals. The results suggest that the inner field effects within the nanocrystal building units cannot be neglected, which play a key role in the polymer-controlled crystallization processes.

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Figure 9. SEM images of calcite mesocrystals obtained on glass slips by gas diffusion reaction after 1 day with different concentrations of Ca2+ and PSS. (a) [CaCl2] = 1.25 mM, [PSS] = 0.1 g L 1. (b) [CaCl2] = 1.25 mM, [PSS] = 1.0 g L 1. (c) [CaCl2] = 5 mM, [PSS] = 0.1 g L 1. (d) [CaCl2] = 5 mM, [PSS] = 1.0 g L 1. Copyright © 2005, American Chemical Society.

On the other hand, self-similar growth of well-defined polyhedral building units can also lead to mesocrystals; this is a growth process, which relies on the shape of the primary building units and some elementary assembly rules. For instance, the construction of a mesocrystal can be provided by geometric packing of the building units, which are aligned by edge-to-edge or face-to-face connections. The driving force for the geometrical orientation is the minimization of the interfacial mismatch energy, through forming a coherent interface and reducing the exposed surface area. Self-similar growth was found for the spontaneous self-assembly of ca. 5 m sized octahedral silica polyhedrons [44] on glass substrates. These crystals are formed by the {111} faces of the cubic phase and can self-assemble in an oriented manner due to the capability of edge sharing (Figure 10). As the primary building blocks are well-defined in size, as well as the contact edges to the next particles, a selfsimilar growth process is induced, leading to a mutual crystallographic orientation of the primary particles and thus to the formation of a mesocrystal. A large variety of arrangements are possible, as the initially formed mesocrystal units can themselves assemble into higherorder structures (Figure 10e, f), because of the octahedral structure of the primary building units, these mesocrystals have porosity due to the structural voids. Larger pores are formed by packing defects and larger cage formation (Figure 10b). The above principle of octahedral edge sharing is learnt from some crystals built by atoms [45] and also by nanoparticles [46], it indicates that the same building principles can be applied regardless of the size of the building units, which is the basis for self-similarity even within one structure.

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Figure 10. Schematic illustrations of edge-sharing stacking: a) primary octahedral units, face-on configurations, b) quartet-octahedron model for the secondary structure, c) tertiary structure with filled corners, d) tertiary structure with unfilled corners, e) a high-order structure from primary octahedra, f) a high-order structure from tertiary units. Reproduced with permission from Wiley-VCH.

Besides the self-similar assembly strategy for mesocrystals, the crystallographic orientation of the nanoparticles building up the mesocrystal can also be achieved by spatial constraints in an already existing mesocrystal. However, the distinction between spatial constraints and self-similar assembly is not easy to make on the basis of the final mesocrystals, and only a time resolved investigation is able to reveal how the mesocrystals are formed. A very recent example of the self-similar assembly of obtained calcite mesocrystals is shown in Figure 11 [47]. In this case, calcite nanoparticles with a characteristic shape aligned to a mesocrystal with a shape resembling that of the primary nanoparticles, with triangular tip structures. The sample consists of a large number of elongated calcite particles with well defined faces and edges (Figure 11a, b, e). Elongation direction of each microparticle is along the caxis, as marked in Figure 11e. The high-magnification SEM images show that the microparticles appear to be self-similar mesoporous superstructures themselves (Figure 11d), formed by aggregation of the primary nanoparticles. The size of those nanoparticles determined from SEM images is ca 23-30 nm, in agreement with the XRD measurement. There are totally eight faces for each microparticle. Two basal faces of each elongated microparticle have a three-fold symmetry (the equilateral triangular crystal planes (Figure 11b & c) have three angles of 60o, that is, the crystallographic planes of each end surface belong to the {001} family), corresponding to the (001) and (00-1) face [48].

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g)

10-1

011 001

1-11

-11-1

00-1

0-1-1 -101 Figure 11. SEM images of the obtained trigonal calcite mesocrystals with triangular capped building blocks in the presence of PS-MA. (a) overall product morphology; (b), (c) and (d) High magnification SEM images showing the basal faces of elongated microparticles; (e) and (f) SEM images showing the lateral faces along c axis; (g) a modelled calcite morphology of a combination of {001} and {011} forms constructed by the Cerius2 software. Grey lines and face indices are those at the back. Polymer: 0.1 g L 1, [CaCl2] = 1.25 mM. Inset in (c) is Sierpinski triangles. (Image reproduced from [47] with permission of Wiley-VCH.)

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a 30 nm

b 100 nm

19

c 300 - 600 nm

d 40 m

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Figure 12. Proposed formation mechanism of hierarchical self-similar calcite mesocrystals made of triangular calcite subunits. (a) nearly spherical CaCO3 nanoparticles formed in the initial reaction stage. (b) crystallization of calcite nanoparticles exposing {011} & {001} faces and their aggregation. (c) further aggregation of the nanoparticles into an aggregate with the shape of its subunits presumably along the {011} faces; (d) formation of large calcite 3D mesocrystals consisting of triangular calcite building blocks via mesoscale transformation. (Image reproduced from [45] with permission of WileyVCH.)

By comparing the measured interfacial angles with the theoretical angles of six lateral faces, it can be concluded that these six lateral faces are (-101) (10-1), (1-1) (-11-1) and (011) (0-1-1) (Figure 1e, g). For example, the isosceles triangular crystal planes (Figure 11e) have angles of 76 1o and 28 1o, that is, the crystallographic planes of this surface and the opposite surface parallel to this surface are ascribed to the {011} family (Figure 11e, g). All eight faces are clearly displayed in Figure 11g, which is a modelled calcite morphology constructed by the Cerius2 software (Accelrys). In fact, the modelling morphology (Figure 11g) for a set of {001} and {011} faces is quite similar to the calcite morphology grown (Figure 11a, e). The side faces from the {011} family are neutral faces, which align according to their surface ion pattern. The oriented self-assembly of subunits toward larger, single crystalline superstructures (Figure 11) can be seen in the framework of self-similar growth of a mesocrystal from particulate subunits. This observation indicates that an overall crystallographic relationship exists among the nanocrystallites that constitute the whole threedimensional mesocrystals. The SEM images depict the formation of a hierarchical structure. This structural hierarchy indicates that both the primary units and the pre-assemlbed intermediates can undergo further oriented attachment, with the larger structures also being able to support larger pores while packing towards the superstructure. The scattering behavior is of a single crystal because the triangle-capped subunits are arranged with the same crystallographic orientation (three edges of each triangle are parallel to those of other triangles). This is schematically visualized in Figure 12. The above example gives clear evidence that mesocrystals can self assemble even from complex shaped subunits via edge or face energy minimization. More recently, crystallization of calcium carbonate in 5 mM CaCl2 solution with PSSMS (PSS-co-poly(maleic acid)) copolymer as additive at a concentration of 0.1 0.25 g L 1 led to the formation of octahedral calcite mesocrystals, as shown in Figure 13 [49]. Figure 13a presents a typical scanning electron microscopy (SEM) image of the obtained product, clearly displaying octahedral morphology, which is unexpected for calcite crystals grown in a

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dilute solution. These Platonic octahedral particles are uniform, with well defined faces and edges, and a size of several tens of micrometers. High resolution SEM images (Figure 13c, d) show that each octahedron consists of a large number of smaller subunits, thus displaying mesocrystal features. Comparable observations were reported for fluoroapatite systems of self-similarity [31] and truncated triangular calcite mesocrystals [29], indicating a mesoscale transformation process occurs for these octahedral calcite aggregates. Figure 13e shows an intermediate of dodecahedra derived from time-dependent experiments performed with different Ca2+ and polymer concentrations. Strikingly, all SEM images of these intermediates suggest a rhombohedral P-surface morphology [50] made up by nanoparticles. Figure 13f shows a unit cell of P-surface. Typically, minimal surfaces observed on the micro- and macroscale occur only for liquid phases with surface tension, such as lipid-water or surfactant-water mixtures as well as copolymers and liquid crystalline mesophases of amphiphiles in water, while on the scale of atoms to few crystalline unit cells, minimal surfaces were also reported for zeolites and equipotential lines in crystal grids [51]. Looking more carefully onto those structures, it is found that there are six smooth and six coarsened facets per dodecahedron. We ascribe the former to the {104} family, thus being the usual exposed faces of calcite, whereas the latter do not look like real faces at all, as they seem to have curvature and blips with a typical nanogranular surface structure. A nonclassical crystallization mechanism is employed for the morphogenesis with elements of liquid and solid behavior resulting in the first observation of a minimal rhombohedral primitive surface in a synthetic crystallization reaction.

a

b

e

c

d

f

Figure 13. SEM images of octahedral calcite mesocrystals. (a-d): [PSS-co-MA] = 0.1 0.25 g L 1, [CaCl2] = 5 mM, 2 weeks; (e): [PSS-co-MA] = 0.0025 g L 1, [CaCl2] = 1.25 mM, 3 days; (f) a unit cell of P-surface with minimal surfaces. (Image reproduced from [47] with permission of Wiley-VCH.)

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1.5. BIO-INSPIRED FUNCTIONAL NANOMATERIALS AND ASSEMBLY Biomaterials naturally have highly organized structures at the nanoscale with a variety of functions. Various bioassemblies are shown to template complex, multidimensional inorganic architectures that are typically not available by our synthetic methods. In addition to the various naturally occurring templates, the powerful techniques developed by life sciences are an interesting tool for engineering strategies for materials science [52]. Besides a structural and morphological control during synthesis, biotemplating approaches may add another dimension to inorganic materials such as biofunctionality. This section is focused on recent advances in the preparation of inorganic materials through use of biomolecule assemblies. We try to elucidate chemical methodologies and presents examples of templates based on protein, lipid and peptide building blocks that were successfully exploited to synthesize inorganic nanostructures. In relation to the surfactant-templated growth of nanostructured materials, the recent use of microorganisms to control inorganic crystal formation has been promoted as genetically engineered polypeptides binding to selected inorganics (GEPIs), such as Au [53] and silica [54]. GEPIs are based on three fundamental principles: molecular recognition, self-assembly and DNA manipulation, and they promise numerous successes in bio-inspired strategies. Although we have not clearly known about the mechanisms of in vivo crystallization processes at a molecular level, proteins play a key role in the formation of inorganic materials. However, this in vivo synthesis is just limited to certain materials such as calcium carbonate, silica, or magnetite. Therefore, the knowledge of biological concepts, functions, and design characteristics has been implemented into approaches for the synthesis of new technologically important materials which have no isomorphous complement in nature. Proteins have many specific recognition capabilities to drive assembly into defined superstructures but are less programmable than the DNA templates. They are, however, based on a very broad platform of chemical diversity. Recent progresses in combinatorial biology allow the identification of amino acid sequences with specific affinity for inorganic crystals, ranging from metal oxides and semiconductors to metals, providing a route to create novel interfaces between biomolecules and inorganic crystals. Besides specific recognition capabilities (e.g., antibody–antigen, biotin–avidin), proteins display various functionalities such as catalytic (e.g., enzymes) and motility functions (e.g., motor proteins) for potential biofunctional inorganic materials. What‘s more, it has been confirmed that nanoparticles may also influence the structure and function of the conjugated biological structures [52]. Some common used biotemplates are schematically summarized in Figure 14 [52]. Biotemplates with well-defined chemical and structural heterogeneity have recently been exploited for the precise control of the size and shape of the formed nanostructures including metal nanostructures of Ag [58], Ni, and Co nanowires [59], Au [60], complex two- or threedimensional assemblies of Au nanoparticles [61] and semiconductor CdS, PbS, ZnS nanostructures [62, 63] A DNA membrane complex as a simplified prototype system can be used as a nanoreactor to template the growth of CdS nanorods [64]. The strong electrostatic interactions within such complexes align the CdS (002) polar planes parallel to the negatively

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charged sugar-phosphate DNA backbone, which suggests that molecular details of the DNA molecule have been replicated onto the inorganic crystal structure. Sastry and co-workers reported a reaction of the extract of the lemongrass plant with aqueous chloroaurate ions, which can produce high yields of thin, flat, single-crystalline gold nanotriangles in a one-step reaction at room temperature [65]. By using a similar method, pure metallic silver and gold nanoparticles and bimetallic Au/Ag nanoparticles can also be obtained through reaction between metallic ions with the extract of Azadirachta indica leaf [66]. Processes based on biotemplates take advantage of the characteristic nanoscale dimensions of the biological specimen and replicate their morphology using inorganic components. Biomolecular components exhibit dimensions from lower nano- to micro-sized ranges with interesting surface features and functionalities. The replication process generates a positive (hollow), negative or exact copy of the template.

Figure 14. Schematic illustration showing the structure and dimensions of several protein assemblies used as biotemplates for materials synthesis. Reproduced from ref. [55] with permission of the American Chemical Society, from ref. [56] with permission of Wiley-Blackwell, and ref. [57] with permission of Wiley-VCH.

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Figure 15. Casting of silver nanowires using peptide nanotubes. (A) The nanowires are formed by the reduction of silver ions within the tubes, followed by enzymatic degradation of the peptide mold. (B) TEM analysis (without staining) of peptide tubes filled with silver nanowires. (C, D) TEM images of silver nanowires that were obtained after proteolyticlysis of the peptide mold. Reprinted with permission from ref [67]. Copyright 2003 American Association for the Advancement of Science.

A very short peptide, the Alzheimer‘s -amyloid diphenylalanine structural motif can be self assembled into discrete and stiff nanotubes. The peptide nanotubes were used to serve as molds for casting metal Ag nanowires (Figure 15a) [67]. The tubes were added to boiling ionic silver solution, and the silver was reduced with citric acid to ensure a more uniform assembly of the silver nanowires. TEM measurements indicate the formation of silver assemblies within the majority of the tubes (Figure 15b). Proteolytic lysis of the peptide mold, by the addition of a proteinase K enzyme to the silver-filled nanotubes solution, resulted in the attainment of individual silver nanowires 20 nm in diameter, as seen by TEM (Figure 15c, d). The diameter of the nanowires is smaller than that of the tubes, which further suggests that casting was done inside the tubular structure. Bio-inspired strategies that use viruses and genetically engineered bacteriophages have been employed to prepare nanometer-sized structures. Some groups have used combinatorial cell surface display or phage display methods to identify peptides that bind strongly to, and in some cases induce the precipitation of, synthetic inorganic materials [68]. Ahmad et al. recently demonstrate that peptides (BT1 and BT2 peptides) identified by phage display biopanning are capable of inducing the rapid, room-temperature formation of tetragonal barium metatitanate, ferroelectric BaTiO3, from an aqueous precursor solution at near neutral pH [69]. Obviously, this bio-inspired synthetic route shows more advantages over conventional synthetic methods. Key to the successful application of nanotechnology on an industrial scale, however, is the ability to manipulate these nano-objects into a spatially ordered pattern. In recent years, interest in using biomolecules, such as crystalline S-layer proteins and ferritin protein cages, as templates to scaffold inorganic nanostructures has arisen [52]. Of the various biomolecules, phospholipids, owing to the strong amphiphilicity stemming from possessing both polar heads and aliphatic tails, are able to spontaneously arrange themselves along a

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phase boundary or external surface. This property permits the generation of a multitude of microscopic structures, that is, micelle, vesicle, bilayer, microtubules, and nanotubes, and would also render the phospholipid an ideal material to drive the assembly of metal nanoparticles. Lipid nanotubes and solid-supported lipid multilayers were used to induce the self-assembly of the metal particles [70]. More recently, Yoon et al. have reported a novel method of using a solid-supported liquid crystalline lipid membrane as a template to synthesize nanometer-sized particles as well as to force the encapsulation of the resulting particles with lipid molecules (Figure 16) [71]. In the multilayer formed by lipids in the liquid crystalline state, the lipid membrane becomes flexible as a result of chain melting. Capitalizing on the flexibility of the liquid-crystalline lipid membrane, when metal film was deposited onto the liquid-crystalline lipid multilayer, the flexible lipid membrane allows penetration of the metal and subsequent formation of metal nuclei within the multilayer membrane. Subsequent spontaneous encapsulation of metal particles by the lipid molecules would limit the further growth of metal particles beyond an equilibrium size by confining the volume and shape of the metal particles. The unique aspect of our approach lies in the fact that the encapsulating lipid molecules are highly mobile and thus able to induce redistribution of metal to produce monodisperse nanoparticles. The mobility of the lipid molecules also expedites the formation of highly ordered nanoparticle superlattices (Figure 16). Dissolving the Ag-embedded lipid membrane in a polar solvent, chloroform, also resulted in the formation of ordered superlattices, but with increased density of mis-registered Ag particles.

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(a)

Figure 16. (a) TEM images of Ag-embedded membrane of lipid DOPC. (b) TEM image of 2D selfassembled Ag nanoparticles formed after dissolving in iso-octane. (c) TEM image of 3D self-assembled Ag nanoparticles formed after dissolving in iso-octane. d) Magnified image of (c). (e) Low- and (f) high-magnification TEM images of the ‗‗honeycomb‘‘ structure formed by dissolving the Agembedded membrane in chloroform. (Image reproduced from [71] with permission from Wiley-VCH.)

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Figure 17. Spontaneous formation of protein–nanoparticle chains by binding of Ni-NTA functionalized Au nanoparticles to a histidine tag modified, stress-related protein oligomer (6His-SP1). Reproduced from ref. [72] with permission of the American Chemical Society.

Besides directly depositing the inorganic material on the biological specimen, biomolecules have been applied to assemble preformed inorganic building blocks into superstructures after binding to biomolecules with specific recognition capabilities. Medalsy et al. have reported the formation of ordered arrays and Au nanoparticle/protein hybrid superstructures using a stress-related stable protein 1 (SP1) [72]. SP1 is a boiling-stable ring protein complex, isolated from aspen plants (populus tremula) expressed during drought, 11 nm in diameter, which self-assembles from 12 identical monomers. SP1 can be utilized to form large ordered arrays; it can be easily modified by genetic engineering to produce various mutants; it is also capable of binding gold nanoparticles (GNPs) and thus forming protein-GNP chains made of alternating SP1s and GNPs. The protein is extremely stable towards high temperature and detergents, e.g., it reveals a melting temperature of 107 oC and resistance to detergents such as sodium dodecyl sulfate (SDS). Large, two dimensionally ordered arrays with a periodicity of 11 nm were assembled by applying the native SP1 oligomer and a phospholipids mixture. The N-termini of the SP1 could be modified by six histidine tags and the mutant could be expressed in E. coli. Histidine tags strongly bind to Ni ions and provide anchor points for Ni–NTA conjugates. 1.8 nm sized Au nanoparticles covered by Ni–NTA ligands could be bound to the inner pore of the histidine tag modified variant. Upon incubation of the Au nanoparticle/6His-SP1 hybrids in buffer solution protein– nanoparticle chains are spontaneously formed (Figure 17). Such particle 6His-SP1 hybrids may serve as potential building blocks for various other nanostructures, e.g., two-dimensional arrays. DNA possesses remarkable molecular recognition properties and structural features, which make it one of the most promising templates to create patterned materials with nanoscale precision. The emerging field of DNA nanotechnology strips this molecule from any preconceived biological role and employs its simple code to produce addressable nanostructures in one, two, and three dimensions. These structures have been used to

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precisely position proteins, nanoparticles, transition metals, and other functional components into deliberately designed patterns [73]. A strategy based on DNA hybridization to link colloids has been demonstrated [74] and was successfully applied in biological sensing [75]. The employment of DNA for materials synthesis and the use of genetically engineered proteins and organisms for inorganic growth and self-assembly opens up new avenues for the design of original nanostructures. During the past few years, two-dimensional (2D) arrays with amazing regularity have been produced using DNA [76]. However, the assembly of 3D dimensional structures has proven difficult. More recently, two groups have independently reported on the DNA-mediated assembly of nanoparticle crystals [71, 72]. Gang et al. have reported the formation of three-dimensional crystalline assemblies of gold nanoparticles mediated by interactions between complementary DNA molecules attached to the nanoparticles’ surface (Figure 18) [77]. It is found that the nanoparticle crystals form reversibly during heating and cooling cycles. Moreover, the body-centeredcubic (bcc) lattice structure is temperature-tunable and structurally open, with particles occupying only 4% of the unit cell volume. It can be expected that this DNA guided crystallization strategy, and the insight into DNA design requirements it has provided, will facilitate both the creation of new classes of ordered multicomponent metamaterials and the exploration of the phase behavior of hybrid systems with addressable interactions. Similar study on DNA-mediated 3D Au nanoparticle assembly was also reported by Mirkin et al. [78].

Figure 18. TEM and SEM images of Au nanoparticles before and after assembly at room temperature. SAXS patterns and corresponding structure factors S(q) for as-assembled systems at room temperature (blue curve) and annealed at Tpm (yellow curve). An illustration of the b.c.c lattice is shown, where the proposed CsCl-type particle arrangement is coded with blue and red colors representing particles with complementary DNA cappings. Reproduced with permission from Nature Publishing Group.

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1.6. CONCLUSION AND OUTLOOK Since organisms have spent millions of years optimizing structural biomaterials for performance, durability, and appearance, it is reasonable that scientists are highly interested in designing functional materials and are developing a curiosity about how Nature has solved problems that are often encountered in materials science and technology. Bioinspired/biomimetic chemistry has now become a separate branch of materials chemistry, wherein lessons learnt from biological systems are implemented into in vitro syntheses. The current chapter shows that a much progress has been made in recent years to transfer biomineralization principles to synthetic materials chemistry. This is triggered by an increased understanding of biomineralization mechanisms as well as an increased understanding of self-assembly and bottom up materials synthesis approaches. Bio-inspired mineralization can now be used for the room temperature synthesis of a large variety of inorganic minerals, metals and organic crystals, producing materials with new and exciting properties. Biomimetic materials and systems such as adaptive materials, nanomaterials, hierarchically structured metamaterials, mesocrystals, materials compatible with ecological requirements, should become a major preoccupation in advanced technologies. It can be expected that this research field will still gain further relevance in the next years because of the importance of hierarchically structured composite materials, one dimensional materials, morphosynthesis approaches, polymorph and size control of crystals and self-organization in materials synthesis. Nature teaches excellent lessons how to achieve these elusive goals and it can be expected that many new and exciting materials can be produced in the future by mimicking biomineralization processes. Bio-inspired selective multifunctional materials with associated properties such as separation, adsorption, catalysis, sensing, biosensing, imaging, multitherapy, will appear in the near coming years. The current overview reveals that much progress has been made in recent years to transfer biomineralization principles into synthetic materials chemistry. Despite the efforts made this past decade to elaborate bio-inspired materials, characterize their structural and physicochemical properties, understand their structure–function relationships and particle mediated assembly of mesocrystals, many unknown mechanisms still need to be investigated in the future study. Prospects for the future include the development of novel transcription methodologies, as well as the creation of new transcriptive templates which present a greater degree of structural complexity or can give rise to extended, complex architectures. One of the principal challenges remaining consists of finding novel methodologies for obtaining transcribed structures with a high degree of order at the multiscale level.

ACKNOWLEDGMENTS Support from the National Natural Science Foundation of China (20671096) and the special funding support from the Centurial Program of CAS is gratefully acknowledged.

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[59] Knez, M.; Bittner, A. M.; Boes, F.; Wege, C.; Jeske, H.; Maiss, E.; Kern, K. Nano Lett. 2003, 3, 1079. [60] Djali, R.; Chen, Y.; Matsui, H. J. Am. Chem. Soc. 2002, 124, 13660. [61] Mirkin, C. A.; Letsinger, R. L.; Mucic, R. C.; Storhoff, J. J. Nature 1996, 382, 607. [62] Shenton, W.; Davis, S. A.; Mann, S. Adv. Mater. 1999, 11, 449. [63] Mao, C.; Flynn, C. E.; Hayhurst, A.; Sweeney, R.; Qi, J.; Georgiou, G.; Iverson, B.; Belcher, A. M. Proc. Natl. Acad. Sci. USA 2003, 100, 6946. [64] Liang, H. J.; Angelini, T. E.; Ho, J.; Braun, P. V.; Wong, G. C. L. J. Am. Chem. Soc. 2003, 125, 11786. [65] Shankar, S. S.; Rai, A.; Ankamwar, B.; Singh, A.; Ahmad, A.; Sastry, M. Nat. Mater. 2004, 3, 482. [66] Shankar, S. S.; Rai, A.; Ahmad, A.; Sastry, M. J. Colloid Interface Sci. 2004, 275, 496. [67] Reches, M.; Gazit, E. Science 2003, 300, 625. [68] (a) Naik, R. R.; Stringer, S. J.; Agarwal, G.; Jones, S. E.; Stone, M. O. Nat. Mater. 2002, 1, 169. (b) Sarikaya, M.; Tamerler, C.; Jen, A. K. Y.; Schulten, K.; Baneyx, F. Nat. Mater. 2003, 2, 577. (c) Slocik, J. M.; Naik, R. R. Adv. Mater. 2006, 18, 1988. [69] Ahmad, G.; Dickerson, M. B.; Cai, Y.; Jones, S. E.; Ernst, E. M.; Vernon, J. P.; Haluska, M. S.; Fang, Y. N.; Wang, J. D.; Subramanyam, G.; Naik, R. R.; Sandhage, K. H. J. Am. Chem. Soc. 2008, 130, 4. [70] Yang, B.; Kamiya, S.; Yoshida, K.; T. Shimizu, Chem. Commun. 2004, 500. [71] Oh, N.; Kim, J. H.; Yoon, C. S. Adv. Mater. 2008, 20, 3404. [72] Medalsy, I.; Dgany, O.; Sowwan, M.; Cohen, H.; Yukashevska, A.; Wolf, S. G.; Wolf, A.; Koster, A.; Almog, O.; Marton, I.; Pouny, Y.; Altman, A.; Shoseyov, O.; Porath, D. Nano Lett. 2008, 8, 473. [73] Aldaye, F. A.; Palmer, A. L.; Sleiman, H. F. Science 2008, 321, 1795. [74] (a) Alivisatos, A. P.; Johnsson, K. P.; Peng, X. G.; Wilson, T. E.; Loweth, C. J.; Bruchez, M. P.; Schultz, P. G. Nature 1996, 382, 609. (b) Mirkin, C. A.; Letsinger, R. L.; Mucic, R. C.; Storhoff, J. J. Nature 1996, 382, 607. (c) Valignat, M. P.; Theodoly, O.; Crocker, J. C.; Russel, W. B.; Chaikin, P. M. Proc. Natl. Acad. Sci. U.S.A. 2005, 102, 4225. (d) Biancaniello, P. L.; Kim, A. J.; Crocker, J. C. Phys. Rev. Lett. 2005, 94, 058302. (e) Maye, M. M.; Nykypanchuk, D.; van der Lelie, D.; Gang, O. Small 2007, 3, 1678. [75] Rosi, N. L.; Mirkin, C. A. Chem. Rev. 2005, 105, 1547. [76] Ding, B.; Seeman, N. C. Science 2006, 314, 1583. Sharma, J.; Ke, Y.; Lin, C.; Chhabra, R.; Wang, Q.; Nangreave, J.; Liu, Y.; Yan, H. Angew. Chem. Int. Ed. 2008, 47, 5157. [77] Nykypanchuk, D.; Maye, M. M.; van der Lelie, D.; Gang, O. Nature 2008, 451, 549. [78] Park, S. Y.; Lytton-Jean, A. K. R.; Lee, B.; Weigand, S.; Schatz, G. C.; Mirkin, C. A. Nature 2008, 451, 553.

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Chapter II

ARTIFICIAL FOSSILIZATION PROCESS: A SHORTCUT TO NANOSTRUCTURED MATERIALS FROM NATURAL SUBSTANCES Jianguo Huang* Department of Chemistry, ZheJiang University, HangZhou, ZheJiang 310027, China

ABSTRACT

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The combination of various synthetic chemical processes and biological assemblies provides a promising strategy for the design and preparation of functional materials with tailored structures and properties. Precise replication of natural substances with inorganic matrices results in artificial materials possessing the initial biological structures and morphologies. An so-called ―artificial fossilization process‖ was developed to achieve inorganic replicas of the biological species which possess the corresponding finest structure details and morphological hierarchies all the way down to nanometer scale. And it was successfully applied to natural cellulosic substances such as filter paper, cotton and cloth to obtain the corresponding metal oxide replicas. The resultant man-made materials are hierarchical ceramics composed of metal oxide nanotubes, which are precise hollow replicas of the initial cellulose nanofibers. This approach has been employed to prepare various nanostructured metal oxide materials as well as metal oxide nanotube-metal nanoparticle hybrid material.

*

Tel. & Fax: +86-571-8795-1202; E-mail: [email protected]

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100 mM in 1:1/v:v toluene/ethanol) was then passed through the filter paper slowly within 2 minutes. Two 20-ml portions of ethanol was immediately filtered to remove the unreacted metal alkoxide, and 20 ml of water was allowed to pass in order to promote hydrolysis and condensation. Finally, the filter paper was dried with air flow. By repeating this filtration/deposition cycle, thin titania gel layers covered the surface of the cellulose fibers. The resultant paper/titania composite was calcined in air at 723 K for 6 hours (heating rate 1 K/min ) to remove the original filter paper. The resultant titania fossil possessed morphological characteristics of the original filter paper except for a little shrinkage in size due to calcination, which is commonly observed after calcination of a sol–gel material (Figures 1a and 1b). The resulted titania sheet is selfsupporting and highly porous with thickness of ~0.22 mm and mass of ~1.5 mg. The sheet size and thickness depend on the original filter paper used. The original morphology of the filter paper was found to be faithfully replicated by titania films, and the cellulose fibers were precisely copied as irregular titania nanotubes as clearly recognized in SEM and TEM images (Figures 1c and 1d). The ―titania paper‖ records the morphological information of the original paper at the nanometer scale. The outer diameter of the tube varies from 30 nm to 100 nm, and the thickness of the tube is uniform along its length with wall thickness of ca. 10 nm. The wall thickness can be controlled by changing the number of deposition cycles of titania layers. The titania nanotube assembly manifests the original morphology of interwoven cellulose fibers, and the nano-branched structure of the initial fibers can be clearly seen (Figure 1c). The SAED pattern from agglomerated titania tubes shows diffraction rings typical of the anatase crystal, and it is revealed by TEM observation that the titania nanotubes are composed of anatase fine particles with sizes of around 10 nm.

Figure 1. Nanoprecise titania replica of filter paper. (a) Photograph of a piece of filter paper after deposition of 10-nm thick titania layer. (b) Photograph of pure titania sheet obtained by calcination of the filter paper sample shown in (a). (c) FE-SEM image of the sample displayed in (b), showing titania nanotube assemblies. (d) TEM image of an individual titania nanotube isolated from the assembly.

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Similar replication processes are readily applied to other natural cellulosic substances like cloth and cotton, resulting in ―titania cloth‖ and ―titania cotton‖ (Figure 2). The hierarchical morphologies of these natural substances are retained in titania films to give macroscopic fossils, in which the structures of the original substance are again faithfully replicated from macroscopic to nanometer scales. The fine titania thread shown in Figure 2a is a copy of an individual fiber that makes up strands in the original cloth; and the fine titania hair displayed in Figure 2b shows the spiral twist of natural cotton lint. The corresponding high magnification SEM images demonstrate that both of them are composed of arrays of tortuous titania nanotubes (insets of Figures 2a and 2b), as precise replicas of cellulose fiber assemblies. The topographic differences among the titania fossils (as shown in Figures 1c, 2a and 2b) mirrors the structural differences of the initial paper, cloth and cotton, although they are all natural cellulosic substances. Among various oxidic nanotubes, titania nanotube is particularly attractive due to its unique electronic, photonic and catalytic properties. The current approach presents a practical and environment friendly approach to produce titania nanotubes. Structural design of the nanotube is achieved by proper selection of template materials. These features are not readily attained by the reported chemical methods such as sol gel template syntheses using porous membranes [17] and polymer fibers [18], or alkali treatment on titania powders [19]. The multi-helical morphology of the titania nanotubes shown in Figure 2b is worthy of mention. Helical inorganic fibers are unique class of advanced functional materials, important and challenging. As clearly shown here, replication of natural helical structures with inorganic matrices can be a shortcut route to helical inorganic materials. It is known that each natural cotton hair is a thin flattened tubular cell with a pronounced spiral twist when it is fully mature and dry, and its length is several centimeters. Precise duplication of cotton hairs with titania via the current petrifaction process gives ―titania cotton‖ composed of multihelical titania nanotubes.

Figure 2. Titania replicas of natural cellulosic substances. (a) SEM image of ―titania cloth‖. (b) SEM image of ―titania cotton‖.

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Figure 3. Electron micrographs of ―zirconia paper‖. Deposition of zirconia thin films was repeated 20 times for this sample. (a) FE-SEM image, showing zirconia nanotube assemblies. (b) TEM image of an individual zirconia nanotube isolated from the assembly.

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2.3. OTHER NANOTUBULAR METAL OXIDE MATERIALS DERIVED FROM CELLULOSIC SUBSTANCES Since the discovery of carbon nanotubes, nanotubular materials have been attracting great attention in both fundamental and industrial studies due to their peculiar properties superior to the corresponding bulk materials and isotropic nanoparticles [20]. In contrast, general and efficient synthetic approaches have not been available for oxidic nanotubes [20a]. As described above, the current ―artificial fossilization‖ process provides a shortcut to produce metal oxide nanotubular materials. Metal oxides other than titania can be similarly employed in the surface sol gel process, and are suitable as replicating matrices. For instance, Figure 3 shows artificial zirconia fossil derived from natural paper by using zirconium n-butoxide (Zr(OnBu)4) as the precursor compound. The initial fiber assembly in paper leads to well aligned zirconia nanotube arrays (Figure 3a), and the zirconia nanotubes are uniform with an ultrathin wall thickness of ca. 10 nm (Figure 3b) and an extremely high aspect ratio (length vs. diameter). Tin oxide nanotubular materials were also prepared by using a natural cellulosic substance (filter paper) as template [13]. Cellulose fibers of filter paper were firstly coated with SnO2 gel layers by the surface sol gel process using Sn(OiPr)4 as precursor compound, followed by calcination in air to give SnO2 nanotubular materials as hollow replicas of natural cellulose fibers. The final resulted material was self-supporting ceramic sheet as being a macroscopic fossil of the template filter paper. For an as-deposited sample prepared by repeating the surface sol gel deposition cycle for twelve times, the SnO2 nanotubes obtained by calcination at 450 ºC were amorphous-like and composed of fine particles with sizes smaller than ca. 5 nm; the outer diameters are tens to two hundred nanometers and wall thicknesses are 10 15 nm. While calcination at 1100 ºC yielded tube-like polycrystalline

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individual ITO nanotube (Figure 7b) clearly show that the tube is composed of nanoparticles. This cage-like nanotube morphology was confirmed by transmission electron microscopy (TEM), and the ITO nanoparticle size is seen to be ca. 10 nm. The selected-area electron diffraction (SAED) pattern indicates the polycrystalline nature of the ITO nanotube. ITO gel layers were deposited on individual cellulose nanofibers by using precursor solutions of indium methoxyethoxide and tetraisopropoxytin with a total concentration of 12 mM with different In/Sn molar ratios. The precursor mixtures of In/Sn ratio of 10/0, 9/1, 2/1, 2/8 and 0/10 were used and named as In10, In9Sn1, In2Sn1, In2Sn8 and Sn10 hereafter, respectively, from the In/Sn ratio in the precursor solution. The practical In/Sn ratio in the nanotube sheets was determined by electron probe micro analysis (EPMA) (Table 1). The observed In/Sn ratio is always smaller than that of the precursor solution. This difference may be caused by the greater reactivity of the indium alkoxide relative to that of the tin alkoxide [11]. The average thickness of ITO sheet, measured by optical microscopy, is in the range of 90-220 m (Table 1). The apparent density of ITO sheet was determined from the nominal volume and weight of 10 10 mm square pieces of the replica. It was very low, being in the range of 1.8 4.7% of the ideal density calculated from the bulk density of In 2O3 and SnO2 and the observed In/Sn ratio (Table 1). This indicates that the replica sheet is highly porous.

Figure 4. XRD patterns of the SnO2 powder obtained by calcination of the as-deposited SnO2 sheet for 3 h at various temperatures.

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Figure 5. Response transients of SnO2 nanotubes to 100 ppm H2, 100 ppm CO and 20 ppm C2H4O at varied temperatures.

Figure 6. Temperature dependence of sensitivities of SnO2 nano-tube sensor to 100 ppm H2, 100 ppm CO and 20 ppm C2H4O.

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The titania thin layer is additionally deposited on the gold nanoparticle, so that the individual nanoparticles are wholly covered by the titania layer. Gold nanoparticles are known to undergo melting at relatively low temperatures [25], and this facilitates fusion of the unprotected nanoparticle. In the case of gold nanoparticles (sizes, 6 1 nm) on carbon nanotube, the fusion was observed after heating for 30 s at 300 C [26]. The titania layers that surround individual gold particles should suppress fusion of adjacent gold particles even at higher temperatures. The gold nanoparticles are protected by coating with 5 titania layers (thickness ~2.5 nm) in the case of sample [(TiO2)15/Au-nanoparticle/(TiO2)5]; and their average size and the standard deviation are 4.9 and 1.4 nm, respectively. In fact, the original size distribution (5 1 nm) is not altered after a long period of calcination (6 h at 450 C). In contrast, the particle fusion was observed when the nanoparticle was not protected by the titania layer. For the hybrid material of titania nanotube and gold nanoparticle, it can be ensured with large surface areas, high and uniform metal loading, as well as enhanced particle stability in the hierarchical morphology. The one-pot fabrication of such complex loading matrices is rendered feasible by appropriate design of hierarchical templates, and should be extremely beneficial from the practical standpoint. Combining the rich varieties of nanoparticles and ceramic nanotubes, the present approach can produce versatile nanoprecision systems with unique physical and chemical functions

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2.5. SUMMARY AND OUTLOOK In summary, a general chemical procedure, artificial fossilization process, was developed for nanoscale to macroscale duplication of the complex hierarchical morphology of natural cellulosic substances with metal oxide matrices. It extends the range of replication techniques that already exist, by allowing hollow replication on both the micron and the nanometer scale simultaneously. This new nano-copying methodology provides replicas (both positive and negative) of targeted objects in nanometer precision. As pointed out by R. A. Caruso [12b], this synthetic procedure provides a pathway to probe structures of biosystems at nanometer scales as well, and obviously is a practical, low-cost and environmentally friendly route to produce nanotubular ceramic materials with unique structural features. And Moreover, this method was extended to fabricate nanostructured conjugated polymer material [27] and functional bioactive material [28] using natural cellulosic substances as scaffolds. The biotemplate-derived functional materials have shown promising potentials for various practical applications due to their unique structures and properties like high inner surface area. The artificial biomineralization science and technology are at the crossing point of biology, chemistry, physics, and materials science; and is a bridge to connect the new-age nanotechnology and classic biological science. Precise replication of the natural threedimensional biological structures at the nanometer spatial scales and further at single molecular level with a certain guest material is still a fairly unexplored field. New methodologies such as nanocopy technique [29] are now in great demand.

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ACKNOWLEDGMENTS Most of J. Huang‘s own research works presented here were done in Prof. Toyoki Kunitake‘s laboratory and under his guidance in RIKEN, Japan. This work was supported by the National Key Project on Basic Research (2009CB930104).

REFERENCES [1] [2] [3] [4] [5] [6] [7] [8]

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[9]

[10] [11]

[12] [13] [14] [15] [16] [17] [18]

Mann, S. Biomineralization: Principles and Concepts in Bioinorganic Materials Chemistry; Oxford University Press: NY, 2001. Sanchez1, C.; Arribart, H.; Guille, M. M. G. Nature Mater. 2005, 4, 277. van Bommel, K. J. C.; Friggeri, A.; Shinkai, S. Angew. Chem., Int. Ed. 2003, 42, 980. Kawano, S.; Tamaru, S.; Fujita, N.; Shinkai, S. Chem. Eur. J. 2004, 10, 343. Numata, M.; Sugiyasu, K.; Hasegawa, T.; Shinkai, S. Angew. Chem., Int. Ed. 2004, 43, 3279. Numata, M.; Li, C.; Bae, A.-H.; Kaneko, K.; Sakurai, K.; Shinkai, S. Chem. Commun. 2005, 4655. Cook, G.; Timms, P. L.; Göltner-Spickermann, C. Angew. Chem., Int. Ed. 2003, 42, 557. Kemell, M.; Pore, V.; Ritala, M.; Leskelä, M.; Lindén, M. J. Am. Chem. Soc. 2005, 127, 14178. Bao, Z.; Weatherspoon, M. R.; Shian, S.; Cai, Y.; Graham, P. D.; Allan, S. M.; Ahmad, G.; Dickerson, M. B.; Church, B. C.; Kang, Z.; Abernathy III, H. W.; Summers, C. J.; Liu, M.; Sandhage, K. H. Nature 2007, 446, 172. Caruso, R. A.; Antonietti, M. Chem. Mater. 2001, 13, 3272. Ichinose, I.; Lee, S.-W.; Kunitake, T. In Supramolecular Organization and Materials Design; Jones, W., Rao, C. N. R., Eds.; Cambridge Univ. Press: Cambridge, UK, 2002; pp 172. (a) Huang, J.; Kunitake, T. J. Am. Chem. Soc. 2003, 125, 11834 11835. (b) Caruso, R. A. Angew. Chem., Int. Ed. 2004, 43, 2746. Huang, J.; Matsunaga, N.; Shimanoe, K.; Yamazoe, N.; Kunitake, T. Chem. Mater. 2005, 17, 3513. Aoki, Y.; Huang, J.; Kunitake, T. J. Mater. Chem. 2006, 16, 292. Huang, J.; Kunitake, T.; Onoue, S. Chem. Commun. 2004, 1008. Weatherspoon, M. R.; Dickerson, M. B.; Wang, G.; Cai, Y.; Shian, S.; Jones, S. C.; Marder, S. R.; Sandhage, K. H. Angew. Chem., Int. Ed. 2007, 46, 5724. (a) Lakshmi, B. B.; Dorhout, P. K.; Martin, C. R. Chem. Mater. 1997, 9, 857. (b) Liu, S. M.; Gan, L. M.; Liu, L. H.; Zhang, W. D.; Zeng, H. C. Chem. Mater. 2002, 14, 1391. Caruso, R. A.; Schattka, J. H.; Greiner, A. Adv. Mater. 2001, 13, 1577.

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[19] (a) Kasuga, T.; Hiramatsu, M.; Hoson, A.; Sekino, T.; Niihara, K. Langmuir 1998, 14, 3160. (b) Chen, Q.; Zhou, W.; Du, G.; Peng, L.-M. Adv. Mater. 2002, 14, 1208. [20] (a) Patzke, G. R.; Krumeich, F.; Nesper, R. Angew. Chem., Int. Ed. 2002, 41, 2446. (b) Xia, Y.; Yang, P.; Sun, Y.; Wu, Y.; Mayers, B.; Gates, B.; Yin, Y.; Kim, F.; Yan, H. Adv. Mater. 2003, 15, 353.

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In: Bio-Inspired Nanomaterials and Nanotechnology Editor: Yong Zhou

ISBN: 978-1-60876-105-0 © 2010 Nova Science Publishers, Inc.

Chapter III

NANO-FABRICATED STRUCTURES FOR BIOMOLECULE ANALYSIS Noritada Kaji1,2,*, Yukihiro Okamoto1,2, Manabu Tokeshi1,2, and Yoshinobu Baba1-5 1

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Department of Applied Chemistry, Graduate School of Engineering, Nagoya University, Nagoya 464-8603, Japan 2 MEXT Innovative Research Center for Preventive Medical Engineering, Nagoya University, Nagoya 464-8603, Japan 3 Plasma Nanotechnology Research Center, Nagoya University, Nagoya 464-8603, Japan 4 Health Technology Research Center, National Institute of Advanced Industrial Science and Technology, Takamatsu 761-0395, Japan 5 Institute for Molecular Science, National Institute of Natural Science, Okazaki 444-8585, Japan

ABSTRACT In this chapter, the recent development of biomolecule analysis, especially biomolecule separation using nano-fabricated structures was reviewed. Fundamental fabrication techniques for micro- and nano-structures on silicon or glass substrates, various approaches for biomolecule separation based on different separation mechanisms, and typical applications such as DNA separation will be included, and practical applications such as DNA separation are described from the aspect of ―nanomaterials and nanotechnology for bio-analytical chemistry‖.

*

Corresponding author: Noritada Kaji, e-mail: [email protected], Fax: +81-52-789-4666; Tel: +81-52789-4611.

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3.1. INTRODUCTION Since the concepts of μTAS was demonstrated in 1990 by Manz [1], micro- and nanofabrication technique have been extensively developed in the semiconductor and MEMS industries, and now, nanofabricated structures are available for many researchers as a novel analytical tool in μTAS. μTAS contains several elements for the acquisition, pretreatment, separation, post-treatment, and detection of samples. In this multi elements system, microfluidics plays a central role to the development of μTAS because these elements must be able to handle liquid or gas samples and be miniaturized and incorporated onto a single chip. To construct the system, various types of components such as valves, pumps, mixers, filters, and interconnects, are still required to transport, mix, and separate samples. Nanofabricated structures have potentials to inspire the current microfluidics and provide extra functions on the components, and explore understanding of a new field, nanofluidics. The early generations of μTAS performed the functions of large analytical devices in small, often disposable, units. The potential benefits of μTAS include reduced consumption of samples and reagents, shorter analysis times, greater sensitivity, portability that allows in situ and real-time analysis, and disposability. As a consequence of these potential benefits, there has been considerable interest in the development of μTAS. In the current nanostructures-equipped μTAS, many researches focus on fundamental understanding of nanofluidics as well as their practical applications. A typical example of practical applications of nanostructures-equipped μTAS is periodical nanoslits for DNA separation. The periodical nanoslits consist of periodical deep and shallow channel to fractionate large DNA molecules which have larger gyration radius than the shallow channel. This entropic barrier generates different duration time for large and small DNA molecules and lead to successful separation. This periodical nanoslits offered a new type of sieving matrix for DNA molecules, in which large DNA molecules migrated faster than small DNA molecules under electrophoresis. This type of separation could not be achieved in the past gel or polymer based separations. In this chapter, the recent development of biomolecule analysis especially biomolecule separation using nano-fabricated structures will be reviewed. Fundamental fabrication techniques for micro- and nanostructures on silicon and glass substrates, various approaches for biomolecule separation based on different separation mechanisms, and practical applications such as DNA separation are described from the aspect of ―nanomaterials and nanotechnology for bio-analytical chemistry‖.

3.2. FABRICATION PROCESSES OF MICRO- AND NANOSTRUCTURES FOR BIOMOLECULE ANALYSIS In this chapter, typical fabrication processes for micro- and nanofluidic devices are described according to the substrate materials.

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3.2.1. Silicon Fabrication

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Since silicon is an indispensable component for the semiconductor and MEMS industries, extensive efforts to develop fabrication techniques have been made. These welldeveloped techniques could directly apply to fabrication of micro- and nanofluidic devices for chemical and biological applications. Two typical fabrication processes, standard photolithographic and sacrificial layer process, are introduced in this section. Microfluidic channels and micron-sized structure could be fabricated on a silicon wafer using standard photolithography and reactive ion etching (RIE) technique as shown in Figure 1. First, photoresist is spin coated on a silicon wafer with a thickness of several tens of micrometers. The required patterns are transferred by UV irradiation through pre-patterned photomask. After the post exposure bake is provided if required, the exposed patterns are developed and hard bake is performed to harden the cross-linking structures in the remaining photoresist. The patterned resist was used as an etch mask in the successive process, RIE, which is most widely used as an anisotropic dry etching process. Through-wafer fluid access holes to the microfluidic channels are then fabricated in precise locations by laser micromachining, sandblaster, and potassium hydroxide (KOH) etching. After this process, in order to provide an electrical insulation layer, the wafers are oxidized by a thermal oxidation process to grow a silicon dioxide layer approximately 100 to 500 nm. This insulation layer is absolutely necessary especially for electrophoresis experiments. The wafers are then bonded to a transparent glass slide to ensure optical observation by anodic bonding. Some research groups use an adhesive silicone, polydimethylsiloxane (PDMS), which was coated on a glass coverslip about 25-μm thick to seal the microchannel [2, 3]. Polypropylene and PDMS tubes are glued above the access holes for liquid operations.

Figure 1. Typical fabrication process of silicon microfluidic channel by photolithography and reactive ion etching. Oxidation process is for growing insulation layer for the following electrophoresis experiments. Pyrex glass is widely used to seal the microfluidic channel ensuring optical path. (Reproduced from Cabodi, M. et al., Electrophoresis 2002, 23, 3496. With permission)

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The other fabrication method is the use of sacrificial layer process which could integrate the floor and ceiling of the microchannels and nano- and micron-sized structures in a single monolithic design [4, 5]. This fabrication method has advantages over earlier methods, in which the ceiling of the microchannels was created by sealing a glass coverslip to opened microchannel fabricated by the above mentioned standard photolithographic technique. Superior uniformity and precise control of the microchannel height is achieved because this dimension is determined by the thickness of the sacrificial layer. The method is also more tolerant of particulate contamination because the structure is disrupted only in the immediate vicinity of a particle. Since the ceiling layer consists of silicon nitride, optical observations are available through the transparent and the negligible autofluorescent ceiling layer. The process steps are outlined schematically in Figure 2. The silicon wafer is thermally oxidized and the layer of silicon dioxide is grown up to 1.0 μm. After the oxidation, silicon nitride is deposited by low-pressure chemical vapor deposition (LPCVD) and form 190 nm of silicon nitride layer. Polysilicon is subsequently deposited at 500 nm and a 100 nm hard-mask layer is thermally grown in the polysilicon layer. Over this oxide hard-mask layer, 40 nm of aluminum was thermally evaporated to assist in the following pattern transfer and provide a conductive substrate for electron beam lithography (EBL). Polymethylmethacrylate (PMMA) is spin coated about 200 nm over the aluminum as an electron beam resist. EBL is carried out to pattern nano-scaled features with an optimal electron dose. The patterned electron beam resist is developed and transferred to the lower aluminum layer by RIE with chlorine (Cl2), boron trichloride (BCl3), and methane (CH4) gas. The patterned aluminum layer is then used as a mask to etch the next SiO2 hard-mask layer by RIE using CF4 gas. As a final etching step, the pattern of the SiO2 hard-mask layer is transferred to the polysilicon sacrificial layer with a three-step RIE with Cl2, BCl3, and H2 gas. At this point, patterning of the sacrificial layer is completed and the required nano- and micron-scale structures such as nanopillar array and microchannel structures are fully constructed. The open space in the sacrificial layer will become isolated nano- and micron-scale structures in the gap between the top ceiling and floor layer after the top layer deposition and the sacrificial layer removal. Silicon nitride was then deposited by low-stress LPCVD by 320 nm thick and the buried silicon nitride in the sacrificial layer will later form nanopillar array structures inside a microchannel. To remove the sacrificial layer, fluid access holes are fabricated by a photolithography and a CF4 RIE. The sacrificial layer was removed by dipping the wafer in a 5% tetramethylammonium hydroxide (TMAH) solution over 30 min. Very low temperature oxide (VLTO) silicon dioxide layer is grown about 2.5 μm over the silicon nitride top layer to reseal the fluid access holes for the sacrificial layer removal. The other fluid access holes for the subsequent separation experiments were fabricated by a photolithography and a CHF3 RIE.

3.2.2. Quarts Fabrication Quartz is one of the most suitable materials for biomolecule analysis because it has excellent optical properties and chemical stability especially at low pH range. Standard silicon process has been basically applied to quartz fabrication so far. However, due to the

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slow etching rate in dry etching process, some modification methods are required especially for etching and bonding process. Three different fabrication processes, standard photolithography, nanoimprint, and focused ion beam (FIB) milling are introduced in this section.

Figure 2. Fabrication process of a sealed monolithic obstacle array using sacrificial layer. (Reproduced from Chou, C. et al., Proc. Natl. Acad. Sci. USA 1996, 96, 13762. With permission)

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When the required structures are over approximately 1.0 μm, standard photolithography and RIE are applicable to quartz substrate as well as silicon. For example, an array of micropillars oriented in a hexagonal lattice whose width was 2 μm and height was 2 μm was fabricated using standard photolithography and RIE [6, 7]. The micropillar arrays were sealed using glass cover slips with a spin coated thin-layer silicone elastomer. To make the silicone elastomer hydrophilic and adhesive to the quartz, the silicone elastomer surface was treated for 1 min in an oxygen plasma. This sealing method through silicone elastomer could be applied for mechanically strong micron-sized pillars but not for more fragile nano-sized pillars. Fluid access holes for buffer solutions and DNA samples were drilled mechanically. When nano-scale and high-aspect-ratio pillar structures are desired to use for separation experiments, more complicated fabrication processes are required including EBL and deep RIE. For example, nanopillar structures with high aspect ratio of 20 were achieved by the following fabrication process [8, 9]; (Figure 3) Thin Cr and Pt layer (~10 nm) were sputtered on the quartz substrate, and then, positive type EB-resist was spin coated about 1.2 m thick on the Cr/Pt layer. The nanopillar pattern was delineated by EBL. To produce high aspect ratio nanopillars, Ni was electroplated into the pattern of nanopillars as a mask for the subsequent SiO2 dry etching using the Cr/Pt layer as cathode in the appropriate nickel sulfate solution. For the microchannel patterning, standard photolithography was employed. The nanopillar and microchannel patterns were etched by deep RIE with a planar-type neutral loop discharge (NLD) plasma of CF4. The surfaces of a quartz cover plate and the patterned plate were treated with a mixed solution of ammonia and hydrogen peroxide and left overnight in contact with each other under pressure at room temperature. After that, they were directly bonded at 1100˚C for 3 hrs without applying pressure. As well as the nanopillar array structures, the nanochannels that could confine a DNA molecule offer novel bio-analytical techniques. The nanochannles that were 100 nm in width with a depth of 200 nm were fabricated on quartz substrate for DNA dynamics study [10] by using the imprinting technique developed by Chou and coworkers [11]. The high-density arrays of nanochannels were fabricated using nanoimprint lithography (NIL). The NIL mold was generated by interferometric lithography (IL) and has 200 nm period gratings over a 100mm-diamter wafer. By using this method, millions of enclosed nanochannels with dimensions smaller than 10 nm have been fabricated. The nanochannels were then bonded with quartz cover plates by a combination of the surface-cleaning protocol(RCA), room-temperature bonding, and annealing at 1000˚C. FIB milling is one of the more recently developed method for a nanochannel fabrication. Nanochannels were prepared by FIB milling after first coating with a 5-nm Au layer on quartz substrate. The gold layer was removed by aqua regia, and then, the device was sealed with a quartz coverslip. After the microchannel fabrication by photolithography and RIE, reservoirs were affixed over the fluid access holes at the ends of the microchannels [12, 13].

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Figure 3. (A) Fabrication process of quartz nanopillar structures. (B-E) Fabricated nanopillar array structures on a quartz substrate before sealing by a cover slip. The scale bars are all 500 nm. (Reproduced from Kaji, N. et al., Anal. Chem. 76, 15, 2004. With permission)

3.2.3. Plastics Fabrication Most microfluidic devices had been fabricated in glass, quartz, or silicon. The microchannels were fabricated in these substrates using photolithography and various etching processes, and then, enclosed by flat substrates using anodic or fusion bonding. Although

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these materials and fabrication methods have been already developed, there are some limitations especially for the rapid development and testing of new concepts in microfluidic devices. The fabrication processes are slow and requires expensive clean-room facilities. The materials are fragile and too expensive for disposable use. In these materials, silicon is relatively easy materials for fabrication but optical and electrical properties are not suitable for certain types of bio-analytical experiments. To compensate these potential defects, plastics are trying to use as an emerging materials for microfluidic devices. Various plastics such as polydimethylsiloxane (PDMS), polymethylmethacrylate (PMMA), and cyclic olefin copolymer (COC), have been applied for microfluidic devices. Here, from the viewpoint of ease of fabrication and disposability, only PDMS device fabrication is described. Negative-type thick photoresist (SU-8) was spin coated onto silicon wafers to create master mold. A microfluidic channel design was printed on a transparent sheet by a highresolution printer and used as a mask in the following photolithography step. After development and post-exposure bake processes, the master molds were placed in a desiccator under vacuum for 2 hrs with a vial containing a few drops of tridecafluoro-1,1,2,2,tetrahydrooctyl-1-trichlorosilane for the surface silanization. This chemical treatment facilitates the removal of the PDMS replica from the master mold. A 10:1 mixture of PDMS prepolymer and curing agent (Sylgard 184) was stirred thoroughly and then degassed under vacuum. The prepolymer mixture was poured onto the master wafer and cured for 1 hr at 65˚C or overnight at room temperature. After curing, the PDMS replica was peeled from the master wafer. For making fluid access holes on the PDMS replica, glass posts were placed on the master wafer before pouring the prepolymer mixture or the cured PDMS replica was punched. To seal the microchannels irreversibly, a PDMS flat plate or a glass plate were contacted immediately after the brief treatment of plasma oxidization. These processes do not require clean room operation except the master molds fabrication, rapid and mass productions are possible. Although there is a size limitation of fabricating structures in this PDMS process (typically 20 μm), transparent and insulating PDMS microchannels are useful for many bioanalytical applications. Novel nanomaterials, which are described in the following section, could be easily packed in the PDMS microchannel and applied for separation experiments.

3.2.4. Nanomaterials Molecular sieving matrices are indispensable for biomolecule separations such as DNA and protein separations. Generally these sieving matrices show high viscosity and it becomes increasingly difficult to load the sieving matrices inside microchannels. To resolve the problem and expand application filed, a new type of nanoparticles called nanoballs was developed [14]. A core-shell type of globular nanoparticles, which was prepared by the multimolecular micellization and subsequent core polymerization of block copolymer of polyethylene glycol(PEG) with polylactic acid(PLA) possessing a methacryloyl group at the PLA end (PEGm-β-PLAn-MA1; Mw(PEG/PLA) = 6,100/4,000, m ≈ 100, n ≈ 40, l ≈ 70) in aqueous medium, was developed. The hydrophobic PLA segments form a spherical core, which is covered by tethered, flexible PEG chains at a fairly high density. The methacryloyl groups located in the particle core were polymerized to form stable core-shell type

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nanospheres having a diameter of 30 nm. The nanospheres have no surface charge, and they have a narrow size distribution and low viscosity in aqueous media (0.94 cP at 1.0%) compared with conventional polymers (e.g., methylcellulose: 8.8 cP at 0.5% and 104 cP at 1.0%), owing to their globular structure. To avoid loading problem of sieving matrices, nanostructures consist of nanomaterials were constructed inside microchannel based on self-assembled mechanism. Self-assembled colloidal arrays [15, 16] and molecular sized cavities interconnected by nanopores [17, 18] have been demonstrated to work as a DNA sieving matrix. In self-assembled colloidal arrays, the colloidal suspension containing monodispersed plain polystyrene (PS) microspheres and silica nanospheres was injected into the microchannel reservoirs. The aqueous solution fills the channels spontaneously, and then, recede of the liquid meniscus accompanying evaporation induced colloidal growth within the microchannels. Before separation experiments, the water in the reservoirs was substituted with the running buffer and equilibrate over 20 min. In molecular sized cavities interconnected by nanopores, a clean glass slip was immersed in an aqueous suspension of negatively charged polystyrene beads. The colloid was deposited on the substrate forming a two-dimensional, hexagonally packed monolayer by slow evaporation under ambient conditions. A 30% acrylamide monomer solution containing 6% bisacrylamide, ammonium persulfate, and tetramethylethylenediamine (TEMED) was introduced into the crystalline lattice and polymerized to form a dense and transparent hydrogel film. PS beads were dissociated in toluene overnight, and finally, a closepacked array of cavities and interconnections of its six nearest neighbors were left behind. The remained gel as shown in Figure 4 was equilibrated in the running buffer and provided for DNA electrophoresis.

3.3. PRACTICAL APPLICATIONS OF MICRO- AND NANOSTRUCTURES In this chapter, practical applications of micro- and nanostructures which were fabricated by above mentioned technologies are described.

3.3.1. Micron-sized Pillars The pioneering work using the device fabricated by microfabrication technique for practical applications was demonstrated by Austin et al [19]. They fabricated the micropillars which are 0.15 μm high, 1.0 μm in diameter, and 1.0 μm spacing on a standard 3 inch diameter silicon wafer. A 0.5 μm SiO2 layer was grown into the surface of the micropillar array for optical observation and electrical insulation. One set of arrays were made with the micropillars on a square lattice, and another set also on a square lattice but with the lattice rotated 45˚. Although the effective pore size of 1.0 μm corresponds to a physically unstable 0.05% agarose gel, they indicated that the micropillar array is capable of length fractionation up to a length of ~100 kbp in a DC field through direct observation of DNA migration in the micropillar array.

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Figure 4. (A) Fabrication process of self-assembled colloidal arrays in a microfluidic channel. (D,E) SEM images of a constructed matrix of 330-nm silica sphere. The scale bars are 200 μm in (D) and 2 μm in (E). (F) SEM image of a hexagonally closed packed 2-μm PS colloidal array. The scale bar is 10 μm. (Reproduced from Zeng, Y. et al., Anal. Chem. 2007, 79, 2289. With permission)

The micropillar array was applied to pulsed-field electrophoresis for long DNA separations [6, 7]. The past pulsed-field electrophoresis in an agarose gel was timeconsuming, typically requiring over 12 hrs, and consumed running buffers more than 1 L. Therefore the micropillar array system was strong candidate to substitute the conventional pulse-field electrophoresis systems. Bakajin et al. demonstrated that T4 (168.9 kbp) and (48.5 kbp) DNAs could be resolved into two clearly separated bands within 10 s by the micropillar array system [7]. This result corresponds to a mass resolution of 6% in 11 min in a 1-cm-long array. In this system, they used entropic focusing method to concentrate and form a thin band of DNA samples for DNA injection at the entrance of the micropillar array. On the other hand, continuous DNA sample injection and separation by pulsed-field electrophoresis were achieved in the DNA prism by Huang et al [6]. In this prism, microfluidic channels surround the micropillar array and connect it to fluid access reservoirs where electric fields are applied. These microfluidic channels provide continuous sample loading and collection ports and create uniform electric fields across the entire micropillar array, whereas conventional pulsed-field electrophoresis systems create uniform electric fields by surrounding the slab gel

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with many electrodes. The prism separated 61-209 kbp DNA molecules in 15 s with 13% resolution.

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Figure 5. The trajectories of different sized DNA fragments, 15 and 33.5 kbp, in the Brownian ratchet array. (Reproduced from Chou, C. et al., Proc. Natl. Acad. Sci. USA 1999, 96, 13762. With permission)

Another important demonstration of micron-sized pillars is Brownian ratchet array which consists of asymmetric micron-sized obstacles. The basic concept is that, by using a regular lattice of asymmetric obstacles to rectify the lateral Brownian motion of the molecules, they follow different trajectories through the device based on the molecular sizes [20]. Compared with conventional gel electrophoresis, one of the advantages of this technique is that sample loading and separation could be achieved continuously as in the DNA prism. In the initial design of the Brownian ratchet array, samples consists of different components were injected in a line stream at the top corner of the device, separated through the array and collected different positions at the bottom edge [4] as shown in Figure 5. This Brownian ratchet achieved a nominal 6% resolution by length of DNA molecules in the size range 15-30 kbp. To produce a fine stream at sample loading, a novel our-of-plane injection scheme was proposed by Cabodi et al [21]. They successfully separated a mixture of T2(164 kbp) and T7(37.9 kbp) coliphage DNA. However, the separation of large DNA molecules in these microfabricated Brownian ratchet array was slow because it totally relies on a diffusion process. Tilted Brownian ratchet array improved the separation resolution and speed by factors of 3 and 10, respectively [22, 23]. Whereas previous Brownian ratchet arrays with no tilt required about 140 min of running time to resolve 48.5 from 164 kbp DNA molecules with resolution 1.4, the same resolution could be achieved in only 14 min at a flow tilt angle of 7.2˚. In this tilted Brownian ratchet array, great reduction of the amount of diffusion required for ratcheting led to faster separation without any loss of resolution.

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As mentioned above, the array of the asymmetric micron-sized obstacles was designed that inherently rely on diffusion for separation. DNA molecules that could be regarded as hydrodynamically equivalent particles have many different migration paths based on different diffusion lengths. Even if the same size of DNA molecule was loaded, the DNA molecule was eluted as a broaden zone by multipath effect such as size-exclusion chromatography. To eliminate this multipath zone broadening, Huang et al. demonstrated a separation process that creates equivalent migration paths for each particle in a mixture [3]. Their separation process uses laminar flow through a periodic array of micron-scale obstacles. The mixture of 0.8-, 0.9-, and 1.0-μm particles was sorted in 40 s with a resolution of 10 nm. Bacterial artificial chromosomes (BAC) of 61 and 158 kbp were separated in 10 min with a resolution of 12%. In this device, the separation is based on a physical displacement, not on a random process such as diffusion. Therefore a much sharper transition occurred when the flow velocity was increased to minimize diffusion effects. In the case of large DNA molecules, higher fields resulted in lower resolution, possibly because of random deformation and stretching of DNA by collisions with the obstacles. Having said that, this separation technique is very useful for relatively rigid spherical particles to achieve faster separation than Brownian ratchet. Using this technique, Davis et al. demonstrated the fractionation of whole blood components and isolation of blood plasma with no dilution [2]. Whole blood components were separated based on their hydrodynamic size, but not their mass, in this device. They successfully separated white blood cells, red blood cells, and platelets form blood plasma at flow velocities of 1000 μm/s and volume rates up to 1 μl/min.

Figure 6. Separation of λ (48.5 kbp) and T4 (165.6 kbp) DNA by the nanopillar chip under DC electric field. (Reproduced from Kaji, N. et al., Anal. Chem. 2004, 76, 15. With permission)

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3.3.2. Nano-sized Pillars An array of nano-sized pillars, nanopillars, was applied for DNA separation by Turner et al. [5] and Kaji et al. [9]. Turner et al. demonstrated that the devices could function as a molecular sieve since they observed a significant mobility difference between two different types of DNA molecules [5]. In the experiments of Kaji et al., the size of pillars and the spacing between pillars are designed as a DNA sieving matrix for optimal analysis of large DNA fragments over a few kbp. DNA fragments ranging from 1 to 38 kbp were separated as clear bands, and furthermore, the mixture of λ (48.5 kbp) and T4(165.6 kbp) DNAs were successfully separated by a 380-μm-long nanopillar channel within only 10 s even under a DC electric field [9] as shown in Figure 6.

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3.3.3. Periodical Nanoslits The microchannel which consists of narrow constrictions and wider regions causes sizedependent trapping of DNA at the onset of a constriction. This process creates electrophoretic mobility differences and enables efficient DNA separation without use of a gel matrix. Surprisingly, longer DNA molecules migrated faster than shorter DNA molecules in this system. This mechanism is explained from the viewpoint of the energy barrier across the narrow constrictions, nanoslits. DNA molecules were entropically trapped at the constriction and escaped with a characteristic lifetime. The difference of mobility comes not from the escaping activation energy barrier, but from the fact that the surface area of a DNA molecule facing the thin slit is different because of the difference in their size. Longer DNA molecules escape faster simply because more monomers are facing the thin slit, and are able to form a beachhead for escape [24-26]. The selectivity of the DNA separation was shown to be dependent on the depth of deep and shallow channel regions, applied electric field, and number of entropic barriers [27]. Recently, Fu et al. expand the application field of the entropic trapping array to shorter DNA and protein separations [28-30]. They showed experimental evidence of the crossover from Ogston-like sieving to entropic trapping, depending on the ratio between nanofilter constriction size and DNA size [29]. The crossover from Ogston sieving to entropic trapping was measured by mobility of DNA of a size ranging from 0.5-8 kbp which corresponds to the gyration radius of 40-220 nm in a 73 nm nanofilter array. Nanofilter arrays with a gap size of 40-180 nm were fabricated and applied for SDSprotein complexes [30]. Separations of SDS-protein complexes and non-denatured proteins are also demonstrated in a microfabricated anisotropic sieving structure consisting of a twodimensional periodic nanofluidic filter array [28] as shown in Figure 7. This device successfully demonstrated high-resolution continuous-flow separation of a wide range of DNA molecules from 50 bp to 23 kbp and proteins from 11 kDa to 400 kDa in just a few minutes.

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Figure 7. Whole design of a microfabricated anisotropic sieving structure consisting of a twodimensional periodic nanofluidic filter array for DNA and protein separations. (Reproduced from Fu, J. et al., Nat. Nanotech. 2007, 2, 121. With permission)

3.3.4. Nanochannels When the polymer that is freely coiled in solution is confined in a nanochannel, a confinement elongation is observed. In the nanochannel, self-avoidance increases the scaling exponent for the contour length because the polymer is prevented from back-folding. Tegenfeldt et al. extended genomic-length molecules sized over 1 Mbp in arrays of imprinted

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nanochannels and measured the length of the extended DNA molecules directly at a single molecule level [10] as shown in Figure 8. They demonstrated that nanochannel-based measurements of DNA length have several advantages over current electrophoresis-based techniques. Using the nanochannel array with diameters of 100-200 nm, restriction mapping of DNA molecules by restriction endonucleases was demonstrated by Riehn et al. [13]. Combining DNA introduction into the nanochannels by electrophoresis and the formation of concentration gradient of magnesium ion (Mg2+) and EDTA in the nanochannels by diffusion, the restriction reactions could be observed at desired locations in the nanochannels. The measurement of the positions of restriction sites were achieved with a precision of 1.5 kbp in 1 min. Wang et al. demonstrated the direct imaging of GFP-LacI repressor proteins bound to bacteriophage λDNA containing a 256 tandem lac operator insertion using the extended DNA molecules in nanochannels [12]. The number of bound protein molecules was counted by using an integrated photon molecular counting method, and then, the locations of the bound protein were determined.

Figure 8. Imprinted nanochannels and the extended DNA molecules in the nanochannels at a single molecule level. (Reproduced from Tegenfeldt, J. et al., Proc. Natl. Acad. Sci. USA 2004, 101, 10979. With permission) Zhou, Yong. Bio-Inspired Nanomaterials and Nanotechnology, Nova Science Publishers, Incorporated, 2009. ProQuest Ebook Central,

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Although the above mentioned nanochannels were fabricated by nanoimprint lithography and FIB milling, soft lithography based on elastomer replicas was used for fabrication of relatively large-scale nanochannels, ~1 μm. This relatively large nanochannel was successfully applied for DNA stretching by using low-ionic-strength buffers. Enzymatic labeling of specific sequences on elongated DNA molecules inside the nanochannels was imaged via fluorescence resonance energy transfer by Jo et al. [31].

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3.3.5. Nanomaterials A microchannel filled with novel nanomaterials, which were fabricated by ―bottom-up approach‖ based on organic chemistry and polymer synthesis, could be also regarded as a new type of nano-fabricated structures in microchannels. A typical example of nanomaterial for DNA separation is so-called nanoball, core-shell type nanospheres, in microchip electrophoresis14. DNA fragments up to 15 kbp were successfully separated within 100 s without observing any saturation in migration rates. DNA fragments migrate in the medium while maintaining their characteristic molecular structure. Although this medium requires little complicate procedures combining pressure and electric field for sample injection, lowviscosity and high-resolution medium could be widely applicable to simple microchannels. Molecular sized cavities interconnected by nanopores were used to investigate confinement effects on long DNA molecules [17, 18]. In this study, they indicated the possibility of DNA separation by the cavities. Self-assembled colloidal arrays, which have contradictory structures of the above mentioned cavities, demonstrated their separation ability as a sieving matrix. The flexibility of pore size enabled by this methodology provided separation of biomolecules with a wide size distribution, ranging from proteins (20-200 kDa) to dsDNA (0.05-50 kbp) [16]. Under moderate electric fields, complete separation can be finished in minutes, with separation efficiency comparable to gel/polymer-filled or micro/nanofabricated microsystems. To realize continuous separation and collection, a two dimensional colloidal self assembly bed surrounded by multiple microchannels was prepared. High-throughput separation of 2-50 kbp DNA was achieved by this 2D microsystem.

3.4. CONCLUDING REMARKS As seen in this chapter, nanofabricated structures allows biomolecule separations based on novel principles such as entropic trapping and pillar arrays that is difficult to achieve using the past gel or polymer based separation. Precise control of nanospaces, which are produced by nanofabricated structures, explores fundamental understanding of nanofluidics and polymer dynamics in confined space. Single molecule approaches such as the nanochannel array approach might be an alternative to achieve high-throughput analytical systems beyond the current μTAS. For construction of integrated systems from cell lysis to biomolecule analysis on a single chip, combining microfluidics and nanofluidics depending on analytes‘ sizes and features are essential. The continued successful development of μTAS

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including nano-fabricated structures is a promising effort to disclose a new insight of chemistry and biology.

REFERENCES [1] [2] [3] [4] [5] [6] [7] [8] [9]

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[10]

[11] [12] [13] [14] [15] [16] [17] [18] [19] [20] [21] [22]

Manz, A.; Graber, N.; Widmer, H. M. Sensors and Actuators B: Chemical 1990, 1, 244. Davis, J. A.; Inglis, D. W.; Morton, K. J.; Lawrence, D. A.; Huang, L. R.; Chou, S. Y.; Sturm, J. C.; Austin, R. H. Proc. Natl. Acad. Sci. U S A 2006, 103, 14779. Huang, L. R.; Cox, E. C.; Austin, R. H.; Sturm, J. C. Science 2004, 304, 987. Chou, C. F.; Bakajin, O.; Turner, S. W.; Duke, T. A.; Chan, S. S.; Cox, E. C.; Craighead, H. G.; Austin, R. H. Proc Natl Acad Sci U S A 1999, 96, 13762. Turner, S. W.; Perez, A. M.; Lopez, A.; Craighead, H. G. J. Vacuum Sc. Technol. B 1998, 16, 3835. Huang, L. R.; Tegenfeldt, J. O.; Kraeft, J. J.; Sturm, J. C.; Austin, R. H.; Cox, E. C. Nat Biotechnol 2002, 20, 1048. Bakajin, O.; Duke, T. A.; Tegenfeldt, J.; Chou, C. F.; Chan, S. S.; Austin, R. H.; Cox, E. C. Anal. Chem. 2001, 73, 6053. Ogawa, R.; Kaji, N.; Hashioka, S.; Baba, Y.; Horiike, Y. Jpn. J. Appl. Phys. 2007, 46, 2771. Kaji, N.; Tezuka, Y.; Takamura, Y.; Ueda, M.; Nishimoto, T.; Nakanishi, H.; Horiike, Y.; Baba, Y. Anal. Chem. 2004, 76, 15. Tegenfeldt, J. O.; Prinz, C.; Cao, H.; Chou, S.; Reisner, W. W.; Riehn, R.; Wang, Y. M.; Cox, E. C.; Sturm, J. C.; Silberzan, P.; Austin, R. H. Proc Natl Acad Sci U S A 2004, 101, 10979. Han, C.; Zhaoning, Y.; Jian, W.; Jonas, O. T.; Robert, H. A.; Erli, C.; Wei, W.; Stephen, Y. C. Appl. Phys. Lett. 2002, 81, 174. Wang, Y. M.; Tegenfeldt, J. O.; Reisner, W.; Riehn, R.; Guan, X. J.; Guo, L.; Golding, I.; Cox, E. C.; Sturm, J.; Austin, R. H. Proc Natl Acad Sci U S A 2005, 102, 9796. Riehn, R.; Lu, M.; Wang, Y. M.; Lim, S. F.; Cox, E. C.; Austin, R. H. Proc Natl Acad Sci U S A 2005, 102, 10012. Tabuchi, M.; Ueda, M.; Kaji, N.; Yamasaki, Y.; Nagasaki, Y.; Yoshikawa, K.; Kataoka, K.; Baba, Y. Nat. Biotechnol. 2004, 22, 337. Zeng, Y.; He, M.; Harrison, D. J. Angew. Chem. Int. Ed. Engl. 2008, 47, 6388. Zeng, Y.; Harrison, D. J. Anal. Chem. 2007, 79, 2289. Zeng, Y.; Harrison, D. J. Electrophoresis 2006, 27, 3747. Nykypanchuk, D.; Strey, H. H.; Hoagland, D. A. Science 2002, 297, 987. Volkmuth, W. D.; Austin, R. H. Nature 1992, 358, 600. Duke, T. A. J.; Austin, R. H. Phys. Rev. Lett. 1998, 80, 1552. Cabodi, M.; Chen, Y. F.; Turner, S. W.; Craighead, H. G.; Austin, R. H. Electrophoresis 2002, 23, 3496. Huang, L. R.; Cox, E. C.; Austin, R. H.; Sturm, J. C. Anal. Chem. 2003, 75, 6963.

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[23] Huang, L. R.; Silberzan, P.; Tegenfeldt, J. O.; Cox, E. C.; Sturm, J. C.; Austin, R. H.; Craighead, H. Phys. Rev. Lett. 2002, 89, 178301. [24] Han, J.; Craighead, H. G. Science 2000, 288, 1026. [25] Han, J.; Turner, S. W.; Craighead, H. G. Phys. Rev. Lett. 1999, 83, 1688. [26] Han, J.; Craighead, H. G. J. Vacuum Sci. Technol. A 1999, 17, 2142. [27] Han, J.; Craighead, H. G. Anal. Chem. 2002, 74, 394. [28] Fu, J.; Schoch, R. B.; Stevens, A. L.; Tannenbaum, S. R.; Han, J. Nat. Nanotechnol. 2007, 2, 121. [29] Fu, J.; Yoo, J.; Han, J. Phys. Rev. Lett. 2006, 97, 018103. [30] Fu , J.; Mao, P.; Han, J. Appl. Phys. Lett. 2005, 87, 263902. [31] Jo, K.; Dhingra, D. M.; Odijk, T.; de Pablo, J. J.; Graham, M. D.; Runnheim, R.; Forrest, D.; Schwartz, D. C. Proc. Natl. Acad. Sci. U S A 2007, 104, 2673.

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In: Bio-Inspired Nanomaterials and Nanotechnology Editor: Yong Zhou

ISBN: 978-1-60876-105-0 © 2010 Nova Science Publishers, Inc.

Chapter IV

BIONIC SUPERHYDROPHOBIC SURFACES BASED ON COLLOIDAL CRYSTAL TECHNIQUE Yue Li,1,* Weiping Cai,1,* Guotao Duan1, Sung Oh Cho2 1

Key Laboratory of Materials Physics, Institute of Solid State Physics, Anhui Key Laboratory of Nanomaterials and Nanotechnology, Chinese Academy of Sciences, Hefei, 230031, Anhui, China 2 Department of Nuclear and Quantum Engineering, Korea Advanced Institute of Science and Technology, Daejeon 305-701, Korea

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ABSTRACT Biomimetic research reveals that superhydrophobicity with a self-cleaning effect of a lotus leaf is ascribed to the combination of both a hierarchical micro-/nanostructure on the surface and a low surface-energy material covering the surface. The syntheses of colloidal crystals and micro/nano structured arrays based on the colloidal crystals have been well developed, a lot of ordered micro- or nano- structured arrays can be prepared using the colloidal monolayer templates. These ordered arrays and the colloidal crystals are rough on the surfaces in the micro- or nanoscale, which gives a good chance to create the superhydrophobicity on the sample surfaces. In this chapter, we review the superhydrophobic surfaces based on the various colloidal crystal techniques. As we know, the micro- or nanostructured arrays by colloidal monolayer templates have important applications in photonics, photoelectronic devices etc. This suggests that nanodevices built from these nanostructured arrays could be waterproof and self-cleaning in addition to their special device functions after possessing the superhydrophobicity.

*

Mailing address: Prof. Weiping Cai, Dr. Yue Li/ Institute of Solid State Physics, Chinese Academy of Sciences, P.O.X 1129, Hefei, 230031, Anhui, China Tel: +86-551-5592747, Fax: +86-551-5591343 ; E-mail: [email protected], [email protected]

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4.1. INTRODUCTION Biomimetic research has recently revealed many interesting phenomena of natural organisms, such as self-cleaning effect of a lotus leaf (so-called lotus effect) that removes contamination on its surface and striking adhesive force of a gecko‘s foot [1]. These unique functionalities are attributed to the combination of hierarchical micro- and nanostructures on the surfaces of the natural organisms. Specially, the lotus effect is related to the superhydrophobicity with a water contact angle (CA) larger than 150° and a sliding angle (SA) less than 10°, which is caused by the combined effect of both a hierarchical micro-/ nanostructure on the surface and a low surface-energy material covering the surface. This superhydrophobic property can be widely used for preventing the adhesion of water or snow to windows, antioxidation coating, self-cleaning utensils, and microfluidic devices [2]. Inspired by the lotus effect, various techniques to synthesize bionic superhydrophobic surfaces have been recently developed [3]. Additionally, theoretic investigation about wettability on rough surfaces was also explored and two famous models were established to explain the wettability phenomena on the rough surface. Generally, when a water droplet dips into the pores or grooves of a rough surface, Wenzel gave a quantitative description of the surface wettability as follows [4]:

cos

r

r cos

(1)

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where r is the roughness factor and is defined as the ratio of the total surface area to the projected area on the horizontal plane. r and are the CAs for the film with rough and smooth surface, respectively. For this Wenzel‘s type surface, obviously, increased roughness can enhance the hydrophobicity and/or hydrophilicity of hydrophobic and/or hydrophilic surfaces. When a liquid droplet contacts with a rough surface and is completely lifted up by the roughness features, or it cannot dip into pores or grooves on the rough surfaces, another model was presented by Cassie as the following equation [5]: cos θr = f1 cos θ - f2 ,

(2)

where f1 and f2 (= 1 - f1) are the area fractions of a water droplet in contact with the solid and air on the rough surface, respectively. Obviously, increasing f2 can lead to larger r . It means that the area fraction on pores or grooves in the surface is important to the hydrophobicity for Cassie‘s type surface. According to these two models, one can find that a superhydrophobic surface should be of two features, the enough roughness and low surface free energy which can produce a hydrophobic property on the native flat surface. Generally, the low surface free energy can be obtained by chemical modification using the low surface free energy materials, such as coating with fluoroalkylsilanes or thiol [3].

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Recently, the syntheses of colloidal crystals and micro/nano structured arrays based on the colloidal crystals have been well developed. Plentiful order micro- or nano- structured array films could be synthesized using the colloidal monolayer as a template, including pore arrays, pillar arrays, hierarchical micro/nano structured arrays etc [6]. The surfaces of these ordered arrays and the colloidal crystals are rough at the microscale or nanoscale. It is expected that such ordered structured arrays could show superhydrophobicity. This means that nanodevices built from these nanostructured arrays could be waterproof and self-cleaning in addition to their special device functions if they possess the superhydrophobicity. In this chapter, we review the recent developments of the bionic superhydrophobic surface based on the colloidal crystal, including synthesis and corresponding theoretic analyses, in the following sections.

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4.2. SUPERHYDROPHOBIC SURFACES OF PLASMA-ETCHED COLLOIDAL MONOLAYERS [7] As we know, the typical colloidal monolayer exhibits a hexagonal-close-packed (hcp) arrangement and can supply the well ordered nanoscaled surface roughness. Therefore, it may give a possibility to induce the superhydrophobicity. However, Chen et al. found that the polystyrene (PS) colloidal monolayer with hcp arrangement and PS-sphere size of 440 nm just exhibited hydrophobicity with a water CA of 132°, after coating a gold layer and modification with octadecanethiol, instead of superhydrophobicity as expected [7]. In order to achieve the superhydrophobicity, they reduced the PS sphere size by the oxygen plasma etching, but the periodicity of the PS colloidal monolayer was kept unchanged. By this route, the hcp colloidal monolayer could be changed to the one with a hexagonal-non-close-packed (hncp) arrangement, which is very helpful to induce the superhydrophobicity after chemical modification. For instance, the PS sphere sizes in the colloidal monolayer can be reduced and well controlled from 400 nm to 190 nm by oxygen plasma etching, as shown in Figure 1 [7]. These samples displayed the increasing hydrophobicity with decrease of the PS phere size after modification with thoil. When the sphere size was less than 330 nm, the array would demonstrate the superhydrophobicity. The Cassie`s model can give a good explanation for the water dewetting behavior on these nanostructured films. With reduction of the sphere size, the liquid-solid contact area will decrease if we assume that the water contact line lies on the upper part of the spheres, the decreasing liquid-solid contact area will result in the enhancement of hydrophobicity according to Cassie`s equation. Additionally, they found that the water CA on the double layer PS sphere arrays had larger value than those on the monolayer, which can be attributed to the less defects of the double layer than the monolayer.

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a

b

c

d

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Figure 1. SEM images of size-reduced PS sphere by reactive ion etching and corresponding water droplet shapes on the sample surfaces. The sizes of PS spheres and water CAs are (a) 400 nm, 135°, (b) 360 nm, 144°, (c) 330 nm, 152°, and (d) 190 nm, 168°. Scale bars: 1 m. Reprinted with permission from Ref. 7, Copyright 2004 America Chemical Society.

Figure 2. Fabrication procedure of binary assemblies by consecutively dip-coating CaCO3-PNIPAM particles and silica or PS colloidal spheres in aqueous solution. Reprinted with permission from Ref. 8, Copyright 2005 America Chemical Society. Zhou, Yong. Bio-Inspired Nanomaterials and Nanotechnology, Nova Science Publishers, Incorporated, 2009. ProQuest Ebook Central,

Bionic Superhydrophobic Surfaces Based on Colloidal Crystal Technique a

b

c

d

63

Figure 3. (a) SEM image of a 2 dimensional CaCO3-PNIPAM particle non-close-packing array produced by first-step dip-coating a 0.1 wt% aqueous particle aqueous suspension. (b) SEM image of binary colloidal assemblies by second-step dip-coating a 1.0 wt % 296 nm silica spheres on the CaCO3PNIPAM particle non-close-packing arrays. Low (c) and high (d) magnification SEM images of binary colloidal assemblies by second-step dip-coating a 2.0 wt% 296 nm silica sphere aqueous suspension on a CaCO3-PNIPAM particle non-close-packing array. Reprinted with permission from Ref. 8, Copyright 2005 America Chemical Society.

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4.3. SUPERHYDROPHOBIC SURFACES FROM BINARY COLLOIDAL ASSEMBLY [8] In order to obtain the superhydrophobic surfaces, enough roughness on the surface is very important. Wang et al. prepared binary structures with hierarchical roughness by twostep consecutive dip-coating (Figure 2). They first synthesized the CaCO3-loaded poly (Nisopropylacrylamide) spheres (denoted as CaCO3-PNIPAM in the following) by in situ mineralization of CaCO3 within PNIPAM gel spheres. These CaCO3-PNIPAM microspheres have uniform sizes and can be easily self-assembled into colloidal monolayers with a nonclose-packed arrangement by dip-coating and subsequently drying with effect of the shrinkage of these hydrogel spheres. The silica or PS spheres were fabricated on the substrate with CaCO3-PNIPAM sphere arrays by second-step dip-coating and the binary packed structures would be achieved, as shown in Figure 3. Additionally, these binary colloidal assemblies demonstrated the enhanced mechanical stability after heating treatment, producing a good durability in applications, for example, in self-cleaning surfaces. After modification with low surface free energy molecules, the as-prepared film with binary colloidal structures displayed an enhanced hydrophobicity. The silicon wafer with CaCO3-PNIPAM microsphere non-close-packed arrays took on hydrophobicity with a water CA of 99o, however, the binary colloidal assembly obtained by second-step dip-coating of 1.0 wt % aqueous suspension of 296 nm silica spheres showed the improved hydrophobicity with water CA of 130o and the one by higher concentration (2.0 wt % silica sphere with the same

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64

Yue Li, Weiping Cai and Guotao Duan

size) led to the superhydrophobicity with water CA of 160o. The enhanced hydrophobicity of binary colloidal assembly was ascribed to the increasing roughness compared with CaCO3PNIPAM microsphere non-close-packed arrays. With increase of concentration of silica sphere aqueous suspension, the roughness on the binary structures will increase in some contents, and finally results in the superhydrophobicity on the surface.

4.4. ORDERED POROUS SEMICONDUCTOR ARRAY FILMS

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4.4.1. Tunable Wettability Caused by the Precursor Concentration [9] As we reported [6], ordered pore array films can be prepared by colloidal monolayer templates and their morphologies are closely dependent on the experimental conditions, for example, precursor concentrations. Recently, we have found that the morphologies of ZnO ordered pore arrays can be well controlled by increasing the precursor concentrations. The surface roughness increases with increase of precursor concentration. Therefore, it is expected to use this phenomenon to control the surface wettability. Figure 4 shows ZnO ordered pore arrays with different morphologies prepared by solution dipping method with different precursor concentrations [6f, 9]. These three kinds of surface microstructures correspond to different precursor (zinc acetate) concentrations. At a low precursor concentration (0.3 M, Figure 4a), the pores in the film demonstrate truncated hollow spheres, and the pore sizes are smaller than the diameter of the colloidal sphere of the template. The depth is also smaller than the radius of the template. With increase of precursor concentration to 0.5 M, each pore looks like a hollow hemisphere and the pore size increases to about the PS diameter (Figure 4b). If the concentration is further increased (1.0 M), the pores show a noncircular shape from the top view. The surface morphology exhibits a hierarchical structure, which is composed of close-packed rough wreaths and some small particles with 30 nm in size, as shown in Figure 4c and d. The wettability of the as prepared ZnO porous arrays with different surface morphologies was by measured the water CA by carefully dropping water droplets upon their surface in a dark chamber. Figure 5 (a-c) gives the photographs of the water droplets on different films and the corresponding water CAs are 125o, 131o, and 143o, respectively. These indicate that such ordered porous structures can effectively increase the hydrophobicity in comparison with relatively flat ZnO films (CA, 109o). Moreover, the wettability shows clearly dependence on surface microstructures, which is determined by the precursor concentration. Moreover, after chemical modification with low surface energy materials, fluoroalkylsilanes, the above-mentioned three samples demonstrate superhydrophobicity and the corresponding water CAs increase to 152o, 156o, and 165o, respectively, as shown in Figure 5 (d-f), where the water droplets display rather spherical shapes.

Zhou, Yong. Bio-Inspired Nanomaterials and Nanotechnology, Nova Science Publishers, Incorporated, 2009. ProQuest Ebook Central,

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a

b

c

d

65

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Figure 4. SEM images of ZnO ordered pore array fims prepared by solution dipping method using different precursor concentration: (a) 0.3 M, (b) 0.5 M, (c d) 1.0 M. Reprinted with permission from Ref. 9, Copyright 2005 Elsevier.

Figure 5. Water droplet shapes on ZnO pore arrays and the corresponding static CAs on the as-prepared samples (a–c), corresponding to Figure 4a–c; (d–f) show the water droplets and CAs on the same samples after chemical modifications, corresponding to (a–c), respectively. Reprinted with permission from Ref. 9, Copyright 2005 Elsevier.

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66

Interestingly, the water droplets on the surfaces of the modified samples shown in Figure 4 a and b did not slide even when the surfaces are almost tilted vertically. But, slightly tilted (less than 5o), the water droplets on the modified surface of Figure 4 c rolled off quickly. This phenomena can be described more accurately by the dynamic CAs, advancing CA ( A ) and receding CA (

R

). The dynamic CAs of the modified ordered pore array films fabricated by

1.0 M precursor are too small to be measured (sliding angle, 150°) with a small H (less than 9°), which corresponds to Cassie

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surface. After modification with lower surface free energy materials, air can be trapped into such ordered porous film. For the modified samples, since the parameter f1 decreases with the precursor concentration, corresponding water CA also increases. With increase of precursor concentration, CA hysteresis decreases for both the asprepared and modified samples, possibly due to the gradual reduction of the adhesive forces between water droplets and films induced by the concentration-dependent roughness. When the precursor concentration is not high (say, 0.3 M or 0.5 M), although the modified samples are of superhydrophobicity, water droplets on their surface do not roll off when the films are tilted. This is mainly attributed to the continuous, stable three-phase (air-liquid-solid) contact line for such net-like ordered pore array films. However, when the precursor concentration is high enough (say, 1.0 M), the surface of the ordered porous film is much rougher and shows hierarchical structure (Figure 4 c-d), which is similar to that of the well-known lotus leaves, leading to superhydrophobicity with large water CA (165°) and small sliding angle (less than 5°). This film could be expected to show self-cleaning effect.

4.4.2. Controlled Superhydrophobicity Based on Structural Periodicities [10] Ordered indium oxide pore array films with different morphologies were prepared by soldipping method using the PS colloidal monolayers as templates. These porous films took on

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hydrophilicity. However, after modification with fluoroalkylsilanes, all of these pore array films displayed superhydrophobicity due to rough surface and low surface free energy materials on their surfaces. Interestingly, with increase of the pore size in the films, the superhydrophobicity could be controlled and was gradually enhanced due to the corresponding increase of roughness caused by nanogaps produced by the thermal stress in the annealing process with increase of film thickness. The ordered pore array films could be fabricated on the substrate after removal of the PSs and annealing at 400 oC in air for 1 hour. Figure 6 shows the SEM images of macropore array films prepared using the colloidal monolayer with different PS sphere sizes ((a), 1 m; (c), 2 m; (e), 4.5 m). Macropores exhibit the orderly hexagonal arrangement, which corresponds well to the colloidal monolayer template. When the PS sphere size is 1 m in the colloidal template, the honeycomb structured, ordered indium oxide macropore array films with hexagonal alignment are fabricated by above-mentioned method, as shown in Figure 6 a and b. If the size of PS sphere increases to 2 m, the pore array film also can be formed and pore shapes are close to hollow hemispheres (Figure 6c and d). However, most walls between the two neighboring pores cracked and many nanogaps were produced, as clearly shown in the magnified image (Figure 6d). Further increasing the PS sphere size to, say, 4.5 m, the macropore shapes were seriously deformed and changed from hollow hemisphere shape to irregular shape. Additionally, many nanogaps with big size also were produced on the walls between the two neighboring pores, as shown in Figures 6 e and f. In the annealing process of film materials, some thermal stress will be generally produced between materials and their supporting substrates. With increase of the film thickness, the influence of thermal stress on the film morphology is becoming much serious, and even the rupture. In this work, with increase of PS sphere size in the colloidal monolayer template, the thickness of macropore array film fabricated using such template will correspondingly increase. When the PS sphere size is relatively small (1 m), the height or thickness of pore array film is about half of sphere size and the thermal stress has nearly no influence on pore array films in the annealing process at 400 oC for 1 hour, and so morphology of the pore array film corresponds perfectly to that of the colloidal monolayer template and exhibits hexagonally arranged regular pores. However, when the PS sphere diameter is of 2 m, which leads to the thicker pore array film, the thermal stress begins to play an important role in formation of pore film and produce many nanogaps on the pore walls. If the thickness of pore array film is further increased by the colloidal template with 4.5 m in sphere size, influence of thermal stress become more serious, resulting in the deformed pore shapes and nanogaps on the pore walls. In addition, for the pore array film, it belongs to a kind of rough surface in essence. According to these results, one can clearly see that roughness of pore array film increases with increase of the PS sphere size in the colloidal monolayer templates due to the effect of thermal stress. Before modification with fluoroalkylsilane, all samples took on the hydrophilicity (water CA