Electrospinning: Nanofabrication and Applications 0128134410, 9780128134412

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Electrospinning: Nanofabrication and Applications

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Electrospinning: Nanofabrication and Applications Edited by Bin Ding Innovation Center for Textile Science and Technology, Donghua University, Shanghai, China

Xianfeng Wang Key Laboratory of Textile Science & Technology, Ministry of Education, College of Textiles, Donghua University, Shanghai, China; Innovation Center for Textile Science and Technology, Donghua University, Shanghai, China

Jianyong Yu Innovation Center for Textile Science and Technology, Donghua University, Shanghai, China

Elsevier Radarweg 29, PO Box 211, 1000 AE Amsterdam, Netherlands The Boulevard, Langford Lane, Kidlington, Oxford OX5 1GB, United Kingdom 50 Hampshire Street, 5th Floor, Cambridge, MA 02139, United States Copyright © 2019 Elsevier Inc. All rights reserved. No part of this publication may be reproduced or transmitted in any form or by any means, electronic or mechanical, including photocopying, recording, or any information storage and retrieval system, without permission in writing from the publisher. Details on how to seek permission, further information about the Publisher’s permissions policies and our arrangements with organizations such as the Copyright Clearance Center and the Copyright Licensing Agency, can be found at our website: www.elsevier.com/permissions. This book and the individual contributions contained in it are protected under copyright by the Publisher (other than as may be noted herein). Notices Knowledge and best practice in this field are constantly changing. As new research and experience broaden our understanding, changes in research methods, professional practices, or medical treatment may become necessary. Practitioners and researchers must always rely on their own experience and knowledge in evaluating and using any information, methods, compounds, or experiments described herein. In using such information or methods they should be mindful of their own safety and the safety of others, including parties for whom they have a professional responsibility. To the fullest extent of the law, neither the Publisher nor the authors, contributors, or editors, assume any liability for any injury and/or damage to persons or property as a matter of products liability, negligence or otherwise, or from any use or operation of any methods, products, instructions, or ideas contained in the material herein. Library of Congress Cataloging-in-Publication Data A catalog record for this book is available from the Library of Congress British Library Cataloguing-in-Publication Data A catalogue record for this book is available from the British Library ISBN: 978-0-323-51270-1 For information on Elsevier publications visit our website at https://www.elsevier.com//books-and-journals

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Contents Contributors ........................................................................................................................................ xix

PART 1 FUNDAMENTALS OF ELECTROSPINNING CHAPTER 1 Introduction and Historical Overview .................................................. 3 Aijaz Ahmed Babar, Nousheen Iqbal, Xianfeng Wang, Jianyong Yu and Bin Ding 1.1 Introduction.................................................................................................................3 1.1.1 Nanofabrication: The Road to Excellence....................................................... 3 1.1.2 Potential Applications of One-Dimensional Nanomaterials ........................... 3 1.1.3 One-Dimensional Nanofabrication Techniques ............................................... 5 1.2 Electrospinning ...........................................................................................................7 1.2.1 Overview........................................................................................................... 7 1.2.2 History of Electrospinning ............................................................................... 8 1.2.3 Modern Electrospinning Technology............................................................. 11 1.3 Nanofibers: Solving Global Issues ...........................................................................12 1.4 Outlook......................................................................................................................14 Acknowledgments.............................................................................................................15 References.........................................................................................................................15

CHAPTER 2 Electrospinning: The Setup and Procedure ....................................... 21 Yun-Ze Long, Xu Yan, Xiao-Xiong Wang, Jun Zhang and Miao Yu 2.1 Basic Electrospinning Setup and Procedure ............................................................21 2.1.1 Applied Voltage .............................................................................................. 22 2.1.2 Tip-to-Collector Distance............................................................................... 23 2.1.3 Solution Flow Rate......................................................................................... 24 2.1.4 Other Parameters ............................................................................................ 24 2.2 Modification of Electrospinning Setup: Collector ...................................................25 2.2.1 Rotating Collectors......................................................................................... 26 2.2.2 Other Rotating Collectors .............................................................................. 27 2.2.3 Static Collectors ............................................................................................. 29 2.2.4 Precision-Deposited Collectors ...................................................................... 30 2.3 Modification of Electrospinning Setup: Spinneret...................................................31 2.3.1 Coaxial Spinneret ........................................................................................... 32 2.3.2 Gas-Assistant Spinneret ................................................................................. 33 2.3.3 Two-Component Spinneret............................................................................. 33 2.3.4 Pointed-Tip Spinneret..................................................................................... 34 2.3.5 Centrifugal Electrospinning Spinneret........................................................... 35 2.3.6 Reciprocating-Type Spinneret........................................................................ 36 2.3.7 Multineedle and Tube Spinnerets .................................................................. 37

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2.4 Portable Electrospinning Setup ................................................................................38 2.4.1 Plug-Dependent Portable Electrospinning Setup........................................... 38 2.4.2 Battery-Operated Portable Electrospinning Setup ......................................... 40 2.4.3 Other Portable Electrospinning Setups .......................................................... 42 2.5 Industrial Electrospinning Setups.............................................................................45 Acknowledgments.............................................................................................................47 References.........................................................................................................................47

CHAPTER 3

Nanofibrous Materials ........................................................................ 53 Zezhou Yang, Ce Wang and Xiaofeng Lu

3.1 Electrospun Polymeric Nanofibers ...........................................................................53 3.1.1 Electrospun Natural Polymeric Nanofibers ................................................... 53 3.1.2 Electrospun Synthetic Polymeric Nanofibers ................................................ 60 3.2 Electrospun Inorganic Nanofibers ............................................................................62 3.2.1 Electrospun Oxide Nanofibers ....................................................................... 62 3.2.2 Carbon-Based and Nitrogen-Based Nanofibers ............................................. 66 3.2.3 Metal Nanofibers ............................................................................................ 68 3.2.4 Multicomponent Inorganic Nanofibers .......................................................... 70 3.3 Electrospun Polymer/Inorganic Composite Nanofibers ...........................................71 3.3.1 Polymer/Metal Composite Nanofibers........................................................... 71 3.3.2 Polymer/Metal Oxide Composite Nanofibers................................................ 75 3.3.3 Polymer/Carbon-Based Composite Nanofibers ............................................. 78 3.3.4 Polymer/Metal Sulfide Composite Nanofibers .............................................. 79 3.3.5 Other Polymer/Inorganic Composite Nanofibers........................................... 80 3.4 Conclusions ...............................................................................................................80 References.........................................................................................................................80

CHAPTER 4

Nanofibrous Structures ....................................................................... 93 Seema Agarwal, Shaohua Jiang and Andreas Greiner

4.1 Introduction...............................................................................................................93 4.2 Nano over Micro, Meso, and Macro ........................................................................95 4.3 Production Methods of Nanofibrous Structures .......................................................96 4.3.1 Bottom-Up Approaches.................................................................................. 96 4.3.2 Top-Down and Special Approaches............................................................. 103 4.3.3 Nanostructures by Electrospinning .............................................................. 106 4.4 Utility of Nanofibrous Structures ...........................................................................117 References.......................................................................................................................119

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PART 2 NANOFABRICATION STRATEGIES FROM ELECTROSPINNING CHAPTER 5 Coaxial Electrospinning.................................................................... 125 Nu¨ Wang and Yong Zhao 5.1 Introduction.............................................................................................................125 5.2 The Structures of Various Fibers............................................................................127 5.2.1 CoreeShell Structure ................................................................................... 127 5.2.2 Hollow Structure .......................................................................................... 131 5.2.3 Side-by-Side Electrospinning....................................................................... 140 5.2.4 Multifluidic Electrospinning ........................................................................ 147 5.2.5 Electrospray Microcapsules ......................................................................... 160 5.3 Applications of Coelectrospun Fibers ....................................................................166 5.3.1 Drug Release ................................................................................................ 166 5.3.2 Tissue Engineering ....................................................................................... 169 5.3.3 Batteries ........................................................................................................ 172 5.3.4 Supercapacitors............................................................................................. 178 5.3.5 Photocatalysts ............................................................................................... 180 5.3.6 Other Applications ....................................................................................... 185 5.4 Conclusion and Future Perspective ........................................................................195 References.......................................................................................................................195

CHAPTER 6 Multineedle Electrospinning ............................................................ 201 Jianxin He and Yuman Zhou 6.1 Introduction to the Multineedle Electrospinning System ......................................201 6.1.1 Principle of Multineedle Electrospinning .................................................... 201 6.1.2 Necessary Conditions for Multineedle Electrospinning .............................. 202 6.2 Multineedle Electrospinning Modes.......................................................................202 6.2.1 Linear Increase in Needle Number .............................................................. 202 6.2.2 Design of Arrangement of Positions for Multiple Needles......................... 203 6.2.3 Improved Multineedle Electrospinning........................................................ 204 6.3 Large-Scale Multineedle Electrospinning ..............................................................209 6.4 Diversified Product Forms of Multineedle Electrospinning ..................................211 6.4.1 Nanofiber Composite Membrane Prepared by Multineedle Electrospinning............................................................................................. 211 6.4.2 Preparation of a Nanofiber Yarn Based on Multineedle Electrospinning............................................................................................. 213 6.5 Conclusion ..............................................................................................................216 References.......................................................................................................................216

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CHAPTER 7

Needle-Less Electrospinning............................................................ 219 Guilong Yan, Haitao Niu and Tong Lin

7.1 Introduction.............................................................................................................219 7.2 Needle-Less Electrospinning With Motionless Spinnerets....................................221 7.2.1 Magnetic Fluid ............................................................................................. 221 7.2.2 Bubble Spinneret .......................................................................................... 221 7.2.3 Cleft Spinneret.............................................................................................. 223 7.2.4 Stepped Pyramid........................................................................................... 224 7.2.5 Wire Spinnerets ............................................................................................ 224 7.2.6 Bowl.............................................................................................................. 227 7.2.7 Plate Edge..................................................................................................... 227 7.2.8 Slit or Slot .................................................................................................... 228 7.2.9 Porous Tube .................................................................................................. 228 7.3 Needle-Less Electrospinning with Moving Spinnerets..........................................230 7.3.1 Cylinder or Drum ......................................................................................... 230 7.3.2 Ball................................................................................................................ 230 7.3.3 Beaded Chain ............................................................................................... 232 7.3.4 Disk............................................................................................................... 232 7.3.5 Coil ............................................................................................................... 233 7.3.6 Cone.............................................................................................................. 233 7.3.7 Wire Frame ................................................................................................... 234 7.3.8 A Summary of Needle-Less Electrospinning Spinnerets ............................ 234 7.4 Needle-Less Electrospinning Enhanced by External Force Fields........................235 7.4.1 Centrifugal Force.......................................................................................... 235 7.4.2 Airflow .......................................................................................................... 236 7.5 Needle-Less Electrospinning Technique Disclosed in Patents ..............................236 7.6 Needle-Less Electrospinning Machines .................................................................242 7.7 Issues Associated with Needle-Less Electrospinning ............................................244 7.8 Summary .................................................................................................................244 References.......................................................................................................................244

CHAPTER 8

Electronetting.................................................................................... 249 Shichao Zhang, Hui Liu, Ning Tang, Jianyong Yu and Bin Ding

8.1 Introduction.............................................................................................................249 8.2 Electronetting Nanotechnology ..............................................................................250 8.2.1 Origin and Definition ................................................................................... 250 8.2.2 Basic Setup ................................................................................................... 251 8.3 Nanofiber/Nanonet Membranes ..............................................................................252 8.3.1 Formation Mechanisms ................................................................................ 252 8.3.2 Structural Properties ..................................................................................... 258 8.3.3 Polymers Used in Electronetting ................................................................. 260

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8.4 Effects of Various Parameters on Electronetting ...................................................264 8.4.1 Solution Properties ....................................................................................... 264 8.4.2 Process Parameters ....................................................................................... 269 8.4.3 Ambient Parameters ..................................................................................... 270 8.5 Application of Nanofiber/Nanonet Membranes .....................................................271 8.5.1 Sensor Application ....................................................................................... 271 8.5.2 Air Filtration................................................................................................. 274 8.5.3 Tissue Engineering ....................................................................................... 275 8.5.4 Other Applications ....................................................................................... 275 8.6 Concluding Remarks and Perspectives ..................................................................276 Acknowledgments...........................................................................................................276 References.......................................................................................................................276

CHAPTER 9 Near-Field Electrospinning ............................................................... 283 Gaofeng Zheng, Jiaxin Jiang, Dezhi Wu and Daoheng Sun 9.1 Introduction.............................................................................................................283 9.2 Mechanism and Jet Behavior of Near-Field Electrospinning................................284 9.2.1 Near-Field Electrospinning With a Solid-Tip Spinneret ............................. 284 9.2.2 Continuous Near-Field Electrospinning With a Hollow Spinneret ............. 286 9.2.3 Control Theory of Near-Field Electrospinning............................................ 287 9.3 Electrohydrodynamic Direct Writing Based on Near-Field Electrospinning........288 9.3.1 MicroeSilk Ribbon Electrohydrodynamic Direct Writing ......................... 288 9.3.2 Drop-on-Demand Electrohydrodynamic Printing........................................ 290 9.3.3 Near-Field Electrospray ............................................................................... 291 9.3.4 Alternating Current Electrohydrodynamic Direct Writing.......................... 291 9.3.5 Multinozzle Electrohydrodynamic Direct Writing ...................................... 293 9.3.6 Mechanoelectrospinning Direct Writing...................................................... 294 9.3.7 Sheath GaseAssisted Electrohydrodynamic Direct Writing ...................... 296 9.3.8 Tip-Induced Electrohydrodynamic Direct Writing...................................... 297 9.4 Electrohydrodynamic Direct-Write Micro/Nanostructures....................................299 9.4.1 Direct-Write Orderly Nanofibers ................................................................. 299 9.4.2 Direct-Write Nanofibrous 2D Patterns......................................................... 300 9.4.3 Direct-Write 3D Structures .......................................................................... 302 9.5 Applications of Near-Field Electrospinning...........................................................304 9.5.1 Nanofluidic Chips......................................................................................... 305 9.5.2 Sensors.......................................................................................................... 305 9.5.3 Energy Generators ........................................................................................ 307 9.5.4 Biotissues...................................................................................................... 308 9.5.5 Flexible Electronics...................................................................................... 310 9.5.6 Optical Devices ............................................................................................ 312 9.6 Summary and Future Work ....................................................................................312 Acknowledgments...........................................................................................................314 References.......................................................................................................................314

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CHAPTER 10

Centrifugal SpinningdHigh Rate Production of Nanofibers......... 321 Chen Chen, Mahmut Dirican and Xiangwu Zhang

10.1 10.2 10.3 10.4

Introduction...........................................................................................................321 A Brief History of Centrifugal Spinning .............................................................322 Fiber Formation ....................................................................................................324 Centrifugal Spinning System................................................................................325 10.4.1 Rotating Spinning Head ...........................................................................325 10.4.2 Nanofiber Collecting System ...................................................................326 10.5 Types of Materials for Centrifugal Spinning .......................................................327 10.5.1 Polymer Fibers .........................................................................................327 10.5.2 Carbon Fibers ...........................................................................................327 10.5.3 Ceramic Fibers .........................................................................................329 10.6 Effect of Processing Parameters on Centrifugally Spun Fiber Structure ............329 10.6.1 Spinning Fluid Properties.........................................................................329 10.6.2 Centrifugal Spinning Processing Conditions ...........................................330 10.7 Application of Centrifugal Spinning Products.....................................................331 10.7.1 Biomedical Applications ..........................................................................331 10.7.2 Energy Storage Applications....................................................................333 10.8 Electrocentrifugal Spinning..................................................................................334 10.9 Summary ...............................................................................................................336 References.......................................................................................................................337

CHAPTER 11

Melt Electrospinning ...................................................................... 339 Weimin Yang, Haoyi Li and Xiaoqing Chen

11.1 Introduction...........................................................................................................339 11.2 Historical Perspective ...........................................................................................340 11.3 Principles of Melt Electrospinning.......................................................................343 11.3.1 Melt Viscosity...........................................................................................343 11.3.2 Load Voltage.............................................................................................343 11.3.3 Jet Characteristics.....................................................................................343 11.3.4 Fiber Fineness...........................................................................................344 11.3.5 Temperature and Humidity of Environment............................................344 11.4 Process Research and Fiber Diameter..................................................................345 11.4.1 Basic Process Parameters .........................................................................345 11.4.2 Material Characteristics............................................................................345 11.4.3 Airflow Auxiliary Parameters ..................................................................346 11.4.4 Laser Auxiliary Parameters......................................................................346 11.5 Configurations of Melt Electrospinning Setups ...................................................347 11.5.1 Single Needle ...........................................................................................348 11.5.2 Multiple Needles ......................................................................................349 11.5.3 Needle-Less Melt Differential Electrospinning .......................................349

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11.6 Industrial Potential Applications of Melt Electrospinning ..................................354 11.6.1 Oil Sorption ..............................................................................................354 11.6.2 Filtration ...................................................................................................354 11.6.3 Biomedical Sciences ................................................................................355 11.6.4 Textiles......................................................................................................355 11.6.5 Other Applications ...................................................................................355 11.7 Conclusions and Future Perspectives ...................................................................356 References.......................................................................................................................356

PART 3 APPLICATIONS OF ELECTROSPUN NANOFIBERS CHAPTER 12

Electrospun Nanofibers for Air Filtration ....................................... 365 Shichao Zhang, Nadir Ali Rind, Ning Tang, Hui Liu, Xia Yin, Jianyong Yu and Bin Ding

12.1 Introduction...........................................................................................................365 12.2 Electrospun Nanofiber Filters...............................................................................366 12.2.1 Structural and Performance Advantages..................................................366 12.2.2 Filtration Mechanisms..............................................................................367 12.3 Polymeric Nanofiber-Based Filters ......................................................................369 12.3.1 Single-Component Polymer Membranes .................................................370 12.3.2 Composite Polymer Membranes ..............................................................371 12.4 Hybrid Nanofiber-Based Filters ...........................................................................373 12.4.1 Polymer/Organic Nanoparticle Membranes.............................................373 12.4.2 Polymer/Inorganic Nanoparticle Membranes ..........................................375 12.5 Nanofiber/Net-Based Filters .................................................................................377 12.5.1 Nanofiber/Net Membranes .......................................................................377 12.5.2 Nanofiber/Net Membranes with Cavity Structures..................................378 12.5.3 Nanofiber/Net Composite Membranes.....................................................381 12.6 Inorganic Nanofiber-Based Filters .......................................................................383 12.7 Concluding Remarks and Perspectives ................................................................385 Acknowledgments...........................................................................................................385 References.......................................................................................................................385

CHAPTER 13

Electrospun Nanofibers for OileWater Separation ....................... 391 Jianlong Ge, Qiuxia Fu, Jianyong Yu and Bin Ding

13.1 Introduction...........................................................................................................391 13.2 Electrospun Nanofibrous Absorbents for Oil-Spill Cleanup ...............................394 13.2.1 Instinctive HydrophobiceOleophilic Polymeric Nanofibrous Mats .......394 13.2.2 Composite Polymeric Nanofibrous Mats .................................................395 13.2.3 Carbon-Based Porous Nanofibrous Mats .................................................396

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13.3 Electrospun Nanofibrous Filter Membranes for OileWater Separation .............399 13.3.1 HydrophobiceOleophilic Membranes for OileWater Separation ..........399 13.3.2 HydrophiliceOleophobic Membranes for OileWater Separation ..........402 13.3.3 Membranes with Controllable Wetting Ability for OileWater Separation .................................................................................................408 13.4 Electrospun Nanofibrous Aerogels for OileWater Separation............................410 13.5 Conclusions and Future Perspectives ...................................................................412 Acknowledgments...........................................................................................................413 References.......................................................................................................................413

CHAPTER 14

Electrospun Nanofibers for Water Treatment ................................ 419 Cheng Cheng, Xiong Li, Xufeng Yu, Min Wang and Xuefen Wang

14.1 Introduction...........................................................................................................419 14.2 Nanofiber Membranes ..........................................................................................421 14.2.1 Affinity Membranes for Adsorption ........................................................421 14.2.2 Nanofiber Membranes for Microfiltration ...............................................425 14.2.3 Nanofibrous Membranes for Membrane Distillation...............................428 14.3 Nanofiber-Based Composite Membranes.............................................................431 14.3.1 Thin-Film Nanofibrous Composite Membranes for Ultrafiltration .........432 14.3.2 Thin-Film Nanofibrous Composite Membranes for Hemodialysis .........436 14.3.3 Thin-Film Nanofibrous Composite Membranes for Nanofiltration or Forward Osmosis .................................................................................439 14.3.4 Thin-Film Nanofibrous Composite Membranes for Pervaporation.........443 14.4 Conclusions...........................................................................................................444 Acknowledgments...........................................................................................................445 References.......................................................................................................................445

CHAPTER 15

Electrospun Nanofibers for Food and Food Packaging Technology ...................................................................................... 455 Jing Tian, Hongbing Deng, Mengtian Huang, Rong Liu, Yang Yi and Xiangyang Dong

15.1 Introduction...........................................................................................................455 15.2 Electrospinning of Biopolymeric Nanofibers in the Food Industry ....................456 15.2.1 Electrospinning of Polysaccharides .........................................................456 15.2.2 Electrospinning of Proteins ......................................................................468 15.2.3 Nanofibers From Other Naturally Occurring Compounds ......................484 15.3 Electrospinning of Synthetic Polymeric Nanofibers in the Food Industry .........487 15.4 Functionalization of Nanofibers ...........................................................................491 15.4.1 Electrospinning of Polymer Blends .........................................................492 15.4.2 Electrospinning of CoreeShell Structures...............................................493 15.4.3 Inclusion of Nano- and Microstructures and Coating Nanofibers ..........494

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xiii

15.5 Application in Food Packaging Technology........................................................494 15.5.1 Food Preservation .....................................................................................495 15.5.2 Preservation From Other Environmental Hazards...................................500 15.5.3 Preservation of Flavor and Masking of Odor ..........................................501 15.5.4 Delivery of Nutraceuticals and Facilitation of Sustained Release ..........501 15.5.5 Harvest and Analyses of Targeted Analytes............................................502 15.5.6 Carriers for Intelligent Sensors ................................................................503 15.6 Conclusions and Perspectives...............................................................................503 References.......................................................................................................................504

CHAPTER 16

Electrospun Nanofibers for Protein Adsorption ............................. 517 Todd J. Menkhaus and Hao Fong

16.1 Introduction...........................................................................................................517 16.2 Fabrication and Bioseparation Studies of Adsorptive Membranes and Felts Made From Electrospun Cellulose Acetate Nanofibers..............................521 16.2.1 Materials and Methods .............................................................................521 16.2.2 Results and Discussion.............................................................................524 16.2.3 Conclusions ..............................................................................................530 16.3 Surface-Functionalized Electrospun Carbon Nanofiber Mats as an Innovative Type of Protein Adsorption or Purification Medium With High Capacity and High Throughput ............................................................................531 16.3.1 Materials and Methods .............................................................................531 16.3.2 Results and Discussion.............................................................................533 16.3.3 Conclusions ..............................................................................................540 Acknowledgments...........................................................................................................540 References.......................................................................................................................541

CHAPTER 17

Electrospun Nanofibers for Waterproof and Breathable Clothing ........................................................................................... 543 Junlu Sheng, Jing Zhao, Xi Yu, Lifang Liu, Jianyong Yu and Bin Ding

17.1 Introduction...........................................................................................................543 17.2 Electrospun Nanofibers for Waterproof and Breathable Clothing.......................545 17.2.1 Polyurethane Electrospun Membranes.....................................................545 17.2.2 Polyvinylidene Fluoride Electrospun Membranes...................................553 17.2.3 Polyacrylonitrile Electrospun Membranes...............................................555 17.2.4 Other Polymer Electrospun Membranes..................................................561 17.3 Conclusion and Future Trends .............................................................................564 Acknowledgments...........................................................................................................564 References.......................................................................................................................564

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CHAPTER 18

Electrospun Nanofibers for Sensors .............................................. 571 Yan Li, Mohammed Awad Abedalwafa, Liqin Tang, De Li and Lu Wang

18.1 Introduction...........................................................................................................571 18.2 How to Design Electrospun NMeBased Sensing Materials ...............................572 18.2.1 Regulating the Constituent Parts of the Polymeric Solution...................572 18.3 Electrochemical Sensors.......................................................................................579 18.3.1 Metal, Metal Oxide, and Ceramic Nanofiber-Based Electrochemical Sensing......................................................................................................579 18.3.2 Conductive Polymeric Nanofiber-Based Electrochemical Sensing.........582 18.4 Optical Sensors .....................................................................................................582 18.4.1 Colorimetric Sensors ................................................................................582 18.5 Resistive Sensors ..................................................................................................586 18.5.1 Inorganic Nanofibers ................................................................................586 18.5.2 Organic Nanofibers...................................................................................587 18.5.3 Hybrid Nanofibers ....................................................................................588 18.6 Mass-Change-Sensitive Sensors (Quartz Crystal Microbalance Sensors)...........588 18.6.1 Mechanism................................................................................................588 18.7 Summary and Perspectives...................................................................................593 Acknowledgments...........................................................................................................594 References.......................................................................................................................594 Further Reading ..............................................................................................................599

CHAPTER 19

Electrospun Nanofibers for Optical Applications .......................... 603 Jianchen Hu and Ke-Qin Zhang

19.1 Optical Properties of Pristine Electrospun Nanofibers and the Corresponding Optical Applications ....................................................................603 19.2 Optical Properties of Doped Electrospun Nanofibers and the Corresponding Optical Applications .............................................................................................604 19.2.1 Photochromic Molecule Doped Electrospun Nanofibers for the Application of Photoswitching.................................................................605 19.2.2 Doping Electrospun Nanofibers to Form Fluorescent Sensors for Nitroaromatic Explosive Detection ....................................................606 19.2.3 Laser Dye Doped Electrospun Nanofibers for Laser Emission ..............609 19.2.4 Dopant Modifying the End of Polymer Chains to Form Fluorescent Aggregation-Induced-Emission Active Polymers for Detection of Oil Absorption .....................................................................................609 19.2.5 Doping Electrospun Nanofibers With Metal Nanoclusters for Selective Heavy Metal Detection.............................................................610 19.3 Optical Applications of Electrospun Nanofibers with Further Treatment ..........611 19.3.1 Optical Properties of Electrospun TiO2 Nanofibers ................................612

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19.3.2 Electrospun GaN, ZnO Nanofibers, and Application in UV Detectors ............................................................................................612 19.3.3 Electrospun Transparent Electrode for the Application of Solar Cells and Photosensors ...................................................................613 19.3.4 Further Treatment of Electrospun Nanofibers for Construction of Structural Color....................................................................................615 19.4 Summary ...............................................................................................................616 References.......................................................................................................................616

CHAPTER 20

Electrospun Nanofibers for Carbon Dioxide Capture..................... 619 Nousheen Iqbal, Aijaz Ahmed Babar, Ghazala Zainab, Bin Ding, Jianyong Yu and Xianfeng Wang

20.1 Introduction...........................................................................................................619 20.2 Traditional Materials for CO2 Capture ................................................................620 20.2.1 Polymeric Membranes..............................................................................620 20.2.2 Ionic Liquids.............................................................................................621 20.2.3 MetaleOrganic Frameworks ....................................................................622 20.2.4 Amine Sorbents ........................................................................................623 20.2.5 Carbon ......................................................................................................624 20.3 Key Issues of Porous Materials............................................................................625 20.4 Electrospun Nanofibers for CO2 Capture.............................................................626 20.4.1 Electrospun Polymer Membrane for CO2 Capture..................................626 20.4.2 Metal-Organic Framework Incorporated Polymer Nanofibers for CO2 Capture .......................................................................................627 20.4.3 Ionic Liquid-Based Nanofibrous Membranes for CO2 Separation .........630 20.4.4 Electrospun Carbon Nanofibers for CO2 Capture ...................................630 20.5 Concluding Remarks and Outlook .......................................................................635 Acknowledgments...........................................................................................................636 References.......................................................................................................................637

CHAPTER 21

Electrospun Nanofiber Electrodes: A Promising Platform for Supercapacitor Applications .................................................... 641 Yue-E Miao and Tianxi Liu

21.1 Introduction...........................................................................................................641 21.2 Electrospun Electrochemical Double-Layer Capacitive Nanomaterials for Supercapacitors .....................................................................................................643 21.2.1 Carbon Nanofibers....................................................................................643 21.2.2 CNF-Based Composite Fibers..................................................................646 21.3 Electrospun Pseudocapacitive Nanomaterials for Supercapacitors .....................648 21.3.1 Conducting Polymer Nanofibers ..............................................................648 21.3.2 Metal Oxide Nanofibers ...........................................................................650

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21.4 Electrospun Nanofiber-Based Composite Electrodes for Supercapacitors..........653 21.4.1 CNF-Based Nanocomposites ...................................................................653 21.4.2 Pseudocapacitive Nanofiber/Carbon Additive Nanocomposites .............661 21.5 Conclusions and Outlook .....................................................................................662 Acknowledgments...........................................................................................................662 References.......................................................................................................................663

CHAPTER 22

Electrospun Nanofibers for Lithium-Ion Batteries ......................... 671 Yunyun Zhai, Haiqing Liu, Lei Li, Jianyong Yu and Bin Ding

22.1 Introduction to Lithium-Ion Batteries ..................................................................671 22.2 Electrospun Nanofiber Anodes.............................................................................672 22.2.1 Carbon Nanofiber-Based Anodes.............................................................672 22.2.2 Silicon-Based Nanofiber Anodes .............................................................674 22.2.3 Metal Oxide Nanofiber Anodes ...............................................................676 22.3 Electrospun Nanofiber Cathodes ..........................................................................677 22.3.1 Lithium Transition Metal Oxide Nanofiber Cathodes.............................677 22.3.2 Transition Metal Oxide Nanofiber Cathodes ...........................................678 22.4 Electrospun Nanofiber Separators ........................................................................680 22.4.1 Electrospun Polymer Nanofiber Separators .............................................681 22.4.2 Electrospun Polymer/Inorganic Nanofiber Separators.............................686 22.5 Conclusions and Outlook .....................................................................................689 Acknowledgments...........................................................................................................689 References.......................................................................................................................689

CHAPTER 23

Electrospun Nanofibers for Catalysts............................................. 695 Ping Lu, Simone Murray and Min Zhu

23.1 Introduction...........................................................................................................695 23.2 Methods for Preparing Nanofibrous Catalysts .....................................................696 23.2.1 Encapsulation Through Electrospinning Process.....................................696 23.2.2 Postelectrospinning Deposition................................................................698 23.3 Electrospun Nanofibers as Catalysts ....................................................................700 23.3.1 Polymer Nanofibers as Catalysts..............................................................700 23.3.2 Metal Nanofibers as Catalysts..................................................................700 23.3.3 Oxide Nanofibers as Catalysts .................................................................701 23.4 Catalysts Supported on Electrospun Nanofibers..................................................702 23.4.1 Biocatalysts on Nanofibers.......................................................................702 23.4.2 Metal Nanoparticles on Nanofibers .........................................................706 23.4.3 Oxide Nanoparticles on Nanofibers .........................................................710 23.4.4 Other Catalyst Nanoparticles on Nanofibers ...........................................712 23.5 Conclusions...........................................................................................................712 References.......................................................................................................................713

CONTENTS

CHAPTER 24

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Electrospun Nanofibers for Tissue Engineering ............................ 719 Xiumei Mo, Binbin Sun, Tong Wu and Dandan Li

24.1 Introduction...........................................................................................................719 24.2 Electrospun Nanofibers for Tendon Tissue Engineering Applications................721 24.2.1 Different Topological Structures of Electrospun Film Scaffolds for Tendon Tissue Engineering ................................................................722 24.2.2 Three-Dimensional Electrospinning Scaffolds for Tendon Tissue Engineering...............................................................................................723 24.2.3 Electrospinning Technology Combined With Textile Structure in Tendon Tissue Engineering..................................................................724 24.3 Electrospun Nanofibers for Vascular Tissue Engineering Applications..............725 24.3.1 Electrospun Single-Layered Vascular Scaffold .......................................726 24.3.2 Electrospun Multilayered Vascular Scaffold ...........................................726 24.3.3 Surface Modification on Electrospun Vascular Scaffold.........................727 24.3.4 Electrospun Vascular Scaffold Loaded With Drugs or Growth Factors.......................................................................................................727 24.4 Electrospun Nanofibers for Nerve Tissue Engineering Applications..................728 24.4.1 Electrospun Nanofiber Scaffold with Aligned Structure.........................728 24.4.2 Electrospun Nanofiber Scaffold Loaded With Growth Factors...............729 24.4.3 Conductive Electrospun Nanofiber Scaffold and Electrical Stimulation for Nerve Tissue Engineering Applications.........................730 24.5 Conclusion ............................................................................................................731 References.......................................................................................................................731

CHAPTER 25

Electrospun Nanofibers for Drug Delivery ..................................... 735 Mary Stack, Deep Parikh, Haoyu Wang, Lichen Wang, Meng Xu, Jin Zou, Jun Cheng and Hongjun Wang

25.1 Introduction...........................................................................................................735 25.2 Approaches to Incorporating Drugs for Release..................................................736 25.2.1 Blending....................................................................................................736 25.2.2 Adsorption ................................................................................................739 25.2.3 Core-Shell .................................................................................................743 25.2.4 Combination With Other Structures ........................................................746 25.3 Types of Drugs for Release ..................................................................................750 25.3.1 Small Molecules .......................................................................................751 25.3.2 Antibiotics ................................................................................................752 25.3.3 Proteins .....................................................................................................752 25.3.4 Multiple Drugs .........................................................................................752 25.4 Medical Applications of Drug-Eluting Fiber Matrices........................................752 25.4.1 Neural Tissue Engineering .......................................................................753 25.4.2 Vascular Tissue Engineering ....................................................................754

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25.4.3 Skin Tissue Engineering...........................................................................755 25.4.4 Bone Tissue Engineering .........................................................................755 25.4.5 Cartilage Tissue Engineering ...................................................................756 25.5 Future Perspectives ...............................................................................................756 25.6 Conclusion ............................................................................................................757 References.......................................................................................................................757

CHAPTER 26

Electrospun Nanofibers for Enzyme Immobilization...................... 765 Dawei Li, Qingqing Wang, Fenglin Huang and Qufu Wei

26.1 Introduction...........................................................................................................765 26.2 Enzyme Immobilization Strategies for Electrospun Nanofibers .........................766 26.2.1 Surface Attachment of Enzymes on Nanofibers......................................766 26.2.2 Encapsulation Immobilization of Enzymes in Nanofibers ......................770 26.3 Application Fields of Enzymes Immobilized by Electrospun Nanofibers ..........771 26.3.1 Enzyme Membrane Bioreactors...............................................................771 26.3.2 Enzyme Biosensors ..................................................................................772 26.3.3 Water Treatment .......................................................................................774 26.4 Summary and Perspectives...................................................................................779 References.......................................................................................................................779 Index ................................................................................................................................................. 783

Contributors Mohammed Awad Abedalwafa Key Laboratory of Textile Science and Technology, Ministry of Education, College of Textiles, Donghua University, Shanghai, PR China; Department of Technical Textile, Faculty of Industries Engineering and Technology, University of Gezira, Wad Madani, Sudan Seema Agarwal University of Bayreuth, Macromolecular Chemistry and Bavarian Polymer Institute, Bayreuth, Germany Aijaz Ahmed Babar Key Laboratory of Textile Science & Technology, Ministry of Education, College of Textiles, Donghua University, Shanghai, China; State Key Laboratory for Modification of Chemical Fibers and Polymer Materials, College of Materials Science and Engineering, Donghua University, Shanghai, China; Innovation Center for Textile Science and Technology, Donghua University, Shanghai, China Chen Chen Fiber and Polymer Science Program, Department of Textile Engineering, Chemistry and Science, North Carolina State University, Raleigh, NC, United States Xiaoqing Chen College of Mechanical and Electrical Engineering, Beijing University of Chemical Technology, Beijing, China Cheng Cheng State Key Laboratory for Modification of Chemical Fibers and Polymer Materials, Donghua University, Shanghai, People’s Republic of China Jun Cheng Department of Chemistry and Chemical Biology, Stevens Institute of Technology, Hoboken, NJ, United States Hongbing Deng Hubei International Scientific and Technological Cooperation Base of Sustainable Resource and Energy, Hubei Key Lab of Biomass Resource Chemistry and Environmental Biotechnology, School of Resource and Environmental Science, Wuhan University, Wuhan, P. R. China Bin Ding Innovation Center for Textile Science and Technology, Donghua University, Shanghai, China Mahmut Dirican Fiber and Polymer Science Program, Department of Textile Engineering, Chemistry and Science, North Carolina State University, Raleigh, NC, United States Xiangyang Dong Hubei International Scientific and Technological Cooperation Base of Sustainable Resource and Energy, Hubei Key Lab of Biomass Resource Chemistry and Environmental Biotechnology, School of Resource and Environmental Science, Wuhan University, Wuhan, P. R. China

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Hao Fong Department of Chemistry and Applied Biological Sciences, South Dakota School of Mines and Technology, Rapid City, SD, United States Qiuxia Fu Key Laboratory of Textile Science and Technology, Ministry of Education, College of Textiles, Donghua University, Shanghai, China Jianlong Ge Key Laboratory of Textile Science and Technology, Ministry of Education, College of Textiles, Donghua University, Shanghai, China Andreas Greiner University of Bayreuth, Macromolecular Chemistry and Bavarian Polymer Institute, Bayreuth, Germany Jianxin He Provincial Key Laboratory of Functional Textile Materials, Zhongyuan University of Technology, Zhengzhou, China Jianchen Hu National Engineering Laboratory for Modern Silk, College of Textile and Clothing Engineering, Soochow University, Suzhou, Jiangsu, PR China Fenglin Huang Key Laboratory of Eco-Textiles, Ministry of Education, Jiangnan University, Wuxi, Jiangsu, China Mengtian Huang Hubei International Scientific and Technological Cooperation Base of Sustainable Resource and Energy, Hubei Key Lab of Biomass Resource Chemistry and Environmental Biotechnology, School of Resource and Environmental Science, Wuhan University, Wuhan, P. R. China Nousheen Iqbal State Key Laboratory for Modification of Chemical Fibers and Polymer Materials, College of Materials Science and Engineering, Donghua University, Shanghai, China; Key Laboratory of Textile Science & Technology, Ministry of Education, College of Textiles, Donghua University, Shanghai, China Jiaxin Jiang Department of Instrumental and Electrical Engineering, Xiamen University, Xiamen, China Shaohua Jiang College of Materials Science and Engineering, Nanjing Forestry University, Nanjing, China Dandan Li State Key Lab for Modification of Chemical Fibers & Polymer Materials, College of Chemistry, Chemical Engineering and Biotechnology, Donghua University, Shanghai, China Dawei Li Key Laboratory of Eco-Textiles, Ministry of Education, Jiangnan University, Wuxi, Jiangsu, China De Li Key Laboratory of Textile Science and Technology, Ministry of Education, College of Textiles, Donghua University, Shanghai, PR China

CONTRIBUTORS

Haoyi Li College of Mechanical and Electrical Engineering, Beijing University of Chemical Technology, Beijing, China; State Key Laboratory of Organic-Inorganic Composites, Beijing, China Lei Li College of Biological, Chemical Sciences and Engineering, Jiaxing University, Jiaxing, China Xiong Li State Key Laboratory for Modification of Chemical Fibers and Polymer Materials, Donghua University, Shanghai, People’s Republic of China Yan Li Key Laboratory of Textile Science and Technology, Ministry of Education, College of Textiles, Donghua University, Shanghai, PR China Tong Lin Institute for Frontier Materials, Deakin University, Geelong, Victoria, Australia Haiqing Liu College of Biological, Chemical Sciences and Engineering, Jiaxing University, Jiaxing, China Hui Liu Key Laboratory of Textile Science & Technology, Ministry of Education, College of Textiles, Donghua University, Shanghai, China; Innovation Center for Textile Science and Technology, Donghua University, Shanghai, China Lifang Liu Key Laboratory of Textile Science & Technology, Ministry of Education, College of Textiles, Donghua University, Shanghai, China Rong Liu Hubei International Scientific and Technological Cooperation Base of Sustainable Resource and Energy, Hubei Key Lab of Biomass Resource Chemistry and Environmental Biotechnology, School of Resource and Environmental Science, Wuhan University, Wuhan, P. R. China Tianxi Liu State Key Laboratory for Modification of Chemical Fibers and Polymer Materials, College of Materials Science and Engineering, Innovation Center for Textile Science and Technology, Donghua University, Shanghai, PR China Yun-Ze Long Collaborative Innovation Center for Nanomaterials & Devices, College of Physics, Qingdao University, Qingdao, China; Industrial Research Institute of Nonwovens & Technical Textiles, Qingdao University, Qingdao, China Ping Lu Department of Chemistry and Biochemistry, Long Island University, Brooklyn, NY, United States Xiaofeng Lu Alan G. MacDiarmid Institute, College of Chemistry, Jilin University, Changchun, PR China Todd J. Menkhaus Department of Chemical and Biological Engineering, South Dakota School of Mines and Technology, Rapid City, SD, United States

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Yue-E Miao State Key Laboratory for Modification of Chemical Fibers and Polymer Materials, College of Materials Science and Engineering, Innovation Center for Textile Science and Technology, Donghua University, Shanghai, PR China Xiumei Mo State Key Lab for Modification of Chemical Fibers & Polymer Materials, College of Chemistry, Chemical Engineering and Biotechnology, Donghua University, Shanghai, China Simone Murray Department of Chemistry and Biochemistry, Long Island University, Brooklyn, NY, United States Haitao Niu Institute for Frontier Materials, Deakin University, Geelong, Victoria, Australia Deep Parikh Department of Biomedical Engineering, Stevens Institute of Technology, Hoboken, NJ, United States Nadir Ali Rind Key Laboratory of Textile Science & Technology, Ministry of Education, College of Textiles, Donghua University, Shanghai, China; Innovation Center for Textile Science and Technology, Donghua University, Shanghai, China Junlu Sheng Key Laboratory of Textile Science & Technology, Ministry of Education, College of Textiles, Donghua University, Shanghai, China Mary Stack Department of Biomedical Engineering, Stevens Institute of Technology, Hoboken, NJ, United States Binbin Sun State Key Lab for Modification of Chemical Fibers & Polymer Materials, College of Chemistry, Chemical Engineering and Biotechnology, Donghua University, Shanghai, China Daoheng Sun Department of Mechanical and Electrical Engineering, Xiamen University, Xiamen, China Liqin Tang Key Laboratory of Textile Science and Technology, Ministry of Education, College of Textiles, Donghua University, Shanghai, PR China Ning Tang Key Laboratory of Textile Science & Technology, Ministry of Education, College of Textiles, Donghua University, Shanghai, China; Innovation Center for Textile Science and Technology, Donghua University, Shanghai, China Jing Tian College of Food Science and Technology and MOE Key Laboratory of Environment Correlative Dietology, Huazhong Agricultural University, Wuhan, P. R. China; Hubei International Scientific and Technological Cooperation Base of Sustainable Resource and Energy, Hubei Key Lab of Biomass Resource Chemistry and Environmental Biotechnology, School of Resource and Environmental Science, Wuhan University, Wuhan, P. R. China

CONTRIBUTORS

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Ce Wang Alan G. MacDiarmid Institute, College of Chemistry, Jilin University, Changchun, PR China Haoyu Wang Department of Chemistry and Chemical Biology, Stevens Institute of Technology, Hoboken, NJ, United States Hongjun Wang Department of Biomedical Engineering, Stevens Institute of Technology, Hoboken, NJ, United States; Department of Chemistry and Chemical Biology, Stevens Institute of Technology, Hoboken, NJ, United States Lichen Wang Department of Biomedical Engineering, Stevens Institute of Technology, Hoboken, NJ, United States Lu Wang Key Laboratory of Textile Science and Technology, Ministry of Education, College of Textiles, Donghua University, Shanghai, PR China Min Wang State Key Laboratory for Modification of Chemical Fibers and Polymer Materials, Donghua University, Shanghai, People’s Republic of China Nu¨ Wang Key Laboratory of Bioinspired Smart Interfacial Science and Technology of Ministry of Education, Beijing Key Laboratory of Bioinspired Energy Materials and Devices, School of Chemistry, Beijing Advanced Innovation Center for Biomedical Engineering, Beihang University, Beijing, PR China Qingqing Wang Key Laboratory of Eco-Textiles, Ministry of Education, Jiangnan University, Wuxi, Jiangsu, China Xianfeng Wang Key Laboratory of Textile Science & Technology, Ministry of Education, College of Textiles, Donghua University, Shanghai, China; Innovation Center for Textile Science and Technology, Donghua University, Shanghai, China Xiao-Xiong Wang Collaborative Innovation Center for Nanomaterials & Devices, College of Physics, Qingdao University, Qingdao, China Xuefen Wang State Key Laboratory for Modification of Chemical Fibers and Polymer Materials, Donghua University, Shanghai, People’s Republic of China Qufu Wei Key Laboratory of Eco-Textiles, Ministry of Education, Jiangnan University, Wuxi, Jiangsu, China Dezhi Wu Department of Mechanical and Electrical Engineering, Xiamen University, Xiamen, China Tong Wu State Key Lab for Modification of Chemical Fibers & Polymer Materials, College of Chemistry, Chemical Engineering and Biotechnology, Donghua University, Shanghai, China

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Meng Xu Department of Chemistry and Chemical Biology, Stevens Institute of Technology, Hoboken, NJ, United States Guilong Yan Institute for Frontier Materials, Deakin University, Geelong, Victoria, Australia Xu Yan Industrial Research Institute of Nonwovens & Technical Textiles, Qingdao University, Qingdao, China Weimin Yang College of Mechanical and Electrical Engineering, Beijing University of Chemical Technology, Beijing, China; State Key Laboratory of Organic-Inorganic Composites, Beijing, China Zezhou Yang Alan G. MacDiarmid Institute, College of Chemistry, Jilin University, Changchun, PR China Yang Yi Hubei International Scientific and Technological Cooperation Base of Sustainable Resource and Energy, Hubei Key Lab of Biomass Resource Chemistry and Environmental Biotechnology, School of Resource and Environmental Science, Wuhan University, Wuhan, P. R. China Xia Yin Key Laboratory of Textile Science & Technology, Ministry of Education, College of Textiles, Donghua University, Shanghai, China; Innovation Center for Textile Science and Technology, Donghua University, Shanghai, China Jianyong Yu Innovation Center for Textile Science and Technology, Donghua University, Shanghai, China Miao Yu Collaborative Innovation Center for Nanomaterials & Devices, College of Physics, Qingdao University, Qingdao, China Xi Yu Key Laboratory of Textile Science & Technology, Ministry of Education, College of Textiles, Donghua University, Shanghai, China Xufeng Yu State Key Laboratory for Modification of Chemical Fibers and Polymer Materials, Donghua University, Shanghai, People’s Republic of China Ghazala Zainab State Key Laboratory for Modification of Chemical Fibers and Polymer Materials, College of Materials Science and Engineering, Donghua University, Shanghai, China; Innovation Center for Textile Science and Technology, Donghua University, Shanghai, China Yunyun Zhai College of Biological, Chemical Sciences and Engineering, Jiaxing University, Jiaxing, China Jun Zhang Collaborative Innovation Center for Nanomaterials & Devices, College of Physics, Qingdao University, Qingdao, China

CONTRIBUTORS

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Ke-Qin Zhang National Engineering Laboratory for Modern Silk, College of Textile and Clothing Engineering, Soochow University, Suzhou, Jiangsu, PR China Shichao Zhang State Key Laboratory for Modification of Chemical Fibers and Polymer Materials, College of Materials Science and Engineering, Donghua University, Shanghai, China; Innovation Center for Textile Science and Technology, Donghua University, Shanghai, China Xiangwu Zhang Fiber and Polymer Science Program, Department of Textile Engineering, Chemistry and Science, North Carolina State University, Raleigh, NC, United States Jing Zhao Key Laboratory of Textile Science & Technology, Ministry of Education, College of Textiles, Donghua University, Shanghai, China Yong Zhao Key Laboratory of Bioinspired Smart Interfacial Science and Technology of Ministry of Education, Beijing Key Laboratory of Bioinspired Energy Materials and Devices, School of Chemistry, Beijing Advanced Innovation Center for Biomedical Engineering, Beihang University, Beijing, PR China Gaofeng Zheng Department of Instrumental and Electrical Engineering, Xiamen University, Xiamen, China Yuman Zhou Provincial Key Laboratory of Functional Textile Materials, Zhongyuan University of Technology, Zhengzhou, China Min Zhu Textile Development and Marketing Department, Fashion Institute of Technology, Manhattan, NY, United States Jin Zou Department of Chemistry and Chemical Biology, Stevens Institute of Technology, Hoboken, NJ, United States

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PART

FUNDAMENTALS OF ELECTROSPINNING

1

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CHAPTER

INTRODUCTION AND HISTORICAL OVERVIEW

1

Aijaz Ahmed Babar1, 2,3, Nousheen Iqbal1, 2, Xianfeng Wang1, 3, Jianyong Yu3, Bin Ding3

Key Laboratory of Textile Science & Technology, Ministry of Education, College of Textiles, Donghua University, Shanghai, China1; State Key Laboratory for Modification of Chemical Fibers and Polymer Materials, College of Materials Science and Engineering, Donghua University, Shanghai, China2; Innovation Center for Textile Science and Technology, Donghua University, Shanghai, China3

1.1 INTRODUCTION 1.1.1 NANOFABRICATION: THE ROAD TO EXCELLENCE Nanofabrication, the technology of the future, is the most advanced manufacturing technology in today’s world. Because this technology lets scientists reach nearly the theoretical limit of accuracy, i.e., the size of a molecule or atom, it is also believed to be the “extreme technology” (Mamalis et al., 2004). Indeed, it is basically the manipulation of matter at the nanoscale, which can develop a variety of materials and devices far superior, in terms of performance, efficiency, and durability, to those produced by conventional processes. This manipulation at the nanoscale alters the material characteristics without compromising the fundamental properties of the substrate and makes them intrinsically different and relatively much better compared with their bulk counterparts (Biswas et al., 2012). In addition, it also meets both of the major demands of the manufacturing industry, i.e., ultraprecision and miniaturization; thus this technology is a roadway to excellence in the field of manufacturing.

1.1.2 POTENTIAL APPLICATIONS OF ONE-DIMENSIONAL NANOMATERIALS Nowadays, nanomaterials are at the center of attention of engineers and scientists because of their ability to alter the performance and capabilities of materials in a number of commercial sectors (Fang et al., 2008, 2011; Zhang and Fang, 2010; Xiao et al., 2011; Wang et al., 2013). Nanomaterials are believed to be at the forefront of the fundamental materials because they provide additional features and aptitudes while maintaining the basic characteristics of the materials. Among all nanomaterials, one-dimensional (1D) nanostructured materials have firmly gained tremendous attraction in recent decades owing to their fundamental features, unique shapes, and potential applications in various fields (Lu et al., 2011; Yuan et al., 2011; Xia et al., 2003). These materials have enough potential to be applied to a very wide range of applications (Fig. 1.1). Their characteristic features, such as high volume-to-surface area, facilitate the production of lighter weight materials, which is one of the key demands of all manufacturing fields; their ability to be Electrospinning: Nanofabrication and Applications. https://doi.org/10.1016/B978-0-323-51270-1.00001-7 Copyright © 2019 Elsevier Inc. All rights reserved.

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FIGURE 1.1 Application potential of 1D nanomaterials. © 2006e14 Royal Society of Chemistry. © 2012 Elsevier. © 2017 John Wiley and Sons. Other sources: www.sigmaaldrich.com, www.greenspec.co.uk, www.pacificbluesolar.com, www.penggagas.com, www.nanodic.com.

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highly hydrophobic and breathable is extremely desired for protective clothing; and their highly porous structure makes them ideal candidates for energy and environmental applications. Hollow fibers with multiple numbers of channels are a value addition in the field of biomedical and tissue engineering. The controlled structures of nanofibers developed from biodegradable and biocompatible sources such as polysaccharides and proteins are very useful in biomedical applications and regulated drug-delivery applications. Moreover, their individual fibers as well as resultant membrane structure can be custom-made to meet the needs of a number of applications. In addition, the growing interest of scientists in using nanofibers in various applications highlights the significance of their potential (Li and Yang, 2016; Kaur et al., 2014), which may also be credited to the easier fabrication with a variety of structural architectures and relatively reasonable production cost.

1.1.3 ONE-DIMENSIONAL NANOFABRICATION TECHNIQUES The 1D nanoscale materials, such as wires, belts, rods, tubes, spirals, and fibers, owe the most vital importance due to their high length-to-width ratio and huge surface area, and can be produced with various commercial fabrication techniques such as template synthesis, drawing, phase separation, selfassembly, hand-spinning, and electrohydrodynamics (EHD) techniques (Lu et al., 2009; Lauhon et al., 2002; Zach et al., 2000; Hu et al., 1999, 2006; Huang et al., 2001; Barth et al., 2010; Xiao et al., 2010; Shojaee et al., 2010; Jiang et al., 2004; Taylor, 1966; Wang et al., 2006, 2008, 2011; Du and Hsieh, 2008; Dzenis, 2004; Sarkar et al., 2010; Kim et al., 2006). The drawing technique of nanofabrication is similar in nature to the traditional dry-spinning technique and capable of producing very long single nanofibers. This technique involves three very simple steps, i.e., (1) a drop of polymer solution nearly 1 mL in volume is placed on the substrate, (2) a micropipette is touched to the edge of the drop, and (3) it is pulled back (Fig. 1.2); this backward motion of the micropipette draws the polymer solution enough to turn it into a nanofiber. The standard speed of the upedown moment of the micropipette is about 104 ms 1, and the diameter of the drawn fiber may range from a few micrometers to several nanometers. Solvent evaporation, polymer nature, and drawing velocity are the parameters that determine the quality of the resultant fibers. The technique is easy to control, is relatively very economical, and also does not require any expert personnel supervision; however, its low production rate and unacceptability for all polymers are major restrictions to its commercial application. A variety of polymeric nanofibers, such as polyvinyl butyl,

FIGURE 1.2 Schematic demonstration of 1D nanofiber fabrication via mechanical drawing. (A) A drop of polymer solution is placed on the substrate, (B) a micropipette is touched to the drop, and (C) the micropipette is pulled back.

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FIGURE 1.3 Schematic demonstration of 1D nanofiber production via template synthesis.

polymethylmethacrylate, polyvinyl alcohol, polycaprolactone, polyethylene oxide, and hyaluronic acid, have been developed via this technique. In addition, nanofibers from melt have also been reported. The template synthesis technique involves nanosized pores and a variety of materials. The solution is forced through a fibril-shaped solid or hollow tubule and is immediately solidified by a solidification solution (Fig. 1.3). The concept of this technique may be credited to the traditional wet spinning technique in which the polymer solution drawn out through spinnerets is directly passed through a trough containing the fiber solidification solution. A variety of materials, including metal oxides (alumina), metals, semiconductors, or carbons, can be processed via this technique for synthesizing membranes for targeted applications, even without any expert supervision. High production times and inability to develop single nanofibers limit its application at the megascale. The phase-separation technique of nanofiber fabrication consists of dissolution, gelation, and extraction using a suitable solvent, followed by freezing and drying techniques. A polymer solution in a Teflon trough is converted into a gel with the help of heat treatment and this resultant gel is dried via a freeze-drying process (Fig. 1.4). Polymer concentration and gelation temperature mostly affect the duration of gel. Low and high gelation temperatures lead to the formation of nanoscale fiber networks and platelet-like structures. Fabricated nanofibers are 50e500 nm in diameter and have a porous

FIGURE 1.4 Schematic demonstration of 1D nanofiber development via the phase-separation technique. Reprinted with permission from He, C., Nie, W., Feng, W., 2014. Engineering of biomimetic nanofibrous matrices for drug delivery and tissue engineering. Journal of Materials Chemistry B 2, 7828e7848. © 2014 Royal Society of Chemistry.

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structure with a network of “endless” filaments. The type of polymer, type of solvent, gelation temperature, gelation duration, and thermal treatment also affect the nanofibers’ morphology. This technique is simple, inexpensive, and widely used for the fabrication of nanofibers. It makes one by one continuous nanofibers, and mass production is also possible through this technique. However, it suffers some major limitations such as a time-consuming process, laboratory-scale production, lack of structural stability, and difficulty in maintaining porosity and is not applicable for all polymers (He et al., 2014).

1.2 ELECTROSPINNING 1.2.1 OVERVIEW Electrospinning is simple, and is potentially the most effective and advanced EHD technique being used for the production of continuous fibers with diameters down to a few nanometers. It shares the characteristic features of two conventional processes, i.e., electrospraying and conventional dry or melt spinning. The process involves the use of high voltage for inducing the formation of a liquid jet, which is soon solidified either by evaporating the solvent or by freezing the melt to ensure nanofiber fabrication (Fig. 1.5). This versatile process can be applied to natural as well as synthetic polymers, polymer alloys, metals, and ceramics (Greiner and Wendorff, 2007). A variety of fibrous architectures, such as porous fibers, coreeshell fibers, hollow fibers, and helical fibers, etc. can be produced by the

FIGURE 1.5 (A) Schematic illustration of the basic electrospinning setup. (B) Schematic drawing of the looping part of the jet showing a sequence of bending instabilities. PPENK, poly(phthalazinone ether nitrile ketone). (A) Reprinted with permission from Wang, G., Zhang, H., Qian, B., Wang, J., Jian, X., Qiu, J., 2015. Preparation and characterization of electrospun poly(phthalazinone ether nitrile ketone) membrane with novel thermally stable properties. Applied Surface Science 351, 169e174. © 2015 Elsevier. (B) Reprinted with permission from Reneker, D.H., Yarin, A.L., 2008. Electrospinning jets and polymer nanofibers. Polymer, 49, 2387e2425. © 2008 Elsevier.

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electrospinning technique using certain special tools. Moreover, the process is capable of producing a diverse range of single-fiber structures as ordered arrangements of fibers. Depending on the physical, biological, or chemical attributes, fibers produced using electrospinning are of great interest for a range of applications, including filtration, biomedicine, sensors, protective clothing, and so on. Since the late 1990s, the electrospinning process has not only been intensively reexamined by laboratories to ensure its acceptability at the megascale, but has also been extensively applied in industry (Thavasi et al., 2008; Dong et al., 2011; Bhardwaj and Kundu, 2010).

1.2.2 HISTORY OF ELECTROSPINNING Electrospinning, also known as electrostatic spinning, is not a very new but yet is a very powerful nanofiber fabrication technique. It is versatile enough to produce fibers on micro- and nanoscales and is believed to be a variant of the electrospray process. The first record claiming the electrostatic attraction of liquids is traced back to the 16th century, reported by Gilbert, the president of the Royal College of Physicians (Wendorff et al., 2012; Tucker et al., 2012). He claimed that if a properly charged piece of amber and a water droplet are brought close enough to each other, the latter would form a cone shape, and small droplets would be ejected from the tip of the cone. About 270 years later, Bose synthesized aerosols using high electric potentials in 1745 (Lin et al., 2012; Greiner and Wendorff, 2007). Later on, in 1882, Rayleigh calculated the maximum amount of charge that causes a liquid drop of definite size to burst by overcoming the surface tension of the droplet. He also explained that the stability of an ascending liquid jet would first increase with increasing electric charge; however, when the electric charge exceeds a certain limit, then it will destabilize the liquid jet (Wang, 2008; De Vrieze and De Clerck, 2009). In the early 20th century, Morton and Cooley demonstrated the phenomenon of the electrospinning process and discovered the possibility of fabricating tiny fibers via electrospinning, and in addition, they first patented the devices using electric charge to spray liquids: four types of indirectly charged spinning heads, a conventional head, a coaxial head, an air-assisted model, and a spinneret featuring a rotating distributor, were proposed (Ding and Yu, 2014; Morton, 1902; Cooley, 1902, 1903). W.B. Wiegand and E.F. Burton further described the relationship between charge and surface tension to examine electrical effects on water streams (Burton and Wiegand, 1912; Wang et al., 2006). John Zeleny, a physicist working at the University of Minnesota, published a series of papers between 1907 and 1920 describing electrical discharge from liquid as well as solid surfaces. He determined that the diameter of the electrodes was the primary factor, rather than the shape of the electrodes, that influenced the discharge current. He also analyzed the effect of humidity and concluded that an increase in humidity required more potential to maintain the predefined current flow. Later on, he examined fluid droplet behavior at the end of metal capillaries and determined the distortion tendency of a hemispherical liquid droplet under high voltage, which helped in developing the mathematical model for determining fluid behavior under electrostatic forces. This is also believed to be the initiation of modern needle electrospinning (Lin et al., 2012; Zeleny, 1914, 1917). K. Hagiwara, Professor at Imperial University Kyoto, reported on the use of electricity for regulating the molecular structure of a colloidal liquid viscose precursor that could align colloidal components leading to highly lustrous fibers, i.e., free of irregular aggregation of the particles. In addition to viscose, other substrates, such cellulose acetate and nitrocellulose, gelatin, albumen, and natural silk solutions, were also run through Professor Hagiwara’s equipment (Kiyohiko, 1929). W. A. Macky from

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New Zealand explained that it is the ionized gas or vapor particles that make a flow of current that breaks the liquid drop during flight, rather than the flight of the charged liquid drop (Macky, 1931, 1937). Further progress toward commercialization was made by A. Formhals (United States), who published a series of 22 patents from 1931 to 1944, which are believed to be a key contribution to the development of electrospinning (Anton, 1934). Anton explained the physical setup for producing polymer filaments using electrostatic force. He intended to develop yarns by gathering up the fibers for further processing, which was a very critical job. Initially, Anton designed a machine having a sawtoothed rotating fiber emitter dipped in a polymer solution trough (Formhals, 1934). Fibers were emitted from the wetted tips with the help of an electric charge toward the rotating collector. Later on, Anton planned tapered nozzleeshaped fiber emitters and aimed to collect short staple fibers; staple fibers with controlled length were produced by disrupting the current flow to the spinning heads of the machine (Formhals, 1937; Anton, 1938). In addition, Anton also proposed cospinning of fibers with opposite charge to produce a product with no net charge, and made serious efforts to devise winding devices to gather up the fiber in a usable form (Anton, 1939, 1940, 1943). Charles Ladd Norton, an American physicist, who had experience of powering Crookes tubes with high voltage, was the first to introduce melt spinning using a combined electrostatic and air-jet assist method, and also made efforts to prepare lofted fibers for insulation or packaging applications (Williams, 1940). Following this, Games Slayter produced glass wool fiber using melt spinning, which was later commercialized by the Owens Corning fiberglass company and was used by naval ships for fire protection (Slayter, 1938). In the 1930s, N.A. Fuchs (USSR) and his coworkers introduced the theory of ultrafine fibrous materials and developed electrospun fibers for filter materials. For this contribution they were awarded the Stalin Prize. Based on their work, a factory was installed to fabricate electrospun fibers for gas masks using cellulose acetate and a solvent mixture of ethanol and dichloromethane. In the 1950s, using the Petryanov filter, a particulate filter mask, the “Lepestok,” was developed for the nuclear industry. B. Vonnegut (Vonnegut and Neubauer, 1952) and V. Drozin (Drozin, 1955; Vonnegut and Neubauer, 1952) investigated liquid jet production under electrostatic force and observed that uniform-sized droplets of about 1 mm were formed during the process, which repelled one another in the case of having like charges. The generation of these droplets was attributed to the dielectric constant of fluid droplets; moreover, dipole moment, conductivity, and refractive index were observed as process-limiting factors. The output of spun filtration materials had reached as much as 20 million m2 per annum by the 1960s. Polymer substrate was pushed through spinnerets under the exceptionally high voltage of 100 kV, and the resulting liquid stream was bifurcated, leading to high volume throughput. It was then Sir Geoffrey Ingram Taylor who made a significant advancement in the theoretical underpinning of the electrospinning process during 1964e65. Taylor designed a mathematical model of the conical shape of the fluid droplet in an electric field. This characteristic droplet shape is now known as the Taylor cone. In 1971 Baumgarten found that fiber diameter depends on process and substrate parameters such as solution viscosity, jet radius and length, and applied voltage. Taylor also reported, with the help of his devised method for photographing electrospun fibers during flight, that only a single fiber is spun at a time and that the filament forms many loops, which fall to the electrical ground. In addition, he described the electrospinning of acrylic, whereas Larrondo and S.J. Manley published a series of articles on the subject of electrospinning of polymer melts. They also launched a melt electrospinner, in which a dead weight was used to push the polymer through a barrel to the spinning tip, which was

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then drawn rapidly using electrostatic force. In the meantime, certain efforts in the commercialization of the electrospinning process were also undertaken in the 1970s. A series of patents was submitted by Simm, from the Bayer Company, on the electrospinning of plastics, and the first practical application advised for electrospinning was for the nonwoven industry. The electrospinning process became popular only after the 1990s, when numerous research groups, especially those of Reneker (University of Akron) and Wendorff, picked up the process. It was then established that several organic polymers could be electrospun into nanofibers. Since then, a number of researchers have entered this field and the quantity as well as quality of research papers has exponentially increased, from just a few papers per annum to over 4000 in 2017 (Fig. 1.6). Increasing numbers of patents, books, and review papers about electrospinning applications have been reported in recent years, and over 500 research institutes and universities around the globe working in the field of electrospinning signify its popularity and provide insight into the most prominent aspects of the process. Increasing interest and active involvement of certain commercial companies, such as Nano Technics, eSpin Technologies, Elmarco Ltd., and Kato Tech, witness the huge impact and significance of electrospinning in the field of materials science. Some companies such as Freudenberg and Donaldson Company have been earning significant capital by reaping the benefits of electrospun nanofibers since the late 1990s or even earlier. In short, this rapid and intensive research carried out since the beginning of the 21st century in the field of electrospinning can be summarized as follows: (1) an extensively enhanced number of polymers and composites are being electrospun; (2) the comprehensive in-depth comprehension of nanofiber fabrication has increased; (3) a very broad range of fiber and membrane structures, including

FIGURE 1.6 Number of publications from 2001 to January 24, 2018, with the keyword “electrospinning.” From ISI Web of Science.

1.2 ELECTROSPINNING

11

those inspired from nature, have been made possible; (4) it is possible to develop multicomponent, composite, and inorganic fibers; and (5) the focus of research has been transformed from fabrication to application, and now it is being centered on the industrialization of this process.

1.2.3 MODERN ELECTROSPINNING TECHNOLOGY The versatile maneuverability of electrospinning to produce nanofibers with regulated individual fibers as well as resultant membrane structures, the controlled inter- and intrafiber porosity, and the ability to produce meticulous fiber orientations and dimensions make it a simple but powerful fiber manufacturing technique. In addition, easier process control and lower production costs are potential reasons for its global attention. Randomly oriented structures of fibers obtained from electrospinning are being utilized in numerous fields. However, this random orientation of electrospun nanofibers limits their broader acceptability in the fields of biomedical and electronic devices (Supaphol et al., 2011). Therefore, it is extremely necessary to fabricate nanofibers with controlled fiber structure and ordered fiber orientation via electrospinning to make full use of their potential and enhance their acceptability in electronic devices and biomedical applications, which require well-arranged fiber alignment and special fiber structures (Greiner and Wendorff, 2007). Many groups are engaged in developing aligned fiber arrays produced via electrospinning with the help of custom-made collectors. As a result, various patterned fiber architectures (Fig. 1.7) have been successfully synthesized by various groups with certain process modifications, such as insulation gaps introduced between conducting collectors demonstrated in uniaxially aligned fiber mats (Rasel, 2015). It is also reported that high-speed rotating rollers can also produce aligned fibers; however, the collector rotation speed and fiber orientation strongly influence the resultant nanofiber properties.

FIGURE 1.7 Schematic demonstration of some electrospun nanofiber architectures and their corresponding applications. Reprinted with permission from Wu, J., Wang, N., Zhao, Y., Jiang, L., 2013. Electrospinning of multilevel structured functional micro-/nanofibers and their applications. Journal of Materials Chemistry 1, 7290e7305. © 2013 Royal Society of Chemistry.

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In addition, this approach facilitates the direct integration of tailorable configured nanofibers, which may be a key support for manufacturing nanofiber-based devices (Wang et al., 2013; Woodruff and Hutmacher, 2010; Pan et al., 2008). In addition to aligned or patterned nanofibers, the process is also capable of producing a very wide range of fibrous structures, including coreeshell (Sun et al., 2003; Zhang et al., 2009), tube-in-tube (Mou et al., 2010), multicore cablelike (Hiroshi et al., 2007), rice grain shape (Shengyuan et al., 2011), helical (Kessick and Tepper, 2004; Shin et al., 2006), ribbon-like (Koombhongse et al., 2001), necklace-like (Jin et al., 2010; Lu et al., 2006), multichannel tubular (Zhao et al., 2007), nanowire-inmicrotube (Chen et al., 2010), firecracker shape (Chang, 2011), and hollow (Li and Xia, 2004) fiber structures (Wu et al., 2013). Moreover, the provision of high specific surface area and feasibility to control pore size along with certain chemical, physical, thermal, and mechanical characteristics of fabricated nanofibers indicate the significance of the process (Huang et al., 2003; Zhu et al., 2008). The versatile nature of the electrospinning process and its ability to synthesize numerous fiber structures from all kinds of materials (i.e., organic as well as inorganic polymers or the combination both) and various input forms, including melts, solutions, emulsions, and mixtures, have recommended it for use in different fields ranging from the hottest fields of energy generation, defense, and security to complicated fields like health care, biomedicine, biotechnology, and environmental engineering (Tran et al., 2011). Several modifications in the basic electrospinning process have been made to meet the desired needs of these applications (Fig. 1.8). These modified electrospinning techniques include tipless electrospinning, edge electrospinning, multijet electrospinning, and electroblowing to enhance the throughput rate of the process to render the electrospun nanofibers acceptable on a larger scale. In addition, the enormous amount of research since 2008 has concluded that nanofibers offer huge potential for various fields; however, fiber diameters yet need to be reduced to tens of nanometers ( spinning voltage > spinning distance > melt temperature. Lyons (2004) studied the electrospinning process of the melt, and pointed out that the diameter of the fiber decreased significantly with an increase in the electrical field strength, and the size of the Taylor cone and the diameter of the fiber decreased as the feed rate of the melt decreased. Zhou et al. (2006) found that as the melt temperature increases, the fiber diameter decreases and the fiber diameter distribution becomes narrower. As the electrical field strength increases, the fiber diameter decreases, but the change was not as obvious as with melt temperature. Rangkupan and Reneker (2003) have confirmed these laws in vacuum electrospinning experiments. In melt electrospinning, the viscosity of the melt is an important parameter for evaluating the spinnability and the diameter of the fiber, because the viscosity of the melt is at least 1 order of magnitude higher than that of the solution electrospinning process. It is an effective way to improve the fineness of the fiber by increasing the spinning voltage, increasing the strength of the electrical field, and reducing the melt supply speed, but this requires one to balance the spinning efficiency.

11.4.2 MATERIAL CHARACTERISTICS The modification of materials to reduce melt viscosity has an important effect on fiber refinement. Tian et al. (2009) developed a melt electrospinning system of CO2 laser heating, adding EVOH copolymer to prepared poly(L-lactic acid) fiber, and found that the fiber diameter decreased from 3 to 1 mm. Dalton et al. (2007) prepared a polyethylene glycolepolycaprolactone block copolymer using a selfmade electrospinning device, and the content ratio of the two components had a direct effect on the fiber diameter. Malakhov et al. (2009) used sodium stearate and oleic acid as plasticizers to reduce the viscosity of nylon 6, with 10% of the plasticizer added to reduce the viscosity by 60 times and the fiber diameter by 40 times. Wang and Huang (2010) added diamethylene phthalate to polymethylmethacrylate, and reduced the fiber diameter apparently from 34.0 to 19.7 mm. The molecular weight and molecular chain vertical structure are important factors in determining the viscosity of melt spinning material and thus influencing the fiber fineness. Lyons (2004) observed the effects of polypropylene of different molecular weights on melt spinning in his experiment. He prepared PP fibers with diameters of 466.15, 10.58, 6.92, and 3.55 mm with average molecular

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weights of 580,000, 190,000, 106,000, and 12,000, respectively. To decrease the molecular weight and refine the fiber, Dalton et al. (2007) added different amounts of chain-cutting agent to polypropylene to regulate the molecular weight of the polypropylene. It was found that adding 1.5% (mass fraction) of a Ciba viscosity-reducing agent could decrease the fiber diameter from 6  1.7 mm to 840  190 nm. Fiber fineness is also greatly affected by the polymer melt conductivity. Typically improving the conductivity of a polymer melt can enhance fiber refinement, which has been a focus of research. Nayak et al. (2012) added oleic acid and NaCl to low-viscosity polypropylene to increase the conductivity from 10e9 S/cm to 10e6 S/cm, and the diameter of the fiber reduced from about 4 mm to about 0.3 mm, excluding the effect of viscosity changes.

11.4.3 AIRFLOW AUXILIARY PARAMETERS The airflow-assisted melt electrospinning process is mainly manifested in two aspects: one is the friction of the air to accelerate the jet to achieve the purpose of jet refinement; the other is the hot airflow on the jet path temperature control effect, extending the distance of refinement and causing a delay in fiber curing. Zhmayev et al. (2010b) studied the effect of auxiliary airflow on the electrospun fiber refinement of a polylactic acid melt (Fig. 11.1) and found that the airflow-assisted diameter was 10% smaller than that of the fibers without airflow, and 20 times fiber diameter refinement would be attained with hot airflow assitance. In addition, the fiber diameter was reduced with air velocity and gas temperature increases. Yang’s team put forward a gas-assisted melt differential electrospinning device (Liu et al., 2015), and Ma’s experiment demonstrated that the fiber diameter decreased with increase in assisting suction wind speed (Ma et al., 2017).

11.4.4 LASER AUXILIARY PARAMETERS In melt electrospinning, to avoid interference between the heating of the electronic control system and the high-voltage static electricity, researchers have adopted a variety of heating methods. The laser heating method is widely used by researchers, because of its energy concentration, instantaneous melting, lack of interference, and other advantages. This type of setup and related studies have also been classified as laser melt electrospinning (Ogata et al., 2007c; Li et al., 2012b, 2014e; Li et al., 2011; Shimada et al., 2012). Laser power and bar feed speed are two main parameters of laser melt electrospinning; in addition, the effect of voltage size on fiber diameter is different compared with other processes. Takasaki et al. (2008) found that the fiber diameter decreased with increasing laser output power. Ogata et al. (2007a) have shown that the fiber diameter decreases exponentially with increasing output power, but the fiber diameter remained constant when the output power increased to a certain extent. As for applied voltage, Takasaki et al. (2008) concluded that the fiber diameter decreases as the voltage decreases, while some reports (Ogata et al., 2007a; Liu et al., 2010) show that the fiber diameter decreases first with an exponential relationship as the voltage increases, then stays at a fixed value. In addition, studies have shown that as the polymer feed rate increases, the fiber diameter decreases first, and then remains at a fixed value, which is in contrast to other melt electrospinning studies. Some researchers have conducted preliminary composite melt spinning experiments: Li et al. (2012b) studied the melt spinning effect of polyethylene terephthalate/SiO2 blends using laser melt

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electrospinning, and prepared microfine fibers of 500 nme7 mm. Cong et al. (2012) revealed that latent heat goes up to 68 J/g when the main polymer contains a mass fraction of 42.5% PEG 4000.

11.5 CONFIGURATIONS OF MELT ELECTROSPINNING SETUPS A basic setup of electrospinning equipment mainly consists of a melt or solution cavity, a spinning head, a high-voltage electrostatic generator, and a receiving device. The high-voltage electrode of most equipment is connected to the spinning head, and the receiving electrode is grounded through a highvoltage electrostatic field between the spinning head and the receiving device acting on the polymer solution or melt in the micro-continuous supply of the polymer solution or melt. However, when the voltage exceeds a certain value to form the Taylor cone, a continuous jet near the receiving plate is solidified into a wire. The melt electrospinning equipment, which is different from solution electrospinning equipment, is characterized by the addition of a precise plasticizing system and by the electrostatic voltage, which is several times more than that of solution electrospinning. The composition of a basic melt electrospinning setup, shown in Fig. 11.4, mainly consists of a plasticizing system (2, 4), a feed system (1), a flow metering system (3), a receiving device (6), and a high voltage power supply (5). There is a lot of melt viscosity electrostatic spinning device research and innovation. According to whether it contains needles, melt electrospinning equipment is divided into single needle, multiple needles, and free needle (so-called needle-less melt differential electrospinning).

FIGURE 11.4 Basic components of a melt electrospinning device. (1) Feed system, (2, 4) plasticizing system, (3) flow metering system, (5) high-voltage power supply, (6) receiving device, (7) liquid jet.

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11.5.1 SINGLE NEEDLE Single-needle electrospinning in a melt spinning device was the earliest method of melt electrospinning. The basic single needle was proposed by Professor Reneker at the University of Akron in the United States (Fig. 11.5). Then Li and Xia (2004) designed a spinning apparatus at the University of Washington for preparing coreeshell-structured nanofibers (Fig. 11.6). Also, Zhao et al. (2007) from

Metal Electrode

Metal Screen Taylor Cone

Air

HV

Fibers

R

Tip of Capillary Tube

FIGURE 11.5 The basic single-needle electrospinning equipment.

FIGURE 11.6 Coreeshell structure nanofiber spinning equipment. PVP, polyvinyl pyrrolidone.

V

11.5 CONFIGURATIONS OF MELT ELECTROSPINNING SETUPS

(A)

inner fluid

349

(B)

outer fluid

10 µm FIGURE 11.7 Schematic of island structure electrospinning. (A) Schematic of island structure electrospinning device and (B) the picture of resultant fiber by this device.

the Chinese Academy of Sciences constructed an island structure electrospinning setup, which produced nanofibers with a multimodal structure (Fig. 11.7). Yang et al. (2007) introduced a magnetic fieldeassisted electrospinning apparatus to promote fiber alignment (Fig. 11.8).

11.5.2 MULTIPLE NEEDLES Dalton et al. (2007) investigated a melt electrospinning setup that used a hot air assist for indirect heating (Fig. 11.9). Shimada et al. (2010) reported a linear laser source for heating a membrane, and a row of jets was generated along with a line of Taylor cones (Fig. 11.10). The yield was relatively improved compared with a single-needle setup. Lin and colleagues (Fang et al., 2012) introduced a disk as a melt electrospinning device, which was suitable only for low-viscosity polymer melts, as shown in Fig. 11.11.

11.5.3 NEEDLE-LESS MELT DIFFERENTIAL ELECTROSPINNING 11.5.3.1 Principles and Process of Melt Differential Electrospinning Aiming at solving the shortage of traditional equipment using needle-like spinnerets, Yang’s team first proposed a melt differential electrospinning method (Weimin and Haoyi, 2014; Li et al., 2014a, 2014b, 2014d, 2014f) that prepared an ultrafine fiber, through which fiber smaller than 1 mm can be produced and a yield of 10e20 g/h can be achieved, using a needle-less nozzle. The method uses an umbellate spinneret (Liu et al., 2012), avoids plugging of the needles by fluid, and can be maintained cost effectively.

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(A)

(B)

syringe solution needle +

c

liquid jet N

b

V

S

high voltage a N

S

– aluminum foil

FIGURE 11.8 Magnetic fieldeassisted electrospinning equipment. (A) Schematic diagram of magnetic fieldeassisted electrospinning equipment; (B) Schematic diagram of the magnetic field of the magnetic system.

FIGURE 11.9 Hot-air-assisted indirectly heated melt electrospinning equipment.

The melt differential electrospinning technique was inspired by waterfalls (shown in Fig. 11.12) and the overflow phenomenon. The liquid overcomes surface tension and flows down uniformly along the edge of the nozzle when fluid exceeds the plane. Through this method, the problem of nonuniform distribution of melt on a free surface is solved.

FIGURE 11.10 Picture of a linear laser melt electrospinning device.

FIGURE 11.11 Pictures of a disk type melt electrospinning device.

FIGURE 11.12 Picture of a natural waterfall.

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FIGURE 11.13 (Left) Photo of melt differential electrospinning process. (Right) Scanning electron micrograph of spun fibers.

The differential melt electrospinning process (shown in Fig. 11.13) is as follows: the micro-flow melt supplied to the nozzle expands to thin, and the melt forms a uniform distribution of dozens of Taylor cones at the lower end of the nozzle surface in the electrical field force. Then the Taylor cones spray into multijets, and the multijets distribute evenly on the bottom edge of the nozzle. In this technology more than 80 jets are ejected from the edge of the single nozzle, and its spinning efficiency is 500e1000 times that of the capillary electrospinning of solution. The voltage electrode is connected to the melt nozzle and the receiver is grounded, which solves the spinning termination due to the electrical breakdown of the high-voltage metal element or heating element. At the same time, the spinning nozzle design includes the inner cone and outer cone (Yuan and Chung, 2012; Li et al., 2014c) specifications. Yang’s team also put forward a gas-assisted melt differential electrospinning setup (Bubakir et al., 2014) and multilevel electrical field relay drawing methods and equipment to solve the microfluidic refinement problem of a highly viscous polymer. As shown in Fig. 11.13, in the center of the cone nozzle airflow is introduced through an airflow duct. When the multiple cones form at the nozzle tip, the assisting air enables a thinner layer of melt and refined fibers can be obtained.

11.5.3.2 Melt Differential Electrospinning Equipment The single-nozzle melt differential electrospinning device is typically divided into five compartments: (1) heating and plasticizing system, (2) electrostatic generator, (3) micro-flow feeding device, (4) melt flow channel, and (5) needle-less differential nozzle. Shown in Fig. 11.14 is a new needle-less melt differential electrospinning apparatus, which consists of variable high-voltage power supply, a needle-less inner-cone nozzle, a heating device, and a cylinder collector. It possesses two attractive advantages: (1) Yang’s team connected the high voltage to the plate electrode and made the nozzle grounded; thus the feeding device can eliminate the disturbance of high voltage. Furthermore, a direct heating method like the heating coil can be used. (2) It breaks through the limitation of capillary electrospinning, brings forth the new idea of melt differential, and uses a needle-less inner-cone nozzle, which can not only eliminate the defect of low

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FIGURE 11.14 Schematic of differential melt electrospinning.

capillary production and easy blocking, but also refine the fiber diameter. The difference between an inner-cone nozzle and a outer-cone nozzle is the structure of the nozzle. The outer-cone nozzle has a thin melt layer on the outer surface of the cone, while the inner-cone nozzle has a thin melt layer on the inner surface of the cone. Both of these two kinds of nozzles could generate multiple jets evenly on the circular edge when enough high electrical strength was loaded. Yang’s team invented four nozzles to test the equipment and the arrayed-type spinning device prototype. The four-nozzle test device consists of four melt differential nozzles, a microextruder, a laying system, and a hot roller. The output of the device could be controlled in the range of 10e100 g/h. The spinning temperature is in the range of 150 Ce300 C. And the device can achieve continuous production of a 220- to 500-mm-width nonwoven fabric. The arrayed spinning device mainly consists of a feeding section, nozzle array, perforated copper electrode, hot roller continuous reception device, and winding device. The feeding section mainly includes twin screw extruders, an automatic screen changer, and a melt metering pump. The specific parameters are as follows: 1. It is constituted by 32 arrayed differential nozzles and each nozzle could generate over 70 jets. 2. It has a width of 0.8 m, a yield of 300e600 g/h, and the average fiber diameter is 200e1000 nm.

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3. The thickness of the nonwoven fabric is adjustable from 10 to 1000 mm, and the working speed is adjustable from 1 to 10 m/min. 4. It has a continuous melt supply with continuous online blending. 5. It could be expanded to a large production line with output of more than 6 kg/h by modular assembly.

11.6 INDUSTRIAL POTENTIAL APPLICATIONS OF MELT ELECTROSPINNING Owing to its high porosity, high surface area, and special physicochemical properties, nanofiber produced by electrospinning is widely used for tissue scaffolding, air filtration, microsensors, etc. As for melt electrospinning, it is considered to have great potential in the application of efficient filtration, textiles, oil sorption, biomedical sciences, microsensors, and catalyst carriers, owing to the absence of solvent and a fiber surface without holes and other defects. And melt electrospun fibers will be active in various applications in the form of composite fibers.

11.6.1 OIL SORPTION Owing to natural disasters or human-made damage, oil spills in the sea occur frequently. When the oil spill contacts the water, the formation of an oil and water emulsion, or floating film, would seriously affect the marine ecological environment. Synthetic organic oil-absorbing materials and ultrafine-fiber absorbent cotton resins are the most widely studied materials for high oil absorption and have good hydrophobicity, good lipophilicity, and high oil absorption rate and can be reused many times (Deng et al., 2013). Polypropylene fiber (Yuan and Chung, 2012) is widely used. Its fiber fineness and porosity affect the oil absorption rate greatly, and it is an ideal oil absorption material (Li et al., 2014b; Bubakir et al., 2014, 2017). The application of solution electrospun ultrafine fibers in this area confirms this advantage (Lin et al., 2012), but solution electrospinning cannot be carried out at room temperature for polypropylene microfine fibers, while melt electrospinning technology overcomes this insufficiency.

11.6.2 FILTRATION Owing to their high flux, fine porous structure, and resistance during filtering, ultrafine fibers produced by melt electrospinning are becoming more important in filtration, including gas and liquid filtration (Brown et al., 2016; Mitchell and Sanders, 2006; Dalton et al., 2005, 2006; Singer et al., 2012; Praeger et al., 2012; Zhao et al., 2012; Li et al., 2015; Cao et al., 2014; Weiss et al., 2016). Furthermore, physical filtering, which has the advantages of saving space, having a simple process, and being environmentally friendly, has developed rapidly in recent years. Yang’s team (Li et al., 2015, 2014b, 2014f) introduced an air-assisted melt differential electrospinning device for preparing polymer fibers applied in the field of filtration. It was revealed that the fiber membranes of various oriented fibers have a smaller pore size than the randomly oriented fibrous membranes, and filtration efficiency reaches more than 90% for 0.5-mm particles, which is better than the random orientation of fibrous membranes, which have 75% filtration efficiency.

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11.6.3 BIOMEDICAL SCIENCES Melt electrospinning technology is a good choice for cell culture or drug delivery without solvent, thereby eliminating the need for a complex solvent mixing and removal process. It is needed only to ensure that the process environment is a sterile clean space or that the fiber is prepared for simple sterilization postprocessing. The diameters of the melt electrospun fibers are mostly distributed within a few micrometers, and the interfiber pores are in the range of several micrometers to several tens of micrometers. The fibers can be randomly formed or constructed by adhesion or mutual support, which are favorable for growth and maturity. In recent years, tissue engineering has attracted great interest from researchers, and studies on tissue engineering and tissue scaffolds from melt electrospinning are relatively hot (Mitchell and Sanders, 2006; Brown et al., 2012; Li et al., 2012c; Mazalevska et al., 2013; Farrugia et al., 2013; Lou et al., 2016). The Dalton group (Dalton et al., 2005, 2006a, 2006b, 2008; Brown et al., 2011) introduced a direct writing mode in melt electrospinning. Through in vitro studies (Muerza-Cascante et al., 2015), it has been proven that scaffolds designed and fabricated via melt electrospinning writing have prospects for support cell attachment, proliferation, extracellular matrix formation, etc. In addition, some experiments showed that direct in vitro electrospinning with polymer melts matched with cells well, and scaffolds designed for specific tissue regeneration strategies performed superbly. Moreover, Hutmacher and coworkers (Brown et al., 2012) presented the growth of three different cell types in vitro by melt-electrospun tube support, which also provided a great hope for tissue engineering.

11.6.4 TEXTILES Melt electrospun fibers can achieve superhydrophobic or superoleophobic effects because of their smaller diameter, allowing gas to pass freely while preventing the passage of liquid. With modification of the material or the surface features of the fiber membrane, textiles can meet the demands of medical protective clothing or military protective clothing (Dasdemir et al., 2013; Cao et al., 2013; Li et al., 2012a; Walker, 2012); they can also be used for athletic wear as thin-sweat textiles. Lee and Obendorf (2007) prepared polypropylene fiber by melt electrospinning that had high barrier and high air permeability. They found that the barrier properties reached more than 90%, and the air permeability was reduced by less than 20%.

11.6.5 OTHER APPLICATIONS In addition to the aforementioned applications, melt electrospun nanofiber as a catalyst carrier could improve the performance of fibers effectively. Zhang et al. (2014) proposed and prepared functionalized carbon nanofiber anodes containing Fe, in which the Fe precursor functions as both catalyst and sacrificial phases. Mu¨ller et al. (2014) successfully prepared silica nanotube-embedded Cr and postmetallocene Fe, and this blend of Cr and Fe was supported on silica nanotubes to produce meltprocessable polyethylene nanocomposites with bimodal molecular weight distribution. Furthermore, Shahgaldi, et al., (2011) investigated polyvinylidene difluoride doping with iron oxide by adding the iron oxide nanoparticles as a catalyst. The result revealed that the characteristics of the carbon fiber for hydrogen storage were improved.

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11.7 CONCLUSIONS AND FUTURE PERSPECTIVES Melt electrospinning enables the preparation of ultrafine fibers in the absence of residual solvents, avoiding a series of problems such as spinning process complexity and product safety, providing opportunities in the areas of controllable tissue scaffolds, sensors, and biomaterials. It has gradually become an important process for green manufacturing of thermoplastic polymers into continuous nanofibers. However, it still has some drawbacks and challenges, such as lower throughput, more complex apparatus, and high cost. 1. Melt electrospinningeenabled fiber diameter is relatively large to some extent. Although the fiber diameters achieved are under 1 mm, they are mostly still above 300 nm. Through optimization, the process can achieve controllable batch preparation of fibers in the hundreds of nanometers; thus with continuing improvements to the air velocity, an increase in the fineness of fibers to under 100 nm is expected. In addition, chemical and physical methods to reduce melt viscosity, such as ultrasonic vibration, microfoaming, and other viscosity reduction methods, are needed to further reduce fiber diameter. 2. Relevant literature on the principles of melt electrospinning is still rare. Further research should be done on the mechanisms of charging and jet refinement, and some work should be done in this field to search for ways to refine the jet in principle. For example, the mechanism of jet formation under the action of electrostatic force explained from the molecular level by means of molecular simulation could provide a theoretical basis for the further refinement of the jet. 3. A series of low-molecular-weight and low-viscosity basic materials for melt electrospinning should be developed to meet the needs of corresponding process research and industrial device development so as to further expand the applications in the areas of controllable tissue scaffolds, sensors, biomaterials, industrial robots, micro/nano manufacturing, and other aspects. 4. Based on a variety of processes of composite technology, the preparation of micro/nanofiber multistage structures should be achieved, for filtering, load sensing, and various other fields. 5. Combined with direct writing technology and 3D printing, it is a development trend to use melt electrospinning to prepare soft materials, multilevel structures, or tiny entities; it can also be applied to accessing templates for complicated micro-flow channels of microfluids.

REFERENCES Agarwal, S., Wendorff, J.H., Greiner, A., 2009. Progress in the field of electrospinning for tissue engineering applications. Advanced Materials 21 (32e33), 3343e3351. Anton F. Process and Apparatus for Preparing Artificial Threads: U.S. Patent 1,975,504. 1934-10-2. Bellan, L.M., Craighead, H.G., 2009. Nanomanufacturing using electrospinning. Journal of Manufacturing Science and Engineering 131 (3), 034001. Bhardwaj, N., Kundu, S.C., 2010. Electrospinning: a fascinating fiber fabrication technique. Biotechnology Advances 28 (3), 325e347. Brown, T.D., Dalton, P.D., Hutmacher, D.W., 2011. Direct writing by way of melt electrospinning. Advanced Materials 23, 5651e5657.

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PART

APPLICATIONS OF ELECTROSPUN NANOFIBERS

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CHAPTER

ELECTROSPUN NANOFIBERS FOR AIR FILTRATION

12

Shichao Zhang1,3, Nadir Ali Rind2, 3, Ning Tang2,3, Hui Liu2, 3, Xia Yin2, 3, Jianyong Yu3, Bin Ding3

State Key Laboratory for Modification of Chemical Fibers and Polymer Materials, College of Materials Science and Engineering, Donghua University, Shanghai, China1; Key Laboratory of Textile Science & Technology, Ministry of Education, College of Textiles, Donghua University, Shanghai, China2; Innovation Center for Textile Science and Technology, Donghua University, Shanghai, China3

12.1 INTRODUCTION Air pollution has become one of the most serious environmental issues, particularly fog and haze pollution, resulting in a growing impact on public health, production efficiency, and even ecosystems. Atmospheric particulate matter (PM) pollution, including solid and liquid particles emitted into the air, is the major noxious form of pollutant, and can cause adverse effects on human health due to its ability to penetrate deep into the lungs and cardiovascular system, causing minor irritation to chronic respiratory and lung cancer and making preexisting heart and lung diseases worse (Kampa and Castanas, 2008; Rodrıguez et al., 2004; Querol et al., 2001). The major sources of PM pollution are industrial emissions (e.g., mineral powder, coal, and carbon powder), combustion from daily life, intensive road transport, secondary nitrates, and secondary sulfates. Furthermore, certain contaminants attached to PM, such as bacteria, pollen, microorganisms, and viruses, may also badly influence the environment and the aforementioned diseases (Montefusco, 2005; Chuanfang, 2012; Peukert, 1998). For example, according to the report provided by the World Health Organization in 2014, PM pollution was the main cause of death around the world for 7 million people (Wang et al., 2016a). The size of PM particles is responsible for various health hazards; for example, thoracic particles (diameter 100 nm and easily collapsed cavity structure. To further decrease the diameters of electrospun nanofibers, Ding et al. have introduced a novel electrohydrodynamic technique called electrospinning/netting, which has evoked great attention by virtue of its ability to fabricate, on a large scale, nanofiber/net materials comprising conventional electrospun nanofibers and two-dimensional (2D) spiderweb-like nanonets with diameters of w20 nm, as shown in Fig. 12.1 (Ding et al., 2006, 2011; Zhang et al., 2015; Wang et al., 2015). The resultant nanofiber/net membranes combine the general properties of conventional nanofibers with some extra outstanding characteristics, like extremely small pore size, high porosity, and enhanced interconnectivity, which allow them to emerge as a promising filtration medium to greatly promote the filtration performance of air filters (Pant et al., 2014; Zhang et al., 2015; Liu et al., 2015a).

12.2.2 FILTRATION MECHANISMS Air filtration media have been in use for a thousand years, but essential studies on filtration mechanisms were proposed only in the past century. According to the mechanism, the filtration process can be classified into two states: steady and unsteady (Qin and Wang, 2006). In the steady state, the filtration efficiency and pressure drop are fixed over time and depend merely on the intrinsic characteristics of the filtration materials, nature of the PM, and rate of the airflow. For the unsteady state, the particle capture capacity and pressure drop change over time with the particles accruing on the filters (Qin and Wang, 2006). Since the unsteady state is a very complex process it still lacks

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FIGURE 12.2 (A) The main filter mechanisms of fiber material. (B) Filtration efficiency for particles with different diameters. (C) The main filtration effect based on particle size. MPPS, most penetrating particle size. (A) Modified with permission from Chuanfang, Y., 2012. Aerosol filtration application using fibrous mediadan industrial perspective. Chinese Journal of Chemical Engineering 20, 1e9. © 2012 Elsevier. (B) and (C) Barhate, R.S., Ramakrishna, S., 2007. Nanofibrous filtering media: filtration problems and solutions from tiny materials. Journal of Membrane Science 296, 1e8. © 2007 Elsevier.

systematical theories to present an accurate prediction for the actual filtration process. The stable state of fiber filtration is further categorized by following five filtration mechanisms (Zhu et al., 2000, 2016; Qin and Wang, 2006; Bull, 2008; Chuanfang, 2012), comprising interception, inertial deposition, diffusion, electrostatic, and gravity effect, as shown in Fig. 12.2.

12.2.2.1 Interception Mechanism There are certain streamlines, for a given particle size, that move close enough to the filter fiber. Then, the particle would touch the fiber and be captured by van der Waals forces, hence being removed from the air streamline (Ramskill and Anderson, 1951). This filtration process is called “interception.” Generally, particles ranging from 0.1 to 1 mm can be captured by interception, and the capture efficiency by interception increases with increasing particle size (Chuanfang, 2012).

12.2.2.2 Inertial Impaction Mechanism With sudden changes in the streamline airflow, particles of larger size are unable to maintain the airflow direction as it changes, due to the action of inertia; they will be separated from the streamline

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and deposited on the filter fibers along their original path. This type of mechanism is most predominant when the particle size is larger than 0.3e1 mm, and it is enhanced with high airflow velocities in the dense-packing fiber filters (Chuanfang, 2012).

12.2.2.3 Diffusion Mechanism The kinetic theory of gases is important for clarifying the capture process of particles by the diffusion mechanism. According to this theory, gas is composed of a large number of small-sized molecules compared with the distances between them. And, these molecules do not travel in continuous streamlines because they are colliding with one another and moving in random paths. This random motion is called Brownian motion. The diffusion mechanism of capturing PM particles is the result of Brownian motion, which can allow the smaller sized particles (0.1 mm) to deviate from their original streamline randomly and enable collisions between particles and fibers, leading to the deposition of the particles on the filter fiber (Chuanfang, 2012). This mechanism is mostly suitable for smaller particles under low airflow velocities (Ramskill and Anderson, 1951).

12.2.2.4 Electrostatic Effect Mechanism Electrostatic interaction has the ability to change the track of the particles and attract them to the surface of the filter fiber electrets, by virtue of the electrostatic adhesion provided by the particles and/ or the fibers. This filtration mechanism is widely used to capture particles having submicrometer diameters (Sahay et al., 2012). The application of electrostatic force can enhance the efficiency of filtration media while maintaining the air resistance of the filters (Wang, 2001).

12.2.2.5 Gravity Effect Mechanism The contribution of gravity to the removal of PM particles is negligible for most of the ultrafine particle sizes; therefore, this effect has not much importance for the high-efficiency air filters (Chuanfang, 2012). According to previous studies, the particulate capturing mechanism by gravity sedimentation will be completely avoided when the particle size is smaller than 0.5 mm. Obvious conclusions can be drawn that the importance of various filtration mechanisms changes with the variation in particle size during the filtration process. And, there would be a most penetrating particle size for particles, which is usually considered to be w300 nm, according to the existing studies from industrial fields and academic circles. Considering the structural features, the separation of PM from the airflow by electrospun nanofiber filters is subject to the integrated effects of inertial deposition, interception, diffusion, electrostatic deposition, and gravity effects.

12.3 POLYMERIC NANOFIBER-BASED FILTERS Electrospun nanofiber air filters have been used as high-performance air filtration media since the late 1980s, because of their tremendous ability, thanks to their large surface area, brilliant surface adhesion, highly porous structure with uniform pore size, and light weight, to capture PM particles from the air (Lu and Ding, 2008; Selvam and Nallathambi, 2015; Thavasi et al., 2008). Electrospun nanofibers have been widely applied in various filtration devices, like vehicle cabin filters, personal respirators, indoor air cleaners, etc. A number of natural and synthetic polymers have been successfully employed to fabricate nanofiber filters with superior performance, including polyacrylonitrile (PAN), polyurethane

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(PU), polyvinyl alcohol (PVA), polyamide (PA), cellulose acetate, chitosan, polysulfone (PSU), and so on. In this portion of the chapter, some typical and advanced polymeric nanofiber filters based on single-component polymers and composite polymer membranes are discussed.

12.3.1 SINGLE-COMPONENT POLYMER MEMBRANES The unique structural features of electrospun nanofibers, such as thinner fiber diameter, smaller pore size, and higher porosity than other fibrous air filters, including melt-blown fibers, spunbonded fibers, and glass fibers, make them fascinating candidates for air filtration. Various electrospun nanofiber filters obtained from single-component polymers have been successfully designed and fabricated for PM filtration. Among them, the robust physical and chemical properties of the polymer PA cause it to be one of the most broadly applied synthetic polymers for fabrication of high-performance air filters. Gibson et al. (2001) first fabricated PA-66 electrospun nanofiber filters, and demonstrated that such filtration medium was efficient for screening ultrafine particles. Ahn et al. (2006) prepared PA-6 nanofiber filters with a fiber diameter of w200 nm, pore size of 0.24 mm, and base weight of w10.75 g/m2. These filters presented superior efficiency (99.993%) compared with the commercialized HEPA (high-efficiency particulate air) filters (99.97%, pore size of 1.7 mm, base weight of 78.2 g/m2) for removing 0.3-mm ultrafine particles, which made them excellent candidates for airborne particles, especially for ultrafine PM filtration. PAN is another commonly used polymer material in air filtration by virtue of its excellent physical and chemical properties. Barhate et al. (2006) for the first time investigated the effects of the process parameters of electrospinning, for instance, applied voltage, collection rate, and electrospinning distance, on the structural and transport properties of PA-6 nanofiber membranes. They concluded that membranes with controllable pore size and distribution could be obtained by regulating the drawing and collecting rates. Electrospun PAN nanofibers with average diameter in the range of 270e400 nm were also prepared by Yun et al. (2007). Moreover, they compared the filtration capability of PAN nanofiber filters and commercial filters for removing differently charged NaCl nanoparticles (NPs), and found that the PAN electrospun filters, with thinner and more uniform fiber diameter, could greatly reduce the penetration of NPs without any relation to particle charge state, indicating that the resultant PAN nanofibers are promising materials for the preparation of high-performance air filters. The polymer PVA, as the largest-volume water-soluble synthetic polymer, is also selected to produce air filters because of its excellent mechanical properties and chemical resistance. Qin and Wang prepared crosslinked electrospun PVA nanofiber membranes and tested their filtration performance (Qin and Wang, 2008). The results showed that the crosslinked PVA nanofibers could remarkably increase the filtration efficiency of melt-blown substrate layers (w30%) and achieve a final removal efficiency of almost 100% for large particles using the as-prepared composite membranes. Polyethylene oxide (PEO) is another typical water-soluble synthetic polymer that has been widely studied for various applications. Leung et al. (2010) prepared nanofiber filters by coating electrospun PEO nanofibers onto a nonwoven microfiber substrate and investigated the effects of material structure and face velocity on the filtration efficiency and pressure drop of the filters. Based on their detailed experiments, they found that the most penetrating particle size for this filter decreased from 140 to 90 nm along with increasing packing density of the nanofiber membranes from 3.9 to 36  10e3, and the removal efficiency decreased as the face velocity increased from 5 to 10 cm/s. In addition to the aforementioned electrospun nanofiber filters, other single-component polymeric membranes, such as

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cellulose nanofibers (Grafe and Graham, 2003b), PU nanofibers (Sambaer et al., 2011), and polyvinyl pyrrolidone nanofibers (Morozov and Mikheev, 2012), have also been fabricated via electrospinning technology and have shown good filtration performance due to their greatly reduced fiber diameters, further confirming that electrospun nanofibers are an excellent candidate material for ultrafine PM filtration.

12.3.2 COMPOSITE POLYMER MEMBRANES Single-component polymeric nanofiber membranes exhibit many remarkable features that make them suitable for air filtration, like small diameter and high porosity. However, the majority of these membranes still lack some other essential properties, such as mechanical strength, chemical resistance, antifouling properties, etc. To overcome the limitations of these materials, two or more polymers with different characteristics are combined to produce composite polymeric nanofiber membranes. In 2013, Wang et al. (2013a) fabricated a tortuously structured polyvinyl chloride (PVC)/PU composite nanofiber filter with robust mechanical properties and filtration performance. The ratio of PVC/PU in solution was optimized and the nanofibers were collected on a traditional filter paper. The authors carefully studied the stressestrain curves of the resultant filters and revealed the three-step break process for fibers upon external stress, as shown in Fig. 12.3A. Owing to the nonbonding structure of PVC nanofibers, the slippage occurred at lower stresses; therefore the single PVC nanofiber membrane exhibited low tensile strength and elongation at break. With the introduction of PU, the mechanical properties gradually increased; and it was observed that all blended samples in the first region followed

FIGURE 12.3 (A) Stressestrain curves of polyvinyl chloride/polyurethane (PU) fibrous membranes. (B) Water and oil contact angles of polyacrylonitrile/PU fibrous membranes with various fluorinated PU (FPU) concentrations. (A) Modified with permission from Wang, N., Raza, A., Si, Y., Yu, J., Sun, G., Ding, B., 2013a. Tortuously structured polyvinyl chloride/polyurethane fibrous membranes for high-efficiency fine particulate filtration. Journal of Colloid and Interface Science 398, 240e246. © 2013 Elsevier. (B) Wang, N., Zhu, Z., Sheng, J., Al-Deyab, S.S., Yu, J., Ding, B., 2014b. Superamphiphobic nanofibrous membranes for effective filtration of fine particles. Journal of Colloid and Interface Science 428, 41e48. © 2014 Elsevier.

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Hook’s law (which states that stress is directly proportional to strain up to its elastic limit), and the membranes displayed a linear curve or elastic behavior under a stress load before reaching the yield point, at which they changed from an elastic to a plastic state. After that, a further increase in stress would result in the breakage of the nanofibers. Wang et al. attributed this phenomenon to a structural transformation based on the coexistence of bonding and nonbonding structures in the composite PVC/ PU nanofiber membranes during the strain process. Significantly, benefiting from the bonding structures, the as-prepared blended nanofiber membranes presented good abrasion resistance (134 cycles) performance and robust air permeability (154.1 mm/s) due to combined 3D porous structure and high tensile strength (9.9 MPa), achieving a high removal efficiency (99.5%) with a low air resistance (114 Pa). New functional nanofiber membranes with high removal efficiency and remarkable antifouling performance, reported by Wang et al., were fabricated by combining PAN/PU composite nanofiber membranes with the new synthesized fluorinated PU (FPU) (Wang et al., 2014b). The PAN/PU composite polymer structure exhibited robust mechanical properties (tensile strength of 12.28 MPa), superior air permeability (706.84 mm/s), and good abrasion resistance. With the addition of FPU, lowsurface-energy and rough nanoscaled structures were created, which endowed the membranes with hydrophobic and oleophobic properties, with water and oil contact angles of 154 and 151 degrees, respectively (Fig. 12.3B). This superamphiphobic property of the membranes can be controlled by altering the ratio of FPU in the precursor solution. In addition, the as-prepared PAN/PU/FPU nanofiber membranes showed better filtration performance toward oil and salt aerosol particles compared with pure PAN/PU membranes, which further confirmed the importance of amphiphobicity to the air filtration application. PAN/polyacrylic acid (PAA) composite nanofiber membranes were also prepared to use as a filtration medium by Liu et al., and they carefully investigated the effects of PAN/PAA ratios of 10:0, 7:3, 6:4, 5:5, and 3:7 on the structure and filtration performance of the resultant membranes (Liu et al., 2015b). The main objective of blending PAA with PAN is to achieve membranes with sufficient tensile strength. With increasing PAA content, the tensile strength of the composite membranes significantly increased from 3.8 to 6.6 MPa, indicating a much higher mechanical property compared with pure PAN membranes. Moreover, they found that the pore size and surface area of the composite nanofiber filters were mainly dependent on the structure of the fiber and composition of the blended solution. The average pore size for different samples was in the range of 16.8e44.4 nm. The composite membrane with the smallest pore size showed high removal efficiency (99.994%) with the lowest air resistance, 160 Pa, against 0.3e0.5 mm NaCl aerosol particles at an airflow velocity of 5.3 cm/s. A novel strategy to develop anti-deformed PEO@PAN/PSU composite nanofiber membranes with binary structure for capturing PM from the air was proposed by Zhang et al. (2016a), who combined multijet electrospinning with a vacuum drying process to form bonded structures of PEO@PAN/PSU membrane filters, as exhibited in Fig. 12.4. The PAN/PSU blended nanofiber membranes with controllable packing density and small pore size were prepared and optimized by varying the jet ratios of PAN and PSU in solution. Then the PEO agent was incorporated into the optimized composite membranes, to create the bonding/nonbonding structure that endowed the membranes with stable cavity structure and anti-deformed features. The resultant PEO@PAN/PSU blended membranes exhibited small pore size, high porosity, and relatively large tensile strength (8.2 MPa) with excellent toughness and modulus (2.44 MJ/m3 and 204 MPa, respectively), and can filter ultrafine particles with a high removal efficiency of 99.992%, low air resistance of 95 Pa, and excellent quality factor of 0.1/Pa.

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FIGURE 12.4 (A) Schematic illustration of the fabrication procedure of polyethylene oxide (PEO)@polyacrylonitrile (PAN)/ polysulfone (PSU) composite membranes. (B and C) The composite membranes (B) before and (C) after heat treatment. (D) Schematic illustration of the filtration process of the anti-deformed PEO@PAN/PSU fiber-based filter. Modified with permission from Zhang, S., Liu, H., Yin, X., Yu, J., Ding, B., 2016a. Anti-deformed polyacrylonitrile/polysulfone composite membrane with binary structures for effective air filtration. ACS Applied Materials & Interfaces 8, 8086e8095. © 2016 American Chemical Society.

12.4 HYBRID NANOFIBER-BASED FILTERS 12.4.1 POLYMER/ORGANIC NANOPARTICLE MEMBRANES Electret fibrous membranes have been proven to be an efficient and promising material for adsorbing airborne particles effectively by long-range electrostatic force because of their ability to quasipermanently reserve abundant charges and create an external macroscopic electric field on the periphery of the fibers. In 2016, Wang et al. prepared novel polyvinylidene fluoride (PVDF)/polytetrafluoroethylene (PTFE) NP electret nanofiber membranes by electrospinning and investigated their application performance in air filtration (Wang et al., 2016b). They first evaluated the effect of PTFE NP concentration on the morphology and structure of the PVDF/PTFE NP membranes, as shown in Fig. 12.5. It was found that the average diameter of the fibers decreased to 380 nm by including 0.05 wt % PTFE NPs compared with the pure PVDF nanofibers (622 nm). With further increase in the concentration of PTFE NPs to 0.1 wt%, the diameter of the fibers increased to 480 nm because of the agglomeration of PTFE NPs. Porosity versus concentration of PTFE NPs revealed that fibrous membranes with 0.05 wt% PTFE NPs possessed smaller pore size and more uniform aperture

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FIGURE 12.5 (A) Scanning electron microscopy and (B) transmission electron microscopy images of polyvinylidene fluoride/ polytetrafluoroethylene (PVDF/PTFE) nanoparticle (NP) electret fibrous membranes. (C) Schematic of electric charge category existing in PVDF/PTFE NP electret fibrous membranes. (D) Filtration efficiency of PVDF/PTFE NP electret fibrous membranes with various basis weights. NF, nanofilter. Modified with permission from Wang, S., Zhao, X., Yin, X., Yu, J., Ding, B., 2016b. Electret polyvinylidene fluoride nanofibers hybridized by polytetrafluoroethylene nanoparticles for high-efficiency air filtration. ACS Applied Materials & Interfaces 8, 23985e23994. © 2016 American Chemical Society.

distribution. The filtration properties of the composite electret membranes were tested with neutralized NaCl monodispersed aerosol particles with size ranging from 0.3 to 0.5 mm. The results showed that the filtration efficiency gradually increased to a maximum value as the concentration of PTFE NPs increased to 0.05 wt%. The filtration efficiency decay test also confirmed that the composite electret membranes with 0.05 wt% PTFE NPs possessed long-term stability of filtration efficiency. To deeply study the electret mechanism of the PVDF/PTFE composite membranes, a linear temperature program was employed to measure the thermally stimulated current. It was found that more interfacial polarization charges were inspired when PTFE NP concentrations reached 0.05 wt%. To further promote the depth of the energy level, various electrospinning voltages ranging from 20 to 50 kV were

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applied to increase the quantity and reinforce the stability of the charge. The results showed that PVDF/PTFE NP membranes prepared under a voltage of 40 kV showed the highest air filtration efficiency (99.972%), which implied that the initial surface potential cloud could be enhanced through increasing voltage.

12.4.2 POLYMER/INORGANIC NANOPARTICLE MEMBRANES In 2014, Wan et al. (2014) described the fabrication of a hierarchically nanostructured and superhydrophobic nanofiber medium for air filtration by electrospinning PSU/titania NP (PSU/TiO2) hybrid nanofibers on a traditional nonwoven substrate, as shown in Fig. 12.6A. They carefully studied the effects of solvent and TiO2 content in the solution on the structure of the PSU/TiO2 hybrid nanofiber membranes. By virtue of the hierarchical roughness caused by the incorporation of TiO2 NPs (indicated by dashed circle), the resultant hybrid nanofiber membranes exhibited a robust hydrophobicity, with a water contact angle of 152 degrees, and could facilely filter ultrafine particles with a high removal efficiency of 99.997% and low air resistance of 45.3 Pa, further indicating the contribution of TiO2 NPs to the filtration performance of the nanofiber filters. Cho et al. (2013) prepared TiO2incorporated PAN nanofibers, which were deposited on a cellulose filtration medium to further increase the filtration performance of the filters. They observed that with increasing content of TiO2 NPs, the numbers of ions and charged particles on the membranes were greatly enhanced, which can be

FIGURE 12.6 (A) Field emission scanning electron microscopy (FE-SEM) images of a polysulfone/TiO2 fibrous membrane. (B) Airflow streamlines around circular and noncircular cross-sectional fibers in the no-slip flow regime. On the right are the corresponding FE-SEM images of relevant fibrous membranes. (A) Modified with permission from Wan, H., Wang, N., Yang, J., Si, Y., Chen, K., Ding, B., Sun, G., EL-Newehy, M., AL-Deyab, S.S., Yu, J., 2014. Hierarchically structured polysulfone/titania fibrous membranes with enhanced air filtration performance. Journal of Colloid and Interface Science 417, 18e26. © 2014 Elsevier. (B) Wang, N., Si, Y., Wang, N., Sun, G., El-Newehy, M., Al-Deyab, S.S.., Ding, B., 2014a. Multilevel structured polyacrylonitrile/silica nanofibrous membranes for high-performance air filtration. Separation and Purification Technology 126, 44e51. © 2014 Elsevier.

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evaluated by thermally stimulated current measurement spectra. Therefore, the filtration performance of hybrid PAN/TiO2 membranes against 100e500 nm aerosol particles showed better efficiency than pristine PAN nanofiber filters, and the air resistance of the hybrid system was much lower than that of single-component PAN filter membranes. Wang et al. (2014a) proposed a powerful yet economic approach to creating high-efficiency filtration media by combining multilevel-structured hybrid nanofiber membranes. The principal feature of this work is that these membranes were fabricated by the accumulation of SiO2 NP-incorporated bimodal-sized PAN nanofibers. The authors observed that the scaffoldlike structure of the as-prepared membranes can be regulated by regulating the jet ratio of the solutions with different PAN concentrations, which has a great influence on the filtration performance of the filters. The addition of SiO2 NPs made the fibrous surface rough, with a noncircular cross section (Fig. 12.6B), and the specific surface area was enhanced by the highly porous structure. Benefiting from the layer-by-layer stacking structures and rough fiber surface, the multilevel PAN/ SiO2 hybrid membranes possessed a high filtration efficiency of 99.989% and low air resistance of 117 Pa compared with single-layer membranes. With the design strategy of capturing PM particles from the air by electrostatic effects, the novel approach of creating electret media provided an efficient way to achieve high removal efficiency with low air resistance. Song et al. (2016) reported an electret filtration medium by incorporating magnetic Fe3O4epolyhedral oligomeric silsesquioxane (POSS) particles with PAN nanofibers. With the help of the hydrosilylation reaction, magnetic Fe3O4ePOSS particles with SiOH were first prepared, and then PAN/Fe3O4ePOSS hybrid nanofiber membranes were deposited on a conventional nonwoven substrate by the electrospinning process. Owing to the incorporation of magnetic particles, a significant increase in the stability of the surface charge was achieved and its retention capacity greatly enhanced compared with pure PAN nanofiber membranes. In 2017, Zhao et al. fabricated a low-resistance hybrid nanofiber-based air filter by electrospinning negative ion powder-doped PVDF hybrid nanofiber membranes, which were capable of releasing negative ions and capturing PM2.5 pollutants from the air effectively (Zhao et al., 2017b). Based on the observation of filters obtained from different polymers (PSU, polyvinyl butyral, and PVDF), they found that a reduction in the fiber diameter (from 1.16 to 0.41 mm) significantly decreased the pressure drop (from 9.5 to 6 Pa) of the membranes, as exhibited in Fig. 12.7. Furthermore, a slower rising rate of air resistance along with a decrease in pore size could be delivered by a thinner fiber diameter of the fibrous membranes. In addition, the PVDF/negative ion particle nanofiber membranes displayed high surface potential, due to the high electronegativity of fluorine, resulting in higher releasing amounts of negative ions, which could be varied by reducing the diameter of the fiber and the negative ion content (from 789 to 2818 ions/cm3). The as-prepared PVDF hybrid nanofiber filters showed a high filtration efficiency of 99.99% at high release amounts of negative ions and a low air resistance of 40.5 Pa. And, this novel air filter with multilayered and cavitylike structures provided a new way to fabricate high-performance filtration media for various air filtration applications. The deadly hazards of air pollution, especially PM pollution, to human health make scientists and engineers eager to develop individual protective materials with several extraordinary features. In 2017, Zhao et al. reported a gradient-structured, hybrid nanofiber-based cleanable air filter with high purification efficiency, low pressure drop, and high moisture vapor transfer rate (Zhao et al., 2017a). These membranes were fabricated by using superhydrophilic PAN/SiO2 fibers, which functioned as a medium to transfer moisture vapor, and hydrophobic PVDF nanofibers, which functioned as a repellent

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FIGURE 12.7 The pressure drops of polysulfone (PSU), polyvinyl butyral (PVB), and polyvinylidene fluoride (PVDF) fibrous membranes (A) with different fiber diameters and (B) at different airflow velocities. Modified with permission from Zhao, X., Li, Y., Hua, T., Jiang, P., Yin, X., Yu, J., Ding, B., 2017b. Low-resistance dual-purpose air filter releasing negative ions and effectively capturing PM2.5. ACS Applied Materials & Interfaces 9, 12054e12063. © 2017 American Chemical Society.

element to restrict the creation of capillary water under high humidity. This gradient structure endowed the membranes with an improved moisture vapor transfer rate of 13,612 g/m2 day, high removal efficiency of 99.99%, and low air resistance of 86 Pa, as shown in Fig. 12.8.

12.5 NANOFIBER/NET-BASED FILTERS 12.5.1 NANOFIBER/NET MEMBRANES Although the electrospun nanofiber-based filters show improved filtration efficiency and enhanced service life compared with microfiber filters, some drawbacks still remain for these filters, including inadequate filtration performance and low quality factor, due to the thick fibers (>100 nm) and relatively large pore size. In 2012, Wang et al. (2012) first presented the fabrication of PA-66 nanofiber/net membranes, which are composed of conventional nanofibers and 2D spiderweb-like nanonets through an advanced hydrodynamic technique called electrospinning/netting. These novel nanonet structures, including coverage rate and pore size, can be facilely controlled by tuning the solution properties (content of additives) and process parameters. Taking advantage of the integrated properties of small diameter, small pore size, and high porosity, the nanofiber/net membranes possessed a high purification efficiency of 99.9% and relatively low air resistance, and can be employed as filtration media in various filtration fields. In 2017, Zhang et al. fabricated novel poly(m-phenylene isophthalamide) (PMIA) nanofiber/net membranes, which were composed of conventional nanofibers and a 2D Steiner tree network (w20 nm), for high-efficiency air filtration by using the electrospinning/netting process for the first time (Zhang et al., 2017a). The bimodal structure of the as-prepared PMIA membranes can be regulated by polymer concentration optimization, dodecyl

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FIGURE 12.8 (A) Schematic illustration of the fabrication process, and (B) filtration performance of gradient composite fibrous membranes. (C) The relationship between moisture vapor transfer rate (MVTR) and pressure drop of the composite membranes and commercial samples. PAN, polyacrylonitrile; PVDF, polyvinylidene fluoride. Modified with permission from Zhao, X., Li, Y., Hua, T., Jiang, P., Yin, X., Yu, J., Ding, B., 2017a. Cleanable air filter transferring moisture and effectively capturing PM2.5. Small 13, 1603306. © 2017 Wiley-VCH.

trimethylammonium bromide additive inspiration, and ambient conditions, especially relative humidity. By virtue of structural features such as extremely small diameter, small pore size, and high porosity, PMIA nanofiber/net membranes showed superlight weight (0.365 g/m2) with ultrathin thickness (w0.5 mm) and robust mechanical property (72.8 MPa), which made them promising candidates for high-performance air filtration against 300e500 nm aerosol particles, with high a removal efficiency of 99.999%, as shown in Fig. 12.9.

12.5.2 NANOFIBER/NET MEMBRANES WITH CAVITY STRUCTURES Nanofiber/net membranes like PA-66 and PMIA have been successfully prepared, and showed relatively high filtration efficiency; however, the pressure drop of these filters was still rather high (PA-66 filter w200 Pa), due to their inadequate and unstable cavity structures and compact packing density. In

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(A)

Filtration

(B)

(C)

(D)

FIGURE 12.9 (A) In situ scanning electron microscopy (SEM) study and (B) 3D model illustration of the filtration process of poly(m-phenylene isophthalamide) (PMIA) nanonet membranes. Field emission SEM images of (C) the top surface and (D) a cross section of a PMIA nanonet after filtration. Modified with permission from Zhang, S., Liu, H., Yin, X., Li, Z., Yu, J., Ding, B., 2017a. Tailoring mechanically robust poly(mphenylene isophthalamide) nanofiber/nets for ultrathin high-efficiency air filter. Scientific Reports 7, 40550.

2015, Liu et al. (2015a) proposed a novel strategy to design bimodal-sized biobased PA-56 nanofiber/ net membranes through the electrospinning/netting technique. These membranes were made of 2D ultrathin nanonets (w20 nm) and stable cavity structures, which are indeed very important for decreasing the air resistance while maintaining high efficiency for the air filters. In this study, the coverage rate of the nanonets was controlled by tuning the solution concentration, while the stable cavity structures were optimized by regulating the HCOOH/CH3COOH weight ratio. With the combined fascinating features of extremely small diameter, high porosity, and boned scaffold, the as-prepared PA-56 membranes displayed a high removal efficiency of 99.995%, low air resistance of 111 Pa, long working life with large dust-holding capacity (49 g/m2), and dust-cleaning regeneration ability compared with commercially used fibrous filters. To construct the stable and large cavity structures, Yang et al. reported a novel strategy to fabricate composite membranes with highly porous

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FIGURE 12.10 (A) Illustration of the concept of a sandwich-structured filtration medium based on polyamide-6 (PA-6) nanonets and polyacrylonitrile (PAN) bead-on-string fiber membranes. (B) Filtration efficiency and pressure drop of PA-6/ PAN/PA-6 composite membranes with various basis weights. Modified with permission from Yang, Y., Zhang, S., Zhao, X., Yu, J., Ding, B., 2015. Sandwich structured polyamide-6/ polyacrylonitrile nanonets/bead-on-string composite membrane for effective air filtration. Separation and Purification Technology 152, 14e22. © 2015 Elsevier.

structures, by using a PA-6/PAN/PA-6 sandwich structure for effective air filtration, as shown in Fig. 12.10 (Yang et al., 2015). The PAN bead-on-string fibers and 2D PA-6 nanonet (w20 nm) in the membrane endowed this newly constructed filter with tunable porous structures, and then obtained high purification efficiency (99.999%), low air resistance (117.5 Pa), and good mechanical properties. To further enlarge the cavity structures, Zhang et al. prepared PA-6/PMIA nanonet membranes with microwave structures by using electrospinning/netting and the staple fiber intercalating process, as shown in Fig. 12.11 (Zhang et al., 2016b). This work aimed to prepare the new filters with low stacking density, small pore size, and microwave fluctuation by combining the PA-6 binary nanofiber/net structures and embedded PMIA staple fibers. The resultant PA-6/PMIA membranes exhibited high removal efficiency of 99.995%, low air resistance of 101 Pa, and the required quality factor against ultrafine airborne particles, together with a relatively high tensile strength (10.7 MPa) and high dustholding capacity (>50 g/m2), which could be attributed to the features of extremely high porosity, small pore size, and large specific surface area. Furthermore, these membranes showed long service life in real applications, due to their high mechanical properties and microwave structure, further confirming that the as-prepared membranes can be strong candidates in the field of high-performance filtration media. In 2017, Zhang et al. described a novel approach to fabricating ripplelike PA-6 nanofiber/net membranes by the combination of electrospinning/netting and receiving substrate design (Zhang et al., 2017b). As shown in Fig. 12.12, the polyethylene terephthalate (PET) framework, with optimized pleat span and pleat pitch, was first designed by regulating the diameter and interval gap of the PET filaments accordingly, to form cavity structures in the "wave-like" zone (indicated between the dashed lines) without damaging Steiner treeelike 2D nanonet assemblies. Based on this regulation, the as-prepared membranes possessed excellent features such as broadly distributed nanonets, fluffy cavity structures, and enlarged frontal surface, which made the ripplelike PA-6

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FIGURE 12.11 Schematic illustration of (A) the fabrication procedure and (B) structures of polyamide-6 (PA-6)/poly(mphenylene isophthalamide) (PMIA) nanonet-based membranes. (C) Schematic illustration of the microwave structured PA-6/PMIA filter for effective air filtration. Modified with permission from Zhang, S., Liu, H., Yu, J., Luo, W., Ding, B., 2016b. Microwave structured polyamide-6 nanofiber/ net membrane with embedded poly(m-phenylene isophthalamide) staple fibers for effective ultrafine particle filtration. Journal of Materials Chemistry 4, 6149e6157. © 2016 Royal Society of Chemistry.

membrane a promising candidate in filtration media to capture ultrafine particles, with a high removal efficiency of 99.996%, low air resistance of 95 Pa, and desired quality factor of more than 0.11/Pa. In addition, the resultant membranes showed a superlight weight (0.9 g/m2) and large dust-holding capacity of >63 g/m2.

12.5.3 NANOFIBER/NET COMPOSITE MEMBRANES Multilevel composite structures give an insight into highly integrated and efficient filtration media for various applications, including personal, industrial, and environmental protection, because of their fascinating features of controllable pore size, gradually varied pore structure, and high porosity. In view of this novel design strategy, Zhang et al. reported the fabrication of highly integrated multilevel PSU/PAN/PA-6 hybrid fibrous membranes to capture airborne particles from polluted air via the sequential electrospinning process, as shown in Fig. 12.13 (Zhang et al., 2016c). Each layer of the integrated PSU/PAN/PA-6 composite membranes possessed the particular diameters and pore sizes for removing particles of different diameters. The PSU microfiber layer, with fiber diameter of w1 mm and pore size of w2.2 mm, was employed to filter particles with diameter of >2 mm; the PAN nanofiber

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FIGURE 12.12 (A) Schematic showing (1) the fabrication of the ripplelike polyamide-6 (PA-6) nanonet membrane and (2) a cross section of the wavelike structure of the PA-6 nanonet filter. (B) Scanning electron microscopy image of the ripplelike PA-6 membrane. Inset is the corresponding image of the cross-sectional view. Scale bar in the inset represents 50 mm. (C) Filtration efficiency, pressure drop, and quality factor (QF) of the ripplelike PA-6 nanonet filters with various basis weights. NF/N, nanofiber/net; PET, polyethylene terephthalate. Modified with permission from Zhang, S., Liu, H., Zuo, F., Yin, X., Yu, J., Ding, B., 2017b. A controlled design of ripple-like polyamide-6 nanofiber/nets membrane for high-efficiency air filter. Small 13, 1603153. © 2017 Wiley-VCH.

layer, with fiber diameter of w200 nm and pore size of w0.55 mm, was used to filter >0.5-mm particles; while the PA-6 nanonet could remove w300-nm particles because of their unique 2D Steiner tree structures with extremely small pore size of 0.27 mm. These orderly assembled layers, with varied diameters, different ranges of pore size, and high porosity, enabled the composite membrane filters to work efficiently and avoid blockage of the pore structures by gradually screening particles with certain sizes. Benefiting from the gradient structure, PSU/PAN/PA-6 membranes can remove 300-nm NaCl aerosol particles with high efficiency of 99.992%, low air resistance of 118 Pa, and high quality factor value, using physical sieving. Furthermore, these composite membranes successfully got rid of the impact of electret failure and high humidity, and displayed robust mechanical (tensile strength of 5.6 MPa) and hydrophobic properties (water contact angle w130 degrees), which made them promising candidates to be used in a broad range of applications for filtration and separation devices.

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FIGURE 12.13 (A) Illustration of the concept of the three layers of the integrated filter. (B) 3D simulation of the filtration process and (C) filtration efficiency of the polysulfone (PSU)/polyacrylonitrile (PAN)/polyamide-6 (PA-6) integrated filter. Modified with permission from Zhang, S., Tang, N., Cao, L., Yin, X., Yu, J., Ding, B., 2016c. Highly integrated polysulfone/ polyacrylonitrile/polyamide-6 air filter for multilevel physical sieving airborne particles. ACS Applied Materials & Interfaces 8, 29062e29072. © 2016 American Chemical Society.

12.6 INORGANIC NANOFIBER-BASED FILTERS The studies discussed previously were focused on air filtration media working at normal temperature or mediumehigh temperature because of the limited thermal stability of those membranes. However, many industrial processes need air filters to operate under high temperatures. For example, most industrial dust-removal processes in cement plants and coal-fired boilers are performed at high temperature in the range of 150 Ce260 C (Ding and Yu, 2014); as a result, conventional polymeric membranes could not fulfill the requirements of air filtration in these fields. To resolve this bottleneck, membranes composed of inorganic electrospun nanofibers are of great interest in air filtration applications under severe conditions because of their high thermal and chemical stability (Li and Xia, 2004; Li et al., 2003, 2012; Mao et al., 2010). Mao et al. (2012) fabricated novel and flexible silica nanofiber membranes with excellent thermal stability by a combination of the solegel process and electrospinning technique. They systematically optimized the flexibility and tensile strength by tuning the composition of the precursor solution and the calcination temperature. Thermogravimetric analysis (with the temperature range of 100 Ce900 C) of various silica membranes showed that no weight loss could be observed, further indicating the outstanding thermal stability of the resultant membranes. And, the as-prepared SiO2 nanofiber membranes maintained their randomly oriented structure and flexibility at calcination temperatures from 600 C to 1000 C. By virtue of their fascinating features, like excellent thermal stability, relatively high tensile strength (5.5 MPa), and remarkable flexibility (0.0156 gf cm), the SiO2 nanofiber membranes can achieve a high removal efficiency of 99.99% and low air resistance of 163 Pa against 300e500 nm NaCl particles. This novel approach may also

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provide a new vision for the design and development of flexible inorganic nanofibrous membranes for various applications and a new strategy for creating high-temperature air filters by using inorganic nanofiber membranes. In 2014, a new approach to constructing high-temperature filtration media for effective air filtration was reported by Wang et al. (2014c). By using the electrospinning technique, they fabricated freestanding g-alumina fibrous membranes that possessed remarkable flexibility, robust tensile strength (2.98 MPa), and excellent thermal stability (w900 C). Moreover, the random arrangement of nanofibers with smaller diameter of w230 nm and high aspect ratio endowed the as-prepared membranes with uniform pore structures and high porosity. And, these features enabled their use as a hightemperature filtration medium against a fine-particulate aerosol (300-nm dioctyl phthalate), which had a high removal efficiency of 99.848% and low air resistance of 239.12 Pa. Another typical study of the fabrication of inorganic nanofibers for high-temperature air filtration was described by Mao et al. (2016). Their group fabricated a flexible and high-temperature-resistant yttria-stabilized zirconia nanofiber membrane by electrospinning followed by a thermal treatment process, as exhibited in Fig. 12.14A and B. By regulating the polymer concentration of the precursor solutions, the morphology and structure of the as-prepared membranes could be effectively controlled to achieve high flexibility and robust mechanical property. As shown in Fig. 12.14C, the resultant membranes with robust bending and heat resistance displayed a high filtration efficiency of 99.996% for 0.3e0.5 mm NaCl particles, which enabled them to be strong candidates for high-temperature filtration in various applications.

FIGURE 12.14 (A) Field emission scanning electron microscopy images of the yttria-stabilized zirconia nanofiber. (B) Optical image of the relevant soft membrane. (C) Filtration efficiency and pressure drop of the yttria-stabilized zirconia nanofiber membranes. Modified with permission from Mao, X., Bai, Y., Yu, J., Ding, B., 2016. Flexible and highly temperature resistant polynanocrystalline zirconia nanofibrous membranes designed for airfiltration. Journal of the American Ceramic Society 99, 2760e2768. © 2016 Wiley-VCH.

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12.7 CONCLUDING REMARKS AND PERSPECTIVES Making our surroundings free from air pollution, especially from fine PM, microorganisms, and volatile organic compounds, has become an important issue to ensure an ecofriendly environment for living beings. In the past few decades, electrospun nanofiber membranes have gained much attention as an effective filter medium to capture airborne particles because of their tremendous advantages of small diameter, high surface area, porosity, etc. A number of polymer materials have been produced for the fabrication of different types of electrospun nanofibers for various air filtration applications. In this chapter, we have covered the latest developments in the fabrication of electrospun nanofiber materials for air filtration through electrospinning, including their structural advantages and filtration mechanisms. Specifically, we have highlighted different types and characters of electrospun nanofiber filters for different roles in filtering PM from polluted air. Interestingly, the nanofiber-based air filters exhibit better performance, such as high removal efficiency, low air resistance, and large clogging capacity, compared with the traditional nonwoven filtration materials, due to the fascinating features discussed herein. Despite remarkable progress in the development of nanofiber filtration media, some challenges still remain unsolved, which restrict their use in large-scale applications. For instance, the mechanical properties are still not sufficiently satisfactory to make nanofiber membranes suitable for practical applications. Therefore, the nanofibers need to be collected on a nonwoven conventional substrate, otherwise, they cannot be used independently in view of their low mechanical properties. This problem, which would endanger personnel and the industrial process if the filter breaks, greatly decreases the service life of nanofiber filters and restricts their future applications. Hence, proper research on the mechanical properties along with the filtration performance of nanofiber filters needs to be carried out. In addition to the experimental research, efforts should be taken to address the issues related to the manufacturing side, such as production rate, which can be effectively increased by developing the industrial equipment. Overall, the continuous efforts of engineers and researchers are expected to deal with the current challenges and promote electrospun nanofiber filters as cost-effective and energy-saving air filtration media in the future.

ACKNOWLEDGMENTS This work was supported by the National Natural Science Foundation of China (Nos. 51473030 and 51673037), the Military Logistics Research Project (No. AWS14L008), the Shanghai Committee of Science and Technology (No. 15JC1400500), the “111 Project” Biomedical Textile Material Science and Technology (No. B07024), the Fundamental Research Funds for the Central Universities, and the “DHU Distinguished Young Professor Program.”

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ELECTROSPUN NANOFIBERS FOR OILeWATER SEPARATION

13

Jianlong Ge1, Qiuxia Fu1, Jianyong Yu2, Bin Ding2

Key Laboratory of Textile Science and Technology, Ministry of Education, College of Textiles, Donghua University, Shanghai, China1; Innovation Center for Textile Science and Technology, Donghua University, Shanghai, China2

13.1 INTRODUCTION With the development of industry and urbanization, the fast-growing demand for petroleum products has significantly intensified the exploitation, processing, and transportation of crude oil and its byproducts (Pintor et al., 2016; Sarbatly et al., 2016). However, due to the frequent oil-spill accidents and enormous release of oily sanitary sewage and industrial wastewater, disastrous oil pollution continues to occur around the world (e.g., in Alaska, the Galapagos Islands, France, the Gulf of Mexico, and Bohai Bay in China) (Peterson et al., 2003; Banks, 2003; Diez et al., 2007; Schrope, 2011; Li et al., 2016). Leaked oils in water mainly consist of petroleum hydrocarbon and other toxic chemicals which cause long-term harm to living things in the water and significantly threaten human health (Peterson et al., 2003; Munilla et al., 2011) (Fig. 13.1). Conversely, the presence of water in fuel oil products is hazardous for automobiles, ships, and even aeroplanes, since the water may breed microbes in the oil pipeline, affect the lubrication properties of fuel, or etch the cylinder, which would destroy the engines (Wang et al., 2017). Thus effective oilewater separation methods are highly desirable to meet the needs of both oily wastewater remediation and fuel oil purification. Many factors need to be taken into consideration to develop an effective oilewater separation method, especially the type of oil/water mixture and its viscosity (Nordvik et al., 1996). Generally, oils form different types of mixtures with water, including oil slicks, oil-in-water emulsions, and water-in-oil emulsions, depending on the oil/water ratio and the forming conditions (Shi et al., 2013). Fig. 13.2 illustrates the process of generating different oil/water mixtures (Nordvik et al., 1996). Conventional oilewater separation methods, such as gravity separation, skimming, and air flotation, are useful for the treatment of oil slicks and unstable emulsions but suffer from limitations of low processing efficiency and high operation cost; these methods are also insufficient to separate stable emulsions. Biological agents and chemical dispersants can break up stable oil/water emulsions, but secondary pollution is often introduced and a relatively long treatment time is needed. Thus alternative methods for oilewater separation are urgently required (Kota et al., 2012; Ma et al., 2016a; Ge et al., 2016; Peng and Guo, 2016).

Electrospinning: Nanofabrication and Applications. https://doi.org/10.1016/B978-0-323-51270-1.00013-3 Copyright © 2019 Elsevier Inc. All rights reserved.

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FIGURE 13.1 (A) Map of the oil-polluted shorelines of the Gulf of Mexico. (B) Digital photos showing volunteers cleaning up the oil spill. (C) Kelp after an oil spill. (D) A bird covered in oil. (A) Reprinted with permission from Schrope, M., 2011. Oil Spill Deep Wounds. Nature 472, 152e154. Copyright © 2011, Nature Publishing Group. (B) Reprinted with permission from Viajero, 2005. Volunteers Cleaning up the Aftermath of the Prestige Oil Spill. http://en.wikipedia.org/wiki/File:PrestigeVolunteersInGaliciaCoast.jpg. Copyright © 2005, GFDL. (C) Reprinted with permission from Svdmolen, 2004. Kelp After an Oil Spill. http://en.wikipedia.org/wiki/Oil_spill. Copyright © National Oceanic and Atmospheric Administration. (D) Reprinted with permission from Brocken, 2007. An Oiled Bird From Oil Spill in San Francisco Bay. http://en.wikipedia.org/wiki/File:Oiled_bird_3.jpg. Copyright © 2005, GNU.

Fibrous absorbents and porous separation membranes are newly developed materials for the treatment of oil/water mixtures, and have the advantages of collection or removal of one phase (oil or water) from a mixture in a simple way at low operational cost (Lim and Huang, 2007; Lee et al., 2013a; Si et al., 2015a). For oil sorbents, several porous materials including natural polymeric materials (Zeiger et al., 2016; Zhang et al., 2014b), inorganic mineral products (Liang et al., 2014), and organic synthetic fibrous mats (Zhao et al., 2013b) have been developed over the past decades. Among these oil-absorbing materials, organic synthetic fibrous adsorbents, such as nonwoven polypropylene (PP) fibrous mats, have been widely applied in the field of oil slick adsorption owing to the hydrophobiceoleophilic surface wettability, open cell structures, and scalable fabrication (Zhao et al., 2013b; Lee et al., 2013b). However, these oil adsorbents often suffer from a relatively low oil adsorption capacity (99%) (Ma et al., 2017b). As shown in Fig. 13.8B, ceramic nanofibrous membranes, such as the F-PBZ/Aluminium oxide nanoparticles (Al2O3 NPs) modified SiO2 nanofibrous membranes, were also fabricated by the in situ polymerization method, and the obtained membranes gave effective gravity-driven separation performance for water-in-oil emulsions with a relative high flux of 892 L/m2 h, and had good antifouling property, thermal stability, and durability (Huang et al., 2013). Apart from in situ polymerization of F-PBZ, other chemicals (e.g., Nafion, beeswax, and n-hexadecyl mercaptan) have successfully been employed to create superhydrophobic and superoleophilic nanofibrous membranes for effective oilewater separation (Li et al., 2014b,c; Reshmi et al., 2017). As shown in Fig. 13.9, a uniform and hierarchical rough layer can be constructed on the surface of single electrospun NF via a simple combination of the amination of PAN NFs (APAN) and immobilization of Ag nanoclusters on the surface of fibers (APANeAg) using an electroless plating technique. This APANeAg nanofibrous membrane was modified with alkyl thiols: the modified nanofibrous membrane was superhydrophobic and superoleophilic, and had an excellent capability for oilewater separation in a hypersaline environment and a broad range of pH conditions (Li et al., 2014c).

13.3.2 HYDROPHILICeOLEOPHOBIC MEMBRANES FOR OILeWATER SEPARATION Membrane separation is the most promising technology to treat oily wastewater, and ultrafiltration and nanofiltration membranes have been used in many industrial oilewater separation processes. Conventional ultrafiltration and nanofiltration membranes have fairly high separation efficiency for oil/water emulsions, but suffer from low flux, which is attributed to their limited permeability and serious surface fouling (Wang et al., 2005). To overcome these problems two aspects should be considered: constructing a hydrophilic surface to avoid oil fouling of the membranes, and increasing the porosity and decreasing the thickness of the separation layer.

13.3 ELECTROSPUN NANOFIBROUS FILTER MEMBRANES FOR OILeWATER

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FIGURE 13.8 (A) Schematic showing the synthesis of CA nanofibrous membranes modified by F-PBZ/SiO2 NPs using in situ polymerization. (B) FE-SEM images and the optical profiles of water droplets of the corresponding membrane. (C) The corresponding optical profilometry images. (D) Photos showing the simple oilewater separation process. (E) Schematic showing the synthesis of F-PBZ/Al2O3 NPs modified SiO2 nanofibrous membranes. (F) FE-SEM image of the corresponding F-PBZ/Al2O3 membrane. (G) Digital photo showing selective wettability for oil (red [dark gray in print version]) and water (blue [light gray in print version]). (H) Digital photos and optical micrographs of the oil/water emulsion and filtrate. (I) Separation flux of F-SNF/Al2O3 membranes for oil/water emulsions with increasing cycle number. (J) WCAs of the F-SNF/Al2O3 membrane after calcination. (D) Reprinted with permission from Shang, Y.W., Si, Y., Raza, A., Yang, L.P., Mao, X., Ding, B., Yu, J.Y., 2012. An in situ polymerization approach for the synthesis of superhydrophobic and superoleophilic nanofibrous membranes for oil-water separation. Nanoscale 4, 7847e7854. Copyright © 2012, Royal Society of Chemistry. (J) Reprinted with permission from Huang, M., Si, Y., Tang, X., Zhu, Z., Ding, B., Liu, L., Zheng, G., Luo, W., Yu, J., 2013. Gravity driven separation of emulsified oilewater mixtures utilizing in situ polymerized superhydrophobic and superoleophilic nanofibrous membranes. Journal of Materials Chemistry 1, 14071. Copyright © 2013, Royal Society of Chemistry.

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FIGURE 13.9 (A) Schematic illustrating the fabrication of superhydrophobic APANeAg nanofibrous membranes. (B) Static contact angles of water and oil on the surface of the fabricated APANeAg membrane. (C) Digital photos showing the oilewater separation process of the APANeAg membranes. (D) Oil flux of the related nanofibrous membrane for separation of different oil/water mixtures. Reprinted with permission from Li, X., Wang, M., Wang, C., Cheng, C., Wang, X.F., 2014c. Facile immobilization of Ag nanocluster on nanofibrous membrane for oil/water separation. ACS Applied Materials & Interfaces 6, 15272e15282. Copyright © 2014, American Chemical Society.

13.3.2.1 Single-Layer SuperhydrophiliceOleophobic Nanofibrous Membranes Inspired by the oil-repellent abilities of creatures in nature (such as fish scales), a superhydrophilic and underwater superoleophobic surface could be constructed by combining a hydrophilic chemical surface and appropriate roughness (Peng and Guo, 2016; Su et al., 2016). Zhang et al. introduced hydrophilic poly(3-hydroxybutyrate-co-4-hydroxybutyrate) into polylactide NFs using the blending electrospinning method. The obtained composite nanofibrous membrane had good hydrophilicity and high water permeability, endowing it with the superior separation performance of oil-in-water emulsion under gravity (Zhang et al., 2015). Besides blending electrospinning, dip coating is also an effect approach to modify nanofibrous membranes. Ahmed et al. employed cellulose regenerated from its ionic liquid solution to coat electrospun PVDF-co-hexafluoropropylene (PVDF-HFP). After modification with cellulose, the membrane had smaller pores with narrower pore size distribution: it

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exhibited superhydrophilicity and underwater superoleophobicity, and was successfully used for oilewater separation with quite high efficiency (Ahmed et al., 2014). Recently, a composite membrane made from graphene oxide coating aminated polyacrylonitrile (GO/APAN) fibers was fabricated by controlled assembly of GO sheets on the surface of electrospun APAN fibers and in the gaps between fibers. This membrane was superhydrophilic, had low oil adhesion, and exhibited ultrahigh flux, a preferable rejection ratio, and remarkable antifouling performance for the separation of oil/water emulsions (Zhang et al., 2017a). Furthermore, as shown in Fig. 13.10, a PAN/GO composite fibrous membrane with spindle-knot structured NFs was fabricated by electrospinning and then hydrolyzed

FIGURE 13.10 (A) Illustration of the preparation of PAN/GO and H-PAN/GO membranes. TEM images of the fabricated membrane: (B) smooth fiber (left) and spindle-knot structure (right), (C) high magnification image of spindle-knot structure, (D) high magnification image of PAN and GO sheets. (E) The water contact angle of PAN/GO and H-PAN/GO membranes (inset is dynamic water contact angle of PAN, H-PAN, and H-PAN/GO membranes). (F) Flux recovery ratio and rejection ratio of PAN/GO and H-PAN/GO membranes for separation of oil/water emulsion, respectively. Reprinted with permission from Zhang, J., Pan, X., Xue, Q., He, D., Zhu, L., Guo, Q., 2017b. Antifouling hydrolyzed polyacrylonitrile/graphene oxide membrane with spindle-knotted structure for highly effective separation of oil-water emulsion. Journal of Membrane Science 532, 38e46. Copyright © 2017, Elsevier.

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(H-PAN/GO) to tailor its chemical features. With the combination of chemical features of hydrolyzed PAN and a spindle-knotted structure, the antifouling performance of the composite membrane was enhanced and it performed well in separating oil/water emulsions (Zhang et al., 2017b). Compared to polymers, ceramic nanofibrous membranes have potential advantages in terms of relatively high surface energy, relatively stable chemical properties, and good antifouling ability. As shown in Fig. 13.11, Yang et al. used electrospun SiO2 NFs (SNFs) as the template, and anchored the SiO2 NPs uniformly on the surface of SNFs through a novel in situ synthesis method to fabricate

FIGURE 13.11 (A) Schematic illustrating the synthesis process of the hierarchical porous SiO2 NP/SNF membranes. (B) Optical photos showing the flexible SNFs and the corresponding SiO2 NP/SNF membranes. (C) SEM images of the related SiO2 NP/SNF membranes. (D) Digital photos of water permeation (top) and underwater oil repelling (bottom) by the SiO2 NP/SNF membranes. (E) Digital photos demonstrating the gravity-driven separation process of oil-in-water emulsions. (F) Permeation flux of different separation cycles. (G) Underwater OCA stability of the related membranes after calcination. Reprinted with permission from Yang, S., Si, Y., Fu, Q., Hong, F., Yu, J., AL-Deyab, S.S., EL-Newehy, M., Ding, B., 2014. Superwetting hierarchical porous silica nanofibrous membranes for oil/water microemulsion separation. Nanoscale 6, 12445e12449. Copyright © 2014, Royal Society of Chemistry.

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flexible, thermally stable, and hierarchically porous structured composite membranes with highly selective wettability of superhydrophilicity and underwater superoleophobicity. With prominent selective wettability and high porosity, the obtained SiO2 NP/SNF composite membranes exhibited an extremely high separation flux up to 2237 L/m2h and high separation efficiency for a surfactantstabilized oil-in-water emulsion (Yang et al., 2014). In further studies, NiFe2O4 NPs were incorporated in SNFs to fabricate hierarchical magnetic nanofibrous membranes: these membranes were able to separate surfactant-stabilized oil-in-water emulsions effectively in a relatively high separation flux (Hong et al., 2015; Si et al., 2015b).

13.3.2.2 Multilayer Structured SuperhydrophiliceOleophobic Nanofibrous Membranes Constructing a separation layer with high selective wettability and small pore size on the surface of electrospun nanofibrous membranes is an effective way to enhance oilewater separation efficiency (Wang et al., 2005; Zhang et al., 2014a). Various composite nanofibrous membranes have been developed for high flux and high efficiency separation of oil-in-water emulsions. The main idea is to deposit a very thin hydrophilic polymeric layer of poly(vinylalcohol) (PVA), chitosan, polyamide, or even ultrafine polysaccharide NFs on to electrospun nanofibrous membranes via physical absorption or interfacial polymerization. As shown in Fig. 13.12, a double-layer separation membrane was fabricated by depositing the PVA NFs on electrospun PAN nanofibrous membranes, and then remelting and cross-linking the PVA nanofibrous layer to construct a nonporous PVA film with a thickness of several micrometers. Using this hydrophilic PVA layer, the composite membrane could effectively separate oil-in-water emulsions with excellent antifouling properties. In general, when a microporous or nonporous coating layer is constructed on the surface of a nanofibrous membrane, the water-permeate flux of the obtained composite membrane will be significantly reduced, which is attributed to the hydraulic resistance of the coating layer (Wang et al., 2010). To address this problem, Raza et al. fabricated superhydrophilic and oleophobic nanofibrous membranes by incorporating a composite layer of polyethylene glycol (PEG) diacrylate NFs on PAN/PEG nanofibrous membranes (x-PEGDA@PG NF) using the in situ cross-linked method. The obtained x-PEGDA@PG NF membranes could be completely wetted by water within a very short time and the superhydrophilic layer could trap a layer of water on the surface of membrane; this blocked the contact of oil with the membrane, thus making the membrane oleophobic. Benefiting from the high selective wettability and high porous structures, the membranes were capable of effectively separating immiscible oil/water mixtures and oil-in-water emulsions with high capacity and robust antifouling property (Raza et al., 2014). Most recently, a novel superhydrophilic and underwater superoleophobic nanofibrous membrane with a hierarchical structured skin for the separation of oil-in-water emulsions was prepared via electrospinning and electrospraying methods. Unlike the conventional nonporous polymeric film or submicro fibrous layer, the hierarchical structured SiO2/PAN microspheres bonded with monofilament significantly enhanced the wetting selectivity and antifouling properties of the composite membranes. With the combination of a superwettable hierarchical structured skin layer and a high-porosity nanofibrous substrate, the membranes performed well in separating microscaled oil-in-water emulsions solely under the driving force of gravity with excellent separation efficiency and high fluxes (Ge et al., 2017).

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FIGURE 13.12 (A) Schematic demonstrating the fabrication process of thin film and nanofiber composite membranes. (B) The permeate flux and corresponding rejection rate of the nanofibrous composite membranes. (C) Schematic indicating the fabrication approaches of x-PEGDA@PG NF membranes. (D) Digital photos showing the separation process of oil/water mixtures using the x-PEGDA@PG NF membranes. (B) Reprinted with permission from Wang, X.F., Zhang, K., Yang, Y., Wang, L.L., Zhou, Z., Zhu, M.F., Hsiao, B.S., Chu, B., 2010. Development of hydrophilic barrier layer on nanofibrous substrate as composite membrane via a facile route. Journal of Membrane Science, 356, 110e116. Copyright © 2010, Elsevier. (D) Reprinted with permission from Raza, A., Ding, B., Zainab, G., ELNewehy, M., AL-Deyab, S.S., Yu, J.Y., 2014. In situ cross-linked superwetting nanofibrous membranes for ultrafast oil-water separation. Journal of Materials Chemistry 2, 10137e10145. Copyright © 2014, Royal Society of Chemistry.

13.3.3 MEMBRANES WITH CONTROLLABLE WETTING ABILITY FOR OILeWATER SEPARATION Beyond nanofibrous membranes with single hydrophobiceoleophilic or hydrophiliceunderwater oleophobic capabilities, smart special wettable membranes with controllable surface wettability have attracted increasing attention for their promising performance in both fundamental studies and industrial applications. In general, an external stimulus such as pH, temperature, light, or even gas were loaded on the solid surface to manipulate its wettability, thus a corresponding stimuliresponsive active material is the key factor for the fabrication of smart wettable surface (Li et al., 2015; Wang et al., 2015a). Consequently, various novel smart nanofibrous membranes with

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FIGURE 13.13 (A) Schematic demonstrating the fabrication process of PMMA-b-P4VP nanofibrous membrane and its switchable wettability performance. (B) Oil (n-hexane is colored with iodine) permeated through the membrane, while water remained above the glass apparatus. (C) Water selectively passed through the acidic water (pH ¼ 3) prewetted membrane. Reprinted with permission from Li, J.J., Zhou, Y.N., Luo, Z.H., 2015. Smart fiber membrane for pH-induced oil/water separation. ACS Applied Materials & Interfaces 7, 19643e19650. Copyright © 2015, American Chemical Society.

switchable wettability have been developed for oilewater separation. As shown in Fig. 13.13, Li et al. created a smart nanofibrous membrane by depositing the electrospun NFs of poly(methyl methacrylate)-block-poly(4-vinylpyridine) (PMMA-b-P4VP) on a substrate (steel mesh). With the combined pH-responsive property of P4VP and the oleophilic/hydrophilic property of PMMA, the resulting membrane exhibited switchable water/oil wettability, and was capable of pH-controllable oil/water separation solely driven by gravity. During separation oil permeated through the membrane and water selectively remained; when the same membrane was prewetted with acidic water, a reverse separating process could be achieved. In addition, the separation was highly efficient, and the membrane could maintain stable smart wettability after numerous separation cycles (Li et al., 2015).

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Wang et al. employed the thermo-responsive poly(N-isopropylacrylamide) (PNIPAAm) to modify an electrospun regenerated cellulose nanofibrous membrane via the surface-initiated atom transfer radical polymerization method. With a temperature-responsive property, the PNIPAAm grafted cellulose nanofibrous membranes exhibited switchable superlyophilic/superlyophobic properties and excellent controllable oilewater separation performance, showing promising application in wastewater treatment and oil purification (Wang et al., 2015b). In further studies, TiO2-doped electrospun PVDF nanofibrous membrane was fabricated, and showed switchable wettability while being stimulated by light (ultraviolet or sunlight) irradiation and heating treatment. The smart membranes could effectively separate oil/water mixtures by selectively allowing water or oil pass through alone (Wang et al., 2016b). More interestingly, a gas (e.g., CO2) could be used as the trigger to control the wettability of the membranes, with less influence on the physical structures of the membranes. Che et al. developed smart nanofibrous membranes by electrospinning the CO2-responsive copolymer PMMA-co-poly(N,N-diethylaminoethyl methacrylate) (PMMA-coPDEAEMA). Due to the tertiary amine groups in PDEAEMA, which could react with CO2 in water to give an extended hydrophilic chain conformation, the PMMA domains in the fibers were covered by the extended PDEAEMA chains with good mobility, thus making the material hydrophilic. If the CO2 was completely removed by N2 bubbling treatment, the hydrophilic surface wettability of the membranes could return to its initial hydrophobicity. With the advantage of CO2-induced selective wetting behavior and rough porous structures, these smart wettable nanofibrous membranes performed well in water and oil purification (Che et al., 2015). Apart from these stimuli-responsive membranes, Janus membranes also show promising application performance for switchable separation of oil-in-water and water-in-oil emulsions. Jiang et al. constructed a Janus membrane by coating a layer of hydrophobic CNTs in a network on the single side of electrospun PAN membranes. The obtained CNTs@PAN membrane exhibited asymmetric wettability on each side: the hydrophilic PAN side was underwater oleophobic, while the hydrophobic CNT side was underwater oleophilic. Thus the membrane showed switchable oil/water separation performance in different operating modes: highly efficient oil-in-water emulsion separation with the PAN side and water-in-oil emulsion separation with the CNT side (Jiang et al., 2017).

13.4 ELECTROSPUN NANOFIBROUS AEROGELS FOR OILeWATER SEPARATION As novel three-dimensional porous bulky materials, aerogels have attracted great attention for various applications. Aerogels are known for their high porosity, low bulk density, high surface area, and tortuous channels, giving them great advantages in oilewater separation (Dorcheh and Abbasi, 2008; Ge et al., 2016). As a result, several functional aerogels have been created for oilewater separation, such as the silica colloid aerogels (Adebajo et al., 2003), CNT aerogels (Hu et al., 2014), graphene aerogels (Li et al., 2014a), and polymeric aerogels (Jiang and Hsieh, 2014). The main two factors in fabrication of an aerogel for oilewater separation are surface wettability, which affects the interception and demulsification of the emulsified liquid droplets, and cellular structure, which dictates the permeation rate of one phase (for example, oil) passing through the aerogel. Exploration of the pertinent technologies to develop a novel oilewater separation technology based on aerogels is promising (Ge et al., 2014; Si et al., 2014).

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FIGURE 13.14 (A) Schematic demonstrating fabrication process of the hierarchical structured functional FIBER aerogels. (B) Digital photo showing the selective permeability of water and oil on the surface of a FIBER aerogel. (CeE) SEM images of FIBER aerogels with different magnifications. (F) The gravity-driven separation process of oil/water emulsions with the FIBER aerogels as filter and the corresponding optical microscopic images of emulsions and filtrate. (G) Change of the flux and flux recovery over 10 cycles. (H) The relative separation flux and driven pressure of selected separation materials. (I) Digital photo showing the apparatus for continuous oil collection from water-in-oil emulsions. Reprinted with permission from Si, Y., Fu, Q.X., Wang, X.Q., Zhu, J., Yu, J.Y., Sun, G., Ding, B., 2015a. Superelastic and superhydrophobic nanofiber-assembled cellular aerogels for effective separation of oil/water emulsions. ACS Nano 9, 3791e3799. Copyright © 2015, American Chemical Society.

Si et al. made impressive progress in the creation of nanofibrous aerogels, as shown in Fig. 13.14. They demonstrated a simple methodology for fabricating a superelastic and superhydrophobic aerogel with hierarchical porous structures consisting of bonded NFs, and called it “fibrous, isotropicallybonded elastic reconstructed” (FIBER) aerogel. As described, electrospun PAN/(benzoxazine, BAa) NFs and SNFs were selected as the major building blocks. The fabrication process started with blending the NFs to homogenize them and give uniform NF dispersion; the prepared dispersion was

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solidified and freeze-dried to produce unbonded nanofibrous aerogels; and the final step was thermo cross-linking the unbonded nanofibrous aerogels to obtain FIBER NFAs. Benefiting from a versatile and simple assembly process, the nanofibrous aerogel exhibits promising properties for various applications and holds great potential for scaling up (Si et al., 2014). Based on this study, an appropriate amount of SiO2 NPs was incorporated into the aerogel to enhance its hierarchical porous structures. Fluoric bifunctional benzoxazine (BAF-a) with low surface energy was employed as the in situ cross-linking agent. The resulting functional nanofibrous aerogels had integral properties of superelasticity, superhydrophobicesuperoleophilic wettability, and high pore tortuosity, which could effectively separate water-in-oil emulsions solely under the drive of gravity. Moreover, permeation fluxes up to 8140  220 L/m2h with a high separation efficiency were obtained, which was significantly higher than conventional filter membranes at the same separating condition, indicating a robust performance in real emulsion separation (Si et al., 2014).

13.5 CONCLUSIONS AND FUTURE PERSPECTIVES This chapter discusses electrospun nanofibrous materials for oilewater separation, including mats, membranes, and aerogels . Electrospinning technology as a versatile and simple approach for the fabrication of nanofibrous materials (polymer, ceramic, carbon, etc.) has gained increasing attention in the field of oilewater separation due to its advantages in controlling the individual fiber structures (diameter, surface energy), porous structures, and wettability of the fiber assembly via variable tuning. Based on these competitive merits, various electrospun nanofibrous oil/water separation media, including oil absorbents with high sorption rate and capacity and membranes or aerogels with high flux and separation efficiency, have been developed; these are crucial for practical applications in oily wastewater treatment and oil purification. Although great achievements have been made in the field of electrospun nanofibrous materials for oilewater separation, several challenges remain to be addressed. In most cases, to realize superwettability of the nanofibrous materials, fine hierarchically rough structures must be constructed on the surface of single NFs; however, these functional layers are quite easily damaged by external influences, and this radically alters the durability of the oilewater separation materials. Thus it is crucial to develop new approaches for the construction of durable, hierarchically rough layers on the surface of NFs to ensure the stability of special wettability. In addition, the current fabricated electrospun nanofibrous materials for oilewater separation still suffer from problems of poor mechanical properties compared with commercial products, and these need to be solved for practical applications. Moreover, the current studies mainly focus on the design and fabrication of various separation materials with different wettability. In fact, the oilewater separation process of nanofibrous materials is quite complex, and involves microfluid mechanics, interfacial chemistry, and even engineering science; however, in-depth investigations are seldom carried out to reveal the fundamental mechanisms and theories of this process. Better understanding is crucial for the improvement of oilewater separation performance. However, further development of electrospun nanofibrous materials for oilewater separation is under way, and we anticipate that electrospinning technology will allow great breakthroughs in the design and fabrication of the next generation of oilewater separation materials in the coming decades.

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ACKNOWLEDGMENTS This work was supported by the National Natural Science Foundation of China (Nos. 51473030 and 51673037), the Military Logistics Research Project (No. AWS14L008), the Shanghai Committee of Science and Technology (No.15JC1400500), “111 Project” Biomedical Textile Material Science and Technology (No. B07024), the Fundamental Research Funds for the Central Universities, and the “DHU Distinguished Young Professor Program.”

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Cheng Cheng, Xiong Li, Xufeng Yu, Min Wang, Xuefen Wang State Key Laboratory for Modification of Chemical Fibers and Polymer Materials, Donghua University, Shanghai, People’s Republic of China

14.1 INTRODUCTION Electrospinning is a very promising fiber-forming technology that can generate continuous ultrafine fibers with diameters ranging from the nano- to the microscale (Reneker and Chun, 1996; Li and Xia, 2010). In general, a typical electrospinning process involves the application of an electrostatic force between a polymer solution kept in a syringe and extruded by a pump and a metal collector as the counterelectrode, such as a flat plate or a rotating drum, kept at a suitable distance. With the application of a sufficiently high electrical field, the electrostatic forces overcome the surface tension of the polymer solution, resulting in the ejection of a thin jet from the pendent drop of polymer solution formed at the tip of the spinneret. The charged jet will first undergo a stable stretching, then bending and whipping, and then deposit randomly on the collector in the form of a textile. Electrospinning has been actively exploited as a facile, versatile, and low-cost technique offering unique capabilities for manufacturing nanofibers from various solutions, melts, emulsions, or suspensions (Subramanian and Seeram, 2013; Wang and Hsiao, 2016). Electrospun nanofibers can be deposited conveniently to form a nonwoven mat possessing an interconnected porous structure, high surface area, high porosity, and light weight. In addition, electrospun nanofibrous membranes (ENMs) can be easily functionalized to meet some certain desirable functions, such as by mixing functional additives into the electrospinning solution (Kaur et al., 2014), or attaching an active species for affinity adsorption (Ma et al., 2006), surface coating (Wang et al., 2006), or interfacial polymerization (IP) (Yoon et al., 2009a), or some other purposeful surface modification (Kaur et al., 2007; Min et al., 2012). These advantages give electrospun nanofibers huge potential for dealing with many arising health, environmental, energy, and other challenges. For example, a nonwoven scaffold of electrospun functional polymer nanofibers can serve as an ideal affinity scaffold for adsorption applications owing to its high porosity and large specific surface area (Min et al., 2012). In general, a large surface-to-volume ratio and many adsorption sites in the matrix are essential for adsorption affinity membranes to remove heavy metal ions or organic dyes from wastewater. Although our planet possesses abundant water resources due to the vastness of the oceans, the lack of adequate and clean drinking water is one of the most severe problems we face today. Global safe water scarcity is being exacerbated gradually with the growing human population, rapid urbanization Electrospinning: Nanofabrication and Applications. https://doi.org/10.1016/B978-0-323-51270-1.00014-5 Copyright © 2019 Elsevier Inc. All rights reserved.

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and industrialization, and climate change. It has been reported that about 50% of the world countries will experience water scarcity by 2025, about 1.2 billion people live without clean drinking water, and 2.6 billion have little or no sanitation (Homaeigohar and Elbahri, 2014). Many emerging water problems caused by urban and industrial pollution are undoubtedly recognized to threaten the environment and human life. For example, toxic heavy metal contaminants from industrial effluents (such as Hg, Pb, Cu, and Cd in wastewater from plating, ceramics, glass, mining, and battery manufacturing) discharged into the surface waters cause the contamination of the groundwater and will threaten human health severely. Industries could be the main sources of heavy metal contamination of the local water supply. As for organic pollutants, the level of concentration that would cause health hazards can be even lower than that of heavy metals, thus becoming even more pressing water issues (Huang et al., 2014). In response to this progressive water scarcity and pollution, there is a demand for the development of new, better, and more efficacious separation technologies for the removal of harmful contaminants (e.g., particles, oily emulsion, dyes, metal ions) from wastewater to ensure clean drinking water. Membrane-based separation technologies have played a dominant role in water purification due to their effectiveness and energy efficiency (Lee et al., 2016a; Dickhout et al., 2017; Wang et al., 2017). Whether membrane adsorption or a filtration process, both are considered effective means to remove unwanted species from contaminated water or prevent harmful contaminants from getting into the environment and the human body. However, progress in making conventional membranes more efficient and less energy consuming to address the pressing water challenges is constrained by their intrinsic limitations. Nanofiber-based membranes could be good alternatives, with better efficiency than the corresponding conventional membranes, for water treatment. The large surface-to-volume ratio and adjustable functionality of nanofibrous membranes render them much more effective for surface adsorption of contaminants from polluted water than conventional porous affinity membranes. Flexible polymeric nanofibrous membranes have high porosity (typically around 80%) and completely interconnected pores, with controllable pore size distribution from micrometers to submicrometers that also makes them very appropriate for a wide range of filtration applications (Wang and Hsiao, 2016). Highly permeable nanofibrous membranes could minimize the pressure drop and enlarge the permeate flux for water filtration compared with the conventional membranes being used (Thavasi et al., 2008; Yoon et al., 2008). Electrospinning turns out to be a remarkably simple and powerful means for fabricating functional nanofibrous scaffolds from a rich variety of materials, including polymers, composites, and ceramics, with the broadest range of functionalities and applications (Li and Xia, 2010; Wang and Hsiao, 2016). In particular, electrospun nanofiber scaffolds with a membrane structure could be used as effective membrane materials for water treatment, benefiting from the high porosity, interconnected pore structure, ease of incorporation of special functionality, and versatility to be used safely. In this chapter, we will focus on advances in functional nanofiber membranes or nanofiber-based composite membranes for several representative water purification systems, including adsorption, filtration, membrane distillation (MD), pervaporation, and hemodialysis. Without question, nonwoven nanofibrous articles and nanofiber-based composites have become an extremely promising and versatile water treatment platform, the potential of which will be further expanded by more innovative nanofiber technologies and modification techniques.

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14.2 NANOFIBER MEMBRANES Electrospinning techniques have gained substantial attention for the construction of nonwoven nanofibrous membranes. Particularly, ENMs possess higher porosity, larger specific surface area (up to 500 m2/g depending on the fiber diameter or a rough and porous structure), lower base weight, and completely interconnected open pore structure, compared with traditional polymeric or ceramic membranes (Barhate and Ramakrishna, 2007). These remarkable structural characteristics can dramatically improve the separation efficiency of membrane-based wastewater treatment, such as the increment of permeate flux, without compromising the contaminant rejection ratio in the liquid filtration. Herein, electrospun nanofibrous nonwoven membranes used as affinity membranes, microfiltration (MF) membranes, and MD membranes will be reviewed and discussed comprehensively.

14.2.1 AFFINITY MEMBRANES FOR ADSORPTION Affinity membranes are a broad class of membranes that can selectively capture targeted molecules. Electrospun nanofibers exhibit several remarkable structural features, including high specific surface area, interconnected pores, facile functionalization, convenient recyclability, and especially safety, which suggest them as promising candidates in the field of adsorption. In general, there are two methods to introduce functional groups onto the electrospun nanofibers: (1) utilizing polymers that contain functional groups for electrospinning the nanofiber and (2) surface modification of the electrospun nanofiber. Chang et al. (2007) demonstrated silk fibroin and keratose/silk fibroin blend electrospun nanofibers for heavy metal ion adsorption. Due to the large surface area and high porosity of the nanofibrous structure, the membranes exhibited a high adsorption capacity for Cu(II) (2.88 mg/mg), which was about 10 times higher than that of common fibrous filters (e.g., wool sliver and filter paper). Moreover, polyelectrolytes with charged and hydrophilic characteristics have been reported as promising and effective nontoxic materials for wastewater treatment. For example, electrospun chitosan nanofiber mats neutralized with potassium carbonate exhibited a certain corrosive stability in water and a high adsorption capacity for Cu(II) of 2.85 mmol/g and Pb(II) of 0.79 mmol/g (Haider and Park, 2009). Polyethylenimine (PEI), with abundant primary, secondary, and tertiary amino groups, has been widely used as an effective chelating agent for heavy metal removal. Based on this material, Min and colleagues (Wang et al., 2011) successfully fabricated PEI nanofibrous affinity membranes with a low loading of polyvinyl alcohol (PVA) as the fiber-forming agent by using a novel wet electrospinning method with a coagulation and crosslinking bath. The results showed that PEI/PVA electrospun membranes could remove heavy metal efficiently, and the equilibrium absorption capacities for Cu(II), Cd(II), and Pb(II) were 1.05, 1.04, and 0.43 mmol/g, respectively, and the resultant membranes could be reused by desorption with EDTA. Meanwhile, polyacrylic acid (PAA) is also an important polyelectrolyte material with abundant carboxyl groups. Thus, PAA-based electrospun nanofibers have received much attention for removing contaminants from water. For example, PVA/PAA (Xiao et al., 2010) and b-cyclodextrin/PAA (Zhao et al., 2015) electrospun nanofiber mats were fabricated to evaluate the adsorption properties toward heavy metals and dyes. The operation process for electrospun functional polymers is a very simple and easy approach. However, many functional polymers cannot be electrospun directly and must be blended with other

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electrospinnable polymers, but this is still followed by sacrificing a substantial portion of the functional groups because they might become embedded inside the nanofibers. In addition, the water solubility of many functional polymers will require that the as-prepared nanofibrous mat be further crosslinked to achieve stability in water, which will also consume functional groups. Therefore, surface modification of nanofibers with various functional groups, such as amino groups, amide groups, carboxylate groups, thiol groups, etc., has been demonstrated to improve the adsorption capability. The nitrile groups of electrospun polyacrylonitrile (PAN) nanofibers could be chemically modified with hydroxylamine (Feng et al., 2011; Saeed et al., 2008), hydrazine (Aoyama et al., 2011), diethylenetriamine (Neghlani et al., 2011; Kampalanonwat and Supaphol, 2010), and PEI (Zhao et al., 2017). For instance, Park and coworkers prepared amidoxime-modified PAN nanofibers by reacting PAN with hydroxylamine hydrochloride, and the resulting membranes could be used for adsorption of Cu(II) and Cd(II) with the saturation adsorption capacities of 3.4482 and 4.5408 mmol/g, respectively, which were about two times higher than that of conventional fibers based on amidoxime PAN (Feng et al., 2011). Wang and coworkers (Zhao et al., 2017) reported that a PAN nanofibrous membrane grafted with a novel branched PEI could be used for the efficient remediation of Cr(VI) from aqueous systems. The adsorbents possessed an excellent adsorption capacity for Cr(VI) of 637.46 mg/g due to the electrostatic interactions and reduction mechanism. Moreover, the residual Cr(VI) concentration could be decreased from 10 mg/L to below 0.05 mg/L (recommended as the standard of drinking water by the WHO) through batch adsorption and dynamic filtration. Ding and coworkers presented an intriguing and economic method for the construction of carbonyl-functionalized hydrolyzing cellulose nanofibrous membranes, in which hydrolytic cellulose acetate (CA) nanofibrous membranes could be grafted with pyromellitic dianhydride (Li et al., 2015b) and maleic anhydride (Ma et al., 2015), which exhibited high efficient adsorption performance of Pb(II) and lysozyme, respectively, and it also showed good regeneration and recycling ability. They also demonstrated another type of carbonylated nanofibrous membrane by in situ functionalizing of electrospun ethyleneevinyl alcohol nanofiber with citric acid; the resulting membranes possessed a relatively high dynamic adsorption capacity of 250 mg/g solely driven by gravity (Fu et al., 2016). Li et al. (2013) successfully fabricated a thioamidegroup-chelating PAN nanofibrous membrane; they found that the as-prepared membranes showed a high removal efficiency for Au(III) ions, and the maximum adsorption capacities were 15.86, 23.50, and 34.60 mmol/g, when the temperature was 298, 323, and 348 K, respectively. The specific surface area is another key factor to influence the adsorption capacity. To obtain a nanofibrous membrane with high specific surface area, it is favorable to introduce roughness and a porous structure onto the nanofiber surface by a chemical or physical method. Wang and colleagues have demonstrated polyethersulfone (PES)/PEI (Min et al., 2012) and PAN/polysulfone (PSF) (Hong et al., 2014) nanofibrous scaffolds with micro/nanostructural protuberances through a microphase separation technique, which is one of the effective methods to achieve a heterogeneous surface morphology. The novel micro/nanostructural PES/PEI affinity membranes were constructed by the electrospinning technique followed by solvent etching in a crosslinking solution. The nanoscaled PEI spherules (w200 nm) were uniformly etched on the nanofiber surface via the phase separation of PEI and PES, as shown in Fig. 14.1. The unique surface morphology could bring a significantly large PEI surface area per unit mass and high performance of adsorption capacity for heavy metal ions and dyes (Min et al., 2012). The research group also reported that PAN/PSF nanofibers with heterogeneous surface morphology could be prepared by electrospinning a PAN and PSF mixed solution, owing to the existence of microphase separation between the two partially miscible polymers in the process of

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FIGURE 14.1 Possible adsorption mechanism for (A) heavy metal ions and (B) anionic dyes with bifunctional amino and imino groups on the micro/nanostructure polyethersulfone/polyethylenimine nanofibrous membranes (Min et al., 2012). Reprinted with permission from Min, M., Shen, L., Hong, G., Zhu, M., Zhang, Y., Wang, X., Chen, Y., Hsiao, B.S., 2012. Micro-nano structure poly(ether sulfones)/poly(ethyleneimine) nanofibrous affinity membranes for adsorption of anionic dyes and heavy metal ions in aqueous solution. Chemical Engineering Journal 197, 88e100. Copyright 2012, Elsevier.

electrospinning. The generated scabrous heterogeneous structure on the PAN/PSF nanofiber surface would be beneficial to polydopamine (PDA) adhesion, and the as-prepared PDA-wrapped PAN/PSF composite nanofiber mat with high specific surface area (27.5 m2/g) showed effective adsorption of La(III) ions and could be desorbed and regenerated easily (Hong et al., 2014). In addition, the fabrication of porous nanofibers with well-distributed nanopores is another approach to increase the specific surface area of electrospun nanofibrous mats. In this sense, researchers have designed a series of mesoporous composite nanofiber mats such as the silane coupling agentsefunctionalized PVA/SiO2 (Irani et al., 2012; Wu et al., 2010; Li et al., 2011), polyvinyl pyrrolidone (PVP)/SiO2 (Teng et al., 2011), and PAA/SiO2 (Xu et al., 2012) nanofiber membranes. Wu et al. (Wu et al., 2010) synthesized a thiol-functionalized mesoporous PVA/SiO2 composite nanofiber membrane by the solegel electrospinning method using PVA, tetraethylorthosilicate, 3-mercaptopropyltrimethoxysilane, and cetyltrimethylammonium bromide. The best-performing membrane exhibited a high surface area of 474.3 m2/g and resulted in an efficient removal of Cr(III) (489.12 mg/g). Hong et al. (2015c) utilized PVP as the pore-forming agent and successfully fabricated a flexible and highly porous PAN nanofibrous membrane by one-step wet electrospinning from a PAN and PVP blended solution with a hot

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(A)

(B)

(C)

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FIGURE 14.2 Field emission scanning electron microscopy images of nanofibrous mats (left, superficial images; right, crosssectional images). Porous polyacrylonitrile nanofibrous membrane (A, B) before and (C, D) after amination (Hong et al., 2015c). Reprinted with permission from Hong, G., Xiong, L., Shen, L., Min, W., Wang, C., Yu, X., Wang, X., 2015c. High recovery of lead ions from aminated polyacrylonitrile nanofibrous affinity membranes with micro/nano structure. Journal of Hazardous Materials 295, 161e169. Copyright 2015, Elsevier.

water bath for in situ leaching of PVP (Fig. 14.2). The entirely porous structure promoted the high amination degree (88%) of PAN nanofiber with diethylenetriamine, which was almost twice the conversion rate from the neat PAN nanofibers reported by Neghlani et al. (2011). The resultant micro/ nanostructured aminated PAN (APAN) nanofibers possessed extremely high extraction capability of 1520.0 mg/g for Pb(II). On the basis of that study, Hong et al. (2015b) constructed a micro/nanostructured p-sulfonatocalixarene complex APAN nanofiber scaffold through electrostatic selfassembly of calix[8] on APAN nanofiber surfaces, which exhibited highly efficient and selective separation of La(III). Si and coworkers (Si et al., 2012a, 2012b; Ren et al., 2012) synthesized a hierarchical, porous, and functional Fe3O4@carbon nanofiber membrane with an extremely high surface area of 1885 m2/g, which possessed highly efficient adsorption of organic dyes as well as facilely magnetic separation properties. Furthermore, various other strategies have also been demonstrated to construct hierarchical nanofibers, such as a mesoporous NiFe@SiO2 nanofiber membrane (Hong et al., 2015a) and ZrO2@SiO2 nanofiber membrane (Wang et al., 2016d) that were fabricated through a combination of electrospun silica nanofibers with dip coating, Fe3O4/PAN composite nanofibers (Liu et al., 2015) and SiO2@g-AlOOH (boehmite) core/sheath nanofibers (Miao et al., 2012) that were prepared by a simple two-step process including electrospinning and a solvothermal method, and

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FIGURE 14.3 Illustration of the fabrication procedure for polyacrylic acid (PAA)eSiO2 hydrogel nanofibers for the adsorption of Ln3þ and its photoluminescent performance (Wang et al., 2016b). Reprinted with permission from Wang, M., Li, X., Hua, W., Shen, L., Yu, X., Wang, X., 2016b. Electrospun poly(acrylic acid)/silica hydrogel nanofibers scaffold for highly efficient adsorption of lanthanide ions and its photoluminescence performance. ACS Applied Materials & Interfaces 8, 23995e24007. Copyright 2016, American Chemical Society.

graphene oxide (GO)/Fe3O4-embedded PAN nanofibrous membranes (Koushkbaghi et al., 2016) that could be synthesized by a facile electrospinning process, and all of them exhibited a relatively high surface area and resulted in excellent performance for contaminants remediation from wastewater. Nevertheless, previous efforts focused excessively on the increase in specific surface area and adsorption sites on the nanofiber surface to promote the recovery efficiency, ignoring the potentially abundant available adsorption sites in the interior of the nanofibers. In 2016, Wang et al. (2016b) described a facile fabrication of a novel PAAeSiO2 hydrogel nanofiber (PAA-S HNF) through moderate thermal crosslinking of a PAA-S electrospun nanofibrous matrix followed by full swelling under water (Fig. 14.3). Due to the penetration of Ln(III) from the HNF surface to the interior through the loose and spongy porous network, the resultant PAA-S HNF scaffold exhibited a high adsorption capacity for Ln(III).

14.2.2 NANOFIBER MEMBRANES FOR MICROFILTRATION MF is a pressure-driven separation process of sieving particle sizes from 0.1 to 10 mm in the liquid environment, which plays an indispensable role in portable water purification and wastewater pretreatment, which requires the removal of waterborne bacteria, suspended microparticles, algae, etc. (Ghayeni et al., 1999; Wang et al., 2008). Compared with the low-flux limitation of traditional porous membranes manufactured by phase inversion, such as the Millipore GSWP MF membrane with a mean

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pore size of 0.22 mm, free-standing ENMs have exhibited great potential in MF because of their attractive structural characteristics such as interconnected open pore structure, high porosity, uniform fiber morphology with controllable pore size, and membrane thickness (Barhate and Ramakrishna, 2007). Gopal et al. (2006, 2007) have exploited the feasibility of developing polyvinylidene fluoride (PVDF) and PSF ENMs with post-heat treatment to enhance the structural integrity for the separation of polystyrene (PS) microparticles. The free-standing PVDF and PSF ENMs could remove more than 90% of 3- to 10-mm PS particles without any permanent fouling, while suffering from fouling irreversibly from 2- and 1-mm particles, with a “cake layer” forming on the membrane surface, and behaving as a depth filter with 0.5- and 0.1-mm particles being adhered on the nanofiber surface. Nylon-6 ENMs with excellent chemical and thermal resistance as well as high wettability have also been prepared and employed for MF. The average separation factor and flux of nylon-6 ENMs with thickness of 600 mm were 99.9% and 374.7 L/m2h for the separation of 1-mm PS microspheres (125 ppm) under an N2 flow of 10 psi (Aussawasathien et al., 2008). In addition, self-supporting nanofibrous polyethylene terephthalate (PET) membranes with attractive structural and mechanical characteristics could be applied to apple juice clarification, and this new clarification process possessed a high permeate flux and was more efficient, more economical, and simpler than the conventional process (Veleirinho and Lopesdasilva, 2009). Most electrospun materials are organic solvent-based polymers, which result in a less environmentally friendly fabrication process. Thus, water-soluble polymers, such as PVA, have become more desirable alternatives for electrospinning. Liu et al. (2013) have demonstrated that PVA ENMs crosslinked with glutaraldehyde (GA) could reject more than 98% of polycarboxylate microspheres with a diameter of 0.21 mm and maintained 1.5e6 times higher permeate flux than the Millipore GSWP membrane. In the electrospinning process, the processing variables, such as solution concentration, applied electric field, solution feeding rate, tip-to-target distance, etc., have a decisive influence on the fiber diameter, fiber morphology, and structural characters, and in turn determine the filtration performance of ENMs (Barhate et al., 2006). The pore size distribution of an electrospun nonwoven layer structure is correlated with the fiber diameter and the bulk porosity, i.e., the mean pore size is about 3  1 times the mean fiber diameter, and the maximum pore size is about 10  2 times the mean fiber diameter (Ma et al., 2011a; Wang et al., 2012). Thus, the membrane pore size can be adjusted by controlling the fiber diameter of electrospun mats, allowing them to satisfy the requirements for various MF applications. Hsiao and coworkers demonstrated the immersion of ultrafine cellulose nanowhiskers into a nanofiber scaffold to modify the effectiveness of MF ENMs for drinking-water purification (Ma et al., 2012b; Wang et al., 2013a). The surface of ultrafine cellulose nanowhiskers is wrapped with negatively charged carboxylate and aldehyde groups, enabling them to adsorb positively charged species, bacteria, viruses, or toxic metal ions. Fig. 14.4 clearly shows the structural morphology of cellulose nanowhiskers and PAN ENMs infused with cellulose nanowhiskers. These modified ENMs could completely remove Brevundimonas diminuta and Escherichia coli by size extrusion, and contained the adsorption capability of 100 mg/g Cr(VI) or 260 mg/g Pb(II), while maintaining a high permeate flux (1300 L/m2hpsi). Meanwhile, Hsiao and colleagues (Ma et al., 2014b) also reported PAN ENMs with improved mechanical properties through the polymerization of dual-vinyl and triple-vinyl monomers on the PAN nanofiber surface. The dual-vinyl monomer with imidazolium cations resulted in the positively charged surface, allowing PAN ENMs not only to create two to three times higher permeate

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FIGURE 14.4 (A) Transmission electron microscopy image of cellulose nanowhiskers fabricated by the 2,2,6,6tetramethylpiperidinooxy (TEMPO)/NaBr/NaClO oxidation method. The inset shows the electron diffraction pattern. Scanning electron microscopy images of cross-sectional views of (B) a polyacrylonitrile (PAN) electrospun nanofibrous scaffold and (C) a cellulose nanowhisker-modified PAN electrospun nanofibrous scaffold. Schematic representations of electrospun nanofibrous scaffolds: (D) scaffold before nanowhisker infusion; (E) infused nanowhiskers forming a loose crosslinked mesh; (F) nanowhiskers collapsed onto the scaffold, forming bundles (Ma et al., 2012b). Reprinted with permission from Ma, H., Burger, C., Hsiao, B.S., Chu, B., 2012b. Nanofibrous microfiltration membrane based on cellulose nanowhiskers. Biomacromolecules 13, 180e186. Copyright 2012, American Chemical Society.

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flux and lower pressure drop, but also to have 99.9999% retention of bacteria and 99.99% retention of bacteriophage compared with commercial MF membranes. Consequently, these results fully demonstrated the great potential of nanofiber membranes for drinking-water purification and as prefilters for ultrafiltration (UF), nanofiltration (NF), reverse osmosis, the food industry, etc.

14.2.3 NANOFIBROUS MEMBRANES FOR MEMBRANE DISTILLATION MD, a new type of highly efficient membrane separation technology, has gained substantial attention. MD is a thermally driven membrane separation process, in which only vapor molecules can transmit through a hydrophobic microporous membrane. The driving force in the MD process is the vapor pressure difference induced by the temperature difference (DT) between the feed and the permeate fluid. Regarding the MD membrane, it has to possess sufficient hydrophobicity, microporosity, and excellent breathability, which are beneficial to the osmotic transmission of vapor molecules. Moreover, the mechanical properties, thermal stability, and corrosion resistance of the membrane are also indispensable prerequisites for practical long-term MD operation (Khayet, 2011). Compared with the traditional hydrophobic MD membranes (flat-sheet membrane and hollow-fiber membrane) with inner disconnected pore structure and poor porosity, which are fabricated by phase inversion, the highly hydrophobic or superhydrophobic ENM exhibits a series of attractive characters, such as a fully interconnected open pore structure, high porosity, uniform fiber morphology with controllable pore size and membrane thickness, etc., which can remarkably improve the drawbacks of low permeate flux and membrane pore wetting during the long-term MD operation process (Tijing et al., 2014). In 2008, Feng et al. (2008) first reported electrospun PVDF nanofibrous membranes for air-gap MD to produce potable water from saline water with NaCl concentrations of 1, 3.5, and 6 wt%. The maximum permeate flux and rejection factor of the PVDF ENMs were 11e12 kg/m2h and 98.7%e99.9% under a DT of 60 C. However, such permeate flux and rejection factor were not very satisfactory, and the relationship between membrane structural characteristics and MD performance were not investigated in detail. Khayet et al. (Essalhi and Khayet, 2013, 2014) first discussed the effects of membrane thickness and PVDF concentration on the ENMs’ structural characteristics and direct contact MD (DCMD) performance. They found an enhancement of the liquid entry pressure of water, a decrease in the mean flow pore (MFP) size, and no significant changes in porosity (85%e93%) with an increase of thickness. The optimized PVDF concentration for nanofiber formation was 25 wt%, and this PVDF ENM exhibited a DCMD permeate flux of 38.9 kg/m2h for 30 g/L NaCl feed solution operating at DT of 60 C, while the NaCl rejection factor was higher than 99.99%. Moreover, Liao et al. (2013) further demonstrated that the heat-press posttreatment could improve the ENM’s integrity and would be beneficial to avoid membrane pore wetting in DCMD operation. The posttreated PVDF ENMs presented a stable permeate flux of about 21 kg/m2h over a testing period of 15 h when using a 3.5 wt% NaCl solution as feed at DT of 30 C, which was better than that of commercial PVDF membranes. To further enhance the efficiency (permeability and selectivity of MD performance) and competitiveness of the MD separation process, various approaches have been implemented for the design and fabrication of modified PVDF ENMs (Prince et al., 2012; Lalia et al., 2013). Liao and coworkers (Yuan et al., 2013) reported an integrally modified superhydrophobic PVDF ENM, including dopamine surface activation, silver nanoparticle (NP) deposition, and surface treatment with low surface energy matter. The DCMD performance exhibited a high and stable permeate flux of

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31.6 L/m2h using a 3.5 wt% NaCl as feed at DT of 40 C. However, the robustness and operability of silver NPs adhered on the PVDF nanofiber surface are perhaps of limited duration, as are those of negatively charged silica NPs grafted on the positively charged PVDF-co-hexafluoropropylene (PVDFeHFP)/cationic surfactant nanofiber surface via electrostatic interaction (Lee et al., 2016b). Therefore, the use of electrospun PVDF or PVDFeHFP solutions incorporated with inorganic nanofillers, such as SiO2 NPs (Li et al., 2015a), carbon nanotubes (Tijing et al., 2016), and graphene (Yun et al., 2016), has been considered an effective method to enhance structural robustness, surface roughness, hydrophobicity, and mechanical properties. Li et al. (2015a) have demonstrated robust superhydrophobic PVDFeSiO2 ENMs with different fiber diameter distributions by controlling the silica particle size, as shown in Fig. 14.5. The fiber diameter distribution has a decisive influence on determining the augmentation of permeate flux through regulating the MFP. The resultant membrane with optimized MFP presented a permeate flux of 41.1 kg/m2h and stable low permeate conductivity (w2.45 ms/cm) (3.5 wt% NaCl salt feed; DT ¼ 40 C) over a DCMD test period of 24 h without membrane pore wetting being detected. Furthermore, PVDFeSiO2 colloid solutions could also be electrosprayed as nanobeads on the surface of a PVDF nanofiber scaffold to form a composite membrane with lotus-leaf-like superhydrophobicity (Liao et al., 2014a, 2014b), like polydimethylsiloxane microspheres electrosprayed on a PVDFeHFP ENM surface (An et al., 2017), which could be beneficial to avoid membrane pore wetting efficiently during the long-term MD operation process. It is noteworthy that PVDF and PVDFeHFP are so far the polymers used mostly in ENMs for MD evaluation, since they can be easily dissolved in common solvents compared with the commercial MD membrane materials polypropylene and polytetrafluoroethylene (PTFE). However, compared with hydrophilic filtration membranes, the electrospinnable polymeric materials with intrinsic hydrophobicity used for MD are extremely limited. Thus, much attention should be paid to developing novel electrospun membrane materials, such as dual-biomimetic superhydrophobic PS ENMs (Li et al., 2014), polyamide (PA) (Guo et al., 2015) and PVA (Dong et al., 2015) ENMs through surface modification with fluoride, PTFE ENMs prepared by sintering the composite nanofibers from electrospinning PTFE/PVA precursor solutions (Tao et al., 2014), polyether imide ENMs doped with hydrophobic organosilica NPs (Hammami et al., 2017), and the well-designed aromatic fluorinated polyazole ENMs (Maab et al., 2012). These have been successfully designed and manufactured for MD performance evaluation. In 2016, Li et al. (2016b) reported a novel profiled PANePS coreeshell nanofiber with peculiar C-shaped groove structure by using an eccentric-axial electrospinning technique, as shown in Fig. 14.6. The unique groove morphology could remarkably increase the membrane porosity, resulting in the promotion of thermal efficiency and enhancing the permeate flux in the MD process. The resultant DCMD performance exhibited a permeate flux of 60.1 kg/m2h and high-quality water permeate (20 g/L NaCl and 1000 ppm sunset yellow FCF aqueous solution as feed, DT ¼ 40 C) over a DCMD test period of 36 h. This study is the first example of an investigation into the effect of profiled and/or macroscopic fiber morphology on MD performance. In addition, Huang et al. (2017) fabricated a SiO2 NP-roughed silica nanofiber by using coaxial electrospinning technology; the fabricated silica ENMs exhibited superamphiphobicity after surface fluorination and robust MD performance in the presence of surfactants. Consequently, superhydrophobic ENMs with appropriate structural characteristics have exhibited their great potential for MD applications.

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FIGURE 14.5 Field emission scanning electron microscopy images of electrospun nanofibrous membranes (AeC) polyvinylidene fluoride (PVDF), (DeF) PVDFeSiO2 (S)-40, (GeI) PVDFeS-167, and (JeL) PVDFeS-210 (Li et al., 2015a). Reprinted with permission from Li, X., Yu, X., Cheng, C., Deng, L., Wang, M., Wang, X., 2015a. Electrospun superhydrophobic organic/inorganic composite nanofibrous membranes for membrane distillation. ACS Applied Materials & Interfaces 7, 21919e21930. Copyright 2015, American Chemical Society.

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FIGURE 14.6 Schematic diagram illustrating the eccentric-axial electrospinning technique for the fabrication procedure of grooved coreeshell nanofibrous membranes for direct contact membrane distillation (Li et al., 2016b). Reprinted with permission from Li, X., Deng, L., Yu, X., Wang, M., Wang, X., Garcı´apayo, C., Khayet, M., 2016b. A novel profiled coreeshell nanofibrous membrane for wastewater treatment by direct contact membrane distillation. Journal of Materials Chemistry A 4, 14453e14463. Copyright 2016, Royal Society of Chemistry.

14.3 NANOFIBER-BASED COMPOSITE MEMBRANES Ideal composite membranes for water treatment should comprise a thin but defect-free skin layer and a supporting layer with high porosity and low tortuosity. An ENM as a supporting layer offers some significant advantages over conventional UF membranes fabricated by a phase-inversion method, such as a highly porous structure and fully interconnected architecture (Wang and Hsiao, 2016). Thus, ENMs are well recognized as a desired substrate for preparing high-performance thin-film nanofibrous composite (TFNC) membranes, which offer relatively low mass transfer resistance and high permeability. The potential of the TFNC membrane is not limited only to aqueous-based media as in UF, NF,

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and forward osmosis (FO) separation processes, but it is also used for hemodialysis and pervaporation processes. In this section, the critical technical challenges that one might encounter in the fabrication process of advanced nanofiber-based composite membranes for different applications and the possible approaches that could be employed to overcome these limitations will be summarized.

14.3.1 THIN-FILM NANOFIBROUS COMPOSITE MEMBRANES FOR ULTRAFILTRATION UF is a process that can filter out a broad spectrum of particulates, such as viruses, emulsions, proteins, and colloids, with sizes ranging from about 1 to 100 nm in the liquid environment (Yoon et al., 2008). Thin-film composite (TFC) membranes have become the most widely used membrane system in the UF process. Such a membrane system comprises three fundamental layers, which include the top ultrathin selective layer, the middle porous support layer, and finally the bottom nonwoven fabric layer. Previously, the conventional middle layer was replaced with ENMs. This whole fibrous membrane structure is termed the TFNC membrane format (Wang and Hsiao, 2016). It has been observed that the flux rate and rejection percentage of the TFNC membranes were higher than those of commercially available UF membranes (Ray et al., 2016; Ma et al., 2016). Therefore, the application of electrospun fibers as the middle layer of TFC membranes has attracted worldwide attention. The demonstrated TFNC UF membranes consist of a three-layered hierarchical structure, as schematically shown in Fig. 14.7 (Wang et al., 2005). The bottom layer is composed of a nonwoven substrate (e.g., PET nonwoven web), which provides the overall mechanical strength to the membrane. The typical thickness of the bottom substrate is in the range of a few hundred micrometers, having fibers with diameters of about 10e30 mm. This nonwoven substrate has a relatively high porosity (about 40%e60%), a Young’s modulus up to 500 MPa, ultimate tensile strength up to 40 MPa, and an

FIGURE 14.7 Schematic representation of a thin-film nanofibrous composite ultrafiltration membrane (Wang et al., 2005). Reprinted with permission from Wang, X., Chen, X., Yoon, K., Fang, D., B.S. Hsiao., Chu, B., 2005. High flux filtration medium based on nanofibrous substrate with hydrophilic nanocomposite coating. Environmental Science & Technology 39, 7684e7691. Copyright 2005, Royal Society of Chemistry.

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elongation-to-break ratio of around 20%, which provides sufficient elastic property to the membrane (Ma et al., 2010a). The middle layer is composed of electrospun nanofibers with high porosity and interconnected pores, instead of the conventional microporous membrane prepared by the phaseinversion method. Typically, PAN (Yoon et al., 2009b; Hao et al., 2013; Khamforoush et al., 2015), crosslinked PVA (Wang et al., 2006; Tang et al., 2009), PVDF (Zhou et al., 2013; Zhang et al., 2013), CA (Huang et al., 2016), and other polymers can be employed to fabricate electrospun nanofibrous scaffolds with a typical layer thickness of w50 mm. The thickness of the barrier layer is typically between 100 and 1000 nm, with the pore size ranging from 5 to 50 nm, depending on the application. It should be noted that based on the concept of TFNC membranes, several breakthroughs in UF performance were made as a result of the following factors: (1) The nanofibers in the midlayer support have unique properties. The typical porosity of an electrospun scaffold is above 80% (it is about three to four times higher than that of conventional membranes prepared by the phase-inversion method). And the pores in an electrospun scaffold, as defined by stacking of nanofibers, are interconnected (Ma et al., 2010a). (2) Directed water channels created through the interface between the nanofiber and the polymer matrix in the barrier layer are implemented in the filtration (as schematically shown in Fig. 14.8) (Ma et al., 2012a). In other words, part of the electrospun scaffold has been incorporated into the barrier matrix. The key feature of this structure is that as the embedded nanofibers are interconnected, the interface between the nanofiber and the barrier matrix can be used to generate water channels for more efficient water transportation. These directed water channels could be of great benefit in increasing the water permeate flux (Ma et al., 2012a). (3) The barrier layer has a unique ultrathin structure. The thickness of the barrier layer in this structure is typically on the order of 50e500 nm, depending on the fabrication process, through coating (Yoon et al., 2006), electrospinning combined with solvent vapor exposure (Wang et al., 2010), electrospraying combined with solution treatment (You et al., 2012), electrospraying combined with hot-pressing treatment (Shen et al.,

FIGURE 14.8 Schematic representation of the nature of water channels in the barrier layer (Ma et al., 2012a). Reprinted with permission from Ma, H., Burger, C., Hsiao, B.S., Chu, B., 2012a. Highly permeable polymer membranes containing directed channels for water purification. ACS Macro Letters 1, 723e726. Copyright 2012, American Chemical Society.

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2016b), or other means. (4) Hydrophilic materials are used as the barrier layer, including crosslinked PVA (Ma et al., 2010b), crosslinked polyethylene glycol (Wang et al., 2013b), CA (Soyekwo et al., 2014), chitosan (Zhao et al., 2012), chitin (Ma et al., 2011c), and polysaccharide ultrafine nanofibers (Cao et al., 2013). The increase in hydrophilicity of the barrier layer was found to enhance the antifouling properties of UF membranes. It is interesting to note that most of the commercial membranes are made of hydrophobic materials, such as PVDF, etc., because these polymers are durable under long-term operation in aqueous solutions. However, the hydrophobic surface has a lower surface energy, which would lead to the adsorption of lower-surface-energy materials and thereby result in fouling and reduce the water permeability during the UF process. Thus, a hydrophilic membrane surface would be of benefit to maintain a high permeate flux. Based on the aforementioned breakthroughs and added advantages in UF performance, TFNC membranes may play an important role in the fabrication of high-flux UF membranes for water purification. Some specific examples of these applications are described in the following. Wang et al. (2005) first reported a high-flux TFNC-based UF membrane system containing a “nonporous” hydrophilic coating that was water permeable (Pebax or PVA), an electrospun nanofibrous support, and a nonwoven microfibrous substrate (Fig. 14.9). The TFNC UF membrane was fabricated by cast coating with the separation material on the electrospun membrane. Results indicated that these unique hydrophilic TFNC membranes exhibited a very high flux (several times higher than conventional TFC-based UF membranes) and comparable high rejection ratio for oil-and-water emulsion separation. Interestingly, the permeability of TFNC UF membranes can be dramatically increased by introducing nanofillers (such as surface-oxidized multiwalled carbon nanotubes [MWCNTs] or cellulose nanofibers [CNs]) as a means to induce water channels in the barrier layer. For example, the addition of 1.3 wt% of CNs or 10 wt% of oxidized MWCNTs could increase permeability by two to three times with the similar rejection ratio of 99.8% for oil/water emulsion (Ma et al., 2010b). Wang et al. (2006) also used electrospinning to fabricate a PVA-based substrate, followed by

FIGURE 14.9 Typical scanning electron microscopy cross-sectional images of composite membranes. (A) Coated with pure Pebax. (B) Coated with a polyvinyl alcohol hydrogel (Wang et al., 2005). Reprinted with permission from Wang, X., Chen, X., Yoon, K., Fang, D., B.S. Hsiao., Chu, B., 2005. High flux filtration medium based on nanofibrous substrate with hydrophilic nanocomposite coating. Environmental Science & Technology 39, 7684e7691. Copyright 2005, American Chemical Society.

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FIGURE 14.10 A three-layered fibrous structure with the barrier layer coated with ultrafine cellulose nanofibers for ultrafiltration (Ma et al., 2011b). Reprinted with permission from Ma, H., Burger, C., Hsiao, B.S., Chu, B., 2011b. Ultrafine polysaccharide nanofibrous membranes for water purification. Biomacromolecules 12, 970e976. Copyright 2011, American Chemical Society.

crosslinking to improve the structural stability and mechanical strength of the membrane. In addition, the applied fabricated membrane seems to be efficient in UF techniques because of low membrane fouling in nature. Yoon et al. (2006) demonstrated that after 24 h of operation, the permeate flux for the electrospun-based membrane remained stable, whereas a commercial UF membrane experienced severe flux decline. The success of the aforementioned TFNC UF membranes could be attributed partially to the unique characteristics of an electrospun scaffold with high porosity and interconnected pores and the utilization of a highly hydrophilic barrier layer (e.g., PVA). In 2011, a unique TFNC UF membrane system with top barrier layers made of ultrafine polysaccharide nanofibers (diameters 5e10 nm) was demonstrated by Chu and Hsiao’s group (Fig. 14.10) (Ma et al., 2011a, 2011b, 2014a). For example, ultrafine CNs with 5-nm mean fiber diameter were prepared from wood pulp. CNs could be formed into a gel and coated on the nanofibrous support. The relatively thin barrier layer thickness provided high water permeability. The flux of the CN-based TFNC UF membrane was 12 times higher than that of a commercial UF membrane for separation of oil emulsions and water, while retaining the same rejection ratio (above 99.5%). In addition, the hydrophilic surface of the CNs (the water contact angle was 10.9 degrees) might be very useful in alleviating fouling of filtration of oil/water emulsions. The preparation of an appropriately thin barrier layer on the highly porous electrospun nanofibrous scaffold often faces a series of challenges. For example, using the conventional casting method, penetration of the coating solution into the nanofibrous scaffold is unavoidable, which would result in a much thicker barrier layer. Various methods, such as adjusting the viscosity of the cast solution through gelation (Wang et al., 2006), soaking the nanofibrous scaffold with coagulant of the cast solution (Yoon et al., 2009b), and photo-crosslinking of a UV-reactive coating solution (e.g., photo-crosslinkable PVA) (Tang et al., 2009), have been demonstrated to minimize the penetration of the cast solution to the electrospun nanofibrous support and to decrease the thickness of the barrier layer. Wang and coworkers demonstrated a unique technique to prepare an ultrathin barrier layer on the nanofibrous scaffold and further improved the performance of the TFNC UF membrane (Wang et al., 2010; You

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et al., 2012; Shen et al., 2016b; Yang et al., 2017b). In their approach, a double-layer electrospun nanofibrous mat, containing a thin hydrophilic nanofibrous top layer and a nanofibrous support layer, was manufactured via electrospinning or electrospraying. The top electrospun layer was subsequently fused using solvent vapor or solvent solution or hot-pressing treatment to form a thin integrated barrier layer without the fibrous structure. An alternative TFNC UF membrane was prepared by remelting the deposited PVA electrospun nanofibers on the PAN electrospun membrane with water vapor (Fig. 14.11A) (Wang et al., 2010). The filtration performances of the PVA/PAN composite membranes were evaluated by the oil/water emulsion separation system, and the highest permeate flux of 210 L/m2h was achieved with the rejection of 99.5% for the composite membrane under the operating pressure of 0.3 MPa. In another of their studies, an electrosprayed PVA top layer was swollen in a water/acetone mixture to form an intact filmlike barrier layer on the electrospun PAN support layer (Fig. 14.11B). The resultant TFNC membrane, containing a crosslinked PVA barrier layer, possessed very high flux (347.8 L/m2h) and a high rejection ratio (99.6%) at low pressure (0.2 MPa) for separation of oil emulsion and water (You et al., 2012). The system was able to maintain a much higher flux value than a typical commercial UF membrane, with no loss of oil rejection capability under a long cross-flow UF operation. To precisely control the compactness of the top-layer film, Wang and coworkers improved the strategy by a hot-press top-layer formation process (Fig. 14.11C) (Shen et al., 2016b). Specifically, the electrosprayed PVA layer was moistened and subsequently softened by hotpressing treatment to form an integrated barrier film on the supporting layer. The obtained TFNC membrane was used to filter molecules smaller than in an oil emulsion, such as in a bovine serum albumin (BSA) solution. Specifically, the optimized PVA/PAN TFNC membrane possessed a high UF performance in BSA filtration tests, with a water flux of 173.0 L/m2h and rejection above 98.0% at a low feeding pressure of 0.3 MPa. A novel three-tiered arrangement of composite membranes consisting of an ultrathin PAN-co-acrylic acid (PANeAA) barrier layer based on a PAN nanofibrous support layer was developed (Fig. 14.11D) (Yang et al., 2017b). Due to its hydrophilic and negatively charged barrier layer, the TFNC UF composite membranes exhibited excellent permeate flux (221.2 L/m2h) and rejection efficiency (97.8%) for BSA aqueous solution at 0.3 MPa. These methods all overcome the typical challenge of easy penetration of the coating solution into the porous substrate and eliminate the bottleneck of the barrier thickness, thus offering great potential in fabrication of TFNC membranes on a large scale.

14.3.2 THIN-FILM NANOFIBROUS COMPOSITE MEMBRANES FOR HEMODIALYSIS Hemodialysis is the process of eliminating toxic metabolites from blood via diffusive or convective transport across the membrane and preventing loss of necessary proteins via pore size exclusion of the membrane. Uremic toxins can be categorized into two groups: small-molecule uremic toxins (200 mm) by a newly developed method and successfully assembled them on a highly porous nanofibrous mat using the vacuum suction method. As shown in Fig. 14.14, the nanochannels, formed by interlayer spacing between adjacent GO nanosheets, enabled an ultrafast water permeability, which is claimed to stem from a quickly passing hydrophilic “gate” (edges of GO nanosheets or defects in the GO) for aggregation and the fast slip flow along the atomically smooth and hydrophobic carbon walls. Moreover, the GO TFNC membranes showed high rejection performance (nearly 100% rejection of Congo red and 56.7% for Na2SO4). In recent years, FO technology has received worldwide attention in the applications of wastewater reclamation, seawater desalination, and energy production because it is energy effective, shows high rejection of contaminants, and causes low pollution (Subramanian and Seeram, 2013; Li et al., 2016a). Unfortunately, most reported FO membranes encounter the internal concentration polarization (ICP) problem, which is mainly responsible for water flux decline, and this seriously occurs in phaseinversion support layers. The ENM with high porosity, low tortuosity, and thin cross section is superior to support layers formed by conventional phase inversion, and it has been demonstrated to be able to break the bottleneck of ICP in the FO process (Bui et al., 2011; Song et al., 2011). In 2011, Song et al. (2011) polymerized a PA barrier layer on top of a hydrophobic PES nanofibrous membrane. This novel TFNC FO membrane exhibited water flux close to 35 L/m2h in the mode of the membrane active layer facing the draw solution, using deionized water as the feed solution and 0.5 M NaCl as the draw solution, which was much higher than that of an FO membrane with a phase-inversion support layer. Subsequently, hydrophobic electrospun PSF (Bui et al., 2011), PVDF (Tian et al., 2013), and PET nanofibers (Hoover et al., 2013) were used as substrates to make high-performance FO membranes. Nevertheless, ICP may be enhanced by the hydrophobic nature of the support layer, and solutes can diffuse only through the wetted porosity of the support, indicating that “wetted porosity” in hydrophilic supporting layers is essential for solute diffusivity and available pathways for water transport. Therefore, some TFNC membranes supported by hydrophilic nanofibers, such as PANeCA (Bui and McCutcheon, 2013) and nylon-6,6 (Huang and McCutcheon, 2014), and hydrophobic/hydrophilic interpenetrating network composite nanofibers, such as PET/PVA (Tian et al., 2014), were fabricated for FO.

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FIGURE 14.14 Schematic illustration of the fabrication of graphene oxide (GO) thin-film nanofibrous composite membranes and conceptual illustration of the hydrophilic “gates” and hydrophobic nanochannels in the GO layer (Wang et al., 2016a). Reprinted with permission from Wang, J., Zhang, P., Liang, B., Liu, Y., Xu, T., Wang, L., Cao, B., Pan, K., 2016a. Graphene oxide as an effective barrier on a porous nanofibrous membrane for water treatment. ACS Applied Materials & Interfaces 8, 6211e6218. Copyright 2016, American Chemical Society.

Furthermore, one of the big challenges that still exists in fabricating TFNC membranes for FO is that the electrospun nanofibrous substrate suffers significant porosity reduction during a necessary posttreatment stage (i.e., bonding the individual nanofibers to achieve reasonably high mechanical strength and integrity). Tian et al. (2015) reported a novel TFNC FO membrane, consisting of a polyetherimide nanofibrous substrate reinforced by functionalized MWCNTs and an ultrathin PA rejection layer. The improved membrane mechanical strength enabled a further improvement of the membrane structure to mitigate the ICP. More recently, silica NPs were also embedded in a polyetherimide nanofibrous support for a TFNC membrane to limit substrate compaction and porosity reduction during the heat-press treatment, thus alleviating undesired ICP during the FO process (Tian et al., 2017). The polyetherimide nanofibrous substrate with the highest silica loading of 1.6 wt% in dope solution had the maximum porosity (83%). The corresponding FO membrane had the smallest structural parameter (w174 mm), and the most promising water flux, as high as 72 L/m2h in the active layer facing the draw solution (1.0 M NaCl) and 42 L/m2h in the active layer facing the feed water (deionized water).

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14.3.4 THIN-FILM NANOFIBROUS COMPOSITE MEMBRANES FOR PERVAPORATION In past decades, pervaporation was considered an efficient membrane process for liquid separation, which has distinct advantages for separating azeotropic and close-boiling mixtures as well as safely handling heat-sensitive and hazardous compounds owing to its mild operation conditions, relative simplicity, environmental benignity, and low energy cost. The success of the pervaporation process largely depends on membranes. Studies have demonstrated that the reduced mass transfer resistance and increased mass transfer efficiency during pervaporation could be achieved by using porous nanofiber-supported composite membranes, resulting in higher flux (Yeh et al., 2012, 2013; Hung et al., 2014). However, it is difficult to fabricate a dense and thin barrier layer on a porous nanofibrous substrate without the coating layer penetrating into the substrate’s three-dimensional channels. Two different strategies are proposed to circumvent this issue: introducing an intermediate transport layer (Yeh et al., 2012, 2013) and IP (Hung et al., 2014). For example, ultrafine CNs (diameter about 5 nm) as a buffer layer on a PAN nanofibrous substrate overcame the PVA intrusion problem (Yeh et al., 2012). The produced TFNC membranes with a dense PVA barrier layer have significantly higher permeate flux than those based on conventional porous membranes fabricated by the conventional phase method for separating ethanol and water. Moreover, the crosslinked PVA layer could be replaced by a thin layer of GO (prepared by spin coating or vacuum filtration), creating a new TFNC membrane system for ethanol dehydration (Yeh et al., 2013). This TFNC membrane exhibited high water permeability (flux of 2.2 kg/m2h) and excellent selectivity (separation factor of 308) for ethanol dehydration at 70 C. In another strategy, Hung et al. (2014) fabricated dual heterogeneous PA layers on an electrospun PAN nanofibrous substrate through IP integrated with spin coating, and various amine and acyl chloride monomers were used to assemble heterogeneous PA layers on the nanofibrous support. Significantly, the resulting ultrathin membranes showed an ideal separation factor of 4499 and permeate flux of 13.9 kg/m2h for 90 wt% ethanol in water at 70 C, which were respectively 19 and 8 times higher than those of the commercial PERVAP 2210 membrane. Research on desalination via pervaporation has become more and more active in recent years; this method has the advantages of a high rejection of salt and the capability of coping with high-salinity solutions (Wang et al., 2016c). Liang et al. (2014) developed a TFNC membrane for pervaporation desalination by sequential deposition employing an electrospraying/electrospinning technique. A crosslinked PVA barrier layer with a thickness of 700 nm on a PAN nanofibrous substrate displayed excellent desalination performance (i.e., high water flux and a salt rejection ratio >99.5%) for different salt concentrations. GO has attracted great attention for membrane separation, as mentioned earlier, especially in the field of pervaporation application. But it is a big challenge to prepare a stable GO layer on a highly porous nanofibrous support because of the undulating surface caused by the fibrous structure. To solve this problem, Cheng et al. (2017) demonstrated a novel TFNC membrane consisting of an electrospun PAN nanofibrous substrate and a robust GO barrier layer for pervaporation desalination application. As shown in Fig. 14.15, the stacked GO nanosheets were successfully interlinked with sufficient bonding by GA with the aid of a flexible connector, PVA, which acted as the spacing bridges to provide adequate stability in a water environment. Thanks to the superiority of a peculiar ultrathin hydrophilic GO skin layer and a fully interconnected porous nanofibrous substrate, the optimized TFNC membranes exhibited an excellent permeate flux of 69.1 L/m2h and a stable high rejection (99.9%) over a testing period of 24 h using an aqueous salt

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FIGURE 14.15 Illustration of the structure design of a graphene oxideepolyvinyl alcoholeglutaraldehyde/ polyacrylonitrile (GO-PVA-GA/PAN) thin-film nanofibrous composite (TFNC) membrane. (A) A digital photo of a GO-PVA-GA/ PAN TFNC membrane. (B) A cross-sectional scanning electron microscopy image of a GO-PVA-GA/PAN TFNC membrane. (C) Schematic representation of the internal architecture and (D) GA chemical crosslinking in the GOePVA barrier layer (Cheng et al., 2017). Reprinted with permission from Cheng, C., Shen, L., Yu, X., Yang, Y., Li, X., Wang, X., 2017. Robust construction of a graphene oxide barrier layer on a nanofibrous substrate assisted by the flexible poly(vinylalcohol) for efficient pervaporation desalination. Journal of Materials Chemistry 5, 3558e3568. Copyright 2017, Royal Society of Chemistry.

solution with NaCl concentration of 35 g/L at 70 C, which was superior to homogeneous membranes and composite membranes applied in pervaporation desalination reported so far.

14.4 CONCLUSIONS Aiming at the increasingly serious issues of water scarcity and pollution, electrospun nanofiber membranes offer an attractive solution for many emerging water purification problems due to the intrinsic versatility of electrospinning for generating highly porous functional nanofiber membranes from various functional materials and adjustable surface functional properties of nanofibrous membranes tailor-designed by various surface treatments, combined with recent innovative and flexible electrospinning technologies. The novel nanofiber-based membranes or composite membranes have been well demonstrated for many water treatment systems, including adsorption, MF, UF, NF, MD, pervaporation, and so on. Undoubtedly, nonwoven nanofibrous articles and nanofiberbased composites have become an extremely promising and versatile water treatment platform,

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where more innovative nanofiber technologies, modification techniques, and surface engineering will further expand the potential of this platform. The development of the cost-effective and energysaving nanofiber-based membrane technology will be anticipated to play an important role in addressing global water scarcity.

ACKNOWLEDGMENTS X.F. Wang would like to acknowledge the financial support from the National Science Foundation of China (51273042 and 21174028) and Program of Shanghai Science and Technology Innovation International Exchange and Cooperation (15230724700).

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Shen, L., Yang, Y., Zhao, J., Wang, X., 2015. High-performance nanofiltration membrane prepared by dopamineassisted interfacial polymerization on PES nanofibrous scaffolds. Desalination and Water Treatment 57, 9549e9557. Shen, L., Cheng, C., Yu, X., Yang, Y., Wang, X., Zhu, M., Hsiao, B.S., 2016a. Low pressure UV-cured CS-PEOPTEGDMA/PAN thin film nanofibrous composite nanofiltration membranes for anionic dye separation. Journal of Materials Chemistry 4, 15575e15588. Shen, L., Yu, X., Cheng, C., Song, C., Wang, X., Zhu, M., Hsiao, B.S., 2016b. High filtration performance thin film nanofibrous composite membrane prepared by electrospraying technique and hot-pressing treatment. Journal of Membrane Science 499, 470e479. Si, Y., Ren, T., Ding, B., Yu, J., Sun, G., 2012a. Synthesis of mesoporous magnetic Fe3O4@carbon nanofibers utilizing in situ polymerized polybenzoxazine for water purification. Journal of Materials Chemistry 22, 4619e4622. Si, Y., Ren, T., Li, Y., Ding, B., Yu, J., 2012b. Fabrication of magnetic polybenzoxazine-based carbon nanofibers with Fe3O4 inclusions with a hierarchical porous structure for water treatment. Carbon 50, 5176e5185. Song, X., Liu, Z., Sun, D.D., 2011. Nano gives the answer: breaking the bottleneck of internal concentration polarization with a nanofiber composite forward osmosis membrane for a high water production rate. Advanced Materials 23, 3256e3260. Soyekwo, F., Zhang, Q.G., Deng, C., Gong, Y., Zhu, A.M., Liu, Q.L., 2014. Highly permeable cellulose acetate nanofibrous composite membranes by freeze-extraction. Journal of Membrane Science 454, 339e345. Subramanian, S., Seeram, R., 2013. New directions in nanofiltration applications-Are nanofibers the right materials as membranes in desalination? Desalination 308, 198e208. Tang, Z., Wei, J., Yung, L., Ji, B., Ma, H., Qiu, C., Yoon, K., Wan, F., Fang, D., Hsiao, B.S., 2009. UV-cured poly(vinyl alcohol) ultrafiltration nanofibrous membrane based on electrospun nanofiber scaffolds. Journal of Membrane Science 328, 1e5. Tao, Z., Yao, Y., Xiang, R., Wu, Y., 2014. Formation and characterization of polytetrafluoroethylene nanofiber membranes for vacuum membrane distillation. Journal of Membrane Science 453, 402e408. Teng, M., Wang, H., Li, F., Zhang, B., 2011. Thioether-functionalized mesoporous fiber membranes: sol-gel combined electrospun fabrication and their applications for Hg2þ removal. Journal of Colloid and Interface Science 355, 23e28. Thavasi, V., Singh, G., Ramakrishna, S., 2008. Electrospun nanofibers in energy and environmental applications. Energy & Environmental Science 1, 205e221. Tian, M., Qiu, C., Liao, Y., Chou, S., Wang, R., 2013. Preparation of polyamide thin film composite forward osmosis membranes using electrospun polyvinylidene fluoride (PVDF) nanofibers as substrates. Separation and Purification Technology 118, 727e736. Tian, E.L., Zhou, H., Ren, Y.W., mirza, Z. a., Wang, X.Z., Xiong, S.W., 2014. Novel design of hydrophobic/ hydrophilic interpenetrating network composite nanofibers for the support layer of forward osmosis membrane. Desalination 347, 207e214. Tian, M., Wang, Y.-N., Wang, R., 2015. Synthesis and characterization of novel high-performance thin film nanocomposite (TFN) FO membranes with nanofibrous substrate reinforced by functionalized carbon nanotubes. Desalination 370, 79e86. Tian, M., Wang, Y.-N., Wang, R., Fane, A.G., 2017. Synthesis and characterization of thin film nanocomposite forward osmosis membranes supported by silica nanoparticle incorporated nanofibrous substrate. Desalination 401, 142e150.

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Tijing, L.D., Choi, J.S., Lee, S., Kim, S.H., Shon, H.K., 2014. Recent progress of membrane distillation using electrospun nanofibrous membrane. Journal of Membrane Science 453, 435e462. Tijing, L.D., Yun, C.W., Shim, W.G., Tao, H., Choi, J.S., Kim, S.H., Shon, H.K., 2016. Superhydrophobic nanofiber membrane containing carbon nanotubes for high-performance direct contact membrane distillation. Journal of Membrane Science 502, 158e170. Veleirinho, B., Lopesdasilva, J.A., 2009. Application of electrospun poly(ethylene terephthalate) nanofiber mat to apple juice clarification. Process Biochemistry 44, 353e356. Wang, X., Hsiao, B.S., 2016. Electrospun nanofiber membranes. Current Opinion in Chemical Engineering 12, 62e81. Wang, X., Chen, X., Yoon, K., Fang, D., Hsiao., B.S., Chu, B., 2005. High flux filtration medium based on nanofibrous substrate with hydrophilic nanocomposite coating. Environmental Science & Technology 39, 7684e7691. Wang, X., Fang, D., Yoon, K., Hsiao, B.S., Chu, B., 2006. High performance ultrafiltration composite membranes based on poly(vinyl alcohol) hydrogel coating on crosslinked nanofibrous poly(vinyl alcohol) scaffold. Journal of Membrane Science 278, 261e268. Wang, Y., Hammes, F., Du¨ggelin, M., Egli, T., 2008. Influence of size, shape, and flexibility on bacterial passage through micropore membrane filters. Environmental Science & Technology 42, 6749e6754. Wang, X., Zhang, K., Yang, Y., Wang, L., Zhou, Z., Zhu, M., Hsiao, B.S., Chu, B., 2010. Development of hydrophilic barrier layer on nanofibrous substrate as composite membrane via a facile route. Journal of Membrane Science 356, 110e116. Wang, X., Min, M., Liu, Z., Yang, Y., Zhou, Z., Zhu, M., Chen, Y., Hsiao, B.S., 2011. Poly(ethyleneimine) nanofibrous affinity membrane fabricated via one step wet-electrospinning from poly(vinyl alcohol)-doped poly(ethyleneimine) solution system and its application. Journal of Membrane Science 379, 191e199. Wang, R., Liu, Y., Li, B., Hsiao, B.S., Chu, B., 2012. Electrospun nanofibrous membranes for high flux microfiltration. Journal of Membrane Science 392e393, 167e174. Wang, R., Guan, S., Sato, A., Wang, X., Wang, Z., Yang, R., Hsiao, B.S., Chu, B., 2013a. Nanofibrous microfiltration membranes capable of removing bacteria, viruses and heavy metal ions. Journal of Membrane Science 446, 376e382. Wang, Z., Ma, H., Hsiao, B.S., Chu, B., 2013b. Nanofibrous ultrafiltration membranes containing cross-linked poly(ethylene glycol) and cellulose nanofiber composite barrier layer. Polymer 55, 366e372. Wang, X., Fang, D., Hsiao, B.S., Chu, B., 2014a. Nanofiltration membranes based on thin-film nanofibrous composites. Journal of Membrane Science 469, 188e197. Wang, X., Yeh, T.-M., Wang, Z., Yang, R., Wang, R., Ma, H., Hsiao, B.S., Chu, B., 2014b. Nanofiltration membranes prepared by interfacial polymerization on thin-film nanofibrous composite scaffold. Polymer 55, 1358e1366. Wang, J., Zhang, P., Liang, B., Liu, Y., Xu, T., Wang, L., Cao, B., Pan, K., 2016a. Graphene oxide as an effective barrier on a porous nanofibrous membrane for water treatment. ACS Applied Materials & Interfaces 8, 6211e6218. Wang, M., Li, X., Hua, W., Shen, L., Yu, X., Wang, X., 2016b. Electrospun poly(acrylic acid)/silica hydrogel nanofibers scaffold for highly efficient adsorption of lanthanide ions and its photoluminescence performance. ACS Applied Materials & Interfaces 8, 23995e24007. Wang, Q., Li, N., Bolto, B., Hoang, M., Xie, Z., 2016c. Desalination by pervaporation: a review. Desalination 387, 46e60.

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Wang, X., Dou, L., Li, Z., Liu, Y., Yu, J., Ding, B., 2016d. Flexible hierarchical ZrO2 nanoparticle-embedded SiO2 nanofibrous membrane as a versatile tool for efficient removal of phosphate. ACS Applied Materials & Interfaces 8, 34668e34676. Wang, J., Zhu, J., Zhang, Y., Liu, J., Van der Bruggen, B., 2017. Nanoscale tailor-made membranes for precise and rapid molecular sieve separation. Nanoscale 2942e2957. Wu, S., Li, F., Wang, H., Fu, L., Zhang, B., Li, G., 2010. Effects of poly(vinyl alcohol) (PVA) content on preparation of novel thiol-functionalized mesoporous PVA/SiO2 composite nanofiber membranes and their application for adsorption of heavy metal ions from aqueous solution. Polymer 51, 6203e6211. Xiao, S., Shen, M., Ma, H., Guo, R., Zhu, M., Wang, S., Shi, X., 2010. Fabrication of water-stable electrospun polyacrylic acid-based nanofibrous mats for removal of copper (II) ions in aqueous solution. Journal of Applied Polymer Science 116, 2409e2417. Xu, R., Jia, M., Li, F., Wang, H., Zhang, B., Qiao, J., 2012. Preparation of mesoporous poly(acrylic acid)/SiO2 composite nanofiber membranes having adsorption capacity for indigo carmine dye. Applied Physics A: Materials Science & Processing 106, 747e755. Yang, Y., Li, X., Shen, L., Wang, X., Hsiao, B.S., 2017a. A durable thin-film nanofibrous composite nanofiltration membrane prepared by interfacial polymerization on a double-layer nanofibrous scaffold. RSC Advances 7, 18001e18013. Yang, Y., Li, X., Shen, L., Wang, X., Hsiao, B.S., 2017b. Ionic cross-linked poly(acrylonitrile-co-acrylic acid)/ polyacrylonitrile thin film nanofibrous composite membrane with high ultrafiltration performance. Industrial & Engineering Chemistry Research 56, 3077e3090. Yeh, T.M., Yang, L., Wang, X., Mahajan, D., Hsiao, B.S., Chu, B., 2012. Polymeric nanofibrous composite membranes for energy efficient ethanol dehydration. Journal of Renewable Sustainable Energy 4, 5e37. Yeh, T.M., Wang, Z., Mahajan, D., Hsiao, B.S., Chu, B., 2013. High flux ethanol dehydration using nanofibrous membranes containing graphene oxide barrier layers. Journal of Materials Chemistry 1, 12998e13003. Yoon, K., Kim, K., Wang, X., Fang, D., Hsiao, B.S., Chu, B., 2006. High flux ultrafiltration membranes based on electrospun nanofibrous PAN scaffolds and chitosan coating. Polymer 47, 2434e2441. Yoon, K., Hsiao, B.S., Chu, B., 2008. Functional nanofibers for environmental applications. Journal of Materials Chemistry 18, 5326e5334. Yoon, K., Hsiao, B.S., Chu, B., 2009a. High flux nanofiltration membranes based on interfacially polymerized polyamide barrier layer on polyacrylonitrile nanofibrous scaffolds. Journal of Membrane Science 326, 484e492. Yoon, K., Hsiao, B.S., Chu, B., 2009b. High flux ultrafiltration nanofibrous membranes based on polyacrylonitrile electrospun scaffolds and crosslinked polyvinyl alcohol coating. Journal of Membrane Science 338, 145e152. You, H., Yang, Y., Li, X., Zhang, K., Wang, X., Zhu, M., Hsiao, B.S., 2012. Low pressure high flux thin film nanofibrous composite membranes prepared by electrospraying technique combined with solution treatment. Journal of Membrane Science 394e395, 241e247. Yu, X., Shen, L., Zhu, Y., Li, X., Yang, Y., Wang, X., Zhu, M., Hsiao, B.S., 2017. High performance thin-film nanofibrous composite hemodialysis membranes with efficient middle-molecule uremic toxin removal. Journal of Membrane Science 523, 173e184. Yuan, L., Wang, R., Fane, A.G., 2013. Engineering superhydrophobic surface on poly(vinylidene fluoride) nanofiber membranes for direct contact membrane distillation. Journal of Membrane Science 440, 77e87. Yun, C.W., Tijing, L.D., Shim, W.G., Choi, J.S., Kim, S.H., Tao, H., Drioli, E., Shon, H.K., 2016. Water desalination using graphene-enhanced electrospun nanofiber membrane via air gap membrane distillation. Journal of Membrane Science 520, 99e110.

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ELECTROSPUN NANOFIBERS FOR FOOD AND FOOD PACKAGING TECHNOLOGY

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Jing Tian1, 2, Hongbing Deng2, Mengtian Huang2, Rong Liu2, Yang Yi2, Xiangyang Dong2 College of Food Science and Technology and MOE Key Laboratory of Environment Correlative Dietology, Huazhong Agricultural University, Wuhan, P. R. China1; Hubei International Scientific and Technological Cooperation Base of Sustainable Resource and Energy, Hubei Key Lab of Biomass Resource Chemistry and Environmental Biotechnology, School of Resource and Environmental Science, Wuhan University, Wuhan, P. R. China2

15.1 INTRODUCTION In recent years nanofibers have drawn considerable attention due to their unique physicochemical properties and characteristics in comparison with analogous materials (Nurfaizey et al., 2012; Kenry and Lim, 2017). Electrospinning, as a versatile and efficient method for the fabrication of slender and continuous nanofibers, possesses various advantages in structure and function (Table 15.1; Bhushani and Anandharamakrishnan, 2014). Thus electrospun nanofibers have been applied in many domains including drug delivery, tissue engineering (Liu et al., 2015b; Lu et al., 2016; Xie et al., 2014; Yoo et al., 2009), energy conversion and storage (Cheng et al., 2011; Hwang et al., 2012; Jeong et al., 2014), sensor carriers (Wang et al., 2013; Yang et al., 2010), environmental governance (Deng et al., 2011; Shang et al., 2013; Zhang et al., 2009), and personal protection (Gugliuzza and Drioli, 2013; Sundarrajan and Ramakrishna, 2007). Moreover, cost-effective electrospun nanofibers also demonstrate huge potential in the food industry in view of these features (Bhushani and Anandharamakrishnan, 2014). The morphologies and characteristics of electrospun nanofibers can be modulated by the optimization of technological parameters (high voltage, spinning distance, flow rate, humidity, and temperature) and aquiform parameters (viscosity coefficient, electrical conductivity, surface tension, electric inductivity, and pH value) (Mazoochi and Jabbari, 2011; Mendes et al., 2017; Pham et al., 2006). Numerous studies have applied electrospun nanofibers in food sector research primarily in food packaging, encapsulation and delivery of nutraceuticals, enzyme immobilization, and filter aids (Bhushani and Anandharamakrishnan, 2014). The materials for electrospinning include natural ingredients, synthetic ingredients, and blends of them (Barnes et al., 2007). Natural polymers, including polysaccharides, proteins, and lipids, exhibit low toxicity, eximious biocompatibility, renewability, and controllable degradability in comparison with synthetic polymers (Doyle et al., 2013). In addition to the unique properties of nanomaterials,

Electrospinning: Nanofabrication and Applications. https://doi.org/10.1016/B978-0-323-51270-1.00015-7 Copyright © 2019 Elsevier Inc. All rights reserved.

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Table 15.1 Advantages of Electrospun Products Structural Advantages

Functional Advantages

Submicrometer and nanometer size Porosity High surface-to-volume ratio Tailored morphology Intertwined fibrous structure

Sustained and controlled release Non-thermally processed products Reduced denaturation Efficient encapsulation Enhanced stability of bioactives Food-grade polymers/biopolymers

Adapted from Bhushani, J.A., Anandharamakrishnan, C., 2014. Electrospinning and electrospraying techniques: potential food based applications. Trends in Food Science & Technology 38, 21e33.

electrospun biopolymers have advantages in safety and degradability (Mendes et al., 2017). Thus there are broad prospects for the application of electrospun nanofibers in the field of food industry. In this chapter, the materials for electrospinning, the functionalization of electrospun nanofibers, and representative results of the application of electrospun nanofibers in the field of food-related research are reviewed.

15.2 ELECTROSPINNING OF BIOPOLYMERIC NANOFIBERS IN THE FOOD INDUSTRY 15.2.1 ELECTROSPINNING OF POLYSACCHARIDES Polysaccharides are one of the most important biopolymers in the food field. They are made of monosaccharides linked together by glycoside linkages (Adams, 1965; Torii et al., 1964). Polysaccharides of specific molecular conformation, physicochemical properties, and functional properties are related to the specific type, number, sequence, and binding of monosaccharide (Heidelberger et al., 1936; Luo et al., 2013). Polysaccharides can be applied in the food industry because of their advantages of nontoxicity (Saha et al., 2017), edibility (Dickinson, 2017), digestibility (Jamroz et al., 2002), biocompatibility, biodegradability (De et al., 2014), and renewability (Bosch, 2009). According to the behavior of polysaccharides in the electrospinning, they can be classified into three categories. The first category is able to produce fibers, the second one is able to produce a jet but no fibers and the third one can not form a jet at all. All charged polysaccharides tested belong to the third category (Stijnman et al., 2011). It has been proved that both the concentration and the shear thinning properties of polysaccharides are important for successful spinning. Concentrations in the range of 10e20 in units of the overlap concentration may result in a sufficiently high Trouton ratio, which is an important condition for spinnability. Device parameters and conductivity are secondary priorities and function in process optimization (Morris, 1990; Stijnman et al., 2011). Polysaccharides have a wide range of molecular properties in weight, structure, carried charges, and solubility, which are important for them to be fabricated into biopolymer-based delivery systems

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(Gao et al., 2009). Delivery systems can be used not only to prevent the degradation of encapsulated components during processing and storage, but also to release them continuously and slowly (Klokkers-Bethke and Fischer, 1991). The sustained release of antimicrobial ingredients during food storage is a good example of such application (Fu et al., 2016). The concentration at which molecular hydrodynamic radius begin to overlap is when the polysaccharides in solution become entangled. The entanglement of random free coil chains at the same concentration leads to the emergence of compact globular polysaccharide chains. The shear thinning behavior reduces the tensile viscosity of the solution, which could prevent the injection and fiber formation because the sprayed liquid is easily broken when it is stretched in an electric field. Therefore, weak shear thinning is necessary for the formation of electrospun polysaccharide (Zhang et al., 2005).

15.2.1.1 Chitosan Chitosan is a cationic alkaline polysaccharide. It can be obtained from the deacetylation of chitin, which is the second largest natural fiber source (Kafetzopoulos et al., 1993; Trang et al., 2015). Chitosan has good film-forming capability, biodegradability, and antibacterial activity. Thus it is an ideal polymer for applications in the food industry (Shahidi et al., 1999). Chitosan is soluble neither in water nor in alkali aqueous solution nor in most mineral acidic solvents except some organic solvents, including acetic and formic acids (Homa et al., 2009). The polycationic nature of chitosan itself and the cationic charge increase the surface tension of the solution, thus chitosan has poor spinnability (Homa et al., 2009). Molecular weight, deacetylation, and solvent have significant impacts on the spinnability of chitosan (Wang and Minato, 2014). Therefore, it is common to mix chitosan with other polymers to improve the electrospinnability. Nevertheless there are also a few reports indicating that pure chitosan dissolved in hexafluoroisopropanol, dichloromethane (DCM), and trifluoroacetate (TFA) has been electrospun to nanofibers (Chen et al., 2008; Ohkawa et al., 2010). The Pakravan group (Pakravan et al., 2011) has prepared composite nanofibrous mats of chitosan with a degree of deacetylation at 97.5% and polyethylene oxide (PEO) at moderate temperature (40
Ce80
C) by electrospinning. The fibers have an average diameter of 60e120 nm. It has been found that the moderate temperature is helpful to get fewer beads of fibers. Upon an increase of chitosan content, the viscosity of the polymer solution decreases and the conductivity increases, thereby leading to a reduction of fiber diameter. Nanofibers with a high content of chitosan can be used for antibacterial packaging. Glucose oxidase (GOD) is a promising material for food packaging and storage, due to the properties of deoxidation (Ge et al., 2012). Electrospun composite nanofibers of chitosan and polyvinyl alcohol (PVA) have been fabricated, on which GOD is deposited to detect glucose (Bhushani and Anandharamakrishnan, 2014; Wu and Yin, 2013). Chitosan and PVA are mixed to fabricate electrospun mats with antimicrobial activity, as shown in Fig. 15.1. Further immobilization of GOD onto the mats exhibits long-term stability, good reproducibility, and absence of interference from other coexisting electroactive species for the glucose detection (Bhushani and Anandharamakrishnan, 2014; Wu and Yin, 2013). The obtained electrospun membranes show 73% deoxidization for the tested sample of jelly and cream cakes. However, the composite nanofibers have less efficacy in the preservation of

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(B)

(A)

(D)

(E)

(C)

(F)

FIGURE 15.1 (A) Scanning electron microscopy image of PVA/CS/tea extract/GOD nanofibers. (B and C) Antibacterial effects of PVA/CS/tea extract/GOD nanofibers on (B) Escherichia coli and (C) Staphylococcus aureus. (D and E) Transmission electron microscopy images of (D) CS/PVA nanofiber without any treatment and (E) CS/PVA nanofibers after being treated for 4 h in 0.5 M NaOH. The elongated surface grooves and pores indicated by the arrow were seen along the fiber direction after the removal of PVA. (F) Impact of pH values on the activity of free and immobilized lipase. CFU, colony-forming unit; CS, chitosan; GOD, glucose oxidase; PVA, polyvinyl alcohol. (C) Adapted from Ge, L., Zhao, Y.S., Mo, T., Li, J.R., Li, P., 2012. Immobilization of glucose oxidase in electrospun nanofibrous membranes for food preservation. Food Control 26, 188e193. (F) Adapted from Torres-Giner, S., Ocio, M.J., Lagaron, J.M., 2009. Novel antimicrobial ultrathin structures of zein/chitosan blends obtained by electrospinning. Carbohydrate Polymers 77, 261e266.

sugar-rich or semisolid food (Ge et al., 2012). Similarly, studies have reported that immobilization of lipase onto chitosan and PVA nanofibrous membranes enhances the storage stability (Huang et al., 2007). Antimicrobial nanofibers prepared by electrospinning of TFA-dissolved chitosan are accepted as food contact materials with specific migration limits (Torres-Giner et al., 2009). Ceylan et al. (2017b) has reported the fabrication of chitosan-based nanofibers (CNs) and liquid smokeeloaded CNs (LSCNs) using 70% TFA and 30% DCM. Then CN and LSCN electrospun nanofibers have been used to coat fish fillets as illustrated in Fig. 15.2. Both of them effectively delay the growth of mesophilic aerobic bacteria, psychrophilic bacteria, yeast and mold. Moreover, the antimicrobial effect of CNs is better than that of LSCNs. In addition, thymol-loaded chitosan electrospun nanofibers (TLCNs) have also been explored to delay the chemical deterioration of sea bream fish meat. It is reported that chitosan (7%, w/v) and thymol (0.4%, w/v) are added into the mixture of 70% TFA and 30% DCM. The prepared nanofibers are cylindrical, smooth, and beadless. The deterioration of total volatile basic nitrogen (TVB-N), trimethylamine (TMA), and thiobarbituric acid (TBA) during cold storage conditions are indicators for evaluation. TLCNs are able to delay the rapid

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FIGURE 15.2 Process of coating fish fillets with electrospun nanofibers. FT-IR, Fourier transform infrared; SEM, scanning electron microscopy. Adapted from Ceylan, Z., Sengor, G.F.U., Sag
dıc¸, O., Yilmaz, M.T., 2017b. A novel approach to extend microbiological stability of sea bass (Dicentrarchus labrax) fillets coated with electrospun chitosan nanofibers. LWT - Food Science and Technology 79, 367e375.

increase of TVB-N, TMA, and TBA during the tested period. Both CNs and TLCNs have a significant effect on delaying the rapid chemical deterioration of fish fillets. And TLCNs are more effective than CNs in preventing the chemical deterioration (Ceylan et al., 2017a). Arkoun et al. (2017) has explored the antibacterial mechanism of electrospun CNs. When the chitosan/PEO formulation is 80/20, the nanofibers show the highest antibacterial activity. The protonation of CN amino groups plays a major role in the antibacterial action of CNs, and the bactericidal effect of CNs is related to the permeabilization of bacterial membranes via pore formation. Thus CNs exhibit bacteriostatic and bactericidal effects. The efficiency of CNs in food preservation and shelf life extension is evaluated using fresh red meat. Because CNs inhibit the growth of microorganisms at pH 5.8, which is lower than the pKa of chitosan, they have a protective effect on the storage of weakly acidic food including dairy and meat products. According to the results of in-situ antibacterial tests, CNs are a promising safe nanomaterial to protect meat from contamination. CNs extend the shelf life of fresh red meat for an extra week. Thus CNs can be used to prolong the shelf life of food, which is of great significance for the production chain.

15.2.1.2 Starch Starch is a natural polymeric carbohydrate consisting of glycosidic bondelinked glucose units. Depending on its botanical source, the size, the shape, and the morphology of starch semicrystalline

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particles are different (Mora´n et al., 2013). Natural starch is cheap and recyclable. It is a raw material for many products in food industrial production. There are two main types of starch polymers: amylopectin and amylose (Fig. 15.3). The ratio of amylose to amylopectin in starch varies due to the botanical origin (Fu et al., 2014). The molecular weight of starch is high, and the intramolecular hydrogen bonds make it difficult for starch to dissolve in most solvents. The literature has reported that only dimethyl sulfoxide (DMSO), 4-methylmorpholine 4-oxide (NMMO), and ionic liquids are able to effectively dissolve starch, of which the most soluble and cheapest is DMSO (Lim, 2011). To expand the applications of starch, it is necessary to chemically modify the raw starch to improve its hydrophobicity, film-forming properties, and tensile resistance (Ren et al., 2016). When the amylose helices are dissolved in a mixture of water and DMSO, the starch maintains a random coil conformation (Vasconcelos et al., 2001). Therefore, DMSO and DMSO/water mixture are applied to dissolve starch for the electrospinning. A study (Kong and Ziegler, 2013b) has analyzed the quantitative relationship between fiber diameter and electrospinning parameters, such as concentration, applied high voltage, spinning distance, and flow rate. According to the response surface analysis, starch concentration has the greatest impact on fiber diameter. The ratios of both the voltage to distance and the polymer concentration to distance are essential to predict the fiber diameter. Under ambient conditions, DMSO does not evaporate sufficiently, so DMSO-dissolved starch solutions fail to deposit on the grounded collector. Wet electrospinning, a modified electrospinning setup, is designed to overcome this problem. Starch is added to a 95% DMSO aqueous solution to form a 15% (w/w) polymer solution. The average diameter of starch fibers by wet electrospinning is on the micrometer scale. To improve the water stability and crystallinity of starch fibers, they are placed into a 50% (v/v) ethanol aqueous solution and then heated at 70
C for 1 h as postspinning treatments. Afterward the fibers are immersed into a 25% (v/v) glutaraldehyde (GTA) aqueous solution to crosslink.

FIGURE 15.3 Structures of amylose (top) and amylopectin (bottom).

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The obtained as-spun starch fibers (Fig. 15.4A) could be used in the food industry (Kong and Ziegler, 2014a). Electrospun nanofibers have been fabricated from high-amylose starch blended respectively with palmitic acid, ascorbyl palmitate, and cetyltrimethylammonium bromide (CTAB). Ascorbyl palmitate is a food additive source of vitamin C and an antioxidant. CTAB is an effective antiseptic agent against bacteria and fungi. The obtained inclusion complexes (Fig. 15.4B) provide a novel medium to encapsulate varied guest compounds, which expands the application of starch-based nanofibrous mats (Kong and Ziegler, 2014b). The Wang group (Wang et al., 2011a) has reported electrospun nanofibers of PVA and oxidized starch (Fig. 15.4C). Oxidized starch is a kind of cationic polysaccharide, which is ionized under acidic and neutral conditions. The addition of oxidized starch affects the conductivity of the solution, and the diameter of the fiber decreases upon the increase in the conductivity of the solution.

15.2.1.3 Alginates Alginate is a polysaccharide polymer of natural origin and is normally extracted from the seaweed Sargassum and other brown algae plants (Mao et al., 2004). Alginate is a food-grade polysaccharide; it is well applied in the food industry (Kasahara and Kobayashi, 1964). Electrospun alginate is a promising carrier to protect and stabilize micronutrients and other bioactive components in food products (Alborzi et al., 2010). Owing to its high viscosity and molecular chain rigidity, pure alginate solution cannot be electrospun. Adding polyvinyl pyrrolidone (PVP) (average relative molecular weight 13  105) or PEO (average relative molecular weight 9  105e15  105) to the alginate can effectively improve the electrospun performance of an alginate solution (Do
gac¸ et al., 2017). Fig. 15.5A and B shows the morphology of nanofibers fabricated from a mixture of PEO/alginate and PVA/alginate, respectively. According to the study of Alborzi et al., solutions of low-viscosity sodium alginate with pectin (LSAP) and medium-viscosity sodium alginate with pectin (MSAP) are prepared by adding 0.01% (w/

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FIGURE 15.4 Scanning electron microscopy images of (A) pure starch fibers, (B) Starchepalmitic acid fibers, and (C) Polyvinyl alcohol/oxidized starch fibers. (A) Adapted from Kong, L., Ziegler, G.R., 2014a. Fabrication of pure starch fibers by electrospinning. Food Hydrocolloids 36, 20e25. (B) Adapted from Kong, L., Ziegler, G.R., 2014b. Formation of starch-guest inclusion complexes in electrospun starch fibers. Food Hydrocolloids 38, 211e219. (C) Adapted from Wang, H., Wang, W., Jiang, S., Jiang, S., Zhai, L., Jiang, Q., 2011a. Poly (vinyl alcohol)/oxidized starch fibres via electrospinning technique: fabrication and characterization. Iranian Polymer Journal (English Edition) 20, 551e558.

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FIGURE 15.5 Scanning electron microscopy images of (A) PEO/alginate nanofibers immobilized with lipase and (B) PVA/ alginate nanofibers immobilized with lipase. (C) The reusability of lipase-immobilized nanofibers. AL, alginate; PEO, polyethylene oxide; PVA, polyvinyl alcohol.

_ Deveci, I., _ Mercimek, B., Teke, M., 2017. A comparative study for lipase immobilization onto (C) Adapted from Dog
ac¸, Y.I., alginate based composite electrospun nanofibers with effective and enhanced stability. International Journal of Biological Macromolecules 96, 302e311.

w) of folic acid into sodium alginate with low or medium viscosity at the ratio of 7:3 (w/w). Then PEO is added into the LSAP and MSAP to obtain an electrospinning solution. Both 4% LSAP/PEO (5:5, w/w) and 5% LSAP/PEO (8:2 to 5:5, w/w) are able to be electrospun to bead-free fibers. And the smallest diameter (about 40 nm) of fibers is observed in the sample of 3% MSAP/PEO (8:2, w/w). The in-vitro release study reveals that the electrospun alginate composite fibers are a potential carrier for folic acid and facilitate sustained release of folic acid in acidic food products (Alborzi et al., 2010, 2014). Electrospun alginate fibers can also be applied as antibacterial food packaging materials (Huq et al., 2014). It has been reported that T4 bacteriophage after being incorporated into the fiber core via electrospinning still exhibits full bacteriophage viability after storage for weeks at 4
C (Korehei and Kadla, 2013). The morphology of electrospun nanofibers from pure PEO and from a PEO/ bacteriophage/alginate emulsion is displayed respectively in Fig. 15.6AeD. The lytic activity of T4 phage after each process of encapsulation is displayed in Fig. 15.6E. According to Fig. 15.6E, T4 phage released from fiber after coaxial electrospinning still maintains full activity. However, T4 phage released from fiber after emulsion electrospinning or after suspension electrospinning or released from Caealginate capsules after emulsification shows a drop in activity.

15.2.1.4 Cellulose and Cellulose Derivatives Cellulose is the most abundant polysaccharide in nature and the primary structural component of the cell wall of plants (De Siqueira and Ferreira Filho, 2010). As a renewable and sustainable raw material, cellulose has excellent biocompatibility as well as thermal and mechanical properties (Brogueira et al., 2011). Because of its crystallinity and extensive hydrogen bonds, cellulose has poor solubility in common organic reagents or water solutions, which limits the fabrication of cellulose nanofibers via electrospinning (Bochek, 2003). Han et al. (2008) have reported that 8% cellulose dissolved in a mixture of potassium thiocyanate and ethylenediamine is successfully electrospun into nanofibers (Fig. 15.7). Another 85 wt% of

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FIGURE 15.6 (A) Scanning electron microscopy (SEM) and (B) transmission electron microscopy (TEM) images of electrospun nanofibers from PEO solution. (C) SEM and (D) TEM images of electrospun nanofibers from PEO/bacteriophage/ alginate emulsion system. (E) Lytic activity of T4 phage after each process. PEO, polyethylene oxide. (E) Adapted from Korehei, R., Kadla, J., 2013. Incorporation of T4 bacteriophage in electrospun fibres. Journal of Applied Microbiology 114, 1425e1434.

NMMO hydrateedissolved cellulose (3e9 wt%, DP 700) has also been reported to be fabricated into nanofibers via electrospinning. Among them, nanofibers fabricated from 7 wt% cellulose solution have the smallest mean diameter of 570  200 nm and the most uniform morphology. Uppal and Ramaswamy (2011) have reported the preparation of cellulose nanofibers with the corresponding polymers dissolved in the mixture of NMMO/N-methyl pyrrolidone/water. The applied voltage of 28 kV, the receiving distance of 25 cm, and the external temperature of 38
C are adopted during the electrospinning process. The average diameter of cellulose nanofibers electrospun from a 1.25 wt% cellulose solution is 207 nm, and the average diameter is increased to 243 nm when the concentration of solution is increased to 2.5%. Montan˜oleyva et al. (2011) have extracted cellulose from wheat straw using chemical methods. Cellulose nanofibers are fabricated by electrospinning with TFA as the solvent. Fig. 15.8A shows the untreated wheat straw (image A), which is composed of cellulose fiber bundles, the cellulose fibers after chemical treatment (image B), and the electrospun cellulose nanofibers with the average diameter of 270  97 nm (image C). Qi et al. (2010) have dissolved cellulose in a mixture of NaOH and urea and found that spherical particles with diameters of 100e300 nm are obtained by electrospinning instead of fibers. The addition

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FIGURE 15.7 Scanning electron microscopy images of (A) CA nanofibers electrospun from 17 wt% CA solution in a mixture of acetic acid/water at a ratio of 75:25. Afterward CA nanofibers were deacetylated using (B) 0.5 M NaOH aqueous solution and (C) 0.5 M KOH/ethanol solution. CA, cellulose acetate. (C) Adapted from Han, S.O., Ji, H.Y., Min, K.D., Kang, Y.O., Park, W.H., 2008. Electrospinning of cellulose acetate nanofibers using a mixed solvent of acetic acid/water: effects of solvent composition on the fiber diameter. Materials Letters 62, 759e762.

of high-molecular-weight polyethylene glycol or PVA auxiliary into the mixture of NaOH and urea reduces the viscosity of the cellulose solution and increased the spinnability of the cellulose, and thereby bicomponent nanofibers with average diameter of 400 nm are produced. Lipase is an enzyme widely used in the production of dairy products, bakeries, fats, and oils (Rezaei et al., 2015). The modification of polyethylene glycol diacylchloride on the nanofiber surface provides more reactive sites for the lipase enzyme, and the immobilization process does not affect its activity (Wang and Hsieh, 2004). Cellulose acetate (CA) nanofibers with immobilization of vitamin A and E have an average diameter of 247e265 nm, with a smooth surface and round cross section. The morphology of the loaded nanofibers is shown in Fig. 15.9. Compared with the cast film of CA, the release of vitamins from electrospun nanofibers is sustained (Taepaiboon et al., 2007). A study (Wongsasulak et al., 2010) has reported the fabrication of electrospun edible nanofibrous films from a blended solution of CA and egg albumen (EA) (Fig. 15.10; Table 15.2). The solvents are 85% acetic acid for the CA and 50% formic acid for the EA. Tween 40 is added as a food-grade nonionic surfactant to modulate the solution properties. To fabricate fibers with antimicrobial activity, Son et al. (2006) have reported the addition of silver nitrate into 10 wt% CA solution dissolved with a mixture of acetone and water (4:1). Afterward the as-prepared fibers are treated by photoreduction, thereby converting the silver ions into silver nanoparticles with antimicrobial activity. And then the silver nanoparticles diffuse to the surface of the nanofibers to form aggregates, which inhibite the growth of various foodborne pathogens, including Staphylococcus aureus, Escherichia coli, Klebsiella pneumoniae, and Pseudomonas aeruginosa. CA (low molecular weight, 30 kDa) is able to be dissolved in a 2:1 (v/v) acetone:dimethylacetamide solvent to form a 17 wt% polymer solution. Curcumin is one of the meat colorants in the food industry and has been applied in a polymer solution at a concentration of 5e20 wt% to fabricate bioactive cellulose nanofibers via electrospinning (Fig. 15.11AeD) (Suwantong et al., 2007).

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FIGURE 15.8 (A) Scanning electron microscopy images, (B) Fourier transform infrared spectra, and (C) thermal gravimetric analysis of (image A) durum wheat straw, (image B) cellulose fibers after chemical treatment, and (image C) cellulose nanofibers. Trace D in (B) is for nanofibers after being exposed to air. TFA, trifluoroacetate. (C) Adapted from Montan˜oleyva, B., Rodriguezfelix, F., Torrescha´vez, P., Ramirezwong, B., Lo´pezcervantes, J., Sanchezmachado, D., 2011. Preparation and characterization of durum wheat (Triticum durum) straw cellulose nanofibers by electrospinning. Journal of Agricultural and Food Chemistry 59, 870e875.

Electrospun cellulose nanofibers containing 5% curcumin have an average diameter of 314  60 nm, while nanofibers containing 20% curcumin have an average diameter of 340  98 nm. The biological activity of curcumin is maintained after the electrospinning process (Suwantong et al., 2007). Chen and Liu (2008) have found that a blend of soybean protein isolate matrix and cellulose enhances the mechanical strength and Young modulus of composite nanofibers (Fig. 15.11G and H). Azeredo has reported that the incorporation of cellulose enhances the tensile strength and Young modulus of mango-puree-based edible films, which also improves the water vapor barrier of the films (Azeredo et al., 2009). These studies expand the applications of nanofibers in the food packaging industry. Moreover, cellulosic nanofibers can be applied as thickeners and texturizing agents for sauces, dairy products, beverages, and meat products (Rezaei et al., 2015). They can mask unpleasant flavors in food. With their good performance in thermal resistance, cellulosic nanofibers can be used as delivery systems to protect heat-sensitive bioactive ingredients.

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FIGURE 15.9 Scanning electron microscopy images of (A) as-spun CA fiber mat from 17% (w/v) CA dissolved in a mixture of acetone/DMAc (2:1, v/v) containing 5 wt% vitamin E, (B) as-cast CA film from 4% (w/v) CA dissolved in a mixture of acetone/DMAc (2:1, v/v) containing 5 wt% vitamin E, (C and D) vitamin Eeloaded as-spun CA fiber mats after immersion in (C) B/T or (D) B/T/M medium for 24 h, as-spun CA fiber mat fabricated from (E) 17% (w/v) and (F) 4% (w/v) of CA dissolved in a mixture of acetone/DMAc (2:1, v/v) containing 0.5 wt% retin-A, and retin-A-loaded as-spun CA fiber mats after immersion in (G) B/T or (H) B/T/M medium for 6 h. CA, cellulose acetate; DMAc, dimethylacetamide; B/T stands for the releasing medium prepared by adding 0.5% of a non-ionic surfactant and Tween 80 to the acetate buffer solution; B/T/M stands for the releasing medium prepared by adding 0.5% of Tween 80 and 10% of methanol in the acetate buffer solution. (H) Adapted from Taepaiboon, P., Rungsardthong, U., Supaphol, P., 2007. Vitamin-loaded electrospun cellulose acetate nanofiber mats as transdermal and dermal therapeutic agents of vitamin A acid and vitamin E. European Journal of Pharmaceutics and Biopharmaceutics 67, 387e397.

15.2.1.5 Dextran Dextran is a natural branched bacterial polysaccharide produced by Leuconostoc mesenteroides that consists of a-1,6-D-glucopyranose with a-1,2, a-1,3, or a-1,4 linked side chains (Jiang et al., 2004). It is soluble in water and organic solvents. It is biodegradable and biocompatible (Groot et al., 2001; Jiang et al., 2007). Dextran dissolves in water to form high-viscosity solution, and it has been widely used as a thickener (Pucci and Kunka, 1989), a suspending agent (Hiler, 1958), a gelling agent (Lobete et al., 2016), and a stabilizer (Aguilera-Miguel et al., 2018; Luo et al., 2018) in the food industry. But there are only a few publications on the application of dextran nanofibers in the food sector. Jiang et al. (2004) have reported on the impact of solvent and polymer concentrations on the electrospinnability of dextran (molecular weight range 64e76 kDa). Among the concentrations of dextran from 500 to 1000 mg/mL, the uniform fibers electrospun from 750 mg/mL dextran have the smallest diameters of 100 nm, and the diameters are dramatically increased to 3 mm when the electrospun solution is 1 g/mL. The addition of 5 wt% bovine serum albumin (BSA) to 750 mg/mL dextran decreases the nanofiber size to 500 nm, because the increased net charges exert higher forces on the jets, which ultimately produces smaller fibers. It is worthy to note that the biological activity of BSA is maintained after electrospinning with dextran. So this study demonstrates the application of nanofibers as a potential carrier for protein-based functional ingredients.

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FIGURE 15.10 Scanning electron microscopy images of the electrospun products from (A) the primary CA solution (20 wt% CA in 85% acetic acid), (B) blend A, (C) blend B, (D) blend C, and (E) the primary EA solution (12 wt% EA in 50% formic acid). Blends A, B, and C are defined in Table 15.2. (F) Fourier transform infrared (FT-IR) spectra and (G) thermal gravimetric analysis (TGA) of the electrospun samples. CA, cellulose acetate; EA, egg albumen. From Wongsasulak, S., Patapeejumruswong, M., Weiss, J., Supaphol, P., Yoovidhya, T., 2010. Electrospinning of food-grade nanofibers from cellulose acetate and egg albumen blends. Journal of Food Engineering 98, 370e376.

Dextran nanofibers have been utilized to encapsulate vitamin E (Fig. 15.12) and fortify cheese (Fathi et al., 2016). Compared with the blank and directly fortified samples, the acceptability and the texture of cheese containing nanofibers perform better, which is attributed to the water-holding properties. The results reveal that electrospun dextran ultrathin fibers could be used to entrap hydrophobic compounds, so dextran fibers have high potential in the design of novel functional food. Table 15.2 Compositions of Electrospun Solutions of Pure Cellulose Acetate, Pure Egg Albumen, and Their Blends Sample

Polymer Weight Ratio

Total Polymer Conc. (% w/w)

Stock CA (g)/Stock EA (g)/Tween 40 (g)

Pure CA Blend A Blend B Blend C Pure EA

100:0 91:9 77:23 66:34 0:100

20.00 19.25 18.25 17.25 12.00

20.00// 3.64/0.36/0.40 3.08/0.92/0.72 2.64/1.36/0.92 e/20.00/e

blend A, blend B, and blend C with the addition of Tween 40 cited from Wongsasulak et al., 2010; CA, cellulose acetate; EA, egg albumen.

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FIGURE 15.11 Scanning electron microscopy (SEM) images of (A) neat electrospun CA fiber mats, (B) solvent-cast CA films, (C) 20% curcumin-loaded electrospun CA fiber mats, and (D) solvent-cast CA films containing 20% curcumin. (E) SEM image of cellulose nanofibers. (F) The diameter distribution of cellulose nanofibers. (G and H) SEM images of (G) the surface of a CNM/SPI composite film with 11% CNM where a few fibers flatly layered indicated with arrows and (H) the surface of a CNM/SPI composite film with 20% CNM. CA, cellulose acetate; CNM, cellulose nanofibrous mat; SPI, soy protein isolate. (D) Adapted from Suwantong, O., Opanasopit, P., Ruktanonchai, U., Supaphol, P., 2007. Electrospun cellulose acetate fiber mats containing curcumin and release characteristic of the herbal substance. Polymer 48, 7546e7557. (H) Adapted from Chen, G., Liu, H., 2008. Electrospun cellulose nanofiber reinforced soybean protein isolate composite film. Journal of Applied Polymer Science 110, 641e646.

15.2.1.6 Mucilage Basil seed mucilage (BSM) extracted from basil (Ocimum basilicum L.) seeds that grows in Asia, Afria and America has been applied to fabricate nanofibers in previous studies (Shamsnejati et al., 2015). Because of the potential applications of the mucilage for thickening and stabilizing, it is a new source of edible hydrocolloid as a functional ingredient in the food industry (Osano et al., 2014). However, BSM cannot be spun alone, and PVA is the most commonly used electrospinning aid agent for BSM electrospinning (Kurd et al., 2017). Mucilage nanofibers can be utilized as a food additive. According to the study (Kurd et al., 2017), water and acetone at the ratio of 7:3 have been used to dissolve 2% BSM. Acetone of good volatility is appropriate for the electrospinning process. A 10% PVA solution is prepared in deionized water. BSM and PVA are mixed at different volume ratios. The results are displayed in Fig. 15.13 and show that (1) viscosity and electrical conductivity of the biopolymer solutions decrease with increase in PVA content, (2) fiber diameter increases with increase in high voltage and PVA content, and (3) PVA addition improves the thermal properties of the nanofibers (Kurd et al., 2017). Therefore, thermostable ultrathin fibers fabricated by electrospinning have potential applications in the food sector, such as in bioactive encapsulation and edible film production.

15.2.2 ELECTROSPINNING OF PROTEINS It is a challenge to obtain protein nanofiber membranes by electrospinning. The reason is that their chemical composition is extremely complex and interactions among biomacromolecules are not

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FIGURE 15.12 Scanning electron microscopy images of (A) fibers prepared with 0.375 g/mL dextran under a voltage of 14 kV, flow rate of 1 mL/h, and distance of 15 cm at 25
C; (B) vitamin E-loaded dextran nanofibers prepared with 1 g/mL dextran solution under 15 kV of voltage; (C) vitamin E-loaded dextran nanofibers prepared with 1.25 g/mL dextran under 13 kV of voltage; and (D) vitamin E-loaded dextran nanofibers prepared with 1 g/mL dextran solution under 13 kV of voltage. (E) The release profile of vitamin E (a-TAC) from dextran nanofibers in gastrointestinal media. (E) Adapted from Fathi, M., Nasrabadi, M.N., Varshosaz, J., 2016. Characteristics of Vitamin E-Loaded Nanofibres From Dextran.

convenient, due to their three-dimensional (3D) structure and polyelectrolytic characteristics (Kriegel et al., 2008). However, coelectrospinning proteins with other polymers and adjusting the solvents are the effective ways to fabricate proteins.

15.2.2.1 Collagen Collagen exists in seven types. Type I mainly presents in bone, skin, tendon, and cornea (Scott and Haigh, 1985), consisting of two identical a1 chains and a different a2 chain (Fullana and Wnek, 2012). Type II primarily exists in cartilage, intervertebral disc, and vitreous. Type III mostly appears in blood vessels, new skin, and scar tissue. Both types II and III are made up of three of the same peptide chains. It is universally acknowledged that the basic structural unit of collagen is tropocollagen (Cox et al., 1967), which is a sort of fibrous protein with the molecular conformation of a triple helix (Smart et al., 2012). The length and the diameter of tropocollagen are about 300 and 1.5 nm, respectively. The triple helix of collagen formed by three chains is constituted by repeated triplets of GlyeXeY, in which X and Y stand for different amino acids, such as proline and hydroxyproline (Fullana and Wnek, 2012). The most common tripeptide unit is GlyeProeHyp, which contributes to the stability of the triple helix. Collagen from bovine has been used in food, cosmetics, and medical materials (Noitup et al., 2005). Collagen in bone tissue is a binder of calcium phosphate, which constitutes the main component of the bones, along with bone collagen. Many efforts have been made to study how to electrospin collagen because of its wide biomedical and food applications. The solvent system is one of the toughest problems to solve before

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FIGURE 15.13 Scanning electron microscopy images of electrospun nanofibers fabricated from BSM and PVA solution (A) at a ratio of 20:80 (v/v) under voltage of 23 kV, (B) at a ratio of 20:80 (v/v) under voltage of 18 kV, (C) at a ratio of 40: 60 (v/v) under voltage of 23 kV, (D) at a ratio of 40:60 (v/v) under voltage of 18 kV, (E) at a ratio of 60:40 (v/v) under voltage of 23 kV, (F) at a ratio of 60:40 (v/v) under voltage of 18 kV, (G) at a ratio of 80:20 (v/v) under voltage of 23 kV, and (H) at a ratio of 80:20 (v/v) under voltage of 18 kV. Thermal gravimetric analyses of (I) BSM, (J) PVA, and (K) nanofibers electrospun from BSM and PVA solution. BSM, basil seed mucilage; PVA, polyvinyl alcohol. From Kurd, F., Fathi, M., Shekarchizadeh, H., 2017. Basil seed mucilage as a new source for electrospinning: production and physicochemical characterization. International Journal of Biological Macromolecules 95, 689e695.

electrospinning (Huang et al., 2001; Matthews et al., 2002). The spinnability of lyophilized type I collagen sponges derived from cow tendon, as the raw material, has been investigated by altering the solvent systems. To increase the rate of evaporation, the proportion of ethanol in the water/ethanol solution is increased. However, the collagen is denatured by the increase in the ethanol concentration. Moreover, collagen solutions at a wide variety of concentrations are prepared in 90% (v/v) aqueous acetic acid solution. The same result is obtained because of the low viscosity (Castilla-Casadiego et al., 2016). Zein from corn and collagen from bovine Achilles tendon are dissolved in aqueous acetic acid (70% v/v) and then coelectrospun. Fig. 15.14 shows that the average diameter of the fibers become bulkier with increased concentration of zein. Furthermore, the tensile strength and the water contact angles of the collagen/zein nanofiber membranes also increase, while the elongation displays the opposite trend. It has been confirmed that the spinnability of collagen is improved with the addition of zein (Lin et al., 2012).

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FIGURE 15.14 Scanning electron microscopy images of electrospun nanofibers fabricated from (A) neat collagen solution (fzein ¼ 0.00), (B) a blend of zein/collagen (fzein ¼ 0.33), (C) a blend of zein/collagen (fzein ¼ 0.50), (D) a blend of zein/collagen (fzein ¼ 0.67), and (E) neat zein solution (fzein ¼ 1.00). (F) The average diameter of electrospun nanofibers produced from blends of zein/collagen with weight ratios of 0, 0.33, 0.5, 0.67, and 1.00. (G) Tensile strength and (H) elongation at break of electrospun nanofibers produced from blends of zein/collagen with weight ratios of 0, 0.33, 0.5, 0.67, and 1.00. Images of the water contact angle of electrospun nanofibers produced from blends of zein/collagen with weight ratios of (I) 0.33, (J) 0.50, and (K) 0.67. (L) The size of water contact angles of electrospun nanofibers produced from blends of zein/collagen with weight ratios of 0.33, 0.50, and 0.67. From Lin, J.T., Li, C.H., Zhao, Y., Hu, J.C., Zhang, L.M., 2012. Co-electrospun nanofibrous membranes of collagen and zein for wound healing. ACS Applied Materials & Interfaces 4, 1050e1057.

15.2.2.2 Silk Silks are divided into mulberry silks and nonmulberry silks. Silkworm silk, an established fiber, is widely used in the textile industry (Kundu et al., 2013). Silk fibroin (SF) is extracted from Bombyx mori silk, which is known as mulberry silk, and consists of two chains (Kundu et al., 2013), a heavy chain and a light one, on which is a glycoprotein, called P25 (Shimura et al., 1976; Tanaka et al., 1999). The proportions of the heavy chain:light chain:P25 are 6:6:1 (Inoue et al., 2000). However, there is a  lack of the light chain and P25 in nonmulberry silks (Sehnal and Zurovec, 2004; Kundu and Kundu, 2010). Great differences are found between the two kinds of silk in bioactivity, mechanical properties, and degradation behavior due to the diversity of their structures. The characteristics of silk are biocompatibility, extensibility, biodegradability, high strength, and minimal production of inflammatory reactions (Yang et al., 2013). SF is extracted from B. mori cocoons. A 4% (w/w) solution is cast on polystyrene petri dishes and dried for 12 h and 4 days respectively to obtain silk films mainly containing nanospheres or nanofilaments. Afterwards they are

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dissolved in 98% formic acid and still remain stable. It has been proved that SF nanofilaments have better spinnability than SF nanospheres due to the lower viscosity. Apart from this, the average diameter of the fibers increases with increasing concentration of SF nanofilaments (Fig. 15.15), which implies that controlling the size of fibers could be easily achieved by changing the SF nanostructure and concentration (Zhang et al., 2012). It has been reported that b-sheet content and fiber hydration have effects on the mechanical properties of an engineered protein named eADF4(C16), which is a spider silk mimic. It has been demonstrated that the aligned fibers fabricated with single proteins have good extensibility and toughness through adequate hydration (Fig. 15.16; Lang et al., 2017).

15.2.2.3 Gelatin Gelatin is the homologous protein of collagen and partially hydrolyzed by it (Daneault et al., 2017). The amino acid composition of gelatin is close to that of collagen. Gelatin molecules can form positive ions, anions or zwitterions in solutions with different pH values. Collagen has a rod-shaped triple-helix structure, which is partially separated and broken when it is hydrolyzed. According to the different preparation methods, gelatin can be divided into two types (Ames, 2010). Type A, derived from hog skin and hydrolyzed by acid, has good plasticity and elasticity. Type B is mainly hydrolyzed by alkali from bones and animal skins and has a good hardness performance. The intensive use of gelatin in food is in gel desserts because of the unique feature of “melting at mouth temperature.” It has also been used in photographic emulsions to protect colloid (Fig. 15.17; Gomes et al., 2013). Thickening and stabilizing are the main applications in food of gelatin as well as electrospun gelatin nanofibers, of which the amounts are smaller and the effects are stronger. The concentration of type B gelatin powder from bovine skin and the applied voltage have been investigated in research by Okutan et al. (2014). It is demonstrated that electrospun gelatin nanofibers are nanoscale only at the concentration of 20% (w/v) gelatin. Furthermore, the lower the applied voltage is, the higher the z potential and diffusion coefficient values for electrospun gelatin nanofiber dispersions are (Okutan et al., 2014). According to a study (Gomes et al., 2013), gelatin from cold-water fish skin is electrospun and then crosslinked. Water or concentrated acetic acid served as electrospinning solvents and both of them are electrospun successfully. There are three different techniques, namely GTA vapor, genipin, and dehydrothermal treatment, applied in the crosslinking process. The results of three studies show that fish gelatin modified with GTA is the most suitable substrate (Gomes et al., 2013).

15.2.2.4 Casein

Casein is the major precipitated protein of milk at the temperature of 20
C and pH value of 4.6. Casein is a phosphorus-containing protein, which has a high content and nutrient value (Horne, 2002). Bovine casein is made up of as1-, as2-, k-, and b-casein (Bonizzi et al., 2009). These four monomers constitute the spatial structure of casein via a-helix, b-sheet, and b-turn. b-Casein from bovine can be used as a superior emulsifier and colloid stabilizer (Dickinson, 2006). The pH value is 4.8, which is the isoelectric point of casein (Liu and Guo, 2008). Casein has excellent steric stability to dispersed oil/fat droplets under neutral pH (Parkinson et al., 2005). A solution containing only casein could not be electrospun due to its strong intermolecular force and 3D structure. However composite dispersions blending PEO or PVA with casein could be electrospun successfully as indicated in Fig. 15.18. The 10% (w/w) casein solutions containing 80% PEO or 50% PVA are used to electrospin with average fiber diameters ranging from 100 to 500 nm (Xie and Hsieh, 2003).

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(F) FIGURE 15.15 (A) Model of electrospun silk fibroin (SF) nanosphere formation. Scanning electron microscopy (SEM) images of SF nanospheres electrospun from regenerated silk nanospheres in 98% formic acid at concentrations of (B) 12 wt % and (C) 6 wt%. (D) Model of electrospun SF nanofilament formation. SEM images of SF nanofilaments electrospun from regenerated silk nanofilaments in 98% formic acid at concentrations of (E) 12 wt% and (F) 6 wt %. (F) Adapted from Zhang, F., Zuo, B., Fan, Z., Xie, Z., Lu, Q., Zhang, X., Kaplan, D.L., 2012. Mechanisms and control of silk-based electrospinning. Biomacromolecules 13, 798e804.

15.2.2.5 Wheat Protein The major components of wheat protein are albumin, globulin, gliadin, gluten, and residue proteins (Scholz et al., 2000). Common wheat protein consists of 20 amino acids. Gliadin and gluten proteins

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FIGURE 15.16 Comparison of construction between natural spider dragline silk and artificial spider silk. Adapted from Lang, G., Neugirg, B.R., Kluge, D., Fery, A., Scheibel, T., 2017. Mechanical testing of engineered spider silk filaments provides insights into molecular features on a mesoscale. ACS Applied Materials & Interfaces 9, 892e900.

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FIGURE 15.17 Scanning electron microscopy images of M1 crosslinked by (A) GTA for 2 h, (B) GTA for 5 h, and (C) DHT, as well as (D) M2 crosslinked by genipin after immersion in Dulbecco’s modified Eagle’s medium (DMEM) for 3 days. (E) Attenuated total reflectance. Fourier transform infrared spectra of electrospun gelatin and crosslinked scaffolds. (F) Weight loss assay performed with gelatin scaffolds crosslinked by DHT with different exposure times and by genipin using different immersion times (1e5 days for M2). DHT, dehydrothermal; GTA, glutaraldehyde. (F) Adapted from Gomes, S.R., Rodrigues, G., Martins, G.G., Henriques, C.M., Silva, J.C., 2013. In vitro evaluation of crosslinked electrospun fish gelatin scaffolds. Materials Science and Engineering: C 33, 1219.

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FIGURE 15.18 Scanning electron microscopy (SEM) images of electrospun mats from 10% polyvinyl alcohol/casein solution at ratios of (A) 100:0 and (B) 70:30. SEM images of electrospun mats from 5% polyethylene oxide/casein solution at ratios of (C) 100:0 and (D) 80:20. (D) From Xie, J., Hsieh, Y.L., 2003. Ultra-high surface fibrous membranes from electrospinning of natural proteins: casein and lipase enzyme. Journal of Materials Science 38, 2125e2133.

are known as storage proteins. Gliadin shows viscous behavior, whereas gluten is elastic (Veraverbeke and Delcour, 2002). Gliadin is a kind of monomer spherical protein with smaller molecular weight. There are no disulfide bonds among the peptides, but hydrogen bonds, hydrophobic bonds, and intramolecular disulfide bonds (Tao et al., 1992). Bread is manufactured with wheat flours derived from hexaploid Triticum aestivum wheats (Veraverbeke and Delcour, 2002). Commercial wheat gluten containing 70.2% or 75% proteins are used as starting material (Fig. 15.19). It is dispersed in 1,1,1,3,3,3-hexafluoro-2-propanol (HFIP) at concentrations ranging from 5% to 10% (w/v) to obtain an electrospun solution. The results indicated that the diameters of the fibers decrease from several micrometers to tens of nanometers. Moreover, the thickness of nanofiber mats is the same as the skin layer but with porous structures.

15.2.2.6 Whey Protein Whey protein consists of a variety of proteins maintained in the supernate after casein precipitation, mainly including b-lactoglobulin and a-lactalbumin (de Wit, 1998). The components of b-lactoglobulin are 162 amino acids in sum. The dominant formation of b-lactoglobulin is a dimer in milk, and the protein is made up of two unit subunits linked with noncovalent bonds. On the other hand, a-lactalbumin is composed of 123 amino acids, including four pairs of disulfide bonds, which play a significant role in stabilizing the structure of protein. Edible films are produced from the mixture of

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FIGURE 15.19 Scanning electron microscopy images of (A) specimen B, which is wheat gluten containing 70.2% protein electrospun onto a rotating and cylindrical mandrel at roughly 5000 rpm; (B) electrospun specimen A, which is wheat gluten containing 75% protein with spherical domains among fibrils existing in the matrix; and (C) a nonwoven mat electrospun from specimen A. (D) Normalized fiber size distribution as a function of fiber diameter group number for specimens A, indicated with hatched green bars (light gray in print version); specimens B, indicated with striped blue bars (gray in print version); and specimens C, indicated with solid red bars (dark gray in print version). From Woerdeman, D.L., Ye, P., Shenoy, S., Parnas, R.S., Wnek, G.E., Trofimova, O., 2005. Electrospun fibers from wheat protein: investigation of the interplay between molecular structure and the fluid dynamics of the electrospinning process. Biomacromolecules 6, 707.

whey protein, oregano, rosemary, and garlic essential oils. And the films have a good antimicrobial property (Seydim and Sarikus, 2007). According to a study (Seydim and Sarikus, 2007), whey protein isolate is mixed with PEO at a ratio of 3:1. Upon the increase in whey protein content, the percentage of a-helices increases and b-turns decreases. The average fiber diameter ranges from 312 to 690 nm. The morphology of nanofibers fabricated from whey protein isolates and PEO, as well as the corresponding diameter analyses, is displayed in Fig. 15.20. According to Fig. 15.20, the addition of whey protein isolate reduces the beaded fibers and improves the smoothness of the fibers (Sullivan et al., 2014). It has also been demonstrated that composite nanofiber mats are produced when the pH value is 2e3. Moreover, when the value of pH is reduced from 7.5 to 2, the percentage of b-sheets decreases and the a-helix percentile increases (Sullivan et al., 2014). Ten percent (w/w) whey protein is dissolved into a 4% (w/w) PEO solution with different values of pH. The PEO solution is prepared by using various solvents, namely, distilled water, aqueous sodium hydroxide, and glacial acetic acid solution. The morphology of the nanofibers produced in distilled water is spherical with a diameter of 2.0  1.0 mm, and the nanofibers electrospun from aqueous sodium hydroxide have an

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FIGURE 15.20 Scanning electron microscopy images and the corresponding fiber diameter analyses of nanofibers electrospun from blended solutions of whey protein isolate and polyethylene oxide at ratios of (A) 0:100, (B) 40:60, (C) 50:50, and (D) 60:40. From Sullivan, S.T., Tang, C., Kennedy, A., Talwar, S., Khan, S.A., 2014. Electrospinning and heat treatment of whey protein nanofibers. Food Hydrocolloids 35, 36e50.

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average diameter of 138  32 mm with spindlelike beads attached. Compared with them the fibers (707  105 mm) in glacial acetic acid are smoother. The reason is that the pH changes the secondary structure of the protein (Vega-Lugo and Lim, 2012).

15.2.2.7 Soybean Protein Nearly 90% of the protein in soybeans is in the form of storage protein. Most of it is b-conglycinin and glycinin, which account for about 30% and 40% of the total protein content, respectively (Utsumi et al., 2002). According to the different settlement coefficients, soybean protein can be divided into four categories, namely 2S, 7S, 11S, and 15S components. The primary constituent of 7S protein is b-conglycinin, which is a kind of glycoprotein (Peng et al., 1984). Glycinin, namely the 11S protein, is a sort of nonglycoprotein consisting of six acid subunits and six alkaline subunits (Saio and Watanabe, 1978). A standard low-lipid diet with the addition of soybean protein plays an important role in reducing cholesterol in patients with type II hyperlipoproteinemia (Verrillo et al., 1985). Soybean protein concentrate blended with glycerol films could be manufactured and used as biodegradable food packaging material (Ciannamea et al., 2014). The polyelectrolytic nature of soy protein isolate (SPI) prevents it from being electrospun. Nevertheless, the addition of PEO improves the spinnability of soybean protein (Fig. 15.21; Xu et al., 2012). The average diameter of the composite is about 200e300 nm. It has been indicated that the nanofiber mats could be used in filtration because of their superhydrophilicity (Xu et al., 2012). The mixture of SPI and PVA has been electrospun to investigate the impacts of pH and SPI contents on the biodegradable and mechanical properties. The results bridge the gap between parameters and mechanical properties. Upon the increase of pH value and SPI content, the mechanical property is lowered. Compared with the degradation time of nanofiber mats containing 50% (w/w) SPI, nanofibers with 25% (w/w) SPI degrade faster, which means the degradation time is related to the proportion of SPI to PVA (Cho and Netravaliba, 2012).

15.2.2.8 Hake Protein

Fish muscle is rich in amino acids and has a high nutritional value (Ya´n˜ez et al., 2010). Consuming a cod-protein diet compared with an equivalent diet of other sources of animal proteins, including pork, lean beef, and eggs, has an advantage in promoting insulin sensitivity (Ouellet et al., 2007). Sarcoplasmic proteins extracted from North Sea cod with a molecular weight of about 200 kDa are dissolved in HFIP for electrospinning. The morphology of the electrospun nanofibers varies with the variety of protein concentrations as shown in Fig. 15.22AeE. No beads or particles are observed in Fig. 15.22D with the protein concentration of 125 mg/mL. Moreover, according to Fig. 15.22F the diameter increases with increasing concentration of protein (Stephansen et al., 2014).

15.2.2.9 Zein The main storage protein in maize is zein, which contains a large area of a-helix in solution. The a-helix is formed by the hydrogen bonds between the hydroxyl groups and imido groups on the main chains of peptides (Shukla et al., 2000). Therefore, zein has a strong hydrophobicity. Zein is divided into four categories of a-, b-, g-, and d-zein based on their different solubilities. a-Zein is made up of two groups, namely 19 kDa (Z19) and 22 kDa (Z22). The content of a-zein is more than 70% of the total prolamins in maize (Matsushima et al., 1997). Edible zein films have been developed to package nuts and tomatoes to retard rancidity, delay weight loss, and slow color changes during storage (Cagri et al., 2004).

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FIGURE 15.21 Diameter analyses of nanofibers electrospun from the blend of SPI and PEO with weight ratios of (A) 16:1, (B) 10: 1, (C) 6:1, and (D) 2:1. (E) Elemental mapping of nitrogen by energy-dispersive X-ray spectrometer (EDS) and (F) scanning electron microscopy (SEM) image of electrospun nanofibers from the blend of SPI and PEO with a weight ratio of 10:1. (G) Elemental mapping of nitrogen with EDS and (H) SEM image of electrospun nanofibers from the blend of SPI and PEO with a weight ratio of 2:1. (I) Elemental mapping of nitrogen with EDS and (J) SEM image of neat PEO nanofibers. PEO, polyethylene oxide; SPI, soy protein isolate. From Xu, X., Jiang, L., Zhou, Z., Wu, X., Wang, Y., 2012. Preparation and properties of electrospun soy protein isolate/polyethylene oxide nanofiber membranes. ACS Applied Materials & Interfaces 4, 4331.

Neo et al. have reported dissolving 25% (w/w) zein (Z3625) into 80% ethanol aqueous solution and further adding 5%, 10% or 20% gallic acid into the mixture to fabricate electrospun nanofibers. The morphology of nanofibers fabricated from zein containing different concentrations of gallic acid is displayed in Fig. 15.23. The diameters of composite fibers range from 327 to 387 nm (Neo et al., 2013a). And further, it is demonstrated that electrospun zein nanofibers as stated above are promising to package dry food because they exhibit stability against heat and chemicals and have low absorbance of water during storage at 21.5
C for 60 days with a relative humidity of approximately 58% (Neo et al., 2013b).

15.2.2.10 Eggshell Membrane Protein The eggshell membrane (ESM), a semipermeable membrane, separates the egg white and the inner surface of the eggshell (Tsai et al., 2006). Biological molecules and protein fibers are the major components of ESM (Yi et al., 2004). The proteins include collagen (types I, V, and X), sialoprotein, and osteopontin (Nys et al., 2004). The complex network of ESM is stable and has a high surface area to be used as an adsorbent (Koumanova et al., 2002). To obtain the electrospinning solution, 40% (w/w) water-soluble ESM powder solution is blended with 5% (w/w) PEO or 10% (w/w) PVA aqueous solutions at different ratios (Fig. 15.24AeD).

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FIGURE 15.22 Scanning electron microscopy images of fibers electrospun from fish sarcoplasmic protein (FSP) at a concentration of (A) 50 mg/mL, (B) 75 mg/mL, (C) 100 mg/mL, (D) 125 mg/mL, and (E) 150 mg/mL. (F) Diameter analysis of FSP fibers. (F) From Stephansen, K., Chronakis, I.S., Jessen, F., 2014. Bioactive electrospun fish sarcoplasmic proteins as a drug delivery system. Colloids and Surfaces B: Biointerfaces 122, 158e165.

The resulting nanofibers are then immersed into a catechin/ethanol solution. It has been reported that the addition of PEO or PVA improves the spinnability of ESM. The best proportions of ESM:PEO and ESM:PVA are 95:5 and 60:40, respectively. After immersion, the solubility is improved by the hydrogen bond interactions between ESM and catechin (Fig. 15.24EeJ) (Kang et al., 2010). Furthermore, coelectrospun nanofibers of ESM and PEO with the modification of methanol and 1,3dicyclohexylcarbodiimide imitate the structure of ESM successfully (Feng et al., 2004).

15.2.2.11 Egg Albumen EA consists of proteins, carbohydrates, lipids, and trace metals. The proteins are divided into four categories, namely, ovalbumin, ovomucoid, ovomucin, and albumin. EA has several advantages in foam forming, water solubility, heat-induced coagulability, and emulsification capacities (Ching-Yung and Holme, 1982). Soluble EA coatings, after being denatured by heating, prevent water loss from raisins. Apart from this, EA is also considered as a coating material to protect food and an emulsion stabilizer (Gennadios et al., 1996; Wongsasulak et al., 2010). Electrospinning solutions with diverse proportions of starting materials have been prepared by mixing CA and EA. CA and EA are dispersed in 85% acetic acid and 50% formic acid, respectively.

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FIGURE 15.23 Scanning electron microscopy images and transmission electron microscopy (insets) images of coelectrospun fibers from mixtures of zein and (A) 0% gallic acid, (B) 5% gallic acid, (C) 10% gallic acid, and (D) 20% gallic acid (w/w in solid fibers). (D) From Neo, Y.P., Ray, S., Jin, J., Gizdavic-Nikolaidis, M., Nieuwoudt, M.K., Liu, D., Quek, S.Y., 2013a. Encapsulation of food grade antioxidant in natural biopolymer by electrospinning technique: a physicochemical study based on zein-gallic acid system. Food Chemistry 136, 1013e1021.

The nanofibers become more continuous with increasing ratio of EA (Wongsasulak et al., 2010). Composite solutions are obtained by dissolving 5% (w/w) EA/PEO into 96% formic acid. The nanofiber diameters show a positive correlation with the proportion of PEO as shown in Fig. 15.25. The melting point of the EA and PEO mixture at the ratio of 1/0.6 is lower than that of pure PEO (Wongsasulak et al., 2007). Both studies suggest that it is easier to fabricate a blend of EA with other polymers compared with the neat EA solution.

15.2.2.12 Bovine Serum Albumin BSA has an overall oblate shape. It is constituted by three domains (I, II, and III). Each of them is linked by disulfide bonds carrying a variety of ionizable groups. The molecular mass of BSA is 66,500 Da and it has 583 amino acid residues. The secondary structure of BSA consists of 10% turn, 23% extended chain, and 67% helix. And no b-sheets exists (Murayama and Tomida, 2004; Weijers, 1977). BSA modifies the tast of tastants by forming a moderately stable complex with the tastant and thereby alters the interaction of the tastant with receptor membranes. (Mudgal et al., 2016). Coreeshell nanofibers have been electrospun, of which BSA is the shell and PVA is the core reported by Won et al. (2012). The results show that the coreeshell nanofibers are excellently formed

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FIGURE 15.24 Scanning electron microscopy (SEM) images of nanofibers electrospun from blends of ESM and PEO at ratios of (A) 95:5, (B) 95:10, (C) 70:30, and (D) 60:40. SEM images of nanofibers electrospun from the blend of S-ESM and PEO at a ratio of 95:5, followed by treatment with 10 wt% catechin/ethanol for (E) 1 day, (F) 3 days, and (G) 7 days. SEM images of nanofibers electrospun from the blend of ESM and PVA at a ratio of 60:40, followed by treatment with 10 wt% catechin/ethanol for (H) 1 day, (I) 3 days, and (J) 7 days. ESM, eggshell membrane; PEO, polyethylene oxide; PVA, polyvinyl alcohol; S-ESM, water-soluble eggshell membrane. From Kang, J., Kotaki, M., Okubayashi, S., Sukigara, S., 2010. Fabrication of electrospun eggshell membrane nanofibers by treatment with catechin. Journal of Applied Polymer Science 117, 2042e2049.

at the ratio of PVA:BSA at 5:5 and applied voltage of 22 kV (Won et al., 2012). Natural, globular, and highly soluble BSA with a molecular mass of 65  103 Da has been electrospun and then reinforced in 15% PEO to obtain insoluble nanofibers (Fig. 15.26). It has been demonstrated that BSA on nanofibers still maintains its native structure (Kowalczyk et al., 2008).

15.2.2.13 Enzymes The elements of enzymes are C, H, O, and N. According to the chemical composition of enzymes, they can be divided into simple enzymes and binding enzymes (Danson and Hough, 1998). The simple enzyme peptides have only amino acid residues (Heck et al., 1996), whereas the binding enzymes consist of enzyme protein and the cofactor (Lee and Lee, 2011). The enzyme protein determines the specificity of reactions (Amin et al., 2013). The cofactor determines the type and nature of reactions, which mostly are small-molecule organic compounds such as ferriporphyrin and the metal ions

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FIGURE 15.25 Scanning electron microscopy images of composite nanofibers of EA and PEO at ratios of (A) 1:0, (B) 1:0.1, (C) 1:0.3, (D) 1:0.6, and (E) 0:1. (F) Viscosity, surface tension, average diameter, and electrical conductivity of composite EA dispersions as a function of EA:PEO ratio with 5 wt% total polymer in formic acid. EA, egg albumen; PEO, polyethylene oxide. From Wongsasulak, S., Kit, K.M., Mcclements, D.J., Yoovidhya, T., Weiss, J., 2007. The effect of solution properties on the morphology of ultrafine electrospun egg albumenePEO composite fibers. Polymer 48, 448e457.

(Yi, 2011). The properties of enzymes are specificity, high efficiency, and diversity. In addition, the conditions for catalytic reaction are mild (Barrett, 1973). Polystyrene, poly(styrene-co-maleic anhydride), and a-chymotrypsin dissolved in organic solvents for electrospinning have been reported. To promote the stability of the enzyme, the nanofibers are modified with GTA in the study (Fig. 15.27; Herricks et al., 2005). And it has been illustrated that the activity of the enzyme on the bulk films is increased (Herricks et al., 2005). Urease (EC 3.5.1.5) is a

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FIGURE 15.26 (A) Atomic force microscopy images of BSA fiber reinforced with the addition of 15% PEO. (B) Temperature denaturation profiles of native BSA (squares) and the blend of BSA/PEO before electrospinning (circles) and after electrospinning (triangles). (C) Electrospun BSA nanofibers stained with fluorescein derivatives. BSA, bovine serum albumin; PEO, polyethylene oxide. From Kowalczyk, T., Nowicka, A., Elbaum, D., Kowalewski, T.A., 2008. Electrospinning of bovine serum albumin. Optimization and the Use for production of biosensors. Biomacromolecules 9, 2087e2090.

catalyst in the hydrolysis of urea. It is immobilized into nanofibers by being mixed into a PVP solution for electrospinning. The enzyme is sensitive to urea and the nonwoven urease/PVP mats display the potential as urea biosensors (Sawicka et al., 2005).

15.2.3 NANOFIBERS FROM OTHER NATURALLY OCCURRING COMPOUNDS 15.2.3.1 Phospholipids Phospholipids are amphiphilic molecules that contain a polar head group and a hydrophobic tail with various degrees of instauration (Kelly et al., 2015). Lecithin contains phospholipids and neutral lipids (Xu and Wu, 2009). Lecithin can form cylindrical and wormlike reverse micelles in nonaqueous

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FIGURE 15.27 Transmission electron microscopy images of (A) as-spun fibers with uniform density and (B) a multiple-neck fiber formed by mechanically stretching the mat. (C) Scanning electron microscopy image of fibers immersed in a buffer solution for 1 week. (D) The impact of glutaraldehyde (GA) treatment on the stability of free a-chymotrypsin (CT) and biocatalytic nanofibers recorded over 1 week. From Herricks, T.E., Kim, S.H., Kim, J., Li, D., Kwak, J.H., Grate, J.W., Kim, S.H., Xia, Y., 2005. Direct fabrication of enzymecarrying polymer nanofibers by electrospinning. Journal of Materials Chemistry 15, 3241e3245.

solutions (Schurtenberger et al., 1991). The main component of lecithin is phosphatidylcholine, which is one of the main components of biofilm. Lecithin is a nontoxic natural surfactant due to its amphiphilic chemical structure (Ahmadi et al., 1994). Lecithin has been applied in health care products in the food industry. Lecithin solution has been reported to be electrospun to obtain nanofibers, for which CHCl3 and N,N-dimethylformamide at a ratio of 7/3 (w/w) were used as the solvent system. Nanofibers of 45 wt% lecithin have an average diameter of 3.3 mm. When the phospholipid concentration increases from 47 to 50 wt%, the average diameter of the fibers increases from 4.2 to 5.9 mm (Mckee and Long, 2006). Huang has reported electrospun phospholipid nanofibers from chemically modified phospholipids (Huang et al., 2010b). The homogeneous nanofibers with diameters ranging from 70 to 120 nm are obtained with a polymer concentration of 8 wt%.

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Jorgensen et al. (Jørgensen et al., 2015) have reported the fabrication of continuous fibers via electrospinning with a mixture of phospholipid and asolectin (Fig. 15.28CeF). The average diameter of as-prepared nanofibers is about 3.3 mm. The impact factors on the morphology and the average diameter of the electrospun phospholipid nanofibers are investigated. The results indicate that the concentrations of the phospholipid and the solvent have an effective impact on the morphology of fibers.

15.2.3.2 DNA DNA is a high-molecular-weight polymer (Bernstein, 1953). Many factors like higher temperatures, organic solvents, acidebase reagents, urea, and amide can cause DNA molecular degeneration (Sorokin et al., 1997). DNA is a nucleic acid containing genetic information. It is crucial substance for the formation and maintenance of all forms of life (Mirsky and Ris, 1951). DNA is composed of nucleotides with a backbone of deoxyribose sugars and phosphate groups linked with ester bonds. The sugar molecules in DNA have one of four different purine or pyrimidine bases attached to them, which contain specific genetic information for each organism (Fraser and Fraser, 1951). However, DNA in aqueous solution is difficult to electrospin (Liu et al., 2007). To improve spinnability and maintain the activity of DNA, DNA may be blended with other polymers like PEO to form an electrospinning solution (Fig. 15.29). Solvents that fit the criteria to

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FIGURE 15.28 Scanning electron microscopy (SEM) images of nanofibers electrospun from (A) 47% (w/w) and (B) 50% (w/w) phospholipid solution. (C) SEM image of electrospun 45% w/w asolectin solution in CHCl3:dimethylformamide (3:2 v/v). SEM images of nanofibers electrospun from (D) 50% (w/w) asolectin in cyclohexane, (E) 60% (w/w) asolectin in limonene, and (F) 60% (w/w) asolectin in isooctane. (AeF) Adapted from Jørgensen, L., Qvortrup, K., Chronakis, I.S., 2015. Phospholipid electrospun nanofibers: effect of solvents and co-axial processing on morphology and fiber diameter. RSC Advances 5, 53644e53652. (B) Adapted from Mckee, M.G., Long, T.E., 2006. Phospholipid nonwoven electrospun membranes. Science 311, 353e355.

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FIGURE 15.29 Scanning electron microscopy (SEM) micrographs of the electrospun fibers from the blends of (A) 3 wt% DNA/3 wt% PEO, (B) 4 wt% DNA/4 wt% PEO, (C) 5 wt% DNA/5 wt% PEO, (D) 3 wt% DNA/7 wt% PEO, (E) 4 wt% DNA/6 wt% PEO, (F) 6 wt% DNA/4 wt% PEO and (G) 7 wt% DNA/3 wt% PEO. PEO, polyethylene oxide. From Liu, Y., Chen, J., Misoska, V., Wallace, G.G., 2007. Preparation of novel ultrafine fibers based on DNA and poly(ethylene oxide) by electrospinning from aqueous solutions. Reactive and Functional Polymers 67, 461e467.

maintain the integrity of DNA exclude a large number of toxic organic solvents. Ten percent (w/w) DNAePEO solutions are prepared by mixing DNA and PEO at ratios of 1:1, 3:7, and 4:6 separately. The corresponding diameter of the nanofibers from the aforementioned polymer solutions ranges from 50 to 250 nm. Increasing the DNA content or decreasing the total amount of polymer in the solution produces fibers with poor morphology, whereas beads disperse all over as indicated in Fig. 15.29 (Liu et al., 2007).

15.3 ELECTROSPINNING OF SYNTHETIC POLYMERIC NANOFIBERS IN THE FOOD INDUSTRY Compared with polymers of natural origin, synthetic polymers have many excellent properties. Firstly the mechanical properties and machinability of natural polymers are generally poor. Secondly, normally natural polymers require purification, which is laborious and expensive. Synthetic polymers with high purity are easy to acquire on a large scale. The application of electrospun nanofibers in the food industry needs to take the following aspects into account. Firstly the polymer itself and the solvent should be nontoxic and have good biocompatibility and biodegradability to ensure that no harm is caused to the human body after being eaten. Secondly the polymer should not have adverse effects, neither on the food composition nor on the edible value of the food, and should not react with the food components. Thirdly nanofibers should not affect the mouthfeel of the food. Because of these demanding requirements, the materials that can be applied to synthesize nanofibers in the food industry are limited, Polycaprolactone (PCL), PVA, PEO, and polylactic acid (PLA) are ideal candidates for the application in food sector.

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Electrospun nanofibers of synthetic polymers are mainly used as carriers to deliver functional ingredients and to facilitate sustained release, which has potential applications in food packaging, targeted elements or compound analyses, filtration aids and biosensors. PCL is a poly-u-hydroxy aliphatic polyester with a certain degree of biodegradability and high biocompatibility, which is widely used as a carrier for functional ingredients delivery. Shaobing Zhou and colleagues have loaded green tea polyphenols into PCL and multiwalled carbon nanotube composite fibers by electrospinning. Studies have shown that the nanofibers loaded with tea polyphenols have fewer side effects on normal osteoblasts, whereas they have a strong inhibitory effect on tumor cells (Shao et al., 2011). PCL nanofibers are also used to inhibit the growth of microorganisms. Idris Cerkez et al. loaded silver chloride into PCL nanofibers. In the antimicrobial experiments, silver chlorideeloaded PCL nanofibers inhibited the growth of 99.9999% of S. aureus and E. coli O157:H7 (Idris et al., 2017), which are main pathogenic bacteria in food. The results are shown in Fig. 15.30. Similarly Adriano Brandelli and colleagues have loaded ketoconazole into PCL nanofibers to obtain PCL nanofibers with antifungal activity. In a test against Aspergillus flavus, inhibition zones ranging from 6 to 44 mm in diameter are observed. This study makes it possible to use nanofiber membranes to inhibit pathogenic fungi in food (Veras et al., 2016). PLA, also known as polylactide, is a new bio-based material made from renewable plant resources such as maize. It has good biodegradability and can be completely degraded to carbon dioxide and water. The products made from PLA are biocompatible, glossy, transparent, and heat resistant.

FIGURE 15.30 Left three columns: photographs of antibacterial test of electrospun PLA nanofiber membrane against Staphylococcus aureus and Escherichia coli. Right two columns: photographs of antibacterial test of electrospun PCL nanofiber membrane against S. aureus and E. coil. CD, cyclodextrin; IC, inclusion complex; PCL, polycaprolactone; PLA, polylactic acid; TR, triclosan. (Left) From Kayaci, F., Umu, O.C., Tekinay, T., Uyar, T., 2013b. Antibacterial electrospun poly(lactic acid) (PLA) nanofibrous webs incorporating triclosan/cyclodextrin inclusion complexes. Journal of Agricultural and Food Chemistry 61, 3901e3908. (Right) From Idris, C., Ayse, S., Sukhwinder K.B., 2017. Fabrication and characterization of electrospun poly(e-caprolactone) fibrous membrane with antibacterial functionality. Royal Society Open Science 4.

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Cyclodextrin (CD) has a unique bowl-like structure (Fig. 15.31A), which can protect small unstable molecules in food; it is widely used in the protection of functional ingredients and the facilitation of sustained release. Tamer Uyar and coworkers have incorporated a triclosan/CD inclusion complex into PLA nanofibers via electrospinning. And the addition of CD enhances the antimicrobial activity of PLA nanofibers containing triclosan, which has a potential application in food packaging

FIGURE 15.31 (A) Approximate dimensions of a-CD, b-CD, and g-CD. Schematic representations of (B) formation of TR/CDIC and (C) electrospinning of PLA/TR/CD-IC solution. CD, cyclodextrin; IC, inclusion complex; PLA, polylactic acid; TR, triclosan. From Kayaci, F., Umu, O.C., Tekinay, T., Uyar, T., 2013b. Antibacterial electrospun poly(lactic acid) (PLA) nanofibrous webs incorporating triclosan/cyclodextrin inclusion complexes. Journal of Agricultural and Food Chemistry 61, 3901e3908.

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(Kayaci et al., 2013b). The fabrication procedure of triclosan-encapsulated CD/PLA nanofibers is displayed in Fig. 15.31B and C , illustrating the unique bowllike structure of CD facilitates the encapsulation of triclosan. PVA is white and water soluble. It has been widely used in the fields of textiles, food, medicine, construction, wood processing, paper, printing, agriculture, steel, and polymer chemicals. Ping Li and colleagues mix GOD into PVA/chitosan/tea extract for electrospinning. Infrared results show that GOD is successfully bonded into the composite nanofibers. The activity of the immobilized enzyme is more than 68% of the free enzyme, and the deoxidization rate of the electrospinning film exceed 73%. Since the growth of food microbes are inhibited when the oxygen content is less than 1%, the electrospun nanofibers, which can remove oxygen, have great potential in the field of food preservation (Ge et al., 2012). Electrospun PVA nanofibers are also widely used to protect and slow the release of food functional constituents. Peter Kingshott and coworkers have incorporated retinyl acetate into a PVA/b-CD polymer solution for electrospinning. The results show that the antioxidant and thermal stability of retinoid acetate in PVA/CD nanofibers are improved (Lemma et al., 2015). Tamer Uyar and colleagues have incorporated geraniol into a PVA solution containing g-CD for electrospinning. The results show that the loss of geraniol in the nanofiber membrane is less than 10% after 2 years of storage. This work have achieved a long-term preservation of the volatile food functional ingredient geraniol (Kayaci et al., 2014). PEO is a thermoplastic water-soluble polymer. The molecular weight of PEO can vary in a wide range. It can form complexes with many organic compounds of low molecular weight, polymers, and inorganic electrolytes. PEO is water soluble, nontoxic, and easy to process. Sachin Talwar and coworkers incorporate whey protein and b-globulin into PEO for electrospinning under acidic conditions, and finally nanofibers with diameters from 312 to 690 nm are obtained. These composite fibers can maintain their morphology at a temperature above the melting point of PEO. Subsequently, the release experiment on flavonoid simulated by rhodamine B shows that whey protein and b-globulin have no impact on the release of flavonoids. The acidic conditions of electrospinning are conducive to improving the shelf life of food (Sullivan et al., 2014). Kamran Ghaedi and colleagues have prepared a branched dextran kefiran/PEO composite nanofiber membrane. Antibacterial experiments show that the composite membrane has antibacterial activity against S. aureus. In addition, the study also explores in-vitro biodegradability of the composite membrane, and the infrared spectrum shows that the kefiran/PEO composite membrane could be biodegraded. This nanofiber membrane can be used in food packaging and food preservation (Jenab et al., 2016). In addition to the aforementioned materials, there are a variety of materials used in the food industry. Polyaniline (PANI) has good thermal stability and an excellent reversible nonredox acid/base doping process. It exists in three well-defined oxidation states: leucoemeraldine base (fully reduced), emeraldine base (EB; half oxidized), and pernigraniline base (fully oxidized). Moreover, EB can exist in the form of emeraldine salt. Polyamide 66 (PA-66) is a white thermoplastic resin, with high mechanical strength and hardness, good heat resistance, and wear resistance. Yanan Wen and colleagues (Wen et al., 2015) prepare PANI/PA-66 blended nanofiber membranes. Surprisingly a large number of beautiful spiderweb-like structures are formed as shown in Fig. 15.32. On this basis, they construct a ready-to-use L-ascorbic acid (L-AA) colorimetric strip, and incubate it with ascorbic acid for 30 min. It has been observed in Fig. 15.33 that the color of the fiber membrane changes from brown to green, and 50 ppb ascorbic acid is sufficient to produce visible color changes. In addition, Bin Ding and coworkers have used a cell phone to take pictures of fiber

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FIGURE 15.32 Scanning electron microscopy (SEM) micrographs of electrospun kefiran/PEO nanofibers at original magnifications of (A) 3000 and (D) 6000. SEM images of PANIeES/PA-66 nanofiber membrane with plotting scales of (B) 1 mm and (E) 100 nm before being treated with KMnO4. SEM images of PANIeES/PA-66 nanofiber membrane with plotting scales of (C) 1 mm and (F) 100 nm after being treated with KMnO4. Insets are their corresponding optical images. ES, emeraldine base; PA-66, polyamide 66; PANI, polyaniline; PEO, polyethylene oxide. (A and D) From Jenab, A., Roghanian, R., Emtiazi, G., Ghaedi, K., 2016. Manufacturing and structural analysis of antimicrobial kefiran/polyethylene oxide nanofibers for food packaging. Iranian Polymer Journal (English Edition) 26, 31e39. (B, C, E, and F) From Wen, Y., Li, Y., Si, Y., Wang, X., Li, F., Yu, J., Ding, B., 2015. Ready-to-use strip for L-ascorbic acid visual detection based on polyaniline/polyamide 66 nano-fibers/nets membranes. Talanta 144, 1146e1154.

membranes, and then RGB parameters are read and statistical analysis is performed to quantify the amounts of ascorbic acid. This qualitative and quantitative detection of L-AA is applied to commercial juice samples. This method avoids a time-consuming sample preparation process and expensive experimental instruments (Wen et al., 2015). Similarly Saverio Mannino and colleagues have loaded polyamide nanofibers onto the surface of a glassy carbon electrode and then equipped it with tyrosinase for the detection of phenols. The detection by this sensor can be limited to 0.05 mM (Arecchi et al., 2010).

15.4 FUNCTIONALIZATION OF NANOFIBERS In general, single-structure electrospun nanofibers lack the desired functional properties, so the application of electrospun nanofibers is limited. To expand the applications of electrospun nanofibers in the field of food, a variety of functional means have been proposed, including blends, coaxial electrospinning, surface grafting and more.

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FIGURE 15.33 (A) UVeVis absorption spectra and optical images of colorimetric strips after being incubated in different concentrations of L-AA aqueous solution for 30 min. (B) UVeVis absorbance spectra and colorimetric responses of strips after being incubated in aqueous solution with 400 ppm of various interfering species for 30 min. (C) Time-dependent UVeVis absorption spectra and digital photographs of colorimetric strips after being incubated in the 4 ppm L-AA aqueous solution. L-AA, L-ascorbic acid; SDA, trisodium citrate. From Wen, Y., Li, Y., Si, Y., Wang, X., Li, F., Yu, J., Ding, B., 2015. Ready-to-use strip for L-ascorbic acid visual detection based on polyaniline/polyamide 66 nano-fibers/nets membranes. Talanta 144, 1146e1154.

15.4.1 ELECTROSPINNING OF POLYMER BLENDS To improve the mechanical strength and enhance biodegradability, biocompatibility, absorbance/ desorption capacity, and hydrophilicity/hydrophobicity, the concept of blended nanofibers is proposed. Blending is a method in which two or more substances are mixed for electrospinning. A mixture of polymers, or of small molecules and polymers, can combine the properties of different materials together. The blending of biocompatible polymers with good mechanics in nanofibers can enhance the mechanical and biocompatibility properties of the fibrous membrane, while blending of functional food components and polymers can impart specific properties to the fibrous membrane. Blending has become an important method of functionalization of nanofibers. Qingrong Huang and coworkers blend kafirin from sorghum with PCL. Compared with pure PCL nanofiber membrane, the hydrophilicity of the composite membrane is improved with increase in PCL content, the fiber membrane becomes more flexible, and the mechanical properties of the composite mats are also improved (Xiao et al., 2016). The composite fiber membrane is able to protect functional ingredients from hazards and slow its release. Tamer Uyar and colleagues have fabricated electrospun nanofibers of polyacrylic acid, quercetin, and CD (Aytac et al., 2016a). De la Torre and coworkers have prepared electrospun nanofibers of PVA, PEO, and carotene (de Freitas Zompero et al., 2015). The fabrication of electrospun composite mats of PVA, dextrins, and retinol acetate has also been reported (Lemma et al., 2015). These studies have found that the incorporation of functional ingredients and CD results in sustained release and further enhances the protective capacity for the ingredients.

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15.4.2 ELECTROSPINNING OF COREeSHELL STRUCTURES To prevent an initial burst release and facilitate a sustained release as well as protecting the substance from harsh environments, nanofibers with coreeshell structures are produced. Korehei and Kadla have used a buffer solution containing T4 bacteriophages as the core and PEO as the shell for electrospinning to fabricate coreeshell structured nanofibers, which are shown in Fig. 15.34 The T4 bacteriophages contained in the coreeshell structure remain stable after storage for several weeks at 4
C. The activity of the T4 bacteriophages in uniaxial fibers produced by electrospinning decreases by more than 5 orders of magnitude, and the activity of the T4 bacteriophages in the nanofibers prepared by emulsion electrospinning decreases by 2 orders of magnitude. Electrospun nanofibers with coreeshell structure could load a large number of T4 bacteriophages, and the activity of the T4 bacteriophages during the storage period is improved. Since T4 bacteriophages can kill microorganisms, the improvement in storage stability of T4 bacteriophages with nanofibers is important to prevent food from corruption caused by microbes (Korehei and Kadla, 2013). Saowakon Wongsasulak and colleagues use CA as the shell and PVA as the core to fabricate coreeshelle structured nanofibers. They have loaded gelatin as a model protein to the core layer to simulate the

FIGURE 15.34 Scanning electron microscopy images of (A) coaxial fibers (PVP/nisin coreePCL sheath), (B) triaxial fibers (PVP/nisin coreePCL intermediateeCA sheath), and (C) electrospun coreeshell PEO fibers. TEM images of (D) coaxial fibers (PVP/nisin coreePCL sheath), (E) triaxial fibers (PVP/nisin coreePCL intermediateeCA sheath), and (F) coreeshell electrospun PEO fibers. CA, cellulose acetate; PCL, polycaprolactone; PEO, polyethylene oxide; PVP, polyvinyl pyrrolidone. (B) From Han, D., Sherman, S., Filocamo, S., Steckl, A.J., 2017. Long-term antimicrobial effect of nisin released from electrospun triaxial fiber membranes. Acta Biomaterialia 53, 242e249. (C) From Korehei, R., Kadla, J., 2013. Incorporation of T4 bacteriophage in electrospun fibres. Journal of Applied Microbiology 114, 1425e1434. (D) From Han, D., Sherman, S., Filocamo, S., and Steckl, A.J., 2017. Long-term antimicrobial effect of nisin released from electrospun triaxial fiber membranes. Acta Biomaterialia 53, 242e249. (F) From Korehei, R., Kadla, J., 2013. Incorporation of T4 bacteriophage in electrospun fibres. Journal of Applied Microbiology 114, 1425e1434.

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functional components of protein foods. The release mechanism of the gelatin in the membrane is anomalous diffusion and the gelatin shows a near zero-order release pattern with a release half-life of about 7.4 days. The fibrous membrane immersed in phosphate-buffered saline is still intact after 20 days without rupture (Sakuldao et al., 2011). Interestingly, according to a report by Andrew J. Steckl and coworkers, with an increase in layer number, nanofibers have an extended release capacity. For instance, the antibacterial effect of none coreeshellestructured nylon-6/PVP nanofibers loaded with nisin lasts for only 1 day, because watersoluble PVP is rapidly dissolved in aqueous solution. However, the replacement of PVP with the waterinsoluble polymer PCL results in fibers without antibacterial effects. In coreeshellestructured nanofibers in which nylon-6 and PVP polymers form the core containing nisin, and PCL is the shell, the antibacterial activity of the fibers is improved compared with the nonecoreeshellestructured nylon-6/PVP nanofibers loaded with nisin. After application of the fibers into a suspension of S. aureus, 99% of the S. aureus is killed. The addition of CA as the outermost layer for coree shellestructured and nisin-loaded nylon-6/PVP/PCL nanofibers further prolongs the antimicrobial effects up to 7 days. And 99.99% of microorganisms are killed in the first 5 days (Han et al., 2017). Transmission electron microscopy images of PVP/nisin coreePCL shell nanofibers and PVP/nisin coreePCL intermediumeCA shell nanofibers are shown in Fig. 15.34.

15.4.3 INCLUSION OF NANO- AND MICROSTRUCTURES AND COATING NANOFIBERS The inclusion of emulsions, microemulsions, and micellar structures into the spinning solution is a feasible means to overcome the instability of encapsulated ingredients and to improve the distribution of the functional components in the fibers. Furthermore the surface of the nanofibers can be modified with physical or chemical methods to expand the application. Dhakate et al. have grafted xylanase onto the surface of polymethylmethacrylate (PMMA) nanofibers (Kumar et al., 2013) . The results show that the binding efficiency of xylanase on PMMA is 90% and the thermal stability is improved. Moreover, the enzyme activity on the composite nanofibers remain 80% after 11 cycles of reactions. Thus the composite material is promising in the food sector due to its thermal stability and enzyme stability. In addition, Garcı´a-Moreno et al. have successfully encapsulated fish oil in PVA by emulsion electrospinning technology. However, it is frustrating that the stability of fish oil in PVA nanofibers is worse than that of nonwrapped fish oil (Garcı´a-Moreno et al., 2016). In the work of Korehei and Kadla mentioned earlier, emulsion electrospinning is also applied, but the activity of encapsulated T4 bacteriophage is still decreased down to two orders of magnitude (Korehei and Kadla, 2013).

15.5 APPLICATION IN FOOD PACKAGING TECHNOLOGY Packaging, as an external component of food, plays an important role in food preservation by protecting the food from spoilage during the distribution to the final consumers (Rooney, 1995). Electrospun fibers for food packaging have structural and functional advantages. Firstly the fibers exhibit advantages in controllable morphology, high porosity, large surface area, and nanoscale effects, which

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are propitious for the enhancement of functionalities (Shen et al., 2011). Secondly high encapsulation efficiency and stability against heat and chemicals make them promising carriers for bioactive ingredients and thereby for its application in food packaging (Bhushani and Anandharamakrishnan, 2014). In general the primary aims of food packaging involve the following aspects: (1) suppression of microorganism growth, resistance to oxidation, and stability against environmental hazards; (2) masking of unpleasant odors and preservation of flavor; (3) delivery of nutriments in a manner of sustained release; (4) assistance in filtration and accumulation of elements; and (5) acting as carriers of biosensors for detection (Deng et al., 2012; Fernandez et al., 2009; Nedovic et al., 2011; Noruzi, 2016).

15.5.1 FOOD PRESERVATION 15.5.1.1 Preservation From Pathogen Contamination Antimicrobial packaging is able to inhibit or kill microorganisms attached to food surface by releasing innoxious antimicrobials from the packaging material, thereby extending the shelf life of food (Chien et al., 2007). There are several types of antibacterial packaging, including edible antibacterial film, antibacterial materials of molecular assembly, and antibacterial materials of nanoscale (Dastjerdi and Montazer, 2010). Among them, antibacterial materials of nanoscale have unique nanoeffects and show some preferable features by comparison with traditional antibacterial materials (Augustin and Hemar, 2009). Ge et al. have prepared PVA/chitosan/tea extract composite fibers with immobilization of GOD via electrospinning. According to Fig. 15.35, the membrane show an obvious antibacterial activity against both gram-positive bacteria and gram-negative bacteria. The activity of immobilized GOD is more

FIGURE 15.35 Antibacterial effects of electrospun membrane on (A) Escherichia coli and (B) Staphylococcus aureus. CFU, colony-forming units; CS, chitosan; PVA, polyvinyl alcohol. (B) From Ge, L., Zhao, Y.S., Mo, T., Li, J.R., Li, P., 2012. Immobilization of glucose oxidase in electrospun nanofibrous membranes for food preservation. Food Control 26, 188e193.

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than 68% by comparison with the free enzyme. Moreover, the result demonstrates a promising application of fibers in food packaging because of the significant antibacterial properties under a lowoxygen-content situation (Ge et al., 2012). A CA nanofiber membrane with negative charge has been fabricated via electrospinning by Huang et al. (2012). Subsequently the mixture of positively charged chitosan/lysozyme/organic rectorite (OREC) and negatively charged sodium alginate are successfully deposited onto the surface of the membrane alternatively by the layer-by-layer self-assembly technique. According to antimicrobial experiments on pork samples, the addition of OREC enhances the inhibitory effects of composite mats on the growth of E. coli and S. aureus. As a result the shelf life of pork could be extended for 3 days. Both pH value and TVB-N remain lower than those of the control group during storage for 21 days (Fig. 15.36). Hence, the nanocomposite is expected to be an effective material for antibacterial packaging in the food sector (Huang et al., 2012). Nanofibers electrospun from a PLA polymer solution with addition of cinnamon essential oil (CEO)/b-CD have higher thermal stability than those electrospun from PLA polymer solution with addition of CEO alone. Meanwhile Wen et al. (2016) have compared the performances of an inclusion complex of PVA with CEO/b-CD (CEO/b-CD-IC) and PVA with CEO in the growth inhibition of E. coli and S. aureus. According to Fig. 15.37 electrospun PLA/CEO/b-CD fibers have a significant improvement in antimicrobial performance for both gram-positive bacteria and gram-negative bacteria. Thus they have a potential application as active food packaging to extend the shelf life of pork by inhibiting the growth of microorganisms (Wen et al., 2016).

FIGURE 15.36 The quantification of (A) pH value and (B) total volatile basic nitrogen (TVB-N) in pork during storage for 21 days. (B) From Huang, W.J., Xu, H.J.L., Xue, Y., Huang, R., Deng, H.B., Pan, S.Y. 2012. Layer-by-layer immobilization of lysozymechitosan-organic rectorite composites on electrospun nanofibrous mats for pork preservation. Food Research International 48, 784e791.

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FIGURE 15.37 (A) The weight loss and (B) the derivative curves of CEO/b-CD-IC, b-CD, and CEO. (C) Inhibition zones of PLA/ CEO and PLA/CEO/b-CD nanofilms on lawns of Escherichia coli and Staphylococcus aureus. (D) Total viable microbial counts of bare pork and pork packaged with fresh-keeping film or PLA/CEO/b-CD nanofilms. b-CD, cyclodextrin; CEO, cinnamon essential oil; CFU, colony-forming units; IC, inclusion complex; PLA, polylactic acid. From Wen, P., Zhu, D.H., Feng, K., Liu, F.J., Lou, W.Y., Li, N., Zong, M.H., Wu, H., 2016. Fabrication of electrospun polylactic acid nanofilm incorporating cinnamon essential oil/beta-cyclodextrin inclusion complex for antimicrobial packaging. Food Chemistry 196, 996e1004.

15.5.1.2 Preservation From Oxidation Eugenol (EG) shows strong antimicrobial and antioxidant properties. Nevertheless, the existence of double bonds makes it an unstable chemical attribute. Accordingly, Kayaci et al. (2013a) manage to improve the thermostability of this component by encapsulating it into an electrospun PVA nanofiber membrane. Artificial CDs of three structures, a-CD, b-CD, and g-CD, have been used to form inclusion complexes of EG/CD to stabilize EG. According to the results shown in Fig. 15.38 the composite containing both b-CD and g-CD enable higher EG stability against heat. In addition, the

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FIGURE 15.38 (A) Thermal gravimetric analysis of EG and the electrospun nanofibers. (B) Peak areas of EG in a head-space gas chromatographyemass spectrometry chromatogram of electrospun nanofibers agitated at 50
C, 75
C, and 100
C for 30 min. CD, cyclodextrin; EG, eugenol; IC, inclusion complex; PVA, polyvinyl alcohol. (B) From Kayaci, F., Ertas, Y., Uyar, T., 2013a. Enhanced thermal stability of eugenol by cyclodextrin inclusion complex encapsulated in electrospun polymeric nanofibers. Journal of Agricultural and Food Chemistry 61, 8156e8165.

release of EG from the composite nanofibers of PVA/EG/b-CD and PVA/EG/g-CD is significantly slowed in comparison with PVA/EG within 30 min at temperatures of 50
C, 75
C, and 100
C, which suggests the vaporization of EG from the nanofibers is abated (Kayaci et al., 2013a). Retinyl acetate (RA) is the ramification of vitamin A and used as an antioxidant in the packaging material. However, it is unstable to oxidation, bright light, and high temperature. Lemma et al. have effectively encapsulated RA into electrospun PVA/CD composite mats to improve its thermal stability via the synthesis of inclusion complexes. The results in Fig. 15.39 indicate that RA/PVA/b-CD fibers show a better stability against oxidation than RA/PVA as well as a higher thermal stability than pure

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FIGURE 15.39 (A) Thermal gravimetric analysis (TGA) for the PVA and RA powders and the electrospun fibers. (B) First-order derivatives of the TGA thermograms for powdered PVA and RA and the electrospun fibers. (C) Differential scanning calorimetry thermograms of powders and electrospun nanofibers. (D) Releasing kinetics monitored by square-wave voltammograms of RA in the nanofibers of (curve A) PVA/RA and (curve B) PVA/RA/b-CD. Inset: the calibration graph of RA and graph equation used to calculate the corresponding concentration signal of RA in the electrochemical cell released from nanofibers of PVA/RA and PVA/b-CD/RA. CD, cyclodextrin; PVA, polyvinyl alcohol; RA, retinyl acetate. (D) From Lemma, S.M., Scampicchio, M., Mahon, P.J., Sbarski, I., Wang, J., Kingshott, P., 2015a. Controlled release of retinyl acetate from beta-cyclodextrin functionalized poly(vinyl alcohol) electrospun nanofibers. Journal of Agricultural and Food Chemistry 63, 3481e3488.

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PVA nanofibers. In this case, electrospinning could be employed as a method to effectively encapsulate food materials, prolong shelf life, and enhance stability against the heat (Lemma et al., 2015).

15.5.2 PRESERVATION FROM OTHER ENVIRONMENTAL HAZARDS Carotenes, which are sensitive to light and heat, are widely applied in the food industry as antioxidants. Zein is an abundant edible biopolymer in corn and can be dissolved in aqueous ethanol solution without the use of toxic reagents such as chloroform and hexafluoroisopropanol. Encapsulation of food ingredients with antioxidant properties, such as b-carotene, into ultrafine zein nanofibers via electrospinning has been successfully effectuated by Fernandez et al. (2009). According to the Raman spectra results in Fig. 15.40 no remarkable variation in the chemical structure of b-carotene enclosed in nanofibers is observed. Meanwhile, there is a stable and extensive distribution of b-carotene inside zein fibers, while some areas are agglomerated. Likewise, the results of UVeVis irradiation exhibit a dramatic enhancement of the light stability of b-carotene encapsulated in fibers. Therefore, electrospun zein prolamine nanofibers have proved to be a ponderable application in protecting the lightsensitive components of food (Fernandez et al., 2009). Fabra et al. have successfully prepared a multilayered membrane based on polyhydroxybutyrateco-valerate in which electrospun zein fibers are wedged as the interlayer. The results show that the electrospun film could reduce the permeability to oxygen and improve the vaporization of water (Table 15.3). Thus the novel materials are expected to be used to enhance the barrier performance of food packaging (Fabra et al., 2013).

FIGURE 15.40 (A) Raman spectra of the as-received b-carotene (dotted line) and of b-carotene recorded inside electrospun zein/ b-carotene fibers (continuous line). (B) Relative decay of the b-carotene concentration versus UVeVis exposure time measured with UVeVis spectroscopy at 466 nm of (C) as-received b-carotene, (B) cast finer b-carotene, and (;) zein/b-carotene fiber mats. (B) From Fernandez, A., Torres-Giner, S., Lagaron, J.M., 2009. Novel route to stabilization of bioactive antioxidants by encapsulation in electrospun fibers of zein prolamine. Food Hydrocolloids 23, 1427e1432.

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Table 15.3 Water Vapor and Oxygen Permeability Values of Multilayer Films With and Without a Zein Interlayer Water Vapor Permeability

Oxygen Permeability 15

(3 10 Zein Deposition Time Control 10 min 20 min 60 min

CompressionMolding 3.83 3.39 4.04 2.35

a,1

(0.08) (0.14)a,1 (0.51)a,1 (0.56)b,1

kg/m Pa s) CompressionMolding

Casting a,2

50.40 (6.00) 14.00 (8.03)b,2 16.00 (8.00)b,2 3.94 (0.05)c,2

a,1

15.40 (0.15) 11.40 (0.41)b,1 6.06 (0.22)c,1 3.73 (0.20)d,1

Casting 14.61 (0.42)a,1 11.10 (0.52)b,1 8.12 (0.15)c,1 6.17 (0.12)d,2

Different superscripts within the same column indicate significant differences among formulations (P < .05). Different superscripts within the same line indicate significant differences due to the method used (P < .05). Adapted from Fabra, M.J., Lopez-Rubio, A., Lagaron, J.M., 2013. High barrier polyhydroxyalcanoate food packaging film by means of nanostructured electrospun interlayers of zein. Food Hydrocolloids 32, 106e114. aed 1e2

15.5.3 PRESERVATION OF FLAVOR AND MASKING OF ODOR Many flavors or fragrances are of a volatile nature and susceptive to oxidation and heat. To overcome these problems, flavoring agents are usually encapsulated to increase the stability against heat and oxygen and to preserve the molecules from evaporation. Nanofibers have been the center of interest for having very high surface areas with variable physicochemical properties and for facilitating the slowed release of loaded ingredients. The incorporation of flavoring agents into a nanofiber matrix may offer practical applications. For instance, b-CD-functionalized poly(ε-caprolactone) and polystyrene nanofibers with efficiency at masking odors have been established (Narayanan et al., 2015; Uyar et al., 2009). Electrospun zein nanofibers have been used to encapsulate u-3 polyunsaturated fatty acids to enhance their bioaccessibility by protecting them from oxidation and undesirable fishy odor (Moomand and Lim, 2015). Vanillin, which has a volatile nature, has been preserved via the electrospinning of vanillin/CD inclusion complex nanofibers (Celebioglu et al., 2016). Similarly, geraniol/CD inclusion complex nanofibers preserve significant amounts of geraniol and show slow release of geraniol, which improves the performance of fragrances and the activity of antibacterials and antioxidants (Aytac et al., 2016b).

15.5.4 DELIVERY OF NUTRACEUTICALS AND FACILITATION OF SUSTAINED RELEASE Electrospun nanofibers have a high ratio of surface to volume and thus the carried substances are released into the medium via a large surface area. The release profiles of substances are connected with diffusion, matrix degradation, and surfaces; thus the composition and morphology of electrospun nanofibers have an impact on the release behavior of the carried substances (Liu et al., 2017; Sharifi et al., 2016). For instance, the hydrophobic surfaces of nanofibers act like a carrier, repelling water molecules and thereby protecting substances from instantaneous burst release into the aqueous system. The addition of amphiphilic or hydrophilic material into the matrix improves the release percentage during the same time period (Lu et al., 2017).

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Bioactive ingredients, including antioxidants, antimicrobial components, vitamins, and probiotics, of living cells have been deposited onto electrospun nanofibers for sustained delivery and improved bioaccessibility. The nutrient vitamin B12 has been incorporated into PCL nanofibers and delivered as a water-soluble energy supplement via the transdermal route (Madhaiyan et al., 2013). The antioxidant carnosic acid has been embedded into hybrid fiber mats of PCL and kafirin protein wherein the amorphous region of kafirin dominates the release rate of carnosic acid and PCL maintains the 3D structure of the nanofibers, improving solvent stability and mechanical properties (Xiao et al., 2016). Similarly, the antioxidant activity of curcumin, the light stability of b-carotene, and the chemical stability of epigallocatechin gallate are improved by being encapsulated into zein nanofibers (Dhandayuthapani et al., 2012; Fernandez et al., 2009; Li et al., 2009). And all of the encapsulated bioactive ingredients undergo sustained release. Apart from the bioactive compounds, it is also feasible to deliver living cells by electrospun nanofibers for food preservation. For instance, bifidobacteria encapsulated by PVA nanofibers have shown prolonged viability at room temperature and refrigeration temperature (Lo´pez-Rubio et al., 2009). In addition, electrospun nanofibers have been developed as a substrate for the immobilization of enzymes to enhance stability toward environmental changes, including pH values and temperature, and to facilitate their controlled release (Lopez-Rubio et al., 2006). For instance, b-galactosidase immobilized on electrospun poly(acrylonitrile-co-methylmethacrylate) has enhanced stability against temperature, pH, and storage compared with the free form (El-Aassar, 2013). Chymotrypsin immobilized on polystyrene nanofibers has structural stability against denaturation and increased half-life in methanol (Jia et al., 2002). Thus the utilization of electrospun nanofibers as carriers for bioactive ingredients displays advantages in sustained release and enhanced stability against environmental hazards, thereby improving the bioaccessibility and efficacy of bioactive ingredients.

15.5.5 HARVEST AND ANALYSES OF TARGETED ANALYTES Analyses of substances from the food matrix are normally performed using liquid chromatographyetandem mass spectrometry, which requires a laborious procedure of sample pretreatments including liquideliquid extraction, solid-phase extraction, molecularly imprinted polymer extraction, and more, to minimize interference. These procedures may require expensive instruments, organic solvents, or long analysis time (Chu et al., 2017). Meanwhile, due to the advantages of electrospun nanofibers, including large surface area-to-volume ratio, chemical and physical stability, inexpensive production, and being exceptionally simple, electrospun nanofibers have received intensive attention and have been utilized as sorbent materials to harvest functional or contraband substances in foodstuffs as well as pollutants in various sample matrices for further analyses. Illegal food additives, including Sudan dyes in chili powder and phthalates, have been effectively extracted by nanofibers for further detection (Cao et al., 2013; Huang et al., 2010a). Packed-fiber solid-phase extraction has been demonstrated to be simple and cost effective to detect six b-agonists from pork samples (Chu et al., 2017). Similarly, harmful residues in food matrices, for instance, pyrethroid insecticides in tea samples, organophosphorus pesticides in cabbage and tap water, chloramphenicol in milk, and residues of diethylstilbestrol, hexestrol, and dienestrol in milk products (Chun-hui et al., 2010; Chun-hui et al., 2009; Maddah et al., 2017; Zheng et al., 2012), have been directly extracted with nanofibers and further analyzed by gas chromatography or high-performance liquid chromatography (HPLC). Pollutants in water, including nitrobenzene,

15.6 CONCLUSIONS AND PERSPECTIVES

503

2-naphthol, benzene, n-butyl p-hydroxybenzoate, naphthalene, and p-dichlorobenzene, have been well-adsorbed with electrospun polymer nanofibers (Qi et al., 2008; Sun et al., 2011). In addition, nanofiber solid-phase extraction has also been established to harvest nutrients such as vitamin B12 in beverages (Fang et al., 2009) as well as fat-soluble vitamins in fortified milk powder and multivitamin tablets (Chen et al., 2011) for quantification, which is a friendly procedure for heatsensitive vitamins and avoids long durations of preconcentration with activated carbon fibers or HPLC. Moreover, nanofiber solid-phase extraction has an advantage in harvesting multiple components in one step, thereby facilitating analyses of multiple components at one setup.

15.5.6 CARRIERS FOR INTELLIGENT SENSORS The electrospun nanofiber characteristics of high surface area, porosity, flexibility, low-cost production, and portable nature make them a good platform for sensors. For instance, L-AA in juices has been visually detected with hybrid nanofibers of PA-66 and PANI (Wen et al., 2015). Lead(II) ions in tap water have been visually detected with pyromellitic dianhydride-grafted cellulose nanofibers (Li et al., 2015). Similarly, formaldehyde in food such as fish slices, silk squid, yuba, and dry seaweed can also be detected by fluorescence from an 8-hydroxyquinoline-based probe on electrospun nanofibers (Liu et al., 2015a). Bioactive aroma compounds have been encapsulated into electrospun nanofibers for humidity-triggered release (Mascheroni et al., 2013). The ethanol content in alcoholic beverages has been estimated from the intensity of fluorescence elicited from terphenyl-ol embedded in nanofibers (Akamatsu et al., 2015). Several scientists have also developed gas sensors using electrospun nanofibers to detect ammonia (Manesh et al., 2007), hydrogen sulfide (Mousavi et al., 2016), hydrogen (Drobek et al., 2016), alcohol vapor (Akamatsu et al., 2015), biogenic amines (Geltmeyer et al., 2016), and more. Apart from chemicals and gases, microorganisms can also be detected by electrospun nanofibers. For instance, S. aureus has been detected with Au nanoparticle/PANI nanofibers by binding of single-stranded capture DNA to the gold nanoparticles (Spain et al., 2011). Cauliflower mosaic virus has been detected via DNA-specific sequences by self-doped PANI nanofibers with deposited Au microspheres (Wang et al., 2011b). Thus novel nanofiber-based biosensors are continually being reported given the possibility that the high surface area of nanofibers increases the sensitivity of biosensors and the fact that the production is exceptionally simple and flexible (Matlock-Colangelo and Baeumner, 2012).

15.6 CONCLUSIONS AND PERSPECTIVES Owing to the advantages of high surface area to volume, continuous nanoscale fibers, flexibility, low cost and exceptionally simple production, portable nature, and excellent accommodation of a wide range of polymers, electrospun nanofibers have received enormous attention regarding applications in the food industry. Herein we have briefly reviewed a variety of diverse polymers for electrospinning fabrication, the functionalization of electrospun nanofibers, and recent developments and representative results of various application in food-related research. In addition to flourishing research in the lab, electrospinning’s versatility has become industrialized. For instance, some leading companies, including DuPont, Ahlstrom, and Donaldson, have manufactured electrospun products, mainly for aiding in filtration (Huang et al., 2003). More than 100 companies are selling

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electrospun-nanofiber-based electrospinning setups or products. However, more intensive efforts are required to commercialize electrospun nanofibers in the food sector. To achieve this goal, the challenges that must be overcome are increasing output production rate of commercial electrospinning setups, improving loading capacity of nanofibers for bioactive ingredients, optimizing controlled-release properties of nanofibers, and enhancing the control of fabrication parameters to produce reproducible and standardized hybrid functional nanofibers. In addition, the application of nanofibers in analytical performance is constantly being improved in reproducibility, detection limits, sensitivity, selectivity, operation range, recovery rate, etc. Furthermore, the individualization of nanofibers for multiple applicable environments or analyses and the integration of multiple functions on a single mat are of interest in the field of functionalizing electrospun nanofibers.

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16

Todd J. Menkhaus1, Hao Fong2

Department of Chemical and Biological Engineering, South Dakota School of Mines and Technology, Rapid City, SD, United States1; Department of Chemistry and Applied Biological Sciences, South Dakota School of Mines and Technology, Rapid City, SD, United States2

16.1 INTRODUCTION Biopharmaceutical therapeutics (i.e., recombinant proteins, monoclonal antibodies, viral vaccines, and plasmid DNA) are quickly becoming major contributors to the fights against life-threatening and debilitating disorders. Prior to 1997 only 6% of approvals by the US Food and Drug Administration (FDA) for new therapies were for biopharmaceuticals. However, between 1997 and 2013 that number grew to 26%, and as of this writing there are thousands of potential drug products of biological origin in clinical development (Reichert, 2004). As encouraging as these figures are, the biopharmaceutical industry is facing enormous pressures from the government and the public to improve the quality of therapeutics and increase the speed to market, while reducing the costs of production (Sundberg, 2003). These demands are particularly relevant to the downstream purification of biological therapies because not only are the separation operations responsible for producing a safe product that meets purity guidelines established by the FDA, but also the economic modeling of processes has shown that a significant percentage (i.e., up to 80%) of the overall manufacturing costs is incurred during downstream purification (Roque et al., 2004). Conventional separation technologies, including packed-bed adsorption and chromatography, membrane filtration, and precipitation and crystallization, have been used for decades for the separation and purification of biologically valuable products. While these techniques have provided acceptable results, they are often inefficient in terms of material and time requirements (Lightfoot and Moscariello, 2004). With these challenges also come opportunities to improve the separation and purification processes of biopharmaceutical therapeutics; herein, new separation and purification media utilizing electrospun cellulose and carbon nanofibers as the supports for chemical ligands within membrane adsorbers are reported. The workhorse for industrial separation and purification processes is selective adsorption and elution of the target molecules within a packed bed of porous resin beads. The operation provides reasonably good purification factors, is reliably scaled between development and manufacturing sizes, and can be easily validated for commercial production (Levison, 2003). Unfortunately, this process also suffers from several major limitations. First, the operational flow rates used during processing must be kept relatively low to maintain acceptable pressure limits, and this often requires reducing the Electrospinning: Nanofabrication and Applications. https://doi.org/10.1016/B978-0-323-51270-1.00016-9 Copyright © 2019 Elsevier Inc. All rights reserved.

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flow rate during processing owing to pressure increases. Similarly, to achieve high binding levels of the target molecules, in terms of bound product per volume of resin, very long residence times are required (again necessitating slow flow rates or cumbersome packing arrangements). These capacity limitations are primarily due to very slow intraparticle diffusion of the relatively large biomolecules to access available binding sites deep within the porous resin beads. Finally, concerns persist regarding the potential for flow channeling and poor dispersion within the packed bed, which lead to inefficient use of expensive resin (Ghosh, 2002; Charcosset, 1998). A promising alternative to packed-bed adsorption, which is gaining support not only from academic researchers but also from industrial practitioners in the biopharmaceutical arena, is the use of membrane or mat adsorption units (also referred to here as adsorptive felts) (Phillips et al., 2005). This style of adsorption utilizes the fibers of a membrane or mat (felt) to act as the support for ligands used during the selective adsorption process. As shown schematically in Fig. 16.1, the most important characteristics of this operation are that first, flow is through micro- and macropores of the felt (as opposed to tightly packed resin beads), and second, adsorption takes place on the surface of the fibers within the felt, where no internal diffusion is required. These factors reduce concerns of high pressure drops with elevated flow rates, and eliminate the slow intraparticle diffusion required for adsorption within resin beads. It has been shown that the binding capacity of biomolecules to currently available adsorption membranes is similar in magnitude to that of resin beads, but can operate at processing flow rates over 10 times faster than packed beds (Ghosh, 2002; Charcosset, 1998). These factors allow for much faster processing times and potentially higher binding levels for purifying valuable biological

Bulk Flow of Feed

Large Micropores Bulk Flow of Feed

Inaccessible Resin Pore

Cross Section of Adsorption Felt

Packed Bed of Resin Beads

“Large” Adsorbed Biomolecule

Adsorption Fiber

FIGURE 16.1 Processing schematics for a packed bed of resin beads (left) and adsorption membrane, felt, or mat (right). The tightly packed column of resin beads requires a much lower operating flow rate to maintain acceptable pressure limits than the more open flow channels within the felt. In addition, the small pores of resin beads are inaccessible for binding large biomolecules (or require extremely long residence times for internal mass transfer), while convective transfer to the surface of adsorbing fibers of the felt makes binding sites readily available for capturing the targeted product.

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products by membrane/mat (felt) adsorption. This is highly desired, especially for large biomolecules (with molecular weight higher than 250 kDa and hydrodynamic diameter of 20e300 nm), because they are very difficult to separate and purify by using traditional chromatography because of severe mass transfer limitations within the small pores of resin beads or particles. Numerous examples of applying membrane/felt/mat adsorption units to the purification of large biomolecules such as plasmid DNA (Endres et al., 2003; Teeters et al., 2003), supercoiled DNA (Haber et al., 2004), and large proteins (Yang et al., 1999, 2002) have appeared since the beginning of the 21st century. The results have been promising, and have repeatedly demonstrated superior performance compared with packed-bed chromatography by providing higher binding capacities and elevated operating flow rates. Several important characteristics of the membrane/felt/mat properties and operating conditions have emerged from the literature that suggest careful consideration of specific characteristics should be made during application. First, to maximize binding capacity, the optimum pore size between fibers should be smaller than w15 mm because of external (liquid phase) convective mass transfer limitations of transporting the target to the surface of adsorbing fibers (Sarfert and Etzel, 1997; Tejeda et al., 1999). Many adsorptive felts utilize pore sizes in the range of 0.5e5 mm. Second, much like packed-bed chromatography, an even inlet flow dispersion and a tight pore size distribution are required for efficient utilization of all available binding sites within the bed. This potential concern can be mitigated by using stacks of felt fibers oriented in random overlay to discourage unimpeded flow (channeling) of the feed solution (i.e., producing an adsorption felt with controlled thickness and firmness, or number of stacked membranes) (Shi et al., 2005). Third, the static and dynamic binding capacities are generally equivalent for felt adsorption, indicating that the binding capacity is independent of feed flow rate (and the corresponding residence time within the adsorption felt) and faster load times should not affect the process performance; this is in sharp contrast to packed beds, which often require slow feed rates or large columns to realize higher binding levels (Yang et al., 1999, 2002). However, it is necessary to note that, in extreme cases of very large target molecules and very short residence times, flow rate must still be considered for membrane adsorption (Haber et al., 2004). Finally and most importantly, the binding capacity of large molecules is often limited by the available surface area of the membrane/felt/mat fibers (Yang and Etzel, 2003; Tennikov et al., 1998). This has been observed through mathematical modeling of experimental results (Yang and Etzel, 2003) and confocal microscopic visualization of protein adsorption on fibers (Reichert et al., 2002; Wickramasinghe et al., 2006). Previous studies have demonstrated the benefits of employing nanofibers, with elevated specific surface area, in other applications. For example, Venugopal and Ramakrishna reported the advantages of polymer nanofibers in biomedical applications of drug delivery and tissue engineering (Venugopal and Ramakrishna, 2005); furthermore, it has been shown that ion-exchange capacity and water uptake can be greatly increased using polymer nanofiber ion exchangers (An et al., 2006). At this writing, the available membranes, felts, and mats are composed of coarse microfibers with diameters in the range of 5e15 mm. This large diameter reduces the specific surface area (i.e., surface area-to-volume ratio) within a membrane bed, and significantly limits the potential binding capacity of the module. Hence, by using thinner-diameter fibers (i.e., “nanofibers” with diameters typically in the range of 10e1000 nm), the available surface area within a given bed volume for potential binding will be significantly increased, by as much as 2 orders of magnitude. Upon controlling the pore size of the manufactured felt, the pressure drop and hydrodynamic flow characteristics can also be tailored and made to be as efficient as microfiber adsorbent felts. Previous research efforts indicated that the

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binding capacity of adsorptive felts made from so-called ultrafine fibers (with diameters of w2 mm) was substantially higher for capturing bovine serum albumin (BSA; a small- to moderate-sized protein with the molecular weight of w66 kDa) (Ma et al., 2005; Bamford et al., 1992). While they were successful at producing the membranes, little was done to suggest improvements beyond a simple demonstration of binding. Furthermore, ultrafine fibers are still significantly thicker than nanofibers. The electrospinning technique utilizes an electric force to drive the spinning process to produce polymer fibers from solutions or melts (Reneker and Chun, 1996; Huang et al., 2003). Unlike conventional spinning techniques (e.g., solution spinning and melt spinning), which are capable of producing fibers with diameters in the micrometer range (c. 5e50 mm), electrospinning is capable of producing fibers with diameters in the nanometer range (c. 10e1000 nm). Electrospun nanofibers possess many extraordinary properties (e.g., small fiber diameter and the concomitant large specific surface area); in addition, the membranes, felts, and mats made of electrospun nanofibers offer the unique capability to control pore size among the nanofibers. Unlike nanorods, nanotubes, and nanowires that are produced by synthetic, bottom-up methods, electrospun nanofibers are produced through a top-down nano-manufacturing process, which results in low-cost nanofibers that are also relatively easy to assemble and process into applications. Cellulose-based materials have been widely adopted in the biopharmaceutical processing industry as a base matrix for adsorbent beads and membranes (Lightfoot and Moscariello, 2004). Cellulose is a natural polymer of particular interest due to abundant availability, biodegradability, compatibility with biological systems, and, most importantly, very low nonspecific binding when used during purification of biopharmaceuticals from complex solutions. Unfortunately, the processing of cellulose is restricted by its limited solubility in common solvents and its inability to melt. The dissolution of cellulose requires the use of a special solvent such as a mixture of N-methylmorpholine-N-oxide and water. Conventionally, cellulose fibers are produced via wet spinning and involve derivatization of the polymer. In wet spinning, a mixture of cellulose (such as wood pulp harvested from tree farms) is fed to the spinneret, which is submerged in a chemical bath, and the fibers are collected as the solvent is removed and the polymer is precipitated and solidified. In this process, cellulose is derivatized into its xanthate form, and a sulfuric acid/zinc mixture is used as a coagulant to regenerate cellulose fibers. Alternatively, cellulose fibers can be produced from cellulose acetate. Unlike cellulose, cellulose acetate is soluble in many common solvents, such as acetone. After cellulose acetate fibers undergo hydrolysis and deacetylation, the regenerated cellulose fibers are produced. Our previous studies have shown that cellulose acetate can also be electrospun into nanofibers; moreover, cellulose acetate nanofibers can be readily converted into regenerated cellulose nanofibers simply upon immersion into a NaOH aqueous solution at room temperature, and the regenerated cellulose nanofibers can be further surface functionalized with a diethylaminoethyl (DEAE) anion-exchange ligand. While cellulose-based separation media have many advantages, they unfortunately suffer from being chemically unstable in strong acids and bases. This limits their use to operations that do not require harsh regeneration conditions, which are often needed in the biopharmaceutical industry to meet the strict cleaning regulations of the FDA. On the other hand, carbon-based adsorptive media are much more chemically robust than cellulose-based media and thus can be used when strong acids and bases are required for cleaning the separation media between uses. It is well known that carbon nanofibers can be made from their electrospun precursors (e.g., polyacrylonitrile nanofibers), and

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electrospun carbon nanofibers may possess even larger specific surface area and higher mechanical properties than electrospun regenerated cellulose nanofibers. However, carbon-based separation media are often susceptible to nonspecific binding of unwanted or unintended contaminants that would require further downstream processing operations for removal from the product (Lightfoot and Moscariello, 2004; van Reis and Zydney, 2001). In this chapter, the preparation, characterization, and bioseparation performance of electrospun cellulose and carbon nanofibers (in the form of membranes, felts, and mats) as an innovative type of adsorption medium or support with high capacity, high throughput, and high selectivity are reported.

16.2 FABRICATION AND BIOSEPARATION STUDIES OF ADSORPTIVE MEMBRANES AND FELTS MADE FROM ELECTROSPUN CELLULOSE ACETATE NANOFIBERS 16.2.1 MATERIALS AND METHODS 16.2.1.1 Materials Cellulose acetate with an average molecular weight of w30,000 g/mol (Cat. No. 180955), NaOH, NaCl, acetone, N,N-dimethylacetamide (DMAc), and N,N-dimethylformamide (DMF) were purchased from SigmaeAldrich Co. (Milwaukee, WI). 2-(Diethylamino)ethyl chloride hydrochloride (DAECH) with a purity of 98% was purchased from Alfa Aesar Co. (Ward Hill, MA). Regenerated cellulose microfiber felts (Contec Bemcot M3 wipes, Cat. No. 18-999-475), bleached absorbent cotton balls (Cat. No. 07-886), and Tris crystallized free base were purchased from Fisher Scientific Co. (Pittsburgh, PA). Commercially available regenerated cellulose adsorptive membranes (47-mm disks with pore size of 0.2 mm and thickness of 186 mm) were obtained from Sartorius (Edgewood, NY, Cat. No. 18407-047N). BSA (with molecular weight of w66 kDa and isoelectric point of 4.9) was purchased from Fisher Bioreagents (Fair Lawn, NJ, Cat. No. BP671-10) as a DNase-, RNase-, and protease-free lyophilized powder. All of the materials were used without further purification.

16.2.1.2 Preparation 16.2.1.2.1 Electrospinning A solution of 15 wt% cellulose acetate in acetone/DMAc (mass ratio of 2:1) was prepared at room temperature. The solution was then transferred to a 30-mL BD Luer-Lok tip plastic syringe having an 18-gauge 90-degree blunt-end stainless-steel needle. The electrospinning setup included a highvoltage power supply (Model No. ES30P), purchased from Gamma High Voltage Research, Inc. (Ormond Beach, FL), and a laboratory-produced roller with a diameter of 10 in. During electrospinning, a positive high voltage of 15 kV was applied to the needle, and a flow rate of 1.0 mL/h was maintained using a digitally controlled, extremely accurate, positive displacement syringe pump (Model No. KDS 200) purchased from KD Scientific, Inc. (Holliston, MA). Cellulose acetate nanofibers were collected as a randomly overlaid felt on the electrically grounded aluminum foil that covered the roller. The rotational speed of the roller during electrospinning was set at 100 rpm. A heating lamp was used to dry the nanofiber felt during electrospinning, and the felt was further dried in a vacuum oven at 80
C for 12 h after electrospinning. The collected cellulose acetate nanofiber felt had a thickness of w225 mm and a mass per unit area of w60 g/m2.

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16.2.1.2.2 Hydrolysis. Deacetylation, and Surface Functionalization The as-electrospun cellulose acetate nanofiber felts were first hydrolyzed and deacetylated by immersion in a 0.05 M NaOH aqueous solution for 24 h. The products (regenerated cellulose nanofiber felts) were then rinsed in distilled water three times and dried in a vacuum oven at 60
C. Subsequently, the samples were immersed in a 15 wt% DAECH aqueous solution for 10 min followed by drying at 60
C. These samples were then immersed in 0.5 M NaOH aqueous solution at 90
C for 10 min. Finally, all samples were rinsed in distilled water three times and then dried at 60
C to yield the DEAE anion-exchange cellulose nanofiber felts. Other adsorption media (including the regenerated cellulose microfiber felts, the bleached absorbent cotton balls, and the commercially available regenerated cellulose adsorptive membranes), which were used for comparison analysis to the cellulose nanofiber felts, were prepared in an identical fashion.

16.2.1.3 Characterization A Zeiss Supra 40VP field emission scanning electron microscope was employed to examine the morphologies of the prepared samples. Prior to scanning electron microscopy (SEM) examination, the specimens were sputter-coated with gold to avoid charge accumulation. Fourier transform infrared spectroscopy (FTIR) spectra of the electrospun nanofibers (including the as-electrospun cellulose acetate nanofibers, the regenerated cellulose nanofibers, and the DEAE surface-functionalized cellulose nanofibers) were obtained using a Bruker Tensor-27 FTIR spectrometer equipped with a liquid nitrogen-cooled mercuryecadmiumetelluride (MCT) detector. The FTIR specimens were prepared by pressing the ground nanofibers with anhydrous KBr. The FTIR spectra were acquired by scanning specimens 64 times in the wave number range from 550 to 4000 cm1 with a resolution of 4 cm1. Nitrogen content analysis was used as an indication of the degree of DEAE surface functionalization and was carried out at Galbraith Laboratories, Inc. (Knoxville, TN).

16.2.1.4 Evaluation 16.2.1.4.1 Batch (Static) Adsorption Batch-adsorption experiments were completed with all four varieties of the prepared DEAE anionexchange adsorption media, to determine Langmuir equilibrium adsorption isotherms. For batch analyses, single layers of the felts were separated; rinsed with 10 mM Tris, pH 8.0, buffer (Tris buffer); cut into approximately 1-cm2 individual pieces; and weighed. Ten individual pieces (w100 mg each piece) were then placed into 15-mL centrifuge tubes. A stock solution of BSA was prepared at 2.0 mg/ mL by mixing a known mass of lyophilized protein with Tris buffer. Appropriate combinations of stock solution and Tris buffer were added to each test tube containing the cut felt pieces to provide a final volume of 14 mL in each tube and an initial protein concentration between 0 and 2.0 mg/mL protein. A 1.0-mL liquid sample from each of the different initial protein concentrations was immediately taken and UV absorbance measured at 280 nm. The samples were then placed on an end-overend mixer rotating at w40 rpm. After mixing for a minimum of 24 h, liquid from each sample was removed and the protein concentration was determined by UV 280-nm absorbance with a Genesys 10 UV spectrophotometer purchased from Thermo Electron Corp. (Madison, WI). By the difference in absorbance, the protein adsorbed to the felt could be calculated. A tube was also prepared with 2.0 mg protein/mL and no felt to evaluate the potential of enzyme adsorbing to the tube surface; no adsorption to the tube surface was found. Likewise, controls were monitored to evaluate the potential of leached

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chemicals that could contribute to the UV 280-nm absorbance and nonspecific binding of BSA to nonderivatized membrane (at the regenerated cellulose stage). No leaching or nonspecific binding was observed for any of the samples. Langmuir adsorption isotherms were then prepared and modeling constants (Qmax and Kd) determined by least-squares regression fit to the following equation: Q¼

Qmax $C Kd þ C

where Q is the equilibrium concentration of BSA adsorbed to the membrane (mg/g), Qmax is the asymptotic maximum saturation capacity, C is the liquid-phase equilibrium concentration of BSA (mg/mL), and Kd is the desorption constant (mg/mL). Following adsorption analysis, the liquid was decanted away and the felts were washed with 14 mL of Tris buffer along with mixing for 1 h. After the wash solution was removed, Tris buffer with 1 M NaCl, pH 8.0, was added to the tubes and mixed for 1 h, and the liquid was sampled for BSA concentration by UV 280-nm absorbance. Elution percentage was calculated based on binding amounts found during the adsorption phase of the study.

16.2.1.4.2 Permeability The permeativity of Tris buffer through the DEAE-functionalized cellulose nanofiber felts and the DEAE-functionalized commercially available regenerated cellulose adsorptive membranes was ¨ KTApurifier (GE Healthcare) with online measurement of pressure. A small-scale measured with an A “coin” membrane adsorption holder from Pall Corp. (Pensacola, FL, Product No. MSTG18H16) was utilized for all experiments. The unit allows for 1.5 cm2 of effective filtration area and is sealed with an O ring to discourage leakage. Initially, the pressure drop of the system only, with the membrane holder in place but with no membrane present, was evaluated at flow rates ranging from 2.0 to 30.0 mL/min. Then layers of each nanofiber felt or commercial membrane were successively added to the unit while measuring the pressure drop at different flow rates. One, three, five, seven, and nine layers were evaluated for each felt or membrane. The system pressure drop was subtracted from the measured pressure drop with the felt or membrane in place to calculate the permeability of the felt or membrane at each flow rate. A minimum of five flow rates and corresponding pressure readings were made with each number of layers.

16.2.1.4.3 System Dispersion System dispersion analysis was performed for the DEAE cellulose nanofiber felts and the DEAE commercial regenerated cellulose adsorptive membranes to determine the degree of axial mixing with different numbers of layers in place. The same arrangement used for permeability analysis was used for system dispersion tests, except the flow rate was maintained at 1.0 mL/min during the entire process. After equilibrating the stack of felts or membranes with Tris buffer, a 1% (volume fraction) solution of acetone in Tris buffer was added to the system. Online absorbance at UV 280 nm was monitored and the resulting curve was analyzed to calculate the Peclet (Pe) number by least-squares fit of the following equation: #) ( " Cout 1 ðPeÞ1=2 ðV  V50 Þ ¼ 1 þ erf 2 Cin 2ðV$V50 Þ1=2

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where Cout is the effluent 280-nm absorbance, Cin is the inlet 280-nm absorbance, V is the volume of acetone solution added, and V50 is the volume when Cout/Cin ¼ 0.50. Larger values of Pe were used to indicate the desirable property of being a closer approximation to plug flow (less axial dispersion).

16.2.1.4.4 Dynamic Breakthrough The Pall Mustang coin holder was used according to the manufacturer’s recommendations for dynamic breakthrough experiments. Nine layers of either the DEAE nanofiber felts or the DEAE commercial ¨ KTApurifier with membranes were used in the analyses. All experiments were operated with an A online measurement of UV 280-nm absorbance, pH, and conductivity and controlled by Unicorn software version 5.01. Fractions were automatically collected by the system in 0.60-mL aliquots (w2 bed volumes). The equilibration and wash buffer (buffer A) was 10 mM TriseHCl, pH 8.0. A minimum of 10 bed volumes were used for equilibration. The elution stock buffer (buffer B) was 10 mM TriseHCl plus 1.0 M NaCl, pH 8.0. Step elution to 100% buffer B was used for each experiment. For all dynamic breakthrough tests the flow rate was maintained at a value of 1.0 mL/min. BSA stock prepared at 1.5 mg/mL in buffer A was loaded until 100% breakthrough was achieved. The felt was then washed with buffer A for a minimum of 10 bed volumes before desorption. All eluent (flow through during load, wash, and elution) was collected, weighed to determine volume, and analyzed for BSA concentration by UV 280-nm absorbance. Protein mass balance was then calculated based on volume loaded and all fractions collected during the process.

16.2.2 RESULTS AND DISCUSSION 16.2.2.1 Preparation and Characterization The SEM images in Fig. 16.2 show the representative morphologies of (Fig. 16.2A) as-electrospun cellulose acetate nanofiber felts, (Fig. 16.2B) regenerated cellulose nanofiber felts, and (Fig. 16.2C) DEAE surface-functionalized cellulose nanofiber felts. As shown in Fig. 16.2A, almost no beads or beaded nanofibers could be microscopically identified in the as-electrospun cellulose acetate nanofibers. The nanofibers, however, were not uniform; the diameters were in the range from tens of nanometers to micrometers. This may have been desirable, however, because the thicker fibers could provide greater mechanical support while the thinner fibers could result in larger specific surface area.

FIGURE 16.2 Scanning electron microscopy images showing the representative morphologies of (A) as-electrospun cellulose acetate nanofiber felts, (B) regenerated cellulose nanofiber felts, and (C) DEAE surface-functionalized cellulose nanofiber felts.

16.2 FABRICATION AND BIOSEPARATION STUDIES OF ADSORPTIVE

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After hydrolysis and deacetylation, the as-electrospun cellulose acetate nanofiber felts were converted into the regenerated cellulose nanofiber felts. As shown in Fig. 16.2B, it appeared that the hydrolysis and deacetylation reaction slightly affected the morphology of the nanofiber felts, and the regenerated nanofibers were slightly curved and fused together, particularly those with relatively small diameters. Furthermore, the hydrolysis and deacetylation appeared to result in the shrinkage of the felts; in other words, the nanofibers in the felts after the reaction seemed to be closer together. After DEAE surface functionalization (Fig. 16.2C), the morphology of the nanofiber felts was well retained; the morphologies of the nanofiber felts before and after the surface functionalization were almost indistinguishable. To better understand the chemical changes involved in the sample preparation, FTIR was employed to study the hydrolysis and deacetylation, and nitrogen content analysis was employed to study the degree of DEAE surface functionalization. As shown in Fig. 16.3, the characteristic ester band (nC¼O at w1700 cm1) completely disappeared after hydrolysis and deacetylation, indicating the reaction successfully converted cellulose acetate into regenerated cellulose. In addition, the hydroxyl band (nOeH at 3300e3500 cm1) was stronger after the reaction, indicating more hydroxyl groups were presented in the regenerated cellulose. The wave number of hydroxyl groups before the reaction (curve A) was higher than that after the reaction (curve B); presumably, this was because the hydroxyl groups in cellulose acetate were less hydrogen bonded than those in cellulose. After DEAE surface functionalization, the FTIR spectrum (curve C) showed no evident difference from that before the surface functionalization (curve B); this was because the attached DEAE group was not sensitive in the FTIR spectrum. To verify the existence and amount of DEAE groups in the functionalized cellulose

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FIGURE 16.3 Fourier transform infrared spectra of (A) as-electrospun cellulose acetate nanofiber felts, (B) regenerated cellulose nanofiber felts, and (C) DEAE surface-functionalized cellulose nanofiber felts.

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CHAPTER 16 ELECTROSPUN NANOFIBERS FOR PROTEIN ADSORPTION

Table 16.1 Nitrogen Contents in Various Samples Before and After the Diethylaminoethyl Surface Functionalization Sample Electrospun cellulose nanofiber felt Commercially available regenerated cellulose adsorptive membrane Regenerated cellulose microfiber felt Cotton ball

Before DEAE Surface Functionalization