Accepted Manuscript Title: Functionalized bacterial cellulose derivatives and nanocomposites Author: Weili Hu Shiyan Chen Jingxuan Yang Zhe Li Huaping Wang PII: DOI: Reference:
S0144-8617(13)01004-7 http://dx.doi.org/doi:10.1016/j.carbpol.2013.09.102 CARP 8191
To appear in: Received date: Revised date: Accepted date:
9-8-2013 23-9-2013 29-9-2013
Please cite this article as: Hu, W., Chen, S., Yang, J., Li, Z., & Wang, H., Functionalized bacterial cellulose derivatives and nanocomposites, Carbohydrate Polymers (2013), http://dx.doi.org/10.1016/j.carbpol.2013.09.102 This is a PDF file of an unedited manuscript that has been accepted for publication. As a service to our customers we are providing this early version of the manuscript. The manuscript will undergo copyediting, typesetting, and review of the resulting proof before it is published in its final form. Please note that during the production process errors may be discovered which could affect the content, and all legal disclaimers that apply to the journal pertain.
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Functionalized bacterial cellulose derivatives and nanocomposites
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Weili Hu, Shiyan Chen*, Jingxuan Yang, Zhe Li, Huaping Wang*
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State Key Laboratory for Modification of Chemical Fibers and Polymer Materials, Key
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Laboratory of High-performance Fibers and Products, Ministry of Education College
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of Materials Science and Engineering, Donghua University, Shanghai, 201620, China;
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Tel: +86-21-67792950; Fax: +86-21-67792726
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E-mail:
[email protected],
[email protected].
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ABSTRACT
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Bacterial cellulose (BC) is a fascinating and renewable natural nanomaterial
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characterized by favorable properties such as remarkable mechanical properties,
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porosity, water absorbency, moldability, biodegradability and excellent biological
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affinity. Intensive research and exploration in the past few decades on BC
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nanomaterials mainly focused on their biosynthetic process to achieve the low-cost preparation and application in medical, food, advanced acoustic diaphragms, and other fields. These investigations have led to the emergence of more diverse potential applications exploiting the functionality of BC nanomaterials. This review gives a
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summary of construction strategies including biosynthetic modification, chemical
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modification, and different in situ and ex-situ patterns of functionalization for the
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preparation of advanced BC-based functional nanomateirals. The major studies being
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directed toward elaborate designs of highly functionalized material systems for 1
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many-faceted prospective applications. Simple biosynthetic or chemical modification
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on BC surface can improve its compatibility with different matrix and expand its
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utilization in nano-related applications. Moreover, based on the construction strategies
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of functional nanomaterial system, different guest substrates including small molecules,
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inorganic nanoparticles or nanowires, and polymers can be incorporated onto the
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surfaces of BC nanofibers to prepare various functional nanocomposites with
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outstanding properties, or significantly improved physicochemical, catalytic,
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optoelectronic, as well as magnetic properties. We focus on the preparation methods,
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formation mechanisms, and unique performances of the different BC derivatives or
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BC-based nanocomposites. The special applications of the advanced BC-based
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functional nanomaterials, such as sensors, photocatalytic nanomaterials, optoelectronic
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devices,
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comprehensively reviewed.
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Key words: bacterial cellulose; modification; nanocomposites; functionalization
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Contents
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1.
Introduction.............................................................................................................4
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2.
Construction strategies of BC–based functional nanomaterials .............................7 2.1.
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Biosynthetic modification.............................................................................8
2.1.1.
Altered BC structure ..........................................................................8
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2.1.2.
Nanocomposites...............................................................................10
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2.2.
Chemical surface modification...................................................................12
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2.3.
In situ formation of nanostructures ............................................................15
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2.3.1.
In situ formation of nanostructures through reduction reaction.......16
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2.3.2.
In situ formation of nanostructures through precipitation reaction .17
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2.3.3.
In situ formation of nanostructures through sol-gel reaction...........19
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2.4.
Ex-situ introduction of components ...........................................................20
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2.5.
Other combined strategies ..........................................................................22
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3.
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Applications of BC-based functional nanomaterials ............................................23 3.1.
Sensors........................................................................................................24
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3.2.
Photocatalytic nanomaterials......................................................................26
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3.3.
Optoelectronics...........................................................................................28
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Electrically conductive films ...........................................................28
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3.3.2.
Optically transparent films...............................................................30
3.3.3.
Flexible displays ..............................................................................31
3.3.4.
Photoluminescent and photochromic films......................................32
3.4.
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Magnetically responsive films....................................................................34
Concluding remarks ..............................................................................................36
Acknowledgement ..................................................................................................38 References ..............................................................................................................38
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1. Introduction
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It is well known that cellulose is a very important and fascinating biopolymer and an
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almost inexhaustible and sustainable natural polymeric raw material, which is of
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special importance both in industries and in daily lives. In the past decade, the design
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and development of renewable resources and innovative products for science,
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medicine and technology have led to a global revival of interdisciplinary research and
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utilization of this abundant natural polymer. Formed by repeated connection of glucose
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building blocks, cellulose possesses abundant surface hydroxyl groups forming
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plentiful inter- and intra-molecular hydrogen bonds, characterized by its hydrophilicity,
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chirality, biodegradability, and broad chemical-modifying capacity(Klemm, Heublein,
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Fink & Bohn, 2005). The properties of cellulose largely depend on the specific
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assembling and supramolecular order controlled by the origin and treatment of
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cellulose(Eichhorn et al., 2010). Bacterial cellulose (BC) has the same molecular formula as plant cellulose, but with unique and sophisticated three-dimensional porous network structures. Intrinsically originated from the unique structure, BC demonstrates a serious of distinguished structural features and properties such as high purity, high
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degree of polymerization (up to 8000), high crystallinity (of 70 to 80%), high water
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content to 99%, and high mechanical stability, which is quite different from the natural
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cellulose(Barud, Regiani, Marques, Lustri, Messaddeq & Ribeiro, 2011). These
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specific parameters are determined by the biofabrication approach of BC (Fig. 1) and
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the controllable shape and supramolecular structure through the alteration of
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cultivation conditions during fermentation. These amazing physicochemical properties
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have attracted significant interest from both research scientists and industrialists. So far,
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BC has wide application in various fields such as medical, food, advanced acoustic
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diaphragms, and so on(Klemm et al., 2011). These research and exploration have led to
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the emergence of more diverse potential applications exploiting the functionality of
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BC nanomaterials.
In view of expanding the scope of BC applications, it is important to take full
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advantage of the unique structure and properties of BC nanomaterials to develop novel
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BC-based nanomaterials with ground-breaking new features. Various modification
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methods have been explored to open up possibilities for endowing BC with new
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functionalities(Klemm et al., 2011). Simple biosynthetic or chemical modification on
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BC surface can improve its compatibility with different matrixes and expand its
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utilization in nano-related applications. Another notable feature of BC is its high aspect
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ratio and abundant active functional hydroxyl groups, which makes it suitable for combination with different nanostructures by providing powerful interaction of BC with surrounding species, such as inorganic and polymeric nanoparticles and nanowires(Huang & Gu, 2011). This novel concept breaks new ground to make
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optimum use of the specific chemical properties of the guest substrates, in association
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with the unique features of renewable BC resources. As a promising template in the
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synthesis of a great variety of nanostructures with designed properties and
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functionalities, BC can play the role of reducing agent, structure-directing agent and 5
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stabilizer(Yang, Xie, Deng, Bian & Hong, 2012). The polymeric or inorganic
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ingredients can be incorporated and cooperatively interact with BC nanofibers to attain
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some functional products. Such new high-value materials are the subject of continuing
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research and are commercially interesting in terms of new products from the
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inexhaustible and sustainable natural polymeric raw material.
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In the last few years, growing worldwide activity can be observed regarding
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extensive scientific investigation and increasing efforts for the practical use of the BC
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materials. There is an increasing annual publication activity on BC (also known as
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microbial cellulose or bacterial nanocellulose) since 2000 as presented in Fig. 2. In
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recent years, the investigation and utilization of BC in functional materials have been
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the focus of research, and a growing number of works have been included in this field.
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Functional BC-based nanomaterials are especially an attractive topic because they
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enable the creation of materials with improved or new properties by mixing multiple
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constituents and exploiting synergistic effects, such as electronic, optical, magnetic,
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catalytic properties or bioactivity, etc. With a special property or several remarkable functions, functional BC-based nanomaterials are a type of high value-added materials possessing potential applications in specific fields. In this review, recent developments on BC-based advanced functional nanomaterials including modified BC nanomaterials,
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functional BC-based nanocomposites and their applications will be discussed and
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reviewed. A variety of surface functionalisation through biosynthetic or chemical
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modification will be considered, which can improve the functionality of BC
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nanomaterials and expand its potential application fields. Various approaches to the 6
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preparation of functional BC-based nanocomposites by incorporating different guest
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substrates including small molecules, inorganic nanoparticles or nanowires, and
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polymers on the surfaces of BC nanofibers are summarized, which mainly focus on the
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preparation methods, performances and some formation mechanisms of specific
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functional nanomaterials.
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2. Construction strategies of BC–based functional nanomaterials
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Although BC nanomaterial has unique physical and chemical characteristics, its
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high degree of crystallinity and sole functional group lead to its poor dissolubility and
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processability, thereby limiting the application fields. BC possesses an abundance of
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hydroxyl groups on the surface, where modification can be easily achieved. It can be
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modified to achieve alternative functional groups and patterns of functionalization
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properties combined with a controlled pre-set functionalization pattern is in the center
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of interest.
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using in situ and ex-situ modification methods as shown in Fig. 3. The properties of BC derivatives are primarily determined by the type of the functional groups. In particular, modified BC with more than one functional group possessing different surface characteristics such as lipophilic-hydrophlic properties, magnetic and optical
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2.1. Biosynthetic modification
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The shape and supramolecular structure of BC can be controlled by the change of
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cultivation conditions such as the type of strain, carbon source, and additives (Hessler
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& Klemm, 2009; Klemm et al., 2006). Studies have demonstrated the potential for
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manipulating the biogenesis of BC in order to produce modified BC nanofibers with a
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controlled composition, morphology, and properties. The inclusion of additives in the
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nutrient media components during biosynthesis can influence the assembly and
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microstructure of BC including the crystallinity, crystalline polymorphism, crystallite
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size and ribbon width. The presence of additives in the media may interfere with the
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bacterial cells or bind directly to the cellulose during production, thereby affecting the
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yield, structure, morphology and physical properties of BC. The in situ generation of
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composites can also be effectively regulated during biosynthesis by the inclusion of
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additives and dispersed particles.
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2.1.1. Altered BC structure
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As a remarkable benefit of BC, the ultrastructure and morphology of BC can be
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altered by introducing additives not specifically required for bacterial cell growth in
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the media. The effects of the water-soluble agents in culture media on the aggregation
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and crystallization of BC microfibrils were intensively studied. Adding various 8
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water-soluble chemical reagents can modify the microfibrillar features of the cellulosic
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ribbons. Adding nalidixic acid or chloramphenicol produced ribbons with an
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apparently larger width, probably because several ribbons from a cluster of cells whose
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dividing process was inhibited, combined or intertwined. While adding dithiothreitol
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produced ribbons with only 45% width of the control ribbons on average(Yamanaka &
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Sugiyama, 2000).
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The pore system and water content of BC can be controlled in situ by the
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incorporation of water soluble polymers such as carboxymethylcellulose (CMC),
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hydroxypropylmethyl cellulose (HPMC), methylcellulose (MC), poly(vinyl alcohol)
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(PVA) and polyethylene glycol (PEG) to the culture medium (Seifert, Hesse, Kabrelian
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& Klemm, 2004). In the presence of these additives, the pore system and elasticity of
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BC, as well as the water adsorption and water holding capacity can be controlled. It has
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been claimed the addition of HPMC, CMC and MC can cause decreased crystallinity
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and crystal size, as well as greater thermal stability and pore size (Chen, Chen, Huang
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& Lin, 2011). Along with the formation of porelike network structure, the water retention ability and the ion absorption capacity increase. The functionalized BC produced with CMC also showed good adsorption performances for copper and lead ions (Chen et al., 2009). Nevertheless the presence of PVA in the culture medium
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results in a reduced water absorption ability and a slightly higher copper ion capacity
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in comparison with original BC. The addition of β-cyclodextrin or PEG 400 causes a
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remarkable pore size increase (Heßler & Klemm, 2009). Surprisingly, these
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co-substrates act as removable auxiliaries not incorporated in the BC samples. 9
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Other polymers such as Tween 80, urea, fluorescent brightener and sodium alginate
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(NaAlg) have also been incorporated into the BC fermentation medium, with the
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observed differences in pore size, degree of polymerization, crystallinity, fibre widths
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and mechanical strength (Huang, Chen, Lin, Hsu & Chen, 2010; Ruka, Simon & Dean,
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2013; Zhou, Sun, Hu, Li & Yang, 2007). The results revealed that the addition of urea
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can increase the mechanical strength. The bundle widths of BC produced with
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fluorescent brightener increased and the cellulose network void grew. BC produced
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with NaAlg added has a lower crystallinity, a smaller crystalline size and an enhanced
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yield.
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2.1.2. Nanocomposites
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In particular, the additives can be in situ incorporated into the growing BC fibrils to
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create a novel type of nanocomposites, which represents a very specific and important modification method of BC. This in situ technique can combine the properties of altered supramolecular structure of BC with those of the incorporated components. So far, researchers have put different efforts to obtain BC nanocomposites by applying
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various additives such as organic compounds, polymers and inorganic substances
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(Klemm et al., 2011).
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The biological fermentation of BC in presence of cationic starch leads to the
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formation of double-network BC composites by incorporation of the starch derivative
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in the BC network (Heßler & Klemm, 2009). This double-stage structure consists of an
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opaque upper and a transparent under part. As shown in Fig. 4a, in case of the upper
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layer, the additive adsorbs at the cellulose fibers and causes an irregular distribution of
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the pore sizes. The under layer indicates the incorporation of the starch into the solvate
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shells of the BC prepolymer, forming an exciting skinny film structure (Fig. 4b). Other
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characteristic examples include the additives of poly(ethylene oxide)(Brown &
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Laborie, 2008), PVA (Gea, Bilotti, Reynolds, Soykeabkeaw & Peijs, 2010) and
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starch(Grande, Torres, Gomez, Troncoso, Canet-Ferrer & Martinez-Pastor, 2009) in
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the media for the formation of nanocomposites with the incorporation of these
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additives into the network of BC. Along with the increase of the additive content, the
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cellulose crystallized into smaller nanofibers, which further bonded together into
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bundles. The BC nanofibers were well dispersed in the composites and the
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nanocomposites typically show significantly improved mechanical properties.
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The inorganic additives can drastically modify the performance of BC.
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BC/multiwalled carbon nanotubes (MWCNTs) composites can be obtained in the presence of MWCNTs in an agitated culture (Park, Kim, Kwon, Hong & Jin, 2009; Yan, Chen, Wang, Wang & Jiang, 2008). Interestingly, a core-shell structure model was demonstrated, with the packed MWNTs attached nascent sub-elementary fibrils as the
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core of the cellulose assemblies as shown in Fig. 4c. While in the static culture method,
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band-like assemblies with sharp bends and rigidness were produced in the presence of
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MWCNTs. Similarly, the crystallinity index, crystallite size, and cellulose Iα content
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also changed, which may be attributed to the interaction between the hydroxyl groups 11
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of treated MWCNTs and the sub-elementary BC fibrils, interfering with the
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aggregation and crystallization of BC microfibrils. By adding silica or titanium
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precursor into the static growth medium, the SiO2 and TiO2 nanoparticles about several
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tens of nanometers in size can be incorporated onto BC microfibrils (Geng, Yang, Zhu,
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Cao, Jiang & Sun, 2011; Yano, Maeda, Nakajima, Hagiwara & Sawaguchi, 2008) (Fig.
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4d). It is inferred that the basic proteins in the outer membrane of bacterium cell act as
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the catalyst for the hydrolysis and condensation of inorganic precursor, the surface of
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both outer membrane and BC nanofibers renders the nucleation and growth sites for
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inorganic nanoparticles. The space-temporal effect endows bacteria the delicate
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control ability over formation of the nanocomposites.
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2.2. Chemical surface modification
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Although the biosynthetic modification of BC is a kind of green sustainable
technology, the strict microbial fermentation environment restricts the introduction of some additives. Other questions involving the interaction mechanism between the additives and microfibrils growth, as well as the structure control of BC nanofibers still
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need to be addressed. While chemically modified methods are not limited by the
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required types of additives. Compared to the biosynthesis method, its more clearly
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defined objectives and principles make it a feasible modified method for BC material.
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Since the chemical composition of BC is similar to plant fibers, it can also be
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carboxymethylated, acetylated, phosphorylated, and modified by other graft
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copolymerization and crosslinking reaction to obtain a series of BC derivatives. The
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introduction of new functional groups to the BC structure can endow BC with various
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features such as hydrophobicity, ions adsorption capacity and optical properties while
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maintaining the unique three-dimensional nano network and excellent mechanical
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properties of BC.
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In most cases, the modification of BC focuses on the improvement of its
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applicability and performance in different application fields. Several methods have
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been employed to achieve BC derivatives with improved metal ions absorption
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capacity. The functionalized diethylenetriamine-BC (EABC), amidoximated BC
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(Am-BC) and phosphorylated BC have been prepared as new adsorbents for metal ions
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(Chen, Shen, Yu, Hu & Wang, 2010; Oshima, Kondo, Ohto, Inoue & Baba, 2008; Shen
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et al., 2009). The experimental data showed that the microporous network structure of
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BC was maintained after the modification and these novel adsorbent showed good
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adsorption performances for different metal ions. To anchor metallic ions on BC nanofibers, the carboxylate groups have been introduced onto the BC nanofiber surface
using
the
2,2,6,6-tetramethylpiperidine-1-oxyradical(TEMPO)-mediated
oxidation system (Ifuku, Tsuji, Morimoto, Saimoto & Yano, 2009). The oxidation can
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proceed under mild aqueous conditions maintaining the crystallinity and crystal size of
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BC nanofibers.
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In order to improve the dispersability and compatibility in different solvents or
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matrices that are suitable in the production of nanocomposites, the acetylation of BC 13
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based on a non-swelling reaction mechanism was reported recently. BC could be
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partially acetylated by the fibrous acetylation method to modify its physical properties,
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while preserving the microfibrillar morphology(Kim, Nishiyama & Kuga, 2002). The
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hydrophobicity of the acetylated surface is advantageous for maintaining a large
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surface area on drying from water and would make the microfibrils compatible with
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other hydrophobic materials. While most of the chemically modified techniques
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require tedious solvent exchanges that strongly diminish the environmental benefit of
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the use of BC. A major thrust of recent BC modification research focused on the design
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of more economical and environmentally friendly method to obtain novel BC
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derivatives. Thus, a solvent-free derivatization system appears as an important goal to
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get BC derivative products with hydrophobic surfaces with a minimum environment
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impact. In line with the solvent-free BC derivatization concept, BC preserving the
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microfibrillar morphology has been partially acetylated using acetic anhydride in the
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presence of iodine as a catalyst (Hu, Chen, Xu & Wang, 2011). Another example is the
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reported vapor-phase technique which has been applied recently for the surface esterification of BC microfibrils with the help of gaseous trifluoroacetic acid mixed with acetylating agents (Berlioz, Molina-Boisseau, Nishiyama & Heux, 2009). Experimental results have shown the acetylation proceeded from the surface to the
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interior crystalline core of BC nanfibers. Hence, for moderate degree of substitution,
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the surface was fully grafted whereas the cellulose core remained unmodified and the
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original fibrous morphology was maintained. The obtained acetylated BC membrane
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shows more hydrophobic surface and good mechanical properties as shown in Fig. 5, 14
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which is in favor of enhancing the hydrophobic non-polar polymeric matrix.
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2.3. In situ formation of nanostructures
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BC has unique micro-nano porous three-dimensional network, which can facilitate
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the penetration of various metal ions into the interior. It also possesses a great deal of
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hydroxyl and ether bonds, forming the effective reactive sites to anchor metallic ions
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on the surface of the nanofibers. Then a variety of inorganic nanoparticles or
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nanowires can be formed through precipitation, oxidation-reduction and sol-gel
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reaction as shown in Fig. 6. Different from the doped nanoparticles into BC matrix, the
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size and morphology of the nanoparticles can be regulated by adjusting the structure of
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BC template and the in situ preparation conditions. At the same time, the nanospace in
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the BC fibers can behave as an effective nanoreactor to prevent the unwanted
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agglomeration phemnonmenon, ensuring the effective dispersion of the formed nanoparticles in the BC matrix. In the process of in situ preparation of BC-based nanocomposites, the second
components such as metal and polymer in the form of particles or fibers can be
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introduced into the BC matrix, while retaining the unique three-dimensional
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nanoporous network of BC. BC can be viewed as a soft template to control the
328
synthesis of desired nano-materials or nano-structure with specific size and shape. This
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can obtain a variety of novel functional materials with unique features and outstanding
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performances. Using BC as a template in the in situ synthesis of nano-materials has the
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following advantages: BC is a kind of environment-friendly and renewable material
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which can be produced from a wide range of raw materials. The shape, structure and
333
properties can be easily adjusted during the biosynthesis, pretreatment and chemical
334
modification process. Furthermore, the BC matrix can be removed by calcination to
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obtain the pure inorganic nanoparticles and nanowires structure.
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Therefore, BC with controllable structure and pore size can provide restrictive
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environment to ensure a variety of nanostructures effectively embedded in the matrix.
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The method does not require severe conditions, and is simple and easy to implement,
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which can obtain the nano-particles with narrow size distribution. So far, this method
340
has been applied to synthesize different nanomaterials such as metal, semiconductor
341
and electrically conducting polymers through different preparation methods.
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2.3.1. In situ formation of nanostructures through reduction reaction
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There have been some reports regarding the application of BC as a soft template to
in situ form different metal nanoparticles by the reduction reaction as shown in Table 1. The size and morphology of the formed nanoparticles can be controlled by using different reducing agents such as sodium borohydride (NaBH4), triethanolamine,
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hydrazine (NH2NH2), hydroxylamine (NH2OH), ascorbic acid, polyethylenimine (PEI)
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and so on (Barud et al., 2008; Yang, Xie, Hong, Cao & Yang, 2012; Zhang et al., 2010).
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These reducing agents can serve as an assistant material for stabilizing nanoparticles,
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preventing their aggregation. In addition, BC itself can be used as a reductant to
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produce metal nanoparticles (Yang, Xie, Deng, Bian & Hong, 2012), without
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introducing additional reducing agent or stabilizing agent, thus avoiding the secondary
353
pollutants and guaranteeing the feasibility of its applications in medical and catalytic
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field.
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2.3.2. In situ formation of nanostructures through precipitation reaction
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Well-separated nanofibrils of BC create an extensive surface area forming active
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sites for metal ion adsorption, and the subsequent introduction of precipitating agent
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will react with the immobilized metal ions to generate the initial nuclei of metal or
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oxide. Then the nuclei continue to grow, thereby further forming functional inorganic
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nanoparticles as shown in Fig. 7. The growth of the nanoparticles was readily
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controlled by repeated alternating dipping of BC membranes in the metal ion and
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precipitating agent solution followed by a rinse step. So it is feasible for BC to serve as
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an excellent matrix in the synthesis of nanoparticles through the in situ precipitation
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reaction.
CdS and CdSe nanoparticles have been synthesized and stabilized on BC nanofibers
using in situ precipitation method (Li et al., 2009a; Yang et al., 2012a). At first, hydroxyl and ether groups of BC anchored Cd2+ or thioglycolic acid capped Cd2+, then
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the anchored Cd2+ reacted with S2- or Se2- to generate CdS or CdSe nanoparticles on
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the BC nanofibers as shown in Fig. 8. The results indicated that nanoparticles with the
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diameter of 20-30 nm deposited on BC nanofibres are well-dispersed in the BC
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nanofibre-network. AgCl nanoparticles with a size of several tenths of nanometers
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have been in situ synthesized in the three-dimensional non-woven network of BC
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nanofibrils (Hu et al., 2009). The growth of the nanoparticles was readily obtained by
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repeated alternating dipping of BC membranes in the solution of silver nitrate or
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sodium chloride followed by a rinse step. The nanopore is essential for introduction of
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silver ions and reaction with Cl− to form AgCl particles into BC fibers and removal of
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the excess chemicals from BC fibers.
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BC can serve as an excellent matrix in the in situ synthesis of the Fe3O4
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nanoparticles through the coprecipitation reaction (Zhang et al., 2011). The ultrafine
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network architecture of BC gives a good tunnel for Fe2+/Fe3+ adsorption and ensures
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the effective anchoring of absorbed ions onto the BC nanofibers through ion–dipole
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interactions. After being rinsed with distilled water to remove the unanchored ions, the
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obtained ions/BC complexes were immersed into the excessive NaOH solution. The
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interaction between Fe2+/Fe3+ and OH− can induce the formation of Fe3O4
385
nanoparticles with the diameter of 80-100 nm as shown in Fig. 9.
387 388 389
M
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Ac ce p
386
an
378
During the in situ formation process of different nanostructures, the size and size
distribution of the formed nanoparticles are controllable by adjusting synthetic parameters such as the concentration of metal ions. When the concentration of metal ions is too high, the BC nanofibers fails to fully immoblize and disperse the large
390
number of metal ions due to the limited hydroxyl reactive sites shown in Fig. 10.
391
Meanwhile, the presence of excess metal ions would result in the agglomeration of the
392
nanoparticles due to the higher aggregation speed than the orientation speed in the
393
particle growth process. Therefore the concentration of the reaction solution needs to 18
Page 18 of 83
be reduced appropriately to ensure the metal ions and nanoparticles effectively
395
dispersed and fixed onto the surface of the nanofibers. However, when the
396
concentration is too low, the loading amount of the nanoparticles will be significantly
397
reduced, which would degrade the performance of the final products. The exploration
398
of optimal reaction conditions would be particularly important to achieve the effective
399
dispersion of nanoparticles with sufficient loading amount, thereby obtaining the
400
functional nanocomposite materials with excellent optical, electrical, magnetic and
401
antibacterial properties.
402
2.3.3. In situ formation of nanostructures through sol-gel reaction
M
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394
Taking advantage of the ultrafine nanofibrous structure, as well as abundant porous
404
channels inside the network, BC nanomaterials can be used as the templates to in situ
405
prepare metal oxides through the sol-gel reaction. The schematic diagram of the in situ
406
preparation process is described in Fig. 11. When immersing BC into the precursor
408 409 410
te
Ac ce p
407
d
403
solution, the inorganic ions can be immobilized onto the BC nanofibers. Then it can be solidified to form the gel through hydrolysis and condensation, and subsequently heated to form the desired oxides. BC template can stabilize and disperse the formed oxides nanoparticles through van der Waals forces and hydrogen bonding interaction,
411
prompting generated nanoparticles distributed along the surface of nanofibers. The BC
412
template can also be removed by calcination to obtain hollow nanofibers with a
413
three-dimensional network structure. TiO2/BC hybrid composites were prepared using BC as a hydrophilic substrate for
414
19
Page 19 of 83
synthesizing TiO2 nanoparticles via sol–gel (Gutierrez, Tercjak, Algar, Retegi &
416
Mondragon, 2012) as shown in Fig. 12. The spherical TiO2 nanoparticles with an
417
average size of 25 nm in diameter were located on the surface of the nanofibers due to
418
hydrogen bonding interactions. The BC template could also be removed in order to
419
change or optimize the properties of the in situ formed nanomaterial. The process also
420
can make use of the synergy between the BC template and inorganic materials to
421
synthesize a variety of nanoscale metal oxides with the hollow network. Mesoporous
422
titania networks consisting of interconnected anatase nanowires typically 20–30 nm in
423
diameter have been synthesized by using BC as natural biotemplates through the
424
sol-gel titania coating (Zhang & Qi, 2005). The titania networks exhibit enhanced
425
photocatalytic activity due to the larger accessible surface areas.
427
2.4. Ex-situ introduction of components
429 430 431
Ac ce p
428
te
426
d
M
an
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415
The functional BC-based nanocomposites can also be formed through the ex-situ
introduction of different components by the solution impregnation method. BC nanocomposites loaded with nano-scale silica nanoparticles have been prepared by
432
immersing BC hydrogel into different concentrations of silica sols, allowing nano scale
433
silica to be trapped and fixed in the spaces between the ribbon-shaped fibrils in the BC
434
hydro-gel matrix (Fig. 13) (Ashori, Sheykhnazari, Tabarsa, Shakeri & Golalipour,
435
2012; Maeda, Nakajima, Hagiwara, Sawaguchi & Yano, 2006; Yano, Maeda,
20
Page 20 of 83
436
Nakajima, Hagiwara & Sawaguchi, 2008). This method improved the modulus and
437
strength compared to the pure BC matrix, but it was difficult to load the BC with more
438
than 10% silica in this way. This strategy also can utilize the nanoeffect of the ribbon-shaped fibrils of BC to
440
prepare optically transparent materials. Transparent poly(L-lactic acid)(PLLA)/BC
441
nanocomposites was prepared by incorporating microbial nanofibrils into PLLA matrix
442
(Kim, Jung, Kim & Jin, 2009). The nanocomposite retained the transparency of the
443
pure PLLA film due to the nanofibrillar structure of the BC, which is smaller than the
444
wavelength of visible rays. An increase in the mechanical properties was observed by
445
incorporating BC as reinforcement into the PLLA matrix. The highly translucent
446
bionanocomposites have been prepared by simply incorporating BC into the pullulan
447
or poly (3-hydroxybutyrate) (PHB) matrix (Trovatti, Fernandes, Rubatat, Freire,
448
Silvestre & Neto, 2012; Zhijiang & Guang, 2011; Zhijiang, Guang & Kim, 2011) (Fig.
449
14a and b), which have improved the mechanical performance and thermal stability of
451 452 453
cr
us
an
M
d
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Ac ce p
450
ip t
439
the nanocomposite films. Novel nanocomposite films based on chitosan and BC were prepared by a fully green procedure by casting a water based suspension of chitosan and BC nanofibrils (Fernandes et al., 2009). They were highly transparent, flexible and displayed better mechanical properties than the corresponding unfilled chitosan films
454
(Fig. 14c). These new renewable nanocomposite materials also presented reasonable
455
thermal stability and low O2 permeability.
21
Page 21 of 83
456
2.5. Other combined strategies In most cases, we can adopt the combined strategies mentioned above to obtain the
458
nanocomposites with designed structures and required performance. The hydrophobic
459
BC-based nanocomposite sheets can be produced by the in situ synthesis of BC/CNTs
460
nanocomposite by biosynthetic modification and the subsequent heterogeneous
461
acetylated surface modification (Adnan & Radiah Awang Biak, 2012). The epoxidized
462
soybean oil (ESO)/BC nanofibers have been prepared through the acetylated
463
modification and the following solution impregnation in the ESO resin (Retegi et al.,
464
2012). An electromagnetic composite of BC has been synthesized by culturing
465
gluconacetobacter xylinus in a medium containing magnetite nanoclusters (MNP) and
466
the subsequently in situ formation of PANI on the magnetic BC fibers by chemical
467
oxidative polymerization with ammonium persulfate (APS) as the oxidant (Fig. 15)
468
(Park, Cheng, Choi, Kim & Hyun, 2013).
te
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cr
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457
Ac ce p
For BC to be suitable as a more effective nanoreactor to control the formation of
473 474
hydrolysis of zinc acetate in a polyol medium to form the ZnO/Am-BC
475
nanocomposites as shown in Fig. 16. The TEMPO-mediated oxidized BC nanofibers
476
has been applied as reaction templates to prepare fine silver nanoparticles with a
477
controlled size distribution (Ifuku, Tsuji, Morimoto, Saimoto & Yano, 2009). The
469 470 471 472
nanomaterials, the chemical functionalization should be employed to modify the BC fibers for imparting BC membranes with new functionalities and producing BC nanocomposites with tailored characteristics. The Am-BC as host matrix can stably anchor numerous zinc ions and then as nanoreactors to fabricate ZnO nanoparticles by
22
Page 22 of 83
478
amidoximation and oxidation of BC can provide more reactive sites to immobilize the
479
formed nanoparticles with high density, which are important factors for electronic,
480
catalytic, and biomedical applications. Thus, the functional nanostructures can be achieved by in situ formation of
482
nanoparticles or ex-situ introduction of components into the biosynthetically or
483
chemically modified BC matrix. At the same time, the fabrication steps can be adjusted
484
or even reversed to construct the BC-based functional nanomaterials with controllable
485
structures and designed functional performance.
an M
486
3. Applications of BC-based functional nanomaterials
d
487
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481
te
488
BC has been recently investigated as an attractive eco-friendly nanomaterial for the
490
preparation of high value-added nanocomposites. The functionalization of BC with
494
Ac ce p
489
495
nanomaterials like antibacterial, optical, electrical and magnetic properties, as well as
496
catalytic and biomedical activity. Several recent reviews giving a summary and
497
discussion regarding the applications of BC-based materials as medical devices or
498
reinforcement materials for polymer nanocomposite have been published (Eichhorn et
491 492 493
inorganic or polymeric materials opens up new pathways for the fabrication of novel multifunctional nanocomposites with promising performances. BC was used as a template for the preparation of novel functional nanocomposites that gather together excellent properties of BC with the ones displayed by typical inorganic or organic
23
Page 23 of 83
al., 2010; Klemm et al., 2011; Petersen & Gatenholm, 2011). Here we reviewed the
500
potential advantages and different applications of BC based functional nanomaterials,
501
which mainly focus on the application such as sensors, photocatalytic nanomaterials,
502
optoelectronic materials and devices, and magnetically responsive membranes.
ip t
499
3.1. Sensors
us
504
cr
503
an
505
BC as a kind of environmentally friendly and low-cost material has high surface
507
area and a large number of surface hydroxyl groups which make it an attractive
508
medium for applications such as sensors. BC can be considered as prime material or
509
substrate material for the construction of high-performance moisture or gas sensor due
510
to their large specific surface areas, a large number of surface hydroxyl groups, good
511
water absorbance and water holding capacity. With the combination of quartz crystal
513 514 515
d
te
Ac ce p
512
M
506
microbalance (QCM) system (Fig. 17), the BC nanofibrous membrane coating can be a good consideration for constructing highly sensitive humidity and gas sensors. The BC dispensed onto the surface of the QCM electrode was installed in the testing chamber. Through injecting water or gas into the chamber, the sensing properties of the sensors
516
were examined by measuring the resonance frequency shifts of QCM due to the
517
additional mass loading. A highly stable and sensitive humidity sensor based on BC
518
coated QCM has been fabricated (Hu, Chen, Zhou, Liu, Ding & Wang, 2011). The
519
results showed that the sensors possessed good sensing characteristics by increasing
24
Page 24 of 83
520
more than two orders of magnitude with increasing relative humidity (RH) from 5 to
521
97%, and the Log(Δf) showed good linearity (20–97% RH). The resultant sensors
522
exhibited a good reversible behavior and good long term stability. By introducing other detective components to the BC substrate, the modified
524
nanoporous BC membranes with the high specific surface area and high porosity
525
would be an ideal candidate for further improving the sensitivity of gas sensors. A
526
formaldehyde sensor based on nanofibrous PEI/BC membranes coated QCM has been
527
fabricated (Hu, Chen, Liu, Ding & Wang, 2011). PEI has been introduced as the
528
detective material to strengthen the interaction between the sensing materials and
529
formaldehyde molecules as shown in Fig. 18. The sensor showed high sensitivity with
530
good linearity and exhibited a good selectivity and repeatability towards formaldehyde
531
in the concentration range of 1-100 ppm at room temperature. It has a detection limit of
532
1 ppm which is enhanced compared to other nanostructured coatings because of the
533
large specific surface area of the sensing materials. This strategy also can be applied in
535 536 537
cr
us
an
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d
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Ac ce p
534
ip t
523
other system to introduce various detective materials combined with BC nanofibers to detect other gases.
BC with the ultrafine nanofibrous network, high stability, excellent biocompatibility
and extensive surface area, would be a very promising support for the construction of
538
biosensors. The gold nanoparticles–BC nanofibers (Au-BC) nanocomposite has been
539
synthesized and used for enzyme immobilization and glucose biosensor fabrication
540
(Wang et al., 2010; Wang et al., 2011; Zhang et al., 2010). The developed biosensor
541
based on horseradish peroxidase (HRP)/Au-BC nanocomposite exhibited excellent 25
Page 25 of 83
performance in terms of a wider linear range, lower limit of detection (lower than 1 μm)
543
and fast response toward hydrogen peroxide (Zhang et al., 2010) (Fig. 19), suggesting
544
the Au–BC nanocomposite a suitable matrix for enzyme immobilization. The
545
biosensor for glucose exhibits a detection limit as low as 2.3 mM with a linear range
546
from 10 mM to 400 mM, which could be used to monitor glucose in real blood samples
547
(Wang et al., 2010).
us
cr
ip t
542
548
3.2. Photocatalytic nanomaterials
an
549
M
550
BC can be viewed as a template with the main controlling sites made up of hydroxyl
552
and ether bonds to guide the synthesis of photocatalytic nanoparticles or
553
nanocomposites. Well-dispersed and regular ZnO nanoparticles with a narrow size
554
distribution of 20–50 nm and high photocatalytic activity have been successfully
556 557 558
te
Ac ce p
555
d
551
synthesized through decomposing BC infiltrated with zinc acetate aqueous solution at high temperature (Hu, Chen, Zhou & Wang, 2010). In addition, the photocatalytic ZnO/BC and ZnO/Am-BC nanocomposite membranes can be synthesized through a facile polyol method using BC or Am-BC as a template (Chen, Zhou, Hu, Zhang, Yin
559
& Wang, 2013). The resulting nanocomposites containing ZnO nanoparticles with a
560
uniform distribution and an average diameter of 50 nm show good mechanical
561
properties and high photocatalytic activity in the degradation of methyl orange.
562
Compared to the ZnO/BC nanocomposite films, the ZnO/Am-BC nanocomposites
26
Page 26 of 83
563
with higher loading content of ZnO nanoparticles displayed a higher decolorization
564
efficiency of 91% at 120 min, indicating an excellent candidate for photocatalyst. The photocatalytic mesoporous TiO2/BC hybrid nanofibers have been fabricated by
566
surface hydrolysis with molecular precision (Sun, Yang & Wang, 2010). The uniform
567
and well-defined nanofibers consist of large quantities of oriented TiO2 nanoparticle
568
arrays implanted on the BC nanofibers with diameters in the 4.3–8.5 nm range. The
569
TiO2/BC nanocomposites were used as photocatalyst for methyl orange degradation
570
under UV irradiation, and they showed higher efficiency than that of the commercial
571
photocatalyst due to the high surface area of 208.17 m2/g. Furthermore, the
572
photocatalytic ability can be improved by dopping with nitrogen. The BC template
573
also can be removed by calcinations to obtain the mesoporous TiO2 networks
574
consisting of interconnected anatase nanowires (Zhang & Qi, 2005). The mesoporous
575
titania networks exhibit a considerably enhanced photocatalytic activity compared
576
with the macroporous titania networks obtained through a similar sol-gel nanocasting
578 579 580
cr
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d
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Ac ce p
577
ip t
565
procedure.
The photocatalytic micrometer-long CdS/BC hybrid nanofibers have been prepared
via a simple hydrothermal reaction at relatively low temperature (Yang, Yu, Fan, Sun, Tang & Yang, 2011). Hexagonal phase CdS nanoparticles were homogeneously
581
deposited onto the hydrated BC nanofibers and stabilized via coordination effect. The
582
CdS/BC hybrid nanofibers demonstrated high-efficiency photocatalysis with 82%
583
methyl orange degradation after 90 min irradiation and good recyclability. The results
584
indicate that the CdS/BC hybrid nanofibers are promising candidate as robust visible 27
Page 27 of 83
585
light responsive photocatalysts.
586
3.3. Optoelectronics
ip t
587
cr
588
BC-based electroconductive composites can be prepared by combining conducting
590
electroactive materials with hydrophilic biocompatible BC. These conductive
591
materials including metal ions and oxides, carbon nanotubes and conducting polymers
592
in the form of nanoparticles or nanowires can be introduced into the BC matrix by in
593
situ synthesis, doping, blending or coating.
594
3.3.1. Electrically conductive films
d
M
an
us
589
The MWCNTs-incorporated BC was prepared by dipping a BC pellicle in an
596
aqueous MWCNT dispersion containing a cetyl trimethyl ammonium bromide (CTAB)
598 599 600 601
Ac ce p
597
te
595
surfactant (Yoon, Jin, Kook & Pyun, 2006). This composite process produced electrically conducting BC pellicles containing well-dispersed and embedded MWCNTs as shown in Fig. 20a. The electrical conductivity of the BC/MWCNT composite was found to be approximately 1.4 ×10-1 S/cm. The transparency of the MWCNTs-incorporated BC can also be improved by incorporating an aqueous silk
602
fibroin solution into BC membranes to obtain an electrically conductive transparent
603
paper (Fig. 20b) (Jung, Kim, Kim, Kwon, Lee & In, 2008). Good optimal transparency
604
and electrical properties were obtained with a transmittance of 70.3% at 550 nm and
605
electrical conductivity of 2.1×10-3 S/cm. This strategy can also be applied in the 28
Page 28 of 83
preparation of single-walled carbon nanotubes (SWCNTs) adsorbed BC membranes to
607
obtain the transparent conducting film with a transmittance of 77.1% and surface
608
resistance of 2.8 kΩ/sq (Kim, Kim, Bak, Yun, Cho & Jin, 2009). The resultant
609
electrically conductive transparent films showed remarkable flexibility without any
610
loss of their initial properties (Fig. 20c). Thus, the prepared electrically conductive
611
paper can be applied as a transparent electrode in photoelectronics such as flexible
612
displays and touch screens.
us
cr
ip t
606
Recently, the polyaniline (PANI)/BC and polypyrrole (PPy)/BC composites have
614
been intensively researched (Hu, Chen, Yang, Liu & Wang, 2011; Lee, Kim & Yang,
615
2012; Marins, Soares, Dahmouche, Ribeiro, Barud & Bonemer, 2011; Muller, Rambo,
616
Recouvreux, Porto & Barra, 2011; Wang, Bian, Zhou, Tang & Tang, 2013). Our group
617
has fabricated conductive PANI/BC nanocomposite membranes by oxidative
618
polymerization of aniline with ammonium persulfate as an oxidant and BC as a
619
template (Hu, Chen, Yang, Liu & Wang, 2011). The schematic diagram of the
621 622 623
M
d
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Ac ce p
620
an
613
formation of PANI/BC nanocomposites is shown in Fig. 21. It was found that the PANI nanoparticles deposited on the surface of BC connected to form a continuous nanosheath by taking along the BC template, and the conductivity could be enhanced when PANI is doped by protonic acids. The resulting PANI-coated BC nanofibrils
624
exhibit excellent electrical conductivity of 5.0×10-2 S/cm and good mechanical
625
properties with Young’s modulus of 5.6 GPa and tensile strength of 95.7 MPa. The
626
electrical conductivity of PANI/BC composites can be further improved to 3.8×10-1
627
S/cm by adopting the interfacial polymerization (Lee, Chung, Kwon, Kim & Tze, 29
Page 29 of 83
2012). Furthermore, by using FeCl3·6H2O as oxidant, the free-standing films of
629
PANI/BC composites with high electrical conductivity values (0.9 S cm-1) and PPy/BC
630
with outstanding electrical conductivities as high as 77 S cm-1 were prepared through
631
the in situ oxidative chemical polymerization of aniline or pyrrole on the surface of BC
632
nanofibers (Müller et al., 2012; Wang, Bian, Zhou, Tang & Tang, 2013). The
633
nanocomposites synergistically combine the electronic characteristics of PANI or PPy
634
with the outstanding mechanical characteristics of the BC matrix, which could be
635
applied in flexible electrodes, flexible displays and other electronic devices.
636
3.3.2. Optically transparent films
M
an
us
cr
ip t
628
BC has been applied as the flexible substrate for the fabrication of organic light
638
emitting diodes (OLEDs) by depositing indium tin oxide (ITO) thin films onto the
639
BC membrane at room temperature using radio frequency magnetron sputtering
640
(Legnani et al., 2008). However, the average transmittance of BC membrane substrates
642 643 644
te
Ac ce p
641
d
637
at 550 nm is only 40%. Thus many researchers have made efforts to improve the clarity of BC. BC comprising weblike networks of ribbon-shaped nanofibers has high tensile strength and low coefficient of thermal expansion (CTE) of less than 0.1 pp/K in the axial direction, which is a perfect candidate to reinforce transparent plastics (Yano et
645
al., 2005). The transparent BC/epoxy resin and BC/acrylic resin composites have been
646
fabricated at a fiber content as high as 70%, which showed a incredibly low CTE of 3
647
pp/K (similar to that of silicon crystal) and a high tensile strength (325 MPa), while
648
maintaining the flexibility and ductility of the resin plastics as shown in Fig. 22 (Yano
30
Page 30 of 83
et al., 2005). Furthermore, due to the nanofiber size effect, the high transparency was
650
demonstrated for several different resins with a wide distribution of refractive indexes,
651
from 1.492 to 1.636 at 20 °C (Shah & Brown, 2005). The optical transparency was also
652
insensitive to temperature up to 80 °C. The hygroscopicity of BC nanocomposites also
653
have been significantly reduced by using acetylated BC nanofibers to improve the
654
dimensional stability against temperature while maintaining optical transparency and
655
thermal stability (Ifuku, Nogi, Abe, Handa, Nakatsubo & Yano, 2007; Nogi, Abe,
656
Handa, Nakatsubo, Ifuku & Yano, 2006). These nanofiber-network-reinforced
657
optically transparent composites are expected to lead the way to a wider use of many
658
different optically transparent polymers in optoelectronic devices such as substrates for
659
flexible displays and components for precision optical devices, which demand high
660
transparency and low thermal expansion.
661
3.3.3. Flexible displays
663 664 665
cr
us
an
M
d
te
Ac ce p
662
ip t
649
Flexible displays have been produced by using BC as the substrate or the basic
optical film to improve their electronic properties. For the fabrication of OLED, the hygroscopicity and surface roughness of BC needs to be adjusted. The Si–O thin film can be deposited on BC surface via plasma enhanced chemical vapor deposition as
666
transparent barrier against water vapor (Ummartyotin, Juntaro, Sain & Manuspiya,
667
2011). The deposited Si–O layer offered uniform and exceptionally smooth surface
668
with the surface smoothness of 15 nm. The surface roughness of BC can be further
669
reduced to 5 nm by using the ferrofluid solution as nano-abrasion media (Ummartyotin,
31
Page 31 of 83
Juntaro, Sain & Manuspiya, 2012b). Nanocomposite film composed of BC (10–50
671
wt.%) and polyurethane (PU) based resin was fabricated and utilized as a substrate for
672
flexible OLED display (Ummartyotin, Juntaro, Sain & Manuspiya, 2012a). The visible
673
light transmittance of the nanocomposite film was as high as 80%. Its thermal stability
674
was stable up to 150 °C while its dimensional stability in terms of CTE was less than
675
20 ppm/K. After OLED was fabricated on the substrate through thermal evaporation
676
technique, the OLED performed highest current efficiency of 0.085 cd/A and power
677
efficiency of 0.021 lm/W at 200 cd/m2 while retaining its flexible feature as shown in
678
Fig. 23, suggesting that BC nanocomposite is a promising material for the
679
development of substrate for flexible OLED display.
680
3.3.4. Photoluminescent and photochromic films
d
M
an
us
cr
ip t
670
For BC to be suitable for more diverse optoelectronic applications, some of its
682
properties need to be modified such as endowing it the photoluminescent and
684 685 686
Ac ce p
683
te
681
photochromic properties. Manipulating the different methods to obtain the functional BC can be a useful approach for finetuning BC properties to satisfy appropriate applications or producing BC composites with tailored characteristics. To design and synthesize BC-based new materials is also more significant in exploring new
687
applications for the unique natural nanofibers. Flexible luminescent CdSe/BC and
688
CdS/BC membranes have been fabricated by the in situ synthesis of the CdSe and CdS
689
nanoparticles on the BC nanofibers (Li et al., 2009b; Yang et al., 2012b). The formed
690
nanoparticles with a size of 20-30 nm were homogeneously dispersed in the BC matrix.
32
Page 32 of 83
The nanocomposites exhibited good photoluminescence properties and excellent
692
mechanical properties as shown in Fig. 24. This work provides an effective method for
693
the construction of flexible BC membranes with photoluminescence properties, which
694
are promising for applications in optics, electronics and biology. The
photochromic
BC
nanofibrous
membranes
containing
cr
695
ip t
691
10,30,30-trimethyl-6-nitrospiro(2H-1-benzopyran-2,20-indoline)
697
successfully prepared by surface modification of BC nanofibers with spiropyran
698
photochromes (Hu, Liu, Chen & Wang, 2011) or in situ incorporation of sol-gel
699
synthesized photoswitchable
700
(Gutierrez, Fernandes, Mondragon & Tercjak, 2012). The modified BC-NO2SP
701
membranes exhibited a pink color and had an absorption peak at 568 nm in the
702
UV–visible spectrum and the color change was reversible due to the reversible
703
isomerization of the spiropyran moieties in BC-NO2SP as shown in Fig. 25.
704
Alternating illumination with UV-and-visible light switches the fluorescence of the
706 707 708
were
an
te
d
M
vanadium nanoparticles using BC as a template
Ac ce p
705
(NO2SP)
us
696
membranes on and off and makes the wetting properties of the membranes controllable, confirming that such modified BC membranes have potential application in sensitive displays and optical devices. The addition of vanadium nanoparticles led to the generation of photochromic nanopaper, which was able to reversibly switch the color
709
from green to dark purple by irradiation under ambient conditions. Moreover, the
710
flexible, conductive, and optically active photochromic hybrid nanopapers maintained
711
the semiconductive properties of the well-dispersed vanadium oxide nanoparticles.
712 33
Page 33 of 83
713
3.4. Magnetically responsive films
714
In recent years, the hybrid nanocomposites consisting of BC nanofibers and
716
magnetic nanoparticles have attracted much attention due to their potential novel
717
applications in biomedicine, security papermaking and packaging. The incorporation
718
of the magnetic nanoparticles with controllable collective magnetic properties can
719
endow BC with new functionalities. The magnetically responsive BC sheets
720
containing Fe3O4 nanoparticles were prepared by using an ammonia gas-enhancing in
721
situ co-precipitation method operated in a closed system without oxygen (Katepetch &
722
Rujiravanit, 2011). The homogeneous dispersion of the magnetic nanoparticles
723
throughout the BC matrix could enhance the percent incorporation of magnetic
724
nanoparticles into BC samples, leading to high and uniform magnetic properties. The
725
saturation magnetization of the magnetic nanoparticle-incorporated BC sheet ranged
726
from 1.92 to 26.20 emu/g with very low remnant magnetization (0.15–2.67 emu/g) and
730
Ac ce p
te
d
M
an
us
cr
ip t
715
731
contact angle (WCA) of 130° and oil contact angle (OCA) of 112°. Additionally, the
732
membranes exhibited the superparamagnetic behavior and can be magnetically
733
actuated. Magnetically responsive BC membrane with hydrophobic and lipophobic
734
surface would have potential applications in electronic actuators, magnetographic
727 728 729
coercive field (40–65 G) at the room temperature. The fluoroalkyl silane (FAS) modification on the magnetic Fe3O4/BC membrane can convert the surface wettability of BC membrane from amphiphilicity to amphiphobicity (Zhang et al., 2011) (Fig. 26). The fluorinated membrane with appropriate roughness showed the highest water
34
Page 34 of 83
735
printing, information storage, electromagnetic shielding coating and anti-counterfeit. The BC-based magnetic nanocomposite has been constructed that enables
737
tunability from highly porous, flexible, magnetically actuating aerogel to solid and stiff
738
magnetic nanopaper with high particle loading as shown in Fig. 27 (Olsson et al.,
739
2010). Magnetic ferromagnetic cobalt ferrite nanoparticles (diameter, 40–120 nm)
740
were precipitated on a BC hydrogel, and the porosity of the resulting dried
741
nanocomposite was controlled from 98% (upon removal of water during freeze-drying)
742
in the aerogel to 10% (upon compaction) in the nanopaper (Fig. 27a and b). The
743
lightweight porous magnetic aerogel can be actuated by a small household magnet as
744
shown in Fig. 27c. Moreover, it can absorb water and release it upon compression (Fig.
745
27d). Owing to their flexibility, high porosity and surface area, these aerogels are
746
expected to be useful in microfluidics devices and as electronic actuators.
te
d
M
an
us
cr
ip t
736
In addition, the magnetically responsive BC can also been provided by precipitation
748
of ferromagnetic Ni nanoparticles with the size of 10 to 60 nm by room temperature
749 750 751 752
Ac ce p
747
chemical reduction of Ni-chloride hexahydrate (Vitta, Drillon & Derory, 2010). The Ni loaded BC exhibits a complex magnetic behavior with characteristics of both superparamagnetism
and
ferromagnetism.
The
magnetization
data
shows
superparamagnetism with a blocking temperature of ~20 K. The hysteresis of
753
magnetization on the other hand, even at room temperature, clearly indicates that a
754
ferromagnetic phase always coexists together with the superparamagnetic phase. This
755
work clearly shows that the Ni loaded BC can be used both for nonbiological
756
applications such as magnetic printing and biological applications such as magnetic 35
Page 35 of 83
757
tissue scaffolds.
758
4. Concluding remarks
ip t
759
cr
760
Looking over the studies summarized in this article, the different biosynthetic or
762
chemical modification and incorporation of various guest substrates into BC matrix
763
bring up new opportunities for the development of novel nanostructured materials with
764
designed functionalities. By combining the unique physicochemical properties of BC
765
and optical, electronic and magnetic features of guest substrates, the significant
766
synergetic effects can be observed, aiming at the advance of a great variety of
767
advanced BC-based nanomaterials. The development and challenges of BC in
768
functional nanomaterial application in the future include several aspects as follows.
te
d
M
an
us
761
Ac ce p
First, it is very meaningful to conduct a systematic study of electrochemical
773 774
nanomaterials according to the application requirements, fulfilling the controllable
775
preparation of different nanostructures on the molecular level. It should be noted that
776
the analysis and investigation of these inherent properties can greatly support and
777
promote their application in the field of functional nanomaterials. Meanwhile, the more
769 770 771 772
and surface properties of BC, especially the investigation of the accurate accessible hydroxyl groups on the surface of BC nanofiber under different conditions. This is an interesting and significant topic deserving study, which can establish the reaction model system to design and simulate the in situ preparation of diversified functional
36
Page 36 of 83
economical and environmentally friendly controllable methods should be developed to
779
modify the surface of nanocellulose while preserving the nanostructural morphology
780
and excellent mechanical properties. The thermal stability of BC can also be improved
781
by choosing the appropriate modification methods, which can facilitate the
782
processability and application of BC on a higher level and broader scope, such as
783
high-temperature filter, catalytic carrier, and porous electrode materials.
us
cr
ip t
778
In addition, all the applications of BC in the field of functional nanomaterials are
785
based on their inherent physicochemical properties, such as remarkable mechanical
786
properties, porosity, water absorbency, moldability, biodegradability, excellent
787
biological affinity, and so on. Therefore, these unique properties should be utilized to
788
the fullest with contributions from multidisciplinary areas like physics, chemistry,
789
materials science and medicine to construct novel advanced functional BC-based
790
nanomaterial system which can greatly expand the application field of BC
791
nanomaterials. In particular, derived from the unique three-dimensional network and
793 794 795
M
d
te
Ac ce p
792
an
784
the interactions between formed nanoparticle and cellulosic nanofibers, BC or modified BC can be potentially useful in the application of some special nanomaterials, such as stimulus-responsive and maintenance-free self-healing materials. However, it should be pointed out that the current research on BC in the field of functional
796
nanomaterials is still largely confined to the laboratory. Increasing attention should be
797
devoted to produce BC in larger quantities at low cost, and to explore various
798
construction strategies that enhance the properties of BC, making it attractive for use in
799
a wide range of industrial sectors. New and emerging industrial processes need to be 37
Page 37 of 83
optimized to achieve more efficient operations and this will require active research
801
participations from the academic and industrial sectors. The next decade should
802
witness the industrial use of a wide spectrum of products derived from this effort.
ip t
800
803
Acknowledgement
cr
804
This work was financially supported by the National Natural Science Foundation of
806
China (51273043), Program of Introducing Talents of Discipline to Universities
807
(B07024) and Shanghai Leading Academic Discipline Project (B603)
an
us
805
M
808
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Communications(21), 2735-2737.
1050
Zhang, T. J., Wang, W., Zhang, D. Y., Zhang, X. X., Ma, Y. R., Zhou, Y. L., & Qi, L. M.
1051
(2010). Biotemplated Synthesis of Gold Nanoparticle-Bacteria Cellulose Nanofiber
1052
Nanocomposites and Their Application in Biosensing. Advanced Functional Materials,
1053
20(7), 1152-1160.
1054
Zhang, W., Chen, S. Y., Hu, W. L., Zhou, B. H., Yang, Z. H., Yin, N., & Wang, H. P.
1056 1057 1058
cr us
an
M
d
te
Ac ce p
1055
ip t
1041
(2011). Facile fabrication of flexible magnetic nanohybrid membrane with amphiphobic surface based on bacterial cellulose. Carbohydrate Polymers, 86(4), 1760-1767.
Zhijiang, C., & Guang, Y. (2011). Optical nanocomposites prepared by incorporating
1059
bacterial cellulose nanofibrils into poly(3-hydroxybutyrate). Materials Letters, 65(2),
1060
182-184.
1061
Zhijiang, C., Guang, Y., & Kim, J. (2011). Biocompatible nanocomposites prepared by
1062
impregnating bacterial cellulose nanofibrils into poly(3-hydroxybutyrate). Current 49
Page 49 of 83
Applied Physics, 11(2), 247-249.
1064
Zhou, L. L., Sun, D. P., Hu, L. Y., Li, Y. W., & Yang, J. Z. (2007). Effect of addition of
1065
sodium alginate on bacterial cellulose production by Acetobacter xylinum. Journal of
1066
Industrial Microbiology and Biotechnology, 34(7), 483-489.
ip t
1063
cr
1067
us
1068 1069
an
1070 1071
M
1072 1073
te
d
1074
1076 1077 1078 1079 1080
Ac ce p
1075
Figure captions
Fig.1. The illustration of biofabrication process of BC. Fig. 2. The illustration of the annual number of publications on BC since 2000 (SciFinder Scholar search system, search term “bacterial cellulose”).
1081
Fig. 3. Schematic illustration of the generalized synthetic routes to modified BC
1082
nanomaterials (Chen, Shen, Yu, Hu & Wang, 2010; Geng, Yang, Zhu, Cao, Jiang &
1083
Sun, 2011; Hessler & Klemm, 2009; Hu, Chen, Xu & Wang, 2011; Ifuku, Tsuji,
1084
Morimoto, Saimoto & Yano, 2009; Oshima, Kondo, Ohto, Inoue & Baba, 2008; Shen 50
Page 50 of 83
et al., 2009; Yamanaka & Sugiyama, 2000).
1086
Fig. 4. SEM images of freeze-dried upper part of BC/starch composite (a), under part
1087
of BC/starch composite (b), middle layer of BC/MWCNTs composites (c), and
1088
BC/SiO2 nanocomposites (d) (Geng, Yang, Zhu, Cao, Jiang & Sun, 2011; Heßler &
1089
Klemm, 2009; Park, Kim, Kwon, Hong & Jin, 2009).
1090
Fig. 5. (a) FE-SEM image of vacuum-dried acetylated BC, and the inset shows the
1091
profile of water droplets on the membrane surface. (b) Tensile stress–strain behaviors
1092
of air dried BC and acetylated BC (Hu, Chen, Xu & Wang, 2011).
1093
Fig.6. Schematic diagram of the in situ preparation of nanoparticles/BC
1094
nanocomposites.
1095
Fig.7. Schematic diagram of the in situ preparation of nanoparticles onto the BC
1096
nanofibers through the precipitation reaction (Hu et al., 2009; Li et al., 2009a; Zhang et
1097
al., 2011).
1098
Fig. 8. Schematic diagram of the formation of CdSe/BC nanocomposite (Yang et al.,
1100 1101 1102
cr
us
an
M
d
te
Ac ce p
1099
ip t
1085
2012a).
Fig. 9. Schematic illustration for the flexible magnetic nanohybrid membrane (Zhang et al., 2011).
Fig. 10. Schematic diagram of the effect of different concentrations of metallic ions on
1103
the size and size distribution of the in situ formed nanoparticles.
1104
Fig. 11. Schematic diagram of the in situ preparation of inorganic hollow nanofibers
1105
using BC as a template through the sol-gel reaction (Hu et al., 2009; Li et al., 2009a;
1106
Zhang et al., 2011). 51
Page 51 of 83
Fig. 12. (a) Illustration of the synthesis of TiO2/BC composites (Gutierrez, Tercjak,
1108
Algar, Retegi & Mondragon, 2012). (b) SEM images of BC–titania hybrid membranes,
1109
and (c) titania networks templated by BC membranes (Zhang & Qi, 2005). The arrow
1110
indicates the twisted structure of ribbon-like nanofibers.
1111
Fig. 13. Preparation process and AFM image of BC/silica composites by the solution
1112
impregnation method (Ashori, Sheykhnazari, Tabarsa, Shakeri & Golalipour, 2012;
1113
Yano, Maeda, Nakajima, Hagiwara & Sawaguchi, 2008).
1114
Fig. 14. FESEM images of PHB/BC nanocomposite: (a) surface morphology, and (b)
1115
cross-section morphology. (c) Image of chitosan and BC nanocomposite films placed
1116
in front of a green plant (Trovatti, Fernandes, Rubatat, Freire, Silvestre & Neto, 2012;
1117
Zhijiang & Guang, 2011; Zhijiang, Guang & Kim, 2011).
1118
Fig. 15. Schematic diagram of the preparation of electromagnetic BC nanocomposites
1119
(Park, Cheng, Choi, Kim & Hyun, 2013).
1120
Fig. 16. Schematic diagram of the formation of ZnO nanoparcticles onto nanofibers of
1122 1123 1124 1125
cr
us
an
M
d
te
Ac ce p
1121
ip t
1107
Am-BC membrane.
Fig. 17. Schematic of the experimental setup for humidity and gas detection (Hu, Chen, Zhou, Liu, Ding & Wang, 2011). Fig. 18. Schematic diagram of the interaction of BC and PEI, and formaldehyde and PEI (Hu, Chen, Liu, Ding & Wang, 2011).
1126
Fig. 19. (a) The SEM image of Au–BC nanocomposite. (b) Amperometric response
1127
curves of HRP/Au–BC/glassy carbon electrode for successive additions of different
1128
concentrations of H2O2 in stirring phosphate buffer (pH 7.4) containing 0.5 mM
1129
hydroquinone (HQ) at the applied potential of −150 mV. The inset shows the 52
Page 52 of 83
amperometric response at time range from 200 s to 350 s (Zhang et al., 2010).
1131
Fig. 20. (a) FESEM image of surface of the MWCNTs-incorporated BC pellicle (Yoon,
1132
Jin, Kook & Pyun, 2006). Optical photographs of (b) electrically conductive
1133
transparent paper, and (c) flexibility of the electrically conductive transparent paper
1134
(Jung, Kim, Kim, Kwon, Lee & In, 2008).
1135
Fig. 21. Schematic diagram of the formation of PANI/BC nanocomposites (Hu, Chen,
1136
Yang, Liu & Wang, 2011).
1137
Fig. 22. (a) Appearance of 65μm thick BC, BC/acrylic resin and BC/epoxy resin sheets.
1138
(b) Flexibility of a 65μm thick BC/acrylic resin sheet (Yano et al., 2005).
1139
Fig. 23. OLED on BC nanocomposite (Ummartyotin, Juntaro, Sain & Manuspiya,
1140
2012a).
1141
Fig. 24. (a) PL spectra of BC and CdS/BC nanocomposite with excitation wavelength
1142
at 370 nm (Li et al., 2009b). (b) Micrograph of the CdSe/BC nanocomposite
1143
membrane with excitation (Yang et al., 2012a). (c)Tensile stress–strain behaviors of
1145 1146 1147
cr
us
an
M
d
te
Ac ce p
1144
ip t
1130
pure BC and CdSe/BC nanocomposite membrane, inset is the photograph of the CdSe/BC nanocomposite membrane (Yang et al., 2012a). Fig. 25. (a) The UV-visible spectra of BC-NO2SP membrane (0.06 wt% SP) under different irradiation time at 365 nm UV light conditions: (a) before UV irradiation, (b)
1148
after UV 10 s, (c) after UV 30 s, (d) after UV 60 s, and (e) after UV 120 s. The inset
1149
shows a sketch of the immobilized spiropyran that undergoes conversions to
1150
merocyanine form and back under UV and visible irradiation, respectively. (b) BC
1151
membrane and the BC-NO2SP membranes (0.06 wt% SP) after being treated by UV 53
Page 53 of 83
irradiation for (b) 0 s, (c) 30 s and (d) 60 s ; e and f were the membrane recovered in
1153
the dark for 10 and 30 minutes after irradiated by UV light for 60 s (Hu, Liu, Chen &
1154
Wang, 2011).
1155
Fig. 26. The flexible magnetically responded Fe3O4/BC nanohybrid membrane with
1156
amphiphobic surface after FAS modification (Zhang et al., 2011).
1157
Fig. 27. (a) Representative SEM image of a 98% porous magnetic aerogel containing
1158
cobalt ferrite nanoparticles after freeze-drying. Insets: photograph and schematic of the
1159
aerogel. Right inset: nanoparticles surrounding the nanofibrils. Left insets: photograph
1160
and schematic of the aerogel. (b) SEM image of a stiff magnetic nanopaper obtained
1161
after drying and compression. Inset: higher magnification image. (c) A piece of
1162
magnetic aerogel is held using tweezers (left panel). Applying a small household
1163
magnet bends the magnetic aerogel upwards (right panel). (d) The magnetic aerogel
1164
bends downwards in response to the magnet and absorbs the water droplet below (left
1165
panel). The aerogel recovers its original shape upon removal of the magnet (right panel)
1167 1168 1169 1170
cr
us
an
M
d
te
Ac ce p
1166
ip t
1152
(Olsson et al., 2010) .
Tables
Table 1 The in situ preparation of metal nanoparticles using BC as the template through reduction reaction.
1171
54
Page 54 of 83
1171
Table 1 The in situ preparation of metal nanoparticles using BC as the template
1172
through reduction reaction.
1173
NH2NH2 NH2 OH
5–14
0.86-1.60
70–10 0
50–70 20–50
>100
40–60 80–100
22.4
4.9 4.9
>100
30–50
—
7.8
25.0
19.6 6.0
NaBH4
(Cai, Kimura, Wada & Kuga, 2009)
NaBH4
(Cai, Kimura, Wada & Kuga, 2009) (Yang et al., 2009)
Pt
(Pinto, Neves, Neto & Trindade, 2012) (Zhou, Wang, Yang, Tang, Sun & Tang, 2012)
Cu
Au
8.0
10-20
—
~8.5
21
M
(Ifuku, Tsuji, Morimoto, Saimoto & Yano, 2009) (Barud et al., 2008)
~9
65.4
7.0±3.2
d
—
NaBH4 KBH4
1174 1175
te
Ac ce p
HCHO
5.6±2.3
19.2
~3.9
14.4%
~36
—
Pd
Application
ip t
Content (w/w%) 1.78
70–100
Oxidized BC triethanol amine PEI
(Zhang et al., 2010)
Particle size (nm) 17.1±5.9
Artificial skin; wound dressings; food packaging; water treatment; anti-counterfeit ing; bio-sensing; catalysis; textiles
cr
Ascorbic acid
(Yang, Xie, Deng, Bian & Hong, 2012) (Yang, Xie, Hong, Cao & Yang, 2012) (Maria, +Gelatin Santos, +PVP Oliveira, +Gelatin Barud, +PVP Messaddeq & Ribeiro, +Gelatin 2009) +PVP
Metal NPs Ag
us
NaBH4
Ref.
an
Reducing agent BC
8.15 %
Biosensing; catalytic; electronic devices; drugs and protein release; smart clothing Catalytic
Antibacterial; catalytic Catalytic
20
55
Page 55 of 83
Functionalized bacterial cellulose derivatives and nanocomposites
1176
1177
Weili Hu, Shiyan Chen*, Jingxuan Yang, Zhe Li, Huaping Wang*
ip t
1175
1178
cr
The construction strategies of BC-based functional nanomateirals were
1179
summarized.
1181
The biosynthetic and chemical modification of BC can expand its application
1182
fields.
1183
Different components can be incorporated in‐situ or ex‐situ for functionalizing
1184
BC.
1185
We focus on the preparation method, mechanism and performance of functional
1186
BC.
1187
The sensor, photocatalytic, magnetic and optoelectronic applications were
1188
reviewed.
1190
an
M
d te
Ac ce p
1189
us
1180
56
Page 56 of 83
Fig.1.
Ac
ce pt
ed
M
an
us
cr
ip t
Fig.1. The illustration of biofabrication process of BC.
57
Page 57 of 83
Fig.2
Fig. 2. The illustration of the annual number of publications on BC since 2000 (SciFinder Scholar search system, search term “bacterial cellulose or microbial
Ac
ce pt
ed
M
an
us
cr
ip t
cellulose or bacterial nanocellulose”).
58
Page 58 of 83
Fig.3.
Fig. 3. Schematic illustration of the generalized synthetic routes to modified BC nanomaterials (Chen, Shen, Yu, Hu & Wang, 2010; Geng, Yang, Zhu, Cao, Jiang & Sun, 2011; Hessler & Klemm, 2009; Hu, Chen, Xu & Wang, 2011; Ifuku, Tsuji,
ip t
Morimoto, Saimoto & Yano, 2009; Oshima, Kondo, Ohto, Inoue & Baba, 2008; Shen
Ac
ce pt
ed
M
an
us
cr
et al., 2009; Yamanaka & Sugiyama, 2000).
59
Page 59 of 83
Fig.4.
Fig. 4.
SEM images of freeze-dried upper part of BC/starch composite (a), under
part of BC/starch composite (b), middle layer of BC/MWCNTs composites (c), and BC/SiO2 nanocomposites (d) (Geng, Yang, Zhu, Cao, Jiang & Sun, 2011; Heßler &
Ac
ce pt
ed
M
an
us
cr
ip t
Klemm, 2009; Park, Kim, Kwon, Hong & Jin, 2009).
60
Page 60 of 83
Fig.5.
Fig. 5. (a) FE-SEM image of vacuum-dried acetylated BC, and the inset shows the profile of water droplets on the membrane surface. (b) Tensile stress–strain behaviors
Ac
ce pt
ed
M
an
us
cr
ip t
of air dried BC and acetylated BC (Hu, Chen, Xu & Wang, 2011).
61
Page 61 of 83
Fig.6.
Fig.6. Schematic diagram of the in situ preparation of nanoparticles/BC
Ac
ce pt
ed
M
an
us
cr
ip t
nanocomposites.
62
Page 62 of 83
Fig.7.
Fig.7. Schematic diagram of the in situ preparation of nanoparticles onto the BC nanofibers through the precipitation reaction (Hu et al., 2009; Li et al., 2009a; Zhang
Ac
ce pt
ed
M
an
us
cr
ip t
et al., 2011).
63
Page 63 of 83
Fig.8.
Fig. 8. Schematic diagram of the formation of CdSe/BC nanocomposite (Yang et al.,
Ac
ce pt
ed
M
an
us
cr
ip t
2012a).
64
Page 64 of 83
Fig.9.
Fig. 9. Schematic illustration for the flexible magnetic nanohybrid membrane (Zhang
us
cr
ip t
et al., 2011).
Ac
ce pt
ed
M
an
.
65
Page 65 of 83
Fig.10.
Fig. 10. Schematic diagram of the effect of different concentrations of metallic ions
Ac
ce pt
ed
M
an
us
cr
ip t
on the size and size distribution of the in situ formed nanoparticles.
66
Page 66 of 83
Fig.11.
Fig. 11. Schematic diagram of the in situ preparation of inorganic hollow nanofibers using BC as a template through the sol-gel reaction (Hu et al., 2009; Li et al., 2009a;
Ac
ce pt
ed
M
an
us
cr
ip t
Zhang et al., 2011).
67
Page 67 of 83
Fig.12.
Fig. 12. (a) Illustration of the synthesis of TiO2/BC composites (Gutierrez, Tercjak, Algar, Retegi & Mondragon, 2012). (b) SEM images of BC–titania hybrid membranes, and (c) titania networks templated by BC membranes (Zhang & Qi, 2005). The arrow
Ac
ce pt
ed
M
an
us
cr
ip t
indicates the twisted structure of ribbon-like nanofibers.
68
Page 68 of 83
Fig.13.
Fig. 13. Preparation process and AFM image of BC/silica composites by the solution impregnation method (Ashori, Sheykhnazari, Tabarsa, Shakeri & Golalipour, 2012;
Ac
ce pt
ed
M
an
us
cr
ip t
Yano, Maeda, Nakajima, Hagiwara & Sawaguchi, 2008).
69
Page 69 of 83
Fig.14.
Fig. 14. FESEM images of PHB/BC nanocomposite: (a) surface morphology, and (b) cross-section morphology. (c) Image of chitosan and BC nanocomposite films placed in front of a green plant (Trovatti, Fernandes, Rubatat, Freire, Silvestre & Neto, 2012;
Ac
ce pt
ed
M
an
us
cr
ip t
Zhijiang & Guang, 2011; Zhijiang, Guang & Kim, 2011).
70
Page 70 of 83
Fig.15.
Fig. 15. Schematic diagram of the preparation of electromagnetic BC nanocomposites
Ac
ce pt
ed
M
an
us
cr
ip t
(Park, Cheng, Choi, Kim & Hyun, 2013).
71
Page 71 of 83
Fig.16.
Fig. 16. Schematic diagram of the formation of ZnO nanoparcticles onto nanofibers of
Ac
ce pt
ed
M
an
us
cr
ip t
Am-BC membrane.
72
Page 72 of 83
Fig.17.
Fig. 17. Schematic of the experimental setup for humidity and gas detection (Hu,
Ac
ce pt
ed
M
an
us
cr
ip t
Chen, Zhou, Liu, Ding & Wang, 2011).
73
Page 73 of 83
Fig.18.
Fig. 18. Schematic diagram of the interaction of BC and PEI, and formaldehyde and
Ac
ce pt
ed
M
an
us
cr
ip t
PEI (Hu, Chen, Liu, Ding & Wang, 2011).
74
Page 74 of 83
Fig.19.
Fig. 19. (a) The SEM image of Au–BC nanocomposite. (b) Amperometric response curves of HRP/Au–BC/glassy carbon electrode for successive additions of different concentrations of H2O2 in stirring phosphate buffer (pH 7.4) containing 0.5 mM
ip t
hydroquinone (HQ) at the applied potential of −150 mV. The inset shows the
Ac
ce pt
ed
M
an
us
cr
amperometric response at time range from 200 s to 350 s (Zhang et al., 2010).
75
Page 75 of 83
Fig.20.
Fig. 20. (a) FESEM image of surface of the MWCNTs-incorporated BC pellicle (Yoon, Jin, Kook & Pyun, 2006). Optical photographs of (b) electrically conductive transparent paper, and (c) flexibility of the electrically conductive transparent paper
Ac
ce pt
ed
M
an
us
cr
ip t
(Jung, Kim, Kim, Kwon, Lee & In, 2008).
76
Page 76 of 83
Fig.21.
Fig. 21. Schematic diagram of the formation of PANI/BC nanocomposites (Hu, Chen,
Ac
ce pt
ed
M
an
us
cr
ip t
Yang, Liu & Wang, 2011).
77
Page 77 of 83
Fig.22.
Fig. 22. (a) Appearance of 65μm thick BC, BC/acrylic resin and BC/epoxy resin
Ac
ce pt
ed
M
an
us
cr
ip t
sheets. (b) Flexibility of a 65μm thick BC/acrylic resin sheet (Yano et al., 2005).
78
Page 78 of 83
Fig.23.
Fig. 23. OLED on BC nanocomposite (Ummartyotin, Juntaro, Sain & Manuspiya,
Ac
ce pt
ed
M
an
us
cr
ip t
2012a).
79
Page 79 of 83
Fig.24.
Fig. 24. (a) PL spectra of BC and CdS/BC nanocomposite with excitation wavelength at 370 nm (Li et al., 2009b). (b) Micrograph of the CdSe/BC nanocomposite membrane with excitation (Yang et al., 2012a). (c)Tensile stress–strain behaviors of
Ac
ce pt
ed
M
an
us
cr
CdSe/BC nanocomposite membrane (Yang et al., 2012a).
ip t
pure BC and CdSe/BC nanocomposite membrane, inset is the photograph of the
80
Page 80 of 83
Fig.25.
Fig. 25. (a) The UV-visible spectra of BC-NO2SP membrane (0.06 wt% SP) under different irradiation time at 365 nm UV light conditions: (a) before UV irradiation, (b) after UV 10 s, (c) after UV 30 s, (d) after UV 60 s, and (e) after UV 120 s. The inset
ip t
shows a sketch of the immobilized spiropyran that undergoes conversions to merocyanine form and back under UV and visible irradiation, respectively. (b) BC
cr
membrane and the BC-NO2SP membranes (0.06 wt% SP) after being treated by UV
us
irradiation for (b) 0 s, (c) 30 s and (d) 60 s ; e and f were the membrane recovered in the dark for 10 and 30 minutes after irradiated by UV light for 60 s (Hu, Liu, Chen &
Ac
ce pt
ed
M
an
Wang, 2011).
81
Page 81 of 83
Fig.26.
Fig. 26. The flexible magnetically responded Fe3O4/BC nanohybrid membrane with
Ac
ce pt
ed
M
an
us
cr
ip t
amphiphobic surface after FAS modification (Zhang et al., 2011).
82
Page 82 of 83
Fig.27
Fig. 27. (a) Representative SEM image of a 98% porous magnetic aerogel containing cobalt ferrite nanoparticles after freeze-drying. Insets: photograph and schematic of the aerogel. Right inset: nanoparticles surrounding the nanofibrils. Left insets:
ip t
photograph and schematic of the aerogel. (b) SEM image of a stiff magnetic nanopaper obtained after drying and compression. Inset: higher magnification image.
cr
(c) A piece of magnetic aerogel is held using tweezers (left panel). Applying a small
us
household magnet bends the magnetic aerogel upwards (right panel). (d) The magnetic aerogel bends downwards in response to the magnet and absorbs the water
Ac
ce pt
ed
M
magnet (right panel) (Olsson et al., 2010) .
an
droplet below (left panel). The aerogel recovers its original shape upon removal of the
83
Page 83 of 83