Nanoparticles meet electrospinning: recent advances and future prospects.

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Nanoparticles meet electrospinning: recent advances and future prospects Chuan-Ling Zhang and Shu-Hong Yu* Nanofibres can be fabricated by various methods and perhaps electrospinning is the most facile route. In past years, electrospinning has been used as a synthesis technique and the fibres have been prepared from a variety of starting materials and show various properties. Recently, incorporating functional nanoparticles (NPs) with electrospun fibres has emerged as one of most exciting research topics in the field of electrospinning. When NPs are incorporated, on the one hand the NPs endow the electrospun fibres/mats novel or better performance, on the other hand the electrospun fibres/mats could preserve the NPs from corrosion and/or oxidation, especially for NPs with anisotropic structures. More importantly, electrospinning shows potential applications in self-assembly of nanoscale building blocks for generating new functions, and has some obvious advantages that are not available by other self-assembly methods, i.e., the obtained free-standing hybrid mats are usually flexible and with large area, which is favourable for their commercial applications. In this critical review, we will focus on the fabrication and applications of NPs–electrospun fibre composites and give an overview on this emerging field combining nanoparticles and electrospinning. Firstly, two main strategies for producing NPs–electrospun fibres will be discussed, i.e., one is preparing the NPs–electrospun fibres after electrospinning process that is usually combined with other post-processing methods, and the other is fabricating the composite nanofibres during the

Received 25th November 2013

electrospinning process. In particular, the NPs in the latter method will be classified and introduced to

DOI: 10.1039/c3cs60426h

show the assembling effect of electrospinning on NPs with different anisotropic structures. The subsequent

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section describes the applications of these NPs–electrospun fibre mats and nanocomposites, and finally a conclusion and perspectives of the future research in this emerging field is given.

1. Introduction Nanoscience and nanotechnology have been a highly innovative area in the recent decades and constitutes two major branches: one is the synthesis of different materials with various morphologies, and the other is the construction/assembly of these as-synthesized nanostructures.1–6 A huge variety of nanocomponents with different structures and dimensionalities (i.e., zero dimensional (0D), one dimensional (1D) and two dimensional (2D)) have been synthesized and their broad applications in physical, chemical and biological fields have been widely investigated.7 Construction of the synthesized nanostructures is necessary with the increasing requirements in energy, material and biomedical fields and the resulting assemblies generally show novel or better performance.8 Selfassembly technique has been demonstrated to be the most effective way, in which the constituent components are

Division of Nanomaterials and Chemistry, Hefei National Laboratory for Physical Sciences at Microscale, Collaborative Innovation Center of Suzhou Nano Science and Technology, University of Science and Technology of China, Hefei 230026, P. R. China. E-mail: [email protected]; Fax: +86 551 63603040

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spontaneously connected and integrated through direct interactions (e.g., van der Waals interaction, electrostatic interaction, molecular surface forces) or indirectly using template or external fields (e.g., carbon nanotubes (CNTs), magnetic, electronic, flow field, and liquid interfaces).9–11 1D nanomaterials have been intensively investigated due to their unique properties and fascinating applications in many areas. Various methods have been developed to fabricate and assemble 1D nanostructures in the form of wires, belts, rods, tubes and rings from many materials.7,12 Among the methods, the electrospinning technique has been rapidly developed in the last decade for the facile preparation of continuous fibres from submicron down to nanometer diameter. Although there are other methods for preparing 1D fibres with high aspect ratio, such as template synthesis, few methods could match electrospinning in terms of its flexibility, versatility and ease of fibre production. Electrospinning was invented at the beginning of the last century (1902), but it was not until the mid-1990s that investigators started to realize the huge capability of the procedure for/of fibre preparation. It is notable that the amount of publications in this field (Fig. 1) is continuously increasing at breakneck speed in the past few years, benefitting from the remarkable

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Fig. 1 Schematic representation of the number of publications per year on electrospinning over the last 14 years following the SciFinder Scholar.

low cost, flexibility, simplicity and potential applications of this technique. So far, electrospinning has been used to fabricate a huge range of nanofibres, such as polymers, metals, ceramics and composites. By simply electrospinning the polymers with no important performance, the obtained fibres/films have no other applications except for using as scaffolds. In order to give full play to the advantages of electrospinning and fabricate functional materials, many functional materials or their precursors have been electrospun. Among the various fibres prepared by electrospinning, NPs–electrospun fibres exhibit huge potential applications, as the composite fibrous mats show flexibility, are free-standing and have applications determined by the polymer and NPs. It is notable that when electrospinning NPs with anisotropic structures such as nanorods (NRs) and nanowires (NWs), the NPs would be aligned within the fibres to a certain extent to reduce Gibbs free energy.13,14 Thus the electrospinning technique is not only a fabrication method, but also could be used as a simple and effective self-assembly method. Because of the weak interactions of self-assembly, it is sensitive to environmental variation, and the assembled structure is usually solution

Chuan-Ling Zhang

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Chuan-Ling Zhang received her PhD in Inorganic Chemistry in 2013 from the University of Science and Technology of China (USTC) under the supervision of Prof. Shu-Hong Yu. Currently, she holds a post-doctoral scholarship at the Hefei National Laboratory for Physical Sciences at Microscale in Prof. Shu-Hong Yu’s group. Her research interests centre on combining and assembling nanoscale building blocks with fibres via electrospinning technique, and investigating the properties of the composites.

or template based. While for electrospinning, as the NPs are stabilized on the surfaces or within the electrospun fibres, it overcomes the disadvantages of the traditional self-assembly methods as described above. Moreover, electrospinning generally does not need any surface functionalization process with high output: it simply requires a suitable solvent in which the NPs can be uniformly dispersed and one kind of polymer that can be dissolved in the solvent. Many excellent review articles on the preparation, alignment and applications of electrospun fibres have been published. Most of them mainly summarized the exciting works emerging in a period of time, the preparation and applications of a specific kind of material, or discussed around particular applications.15–18 However, review articles focused on NPs–electrospun fibres, especially about directly fabricating NPs–electrospun fibres during electrospinning process, are as yet not fully realized. Most review articles just generally introduce this area, which, however, is of high importance and more articles are needed to further promote electrospinning-related research. Recently, the Popa group well summarized the important results and discussed the perspectives of electrospinning of organic or/and inorganic dispersions for the synthesis and characterization of multicompartment fibres.19 However, up to now, electrospinning of NPs with different anisotropic structures has been relatively neglected, though of great significance because electrospinning shows potential applications in self-assembly of anisotropic NPs for generating new functions. Besides, the applications of the produced various fibre composites are little mentioned in literature. In this critical review, we will focus on the fabrication and applications of NPs–electrospun fibre composites. First, two main strategies for producing NPs–electrospun fibres will be

Shu-Hong Yu received his PhD in Inorganic Chemistry in 1998 from the University of Science and Technology of China (USTC). From 1999 to 2001, he worked in Tokyo Institute of Technology as a Postdoctoral Research Fellow. From 2001 to 2002, he was as an Alexander von Humboldt Research Fellow in the Max Planck Institute of Colloids and Interfaces, Potsdam, Germany. He was appointed as a full Shu-Hong Yu professor in 2002 and the Cheung Kong Professorship in 2006 by the Ministry of Education in the Department of Chemistry, USTC. His research interests include bio-inspired synthesis and self-assembly of new nanostructured materials and nanocomposites, and their related properties. He serves as an editorial advisory board member of several journals. His recent awards include Chem. Soc. Rev. Emerging Investigator Award (2010) and Roy-Somiya Medal of the International Solvothermal and Hydrothermal Association (ISHA) (2010).


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discussed: one is preparing NPs–electrospun fibres after electrospinning that is usually combined with some other post-processing methods, and the other is fabricating the composite nanofibres during the electrospinning process. Specifically, the NPs in the latter method will be classified and introduced to show the assembly effect of electrospinning on NPs with anisotropic structures. Then, we will overview the applications of these hybrid fibres/mats, and finally give our conclusions and perspectives of this emerging research field on the fabrication and applications of NPs–electrospun fibres.

2. Fabrication of nanoparticles– electrospun fibres 2.1

Basic setup and principles

Electrospinning is a simple and versatile technique that utilizes high electrostatic forces for fibre production. Four major components are required for electrospinning: a direct current power supply, a metallic needle with blunt-tip, a syringe for containing the electrospun solution, and a grounded conductive collector (Fig. 2). Although the setup for electrospinning is very simple, there are several influencing factors during the fibres preparing process, such as polymer concentration, solution viscosity and flow rate, electric field intensity, the work distance (distance between collector and tip of the needle), air humidity, etc.15,17–22 Generally, increasing the polymer concentration or adding a pinch of surfactant to the electrospun solution could decrease the number of beads, which is a common problem in electrospun fibres.22 The reservoir containing the electrospun solution can be placed horizontal or vertical: the solution flow rate is controlled by a syringe pump in the former set-up, while mainly through the gravity of the solution itself for the latter mode to supply the liquid at the tip. When the electrospun solution is hosted in a syringe and is ejected at a controlled rate by a syringe pump, the solution will emerge from the spinneret connected to the syringe, and a droplet would form due to the confinement of

Fig. 2 Schematic representation of the laboratory setup for electrospinning. The inset is a sketch of the electrified Taylor cone.


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surface tension. When a high voltage is supplied to the spinneret, charges are built on the surfaces of the droplet, and a Taylor cone will be formed once the repulsion among the charges is sufficiently strong to overcome the surface tension, followed by a liquid jet directed towards the grounded collector. Before reaching the collector, the jet is stretched by electrostatic repulsion, and the solvent is evaporated rapidly at the same time. Finally, the jet will be solidified and fibres will be formed and deposited on the collector. In general, nearly all soluble polymers can be processed into fibres by electrospinning, provided that a large set of parameters that influence electrospinning are correctly adjusted. For example, the properties of the polymer itself (such as molecular weight, molecular-weight distribution, solubility, melting point, glass-transition temperature and solubility), the properties of the polymer solution (such as viscosity, concentration, surface tension, temperature), and the parameters of electrospinning process (substrate, electric field, feed rate and relative humidity) play a major role in fiber formation. Different electrospinning parameters can lead to fibers with very different shapes and dimensions.17,22 The viscosity of polymer solution is very important as it determines whether fibers can be electrospun or not, which is affected by the molecular weight and concentration of polymer, temperature and relative humidity of the surroundings. In principle, high molecular weight is in favor of increasing the viscosity of the solution. If the molecular weight of the polymer is too low, the viscosity of the polymer solution still be very low even with a large concentration, the surface tension will make the polymer break up into droplets before reaching the collector. Conversely, if excess polymer with high molecule weight is added, the viscosity of solution is so high that it is impossible to spin, and so unable to form fibres. All polymers therefore have an optimal molecular weight/concentration range in which they can be electrospun. Within this optimal range, higher molecular weight leads to increased fiber diameters. It is not possible to make a general recommendation for particular molecular weights/concentrations and the resulting viscosities and surface tensions, because the ideal values of these parameters vary considerably with the polymer–solvent system. Although electrospinning can be used to fabricate a huge range of polymer fibres, not all polymers can be electrospun into fibres easily, such as alginate. Because the gelation of alginate solution starts to occur at very low polymer concentrations (about 2 wt% polymer solution in deionized water), this leads to solution containing insufficient material to form fibre structures or just sprayed droplets, while at higher concentration, the solution becomes so viscous that it can not be injected. In this case, the way to solve the problem is to incorporate a fraction of copolymer, and to apply surfactant or/and cosolvents into the polymer solution.23,24 In some cases, for example, when used for catalysis and sensors, materials with high specific surface areas are needed. The solid electrospun nanofibres have high surface to volume areas, and this can be further enhanced by coaxial electrospinning (co-electrospinning), adding appropriate amounts of other materials

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that could be subsequently removed, or electrospinning into a cryogenic liquid. So far, electrospun fibres with hollow,22,25 multichannel,26–28 porous29 and thin-wall assembled structures30 have been successfully prepared, as shown in Fig. 3. In particular,

Fig. 3 (a) Schematic illustration of the hollow fibre fabrication system. The spinneret consists of two coaxial capillaries, through which the immiscible inner (mineral oil) and outer fluids (an ethanol solution containing poly(vinyl pyrrolidone) (PVP) and titanium tetraisopropoxide) were ejected into the individual capillaries. (b) TEM image of TiO2 hollow fibres, which were fabricated by calcining the as-spun fibres at 500 1C after the oil cores were extracted with octane. Reprinted with permission from ref. 22. Copyright 2004 Wiley. (c) Schematic illustration for the fabrication of a three-channel tube. The inner and outer fluids (red for mineral oil and blue for titanium tetraisopropoxide solution) were separately injected to the individual syringe needles. (d) SEM image of the three-channel tubes in which the channels were divided into three independent parallel parts. Reprinted with permission from ref. 26. Copyright 2007 American Chemical Society. (e) A modified electrospinning setup with liquid nitrogen as collector. (f) TEM images of the porous polymer fibres with insets at higher magnification. Reprinted with permission from ref. 29. Copyright 2006 American Chemical Society. (g) Schematic illustration of fabrication of as-spun and calcined SnO2 fibres prepared at a flow rate of 25 mL min1. (h) Magnified SEM image of fibrous thin-wall assembled SnO2 fibres. Reprinted with permission from ref. 30. Copyright 2012 Wiley.

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a variety of functional composite materials could be prepared by co-electrospinning technique, which is derived from electrospinning. Co-electrospinning is based on a spinneret consisting of two coaxial capillaries with different diameters. By co-electrospinning two different fluids with this spinneret, fibres with hollow or core–sheath structures could be fabricated. For example, the curing agent of poly(dimethylsiloxane) (PDMS) could be fixed within poly(vinyl pyrrolidone) (PVP) electrospun fibres, and the composite film immersed in a PDMS template. When the PDMS template is destroyed, the curing agent could outflow from the polymer fibres and crosslink the PDMS across the incision, thus the material could selfheal.31 Inspired by feathers of many birds, Jiang and co-workers fabricated multichannel fibres by a multifluidic compound-jet electrospinning technique for the first time (Fig. 3c and d).26 Furthermore, emulsion electrospinning technique has been developed because it can encapsulate functional materials within fibres to form porous or core–shell structures without the need of a complex spinneret.27 Besides the controllable morphology for the fibres, electrospun fibres could also be aligned by either rotating collectors or patterned electrodes (Fig. 4).32–37 The ability to control the arrangement of fibres is critical to achieve the desirable functions, particularly for mechanical enhancement and tissue engineering. By changing the grounded collector to two pieces of conductive (silicon or Au) strips separated by a void gap, the electrospun fibres could be aligned across the gap (Fig. 4a and b). It has been reported that arrayed crossbar junctions could be readily obtained by transferring uniaxially aligned nanofibres onto the same substrate in a layer-by-layer fashion. With this similar method, Xia’s group successfully stacked aligned nanofibres into multilayered films with controllable hierarchical structures (Fig. 4c and d).35 In addition to using an electric field, magnetic fibres could also be aligned by producing an external magnetic field at the collector region. In 2010, Yang et al. demonstrated that polymer nanofibres without magnetic NPs also could be well-ordered over a large area and thick fibrous mats could be obtained (Fig. 4e and f).36 Furthermore, if collected by a rotating drum, the fibres also could be aligned through use of a mechanical field (Fig. 4g and h).37 Therefore, electrospinning technique is not only a facile method for preparing ultralong fibres with specific structures, but also a convenient technique to assemble nanofibres at large-scale, while for other methods this is difficult. However, if just electrospinning the polymers with no important performance, the obtained fibres/films have no other applications except using as scaffolds. In order to give full play to the advantages of electrospinning and fabricate functional materials, many functional materials or their precursors have been electrospun. Among the various fibres prepared by electrospinning, the NPs–electrospun fibres exhibit a huge variety of potential applications, as the composite fibrous mats show flexibility, are freestanding and will show applications determined by the polymer and NPs. Until now, two major ways have been used to fabricate NPs–electrospun fibres.


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2.2.1 NPs formed on the surfaces of electrospun fibres 2.2.1.1 Surface treatment. Surface treatment of the electrospun fibres is the simplest way to prepare NPs–electrospun composite fibres by just immersing the surface-treated electrospun fibres/films into the colloidal solution, through which the as-prepared NPs could be adsorbed on the surfaces of fibres by electrostatic force, hydrogen bonding or the interactions among functional groups. Many kinds of NPs, such as metal, metal oxide, carbon, polymer, fluorescent NPs, and even cells, have been successfully combined with electrospun fibres and form composite fibres.38–41 For example, our group fabricated AuNPs–poly(vinyl alcohol) (PVA) water-stable nanofibrous mats, as shown in Fig. 5a, in which the surfaces of the fibres were pretreated with 3-mercaptopropyltrimethoxysilane (MPTES), and AuNPs were adsorbed on the surfaces via Au–S bonds (Fig. 5b).42 Kim and co-workers found a novel method for applying reduced graphene oxide (RGO) directly to prepare electronic textiles from yarns to fabrics.43 First, the electrospun Nylon-6 fibres were functionalized with bovine serum albumin (BSA) molecules via a simple dipping process to make the surface of fibres positive charged, and then a uniform GO coating was formed on the surfaces of the fibres via electrostatic self-assembly, finally conductive RGO/nanoyarns were obtained after a low-temperature

Fig. 4 (a) Schematic illustration of a pattern that used to align electrospin fibres. The collector was composed two gold electrodes separated by a void gap. (b) Dark-field optical micrographs of aligned PVP nanofibres collected between the two gold electrodes patterned on a quartz wafer. (c) Schematic illustration of the patterns composed of six electrodes deposited on quartz wafers. (d) Optical micrograph of a trilayer mesh of PVP fibres collected in the center area of the electrodes shown in (c). Alternating layers with their fibres rotated by B601 were collected by the electrode pairs (1–4, 2–5, 3–6) sequentially grounded for 5 s. Reprinted with permission from ref. 35. Copyright 2004 Wiley. (e) Schematic illustration of the setup using for the magnetic-field-assisted electrospinning (MFAES) to align polymer fibres. (f) SEM image of the aligned poly(lactic-co-glycolic) acid (PLGA) fibres fabricated by the MFAES. Reprinted with permission from ref. 36. Copyright 2010 Wiley. (g) Schematic diagram of modified collector to align electrospun fibres. The collector was an insulated roller fixed with parallel sticks. (h) Fibres collected by a roller with parallel sticks and a baffle. Reprinted with permission from ref. 36. Copyright 2007 Wiley.

2.2 Indirect fabrication of NPs–electrospun fibres after electrospinning process If the NPs are not well dispersed within the electrospun solution, or can not be fabricated on a large scale, NPs–electrospun fibres can be directly fabricated by handling the fibres with some other methods, such as surface treatment, hydrothermal, sputtering etching, or gas–solid reaction. Based on different posttreatment methods, NPs can be formed on the surfaces or within the fibres.


Fig. 5 (a) Schematic illustration of water-stable functional AuNPs-PVA/ GA electrospun fibres, which is also a universal route for preparing hybrid fibres by surface treatment. (b) High magnification SEM image of RGO/ Nylon-6 fibres. Numerous rumples were observed, indicating that RGO was well wrapped around the fibres. Reprinted with permission from ref. 42. Copyright 2012 American Chemical Society. (c) TEM image of AuNPs assembled on Nylon-6 fibres at pH 5. Reprinted with permission from ref. 43. Copyright 2013 Wiley. (d) SEM image of TiO2 hollow fibres whose inner and outer surfaces were derivatized with CH3– and NH2– terminated silanes, respectively, followed by incubation with Au colloids, and found that AuNPs were only adsorbed on to the outer surfaces. (e) SEM image of hollow fibres whose inner and outer surfaces were both treated with an NH2-terminated silane, and then immersed in Au colloids. In this case, the AuNPs were adsorbed on both surfaces. Reprinted with permission from ref. 44. Copyright 2005 Wiley.

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vapour reduction process using hydriodic acid as reducing agent (Fig. 5c). Interestingly, the RGO/nanoyarns show a high electrical conductivity (41000 S m1) even under real-world conditions, such as washing, low and high temperatures, and a large number of repetitive bending cycles. Besides solid fibres, the NPs also could be adsorbed on the inner or outer surfaces of hollow fibres, such as Au–TiO2 hybrid fibres (Fig. 5d and e).44 2.2.1.2 In situ reduction. Sometimes, the surfaces of fibres are stable and do not readily adsorb NPs. In such instances, NPs may attach on the surfaces by an in situ reduction method. Upon immersion of the electrospun mats into the precursor solution, then NPs can be formed on the surfaces by a reducing agent: the as-prepared electrospun fibres are first immersed into an aqueous solution containing the precursor metal ion complex to coordinate with the groups on the fibres and then the NPs are synthesized by reducing the complex by photoreduction or a reducing agent. For the case of preparing metal NPs (Au, Ag, etc.), a vacuum drying process at room temperature is often needed to prevent reoxidation. For example, Xia and co-workers demonstrated that Au–TiO2 composite nanofibres could be synthesized by UV photoreduction of HAuCl4 in the presence of TiO2 nanofibres and organic stabilizers. Depending on the type and concentrations of organic stabilizers, different shapes of Au NPs, such as nanospheres, fractal nanosheets or nanowires, could be synthesized on the surfaces of TiO2 nanofibres. Such a technique could also be applied in the formation of other metal nanostructures on TiO2 nanofibres, which might be used for chemical or biological sensing.45 This method is a compact way to fabricate NPs–polymer hybrid fibres, and many NPs have been synthesized on the electrospun fibres, such as Au, Ag, Pd, Pt, TiO2, WO3, SnO2.46–51 A requirement is the electrospun polymer fibres does not dissolve in the colloidal solution. 2.2.1.3 Hydrothermal-assisted process. Novel hierarchical heterostructures, in which the major 1D cores and branches consist of different materials, are more attractive in many nanoscale photonic and electron-optical device applications. The hydrothermal process is a well-known method for preparing nanomaterials,52–55 and when it is combined with electrospinning, many kinds of NPs with different morphologies (e.g., sphere, plate and rod) can be synthesized on the surfaces of electrospun fibres. This strategy for preparing NPs–electrospun fibres has several favorable merits including: (i) both the electrospinning and hydrothermal method have been proven to be a comparatively versatile, low cost, applicable and environmentally friendly technique; (ii) this method could be extended to the fabrication of various metal oxides (MOs) (e.g., TiO2, WO3, CuO, Co3O4, Fe2O3, Fe3O4, etc.) on electrospun nanofibres to form MOs–electrospun hierarchical heterostructures; (iii) the synergistic effects of nanoscale building blocks as well as the unique hierarchical heterostructures may contribute to the improved performances. With this strategy, ZnONWs–polystyrene (PS) hybrid nanofibres with ‘‘nanobrush’’ structure were fabricated by immersing the PS electrospun fibres into ZnO NPs solution to lead to the NPs adsorbing on the fibre surfaces, and then ZnO nanowires were grown from the seeds through a

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simple hydrothermal process. The fabricated composite fibres with special heterostructures showed high performance for strain sensing.56 Zhang and co-workers also controlled synthesized metal oxide–TiO2 hierarchical heterostructures with improved lithiumion battery performances by using this method.57 Other composite fibres, such as SnO–C, In2O3–C, hydroxyapatite–C, TiO2–WO3 and WO3–TiO2, also have been successfully fabricated.58–61 With this similar method, TiO2–Pt nanocomposite fibres can be prepared by a combination of microwave irradiation and electrospinning technique, and show improved electrical catalytic activity.62 In addition to the commonly used methods, ultrasonication and sputtering etching are also applied to fabricate NPs on the surfaces of electrospun fibres.39,40,63 2.2.2 NPs formed within electrospun fibres. Besides on the surfaces, NPs could also be synthesized within the fibres by first electrospinning a solution containing metallic or ceramic acetate precursor, and then applying other treatments such as gas–solid reaction, calcination or laser ablation. 2.2.2.1 Gas–solid reaction. Doped NPs within electrospun fibres through gas–solid reaction was first reported by Wang et al.,64 and a series of composite fibres have been fabricated. For this method, precursor nanofibres with metallic or ceramic acetate dissolved in polymer are electrospun, and then exposed to a special gas atmosphere, such as H2S, to form NPs. Especially, semiconductor NPs with diameters below 10 nm can be readily synthesized with this method. Wang and co-workers successfully fabricated PbS–PVP and CdS–PVP composite fibres by using lead acetate and cadmium acetate as precursors, respectively, and finally exposed the fibres to H2S gas.64 Similarly, Yang and co-workers fabricated AgCl–poly(acrylonitrile) (PAN) composite nanofibres by exposing AgNO3–PAN electrospun fibres to HCl atmosphere, by which AgCl NPs both on the surfaces and interior of the fibres can be synthesized.65 In addition to the metallic compounds, purely metallic nanofibres have also been fabricated with this method. For example, Greiner and co-workers fabricated Cu NWs by electrospinning copper nitrate within poly(vinyl butyral) (PVB) nanofibres, and then heating the fibres in an air atmosphere to 450 1C for 2 h to remove PVB; further heating in a hydrogen atmosphere at 300 1C for 1 h was then applied and resulted in copper fibres.66 Recently, the Cui group electrospun PVA nanofibres containing copper acetate onto a glass substrate, then the fibres were calcined at 500 1C in air for 2 h to remove PVA and transformed the composite nanofibres to CuO nanofibres. Finally, the CuO nanofibres were reduced into red Cu nanofibres after annealing in an H2 atmosphere at 300 1C for 1 h. The copper nanofibres showed great flexibility and stretchability, and glass substrates coated with Cu nanofibres showed high transmittance at low sheet resistance.67 2.2.2.2 Calcination. Calcination of the electrospun fibres containing metallic or metal/ceramic acetate precursors is another method to prepare NPs–electrospun composite materials, and the NPs are usually well dispersed within the electrospun fibres. Its basic principle is combining electrospinning technique with the traditional sol–gel method to prepare precursor fibres, and then calcining the as-prepared composite fibres to prepare NPs


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within the fibres. With this method, different kinds of metal, metal oxide and their carbon composite fibres have been prepared. The use of water-based polymer and metallic precursor for fabricating metallic fibres sometimes limits the measurements in terms of magnetic and electrical properties owing to low loading of metal precursors, while the metal/ceramic acetate precursors could overcome the above shortcomings and have been widely used. Joo and co-workers electrospun acetates of Cu, Ni, Fe and Co within PVA fibres, calcined the fibres at 400 1C in an Ar atmosphere, and the synthesized NPs were uniformly distributed within the fibres.68 Wang and co-workers fabricated Sb–C fibres via electrospinning SbCl3 with PAN. After a calcination process, SbNPs (B30 nm) were uniformly distributed within the fibres and the fibrous mat can be used as a stable and fast sodium-ion battery anode.69 Similarly, Li3N–C composite fibres were fabricated and show advanced hydrogen storage performance.70 If different precursors were added into the electrospun solution, sometimes composite fibres with two or three kinds of metal oxide could be synthesized, such as ZnO– SnO2,71 TiO2–V2O572 and Al2O3–ZnO.73 Interestingly, the V2O5 NPs were not uniformly distributed within the fibres with TiO2, but found on the surfaces of the fibres in rod form.72 Besides spherical structure, NPs with other special structures could also be fabricated. For example, Co3O4 hollow NPs were fabricated within carbon nanofibres (CNFs), and the formation of hollow NPs can be easily understood and explained by the well-known Kirkendall effect during the transformation from Co NPs into Co3O4 hollow NPs.74 The Marier group reported on Sn–C nanofibres fabricated by forming Sn NPs encapsulated in porous multichannel carbon microtubes via electrospinning and subsequent calcination (Fig. 6).75 In this way, an appropriate

Fig. 6 (a) Schematic diagram of the electrospinning process of a poly(methyl methacrylate) (PMMA) PAN–tin octoate mixture in dimethylformamide (DMF). The inset shows a magnified view of the solution in the needle. (b) SEM image of the as-synthesized core–shell nanofibres obtained via electrospinning. (c) Proposed synthetic scheme for Sn NPs encapsulated in porous multichannel carbon microtubes. (d) High-magnification crosssectional image of PMMA–PAN–tin octoate nanofibres after carbonization that reveals the multichannel tubular structure of the fibres. Reprinted with permission from ref. 75. Copyright 2009 American Chemical Society.


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cooperation between a high filler density of Sb NPs and sufficient space to buffer the volume change can be realized. In 2011, the same group reported single-crystalline carboncoated LiFePO4 nanowires synthesized by a similar method. The obtained nanowires not only differ from reported analogues in crystallinity, but they were also substantially thinner.76 2.3 Direct fabrication of NPs–electrospun fibres during electrospinning process The as-prepared NPs can be directly added into the polymer solution, by which the composite fibres could be synthesized in one-pot by electrospinning the above solution. This is the most straightforward strategy to fabricate composite electrospun fibres and has been widely used. Until now, various NPs have been successfully synthesized, and the NPs–polymer electrospun fibres could be easily prepared as long as the NPs are uniformly distributed within the polymer solution. By varying the amount of NPs added into the electrospun solution, the density of NPs within the polymer fibres can be easily controlled, and thus the properties of the composites could be tuned. Especially for anisotropic NPs, such as polymer chains, NRs and NWs, electrospinning exhibits its assembling effect with several advantages, i.e., the NPs can be readily assembled on a large scale and a freestanding film can be fabricated. With years of development of electrospinning, different kinds of NPs have been directly electrospun. Here, we divide these NPs into four main classifications, i.e., 0D NPs, 1D NPs, 2D NPs, and other organic or biomolecules. 2.3.1 0D NPs–electrospun fibres. Inorganic NPs have higher electronic density than the polymers, as a result of which the NPs could be encapsulated within the polymer fibres by electrospinning technique. Compared to the film composed of spherical NPs, the morphology of the electrospun fibres do not have a regular hexagonal structure, which is caused by the different time scale: the film formation is measured in hours, whereas the electrospun fibre is formed within milliseconds. Up to now, a variety of 0D nanomaterials, such as metal, metal oxide, quantum dots and polymer spheres, have been prepared and show advanced properties in various areas. When these materials are electrospun, the NPs are usually randomly dispersed within the polymer fibres and functionalize the fibres. Sometimes, the NPs can not be well dispersed in the electrospun solution, and cluster like structures form in the obtained composite electrospun fibres. In such cases, some surface treatments of the particles are needed to make the NPs uniformly distributed. 2.3.1.1 Metal and metal oxide NPs. Zero-valent NPs have great potential applications in material engineering. NPs with different diameters have been successfully electrospun within polymers, such as water-soluble polymers (poly(ethylene oxide) (PEO), PVA, PVP) and non-water-soluble polymers (PAN, poly(L-lactide) (PLLA), etc.). This is the simplest method for preparing composite fibres: just adding the NPs into the electrospun solution with appropriate polymer concentration, stirring it to make the NPs uniformly distributed, and finally electrospinning the above solution. If both NPs and polymer dissolve in the solvent, the NPs are generally randomly distributed within the

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electrospun fibres. Sometimes we need to combine the performances of both NPs and functional polymers together even if the two components can not be dissolved in the same solvent. In that case, two strategies can be used, the first one is dissolving the NPs and polymers separately in different solvents, then mixing the two solutions, the premise of this method is the solvents are miscible; the other strategy is modifying the surfaces of the NPs using surfactants, such as PVP and thionaphthol, to make the NPs evenly dispersed. Gold NPs have special plasmonic properties and have been widely electrospun.77,78 After electrospinning, the AuNPs–polymer composite fibres have better conductivity than the pure polymer fibres, and could be used as scaffolds for tissue engineering. Based on the plasmonic property of AuNPs, the electrospun fibres can also be used for SERS and photothermal applications. For the latter application, the fibres are not only used for cancer therapy but also used as localized sources for surface plasmon resonancemediated heating, which could lead to the melting of polymers or altering their nanostructures.79 Recently, a novel flexible nonvolatile flash transistor memory device based on poly(ethylene naphthalate) (PEN) substrate using 1D electrospun nanofibres of poly(3-hexylthiophene) (P3HT):AuNPs hybrid as the channel was presented. In this device, AuNPs were employed as localized charge traps across the nanofibre channel and could program/ erase the device towards low conductance (OFF)/high conductance (ON) states under an applied electrical field.80 The electrospinning process was also successfully used to fabricate polymer nanofibres containing one-dimensional arrays of AuNPs. The intrinsic nature of semicrystalline PEO was used as a template to arrange the AuNPs within the fibres during electrospinning. The TEM image revealed that AuNPs form quite long and one dimensionally arranged chainlike arrays within the electrospun fibres.81 Silver NPs are another important metallic material for their plasmonic and antimicrobial properties. Different diameters of AgNPs have been widely electrospun. Interestingly, AgNPs tend to aggregate and form chain-like structures after vigorous stirring of AgNPs–PVA solution in a shaker at 40 1C for 5 h, as shown in Fig. 7. The number of NPs in the chain is determined by the concentration of AgNPs. After electrospinning, the NP chains were confined and aligned within PVA fibres. The obtained free-standing electrospun mat could be used as a SERS substrate with high sensitivity and reproducibility.82 Some other zero-valent NPs, such as nanodiamond, Si, Zn, Co, Ti and Cu, have also been successfully electrospun.83–85 Especially for electrospinning CuNPs, the NPs were fixed within PVA fibres, and form cable-like structures, which may be related to the polarization of the NPs. Different kinds of ceramic NPs, such as SiO2, TiO2, MgO, ZnO and ZrO2, have been electrospun. The NPs with dimensions ranging from nanometers to micrometers have been electrospun into hydrophobic or hydrophilic polymers. For example, when SiO2 NPs were electrospun, the obtained film was usually superhydrophobic with mesoporous structure. The state of the NPs in the polymer fibres is diameter-related: when the diameter of NPs is far below the fibres, the NPs could be randomly dispersed

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Fig. 7 (a) Photograph of AgNPs/PVA fibrous mat by electrospinning for 1 h, the molar ratio of PVA/Ag is 530 : 3. (b) Schematic illustration of formation of Ag NP aggregates with chain-like structures within PVA fibres. (c–f) TEM images of AgNPs/PVA fibres with different PVA/Ag molar ratios of 530 : 1, 530 : 2, 530 : 3 and 530 : 4. Reprinted with permission from ref. 82. Copyright 2009 American Chemical Society.

within the fibres (Fig. 8A),86 while when over 140 nm, the NPs tend to aggregate,87 and are aligned within the polymer fibres with necklace-like structure once increasing to over 260 nm (Fig. 8B).88 If Rhodamine B is electrospun with SiO2 NPs at the same time, the SiO2–PMMA composite fibres could be used for white light emission.89 Cai and co-workers found that the position of SiO2 NPs within the polymers is also related to the mixed solvent used for dissolving the polymers. When decreasing the vapor pressure of tetrahydrofuran/dimethylformamide (THF/DMF), the SiO2 NPs transferred from the middle of the

Fig. 8 (A) SEM image of Ag2S–PVP composite fibres fabricated by electrospinning PVP solution containing Ag2S colloid. Reprinted with permission from ref. 86. Copyright 2009 Wiley. (B) PVA/SiO2 (910 nm) electrospun fibres with necklace-like structure. PVA : SiO2 = 300 : 700, PVA 12 wt%, voltage 15 kV. Reprinted with permission from ref. 88. Copyright 2010 American Chemical Society. (C) HRTEM image of MRF–PET composite fibres fabricated by co-electrospinning. The mass ratio of solution was 1.5 : 1, the flow rate of both core and sheath was 5 mL min1. Reprinted with permission from ref. 98. Copyright 2012 Elsevier. (D) HRTEM image of a self-assembled array of FePt NPs encapsulated within PCL nanofibres by co-electrospinning. Reprinted with permission from ref. 99. Copyright 2005 Elsevier.


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fibres to the surfaces slowly.90 Also for the case of TiO2 NPs, TiO2–polymer electrospun fibres showed better photocatalytic performance than TiO2 NPs, the authors explained that the photocatalytic superiority of TiO2 nanofibres is attributed to effects of mesoporosity and nanoparticle alignment, which could cause efficient charge separation through interparticle charge transfer along the nanofibre framework.91 Another important species in metal oxide NPs is magnetic NPs. Magnetic composite nanofibres, in which magnetic NPs are embedded, are expected to exhibit interesting magnetic fielddependent mechanical behavior with potential applications in a range of areas. For example, the composite nanofibres could be used as ‘‘intelligent’’ fibres or fabrics for defensive clothing for military use, and in health care. Due to their high surface-tovolume ratio, magnetic NPs tend to agglomerate with each other to reduce energy. To overcome this problem, the use of stabilizer, electrostatic surfactant, and steric polymers have been proposed.92–95 Rutledge and co-workers first prepared superparamagnetic nanofibres containing stably-dispersed superparamagnetic NPs via electrospinning. The NPs were synthesized in the presence of a polymer that attached to the particle surfaces and conferred steric stabilization to the nanoparticle dispersion in the polymer solution. The NPs were observed to line up within the fibres in columns parallel to the fibre axis direction, apparently induced by the electrospinning process.96 Ferrofluids are a colloidal suspension of magnetic particles, and magnetic NPs in ferrofluids are coated with an organic material, thus they are free from agglomeration to some extent. It is well known that fibres with core–shell or hollow structures could be fabricated by coaxial electrospinning. Similarly, by adding mineral oil into a ferrofluid and coaxial electrospinning the composite solution, magnetic NPs could be uniformly aligned on the inner wall of hollow fibres, and the obtained mats/fibres possess magnetic properties.97 For example, Fe3O4–poly(ethylene terephthalate) (PET) magnetic composite nanofibres have been fabricated by coaxial electrospinning. Interestingly, the NPs could align in the middle of the fibres and form cable-like structures (Fig. 8C).98 The magnetic properties as well as mechanical properties with applied magnetic field were distinctly increased due to the dipole–dipole interaction between magnetic NPs in the magneto-rheological fluid (MRF). Self-assembled FePt NPs have also been encapsulated within poly(e-caprolactone) (PCL) nanofibres by coaxial electrospinning, assisted by electrostatic interactions, the encapsulated array of the discrete FePt NPs can reach as long as 3 mm along the fibre axis (Fig. 8D).99 Other magnetic composite fibres with core–sheath structure, such as polyarylene ether nitriles (PEN)–Fe-phthalocyanine–Fe3O4, Fe3O4–PEO and Fe3O4–PET, have also been fabricated by a similar method.98,100,101 Thus the coaxial electrospinning technique is a simple and effective method for separating magnetic NPs. In these composite nanofibres, Fe3O4 NPs were all in linear arrangement in the middle of the fibres, and the magnetic performance of the mats as a function of the orientation of the fibres has been investigated. In addition, electrospinning a copolymer rather than a homopolymer, the polymers would self-assemble and form coaxial


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structures within the fibres.102 Surface treatment of NPs could control the spatial location of NPs in polymer matrices.103 For example, coaxial nanofibres with poly(styrene-block-isoprene) (PS-b-PI)/magnetite NPs as core and silica as shell were fabricated using electrospinning technique, in which the magnetite NPs were surface coated with oleic acid to provide marginal selectivity towards an isoprene domain, the TEM images of the composite fibres confirmed that the NPs were uniformly distributed within the PI domain.103 2.3.1.2 Fluorescing NPs of quantum dots and rare-earth metals. Many metal and inorganic fluorescing NPs have been electrospun and their luminescent properties investigated.104,105 These phosphor NPs include boron carbonitride (BCN) oxide, sodium yttrium fluoride, quantum dots and rare-earth metals, etc. It is well known that the rare-earth elements have good luminescent properties, and various composite electrospun fibres of rare-earth NPs and polymers or ceramics have been prepared, such as Sm3+/TiO2, NaYF4:Yb3+, Er3+/SiO2, YVO4:Eu3+/PEO, X1–Y2SiO5:Ce3+/Tb3+.106–109 Besides good luminescent properties, the composite fibres also have certain applications in controlled drug release. Quantum dots, such as CdTe and CdS, also have been successfully electrospun within polymer fibres, the fabricated mats/fibres exhibit corresponding luminescent property, and photoluminescence spectra of the electrospun fibres are blue-shifted compared to the quantum dot solutions, this is because the NPs have different quantum confinement effects in different media.110,111 Fluorescent molecules also have been electrospun. For example, a thermoplastic polyurethane (TPU)–fluorescein composite nanofibrous mat is used to identify latent fingerprints on various substrates within 30 s and produce inkjet-printed patterns. In contrast to classical approaches, the method is easyto-operate, environmentally friendly, and has implications in other applied systems including chemical sensors, drug delivery, biological detection and microreactors (Fig. 9).112 2.3.1.3 Polymer NPs. Recently, considerable research has been focused on the therapeutic applications of electrospun fibres for gene, drug or protein delivery in tissue engineering, due to the similarity between a porous nanoscale electrospun matrix and the naturally occurring fibrillar extracellular matrix (ECM). The encapsulation of bioactive compounds, such as drug or proteins, using NPs is widely investigated as a delivery method because NPs can pass through cell membranes, allowing for facilitated delivery of bioactive compounds to cells. There are several methods for encapsulating bioactive compounds, and the simplest way is encapsulating the compounds within polymer NPs, followed by hardening of the formed NPs via electrospinning. Besides, crosslinking among or within the particles could further improve the mechanical properties and thermal stability of the fibres. Microgels have been widely investigated because of their conformation state: this can control the release of the materials confined within the gels by applying an external stimulus. Poly(NIPAM-co-AA-co-MBAAm) microgel, which is sensitive to

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Fig. 9 (A) Representation of the electrospinning setup using a rotated drum collector. (B) SEM image of the TPU–fluorescein composite nanofibres, inset is the photograph of the composite fibrous mat. (C) Bright field image of a fingerprint on the hybrid mat touched by a finger and then developed with hot air for 5 s. Reprinted with permission from ref. 112. Copyright 2011 Wiley.

temperature and pH, was electrospun in cross-linked PVP fibres.113 Piperno and co-workers found that the microgel could be electrospun without polymers as long as the concentration reached a certain concentration, and this process was not limited to the electrospinning polymer particles distributed in the polar medium.114 PMMA–PVA composite fibres also have been fabricated, and this study results from the fact that swollen polymeric colloids can be localized in the core region of electrospun fibres and packed close to each other to form a continuous core (Fig. 10a).115 Besides, core–sheath fibres consisting of a PCL sheath and PNIPAM microgel particles in the core have been synthesized (Fig. 10b). PS NP is another kind of often used polymer spheres in electrospinning for delivery (Fig. 10c),107 and other polymer NPs, such as polyacrylamide, PMMA/PAN, poly(lactide-co-glycolide), polyurethane (PU) and poly(hexamethylene adipate)-PEO block copolymers (PHA-b-PEO), have also been electrospun.116–121 Most research uses electrospinning for delivery focuses on the encapsulation and release of one component, while for many applications, multiple compounds are needed. Recently, Wnek et al. developed a novel method for the fabrication of biocompatible polymeric fibres with the encapsulation of two distinct biological components, as shown in Fig. 10d.122 Two-luminophore nanowires with a polyphenylenevinylene (PPV) shell and (n-Bu4N)2(Mo6Br8F6)@PMMA core were synthesized by coaxial electrospinning. The two luminophores, (n-Bu4N)2[Mo6Br8F6] compound and PPV, were selected to restrain the absorption and emission spectral overlapping, while a spatial separation was achieved by the coaxial geometry.

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Fig. 10 (a) Schematic diagram for producing core–sheath nanofibres that contain an array of colloids in the core. (b) Combination of optical and fluorescence mode images of the core–sheath fibres, consisting of a PCL sheath and PNIPAM microgel particles in the core. Reprinted with permission from ref. 115. Copyright 2009 Wiley. (c) Two-color stimulated emission depletion microscopy (STED) image of PVA fibres immobilized with PS NPs. Two fluorophores, B504-MA and PTCA, were used to label PS NPs and PVA, respectively. Reprinted with permission from ref. 107. Copyright 2011 American Chemical Society. (d) Fluorescence image of the PU electrospun fibres containing PVA/EGF-AF488 and PVA/BSA-TR particles with three times more EGF-AF488 and BSA-TR loaded during particle synthesis. Reprinted with permission from ref. 122. Copyright 2009 Wiley.

Other NPs, such as molecular imprinted polymers and PVA NPs loaded with protein, have also been electrospun.122,123 2.3.2 1D NPs–electrospun fibres 2.3.2.1 Organic nanochains. Materials with anisotropic structure, such as NRs and NWs, have a large surface-to-volume ratio and two-dimensional confinements. 1D materials have been widely investigated for their unique optical, electrical and magnetic properties.12 When these oriented materials are electrospun, electrospinning shows its potential ability for assembly: the nanocomposites are usually aligned along the long axial direction of the electrospun fibres. Ranging from organic molecule chains to inorganic 1D nanomaterials, even big biological molecules, could be aligned. Especially for those materials whose properties are determined by their aggregation or orientation state, such as AuNRs, electrospinning could facilely control the spaces among particles at the same time, just by varying the amount of NPs directly added into the electrospun solution. Rabolt and co-workers reported for the first time that an electric field can be used to macroscopically align polymer nanofibres and the polymer chains are parallel to the fibre axis. This important result indicates that anisotropic structural properties can be induced in polymer nanofibres during the electrospinning process.124 In 2008, the Chirachanchai group induced the electrospinning technique for controlling the crystal orientations the poly(oxymethylene) (POM) fibres. The author emphasized that this finding opened a new research field in polymer science where we can understand the relationship between structure at the molecular level and the properties


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Fig. 11 (a) SEM image of a single polyoxymethylene (POM) nanofibre collected at 630 m min1. (b) Schematic illustrations of nanofibril within a single POM fibre. (c–e) Schematic illustration of the crystal orientation at 0 m min1, 630 m min1 and 1890 m min1, respectively. (f) Crystal confirmation of the helical structure of the POM chain. Reprinted with permission from ref. 125. Copyright 2008 American Chemical Society.

at the macroscopic level (Fig. 11).125 Becker and co-workers used a primary amine-derivatized 4-dibenzocyclooctynol (DIBO) to initiate the ring-opening polymerization of poly(g-benzylL-glutamate) (DIBO-PBLG). This initiator yields well-defined PBLG polymers functionalized with DIBO at the chain termini. The DIBO end group further survives an electrospinning process that yields nanofibres that were then available for post-assembly functionalization with any number of azidederivatized molecules.126 Soft self-assembled photonic materials such as cholesteric liquid crystals are attractive due to their multiple unique and useful properties, and a short-pitch cholesteric confined in the core of polymer fibres produced by coaxial electrospinning, showing that the selective reflection arising from the helical photonic structure of the liquid crystal is present even when its confining cavity is well below a micrometer in thickness, allowing as little as just half a turn of the helix to develop. At this scale, small height variations result in a dramatic change in the reflected color, in striking difference to the bulk behavior. Liquid crystals have also been electrospun within polymer fibers.127 It is notable that if monomers are aligned within the fibre, the polymerized chains can also be arranged. Recently, hybrid mechanoresponsive polymer wire composed of polydiacetylene (PDA) and PEO have been fabricated. Because the attractive forces among diacetylene molecules were much larger than those between the monomers and the matrix, the monomers self-assembled within the fibres and oriented along the stretching direction. Finally, mechanoresponsive fibres were fabricated through exposure to UV light within which the polymer chains of PDA were aligned.128


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Fig. 12 (a) Typical SEM image of the Au/PVA nanofibre mat. (b) Photograph of AuNRs/PVA fibrous mat fabricated by electrospinning for 1 h with AuNRs particle concentration of 200 nM. (c) High magnification TEM image of the composite fibre in (a), in which the NRs were all aligned along the axes direction of the electrospun fibres. (d) The UV-Vis-NIR absorption spectra of the Au/PVA electrospun mats with different AuNRs concentrations. Reprinted with permission from ref. 14. Copyright 2011 Wiley.

2.3.2.2 Inorganic 1D NPs. AuNR is one important kind of NP for its localized surface plasmon resonance (LSPR) performance, and have been widely investigated in the fields of catalysis, biomedicine and sensors. Several works have been reported that AuNRs can be electrospun within the fibres.14,129,130 Our group applied the electrospinning technique to readily assemble AuNRs on a large scale, which has the advantage of simple and convenient preparation, and the fabricated free-standing film is flexible and stable (Fig. 12a and b). To investigate the assembly effect on the properties of the mat, the optical and SERS property are investigated. By controlling the amount of the NRs added into the electrospun solution, the side-to-side and tip-to-tip distances can be adjusted, and therefore the optical properties of the composite mats could be controlled. Compared with the AuNRs solution and corresponding casting films, both the transverse plasmon band (TPB) and the longitudinal plasmon band (LPB) of the electrospun mat red shifted; when increasing the AuNRs concentrations, the LPB of the electrospun mats blue shifted and broadened, while there is no significant change on TPB, which confirms the effect of assembly on their optical properties. In addition, the electrospun mat could be used as a surface enhanced Raman scattering (SERS) substrate. The resulting electrospun mat makes a significant SERS enhancement to 3,3 0 -diethylthiatricarbocyanine iodide (DTTCI) molecules with large-area uniformity and good reproducibility. NWs with high aspect ratio, such as calcium silicate hydrate (Fig. 13a),131 CNTs (Fig. 13b),132 AgNWs (Fig. 13c and d),133 and AuNR–AgNW assemblies (Fig. 13e and f),134 have been electrospun, which play a critical role in the mechanical and electronic reinforcement of the polymers. For example, CNTs consist of rolled graphene sheets as first reported by Oberlin and Endo,135 and have been electrospun within polymer fibres. The purpose of electrospun CNTs is to functionalize the polymer fibres (in terms of conductivity, mechanical performance or thermal conductivity)

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Fig. 13 (a) TEM image of a hybrid electrospun fibre with 10 wt% calcium silicate hydrate (CSH) NWs. Reproduced from ref. 131. (b) The high magnification bright field TEM images of electrospun MWNT–PAN hybrid nanofibres containing 10 wt% oxidized MWNTs. Reprinted with permission from ref. 132. Copyright 2004 American Chemical Society. (c) Photograph of AgNWs/PVP electrospun mat with a AgNW concentration of 15 mg mL1 and an electrospinning time of 15 min fabricated by magnetic-assisted electrospinning technique, by which the composite nanofibres were all parallel aligned in the whole mat. (d) High-magnified TEM image of the AgNWs/PVP electrospun fibres, in which more than ten AgNWs could be assembled within a single fibre and aligned along electrospun fibres. Reprinted with permission from ref. 133. Copyright 2012 Wiley. (e) Photograph of AuNR– AgNW/PVA electrospun mat. The mass concentrations of AgNWs and particle concentration of AuNRs in the electrospun solution were 4 mg mL1 and 20 nM, respectively. (f) The corresponding TEM image of the fibres containing AuNR–AgNW nanocomposites. Reproduced from ref. 134.

or make CNTs align within the fibres. At present CNTs have been electrospun within many different polymers, such as PAN, PEO, PVA, PLA, PMMA. It has been demonstrated that when well dispersed in the polymer fibres, CNTs can fully complex with polymer and be aligned within the axial direction of the fibres. Unexpectedly, when calcining the CNTs/PAN fibres to make polymer transferred into carbon, CNTs aligned on the surface of the carbon fibres with nanobrush-structure.136 Recently, both TiO2 NRs and CNTs were electrospun within fibres. After calcining, the composite showed high electronic collection capacity when used as an electrode of a dye fuel cell.137 Our group has confirmed that AgNWs could be aligned on the macroscopic scale by using the magnetic-field-assisted electrospinning technique (Fig. 13c and d).133 With this method, ultralong AgNWs can be assembled within the PVP electrospun fibres and arranged in parallel. The number of AgNWs confined within the PVP fibres can be controlled by

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changing the concentration of NWs in the electrospun solution while the polymer nanofibres can also be arranged by the magnetic-assisted-field electrospinning technique and the AgNWs manipulated to align parallel to each other throughout the entire film. AgNWs can be further aligned as a result of the flexibility of the polymer fibres, and mats with variable cross angle and the fibre assemblies with hierarchical structures can be prepared. Other complex 1D nanostructures, such as AuNR– AgNW assemblies that were prepared by electrostatic adsorption, also have been assembled by electrospinning.134 Compared with the normal casting films composed of randomly dispersed AuNRs and AgNWs, or electrospun mats with monometallic components, the resulting AuNR–AgNW/PVA electrospun mats show red-shifted and broader absorption bands, also higher SERS performances, due to the ordered alignment of AuNR– AgNW nanocomposites on a large scale. Other NWs, such as cellulose nanowhiskers,138 and halloysite nanotubes,139 also have been electrospun and greatly improved the mechanical performance of the composite fibres. 2.3.3 2D NPs–electrospun fibres 2.3.3.1 Clay nanosheets. Clay has a sheet structure, and itself has good mechanical properties and thermal stability. The addition of layered nanostructures enhances the mechanical properties, thermal stability and barrier properties of polymers. This is attributed to the high aspect ratio and available surface area of the nanosheets. Theoretical analysis has shown that addition of materials with high aspect ratio could highly enhance the mechanical properties of the composites. Three methods have been investigated for preparing traditional sheets/polymer nanocomposite materials: melt mixing, liquid phase polymerization and liquid-phase-coordination. Much work have been done to investigate whether the electrospinning technique could be used to prepare fibres containing plates rather than 0D or 1D NPs. The main problem for electrospinning 2D NPs is how to make the clay sheets disperse in the electrospun solution uniformly. Without sufficient stripping, clay sheets will deposit in the bottom of the electrospinning solution or form irregular electrospun fibres with ridge or bead structure, and thus affecting its performance. To solve this problem, there are two main methods: one is by using one of the three methods abovementioned for preparing sheet/polymer composite material, and then electrospinning; the other is to dissolve clay tablets and polymer in a particular solvent, or mix two different solutions together that dissolve the polymer and clay tablets separately. In the former case, the dispersion of the clay tablets mainly depends on the preparation process, while the second method mainly depends on the dispersion state of NPs in the solvent. Many related research works have been published, and clay tablets are coated in a variety of electrospun fibres. For example, after modification with a quaternary ammonium salt, organiclay/polyamide-66 (PA66) electrospun fibres with organoclay content as high as 7.5 wt% could be obtained (Fig. 14a).140 In the choice of polymer, in addition to considering the compatibility with clay tablets, polymers with good mechanical properties are considered, such as PMMA, PS, PU, PAA, polyamide-6


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Fig. 14 (a) TEM image of organoclay–PA66 composite nanofibres with organoclay content of 7.5 wt%. Reprinted with permission from ref. 140. Copyright 2012 Springer. (b) TEM image of montmorillonite (MMT)/Ag/PVA nanofibres with MMT and Ag contents of 5 wt% and 1 wt%. Reprinted with permission from ref. 145. Copyright 2010 Springer. (c) HRTEM image of GO/PNA nanofibre, in which GO nanosheets were modified by 1-pyrenebutanoic acid, succinimidyl ester. The arrows indicate the flakes inside the fibres. Inset is an enlarged image of GO embedded within the sidewall of a fibre. Reprinted with permission from ref. 151. Copyright 2010 Wiley. (d) SEM image of graphene oxide-templated PAN nanofibres, inset is an SEM image of a thicker area showing wrinkled structure. Reprinted with permission from ref. 155. Copyright 2013 Wiley.

(PA6) and PA66.140–144 By electrospinning AgNPs and montmorillonite clay tablets together (MMT), the Ag–MMT–PVA composite nanofibres show good mechanical performance and bactericidal effect (Fig. 14b).145 2.3.3.2 Graphene and graphene oxide. Due to its special structure (consisting of a single layer of carbon atoms in a sheet), graphene nanosheets (GNS) which has the strongest interlayer force among all the materials, also has excellent thermal and electrical conductivity, making it among the best candidates for nanofillers.146 It has been demonstrated that, with the same loading, the graphene-reinforced composites significantly mechanically out-perform their counterparts with singlewalled/multi-walled carbon nanotubes as nanofillers. Thermal stability can also be greatly improved as evidenced by the 30 1C increase in glass transition temperature with only 0.05 wt% of graphene in PMMA.147 Conjugated graphene or graphene oxide sheets can be readily prepared by non-covalent p–p stacking or covalent coupling reaction between C–C bonds. Different functionalizing organic groups can be dissolved in different solvents and can complex with graphene and form donor–acceptor composites, which can adjust the conductivity, optical and optoelectronic properties. It can be expected that based on the various excellent performance of graphene and its derivatives, significant improvement of the viscosity, electrical, mechanical and thermal properties of graphene–polymer composites can be achieved. The feature of graphene sheets provides maximum surface area of p–p stacking with the polymer, thus it can be well dispersed in


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the polymer matrix. Carboxylic acid and hydroxyl functional groups on the edge of graphene can also be used as crosslinking agents and connect with the organic portion of the polymer, thereby enhancing the mechanical properties.147,148 In addition to the traditional methods for preparing graphene– polymer composites, graphene can be fixed to the polymer fibres by the electrospinning technique, and various functional composite fibres could be fabricated. The main polymers that have been used are PVA, PAN, and PMMA. By changing the added amount of graphene, the mechanical and electrical properties of the composite fibre can be well adjusted.146,148,149 For example, when adding 4 wt% of graphene, the Young’s modulus of the PAN electrospun fibres can be doubled. Similarly, the distribution state of graphene in the fibres will have a significant impact on the mechanical properties: when uniformly dispersed, the modulus of PVA electrospun fibre can be increased 205%.150 It has been shown that graphene can accelerate the photon movement within PVA fibres, and thus increase their conductivity. A small loading (0.07 wt%) of functionalized graphene enhanced the total optical absorption of PVA by 10 times (Fig. 14c).151 In addition to the mechanical and electrical aspects of applications, graphene– PVA electrospun fibres can also be used as scaffolds in tissue engineering aspects.152 Furthermore, when graphene was electrospun with another substance at the same time, such as chitosan, besides improving the mechanical strength, the composite fibres can also be used in biosensor applications.153 Graphite layers could be separated through intercalation and exfoliation, and thin nanoplates can be formed. It has been reported that by electrospinning graphite nanoplates with polyacrylonitrile, fibres with an average diameter of about 300 nm could be fabricated. The composite nanofibres demonstrate a modest increase in thermal stability with increasing wt% graphite nanoplatelets, and the Young’s modulus doubled with only 4 wt% incorporation of graphite nanoplates.146 Besides, by a combination of electrospinning and thermal treatment, free standing CNF–GNS composite paper is prepared and the paper exhibits a large specific capacitance, about 24% higher than that of pure CNF paper.154 Continuous carbon fibres present an attractive building block for a variety of multifunctional materials and devices. However, the carbonization of PAN precursors usually results in CNFs with weak graphic structures and thus modest properties. The Dzenis group found that the graphitic structure of carbon nanofibres can be improved with an addition of a small amount of graphene oxide into PAN prior to processing, and continuous CNFs with 1.4 wt% of graphene oxide NPs were prepared from PAN solutions (Fig. 14d).155 After calcining the graphene–polymer electrospun fibres, graphene–metal oxide composite fibres could also be obtained. For example, Ramakrishna and co-workers electrospun graphene and tetra-n-butyl in the polymer fibres, and then prepared graphene–TiO2 composite fibres by calcination. Experiments confirmed that, compared to TiO2 fibres, graphene–TiO2 composite fibres showed a better performance of superior photovoltaic parameters in the dye cell, and can photocatalyze methyl orange more rapidly.156 Furthermore, such material can also be used as the anode electrode in lithium ion batteries and dye-sensitized solar cells.157,158 If the polymer is not

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removed during the calcination process and is converted to carbon, then graphene–C composites can be easily fabricated. Other composite fibre materials, such as graphene–SiC, graphene–Li4Ti5O12, graphene–C–TiO2, and graphene–MnO2 have been successfully prepared, and show excellent energy conversion and storage performance in catalysis, batteries and supercapacitors, etc.149,159–163 2.3.4 Other organic or biological NPs–electrospun fibres. Controlled release of therapeutically active agent through polymers is an established technique, and has been in clinical use. Electrospun fibres have important applications in drug release and tissue engineering for their structural and morphological similarly to the native ECM. Therefore, in this section, we will focus on the electrospinning of organic molecules and biological NPs to prepare functional composite fibres. Packaging can accordingly improve the efficacy of drugs, specificity, tolerability and therapeutic index. Understanding the mechanism of controlled release is an important factor to design effective release systems. Drugs can be uniformly dispersed in the polymer fibres, and also can be encapsulated in the core. Much work has been done for controlled release of drugs through electrospun fibres, and several relevant reviews have also been published.164–168 In addition to controlling drug release, various active cells have also been successfully electrospun.169–175 When active bacteria are electrospun, a requirement of the polymer is that it is water-soluble and with good oxygen barrier properties after drying, such as PVP, PEO and PVA. In 2004, Belcher and co-workers electrospun M13 virus within PVP fibres for the first time, and established virus infection of bacterial hosts coming from the virus within the electrospun membrane.176 Then in 2006, Zussman and co-workers successfully prepared E. coli and Staphylococcus aureus-containing PVA electrospun fibres, and experiments showed that the bacteria in the fibres showed no loss of biological function.177 Because of the formation process of electrospun fibres, the solvent (water) evaporates quickly, and this will change the osmotic pressure of bacteria and affect its activity. Therefore, Greiner and co-workers dissolved water-soluble polymer in an aqueous solution containing active bacteria (Micrococcus luteus that can still survive when moisture content is very low, as well as relatively weak Gramnegative E. coli) and subjected to electrospinning; composite fibres were prepared and the survival of different bacteria within the polymer were investigated.173 It was found that these bio-composite materials could be used as ‘‘active membranes’’ in separation, catalysis and tissue engineering (Fig. 15a). In addition, various proteins also could be electrospun within polymer fibres and have important applications in biological tissue engineering.172,175 The researchers found that it is conducive to neuronal cells adsorbed on the fibres when the protein-containing polymeric nanofibres were aligned.171 Biological enzymes are another kind of active substances, and their immobilization has attractive sustained attention in fine chemicals, bio-medicine and bio-sensing areas. The performance of immobilized enzymes largely depends on the support structure, and nanocarriers are considered to be able to retain catalytic activity and to ensure high efficiency of enzyme immobilization.

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Fig. 15 (a) SEM image of M. luteus–PEO composite fibres. Inset is the optical fluorescence of the composite fibres under UV illumination. Reprinted with permission from ref. 173. Copyright 2007 Wiley. (b) SEM image of the chitosan/ siRNA-Cy5NP-encapsulated PLGA fibres and inset is the fluorescence microscopy image. Reprinted with permission from ref. 170. Copyright 2012 American Chemical Society. (c) Fluorescence microscopy image of stretched DNA molecules embedded in PEO fibres that of two overlapping ‘‘ribbons’’ formed from 100 mg DABCO solution when the electrospinning jet became unstable and spurted. Reprinted with permission from ref. 169. Copyright 2006 American Chemical Society. (d) Living scaffold encapsulating GFP expressing N2A cells generated by cell electrospinning. Reprinted with permission from ref. 181. Copyright 2013 Wiley.

Electrospinning provides a simple and flexible way to produce nanofibrous scaffolds. Compared with other nanostructure supports (such as a porous silica, NPs), nanofibre scaffolds show a number of advantages in terms of their high porosity and interconnectivity. Based on this, numerous articles have been published on the use of electrospinning to immobilize enzymes, along with relevant reviews. Currently, the main research focus in this area is the use of biological enzymes for catalysis and sensing research.42,153 RNA interference (RNAi) is a promising method for interrupting gene expression, and siRNA release through electrospun nanofibres has been demonstrated successfully using PCL and PEG. Until 2012, the mechanism of release was fully investigated by Besenbacher and co-workers (Fig. 15b).170 Chitosan/ siRNA NPs were encapsulated in PLGA by electrospinning, and the composite nanofibres were successfully prepared. Through the study of acidic/alkaline hydrolysis and bulk/surface degradation mechanism the optimal release process can be ascertained in order to achieve long-term and effective gene control. Previous studies have proven electrospinning technology can fix the DNA as well as part stretched large DNA (200–900 KBP) within the interior of polymer fibres.178,179 In the first study, DNA molecules that were much larger than single molecules were directly fixed within the polymer fibres, while in the second study, large single DNA molecules were deposited on the surface of mica and then electrospun. Another study confirmed that DNA molecules released from the electrospun film also maintained structural integrity and biological activity.180


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Craighead and co-workers prepared polymer fibres containing stretched isolated DNA molecules in 2006, and discovered for the first time that the DNA molecule within a single electrospun fibre was stretched and arranged along the fibre orientation (Fig. 15c).169 Recently, Robertson et al. has demonstrated that neither cell electrospinning (CE) nor aerodynamically assisted biothreading (AABT) have any detectable effect on the in vitro or in vivo growth of the cells (Fig. 15d).181 Conjugated polydiacetylene (PDA)-embedded electrospun mats can be used to detect adulterated gasoline.182 Cyclodextrins are electrospun within the polymer fibres to filter the molecules.183 Another report found that spiropyran–cyclodextrin functionalized fibres show photoresponsive behaviour.184 Effects of nanoclay on the thermal, mechanical and crystallization behaviour of nanofibre webs of Nylon-6 has also been investigated.185

3. Applications of NPs–electrospun fibres 3.1

NPs–electrospun fibres for energy

With the development of industry, new energy sources is one of the most important issues that need to be solved, and many scientists are seeking for appropriate strategies or materials to solve this problem. Nowadays, miniaturization, multifunctionally, flexibility and low energy consumption have become the development trend of electronic devices. In earlier reports, electrospun nanofibres have been used to prepare electrodes either by milling the nanofibre meshes and mixing them subsequently with carbon black and binder to form the electrode, or they were directly used as self-supporting electrodes without any additives. 3.1.1 Supercapacitors. Supercapacitors have been intensively investigated as one of the main energy storage systems because they can carry high power density and have long life-cycles, rapid charging–discharging capacity and potential applications in many fields, such as mobile devices and electric vehicles. Many carbon nanostructures have been proven to be supercapacitor electrode materials. Among them, 1D carbon nanotubes have attracted substantial attention, because they have large accessible surface area and relatively high electrical conductivity, which make them likely to become the dominant electrode material of high-power supercapacitors.186 Currently, the problems for using carbon nanotubes as the electrode material are their low effective surface area, the limited charge capacity and higher costs compared with other materials. These all limit CNTs being used as an electrode material of supercapacitors.186,187 Alternatively, carbon fibres can be prepared by the combination of electrospinning and calcination, which are mainly used as a capacitor electrode material, by controlling the pore size of the fibre and loading metal NPs on/within the fibres in order to increase the standard capacitance. Carbon nanofibres prepared by electrospinning have been widely studied, because of their free-standing nature, high mat porosity and low fibre diameter. Carbonized electrospun fibres are expected to be an excellent alternative material for making electrodes.188 A further report found similar results.189


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The electrospun fibres based on polymer (such as PAN, PI, etc.) substrate can be used as electrodes of electric double layer capacitors prepared after stabilization, carbonization and activation at high temperature. With the development of capacitors, it has been found that the specific surface area and pore-size distribution of the carbon fibres are the major factors which influence the electric performance. For example, after activation under different temperatures (70% N2/30% water steam), the PAN-based electrospun fibres transform into carbon fibres, and the capacitance of fibres activated under 700 1C is 173 F g1 under low current density (10 mA g1), while the capacitance of fibres activated under 800 1C is 120 F g1 under high current density (1000 mA g1): the specific surface area of the former is 1230 m2 g1 and of the latter is 850 m2 g1.187 The importance of pores presented within the carbon fibres on their electronic properties also has been investigated. Various additives, such as cellulose acetate and polymers that can be removed, have been electrospun within the fibres. After the activation process, pores would form within the carbon fibres and thus the specific surface area increased. Moreover, inorganic metal salts could influence the pore structure within the formed carbon fibres. For example, Yang and co-workers prepared self-sustained thin webs consisting of porous carbon fibres via electrospinning of PAN solutions containing ZnCl2, that can be removed by HCl and a porous structure can be formed without any activation process. Carbon nanofibres with 5 wt% ZnCl2 exhibited the highest surface area, the highest capacitance, and good rate capability because of their high surface area and smaller fibre diameters.189 Liu and co-workers used KOH to tune the fibre diameter and to improve porous texture. By adjusting KOH content in the spinning solution, the fibre diameter, the microporous volume and specific surface areas could be greatly improved.190 In order to increase the pseudo-capacitance in the doublelayer capacitor, many kinds of NPs, such as CNTs and metal oxides, have been electrospun within carbon fibres. For example, hybrid carbon nanofibres containing multiwalled carbon nanotubes (MWCNTs) were produced by electrospinning CNTs suspended in a PAN–DMF solution, followed by carbonization and activation using a H2O2–water steam mixture at 650 1C. The specific capacitance of electric double-layer capacitors reached 310 F g1, which was almost double that obtained for capacitors containing pristine carbon fibres as electrodes (170 F g1).191 Moreover, flexible films derived from electrospun carbon nanofibres incorporated with Co3O4 hollow NPs as self-supported electrodes for electrochemical capacitors were prepared. Benefiting from intriguing structural features, the unique binder-free hybrid film electrodes exhibited high specific capacitance of 556 F g1 with loading of 35.9 wt% Co3O4 at a current density of 1 A g1. Remarkably, almost no decay in specific capacitance was found after continuous charge–discharge cycling for 2000 cycles at 4 A g1. Co3O4 hollow NPs/CNF shows excellent rate capability and cycling stability (Fig. 16).192 3.1.2 Lithium-ion batteries. For the past few decades lithiumion batteries have been regarded as one of the most useful energy sources for mobile electric devices and hybrid vesicles because of

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Fig. 16 (a–c) CV curves of Co3O4/CNFs film electrodes with different amounts of Co3O4 in 6 M KOH aqueous solution. (d) Specific capacitance of Co3O4–CNFs film electrodes in (a) as a function of current density. (e) Cycling performance and coulombic efficiency of the Co3O4–CNFs3 film electrode at a current density of 4 A g1. Reprinted with permission from ref. 192. Copyright 2013 Wiley.

their high energy density and long cycle lifetime. To serve as cathodes or anodes for lithium ion batteries, nanostructured active materials should have a short Li-ion insertion/extraction distance, facile strain relaxation upon electrochemical cycling, and very large surface-to-volume area to contact with the electrolyte, which can improve the capacity and cycle life of lithium ion batteries. However, nanomaterials tend to aggregate, owing to their high surface energy and reduce the effective contact areas of active materials, electrolyte and conductive additives. The hierarchical structures of materials, such as hollow, porous, and nanowire-on-nanowire can ensure that the surface remains uncovered to keep the effective contact areas large even if a small amount of self-aggregation occurs. Moreover, when ID nanomaterial are up to several hundred micrometers or even several millimeter scale, such as for ultralong NWs or nanobelts, self-aggregation of the nanomaterials could be effectively prevented. Therefore, ultralong hierarchical nanowires are one of the most favourable structures as cathode/anode materials for high performance. Among the variously of nanomaterials used for battery electrodes, the phosphoolivines with the chemical formula LiMPO4 (M = Fe, Co, Ni, Mn) have attracted great interest due to their high cell voltage, cycling stability and capacity. In the case of Mn and Fe, the materials are non-toxic and can be produced in large volumes at low cost, which make them highly attractive for industrial applications. However, their intrinsic electrical conductivity is poor, which makes a coating with conductive matrices (e.g., carbon) necessary. The olivine-structured LiFePO4 has high theoretical capacity (170 mA h g1) and acceptable operating voltage (3.4 V vs. Li+/Li). LiFePO4 NWs offer a better percolation behavior than NPs, but to date there are only a few reports on the synthesis of LiFePO4 NWs, and electrospinning has been confirmed as one powerful method to synthesize the NWs. Hosono and co-workers synthesized carbon-coated LiFePO4 NWs as well as triaxial NWs with a carbon nanotube and a carbon shell. However, the large diameter of LiFePO4 NWs (4500 nm) constrains their performance.193 Thinner NWs

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Fig. 17 (a) Schematic diagram for the preparation of single-crystalline LiFePO4 NWs by electrospinning. (b) SEM image of as-prepared LiFePO4 NWs. (c) Cyclic voltammetry of LiFePO4 NWs with a scan rate of 0.1 mV s1. (d) Cycling stability of LiFePO4 NWs at 1 C rate at 25 1C and 60 1C, respectively. Reprinted with permission from ref. 76. Copyright 2011 Wiley.

are needed for the applications in lithium batteries. Maier and co-workers using LiH2PO4, Fe(NO3)3 and PEO in the electrospinning process, fabricated single-crystalline carbon-coated LiFePO4 NWs with diameter of 100 nm, which showed very good rate performance and cycling capability (Fig. 17).76 The addition of Mn to form LiFe1yMnyPO4 solid-solutions increases the cell voltage and thereby the energy density of the cell. There have been many efforts to improve the more ionic members of this family, and electrospinning represents a promising approach to prepare LiFe1yMnyPO4 nanofibres with control over their nanostructure and allowing in situ incorporation of carbon into the structure. For example, Mathur and co-workers fabricated LiFe1yMnyPO4–C nanofibre composites by electrospinning of commercially available precursors, and the nanofibres were then calcined at 850 1C under Ar/H2 atmosphere.194 The fibres are directly used as self-supporting electrodes without any conductive additive or polymer binder. 3.1.3 Dye-sensitized solar cells. Dye-sensitized solar cells (DSSCs) are promising low-cost, high-efficiency photovoltaic devices for solar energy conversion. However, nanoparticle-based devices result in a lower efficiency because electron transport is limited by a trap-limited diffusion process, and the slow charge diffusion increases the probability of recombination. Moreover, the grain boundaries encountered during electron transport lead to fast recombination prior to their collection at the electrode. To solve the above problem, one strategy is by using 1D nanostructures to replace the NPs, which provides direct pathways for collection of charges generated throughout the device. It has been confirmed that electron transport in 1D nanostructures is several orders of magnitude faster than that of NPs. Another approach is to incorporate highly electrically conductive materials in photoanodes, such as carbon tubes and graphene. The presence of conductive materials in a photoanode is expected to improve the charge transport properties and extend the electron lifetime, thereby improving the performance of the device. For example, Leung and co-workers


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Fig. 18 Electron transport across a photoanode of CNTs totally surrounded/ embraced by (a) TiO2 NPs, (b) NRs and (c) CNTs inside TiO2 NRs. The thickness of the arrow represents the electron transport speed, the thicker arrow representing faster electron transport. (d) J–V characteristics of different photoanodes, (e) the effect of MWCNT concentration in the precursor solution on Jsc and fill factor (FF). Both in (d) and (e): hollow and solid symbol curves represent thinner (6.6 0.7 mm) and thicker (14.3 0.3 mm) photoanodes, respectively. Reprinted with permission from ref. 137. Copyright 2013 Wiley.

introduced MWCNTs within TiO2 NRs via electrospinning.137 This configuration was successfully applied in a photoanode to improve the performance of a DSSC device. When 0.1% MWCNTs was incorporated, the DSSC device exhibited the highest efficiency of 10.24%, with a high Jsc of 18.53 mA cm2 and a fill factor (FF) of 74% (Fig. 18). These positive results confirm that incorporate CNTs in the electrospun NTs provides an effective strategy to improve the charge transport performance for realizing solar energy conversion in the future. 3.1.4 Catalysis. The use of electrospun fibres/mats for catalysts is also an upcoming field of interest. The surface area is the dominant factor that decides the extent of catalysis and its efficiency, and catalyst material having fibrous morphology is superior to particles as far as the recycling and aggregation are concerned. Fibres with hollow, porous or multichannel structures have been readily fabricated by electrospinning as previously discussed (Fig. 3), which further improve the specific area of the fibres, and thus their catalytic performance. Ceramic nanofibres have been widely used in photocatalysis for the degradation of organic pollutants with high activities, durability and low cost.195,196 Among various oxide and nonoxide photocatalysts, titania has proven to be the most versatile material because of its high catalytic activity and long-term stability.197 The electrospinning of concentrated TiO2 nanoparticle dispersions in PVA matrices followed by calcination allowed the fabrication of nanofibres with narrow polydispersity, displaying tunable mesopores volumes and diameters. This approach is more flexible than the electrospinning of sol–gel precursors and therefore it is relevant for the production of easy-to-handle and versatile catalytic substrates.198 Besides, TiO2 hollow fibres with mesoporous walls or rice grain-shaped


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mesostructures were fabricated using electrospinning combined sol–gel method, and exhibited enhanced photocatalytic activities compared to commercial TiO2 NPs (P25) and mesoporous TiO2 powers.199–201 Furthermore, the photocatalytic activity of multichannel TiO2 hollow fibres achieved by multifluidic electrospinning method have also been investigated. A quite interesting phenomenon is that the percentage increase for photocatalytic activity is higher than that for surface area. The authors proposed that multichannel hollow structures offered a cooperative effect of trapping more gaseous molecules inside the channels and multiple reflection of incident light, which further improved the photocatalytic activity of TiO2 hollow fibres.202 Because of the fast recombination rate of photogenerated electron–hole pairs in the bulk ceramic materials lowers their photocatalytic efficiency, composite materials are being fabricated in order to extend the light absorption spectrum as well as suppressing the recombination of photogenerated electrons. The metal–metal oxide composite materials will serve as a better catalyst, with the ceramic as the substrate material and the metal acting as the reactive sites. For example, Xia’s group coated the surfaces of TiO2 fibres with Pt NPs 2–5 nm in size using a simple chemical reduction process. The Pt NPs could also serve as seeds for growing Pt NWs. The composite fibre membranes exhibited excellent catalytic activity for the hydrogenation of azo bonds in methyl red, which could be prepared in a continuous mode by passing the solution through the membrane at a flow rate of 0.5 mL s1.203 Similarly, the surfaces of TiO2 and ZrO2 nanofibres could also be functionalized with Pt, Pd, and Rh NPs via the similar method, and the composites could be used to catalyse the Suzuki coupling reactions and be operated in a continuous flow fashion.204 Other fibre heterostructures achieved by electrospinning technique with high photocatalytic activity, such as SrTiO3/ TiO2,205 Ag/TiO2,206 Ag/SiO2,207 ZnO/BaTiO3,208 and rare-earth doped TiO2,209 have also been reported. Besides using for the degradation of organic pollutants, the composite fibres also exhibit high catalytic performance for the reduction of 4-nitrophenol (4-NP) with NaBH4,210 preferential CO oxidation (CO-PROX),211 and oxygen reduction reaction (ORR).212 The Shao group fabricated several nanocomposite catalysts for 4-NP by combining electrospinning and an in situ reduction approach.213–216 For example, tubular nanocomposite catalysts with size-controlled and highly dispersed AgNPs assembled on the inner and outer surfaces of SiO2 NTs were successfully fabricated and the composites exhibited excellent catalytic performance for the reduction of 4-NP. Besides their high specific surface area, the authors believed that the size effect of AgNPs played a leading role in enhancing the catalytic activity. Furthermore, the nanocomposite catalysts could be easily recycled without a decrease of the catalytic activity because of their 1D nanostructure property.217 Pt–CeO2 composite oxide catalysts are promising candidates for the COPROX owing to the strong metal–support interaction effects with potential to enhance the catalytic activities. However, they show low thermal stability and loss of catalytic activity owing to sintering. Recently, CeO2 hollow fibres with PtNPs embedded

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in the inner surfaces were fabricated by the Xia group via using PS electrospun fibres as the template. This catalytic system could resist thermal sintering up to 700 1C and the turnover frequency (TOF) for CO oxidation was 2–3 orders of magnitude higher than those based on the conventional Pt–SiO2 system, which was the result of a unique structure and composition for maximizing the Pt/CeO2 interface.218 For proton exchange membrane fuel cells, the short life and high cost are two main problems of carbon-supported Pt nanoparticle catalysts (Pt/C). Porous Pt alloy NWs fabricated by dealloying have more durability and catalytic activity than Pt/C, but the process of porosity formation is difficult to control. Shui and co-workers fabricated long, thin and yet nano-porous Pt–Fe alloy NWs via electrospinning and chemical dealloying techniques.219 It was found that non-uniform composition in the precursor PtFe5 alloy NWs benefited the formation of a nanoporous structure. The overall wire diameter is about 10–20 nm and the ligament diameter only 2–3 nm. These porous long nanowires interweave to form a self-supporting network with a high specific activity, 2.3 times that of conventional Pt/C catalysts, and also have better durability. 3.2

Chem Soc Rev

Fig. 19 (a) Stress–strain curves of MWCNT/PMIA nanofibres with CNT contents of 0, 0.5, 1 and 1.5 wt%. (b) Photographs showing the as-prepared nanofibrous membrane (MWCNT content was 1.5 wt%) supporting a weight of 5 kg, the membrane has 20 mm width and 1.8 mm thickness. Reproduced from ref. 221. (c) Schematic illustration of the fabrication process. (d) Digital image of the as-obtained composite bulks with different NW contents. (e, f) TEM images of ultrathin sections of the planes as indicated in (d). (g) Three-point bending strength and corresponding modulus of the composite bulks. The error bars represent the standard deviation of the mean. (h) Typical stress–strain curves of composite bulks with different nanowire contents in three-point bending tests. Reproduced from ref. 131.

NPs–electrospun fibres for mechanical enhancement

The research on mechanical enhancement of composite nanofibre/membrane prepared by electrospinning can be divided into two types: one is to reinforce the composite nanofibre itself, and the other is to reinforce the other substrate material, i.e., fibre reinforced composite material. The mechanical properties of composites mainly depend on the nanofibres’ composition, size, fibre orientation, molecular structure, the arrangement of fibres and some post-processing conditions, such as calcination temperature and time. Engineering plastic itself can lead to good mechanical properties; the electrospun nanofibres tend to have better mechanical properties, because the fibres are formed through the formation of self-organized control in the electrospinning process. It is well known that decreasing the diameter of fibres can increase their strength though often at expense of their toughness. Recently, it has been reported that both the strength and toughness of PAN electrospun fibres were increased.220 When the diameter of the fibres was less than 250 nm, the fibres became tougher, but did not lose their strength. The fabricated fibres were up to 10-times tougher and stronger than the best commercial fibres. The authors suggested that the toughness was possible as the less crystalline of the nanofibres than the coarse ones. Moreover, adding nano-additives into the fibres could further improve their mechanical properties. For example, by electrospinning the solution of CNTs and polymers, such as PAN, PU, PS, PLA, poly(m-phenylene isophthalamide) (PMIA), etc., the mechanical property of composite fibres could be greatly enhanced.13,221–223 Highly aligned MWCNT–PMIA composite fibres were prepared with robust mechanical strength, and the mats with a MWCNT content of 1.5 wt% exhibited the highest tensile strength of 316.7 MPa (Fig. 19a and b).221 In addition, the polymer could be transferred into carbon after calcination, which could further enhance the mechanical performance of the fibres.

Chem. Soc. Rev.

Carbon materials with 2D structure, such as graphene, also could enhance the mechanical properties.146,148,224 Besides carbon materials, different drugs could be incorporated within the fibres by emulsion electrospinning, and the mechanical properties of the single electrospun fibre could be affected. When 10–20 wt% retinoic acid was encapsulated, the fibre’s mechanical strength was enhanced, while the fibre’s strength decreased if 10–20 wt% bovine serum albumins was encapsulated. Alignment of nanofibres will greatly increase their hardness to the orientation direction and improve the anisotropy of the stent. This is an important analog function similar to anisotropic materials, such as tendons, fibrous rings and myocardial tissue. Various silicates, such as hydroxyapatite, tubular clay, montmorillonite, etc., can improve the mechanical properties of fibres.138,142,143,225 Combining nanofillers and polymers leads to substantial improvements in the mechanical, thermal, antifouling and electric properties of polymer nanocomposites. Many researches in industry and academia use microfibres as filler material, thereby creating a microcomposite. Comparing with a microfibre, nanofibres are several orders of magnitude lower in size, thus making them ideal as nanofillers because of their availability of very high surface area for interfacial interaction. The mechanical properties of fibre-reinforced composite materials are mainly controlled by the mechanical properties and the aspect ratio of the fibre itself, and the mechanical coupling between the fibre and matrix. In composite applications, if there is a refractive index difference between matrix and fibre, the resulting composite becomes opaque or non-transparent due to light scattering. One possible way of circumventing this limitation would be to use fibres with a diameter significantly smaller than the wavelength of visible light. Owing to their small diameter, nanofibres cause only negligible refraction of light so that a nanofibre-reinforced transparent matrix can maintain the transparency even when


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the refractive index of the fibre and matrix do not match. Finally, a large surface area between the nanofibres and matrix increases the relaxation process, thereby improving the impact strength of the reinforcing matrix. Chang and co-workers first electrospun calcium silicate hydrate within the polymer, and aligned the composite fibres to a certain orientation through the collection device, finally obtaining bulk composites after hot-pressing,131 as shown in Fig. 19c–h. It was found that with calcium silicate hydrate at 20 wt%, the bending strength of the composite material reached up to 188 MPa, which was much higher than the pure polymer (86 MPa).131 Li and co-workers prepared a PDMS hybrid silica aerogel (xerogel) thin film reinforced by continuous electrospun nanofibres for the first time.226 The nanofibre reinforcement was accomplished through the electrospinning of nanofibres into the sol whose gelation kinetics were delayed so that the nanofibres could easily penetrate the cast wet sol film while it still exhibited low viscosity. The controlled gelation kinetics of the sol ensured that nanofibres are embedded in the gel film or at least anchored to it, creating continuous nanofibre reinforcement to improve the mechanical performance. Other electrospun fibres composed of polymer or inorganic/polymer, such as PVA, Nylon-4,6, PDMS/PEO, PMMA/PDMS, CNTs/PU and MWCNTs/poly(styrene-co-glycidylmethacrylate) (P(St-co-GMA)), also have been used to reinforce the epoxy or silicon matrix.226–231 3.3

Review Article

concentration in important biological processes. Using electrospun nanofibrous membranes as highly responsive fluorescence quenching-based optical sensors for metal ions (Fe3+ and Hg2+) and 2,4-dinitrotoluene (DNT) for the first time is reported by Sarnuelson and co-workers.233 Poly(acrylic acid)poly(pyrene methanol) (PAA-PM), a fluorescent polymer, was used as a sensing material. Another type of chemical sensors, optical sensors, were fabricated by electrospinning PAA-PM and thermally crosslinkable polyurethane latex mixture solutions, which showed high sensitivities compared to traditional sensors due to the high surface area-to-volume ratio of the nanofibrous membrane, which is in favor of the target molecules transferring to the active point of the fibres. Tian et al. fabricated low-cost fully transparent ultraviolet photodetectors based on electrospun ZnO–SnO2 heterojunction nanofibres (Fig. 20).71 The photodetector showed excellent operating characteristics: high UV-sensitivity and photo-dark current ratio, and fast response speed when used as a visible-blind UV-light sensor within an optoelectronic circuit. Among the chemical sensors, humidity nanosensors are very important for their practical applications in environment monitoring, our daily life and industrial process. Many humidity nanosensors have been successfully fabricated. However, the sensing

NPs–electrospun fibres for sensors

Design and fabrication of chemical sensors has become one of the most active research fields due to their diverse practical and potential applications. To improve the sensing characteristics, nanoscale sensors have been widely investigated for their large surface areas. In particular, nanosensors based on 1D materials are of great interest because of their high surface to volume ratio and special physical and chemical properties. Here, several kinds of sensors are briefly introduced. Solid-state real-time chemical sensors are important for biomedical research and medicine. Traditionally, the target molecules are dissolved in a liquid for detection, and then react with the target or the cell that release the chemicals, then the colour or fluorescence changes. On the other hand a solidstate sensor is non-invasive and it transforms the real-time chemical concentration into electronic signals without interference with the biological processes under study. Zinc ion play key roles in normal physiology and is the second most abundant transition metal ion in the human body after iron. In order to study the dynamics of biological processes such as a stroke, it is important to detect the zinc ion concentration every few minutes, while the real-time detection is impossible for traditional molecular probes dissolved in solution. Recently, Wang and co-workers dispersed a probe molecule for Zn2+ in polymer host by electrospinning, with the fibrous film detecting zinc ions with a concentration as low as 106 M with timeresolution of 5 min and showing high stability as well as unchanged sensing function in highly acidic conditions.232 The sensitivity and response speed make it possible for a non-invasive and real-time study for the variation of zinc ion


Fig. 20 (a) Spectroscopic photoresponse of the ZnO–SnO2 heterojunction nanofibres measured at a bias of 5.0 V at different wavelengths ranging from 250 to 630 nm. The inset is the optical absorption spectrum. (b) I–V spectra of the device illuminated with a light of different wavelengths, and under dark conditions. (c) Logarithmic plot of (b). (d) Photocurrent as a function of light intensity and corresponding fitting curve using the power rule under 300 nm light illumination. (e) I–V curves of the device under 300 nm wavelength light illumination measured under air and vacuum conditions. Inset is I–V curves under dark. (f) Time-dependent response of the device measured under air conditions at a bias of 9.0 V under 300 nm light illumination. Reprinted with permission from ref. 71. Copyright 2013 Wiley.

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characteristics, such as their responsiveness, reproducibility and stability, still need to be improved. A highly sensitive and stable humidity nanosensor based on LiCl-doped TiO2 nanofibres through electrospinning and calcination has been reported.234 The sensor exhibited ultrafast responsive and recovery behavior, which are important in humidity detection and control. It also provides a useful platform to design and construct effective humidity nanodetectors. Additionally, KCldoped TiO2 nanofibres also show similar results.235 Further, Lei’s group reported the fabrication of a novel fluorescent nanofibrous membrane via electrospinning for the direct naked eye vapor detection of ultra-trace nitro explosive and buried explosives under UV excitation, and this is a new report about the detection of buried explosives without the use of any advanced analytical instrumentation.236 Strain sensors with ultrahigh flexibility and stretchability have great potential applications in personal health monitoring, human-benign devices, and highly sensitive robot sensors, etc. A novel strain sensor prototype based on ZnO NW–PS NF hybrid structure coated on a poly(dimethylsiloxane) (PDMS) film has been demonstrated.56 The device can withstand strain up to 50%, with high durability and high gauge factors. The device also demonstrated a good performance on small and rapid human

Chem Soc Rev

motion measurements (Fig. 21). With flexibility and stretchability, the device will have potential applications in nanosensor systems for precision measurements, human skin, and personal health monitoring. Moreover, the device can be driven by solar cells and has potential applications as an outdoor sensor system. The composite nanofibres also could be used as biosensors, which are defined as devices that can transform biological signals into electric output signals and have a biological recognition mechanism. A variety of enzymes can be encapsulated within fibres via electrospinning, and the high surface-to-volume area favors the enhancement of adsorption rate and reduction of the response time. The Wei group electrospun horseradish peroxide enzymes within porous silicon nanofibres. The high surface area and freedom of encapsulating various enzymes make porous silica nanofibres excellent biosensors.237 Zhao and co-workers demonstrated a new CTC-capture platform that combined a highaffinity cell enrichment assay based on electrospun nanofibresdeposited substrate coated with cell-capture agent. This substrate may have potential applications in isolating rare populations of cells that can not be easily realized using current technologies.238 Other NPs, such as Au and graphene, also could be used for biosensors.42,239

4. Concluding remarks

Fig. 21 Electromechanical behaviors of the strain sensor. (a) The I–V curves of the device under different strain. (b) The relative change in resistance of the device in (a). The inset of (a) is the schematic illustration of the strain sensor. (c, d) Current response of the strain sensor device at different static strain state under a fixed bias of 10 V. (e) The current–time response curve of the strain sensor device that was fixed on a finger at four different bending and release motions under a fixed bias of 10 V. The upper insets I, II, III and IV display the four different finger motion states. Reprinted with permission from ref. 56. Copyright 2011 Wiley.

Chem. Soc. Rev.

In summary, recent advances have clearly demonstrated that the electrospinning technique affords us a remarkably simple and versatile method for the preparation of NPs–electrospun fibres with different functionalities. The composite fibres fabricated by a post-treatment of the as-prepared electrospun fibres or directly adding NPs into electrospun solution exhibit many superior properties. In particular, the applications in energy, sensor and mechanical enhancement have been highlighted in this review. The direct fabrication of composite fibres by electrospinning is facile and versatile, however, there are still several challenges in this field. For example, a large amount of NPs is needed to make them encapsulated within each fibre, while only some NPs can be synthesized on a large scale so far. Besides, as the NPs are encapsulated within polymer fibres, the NPs can be more stable, however, this also limits their applications, for example, for catalysis or sensing. Some rational methods are still needed to remove the outer shell and yet not disturb the assembled state at the same time. Besides, recently emerging primary research works on the self-assembling effect of electrospinning on the ordering of NPs has appeared, which is of high value. So far, although the nanomaterials have superior properties to those of their bulk counterparts, it still remains a challenge in assembling these individual nanocomponents into macroscopic materials on a large scale and retain the stability of the assemblies. Electrospinning fits well with the above conditions and has several advantages, i.e., the NPs can be readily assembled on a large scale and a free-standing film with high stability could be obtained. Until now, in nearly all reports, only one kind of 1D nanomaterial has been electrospun


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within the same fibre to investigate the assembly effect of electrospinning. Further work is needed to study the affect on the performance of the composite fibres when more than one kind of NPs are assembled via electrospinning, because a synergistic effect may occur among the NPs and also the matrix, and new or enhanced performance could be realized. Moreover, only some 1D NPs have been electrospun so far, and the scope needs to be extended further, and thus more functional or multifunctional nanocomposite materials are expected to be produced. In addition to enhancing and improving the device performance by controlling their morphology and composition, the state of the NPs (randomly or aligned) in the device should also be considered. Especially for energy and sensor applications, assembling of NPs may be conductive to the electron transport. For mechanical enhancement, further work needs to be done to create the interactions between NPs and matrix (as ‘‘binder’’). For example, organic molecules could be used to functionalize the NPs and act as a ‘‘bridge’’ to enhance the force between NPs and the matrix. Moreover, further textiling the fibres into bundles may increase the mechanical strength of composites. Although many technical issues still need to be resolved or improved upon, there is no doubt that electrospinning utilizing NPs can become one of the most powerful tools for fabricating functional 1D nanomaterials and composite materials with ideal compositions and structures. Combining and assembling NPs with fibres via the electrospinning technique will introduce a new field on fabrication of diverse multifunctional materials in the future.

Acknowledgements This work is supported by the National Basic Research Program of China (Grants 2010CB934700, 2013CB933900, 2014CB931800, 2012BAD32B05-4), the National Natural Science Foundation of China (Grants 91022032, 91227103, 21061160492, J1030412), the Chinese Academy of Sciences (Grant KJZD-EW-M01-1), the Fundamental Research Funds for the Central Universities, and the China Postdoctoral Science Foundation.

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