Hindawi Publishing Corporation

Advances in Materials Science and Engineering

Volume 2015, Article ID 356286, 19 pages
http://dx.doi.org/10.1155/2015/356286

Review Article
A Review of Electrospun Conductive Polyaniline Based
Nanofiber Composites and Blends: Processing Features,
Applications, and Future Directions

Saiful Izwan Abd Razak,1,2 Izzati Fatimah Wahab,2 Fatirah Fadil,3

Farah Nuruljannah Dahli,4 Ahmad Zahran Md Khudzari,1,2 and Hassan Adeli5
1
IJN-UTM Cardiovascular Engineering Centre, Institute of Human Centered Engineering, Universiti Teknologi Malaysia,
81310 Skudai, Johor, Malaysia
2
Faculty of Bioscience and Medical Engineering, Universiti Teknologi Malaysia, 81310 Skudai, Johor, Malaysia
3
Department of Chemistry, Faculty of Science, Universiti Teknologi Malaysia, 81310 Skudai, Johor, Malaysia
4
Department of Bioprocess and Polymer Engineering, Faculty of Chemical and Energy Engineering,
Universiti Tun Hussein Onn Malaysia, 86400 Batu Pahat, Johor, Malaysia
5
Department of Chemical Engineering, University of Mazandaran, Babolsar, Iran

Correspondence should be addressed to Saiful Izwan Abd Razak; saifulizwan@utm.my

Received 18 October 2015; Revised 9 December 2015; Accepted 10 December 2015

Academic Editor: Giorgio Pia

Copyright © 2015 Saiful Izwan Abd Razak et al. This is an open access article distributed under the Creative Commons Attribution
License, which permits unrestricted use, distribution, and reproduction in any medium, provided the original work is properly
cited.

Electrospun polymer nanofibers with high surface area to volume ratio and tunable characteristic are formed through the
application of strong electrostatic field. Electrospinning has been identified as a straight forward and viable technique to produce
nanofibers from polymer solution as their initial precursor. These nanofiber materials have attracted attention of researchers due
to their enhanced and exceptional nanostructural characteristics. Electrospun polyaniline (PANI) based nanofiber is one of the
important new materials for the rapidly growing technology development such as nanofiber based sensor devices, conductive
tissue engineering scaffold materials, supercapacitors, and flexible solar cells applications. PANI however is relatively hard to
process compared to that of other conventional polymers and plastics. The processing of PANI is daunting, mainly due to its rigid
backbone which is related to its high level of conjugation. The challenges faced in the electrospinning processing of neat PANI have
alternatively led to the development of the electrospun PANI based composites and blends. A review on the research activities of
the electrospinning processing of the PANI based nanofibers, the potential prospect in various fields, and their future direction are
presented.

1. Introduction of quite extensive studies worldwide, because they represent

the smallest dimensional structures with high aspect ratios
Nanostructures, structures of 100 nanometers (nm) or less and degrees of flexibility. Hence, these 1D nanostructures can
in size, have been at the forefront of much modern engi- be exploited as elements in nanodevices with various applica-
neering and science research due to their novel, significantly tions, such as supercapacitors, chemicals, and biosensors, in
improved biological, chemical, and physical properties. There optoelectronics and tissue engineering [1–7].
are mainly four types of nanostructures: zero- (0D) (e.g., Nanoapplication and nanotechnology development con-
nanoparticles), one- (1D) (e.g., nanowires, nanotubes, and tinues to stir the scientists’ and engineers’ interest in explor-
nanofibers), two- (2D) (e.g., films and layers), and three- (3D) ing the best combination of materials and 1D nanostruc-
dimensional structures (e.g., polycrystals). Among them, tures. A few examples of such combinations are carbon
one-dimensional (1D) nanostructures have been in the focus nanotubes, inorganic semiconductor nanowire/nanobelts,
2 Advances in Materials Science and Engineering

Publications on polyaniline nanofibers by electrospinning Publications from top 10 countries

180
China
160
United States
140 India
120 South Korea
100 Australia
80 Taiwan
60 Iran
40 Brazil
20 Japan
0 Singapore
2005 2006 2007 2008 2009 2010 2011 2012 2013 2014 0 100 200 300 400 500
(a) (b)

Figure 1: (a) Comparison of the annual number of scientific publications related to electrospinning polyaniline nanofibers. (b) Publication
distribution around the world based on data analysis carried out using the Scopus search system with the term “polyaniline nanofibers” as of
30 March 2015.

metallic nanotubes/nanowires, and conjugated polymer regeneration [14], sensory and electronic devices [15, 16],
nanofibers/nanotubes. The conjugated polymer 1D nanos- smart textiles [17], and membrane filtration technology [18].
tructures possess certain highly attractive characteristics, The viability of this technology is evidenced by the easiness
such as an easily controllable bandgap, high mechanical of the spinning procedure and the simplicity in the cus-
flexibility, and greater biocompatibility than that of many tom setting of the electrospinning machine [19]. The most
inorganic materials. In addition, conjugated polymers are amazing characteristics of electrospun nanofibers include
also called conducting polymers; in their neutral states their high surface area-to-volume ratio of well-interconnected
conductivity typically ranges from 10−10 to 10−5 Scm−1 . How- nanofiber webs, flexibility in surface functionalities, tuneable
ever, the conductivity can be enhanced into semiconductor surface properties, high permeability, and good mechanical
or conductor states via chemical or electrochemical redox performance [20].
reactions. Among the various classes of conducting polymers, The principle of the electrospinning technique was first
polyaniline (PANI) is one of the most widely investigated described by Zeleny in his published work on the behavior
materials due to its easy synthesis, excellent optical and mag- of fluid droplets at the end of metal capillaries [12, 21]. Later
netic properties, and environmental stability. Furthermore, on, in 1934, Formhals established the commercialized system
the electrical properties can be controlled by the oxidation of electrospinning for the fabrication of textile yarns in the
and protonation state [8–10]. U.S. Patent number 195704 [22]. Further development of the
Up to now, a large number of approaches have already electrospinning from a polymer melt rather than a polymer
been demonstrated for preparing PANI 1D nanostructures, solution using an air-blast to assist the fiber formation was
including chemical routes, such as hard physical template- patented by Norton in 1936 [23]. The conceptual principle of
guided synthesis and soft chemical template synthesis (inter- electrospinning is based on the employment of the external
facial polymerization, template-free method, dilute polymer- electric field to induce polymer fluid transformation by
ization, and reverse emulsion polymerization), and a variety the elongation and whipping of the jet. In 1969, Taylor
of lithography techniques [11]. However, the drawbacks of introduced the concept of Taylor cone, which is associated
these techniques, such as required postsynthesis process, with the deformation of a liquid surface into a characteristic
relatively poor control of the size and morphology unifor- shape induced by electric fields. The theoretical analysis of
mity, poorly oriented nanostructure arrays, and high costs, the disintegration of liquid drops under electric fields, the
may limit their production on a large scale. Thus, physical calculation of the electric force acting on fine jets, of the
approaches such as electrospinning are more advantageous critical voltage for inducing jet elongation, and of the voltage
compared to the above-mentioned techniques, especially for values at which Taylor cones are formed have been described
the mass production of continuous nanofibers. in detail elsewhere [24]. Recently, the electrospinning process
has gained huge popularity and the publication of works
related to electrospinning increased over the years. A survey
2. Electrospinning Theory and Principle of the publications related to electrospun PANI nanofibers
in the past 10 years is given in Figure 1(a), whereas the
Electrospinning is a fiber-spinning technology used to pro- distribution of these published works among the 15 top
duce long, three-dimensional, ultrafine fibers [12]. Accord- countries is shown in Figure 1(b). These literature data were
ing to Frenot and Chronakis, nanofibers produced by the obtained based on the Scopus search system.
electrospinning technique have diameters in the range of The data clearly demonstrate the progression of research
100 nm to 1 𝜇m and lengths of up to kilometers [13]. Elec- works associated with PANI nanofibers obtained by means
trospun nanofibers present high attractiveness for diverse of electrospinning. The increase in the number of patents for
applications, such as in biomedical engineering and tissue diverse applications gives an insight into the recognition of
Advances in Materials Science and Engineering 3

the electrospinning process. A few companies, such as Espin

Technologies, Nano Technics, and KATO Tech, are constantly
benefited by the utmost features of nanofibers derived by elec- High voltage
trospinning, while companies such as Donaldson Company − +
and Freudenberg have already applied the electrospinning Syringe
process in their air filtration products [25, 26]. pump
Solution
In this report, a comprehensive review is made on the
research and developments related to PANI electrospun
nanofibers, including a discussion of their processing char-
Needle
acteristics, physical and mechanical properties, and applica-
tions. Other basic issues regarding the processing limitations
of the materials and research challenges are also considered
in detail.

3. Technical Aspects in Electrospinning

Collector
From the technical point of view, the electrospinning
equipment is an inexpensive, robust, and straight forward
nanofiber production machine. The standard laboratory elec-
trospinning unit generally consists of a metallic capillary
Figure 2: Schematic diagram of an electrospinning setup.
needle, which is connected to a high voltage power supply
and a grounded collector. The droplet of the spinning solution
is discharged from the capillary needle in a continuous flow
and the feed rate of the discharged droplet is adjusted using
solution depends on several aspects during the electrospin-
an infusion syringe pump. The electric field is applied at the
ning process [28]. These parameters can be classified into
tip of the capillary needle attached to the syringe, which is
major and minor influences. Major parameters comprise the
loaded with a spinning polymer solution, to induce an electric
viscosity, elasticity, and surface tension of the solution, the
charge on the surface of the hanging droplet. The hanging
hydrostatic pressure of the solution in the capillary needle,
droplet of the spinning solution, which is held by its surface
the electric potential, and the distance from the capillary tip
tension, will then develop mutual charge repulsion due to
to the collector, while ambient influences, such as solution
excess pressure. The contraction of the surface charges to the
temperature, humidity, and air velocity, can be classified as
counter electrode is caused by a force directly opposite to the
minor parameters [29].
surface tension of the liquid droplet [27].
The difference in the viscosities and surface tension of
The hemispherical droplet gets distorted into a conical
the spinning solution will directly influence the morphol-
shape, also called Taylor cone, under the simultaneous
ogy of the nanofibers formed either as an ultrathin or as
actions of the repulsive force. Additional increases in the
beaded fibrous structure. A spinning solution with viscosity
electric field intensity will lead to reaching a critical value,
values of 1–20 poises and surface tension values of 35–55
whereby the repulsive electrostatic force overcomes the sur-
dynes/cm2 is regarded as a suitable solution for inducing fiber
face tension of the charged droplet to initiate the polymer jet
formation. Meanwhile, at viscosities of above 20 poises, the
formation. The initiation of the polymer jet then undergoes
electrospinning is hindered because of the flow instability
an instability and elongation process simultaneously with the
caused by the high cohesiveness of the solution. As the
evaporation of the solvent.
viscosity of the solution is increased, the beads become larger,
A grounded collector, which receives the deposition of
the average distance between beads is longer, and enlarged
the electrospun nanofibers from the charged jets, is placed
fiber diameters are observed. The formation of beaded fibers
at a certain distance from the metallic capillary needle [20].
occurs because the fluid jet breaks up to droplets when the
A schematic diagram of the electrospinning system with the
viscosity of the spinning solution is below 1 poise [30].
basic setup of a syringe pump attached to a disposable syringe
The need for a volatile solvent as carrier of a particular
with a metallic needle, a high voltage power supply unit, and
polymer in solution spinning is one of the fundamental
a collector is Figure 2.
criteria in electrospinning. Solvent selection is of paramount
Multiple aspects of the electrospinning process have been
importance in determining the critical minimum solution
demonstrated to affect the morphology of the nanofibers [27].
concentration to allow the transition from electrospray-
The major factors involved in the electrospinning process,
ing to electrospinning, thereby significantly affecting solu-
including the polymer solutions, needles, and collectors, are
tion spinnability and the morphology of the electrospun
discussed below [20].
nanofibers [31]. Bahrami and Gholipour Kanani studied the
effects of using solvents of different polarity, such as glacial
3.1. Polymer Solutions. When dealing with the spinning acetic acid, dimethyl formamide, glacial formic acid, and
solution of various polymers, the spinnability of the polymer acetone, on the morphology of polymer nanofibers. It was
solution becomes an utmost issue. The spinnability of a reported that the use of glacial acetic acid as solvent resulted
4 Advances in Materials Science and Engineering

nanofiber structures can be obtained if the nonuniform

deposition of the fiber can be taken under control [35]. Bi-
component mingled structured fibers have been obtained
by merging side-by-side two spinnerets fed by separate
reservoirs [36, 37]. In spite of the fact that the electrospinning
of polymer blends can be achieved by spinning a mixture
Polymer A Polymer A Polymer A of polymers or polymer solutions through a single needle,
Nanoparticles Polymer B Polymer B the electrospinning setup can be designed as having two
(a) (b) (c)
separate reservoirs to avoid some limitations related to the
physics of polymeric blends. Indeed, with a single spinneret,
Figure 3: Schematic cross-sectional view of different structures a homogenous, thermodynamically stable mix of polymers
of composite nanofibers: (a) randomly blended; (b) core-shell; (c) is required and, in the case of solution electrospinning, the
mingled structure generated from various capillary needle nozzles. interactions between the polymer and the solvent of the
opposing pair become crucial. With two separate spinnerets,
the polymer solutions will come in contact only at the end of
the spinning head, allowing for the formation of blend fibers
in the formation of fibers with nonuniform diameter distri- that are otherwise difficult to produce [35].
bution compared to other solvents. This was due to the high
dielectric constant value of acetic acid [32]. Generally, the net 3.3. Collectors. The collector used during the electrospinning
volume charge density is proportional to the conductivity of process would also have an influence on the morphology of
the solution. As the net charge density increases, the beads the electrospun nanofibers. At first, the randomly oriented
become smaller and change more into a spindle-like shape, nanofibers were obtained by collecting the fibers from a
while the fibers become thinner. The neutralization of the stationary collector covered with aluminium foil [38]. An
charge carried by the polymer jet favors the formation of attempt to obtain the deposited nanofibers in aligned orienta-
beads, as the tension in the fiber depends on the net charge tion has been made as the electrospinning jet was shown to be
repulsion and the interaction of the net charge with the sensitive to small differences in the electric field profile [39]. A
electric field [30]. One of the effective approaches to adjust conductive collecting substrate with textured surface, such as
the solution conductivity is by introducing a conductive wire mesh or grid, has been used to form patterned/textured
nanoparticle filler or soluble salt into the solution. The nanofibers due to its electric field profile. The results show
incorporation of salt increased the stretching of the polymer that, in a typical square grid substrate, the nanofibers are
jet and reduced the incident of bead formation by distributing preferentially deposited along the wire. As the wires forming
the charges consistently to control the fiber uniformity [33]. the grid are narrow, nanofibers tend to align along the length
of the wire. Despite the preferential deposition of fibers on
3.2. Needles. Typical electrospun nanofibers represent solid the wire, the presence of numerous neighbouring wires often
fibers, with a smooth surface and circular cross section. diverts the electrospinning jet to the adjacent wire. This
However, electrospun nanofibers can also be designed and results in a higher density of the nanofibers that are aligned
fabricated in hybrid form of either basically random blending on the wire, compared to the randomly oriented, scattered
with nanoparticles or ordered structures, such as core-sheath nanofibers at the gaps between the wires. Where the distance
and mingled nanofibers, which consist of two distinctive between the wires gets larger, the difference in the density of
polymers. Various designs of electrospun nanofibers were the nanofibers on the wires as well as the space between the
successfully created by the use of different needles (coelec- wires also increases. There are differences in fiber diameter
trospinning). These needles allow the formation of different as well whereby the nanofibers deposited on the wire are
fiber morphologies that could be exploited for specific uses larger than the nanofibers deposited within the space between
[34]. Cross-sectional views of electrospun nanofibers in the the wires. This has been attributed to a greater stretching
forms of randomly blended structure, core-shell structure, of the nanofibers when they span across the space [40].
and mingled structure are illustrated in Figure 3. Other forms of patterned substrates have also been used to
For the randomly blended nanofibers, a standard needle collect the deposited nanofibers. Grids made out of parallel
with only one capillary opening is used. However, core-shell wires close to one another have been shown to form dense,
and mingle structured nanofibers need to be spun using a aligned nanofibers between the wires, while a substrate with
special needle, respectively, as shown in Figure 4. For the arrayed pins gives rise to a membrane made out of radiating
core-sheath needle, two capillary openings are combined: nanofibers between the pins [41]. More grid variations, such
the outer capillary opening is for the sheath and the inner as having a pin in the center of a round space, form a pattern
capillary opening is for the core, while for mingled structured where fibers radiate from the central pin to the edges.
nanofiber formation, the use of a coaxial needle with double Furthermore, aligned nanofibers have also been success-
or multiple capillary openings in a parallel axis is needed. fully collected by using a rotating disk collector instead of the
Nonetheless, the same difficulties in optimizing the solu- stationary collector. The adjustment of the take-up velocity of
tion properties are still present and affecting the multineedle the rotating disk collector proved that the fiber alignment was
design of the electrospinning system. However, since the a direct function of the velocity of the rotating disk. Adding
throughput is not a primary concern in this case, interesting to this, as the take-up velocity increased from 630 m/min
Advances in Materials Science and Engineering 5

(a) (b)

(c) (d)

(e)

Figure 4: Various needles used in electrospinning: (a) standard needle; (b) inlet of standard needle; (c) core-sheath needle; (d) inlet of core-
sheath needle; (e) coaxial needle with double or multiple capillary openings.

to 1890 m/min, the average diameter of aligned nanofibers for the industrial production scale. This employs a new design
decreased from 700 to 350 nm [42]. for supplying the solution to a metal roller spinneret. The
advantage of this setup is its ease of scaling-up for increased
3.4. Configurations. There have been established several output. By using the needleless electrospinning system, the
types of arrangement of the electrospinning setup, such as possibilities to increase the production rate of nanofibers are
horizontal, vertical upward, and vertical downward. For each higher due to the use of multiple jets. The productivity rate
type of arrangement, the positioning of the components is has been significantly enhanced within 30 times higher than
different. Figure 5 illustrates the arrangement of the electro- that of the single needle electrospinning [43].
spinning in detail. For a small laboratory scale use, the vertical The setup of the needleless electrospinning system was
downward setup of the electrospinning is the most favorable firstly introduced in 2010 by Tang et al., whereby the nano-
due to the simple optimization and operational monitoring. fibers could be produced in abundance by applying the
Most of the research works on the electrospinning method of splashing the polymer solution onto the surface
nanofibers completed in the universities are based on the of a metal roller spinneret [44]. In this method, the setup
conventional single needle. One of the main issues to over- consisted of a metal roller spinneret as the positive electrode
come in this case is the limited throughput obtainable by connected to a high voltage power supply. The polymer
the use of such a system. For industrial applications, despite solution droplets were splashed onto the surface of the metal
the commonly high added value of the products, the low roller spinneret through the holes of the solution distributor,
production rate (in the order of tenths of grams per hour) which was located above the spinneret. Niu et al. [45] impro-
of the basic electrospinning apparatus represents a severe vised the system by inventing a spiral coil setup and proved
restraint. Therefore, the mass production of the nanofibers by that this method had a higher fiber production rate and
using the conventional single needle is tough and challenging. allowed better control of fiber morphology. Considering the
For this reason, the vertical upward setup has been selected above information, one can conclude that various designs and
6 Advances in Materials Science and Engineering

Solution tank/bath

(a) (b) (c)

Figure 5: Common configuration of electrospinning: (a) vertical; (b) horizontal; (c) needleless.

dimensions of nanofibers can be tailored by using different applications, including electric devices, flash welding [55],
kinds of electrospinning setup. sensors and actuators [57, 58], rechargeable batteries [59],
electromagnetic shielding devices, and anticorrosion coating
[60].
4. Basics of PANI The neutral intrinsic redox states of PANI can vary from
Conducting polymers are the fourth generation of polymeric that of the fully reduced leucoemeraldine to that of the
materials. This type of nanofibers has become the source for fully oxidized pernigraniline. The 50% intrinsically oxidized
the development of films, membranes, and nanoelectrodes polymer has been termed emeraldine and the 75% intrinsi-
for sensor applications. Their electrical conductivities can be cally oxidized polymer is called nigraniline [61]. Unlike most
increased by many orders of magnitude from 10−10 –10−5 to other polyaromatics, the fully oxidized state of PANI is not
102 –105 Scm−1 upon doping [8, 46], which covers the whole conducting. PANI becomes conducting when the emeraldine
insulator-semiconductor-metal range. A variety of con- base is protonated and charge carriers are generated. The
ducting polymers, such as polyaniline (PANI), polypyrrole process is generally termed protonic acid doping. The green
(PPY), poly(p-phenylene-vinylene) (PPV), poly(3,4-ethylene protonated emeraldine salt has conductivity by many orders
dioxythiophene) (PEDOT), and other polythiophene deriva- of magnitude higher than that of common polymers, but
tives, which can be synthesized into 1D nanostructures, such lower than that of typical metals.
as nanotubes and nanowires, have recently attracted attention PANI is composed of the repeating units of the aniline
in the areas of nanoscience and nanotechnology due to their monomer connected to form a backbone. The existence
special conduction mechanism, unique electrical properties, of a nitrogen atom lying between the phenyl rings allows
reversible doping/dedoping process, controllable chemical the formation of different oxidation states, which can affect
and electrochemical properties, and processability [46–48]. its physical properties. PANI has an inherently unstable
PANI has the longest history among the intrinsically backbone, resulting from the formation of alternate single
conducting polymers. It is one of the oldest artificial con- and double bonds along the monomer units during polymer-
ducting polymers and its high electrical conductivity among ization. The delocalized 𝜋 bonding electrons, produced across
organic compounds has attracted continuing attention. PANI the conjugated backbone, provide an electrical pathway for
has become one of the most attractive conducting poly- mobile charge carriers, which are introduced through doping.
mers due to its high stability, ease of synthesis, feasibility Consequently, the electronic properties of PANI, as well as
of electrical conductivity control by changing either the many other physicochemical properties, are determined by
protonation state or the oxidation state, and the low cost of the structure of the polymer backbone and the nature and
the aniline monomer [49–51]. PANI is either totally insulating concentration of the dopant ions [62–64].
or electrically conductive, depending on the oxidation state
and protonation level. Only in the intermediate oxidation 4.1. PANI Nanostructures. Much effort has been put in
state, the protonated emeraldine form is conductive. The fully synthesizing nanostructured PANI, owing to its novel phys-
reduced leucoemeraldine and fully oxidized pernigraniline ical properties and potential applications. Generally, PANI
are insulating materials [52]. PANI nanofibers have received nanostructures can be synthesized using hard- and soft-
much attention due to their superior properties compared template methods. In the hard-template method, which
to the conventional bulk PANI or PANI film [53–55]. PANI typically employs an aluminium oxide or track-etched mem-
nanofibers show enhanced water processability [56] and brane as a template for synthesizing the desired material,
improved acid-base sensitivity and response time when they polymerization occurs within the pores of the membrane
are exposed to chemical vapor, due to their large surface area. [65]. However, a rather tedious postsynthesis process is often
On the other hand, PANI nanofibers have numerous potential required to remove the template used [66]. Additionally,
Advances in Materials Science and Engineering 7

preformed PANI nanostructures may be destroyed or form to test suitability of electrospinning PANI solutions with-
undesirable aggregates when released from the template. The out blending with other polymers. Being an intrinsically
soft-template method uses micelles, emulsions, liquid crys- conducting polymer, PANI is available only in the form of
tals, and surfactant gels to synthesize PANI nanostructures low molecular weight, which thus makes the elasticity of
[67]. its solution insufficient for being electrospun directly into a
The soft-template method [68, 69] is a simple self-assem- fiber form [79]. Previous studies reported the formation of
bly method. By controlling the synthesis conditions, includ- structures like beads and droplets in electrospun fibers with
ing the temperature and molar ratio of monomer to dopant, low elasticity [92]. The use of an insulating copolymer with
PANI nanostructures can be prepared by in situ doping poly- high molecular weight is expected to act as a remedy, impart-
merization in the presence of protonic acids as dopants. In the ing greater elasticity. However, the presence of a noncon-
self-assembled formation mechanism using this approach, ducting polymer strongly modifies the physical and chemical
the micelles formed by dopant and/or monomer dopant act properties of PANI and eventually limits its applications
as soft templates in the process of forming tubes/wires [68]. [93]. The fabrication of one-dimensional polymer field effect
Up to now, a variety of PANI micro/nanostructures, such as transistor using PANI nanofibers blended with polyethylene
micro/nanotubes [70, 71], nanowires/fibers [72–74], hollow oxide (PEO) in a 1 : 1 ratio has been reported. The addition
microspheres [75, 76], nanotube junctions, and dendrites of PEO resulted in a drastic decrement of conductivity. The
[77], have been prepared by the soft-template method. How- results of the study allow the conclusion that reducing the
ever, this method allows for a limited range of chemicals to PEO content will enhance the conductivity of the electrospun
be used. Moreover, to date, some novel template-less methods PANI by one or several orders of magnitude [94].
have also been developed to shape PANI into nanofibers, such In order to improve the electrospinning processing of
as electrospinning [78]. The nanofibers can be synthesized by PANI, a few routes have been developed to find the optimum
deposition directly onto aluminium foil, wire mesh, cotton, conditions to produce pure PANI nanofibers with smooth
silk, wool, and Si wafer. fiber surface and to minimize the limitations caused by
Many other methods have also been recognized for nonconducting copolymers. The objective was not only to
the synthesis of PANI nanostructures. PANI films with a produce neat PANI, but also to obtain fibers that would raise
thickness of 20 to 100 nm have been prepared using vapor- no concern about side reactions with the doping agents when
phase polymerization [79]. Vapor-phase polymerization is exposed to external agents, such as radiation or gases [93].
one of the self-assembling techniques that do not require a In their study, Cárdenas and coworkers proposed using a
host polymer. It utilizes the organic arrangement of macro bath collector in the fabrication of PANI nanofibers by the
molecules to fabricate nanolayered aggregates and thin films electrospinning procedure. In this research work, the fibers
[80, 81]. There are also reports on the preparation of nanos- were not deposited on the grounded collector, but instead
tructured PANI through polymerization in the presence of they were collected in an acetone bath placed on an electrode
functionalized protonic acid, which acts as both surfactant [95]. Although there are a number of literature sources [96–
(and emulsifier) and protonating agent. This process is called 98] describing the preparation of PANI based nanofibers by
emulsion polymerization and it combines aniline, protonic electrospinning, the reports on the fabrication of neat PANI
acid, and oxidant with a mixture of water and a nonpolar or nanofibers by electrospinning are quite rare. In an earlier
weakly polar liquid [82–84]. In order to avoid agglomeration study, MacDiarmid et al. [99] produced nanofibers of PANI
of thick fibers into irregularly shaped particles, Jing and by a method similar to the one used by Cárdenas et al. and
coworkers have employed ultrasonic radiation to assist the achieved fibers with an average diameter of 139 nm by placing
conventional dropwise addition of the oxidant solution to a 20 wt% solution of PANI in 98% sulfuric acid in a glass
form PANI nanofibers [85]. Ultrasound assistance through pipette fixed above a copper cathode immersed in pure water.
the sonochemical method is an important approach, allow- However, in contrast to this work, Cárdenas and coworkers
ing researchers to obtain uniform dispersion of inorganic used a solution with a considerably lower concentration of
nanoparticles in the polymer matrix [86, 87]. Another popu-
PANI (1 wt%) in acetone [95].
lar method to obtain PANI nanofibers is through interfacial
By using the acetone bath as collector, the excess solvent
polymerization, which involves step polymerization of two
was diffused into acetone and allowed fiber formation. Thus,
reactive agents or monomers that dissolve in two immis-
the researchers have successfully fabricated neat submicron
cible phases. This technique allows the synthesis of high
crystallinity PANI nanofibers without depending on specific PANI using the electrospinning technique without incor-
templates and solvents [60, 88–90]. poration of high molecular weight polymer blending for
gaining jet stability to form fibers. The fibers produced by this
technique were observed and characterized under scanning
5. PANI Nanofibers via Electrospinning electron microscope (SEM). These fibers appeared as isolated
5.1. Neat PANI Nanofibers. Producing neat PANI nanofiber on the substrates, which is favorable for electrical charac-
by electrospinning has been a great challenge for researchers. terization. The conductivity of these fibers was measured
The poor solubility of PANI in common solvents interferes to be in the order of 10−3 to 102 Scm−1 , with a significant
with electrospinning into uniform fibers, requiring it to be increasing trend in the conductivity values. Nanofibers with
doped by organic acids in order to increase its solubility in larger diameters have higher values of conductivity due to
organic solvents [91]. Various methods have been attempted their increased volume to surface ratio. A higher volume
8 Advances in Materials Science and Engineering

to surface ratio causes a relatively slower loss of solvent by doped Si/SiO2 wafer. The average diameter of these fibers was
evaporation and consequently the fibers will be more partially measured to be approximately 10 𝜇m.
doped and conductive.
The method of utilizing the copper cathode immersed 5.2. Blend PANI Nanofibers. Since PANI is difficult to be elec-
in pure water was done by placing 20 wt% of PANI solution trospun, most researchers tend to incorporate it with other
with 98% sulfuric acid inside the glass pipette. The fibers polymers to make it electrospinnable. Chen and coworkers
were collected either in or at the surface of water. In this studied the incorporation of PANI into poly(𝜀-caprolactone)
study, the authors reported having successfully produced and gelatin for orthotopic photothermal treatment of tumors
fibers with average diameters of 139 nm, the conductivity of in vivo [102]. Apart from having the right polymer can-
single fibers being of about ∼0.1 Scm−1 . It is noticeable that didates to be blended with, the solvent used during the
the morphology of the nanofibers deposited in the solution electrospinning solution preparation is also an important
bath was less smooth as compared to that of the nanofibers parameter to consider. Fryczkowski et al. investigated the
collected from the conductive surface [99]. effect of two different solvents in preparing electrospinning
Another study by Shie et al. [100] reported the use of solution of poly(3-hydroxybutyrate) (PHB) with PANI [103].
the emeraldine base of poly(o-methoxyaniline) powder dis- Their study showed potential biodegradability and interest-
solved in a mixture of tetrahydrofuran/dimethylformamide ing electrical properties for PHB-PANI. THF gave better
(THF/DMF) for the preparation of the electrospinning solu- nanofiber structure and properties than chloroform. PHB-
tion. A 5 wt% solution of PANI was loaded into a plastic PANI in THF solvent produces smooth, uniform, and fine
syringe connected with a metallic needle, with a rate of nanofibers with enhanced hydrophilicity and low electrical
0.02 to 0.04 mL/min. PANI nanofibers were collected on resistance.
an aluminum plate placed 8 to 14 cm below the tip of the Karim utilized dual approach to incorporate PANI
needle. The samples were then heated at 100∘ C for an hour. aligned nanofibers [104]. The first step was to synthesize PANI
The biocompatibility of these fibers was examined through copolymer with o-aminobenzenesulfonic acid (PANI-co-
various analyses: agar diffusion test, nitric oxide assay, and PABSA) by in-situ polymerization. Then, polyvinyl alcohol
cell and protein adsorption. The study found that the PANI (PVA) and chitosan oligosaccharide (COS) were blended into
nanofibers fabricated by electrospinning possessed negligible the PANI copolymer to make the electrospinning solution.
cytotoxicity, low serum protein absorption, and low myoblast XRD and FTIR data from this study displayed the existence
cell attachment. The percentage of myoblast cell attachment of hydrogen bonds between PANI-co-PABSA, COS, and PVA
to PANI was measured to be 40.7% ± 6.1%, while cell molecules that may possibly cause weak interaction in COS.
adhesion to electrically stimulated PANI was 53.6% ± 9.9%, Sharma et al. made use of electrospun fibers to fabri-
which was slightly higher. The results indicated that the cate scaffolds for three-dimensional cell culture [105]. They
cells on the PANI fibers exhibited a significantly extended reported fabrication of PANI/poly(N-isopropyl acrylamide-
lag phase of growth, while after electrical stimulation, the co-methacrylic acid) (PANI-CNT/PNIPAm-co-MAA) com-
cells demonstrated higher proliferation on PANI fibers. The posite by 1 : 1 ratio of polymers. This study suggests possible
decreased cell growth on PANI fibers was probably caused by use of conducting nanofibers as scaffolds after obtaining
the hydrophobic surface of the fibers. A significant increase better cell survival than the control and PNIPA-co-MAA
of nitric oxide secretion by PANI nanofibers shows that without PANI. An effort to replace high cost platinum for
oxygen reduction reaction catalyst in polymer electrolyte fuel
these fibers may elicit a slight inflammatory response after
cells with cheaper material has been done by Zamani et al.
implantation in the human body. The response index of the
by having metal-polymer blend [106]. 10 wt% PANI addition
toxicity assay of PANI to myoblast cells was nearly zero
into polyacrylonitrile (PAN) was firstly used as nanofiber
after electrical stimulation. Thus, it is suggested that PANI
catalysts and it provides improvements to half-wave potential
nanofibers can be considered to be very suitable for biosensor and ORR onset potential by 70 and 100 mV.
applications. PANI nanofibers with the least amount of PEO, within
A study of electrospun PANI doped with 2-acrylomido- the range of 1 to 0% w/w of PEO content, with respect to
2-ethyl-1-propanesulfonic acid (AMPSA) indicated a con- the amount of PANI, revealed some defects such as bead
ductivity of 0.76 Scm−1 , suggesting that the fibers were in formation [94]. Such drawbacks could be explained by the
the metallic regime of charge transport since there was no capillary instability of the spinning jet by surface tension as
apparent field effect [101]. Pure PANI fibers were prepared by PEO concentration was decreased [107]. Compared to PEO,
dissolving 900 mg of PANI-AMPSA into 3 mL concentrated polystyrene (PS) was found to be more suitable for blending
sulfuric acid and manually stirring the solution with a glass with PANI, as it led to the formation of thinner PANI
rod for 30 min. The solution was then held in a glass pipette nanofibers, with a diameter below 100 nm. Thus, electro-
placed vertically 3 cm over a beaker of water acting as spinning the blended polymer solution produced nanofibers
cathode. A value of 15 kV of spinning voltage was successfully with fewer defects, as compared to PEO, and presented
applied, leading to the deposition of fine fibers in an erratic better homogeneity of the blend in chloroform. Since the
manner due to the spark formation at the end of the pipette. conductivity of PANI is very sensitive to the amount of
Fibers deposited on the surface of water and were left in defects, it is reasonable to suggest that the overall conductivity
the water for 12 h to allow the sulfuric acid to dissolve in of the PANI/PS blends will be higher than that of the
the water. The fibers were then collected on a degenerately PANI/PEO blends [91].
Advances in Materials Science and Engineering 9

5.3. Core/Shell PANI Nanofibers. As mentioned earlier, coax- in the array form and able to emit green light under ultraviolet
ial spinning can also be used in producing PANI nanofibers. light due to the sheath properties. The ability to tune the
This kind of electrospinning uses two spinnerets, which emission colour of their belt-shaped coaxial microcables by
allows the low elastic fluid to elongate along with the changing the europium/terbium (Eu+ /Tb+ ) molar ratio in
electrospinnable fluid [108]. This process will result in the sheath was shown by Shao et al. [114]. They were able to
nanofiber with continuous core-sheath morphology. Zhang tune the emission in wide range of red-yellow-green colour
and Rutledge produced fibers of PANI with a dopant of by adjusting the mass ratio of Eu+ /Tb+ complexes, PANI
camphor-10-sulfonic acid (HCSA) blended with poly(methyl content, or Fe3 O4 nanoparticles content.
methacrylate) (PMMA) in core-sheath form through coaxial An attempt by Liu et al. has led to an interesting way to
electrospinning. 100% doped-PANI fibers were then obtained fabricate PANI nanofiber with hollow structure through elec-
by immersing the fiber blend into isopropyl alcohol. This trospinning [115]. Instead of using the conventional coaxial
was done to remove the PMMA shells and thereby release electrospinning, the technique did not require the prepa-
the doped PANI cores. Due to the removal, the diameters ration of two different electrospun solutions. The one-
of the fibers decreased from 1440 ± 200 to 620 ± 160 nm pot electrospinning requires only one mixture of solu-
[109]. tion favored for the shell and eventually directly produces
Coaxial electrospinning is also useful in incorporating hollow electrospun nanofibers. Liu et al. fabricated tri-
conducting material onto nonconducting fibers (or vice functional (photoluminescent-electrical-magnetic) flexible
versa) with distinct layers. Sarvi et al. applied this approach Eu(BA)3 phen/PANI/Fe3 O4 /PVP hollow nanofibers. The hol-
to develop nanofibers with superior piezoelectricity [110]. The low nanofibers had outer diameters of 305 nm and inner
nanofibers consist of poly(vinylidene fluoride) (PVDF) as the diameters of about 140 nm. The team also used the same
core and blends of PVA, PANI, and multiwalled carbon nan- procedure to produce Tb(BA)3 phen/PANI/Fe3 O4 /PVP hol-
otube (MWCNT) as the shell. Low concentration of PVDF low nanofibers with 238 nm outer diameters and 80 nm
was added to PANI solution in order to increase the viscosity inner diameters [116]. There are also few studies that
of relatively dilute PANI solution so that it is electrospinnable. reported hollow structured PANI nanofibers. However, those
MWCNT was reported to significantly reduce the electrical nanofibers were obtained by the in situ polymerization of
percolation concentration in the polymer matrix. It was aniline on the surface of as-prepared electrospun nanofibers
shown that the presence of PANI component increased the and followed by removal of the electrospun template
PVA nanofibers mat formation by bridging the nanofibers [117].
and eliminating air resistance. Lv and coworkers applied electrospinning using spe-
An attempt to develop a material extraction for in vivo, cially designed parallel dual spinnerets to prepare bistrand-
semisolid tissue extraction has been done by preparing aligned nanobundles [118]. These nanobundles consisted
polystyrene (PS)/PANI core-sheath electrospun nanofibers of PANI/PVP as one strand and Eu(BA)3 phen/PVP as
[111]. Wu et al. have previously worked on PS/crosslinked another strand, where the PVP acted as template. They are
collagen core-sheath for same purpose but the PS core the first to record this procedure of obtaining bistrand-
showed poor extraction ability to very polar and ionic aligned nanostructures. They suggested better connectivity of
compounds [112]. The collagen sheath failed to extract and PANI in the nanofibers and effective isolation of rare earth
therefore would prevent the permeation of tissue matrix into complex.
PS core. Later, they successfully modified the fabrication The mechanical properties of electrospun nanofibers are
of core-sheath by having PANI sheath. Neat PANI sheath generally considered important for many applications. In
was obtained after selectively removed collagen content in spite of this, nanofibers are often found to be mechanically
as-spun collagen/PANI sheath. This approach gave better
poorer than their corresponding textile fibers made from the
efficiency for acidic phytohormones and high extraction
same polymers. Their tensile strength and Young’s modulus
capacity.
were reported below 300 MPa and 3 GPa, respectively [119–
5.4. Modified Core/Shell PANI Nanofibers. Various modifi- 121]. To produce high-performance electrospun nanofibers,
cations can also be done to produce variations of core/shell the molecular structures of the nanofibers should be oriented
structure. Spinneret of coaxial electrospinning can be with chain extension along the fiber axis [122]. PANI on the
designed to produce novel ribbon-shaped flexible nanocables other hand usually acts as conducting particulate filler in
[113]. The modified spinneret allows the core for the nanorib- a suitable matrix, which provides the required mechanical
bon to be adjusted to achieve preferred diameters by changing properties. Valentová and Stejskal [123] characterized the
the inner needle. This novel technique could fabricate tunable mechanical properties of PANI on compressed circular pel-
core composed of PANI, magnetic Fe3 O4 nanoparticles, and lets instead of conventional rectangular or dumbbell shaped
PMMA template with photoluminescent Tb(BA)3 phen (BA samples. It was found out that the dynamic mechanical prop-
= benzoic acid, phen = 1,10-phenanthroline). The nanoca- erties of the compressed pellets depended on the compres-
ble array produced electrically conductive-magnetic bifunc- sion pressure. The samples became brittle at a compression
tional core with insulating photoluminescent sheath. The pressure above 800 MPa. PANI fibers were wet spun from a
conductive cores possessed electrical conductivity at the solution of PANI containing solvent. It was found that the
order of 10−2 Scm−1 and may give response in magnetic field. coagulant temperature had significant effects on the strength
These conductive nanocables were insulated with each other and modulus of the spun fibers [124].
10 Advances in Materials Science and Engineering

6. Applications of Electrospun such as tensile strength, elasticity, conductivity, and electron

PANI Nanofibers microscopy analysis, have confirmed the uniformity of the
ensued PANI nanofibers.
Researchers have been investigating the potential applica- The preliminary study of PANI nanofibers in tissue engi-
tions of electrospun PANI nanofibers in various fields. This neering [143] was then supported with the research works
section highlights opportunities of using electrospun PANI developed by Ghasemi-Mobarakeh et al. It was explained
nanofibers for emerging technology development, such as in that the surface modification and functionalization of PANI
the conductive scaffolding materials, controlled drug delivery with different biomolecules or dopants allowed them to be
systems, nanofiber based sensor devices, supercapacitors, and modified with biological sensing elements and to exhibit dif-
flexible solar cells. ferent signaling pathways required for cellular processes. In
this way, significant enhancement in cell proliferation and dif-
6.1. Tissue Engineering and Drug Delivery. Tissue engineering ferentiation was observed through the employment of PANI.
is a method to fabricate new tissue from cultured cells. Thus, conductive PANI provides an excellent opportunity for
Biodegradable scaffolds can act as transplant vehicles for the fabrication of highly selective, biocompatible, specific,
cultured cells and templates to guide tissue regeneration [125, and stable nanocomposite scaffolds for tissue engineering of
126]. The scaffolds should be biocompatible, biodegradable, different organs [146].
and highly porous; they should have large surface area and The effect of PANI nanofiber orientation on electrical
highly interconnected pore structure with suitable pore size conductivity has also been reported by Ku et al. [147] who
[127, 128]. PANI is one of the conducting polymers that prepared electrospun PANI nanofibers in a PCL blend.
have the potential to be applied as conductive substrates for Rotor speed was adjusted to 100 rpm for collecting ran-
tissue engineering [129–131]. It is only quite recently that dom nanofibers and to 900 rpm for aligned nanofibers. The
the tunable electroactivity of PANI has been explored in the electrospun PANI/PCL nanofibers with both random and
area of diverse biological applications, such as for scaffolds in aligned orientation were used as scaffolds for the growth of
the tissue engineering field. Still, the investigation of PANI C2C12 mouse myoblast cells. The diameter of the blended
for such applications has already brought evidence of the PANI-PCL nanofibers was reduced through the increasing
ability of PANI and blended PANI to support cell growth. The concentration of PANI as a result of the increment in
ability of varying the oxidative state of PANI has increased the electrical conductivity of the spinning solution.
research development in biomedical research fields [132]. The seeded cells grown on randomly oriented nanofiber
Although, like many conductive polymers, PANI is matrixes were observed as having flat and multipolar mor-
not inherently biodegradable [133], it can be made so by phologies. In contrast, the seeded cells grown on aligned
(1) blending it with a biodegradable polymer to form a nanofibers were found to possess bipolar morphologies.
composite [134]; (2) modifying the backbone to render PANI loading significantly affected the number of myotubes
degradability [135]; and (3) synthesizing short PANI chains formed on the PANI-PCL nanofibers. The number of the
that can undergo gradual erosion [136]. Electrospinning myotubes formed increased with increasing PANI concen-
a biocompatible or a biodegradable polymer filled with tration. The results suggested that both conductivity and
PANI could produce nanofibers that may be useful in tis- alignment of nanofibers can synergistically stimulate the
sue engineering applications. Studies on electrospun PANI myogenic differentiation.
blends for tissue engineering include reports on PANI/ The development of effective methods to deliver drugs
chitosan [137], PANI/poly(glycerol-sebacate) (PGS) [138], has attracted wide attention, primarily in the fields of tumor
PANI/nanohydroxyapatite/gelatin [139], PANI/PLA [140], therapy and tissue engineering. A controlled drug delivery
PANI/PPY [141], and PANI/PCL [142]. system can release quantitatively drugs at a specific time
Previous research conducted by Li et al. [143] described and position, especially the stimuli-response drug delivery
the use of composite nanofibers of PANI blended with gelatin systems [148–150]. Conducting polymers exhibit a reversible
as a conductive scaffold in the proliferation of myoblast electrochemical response; they will contract upon reduction
cell tissue culture. Gelatin is a frequently used biomaterial and expand after oxidation. This feature permits the con-
for tissue engineering applications, especially in cardiac trolled release of various kinds of drugs [151, 152]. Electrospun
tissue engineering [144, 145]. Gelatin, which is less than 3% conducting polymer nanofibers provide high surface area,
compared to the amount of PANI in total weight, leads to an which can be easily loaded with drugs. It has been shown that
alteration of the physicochemical properties of the nanofiber the release of dexamethasone from electrospun PEDOT/PGA
composites. The decrement in the diameter of the composite nanofibers can be controlled by using electrical stimulation,
nanofibers was found with the increment of the PANI to which is due to the contraction and expansion of the PEDOT
gelatin volume ratio. The cells initially displayed different [153]. Up to date, there have been a few works reported
morphologies, depending on the concentration of PANI. The focusing on electrospun PANI blends for controlled drug
cells cultured on the smaller diameter PANI-gelatin nanofiber release [154].
blend were more spread out and exhibited a smooth muscle-
like morphology, similarly to the cells growing on a glass 6.2. Sensors. PANI nanofibers are a promising candidate for
cover surface (control sample). However, cell growth on designing sensors since their conductivity is highly sensitive
larger diameter nanofiber substrates presented low density to chemical vapors due to their high surface area [155].
and pseudopodia. Other physicochemical measurements, Synthetically prepared PANI nanofibers [156, 157] and PANI
Advances in Materials Science and Engineering 11

nanocomposites with carbon nanotubes (CNTs) [158], WO3 with good sensitivity. The high surface-to-volume ratio and
[159], and TiO2 [160] have been investigated for gas sensing the improved mass load, electroacoustic load, and viscoelastic
applications towards humidity and vapors and as biosensor load of the core-sheath structured composite nanofibers of
devices, thus making use of the conductivity changes upon PANI blended with PVB were responsible for the attractive
oxidation or reduction [161]. This specific feature was targeted humidity sensing properties of the SAW sensors [178].
to amplify signals transduced by the electrochemical reac- It is known that the detection stability is an important
tions [162–164]. parameter for a sensor. Doped PANI generally shows unsat-
Electrospun PANI nanofibers and their polymeric blends isfying stability in the open air, leading to a short lifetime
have recently captured the researchers’ interest due to of PANI based sensors. However, the PANI/PVB nanofiber
their ease of processing, excellent detection, and uniform sensor showed good stability, owing to the rheology of the
nanofiber formation, compared to other chemical or template electrospun composite nanofibers, which exhibited a core-
routes. Several types of electrospun PANI nanofiber based sheath structure, with PANI inner core, covered by PVB
sensors have been developed lately. Recent reports include the outer sheath, instead of being directly exposed to the open
electrospun nanofiber of graphene/PANI/PS for the detection air. The chemically stable and relatively hydrophobic PVB
of Pb and Cd ions [165], PANI/SnO2 for H2 gas sensing [166], could effectively protect the PANI core. The investigation on
multiple detections using PANI/PCL [167], PANI/polyamide the long-term stability of the SAW sensor based on PANI-
66 for colorimetric sensing [168], PANI/poly(ethylene oxide) PVB nanofibers found that the sensor maintained its high
gas sensor [169], PANI/ZnO for chemiresistor sensor [170], sensitivity after storing for more than seven months [178].
PANI/PLA to sense alcohol vapors of increasing molecular Electrospun PANI nanofibers doped with palladium
size [171], PANI/polyvinylidene fluoride as strain sensor nanoparticles have been demonstrated efficient for hydrogen
[172], high sensitivity gas sensors based on PANI/PMMA sensing application in the fabrication of a chemiresistor [179].
[173], PANI/fibroin microfibrous mat as reactive sensor [174], The nanofibers were deposited as single nanofibers across
and PANI/polyamide 6/TiO2 nanofibers for ammonia sensor two gold electrodes by means of near-field electrospinning
[175, 176]. without using the conventional lithography process. These
The humidity response of the PANI nanofiber sensors palladium nanoparticle based chemiresistors recorded 1.8%
was determined by measuring their impedance at different resistance change in the environment with 0.3% hydrogen
humidity levels at room temperature. According to Lin et concentration. These nanofiber sensors are flexible, have
al. [177], the rheology of PANI nanofibers presenting the good reversibility, and are potentially versatile by doping with
formation of some beads exhibited improved adhesion to the different particles.
electrode receptor and better electrical contact due to the high
Further, the study of Zhang and Rutledge reported the
surface area, which in turn enhanced their sensing properties,
fabrication of electrospun fibers of PANI and poly(3,4-ethyl-
compared with the nanofibers without beads. The blending
enedioxythiophene) (PEDOT), blended with poly(ethylene
of PEO into PANI greatly modified the hydrophilicity of
oxide) (PEO) and poly(methyl methacrylate) (PMMA) over
the PANI-PEO nanofibers, and their humidity response. The
a range of compositions, using template synthesis. Fibers
PANI-PEO nanofiber based sensors revealed much higher
of neat PANI doped with camphor sulfonic acid (CSA)
sensitivity than their film counterparts with the impedance
were successfully fabricated for the first time by coaxial
increasing by three orders of magnitude from 20% to 90%
electrospinning and subsequent removal of the PMMA shell
RH. Humidity sensors based on PANI nanofibers with some
by dissolution. This allowed for the electrospun PANI/CSA
beads and a small content of PEO revealed high sensitivity,
fibers to be tested for electrical performance and its enhance-
fast response, and small hysteresis [177].
ment, as well as for gas sensing application. The conductivities
Lin et al. [178], on the other hand, developed humidity
of the PANI-blend fibers were found to increase exponentially
sensors of surface acoustic wave (SAW) based on electro-
with the weight percent of doped PANI in the fibers, to as
spun core-sheath structured polyaniline/poly(vinyl butyral)
PANI/PVB blended nanofibers. It was reported that nanos- high as 50 ± 30 Scm−1 for the as-electrospun fibers of 100%
tructured PANI exhibited an excellent chemical sensitive PANI/CSA. The conductivity of the neat doped PANI fibers
response, with higher sensitivity and faster response com- was found to increase to 130 ± 40 Scm−1 with increasing
pared to those of bulk PANI. As it was not possible to molecular orientation, achieved through solid state drawing
prepare nanostructured PANI directly from its solution, it [100].
was blended with PVB. The results indicated that PVB was The experimental results thus support those described
suitable to be coelectrospun with PANI to form a core-sheath previously; that is, enhanced molecular alignment within
structure. electrospun fibers, during both the electrospinning pro-
It was demonstrated that high performance SAW humid- cess and subsequent posttreatment, contributes positively to
ity sensors could be successfully fabricated based on elec- increasing the electrical conductivity of conductive polymers.
trospun core-sheath PANI-PVB composite nanofibers. The Neat PANI fibers with different levels of doping were also fab-
composite nanofiber SAW sensor revealed very high sensitiv- ricated by coaxial electrospinning and subsequent removal of
ity of 75 kHz/% RH over a wide humidity range, good sensing the shell by dissolution and shown to exhibit a large range of
linearity, and very fast response in both humidification and fiber electrical conductivities, which increased exponentially
desiccation processes. Moreover, PANI-PVB nanofiber based with the increasing ratio of dopant to PANI. These fibers
sensors could be used to detect humidity as low as 0.5% RH were found to be very effective in nanoscale chemiresistive
12 Advances in Materials Science and Engineering

sensors for both ammonia and nitrogen dioxide gases, thanks in propylene carbonate (PC)) electrolyte is compared with
to this large range of available electrical conductivities. Both that of PANI powder. The specific capacitance of a PANI
sensitivity and response times were shown to be excellent, nanofiber web in 1 M H2 SO4 is 267 F⋅g−1 at a current density
with response ratios up to 58 for doped PANI sensing of of 0.35 A⋅g−1 , which is higher compared to that of PANI
ammonia and up to more than 105 for nitrogen dioxide powder (208 F⋅g−1 ). PANI nanofiber webs demonstrate supe-
sensing by undoped PANI fibers. The characteristic times for rior stable performance compared to that of PANI powder
the gas sensing were shown to be on the order of 1 to 2 min. and deliver a specific capacitance of 230 F⋅g−1 at the 1000th
Using a model that accounted for the effects of intrinsic fiber cycle. Over 86% of its original capacitance is retained after
conductivity (including both composition and molecular 1000 cycles; meanwhile, the capacitance retention of the
orientation), mat porosity, and fiber orientation distribution PANI powder is observed to be 48% after 1000 cycles.
within the mat, calculated mat conductivities were obtained This observation indicated that PANI nanofiber webs have
in quantitative agreement with the mat conductivities mea- better cycle stability than PANI powder. Smaller individual
sured experimentally [100]. Electrospun PANI/PS nanofiber impedance parameters in the case of PANI nanofiber webs
showed excellent detection of H2 O2 compared to that of its were also observed, indicating the availability of electroactive
corresponding thin film form. The H2 O2 is useful because it
sites needed for charge transfer reaction and counterion
is often a product of enzymatic reactions. It is generated in
diffusion. Given these characteristics demonstrated by PANI
the reaction between glucose and oxygen in the presence of
nanofibers synthesized through electrospinning, it was found
glucose oxidase (GOX). The large surface area promotes more
to be a suitable candidate for being used as electrode material
GOX immobilization than the film sensor [176].
in high performance supercapacitors [186].
6.3. Supercapacitors. The increasing energy need of the world
population has urged for new discoveries and manipulation 6.4. Solar Cells. Dye-sensitized solar cells (DSCs) have
of renewable energy sources. However, it is crucial to find turned out to be the centre of interest in nowaday’s low-
ways to store energy in order to keep getting sufficient carbon economy as a solution to the energy requirements
supply and to secure energy distribution. For this reason, [191, 192]. They have been recognized as a potential alternative
the supercapacitor has been a phenomenal discovery with its to silicon based solar cells due to their simple fabrication
low internal resistance, fast charge and discharge, high power procedures, high catalytic performance, and cheap counter
density, high energy density, and long life cycle, compared electrodes [193]. Counter electrodes of DSCs function as
to conventional capacitors and batteries [180–184]. The key collecting electrons, catalyzing the redox couple regeneration.
technology that underlies its performance and has turned it These are usually made of Pt films, but despite their good
into a research hotspot is the electrode material. Electrode performance, Pt based solar cells need high temperature of
materials mainly include carbon material, metal oxide, and operation and complex facilities, which limits their appli-
conducting polymers. Among conductive polymers, PANI cation [194–196]. For these reasons, recent decades have
has become the preferred material due to its high capacitance, witnessed the development of PANI usage in this area in order
desirable chemical stability, and multiple intrinsic redox to enhance cell stability, reduce fabrication cost, and simplify
states [185]. the preparation techniques, compared to previously made
Given all these excellent advantages of PANI, it has solar cells [197]. Duan et al. also proved that a solid PANI
become a strong competitor as a material for building pow- electrolyte has the ability of catalyzing tri-iodide species,
erful supercapacitors. Recently, researchers have recognized shortening the charge diffusion path length, and recovering
this potential of electrospun PANI through their studies dye molecules at the anode/electrolyte interface [197]. The
on electrospun PANI nanofiber web electrodes [186], three- focus of the research on utilizing PANI as a component in
dimensional porous PANI/polyacrylonitrile core-shell [187], DSCs has widened by the effort of certain research teams in
sandwiched symmetric supercapacitor consisting of flexible studying electrospun PANI nanocomposites. However, this
PANI/carbonized polyimide [188], PANI/GO/PVDF [189], is still an emerging topic of research; therefore the studies
hollow-structured PANI [91], and PANI/vanadium pentoxide dedicated to it seem scarce, regardless of the potential shown.
as asymmetric supercapacitor [190]. An investigation on electrospun PLA/PANI nanofibers for
Chaudhari and his coworkers [186] initiated a study rigid and flexible DSCs [198] and another on PANI/MWCNT
on applying PANI nanofiber webs as electrode materials counter electrodes [199] are among the few studies con-
for supercapacitors. Since the direct processing of PANI by ducted.
electrospinning is a difficult task, they proposed the fabri- A simple electrospinning method was developed [198] to
cation of high aspect ratio PANI (>50) by electrospinning directly deposit conductive 10-camphorsulfonic acid (CSA)
a polymeric blend of PANI with polyethylene oxide (PEO). doped PANI blended with polylactic acid (PLA) composite
The fabrication of electrodes took 75 wt% of PANI, 15 wt% of films (PANI/CSA-PLA) on flexible indium tin oxide-coated
carbon black, 10 wt% of polyvinylidene fluoride, and a few polyethylenenaphthalate (PEN) and rigid fluorine doped tin
drops of N-methylpyrrolidinone to form a uniform slurry. oxide (FTO) substrates. The result revealed that the catalytic
The resulting slurry was then used to coat a graphite substrate, performance of the PANI/CSA-PLA film was higher than that
which served as current collector. of the PANI-PLA film, which was ascribed to the contribution
The performance of electrospun nanofibers produced in of the high surface area of PANI/CSA-PLA nanofibers and
aqueous electrolyte (1 M H2 SO4 ) and organic (1 M LiClO4 relatively high electrical conductivity of the PANI/CSA-PLA
Advances in Materials Science and Engineering 13

film. The preparation process of the PANI/CSA-PLA counter (7) In addition to issues on extended response/recovery
electrodes suggested in this experiment was simple and capabilities, the viability of PANI nanofibers is also
cost-efficient, and thus feasible for large-scale and flexible being held back by poor mechanical properties as
application of DSCs. PANI tends to be a brittle material with poor ductility.
The flexible and rigid DSCs demonstrate high photo- Although films of PANI nanofibers can be deposited
voltaic efficiency, close to that of sputtered Pt based DSCs. onto many substrates, it is not possible to produce
It was found that the flexible DSC based on the PANI/CSA- PANI nanofiber films, which are mechanically robust
PLA counter electrode can exhibit a conversion efficiency enough to be free-standing. A new method towards
of 3.1%, while the PANI/CSA-PLA film on FTO glass as free-standing PANI nanofibers by improving the
counter electrode exhibits a reasonably high photoelectrical mechanical properties is another way to enhance the
conversion efficiency of 5.30%. Although these results are a usability of PANI nanofibers for a wide range of future
little lower than those for the sputtered Pt based DSC (flexible biomedical, energy, and biotechnology applications.
= 4.39%, rigid = 6.51%), they demonstrate that PANI can
be used as counter electrode material for DSCs, providing
efficiency values comparable to those of the Pt based device
Conflict of Interests
[198]. The authors declare that there is no conflict of interests
regarding the publication of this paper.
7. Conclusions
(1) Electrospinning is a simple, versatile, and rapid tech- Acknowledgments
nology advancement, which can generate nonwoven The authors would to express their gratitude to Universiti
nanofibers with high surface area to volume ratio and Teknologi Malaysia for the Potential Academic Staff (PAS)
tunable porosity and can be designed into various scheme, IJN-UTM Cardiovascular Engineering Centre, Fac-
fiber forms. Due to these interesting properties, the ulty of Bioscience and Medical Engineering, and also Depart-
electrospinning process seems to be a promising ment of Chemistry, Faculty of Science, Universiti Teknologi
method for various applications. Malaysia.
(2) The polymer solution, the needle, and the collector
used in electrospinning have significant effects on
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