1. Introduction

It is commonly expected that the future progress in such fields of technology as microelectronics, sensorics and biosensorics, and chemical and biochemical engineering will be concerned with new knowledge-based nanostructurized materials. Conducting and electroactive polymers are relatively new materials, extensively studied during the past two decades. Recently, nanostructurization of conducting polymers and their composites emerged as a new field of research and development, directed to creation of new smart materials for use in modern and future technologies. In particular, electrochemistry-related aspects of nanostructurized conducting polymers have recently attracted a great deal of interest. The main expected electrochemistryrelated fields of application for nanostructurized conducting polymers are electric energy storage systems, chemical-

4 Address for correspondence: Institute of Chemistry, Gostautoˇ Street 9, LT-01108 Vilnius, Lithuania.

to-electric or vice versa energy conversions, sensors and biosensors, and materials for corrosion protection. The present concise review deals with electrochemistry-related aspects of nanostructurized conducting polymers and their composites. To our knowledge, this is a first attempt to give a general overview on the topic bordering three disciplines—electrochemistry, nanoscience, and conducting polymer chemistry.

2. Electrochemical synthesis of conducting polymer nanostructures

Electrochemical synthesis, like chemical polymerization, is a very frequently used technique for obtaining conducting polymers. At a sufficiently high positive (i.e. anodic) electrode potential, some monomers like aniline or pyrrole undergo electrochemical oxidation yielding cation radicals or other reactive species. Once formed, these species trigger the polymerization process. As a result, oligomers and/or

polymers, derived from the corresponding monomers, are formed. Depending on plenty of experimental variables available, like the monomer concentration, electrolyte used, and electrode potential, different structures of conducting polymers can be obtained, ranging from micrometre or submicrometre (colloid)-sized particles to thick compact deposits on the electrode surface.

Three main strategies for obtaining conducting polymer nanostructures are used:

(1)Templateless synthesis. Following this route, the nanostructures are obtained by choosing the appropriate conditions of electrosynthesis at simple chemically inert electrodes.

(2)Template-assisted synthesis. Nanostructured templates are created on the electrode surface, and electropolymerization occurs within the channels, holes, cavities, or related nanosized structural units of a template.

(3)Molecular template-assisted synthesis. As a template, some molecules having nanosized cavities, and arranged in a distinct way on the electrode surface, can be used. Again, electropolymerization proceeds within these cavities, leading to nanostructures of the resulting polymers.

Most of the works in the field have been done using templateor molecular template-assisted electrosynthesis; however, some attention has been paid to templateless synthesis as well.

Many aspects of electrochemical synthesis of conducting polymer nanostructures have been reviewed recently. Bao and Xu reviewed template synthesis of nanostructures consisting of metals, carbon, semiconductors, and polymers [1]. Synthetic approaches for obtaining nanosized conducting polymer structures have been discussed by Wallace and Innis [2]. Different kinds of techniques for the preparation of polypyrrole nanowires and nanotubules have been reviewed by Demoustier-Champagne et al [3] and Ge et al [4].

2.1. Templateless electrosynthesis

It is well known that conducting polymer colloids can be prepared via simple electropolymerization at optimized conditions as regards monomer concentration, polymerization time, and working electrode porosity and surface area [5]. More controlled synthesis, however, can be achieved at more specified electropolymerization conditions. With the use of a pulse potentiostatic method, polyaniline nanoparticles have been prepared at a highly ordered pyrolytic graphite (HOPG) electrode from low concentration aniline solution [6]. The dimensions of these disc-shaped nanoparticles obtained varied from 20 to 60 nm over the range of electropolymerization charge applied of 5.7 to 19.3 µC cm−2. Besides simple colloid particles, some more complex structures have been obtained using templateless electrosynthesis, performed at well specified conditions. Large arrays of uniformly oriented polyaniline nanowires have been prepared via a controlled nucleation and growth process [7]. At a high initial current density, a large number of polyaniline nuclei are deposited onto a substrate electrode, and this is followed by the growth of oriented nanowires from the nucleation sites, achieved by

stepwise reduction of the polymerization current density [7]. Similarly, polypyrrole nanowires have been obtained by instantaneous 2D nucleation and following 1D growth at a graphite–paraffin composite electrode [8]. Nanostructures of polypyrrole about 10 nm in size have been prepared by electropolymerization at naturally occurring step defects, and artificially formed pit defects of HOPG [9]. Under controlled potential electrolysis conditions, wire-shaped growth has been observed for a short electropolymerization time. After synthesis, the nanoparticles obtained can be removed from the electrode surface by a sonication procedure.

The localization of polymer growth sites on the electrode on the nanometre scale can be attained by artificial means as well. Nanofabrication of polyaniline and polypyrrole has been done with the use of atomic force microscopy (AFM) tip–sample interactions, while electropolymerization could be either blocked on the bare HOPG surface, or enhanced at the as-polymerized film [10]. As a result, this nanolocalized electropolymerization could allow manipulation with nanoscaled lines, square platforms, or hollows at the desired surface. This approach allows one to use AFM as a tool for localized nanolithography [11]. Similarly, nanolocalized electropolymerization of aniline at a graphite surface, with the use of a platinum tip of a scanning tunnelling microscope, has been demonstrated [12]. For this, a two-voltage pulse technique has been employed, where the first pulse results in pit formation, and the second one, in oxidation and further nanolocated polymerization of aniline monomer, leading to polyaniline particles 10–60 nm in diameter and 1–20 nm in height.

It is well known that the morphology and properties of electrochemically generated conducting polymers are greatly influenced by the nature of the anion present in the electropolymerization solution. In their conducting form, these polymers are usually proton and anion doped, whereas reversible expulsion and bonding of anions proceed during electrochemical reduction and oxidation processes. It has been shown recently that the presence of suitable dopant anions leads to the formation of well defined nanostructures of conducting polymers. Tubules of polypyrrole, 0.8–2.0 µm in diameter and 15–30 µm in length, were synthesized by an electrochemical template-free method in the presence of β -naphthalenesulfonic acid as a dopant on a stainless steel electrode [13]. Polypyrrole nanotubules with diameter ranging from 50 nm to 2 µm were synthesized electrochemically in the presence of either β -naphthalenesulfonic acid, or p-toluenesulfonic acid as a dopant [14]. It was proposed that the micelles of dopant or pyrrole–dopant clusters act like templates on the electrode surface, enabling polypyrrole to grow in a tubular form.

Electrochemical assembly as a method for preparing nanoordered conducting polymer structures has been reported [15]. Following this method, a close packed nanostructured polyaniline film has been prepared by electropolymerization of aniline in the presence of p-aminobenzenethiol. A similar electrochemical assembly technique has been applied in obtaining nanoscaled dot or line arrays of poly(o-phenylenediamine) at a bare Au(111) surface and at a p-aminobenzenethiol-modified Au surface [16].

2.2. Template-assisted electrosynthesis

Following template-assisted synthesis, the electrode surface is first covered with any electrically insulating template possessing pores. The design of the pores must allow for the solute species to pass to the electrode surface. After applying a suitable electrode potential, the electropolymerization starts at the electrode surface within the pores, and propagates through the pores towards the outside solution. Both the diameter and the depth of the pores are subject to experimental variation, so nanowires or nanofibrils of different length, thickness, and shape can be obtained. An essential point as regards template synthesis is a suitable nanosized template. Because of experimental difficulties, there are a limited number of nanosized templates and/or techniques for their preparation available. The two most popular architectures are as follows:

(1)Track-etched polymer membranes. These are produced by the bombardment (irradiation) of a membrane target with high energy heavy ions, followed by chemical etching. The number of pores per square unit, i.e., the pore density, depends on the intensity and duration of the irradiation, whereas the diameter of the resulting pores depends mainly on the intensity of the etching process. Although expensive, this technique enables one to obtain well suited and desired nanoporous templates with well designable parameters.

(2)Alumina (aluminium oxide) templates. In an ambient atmosphere, a freshly cut surface of aluminium metal is immediately covered with a very thin dense layer of aluminium oxide (barrier layer). For many technical purposes, in the first line for increase of its corrosion resistance, the aluminium surface is additionally covered with a thicker layer of oxide by the use of an anodization (passivation) process. This process, performed usually by applying an anodic (oxidation) current in a suitable acidic electrolyte, results in a dense, but porous, relatively thick alumina layer [17]. The pores of a porous alumina layer have a mean diameter of a few nm; however, the use of specially designed electrolytes and treatment conditions enable one to obtain controlled-size pores with a diameter ranging from a few nm to a few tens of µm [18]. As a result, a template consisting of an electrically nonconducting nanoporous alumina coating can be obtained at the aluminium surface, suitable for electrosynthesis of nanosized structures of conducting polymers.

The preparation of track-etched templates allows many possible modifications. In [19], spin-coated thin polycarbonate plastic film has been irradiated with Ar(9+) ions at an energy of 220 MeV, UV irradiated, and chemically etched in a

a few µm, and a pore size between 15 and 100 nm. This template has been used to prepare electrochemically copper nanowires and polypyrrole nanotubes [19]. Similarly, polypyrrole doped with either perchlorate or poly(sodium 4- styrenesulfonate) anions has been electrosynthesized within a nanoporous polycarbonate track-etched membrane [20, 21]. The influence of polymerization conditions on the synthesis of polypyrrole nanotubules has been studied; in particular, an

increase of electropolymerization rate at a controlled electrode potential with increase of the feed solution concentration has been shown [21]. The thickness of polypyrrole nanotubules was shown to depend on the pore size and on the electrolyte used [22]. The electric conductivity of polypyrrole nanotubules appears to be higher than for the bulk material, which is probably consistent with an increased conjugation length in polypyrrole molecules packed into nanotubules, as has been shown with the use of Raman spectroscopy [22]. In addition to electrochemical synthesis, chemical preparation of perfectly cylindrical polypyrrole tubules 15 nm in diameter within the same track-etched polycarbonate membranes has been reported [23].

A detailed scanning electron microscopy study of chemical or electrochemically prepared polyaniline within the pores of track-etched membrane has shown that hollow tubules are formed, because polyaniline deposits during polymerization on the surface of the pore walls [24]. A preferential deposition of some conducting polymers on the surface of polycarbonate membranes has been shown by means of chemical polymerization [25]. For polyaniline, the polymer is first formed in a polymerization solution, and then it precipitates on the membrane, while for poly(2- methoxyaniline) the oxidized monomer is first adsorbed onto a polycarbonate surface, and then it recombines to yield the polymer [25]. Besides polyaniline and polypyrrole, some other less common conducting polymers, e.g. poly(3,4- ethylenedioxythiophene), can be prepared by electrochemical polymerization within a track-etched porous membrane [26]. Many authors point out an increased electric conductivity of nanotubules or nanowires prepared within the pores of tracketched membranes as compared to that for the same bulk polymerized materials [22, 24]. A probable reason for this is that the polymerization inside the confined space of the pores, combined with electrostatic interaction, ensures the alignment of the resulting polymers on the walls of the pores [27].

Another frequently used template for electrosynthesis of nanostructured conducting polymers is nanoporous alumina. This template can be easily prepared by electrochemical (anodic) treatment of aluminium metal. As-prepared, alumina template appears tightly adherent to the metal surface, and can hardly be removed from it. Copolymer (polyaniline and polypyrrole) nanofibrils have been prepared within the pores of alumina template, and it has been shown, by means of scanning and transmission electron microscopy, that nanofibrils have uniform and well aligned structure, whereas their diameter and length can be controlled by changing the aspect ratio of the pores of the alumina template [28]. Similarly, copolymers of pyrrole and thiophene have been prepared in the form of uniform and aligned nanofibrils by electrochemical copolymerization within the pores of alumina template [29, 30]. It has been revealed by IR spectroscopy and cyclic voltammetry that both comonomer units are present in a copolymer structure, and it has been shown that the ratio of the comonomer units in a resulting copolymer can be simply varied by changing the electropolymerization potential [29, 30]. Aligned nanotubular heterojunctions of poly( p-phenylene) and polythiophene have been prepared

diameter [31]. Nanotubes and nanowires of polypyrrole, polyaniline, and poly(3,4-ethylenedioxythiophene) have been prepared by electropolymerization within pores of an alumina template 200 nm in diameter, and it has been shown that the length and the wall thickness of the nanotubes can be controlled by varying of electropolymerization current, the duration, and the nature of the dopant anion present in the electropolymerization solution [32].

A combined two-step method of preparing polypyrrole nanowires was reported, consisting of (i) electrochemical grafting of a thin polyacrylate film at a carbon electrode and (ii) electropolymerization of pyrrole, resulting in polypyrrole wires, 600 nm in diameter and 300 µm in length, growing through the pores of a polyacrylate film [33].

2.3. Molecular template-assisted electrosynthesis

Although this technique has not found wide application yet, it presents a simple and well controllable approach for preparing conducting polymer nanostructures. Sometimes, it is difficult to draw a clear boundary between templateless and molecular template-assisted electrosynthesis, since some of the dopant anions or other active species used in a ‘templateless’ synthesis can themselves act like templates of molecular dimensions. Thus, some methods of ‘templateless’ synthesis described above [13, 14] could be ascribed to molecular templateassisted techniques as well. Here we describe some ‘true’ molecular template syntheses, in which the electrode surface is specifically modified with some kind of adsorbate directing the electropolymerization to proceed in a template-like manner.

Molecular template-directed growth of polyaniline nanostructures has been achieved at a gold electrode modified with two component self-assembled monolayers consisting of p- aminobenzenethiol and n-octadecanethiol, where nanodimensional polyaniline features are formed on p-aminobenzenethiol islands embedded into an octadecanethiol layer [34]. Polypyrrole films have been grown galvanostatically in acetonitrile solution at a gold electrode modified with thiolated

β-cyclodextrin self-assembled monolayers, and it has been shown that spectroscopic characteristics reveal a nanosized fibre structure of the resulting polymer [35]. The thiolated

β-cyclodextrin acts as a molecular template restricting the polymer growth sites to within the β -cyclodextrin cavities, as was shown by scanning electron microscopy [35]. Similarly, polyaniline nanowires and nanorings have been synthesized at a gold electrode modified with self-assembled monolayers of thiolated cyclodextrin, embedded into an alkanethiol layer, where thiolated aniline derivative has been anchored to the surface within the cyclodextrin cavity forming an initiation point for aniline electropolymerization [36]. Nanosized dots and wires of polypyrrole and polyaniline have been obtained at a gold electrode modified with self-assembled monolayers of either β -cyclodextrin or p-aminobenzenethiol embedded into hexadecanethiol surroundings [37].

3. Metal–conducting polymer nanocomposites

There are two main kinds of nanosized composites of conducting polymers with metals:

(1)Metal core nanoparticles, covered with a conducting polymer shell. These composites are usually prepared by the chemical or electrochemical polymerization of a thin, nanometre-sized layer of a conducting polymer onto colloid metal particles. There are many techniques known for the deposition of nanometre-sized conducting polymer layers onto different substrates, including nanosized ones [38].

(2)Metal nanoparticles, embedded into a conducting polymer matrix. These composites can be easily obtained by the chemical reduction of metal ions from their salt solution at the conducting polymer/solution interface. Many conducting polymers, when present in their reduced form, have a sufficiently high reducing power with respect to some metal ions; thus, some metal ions having a relatively high positive redox potential, e.g. gold, silver, platinum, and copper, can be reduced at a layer of conducting polymer, forming clusters or small particles within a porous conducting polymer layer. Because of the relatively high metal surface area, these composites are often expected to be of use in various electrocatalytic systems for the electrochemical conversion of solute species. This field was reviewed earlier [39]. Many works in the field do not attempt to indicate or study the size of the metal particles embedded in the polymer matrix; therefore, we describe here only those works where nanodimensions of the metal particles are clearly indicated.

Nanostructured composites of polyaniline with gold clusters have been prepared by the spontaneous chemical reduction (i.e. cementation) of gold from tetrabromoaurate ions in an acidic solution onto a polyaniline layer, electropolymerized at a platinum electrode [40]. It has been pointed out that although the amount and size distribution of the gold clusters vary with the reaction time, it is difficult to control both of these parameters. Composite nanoparticles of gold with polypyrrole have been prepared by the oxidation of pyrrole with ferric chloride yielding polypyrrole colloids, and subsequently reacting them with chloroauric acid, or, alternatively, by the direct oxidation of pyrrole with chloroauric acid in the presence of sodium dodecylbenzenesulfonate [41]. Obviously, in the last case chloroauric acid serves both as an oxidizer for pyrrole polymerization and as a source of gold in the subsequent cementation of gold metal on polypyrrole colloids. Both methods of preparation gave irregularly shaped nanocomposite particles approximately 500 nm in size; however, the gold spheres present in nanocomposites varied in size from 13 nm for direct synthesis with chloroauric acid to 400 nm for synthesis using ferric chloride as the oxidant [41]. Gold– polypyrrole nanocomposites with a diameter of less than 8 nm have been prepared electrochemically, exploiting the autopolymerization of pyrrole on gold nanoparticles [42]. Polyaniline has been doped with carboxy-modified gold nanoparticles to yield layer-by-layer assembled multilayer films with extended polyaniline redox activity up to neutral pH values, valuable for biosensing applications [43].

Silver–polyphenylpyrrole core–shell nanoparticles have been prepared by a combined electrochemical–chemical method at the liquid/liquid interface [44]. The inclusion of

silver nanoparticles into an organic composite consisting of polyaniline and 2,5-dimercapto-1,3,4-thiadiazole (DMcT) has been reported to improve charge–discharge characteristics of the composite as a cathode material for lithium rechargeable batteries [45]. In this composite, a strong interaction of silver nanoparticles with DMcT, but weak interaction with polyaniline, has been suggested by UV–vis spectroscopy. Following both galvanostatic and potentiostatic operation modes, copper nanostructures including fractals, nanowires, and cubic nanocrystals were obtained at a polypyrrole film electropolymerized at a gold electrode [46]. In accordance with the instantaneous growth mechanism, a homogeneous distribution of copper nanostructures has been attained with over-peak potentiostatic electrodeposition.

Palladium nanoparticles of mean diameter 5 nm have been prepared electrochemically and embedded into electrochemically prepared polypyrrole thin film, yielding a composite that shows electrochemical activity related to both polypyrrole and palladium particles [47]. The resulting composite films have good electric conductivity [48]. A detailed study on the electrochemical deposition of palladium nanoparticles in poly(3,4-ethylenedioxythiophene) films gave evidence for three-dimensional growth of metal clusters at an initial stage [49]. The resulting nanocomposite shows a high electrocatalytic activity with respect to hydrogen adsorption. In the search for preparation of platinumcontaining nanocomposites, PtCl42− anions have been trapped into a polypyrrole film during its electrochemical preparation, and, after solution exchange, these anions have been reduced cathodically to yield platinum metal particles of average size 10 nm, included in a 3D polypyrrole matrix [50]. The resulting composite film showed an electrocatalytic effect on the electrochemical methanol oxidation. Platinum nanoparticles have been incorporated into a polyaniline matrix, and the electrochemical process of hydrogen evolution has been studied at these composite electrodes [51]. Nanocomposites consisting of platinum–ruthenium alloy and either poly(N - vinylcarbazole) or poly(9-(4-vinylphenyl)carbazole) have been prepared electrochemically, and electrocatalytic activity of the resulting nanocomposites as regards anodic oxidation of methanol in an acidic solution has been shown [52].

An original ‘second-order-template’ method has been proposed for the fabrication of metal and conducting polymer composite nanowires. Following this route, an array of polyaniline nanotubules has been synthesized within the pores of an alumina template, and then nickel nanowires were electrodeposited into these nanotubules, forming resulting nickel nanowires, enveloped in polyaniline nanotubules [53]. Also, gold [54, 55] and silver [55] nanowire arrays coated with polythiophene layers were prepared following this route. The diameter of the composite nanowires was 200 nm, and the diameter of a gold nanowire was 100 nm [54, 55].

Since the discovery in the 1970s, surface enhanced Raman spectroscopy emerged as a unique tool for investigation of various adsorbed species, even at a monolayer level, at gold, silver, and copper electrodes. A prerequisite for obtaining enhanced Raman spectra is electrochemical roughening of the electrode surface, consisting in repeated oxidation–reduction cycling in an electrochemical system. Although not fully understood yet, this roughening procedure enables one to

obtain an enormously high enhancement, by a factor up to 106, of Raman spectra from a monolayer of adsorbate. Recently, some indirect evidence of the formation of nanosized complexes during electrochemical cycling has been discussed. Gold complexes with the perchlorate anion Au(ClO4)4− with a grain size less than 100 nm have been claimed to form during the oxidation–reduction cycling of a gold electrode [56]. Pyrrole has been found to autopolymerize on this roughened gold surface due to the electrochemical activity of the complexes, acting as an oxidant, resulting in a nanosized gold–polypyrrole complex [56]. Electrocatalytic activity of chloride-containing gold nanocomplexes, obtained by redox cycling of a gold electrode in sodium chloride solution, as regards the electropolymerization of pyrrole has been shown, and a surface enhanced Raman scattering effect has also been shown [57]. The characteristics of polypyrrole, electrodeposited at a nanoroughened gold surface, were shown to differ from those for the same polymer deposited at unroughened electrodes, as regards a surface enhanced Raman scattering effect, a higher electrical conductivity, and an increase of oxidation level of the polymer [58]. Similarly, silver chloride nanocomplexes with grain size less than 100 nm have been prepared by oxidation–reduction cycling of a silver electrode in potassium chloride solution, and autopolymerization of pyrrole has been shown to proceed at this electrode, as confirmed with FTIR spectroscopy [59]. Nanostructured thin amorphous films of platinum and palladium have been prepared by means of UV photolysis of a film of organic complexes of the corresponding metals, and, after oxidation–reduction cycling, these films were shown to electrocatalyse the electropolymerization of aniline [60, 61].

4. Carbon–conducting polymer nanocomposites

Most of the work done on carbon–conducting polymer composites relates to carbon nanotubes or nanofibrils coated with nanosized layers of conducting polymer, and to the study of diverse possible applications of these novel composite materials. Carbon nanotubes have been coated with a

polypyrrole coating on the carbon nanotubes can be easily controlled by means of the electropolymerization conditions (electropolymerization charge), and very uniform polypyrrole coatings on nanotubes over the entire length can be obtained, even at a relatively thick coverage [63]. Due to the high surface area of the polypyrrole-coated carbon nanotubes in the aligned arrays, the redox performance of these electrodes has been significantly improved as compared to the flat electrodes [64].

Alternatively, a composite consisting of polyaniline and carbon nanotubes can be prepared by electrochemical polymerization of aniline from a solution containing well dispersed single-walled carbon nanotubes. The resulting composite film shows enhanced electrochemical activity and electric conductivity [65]. Also, a single-walled carbon nanotube mat has been coated with polyaniline, yielding a novel actuator material [66]. A new electrode material, consisting of a composite of polyaniline with dispersed