Csaba Visy

Conducting polymer-based hybrid assemblies for electrochemical sensing: A materials science perspective

In this topical review, progress achieved in amperometric sensing of different analytes over conducting polymer-based hybrid electrocatalysts is summarized. We report a variety of synthetic methods and the resulting hybrid assemblies, with the effectiveness of such strategies, for designing conjugated polymer-based hybrids as robust sensors for amperometric detection. Beyond incorporation of metal nanoparticles, metal-oxide and non-oxide semiconductors, carbon-based nanomaterials (nanotubes, graphene, and graphene oxide), and special dopant ions are also discussed. Moreover, some particularly interesting miscellaneous approaches, for example photo-amperometric sensing or use of overoxidized polymers, are also emphasized. Determination of dissolved gases (for example O2, NO, and NO2), ions (sulfite, nitrite, nitrate, chlorate, bromate, and iodate) and smaller and larger molecules (for example H2O2, ascorbic acid (AA), dopamine (DA), urea (UA), amino acids, hydrazine, NADH, serotonin, and epinephrine) is discussed. These achievements are reviewed from the materials perspective, addressing both synthetic and electrocatalytic aspects of the polymer-based modified electrodes. Beyond simple or more sophisticated mixing, a wide range of methods of preparation is presented, including chemical (one-pot polymerization, impregnation), electrochemical (co-deposition, doping type inclusion, etc.) and combined strategies. Classification of such synthetic routes is also included. However, it is important to note that we omit studies in which conducting polymers alone were used for determination of different species. Furthermore, because excellent reviews—cited in this work also—are available on immobilization of biomolecules (for example enzymes) for biosensing purposes, this topic, also, is excluded.

A thermodynamic approach to the interpretation of anodic and cathodic doping of poly(3-methylthiophene)

Abstract Anodically and cathodically conducting poly(3-methylthiophene) have been studied by transient and steady-state spectroelectrochemical methods in the presence of different anions (ClO−4, PF−6) and cations (BU4N+, Dec4N+) in acetonitrile solution of low water content. Spectral differences obtained with different ions are interpreted by assuming segments of different effective conjugation lengths in the neutral polymer determined by an interaction between the anions and the chains during the electrochemical preparation of the film. A redox mechanism based on thermodynamic considerations is suggested to describe the anodic and cathodic doping of thiophene-type conducting polymers. Both dopings start with the electrochemical transformation of species of longer effective conjugation length. Cyclic measurements are strongly controlled by non-equilibrium or quasi-reversible effects explained by the assumption of a two-phase system resulting in the undoping not being the simple inverse process of doping.

Electrochemical grafting of poly(3,4-ethylenedioxythiophene) into a titanium dioxide nanotube host network.

This study focuses on electrodeposition for infiltrating in situ a conducting polymer such as poly(3,4-ethylenedioxythiophene) (PEDOT) into a host titanium dioxide (TiO(2)) nanotube array (NTA) framework. The TiO(2) NTA was electrosynthesized on titanium foil in turn by anodization in a fluoride-containing medium. The PEDOT layer was electrografted into the TiO(2) NTA framework using a two-step potentiostatic growth protocol in acetonitrile containing supporting electrolyte. The nanoscopic features of oligomer/polymer infiltration and deposition in the NTA interstitial voids were monitored by field-emission scanning electron microscopy. Systematic changes in the nanotube inner diameter and the wall thickness afforded insights into the evolution of the TiO(2)NTA/PEDOT hybrid assembly. This assembly was subsequently characterized by UV-visible diffuse reflectance, cyclic voltammetry, and photoelectrochemical measurements. These data serve as a prelude to further use of these hybrids in heterojunction solar cells.

Cyclic spectrovoltammetry: a new method to study the redox processes in conductive polymers

Abstract We describe a new spectroelectrochemical method called cyclic spectrovoltammetry. The essence of the method is the application of factor analysis to the spectra recorded during a voltammetric measurement. Factor analysis enables us to use all the information contained in the spectra, a considerable improvement to former techniques employing data at only some wavelengths. When applied to poly(3-methylthiophene), two successive oxidation processes can be observed during the single oxidation peak in the cyclic voltammogram. The usefulness of the method in cases where voltammetric measurements are difficult to interpret is also demonstrated.

EQCM and in situ conductance studies on the polymerisation and redox features of thiophene co-polymers

3-Thiophene-acetic acid and 3-methylthiophene co-polymers have been prepared with the consideration that the solvation behaviour of the film could be modified. The modification is coupled with the observed splitting of the oxidation peak on the cyclic voltammograms. The redox transformation of the films was studied by electrochemical quartz crystal microbalance (EQCM) and in situ conductance (ISC) techniques. Mass changes during the electrochemical processes showed that the cation is not involved into the charge balancing of the film. Manifestation of self-doping was excluded by analogous measurements with an aprotic co-polymer of methylthiophene. Results completed with in situ conductance and spectroelectrochemical observations confirmed the assumption of a chemical step taking place after the first oxidation. In this process desolvation of the oxidised intermediate occurs, which is a necessary step for the achievement of the quasi-metallic state.

Conducting Polymer Based Electrochemical Sensors on Thick Film Substrate

New types of electroconducting conjugated polymer (ECP) based enzymatic and gas sensors were developed by using thick film technology. In order to prepare a stable polymeric film which is suitable for the enzyme immobilization and for gas detection itself, a special conducting salt (sodium dodecyl sulfate, SDS) was used. This combination resulted in sensors with favorable properties, i.e., higher sensitivity and longer operation stability. In the case of the ECP based uric acid biosensor a new layout was developed for the measurement of the current difference between the polymer and the enzyme electrode by using two working electrodes identically prepared (sensor and polymer electrodes) at the same time in the same solution. The sensor gives an easily measurable stationary response in the physiological concentration range of uric acid. Using a similar electropolymerization method, a selective ECP based sensor was prepared as part of a sensor array for intelligent gas monitoring. The changes in conductivity of the polypyrrole film in the presence of different gases were registered. A reversible sensor response of about 30 % in saturated methanol=air mixture and 50 % at 1000 ppm ammonia in air was obtained at room temperature. For NH3 a linear response in the 40‐1000 ppm concentration range could be registered while almost no response was obtained for ethanol, NO and CO. The sensor proved to be insensitive for H2.

Evolutionary spectral factor analysis of doping-undoping processes of thin conductive polymer films

Abstract Mathematical methods based on evolutionary factor analysis are applied to the doping-undoping processes of poly(3-methylthiophene). Two new methods are developed in this work: differential evolutionary factor analysis (DEFA), which is sensitive to changes in the composition of the mixture during the evolution process, and the projection matrix method, which can be used to annihilate the influence of a known spectrum from a set of measured spectra in order to obtain information about the unknown components.

Polythiophene puzzle—a plausible solution

Previous in situ spectroelectrochemical studies on the redox transformation of poly(3-methylthiophene) have been completed by further electrochemical, EQCM and in situ conductance measurements. In the case of thicker films, the generally observed single, wide anodic voltammetric wave could be separated into two oxidation steps, which are not consecutive, but complementary processes. The mass vs. potential curve from the EQCM measurements also reflects the double pattern of the electronic process. The virtual stoichiometric factor per electron of the anion during the first oxidation is lower than 0.5, and it is close to 1 in the second oxidation region. The experimentally obtained smaller values are explained by the parallel removal of solvent. The conductance vs. logarithm of charge curves at small sweep rate show no hysteresis. However, during the first oxidation step the film is nonconducting; its transformation to conducting film is coupled with the second oxidation step. On the basis of the experimental observations, a detailed mechanism of the redox transformation is suggested in which the roles of the solvated and nonsolvated parts of the neutral film are distinguished. The achievement of the quasi-metallic state is connected to a capacitive process in which an intermediate of uncompensated charge is involved.

Electrochemical polymerization of the tetrathienyl derivatives of the carbon group elements

The electropolymerization of four tetrathienyl compounds of the carbon group elements (T4M, where M ≡ Si, Ge, Sn or Pb) was studied on indium tin oxide electrodes in three organic solvents with low water content (acetonitrile, propylene carbonate and nitrobenzene). Only poorly conducting deposits were obtained in acetonitrile, but polymerization in propylene carbonate yielded partially electroactive films. The best polymers were formed in nitrobenzene, but none of the tetrathienyls tested produced polythiophene films superior to those prepared using thiophene as the monomer with the same experimental conditions. The polymerization mechanism of the thienylmetals is different from that of purely organic thiophene derivatives, as the oxidation produces neutral thiophene radicals in the former case, instead of radical cations. The results suggest that the reactive neutral thiophene radical can abstract a hydrogen atom from solvents such as acetonitrile, explaining the apparent contradiction in the polymerization efficiency of tetrathienyls and thiophene in this solvent.

Reasons behind the improved thermoelectric properties of poly(3-hexylthiophene) nanofiber networks

Enhanced thermoelectric properties of poly(3-hexylthiophene) nanofiber networks, doped in their reaction with silver cations, are presented. The role of charge carrier concentration and mobility (influenced by the supramolecular structure and nanoscale morphology) is discussed. The nanonet structure leads to a six fold increase in the ZT value compared to the bulk polymer counterpart.