Deposition electropolymerization

One-step electropolymerization deposition processes are envisaged as the most interesting deposition methods in view of potential technological applications. For this reason, there is a growing interest in the integration of materials obtained in such way to obtain multifunctional nanostructured composites and hybrid materials. The selective localization through electrophoretic deposition of nanotubes at the interface of polymers to obtain flexible, transparent and conductive materials could represent a novel approach. 

Although many scientists have put enormous effort into studying how the electropolymerization-deposition process exactly occurs [2, 7, 9], some aspects... 

Entrapment of biochemically reactive molecules into conductive polymer substrates is being used to develop electrochemical biosensors (212). This has proven especially useful for the incorporation of enzymes that retain their specific chemical reactivity. Electropolymerization of pyrrole in an aqueous solution containing glucose oxidase (GO) leads to a polypyrrole in which the GO enzyme is co-deposited with the polymer. These polymer-entrapped GO electrodes have been used as glucose sensors. A direct relationship is seen between the electrode response and the glucose concentration in the solution which was analyzed with a typical measurement taking between 20 to 40 s. 

Example 2-3 The electropolymeric growth of 2 ng polyphenol onto a gold QCM crystal A = 1 cm2 /0 = 5 MHz) resulted in a frequency change of 12 Hz. Calculate the frequency change associated with the deposition of 4ng polyphenol onto a 0.5 cm2 crystal (/0 = 8 MHz). 

Despite the vast quantity of data on the chemistry of electropolymerization, relatively little is known about the processes involved in the deposition of polymers on the electrode, i.e. the heterogeneous phase transition. Research — voltammetric... 

Electron transfer processes leading to a product adsorbed in the interfacial region o are of practical interest. These processes include the deposition of a metal such as Cu or Pd at ITIES, the preparation of colloidal metal particles with catalytic properties for homogeneous organic reactions, or electropolymerization. 

Electrochemistry is one of the most promising areas in the research of conducting polymers. Thus, the method of choice for preparing conducting polymers, with the exception of PA, is the anodic oxidation of suitable monomeric species such as pyrrole [3], thiophene [4], or aniline [5]. Several aspects of electrosynthesis are of relevance for electrochemists. First, there is the deposition process of the polymers at the electrode surface, which involves nucleation-and-growth steps [6]. Second, to analyze these phenomena correctly, one has to know the mechanism of electropolymerization [7, 8]. And thirdly, there is the problem of the optimization of the mechanical, electrical, and optical material properties produced by the special parameters of electropolymerization. 

Despite the vast quantity of data on electropolymerization, relatively little is known about the processes involved in the deposition of oligomers (polymers) on the electrode, that is, the heterogeneous phase transition. Research - voltammetric, potential, and current step experiments - has concentrated largely on the induction stage of film formation of PPy [6, 51], PTh [21, 52], and PANI [53]. In all these studies, it has been overlooked that electropolymerization is not comparable with the electrocrystallization of inorganic metallic phases and oxide films [54]. Thus, two-or three-dimensional growth mechanisms have been postulated on the basis that the initial deposition steps involve one- or two-electron transfers of a soluted species and the subsequent formation of ad-molecules at the electrode surface, which may form clusters and nuclei through surface diffusion. These phenomena are still unresolved. 

An alternate method to produce templated electrodes is the use of chemical reduction of the monomer in the presence of a track-etched or alumina membrane. Parthasarathy et al. [46] have produced enzyme-loaded nanotubules by a combination of both electrochemical and chemical deposition. Initially, the alumina membrane was sealed at one end with a thick Au film (Figure 1.9a), after which the membrane was placed into a mixture of pyrrole and Et4NBF4. The pyrrole was then electropolymerized to form a small plug of polypyrrole at the closed end of the alumina membrane (Figure 1.9b). Subsequently, the membrane was placed into a... 

Most suitable for electrically conducting materials such as carbon fibers, the electrochemical processes involve deposition of polymer coatings on the fiber surface through electrodeposition or electropolymerization techniques. The major advantage of these processes is that a uniform layer of controlled thickness and variable polymer structure and properties can be obtained by controlling the current and the solution concentration. 

Another form of electro-decoration is electropolymerization of the MPc complex, especially the MTAPc complex such as the NiTAPc onto CNT-modified electrodes [13], The beauty of this technique is that the thickness and morphology of the resulting MPc polymeric film may be easily controlled by manipulating the deposition voltage, the number of cycle scans and concentration of the MPc solution. 

It is possible to inhibit reaction (5) by employing spray pyrolysis to deposit a compact layer of 2 on the Sn02 substrate before deposition of the nanocrystalline 2 film [9,10]. It is, however, relatively difficult to achieve a pinhole-free film. We chose to investigate the electropolymerization of an insulating film on the exposed parts of the Sn02 after the nanocrystalline 2 film has been depos-... 

Fig. 9.9 CV and frequency-potential curves for the oxidation and re-reduction processes of the electropolymerized polyaniline film [26] (a) in 0.5 M LiCICVAN, and (b) in aqueous 0.5 M NaCl04+HCIO4 (pH = 1). Voltage sweep rate 5 mVs-1, quantity of film deposition 0.4Ccm-2, and SSCE = saturated NaCl calomel electrode.

Such bilayers can conveniently be built up by successive electropolymerization of complexes containing ligands with vinyl substituents, e.g. 4-vinylpyridine or 4-vinyl-4 -methyl-2,2 -bipyridyl. The films may be deposited on metallic or semiconductor electrodes (e.g. Pt, glassy carbon, Sn02, Ti02). More efficient metailation of the films is obtained by polymerization of coordinated ligand than by subsequent metailation of a preformed polymer film. An alternative to discrete films would be a copolymer with distinct redox sites, or a combination of a single polymer film with a copolymer film in a bilayer device. 

Biogenic amines, such as histamine [131], adenine [132], dopamine [133] and melamine [134], have been determined using chemosensors combining MIP recognition and PM transduction at QCM. Electronically conducting MIPs have been used in these chemosensors as recognition materials. Initially, functional electroactive bis(bithiophene)methane monomers, substituted either with the benzo-18-crown-6 or 3,4-dihydroxyphenyl, or dioxaborinane moiety, were allowed to form complexes, in ACN solutions, with these amines as templates. Subsequently, these complexes were oxidatively electropolymerized under potentiodynamic conditions. The resulting MIP films deposited onto electrodes of quartz resonators were washed with aqueous base solutions to extract the templates. 

The histamine [131], adenine [132] and dopamine [133] amines are electroactive in the positive potential range, in which the thiophene is electropolymerized. Therefore, these amines could be oxidized at the electrode surface in the course of deposition of the MIP film. That way, products of these oxidations might be available in the electrode vicinity for imprinting rather than the desired pristine... 

A nitrate-selective potentiometric MIP chemosensor has been devised [197, 198]. For preparation of this chemosensor, a polypyrrole film was deposited by pyrrole electropolymerization on a glassy carbon electrode (GCE) in aqueous solution of the nitrate template. Potentiostatic conditions of electropolymerization used were optimized for enhanced affinity of the resulting MIP film towards this template. In effect, selectivity of the chemosensor towards nitrate was much higher than that to the interfering perchlorate ( o3 cio4 = 5.7 x 10-2) or iodide ( N03, r = x 10 2) anion. Moreover, with the use of this MIP chemosensor the selectivity of the nitrate detection has been improved, as compared to those of commercial ISEs, by four orders of magnitude at the linear concentration range of 50 pM to 0.5 M and LOD for nitrate of (20 10) pM [197]. 

An alkaloid pain reliever, morphine, is an often abused drug. Chronoampero-metric MIP chemosensors have been devised for its determination [204]. In these chemosensors, a poly(3,4-ethylenedioxythiophene) (PEDOT) film was deposited by electropolymerization in ACN onto an ITO electrode in the presence of the morphine template to serve as the sensing element [204], Electrocatalytic current of morphine oxidation has been measured at 0.75 V vs AglAgCllKClsat (pH = 5.0) as the detection signal. A linear dependence of the measured steady-state current on the morphine concentration extended over the range of 0.1-1 mM with LOD for morphine of 0.2 mM. The chemosensor successfully discriminated morphine and its codeine analogue. Furthermore, a microfluidic MIP system combined with the chronoamperometric transduction has been devised for the determination of morphine [182] with appreciable LOD for morphine of 0.01 mM at a flow rate of 92.3 pL min-1 (Table 6). 

An imprinted poly[tetra(o-aminophenyl)porphyrin] film, deposited on a carbon fibre microelectrode by electropolymerization, was used for selective determination of dopamine [208] in the potential range of —0.15 to 1.0 V. This chemosensor has been used successfully for dopamine determination in brain tissue samples. The dopamine linear concentration range extended from 10 6 to 10-4 M with LOD of 0.3 pM. However, this LOD value is very high compared to that of the dopamine voltammetric detection using polyaminophenol MIPs prepared by electropolymerization [209]. Dopamine was determined by CV and DPV at concentrations ranging from 2 x 10 s to 0.25 x 10 6 M with LOD of 1.98 nM. This LOD value is lower than that of PM dopamine detection [133]. 

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