Formation and redox behaviour of polycarbazole prepared by electropolymerization of solid carbazole crystals immobilized on an electrode surface

Carbazole solid crystals have been mechanically attached to platinum or gold electrodes by two different methods and investigated by cyclic electrochemical quartz crystal microbalance measurements in the presence of aqueous acidic media. It was demonstrated that oxidative dimerization and polymerization can also be accomplished under such conditions. During electropolymerization, anions and water molecules enter the surface layer; however, these species leave the film after the reduction of the polymer formed. The mass changes observed in the course of the redox transformations of polycarbazole film can be explained by potential- and time-dependent sorption/desorption of H⁺ and ClO₄ ⁻ ions. The electroactivity of the polymer diminishes in more dilute acid media; however, it is recovered again in concentrated HClO₄ solutions. In more dilute acid solution the extent of the water sorption (film swelling) increases.     

Thin-film electrochemical sensor for diphenylamine detection using molecularly imprinted polymers

This work reports on the development of a new voltammetric sensor for diphenylamine based on the use of a miniaturized gold electrode modified with a molecularly imprinted polymer recognition element. Molecularly imprinted particles were synthesized ex situ and further entrapped into a poly(3,4-ethylenedioxythiophene) polymer membrane, which was electropolymerized on the surface of the gold electrode. The thickness of the polymer layer was optimized in order to get an adequate diffusion of the target analyte and in turn to achieve an adequate charge transfer at the electrode surface. The resulting modified electrodes showed a selective response to diphenylamine and a high sensitivity compared with the bare gold electrode and the electrode modified with poly(3,4-ethylenedioxythiophene) and non-imprinted polymer particles. The sensor showed a linear range from 4.95 to 115 μM diphenylamine, a limit of detection of 3.9 μM and a good selectivity in the presence of other structurally related molecules. This sensor was successfully applied to the quantification of diphenylamine in spiked apple juice samples.     

Adsorption of SDS and PEG on calcium fluoride studied by sum frequency generation vibrational spectroscopy

The adsorption of sodium dodecyl sulfate (SDS) from aqueous solution onto a calcium fluoride substrate (CaF(2)), in the presence of polyethylene glycol (PEG) of different molecular weights, has been investigated using the interface specific nonlinear optical technique of sum frequency generation (SFG) vibrational spectroscopy. Spectra of adsorbed SDS (in the C-H stretching region) were recorded at the surface of a CaF(2) prism in contact with SDS solutions at concentrations up to the cmc (8 mM) of the pure surfactant and in contact with binary solutions containing SDS and PEG with molecular weights from 400 to 12 000. In contrast with SFG spectra from the same combinations of surfactant and polymer on a hydrophobic surface, there was no evidence of spectra arising from the actual polymer adsorbed on CaF(2) at any polymer molecular weight either in the absence or presence of surfactant. However, there was indirect evidence for the presence of adsorbed polymer from changes in the SDS SFG spectra in the presence of polymer compared with spectra when the polymer was absent. The SFG spectra of SDS at 0.8 mM were closely similar to each other at all polymer molecular weights and different from the spectra in the absence of the polymer. The spectral differences between the polymer present and polymer absent was much smaller when the solution concentration of surfactant was 8 mM.     

Use of linear polymers to control the preparation of luminescent organic microcrystals

Microcrystals of an organic fluorescent dye, 4-octylamino-7-nitrobenz-2-oxa-1,3-diazole, were generated using the reprecipitation method, which is a solvent exchange process. In the presence of polymers, namely, poly(acrylic acid), molecular weight 5100 g mol(-1) and 15 000 g mol(-1), and poly(acrylic acid-co-maleic acid), average molecular weight about 50 000 g mol(-1), used as their sodium salts, the reprecipitation process was strongly accelerated. The reprecipitation kinetics was monitored by UV/vis absorption spectroscopy and revealed a three-step mechanism, each step being influenced by the polymer. The size and shape of the microcrystals were analyzed by fluorescence microscopy. The microcrystals obtained in the presence of polymers were smaller and more regular than those prepared in water alone and were not agglomerated. When the polymer was placed in the reprecipitation medium before introducing the dye solution, the microcrystals displayed a rectangular shape. When the polymer was introduced 20 min after the beginning of the reprecipitation process, intricately structured flowerlike microcrystals were observed. Microanalysis revealed that the microcrystals contained noticeable amounts of polymer. The measurement of the surface electric zeta potential suggested that a proportion of the polymer was present at the microcrystal surface. This work gives a thorough insight into a field where trials have until now been performed in an empirical way. It opens new perspectives to produce low-cost organic microcrystals, potentially useful in optics or pharmaceutical sciences.     

Influence of aqueous electrolytes on the wetting behavior of hydrophobic solid polymers-low-rate dynamic liquid/fluid contact angle measurements using axisymmetric drop shape analysis

The interaction of inorganic ions with low-energy hydrophobic surfaces was examined using model systems of solid polymers without ionizable functional surface groups in aqueous electrolyte solutions. Low-rate dynamic contact angle measurements with captive bubbles in conjunction with axisymmetric drop shape analysis (ADSA) were performed to study the influence of electrolyte ions (in the aqueous test solutions) on the wettability of the polymers. When various types of ions were used, no significant change in advancing and receding contact angles was observed. The contact angle hysteresis was small. The zeta potential of the model polymers in aqueous electrolyte solutions was determined from streaming potential measurements. The variation of the zeta potential at different pH levels indicates preferential adsorption of hydroxyl ions at this interface. However, the presence of electrolytes at the interface between water and the different model polymers did not influence the macroscopic contact angle. The results may suggest the absence of any specific interaction between the ions and the solid polymer, as this should result in changes of hydrophobicity. Similar to the air/water interface, the composition and the potential of the polymer/water interface are obviously determined predominantly by the aqueous phase with only slight influence from the solid phase.     

Microgravimetric study of electrochemically controlled nucleophilic addition of sulfite to polyaniline

The sulfonation of polyaniline (PANI) films by nucleophilic addition of sulfite ion has been controlled through the polymer oxidation state under electrochemical control. The process was monitored by in situ electrochemical quartz crystal microbalance (EQCM), and the polymer oxidation was accomplished by electrode potential steps in sulfite aqueous solutions. The nucleophilic addition of sulfite to PANI only takes place on the oxidized polymer. From the ratio of added mass to the injected charge, the degree of sulfonation has been obtained with a yield as high as 50%. It has been observed that the ion-exchange mechanism during the oxidation-reduction process in the resulting sulfonated polymer is analogous to the polymer produced by electrophilic sulfonation of polyaniline or by copolymerization of aniline with aminosulfonic acids, unlike the ionic exchange observed for unmodified PANI.     

Mechanistic – Kinetic Scheme of Oxidation/Reduction of TEMPO Involving Hydrogen Bonded Dimer. RDE Probe for Availability of Protons in Reaction Environment

Mechanistic – kinetic studies on the electrochemical oxidation/reduction process of radical TEMPO (2,2,6,6‐tetramethylpiperidine‐1‐oxyl) under ionic strength (0.1 M, 1.0 M) and pH (0, 7) of aqueous perchlorate electrolyte (NaClO₄‐HClO₄) have been undertaken. Analytical and/or digital simulation methods for voltammetry at stationary (CV) and rotating electrode (RDE) have allowed one to determine numerical values of twelve parameters characterizing two electrode reactions (oxidation and reduction of the radical) and three chemical reactions (protonation, disproportionation, dimerization involving the radical and/or electrogenerated species). A potential window of the measurements was 0.6 V and it corresponded to that where the oxidation wave of TEMPO in neutral aqueous solution is situated. To account for the observed pH effect, the hydrogen bonded dimer resulting from the radical reactant and the protonation product of its reduction has been postulated to form in solution near the electrode surface. The RDE voltammetric discernables of the TEMPO process (i.e., absolute RDE wave current, zero RDE current potential, oxidation and reduction limiting RDE currents) can be considered good candidates for a use to follow acidity of complex reactive media.     

Surface electrochemistry of the oxidation reactions of α- and β-alanine at a platinum electrode

The electrochemical oxidation reactions of α- and β-alanine at a Pt electrode were investigated in aqueous solutions at pH 1, 7, and 13 using steady-state current-potential measurements, cyclic voltammetry, and open circuit potential decay. The capacitance behaviour and the high Tafel slopes suggest the production of free radicals at the surface of the electrode accompanied by a second reaction involving loss of CO₂ which is the rate determining step. In the surface electro-oxidation of α-alanine, it appears that the adsorbed intermediate species is either hydrolyzed anodically to acetaldehyde and ammonia, or is oxidized to a carbonium ion which is subsequently hydrolyzed to acetaldehyde and ammonia in solution, analogous to the behaviour observed for glycine [D.G. Marangoni, R.S. Smith and S.G. Roscoe, Can. J. Chem., 67 (1989) 921]. The mechanisms for β-alanine would be similar except carbonium ion formation would probably be accompanied by a hydride transfer to form acetaldehyde. No dimerized products were detected by gas chromatography. These mechanisms differ from the dimerization process typical of the radical reactions associated with the Kolbe mechanism.     

Reduction of friction at oxide interfaces upon polymer adsorption from aqueous solutions

Reduction of the interfacial friction for the contact of a silicon oxide surface with sodium borosilicate in aqueous solutions has been accomplished through the adsorption of poly(L-lysine)-graft-poly(ethylene glycol) on one or both surfaces. Spontaneous polymer adsorption has been achieved via the electrostatic attraction of the cationic polylysine polymer backbone and a net negative surface charge, present for a specific range of solution pH values. Interfacial friction has been measured in aqueous solution, in the absence of wear, and on a microscopic scale with atomic force microscopy. The successful investigation of the polymer-coated interfaces has been aided by the use of sodium borosilicate microspheres (5.1 microm diameter) as the contacting probe tip. Measurements of interfacial friction as a function of applied load reveal a significant reduction in friction upon the adsorption of the polymer, as well as sensitivity to the coated nature of the interface (single-sided versus two-sided) and the composition of the adsorbed polymer. These measurements demonstrate the fundamental opportunity for lubrication in aqueous environments through the selective adsorption of polymer coatings.     

Silver nanoparticle decorated poly(2-aminodiphenylamine) modified carbon paste electrode as a simple and efficient electrocatalyst for oxidation of formaldehyde

This work describes the promising activity of silver nanoparticles on the surface of a poly(2-amino diphenylamine) modified carbon paste electrode (CPE) towards formaldehyde oxidation. Electrodeposition of the conducting polymer film on the CPE was carried out using consecutive cyclic voltammetry in an aqueous solution of 2-aminodiphenylamine and HCl. Nitrogen groups in the polymer backbone had a Ag ion accumulating effect, allowing Ag nanoparticles to be electrochemically deposited on the surface of the electrode. The electrochemical and morphological characteristics of the modified electrode were investigated. The electro-oxidation of formaldehyde on the surface of electrode was studied using cyclic voltammetry and chronoamperometry in aqueous solution of 0.1 mol/L NaOH. The electro-oxidation onset potential was found to be around -0.4 V, which is unique in the literature. The effect of different concentrations of formaldehyde on the electrocatalytic activity of the modified electrode was investigated. Finally, the diffusion coefficient of formaldehyde in alkaline media was calculated to be 0.47 × 10^-6 cm²/s using chronoamperometry.     

Oxidation of 2-mercaptobenzoxazole in aqueous solution: solid phase formation at glassy carbon electrodes

The redox reactions of 2-mercaptobenzoxazole (MBO) have been investigated by cyclic voltammetry at glassy carbon electrodes in aqueous solution. Four anodic and three cathodic processes could be identified. A more detailed analysis of the oxidation processes up to a potential of +0.6 V (SCE) and the corresponding reduction signals showed that the oxidation leads to bis(benzoxazolyl) disulfide (BBOD). Owing to its low solubility, the oxidation product remains at the electrode surface. This product has been identified by ex situ FTIR and XPS analysis. During the reduction of BBOD, mainly MBO is formed. The remarkable lower solubility of BBOD in aqueous solutions compared to MBO allows preparation of layers of BBOD in situ and to control the amount of deposited BBOD via the MBO solution concentration and electrolysis time. The peak potential and peak shape of the reduction signals change remarkably as the amount of BBOD increases from submonolayer coverage to coverages that correspond to multilayers. The behavior can be explained by assuming an electrochemical conversion of BBOD microcrystals, which are deposited on the electrode surface, if the amount of BBOD formed during the MBO oxidation exceeds one monolayer. Received: 21 January 1999 / Accepted: 15 March 1999     

Effect of anionic surfactant and short-chain alcohol mixtures on adsorption at quartz/water and water/air interfaces and the wettability of quartz

Measurements of the advancing contact angles for aqueous solutions of sodium dodecyl sulfate (SDDS) or sodium hexadecyl sulfonate (SHS) in mixtures with methanol, ethanol, or propanol on a quartz surface were carried out. On the basis of the obtained results and Young and Gibbs equations the critical surface tension of quartz wetting, the composition of the surface layer at the quartz-water interface, and the activity coefficients of the anionic surfactants and alcohols in this layer as well as the work of adhesion of aqueous solutions of anionic surfactant and alcohol mixtures to the quartz surface were determined. The analysis of the contact angle data showed that the wettability of quartz changed visibly only in the range of alcohol and anionic surfactant concentration at which these surface-active agents were present in the solution in the monomeric form. The analysis also showed that there was a linear dependence between the adhesion and the surface tension of aqueous solutions of anionic surfactant and alcohol mixtures. This dependence can be described by linear equations for which the constants depend on the anionic surfactant and alcohol concentrations. The slope of all linear dependence between adhesion and surface tension was positive. The critical surface tension of quartz wetting determined from this dependence by extrapolating the adhesion tension to the value equal to the surface tension (for contact angle equal zero) depends on the assumption whether the concentration of anionic surfactant or alcohol was constant. Its average value is equal to 29.95mN/m and it is considerably lower than the quartz surface tension. The positive slope of the adhesion-surface tension curves was explained by the possibility of the presence of liquid vapor film beyond the solution drop which settled on the quartz surface and the adsorption of surface-active agents at the quartz/monolayer water film-water interface. This conclusion was confirmed by the work of adhesion of aqueous solutions of anionic surfactants and short-chain alcohol mixtures to the quartz surface determined on the basis of the contact angle data and molar fraction of anionic surfactants and alcohols and their activity coefficient in the surface layer.     

Wetting characteristics of aqueous rhamnolipids solutions

The wetting properties of surfactants on solid surfaces form the basis of many industrial and biological processes. The preferential adsorption of the surfactants from aqueous solutions onto solid surfaces alter the adhesion tension of the surface and this behavior may cause partial to complete wetting of the surfaces by the aqueous surfactant solutions. However, different types of surfactants show different wetting characteristics. To study the wetting properties of biologically produced rhamnolipids (RL), advancing contact angles of the aqueous solutions of the RL mixture of R1 and R2 in a ratio of R2/R1=1.1 were measured as a function of surfactant concentration. For a comparison of the wetting performance, sodium dodecyl sulfate (SDS) was chosen as the reference surfactant. A hydrophilic glass surface, a hydrophobic polymer, polyethylene terephthalate (PET), and gold surface were used as the solid surfaces to determine the wetting characteristics of rhamnolipids. At low surfactant concentrations (RL concentration <3x10(-5)M, SDS concentration<3x10(-4)M) contact angle (Theta) varied in a certain range depending on the character of the surfactant interactions with the surface. This was followed by a decrease in contact angle. Parallel to this behavior, at low surfactant concentrations the adhesion tension decreased, then remained constant and an increase at higher surfactant concentrations was obtained on hydrophobic surfaces. On hydrophilic surfaces a steady decrease in adhesion tension was observed with both surfactant solutions.     

Monitoring of the electrochemical degradation of PEDOT films on gold using the bending beam method

Electrochemical and mechanical properties of thin poly(3,4-ethylenedioxythiophene) films deposited on gold have been investigated in aqueous sulfuric acid and sodium-sulfate solutions. It has been shown that at sufficiently positive electrode potentials, overoxidation of the polymer takes place. In some cases, only small changes could be observed in the shape of cyclic voltammograms taken in the “stability region??before and after overoxidation. In contrast to this, the impedance spectra recorded after overoxidation differed considerably from the impedance spectra of a freshly made electrode. Morphological changes of the polymer caused by overoxidation (degradation) could be detected by using the bending beam method.     

Electrodeposition of poly(N-vinylcarbazole) at the three-phase junction. Formation of very different polymer structures

Poly(N-vinylcarbazole) films can be deposited at the three-phase boundary when the organic phase contains only monomer, N-vinylcarbazole, while the aqueous phase contains supporting electrolyte. A cylindrical platinum microelectrode is immersed into the two-liquid system in such a way that a part of it is located in one liquid and the other part resides in the second liquid. The thickness of the reaction layer, or the width of the microelectrode zone where the polymer grows, depends on the kind of ions present in the aqueous phase and the time of the experiment. The structure of the deposited polymer may be very different and depends on the distance from the three-phase boundary and the type of the anion present in the aqueous phase. The key parameters here are the local electrode potential and the local concentration of the anions. The list of obtained polymer structures includes, among others, arrays of perfectly arranged deep oval channels and groups of microcrystals. The porosity of the polymer deposits increases with the distance from the aqueous phase.     

Contact mechanics studies with the quartz crystal microbalance: origins of the contrast factor for polymer gels and solutions

A contact mechanics methodology utilizing the quartz crystal microbalance (QCM) has been applied to study the spreading behavior of polymer solutions and gels. Changes in the resonant frequency and in the dissipation are monitored as these materials are brought into contact with the electrode surface of the QCM. The primary application is in studies of elastic polymer gels, where spreading over the surface of the QCM is limited by the elasticity of the gel. Simultaneous measurement of the applied loads and displacements, along with measurement of the QCM/gel contact area, the frequency shift, and the dissipation, enable us to calibrate the QCM as a contact sensor. While changes in the frequency and dissipation both depend linearly on the contact area, measurements of the dissipation provide a more reliable indicator. The relationship between the dissipation and the contact area is determined by the solvent viscosity and by the high-frequency intrinsic viscosity of the system of interest. This result is consistent with previous results on the high-frequency rheological behavior of polymer solutions.     

The effects of cross-linking and anodic surface roughening on quinone polymer/carbon electrodes

A polymer in which anthraquinone-2-carbonyl groups were bound to polyethyleneimine was coated onto a glassy carbon electrode. Electrodes of this kind were studied using cyclic voltammetry and pH 7 aqueous solutions. At pH <10 only those quinone units in contact with the carbon surface are electroactive. It was shown that anodic surface roughening increased the limited number of electroactive groups in the polymer film and gave more stable activity and narrower voltammetric peaks. Above pH 10 redox propagation through the layer is more rapid but the anionic product desorbs. This desorption was inhibited by cathodically cross-linking a layer of mixed polymers on a polyethyleneimine backboned polymer containing fluorenone units as well as anthraquinone units.     

Electrochemical Detection of Melamine

Poly(caffeic acid) polymer was immobilized onto the surface of a glassy carbon electrode via electropolymerization. Voltammetry shows a signal related to the two‐electron oxidation of the immobilized hydroquinone groups in the caffeic acid monomer units. The modified electrode in aqueous solution shows complexation of the electrogenerated o‐quinone species with melamine thus allowing in the electrochemical detection of melamine by recording the shift in potential of the oxidation signal of the polymer. Melamine detection was investigated in pure aqueous solutions and in the presence of milk powder solutions and the proposed analytical method of melamine detection in milk powder was applied successfully with an average recovery of ca. (91±7.9)%.     

Monitoring of the electrochemical degradation of PEDOT films on gold using the bending beam method

Electrochemical and mechanical properties of thin poly(3,4-ethylenedioxythiophene) films deposited on gold have been investigated in aqueous sulfuric acid and sodium-sulfate solutions. It has been shown that at sufficiently positive electrode potentials, overoxidation of the polymer takes place. In some cases, only small changes could be observed in the shape of cyclic voltammograms taken in the “stability region” before and after overoxidation. In contrast to this, the impedance spectra recorded after overoxidation differed considerably from the impedance spectra of a freshly made electrode. Morphological changes of the polymer caused by overoxidation (degradation) could be detected by using the bending beam method.

     

Graphoepitaxial deposition of cationic polymer micelles on patterned SiO2 surfaces

We have studied the deposition of polymer micelles formed from poly(styrene)-block-poly(2-vinylpyridine) (PS-PVPH+) from room-temperature aqueous solutions at pH 1 onto a hydrophilic Si/SiO2 surface with a relief pattern 100 nm deep with variable widths. It has been found that the micelle density is substantially higher and the ordering of the micelles is improved for micelles that adsorb in the 100 nm depressions in the width range of ca. 500-5000 nm. We ascribe these effects to capillary forces that pull the aqueous solution into the canyons where the micelles can be trapped. While the ordering of the micelles can be substantial, they do not form a perfect hexagonal crystal. If the surface is chemically modified by a Au coating, the micelle-surface interaction is strengthened and the degree of ordering is diminished. These results demonstrate that a combination of graphoepitaxy and processing conditions (speed of substrate withdrawal or evaporation of solvent) can be used to make fairly ordered polymer micelle arrays over a space of (at least) several millimeters.