Raman spectroelectrochemical study of Toluidine Blue,adsorbed and electropolymerized at a gold electrode

The redox dye Toluidine Blue (TB), adsorbed and electropolymerized at a roughened gold electrode, has been studied by Raman spectroscopy at λ_ex of 676.4 nm in an electrochemical cell. Most of the spectral bands have been assigned based on density functional theory (DFT) calculations. The number and position of the bands, as well as their intensity depend on electrode potential and solution pH, allowing one to discern different redox forms of TB or its polymer. Raman spectra, obtained in an acidic solution (pH 1.0) at a low electrode potential (−0.2 to 0.0 V vs. Ag/AgCl) are of low intensity, and correspond to the reduced (leucoform) of TB. At a higher potential (0.2–1.0 V), the intensity increases significantly, presumably due to resonance enhancement, and new bands appear, corresponding to oxidized form of TB. In a pH-neutral solution, the changes in spectra with varying electrode potential are less expressed. The observed changes in band positions and intensities have been analyzed.     

Electrochemical manipulation of localised pH: application to electroanalysis

The electrochemical manipulation of the local pH at a polymer functionalised electrode has been achieved in order to enhance the electrochemical response to cationic analytes. The changes in pH have been shown to provide a method for significantly enhancing the analytical signal towards the model compounds, dopamine and p-aminophenol. The procedure was found to operate irrespective of the electrical properties of the film. The main requirement for this electroanalytical system is that the film contains acidic groups within the polymer backbone. In the carboxylic acid functionalised polypyrrole film studied here, the performance was found to be greatest when the bulk solution pH was less than the pK_a of the acid groups. The mechanism attributed to the enhanced response is elucidated and the limitations of the technique are assessed.     

X-ray photoelectron and in situ and ex situ resonance Raman spectroscopic investigations of polythiophene overoxidation

Polythiophene films have been electrochemically synthesized on platinum electrodes by anodic oxidation of thiophene in CH₃CN-LiClO₄ organic solution, then galvanostatically polarized in the same but monomer-free electrolytic medium to obtain the reduced, oxidized, and overoxidized states of the polymer. X-ray photoelectron spectroscopy (XPS) measurements were performed to investigate the changes within the chemical composition accompanying the overoxidation of the polythiophene (PT) film and connected to degradation mechanisms reported in the literature. Ex situ resonance Raman spectroscopy (RRS) studies of the three oxidation states, with red and near-infrared excitation laser lines, lead to complementary insights about the degradation process of this polymer. Particularly, marked modifications of the positions, widths, and relative intensities of the Raman bands are attributed to structural transformations and the appearance of defects in the polymer chains. Moreover, in situ RRS experiments, during the gradual transition between the reduced and overoxidized states, allow to determine the electrochemical stability threshold of the PT. In the reversibility domain, the quinoid to aromatic intensity ratio of the ring C=C symmetric stretching obeys a Nernst-like equation when the applied potential to the PT/Pt working electrode is varied.     

Raman spectroelectrochemical study of electrode processes at Neutral red- and poly(Neutral red) modified electrodes

The redox dye Neutral red (NR), adsorbed and electropolymerized at a roughened gold electrode, has been studied by Raman spectroscopy at λ_ex of 676.4 nm in an electrochemical cell. Spectral bands have been assigned based on density functional theory (DFT) calculations. The number and position of the bands, as well as their intensity depend on electrode potential, allowing one to discern different redox forms of NR or its polymer. The observed changes in band positions and intensities have been analyzed. Electrooxidation of hydroquinone and ascorbic acid at a gold electrode modified with adsorbed or electropolymerized layer of NR has been studied with in situ Raman spectroelectrochemical technique. During electrooxidation of solution species, NR layer contains both oxidized and reduced forms of this modifier. It has been shown that the relative content of a reduced form of NR at electrode surface increases with increasing concentration of any of oxidizable species used. It has been concluded that anodic oxidation of ascorbic acid or hydroquinone at NR or polyNR modified electrode proceeds within the modifier layer rather than at a modifier/electrolyte interface. In this respect, electrooxidation follows a redox mechanism.     

Ascorbate amperometric determination using conducting copolymers from aniline and N-(3-propane sulfonic acid)aniline

The sequential electrochemical polymerization of aniline and N-(3-propane sulfonic acid)aniline (PSA) is proposed to construct a sensor able to detect ascorbate at physiological conditions. Compared to poly(aniline) modified electrode, a device with improved conducting and electrochemical properties at neutral pH is obtained. The electrochemical copolymerization of the same starting materials is also carried out. For a PSA:aniline ratio of 10:90, a polymer with a similar electrochemical behavior to the one grown in the sequential mode is observed.The detection of ascorbate was tested for both configurations at pH 7.2, the modified electrode is able to determine ascorbate at 0 mV versus Ag/AgCl; an optimized sensor constructed by sequential polymerization can easily detect ascorbate concentrations with a detection limit of 2.2 μM. Uric acid and dopamine does not interfere in the ascorbate determination.     

Copper recovery by polymer enhanced ultrafiltration (PEUF) and electrochemical regeneration

In this work, the removal of Cu²⁺ from a synthetic effluent has been tested by means of polymer enhanced ultrafiltration (PEUF), using partially ethoxylated polyethylenimine (PEPEI) as water-soluble polymer. Overall, the two necessary steps of a hypothetical continuous process, metal retention (in total recirculation and discontinuous mode) and polymer regeneration (in discontinuous mode), have been confronted individually. On the one hand, the values of temperature (T), transmembrane pressure (ΔP), metal–polymer ratio and pH that maximize both, permeate fluxes and rejection coefficients, have been obtained by ultrafiltration tests, reaching Cu²⁺ retention coefficients higher than 97%. On the other hand, the polymer regeneration step has been carried out by the electrochemical technique, which consists in the metal electrodeposition on the cathode of an electrochemical cell. In a first step, cyclic voltammetries have been carried out to assure the polymer does not suffer any oxidation or reduction process. From these tests, a cathodic working potential has been selected to minimize hydrogen evolution reaction (−0.7 V vs. Ag/AgCl). Working at this voltage in deposition tests, a pH of 3.3 has been selected from experiments at different pH values. This pH is less extreme than the pH necessary if this step was carried out chemically (pH 2).     

Three-dimensional imaging of proton gradients at microelectrode surfaces using confocal laser scanning microscopy

The first use of fluorescence confocal laser scanning microscopy (CLSM) to image three-dimensional pH gradients at electrode surfaces is described, using the reduction of benzoquinone (BQ) to hydroquinone in aqueous solution as an example. The associated local pH changes accompanying the process have been imaged using a trace amount of fluorescein, which has a pH-dependent fluorescent signal. Images recorded in x–y–z space, allow pH profiles to be obtained as a function of applied electrode potential. Experimentally determined profiles measured when BQ is reduced at a diffusion-limited rate are shown to be in good agreement with predictions from numerical simulation. Future applications of CLSM for pH imaging at electrode surfaces and its use in scanning electrochemical microscopy (SECM) are highlighted briefly.     

The effects of abrasive particles on the electrochemical behavior of adrenaline at different electrodes

Cyclic voltammetry (CV), double-potential step chronocoulometry (DPSCC), and electrochemical impedance spectroscopy (EIS) techniques have been performed to study the effects of abrasive particles on the electrochemical reaction of adrenaline at glassy carbon electrode (GCE) and platinum electrode in 0.5 mol/L H₂SO₄ solution. For the electrochemical reaction of adrenaline, it was shown that abrasive particles have a more marked electrocatalytic effect at GCE compared to that at platinum electrode. The electrocatalytic effect of SiC coated GCE is more obvious comparing to that of Al₂O₃ coated GCE. With the coarse degree of the abrasive paper increasing, the peak current (i _p) increases significantly and the peak-to-peak potential separation (ΔE _p) changes a little at the pretreated GCE. The electron transfer process of adrenaline at the different pretreated GCE is controlled by the diffusion in this system.     

Electrochemical monitoring of photoelectrocatalytic degradation of rhodamine B using TiO2 thin film modified graphite electrode

A commercially available TiO₂ powder (Degussa P25) has been used to prepare thin films on graphite plates. The photoelectrochemical degradation of rhodamine B was investigated using this photoelectrode. The effects of applied potential, pH, and initial rhodamine B concentration on the photoelectrocatalytic (PEC) degradation of rhodamine B using ultraviolet illuminated TiO₂/graphite (TiO₂/C) thin film electrode were examined and discussed. Also, direct photolysis, electrochemical oxidation, photocatalytic, and PEC degradation of rhodamine B were compared. Results show that the best responses for PEC are obtained at applied potential of 1.2?V vs. reference electrode, pH?4.0, and initial rhodamine B concentration of 4.2?mg?L^?1.     

Switchable electrode controlled by Boolean logic gates using enzymes as input signals

Application of Boolean logic operations performed by enzymes to control electrochemical systems is presented. Indium–tin oxide (ITO) electrodes with the surface modified with poly-4-vinyl pyridine (P4VP) brush were synthesized and used as switchable electrochemical systems. The switch ON and OFF of the electrode activity were achieved by pH changes generated in situ by biocatalytic reactions in the presence of enzymes used as input signals. Two logic gates operating as AND/OR Boolean functions were designed using invertase and glucose oxidase or esterase and glucose oxidase as input signals, respectively. The electrode surface coated with a shrunk P4VP polymer at neutral pH values was not electrochemically active because of the blocking effect of the polymer film. The positive outputs of the logic operations yielded a pH drop to acidic conditions, resulting in the protonation and swelling of the P4VP polymer allowing penetration of a soluble redox probe to the conducting support, thus switching the electrode activity ON. The electrode interface was reset to the initial OFF state, with the inhibited electrochemical reaction, upon in situ pH increase generated by another enzymatic reaction in the presence of urease. Logically processed biochemical inputs of various enzymes allowed reversible activation–inactivation of the electrochemical reaction.     

Voltammetric Characterization of Grafted Polymer Modified with ZnO Nanoparticles on Glassy Carbon Electrode

In this study, a grafted polymer (GP) with ZnO nanoparticles (GP/ZnO NPs) was attached on the surface of glassy carbon electrode (GCE), in order to produce a new modified electrode (GP/ZnO NPs-GCE). The gamma irradiation method was used to grafted polystyrene (polymer) with acrylonitrile (monomer), while slow evaporation process was used to prepare the new modified electrode. The cyclic voltammetry (CV) of K₄[Fe(CN)₆] was used to study the electrochemical properties GP/ZnO NPs-GCE. The peak separation (ΔE_pa-c) was 500 mV between the redox peaks of Fe(II)/Fe(III) in an aqueous solution of 1 M KCl and the current ratio of redox current peaks (I_pa/I_pc) was ≈ 1 for the modified electrode. This indicated that the modified electrode has s good reversibility and conductivity, wherefore; it was applied in the voltammetric filed. It was found that the modified electrode GP/ZnO NPs-GCE have a reasonable solubility and stability at various pH medium. Additionally, the sensitivity of the electrochemical analysis by cyclic voltammetric (CV) method is extensively subjected to the pH medium and the scan rate (SR). A couple of redox current peaks of K₄[Fe(CN)₆] in KCl solution was observed with a reversible process: Fe³⁺/Fe²⁺. Finally a good diffusion coefficient of electroactive species (D) for the new modified electrode was found in this study by chronoamperometry method using Cottrell equation.     

Electrochemical behavior of mesidine in aqueous solutions

The electrochemical oxidation of mesidine has been investigated in the pH range 0.7–11 at a platinum electrode with periodical renewal of the diffusion layer and at a stationary carbon paste electrode. A E_1/2/pH relation for the platinum electrode has been derived in the pH range 0–7. It has been indicated that the only oxidation products of mesidine are 2,6-dimethylbenzoquinone-4-(2′,4′,6′-trimethyl)-anil at pH <8 and 2,4,6,2′,4′,6′-hexamethylazoxybenzene at pH>8. Oxidation mechanisms consistent with the kinetic parameters and the oxidation products are discussed.     

Polymer/Iron Oxide Nanoparticle Modified Glassy Carbon Electrodes for the Enhanced Detection of Epinephrine

Novel electrochemical sensors for epinephrine (EP) based on a glassy carbon electrode (GCE) modified with a redox polymer film and iron (III) oxide nanoparticles (Fe₂O₃NP) have been developed. Two redox polymers‐poly(brilliant cresyl blue) (PBCB) and poly(Nile blue) (PNB), and two different architectures‐polymer/Fe₂O₃/GCE and Fe₂O₃/polymer/GCE were investigated. The electrochemical oxidation of epinephrine at the modified electrodes was performed by differential pulse voltammetry (DPV), in pH 7 electrolyte, and the analytical parameters were determined. The results show enhanced performance, more sensitive responses and lower detection limits at the modified electrodes, compared to other electrochemical epinephrine sensors reported in the literature. The best voltammetric response with the lowest detection limit was obtained for the determination of epinephrine at PBCB/Fe₂O₃/GCE. The novel sensors are reusable, with good reproducibility and stability, and were successfully applied to the determination of epinephrine in commercial injectable adrenaline samples.     

Degradation of perfluorinated sulfonic acid films: An in-situ infrared spectro-electrochemical study

The in-situ Fourier transform infrared (FTIR) spectro-electrochemical method was used to evaluate the degradation mechanism of perfluorinated sulfonic acid (PFSA) polymer for the first time. The effect of H₂O₂ concentration in Fenton's reagent on the PFSA film degradation was studied. It was found that the effect of the H₂O₂ concentration on the chemical degradation of the PFSA film was limited, whereas, the electrochemical degradation was obviously enhanced at higher H₂O₂ concentrations. The chemical degradation of PFSA film in Fenton's reagent was similar to that in pure H₂O₂ solution. However, the results indicated that the electrode potential is the dominating factor affecting the degradation of the PFSA film. It is clear that the electrochemical degradation of PFSA film in Fenton's reagent is more serious than that in pure H₂O₂ solution. Moreover, it is indicated that the main cause of the membrane degradation is the instability of the backbone of the polymer chain under electrochemical conditions and secondarily the loss of sulfuric groups in the side-chains. It is concluded that the in-situ FTIR spectro-electrochemical method could be used to evaluate more objectively the degradation of the polymer film.     

Real-time surface-enhanced Raman spectroscopy monitoring of surface pH during electrochemical melting of double-stranded DNA

The application of a negative potential ramp at a double-stranded DNA (dsDNA) functionalized electrode surface results in the gradual denaturation of the DNA in a process known as electrochemical melting. The underlying physical chemistry behind electrochemically driven DNA denaturation is not well understood, and one possible mechanism is a change in local pH at the electrode surface. We demonstrate that by coimmobilization of p-mercaptobenozic acid at a dsDNA-functionalized electrode surface, it is possible to monitor both DNA denaturation and the local pH simultaneously using surface-enhanced Raman spectroscopy. We find that the local pH at the electrode surface does not change as the applied potential is scanned negative and the dsDNA denatures. We therefore conclude that in these experiments electrochemical melting is not caused by electrochemically driven local pH changes.     

Long-term equilibrium potential and electrochemical impedance study of Ag/AgCl electrodes used in Harned Cell measurements of pH

A long term study of the voltage and electrochemical impedance characteristics of Ag/AgCl electrodes used in Harned Cell measurement of pH is presented. By all the measures investigated the electrodes are shown to degrade only slowly until approximately 200 days after manufacture, after which time the rate of degradation and critical failure of the electrodes increases. The absolute voltage drift of the electrodes may not be easily measured, so parameters determined directed or indirectly by electrochemical impedance spectroscopy have been assessed as a method to produce an alternative indication of electrode integrity. In this respect, resistance to charge transfer has been shown to be a very sensitive measure of changes in the characteristics of the electrodes, and the most closely related to the observed changes in voltage. Evidence is presented to support the hypothesis that the majority of electrode degradation (excluding critical failure) comes from the increased blocking of the microporous structure of the electrodes.     

Voltammetric properties at pH 14 of electrodes modified by azobenzene polymers

Azobenzene polymers were prepared by condensation of p-phenylazobenzoyl chloride and poly(ethylenimine). Their loadings in electroactive sites range from 5 to 95%. They were adsorbed on a glassy carbon electrode or on a hanging mercury drop electrode (HMDE) whose area could be expanded after the adsorption. The voltammetric behavior of the polymeric films is described at pH 14. The azobenzene sites which are in the vicinity of the electrode surface are electroactive, but the electrochemical reaction does not propagate to the bulk of the coating. When the loading of the polymer is not too high, the expansion of the HMDE causes an increase in the number N of azobenzene double bonds which are reduced, N remaining proportional to the drop area A, because-the film is sufficiently flexible to cover the new surface which appears (soap bubble effect). For the highly loaded polymers (loading larger than about 50%), conversely, N remains nearly constant, owing to the rigidity of the film, which causes it to break up when the drop is expanded. The reversibility of the electrochemical reaction depends both on the loading and on the expansion, which could be due to changes in the orientation of the azobenzene molecules at the surface of the electrode.     

Electrochemical characterization of the redox couple of Fe(III)/Fe(II) mediated by grafted polymer electrode

A new grafted polymer electrode (GPE) (polystyrene as polymer) was grafted with acrylonitrile as a monomer using gamma irradiation to produce a new grafted polymer. The redox process of K₃Fe(CN)₆ during cyclic voltammetry was studied by the new GPE. The ratio of Ipc/Ipa >1 of GPE to GCE Ipc/Ipa = 1.7, indicating that this electrode is a reversible electrode and can be used in conductivity studies by voltammetric analysis. The physical properties of the new electrode GP have good hardness, insolubility, and stability at different high temperatures and at different pH. Also, the sensitivity under conditions of cyclic voltammetry is significantly dependent on pH, electrolyte, and scan rate. At different scan rates, two oxidation peaks and two reduction peaks of Fe(III) were observed in a reversible process: Fe(III) Fe(II), and Fe(II) Fe(0). Interestingly, the redox reaction of Fe(III) solution using GPE remained constant even after 15 cycles. It is therefore evident that the GPE possesses some degree of stability. The potential use of the grafted polymer as a useful electrode material is therefore clearly evident.     

Stability of conducting polyester/polypyrrole fabrics in different pH solutions. Chemical and electrochemical characterization

The stability of conducting fabrics of polyester (PES) covered with polypyrrole/anthraquinone sulfonic acid (AQSA) has been tested in different pH solutions (1, 7, 13) and after washing tests. It is important to determine the stability of the counter-ion in the polymer matrix, since its loss causes the decrease of the conducting properties of the fabrics. X-ray photoelectron spectroscopy (XPS) studies were done to quantify the amount of counter-ion in the polymer and to obtain the doping level (N^δ+/N). Surface resistivity changes after the different tests were measured by electrochemical impedance spectroscopy (EIS). An increase in the solution pH caused a decrease of the doping level (N^δ+/N), the release of part of the counter-ions and an increase in the surface resistivity. Cyclic voltammetry (CV) measurements showed a gradual loss of electroactivity as pH increased. The influence of the scan rate on the characterization of conducting fabrics has been also demonstrated by CV. Lower scan rates produce a more characteristic response than higher ones. Scanning electrochemical microscopy (SECM) measurements showed a loss of electroactivity when the sample was tested in the pH 13 solution, although the material continued being electroactive.     

Electrochemical Monitoring of Photoelectrocatalytic Degradation of 3,4‐Dichlorophenol Using TiO2 Thin Film Modified Graphite Electrode

A novel electrode was prepared by forming TiO₂ thin films using a commercially available TiO₂ powder (Degussa P25) on graphite plates for water photoelectrocatalytic decontamination. In addition to, for the first time the photoelectrochemical degradation of 3,4‐dichlorophenol was investigated. The effects of applied potential, pH, and initial 3,4‐dichlorophenol concentration on the photoelectrocatalytic (PEC) degradation of 3,4‐dichlorophenol using ultraviolet (UV) illuminated TiO₂/graphite (TiO₂/C) thin film electrode was examined and discussed. Also, direct photolysis (DP), electrochemical oxidation (EC), photocatalytic (PC) and photoelectrocatalytic (PEC) degradation of 3,4‐dichlorophenol were compared. Results show that the best responses for PEC are obtained at applied potential 1.2 V versus reference electrode, pH 8.0 and initial 3,4‐dichlorophenol concentration 6.7 mg L^?1.