Abstract: Porous sulfonic acid functionalized silica spheres have been prepared by oxidation of thiol-functionalized mesoporous silica samples obtained by co-condensation of (mercaptopropyl)trimethoxysilane and tetraethoxysilane in the presence of cetyltrimethylammonium as a template. The physicochemical characteristics of the resulting ion exchangers have been analyzed by various techniques and discussed with respect to the amount of functional groups in the materials. Their ion exchange behavior was then studied from batch experiments (determination of cation exchange capacities) and by electrochemistry at carbon paste electrodes modified with these solids. In particular, ion exchange voltammetry applied to two model electroactive cations, Cu2+ and Ru(NH3)63+, has pointed out the key role played by the content of organofunctional groups in the materials (which strongly affects their structure and porosity) on their performance as electrode modifiers for preconcentration of target analytes prior to electrochemical...

Abstract: The bismuth film electrode (BiFE) is presented for use in both batch voltammetric and flow injection (FI) amperometric detection of some nitrophenols (2-nitrophenol, 2-NP; 4-nitrophenol, 4-NP; 2,4-dinitrophenol, 2,4-DNP). The bismuth film was deposited ex situ (batch measurements) and in-line (FI) onto a glassy carbon substrate electrode. Batch analysis of the nitrophenols was carried out in 0.04 M Britton Robinson (BR) buffer pH 4, while for FI measurements, a carrier/electrolyte solution composed of 0.1 M BR buffer pH 4 mixed with methanol (20+80, v/v%) was employed to resemble media used in preconcentration/clean-up and flow separation sample pretreatment procedures. Under batch conditions, the voltammetric behavior of the nitrophenols was examined for dependence on medium pH in the range of 2 to 10. Employing the square-wave voltammetry mode, the limits of detection were 0.4 μg L−1, 1.4 μg L−1, and 3.3 μg L−1 for 2-NP, 4-NP, and 2,4-DNP, respectively. Under flow conditions, a simple in-line electrochemical bismuth film renewal procedure was tested and shown to provide very good inter- and intra-electrode reproducibility of the current signals at low μg L−1 analyte concentrations. The limits of detection for 2-NP, 4-NP and 2,4-DNP obtained using FI and amperometric detection at −1.0 V (vs. Ag/AgCl) were 0.3 μg L−1, 0.6 μg L−1 and 0.7 μg L−1, respectively, with linear ranges extending up to 20 μg L−1. The attractive performance of the BiFE under flow analysis conditions offers great promise with respect to its detection capability and to its use for a prolonged period of time with no need for inconvenient removal of the electrode from the system for mechanical surface treatment.

Abstract: The aggregation of α-synuclein, a 14 kDa protein, is involved in several human neurodegenerative disorders, including Parkinson's disease. We studied native and in vitro aggregated α-synuclein by circular dichroism (CD), atomic force microscopy (AFM) and electrochemical methods. We used constant current chronopotentiometric stripping analysis (CPSA) to measure hydrogen evolution catalyzed by α-synuclein (peak H) at hanging mercury drop electrodes (HMDE) and square-wave stripping voltammetry (SWSV) to monitor tyrosine oxidation at carbon paste electrodes (CPE). To decrease the volume of the analyte, most of the electrochemical measurements were performed by adsorptive transfer (medium exchange) from 3-6 L drops of α-synuclein samples. With both CPE and HMDE we observed changes in electrochemical responses of α-synuclein corresponding to protein fibrillization detectable by CD, fluorescence and AFM. Aggregation-induced changes in peak H at HMDE were relatively large in strongly aggregated samples, suggesting that this electrochemical signal may find use in the analysis of early stages of α-synuclein aggregation. This assumption was documented by marked changes in the peak H potential and height in samples withdrawn at the end of the lag and the beginning of the elongation phase. Native α-synuclein can be detected down to subnanomolar concentrations by CPSA.

Abstract: We report the use of electrolyte variations as a means to actively control redox and n-type doping processes in π-conjugated electroactive polyheterocycles. Specifically, a series of donor−acceptor bis-EDOT-pyridine polymers, poly[2,5-bis-(2,3-dihydro-thieno[3,4-b][1,4]dioxin-5-yl)-pyridine] (PBEDOT-Pyr) and poly[5,8-bis(3-dihydro-thieno[3,4-b][1,4]dioxin-5-yl)-2,3-diphenyl-pyrido[3,4-b]pyrazine (PBEDOT-PyrPyr(Ph)2), have been investigated. The use of soft, bulky cations, such as n-alkylammonium, allows true n-type doping to be observed in cyclic voltammetry, differential-pulse voltammetry, and in situ conductance experiments. Color changes are also observed upon conversion of the polymers from their neutral to reduced states. Hard, electrophilic cations, such as lithium and sodium, induce minimal current and conductivity responses for these polymeric systems. This “pinning” of the cation−anion pair causes a decrease in the electrochemical and conductivity response. Optical changes upon reduction are obse...

Abstract: The technique of Fourier transform large-amplitude alternating current (ac) cyclic voltammetry has been applied to a study of the electrochemistry of surface-bound azurin (Az) at a paraffin-impregnated graphite electrode. With the methodology used, the total current, as well as the dc component, and the ac harmonics can be obtained from a single experiment. For the dc (and fundamental harmonic cases), the background current dominates the voltammetry, so that quantitatively useful data only can be obtained after employment of an empirical background correction procedure. In contrast, the capacitance current does not contribute to the second and higher harmonic voltammograms. Furthermore, the higher harmonic Faradaic currents are greatly amplified when large-amplitude sinusoidal perturbations are employed as an alternative to the traditionally used small-amplitude ac methods. Results obtained from numerical simulations are presented for a quasi-reversible process (Butler−Volmer model) in order to illustrate...

Abstract: Ordered intermetallic phases of PtBi and PtPb have been prepared and studied [using cyclic voltammetry, rotating disk electrode voltammetry, and Fourier transform infrared (FTIR)] as potential fuel cell electrocatalysts using formic acid as a model system, elucidating electrochemical activity, reaction kinetics, and resistance to CO surface poisoning. Both electrodes displayed activity superior to that of pure Pt under any conditions tested but were found to roughen at significantly positive potentials (typically above +400 mV vs. Ag/AgCl) in acidic media, likely due to preferential dissolution of the p-block metal (Bi, Pb). Each electrode was subjected to a variety of surface treatments to obtain suitable cleaning methods. Mechanical polishing with alumina was found to be the best treatment for PtBi, while PtPb was fairly insensitive to treatment procedures and parameters. Using the rotating disk electrode method, a value of 1.4 x 10 -4 cm s -1 was obtained for the heterogeneous charge-transfer rate constant for PtPb in formic acid. Both PtBi and PtPb exhibited high tolerance to CO poisoning; no evidence of CO adsorption at any potential was found via FTIR.

Abstract: The steady-state voltammetric oxidation of hexachloroiridate(III), IrCl6(3-) (1-5 mM), in the presence and absence of an excess supporting electrolyte was investigated at disk- and hemispherical-shaped Pt electrodes with radii ranging from 48 nm to 12.5 microm. Thermodynamic, kinetic, and transport parameters that define the shape and magnitude of the voltammetric wave exhibit a complex dependence on whether a supporting electrolyte is present in the solution. First, the half-wave potential, E1/2, for oxidation of IrCl6(3-) shifts to more positive potentials in the presence of a supporting electrolyte, a consequence of the relative difference in the strength of ion pairing of IrCl6(3-) and IrCl6(2-) by the supporting electrolyte cation. E1/2 increases in the order no electrolyte < n-tetrabutylammonium < Na+ approximately K+ approximately Ca2+, but is independent of the supporting electrolyte anion (Cl-, NO3-, PF6-). Second, the heterogeneous electron-transfer rate constant for oxidation of IrCl6(3-) increases by approximately an order of magnitude in the presence of a supporting electrolyte. Third, in the absence of electrolyte, mass transport limited currents deviate significantly from predicted values based on the Nernst-Planck equation, but only when the electrode radius is smaller than ca. 1 microm. The latter two effects (Frumkin and dynamic diffuse layer effects) result from the dependence of interfacial electrical fields and, thus, the rates of electron-transfer and ion migration, on the supporting electrolyte concentration. We also demonstrate that the theoretical shape of the voltammetric response for oxidation or reduction of a highly charged redox species (e.g., IrCl6(3-)) is essentially independent of whether a supporting electrolyte is present in the solution. This finding can greatly simplify the analysis of heterogeneous electron-transfer rates using steady-state voltammetry in low ionic strength solutions.

Abstract: Electrochemical and electrocatalytic properties of heme proteins, including myoglobin (Mb), hemoglobin (Hb), and horseradish peroxidase (HRP), in layer-by-layer films assembled with SiO2 nanoparticles were investigated at pyrolytic graphite (PG) electrodes. The stable layer-by-layer {SiO2/protein}n films showed a pair of well-defined, nearly reversible cyclic voltammetric peaks at about −0.25 V (vs. SCE) at pH 5.5, characteristic of the protein heme FeIII/FeII redox couples. The proteins in the films displayed good electrocatalytic activities toward various substrates such as oxygen, hydrogen peroxide, trichloroacetic acid, and nitrite, and showed a potential applicability in fabricating the new kind of biosensors without using mediators. Electrochemical parameters of the protein films such as apparent heterogeneous electron transfer rate constant (ks) and formal potential (E°′) were estimated by square-wave voltammetry and nonlinear regression analysis. The influencing factors like the number of bilayer and the pH of external solution were also investigated.

Abstract: Liquid mercury free polished silver solid amalgam electrode (p‐AgSAE) proved experimentally a suitable substitute for the hanging mercury drop electrode in cathodic stripping voltammetric (CSV) analyses of purine bases and of acid treated DNA. The analyses are done in alkaline solution in the presence of copper(II) ions. With p‐AgSAE using direct current voltammetry (DCV) the limits of detection for purine bases as well as for two types of DNA were on nanomol level. Low levels of RSD (1–3%, n = 11) of repeated measurements confirmed that the used mode of electrochemical regeneration of p‐AgSAE was sufficient, and it provided reliable measurement results. The mentioned working electrode is suitable for a sensitive electrochemical detection of acid‐hydrolyzed DNA, and may be applied in DNA hybridization assays as an alternative to working electrodes based on liquid mercury [hanging mercury drop electrode (HMDE), meniscus AgSAE].