Determination of the interfacial acidity of the functional alkanethiols self-assembly membrane by capacitance measurement

The acid-base equilibrium of self-assembly monolayer(SAM) of mercaptopropionic acid on gold electrode was studied by capacitance titration. The change of capacitance was due to protonation and deprotonation of the modified electrode surface. This method can be used to measure pH value in solution and capacitance of solid/liquid on SAMs surface synchronously. The shift of pK(a) was observed during the experiment as previous reports. The factors that affect the capacitance measurement were discussed in detail.

Amperometric quantification of polar organic solvents based on a tyrosinase biosensor

A novel amperometric biosensor for quantification of the electrochemically inert polar organic solvents based on tyrosinase electrode was preliminarily reported. The biosensor was fabricated by simply syringing an aqueous solution of tyrosinase/PVAVP (PVAVP: copolymer of poly(vinyl alcohol) grafting with 4-vinylpyridine) onto glassy carbon electrode surface followed by drying the modified electrode at +4 degrees C in a refrigerator. The current generated from electrochemical reduction of quinone is a probe signal. The biosensor can be used for quantification of polar organic solvents, and its mechanism was characterized with in situ steady-state amperometry-quartz crystal microbalance experiments. The detection limit, sensitivity, and dynamic range for certain organic solvents are dependent on the kind and concentration of the substrate probe and the hydrophobicity of the immobilization matrix. The response time for all the tested organic solvents is less than 2 min.

Sol-gel-derived amperometric biosensor for hydrogen peroxide based on methylene green incorporated in Nafion film

A hydrogen peroxide biosensor was fabricated by coating a sol-gel-peroxidase layer onto a Nafion-methylene green modified electrode. Immobilization of methylene green (MG) was attributed to the electrostatic force between MG(+) and the negatively charged sulfonic acid groups in Nafion polymer, whereas immobilization of horseradish peroxidase was attributed to the encapsulation function of the silica sol-gel network. Cyclic voltammetry and chronoamperometry were employed to demonstrate the feasibility of electron transfer between sol-gel-immobilized peroxidase and a glassy carbon electrode. Performance of the sensor was evaluated with respect to response time, sensitivity as well as operational stability. The enzyme electrode has a sensitivity of 13.5 mu A mM(-1) with a detection limit of 1.0 x 10(-7) M H2O2, and the sensor achieved 95% of the steady-state current within 20 s. (C) 2000 Elsevier Science B.V. All rights reserved.

Protein structural characterization by scanning tunneling microscopy with electrochemistry

The structure characterization of proteins or enzymes by STM on electrochemically prepared HOPG surface studied in this laboratory is reviewed. The serial structures of Hb were observed. The differences between the denaturation and inactivation of HRP were investigated by in situ and ex situ STM. The structural variation of Hb in an organic solvent was imaged while protein denaturation was easily observed in a polar solvent.

Electric field-induced conformational transition of bovine serum albumin from alpha-helix to beta-sheet

The irreversible conformational transition of bovine serum albumin (BSA) from alpha-helix to beta-sheet, induced by electric field near the electrode surface, was monitored by circular dichroism (CD) with a long optical path thin layer cell (LOPTLC).

Electrochemical synthesis of polyaniline nanoparticles

Polyaniline nanoparticles were prepared on a highly oriented pyrolytic graphite (HOPG) surface from dilute polyaniline acidic solution (1 mM aniline + 1 M HClO4) using a pulsed potentiostatic method. Electrochemistry, Fourier transform infrared external reflection spectroscopy (FT-IR-ERS), X-ray photoelectron spectroscopy (XPS) and tapping-mode atomic force microscopy (TMAFM) were: used to characterize the composition and structure of the polyaniline nanoparticles. FT-IR-ERS and XPS results revealed that the polyaniline was in its emeraldine form. TMAFM measurement showed that the electropolymerized polyaniline nanoparticles dispersed on the:HOPG surface with a coverage of about 10(10) cm(-2). These nanoparticles were disk-shaped having a height of 10(-30) Angstrom and an apparent diameter varying from 200 to 600 Angstrom. The particle dimensions increased with the electropolymerization charge (Q) over the interval from 5.7 to 19.3 mu C cm(-2) (C) 2000 Elsevier Science S.A. All rights reserved.

Circular dichroism spectro electrochemical and voltammetric studies of vitamin B-2

Electroreduction of vitamin B-2 (VB2) was studied by in situ circular dichroism (CD) spectroelectrochemistry (SEC) with a long optical path length thin layer cell (LOPLTLC). The results showed that the electroreduction of VB2 in phosphate buffer solution (PBS) (PH 6.8) was a two-electron electrochemical process with weak adsorption of the reactant at the glassy carbon (GC) electrode surface. The CD spectra change of VB2 in the reduction process was explained with the theory of electronic states. We also treated the CD spectra with a singular value decomposition least square (SVDLS) method, and have found not only the number of components and their spectra, but also the fraction distribution of each component in the electroreduction process of VB2.

A novel electrochemical SPR biosensor

In this paper, we demonstrate for the first time that upon electrochemical oxidation/reduction, the transition in the conductivity of polyaniline (PAn) film on gold electrode surface leads to a large change of surface plasmon resonance (SPR) response due to a change in the imaginary part of dielectric constant of PAn film. Based on the amplifying response of SPR to the redox transformation of PAn film as a direct result of the enzymatic reaction between horseradish peroxidase (HRP) and PAn in the presence of H2O2, a novel PAn-mediated HRP sensor has been fabricated. The electrochemical SPR biosensor, unlike a usual binding assay with SPR, can afford a larger SPR response, and can also be reused by reducing the PAn film electrochemically to its reduced state. This method opens up a new route to the fabrication of SPR biosensor. (C) 2001 Elsevier Science BN. All rights reserved.

Formation of a self-assembled monolayer of 2-mercapto-3-n-octylthiophene on gold

Monolayer assembly of 2-mercapto-3-n-octylthiophene (MOT) having a relatively large headgroup onto gold surface from its dilute ethanolic solutions has been investigated by electrochemistry. An electrochemical capacitance measurement on the permeability of the monolayer to aqueous ions, as compared with its alkanethiol counterpart [CH3(CH2)(9)SH (DT)] with a similar molecular length, shows that the self-assembled monolayers (SAMs) of MOT can be penetrated by aqueous ions to some extent. Furthermore, organic molecular probes, such as dopamine, can sufficiently diffuse into the monolayer because a diffusion-limited current peak is observed when the dopamine oxidation reaction takes place, showing that the monolayer is loosely packed or dominated by defects. But the results of electron transfer to aqueous redox probes (including voltammetry in Fe(CN)(6)(3-/4-) solutions and electrochemical ac impedance spectrum) confirm that the monolayer can passivate the gold electrode surface effectively for its very low ratio of pinhole defects. Moreover, a heterogeneous patching process involving addition of the surfactants into the SAMs provides a mixed or hybrid membrane that has superior passivating properties. These studies show that the MOT monolayer on the electrode can provide an excellent barrier for hydrated ionic probe penetration but cannot resist the organic species penetration effectively. The unusual properties of the SAMs are attributed to the entity of the relatively large thiophene moiety between the carbon chain and the thiol group.

Capacitive detection of glucose using molecularly imprinted polymers

A novel glucose biosensor based on capacitive detection has been developed using molecularly imprinted polymers. The sensitive layer was prepared by electropolymerization of o-phenylenediamine on a gold electrode in the presence of the template (glucose). Cyclic voltammetry and capacitive measurements monitored the process of electropolymerization. Surface uncovered areas were plugged with 1-dodecanethiol to make the layer dense, and the insulating properties of the layer were studied in the presence of redox couples. The template molecules and the nonbound thiol were removed from the modified electrode surface by washing with distilled water. A capacitance decrease could be obtained after injection of glucose. The electrode constructed similarly but with ascorbic acid or fructose only showed a small response compared with glucose. The stability and reproducibility of the biosensor were also investigated. (C) 2001 Elsevier Science B.V. All rights reserved.

Effects of heteropoly acids and surfactant on electrochemiluminescence of tris(2,2 '-bipyridine) ruthenium(II)

The effects of heteropoly acids and Triton X-100 on electrochemiluminescence (ECL) of Ru(bpy)(3)(2+) are investigated. Triton X-100 prevents the oxidation of oxalate and results in an increase of the ECL signal. H5SiW11VO40 prevents the direct oxidation of oxalate and makes the electrochemical behavior of Ru(bpy)(3)(2+) less reversible, which leads to a decrease of the ECL signal. In contrast, H3PMo12O40 has negligible effect on ECL intensity. Some possible reasons for the effects on the ECL of Ru(bpy)(3)(2+) are discussed based on the adsorption of SiW11VO405- on electrode surface and the ion association between SiW11VO405- and Ru(bpy)(3)(2+). The signal of ECL decreases linearly with the concentration of heteropoly acid in the range from 2x10-6 to 1x10(-4) mol l(-1). The results indicate that ECL of RU(bpy)(3)(2+) is a potential sensitive and selective detection method for heteropoly acids and hence for the elements comprised in them.

Investigation of the self-accelerating process of ascorbic acid by in-situ circular dichroism spectroelectrochemistry

The electrode reaction process of ascorbic (Vc) was studied by in-situ circular dichroic(CD) spectroelectrochemistry with a long optical path thin layer cell on glassy carbon(GC) electrode. The spectroelectrochemical data were analyzed by the double logarithmic method together with nonlinear regression. The results suggested that the mechanism of Ve in pH 7.0 phosphate buffer solution at GC electrode was a two-electron irreversible electrooxidation followed by adsorption of the oxidation product. That is a self-accelerated process. Some kinetic parameters at free and at adsorbed electrode surface, i.e, the formal potentials, E-0' = 0.09 V, E-a(0') = 0.26 +/- 0.02 V; the electron transfer coefficient and number of transfered electron, alpha n = 0.41, alpha(a)n = 0.07;the standard heterogeneous electron transfer rate constant, k(0) = 8.0 x 10(-5) cm.s(-1), k(a)(0) = 1.9 x 10(-4) cm.s(-1) and adsorption constant, beta = 102.6 were also estimated.

Molecular characterization of beef liver catalase by scanning tunneling microscopy

Beef liver catalase molecules can stick tenaciously to the highly oriented pyrolytic graphite (HOPG) surface which has been activated by electrochemical anodization. The immobilized sample is stable enough for high resolution scanning tunneling microscope (STM) imaging. When the anodized conditions are controlled properly, the HOPG surface will be covered with a very thin oxide layer which can bind the protein molecules. Individual molecules of native beef liver catalase are directly observed in detail by STM, which shows an oval-shape structure with a waist. The dimensions of one catalase molecule in this study are estimated as 9.0 x 6.0x 2.0 nm(3), which are in good agreement with the known data obtained from X-ray analysis, except the height can not be exactly determined from STM. Electrochemical results confirm that the freshly adsorbed catalase molecules maintain their native structures with biological activities. However, the partly unfolding structure of catalase molecules is observed after the sample is stored for 15 days, this may be caused by the long-term interaction between catalase molecules and the anodized HOPG surface.

The electrochemistry of cytochrome c at a viologen-thiol self-assembled monolayer

At the self-assembled monolayer (SAM) of a thiol-functionalized viologen modified gold electrode, cytochrome c (cyt c) exhibits a quasi-reversible electrochemical reaction. The heterogeneous electron transfer rate constant of cyt c in 0.1 mol/L phosphate buffer solution(pH 6.96) is 0.164 cm.s(-1) at 500 mV/s. The adsorbed cyt c on the viologen SAM forms a closely packed monolayer, whose average electron transfer rate is 4.85 s(-1) in the scan range of 50 to 500 mV/s. These results suggest that the SAM of viologen-thiol is a relatively stable, ordered and well-behaved monolayer from an electrochemical standpoint and it promotes the electron transfer process of biomolecules on electrode surface well.

Electrochemical quartz crystal microbalance study of the electrochemical behavior of riboflavin at gold electrodes

The electrochemical and adsorption behaviors of riboflavin (RF) at gold electrodes has ken studied by using an electrochemical quartz crystal microbalance (EQCM). Useful information is obtained not only about electrochemical behavior but also about mass changes on the electrode surface. The electrochemical properties and frequency shifts were investigated in RF solutions at different pH values, concentrations and scan rates. Reversible voltammograms were observed for pH less than or equal to 9.71. There was no electrochemical reaction for pH > 9.71. The maximum current response was obtained at about pH 8. The current response was proportional to the square root of scan rates when the concentration of RF was lower than 1.0 x 10(-4) mol L-1 (pH 6.92). On the contrary, at concentrations higher than 1.0 x 10(-4) mol L-1 (pH 6.92), it was proportional to the scan rates.