Self-assembled monolayers of thiols on gold electrodes for bioelectrochemistry and biosensors

Monolayers of biological compounds including redox proteins and enzymes, and phospholipids have been immobilized on a gold electrode surface through self-assembling. These proteins and enzymes, such as cytochrome c, cytochrome c oxidase and horseradish peroxidase (HRP), immobilized covalently to the self-assembled monolayers (SAMs) of 3-mercaptopropionic acid on a gold electrode, communicate directly electrons with the electrode surface without mediators and keep their physiological activities. The electron transfer of HRP with the gold electrode can also be mediated by the alkanethiol SAMs with electroactive group viologens on the gold electrode surface. All these direct electrochemistries of proteins and enzymes might offer an opportunity to build a third generation of biosensors without mediators for analytes, such as H2O2, glucose and cholesterol. Monensin and valinomycin have been incorporated into the bilayers on the gold electrode consisting of the SAMs of alkanethiol and a lipid monolayer, which have high selectivity for monovalent ions, and the resulting Na+ or K+ sensor has a wide linear range and high stability. These self-assembly systems provide a good mimetic model for studying the physiological function of a membrane and its associated enzyme. (C) 1997 Elsevier Science S.A.

Preparation of highly dispersed gold microparticles and their electrocatalytic activity for the oxidation of formaldehyde

The electrochemical preparation of highly dispersed Au microparticles on the surfaces of glassy carbon (GC) electrodes and their electrocatalytic activities for the oxidation of formaldehyde were studied. It was found that the reduction of Au3+ to Au is controlled by diffusion and the formation mechanism of Au microparticles on the GC surfaces corresponds to an instantaneous nucleation and diffusion-controlled three dimensional growth process. The particle size is about 80-90 nm in diameter after the electrochemical ageing treatment. These highly dispersed Au microparticles have high surface areas and exhibit better electrocatalytic activity than that of bulk-form Au toward the electrochemical oxidation of formaldehyde in alkaline media.

Electrochemical behavior of FAD at a gold electrode studied by electrochemical quartz crystal microbalance

The electrochemical behavior of flavine adenine dinucleotide (FAD) at a gold electrode involving adsorption of the reduced form FADH(2) and desorption of the oxidized form FAD has been studied by using electrochemical quartz crystal microbalance (EQCM). EQCM can present information not only about the electrochemical behavior but also about the mass changes on the electrode surface. The electrochemical properties and frequency shifts were investigated in FAD solutions at different pH values, concentrations and scan rates. Reversible voltammograms were observed when pH<4.5 and irreversible voltammograms were found when pH greater than or equal to 4.5. It is found to be a diffusion controlled process when the concentration of FAD is lower than 2x10(-4) moll(-1) (pH 1.5). On the contrary, at concentrations higher than 2x10(-4) moll(-1) (pH 1.5), it is found to be an adsorption controlled process.

Viologen-thiol self-assembled monolayers for immobilized horseradish peroxidase at gold electrode surface

A stable, well-behaved self-assembled monolayer (SAM) of viologen-functionalized thiol was used to immobilize and electrically connect horseradish peroxidase (HRP) at gold electrode. Viologen groups in SAMs facilitated the electron transfer from the electrode to the protein active site so that HRP exhibited a quasi-reversible redox behavior. HRP adsorbed in the SAMs is very stable, and close to a monolayer with the surface coverage of 6.5 x 10(-11) mol/cm(2). The normal potential of HRP is -580 mV vs Ag/AgCl corresponding to ferri/ferro active center and the standard electron transfer rate constant is 3.41 s(-1) in 0.1 M phosphate buffer solution (pH 7.1). This approach shows a great promise for designing enzyme electrodes with other redox proteins and practical use in tailoring a variety of amperometric biosensor devices. Copyright (C) 1997 Elsevier Science Ltd.

Permselectivity of 4-aminophenol, paracetanol and phenacetin on the self-assembled n-alkanethiol monolayer modified gold electrode

The gold electrodes coated by n-alkanethiol with various chain lengths were used to study the permeability of uric acid, ascorbic acid, 4-aminophenol, paracetanol and phenacetin by means of linear sweep voltammetry. The results show that the optimum chain length is n=10. The improvements in the selectivity and the stability of the amperometric detection of these compounds in a flow stream were obtained by n-alkanethiol self assembled monolayers modified electrodes based on their differences in the hydrophobicity and the permeability.

Direct electron transfer to cytochrome c oxidase in self-assembled monolayers on gold electrodes

The monolayer of cytochrome c oxidase maintaining physiological activity and attached covalently to the self-assembled monolayers of 3-mercaptopropionic acid (MPA) on a gold electrode was obtained. The results of cyclic voltammetry show that direct electron transfer between cytochrome c oxidase and the electrode surface is a fast and diffusionless process. MPA has a dual role as both electrode modifier and the bridging molecule which: keeps cytochrome c oxidase at an appropriate orientation without denaturation and enables direct electron transfer between the protein and the modified electrode. Immobilized cytochrome c oxidase exhibits biphasic phenomena between the concentration of the electrolyte and the normal potentials; meanwhile its electrochemical behavior is also influenced by the buffer components. The quasi-reversible electron transfer process of cytochrome c oxidase with formal potential 385 mV vs. SHE in 5mM phosphate buffer solution (pH 6.4) corresponds to the redox reaction of cyt a(3) in cytochrome c oxidase, and the heterogeneous electron transfer rate constant obtained is 1.56 s(-1). By cyclic voltammetry measurements, it was observed that oxidation and reduction of cytochrome c in solution were catalyzed by the immobilized cytochrome c oxidase. This cytochrome c oxidase/MPA/Au system provides a good mimetic model to study the physiological functions of membrane-associated enzymes and hopefully to build a third-generation biosensor without using a mediator.

The direct electrochemistry of cytochrome c at the nanometer-sized rare earth element oxide particle-modified gold electrodes

The direct electrochemistry of cytochrome c was studied at nanometer-sized rare earth element dioxide particle-modified gold electrodes. It was demonstrated that rare earth element oxides can accelerate the electrochemical reaction of cytochrome c and the reversibility of the electrochemical reaction of cytochrome c was related to the size of rare earth element oxide particles.

Synthesis of N-(n-octyl)-N'-(10-mercaptodecyl)-4,4'-bipyridinium dibromide and electrochemical behaviour of its monolayers on a gold electrode

A new viologen derivative of N-(n-octyl)-N'-(10-mercaptodecyl)-4,4'-bipyridinium dibromide has been prepared and characterized by elemental analysis, IR, H-1 NMR, MS and TG-DTA. X-Ray photoelectron spectroscopy, cyclic voltammetry and chronoamperometry have been used to characterize the monolayers formed by this compound on the bulk gold electrodes by self-assembly.

PROMOTER EFFECT OF POLY(4-VINYLPYRIDINE) ON THE DIRECT ELECTRON-TRANSFER BETWEEN CYTOCHROME-C AND GOLD ELECTRODE

The electrochemistry of cytochrome c was studied at the PVP-modified gold electrode. It was found that the promoter effect is related to the amount of PVP at the gold electrode. From our results, it can be seen that the nitrogen element in the polymer is important for accelerating the electron transfer of cytochrome c.

PROMOTER EFFECT OF HALOGEN ANIONS ON THE DIRECT ELECTROCHEMICAL REACTION OF CYTOCHROME-C AT GOLD ELECTRODES

The promoter effect of halogen anions for heterogeneous electron transfer between cytochrome c and a gold electrode was studied. It was found that the order of the promoter ability of halogen anions is I- > Br- > Cl- > F-. In addition, factors which can affect the promoter effect were discussed.

BINDING OF CYTOCHROME-C WITH 4-PYRIDYL DERIVATIVES MODIFIED ON THE GOLD ELECTRODE

In this work, the adsorption or binding of cytochrome c with 4-pyridyl derivatives modified on the gold electrode was studied. It was found that the concentrations of electrolyte had much influence on the adsorption of cytochrome c. At lower concentration

ELECTROCHEMICAL REACTION OF CYTOCHROME-C AT GOLD ELECTRODES MODIFIED WITH THIOPHENE CONTAINING ONE FUNCTIONAL-GROUP

The electrochemical reactions of cytochrome c were studied at a thiophene-modified gold electrode. It was demonstrated that thiophene is an effective promoter, although there is only one functional group in the molecule. Based on this result, the mechanis

ELECTROOXIDATION OF ASCORBIC-ACID AT THE BIS(4-PYRIDYL) DISULFIDE MODIFIED GOLD ELECTRODE

The electrooxidation of ascorbic acid (AA) at the bis(4-pyridyl)disulfide (PySSPy) modified gold electrode was studied. The results showed that the oxidation process was pH-dependent. It was mainly due to the static interaction between AA and the modified

EFFECT OF PH ON THE ELECTRON-TRANSFER OF CYTOCHROME-C ON A GOLD ELECTRODE MODIFIED WITH BIS(4-PYRIDYL)DISULPHIDE

The adsorption of bis(4-pyridyl)disulphide (PySSPy) and 4.4'-bipyridyl (PyPy) on a gold electrode was studied using cyclic voltammetry. The adsorption isotherms and equilibrium constants (1 X 10(6) mol-1 l for PyPy and 6 x 10(6) Mol-1 l for PySSPy) were determined. The effect of pH on the electrochemical behaviour of cytochrome c was studied on the PySSPy-modified gold electrode. The results show that cytochrome c can only transfer electrons on a deprotonated electrode surface. When the pH is decreased, the standard heterogeneous rate constant of cytochrome c on the modified gold electrode decreases and the electrochemical behaviour changes from a quasi-reversible to an irreversible process.