Charge Transport Processes of Immobilized Heteropolyanions at Self-assembled Monolayer Modified Gold Electrode by Electrochemical Quartz Crystal Microbalance Measurement

The in situ electrochemical quartz crystal microbalance(EQCM) technique was used to investigate the ion transport of immobilized heteropolyanions at a self-assembled monolayer(SAM) modified gold electrode during electrochemical redox process. A mixed transfer method was presented to analyse the abnormal change of resonant frequency based on the simultaneous insertion/extraction of different ions. The results indicate that the migration of HSO4- anions was indispensable in the redox process of the heteropolyan ions in a I mol/L H2SO4 solution and played a key role in the abnormal change of the resonant frequency. Such a change was attributed to different packing densities derived by means of differently immobilized methods.

Simultaneous determination of dopamine and ascorbic acid at an in-site functionalized self-assembled monolayer on gold electrode

The in-site functionalization of 4-aminothiophenol (4-ATP) self-assembled monolayer on gold electrode at physiological pH yields a redox active monolayer of 4'-mercapto-N-phenylquinone diimine (MNPD). The functionalized electrode exhibits excellent electrocatalytic responses towards dopamine (DA) and ascorbic acid (AA), reducing the overpotentials by about 0.22 V and 0.34 V, respectively, with greatly enhanced current responses. Due to its different catalytic activities toward DA and AA, the modified electrode resolves the overlapping voltammetric responses of DA and AA into two well-defined voltammetric peaks by differential pulse voltammetry (DPV), which can be used for the simultaneous determination of these species in a mixture. The catalytic peak current obtained from DPV was linearly related to DA and AA concentration in the ranges of 5.0 x 10-6 - 1.25 x 10-4 M and 8.0 x 10-6 - 1.3 x 10-4 M with correlation coefficient of 0.999 and 0.998, respectively. The detective limits (3sigma) for DA and AA were found to be 1.2 x 10-6 M and 2.4 x 10-6 M, respectively.

A novel nanometer-scale "gold electrode-molecular wires-silver" junction: An efficient test bed for electrochemical investigation of electron transportation through molecular wires in self-assembled monolayers

A novel "gold electrode-molecular wires-silver" junction was facilely fabricated for electrochemical study on the electron transportation through molecular wires. Rapid electron transportation through this sandwich-like structure was indeed observed by cyclic voltammograms and ac impedance measurements. Since rather reproducible and reliable results are easily available by electrochemical techniques, it would be an efficient and reliable test bed for electrochemical investigation of charge transportation through molecular wires in self-assembled monolayers on electrodes.

A novel hydrogen peroxide sensor based on horseradish peroxidase immobilized in DNA films on a gold electrode

A novel third-generation hydrogen peroxide (H2O2) biosensor was developed by immobilizing horseradish peroxidase (HRP) on a biocompatible gold electrode modified with a well-ordered, self-assembled DNA film. Cysteamine was first self-assembled on a gold electrode to provide an interface for the assembly of DNA molecules. Then DNA was chemisorbed onto the self-assembled monolayers (SAMs) of cysteamine to form a network by controlling DNA concentration. The DNA-network film obtained provided a biocompatible microenvironment for enzyme molecules, greatly amplified the coverage of HRP molecules on the electrode surface, and most importantly could act as a charge carrier which facilitated the electron transfer between HRP and the electrode. Finally, HRP was adsorbed on the DNA-network film. The process of the biosensor construction was followed by atomic force microscopy (AFM). Voltammetric and time-based amperometric techniques were employed to characterize the properties of the biosensor derived. The enzyme electrode achieved 95% of the steady-state current within 2 s and had a 0.5 mu mol l(-1) detection limit of H2O2. Furthermore, the biosensor showed high sensitivity, good reproducibility, and excellent long-term stability.

Self-assembly of 4-ferrocene thiophenol capped electroactive gold nanoparticles onto gold electrode

Gold nanoparticles capped by 4-ferrocene thiophenol with an average core size of 2.5 nm and surface plasmon absorbance at 522 nm were place-exchanged with 1,8-octanedithiol, and then self-assembled onto the gold electrode via tail SH group. The self-assembly was characterized by X-ray photoelectron spectroscopy. Cyclic voltammograms examined the coverage fraction of the self-assembled monolayers of the electroactive gold nanoparticles and the formal potential of the indicated SAMs. Further experiments exhibited that the electrode process was controlled by surface confined faradic reactions.

Toluidine blue modified self-assembled silica gel coated gold electrode as biosensor for NADH

A toluidine blue modified gold electrode was constructed using self-assembled silica gel technique. Firstly, toluidine blue was encapsulated within 3D network of silica self-assembly monolayer on the surface of gold electrode. Secondly, another layer of silica sol was further assembled to protect from leaching of mediator or possible contamination. The electrochemical characteristics of toluidine blue immobilized within self-assembled silica gel were studied in detail. The modified electrode was applied for electrochemical oxidation of NADH with satisfactory results.

Electrochemical and Raman studies of the biointeraction between Escherichia coli and mannose in polydiacetylene derivative supported on the self-assembled monolayers of octadecanethiol on a gold electrode

Here, we describe a new method to study the biointeraction between Escherichia coli and mannose by using supramolecular assemblies composed of polydiacetylene supported on the self-assembled monolayer of octadecanethiol on a gold electrode. These prepared bilayer materials simply are an excellent protosystem to study a range of important sensor-related issues. The experimental results from UV-vis spectroscopy, resonance Raman spectroscopy, and electrochemistry confirm that the specific interactions between E. coli and mannose can cause conformational changes of the polydiacetylene backbone rather than simple nonspecific adsorption. Moreover, the direct electrochemical detection by polydiacetylene supramolecular assemblies not only opens a new path for the use of these membranes in the area of biosensor development but also offers new possibilities for diagnostic applications and screening for binding ligands.

Simultaneous determination of both the calibration constant in an electrochemical quartz crystal microbalance and the active surface area of a polycrystalline gold electrode

A novel method is employed for the simultaneous determination of both the calibration constant of an electrochemical quartz crystal microbalance (EQCM) and the active surface area of a polycrystalline gold electrode. A gold electrode: is immersed into a 1 mM KI/1 M H2SO4 solution and on which forms a neutral monolayer. The adsorbed iodine can then be completely oxidized into IO3-. The active surface area of a gold electrode can be obtained from the net electrolytic charge of the oxidation process, and the calibration constant in the EQCM can be calculated from the corresponding frequency shift. The result shows that this method is simple, convenient and valid. (C) 2000 Elsevier Science S.A. All rights reserved.

Formation and characterization of heteropolyacid/polycation multilayer films on gold electrode

Ultrathin multilayer films of a polybasic lanthanide heteropoly tungstate-molybdate complex and a cationic polymer of quaternized poly(4-vinylpyridine) partially complexed with osmium bis(2,2'-bipyridine) have been fabricated on a gold electrode precoated with a cysteamine self-assembled monolayer. The multilayer films have been characterized by optical spectroscopy, small-angle X-ray diffraction, and electrochemical methods (cyclic voltammetry and electrochemical impedance). Especially, the electrochemical impedance spectroscopy is developed to monitor the layer deposition processes. It provides important information such as double-layer capacitance and charge-transfer resistance. All obtained results reveal regular film growth with each layer adsorption. (C) 2001 The Electrochemical Society.

Technique for determination of gold electrode area and its application of characterization in self-assembling process

Underpotential deposition(UPD) of copper at polycrystal gold surface under different concentrations has been studied, and its reversibility and stability in high concentration (0.2M CuSO4+0.IM H2SO4) have been demonstrated by cyclic voltermmetry and EQCM. A Valid approach to determine the gold electrode area in presence of adsorbed species has been provided by using Cu UPD method. Further, the growth kinetics of decane thiol on gold has also been investigated based on such a Cu UPD technique.

Spontaneous assembly of the inclusion complexes of viologen-attached alkanethiols and alpha-, beta-cyclodestrin on gold electrode

Electroactive self-assembled monolayers (SAMs) with well-defined electrochemical responses were prepared by spontaneous assembly of the inclusion complexes (CD/C8VComegaSH) of viologen-attached alkanethiols (C8VComegaSH) and alpha- and beta-cyclodextrin (CD). They were characterized by X-ray photoelectron spectroscopy and cyclic voltammetry. The results demonstrate that the chemisorption process of CD/C8VComegaSH on gold substrate occurs through S-Au bonds, and that the redox sites in SAMs of CD/C8VComegaSH are in a much more uniform environment than those in SAMs of C8VComegaSH.

Interfacial characteristics of the self-assembly system of poly-L-lysine 3-mercaptopropionic acid gold electrode

The interfacial characteristics of poly-L-lysine (PL) attached on self-assembled monolayers (SAMs) of 3-mercaptopropionic acid (MPA) were studied by an electrochemical method. The results indicated that PL\MPA layer inhibited partly the diffusion process of redox species in solution, and the electrode surface behaved like a microelectrode array. Its permeation effect was also strongly affected by Mg2+. The more Mg2+ ions were added into the electrolyte solution, the greater the difficulty with which the electron transfer of potassium ferricyanide took place. The three different conformations of PL on the electrode surface had different influences on the electron transfer processes of ferricyanide. PL in random coil state hindered most strongly the electron transfer behavior of ferricyanide,while the alpha-helical PL had nearly no effect and the effect of the beta-sheet state PL was intermediate of these. (C) 1997 Elsevier Science S.A.

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.