Formation of a supported hybrid bilayer membrane on gold: A sterically enhanced hydrophobic effect

Adsorption of a monolayer of didecanoyl-L-alpha-phosphatidylcholine (DDPC) from dispersions of small unilamellar vesicles onto hydrophobic surfaces was investigated by mean of cyclic voltammetry and impedance spectroscopy. The hydrophobic surfaces were self-assembled monolayers of 2-mereapto-3-n-octylthiophene (MOT) on gold. One characteristic of the MOT monolayer is its permeability to organic molecules in aqueous solution, thus providing a more energetically favorable hydrophobic surface for the addition of phospholipid vesicles. The kinetics of the lipid monolayer formation were followed by measuring the time-dependent interfacial capacitance. Unusual values of thickness and capacitance of the MOT/ DDPC bilayers were observed. An interdigitating conformation of the bilayer structure was proposed to interpret the experimental results, The horseradish peroxidase reconstituted into the bilayer demonstrated the expected protein activity, showing practical use in research and in biosensor application.

A method to construct a third-generation horseradish peroxidase biosensor: Self-assembling gold nanoparticles to three-dimensional sol-gel network

A novel method for fabrication of horseradish peroxidase biosensor has been developed by self-assembling gold nanoparticles to a thiol-containing sol-gel network. A cleaned gold electrode was first immersed in a hydrolyzed (3-mercaptopropyl)-trimethoxysilane (MPS) sol-gel solution to assemble three-dimensional silica gel, and then gold nanoparticles were chemisorbed onto the thiol groups of the sol-gel network. Finally, horseradish peroxidase (HRP) was adsorbed onto the surface of the gold nanoparticles. The distribution of gold nanoparticles and HRP was examined by atomic force microscopy (AFM). The immobilized horseradish peroxidase exhibited direct electrochemical behavior toward the reduction of hydrogen peroxide. The performance and factors influencing the performance of the resulting biosensor were studied in detail. The resulting biosensor exhibited fast amperometric response (2.5 s) to H2O2. The detection limit of the biosensor was 2.0 mumol L-1, and the linear range was from 5.0 mumol L-1 to 10.0 mmol L-1. Moreover, the studied biosensor exhibited high sensitivity, good reproducibility, and long-term stability.

Self-assembled monolayer of polyoxometalate on gold surfaces: Quartz crystal microbalance, electrochemistry, and in-situ scanning tunneling microscopy study

A new kind of inorganic self-assembled monolayer (SAM) was prepared by spontaneous adsorption of polyoxometalate anion, AsMo11VO404-, onto a gold surface from acidic aqueous solution. The adsorption process, structure, and electrochemical properties of the AsMo11VO404- SAM were investigated by quartz crystal microbalance (QCM), electrochemistry, and scanning tunneling microscopy (STM). The QCM data suggested that the self-assembling process could be described in terms of the Langmuir adsorption model, providing the value of the free energy of adsorption at -20 KJ mol(-1). The maximum surface coverage of the AsMo11VO404- SAM on gold surface was determined from the QCM data to be 1.7 x 10(-10) mol cm(-2), corresponding to a close-packed monolayer of AsMo11VO404- anion. The analysis of the voltammograms of the AsMo11VO404- SAM on gold electrode showed three pairs of reversible peaks with an equal surface coverage of 1.78 x 10(-10) mol cm(-2) for each of the peaks, and the value was agreed well with the QCM data. In-situ STM image demonstrated that the AsMo11VO404- SAM was very uniform and no aggregates or multilayer could be observed. Furthermore, the high-resolution STM images revealed that the AsMo11VO404- SAM on Au(lll) surface was composed of square unit cells with a lattice space of 10-11 Angstrom at +0.7 V (vs Ag\AgCl). The value was quite close to the diameter of AsMo11VO404- anion obtained from X-ray crystallographic study. The surface coverage of the AsMo11VO404- SAM on gold electrode estimated from the STM image was around 1.8 x 10(-10) mol cm(-2), which was consistent with the QCM and electrochemical results.

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.

Controlled nucleation and growth of surface-confined gold nanoparticles on a (3-aminopropyl)trimethoxysilane-modified class slide: A strategy for SPR substrates

The thickness of the gold film and its morphology, including the surface roughness, are very important for getting a good, reproducible response in the SPR technique. Here, we report a novel alternative approach for preparing SPR-active substrates that is completely solution-based. Our strategy is based on self-assembly of the gold colloid monolayer on a (3-aminopropyl)trimethoxysilane-modified glass slide, followed by electroless gold plating. Using this method, the thickness of films can be easily controlled at the nanometer scale by setting the plating time in the same conditions. Surface roughness and morphology of gold films can be modified by both tuning the size of gold nanoparticles and agitation during the plating. Surface evolution of the Au film was followed in real time by UV-vis spectroscopy and in situ SPRS. To assess the surface roughness and electrochemical stability of the Au films, atomic force microscopy and cyclic voltammetry were used. In addition, the stability of the gold adhesion is demonstrated by three methods. The as-prepared Au films on substrates are reproducible and stable, which allows them to be used as electrodes for electrochemical experiments and as platforms for studying SAMs.

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.

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.

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

ELECTROCHEMICAL-BEHAVIOR OF HEXACYANOFERRATE(II) ON A GOLD ELECTRODE MODIFIED WITH BIS(4-PYRIDYL)DISULFIDE

The electrochemical behaviour of hexacyanoferrate(II) has been studied by using a bis(4-pyridyl)disulfide modified gold electrode. On the protonated electrode surface, hexacyanoferrate(II) can transfer an electron reversibly but no apparent adsorption was detected. On the deprotonated electrode surface, electron transfer by hexacyanoferrate(II) was more difficult. The electrochemical reversibility varied with the pH of the solution. Relationships between the currents or the standard heterogeneous rate constants and pH were derived.

ELECTROCHEMICAL-BEHAVIOR OF N-METHYL-N'-HEXADECYLVIOLOGEN MONOLAYERS ON SILVER ELECTRODES - EFFECTS OF COUNTERIONS

N-Methyl-N'-hexadecylviologen (C16MV) has been the subject of several electrochemical and spectroelectrochemical studies which characterized the species present in various redox states for C16MV monolayers on silver electrode surfaces. Both self-assembled monolayers (SA) and Langmuir-Blodgett (LB) transferred systems have been studied. These indicated inconsistencies regarding the presence or absence of splitting of the first reduction peak in its cyclic voltammogram (CV). The present study demonstrates the important influence of the specific anionic species present in the supporting electrolyte. Splitting may or may not take place, depending on the size and relative strength of the adsorption of specific anions contributed by the supporting electrolyte. Small, strongly adsorbing anions such as iodide produced peak splitting in the CV of C16MV monolayers; bulky but weakly adsorbing anions such as perchlorate may disrupt the ordered structure of monolayers but produce no splitting. Ancillary data provided by surface enhanced Raman spectroscopy (SERS) was consistent with the electrochemical measurements.

A NOVEL DESIGN OF LIGHT-TRANSPARENT GOLD-MINIGRID THIN-LAYER FTIR SPECTROELECTROCHEMICAL CELL WITH VERTICAL CONFIGURATION

A Teflon bridge/edge-eliminator is designed to connect a glass container and a light-transparent gold-minigrid NaCl thin-layer cell to form a vertically configured in-situ FTIR spectroelectrochemical cell. The bridge/edge-eliminator sets an internal reference point for accurate potential control. The size of the thin-layer chamber is 5 X 5 X 0.11mm. A 1900-omega formal resistance of the thin-layer cell was measured in CH2Cl2/0.1 M TBAP solution. Well defined thin-layer cyclic voltammograms and IR spectral changes for ferrocene oxidation were obtained.

Study of the Adsorption of Fibrinogen on Gold-Coated Silicon Wafer by an Impedance Method

In 0.1 mol/l KH2PO4–Na2HPO4 (pH 7.80) buffer solution, the potential of zero charge (PZC) and the open circuit potential of gold-coated silicon were determined to be about −0.6 and +0.10 V (vs SCE), respectively. The open circuit potential was higher than the PZC, which indicated that the surface of the gold-coated electrode had a positive charge. The ellipsometry experiment showed that the adsorption of fibrinogen onto the gold-coated silicon wafer surface arrived at a saturated state when the adsorption time exceeded 50 min. The percentage of surface without adsorbed protein, θ, was about 63%. This means that the proportion of surface actually occupied by fibrinogen was only about 37% after the adsorption arrived at saturation. The solution/protein capacitance value was determined in an impulse state around −0.59 V (vs SCE) and was stable (4.2×10−5 F) at other potentials.

Influence of Electrostatic Interaction on Fibrinogen Adsorption on Gold Studied by Imaging Ellipsometry Combined with Electrochemical Methods

Imaging ellipsometry was combined with electrochemical methods for studying electrostatic interactions of protein and solid surfaces. The potential of zero charge for gold-coated silicon wafer/solution interfaces wad determined by AC impedance method. The potential of the gold-coated silicon wafer was controlled at the potential of zero charge, and the adsorption of fibrinogen on the potential-controlled and non-controlled surfaces was measured in real time at the same time by imaging ellipsometry The effect of electrostatic interaction was studied by comparing the difference between the potential of controlled adsorption and the Potential of noncontrolled adsorption. It was shown that the rate of fibrinogen adsorption on the potentiostatic surface was faster than that on the nonpotentiostatic surface. The electrostatic influence on fibrinogen adsorption on the gold-coated silicon wafer was weak, so the hydrophobic interaction should be the major affinity.

Adsorption of human serum albumin onto gold: a combined electrochemical and ellipsometric study

Human serum albumin adsorption onto gold surfaces was investigated by electrochemical and ellipsometric methods. Albumin adsorption onto gold was confirmed by the change of the open circuit potential of gold and by the ellipsometric parameter variation during albumin immobilization. In both experiments the parameters reached stable values within 10-15 min. The albumin adsorption layer thickness measured with the ellipsometer was about 1.5 nm. The adsorption of albumin Under applied potential was also investigated and it was found that both positive and negative applied potential promote albumin adsorption. Changes in the optical parameters of bare gold and albumin adsorbed onto gold surface under applied potential were investigated with in Situ ellipsometry. The similarity and reversibility of the optical changes showed that adsorbed albumin was stable on the gold surface Under the applied potential range (-200-600 mV). The cyclic voltammograms of K3Fe(CN)(6) on the modified gold surface showed that albumin Could partly block the oxidation and reduction reaction. (C) 2004 Elsevier Inc. All rights reserved.

Electro-optic integration of embedded electrodes and waveguides in LiNbO3 using a femtosecond laser

We describe the fabrication of a Mach-Zehnder optical modulator in LiNbO3 by femtosecond laser micormachining, which is composed of optical waveguides inscripted by a femtosecond laser and embedded microelectrodes subsequently using femtosecond laser ablation and selective electroless plating. A half-wave voltage close to 19 V is achieved at a wavelength of 632.8 nm with an interaction length of 2.6 mm. This simple and cost-effective technique opens up new opportunities for fabricating integrated electro-optic devices. (C) 2008 Optical Society of America