Oxidation of ascorbic acid by rutin at a glassy carbon electrode modified with lipid films

A stable film was prepared by casting dipalmitoylphosphatidylcholine (DPPC) and rutin onto the surface of a glassy carbon (GC) electrode. The electrochemistry behavior of rutin in the DPPC film was investigated. The modified electrode coated with rutin shows a quasi-reversible reduction-oxidation peak on the cyclic voltammogram in phosphate buffer (pH 7.4). This model of biological membrane was not only used to provide biological environment but also to investigate the oxidation of ascorbic acid by rutin. The DPPC-rutin modified electrode behaves as electrocatalytic oxidation to ascorbic acid. The oxidation peak current of ascorbic acid increases drastically and the peak potential of 4 x 10(-4) mol L-1 ascorbic acid shifts negatively about 100 mV compared with that obtained at a bare glassy carbon electrode. The catalytic current increased linearly with the ascorbic acid concentration in the range of 2 x 10(-4) mol L-1 and 1.4 x 10(-3) mol L-1 at a scan rate of 50 mV s(-1).

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.

Photoreduction behaviors of microperoxidase-11 adsorbed on the roughened silver electrodes

It was found that microperoxidase-ll (MP-II) can undergo photoreduction at the bale roughened silver electrode. No photoreduction happens at the roughened silver electrode modified with mercaptoundecanoic carboxylic acid/poly-lysine. The photoreduction mechanism is discussed.

Self-gelatinizable copolymer immobilized glucose biosensor based on Prussian Blue modified graphite electrode

A novel poly(vinyl alcohol) grafting 4-vinylpyridine self-gelatinizable copolymer was adapted to immobilize glucose oxidase. The reduction of hydrogen peroxide (H2O2) was detected at a Prussian Blue (PB) modified graphite electrode. A stable and sensitive glucose amperometric biosensor is described. The copolymer is a good biocompatible polymer in which the glucose oxidase retains high activity. Moreover, the copolymer can adhere firmly to the inorganic PB membrane. The sensor showed an apparent Michaelis-Menten constant of 18 +/- 0.2 mM and a maximum current density of 1.14 mu A cm(-2) mM(-1). The linear range is from 5 mu M to 4.5 mM glucose and the detection limit is 0.5. mu M glucose. The catalytic efficiency of PB for the reduction of H2O2 is higher than that for the oxidation of H2O2. Glucose concentrations in serum samples from healthy persons and diabetic patients were determined using the sensor. The results compared well with those provided by the hospital using a spectroscopy method.

Unidirectional current flow of reversible redox couples on a MoO3 film-modified microelectrode

In this paper, we have investigated the reactivity of the molybdenum oxide film toward some standard redox systems (e.g., ferrocene (Fc) and its derivatives) and observed a few interesting phenomena. The results demonstrate that the electrochemical behaviour of Fc and its derivatives at the oxide-modified carbon fiber (CF) microelectrode differs from that at a bare CF microelectrode, The conductivity of the molybdenum oxide film is seriously affected by the range and the direction of the potential scan, which influences the electrochemical behaviour of these redox systems at the film electrode. If the cycling potential is more positive than the reduction potential of the molybdenum oxide film, the reduction and oxidation peak currents of Fc and its derivatives could not be observed. The result indicates that the molybdenum oxide film on a microelectrode surface cannot transfer electrons between the surface of the electrode and Fc or its derivatives due to the existence of a high resistance between the interface in these potential ranges. On the other hand, if the lower limit of the scan potential was extended to a potential more negative than the reduction peak potential of the film, the oxidation peak of Fc or its derivatives appeared at about the potential relative to E-0 of Fc or its derivatives on the bare electrode, and the peak current is proportional to the concentration of these couples in the electrolyte. To our surprise, the peak height on the modified electrode is much larger than that on the bare CF microelectrode under the same conditions in the range of low concentration of these couples, and the oxidation peak potential of these couples is more negative than that on the bare CF microelectrode. On the basis of the experimental observation, we propose that these redox couples may undergo an interaction with the reduction state of the molybdenum oxide film. The new phenomena that we observed have been explained by using this interaction. (C) 1997 Elsevier Science S.A.

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.

Detection of hydrazine, methylhydrazine, and isoniazid by capillary electrophoresis with a palladium modified microdisk array electrode

A palladium particle-modified carbon fiber microdisk array electrode was designed and employed in capillary electrophoresis for the simultaneous detection of hydrazine, methylhydrazine, and isoniazid. The Pd-modified microdisk electrode had high catalytic ability for hydrazines and exhibited good reproducibility and stability. The response for hydrazine was linear over 3 orders of magnitude with a correlation coefficient of 0.993. The detection limits far hydrazine, methylhydrazine, and isoniazid were 1.2, 2.1, and 6.2 pg, respectively.

Acetylcholinesterase amperometric detection system based on a cobalt(II) tetraphenylporphyrin-modified electrode

An acetylcholinesterase (AChE) activity detection system was fabricated based on the electrocatalysis of cobalt(II) tetraphenylporphyrin of the electrooxidation of thiocholine chloride, which is the product of the hydrolysis of acetylthiocholine chloride by AChE. A simple modified method was used to form the base electrode. AChE was cross-linked on the base electrode by glutaraldehyde. The optimum working conditions are discussed and the characteristics of the detection system are evaluated.

Electrochemical preparation of microelectrodes modified with non-stoichiometric mixed-valent molybdenum oxides

A compact non-stoichiometric molybdenum (VI, V) oxide of blue film was grown on carbon fiber (CF) microelectrode surface be cycling the potential between + 0.2V and - 0.70V in a freshly prepared Na2MoO4 solution containing 5 x 10(-3) mol/L H2SO4. The quantity-of the oxide is controlled by the charge passing the electrode. The electrochemical pretreatment of CF microelectrode not only mises the deposition velocity of molybdenum oxide on CF surface, but also improves greatly the cyclic voltammetric behavior of the molybdenum oxide film prior to the electrodeposition. The cathodic processes are believed to yield the hydrogen molybdenum oxide bronzes HxMoO3(0 < x < 2), or substoichiometric lower molybdenum oxides with the formula MoO3-y(0 < y < 1). The anodic response results from the reversible oxidation of molybdenum bronze/Mo(V) centers [or perhaps Mo(IV) in more reduced coatings], to Mo(VI). Further information was gained about the chemical composition and valent state of Mo from XPS and SEM.

Improvements in the selectivity of electrochemical detectors for liquid chromatography and flow injection analysis using the self-assembled n-alkanethiol monolayer-modified au electrode

4-Aminophenol (4-AP), paracetamol (PRCT), norepinephrine (NE), and dopamine (DA) (all somewhat hydrophobic compounds) were HPLC electrochemically detected while the signals from uric acid (UA) and ascorbic acid (AA) (both hydrophilic compounds at the pH studied) were minimized, taking advantage of the permselectivity of the self-assembled n-alkanethiol monolayer (C-10-SAM)-modified Au electrodes based on solute polarity, The effects of various factors, such as the chain length of the n-alkanethiol modifier, modifying time, and pH value, on the permeability of C-10-SAM coatings were examined, The calibration curves, linear response ranges, detection limits, and reproducibilities of the EC detector for 4-AP, PRCT, NE, and DA were obtained, The result shows that the EC detector can be applied in the chromatographic detection of 4-AP, PRCT, NE, and DA in urine, effectively removing the influence of UA and AA in high concentrations existing in biological samples. As a result, a great improvement in the selectivity of EC detectors has been achieved by using Au electrodes coated with neutral n-alkanethiol monolayer.

KINETIC-STUDY OF ELECTROCATALYTIC REDUCTION OF HYDROGEN-PEROXIDE AT IRON(III) PORPHYRIN MODIFIED GLASSY-CARBON ELECTRODE IN ALKALINE-MEDIUM USING THE ROTATION-SCAN METHOD

Reduction of hydrogen peroxide at a glassy carbon (GC) electrode modified with sigma-bonded pyrrole iron(III) octaethylporphyrin complex, (OEP)Fe(Pyr), was studied by cyclic voltammetry and a rotating disk electrode. In 0.1N NaOH solution, it is shown that such an (OEP)Fe(Pyr)/GC electrode has a significant catalytic activity towards hydrogen peroxide reduction (E(D) = -0.80 V, k = 0.066 cm s(-1)); however, the electrode stability is low. The deactivation is observed when the reaction charge (Q) is passing through the (OEP)Fe(Pyr)/GC disk electrode. A linear rotation scan method is applied to study the kinetic process by determining the disk electrochemical response (i(D)) to rotation rate (omega) at a definite disk potential (E(D)). Considering that the number of adsorbed electroreduced catalyst molecules (Red) varies according to the disk potential, a factor theta(= Gamma(Red)/(Gamma(Red) + Gamma(Ox))) is introduced to describe the electrode surface area fraction for electroreduced species. The obtained Koutecky-Levich equation is applicable whatever the potential is.

IN-SITU SCANNING-TUNNELING-MICROSCOPY IMAGE OF A HIGHLY ORIENTED PYROLYTIC-GRAPHITE ELECTRODE MODIFIED WITH POTASSIUM BIS(11-MOLYBDOSILICATO) DYSPROSATE(III) IN SODIUM-SULFATE SOLUTION

The variation in molecule adsorption mode on pretreated highly oriented pyrolytic graphite electrodes, modified with the title complex K10H3[Dy(SiMo11O39)(2)] by cyclic voltammetry in the title complex solution, was observed in situ by electrochemical scanning tunnelling microscopy (ECSTM) with molecular resolution in sodium sulphate solution. According to the ECSTM images and the known molecular structure we conclude that the adsorption mode of the title complex modified electrode changed during potential cycling from ''vertical'' to ''inclined'' and then ''horizontal'' or ''flat'' mode, i.e. the title complex adsorbed on the surface of electrode by one ligand of the complex at first, then began to incline and was finally adsorbed by two ligands of the complex. This result indicates that the adsorption mode on the modified electrode surface changed during potential cycling in the sulphate solution and a much more stable molecular layer was formed. The change in adlattice of adsorbates on the modified electrode surface from hexagonal to rectangular was also observed by ECSTM. A plausible model was given to explain this process.

SURFACE MODIFICATION OF ELECTRODES FOR AMPEROMETRIC BIOSENSORS IN ANALYTICAL-CHEMISTRY

Amperometric biosensors based on surface modifications of electrodes are described. Cobalt porphyrins modified on glassy carbon and carbon fiber electrodes can greatly decrease the overpotential and increase the sensitivity of detection due to EC electroc

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

GAS PERMEABILITIES OF POLY(TRIMETHYLSILYLPROPYNE) MEMBRANES SURFACE MODIFIED WITH CF4 PLASMA

Surface fluorination of poly (trimethylsilylpropyne) (PTMSP) membranes by CF4 plasma was studied. The surface fluorination of the membranes was carried out in an atmosphere of CF4 in a capacitively coupled discharge apparatus with external electrodes. Dramatic increase in selectivity (P(O2)/P(N2)) was observed. The effect of fluorination conditions such as duration of treatment and discharge power on the permeabilities of the membranes was studied. X-ray photoelectron spectrometric data of modified PTMSP membranes showed a drastic alternation in the surface layer. The P(O2) and P(O2)/P(N2) of the membranes were observed to be dependent on the F/C atomic ratio. At F/C > 1, the P(O2/P(N2) value of the membranes could be more than four.