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.

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.

SENSITIVE AMPEROMETRIC DETECTION OF GLUCOSE BY REVERSED PHASE LIQUID-CHROMATOGRAPHY AT A PRUSSIAN BLUE CHEMICALLY MODIFIED ELECTRODE OF NOVEL CONSTRUCTION

A new liquid chromatography electrochemical (LCEC) scheme for glucose sensing has been developed on the basis of a Prussian Blue chemically modified electrode (CME) of novel construction and characterized in terms of various experimental parameters by the flow injection analysis (FIA) technique. Unique hydrodynamic voltammograms were obtained for the first time at the CME in the flow-through amperometric detection of glucose, and subsequently both anodic and cathodic peaks could be expected on monitoring the operating potential in the modest positive or negative region. The unique pH dependence on the CME response towards glucose makes it perfectly compatible with conventional reversed phase liquid chromatography systems. On the basis of these features, practical application in glucose LCEC detection has been effectively performed; a linear response range over three orders of magnitude and a detection limit of subpicomole level were readily obtained. The capability of the established LCEC mode in the direct sensing of urinary glucose has been demonstrated.

REEVALUATION OF 2,2-BIPYRIDINE AND PYRAZINE AS PROMOTERS FOR DIRECT ELECTRON-TRANSFER BETWEEN CYTOCHROME-C AND METAL-ELECTRODES

Results from previous electrochemical studies have indicated that 2,2'-bipyridine and pyrazine do not function as promoters for heterogeneous electron transfer between cytochrome c and metal electrodes. Their lack of activity was attributed to the improper positioning of the two functional groups in 2,2'-bipyridine and the inefficient length of pyrazine. In the present study it was determined that both 2,2'-bipyridine and pyrazine act as promoters when self-absorbed over a sufficiently long dipping time or at roughened electrodes. The promoter characteristics of these two molecules were studied and compared with those of 4,4'-bipyridine. The difference in their promoter behavior appears to result primarily from their different strengths of adsorption and not because electrodes modified with 2,2'-bipyridine or pyrazine are unsuitable for accelerating direct electron transfer reactions in cytochrome c. These results have implications regarding the mechanism(s) of promoter effects in electrochemical reactions of cytochrome c.

THE IMPEDANCE STUDY OF MODIFIED PEO POLYMER ELECTROLYTE

The ultra-thin modified PEO (polyethylene oxide)-LiClO4 polymer electrolyte film (50-mu-m) was obtained by solution-casting technique. Impedance spectra were taken on the cells consisting of above PEO film electrolyte and ion-blocking or nonblocking electrodes. The ambient conductivity as high as 1.33 X 10(-4)S cm-1 could be achieved for PEO electrolyte modified by the crosslinking. It was shown that the resistance at the interface between solid polymer electrolyte and lithium electrode is growing with increasing the storage time. At high temperature, as 96-degrees-C, the ionic transport is clearly controlled by diffusion.

ELECTROCATALYTIC OXIDATION OF ASCORBIC-ACID AT A PRUSSIAN BLUE FILM MODIFIED MICRODISK ELECTRODE

The preparation and the behaviour of a Prussian Blue (PB) film on a platinum microdisk electrode has been described. Electrocatalytic oxidation of ascorbic acid has occurred at the PB film modified microelectrode. This shows a typical example of a modified microelectrode in electrocatalysis following our previous theoretical studies (J. Electroanal. Chem., 309 (1991) 103) and the related catalytic reaction rate constant was determined.

ELECTROCATALYSIS AT A MICRODISK ELECTRODE MODIFIED WITH A REDOX SPECIES

The current equation of the electrocatalytic reaction at a microdisk electrode modified with redox species has been described and verified experimentally. There exists a linear relationship between plateau limiting current and the radius of the microdisk electrode for a catalytic process. The influence of the dimensions of the microdisk electrode on catalytic efficiency is discussed. The polyvinylferrocene (PVFc)-modified microdisk electrode prepared by the coating method was taken as a typical example, on which the electrocatalytic oxidation of ascorbic acid could be studied. The catalytic reaction rate constants were determined as an average value of 1.5 X 10(-7) cm3/mol s by this method, and are consistent with those obtained at a conventional electrode.

ANODIC-OXIDATION OF HYDRAZINE ON GLASSY-CARBON MODIFIED BY MACROCYCLIC TRANSITION-METAL COMPLEXES .1. COBALT PROTOPORPHYRIN DIMETHYL ESTER MODIFIED ELECTRODE

The electrocatalytic oxidation of hydrazine (N2H4) on a glassy carbon electrode (GC) modified by monolayer and polymer films of cobalt protoporphyrin dimethyl ester (CoPP) has been studied. Both the monolayer and polymer films of CoPP are very active to the anodic oxidation of N2H4. The activity of CoPP for the anodic oxidation of N2H4 is dependent on the pH of the solution, and the thickness of polymerized CoPP film. The oxidation kinetics were examined by methods of cyclic voltammetry, rotating disc electrodes and steady-state polarization measurement.

LIQUID-CHROMATOGRAPHY AMPEROMETRIC DETECTION OF CARBOXYLIC-ACIDS AND PHENOLIC-ACIDS WITH A COPPER-BASED CHEMICALLY-MODIFIED ELECTRODE

A copper-based chemically-modified electrode has been constructed and characterized by various experimental parameters in flow-through amperometric detection of carboxylic acids and phenolic acids. Novel hydrodynamic voltamperograms were first obtained in flow-through amperometric detection with the Cu-based CME and subsequently negative and positive peaks were observed in a single chromatogram. This unique and flexible potential dependence could be of great benefit in chromatographic speciation and quantification. These observations suggest that the detector response was governed by the complexation reaction of copper ions with the solutes.

Fabrication of Bismuth/Multi-walled Carbon Nanotube Composite Modified Glassy Carbon Electrode for Determination of Cobalt

A bismuth/multi-walled carbon nanotube (Bi/MWNT) composite modified electrode for determination of cobalt by differential pulse adsorptive cathodic stripping voltammetry is described. The electrode is fabricated by potentiostatic pre-plating bismuth film on an MWNT modified glassy carbon (GC) electrode. The Bi/MWNT composite modified electrode exhibits enhanced sensitivity for cobalt detection as compared with the bare GC, MWNT modified and bismuth film electrodes. Numerous key experimental parameters have been examined for optimum analytical performance of the proposed electrode. With an adsorptive accumulation of the Co(II)-dimethylglyoxime complex at -0.8 V for 200 s, the reduction peak current is proportional to the concentration of cobalt in the range of 4.0x10(-11)-1.0x10(-7) mol/L with a lower detection limit of 8.1x10(-11) mol/L. The proposed method has been applied Successfully to cobalt determination in seawater and lake water samples.

Development of polyaniline microarray electrodes for cadmium analysis

Disposable screen-printed electrodes (SPCE) were modified using a cosmetic product to partially block the electrode surface in order to obtain a microelectrode array. The microarrays formed were electropolymerized with aniline. Scanning electron microscopy was used to evaluate the modified and polymerized electrode surface. Electrochemical characteristics of the constructed sensor for cadmium analysis were evaluated by cyclic and square-wave voltammetry. Optimized stripping procedure in which the preconcentration of cadmium was achieved by depositing at –1.20 V (vs. Ag/AgCl) resulted in a well defined anodic peak at approximately –0.7 V at pH 4.6. The achieved limit of detection was 4 × 10−9 mol dm−3. Spray modified and polymerized microarray electrodes were successfully applied to quantify cadmium in fish sample digests.

Biosensor based on multi-walled carbon nanotubes paste electrode modified with laccase for pirimicarb pesticide quantification

This study focused on the development of a sensitive enzymatic biosensor for the determination of pirimicarb pesticide based on the immobilization of laccase on composite carbon paste electrodes. Multi- walled carbon nanotubes(MWCNTs)paste electrode modified by dispersion of laccase(3%,w/w) within the optimum composite matrix(60:40%,w/w,MWCNTs and paraffin binder)showed the best performance, with excellent electron transfer kinetic and catalytic effects related to the redox process of the substrate4- aminophenol. No metal or anti-interference membrane was added. Based on the inhibition of laccase activity, pirimicarb can be determined in the range 9.90 ×10- 7 to 1.15 ×10- 5 molL 1 using 4- aminophenol as substrate at the optimum pH of 5.0, with acceptable repeatability and reproducibility (relative standard deviations lower than 5%).The limit of detection obtained was 1.8 × 10-7 molL 1 (0.04 mgkg 1 on a fresh weight vegetable basis).The high activity and catalytic properties of the laccase- based biosensor are retained during ca. one month. The optimized electroanalytical protocol coupled to the QuEChERS methodology were applied to tomato and lettuce samples spiked at three levels; recoveries ranging from 91.0±0.1% to 101.0 ± 0.3% were attained. No significant effects in the pirimicarb electro- analysis were observed by the presence of pro-vitamin A, vitamins B1 and C,and glucose in the vegetable extracts. The proposed biosensor- based pesticide residue methodology fulfills all requisites to be used in implementation of food safety programs.

Novel biosensing device for point-of-care applications with plastic antibodies grown on Au-Screen Printed Electrodes

A gold screen printed electrode (Au-SPE) was modified by merging Molecular Imprinting and Self-Assembly Monolayer techniques for fast screening cardiac biomarkers in point-of-care (POC). For this purpose, Myoglobin (Myo) was selected as target analyte and its plastic antibody imprinted over a glutaraldehyde (Glu)/cysteamine (Cys) layer on the gold-surface. The imprinting effect was produced by growing a reticulated polymer of acrylamide (AAM) and N,N′-methylenebisacrylamide (NNMBA) around the Myo template, covalently attached to the biosensing surface. Electrochemical impedance spectroscopy (EIS) and cyclic voltammetry (CV) studies were carried out in all chemical modification steps to confirm the surface changes in the Au-SPE. The analytical features of the resulting biosensor were studied by different electrochemical techniques, including EIS, square wave voltammetry (SWV) and potentiometry. The limits of detection ranged from 0.13 to 8 μg/mL. Only potentiometry assays showed limits of detection including the cut-off Myo levels. Quantitative information was also produced for Myo concentrations ≥0.2 μg/mL. The linear response of the biosensing device showed an anionic slope of ~70 mV per decade molar concentration up to 0.3 μg/mL. The interference of coexisting species was tested and good selectivity was observed. The biosensor was successfully applied to biological fluids.