Chuan-sin Cha

Detection of dopamine in the presence of a large excess of ascorbic acid by using the powder microelectrode technique

Abstract The powder microelectrode technique is employed in the detection of ultra-low concentrations of dopamine (DA), which can be adsorbed and thus preconcentrated on the inner surface of a carbon black packed powder microelectrode. The apparent reversibility and kinetics of the oxidation of DA were found to improve significantly, and separation of the oxidation current peaks of DA and ascorbic acid (AA) reaches about 300 mV. In the presence of 0.6 mM AA in solution, the oxidation current peak due to 20 nmol of DA remains clearly evident. After correction of the background current of AA, the height of the oxidation current peak of DA oxidation is proportional to the concentration of DA in solution, and is not affected by the presence of large excess of AA. The low limit of DA detection is ca. 20 nM in acidic solution, and less than 1 μM in neutral solution. Detection of DA in concentrations as low as 5 nM can be achieved after prolonged immersion of powder microelectrode in solution.

Electro-oxidation of glucose on platinum in alkaline solution and selective oxidation in the presence of additives

Abstract The electrochemical behavior of glucose at Pt electrodes was investigated in alkaline solutions using cyclic voltammetry and the modulated potential-time waveform (MPTW) technique. It was found that suitable MPTWs can produce reproducible results after the elimination of the absorbed “poison” formed during glucose oxidation, leaving a clean electrode surface. The effects of the experimental conditions on the voltammetric characteristics of glucose oxidation were studied systematically. These results were explained by an oxidation mechanism via the rate-determining decomposition of adsorbed glucose with the release of a hydrogen atom. The applicability of MPTW amperometry and coulometry to the measurement of glucose concentration was demonstrated. Glucose can penetrate the Nafion ® membrane which modifies the Pt electrode surface. By making use of Nafion ® -modified Pt electrodes, it was found that the interference of several biologically active small organic molecules, such as ascorbic acid, cysteine and uric acid, with the oxidation of glucose was suppressed effectively, forming the basis for studies of practical electrochemical sensors.

Elimination of the interference of ascorbic acid in the amperometric detection of biomolecules in body fluid samples and the simple detection of uric acid in human serum and urine by using the powder microelectrode technique

Abstract A study of the electrochemical behaviour of ascorbic acid (AA) at the powder microelectrode revealed that exhaustive electrolysis of AA could be achieved within a narrow range of potential. As a result, the interference of AA in the amperometric detection of bioactive molecules in body fluid samples can be practically eliminated by employing the powder microelectrode technique. Elimination of the interference of AA can be further improved if the monitored biomolecules are strongly adsorbed on the surface of carbon powder. A good example of such a bioactive compound is uric acid (UA), which can be strongly adsorbed on the surface of acetylene black powder. In a mixture containing 0.8 μM–0.4 mM UA and a large excess of AA in 0.1 M phosphate buffer solution, the separation between the potentials of oxidation current peaks of UA and AA reaches 400 mV, and the height of the UA peak is linear with respect to the UA concentration. A very simple procedure for the detection of UA in human serum and urine samples has been worked out to illustrate the above-stated principle. Application of the same principle in the amperometric detection (and probably simultaneous determination) of other electroactive biomolecules also seems promising.

Study of the powder/membrane interface by using the powder microelectrode technique I. The Pt-black/Nafion® interfaces

The polarization behavior of a Pt microdisk electrode, a platinized Pt microdisk electrode and a Pt-black powder microelectrode immersed in sulfuric acid solution, in contact with a moist Nafion® membrane were studied by cyclic voltammetry. The experimental results reveal that the surface diffusion of the adsorbed species plays an important role in determining the kinetics of interfacial reactions taking place at the powder/Nafion® membrane interface. The weakly adsorbed hydrogen atoms diffuse significantly faster than the strongly adsorbed hydrogen and the oxygen-containing surface species. Surface active sites in the range of tens of microns from the powder/Nafion® interface can be involved in interfacial reactions. The apparent coefficients of diffusion of various surface species were estimated from the variation of peak current with scan rate of potential. The apparently higher irreversibility of electrode reaction observed at the powder/membrane composite electrode is explained by considering mechanism of current generation of this type of electrode.

Application of time-resolved EQCM to the study of the mechanism of silver(I) oxide formation on a polycrystalline silver electrode in alkaline solution

Abstract The time-resolved EQCM was used to investigate the mechanism of the electrochemical formation of silver(I) oxide on a polycrystalline silver electrode in 0.1 M NaOH. The mass change was monitored in a series of potentiodynamic experiments. The first stage of the oxidation can be unambiguously attributed to the formation of a monolayer of Ag 2 O. In the second stage of the oxidation, a very small amount of Ag or its oxide dissolves into the solution and the Ag 2 O layer thickens gradually. No species other than Ag 2 O was detected during the whole oxidation process. The structure of the Ag 2 O layers formed is different at different stages of the oxidation. Compact Ag 2 O layers are formed in the first two stages, while Ag 2 O formed at more positive potentials is porous. The roughening of the electrode during the formation and reduction of the oxide is also discussed on the basis of the mass changes.

An EQCM investigation of oxidation of formic acid at gold electrode in sulfuric acid solution

The oxidation of HCOOH at Au electrode in sulfuric acid solution was investigated using the electrochemical quartz crystal microbalance (EQCM). The potentiodynamic mass responses in HCOOH-containing solution were compared to that in the background solution. This comparison provided valuable information on the mechanism of HCOOH oxidation. The effect of the potential scan rate on the current peak corresponding to oxide reduction was also discussed.

Characteristics of the glucose oxidase at different surfaces

Abstract By adsorption or chemical bonding, glucose oxidase (GOD) molecules are immobilized to different surfaces, including bare Pt and Au, alkanethiols self-assembled monolayers, and ω-carboxylic acid thiols self-assembled monolayers. Except Au and reduced Pt surfaces, GOD can be immobilized on all the surfaces tested. The most durable immobilization is achieved by covalent bonding GOD to carboxylic terminated SAM. In most cases the immobilized GOD retains its native enzymatic activity. A chain length dependence of the apparent Michealis constant is found for the GOD adsorbed at carboxylic group terminated SAM and the possible reasons are discussed.

Improvement of the output characteristics of amperometric enzyme electrode by using the powder microelectrode technique

Application of the powder microelectrode technique to the fabrication of enzyme electrodes can result in significant improvement of the output behaviours of an amperometric enzyme electrode. The theoretical kinetic equation of the powder enzyme electrode is derived and compared with that of the planar enzyme electrode, together with a simple method for the estimation of apparent Michaelis constant Km′ from the response of the powder enzyme electrode. The glucose oxidase (GOD) electrode is used as the model to verify the theoretical equations. Values of Km′ estimated from experimental data obtained with powder electrodes are closer to the intrinsic values of the same enzyme system in bulk phase. The working life of the enzyme electrode was found to improve and the interfering effects of various bioactive molecules were found to be less in the case of powder enzyme electrodes.

The time-resolved EQCM and study of the kinetics of silver(I) oxide formation on a polycrystalline silver electrode in alkaline solution

The time-resolved EQCM method is described and a theoretical analysis of the relation between the frequency resolution and the sampling rate (time resolution) is presented. It is shown that the time-resolved EQCM can fulfil simultaneously good frequency resolution and time resolution. The kinetics of the oxidation of Ag to Ag2O are studied using this method combined with the potentiostatic technique. The monolayer Ag2O formed at the first stage shows a potential dependent coverage. In the first two stages, the Ag2O layer thickens layer by layer with a rate-determining step of diffusion of Ag+ to the surface, where it forms an Ag2O base layer. The frequency transients associated with the formation of the bulk phase Ag2O are well consistent with the current transients, and both indicate an instantaneous nucleation and 3D growth mechanism. A relatively accurate method to estimate the mean height h of the cavities at the rough surface is also presented based on the frequency transient and charge density, and a linear increase of h with anodic charge is observed during the potentiostatic process. It has been demonstrated that the time-resolved EQCM can provide valuable information on the transient change of the morphology at the electrode|electrolyte interface.

SPE composite microdisk electrodes used in organic electrosynthesis

Abstract An SPE composite microdisk electrode is made by pressing a microdisk electrode on a piece of SPE membrane. It has a small ohmic drop and shows good reproducibility and some well-developed cyclic voltammograms and steady-state polarization curves are obtained. The electrochemical behavior of nitrobenzene and ferrocene at different microelectrode¦SPE membrane interfaces is investigated. It is found that electroactive compounds in the vicinity of the interface diffuse from the bulk solution to the electrode surface through the SPE membrane. This technique is suitable for investigating the electrochemical behavior of gases at the metal¦SPE interface or organic electroactive compounds poorly soluble in aqueous solution without addition of supporting electrolyte.