Sompong Thongngamdee

Effect of Surface-Active Compounds on the Stripping Voltammetric Response of Bismuth Film Electrodes

The influence of model surface-active macromolecules on the stripping voltammetric response of bismuth film electrodes was evaluated. Gelatin, albumin, humic acid, and Triton X-100 were used as representative surfactants, while lead and cadmium were employed as test metal ions. The adsorption of these surfactants onto the bismuth electrode caused a gradual decrease of the stripping peaks. The faster and larger suppressions observed at the bismuth electrode (compared to the mercury one) in the presence of albumin and Triton X-100 reflect differences in the adsorption kinetics and equilibrium of these surfactants at these electrodes. The surfactant interference at bismuth stripping electrodes can be circumvented using a permselective/protective Nafion coating. The effective resistance to surfactant effects makes the Nafion-coated bismuth electrode very attractive for practical stripping applications. A Nafion-modified stripping electrode is applied for measurements at large sample-instrument distances.

Electrically Heated Bismuth-Film Electrode for Voltammetric Stripping Measurements of Trace Metals

Bismuth-coated carbon paste electrodes display an attractive stripping voltammetric behavior which improves greatly upon heating the electrode during the deposition step. Such “hot-electrode” operation leads to a dramatic enhancement of the stripping peaks of lead, cadmium and zinc, while retaining low background currents. The influence of relevant parameters, including the heating current, deposition potential and deposition time is examined. The stripping signals for selected heavy metals result in sharp, well defined and undistorted peaks, with favorable reproducibility (5%). Highly linear calibration plot (correlation coefficient, 0.9986) is observed over the 20–140 μg/L lead range. An estimated detection limit of 3.16 μg/L lead could be observed on the basis of signal-to-noise ratio.

Catalytic adsorptive stripping voltammetric measurements of trace vanadium at bismuth film electrodes.

Bismuth-coated glassy carbon electrodes have been successfully applied for catalytic adsorptive stripping voltammetric measurements of low levels of vanadium(V) in the presence of chloranilic acid (CAA) and bromate ion. The new protocol is based on the accumulation of the vanadium-chloranilic acid complex from an acetate buffer (pH 5.5) solution at a preplated bismuth film electrode held at -0.35V (versus Ag/AgCl), followed by a square-wave voltammetric scan. Factors influencing the adsorptive stripping performance, including the CAA and bromate concentrations, solution pH, and accumulation potential or time have been optimized. The response compares favorably with that observed at mercury film electrodes. A linear response is observed over the 5-25mug/L concentration range (2min accumulation), along with a detection limit of 0.20mug/L vanadium (10min accumulation). High stability is indicated from the reproducible response of a 50mug/L vanadium solution (n=25; R.S.D.=3.1%). Applicability to a groundwater sample is illustrated.

Mixed enzyme (laccase/tyrosinase)-based remote electrochemical biosensor for monitoring phenolic compounds

An electrochemical biosensor for remote continuous monitoring of phenolic compounds in environmental analysis is described. The probe relies on rapid and sensitive amperometric detection at a submersible biosensor assembly, connected to a 50 ft long shielded cable. The enzymes laccase and tyrosinase were used as individual sensors and also as a bienzymatic sensor; these enzymes were immobilized chemically on the carbon fiber transducer. The analysis was based on the amperometric detection of the enzymatic products at a potential of −0.10 V vs. Ag/AgCl. Operational conditions were optimized to meet the requirements of remote operations. Tests with untreated river water spiked with phenolic compounds gave results similar to those obtained with synthetic buffer solutions. The remote laccase biosensor allowed the convenient quantification of guaiacol and chloroguaiacol at levels down to 22 and 9 nmol L−1, respectively. The co-immobilization of laccase and tyrosinase allowed the efficient detection of a larger group of phenolic compounds.