Peter Christian Klitgaard

Bioelectrode response modulation caused by periodical carbon paste filling process

Abstract Carbon paste electrodes (CPEs) doped with glucose oxidase showed periodical variation of responses when new surfaces of the electrode were exposed to glucose solution. The periodicity of the responses was analyzed by discrete Fourier transformation. At an enzyme activity in carbon paste (CP) of 0.11–0.18 U mg −1 the main harmonic was 3.7–6.5 mm in length (12.4–21.6 slices of the electrode). The periodical change of response correlated with the periodicity of the CPE filling process. The modulation factor, expressed as modulation amplitude divided by the mean value of the parameter, depended on enzyme activity in the CP matrix. It was 15.1% and 2.8% at enzyme activities of 0.16 and 1.28 U mg −1 , respectively. The ohmic resistance of the CPEs measured by mercury electrode also changed periodically. The modulation amplitude of resistance was almost independent of enzyme activity and it was 2.0 Ω when CP contained 0.16 and 1.28 U mg −1 enzyme activity, respectively. The periodicity of the resistance modulation also correlated with the periodicity of the electrode filling process. To explain the periodical changes of the responses of the CPEs and factors influencing the modulation factor, the bioelectrode response dependence on enzyme activity was determined and a macrokinetic model was build. The CPE responses exhibit a saturating dependence on enzyme activity, which was explained by external diffusion limitation of the process. The model of external diffusion predicts that: (i) periodical CPE response variation is determined by biocatalytical activity modulation of the CPE surface; (ii) the modulation factor is a function of enzyme activity in CP matrix. The surface activity variation is related to pinhole formation on the electrode surface and coverage of the electrode by the pasting liquid. The surface coverage depends on the variation of local CP composition caused by periodical CP pressing, during the filling process.