Solvent effects on the reactivities of an amperometric glucose sensor

Abstract

Reactivities of organic phase biosensors containing 5.1 pmol cm−2 glucose oxidase (GOx) on glassy carbon (GC) or Pt electrode surfaces (0.071 cm2 in area) were evaluated in acetonitrile, acetone, butan-2-ol, tetrahydrofuran and 0.1 M phosphate buffer(pH 7.0). Each of the organic media contained 10% v/v of water. Ferrocenemonocarboxylic acid was used as a soluble electron transfer mediator for the detection of glucose in these solvents. Tafel analyses of the cyclic voltammograms (CVs) of the electrocatalytic reaction gave Tafel slopes of between 103 and 129 mV decade−1, which are in good agreement with the theoretical value of 118 mV decade−1. Constant-potential amperometric studies on GOx-modified rotating Pt disc electrodes (RDEs) were carried out at 0.45 V, a potential dictated by the limiting catalytic currents IK of the CV experiments. The apparent turnover rate constant k′cat of GOx in the biosensor and its catalytic efficiency k′catK′m were estimated from the results of the RDE experiments. Changing from the aqueous buffer to organic media produced a drastic decrease in k′cat, which is more than two orders of magnitude lower in butan-2-ol. This sensor characteristic is related to the lower solvent-dependent diffusibility of glucose in the sensor for the organic systems vis-a-vis phosphate buffer. The normalized catalytic efficiencies, (k′cat/K′m)org solv/(k′mcat/K′m)buffer show an enhancement of biosensor efficiency on changing from phosphate buffer to polar organic solvents. The k′catK′m values are indicators of the degree of activation of the biosensor's electrocatalytic reaction. Greater stabilization of the transition state of the electroenzymatic process by organic phases relative to phosphate buffer was ascertained from the normalized catalytic efficiency. The enhanced catalytic efficiency of the organic phase sensors is attributed solely to the activation of the catalytic reaction of GOx and β-d-glucose.