SOLVENT EFFECTS ON THE REACTIVITIES OF AN AMPEROMETRIC GLUCOSE SENSOR

Reactivities of organic phase biosensors containing 5.1 pmol cm(-2) gl ucose oxidase (GOx) on glassy carbon (GC) or Pt electrode surfaces (0. 071 cm(2) in area) were evaluated in acetonitrile, acetone, butan-2-ol , tetrahydrofuran and 0.1 M phosphate buffer (pa 7.0). Each of the org anic media contained 10% v/v of water. Ferrocenemonocarboxylic acid wa s used as a soluble electron transfer mediator for the detection of gl ucose in these solvents. Tafel analyses of the cyclic voltammograms (C Vs) of the electrocatalytic reaction gave Tafel slopes of between 103 and 129 mV decade(-1), which are in good agreement with the theoretica l value of 118 mV decade(-1). Constant-potential amperometric studies an GOx-modified rotating Pt disc electrodes (RDEs) were carried out at 0.45 V, a potential dictated by the limiting catalytic currents I-K O f the CV experiments. The apparent turnover rate constant K-cat' of GO x in the biosensor and its catalytic efficiency k(cat)'/K-m' were esti mated from the results of the RDE experiments. Changing from the aqueo us buffer to organic media produced a drastic decrease in k(cat)', whi ch is more than two orders of magnitude lower in butan-2-ol. This sens or characteristic is related to the lower solvent-dependent diffusibil ity of glucose in the sensor for the organic systems vis-g-vis phospha te buffer. The normalized catalytic efficiencies, (k(cat)'/K-m')(org) (solv)/(K-cat'/K-m')(buffer) show an enhancement of biosensor efficien cy on changing from phosphate buffer to polar organic solvents. The K- cat'/K-m' values are indicators of the degree of activation of the bio sensor's electrocatalytic reaction. Greater stabilization of the trans ition state of the electroenzymatic process by organic phases relative to phosphate buffer was ascertained from the normalized catalytic eff iciency. The enhanced catalytic efficiency of the organic phase sensor s is attributed solely to the activation of the catalytic reaction of GOx and beta-D-glucose.