Direct electrochemistry of glucose oxidase immobilized on carbon nanotube for improving glucose sensing

In this research, we suggest an enzyme immobilization structure that glucose oxidase (GOx) is coated on carbon nanotube (CNT) and quantify an optimal condition for the immobilization. Physical adsorption of GOx to CNT is used as the method for GOx immobilization on CNT (GOx/CNT). Cyclic voltammetry (CV) is served to evaluate catalytic activity and direct electrochemistry, while SEM is used to confirm the formation of GOx/CNT. To investigate the catalytic activity and the optimal loading of GOx, its peak current and electron transfer rate constant, k_s, are measured. In both ways, 2 mg mL^-1 GOx shows best results and its k_s is 1.14s^-1. From the relationship between scan rate and peak current, it is also revealed that this structure is (i) controlled by surface reaction and (ii) quasi-reversible. Regarding redox reaction of GOx, peak potential is linearly varied with pH with slope of -51 mV/pH. The slope indicates a typical two-electron reaction that is a desirable reaction pathway. GOx-catalyzed glucose oxidation reaction (GOR) is also investigated by reacting different concentrations of glucose with GOx/CNT layer. Peak current for GOR linearly increases with glucose concentration, proving that increase in glucose concentration promotes GOR. Therefore, GOx/CNT leads to high sensitivity of glucose (53.5 μAmM^-1cm^-2). When it comes to long term stability, activity of GOx/CNT is measured and 86% of the activity is maintained even after two weeks, indicating that long term stability of GOx is excellent.