A Water-Soluble Tetramethylbenzidine-2-Hydroxypropyl-β-Cyclodextrin Inclusion Complex as an Efficient Mediator for Oxidoreductases

A water soluble 3,5,3′,5′-tetramethylbenzidine-2-hydroxypropyl-β-cyclodextrin (TMB-hpβCyD) inclusion complex was demonstrated for the first time as an efficient mediator for glucose oxidase, xanthine oxidase and glutamate dehydrogenase. At pH 5, the cyclic voltammogram of the TMB-hpβCyD complex exhibited two reversible oxidation waves with potential peak values (E_pa) of 384mV (TMB → TMB^+·) and 496mV (TMB^+· → TMB²⁺) at a glassy carbon electrode (vs. Ag/AgCl). The E_pa value of the former was pH-independent, whereas that of the latter was strongly pH dependent. However, at pH 7 the inclusion complex exhibited only one reversible oxidation wave with E_pa of 396mV. Based on cyclic voltammetric measurements, the second rate constant (k) for the TMB^+· (glucose oxidase) reaction was determined as 6.7 × 10⁵ M^-1s^-1 which compares favorably with that of ferrocene (2.6 × 10⁴ M^-1s^-1), one of the most common mediators. The inclusion complex was then investigated for bioelectrocatalysis at membrane electrodes of glucose oxidase and xanthine oxidase. The response of the hypoxanthine electrode was linear up to 40 μM; its sensitivity was 2.8 μA mM^-1 and a response time was smaller than 2 min. A similar response time was also observed for the glucose electrode, however, the linear range and sensitivity of the glucose electrode were 0-3 mM and 0.6 μA mM^-1, respectively. The capability of TMB^+· to oxidize NADH formed from a glutamate dehydrogenase:NAD/glutamate reaction, was also demonstrated, as illustrated by a well-defined catalytic wave for NADH oxidation.