Transition metal half-sandwich complexes as redox mediators to glucose oxidase

Abstract

Chromium and manganese half-sandwich complexes are evaluated as mediators to glucose oxidase (GOx) since they are of similar size to ferrocene derivatives (sandwich complexes) and contain a single π-ligand for interaction with the enzyme co-factor. A series of seven amino derivatives of [(η-C 6H 6)Cr(CO) 3] were investigated of which only [{η-C 6Me 4(NH 2) 2}Cr(CO) 3] (7), with the lowest oxidation potential of +40 mV (versus SCE), was found to display reversible electrochemistry. Small catalytic currents were recorded in the presence of GOx and glucose when complex (7) was incorporated in a screen-printed carbon electrode. Manganese cyclopentadienyl (Cp) half-sandwich complexes were found to be more effective GOx mediators and comparable in efficacy to ferrocene derivatives. A mediator rate constant k M of 2.1×10 5 M −1 s −1 was determined for the water-soluble complex [(η-MeC 5H 4)Mn(NO)(CN) 2]Na (11) compared to a range of 3×10 4 to 8×10 6 M −1 s −1 previously determined for ferrocenes under the same experimental conditions. β-Cyclodextrin (β-cd) was found to be helpful in solubilising hydrophobic complexes such as [(η-MeC 5H 4)Mn(NO)(S 2CNMe 2)] (15) and the neutral oxidised form of [MeCpMn(NO){(SCCN) 2}]NEt 4 (14), either directly as an inclusion adduct or in situ during cyclic voltammetry. Screen-printed amperometric electrodes, containing a mediator and GOx immobilised in an organic conducting carbon layer, were useful in assessing the mediation ability of complex (15) where aqueous insolubility precluded any kinetic studies with GOx in solution. This work was briefly extended to other oxidoreductase enzymes apart from GOx. Thus, rotating ring-disk voltammetry demonstrated that the β-cd complex of compound (15) is also a useful mediator to Horseradish peroxidase (HRP) since it displays an identical catalytic current to the ferrocene ethanolamine derivative (1) used in the MediSense ® ExacTech™ and Precision QID™ blood glucose biosensor electrodes.