Improved glucose oxidation catalytic current generation by an FAD-dependent glucose dehydrogenase-modified hydrogel electrode, in accordance with the Hofmeister effect

Herein, we describe the effect of varying anions in an electrolyte solution on current generation by a redox hydrogel electrode. The electrode surface is coated with a thin film of hydrogel matrix, consisting of an osmium (Os) redox polymer including tethered Os complexes, polymer backbone, and a redox enzyme. In this case, the enzymes employed are flavin adenine dinucleotide-dependent glucose dehydrogenase (FAD-GDH), which catalyzes glucose oxidation, and the result was compared with that reported earlier for glucose oxidase (GOx). The hydrogel matrix facilitates efficient electron transfer from glucose to the electrode via collision of the Os complexes and thus acts as a mediator. The degree of impact of anions on current generation is characteristic of the Hofmeister series. Chaotropic anions, such as nitrate and chloride, increase and decrease the catalytic current produced by FAD-GDH and GOx hydrogel electrodes, respectively. Such anions can adsorb onto the cationic region of the FAD-GDH surface and induce a negative charge, which enhances electrostatic interactions between the enzyme and the positively charged Os polymer. Kosmotropic anions, such as sulphate and phosphate increase the catalytic current due to hydrogel shrinkage, which increases the relative concentrations of both enzyme and mediator within the hydrogel architecture due to an increase in density. High-performance electrode design depends on understanding the impact of ion identity on catalytic current responses of redox hydrogel electrodes.