Electrocatalytic reduction of oxygen by bilirubin oxidase in hydrophobic ionic liquids containing a small quantity of water

We have examined a direct electron transfer reaction of bilirubin oxidase (BOD) at a gold nanoparticle (AuNP) electrode in the mixtures of ionic liquid (IL) and water. No catalytic current induced by BOD was observed when neat ILs were used as an electrolyte solution. However, BOD-induced catalytic currents were observed when small quantities of water were added to hydrophobic ILs. The BOD-modified AuNP electrode provided a current density as high as 218 μA cm−2 in 1-(2-hydroxyethyl)-3-methylimidazolium bis(trifluoromethane-sulfonyl)imide ([C2OHmim][Tf2N]) containing 7.4 wt% H2O. To investigate the effect of the added water to ILs on the enzymatic activity we compared the physicochemical properties of the IL/H2O mixtures. There was a certain relation between the water activity (aW) of the IL/H2O mixtures and the current density of BOD. The BOD was confirmed to catalyze the four-electron reduction of O2 to H2O in the [C2OHmim][Tf2N]/H2O mixture by the data with cyclic voltammetry measurements using a rotating ring-disk electrode.