or paraformaldehyde as substrate (Figure and). The currents obtained under inert atmosphere were used for normalization of the current response under air and in the presence of the O2 scavenger (Figure and). In the absence of O2 scavenger, O2 interference is observed in the whole glucose concentration range as indicated by catalytic currents at least 50% lower compared to measurements taken under N2 (Figure and). The O2 interference is most pronounced at glucose concentrations in the low millimolar range since O2 itself is present at the sub-millimolar concentration in aqueous solutions exposed to air. Nevertheless, even at high concentrations of glucose, the decrease is more than the ratio of concentrations of O2 to glucose. When both the electron mediator and GOx are freely diffusing in solution (Figure ), the large loss in current is explained through the continuous transport of O2 from ambient air into the bulk of the electrolyte where the reaction between glucose and O2 takes place. In the case of GOx entrapped within the Os-complex modified redox hydrogel films (Figure ), the reactions are confined to the volume of the film on the electrodes. Mano et al previously reported that O2 interference is most pronounced in thicker films where electron transfer limits the catalytic current. In these films, the GOx in the outer layer of the film is not accessible for charge transfer with the electrode via the electron mediator, leading to favored electron transfer from reduced GOx to O2. Differences in the diffusion coefficients of O₂ and of glucose result in reduced flux of glucose to the inner layer, thereby limiting the catalytic current.