We screened several alcohol oxidases (AOx) from different organisms, and selected the one from Pichia pastoris because of its superior activity (Table ). This enzyme exhibited the highest catalytic turnover (kcat) with methanol (161.7 ± 7.9 s -1 ) as the substrate. The kcat decreased with increasing alkyl chain length of the alcohols (e.g.: kcat = 121.3 ± 11.8 s -1 for ethanol). However, due to their volatility, short chain alcohols are not practical as reducing agents for the long-term storage of the sensors. AOx also catalyzes the reaction of formaldehyde with O2 to produce formate (kcat = 13.5 ± 0.1 s -1 , Table ). While formaldehyde is also highly volatile, its polymeric form, paraformaldehyde, is a solid that rapidly hydrolyzes to formaldehyde upon addition of an aqueous solution. Hence, it can serve as a non-volatile precursor for formaldehyde for the AOx-based O2-scavenger. The effect of the paraformaldehyde concentration on the reaction rate of O2 removal was evaluated using a carbon microelectrode to monitor the O2 concentration. The O2 level of the aerated solutions before AOx addition was approximately 0.25 mM. Upon addition of AOx into solutions containing paraformaldehyde and catalase, the O2 reduction current rapidly decreased until complete O2 depletion, whereby depletion rates increased as the paraformaldehyde concentration increased (Figure ). The time required for the O2 reduction current to disappear decreased from 389 to 67 s when increasing the concentration of paraformaldehyde from 0.5 mg mL -1 to 2 mg mL -1 (Figure ). At higher paraformaldehyde concentrations, the time for O2 depletion decreased only moderately (35 s for complete O2 removal at 5 mg mL -1 paraformaldehyde) indicating that saturation of paraformaldehyde was reached. At saturating paraformaldehyde concentrations, the rate of O2 reduction catalyzed by AOx is similar to that obtained with 10 mM formaldehyde (Figure ). This rapid O2 depletion indicates that the rate of hydrolysis of paraformaldehyde and the subsequent oxidation of formaldehyde are fast, rendering paraformaldehyde a highly compatible reducing agent for the AOx-based O2 scavenger.