Interestingly, reoxygenating the reaction media under normoxic conditions (21% O2) for 30 mins led to 82% fluorescence recovery, compared to the parent DDAO (Figure ). The reduction of the DDAO did not occur under normoxic conditions (Figure ) and was not promoted by β-NADPH alone (Figure ). These data indicate that compound 1 can undergo bioreduction to produce DDAO, but the dye is also reduced to 13 under the same conditions. However, the reversibility of this reaction allows recovery of fluorescence in normoxia, meaning that this probe might be useful in certain circumstances.