We assumed that this selective activation of T-azoJSiR640 around 1% oxygen concentration is due to the high rate of reverse oxidation of the azo anion radical produced in the first reduction step (Figure ). This would explain why the fluorescence signal of T-azoJSiR640 is suppressed around 20-3% oxygen concentration, but is activated in extremely low oxygen concentration (around 1%), thereby allowing the selective detection of severe hypoxia.