O2 reduction appears at a potential lower than -0.1 V vs SHE on the GCE as a large irreversible cathodic current response (Figure and Figure ). After addition of the O2 scavenging system into the solution, the O2 reduction signal disappears which confirms the effective O2 removal (Figure and Figure ). With 5 mg L -1 paraformaldehyde, the anaerobic condition was maintained for at least 4.5 hours in solutions exposed to air, as demonstrated by the absence of any O2 reduction signal in the 20 successive CVs (Figure ). Similar results were obtained using EtOH as the reducing agent instead of paraformaldehyde (Figure ). AOx and CAT (red trace) and 20 successive CVs upon addition of paraformaldehyde (5 mg mL -1 , blue trace). e)