time-dependent absorbance changes of the TMB–H2O2 system under
different conditions. A small absorbance change in the system can
be observed due to the slow reaction rate between TMB and H2O2
(Fig. 5B, curve a) [66]. Upon adding GO, the absorbance intensity
of the system increased due to the intrinsic peroxidase property of
GO, which can catalyze H2O2-mediated oxidation of TMB and produced
the oxidation product with color change (Fig. 5B, curve c).
GQDs have increased the absorbance intensity of the system (Fig. 5B,
curve e) with the concentration of H2O2 from 10 nM to 0.5 mM. It
is known that GSH can be converted into glutathione disulfide (GSSG)
by reacting with H2O2. Using this strategy, GSSG was detected in the
range 0.5–100 μM [66].