The influence of the pH on this system was investigated in pH
range of 6.0–11.0 (Fig. 3a). At pH lower than 8, the DI decreased
because the S2O3
2 was not stable and broke down into sulfate,
sulfide, sulfite tetrathionate, trithionate, polythionates and polysulfides
[31]. However, in the pH range of 9–12, the complex Cu
(NH3)4
2+ concentration increased with increasing NH3 concentration,
enabling the oxidation of AgNPls by Cu(NH3)4
2+. The intensity
color of modified AgNPls was almost constant above pH 11 in the
presence Cu2+. Therefore, pH 11 was selected as the optimal value
for all experiment.