In this section, various optimization parameters including modifier ratio, pH, and
reaction time were examined. The resulting aggregation capability between Ni2+ and GSHCys-
AgNPls in each condition was observed using the absorbance measurement at 618 nm.
First, the modifier concentration in different proportion (0.5-5 μM) was investigated.
Interestingly, the GSH and Cys concentration over 5 μM resulted in the self-aggregation of
AgNPls without the presence of Ni2+ (data not shown), leading to the decrease in sensitivity
of Ni2+ detection. In Fig. 3A, the highest absorbance at 618 nm was obtained using GSH and
Cys concentration ratio of 1:5, distinguishing clearly from the other GSH and Cys
proportions. We believe that this optimal GSH and Cys ratio provides greater aggregation via
the interaction between Ni2+ and an appropriate surface modification of GSH-Cys-AgNPls,
similarly to those observed for Mn2+ in the different work reported previously [16].