There are several studies to detect heavy metal ions based on
corrosion mechanism of AgNPs uncovered before [16,34,35]. In this
study, we designed a rapid and low-cost detection system based
on starch-stabilized AgNPs to detect Cu2+ in aqueous solutions.
The specific mechanism is shown in Scheme 1(a) comparing with
starch-AuNPs system (Scheme 1(b)). Starch can stabilize the AgNPs
and match with Cu2+ to form a chelate [36]. In order to seek out the
optimum pH value, hydrochloric acid was used to adjustthe acidity.
The standard potential of Ag+/Ag reduces from 0.7996V to 0.222V
(AgCl/Ag) in Eq. (1) due to the existence of chloride ions. At the
same time, the standard potential of CuCl2/CuCl is 0.57V, which
enables the redox reaction to carry through easily. AgNPs can be
easily oxidized by Cu2+ and Ag+ reacts with Cl− to form AgCl. In
addition, the color of the nanoparticle dispersions changes from
deep yellow to colorless. However, as we all know, the standard
potential of HgCl2/Hg2Cl2 is 0.63V (see Supplementary data, Table
S1), which is higher than that of CuCl2/CuCl, and this means that
Hg2+ can oxidized AgNPs easier in theory. Nevertheless, Cu2+ has
been found to have an intensively catalytic effect on noble metal
[37]. Taking all these factors, Cu2+ can oxidize AgNPs forming Cu+,which is unstable and some ofthemmay be oxidized to Cu2+ by oxygen
existed in the dispersion solution. The electron arrangement of
Cu2+ is 3d94s0, which endow it sp3 hybridization with oxygen to
form Cu2+ which was shown in Eq. (2).