Upon the addition of S2O3
2 ions to CTAB/AgNPls the AgNPls can
beoxidizedbydissolvedO2 leadingtoreductionof theparticle sizeof
AgNPls (SchemeS1). However, itwas found that this reaction is very
slow because the Ag(S2O3)2
3 complexes immediately generated a
passive layer on the surface AgNPls. By adding Cu2+, the Cu2+ in the
0.1M ammonia buffer at pH 11 forms the Cu(NH3)4
2+ complex and
the standard potential of Cu(NH3)4
2+/Cu2+ in the presence of S2O3
2
was increased (Eq. (2)). The Cu2+ could accelerate the etching rate of
the AgNPls by forming Cu(S2O3)3
5 and the complexes could also be
oxidized to Cu2+ by dissolved oxygen (Eq. (3)). The etching of S2O3
2/
CTAB/AgNPls increased with increasing concentration of Cu2+. As a
result, the color solution changes from violet-red to colorless.
Therefore, the colorimetric detection based on the catalytic etching
of modified AgNPls provides a simple and sensitive method for the
measurement of trace Cu2+.