Fig. 8. The absorbance ratio of A640/520 (nm/nm) for the AuNPs–Cu2+ sensing system at pH 7 over the concentration range (0.005–1.0 μM). The inset shows the magnified representation of A640/520 ratio from 0.005 μM to 0.1 μM. The right pictures show the progressive colorimetric changes of the AuNPs–Cu2+ sensing system with increasing Cys concentration (0–2.0 μM).
