The absorbance at 451 nm decreases gradually and the absorption peak has a 9 nm red shift upon the succedent addition of copper ions. The reaction reaches to saturation upon the addition of 1.0 equiv. Cu2+. To compound 2, with the addition of the Cu2+, the original absorption peak at 429 nm decreases gradually, at the same time, a newpeak at 464 nmattends and increases gradually. The detection limits [23,24] (Fig. S2) aremeasured to be 0.4 μM(1) and 0.15 μM (2), respectively