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 nm attends and increases gradually.The detection limits [23,24] (Fig. S2) aremeasured to be 0.4 μM(1) and
0.15 μM (2), respectively