The initial exponential increase in the fluorescence intensity was observed up to 1 μM concentration of Cu 2+ , then further addition more than 6 μM leads to the saturation in fluorescence intensity (Figure ). The plot between the I-I 0 v/s concentrations of Cu 2+ [M] ions provide very good linear correlations for two concentrations of Cu 2+ in the range of 0 μM -0.5 μM (R 2 = 0.997) and 0.5 μM -1 μM (R 2 = 0.995) with detection limits for both concentrations 15.6 nM and 3.6 nM, respectively (Figure S2a, ESI).