Off–on and on–off chemosensors for ultratrace mercury(II) and copper(II) using functionalized thiazole and cadmium sulfide nanoparticles fluorophores

We have developed “off–on” and “on–off” chemosensors for Hg(II) and Cu(II) using 2-mercapto-4-methyl-5-thiazoleacetic acid (MMTA) and MMTA capped CdS nanoparticles (MMTA–CdS NPs) as fluorophores, respectively. MMTA exhibits absorption maximum at 318 nm and emission maximum at 430 nm (λex: 318 nm). While adding 10 nM Hg(II) into MMTA, the emission intensity was enhanced whereas 20,000-fold higher concentrations of common interferences including Cu(II) does not alter it. Based on the enhancement of emission intensity, the concentration of Hg(II) was determined. A good linearity was observed from 1 × 10−3 to 10 × 10−9 M of Hg(II) (R2 = 0.9986) and a detection limit was found to be 100 pg/L (S/N = 3). The MMTA–CdS NPs show emission maximum at 660 nm (λex: 368 nm). Interestingly, the emission intensity at 660 nm was decreased while adding 10 pM Cu(II) whereas 10,000-fold higher concentrations of common interferences including Hg(II) does not affect the emission intensity. The observed fluorescence quenching was due to the aggregation of CdS NPs. A good linearity was observed from 100 × 10−3 to 200 × 10−12 M of Cu(II) (R2 = 0.9978) and the detection limit was found to be 0.5 pg/L (S/N = 3). The proposed method was used to determine Hg(II) and Cu(II) in certified materials and water samples.