A ratiometric nanoprobe based on silver nanoclusters and carbon dots for the fluorescent detection of biothiols

Ratiometric fluorescent probes could eliminate the influence from experimental factors and improve the detection accuracy. In this article, a ratiometric nanoprobe was constructed based on silver nanoclusters (AgNCs) with nitrogen-doped carbon dots (NCDs) and used for the detection of biothiols. The fluorescence peak of AgNCs was observed at 650   nm with excitation wavelength at 370   nm. In order to construct the ratiometric fluorescent probe, NCDs with the excitation and emission wavelengths at 370   nm and 450   nm were selected. After adding AgNCs, the fluorescence of NCDs was quenched. The mechanism of the fluorescence quenching was studied by fluorescence, UV–Vis absorption and the fluorescence lifetime spectra. The results indicated that the quenching could be ascribed to the inner filter effect (IFE). With the addition of biothiols, the fluorescence of AgNCs at 650   nm decreased due to the breakdown of AgNCs, and the fluorescence of NCDs at 450   nm recovered accordingly. Thus, the relationship between the ratio of the fluorescence intensities (I 450 /I 650 ) and biothiol concentration was used to establish the determination method for biothiols. Cysteine (Cys) was taken as the model of biothiols, and the working curve for Cys was I 450 /I 650   =   0.60C Cys   −   1.86 (C Cys : μmol/L) with the detection limit of 0.14   μmol/L (S/N   =   3). Then, the method was used for the detection of Cys in human urine and serum samples with satisfactory accuracy and recovery ratios. Furthermore, the probe could be applied for the visual semi-quantitative determination of Cys by naked eyes.