Cu2+ modulated silver nanoclusters as an on–off–on fluorescence probe for the selective detection of l-histidine

In the present study, a new strategy based on Cu 2+ mediated DNA-templated silver nanoclusters (DNA-Ag NCs) was developed, as a label-free, on–off–on fluorescent probe for the detection of l -histidine. Eight synthesized DNA oligonucleotides (D1–D8) were experimentally tested, and D5-Ag NCs was finally selected for l -histidine detection due to its best fluorescent properties. The fluorescence emission of D5-Ag NCs could be quenched by Cu 2+ via electron or energy transfer. Upon addition of l -histidine, the chelation between Cu 2+ and the imidazole group of l -histidine leads to Cu 2+ liberation from D5-Ag NCs, and subsequently results in a dramatic fluorescence enhancement of the probe. The method displayed a good selectivity toward l -histidine over all the other amino acids , with a linear relationship in the range of 0.20–80   μM, and a limit of detection (LOD) of 4.3   nM. The strategy was also successfully applied to detect l -histidine in diluted human urine, exhibiting great opportunities for practical application in biological system.