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 Cu2+ 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 Cu2+ via electron or energy transfer. Upon addition of l-histidine, the chelation between Cu2+ and the imidazole group of l-histidine leads to Cu2+ 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.