Intriguing manipulation of metal-enhanced fluorescence for the detection of Cu(II) and cysteine.

Commercially available salicylaldehyde, in alkaline medium, exhibits strong fluorescence after one hour of UV exposure in the presence of Ag(I) . The phenolic group of salicylaldehyde is converted into the quinone form under alkaline conditions in the presence of AgNO3 , resulting in aggregated Ag(0), which causes approximately 250 times fluorescence enhancement of the in situ produced quinone. Such high silver-enhanced-fluorescence (SEF) is selectively quenched by cysteine, arginine, histidine, methionine, and tryptophan. In contrast to the other amino acids, ageing brings selectivity of the cysteine-induced quenching effect. Interestingly, Cu(II) is found to be the only metal ion that exclusively regenerates the lost fluorescence. Thus, quenching and recovery of fluorescence (Turn Off/On) can be used for the selective and sensitive detection of cysteine as well as Cu(II) ions in one pot. Alteration of the electric field density around the fluorophore (lightening rod effect) and scattering/absorption cross-section have been proposed to account for the Off/On fluorescence.