Sensitive Detection of NAD+/NADP+ via Strong Coupling Fluorescence from Silver Nanoclusters

Abstract

The pyridine nucleotides (NAD+, NADP+) are the major coenzymes participate in multiple redox processes in living cells. Both NAD+ and NADP+ are not fluorescent and almost structurally identical, so it is difficult to directly distinguish NAD+ or NADP+ via optical methods (such as fluorescence and Raman spectroscopy). We report here a sensitive probe of NAD+/NADP+ based on fluorescent silver nanoclusters with dual emission band. The silver nanoclusters with an initial fluorescent emission peak at 410 nm were synthesized by etching large size silver nanoparticles. With the addition of NAD+/NADP+ solution, due to the strong coupling (charge-transfer) between silver nanoclusters and ligands (NAD+/NADP+), a new fluorescence emission peak of the silver nanoclusters was found and raised at 550 nm and the fluorescence intensity was dependent on the ratio between NAD+ and NADP+. The time-resolved fluorescence decay (at 550 nm) of silver nanoclusters showed a single-exponential decay lifetime of 3.9 ns caused by the strong coupling between silver nanoclusters and ligands (NAD+/NADP+). Meanwhile, the 410 nm emission was also selectively enhanced by the different ratio of NAD+ /NADP+ molecules. The intensity ratio of fluorescence emission at 410nm and 550nm may be useful to monitor the levels of NAD+ /NADP+ in aqueous solutions, cellular extracts and living cells. Candidate mechanisms and the analysis of time resolved emission spectra will be discussed.