Polyethylenimine-capped silver nanoclusters as fluorescent sensors for the rapid detection of ellagic acid in cosmetics.

Polyethylenimine-capped silver nanoclusters (PEI-AgNCs) are being developed as fluorescent sensors because of their high fluorescence, excellent photostability and large Stokes shift. In this study, PEI-AgNCs demonstrate characteristic absorption peaks at 268 and 360 nm with the strongest blue fluorescence emission at 436 nm. The PEI-AgNCs are spherical, and their average diameter is 3.7 ± 1.8 nm. In addition, the quantum yield of the PEI-AgNCs is 3.03%. A sensitive, selective and rapid detection technique for ellagic acid has been developed utilizing a fluorescent PEI-AgNC sensor. Ellagic acid exhibits tyrosinase inhibition and antioxidative properties and is hence always added in cosmetics as a skin whitening ingredient. The reaction can be completed within 60 s in Britton-Robinson buffer at pH 11. The fluorescence quenching mechanism is assumed to be static quenching because the fluorescence lifetime of PEI-AgNCs without and with ellagic acid did not obviously change. Moreover, the quenching intensity of the PEI-AgNCs is linearly related to the ellagic acid concentration over the range from 0.1 μg/mL to 30.0 μg/mL with good linear correlation (r = 0.999), and the detection limit is 33.5 ng/mL. Finally, PEI-AgNCs have also been applied in the detection of a cosmetics containing ellagic acid.