Copper nanoclusters as an on-off-on fluorescent probe for ascorbic acid

We report that copper nanoclusters (CuNCs) represent a viable fluorescent probe for ascorbic acid. The CuNCs were stabilized by tannic acid and characterized by high-resolution transmission electron microscopy, X-ray photoelectron spectroscopy, UV-vis, time-correlated single photon counting and fluorescence spectroscopy. It is found that the fluorescence of CuNCs (at excitation/emission peaks of 355/430 nm) is quenched by ferric ion due to electron-transfer between CuNCs and Fe(III) ions. If ascorbic acid is added, Fe(III) is reduced to Fe(II) and quenching is suppressed. The process is completed within 5 min. Under optimal conditions, a linear relation exists between fluorescence recovery and the concentration of ascorbic acid in the 0.5 mu M to 10 mu M concentration range. The detection limit is 0.11 mu M. The method was applied to the determination of ascorbic acid in (spiked) fruit and vegetables and gave recoveries ranging from 89 to 110 %.