Electrostatically confined in-situ preparation of stable glutathione-capped copper nanoclusters for fluorescence detection of lysozyme

The fundamental challenges in application of copper nanoclusters (CuNCs) to fluorescence sensing are their poor stability, weak fluorescence intensity, and time-consuming preparation. In this work, an efficient and green method was developed by using a positively charged natural polysaccharide, chitosan (CS), as confining agent to electrostatically trap the negatively charged glutathione (GSH) as reducing-cum-protecting ligand for the in-situ synthesis of CuNCs. The synthesis can be completed within 1 min and CS-GSH-CuNCs emits intense orange fluorescence with an emission peak at 574 nm, which is much higher than that of GSH-CuNCs prepared in the absence of CS. Meanwhile, owning to both confinement and antioxidant effects of CS, CS-GSH-CuNCs shows excellent stability against time, temperature, salt, UV irradiation, and pH. Furthermore, the emission intensity of CS-GSH-CuNCs can be quenched by lysozyme (Lys) through the enzymatic hydrolysis of CS by Lys. Based on that, a fluorescence method was developed for the Lys determination with a linear range of 5–110 nM and a detection limit of 1.6 nM. Moreover, this approach shows high selectivity to Lys over various common substances in the biological environment, and thus can be used to determine Lys in human serum.