pH-Dependent reversible emission behaviour of lysozyme coated fluorescent copper nanoclusters

An environment-friendly method is used to prepare lysozyme coated fluorescent copper nanoclusters (CuNCs) of size 2.87 +/- 0.09 nm as obtained from the transmission electron microscopy. The CuNCs show UV-vis absorption at around 365 nm and the corresponding fluorescence emission is obtained at around 449 nm. X-ray photoelectron spectroscopy study confirms that the valency of the prepared CuNCs lies between 0 and + 1. The pH dependent fluorescence emission behaviour of as-synthesized ultra-small CuNCs is studied in buffer solution within the pH range, approximately from 2.5 to 7.5. The CuNCs show excellent stability in the whole pH range and show highly reversible emission behaviour with pH variation. For having such superior property, the lysozyme coated CuNCs can be used as a potential candidate for pH sensing. Fourier transformed infrared spectroscopy study confirms that the structural modification of lysozyme protein is responsible for showing such reversible optical behaviour. The mechanism of such reversible emission behaviour with pH variation is also proposed.