Fluorescence emission and interaction mechanism of protein-coated gold and copper nanoclusters as ion sensors in different ionic environments

Abstract

Lysozyme protein-induced luminescent gold and copper nanoclusters (AuNCs and CuNCs) are prepared and their sensing capability is studied in the presence of different valent ions (Na+, Ca2+, Hg2+, Pb2+, Fe3+ and La3+ ions) in aqueous solutions of up to 100 µM salt concentration. Both types of nanocluster show good stability in aqueous conditions and, among all the chosen valent ions, it is found that maximum fluorescence quenching is observed in the presence of Hg2+ and Pb2+ ions for AuNCs and CuNCs, respectively. From steady-state fluorescence study and by using Stern–Volmer plots, the Stern–Volmer constant and other important thermodynamical parameters, i.e. changes in enthalphy, entropy and Gibbs free energy, are calculated. We propose a two step interaction process between the nanoclusters and the dissolved ions: first the interaction occurs between the protein-coated nanocluster surface and the ions, followed by the interaction of the metallic nanocluster core with the ions. Although thermodynamic investigation implies that both types of interaction are electrostatic in nature, selective fluorescence quenching of AuNCs and CuNCs in the presence of Hg2+ and Pb2+ ions occurs due to noncovalent metallophilic interactions between the cluster core and the dissolved ions.