Fluorescence sensing of mercury(ii) and melamine in aqueous solutions through microwave-assisted synthesis of egg-white-protected gold nanoclusters

Abstract

In this study, a microwave-assisted synthesis of fluorescent egg-white-protected gold nanoclusters (ew–AuNCs) for sensing Hg²⁺ ions and melamine was developed, optimized, and evaluated. By conducting infrared spectrometry and matrix-assisted laser desorption/ionization time-of-flight mass spectrometry, we confirmed that clusters consisting of 22 Au atoms were successfully embedded in egg white. Transmission electron microscopy measurements revealed that the ew–AuNCs were well dispersed with an average diameter of 2.7 ± 0.7 nm. The ew–AuNCs exhibited red fluorescence emission (λ_em 648 nm) with high quantum yield (2.37%). In the presence of Hg²⁺ ions, the fluorescence of these ew–AuNCs was quenched because of the Hg²⁺–Au⁺ interaction. The relative change in fluorescence intensity at 648 nm was dependent on the concentration of Hg²⁺ ions. Detection of melamine was based on the diminished fluorescence quenching of ew–AuNCs by Hg²⁺ when melamine was added along with Hg²⁺. The limit of detection for Hg²⁺ ions and melamine was 0.89 nM and 0.46 μM, respectively. Determination of Hg²⁺ ions in a pond water sample and of melamine in milk and dog food samples was performed, and the recoveries from the developed method were >92.9%, confirming its potential for sensor applications.