Dual-emitting zein-protected gold nanoclusters for ratiometric fluorescence detection of Hg2+/Ag+ ions in both aqueous solution and self-assembled protein film

The use of plant protein for the preparation of fluorescent gold nanoclusters (FGCs) is highly desirable owing to its abundance, low-cost, sustainability, and hypoallergenicity compared with protein from animals or microorganisms. In this work, zein, a protein derived from corn, was used as both a protecting ligand and reducing agent for FGC synthesis. Owing to the unique solubility of the protein, the zein-protected FGCs (Z-FGCs) can be separated from unreacted HAuCl4 and NaOH by simply mixing with an acidic buffer for several minutes. The Z-FGCs exhibit dual emission at 442 and 655 nm under excitation at 365 nm. Upon the introduction of Ag+/Hg2+ ions, the Z-FGC emission at 655 nm can be significantly enhanced/quenched while leaving the fluorescence at 442 nm almost unchanged. Accordingly, Z-FGCs can be employed as fluorescent ratiometric probes for the detection of Ag+/Hg2+ ions. The Z-FGC probes function well under a broad range of pH (pH 6–10) and temperatures (20–60 °C) and can thus be applied to determine Ag+/Hg2+ ions in tap water. Furthermore, the self-assembled film-forming property of zein is preserved after the FGC synthesis, which allows for the facile preparation of Z-FGC films for real-time visual sensing of Ag+/Hg2+ ions.