Acid-Triggered Self-Assembled Egg White Protein-Coated Gold Nanoclusters for Selective Fluorescent Detection of Fe³⁺, NO₂^–, and Cysteine

Herein, we present a simple and economical synthesis for the first multianalyte probe able to selectively quantify the concentrations of Fe3+, NO2–, and cysteine. It comprises H+-triggered self-assembled gold nanoclusters (AuNCs@EW/H+, AuEHs), showing enhanced red fluorescence at 640 nm. The AuEH is a good fluorescent nanosensor for Fe3+ and NO2– with detection limits of 1.40 and 2.82 nM, respectively. Iron detection, through fluorescence quenching, occurs because of nanocluster aggregation elicited by the complexation of Fe3+ with amino acids on the surface of AuEH; nitrite detection likely proceeds through fluorescence quenching via the disassembly of the nanoclusters following irreversible oxidation by nitrite. This selectivity is good enough that it can be used to quantify the nitrite concentration in commercially available processed meat. Cysteine detection occurs through the restoration of fluorescence of iron-quenched samples; similar molecules including homocysteine and glutathione are unable to restore fluorescence, showing the specificity of the interaction. Applications, including as a detecting ink and as a biocompatible probe, show promise because of the lack of observable toxicity of the AuEHs, demonstrating their promise as specific and sensitive biosensors.