This study reports a novel colorimetric assay for Ag+ and L-cysteine (L-Cys) detection by Ag+-mediated aggregation of gold nanoparticles (AuNPs) and gold nanoclusters (AuNCs). L-histidine-stabilized AuNCs (His@AuNCs) and EDTA-stabilized AuNPs (EDTA@AuNPs) were synthesized. Upon introduction of Ag+, the induced of AuNCs- Ag+-AuNPs aggregates resulted in a distinct color shift from red to blue. Quantitative analysis showed that the absorbance ratio of the aggregation system was linearly correlated with the concentration of Ag+ (8-20 μM) (R2>0.998), and the recovery rate was 97.6-103.1% in river water samples. The -SH of L-Cys competitively complexed with Ag+, effectively disintegrating the nanomaterial aggregates and restoring the solution color and absorption spectra. The standard curve of 1-7 μM with recovery rates of 97.5-106.7% was established, and the quantification of L-Cys in milk was realized. The developed method features simple operation, high sensitivity, and good selectivity, providing new ideas for detecting metal ions and biomarkers using gold nanomaterials. Abstract text goes here. The abstract should be a single paragraph that summarises the content of the article.

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