The effect of pH and transition metal ions on cysteine-assisted gold aggregation for a distinct colorimetric response

Colorimetric detection is a promising sensing strategy that is applicable to qualitative and quantitative determination of an analyte by monitoring visually detectable color changes with the naked eye. This study explored the cysteine (Cys)-induced aggregation of gold nanoparticles (AuNPs) in order to develop a sensitive colorimetric detection method for Cys. For this purpose, we systematically investigated the colorimetric response of AuNPs to Cys with varying particle sizes and concentrations. The AuNPs with various diameters ranging from 26.5 nm to 58.2 nm were synthesized by the citrate reduction method. When dispersed in water to have the same surface area per unit volume, the smaller AuNPs (26.5 nm) exhibited a more sensitive response to Cys compared to a larger counterpart (46.3 nm). We also examined the effect of divalent first-row transition metal ions (Mn²⁺, Fe²⁺, Co²⁺, Ni²⁺, Cu²⁺, and Zn²⁺) on the Cys-induced aggregation of AuNPs. Among the tested metal ions, the addition of Cu²⁺ provided the highest enhancement in sensitivity to Cys regardless of pH between 3.5 and 7. The significant increase in the sensitivity caused by Cu²⁺ could be attributed to the capability of Cu²⁺ to form a highly stable chelate complex with surface-immobilized Cys, facilitating the aggregation of AuNPs. For the AuNPs-Cu²⁺ system at pH 7, the detection limit for Cys was determined to be 5 nM using UV-vis spectroscopy. The reported strategy showed the potential to be used for a rapid and sensitive detection of Cys and also metal ions that can facilitate Cys-mediated aggregation of AuNPs.