Preparation of 6, 8-Dithiooctanoic Acid-Functionalized Copper Nanoclusters with Near-Infrared Emission: Application for Determination of Melamine in Food Matrices

Abstract

In this study, a novel near-infrared (NIR)-emissive probe was developed for the detection of melamine, based on 6, 8-dithiooctanoic acid-functionalized copper nanoclusters (DAC@Cu NCs). The probe exhibited a strong emission peak at 712 nm under excitation at 330 nm. Interestingly, the addition of melamine alone did not alter the fluorescence emission of DAC@Cu NCs, indicating the necessity of an intermediate quencher for effective detection. To address this, Hg²⁺ ions were employed as a quencher, effectively reducing the fluorescence emission of DAC@Cu NCs through a static quenching mechanism. Upon introducing melamine, coordination between melamine and Hg²⁺ resulted in the removal of Hg²⁺ from the probe’s surface, restoring its fluorescence emission. Under optimized conditions, the ratio of fluorescence intensity before (F⁰) and after the addition of melamine (F) exhibited a linear relationship with melamine concentration over the range of 0.01–110 µM, with an impressive limit of detection (LOD, S/N = 3) of 0.0015 µM. The system demonstrated excellent selectivity for melamine detection, even in the presence of amine-containing compounds, confirming its robustness. Furthermore, the DAC@Cu NCs + Hg²⁺ system was successfully applied to real-world samples, including infant formula and commercial milk, achieving recovery rates of 98.7–103.5% with relative standard deviations (RSDs) between 2.43% and 3.78%. These findings highlight the practical utility of the proposed probe for rapid, accurate, and cost-effective melamine detection. The system’s simplicity, high sensitivity, and outstanding selectivity position it as a promising candidate for routine monitoring in food safety and quality control. Additionally, its versatility suggests potential for expansion into other sample matrices and on-site analysis applications.