Developing a switch "OFF-ON" fluorescent probe for detection of melamine based on doubly-protected red emissive copper nanoclusters mediated by Hg2+ ions.

Melamine, often used as an adulterant in infants' formula due to its high protein content, can be harmful when ingested in large amounts, leading to the formation of cyanurate-melamine co-crystals in infants and potentially causing kidney damage. In this study, we introduce a fluorescent method for the selective and reliable detection of melamine in milk and infants' formula. The fluorescent probe comprises copper nanoclusters (Cu NCs) functionalized with thiosalicylic acid (TSA) and polyvinylpyrrolidone (PVP) as double-protecting ligands. Upon the addition of Hg²⁺, the fluorescence emission of TSA-PVP@Cu NCs is diminished due to static quenching. Subsequently, the fluorescence emission of the TSA-PVP@Cu NCs + Hg²⁺ probe is restored upon the introduction of melamine, facilitated by the coordination interaction between melamine and Hg²⁺ and the formation of a stable chelate between them. Under optimized conditions, the fluorescence emission was recorded initially for the TSA-PVP@Cu NCs + Hg²⁺ probe (F°) and after melamine addition (F). The (F/F°) ratio increased with rising melamine concentrations within the range of 0.025-65 µM. The detection limit, calculated using a signal-to-noise ratio of 3, was determined to be 8.0 nM. The TSA-PVP@Cu NCs + Hg²⁺ probe was successfully employed to detect melamine in milk and infants' formula, yielding acceptable recovery percentages and relative standard deviations. These results underscore the reliability and efficacy of the proposed probe for the fluorometric detection of melamine in real-world samples.