Rapid dual-readout colorimetric detection of melamine in milk based on silver nanoparticles

Milk is an indispensable component of the human diet and is therefore an important target matrix for rapid food safety screening. Melamine adulteration of dairy products poses a potential health risk, highlighting the need for simple and reliable analytical methods for rapid screening. Herein, a thymine-derivative-functionalized silver nanoparticle-based colorimetric sensing platform was developed for the rapid detection of melamine in milk. The thymine moieties provide complementary hydrogen-bonding recognition sites for melamine, while the silver nanoparticles serve as plasmonic signal transducers. Melamine-induced interparticle association produces concentration-dependent nanoparticle aggregation and changes in localized surface plasmon resonance, resulting in a readily visible color transition from yellow to purplish red. The sensing response was interrogated through two complementary readout formats: image-based colorimetric screening using visual observation/RGB analysis and ratiometric UV-vis spectroscopy for quantitative determination. Under optimized conditions, the method exhibited excellent linearity over the concentration range of 0.16-0.28 µM (R² = 0.996) and achieved a calculated detection limit of 3.52 nM. A visible response was obtained within 90 s, and common milk-associated interferents at more than 1000-fold molar excess relative to melamine caused negligible changes in the analytical response. For independent validation using three representative commercial milk products, satisfactory recoveries of 98.6-102.9% were obtained, with relative standard deviations of 1.1-3.2%. The proposed platform integrates molecular recognition with a simple optical readout, providing a rapid approach to the screening and quantitative determination of melamine in dairy matrices.