LSPR-Driven Dual-Mode Urea Sensor for Milk Adulteration Detection Using Gold Nanobipyramids and Upconversion Nanoparticles

Ensuring food quality and safety is crucial, as the consumption of contaminated or substandard food poses serious risks to public health and imposes a substantial economic burden. Among various food safety concerns, milk adulteration with urea has emerged as a significant issue. This study presents a highly sensitive dual-mode colorimetric and fluorometric urea sensor for milk adulteration detection, utilizing enzymatic hydrolysis with Fenton-mediated plasmonic etching. The urease-catalyzed hydrolysis of urea produces ammonia, which inhibits the Fenton reaction, thereby modulating the localized surface plasmon resonance (LSPR) of gold nanobipyramids (AuNBPs). The resulting LSPR shift (Δλ > 150 nm) generates a distinct colorimetric response across a broad urea concentration range (0–5 mM), with a low limit of detection (LoD) of 0.09 μM. Moreover, the tunable LSPR of AuNBPs serves as a dynamic quencher for upconversion nanoparticles (UCNPs) via Förster resonance energy transfer (FRET), enabling ratiometric fluorometric sensing with high sensitivity (LoD of 0.056 μM). The spectral overlap between the LSPR of AuNBPs and the green/red emission of UCNPs facilitates a dynamic “Off/On–On/Off” response, ensuring accurate urea quantification in fluorescence mode. Furthermore, a smartphone-based RGB analysis enhances accessibility for on-site monitoring of urea levels, making this sensor a promising tool for real-world applications.