Ratiometric fluorescent probe for continuous detection of copper ions and lipoic acid in spinach using gold-silver bimetallic nanoclusters grown in situ on Zn MOF

Accurate monitoring of copper ions (Cu 2+ ) and lipoic acid (LA), essential trace elements and antioxidants in vegetables, is critical for food safety and human health. Here, we developed mercaptopurine-stabilized gold-silver bimetallic nanoclusters (MP-AuAg NCs) as a fluorescence 'turn-off' probe for Cu 2+ detection, achieving a linear response of 0.05–20 μM with a detection limit of 23 nM (LOD). To enhance sensitivity, a ratiometric probe (Zn MOF@MP-AuAg NCs) was engineered by in situ growth of zinc metal organic frameworks (Zn MOF) in the nanoclusters, which reduced Cu 2+ LOD to 4 nM (linear range: 0.015–0.50 μM). Interestingly, the Cu 2+ quenched system functioned as a 'turn on' sensor for LA detection, offering a dual range response: exponential correlation from 0.3 to 80 μM (R 2  = 0.997) and linear quantification from 0.3 to 25 μM (LOD = 94 nM, R 2  = 0.998). Validated in spinach samples, the method demonstrated exceptional precision (recoveries: 96.5–102 %) and precision (RSD <3.12 %), confirming its reliability for sequential Cu 2+ -LA analysis in complex food matrices. This work establishes a versatile sensing platform with enhanced sensitivity and practical utility for food quality monitoring.