Fluorescence sensor based on IFE between AEP@UCNPs and oxTMB for the determination of Cd²⁺ in paddy rice combined with HRP enzyme inhibition mechanism

Cadmium ion (Cd²⁺) detection technology plays a prominent role in food safety and human health. Herein, we designed and constructed an 2-aminoethyl dihydrogen phosphate (AEP)@upconversion nanoparticles (UCNPs) fluorescence sensor for quantitative detection of Cd²⁺ in paddy rice based on inner filter effect (IFE) combined with enzyme inhibition mechanism. The AEP modification UCNPs can offer a stable fluorescence donor at 658 nm and be quenched by the oxidized tetramethylbenzidine (oxTMB) catalyzed by horseradish peroxidase (HRP) enzymes. Without addition of Cd²⁺, the fluorescence of AEP@UCNPs fluorescence sensor was weaken due to the IFE between AEP@UCNPs and oxTMB. With addition of Cd²⁺, HRP enzyme activity was inhibited by Cd²⁺, leading to the decreased oxTMB, resulting in the enhance upconversion fluorescence intensity. As a result, the fluorescence intensity signal at 658 nm of the IFE-based AEP@UCNPs fluorescence sensor increased linearly with the increase in Cd²⁺ in a wide range from 0.5 μmol/L to 6 μmol/L and the limit of detection (LOD) was 24.6 nmol/L. In addition, our proposed IFE-based AEP@UCNPs fluorescence sensor can achieve Cd²⁺ detection in paddy rice in 30 min.