Tracking Cadmium Transfer from Soil to Cup: An Electrochemical Sensing Strategy Based on Bi³⁺-Rich MOFs for Tea Safety Monitoring

Tea is one of the most widely consumed beverages worldwide, yet increasing environmental cadmium (Cd²⁺) contamination poses a serious threat to consumer safety. Understanding the migration pathway of Cd²⁺ from contaminated soils through tea plants into brewed infusions is essential for comprehensive risk assessment across the entire tea supply chain. However, conventional analytical methods for Cd²⁺ detection are often time-consuming, labor-intensive, and unsuitable for rapid or on-site monitoring. In this study, we developed a facile, sensitive, and selective electrochemical sensing platform based on a Bi³⁺-rich metal–organic framework (MOF(Bi)) for reliable Cd²⁺ quantification in various tea-related matrices. The MOF(Bi) was synthesized via a solvothermal method and directly immobilized onto a glassy carbon electrode (GCE) in a one-step modification process. To enhance Cd²⁺ preconcentration, cysteine was introduced as a complexing agent, while Nafion was employed to stabilize the sensing interface and improve reproducibility. The resulting Nafion/cys/MOF(Bi)/GCE sensor exhibited excellent sensitivity with a wide linear range from 0.2 and 25 μg/L, a low detection limit of 0.18 μg/L (S/N = 3), high selectivity against common interfering ions, and good stability. This platform enabled accurate tracking of Cd²⁺ transfer from polluted garden soil to raw tea leaves and finally into tea infusions, showing strong correlation with ICP-MS results. Our strategy not only offers a practical tool for on-site food safety monitoring but also provides new insights into heavy metal transfer behavior during tea production and consumption.