Smartphone-integrated sulfur-functionalized polymer/Cu–BDC MOF hybrid sensor for on-site mercury monitoring in natural waters with cross-validation in serum

Reliable and rapid detection of mercury in natural waters and biological fluids remains a critical analytical challenge due to its extreme toxicity and environmental persistence. This study reported a disposable smartphone-integrated electrochemical sensing platform based on a hybrid of a sulfur-functionalized polymer and a copper–benzene dicarboxylate (Cu–BDC) metal–organic framework (MOF) for the sensitive and selective detection of Hg 2+ . The structural and functionalized integration of sulfur moieties within the Cu–BDC framework significantly enhanced the electron transfer rate and provided strong affinity toward mercury, as confirmed by electrochemical impedance spectroscopy and cyclic voltammetry. Compared to the bare electrode, the modified electrode exhibited a 64.9% increase in electroactive surface area and a 3.0-fold higher heterogeneous electron transfer rate constant. Under optimized conditions, the sensor demonstrated a linear response from 1 μmol/L to 500 μmol/L, a limit of detection of 0.324 μmol/L, and a sensitivity of 2.601 μA⸱L/(cm 2 ⸱μmol). Following hard-soft acid-base interactions, the platform exhibited excellent selectivity against common interfering metal ions (Zn 2+ , Cd 2+ , Pb 2+ , and As 3+ ) and organic compounds (humic acid, cysteine, and glutathione) and maintained long-term stability for over 16 d. Real-sample analyses of groundwater, canal water, and human serum yielded recovery rates between 96.2% and 106.8%, confirming the applicability of the device for environmental and biomedical mercury monitoring. This study establishes a portable, cost-effective, and highly sensitive electrochemical route for Hg 2+ detection, paving the way toward on-site environmental surveillance and clinical exposure assessment using smartphone-assisted diagnostics.