Ultra-sensitive and stable N-doped carbon dots for selective detection of uranium through electron transfer induced UO₂⁺(V) sensing mechanism

Unintended leakage of toxic and radioactive uranyl ion (UO₂²⁺) ions poses high harmful to human health and environment, hence its monitoring and detection is of utmost significance. Here, we developed a mild synthetic route to prepare stable N-doped carbon nanodots (CDs) with special hydrophilicity functional groups and graphitic N by polymerization and carbonization of novel norfloxacin precursor with nitrogen heterocyclic structure. The morphology, chemical structure, fluorescent properties, sensing competition and selectivity behaviors of CDs-UO₂²⁺ were systematically analyzed. Controllable experiments including low and high concentration CDs-UO₂²⁺ and another type of carbon dots (TEA-CDs) with low graphitic N amount using triethanolamine (TEA) precursor without nitrogen heterocyclic structure were designed, which further confirmed the effective coordination and interaction mechanism between CDs and UO₂²⁺ that graphitic N in CDs induced the formation UO₂⁺(V) by electron transfer. It is very suitable for on-site and real-time monitoring of UO₂²⁺ with an ultra-fast response time (∼20 s) and low detection limit of 20.38 nM (4.7 μg L⁻¹), which is lower than the permissible limits (30 μg L⁻¹) defined by the United States Environmental Protection Agency (EPA). More importantly, different form the liquid sensor, solid sensors (printed test strip and hydrogel) were also successfully employed to monitor UO₂²⁺ targets quickly, broadening the potential application for the advanced image encryptions.