Green hydrothermal synthesis of multifunctional carbon dots from cassava pulps for metal sensing, antioxidant, and mercury detoxification in plants

Carbon dots (CDs) have been attracted to nanocarbon materials for metal ion sensing, biological activity, and plant phytotoxicity due to their excellent photophysical properties, such as low cytotoxicity, high quantum yield, tunable fluorescence emission, and biocompatibility. Cassava pulp, which consists mainly of starch, has been identified as a low-cost biomass waste from the cassava starch industry. Therefore, this research developed CDs and nitrogen-doped CDs (NCDs) from cassava pulp using a one-step hydrothermal process in deionized water at 200 °C. The effects of the synthesis conditions, including reaction time (6–24 h) and the nitrogen doping derivatives, were also investigated. CDs and ethylenediamine doped-NCDs exhibited tunable fluorescence emission, strong quantum yield, high photostability, and tolerance to photobleaching. Furthermore, the potential applications of CDs-12 h were demonstrated such as fluorescent sensors for metal ion sensing, antioxidant activity, and mercury detoxification in plants. Fluorescence quenching of the CDs-12 h via both static and dynamic quenching mechanisms was observed in the presence of several metal ions such as Hg²⁺, Cu²⁺, and Fe³⁺ with the detection limit in micromolar levels and further applied to real water samples with good recovery and acceptable relative standard derivation. The paper test strip coated with CDs-12 h could also detect these metal ions under UV light. CDs and NCDs-EDA also showed potential DPPH radical scavenging activity and alleviated mercury toxicity in the Chinese cabbage seedlings with the incubation of CDs-12 h and NCDs-EDA-12 h (30 mg/L).