Microflow Synthesis of Fluorescent Carbon Dots for Selective Co²⁺ Detection

Green continuous synthesis of fluorescent carbon dots (CDs) from natural resources is a promising but challenging alternative to batch processing. Herein, a microflow approach was used to prepare CDs from a lemon juice precursor in one step. Through optimization of the reaction conditions, CDs prepared at 160 °C and 30 min of residence time were selected as a study model. Their structures, morphology, size, chemical composition, and optical properties were comprehensively analyzed. The results show small-sized fluorescent CDs with a high quantum yield and narrow size distribution, which are composed of a mixture of sp²- and sp³-hybridized carbon atoms and functional groups. The CDs show excellent photoluminescence properties and fluorescence stability at different pH values (pH: 5–11) and salt levels (KCl concentration: 0.25–4 M). Relying on the high binding affinity toward Co²⁺ ions, the microflow-synthesized CDs exhibited good selectivity and sensitivity for Co²⁺ sensing, with a detection limit down to 318 nM. The microflow approach was compared with existing techniques for the synthesis of CDs to show its advantages. In addition, a plausible Co²⁺-induced fluorescence quenching process and scale-up strategy for CDs synthesis are discussed.