Screening synthetic cathinones mixed with instant coffee and fruit juice powder using hydrophobic carbon dot probes

Synthetic cathinones, a category of new psychoactive substance (NPS) with diverse structures and appearances in forms such as powder, tablets, and pills, have caused severe social problems. To make them attractive to teenagers, synthetic cathinones may be mixed with instant coffee and fruit juice powders. Difficulties in detecting these drugs in complex matrices complicate seizure of synthetic cathinones. Herein, a novel probe kit comprising hydrophobic tryptophan-carbon dots (htC-dots) with a liquid–liquid extraction procedure was used to detect 4-chloromethcathinone (4-CMC) and other synthetic cathinones mixed into instant coffee and fruit juice powder. Additionally, a self-made fluorescence detector controlled with an Arduino system was developed for the htC-dot probe kits. For 4-CMC, the dynamic ranges and detection limits were assessed as 0.125–0.75 mM and 0.08 mM, respectively, for the microplate reader and as 0.625–5 mM and 0.42 mM, respectively for the self-made fluorescence detector. The sensory mechanism of the probe kit involved photoluminescence (PL) quenching of htC-dots through electron and hot electron transfer under an excitation wavelength of 302 nm. The matrix effects of five brands of instant coffee and fruit juice powder were evaluated, and a cut-off htC-dot solution was prepared to assess htC-dot probe kit accuracy. Finally, 24 real-world samples were tested using the htC-dot probe kits, and the results were confirmed using gas chromatography–mass spectrometer (GC–MS). The results suggested that the htC-dot probe kits have considerable potential for on-site screening of synthetic cathinones mixed into instant coffee and fruit juice powder.