Sulfur and nitrogen co-doped carbon quantum dots as fluorescent probes for the determination of some pharmaceutically-important nitro compounds

In this study, highly fluorescent sulfur and nitrogen co-doped carbon quantum dots (SN-CQDs) were synthesized by a simple one-pot hydrothermal method using thiosemicarbazide and citric acid as starting materials. Various spectroscopic and microscopic techniques were applied to characterize the prepared SN-CQDs. The synthesized SN-CQDs’ maximum fluorescence emission was obtained at 430 nm after excitation at 360 nm. Rifampicin (RFP), tinidazole (TNZ), ornidazole (ONZ), and metronidazole (MNZ) all quantitatively and selectively quenched the SN-CQDs’ native fluorescence, which was the base-for their-spectrofluorimetric estimation without the need for any tedious pre-treatment steps or high-cost instrumentation. SN-CQDs demonstrated a “turn-off” fluorescence response to RFP, TNZ, ONZ, and MNZ over the ranges of 1.0–30.0, 10.0–200.0, 6.0–200.0, and 5.0–100.0 μM with detection limits of 0.31, 1.76, 0.57, and 0.75 μM and quantitation limits of 0.93, 5.32, 1.74, and 2.28 μM respectively. The suggested method was successfully used to determine the investigated drugs in their commercial dosage forms. The method was further extended to their determination in spiked human plasma samples, with satisfactory mean % recoveries (99.44–100.29) and low % RSD values (< 4.52). The mechanism of fluorescence quenching was studied and discussed. The suggested method was validated in accordance with ICH recommendations.