Multifunctional N,S co-doped carbon dots for sensitive probing of temperature, ferric ion, and methotrexate

In this paper, we have presented a facile method to fabricate nitrogen and sulfur co-doped carbon dots (N,S-CDs) for blood methotrexate (MTX) sensing applications. The N,S-CDs with quantum yield up to 75% were obtained by one-step hydrothermal carbonization, using reduced glutathione and citric acid as the precursors. With this approach, the formation and the surface passivation of N,S-CDs were carried out simultaneously, resulting in intrinsic fluorescence emission. Owing to their pronounced temperature dependence of the fluorescence emission spectra, resultant N,S-CDs can work as versatile nanothermometry devices by taking advantage of the temperature sensitivity of their emission intensity. In addition, the obtained N,S-CDs facilitated high selectivity detection of Fe³⁺ ions with a detection limit as low as 0.31 μM and a wide linear range from 3.33 to 99.90 μM. More importantly, the added MTX selectively led to the fluorescence quenching of the N,S-CDs. Such fluorescence responses were used for well quantifying MTX in the range of 2.93 to 117.40 μM, and the detection limit was down to 0.95 μM. Due to "inert" surface, the N,S-CDs well resisted the interferences from various biomolecules and exhibited excellent selectivity. The proposed sensing system was successfully used for the assay of MTX in human plasma. Due to simplicity, sensitivity, selectivity, and low cost, it exhibits great promise as a practical platform for MTX sensing in biological samples. Graphical Abstract.