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As one of the main ingredients in the traditional Chinese medicine Gastrodia elata, p-hydroxybenzaldehyde (PHBA) has many pharmacological effects. Therefore, the monitoring of PHBA level is of great significance to exact valuation of the pharmacokinetics and toxic effect of PHBA in body fluids. In order to achieve the sensitive detection of PHBA, nitrogen-doped carbon dots (NCDs) have been designed. NCDs were successfully prepared through a hydrothermal strategy with chrysanthemum as the precursor. The transmission electron microscopy, Fourier transform infrared spectroscopy, X-ray photoelectron spectroscopy and X-ray diffraction pattern were used for the characterization of NCDs. The as-prepared NCDs showed bright fluorescence with excitation/emission maxima of 290/421 nm and exhibited a fluorescence quenching response to PHBA under the optimal conditions. A good linear response was constructed at the PHBA concentration, ranging 10.0–150.0 μM with a detection limit of 63 nM based on the mechanisms of static quenching and inner filter effect. The proposed probe has been successfully applied to the detection of PHBA in blood and urine samples. In addition, the as-prepared NCDs with good biocompatibility were utilized for cell imaging. These results suggested that NCDs were promising probes for biosensing and bioimaging.

In this present work, nitrogen and sulfur co-doped carbon dots (NSCDs) were prepared from thiomalic acid as the carbon and sulfur source and NSCDs were used conveniently and sensitively to monitor Hg2+ and glutathione, regarding as a switching mode (turn-on-off) fluorescent sensing probe. The constructed NSCDs were characterized with their morphology and surface functional groups by TEM, XPS and XRD. The average particle size of the as-synthesized NSCDs is 4.4 nm. The fluorescence of NSCDs was efficiently quenched by Hg2+ with excitation and emission wavelength at 380/568 nm, respectively. The sensitive detection method of Hg2+ was established in 0.00–32.0 μM concentration range. The quenched fluorescence of NSCDs@Hg2+ could be recovered after adding GSH. As a result, the concentration of GSH at 0.50–34.0 μM could linearly relate to the change in fluorescence of NSCDs@Hg2+. The Hg2+ and GSH in human urine and serum samples were detected with the recoveries of 98.5%–104.5 and 98.5–103.5%, respectively. Owing to NSCDs favorable biocompatibility and low cytotoxicity, they were successfully applied to detect Hg2+ and GSH in HepG2 cells, indicating that NSCDs have potential applications in biology.