Synthesis of highly fluorescent carbon dots as a dual-excitation rationmetric fluorescent probe for the fast detection of chlorogenic acid.

Nitrogen and sulfur co-doped carbon dots (abbreviated as N,S-CDs) were obtained by two-step hydrothermal reactions using citric acid/sulfamic acid as precursors, polyethyleneimine (PEI) as passivation agent. It was found that the PEI modified CDs with a fluorescence quantum yield of up to 29.1%, showed an obviously enhanced photoluminescence (PL) compared to the initial CDs. Interestingly, when monitored at the fluorescence emission wavelength of 460 nm, the dispersed N,S-CDs solution exhibits only one excitation band peaked at 355 nm, while one aggregated N,S-CDs solution with good water solubility and excellent fluorescence stability possesses two well-separated excitation bands centered at 310 nm/397 nm. When chlorogenic acid (CGA) was added to this aggregated N,S-CDs solution, the excitation peak at 310 nm was obviously reduced due to the inner filter effect (IFE), whereas another peak at 397 nm almost remained constant. Based on the above phenomenon, a dual-excitation ratiometric fluorescent probe for CGA assay was constructed. Under the optimized conditions, the logarithm of the fluorescence intensity ratios (F/F) exhibited a good linear correlation with the CGA concentration over a range from 0.33 to 29.70 μg/mL with a detection limit of 0.12 μg/mL. Moreover, the proposed sensing system was applied to determine CGA content in real samples with satisfactory results. The proposed sensing platform provides a new method for the detection of CGA.