Wild jujube-based fluorescent carbon dots for highly sensitive determination of oxalic acid.

Fluorescent carbon dots (CDs) were synthesized by a one-step hydrothermal treatment of wild jujube and dl-tryptophan. The structure and properties of the CDs were confirmed by transmission electron microscopy, X-ray photoelectron spectroscopy, ultraviolet visible absorption spectroscopy, fluorescence spectroscopy and so on. The as-prepared CDs exhibit excellent excitation-independent but pH-dependent (4.0-12.0) fluorescent features and emit blue strong fluorescence under 365 nm light. Hg can decrease the fluorescence intensity of the CDs through static quenching, while the addition of oxalic acid (OA) recovers it owing to the coordination binding between oxalic acid and Hg. Based on this, the as-prepared CDs were used as a new "off-on" fluorescent probe for highly sensitive determination of oxalic acid with a wide linear detection range of 0.1-20 mg L and a low detection limit of 0.057 mg L. Moreover, the fluorescent probe was successfully applied to detect oxalic acid in tomato and cherry tomato samples with satisfactory results.