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. Hg2+ 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 Hg2+. 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-1 and a low detection limit of 0.057 mg L-1. Moreover, the fluorescent probe was successfully applied to detect oxalic acid in tomato and cherry tomato samples with satisfactory results.
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