A fluorescent turn-off/on method for detection of Cu2+and oxalate using carbon dots as fluorescent probes in aqueous solution

• CDs obtained from the activated carbon were synthesized.
• The fluorescence intensity of CDs was effectively quenched by Cu2+ (turn-off).
• The fluorescence intensity of CDs restored reversibly by oxalate (turn-on).
• The CDs could be used for the quantitative determination of Cu2+ and oxalate.

Carbon dots (CDs) were synthesized by the nitric acid oxidation of activated carbon. The experimental measurements showed that the resulting particles exhibited an average diameter of 50 nm and tunable emission wavelength. The fluorescence intensity of CDs was effectively quenched by copper ion (Cu2+) (turn-off) and restored reversibly by the addition of oxalate (turn-on). Based on this phenomenon, an “off–on” approach for detection of Cu2+ and oxalate using CDs as fluorescent probes was performed. The quantitative detection of Cu2+ ranged from 1.0×10−5 to 9.0×10–5 mol/L was allowed, and the concentrations of oxalate in the range of 1.0×10−5–7.0×10–5 mol/L exhibited a linear relationship with the relative fluorescence intensity with a correlation coefficient R2=0.9945. Furthermore, the detection limit for oxalate was as low as 1.0×10−6 mol/L, and the influences of 14 kind potential interference ions on oxalate determination were examined.