Red-emission carbon dots as fluorescent "on-off-on" probe for highly sensitive and selective detection of Cu²⁺ and glutathione

A highly sensitive and selective fluorescent "on-off-on" strategy is established for the synchronous detection of Cu²⁺ and glutathione in aqueous solution. Red carbon dots (R-CDs) were prepared by using precursors of 4,5-difluoro-1,2-phenylenediamine and citric acid via a one-step hydrothermal strategy. R-CDs show a relatively long fluorescence lifetime of 3.47 ns under 455 nm excitation and high absolute fluorescent quantum yield of 20.1% with an excitation wavelength of 550 nm. R-CDs exhibit a marked pH-responsive fluorescence property with no significant perturbation from pH 4 to pH 13 even after five cycles. R-CDs with higher concentration of 750 μg·mL^-1 exhibit no significant cytotoxicity and good biocompatibility on HeLa cells and A549 cells after incubation for 48 h. The fluorescence of R-CDs at 619 nm (excited at 550 nm) is quenched statically by Cu²⁺ and recovered by glutathione subsequently, resulting in a fluorescent "on-off-on" assay for the synchronous detection of Cu²⁺ and glutathione. Under optimal conditions, the linear response covers the Cu²⁺ concentration range of 1 to 50 μM and the glutathione concentration range of 1 to 70 μM. Detection limits of Cu²⁺ and glutathione are 0.16 and 0.41 μM, respectively. This fluorescent probe is applied to the determination of Cu²⁺ and glutathione in authentic samples with satisfying results. Such an assay broadens the potential application of CDs in environmental areas and clinical therapy fields.