Bright Mn-doped carbon dots for the determination of permanganate and L-ascorbic acid by a fluorescence on-off-on strategy

A one-pot hydrothermal synthesis of manganese-doped carbon dots (Mn-CDs) is reported for fluorescent "on-off-on" determination of Mn(VII) and L-ascorbic acid (L-AA) in aqueous solution and living cells. Mn-CDs were prepared by using sulfanilic acid, tetrakis(hydroxymethyl)phosphonium chloride, and Mn(II) chloride as precursors. Mn-CDs were characterized by several spectroscopic methods and microscopic techniques. Mn-CDs show distinctly long fluorescence lifetime (12.39 ± 0.07 ns) and high absolute fluorescence quantum yield (around 37%) with excitation and emission wavelengths of 362 and 500 nm, respectively. Mn-CDs exhibit no significant cytotoxicity to human cervical carcinoma HeLa cells and human embryonic kidney HEK-293T cells at 200 μg mL^-1 level after 48 h incubation. The fluorescence of Mn-CDs at 500 nm (excited at 362 nm) is quenched efficiently by Mn(VII) and can be further recovered after the addition of L-AA, resulting in a fluorescent "on-off-on" assay for the determination of Mn(VII) and L-AA. Under optimal experimental conditions, the linear response covers the 3 to 150 μM Mn(VII) concentration range and the 3 to 140 μM L-AA concentration range. This method offers relatively low detection limits of 0.66 μM for Mn(VII) and 0.90 μM for L-AA. This strategy was applied to visual determination of Mn(VII) and L-AA in living HeLa cells with satisfying results. Graphical abstract Schematic presentation of bright Mn-CD-based fluorescence "on-off-on" assay for both Mn(VII) and L-AA. This fluorescent assay possessed low detection limit of 0.66 μM for Mn(VII) and 0.90 μM for L-AA. This strategy was applied for visual determination of Mn(VII) and L-AA in living HeLa cells with satisfying results.