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 (12.390.07 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 mu g 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 mu M Mn(VII) concentration range and the 3 to 140 mu M L-AA concentration range. This method offers relatively low detection limits of 0.66 mu M for Mn(VII) and 0.90 mu M for L-AA. This strategy was applied to visual determination of Mn(VII) and L-AA in living HeLa cells with satisfying results. 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 mu M for Mn(VII) and 0.90 mu M for L-AA. This strategy was applied for visual determination of Mn(VII) and L-AA in living HeLa cells with satisfying results.