Green synthesis of nitrogen-doped carbon dots from konjac flour with “off–on” fluorescence by Fe³⁺ and L-lysine for bioimaging

Highly nitrogen-doped carbon dots (N-CDs) are prepared by the pyrolysis of konjac flour under mild conditions followed with a simple extraction by ethanol and water. The N-CDs exhibit excellent pH-switched photoluminescence (PL), and their PL intensity can be facilitated by either mixing with NaOH and basic amino acids or by surface passivation with non-amine-terminated polyethylene glycols of different molecular weights. Further, the fluorescence of N-CDs can be quenched with Fe³⁺ and recovered with L-lysine, accompanied with a red-shift of emission wavelength. In addition, the low toxicity and strongly fluorescent N-CDs are applied for cell imaging, and the quenched fluorescence by Fe³⁺ can be recovered inside the living cells.