Nitrogen-doped carbon dots for sensitive detection of ferric ions and monohydrogen phosphate by the naked eye and imaging in living cells

Nitrogen doped carbon dots (N-CDs) have been prepared via a one-pot hydrothermal method by using formamide and o-phenylenediamine as the carbon precursors. The as-fabricated N-CDs display excellent water dispersibility, good biocompatibility and anti-photobleaching properties. A strong emission band with an emission maximum (λ(fl)(max)) of 556 nm is observed under 450 nm excitation, and a large Stokes shift of 106 nm is presented. However, the fluorescence is quenched by the addition of Fe(3+); a good linearity is shown in the range of 0–65 μM with a detection limit as low as 0.85 μM. Fortunately, the quenched fluorescence could be recovered rapidly by the addition of monohydrogen phosphate (HPO(4)(2−)) due to the formation of the stable [N-CDs–Fe(3+)–HPO(4)(2−)] complex, and a good linearity is exhibited in the range of 0–60 μM with a low detection limit of 0.80 μM for HPO(4)(2−). A novel “on–off–on” fluorescence response is seen with an obvious color change from yellow-crimson-yellow by the naked eye. In addition, the confocal microscopy images suggest that the as-synthesized N-CDs could serve as a sensitive nanosensor for Fe(3+) and HPO(4)(2−) detection, implying the diverse potential application of N-CDs in the biomedical field.