Fluorescent Carbon Dots as Nanosensors for Monitoring and Imaging Fe³⁺ and [HPO₄]^2– Ions

Nitrogen(N) and sulfur(S) codoped carbon dots (N/S-CDs) are facilely developed by a one-pot hydrothermal process from thiourea and o-phenylenediamine as carbon precursors. The as-fabricated N/S-CDs reveal a high quantum yield of 26% and a large Stokes shift up to 106 nm under the excitation of 450 nm with good water dispersibility as well as antiphotobleaching. With addition of Fe3+, fluorescence quenching happens with a good linearity within 0.3–70 μM and a low detection limit of 0.19 μM. However, the quenched fluorescence could be rapidly recovered with the addition of monohydrogen phosphate ([HPO4]2–) due to formation of the stable complex [Fe(HPO4)]+ by a competitive reaction between the N/S-CDs-Fe3+ complex and [HPO4]2–, a good linearity is in the range of 0.3–60 μM with a low detection limit of 0.27 μM. Thus, a significant “on–off–on” fluorescence response is performed with obvious color change from yellow-dark brown-yellow. In addition, the practical application and the confocal microscopic images all suggest that the N/S-CDs could target both Fe3+ and [HPO4]2– in living cells, implying that the N/S-CDs possess the diverse applications for Fe3+ and [HPO4]2– recognitions in biomedical fields.