Nitrogen and phosphorus dual doped-fluorescent carbon quantum dots: An efficient nanoprobe sensor for sensitive and selective detection of ferric and mercuric ions

Abstract

The work uses phosphorous and nitrogen-co-doped carbon quantum dots (PN-CQDs) to present the effective and precise metal ion sensing for Fe (III) and Hg (II) in aqueous environments. Fluorescent PN-CQDs were synthesized using a microwave method with Citrus limetta juice as the carbon source, phosphoric acid (85 %) as the phosphorus source, and ethylenediamine for nitrogen. Structural analysis confirmed 4 nm spherical particles with an amorphous carbon core and rich surface functionalities. XPS revealed 6.63 % phosphorus and 15.48 % nitrogen doping. Optical studies showed excitation-dependent photoluminescence (PL) in the UV–visible range, with maximum emission at 550 nm under 440 nm excitation. The PN-CQDs exhibited a high quantum yield (56 %) and strong photostability under UV and visible light. For heavy metal ion sensing, the PN-CQDs showed high selectivity towards Hg(II) and sensitivity for Fe(III) among other metal ions, achieving nanomolar detection limits (1.35 nM for Fe(III) and 5.23 nM for Hg(II)). The linear response range and low detection limits highlight the potential of PN-CQDs as reliable fluorescent probes for the selective detection of Fe(III) and Hg(II) metal ions. The quenching mechanism was attributed to static quenching through ground state complex formation. The presence of phosphorus and pyridinic-N groups enabled selective Fe(III) and Hg(II) detection in real water samples with reliable recovery rates (96–100 %). These results underscore the high sensitivity and selectivity of PN-CQDs for detecting Fe(III) and Hg(II) ions, making them highly effective for environmental monitoring and bioanalytical applications.