Fluorometric sensing and nanomolar level detection of heavy metal ions using nitrogen doped carbon dots

The contamination of the environment with heavy metals due to industrial and human activities has become a major concern, posing significant health risks. In this paper, a facile and environmentally sustainable synthesis method is employed to prepare nitrogen-doped carbon dots (NCDs) and used as efficient fluorescent probe for the detection of dual heavy metals including Fe(III) and Hg(II) in real samples. Further, the metal-ion sensing is also validated by electrochemical Cyclic voltammetry (C-V) measurements. Various characterization techniques, including Transmission Electron Microscopy, X-ray diffraction, X-ray photoelectron spectroscopy, and Fourier-transform infrared spectroscopy, were employed and confirmed the formation of 2–3 nm N-doped carbon quantum dots with spherical morphology and various surface functional groups. Under irradiation with 440 nm visible light, the synthesized NCDs demonstrated narrow and intense green fluorescence. Leveraging the hydrophilic optical properties, the NCDs were utilized for the detection of Fe(III) and Hg(II) ions in an aqueous environment. The fluorescence of the NCDs was found to be quenched in the presence of Fe(III) and Hg(II), and the quenching rate showed a linear correlation with the concentration of the metal ions. The limit of detection (LOD) for Fe(III) and Hg(II) ions was determined using the Stern-Volmer relationship, resulting in LOD values of 35.8 nM and 6.8 nM respectively. The fluorescence quenching is attributed to the electron transfer between NCDs to Fe(III) and Hg(II) ions, Further, the high sensitivity and selectivity of NCDs for Hg(II) was observed by fluorometric sensing and electrochemical measurements. Real water sample analysis in pond water, sea water and well water without any pretreatment showed 90–100% recovery. Above mentioned findings lay the foundation for the potential application of prepared NCDs as fluorescence sensors in aqueous media, offering a promising avenue for future development in this field.