Green synthesis of nitrogen and sulphur-doped carbon quantum dots for dual sensing of brilliant green dye and Cu2+ ions in environmental samples

Nitrogen and sulfur-doped carbon quantum dots (N,S-CQDs) were successfully synthesized utilizing Aegle Marmelos as an environmentally friendly precursor via an innovative method. Various spectroscopic and microscopic techniques, including fluorescence spectroscopy, XPS and HR-TEM, were employed to analyse the N,S-CQDs. The material demonstrates excitation-dependent emission behaviour, emitting a blue light under UV lamp illumination. Notably, the emission remains pH-independent. The synthesized N,S-CQDs exhibit a high quantum yield of 31.42%. The resulting blue-emitting N,S-CQDs were utilized for both quantitative and qualitative determination of brilliant green (BG) dye and Cu2+ ions in aqueous environmental samples, with drinking water and treated wastewater as representative real samples, demonstrating a robust recovery of BG dye of 99.6-101.06% for drinking water and 102.4-105.8% for treated wastewater samples and favourable recovery for Cu2+ ions of 99.2-104% for drinking water and 100.8-107.4% for treated wastewater samples. The fluorescence of N,S-CQDs was influenced by BG dye/Cu2+ ions through the PET mechanism, causing fluorescence quenching/enhancement. Fluorescence lifetime decay analysis confirmed dynamic quenching with BG dye and aggregation-induced high fluorescence emission with Cu2+ ions.