Eco-friendly synthesis of N-doped carbon quantum dots from Bombax ceiba stem: A photophysical study with exploratory sensing of Fe3+/Cd2+ and dye degradation

This study reports a green, rapid, and sustainable continuous hydrothermal synthesis of nitrogen-doped carbon quantum dots (N-CQDs) derived using the tropical biomass Bombax ceiba as a new carbon precursor along with urea as a nitrogen source. The N-CQDs showed high blue fluorescence emissions when excited with 365 nm UV light and can be used in fluorescence sensing and photocatalysis. High-resolution spectroscopy, microscopy, and surface analysis were adequate to characterize N-CQDs and N-CQDs/TiO₂ comprehensively to confirm their optical characteristics, morphology, elemental composition, and surface polarity. N-CQDs were mixed with TiO₂ to attain N-CQDs/TiO₂ composites at ambient conditions under visible light to achieve photocatalytic responses. These N-CQDs achieved specific targeting of the Fe³⁺ and Cd²⁺ ions by fluorescence quenching, whose sensitivity had the strongest response to the tested Fe³⁺ and lowest limit of detection (LOD) of 1.9 μM, which is far below the guideline threshold set out by WHO, indicating their possible applicability in environmental monitoring. Methylene Blue (MB) was used as a common organic dye as a model pollutant to measure photocatalytic activity, with degradation being 88 % under visibly illuminated N-CQDs/TiO₂ composite, degraded better than both pure TiO₂ (44 %) and pure N-CQDs (75 %) under visible light. This research highlights the dual functionality of N-CQDs; firstly, as sensors to sense heavy metal ions and secondly as photocatalysts in decomposing dyes, offering a prospective application to environmental remediation.