Enhanced fluorescence and heavy metal ion sensing using nitrogen-doped carbon quantum dots synthesized from banana peel waste

• Nitrogen-doped carbon quantum dots (N-CQDs) were synthesized from banana peel waste using a hydrothermal–microwave route. • Sub-nanometer N-CQDs (0.879 nm) with tunable band gap (5.38–4.71 eV) were achieved by varying L-aspartic acid concentration. • Excitation-dependent emission, heatmaps, and 3D surface mapping revealed systematic modulation of emissive trap states. • Nitrogen doping enhanced PL intensity, quantum yield (up to 27%), and induced red-shifted emission behavior. • BP-NCQDs demonstrated strong selectivity and sensitivity toward Cr³⁺ and Fe³⁺ as fluorescent quenching probes. • Study provides a comprehensive structure–optical–thermal correlation for sustainable fluorescence sensors. This study reports the green synthesis of nitrogen-doped carbon quantum dots (N-CQDs) derived from banana peel biomass using L-aspartic acid as a nitrogen precursor to enhance their optical and sensing performance. The BP-NCQDs were prepared through a hydrothermal–microwave route and systematically characterized using UV–Vis, photoluminescence (PL), FTIR, HRTEM/FFT, particle-size analysis, and DSC–TGA. The as-prepared NCQDs exhibited sub-nanometer average size (2.66 ± 1.09 nm), semi-amorphous carbon structures, and abundant oxygen- and nitrogen-containing surface functionalities. Increasing L-aspartic acid concentration progressively modulated the absorption behavior, narrowed the optical band gap (5.50 to 3.55 eV), and strengthened excitation-dependent emission. PL analyses revealed that moderate nitrogen doping (0.3 M) provided the optimal balance between radiative recombination and defect passivation, yielding the highest emission intensity, while heavy doping (0.5 M) achieved the highest quantum yield (27%) with red-shifted emission. Thermal analysis confirmed multistep decomposition associated with dopant-derived functional groups and demonstrated increased structural transitions at higher doping levels. The sensing capability of BP-NCQDs toward Pb²⁺, Cr³⁺, and Fe³⁺ ions showed strong fluorescence quenching, with the highest Stern–Volmer constants observed for Cr³⁺ and Fe³⁺, indicating superior selectivity and sensitivity for these ions. Overall, the results highlight BP-NCQDs as low-cost, sustainable fluorescent nanoprobes suitable for environmental monitoring and rapid detection of toxic metal ions