Biomass-derived carbon dots and N-doped carbon dots from candlenut shells as fluorescent probes for candida albicans imaging

Carbon dots (CDs) and nitrogen-doped carbon dots (N-CDs) were synthesized from candlenut shell biomass via a hydrothermal method to evaluate the effect of nitrogen incorporation on physicochemical properties and fungal bioimaging performance. The materials were prepared at 230̊C for 6 h using ethylenediamine (EDA) as a nitrogen source and characterized by HR-TEM, UV-Vis, photoluminescence (PL), Fourier transform infrared spectroscopy (FTIR), XPS, Raman spectroscopy, and XRD. Both samples formed quasi-spherical nanoparticles, with the average diameter increasing from 1.464 nm CDs to 2.012 nm N-CDs after nitrogen doping. Optically, CDs exhibited a π–π* absorption band at 273 nm, while N-CDs showed a red-shift and an additional n–π* transition, indicating nitrogen-related surface states. Nitrogen doping resulted in a red-shift of the emission peak from 498 nm CDs to 515 nm N-CDs, accompanied by an increase in quantum yield (QY) from 25.36% to 28.60%. Structural analyses confirmed an amorphous graphitic framework with abundant surface functionalities. Confocal microscopy demonstrated dye-free fluorescence visualization of Candida albicans , with CDs providing higher imaging contrast. These results highlight the potential of candlenut shell-derived carbon dots as sustainable fluorescent probes for qualitative fungal bioimaging.