Neem seed derived green C-dots: A highly sensitive luminescent probe for aqueous Au³⁺ ions and nurtures green gold recovery

Highly luminescent green C-dots (NeCDs) were synthesized using neem seed kernel as the carbon precursor via one step microwave assisted pyrolysis. The brown NeCDs powder ensued after purification readily disperses in water (avg. particle size ~ 2 nm) and exhibits extensive surface functionalization of the carbon core by carboxyl and poly-hydroxyl functional groups. The aqueous solution of NeCDs is stable, has long shelf life and exhibit high photostability in different environments. In addition, NeCDs is found to be harmless and nontoxic towards A549 cells, and hence it was employed as a luminescent probe for multicolor cell imaging. Moreover, the photoluminescence (PL) of NeCDs solution (φf ~ 12.5%) is significantly/selectively quenched in the presence of Au3+ ions via two distinct deactivation processes viz donor excited photo-induced electron transfer and co-ordination induced aggregation. As a result, NeCDs exhibit the lowest ever reported limit of detection (LOD) of ~16 nM for aqueous Au3+ ions. Prolonged exposure (~ 24 h) to the poly-hydroxyl surface group of NeCDs chemically reduces the co-ordinated Au3+ ions at room temperature even in dark and produces urchin-like gold nanoparticles (UL-Au NPs), a simple and greener way to recover gold from aqueous medium.