Mustard oil-derived carbon dots for ultra trace Pb²⁺ detection and cell bioimaging applications

Water-soluble blue fluorescent carbon soot-carbon dots (CS-CDs) were synthesized using mustard oil through a flame deposition method. This work investigates the effect of a confined combustion environment on particle formation. While particle size remained same even when the container volume was increased fourfold, a significant increase in defect density was observed. An intermediate volume of 1 L produces blue-shifted emission at 422 nm, compared to 430 and 428 nm for 0.5 L and 2 L, respectively, upon excitation at 350 nm. Microscopic and spectroscopic analyses revealed that as-synthesized CS-CDs possess a partially graphitized nanocrystalline core embedded within a highly disordered, defect-dense amorphous matrix. CS-CDs obtained from within the 2 L volume demonstrated sensitive and selective detection of Pb²⁺, achieving a detection limit as low as 1.57 nM in the linear range of 0-17 nM without surface functionalization despite their low quantum yield of 4%. Furthermore, the adsorption studies revealed multiple rate-limiting mass transfer processes, with an equilibrium adsorption capacity of 5.72 × 10^-4 mg g^-1 when added to 3.20 mg mL^-1 CDs. MTT assays confirmed the non-cytotoxic nature of CS-CDs in C2C12 cell lines, validating their biocompatibility. These findings demonstrate a simple route to tune and tailor the characteristics of CS-CDs for designing eco-friendly, selective and biocompatible sensors for environmental monitoring.