Fluorescent zinc doped carbon quantum dots as active fillers in carboxymethyl cellulose/whey protein films for selective microbial sensing and enhanced apple packaging

This study reports dual-functional active/intelligent composite films based on carboxymethyl cellulose (CMCe) and whey protein (WP) reinforced with fluorescent, zinc-doped carbon quantum dots (Zn-CQDs) derived from sustainable sugarcane bagasse. Microwave-assisted synthesis yielded ultra-small spherical CQDs (2.35-2.90 nm) with diffuse amorphous SAED rings, which shifted into template-coordinated crystalline Zn-CQDs (1.80-1.85 nm) with sharper diffraction profiles. Liquid-phase CFU assays demonstrated powerful concentration-dependent antimicrobial kinetics: lower loadings (5 wt%) optimized prokaryotic mitigation, reducing Escherichia coli by 88.90% and Staphylococcus aureus by 84.58%, whereas eukaryotic Candida albicans inhibition peaked at 70.21% under 15 wt% loading (CW-Q15) due to dense chitin-wall disruption. Under 15 wt% loading, film tensile strength shifted from 28.99 to 19.66 MPa and water permeability rose from 384.09 to 441.25 g/day m². Density Functional Theory (DFT) calculations validated optical sensitivity, showing a narrowed energy gap (E_g) from 3.0748 to 2.7216 eV. Crucially, CW-Q15 functioned as a selective biosensor, showing target-specific fluorescent fingerprints: green dots for E. coli, green threads for S. aureus, and red holed circles for C. albicans. Real-world fresh-cut apple storage trials confirmed robust active preservation, offering a sustainable, biodegradable smart-packaging solution.