Fluorescent Poly(vinyl alcohol) Composite Films Incorporating Doped Carbon Dots for Heavy Metal Detection in Smart Packaging Applications

Fluorescent poly-(vinyl alcohol) (PVA) composite films incorporating heteroatom-doped carbon dots (CDs) were developed for heavy metal sensing and packaging applications in the food industry. Uniformly dispersed nitrogen-doped (N-doped), fluorine and nitrogen codoped (F-N-doped), and sulfur and nitrogen codoped (S-N-doped) CDs retained strong photoluminescence in the PVA composite films, with their emission characteristics tuned through heteroatom doping. Mechanical characterization revealed that the N-doped film exhibited the highest elastic modulus across films, whereas the S-N-doped film demonstrated enhanced compliance and elongation. Wide-angle X-ray scattering confirmed homogeneous nanoparticle distribution and the promotion of tensile stress via chain realignment. Ion sensing studies highlighted dopant-dependent selectivity, with the S-N-doped film exhibiting the broadest sensitivity enhancement across films, achieved limits of detection of 0.4561, 0.4937, 0.6557, and 0.6817 ppm for cobalt-(II), iron-(II), copper-(II), and nickel-(II), respectively. The incorporation of S-N-doped CDs yielded a film with a more porous, uniform morphology and improved ductility, facilitating analyte diffusion and contributing to superior heavy metal ion sensing performance. X-ray absorption near-edge structure measurements confirmed dominant first-shell metal-O coordination upon adsorption. Overall, the S-N-doped film offered the best balance between mechanical robustness and fluorescence, demonstrating its potential as a candidate material for reliable heavy metal detection in smart packaging applications.