Amylopectin xerogel with onion based sulfur nitrogen doped carbon quantum dots as a chemosensor for chromium and biosensor for microbial spoilage in tomatoes

This study presents the development of a multifunctional, biodegradable xerogel film based on amylopectin and poly(N-isopropylacrylamide) (poly(NIPAm)) incorporating sulfur and nitrogen-doped carbon quantum dots (S, N-CQDs) derived from red onion peels (ROP). The formation mechanism and stability of the composite film were investigated using DFT calculations, revealing enhanced interactions and stability in the S, N-CQDs-containing film (amylopectin-S, N-CQDs15). FTIR and SEM analyses confirmed the successful incorporation of S, N-CQDs and revealed a tighter pore structure in the composite film, leading to increased surface area. The amylopectin-S, N-CQDs15 film exhibited significantly improved antibacterial activity, with inhibition rates of 95.25% against Escherichia coli, 99.12% against Staphylococcus aureus, and 99.23% against Candida albicans. These findings were supported by molecular docking studies indicating strong binding affinities. Furthermore, the film demonstrated its potential as a smart sensor through distinct fluorescence responses to these microorganisms: it showed mixed green and red fluorescence with E. coli, blue dots with S. aureus, and a change from large red regions to numerous green dots with C. albicans. The film also exhibited a fluorescence shift from red to blue upon exposure to Cr(VI). Notably, the film displayed pH-responsive color transitions relevant to monitoring tomato spoilage. These findings highlight the potential of this bio-based composite film, prepared from a waste resource, as a sustainable and effective solution for active food packaging, offering antimicrobial properties and detection of spoilage and contamination.