Made from cost-effective, environmentally friendly starting materials, the sensor can visually indicate the presence of spoilage microorganisms and chemical contaminants like hexavalent chromium (Cr(VI)).
The sensor is an answer to global food safety issues of spoilage and contamination. Current detection methods are largely dependent on slow, lab-based analysis, driving interest in smart packaging systems that offer real-time insights into food quality. Fluorescence-based sensors have emerged as promising tools thanks to their sensitivity and ability to signal changes visibly.
Carbon quantum dots (CQDs) are particularly appealing for such applications due to their luminescent properties, biocompatibility, and environmental safety. When doped with heteroatoms like sulfur and nitrogen, their performance is further improved, enhancing sensitivity and surface reactivity.
Embedding these doped CQDs into a matrix like amylopectin allows the film to respond to chemical and biological changes with observable color or fluorescence shifts.
The Study
To create the sensor, researchers used a hydrothermal process to synthesize S, N-CQDs from red onion peels.
First, red onion peels were mixed with sodium hydroxide, thiourea, and water to produce a consistent mixture. Thiourea was added to improve the solubility of the cellulosic material in onion peel, as cellulose is frequently insoluble in larger polymer chains. The resulting solution was treated via freezing, sonication, and microwave irradiation to produce sulfur, nitrogen-doped carbon dots.
The amylopectin was polymerized at 60 °C with N-isopropylacrylamide, N,N'-methylenebis(acrylamide), sodium methacrylate, ammonium persulfate, and tetramethylethylenediamine.
The quantum dots were then added to the amylopectin matrix and dried to form a biodegradable, flexible xerogel film.
The team tested the film under various conditions, including exposure to pH changes, Cr(VI) solutions, and spoiled tomatoes. Color and fluorescence changes were monitored under UV light and in ambient conditions, allowing researchers to evaluate its efficacy in detecting different spoilage indicators.
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The amylopectin-S, N-CQDs film responded clearly to environmental stimuli related to food degradation. When exposed to acidic or alkaline conditions, the film’s color shifted with changes to the quantum dots’ electronic properties.
Importantly for spoilage and contamination detection, the film turned from brown to yellow when in contact with metabolites from bacteria and fungi, clearly indicating microbial spoilage. This color change was linked to interactions between microbial by-products and the CQDs’ surface groups, which altered their light-emitting behavior.
The film also reacted to Cr(VI), turning a vivid pomegranate red after several days. This response was caused by a direct chemical interaction between the contaminant and the CQDs or polymer matrix. Visual changes were easily interpreted and confirmed through fluorescence analysis, making the sensor well-suited for quick, on-site food assessments.
Future Outlook
While the sensor offers strong visual cues and reliable detection, its current xerogel form is slightly brittle, which could limit its usability in real-world packaging. Future work will explore ways to enhance flexibility without compromising sensitivity.
Still, the combination of microbial and chemical detection, biodegradability, and affordability positions this film as an interesting step forward in smart packaging. Its real-time spoilage detection using materials derived from food waste supports broader sustainability goals.
Journal Reference
Tohamy H.A.S. (2025). Amylopectin xerogel with onion-based sulfur nitrogen-doped carbon quantum dots as a chemosensor for chromium and biosensor for microbial spoilage in tomatoes. Scientific Reports 15, 32667. DOI: 10.1038/s41598-025-19875-x, https://www.nature.com/articles/s41598-025-19875-x