Easy-to-Use Visual Sensing System for Milk Freshness, Sensitized with Acidity-Responsive N-Doped Carbon Quantum Dots

Abstract

This study established a flexible and eye-readable sensing system for the easy-to-use, visual detection of milk freshness, using acidity-responsive N-doped carbon quantum dots (N-CQDs). N-CQDs, rich in amino groups and with characteristic acidity sensitivity, exhibited high relative quantum yields of 25.2% and an optimal emission wavelength of 567 nm. The N-CQDs fluorescence quenching upon the dissociated hydrogen ions (H+) in milk and their reacting with the amino groups produced an excellent linear relation (R2 = 0.996) between the fluorescence intensity and the milk acidity, which indicated that the fluorescence of the N-CQDs was highly correlated with milk freshness. Furthermore, a fluorescence sensor was designed by depositing the N-CQDs on filter-papers and starch-gel films, to provide eye-readable signals under UV light. A fluorescence colorimetric card was developed, based on the decrease in fluorescence brightness as freshness deteriorated. With the advantages of high sensitivity and eye readability, the proposed sensor could detect spoiled milk in advance and without any preprocessing steps, offering a promising method of assessing food safety.

Zein/MnO2 Nanosheet Composites Integrated with a Smartphone for Colorimetric Sensors for On-Site Detection of Adulterants in Milk

“…Additionally, when one evaluates the quality of milk in relation to freshness, an important indicator is the presence of lactic acid. 15 The production of acidifying metabolites in milk is primarily caused by the activity of mesophilic bacteria, which grow well at ambient temperatures. 16 Prompt and effective cooling of milk after milking can significantly reduce the growth of mesophilic bacteria.…”

“…Investigations have shown that exposure to H 2 O 2 in both short- and long-term can lead to cellular damage and is associated with several diseases, including diabetes, cancer, and cardiovascular and degenerative diseases. − Therefore, it is crucial to develop analytical methods that are both simple and affordable while also providing rapid detection of H 2 O 2 . Additionally, when one evaluates the quality of milk in relation to freshness, an important indicator is the presence of lactic acid . The production of acidifying metabolites in milk is primarily caused by the activity of mesophilic bacteria, which grow well at ambient temperatures .…”