Multifunctional Starch-Based Composite Films Embedded with Carbon Dots for pH-Responsive Sensing and Monitoring of Food Freshness

This study presents the development of a starch-based smart film enhanced with carbon dots (CDs), which exhibits high pH sensitivity for the rapid assessment of liquid food quality. CDs were synthesized from ellagic acid and urea as precursors and incorporated into amylopectin (CA) and whole-component starch (CS) matrices, yielding a range of starch-based/CDs composite films. Structural analyses indicated that hydrogen bonding interactions between the CDs and starch molecules not only provided a nano-plasticizing effect-significantly improving film flexibility, as evidenced by a 29.6-fold increase in the elongation at break of the CS/CDs-2 film-but also slightly enhanced the crystallinity by acting as a nucleating agent. The incorporation of CDs markedly improved the electrochemical properties of the films, reducing the electrical impedance by several orders of magnitude and imparting intense blue fluorescence. The fluorescence intensity and current response of the composite film were found to be highly sensitive to pH variations. A strong linear positive correlation (<i>R</i>² > 0.97) was observed between the current output and pH over the range of 2 to 7. In a simulated orange juice spoilage test, the film effectively distinguished between fresh (pH = 3.38) and spoiled (pH = 2.68) samples based on current signal differences, demonstrating its practical utility. This work offers a novel approach for designing low-cost, biodegradable sensing materials for smart packaging, with promising potential in real-time food freshness monitoring.