Garlic carbon dot doped amphiphilic whey protein alginate nanoemulsion sensor for intelligent apple preservation

A novel, dual-functionality intelligent food packaging nanocomposite was successfully developed by incorporating nitrogen-doped carbon dots (CDs) into a whey protein-alginate (WPC-ALG) biopolymer emulsion. This study aimed to create a material with both real-time spoilage sensing and active antimicrobial properties. The CDs, prepared from red garlic peels (RGP), demonstrated inherent optical properties with dual fluorescence emission peaks at approximately 445 nm and 520 nm, showing a high sensitivity to pH changes, which was confirmed by shifts in their CIE coordinates. TEM analysis further confirmed the successful emulsification, showing a dramatic decrease in particle size from 119.70 to 142.59 nm (WPC-ALG) to a stabilized nanocolloidal range of 27.26–36.99 nm (WPC-ALG-CDs). The successful incorporation of the nitrogen doped CDs was validated by FTIR results, which showed an additional peak at 632 cm–1 related to C–N bonds, and an increased LOI from 0.90 to 1.25, indicating enhanced interaction. The DFT and the ESPM results showed that the WPC-ALG-CDs film had a significantly lower energy gap (3.7799 eV), a higher dipole moment (56.51 Debye), and a positive electrophilicity index (3.1068 eV), making it more reactive than the WPC-ALG control. The ESPM visually confirmed the presence of highly nucleophilic sites around the nitrogen atoms, which are key for targeted interactions. This foundational reactivity enabled the material’s two primary functions. The CFU assay revealed remarkable antimicrobial activity, with a 96.13% inhibition against Candida albicans, far surpassing the control’s 70.60% and demonstrating the film’s effectiveness in apple preservation. The sensing study demonstrated a visual indicator of spoilage: a red spot appeared on apples coated with the WPC-ALG-CDs composite, in stark contrast to the WPC-ALG control. This change was a result of the CDs’ responsivity altering in response to the acidic byproducts of microbial spoilage. SEM analysis revealed that the WPC-ALG-CDs film had a larger average pore size (3.31–8.13 μm), which surprisingly enhanced its performance by facilitating the rapid diffusion of microbial byproducts and ensuring greater contact with the active sites.