pH-sensitive biodegradable nanocomposite films incorporating chitosan and red cabbage anthocyanins.

The growing demand for sustainable and smart food packaging has highlighted the need for biodegradable materials that not only preserve food quality but also enable real-time monitoring of freshness. However, many existing biopolymer-based films lack sufficient mechanical strength, moisture resistance, and smart responsiveness. In this study, we developed pH-sensitive biodegradable films using a solvent casting method, using chitosan (CS) as a polymer matrix reinforced with cellulose nanofibers (CNF) with a high crystallinity index (91 %). Anthocyanins extracted from red cabbage (RCA) were incorporated as natural pH-sensitive colorimetric indicators at different concentrations (2 %, 4 %, 6 %, 10 %, and 15 % by weight), combined with 5 % CNF. Structural and morphological analyses confirmed uniform dispersion and high compatibility within the CS matrix. Among all formulations, the CS-CNF-4 % RCA film showed the most balanced and improved performance. Its tensile strength increased from 26.71 MPa (pure CS) to 32.01 MPa, along with a reduction in moisture absorption (from 23.02 % to 20.7 %), water absorption (from 109.4 % to 101.71 %), solubility (from 13.96 % to 8.37 %), and water vapor transmission rate (from 728.90 to 682.2 g/m·day). The film also showed improved antioxidant activity and antimicrobial zones of 24 mm (S. aureus), 27 mm (K. rhizophilia), and 10 mm (E. coli). Clear color changes depending on pH were observed, with ΔE values ranging from 31.93 to 38.63 and WI values ranging from 28.57 to 40.46 for a pH between 2 and 12. The film achieved complete biodegradation in 221 days. These results demonstrate the film's potential as a multifunctional, eco-friendly, and smart packaging solution for sustainable food preservation.