Design of responsive films using phenylurea modified chitosan and Viburnum tinus L. extract as natural indicator

Chitosan (Cs) films are attractive for sustainable packaging due to biodegradability and intrinsic antimicrobial activity, yet their barrier and mechanical performances are often insufficient for demanding applications. Here, we report a two-step strategy combining covalent phenylurea grafting on chitosan (Cs-PU) with the incorporation of a Viburnum tinus L. fruit ethanolic extract (VTE) as a natural, pH-responsive indicator. Phenylurea grafting was designed to introduce strong, directional intermolecular interactions within the polymer matrix and led to marked improvements in film performance: compared with pristine Cs, Cs-PU showed ∼25% higher tensile strength, a higher surface hydrophobicity (water contact angle: 81.8° → 101.2°), and a lower water vapor permeability (3.5 → 2.6 × 10⁻¹¹ g·m⁻¹·s⁻¹·Pa⁻¹). VTE, not previously reported as an additive for biopolymer film modification, endowed the films with pH-responsive optical functionality, producing a clear and reversible color shift over pH 0–14, while maintaining good structural and barrier properties. As a proof-of-concept application, Cs-PU@VTE films were used as headspace indicator labels for Atlantic cod (Gadus morhua): the labels exhibited rapid and sensitive colorimetric responses to spoilage-related volatile amines, with visible changes within 24 h at room temperature, enabling real-time freshness monitoring. Overall, this work demonstrates an integrated approach to obtain bio-based films that combine improved mechanical/barrier properties with smart colorimetric functionality for potential packaging applications.