A pH-responsive fluorescent multifunctional pectin films based on imine and hydrogen bond dynamic locking for...

Polysaccharide-based films are generally functionalized with active substances through physical blending to impart biological activity. However, its preparation process, cost, compatibility and functional activity are yet to be improved. Herein, histidine (His) was reversibly grafted onto oxidized pectin (OLAP) with good film-forming properties through imine and hydrogen bond dynamic locking to prepare fluorescent films. Structural analysis confirmed that the carbonyl group of OLAP was connected to the α-amino group of His via CN bonds. Concurrently, the -NH- group on the imidazole ring formed hydrogen bond with the -COO- group on OLAP, generating fluorescence through intramolecular charge transfer. Besides, the CN cross-linking further stabilize the matrix, yielding compact structures that enhance thermal stability and oxygen barrier performance (58.14-73.88 g/m²/h). Compared with pristine OLAP film, the films formed by OLAP and His exhibited superior elasticity and degradability. Benefiting from the imidazole ring and CN bond structure, the film had exceptional UV shielding ability, antioxidant activity (ABTS and DPPH scavenging up to 85.05 % and 53.58 %) and antibacterial activity against E. coli. The films exhibited pH-responsive fluorescence to the acidic gases produced by fruit respiration, enabling fresh-cut kiwifruit freshness monitoring. Furthermore, the films can retard kiwifruit softening by 1.41-2.26-fold.