Colorimetric pH-Responsive Biomaterials Based on Pyranoflavylium-Biopolymer Hybrid Conjugates

Abstract

The conjugation of functional molecules into marine-origin biopolymers has been extensively used toward the creation of new (bio)­materials for applications in food and biomedical areas. Herein, two chitosan (CHT) and alginate (ALG) biobased conjugates functionalized with pH-dyes were successfully developed to be integrated as smart labels and tags in biocompatible packaging materials for food spoilage sensing. For that, two pyranoflavylium-based pH-indicator dyes, Pyflav (4) and Pyflav (5), were designed, synthesized, and then grafted onto CHT and ALG scaffolds via EDC/NHS carbodiimide coupling chemistry with substitution degrees of 14 and 1.2%, respectively. Afterward, the conjugates were fully characterized by Fourier transform infrared, 1H NMR, and dynamic light scattering techniques, and their acidity constants were determined by UV–vis spectroscopy, yielding a great color variation (from orange to purple, pK a 6.43 ± 0.04, for Pyflav-CHT (6) and from pink to purple, pK a 7.17 ± 0.02, for Pyflav-ALG (7) and compatible with the food spoilage pH range of many perishable foods (pH between 5.8 and 7.4).

Acidochromic Free-Standing Multilayered Chitosan-Pyranoflavylium/Alginate Membranes toward Food Smart Packaging Applications

“…The presence of an extra pyranic ring at the flavylium core, blocking the C-4 position, prevents the hydration reaction for the formation of colorless hemiketal species, thus avoiding tautomerization and isomerization reactions. , Therefore, this type of dye undergoes only acid–base reactions, shifting from the yellow-orange acidic flavylium cation species to pink-blue neutral and anionic quinoidal base species. − In a recent work, the design of one appropriate pyranoflavylium (Pyflav) dye and its noncovalent immobilization in an optimized cellulose acetate-based formulation was revealed to be crucial for achieving significant pH-dependent color variation without compromising stability . On the other hand, it was also demonstrated that Pyflav dyes could be chemically grafted onto marine-origin polysaccharides (alginate, chitosan) using carbodiimide coupling chemistry, offering a valuable strategy for developing pH-responsive bioconjugates. − In this work, the well-known and efficient “click chemistry” was considered instead to covalently attach a pyranoflavylium-based dye onto a chitosan derivative . Therefore, an azide-containing Pyflav dye (PyFlav-N 3 ) was first designed and synthesized and further functionalized to an alkyne-modified chitosan (CHT-Alk) via copper-catalyzed azide–alkyne cycloaddition reaction (CuAAC).…”

“… 22 On the other hand, it was also demonstrated that Pyflav dyes could be chemically grafted onto marine-origin polysaccharides (alginate, chitosan) using carbodiimide coupling chemistry, offering a valuable strategy for developing pH-responsive bioconjugates. 23 − 25 In this work, the well-known and efficient “click chemistry” was considered instead to covalently attach a pyranoflavylium-based dye onto a chitosan derivative. 26 Therefore, an azide-containing Pyflav dye (PyFlav-N 3 ) was first designed and synthesized and further functionalized to an alkyne-modified chitosan (CHT-Alk) via copper-catalyzed azide–alkyne cycloaddition reaction (CuAAC).…”