High performance of alginate/polyvinyl alcohol composite film based on natural original melanin nanoparticles used as food thermal insulating and UV-vis block

Light is a major factor in promoting food aging and deterioration, especially for ultraviolet (UV) light. Herein, bioinspired dopamine-melanin solid nanoparticles with strong absorption at a wide range of 200-2500 nm were first incorporated into alginate/polyvinyl alcohol to fabricate film materials in this work for UV-vis block, and this also brings excellent thermal insulating properties to the materials. In addition, in order to obtain a material with excellent performance, particles of uniform size of about 100 nm are obtained by fractional centrifugation. It was found the mechanical, UV-vis block and thermal insulating properties were improved significantly compared with the control samples. This study provides a strategy to design a non-polluting, biodegradable, biocompatible film with excellent mechanical properties that can be used in UV-vis barriers and has potential applications in thermal insulating materials for food preservation.

pH-Responsive Dipeptide-Based Dynamic Covalent Chemistry Systems Whose Products and Self-Assemblies Depend on the Structure of Isomeric Aromatic Dialdehydes

Facile control over preparation of organic building blocks and self-assembled aggregations to construct the desired materials remains challenges. This article reports selective dynamic covalent bonds formation and the corresponding self-assembly behaviors by using a dipeptide, glycylglycine (GlyGly), reacting with isomeric aromatic dialdehydes o-phthalaldehyde (OPA), p-phthalaldehyde (PPA), and m-phthalaldehyde (MPA) to demonstrate diversified aggregation forms caused by structure topology variations. Under alkaline condition, the aldehyde groups of phthalaldehydes can be connected with the amino groups of GlyGly by imine bonds as the dynamic chemical bonds. Owing to the fact that formation and dissociation of the imine bonds were reversibly pH-responsive, the reactions and aggregates assembled by their products were also reversibly controlled by changing pH. Three products, including two-armed product (OPGG, in which two GlyGly molecules were connected with one OPA molecule), single-armed product (PPG, in which only one GlyGly molecule was connected with a PPA molecule), and a mixture product (MPGG and MPG), as well as their different self-assembly behaviors, were obtained from OPA/GlyGly, PPA/GlyGly, and MPA/GlyGly systems, respectively, at the same condition of pH 8.6 in 90% methanol aqueous solution. However, for OPA/GlyGly system, another different type of product with benzopyrrole structure (OPG) was obtained by nucleophilic substitution via mixing OPA and GlyGly in water, which generated organic nanoparticles. Based on the results above, we conjectured the differences in dynamic covalent bond formation and supramolecular assembly clearly were influenced by the structure topologies of phthalaldehydes (OPA, PPA, and MPA). The experimental phenomenon verified the hypothesis as well, which may guide us to realize facile construction of selective reaction products and intelligent reversibly responsive materials with diverse morphologies and functions.