Bifunctional metavanadate promoted chitosan/cassava biopolymer films with photo-switchable wetting properties: unveiling the surface restructuring mechanism

Abstract

Biopolymer films derived from starch and chitosan were soaked in vanadium salt solutions to produce vanadium metallopolymer films. Visible light irradiation induces significant color shifts from yellow to green due to changes in the oxidation state of vanadium. The material was observed to undergo dramatic structural changes upon incorporation of vanadium, with further restructuring occurring after visible light illumination. Metallopolymer films exhibited enhanced hydrophobic properties, which were further amplified when the material was irradiated with visible light, resulting in water contact angles up to 103°. X-ray photoelectron spectroscopy (XPS) measurements reveal that photoirradiation reduces vanadium metal from the 5+ (VO₃⁻) oxidation state to lower oxidation states. Initially, V⁵⁺ (VO₃⁻) interacts electrostatically with –NH₃⁺ moieties in chitosan. These interactions were diminished following photoreduction as the formation of reduced species such as V⁴⁺ (VO²⁺) decreases the interaction of vanadium (previously V⁵⁺) with –NH₃⁺. As the biopolymer chain breaks free from vanadium, interactions between neighboring polymer strands increase, leading to significant shifts in biopolymer surface structuring. Atomic force microscopy (AFM) measurements showed high root mean square (RMS) roughness values in starch-chitosan control films due to free interactions between biopolymer chains. Upon vanadium soaking, the chains were pulled inward by electrostatic attraction, which created a constraint that reduced the configurational states of the polymer and prevented the chains from interacting with neighboring polymer chains, significantly lowering RMS roughness. After photoirradiation, the electrostatic forces became repulsive, which released the polymer from this constraint and led to a slight increase in RMS roughness. The newly structured surface, dominated by high-frequency features, aligns well with the hydrophobicity model being developed in this work. To verify the reversible nature of the film's surface properties, irradiation and oxidative treatment cycles were conducted, and the contact angle of water was shown to drastically cycle from >100° following irradiation to ≈60° after oxidative treatments. This reversible property provides prospects and design parameters for the fabrication of future smart photo-switchable biopolymer films.