Bioionic Liquid-Assisted Transparent Sodium Alginate-κ-Carrageenan-Lignin Composite Films with Excellent Ultraviolet Shielding, Antioxidant, and Antibacterial Properties

With growing awareness of eco-sustainability, health hazards, and material degradation linked to ultraviolet (UV) exposure, UV shielding films have garnered importance across industries like packaging, cosmetics, and photothermal materials. Consequently, the demand for utilizing renewable biobased resources to create sustainable, environmentally friendly, and economically viable materials has intensified. Responding to this, we have developed a novel composite film through the synergic combination of biopolymers: sodium alginate (SA), κ-carrageenan (kC), lignin (LG), and a bioionic liquid (IL). Choline indole-3-acetate, the used IL, has been strategically employed to confer antioxidant, antibacterial, UV shielding, and plasticizing properties to the SAkC composite films. The native SAkC film exhibits only 28% radical scavenging activity and lacks antibacterial and UV shielding capabilities. Intriguingly, IL doping significantly enhances antioxidant activity to 65%, completely inhibits bacterial growth, and increases the UV-blocking ability to 92%. The addition of a low loading of lignin (0.9 wt % with respect to polymer) proves sufficient for achieving complete UV blocking without compromising other properties. A 50 wt % IL loading with respect to the polymer matrix enhanced the mechanical strength of the film with elongation (69%) and tensile strength (32.5 MPa), obviating the need for additional cross-linkers and plasticizers. The antibacterial properties of the synthesized IL and developed SAkC film were ascertained using E. coli and S. aureus bacteria. The film shows excellent UV shielding in a wide temperature range (20–80 °C) and remains stable during continuous exposure to UV–A radiation of 390 nm for 24 h. UV–A blocking was also confirmed by testing the shielding of Rhodamine dye (RhB) degradation covered with the SAkC-IL-LG film where only 4.1% of RhB degradation was observed for 1 h of exposure.