Structure-controlled lignin complex for PLA composites with outstanding antibacterial, fluorescent and photothermal conversion properties

Abstract

Polylactic acid (PLA) is increasingly being considered as an ideal biodegradable candidate to replace petroleum-based polymers. However, its practical applications are often hampered by the poor mechanical robustness and solo functionality. Herein, based on the mechanical property improvement of PLA we proposed a simple process of assembling lignin-hybridized modifier and PLA matrix, as opposed to the traditional trade-off between mechanical strength and functionality, while anchoring a biofluorescent moiety onto lignin surfaces. Specifically, the fluorophore group could act as interfacial compatibilizer of complex and facilitate the shape-tailored hybrids for promoting functionality flexibility. With the bimetallic hetero-particles, the preferable lignin-assembled complex could be controllably configured as an antibacterial, fluorophore and photothermal agent. Thus, mechanical enhancement, fluorescence introduction and favorable photothermal ability of the resulting PLA composites were successfully achieved for integrated unification of structural robustness, geometric integrity and functional multiplicity, which was never seen in the other reports. The results showed that PLA composites containing 5 wt% modified lignin, 10 wt% zinc oxide, and 5 wt% silver presented excellent mechanical, fluorescent, photothermal conversion properties. By controlling the ZnO content and morphology, strong inhibition of Escherichia coli (Gram-negative) than that of Staphylococcus Aureus (Gram-positive) was also observed. The flake-shaped ZnO /Ag hybrids contributed to better overall performance of PLA composites than the rod-shaped ZnO/Ag. In this work we developed a facile strategy to assemble a bioderived fluorophore with lignin particles for constructing a structure-controlled complex as a multitasking modifier, featuring mechanical unity and functional adaptability. Specifically, the lignin reinforcement and bimetallic hybrids with different morphologies were explored as an effective fluorophore, antibacterial and photothermal agent. Through multiple dehydration reactions, a conjugating fluorophore was successfully grafted on lignin surfaces to serve as an interface modifier without physical changing its structural robustness. And morphology-tailored hybrid was advantageously immobilized on predefined hetero-particle carrier of fluorescent lignin and endowed composites with desirable antimicrobial properties. The developed strategy would expand the functional applications of PLA materials in food packaging, biopharmaceuticals and simple fluorescent anti-counterfeiting.

Keywords: Antibacterial; Fluorescence; Lignin; Photothermal property; Polylactic acid.