Synthesis and structural design of microspheres comprising cellulose nanofibers and artificial lignin polymer by enzyme-mediated Pickering emulsion templating

Abstract

With the rapid advancement of plastics, microparticles and microbeads are becoming an integral part of various applications such as detergents, cleaning products, and cosmetics. This has resulted in the severe and irreversible contamination of ecosystems, especially the marine environment. Woody polysaccharides and lignin are biodegradable and can be decomposed by marine microorganisms. In the present study, we synthesized an artificial lignin polymer (ALP) that was densely enveloped in cellulose nanofibers (CNFs) by enzyme-mediated Pickering emulsion templating. We used isoeugenol, which is a structural analogue of lignin precursors, as the oil phase, and either native CNFs or CNFs surface-modified with carboxy, sulfate, or phosphate groups as Pickering emulsion stabilizers. Dehydrogenative polymerization was initiated by horseradish peroxidase and hydrogen peroxide. The resulting microspheres were 1–2 μm in diameter and had a sphericity of more than 98%. The yield was high (51–81%). Scanning electron microscopy and toluidine blue staining revealed that the shell layers of the microspheres comprised dense nanofiber networks of various CNFs, whereas the core components comprised a β-5-rich lignin-like structure, as determined by nuclear magnetic resonance analysis. We monitored the pH-dependent adsorption and desorption behaviors of the toluidine blue dyes, which changed according to the functional groups introduced on the native CNF. This facile strategy will enable the development of versatile and sustainable microparticles enveloped in CNFs, which are potentially useful as marine-biodegradable cosmetic microparticles.