Synthesis of pH-Responsive Lignin-Based Nanocapsules for Controlled Release of Hydrophobic Molecules

Lignin-based pH-responsive nanocapsules were successfully fabricated via an interfacial miniemulsion polymerization. Lignin was first grafted with allyl groups through etherification and further dispersed in an oil-in-water (O/W) miniemulsion system via ultrasonication. Then allyl-functionalized lignin was reacted with a thiol-based cross-linking agent in the interfaces of miniemulsion droplets to form nanocapsules via a thiol–ene radical reaction. The FTIR and 1H NMR spectra indicated the successful grafting of allyl groups on lignin. TEM images showed that lignin nanocapsules had particle sizes ranging from 50 to 300 nm. These newly synthesized nanocapsules could be readily loaded with hydrophobic coumarin-6 during the preparation of a miniemulsion system with 0.713 mmol/g entrapment efficiency. Moreover, the release of encapsulated coumarin-6 could be controlled by varying pH in the solution due to the existence of acid-labile β-thiopropionate cross-linkages in the capsule shell. An approximately linear release profile was observed at pH 7.4, whereas the release followed a Korsmeyer–Peppas profile at pH 4. The syntheses of lignin-based nanocapsules not only provide a facile approach to utilize the waste biomaterials from biorefinery industries, but also have great potential for applications in a controllable delivery of hydrophobic molecules such as drugs, essential oils, antioxidants, etc.