Development of flexible lignin nanotubes for the smart delivery of therapeutic agents

Development of high-value co-products has the potential to increase the economic sustainability of biorefineries that convert lignocellulosic biomass to fuels and bulk chemicals. We synthesized flexible lignin nanotubes in a sacrificial alumina membrane template using lignin from the waste stream of a biorefinery. A base layer of lignin was crosslinked to the inner walls of an activated alumina membrane and layers of dehydrogenation polymer were added to the base layer via a peroxidase-catalyzed reaction. The lignin nanotubes were released by dissolving the membrane in phosphoric acid. By using different phenolic monomers, we were able to synthesize nanotubes with a wall thickness of 15-45 nm, or nanowires with a diameter of 200 nm. Optical properties could also be varied. Electron microscopy and nano-indentation studies revealed that morphology and mechanical properties of the nanotubes depend on the biomass species (hardwood, softwood, grass) and lignin isolation procedure (NaOH, sulfuric acid, thioglycolic acid). Based on cytotoxicity experiments with human cell cultures (HeLa cells), lignin nanotubes have lower cytotoxicity than multiwalled carbon nanotubes. Furthermore, lignin nanotubes are able to deliver DNA without the need for auxiliary agents, as demonstrated by the expression of green fluorescent protein (GFP) following transfection of HeLa cells with plasmid DNA encoding GFP. The ability to select physico-chemical properties and the ease with which lignin nanotubes can be functionalized make them very attractive candidates for the delivery of DNA in gene therapy, as well as for the smart delivery of pharmacological agents. Funding provided by USDA-BRDI project 2011-10006-303508.