Cellulose nanocrystals and self-assembled nanostructures from cotton, rice straw and grape skin: a source perspective

Cellulose nanocrystals (CNCs) have been derived by sulfuric acid hydrolysis (64–65 wt% H2SO4, 10 mL/g cellulose, 45 °C) of pure cellulose isolated from cotton, rice straw and grape skin, producing relatively consistent products in 60, 45 and 30 min, respectively, and generally reflecting the extent of crystallinity and crystallite sizes of these cellulose sources. CNCs in nanorod forms are observed from all three cellulose sources and, in the case of cotton and grape skin, in the presence of more dominant forms of nanoparticles. Cotton CNCs are <10-nm-wide nanorods at up to 40 aspect ratios, whereas rice straw CNCs are flat ribbon cross-sectional shaped in 10:2:1–44:5:1 length/width/thickness ratios, and those from grape skin are abundant nanoparticles but fewer nanorods, all of very different nanoscale dimensions. Freezing (−196 °C) and freeze-drying (−50 °C) of dilute CNC suspensions induce self-assembling of these CNC populations into yet further distinctly different morphologies. Self-assembled cotton CNCs are loosely organized nanorods and nanospheres, whereas grape skin CNCs are mainly nanospheres of 5-nm-sized nanoparticles clusters around nanorod cores. Uniquely, rice straw CNCs assembled anisotropically into ultra-thin non-porous fibers. These source-linked unique CNC geometries and the ability of CNCs to self-assemble into different morphologies present wide ranging dimensions of these renewable cellulose nanomaterial building blocks from by-products of the world largest fiber, cereal and fruit crops.