Reversing the structural chirality of cellulosic nanomaterials

Nature organizes cellulose, a linear polysaccharide of D-glucose and an important component of plants and trees, into intricate structures with twists in the trunks of trees, microfibrils within cell walls, and at the nanoscale. Manipulating the hierarchical organization of materials requires control down to the molecular level. In computational models cellulose nanocrystals twist, and Quantum Mechanical models have shown recently that chains at the surface of nanocrystals are right-handed, while the interior chains are mostly left-handed. Here we provide experimental evidence showing the induced circular dichroism of two optical dyes reverses when adsorbed onto thin cellulose nanocrystals. The reversal in optical activity is consistent with earlier TD-DFT B3LYP 6-31G calculations of the induced optical activity of Congo red adsorbed onto twisted 1 0 0 crystal surfaces of cellulose and demonstrates control of the chiral molecular interactions at the nanocrystal surface. The results suggest it may be possible to reverse the structural twist handedness of the nanocrystal itself and build chirality-dependent hierarchical supramolecular structures from cellulose.