Molecular imaging of single cellulose chains aligned on a highly oriented pyrolytic graphite surface

Individual cellulose macromolecules were successfully visualized on a highly oriented pyrolytic graphite (HOPG) surface by tapping-mode atomic force microscopy under ambient condition. Monomolecular-level dispersion of cellulose chains was achieved through the momentary contact of dilute cellulose/cupri-ethylenediamine (Cu-ED) solution onto the HOPG substrate. Both concentrations of cellulose and Cu-ED provided critical impacts on the topographical images. Single cellulose chains with molecular height of ca. 0.55 nm could be observed under the optimal conditions, showing rigid molecular rods with a unique morphology of hexagonal regularity. It was strongly suggested that the cellulose chains were aligned along the HOPG crystal lattice through a specific attraction, possibly due to a CH-pi interaction between the axial plane of cellulose and the HOPG pi-conjugated system. These phenomena would imply the potential applications of an HOPG substrate for not only nano-level imaging, but also for molecular alignment of cellulose and other structural polysaccharides.