Self-Assembling Halloysite Nanotubes into Concentric Ring Patterns in a Sphere-on-Flat Geometry.

Highly ordered and concentric ring patterns consisting of halloysite nanotubes (HNTs) with hierarchical cholesteric architectures are prepared by evaporation-induced self-assembly in a sphere-on-flat geometry. The structure and properties of HNTs are investigated. HNTs show a perfect tubular morphology on the nanoscale with high dispersion stability in water. Upon drying the HNTs aqueous suspension in a sphere-on-flat confined space, regular concentric HNTs rings are formed on the substrate via a self-assembly process. The widths of the inner and outer rings and the spacing between the adjacent rings increase with an increase in the concentration of the HNTs suspension. The highly ordered and concentric HNTs rings show a pronounced Maltese cross-like pattern under crossed polarizers, which suggests the formation of hierarchical cholesteric architectures. Scanning electron microscopy and atomic force microscopy observations show a disclination alignment of HNTs in the ring strips, especially with a high concentration of the HNTs suspension. The patterned rough surfaces of the HNTs show low cytotoxicity and can be used as a cell-supporting scaffold. The HNTs rings can guide the growth and orientation of C2C12 myoblast cells perpendicular to the rings. This work provides a simple, repeatable, mild, and high-efficiency method for obtaining HNTs with hierarchical architectures, which show potential for a large variety of applications, for example, in vascular grafts and skin regeneration.