There is an increasing interest in using nanoparticles as building blocks for well-defined structures that have practical applications owing to the various novel properties of nanoparticles. However, their assembly is a challenging task. Methods based on surface functionalization, andor template patterning have been used for this purpose, but both of these processes can be rather complicated. Thus, there is a continuing need for a simple method for synthesizing high aspect ratio microstructures constituted of nanoparticle building blocks. This invention is directed to a novel freeze-drying method that assists in the self-assembly of nanoparticles into controlled microstructures including capped tubes, sheets, or spheres. Capped microtubes constituted of titanium dioxide nanoparticle building blocks were synthesized for the first time using this novel freeze-drying process. This process has great significance because of its simplicity, its ability to enable the self-organization of weakly interacting nanoparticles into various microstructures, and its likely general applicability to any and all nanoparticles of sufficiently small size and narrow size distribution.