Evaporation-Induced Self-Assembly: Nanostructures Made Easy

As we look toward the next millennium, we envision new technologies based on nanoscale machines and devices. Key to the realization of this nanotech world are simple, efficient methods of organizing materials (molecules, molecular clusters, polymers, or, generally speaking, building blocks) into precise, predetermined nanostructures that can be preserved in a robust engineering form. Marine organisms like diatoms and radiolaria provide us with many examples of intricately organized architectures preserved in silica or calcium carbonate. Such natural microstructures are formed by biomineralization, a templated self-assembly process in which preorganized organic surfaces regulate the nucleation, growth, morphology and orientation of inorganic crystals. To date, a variety of synthetic pathways that mimic aspects of biomineralization have been explored to prepare patterned ceramic materials. In an early attempt to achieve antigen/ antibodyselectivity inaporousadsorbent,Dickey prepared silicagels inthepresenceofthetargetmoleculetobeadsorbed (in this case methyl orange). After methyl orange extraction, the resulting templated silicas showed preferential selectivity for methyl orange over its alkyl orange homologues. In the 1960s researchers at the Mobil Oil Corporation used alkylammonium ions as templates to control the pore size, shape and periodicity of zeolites, crystalline solids that define 1-, 2-, or 3-dimensional (1-, 2-, or 3-D, respectively) networks of microporous channels. More recently Kresge and colleagues at Mobil used longer-chain alkylammonium ions in an attempt to increase the maximum pore size of zeolites beyond ~1.2 nm. They observed honeycomb-like arrays of ~4 nm pores and, based on analogies with hexagonal liquidcrystalline systems, proposed a supramolecular liquid-crystalline templating mechanism. Although excellent progress has been made in the preparationofawidevarietyofpatternedceramicmaterials, current synthetic methods have several inherent drawbacks fromthestandpointofnanotechnology:First,mosttemplating procedures are conducted in time-consuming batch operations often employing hydrothermal processing conditions. Second, the resultant products are typically ill-defined powders, precluding their general use in thin film technologies. Third, procedures developed to date are often limited to forming patterns of pores. For many envisioned nanotechnologies, it would be desirable to create patterned nanocomposites consisting of periodic arrangements of two or more dissimilar materials. This article summarizes a simple evaporation-induced self-assembly (EISA) process, that enables the rapid production of patterned porous or nanocomposite materials in the form of films, fibers, or powders.