Self-Assembled Materials
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Self-Assembled Materials W.M. Tolles Any intelligent fool can make things bigger, more complex, and more violent. It takes a touch of genius— and a lot of courage—to move in the opposite direction. Albert Einstein (1879–1955) Doing engineering is practicing the art of the organized forcing of technological change. Gordon Spencer Brown Electronics; Engineer-Scientist (p. 53) Volume 32, Number 47 20 November 1959
Self-assembled materials in the 21st century may represent building blocks comparable to those of alloys, plastics, and semiconductors in the 20th century. These materials are formed from interatomic and intermolecular interactions other than the traditional covalent, ionic, and metallic bonding forces. Opportunities offered by self-assembled materials are becoming a significant factor in current research directions. Biological life forms demonstrate an intricate pattern of macroscopic structures and functions formed through a hierarchical series of forces. A consequence of this “intelligent self-assembly” is the functional utility of self-replication and self-repair. Until recently, the term “self-assembly” was applied almost exclusively to biological structures. Research publications today reveal a host of important self-assembled materials beyond those in biology. Figure 1 displays the number of times that the term “self-assembly” appears in titles of articles cataloged by Science Citation Index over the past 26 years. These data include only those articles that have the term in the title; many additional publications include self-assembly concepts. Clearly, a great deal of interest has been generated in the subject recently. Self-assembly involves forces such as hydrogen bonding, dipolar forces, other van der Waals forces, hydrophilic or hydrophobic interactions (all frequently referred to as “supramolecular interactions”), chemisorption, surface tension, and gravity. Forces involving ions and ligands (the “coordinate-covalent bond”) have resulted in supramolecular structures. These interactions lead to atomic aggregates that are typically larger than conventional molecular species, are regular in form and appearance, and have some properties unlike either their con36
stituents or bulk properties of similar materials. The assembly of clusters in a “superlattice” is included in this category. Biological structures formed by selfassembly include membranes, vesicles, tubules, DNA, and a wide variety of structures in a cell. The double helix of DNA, with major contributions by a 1962 Nobel Prize discovery by Watson, Crick, and Wilkins, results from the attraction between two separate molecular strands primarily through hydrogen-bonding interactions. Protein-folding of biological molecular structures results from these hydrogen-bonding forces and other dipolar interactions throughout the molecule, balanced by the potential-energy surfaces associated with changes in molecular conformation. Membrane materials form through the intermolecular attraction of like ends of amphiphilic molecules, forming hydrophilic
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