July 19, 1999 Physicists from the Institute for Medicine and Engineering (IME) at the University of Pennsylvania have found a new class of materials that self-assemble into flat, two-dimensional "crystallites" made from tiny plastic beads the size of bacteria. Laurence Ramos, PhD, a postdoctoral fellow in the physics department, and her colleagues at IME and the University of Delaware used membranes similar to soap bubbles as templates to direct the assembly of clusters of the beads into an intriguing new microstructured material. Along the way, the team uncovered a surprise: Under the right conditions, their beads seemed to defy the basic physical principle that oppositely charged objects attract. Their findings appear in this week's issue of Science.
Electrostatic self-assembly of objects onto membranes is a relatively new technique with such potential biological applications as DNA and protein chips, gene-delivery vehicles, and industrial catalysts. The researchers had long-studied the components of their new structures separately, but nothing in their experience prepared them for the highly organized structures Ramos saw when the two were mixed. "The controlled manufacture of these microarrays could mimic and exploit the remarkable organization seen in many natural biomaterials," says coauthor Philip Nelson, PhD, a professor of physics at Penn.
The membrane portion of the material is essentially a thin, positively charged soaplike bubble. The surface of the bubble serves as a temporary template on which the raft of negatively charged spheres assembles. In many cases, the positively charged membrane attracted only a few dozen negatively charged spheres, then repelled all others. "At this point there was a lot of head-scratching, to put it mildly," says Nelson. "Every high-school student is taught that oppositely charged objects attract -- so how could the membrane switch from attracting to repelling the beads?" The key to the puzzle, say the researchers, is to remember that objects in water, such as the plastic beads, are surrounded by an invisible cloud of ions. Under the experiment's conditions, these ions spontaneously migrate in such a way as to overwhelm the membrane's own positive charge, and effectively reverse it in the region not covered by beads. The raft of attached beads then has a definite size, determined ultimately by the membrane's chemical composition. Far from being just an obscure footnote, the fact that the particle arrays can be self-limiting in this way seems to be crucial for the ultimate formation of the "crystallites."
More generally, understanding the basic strategies of self-assembly holds out the promise of far-reaching consequences in the design of future microstructured materials, for example in biosensors, drug carriers, and smart materials that respond to their environment.
Coauthors on the paper are Yi Chen, Tom C. Lubensky, and David A. Weitz from Penn, and Nily Dan and Helim Aranda-Espinoza from the University of Delaware. (Ramos now works at the Universite de Montpellier, France). The research was supported in part by grants from the National Science Foundation.
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