| RESEARCH |
Method imprints features of biological molecules on inorganic crystals MITCH JACOBY Structural features of biological molecules can be imprinted on inorganic crystals using a biomineralization procedure based on synthetic biomolecular templates [J. Am. Chem. Soc., 125, 11786 (2003)]. The technique, which was developed at the University of Illinois, Urbana-Champaign (UIUC), may lead to custom-designed crystals with useful electronic, magnetic, and optical properties. IN CONTROL Templates made of DNA (blue and purple strands) and lipid layers guide Cd2+ ions (red) into interhelical pores. Treatment with H2S (yellow) produces CdS nanorods that are endowed with unique crystallographic properties. At the heart of the new crystal-growth method are electrostatic forces that drive the material's synthesis steps. To prepare the templates, the UIUC researchers combine anionic DNA and cationic membranes that are composed of a mixture of lipids. The components self-assemble in solution, forming lamellar structures (stacks of sheets) in which DNA strands are confined between the lipid layers. The team, which includes assistant professor of materials science and physics Gerard C. L. Wong, Hongjun Liang, Paul V. Braun, and coworkers, notes that adding Cd2+ ions to the solution causes the cations to assemble in the interhelical pores between the charged DNA molecules. After the cations are in place, the group exposes the complex to hydrogen sulfide, thereby forming CdS nanorods, which are readily separated from the templates. The separation between DNA strands and the number of Cd2+ ions condensed in the template are easily controlled by adjusting the Cd2+ concentration, the group points out. In addition, the widths of the nanorods can be tailored by adjusting the lipid mixture. Wong and coworkers stress that unlike conventionally prepared II-VI semiconductor nanorods, in which the (002) crystallographic planes are perpendicular to the rod axis, the UIUC crystals are unique in that the (002) planes are tilted 60° away from the rod axis. That feature can be related to the orientation of the negatively charged sugar-phosphate DNA backbone, which is tilted roughly 60° relative to the DNA helix axis, they say. "We have demonstrated that it's possible to gain crystallographic control of a biomineralized phase by directly imprinting molecular details from a biological molecule onto an inorganic crystal," Wong says. He adds that the method enables the shape of a nanoparticle to be controlled independently from its crystallographic orientation. Michael D. Ward, a professor of chemical engineering and materials science at the University of Minnesota, Twin Cities, comments that the "really neat" feature of the work is the hierarchical organization of 1-D DNA in a 2-D lipid bilayer in a way that provides control over the size and shape of CdS crystals and the direction in which they grow. He adds, "It's a level of control that's quite exquisite." |
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