RNA sequences have been discovered that mediate the growth of hexagonal palladium nanoparticles. In vitro selection techniques were used to evolve an initial library of approximately 10(14) unique RNA sequences through eight cycles of selection to yield several active sequence families. Of the five families, all representative members could form crystalline hexagonal palladium platelets. The palladium particle growth occurred in aqueous solution at ambient temperature, without any endogenous reducing agent, and at low concentrations of metal precursor (100 micromolar). Relative to metal precursor, the RNA concentration was significantly lower (1 micromolar), yet micrometer-size crystalline hexagonal palladium particles were formed rapidly (7.5 to 1 minutes).

RNA sequences previously isolated by in vitro selection were further characterized for their ability to control palladium particle growth. Five pyridyl-modified RNA sequences (Pdases) representing each of the different evolved families were found to form hexagonal plates with a high degree of shape specificity. However, a sixth nonrelated pyridyl-modified RNA sequence was found to form exclusively cubic particles under identical conditions. Replacing pyridyl-modified RNA with native RNA resulted in a complete loss of RNA function. Removing the 3'-fixed sequence region from the Pdase had little effect on particle growth; however, further truncations into the variable region resulted in a significant loss of activity and particle shape control. These Pdases were selected using the organometallic precursor complex tris(dibenzylideneacetone) dipalladium(0) ([Pd2(DBA)3]). Changing the metal center and ligand of the group VIII organometallic precursor complex revealed a strong dependence of particle growth and shape on the DBA ligands. Changing the metal center from Pd to Pt while retaining the DBA ligands gave predominantly hexagonal Pt, but with a decrease in shape control. Taken together, the results of this study suggest that the full-length Pdases contain active sites capable of highly specific molecular recognition of organometallic complexes as particle formation reagents.

RNA catalysts for the shape-controlled synthesis of Pd particles from the precursor complex trisdibenzylideneacetone dipalladium ([Pd2(DBA)3] were recently discovered in our laboratory (J. Am. Chem. Soc. 2005, 127, 17814-17818). In the work described here, RNA codes for hexagonal Pd platelets and Pd cubes were covalently immobilized on gold surfaces and evaluated for their activity toward particle synthesis. When coupled to gold via oligoethylene glycol linkers, both RNA sequences were able to catalyze the formation of Pd particles with the same shape control previously observed in solution. For low surface coverages, the average distance between RNA molecules on the surface was estimated at ca. 300 nm, yet large (e.g., dimensions of hundreds of nanometers) Pd hexagons and cubes still formed. This surprising result suggests that a single RNA molecule may be sufficient for nucleating and controlling the shapes of these particles. Finally, the use of surface-bound RNA as a tool for directing the orthogonal synthesis of materials on surfaces was demonstrated. Patterning the RNA code for Pd hexagons next to the code for Pd cubes, followed by incubation in a solution containing [Pd2(DBA)3], resulted in the spontaneous formation of spatially distinct spots of hexagonal and cubic particles.

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