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42150-G10
Biofunctionalized Nanoparticle Superlattices

David S. Ginger, University of Washington

Narrative Report for 42150-G10 “Biofunctinoalized Nanoparticle Superlattices”

The goal of this project is to use biological motifs to organize inorganic nanoparticles and chromophores into supramolecular structures with well-defined geometries and spacings. In addition to developing general methods for assembling nanostructured materials, a major motivation of this work is to use the distance and orientational control achievable with DNA linkers to study the emergence of collective optical properties in nanoparticle clusters, particularly in assemblies of plasmon resonant metal nanoparticles coupled with molecular dyes. We have therefore focused our initial experiments on assembling metal nanoparticle/DNA/dye clusters immobilized on surfaces in order to study the effects of local electromagnetic field enhancements surrounding the nanoparticles on the optical properties of the dye (via single-particle darkfield scattering and fluorescence). The general approach we have taken is depicted schematically in the figure. First, silver nanoparticles are synthesized using different colloidal synthetic routes to produce particles with scattering maximum spanning across the visible spectrum. Second, the silver particles are affixed to a silanized substrate, and the substrate is subsequently blocked chemically to prevent non-specific absorption of DNA. Next, single-stranded thiolated DNA is attached to the silver particles, after which a complementary strand of DNA that has been functionalized with a fluorescent dye is hybridized to the first strand. We have verified that this method yields nanoparticle/DNA/dye clusters with dye that is specifically attached to the plasmon resonant silver particles through a double-stranded DNA linkage. We have demonstrated that very little non-specific adsorption of the dye to the particles occurs, and we are able to measure both darkfield and fluorescence spectra as well as take widefield images of both darkfield and fluorescence in order to correlate properties over large numbers of individual particles. We are currently using this system to study the distance dependent fluorescence of the dye from the metal nanoparticle surfaces. In addition, we have made a successful study of the dependence of the fluorescence intensity on the spectral overlap between the absorption/emission spectra of the dye and the resonant scattering spectra of the particles, and have studied the dependence of the plasmon linewidth on the metal nanoparticle size and resonance position. The grant has also supported our modeling efforts using finite-difference time-domain calculations. This grant has led directly to two publications (Chen, Munechika, Ginger, Nano Letters, 7, 690-696 (2007), and Munechika, Smith, Chen, Ginger, J. Phys. Chem. C., submitted) that contain the details of these experiments. This grant has served as the early nucleus supporting our research into bioinspired materials assembly and nanophotonics, and has enabled us to obtain funding from both DoD and NSF to support our research in this area.

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