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Structure and Dynamics of Molecularly engineered Biomimetic Nano-assemblies
We are collaborating with Jennifer S. Martinez, Center for Integrated Nanotechnologies, Los Alamos National Laboratory, and involved in her big project to study molecularly engineered biomimetic nanoassemblies. The overall goal of this project is to produce functional materials with characteristics found in, and inspired by, complex assemblies in Nature. Nature produces assemblies of functional components with both tunable and adaptable behavior. To reproduce these behaviors, we aim to develop nanoscale components whose interactions can be controlled and systematically tuned. These components will then be brought together through self-assembly and biologically-assisted assembly methods, leading to production of materials with functional responses related to energy transduction, particularly the control of photophysical responses. Classes of materials studied are chosen to span several important areas of nanoscience in order to lead to improved understanding of different types of complex nanoassemblies. Thus, nanoscale components to be investigated are conjugated polymers (and conjugated polyelectrolytes), carbon-based nanomaterials, noble metal clusters and particles, and biological membranes. Each of these components will be developed so as to have built-in ability to interact with one another in such a way that larger-scale assemblies with tunable behavior can be produced.
Interwoven through the entire project is a characterization effort that addresses both functional and structural characterization of the resultant materials. For structural characterization, we utilize small angle X-ray scattering (SAXS) and neutron scattering (SANS). Dynamical information is another key effect to improve our basic understanding of assembly processes. For dynamical study, X-ray photon correlation spectroscopy (XPCS) and neutron spin-echo (NSE) are employed. Detailed information for involved scattering techniques can be found in the "Scattering" section.
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The specific system we have worked on during past several years is DNA-Templated Silver Nanoclusters. Due to the high affinity of silver cations (Ag+) for DNA bases and following reduction of the Ag+, silver atoms may form short oligonucleotide-encapsulated Ag nanoclusters (NC) (<1 nm) without formation of large particles. Such DNA-templated silver nanoclusters have received significant attention as potential fluorescent labels due to their useful properties, including high molar absorptivities, good quantum yields and photostability, and small size. Their potential use may range from biology to nanoscience. For example, they are promising biological fluorescence probes due to their fluorescence properties dependence to DNA template sequence. One can tune the emission spectrum of the NCs through sequence changes in the DNA template. Despite the ongoing synthetic development and the attempt to use them in real systems, some basic features, like the structure of the DNA/Ag complex, are still unclear. With different experimental techniques, the measured sizes of the Ag NCs can vary from several atoms to tens atoms. We have conducted both SANS and SAXS experiments to investigate the formation of the Ag NCs. By comparing the SANS data from conjugated samples, pure DNA and DNA/Ag complex, we can characterize the size and position of the Ag clusters along the DNA strand. Fitting SAXS data with wormlike chain model gives very good agreements and we can then reconstruct the NCs. Although we have demonstrated that scattering technology is valid for getting the structure of these clusters, the formation and time evolution of DNA/Ag NCs strongly depends on the sequence of the DNA and it makes things very complicated. One have to study them case by case.
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