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Research progress:

The goal of this research project is to develop a nanomotor system capable of directional motion at the nanoscale.  The two key components of the nanomotor system are: 1. Nanostructures with tailored functional groups to perform directional motion.  2. A surface patterned with different electroactive functional groups under electrochemical control.  The electrode potential will be controlled to selectively reduce and oxidize functional groups in certain surface regions, resulting in dynamic affinities with the nanostructures.   

First is the development of a novel patterned reconfigurable surface, whose interactions with nanostructures, such as dendrimers, biomolecules and nanoparticles can be switched on demand.  Using nanografting, we are able to successfully introduce a broad range of chemical functional groups onto specific locations of a surface, with precision down to single molecules.  The ability to manipulate single biomolecules promises to enable analogous insights at biologically relevant interfaces.  Most of the existing biomolecular patterning techniques pattern bundles of molecules.  The question of how to pattern biomolecules with single molecule precision remained largely unaddressed until a novel work by Kufer et al..¹  However, the approach has a few limitations, including the inability to use high resolution imaging to verify the single molecule patterns and uncontrolled nanoscale environments.  We have developed a novel technique that successful positioned individual DNA within a controlled nanoscale environment and observed these molecules in situ.²   Such molecular patterns will allow us to understand and potentially decouple the heterogeneity caused by the local environment from the intrinsic properties in single-molecule biophysical measurements. Additionally, our approach can potentially be extended to the molecule-by-molecule assembly of larger artificial test structures of nucleic acids or proteins.  In addition, we have also developed a simple technique that generates chemical gradients ranging from 50 nm to 200 nm.  This is achieved by varying the density of the lines drawn by AFM.

We have also studied the electrochemical properties of several electroactive molecules.³  The electrochemical reactions suitable for patterned electroactive surface must be reversible and occur in a moderate potential range.  We have investigated the reversibility of a ferrocene SAM on gold.  The oxidation into ferrocenium renders the surface positively charged, providing a facile method to change the surface charge density that could guide nanoscale motion.  However, we found that the ferrocenium groups undergo partial decomposition within 10 min. In collaboration with the Stoddart group at Northwestern, we have identified molecules that or change its charge state under working conditions.  We found that an alkanethiol molecule with the tetrathiafulvalene (TTF) group, can undergo many cycles of charge state changes. With both nanoscal chemical patterning and resersible elctroactive surfaces in place, we are in a position to realize directional nanoscale motion.   

GaSe nanoparticles, developed by the David Kelley group, have unique photophysical properties due to the unique disclike shape.   We have developed an approach to deposit onto gold surface GaSe nanoparticles with well-defined orientations and coverages that are optimal for AFM investigation, which has yielded valuable structural information of single nanoparticles, complementing the Kelley group’s ensemble spectroscopic investigation.⁴  We have found that once deposited on gold surface in an isolated form, GaSe NPs are adsorbed flat on the surface due to strong interaction between Se and gold.  We have gained real space understanding on how the ligands decorating the edges of the NPs play significant role in the aggregation NPs.  Our investigation points to a new strategy that may allow us to assemble nanoparticles aggregates normal to the surface, facilitating efficient light capture, charge separation and transport for solar energy conversion. 

Training of students:

The PRF grant has provided partial support to three graduate students, Janice Cosio, Eric Josephs and Jingru Shao.  In less than two years here, both PhD students, Shao and Josephs,  have published first-author papers in ACS journals.  Eric Josephs won a Best Student Presentation award at the 2010 UC Systemwide Bioengineering Symposium. He has also won the UC Merced Faculty Mentor Program Fellowship.

The two year support of this PRF grant has been highly valuable in the starting phase of my career.  It supplements my start up grant, which has a number of restrictions on personnel support.  It significantly advanced the original research goal of nanomotors.  In addition, the techniques we developed for the project also enabled two new research directions, the manipulation of single DNA molecules in well defined chemical environments and surface self-assembly of anisotropic quantum dots.

References:

1. Kufer, S. K., Puchner, E. M., Gumpp, H., Liedl, T., Gaub, H. E. Single-molecule cut-and-paste surface assembly. Science, 319, 594 (2008).
2. Josephs, E. A., Ye, T.  Nanoscale positioning of individual DNA molecules by an atomic force microscope. Journal of the American Chemical Society, 132, 10236–10238 (2010).
3. Zhang, L., Kerszulis, J. A., Clark, R. J., Ye, T., Zhu, L.  Catechol boronate formation and its electrochemical oxidation. Chemical Communications, 2009, 2151–2153 (2009).
4. Shao, J., Mirafzal, H., Petker, J. R., Cosio, J. L. S., Kelley, D. F., Ye, T.  Nanoscale organization of GaSe quantum dots on a gold surface. Journal of Physical Chemistry C, 113(44), 19102–19106 (2009).