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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.  Over the past year, we are able to make progress on both fronts.

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.  The nanografting method starts with a preformed host alkanethiol self-assembled monolayer(SAM) on gold immersed under a solution containing the thiol moleculed to be patterned (1).  When the atomic force microscope (AFM) tip applies a sufficient pressure on the SAM, the host molecules desorb and the guest molecules quickly backfill the area (1).  Therefore the AFM tip can be use to write chemical patterns on a preformed self-assembled monolayer.  We have custom built a fluidic system that allows us to generate multiplex patterns with a spatial resolution approaching 10nm.  We can now carry out multi-step surface reactions under electrochemical control.  Such nanoscale chemical patterns will be used to create dynamic potential energy gradients that will direct the motion of nanostructures. 

We have also studied the electrochemical properties of several electroactive molecules.  The electrochemical reactions suitable for patterned electroactive surface must be reversible and occur in a moderate potential range.  Using combined electrochemical and fluorescence measurements, we have found that catachol is covalently attached to a boronic acid group (2).  After catachol is oxidized into quinone, the boronic acid group is released.  Partial reversibility in the solution phase was observed because some of the quinone groups dimerized (2).  The reversibility is expected to increase when the catachol groups are immobilized and spatially separated on a surface.  We have also investigated the reversibility of electrochemical activity 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 underwent partial decomposition within 10 min.  We are currently developing other surfaces that can undergo reversible change of charge state. 

We are also investigating nanostructures with controlled orientations and interactions with the surface.  Of particular interests are disc shaped nanoparticles that can have more controllable interactions with the surface.  GaSe nanoparticles, developed by the Kelley group at Merced, have unique photophysical properties due to the unique disc shape.   Although the layered Se-Ga-Ga-Se structure was inferred from other measurements, there had been no direct real space evidence.  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.   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 (3). We have gained real space understanding on how the ligands decorating the edges of the NPs play significant role in the aggregation NPs.  These anisotropic nanoparticles not only have potentials in nanomotors.  With their controlled orientation and electronic coupling, they can facilitate efficient charge separation and transport important for solar energy conversion.  We have also synthesized gold nanoplates with specific chemical functionalities.  Compared to dendrimers, which require extensive efforts to tune the surface functional gropus, the functional groups on these nanoplates can be changed more easily utilizing the Au-thiol surface chemistry.  

In conclusion, over the past year, we have focused on the two key components required for the operation of nanomotors,  1. nanostructures with tailored functional groups to perform directional motion, and 2. nanoscale electroactive surface patterns.  In the next step, we plan to integrate the two components, and demonstrate that the motions of nanostructures can be triggered by controlling the electrode potential.

References:

(1) Xu, S.; Miller, S.; Laibinis, P. E.; Liu, G. Y. Fabrication of nanometer scale patterns within self-assembled monolayers by nanografting; Langmuir 1999, 15, 7244-7251.

(2) Lu Zhang, Justin A. Kerszulis, Ronald J. Clark, Tao Ye , Lei Zhu Catechol boronate formation and its electrochemical oxidation; Chemical Communications 2009, 5491-5495.

(3) Jingru shao, Hoda Mirafzal, Jared R. Petker, Janice Cosio, David F. Kelley, and Tao Ye Nanoscale organization of GaSe quantum dots on surface; (submitted to Journal of Physical Chemistry C).