Cells regulate active transport of intracellular cargo using motor proteins. Recent nanobiotechnology
efforts aim to adapt motor proteins to power the movement and assembly of synthetic materials. A motor-protein-based nanoscale transport system (molecular shuttle) requires that the motion of the shuttles be
guided along tracks. This study investigates the principles by which microtubules, serving as shuttle units,
are guided along micrometer-scale kinesin-coated chemical and topographical tracks, where the efficiency
of guidance is determined by events at the track boundary. Thus, we measure the probability of guiding
as microtubules reach the track boundary of (1) a chemical edge between kinesin-coated and kinesin-free
surfaces, (2) a topography-only wall coated completely with kinesin, and (3) a kinesin-free wall next to a
kinesin-coated bottom surface (topography and chemistry combined). We present a guiding mechanism
for each surface type that takes into account the physical properties of microtubule filaments and the
surface properties (geometry, chemistry), and elucidate the contributions of surface topography and
chemistry. Our experimental and theoretical results show that track edges that combine both topography
and chemistry guide microtubules most frequently (approximately 90% of all events). By applying the
principles of microtubule guidance by microfabricated surfaces, one may design and build motor-protein-powered devices optimized for transport.
The ability to precisely place nanomaterials at predetermined locations is necessary for realizing applications using these new materials. Using an organic template, we demonstrate directed growth of zinc oxide (ZnO) nanorods on silver films from aqueous solution. Spatial organization of ZnO nanorods in prescribed arbitrary patterns was achieved, with unprecedented control in selectivity, crystal orientation, and nucleation density. Surprisingly, we found that caboxylate endgroups of omega-alkanethiol molecules strongly inhibit ZnO nucleation. The mechanism for this observed selectivity is discussed.
The integration of active transport into nanodevices greatly expands the scope of their applications. Molecular shuttles represent a nanoscale transport system driven by biomolecular motors that permits the transport of molecular cargo under user-control and along predefined paths. Specifically, we utilize functionalized microtubules as shuttles, which may be transported by kinesin motor proteins along photolithographically defined tracks on a surface. While it was thought that efficient guiding along these tracks requires a combination of surface chemistry and topography, we show here that channel-like tracks with a particular wall geometry can be created to efficiently guide microtubules in the absence of selectively adsorbed motor proteins. This new wall geometry consists of an undercut 200 nm high at the bottom of the channel wall fabricated by image reversal photolithography using AZ5214 photoresist. Microtubules move unencumbered in the undercut, suggesting applications for nanofluidic systems and for in vitro motility assays mimicking the restricted environment characteristic of intracellular transport. Because adsorbed kinesin supports motility on top and bottom surfaces of the guiding channels, this guiding mechanism may serve as a first step toward the development of three-dimensional architectures.
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