“One major focus of our research is to define ‘tracks’ for our shuttles, either by chemical changes of the surface, or by defining a specific topography, or by combining the two approaches,” Henry Hess told nanotechweb.org. “We always liked to work with the topography approach alone, since the creation of guiding channels is very straightforward and results in a surface with good long-term stability. However, shuttles frequently climbed the sidewalls of the guiding channels, permitting only very simple track arrangements such as arrays of parallel tracks.”
So the researchers began to work on combining surface chemistry and topography. “It was an almost serendipitous finding when we discovered that a plasma treatment of our surfaces, which completely erased the chemical patterning, did not change their capability to guide the shuttles,” said Hess. “Further investigation using SEM [scanning electron microscopy] revealed that we had created guiding channels with a unique geometry of the sidewall: a large undercut prevented the shuttles from climbing the sidewalls.”
Hess and colleagues created the channels by image-reversal photolithography on a glass substrate. The resulting structures had walls 1 micron high with an undercut that was 200 nm high and 1 micron deep. To carry out the process, the scientists added a layer of AZ5214 photoresist to glass wafers and exposed certain areas of the photoresist to ultraviolet light. Then they baked the wafers to cross-link the exposed photoresist before again treating the wafer with ultraviolet light. Finally, treatment with a developer solution removed the regions of photoresist that weren’t cross-linked, and an oxygen plasma cleaned the glass surface and oxidized the photoresist. The scientists believe that the undercut formed because the photoresist was not fully exposed and remained slightly soluble in the developer. Indeed, the size of the undercut depended on the development time.
The oxygen plasma treatment also made the photoresist surface hydrophilic, so that it had a similar affinity to motor protein adsorption as the glass surface at the bottom of the channel.
The researchers adsorbed kinesin motor proteins onto the channel surfaces, and used the proteins to transport microtubules - hollow filaments with an outer diameter of 30 nm assembled from the protein tubulin. The microtubules were functionalized with fluorescent dyes to enable their observation by microscopy, and with biotin linkers to enable selective loading of the “shuttles”. Epifluorescence microscopy of 43 microtubules revealed that all of them travelled successfully down the channels, without climbing up to the top surface.
“Our findings not only simplify the experimental system, but also permit us to do new things in the future, for example observing molecular scale transport in the small space defined by the undercut, which has dimensions similar to those of the axons of nerve cells,” explained Hess. “It will also become possible to guide shuttles on different planes above the surface, which gets us from an essentially two-dimensional transport system to a more three-dimensional one.”
According to Hess, the team’s nanoscale transporters could make an impact in the development of biosensors, the assembly of nanomaterials, and in molecular electronics.
The scientists reported their work in Nano Letters.