"Our method can potentially contribute to a new microfabrication process for a wide range of micro- and nano-devices which need nanostructures to increase their surface area or change their surface properties," explains Faheng Zang, lead author of the Nanotechnology paper. "Applications include energy storage devices, biochemical sensors, microscale heaters and coolers, superhydrophobic or superhydrophillic surfaces, etc."

Photolithography using photoresists is a well-established technique for micropatterning a substrate. Now, Faheng Zang and colleagues at the University of Maryland have found a way to make the photoresist perform a dual role, as Zang explains: "Instead of using the photoresist just as a photographic material, we wanted to explore its functions as a microfluidic channel to further fabricate nanostructure-textured microdevices." Combined with a biofabrication process using particles of TMV, the researchers used their technique to produce a supercapacitor with electrodes that have a significantly greater surface area than a planar equivalent.

To create the microfluidic channels, Zang and his team, led by Reza Ghodssi, used photolithography and a standard photoresist to shape an interlocking pair of comb-like chromium–gold (Cr–Au) electrodes on a substrate. Usually the photoresist would be stripped away once it has served this purpose, leaving the electrodes standing proud of the substrate. Instead of taking this step immediately, the researchers kept the photoresist in place to form the sides of open channels, the bottoms of which comprised the Cr–Au electrode layer. The channels were then subjected to plasma treatment to make their bottom and side surfaces hydrophilic, which causes fluids to be drawn through the channels by capillary action.

TMV nanorods

Next, the microfluidic channels were perfused with a suspension of TMV particles. TMV is a rod-shaped virus that can be genetically modified to express an amino acid, cysteine, on its surface. A compound in cysteine binds to gold, and the precise arrangement of the cysteine on the virus causes particles of TMV to self-assemble end-on to a gold surface. The researchers found that one hour of evaporation at room temperature was enough to result in a layer of TMV nanorods on the gold electrodes. By keeping fluid velocities low, and channels short and narrow, the group was able to ensure an even distribution of virus particles with a high porosity.

Once deposited, the researchers used an electroless plating process, followed by heating, to coat the virus particles with nickel oxide. When the photoresist was finally removed, what remained was a sharp-edged pair of nanostructured, interdigitated electrodes—NiO on a Cr–Au base—with a width and spacing of 4 µm, an overall area of 4 mm2, and an exceptionally large surface area.

Supercapacitors and more

Zang and colleagues tested the performance of their electrodes in a three-electrode experiment, and found a 3.6-fold improvement in areal capacitance over a planar equivalent, with excellent stability over 30 charge–discharge cycles. When the electrodes were tested symmetrically in the presence of an electrolyte, the device's performance characterized it as a supercapacitor, and showed no significant deterioration over 70 cycles.

As effective as the new method is for making supercapacitors, Zang stresses the more general advantages over existing microfabrication techniques: "Combining capillary microfluidics and TMV, we are able to fabricate well-patterned and nanostructured electronic devices in a simple and rapid fashion without the need for complex nanomaterial synthesis and surface chemistry." Furthermore, because the use of photoresists is already so common, the approach can be easily integrated into current processes.

Roger Howe, a professor at Stanford University who was not involved in the research, described the method as "a new and highly promising technique for making reproducible, high-surface-area templates for supercapacitor electrodes and other applications," and explained that, "since maskless 3D optical lithography tools are now available, the microfluidic channels can be printed onto curved or flexible substrates, which will allow the technique to be used for a wide range of device topologies."

Next, Ghodssi's team intends to "further explore the capability of temporary capillary microfluidics in the fabrication or functionalization of multiplexed biological or chemical sensors on-chip. The use of this technology can not only create precisely patterned and functionalized device surfaces, but also limit the use of sample volume, which will be critical in the development phase of most biosensors."

Full details of the research are reported in Nanotechnology.