Proton-based Transistor Could Provide Viable Bio-interface
by Tim Palucka
Materials Research Society | Published: 25 May 2012
Researchers at the University of Washington in Seattle and collaborators at the University of Waterloo in Canada have developed a protonic field effect transistor (H+-FET) that controls the flow of protons instead of electrons, making it a good potential starting point for bio-interface devices. In biology, it’s generally the movement of ions (H+, Na+, K+, and Ca2+), instead of electrons, that controls processes such as ATP synthesis, neuronal signaling, and cell communication. Protons specifically play a key role in biological energy transduction mediated by ATP.
Lead author Marco Rolandi of the University of Washington coined the word “bionanoprotonics” in a recent paper in Nature Communications to describe this novel field, complementing research being done in bionanoelectronics. “We had to learn and, at times, make up all new terminology because all of a sudden there are no electrodes, there are ‘protodes’ for contacts, and there’s no electronic current, there’s protonic current,” he says.
The H+-FET consists of maleic chitosan nanofibers bridging the source and drain of the transistor, which are made of proton-conducting PdHx. The prototype is built on a traditional Si/SiO2 substrate, which would have to be replaced with a biocompatible and flexible material if these devices are ever used in biological systems. Maleic chitosan is a biodegradable, non-toxic polysaccharide chitin derivative that forms many hydrogen bonds when hydrated. When an electrostatic potential is applied between source and drain, the protons dissociated from the maleic acid groups “hop” along the hydrogen bond network as described by the Grotthus mechanism. This hopping results in a protonic current from source to drain, which can be modulated by a voltage applied to the gate. The measured mobility of this current is 4.9 x 10-3 cm2 V-1 s-1. “We think it is actually a molecular level process rather than protons [as hydronium ions] just diffusing between the water molecules, pushing them around,” Rolandi says. Because the mechanism is specific to protons, this device will not be suitable for controlling Na+, K+, or Ca2+. “We wish we could work with those ions, but we’re happy with protons for now,” he says.
Future work will attempt to make a truly nanoscale device; the prototype is a microscale device with nanoscale fibers. Rolandi would like to bridge the source and drain contacts with a single nanofiber of maleic chitosan to see whether that improves the on/off ratio of the H+-FET, which is currently low compared to traditional semiconductors. A further goal is to interface these transistors with cell cultures. Ultimately, in the distant future, the goal is to optimize the materials and performance of H+-FETs in physiological conditions so that in vivo sensing and stimulation of proton-selective ion channels could become possible.
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