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New Microscopy Technique Measures Properties of Biocircuits

Photonics.com
Jun 2011
PHILADELPHIA, June 10, 2011 — University of Pennsylvania researchers have developed a way to form biological molecules that can be directly integrated into electronic circuits, and they have developed a new microscopy technique that can measure the electrical properties of these and similar devices.

The development involves bundles of peptide helices with a photoactive molecule inside. These artificial proteins are arranged on electrodes that transmit electrical charges between metallic and nonmetallic elements. When light is shined on the proteins, they convert photons into electrons and pass them to the electrode.

"It's a similar mechanism to what happens when plants absorb light, except in that case the electron is used for some chemistry that creates energy for the plant," said Dawn Bonnell, director of the university’s Nano/Bio Interface Center. "In this case, we want to use the electron in electrical circuits."

Similar peptide assemblies had been studied in solution before by several groups and been proved to react to light. But there was no way to quantify their ambient electrical properties, particularly capacitance.

"It's necessary to understand these kinds of properties in the molecules in order to make devices out of them. We've been studying silicon for 40 years, so we know what happens to electrons there," Bonnell said. "We didn't know what happens to electrons on dry electrodes with these proteins; we didn't even know if they would remain photoactive when attached to an electrode."

Designing circuits and devices with silicon is inherently easier than with proteins. The electrical properties of a large chunk of a single element can be measured and then scaled down, but complex molecules like these proteins cannot be scaled up. Diagnostic systems that could measure their properties with nanometer sensitivity simply did not exist.

Bonnell and her colleagues therefore needed to invent both a new way of a measuring these properties and a controlled way of making the photovoltaic proteins that would resemble how they might eventually be incorporated into devices in open-air, everyday environments, rather than swimming in a chemical solution.

To solve the first problem, the team developed a new kind of atomic force microscope technique, dubbed torsional resonance nanoimpedance microscopy. Atomic force microscopes operate by bringing an extremely narrow silicon tip very close to a surface and measuring how the tip reacts, providing a spatial sensitivity of a few nanometers down to individual atoms.

"What we've done in our version is to use a metallic tip and put an oscillating electric field on it. By seeing how electrons react to the field, we're able to measure more complex interactions and more complex properties, such as capacitance," Bonnell said.

Researchers led by Bohdana Discher of the university’s Perelman School of Medicine designed the self-assembling proteins much as they had done before but took the additional step of stamping them onto sheets of graphite electrodes. This manufacturing principle and the ability to measure the resulting devices could have a variety of applications.

“Photovoltaics — solar cells — are perhaps the easiest to imagine, but where this work is going in the shorter term is biochemical sensors,” Bonnell said.

Instead of reacting to photons, proteins could be designed to produce a charge when in the presence of a certain toxins, either changing color or acting as a circuit element in a human-scale gadget.

For more information, visit: www.upenn.edu  


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