Rendering of protein assemblies under an atomic force microscope. Reprinted with permission from “Direct Probe of Molecular Polarization in De Novo Protein–Electrode Interfaces,” Kendra Kathan-Galipeau, Sanjini Nanayakkara, Paul A. O’Brian, Maxim Nikiforov, Bohdana M. Discher, Dawn A. Bonnell, ACS Nano, Copyright 2011 American Chemical Society |
Electrical
engineers have long been toying with the idea of designing biological
molecules that can be directly integrated into electronic circuits.
University of Pennsylvania researchers have developed a way to form
these structures so they can operate in open-air environments, and, more
important, have developed a new microscope technique that can measure
the electrical properties of these and similar devices.
The
research was conducted by Dawn Bonnell, Trustee Chair Professor and
director of the Nano/Bio Interface Center, graduate students Kendra
Kathan-Galipeau and Maxim Nikiforov and postdoctoral fellow Sanjini
Nanayakkara, all of the Department of Materials Science and Engineering
in Penn’s School of Engineering and Applied Science. They collaborated
with assistant professor Bohdana Discher of the Department of Biophysics
and Biochemistry at Penn’s Perelman School of Medicine and Paul A.
O’Brien, a graduate student in Penn’s Biotechnology Masters Program.
Their work was published in the journal ACS Nano.
The
development involves artificial proteins, bundles of peptide helices
with a photoactive molecule inside. These proteins are arranged on
electrodes, which are common feature of circuits that transmit
electrical charges between metallic and non-metallic 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,” Bonnell said. “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 had been tested to show that they indeed react to light. But
there was no way to quantify their ambient electrical properties,
particularly capacitance, the amount of electrical charge the assembly
holds.
“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.
The
researchers 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, known as 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.
Bohdana
Discher’s group 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.
This research was supported by the Nano/Bio Interface Center and the National Science Foundation.
