Engineers Building “Erasable” Detectors, “Nanobrushes” and DNA “Highrises”

A Duke University engineering group is doing pioneering work at very diminutive dimensions. Their basic studies could lead to genetically engineered proteins that can form erasable chemical detectors; self-grown forests of molecular "bottlebrushes" that keep themselves contamination-free; and auto-assembled DNA "towers" that could become anchors for the tiniest of devices.

The proposed erasable detectors are made of artificial elastin-like polypeptides (ELPs), which are short segments of proteins normally soluble in water. Crafted through genetic engineering with the aid of bacteria, such ELPs have the useful property of coming out of a solution to form a solid whenever a slight temperature increase or other alterations to the water induces a phase change.

After dotting such a slide with microscopic amounts of surface-bound ELPs, the researchers discovered that dissolved fusion proteins would selectively attach to those microdots upon leaving the solution.

They also found the "captured" fusion proteins could pull other select proteins from solution so those could be chemically identified. Finally, they confirmed that microdot array could then be wiped clean of all attached proteins simply by "reversing the phase transition," said Ashutosh Chilkoti, professor of biomedical engineering at Duke's Pratt School of Engineering

In this case, the researchers added salt to the solution to induce the same kind of phase changes as does raising the water temperature.

"It's a way of creating what I would call a cleanable surface for sensing," Chilkoti said. "We can create a surface for a sensor, do a binding reaction, detect a signal, and then release everything. Then we could repeat the same process with the same fusion protein, or a different one."

But the dots used in that experiment were "microns" wide -- at the millionths of a meter scale. Chilkoti's team wondered if the process would also work at the thousand-times-smaller "nanometer" scale (billionths of a meter) to capture a few hundred individual molecules.

So they collaborated with Stefan Zauscher, a Duke assistant professor of mechanical engineering and materials science whose group has an Atomic Force Microscope that can deposit nanoscale amounts of material through a process called "dip pen nanolithography" (DPN).

Instead of using a glass slide, that collaboration fabricated a gold surface on which to bind ELP nanodots because "DPN really works well on gold," Chilkoti said. Repeating the reversible phase change experiments to draw proteins from solution for detection, "we found it worked even better at the nanoscale," he added.

A major reason for their improved success is that the gold surface was specially modified to prevent stray proteins from attaching to the experimental array, he said. "There was nothing binding in the background, so we could get extraordinary reversibility. We would have a clean surface, and we could do it over and over."

The goal of keeping away stray proteins also motivated Chilkoti's group to grow forests of special 15-nanometer-high polymer brushes with fuzzy branches that could act as raised platforms on which to locate ELP protein sensors or other molecular sized devices.

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