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|Scanning electron microscope photss of polystyrene spheres distributed on an array of nanofabricated silicon cantilevers, where they adhere by electrostatic forces. Making the cantilevers vibrate violently up and down won't shake such materials off, but shaking from side to side will.|
Tiny vibrating silicon resonators are of intense interest in nanotechnology circles for their potential ability to detect bacteria, viruses, DNA and other biological molecules.
Cornell researchers have demonstrated a new way to make these resonators vibrate "in the plane" -- that is, side to side -- and have shown that this can serve a vital function: shaking off extraneous stuff that isn't supposed to be detected.
The research is reported in the July 14 online version of the journal Nano Letters and in the August print edition.
The typical resonator is a cantilever -- a narrow strip of silicon a few millionths of a meter long that can be made to vibrate up and down like a diving board just after someone jumps off. In research aimed at building the much-sought "lab on a chip," Professor Harold Craighead's group at Cornell and other researchers have shown that by binding antibodies to such resonators they can cause pathogens to attach to them. At the nanoscale, just adding the mass of one bacterium, virus or large molecule is enough to change the resonant frequency of vibration of the cantilever by a measurable amount, thereby signaling the presence of the pathogen.
But "If, for example, you are trying to detect E. coli, there will be
more things in the fluid than E. coli, and they can weakly absorb on
the detector by electrostatic forces. This is a problem in any sort
of biodetection," explained B. Rob Ilic, a researcher in the Cornell
NanoScale Facility. The answer, he said, is to make the resonator
vibrate from side to side. This will shake off loosely adhered
materials, while whatever is tightly bound to an antibody will stay
Ilic and colleagues made cantilevers about a micron (millionth of a
meter) wide, 5 or 10 microns long and 200 nanometers (billionths of a
meter) thick, suspended over an empty space about a micron deep. When
energy was pumped in from a laser or by an attached vibrating
piezoelectric crystal, the cantilevers vibrated up and down at a
resonant frequency that depended on their dimensions and mass.
Then the researchers demonstrated that in-plane motion can be created
by hitting the base of the cantilever with a laser pulsed at the
resonant frequency of the cantilever's in-plane vibration, which is
different from the resonant frequency of its vibration perpendicular
to the plane. To measure in-plane motion the researchers shined
another laser on the free end of the cantilever and detected the
chopping of the beam as the cantilever moved from side to side.
To show that in-plane motion could shake unwanted materials off of
biosensors, the researchers distributed polystyrene spheres ranging
from half a micron to a micron in diameter onto an array of
cantilevers. The spheres, which attached themselves by electrostatic
attraction, were removed by in-plane shaking. But when the
cantilevers were made to vibrate more intensely up and down -- even
so far that they bumped the "floor" below -- the spheres did not
budge, nor did they during spinning of the entire chip.
In-plane vibration also could be used to determine how strongly
particles are bound to the surface by observing how hard they need to
be shaken to come loose, Ilic said. The ability to excite in-plane
motion also has applications in making nanoscale gyroscopes, in nano
optics and for basic physics experiments, he added.
Co-authors with Ilic and Craighead, who is the Charles W. Lake Jr.
Professor of Engineering and professor of applied and engineering
physics at Cornell, are Slava Krylov, professor in the Department of
Solid Mechanics, Materials and Systems at Tel Aviv University, and
Marianna Kondratovich, an undergraduate researcher in Cornell's
Department of Mechanical and Aerospace Engineering.
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