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Photograph of the microsensor
chip with four disk-type microresonators in the center. The size
of the chip is 3.5 millimeters by 3.5 millimeters.
Oliver Brand, an associate professor in Georgia
Tech's School of Electrical and Computer Engineering, and graduate
student Jae Hyeong Seo look at a circuit board used to operate the
microsensor chip. The monitor shows a photograph of the four
disk-type microstructures combined on a single silicon chip.
Boris Mizaikoff, an associate professor in Georgia
Tech's School of Chemistry and Biochemistry, and graduate student
Yuliya Luzinova inspect a microsensor chip coated with polymer
layers under a microscope.
All Fotos � by Gary Meek
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�When pollutant chemicals get adsorbed to the
surface of the sensor, a frequency change of the vibrating
microbalance provides a measure of the associated mass change,� said
Oliver Brand, associate professor in Georgia Tech�s School of
Electrical and Computer Engineering.
Cantilever-type balances, which move up and down like a diving board,
are common when measuring the amount of a chemical in the gas phase.
However, the mechanical vibrations of the balance used to detect the
mass changes are damped in liquids, causing the sensitivity of the
balance to decrease. Thus, Brand and graduate students Jae Hyeong Seo,
Stuart Truax and Kemal Safak Demirci searched for structures whose
vibrations were less affected by the surrounding medium.
The researchers chose a silicon disk platform for the sensor. The disk
shears back and forth around its center with a characteristic
resonance frequency between 300 and 1,000 kHz, depending on its
geometry. With proper actuation and sensing elements integrated onto
the microstructures, Brand can electrically excite the resonator and
sense these rotational oscillations.
Since each sensor has a diameter of approximately 200-300 microns, or
the average diameter of a human hair, an array of a dozen sensors is
only a few millimeters in size.
To determine how to selectively detect multiple pollutants in the same
sample, Brand began collaborating with Boris Mizaikoff, an associate
professor in Georgia Tech�s School of Chemistry and Biochemistry and
director of its Applied Sensors Laboratory.
Mizaikoff and graduate students Gary Dobbs and Yuliya Luzinova
selected commercially available hydrophobic polymers and deposited
them as thin film membranes on the sensor surface. They continue to
investigate innovative ways to consistently deposit the polymers at
the disk surface, while ensuring sufficient adhesion for long-term
field applications.
�By modifying the silicon transducer surface with different polymer
membranes, each sensor becomes selective for groups of chemicals,�
explained Mizaikoff.
An array of these sensors, each sensor with a different chemically
modified transducer surface, can sense different pollutants in a
variety of environments ranging from industrial to environmental and
biomedical monitoring applications.
Brand and Mizaikoff aim to detect volatile organic compounds (VOCs) in
aqueous and gaseous environments. VOCs are pollutants of high
prevalence in the air and surface and ground waters. They are emitted
from products such as paints, cleaning supplies, pesticides, building
materials and furnishings, office equipment and craft materials.
A common VOC is benzene, with a maximum contaminant level set by the
Environmental Protection Agency (EPA) at five micrograms per liter in
drinking water. Many VOCs are present at similar very low
concentrations, so effective sensors must accurately measure and
discriminate very small mass changes.
�We�ve been able to measure concentrations among the lowest levels
that have been achieved using this type of resonant microsensor,�
noted Brand. �While we have not achieved the required sensitivity yet,
we are constantly making improvements.�
Brand and Mizaikoff have tested their sensor device in the laboratory
by pumping water with specific pollutant concentrations through a
simple flow cell device attached to the sensor.
A typical test begins by flowing a water sample containing a known
amount of pollutant over a sensor coated with a polymer membrane. When
the sample flows through the cell, the mass of the microstructure
increases, causing its characteristic vibration frequency, or
resonance frequency, to decrease. By monitoring this resonance
frequency over time, Brand and Mizaikoff can detect the amount of
aromatic hydrocarbons such as benzene present in water.
The researchers plan to run field trials to investigate the use of
this new microsensor in aqueous and gaseous environments for rapid
on-site screening of multiple pollutants.
�With benzene and other VOCs high on the EPA priority pollutant list,
it would be a major advantage to get a rapid reading of VOC
concentrations directly in the field,� said Mizaikoff.
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