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New Microsensor Measures Volatile Organic Compounds In Water And Air On-Site
~ researchers at the Georgia Institute of Technology developed a miniature sensor
~ it uses polymer membranes deposited on a tiny silicon disk to measure pollutants present in aqueous or gaseous environments
~ an array of these sensors with different surface coatings could be used during field-testing to rapidly detect many different chemicals
~ this new sensor allows water and air samples to be analyzed in the field
~ it is an improvement over classical techniques that require samples be carried back to the laboratory for analysis
~ the research, funded by the National Science Foundation, presented at the American Chemical Society's 234th National Meeting
~ the heart of the disk-shaped sensor is a microbalance that measures the mass of pollutant molecules
~ 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
~ 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
~ researchers searched for structures whose vibrations were less affected by the surrounding medium
~ they 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,
~ they 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,
~ they began collaborating with Boris Mizaikoff and director of its Applied Sensors Laboratory
~ Mizaikoff and students 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
~ 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
~ they 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
~ effective sensors must accurately measure and discriminate very small mass changes
~ have measured concentrations among the lowest levels that have been achieved using this type of resonant microsensor
~ while not achieved the required sensitivity yet, are constantly making improvements
~ 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, they can detect the amount of aromatic hydrocarbons such as benzene present in water
~ 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