Miniature Sensor Measures Pollutant VOCs

A miniature sensor that uses polymer membranes deposited on a tiny silicon disk can measure impurities present in aqueous or gaseous environments.

Researchers at the Georgia Institute of Technology (Atlanta, USA) have developed a miniature sensor that detects volatile organic compounds (VOCs). The heart of the disk-shaped sensor is a microbalance that measures the mass of pollutant molecules present in air or water. The disk shears back and forth around its center with a characteristic resonance frequency between 300 and 1,000 kHz, depending on its geometry. Actuation and sensing elements integrated onto the microstructures electrically excite the resonator and sense these rotational oscillations. By modifying the silicon transducer surface with different polymer membranes, the sensors become selective for different groups of chemicals. An array of these sensors, each 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. Since each sensor has a diameter of approximately 200-300 microns--the average diameter of a human hair--an array of a dozen sensors is only a few millimeters in size.

The researchers tested the device in the laboratory by pumping water with specific pollutant concentrations through a simple flow cell device attached to the sensor; when the sample flows through the cell, the mass of the microstructure increases, causing the resonance frequency to decrease. By monitoring this resonance frequency over time, the researchers can detect the amount of aromatic hydrocarbons such as benzene present in water.

"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. "We've been able to measure concentrations among the lowest levels that have been achieved using this type of resonant microsensor. While we have not achieved the required sensitivity yet, we are constantly making improvements.”

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.


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Georgia Institute of Technology