Microsensors target VOCs

Depositing polymer membranes on tiny resonating silicon disks yields microbalances that will allow field measurements of a variety of volatile organic compounds (VOCs) or other substances at one time on a single chip, believe researchers at the Georgia Institute of Technology, Atlanta.

The disks resonate at a characteristic frequency of from 300 kHz to 1,000 kHz, depending upon their geometry. “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,” notes Oliver Brand, an associate professor in the School of Electrical and Computer Engineering.

Figure 1. This 3.5-mm × 3.5-mm chip includes four microresonators, each covered with an adsorbent polymeric membrane. Source: Georgia Institute of Technology.
Figure 1. This 3.5-mm x 3.5-mm chip includes four microresonators, each covered with an adsorbent polymeric membrane. Source: Georgia Institute of Technology.

“By modifying the silicon transducer surface with different polymer membranes, each sensor becomes selective for groups of chemicals,” explains Boris Mizaikoff, an associate professor in the School of Chemistry and Biochemistry and director of its Applied Sensors Laboratory. “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,” he notes. “We’re aiming for microsensors that can monitor five to 10 compounds, but more may be feasible.”

The current CMOS-based chip contains four resonators, each with a different membrane; use of more resonators is possible. The chip is made via conventional microelectronics fabrication technology and relatively simple surface chemistry, notes Brand, enabling economic manufacturing, particularly for devices for volume applications like VOC monitoring.

The researchers deposit commercially available hydrophobic polymers on the surface of the sensor, which generally has a diameter of only 200 microns to 300 microns. The ability to easily customize a device for a particular application is a major strength of the approach, says Brand.

If a membrane fouls, it’s easy to swap chips, adds Mizaikoff. He expects the membranes to last up to a couple of years, depending upon application and membrane.

The researchers are first targeting the monitoring of VOCs, both in gases and liquids, because it’s a big potential market. The devices, for instance, might be used in plants to monitor workplace safety. “The tricky part is liquids,” notes Mizaikoff. So, adds Brand, they want to particularly pursue such applications because fewer options are available for checking liquids in the field.

They already have tried the technology on gases containing xylenes, benzene, toluene and other hydrocarbons, and for a liquid with m-xylene. Measurements, which involve adsorption, desorption and analysis, take a couple of seconds for gases, and a couple of minutes for liquids, Mizaikoff says.

Compared to other techniques for gas measurement, the devices — given the right choice of membranes — should offer advantages in sensitivity and discrimination between similar molecules, he believes. For liquids, the measurement times certainly beat the usual approach of sending samples for offline laboratory analysis. “A couple of minutes is good except for control applications,” he notes.

Mizaikoff sees the devices serving in the field to monitor for VOC thresholds, triggering laboratory analyses only when necessary.

The reusable sensors haven’t yet been optimized but should be able to provide measurements to the single-digit ppm levels, he expects, with a hoped for reproducibility of a couple of percent.

Field trials for monitoring multiple pollutants should begin within two years, with commercialization of the devices in around five years, the researchers hope.

The technology isn’t limited to chemicals’ detection. Indeed, it has promise for medical diagnostics. Work already has started on functionalizing surfaces with enzymes to allow monitoring of antibody concentration, says Brand.

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