Acoustic Chemical Sensors
What is a Acoustic Chemical Sensors?
Bulk acoustic wave sensors (BAW) and surface acoustic waves sensors (SAW), can be used as chemical sensors if the acoustic wave travels through a chemical sensing layer that affects the sound propagation, when the molecules of the analyte react with this sensing layer.
Acoustic wave devices with an applied sorbent layer as vapor sensor represent one of the first applications of acoustic wave chemical sensors. King used quartz crystal microbalance (QCM) in 1964. In 1979 acoustic sensors were extended to surface acoustic wave devices.
Acoustic Chemical Sensors Structure
The sorbent layer, often a polymer layer, acts to collect and concentrate vapor molecules from the gas phase onto the sensor surface. The adsorbed molecules are then detected by their effect on the velocity of the acoustic wave (i.e. added mass and viscoelastic effects on the surface layer). The partition coefficient quantifies the distribution of vapor molecules between the gas phase and the sorbent phase,
K = (cs / cv)
where cs is the analyte concentration in the surface sensing layer and cv represents the concentration in the vapor. The sensor responds to the analyte molecules in the sorbent phase and not in the gas phase; therefore the acoustic device detects cs. It is necessary to calibrate the sensor against the gas phase with known cv and thus the partition coefficient is the calibration parameter.
A number of analytes have been sensed with acoustic wave devices such as water, sulphur dioxide, ammonia, aliphatic and aromatic hydrocarbons, hydrogen sulphide, mercury, carbon monoxide, etc. An interesting example is the self-assembled monolayers coupled to SAW devices used effectively as chemical sensors for the detection of vapor phase of pesticides and chemical warefare such as organophosphonates, which proves the high sensitivity of acoustic detection.
Piezoelectric sensors have also been employed in contact with liquid samples in flow injection analysis, immunosensors and DNA hybridization. However, in this case the attenuation of the acoustic wave by the viscous liquid also enters into the generation of the acoustic chemical sensors response.