SAW devices coated with chemically selective films have been used as chemical vapor sensors for more than 20 years. Work in the 1980’s by Hank Wohltjen and colleagues at the U. S. Naval Research Labs established the high sensitivity of these devices to modifications of materials at the surface of the device, and recognized the potential to monitor film changes including transitions and vapor absorption.
Further work led to commercialization of SAW chemical sensors for chemical agent detection and volatile organic chemical monitoring. Examples of SAW-based chemical vapor systems that are commercially available include the HAZMATCAD® and HAZMATCAD® Plus, and the VaporLab®, products of Microsensor Systems Inc., a subsidiary of MSA.
Polymers, metal oxides, and nanomaterials have all been used to form chemically sensitive films on the surface of SAW devices. SAW chemical sensors have been demonstrated using both resonant and delay line devices. Chemical vapors in the environment of the sensor reach an equilibrium partitioning between the solid (film) phase and the vapor phase, based on the concentration of the chemical in the vapor phase. Diffusion time into and out of the film to establish equilibrium dictates the response time of the sensor, as the SAW device responds real-time (i.e. within microseconds) to changes in the film. Deposition of the selective film itself causes changes in SAW device performance, which may be temperature dependent. The coated device performance becomes a baseline for sensor response, and changes in response with exposure to various vapors is measured relative to this coated baseline response. Absorption of the target vapor can cause mass loading, changes in film viscoelastic properties, or changes in film electrical conductivity that can be measured.
The chemical selectivity of the sensor is dictated by the selectivity of the coating used. In general, chemical vapor sensors are not as selective as biosensors, which use very specific targeted biomolecular interactions (such as antibody/antigen binding) to detect desired analytes. Chemical sensors, by comparison, normally respond more or less strongly to classes of molecules with general chemical functionalities (such as alcohols, ketones, amines, etc.). Sets of sensors responding to different functionalities can be used with pattern recognition to generate a “fingerprint” response that is characteristic of individual vapors. This technique was used with polymer coated SAW sensors in the 1990’s by Dr. Hines and fellow researchers at Sawtek Inc. to demonstrate simultaneous identification and quantitation of low levels of vapors in mixtures, such as TCE (trichloroethylene) and PCE (perchloroethylene) with humidity in headspace for well monitoring for environmental cleanup. ASR&D has focused on the use of very thin films and nanostructured coatings, some with selectivity that is materials-based, to develop rapid, reversible chemical vapor sensors for specific analytes. Read more about ASR&D Chemical Sensor Work here.