product review
The new era of SAW devices Commercial SAW sensors move beyond military and security applications. James P. Smith and Vicki Hinson-Smith
W
hen surface acoustic wave (SAW) instruments were last reviewed by Analytical Chemistry (2003, 75, 355 A– 358 A), expectations were high that the technology would continue to evolve as an analytical technique for environmental, security, and process monitoring. However, the scenario has unfolded a bit differently than expected because competing analytical technologies such as fluorescent labeling, ion mobility spectroscopy, and enzyme-based electrochemical methods have been difficult to displace. Military applications remain critical, but a SAW device is often one unit in a multisensor hybrid system designed for several analytical tasks. For example, a single security monitoring device must search for chemical, biological, and nuclear hazards simultaneously. SAW devices have been incorporated into a commercial electronic nose that conducts analyses by fast GC. The electronic nose simulates animal olfactory responses and can be programmed for specific monitoring tasks, such as testing fragrances or detecting explosives. Electronic noses react to multiple analytes simultaneously, producing characteristic patterns that can be converted into high-resolution visual images or other convenient formats. Selected commercial analytical products based on SAW sensors are summarized in Table 1 (p 3506). The majority are monitoring devices for military applications or for use by first responders. Table 1 is meant to be representative, not comprehensive; other products may be available. On the research side, the interesting new developments are in SAW devices for analyzing aqueous samples. One team is driving down sample sizes by fabricating lab-on-a-chip devices, while another is combining SAW sensors with surface plasmon resonance (SPR). Both developments are new examples of a long-standing interest in applying SAW sensors to liquid samples rather than the more common gas-phase ones. Despite these advances, SAW technology still has some hurdles to clear. Hank Wohltjen of Microsensor Systems, Inc. (MSI), who has developed SAW-based commercial and military sensors for >25 years, says that the high sensitivity of the SAW technology to many physical variables, such as temperature, mechanical stress, and ionic strength, presents significant © 2006 AMERICAN CHEMICAL SOCIETY
engineering challenges to the development of practical sensors for liquid-phase analytes. “SAW has great potential but is not heavily exploited as an analytical tool for liquid samples because of practical handling and procedural problems—a lot of technology still needs to be worked out,” he says.
Defense applications Many SAW devices have been developed with military and defense applications in mind. The company best known for delivering SAW-based analyzers has been MSI, which was incorporated in 1985. Several MSI systems are in use to detect low levels of nerve and blister agents, including sarin (often known by the abbreviation GB), soman (GD), tabun (GA), and mustard agents (HD), as well as a variety of widely available industrial compounds. MSI systems currently monitor critical infrastructure systems in major metropolitan areas around the world. SAW devices also are being explored for on-the-spot vapor-phase detection of plastic explosives that contain nitro groups; the best known examples are trinitrotoluene (TNT) and cyclotrimethylenetrinitramine (RDX). In April 2004, MSI signed an agreement with Mine Safety Appliances (MSA) to distribute the HAZMATCAD product line for the detection of chemical warfare agents. In September 2005, MSA acquired MSI. Crucial funding to support research for homeland security, J U N E 1 , 2 0 0 6 / A N A LY T I C A L C H E M I S T R Y
3505
product review
Table 1. Selected commercial SAW-based sensors.1 Product/Company
Analytes
ZNose Comprehensive, real-time analysis of any Electronic Sensor chemical, vapor, or toxin in
