Military training and testing can often lead to disturbances in and around stream and riparian areas as well as emissions of regulated air pollutants. To detect air- and water-quality parameters, there is a need for innovative, miniaturized, accurate, and robust sensors that have long lifetimes in the field and can withstand environmental extremes and possible disruption due to military activities. Ionic liquid-coated microcantilever transducers have shown promise in developing miniaturized, highly sensitive, and selective micro-electro-mechanical systems (MEMS). Microcantilevers respond to stimuli that affect the micro-mechanical characteristics of the transducer in such a way that the resulting change can be measured using optical, electronic, or other means. Microcantilevers have been used to detect molecular adsorbates with masses in the femtogram (10-15) and, more recently, attogram (10-18) range. Compared to more common gravimetric sensors, microcantilever transducers offer mass sensitivity that is better by many orders of magnitude. Microcantilever transducers are equally functional in contact with gases and liquids. In addition, transducers based on the MEMS platform are fabricated using cost-efficient integral technologies.
The objectives of this SERDP Exploratory Development (SEED) project were to (1) show that ionic liquids can be used to increase the sensitivity of microcantilever sensors toward selected air-quality parameters, such as concentrations of nitrogen and sulfur oxides, ozone, and fine particles; (2) show that specific ionic liquids can be used as microcantilever coatings to enhance selectivity of one analyte over the others; (3) demonstrate the potential use of microcantilever sensors coated with ionic liquids to monitor water-quality parameters; and (4) demonstrate operation of the sensing device at extreme environmental conditions.
Researchers reviewed the ionic-liquid literature to identify cations and anions that have chemical affinity with analytes relevant to environmental parameters. Microcantilever transducers of special geometries were then designed and fabricated. The sensing capabilities of the microcantilevers were evaluated for various analyte mixtures toward selected air-quality parameters. Microcantilever sensors coated with ionic liquids also were used to monitor water-quality parameters, such as metal ions and soluble organics. In addition, their use as sensor devices at extreme environmental conditions (e.g., high temperatures) was demonstrated.
The results of this project revealed that ionic-liquid coatings generate a desirable response for microcantilevers and may be suitable for use in detection of vapor and aqueous analytes. Coatings are also stable in various environments, including atmospheric conditions and high temperature vapors. The biggest practical appeal of microcantilever sensors is that they can provide sensitive and rapid detection of chemical and biochemical species directly because of changes in intrinsic stresses or mass-loading effects associated with bulk (absorption) or interfacial (adsorption-desportion) processes. Their performance makes them good candidates for MEMS-based sensor applications to detect and quantify both air- and water-quality parameters that are relevant to military activities. (SEED Project Completed - 2007)