Embedded Piezoresistive Microcantilever Sensors: Materials for Sensing Chemical and Biological Analytes
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Embedded Piezoresistive Microcantilever Sensors: Materials for Sensing Chemical and Biological Analytes Timothy L. Porter, William Delinger, Robert L. Gunter Dept. of Physics Northern Arizona University Flagstaff, AZ 86011 ABSTRACT We have used embedded piezoresistive microcantilever (EPM) sensors in the detection of volatile organic compounds, industrial chemicals, and biological molecules. The EPM design incorporates a tiny piezoresistive microcantilever embedded or partially embedded into a material sensing layer. The sensing layer material is chosen so as to react to the desired analyte(s), and undergo a minute volumetric or vertical shift in dimension. In the present paper, we present results on the sensing of various VOC’s, vaccinia virus in solution, carbon tetrachloride in vapor, and osmolality in human subjects. INTRODUCTION Embedded piezoresistive microcantilever (EPM) sensors combine many of the positive features of coated-cantilever sensor designs and standard chemiresistor sensors [1-3]. In the EPM design, a tiny piezoresistive microcantilever, of approximate length 200 microns, is embedded or partially embedded into a “sensing material”. This sensing material may be polymeric, functionalized polymeric, biological, or a composite material consisting of a polymer with biological molecules. Depending on the sensing material, analyte molecules may adsorb on the surface, physically partition into, covalently bond to, or otherwise incorporate themselves into the material layer. This action results in a tiny volumetric or vertical shift in the sensing material layer, which in turn results in a tiny strain in the piezoresistive microcantilever (Fig. 1). Strains of only a few Angstroms may be measured. The piezoresistive cantilever strain manifests as a change in resistance of the cantilever, which may be measured by a multimeter or a bridge circuit. These EPM sensors are tiny, only a few mm in dimension, and require only simple electronics to operate. They are also very robust, as the cantilever and sensing material form a single, rigid unit. Depending on the sensing material used, the devices may be designed to sense volatile organic compounds (VOC’s), environmentally sensitive chemicals such as TCE, MTBE and carbon-tetrachloride, or biological molecules such as proteins, DNA and viruses. EXPERIMENTAL Individual sensing units (Fig. 1) are assembled beginning with a liquid polymer or composite material deposited onto a suitable substrate such as a Si wafer. The piezoresistive microcantilever is then physically embedded into the liquid material either
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Fig. 1. Schematic diagram of embedded piezoresistive microcantilever sensor unit. The cantilever is partially embedded into the sensing material layer. from the side or from the top. After embedding, epoxy is used to immobilize the cantilever to the substrate. The assembly is then allowed to dry (both polymer and epoxy). The units formed are very small, only a few mm in dimension, and very rigid and immune to handling and vibration. The canti
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