Novel Differential Surface Stress Sensor for Detection of Chemical and Biological Species
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Novel Differential Surface Stress Sensor for Detection of Chemical and Biological Species Kyungho Kang, and Pranav Shrotriya Mechanical Engineering Department, Iowa State University, 2025 Black Engineering Building, Ames, IA, 50011 ABSTRACT A miniature sensor consisting of two adjacent micromachined cantilevers (a sensing/reference pair) is developed for detection of chemical and biological species. A novel interferometric technique is utilized to measure the differential bending of sensing cantilever with respect to reference. Presence of species is detected by measuring the differential surface stress associated with adsorption/absorption of chemical species on sensing cantilever. Surface stress associated with formation of alkanethiol self-assembled monolayers (SAMs) on the sensing cantilever is measured to characterize the sensor performance. INTRODUCTION Microcantilever based sensors are increasingly being investigated to detect the presence of chemical and biological species in both gas and liquid environments. Thundat et. al. [1] reported the static deflection of microcantilevers due to changes in relative humidity and thermal heating and thus opened a myriad of possibilities for the use of AFM cantilever deflection technique for chemical and biological sensing. Cantilever based sensors have been successfully demonstrated for DNA sequence recognition and as electrical noses to detect chemical mixtures [2]. In majority of the current state of art sensors, molecule absorption induced surface stress change is inferred from the deflection of a single or multiple laser beams reflected from the sensing surface. A large optical path is required between sensitized surface and position sensitive detectors to achieve high sensitivity in surface stress measurement. As a result, it is difficult to implement the sensing scheme into a single micro-fabricated device. In the current paper, we report a novel differential surface stress sensor that utilizes a single-mode fiber based MachZehnder interferometer for measuring cantilever deflection and consequently, the detection of chemical and biological species. The interferometric technique is amenable to miniaturization and may facilitate the integration of all components of sensors into a single microfabricated chip. Measurement of surface stress associated with formation of alkanethiol self assembled monolayer (SAM) on a gold surface is utilized to characterize the performance of differential surface stress sensor. Berger et. al. [3] reported the generation of compressive stresses on the order of 0.1-0.5 N/m during the formation of alkanethiol self-assembled monolayer on the cantilever’s surface and also reported that the magnitude of surface stress increased linearly with the carbon chain backbone of the monolayer. Since the first report by Berger et. al. [3], SAMs have been used as test system for almost all cantilever based sensing techniques [4-6]. This is because they are relatively easy to prepare, form well-ordered close packed films and offers limitles
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