A Self-Locking Technique With Fast Response and High Sensitivity for Micro-Cantilever Based Sensing of Analytes
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A SELF-LOCKING TECHNIQUE WITH FAST RESPONSE AND HIGH SENSITIVITY FOR MICRO-CANTILEVER BASED SENSING OF ANALYTES A. Mehta, G. Muralidharan, A. Passian, S. Cherian T.L. Ferrell and T. Thundat. Life Sciences Division, Oak Ridge National Lab, Oak Ridge, TN 37831
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ABSTRACT MEMS based microcantilevers have been employed as sensors in both liquid and ambient conditions. One scheme for detection is based upon monitoring the change in microcantilever resonant frequency as a function of the adsorbed analyte concentration. However, the sensitivity is limited by the accuracy of the frequency measurements, which is a function of the Q-factor of the vibrating element and the measurement bandwidth. In this paper, we present a feedback scheme for self-locking amplification of the small-amplitude thermal oscillations of the microcantilever. Using this approach, we demonstrate an improvement in the Q-factor by two to three orders of magnitude as compared to that of the undriven microcantilever. Use of this technique eliminates the need for lock-in detection and results in improved response times for sensor applications. Experiments using the proposed feedback amplification technique show improved sensitivity for the detection of biological molecules in liquids, and for adsorbed vapors under ambient conditions. INTRODUCTION The field of chemical and biological sensing is increasingly turning to microelectromechanical systems (MEMS) based devices to perform rapid measurements of specific chemical species with high selectivity and sensitivity. Microcantilevers used for imaging in atomic force microscopes show a response because of interatomic forces between the surface and the cantilever tip. This phenomenon has been exploited to use the cantilever as a sensor/transducer giving birth to a new class of chemical and biological sensors [1,2,3,4]. In such sensors, the microcantilever surface is treated with a specific coating or receptor molecules. On exposure to the sample, the target molecules bind to the surface. This step can be detected by either monitoring the deflection of the cantilever (DC detection) or by measuring the change in its resonant frequency (AC detection). Various techniques such as optical beam deflection, variation in piezoresistive, capacitive or piezoelectric response are used to quantify the amount analyte bound. Microcantilever sensors developed for gas phase environments have been demonstrated to have a picogram mass resolution [5]. For many applications, especially for biological samples, sensors need to function in a liquid environment. The development of microcantilever based sensors for detection in liquid faces two drawbacks: (i) a drift in the cantilever deflection due to thermal fluctuations in the flow chamber[6] and (ii) the reduction in the quality factor (Q) of the microcantilever when it is immersed in liquids [7,8] Cantilevers exhibit a measurable amplitude of vibrations due to thermal or ambient induced oscillations [9,10]. With an increasing trend towards miniaturization, manipulation and contro
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