Design and Analysis of Microcantilevers for Biosensing Applications
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Design and Analysis of Microcantilevers for Biosensing Applications
Xuan Zhang, Mo Yang and Cengiz S. Ozkan Mechanical Engineering Department, University of California, Riverside, CA 92521 ABSTRACT The primary deflection due to the chemical reaction between the analyte molecules and the receptor coating, which produces surface stresses on the receptor side is analyzed. The resonance frequency of microcantilevers is very sensitive to the properties of the microcantilever surface. Biosensing experiments based on resonance frequency shift are presented, which show that the results strongly depend on the interaction of specific analyte molecules with the receptor surface.
INTRODUCTION Recent advances in biochips have shown that sensors based on the bending of microfabricated cantilevers have potential advantages over previously used detection methods. Biochips with mechanical detection systems use microcantilever bi-material (e.g. Au-Si) beams as sensing elements. The Au side is usually coated with a certain receptor. Upon the binding of the analyte (e.g. biological molecules such as proteins or biological agents) with the receptor (each individual protein interacts with a unique receptor), the receptor surface is either tensioned or relieved. This causes the microcantilever to deflect and the deflection was found to be proportional to the analyte concentration. The resonance frequency of microcantilevers is very sensitive to the properties of the microcantilever surface. Changes in the surface properties of the microcantilever through binding or hybridization of analytes to receptor molecules will directly influence its resonance frequency by changing the overall cantilever mass and the thickness of the binding layer (see [Lu (2001)]).
EXPERIMENTAL PROCEDURES Microcantilevers are mounted to a cartridge with a piezoresistive film stack for operation in the non-contact or tapping mode. An optical detection system with a fourquadrant photodetector is used to detect the cantilever detection and the resoance frequency. The ultralevers have a gold coating on the backside for enhanced surface reflectivity, which is also useful for chemical modification to obtain selective binding of specific analytes. For our experiments, self assembled monolayers (SAM) of aminoethanethiol and dodecanethiol were utilized as receptor molecules to modify the cantilever surface. The sulfur group in the thiol chain has high affinity for binding to gold surfaces and hence well defined monolayers are generated, which are dense and stable [Fritz (2000), Ulman (1996)]. Before doing the chemical modification, we have measured
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the blank cantilever’s frequency response (first order resonance) using our AFM in the non-contact mode. Chemical modification was achieved by dipping the cantilevers into saturated solutions of aminoethanethiol and dodecanethiol for a duration of 12 hours, followed by a 24 hour drying process. After that, the modified cantilever's frequency response was recorded. The height of the aminoethanethiol and do
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