Optical Interferometric Biosensor with PMMA as Functional Layer
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Optical Interferometric Biosensor with PMMA as Functional Layer Wenhui Wang1, Xiaodong Ma1, Lisa-Jo Clarizia2, Xingwei Wang1, and Melisenda J. McDonald2 1 Department of Electrical Engineering and 2Department of Chemistry, UMass Lowell, One University Ave. Lowell, MA 01854 ABSTRACT Silica-on-silicon label-free biosensor with PMMA (Poly(methyl methacrylate), as the functional layer was designed, fabricated, and tested. The sensor is based on Fabry Perot (FP) interferometry. Specific binding was tested with Human IgG and anti-Human IgG. Non-specific binding was tested with Human IgG and Mouse IgG. The testing results show that the sensor has a nearly six-fold greater response upon specific binding than upon non specific binding. Thermal and long term stability experiments show that the sensor is insensitive to the environment fluctuation. The fabrication process is simple without special surface treatment. In addition, this biosensor is inexpensive and easy to use. INTRODUCTION The Fabry-Perot (FP) interferometer is well known for its simple structure, high sensitivity to change of optical path difference (OPD), as well as capability of continuous monitoring and label free sensing. A few label free optical biosensors based on FP interferometer have been developed to detect biomaterials. Various configurations have been reported, such as using porous silicon[1, 2] optical fiber tip [3-5], single layer or multi-layer plane substrates [6-8]. Some configurations are integrated with micro-fluidic channels [9] or combined with other analysis methods [10]. In order to minimize noise from the environment fluctuation and achieve better signal stability, the material for FP cavity construction must be carefully chosen. Silica is an ideal material due to its low thermal expansion coefficient and good chemical stability [5, 11]. Silanization is usually necessary for the surface treatment in order to enhance the immobilization of the probe biomaterial on the sensor surface [7, 12]. PMMA (poly(methyl methacrylate) has been widely used in the biosensor design due to its potential for surface modification and its low cost [13] . Recently, Clarizia and coworkers have reported that that an immunoassay substrate consisting of a Protein G’- PMMA complex consistently yielded a higher percentage of correctly-oriented primary antibody than that of a polystyrene substrate control [14]. This could lead to a more highly sensitive immunosensor than is presently available [15]. In this paper, a low cost optical biosensor with PMMA as the functional layer was fabricated and tested. PRINCIPLE AND FABRICATION Basically, the thin films on the silicon substrate form the Fabry-Perot interferometer. The bare sensor is a three-layer structure as shown in Fig.1. The top layer is PMMA and serves as the functional layer for binding to biomaterials; the middle layer is Silica the main part of the FP cavity and silicon is the bottom to hold both top and middle layers. Because the optical refractive index difference between PMMA and Silica is muc
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