Evanescent Field Response to Patterned Features on a Planar Waveguide Biosensor
Matthew D. Stephens1, David S. Dandy1, Kevin L. Lear1, Guangwei Yuan1, Jonathan K. Gerding2, Alan Van Orden1, Charles S. Henry3, and Xinya He3. (1) Colorado State University, 100 Glover Building, Fort Collins, CO 80523-1370, (2) Dept. of Chemistry, Colorado State University, B101 Chemistry, Fort Collins, CO 80523, (3) Department of Chemistry, Colorado State University, Fort Collins, CO 80523
The need for a selective, multianalyte biosensor capable of detecting target molecules with high sensitivity has long been recognized. In this project, a novel means of detecting probe-analyte interactions is under development, whereby a shift in the evanescent wave surrounding the core of an optical waveguide is monitored. The bound analytes cause a refractive index change on the surface of the waveguide core, which in turn causes the evanescent wave to shift in intensity. To detect the evanescent wave and any subsequent changes resulting from analyte binding, it must extend a significant distance away from the core. Because of this, the optimal thickness of the waveguide core must be a fraction of a micrometer. The biosensor that is being studied is based on a waveguide fabricated from a high refractive index silicon nitride thin film. The film forms the core of the waveguide, which is surrounded by a lower cladding of silicon dioxide and an upper cladding of air. The detection of bound analyte is determined by observing the intensity of the evanescent field at the core/upper cladding interface. The binding of analyte is inferred from the two-dimensional light intensity plot generated by a Near Field Scanning Optical Microscope (NSOM) as it is rastered across the surface of the waveguide. The probe/analyte regions are physically simulated using several techniques. The techniques used in this study to form the adlayers are: (1) direct contact printing of proteins or polystyrene spheres, (2) capture of conjugated nanoparticles using avidin-biotin interaction, and (3) the use of flow focusing printing. The evanescent field response characteristics of the sensor to these features determined using NSOM will be presented.