Functionalized Waveguides for Biological Assays
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0950-D04-38
Functionalized Waveguides for Biological Assays Aaron S. Anderson, Andrew M. Dattelbaum, Gabriel A. Montaño, Jurgen G. Schmidt, Jennifer S. Martinez, W. Kevin Grace, Karen M. Grace, and Basil I. Swanson Los Alamos National Laboratory, MS K771, Los Alamos National Lab, Los Alamos, NM, 87545
ABSTRACT We report here a procedure for the functionalization of SiO2-coated, SiONx waveguides for biological assays. Surface functionalization occurs by self-assembly of an amine-terminated silane monolayer on the waveguide, followed by partial chemical modification with functionalized polyethylene glycol (PEG) groups. Functionalized surfaces were characterized by atomic force microscopy and contact angle measurements. When combined with a BSA blocking step, these functional PEG surfaces significantly reduced non-specific binding and allowed for specific binding to occur. An antibody sandwich assay for detection of Bacillus anthracis protective antigen was used to validate these surfaces for sensing applications. INTRODUCTION Surfaces that support the conjugation of a recognition ligand while minimizing non-specific adsorption of biomolecules are important for biofouling resistance and biosensor applications.1 In our laboratories, phospholipid membranes on waveguides are an effective platform to bind ligands specifically to surfaces while suppressing non-specific binding.2-4 The waveguide (from nGimat, Figure 1) consists of a high index of refraction, dielectric SiONx film sandwiched between a planar substrate and a silicon dioxide coating (SiO2), both with lower indices of refraction than the waveguiding material. Because of the difference in refractive index, the Figure 1. Idealized schematic of a single-mode waveguide excitation laser light propagates along the that is coated by a thin film containing biotin used for a high index material with approximately sandwich-type assay. 120 reflections per millimeter providing a strong interaction between the guided light and the surface. The intensity of the portion of light that extends beyond the sensor surface, called the evanescent field, decreases exponentially with distance from the surface. Therefore, the bulk solution volume is not irradiated while the field intensity at the surface is still strong. This intensity gradient is important for detecting small amounts of analyte bound to a surface while minimizing background fluorescence from the surrounding environment.
Phospholipid membrane coatings on waveguides used previously by us generally impart excellent resistance to non-specific binding and give no background fluorescence; however, these surfaces can become unstable under a variety of environmental conditions.2 Further, in assays done in complex solutions where significant non-specific binding is observed, strong rinsing agents, such as Tween-20, cannot be used because detergents disrupt the bilayer. These issues led us to examine other waveguide surface coatings that are more flexible with respect to the receptors presented at the surface and are more st
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