SGER: Capillary Affinity Sensors Based on Surface Plasmon Resonance

University Of Washington, Seattle WA

Investigators

Abstract

The high sensitivity and label-free nature of surface plasmon resonance (SPR) sensors make SPR an excellent technology for affinity sensing. SPR sensors are typically built using a prism with one gold coated surface onto which a flow cell is attached. This prism-based SPR sensor design is fairly complex and has several drawbacks, including awkward fluid handling, optical inflexibility, and the need for index matching. To overcome these limitations, a simple, one-piece sensor design based on capillary tubes will be developed. A TM-polarized, collimated beam from a diode laser passes through a converging lens with its focus near the capillary surface. The beam hits the gold-coated internal surface of the capillary at angles greater than the critical angle, and is reflected by this diverging mirror surface. The reflected light exits the capillary and strikes a diode array detector. Since each location on the diode array will correspond to a different angle of incidence, a reflectivity vs. angle measurement results. The capillary design has simple construction, no index matching, an ideal flow geometry, a protected sensor surface, and optical versatility. To further develop this promising sensor concept, research in capillary optics and fabrication techniques will be conducted. In the capillary SPR sensor, information is encoded in the reflectivity vs. angle spectrum of the metal coated capillary interior surface. SPR reflection spectra display a dramatic dip centered at a certain angle, and the sharpness of this dip determines in large part the resolution of the measurement. For this reason, research will focus on elements of sensor design which contribute to this sharpness, including choice of beam divergence, diffraction, plasmon effects, and choice of capillary dimensions and refractive index. With improved understanding of the capillary response, the use of simplified light sources and interference effects may be used to enhance the capillary sensor. To achieve high quality data from a capillary SPR sensor, high quality gold layers must be deposited inside capillaries. The thermal wire deposition method used to date is inadequate: capillary length is limited, capillary ID is constrained, and deposition is nonuniform. As an alternative, a method of wet chemical gold deposition will be investigated. This procedure will be used to internally coat capillaries with gold, using an experimental technique in which the necessary reagents are pumped back and forth through capillaries using air pressure. Since the capillaries have a small internal volume, only a small amount of reagent will be necessary, reducing costs and simplifying waste disposal.