Self-Referencing Surface-Plasmon Sensors

Self-referencing Surface-Plasmon Resonance Sensors J. Todd Hastings, University of Kentucky Background and Research Goals Over the past decade surface-plasmon resonance (SPR) has become a widely accepted and commercialized optical technique for studying bio-chemical interactions in the laboratory. As a result, there has been great interest in developing field deployable SPR sensors, and great progress has been made in miniaturizing these sensors using optical fibers and planar waveguides. Nevertheless, mobile sensors that can function in complex analytes remain in the research phase because of the difficulty in distinguishing the interaction of interest from interferents and environmental variations. The proposed research seeks to solve this problem by developing a novel selfreferencing SPR sensor based on simultaneous coupling to long- and short-range surface plasmon waves. The proposed sensor offers self-referenced measurements for each recording channel to minimize residual errors and provide high channel density. In addition, our approach is easily adaptable to fiber and waveguide SPR sensor designs. Surface-plasmons are bound electromagnetic waves that propagate along the interface of two materials with real dielectric constants of opposite signs, i.e. a dielectric and certain metals such as gold or silver. SPR sensors use these waves to detect changes in refractive-index of the medium immediately adjacent to the metal layer. The field of a surface-plasmon wave at the metal surface is significantly stronger than a typical evanescent field; as a result, SPR sensors are particularly sensitive to surface binding or to changes in the refractive index of surface sensing layers. Surface-plasmon waves are not phase-matched to free space waves; therefore, one must couple to them using evanescent fields. The most common SPR sensor configuration, similar to that shown in figure I(a), uses a prism to couple light from an appropriate source to the plasmon mode. One observes a minimum in the reflected intensity at angles and wavelengths where light couples strongly to the surface-plasmon wave. SPR sensors measure refractive index changes by detecting shifts in the angle or wavelength of this reflection minimum. Despite the success of surface-plasmon resonance as a laboratory tool, implementation of SPR sensors in the field has been far more difficult. Several groups have successfully developed mobile sensors based on miniaturized optics, optical fibers, and planar waveguides.[I] Nevertheless, a key challenge remains unresolved: How does one differentiate between changes in the environment (temperature, background concentration, and the presence of interferents) and detection of the target substance? Experimental sensors currently use one of two approaches: interrogation of a separate reference region[2] or sensing at multiple angles[ I]. Practical implementations of these approaches require sensing to occur at spatially separated locations on the device. Thus, these techniques reduce channel density and introduce residual errors from the different conditions at each location. Self-referencing Surface-Plasmon Resonance Sensors J. Todd Hastings, University of Kentucky Background and Research Goals Over the past decade surface-plasmon resonance (SPR) has become a widely accepted and commercialized optical technique for studying bio-chemical interactions in the laboratory. As a result, there has been great interest in developing field deployable SPR sensors, and great progress has been made in miniaturizing these sensors using optical fibers and planar waveguides. Nevertheless, mobile sensors that can function in complex analytes remain in the research phase because of the difficulty in distinguishing the interaction of interest from interferents and environmental variations. The proposed research seeks to solve this problem by developing a novel selfreferencing SPR sensor based on simultaneous coupling to long- and short-range surface plasmon waves. The proposed sensor offers self-referenced measurements for each recording channel to minimize residual errors and provide high channel density. In addition, our approach is easily adaptable to fiber and waveguide SPR sensor designs. Surface-plasmons are bound electromagnetic waves that propagate along the interface of two materials with real dielectric constants of opposite signs, i.e. a dielectric and certain metals such as gold or silver. SPR sensors use these waves to detect changes in refractive-index of the medium immediately adjacent to the metal layer. The field of a surface-plasmon wave at the metal surface is significantly stronger than a typical evanescent field; as a result, SPR sensors are particularly sensitive to surface binding or to changes in the refractive index of surface sensing layers. Surface-plasmon waves are not phase-matched to free space waves; therefore, one must couple to them using evanescent fields. The most common SPR sensor configuration, similar to that shown in figure I(a), uses a prism to couple light from an appropriate source to the plasmon mode. One observes a minimum in the reflected intensity at angles and wavelengths where light couples strongly to the surface-plasmon wave. SPR sensors measure refractive index changes by detecting shifts in the angle or wavelength of this reflection minimum. Despite the success of surface-plasmon resonance as a laboratory tool, implementation of SPR sensors in the field has been far more difficult. Several groups have successfully developed mobile sensors based on miniaturized optics, optical fibers, and planar waveguides.[I] Nevertheless, a key challenge remains unresolved: How does one differentiate between changes in the environment (temperature, background concentration, and the presence of interferents) and detection of the target substance? Experimental sensors currently use one of two approaches: interrogation of a separate reference region[2] or sensing at multiple angles[ I]. Practical implementations of these approaches require sensing to occur at spatially separated locations on the device. Thus, these techniques reduce channel density and introduce residual errors from the different conditions at each location. II. Research Outcomes and Relevance We propose the development ofa novel SPR sensor, shown in figure l(a), that alleviates these problems by providing a selfreferenced measurement for each sensing channel. By using a thin metal film with a buffer layer whose refractive index is matched to the analyte or sensing layer, one can simultaneously couple to both longrange[ 3] and short-range surface-plasmon modes. These modes exhibit different dispersion relations and different field penetration depths into the analyte. By measuring shifts in the resonance wavelength associated with each mode, we can simultaneously measure surface binding and bulk refractive index changes at exactly the same location on the sensor. The proposed effort seeks to quantify design trade-offs between target detection and self- 800 1000 1200 1400 Wavelength (nm)