Optics: measuring and testing – Of light reflection
Reexamination Certificate
1999-11-23
2002-10-15
Stafira, Michael P. (Department: 2877)
Optics: measuring and testing
Of light reflection
Reexamination Certificate
active
06466323
ABSTRACT:
FIELD OF THE INVENTION
This invention relates to a surface plasmon resonance sensor or probe used in biochemical, chemical, biological or other applications.
BACKGROUND OF THE INVENTION
Surface plasmon resonance (“SPR”) is an optical phenomenon caused by the interaction between light or other electromagnetic radiation and several different types of materials, usually comprising a dielectric material and a conductive material arranged in a multi-layer stack of thin films. Technical details describing this optical phenomenon are set forth in various publications, such as one by Schwotzer, et al., titled Fiber Optic. Sensor for Adsorption Studies Using Surface Plasmon Resonance, vol. 2508, Institute of Radio Engineering & Electronics, pp. 324-33, and patents, including U.S. Pat. Nos. 4,997,278 and 5,485,277, each of which documents are incorporated herein by reference.
Basically, however, SPR is an optical phenomenon that occurs when light is shined at a certain angle into a prism that has upon one surface a thin coating comprising one or more conductive or dielectric layers. If the light is shined into the prism at a particular “critical angle,” the light may totally internally reflect within the prism so that it does not escape that side of the prism. The critical angle depends upon the characteristics of the prism, the layer(s) or the environment surrounding the entire structure. For instance, an everyday example of total internal reflection occurs when you peer into a clear glass of water. As you change the angle and orientation of the glass relative to your line of sight, at some point you will see the sides of the glass turn opaque or silver. Even though you can normally see through the water in the glass, at the critical angle at which the sides turn silver or opaque, the light is totally internally reflected within the glass and water therein because of the different refractive indices of the water, glass and surrounding air.
Light that totally internally reflects within a coated prism forms an electromagnetic wave that propagates along the conductive (i.e., metal) layer boundary. This wave is known as a surface plasmon. The surface plasmon wave is optically excited at the interface between a conductor or semiconductor, e.g., a metal surface and a dielectric. The optical excitation takes place by an evanescent field, created when light undergoes total internal reflection, for example, off the base of a prism. This evanescent field penetrates the metal and excites a surface plasmon wave where the metal meets the dielectric.
It takes energy to create the surface plasmon. The energy forming the surface plasmon is removed, at a specific frequency or wavelength, from the light that hits the interface between the prism and its coating. Thus, the resulting reflected light beam lacks the removed energy. If you examined the energy in the reflected beam across a spectrum of frequencies or wavelengths, you would see a dip or drop in the energy at a particular “plasmon resonance wavelength,” which is the wavelength at which the surface plasmon removes energy from the reflected beam. The plasmon resonance wavelength is determined by a number of factors, including: the thickness, composition and number of the conductive or dielectric layers, as well as the incidence angle of the light upon the substrate, and the interaction between the metal or dielectric layers and the ambient environment.
This surface plasmon resonance (“SPR”) phenomenon can be and has been used to create sensors that sense the presence of certain chemical, biological or biochemical agents. For instance, incorporating a particular dielectric or other transducing layer whose permittivity and/or thickness varies in response to chemicals (analytes) of interest results in a sensor whose normal SPR frequency changes with that variation. By analyzing the degree and type of the change, one can determine the presence and/or quantity of a particular analyte of interest. SPR sensors are generally based on bulk optical components (prisms, polarizers, etc.) that yield high quality resonances but which are very difficult to miniaturize into suitable probes for remote sensor applications.
Examples of such SPR sensors are described in U.S. Pat. Nos. 5,485,277 or 4,977,278 or in the Schwotzer, et al. publication cited above. These sensors usually work by shining a collimated light through focusing lenses into a prism or other high refractive index material and then detecting and analyzing the reflected light with a spectrum analyzer. A baseline surface plasmon resonance frequency is found for the particular sensing medium, such as a metallic or other layer of material, coating the prism. The addition of an analyte to the sensing medium changes the SPR frequency. A detector analyzes the reflected light to detect the new SPR frequency. By comparing the new versus baseline frequency, the analyte and/or its quantity can be identified and detected. Such sensors are usually bulky and difficult to keep properly calibrated during their employment.
Indeed, U.S. Pat. No. 4,997,278 to Finland, et al. itself recognizes that one problem with sensors that use a prism or the like is that slight movements of the prism or light source result in changes to the incidence angle, which in turn changes the SPR frequency. That means that the changes to SPR frequency detected by the sensor will be rendered inaccurate or less accurate since variables (e.g., the movement) other than just the presence and amount of analyte will alter the relationship between the baseline SPR frequency and the SPR frequency obtained with the analyte. Prior sensors, including Finland, et al.'s, also use a variety of optics in order to collimate, focus and guide the incident and reflected light beams. These optics contribute to the bulkiness of the probe sensors, rendering them both more expensive to build or maintain and less versatile during use.
Another problem with these conventional SPR probes is that they normally use only one area upon the surface of the probe as the sensing medium. For instance, the Finland, et al. patent applies a sensing layer to the rear, planar surface of an optically transmissive component formed of a slide in contact with a cylindrical lens. Finland, et al., then passes a collimated beam of non-coherent light through the hemispherical portion of the lens so that the light impacts upon the flat surface of the slide upon which the metallic film has been formed. Finland describes the non-coherent light that it shines upon the sensing medium as a fan or cone shaped beam of light. Finland proposes that the advantage of such a fan or cone shaped beam of light is that the range of angles of incidence of the light at the intersection point spans the angle which excites a SPR in the film. Although this allows Finland, et al. to use several beams of light to impact the sensing medium, Finland, et al.'s probes are like prior probes that still use only one portion as a sensing medium and still require monochromatic operation (e.g., use of a particular single wavelength of light) and focusing optics.
SUMMARY OF THE INVENTION
The present invention is an SPR probe that has a substrate with a generally curved reflecting surface. In the present invention, light is input through the substrate to the generally curved reflecting surface where it interacts with one or, optionally, multiple, sensing areas coated with the same or different sensing mediums. By causing the light first to impact against the curved reflecting surface, the light may be reflected from a first impact area to a second, third, etc. impact area. That is because the radius of curvature of the substrate causes the light incident upon the first impact area to reflect to another portion of the substrate with the same incident angle. Thus, for each of the impacts of light on the different portions of the substrate the incident angle remain constant. The number of reflections, and thus the number of light impact and potential sensing areas, can be adjusted by modifying the shape, size and cur
Anderson Brian Benjamin
Nave Stanley Eugene
McNair Law Firm, P.A.
Stafira Michael P.
Westinghouse Savannah River Company, L.L.C.
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