Optics: measuring and testing – Of light reflection
Reexamination Certificate
2002-09-16
2004-03-30
Font, Frank G. (Department: 2877)
Optics: measuring and testing
Of light reflection
C356S630000
Reexamination Certificate
active
06714303
ABSTRACT:
BACKGROUND OF THE INVENTION
1. Field of the Invention
The present invention relates to a method and an apparatus for optical surface analysis of a sample area on a sensor surface.
2. Description of the Related Art
The interest for surface sensitive measuring techniques has increased markedly recently as several optical techniques have been developed for identifying and quantifying molecular interactions, which techniques do not require labelling. The most used optical technique so far is based on surface plasmon resonance, hereinafter frequently referred to as SPR.
The phenomenon of surface plasmon resonance, or SPR, is well known. In brief, SPR is observed as a dip in intensity of light for a specific wavelength reflected at a specific angle (as measured by, e.g., a photodetector) from the interface between an optically transparent material, e.g., glass, and a thin metal film, usually silver or gold, and depends on among other factors the refractive index of the medium (e.g., a sample solution) close to the metal surface. A change of the real part of the complex refractive index at the metal surface, such as by the adsorption or binding of material thereto, will cause a corresponding shift in the angle at which SPR occurs, the so-called SPR-angle. For a specific angle of incidence, the SPR is observed as a dip in intensity of light at a specific wavelength, a change in the real part of the refractive index causing a corresponding shift in the wavelength at which SPR occurs.
To couple the light to the interface such that SPR arises, three alternative arrangements may be used, viz., either a metallized diffraction grating (see H. Raether in “Surface Polaritons”, Eds. Agranovich and Mills, North Holland Publ. Comp., Amsterdam, 1982), a metallized glass prism (Kretschmann configuration) or a prism in close contact with a metallized surface on a glass substrate (Otto configuration). In a SPR-based assay, for example, a ligand is bound to the metal surface, and the interaction of this sensing surface with an analyte in a solution in contact with the surface is monitored.
Originally, collimated light was used for measuring the SPR-angle, the sensing area being restricted to the intersection of the collimated light beam and the metal surface. The apparatus used was based on mechanical goniometry with two movable mechanical axes carrying the illumination and detecting components, the rotational center of which was placed in the center of the sensor area, i.e., one axis for the incident light and another axis for the reflected light which was detected by a single photodetector. A plane-sided coupling prism at total internal reflection condition was used to preserve the collimated beam inside the prism, which, however, introduced a refraction at non-orthogonal incidence at the prism, thereby introducing a beam-walk at the metal surface. A half-cylindrical coupling lens with orthogonal incidence of the optical axis of the light gave a fixed sensor area, however, introduced a beam-convergency, i.e., a non-collimated or quasi-collimated beam inside the prism.
In a development, the movable opto-mechanical axis on the illumination side was eliminated by using a focused incident beam, so-called focused attenuated total reflection (focused ATR), as described by Kretschmann, Optics Comm. 26 (1978) 41-44, the whole angular range simultaneously illuminating a focal line or point of a given sensing surface. The use of a detector matrix for detecting the reflected light eliminated the movable opto-mechanical axis also on the reflectance side, providing a faster SPR-detection than that of the prior art. Such systems are described in, e.g., EP-A-305 109 and WO 90/05295. In the latter, light beams reflected from a specific sub-zone at the sensor area are imaged anamorphically so that beams in one plane (the sagittal plane) create a real image on a specific detector-pixel row of a matrix detector, permitting the occurrence of a local surface binding reaction to be identified, while quantification of the reaction is obtained via angular data for a reflectance curve measured along the same pixel row, where the reflectance curve is created by beams in a plane (the meridian plane) normal to the first-mentioned plane. Thus, one dimension of the matrix detector is used for real imaging simultaneously as the other dimension is used for only angular measurement. This permits only sub-zones arranged in a row to be simultaneously monitored and imaged.
In a variation of goniometry, the bulky mechanical axes were replaced by rotating or vibrating mirrors, respectively. A disadvantage of that approach is, however, that when scanning the incident angle by means of a plane mirror, the point where the light beam hits the sensor surface will move along the internal reflection surface of the prism. This problem was avoided by using a combination of rotating mirror and focused SPR, e.g., as described by Oda, K., Optics Comm. 59 (1986) 361. In this construction, a first collimated beam of about 1 mm diameter impinges on the rotational center of a rotating mirror placed at the focal length of a focusing lens, thus producing a second quasi-collimated beam, the distance of which to the optical axis depends on the reflecting angle of the mirror. The second collimated beam is focused by a second focusing lens onto a prism base at total internal reflection conditions. During the rotation of the mirror, the angle of incidence at the approximately fixed sensor area is scanned for the quasi-collimated beam.
Another approach to obtain a fixed and also enlarged sample spot is proposed by Lenferink et al., “An improved optical method for surface plasmon resonance experiments”, vol. B3 (1991) 261-265. This technique uses the combination of a collimated light beam illuminating a plane rotating mirror, a focusing cylindrical (convex) lens after the mirror and a half-cylinder lens for coupling the light to the sensing surface. By making the cylindrical lens focus the light on the focal surface of the coupling half-cylinder, using a relatively complex lens system, a collimated beam is obtained inside the coupling prism.
Other optical techniques similar to SPR are Brewster angle reflectometry (BAR) and critical angle reflectometry (CAR).
When light is incident at the boundary between two different transparent dielectric media, from the higher to the lower refractive index medium, the internal reflectance varies with angle of incidence for both the s- and p-polarized components. The reflected s-polarized component increases with the angle of incidence, and the p-polarized component shows a minimum reflectance at a specific angle, the Brewster angle. The angle at which both s- and p-polarized light is totally internally reflected is defined as the critical angle. For all angles of incidence greater than the critical angle, total internal reflection (TIR) occurs.
Schaaf et al., Langmuir, vol. 3 (1987) describes Brewster angle reflectometry using a micro-controlled rotation table and a movable detector in scanning angle reflectometry around the internal Brewster angle to study a protein (fibrinogen) at a silica/solution interface. The use of movable optomechanical axes and a rotation table gives a slow measuring procedure, and the sensor area is restricted to the cross-section of the collimated light beam with the sensor surface being limited by the need for non-beam-walking.
A focusing critical angle refractometer, based on a wedge of incident light which strikes the line of measurement, including the critical angular interval to be measured, and which measures the one-dimensional refractive index profile along a focused line immediately adjacent to the glass wall of a liquid container is described by Beach, K. W., et al., “A one-dimensional focusing critical angle refractometer for mass transfer studies” Rev. Sci. Instrum., vol. 43, 1972. This technique is limited in that it enables only a one-dimensional sensor area, which is restricted to the cross-section of the light line with the sensor area.
In all the above described prior art method
Biacore AB
Font Frank G.
Merlino Amanda
Seed IP Law Group PLLC
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