Optics: measuring and testing – Of light reflection
Reexamination Certificate
2003-12-22
2004-12-14
Smith, Zandra V. (Department: 2877)
Optics: measuring and testing
Of light reflection
Reexamination Certificate
active
06831748
ABSTRACT:
BACKGROUND OF THE INVENTION
The invention relates to a plasmon resonance sensor, in particular for biosensor technology, with an optically transparent body, in particular a glass prism, a reflective metal layer or semiconductor layer which is applied to one face of the body and has a surface sensitive to molecules to be detected, which forms a measurement cell in conjunction with a cuvette, a light source for emitting a divergent light pencil or beam path through the optically transparent body onto the inner face of the layer, and a detector which is assigned to the emerging beam path reflected by the layer and registers as a function of time the angle of incidence of the light, which changes as a result of molecule buildups on the sensitive surface, at which an intensity minimum of emerging light occurs owing to resonance.
Such a plasmon resonance sensor, with a glass prism and a gold layer, is known from U.S. Pat. No. 4,844,613. The light source used there is a laser diode, which emits in a pointlike fashion with an angle range which directly covers all the angles of incidence relevant to resonance detection.
For plasmon resonance determination, in simple terms, the intensity of the light reflected by the gold layer is detected. In order to find the resonance, either the angle of incidence of the light on the gold layer is scanned at a constant wavelength, or the wavelength is scanned at a constant angle of incidence. Surface plasmon resonance is identified by reduced reflectivity with a particular combination of angle of incidence and wavelength of the light. From the position of the resonance condition, it is possible to determine the refractive index of the layer, for example of a sample liquid, located on the gold layer. The interaction range, starting from the gold layer and extending into the layer to be determined, is restricted to a thickness of about one wavelength of the light.
In order to obtain the best possible resolution, either monochromatic light or a very accurately defined angle of incidence is necessary. WO 96 02 823 proposes the use of a monochromatic light source, the angle of incidence being scanned using a rotating mirror. However, moving parts are necessary to do this, which is found to be a disadvantage. Conversely, according to EP 305 109 B1, the corresponding angle range is generated optically by a beam fan. In both embodiments, the place of the reflection on the gold layer is very accurately localized. This can lead to undesirable heating of the gold layer, because the energy of the plasmons which are generated is dissipated in the gold layer. The refractive index of liquids, the medium most commonly to be measured in biosensors, depends strongly on temperature, so that falsifications of the measurement may arise because of the heating. The accurate localization of the reflection on the gold layer furthermore leads to problems in terms of measurement accuracy in the event of even very minor inhomogeneities of the gold layer, for example the small holes (pinholes) which are generated during thermal evaporation coating of gold layers.
SUMMARY OF THE INVENTION
The problem of the gold layer being heated is circumvented by the plasmon resonance sensor described in the introduction. There is no focus on the gold layer owing to the divergently incident light, but instead the various angles of incidence are each reflected at a different point on the gold layer. This avoids heating of the gold layer. However, the problem due to inhomogeneities in the gold layer is exacerbated here because different angles of incidence are influenced varyingly by them. The physical size of the laser diodes used as a light source may furthermore be a hindrance, because miniaturization and parallelization of the measurement channels is of crucial importance in modern instruments. Lastly, it is also to be regarded as disadvantageous that a point light source, namely a laser diode, needs to be used in the interest of good resolution. A light-emitting diode, which typically has an emission face with a diameter of 150 &mgr;m, cannot be used because of its extended emission and also because of the polychromatic emission.
It is an object of the invention to improve the plasmon resonance sensor described in the introduction, in such a way that the effect of inhomogeneities of the metal layer is suppressed while maintaining the simple design and substantial freedom from heating of the gold layer, and exact measurement results are therefore achieved.
This object is achieved according to the invention by the fact that the light source is assigned a non-pointlike emission face with an extent of at least 10 &mgr;m, and that a collimation lens is arranged in the emerging beam path between the optically transparent body and the detector.
The invention hence operates with a spatially extended light source, or emission face. This leads to an inherently undesirable significant broadening of the detected plasmon, because the angle information becomes blurred owing to the spatial extent of the light source, which can be regarded as a multiplicity of point light sources in spatial proximity. This apparently disadvantageous result is nevertheless compensated for according to the invention by the collimation optics which are provided.
Since the spatially extended light source which is used corresponds to a multiplicity of conically emitting point light sources in close proximity, light beams with the same angle of incidence strike the gold layer at different points, or over a wide region. Put another way, a plurality of different angles of incidence will be encountered at one place on the gold layer. The same angles of incidence of different places on the gold layer are then sent, having been recombined by the collimation lens provided according to the invention, to the detector where they are detected. This affords the substantial advantage that averaging takes place over a wide region of the gold layer during measurement of the plasmon generation, so that inhomogeneities of the gold layer now become unimportant, or almost so.
Expediently, the emission face is arranged at one focal point and the illuminated face of the detector is arranged at the other focal point of the collimation lens. Such a design fully compensates for the aforementioned broadening affect due to the extended emission face. Concerning the focal position of the emission face, it should in this case be borne in mind that the beam path is refracted in the prism, so that the focal position may need to be corrected.
In a particularly advantageous embodiment, the emission face is formed by the end of an optical fiber next to the optically transparent body. Optical fibers provide the opportunity to deliver the light close up to the glass body in a straightforward way, in which case the light source itself may be arranged some distance away, which offers design advantages and favors the aforementioned miniaturization. It was not previously possible to use optical fibers, or it was possible to use them only with great restrictions, because they do not have the emission face which is pointlike owing to the desired resolution. Although there are optical fibers with very small core diameters, namely single-mode fibers with a core diameter of from 2 to 9 &mgr;m, these are not in fact true point light sources and, in particular, they are difficult to work with. Elaborate input optics in conjunction with a separate light source for each individual optical fiber are needed. Technical implementation is therefore difficult and expensive. The same correspondingly applies to multi-mode fibers with a core diameter of from 50 to 150 &mgr;m.
To carry out the invention using optical fibers, ones with core diameters in the range of from 300 to 700 &mgr;m are therefore preferred, because the invention allows their use and correspondingly thick optical fibers are much easier to work with, especially in terms of light input. For example, the light from a single light source may be put into a plurality of such thick optical fibers. To this end, although
Luethy Carsten
Tittel Jakob
Jandratek GmbH
Smith Zandra V.
Stock, Jr. Gordon J.
Striker Michael J.
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