Abstract:
The plasmon resonance sensor ( 1 ) comprises a chip ( 2 ) of transparent plastic with a gold layer ( 3 ) made up of narrow part surfaces ( 4 ), arranged in a row, on the inner side of which light from a planar light source ( 12 ) is convergently guided. A Fourier lens ( 24 ) integrated in the chip ( 2 ) forms the reflected angular spectrum on the detector ( 23 ), arranged at a focal separation (F) from the integrated Fourier lens ( 24 ) for temporal determination of the incident angle with a resonant intensity minimum of reflected light. An incident lens ( 13 ) brings about imaging of the planar light source ( 12 ) in the form of lines of light on the part surfaces ( 4 ), which are further imaged in the form of lines of light on the detector ( 23 ).

Description:
CROSS-REFERENCE TO RELATED APPLICATION 
     The invention described and claimed hereinbelow is also described in German Patent Application DE 10 2006 041 338.5 filed on Sep. 1, 2006. This German Patent Application, whose subject matter is incorporated here by reference, provides the basis for a claim of priority of invention under 35 U.S.C. 119(a)-(d). 
     BACKGROUND OF THE INVENTION 
     The invention relates to a plasmon resonance sensor, in particular for biosensor technology, comprising a light-transmissive body, a—applied to an area of the body—reflective metal layer or semiconductor layer having a surface which can be sensitized to molecules to be detected, which comprises partial areas arranged alongside one another in a series and serving for forming a respective one of a plurality of measurement cells, comprising an areal light source for producing a beam path incident on the inner side of the layer through the body, and comprising a detector, which captures the reflected emergent beam path and ascertains in a time-dependent manner the angle of incidence of the light which changes as a result of molecular attachments to the sensitive surface and in the case of which an intensity minimum of emergent light occurs owing to resonance, a Fourier lens being arranged in the emergent beam path and the detector being arranged at the focal point distance with respect to the Fourier lens. 
     The phenomenon of surface plasmon resonance (SPR) involves a collective excitation of the electrons at the surface of a layer having free electrons. The resonant frequency of the surface plasmons is highly sensitive to the refractive index of the medium adjacent to the sensitive surface. This can be used to measure thin layers with regard to the refractive index or the layer thickness (up to approximately one light wavelength). In biosensor technology, in particular, this effect is used to examine the kinetics of attachment of biomolecules from a sample liquid to a functionalized metal surface. For this purpose, the resonance condition of the surface plasmons is detected in time-resolved fashion. The surface plasmons of the thin metal layer are excited by light that falls onto the metal layer at a specific angle or angular range. The resonance condition is then met for a specific combination of wavelength and angle of incidence. Under this resonance condition, the intensity of the light reflected at the metal layer is significantly reduced on account of the generation of surface plasmons. In order to find the resonance condition, either the angle of incidence (given a constant wavelength) or the wavelength (given a constant angle of incidence) can be tuned and the intensity of the reflected light can be detected. A gold-coated glass body (prism) and light having a constant wavelength, which impinges on the gold layer at different angles of incidence, are generally employed. 
     It is known from EP 305 109 B1 in the case of angle-resolved measurement, to produce the corresponding angular range optically by means of a beam fan which is focused onto the metal layer by means of a hemispherical glass lens. The focusing enables only a single measurement, after which the layer which is sensitive to molecules has to be regenerated again before a further measurement becomes possible. Moreover, owing to the focusing, there is the risk of local heating of the metal layer and, as a result of this, the possibility of corruption of the measurement values. 
     DE 100 23 363 C1 has already disclosed realizing different angles of incidence of the light by irradiating the metal layer with a divergent light beam which issues from a point light source in the form of a laser diode. In this case, simultaneous multiple measurements of different samples are made possible by virtue of the fact that the incident light is fed divergently only in one direction (in the incidence plane), while it is collimated by means of a cylindrical lens in the direction perpendicular thereto, a plurality of measurement cells being arranged in a series transversely with respect to the incidence plane on the metal layer. Harmful heating of the metal layer is prevented owing to the divergent irradiation. Within the incidence plane or divergence plane, different angles of incidence are present in each case at a different location of the metal layer. This leads to an influencing of the measurement values owing to inhomogeneities of the metal layer. Furthermore, the still divergently emergent beam path is detected directly, which necessitates large detectors that are each assigned to a single measurement cell. This leads to a complicated and spatially extended device which enables only a small number of measurement cells and thereby simultaneous measurements. 
     DE 100 55 655 C2 discloses the plasmon resonance sensor comprising a plurality of measurement cells as described in the introduction. In this case, each measurement cell or partial area of the metal layer is assigned a dedicated light source formed by an optical waveguide having an extended and not point-type emission area, the optical waveguides being directly connected to the light-transmissive body and a Fourier lens and a cylindrical lens being arranged in the emergent beam path, which permits a common detector. The areal light sources, which are in each case comparable to a multiplicity of adjacent point light sources, have the consequence that the range of different angles of incidence is present at all locations of the metal layer. The Fourier lens is arranged for the purposes of a Fourier imaging of a 2 f arrangement between the emission areas of the light sources and the detector, the emergence optical unit ensuring that identical angles of incidence are combined on the detector and are separated there from light based on other angles of incidence. It is thus possible to obtain exact averaged measurement results which are free from unfavorable influences due to heating or inhomogeneities of the metal layer. 
     What is disadvantageous about this known plasmon resonance sensor, however, is that each measurement cell or partial area of the metal layer is assigned a dedicated areal light source, which greatly limits the possible number of measurement cells. Moreover it is not easy to obtain an angular spectrum which encompasses the entire angular range to be measured and which is dependent on the distance between the optical fiber and the metal surface, the numerical aperture of the fiber and the fiber diameter. In addition, the divergent emission of the metal layer requires a comparatively extended optical unit with a large Fourier lens which captures the emergent beam path. That, too, is detrimental to a compact and uncomplicated design. 
     SUMMARY OF THE INVENTION 
     Accordingly, the invention is based on the object of improving this plasmon resonance sensor such that, whilst maintaining the advantages outlined, it is simpler and more compact and has greater performance. 
     Proceeding from the sensor described in the introduction, this object is achieved according to the invention by virtue of the fact that an incidence optical unit is provided between the light source and the layer, and it collimates the incident beam path in the series direction of the partial areas and directs it onto the layer convergently in the incidence plane running perpendicular to the series direction of the partial areas, such that the single areal light source is imaged on each partial area of the layer in the form of a luminous line. 
     The convergent irradiation of the metal layer enables a compact design with small components or optical elements both on the light incidence side and on the light emergence side, in which case very narrow partial areas which are closely adjacent to one another can also be employed in the interests of a large number of measurement cells. 
     Expedient configurations and developments of the invention are apparent from the dependent claims. In this case, in particular the use of a chip composed of a light-transmissive polymeric plastic as light-transmissive body contributes to simplification and increase in performance since it can be produced inexpensively and simultaneously by corresponding shaping also with optical functions for the light guiding on the emergence side, and if appropriate also on the incidence side, by way of injection molding, as is known in principle from DE 103 24 973 B4. This affords the possibility of designing the chip as a replaceable disposable article, such that regeneration measures become superfluous and not only can a plurality of measurements be carried out simultaneously but the multiple measurements can also be performed in rapid succession. A comparatively expensive optical component is obviated as a result of the integration of the collimation lens into the chip. Furthermore, the insertion of the chip into the plasmon resonance sensor results in an optical coupling between the illumination optics and the detection optics without a disturbance-prone coupling by means of immersion media. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
       An exemplary embodiment of the invention is explained in more detail below with reference to a schematic drawing, in which: 
         FIG. 1  shows the parts of the plasmon resonance sensor which are essential for the invention, in a side view; 
         FIG. 2  shows the arrangement according to  FIG. 1  in plan view, but with light incidence and light emergence sides pivoted up in coaxial alignment and adapted to the side view width; 
         FIG. 3  shows the right-hand part according to  FIG. 1  in a somewhat enlarged and overall pivoted illustration; 
         FIG. 4  shows an image of the irradiated partial areas that is produced on the detector; 
         FIG. 5  shows the measured angle-dependent intensity distribution along a partial area; 
         FIG. 6  shows the temporal development of the position of the minimum of the resonance for a sample measurement and a reference measurement; 
         FIG. 7  shows the time-dependent profile of the difference signal obtained from the two curves according to  FIG. 6 . 
     
    
    
     DESCRIPTION OF THE PREFERRED EMBODIMENTS 
     The plasmon resonance sensor  1 , the essential parts of which are illustrated in  FIGS. 1 and 2 , has a light-transmissive body  2 , which is a chip composed of a polymeric plastic and produced by injection molding. This chip  2  bears on its top side  6  a light-reflecting thin metal layer  3 , which is preferably composed of gold. By means of a chemical modification of the gold surface, the latter acquires a structure wherein individual narrow partial areas  4  which are arranged closely adjacent to one another and are spaced apart by intermediate strips  5  are arranged alongside one another in a series ( FIG. 2 ). The chip  2  has a planar top side  6  and a planar underside  7  parallel thereto, which are connected to one another by likewise planar end sides  8  and  9 . By contrast, the side faces  10  and  11 , as can be seen from  FIGS. 1 and 3 , are cylindrically curved. 
     The chip  2  is assigned a light source  12  and an incidence optical unit  13 , whereby an incident beam path  14  with optical incidence axis  15  is produced, said beam path penetrating into the chip  2  through the side face  10  and being incident on the inner side of the reflective metal layer  3 . 
     The light source  12 , as indicated in the illustration, is an areal or extended light source, the emission area of which comprises as it were a multiplicity of adjacent point light sources which each emit within a specific angular range. By way of example, the end face of a so-called multimode fiber or LED is used as such a light source. 
     The incidence optical unit  13  comprises, as viewed in the direction of incidence, a spherical lens  16 , which firstly collimates the incident beam path  14 , that is to say parallelizes the light beams, and also a wavelength filter  17 , a polarization filter  18  and a cylindrical lens  19 , which converts the incident beam path  14  into a beam bundle that is convergent in the incidence plane ( FIG. 1 ), but in the direction perpendicular thereto, that is to say in the series direction of the partial areas  4 , leaves it in the parallel orientation ( FIG. 2 ). With the curved side face  10  of the chip  2  it is also possible, if appropriate, for a part of the incidence optical unit  13  to be integrated into the chip  2 . The incidence optical unit  13  described has the effect that the light source is imaged in the direction of the incidence plane on the partial areas  4 , while in the direction perpendicular to the incidence plane the partial areas arranged alongside one another are illuminated in identical fashion (in collimated fashion). 
     The reflection of the incident beam path  14  at the metal layer  3  or the partial areas  4  thereof gives rise to an emergent beam path  20  with the optical emergence axis  21 , which beam path emerges through the side face  11  of the chip  2  and is directed onto a detector  23  by means of an emergence optical unit  22 . The detector  23  can be a CCD sensor (CCD chip) in the manner of a 2D camera and continuously measures, separately for each partial area  4 , the angle-of-incidence-specific intensity of the reflected light. 
     The emergence optical unit  22  includes a cylindrical Fourier lens  24 , which is formed by the cylindrically curved side face  11  of the chip  2  and is therefore integrated into the chip, and also a larger cylindrical lens  25  and a smaller cylindrical lens  26 , which bring about an imaging of the partial areas  4  or of the luminous lines present thereon on the detector  23 . 
       FIG. 3  makes it clear that the radius R of curvature of the side face  11  or of the integrated Fourier lens  24  distinctly exceeds the distance thereof from the partial areas  4 . By way of example, given a distance of 5 mm from the gold layer  3 , the radius R of curvature is 28 mm, that is to say five to six times the distance. The detector  23  is arranged at the focal distance F with respect to the integrated Fourier lens  24 , such that a so-called “Fourier imaging” arises on it and, for each illuminated partial area  4 , the rays incident thereon at the same angle in distributed fashion are combined at a point on the detector  23 , as is illustrated for two points in  FIGS. 1 and 3 . 
     Although the 2 f arrangement, which is traditional for a Fourier imaging and in which the object to be imaged is also arranged at the focal point distance with respect to the lens, cannot be realized with the integrated lens  24 , a separate Fourier lens would have to be of a considerable size and would be correspondingly voluminous, heavy and expensive. The radius R of curvature of the integrated Fourier lens  24  thus determines the distance from the detector  23  at which the image of the angular spectrum arises (image distance), and also the size of the image. The cylindrical lenses  25  and  26  provided in the emergent beam path counteract blurring of the imaging of the partial areas  4  on the detector  23 . Such blurring results from the area extent of the light source  12  and a divergence of the emergent light bundle in the series direction and also from diffraction effects at the edges of the partial areas  4 . 
     In order to illustrate the optical conditions of the plasmon resonance sensor  1 ,  FIGS. 1 to 3  depict the course of a plurality of rays from the light source  12  as far as the detector  23 , to be precise—from the standpoint of the incidence side—the upper limiting ray  27 , the lower limiting ray  28 , the left-hand limiting ray  29  and the right-hand limiting ray  30  and also a central ray  31 . It should be noted in this respect that the corresponding ray angles are present over the entire light beam cross section owing to the special light source  12  having an areal emission area within the incident beam path  14  and also the chip  2 .  FIGS. 1 and 3  show the ray refraction—which only occurs in the incidence plane—by the Fourier lens  24 , integrated into the chip  2 , with a non-telecentric—that is to say not parallel to the optical emergence axis  21 —concentration of the rays having an identical angle of incidence. 
       FIG. 4  shows the intensity distribution on the detector  23 . The dark stripes A and the bright stripes B arise as a result of the fact that the layer  3  on the chip  2  was wetted with water. In the uncoated intermediate strips  5 , the light coming from the light source  12  is completely reflected (total reflection), whereby the bright stripes B arise. In the regions formed by the metal-coated partial areas  4 , by contrast, the plasmon resonance effect is observed, which is reproduced by the dark stripes A. 
     The intensity distribution along such a stripe A that is measured at a specific point in time can be seen from  FIG. 5 . In this case, the dashed line highlights the position of the intensity minimum corresponding to a specific angle of incidence. This minimum position, which shifts in a time-dependent manner with increasing attachment of molecules, is determined precisely. 
       FIG. 6  shows the temporal development of the position of the minimum of the resonance for two different partial areas  4  on the chip. One partial area, a reference partial area, essentially only comprises the gold coating, while the other partial area, a sample partial area, was immobilized with protein A. In the experiment, antibody solutions and water were alternately directed over the partial areas  4 , and the concentration of the antibody solution was increased from step to step. The specific binding of the antibodies to the sample partial area leads to the observed association (wetting with antibody solution) or disassociation (wetting with water) of the antibodies at the binding surface. A signal is observed at the reference partial area only in the region of high antibody concentration, said signal being attributable to the increased refractive index of this solution. 
     The measurements in accordance with  FIG. 6  were carried out simultaneously and for relatively inhomogeneous illumination with an intensity distribution superposing the resonance curves.  FIG. 7  shows the profile of the difference signal that results from the two curves in  FIG. 6 . The irregular form of the binding curve at the highest antibody concentration is corrected in this case. In addition, noise that affects both partial areas  4  equally can be reduced by the difference formation. A further advantage of the reference measurement is that the influence of non-specific bindings can be corrected. 
     In accordance with the simultaneous measurement of sample values and assigned reference value, the partial areas  4  are divided into measurement areas for the samples and into at least one reference area. Each measurement area can also be assigned a dedicated reference area. 
     The formation of the partial areas can be obtained by a corresponding structuring of the layer  3  or else by a chemical modification of the surface thereof. In this case, it may be advantageous firstly to form partial areas of identical type and then to subdivide them into measurement areas and into reference areas by means of a chemical surface modification.