Abstract:
A surface plasmon resonance device includes a metal thin film, a dielectric thin film on an optical member and, in addition, an electro-optically active thin film either between the metal thin film and the dielectric thin film or between the optical member and the metal thin film. The electro-optically active thin film is subject to a voltage which is varied to tune the resonance condition of the device.

Description:
FIELD OF THE INVENTION 
     The invention relates to surface plasmon resonance devices and in particular to application of surface plasma resonance devices to biosensors. 
     BACKGROUND 
     Surface plasmon resonance (SPR) is used to examine the optical properties of a thin film which is under investigation. Surface plasmon resonance has been used, for example, in biosensor apparatus for reading thickness changes in binding processes on microarray biochips. The use of SPR permits these changes to be detected as changes in local optical reflectivity. 
     SPR is a useful technique because the effect being studied only exists at surfaces, that is, at interfaces between a metal and dielectric materials. 
     Known SPR devices, comprising a metallic layer and a recognition layer deposited on a prism or waveguide, detect changes in the plasmon resonance angle when an analyte changes the recognition layer&#39;s refractive index. 
     Known SPR apparatus has certain problems. One problem is that the apparatus implements an SPR technique which is a steady state technique. That is, the apparatus looks for a dark spot in reflectivity by moving a light source or by implementing multiple angles. Known SPR techniques have limited dynamic range and must be operated within limited refractive index values. The fundamental SPR event in known devices also cannot be modulated. Their sensitivity is limited. 
     SUMMARY OF THE INVENTION 
     The invention is an improvement in SPR apparatus and methods. The invention resides in the inclusion of, for example, an electro-optically active or thermally controlled layer that can be used to “tune” the plasmon resonance, to modulate it, or to gate it. The invention is applicable to surface plasmon resonance biosensing by active multilayer detection. Active multilayer devices using the principles of the invention enable the use of SPR techniques in numerous applications, for example, in micro-assays for diagnostics, for drug development or for biotechnology experiments, by enhancing sensitivity and range of operation while simultaneously shrinking the size requirements of SPR devices. Other controlled layers may be any such layer the refractive index of which can be controlled. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     FIG. 1 shows a prior art surface plasmon resonance structure. 
     FIG. 2 is a typical SPR sensitivity curve; and 
     FIG. 3 shows a surface plasmon resonance structure which embodies the invention. 
     FIG. 4 shows an alternative surface plasmon resonance structure which embodies the invention. 
    
    
     DETAILED DESCRIPTION 
     FIG. 1 shows a type of surface plasmon resonance device of the prior art. A total internal reflection (TIR) prism  10  has on it a metal thin film  12  and a dielectric thin film  14 . Since surface plasmon waves propagate in TM (transverse magnetic) mode, the light beam  16 A is TM-polarized light, entering at an angle to produce total internal reflection and an associated evanescent wave. Variations of this device often use metal coated glass slides placed against the prism, so that the surface of the slide is the TIR surface rather than the prism surface itself as the TIR surface. The technology of prior art surface plasmon resonance (SPR) has been developed and published in the literature, is known to those skilled in the art and need not be recited in detail here other than as needed to understand the present invention. 
     SPR devices are used to measure refractive index and refractive index changes of a thin layer near a solid surface. In the context of biosensors, the biological material is placed on a surface defining a thin layer referred to as a biorecognition layer. The goal is to measure changes in thickness of biological material which comprises the biorecognition layer. During the biosensing procedure, the biological material which constitutes the biorecognition layer undergoes changes in properties, the specifically relevant changes being in optical thickness. SPR is sensitive to changes in optical thickness, or refraction index changes. Therefore SPR devices are useful for biosensing. However, SPR devices in accordance with the principles of this invention are more sensitive than prior art SPR devices and thus can measure relatively smaller changes in refractive index than can be measured by prior art devices. The smaller the change in refractive index that can be measured, the smaller are the changes in the biorecognition layer that can be measured. Also prior art devices are limited in the frequency in which sequential measurements can be made. 
     A typical SPR sensitivity curve is shown in FIG. 2; where the horizontal axis can be refractive index, light incidence angle or wavelength, and the vertical axis is reflected light intensity. 
     By using the principles of the present invention the ability of SPR devices to collect biological information in a biological binding process can be increased. In particular, due to the greater sensitivity and range being available, relatively small changes in optical thickness can be detected, greater accuracy of optical thickness can be measured and a wider range of optical thicknesses can be measured. 
     The technical problem addressed by the present invention is to extend the sensitivity of SPR devices by providing a control to modulate the refractive index of the SPR. According to one embodiment of the invention, an electro-optically active layer is employed the refractive index of which is modulated according to an impressed voltage. The voltage can range from DC to AC up to any practical frequency such as into the gigahertz range. The sensitivity of SPR devices is increased by modulating the voltage to vary the refractive index around a set point. It is advantageous to modulate the voltage about some selected or variable level of AC voltage. 
     In addition to increasing sensitivity of an SPR sensing device, apparatus in accordance with the present invention can increase range of the SPR device. To increase range, as the refractive index starts to change, we take advantage of the fact that the reflectance (R) starts to change with it. A voltage applied to the electro-optically active layer of the invention shifts the curve  19  of FIG.  2 . Therefore it is possible by applying a voltage signal which shifts the curve in a manner to maintain the resonance condition that gives the highest sensitivity. 
     Also according to the principles of the invention, it is possible to do null point detection. That is to say, it is possible to set the apparatus to start at a given point on the refractive index line of FIG. 2, where there is a minimum or no light. As light begins to appear due to changes in the biorecognition layer, it is possible to change the refractive index of the SPR device by changing the voltage impressed on the electro-optically active layer to come back to the minimum reflectivity condition. 
     Also, the voltage change that produces the null condition is a direct measurement of the change in refractive index that has occurred in the biorecognition layer, that is the change in thickness of the biological material. In this way, the change in thickness can be measured continuously or at intermediate times in the biological experiment, and the time rate of change can be calculated, all of which can enhance the biological information available. 
     Also, if the electro-optically active layer is set up to detect minimum light, that is its refractive index is controlled via a DC voltage to get minimum light, and a small modulation voltage is added, oscillating at frequency f, the detector will see a signal at 2 f (according to the principle of frequency doubling). If this signal is used in a feedback loop to maximize the 2 f signal, the result will be very fast automatic locking at the minimum light condition, and the voltage change necessary to achieve the locking will be a direct measurement of the change in biological thickness. Similarly the rate of change can be measured in very small increments. 
     FIGS. 3 and 4 show structures that implement the invention. Specifically, FIG. 3 shows a TIR structure, shown as a TIR prism  20 , which has on its surface  22  a metal thin film  24 , an electro-optically active thin film  26 , dielectric thin film  28 , also referred to as a chemical recognition layer  28 , and an electric field generator  30 . Generator  30  is attached by lines  32 ,  34  to the electro-optically active thin film  26 . FIG. 4 shows a similar structure, except that the electro-optically active thin film  26  is between the TIR prism  20  and the metal thin film  24 . In each of the embodiments of FIGS. 3 and 4 the electro-optically active thin film  26  is adjacent the metal thin film  24 . The dielectric thin film  28  (chemical recognition layer) is distal from the TIR prism  20 , that is with both the metal thin film  24  and the electro-optically active thin film  26  between the TIR prism  20  and the dielectric thin film  28 . 
     Although the TIR prism  20  is shown as the source of TIR incident light, other means of providing a totally internally reflected beam and an evanescent wave could be employed. 
     The electric field generator  30  is preferably selectably or controllably variable so that by varying the electric field on the electro-optically active thin film  26  the plasmon resonance can be modulated. FIG. 3 shows a controller  35  for controlling generator  30 . Controller  35  may be any such device as, for example, a microprocessor for controlling the electric field. 
     The effect of adding the electro-optically active thin film adjacent to the metal thin film is to allow a resonance condition to be scanned or modulated without the need for angle or wavelength scanning. This makes DC null-point detection possible as the sensor could be maintained in the plasmon resonance condition by controlling the refractive index of the active layer in a feedback loop whose “error voltage” would give direct indication of changes in the refractive index of the dielectric layer. 
     Alternatively, the index of the modulation layer could be scanned at a fixed or variable frequency, and changes in the refractive index of the dielectric layer could be detected as changes in the amplitude of the second harmonic light signal. 
     Another advantageous use of the invention is that it allows for tuning the plasmon sensor response to the refractive index range of interest. For example refractive indices of chemical recognition systems cover a wide range of values, and, ordinarily, changing recognition layers would necessitate the engineering of a new metal film, or even the use of a new prism. Use of the intermediate tunable electro-optically active layer alleviates this problem, since application of a DC voltage to the electro-optically active layer biases the multilayer structure in such a way as to compensate for changes in the basic refractive index of the recognition layer. 
     The preferred materials for the electro-optically active layer are: 
     Polyimide 
     Polyaniline 
     Poly (N-benzyl aniline) 
     Poly (N-napthyl aniline) 
     Poly (2-ethoxyaniline) 
     Poly (diarylaniline) 
     Poly (DR1-MMA) 
     The most preferred material is poly (DR1-MMA) which is an abbreviated term for poly (disperse red 1 methacrylate-co-methyl methacrylate). 
     While there are shown and described herein certain specific combinations embodying the invention for the purpose of clarity of understanding, the specific combinations are to be considered illustrative in character, it being understood that only preferred embodiments have been shown and described. It will be manifest to those of ordinary skill in the art that certain changes, various modifications and rearrangements of the features may be made without departing from the spirit and scope of the underlying inventive concept and concepts and that the present invention is not limited to the particular forms herein shown and described except insofar as indicated by the scope of the appended claims and equivalents thereof.