Abstract:
An improved method of analyzing target analytes in a sample is described. The method is based on anti-resonant guided optical waveguides which enables a strongly improved light-target interaction since the light can be guided within the target-containing medium. The light-target interaction can be monitored by many different means to determine characteristics of the target analyte. The anti-resonant waveguide concept is suitable for a large variety of characterization methods and combinations of them, since it is relatively unaffected by changes to both wavelength and film thickness.

Description:
BACKGROUND  
       [0001]     The detection of micro-organisms for medical treatments and security systems has taken on increased importance in recent years. Modern medical systems as well as security systems depend on the detection and identification of microorganisms, including bioagents or toxins in the air, food, water, blood or other specimens.  
         [0002]     Conventional detection is usually done in the laboratory. Laboratory testing typically uses skilled personnel in a time consuming process. Portable versions of laboratory PCR (polymerase chain reaction) have been developed, however, these devices are bulky and not cost effective.  
         [0003]     Optical systems for detecting and identifying micro-organisms provide numerous advantages over chemical and other analysis techniques. For example, optical systems can reduce or eliminate the need for field workers to use chemical reactions to detect elements. Optical systems are also often nondestructive to the sample being analyzed.  
         [0004]     Most optical biosensor designs rely on interactions between light and a biological sample to provide information on sample characteristics. However, the interaction between light and biological elements in the sample is typically weak. Thus without amplification of the interaction, a large quantity of analyte may be needed. Obtaining such large sample sizes may not be practical for many applications.  
         [0005]     In order to increase the interaction between light and biological elements in the sample, optical waveguides may concentrate the intensity of light on the sample. In one use, microorganisms in the sample reside in liquid immediately adjacent to a waveguide surface. Evanescent waves from the waveguide interact with the molecules of the biological elements. However, the interaction between the evanescent waves and the biological elements is still weaker than desired.  
         [0006]     Thus an improved system for microorganism detection and identification is needed.  
       SUMMARY  
       [0007]     A method of analyzing a sample is described. The sample includes a medium (e.g., gas, aerosol or fluid) carrying certain target analytes (e.g., toxins, bacteria or their spores), viruses, mammalian or insect cells, parasites, oocytes, or certain chemicals). The method places the sample to be analyzed between a first layer/medium and a second layer/medium. The sample has a sample index of refraction that is less than the indexes of refraction of the first and second layer/medium. A beam of light enters the sample at an angle such that an anti-resonant guided optical waveguide (ARGOW) mode propagates through the sample. Anti-resonance waveguides enable a strongly enhanced interaction between light and analyte. This is useful for many different characterization methods. The interaction between photons in the anti-resonant mode and target analyte (e.g. biological molecules) in the sample is monitored to determine a characteristic of molecules in the sample. 
     
    
     BRIEF DESCRIPTION OF THE DRAWINGS  
       [0008]      FIG. 1  shows a side sectional overview of an analysis system.  
         [0009]      FIG. 2  shows an expanded side sectional view of a waveguide receiving an input light beam with a target-containing sample as a core.  
         [0010]      FIG. 3  is a table showing example incidence angles for different analytes surrounded by a glass cladding.  
         [0011]      FIG. 4  is a chart that plots an angle of incidence into the waveguide structure of  FIG. 2  as a function of the index of refraction of the sample.  
         [0012]      FIG. 5  shows a side sectional view of a waveguide with a biological sample as a core and with a tilted entrance facette.  
         [0013]      FIG. 6  shows an intensity profile of various anti-resonant modes in an example analyte cross section.  
         [0014]      FIG. 7  shows the fluorescence intensity as a function of the coupling angle of the excitation light.  
         [0015]      FIG. 8  shows a top view of a system to process in parallel different tests on a sample to determine the presence of a target analyte  
         [0016]      FIGS. 9-14  show sample index profiles of a sample and cladding immediately adjacent the sample. 
     
    
     DETAILED DESCRIPTION  
       [0017]     An improved sensor that enhances interaction between light and target analytes in a sample is described. Light from a light source is coupled into a sensor chamber, such as a microfluidics channel filled with the sample. By controlling the angle of light entry into the sensor chamber, anti-resonant modes are generated in the sample. The anti-resonant modes allow the sample itself to serve as an optical waveguide resulting in increased interaction between the target molecules and the light.  
         [0018]      FIG. 1  shows a side view of one embodiment of the optical sensing system  100 . In  FIG. 1 , a light source  104  and/or a lens system  108  directs a light beam  112  into a sample  116 . Depending on the test being conducted, light in light beam  112  may be of coherent or incoherent. When coherent light is used, light source  104  is typically a laser. In other cases white light or light emitting diodes may be used.  
         [0019]     Light beam  112  enters sample  116  at an angle of incidence  120 . As used herein, reference to the word “light”, “light beam” and “optical” is should be broadly interpreted to include a broad range of frequencies including ultraviolet, visible, infrared, and far infrared radiation as well as terahertz radiation. As used herein, the angle of incidence is the angle with respect to a normal  124  of the surface  128 . The angle of incidence is carefully selected such that an anti-resonant guided optical wave (ARGOW) or mode of light can be set up within sample  116 .  
         [0020]     Sample  116  is typically a thin film of liquid carrying the target analyte (e.g., biological molecules) to be analyzed. Sample  116  may also be a gas or an aerosol carrying the analyte to be analyzed. If the sample is a gas or aerosol, sealing materials around the perimeter of the chamber containing the sample keeps the gas between substrate  132  and covering layer  136 . Sample  116  thickness is usually kept larger than the wavelength of light being used to analyze the sample.  
         [0021]     Substrate  132  and covering layer  136  border sample  116  sides. Substrate  132  and covering layer  136  are typically made from a transparent material such as glass. In one embodiment, glass slides are used for substrate  132  and covering layer  136 . The index of refraction of the substrate and covering layer are slightly higher than that of the sample  116  to facilitate generation of an anti-resonant wave in sample  116 . An example index of refraction of substrate  132  and covering layer  136  might be between 1.4 and 1.8 while the index of refraction of a liquid sample  116  might be between 1.2 and 1.4 although as will be explained, a wide range of other indices are also possible.  
         [0022]     The actual conditions used to create an anti-resonant guided optical wave (ARGOW) propagating through a sample sandwiched between two higher index materials may be found by computing the Eigensolutions of the Helmholtz equation for a plane wave propagating along a slab waveguide structure. A general Helmholtz equation for the electric field E is given by: 
 
( ∇   2   +|{right arrow over (k)}|   2   E= 0; |{right arrow over (k)}|=|{right arrow over (k)} 0 ·n  (Eq. 1) 
 
         [0023]     Assuming a plane wave that propagates along a x-direction within a slab waveguide structure, and confining the wave with respect to the z-orientation results in the following solution to the Helmholtz equation:  
                     E   =         E   ~     ⁡     (   z   )       ·     ⅇ     ⅈ   ⁡     (         k   x     ⁢   x     -     ω   ⁢           ⁢   t       )             ;               ∂   E       ∂   y       =   0                 (     Eq   .           ⁢   2     )             
 
 where E denotes the electric field, {tilde over (E)}(z) its z-dependence, k x  the x-component of the wavevector. {right arrow over (k)} 0  is the lights vacuum wavevector and n the materials refractive index. 
 
         [0024]     In this case the Helmholtz equation reduces to:  
                 (           ∂   2     ⁢   E       ∂     z   2         +       k   0   2     ·       n   2     ⁡     (   z   )           )     ⁢           ⁢       E   ~     ⁡     (   z   )         =         k   x   2     ⁡     (   z   )       ·         E   ~     ⁡     (   z   )       .               (     Eq   .           ⁢   3     )             
 
         [0025]     The Eigensolutions {tilde over (E)}(z) can be characterized by k x , or for convenience by a so called effective refractive index n eff  defined as:  
               n   eff     ≡       k   x              k   →     0                    (     Eq   .           ⁢   4     )             
 
         [0026]     In the previously described slab index guided waveguide structure, the equations above can be numerically solved resulting in a large number of Eigensolutions {tilde over (E)}(z). These Eigensolutions are called optical modes. Equations 3 and equation 4 also enable computation of the respective refractive indices n eff  and modal confinement factors Γ of these modes.  
         [0027]      FIG. 6  shows examples of optical modes. In  FIG. 6 , anti-resonant intensity patterns  612 ,  616 ,  620  are plotted across a cross section of a liquid sample  600  placed between glass plates  602 ,  604 . Typical indexes of refraction across the sample are provided along y axis  606 . A distance along sample  600  is provided on x axis  608 . An example first optical mode is shown by normalized intensity pattern  612 , a second optical mode is shown by normalized intensity pattern  616  and a third optical mode is shown by normalized intensity pattern  620 .  
         [0028]     A confinement factor Γ corresponds to the fraction of the light intensity confined in the waveguide core. For maximum interaction between target molecules in the sample and the light beam, the sample or analyte itself serves as the waveguide core. The core is surrounded by a cladding layer, typically the portion of the medium immediately adjacent to the sample. In future references to the cladding, the “cladding layer” shall refer to a portion of the medium that lies immediately on either side of the sample. The thickness of the cladding layer can be chosen within a wide range but the typical thickness is a several wavelengths of the light propagating in the medium.  
         [0029]     In the case of “anti-resonant” waveguides, herein defined to be a waveguide in which the core has a lower refractive index than the cladding layer, a number of optical modes with reasonably large confinement factors, up to and past 90%, can be found. These modes (or Eigensolutions) are characterized by effective refractive indices n eff  close to (typically slightly smaller than) the refractive index n of the core layer material. When the core thickness is large compared with the wavelength of propagating light, the n eff  of these modes of interest, approaches the refractive index of the core n. 
 
d core &gt;&gt;λ         n eff ≈n  (Eq. 5) 
 
         [0030]     Each Eigenmode can be excited by directing a beam of light at the waveguide at a specific angle of incidence. The angle of incidence corresponds to the effective refractive index n eff .  FIG. 2  shows one geometry of a slab waveguide  200  where the refractive index of the analyte  204  is n, the refractive index of substrate  208  and cover layer  212  are n′ and the refractive index of the surroundings  216  is n″. The optimum angle of incidence γ(n eff )  220  for the structure of  FIG. 2  may be derived as follows:  
                       sin   ⁡     (   φ   )       =         k   x     k     =       n   eff     n         ;                   sin   ⁡     (     φ   ′     )       =         n     n   ′       ⁢           ⁢     sin   ⁡     (   φ   )         =       n   eff       n   ′           ;                   cos   ⁡     (     γ   ′     )       =       cos   ⁡     (       90   o     -     φ   ′       )       =     sin   ⁡     (     φ   ′     )           ;                   γ   ′     =     arccos   ⁡     (       n   eff       n   ′       )         ;                   sin   ⁢           ⁢     γ   ″       =         n   ′       n   ″       ⁢           ⁢   sin   ⁢           ⁢     γ   ′         ;                   γ   ″     =     arcsin   (         n   ′       n   ″       ⁢           ⁢     arccos   ⁡     (       n   eff       n   ′       )         )       ;                 (     Eq   .           ⁢   6     )             
 
         [0031]     When analyte  204  thickness  220  (typically waveguide core diameter d core ≈10 . . . 100 μm) is large compared with the wavelength of the incident light (=0.3 . . . 2 μm) the approximation of (Eq. 5) is acceptable. Using the equation 4 approximation allows substitution of analyte refractive index n for effective refractive index n eff . The substitution results in an angle of incident that depends only on the refractive indices of the analyte, the core layer and the outside world:  
                 γ   ″     =     arcsin   (         n   ′       n   ″       ⁢           ⁢     arccos   ⁡     (     n     n   ′       )         )       ;           (     Eq   .           ⁢   7     )             
 
         [0032]     An example of a typical set of refractive indices might be an analyte of water with an n=1.34, a glass cladding layer with an n′=1.5 and an air or vacuum surrounding with n″=1. Using a glass cladding in an air surrounding for an example, the table in  FIG. 3  lists appropriate angles of incident γ″ in order to generate an ARGOW mode based on the sample or analyte refractive indexes.  
         [0033]      FIG. 4  plots the data shown in  FIG. 3 . As shown in curve  404  of  FIG. 4 , the angle of incidence increases with decreases in the sample refractive index. At sample refractive indices less than 1.15 (n&lt;1.15), it is very difficult to couple light into the waveguide facette and generate desired anti-resonant modes. Even for n&gt;1.15, the optimum angles for generating anti-resonant modes are still larger than what may be suitable for coupling large amounts of light into the sample. Large angles create difficulties because these angles force the use of smaller diameter beams to hit the facette at the large angles. Furthermore, the use of large angles substantially increases reflection losses.  
         [0034]      FIG. 5  shows an alternate structure of  FIG. 2  that minimizes losses caused by large incident angles. In  FIG. 5 , the entrance facette  504  is tilted. Reflections at the facette are minimized when incidence beam  508  perpendicularly enters entrance facette  504 . By adjusting the tilt angle γ′ such that a beam perpendicularly enters facette  504  and still strikes the cladding and sample interface  506  at an angle φ′ suitable to create an anti-resonant mode, reflections from the facette can be minimized while still generating the desired anti-resonant modes.  
         [0035]     Table 3 shows tilt angles γ′ for the structure of  FIG. 5  that corresponds to various analyte refractive indexes. By tilting the entrance facette  504 , generation of anti-resonant optical waves in analytes with refractive indices that range down to n=1 becomes possible. Generating anti-resonant optical waves in low index samples enables the use of gas and aerosol samples. Note that in this case the refractive index of the surrounding medium n″ might be chosen smaller than the refractive index of the medium n″ in order to also allow higher anti-resonant waveguide modes to be guided with reasonable leakage loss.  
         [0036]     Although two geometries and end facette designs have been provided in  FIG. 2  and  FIG. 5 , these geometries are provided for example only. It is possible to use other geometries and end facette designs to couple light into an anti-resonant propagating wave. Examples of other geometries include curved end facettes and cylindrical sample shapes rather than the angular end facettes and slab structures described. How to couple light into these other geometries in order to generate an anti-resonant wave in the sample can be determined by solving, either mathematically or numerically the general Helmholtz equation for these geometries. Such calculations are known to those of skill in the art. Thus the scope of the invention should not be limited to the particular example analyzed herein.  
         [0037]      FIG. 7  is a plot of the actual florescent intensity output from a sample as a function of a coupling angle of excitation light into the sample. As will be described, the experimentally generated results of  FIG. 7  match closely the theoretical expected coupling efficiencies at various angles of light input.  
         [0038]     In order to generate the graph of  FIG. 7 , excitation light from a single blue high powered LED was coupled at various angles into a side of a liquid film placed between two glass slides. The excitation light excited a fluorescein dye in the liquid film and resulted in fluorescence throughout the entire film area (an area of 25×75 mm 2 ). The resulting fluorescence was then measured.  
         [0039]     In the measurements, the measured fluorescence intensity per unit area was similar to that which has been obtained by perpendicularly (from the top) focusing the total excitation power from the LED onto a small spot (e.g. 3×3 mm 2 ) in the sample. The improved fluoresce results from a more efficient use of the excitation light by coupling the light into an ARGOW, in particular, guiding the light between the glass slides. This compares favorably to regular fluorescence detection when the excitation light is input perpendicular to the sample plane and results in transmission of most of the light. Using anti-resonant waveguide excitation the sample itself guides the excitation light between the glass slides providing a long interaction length between light and fluorescent molecules.  FIG. 7  plots the fluorescence intensity as a function of the coupling angle of the excitation light. The experimental value for optimum coupling efficiency is in excellent agreement with the theoretically predicted value.  
         [0040]      FIG. 6  shows the refractive index profile and the normalized mode intensity of 3 anti-resonant modes of a glass/water/glass anti-resonant waveguide. The anti-resonant modes are calculated assuming 480 nm wavelength light and a 15 μm thick liquid film between two glass slides. The predicted confinement factors for these modes within the liquid film are quite large. For the first three modes confinement factors of Γ=0.9, 0.8 and 0.55 respectively were obtained.  
         [0041]     Each mode can be specifically excited by adjusting the incidence angle φ (the angle  120  of  FIG. 1 ). The anti-resonant modes with the highest confinement factors can be excited at a glancing angle φ=46.50. Glass cladding thickness variations will usually not affect this angle because glass thicknesses are large compared with the wavelength of the propagating light (even if infrared light is used). Changes in liquid film thickness can change the optimum incidence angle; however, calculations show that the effect is very small. Reducing the thickness of the liquid film from 15 μm to 5 μm changes the optimum glancing angle φ from about 46.5° to only about 46.6°. Because within a window of about 0.5 degree, there is available a number of modes with reasonably high confinement factors, the slight change in optimum glancing angle does not present difficulties for the actual system.  
         [0042]     Changes in light wavelength also produces slight changes in optimum incidence angle. For example, substituting infrared light (˜1500 nm) for blue light (˜480 nm) only changes the optimum incidence angle by about 1.8°. The difference in the dispersion of glass and water has a larger influence compared to the different confinement conditions for the different wavelengths which have only small impact on incidence angle.  
         [0043]     The ability of the overall system to accommodate changes in light frequency and sample thickness makes it ideal for use in parallel analytic techniques. These are particularly useful in sophisticated systems where several different tests are to be conducted in parallel to determine the composition or presence of various target analytes.  FIG. 8  shows a top view of a sample  800  receiving several frequencies of light  804 ,  808 ,  812  at once. Each frequency of light could correspond to a different test to be performed on the sample.  
         [0044]     In the preceding discussion, analysis has been done on step index profiles such as that shown in  FIG. 9 . However, the generation of ARGOWs should not be limited to such index profiles.  FIGS. 9-14  show other index profiles where an index of refraction through the cladding and sample is plotted along a vertical axis and the distance along a cross section of the cladding and sample is plotted along a horizontal axis. As was previously explained, the thickness of the cladding layers is not critical and can be chosen within a wide range. Depending on the application and method of forming the cladding, the thickness of the cladding in one example embodiment is approximately 1 mm (e.g. if glass slides are used). In other cases the cladding may be chosen very thin, not more than three or four wavelengths of the propagating light.  
         [0045]      FIG. 10  shows a two step function where cladding region  1004  surrounding sample region  1008 . Cladding region  1004  includes two steps in the index of refraction. Systems where a coating is used to prevent sticking of the analyte or other parts of the sample to the sample chamber or medium walls might exhibit such an index of refraction profile. For example, a teflon coating used in cladding region  1004  to coat a glass medium might be a typical example. Teflon has an index of refraction of 1.38 between the glass medium  1012  index of refraction (about 1.44) and a water based sample index of refraction.  
         [0046]      FIG. 11  shows that the sample itself does not have to have a constant index of refraction.  FIG. 11  shows a parabolic index of refraction profile that may be exhibited by a fluid sample flowing at different speeds through a medium (e.g. causing phase separation of a mixture). Other monotonically increasing indexes of refraction (monotonically increasing from the edge of the sample through the cladding layer) are shown in  FIG. 12-14 . Monotonically increasing indexes of refraction through the cladding region minimizes reflections that may occur from the cladding layers.  
         [0047]     Returning to  FIG. 1 , once an ARGOW propagating wave is generated in the sample, the resulting interaction of the light with the sample contents may be analyzed for information. In one embodiment, a detector  140  of  FIG. 1  detects the light that propagates through the sample. In an alternate embodiment, a detector  144  of  FIG. 1  detects light that is scattered or refracted by the sample. Depending on the target (e.g. bioagent) to be detected and the particular detection technique to be used, detectors  140 ,  144  may include wavelength sensitive elements such as gratings, prisms, Bragg reflectors or resonators.  
         [0048]     Wavelength sensitive elements enable identification of signatures and specific biological or chemical agents. Detectors  140 ,  144  may also integrate the wavelength sensitive elements with conventional optics or micro-optics components including mirrors and lenses. In some embodiments, the detectors may include a means for converting the optical signal to an electrical signal. Such conversions may be achieved using a charge coupled device, a photosensor, or any of a variety of conversion devices. Once converted to an electrical signal, detector  140 ,  144  output can be analyzed using electric processors, such as microprocessors (not shown).  
         [0049]     Detector  140  of  FIG. 1  detects light transmitted by sample  116 . In one embodiment, the light transmitted by sample  116  is analyzed by processors coupled to the detector to determine the presence or absence of chemical, environmental or biological molecules in sample  116 . The output of detector  140  may also be used to analyze the characteristics of molecules in sample  116 . An example of using detectors to detect light transmitted by a sample and a processor to analyze the detector output is provided in U.S. Pat. No. 6,603,548 entitled “Biosensor” by Church et al. which is hereby incorporated by reference in its entirety.  
         [0050]     In an alternate embodiment, instead of detecting light that is transmitted, a second detection system such as detector array  144  may detect light that is scattered or otherwise output by sample  116 . Scattered light may be caused by reflection or refraction of light by molecules in sample  116 . Example scattering techniques include elastic and inelastic light scattering spectroscopy as described in Introduction to Biophotonics, by Paras N. Prasad ISBN 0-471-28770-9, Wiley-Interscience  2003 ) which is hereby incorporated by reference in its entirety.  
         [0051]     In still another embodiment, light output from sample  116  may be caused by fluorescence that results from binding of chemical elements in the sample to biological materials. The binding results in fluorescence when an excitation source, such as the anti-resonant light propagating in the sample is present. U.S. Pat. No. 6,577,780 by Lockhart entitled Cell Designs for Optical Sensors describes using antigens that attach to antibodies resulting in a structure that fluoresces in the presence of an evanescent field. U.S. Pat. No. 6,577,780 by Lockhart is hereby incorporated by reference in its entirety. By using anti-resonant waves propagating through the sample instead of evanescent fields, the sensitivity of the system can be improved.  
         [0052]     Besides the examples given, many other optical detection and sensing techniques may be used with sensors  140  and  144 . Those techniques include, but are not limited to single or multi-color light-induced intrinsic fluorescence or fluorescence from tagged molecules and applications derived from the manipulation of the fluorescent lights such as fluorescence lifetime imaging microscopy (FLIM), fluorescence resonance energy transfer (FRET), fluorescence correlation spectroscopy (FCS), etc., light scattering or vibrational spectroscopy (Raman, IR) or spectroscopic applications utilizing optical activity of chiral media such as circular dichroism (CD), among others. A more detailed description of various detection techniques utilizing photon interactions is provided in Chapter 4 of “ Introduction to Biophotonics ” by Paras N. Prasad, ISBN 0-471-28770-9, Wiley-Intersicence  2003 ) which is hereby incorporated by reference.  
         [0053]     Although optical detection techniques have been described, other methods of detecting the enhanced light-target interaction may be used. For example thermal detection techniques may be used. Predetermined light wavelengths may initiate a specific exothermic or endothermic chemical reaction which causes a temperature change. The detected temperature change indicates the presence of the reaction and thus the presence of compounds needed to create the reaction. Other example detection techniques include, but are not limited to, ARGOW induced photo ionization or photo fractionation. The photo ionization or photo fractionation generates charged particle which can be detected by known means such as a Coulter Counter.  
         [0054]     In order to speed up analysis of the samples, parallel processing of a sample may occur. Thus the techniques described are not mutually exclusive and may be used in conjunction or in parallel to yield rapid detailed analysis of molecules in the sample.  
         [0055]     A number of example geometries for a sample geometry and analysis techniques have been provided. However, the details provided have been provided as examples to facilitate understanding of the invention, and to provide sample calculations. However, the scope of the invention should not be limited to these geometries nor the particular analysis techniques described. Instead, the invention should only be limited by the claims, as originally presented and as they may be amended to encompass variations, alternatives, modifications, improvements, equivalents, and substantial equivalents of the embodiments and teachings disclosed herein, including those that are presently unforeseen or unappreciated, and that, for example, may arise from applicants/patentees and others.