Molecular interferometric imaging process and apparatus

A molecular interferometric imaging system for detecting an analyte in a sample, that includes an illumination source providing a beam of radiation; a pixel array for detecting radiation in an image plane; a biolayer designed to react to the analyte when it comes in contact with the sample; a substrate designed to convert phase modulation into intensity modulation which can be detected and imaged directly by the pixel array, the biolayer being on the substrate; a reference surface; an image switching means for switching between a first position for collecting a sample image of the biolayer, and a second position for collecting a reference image of the reference surface; and a processing means for producing a composite image using the sample image and the reference image for illumination normalization.

BACKGROUND AND SUMMARY

The present invention generally relates to obtaining direct images of biological molecules distributed on surfaces designed to convert molecular phase to reflected intensity. The reflected intensity is linearly proportional to protein density. Normally invisible biological molecules are made visible by the condition of in-line interferometric quadrature established by the substrate that transduces phase to intensity. The basic principle of operation is shearing in-line common-path interferometry in which a digital interferometric image of patterns of biological molecules is acquired and referenced to a reference surface by two image acquisitions. The technique has the advantage of high speed, high sensitivity and high-resolution optical detection of biological molecules.

The Quadraspec biological compact disc system described in U.S. Pat. No. 6,685,885 requires all serial data to be acquired on a single channel. However, it may also be advantageous in signal-to-noise (and hence sensitivity) applications to acquire many channels at the same time. With the present technique, a pixel array captures a plurality of pixel readings for each image. Moreover, while conventional laser scanning techniques are time-consuming when obtaining high-resolution scans of protein spots, as well as incompatible with disc wobble when scanning spots under high magnification, the present system minimizes these problems by acquiring numerous pixels in a single exposure. In addition, the focus of the microscope can be adjusted for each well, and even at a lower magnification, an entire “well” of spots can be seen in the field of view. As such, all the protein spots are acquired at the same time and under the same conditions.

Conventional laser scanning interferometric approaches are also incompatible with real-time kinetic captures from wet samples, particularly as flow-cell plumbing is impossible, except for the use of centrifugal force to move fluids. The present system can image through a flow-cell and system, thereby making it much more like surface plasmon resonance (“SPR”) systems.

However, there are several advantages of the present system over SPR techniques, particularly as the present technology is easier to implement and has a higher sensitivity than SPR technology. The present system uses a non-resonant quadrature condition, thus the operating condition is relatively insensitive to spacer thickness or wavelength. SPR systems, on the other hand, are sensitive to thicknesses and require tightly constrained wavelengths and angles. The goal of quadrature detection is to suppress noise rather than to boost signal which frees it from operating-point drift and allows it to be multiplexed over large areas. The present system also has minimal restrictions on operating wavelength or angle. The quadrature conditions can be achieved at either surface-normal or higher angles. Operation at 30° is achievable without loss in sensitivity. The optimal wavelength is also defined within a relatively broad range of tens of nanometers.

Because the operation of the present system is so robust, the noise is very low, thereby giving higher signal-to-noise ratios than SPR approaches. It is anticipated that molecular interferometric imaging will have a surface mass sensitivity of one to two orders of magnitude better than SPR. In addition, the thickness of the spacer that establishes the quadrature condition does not have to be tightly constrained. A 20% drift in thickness across a platform causes almost no change in operating sensitivity. Moreover, the loose requirements on spacer thickness and operating wavelength allows a large area to be manufactured that does not have significant sensitivity drift across the platform. This allows large-area multiplexing.

Additional features and advantages of the invention will become apparent to those skilled in the art upon consideration of the following detailed description of illustrated embodiments.

DETAILED DESCRIPTION OF EXEMPLARY EMBODIMENTS

This application is related to U.S. patent application Ser. No. 10/726,772, entitled “Adaptive Interferometric Multi-Analyte High-Speed Biosensor,” filed Dec. 3, 2003 (published on Aug. 26, 2004 as U.S. Pat. Pub. No. 2004/0166593), which is a continuation-in-part of U.S. Pat. No. 6,685,885, filed Dec. 17, 2001 and issued Feb. 3, 2004, the disclosures of which are all incorporated herein by this reference. This application is also related to U.S. patent application Ser. No. 11/345,462 entitled “Method and Apparatus for Phase Contrast Quadrature Interferometric Detection of an Immunoassay,” filed Feb. 1, 2006; and also U.S. patent application Ser. No. 11/345,477 entitled “Multiplexed Biological Analyzer Planar Array Apparatus and Methods,” filed Feb. 1, 2006; and also U.S. patent application Ser. No. 11/345,564, entitled “Laser Scanning Interferometric Surface Metrology,” filed Feb. 1, 2006; and also U.S. patent application Ser. No. 11/345,566, entitled “Differentially Encoded Biological Analyzer Planar Array Apparatus and Methods,” filed Feb. 1, 2006, the disclosures of which are all incorporated herein by this reference.

Prior to describing various embodiments of the invention the intended meaning of quadrature in the interferometric detection system(s) of the present invention is further explained. In some specific applications quadrature might be narrowly construed as what occurs in an interferometric system when a common optical “mode” is split into at least 2 “scattered” modes that differ in phase by about N*π/2 (N being an odd integer). However, as used in the present invention (and the previously referred to issued patents and/or pending applications of Nolte et al.) an interferometric system is in quadrature when at least one mode “interacts” with a target molecule and at least one of the other modes does not, where these modes differ in phase by about N*π/2 (N being an odd integer). This definition of quadrature is also applicable to interferometric systems in which the “other mode(s),” referring to other reference waves or beams, interact with a different molecule. The interferometric system may be considered to be substantially in the quadrature condition if the phase difference is π/2 (or N*π/2, wherein N is an odd integer) plus or minus approximately twenty or thirty percent. The phrase “in-phase” is intended to describe in-phase constructive interference, and “out of phase” is intended to describe substantially 180-degree-out-of-phase destructive interference. This is to distinguish these conditions, for both of which the field amplitudes add directly, from the condition of being “in phase quadrature” that describes a relative phase of an odd number of π/2.

FIG. 1shows a basic schematic of an embodiment10of a molecular interferometric imaging system viewing a sample30. The sample30is placed on a stage20of the system10. The sample30is not shown to scale inFIG. 1to ease viewing and description. The sample30includes a biolayer32that is located on a substrate34. In some embodiments a spacer36is located between the biolayer32and the substrate36.FIG. 2shows a top-view of a portion of the sample30to be characterized by the system10. The sample30includes the biolayer32that is to be analyzed by the system10and a land33that acts as a reference surface. When there is a spacer36, the spacer36can act as the land33; and when the biolayer32is applied directly to the substrate34, the substrate34can act as the land33. As an example, one embodiment of the sample30can include silicon as the substrate34with an oxide layer as the spacer36, and the oxide thickness selected to put the system in an in-line quadrature condition (typically 120 nm). In this embodiment, the biolayer32can include a plurality of spots containing a capture antibody deposited on the spacer36, and when a specimen containing the analyte is applied to the sample30, the analyte is captured by the antibody in the biolayer32.

A radiation beam from an illumination source12passes through illumination filters14and into a beam splitter16which directs the incident beam through an objective lens18on onto the sample30. The reflected beam from the sample30passes back through the objective lens18and the beam splitter16. The reflected beam then passes through detection filters22and onto a pixel array camera24. The pixel array camera24is connected to a computer26which stores the reflected image of the sample30. If the light source12is a laser tuned to the appropriate quadrature condition for the spacer36of the sample30, then either or both of the filters14and22are optional. However, if the light source12is a broad-band source (such as an incandescent light or a halogen lamp) then at least the illumination filters14would be necessary.

A differential composite image is obtained by acquiring an image of the biolayer32and an image of the land33, and then differencing the two images. The adjacent land acts as the reference surface for illumination normalization.

The substrate34and spacer36are configured to convert phase modulation to reflected intensity so that it can be detected and imaged directly by the pixel array24. This phase-to-intensity conversion takes place through in-line quadrature interferometry which is described in U.S. patent application Ser. No. 11/675,359, entitled “In-Line Quadrature and Anti-Reflection Enhanced Phase Quadrature Interferometric Detection,” which was filed on Feb. 15, 2007, and is hereby incorporated herein by reference. The light reflected from the biological molecules has a quadrature condition relative to light reflected from an in-line reference surface. This converts the phase modulation caused by the light interacting with the molecular dipoles to interfere in the far-field with the reference light to create the intensity modulation that is proportional to the phase modulation. The equation describing this process is:
ΔI=2√{square root over (IrefIsignal)}Δφ  (1)
where the phase modulation caused by the molecules is:

Δ⁢⁢ϕ=4⁢⁢πλ⁢(nb-nm)⁢d(2)
where d is the effective thickness of the biolayer, nbis the refractive index of the biolayer, and nmis the refractive index of the surrounding medium. From a molecular point of view there is not a biolayer but rather a scattered distribution of molecules on the surface. Then the modulated phase is:

Δ⁢⁢ϕ=4⁢⁢πλ⁢(nb-nm)⁢2⁢⁢π⁢⁢rm33⁢rs2(3)
where rmis the molecular radius of gyration, and rsis the average molecular separation on the surface. The refractive index in this case is the refractive index associated with the individual molecules.

The intensity modulation ΔI caused by the biolayer32is often small, in the range of a few percent of the total intensity. Therefore, spatial variations in the illumination can be nearly as large as the protein signal. The land33can be used to normalize this background variation and make the protein structures clear. The land33has substantially no protein on it and acts as a normalization surface.

FIG. 3shows one example of normalizing the background variation with a differential image. In this example, a portion of a biological compact disc40is shown that has a protein spot44, a lower adjacent land43, and an upper adjacent land45. A first image41is taken of the protein spot44using the imaging system10which includes the lower adjacent land43. The disc40is shifted in the field of view of the system10and a second image42is taken of the protein spot44using the imaging system10which includes the upper adjacent land45. The normalization procedure uses the two images41and42. A composite differential image47is computed on a pixel-by-pixel basis as:

IDiff=2⁢(IA-IB)(IA+IB)(4)
where IAis a pixel value from the second image42and where IBis the corresponding pixel value from the first image41. The composite differential image47includes two versions of the protein spot44: a negative difference version46(the land45of image42minus the protein spot44of image41) and a positive difference version48(the protein spot44of image42minus the land43of image41). The spot information in the image pair46,48is the same, but the background is different. The difference compensates for the spatial variations in the illumination, and can be used to produce an image of the protein spot44. It is preferable to use a combination of the image pair46,48in subsequent data analysis to provide for an average of the protein spot44, but either46or48can be used alone. The magnitude of the spot height in the difference images46,48are proportional to the amount of protein present in the spot44.

The land adjacent to a biological spot should be flat and clean to provide a good normalization surface. The land on both sides of a spot can be used for a single shift in one direction, but multiple shifts could also be used to try to balance directional systematics on the disc or wafer. For a single difference image of a spot and adjacent land, no registration is needed since the land is generally homogeneous. However, when taking many difference images and averaging them, then registration of the multiple difference images is preferred. Algorithms and software packages are commercially available to register images in microscopy. The normalization surface is not an interferometric reference surface. The interferometric reference surface is in-line, not lateral. The normalization surface takes effect after the reference-surface has already converted phase to intensity. The normalization removes spatial variations in the illumination.

The following description provides greater detail on the eight basic elements of the embodiment of the in-line molecular interferometric imaging system10shown inFIG. 1. There are many possible alternative embodiments for each of these elements.

The illumination source12could be any one of numerous illumination sources known in the art (e.g., incandescent; halogen; LED). The light source12can be coherent or incoherent, and single color or multiple color. High photon flux is provided by an LED or a superluminescent diode, but a more basic embodiment would be a white light source that is filtered.

The illumination filters14can be any one of numerous illumination filters known in the art (e.g., color; polarization; Fourier and image masks). Illumination filters can convert white light into single color or multiple color light. Multiple colors could be selected to coincide with the two opposite in-line quadrature conditions set by the substrate. By matching the detection filters22to the illumination filters14differential color composite images can be composed to isolate protein relative to scattered light or absorbed light. If the filters are in the UV, then protein or DNA spectroscopy becomes possible because of the optical transitions in the UV. The combination of in-phase with quadrature information in interferometry provides a complete picture of the material optical transitions (refractive index and absorption).

The illumination filters14can also be used to provide Fourier filtering of the beam from the illumination source12. This could be used, for example, to present illumination that selects phase contrast on the disc or plate. If the disc or plate at a selected wavelength is in the anti-node condition (maximum field at the substrate surface), then phase contrast images can be acquired at that wavelength. If multiple wavelengths are used, then the phase contrast image can be combined with the quadrature images obtained at other wavelengths.

The illumination filters14can also be used to provide polarization of the light from the source12which can be informative if the molecules are oriented on the substrate.

The objective lens18could be any one of numerous objective lens systems known in the art (e.g., coverslip corrected; coverslip uncorrected; long working distance). The objective lens18is the imaging element in the system. It can be configured to work with or without coverslips. In the case of microfluidic systems, the objective should have a working distance that is compatible with the coverings over the microfluidic systems. In the case of conventional 96-well plate, the objective lens should have a long working distance. This can sometimes reduce the magnification, but a large numerical aperture (NA) system can retain high magnification even for long working distance.

The substrate34could be composed of numerous materials (e.g., quadrature conditions: 120 nm oxide on silicon, 100 nm oxide on silicon, 80 nm oxide on silicon; SiN on silicon, anti-reflective (AR) coatings on glass, dielectric stacks on glass; Substrate formats: Quadraspec biological compact disc substrates, 96, 384, 1536-well plate substrates; and microfluidics). The substrate converts phase modulation to intensity modulation by interference effects set up by the substrate structure. This can be accomplished by a wide range of structures that have multiple layers ranging from a single layer to possibly hundreds.

One embodiment uses a substrate of thermal oxide grown on silicon. Thicknesses of 120 nm and 80 nm provide opposite quadrature at a wavelength of 635 nm. A thickness of 100 nm provides for phase-contrast imaging if a Fourier filter is used in the illumination and detection Fourier planes. Shifting of quadratures is also possible by choice of wavelength. Therefore, any multilayer substrate that produces partial reflections that may differ in phase by substantially π/2 will produce the appropriate phase-to-intensity conversion that is needed. An antireflection structure tuned near quadrature, or more generally dielectric stacks, can be used.

Substrate formats can be highly varied. A Quadraspec biological compact disc system format is possible, with direct imaging of protein spots in the wells. Or conventional 96-well plates can be used with protein spots printed onto an optically flat bottom that has been coated with dielectric layers that provide the quadrature condition. The substrates also can consist of microfluidic systems that deliver sample to the protein spots in real time. The molecular interferometric imaging process works when the system is immersed in water or biological fluids. The effects of the fluid matrix are cancelled by comparing the mass increase of a specific spot to land and also to non-specific spots. Therefore, the near-surface sensitivity of SPR and BioLayer Interferometry (“BLI”) are not necessary because the full-field image allows reference values to be acquired simultaneously by which the matrix effects are subtracted.

The biolayers32can be structured in any of numerous ways known in the art (e.g., spots; ridges; checkerboard). The biological molecules can be patterned on the disc in many possible configurations. The most common are spots, ridges and checkerboards. Periodic ridges enable Fourier image processing techniques in one-dimension, and checkerboard patterns allow Fourier image analysis in two-dimensions. Alternating ridges of specific and non-specific molecules constitute an embodiment of differential encoding.

The stage20can also be structed in various embodiments (e.g., rotation; translation; dither). The stage motion enables normalization. Shifts of the stage20can take many formats that are chosen to be optimal for the different substrate formats. A rotation stage is perhaps most compatible with compact disc systems, while X-Y translation is most compatible with 96-well plates.

Dithering, which is another option for stage motion, is a periodic shifting back and forth. This might be used during kinetic binding experiments to better track the added mass. Dithering combined with synchronized pixel array image acquisition can be considered to be a type of pixel array lock-in approach.

The detection filters22can be any one of numerous detection filters known in the art (e.g., color; polarization; Fourier and image masks; phase contrast). The detection filters are placed before the pixel array24. They may reside on image planes or Fourier planes. If the detection filters22are in the Fourier plane, they may include phase and amplitude masks. These masks can perform important functions such as phase contrast imaging. In this case, a π/2 mask on the Fourier plane can produce a phase contrast image on the pixel array24.

Other detection filters that may reside on or off the image or Fourier planes would be wavelength and polarization filters that are matched to the respective illumination filters. These can allow multi-wavelength operation, for instance, or single wavelength operation. It would also be possible to place dichroic beamsplitters before the image detection to separate spatially images of different colors. Multiple dichroic beamsplitters would enable multiple different color images that could all be detected individually with individual cameras. Alternatively, a rotating filter wheel could sequentially switch color filters synchronized with the camera acquisition. This would enable multiple wavelength images to be acquired using only a single camera.

The pixel array24can be any of numerous image detectors known in the art (e.g., CCD; complementary metal oxide semiconductor (“CMOS”); pixel arrays; red, green and blue (“RGB”); megapixel; synchronization). Many formats are possible for the image detection. In one embodiment, the image detection is through a CCD or CMOS or pixel array device. Any device that has separate spatial channels to detect light at multiple locations on the image plane would be applicable. A pixel format having a high pixel density can be used, resulting in, for example, from 1 megapixel images up to 15 megapixel or greater images. The “dead” space between pixels can be small. The pitch between pixels can also be small to reduce the requirements for high magnification.

Synchronization of the camera with an external trigger can be used to capture sequential images as some property is changed in the detection mode. For instance, synchronizing the camera with switching color filters, or synchronizing the camera with platform displacement or dither.

The camera24can be monochrome, using multiple color filters to acquire multicolor data—or the camera24can be a 3-color-channel array that detects red, green and blue individually. The oxide thickness of the substrate34can be changed to match the two quadrature conditions of the substrate to the red and blue channels on the camera, with the green channel representing the null condition in-between. This would allow full detection sensitivity for the red and blue, and enable full differential sensitivity for the green channel.

Advantages and improvements of the methods of the present invention are demonstrated in the following examples. The examples are illustrative only and are not intended to limit or preclude other embodiments of the invention.

Images of proteins on thermal oxide on silicon in the quadrature condition using a color filter on a conventional microscope have been acquired. These images were near the quadrature condition. First and second images were acquired with the platform displaced in-between acquisitions. The differential composite exhibited high sensitivity to protein and low sensitivity to background effects.

A direct image under 40× magnification of a protein is shown inFIG. 4as a grayscale. The protein was IgG printed on 120 nm oxide on silicon. The spot was printed using a Scienion printer with approximately 100 picoliters of liquid volume. The substrate is an in-line quadrature Quadraspec biological compact disc with functionalized surface chemistry. The full range of the color bar is approximately 4 nanometers. The image was acquired with a 12-bit CCD camera. The intensity modulation caused by the protein is generally a few percent. The protein height is approximately a nanometer. The brighter tail on the lower left is the wash-off tail which is several nanometers high. The variability in the image is mostly caused by inhomogeneous illumination and also by dust in the optics. The background variability is smaller than, but still comparable to, the magnitude of the protein signal.

The protein spot after execution of the platform shift and the calculation of the differential composite image is shown inFIG. 5. Most of the background variability is removed by the normalization procedure of Equation 4. The protein heights in the image are several nanometers.

FIG. 6shows a high resolution protein image of a uniformly printed 120 micron diameter protein spot. The protein height is approximately 1-2 nanometers. The full range scale is −2 nm to 2 nm.

A photo gallery of many differential composite images for many types of spot morphologies is shown inFIG. 7. The protein spots are all approximately 100 microns in diameter. The images were acquired from many different wafers using many different chemistries. The protein spot heights in all cases are from about half a nanometer to several nanometers.

FIG. 8shows a pseudo three-dimensional image of surface morphology of a printed spot with a pronounced outer ring. The high circular rim is caused by preferential protein deposition as the spot evaporates. The ring height is approximately 0.7 nm.

Repeatability experiments were performed in which pixel variability was measured as a function of the number of frames that were acquired and averaged. The height repeatability (standard deviation) is plotted inFIG. 9as a function of the number of acquisitions for both 40× and 4× objective magnification. The standard deviation of the differenced images decreases inversely with the square root of the number of acquisitions up to approximately 1024 images. For more images than this, long-term drift begins to dominate, representing 1/f noise. The minimum standard deviation was 20 picometers per pixel. The fluctuations for 4× are not much higher. The pixel size in the case of 40× is 0.5 microns, and for 4× is 5 microns. When the platform shift is used to normalize the pixel values, the standard deviation increases by about a factor of 3 to 60 picometers. The equivalent number of IgG molecules that this corresponds to is approximately 100 molecules. This is illustrated inFIG. 10, in which the pixel-to-pixel fluctuations are at the level of approximately 100 molecules on the edge of the printed protein spot.

Embodiments of the present invention can also operate under water. The protein differential composite image is shown inFIG. 11under the two conditions of dry and wet. The protein was under a glass coverslip. Water was introduced between the slip and the disc surface. The protein is still visible, with approximately a factor of 3 reduction in the signal intensity. At the same time, dust and other background noise also decreased by about a factor of 3 keeping the signal-to-noise ratio approximately constant. Detection sensitivity is set by the signal-to-noise ratio. Therefore, operation of the molecular interferometric imaging under water is feasible. This enables kinetic capture experiments in which binding could be tracked in real time. To detect real time binding, either the platform is dithered with synchronized image acquisition, or else successive images would be differenced and normalized to detect the binding.

These data also demonstrate the ability to use the known refractive index of water to measure the refractive index of the protein. The protein signal under water is still a positive signal. This requires that the refractive index of the protein be larger than the refractive index of water. The refractive index of the protein is calculated by solving the equation:

n-1n-1.33=Δ⁢⁢IdryΔ⁢⁢Iwet(6)
The refractive index measured for the protein layer in this way is approximately n=1.5.

The capability to directly image through water using molecular interferometric imaging enables real-time binding experiments. A simplified diagram of the experimental arrangement is shown inFIG. 12. The optical arrangement is similar to that ofFIG. 1, and for clarity only the objective lens118is shown. However, now a sample130is to be characterized that has an active flow of liquid in the direction of arrow133over antibody spots134on a support layer138. The lower white part of the spots134represents the antibody and the upper dark part of the spots134represents the captured analyte. The support layer138can be the top surface of a substrate or oxide layer.

The imaging is performed through a top glass coverslip136and through the liquid. The liquid is a potential source of background signal because it contains the analytes that are being captured out of solution by the antibody spots134. However, the captured mass is enhanced relative to the background analyte by the anti-node condition that is at the surface of the support layer138. The field strength is twice as high at the protein spot134compared to the average over the liquid volume. Furthermore, in molecular interferometric imaging, continual image-pair acquisition is taking place that compares the mass over the spot134to the mass captured by adjacent land138. The overlying fluid remains the same in both images and hence is subtracted. Another approach to ameliorate the background in the liquid could be to periodically flush the system with buffer, during which image pairs are acquired. In this case, the overlying liquid is free of the analyte. A combination of both approaches might give the best balance in terms of sensitivity to bound analyte.

The in-line approach also makes it possible to interrogate the proteins without going through the liquid. One embodiment of this is shown inFIG. 13. The imaging geometry is the same as that ofFIG. 12, but now the light passes through the upper glass slide238that carries the antibody spots and the bound analyte234. The in-line quadrature condition is established by appropriate dielectric layers232. A top reflection can be the reference wave, and the reflection off the protein-carrying surface can be the signal wave. The addition of mass on the protein spot234alters the phase of the reflected light that is detected through the in-line quadrature interfeormetry at the camera. This approach has the advantage that the light does not pass through the liquid layer containing the analyte.

FIG. 14is a graph of experimental data from real-time binding showing an increase in the spot height for the specific spots, and a decrease in the spot height for the non-specific reference spots. The decrease in spot height for the non-specific reference spots is due to wash-off. The time between frames is 40 seconds. The analyte concentration was 40 ug/ml. During the collection of 28 images (1,080 seconds or 18 minutes) the relative binding on the specific spots increased from approximately 0.0001 to 0.0020 while the relative binding on the non-specific spots decreased from approximately 0.0001 to −0.0004.

The sensitivity of the molecular interferometric imaging approach can be enhanced by increasing the data acquisition rate, especially with respect to disc translation from spot to spot. A simple embodiment is to utilize a slowly spinning disc or translating plate as shown inFIG. 15. The pixel array of the system has a field of view250and the sample has a protein spot254. In part (A) ofFIG. 15, the camera shutter is opened and closed on a time scale that is short relative to the motion of the protein spot254. This results in a multiple exposure of the same protein spot254as it travels across the field of view250. In the case shown inFIG. 15(A), an image with six multiple exposures of the protein spot are captured at different positions in the field of view250. For normalization purposes, the same operation can be done for the adjacent land as shown in part (B) ofFIG. 15. In the case shown inFIG. 15(B), an image with six multiple exposures of the land, which is substantially clean and uniform, is captured at different positions in the field of view250. Different magnifications can be used to capture more or less protein spots in the field of view.

Each exposure is of the same protein spot254, providing averaged detection statistics, and the multiple exposure image of the protein spot is referenced to a multiple exposure image of different parts of the land, providing averaging over the land topology which can be a limiting factor in single-pair molecular interferometric imaging. This approach would not necessarily need a larger memory, because the shutter could open and shut many times prior to reading out the digital image. The data in this case is a repeated exposure. Many images of the spot and land are acquired in only one multiple exposure image.

Another embodiment that utilizes a continuously spinning disc or translating plate, and takes the last embodiment to its limiting behavior, is time-lapse exposure while the disc is continuously spinning or plate is continuously moving as shown inFIG. 16. The pixel array of the system has a field of view260and the sample has a protein spot264. In part (A) ofFIG. 16, the camera shutter is opened when the spot264is at position266at the edge of the field of view260. The shutter remains open while the protein spot264crosses the field of view260and finally closes when the spot264reaches position268at the other side of the field of view260. For normalization purposes, the shutter remains open for the same time period as the adjacent land as shown in part (B) ofFIG. 16traverses the field of view260in the direction of arrow262.

While the spot264is moving across the field of view260, the pixel array records an average intensity that is a combination of the protein spot264and the adjacent land on the trailing side and appears as a “swath”. The average intensity over the swath provides a means for averaging spatial illumination drift, which can be a limiting factor in molecular interferometric imaging.

Rapid acquisition of the reference surface improves the results of the molecular interferometric imaging system. The idea of disc translation, taken to the limiting case of having the disc spin, is one type of approach, but other approaches are also possible. The purpose of the reference surface in molecular interferometric imaging is to provide intensity normalization. This is a relatively easy requirement that is much simpler than the reference surfaces that are required for non-common-path interferometers in which the reference surface distance must be stabilized to within a small fraction of a wavelength. Therefore, all that is needed is a means of introducing the reference surface as a separate image to be acquired.

One embodiment is to have a high-speed mirror that rapidly switches back and forth between a protein spot and a physically separated reference surface. The image acquisition by the camera can be synchronized with the mirror motion.

Another embodiment is to have a mirror on a spinning disc that is between the protein layer and the objective lens. The disc would have a clear aperture to image the protein spot, then the mirror moves between the lens and the protein spot. A reference image is acquired at the moment the mirror is between the lens and the protein spot. A similar embodiment uses an oscillating galvonometer that switches the image between the reference surface and the protein spot.

FIG. 17shows an embodiment that requires no moving parts. This embodiment uses beam polarization control by passing the beam through an electro-optic modulator in a half-wave configuration. The polarization is switched back and forth between orthogonal polarizations. The polarized beam passes through the objective lens278and enters the polarizing beam splitter272which redirects the illuminating beam back and forth, depending on its vertical and horizontal polarization either to the protein spot274, or to the reference surface276. Image acquisition would be synchronized with the polarization.