US 20090269244 A1
An apparatus and method for detection of peak wavelength values of colorimetric resonant optical biosensors using tunable filters and tunable lasers is provided. Biomolecular interactions may be detected on a biosensor by directing collimated white light towards a surface of the biosensor. Molecular binding on the surface of the biosensor is indicated in a shift in the peak wavelength value of the reflected or transmitted light from the biosensor, while an increase in the wavelength corresponds to an increase in molecular absorption. A tunable laser light source may generate the collimated white light and a tunable filter may receive the reflected or transmitted light and pass the light to a photodiode sensor. The photodiode sensor then quantifies an amount of the light reflected or transmitted through the tunable filter as a function of the tuning voltage of the tunable filter.
1. A measuring apparatus for detecting a biochemical interaction on a biosensor, the measuring apparatus comprising: a tunable laser light source for generating light having a tunable laser wavelength and for directing the light towards a surface of the biosensor, wherein the tunable laser wavelength is tuned by adjusting a tuning voltage; and a photodiode detector operable to receive light transmitted through the biosensor and to detect a broad band of wavelengths encompassing a wavelength passband, wherein an output of the photodiode detector quantifies an amount of the light transmitted through the biosensor as a function of the tuning voltage.
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The present invention generally relates to a method and system for detecting biomolecular interactions, and more particularly to the detection of calorimetric resonant optical biosensor peak wavelength values using tunable filters and tunable lasers.
There have recently been rapid advances in molecular biology. For example, one significant accomplishment was the completion of the sequencing of the human genome. Presently, another challenge is to understand how the many protein targets encoded by the DNA interact with other proteins, small molecule pharmaceutical candidates, and a large host of enzymes and inhibitors.
To determine such interactions, assays may be completed using biosensors. Biosensors have been developed to detect a variety of biomolecular interactions including antibody-antigen, hormone-receptor, and enzyme-substrate interactions. Biosensors include a highly specific recognition element and a transducer that converts a molecular recognition event into a quantifiable signal. Signal transduction has been accomplished by many methods including fluorescence, interferometry, and gravimetry, for example.
For the majority of assays for genomics, proteomics, pharmaceutical compound screening, and clinical diagnostic applications completed using biosensors, fluorescent or calorimetric chemical labels are commonly attached to the molecules under study so these molecules may be readily visualized. However, attachment of a label substantially increases assay complexity and possibly alters functionality of the molecules through conformational modification or epitope blocking.
Detection of biomolecular interactions may be accomplished using label free detection techniques. For example, label free detection techniques include measuring changes in mass, changes in microwave transmission line characteristics, microcantilever deflection, or optical density detection.
Unfortunately, however, the widespread commercial acceptance of label free biosensor technologies has been limited by the lack of ability to provide high detection sensitivity in a format that is inexpensive to manufacture and package. For example, label free detection biosensors fabricated upon semiconductor or glass wafers are costly to produce and package if the sensor area is large enough to contain a number of parallel assays. Similarly, providing for electrical connections to individual biosensors in an array of biosensors poses difficult challenges in packaging, and compatibility with fluid exposure. In addition, many label free biosensor transduction methods (e.g., surface plasmon resonance (“SPR”), output grating coupling, ellipsometry, evanescent wave detection, and reflectance interference spectroscopy (“RIS”)) are rather slow and can be very expensive. Furthermore, some of these label free detection methods are limited to dry samples, and thus are not suited for samples immersed in fluid. This substantially limits applications for these biosensors.
As the industry evolves from the detection of genes towards identification of protein interactions for example, the emphasis shifts from simply identifying structure to identifying both structure and function. Also, there are many more proteins than genes, which increases difficulty in the identification process. Indeed, the use of labels, such as calorimetric or fluorescent tags, for genomic investigations is known to adversely affect the structure and function of some proteins. In addition, the limitations of existing label free technologies present too many obstacles to overcome. Therefore, there is a need for a new sensing mechanism that can monitor assays without the use of labels, is amenable to ultra high throughput, and can lower the cost per assay performed.
In an exemplary embodiment of the present invention, a measuring apparatus for detecting a biochemical interaction on a biosensor is provided. The measuring apparatus comprises a light source that generates collimated white light and directs the collimated white light towards a surface of the biosensor. The apparatus also includes a tunable filter that receives light transmitted through the biosensor and passes a narrow band of light that has wavelengths substantially centered at a passband wavelength and reflects substantially all other wavelengths. The passband wavelength may be adjusted according to a tuning voltage of the tunable filter.
In another embodiment, the measuring apparatus includes a tunable laser light source and a photodiode detector. The tunable laser light source generates light that has a tunable laser wavelength and directs the light towards a surface of the biosensor. The tunable laser wavelength is tuned by adjusting a tuning voltage. The photodiode detector receives light transmitted through the biosensor and detects a broad band of wavelengths encompassing a wavelength passband. An output of the photodiode detector quantifies an amount of the light transmitted through the biosensor as a function of the tuning voltage.
In still another embodiment, an apparatus for detecting a maximum wavelength of reflected light is provided. The apparatus comprises a light source that generates collimated white light and a power splitter that directs the collimated white light towards a surface of a sensor. The apparatus further comprises a tunable detector that receives light reflected by the sensor and measures a radiation spectrum of the light reflected from the sensor. The tunable detector filters a narrow band of light that has wavelengths substantially centered at a passband wavelength from the light reflected by the sensor and this passband wavelength is adjusted according to a tuning voltage of the tunable detector.
In yet another embodiment, the apparatus for detecting a maximum wavelength of reflected light comprises a tunable laser light source, an optical circulator, and a photodiode detector. The tunable laser light source generates a tunable single wavelength of light and can be tuned by adjusting a tuning voltage. The optical circulator directs the tunable single wavelength of light towards a surface of a sensor. In turn, the photodiode detector receives light reflected from the biosensor and detects a broad band of wavelengths encompassing a wavelength passband. An output of the photodiode detector quantifies an amount of the light reflected from the biosensor as a function of the tuning voltage.
These as well as other features and advantages will become apparent to those of ordinary skill in the art by reading the following detailed description, with appropriate reference to the accompanying drawings.
Exemplary embodiments of the present invention are described herein with reference to the drawings, in which:
The present invention generally relates to a method and system for detecting biomolecular interactions. These biomolecular interactions may occur on a surface of a biosensor. In one embodiment, the biosensor is a calorimetric resonant optical biosensor embedded into a surface of a microarray slide, microtiter plate or other device.
The colorimetric resonant optical biosensor allows biochemical interactions to be measured on the sensor's surface without the use of fluorescent tags or colorimetric labels. Thus, the calorimetric resonant optical biosensor provides for label free detection of these interactions. For more information concerning label-free detection, reference is made to the following commonly owned U.S. Patent Applications, which are all fully incorporated herein by reference: “Method And Instrument For Detecting Biomolecular Interactions,” as described in U.S. patent application Ser. No. 10/180,374, filed on Jun. 26, 2002; “Method And Apparatus For Detecting Biomolecular Interactions,” as described in U.S. patent application Ser. No. 10/180,647, filed on Jun. 26, 2002; and “Label-Free Methods For Performing Assays Using A Colorimetric Resonant Optical Biosensor,” as described in U.S. patent application Ser. No. 10/237,641, filed on Sep. 9, 2002.
Generally, to detect a biochemical interaction on a biosensor, an optical structure on a surface of the calorimetric resonant optical biosensor is illuminated with collimated white light. The optical structure is designed to reflect only a narrow band of wavelengths, which is described as a wavelength “peak.” The peak wavelength value (“PWV”) changes when biological material is deposited or removed from the biosensor surface.
According to an exemplary embodiment of the present invention, a tunable laser may illuminate the biosensor. Biochemical interactions may then be detected using a tunable detector or a tunable filter. A spectrum interpretation can then be transduced to a dynamic protein characteristic that is attached to the biosensor.
Referring now to the figures,
Illuminating fiber 106 directs an area of collimated light, via the collimating lens 112, on surface 124, which is preferably a bottom surface of biosensor 118. Illuminating fiber 106 is bundled along with detecting fiber 114 in a unitary optical fiber probe. Detecting fiber 114 is utilized to collect light reflected from biosensor 118. Detecting fiber 114 channels the reflected light to spectrometer 116, which is preferably a wavelength spectrometer. Spectrometer 116 processes the reflected light to determine characteristics of the reflected light.
In particular, measuring apparatus 102 collects light reflected from the illuminated biosensor surface 124 and scans detection head 110 across the biosensor surface. White light source 104, for example, illuminates about a ˜1 millimeter (mm) diameter region of surface 124 through about a 400 micrometer (μm) diameter fiber optic and collimating lens 112 at nominally normal incidence through the bottom of a microtiter plate, for example. Such a microtiter plate could have a standard 96-well, 384-well, or 1526-well microtiter plate format, but with a biosensor attached to the bottom, for example.
When surface 124 is illuminated with collimated white light generated by white light source 104, surface 124 reflects only a narrow band of wavelengths. This narrow band of wavelengths is referred to as a wavelength peak. The “peak wavelength value” (PWV) changes when biological material is deposited or removed from surface 124. That is, the PWV changes when a biological material is deposited between structures 120 and 122. Based on light reflected off surface 124, measuring apparatus 102 can measure certain values, such as the PWV's of a plurality of locations within biosensor 118. Collected reflected light is gathered into spectrometer 116 for processing. For example, spectrometer may then generate a PWV for the biosensor 118.
Optical device 100 provides for determining the PWV of a colorimetric resonant optical biosensor by simultaneous illumination of a biosensor at normal incidence with a broad band of wavelengths, and simultaneous detection of a broad band of reflected or transmitted wavelengths. The illuminating light should be substantially collimated (e.g., parallel) in order to produce a narrow resonance band on the reflected or transmitted radiation spectrum. The illuminating radiation can be provided by a white light source, such as a light bulb, or a light emitting diode (LED) with a radiating spectrum that encompasses the resonance wavelength, for example.
When a resonance wavelength is within the band of illuminating wavelengths, a high reflected intensity (or low transmitted intensity) is obtained only at the resonance wavelength. When the reflected (or transmitted) light is gathered into a spectrometer that accepts light in a range of wavelengths containing the resonance wavelength, the spectrometer can measure the reflected (or transmitted) intensity as a function of wavelength. The extent of the shift of reflected wavelength can be used to determine an amount of bound molecules in the sample and the chemical affinity between receptor molecules and an analyte. For example, when molecules are attached to the surface of the biosensor, the reflected wavelength is shifted due to the change of the optical path of light within the grating. Consequently, by linking receptor molecules to the grating surface, complimentary binding molecules (e.g., analytes) can be detected without the use of any kind of fluorescent probe or particle label.
In one embodiment, biosensor 118, when illuminated with white light, is designed to reflect only a single wavelength. Subwavelength structured surfaces (SWS) are generally an unconventional type of diffractive optic that can mimic the effect of thin-film coatings. These structures may be used as antireflection filters, polarizing filters, and narrowband filters. See, e.g. S. Peng and G. M. Morris, “Resonant scattering from two-dimensional gratings,” J. Opt. Soc. Am. A, Vol. 13, No. 5, p. 993, May 1996; R. Magnusson, and S. S. Want, “New principle for optical filters,” Appl. Phys. Lett., 61, No. 9, p. 1022, August, 1992; and S. Peng and G. Morris, “Experimental demonstration of resonant anomalies in diffraction from two-dimensional gratings,” Optics Letters, Vol. 21, No. 8, p. 549, April, 1996, the full disclosures of which are herein incorporated by reference.
An SWS structure contains a surface-relief grating in which the grating period is small compared to the wavelength of incident light. In this manner, no diffractive orders other than the reflected and transmitted zeroth orders are allowed to propagate. To design a SWS surface narrowband filter, a two-dimensional grating is sandwiched between a substrate and a cover layer that fills the grating grooves. When the effective index of refraction of the grating region is greater than the substrate or the cover, a waveguide-like region is created. When the filter is designed properly, incident light passes into the waveguide region and propagates as a leaky mode. The grating structure selectively couples light at a narrow band of wavelengths into a mode that can travel laterally across the grating surface. The light propagates only a short distance (on the order of 10-100 micrometers), undergoes scattering, and couples with the forward- and backward-propagating zeroth-order light. This sensitive coupling condition can produce the resonant grating effect on the reflected radiation spectrum, resulting in a narrow band of reflected or transmitted wavelengths.
When specific binding substances are attached to the surface of biosensor 118, the reflected wavelength (e.g., color) is shifted due to the change of the optical path of light that is coupled into the grating. By linking specific binding substances to a biosensor surface, complementary binding partner molecules can be detected without the use of any kind of fluorescent probe or particle label. The detection technique is capable of resolving changes of, for example, ˜0.1 nanometer (nm) thickness of protein binding, and can be performed with the biosensor surface either immersed in fluid or dried. A more detailed description of these binding partners is provided in related and commonly assigned U.S. patent application Ser. No. 09/930,352, entitled “Label-Free High-Throughput Optical Technique For Detecting Biomolecular Interactions,” filed on Sep. 12, 2002, herein entirely incorporated by reference and to which the reader is directed for further information.
Optical device 100 utilizes a change in the refractive index upon a surface to determine when a chemically bound material is present within a specific location. For more information concerning calorimetric resonant optical biosensor detection, the reader is referred to “A Plastic Colorimetric Resonant Optical Biosensor for Multiparallel Detection of Label-Free Biochemical Interactions,” B. T. Cunningham, B. Lin, J. Qiu, P. Li, J: Pepper, and B. Hugh, Sensors and Actuators B, Vol. 85, number 3, November 2002; and “Colorimetric Resonant Reflection as a Direct Biochemical Assay Technique,” B. T. Cunnigham, P. Li, B. Lin, and J. Pepper, Sensors and Actuators B, Volume 81, p. 316-328, January 2002; the full disclosures of which are herein incorporated by reference.
Measuring apparatus 102 includes a single-point optical spectrometer included in spectrometer 116 to identify biochemical interactions at the resonance wavelength. The spectrometer may be any optical device that is capable of measuring the wavelength as a function of intensity of an incoming light source. Typically, this function is performed by allowing an incoming point light source to reflect against the surface of a diffraction grating. Light of different wavelengths will reflect from the grating surface at different angles to spread the incident point source light according to its wavelength content. Typically, a linear array of optical sensors is positioned to gather the grating-reflected light, where resolvable wavelength illuminates a single element in the optical sensor array. By plotting the output of each optical sensor in the array, and by assigning each optical sensor to a particular wavelength value according to the angle of its reflection of the grating, the intensity as a function of wavelength of the incoming light source may be accurately determined. The resolution of the wavelength determination is a function of the number of optical sensors in the linear array, the distance between the diffraction grating and the sensor array, and the period/depth/accuracy of the diffraction grating.
In the exemplary embodiment, measuring apparatus 102 may include tunable components. For example, spectrometer 116 may be replaced with a narrowband transmission filter whose transmission wavelength can be tuned over the band that encompasses the biosensor resonance wavelength. The tunable filter may be a micro-electro-mechanical (MEM) cantilever containing a Bragg reflector. In addition, in another embodiment, white light source 104 can be replaced with a laser light source (emitting one wavelength at a time) whose wavelength can be tuned over the band that encompasses the biosensor resonance wavelength. For example, the tunable laser may be a vertical cavity surface emitting laser (“VCSEL”).
Incorporating tunable components into measuring apparatus 102 allows measuring apparatus to adapt to many types of biosensors to measure biochemical interactions therein. For instance, using spectrometer 116, reflected (or transmitted) light is gathered into spectrometer, 116 only when the light is in a range of predetermined (and non-tunable) wavelengths. As such, using a fixed wavelength light source limits operation of measuring apparatus 102 since a high reflected intensity (or low transmitted intensity) is only obtained when a resonance wavelength of the biosensor is within the band of the illuminating radiation form the light source. Therefore, providing a tunable light source and a tunable detector provides for greater flexibility within measuring apparatus 102.
Furthermore, spectrometers are often very expensive, and tend to be bulky. On the other hand, tunable detectors can be designed to be very small (e.g., on the order of 0.1 cm), and can be manufactured at a cost of a fraction of a typical spectrometer.
A tunable light source and a tunable detector may be provided using a number of components. In the exemplary embodiment, the tunable detector and the tunable light source include a VCSEL. A VCSEL is a type of semiconductor diode laser whose emission cavity is perpendicular to the wafer plane. Thus, the VCSEL emits an output optical beam in the vertical direction, with respect to the wafer substrate. A VCSEL cavity may include two distributed Bragg reflectors (“DBRs”) with an active region positioned in-between. Typically, the entire cavity is grown in one single-step epitaxy. Subsequent processing is used to create any necessary current and optical confinements. This unique topology also facilitates simple fabrication of arrays and integration of detectors.
Top DBR 302 may comprise a p-doped DBR (“p-DBR”), such as an AlGaAs/GaAs composition. Likewise, contact 310 may be a p-doped contact. Bottom DBR 306 may comprise an n-doped DBR (“n-DBR”), such as an n-GaAs-AlGaAs composition. Contact 314, therefore, may be an n-doped contact. Top DBR 302 and bottom DBR 306 comprise alternating layers of semiconductor materials lattice matched to substrate 308, allowing “monolithic” formation of the entire structure during a single epitaxy stage of device fabrication.
VCSEL300 emits light from a cavity 316 of top DBR 302. VCSEL300 emits light when a voltage is applied across contacts 310 and 314. The wavelength of the light emitted depends on the amount of voltage applied to the contacts 310 and 314. As one example, for a 1.55 μm wavelength VCSEL, tunable diode laser 300 comprises an active region that is an epitaxial DBR with about 35 layer-pairs of InGaAlAs/InAlAs, and a bottom DBR 300 comprising a dielectric layer stack as a DBR in conjunction with a gold termination. With an applied voltage drop across contacts 310 and 314 of about 0.9V, VCSEL300 emits the 1.55 μm wavelength. It should be understood that the VCSEL 300 may contain any number of layers within the top and bottom DBRs, and within the active region, and may also comprise other materials than presented herein to produce other desired wavelengths than described herein. In addition, a VCSEL 300 may be made tunable by many different designs. A few examples include a cantilever VCSEL, a membrane VCSEL, and a half-symmetric cavity VCSEL.
Since a portion of top mirror 410 suspends above laser drive contact 420, airgap 414 can be adjusted continuously. For example, a continuous tuning range of approximately 32 nm may be achieved with VCSELs centered at a wavelength of 950 nm, with output power greater than 1 mW throughout the tuning range. As tuning is adjusted by applying a voltage to move cantilever 424, the emission, speed is limited by the response of cantilever 424, which is typically in the range of microseconds.
In one application, tunable VCSEL 400 may tune over 22 nm continuously and emit a single-mode output with a side mode suppression ratio greater than 45 dB. Tunable VCSEL may transmit at 2.5 giga-bits per second (Gbps) over 100 km single mode fiber. Using similar compound material systems, tunable VCSELs can be made with the center wavelength range from 0.65-1 micron and 1.5-1.6 micron. For more information regarding tunable VCSELs, the reader is referred to C. J. Chang-Hasnain, “Tunable VCSEL,” IEEE Journal of Selected Topics in Quantum Electronics, 6 (2000), the full disclosure of which is herein incorporated by reference.
The major advantages of VCSELs are low-cost manufacturing and high spectral performance. Testing and packaging constitute a large portion of the manufacturing cost of optoelectronic devices. However, in an equivalent topology with light emitting diodes (LEDs), VCSELs can be tested and packaged in similar low-cost manufacturing processes. For example, large size VCSELs emitting at wavelengths of 850 nm, with a typical dimension of 15-20 micron diameter, can be manufactured in volume at low costs for gigabit Ethernet and other low-cost LAN applications.
Spectral advantages arise because a VCSEL has an ultra short cavity that supports a single Fabry-Perot mode. Thus, a small diameter VCSEL can be made into a single-wavelength single-mode laser. In addition, with a small variation of the cavity thickness, the emission wavelength can tune over a large range.
In one embodiment, tunable VCSEL 400 can be readily translated into a tunable detector or a tunable filter with surface normal topology and array fabrication capabilities. Micromechanically actuated filters can exhibit wide tuning. For example, the tuning range of a MEMS-VCSEL is governed by the wavelength difference resulted from maximum deflection of the cantilever. However, with increases in applied voltage, the cantilevers are pulled onto the substrate, which may result in device damage and reliability issues. A simple analytic approximation of a maximum deflection of the cantilever as well as a capacitive nature of the attractive force is given below in Equation 1.
d airgap size without applied voltage
z cantilever displacement
V applied voltage
E bulk modulus
τ radius of the laser
l length of the cantilever
ω width of the cantilever
t thickness of the cantilever
Solving this equation, the maximum deflection of the cantilever, and thus the tuning range, approximates ⅓ of the airgap size. As one example, a wide tuning range of 31.6 nm centered at a wavelength of 950 nm may be achieved with tunable VCSEL 400 operating at room temperature. The tuning voltage to achieve this is about 26V.
Tunable detector 500 may be a Fabry-Perot filter, with top and bottom DBR mirrors 510 and 506 forming an optical resonator, and an intrinsic-absorption layer under the resonator. Due to multiple reflectances and interferences, only light with a resonant wavelength will be transmitted through tunable filter 500. All other wavelengths are reflected and the multiple reflections cause constructive and destructive interference. By changing the resonator length through deflection of one of the two mirrors, the transmission peaks can be shifted to the wavelength of a desired length.
Top and bottom DBR mirrors 510 and 506 may include Bragg gratings. When a half-wave shift is introduced in an otherwise uniform Bragg grating, the resultant structure behaves as an optical resonator, similar to a Fabry-Perot cavity or a ring resonator. The gratings may be long, uniform structures that perform as wavelength-selective reflectors. Depending upon the characteristics of the Bragg grating, the filter can be configured to perform many different functions. For example, by appropriately selecting the length and depth of the Bragg grating, the reflection spectral response can be made to have a bandpass shape. With this configuration, the device performs as an add/drop filter: one wavelength is transmitted by the gratings, while all other channels are reflected. Another useful device may be realized by making the Bragg grating very shallow, such that peak reflectivity is small.
The light transmitted through biosensor 700 next illuminates an input of tunable detector 702. Tunable detector 702 a tunable filter 704, a photodiode sensor 706, a top DBR 708 and a bottom DBR 710. Tunable filter 704 provides a high degree of light transmission at a narrow band of wavelengths centered at a passband wavelength, λf, but reflects all other wavelengths. The passband wavelength, λf, can be controlled over a range of wavelengths by adjustment of tunable filter's 704 tuning voltage, Vtun. The passband adjustment range, λf(min) to λf(max), should encompass a biosensor resonant wavelength, λp. The tuning voltage is continuously adjusted as a function of time, so as to sweep the passband wavelength from λf(min) to λf(max).
When tunable filter 704 passband does not coincide with the biosensor resonant wavelength, a high intensity of light is transmitted through tunable filter 704. When tunable filter 704 passband does coincide with the biosensor resonant wavelength, a low intensity of light is transmitted through tunable filter 704. A minima in transmitted intensity through tunable filter 704 is obtained at the tuning voltage that produces a passband wavelength that is the same as the resonant wavelength of biosensor 700. To detect the light that is transmitted through tunable filter 704, photodiode sensor 706 is positioned beneath tunable filter 704 as shown in
In an exemplary embodiment, multiple tunable filters can be integrated into a single surface, where each tunable filter is used to measure transmitted light from a different region on a biosensor surface. Each region of the biosensor surface may be activated with a different immobilized molecular receptor (such as different proteins, antibodies, proteins, or DNA). In this manner when the entire biosensor surface is exposed to a test sample, different biosensor regions produce shifts in the resonant wavelength corresponding to the amount of material from the test sample that is gathered onto each individual region. Using any number of microfabrication methods, a linear array of tunable filters, or an x-y grid of identical tunable filters can be produced on a single semiconductor chip.
In the exemplary embodiment, tunable laser 1002 is a vertical cavity surface emitting laser (VCSEL) because VCSELs may be fabricated in integrated arrays with many VCSELs on a single semiconductor chip. In one embodiment, multiple VCSELs can be integrated into a single surface, where each tunable VCSEL is used to illuminate a different region on the biosensor surface. Each region of the biosensor surface may activate with a different immobilized molecular receptor (such as different proteins, antibodies, proteins, or DNA), so that when the entire biosensor surface is exposed to a test sample, different biosensor regions produce shifts in the resonant wavelength corresponding to the amount of material from the test sample that is gathered onto each individual region. Using microfabrication methods known in the art, a linear array of tunable VCSELs, or an x-y grid of identical tunable filters can be produced on a single semiconductor chip or assembled onto a surface such as a circuit board.
In the exemplary embodiment, a tunable filter and a tunable laser source are used within calorimetric resonant optical detection. As discussed above, the tunable filter and tunable laser source (e.g., VCSEL), and a pin detector may replace a light source, spectrometer, and a detector array as used in existing implementations (e.g., See
As illustrated, biosensor resonance measured with laser excitation produces a more narrow resonant peak than the same biosensor measured by the conventional method (white light illumination and spectrometer detection). The improved behavior may be a result of the more highly collimated and coherent nature of laser light compared to White light. A more narrow resonance can measure small shifts in PWV with finer resolution, thus improving the signal-to-noise ratio for determining shifts in PWV. The higher resolution results in a sensor instrument with the ability to measure lower concentration analytes in test solutions, or smaller molecules with enhanced resolution.
In addition, as described above, the wavelength output of a tunable laser may be continuously adjusted with fine control of the tunable laser's tuning voltage. Wavelength tuning accuracy proportional to the linewidth of the VCSEL may be obtained for such systems. By contrast, spectrometer systems measure the intensity of each wavelength in the spectrum in discrete bins, whose resolution is determined by the period of a reflection grating inside the spectrometer, the size of the spectrometer, and the spatial separation (resolution) of pixels within a linear array of photodiodes that interface to the spectrometer output. For inexpensive spectrometers, the separation between wavelength bins is 0.14 nm, while for large, expensive instruments, the separation between wavelength bins is 0.06 nm. Using these spectrometer instruments, current PWV-shift resolution is approximately 0.001 nm. Therefore, the use of a tunable laser illuminating source can potentially increase PWV detection resolution by an order of magnitude without incurring the cost and size disadvantages of high resolution spectrometer instruments. It is important to note that the resolution advantage may be obtained without making any change to the biosensor.
In the exemplary embodiment, the size of a colorimetric resonant optical biosensor readout instrument based on tunable laser or tunable filter readout can be very small. For tunable VCSEL illumination, a laser power supply and voltage tuning circuit are included. For a tunable filter output, only a low voltage tuning circuit is needed. The tuning circuits can be built using integrated circuit technology to make a biosensor readout interface that is small enough to be handheld. Handheld instruments may be useful in medical diagnostics (such as emergency room or intensive care units), outdoor applications such as environmental monitoring, or placement of self-contained sensor systems within building air ducts.
While the tunable detector detection method described above utilizes detection of a minimum wavelength of transmitted light, the configuration shown in
A peak reflecting wavelength can be determined by maximizing PIN detector 1412 signal while varying tunable VCSEL 1402 emission wavelength. As tunable VCSEL 1402 wavelength is tuned, only the wavelength that corresponds to the resonance wavelength of biosensor 1410 resonant will be reflected from biosensor 1410, which performs as a narrow passband reflector. The resolving power, or resolution, of a detector is a measure of how well the detector can distinguish between two slightly different wavelengths. In this case, the resolution will be the laser linewidth. Typical VCSEL linewidth is on the order of 100 MHz, which is about 200 fm at 850 nm.
While exemplary embodiments have been described, persons of skill in the art will appreciate that variations may be made without departure from the scope and spirit of the invention. This true scope and spirit is defined by the appended claims, which may be interpreted in light of the foregoing.