US 20060196253 A1
The disclosed sensor chip includes a substrate and a moving member coupled to the substrate and disposed for movement relative to the substrate. The moving member moves relative to the substrate in a first direction and in a second direction in response to movement of the substrate. The first direction is different than the second direction. The moving member includes a plurality of receptors. The receptors are configured for selectively binding to a first measurand.
41. An apparatus for determining a property of a sample, comprising:
a moving member
i) coupled to the substrate by a plurality of connections, and
ii) moving relative to the substrate; and
at least one receptor area located on the moving member for selectively binding to a first measurand.
42. The apparatus of claim 1, comprising a plurality of tethers for coupling the substrate to the moving member.
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63. A method for determining a property of a sample, comprising:
exposing a first receptor area of a member to a first measurand;
moving a substrate that is coupled to the member by a plurality of connections; and
monitoring movement of the member relative to the substrate.
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82. An apparatus for determining a property of a sample, comprising:
means for exposing a first receptor area of a member to a first measurand;
means for moving a substrate that is coupled to the member by a plurality of connections; and
means for monitoring movement of the member relative to the substrate.
This application is related to, and claims the priority of, U.S. Provisional Patent Application No. 60/406,808, which was filed on Aug. 29, 2002, and which is entitled DEVICE, APPARATUS, AND METHOD FOR SENSITIVE & ACCURATE DETECTION OF INTERACTING CHEMICAL AND BIOLOGICAL MATTER, and which is hereby incorporated by reference.
The present invention relates to a sensing system. More specifically, the present invention relates to a MEMS (microelectromechanical system) based sensing system for measuring the properties of a sample, including detecting the presence of a measurand in a sample. As used herein, the term “measurand” means a particular kind of matter, organic or inorganic, such as a particular chemical, protein, virus, allergen, pathogen, molecule, analyte, . . . etc., that is to be detected and/or measured.
Various types of sensing systems for measuring the properties of fluids (i.e., liquids and gases) are known in the art, such as, for example, systems for measuring the viscosity of oil. Additionally, various types of sensing systems for detecting the presence of a particular measurand are known in the art, such as, for example, systems for detecting the presence of protein molecules in a sample. MEMS based sensing systems have been developed. However, there remains a need for improved accurate, sensitive, reliable, inexpensive sensing systems.
These and other objects are provided by an improved MEMS based resonant sensor, which can exhibit two or more resonant modes. A plurality of such resonant sensors can be incorporated onto a single sensor chip. Activity of the resonant sensors can be sensed optically.
Still other objects and advantages of the present invention will become readily apparent to those skilled in the art from the following detailed description wherein several embodiments are shown and described, simply by way of illustration of the best mode of the invention. As will be realized, the invention is capable of other and different embodiments, and its several details are capable of modifications in various respects, all without departing from the invention. Accordingly, the drawings and description are to be regarded as illustrative in nature, and not in a restrictive or limiting sense, with the scope of the application being indicated in the claims.
For a fuller understanding of the nature and objects of the present invention, reference should be made to the following detailed description taken in connection with the accompanying drawings in which the same reference numerals are used to indicate the same or similar parts wherein:
The operation and construction of sensing system 100 will be discussed in greater detail below. Briefly, sensing system 100 accurately detects the presence of a measurand in a sample. In operation, a liquid or gas sample flows through sensing system 100, entering system 100 at an inlet 110 and exiting system 100 at an outlet 112. Flowing the sample through system 100 exposes the sample to a sensor chip 120. As shown in
By way of example, resonant sensors constructed according to the invention are capable of detecting a mass change of 0.05 pico grams (i.e., the system can detect the event of 0.05 pico grams of measurand binding to the receptor site of a resonant sensor). A single protein molecule is on the order of 3×10−7 pico grams. Accordingly, measurement systems constructed according to the invention are capable of usefully detecting minute amounts of material (e.g., 150,000 protein molecules) in a sample.
As shown in
In operation, when actuator 160 is activated, the actuator 160 moves one corner 122 of sensor chip 120 by a controlled minute amount in a direction suggested by the arrow A-A shown in
The optical system 180 includes a light generator (e.g., a laser) 182 and a light detector (e.g., a photodiode) 184. The optical system 180 may be moved so as to address the plurality of resonant sensors 130 one at a time. When one of the resonant sensors 130 has been so addressed, light emitted by light generator 182 is incident on the resonant sensor and light reflected by the resonant sensor is received by light detector 184. The light detector 184 generates an output signal representative of the light received by the detector. Sensing system 100 measures the motion of the moving element of the resonant sensor by monitoring the output signal generated by light detector 184.
As shown in
Resonant sensor 130 includes a central moving member 132 and four anchors 134. The left pair of anchors 134 are connected by a tether 135 that extends in a Y direction (the Y direction being indicated in
As shown in
The upper surface of central moving member 132 defines a receptor area 140. The construction and operation of receptor area 140 will be discussed further below. Briefly, receptor area 140 is configured to provide binding, or receptor, sites for a particular measurand.
As shown in
It will be appreciated that the dimensions and measurements discussed above merely provide an example embodiment and that considerable variation is possible.
In operation, the substrate may be moved in an oscillatory fashion at a low frequency, such as between 0.1 and 10 MHz, by controlled minute amounts (e.g., by the actuator 160 shown in
in which M is the mass of central member 132, ΔM is the change in mass when the measurand binds to the central member 132, fresonance is the frequency at which the central member 132 exhibits resonance when no measurand is bound to the receptor area 140, and Δfresonance is the change in resonance frequency caused by having a measurand (of mass ΔM) bound to the receptor area 140. Equation (1) provides an idealized expression relating mass change to frequency change, and such an idealized viewpoint further includes a similar expression for each mode of the resonator.
The moving member 132 of the sensor 130 shown in
By way of example, tethers that extend in the Y direction (135, 136) exhibit distinct changes in structural response due to thermal influences as compared with the tethers that extend in the X direction (137, 138). Both reflect the same changes in stiffness due to modulus changes induced by temperature change. However, tethers 135, 136 are effectively clamped to the substrate at either end, and are subject to changing tension (or compression) that results from differential expansion between the substrate and the tether material. This stress is relieved in tethers 137, 138 by the bending of tethers 135, 136. Thus the Y translation mode of the moving member 132 will have larger relative shift in resonance frequency due to temperature change than X translation mode. Both will yield the same relative shift due to measurand mass loading. So, by measuring both resonant frequency changes (i.e., changes in resonant frequency of the moving member 132 in both the X and Y directions) the influence of the measurand can be distinguished from temperature induced effects.
For convenience of illustration,
Ideally, only a preselected measurand will bind to the binding sites 410 and nothing in the sample will bind to areas of the moving member 132 other than the binding sites 410. However, unwanted material from the sample (e.g., protein) typically will bond to the moving member 132 at sites other than the binding sites 410. Blocking sites 420 are included to reduce the amount of material from the sample that binds to the moving member 132 at sites other than the receptor sites 410. Again, ideally, nothing will bind to the blocking sites 420. However, although the blocking sites 420 reduce the amount of material that binds to areas of the moving member 132 other than the receptor sites 410, some material from the sample still generally binds to the blocking sites 420.
In operation, if unwanted material binds to the blocking sites 420 in the first receptor area 140A, the same type of unwanted material will also likely bind to the second receptor area 140B. Due to its distance from the center of the moving member 132, the mass of material binding to the second receptor area 140B will have a pronounced effect on the rotational resonant mode of the sensor 130 (i.e., oscillation of the central moving member in the theta direction). This allows the measurement system to distinguish between the measurand binding to the binding sites 410 and unwanted material binding to the blocking sites 420.
A brief discussion of four possible cases illustrates generally how the receptor area configuration shown in
As has been discussed above, sensing system 100 can detect the presence of a particular measurand in a sample by monitoring the resonant frequencies of the resonant sensors 130 after the system has been exposed to the sample. The simple method of monitoring only the resonant frequency of the resonant sensors is an effective way to use sensing system 100. However, it also represents a simple, degenerate, case of the information that may be obtained from sensing system 100. A more complete representation of this information is shown in
Referring back to
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As noted above, presence of the measurand in the sample can be detected simply by monitoring the resonant frequencies of the moving member 132 of the resonant sensor 130. Similarly, presence of the measurand in the sample can be detected simply by monitoring the frequency response of the resonant sensor 130 at a particular oscillation frequency (e.g., by oscillating the sensor chip at a particular, constant, frequency, and by monitoring the frequency response of the resonant sensors at that frequency).
However, instead of monitoring the frequency response at a particular, constant, frequency, it may be advantageous to monitor the frequency response of the resonant sensors over a range of frequencies. This advantageously allows a decision as to whether the measurand is present in the sample to be based on a collection of data points rather than a single, or a small number of, data points. Or in other words, instead of examining isolated data points, this technique allows examination of functions over a range. For example, the amplitude and phase of the frequency response of a resonant sensor can be measured over a range of frequencies and that measured response can be compared with one or more sets of previously determined frequency responses, or curve fit functions thereof. Each of the frequency responses is a function of the oscillation frequency of the substrate and other variables. The previously determined frequency responses can represent a variety of cases, such as the case in which (1) the sensing system has not been exposed to any sample and is in a “just manufactured” state; (2) the sensing system has been exposed to a sample containing the measurand, and in which the sample is of the type that does not cause binding to the blocking sites of the receptor area; and (3) the sensing system has been exposed to the sample containing the measurand, and in which the sample is of the type that does cause binding to the blocking sites of the receptor area. It will be appreciated that frequency responses, or curve fit functions thereof, for many other cases may be prepared in advance and that in operation the measured frequency response can be compared against these responses and functions. The decision about which case actually represents the conditions experienced by the sensing system may be made by comparing the measured frequency response with that of the previously determined cases and selecting the best match.
It will also be appreciated that the previously determined frequency responses can be prepared using one or more control resonant sensors. The control resonant sensors may be manufactured by the same process used to produce resonant sensors that are later used to test for the presence of a particular measurand such that they exhibit substantially similar frequency responses. Alternatively, such control resonant sensors need not be used and the previously determined responses can be measured using the same sensors that are later used to detect the presence or absence of a measurand in a sample.
It will also be appreciated that multiple sensors can be configured with different receptor molecules for the same measurand. For example, different antibodies specific for different epitopes of a single measurand can be employed. This is useful for increasing the confidence in the detection of the measurand.
Numerous methods for using sensing system 100 to detect the presence of a measurand in a sample have been discussed above. In addition to detecting the presence of a measurand, sensing system 100 may be used more generally to measure properties of a sample. For example, if sensing system 100 is submerged in an oil, sensing system 100 may be used to determine the oil's viscosity. Whereas measurand binding to the receptor area adds mass to a moving member 132 of a resonant sensor 130, oil surrounding a resonant sensor 130 provides resistance to the motion of the central moving member 132 (even if none of the oil binds to the sensor, and even if the sensor does not include a receptor area for selective binding). The amount of resistance added by the oil is a function of the oil's viscosity. So, the oil's viscosity can be measured by monitoring the manner in which motion of the moving member is affected by presence of the oil. In general, sensing system 100 may be used to measure interaction between a sample (e.g., oil, a gas that may contain a particular measurand, etc.) and one or more resonant sensors 130.
The first step 510 in process 500 is to provide a silicon-on-insulator (SOI) wafer. Such a SOI wafer 610 is shown in
The second step 512 in process 500 is to form raised areas in the device layer.
The next step 516 in process 500 is to form metallic structures on structure 620. The result of this step is shown in
The next step 518 in process 500 is to form metallic structures on a glass substrate. The result of this step is shown in
The next step 520 in process 500 is to invert the structure produced at the end of step 516 (i.e., and shown in
The next step 522 in process 500 is to remove the “handle wafer” from the SOI structure. The result of this process is shown in
The next step 524 in process 500 is to remove the dielectric layer 614 from what remains of the SOI wafer. The result of this step is shown in
The next step 526 in process 500 is to form the receptor area 622 on the moving member 620. The process of forming the receptor area 622 will be discussed further below. However, it may include depositing layers of titanium and gold on the moving member structure 620. Gold is useful for forming receptor sites and titanium is useful for binding gold to the silicon structure 622.
Although the steps of process 500 have been discussed in connection with forming a single sensor chip, which has a single resonant sensor on it, it will be appreciated that process 500 may be used to form a plurality of sensor chips, and each of the chips may include a plurality of resonant sensors. After wafer processing has been completed, the wafer is diced up into individual chips, typically by using a die saw. Moving microstructures are preferably protected during wafer dicing. A layer of photoresist that is subsequently removed is generally adequate for this purpose.
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It is also possible to fabricate the moving member, anchors, and tethers from a metal, such as, nickel, iron, iron-nickel alloys, copper or gold. Plating processes are advantageous for forming these structures since the metal layers produced tend to have low stress levels and predictable properties. The fabrication process starts with a planar substrate, such as a silicon, ceramic (e.g., aluminum nitride) or glass (e.g., Pyrex) wafer. A thin conductive seed layer is deposited on the substrate. For example, 500 Angstroms of chromium followed by 2000 Angstroms of gold is adequate. Other metal layer combinations and thicknesses can be used. Additionally, the layer can be patterned using standard photolithographic methods if desired. A layer of photoactivated polymer (e.g., photoresist, polyimide) is then deposited over the seed layer and then patterned to define the locations of the anchors. A metal, such as nickel, is then formed by electroplating so as to fill the holes defined by the patterned photoresist with metal. This metal is formed to the desired thickness to define the height of the anchors and accordingly the gap between the moving member and the substrate. This thickness matches approximately the height of the polymer layer. Subsequently, the metal anchors and polymer can be planarized if desired. A second seed layer of metal is then formed over the anchors and polymer photoresist. This second seed layer is then patterned to define the shape of the tethers and the central moving member. A second layer of polymer photoresist is then deposited over this second seed layer. The second layer of photoresist is then patterned to expose the second seed layer (i.e., to expose the sites where the tethers and moving member will be formed). The second layer of patterned photoresist effectively acts as a mold for forming the tethers and moving member. Electroplating is then used to from a thicker metal layer over the second seed layer. This thicker metal layer forms the tethers and the central moving member. The plating time is controlled to insure that the tethers and central moving member have the desired thickness. The top surface can be planarized to reduce the surface roughness if desired. A final metal layer (e.g., gold) can patterned on top of the central moving member for attachment of receptors and/or to facilitate optical sensing, as described in for previous fabrication methods. A solvent is then used to remove the photoresist and release the metallic central moving member for movement with respect to the substrate. The process described here for producing a moving member constructed from metal is only one example. More advanced plating methods include LIGA processing as well as the Efab process available from MEMGen, Burbank, Calif.
As discussed above in connection with
Other, non-optical, approaches have been used in the prior art to monitor the position of a moving member in a MEMS device, such as capacitive, piezoresistive, and piezoelectric methods. These methods can be employed in the current invention. However, optical monitoring is advantageously unaffected by the presence of a conducting solution (e.g., salts in the sample).
As shown in
In operation, sensor 900 is illuminated from above and photodiodes receive light that passes through apertures 902 between the moving member and the anchors. As the moving member moves, the sizes of apertures 902 change thereby increasing or reducing the amount of light incident on the photodiodes. The photodiodes generate output signals representative of the amount of light incident on the photodiodes. The output signals generated by the photodiodes are accordingly indicative of the instantaneous position of the moving member 880. Monitoring these signals over time allows the measurement system to measure the oscillation frequency of the moving member.
In one embodiment, the width of the apertures 902 is nominally (i.e., when the moving member is stationary) five microns. In this embodiment, displacement of the moving member by one nanometer changes the size of the apertures 902 by 0.02%. The output signals generated by the photodiodes are generally out of phase and can be detected differentially to improve the signal-to-noise ratio. Photodiodes configured as shown in
The output signals generated by the four photodiodes may be combined in various ways to produce signals indicative of position of the moving member 880 in the X, Y, and theta directions. In general, the difference of the output signals generated by photodiodes A and B is indicative of position of moving member 880 as measured in the X direction (i.e., X position indicated by A−B); and the difference between the output signals generated by photodiodes C and D is indicative of the position of the moving member as measured in the Y direction (i.e., Y position indicated by D−C). Finally, the difference between the sum of the output signals generated by photodiodes C and D and the sum of the output signals generated by the photodiodes A and B is indicative of the rotational orientation of the moving member 880 (i.e., Rotation position indicated by (A+B)−(C+D)). Monitoring these signals over time provides measurement of the oscillation frequencies of the moving member 880 in the X, Y, and theta directions.
The structure shown in
Another alternative approach for optically monitoring the position and oscillation frequency of the central moving member of a resonant sensor is to illuminate the sensor with a pulsed source (or strobed source) and to image light reflected from the moving member with a phase-locked imaging system. The light pulses are of a duration sufficiently short to effectively freeze the motion of the central moving member. Several images recorded at the same phase can be averaged to reduce noise. For moving members that have an amplitude of about ten nanometers and a frequency of about one MHz, a pulse duration of about ten nanoseconds is used to resolve a one nanometer displacement of the moving member. This is achievable with a variety of laser sources (e.g., visible semiconductor lasers). The reflected light can be imaged onto a CCD camera. Imaging systems based on this approach are commercially available, for example, the MMA G2™ MEMS Motion Analyzer from UMech Technologies of Watertown, Mass. Reflective features can be included on the moving and stationary portions of the sensor chip to facilitate referenced displacement measurements. This facilitates rejecting bulk movement of the assembly relative to the imaging system.
Regardless of which configuration is used, the actuators may excite (or move) the sensor chip over a frequency range of about 0.1-10 MHz. The inertia of the central moving members of the resonant sensors causes excitation of the resonant sensors when the substrate of the sensor chip 120 is excited (or moved).
It may be advantageous to encapsulate the substrate of the sensor chip 120 and the actuator(s) 160 in a compliant sealant. Such encapsulation may protect the electrical drive of the actuator from exposure to a liquid/fluid sample.
In all of the configurations discussed above, motion of the substrate of the sensor chip 120 simultaneously excites all of the resonant sensors. This implementation has several benefits (i.e., as opposed to independently moving each of the resonant sensors). First, each resonant sensor is excited by a like source so that systematic influences of excitation authority and phase lags are common to all measured resonant sensor responses. As a result, comparison of responses of the resonant sensors is more accurate. Second, the excitation source is isolated from the resonant sensors, thereby eliminating potential sources of electrical cross talk and noise. Also, when driving large arrays of resonant sensors, the use of a limited number of excitation sources greatly reduces the overhead of routing power carrying electrical connections.
It may be useful to fabricate sensor chips such that the receptor areas on every resonant sensor on the chip selectively bind to the same measurand. Such sensor chips can enhance accuracy by providing redundant measurements. Alternatively, it may also be useful to fabricate sensor chips in which each resonant sensor is used to detect the presence of a unique measurand. In such sensor chips, the receptor area on each resonant sensor selectively binds to a particular measurand that is different than the measurands detected by the other resonant sensors. If such a sensor chip has n resonant sensors, where n is an integer (e.g., n=20), the chip can be used to detect the presence of n different measurands in a sample or series of samples. As yet another alternative, sensor chips may be constructed according to the invention that have n resonant sensors and that are used to detect the presence of m different measurands, where m is an integer less than n. In such sensor chips, two or more resonant sensors may be devoted to detecting a particular measurand. Also, some of the resonant sensors on the chip may be control sensors that are never exposed to a sample. Such a control sensor may be produced by placing a covering over the sensor that prevents the sensor from being exposed to the sample and yet allows other resonant sensors on the chip to be exposed to the sample. Such control sensors may be used to provide a reference for calibrating other sensors on the chip and for compensating for environmental factors. Such a covering 101, for protecting two of the resonant sensors 130 from the sample, is shown in
In some environments it may be necessary to couple the inlet 110 and the outlet 112 to sample sources and waste disposals, respectively, via piping or conduits. It may be advantageous to use conduits made from compliant material (such as PDMS, which is commercially available from Dow Corning, Midland, Mich.) to minimize mechanical coupling between the sensor chip and the sample source.
The resonant sensor can be used in a variety of applications, including those normally associated with other acoustic or resonant sensors. Particularly useful applications fall into three main categories: (1) fluid characterization; (2) small chemical species detection; and (3) biological analyte detection. A review of the various sensor applications for traditional resonant or acoustic sensors (e.g., quartz crystal microbalances and surface acoustic waves devices) can be found in Balantine et al., Acoustic Wave Sensors: Theory, Design, and Physico-Chemical Applications, Academic Press, San Diego, 1997. The nature of the preparation of the receptor area depends on the application for which the resonant sensor is intended. For sensing physical properties of fluids (e.g., viscosity or density of liquids) no additional processing may be required. For detection of chemical species or biological analytes, however, the surface treatment will depend on the specific measurand to be monitored.
Thus, in some embodiments, the resonant sensor is used for fluid characterization to measure the density or viscosity of a gas or liquid. Similarly, the viscoelastic properties of gels and polymers can be measured. In these applications, no specific receptor molecules need to be bound to the receptor area of the sensor. Rather, the non-specific physical interaction of the fluid with the receptor area can have a detectable effect on the moving member.
In other embodiments, the resonant sensor is used for detecting chemical species present in fluids such as vapors, gases, or liquid solutions. In these embodiments, an adsorbent layer can be applied to the sensor surface which adsorbs the measurand to be detected. The adsorbent layer can be a thin polymer layer, the properties of which can vary with the nature and the amount of the measurand to be adsorbed. For example, the adsorbent layer can be chemically reactive with the measurand such that the measurand becomes covalently bound to the layer. Alternatively, the adsorbent layer can adsorb the measurand through non-covalent interactions (e.g., electrostatically or through hydrophobic or hydrophilic interactions). As a result of adsorbing the measurand, the mass of the moving member will be changed and, therefore, its resonant characteristics will be affected.
A large variety of chemical species can be sensed by employing thin layers of polymers that adsorb the measurand. Families of such chemo-selective polymers have been developed for detecting different classes of chemical vapors, including hydrocarbons, chemical warfare agents and explosives (e.g., McGill et al., “Choosing Polymer Coatings for Chemical Sensors,” Chemtech, 1994, p. 27; Houser et al., “Rational Materials Design of Sorbent Coatings for Explosives: Applications With Chemical Sensors,” Talanta, 54 (2001), pp. 469-485. Polymer layer thicknesses typically range from between 10 nm-1 μm. In order to deposit the layer, the polymer can be dissolved in a solvent, sprayed on the surface and allowed to dry or, in some embodiments, laser deposition techniques can be employed. A mask can be used to confine the coating to all or some of the receptor area.
Alternatively, the resonant sensor can be used to detect chemical species which react with the receptor area by, for example, causing oxidation, reduction, dissolution, or corrosion of a chemical or material deposited onto the receptor area. For example, the presence and concentration of corrosive vapors can be detected by coating the surface with a material known to react with vapors of interest. Metals are commonly used, and most can be evaporated or sputtered onto the surface as a thin film, using a mask to confine the coating to specific areas of the sensor. For example, and without limitation, a layer of copper or silver can be used to detect the presence of hydrogen sulfide (H2S). Alternatively, the sensor can be used to evaluate the corrosion resistance of materials. For example, the sensor can be coated with the material of interest and the change in signal monitored during exposure to a known amount of corrosive vapor. Studies in liquid environments are also possible. By reacting with the polymer layer, the measurand can alter the mass of the moving member without being adsorbed thereto.
Chemical interactions with the polymer also can alter properties of the layer other than mass (e.g., viscoelastic properties, internal strain) and these can be detected in conjunction with or independently of the mass change. Moreover, the resonant sensor can be designed such that the altered properties result in a change in the resonant characteristics. For example, the tethers can be coated with the polymer, or the structure can be designed so that stresses induced in the central element are transferred to the tethers.
The resonant sensors of the invention can also be used to detect biological measurands, for example, whole cells (e.g., bacteria, yeast, fungi, blood cells, dissociated tissue cells), spores (e.g., fungal or yeast), viruses (e.g., HIV), proteins (e.g., growth factors, cytokines, and prions), lipids (e.g., cholesterol), carbohydrates (e.g., sugars, glycosaminoglycans, lipopolysaccharides), nucleic acids (e.g., DNA or RNA), and various small molecules (e.g., hormones, pharmaceutical drugs). Such biosensor devices are useful in a wide array of applications, including cytometry, toxicology, diagnostics, genomic profiling and forensic identification.
Detection of such biological measurands requires immobilizing a receptor molecule on the receptor area which is specific for the measurand. Generally, the goal is to maximize the specificity of binding for the target while minimizing non-specific binding of other molecules (e.g., interferents). In many cases, complete specificity will be unachievable and, in some cases, it will be unnecessary. Rather, the degree of specificity required will depend upon the relative amounts of the measurand and interferents in the sample, as well as the ability to distinguish amongst them by other means. In some embodiments, the measurands will be a class of molecules and, therefore, specificity for the class will be desirable whereas specificity for molecules within the class will not.
The receptor molecules useful in specific embodiments will depend upon the nature of the measurand. Useful receptor molecules include antibodies, polynucleotides, aptamers, cell surface receptors, cytoplasmic receptors, binding domains, small molecule ligands, sugars, polysaccharides, glycans, glycoproteins and the like.
For example, antibodies can be used as receptors in the resonant sensors of the invention. As used herein, the term “antibody” is intended to embrace naturally produced antibodies, recombinantly produced antibodies, monoclonal antibodies, and polyclonal antibodies, as well as antibody fragments such as Fab fragments, F(ab′)2 fragments, Fv fragments, and single-chain Fv fragment (scFv). Useful antibody receptors include all immunoglobulin classes, such as IgM, IgG, IgD, IgE, IgA and their subclasses. Antibodies may be produced by standard methods, well known in the art. See, e.g., Pluckthun, Nature 347:497-498 (1990); Huse et al., Science 246:1275-1289 (1989); Chaudhary et al., Proc. Natl. Acad. Sci. USA 87:1066-1070 (1990); Mullinax et al., Proc. Natl. Acad. Sci. USA 87:8095-8099 (1990); Berg et al., Proc. Natl. Acad. Sci. USA 88:4723-4727 (1991); Wood et al., J. Immunol. 145:3011-3016 (1990); and references cited therein. Antibody receptors are particularly useful for capturing measurands such as whole cells (e.g., cells bearing a cell surface molecule which binds the antibody), spores (e.g., fungal or yeast), viruses (e.g., HIV), and proteins (e.g., growth factors, cytokines, and prions).
Polynucleotides are also useful as receptor molecules in the present invention. As used herein, the term “polynucleotide” means any molecule comprising a sequence of covalently joined nucleoside-like chemical units which has selective binding affinity for a naturally-occurring nucleic acid of complementary or substantially complementary sequence under appropriate conditions (e.g., pH, temperature, solvent, ionic strength, electric field strength). Polynucleotides include naturally-occurring nucleic acids as well as nucleic acid analogues with modified nucleosides or internucleoside linkages, and molecules which have been modified with linkers or detectable labels which facilitate immobilization on a substrate or which facilitate detection.
The polynucleotide receptors can be DNA molecules, RNA molecules, or polynucleotides having modified nucleoside bases or modified internucleoside linkages. Useful modifications include, but are not limited to, those which provide other chemical groups that incorporate additional charge, polarizability, hydrophobicity, hydrogen bonding, or electrostatic interaction.
Examples of modified nucleoside bases include, without limitation, the modified bases described in WIPO Standard ST.25 (1998), Appendix 2, Table 2, the entire disclosure of which is incorporated by reference herein (see also 37 C.F.R. 1.821-1.825). Examples of nucleoside base modifications include, without limitation, 5-position pyrimidine modifications, 8-position purine modifications, modifications at exocyclic amines, and the like. Specific modifications include, without limitation, modified cytosine bases including isocytosine, 5-methylcytosine, 4-acetylcytosine, 3-methylcytosine, 5-hydroxymethyl cytosine, 2-thiocytosine, 5-halocytosine, 5-propynyl cytosine, 6-azocytosine, 5-trifluoromethylcytosine, N4,N4-ethanocytosine, phenoxazine cytidine, phenothiazine cytidine, carbazole cytidine or pyridoindole cytidine; modified guanine bases including isoguanidine, 6-methylguanine, 1-methylguanine, 2,2-dimethylguanine, 2-methylguanine, 7-methylguanine, 2-propylguanine, 6-propylguanine, 8-haloguanine, 8-aminoguanine, 8-sulfhydrylguanine, 8-thioalkylguanine, 8-hydroxylguanine, 7-methylguanine, 8-azaguanine, 7-deazaguanine or 3-deazaguanine; modified adenine bases including 6-methyladenine, N6-isopentenyladenine, N6-methyladenine, 1-methyladenine, 2-methyladenine, 2-methylthio-N-6-isopentenyladenine, 8-haloadenine, 8-aminoadenine, 8-sulfhydryladenine, 8-thioalkyladenine, 8-hydroxyladenine, 7-methyladenine, 2-haloadenine, 2-aminoadenine, 8-azaadenine, 7-deazaadenine or 3-deazaadenine; modified thymine bases including dihydrothymine, 1-methylpseudothymine, 2-thiothymine, 4-thiothymine, pseudothymine, 2-thiothymine, 3-(3-amino-3-N-2-carboxypropyl)thymine, 6-azothymine, or 4-thiothymine; and modified uracil bases including 5-halouracil, 5-(carboxyhydroxylmethyl)uracil, 5-carboxymethylaminomethyl-2-thiouracil, 5-carboxymethylaminomethyluracil, dihydrouracil, 1-methylpseudouracil, 5-methoxyaminomethyl-2-thiouracil, 5′-methoxycarbonylmethyluracil, 5-methoxyuracil, 5-methyl-2-thiouracil, 2-thiouracil, 4-thiouracil, 5-methyluracil, uracil-5-oxyacetic acid methylester, uracil-5-oxyacetic acid, pseudouracil, 5-methyl-2-thiouracil, 2-thiouracil, 3-(3-amino-3-N-2-carboxypropyl)uracil, 5-methylaminomethyluracil, 5-propynyl uracil, 6-azouracil, or 4-thiouracil. Also useful are the modified bases described in U.S. Pat. Nos. 3,687,808, 3,687,808, 4,845,205, 5,130,302, 5,134,066, 5,175,273, 5,367,066, 5,432,272, 5,457,187, 5,459,255, 5,484,908, 5,502,177, 5,525,711, 5,552,540, 5,587,469, 5,594,121, 5,596,091, 5,614,617, 5,645,985, 5,830,653, 5,763,588, 6,005,096, and 5,681,941.
Examples of modified internucleoside linkages include, without limitation, modifications of the ribosyl or deoxyribosyl units such as halogenation, alkylation, alkoxylation and the like, modification or replacement of the phosphodiester linkages (e.g., substitution with phosphorothioates or alkyl phosphates), or modification or replacement of both the (deoxy)ribosyl and phosphate backbone (e.g., substitution with peptide nucleic acid (PNA) linkages). One or more of the termini of such polynucleotides also can be modified (e.g., 5′ or 3′ inverted residues or caps). Modified internucleoside linkages can be useful to protect against degradation by nucleases present in a sample. See, for example, Wetmur, Crit. Rev. Biochem. Mol. Biol. 26:227-259 (1991); Moody et al., Nucleic Acids Res. 17:4769-4782 (1989); Iyer et al., J. Biol. Chem. 270:14712-14717 (1995); Nielsen et al., Science 254:1497-1500 (1991); Ortigao et al., Antisense Res. Devel. 2:129-146 (1992)); Sinha et al., Nucleic Acids Res. 12:4539-4557 (1984)). Polynucleotides with naturally-occurring bases and linkages can be produced synthetically or by organisms (e.g., bacteria) genetically engineered to produce them as transcription products. Polynucleotides with modified bases and linkages can be produced synthetically.
Examples of modified internucleoside linkages known in the art include, without limitation, those having, e.g., 2′ ribosyl substituents such as F, Cl, Br, CN, SH, OCH3, SCH3, OCN, CF3, OCF3, SOCH3, SO2, CH3, ONO2, NO2, N3, NH2, OCH2CH2OCH3, O(CH2)2ON(CH3)2, OCH2OCH2N(CH3)2, O(C1-10 alkyl), O(C2-10 alkenyl), O(C2-10 alkynyl), S(C1-10 alkyl), S(C2-10 alkenyl), S(C2-10 alkynyl), NH(C1-10 alkyl), NH(C2-10 alkenyl), NH(C2-10 alkynyl), and O-alkyl-O-alkyl, OCH2CH2CH2NH2, CH2—CH═CH2, and O—CH2—CH═CH2. The 2′-substituent may be in the arabino (up) position or ribo (down) position.
In some embodiments, polynucleotide receptor molecules have a length of between 15 and 200 bases. In certain embodiments, the polynucleotide receptor molecules have a length between 15 and 50 bases, between 50 and 80 bases, between 80 and 110 bases, between 110 and 140 bases, between 140 and 170 bases, or between 170 and 200 bases. Substantially longer polynucleotides also can be used.
Polynucleotide receptor molecules can be directed to sequences known to include polymorphisms in a particular population, including single nucleotide polymorphisms, deletions or insertions, or regions of microsatellite instability. In particular, polynucleotide receptor molecules can be directed to allelic sequences known to include sites of deleterious mutations for purposes of genotyping, or can be directed to sequences characteristic of the genome of a particular species (e.g., a pathogen) for purposes of diagnosing disease or detecting contamination.
Receptor molecules useful in the invention also include aptamers or “nucleic acid ligands.” As used herein, the term “aptamer” means any polynucleotide having selective binding affinity for a non-polynucleotide molecule via non-covalent physical interactions. An aptamer is a polynucleotide that binds to a ligand in a manner analogous to the binding of an antibody to its epitope.
The aptamer polynucleotide sequences can be developed and selected by methods well known in the art (see, e.g., Tuerk et al. (1990), Science 249:5050; Joyce (1989), Gene 82:83-87; Ellington et al. (1990), Nature 346:818-822; Klug et al. (1994), Mol. Biol. Reports 20:97-107), and can be used as receptor molecules against many kinds of analytes, including proteins, carbohydrates and small organic molecules. Aptamers can be produced by any known method of producing polynucleotides, and can include modified nucleoside bases or modified internucleoside linkages as discussed above with respect to polynucleotides. See also U.S. Pat. No. 5,660,985.
Other receptor molecules useful in the invention include cell surface receptors (e.g., immune system molecules such as MHC antigens, T cell receptors and CD antigens; protein and peptide hormone receptors such as the thyroid-stimulating hormone, luteinizing hormone, follicle-stimulating hormone, calcitonin, somatotropin, vasopressin, parathyroid hormone, insulin and glucagon receptors; catecholamine receptors such as the dopamine, epinephrine and norepinephrine receptors; eicosanoid receptors such as the prostaglandin receptors; folic acid receptors), cytoplasmic receptors (e.g., thyroid hormone receptor, peroxisome proliferator-activator receptors (PPARs), and steroid receptors such as the estrogen, androgen, mineralocortocoid and glucocorticoid receptors), binding domains (e.g., DNA, RNA, metal, glycosaminoglycan, ubiquitin, cofactor and other ligand binding domains of various proteins), small molecule ligands (e.g., nucleotide mono-, di- and triphosphates, combinatorial chemistry libraries, peptide libraries), sugars (e.g., lactose, trehalose, L-arabinose, D-maltose), polysaccharides (e.g., bacterial endotoxin, mannan, pullulan, amylopectin, dextran), glycans (e.g., glycosaminoglycans (GAGs), glycosylation structures), and glycoproteins (e.g., lectins).
Finally, for blocking sites, blocking molecules may be chosen from a wide variety of molecules which have low affinity for binding, adhering, or adsorbing biological materials. Such groups include, without limitation, polar but uncharged groups (e.g., polyethylene glycol (PEG)) and hydrophilic groups (e.g., mannitol). In the discussion below, blocking molecules will be treated as a subset of receptor molecules, and will not be discussed separately.
Receptor molecules can be bound or attached to the surface of the receptor area using standard chemistries for the reactive groups already present on the receptor molecules and receptor area, or after derivatizing the receptor molecules or receptor area to produce or introduce desired reactive groups.
As noted above, the receptor area is a portion of the central moving member which may be produced from silicon. The receptor area can, however, be derivatized by depositing layers of titanium and gold on the moving member structure. Alternatively, other metals or metal oxides can be sputtered onto the surface. Such metal surfaces can be further derivatized with adaptor molecules which facilitate attachment of the receptor molecules.
Adaptor molecules can include an alpha reactive group which binds to the receptor area surface and an omega reactive group which can be reacted with the receptor molecules. For example, thiols bind strongly to gold surfaces and therefore adaptor molecules with alpha thiol reactive groups can be used with gold-coated receptor areas. Similarly, siloxanes bind to silicon surfaces, and fatty acids have been shown to bind to metal oxides. Therefore, siloxanes and fatty acids can be used as alpha reactive groups with silicon and metal oxide surfaces, respectively. For blocking sites, an adaptor molecule can be used in which the omega group is not reactive and terminates in a polar uncharged group (e.g., polyethylene glycol (PEG)) or hydrophilic group (e.g., mannitol).
Omega reactive groups for adaptor molecules can be chosen to complement the reactive groups available on the receptor molecules. For example, carboxyl groups can be reacted with amine groups using carbodiimide conjugation reactions (e.g., 1-ethyl-3(3-dimethylamino propyl) carbodiimide (EDC)); primary amines can be reacted with other amine groups using glutaraldehyde; CNBr treatment can convert hydroxyl groups to cyanate ester or imidocarbonate groups which can be reacted with primary amines; and cyanuric chloride treatment can convert primary amines to chlorotriazines which can be reacted with primary amines or thiols. For a review of useful conjugation reactions, see, e.g., Wong, ed., Chemistry of Protein Conjugation and Cross-Linking, CRC Press, Boca Raton, Fla. (1993).
Alternatively, adaptor molecules can include one member of an affinity binding pair and the receptor molecules can be conjugated to the other member of the binding pair such that the receptor molecules can be attached to the receptor area through the binding pair. Affinity binding pairs useful in this context include, without limitation, the biotin and streptavidin binding pair and the digoxigenin and antidigoxigenin binding pair. Thus, for example, and without limitation, adaptor molecules can be conjugated to avidin or streptavidin to cause immobilization of biotinylated receptor molecules. For antibody receptor molecules, the antibody itself can serve as an affinity binding partner with Protein A, which can be immobilized on the receptor area.
Patterns of receptor molecules, including blocking molecules, can be formed with or without adaptor molecules, using chemical printing or stamping methods, or photoreactive molecules using irradiative patterning with masks.
In some embodiments, the receptor molecules, including blocking molecules, are attached to the receptor area using self-assembled monolayers (SAMs). SAMs are formed by a particular form of adaptor molecule in the central portion of each molecule (i.e., the portion between the alpha and omega groups) interacts with the central portion of neighboring molecules in the monolayer to form a relatively ordered array. SAMs have been formed on a variety of surfaces or substrates including, but not limited to, silicon dioxide, gallium arsenide and gold. SAMs are applied to surfaces in predetermined patterns by a variety of techniques well known in the art, including simple flooding of a surface and more sophisticated methods such as microstamping and irradiative patterning. Monolayers may be produced with varying characteristics and with various omega reactive groups at the free end of the molecules which form the SAM. Therefore, by conjugating various receptor molecules to the SAMs through the omega reactive groups, SAMs with very specific binding affinities can be produced. This also allows for the production of patterned SAMs in which a plurality of receptor sites can differ from each other in the specific receptor molecules presented, and/or blocking sites can be interspersed with receptor sites.
For example, U.S. Pat. No. 5,512,131, incorporated herein by reference, discloses methods for the formation of microstamped patterns of SAMs on surfaces. In accordance with these methods, SAM patterns can be applied to the receptor area using a stamp in a “printing” process in which the “ink” consists of a solution including a compound capable of chemisorbing to form a SAM. The ink is applied to the surface using the stamp and deposits a SAM on the surface in a pattern determined by the pattern on the stamp. The surface may be stamped repeatedly with the same or different stamps in various orientations and with the same or different SAM-forming solutions. In addition, after stamping, the portions of the surface which remain bare or uncovered by a SAM may be derivatized with blocking molecules. Thus, for example, a grid pattern may be created in which the square regions of the grid bind different molecules or pathogens of interest but the linear regions of the grid bind different cells or molecules of interest. See, U.S. Pat. No. 6,368,838, incorporated herein by reference.
A wide variety of surface materials and SAM-forming compounds are suitable for use in the present invention. Useful combinations of surface materials and alpha reactive groups include, without limitation, metals such as gold, silver, copper, cadmium, zinc, palladium, platinum, mercury, lead, iron, chromium, manganese, tungsten, and any alloys of the above when employed with sulfur-containing alpha reactive groups such as thiols, sulfides, disulfides, and the like; doped or undoped silicon employed with silanes and chlorosilanes; metal oxides such as silica, alumina, quartz, glass, and the like employed with carboxylic acids; platinum and palladium employed with nitrites and isonitriles; and copper employed with hydroxamic acids. Additional suitable alpha reactive groups include acid chlorides, anhydrides, sulfonyl groups, phosphoryl groups, hydroxyl groups and amino acid groups. Additional surface materials include germanium, gallium, arsenic, and gallium arsenide. Additionally, epoxy compounds, polysulfone compounds, plastics and other polymers may find use as the surface material in the present invention. Polymers used to form bioerodable articles, including but not limited to polyanhydrides, and polylactic and polyglycolic acids, are also suitable. Additional materials and functional groups suitable for use in the present invention can be found in U.S. Pat. No. 5,079,600, incorporated herein by reference.
The central portion of the molecules comprising the SAM-forming compound may include a spacer functionality connecting the alpha reactive group and the omega reactive group. Alternatively, the spacer may essentially comprise the omega group, if no particular reactive group is required (e.g., for blocking molecules). Any spacer that does not disrupt SAM packing and that allows the SAM layer to be somewhat impermeable to organic or aqueous environments is suitable. The spacer may be polar, non-polar, halogenated (e.g., fluorinated), positively charged, negatively charged, or uncharged. For example, a saturated or unsaturated, linear or branched alkyl, aryl, or other hydrocarbon spacer may be used.
In one nonlimiting embodiment, the receptor area of a resonant sensor is coated with gold and alkyl-thiol SAMs are bound to the gold surface. Because the degree of ordering improves as the grain size of the gold film increases, vacuum deposition processes such as electron beam evaporation can be used to produce oriented surfaces with grain sizes on the order of tens of nanometers. The alkyl chain length of the SAM layer is chosen to be approximately 1-50 units (e.g., 10, 20, 30 or 40 units), which represents a balance between the degree of ordering of the layer and the cost of synthesis. Different SAMs with different omega groups are patterned onto the surface to form arrays of receptor sites. The reactive groups of the receptor molecules are then reacted with the omega groups of the SAMs to form the completed receptor areas. The surfaces of the resonant sensor that are distinct from receptor area (i.e., the silicon microstructure) can also be treated, for example, to prevent non-specific binding. For example, a PEO-terminated SAM layer that has a siloxane group for attachment to the silicon surface can be used to prevent non-specific binding to the silicon microstructure.
To attach receptor molecules to the SAM, standard activation-binding-inactivation protocols are used. For example, carboxyl-terminated SAM molecules can be activated by soaking the sensor surface in a solution of EDC (e.g., 0.01-0.05M) and N-hydroxysuccinimide (NHS) (e.g., 0.04-0.20M) in deionized water or buffer at pH 4-7 for 30 minutes. Next, a buffer solution (e.g., phosphate buffered saline (PBS)) containing the receptor molecules, at, e.g., about 0.1 μg/ml to 250 μg/ml, can be spotted onto the sensor surface using a nanoliter dispensing system to deliver a volume sufficient to cover the sensor surface (e.g., several nanoliters). Coupling can proceed for a period of 1-2 hours in a humidity-controlled environment to minimize the effects of evaporation. The sensor surface can then be washed with PBS, to remove adsorbed material that is not covalently bonded. Finally, the sensor surface can be immersed in an inactivation solution of sodium phosphate to quench any unreacted carboxyl groups. This process takes about 20 minutes. The sensor then can be washed and stored in PBS, or a different protocol can be employed to react different receptors molecules to different omega groups attached to different receptor sites.
Since certain changes may be made in the above apparatus without departing from the scope of the invention herein involved, it is intended that all matter contained in the above description or shown in the accompanying drawing shall be interpreted in an illustrative and not a limiting sense.