Process for detecting or quantifying a biological rreaction using superparamagnetic label

A method for detecting the occurrence of a biological reaction or quantitating its result employing superparamagnetic particles is disclosed. The particles are first conjugated or adsorbed to identical biomolecules which are members of a biological binding pair and the conjugates or adsorbates so formed are then contacted with a liquid or solid sample known to contain, or suspected of containing, molecules that are the biological binding partners of the biomolecules in the conjugates or adsorbates. The conjugates or adsorbates are digested with the liquid or solid sample for a time sufficient to enable the formation of a tightly bound, three-dimensional mass comprised of interlinked biomolecules and bound superparamagnetic particles. The mass is exposed to a magnetic filed for the shortest period necessary to induce magnetization of the superparamagnetic particles, whereupon the magnetic field is immediately removed. It has been found that the mass exhibits, in concert, a measurable nonpermanent magnetization which persists for at least 20 minutes following exposure to the magnetic field. If it is desired only to confirm that reaction occurred, this may be done by confirming the existence of a magnetic signal with a suitable instrument. If a quantitative result is desired, the intensity of the magnetization may be measured over time and correlated to the quantitative concentration, or known number, of one of the biomolecules in the biological binding pair.

DETAILED DESCRIPTION OF THE INVENTION According to the present invention, superparamagnetic particles which are individually too small to maintain any degree of magnetization after exposure to the action of a magnetic field of the strength of about 10,000 Gauss for a period as short as possible, preferably 10 seconds or less, and not more than 30 seconds, have been shown to acquire measurable non-permanent aggregative magnetization—i.e., collective magnetization of an aggregated, interacted three dimensional biomass—when closely incorporated into a tightly packed three-dimensional mass with agglomerated biological material such as, e.g., a mass of labeled superparamagnetic antibody:antigen:immobilized antibody “sandwiches”, a clotted mass of labelled blood platelets, a mass of chromatographically separated protein, and the like. Use of superparamagnetic particles as labels for biological, including biochemical, reactions offers substantial advantages over many of the labels now used, e.g., in various assay systems. For example, superparamagnetic particles, in contrast to ferromagnetic particles, do not display remanent magnetization and have no magnetic properties until subjected to the influence of a magnetic field. They are accordingly virtually unlimitedly shelf-stable in contrast to many of the labelling materials in common use, including colloidal metals, enzymes, chemiluminescent agents, radioactive tracers etc. Their stability renders them easy to mix with other substances, to suspend freely in liquids and otherwise to work with, so long as they are not exposed to magnetic fields of sufficient intensity to excite magnetization. Nonpermanent aggregative magnetization as observed in the context of the present invention is a measurable phenomenon which is a straight line function of the concentration, or number, of target biomolecules in a test sample. However, care must be taken to measure the nonpermanent aggregative magnetization at the same time interval after removal of the magnetic field that causes this magnetization if one is to achieve comparable results in a series of tests—e.g., tests conducted at different concentrations of a target analyte molecule, tests undertaken to construct a standard curve, tests undertaken with the intent to rely on an already constructed standard curve to determine concentration present in a sample, etc. It is believed that particle size, surface features of the particles, magnetic field strength and time employed in the magnetization step, as well as the mean distance between the magnetized particles trapped in the end product mass of bioorganic material and bound particles, will all play a role in the length of time within which nonpermanent aggregative magnetization persists and the rate at which it decays. It also appears that the decay, at least in systems so far tested, occurs at a rate such that correlation of the number of bound particles with the concentration of a target analyte or other target molecule can be achieved when total magnetization measurements are made not only at the 5 minute interval following the magnetization step that was chosen for the work underlying this invention but at some other uniform interval from that step. Care must be taken, of course, that the measurements are taken at an interval such that measurement of total magnetization yields readings that are in excess of the reading for any background magnetization that may need to be deducted, depending upon the “platform” or biological matrix that may be present. Furthermore, before selecting a different interval for measurement of total magnetization, one needs to ensure that the rate of decay of non-permanent aggregative magnetization follows a consistent pattern for superparamagnetic particles that have the same treatment history. As used herein, “superparamagnetic particles” refers to particles that are magnetizable but retain no permanent magnetization when tightly packed together in close association in a mass of inter-reacted bio-organic materials and that, when measured individually after attempted magnetization, exhibit no remanent magnetization. These particles may comprise pure metals such as, Fe, Co, Cd, Ru, Mg, Mn, etc. that are known to be readily magnetizable, iron oxide, CoFe 2 O 4 , MgFe 2 O 4 and oxides of other metals that are known readily to be magnetizable when a mass thereof is subjected to the influence of a magnetic field, such as nickel, cadmium, cobalt, ruthenium, etc. Also usable are, e.g. Fe-Ru and its oxides and other metal combinations, and oxides thereof, that exhibit spinel structure upon examination by X-ray diffraction and Transmission Electron Microscopy. It should be noted that pure metals are superparamagnetic only within a physical size range wherein the average mean diameters are confined with a few nanometers, usually less than 5 nm. Superparamagnetic particles of pure metals are also chemically unstable whereas oxides of corresponding superparamagnetic particles are relatively inert and maintain their superparamagnetism within a broader physical size range. These superparamagnetic particles are not intrinsically reactive with bio-organic materials and often are desirably coated with a substance that enables them to react with a binding partner of the target molecule which is to be monitored, assayed for, or otherwise located and quantitated. Various methods of and materials for such coating are known and have been used in the past for coating polymers or glass, including glass beads and solid polymer—comprising inserts or “dipsticks” that have been used in various assay systems. The same coating methods and materials are useful in coating superparamagnetic particles to be used in detecting end products of biological, including biochemical, reactions. Various methods of adsorption are also well known wherein proteins and the like are directly adsorbed on, e.g. iron oxides and the like and they also may be utilized in this invention to improve the reactivity of the particles. Superparamagnetic particles are distinguished from both ferromagnetic, including (ferrimagnetic), particles, which acquire permanent remanent magnetization upon exposure to an external magnetic field, and also from paramagnetic materials, which have a positive magnetic susceptibility less than 0.001 times that of ferromagnetic materials. The magnetic susceptibility of superparamagnetic materials lies between that of ferromagnetic materials and particles is intermediate that of ferromagnetic and paramagnetic particles. Ideal superparamagnetic systems, at temperatures equal to or below their critical blocking temperature exhibit a slow relaxation time—i.e., they revert from a magnetized state to a non-magnetized state slowly. The particles used in the work underlying this application were of 5-15 nm average mean diameter as measured by X-ray diffraction and Transmission Electron Microscopy and exhibited a blocking temperature slightly above room temperature, i.e., slightly above about 25° C. They were of pure Fe 3 O 4 having spinel structure, as confirmed by X-ray diffraction and transmitted electron microscopy. Superparamagnetic materials are known by physicists not to exhibit remanent magnetization. The hysteresis loop of superparamagnetic materials (i.e., the plot obtained by plotting magnetization against magnetic field strength) is curve-like and it typically resembles those shown in FIGS. 2 and 2 A hereof. This is in contrast to the typical hysteresis loop of ferromagnetic materials ( FIG. 1 ) and the linear hysteresis plot obtained with paramagnetic materials. Most usually, superparamagnetic particles have a small average mean diameter in the order of less than 50 nm, often 30 nm or less, in physical size as measured by X-ray diffraction and Transmission Electron Microscopy—although in some systems larger sizes of particles with superparamagnetic properties have been observed. (It is noted parenthetically that both their size and behavior after removal from a magnetic field suggest that the 800 nm and larger particles referred to in the Adelmann et al. article were ferromagnetic and not superparamagnetic). Still further, it is typical of superparamagnetic particles that the magnetization they may exhibit when subjected to a magnetic field, decays with time until it dissipates altogether. Finally, superparamagnetic particles possess a degree of magnetic ordering—i.e., they have what is called a subdomain structure consisting of clusters of varying sizes containing some atoms with unpaired electrons in the unmagnetized state, but the clusters are small and scattered in comparison to the “domain structure” of ferromagnetic materials which are characterized by larger clusters called domains in which each atom or other structural unit has unpaired electrons that impart a net magnetic moment. In these latter materials, each domain exhibits a directional magnetic effect which is the vector sum of all unpaired electrons present in that domain. In sum, ferromagnetic materials have strong magnetic ordering, superparamagnetic materials have some magnetic ordering, but much less than ferromagnetic materials. The presence of magnetic ordering in superparamagnetic materials has been confirmed by neutron diffraction measurements. See Chen et al., “Synthesis of Paramagnetic MgFe 2 O 4 Nanoparticles by Coprecipitation”, J. Magnetism and Magnetic Materials , vol. 94, pp. 1-7 (1999). Paramagnetic materials have no magnetic ordering. For a good technical description of similarities and differences physicists recognize among “ferromagnetic”, “superparamagnetic”, and “paramagnetic” materials, see Chen et al., “Size-dependent Superparamagnetic Properties of Mg 2 Fe O 4 Spinel Ferrite Nanocrystallites”, Appl. Phys. Letters , vol. 73, pp. 3156-8 (1998). It is possible that the ability to measure non-permanent aggregative magnetization in agglomerated three-dimensional reaction products comprising bio-organic materials in close association with superparamagnetic particles is attributable to the relatively stable macrostructure of these reaction products, which macrostructure holds the incorporated particles in place and prolongs the decay, of the magnetization imparted by exposure to a magnetic field. Applicants, however, have not established this or any other scientific explanation for the reproducible phenomenon observed in the experimental work relating to this invention and hence do not intend to bind themselves to any particular explanation. The present invention provides enhancements in a virtually unlimited spectrum of in vitro biological reactions including at least immunoassays, DNA probes, oligonucleotide probes, chromatographic molecular separations and other biological reactions where it is desirable to quantitate the amount of a target molecule present. It is believed that this invention may also be useful in monitoring certain in vivo biological reactions. The detection system of this invention is beneficially employed in immunoassays generally, but especially in immunochromatographic and other “lateral flow” assays and in so-called “flow through” assays—i.e., those involving vertical flow steps in which the reactants are brought together. The Midwest Scientific Co. newsletter, Shark Bytes, for October 2000 describes a form of assay now in development at Ohio State University wherein a compact disc (“CD”) rotated by a compact disc player is equipped with tiny reservoirs and channels that cause medical samples suspected of containing target analyte to mix with tiny pools of test reagents. Including superparamagnetic particle tagged binding partners for analytes suspected of being present in tiny pools on such test platforms would lead to very useful assays capable of being rapidly performed and rapidly evaluated via the computer anticipated to be included in the CD player of the Ohio State system. This computer could readily be programmed to read non-permanent aggregative magnetization imparted by an external magnetic field in digital form and to correlate this reading to stored information corresponding to a standard curve. As those skilled in immunoassays will recognize, one CD with associated CD player-computer combination could readily be adapted to perform several assays simultaneously on portions of a single test sample by providing, e.g., different antibodies for different target analytes conjugated to superparamagnetic particles placed in different “pool” regions of the CD. In addition to CD's, other “platform” materials upon which superparamagnetic particle-tagged biomolecules may be reacted with target biomolecules in a test sample are contemplated to be useful in work performed within the scope of this invention. Possibilities specifically explored in preliminary work, in addition to what the specific examples below show, are balsa wood and glass. With balsa wood, it was found that even though the material is intrinsically non-magnetic and non-magnetizable its capillarity may lead to readings of non-permanent aggregative magnetization that exhibit a very large standard deviation. It is believed that filling these capillaries with non-magnetizable plastic or with a substance such as bovine serum albumin, other proteins, polyethyleneglycol or other substances well known for blocking capillarity in “dipstick” type immunoassay devices described in the prior art would render balsa wood more acceptable as a platform. Glass was found to avoid the capillarity problem and to be a satisfactory platform material, provided that appropriate background readings are obtained, allowing one to compensate for the fact that most glass slides contain sufficient iron to be magnetizable to a low degree upon exposure to a magnetic field. This makes it necessary to determine the background signal and subtract it from sample readings whenever biological reactions wherein one reactant is labelled with superparamagnetic particles according to this invention are run on glass as a platform. To measure non-permanent aggregative magnetization, various instruments may be used. In this regard, several different research and development groups are in process of developing relatively low cost measuring instruments which apply knowledge gleaned from high resolution magnetic recording technology and computer disk drive technology. One of these is the Ericomp Maglab 2000, at least one early prototype version of which is illustrated in the Adelman J. Assn. for Lab. Automation article cited hereinabove. Another is the Quantum Design, Inc. instrument described in U.S. Pat. No. 6,046,585 issued Apr. 4, 2000. The following specific examples illustrate the substitution of superparamagnetic labelling according to this invention for colloidal gold in an immunochromatographic (“ICT”) assay for Canine Heart Worm that is commercially available from Binax, Inc, assignee of this patent application. 
 EXAMPLE 1 
 Selection of Coating Agent for Superparamagnetic Particles Superparamagnetic particles can be coated with a reagent that has free carboxyl functional groups in order to be capable of covalent coupling to a particular antibody, such as the antibody Canine Heart Worm (“CHW”). Initial work was accordingly performed to ascertain the coating material of choice for this purpose. Because rheological properties are important to the successful operation of ICT assays and past experience with colloidal gold labels has shown smaller particles to be rheologically superior, 10 nm diameter Fe 3 O 4 particles were chosen for this work. Their size was confirmed by X-ray diffraction and by Transmission Electron Microscopy. Two polymeric coating materials were tested on separate lots of superparamagnetic particles using the coating method described in U.S. Pat. No. 5,547,682. One lot of these particles was coated with Chondroitin Sulfate A (“CSA”) and the other with small polyacrylic acid (“sPAA”). The coated particles in suspension, in each instance had a diameter of 40 to 60 nm, as determined by dynamic light scattering. Both the CSA-coated and the sPAA coated superparamagnetic particles were further tested and it was shown thereby that CSA-coated particles were superior in stability and ability to bind to antibodies. The hysteresis loop as measured by Vibrating Sample Magnetometer (“VSM”) tests for three sets of particles having varying iron concentrations, all of which were sPAA coated and had the commercially available antibody bethyl bonded thereto is shown in FIG. 2 . These particles in uncoated form are identical to those which were the starting materials for Example 3 . FIG. 2 shows the typical hysteresis curve shape of superparamagnetic materials and also confirms that these particles had no remanent magnetization. FIG. 2A shows hysteresis measurements made with coated particles of superparamagnetic 10 nm Fe 3 O 4 with a SQUID instrument (curve with black squares) and by VSM (curve with white circles). Also, shown on FIG. 2A is a plot of hysteresis measurements made by VSM of the same coated particles to which a carboxy polysaccharide antibody was conjugated. All three again exhibit the typical shape of superparamagnetic material hysteresis behavior and confirm the particles lack of remanent magnetization. 
 EXAMPLE 2 
 Preparation of Superparamagnetic Particle-Labelled Antibodies CSA-coated superparamagnetic particles prepared as in Example 1 were covalently coupled to anti-Canine Heart Worm (“CHW”) antibody identical to the anti-CHW antibody used in the manufacture of the commercially available Binax ICT test for CHW antigen and were then suspended in phosphate-buffered saline solution containing 10 mg/ml of bovine serum albumin pending their use as in Example 3. 
 EXAMPLE 3 
 Performance of ICT Assay for CHW Using Superparamagnetic Labels ICT flow path test strips of nitrocellulose were treated in the same manner as those used in the Binax commercially available ICT assay for CHW. These strips were incorporated into dipstick-type devices by the lamination of absorbent pad components overlapping opposite ends of the flow path test strip. A series of solutions containing CHW antigen were prepared with antigen concentrations ranging from 100 pg/ml to 200 ng/ml. 190 &mgr;l of each of these antigen solutions were dispensed into separate wells of a new polystyrene 96 well microtiter plate. To these samples were added 5 &mgr;l of the CSA-coated superparamagnetic particle-labelled anti-CHW antibodies of example 2. Labeled antibody/antigen solution mixtures were incubated for 15 minutes at room temperature. The sample receiving end of a dipstick device was added to each labeled antibody/antigen solution mixture immediately following this incubation, causing the mixture to flow into the capture zone. Immobilized unlabelled rabbit polyclonal anti-CHW antibodies bound to the strip in the capture zone thereupon reacted with antigen:superparamagnetic labelled antibody conjugates to form immobilized antibody:antigen:superparamagnetic labelled antibody “sandwiches” along the capture line. The ICT strips were removed from the ICT devices after 15 minutes, and exposed to a magnetic field of 10,000 Gauss for 10 seconds each. After 5 minutes from the removal of the magnetic field, each strip was placed in an Ericomp Maglab 2000 instrument with the capture line in the field of view of the detector and its non-permanent aggregative magnetization was read. Each antigen solution was tested in duplicate in the ICT test as described. The readings of non-permanent aggregative magnetization for both series of sample having known antigen concentrations above 1 ng/ml have been graphed in FIG. 3 against antigen concentration. The Fe content of the immune complexes at the capture lines of ICT devices used in the duplicate series of tests was determined by a chemical calorimetric procedure using a commercial ferrizine test from Sigma Chemical Co. It was found that the calorimetric chemical test results and the magnetization readings correlated well, as shown in FIG. 4, a plot of iron concentration calculated as Fe 3 O 4 , in &mgr;g/ml, against antigen concentration in ng/ml. In the ICT tests as performed in this example, any labelled antibody initially deposited at the flow path threshold that fails to react with antigen in the sample flows past the capture zone and into another pad positioned upstream from that zone. These unreacted paramagnetic particle-labelled antibodies were subjected to the effect of a magnetic field of 10,000 Gauss for 10 seconds, set aside for 5 minutes and then placed in the sensor area of the Ericomp Maglab 2000 instrument and found to exhibit no measurable magnetization. From the results of the foregoing examples, it was determined that the relationship between magnetic reading in relative magnetic units (“RMU”) and concentration of antigen (the target analyte) is an essentially linear function in the range of 1 ng antigen/ml to 150 ng antigen/ml. See FIG. 3, a plot of relative magnetic units against antigen concentration in ng per ml for each of the two series of CHW assays performed. The instrument noise, however, caused large standard deviations in the readings of samples having concentrations of antigen below 1 ng/ml. This is illustrated in FIG. 5, a plot of measured values in relative magnetic units against antigen concentration in pg per ml. The fact that ICT strips having 200 pg/ml of added antigen had non-permanent aggregative magnetization that could be read when that antigen was incorporated in labelled antibody:antigen: immobilized antibody sandwiches collected in a mass, subjected to 10,000 Gauss of magnetic field for 10 seconds, and then set aside for five minutes is an indication nonetheless that the sensitivity of the test is significantly enhanced by substituting superparamagnetic labels for colloidal gold labels. With an improved instrument having reduced noise, it is clear that the physical sensitivity of superparamagnetic labelling as described approaches 0.1 ng of Fe calculated as Fe 3 O 4 or about 10 −18 per mole, while the broad dynamic sensitivity range will fall between about 1 and 10 6 relative units and has potentially high tolerance to interference from various biological matrices that may present. While the invention has been exemplified in the context of a well known immunodiagnostic system specific to the antigen of the causative agent for the canine disease Dirofilaria immitis, the vast range of applications in which it will produce greatly improved results or will enable precise quantitative measurement of observed phenomena previously deemed to be difficult to impossible to measure will be readily apparent to those ordinarily skilled in immunochemistry and/or biology. It is accordingly intended that the scope of this invention be limited only to the extent of the scope of the appended claims.