Giant magnetoresistance-based biosensors

Abstract

Disclosed is a system for detecting one or more target analytes which includes a resistor structure comprised of a substrate, a graphene-based nanocomposite material located on a surface of the substrate with the graphene-based nanocomposite material exhibiting one or more magnetoresistance properties. A surface of the nanocomposite material includes molecular sensing elements bound thereto which exhibit an affinity for binding with the target analytes. Electrodes are connected to the resistor structure connectable to a power source and a device for measuring a resistance across the resistor structure for sensing a giant magnetoresistance (GMR) value of the resistor structure. Included are magnetic colloidal nanoparticles exhibiting preselected magnetic properties with an outer surface of the magnetic colloidal nanoparticles being modified to allow interaction with the surface of the resistor structure resulting in a change in the GMR value of the resistor structure. The resistor structure is configured to be operably connected to a magnetic field generating device configured to apply a magnetic field to the graphene-based nanocomposite wherein the field has a magnitude in a range from greater than 0 to about 5 Tesla. A presence of target analytes in a vicinity of the surface of the resistor structure induces the interaction to occur by binding of the target analytes to the molecular sensing elements bound thereto causing a change in GMR value of the resistor structure.

Claims

1. A system for detecting one or more target analytes, comprising: a) a first resistor structure comprising a substrate, a graphene-based nanocomposite material located on a surface of said substrate, said graphene-based nanocomposite material exhibiting one or more magnetoresistance properties, a surface of said graphene-based nanocomposite material including molecular sensing elements bound thereto which exhibit an affinity for binding with said target analytes, and electrodes connected to said resistor structure connectable to a power source and a device for measuring a resistance across said resistor structure for sensing a giant magnetoresistance (GMR) value of said resistor structure; b) magnetic colloidal nanoparticles exhibiting preselected magnetic properties, an outer surface of said magnetic colloidal nanoparticles being modified to allow interaction with said surface of the resistor structure resulting in a change in the GMR value of said resistor structure; c) said resistor structure configured to be operably connected to, a magnetic field generating device configured to apply a magnetic field to said graphene-based nanocomposite wherein the field has a magnitude in a range from greater than 0 to about 5 Tesla; wherein a presence of target analytes in a vicinity of the surface of said resistor structure induces said interaction to occur by binding of said target analytes to said molecular sensing elements bound thereto causing a change in said GMR value of the resistor structure.

2. The system according to claim 1, wherein the magnetic colloidal nanoparticles have a diameter in a range from about 1 nm to about 1000 nm.

3. The system according to claim 1, wherein the magnetic colloidal nanoparticles are any one of Fe, Co, Ni, Fe.sub.xCo.sub.y (x+y=100), Fe.sub.xNi.sub.y (x+y=100), FePt, EuO, Eu.sub.1-xGd.sub.xSe (0.02≤x≤0.8), or Gd.sub.3-xS.sub.4 (0≤x≤0.8) based nanoparticles.

4. The system according to claim 1, wherein the magnetic colloidal nanoparticles are any one of nanoparticles containing Fe, Co, or Ni, or core-shell magnetic nanoparticles including silica coated magnetic nanoparticles, gold coated magnetic nanoparticles, or chitosan-coated magnetic nanoparticles.

5. The system according to claim 1, further comprising two or more additional resistor structures mounted on said substrate and connected in series with said first resistor structure and said power supply and said device for measuring resistance and magnetoresistance, and wherein said magnetic field generating device is configured to apply the same magnetic field to all of said resistor structures.

6. The system according to claim 1, wherein said molecular sensing elements bound to said surface of said graphene-based nanocomposite material include any one or combination of a glucose binding protein, Concanavalin A, glucose oxidase enzyme, boronic acid, antibody, DNA sequences, and amyloid-β-derived diffusible ligands (ADDLs).

7. The system according to claim 1, wherein the one or more target analytes being detected for include any one or combination of glucose, DNA, proteins, lipids, or microbes.

8. The system according to claim 1, wherein a thickness of the graphene-based nanocomposite is in a range from about 100 nm to about 5 mm.

9. The system according to claim 1, wherein the magnetic colloidal nanoparticles and graphene-based nanocomposites are selected such that the interaction between the colloidal magnetic nanoparticles and graphene-based nanocomposites changes the magnetic field in a range from about 5 Oe to about 30 KOe.

10. The system according to claim 1, wherein in use the mixture containing magnetic colloidal nanoparticles are introduced onto the surface of the resistor structure by a liquid dispenser which can be any one of a pipettor, a pump-connected microfluidic system, or a fluidic loop system.

11. The system according to claim 1, wherein in use the magnetic colloidal nanoparticles which are not bound on the surface of resistor structure after being exposed thereto are removed from the surface of the resistor structure by any one of aqueous-based washing solution, or a pumping solvent removal system, or vortex microfluidic technology.

12. The system according to claim 1, wherein during preparation of the resistor structure, a thickness of the graphene-based nanocomposite material is controlled by use of a hydraulic press, a method of physical deposition, a method of chemical coating or a method of 3-D printing.

13. The system according to claim 1, wherein the magnetic colloidal nanoparticles have a diameter in a range from about 1 nm to about 1000 nm.

14. The system according to claim 1, wherein the system is configured to be connected with a wireless system for real-time and remote detection.

15. The system according to claim 1, wherein said outer surface of said magnetic colloidal nanoparticles are modified to include molecular sensing elements bound thereto which exhibit an affinity for binding with said target analytes, and wherein when target analytes are present in a vicinity of the magnetic colloidal nanoparticles they bind to said molecular sensing elements on said magnetic colloidal nanoparticles, and when said magnetic colloidal nanoparticles with target analytes bound thereto are in a vicinity of said surface of said resister structure, the interaction with the surface is binding of the target analytes, bound to their respective magnetic colloidal nanoparticles, to said molecular sensing elements bound to said surface of said resistor structure.

16. The system according to claim 15, wherein the molecular sensing elements bound to the surface of the magnetic colloidal nanoparticles which exhibit an affinity for binding with said target analytes include functional groups selected from the group consisting of hydrogen, hydroxyl, carboxyl, amine, amide, phosphate, thiol, methyl, and polyethylene glycol (PEG) derivatives.

17. The system according to claim 1, wherein said outer surface of said magnetic colloidal nanoparticles are modified to include competing molecules bound thereto which exhibit an affinity for binding with said molecular sensing elements bound to said surface of said resistor structure, and wherein when said target analytes are in a vicinity of said surface of said resister structure, the interaction with the surface is binding of the target analytes is displacement of the bound magnetic colloidal nanoparticles and binding of said target analytes to said molecular sensing elements bound to said surface of said resistor structure.

18. The system according to claim 17, wherein said competing molecules bound to the surface of the magnetic colloidal nanoparticles which exhibit an affinity for binding with said molecular sensing elements bound to said surface of said resistor structure include functional groups selected from the group consisting of hydrogen, hydroxyl, carboxyl, amine, amide, phosphate, thiol, methyl, and polyethylene glycol (PEG) derivatives.

19. The system according to claim 1, wherein the substrate is comprised of any one of a ceramic, a polymer or a metal.

20. The system according to claim 9, wherein the ceramic comprises of any one of SiC or glass, and wherein the polymer comprises any one of polydimethylsiloxane (PDMS) or biopolymers.

21. The system according to claim 1, wherein the electrodes are comprised of metals or carbon-based materials.

22. The system according to claim 21 wherein the metals comprise any one of gold (Au), tungsten (W), platinum (Pt).

23. The system according to claim 1, wherein graphene-based nanocomposites are comprised of graphene nanosheets loaded with magnetic nanocrystals.

24. The system according to claim 23, wherein a weight ratio of said graphene to said magnetic nanocrystals is in a range from about 2:98 to about 98:2.

Description

BRIEF DESCRIPTION OF THE DRAWINGS

(1) Embodiments will now be described, by way of example only, with reference to the accompanying drawings, in which:

(2) FIG. 1A is a graphical depiction of the magnetoresistance phenomenon related to the structure of the electrical orbitals at the Fermi surface.

(3) FIG. 1B is a schematic of multilayer magnetic films with giant magnetoresistance effect as disclosed herein.

(4) FIG. 2A is a schematic view of a multilayer giant magnetoresistance system (Prior Art)

(5) FIG. 2B is a schematic view of a spin valve giant magnetoresistance system (Prior Art)

(6) FIG. 2C is a schematic view of a pseudo-spin giant magnetoresistance system (Prior Art)

(7) FIG. 2D is a schematic view of a granular giant magnetoresistance system (Prior Art)

(8) FIG. 3A is a schematic of the deposition process of Graphene-FexCoy nanocomposites made by an electroless deposition

(9) FIG. 3B is a graphical illustration of the change in magnetoresistance of graphene deposited with field size of FeCo nanocrystals measured by a vibrating sample magnetometer.

(10) FIG. 3C is a TEM micrograph of the graphene-based nanocomposite.

(11) FIG. 4A is a schematic of a MAPLE process for depositing nanoparticles on a graphene sheet.

(12) FIG. 4B is a view of the graphene sheet from FIG. 4A.

(13) FIG. 4C is a TEM micrograph of graphene-Fe nanocomposite by MAPLE for 60 min.

(14) FIG. 5 is an Illustration of the graphene-based GMR biosensor device.

(15) FIG. 6A is a schematic diagram of the direct measurement process using the GMR biosensor device

(16) FIG. 6B is a schematic diagram of the competitive measurement process using the GMR biosensor device.

(17) FIG. 7A is a schematic of current applied to a graphene sheet where Fe50Co50 crystals are randomly distributed on the sheet.

(18) FIG. 7B is a graphical representation of the magnetoresistance of various graphene sheet samples at room temperature

(19) FIG. 7C is a schematic of a current applied to a graphene sheet where the spins of the sheet/crystals are unaligned and the applied magnetic field has a zero magnitude.

(20) FIG. 7D is a schematic of a current applied to a graphene sheet where the spins of the sheet/crystals are aligned and the applied magnetic field has a positive magnitude.

(21) FIG. 8 is a graphical illustration of the relative GMR value and its relation to the concentration of targeted biomolecules.

(22) FIG. 9 is a schematic diagram of the integration of GMR biosensor by using graphene nanocomposites into wireless system.

DETAILED DESCRIPTION

(23) Various embodiments and aspects of the disclosure will be described with reference to details discussed below. The following description and drawings are illustrative of the disclosure and are not to be construed as limiting the disclosure. The figures are not to scale. Numerous specific details are described to provide a thorough understanding of various embodiments of the present disclosure. However, in certain instances, well-known or conventional details are not described in order to provide a concise discussion of embodiments of the present disclosure.

(24) As used herein, the terms, “comprises” and “comprising” are to be construed as being inclusive and open ended, and not exclusive. Specifically, when used in the specification and claims, the terms “comprises” and “comprising” and variations thereof mean the specified features, steps or components are included. These terms are not to be interpreted to exclude the presence of other features, steps or components.

(25) As used herein, the term “exemplary” means “serving as an example, instance, or illustration,” and should not be construed as preferred or advantageous over other configurations disclosed herein.

(26) As used herein, the terms “about” and “approximately” are meant to cover variations that may exist in the upper and lower limits of the ranges of values, such as variations in properties, parameters, and dimensions. In one non-limiting example, the terms “about” and “approximately” mean plus or minus 10 percent or less.

(27) As used herein, the phrase “Giant magnetoresistance (GMR) effect” refers to the significant change (>3%) of electrical resistance of a material or a device under a magnetic field.

(28) As used herein, the phrase “graphene-based nanocomposite” refers to a composite made of graphene and hybrid graphene, and magnetic nanostructures.

(29) As used herein, the phrase “sensing elements” refers to the chemical molecules, or biomolecules, which is able to involve in a reaction with targeted molecule.

(30) Unless defined otherwise, all technical and scientific terms used herein are intended to have the same meaning as commonly understood to one of ordinary skill in the art.

(31) Methodology

(32) The currently disclosed process involves the loading of magnetic nanocrystals into graphene sheets. Several different embodiments of the currently claimed process involve different wet chemical processes which will incorporate the magnetic nanocrystals into graphene-based sheets.

(33) In an embodiment, a method is applied to produce iron (Fe)-loaded graphene sheets. The method of this embodiment is a specific chemical process that is applicable for generating graphite oxide through the addition of potassium permanganate to a solution of graphite, sodium nitrate, and sulfuric acid. It may be utilized for producing quantities of graphite oxide. In this embodiment, graphite is oxidized to graphene oxide (GO) by potassium permanganate and sulfuric acid. At a neutral pH value, the graphene oxide is reduced to graphene via Fe powder. 1 g of Fe powder (average particle size: 10 μm.) and 20 mL of HCl (35 wt %) is directly added into 100 mL of GO suspension (0.5 mg/mL) at ambient temperature. The mixture is then stirred for 30 min and then maintained for a period of time. After reduction, 15 mL of HCl (35 wt %) is added into the GO solution in order to fully remove excess Fe powder. Finally, the graphene sheets is collected through filtration, and are washed with pure water and ethanol several times, and dried at 100° C. for 12 hours (h) in a vacuum oven.

(34) In an additional embodiment of the process for forming loaded graphene sheets, an electrolytic process is applied to deposit magnetic nanoparticles (Fe.sub.xCo.sub.y nanocrystals, etc.) onto graphene sheets as shown in FIG. 3A. In this embodiment, magnetic graphene/Fe.sub.50Co.sub.50 hybrid nanosheets are synthesized by growing Fe.sub.50Co.sub.50 crystals onto graphene nanosheets via a facile polyol process. First Fe.sub.50Co.sub.50 crystals were synthesised through a modified polyol process according to reference (Kodama D, et al. Adv Mater 18, 3154-3159 (2006). Briefly, 2.5 mmol FeCl.sub.2.4H.sub.2O and 2.5 mmol Co(Ac).sub.2.4H.sub.2O metal salts precursors were mixed with 200 mmol sodium hydroxide in 100 mL ethylene glycol. The mixture was stirred under nitrogen gas protection and heated to 130° C. After 1-hour reaction at 130° C., the black mixture was collected by magnet and washed by pure ethanol. The crystals were dried in vacuum under room temperature.

(35) Graphene/Fe.sub.50Co.sub.50 hybrid nanosheets were synthesizing by the following procedure, varied graphene powder (30 mg, 60 mg and 120 mg, samples namely G.sub.9.5FeCo.sub.90.5, G.sub.17.3FeCo.sub.82.7 and G.sub.29.5FeO.sub.70.5 respectively, the subscript is weight ratio percentage), metal precursors (2.5 mmol FeCl.sub.2.4H.sub.2O and 2.5 mmol Co(Ac).sub.2.4H.sub.2O) and 200 mmol sodium hydroxide was mixed in 100 mL ethylene glycol and reacted at 130° C. for 1 hour under nitrogen gas protection. In this polyol process, the Fe.sub.50Co.sub.50 crystals were deposited on the graphene nanosheets surface. The Graphene/Fe.sub.50Co.sub.50 hybrid nanosheets were collected by magnet and washed by ethanol and freeze-dried. Graphene/Fe.sub.50Co.sub.50 nanosheets powder was pressed into thin film pellets for 4-probe magnetoresistance measurement. To control the thickness of the graphene-based GMR sensor, different methods can be used including hydraulic press, or a method of physical deposition, a method of chemical coating, or a method of 3-D printing. The graphene and Fe.sub.50Co.sub.50 crystals weight ratio are calculated using nominal adding chemical amount.

(36) The Fe.sub.50Co.sub.50-graphene nanocomposites show strong ferromagnetic properties with high saturation magnetization. An unexpected result of this particular embodiment of the method is that the prepared graphene/Fe.sub.50Co.sub.50 hybrid nanosheets show a magnetoresistance effect as high as 7.8±0.5% at low magnetic field of 9.5 kOe at room temperature. This phenomenon is further displayed in FIGS. 3A and 3B. In this non-limiting experimental example, the magnetoresistance of graphene which was deposited with FeCo nanocrystals, shown in FIG. 3A was measured using a vibrating sample magnetometer (VSM). The hybrid nanosheet pellets exposed in this micrograph also demonstrate magnetic anisotropic properties as is demonstrated by the measured magnetoresistance as shown in the TEM micrograph of the graphene based nanocomposites of FIG. 3B. The GMR effect of graphene nanocomposites is likely related to the high spin carrier/mobility.

(37) In an embodiment, the synthesis of magnetic nanoparticles that physically deposit on graphene sheets is controlled through a laser-based fabrication device. A laser-assisted fabrication system is utilized to achieve the physical deposition of the magnetic nanoparticles. The pulsed-laser fabrication system is equipped with an optical parametric oscillator (OPO) which enables fine tuning of the wavelength of the laser from 532 to 1900 nm. This embodiment utilizes the process of matrix assisted pulsed laser evaporation (MAPLE). MAPLE provides a more moderate and protective method to deposit organic and inorganic nanoparticles on various substrates. MAPLE is a contamination free surface modification system. In this embodiment, the target material is normally dissolved or suspended in solvent which is highly volatile. The solution is then frozen by liquid nitrogen, and the frozen solution (target) is irradiated by a pulsed laser beam.

(38) In an embodiment, the entire process is performed under vacuum (1×10.sup.−6 Torr) where solvent molecules are pumped away, and the target nanoparticles, which are heavier than the solvent molecules, remain in the chamber and are deposited on the surface of the substrate. Generally, the Nd:YAG laser (wavelength 532 nm) has 10 Hz frequency, and T.sub.fwhm≅200 μs. The resulting laser fluence is 0.120 J/cm2 with a laser spot size diameter of 0.56 cm and will result in the deposition of hybrid nanostructures on the surface of the substrate

(39) Referring to FIG. 4A and FIG. 4B, the currently claimed embodiment of the physical process involves deposition of Fe-based nanoparticles 416 on graphene sheets 418 by the MAPLE process. The substrate holder 400 contains mounted graphene sheets 412 which are coated by a ceramic substrate 410. This coating by the ceramic substrate may be achieved though dip-coating and/or spin coating at room temperature. The magnetic powder 402 is suspended in a suitable solvent 404 and is then introduced into the target holder 406 cooled by liquid nitrogen, which is filled in the sidewall of the target holder. In the MAPLE process, the Fe-based powder is ablated by the laser irradiation 408 due to the photon-electron interaction.

(40) Under the high vacuum, the ablated nanoparticles can be transported with the evaporated solvent to the graphene sheets in a contamination-free manner, and without damaging the graphene sheet as the solvent is pumped away during the deposition process. Energy dispersive X-ray analysis (EDX) with a TEM was carried out for the element analysis was carried out by and confirmed the presence of Fe. FIG. 4C shows the TEM micrograph of graphene sheet with the deposition of Fe nanoparticles under laser irradiation for 60 minutes.

(41) To achieve the GMR effect in graphene nanocomposites, the interaction between graphene and magnetic nanoparticles should preferably be closely and accurately controlled. Magnetic properties of the resulting sheets are particularly affected by these interactions. The GMR effect of different designs of graphene nanocomposites is varied by utilizing various magnetic nanocrystals and by arranging the magnetic nanocrystals on or within the targeted graphene sheet(s). The magnetic properties will vary as a function of temperature from 80K to 300K under a magnetic field from 100 Oe to 10 KOe.

(42) The magnetoresistance curves may be measured by an accessory 4-probe detecting device of a VSM, the sample plane and a current applied parallel to the applied magnetic field. In this experimental setup, the magnetoresistance value is determined by analysing changes in the resistance at an instance of zero magnetic field application and any other instance where a field is applied. The MR value is highly dependent on the distance between magnetic nanocrystals on/in the graphene sheets, the size of magnetic nanocrystals, and the type of magnetic crystals.

(43) In addition to the various embodiments of the fabrication process, the effect of the ratio of nanoparticles and graphene matrix on GMR effect have been systematically studied in the range of 100 Oe to 10 KOe to establish a viable detection range for the GMR sensor. Bio-conjugation of the nanocomposite GMR leads in this sensor enables the devices capability to detect various biomolecules-involved in the reaction. The graphene-based GMR sensor is advantageous for three significant reasons. First, the sensor can be utilized as a platform for diagnosis of various diseases. The sensor can also be used for detecting biomolecules-involved processes in various body fluids including but not limited to, blood, tears, saliva, and urine. Lastly, the graphene based GMR sensors are also useful for detecting changes in biomarker levels within bodily fluid through detecting corresponding changes in the electrical resistance of the GMR sensor. The detected changes in the biomarker level can be transferred through computer/wireless system for real-time and remote diagnosis.

(44) Design of GMR Biosensor

(45) Referring to FIG. 5 a biosensor device 500 is shown that employs the graphene-based nanocomposite 504 or multiple graphene-based nanocomposies in a designed electrical circuit to have the GMR effect and electrodes 502 (where the electrode is composed of metals including but not limited to Au and Pt, or carbon-based materials) onto a mounting substrate 506 (where the substrate is composed of ceramics including but not limited to SiC and glass, or polymer-based materials) for detecting the electrical resistance while the magnetic field is changed in the range of 10 Oe to 30 KOe.

(46) Referring to FIGS. 6a and 6b, the currently claimed system utilizes methods (two or more electrodes) for measurement within a GMR based biosensor; direct measurement, and in-direct measurement, i.e. competitive measurement, for detecting biomolecules-involved reactions.

(47) FIG. 6a is an illustration showing the direct method. The targeted molecules interact with the magnetic colloidal nanoparticles modified with functional group, e.g. hydroxyl, carboxylic acid, amine, amide, phosphate; following that, the magnetic nanoparticles binding with target molecules or target analytes can directly interact with the sensing elements on the surface of graphene-based GMR sensor. The magnetic colloidal nanoparticles binding with target analytes will be introduced on to the graphene-based GMR sensor by a liquid dispenser by applying any one of the techniques; e.g. pipettor, a pump-connected microfluidic system, or a fluidic loop system. The GMR or MR value will change as a function of the amount of magnetic nanoparticles binding with targeted molecules.

(48) FIG. 6b is an illustration showing the in-direct method. The magnetic colloidal nanoparticles binding with competing molecules interact with the surface of the graphene based GMR sensor, resulting in the changes of the GMR or MR value of the graphene based GMR sensor. The magnetic colloidal nanoparticles binding with competing molecules will be introduced on to the graphene-based GMR sensor by a liquid dispenser by applying any one of the techniques; e.g. pipettor, a pump-connected microfluidic system, or a fluidic loop system. The competing molecules, e.g. β-cyclodextrin (β-CD), has a weaker interaction with the sensing element modified on the graphene-based nanocomposites as compared to the targeted molecules, e.g. glucose. In the presence of targeted molecules, the targeted molecules will compete with the magnetic nanoparticles binding with competing molecules to bind on the graphene-based nanocomposites; and the replaced magnetic colloidal nanoparticles will be removed by an aqueous-based washing solution which can be cleaned by any one of the following methods, pumping solvent removal system, or a vortex microfluidic technology pumping solvent removal system. Therefore, the signal of the graphene based GMR sensor with be restored with increasing the targeted molecules.

(49) Current experiments completed using these two methods of measurement for the GMR sensing of biomolecules-involved reaction have employed glucose and DNA in the sensing process. The sensing process could additionally utilize other molecular structure including but not limited to antibody, antigen, growth factor, etc.

(50) Referring to FIG. 7A the current is applied parallel to applied magnetic field on graphene-based nanocomposites made of graphene and Fe.sub.50Co.sub.50 nanocrystals. FIG. 7C illustrates the resulting samples where the resulting magnitude of the applied magnetic field is zero, and the spins of the electrons in the crystals and graphene sheets are unaligned. FIG. 7D illustrates the resulting samples where the magnitude of the applied field is greater than zero, and the spins of the current electrons and the electrons in the magnetic nanocrystals and graphene sheets are aligned. These schematics illustrate the magnetic field effect of the samples resulting from the various deposition processes described previously. The magnetoresistance of samples at room temperature for various combinations was analyzed and the resulting MR of pure graphene (G) sheets, G9.5FeCo90.5, G17.3FeCo82.7 and G29.5FeCo70.5 hybrid nanosheets is displayed in FIG. 7B.

(51) The magnetoresistance nanocomposites made of magnetic nanoparticles-loading graphene sheets are produced to have a thickness in the range of 100 nm to 500 μm.

(52) In the various embodiments disclosed herein, surface modification of the graphene-based nanocomposite with GMR properties may be achieved through a variety of modification methods.

(53) In a first embodiment of the modification method, 3-Aminopropyl-triethoxysilane (APTES) is utilized to modify the surface of the GMR sensor. In this modification method, the surface of the graphene-based nanocomposite patch is first cleaned through a purified water wash and is then dried by applying a stream of nitrogen gas to the nanocomposite patch. 3-Aminopropyl-triethoxysilane (APTES) solution in DMSO (5%, w/v, 2 μL) is then applied to the surface of the FeCo nanoparticles-loading graphene sheets. The nanocomposite patch with surface modification and kept for between 3 and a half to 4 and a half hours at room temperature. Once again, the surface is washed by water and dried by nitrogen gas. Lastly, glutaraldehyde (10%, 2 μL) is applied to the surface for 2 hours followed by a final water wash and nitrogen gas drying step.

(54) In a second embodiment of the modification method Con A is utilized to modify the surface of the modification of GMR sensor. In this embodiment, the Con A solution (1 mg/mL) with a volume of 2 μL was applied to the nanocomposite surface for 2 hours at a temperature of approximately 4 degrees C. The nanocomposite surface is then water washed, dried and stored in a concealed space at approximately −20 degrees C.

(55) In one non-limiting example of the surface modification method using Con A, or other sensing elements for detecting glucose, a colloidal magnetic nanoparticle with suitable surface modification is prepared. To prepare this exemplary colloidal magnetic nanoparticle, iron oxide (with a core size of approx. 7 nm) silica (with a shell size of approx. 22 nm) are prepared as follows. A one-pot reaction incorporates base-catalyzed oxidization of iron chlorides (FeCl.sub.2/FeCl.sub.3) followed a condensation, polymerization of tetraethylorthosilicate (TEOS) for silica coating on iron oxide, and encapsulation of organic dye in the shell. First 7.3 g of cetyltrimethyl ammonium bromide (CTAB) was added in 150 mL of toluene. The mixture was stirred at 600 rpm for 4 h, followed by slowly adding the aqueous FeCl.sub.2/FeCl.sub.3 solution (0.2 g/0.5 g, 7.2 mL) under nitrogen (N.sub.2) atmosphere.

(56) Upon completion, the reaction mixture was purged with N.sub.2 for 2 h before it was stirred vigorously for 8 h. An ammonium hydroxide solution (35% NH.sub.4OH in water, 1.0 mL) was then dropped in the solution under N2 protection. The solution was continuously stirred for another 4 h. After then, 7.4 mL of TEOS and 20 mL of toluene were added dropwise. 0.8 mL of ammonium hydroxide solution was then mixed in the one-pot solution under N.sub.2 atmosphere. The mixture was continuously stirred for 5 days under N.sub.2 atmosphere. The pH of mixture was maintained at 8.5-9. The reaction was stopped by an addition of ethanol. The brownish surfactant was removed through centrifuging the solution. The residue was dissolved in ethanol (200 mL), which was refluxed for 15 h at 78° C. before it was cooled down to room temperature. The dark brown precipitates were washed by the mixture of ethanol, water, and acetone with volume ratio of 1:1:1 for three times and collected with magnets. The final product was then freeze-dried and stored as fine powder (reference: Jin Zhang, et al. *, Nanoscale Res. Lett., 4. 1297, 2009).

(57) In a next step of this non limiting example, 3-Glycidyloxypropyltrimethox-ysilane (GLYMO, 98%) and 3-aminophenylboronic acid monohydrate (APB, 98%) are mixed to form GLYMO-APB (GA). In a next step, approximately 5 mL of the formed GA solution is mixed with 20 mg iron oxide/silica through controlled stirring at 75 degrees Celsius for approximately 2 hours. The mixture of the iron oxide/silica and GA solution is then centrifuged and an additional 5 mL GA is added to the product for modification. The final product of this mixing is then centrifuged and washed.

(58) In an embodiment, the sensor performance may be evaluated to determine accuracy and repeatability. In a non-limiting example, glucose solution (1 mg/mL (5.5 mM), 10 mL) is mixed with approx. 1 mg of the previously discussed colloidal magnetic nanoparticles with surface modification (The discussed iron oxide/silica-GA particles). The magnetic colloidal nanoparticles binding with the target analytes, e.g. glucose, may be introduced on to the surface of the graphene-based GMR sensor by a liquid dispenser by applying any one of the following techniques; pipettor, a pump-connected microfluidic system, or a fluidic loop system. In this same non-limiting example, the concentration of the glucose-binding magnetic colloidal nanoparticles was actively sensed over a range from 0.2 mg/mL to 1 mg/mL with a sensing interval 0.2 mg/ml The sensor performance results of this example are shown in FIG. 8 whereby a relatively strong correlation curve was determined between the sensed concentration of biomarkers and the sensed relative magnetoresistance value.

(59) The GMR biosensor disclosed herein has numerous potential applications in detecting target molecules for diagnosis of diseases such as various forms of cancer and diabetes. Current test procedures have focused on applying the GMR biosensor to detect targeted molecules, glucose, DNA, ADDLs (amyloid-β-derived diffusible ligands), and other relevant biomarkers for diagnosis of diseases, e.g. diabetes, cancer, Alzheimer's disease. The results of these studies, as shown in FIG. 8 indicate that a larger magnitude of the relative GMR value will result in a significant increase in the concentration of targeted biomolecules that are registered by the biosensor.

(60) In an additional embodiment, the nanocomposite GMR biosensor may be integrated into a wireless communications system. In this embodiment, a Digi XBee™ unit is linked with the GMR sensor as shown in the FIG. 9. A computer processing unit with user control channels can then be used to control the input/output signals from the biosensor to enable the sensor to be utilized for real-time and remote diagnosis. The GMR biosensor may be connected to an electric resistance signal transistor and amplifier device which in turn may be connected to a wireless GMR node which will relay data from the sensor to a wireless receiver in a PC or smart phone device.

(61) A significant advantage of the sensor disclosed herein is that the designed graphene-based nanocomposite shows GMR signal at a magnetic field<5 Tesla, while most reported GMR materials cannot show GMR phenomenon at such low magnetic fields.

(62) In summary, the present disclosure provides a system for detecting one or more target analytes. The system includes one or multiple resistor structures. A resistor structure comprises a substrate and coated on the substrate is a graphene-based nanocomposite material located on a surface of the substrate. The graphene-based nanocomposite material exhibiting one or more magnetoresistance properties. The composite is made of magnetic nanoparticles-loaded graphene sheets. The magnetic nanoparticles may have a size in a range from about 1 nm to about 1000 nm. A preferable size range of the thickness of the graphene-based nanocomposites is from about 100 nm to about 5 mm.

(63) Non limiting examples of the types of magnetic nanoparticles that can be used include any one of nanostructures made of iron (Fe), or cobalt (Co), or nickel (Ni), or alloys and/or compounds containing iron (Fe), and cobalt (Co), and nickel (Ni), platinum (Pt), and rare-earth, for instance, Fe.sub.3O.sub.4, CoO, NiO, Fe.sub.xCo.sub.y (x+y=100), Fe.sub.xNi.sub.y (x+y=100), FePt, FePtCo, FePtNi, EuO, Eu.sub.1-xGd.sub.xSe (0.02≤x≤0.8), or Gd.sub.3-xS.sub.4 (0≤x≤0.8), to mention a few.

(64) Methods of loading the nanoparticles may include a polyol process, electrochemical plating, chemical vapor deposition, physical deposition, laser-assisted deposition. The interaction between magnetic nanoparticles and graphene sheets can be physical and chemical bonding. The weight ratio of magnetic nanoparticles to graphene is preferably in a range from about 98:2 to about 20:80.

(65) The polyol process involves a polyol solution (e.g. ethylene glycol and sodium hydroxide) used as a reductive agent and surfactant for reducing metallic compounds to magnetic nanocrystals and depositing the magnetic nanocrystals on graphene sheets. The reaction temperature is in a range of 100° C. to 300° C.

(66) A surface of the graphene-based nanocomposite material includes molecular sensing elements bound thereto which exhibit an affinity for binding with the target analytes. Electrodes are connected to the resistor structure to enable electrical connection to a power source and a device for measuring the resistance across the resistor structure for sensing a giant magnetoresistance (GMR) value of the resistor structure.

(67) The system includes magnetic colloidal nanoparticles exhibiting preselected magnetic properties with an outer surface of the magnetic colloidal nanoparticles being modified to allow interaction with the surface of the resistor structure resulting in a change in the GMR value of the resistor structure. The system includes a magnetic field generating device configured to apply a magnetic field to the graphene-based nanocomposite wherein the field has a magnitude in a range from greater than 0 to about 5 Tesla.

(68) The interaction between the magnetic colloidal particles and the surface of the resistor structure having the molecular sensing elements bound thereto is mediated through the target analytes when these are present in a sample being tested binding to the molecular sensing elements. It is this binding of the target analytes to the sensing elements which cause the interaction leading to a change in the GMR value of the resistor structure.

(69) In one embodiment, the outer surface of the magnetic colloidal nanoparticles are modified to include molecular sensing elements bound thereto which exhibit an affinity for binding with the target analytes. When target analytes are present in a vicinity of the magnetic colloidal nanoparticles they bind to the molecular sensing elements on the magnetic colloidal nanoparticles, and when the magnetic colloidal nanoparticles with target analytes bound thereto are in a vicinity of the surface of the resister structure, the interaction with the surface is binding of the target analytes, bound to their respective magnetic colloidal nanoparticles, to the molecular sensing elements bound to the surface of the resistor structure. This results in a permanent change in the GMR value of the resistor structure. The change of GMR value is proportional to the concentration of magnetic colloidal nanoparticles that interact or bind with the graphene-based nanocomposites through the bond between sensing elements and target analytes. Consequently, the concentration of target analytes binding with sensing elements can be obtained by evaluating the change of GMR value.

(70) In another embodiment, the outer surface of said magnetic colloidal nanoparticles are modified to include molecular sensing elements bound thereto which exhibit an affinity for binding with the molecular sensing elements bound to the surface of the resistor structure, and when this binding occurs a change in the GMR value of the resistor structure occurs. When the target analytes are in a vicinity of the surface of the resister structure, the interaction with the surface occurs by the target molecules displacing the bound magnetic colloidal particles bound to the molecular sensing elements which then preferentially bind to the molecular sensing elements, resulting in the GMR value of the resistor structure returning to the same value it had before the magnetic nanoparticles were bound to the surface of the resistor surface. The degree or amount of the restoration of GMR value is depends on the amount of target analytes replacing the magnetic colloidal nanoparticles. Full restoration to the original GMR value occurs when all the magnetic colloidal nanoparticles are displaced by the target analytes.

(71) This description is exemplary and should not be interpreted as limiting the invention or its applications. Specific parts or part numbers mentioned in the description may be substituted by functional equivalents.