Abstract:
The present invention provides a device and methods for the detection and quantification melamine in a sample by rapid and specific electrochemical detection. The present invention includes using a field-effect transistor (FET) biosensor having an open Si channel with a melamine antigen, or hapten, or an antibody, anchored via a linker molecule such as self assembled monolayer to the surface of the gate dielectric of the said open Si channel. The anchoring molecule having the capability of detecting melamine directly or indirectly by selectively binding melamine antibodies, which changes a field-effect on a Si channel, causing a change in conductivity of the FET. This change in conductivity can be measured and is used to determine the presence or absence of melamine in a sample compared to a standard signal or pre-measured database.

Description:
CROSS-RELATED TO RELATED APPLICATIONS 
       [0001]    This application claims the benefit of U.S. Provisional Application No. 61/545,239, filed Oct. 10, 2011. 
     
    
     FIELD OF THE INVENTION 
       [0002]    The present invention relates generally to detection of melamine, and more particularly, to a sensor capable of immediate detection of melamine using a field-effect transistor (FET), and methods of detecting melamine using an FET. 
       BACKGROUND OF THE INVENTION 
       [0003]    Melamine (1,3,5-Triazine-2,4,6-triamine) has uses in several industrial areas, including the making of pesticides, fire retardants, concrete, and resins. While in low doses, melamine is non-toxic, in higher doses, melamine has been shown to be toxic in animals. Studies have shown that melamine causes skin irritation, renal failure, kidney stones, bladder stones, and reproductive damage. 
         [0004]    While melamine can enter food sources by industrial leaching into the food supply, melamine has also been used as an adulterant in foodstuffs due to its nitrogen content, which yields false high protein readings when food is tested. This adulteration has led to several scandals involving melamine contamination. These scandals include the 2007 Chinese animal feed recall and the 2008 Chinese contaminated infant formula recall, where several children died from drinking milk contaminated with melamine. Melamine has been used as an adulterant because the nitrogen in melamine gives false high protein content readings, and companies wishing to increase the perceived protein content may add melamine to the foodstuff instead of actually increasing the protein content. Thus, the ability to detect the presence and amount of melamine has high importance in the food industry. 
         [0005]    To date, there have been a number of methods to detect melamine, including the use of various mass spectrometry (MS) techniques, including High Performance Liquid Chromatography (HPLC), gas chromatography (GC), and Ultraviolet mass spectrometry (UVMS). Enzyme-linked immunosorbent assays (ELISA), and enzymatic detection methods are other methods currently used to detect melamine. However, mass spectrometry can be very expensive and time consuming. ELISA assays for melamine are time consuming, take several steps before melamine can be detected, and suffer from low accuracy of detection due to the low molecular weight of melamine. These techniques are often not conducive to quick accurate on-site detection of melamine, which is crucial, due to the short shelf life of milk products and other foodstuffs. 
         [0006]    Common to almost all melamine tests is the use of a melamine antibody that binds to a melamine molecule. To produce a melamine antibody, melamine can be injected into a host animal. However, melamine is a small molecule and a weak melamine antigen which generates a weak or no immune response when injected into a host animal by itself. To overcome this lack of immune response, to generate high quality melamine antibodies, melamine is first attached to a hapten such as bovine serum albumin (BSA) to form a more powerful melamine antigen which then produces a melamine antibody. When the BSA-melamine protein is injected into a host animal, the immune system generates a vigorous response to the BSA-melamine antigen thereby generating high quality antibodies. Antibodies generated in this manner typically bind with high selectivity and specificity to the BSA-melamine antigen to form an antibody plus hapten-antigen complex. This antibody also binds with high selectivity and specificity to free melamine molecules to form the antibody plus antigen complex. Even though other similar molecules may be present in the sample, the melamine antibody binds selectively to only the melamine molecule. One type of melamine hapten based on BSA is Bovine Serum Albumin Sulfamethazine (BSA-SM2). 
         [0007]    While there have been several methods and devices to detect small molecules, electronic sensors such as bio-FETs have shown great potential to achieve inexpensive and portable detection methods. An FET sensor works to detect biomolecules by using an electric field to control the charge carrier density on a semiconducting channel of the FET device. The key difference of an FET sensor from a typical FET device is that the top gate is removed so as to expose the semiconducting channel with gate dielectrics to the target sample, such as milk to be tested. Immobilized onto the surface of gate dielectrics (typically silicon dioxide) around the semiconducting channel of the FET sensor are probe molecules specific to target molecules. The target molecules bound to the channel of FET sensor can modulate the charge carrier density of the channel and therefore the change the conductance of the FETs via field-effects. A change in conductivity therefore indicates the presence of particular target molecules that bind to the probe molecules anchored on the surface of the semiconducting channel of the FET sensor. 
         [0008]    One advantage of an FET sensor compared to other methods to detect biomolecules is that the small size of the semiconducting channel of the FET sensors provides higher detection sensitivity as it requires less target molecules to yield a measurable signal. For example, FET sensors with nanoscale channels have been proven to provide extremely high sensitivity in biochemical detection. An example of a nanoscale FET sensor in a device is a bio-fin-shaped Field Effect Transistor (bio-finFET) such as the one disclosed in PCT Application Publication No. WO 2012/050873 to Hu et al., incorporated herein by reference in its entirety. 
         [0009]    Another type of nanoscale FET sensor devices is described as a nanogrid finFET in U.S. patent application Ser. No. 13/590,597 to Wu, incorporated herein by reference in its entirety. Nanoscale FET sensors such as finFET biosensors have been shown to be capable of measuring the concentration of proteins in solution down to the femto molar range. The fin channels of the finFET transistor have a high surface area which provides a high transistor channel area and high sensor sensitivity. A thin layer of SiO 2  as a gate dielectric is grown around the fins. An antibody to a target molecule may be attached to the gate dielectric covering the surface of the finFET transistor channel forming a sensor area. When the sensor area of the finFET transistor is immersed in a sample containing the target molecule, the target molecule binds to the antibody forming an antibody-target molecule complex. The change in charge caused by the formation of the antibody-target molecule complex changes the charge on a gate of the finFET transistor resulting in a change in conductance of the finFET transistor channel. The change in finFET transistor conductance may be measured by monitoring a transistor signal such as drive current (I ds ) and may be correlated to the amount of target molecule that is bound to the antibody on the gate. A sample with a low concentration of the target molecule will form few antibody-target molecule complexes resulting in a small change in the finFET transistor signal whereas a sample with a high concentration of the target molecule will form many antibody-target molecule complexes resulting in a large change in the finFET transistor signal. 
         [0010]    Despite the available methods and devices to currently detect melamine, portable low cost sensors and methods to accurately detect low concentrations of are still desired. 
       SUMMARY OF THE INVENTION 
       [0011]    Embodiments of the present disclosure relate to devices and methods of melamine detection and/or quantification. Briefly described, embodiments of the present disclosure can include devices and methods of using a nanoscale silicon FET sensor, for detection of melamine based on a competitive antibody binding assay and a direct assay using antibody or aptamer. 
         [0012]    In one embodiment of the invention, the sensor and the methods of detection of melamine involve a bio-FET such as a finFET biosensor. The FET biosensor comprises a semiconducting substrate and at least one open silicon channel on the semiconducting substrate. Attached to the gate dielectric on the silicon channel is a linker molecule (such as a silane based linker molecule) which is attached to the gate dielectric. The linker molecule (which may be a complex molecule formed via multiple steps of surface treatment) is also attached to a melamine antigen and forms the sensor area for measuring a presence of a target molecule that binds to the melamine antigen. In one embodiment, the melamine antigen is a melamine molecule, while in another embodiment the melamine antigen is a hapten melamine molecule such as BSA-SM2. The biosensor has circuitry arranged to measure a change in electrical signals passing through the FET. In one embodiment the biosensor is a finFET and in still another embodiment the biosensor is a nanogrid finFET. 
         [0013]    When the target molecule binds to the melamine antigen on the gate dielectric, a change of the charge carrier density of the open silicon channel occurs, which changes the conductance of the FET biosensor via field effects. By changing the conductance of the FET, this allows the user of the device to determine the presence of melamine by measuring the change in the conductance of the FET. 
         [0014]    In another embodiment of the present invention, instead of a melamine antigen bound to the gate dielectric (via the linker molecule) a melamine antibody is bound to the gate dielectric. The melamine antibody can bind melamine or a melamine hapten, and this sensor can be used to measure melamine concentration with a direct assay or competitive binding assay. 
         [0015]    Embodiments for methods of detecting melamine are disclosed. In one embodiment, a competitive binding assay to detect melamine is used to determine the concentration of melamine. The concentration of melamine is determined by mixing a standard sample of known concentration of melamine antibody with a target sample having an undetermined concentration of melamine. This mixture is the testing sample which is immersed on the sensor area. An electrical transistor signal is measured through the FET biosensor to determine a melamine concentration by comparing the testing sample signal to a standard signal. In other embodiments, the sensor is immersed in a standard sample and a standard sample is determined instead of merely referencing a standard signal. 
         [0016]    In one embodiment of a method to determine the concentration of melamine in a sample using a competitive binding assay, a melamine analog is anchored and immobilized to the gate dielectric of the sensor and used as a probe for melamine antibodies. The immobilized probe may be a melamine hapten such as BSA-SM2, which has a chemical group (such as sulfamethazine) that mimics melamine (both BSA-SM2 and melamine have an NH 2  group off of a benzene ring) and therefore can bind with added melamine antibodies in the solution. The immobilized probe molecule, when not bound to melamine antibodies produces a measurable current or voltage in the FET sensor as a baseline signal, and produces a different measurable signal when bound to melamine antibodies forming a complex. This occurs due to charge differences between the bound and unbound immobilized molecule on the gate dielectric. The different field-effects created by either bound or unbound probe molecules on the gate dielectric produce different electrical signals when current passes through the FET. 
         [0017]    A change of the number of melamine antibodies capable of binding to the immobilized molecule is directly proportional to the number of melamine molecules in a solution of melamine and melamine antibodies. By detecting a change in the electrical signals of the finFET sensor, the presence of melamine antibodies is determined. Due to competitive binding, when the presence of melamine antibodies is determined, the presence and concentration of melamine is also determined. 
         [0018]    In another embodiment of detecting the presence of melamine, the binding assay has a step of mixing a first sample with a melamine antibody solution. The first sample has no presence of melamine and is used as a reference sample to obtain a baseline measurement. A second sample (having an undetermined amount of melamine), is mixed with the same melamine antibody solution as the test sample. The reference sample solution is applied to the sensing surface of the device and a baseline measurement of an electrical signal across the FET is obtained. The sensor is rinsed of the reference solution, and the second sample is applied to the sensing surface. A final measurement of an electrical signal of the FET is obtained for this sample to determine the presence of melamine. The presence of melamine in the test solution is determined by comparing the electrical signal obtained from the reference solution and the electrical signal after the test solution is applied. If there is no change in the electrical signal there is no presence of melamine in the test solution. If there is a difference between the two electrical signals (such as conductance), melamine is present in the test solution. A quantitative measurement of the concentration of melamine can be obtained by comparing the change of the FET signal to a pre-measured standard curve of signal levels vs. amount of melamine. 
         [0019]    In another object of the invention, a similar competitive assay can be achieved by exchanging the roles of the probe molecule and the melamine antibody from the previously sensor embodiment. In other words, the melamine antibody can be anchored on the FET surface, while a known concentration of BSA-SM2 can be added to both the reference and the test solutions. Then melamine and BSA-SM2 will compete for binding with the antibodies on the FET surfaces. Since melamine is not charged, higher melamine concentration in the test solution can reduce binding of BSA-SM2 with the antibodies causing a larger change of FET signals from baseline. Lower melamine concentration yields smaller change from the baseline signals. 
         [0020]    In yet another object of the invention, a direct detection assay can be achieved by using the FET sensor coupled with melamine antibodies to directly detect melamine. The binding of melamine to the antibody on the FET changes the charge of the antibody itself, causing a change of channel conductance of the FET sensor. Compared to competitive assay method, the direct assay is simpler but less sensitive. 
         [0021]    Accordingly, an object of the invention is to provide a field-effect transistor device capable of detecting the presence of melamine and another object is to provide methods to detect the presence of melamine. 
     
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         [0022]    These and other features and advantages of the present invention will become appreciated as the same becomes better understood with reference to the specification, claims and drawings wherein: 
           [0023]      FIG. 1A  is a perspective view of a nanoscale FET sensor in the prior art. 
           [0024]      FIG. 1B  is a cross sectional view of antibodies binding to a target molecule on a gate dielectric covering the surface of a finFET. 
           [0025]      FIG. 2  is an illustration of melamine antibodies, antigens, and non-melamine structures. 
           [0026]      FIG. 3A  is an illustration of a melamine antibody forming a hapten-antigen complex. 
           [0027]    FIG. is an illustration of a melamine antibody forming the antibody plus antigen complex. 
           [0028]      FIG. 4A  is a cross sectional view of an FET sensor in a direct binding assay with a low concentration of melamine. 
           [0029]      FIG. 4B  is a cross sectional view of an FET sensor in a direct binding assay with a high concentration of melamine. 
           [0030]      FIG. 5A  is a cross sectional view of an FET sensor during a competitive binding assay according to an antigen anchored embodiment of the invention with a low concentration of melamine. 
           [0031]      FIG. 5B  is a cross sectional view of an FET sensor during a competitive binding assay according to an antigen anchored embodiment of the invention with a high concentration of melamine. 
           [0032]      FIG. 6A  is a cross sectional view of an FET sensor during a competitive binding assay according to a hapten anchored embodiment of the invention with a low concentration of melamine. 
           [0033]      FIG. 6B  is a cross sectional view of an FET sensor during a competitive binding assay according to a hapten anchored embodiment of the invention with a high concentration of melamine. 
           [0034]      FIG. 7  is a graph of a melamine assay standard curve for determining melamine concentration using a finFET. 
           [0035]      FIG. 8A  is a graph showing experimental results of a BSA-SM2 treated finFET showing monotonic dependence of sensor signals vs. antibody concentration. 
           [0036]      FIG. 8B  is a graph showing competitive assay results of the finFET sensor of  FIG. 8A . 
           [0037]      FIG. 9A  is a graph showing the experimental results of direct melamine detection using an antibody anchored finFET sensor. 
           [0038]      FIG. 9B  is a graph of a standard curve obtained for a direct assay of melamine. 
       
    
    
     DETAILED DESCRIPTION OF THE EMBODIMENTS 
       [0039]    It is to be understood that this disclosure is not limited to the particular embodiments described. It is also to be understood that the terminology used is for the purpose of describing particular embodiments only, and is not intended to be limiting, since the scope of the present disclosure will be limited only by the appended claims. 
         [0040]    All publications and patents cited in this specification are herein incorporated by reference as if each individual publication or patent were specifically and individually indicated to be incorporated by reference and are incorporated herein by reference to disclose and describe the methods and/or materials in connection with which the publications are cited. The citation of any publication is for its disclosure prior to the filing date and should not be construed as an admission that the present disclosure is not entitled to antedate such publication by virtue of prior disclosure. Further, the dates of publication provided could be different from the actual publication dates that may need to be independently confirmed. 
         [0041]    The following examples are put forth so as to provide those of ordinary skill in the art with a complete disclosure and description of the device and how to perform the methods of detecting melamine. Unless otherwise stated, parts are by weight, temperatures in degrees Celsius (C.), and pressure is at or near atmospheric pressure. Standard temperature and pressure are defined as 20° C. and 1 atmosphere. 
         [0042]    It must be noted that, as used in the specification and the appended claims, the singular forms “a,” “an,” and “the” include the plural references unless the context clearly dictates otherwise. 
         [0043]    Unless defined otherwise, all terms used herein have the same meaning as commonly understood by one of ordinary skill in the art of which this disclosure belongs. Although any methods and material similar or equivalent to those described herein can also be used in the practice or testing of the present disclosure, the preferred methods and materials are now described. 
         [0044]    The term “sample channel” refers to the area into which a sample is placed to come into contact with the sensor area of the finFET biosensor transistor. The sample channel may be a pipe like structure thorough which a sample flows or may be a sample well that may be filled with sample solution. A sample solution to be tested for melamine may flow through a conduit like sample channel and over the sensor area or a finFET biosensor device containing an opening such as a sample well may be immersed in a sample to be tested for melamine. 
         [0045]    The term “finFET signal” refers to both the directly measured finFET transistor electrical parameters and also refers to parameters which may be derived from the measured finFET transistor electrical parameters. The detected signals of the transistor biosensor can be in many forms. For directly measured finFET signals, there can be several different biasing and configurations. One method is to bias the source and drain with a known voltage and also bias a gate electrode with another known voltage, measure the drain current during sensing experiments. Another method is to bias the source and drain with a current source and bias a gate electrode with a known voltage, and measure the drain voltage during sensing. A third method is to bias the source and drain with a known voltage, sweep the voltage of gate electrode in a chosen voltage range, to simultaneously measure the drain current, and to generate a standard transistor current versus voltage (I-V) plot. 
         [0046]    The term “standard sample” or “standard solution” refers to a sample with a known concentration of melamine. The known concentration may be zero mg/ml or may be a nonzero mg/ml. 
         [0047]    The term “reference sample” or “reference solution” refers to a sample solution with a known concentration of melamine antibody for the embodiment of competitive assays with melamine-hapten anchored sensor; or with a known concentration of melamine-hapten such as BSA-SM2 for the embodiment of competitive assays with melamine-antibody anchored sensor. The reference sample solution may have no melamine. 
         [0048]    The term “target sample” refers to a sample with an unknown concentration of melamine. 
         [0049]    The term “target signal” refers to a finFET signal measured either when the sensor area of a finFET biosensor transistor is immersed in a target sample or after the sensor area of a finFET biosensor transistor was immersed in a target sample. 
         [0050]    The term “standard signal” refers to a finFET signal measured either when the sensor area of a finFET biosensor transistor is immersed in a standard sample or after the sensor area of a finFET biosensor transistor was immersed in a standard sample. 
         [0051]    A finFET biosensor according to an embodiment is illustrated in  FIG. 1A . The finFET biosensor transistor  98  consists of a source electrode  106 , a drain electrode  104 , with multiple silicon channel fins  108  forming parallel transistor channels between the source  106  and drain  104 . The finFET biosensor is formed on a semiconductor on insulator (SOI) which consists of a substrate  100  which may be silicon, with a buried oxide (BOX)  102  electrically isolating the finFET biosensor from the substrate  100 . A thin layer of SiO 2  or nitride SiO 2  as gate dielectrics  110  is grown around the fins  108 . The probe molecule  112  is attached to the gate dielectrics  110  via a linker molecule  122 . Silane based self assembled monolayers (SAMs) are often used as the linker molecule. Many times, multiple surface treatment processes may be needed to make the linker molecule or a complex to have desired functions for attaching probes. For simplicity, in the following figures and descriptions, the linker molecules  122  are not shown or described in subsequent illustrations and descriptions. 
         [0052]    Contacts are formed to the source  106  and drain  104  of the finFET biosensor to measure an electrical property or signal of the finFET biosensor transistor such as drive current (I ds ). A sample solution flows over the channel area of the finFET biosensor. Surface areas in the sample channel outside the sensor area may be coated with anti-adhesion protective molecules such as polyelthylene glocol (PEG) terminated self assembled monolayers (SAMs), benzene terminated SAMs, fluorocarbon silanes, etc., which prevent melamine in the sample from adsorbing to non-sensor areas causing a change in the melamine concentration. 
         [0053]    As shown in  FIG. 1B , a thin layer of SiO 2  as gate dielectrics  110  is grown around the fins  108 . Then an antibody  112  to a target molecule  118  may be attached to the gate dielectric  110  covering the surface of the finFET transistor channel  108  forming a sensor area. When the sensor area of the finFET transistor is immersed in a sample containing the target molecule  118 , the target molecule binds to the antibody forming an antibody-target molecule complex  120 . The change in charge caused by the formation of the antibody-target molecule complex  120  changes the charge on a gate of the finFET transistor resulting in a change in conductance of the finFET transistor channel. The change in finFET transistor conductance may be measured by monitoring a transistor signal such as drive current (I ds ) and may be correlated to the amount of target molecule that is bound to the antibody on the gate. A sample with a low concentration of the target molecule will form few antibody-target molecule complexes resulting in a small change in the finFET transistor signal whereas a sample with a high concentration of the target molecule will form many antibody-target molecule complexes resulting in a large change in the finFET transistor signal. 
         [0054]    The finFET signal may also be indirect measurements or parameters derived from directly measured finFET transistor electrical parameters as outlined above. For example, the change in one of the measured finFET transistor electrical parameters may be derived by subtracting the initial measured finFET transistor electrical parameter measured before the sample is introduced into the sensor area from the finFET transistor electrical parameter measured after the sample is introduced into the sample area. A percentage change may additionally be derived by dividing the relative change by the initial value. Alternatively, the transistor conductance may be derived by dividing the measured finFET transistor drain current by the finFET transistor drain voltage, or the trans-conductance of the finFET transistor may be derived by dividing the measured finFET transistor drain current by the voltage of gate electrode. With the measured I-V curve of the finFET transistor, the finFET transistor threshold voltage (Vt) or change in Vt or shift in Vt, etc may also be extracted. These direct or indirect finFET biosensor transistor signals are examples of biosensor signals that may be used to analyze results and may be correlated to the concentration of melamine in the sample. Conductance of the transistor as an exemplary signal of the sensor device in the following embodiments, but other measurements of the transistor signals may be used to determine concentration of melamine. 
         [0055]      FIG. 2  and  FIG. 3  illustrate representations of biomolecules in the detection of melamine. Typically, to produce a high quality melamine antibody, melamine  204  is first attached to a hapten  206  such as bovine serum albumin (BSA) to form a more powerful melamine antigen. When the BSA-melamine protein  208  is injected into a host animal, the immune system generates a vigorous response to the BSA-melamine antigen  208  generating high quality antibodies. Antibodies  202  generated in this manner typically bind with high selectivity and specificity to the BSA-melamine antigen  208  to form the antibody plus hapten-antigen complex  302  and also binds with high selectivity and specificity to free melamine molecules  204  to form the antibody plus antigen complex  304 . Even though other similar molecules  209 ,  210 ,  211 ,  213 ,  214  may be present in the sample, the melamine antibody binds selectively to only the melamine molecule. 
         [0056]    In one embodiment of bio-finFET sensors,  FIG. 4A  and  FIG. 4B  shows sensor areas  403 ,  405  and for detecting melamine using a direct detection assay. Antibodies  404  to melamine  402  are anchored to the gate dielectric  110  on the finFET sensor areas  403 ,  405 . When a sample containing melamine comes into contact with the sensor area, the melamine antibody  404  immobilized on the finFET transistor channel  108  binds to the melamine molecule  402  forming a melamine antibody-melamine complex  406  that causes a change in channel conductance. If the sample contains a high concentration of melamine  404  as shown in  FIG. 4B , more melamine antibody-melamine complexes  406  form on the finFET biosensor channel  108  causing a larger change in channel conductance. While this direct binding embodiment may be sufficient to detect whether melamine is present or absent in a sample, because melamine is a small uncharged molecule, the change in fin channel conductance when a melamine molecule  406  is bound by the immobilized melamine antibody  404  may be small and therefore the detection has a poor sensitivity in comparison to the competitive assay method. However, the direct detection method is a simpler method. 
         [0057]    In an embodiment of sensors used for a competitive binding assay, as shown in  FIG. 5A  and  FIG. 5B , a known concentration of hapten-melamine molecules such as BSA-SM2  508  may be added to the sample prior to immersing the finFET biosensor sensor area containing immobilized melamine antibody  504  in a sample solution.  FIG. 5A  illustrates a sensor area  507  with antibodies anchored to the gate dielectric  110  having a low concentration of melamine, while  FIG. 5B  illustrates a sensor area  509  with melamine antibodies  504  anchored to the gate dielectric  110  having a high concentration of melamine. In this embodiment, the hapten-melamine molecules  508  compete with the melamine molecules  502  in the sample solution for binding sites on the immobilized melamine antibody  504 . If there is a low concentration of melamine molecules  502  in the sample solution, then most of the melamine antibody sites  504  will be bound to hapten-melamine molecules  510  as shown in  FIG. 5A . 
         [0058]    If, however, there is a high concentration of melamine molecules  502  in the sample solution, as in  FIG. 5B , then most of the melamine antibody binding sites  504  will be bound to melamine molecules  506 . Since the hapten-melamine molecule  508  carries significant charge, differences in the number of bound hapten-melamine molecules causes a larger change in finFET channel conductance than differences in the number of bound melamine molecules. Competitive binding of the hapten-melamine molecule thus increases the sensitivity of the finFET biosensor melamine assay. 
         [0059]    In another embodiment of a sensor and method depicted in  FIGS. 6A and 6B , a competitive binding assay provides increased sensitivity for the detection of melamine by binding a molecule with a significant amount of charge to the finFET biosensor transistor channel  108 .  FIG. 6A  illustrates a sensor area  609  during conditions of a low concentration of melamine and  FIG. 6B  illustrates a sensor area  611  during conditions of a high concentration of melamine. In this competitive binding embodiment the hapten-melamine molecule BSA-SM2  608  is be anchored to the finFET channel  108 . A known concentration of melamine antibody  604  is added to a sample of unknown concentration of melamine to create a testing solution. The melamine antibodies  604  in the testing solution will competitively bind to the melamine molecules  602  in solution to form complex  606 , and to the hapten-melamine molecules  608  immobilized on the finFET transistor channel  108  to form complex  612 . If there is a low concentration of melamine  602  in the sample as shown in  FIG. 6A  a large number of the melamine antibody molecules  604  will bind to the hapten-melamine molecules immobilized on the finFET transistor channel  108  producing a particular electrically measured signal. 
         [0060]    If, however, there is a high concentration of melamine  602  in the sample as shown in  FIG. 6B , most of the melamine antibody molecules  604  will bind to melamine molecules  602  to form complex  606  in solution and few of the melamine antibody molecules  604  will be available to bind to the immobilized hapten-melamine molecules  608  to form a BSA-SM2 antibody complex  610 . Since the melamine antibody  604  has significantly more charge than the melamine molecule, a change in amount of the melamine antibody  610  bound to the finFET transistor channel  108  causes a significantly larger change in channel conductance than does a change in the amount of melamine molecules bound to the finFET transistor channel  108 . The number of BSA-SM2 antibody complexes  610  is inverse proportional to the amount of melamine in the testing solution, and can be measured via changes in field effects that occur when BSA-SM2 forms the complex  608  with melamine antibodies  604 . Competitive binding of the melamine molecule thus increases the sensitivity of the finFET biosensor melamine assay 
         [0061]    As shown in  FIG. 7 , a series of standard solutions with standard concentrations of melamine  702  may be used to generate a standard curve  704  of melamine concentration vs finFET transistor drive current. The concentration of melamine in an unknown sample  708  may then be determined by reading the drive current  706  from a sample off the standard curve  704 . 
         [0062]      FIG. 8A  shows the experimental results of BSA-SM2 treated finFET to different concentrations of melamine antibodies from 0.2 pM to 200 pM showing monotonic dependence of sensor signals vs. antibody concentration. This result demonstrates good binding between BSA-SM2 and melamine antibody.  FIG. 8B  shows the competitive assay results using the same finFET sensor of  FIG. 8A . 200 pM of melamine antibodies is added to two target sample solutions (one with 20 pM melamine and one with 200 pM melamine). Test solution of 20 pM melamine (MLa) yields a small signal change from baseline while 200 pM melamine yields higher signal changes, demonstrating successful detection of melamine at a low detection limit (high sensitivity) using the competitive assay method. The assay sensitivity (limit of detection or LOD) achieved using this competitive assay is several orders of magnitude higher (lower for LOD) than conventional methods such as ELISA or mass spectrometry. 
         [0063]      FIG. 9A  shows the experimental results of direct detection of melamine using antibody anchored finFET sensor devices. A change of finFET current is found monotonic to the concentration of melamine, with higher melamine concentrations yield larger signal changes. It is noted that the solution of melamine concentration of 2 uM gives a very small signal, in comparison to  FIG. 8B , showing the competitive assay provides much higher detection sensitivity than the direct detection method.  FIG. 9B  shows a standard curve obtained for direct assay experiments for the detection of melamine. 
         [0064]    A method of detecting a presence of melamine in a sample using the modified sensors is now described. A sample known to have no presence of melamine is mixed and diluted into a 1 mM TRIS-HCL buffer pH 7.5 to produce a reference sample solution having no melamine. The solution is filtered through a 0.2 μm filter. Milk, or other foodstuff with possible but undetermined amount of melamine is mixed and diluted with the same amount of 1 mM TRIS-HCl buffer pH 7.5 solution that the baseline reference solution was mixed and diluted with to produce a test solution. For a competitive assay method, a known concentration of antibody (e.g. 200 pM) is added to both the reference and test solutions. As shown in  FIG. 8B , first, the reference sample solution with 200 pM antibody is applied to the sensing surface of the FET with BSA-SM2 attached to the fin surfaces, and a first electrical signal is measured as a baseline. Then, the test solution is applied to the sensing surface and a second electrical signal is measured on the FET device. The presence of melamine in the test solution is determined by comparing the baseline reference measurement and the testing solution measurement. The difference in the first and second electrical signals in the presence of melamine is due to the competitive binding of melamine and the immobilized molecule (BSA-SM2) to melamine antibodies. When melamine binds to the melamine antibodies, melamine prevents the melamine antibodies from binding to the immobilized molecule (BSA-SM2). Since the immobilized molecule produces a different field-effect on the silicon nanochannel compared to when the immobilized molecule is bound to melamine antibodies, the conductivity and the electrical signals, such as drain current, as measured by the FET device, changes. As shown in  FIG. 8B , 200 pM melamine causes higher change of current from the baseline than the 20 pM melamine to approve the feasibility of this method. 
         [0065]    Because of the reproducibility of the finFET biosensor technology, a signal may be measured from a standard sample and the value of that signal may be stored in a data base and used as the reference value. For example, a target signal from a target sample containing an unknown amount of melamine may be compared with a standard signal from a database to determine the concentration of the melamine in the target sample without actually generating a standard signal by measuring a standard sample in the field. 
       Sensor Preparation 
       [0066]    The preparation of the sensor on the device to detect melamine is illustrated in the proceeding examples. Materials used in the preparation of the sensor are as follows: 
         [0000]    
       
         
               
               
               
               
             
           
               
                   
                   
               
               
                   
                 Chemical 
                 Vendor 
                 CAS/Cat 
               
               
                   
                   
               
             
             
               
                   
                 3-aminopropyltriethoxysilane 
                 Sigma-Aldrich 
                 919-30-2 
               
               
                   
                 (APTES, &gt;98%) 
                   
                   
               
               
                   
                 Triethoxysilyl undecanal 
                 Gelest 
                 116047-42-8 
               
               
                   
                 (TESU, &gt;90%) 
                   
                   
               
               
                   
                 11- 
                 Gelest 
                 116821-45-5 
               
               
                   
                 Aminoundecyltriethoxysilane 
                   
                   
               
               
                   
                 (AUTE &gt;95%) 
                   
                   
               
               
                   
                 Anhydrous toluene (&gt;99.8%) 
                 Sigma-Aldrich 
                 108-88-3 
               
               
                   
                 Anhydrous ethanol (&gt;99.8%) 
                 Sigma-Aldrich 
                 64-17-5 
               
               
                   
                 Triethylamine (&gt;99.8%) 
                 Sigma-Aldrich 
                 121-44-8 
               
               
                   
                 PEG-silane (MW = 2000) 
                 Nanocs 
                 PEG6-0102 
               
               
                   
                 Sodium cyanoborohydride 
                 Sigma-Aldrich 
                 25895-60-7 
               
               
                   
                 Ethanol amine 
                 Sigma-Aldrich 
                 141-43-5 
               
               
                   
                   
               
             
          
         
       
     
         [0067]    Below, an example of a proven surface chemistry to prepare the finFET sensor for melamine detection is described in detail.  FIG. 6  shows the functionalized sensor device for a competitive assay to detect melamine. The sensor comprises a silicon finFET with a fin surface modified to detect melamine in a sample. The surface of a gate dielectric (typically SiO 2 ) of Si fins is modified with silane molecules as linker molecule such as (3-aminopropyltriethoxysilate) (APTES) or Triethoxysilyl undecanal (TESU) to activate the fin surface for antibody immobilization. The silane molecules are attached to the sensing areas of the devices including the fins and the surrounding area of SiO 2 . The channel or fin area is first cleaned with fresh piranha solution, a mixture of sulfuric acid (H 2 SO 4 ) and hydrogen peroxide (H 2 O 2 ) with a ratio of 1:1 for example, for 30 seconds or longer. The piranha cleaned chip can be stored in deionized (DI) water to maintain the surface cleanness and surface hydrophilicity for more than one month without any dissolution of oxide. An anhydrous solution with 0.1% TESU is mixed and ultrasonicated for 1 minute. The chip having the sensor is immersed in 0.1% anhydrous toluene solution for 1.5 hours. The sensor is rinsed with an excess of anhydrous solution. Melamine hapten BSA-SM2 is immobilized onto the fin surface for the first competitive assay ( FIG. 6 ) by immersing the TESU functionalized fin surface in 50 mg/ml BSA-SM2 buffer solution (1 mM NaCNBH 3  in 2 mM potassium phosphate buffer pH 7.4) for 3 hours. 
         [0068]    The same process can be used to anchor melamine antibody to the finFETs for another embodiment of a competitive assays (see description of  FIG. 5 ). The modified silicon nanochannel or fins is rinsed in 2 mM potassium phosphate buffer pH 7.4 solution for 5 minutes to remove physically adsorbed antibody. The silicon finFETs is immersed in a 50 mM ethanolamine buffer solution (100 mM NaCNBH 3  in 2 mM potassium Phosphate buffer pH 7.4:5 mM ethanolamine at a 1:100 ratio) for 3 hours to passivate the unreacted aldehyde groups. The modified silicon fins are rinsed in 2 mM potassium phosphate buffer pH 7.4 for 5 minutes to remove physically adsorbed molecules. 
         [0069]    While various embodiments have been described above, they are presented by way of example only and are not to be construed as a limitation of the invention. Numerous changes to the disclosed embodiments can be made without departing from the scope of the invention. The scope of the invention is defined in accordance with the following claims and their equivalents.