WO2005036156A1

WO2005036156A1 - Detection of molecular interactions using a metal-insulator-semiconductor diode structure

Info

Publication number: WO2005036156A1
Application number: PCT/GB2004/004271
Authority: WO
Inventors: Piero Migliorato; Pedro Miguel De Lemos Correia Estrela; Feng Yan
Original assignee: Cambridge University Technical Services Limited
Priority date: 2003-10-10
Filing date: 2004-10-08
Publication date: 2005-04-21
Also published as: GB0323802D0

Abstract

A sensor for the detection of a molecular interaction comprising a metal insulator semiconductor (MIS) diode structure having a back contact and an exposed metal sensor electrode on which probe molecules can be immobilized, wherein the sensor is operative to produce a change in a capacitance-voltage (C-V) characteristic of the MIS diode structure in response to molecular interaction at the exposed surface of the metal sensor electrode. The sensor is particularly suitable for detecting biomolecular interactions such as the hybridization of DNA, when the sensor is prepared with suitable probe molecules immobilized on the exposed metal electrode.

Description

DETECTION OF MOLECULAR INTERACTIONS USING A METAL-INSULATOR-SEMICONDUCTOR DIODE STRUCTURE Field of the Invention The present invention relates to the detection of molecular interactions, particularly the hybridization of DNA, by means of an MIS diode structure with a functionalized metal electrode. Background to the Invention The detection of molecular interactions is important for analyzing the chemistry or biochemistry of such interactions and may also be used for identifying certain species participating in the interactions. A range of interactions may be detected when a first type of molecules (probe molecules), that are attached to a metal, are exposed to other molecules (target molecules). A good example of this type of interaction is where DNA probe oligomers with A bases attach to DNA target oligomers with T bases. The ability to detect such a reaction is essential in the field of genomics. One commonly employed method in monitoring the interaction is optical detection. Here, known DNA strands are immobilised at selected locations and the target is labelled with fluorophors. Evidence of the hybridization of a target with a complementary probe is evinced from the presence of fluorescence at the location of the probe. However, the method is expensive and difficult to implement in portable instrumentation. An alternative approach, which aims to overcome these drawbacks, uses the field effect transistor (FET) for label-free, electrical detection. DNA hybridisation has been detected by this technique. In one reported device a structure was employed that did not have a metal gate, the voltage being applied via an electrolyte. In this example, the probes were immobilised onto silicon or silicon based insulators, such as silicon dioxide and silicon nitride. The presence of chemical or biological molecules immobilized on the gate results in a change of the interfacial dipole affecting the potential drop across the electrochemical double layer. This modulates the voltage applied to the gate of the devices, resulting in a change of the characteristics of the FET. However, when an electrolyte is placed directly in contact with silicon based insulators or other commonly used gate dielectric materials such as metal oxides or semiconductor oxides, problems such as adsorption of hydrogen or other ions, hydration or even superficial migration of ions occur at the surface of the gate dielectric. Depending on the material used, these processes often render the device unstable for operation in a liquid environment or dependent on the concentration of hydrogen (pH dependence) or other ions present in the electrolyte. The immobilization of biomolecules on silicon-based substrates requires that several (bio)chemical processes or reactions be performed on the surface. An example is silanization of the substrate and subsequent immobilisation of an intermediate molecule, prior to the immobilisation of the chemically modified biomolecule. As a consequence of applying multiple processes, the surfaces so produced are often irreproducible, and it is difficult to control the formation of monolayers of biological molecules. Furthermore, semiconductors and insulator surfaces, such as silicon, silicon oxide, and silicon nitride are subject to uncontrolled modifications and contaminations, which add to the problem of achieving reproducible assays. In contrast, the formation of self-assembled monolayers onto gold (Au) substrates via thiolated CH₂ chains is a well-known chemistry and can be achieved with a single biochemical step. Biomolecules modified with a thiol group can easily be assembled onto Au substrates, simply by placing a solution containing the modified biomolecules in contact with the gold substrate for a certain period of time. The time required to form a monolayer, and the concentration of probe molecules, can be controlled by applying a voltage between the Au substrate and the solution. The result of the process is the reproducible formation of monolayers of biomolecules. Furthermore, metals such as gold (Au) or platinum (Pt) are immune to oxidation and their surface can be rendered clean and reproducible by a variety of techniques, including chemical etching, chemical or plasma cleaning and thermal annealing. A DNA sensor has been proposed that uses a thin film transistor (TFT) with Au metal gate, on which a probe can be immobilized in the manner described above. The device comprises a conventional polycrystalline silicon thin film transistor (PTFT) with an Au layer fabricated on top of the TFT channel area. Thus the device combines the advantages of an electrical detector, having internal amplification, with the known chemistry/biochemistry of molecular immobilization on gold substrates. However, the above device configuration has a number of drawbacks, perhaps the most important of which is the disadvantage of having the functionalized metal sensing area, where voltage modulation occurs, in close proximity to the field effect transistor, where amplification occurs. This leads to difficulty in isolating the device, both chemically and electrically, particularly when in contact with an electrolyte. A further drawback is the relative complexity of the three-terminal FET device structure, which makes large-scale production of the sensor more expensive. Simpler semiconductor device structures have been proposed for use as sensors. For example, a metal-insulator-semiconductor (MIS) device has been proposed for detecting certain components in a reactive gas mixture. In particular, hydrogen-containing gases react on the metal surface to liberate surface hydrogen atoms, which then diffuse through the metal and become trapped at the metal-insulator interface, where they result in measurable changes in the electronic properties of the device. However, devices have yet to be developed which permit the detection of a broad range of species or interactions, and particularly biomolecular interactions. Summary of the Invention According to a first aspect of the present invention, there is provided a sensor for use in the detection of a molecular interaction comprising a metal insulator semiconductor (MIS) diode structure having a back contact and an exposed metal sensor electrode on which probe molecules can be immobilized, wherein, in use, the sensor is operative to produce a change in a capacitance-voltage (C-V) characteristic of the MIS diode structure in response to molecular interaction at the exposed surface of the metal sensor electrode. The present invention provides a sensor capable of label-free, electrical detection of a range of molecular interactions, depending on the probe species employed. The simplicity of the MIS diode structure means a much less complex fabrication procedure is required, as compared to FET devices, with the attendant reduction in cost per device. The change in the capacitance-voltage characteristic of the MIS diode structure is generally dependent on a voltage difference applied across a part of the diode structure. It is therefore preferred that a predetermined bias voltage is applied to the MIS diode structure, the sensitivity of the sensor being optimum at the predetermined bias voltage. The particular C-V characteristic of the MIS diode will depend on the constituent materials used in the fabrication of the sensor. It is therefore preferred that the predetermined bias voltage for optimum sensor sensitivity is in dependence on the concentration of a dopant in a part of the MIS diode structure. Preferably, the sensor further comprises detection circuitry, wherein the predetermined bias voltage is matched to the detection circuitry. This ensures optimum performance of the whole sensing module. In the present invention a metal gate is used to passivate the underlying diode structure, allowing its use in a range of aqueous environments. The metal gate may comprise one or more layers of metal. In contrast to prior art devices, the diode semiconductor or insulating material is protected from hydration or other ionic diffusion processes by the metal, thereby maintaining its dielectric properties. Metals such as gold are inert to most electrolytes of interest and their conductive properties are not affected by the ionic content of an electrolyte. The MIS diode characteristics are therefore well defined, stable in an aqueous environment and independent of the ionic strength and pH of an electrolyte in contact with the exposed sensor metal. Preferably, the sensor electrode comprises one or more metals selected from a group which includes gold, chromium and platinum. Gold provides a particularly good metallic surface on which a wide range of molecules may be immobilized by means of a range of known chemical and biochemical processes. Typically, the back contact comprises a layer of a metal such as aluminium. Preferably, the sensor further comprises means for electrical connection to at least one of the sensor electrode and back contact. Although many semiconductor materials are possible, it is preferred that the semiconductor of the MIS diode structure is selected from a group which includes single crystal silicon, amorphous or polycrystalline silicon and organic semiconductor. Preferably, the sensor further comprises a passivation layer for passivating a portion of the MIS diode structure. Preferably, the passivation layer is formed from polyimide, although other materials such as silicone rubber, BCB and Si₃N₄ are possible, as are multiple layers of different materials such as SiO₂ and Si₃N₄. It is preferred that the sensor further comprises a reference electrode. Preferably, means are provided for applying a voltage difference between a part of the MIS diode structure and the reference electrode. In particular, a voltage difference may be applied between the reference electrode and sensor electrode, in order to influence molecular immobilization or interaction time. Alternatively, a voltage difference may be applied between the reference electrode and the back contact. Preferably, the sensor further comprises a counter electrode. This permits another circuit to be formed with the MIS diode structure for monitoring voltages and currents. In prior art devices, separate electrical connection may not be provided to the Au electrode and, indeed, the designs employed often make this difficult. Provision of a connection enables the application of a voltage difference between a reference electrode and the Au sensor electrode, or alternatively the back contact. In the absence of such an applied voltage, many of the necessary chemical/biochemical processes can take a long time. This includes not only the processes required in the preparation and fabrication of the device, such as immobilization of probe molecules, but also the molecular interaction of interest, such as the hybridization of a complementary target. Without the facility for independent electrical control, the system is unsuitable for mass production and high throughput screening. Preferably, the sensor further comprises at least one probe molecule immobilized on the exposed metal sensor electrode. A range of both chemical and biochemical interactions may be detected with the sensor, depending on the nature of the probe molecule. Preferably, the probe molecule is selected from a group which includes proteins, antibodies and antigens, vitamins, peptides, sugars and oligonucleotides, including DNA, RNA and PNA. Preferably, the sensor further comprises an electrolyte in contact with the probe molecule. The electrolyte can serve as a suitable host for target molecules and also complete an electrical circuit between the sensor electrode and the reference electrode. In addition to an individual sensor, there is provided a sensor array comprising a plurality of sensors, each sensor being in accordance with the first aspect of the present invention. The sensor array may be a 1-dimensional (linear) array or a 2-dimensional array. The array may be provided with additional circuitry for control or data capture, giving additional functionality in monitoring the interaction. In addition to characterizing a particular interaction, the sensor may be used to identify a particular species associated with the interaction. Preferably, the sensor or sensor array is used for the identification of a target molecule. For certain types of interaction it is preferred that the target molecule is a bioconjugate of a probe molecule. According to a second aspect of the present invention, there is provided a method for detecting a molecular interaction comprising the steps of: immobilizing at least one probe molecule on an exposed metal sensor electrode which forms an upper part of a metal insulator semiconductor diode structure; placing an electrolyte containing at least one target molecule in contact with the probe molecule; and, detecting a change in a capacitance-voltage characteristic of the MIS diode structure in response to a molecular interaction between the probe molecule and the target molecule at the exposed surface of the metal sensor electrode. The method provides a simple way to detect interactions between two types of molecules resulting in a characteristic electrical signal, which can be monitored and processed as required. Preferably, the step of detecting the change in a capacitance-voltage characteristic includes the step of applying a voltage difference between a part of the MIS diode structure and a reference electrode which is in contact with the electrolyte. By applying a voltage difference in this way, both the rate of immobilization and the resulting density of probe molecules may be controlled. Furthermore, the molecular interaction rate may also be increased, thereby permitting data collection at near real-time speeds. Sometimes the density of probe molecules is not sufficient to adequately cover the sensor electrode. It is therefore preferred that the method further comprises the step of positioning spacer molecules between probe molecules on the sensor electrode, the spacer molecules being substantially inert to the target molecules. The detection method can be improved by suitable labelling of the interaction species, particularly if the change in MIS diode capacitance-voltage characteristic is enhanced. Preferably, the method further comprises the step of labelling the target molecules with electrically charged molecules. Preferably, the method further comprises the step of providing electrically charged molecules that bind to a product of the molecular interaction between the probe molecules and the target molecules. As a result of the ability to detect a specific molecular interaction, the method may also be used to identify a specific species associated with the interaction. In particular, it is preferred that the method is used for identifying DNA by detecting the hybridization of DNA. According to a third aspect of the present invention, there is provided a method for fabricating a metal insulator semiconductor (MIS) diode structure for use in a sensor, the sensor being operative to produce a change in a capacitance-voltage characteristic of the

MIS diode structure in response to a molecular interaction occurring near a surface of a metal electrode of the MIS structure, comprising the steps of: forming a semiconductor substrate; forming a layer of an insulator on the semiconductor layer; and, forming the metal electrode on the insulator layer; the method further comprising the step of doping a part of the MIS diode structure such that a predetermined bias voltage is required to achieve a maximum change in the capacitance-voltage characteristic of the MIS diode structure. In summary, the present invention provides a versatile electrical sensor and sensing method that can be used to monitor a wide variety of molecular interactions and thereby also be used in the identification of particular target species. A particularly important application is in the identification of DNA by detecting the hybridization process. The use of a MIS diode provides a sensor that is simple to manufacture and operate, and which may be passivated to provide electrical and chemical isolation of all but a part of the exposed metal sensor electrode. A separate electrical connection to the metal sensor electrode may be supplied, as may one or more reference/counter electrodes, for the application of control signals and the detection of changes in the device behaviour. An applied voltage allows control of the probe immobilization process for sensor electrode functionalization and also control over the subsequent interaction with target molecules contained within an electrolyte. Increased speed can be achieved in this way. Maximum sensitivity can be assured by applying the optimum voltage bias to the diode, the optimum voltage depending on the particular sensor and the interaction to be detected. This optimum voltage bias can be tuned during device fabrication by various processes, including doping, thereby allowing the optimum operating voltage for the MIS diode to be matched to the requirements of the control and detection circuitry. The single sensor device is easily extended to an integrable array of sensors, which can provide greater device functionality and monitoring capability.

Brief Description of the Drawings Examples of the present invention will now be described in detail with reference to the accompanying drawings, in which: Figure 1 shows the C-V characteristics of a known MIS device; Figure 2 shows a cross-section through the MIS diode structure of a sensor according to the present invention; Figure 3 shows an MIS diode sensor having a functionalized sensor electrode in contact with an electrolyte; Figure 4 shows the capacitance-voltage characteristics of an MIS diode sensor with immobilized ssDNA probe and then after exposure to non-complementary and complementary DNA strands; and, Figure 5 shows the capacitance-voltage characteristics of an MIS diode sensor with immobilized ssDNA probe and then after exposure to DNA strands having one base mismatch and a full complementary match.

Detailed Description The present invention is directed to the detection of chemical, biochemical or biological interactions, which result in a change of the electric potential distribution at the interface between the functionalized metal electrode (gate) of an metal-insulator- semiconductor (MIS) structure and an electrolyte. In contrast to field effect transistor (FET) based sensors, a sensor according to the present invention is less complex and is cheaper to manufacture. In a typical MIS structure, a metal (sometimes called the gate) is deposited on a region of a layer of an insulator, which in turn is formed on the upper surface of a semiconductor substrate. An ohmic contact (back-contact) is fabricated on the semiconductor, for instance by depositing an electrically conductive material. This device is known as MIS device or MIS diode and its properties are described in the general literature of the semiconductor device field. The capacitance of the MIS diode depends on the voltage applied between the gate and the back contact in a well-known way. As such, the device may be used as a capacitor with either a fixed or variable capacitance, depending upon the dc bias voltage applied. Figure '1 shows a typical capacitance-voltage (C-V) characteristic for a device in which the semiconductor is p-type Silicon, with 10 ohm-cm resistivity, and the insulator is thermally grown silicon dioxide. The characteristic is measured by probing with a small ac signal at a particular frequency (1kHz here), whilst sweeping the applied dc bias voltage. Inset A in Figure 1 shows the equivalent circuit for the MIS capacitor, comprising a fixed capacitance (Cj) arising from the insulator, and a variable capacitance (C_d) arising from the semiconductor, in particular the semiconductor-insulator interface. In practice, electric current is measured in order to calculate the complex impedance of the device, which generally includes not only the capacitative component but also a resistive component. By measuring the behaviour at several frequencies, the resistive contribution may be deduced, thereby permitting calculation of the total device capacitance. As can be seen from in Figure 1 , as the applied voltage is swept from reverse bias through to forward bias, the behaviour of the MIS capacitor changes from accumulation (C/Cj = 1), through depletion to inversion. In ideal behaviour, the capacitance remains constant in accumulation, decreases in depletion, before again remaining constant in inversion at a lower value. Small deviations from this ideal may arise as a result of impurities within the MIS structure. However, once the bias is above a certain threshold voltage for inversion behaviour, the device capacitance may increase again in a manner that is frequency dependent, as can be seen from inset B of in Figure 1. This effect is particularly pronounced at low frequency. The present invention makes use of an MIS diode structure having a suitable metal electrode (gate) on which probe molecules are immobilized. Figure 2 shows an example of an MIS diode structure 20 for a sensor according to the present invention. The device comprises a metal sensing electrode 21 deposited on an insulator layer 22, which is located on a semiconductor substrate 23. The semiconductor 23 may be a bulk semiconductor such as single crystalline silicon, a thin film of semiconductor, including single crystal silicon (as in silicon-on-insulator, SOI), amorphous or polycrystalline silicon, an organic semiconductor, or any other suitable type of substrate. The insulator layer 22 may be formed by any suitable technique, including thin film insulator fabrication such as thermal growth, chemical vapor deposition, sol-gel, spin on, ink-jet, etc. The sensor (gate) electrode 21 should be of a type of metal that enables the chemical or biochemical immobilization of probe molecules. A good example is an electrode comprising a layer of chromium followed by gold, for the immobilization of thiolated biomolecules. A metallized region 24 on the semiconductor provides an ohmic back contact. When in use, some regions of the insulator might be exposed to the electrolyte, and so protection of these regions may be necessary by deposition of a thick passivating material such as polyimide, silicone rubbers or any other chemically inert and electrically insulating material. In order to avoid penetration of the electrolyte at the edges of the interface between the metal gate and the insulator, the passivation layer 25 may also partially cover this region, as shown in Figure 2. The reduced exposed region 26 of the metal electrode acts as the sensing area, on which probes molecules will be immobilized. Furthermore, the passivation layer may be extended to passivate the sides of the MIS structure. If desired, a direct electrical connection can be provided to the metal gate beneath the passivation layer 25 in order to allow the application of a control voltage. When the gate metal (sensor electrode) of the MIS diode is placed in contact with an electrolyte, and the voltage is measured with respect to a reference electrode, which is also in contact with the electrolyte, a change in the surface dipole magnitude (χ) occurs at the interface between the gate and the electrolyte. This is accompanied by changes in potential differences within the device, including the potential (φo) across the electrochemical double layer. As a consequence of these changes, the C-V characteristic of the MIS diode will shift along the voltage axis. This shift may be calibrated for different types of electrolyte. Figure 3 shows an experimental arrangement for monitoring the change in C-V characteristic in the presence of an electrolyte. The sensor comprises an MIS diode structure 30 having a back contact 31 , a semiconductor substrate 32, an insulator layer 33 and a metal sensor electrode 34. An electrolyte 37 is brought into contact with a portion of the sensor electrode 34 and potential differences are applied and measured by means of a reference electrode 38 and a counter electrode 39, which are immersed in the electrolyte 37. These electrodes (38,39), together with the back contact 31, are connected to suitable readout apparatus 40, such as a potentiostat, thereby completing the circuit. The immobilization or interaction of electrically charged molecules, such as DNA, can be affected by applying a voltage between the metal gate 34 or the back-contact 31 and an electrode 38,39 in the electrolyte solution 37 containing the biological molecules. The applied voltage must be attractive for the charges in the biomolecules. Thus, by applying a control signal to the sensor electrode 34, the immobilization time and concentration of immobilized probe molecules on the electrode may be controlled. Similarly, the speed and effectiveness of any subsequent interaction mechanism, such as hybridization, may also be controlled. In the present invention, one or more types of biological, organic or other types of molecules, here termed probe molecules, are immobilized on the metal gate via some chemical or biochemical process. Particular examples of probe molecules include proteins, antibodies and antigens, vitamins, peptides, sugars and oligonucleotides including DNA, RNA and PNA. The modified metal electrode of the MIS diode is then described as a functionalized electrode (or gate). The presence of immobilized chemical species leads to a further change of χ, brought about by various microscopic phenomena, including the charge distribution of the immobilized chemical species and interactions between the functionalized gate and the electrolyte, such as chemisorption or physisorption of electrolyte molecules. The corresponding effect on the potential φo leads a to change in the C-V characteristic of the MIS diode, which can broadly be described as a shift along the voltage axis, as compared to the MIS diode with an unmodified gate. Further changes in χ and φ₀ occur when the probe molecules interact with other species present in the electrolyte. In particular, these species will have been intentionally introduced into the electrolyte and are thus termed target molecules. The change may be especially marked if the target molecule is the bioconjugate of the probe molecule. For example, when a gate functionalized with a given strand of DNA probe is exposed to a target with the complementary strand, hybridization occurs. Since the total negative charge carried by the hybridized molecule is twice that of the single stranded oligomer, χ and φ₀ change. By contrast when the functionalized gate is exposed to a non- complementary strand, no binding occurs and the above parameters are unchanged. Thus the shift, or any other change in the C-V characteristic, can be used to detect DNA hybridization. The method may be extended to other chemical and biochemical systems, such as proteins and cells. In order to amplify, or indeed induce, the change of χ and φo change upon interaction between the probe and target molecules, the target molecules can be biochemically labelled with electrically charged molecules. Alternatively, electrically charged molecules that bind specifically to the bioconjugate probe-target specie can be added to the system to enhance or induce the changes. If there are areas of the sensor electrode metal that are not covered by the probe molecules, and are therefore exposed to the electrolyte, the effectiveness of the method can be reduced. For this reason, molecules that are inert to the target and carry a much lower charge can be used to passivate these areas. Such molecules are usually termed spacer molecules. The effectiveness of the method is also reduced if the distance between probe molecules is larger that the characteristic Debye length in the electrolyte. The density of probe molecules may be controlled by applying a voltage between the gate metal and the reference electrode, whilst the Debye length in the electrolyte can be controlled by changing the ionic concentration of the solution. With reference again to Figure 3, a detailed example of the structure of an MIS diode sensor and the procedure for the chemical/biochemical preparation of the sensor for detecting DNA hybridization is as follows. The MIS diode structure 30 comprises a p-type boron-doped silicon semiconductor (p-Si) substrate 32 with a resistivity of 6-12 Ωcm, a thermally grown 50 nm thick layer of silicon dioxide (SiO₂) insulator 33, a metal gate 34 comprising a 10 nm thick layer of evaporated chromium (Cr) 35 followed by a 100 nm thick layer of evaporated gold (Au) 36 and a back contact 31 comprising a 100 nm thick layer of evaporated aluminium (Al). The electrolyte 37 used comprises a 50 mM potassium phosphate buffer containing 50 mM sodium chloride (NaCI) with pH 7.0. A dc bias voltage of between -1 V and +3 V is applied to the back contact 31 with respect to a silver/silver chloride (Ag/AgCI) reference electrode 38 immersed in the electrolyte 37 and a small 20 mV ac voltage is applied on top of the bias voltage in order to allow determination of the impedance and hence the capacitance of the device. It is preferable to avoid the low frequency effects that occur when the MIS diode is biased into the inversion regime. Referring to Figure 1 , it can be seen from the second inset that more "ideal" C-V behaviour may be obtained by operating with an ac voltage frequency of at least several hundred Hertz. In the present example, an operating frequency of 1kHz was employed. The measurement is effected by monitoring the electric current with the help of platinum wire 39, which acts as a counter electrode in a 3- electrode electrochemical arrangement. Control of the applied voltage, as well as the measurement of current and calculation of the complex impedance, is performed using a potentiostat 40 connected to a computer. This allows fast scanning and data processing, with the possibility of obtaining time-resolved and near real-time measurements. Single stranded DNA (ssDNA) consisting of 20 base pairs of Adenine and modified on the 5' end by HS-(CH₂)₆-PO -(CH₂CH₂O)₆-ssDNA was immobilized on the gold electrode 36 using a concentration of 1 μM in a 1 M potassium phosphate buffer with pH 7.0 containing 1 M NaCI and 1 mM ethylene diamine tetraacetic acid (EDTA). A potential of +0.3 V was applied to the Al back contact 31 with respect to a platinum wire 39 immersed in the solution containing the modified DNA. The immobilization was performed over a period of 3 hours, after which the substrate was washed with pure H₂0 and 10 mM NaCI containing 10 mM EDTA. In order to create a spacer between the DNA molecules, mercaptohexanol, HS-

(CH₂)₆-OH, was subsequently immobilized on the gold electrode 36 over a 1 hour period, using a concentration of 1 mM in 1 M potassium phosphate buffer with pH 7.0, containing 1 M NaCI and 1 mM EDTA. After immobilization of the spacer molecules, the substrate was washed with H₂O and NaCI/EDTA. A complementary DNA strand consisting of 20 base pairs of Thymine in a concentration of 1 μM in 1 M NaCI containing 10 mM Tris and 1 mM EDTA was deposited on top of the substrate for 1 hour using a +0.3 V potential as before. The substrate was again washed with H₂O. For comparison, a control was set up, which comprised substrates with immobilized probe ssDNA and spacer molecules under the same conditions, and on which was deposited a non-complementary DNA strand consisting of

20 base pairs of Adenine in the same concentration and using the same buffer conditions. Figure 4 shows the measured C-V characteristics for three cases: (1) ssDNA with spacers; (2) ssDNA with spacers after exposure to a non-complementary target; and, (3) ssDNA with spacers after exposure to a complementary target. The voltage is applied between the back contact and the Ag/AgCI reference electrode and referred to the latter. It should be noted that, as a consequence of making measurements relative to the reference electrode, the positive and negative voltages shown in Figure 4 relate to reverse bias and forward bias, respectively. Therefore, the C-V characteristic shown in Figure 4 appears reversed, as compared to that shown in Figure 1. As can be seen, the characteristic displays negligible change upon exposure to the non-complementary strand, when compared to the single-stranded case. However, upon hybridization with the complementary strand, there is a significant shift in the characteristic of approximately 150 mV in the direction of negative voltages. This is as expected, given the significant increase of negative charges on the metal gate arising from the hybridization. Single base pair mismatches can also be detected by the magnitude of the shift observed. To demonstrate this experimentally, single stranded DNA (ssDNA) consisting on 18 base pairs and sequence 5'-ACCATTTCAGCCTGTGCT modified at the 5' by HS- (CH₂)₆-PO₄-(CH₂CH₂O)<3-ssDNA was immobilized on the metal gate together with spacer molecules consisting of mercaptohexanol, HS-(CH₂)e-OH in a molar ratio of 1:1. A total concentration of 2 μM was used in a 1 M potassium phosphate buffer pH 7.0 containing 1 M NaCI, 5 mM MgCI and 1 mM EDTA. After immobilization the substrate was washed with pure H₂O. Complementary DNA strands with sequence 3'- TGGTAAAGTCGGACACGA and non-complementary strands with only 1 base mismatch (sequence 3'-TGGTAAAGTCAGACACGA) were used in a concentration of 1 μM in 1 M phosphate buffer pH 7.0 with 1 M NaCI. After interaction, the substrate was again washed with H₂O. Figure 5 shows the measured C-V characteristics of a MIS diode functionalized with ssDNA and spacers for three cases: (1) before exposure to target; (2) after exposure to a target containing a single base mismatch; and (3) after the subsequent exposure to a fully complementary target. Upon exposure to a target with only one base pair mismatch, a partial hybridization process is expected to occur. Figure 5 shows a significant shift of the characteristics after interaction with the target with one mismatch, indicating a partial hybridization process. Upon exposure to the complementary target, the un-interacted ssDNA probes hybridize and a further shift in the characteristics is observed. The value of the shifts observed upon interaction with non-complementary strands can be compared to the value expected for the interaction with the fully complementary target in order to determine the efficiency of the hybridization. This in turn is related to the number of mismatches present in the target. It is apparent from the results of Figures 4 and 5 that the fractional change in capacitance (ΔC/C) induced by the hybridization process, and hence the sensitivity of the technique, is dependent on the applied bias voltage. Thus, by applying the optimum bias, the sensitivity of the device can be maximized for any given measurement. Furthermore, the sensor may be calibrated in order that particular interactions or molecular species may be distinguished according to the measured change in capacitance. There may be situations where the additional control and detection circuitry needs to operate at a particular voltage, in which case it is desirable to adjust the threshold voltage of the MIS sensor such that the bias required to achieve maximum sensitivity coincides with that applied by the detection and control circuit. Fortunately, there are many known techniques, such as doping a part of the MIS structure, by which the fundamental characteristic of the MIS diode device can be tuned during fabrication. Particular examples include ion implantation into the semiconductor substrate through the surface and the introduction of charges into the insulator. This facility of the MIS sensor provides great flexibility, whereby the required bias can be tuned in a predetermined manner according to the requirements of the particular molecular interaction to be detected and also the associated control and detection circuitry. Of course, much of the control and detection circuitry may be integrated with the MIS structure in a single sensor module. Although the examples given above have only dealt with single sensor devices, it is quite possible to extend the MIS detector to an array of sensors. In this way, greater areal coverage may be achieved and spatial resolution across the array becomes possible. Both one-dimensional and two-dimensional array of MIS diode sensors may be employed, with scan and sensing circuitry connected to each sensor element in the array. Again, the control and detection circuitry can be located on an external microchip or could be monolithically integrated with the sensor array.

Claims

1. A sensor for use in the detection of a molecular interaction comprising a metal insulator semiconductor (MIS) diode structure having a back contact and an exposed metal sensor electrode on which probe molecules can be immobilized, wherein, in use, the sensor is operative to produce a change in a capacitance- voltage (C-V) characteristic of the MIS diode structure in response to molecular interaction at the exposed surface of the metal sensor electrode.

2. A sensor according to claim 1, wherein a predetermined bias voltage is applied to the MIS diode structure, the sensitivity of the sensor being optimum at the predetermined bias voltage.

3. A sensor according to claim 2, wherein the predetermined bias voltage for optimum sensor sensitivity is in dependence on the concentration of a dopant in a part of the MIS diode structure.

4. A sensor according to claim 2 or claim 3, further comprising detection circuitry, wherein the predetermined bias voltage is matched to the detection circuitry.

5. A sensor according to any of claims 1 to 4, wherein the sensor electrode comprises one or more metals selected from a group which includes gold, chromium and platinum.

6. A sensor according to any preceding claim, wherein the semiconductor of the MIS diode structure is selected from a group which includes single crystal silicon, amorphous or polycrystalline silicon and organic semiconductor.

7. A sensor according to any preceding claim, further comprising a passivation layer for passivating a portion of the MIS diode structure.

8. A sensor according to claim 7, wherein the passivation layer is formed from at least one material selected from a group which includes polyimide, silicone rubber, BCB, SiO₂ and Si₃N .

9. A sensor according to any preceding claim, further comprising a reference electrode.

10. A sensor according to claim 9, further comprising means for applying a voltage difference between a part of the MIS diode structure and the reference electrode.

11. A sensor according to any preceding claim, further comprising a counter electrode.

12. A sensor according to any preceding claim, further comprising at least one probe molecule immobilized on the exposed metal sensor electrode.

13. A sensor according to claim 12, wherein the probe molecule is selected from a group which includes proteins, antibodies and antigens, vitamins, peptides, sugars and oligonucleotides, including DNA, RNA and PNA.

14. A sensor according to claim 12 or claim 13, further comprising an electrolyte in contact with the probe molecule.

15. A sensor array comprising a plurality of sensors, wherein each sensor is in accordance with any preceding claim.

16. The use of a sensor according to any of claims 12 to 14 or a sensor array according to claim 15 for the identification of a target molecule.

17. A use of a sensor or sensor array according to claim 16, wherein the target molecule is a bioconjugate of a probe molecule.

18. A method for detecting a molecular interaction comprising the steps of: immobilizing at least one probe molecule on an exposed metal sensor electrode which forms an upper part of a metal insulator semiconductor (MIS) diode structure; placing an electrolyte containing at least one target molecule in contact with the probe molecule; and, detecting a change in a capacitance-voltage characteristic of the MIS diode structure in response to a molecular interaction between the probe molecule and the target molecule at the exposed surface of the metal sensor electrode.

19. A method according to claim 18, wherein the step of detecting a change in the capacitance-voltage characteristic includes applying a voltage difference between a part of the MIS diode structure and a reference electrode which is in contact with the electrolyte.

20. A method according to claim 18 or claim 19, further comprising the step of positioning spacer molecules between probe molecules on the sensor electrode, the spacer molecules being substantially inert to the target molecules.

21. A method according to any of claims 18 to 20, further comprising the step of labelling the target molecules with electrically charged molecules.

22. A method according to any of claims 18 to 21, further comprising the step of providing electrically charged molecules that bind to a product of the molecular interaction between the probe molecules and the target molecules.

23. A method for identifying DNA comprising the step of detecting the hybridization of DNA using the method of any of claims 18 to 22.

24. A method for fabricating a metal insulator semiconductor (MIS) diode structure for use in a sensor, the sensor being operative to produce a change in a capacitance- voltage characteristic of the MIS diode structure in response to a molecular interaction occurring at a surface of an exposed metal sensor electrode of the MIS structure, comprising the steps of: forming a semiconductor substrate; forming a layer of an insulator on the semiconductor layer; and, forming the metal electrode on the insulator layer; the method further comprising the step of doping a part of the MIS diode structure such that a predetermined bias voltage is required to achieve a maximum change in the capacitance-voltage characteristic of the MIS diode structure.