Microsystem liquid gate field effect sensor chip, manufacturing method and application thereof
By introducing self-assembly functional factors and agarose gel electrolyte into a graphene-based field-effect transistor sensor, the detection system was optimized, solving the problems of sensor selectivity and stability, and realizing the detection of micro-systems with high sensitivity and high selectivity.
Patent Information
- Authority / Receiving Office
- CN · China
- Patent Type
- Patents(China)
- Current Assignee / Owner
- HEFEI UNIV OF TECH
- Filing Date
- 2023-03-22
- Publication Date
- 2026-07-14
AI Technical Summary
Existing graphene-based field-effect transistor sensors lack selectivity during detection and have unstable detection systems, making them difficult to apply in traditional detection media.
A micro-system liquid gate field-effect sensor chip is designed, which uses a monolayer graphene channel and a gate modified with self-assembled functional factors, combined with an agarose gel solid electrolyte, to specifically identify and capture target molecules, thereby optimizing the detection system.
It improves the sensitivity and stability of detection, enhances selectivity and repeatability, and achieves high sensitivity, high stability and high selectivity for micro-system detection.
Smart Images
Figure CN116297773B_ABST
Abstract
Description
Technical Field
[0001] This invention belongs to the field of field-effect sensor chip technology, specifically relating to a micro-system liquid gate field-effect sensor chip, its fabrication method, and its application. Background Technology
[0002] A field-effect transistor (FET) is a semiconductor device that controls the output current by controlling the electric field effect of the input circuit. It has become one of the most important components in the microelectronics industry. Graphene's single-atom-layer structure and ultra-high carrier mobility make it extremely responsive to input voltage, making it ideal as a short-channel material for high-speed FETs and promising for applications in both analog and digital circuits.
[0003] In recent years, electrochemical-based analytical methods have attracted great interest from scientists due to their simplicity, rapid reaction, and low consumption of toxic chemicals. Against this backdrop, sensors based on electrolyte-gate-controlled graphene field-effect transistors (SGGTs) have shown strong potential, offering significant advantages in miniaturization, low cost, and high sensitivity, which can be helpful in food quality and safety detection.
[0004] While SGGT possesses some signal amplification capabilities, its lack of selectivity is the biggest obstacle preventing its widespread application. In recent years, many researchers have attempted to endow SGGT with selectivity through interfacial functionalization, including modifications to carbon materials, enzymes, and nucleic acid functional factors. However, these functional molecules have not performed as expected in traditional detection environments.
[0005] Surface-specific functionalized sensors have been widely used in traditional electrochemistry, but their application in SGGT sensors and other field-effect transistors has yielded limited success. This is because field-effect transistor methods and systems are still immature, the parameters of the entire detection system significantly impact detection performance, and the instability and difficulty in achieving stability in traditional detection media remain unresolved. Therefore, it is necessary to design and optimize the detection system to allow the specific functional factors to fully play their role in SGGT sensors, and then extrapolate their application to other types of field-effect transistors. Summary of the Invention
[0006] The purpose of this invention is to provide a micro-system liquid gate field effect sensor chip, its fabrication method, and its application in order to solve the above-mentioned problems.
[0007] The present invention achieves the above objectives through the following technical solutions:
[0008] This invention provides a micro-system liquid gate field-effect sensor chip, comprising a substrate, on which a source, a drain, and a gate are disposed, and a monolayer of graphene is disposed in the channel between the source and the drain. The substrate also has a PDMS layer for encapsulating the source, drain, and gate, which forms a PDMS trench after encapsulation. The PDMS trench is further filled with a solid gel electrolyte, and the gate is modified with self-assembled functional factors for specifically recognizing and capturing target molecules.
[0009] As a further optimization of the present invention, the oligonucleotide sequence fragment of the functional factor is: 5′-GTTTGTGTATTACAGTTATGTTACCCTCATTTTTCTGAAC-3′.
[0010] As a further optimization of the present invention, the solid gel electrolyte is specifically an agarose gel with a volume of 200 μL.
[0011] This invention provides a method for fabricating the above-mentioned microsystem liquid-gate field-effect sensor chip, comprising the following steps:
[0012] (1) Patterned Cr / Au electrodes are sequentially deposited on a clean substrate by magnetron sputtering. The electrodes include a source, a drain, and a gate.
[0013] (2) Graphene was transferred to a patterned channel between the source and drain electrodes in deionized water using a wet chemical method.
[0014] (3) Electrochemically activate the gate after magnetron sputtering, measure the target functional factor modified with thiol, drop it onto the electrode surface and spread it evenly, incubate it overnight under room temperature and light protection, then rinse it with pure water, air dry it naturally, add 6-mercaptohexyl-1-ol MCH to seal the gate, and then rinse off the excess MCH with pure water to obtain the gate modified with self-assembled functional factor.
[0015] (4) Use an organosilane layer to encapsulate the metal connection lines of the source, drain and gate on the upper part of the sensor to form a PDMS trench;
[0016] (5) Prepare a 0.5% concentration agarose gel as a solid electrolyte, take 200 μL and add it to the PDMS tank. After cooling to room temperature and solidifying, the finished product is obtained.
[0017] As a further optimization of the present invention, step (1) specifically involves: using a magnetron sputtering RF magnetron to sequentially sputter and deposit patterned Cr / Au on a substrate, with dimensions of 10nm / 100nm, and sputtering parameters of: Cr, power 50W, pressure 0.3Pa, Au, power 35W, pressure 0.8Pa.
[0018] As a further optimization of the present invention, the graphene preparation process in step (2) is as follows:
[0019] A monolayer of graphene was synthesized on a 25μm thick copper foil at 1000℃ using a mixture of CH4 and H2 gases. Polymethyl methacrylate was uniformly spin-coated onto the graphene using a spin coater. The copper foil substrate was then etched with an etching solution and repeatedly rinsed with deionized water.
[0020] Graphene was cut into appropriately sized pieces and transferred to a patterned channel between the source and drain electrodes in deionized water using a wet chemical method. It was then annealed on a hot plate at 125°C for 20 minutes and then soaked in acetone at 55°C for 3 hours, with the acetone being replaced every hour to completely remove PMMA.
[0021] As a further optimization of the present invention, the electrochemical activation in step (3) is specifically as follows: the gate is scanned sequentially in 0.5M NaOH and 0.5M H2SO4 using cyclic voltammetry, with 2-4 scans each time until each cycle overlaps, rinsed with pure water, and dried with high-purity nitrogen.
[0022] As a further optimization of the present invention, the preparation process of the 0.5% concentration agarose gel in step (5) is as follows: prepare 0.1X PBS buffer, weigh 0.05mg of agarose powder into 10mL of 0.1X PBS, then heat it on a heating plate until the agarose powder is completely dissolved, and then cool it to about 30°C to obtain the gel.
[0023] As a further optimization of the present invention, the graphene channel between the source and drain in step (2) has a width of 0.22 mm and a length of 3 mm.
[0024] This invention provides an application of the aforementioned microsystem liquid gate field effect sensor chip in the field of tetracycline detection.
[0025] This invention provides a method for detecting tetracycline using the above-mentioned microsystem liquid-gate field-effect sensor chip, comprising the following steps:
[0026] (1) Using two source meters, connect the source, drain and gate of the sensor chip. After the gate voltage and source-drain voltage parameters of the device are stable, fix the sampling interval at 2 μL of tetracycline standard solution to be added to the PDMS bath containing 200 μL of gel solid electrolyte every 500 s, and record the real-time current curve.
[0027] (2) The first 100 seconds before each sample addition was taken as the steady signal of the tetracycline concentration standard. The correspondence between time and channel current was obtained. The logarithm of the channel current change and the tetracycline concentration was extracted and linearly fitted to obtain the linear relationship between channel current and tetracycline concentration.
[0028] (3) Add 2 μL of the analyte solution to the PDMS cell of the sensor chip at a fixed sampling interval of every 500 s, and record the real-time current curve to obtain the channel current. Substitute the channel current into the relevant equation in step (3) to obtain the tetracycline concentration in the sample to be tested.
[0029] As a further optimization of the present invention, the gate voltage VGS = 0.1V and the source-drain voltage VDS = 0.05V in step (2).
[0030] As a further optimization of the present invention, the linear relationship between the channel current and the logarithm of the tetracycline concentration is y = 1.43025x + 15.63456, where x represents the logarithm of the tetracycline concentration, y is the channel current in μA, and the goodness of fit R0 is 1.43025x + 15.63456. 2 =0.98995.
[0031] This sensor consists of a single monolayer graphene channel and a gate. The gate is modified with self-assembled functional factors that can specifically recognize and capture target molecules. The basic principle of SGGT is to use the gate voltage to regulate the channel current. Therefore, when the functional factors recognize and bind to the target molecules, it causes a change in the channel current. By determining the correlation between the current change and the concentration, the goal of rapid detection can be achieved. Detection in a 200μL microsystem enhances the gate's ability to regulate the channel current and the efficiency of the functional factors in capturing the target, significantly improving the sensitivity of the detection system. Using agarose gel solid electrolyte instead of traditional liquid buffer solution avoids the impact on the system during sample addition, preventing the originally stable electrical signal from shifting and generating "false signals." Furthermore, the strong water-retention properties of the gel reduce the evaporation of water in the 200μL detection system, ensuring that the ion concentration in the detection environment remains constant, thus greatly improving the stability of the detection system. However, due to the filtering effect of the gel pore size, the time for the target material to reach the gate surface is prolonged, and no obvious "step-like" response can be seen in the traditional continuous IT curve. Therefore, this invention innovatively improves the specific detection method by setting the final response of the concentration to 100s before sample addition, so that the micro system IT has an obvious step-like response, which is beneficial for subsequent data observation and processing.
[0032] Therefore, the beneficial effects of the present invention are as follows:
[0033] 1. In a small 200μL system, the required sample volume is small, while the sensitivity of the detection system is improved. It is simple to operate, economical and portable.
[0034] 2. Using a gel solid electrolyte instead of a traditional liquid buffer avoids the impact on the system during sample addition, thus preventing "false signals" and improving the stability of the detection system. The data processing procedure has been innovatively modified, capturing the stable response at a given concentration for the first 100 seconds after sample addition, which is beneficial for subsequent data observation and processing.
[0035] 3. Specific functionalization of the gate endows the field-effect transistor detection system with higher selectivity and repeatability, making it universal and enabling real-time monitoring. In summary, this application develops a micro-system detection strategy with high sensitivity, high stability, and high selectivity. Attached Figure Description
[0036] Figure 1 This is a schematic diagram of the microsystem liquid gate field effect sensor chip structure of the present invention;
[0037] Figure 2 This is a schematic diagram of the gate functionalization self-assembly modification process;
[0038] Figure 3 Comparison of detection performance between gel solid electrolytes and traditional liquid buffer solutions
[0039] Figure 4 Sensing performance results of functionalized modified gate SGGT sensor.
[0040] Illustration: 1. Substrate; 2. Gate; 3. Source; 4. Drain; 5. Monolayer graphene; 6. Solid gel electrolyte; 7. PDMS tank. Detailed Implementation
[0041] The present application will now be described in further detail with reference to the accompanying drawings. It should be noted that the following specific embodiments are only used to further illustrate the present application and should not be construed as limiting the scope of protection of the present application. Those skilled in the art can make some non-essential improvements and adjustments to the present application based on the above application content.
[0042] 1. Materials
[0043] Unless otherwise specified, all methods used in this invention are conventional methods known to those skilled in the art, and all reagents used are commercially available products unless otherwise specified.
[0044] 2. Method
[0045] 2.1 Microsystem Liquid Gate Field Effect Sensor Chip SGGT
[0046] like Figure 1As shown, the micro-system liquid gate field effect sensor chip of the present invention includes a substrate, on which a source, a drain and a gate are provided. A monolayer of graphene is provided in the channel between the source and the drain. A PDMS layer for encapsulating the source, drain and gate is also provided on the substrate. After encapsulation, the PDMS layer forms a PDMS trench. The PDMS trench is also filled with a solid gel electrolyte. The gate is modified with self-assembled functional factors for specifically recognizing and capturing target molecules.
[0047] In this embodiment, the sequence of the self-assembled functional factor is as follows: 5′-GTTTGTGTATTACAGTTATGTTACCCTCATTTTTCTGAAC-3′. This sequence was ordered from Shanghai Sangon Biotech Co., Ltd. It can specifically recognize TC and does not bind to other tetracycline molecules such as oxytetracycline (OTC) and chlortetracycline (CTC). The 3′ is modified with C6-SH, which can form a stable Au-S bond with the gate gold, thus enabling stable self-assembly on the gate surface.
[0048] 2.2 Fabrication of the SGGT Sensor
[0049] (1) Fabrication and cleaning of glass substrate
[0050] First, use a glass cutter to cut the glass slide into small glass pieces 1cm wide and 1.2cm long to be used as the substrate of the sensor. Then, use acetone, anhydrous ethanol and deionized water to ultrasonically clean twice, 20 minutes each time. Next, blow dry with high-purity nitrogen and continue to bake on a heating table at 120°C for 15 minutes, ensuring that the substrate surface is clean and free of water stains.
[0051] (2) Fabrication of patterned gold electrodes
[0052] Clean and cooled glass slides were fixed to a custom mask with high-temperature tape. Using a magnetron sputtering RF magnetron, patterned Cr / Au (10nm / 100nm) electrodes were sequentially sputtered onto the glass slide attached to the mask. The sputtering parameters were Cr (power: 50W; pressure: 0.3Pa) and Au (power: 35W; pressure: 0.8Pa).
[0053] (3) Preparation of monolayer graphene layers
[0054] A monolayer of graphene was synthesized on a 25 μm thick copper foil at 1000 °C using a mixed gas (CH4 and H2) via chemical vapor deposition (CVD). Polymethyl methacrylate (PMMA) was uniformly spin-coated onto the graphene using a spin coater (two-step spin coating method: 800 rpm, 10 s; 2000 rpm, 20 s). The copper foil substrate was then etched using an etching solution (CuSO4:HCl:H2O = 10 g:50 mL:50 mL), and the substrate was rinsed repeatedly with deionized water 3–5 times.
[0055] (4) Transfer of graphene
[0056] Graphene was cut into appropriately sized pieces and transferred to a patterned channel between the source and drain electrodes using a wet chemical method in deionized water. It was then annealed on a hot plate at 125°C for 20 minutes, followed by immersion in acetone at 55°C for 3 hours, with the acetone being replaced hourly to completely remove PMMA.
[0057] (5) Gate interface functionalization self-assembly modification
[0058] like Figure 2 As shown, the magnetron sputtered gate was first electrochemically activated, and cyclic voltammetry was performed sequentially in 0.5M NaOH and 0.5M H₂SO₄, with 2-4 scans per cycle until each cycle overlapped. The electrode was rinsed with pure water and dried with high-purity nitrogen. Then, 10 μL of a 10 μM thiol-modified target functional molecule was dropped onto the electrode surface and spread evenly. The electrode was then incubated overnight (8-12 h) at room temperature in the dark. After rinsing with pure water and air drying, 10 μL of 5 μM 6-mercaptohexyl-1-ol (MCH) was added to seal the electrode, reducing or preventing non-specific adsorption. After sealing for 30 min, excess MCH was rinsed off with pure water, and the electrode could then be connected to a 2400 source meter for electrochemical detection.
[0059] (6) Sensor Packaging
[0060] Finally, an organosilane layer is used to protect the metal interconnects of the source, drain, and gate electrodes on the top of the sensor to prevent direct contact between the electrodes and the electrolyte during testing. The sensor consists of a drain (D), a source (S), and a gate (G). The graphene channel between the source and drain electrodes is 0.22 mm wide and 3 mm long.
[0061] (7) Preparation of gel solid electrolytes
[0062] When the agarose gel concentration was 0.075%, the average pore size was 800 nm. When the agarose gel concentration was 0.16%, the average pore size was 500 nm. When the agarose gel concentration was 1%, the average pore size was 150 nm. Based on the gel concentration optimization experiments, a 0.5% concentration gel was found to have good electrochemical performance and water retention.
[0063] First, prepare 0.1XPBS buffer by weighing 0.05 mg of agarose powder into 10 mL of 0.1XPBS. Then, heat the buffer until the agarose powder is completely dissolved. After cooling to about 30°C, add 200 μL to the PDMS bath. After cooling to room temperature and solidifying, it can be used for electrochemical detection.
[0064] 2.3 SGGT Chip Performance Characterization
[0065] Before each measurement, the SGGT device was rinsed with deionized water and 0.001M PBS buffer to remove residual contaminants from the graphene channel. Taking tetracycline as an example, in 200 μL of gel solid electrolyte, two source meters (Keithley 2400) controlled by LabVIEW were connected to the source, drain, and functional material-modified gate, and the sensor performance was characterized by the transfer characteristic curve (IDS-VG) and the channel current real-time response curve (IDS-Time).
[0066] (1) Determination of transfer characteristic curve
[0067] The transfer characteristic curves were measured under the conditions of a scan rate of 0.01V / s, a fixed source-drain voltage (VDS = 0.05V), and a gate voltage (VG) range of -0.2 to 0.5V.
[0068] (2) Determination of the real-time response curve of the channel current
[0069] After the device stabilized at a gate voltage (VGS) of 0.1V and a fixed source-drain voltage (VDS) of 0.05V, 2μL of tetracycline standard solution was added to a PDMS bath containing 200μL of gel solid electrolyte every 500s, and the real-time current curve was recorded.
[0070] (3) The first 100 seconds before each sample addition is taken as the steady signal of the tetracycline concentration standard. The correspondence between time and channel current is obtained. The logarithm of the channel current change and the tetracycline concentration is extracted and linearly fitted to obtain the linear relationship between channel current and tetracycline concentration.
[0071] like Figure 3As shown, the left side displays the IT curve of SGGT measured in a traditional liquid buffer. It is evident that the 200 μL microsystem is unstable, and the impact of sample addition causes "false signals." Furthermore, due to the evaporation of the electrolyte solution, the system remains in a metastable state, making it difficult to determine the stable signal value after sample addition, severely impacting the accuracy and repeatability of the detection. In contrast, the gel solid electrolyte (right figure) maintains a stable state for a long time due to the strong water-retention properties of the gel. The impact of sample addition is small, the response is stable, and no false positive signals are generated. However, due to the filtering effect of the gel pore size, the response time is prolonged due to the slow release of the response factor; therefore, the traditional "stepped" response is not observed in the IT curve. To facilitate data observation in the early stages and result processing in the later stages, this invention innovatively improves the specific detection method for the microsystem.
[0072] The results are as follows Figure 4 As shown, the point with relatively large transconductance is found based on the transfer curve. This is determined by the transfer curves of different detection media, as well as the water retention, conductivity, and pore size of gels at different concentrations, and the properties of the target substance. grain The optimal gel concentration, VGS, VDS, and loading interval were determined by analyzing the diameter and redox potential. Taking tetracycline as an example, the optimal gel concentration was 0.5%, and the gate voltage VGS = 0.1V. The loading interval was 500s, and the first 100s of loading was taken as the stable signal for that concentration. The time-channel current correlation curve (IT curve) was obtained, clearly showing that the sensor has good stability. Furthermore, as the tetracycline concentration increases, the change in channel current also increases accordingly, exhibiting a good linear relationship. According to the linear fitting curve, the logarithm of the channel current and concentration satisfies the equation y = 1.43025x + 15.63456, and the goodness of fit R0 is high. 2 =0.98995, detection limit is 100 pM, and detection linear range is 100 pM to 1 μM, indicating that the present invention is feasible for tetracycline detection. In actual operation for tetracycline detection, a fixed sampling interval of 2 μL of the analyte solution is added to the PDMS tank of the sensor chip every 500 s, and the real-time current curve is recorded to obtain the channel current. Substituting the channel current into the equation y = 1.43025x + 15.63456, and after logarithmic transformation, the tetracycline concentration in the analyte solution is obtained.
[0073] The embodiments described above are merely examples of several implementations of the present invention, and while the descriptions are specific and detailed, they should not be construed as limiting the scope of the present invention. It should be noted that those skilled in the art can make various modifications and improvements without departing from the concept of the present invention, and these modifications and improvements all fall within the scope of protection of the present invention.
Claims
1. A micro-system liquid-gate field-effect sensor chip, comprising a substrate, characterized in that, The substrate has a source, a drain, and a gate. A monolayer of graphene is disposed in the channel between the source and the drain. The substrate also has a PDMS layer for encapsulating the source, drain, and gate. The PDMS layer forms a PDMS trench after encapsulation. The PDMS trench is filled with a solid gel electrolyte. The gate is modified with self-assembled functional factors, specifically oligonucleotide sequence fragments, for the specific recognition and capture of target molecules.
2. The micro-system liquid-gate field-effect sensor chip according to claim 1, characterized in that, The oligonucleotide sequence fragment of the functional factor is: 5′-GTTTGTGTATTACAGTTATGTTACCCTCATTTTTCTGAAC-3′.
3. The micro-system liquid-gate field-effect sensor chip according to claim 1, characterized in that, The solid gel electrolyte is specifically an agarose gel with a volume of 200 μL.
4. A method for fabricating a micro-system liquid-gate field-effect sensor chip as described in any one of claims 1-3, characterized in that, Includes the following steps: (1) Patterned Cr / Au electrodes are sequentially deposited on a clean substrate by magnetron sputtering. The electrodes include a source, a drain, and a gate. (2) Graphene was transferred to the patterned channel between the source and drain electrodes in deionized water using a wet chemical method; (3) Electrochemically activate the gate after magnetron sputtering, measure the target functional factor modified with thiol, drop it onto the electrode surface and spread it evenly, incubate it overnight under room temperature and light protection, then rinse it with pure water, air dry it naturally, add 6-mercaptohexyl-1-ol MCH to seal the gate, and then rinse off the excess MCH with pure water to obtain the gate modified with self-assembled functional factor. (4) Use an organosilane layer to encapsulate the metal connection lines of the source, drain and gate on the upper part of the sensor to form a PDMS trench; (5) Prepare a 0.5% concentration agarose gel as a solid electrolyte, take 200 μL and add it to the PDMS tank. After cooling to room temperature and solidifying, the finished product is obtained.
5. The method for fabricating a micro-system liquid-gate field-effect sensor chip according to claim 4, characterized in that, The specific step (1) is as follows: using a magnetron sputtering RF magnetron, patterned Cr / Au is sequentially sputtered and deposited on the substrate with a size of 10 nm / 100 nm. The sputtering parameters are: Cr, power 50 W, pressure 0.3 Pa; Au, power 35 W, pressure 0.8 Pa.
6. The method for fabricating a micro-system liquid-gate field-effect sensor chip according to claim 4, characterized in that, The graphene preparation process in step (2) is as follows: A monolayer of graphene was synthesized on a 25μm thick copper foil at 1000℃ using a mixture of CH4 and H2 gases. Polymethyl methacrylate was uniformly spin-coated onto the graphene using a spin coater. The copper foil substrate was then etched with an etching solution and repeatedly rinsed with deionized water. Graphene was cut into appropriately sized pieces and transferred to a patterned channel between the source and drain electrodes in deionized water using a wet chemical method. It was then annealed on a hot plate at 125°C for 20 min and then soaked in acetone at 55°C for 3 h, with the acetone being replaced every hour to completely remove PMMA.
7. The micro-system liquid-gate field-effect sensor chip according to claim 4, its fabrication method, and its application, characterized in that, The preparation process of the 0.5% concentration agarose gel in step (5) is as follows: Prepare 0.1X PBS buffer, weigh 0.05 mg of agarose powder into 10 mL of 0.1X PBS, then heat it on a heating plate until the agarose powder is completely dissolved, and then cool it to about 30°C to obtain the gel.
8. The application of a microsystem liquid-gate field-effect sensor chip as described in any one of claims 1-3 in the field of tetracycline detection.
9. A method for detecting tetracycline using a microsystem liquid-gate field-effect sensor chip according to any one of claims 1-2, characterized in that, Includes the following steps: (1) Use two source meters to connect the source, drain and gate of the sensor chip. After the gate voltage and source-drain voltage parameters of the device are stable, fix the sampling interval to add 2 μL of tetracycline standard solution to the PDMS bath containing 200 μL of gel solid electrolyte every 500 s, and record the real-time current curve. (2) The first 100 seconds before each sample addition was taken as the steady signal of the tetracycline concentration standard. The relationship between time and channel current was obtained. The logarithm of the channel current change and the tetracycline concentration was extracted and linearly fitted to obtain the linear relationship between channel current and tetracycline concentration. (3) Add 2 μL of the analyte solution to the PDMS cell of the sensor chip at a fixed sampling interval of every 500 s, and record the real-time current curve to obtain the channel current. Substitute the channel current into the relevant equation in step (3) to obtain the tetracycline concentration in the analyte solution.
10. The method for detecting tetracycline according to claim 9, characterized in that, The linear relationship between the channel current and the tetracycline concentration is as follows: , The logarithm represents the concentration of tetracycline. Channel current, in units of goodness of fit .