CN115061523A - Portable high-resolution constant potential system - Google Patents
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Abstract
The invention relates to the technical field of electrochemical detection, and discloses a portable high-resolution constant potential system, which comprises a power supply management unit, a control unit, a constant potential unit, a TIA unit, a filtering unit and an ADC unit; the constant potential unit comprises an operational amplifier U1, an operational amplifier U2, an operational amplifier U3 and a screen printing three-electrode, wherein the step voltage output by the control unit is connected with the non-inverting input end of the operational amplifier U1, and the output end of the operational amplifier U1 is connected with the inverting input end of the operational amplifier U2; the output end of the operational amplifier U1 and the output end of the operational amplifier U2 are connected with the counter electrode CE; the non-inverting input end of the operational amplifier U3 is connected with the reference electrode RE, the inverting input end is connected with the output end, and the output end is connected with the inverting input end of the operational amplifier U2; the invention improves the sensitivity, does not need a complex excitation signal generating unit, greatly reduces the complexity of the circuit and improves the maintainability of the circuit.
Description
Technical Field
The invention relates to the technical field of electrochemical detection, in particular to a portable high-resolution potentiostat, which integrates a three-electrode test system and an embedded technology, and has the advantages of low power consumption and portability while quickly and accurately detecting the concentration of a sample to be detected.
Background
With rapid progress in economy and science and technology, the characterization requirements of scientific research and industrial production on the aspects of the property composition and the like of substances are also rapidly increased. The electrochemical analysis method is a method for analyzing the electrochemical properties of substances, can obtain parameters such as the conductance, the potential, the electric quantity and the like of a measured object, and further can deduce important data such as the concentration, the chemical properties and the like of the measured substance, thereby being widely applied. The three-electrode detection system is one of important means of an electrochemical analysis method, has good universality and stability, and can obtain accurate and stable test results. The potentiostat is an important interface of a three-electrode detection system and a rear-end analysis instrument, and the response speed and resolution directly determine the overall performance of the system. However, the traditional potentiostat has large volume, high manufacturing cost, complex function and inconvenient operation. In order to meet the requirement of modern rapid detection, the portable potentiostat is produced, the system volume is greatly reduced, and the test is easier and faster. At present, a portable potentiostat mostly adopts a cyclic voltammetry scanning method, and triangular waves are required to be applied between a working electrode and a reference electrode for scanning, so that the complexity of circuit design and data analysis is increased, and the later maintenance is not facilitated; in addition, in the I/V conversion unit of the key part of the system, a multistage amplification mode is adopted to convert and amplify the tiny signals, so that larger system noise is brought, and meanwhile, the design difficulty of a post-stage filtering unit and an ADC (analog-to-digital converter) acquisition unit is brought; because these portable potentiostats are mostly mounted on a bare computer Microcontroller (MCU), the defects of untimely signal acquisition, untimely user interaction task and the like are caused.
Therefore, it is necessary to invent a portable high-resolution constant potential system with simple operation and low cost, which can satisfy the requirements of portability and low power consumption and simultaneously ensure the detection accuracy and sensitivity.
Disclosure of Invention
The invention overcomes the defects of the prior art, and solves the technical problems that: the portable high-resolution potentiostat system has the advantages of low consumption, high resolution, high sensitivity, good repeatability of detection results and low cost.
In order to solve the technical problems, the invention adopts the technical scheme that: in order to solve the technical problems, the invention adopts the technical scheme that: a portable high-resolution constant potential system comprises a power supply management unit, a control unit, a constant potential unit, a TIA unit, a filtering unit and an ADC unit;
the constant potential unit comprises an operational amplifier U1, an operational amplifier U2, an operational amplifier U3 and a screen printing three-electrode, wherein the screen printing three-electrode comprises a counter electrode CE, a reference electrode RE and a working electrode WE; the step voltage output by the control unit is connected with the non-inverting input end of an operational amplifier U1, and the output end of an operational amplifier U1 is connected with the inverting input end of an operational amplifier U2; the output end of the operational amplifier U1 and the output end of the operational amplifier U2 are connected with the counter electrode CE; the non-inverting input end of the operational amplifier U3 is connected with the reference electrode RE, the inverting input end is connected with the output end, and the output end is connected with the inverting input end of the operational amplifier U2;
the TIA unit comprises an operational amplifier U4, a feedback resistor Rf and a capacitor Cf, wherein the reverse input end of the operational amplifier U4 is connected with the working electrode WE, the output end of the operational amplifier U4 is connected with the reverse input end through the feedback resistor Rf and the capacitor Cf which are connected in parallel, and the output end of the operational amplifier U4 is also connected with the input end of the filtering unit;
the filtering unit comprises an operational amplifier U5, a resistor R3, a resistor R4 and a capacitor C2, wherein the non-inverting input end of the operational amplifier U5 is connected with the output end of the operational amplifier U5 sequentially through a resistor R4 and a resistor R3, one end of the capacitor C2 is connected with the output end of the operational amplifier U5, the other end of the capacitor C2 is connected with the non-inverting input end of the operational amplifier U5 through the resistor R4, the output end of the operational amplifier U5 is connected with the ADC unit, and the output end of the ADC unit is connected with the input end of the control unit.
The operational amplifier U1, the operational amplifier U2 and the operational amplifier U3 are three of four-channel operational amplifiers OPA 4330.
The model of the operational amplifier U4 is LMC6001, the capacitance Cf is 10pF, and the Rf is 10M omega.
In the portable high-resolution constant potential system, a circuit is arranged on a PCB, and the PCB is provided with an annular island at the input end of an operational amplifier U4 through a lead, and the annular island covers the inverted input terminal of the operational amplifier U4 and is connected with the ground;
the working electrode WE is connected with the inverting input terminal of the operational amplifier U4 by three coaxial wires.
In the filtering unit, R3 = R4 = 510 Ω, C1 = 10uF, and C2= 22 uF.
In the TIA unit, the output end of the operational amplifier U4 is lapped with the feedback resistor Rf in a floating manner.
The portable high-resolution constant potential system adopts a chronoamperometry as a three-electrode detection method.
The portable high-resolution constant potential system further comprises a user interaction unit, wherein the user interaction unit is connected with the control unit and comprises a key module, an OLED display module and a USART serial port module.
The power supply management unit comprises a first power supply module, a second power supply module and a third power supply module;
the first power supply module is used for converting the battery voltage into DC5V and then supplying power to the operational amplifier;
the second power supply module is used for converting the battery voltage into DC3.3V and supplying power to the control unit;
the third power supply module is used for converting DC5V direct-current voltage into DC +2.5V and DC-2.5V and then supplying power to the operational amplifier U1, the operational amplifier U2, the operational amplifier U3, the operational amplifier U4 and the operational amplifier U5; the power supply comprises an operational amplifier U6, a resistor R5 and a resistor R6, wherein the resistors R5 and R6 are the same in resistance value, one end of a resistor R5 is connected with the positive electrode of a power supply, the other end of the resistor R6 is connected with the negative electrode of the power supply, the non-inverting input end of the operational amplifier U6 is connected with the other end of the resistor R5, the inverting input end of the operational amplifier U6 is connected with the output end of the operational amplifier, the output end of the operational amplifier U6 is grounded, and the positive power supply end and the negative power supply end of the operational amplifier U6 are respectively connected with the positive electrode of the power supply and the negative electrode of the power supply.
The resistor R5 and the resistor R6 are 0.1% precision resistors of 100K.
Compared with the prior art, the invention has the following beneficial effects:
the invention provides a portable high-resolution potentiostat system, which realizes constant potential control of a circuit through a constant potential unit, can adopt a selected time-counting current method as a three-electrode system detection scheme, improves the sensitivity, does not need a complex excitation signal generation unit, greatly reduces the complexity of the circuit and improves the maintainability of the circuit; in the conventional micro current conversion scheme, in order to avoid Johnson noise caused by a high-resistance resistor, a scheme of proportional multi-stage amplification of an operational amplifier is adopted, a single-pole TIA circuit is selected as an I/V conversion unit, and a high-voltage insulation resistor with a high resistance value is selected to bring high single-pole gain to a system, so that system noise caused by multi-stage amplification is avoided; the selected IC device and the power supply scheme well meet the requirements of portability and low power consumption; the invention selects the mu C/OS-II operating system, improves the real-time property and the response speed of the system, and simultaneously, the development idea of the mu C/OS-II modularization is beneficial to the maintenance and the transplantation of software.
Drawings
Fig. 1 is a schematic block circuit diagram of a portable high-resolution potentiostat system according to an embodiment of the present invention;
fig. 2 is a schematic circuit diagram of a potentiostat unit, a TIA unit, and a filter unit in a portable high-resolution potentiostat system according to an embodiment of the present invention;
fig. 3 is a schematic diagram of a circular input island disposed on a TIA cell in an embodiment of the invention;
FIG. 4 is a schematic circuit diagram of a third power module in an embodiment of the invention;
FIG. 5 is a V-T plot of the converted current flowing into the working electrode versus time according to the present invention;
FIG. 6 is a V-C linear relationship diagram of the relationship between the voltage and the concentration of the working electrode after the current conversion at the 20 th time is selected.
Detailed Description
In order to make the objects, technical solutions and advantages of the embodiments of the present invention clearer, the technical solutions in the embodiments of the present invention will be clearly and completely described below, and it is obvious that the described embodiments are some embodiments of the present invention, but not all embodiments; all other embodiments, which can be derived by a person skilled in the art from the embodiments given herein without making any creative effort, shall fall within the protection scope of the present invention.
As shown in fig. 1, an embodiment of the present invention provides a portable high-resolution constant potential system, which includes a power management unit, a control unit, a constant potential unit, a TIA unit, a filtering unit, and an ADC unit. TIA refers to transimpedance amplifiers, all known as trans-impedance amplifiers.
As shown in fig. 2, in the present embodiment, the potentiostatic unit includes an operational amplifier U1, an operational amplifier U2, an operational amplifier U3, and a screen-printed three-electrode including a counter electrode CE, a reference electrode RE, and a working electrode WE; the step voltage output by the control unit is connected with the non-inverting input end of an operational amplifier U1, and the output end of the operational amplifier U1 is connected with the inverting input end of an operational amplifier U2 through a resistor R1; the output end of the operational amplifier U2 is connected with the counter electrode CE; the non-inverting input end of the operational amplifier U3 is connected with the reference electrode RE, the inverting input end is connected with the output end, and the output end is connected with the inverting input end of the operational amplifier U2 through a resistor R2.
In this embodiment, a screen printing flexible three-electrode is selected as a test system, and the three-electrode is a counter electrode, a reference electrode and a working electrode. Compared with a double-electrode system, the three-electrode system is additionally provided with a reference electrode, and the potential deviation condition of a working electrode and a counter electrode in the double-electrode system is improved. The requirements of the three-electrode system on the electrode material are as follows: the reaction of the electrode does not influence the electrochemical reaction; the electrodes must remain stable in an electrochemical environment; the electrode surface must be homogeneous and the electrode area should not be too large.
Specifically, in the present embodiment, the control unit includes a Microcontroller (MCU). The Microcontroller (MCU) with built-in DAC function can greatly simplify the design of an external circuit and reduce the cost. Specifically, the Microcontroller (MCU) selects STM8L151, and only 0.35 muA is needed in the lowest power consumption mode. The 2KRAM meets the requirement of the operating system mu C/OS-II selected by the invention, and the external 16M crystal oscillator provides a system clock for the operating system mu C/OS-II. STM8L151 embeds 12bit DAC, has simplified external circuit design and reduced cost greatly. In order to save the PCB space and ensure the system performance, the operational amplifier U1, the operational amplifier U2, and the operational amplifier U3 are selected from three of the four-channel operational amplifiers OPA 4330. The offset voltage of the four-channel precision operational amplifier OPA4330 is only 8 muV, the bias current is only +/-70 pA, and the static working current is only 35 muA while the performance is excellent.
Specifically, in this embodiment, a built-in DAC of a software-controlled Microcontroller (MCU) outputs a step voltage of 0.31V, and a channel of an OPA4330 is selected as an operational amplifier U1 to construct a voltage follower as a buffer of the step voltage, as shown in fig. 2. The magnitude of the step voltage depends on the position of an oxidation peak or a reduction peak in the cyclic voltammetry scan curve of the sample to be tested. In this embodiment, the constant potential unit includes an operational amplifier, a resistance-capacitance element and a screen printing three-electrode, and under the depth negative feedback regulation of the operational amplifier U2 and the operational amplifier U3, the potential difference of the working electrode WE with respect to the reference electrode RE is always equal to the control voltage. Depending on the requirements of the electrochemical test for three electrodes, it may be desirable to select electrodes composed of "inert" solid electrode materials. In the flexible screen printing three-electrode system selected by the invention, the counter electrode and the working electrode are made of gold, and the reference electrode is made of silver chloride.
As shown in fig. 2, in this embodiment, the TIA unit includes an operational amplifier U4, a feedback resistor Rf, and a capacitor Cf, an inverting input terminal of the operational amplifier U4 is connected to the working electrode WE, an output terminal of the operational amplifier U4 is connected to the inverting input terminal through the feedback resistor Rf and the capacitor Cf connected in parallel, and the output terminal is further connected to the input terminal of the filtering unit.
In the TIA cell of this embodiment, the "virtual off" and "virtual ground" characteristics of the operational amplifier are obtained: the output voltage of the operational amplifier U4 is:
U4out = -I*Rf；（1）
in the test method, the glutamic acid solution of the sample to be tested is subjected to oxidation reaction on the working electrode, so that the current flows into the working electrode, the output voltage of the operational amplifier U4 is a positive value, and in order to meet the requirements of different samples to be tested, the condition that the output voltage of the operational amplifier U4 is a negative value must be considered, so that the power supply voltage of the operational amplifier U4 is +/-2.5V. In the present invention, the resistor Rf is a 10M Ω high voltage insulation resistor to obtain a unipolar high gain. The voltage density of resistive thermal noise is a frequency-independent quantity:
t = 300K (at room temperature of 27 ℃), is substituted with formula (2) to obtain:
it can be seen that the noise is proportional to the half power of the internal resistance, and the range and the gain are proportional to the first power of the internal resistance, so that the high gain caused by the high resistance is a scheme with high cost performance. The conventional tiny current conversion scheme mostly adopts a scheme of proportional multistage amplification of an operational amplifier to avoid Johnson noise caused by high-resistance, the scheme has extremely high requirements on output noise of a first-stage operational amplifier, the first-stage voltage gain is assumed to be A1, the second-stage voltage gain is assumed to be A2, the total gain is A1A 2, in the amplification of tiny signals, in order to ensure the quality of the signals, the maximum amplification factor of the first stage does not exceed 1000, higher gain is required to be A2, if the output noise of the first stage is u, the noise after the second-stage amplification is A2 u, higher performance requirements are provided for a post-stage filtering unit, and higher development cost is brought at the same timeResistance-induced unipolar high-gain schemes. Because the response current of the sample to be tested is as low as pA level, the bias current of the operational amplifier is large to cause obvious shunt, so that the test result deviates from normal data, and the operational amplifier with low bias current is selected as the core device of the TIA unit.
In the TIA unit, since the current to be measured is as low as pA level, if the bias current of the operational amplifier is close to pA, the result is obviously influenced, so the U4 selects the low bias current operational amplifier LMC6001, and the bias current of the operational amplifier LMC6001 is not more than 2 fA. The input capacitance of the operational amplifier affects the stability of the circuit, and forms a hysteresis network with the feedback resistance, when the frequency of the input signal is very high, the frequency response of the operational amplifier is deteriorated due to the existence of the input capacitance, but in the invention, the voltage signal output by the operational amplifier is similar to a direct current signal, and in order to improve the response speed, a CBB capacitance of 10pF is selected.
Specifically, in the TIA unit of this embodiment, since the current to be measured is as low as pA, if the bias current of the operational amplifier is close to pA, the result will be obviously affected, and therefore the operational amplifier U4 in the TIA unit selects the low bias current operational amplifier LMC6001, and the bias current thereof does not exceed 2 fA. The input capacitance of the operational amplifier affects the stability of the circuit, and it forms a hysteresis network with the feedback resistance Rf, when the frequency of the input signal is very high, the frequency response of the operational amplifier will be deteriorated due to the existence of the input capacitance Cf, but in the present invention, the voltage signal output by the operational amplifier U4 is similar to a dc signal, and in order to increase the response speed, a CBB capacitance of 10pF is selected. The feedback resistor Rf is a high voltage insulation resistor with a high resistance (greater than 1M Ω), and in this embodiment, the resistance of the feedback resistor Rf is 10M Ω.
Because the detected current signal is as low as pA level, the PCB wiring of TIA part is very critical, and good wiring can restore the signal to the maximum extent and eliminate interference. As shown in fig. 3, in the embodiment of the present invention, the circuit is disposed on a PCB board, and the PCB board is disposed with a ring island at the input terminal of the operational amplifier U4 through a conducting wire, specifically, as shown in fig. 2, since the non-inverting input terminal of the operational amplifier U4 is grounded, a ring island is surrounded by a conducting wire near the inverting input terminal and is connected to the ground terminal of the non-inverting input terminal, so that an equipotential surface is formed near the input terminal of the signal, and the current leakage current is reduced. The working electrode WE is connected to the inverting input terminal of the operational amplifier U4 by three-coaxial wiring. Compared with the common BNC wiring, the three-coaxial wiring mode greatly reduces the loss in the current transmission process because the equipotential surface is wrapped on the current transmission path. At the output end of the operational amplifier U4, the pin OUT of the operational amplifier U4 is directly overlapped with the feedback resistor Rf in a hanging manner, thereby reducing unnecessary leakage current paths.
As shown in fig. 2, in this embodiment, the filtering unit includes an operational amplifier U5, a resistor R3, a resistor R4, and a capacitor C2, a non-inverting input terminal of the operational amplifier U5 is connected to an output terminal of the operational amplifier U5 sequentially through a resistor R4 and a resistor R3, one end of the capacitor C2 is connected to an output terminal of the operational amplifier U5, the other end of the capacitor C2 is connected to a non-inverting input terminal of the operational amplifier U5 through the resistor R4, an output terminal of the operational amplifier U5 is connected to the ADC unit, and an output terminal of the ADC unit is connected to an input terminal of the control unit.
Specifically, in the filtering unit of the present embodiment, an active filter is selected as a filtering scheme, and as shown in fig. 2, the capacitors C1 and C2 and the resistors R3 and R4 determine the pole frequency fp of the filtering circuit:
the Q value represents the quality factor, and when the Q value is equal to 0.707, the filter is called a butterworth filter, the attenuation of which is zero in most of the area within the pass band. When the Q value is greater than 0.707,
where f0 is the resonance frequency at which the peak appears, fc is the cut-off frequency, f3db is the 3db frequency, and K0, Kc, and K3 respectively represent coefficient values corresponding to the resonance frequency and the cut-off frequency and the 3db frequency.
In this embodiment, the low voltage rail-to-rail operational amplifier OPA333 is selected as the core element of the active filter. When R3 = R4 = 510 Ω, C1 = 10uF, C2= 22uF is selected, Q = 0.74 can be obtained by substituting formula (4), and the pole frequency fp = 21Hz is calculated, and the resonance frequency f at which the peak occurs 0 = 6.9Hz, cut-off frequency fc = 9.9Hz, 3db frequency f 3db = 22.2Hz, and the maximum attenuation Ap of the pass-band = 0.05 dB. The filter is close to the Butterworth filtering characteristic, passband attenuation cannot be generated on low-frequency signals lower than 10Hz, background noise of the system is effectively inhibited, and the anti-interference performance of the system is improved.
Specifically, in this embodiment, a chronoamperometry is employed as the three-electrode detection method. A first-step signal is applied to a loop formed by the working electrode WE and the reference electrode RE to serve as an excitation signal, current which is in direct proportion to the concentration of a sample to be detected flows into or out of the working electrode WE, and the potential of the working electrode relative to the reference electrode is guaranteed not to deviate along with the polarization of the electrode through the deep negative feedback of the operational amplifier U3. The function of the reference electrode RE is to provide and maintain a fixed reference potential, and if current flows through the reference electrode RE, the potential will change, so the reference electrode RE should be connected to the input end of the voltage follower, and the current flowing through the reference electrode is very weak due to the high input impedance of the voltage follower. The working electrode WE is connected to the inverting input end of the TIA unit operational amplifier U4, the non-inverting input end is grounded, and the potential of the working electrode WE is almost equal to 0 potential according to the virtual ground characteristic of the operational amplifier, so that the anti-interference performance of the system can be greatly enhanced. Implementation of 'constant potential' and operational amplifier U 1 、U 2 、U 3 And a resistance R 1 、R 2 In this regard, the derivation process is as follows:
according to the 'virtual short' characteristic of the operational amplifier, the following characteristics are provided:
(U 1out - U 2IN+ )/R 1 = (U 2IN+ -U 3out )/R 2 ；（6）
to obtain:
U 1out = -U 3out = -U RE = 0 - φ RE ；（7）
0-phi since the potential of the working electrode WE is almost equal to 0 potential RE = φ WE - φ RE (ii) a Wherein phi RE Is the potential of a reference electrode, [ phi ] WE Is the potential of the working electrode.
Thus, the operational amplifier U 1 Output voltage U of 1out = φ WE - φ RE And an operational amplifier U 1 The output voltage of (2) is the step voltage output by the Microcontroller (MCU), i.e. the Microcontroller (MCU) controls the potential of the working electrode relative to the reference electrode to be equal to the control voltage. In the chemical reaction process, if the potential of the working electrode is reduced due to polarization, the potential of the working electrode can be equivalently regarded as the potential increase of the reference electrode, and since the control voltage is not changed, the cathode polarization current flowing through the working electrode is reduced, so that the potential of the working electrode is further increased, the negative feedback process is completed, and the potential of the working electrode is kept stable.
Further, as shown in fig. 1, in this embodiment, the power management unit includes a battery, a first power module, a second power module, and a third power module; the first power supply module is used for converting the battery voltage into DC5V and then supplying power to the operational amplifier; the second power supply module is used for converting the battery voltage into DC3.3V and supplying power to the control unit; as shown in fig. 4, the third power module is configured to convert DC5V DC voltage into DC +2.5V and DC-2.5V, and then supply power to the operational amplifier U1, the operational amplifier U2, the operational amplifier U3, the operational amplifier U4, and the operational amplifier U5; the power supply comprises an operational amplifier U6, a resistor R5 and a resistor R6, wherein the resistors R5 and R6 are the same in resistance value, one end of a resistor R5 is connected with the positive electrode of a power supply, the other end of the resistor R6 is connected with the negative electrode of the power supply, the non-inverting input end of the operational amplifier U6 is connected with the other end of the resistor R5, the inverting input end of the operational amplifier U6 is connected with the output end of the operational amplifier, the output end of the operational amplifier U6 is grounded, and the positive power supply end and the negative power supply end of the operational amplifier U6 are respectively connected with the positive electrode of the power supply and the negative electrode of the power supply. The resistor R5 and the resistor R6 are 0.1% precision resistors of 100K.
In this embodiment, in order to provide sufficient margin for different types of sensors, the output amplitude of the operational amplifier U4 in the TIA unit should meet the negative voltage requirement, and therefore, the voltage of 5V is divided into ± 2.5V by the third power supply module through one channel of the four-channel operational amplifier, i.e., the operational amplifier U6 and two 0.1% precision resistors R5 and R6 of 100K, as the power supply voltage of all the operational amplifiers.
Specifically, one of the four-channel precise operational amplifiers is selected as the operational amplifier U6, the operational amplifier U1, the operational amplifier U2 and the operational amplifier U3, so that the system performance is guaranteed while the PCB space is saved, and the four channels are used as follows: the voltage-controlled type sensor comprises a partial voltage follower (U6), a control voltage follower (U1), a reference electrode voltage follower (U3) and a voltage comparator (U2), wherein the partial voltage follower converts 5V voltage into +/-2.5V voltage to supply power for a subsequent negative feedback unit, so that sufficient allowance is provided for sensors of different types, the universality and expandability of the system are improved, the control voltage follower can enable an excitation signal output by a Microcontroller (MCU) to be reduced in fluctuation, and the quick response of the system is guaranteed.
Specifically, in this embodiment, the power management unit selects the rechargeable lithium-ion button battery to supply power to the system. The full-power voltage of the rechargeable lithium ion button battery is 4.2V and slowly drops along with the power consumption of the system, in order to guarantee stable power supply of the system, the DC-DC boost IC is selected to meet the power consumption requirement of 5V of an operational amplifier in the system, and the LDO is selected to meet the power consumption requirement of 3.3V of a Microcontroller (MCU).
Further, in order to stabilize the power supply of all ICs in the system, a 0.1uF capacitor is connected across each IC near the power supply pin to reduce the power supply ripple.
Specifically, in this embodiment, the ADC unit is composed of an analog-to-digital conversion chip AD7787 and a reference source ADR 431. The AD7787 has a static current of only 75uA, fully meets the requirements of low power consumption design, has a maximum resolution of 24bit and an effective resolution of 22bit, and has a 1LSB of 2.5V/2^24 = 0.15uV when a power supply voltage is 5V and a reference voltage is 2.5V, and a corresponding current resolution of 0.15uV/10M Ω = 0.015pA due to the fact that a feedback resistor is selected to be 10M Ω. The performance of the reference source directly determines whether the performance of the analog-digital conversion chip can be fully exerted, the ADR431 is an ultra-low noise XFET reference voltage source, the noise peak value is only 3.5uV, and high-resolution acquisition of system data can be ensured by matching with good PCB layout and decoupling.
Further, as shown in fig. 1, the portable high-resolution constant potential system according to this embodiment further includes a user interaction unit, where the user interaction unit is connected to the control unit and includes a key module, an OLED display module, and a USART serial port module. In addition, in order to timely and accurately reflect the change of the collected voltage data and timely process the interaction request of the user, in this embodiment, the control module adopts a μ C/OS-II operating system to schedule tasks.
Experimental example 1:
in the experimental example, seven kinds of glutamic acid solutions with different concentrations are prepared, a standard concentration curve is drawn, a sample to be tested with different concentrations is prepared, and whether the relation described by the standard concentration curve is met or not is verified. The method comprises the following specific steps:
1. a1 mmol/L glutamic acid solution was prepared, and the resulting solution was diluted with PBS solution to give buffers having concentrations of 750. mu. mol/L, 500. mu. mol/L, 250. mu. mol/L, 100. mu. mol/L, 50. mu. mol/L and 10. mu. mol/L, respectively. And dropwise adding the prepared buffer solution on a screen printing three-electrode to perform a timing current test, wherein the step control voltage is 0.31V, the response current of the working electrode is the largest under the control voltage, and for the expandability of the system, a user can interact with a Microcontroller (MCU) through keys to change the magnitude of the control voltage.
2. When a solid electrode is used, the necessary electrode pretreatment step can ensure the reproducibility of the chemical reaction process. In order to enable the buffer solution to be better soaked into the electrode, firstly, electrode pretreatment is carried out, and the specific process comprises the following steps: applying control voltage to make buffer solution produce one-minute oxidation reaction on working electrode, controlling Microprocessor (MCU) to stop outputting control voltage, standing for one minute and making formal test, and making time control of whole test process pass through microprocessorThe timer in the (MCU) is used for timing, and the test result is displayed on the PC end through a serial port (USART) or on the portable constant potential system through an OLED. The ADC acquisition results recorded at the PC end are analyzed and processed, and as shown in FIG. 5, the C1-C7 curves correspond to concentrations of 10umol/L, 50umol/L, 100umol/L, 250umol/L, 500umol/L and 750umol/L respectively. The different concentrations of the prepared glutamic acid solution can be distinguished in the 20 th s of the formal test, the voltage value collected by the system in the 20 th s is taken to obtain the relation between the voltage and the concentration as shown in fig. 6, and repeated experiments show that the linear relation between the concentration (C) and the voltage (V) is V = 1.00565C + 112.42613, the linear relation is 0.99688, wherein the unit of the voltage is mV, and the unit of the concentration is umol/L. The relationship between the voltage value V collected in the invention and the current I flowing into the working electrode is as follows: v = I R f The further conversion yields a concentration (C) versus current (I) in the range of I = 1.00565E-4C + 112.42613E-4, where current is in nA and concentration is in umol/L, which is a linear range from 10 to 1000 umol/L.
3. Diluting 1mmol/L glutamic acid solution to 400umol/L by PBS buffer solution, dripping the solution on a screen printing three-electrode for testing, wherein the AD acquisition result is 516.36mv, and the relation error of the reaction with a standard curve is 3.2 per mill.
Finally, it should be noted that: the above embodiments are only used to illustrate the technical solution of the present invention, and not to limit the same; while the invention has been described in detail and with reference to the foregoing embodiments, it will be understood by those skilled in the art that: the technical solutions described in the foregoing embodiments may still be modified, or some or all of the technical features may be equivalently replaced; and the modifications or the substitutions do not make the essence of the corresponding technical solutions depart from the scope of the technical solutions of the embodiments of the present invention.
Claims (10)
1. A portable high-resolution constant potential system is characterized by comprising a power supply management unit, a control unit, a constant potential unit, a TIA unit, a filtering unit and an ADC unit;
the constant potential unit comprises an operational amplifier U1, an operational amplifier U2, an operational amplifier U3 and a screen printing three-electrode, wherein the screen printing three-electrode comprises a counter electrode CE, a reference electrode RE and a working electrode WE; the step voltage output by the control unit is connected with the non-inverting input end of an operational amplifier U1, and the output end of an operational amplifier U1 is connected with the inverting input end of an operational amplifier U2; the output end of the operational amplifier U1 and the output end of the operational amplifier U2 are connected with the counter electrode CE; the non-inverting input end of the operational amplifier U3 is connected with the reference electrode RE, the inverting input end is connected with the output end, and the output end is connected with the inverting input end of the operational amplifier U2;
the TIA unit comprises an operational amplifier U4, a feedback resistor Rf and a capacitor Cf, wherein the reverse input end of the operational amplifier U4 is connected with the working electrode WE, the output end of the operational amplifier U4 is connected with the reverse input end through the feedback resistor Rf and the capacitor Cf which are connected in parallel, and the output end of the operational amplifier U4 is also connected with the input end of the filtering unit;
the filtering unit comprises an operational amplifier U5, a resistor R3, a resistor R4 and a capacitor C2, wherein the non-inverting input end of the operational amplifier U5 is connected with the output end of the operational amplifier U5 sequentially through a resistor R4 and a resistor R3, one end of the capacitor C2 is connected with the output end of the operational amplifier U5, the other end of the capacitor C2 is connected with the non-inverting input end of the operational amplifier U5 through the resistor R4, the output end of the operational amplifier U5 is connected with the ADC unit, and the output end of the ADC unit is connected with the input end of the control unit.
2. The portable high-resolution potentiostat system of claim 1, wherein the operational amplifiers U1, U2, U3 are three of four-channel operational amplifiers OPA 4330.
3. The portable high-resolution potentiostat system of claim 1, wherein the operational amplifier U4 is of the type LMC6001, the capacitor Cf is of the capacitance value 10pF and the resistance Rf is 10M Ω.
4. The portable high-resolution constant potential system according to claim 1, wherein the circuit is arranged on a PCB board, the PCB board is provided with a ring-shaped island at the input end of the operational amplifier U4 through a conducting wire, the ring-shaped island covers the inverted input terminal of the operational amplifier U4 and is connected with the ground;
the working electrode WE is connected with the inverting input terminal of the operational amplifier U4 by three coaxial wires.
5. The portable high-resolution potentiostat system of claim 1, wherein in the filter unit, R is 3 ＝R 4 ＝510Ω，C 1 ＝10uF，C 2 ＝22uF。
6. The portable high-resolution constant potential system according to claim 1, wherein in the TIA unit, the output end of an operational amplifier U4 is overlapped with the feedback resistor Rf by hanging.
7. A portable high resolution potentiostat system according to claim 1, characterized in that chronoamperometry is used as the three-electrode detection method.
8. The portable high-resolution constant potential system according to claim 1, further comprising a user interaction unit, wherein the user interaction unit is connected with the control unit and comprises a key module, an OLED display module and a USART serial port module.
9. The portable high-resolution potentiostat system of claim 1, wherein the power management unit comprises a first power module, a second power module, and a third power module;
the first power supply module is used for converting the battery voltage into DC5V and then supplying power to the operational amplifier;
the second power supply module is used for converting the battery voltage into DC3.3V and then supplying power to the control unit;
the third power supply module is used for converting DC5V direct-current voltage into DC +2.5V and DC-2.5V and then supplying power to the operational amplifier U1, the operational amplifier U2, the operational amplifier U3, the operational amplifier U4 and the operational amplifier U5; the power supply comprises an operational amplifier U6, a resistor R5 and a resistor R6, wherein the resistors R5 and R6 are the same in resistance value, one end of a resistor R5 is connected with the positive electrode of a power supply, the other end of the resistor R6 is connected with the negative electrode of the power supply, the non-inverting input end of the operational amplifier U6 is connected with the other end of the resistor R5, the inverting input end of the operational amplifier U6 is connected with the output end of the operational amplifier, the output end of the operational amplifier U6 is grounded, and the positive power supply end and the negative power supply end of the operational amplifier U6 are respectively connected with the positive electrode of the power supply and the negative electrode of the power supply.
10. The portable high resolution potentiostat system of claim 9, wherein the resistors R5 and R6 are 0.1% precision resistors of 100K.
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