CN111781267A - Arduino-based excitation signal generation circuit for realizing time current method and cyclic voltammetry and electrochemical detection device - Google Patents
Arduino-based excitation signal generation circuit for realizing time current method and cyclic voltammetry and electrochemical detection device Download PDFInfo
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Abstract
The invention provides an excitation signal generating circuit and an electrochemical detection device based on an Arduino implementation time current method and a cyclic voltammetry method, wherein the electrochemical detection device comprises an excitation signal generating circuit, a three-electrode system circuit, a test electrode, a signal measuring circuit and a power supply signal circuit; the excitation signal generating circuit generates an excitation signal through the received Arduino digital signal; the three-electrode system circuit generates a constant voltage signal or a triangular wave voltage signal between a working electrode and a reference electrode of a test electrode through an excitation signal; the signal measurement circuit converts the collected polarization current in the working electrode and counter electrode loops of the test electrode into digital signals, and the digital signals are fed back to Arduino. The invention has excellent detection performance, one percent of cost of commercial electrochemical equipment and one sixth of volume and mass, and provides a new device for low cost, portability and wearability of the electrochemical monitoring equipment.
Description
Technical Field
The invention belongs to the field of electrochemical detection, and particularly relates to an excitation signal generation circuit for realizing a time current method and a cyclic voltammetry method based on Arduino and an electrochemical detection device.
Background
In recent years, the development of new electrochemical and biological sensing systems has attracted great attention because of their wide application in the fields of healthcare, environmental monitoring, national defense, and the like. The low-cost portable electrochemical detector can provide another way for the fields of disease diagnosis, food industry, agriculture, environment monitoring and the like, and provides a new way for the fields of advanced medical treatment, wearable medical detection equipment, advanced agricultural environment detection, quality control and the like.
At present, special Instruments and equipment are needed for the development of the detection in the fields of production, experiments, scientific research, use, maintenance and the like, commercial companies for producing the test equipment include CH Instruments, Ivium, PalmSens, Metrohm, Gamry and the like, and the equipment produced by the Instruments, the Ivium, the PalmSens, the Metrohm, the Gamry and the like has complete functions, but is huge in volume and expensive in cost. Two methods which are the most basic and the most common in teaching, scientific research and production are a time current method (CA) and a Cyclic Voltammetry (CV), and a commercial potentiostat or an electrochemical workstation is a universal detection device which can realize a plurality of detection technologies, so that detection resources are wasted in many cases. There are currently a number of documents and designs for miniaturizing, reducing, and tailoring Electrochemical detection devices, among which (p.m.levine, p.gong, r.levick, and k.l.shepard, "Active CMOS Sensor Array for Electrochemical biomolecular emission," IEEE j.solid-st.circuit, vol.43, No.8, pp.0-1871,2008.), (a.a.rowet., "Open-Source," Do-It-your electric, "conductive for organic electronic Applications," os One, vol.6, No.9, p.23783, 2011., (j.r.black, "Design" for Electrochemical emission, "plasma, ph.3, plasma, ph.r.p.n.p.n.r.p.n.p.n.r.p.n.p.n.p.n.p.n.p.n.p.n.p.n.r.p.n.p.n.r.p.n.p.n.p.n.p.12, phosphor, n.p.p.n.n.p.n.p.n.n.p.n.n.p.n.p.n.p.n.n.p.p.n.12, phosphor, emission, phosphor, p.p.p.p.p.p.p.p.p.p.p.p.p.p.n.p.p.p.p.p.n.n.p.12, 12, phosphor, no.7, pp.1028-1036,2014), "(A.Muid, M.Djamal, and R.Wirawan," Development of a low cost market using ATXMEGA32, "AIPConf.Proc., vol.1589, p.124,2014.), (M.D.M. Dryden and A.R.Wheeler," DStat: AVerstate, Open-Source market for electric Analysis and Integration, "ploson, vol.10, No.10, p.E. 0140349,2015," (A.V.Gossaging and D.Russell, "insulation-capable environment and calibration for simulation environment and Integration," feedback detector ", S.A.V.Gossaging and D.S.P.J., and" detection instrument ", S.83, S.B.A.A.B.A.A.S. for detection and B.A.A.A.A.A.A.M.D.A.A.A.D.M.M. Dryden, and A.D.R.W.Wheastern. noise detection instrument for detection and B.A.A.A.A.A.A. detection instrument, B.A.A.A.A.A. is a" detection instrument for detection instrument and B.A.A.A.A.A.A.A.A. detection instrument for detection of environment, B.S. A.S. A.A.A. A. 1, B.S. A. is a high efficiency, and detection of detection. Some electrochemical detectors are based on either the evolutionary version Of Arduino or the evolutionary version Of Arduino, wherein (P.Joshi, S.D.S., and R.A.S., "Development Of A Cyclic Voltage measurement System By Designing a Low Cost," int.J.Curr.Res, vol.9, pp.51072-51075, 05/312017.), (H.Shamkhalic nano-red J. -W.Choi., "An input-printed non-enzymatic hydrogen sensor on paper," J electric chemistry, Soc., 164, No.5, pp.B3101-B3106,2017., (A.New electronic, "Universal electronic detector", P.S.A.J.12. electronic detector, U.S., "Development Of No.5, P.B.3101-B3106,2017.," A.S. 12. electronic detector, U.S. Pat. No.5, U.S. Pat. No.5, U.S. Pat. 4. No. 11, U.S. 4. U.A.S. 1-No. 9, U.S. 11, U.S. Pat. 1-No. 11, U.A.S. 4, U.S. 4. A.A.S. 4. environmental detector, U.S. 4. A.S. 4, No. 4. A.S. 4. A.A.A.S. 4, U.S. A.S. No.4, No. 4. A.A.A.S. 4, No.4, U.S. A.A.A.A.S. A.A.S. 4, it is also difficult for researchers across professional domains to implement such designs; some documents only give experimental results, but the experimental results are not compared with commercial electrochemical detection equipment, so that the reliability of the experimental results is reduced; while (a.ainla et al, "Open-Source positional state for wireless electrochemical Detection with Smartphones," anal. chem., vol.90, pp.6240-6246,2018.), its hardware and software design is Open-Source and the experimental results are compared with the existing commercial electrochemical Detection equipment, but it adopts RFDUINO as microcontroller, RFDUINO is the combination of Arduino and bluetooth module, it is developed for wireless scene application, its price is ten times of that of ordinary Arduino, so the design is suitable for wireless scene, and the price is expensive; furthermore, (Meloni and G.N., "Building a Microcontroller Based position for Electrochemical Modulation," J.chem.Educ., vol.93, pp.1320-1322,2016.), (L.Y.C.et., "easy random Low-code position with User-Friendly software integration devices," J.chem.Educ., vol.95, 1658-1661, 2018.), whose hardware design is open but whose internal DAC-generated Pulse Width is compared with DAC-generated DAC-DAC function, but no internal DAC-Based PWM function is used to detect whether the DAC-Based PWM function is used for the internal PWM function, and thus the accuracy of the experimental results may also be reduced.
Disclosure of Invention
The invention discloses an excitation signal generating circuit for realizing a time current method and a cyclic voltammetry method based on Arduino and electrochemical detection equipment.
In order to achieve the purpose, the invention adopts the following technical scheme:
an excitation signal generating circuit based on Arduino comprises a first analog-to-digital conversion unit, a second analog-to-digital conversion unit and an inverse circuit unit;
each analog-to-digital conversion unit converts the digital signals received by the Arduino into analog signals and respectively sends the analog signals to the reverse circuit unit; the reverse circuit unit generates an excitation signal according to the received analog signal.
Furthermore, the digital filter further comprises a second-order low-pass filtering unit which is arranged between the first analog-to-digital conversion unit and the reverse circuit unit.
Further, in the excitation signal generating circuit, the Vdd terminal of the first adc and the Vref terminal of the first adc are respectively connected to the +2.5V power supply, the a0 terminal of the first adc is connected to the +5V reference potential, the GND terminal of the first adc is grounded, the signal input terminal of the first adc is connected to the signal output terminal of Arduino, the output terminal of the first adc is connected to one terminal of a resistor R1, the other terminal of the resistor R1 is respectively connected to one terminal of a resistor R2 and one terminal of a capacitor C1, the other terminal of the resistor R2 is respectively connected to one terminal of a capacitor C2 and the positive input terminal of the first operational amplifier, the other terminal of the capacitor C2 is grounded, the other terminal of the capacitor C1 is respectively connected to the negative input terminal of the first operational amplifier, the output terminal of the first operational amplifier and one terminal of a resistor R3, the other terminal of the resistor R3 is respectively connected to one terminal of a resistor R4 and the negative input terminal of the second operational amplifier, the Vdd end of the second analog-to-digital converter and the Vref end of the second analog-to-digital converter are respectively connected to a +2.5V power supply, the A0 end of the second analog-to-digital converter and the GND end of the second analog-to-digital converter are respectively grounded, the signal input end of the second analog-to-digital converter is connected to the signal output end of the Arduino, the output end of the second analog-to-digital converter is connected to the positive input end of the second operational amplifier, and the other end of the resistor R4 is connected to the output end of the second operational amplifier.
Further, the model of the first and second analog-to-digital converters includes MCP4725, and the model of the first and second operational amplifiers includes OP 297.
Further, the addresses of the first analog-to-digital converter and the second analog-to-digital converter are different.
An electrochemical detection device for realizing a time current method and a cyclic voltammetry method based on Arduino comprises an excitation signal generation circuit, a three-electrode system circuit, a test electrode, a signal measurement circuit and a power supply signal circuit, wherein the excitation signal generation circuit, the three-electrode system circuit, the test electrode, the signal measurement circuit and the power supply signal circuit are obtained by adopting the method;
the excitation signal generating circuit generates an excitation signal through the received Arduino digital signal;
the three-electrode system circuit generates a constant voltage signal or a triangular wave voltage signal between a working electrode and a reference electrode of a test electrode through an excitation signal;
the signal measuring circuit converts the collected polarization current in the working electrode and counter electrode loops of the test electrode into digital signals and feeds the digital signals back to Arduino;
the power supply signal circuit supplies power to the excitation signal generating circuit, the three-electrode system circuit and the signal measuring circuit.
Further, in the three-electrode system circuit, one end of a resistor R5 is connected to the output end of the excitation signal generating circuit, the other end of the resistor R5 is connected to the negative input end of the third operational amplifier and one end of a resistor R6, respectively, the positive input end of the third operational amplifier is grounded, the output end of the third operational amplifier is connected to the counter electrode of the test electrode, the other end of the resistor R6 is connected to the negative input end of the fourth operational amplifier and the output end of the fourth operational amplifier, the positive input end of the fourth operational amplifier is connected to the reference electrode of the test electrode, the working electrode of the test electrode is connected to the negative input end of the fifth operational amplifier, one end of a resistor R7, one end of a resistor R8 and one end of a capacitor C3, the other end of a resistor R8 is connected to a-2.5V power supply, the positive input end of the fifth, the output end of the fifth operational amplifier is respectively connected with the other end of the resistor R7 and the other end of the capacitor C3; the model of the third operational amplifier, the fourth operational amplifier and the fifth operational amplifier comprises an OP 297.
Furthermore, in the signal measuring circuit, one end of a resistor R9 is connected to the output end of a fifth operational amplifier of the three-electrode system unit, the other end of the resistor R9 is respectively connected with one end of a resistor R10, one end of a capacitor C4 and one end of a capacitor C5, the other end of a resistor R10 is connected with the positive input end of a sixth operational amplifier, the other end of a capacitor C4 is grounded, the other end of a capacitor C5 is respectively connected with the negative input end of the sixth operational amplifier, the output end of the sixth operational amplifier and the A1 end of a digital-to-analog converter, the Vcc end of the digital-to-analog converter is connected with a +5V reference power supply, the Addr end of the digital-to-analog converter is respectively grounded with the Gnd end of the digital-to-analog converter, and the IIC; the model of the sixth operational amplifier includes OP 297.
Furthermore, the power supply signal circuit comprises circuits required by Arduino and operational amplifier, circuits required by analog-to-digital converter and digital-to-analog converter in the device and a bias power supply circuit;
in the circuit required by the Arduino and the operational amplifier, one end of a capacitor C6, one end of a capacitor C9 and a Vin end of a first voltage stabilizing chip are respectively connected to a +12V power supply, the other end of a capacitor C6, the other end of a capacitor C9, a Gnd end of the first voltage stabilizing chip, one end of a capacitor C7, one end of a capacitor C8 and an anode input end of a seventh operational amplifier are respectively grounded, a Vout end of the first voltage stabilizing chip is respectively connected with the other end of a capacitor C8 and one end of a resistor R11, the other end of the resistor R11 is respectively connected with one end of a resistor R12 and a cathode input end of the seventh operational amplifier, and an output end of the seventh operational amplifier is respectively connected with the other end of a resistor R12 and the other end of a capacitor C7; the output voltage of the Vout end of the first voltage stabilizing chip is +5V, and the output voltage of the seventh operational amplifier is-5V; the first voltage stabilizing chip comprises an LM 2950;
in the circuits required by the analog-digital converter and the analog-digital converter in the device, one end of a capacitor C10 and the Vin end of a second voltage stabilization chip are respectively connected to +12V voltage, the other end of a capacitor C10, the Gnd end of the second voltage stabilization chip, one end of a capacitor C12, one end of a capacitor C13 and the Gnd end of a third voltage stabilization chip are respectively grounded, and the Vout end of the second voltage stabilization chip is respectively connected with the other end of the capacitor C12, the other end of the capacitor C13 and the Vin end of the third voltage stabilization chip; the output voltage of the Vout end of the second voltage stabilizing chip is +5V, and the output voltage of the Vout end of the third voltage stabilizing chip is + 2.5V; the model of the second voltage stabilizing chip comprises ADR425, and the model of the third voltage stabilizing chip comprises ADR 421;
in the bias power supply circuit, one end of a capacitor C14 and one end of a resistor R13 are respectively connected with the Vout end of a third voltage stabilizing chip, one end of a capacitor C11, the other end of a capacitor C14, one end of a capacitor C15 and the positive input end of an eighth operational amplifier are respectively grounded, the other end of a resistor R13 is respectively connected with one end of a resistor R14 and the negative input end of the eighth operational amplifier, and the output end of the eighth operational amplifier is respectively connected with the other end of a resistor R14, the other end of a capacitor C11 and the other end of a capacitor C15; the output voltage of the output end of the eighth operational amplifier is-2.5V; the eighth operational amplifier includes an OP 297.
Further, the resistance is a parameter symmetric resistance, and the capacitance is measured by a capacitance bridge.
The method provided by the invention has the following advantages and effects:
1. an electrochemical analysis device manufactured by taking Arduino as a platform is developed, and the detection performance of the CV and CA detection method is highly consistent with that of a commercial electrochemical device (CHI660E), the cost of the detection device is one percent of that of the commercial electrochemical device, and the volume and the mass of the detection device are one sixth of that of the commercial electrochemical device. Provides a new way for the low cost, portability and wearability of the electrochemical monitoring device.
2. In the development process, the problems of low precision, poor stability and difficult and effective control of forming an excitation signal by using a Pulse-width modulation (PWM) waveform of the Arduino are found.
3. Key designs of the electrochemical analysis equipment designed by taking Arduino as a platform are pointed out, such as excitation signal generation design, three-electrode system circuit design, signal sampling circuit design, power supply design and chip selection. And software and hardware resources are completely opened, so that the popularization of the development is more convenient.
4. The invention adopts common electronic components and reasonable software and hardware design, develops all the contents of the invention and provides electrochemical detection equipment which is low in cost, portable and high in precision.
Drawings
FIG. 1 is a block diagram of a system hardware implementation of the present invention.
FIG. 2 is a circuit diagram of the hardware design of the present invention.
Fig. 3 is a circuit diagram of a prior art PWM implementation of an analog voltage.
Fig. 4 is a circuit diagram of the excitation signal generation of the present invention.
Fig. 5A is a prior art three-electrode architecture circuit diagram.
Fig. 5B is a circuit diagram of another prior art improved three-electrode architecture.
Fig. 5C is a circuit diagram of a three-electrode architecture of the present invention.
Fig. 6 is a signal sampling circuit diagram of the present invention.
Fig. 7A is a circuit diagram of the power supply required by Arduino and op-amp of the present invention.
Fig. 7B is a circuit diagram of the power supply for the ADC and DAC and the DAC reference power supply of the present invention.
Fig. 7C is a bias power supply circuit diagram of the current-voltage conversion circuit of the present invention.
Fig. 8A is a circuit diagram of another prior art PWM wave to derive an excitation signal.
FIG. 8B is a schematic diagram of another prior art PWM-based triangular step voltage of-1 v to 1 v.
FIG. 8C is an enlarged view of FIG. 8B from 0V to-0.5V.
FIG. 8D is a schematic diagram of the present invention obtaining a-1 v to 1v triangular step voltage based on PWM.
FIG. 8E is an enlarged view of FIG. 8D from 0V to-0.5V.
FIG. 9A is a CV comparison plot of 50mv/s scan rate for the present invention and CHI660E
FIG. 9B is a CV comparison plot of 80mv/s scan rate for the present invention and CHI660E
FIG. 9C is a CV comparison plot of 100mv/s scan rate for the present invention and CHI660E
FIG. 9D is a CV comparison of the present invention and CHI660E at a scan rate of 160mv/s
Fig. 9E is a graphical illustration of a linear fit of the peak current to the square root of the scan rate of fig. 9A-D.
FIG. 10A is a time-current graph of continuous addition of hydrogen peroxide in accordance with the present invention.
FIG. 10B is a schematic of a calibration curve for determining hydrogen peroxide from FIG. 10A.
Detailed Description
In order to make the objects, technical solutions and advantages of the present invention more clear, the present invention is further described in detail below with reference to specific embodiments and accompanying drawings. It should be understood that the specific embodiments described herein are merely illustrative of the invention and are not intended to limit the invention.
The electrochemical detection device for realizing the time current method and the cyclic voltammetry based on the Arduino is built by utilizing an Arduino platform and comprises the steps of generating an excitation signal, designing a three-electrode system circuit, sampling a signal, processing a power supply signal and selecting a device.
1. Measurement platform built by Arduino
Arduino is a very popular soft (MIT or BSD license) hardware integrated platform in the present, and compared with a traditional single chip microcomputer, the integrated platform has the characteristics of simplicity and easiness in development, low cost, rapidness in implementation, flexibility in selection and convenience in adjustment.
The Arduino hardware platforms of various models can be selected according to different task execution complexity, calculated amount, application scenes and the like, wherein the minimum Arduino (atto Arduino) plate area is only 10.3mm x 11.5 mm. An Integrated Development Environment (IDE) of Software is developed by using C language, a development board communicates with a computer through a serial port, and is provided with open source library files which are updated in real time and are convenient to operate for various complex modules such as sensors, communication, sampling, AD/DA (analog to digital) and monitoring, the libraries are basically based on Berkeley Software release (BSD) or Massachu-setts institute of Technology (MIT), and the libraries are the most loose Software use license so as to meet the open source of the Software to the maximum extent. Arduino provides a serial port tool, and is convenient for developers to control and display data in real time. By utilizing the powerful software and hardware platform of Arduino, the equipment needed by the manufacture can be manufactured more quickly, easily and more cost-effectively.
The electrochemical detection equipment realized by the invention has low requirements on the calculation, storage, volume and peripheral equipment of Arduino, so the invention selects the most commonly used Arduino Uno as a control measurement platform. And secondly, the serial port tool provided by Arduino can be used for carrying out later-stage data analysis, so that the development time can be saved.
2. System design
The invention samples the three-electrode system, realize the instrument design of time current method and cyclic voltammetry in the control potential method, the apparatus used for carrying on electrochemical measurement usually includes three major parts, one is the signal generator (signal generator) producing the necessary excitation signal; the second is the control and measurement part of the signal; and thirdly, recording and displaying the current and voltage signals. The three parts of instruments are connected with each other, and the tester is connected with the electrolytic cell, so that the control and measurement of signals such as current, voltage and the like in the electrochemical system are realized. Wherein the control and measurement part of the signal is the core part of the whole set of equipment.
FIG. 1 is a block diagram of the hardware implementation of the system of the present invention, in order from left to right, the leftmost dashed box represents the control, recording and display part of the current and voltage signals, the computer is connected with Arduino through USB to complete the control, recording and display of the current and voltage signals, and the main work of the part is code programming and data processing; the middle left dotted line frame is an excitation signal generating circuit part and comprises a DAC0, a DAC1, an LPF0 and an inverting circuit, wherein the Arduino controls the DAC0 and the DAC1 to provide input signals for the inverting arithmetic circuit, and the inverting circuit converts the two signals to obtain an excitation signal; the middle dotted line frame close to the right is a signal loading part, which comprises a three-electrode system circuit and a current-voltage conversion circuit and is used for loading an excitation voltage between a working electrode and a reference electrode and sampling a polarization current from a working electrode loop and a counter electrode loop; the middle lower dotted line frame is a signal measurement part which comprises a current-voltage conversion circuit, an LPF1 and an ADC and is used for sampling the polarization current and transmitting the polarization current to Arduino; the middle lower dotted line frame is a power signal processing part and is used for converting a +/-12V input power supply into a power supply required by each module. The rightmost dashed box is the test electrode and test solution system. The work done by the present invention is shown in italics in fig. 1, and the ready-to-use test electrode and solution system is shown in italics plus the underline.
Referring to the black block diagram in fig. 1, a specific design implementation circuit is shown in fig. 2, where each dotted block diagram in fig. 2 corresponds to a dotted block diagram portion in fig. 1, we (working electrode) in fig. 2 is a working electrode, re (reference electrode) is a reference electrode, and ce (counter electrode) is a counter electrode.
2.1 Generation of excitation signals
Both a time current method (CA) and a cyclic voltammetry method (CV) need appropriate excitation signals, the requirements on the excitation signals are that the excitation signal range is required, a test range of-2.5V-2.5V can meet the requirements of most test voltage widths, the excitation signal precision and resolution are required, the signals are required to be stable enough, the resolution within 1mV is required, and the communication speed requirement is met. The present invention requires the design of the excitation signal to meet the three above requirements.
The invention uses an operational amplifier (OP), an analog-to-digital converter (DAC) to be connected to Arduino to realize the generation of the excitation signal. Although Arduino has many advantages for development, there is no DAC inside Arduino, and in order to realize the analog output of Arduino, it is the simplest way to convert the Pulse Width Modulation (PWM) signal it generates into an analog excitation signal.
A circuit diagram of a prior art PWM for implementing analog voltage is shown in fig. 3, in which a PWM signal is first passed through an RC low-pass circuit (with a time constant μ ═ 4.7us), and then through a voltage follower circuit composed of operational amplifiers, so as to adjust square waves with different duty ratios into different analog voltage signals. However, the method cannot meet the requirement of generating resolution within 1mV, especially in CV experiments, triangular wave linear voltage needs to be generated, and the DAC function realized by PWM cannot be met.
According to the invention, the excitation signal is generated by adding the external DAC module to Arduino, so that the problem that the DAC function cannot meet the experimental requirement by utilizing PWM (pulse-width modulation) waves is solved. FIG. 4 is a circuit diagram of the excitation signal generation circuit of the present invention, in which two DAC modules are added in the excitation signal generation process, the DAC module is a 12-bits DAC module MCP4725, and the MCP4725 module is a digital-to-analog converter (DAC) with low power consumption, high precision, single channel, and 12-bit buffer voltage output, and has a non-volatile memory (EEPROM). Second 12bits DAC chip with LSB ═ Vref/4096 wherein VrefWhen set to 2.5V, its LSB is 0.6mV (2.5/4096, error 0.3mV), so LSB<1mV meets the precision requirement.
It carries out IIC communication with the singlechip, and the communication speed has 100kbps, 400kbps, 3.4 mbps. Setting a harsher CV scanning triangular wave voltage with a voltage width of 0.5V, a scanning rate of 5V/S, a scanning interval voltage of 5mV, and a scanning time in one voltage period
0.5/5＝0.1S
The number of scanning times of one voltage period is
0.5/0.005＝100
Then the scanning frequency is
1/0.1/100＝1000Hz
The required bandwidth per second is
1000*12＝12kbps
Therefore, three communication rates fully satisfy the requirements of the present invention. The invention needs 2 MCP4725 modules, and when in use, the communication addresses of the IICs are set to be different.
In FIG. 4, Arduino controls two DACs (DAC0 and DAC1) via the IIC data line, and then switches the two DACs into a 2.5V reference voltage so that their respective outputs can output voltages in the range of 0-2.5V; DAC0 is followed by a second order low pass filter, which is a second order Sallen-Key filter. The second order low pass filter is placed after the output of the first DAC block and not after the output generated by the entire excitation signal because the amount of conversion is Vi and Vdac1 is fixed when the CV triangular wave voltage is generated using this circuit, so it is placed after DAC0, mainly to filter the higher harmonics of the triangular wave signal generated by DAC 0; finally, the signal passes through an inverter circuit (OP1), and the input and output equations are as follows:
setting R1 ═ R2, one can obtain:
V0＝2VDAC1-Vi
where Vi is derived from Arduino control DAC0 and Vdac1 is derived from Arduino control DAC1, both of which may output voltages in the range of 0-2.5V, so V0The voltage in-2.5V to 5V can be obtained, and the voltage range of the excitation signal +/-2.5V is met. In addition, another ingenious point of the excitation signal circuit is that a wider voltage range can be obtained by changing the ratio relation of R1/R2 (note that the output voltage range of the operational amplifier is smaller than the width of the power supply).
One specific embodiment of the present invention is:
the Vdd end of the first analog-to-digital converter and the Vref end of the first analog-to-digital converter are respectively connected to a +2.5V power supply, the A0 end of the first analog-to-digital converter is connected to a +5V power supply, the GND end of the first analog-to-digital converter is grounded, the signal input end of the first analog-to-digital converter is connected to the signal output end of Arduino, the output end of the first analog-to-digital converter is connected to one end of a resistor R1, the other end of the resistor R1 is respectively connected to one end of a resistor R2 and one end of a capacitor C1, the other end of a resistor R2 is respectively connected to one end of a capacitor C2 and the positive input end of a first operational amplifier, the other end of a capacitor C2 is grounded, the other end of a capacitor C1 is respectively connected to the negative input end of the first operational amplifier, the output end of the first operational amplifier and one end of a resistor R46, the Vdd end of the second analog-to-digital converter and the Vref end of the second analog-to-digital converter are respectively connected to a +2.5V power supply, the A0 end of the second analog-to-digital converter and the GND end of the second analog-to-digital converter are respectively grounded, the signal input end of the second analog-to-digital converter is connected to the signal output end of Arduino, the output end of the second analog-to-digital converter is connected to the positive input end of the second operational amplifier, and the other end of the resistor R4 is connected to the output end of the second operational amplifier. The model of the first analog-to-digital converter and the model of the second analog-to-digital converter comprise MCP4725, the system addresses of the first analog-to-digital converter and the second analog-to-digital converter are different, and the model of the first operational amplifier and the second operational amplifier comprise OP 297; when setting the frequency signal below 1kHZ to pass, the parameters may preferably be set to: r1 ═ 1.2k Ω, R2 ═ 1.2k Ω, R3 ═ 1k Ω, R4 ═ 1k Ω, C1 ═ 100nF, and C2 ═ 10 nF.
Thus, in the CA experiment, when 0.5V is output by DAC1 and 0.4V is output by DAC0, a constant voltage signal of 0.6V is applied between the working electrode and the reference electrode.
Thus, in the CV experiment, when DAC1 outputs 0.5V and DAC0 outputs 0-1.4V, a triangular wave voltage signal of-0.4V-1V is generated between the working electrode and the reference electrode.
2.2 three-electrode System Circuit design
The present invention uses a three-electrode system, the test requiring that an excitation signal (voltage) be applied between the working electrode and the reference electrode. Secondly, no current flows into the reference electrode, and thirdly, the polarization current is measured between the working electrode and the counter electrode.
The three-electrode design is the most important part in the design of the electrochemical detection equipment, and fig. 5A is a basic three-electrode system design circuit, wherein an excitation signal is input to a positive input end of an operational amplifier,
VRE＝VD，VWE＝0；
then:
VWR＝-VD；
the basic three-electrode system design circuit can obtain the excitation voltage signal to be loaded only by inputting the reverse excitation signal.
According to the virtual break, virtual short characteristics of the operational amplifier, OP2 in fig. 5A can obtain:
IW＝(VADC-0)/RS；
the basic three-electrode system design circuit specifies that the direction of current flowing into the working electrode is positive, a current-voltage conversion circuit is used, a sampling resistor is added in the circuit, and the current is converted into a voltage signal which can be identified by an analog-digital converter, so that the polarization current between the working electrode and the counter electrode is calculated. Therefore, through the design of a basic three-electrode system, an excitation signal can be loaded on the working electrode and the reference electrode according to requirements, and polarized current can be sampled from a loop of the working electrode and the counter electrode.
Fig. 5B is a modified three-electrode system circuit, which adds a unit gain circuit to the reference electrode compared to fig. 5A, and can also obtain:
VRE＝VD，VWE＝0；
VWR＝-VD；
IW＝(VADC-0)/RS；
the processing and calculation method of the method is the same as a basic three-electrode system circuit, but a unit gain circuit is added at the reference electrode end, so that current does not flow into the reference electrode end, and the voltage stability is higher;
fig. 5C is a three-electrode system circuit design employed by the present invention, which can be derived:
VO1/R2＝-VD/R1；
VO1＝VRE；
VWR＝-VO1；
IW＝(VADC-0)/RS；
so that:
VO1＝-VDR2/R1；
let R1＝R2The following can be obtained:
VWR＝VD；
therefore, the excitation signal can be directly loaded between the working electrode and the reference electrode without negative processing, and a unit gain operational amplifier is arranged at the reference electrode end, so that no current flows into the reference electrode end, and the current is stabilized.
One specific embodiment of the present invention is:
one end of a resistor R5 is connected to the output end of the excitation signal generating unit, the other end of the resistor R5 is respectively connected with the negative input end of the third operational amplifier and one end of a resistor R6, the positive input end of the third operational amplifier is grounded, the output end of the third operational amplifier is connected with the counter electrode of the test electrode, the other end of the resistor R6 is respectively connected with the negative input end of the fourth operational amplifier and the output end of the fourth operational amplifier, the positive input end of the fourth operational amplifier is connected with the reference electrode of the test electrode, the working electrode of the test electrode is respectively connected with the negative input end of the fifth operational amplifier and one end of the resistor R7, one end of the resistor R8 and one end of the capacitor C3, the other end of the resistor R8 is connected to a-2.5V power supply, the anode input end of the fifth operational amplifier is grounded, and the output end of the fifth operational amplifier is respectively connected with the other end of the resistor R7 and the other end of the capacitor C3. The model of the third operational amplifier, the fourth operational amplifier and the fifth operational amplifier comprises OP 297; according to the excitation signal generating unit, the parameters of the three-electrode system unit are preferably set as follows: r5 ═ 1k Ω, R6 ═ 1k Ω, R7 ═ 10k Ω, R8 ═ 10k Ω, C1 ═ 100nF, and C2 ═ 10nF, where R7 and R8 are sampling resistors, and the span of the sampling current in this embodiment is ± 250 uA.
2.3 Signal sampling
The signal sampling circuit needs to sample the polarized current in the working electrode and counter electrode loops, and a resistor is added to convert the current into voltage in the current-voltage conversion circuit, so that a current signal is converted into a signal which can be identified by an analog-to-digital converter (ADC). In order to correctly sample the current, the characteristics of the sampled current need to be analyzed, firstly, in a CV experiment, the current has positive and negative, and the width requirement of the sampled current needs to be met; the precision requirement of the second sampling current is less than 10nA, and the sampling interval time is more than 10 mS; and thirdly, filtering out clutter signals in the sampling current.
Fig. 6 is a signal sampling circuit of the present invention, which is, in order from right to left, an I/V conversion circuit, a second-order low-pass filter, and an analog-to-digital conversion circuit.
The rightmost side of fig. 6 is an I/V conversion circuit (OP4), since the sampling current has positive and negative, but the ADC adopted in the present invention is a 16-bit unipolar ADS115, which can only sample positive voltage signals, and at present there is also a bipolar ADC capable of sampling positive and negative voltage signals, but it is also necessary to provide positive and negative reference voltages for the ADC. Due to the fact that
This gives:
the reason why the bias voltage is set to-2.5V here is that VOIn the range of 0-5V, such that iwIs in the range of (-2.5/Rs) - (2.5/Rs) A, so that the sampling current can simultaneously reach the maximum range of a positive half shaft and a negative half shaft,meanwhile, the size of Rs can be adjusted to correspond to different current sampling ranges.
The left-most side of fig. 6 is the ADS1115ADC of the invention selected with 16 bits, the ADS1115 is a precision analog-to-digital converter (ADC), 16 bits of resolution, with an internal programmable gain amplifier, providing an alternative gain control scheme to that of fig. 6, with LSBs (least significant bits) ranging from 0.007815mv to 0.1875mv at different gains, as shown in the span selection table of table 1, although higher gains are selected to maximize resolution, the higher resolution, the smaller the voltage range of measurement, so that reasonable gains and offset voltages are selected according to the test requirements to achieve the required measurement accuracy and range.
| Different gains | Measuring voltage range | Precision (LSB) (mv) |
| 2/3x | 6.144 | 0.1875 (default) |
| 1x | 4.096 | 0.125 |
| 2x | 2.048 | 0.0625 |
| 4x | 1.024 | 0.03125 |
| 8x | 0.512 | 0.015625 |
| 16x | 0.256 | 0.0078125 |
TABLE 1
The ADS1115 has the sampling rate of 8, 16, 32, 64, 128(default), 250, 475 and 860sps, but the invention provides that the minimum sampling interval time reaches 10mS, and 100 times of sampling can be carried out in one second, so that the sampling rate of more than 128 can meet the requirement.
In the middle of fig. 6, a 2 nd order low-pass Sallen-key filter is arranged, in the CV experiment, the polarization current also changes along with the change of the excitation voltage, and a low-pass filter is added to filter higher harmonics in the signal and set the cut-off frequency to be about 1 kHz.
One specific embodiment of the present invention is:
one end of a resistor R9 is connected to the measuring end of the three-electrode system unit, the other end of the resistor R9 is connected with one end of a resistor R10, one end of a capacitor C4 and one end of a capacitor C5 respectively, the other end of the resistor R10 is connected with the anode input end of a sixth operational amplifier, the other end of the capacitor C4 is grounded, the other end of the capacitor C5 is connected with the cathode input end of the sixth operational amplifier, the output end of the sixth operational amplifier and the A1 end of a digital-to-analog converter respectively, the Vcc end of the digital-to-analog converter is connected with a +5V power supply, the Addr end of the digital-to-analog converter and the Gnd end of the digital-to-analog converter are grounded respectively, and the IIC end. Preferably, the model of the sixth operational amplifier includes OP297, and the model of the digital-to-analog converter includes ADS1115, R9 ═ 10k Ω, R10 ═ 29.4k Ω, C4 ═ 10nF, and C5 ═ 100 nF.
2.4 Power Signal processing
The processing of the power supply signal is required to meet two requirements, the first is to meet the power supply requirements and design requirements of each used device, and the second is to test the reference voltage with sufficient accuracy and stability. The first requirement is to meet the requirements necessary for system operation, the second is to meet the inevitable requirements for small signal measurements, and for small signal measurements, the accuracy and stability of the reference voltage is directly related to the accuracy of the final measurement. The above two requirements are the objects to be achieved by the present invention, and especially the requirement for sufficiently high accuracy and stability of the reference signal.
TABLE 2
As can be seen from the required power supplies in Table 2, the present invention requires power supplies designed for Arduino Uno (5V), OP297(5V, -5V,12V, -12V), ADC (5V), DAC (2.5V), and current-to-voltage conversion circuit (-2.5V). The design idea of the invention is to select a +/-12V power supply produced by a bright weft power supply as a source power supply, and then obtain the power supply required by the table 2 through a high-precision voltage stabilizing chip and a reverse operational amplifier circuit.
Fig. 7A is a layout diagram of the power supply required for the Arduino and the operational amplifier, and the LM2950 chip is used to convert the 12V voltage to 5V, and then the-5V voltage is obtained through the inverse operation circuit (OP6), where it is noted that a resistor above 10k Ω is selected, otherwise the operational amplifier and the Arduino cannot be driven to operate. The LM2950 is a linear dropout regulator (LDO), has the characteristics of high output voltage precision (0.5%), good stability and low cost, is inferior to the high precision (0.04%) and high stability of the ADR425, but has much higher precision (2%) and stability than AMS1117 of Arduino, and can meet the operation of Arudino and operational amplifier. Furthermore, the power supply of the operational amplifier is related to the output range and stability of the operational amplifier, the output range of the operational amplifier is smaller than the power supply range, the more stable the power supply of the operational amplifier is, the more stable the output is, so that the high-precision LM2950 chip with 5V output is selected.
One specific embodiment of the present invention is:
one end of a capacitor C6, one end of a capacitor C9 and a Vin end of a first voltage stabilization chip are respectively connected to a +12V power supply, the other end of a capacitor C6, the other end of a capacitor C9, a Gnd end of the first voltage stabilization chip, one end of a capacitor C7, one end of a capacitor C8 and a positive electrode input end of a seventh operational amplifier are respectively grounded, a Vout end of the first voltage stabilization chip is respectively connected with the other end of a capacitor C8 and one end of a resistor R11, the other end of the resistor R11 is respectively connected with one end of a resistor R12 and a negative electrode input end of the seventh operational amplifier, and an output end of the seventh operational amplifier is respectively connected with the other end of the resistor R12 and the other end of the capacitor C7. The output voltage of the Vout end of the first voltage stabilizing chip is +5V, and the output voltage of the seventh operational amplifier is-5V; preferably, the first voltage stabilizing chip includes an LM2950, and the seventh operational amplifier includes an OP297, where R11 ═ 10k Ω, R12 ═ 10k Ω, C6 ═ 10nF, and C7 ═ C8 ═ C9 ═ 100 nF.
FIG. 7B shows the design circuit of the power supply of the ADC and the DAC and the reference power supply of the DAC. The ADR425 chip is used to convert the 12V voltage to 5V, and the 5V voltage is used as the power supply for the DAC and ADC. The 5V voltage is then converted to 2.5V using the ADR421 regulator chip, this 2.5V serving as the reference supply for the DAC and the input supply for the 2.5V to-2.5V circuit. ADR425 and ADR421 are one of the most accurate (0.04%) and stable voltage regulation chips at present, and although the cost is hundreds of times higher than that of the common voltage regulation chip, the measurement of the small signal is very necessary for the invention, especially for the reference voltage of DAC. According to the invention, the reference voltage of the DAC is obtained without converting 12V voltage into 2.5V by using the ADR421, but the 12V voltage is converted into 5V by using the ADR425, and then the 5V voltage is used as an input power supply of the ADR421, so that the advantages that the first ADR425 and the ADR421 are LDO (low dropout), the lower the linear voltage drop is, the better the voltage stabilization effect is, the better the ripple suppression ratio of the parameter power supply voltage of the second voltage stabilization chip is, namely the suppression of the output signal relative to the noise of the input signal, the more the primary voltage stabilization chips are, the more the suppression of the ripple of the input signal is, and the better effect of voltage stabilization of the reference voltage is achieved.
One specific embodiment of the present invention is:
one end of the capacitor C10 and the Vin end of the second voltage stabilization chip are respectively connected to +12V voltage, the other end of the capacitor C10, the Gnd end of the second voltage stabilization chip, one end of the capacitor C12, one end of the capacitor C13 and the Gnd end of the third voltage stabilization chip are respectively grounded, the Vout end of the second voltage stabilization chip is respectively connected to the other end of the capacitor C12, the other end of the capacitor C13 and the Vin end of the third voltage stabilization chip, and the output voltage of the Vout end of the third voltage stabilization chip is + 2.5V. Preferably, the model of the second zener chip includes ADR425, and the model of the third zener chip includes ADR421, C10 ═ 1uF, C12 ═ 100nF, and C10 ═ 10 uF.
Fig. 7C is a bias power supply design circuit of the current-voltage conversion circuit, in which the operational amplifier circuit is a unit inverting operational amplifier circuit, and the input of the operational amplifier is 2.5V, and a-2.5V circuit can be obtained through the circuit.
One specific embodiment of the present invention is: one end of a capacitor C14 and one end of a resistor R13 are respectively connected with a +2.5V power supply, one end of a capacitor C11, the other end of a capacitor C14, one end of a capacitor C15 and the positive input end of an eighth operational amplifier are respectively grounded, the other end of a resistor R13 is respectively connected with one end of a resistor R14 and the negative input end of the eighth operational amplifier, and the output end of the eighth operational amplifier is respectively connected with the other end of a resistor R14, the other end of the capacitor C11 and the other end of a capacitor C15. Wherein, the output voltage of the output end of the eighth operational amplifier is-2.5V. Preferably, the eighth operational amplifier comprises OP297, R11 ═ R12 ═ 5.6k Ω, C11 ═ 10uF, and C14 ═ C15 ═ 100 nF.
In the invention, other chips are used for selecting the voltage stabilizing chip, the output effect of the voltage stabilizing chip is more different from the detection effect of a commercial electrochemical workstation, and the table 3 is used for comparing the output precision of each voltage stabilizing chip.
| Voltage stabilizer | Output (V) | Precision (Max) | Maximum fluctuation (mv) |
| 7905,7805 | -5,+5 | 4％ | 200 |
| |
5 | 1％ | 50 |
| AMS1117 | 2.5,3.3,5 | 2％ | 100 |
| TPS79933 | 3.3 | 2％ | 66 |
| LM2950(LDO) | 5 | 0.5％ | 25 |
| |
5 | 0.2％ | 10 |
| |
5 | 0.06％ | 2.5 |
| ADR03B | 2.5 | 0.1％ | 2.5 |
| ADR421B | 2.5 | 0.04％ | 1 |
| |
5 | 0.04％ | 2 |
TABLE 3
2.5 device selection
The invention has been explained from the aspects of excitation signal generation, three-electrode system circuit design and signal sampling, but the specific realization of the circuit needs specific analog devices, the analog devices are instrument devices capable of processing continuous electric signals, and the selection of the analog devices needs to be considered according to the use purpose, scene, difficulty in realization, volume, cost and the like. The analog devices used in the present invention include Arduino, operational amplifier, DAC, ADC, voltage stabilization chip, resistor and capacitor, and the selection of Arduino, DAC, ADC and voltage stabilization chip is described above. For the selection of the resistor and the capacitor, the invention selects the common plug-in resistor and capacitor, the resistors are symmetrical based on the use of the invention, the resistors with symmetrical parameters are selected after the universal meter is used for measurement, and the capacitor is used after the capacitor is used for measurement by using a capacitor bridge.
In the introduction of the background of the invention, it was stated that a sufficiently high input impedance is required in the control of the electrode potential, and the generation of the excitation voltage, the design of the three-electrode system circuit and the planning of current sampling are realized by depending on the operational amplifier circuit, all of them are based on the ideal 'virtual short (equal input end voltage)' and 'virtual break (infinite input end resistance)' of operational amplifier, however, the actual operational amplifier has a certain offset voltage (the voltages at the two ends of the operational amplifier are unequal) and bias current (the current flowing into the operational amplifier), which are not true virtual short and virtual break, and for general operational amplifiers, such as the LM324 operational amplifier, the offset voltage is 3mv, the bias current is 20nA, the voltage precision of the excitation signal is less than 1mv, and the sampling current precision of the signal sampling is less than 10nA, so that the precision requirement cannot be completed by selecting a universal operational amplifier. The invention selects the low-temperature-drift type precision operational amplifier which has the characteristics of high precision, low temperature drift, high open-loop gain, low bias current and the like, and is very suitable for the design requirement of the circuit. The bipolar op297 adopted by the invention has the bias current less than 100pA and the offset voltage less than 25uV (the precision is improved by 120 times compared with 324 operational amplifier), and meets the design requirement.
Table 4 is a list of components used in the present invention, whose 190-tuple cost is equivalent to one percent of CHI 660E.
TABLE 4
The ± 12V power source was purchased from guangxi electronics ltd, guangzhou.
3. Procedure of experiment
3.1 apparatus and reagents
A Keylight MSOX3022T oscilloscope was purchased from Tektronix (China) Inc. A 3D printing device (Vistar SL500) was purchased from Vistar (xiamen) science and technology limited. A CHI660E electrochemical workstation was purchased from chenhua equipment limited and used for comparison with the system of the present design.
Commercial screen-printed electrodes are available from Gwent electronics materials ltd (pomtepl, uk). Silver chloride electrodes, glassy carbon electrodes and platinum electrodes were purchased from Colosse Chong electron technology development, Inc. Solid hydrogen peroxide (H2O2) was purchased from Sigma-Aldrich, Inc. (Beijing). Sodium dihydrogen phosphate was purchased from Tianjin Fuzhen chemical reagent factory (Tianjin). Sodium hydroxide was purchased from a Beijing chemical plant (Beijing). From Shanghai Meilin Biochemical Co., Ltd, Potassium chloride was purchased from Xilong science Co., Ltd as potassium ferricyanide.
Arduino Uno, ADR425B, and ADR421B were ordered from shenzhencology aeolian limited (shenzhen). LM2950 was ordered from Shenzhen, Tianyuhao science and technology Limited (Shenzhen, China). Power supply of 12V was ordered from mingwei electronics, inc (guangzhou). OP297 is ordered from Shenzhen, Chuanlan electronics technology Limited (Shenzhen). 12-bit MCP4725DAC module and 16-bit ADS1115ADC module were ordered from Shenzhen, North Korea limited (Shenzhen, China). A pack of commonly used resistors and a pack of commonly used capacitors are ordered from shenzhen, baijia technology limited corporation (shenzhen, china). A bread board was ordered from Shenzhen Jieshen electronics Limited (Shenzhen, China). One printed circuit board was ordered from agile corporation (hangzhou).
3.2 Process of development of this tester
First, a schematic of the circuit was designed using Fritzing (version 0.9.3). Then, a schematic circuit diagram was constructed with a bread board, and the performance of CA and CV was tested. This circuit schematic is then designed into a 2-layer PCB. The physical 3D housing was then designed using Cinema 4DR20 and material Magics 21.0 and printed using a Vistar SL 5003D printer. And finally, assembling the PCB, the Arduino-Uno and the power supply into a 3D shell to form an integrated system.
The code for CV and CA experiments was written in Arduino software v1.8.10. Next, the code is uploaded to Arduino Uno and the data is acquired through the Arduino serial port. In addition, the data were exported to Microsoft Excel (version 15.37) or Matlab R2016b for analysis.
3.3 testing
The electrochemical detection equipment is used for generating triangular wave voltage of-1 v to 1v, comparing the performance of the triangular wave voltage with the triangular wave voltage formed by PWM waveform, and acquiring and analyzing data by using an oscilloscope.
The CV performance of the electrochemical test device was studied and compared with that of a commercially available CHI660E electrochemical workstation. The measurement was performed with a glassy carbon electrode as the working electrode, an Ag/AgCl electrode as the reference electrode, and a platinum electrode as the counter electrode. DAC0 was adjusted to output a voltage of 0-1.5V, DAC1 was adjusted to output a voltage of 0.5V, allowing potential sweep to range from-0.5V to 1V, DAC0 was adjusted for cycle, rate, gap voltage was adjusted to allow gap voltage of 20mV, sweep rates of 0.05, 0.08, 0.1 and 0.16V/S, respectively, and the electrodes were placed in a solution containing 19.35mM potassium ferricyanide and 0.1M potassium chloride. And converting the voltage sampled by the sampling circuit into sampling current, sending the current signal to a computer through an Arduino serial port tool, and displaying through Microsoft Exell of the computer.
The electrochemical test device was investigated for CA performance and compared with a commercial electrochemical workstation CHI 660E. Using a commercial printed electrode, a potential of 600mV was applied between the working and reference electrodes at room temperature (24 ℃ C.) with a 0.4V output from DAC0 and a 0.5V output from DAC 1. 50uL of a buffer (100mM phosphate buffer, pH7.2) was dropped on the electrode, and 5uLH2O2 solutions at concentrations of 20mM, 40mM, 80mM, 120mM, 160mM, and 200mM were sequentially injected into the buffer. And converting the voltage sampled by the sampling circuit into sampling current, sending the current signal to a computer through an Arduino serial port tool, and displaying the current signal through Microsoft Exell of the computer.
The above experiment gave the following experimental results
4.1 comparison of two methods of generating test scanning voltage by Arduino because the prior art does not have DAC chip to generate analog voltage in Arduino, and the method is realized by PWM wave output by Arduino, when no external DAC module is added, the method is natural, and under the condition of no DAC module, the method is the only method for converting digital signal output into analog voltage output; the analog voltage of the present invention is generated by the Arduino control external DAC module and gives the benefit of doing so. FIGS. 8A-E are graphs of a prior art (Meloni and G.N., "Building a Microcontroller base termination: A Inexpensive and vertical Platform for the teaching electronics and Instrumentation," J.chem.Educ., vol.93, pp.1320, 1322,2016.) and a comparison with the present invention, based on two methods, respectively, to generate triangular step voltages ranging from-1 v to 1v with a separation voltage of 20mV and a scan speed of 200mV/s, which are commonly used as excitation voltages for testing. Fig. 8A is a circuit diagram of a prior art PWM wave to obtain an excitation signal, the circuit is composed of two parts, a broken line block diagram of a first part is a circuit for converting the PWM wave into an analog voltage, and includes an RC low pass filter and a follower amplifier, the first part can obtain a voltage of 0-5V (Arduino power voltage), a broken line block diagram of a second part can convert 0-5V into a triangular voltage range of-1-1V (offset operation circuit), fig. 8B is a prior art PWM-based triangular step voltage of-1V to 1V, fig. 8C shows an enlarged view of fig. 8B from 0V to-0.5V, and the interval voltage is 10 mv. FIG. 4 shows a circuit diagram of the excitation signal generation of the present invention, and similarly, FIG. 8D is a diagram of the present invention for obtaining a triangular step voltage of-1V to 1V, and FIG. 8E is an enlarged view of FIG. 8D from-0.5V to 0V. By comparison, both methods produce the required voltage range, but this prior art technique, using which the voltage and the gap voltage cannot be controlled precisely, is far less accurate than the present invention as seen in fig. 8A-E. The present invention can control the excitation voltage and the scanning voltage interval very precisely. A small error in the excitation signal may cause a large error (0.5V/10k Ω — 50uA) in detection of a small current. The correct use of an external DAC is very important for detection accuracy for Arduino-based electrochemical detection devices.
4.2 CV experiment
Electrochemical test device the performance of the electrochemical test device according to the invention was compared with that of the CHI660E electrochemical workstation under the same test conditions. The test conditions are that a glassy carbon electrode is taken as a working electrode, an Ag/AgCl electrode is taken as a reference electrode, and a platinum electrode is taken as a counter electrode for measurement. The potential sweep range was-0.5V to 1V, the interval voltage was 20mV, the sweep rates were 0.05V/S (FIG. 9A), 0.08V/S (FIG. 9B), 0.1V/S (FIG. 9C) and 0.16V/S (FIG. 9D), respectively, and the experimental data shown are for the second sweep cycle at each sweep rate for each apparatus, and the electrodes were placed in a solution containing 19.35mM potassium ferricyanide and 0.1M potassium chloride. From FIGS. 9A-D, the shape of the curve obtained for the electrochemical detection device of the present design is highly consistent with the shape of the curve obtained for the commercial CHI660 electrochemical workstation at each scan rate. FIG. 9E is a linear fit of the peak current to the square root of the scan rate of FIGS. 9A-D, showing the relationship of the peak current (cathode) to the square root of the scan rate, and the diffusion of the two relationships to the Randles-Sevcik equation (D.M.R.D.Rooij, "Electrochemical Methods: Fundamentals and Applications," Anti-corros.M., vol.50, No.5, pp.580-632,1980.).
Randles-Sevcik equation, where iPIs the peak current, n is the number of electrons participating in the reaction, A is the electrode area, C is the concentration of the electroactive species, v is the scan rate, D is the diffusion coefficient, where iPIs the peak current and v is the square root of the scan rate. The results show that the Arduino-based electrochemical detection device has good cyclic voltammetry performance, and the results are highly consistent with those of the commercial electrochemical detection device.
4.3 CA experiment
Electrochemical test device the performance of the electrochemical test device according to the invention was compared with that of the electrochemical workstation of CHI660E under the same test conditions, wherein the relevant experiments according to the invention were carried out twice and CHI660E was carried out once. The test conditions were that using a commercial printed electrode, 50uL of buffer was dropped onto the electrode, 5uL of hydrogen peroxide solution was added to the buffer solution at various times, the working electrode was at 0.6V to the reference electrode, and the polarization current was measured. FIG. 10A is a time-current graph of continuous addition of hydrogen peroxide according to the present invention, wherein C1:1.82mM, C2:5mM, C3:10.77mM, C4:18.57mM, C5:28mM, C6:38.75mM, and FIG. 10B is a graph showing a calibration curve for determination of hydrogen peroxide according to FIG. 10A. As shown in fig. 10A and 10B, when the hydrogen peroxide solution was added to the buffer solution, the polarization current increased and stabilized after about 1 minute, where C1, C2, C3, C4, C5 and C6 are the concentration values of the hydrogen peroxide solution at different times, and the polarization current at that time is also taken as the polarization current at different concentrations of the hydrogen peroxide solution, and by comparing the two kinds of test devices, it can be found that the test signal of the electrochemical detection device of the present design highly coincides with the signal of the CHI660E electrochemical workstation. Furthermore, a calibration curve between the polarization current response and the hydrogen peroxide concentration was also plotted, and as shown in fig. 10A and 10B, a good linear relationship between the polarization current response and the solubility of the hydrogen peroxide solution was observed, and the slope was also almost the same as that of the commercial electrochemical detection device. The results show that the CA measurement result of the electrochemical detection device of the design is highly consistent with the measurement result of the commercial electrochemical detection device.
Electrochemical detection device 4.4 Performance comparison with commercial CHI electrochemical workstation
The performance of the electrochemical detection device of the invention and of CHI660E (laboratory instruments),
as shown in table 5, the platform has greater advantages in volume, price and cost than the commercial electrochemical workstation CHI 660E.
Electrochemical detection device
TABLE 5
Electrochemical detection equipment
The above embodiments are only intended to illustrate the technical solution of the present invention and not to limit the same, and a person skilled in the art can modify the technical solution of the present invention or substitute the same without departing from the spirit and scope of the present invention, and the protection scope of the present invention shall be subject to the claims.
Claims (10)
1. An excitation signal generating circuit for realizing a time current method and a cyclic voltammetry method based on Arduino comprises a first analog-to-digital conversion unit, a second analog-to-digital conversion unit and a reverse circuit unit;
each analog-to-digital conversion unit converts the digital signals received by the Arduino into analog signals and respectively sends the analog signals to the reverse circuit unit; the reverse circuit unit generates an excitation signal according to the received analog signal.
2. The excitation signal generating circuit according to claim 1, further comprising a second-order low-pass filtering unit disposed between the first analog-to-digital converting unit and the inverting circuit unit.
3. The stimulus signal generation circuit of claim 2, wherein in the stimulus signal generation circuit, a Vdd terminal of the first analog-to-digital converter and a Vref terminal of the first analog-to-digital converter are connected to a +2.5V power supply, respectively, an a0 terminal of the first analog-to-digital converter is connected to a +5V reference potential, a GND terminal of the first analog-to-digital converter is grounded, a signal input terminal of the first analog-to-digital converter is connected to an Arduino signal output terminal, an output terminal of the first analog-to-digital converter is connected to one terminal of a resistor R1, the other terminal of the resistor R1 is connected to one terminal of a resistor R2 and one terminal of a capacitor C1, respectively, the other terminal of the resistor R2 is connected to one terminal of a capacitor C2 and a positive input terminal of the first operational amplifier, the other terminal of the capacitor C2 is grounded, the other terminal of the capacitor C1 is connected to a negative input terminal of the first operational, the other end of the resistor R3 is connected with one end of the resistor R4 and the negative input end of the second operational amplifier respectively, the Vdd end of the second analog-to-digital converter and the Vref end of the second analog-to-digital converter are connected to a +2.5V power supply respectively, the A0 end of the second analog-to-digital converter and the GND end of the second analog-to-digital converter are grounded respectively, the signal input end of the second analog-to-digital converter is connected to the signal output end of Arduino, the output end of the second analog-to-digital converter is connected to the positive input end of the second operational amplifier, and the other end of the resistor R4 is connected with the.
4. The excitation signal generation circuit of claim 3, wherein the first and second analog-to-digital converters are of a type including MCP4725, and the first and second operational amplifiers are of a type including OP 297.
5. The excitation signal generation circuit of claim 3, wherein the first analog-to-digital converter and the second analog-to-digital converter have different addresses.
6. An electrochemical detection device based on Arduino for realizing a time current method and a cyclic voltammetry method, which comprises an excitation signal generation circuit, a three-electrode system circuit, a test electrode, a signal measurement circuit and a power supply signal circuit, wherein the excitation signal generation circuit, the three-electrode system circuit, the test electrode, the signal measurement circuit and the power supply signal circuit are obtained by adopting the method in any one of claims 1 to 5;
the excitation signal generating circuit generates an excitation signal through the received Arduino digital signal;
the three-electrode system circuit generates a constant voltage signal or a triangular wave voltage signal between a working electrode and a reference electrode of a test electrode through an excitation signal;
the signal measuring circuit converts the collected polarization current in the working electrode and counter electrode loops of the test electrode into digital signals and feeds the digital signals back to Arduino;
the power supply signal circuit supplies power to the excitation signal generating circuit, the three-electrode system circuit and the signal measuring circuit.
7. The electrochemical detection device according to claim 6, wherein in the three-electrode system circuit, one end of a resistor R5 is connected to the output end of the excitation signal generating circuit, the other end of a resistor R5 is connected to the negative input end of a third operational amplifier and one end of a resistor R6, respectively, the positive input end of the third operational amplifier is grounded, the output end of the third operational amplifier is connected to the counter electrode of the test electrode, the other end of a resistor R6 is connected to the negative input end of a fourth operational amplifier and the output end of the fourth operational amplifier, the positive input end of the fourth operational amplifier is connected to the reference electrode of the test electrode, the working electrode of the test electrode is connected to the negative input end of a fifth operational amplifier, one end of a resistor R7, one end of a resistor R8 and one end of a capacitor C3, the other end of a resistor R8 is connected to a-2.5V power supply, the positive electrode input end of the fifth operational amplifier is grounded, and the output end of the fifth operational amplifier is respectively connected with the other end of the resistor R7 and the other end of the capacitor C3; the model of the third operational amplifier, the fourth operational amplifier and the fifth operational amplifier comprises an OP 297.
8. The electrochemical detection device according to claim 7, wherein in the signal measuring circuit, one end of a resistor R9 is connected to an output end of a fifth operational amplifier of the three-electrode system unit, the other end of the resistor R9 is connected to one end of a resistor R10, one end of a capacitor C4 and one end of a capacitor C5 respectively, the other end of a resistor R10 is connected to a positive input end of a sixth operational amplifier, the other end of a capacitor C4 is grounded, the other end of a capacitor C5 is connected to a negative input end of the sixth operational amplifier, an output end of the sixth operational amplifier and an a1 end of a digital-to-analog converter respectively, a Vcc end of the digital-to-analog converter is connected to a +5V reference power supply, an Addr end of the digital-to-analog converter is connected to a Gnd end of the digital-to-analog converter respectively, and an ii; the model of the sixth operational amplifier includes OP 297.
9. The electrochemical detection device of claim 8, wherein the power signal circuit comprises Arduino and operational amplifier circuits, analog-to-digital converter and digital-to-analog converter circuits, and bias power circuit;
in the circuit required by the Arduino and the operational amplifier, one end of a capacitor C6, one end of a capacitor C9 and a Vin end of a first voltage stabilizing chip are respectively connected to a +12V power supply, the other end of a capacitor C6, the other end of a capacitor C9, a Gnd end of the first voltage stabilizing chip, one end of a capacitor C7, one end of a capacitor C8 and an anode input end of a seventh operational amplifier are respectively grounded, a Vout end of the first voltage stabilizing chip is respectively connected with the other end of a capacitor C8 and one end of a resistor R11, the other end of the resistor R11 is respectively connected with one end of a resistor R12 and a cathode input end of the seventh operational amplifier, and an output end of the seventh operational amplifier is respectively connected with the other end of a resistor R12 and the other end of a capacitor C7; the output voltage of the Vout end of the first voltage stabilizing chip is +5V, and the output voltage of the seventh operational amplifier is-5V; the first voltage stabilizing chip comprises an LM 2950;
in the circuits required by the analog-digital converter and the analog-digital converter in the device, one end of a capacitor C10 and the Vin end of a second voltage stabilization chip are respectively connected to +12V voltage, the other end of a capacitor C10, the Gnd end of the second voltage stabilization chip, one end of a capacitor C12, one end of a capacitor C13 and the Gnd end of a third voltage stabilization chip are respectively grounded, and the Vout end of the second voltage stabilization chip is respectively connected with the other end of the capacitor C12, the other end of the capacitor C13 and the Vin end of the third voltage stabilization chip; the output voltage of the Vout end of the second voltage stabilizing chip is +5V, and the output voltage of the Vout end of the third voltage stabilizing chip is + 2.5V; the model of the second voltage stabilizing chip comprises ADR425, and the model of the third voltage stabilizing chip comprises ADR 421;
in the bias power supply circuit, one end of a capacitor C14 and one end of a resistor R13 are respectively connected with a Vout end of a third voltage stabilizing chip, one end of a capacitor C11, the other end of a capacitor C14, one end of a capacitor C15 and the positive input end of an eighth operational amplifier are respectively grounded, the other end of a resistor R13 is respectively connected with one end of a resistor R14 and the negative input end of the eighth operational amplifier, and the output end of the eighth operational amplifier is respectively connected with the other end of a resistor R14, the other end of the capacitor C11 and the other end of a capacitor C15; the output voltage of the output end of the eighth operational amplifier is-2.5V; the eighth operational amplifier includes an OP 297.
10. The electrochemical test device of claim 9, wherein the resistance is a parametrically symmetric resistance and the capacitance is measured by a capacitance bridge.
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