Introduction to Electrochemical Sensors

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Ⅰ Introduction

Electrochemical sensors are made on the basis of ion conduction. According to the formation of their electrical characteristics, electrochemical sensors can be divided into potential sensors, conductivity sensors, electricity sensors, polarographic sensors, and electrolytic sensors. Electrochemical sensors are mainly used to analyze gas, liquid, or solid components dissolved in liquids, the measurement of liquid pH, conductivity, and oxidation-reduction potential.

With the development of electronic technology and detection technology, various types of sensors are increasingly used in various industries. Sensors based on electrochemical principles have been widely used in the detection of harmful gases in chemical, coal, environmental protection, and health departments. Because electrochemical sensors can respond to a variety of harmful gases and are simple in structure and low-cost, they have shown an important position in the detection of harmful gases.

Ⅱ Principles of electrochemical sensors

The so-called electrochemical sensor means that when the measured gas enters the sensor, an electrochemical reaction occurs inside it, thereby converting the measured gas side content into a current (or voltage) signal output. However, even if the sensor is in clean ambient air, the output signal of the sensor is often not zero. Normally, this kind of current signal is called a background current or initial current. This is a randomly changing signal, and its instability is firstly affected by temperature changes and secondly related to the action time. The value is sometimes tens of times larger than the detected signal. Therefore, eliminating the influence of the sensor background current is an important part of improving the measurement accuracy, and we must pay attention to it.

There are many ways to eliminate the influence of the background current of the electrochemical sensor. In some pocket-sized and portable harmful gas detectors, hardware compensation methods are usually used, that is, an appropriate compensation circuit is added to the detection circuit of the sensor.

The electrochemical sensor works by reacting with the measured gas and generating an electric signal proportional to the gas concentration. A typical electrochemical sensor consists of a sensing electrode and a counter electrode and is separated by a thin electrolytic layer.

Electrochemical sensor structure diagram

The gas first reacts with the sensor through the tiny capillary-shaped opening, then the hydrophobic barrier layer, and finally reaches the electrode surface. Using this method can allow an appropriate amount of gas to react with the sensing electrode to form a sufficient electrical signal while preventing electrolytes from leaking out of the sensor.

The gas diffused through the barrier reacts with the sensing electrode, and the sensing electrode can adopt an oxidation mechanism or a reduction mechanism. These reactions are catalyzed by electrode materials designed for the gas being measured.

Through the resistor connected between the electrodes, a current proportional to the concentration of the gas to be measured will flow between the positive electrode and the negative electrode. The gas concentration can be determined by measuring this current. Because of the electric current generated in this process, electrochemical sensors are often called current gas sensors or micro fuel cells.

In practice, due to the continuous electrochemical reaction on the surface of the electrode, the potential of the sensing electrode cannot be kept constant. After a long period of time, it will cause the performance of the sensor to degrade. In order to improve the performance of the sensor, a reference electrode was introduced.

The reference electrode is installed in the electrolyte, adjacent to the sensing electrode. A fixed stable constant potential acts on the sensing electrode. The reference electrode can maintain this fixed voltage value on the sensing electrode. No current flows between the reference electrodes. The gas molecules react with the sensing electrode and the counter electrode is measured at the same time. The measurement result is usually directly related to the gas concentration. The value of the voltage applied to the sensing electrode can make the sensor target the target gas.

Ⅲ Composition of electrochemical sensors

The electrochemical sensor contains the following main components:

Electrochemical carbon monoxide gas sensor structure

A. Breathable membrane (also called hydrophobic membrane): The breathable membrane is used to cover the sensing (catalytic) electrode, and in some cases, it is used to control the molecular weight of the gas reaching the electrode surface. Such barriers are usually made of low-porosity Teflon films. This type of sensor is called a coated sensor. Alternatively, it can be covered with a high-porosity Teflon film, and the molecular weight of the gas reaching the electrode surface can be controlled with a capillary tube. This type of sensor is called a capillary sensor. In addition to providing mechanical protection for the sensor, the membrane also has the function of filtering out unwanted particles. In order to transmit the correct gas molecular weight, the correct membrane and capillary pore size must be selected. The pore size should allow a sufficient amount of gas molecules to reach the sensing electrode. The pore size should also prevent leakage or rapid drying of the liquid electrolyte.

B. Electrode: The choice of electrode material is very important. The electrode material should be a catalytic material that can perform semi-electrolytic reactions over a long period of time. Generally, electrodes are made of precious metals, such as platinum or gold, which react effectively with gas molecules after catalysis. Depending on the design of the sensor, the three electrodes can be made of different materials to complete the electrolysis reaction.

C. Electrolyte: The electrolyte must be able to carry out the electrolysis reaction and effectively transfer the ionic charge to the electrode. It must also form a stable reference potential with the reference electrode and be compatible with the materials used in the sensor. If the electrolyte evaporates too quickly, the sensor signal will weaken.

D. Filter: Sometimes a scrubber filter is installed in front of the sensor to filter out unwanted gas. The selection of filters is limited, and each filter has a different degree of efficiency. The most commonly used filter material is activated carbon. Activated carbon can filter out most chemical substances, but not carbon monoxide. By choosing the right filter material, the electrochemical sensor can have a higher selectivity for its target gas.

Ⅳ Characteristics of electrochemical sensors

Electrochemical sensor

There are many ways to manufacture electrochemical sensors, ultimately depending on the gas to be detected and the manufacturer. However, the main characteristics of the sensors are very similar in nature. Here are some common characteristics of electrochemical sensors:

1. On a three-electrode sensor, a jumper wire is usually used to connect the working electrode and the reference electrode. If it is removed during storage, it will take a long time for the sensor to remain stable and ready for use. Some sensors require a bias voltage between the electrodes, and in this case, the sensor is shipped with a nine-volt battery-powered electronic circuit. It takes 30 minutes to 24 hours for the sensor to stabilize and three weeks to continue to remain stable.

2. Most toxic gas sensors require a small amount of oxygen to maintain normal function. There is a vent hole on the back of the sensor for this purpose. It is recommended to perform a recheck with the manufacturer in applications where a non-oxygen background gas is used.

3. The electrolyte of the battery in the sensor is a water solvent, which is isolated by a hydrophobic barrier, which has the function of preventing the leakage of the water solvent. However, like other gas molecules, water vapor can pass through a hydrophobic barrier. Under conditions of high humidity, prolonged exposure may cause excessive moisture accumulation and cause leakage. In low humidity conditions, the sensor may dry out. The sensor designed to monitor high-concentration gas has a lower porosity barrier to limit the molecular weight of the gas passing through, so it is not affected by humidity. Like the sensor used to monitor low-concentration gas, this sensor has a higher porosity barrier and allows gas molecules to flow freely.

Ⅴ Advantages and disadvantages of electrochemical sensors

1. Advantages of electrochemical sensors

The use of electrochemical sensors can accurately measure some odors, analyze proteins, and can also be used for new drug development and disease treatment. These are all possible. And the detection speed is relatively fast, very simple, convenient. The most important thing is that the cost is relatively low, and it can be used in conjunction with other technologies, so electrochemical sensors have become a research hotspot. Electrochemical sensors are useful in many aspects, such as medicine and industrial analysis.

2. Disadvantages of electrochemical sensors

The main disadvantage of electrochemical sensors is that they have a short lifespan. They usually need to be replaced every one to three years. Of course, this is not all of them. It also depends on the brand and the environment in which it is used. If the protection is good, the lifespan can be extended. of. Also, some brands use electrolytic solutions. If you use an electrolytic solution, you need to replenish it regularly, which is a bit troublesome. However, there are many advantages of electrochemical sensors, so there are still many people who will buy electrochemical sensors.

Ⅵ Application of electrochemical sensors

 1. Humidity sensor

Humidity is an important indicator of the air environment. There is a close relationship between the humidity of the air and the heat of evaporation of the human body. When in an environment of high temperature and high humidity, the human body feels stuffy due to the difficulty of evaporating water from the human body. When in an environment of low temperature and high humidity, the heat dissipation process of the human body is intense. The most suitable temperature for the human body is 18~22℃, and the relative humidity is 35%~65%RH. In environmental and sanitation monitoring, it is often used to measure air humidity with instruments such as wet bulb thermometers and hygrometers, hand-operated wet thermometers, and ventilation and humidity thermometers. In recent years, a large number of literature reports using sensors to measure air humidity. The sensor used to measure relative humidity is coated with piezoelectric quartz crystal, which is made of small quartz crystal by photolithography and chemical etching technology. The AT-cut 10MHZ quartz crystal is coated with 4 substances, which has a high humidity Quality sensitivity. The crystal is a resonator in an oscillating circuit. Its frequency changes with quality. Choosing an appropriate coating, the sensor can be used to determine the relative humidity of different gases. The sensor's sensitivity, response linearity, response time, selectivity, Hysteresis, and service life depend on the nature of the coating chemicals.

2. Nitrogen oxide sensor

Nitrogen oxide is the general term for a gas mixture composed of various oxides of nitrogen, often expressed as NOX. Among nitric oxide, the chemical stability of different forms of nitric oxide is different. The normal air is nitric oxide and nitrogen dioxide with relatively stable chemical properties. Their hygienic significance is more important than other forms of nitric oxide. In environmental analysis, nitrogen oxide generally refers to nitrogen monoxide and nitrogen dioxide. The standard method for monitoring nitric oxide is the naphthalene ethylene diamine hydrochloride colorimetric method. The sensitivity of the method is 0.25ug/5ml. The method conversion coefficient is affected by the composition of the absorption liquid, the concentration of nitrogen dioxide, the gas recovery rate, the structure of the absorption tube, the coexisting ions, and the temperature. The influence of various factors is not completely unified. Sensor measurement is a new method developed in recent years. It is reported in the literature that a new type of gas-sensitive microsensor is obtained by combining a microelectronic integrated circuit with interdigitated gate electrode field-effect transistors and chemically active electron beam evaporation phthalocyanine bronze film, which can selectively detect mg/m3 level of dioxide Nitrogen and Diphenylene Methyl Phosphonate (DIMP).

3. Hydrogen sulfide gas sensor

Hydrogen sulfide is a colorless combustible gas with a special rotten egg odor. It is irritating and suffocating and is harmful to the human body. Colorimetry and gas chromatography are mostly used to determine hydrogen sulfide in the air. The measurement of air pollutants whose content is often as low as mg/m3 is a major application of gas sensors. However, in a short period of time, semiconductor gas sensors cannot meet the sensitivity and selectivity requirements for monitoring certain pollutant gases. The silver-doped thin-film sensor array is composed of four sensors. Through the universal analysis device based on library titration and the signal of the semiconductor gas sensor array, the concentration of sulfur dioxide and hydrogen sulfide are recorded at the same time.

Electrochemical hydrogen sulfide sensor

4. Sulfur dioxide sensor

Sulfur dioxide is one of the main substances that pollute the air. The attempt to detect sulfur dioxide in the air is a regular task of air inspection. Sensors are used to monitor sulfur dioxide. From shortening the detection time to lowering the detection limit, it shows great advantages. A solid polymer is used as an ion-exchange membrane. One side of the membrane contains the internal electrolyte of the counter electrode and the reference electrode, and the other side is inserted with a platinum electrode to form a sulfur dioxide sensor. The sensor is installed in a flow cell and oxidizes sulfur dioxide at 0.65V. The batch indicates the amount of sulfur dioxide. The sensing device has high current sensitivity and good stability. Its response time is short and the background noise is low. The linear range is 0.2mmol/L, the detection limit is 8*10-6mmol/L, and the signal-to-noise ratio is 3. The sensor can not only measure sulfur dioxide in the air but also measure sulfur dioxide in liquids with low conductivity. The gas-sensitive coating of the organic modified silicate thin film sulfur dioxide gas sensor is made by the sol process and spin technology. It has good reproducibility and reversibility for the determination of sulfur dioxide. The response time is less than 20S. The sympathy is small, and it is less affected by temperature and humidity.

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