Basic principles Chemical sensors are mainly composed of two parts: identification system; Conduction or conversion system. The recognition system relates a chemical parameter (usually concentration) of the analyte to the conduction system. It has two main functions: it selectively interacts with the measured object and converts the measured chemical parameters into signals that can be generated by the conduction system. Molecular recognition system is the key factor that determines the whole chemical sensor. Therefore, the main problem of chemical sensor research is the selection of molecular recognition system and how to connect the anti-molecular recognition system with the appropriate conduction system. The conductive system of the chemical sensor receives the response signal of the identification system, and transmits the response signal to the electronic system in the form of voltage, current or light intensity through electrodes, optical fibers or quality sensitive elements for amplification or conversion output, so that the response signal of the identification system is finally converted into a signal that people can use for analysis, thereby detecting the amount of analyte in the sample. Application of chemical sensors in environmental and health monitoring (I) Air detection 1. Humidity sensor Humidity is an important index of air environment, and air humidity is closely related to the heat of vaporization of human body. When the temperature is high and the humidity is high, people feel sultry because the water is difficult to evaporate, while when the temperature is low and the humidity is high, the heat dissipation process of the human body is intense, which is easy to cause colds and frostbite. The optimum temperature for human body is 18~22℃ and the relative humidity is 35% ~ 65% RH. In environmental and health monitoring, wet bulb thermometer, manual wet thermometer and ventilation wet thermometer are often used to measure air humidity. In recent years, a large number of documents have reported the measurement of air humidity with sensors. A relative humidity sensor with a coated piezoelectric crystal is made of a small transistor by photolithography and chemical etching. AT-cut 10MHZ crystal is coated with four substances, which has high quality sensitivity to humidity. Crystal is a * * * oscillator in oscillator circuit, and its frequency varies with mass. The sensor can be used to measure the relative humidity of different gases if a suitable coating is selected. The sensor has sensitivity, linearity and response time. 1986, ErbenUwe [Germany] proposed a sensor for measuring humidity and obtained a patent. The sensor adopts a metal-insulator-semiconductor (MIS) structure based on silicon. The MIS structure is coated with silicon dioxide and a humidity-sensitive layer, and the material of the humidity-sensitive layer contains metal oxides, oxides and polymers with low polarity components. The water absorption of humidity-sensitive materials is related to the change of relative dielectric constant of each wet material. The sensor has two measuring methods: alignment method and support method, but the former is more convenient and labor-saving than the latter, and can monitor, control and adjust humidity in air conditioning system, construction site and daily living environment. Chinese scientists and technicians have developed a universal humidity controller with good stability and convenient adjustment by using the newly developed tantalum oxide film humidity sensitive capacitor. This kind of sensor can be used for air humidity monitoring in many occasions, such as constant humidity box, computer room, anti-humidity machine and so on. It is a universal humidity sensor with high cost performance. Someone developed a humidity sensor with very reliable performance by using the humidity sensitivity of phosphate coating. Its main electrode is a stainless steel wire with a diameter of 0.4~ 1.0mm, and its surface is coated with a phosphating film. A layer of gold-plated wire is wound around the film as the counter electrode of the main electrode, and there is only a coating with a thickness of 20~50um between the two electrodes, which is much smaller than the general humidity sensor, so the response speed is improved. Various humidity sensors with different characteristics can be made by replacing the phosphate coating. During the working process of the sensor, ions generated by moisture adsorbed on the surface of phosphate coating move back and forth between electrodes, which leads to the change of conductivity, thus showing humidity sensitivity. If an AC load is applied to the sensor element, air humidity can be measured by detecting a change in impedance. The sensor is small in size, can be sealed in the gap of syringe needle, and can be inserted into a narrow measuring part, which is convenient to use and can be used for dew point measurement. At present, more than 30 companies in Japan manufacture and sell humidity sensors and humidity measurement and control instruments. There are many kinds of temperature sensors, and the humidity-sensitive materials used are electrolyte ceramics and organic polymer films. Most of them have high detection accuracy, simple structure, ultra-miniaturization and integration. 2. NOX sensor Nitrogen oxides are the general name of gas mixtures composed of various oxides of nitrogen, often expressed as NOx. Among nitrogen oxides, the chemical stability of different forms of nitrogen oxides is different. Nitric oxide and nitrogen dioxide, which are relatively stable in chemical properties, are common in the air, and their sanitary significance is more important than other forms of nitrogen oxides. In environmental analysis, nitric oxide generally refers to nitric oxide and nitrogen dioxide. The standard method for monitoring nitrogen oxides in China is naphthyl ethylenediamine hydrochloride colorimetry with a sensitivity of 0.25ug/5ml. The conversion coefficient of the method is influenced by many factors, such as the composition of the absorption liquid, the concentration of nitrogen dioxide, the gas production speed, the structure of the absorption tube, the ions stored in the * *, the temperature and so on, and it is completely unified at present. Sensor measurement is a new method in recent years. It is reported in the literature that a new type of gas-sensitive microsensor is obtained by combining the microelectronic integrated circuit of interdigital gate electrode field effect transistor with chemically active electron beam evaporation of copper phthalocyanine thin film, which can selectively detect nitrogen dioxide and DIMP at mg/m3 level. It uses voltage pulse to excite the sensor and measure the response in time domain and frequency domain. The measured peak shape is related to the normalized differential Fourier transform spectrum, which can clearly distinguish the responses of nitrogen dioxide and DIMP, and each peak area can correspondingly reflect the sensitivity of the sensor to a specific gas concentration. Scientists and technicians have studied a high-frequency surface acoustic wave (SAW) gas sensor with a working frequency of 600MHZ. The device consists of three independent SAW delay lines, which are the frequency measuring elements of the oscillator circuit. An organic film is coated on the surface of SAW delay line as a gas adsorbent. The film is a lead phthalocyanine film with a thickness of 1~ 15nm or an LB(Langmuir-Blodgett) film composed of soluble iron phthalocyanine derivatives. In the process of adsorption, the film quality increases, which leads to the decrease of surface wave velocity and oscillation frequency, thus achieving the purpose of measuring nitrogen dioxide concentration. Tin is deposited at a temperature above the melting point, while cadmium is deposited at room temperature. Tin dioxide films doped with cadmium 1%~6% can be prepared by a new method of heating evaporation. When the film is slowly oxidized at 520℃, polycrystals of tin dioxide and cadmium oxide are formed, and low concentrations of nitrogen oxides and nitrogen dioxide are adsorbed on the surface of the film. At 300℃, the membrane has the highest sensitivity to 10g/m3 of nitric oxide and nitrogen dioxide, and the relative change percentages are 10000% and 400% respectively. Under the same conditions, the sensitivity to 0.0 1% carbon monoxide, methane, butane and hydrogen in the air is below 300%. The conductivity of semiconductor cyanine film has excellent sensitivity to electron acceptor gas, which provides a theoretical basis for manufacturing nitrogen dioxide sensor system with low cost, low energy consumption and small volume. However, when used in sensors, such films also have disadvantages, such as slow response and reversible decline in response under wet conditions. Therefore, WilsonA et al. developed a microprocessor-controlled sensing system. By controlling the sampling and sensor working conditions, the system can obtain a reproducible dynamic process, thus reducing the influence of the above shortcomings to the lowest point. 3. Hydrogen sulfide gas sensor Hydrogen sulfide is a colorless flammable gas with a special smell of rotten eggs, which is irritating, suffocating and harmful to human body. At present, the determination of hydrogen sulfide in air mostly adopts colorimetry and gas chromatography. It is one of the main applications of gas sensors to measure air pollutants whose content is often as low as mg/m3, but semiconductor gas sensors can not meet the requirements of sensitivity and selectivity for monitoring some polluted gases in a short time. He proposed to use silver-doped thin film sensor to monitor hydrogen sulfide in laboratory and urban air. The sensor array is composed of four sensors, and the concentrations of sulfur dioxide and hydrogen sulfide are recorded simultaneously through the signals of the universal analysis device based on library titration and the semiconductor gas sensor array. The practice shows that the silver-doped thin film sensor with attenuation at the constant temperature of 65438 050℃ is effective in monitoring the hydrogen sulfide content in urban air. Yomogoe N has improved and studied the semiconductor gas sensor, which overcomes its shortcomings in detecting hydrogen sulfide and other gases. He improved the sensing performance of the tin dioxide semiconductor gas sensor by controlling the basic factors affecting the receiving and conversion functions. He found that the transfer function is closely related to the microstructure of elements, such as the particle size of tin dioxide (D) and the thickness of surface space charge layer (L). When D≤2L, the sensitivity of the sensor is greatly improved. The introduction of other receptors on the surface of tin dioxide greatly improves the receptor function of the sensor, especially when silver and palladium are used as cocatalysts, the oxide formed in the air interacts with the surface of tin disulfide, resulting in the substantial problem charge of insufficient electrons, which greatly improves the sensitivity of detecting gas. Using CaO-SnO2 element, hydrogen sulfide in air can be detected very sensitively. 4. Sulfur dioxide sensor Sulfur dioxide is one of the main substances that pollute the air, and trying to detect sulfur dioxide in the air is a routine work of air detection. Use sensors to monitor sulfur dioxide. From shortening the detection time to reducing the detection limit, it shows great advantages. A solid polymer is used as an ion exchange membrane. One side of the membrane contains internal electrolyte of a counter electrode and a 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 a voltage of 0.65V V. Indicates the amount of sulfur dioxide. The sensing device has high current sensitivity. Short response time, good stability, low background noise, linear range of 0.2mmol/L, detection limit of 8* 10-6mmol/L and signal-to-noise ratio of 3. The sensor can not only determine sulfur dioxide in air, but also be used to determine sulfur dioxide in low conductivity liquid. The gas sensitive coating of organic modified silicate thin film sulfur dioxide gas sensor is made by sol method and spin coating technology, which has good reproducibility and reversibility for the determination of sulfur dioxide, response time is less than 20S, resonance with other gases is small, and it is less affected by temperature and humidity. Xue et al., Changchun Institute of Applied Chemistry, Chinese Academy of Sciences, have successfully developed a small sulfur dioxide concentration sensor with a wide detection range, which can be used to assemble a small, portable and cheap pickup sulfur dioxide gas concentration detector. It can be used to directly detect the concentration of sulfur dioxide gas on site without separate sampling. The sensor adopts the principle of controlling electrode electrolysis. The gas to be measured is oxidized at a certain control potential on the working electrode of the sensor. When the potential control is positive enough and the catalytic activity of the electrode is high enough, the oxidation reaction proceeds rapidly. The total speed of the process is determined by the step of sulfur dioxide diffusion, and the signal current generated is proportional to the concentration of sulfur dioxide. The sensor responds quickly, and the response time is less than 30S. In a wide range of sulfur dioxide concentration, it has a good linear relationship and a small linear error.
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