Introduction to VOC Sensor

What is VOC?

VOC, namely Volatile Organic Compounds. The U.S. Environmental Agency (EPA) defines VOC as: in addition to carbon monoxide, carbon dioxide, carbonic acid, metal carbides, carbonates, and ammonium carbonate, any carbon-containing compound that participates in photochemical reactions in the atmosphere.

On average, each person inhales about 15kg of air per day, 80% of which is indoor air. Outdoor air quality is generally monitored by government agencies, while indoor air quality (IAQ) monitoring is the responsibility of building operators or occupants-provided they are willing to actually implement it. Now, a new generation of small surface-mount low-power volatile organic compound VOC sensor has been launched. Distributed and local IAQ monitoring functions can be realized through small, low-cost components.

The hazards of VOC

When VOC reaches a certain concentration in the room air, people will start to feel headaches, nausea, and limb weakness. If they continue to stay for a long time, they will damage the liver, kidney, brain, and nervous system, and may even cause convulsions, coma, and memory loss.

VOC is so serious, but where does it come from? Experts pointed out that building materials such as walls, ceilings, and floors, as well as decorative materials such as latex paint, wallpaper, thermal insulation materials, and adhesives, are the main sources of VOC. It needs to be pointed out that: The VOC of latex paint most commonly used in decoration mainly comes from the VOC of raw materials, including formaldehyde, ammonia, ethylene glycol, etc., so it is very important to control the VOC content of raw materials!

The working principle of the VOC sensor

The VOC sensor itself can detect a variety of VOCs and provide a relative output corresponding to changes in the VOC concentration. When equipped with an onboard processor, the VOC sensor can also calculate the equivalent relative value of a variety of VOCs. Since the output of these components is relative, calibration is not required.

VOC sensor principle

In addition, there is a type of absolute output gas sensor that is ideal and necessary. In these applications, high concentrations of certain gases pose a direct threat to life or health. This kind of absolute output element is usually: relatively expensive; it can only detect one kind of gas; it needs to be calibrated regularly to provide accurate output data.

The VOC sensor is a supplement to this important but limited absolute measurement source: this sensor can detect a variety of VOCs, so it can be used to detect changes in indoor air quality caused by one or more VOC compounds.

In IAQ monitoring applications, the VOC sensor can be used with the absolute output CO2 sensor to provide an exact reference for the CO2 concentration at any time. The VOC sensor enhances the measurement of absolute CO2 and collects other data related to VOC events. These data are not necessarily directly related to the occupants (usually the main reason for the increase in CO2 concentration), as shown in the figure below.

IAQ monitoring

In the figure, during the period when the VOC sensor indicates that the air quality is degrading, the CO2 sensor did not move at all. This may be caused by the use of cleaning chemicals during the recess of the meeting or the emissions from equipment such as printers and copiers. In response to the output of the VOC sensor (not the indication of the CO2 sensor), there is usually better ventilation. Therefore, in this example, during the VOC event, the air quality in the room will be improved for the occupants.

Classification of VOC sensors

Common VOC sensors are mainly divided into three categories according to their working principles: electrochemical gas sensors (such as resistance, current, impedance, potential, etc.), optical sensors (including spectral absorption, fluorescence, visualization, etc.), and mass gas sensors (such as quartz crystal microbalances and surface acoustic wave gas sensors), etc.

According to the gas-sensitive materials, it can be divided into semiconductor metal oxide materials, organic polymer materials, inorganic-organic composite materials and so on. In recent years, the development trend of gas sensors is miniaturization, intelligence, and multi-function.

1. Electrochemical VOC sensor

The detection principle of the electrochemical VOC sensor is that the VOC gas and the surface of the gas-sensitive material produce adsorption or reaction (physical adsorption or chemical adsorption), thereby causing changes in its electrical properties (such as resistance, current, impedance, potential, etc.).

Among them, the conductivity type VOC sensor based on semiconductor metal oxide is the most widely used, and it occupies an important position in the current gas sensing field. According to the gas electrical detection device, it can be divided into the common two-electrode conductivity type detection system and the three-electrode field-effect tube detection system. According to VOC electrical gas-sensitive materials, they can be divided into semiconductor metal oxides, conductive polymers, nanomaterials (typical nanomaterials such as zero-dimensional gold nanoclusters, one-dimensional carbon nanotubes or silicon nanowires, and multidimensional graphene, etc.), and porous materials.

(1) Semiconductor metal oxide conductivity sensor

The semiconductor metal oxide gas sensor uses the characteristic that the resistance or the work function of the semiconductor changes when it contacts the gas to realize the detection of the gas. The semiconductor sensor is the earliest gas sensor.

As early as 1936, it was discovered that the conductivity of Cu20 changed after adsorbing water vapor. So far, the semiconductor gas sensor has developed into a large system due to its simple structure, rapid sensitivity, low cost and stability, and simple circuit. Among them, the research of ZnO and Sn02 is the most mature.

However, the disadvantages of semiconductor metal oxide gas sensors are their high operating temperature, poor gas selectivity, and prone to poisoning. Therefore, some new types of metal-organic compounds and heavy metal-doped semiconductor gas sensors have been developed and applied.

(2) Zero-dimensional nanomaterial conductivity sensor

As we all know, nanostructures are very sensitive to chemical environments and can be used as ultra-sensitive gas sensing materials. Zero-dimensional gold nanoclusters have attracted widespread attention in the field of sense due to their special physical and chemical properties.

Gold nanoclusters not only have the quantum dot behavior of zero-dimensional nano-sized metalcore but also can interact with ligands on the surface. The inner gold provides a conductive channel for electrons, and the outer organic shell serves as an insulating layer to provide a selective adsorption interface for VOC. After the adsorption of VOC, the single-layer gold nanocluster expands and the distance between the gold cores is increased, which causes the conductivity to decrease and the resistance value to increase. The spraying method is usually used to deposit a single layer of gold nanoclusters on the integrated electrode.

The electrical response characteristics of single-layer gold nanoclusters to VOC are not only related to the change in electronic conductivity between gold nuclei after VOC adsorption, but also to the activation energy. The activation energy is related to the charging process between the gold nanoclusters and is closely related to the dielectric constant of the VOC.

(3) Conductivity gas sensor based on nanoporous materials

Porous materials tend to have good gas adsorption capacity due to their own structural characteristics. For example, due to its huge specific surface area and micro-nano size effect, nanoporous silicon photonic crystals have a good adsorption capacity for VOC gas. At the same time, porous silicon has good optical and electrical properties and shows strong advantages in the field of VOC sensors.

(4) Conductivity sensors based on polymer materials

Conductive polymer materials not only have the electrical and optical properties of metals and semiconductors, but also have the flexibility and mechanical properties of organic polymers, as well as electrochemical redox properties, so they are often used as gas-sensitive materials in the sensor field.

Conductive polymer gas-sensitive materials mainly include phthalocyanine polymer, polypyrrole, polyaniline, porphyrin and metalloporphyrin complexes, and other conjugated polymer materials, which can generate gain and loss electron relationships with adsorbed gas molecules and cause changes in doping level and physical properties. That makes the resistance or work function of the conductive polymer gas-sensitive material respond to the adsorbed gas.

However, for most VOC gases, it is difficult for chemical reactions based on electron transfer to occur with conductive polymer gas-sensitive materials, but weak physical interaction forces. The principal component analysis method is used to analyze the cross-response results to realize the identification and differentiation of VOCs.

2. Optical VOC sensor

The gas sensor based on optical signal has the advantages of strong resistance to electromagnetic field interference, fast and sensitive, and easy to realize the online monitoring mode of organic gas.

According to the working principle, the types of optical sensors include reflection interferometry, ultraviolet-visible absorption photometry, visualization method based on color change, fluorescence method, surface plasmon resonance method, and optical fiber sensing technology. Optical gas-sensitive materials include traditional porphyrins and metalloporphyrins, fluorescent dye molecules, pH indicators, and new biomimetic photonic crystals.

(1) Sensor based on the principle of light absorption

The spectral absorption gas sensor detects the V0C gas based on the intensity or displacement change of the absorption spectrum of the gas-sensitive material after the gas is adsorbed. Commonly used gas-sensitive materials include pH indicators, lyotropic dyes, and metalloporphyrins.

(2) Visual sensor based on color change

The visual gas sensor is a new type of optical sensing technology, and it is also one of the important trends in the development of sensor technology. The characteristic information of smell is represented in the form of images, also known as visual smell.

Compared with traditional electrochemical and fluorescent sensing signals, this colorimetric signal output mode is the simplest sensing platform for developing naked-eye detection technology, minimizing the need for signal conversion equipment modules. It can provide on-site actual testing for non-technical personnel or end-users. The sensor materials that have been reported for VOC visualization include polydiacetylene paper chips, methyl yellow nylon 6 nanofibers (iv), Fabry interference type microporous polymers, and supramolecular host-guest complexes.

(3) VOC sensor based on the principle of light interference

Photonic crystal (CP for short) is a dielectric material whose refractive index changes periodically in space, and its change period is the same order of magnitude as the wavelength of light. The main feature of photonic crystals is the existence of photonic conduction band and photonic forbidden band in its energy band spectrum, which is also called periodic artificial microstructure with photonic bandgap (abbreviated as PBG).

When electrons in a semiconductor material propagate in the periodic potential field of the crystal lattice, an energy band structure is formed due to Bragg scattering, and a bandgap appears between the bands. If the energy of the electron wave falls in the bandgap, propagation is prohibited.

Similar to the modulation of the electronic wave function in a semiconductor lattice, the refractive index of light in a photonic crystal changes periodically, and a band gap structure of light also appears when electromagnetic waves propagate in it. Light waves with energy in the photonic band gap are forbidden to propagate. In principle, people can control the motion behavior of photons by designing and manufacturing photonic crystals and their devices, which is of great significance in the development of various optical devices, optical fiber communications, and photonic computers. Simply put, the photonic crystal has the function of filtering, which can selectively let the light of a certain wavelength pass through and block the light of the other wavelengths.

(4) VOC sensor based on the principle of fluorescence emission

Fluorescent gas sensors are a major development in analytical chemistry. They have the characteristics of high sensitivity, good selectivity, and strong ability to resist electromagnetic interference. However, they often have problems such as difficult labeling and poor repeatability. The external environment such as humidity, polarity, pH, etc. of the fluorescent molecule will affect its structure, three-dimensional conformation, and fluorescence efficiency, thereby affecting the shape and intensity of its fluorescence spectrum.

(5) VOC sensor based on the principle of surface plasmon resonance

Surface Plasmon Resonance (SPR) is a physical optical phenomenon of the evanescent field. It is an evanescent wave that penetrates into the metal film when light undergoes total internal reflection at the interface of glass and metal film, which can trigger free electrons on the metal surface to generate surface plasma waves.

When the incident angle or wavelength is an appropriate value, the frequency and wavenumber of the surface plasmon wave and the evanescent wave resonate, the incident light is absorbed, and a resonance peak appears in the reflection spectrum. The gas adsorbs on the surface of the metal film to change its thickness or refractive index so that its resonant peak (resonance angle or resonant wavelength) changes. Surface Plasmon Resonance Technology (SPR) is a new type of gas detection method, which has the advantages of simple structure, high sensitivity, and wide detection range.