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Overview
With the implementation of China’s carbon peak and carbon neutrality policies, environmental quality has received increased attention. Practical environmental monitoring often requires portable instruments capable of continuous, multi-target measurements. Advances in sensor technology help meet these requirements.
Sensor systems generally consist of two parts: the sensing element that interacts with the analyte, and the signal transducer. The transducer converts changes produced by the analyte interaction into electrical or optical signals that instrument electronics can process.
By detection method, sensors include optical and electrochemical types. By reaction mechanism, they include enzymatic biosensors, immunosensors, and chemical sensors. By target medium, sensors are classified as liquid or gas sensors.
1. Sensors for Nitrogen Oxides
Nitrogen oxides are common atmospheric pollutants and major contributors to photochemical smog, posing risks to human health. They primarily originate from combustion of fossil fuels and can form environmentally harmful NO2 under sunlight. Some sensors detect nitrogen oxides by generating bacteria that consume nitrite at an oxygen electrode and calculating NOx concentration from dissolved oxygen changes.
Because the generated bacteria use nitrate as an energy source exclusively, the method shows specificity and is less affected by interference from other substances. Researchers abroad have further refined membrane-based approaches to indirectly measure very low atmospheric NO2 concentrations.
2. Sulfite/Sulfur Dioxide Sensors
Besides nitrogen oxides, sulfur compounds are significant atmospheric pollutants. SO2 contributes to acid rain and acid mist. Traditional SO2 measurement methods can be complex and lack accuracy.
Recent studies show certain sensors oxidize sulfite, consuming oxygen during the oxidation process and producing a measurable decrease in dissolved oxygen that generates a current response. Such sensors can provide rapid and reliable sulfite measurements.
3. Carbon Dioxide Sensors
Carbon dioxide is a primary greenhouse gas. Traditional CO2 monitoring can be affected by interfering ions and volatile acids, leading to inaccurate readings. Researchers including Lu Zhongming developed an indicator-based method capable of accurately measuring CO2 concentrations below 190 ppm. These sensors are compact and low-power, addressing the need for long-term automated monitoring in engineering applications and improving operational efficiency.
4. Sensors for Other Gases
Other common gases such as formaldehyde, methane, and ammonia also affect daily life. These gases are often present at low concentrations and can be difficult to detect. Researchers including Zhu Yantao reviewed formaldehyde sensor technologies, identified existing issues, and forecasted development trends. Other groups have used specific bacteria as sensing matrices to enable continuous monitoring of atmospheric pollutants.
Electrochemical Sensing
1. Characteristics
Electrochemical sensors are relatively insensitive to pressure changes, but internal pressure differentials can damage them, so maintaining pressure balance is essential for normal operation. They are temperature sensitive: readings tend to be higher above 25°C and lower below 25°C. Electrochemical sensors exhibit strong selectivity toward target analytes; sensor type, sample concentration, and target gas influence selectivity. Highly selective, reliable oxygen sensors are less prone to interference than other types.
2. Measurement Principle
Electrochemical sensors detect target gases through chemical reactions that produce an electrical signal proportional to gas concentration. Typical components include a gas-permeable membrane, electrodes, an electrolyte, and filters.
3. Applications
Common electrochemical devices include humidity sensors, nitrogen oxide sensors, and sulfur compound sensors. Traditional humidity instruments include wet-bulb thermometers and ventilated thermometers, while electrochemical humidity sensors offer greater sensitivity. Sensors based on coated quartz crystal microbalance are widely used for humidity measurement: a piezoelectric quartz crystal serves as the resonator, and its resonant frequency changes with mass loading when coated with humidity-sensitive materials; different coatings enable detection of different gases or relative humidity ranges.
SO2 detection can shorten analysis time and improve accuracy. SO2 sensors may use an ion-exchange polymer membrane; one side contains internal electrolytes for working and reference electrodes, while the other side interfaces with a platinum electrode to form an SO2 sensing cell.
Biosensing
1. Characteristics
With advances in biotechnology and electronics, biosensing has grown widely. Biosensors are typically integrated, miniaturized, and highly automated, enabling rapid, effective sample analysis. They are applied in areas such as biopharmaceuticals, environmental monitoring, and the food industry.
2. Detection Principle
Biosensors combine a molecular recognition unit with a transducer. Biological molecules recognize the analyte, and the resulting physical or chemical changes are converted into electrical signals, which an analytical system then processes and displays.
3. Applications
Compared with traditional electrochemical methods, biosensors can offer greater sensitivity. For example, traditional methods for detecting organophosphates and carbamates have minimum detection limits around 400 ng/kg, while enzyme sensors using immobilized aldehyde dehydrogenase can reduce the detection limit to about 9 ng/kg.
Cell-based fluorescence assays using oocytes from a Chinese rodent species have been explored for detecting environmental toxins. Continued development in techniques such as PCR and microarray chips has led to portable, fast, and accurate biochemical analyzers and biosensor chips, expanding the role of biosensing in environmental monitoring.
Fiber-Optic Sensing
1. Characteristics
Fiber-optic sensing evolved from fiber-optic communication technology and is applicable to atmospheric, water quality, and ecological monitoring. It features small size, high sensitivity, strong anti-interference capability, and corrosion resistance, making it suitable for toxic, flammable, or explosive environments. Fiber-optic sensors support real-time online and distributed remote monitoring, meeting demands for rapid, efficient environmental measurements.
2. Detection Principle
Fiber-optic sensors rely on interactions between light and the pollutant or on indirect changes in optical signal properties due to other material interactions. A typical system includes a light source, optical fiber, sensitive element, and signal detection/analysis equipment.
3. Applications
Fiber-optic pH sensors detect water pH using dyed materials that act as colorimetric indicators; these are formed into transmissive or reflective films and analyzed spectrally. Fiber-optic ion sensors combine flow injection analysis, laser-excited fluorescence, fiber-optic conduction, and CCD imaging to rapidly detect copper, zinc, cadmium, and nickel ions. Optical turbidity sensors use laser backscatter to assess decreases in sample transparency. Research in China on fiber-optic sensing began relatively later, but decades of development have produced notable results. For example, the Shandong Institute of the Chinese Academy of Sciences developed a fiber-optic gas sensor and online detection system that uses infrared absorption spectroscopy and optical techniques to detect flammable gases such as methane accurately.
Conclusion
As economies develop and living standards improve, environmental issues have gained broad attention. Monitoring helps assess current conditions and provides technical guidance for remediation and improved living conditions. Ongoing technological advances will continue to optimize environmental sensing technologies and equipment toward faster, more efficient, and more accurate measurements.
