Editorial Feature

Engineering Gas Sensors with Conducting Polymers

Conducting polymers have conventionally attracted huge interest in gas sensing applications due to their easy synthesis, versatile nanostructure, good environmental stability, and superior sensing capability at room temperature.2

Scientist hold novel flexible material with new properties

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Introduction to Gas Sensors with Conducting Polymers

Conducting polymers like polythiophene (PTh), polyaniline (Pni), polypyrrole (PPy), and poly(3,4-ethylenedioxythiophene) (PEDOT) have been used as active gas sensor layers over the last few decades.

Sensors fabricated using conducting polymers possess many enhanced features compared to metal oxide-based commercially available sensors that operate at high temperatures. For instance, the conducting polymer sensors have shorter response times and higher sensitivities, even at room temperatures.

Lightweight and cost-effective, conducting polymers can be readily made through electrochemical or chemical methods. Their molecular chain structure can be conveniently modified by structural derivations or copolymerization. Additionally, conducting polymers possess good mechanical properties, allowing for simple sensor fabrication.

Moreover, the low-temperature fabrication process, print deposition, and excellent flexibility of conducting polymers facilitate the large-scale development of advanced and flexible gas sensors to detect carbon dioxide, ammonia, carbon monoxide, and volatile organic compounds (VOCs), which are hazardous and toxic to health. Thus, conducting polymers can be used to develop gas sensors that display gas-material interactions at room temperature without requiring a microheater and generate a readout signal instantly upon target gas molecule detection.1,2,3

Properties and Characteristics of Conducting Polymers

The primary chains of conducting polymers consist of alternating double and single bonds, which result in a broad π-electron conjugation. This π-electron conjugation system along the polymer chain can be attributed to the unique physical and chemical characteristics of conducting polymers as the system allows delocalized electronic states' formation, leading to a resonance-stabilized structure of the polymer.

Conducting polymers possess optical and electrical properties similar to inorganic semiconductors or metals while retaining attractive processing advantages and mechanical properties of polymeric materials and synthetic metals.

Specifically, nanostructured conducting polymer-based sensors are used as high-performance signal transducers with improved sensing capability compared to conventional bulk-scale materials owing to their superior physical and electrical properties, multidimensional architectures, easy functionalization with different functional groups for sensing trace amounts of target species, and high surface-to-volume ratios.4

Moreover, the sensor's signal intensity can be improved by controlling the shapes due to the one-directional signal pathway of the conducting polymer nanostructure. However, the low conductivity of pure conducting polymers at less than 10-5 S cm-1 is a major disadvantage. Thus, doping is essential to realize highly conductive polymers.

Conducting polymers are doped by protonation or redox reaction. The oxidation level realized by electrochemical and chemical de-doping/doping mechanisms can lead to a rapid and sensitive response to an analyte at room temperature. Issues like surface modification, size control, morphological design, and conductivity are critical in improving response times and sensitivity.3

Working Principle of Conducting Polymer Gas Sensors

Undoping and doping play a critical role in the gas-sensing mechanism of conducting polymer sensors. The doping levels of conducting polymers are readily regulated at room temperature by chemical reactions with different target gases. Most conducting polymers are undoped/doped through redox reactions.

Doping a conducting polymer with an oxidizing agent adds charge carriers, making it a p-type conductor. Conversely, n-type doping is achieved when doped with a reducing agent. The conducting polymers' doping level is controlled by transferring charge carriers to or from the target gases.

Electron transfer changes the conducting polymers' conductivity and affects the material's work function. For instance, electron acceptors like nitrogen dioxide remove the electron from conducting polymers. The conductivity and doping level of the conducting polymers are improved when these gases react with p-type conducting polymers, while the reverse happens upon detecting an electron-donating gas.2

However, many organic analytes like toluene, benzene, and other VOCs do not react under mild conditions and at room temperature. Although detecting them based on their chemical reactions with sensing polymers is extremely difficult, they have weak physical interactions with the conducting polymers, which involve swelling or absorbing the polymer matrices.

These interactions affect the sensing materials' properties and make the gases detectable. Absorption of analyte molecules on the sensing film surface is used extensively in gas sensing. Additionally, conducting polymers swell in various organic solvents, which is detected using AFM.3

Fabrication Methods and Sensor Design

Different methods have been developed to fabricate conducting polymer films that adapt to various sensing materials and sensor configurations. Electrochemical deposition, dip-coating, spin-coating, the Langmuir-Blodgett technique, the layer-by-layer self-assembly technique, vapor deposition polymerization, drop-coating, and electric field-induced electrochemical polymerization are used to deposit conducting polymer films.

Among these methods, electrochemical deposition is the most suitable for depositing conducting polymer films as the film thickness can be controlled by the total charge that passes through the electrochemical cell during film growth. Additionally, this method allows the deposition of films on patterned microelectrodes.

Conducting polymer-based gas sensor devices are classified into electrochemical device sensors, such as amperometric sensors and potentiometric sensors; electrical device sensors, such as polymer-absorption sensors/chemiresistors; optical device sensors, such as surface plasmon resonance optical sensors; fiber-optic devices, UV-vis and infrared sensors, and mass-sensitive device sensors, such as quartz crystal microbalance and surface acoustic wave sensors, based on signal transduction.

Chemiresistors are the most common sensor type, as they are fabricated conveniently and cheaply. The electrical resistance of a chemiresistor is sensitive to the chemical environment. Thus, thin films, bulk materials, and fibers can be utilized as chemiresistors' sensing elements, while resistances serve as output signals.3,4

Applications in Environmental Monitoring and Industrial Safety

Conducting polymer-based gas sensors are increasingly gaining importance due to their rising application in industrial safety control, security, and online environmental monitoring. These sensors can detect different pollutants like nitrogen oxides, ammonia, and hydrogen sulfide with high reliability, making them suitable for industrial process control and air quality monitoring stations.4

Future Directions and Research Challenges

The parameters influencing the conducting polymer-based gas sensor performance include sensing materials, device fabrication, and working environment. These parameters must be factored in while developing polymer-based sensors to achieve higher sensitivity, quicker response time, and long-term stability. Specifically, more research is required to mitigate the issues of long-time instability and low selectivity in these sensors.3

Overall, conducting polymers are a promising material for gas sensors and advantageous over traditional sensors. They are effective for applications in environmental monitoring and industrial safety.

References and Further Reading

  1. Dam. S. (2022). Future Outlook on Conducting Polymer Gas Sensors [Online] Available at https://www.azonano.com/news.aspx?newsID=38924 (Accessed on 13 March 2024)
  2. Liu, X., Zheng, W., Kumar, R., Kumar, M., Zhang, J. (2022). Conducting polymer-based nanostructures for gas sensors. Coordination Chemistry Reviews, 462, 214517. https://doi.org/10.1016/j.ccr.2022.214517
  3. Bai, H., Shi, G. (2007). Gas Sensors Based on Conducting Polymers. Sensors (Basel, Switzerland), 7(3), 267-307. https://www.ncbi.nlm.nih.gov/pmc/articles/PMC3756721/
  4. L. Torad, N., M. Ayad, M. (2020). Gas Sensors Based on Conducting Polymers. IntechOpen. https://doi.org/10.5772/intechopen.89888

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Samudrapom Dam

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Samudrapom Dam

Samudrapom Dam is a freelance scientific and business writer based in Kolkata, India. He has been writing articles related to business and scientific topics for more than one and a half years. He has extensive experience in writing about advanced technologies, information technology, machinery, metals and metal products, clean technologies, finance and banking, automotive, household products, and the aerospace industry. He is passionate about the latest developments in advanced technologies, the ways these developments can be implemented in a real-world situation, and how these developments can positively impact common people.

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