In a recent article published in the journal Sensors, researchers discussed the development of a novel gas sensor for detecting an ammonia (NH3) gas based on a grafted Polyaniline (PANI) film grown on a polyethylene terephthalate (PET) substrate. The sensor aims to improve gas-sensing properties by utilizing a unique PANI graft film structure, enhancing sensitivity and selectivity in NH3 gas detection applications.
Illustrations of (a) graft polymer conformations, (b) grafted PANI chains, and (c) the diffusion of ammonia molecules within the PANI graft chains. Image Credit: https://www.mdpi.com/1424-8220/24/11/3695
Background
Traditionally, gas sensors have focused on complexing PANI with nanomaterials to enhance sensitivity. However, this study highlights the importance of structure-controlled PANI films with large surface areas, such as nanofibers, nanotubes, and networks, in improving gas diffusivity and sensor properties.
The PANI film structure offers advantages in promoting gas diffusion between PANI chains, leading to improved sensing characteristics and mechanical stability.
The Current Study
First, a Polydopamine (PD) layer was formed on the PET film by allowing dopamine monomers to self-polymerize under alkaline conditions. The alkaline environment facilitated the conversion of catechol into a quinone, enabling the formation of covalent bonds with nucleophiles like amines. This step aimed to create a suitable surface for subsequent polymerization.
The PD-coated film was then immersed in a p-phenylenediamine solution to increase the number of amino groups on the surface. These amino groups served as initiation points for the surface-initiated polymerization process, crucial for the subsequent growth of PANI chains.
The aniline monomer was subjected to oxidative graft polymerization according to well-known methods. The PET substrate, which had been modified with amino groups, was submerged in an acidic solution comprising aniline monomer. Subsequently, an aqueous ammonium persulfate (APS) solution was added, and the reaction mixture was stirred at 30 °C for 24 hours to facilitate the growth of PANI chains on the PD surface.
To remove any non-grafted PANI, any deposited PANI that was not chemically bonded to the PET film was subjected to de-doping and treated with N-methyl-2-pyrrolidone (NMP). The NMP dissolves the dedoped PANI, and the chemically bonded graft PANI remains intact.
The PANI-grafted PET film underwent immersion and redoping with sulfuric acid. The film was then washed with acetone and dried at room temperature to ensure its stability and functionality. The sensor was assembled by depositing interdigitated gold electrodes on the PANI film using a vacuum deposition process, thus completing its construction and preparing it for gas-sensing measurements and characterization.
Results and Discussion
The study conducted surface and cross-sectional scanning electron microscope (SEM) observations to investigate the growth process and morphology of the PANI film on the PET substrate. The images revealed the development of PANI chains on the PET substrate, showcasing the successful formation of the PANI film.
The surface SEM results indicated visible roughness after the PD coating, with larger irregularities spreading in a patchy manner post-polymerization of PANI. Cross-sectional SEM observations further confirmed the uneven growth of the PANI layer on the PD-coated gold film sputtered on the silicon substrate, with a calculated degree of polymerization of approximately 120. These findings align with the Atomic Force Microscopy (AFM) images and support the structural integrity and growth of PANI chains on the substrate.
The sensor utilizing PANI film exhibited promising characteristics in detecting NH3 gas. The sensor responded reversibly to NH3 gas at both 30 °C and 50 °C, showcasing operational efficiency at room temperature. The response values and recovery rates indicated successful NH3 gas detection, with response values of 32 at 30 °C and 35 at 50 °C.
The high recovery rates of 89 % at 30 °C and 90 % at 50 °C, along with response times of 7 minutes at 30 °C and 11 minutes at 50 °C, demonstrated the sensor's reliability and efficiency in detecting NH3 gas. The minimal impact of temperature on sensor characteristics around room temperature further validated the sensor's stability and performance in gas-sensing applications.
The study explored the potential of the PANI-based sensor as a flexible gas sensor, highlighting its mechanical strength and adaptability to bending or stretching. Bending affected the response value due to poor contact between the PANI film and electrodes, but the response time and recovery rate remained unaffected, indicating the sensor's structural integrity.
Future improvements may focus on optimizing the sensor structure, including electrode placement, to enhance overall performance. The fabrication technology presented in the study offers a versatile approach for developing flexible sensors with excellent mechanical properties, paving the way for diverse gas sensing applications.
Conclusion
The research presents a novel approach to gas sensing using a PANI-based sensor, demonstrating improved NH3 gas detection performance. The study highlights the potential of the PANI film structure in enhancing gas-sensing properties and offers a versatile method for fabricating gas sensors on various substrate materials. The findings contribute to the advancement of flexible and efficient gas-sensing technologies for diverse applications.
Journal Reference
Matsuguchi M., Horio K., et al. (2024). A Flexible Ammonia Gas Sensor Based on a Grafted Polyaniline Grown on a Polyethylene Terephthalate Film. Sensors, 24(11):3695. https://doi.org/10.3390/s24113695, https://www.mdpi.com/1424-8220/24/11/3695