Polyvinyl Fluoride/Ionic Liquid Blend-Based Sensors for Volatile Organic Compounds Detection

Indoor air pollution is a serious health threat, linked to over 3 million deaths per year. Therefore, monitoring indoor pollutants, in particular volatile organic compounds (VOCs) arising from building materials, is essential to take proper health measures in due time. Among the existing gas sensor technologies, metal oxide semiconductors are at the forefront due to their low operating temperature, high sensitivity and affordability, but still their selectivity and processability continue to be their main drawback, especially when comparing to polymers.¹On the other hand, the performance of polymers for gas sensing is undoubtedly far from being competitive, in particular due to their highly resistive nature.<br/>The use of ionic liquids (ILs) can overcome this technological barrier, as ILs display high ionic conductivity, low volatility and high chemical/thermal stability. In addition, the combination of different IL’s cations and anions enables precise adjustment of their physical-chemical properties for selective gas interaction and improved electrical output.² Nevertheless, most ILs remain liquid at room temperature, challenging their processability. The encapsulation of ILs into polymer matrices can create robust, versatile, and easily manufacturable materials. Among the different polymers, polyvinylidene fluoride (PVDF), an electroactive polymer with high thermal, chemical and radiation resistance, offers a suitable dipolar-ion interaction with IL ions, enabling a high magnitude of response. The combination of PVDF and ILs allows their processing via printing technologies, giving rise to miniaturized and flexible devices at a low cost. Further, these particle-free inks offer several advantages, as they can be easily formulated and printed, preventing clogging issues. Overall, the process provides a platform to develop transparent and cost-effective sensors which can be printed on flexible substrates.³<br/>This study reports on a novel platform for printed robust and flexible gas sensors by exploring the performance of PVDF/ILs sensors and by investigating the physical-chemical interactions between PVDF and ILs. Accordingly, the wettability, structural, thermal, morphological, and electrical properties of the PVDF/IL films were evaluated to investigate the effects of incorporating various loadings of ILs with different anions (TFSI, N(CN)₂, SCN, FeCl₄, NiCl₄, Co(SCN)₄) and similar cations (Bmim, Emim) into the PVDF matrix. In addition, the sensor’s sensitivity and selectivity to humidity and VOCs including ethanol, acetone and isopropanol (IPA) were evaluated.<br/>The variation in capacitance (ΔCp) revealed that the response of the IL alone differs from the response of the same IL when encapsulated in PVDF, indicating that PVDF not only serves as a host but also influences the sensor's sensitivity and selectivity. Additionally, it was found that the key factors affecting the sensor's ΔCp are the film's ionic conductivity, wettability, and the ILs' polarity and volume, rather than the film's morphology. The PVDF/IL sensors’ response yielded maximum sensitivities of 6×10⁻³, 3×10⁻⁴, 9×10⁻⁵ and 9×10⁻⁵ pF/ppm for humidity, ethanol, acetone, and IPA, respectively. The minimum detection limits for these substances were ~200, ~850, ~2.8k and ~650 ppm for humidity, ethanol, acetone and IPA. Thus, our experimental setup can effectively detect IPA and ethanol at relevant concentrations, as IPA becomes harmful above 3k ppm and ethanol begins to irritate the eyes above 1k ppm. Given the promising results of the individual sensors, a cross-selectivity system was implemented to enhance selectivity and provide a fast and accurate sensor capable of promptly detect indoor excessive VOC concentrations, safeguarding human health.<br/><b>1</b> Y. Zhang, et al., RSC Adv 14 (2024) 3044–3051.<br/><b>2</b> B. F. Gonçalves, et al., J. Mater. Chem. A., 12 (2024) 14595–14607.<br/><b>3</b> W. Yang, et al., J. Mater. Chem. C Mater 7 (2019) 15098–15117.