Polymer Ionic Liquids As Perspective Materials for Chemiresistors and QCM Sensors
Authors: Sipka Richard , Vlcek Jan , Otta Jaroslav , Hruska Martin , Tomecek David , Fitl Premysl , Vrnata Martin 
Published: May 2020

Introduction

Polymerized ionic liquids (PILs) have been reported for the first time in 1998, and a brief overview of their properties can be found e.g. in [1]. Their electrotransport properties are unique (when compared with other organic substances), as they are purely ionic conductors. Moreover, majority of them can be considered as single-ion conductors. PILs are characterized by large capacity to absorb analytes with small molecules – especially CO2 and water. When such absorption, the internal volume of polymer is modified, hence the mobility of ions changes [2]. Such a phenomenon could work as a prospective transducer mechanism in chemiresistors. This property of PILs is very valuable, because namely CO2 and numerous other analytes (whose molecule has neither redox properties, nor dipole moments) cannot be detected on chemiresistors by any other mechanism.
In the field of chemical sensing, PILs are often employed in electrochemical sensors, but much rarely for sorbents in QCM sensors [3]. Surprising still, their applications in chemiresistors are at a pioneering stage [4]. This contribution presents the research of chemiresistors and QCM sensors with sensitive layers based on poly(tetrabutylphosphonium 3-sulfopropylacrylate) marked as poly(P4,4,4,4SPA) and poly(tributyl-octylphosphonium 3-sulfopropylacrylate) marked as poly(P4,4,4,8SPA). The response of prepared sensors to noble gases, hydrogen, carbon dioxide, alkanes and methanol vapours was evaluated by impedance spectroscopy. The purpose was to study the consequences between the molecular weight and/or chemical reactivity of the analyte on one side and the sensing mechanism (modulation of electrotransport parameters of PIL in chemiresistor; modulation of the amount of analyte absorbed in PIL in QCM) on the other side.

Experimental

Both the monomers P4,4,4,4SPA and P4,4,4,8SPA were synthesized as follows: the respective chlorides P4,4,4,4Cl and P4,4,4,8Cl were dissolved in distilled water and subsequently the 1.5 molar equivalent of potassium 3- sulfopropylacrylate was added dropwise; this solution was continuously stirred at 30°C for 18 h. Hence the ion exchange proceeded. Subsequently the water was evaporated in a high vacuum and the crystallic phase was dissolved in ethylacetate and filtered. The monomers were obtained after evaporation of ethylacetate. Subsequently 2% molar equivalent of phenylbis(2,4,6-trimethylbenzoyl)phosphine oxide (initiator of polymerization) and 1% of thimethylolpropane ethoxylate triacrylate (cross-linking agent) in distilled water were added, the resulting mixture was dropped to sensor substrates. For chemiresistors we used glass sensor platforms with gold electrodes and in case of QCM sensors directly quartz crystal with gold electrode was applied. The photopolymerization was carried out by exposition to a source of "white" light Leica LMI-6000 Fiber-Lite for 45 min, thus obtaining final sensors – chemiresistors or QCM with sensitive layers based on two PILs - poly(P4,4,4,4SPA) and poly(P4,4,4,8SPA).
The responses of both chemiresistors and QCM sensors to 10 000 ppm of noble gases (He, Ne, Ar, Kr), carbon dioxide, hydrogen, alkanes (n- penthane, n- hexane, n-heptane) and methanol vapours were tested. As a reference gas we used synthetic air. The impedance measurements of chemiresistors were carried out with GAMRY Reference 600 apparatus in the range of frequencies from 10 mHz to 1 MHz and their results evaluated as Nyquist plots. For QCM sensors the Agilent 4294 impedance analyzer operating in frequency range 1 – 100 MHz was used and the shift of the resonance frequency observed.

Results and discussion

An example of Nyquist plot of the chemiresistor with poly(P4,4,4,8SPA) is presented in Fig.1. The upper part corresponds to sensor exposed to synthetic air and the downer part to sensor in 10 000 ppm of CO2 in synthetic air. The experimental data (blue line) were fitted by Randles circuit (red line). In both plots two characteristic regions can be recognized – low frequency "tail", which can be described as the Warburg impedance and, further, high frequency semicircle attributed to charge transfer phenomena between PIL and the gold electrode of chemiresistor. On exposure to CO2 the value of Warburg impedance (Zw) decreased from (2.43 x 1011 Ω.s1/2) to (2.16 x 1011 Ω.s1/2). Also, on exposure to CO2 the resistance between PIL and gold electrode (corresponding to semicircle diameter) was lowered from 10.1 to 5.9 MΩ.
Amongst the noble gases it was revealed that when detected on poly(P4,4,4,8SPA) the Warburg impedance increases monotonously with increasing molecular (in fact atomic) weight. When expressed relatively and taking the Warburg impedance in krypton Zw (Kr) as 100%, then Zw (He) ≈ 50%, then Zw (He) ≈ 50%, Zw (Ne) ≈ 60%, and Zw (Ar) ≈ 85%. The possibility to detect noble gases by chemiresitors is rather unique.
The exposure of QCM sensors with poly(P4,4,4,4SPA) and poly(P4,4,4,8SPA) to the above mentioned analytes lead to change in resonance frequency by 101 – 102 Hz.

References

[1] N. Nishimura, H. Ohno, 15th anniversary of polymerized ionic ionic liquids, Polymer 55 (2014) 3289—3297. doi:https://doi.org/10.1016/j.polymer.2014.02.042.
[2] J. Yuan, D. Mecerreyes, M. Antonietti, Poly(ionic liquid)s: an update, Prog. Polym. Sci. 38 (2013) 1009–1036. doi:https://doi.org/10.1016/j.progpolymsci.2013.04.002.
[3] X. Jin, L. Yu, D. Garcia, R.X. Ren, X. Zeng, Ionic liquid high-temperature gas sensor array, Anal. Chem. 78 (2006) 6980–6989. doi:10.1021/ac0608669.
[4] E. Marešová, D. Tomeček, P. Fitl et al., Textile chemiresistors with sensitive layers based on polymer ionic liquids: applicability for detection of toxic gases and chemical warfare agents, Sens. Actuators B 266 (2018) 830-840. doi:https://doi.org/10.1016/j.snb.2018.03.157.

Figure 1