Synthesis and Characterization of MXene-Modified PNIPAAm Hydrogel for VOC Gas Sensing

MXene, as a novel two-dimensional (2D) material, has gained tremendous attention due to its unique chemical and physical properties. Especially in gas sensing, the large specific surface area and potential for tailoring by means of surface functionalization are very favorable. To date, most of the sensing devices are based on interdigitated electrodes (IDEs) where the metallic conductivity and p-type sensing behavior of the MXene are harnessed [1]. However, the conventional drop-casting method for material fabrication on IDEs generally creates a compact and dense layer of MXene flakes stacked together which reduces the number of active sites and decelerates gas adsorption and desorption.<br/><br/>In our work, we aim at combining the favorable properties of MXene with smart hydrogels to create fully three-dimensional, highly sensitive and selective composite materials specifically tailored for detection of volatile organic compounds (VOCs). The role of the MXene is twofold: (i) Enhancement of gas sorption and swelling response of the hydrogel and (ii) providing a means for direct transduction of the hydrogel’s analyte-dependent swelling-state, constituting a sensing material with integrated transduction.<br/><br/>As a first step, we focused on synthesizing a truly 3D Ti₃C₂T_x bulk MXene by ice-templating and subsequent cation-induced stabilization based on [2]. The gas sorption ability was studied by gravimetric experiments in varying relative humidity from 2% to 100% and different acetone and isopropanol concentrations as test VOCs. In the weighing studies, 3D MXene demonstrated a stable sorption for both analytes without being severely affected by changing humidity. In order to compare the response of our 3D MXene to conventionally drop-cast-fabricated ones, we furthermore synthesized both types on IDEs and tested them in the same gas environments. This revealed a faster response of 3D MXene for both test analytes. Images acquired by scanning electron microscopy (SEM) verified the successful fabrication of a 3D porous MXene. However, the 3D structure oxidized rapidly, leading to material degradation and a significant increase in resistance after few tests.<br/><br/>In a second step, we incorporated the MXene into a smart poly(<i>N</i>-isopropylacrylamide) (PNIPAAm) hydrogel. We have recently demonstrated that this type of smart polymer is well suited for VOC detection and which fabrication steps are necessary to achieve a stable 3D porous structure for a fast response [3].<br/>Composite fabrication was achieved by mixing Ti₃C₂T_x MXene suspension with NIPAAm pregel solution containing poly(ethylene glycol) (PEG) as porogen. Leaching of PEG and subsequent freeze-drying of the swollen gel create a porous structure [3]. In order to explore the gas sorption ability of the composite material, gravimetric measurements were carried out in the same manner as described above.<br/>SEM analysis and energy-dispersive X-ray spectroscopy confirmed the formation of a highly porous composite matrix with homogeneous MXene distribution. However, the weighing results did not indicate an improvement of the gas sorption ability (amount of swelling, time constants) of the MXene-enhanced PNIPAAm composite compared to the unmodified PNIPAAm. On the contrary, the achievable delta-weight change was reduced which we ascribe to the fast degradation of the MXene and its, yet unknown, effects on the hydrogel matrix. This will be studied further in fundamental investigations of the interactions between polymer matrix and MXene in order to combine the favorable properties of both for future applications.<br/><br/>References<br/>[1] K. Deshmukh et al., <i>Coord. </i><i>Chem. Rev.</i>, Bd. 424, S. 213514, 2020<br/>[2] H. Chen et al., <i>ACS Nano</i>, Bd. 14, Nr. 8, S. 10471–10479, 2020<br/>[3] S. Wang et al., <i>Tagungsband 16. Dresdner Sensor-Symposium</i>, pp.109-114, 2022