A 3D porous MXene/PNIPAAm hydrogel composite with advanced degradation stability and control of electronic properties in air

This study reports the fundamental investigation of a novel composite material consisting of MXene (Ti$_3$C$_2$T$_x$) and a stimulus-responsive hydrogel (Poly(N-isopropylacrylamide) - PNIPAAm). In contrast to previously reported integration of MXene and hydrogels, the material described here offers a tunable porous structure for volatile organic compound (VOC) sensing that maintains its stability and responsiveness in any kind of gaseous environment and in a dried state. Furthermore, its synthesis is significantly simpler and more efficient compared to other studies on porous MXene composites. The presented study focuses on a fundamental investigation of the synthesized MXene/PNIPAAm composite's properties. Thereby, the fabrication and comparison of pure MXene and composite samples featuring either a compact or a highly porous three-dimensional microstructure, reveal unique properties with respect to: (i) controllable three-dimensional spatial arrangement of MXene instead of the prevalent stacked-sheet structure, (ii) reduction of oxidation-induced degradation of MXene and substantially enhanced stability over the course of three months, and (iii) tunable electronic states in response to gas interactions. Material characterization is conducted by scanning electron microscopy and rheology to assess the microstructural and mechanical properties, and in a chemiresistive measurement setup for determination of electrical properties and the evaluation of the composite's potential for VOC sensing in a gaseous environment with the test analyte acetone. These investigations reveal fundamentally novel material effects and properties that address some of the key MXene-related challenges. Additionally, the interplay between the MXene and the hydrogel enables unprecedented opportunities for enhancing the sensing potential of stimulus-responsive hydrogels, specifically in gaseous environments.