Metal Ion-Driven Gelation in MXenes for Multiproperty and Multifunctional Hydrogels and Aerogels

To date, MXenes include over fifty different structures and compositions, with more than a hundred compositions predicted computationally (more than a thousand if surface terminations are also taken into account), being the largest known family of 2D materials. Owing to their extremely versatile structural and chemical composition, as well as surface chemistry, MXenes are one-of-a-kind materials in many research fields, embodying a true revolution in materials science. They introduce a large number of 2D building blocks in materials science, providing an exciting and wide-ranging portfolio of physicochemical properties and promoting the production of several MXenes-based hybrid systems for numerous applications.<br/>Here, we will discuss the production of all-MXene hydrogels and aerogels, where the gelation process is induced by metal ions. Taking advantage of the coordination chemistry between -OH groups on the surface of MXene flakes (resulting from the wet chemical etching steps) and metal ions, the gelation (in aqueous environment) is very fast (&lt; 5 min) and the physicochemical properties of the resulting hybrid systems are strongly related to the nature of the MXene/metal ion dyad. Rheological measurements show a strong relationship between the metal ion and the loss tangent <i>tan</i><i>δ</i> (equal to the ratio between loss Modulus and storage Modulus, namely viscous modulus and elastic modulus) of the related hydrogel, where the main parameter seems to be represented by the charge density of the selected ion. Such a dependence is also observed in the surface area of corresponding aerogels (prepared from hydrogel by means of freeze-drying), where metal ions with higher charge density return lower surface area, glimpsing the chance to tune the latter parameter based on the envisaged final applications. In this regard, pore shape and aerogel structure are strongly affected by the chemical nature of the metal ions too, as a result of the different coordination geometry. Considering the extremely high versatility of such all-MXene hydrogels and aerogels (in terms of chemical and structural composition, as well as physicochemical properties), many different applications are being tested. For instance, the different and tunable electrostatic behavior of MXene aerogels could result crucial to produce gas and/or particle capture and sensing systems, whose regeneration <i>via</i> Joule heating might be possible because of their high electrical conductivity and versatile thermal properties. Also, the metal ions trapped within the MXene porous structure can be converted to metal clusters by simple thermal annealing (performed at 300°C, under Ar/H₂), allowing the formation of ferromagnetic particles for electromagnetic sensing and other related applications. Moreover, due to their tunable porosity, mechanical properties, and composition, MXene aerogels are being tested for acoustic insulation/damping and sensing as well. Finally, thanks to the abovementioned gelation process, MXene hydrogels and aerogels can be shaped in many forms, including fibers that might be use for bioelectronics and sensing. Finally, this versatile approach can be employed for several metal ions and MXene structures, leading to the production of hybrid systems with tunable properties according to envisaged applications.