Salt, e.g., sodium chloride, is essential for numerous biological functions, from fluid balance to hydration. In higher concentrations, however, salt can cause dehydration, which may lead to health issues like high blood pressure and kidney disease. The “salt-out” effect, used in the meat processing industry to draw out moisture from proteins, also serves as a key principle in our salt-assisted assembly technology.

MXenes, two-dimensional (2D) carbides and nitrides of transition metals, have outstanding properties. In 2024, they were listed by IUPAC among its Top 10 Emerging Technologies In Chemistry with a true potential to transform our world. However, 2D flakes must be assembled into films, coatings, or 3D strictures for practical applications. We applied salt-induced dehydration to water-dispersed nanometer-thin MXene flakes, achieving controllable deposition and producing high-quality films. By “salting out” MXene, we enabled the deposition of MXene coatings on various polymer substrates, including hydrophobic or chemically inert ones. This innovation tackles a significant challenge in nanomaterial assembly: attaching hydrophilic MXene to hydrophobic polymer surfaces, where water at the interface typically destabilizes bonding. Adding salts reshapes the interactions among water, MXene, and polymers, making MXene deposition on polymers more energetically favorable. By selecting different salts, we unlocked full control over the dehydration efficiency, assembly speed, and properties of the final product.

This advancement paves the way for MXene-based wearables to be used in extreme environments like intense heat, cold, and radiation, where traditional polymer substrates fall short. For instance, polymers like polyethylene terephthalate (PET) have limited thermal stability, restricting their use in high-temperature environments. While high-performance polymers like polyetheretherketone (PEEK), poly(tetrafluoroethylene) (PTFE), and poly-paraphenylene terephthalamide (Kevlar) offer greater heat resistance and mechanical strength, their hydrophobic or chemically resistant nature makes MXene adhesion difficult. Traditional solutions often involve harsh chemical or physical treatments, such as plasma, which can compromise both MXene and polymer performance. In contrast, our eco-friendly, energy-efficient method maintains the integrity of both materials.

Imagine, for example, a future where you are part of the first generation living on Mars, a planet with extreme temperature fluctuations. You’d need a lightweight, flexible spacesuit that handles all weather conditions. Such a suit, made from high-performance polymers, would provide strength and thermal stability while being flexible and comfortable to wear. An electrically conductive, mechanically strong, and thermally insulating MXene coating could generate heat at low temperatures by joule heating (powered by a small battery), reflect infrared radiation to keep you warm, and protect you from space dust and harmful radiation.

This research was funded by the U.S. National Science Foundation and the Department of Energy, Office of Science. The research team filed one US patent (2023/0286015 A1) in November 2022 and a US provisional patent in January 2024.

For more details, refer to our paper, “Universal Salt-Assisted Assembly of MXene from Suspension on Polymer Substrates.” https://www.nature.com/articles/s41467-024-53840-y