Ti 3 C 2 T x belongs to the family of MXenes, 2D materials with an attractive combination of functional properties suitable for applications such as batteries, supercapacitors, and strain sensors. However, the fabrication of devices and functional coatings based on Ti 3 C 2 T x remains challenging as they are prone to chemical degradation by their oxidation to TiO 2. In this paper, we examine the oxidation of Ti 3 C 2 T x in air, liquid, and solid media via conductivity measurements to assess the shelf life of Ti 3 C 2 T x MXenes. The oxidation of Ti 3 C 2 T x was observed in all the media used in this study, but it is fastest in liquid media and slowest in solid media (including polymer matrices). We also show that the conventional indicators of MXene oxidation, such as changes in color and colloidal stability, are not always reliable. Finally, we demonstrate the acceleration of oxidation under exposure to UV light.

Bendable, stretchable MXene multilayer coatings are deposited onto a wide variety of surfaces, rendering them conductive and responsive.

Inkjet-printable aqueous suspensions of conjugated polymer precursors are developed for fabrication of patterned color images on paper substrates. Printing of a diacetylene (DA)-surfactant composite ink on unmodified paper and photopaper, as well as on a banknote, enables generation of latent images that are transformed to blue-colored polydiacetylene (PDA) structures by UV irradiation. Both irreversible and reversible thermochromism with the PDA printed images are demonstrated and applied to flexible and disposable sensors and to displays.

The rapidly growing fields of noncontact medical diagnosis, noninvasive epidermal sensing, and environmental monitoring bring forward the need for fast humidity sensors. However, achieving a rapid response to dynamic changes in humidity, such as for human respiration, is challenging. This is because the response can be limited by the diffusion of water, the sorption of water in the material, and the sensing method itself. Here, the water sorption and response mechanism for multilayer assemblies made from MXene nanosheets and polyelectrolytes for ultrafast humidity sensing are described. MXenes are a class of two-dimensional transition metal carbides (e.g., Ti 3 C 2 ) possessing hydrophilicity and metal-like conductivity. Herein we show that MXene/polyelectrolyte multilayer films prepared using layer-by-layer (LbL) assembly exhibit response and recovery times exceeding those of most humidity sensors. Quartz crystal microbalance and ellipsometry support the mechanism that, upon changing humidity, water molecules intercalate into (or deintercalate from) the MXene/polyelectrolyte multilayer, resulting in an increase (or a decrease) in the thickness and sheet-to-sheet distance, which then changes the tunneling resistance between MXene sheets. The ultrafast response was further demonstrated by monitoring real-time human respiration using a portable microcontroller for wireless sensing.

As they have been designed to undergo colorimetric changes that are dependent on the polarity of solvents, the majority of conventional solvatochromic molecule based sensor systems inevitably display broad overlaps in their absorption and emission bands. As a result, colorimetric differentiation of solvents of similar polarity has been extremely difficult. Here we present a tailor-made colorimetric and fluorescence turn-on type solvatochromic sensor that enables facile identification of a specific solvent. The sensor system displays a colorimetric transition only when a thin protective layer, which protects the solvatochromic materials, is destroyed or disrupted by a specific solvent. The versatility of the strategy is demonstrated by designing a sensor that differentiates chloroform and dichloromethane colorimetrically and one that performs sequence selective colorimetric sensing. In addition, the approach is employed to construct a solvatochromic molecular AND logic gate. The new strategy could open new avenues for the development of novel solvatochromic sensors.