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 the demand for wearable electronic devices increases, interest in small, light, and deformable energy storage devices follows suit. Among these devices, wire-shaped supercapacitors (WSCs) are considered key components of wearable technology due to their geometric similarity to woven fiber. One potential method for creating WSC devices is the layer-by-layer (LbL) assembly technique, which is a "bottom-up" method for electrode fabrication. WSCs require conformal and adhesive coatings of the functional material to the wire-shaped substrate, which is difficult to obtain with other processing techniques such as vacuum filtration or spray-coating. However, the LbL assembly technique produces conformal and robust coatings that can be deposited onto a variety of substrates and shapes, including wires. In this study, we report WSCs made using the LbL assembly of alternating layers of positively charged reduced graphene oxide functionalized with poly-(diallyldimethylammonium chloride) and negatively charged Ti 3 C 2 T x MXene nanosheets conformally deposited on activated carbon yarns. In this construct, the added LbL film enhances capacitance, energy density, and power density by 240, 227, and 109%, respectively, relative to the uncoated activated carbon yarn, yielding high specific and volumetric capacitances (237 F g −1 , 2193 F cm −3 ). In addition, the WSC possesses good mechanical stability, retaining 90% of its initial capacity after 200 bending cycles. This study demonstrates that LbL coatings on carbon yarns are promising as linear energy storage devices for fibrous electronics.
Two-dimensional transition metal carbide and nitride nanomaterials, known as MXenes, exhibit low chemical stability in aqueous environments; they tend to oxidize and react with water molecules, resulting in structural degradation and decreased electrical conductivity. This significantly limits their storage lifetime and potential use in the presence of water, particularly in nanosheet-assembled films for battery electrodes and functional coatings. Here we demonstrate that thermal annealing of Ti3C2T z films at elevated temperatures (∼600 °C) causes changes in the termination distribution as well as the formation of a protective layer of TiO2 on the outermost layer of films. The induced chemical and structural changes during thermal treatment arrest MXene oxidation and enable the MXene films to be stable in aqueous solutions for over 10 months.
Conventional wastewater treatment systems generally require multiple steps and complex procedures to remove aqueous pollutants and oil contaminants from polluted water. Herein, we fabricate an underwater superoleophobic membrane by cross-linking konjac glucomannan on pristine fabrics, demonstrating that the concept of oil–water separation and the principle of aqueous pollutant removal can be integrated. Such biopolymer-modified fabric not only separates oil–water mixtures with high efficiency (up to 99.9%), but also exhibits the intriguing characteristic of removing water-soluble pollutants (including polyaromatic dyes and heavy metal ions). As a proof of concept, the synthetic wastewater purified with biopolymer membranes was used to cultivate and irrigate pinto beans, causing no observable deleterious effect on seed germination and growth. These results further confirm the biocompatibility and effectiveness of biopolymer membranes, offering an encouraging solution to challenges including wastewater treatment and cleanup of oil spills.
Highly pH sensitive polycation/MXene multilayers were assembled, and sensitivity was enhanced by varying the polycation.
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