Ultrastable Organic Anode Enabled by Electrochemically Active MXene Binder toward Advanced Potassium Ion Storage.

Conjugated carbonyl compounds are regarded as promising organic anode materials for potassium ion batteries (PIBs) due to their rich redox sites, excellent reversibility, and structural tunability, but their low electrical conductivity and severe solubility in organic electrolytes have substantially restricted their practical application. Herein, 2D MXene is utilized as an electrochemically active binder to fabricate perylene-3,4,9,10-tetracarboxylic dianhydride (PTCDA) electrodes for high-performance PIBs. MXene, coupled with Super-P particles, served as a binder and conductive matrix to facilitate rapid ion and electron transport, restrain the solubility of PTCDA, promote potassium adsorption, and alleviate the volume expansion of PTCDA during potassiation. Consequently, the PTCDA electrode bonded by the MXene/Super-P system delivers excellent potassium storage performance in terms of a high capacity of 462 mAh g-1 at 50 mA g-1, superior rate capability of 116.3 mAh g-1 at 2000 mA g-1, and stable cycle performance over 3000 cycles with a low capacity decay rate of ∼0.0033% per cycle. When configured with the PTCDA@450 cathode, an all-PTCDA potassium ion full cell delivers a maximum energy density of 179.5 Wh kg-1, indicating the superiority of MXene as an electrochemically active binder to promote the practical application of organic anodes for PIBs.

Ultralong Stability of Ti3 C2 Tx -MXene Dispersion Through Synergistic Regulation of Storage Environment and Defect Capping with Tris-HCl Buffering.

Aqueous MXene dispersion suffers from a bottleneck issue of oxidation, leading to its gradual deterioration and ultimately compromised physicochemical characteristics. Herein, Tris-HCl buffer is employed to stabilize the diluted Ti3 C2 Tx -MXene dispersion (0.05 mg mL-1 ) through the synergy of its potent pH-regulation capability and capping effect toward oxidation-susceptible defects/edges. Tris-HCl functionalized Ti3 C2 Tx maintained its original morphology, structure, and favorable dispersity even after 150 days of aging under naturally aerated conditions. The pH-regulation nature of Tris-HCl is elucidated through solution monitoring of Ti3 C2 Tx dispersion, while the adsorption of Tris-HCl onto defects/edges is revealed by spectral analysis and multi-scale simulations. Tris-HCl at the neutral pH can bind to the negatively charged basal plane of Ti3 C2 Tx via + HTris moiety, while the other moiety (Tris) interacts with the exposed edge-based Ti atoms and/or intrinsic defects, forming a Ti─N bond that prevents MXene from attack by H2 O and O2 . Besides, Tris-HCl stabilized Ti3 C2 Tx exhibited nearly identical capacitive characteristics to its freshly-etched counterpart, indicating the minimal impact of Tris-HCl on electrochemical performance of Ti3 C2 Tx during long-term storage. This study provides practical guidance for stabilizing MXene in their native aqueous dispersion without compromising the inherent properties.

Achieving Stable and Ultrafast Potassium Storage of Antimony Anode via Dual Confinement of MXene@Carbon Framework.

Antimony-based anode materials are recognized for their high potassium storage capacities and appropriate operating potentials. However, the large volume expansion of Sb during the potassiation/depotassiation process, which results in a quick capacity decay, severely limits its practical application in potassium-ion batteries (PIBs). Here, a carbon-coated Sb/MXene heterostructure composite (CSM) is synthesized by adsorption of Sb3+ on MXene nanosheets via Sb-O-Ti bonds followed by carbothermic reduction to construct dual-confined MXene@carbon conductive framework capable of withstanding high volume expansion of Sb and conducive to enabling accelerated electron transfer kinetics. The CSM composite, particularly CSM-700, when configured as an anode for PIBs, realized high capacity (484.4 mAh g-1 at 0.1 A g-1 ), an ultra-stable cycling performance with a high reversible capacity of 435.9 mAh g-1 at 0.1 A g-1 after 100 cycles corresponding to a capacity retention rate of 90.0%, and superior rate performance of 323.0 mAh g-1 at 1 A g-1 . The proposed strategy offers a simple route to construct high-performance Sb-based anodes for advanced PIBs.

Progression in the Oxidation Stability of MXenes.

MXenes are under the spotlight due to their versatile physicochemical characteristics. Since their discovery in 2011, significant advancements have been achieved in their synthesis and application sectors. However, the spontaneous oxidation of MXenes, which is critical to its processing and product lifespan, has gotten less attention due to its chemical complexity and poorly understood oxidation mechanism. This perspective focuses on the oxidation stability of MXenes and addresses the most recent advancements in understanding and the possible countermeasures to limit the spontaneous oxidation of MXenes. A section is dedicated to the presently accessible methods for monitoring oxidation, with a discussion on the debatable oxidation mechanism and coherently operating factors that contribute to the complexity of MXenes oxidation. The current potential solutions for mitigating MXenes oxidation and the existing challenges are also discussed with prospects to prolong MXene's shelf-life storage and expand their application scope.

Effect of Sodium Dodecyl Sulfate on Stability of MXene Aqueous Dispersion.

MXenes suffer from severe oxidation and progressive degradation in aqueous media due to its poor chemical stability. Herein, sodium dodecyl sulfate (SDS) is employed as an efficient protectant for long-term storage of Ti3 C2 Tx -MXene aqueous dispersion. Experimental data support SDS's capability to protect oxidation-prone sites on Ti3 C2 Tx nanosheets, providing extended colloidal stability of up to 213 days. Concentration-dependent anti-oxidation effect articulates that 1.5 mg mL-1 is deemed as an ideal SDS dose for Ti3 C2 Tx to achieve optimal oxidation-resistance in aqueous solution. Additionally, a chroma strategy is developed to instantly and precisely measure the oxidation degree of Ti3 C2 Tx . Adsorption-driven anti-oxidation efficacy of SDS is further confirmed by optimized conformations with interaction energies of SDS on termination-free and surface-defective Ti3 C2 Tx through multiscale simulations. This proposed route is a step forward in broadening the horizons of experimental and theoretical investigations of MXenes with promising implications for long-term storage and reliable applications.

Natural oxidation of Ti3C2Tx to construct efficient TiO2/Ti3C2Tx photoactive heterojunctions for advanced photoelectrochemical biosensing of folate-expressing cancer cells.

The rapid-charge carrier recombination and low conductivity are critical in devising an efficient photoelectrochemical (PEC) sensor. Herein, we propose partial oxidation of few-layered MXene (Ti3C2Tx) to construct a photo-active TiO2/Ti3C2Tx platform that could be configured for PEC sensing of folate receptors (FR), particularly, FR-expressing breast cancer cells (MDA-MB-231). MXene-Ti3C2Tx dispersion was oxidized in natural-open air conditions, where continuous exposure for six (06) days allowed for homogeneous in-situ growth of TiO2 over MXenes nanosheets (MX-06). This exposure enabled partial oxidation of MXene-sheets with a balanced TiO2 to MXene content that could exhibit improved photoresponsive characteristics owing to the synergism of redox-active TiO2 and highly conductive underlying Ti3C2Tx. The photoelectrode was then adapted for biorecognition by conjugating chitosan and folic acid (FA) networks, which permitted selective detection of FR-expressed cells with significant antifouling capabilities against common proteins such as bovine serum album (BSA), hemoglobin, and immunoglobulin G. (Ig G). The detection mechanism relies on FA's strong affinity for cancer cell folate receptors, which proportionally inhibited the photoelectrodes PEC oxidation response to ascorbic acid (AA)(mediator). The proposed inhibition strategy enabled sensitive detection of FR-expressed MDA-MB-231 cells in the concentration range of 1 × 102 to 2 × 107 cells/mL with a detection limit of 1.01 cells/mL (S/N = 3).

3D Porous Co3O4/MXene Foam Fabricated via a Sulfur Template Strategy for Enhanced Li/K-Ion Storage.

Co3O4 is a potential high-capacity anode material for lithium-ion batteries (LIBs) and potassium-ion batteries (PIBs), but the poor electrical conductivity and large volume fluctuations during long-term cycling severely limit its cycle durability and rate capabilities, especially for PIBs with large K-ion size. Here, we propose a sulfur template route to fabricate an integral 3D porous Co3O4/MXene (Ti3C2Tx) foam using simple vacuum co-filtrating an aqueous dispersion of Co3O4, S and MXene followed by calcining to remove the S template. The 3D porous structure can easily accommodate the large volume changes of Co3O4 while maintains electrode structural integrity, allowing to realize outstanding long-term cycle stability when tested as anodes for both LIBs (620.4 mA h g-1 after 1000 cycles at 1 A g-1) and PIBs (134.1 mA h g-1 after 1000 cycles at 0.5 A g-1). The high metallic conductivity of the 3D porous MXene network further facilitates the electron/ion transmission, resulting in an improved rate capability of 390 mA h g-1 at 13 A g-1 for LIBs and 125.3 mA h g-1 at 1 A g-1 for PIBs. The robust performance of the 3D porous Co3O4/MXene foam reflects its perspective as a high-performance anode material for both LIBs and PIBs.

Advances in the Synthesis of 2D MXenes.

2D transition metal carbides, nitrides, and carbonitrides, also known as MXenes, are versatile materials due to their adjustable structure and rich surface chemistry. The physical and chemical diversity has recognized MXenes as a potential 2D material with a wide spectrum of application domains. Since the discovery of MXenes in 2011, a wide variety of synthetic routes has been proposed with advancement toward large-scale preparing methods for MXene nanosheets and derivative products. Herein, the critical synthesis aspects and the operating conditions that influence the physical and chemical characteristics of MXenes are discussed in detail. The emerging etching methods including HF etching methods, in situ HF-forming etching methods, electrochemical etching methods, alkali etching methods, and molten salt etching methods, as well as delamination strategies are discussed. Considering the future developments and practical applications, the large-scale synthesis routes and the antioxidation strategies of MXenes are also summarized. In summary, a generalized overview of MXenes synthesis protocols with an outlook for the current challenges and promising technologies for large-scale preparation and stable storage is provided.

L-cysteine protected copper nanoparticles as colorimetric sensor for mercuric ions.

This report demonstrates a novel, simple and efficient protocol for the synthesis of copper nanoparticles in aqueous solution using L-cysteine as capping or protecting agent. UV-visible (UV-vis) spectroscopy was employed to monitor the LSPR band of L-cysteine functionalized copper nanoparticles (Cyst-Cu NPs) based on optimizing various reaction parameters. Fourier Transform Infrared (FTIR) spectroscopy provided information about the surface interaction between L-cysteine and Cu NPs. Transmission Electron Microscopy (TEM) confirmed the formation of fine spherical, uniformly distributed Cyst-Cu NPs with average size of 34 ± 2.1 nm. X-ray diffractometry (XRD) illustrated the formation of pure metallic phase crystalline Cyst-Cu NPs. As prepared Cyst-Cu NPs were tested as colorimetric sensor for determining mercuric (Hg(2+)) ions in an aqueous system. Cyst-Cu NPs demonstrated very sensitive and selective colorimetric detection of Hg(2+) ions in the range of 0.5 × 10(-6)-3.5 × 10(-6) mol L(-1) based on decrease in LSPR intensity as monitored by a UV-vis spectrophotometer. The developed sensor is simple, economic compared to those based on precious metal nanoparticles and sensitive to detect Hg(2+) ions with detection limit down to 4.3 × 10(-8) mol L(-1). The sensor developed in this work has a high potential for rapid and on-site detection of Hg(2+) ions. The sensor was successfully applied for assessment of Hg(2+) ions in real water samples collected from various locations of the Sindh River.