Oxidation of Molybdenum-Based Single-Metallic/bimetallic Carbide MXenes in Aqueous Suspensions: Mechanistic Insights

Molybdenum carbide MXenes have garnered considerable attention in electronics, energy storage, and catalysis. However, they are prone to oxidative degradation, but the associated mechanisms have not been systematically explored. Therefore, the oxidation mechanisms of Mo-based single-metallic/bimetallic carbide MXenes including Mo2CTx, Mo2TiC2Tx, and Mo2Ti2C3Tx in aqueous suspensions were investigated for the first time in this study. Similar to Ti3C2Tx MXene, Mo-based MXenes were found to undergo oxidative degradation in their aqueous dispersions, leading to the disruption of their crystal structure and subsequent loss of optical and electronic properties. Notably, the Mo2CTx MXene deviated from this typical oxidation behavior as it produced an amorphous product with Mo ions instead of highly crystalline Mo-oxides during oxidation. Similarly, the Mo2TiC2Tx and Mo2Ti2C3Tx MXenes did not yield crystalline Mo-oxides; instead, they produced highly crystalline anatase TiO2 and a Mo-ion-containing amorphous product simultaneously. Furthermore, high-temperature annealing of the oxidized Mo2CTx MXene powder at 800 °C transformed the amorphous Mo-containing product into highly crystalline MoO2 crystals. These findings highlight the unconventional oxidation behavior of Mo-based MXenes, which suggests that the formation of crystalline Mo-based oxides requires a higher activation energy during oxidation than that of TiO2. The unique oxidative pathway reported herein can help elucidate the oxidation mechanisms of Mo-based MXene dispersions and their products. The insights from this study can pave the way for fundamental studies in academia as well as broaden the applications of Mo-based MXenes in various industries.

Ultrahigh Nonlinear Responses from MXene Plasmons in the Short-Wave Infrared Range

Surface plasmons in 2D materials such as graphene exhibit exceptional field confinement. However, the low electron density of majority of 2D materials, which are semiconductors or semimetals, has limited their plasmons to mid-wave or long-wave infrared regime. This study demonstrates that a 2D Ti3C2Tx MXene with high electron density can not only support strong plasmon confinement with an acoustic plasmon mode in the short-wave infrared region, but also provide ultrahigh nonlinear responses. The acoustic MXene plasmons (AMPs) in the MXene (Ti3C2Tx)-insulator (SiO2)-metal (Au) nanostructure generate in the 1.5-6.0 µm wavelength range, exhibiting a two orders of magnitude reduction in wavelength compared to wavelength in free space. Furthermore, AMP resonators with patterned Au rods exhibit a record-high nonlinear absorption coefficient of 1.37 × 10-2 m W-1 at wavelength of 1.56 µm, ≈3 orders of magnitude greater than the highest value recorded for other 2D materials. These results indicate that MXenes can overcome fundamental plasmon wavelength limitations of previously studied 2D materials, providing groundbreaking opportunities in nonlinear optical applications, including all-optical processing and ultrafast optical switching.

Asymmetric Laser Field Interaction with MXene Coated on the Side Surface of Optical Fibers for Ultrafast Nonlinear Switches

In recent years, there has been significant interest in researching ultrafast nonlinear optical phenomena involving light-matter interactions in two-dimensional (2D) materials, owing to their potential applications in optics and photonics. MXene, a recently developed 2D material, has garnered considerable attention due to its graphene-like properties and highly tunable electronic/optical characteristics. Herein, we demonstrate ultrafast all-optical switches based on four-wave-mixing (FWM) utilizing the nonlinear optical property of MXene Ti3C2Tx. In order to realize the device, we deposited multilayered Ti3C2Tx in the form of a supernatant solution onto the polished surface of a side-polished optical fiber, enabling the interaction of Ti3C2Tx with the asymmetric evanescent field of the incident input. We systematically characterized the nonlinear optical responses derived from the Ti3C2Tx layers. The fabricated device exhibits notable performance metrics, an enhancement of the extinction ratio, and a conversion efficiency of the newly generated signal, displaying 5.3 and 5.2 dB, respectively. Additionally, the device operates at high modulation frequencies, reaching up to 20 GHz, and demonstrates high-resolution detuning with channel distances of up to 15 nm. Our findings highlight the potential of MXene-based materials for ultrafast optical data management systems.

Tailoring Built-In Electric Field in a Self-Assembled Zeolitic Imidazolate Framework/MXene Nanocomposites for Microwave Absorption

Heterointerface engineering, which plays a pivotal role in developing advanced microwave-absorbing materials, is employed to design zeolitic imidazolate framework (ZIF)-MXene nanocomposites. The ZIF-MXene composites are prepared by electrostatic self-assembly of negatively charged titanium carbide MXene flakes and positively charged Co-containing ZIF nanomaterials. This approach effectively creates abundant Mott-Schottky heterointerfaces exhibiting a robust built-in electric field (BIEF) effect, as evidenced by experimental and theoretical analyses, leading to a notable attenuation of electromagnetic energy. Systematic manipulation of the BIEF-exhibiting heterointerface, achieved through topological modulation of the ZIF, proficiently alters charge separation, facilitates electron migration, and ultimately enhances polarization relaxation loss, resulting in exceptional electromagnetic wave absorption performance (reflection loss RLmin = -47.35 dB and effective absorption bandwidth fE = 6.32 GHz). The present study demonstrates an innovative model system for elucidating the interfacial polarization mechanisms and pioneers a novel approach to developing functional materials with electromagnetic characteristics through spatial charge engineering.

Effect of Substitutional Oxygen on Properties of Ti3 C2 Tx MXene Produced Using Recycled TiO2 Source

MXenes are an emerging class of 2D materials with unique properties including metallic conductivity, mechanical flexibility, and surface tunability, which ensure their utility for diverse applications. However, the synthesis of MXenes with high crystallinity and atomic stoichiometry in a low-cost process is still challenging because of the difficulty in controlling the oxygen substitute in the precursors and final products of MXenes, which limits their academic understanding and practical applications. Here, a novel cost-effective method is reported to synthesize a highly crystalline and stoichiometric Ti3 C2 Tx MXene with minimum substitutional oxygen impurities by controlling the amount of excess carbon and time of high-energy milling in carbothermal reduction of recycled TiO2 source. The highest used content (2 wt%) of excess-carbon yields TiC with the highest carbon content and minimal oxygen substitutes, which leads to the Ti3 AlC2 MAX phase with improved crystallinity and atomic stoichiometry, and finally Ti3 C2 Tx MXene with the highest electrical conductivity (11738 S cm-1 ) and superior electromagnetic shielding effectiveness. Additionally, the effects of carbon content and substitutional oxygen on the physical properties of TiC and Ti3 AlC2 are elucidated by density-functional-theory calculations. This inexpensive TiO2 -based method of synthesizing high-quality Ti3 C2 Tx MXene can facilitate large-scale production and thus accelerate global research on MXenes.

Biocompatible and Oxidation-Resistant Ti3 C2 Tx MXene with Halogen-Free Surface Terminations

Surface chemistry influences not only physicochemical properties but also safety and applications of MXene nanomaterials. Fluorinated Ti3 C2 Tx MXene, synthesized using conventional HF-based etchants, raises concerns regarding harmful effects on electronics and toxicity to living organisms. In this study, well-delaminated halogen-free Ti3 C2 Tx flakes are synthesized using NaOH-based etching solution. The transversal surface plasmon mode of halogen-free Ti3 C2 Tx MXene (833 nm) confirmed red-shift compared to conventional Ti3 C2 Tx (752 nm), and the halogen-free Ti3 C2 Tx MXene has a different density of state by the high proportion of -O and -OH terminations. The synthesized halogen-free Ti3 C2 Tx exhibits a lower water contact angle (34.5°) and work function (3.6 eV) than those of fluorinated Ti3 C2 Tx (49.8° and 4.14 eV, respectively). The synthesized halogen-free Ti3 C2 Tx exhibits high biocompatibility with the living cells, as evidenced by no noticeable cytotoxicity, even at very high concentrations (2000 µg mL⁻1 ), at which fluorinated Ti3 C2 Tx caused ≈50% reduction in cell viability upon its oxidation. Additionally, the oxidation stability of halogen-free Ti3 C2 Tx is enhanced unexpectedly, which cumulatively provides a good rationale for pursuing the halogen-free routes for synthesizing MXene materials for their uses in biomedical and therapeutic applications.

Texture Regulation of Metal-Organic Frameworks, Microwave Absorption Mechanism-Oriented Structural Optimization and Design Perspectives

Texture regulation of metal-organic frameworks (MOFs) is essential for controlling their electromagnetic wave (EMW) absorption properties. This review systematically summarizes the recent advancements in texture regulation strategies for MOFs, including etching and exchange of central ions, etching and exchange of ligands, chemically induced self-assembly, and MOF-on-MOF heterostructure design. Additionally, the EMW absorption mechanisms in approaches based on structure-function dependencies, including nano-micro topological engineering, defect engineering, interface engineering, and hybrid engineering, are comprehensively explored. Finally, current challenges and future research orientation are proposed. This review aims to provide new perspectives for designing MOF-derived EMW-absorption materials to achieve essential breakthroughs in mechanistic investigations in this promising field.

Universal Ligands for Dispersion of Two-Dimensional MXene in Organic Solvents

Ligands can control the surface chemistry, physicochemical properties, processing, and applications of nanomaterials. MXenes are the fastest growing family of two-dimensional (2D) nanomaterials, showing promise for energy, electronic, and environmental applications. However, complex oxidation states, surface terminal groups, and interaction with the environment have hindered the development of organic ligands suitable for MXenes. Here, we demonstrate a simple, fast, scalable, and universally applicable ligand chemistry for MXenes using alkylated 3,4-dihydroxy-l-phenylalanine (ADOPA). Due to the strong hydrogen-bonding and π-electron interactions between the catechol head and surface terminal groups of MXenes and the presence of a hydrophobic fluorinated alkyl tail compatible with organic solvents, the ADOPA ligands functionalize MXene surfaces under mild reaction conditions without sacrificing their properties. Stable colloidal solutions and highly concentrated liquid crystals of various MXenes, including Ti2CTx, Nb2CTx, V2CTx, Mo2CTx, Ti3C2Tx, Ti3CNTx, Mo2TiC2Tx, Mo2Ti2C3Tx, and Ti4N3Tx, have been produced in various organic solvents. Such products offer excellent electrical conductivity, improved oxidation stability, and excellent processability, enabling applications in flexible electrodes and electromagnetic interference shielding.

Fast and High-Yield Anhydrous Synthesis of Ti3 C2 Tx MXene with High Electrical Conductivity and Exceptional Mechanical Strength

2D transition metal carbides or nitrides (MXenes) have attracted considerable attention from materials scientists and engineers owing to their physicochemical properties. Currently, MXenes are synthesized from MAX-phase precursors using aqueous HF. Here, in order to enhance the production of MXenes, an anhydrous etching solution is proposed, consisting of dimethylsulfoxide as solvent with its high boiling point, NH₄ HF₂ as an etchant, CH₃ SO₃ H as an acid, and NH₄ PF₆ as an intercalant. The reaction temperature can be increased up to 100 °C to accelerate the etching and delamination of Ti₃ AlC₂ MAX crystals; in addition, the destructive side reaction of the produced Ti₃ C₂ T_x MXene is suppressed in the etchant. Consequently, the etching reaction is completed in 4 h at 100 °C and produces high-quality monolayer Ti₃ C₂ T_x with an electrical conductivity of 8200 S cm^-1 and yield of over 70%. The Ti₃ C₂ T_x MXene fabricated via this modified synthesis exhibits different surface structures and properties arising from more F-terminations than those of Ti₃ C₂ T_x synthesized in aqueous HF₂ T. The atypical surface structure of Ti₃ C₂ T_x MXene results in an exceptionally high ultimate tensile strength (167 ± 8 MPa), which is five times larger than those of Ti₃ C₂ T_x MXenes synthesized in aqueous HF solution (31.7 ± 7.8 MPa).

Controllable Surface-Grafted MXene Inks for Electromagnetic Wave Modulation and Infrared Anti-Counterfeiting Applications

Two-dimensional transition metal carbide/nitride (MXene) conductive inks are promising for scalable production of printable electronics, electromagnetic devices, and multifunctional coatings. However, the susceptible oxidation and poor rheological property seriously impede the printability of MXene inks and the exploration of functional devices. Here, we proposed a controllable surface grafting strategy for MXene flakes (p-MXene) with prepolymerized polydopamine macromolecules to protect against water and oxygen, enrich surface chemistry, and significantly optimize the rheological properties of the inks. The obtained p-MXene inks can adapt to screen-printing and other high-viscosity processing techniques, facilitating the development of patterned electromagnetic films and coatings. Interestingly, the printed MXene polarizer can freely switch and quantitatively control microwave transmission, giving an inspiring means for smart microwave modulation beyond the commonly reported shielding function. Moreover, the introduction of polydopamine nanoshell enables the infrared emissivity of MXene coating to be adjusted to a large extent, which can produce infrared anti-counterfeiting patterns in a thermal imager. Therefore, multifunctional antioxidant p-MXene inks will greatly extend the potential applications for the next-generation printable electronics and devices.

Tunable Ti3C2Tx MXene-Derived TiO2 Nanocrystals at Controlled pH and Temperature

While two-dimensional (2D) Ti₃C₂T_x MXene in aqueous dispersions spontaneously oxidizes into titanium dioxide (TiO₂) nanocrystals, the crystallization mechanism has not been comprehensively understood and the resultant crystal structures are not controlled among three representative polymorphs: anatase, rutile, and brookite. In this study, such control on the lattice structures and domain sizes of the MXene-derived TiO₂ crystallites is demonstrated by means of the oxidation conditions, pH, and temperature (3.0-11.0 and 20-100 °C, respectively). It is observed that the formation of anatase phase is preferred against rutile phase in more basic and hotter oxidizing solutions, and even 100% anatase can be obtained at pH 11.0 and 100 °C. At lower pH and temperature, the portion of rutile phase increases such that it reaches ∼70% at pH 3 and 20 °C. Under certain circumstances, small portion of brookite phase is also observed. Smaller domain sizes of both anatase and rutile phases are observed in more basic oxidizing solutions and at lower temperatures. Based on these experimental results, we propose the crystallization mechanism in which the oxidative dissociation of Ti₃C₂T_x first produces Ti ions as the intermediate state, and they bind to abundant oxygen in the aqueous dispersions, and nucleate and crystallize into TiO₂.

Flexible and Transparent Electrode of Hybrid Ti3C2TX MXene-Silver Nanowires for High-Performance Quantum Dot Light-Emitting Diodes

The development of electrodes with high conductivity, optical transparency, and reliable mechanical flexibility and stability is important for numerous solution-processed photoelectronic applications. Although transparent Ti₃C₂T_X MXene electrodes with high conductivity are promising, their suitability for displays remains limited because of the high sheet resistance, which is caused by undesirable flake junctions and surface roughness. Herein, a flexible and transparent electrode has been fabricated that is suitable for a full-solution-processed quantum dot light-emitting diode (QLED). An MXene-silver nanowire (AgNW) hybrid electrode (MXAg) consists of a highly conductive AgNW network mixed with solution-processed MXene flakes. Efficient welding of wire-to-wire junctions with MXene flakes yields an electrode with a low sheet resistance and a high transparency of approximately 13.9 Ω sq^-1 and 83.8%, respectively. By employing a thin polymer buffer layer of poly(methyl methacrylate) (PMMA), followed by mild thermal treatment, a hybrid PMMA-based MXene-AgNW (MXAg@PMMA) electrode in which the work function of an MXAg hybrid FTE physically embedded in PMMA (MXAg@PMMA) can be tuned by controlling the amount of MXene in the hybrid film facilitates the development of a high-performance solution-processed QLED that exhibits maximum external quantum and current efficiencies of approximately 9.88% and 25.8 cd/A, respectively, with excellent bending stability. This work function-tunable flexible transparent electrode based on solution-processed nanoconductors provides a way to develop emerging high-performance, wearable, cost-effective, and soft electroluminescent devices.

Using the lactate-to-albumin ratio to predict mortality in patients with sepsis or septic shock: a systematic review and meta-analysis

OBJECTIVE

This study aimed to investigate whether the lactate-to-albumin ratio (LAR) can predict mortality in patients with sepsis or septic shock.

PATIENTS AND METHODS

A systematic search of the PubMed, EMBASE, Web of Science, and Google Scholar databases was conducted on December 16, 2021, for relevant articles that provided the predictive performance of LAR for mortality in patients with sepsis or septic shock.

RESULTS

Eight studies encompassing a total of 4,723 patients were included in this paper. The pooled sensitivity, specificity, and diagnostic odds ratio of the LAR for predicting mortality were 0.71 (95% confidence interval [CI]: 0.54-0.84), 0.68 (95% CI: 0.58-0.76) and 5.23 (95% CI: 2.62-10.45), respectively. The area under the summary receiver operating characteristic curve was 0.74 (95% CI: 0.70-0.78).

CONCLUSIONS

The current evidence suggests that LAR is moderately predictive of mortality among patients with sepsis or septic shock and may be beneficial to identify high-risk patients.

Neutrophil-to-lymphocyte ratio for the diagnosis of pediatric acute appendicitis: a systematic review and meta-analysis

OBJECTIVE

Acute appendicitis (AA) is one of the most common surgical emergencies and causes of acute abdominal pain in the pediatric population. However, it can be difficult to diagnose in children. We aimed to provide updated evidence on the diagnostic utility of the neutrophil-to-lymphocyte ratio (NLR) for AA, along with other conventional biomarkers, in pediatric patients.

MATERIALS AND METHODS

We searched the PubMed, Embase, Cochrane Library, and Web of Science databases for eligible articles published up to May 16, 2021.

RESULTS

We included 19 studies comprising a total of 5,974 pediatric cases. The overall sensitivity and specificity of the NLR were 0.82 (95% confidence interval [CI]: 0.79-0.85) and 0.76 (95% CI: 0.69-0.81), respectively. The overall diagnostic odds ratio was 14.34 (95% CI: 9.05-22.73). The area under the summary receiver operating characteristic curve was 0.86 (95% CI: 0.83-0.89). The pooled sensitivity and specificity of other biomarkers were as follows: 0.79 (95% CI: 0.71-0.86) and 0.66 (95% CI: 0.54-0.77) for the white blood cell count, 0.73 (95% CI: 0.69-0.77) and 0.68 (95% CI: 0.55-0.79) for the C-reactive protein level, 0.75 (95% CI: 0.65-0.82) and 0.78 (95% CI: 0.72-0.83) for the absolute neutrophil count, and 0.83 (95% CI: 0.79-0.87) and 0.68 (95% CI: 0.53-0.80) for the neutrophil percentage, respectively.

CONCLUSIONS

The NLR has moderate predictive power for AA and can be used as a simple, auxiliary tool for diagnosis. NLR can also help clinicians decide whether to perform imaging testing when the clinical symptoms or physical examination findings are vague.

Multidimensional Ti3C2Tx MXene Architectures via Interfacial Electrochemical Self-Assembly

An effective pathway to build macroscopic scale functional architectures bearing diverse structural dimensions is one of the critical challenges in the two-dimensional (2D) MXene research area. Unfortunately, assembling MXene without adhesive binder is largely limited due to its innate brittle nature and the relatively weak inter-flake van der Waals contact, in contrast to other mechanically compliant 2D materials such as graphene. Herein, an electrochemical self-assembly of pure Ti₃C₂T_x MXenes is presented for functional multidimensional MXene structures, effectively driven by layer-by-layer spontaneous interfacial reduction at metal template surfaces and subsequent defunctionalization. A three-dimensional open porous aerogel as well as 2D highly stacked thin film structures could be readily obtained in this approach, along with largely enhanced electrical properties induced by spontaneous removal of charge-trapping oxygen functional groups. Accordingly, supercapacitors and electromagnetic interference shielding films based on the multidimensional assembly demonstrate excellent performances.

Polymer-Laminated Ti3C2TX MXene Electrodes for Transparent and Flexible Field-Driven Electronics

MXenes (Ti₃C₂T_X) are two-dimensional transition-metal carbides and carbonitrides with high conductivity and optical transparency. However, transparent MXene electrodes with high environmental stability suitable for various flexible organic electronic devices have rarely been demonstrated. By laminating a thin polymer film onto a solution-processed MXene layer to protect the MXene film from harsh environmental conditions, we present transparent and flexible MXene electronic devices. A thin polymer layer spin-coated onto a transparent MXene electrode provides environmental stability even under air exposure longer than 7 d at high temperatures (up to 70 °C) and humidity levels (up to 50%) without degrading the transparency of the electrode. The resulting polymer-laminated (PL) MXene electrode facilitates the development of a variety of field-driven photoelectronic devices by exploiting the electric field exerted between the MXene layer and the counter electrode through the insulating polymer. Field-induced electroluminescent displays, based on both organic and inorganic phosphors, with PL-MXene electrodes are demonstrated with high transparency and mechanical flexibility. Furthermore, our PL-MXene electrode exhibits high versatility through successful implementation in capacitive-type pressure sensors and triboelectric nanogenerators, resulting in field-driven sensing and energy harvesting electronic devices with excellent operation reliability.

Mechanism and Kinetics of Oxidation Reaction of Aqueous Ti3C2Tx Suspensions at Different pHs and Temperatures

Understanding the oxidation reaction of aqueous Ti₃C₂T_x MXene suspensions is very important for fostering fundamental academic studies as well as widespread industrial applications. Herein, we investigated the mechanism and kinetics of the oxidation reaction of aqueous Ti₃C₂T_x suspensions at various pH and temperature conditions. Through comprehensive analysis, the mechanism of the chemical oxidative degradation of aqueous Ti₃C₂T_x colloids was established. Chemical oxidation produces solid products such as TiO₂ and amorphous carbon as well as various gaseous species including CH₄, CO, CO₂, and HF. Additionally, our comprehensive kinetic study proposes that aqueous Ti₃C₂T_x dispersions are degraded via an acid-catalyzed oxidation reaction, where, under acidic conditions, the protonation of the hydroxyl terminal groups on the Ti₃C₂T_x flakes induces electron localization on titanium atoms and accelerates their oxidation reaction. In contrast, under basic conditions, the electrostatically alkali-metalized hydroxyl intermediates forming a bulky solvent cage results in less electron localization on titanium atoms, and thus retards their oxidative degradation.

Reduction of Electrochemically Exfoliated Graphene Films for High-Performance Electromagnetic Interference Shielding

Two-dimensional graphene is of great interest for electromagnetic interference (EMI) shielding owing to its inherent electrical conductivity, lightweight, and excellent mechanical flexibility even at minor thicknesses. However, the complex synthesis and quality-control difficulties limit its application. In this study, we demonstrate that electrochemically exfoliated graphene (EEG) with post-reduction treatment is a promising candidate for lightweight EMI shielding materials. A facile electrochemical exfoliation approach produces a high-quality multilayer graphene with a high electrical conductivity of ∼600 S cm^-1, owing to its low degree of oxidation. The reduction of EEG by three different methods, including chemical, thermal, and microwave treatments, causes the removal of surface functional groups as well as significant changes in the microstructure of the final films. The reduced graphene films by microwaves, which are driven by the improved electrical conductivity and large volume expansion, exhibit an EMI shielding effectiveness of 108 dB at a thickness of 125 μm, one of the largest EMI shielding values ever reported for graphene at comparable thicknesses.

Improving oxidation stability of 2D MXenes: synthesis, storage media, and conditions

Understanding and preventing oxidative degradation of MXene suspensions is essential for fostering fundamental academic studies and facilitating widespread industrial applications. Owing to their outstanding electrical, electrochemical, optoelectronic, and mechanical properties, MXenes, an emerging class of two-dimensional (2D) nanomaterials, show promising state-of-the-art performances in various applications including electromagnetic interference (EMI) shielding, terahertz shielding, electrochemical energy storage, triboelectric nanogenerators, thermal heaters, light-emitting diodes (LEDs), optoelectronics, and sensors. However, MXene synthesis using harsh chemical etching causes many defects or vacancies on the surface of the synthesized MXene flakes. Defective sites are vulnerable to oxidative degradation reactions with water and/or oxygen, which deteriorate the intrinsic properties of MXenes. In this review, we demonstrate the nature of oxidative degradation of MXenes and highlight the recent advancements in controlling the oxidation kinetics of MXenes with several promising strategic approaches, including careful control of the quality of the parent MAX phase, chemical etching conditions, defect passivation, dispersion medium, storage conditions, and polymer composites.

Point-of-care testing for the detection of SARS-CoV-2: a systematic review and meta-analysis

OBJECTIVE

To evaluate the diagnostic accuracy of the Food and Drug Administration Emergency Use Authorization (FDA-EUA) authorized point-of-care tests (POCTs) for the detection of severe acute respiratory syndrome coronavirus 2 (SARS-CoV-2).

MATERIALS AND METHODS

A systematic literature search was conducted using the PubMed, Embase, and Web of Science databases for articles published till August 10, 2020. We included studies providing information regarding diagnostic test accuracy of FDA-EUA POCTs for SARS-CoV-2 detection. The methodologic quality was assessed using the Quality Assessment of Diagnostic Accuracy Studies-2 tool. The review protocol is registered in the International Prospective Register of Systematic Reviews (protocol number CRD42020202248).

RESULTS

We included 26 studies describing a total of 3242 samples. The summary sensitivity and specificity were 0.94 [95% confidence interval (CI): 0.88-0.97] and 1.00 (95% CI: 0.99-1.00), respectively. The area under the summary receiver operating characteristic curve was 1.00 (95% CI: 0.99-1.00). A pooled analysis based on the index test revealed a summary sensitivity and specificity of Cepheid Xpert Xpress SARS-CoV-2 [0.99 (95% CI: 0.97-1.00) and 0.99 (95% CI: 0.94-1.00, respectively)] and ID NOW COVID-19 [0.78 (95% CI: 0.74-0.82) and 1.00 (95% CI: 0.98-1.00), respectively].

CONCLUSIONS

FDA-EUA POCTs, especially molecular assays, have high sensitivity, specificity, and overall diagnostic accuracy for detecting SARS-CoV-2. If approved, FDA-EUA POCTs can provide a rapid and practical way to identify infected individuals early on and help to limit the strain on the healthcare system. However, more high-quality clinical data are required to support our results.

2D Transition Metal Carbides (MXenes): Applications as an Electrically Conducting Material

Since their discovery in 2011, 2D transition metal carbides, nitrides, and carbonitrides, known as MXenes, have attracted considerable global research interest owing to their outstanding electrical conductivity coupled with light weight, flexibility, transparency, surface chemistry tunability, and easy solution processability. Here, the promising abilities of 2D MXenes, from both experimental and theoretical perspectives, for designing conductive materials for a range of applications, including electromagnetic interference shielding, flexible optoelectronics, sensors, and thermal heaters, are presented.

Mussel Inspired Highly Aligned Ti3C2Tx MXene Film with Synergistic Enhancement of Mechanical Strength and Ambient Stability

Two-dimensional (2D) MXene has shown enormous potential in scientific fields, including energy storage and electromagnetic interference (EMI) shielding. Unfortunately, MXene-based material structures generally suffer from mechanical fragility and vulnerability to oxidation. Herein, mussel-inspired dopamine successfully addresses those weaknesses by improving interflake interaction and ordering in MXene assembled films. Dopamine undergoes in situ polymerization and binding at MXene flake surfaces by spontaneous interfacial charge transfer, yielding an ultrathin adhesive layer. Resultant nanocomposites with highly aligned tight layer structures achieve approximately seven times enhanced tensile strength with a simultaneous increase of elongation. Ambient stability of MXene films is also greatly improved by the effective screening of oxygen and moisture. Interestingly, angstrom thick polydopamine further promotes the innate high electrical conductivity and excellent EMI shielding properties of MXene films. This synergistic concurrent enhancement of physical properties proposes MXene/polydopamine hybrids as a general platform for MXene based reliable applications.

Anomalous absorption of electromagnetic waves by 2D transition metal carbonitride Ti3CNT x (MXene)

Lightweight, ultrathin, and flexible electromagnetic interference (EMI) shielding materials are needed to protect electronic circuits and portable telecommunication devices and to eliminate cross-talk between devices and device components. Here, we show that a two-dimensional (2D) transition metal carbonitride, Ti₃CNT _x MXene, with a moderate electrical conductivity, provides a higher shielding effectiveness compared with more conductive Ti₃C₂T _x or metal foils of the same thickness. This exceptional shielding performance of Ti₃CNT _x was achieved by thermal annealing and is attributed to an anomalously high absorption of electromagnetic waves in its layered, metamaterial-like structure. These results provide guidance for designing advanced EMI shielding materials but also highlight the need for exploring fundamental mechanisms behind interaction of electromagnetic waves with 2D materials.

Anomalous absorption of electromagnetic waves by 2D transition metal carbonitride Ti3CNT x (MXene)

Lightweight, ultrathin, and flexible electromagnetic interference (EMI) shielding materials are needed to protect electronic circuits and portable telecommunication devices and to eliminate cross-talk between devices and device components. Here, we show that a two-dimensional (2D) transition metal carbonitride, Ti₃CNT _x MXene, with a moderate electrical conductivity, provides a higher shielding effectiveness compared with more conductive Ti₃C₂T _x or metal foils of the same thickness. This exceptional shielding performance of Ti₃CNT _x was achieved by thermal annealing and is attributed to an anomalously high absorption of electromagnetic waves in its layered, metamaterial-like structure. These results provide guidance for designing advanced EMI shielding materials but also highlight the need for exploring fundamental mechanisms behind interaction of electromagnetic waves with 2D materials.

Beyond Ti3C2Tx: MXenes for Electromagnetic Interference Shielding

New ultrathin and multifunctional electromagnetic interference (EMI) shielding materials are required for protecting electronics against electromagnetic pollution in the fifth-generation networks and Internet of Things era. Micrometer-thin Ti₃C₂T_x MXene films have shown the best EMI shielding performance among synthetic materials so far. Yet, the effects of elemental composition, layer structure, and transition-metal arrangement on EMI shielding properties of MXenes have not been explored, despite the fact that more than 30 different MXenes have been reported, and many more are possible. Here, we report on a systematic study of EMI shielding properties of 16 different MXenes, which cover single-metal MXenes, ordered double-metal carbide MXenes, and random solid solution MXenes of M and X elements. This is the largest set of MXene compositions ever reported in a comparative study. Films with thicknesses ranging from nanometers to micrometers were produced by spin-casting, spray-coating, and vacuum-assisted filtration. All MXenes achieved effective EMI shielding (>20 dB) in micrometer-thick films. The EMI shielding effectiveness of sprayed Ti₃C₂T_x film with a thickness of only ∼40 nm reaches 21 dB. Adjustable EMI shielding properties were achieved in solid solution MXenes with different ratios of elements. A transfer matrix model was shown to fit EMI shielding data for highly conductive MXenes but could not describe the behavior of materials with low conductivity. This work shows that many members of the large MXene family can be used for EMI shielding, contributing to designing ultrathin, flexible, and multifunctional EMI shielding films benefiting from specific characteristics of individual MXenes.

Electromagnetic Shielding of Monolayer MXene Assemblies

Miniaturization of electronics demands electromagnetic interference (EMI) shielding of nanoscale dimension. The authors report a systematic exploration of EMI shielding behavior of 2D Ti₃ C₂ T_x MXene assembled films over a broad range of film thicknesses, monolayer by monolayer. Theoretical models are used to explain the shielding mechanism below skin depth, where multiple reflection becomes significant, along with the surface reflection and bulk absorption of electromagnetic radiation. While a monolayer assembled film offers ≈20% shielding of electromagnetic waves, a 24-layer film of ≈55 nm thickness demonstrates 99% shielding (20 dB), revealing an extraordinarily large absolute shielding effectiveness (3.89 × 10⁶ dB cm² g^-1 ). This remarkable performance of nanometer-thin solution processable MXene proposes a paradigm shift in shielding of lightweight, portable, and compact next-generation electronic devices.

Nonpolar Organic Dispersion of 2D Ti3C2Tx MXene Flakes via Simultaneous Interfacial Chemical Grafting and Phase Transfer Method

Herein, we demonstrate a simple and versatile way for preparing stable Ti₃C₂T_x MXene dispersions in nonpolar organic solvents through a simultaneous interfacial chemical grafting reaction and phase transfer method. Alkylphosphonic acid ligands were chemically grafted on the hydroxyl terminal groups of Ti₃C₂T_x flakes at the liquid-liquid interface between water and water-immiscible organic medium to form a covalent Ti-O-P bond via interfacial nucleophilic addition and sequential condensation reaction at room temperature; the surface-functionalized Ti₃C₂T_x flakes concurrently migrated from the aqueous phase to the organic phase. Unlike conventional surface chemical modification methods that require many complex and tedious steps, this is a simple and easy process for fabricating a Ti₃C₂T_x organic dispersion in various organic solvents, from highly polar to nonpolar. The nonpolar Ti₃C₂T_x dispersion in chloroform also exhibits strong oxidation resistance and stable long-term storage. This approach provides an opportunity for preparing MXene nanocomposites with nonpolar polymeric matrices that are soluble in organic media for future applications such as stretchable electrode.

Ultralight and Mechanically Robust Ti3C2Tx Hybrid Aerogel Reinforced by Carbon Nanotubes for Electromagnetic Interference Shielding

Lightweight materials with high electrical conductivity and robust mechanical properties are highly desirable for electromagnetic interference (EMI) shielding in modern portable and highly integrated electronics. Herein, a three-dimensional (3D) porous Ti₃C₂T_x/carbon nanotube (CNT) hybrid aerogel was fabricated via a bidirectional freezing method for lightweight EMI shielding application. The synergism of the lamellar and porous structure of the MXene/CNT hybrid aerogels contributed extensively to their excellent electrical conductivity (9.43 S cm^-1) and superior electromagnetic shielding effectiveness (EMI SE) value of 103.9 dB at 3 mm thickness at the X-band frequency, the latter of which is the best value reported for synthetic porous nanomaterials. The CNT reinforcement in the MXene/CNT hybrid aerogels enhanced the mechanical robustness and increased the compressional modulus by 9661% relative to that of the pristine MXene aerogel. The hybrid aerogel with high electrical conductivity, good mechanical strength, and superior EMI shielding performance is a promising material for inhibiting EMI pollution.

Precision Interface Engineering of an Atomic Layer in Bulk Bi2Te3 Alloys for High Thermoelectric Performance

Grafting nanotechnology on thermoelectric materials leads to significant advances in their performance. Creation of structural defects including nano-inclusion and interfaces via nanostructuring achieves higher thermoelectric efficiencies. However, it is still challenging to optimize the nanostructure via conventional fabrication techniques. The thermal instability of nanostructures remains an issue in the reproducibility of fabrication processes and long-term stability during operation. This work presents a versatile strategy to create numerous interfaces in a thermoelectric material via an atomic-layer deposition (ALD) technique. An extremely thin ZnO layer was conformally formed via ALD over the Bi_0.4Sb_1.6Te₃ powders, and numerous heterogeneous interfaces were generated from the formation of Bi_0.4Sb_1.6Te₃-ZnO core-shell structures even after high-temperature sintering. The incorporation of ALD-grown ZnO into the Bi_0.4Sb_1.6Te₃ matrix blocks phonon propagation and also provides tunability in electronic carrier density via impurity doping at the heterogeneous grain boundaries. The exquisite control in the ALD cycles provides a high thermoelectric performance of zT = 1.50 ± 0.15 (at 329-360 K). Specifically, ALD is an industry compatible technique that allows uniform and conformal coating over large quantities of powders. The study is promising in terms of the mass production of nanostructured thermoelectric materials with considerable improvements in performance via an industry compatible and reproducible route.

Shape-Adaptable 2D Titanium Carbide (MXene) Heater

Prior to the advent of the next-generation heater for wearable/on-body electronic devices, various properties are required, including conductivity, transparency, mechanical reliability, and conformability. Expansion to two-dimensional (2D) structure of metallic nanowires based on network- and mesh-type geometries has been widely exploited for realizing these heaters. However, the routes led to many drawbacks such as the low-density cross-bar linking, self-aggregation of wire, and high junction resistance. Although 2D carbon nanomaterials such as graphene and reduced graphene oxide (rGO) have shown their potentials for the purpose, CVD-grown graphene with sufficiently high conductivity was limited due to its poor processability for large-area applications, while rGO fabricated with a complex reduction process involving the use of toxic chemicals suffered from a low electrical conductivity. In this study, we demonstrate a simple and robust process, utilizing electrostatic assembling of negatively charged MXene flakes on a positively treated surface of substrate, for fabricating a metal-like 2D MXene thin film heater (TFH). Our TFH showed a high optical property (>65%), low sheet resistance (215 Ω/sq), fast electrothermal response (within dozens of seconds) with an intrinsically high electrical conductivity, and mechanical flexibility (up to 180° bending). Its capability for forming a firm and stable ionic-type interface with a counterpart surface allows us to develop a shape-adaptable and patchable thread heater (TH) that can be shaped on diverse substrates even under harsh conditions of conventional sewing or weaving processes. This work suggests that our shape-adaptable MXene heaters are potentially suitable not only for wearable devices for local heating and defrosting but also for a variety of emerging applications of soft actuators and wearable/flexible healthcare monitoring and thermotherapy.

Mechanical Fatigue Resistance of Piezoelectric PVDF Polymers

The fatigue resistance of piezoelectric PVDF has been under question in recent years. While some report that a significant degradation occurs after 10⁶ cycles of repeated voltage input, others report that the reported degradation originates from the degraded metal electrodes instead of the piezoelectric PVDF itself. Here, we report the piezoelectric response and remnant polarization of PVDF during 10⁷ cycles of repeated compression and tension, with silver paste-based electrodes to eliminate any electrode effect. After applying repeated tension and compression of 1.8% for 10⁷ times, we do not observe any notable decrease in the output voltage generated by PVDF layers. The results from tension experiments show stable remnant polarization of 5.5 μC/cm², however, the remnant polarization measured after repeated compression exhibits a 7% decrease as opposed to the tensed PVDF. These results suggest a possible anisotropic response to stress direction. The phase analyses by Raman spectroscopy reveals no significant change in the phase content, demonstrating the fatigue resistance of PVDF.

Shaping micro-clusters via inverse jamming and topographic close-packing of microbombs

Designing topographic clusters is of significant interest, yet it remains challenging as they often lack mobility or deformability. Here we exploit the huge volumetric expansion (up to 3000%) of a new type of building block, thermally expandable microbombs. They consist of a viscoelastic polymeric shell and a volatile gas core, which, within structural confinement, create micro-clusters via inverse jamming and topographical close-packing. Upon heating, microbombs anchored in rigid confinement underwent balloon-like blowing up, allowing for dense clusters via soft interplay between viscoelastic shells. Importantly, the confinement is unyielding against the internal pressure of the microbombs, thereby enabling self-assembled clusters, which can be coupled with topographic inscription to introduce structural hierarchy on the clusters. Our strategy provides densely packed yet ultralight clusters with a variety of complex shapes, cleavages, curvatures, and hierarchy. In turn, these clusters will enrich our ability to explore the assemblies of the ever-increasing range of microparticle systems.

Self-assembled systems are normally composed of incompressible building blocks, which constrain their space filling efficiency. Yu et al. show programmable, densely packed clusters using thermally expandable soft microparticles, whereby the self-assembling process is realized via a jamming transition.

Density-tunable lightweight polymer composites with dual-functional ability of efficient EMI shielding and heat dissipation

Lightweight dual-functional materials with high EMI shielding performance and thermal conductivity are of great importance in modern cutting-edge applications, such as mobile electronics, automotive, aerospace, and military. Unfortunately, a clear material solution has not emerged yet. Herein, we demonstrate a simple and effective way to fabricate lightweight metal-based polymer composites with dual-functional ability of excellent EMI shielding effectiveness and thermal conductivity using expandable polymer bead-templated Cu hollow beads. The low-density Cu hollow beads (ρ ∼ 0.44 g cm^-3) were fabricated through electroless plating of Cu on the expanded polymer beads with ultralow density (ρ ∼ 0.02 g cm^-3). The resulting composites that formed a continuous 3D Cu network with a very small Cu content (∼9.8 vol%) exhibited excellent EMI shielding (110.7 dB at 7 GHz) and thermal conductivity (7.0 W m^-1 K^-1) with isotropic features. Moreover, the densities of the composites are tunable from 1.28 to 0.59 g cm^-3 in accordance with the purpose of their applications. To the best of our knowledge, the resulting composites are the best lightweight dual-functional materials with exceptionally high EMI SE and thermal conductivity performance among synthetic polymer composites.

Sulfonated Copper Phthalocyanine/Sulfonated Polysulfone Composite Membrane for Ionic Polymer Actuators with High Power Density and Fast Response Time

Ionic polymer composite membranes based on sulfonated poly(arylene ether sulfone) (SPAES) and copper(II) phthalocyanine tetrasulfonic acid (CuPCSA) are assembled into bending ionic polymer actuators. CuPCSA is an organic filler with very high sulfonation degree (IEC = 4.5 mmol H⁺/g) that can be homogeneously dispersed on the molecular scale into the SPAES membrane, probably due to its good dispersibility in SPAES-containing solutions. SPAES/CuPCSA actuators exhibit larger ion conductivity (102 mS cm^-1), tensile modulus (208 MPa), strength (101 MPa), and strain (1.21%), exceptionally faster response to electrical stimuli, and larger mechanical power density (3028 W m^-3) than ever reported for ion-conducting polymer actuators. This outstanding actuation performance of SPAES/CuPCSA composite membrane actuators makes them attractive for next-generation transducers with high power density, which are currently developed, e.g., for underwater propulsion and endoscopic surgery.

Multidirectional Wrinkle Patterns Programmed by Sequential Uniaxial Strain with Conformal yet Nontraceable Masks

Surface wrinkling is a promising route to control the mechanical, electrical, and optical properties of materials in a wide range of applications. However, previous artificial wrinkles are restricted to single or random orientation and lacks selectivity. To address this challenge, this study presents multidirectional wrinkle patterns with high selectivity and orientation through sequential uniaxial strain with conformal polymeric shadow masks. The conformal but nontraceable polymeric stencil with microapertures are adhered to a flat substrate prior to oxidation, which forms discrete and parallel wrinkles in confined domains without any contamination. By fully investigating the process, this study displays compound topography of wrinkles consisting of wrinkle islands and surrounding secondary wrinkles on the same surface. With this topography, various diffusion properties are presented: from semi-transparent yet diffusive films to multidirectional diffusive films, which will be available for new types of optical diffuser applications.

Synthesis of Multifunctional Electrically Tunable Fluorine-Doped Reduced Graphene Oxide at Low Temperatures

Doping with heteroatoms is a well-established method to tune the electronic properties and surface chemistry of graphene. Herein, we demonstrate the synthesis of a fluorine-doped reduced graphene oxide (FrGO) at low temperatures that offers multiple opportunities in applied fields. The as-synthesized FrGO product shows a better electrical conductivity of 750 S m^-1 than that of undoped rGO with an electrical conductivity of 195 S m^-1. To demonstrate the multifunctional applications of the as-synthesized FrGO, it was examined for electromagnetic interference shielding and electrochemical sensing of histamine as an important food biomarker. A laminate of FrGO delivered an EMI shielding effectiveness value of 22 dB in Ku band as compared with 11.2 dB for an rGO laminate with similar thickness. On the other hand, an FrGO modified sensor offered an excellent sensitivity (∼7 nM), wide detection range, and good selectivity in the presence of similar biomarkers. This performance originates from the better catalytic ability of FrGO as compared with rGO, where fluorine atoms play the role of catalytic active sites owing to their high electronegativity. The fluorination reaction also helps to improve the reduction degree of the chemically synthesized graphene, consequently enhancing the electrical conductivity, which is a prime requirement for increasing the electromagnetic and electrochemical properties of graphene.

Electromagnetic interference shielding with 2D transition metal carbides (MXenes)

Materials with good flexibility and high conductivity that can provide electromagnetic interference (EMI) shielding with minimal thickness are highly desirable, especially if they can be easily processed into films. Two-dimensional metal carbides and nitrides, known as MXenes, combine metallic conductivity and hydrophilic surfaces. Here, we demonstrate the potential of several MXenes and their polymer composites for EMI shielding. A 45-micrometer-thick Ti3C2Tx film exhibited EMI shielding effectiveness of 92 decibels (>50 decibels for a 2.5-micrometer film), which is the highest among synthetic materials of comparable thickness produced to date. This performance originates from the excellent electrical conductivity of Ti3C2Tx films (4600 Siemens per centimeter) and multiple internal reflections from Ti3C2Tx flakes in free-standing films. The mechanical flexibility and easy coating capability offered by MXenes and their composites enable them to shield surfaces of any shape while providing high EMI shielding efficiency.

Multifunctional Mesoporous Ionic Gels and Scaffolds Derived from Polyhedral Oligomeric Silsesquioxanes

A new methodology for fabrication of inorganic-organic hybrid ionogels and scaffolds is developed through facile cross-linking and solution extraction of a newly developed ionic polyhedral oligomeric silsesquioxane with inorganic core. Through design of various cationic tertiary amines, as well as cross-linkable functional groups on each arm of the inorganic core, high-performance ionogels are fabricated with excellent electrochemical stability and unique ion conduction behavior, giving superior lithium ion battery performance. Moreover, through solvent extraction of the liquid components, hybrid scaffolds with well-defined, interconnected mesopores are utilized as heterogeneous catalysts for the CO₂-catalyzed cycloaddition of epoxides. Excellent catalytic performance, as well as highly efficient recyclability are observed when compared to other previous literature materials.

Nonlinear Frameworks for Reversible and Pluripotent Wetting on Topographic Surfaces

Soft, ultrathin frameworks nonlinearly organized in tandem are presented to realize both reversible and pluripotent wetting on topographic surfaces. A design rule is introduced by establishing and proving the theoretical model upon hierarchical textures. Nonlinear frameworks can be conformally and reversibly wet upon complex topography in nature, thereby overcoming the wetting problems in previous conventional solid systems.

Facilitated Ion Transport in Smectic Ordered Ionic Liquid Crystals

A novel ionic mixture of an imidazolium-based room-temperature ionic liquid containing ethylene-oxide-functionalized phosphite anions is fabricated, which, when doped with lithium salt, self-assembles into a smectic-ordered ionic liquid crystal through Coulombic interactions between the ion species. Interestingly, the smectic order in the ionic-liquid-crystal ionogel facilitates ionic transport.