Multimodal biosensing systems based on metal nanoparticles

Biosensors are currently among the most commonly used devices for analysing biomarkers and play an important role in environmental detection, food safety, and disease diagnosis. Researchers have developed multimodal biosensors instead of single-modal biosensors to meet increasing sensitivity, accuracy, and stability requirements. Metal nanoparticles (MNPs) are beneficial for preparing core probes for multimodal biosensors because of their excellent physical and chemical properties, such as easy regulation and modification, and because they can integrate diverse sensing strategies. This review mainly summarizes the excellent physicochemical properties of MNPs applied as biosensing probes and the principles of commonly used MNP-based multimodal sensing strategies. Recent applications and possible improvements of multimodal biosensors based on MNPs are also described, among which on-site inspection and sensitive detection are particularly important. The current challenges and prospects for multimodal biosensors based on MNPs may provide readers with a new perspective on this field.

Effect of Surface Functional Groups on the Electronic Behavior and Optical Spectra of Mn2N Based MXenes

The extensive applications of MXenes, a novel type of layered materials known for their favorable characteristics, have sparked significant interest. This research focuses on investigating the influence of surface functionalization on the behavior of Mn2NTx (Tx=O2, F2) MXenes monolayers using the "Density functional theory (DFT) based full-potential linearized augmented-plane-wave (FP-LAPW)" method. We elucidate the differences in the physical properties of Mn2NTx through the influence of F and O surface functional groups. We found that O-termination results in half-metallic behavior, whereas the F-termination evolves metallic characteristics within these MXene systems. Similarly, surface termination has effectively influenced their optical absorption efficiency. For instance, Mn2NO2 and Mn2NF2 effectively absorb UV light ~50.15×104 cm-1 and 37.71×104 cm-1, respectively. Additionally, they demonstrated prominent refraction and reflection characteristics, which are comprehensively discussed in the present work. Our predictions offer valuable perspectives into the optical and electronic characteristics of Mn2NTx-based MXenes, presenting the promising potential for implementing them in diverse optoelectronic devices.

Recent Advances and Perspectives of Lewis Acidic Etching Route: An Emerging Preparation Strategy for MXenes

Since the discovery in 2011, MXenes have become the rising star in the field of two-dimensional materials. Benefiting from the metallic-level conductivity, large and adjustable gallery spacing, low ion diffusion barrier, rich surface chemistry, superior mechanical strength, MXenes exhibit great application prospects in energy storage and conversion, sensors, optoelectronics, electromagnetic interference shielding and biomedicine. Nevertheless, two issues seriously deteriorate the further development of MXenes. One is the high experimental risk of common preparation methods such as HF etching, and the other is the difficulty in obtaining MXenes with controllable surface groups. Recently, Lewis acidic etching, as a brand-new preparation strategy for MXenes, has attracted intensive attention due to its high safety and the ability to endow MXenes with uniform terminations. However, a comprehensive review of Lewis acidic etching method has not been reported yet. Herein, we first introduce the Lewis acidic etching from the following four aspects: etching mechanism, terminations regulation, in-situ formed metals and delamination of multi-layered MXenes. Further, the applications of MXenes and MXene-based hybrids obtained by Lewis acidic etching route in energy storage and conversion, sensors and microwave absorption are carefully summarized. Finally, some challenges and opportunities of Lewis acidic etching strategy are also presented.

How titanium and iron are integrated into hematite to enhance the photoelectrochemical water oxidation: a review

Hematite has been considered as a promising photoanode candidate for photoelectrochemical (PEC) water oxidation and has attracted numerous interests in the past decades. However, intrinsic drawbacks drastically lower its photocatalytic activity. Ti-based modifications including Ti-doping, Fe₂O₃/Fe₂TiO₅ heterostructures, TiO₂ passivation layers, and Ti-containing underlayers have shown great potential in enhancing the PEC conversion efficiency of hematite. Moreover, the combination of Ti-based modifications with various strategies towards more efficient hematite photoanodes has been widely investigated. Nevertheless, a corresponding comprehensive overview, especially with the most recent working mechanisms, is still lacking, limiting further improvement. In this respect, by summarizing the recent progress in Ti-modified hematite photoanodes, this review aims to demonstrate how the integration of titanium and iron atoms into hematite influences the PEC properties by tuning the carrier behaviours. It will provide more cues for the rational design of high-performance hematite photoanodes towards future practical applications.

Two-dimensional transition metal carbides and nitrides (MXenes) based biosensing and molecular imaging

As a "star material", 2D transition metal carbides and/or nitrides (MXenes) have tremendous potential applications in biosensor development and molecular imaging. MXenes have a lot of advantages due to their large specific surface, excellent electrical conductivity, adjustable band gap, and easy modification. MXenes that immobilized with DNA strands, proteins, enzymes, or other bioluminescent materials on the surface, have been used to measure small molecules with extraordinary sensitivity and remarkable limit of detection. This review provides an overview of most recent development in the synthesis, fundamental properties, biosensing, and molecular imaging applications of MXenes. We focused on molecular detection through MXene-based electrochemical properties their challenges and novel opportunities of MXenes in biological applications. This article will provide a guide for researchers who are interested in the application of MXenes biosensors.

Surface potential-determined performance of Ti3C2T2 (T = O, F, OH) and Zr3C2T2 (T = O, F, OH, S) MXenes as anode materials of sodium ion batteries

Sodium ion batteries (SIBs) have attracted increasing attention due to their low cost and abundant reserves of sodium, but their ideal anode materials still need to be explored. MXenes could be candidate electrode materials due to their excellent electrical conductivity and large specific surface area. In this work, the theoretical performance of Ti- and Zr-containing MXenes Ti₃C₂T₂ (T = O, F, OH) and Zr₃C₂T₂ (T = O, F, OH, S) as SIB anode materials is investigated. The influence of the Hubbard U correction is discussed, and the behaviour at the MXene surface with the partial occupation of sodium atoms is considered. Including the weight and volume of adsorbed sodium atoms, Ti₃C₂O₂ presents the best performance among the seven MXenes studied. Its mass and volumetric capacities are 299 mA h g^-1 and 993 mA h cm^-3 respectively, and the migration barrier and open circuit voltage are 0.138 eV and 0.421 V. Both Zr₃C₂O₂ and Zr₃C₂S₂ can adsorb double layers of sodium atoms on both sides, and the former shows a higher capacity because of its lower weight and smaller volume. The mass and volumetric capacities of Zr₃C₂O₂ are 254 mA h g^-1 and 913 mA h cm^-3 respectively. More importantly, the surface potential is determined to be an effective descriptor for selecting electrode materials. The migration barrier is proportional to the fluctuation amplitude of the surface potential. A low surface potential generally implies a high capacity. A large open circuit voltage is prone to appear in the structure with a large fluctuation amplitude and a low average value of its surface potential.

Computational screening of functionalized MXenes to catalyze the solid and non-solid conversion reactions in cathodes of lithium-sulfur batteries

The poor cycling abilities of S cathodes due to the dissolution of high-order lithium polysulfides and sluggish reaction kinetics of low-order solid Li₂S hinder the commercial application of lithium-sulfur batteries. Although many hosts have been introduced into S electrodes to anchor high-order polysulfides, an effective procedure to select the hosts to improve the conversion kinetics of solid Li₂S is scarce. Using density functional theory calculations, we proposed a procedure to screen catalytic hosts for solid and non-solid reactions of Li₂S₂/Li₂S by employing the available functionalized Ti₃C₂T₂ MXenes (T = H, O, F, S, Cl, Se, Te, Br, OH, and NH), under the precondition of good anchoring abilities for high-order polysulfides. For the solid-state reactions, it was found that Ti₃C₂Se₂ is the optimal candidate for improving the reaction kinetics of solid Li₂S. Suitable catalysts for different reaction processes between molecular Li₂S₂ and Li₂S have also been proposed. We also proposed that sulfur cathodes doped with heavy atoms (Se or Te) belonging to the main group VI may significantly modify the reaction kinetics of Li₂S. These results provide guidance on synthesizing MXenes with the given surface groups as the hosts and can accelerate the step of finding out other suitable host materials.

Hierarchical crumpled NiMn2O4@MXene composites for high rate ion transport electrochemical supercapacitors

MXenes have received great attention due to their excellent features such as metal-like electronic conductivity, hydrophilic surface groups, and high volumetric capacitance. However, many performances of MXenes are still unsatisfactory due to their low energy density and easy horizontal stacking. In this work, an NiMn2O4@MXene composite with a crumpled surface was fabricated by a hydrothermal method and a developed dip-coating method. The maximum specific capacitance of the electrode is about 1.52 times that of NiMn2O4. Besides, it delivers a retention rate of 93.3% after 4000 cycles due to the increased transport of ions and electrons by the crumpled surface. An asymmetrical device based on the crumpled NiMn2O4@MXene composite and AC was also assembled, which shows an ultra-high energy density. This work provides an effective strategy to solve the vertical stacking problem of MXenes, which can open new avenues for large-scale applications of MXenes in energy storage.

Rational Design of Pillared SnS/Ti3C2Tx MXene for Superior Lithium-Ion Storage

MXenes have been widely explored in energy storage because of their extraordinary properties; however, the majority of research on their application was staged at multilayered MXenes or assisted by carbon materials. Scientifically speaking, the two most distinctive properties of MXenes are usually neglected, composed of large interlayer spacing and abundant surface chemistry, which distinguish MXenes from other two-dimensional materials. Herein, few-layered MXene (f-MXene) nanosheet powders can be easily prepared according to the modified solution-phase flocculation method, completely avoiding the restacking phenomenon of f-MXene nanosheets in preparation and oxidation issues during the storage process. Via further employing the solvothermal reaction and annealing treatment, we successfully constructed pillared SnS/Ti₃C₂T_x composites decorated with in situ formed TiO₂ nanoparticles. In the composites, MXenes can play the role of a conductive network, a buffer matrix for volume expansion of SnS, while the active SnS nanoplates can fully deliver the advantage of high capacity and further induce interlayer engineering of Ti₃C₂T_x during cycling. As a result, the pillared SnS/Ti₃C₂T_x MXene composites exhibit obvious improvement in electrochemical performance. Interestingly, there is an apparent enhancement of capacity at succedent cycling, which can be ascribed to the "pillar effect" of Ti₃C₂T_x MXenes. The efforts and attempts made in this work can further broaden the development of pillared MXene composites.