First-Principles Calculations on the Adsorption Behavior of Amino Acids on a Titanium Carbide MXene

The nature of the electronic ground state of M2C (M = Ti, V, Cr, Zr, Nb, Mo, Hf, Ta, and W) MXenes

A systematic computational study is presented aimed at accurately describing the electronic ground state nature and properties of M2C (M = Ti, V, Cr, Zr, Nb, Mo, Hf, Ta, and W) MXenes. Electronic band structure calculations in the framework of density functional theory (DFT), carried out with different types of basis sets and employing the generalized gradient approach (GGA) and hybrid functionals, provide strong evidence that Ti2C, Zr2C, Hf2C, and Cr2C MXenes exhibit an open-shell conducting ground state with localized spins on the metal atoms, while V2C, Nb2C, Mo2C, Ta2C, and W2C MXenes exhibit a diamagnetic conducting ground state. For Ti2C, Zr2C, Hf2C, and Cr2C, the analysis of the low-lying spin polarized solutions with different spin orderings indicates that their ground states are antiferromagnetic (AFM), consisting of two ferromagnetic (FM) metal layers coupled antiferromagnetically. For the diamagnetic MXenes, the converged spin polarized solutions are significantly less stable than the closed shell solution except for the case of V2C and Mo2C where those excited open shell solutions can be thermally accessible (less than 300 meV per formula unit). The analysis of charge and spin density distributions of the ground state of the MXenes reveals that, in all cases, the metal atoms have a net charge close to +1 e and C atoms close to -2 e. In the case of diamagnetic MXenes, the electronic structure of V2C, Nb2C, and Ta2C is consistent with metal atoms exhibiting a closed-shell s2d2 configuration whereas for Mo2C, and W2C is consistent with a low-spin s1d4 configuration although the FM solution is close in energy for V2C and Mo2C suggesting that they may play a role in their chemistry at high temperature. For the open shell MXenes, the spin density primarily located at the metal atoms showing one unpaired electron per Ti+, Zr+, and Hf+ magnetic center, consistent with s2d1 configuration of the metal atom, and of ∼3.5 unpaired electrons per Cr+ magnetic center interpreted as a mixture of s2d3 and high-spin s1d4 configuration. Finally, the analysis of the density of states reveals the metallic character of all these bare MXenes, irrespective of the nature of the ground state, with significant covalent contributions for Mo2C and W2C.

Exploring the Synergies of Single-Molecule Fluorescence and 2D Materials Coupled by DNA

The world of 2D materials is steadily growing, with numerous researchers attempting to discover, elucidate, and exploit their properties. Approaches relying on the detection of single fluorescent molecules offer a set of advantages, for instance, high sensitivity and specificity, that allow the drawing of conclusions with unprecedented precision. Herein, it is argued how the study of 2D materials benefits from fluorescence-based single-molecule modalities, and vice versa. A special focus is placed on DNA, serving as a versatile adaptor when anchoring single dye molecules to 2D materials. The existing literature on the fruitful combination of the two fields is reviewed, and an outlook on the additional synergies that can be created between them provided.

High-capacity/high-rate hybrid column for high-performance ion exchange

Herein, a fixed-bed high-capacity/high-rate (HC/HR) hybrid column was developed using commercial ion-exchange beads (IEBs) and ion-exchange fibers (IEFs). The as-fabricated HC/HR hybrid column exhibited excellent breakthrough bed volume (BV) and utilization efficiency of capacity (UEC) at a high service flow rate (SFR) for the adsorption of Cd(II). The IEBs displayed a high adsorption capacity of 235.2 ± 9.8 mg g^-1 and slow adsorption kinetics (k₂ = 0.0001 g mg^-1 min^-1) for the sorption of Cd(II); meanwhile, the IEFs showed a maximum adsorption capacity of only 146.3 ± 7.5 mg g^-1, which is lower than that of the IEBs, but fast kinetics (k₂ = 0.0130 g mg^-1 min^-1). At an SFR of 104.23 BV h^-1, the HC/HR hybrid column showed excellent performance for the sorption of Cd(II), having a high breakthrough BV of 1009.11 and a UEC of 92.86%; these values are much higher than those of the IEB-packed column. Furthermore, at an increased SFR (318.47 BV h^-1), the HC/HR hybrid column maintained its high performance, demonstrating a breakthrough BV of 568.80 and UEC of 83.90%. The regeneration experiment indicates that 97% of the initial capacity was retained. Thus, the HC/HR hybrid column could easily be applied to existing column systems and shows promising performance in ion-exchange processes.

Theoretical Analysis of Magnetic Coupling in the Ti2C Bare MXene

The nature of the electronic ground state of the Ti₂C MXene is unambiguously determined by making use of density functional theory-based calculations including hybrid functionals together with a stringent computational setup providing numerically converged results up to 1 meV. All the explored density functionals (i.e., PBE, PBE0, and HSE06) consistently predict that the Ti₂C MXene has a magnetic ground state corresponding to antiferromagnetic (AFM)-coupled ferromagnetic (FM) layers. A spin model, with one unpaired electron per Ti center, consistent with the nature of the chemical bond emerging from the calculations, is presented in which the relevant magnetic coupling constants are extracted from total energy differences of the involved magnetic solutions using an appropriate mapping approach. The use of different density functionals enables us to define a realistic range for the magnitude of each of the magnetic coupling constants. The intralayer FM interaction is the dominant term, but the other two AFM interlayer couplings are noticeable and cannot be neglected. Thus, the spin model cannot be reduced to include nearest-neighbor interactions only. The Néel temperature is roughly estimated to be in the 220 ± 30 K, suggesting that this material can be used in practical applications in spintronics and related fields.

MXenes à la Carte: Tailoring the Epitaxial Growth Alternating Nitrogen and Transition Metal Layers

A high-throughput analysis based on density functional simulations underscores the viable epitaxial growth of MXenes by alternating nitrogen and metal adlayers. This is supported by an exhaustive analysis of a number of thermodynamic and kinetic thresholds belonging to different critical key steps in the course of the epitaxial growth. The results on 18 pristine N- and C-based MXenes with M₂X stoichiometry reveal an easy initial N₂ fixation and dissociation, where N₂ adsorption is controlled by the MXene surface charge and metal d-band center and its dissociation controlled by the reaction energy change. Furthermore, formation energies indicate the plausible formation of N-terminated M₂XN₂ MXenes. Moreover, the further covering with metal adlayers is found to be thermodynamically driven and stable, especially when using early transition metal atoms. The most restrictive analyzed criterion is the N₂ adsorption and dissociation at nearly full N-covered adlayers, which is yet achievable for almost half of the explored M₂X seeds. The present results unfold the possibility of expanding, controlling, and tuning the composition, width, and structure of the MXene family.

Advances and emerging challenges in MXenes and their nanocomposites for biosensing applications

Two-dimensional materials have unique properties and their better functionality has created new paradigms in the field of sensing. Over the past decade, a new family of 2D materials known as MXenes has emerged as a promising material for numerous applications, including biosensing. Their metallic conductivity, rich surface chemistry, hydrophilicity, good biocompatibility, and high anchoring capacity for biomaterials make them an attractive candidate to detect a variety of analytes. Despite such notable properties, there are certain limitations associated with them. This review aims to present a detailed survey of MXene's synthesis; in particular, their superiority in the field of biosensing as compared to other 2D materials is addressed. Their low oxidative stability is still an open challenge, and recent investigations on MXene's oxidation are summarized. The hexagonal stacking network of MXenes acts as a distinctive matrix to load nanoparticles, and the embedded nanoparticles can bind an excess number of biomolecules (e.g., antibodies) thereby improving biosensor performance. We will also discuss the synthesis and corresponding performance of MXenes nanocomposites with noble metal nanoparticles and magnetic nanoparticles. Furthermore, Nb and Ti2C-based MXenes, and Ti3C2-MXene sandwich immunoassays are also reviewed in view of their importance. Different aspects and challenges associated with MXenes (from their synthesis to final applications) and the future perspectives described give new directions to fabricate novel biosensors.

Nitrogen-Containing Gas Sensing Properties of 2-D Ti2N and Its Derivative Nanosheets: Electronic Structures Insight

In this work, the potentials of two-dimensional Ti2N and its derivative nanosheets Ti2NT2(T=O, F, OH) for some harmful nitrogen-containing gas (NCG) adsorption and sensing applications have been unveiled based on the quantum-mechanical Density Functional Theory calculations. It is found that the interactions between pure Ti2N and NCGs (including NO, NO2, and NH3 in this study) are very strong, in which NO and NO2 can even be dissociated, and this would poison the substrate of Ti2N monolayer and affect the stability of the sensing material. For the monolayer of Ti2NT2(T=O, F, OH) that is terminated by functional groups on surface, the adsorption energies of NCGs are greatly reduced, and a large amount of charges are transferred to the functional group, which is beneficial to the reversibility of the sensing material. The significant changes in work function imply the good sensitivity of the above mentioned materials. In addition, the fast response time further consolidates the prospect of two-dimensional Ti2NT2 as efficient NCGs' sensing materials. This theoretical study would supply physical insight into the NCGs' sensing mechanism of Ti2N based nanosheets and help experimentalists to design better 2-D materials for gas adsorption or sensing applications.

Lysozyme Adsorption on Different Functionalized MXenes: A Multiscale Simulation Study

Recently, MXenes, due to their abundant advantages, have been widely applied in energy storage, separation, catalysis, biosensing, et al. In this study, parallel tempering Monte Carlo and molecular dynamics methods were performed to investigate lysozyme adsorption on different functionalized Ti₃C₂T_x (-O, -OH, and -F). The simulation results show that lysozyme can adsorb effectively on Ti₃C₂T_x surfaces, and the order of interaction strength is Ti₃C₂O₂ > Ti₃C₂F₂ > Ti₃C₂(OH)₂. Electrostatics together with van der Waals interactions control protein adsorption. The orientation distributions of lysozyme adsorbed on the Ti₃C₂O₂ and Ti₃C₂F₂ surfaces are more concentrated than that on the Ti₃C₂(OH)₂ surface. During adsorption, the conformation of lysozyme remains stable, suggesting the good biocompatibility of Ti₃C₂T_x. Besides, the distribution of the interfacial water layer on the Ti₃C₂T_x surface has a certain impact on protein adsorption. This study provides theoretical insights for understanding the biocompatibility of 2D Ti₃C₂T_x materials and may help us evaluate the engineering of their surfaces for future biorelated applications.

Exfoliation Energy as a Descriptor of MXenes Synthesizability and Surface Chemical Activity

MXenes are two-dimensional nanomaterials isolated from MAX phases by selective extraction of the A component-a p-block element. The MAX exfoliation energy, Eexf, is considered a chemical descriptor of the MXene synthesizability. Here, we show, by density functional theory (DFT) estimations of Eexf values for 486 different MAX phases, that Eexf decreases (i) when MAX is a nitride, (ii) when going along a metal M component d series, (iii) when going down a p-block A element group, and (iv) when having thicker MXenes. Furthermore, Eexf is found to bias, even to govern, the surface chemical activity, evaluated here on the CO2 adsorption strength, so that more unstable MXenes, displaying larger Eexf values, display a stronger attachment of species upon.

Theoretical Insight on the Biosensing Applications of 2D Materials

The research on the design of efficient, reliable, and cost-effective biosensors is expanding given its high demand in various fields such as health care, environmental surveillance, agriculture, diagnostics, industries, and so forth. In the last decade, various fascinating and interesting 2D materials with extraordinary properties have been experimentally synthesized and theoretically predicted. 2D materials have been explored for the sensing of different biomolecules because of their large surface area and strong interaction with different biomolecules. Theoretical simulations can bring important insight on the interaction of biomolecules on 2D materials, charge transfer, orbital interactions, and so forth and may play an important role in the development of efficient biosensors. Quantum simulation techniques, such as density functional theory (DFT), are very powerful and are gaining popularity especially with the advent of high-speed computing facilities. This review article provides theoretical insight regarding the interaction of various biomolecules on different 2D materials and the charge transfer between the biomolecules and 2D materials leading to electrochemical signals, which can then provide experimentalists the useful design parameters for fabrication of biosensors. It also includes an overview of quantum simulations, use of the DFT method for simulating biomolecules on 2D materials, parameters obtained from theoretical simulations and sensitivity, and limitations of computational techniques for sensing biomolecules on 2D materials. Furthermore, this review summarizes the recent work in first-principles investigation of 2D materials for the purpose of biomolecule sensing. Beyond the traditional graphene or 2D transition-metal dichalcogenides, some novel and recently proposed 2D materials such as pentagraphene, haeckelite, MXenes, and so forth which have exhibited good sensing applications have also been highlighted.