All-MXene electronic tongue for neurotransmitters detection.

Neurotransmitters (NTs) are molecules produced by neurons that act as the body's chemical messengers. Their abnormal levels in the human system have been associated with many disorders and neurodegenerative diseases, which makes the monitoring of NTs fundamentally important. Specifically for clinical analysis and understanding of brain behavior, simultaneous detection of NTs at low levels quickly and reliably is imperative for disease prevention and early diagnosis. However, the methods currently employed are usually invasive or inappropriate for multiple NTs detection. Herein, we developed a MXene-based impedimetric electronic tongue (e-tongue) for sensitive NT monitoring, using Nb2C, Nb4C3, Mo2C, and Mo2Ti2C3 MXenes as sensing units of the e-tongue, and Principal Component Analysis (PCA) as the data treatment method. The high specific surface area, distinct electrical properties, and chemical stability of the MXenes gave rise to high sensitivity and good reproducibility of the sensor array toward NT detection. Specifically, the e-tongue detected and differentiated multiple NTs (acetylcholine, dopamine, glycine, glutamate, histamine, and tyrosine) at concentrations as low as 1 nmol L-1 and quantified NTs present in a mixture. Besides, analyses performed with interferents and actual samples confirmed the system's potential to be used in clinical diagnostics. The results demonstrate that the MXene-based e-tongue is a suitable, rapid, and simple method for NT monitoring with high accuracy and sensitivity.

Comprehensive synthesis of Ti3C2Tx from MAX phase to MXene.

MXenes are a large family of two-dimensional materials that have attracted attention across many fields due to their desirable optoelectronic, biological, mechanical and chemical properties. There currently exist many synthesis procedures that lead to differences in flake size, defects and surface chemistry, which in turn affect their properties. Herein, we describe the steps to synthesize Ti3C2Tx-the most important and widely used MXene, from a Ti3AlC2 MAX phase precursor. The procedure contains three main sections: synthesis of Ti3AlC2 MAX, wet chemical etching of the MAX in hydrofluoric acid/HCl solution to yield multilayer Ti3C2Tx and its delamination into single-layer flakes. Three delamination options are described; these use LiCl, tertiary amines (tetramethyl ammonium hydroxide/ tetrabutyl ammonium hydroxide) and dimethylsulfoxide respectively. These procedures can be adapted for the synthesis of MXenes beyond Ti3C2Tx. The MAX phase synthesis takes about 1 week, with the etching and delamination each requiring 2 d. This protocol requires users to have experience working with hydrofluoric acid, and it is recommended that users have experience with wet chemistry and centrifugation; characterization techniques such as X-ray diffraction and particle size analysis are also essential for the success of the protocol. While alternative synthesis methods, such as minimally intensive layer delamination, are desirable for certain MXenes (such as Ti2CTx) or specific applications, this protocol aims to standardize the more commonly used hydrofluoric acid/HCl etching method, which produces Ti3C2Tx with minimal concentration of defects and the highest conductivity and serves as a guideline for those working with MXenes for the first time.

Pristine Ti3C2Tx MXene Enables Flexible and Transparent Electrochemical Sensors.

In the era of the internet of things, there exists a pressing need for technologies that meet the stringent demands of wearable, self-powered, and seamlessly integrated devices. Current approaches to developing MXene-based electrochemical sensors involve either rigid or opaque components, limiting their use in niche applications. This study investigates the potential of pristine Ti3C2Tx electrodes for flexible and transparent electrochemical sensing, achieved through an exploration of how material characteristics (flake size, flake orientation, film geometry, and uniformity) impact the electrochemical activity of the outer sphere redox probe ruthenium hexamine using cyclic voltammetry. The optimized electrode made of stacked large Ti3C2Tx flakes demonstrated excellent reproducibility and resistance to bending conditions, suggesting their use for reliable, robust, and flexible sensors. Reducing electrode thickness resulted in an amplified faradaic-to-capacitance signal, which is advantageous for this application. This led to the deposition of transparent thin Ti3C2Tx films, which maintained their best performance up to 73% transparency. These findings underscore its promise for high-performance, tailored sensors, marking a significant stride in advancing MXene utilization in next-generation electrochemical sensing technologies. The results encourage the analytical electrochemistry field to take advantage of the unique properties that pristine Ti3C2Tx electrodes can provide in sensing through more parametric studies.

Synthesis of Three Families of Titanium Carbonitride MXenes.

Layered MAX phases and two-dimensional (2D) MXenes derived from them are among the most studied materials due to their attractive properties and numerous potential applications. The tunability of their structure and composition allows for every property to be modulated over a wide range. Particularly, elemental replacement and formation of a solid solution without changing the structure allow fine-tuning of material properties. While solid solutions on the M (metal) site have received attention, the partial replacement of carbon with nitrogen (carbonitrides) has received little attention. By applying this concept, herein we report the synthesis of three families of titanium carbonitride Tin+1Al(C1-yNy)n MAX phases and Tin+1(C1-yNy)nTx MXenes with one, two, and three C/N layers. This greatly expands the variety of known MAX phases and MXenes to encompass 16 titanium carbonitrides with tunable X-site chemistries and different 2D layer thicknesses, including MXenes in the Ti4(C1-yNy)3Tx system, which have not been previously reported. We further investigated the relationship among the composition, structure, stability, and synthesis conditions of the MXenes and their respective Al-based MAX phases. This range of materials will enable fundamental studies of the N/C ratio effect on optoelectronic, electromagnetic, and mechanical properties of MXenes, as well as tuning those properties for specific applications.

M5X4: A Family of MXenes.

MXenes are two-dimensional (2D) transition metal carbides, nitrides, and carbonitrides typically synthesized from layered MAX-phase precursors. With over 50 experimentally reported MXenes and a near-infinite number of possible chemistries, MXenes make up the fastest-growing family of 2D materials. They offer a wide range of properties, which can be altered by their chemistry (M, X) and the number of metal layers in the structure, ranging from two in M2XTx to five in M5X4Tx. Only one M5X4 MXene, Mo4VC4, has been reported. Herein, we report the synthesis and characterization of two M5AX4 mixed transition metal MAX phases, Ti2.5Ta2.5AlC4 and Ti2.675Nb2.325AlC4, and their successful topochemical transformation into Ti2.5Ta2.5C4Tx and Ti2.675Nb2.325C4Tx MXenes. The resulting MXenes were delaminated into single-layer flakes, analyzed structurally, and characterized for their thermal and optical properties. This establishes a family of M5AX4 MAX phases and their corresponding MXenes. These materials were experimentally produced based on guidance from theoretical predictions, leading to more exciting applications for MXenes.

V4 C3 MXene Immune Profiling and Modulation of T Cell-Dendritic Cell Function and Interaction.

Although vanadium-based metallodrugs are recently explored for their effective anti-inflammatory activity, they frequently cause undesired side effects. Among 2D nanomaterials, transition metal carbides (MXenes) have received substantial attention for their promise as biomedical platforms. It is hypothesized that vanadium immune properties can be extended to MXene compounds. Therefore, vanadium carbide MXene (V4 C3 ) is synthetized, evaluating its biocompatibility and intrinsic immunomodulatory effects. By combining multiple experimental approaches in vitro and ex vivo on human primary immune cells, MXene effects on hemolysis, apoptosis, necrosis, activation, and cytokine production are investigated. Furthermore, V4 C3 ability is demonstrated to inhibit T cell-dendritic cell interactions, evaluating the modulation of CD40-CD40 ligand interaction, two key costimulatory molecules for immune activation. The material biocompatibility at the single-cell level on 17 human immune cell subpopulations by single-cell mass cytometry is confirmed. Finally, the molecular mechanism underlying V4 C3 immune modulation is explored, demonstrating a MXene-mediated downregulation of antigen presentation-associated genes in primary human immune cells. The findings set the basis for further V4 C3 investigation and application as a negative modulator of the immune response in inflammatory and autoimmune diseases.

Recent Advances in 2D Material Theory, Synthesis, Properties, and Applications.

Two-dimensional (2D) material research is rapidly evolving to broaden the spectrum of emergent 2D systems. Here, we review recent advances in the theory, synthesis, characterization, device, and quantum physics of 2D materials and their heterostructures. First, we shed insight into modeling of defects and intercalants, focusing on their formation pathways and strategic functionalities. We also review machine learning for synthesis and sensing applications of 2D materials. In addition, we highlight important development in the synthesis, processing, and characterization of various 2D materials (e.g., MXnenes, magnetic compounds, epitaxial layers, low-symmetry crystals, etc.) and discuss oxidation and strain gradient engineering in 2D materials. Next, we discuss the optical and phonon properties of 2D materials controlled by material inhomogeneity and give examples of multidimensional imaging and biosensing equipped with machine learning analysis based on 2D platforms. We then provide updates on mix-dimensional heterostructures using 2D building blocks for next-generation logic/memory devices and the quantum anomalous Hall devices of high-quality magnetic topological insulators, followed by advances in small twist-angle homojunctions and their exciting quantum transport. Finally, we provide the perspectives and future work on several topics mentioned in this review.

Systematic Study of the Multiple Variables Involved in V2AlC Acid-Based Etching Processes, a Key Step in MXene Synthesis.

The realization of the broad range of application of MXenes relies on the successful and reproducible synthesis of quality materials of tailored properties. To date, most MXenes have been produced making use of acid-based etching methods, yet an in-depth understanding of etching processes is lacking. Herein, we have engaged in a comprehensive study of the multiple variables involved in the synthesis of V2CTx with focus on the properties of etched materials. Two main sets of experiments were considered, each using a different V2AlC precursor and a range of synthesis variables including reaction time and temperature, mixing rate, and type of acid. Correlations of synthesis conditions-materials properties were investigated using a broad range of characterization techniques including analytical methods, scanning and transmission electron microscopy, X-ray diffraction (XRD), and X-ray photoelectron spectroscopy (XPS). Findings indicated the crucial relevance of properties of the MAX precursor such as elemental composition, particle size, and crystal structure on etching processes and properties of etched materials. Particularly, depending on the MAX precursor, two etching patterns were identified, core-shell and plate-by-plate, the latter describing a more efficient etching. Combined studies of elemental composition, crystal structure, and yield quantification allowed us to evaluate the effectiveness of etching processes. XRD studies revealed key crystal-structure-type of acid correlations showing advantages of using a HF/HCl mix over only HF. Analytical methods XRD and XPS delivered insights into undergoing chemical processes and their influence on bulk and surface chemistry of etched materials. The relevance for reaction kinetics of highly correlated variables such as reaction vessel dimensions, mixing efficiency, and reaction temperature was recognized. For the first time, a MXene synthesis has been investigated comprehensively highlighting its multivariable nature and the high variable intercorrelation, opening up venues for further investigation on MAX and MXene synthesis.

MXene Aerogel Derived Ultra-Active Vanadia Catalyst for Selective Conversion of Sustainable Alcohols to Base Chemicals.

Selective oxidation reactions are an important class of the current chemical industry and will be highly important for future sustainable chemical production. Especially, the selective oxidation of primary alcohols is expected to be of high future interest, as alcohols can be obtained on technical scales from biomass fermentation. The oxidation of primary alcohols produces aldehydes, which are important intermediates. While selective methanol oxidation is industrially established, the commercial catalyst suffers from deactivation. Ethanol selective oxidation is not commercialized but would give access to sustainable acetaldehyde production when using renewable ethanol. In this work, it is shown that employing 2D MXenes as building blocks allows one to design a nanostructured oxide catalyst composed of mixed valence vanadium oxides, which outperforms on both reactions known materials by nearly an order of magnitude in activity, while showing high selectivity and stability. The study shows that the synthesis route employing 2D materials is key to obtain these attractive catalysts. V4C3Tx MXene structured as an aerogel precursor needs to be employed and mildly oxidized in an alcohol and oxygen atmosphere to result in the aspired nanostructured catalyst composed of mixed valence VO2, V6O13, and V3O7. Very likely, the bulk stable reduced valence state of the material together coupled with the nanorod arrangement allows for unprecedented oxygen mobility as well as active sites and results in an ultra-active catalyst.

Vanadium and Niobium MXenes-Bilayered V2 O5 Asymmetric Supercapacitors.

MXenes offer high metallic conductivity and redox capacitance that are attractive for high-power, high-energy storage devices. However, they operate limitedly under high anodic potentials due to irreversible oxidation. Pairing them with oxides to design asymmetric supercapacitors may expand the voltage window and increase the energy storage capabilities. Hydrated lithium preintercalated bilayered V2 O5 ( δ-Lix V2 O5 ·nH2 O) is attractive for aqueous energy storage due to its high Li capacity at high potentials; however, its poor cyclability remains a challenge. To overcome its limitations and achieve a wide voltage window and excellent cyclability, it is combined with V2 C and Nb4 C3 MXenes. Asymmetric supercapacitors employing lithium intercalated V2 C (Li-V2 C) or tetramethylammonium intercalated Nb4 C3 (TMA-Nb4 C3 ) MXenes as the negative electrode, and a δ-Lix V2 O5 ·nH2 O composite with carbon nanotubes as the positive electrode in 5 m LiCl electrolyte operate over wide voltage windows of 2 and 1.6 V, respectively. The latter shows remarkably high cyclability-capacitance retention of ≈95% after 10 000 cycles. This work highlights the importance of selecting appropriate MXenes to achieve a wide voltage window and a long cycle life in combination with oxide anodes to demonstrate the potential of MXenes beyond Ti3 C2 in energy storage.

Electrochemically modulated interaction of MXenes with microwaves.

Dynamic control of electromagnetic wave jamming is a notable technological challenge for protecting electronic devices working at gigahertz frequencies. Foam materials can adjust the reflection and absorption of microwaves, enabling a tunable electromagnetic interference shielding capability, but their thickness of several millimetres hinders their application in integrated electronics. Here we show a method for modulating the reflection and absorption of incident electromagnetic waves using various submicrometre-thick MXene thin films. The reversible tunability of electromagnetic interference shielding effectiveness is realized by electrochemically driven ion intercalation and de-intercalation; this results in charge transfer efficiency with different electrolytes, accompanied by expansion and shrinkage of the MXene layer spacing. We finally demonstrate an irreversible electromagnetic interference shielding alertor through electrochemical oxidation of MXene films. In contrast with static electromagnetic interference shielding, our method offers opportunities to achieve active modulation that can adapt to demanding environments.

4D printing of MXene hydrogels for high-efficiency pseudocapacitive energy storage.

2D material hydrogels have recently sparked tremendous interest owing to their potential in diverse applications. However, research on the emerging 2D MXene hydrogels is still in its infancy. Herein, we show a universal 4D printing technology for manufacturing MXene hydrogels with customizable geometries, which suits a family of MXenes such as Nb2CTx, Ti3C2Tx, and Mo2Ti2C3Tx. The obtained MXene hydrogels offer 3D porous architectures, large specific surface areas, high electrical conductivities, and satisfying mechanical properties. Consequently, ultrahigh capacitance (3.32 F cm-2 (10 mV s-1) and 233 F g-1 (10 V s-1)) and mass loading/thickness-independent rate capabilities are achieved. The further 4D-printed Ti3C2Tx hydrogel micro-supercapacitors showcase great low-temperature tolerance (down to -20 °C) and deliver high energy and power densities up to 93 µWh cm-2 and 7 mW cm-2, respectively, surpassing most state-of-the-art devices. This work brings new insights into MXene hydrogel manufacturing and expands the range of their potential applications.

Controlling morphology in electrosprayed methylcellulose nanowires via nanoparticle addition: coarse-grained modeling and experiments.

Electrospray deposition (ESD) has shown great promise for manufacturing micro- and nanostructured coatings at scale on versatile substrates with complex geometries. ESD exhibits a broad spectrum of morphologies depending upon the properties of spray fluids. Among them are nanowire forests or foams obtained via the in-air gelation of electrospray droplets formed from methylcellulose (MC) solutions. In this study, we explored MC ESD loaded with nanoparticles of various shapes and uncovered the effects of particle fillers on morphology evolution using coarse-grained simulations and physical experiments. Utilizing electrostatic dissipative particle dynamics, we modeled the electrohydrodynamic deformation of particle-laden MC droplets undergoing in-flight evaporation. The simulations quantitatively predict the suppression of droplet deformation as the size or concentration of spherical nanoparticles increases. While small particles can be readily encapsulated into the nanowire body, large particles can arrest nanowire formation. The model was extended to nanoparticles with complex topologies, showing MC nanowires emerging from particle edges and vertices due to curvature-enhanced electric stress. In all cases, strong agreements were found between simulation and experimental results. These results demonstrate the efficacy of the coarse-grained model in predicting the morphology evolution of electrosprayed droplets and lay the groundwork for employing MC nanowires for developing nanostructured composites.

Immune Profiling and Multiplexed Label-Free Detection of 2D MXenes by Mass Cytometry and High-Dimensional Imaging.

There is a critical unmet need to detect and image 2D materials within single cells and tissues while surveying a high degree of information from single cells. Here, a versatile multiplexed label-free single-cell detection strategy is proposed based on single-cell mass cytometry by time-of-flight (CyTOF) and ion-beam imaging by time-of-flight (MIBI-TOF). This strategy, "Label-free sINgle-cell tracKing of 2D matErials by mass cytometry and MIBI-TOF Design" (LINKED), enables nanomaterial detection and simultaneous measurement of multiple cell and tissue features. As a proof of concept, a set of 2D materials, transition metal carbides, nitrides, and carbonitrides (MXenes), is selected to ensure mass detection within the cytometry range while avoiding overlap with more than 70 currently available tags, each able to survey multiple biological parameters. First, their detection and quantification in 15 primary human immune cell subpopulations are demonstrated. Together with the detection, mass cytometry is used to capture several biological aspects of MXenes, such as their biocompatibility and cytokine production after their uptake. Through enzymatic labeling, MXenes' mediation of cell-cell interactions is simultaneously evaluated. In vivo biodistribution experiments using a mixture of MXenes in mice confirm the versatility of the detection strategy and reveal MXene accumulation in the liver, blood, spleen, lungs, and relative immune cell subtypes. Finally, MIBI-TOF is applied to detect MXenes in different organs revealing their spatial distribution. The label-free detection of 2D materials by mass cytometry at the single-cell level, on multiple cell subpopulations and in multiple organs simultaneously, will enable exciting new opportunities in biomedicine.

Ultralarge Flakes of Ti3C2Tx MXene via Soft Delamination.

Two-dimensional (2D) titanium carbide MXene (Ti3C2Tx) has attracted significant attention due to its combination of properties and great promise for various applications. The size of the 2D sheets is a critical parameter affecting multiple properties of assembled films, fibers and 3D structures. The increased lateral size of MXene flakes can benefit not only their assemblies by improving the interflake contacts and alignment but also fundamental studies at the individual flake level, allowing for facile patterning and investigation of intrinsic physical properties of MXenes. Increasing the average size of the parent MAX phase is one of the strategies previously used to increase the flake size of the resultant MXene. Here, we show that the protocol used for the next step of the synthesis procedure, delamination of multilayer MXene into individual nanosheets, significantly affects the lateral size of the resultant flakes. We developed a soft delamination approach, which prevents fracture of flakes and preserves their size. Combining this approach with the large-grain Ti3AlC2 MAX phase precursor, we achieved individual flakes of up to 40 µm in lateral size. These flakes can be used for patterning multiple contacts and fabrication of field-effect transistors for multiprobe electrical characterization and other measurements. These findings indicate the importance of controlling the delamination process in order to achieve large MXene flakes and improve properties of MXene-based materials and devices.

MXene chemistry, electrochemistry and energy storage applications.

The diverse and tunable surface and bulk chemistry of MXenes affords valuable and distinctive properties, which can be useful across many components of energy storage devices. MXenes offer diverse functions in batteries and supercapacitors, including double-layer and redox-type ion storage, ion transfer regulation, steric hindrance, ion redistribution, electrocatalysts, electrodeposition substrates and so on. They have been utilized to enhance the stability and performance of electrodes, electrolytes and separators. In this Review, we present a discussion on the roles of MXene bulk and surface chemistries across various energy storage devices and clarify the correlations between their chemical properties and the required functions. We also provide guidelines for the utilization of MXene surface terminations to control the properties and improve the performance of batteries and supercapacitors. Finally, we conclude with a perspective on the challenges and opportunities of MXene-based energy storage components towards future practical applications.

Modified MAX Phase Synthesis for Environmentally Stable and Highly Conductive Ti3C2 MXene.

One of the primary factors limiting further research and commercial use of the two-dimensional (2D) titanium carbide MXene Ti3C2, as well as MXenes in general, is the rate at which freshly made samples oxidize and degrade when stored as aqueous suspensions. Here, we show that including excess aluminum during synthesis of the Ti3AlC2 MAX phase precursor leads to Ti3AlC2 grains with improved crystallinity and carbon stoichiometry (termed Al-Ti3AlC2). MXene nanosheets (Al-Ti3C2) produced from this precursor are of higher quality, as evidenced by their increased resistance to oxidation and an increase in their electronic conductivity up to 20 000 S/cm. Aqueous suspensions of stoichiometric single- to few-layer Al-Ti3C2 flakes produced from the modified Al-Ti3AlC2 have a shelf life of over ten months, compared to 1 to 2 weeks for previously published Ti3C2, even when stored in ambient conditions. Freestanding films made from Al-Ti3C2 suspensions stored for ten months show minimal decreases in electrical conductivity and negligible oxidation. Furthermore, oxidation of the improved Al-Ti3C2 in air initiates at temperatures that are 100-150 °C higher than that of conventional Ti3C2. The observed improvements in both the shelf life and properties of Al-Ti3C2 will facilitate the widespread use of this material.

2D MXenes with antiviral and immunomodulatory properties: A pilot study against SARS-CoV-2.

Two-dimensional transition metal carbides/carbonitrides known as MXenes are rapidly growing as multimodal nanoplatforms in biomedicine. Here, taking SARS-CoV-2 as a model, we explored the antiviral properties and immune-profile of a large panel of four highly stable and well-characterized MXenes - Ti3C2Tx, Ta4C3T x , Mo2Ti2C3T x and Nb4C3T x . To start with antiviral assessment, we first selected and deeply analyzed four different SARS-CoV-2 genotypes, common in most countries and carrying the wild type or mutated spike protein. When inhibition of the viral infection was tested in vitro with four viral clades, Ti3C2T x in particular, was able to significantly reduce infection only in SARS-CoV-2/clade GR infected Vero E6 cells. This difference in the antiviral activity, among the four viral particles tested, highlights the importance of considering the viral genotypes and mutations while testing antiviral activity of potential drugs and nanomaterials. Among the other MXenes tested, Mo2Ti2C3T x also showed antiviral properties. Proteomic, functional annotation analysis and comparison to the already published SARS-CoV-2 protein interaction map revealed that MXene-treatment exerts specific inhibitory mechanisms. Envisaging future antiviral MXene-based drug nano-formulations and considering the central importance of the immune response to viral infections, the immune impact of MXenes was evaluated on human primary immune cells by flow cytometry and single-cell mass cytometry on 17 distinct immune subpopulations. Moreover, 40 secreted cytokines were analyzed by Luminex technology. MXene immune profiling revealed i) the excellent bio and immune compatibility of the material, as well as the ability of MXene ii) to inhibit monocytes and iii) to reduce the release of pro-inflammatory cytokines, suggesting an anti-inflammatory effect elicited by MXene. We here report a selection of MXenes and viral SARS-CoV-2 genotypes/mutations, a series of the computational, structural and molecular data depicting deeply the SARS-CoV-2 mechanism of inhibition, as well as high dimensional single-cell immune-MXene profiling. Taken together, our results provide a compendium of knowledge for new developments of MXene-based multi-functioning nanosystems as antivirals and immune-modulators.

Extremely hard and tough high entropy nitride ceramics.

Simultaneously hard and tough nitride ceramics open new venues for a variety of advanced applications. To produce such materials, attention is focused on the development of high-entropy ceramics, containing four or more metallic components distributed homogeneously in the metallic sublattice. While the fabrication of bulk high-entropy carbides and borides is well established, high-entropy nitrides have only been produced as thin films. Herein, we report on a newel three-step process to fabricate bulk high-entropy nitrides. The high-entropy nitride phase was obtained by exothermic combustion of mechanically-activated nanostructured metallic precursors in nitrogen and consolidated by spark plasma sintering. The fabricated bulk high-entropy nitride (Hf0.2Nb0.2Ta0.2Ti0.2Zr0.2)N demonstrates outstanding hardness (up to 33 GPa) and fracture toughness (up to 5.2 MPa∙m1/2), significantly surpassing expected values from mixture rules, as well as all other reported binary and high-entropy ceramics and can be used for super-hard coatings, structural materials, optics, and others. The obtained results illustrate the scalable method to produce bulk high-entropy nitrides with the new benchmark properties.

Tailoring Electronic and Optical Properties of MXenes through Forming Solid Solutions.

Alloying is a long-established strategy to tailor properties of metals for specific applications, thus retaining or enhancing the principal elemental characteristics while offering additional functionality from the added elements. We propose a similar approach to the control of properties of two-dimensional transition metal carbides known as MXenes. MXenes (Mn+1Xn) have two sites for compositional variation: elemental substitution on both the metal (M) and carbon/nitrogen (X) sites presents promising routes for tailoring the chemical, optical, electronic, or mechanical properties of MXenes. Herein, we systematically investigated three interrelated binary solid-solution MXene systems based on Ti, Nb, and/or V at the M-site in a M2XTx structure (Ti2-yNbyCTx, Ti2-yVyCTx, and V2-yNbyCTx, where Tx stands for surface terminations) showing the evolution of electronic and optical properties as a function of composition. All three MXene systems show unlimited solubility and random distribution of metal elements in the metal sublattice. Optically, the MXene systems are tailorable in a nonlinear fashion, with absorption peaks from ultraviolet to near-infrared wavelength. The macroscopic electrical conductivity of solid solution MXenes can be controllably varied over 3 orders of magnitude at room temperature and 6 orders of magnitude from 10 to 300 K. This work greatly increases the number of nonstoichiometric MXenes reported to date and opens avenues for controlling physical properties of different MXenes with a limitless number of compositions possible through M-site solid solutions.