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

Ramanavicius S., Jagminas A., Ramanavicius A.

Nanostructured titanium compounds have recently been applied in the design of gas sensors. Among titanium compounds, titanium oxides (TiO2) are the most frequently used in gas sensing devices. Therefore, in this review, we are paying significant attention to the variety of allotropic modifications of titanium oxides, which include anatase, rutile, brukite. Very recently, the applicability of non-stoichiometric titanium oxide (TiO2−x)-based layers for the design of gas sensors was demonstrated. For this reason, in this review, we are addressing some research related to the formation of non-stoichiometric titanium oxide (TiO2−x) and Magnéli phase (TinO2n−1)-based layers suitable for sensor design. The most promising titanium compounds and hetero- and nano-structures based on these compounds are discussed. It is also outlined that during the past decade, many new strategies for the synthesis of TiO2 and conducting polymer-based composite materials were developed, which have found some specific application areas. Therefore, in this review, we are highlighting how specific formation methods, which can be used for the formation of TiO2 and conducting polymer composites, can be applied to tune composite characteristics that are leading towards advanced applications in these specific technological fields. The possibility to tune the sensitivity and selectivity of titanium compound-based sensing layers is addressed. In this review, some other recent reviews related to the development of sensors based on titanium oxides are overviewed. Some designs of titanium-based nanomaterials used for the development of sensors are outlined.

Seredych M., Maleski K., Mathis T.S., Gogotsi Y.

Delamination of two-dimensional materials is a requisite step for exploiting their unique properties. Herein, we report on the formation of stable colloidal solutions of Ti 3 C 2 and Mo 2 Ti 2 C 3 MXene nanosheets by the dispersion and electrostatic exfoliation of stacked multilayer MXenes in the presence of albumin. Delamination of multilayered MXenes into atomically thin nanosheets resulted in stable colloidal solutions with good stability due to the adsorption of albumin, which prevents re-aggregation of the few-layer nanosheets. Cascading centrifugation was used to produce monodisperse colloids. This study demonstrates a processing method that can be utilized to prepare MXenes for biomedical applications. Since protein adsorption onto nanomaterial surfaces plays a significant role in many fields, including medicine, biology, pharmaceuticals, and environmental engineering, albumin coated MXenes may find many applications.

Choi E., Lee J., Kim Y., Kim H., Kim M., Hong J., Kang Y.C., Koo C.M., Kim D.W., Kim S.J.

MXenes have recently attracted significant interest owing to their outstanding properties and performance. However, their hydrophilic and metastable surfaces make most MXenes prone to oxidation, which can greatly degrade their properties and hinder their practical applications. Here, we enhanced the stability of Ti 3 C 2 T x MXene films by coating a continuous zeolitic imidazolate framework-8 (ZIF-8) layer. The high-density oxygen functional groups of MXene, which are crucial for inducing the nucleation of ZIF-8 crystals, were merged into a continuous layer on the MXene surface. 98% of the original electromagnetic interference shielding effectiveness of ZIF-8/Ti 3 C 2 T x MXene was retained even after 4 days of harsh oxidation treatment at 85 °C and 85% RH. The enhanced stability could be attributed to the hydrophobic microporous structure of ZIF-8, which effectively hindered the permeation of water molecules in addition to terminating the dangling bonds of MXene with Zn ions.

Su T., Ma X., Tong J., Ji H., Qin Z., Wu Z.

This paper reviews the surface engineering, including surface termination groups, surface functionalization, surface defects and surface oxidation, of MXenes, and their impact on energy and environmental applications of MXenes.

Li X., Huang Z., Shuck C.E., Liang G., Gogotsi Y., Zhi C.

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. Dramatic innovations in surface and bulk chemistry enable MXenes to flourish in electrochemical applications. This Review analyses the recorded footprints of MXene components for energy storage, with particular attention paid to a coherent understanding of the fundamental relationship between MXene components and their qualified roles from a nuanced chemical perspective.

Bhardwaj S.K., Singh H., Khatri M., Kim K., Bhardwaj N.

Over the last decade MXenes have become a hotspot of materials science as one of the newest 2-dimensional (2D) materials. Upon the recognition of their distinctive features (e.g., superior optical characteristics, large surface area, excellent hydrophilicity, biocompatibility, ease of surface functionalization, and high conductivity), their potential in biosensing applications has also gained considerable attention. With versatility in MXene synthesis methods and suitable etching, MXenes can be easily transformed into quantum dots, nanosheets, and MXenes composites. As such, during the last decade optical biosensing platforms-based on MXenes have emerged along with electrochemical sensors and wearable sensors built from MXenes. Herein, we present a broad perspective on the optical properties of MXenes alongside recent findings on their biosensing applications, which are based on different optical transduction principles (e.g., photoluminescence, colorimetry, surface plasmon resonance, surface-enhanced Raman scattering, and electro chemiluminescence). Furthermore, the future perspective and challenges concerning MXenes-based optical sensing techniques are discussed.

Wang K., Jin H., Li H., Mao Z., Tang L., Huang D., Liao J., Zhang J.

The recently discovered surface-group-dependent superconductivity in Nb$_2$C-MXene fabricated by the molten salts method is attracting wide attention. However, regarding the superconductivity of Nb$_2$C-MXene with functional F groups (Nb$_2$CF$_x$), there were some conflicting results in experimental and theoretical studies. Herein, we systematically carried out experimental and theoretical investigations on the superconductivity in Nb$_2$C-MXene with the Cl functional group (Nb$_2$CCl$_x$) and Nb$_2$CF$_x$. The experimental results of the Meissner effect and zero resistivity have proved that Nb$_2$CClx is superconducting with the transition temperature (Tc) ~ 5.2 K. We extract its superconducting parameters from the temperature dependence of resistivity and the field dependence of the magnetization. The Ginzburg-Landau parameter K$_G$$_L$ is estimated to be 2.41, indicating that Nb$_2$CClx is a typical type-II superconductor. Conversely, both magnetic and electrical transport measurements demonstrate that Nb$_2$CF$_x$ is not superconducting. The first-principles density functional theory (DFT) calculations show that the Tc of Nb$_2$Cl$_x$ is ~ 5.2 K, while Nb$_2$CF$_x$ is dynamically unstable with imaginary frequency in phonon spectrum, which is in good agreement with the experimental results. Our studies not only are useful for clarifying the present inconsistency but also offer referential significance for future investigations on the superconductivity of MXenes.

Ibragimova R., Rinke P., Komsa H.

Two-dimensional MXenes have recently received increased attention due to their facile synthesis process and extraordinary properties suitable for many different applications. During the wet etching synthesis of MXenes, native defects, such as metal and carbon or nitrogen vacancies, are produced, but the underlying defect formation processes are poorly understood. Here, we employ first-principles calculations to evaluate formation energies of Ti, C, and N vacancies in Ti3C2 and Ti2N MXenes under etching conditions. We carefully account for the mixed functionalization of the surfaces as well as the chemical environment in the solution (pH and electrode potential). We observe that the formation energies of the metal vacancies differ significantly for different types of surface functionalization as well as for different local and global environments. We attribute these differences to electrostatic interactions between vacancies and the surrounding functional groups. We predict that Ti vacancies will be prevalent on bare or OH-functionalized surfaces but not on O-functionalized ones. In contrast, C and N vacancies are more prevalent in O-functionalized surfaces. In addition, our results suggest that the pH value of the etching solution and the electrode potential strongly affect vacancy formation. In particular, the predicted conditions at which abundant vacancy formation is expected are compared to experiments and found to coincide with conditions at which MXenes oxidize readily. This suggests that Ti vacancy formation is a crucial step in initiating the oxidation process.

Shevchuk K., Sarycheva A., Gogotsi Y.

This initial report demonstrates the family of two-dimensional transition-metal carbides and carbonitrides (MXenes) as promising plasmonic materials for surface-enhanced Raman spectroscopy (SERS), allowing sensitive detection of analyte molecules. Their surface plasmon resonances, metallic characters, and rich surface chemistries enable both SERS electromagnetic and chemical enhancements. We have synthesized seven MXenes—Nb2C, Mo2C, Ti2C, V2C, Ti3C2, Mo2TiC2, and Ti3CN, of which only Ti3C2 and Nb2C have been explored as pristine substrates for SERS applications. All explored MXenes could detect the 10–7 M concentrations of Rhodamine 6G dye. Ti3C2 and Ti2C show the highest enhancement and were compared to the commercially available substrates, where Ti3C2 outperformed gold nanoparticles. Additionally, the best-performing MXene was tested with a variety of dyes, suggesting the versatility of MXene sensing. Due to high charge carrier densities and tunable surface chemistries, MXenes show potential for SERS sensors that are inexpensive, easy to fabricate and optimize for specific applications. Surface-enhanced Raman spectroscopy is a powerful, non-destructive analytical tool that provides high selectivity and sensitivity and could be used for single-molecule characterization. It allows for fast detection of environmental pollutants and toxins, as well as biosensing and environmental monitoring. However, the prevalence of the application is limited by the cost and stability of commercially available substrates that employ noble metal nanostructures (Ag, Au, Pt). Recent research activities have focused on discovering new enhancing substrates: the Ti3C2 MXene has been gaining interest as a SERS substrate due to its surface plasmon resonance, metallic behavior, and surface terminations, achieving the record-setting enhancement factors. In contrast, this study elucidates the capability of MXenes beyond Ti3C2 to perform as SERS substrates. We showed that Nb2C, Mo2C, Ti2C, V2C, Ti3C2, Mo2TiC2, and Ti3CN MXenes could detect low concentrations of dye molecules. Furthermore, compared to the commercial gold nanoparticle substrate, Ti3C2 showed advantageous performance. There are approximately 50 MXenes that have been synthesized to date. Their high charge carrier densities and tunable surface chemistries open the avenues of SERS detection of various analytes, with the perspective of replacing noble metals with materials made of earth-abundant elements. This would make SERS more accessible for biomedical, environmental, and forensic applications.

Zahra S.A., Rizwan S.

Utilization of cost-effective, bifunctional, and efficient electrocatalysts for complete water splitting is desirable for sustainable clean hydrogen energy. In last decade, MXenes, a family of emerging two-dimensional (2D) materials with unique physiochemical properties, enticed scientists because of their use in different applications. However, insufficient electron transport, lower intrinsic chemical activity and limited active site densities are the factors inhibiting their use in electrocatalytic cells for hydrogen production. Here, we have presented material design to address this issue and introduced carbon nanotubes (CNTs) on V2CT x MXene sheets for conductive network channels that enhance the ion diffusion for enhanced electrochemical activity. The SEM reveals the uniform dispersion of the MWCNTs over the MXene surface that resulted in the formation of conductive network channels and enhances reaction kinetics. The as-synthesized electrocatalyst was subjected to linear sweep voltammetry (LSV) measurements for hydrogen evolution reaction (HER) and oxygen evolution reaction (OER). The hybrid catalyst M2 exhibited an enhanced HER activity with a lower over-potential of 27 mV which is comparable to commercially available Pt-based catalysts (32 mV). Similarly, an enhanced OER activity was observed with a lower over-potential of 469 mV as compared to pristine V2CT x MXene. The electrocatalyst was subjected to a durability test through chronoamperometry and was observed to be stable for 16 hours. Hence, this study opens a new avenue for future cost-effective efficient catalysts for overall water splitting as a solution to produce clean hydrogen.

Chaudhary V., Kaushik A., Furukawa H., Khosla A.

Sensors are considered to be an important vector for sustainable development. The demand to meet the needs of future generations is accelerating the development of intelligent sensor-systems integrated with internet of things (IoTs), fifth generation (5G) communication, artificial intelligence (AI) and machine learning (ML) strategies. The inclusion of 2D nanomaterials with the IoTs/AI/ML has revolutionized the diversified applications of sensors in healthcare, wearable electronics, safety, environment, defense, and agriculture. Owing to their unique physicochemical characteristics and surface functionalities, borophene and MXenes have emerged as advanced 2D-materials (A2M) to architect future-generation sensors. ML-AI based theoretical modeling has guided the research and development of A2M-sensors economically by reducing cost, human resources, and contamination. A2M-sensors are flexible, wearable, intelligent, biocompatible, portable, energy-efficient, self-sustained, point-of-care, and economical, which can drastically transform the conventional sensing strategies. This review provides an insight in to the state-of-the-art A2M-based physical, chemical, and biosensor to efficiently detect chemical species, gases/vapors, drugs, biomarkers/pathogens, pressure, metal ions, radiations, temperature, light, and humidity. Besides the fundamental challenges creating a gap between theoretical predictions, practical-evaluations, in-lab-technology, and commercial viability, their potential solutions, field-deployable prospects are addressed to realize commercialization, thereby ensuring ability of future generations to maintain sustainable communities.

Rhouati A., Berkani M., Vasseghian Y., Golzadeh N.

Since the discovery of MXenes at Drexel University in the United States in 2011, there has been extensive research regarding various applications of MXenes including environmental remediation. MXenes with a general formula of Mn+1XnTx are a class of two-dimensional (2D) transition metal carbides, carbonitrides, and nitrides with unique chemical and physical characteristics as nanomaterials. MXenes feature characteristics such as high conductivity, hydrophobicity, and large specific surface areas that are attracting attention from researchers in many fields including environmental water engineering such as desalination and wastewater treatment as well as designing and building efficient sensors to detect hazardous pollutants in water. In this study, we review recent developments in MXene-based nanocomposites for electrochemical (bio) sensing with a particular focus on the detection of hazardous pollutants, such as organic components, pesticides, nitrite, and heavy metals. Integration of these 2D materials in electrochemical enzyme-based and affinity-based biosensors for environmental pollutants is also discussed. In addition, a summary of the key challenges and future remarks are presented. Although this field is relatively new, future research on biosensors of MXene-based nanocomposites need to exploit the remarkable properties of these 2D materials.

Wu J., Yu Y., Su G.

MXenes, representing a new class of two-dimensional nanomaterial, have attracted intense interest in a variety of fields as supercapacitors, catalysts, and sensors, and in biomedicine. The assessment of the safety of MXenes and related materials in biological systems is thus an issue that requires significant attention. In this review, the toxic effects of MXenes and their derivatives are summarized through the discussion of current research into their behaviors in mammalian cells, animals and plants. Numerous studies have shown that MXenes have generally low cytotoxicity and good biocompatibility. However, a few studies have indicated that MXenes are toxic to stem cells and embryos. These in vitro and in vivo toxic effects are strongly associated with the dose of material, the cell type, the mode of exposure, and the specific type of MXene. In addition, surface modifications alter the toxic effects of MXenes. The stability of MXenes must be considered during toxicity evaluation, as degradation can lead to potentially toxic byproducts. Although research concerning the toxicity of MXenes is limited, this review provides an overview of the current understanding of interactions of MXenes with biological systems and suggests future research directions.

Zou J., Wu J., Wang Y., Deng F., Jiang J., Zhang Y., Liu S., Li N., Zhang H., Yu J., Zhai T., Alshareef H.N.

2D carbides and nitrides of transition metals, also known as MXenes, are an emerging class of 2D nanomaterials that have shown excellent performances and broad application prospects in the fields of energy storage, catalysis, sensing, electromagnetic shielding, electronics and photonics, and life sciences. This unusual diversity of applications is due to their superior hydrophilicity and conductivity, high carrier concentration, ultra-high volumetric capacitance, rich surface chemistry, and large specific surface area. However, it is difficult to make MXenes with the desired surface functional groups that deliver high reactivity and high stability, because most MXenes are extracted from ceramics (MAX phase) by an etching process, where a large number of metal atoms are inevitably exposed on the surface, with other anions and cations embedded uncontrollably. The exposed metal atoms and implanted ions are thermodynamically unstable and readily react with trace oxygen or oxygen-containing groups to form the corresponding metal oxides or degrade chemically, resulting in a sharp decline in activity and loss of excellent physicochemical properties. The addition of certain synergistic additives during the intercalation and chemical modification of surface functional groups under non-hazardous conditions can result in stable and efficient MXene-based materials with exceptional optical, electrical, and magnetic properties. This review discusses several such methods, mainly additive-mediated intercalation and chemical modification of the surface functional groups of MXene-based materials, followed by their potential applications. Finally, perspectives are given to discuss the future challenges and promising opportunities of this exciting field.

Hu K., Cheng J., Wang K., Zhao Y., Liu Y., Yang H., Zhang Z.

Cytokeratin fragment antigen 21-1 (CYFRA21-1) is a sensitive marker for detecting non-small cell lung cancer (NSCLC). Ti3C2Tx modified by gold nanoparticles (AuNPs) and molybdenum disulfide (MoS2) were synthesized for the first time to obtain the AuNPs@MoS2@Ti3C2Tx composites, which have large specific surface area and good electrocatalytic properties. A novel electrochemical immunoassay for sensitive detection of CYFRA21-1 was developed by loading a large quantity of secondary antibodies (Ab2) and toluidine blue (TB) on the surface of the material as signal probe, and Nafion-AuNPs mixture as electrode material. When the electrochemical response value of CYFRA21-1 increased linearly within the concentration range of 0.5 pg mL-1-50 ng mL-1, the detection limit can reach as low as 0.03 pg mL-1. In addition, the experimental results showed that the biosensor had the potential to rapidly detect CYFRA21-1 in the complex samples such as patient serum, and had a broad application prospect in the early diagnosis and monitoring of NSCLC.

Rana I., Malakar V.K., Ranjan K.R., Verma C., AlFantazi A., Singh P., Kumari K.

Kumar Y.A., Ramachandran T., Ghosh A., Al-Sehemi A.G., Reddy N.P., Moniruzzaman M.

Moshkriz A., Darvishi R., Barati A., Tafazoly M.

Kumar B., Jangra S., Goyat M.S., Mishra Y.K., Kheir M., Halder S., Das S.

AbstractAchieving high electromagnetic interference (EMI) shielding effectiveness alongside outstanding mechanical properties remains a notable challenge for carbon fiber‐reinforced epoxy (CFRE) composites. Herein, an efficient method for interface modification is proposed to produce hybrid CFREs with enhanced EMI shielding capabilities and exceptional mechanical properties. This study introduces the fabrication of CFRE composites incorporating APTES‐modified MXene sheets into the epoxy matrix (AM‐CFRE), demonstrating significant improvements in structural integrity and EMI‐shielding performance. Compared to pristine CFRE (P‐CFRE), AM‐CFRE exhibits enhanced mechanical properties with a 39.63%, 35.41%, 65.01%, 29.71%, 34.9%, and 34.21% increase in tensile strength, Young's modulus, tensile toughness, flexural strength, flexural modulus, and short beam strength, respectively. Furthermore, AM‐CFRE demonstrates a ~50% improvement in EMI shielding effectiveness in the X‐band compared to P‐CFRE. The lightweight AM‐CFRE structure, with strong mechanical strength and remarkable EMI shielding, shows great potential for various applications, particularly in protecting avionics within the aerospace and defense industries.Highlights MXene was synthesized via HF etching, and its surface was modified with APTES. Hybrid CFRE was made with an epoxy system containing surface‐modified MXene. Exceptional mechanical and thermo‐mechanical properties were achieved in Hybrid CFRE. AM‐CFRE achieves EMI shielding effectiveness of 25.36 dB in the X‐band.

Khalid M.Y., Umer R., Zweiri Y.H., Kim J.

Kulandaivel S., Keng N.W., Samykano M., Suraparaju S.K., Ghazali M.F., Rajamony R.K., Abd Ghafar N.S.

G S.C., Gokavi L., Ravikumar C.H., Balarkishna R.G.

Babar Z.U., Iannotti V., Rosati G., Zaheer A., Velotta R., Della Ventura B., Álvarez-Diduk R., Merkoçi A.

This review serves as a tutorial on MXene synthesis, outlining laboratory practices and linking them to core scientific concepts. It also examines healthcare applications, computational aspects, and the role of AI technologies in advancing MXene research.

Banjare M.K., Behera K., Banjare R.K., Pandey S., Ghosh K.K.

Ullah M., Yaseen M.

Jiao J., Yin M., Wang Z., Hu B., Chi J., Lu L., Dai F., Xue L., Wang T., Wang X., Zhao J., Zhao L., Chen Q.

Vojoudi H., Soroush M.

AbstractBiomolecule isolation is a crucial process in diverse biomedical and biochemical applications, including diagnostics, therapeutics, research, and manufacturing. Recently, MXenes, a novel class of two‐dimensional nanomaterials, have emerged as promising adsorbents for this purpose due to their unique physicochemical properties. These biocompatible and antibacterial nanomaterials feature a high aspect ratio, excellent conductivity, and versatile surface chemistry. This timely review explores the potential of MXenes for isolating a wide range of biomolecules, such as proteins, nucleic acids, and small molecules, while highlighting key future research trends and innovative applications poised to transform the field. This review provides an in‐depth discussion of various synthesis methods and functionalization techniques that enhance the specificity and efficiency of MXenes in biomolecule isolation. In addition, the mechanisms by which MXenes interact with biomolecules are elucidated, offering insights into their selective adsorption and customized separation capabilities. This review also addresses recent advancements, identifies existing challenges, and examines emerging trends that may drive the next wave of innovation in this rapidly evolving area.

Dhariwal N., Yadav P., Kumari M., Akanksha, Sanger A., Kang S.B., Kumar V., Thakur O.P.

Tariq M., Sandhu Z.A., Tariq A., Raza M.A., Ashraf S., Ashraf H., Raza H., Al-Sehemi A.G.

Iravani S., Khosravi A., Nazarzadeh Zare E., Varma R.S., Zarrabi A., Makvandi P.

Exploring the symbiotic relationship between MXenes and AI, this highlight focuses on recent advancements pertaining to the prediction and optimization of properties, synthesis routes, and diverse applications of MXene materials.