2D MXene-Based Biosensing: A Review

Nanomaterials revolutionize biosensing: 0D-3D designs for ultrasensitive detection of microorganisms and viruses

Various diseases caused by harmful microorganisms and viruses have caused serious harm and huge economic losses to society. Thus, rapid detection of harmful microorganisms and viruses is necessary for disease prevention and treatment. Nanomaterials have unique properties that other materials do not possess, such as a small size effect and quantum size effect. Introducing nanomaterials into biosensors improves the performance of biosensors for faster and more accurate detection of microorganisms and viruses. This review aims to introduce the different kinds of biosensors and the latest advances in the application of nanomaterials in biosensors. In particular, this review focuses on describing the physicochemical properties of zero-, one-, two-, and three-dimensional nanostructures as well as nanoenzymes. Finally, this review discusses the applications of nanobiosensors in the detection of microorganisms and viruses and the future directions of nanobiosensors.

V-MXenes for Energy Storage/Conversion Applications

MXenes, a two-dimensional (2D) material, exhibit excellent optical, electrical, chemical, mechanical, and electrochemical properties. Titanium-based MXene (Ti-MXene) has been extensively studied and serves as the foundation for 2D MXenes. However, other transition metals possess the potential to offer excellent properties in various applications. This comprehensive review aims to provide an overview of the properties, challenges, key findings, and applications of less-explored vanadium-based MXenes (V-MXenes) and their composites. The current trends in V-MXene and their composites for energy storage and conversion applications have been thoroughly summarized. Overall, this review offers valuable insights, identifies potential opportunities, and provides key suggestions for future advancements in the MXenes and energy storage/conversion applications.

The potential of MXene-based materials in fluorescence-based sensing/biosensing of ionic and organic contaminants in environment and food samples: Recent advancements and challenges

MXenes belong to one of the recently developed advanced materials with tremendous potential for diverse sensing applications. To date, various types of MXene-based materials have been developed to generate direct/indirect ultrasensitive sensing signals against various forms of analytes via fluorescence quenching or enhancement. In this work, the fluorescence sensing/biosensing capabilities of the MXene-based materials have been explored and evaluated against a list of ionic/emerging pollutants in environment and food matrices. The suitability of an MXene-based sensing approach is also validated through the assessment of the performance based on the basic quality assurance parameters, e.g., limit of detection (LOD), sensing range, and response time. Accordingly, the best performing MXene-based materials are selected and recommended for the given target(s) to help facilitate their scalable applications under real-world conditions.

CuxO decorated Ti3C2Tx MXene composites for non-enzymatic glucose sensing with large linear ranges

Ti3C2Tx MXene/CuxO composites were prepared by acid etching combined with electrochemical technique. The abundant active sites on the surface of MXene greatly increase the loading of CuxO nanoparticles, and the synergistic effect between the different components of the composite can accelerate the oxidation reaction of glucose. The results indicate that at the working potential of 0.55 V (vs. Ag/AgCl), the glucose sensor based on Ti3C2Tx MXene/CuxO composite presents large linear concentration ranges from 1 µM to 4.655 mM (sensitivity of 361 µA mM-1 cm-2) and from 5.155 mM to 16.155 mM (sensitivity of 133 µA mM-1 cm-2). The limit of detection is 0.065 µM. In addition, the sensor effectively avoids the oxidative interference of common interfering species such as ascorbic acid, dopamine and uric acid. The sensor has good reproducibility, stability and acceptable recoveries for the detection of glucose in human sweat sample (97.5-103.3%) with RSD values less than 4%. Based on these excellent properties it has great potential for the detection of glucose in real samples.

Polyaromatic Hydrocarbon-Functionalized 2D MXene-Based 3D Porous Antifouling Nanocomposite with Long Shelf Life for High-Performance Electrochemical Immunosensor Applications

Affinity-based electrochemical (AEC) biosensors have gained more attention in the field of point-of-care management. However, AEC sensing is hampered by biofouling of the electrode surface and degradation of the antifouling material. Therefore, a breakthrough in antifouling nanomaterials is crucial for the fabrication of reliable AEC biosensors. Herein, for the first time, we propose 1-pyrenebutyric acid-functionalized MXene to develop an antifouling nanocomposite to resist biofouling in the immunosensors. The nanocomposite consisted of a 3D porous network of bovine serum albumin cross-linked with glutaraldehyde with functionalized MXene as conductive nanofillers, where the inherited oxidation resistance property of functionalized MXene improved the electrochemical lifetime of the nanocomposite. On the other hand, the size-extruded porous structure of the nanocomposite inhibited the biofouling activity on the electrode surface for up to 90 days in real samples. As a proof of concept, the antifouling nanocomposite was utilized to fabricate a multiplexed immunosensor for the detection of C-reactive protein (CRP) and ferritin biomarkers. The fabricated sensor showed good selectivity over time and an excellent limit of detection for CRP and ferritin of 6.2 and 4.2 pg/mL, respectively. This research successfully demonstrated that functionalized MXene-based antifouling nanocomposites have great potential to develop high-performance and low-cost immunosensors.

M4X3 MXenes: Application in Energy Storage Devices

MXene has garnered widespread recognition in the scientific community due to its remarkable properties, including excellent thermal stability, high conductivity, good hydrophilicity and dispersibility, easy processability, tunable surface properties, and admirable flexibility. MXenes have been categorized into different families based on the number of M and X layers in Mn+1Xn, such as M2X, M3X2, M4X3, and, recently, M5X4. Among these families, M2X and M3X2, particularly Ti3C2, have been greatly explored while limited studies have been given to M5X4 MXene synthesis. Meanwhile, studies on the M4X3 MXene family have developed recently, hence, demanding a compilation of evaluated studies. Herein, this review provides a systematic overview of the latest advancements in M4X3 MXenes, focusing on their properties and applications in energy storage devices. The objective of this review is to provide guidance to researchers on fostering M4X3 MXene-based nanomaterials, not only for energy storage devices but also for broader applications.

Skin-inspired, sensory robots for electronic implants

Drawing inspiration from cohesive integration of skeletal muscles and sensory skins in vertebrate animals, we present a design strategy of soft robots, primarily consisting of an electronic skin (e-skin) and an artificial muscle. These robots integrate multifunctional sensing and on-demand actuation into a biocompatible platform using an in-situ solution-based method. They feature biomimetic designs that enable adaptive motions and stress-free contact with tissues, supported by a battery-free wireless module for untethered operation. Demonstrations range from a robotic cuff for detecting blood pressure, to a robotic gripper for tracking bladder volume, an ingestible robot for pH sensing and on-site drug delivery, and a robotic patch for quantifying cardiac function and delivering electrotherapy, highlighting the application versatilities and potentials of the bio-inspired soft robots. Our designs establish a universal strategy with a broad range of sensing and responsive materials, to form integrated soft robots for medical technology and beyond.

Staphylococcus aureus epidemiology, pathophysiology, clinical manifestations and application of nano-therapeutics as a promising approach to combat methicillin resistant Staphylococcus aureus

Staphylococcus aureus is a Gram-positive bacterium and one of the most prevalent infectious disease-related causes of morbidity and mortality in adults. This pathogen can trigger a broad spectrum of diseases, from sepsis and pneumonia to severe skin infections that can be fatal. In this review, we will provide an overview of S. aureus and discuss the extensive literature on epidemiology, transmission, genetic diversity, evolution and antibiotic resistance strains, particularly methicillin resistant S. aureus (MRSA). While many different virulence factors that S. aureus produces have been investigated as therapeutic targets, this review examines recent nanotechnology approaches, which employ materials with atomic or molecular dimensions and are being used to diagnose, treat, or eliminate the activity of S. aureus. Finally, having a deeper understanding and clearer grasp of the roles and contributions of S. aureus determinants, antibiotic resistance, and nanotechnology will aid us in developing anti-virulence strategies to combat the growing scarcity of effective antibiotics against S. aureus.

Vertically aligned MXene bioelectrode prepared by freeze-drying assisted electrophoretic deposition for sensitive electrochemical protein detection

Two-dimensional (2D) carbides, MXenes, have attracted attention as electrode materials of electrochemical biosensors because of their metallic conductivity, hydrophilicity, and mechanical stability. However, when fabricating electrodes, the nanosheets tend to re-stack and generally align horizontally with respect to the current collector due to the highly anisotropic nature of MXene, resulting in low porosity and poor utilization of the MXene surface. Here we report the electrochemical biosensing of antibody-antigen reactions with a vertically aligned Ti3C2Tx MXene (VA-MXene) electrode prepared by freeze-drying assisted electrophoretic deposition. The macroporous VA-MXene electrode exhibited a better electrochemical response towards the immunoreaction between the allergenic buckwheat protein (BWp16) and the antibody compared to a non-porous, horizontally (in-plane) stacked MXene (HS-MXene) and the sensors reported in the literature. The sensor responsiveness, defined as the ratio of the obtained current density of the electrode to the antigen concentration, was much higher for the VA-MXene electrode (238 μA cm-2 (ng mL-1) -1) than for the HS-MXene electrode. The proposed technique is applicable to other exfoliated nanosheets, and will open a new avenue for porous nanosheet electrodes to improve the sensing characteristics of electrochemical biosensors.

Conductive and Enhanced Mechanical Strength of Mo2Ti2C3 MXene-Based Hydrogel Promotes Neurogenesis and Bone Regeneration in Bone Defect Repair

Bone defects are common with increasing high-energy fractures, tumor bone invasion, and implantation revision surgery. Bone is an electroactive tissue that has electromechanical interaction with collogen, osteoblasts, and osteoclasts. Hydrogel provides morphological plasticity and extracellular matrix (ECM) 3D structures for cell survival, and is widely used as a bone engineering material. However, the hydrogels have poor mechanical intensity and lack of cell adhesion, slow gelation time, and limited conductivity. MXenes are novel nanomaterials with hydrophilic groups that sense cell electrophysiology and improve hydrogel electric conductivity. Herein, gelatin had multiple active groups (NH2, OH, and COOH) and an accelerated gelation time. Acrylamide has Schiff base bonds to cross-link with gelatin and absorb metal ions. Deacetylated chitosan improved cell adhesion and active groups to connect MXene and acrylamide. We constructed Mo2Ti2C3 MXene hydrogel with improved elastic modulus and viscosity, chemical cross-linking structure, electric conductivity, and good compatibility. Mo2Ti2C3 MXene hydrogel exhibits outstanding osteogenesis in vitro. Mo2Ti2C3 MXene hydrogel promotes osteogenesis via alkaline phosphatase (ALP) and alizarin red S (ARS) staining, improving osteogenic marker genes and protein expressions in vitro. Mo2Ti2C3 MXene hydrogel aids new bone formation in the in vivo calvarial bone defect model via micro-CT and histology. Mo2Ti2C3 MXene hydrogel facilitates neurogenesis factors nerve growth factor (NGF) and brain-derived neurotrophic factor (BDNF) expression, and aids newly born neuron marker Tuj-1 and sensory neuron marker serotonin (5-HT) and osteogenesis pathway proteins, runt-related transcription factor 2 (Runx2), osteocalcin (OCN), SMAD family member 4 (SMAD4), and bone morphogenetic protein-2 (BMP2) in the bone defect repair process. Mo2Ti2C3 MXene hydrogel promotes osteogenesis and neurogenesis, which extends its biomedical application in bone defect reconstruction.

Green Synthesis and Biosafety Assessment of MXene

The rise of MXene-based materials with fascinating physical and chemical properties has attracted wide attention in the field of biomedicine, because it can be exploited to regulate a variety of biological processes. The biomedical applications of MXene are still in its infancy, nevertheless, the comprehensive evaluation of MXene's biosafety is desperately needed. In this review, the composition and the synthetic methods of MXene materials are first introduced from the view of biosafety. The evaluation of the interaction between MXene and cells, as well as the safety of different forms of MXene applied in vivo are then discussed. This review provides a basic understanding of MXene biosafety and may bring new inspirations to the future applications of MXene-based materials in biomedicine.

Charge Carriers Localization Effect Revealed through Terahertz Spectroscopy of MXene: Ti3C2Tx

The transport properties of charge carriers in MXene, a promising material, have been studied using terahertz time-domain spectroscopy (THz-TDS) to examine its potential applications in optical and electronic devices. However, previous studies have been limited by narrow frequency ranges, which have hindered the understanding of the intrinsic mechanisms of carrier transport in MXenes. To address this issue, ultrabroadband THz-TDS with frequencies of up to 15 THz to investigate the complex photoconductances of MXene (Ti3C2Tx) films with different thicknesses are employed. The findings indicate that the electronic localization is substrate-dependent, and this effect decreases with an increase in the number of layers. This is attributed to the screening effect of the high carrier density in Ti3C2Tx. Additionally, the layer-independent photocarrier relaxations revealed by optical pump THz probe spectroscopy (OPTP) provide evidence of the carrier heating-induced screening effect. These results are significant for practical applications in both scientific research and various industries.

Advances in MXene-based luminescence sensing strategies

MXenes have attracted the attention of many researchers as one of the latest two-dimensional (2D) materials over the last decade. Their great potential for biosensing has also been fully exploited after the discovery of their unique properties such as superior optical properties, excellent hydrophilicity, good thermal stability, excellent mechanical property, high electrical conductivity, biocompatibility, large surface area, and ease of surface functionalization. In the MXene-based luminescence sensing strategy, MXenes typically appear in the form of nanosheets, quantum dots and modified MXene nanocomposites, and they are utilized as different sensing platforms or as a luminescence source. In this review, we focused on the MXene-based luminescence sensing strategies, including fluorescence, electrochemiluminescence and chemiluminescence sensors and the comparison of their performance. Finally, the perspectives of the MXene-based luminescence sensors are discussed.

Mo-based MXenes: Synthesis, properties, and applications

Ti-MXene allows a range of possibilities to tune their compositional stoichiometry due to their electronic and electrochemical properties. Other than conventionally explored Ti-MXene, there have been ample opportunities for the non-Ti-based MXenes, especially the emerging Mo-based MXenes. Mo-MXenes are established to be remarkable with optoelectronic and electrochemical properties, tuned energy, catalysis, and sensing applications. In this timely review, we systematically discuss the various organized synthesis procedures, associated experimental tunning parameters, physiochemical properties, structural evaluation, stability challenges, key findings, and a wide range of applications of emerging Mo-MXene over Ti-MXenes. We also critically examined the precise control of Mo-MXenes to cater to advanced applications by comprehensively evaluating the summary of recent studies using artificial intelligence and machine learning tools. The critical future perspectives, significant challenges, and possible outlooks for successfully developing and using Mo-MXenes for various practical applications are highlighted.

MXene: A wonderful nanomaterial in antibacterial

Increasing bacterial infections and growing resistance to available drugs pose a serious threat to human health and the environment. Although antibiotics are crucial in fighting bacterial infections, their excessive use not only weakens our immune system but also contributes to bacterial resistance. These negative effects have caused doctors to be troubled by the clinical application of antibiotics. Facing this challenge, it is urgent to explore a new antibacterial strategy. MXene has been extensively reported in tumor therapy and biosensors due to its wonderful performance. Due to its large specific surface area, remarkable chemical stability, hydrophilicity, wide interlayer spacing, and excellent adsorption and reduction ability, it has shown wonderful potential for biopharmaceutical applications. However, there are few antimicrobial evaluations on MXene. The current antimicrobial mechanisms of MXene mainly include physical damage, induced oxidative stress, and photothermal and photodynamic therapy. In this paper, we reviewed MXene-based antimicrobial composites and discussed the application of MXene in bacterial infections to guide further research in the antimicrobial field.

MXene-based electrochemical devices applied for healthcare applications

The initial part of the review provides an extensive overview about MXenes as novel and exciting 2D nanomaterials describing their basic physico-chemical features, methods of their synthesis, and possible interfacial modifications and techniques, which could be applied to the characterization of MXenes. Unique physico-chemical parameters of MXenes make them attractive for many practical applications, which are shortly discussed. Use of MXenes for healthcare applications is a hot scientific discipline which is discussed in detail. The article focuses on determination of low molecular weight analytes (metabolites), high molecular weight analytes (DNA/RNA and proteins), or even cells, exosomes, and viruses detected using electrochemical sensors and biosensors. Separate chapters are provided to show the potential of MXene-based devices for determination of cancer biomarkers and as wearable sensors and biosensors for monitoring of a wide range of human activities.

Ultrasensitive biosensing platform for Mycobacterium tuberculosis detection based on functionalized graphene devices

Tuberculosis (TB) has high morbidity as a chronic infectious disease transmitted mainly through the respiratory tract. However, the conventional diagnosis methods for TB are time-consuming and require specialists, making the diagnosis of TB with point-of-care (POC) detection difficult. Here, we developed a graphene-based field-effect transistor (GFET) biosensor for detecting the MPT64 protein of Mycobacterium tuberculosis with high sensitivity as a POC detection platform for TB. For effective conjugation of antibodies, the graphene channels of the GFET were functionalized by immobilizing 1,5-diaminonaphthalene (1,5-DAN) and glutaraldehyde linker molecules onto the graphene surface. The successful immobilization of linker molecules with spatial uniformity on the graphene surface and subsequent antibody conjugation were confirmed by Raman spectroscopy and X-ray photoelectron spectroscopy. The GFET functionalized with MPT64 antibodies showed MPT64 detection with a detection limit of 1 fg/mL in real-time, indicating that the GFET biosensor is highly sensitive. Compared to rapid detection tests (RDT) and enzyme-linked immunosorbent assays, the GFET biosensor platform developed in this study showed much higher sensitivity but much smaller dynamic range. Due to its high sensitivity, the GFET biosensor platform can bridge the gap between time-consuming molecular diagnostics and low-sensitivity RDT, potentially aiding in early detection or management of relapses in infectious diseases.

3D nanocake-like Au-MXene/Au pallet structure-based label-free electrochemical aptasensor for paraquat determination

3D nanocake-like Au-MXene and Au pallet (Au-MXene/AuP) nanocomposite-modified screen-printed carbon electrodes (SPCEs) were utilized to construct an ultrasensitive label-free electrochemical aptasensor through a self-assembly procedure for trace paraquat (PQ) residue detection. Benefiting from the excellent electrochemical (EC) performances (e.g., high conductivity and large surface area) of Au-MXene nanocomposites and AuP substrate, the developed Apt/Au-MXene/AuP/SPCE-based EC aptasensor displayed excellent specificity and anti-interference ability, good repeatability, and stability. A linear relationship between the log value of the change in current intensity [lg (ΔI)] and the log value of the concentration of PQ [lg (CPQ)] was obtained in the range 0.05-1000 ng/mL. The limit of detection was 0.028 ng/mL, and the sensitivity was 255.5 μA/(μM·cm2). Practical applications in malt and mint samples confirmed the accuracy of the EC aptasensor in complex matrices for PQ detection, providing a universal analytical tool for other trace pesticides in different food samples by simply replacing the corresponding aptamers.

Engineering the surface of Nbn+1CnTx MXenes to versatile bio-activity towards microorganisms

Two-dimensional (2D) transition metal carbides/nitrides (MXenes) are potential antibacterial agents. However, their activity against microorganisms is not fully understood. It could relate to MXenes' surface which further influences their biocidal action. Herein, we report no continuous biocidal activity for delaminated 2D niobium-based MXenes (Nbn+1XnTx) such as Nb2CTx and Nb4C3Tx prepared with HF/TMAOH protocol. Biocidal activity towards Bacillus subtilis and Staphylococcus aureus microorganisms was achieved by surface-functionalization with lysozyme macromolecule. MXenes' engineering with lysozyme changed MXene's surface charge from negative into positive thus enabling the elimination of bacteria cells during 48 h of incubation. In contrast, Nb4C3Tx functionalized with collagen stimulated the growth of Bacillus subtilis by 225 %, showing MXene's biocompatibility towards this particular strain. Altogether, our results show that MXenes are incredibly bio-tunable. Opposing bio-effects such as antimicrobial or growth-stimulating can be achieved towards various microorganisms with rational surface engineering.

Recent Advances in Two-Dimensional MXene-Based Electrochemical Biosensors for Sweat Analysis

Sweat, a biofluid secreted naturally from the eccrine glands of the human body, is rich in several electrolytes, metabolites, biomolecules, and even xenobiotics that enter the body through other means. Recent studies indicate a high correlation between the analytes' concentrations in the sweat and the blood, opening up sweat as a medium for disease diagnosis and other general health monitoring applications. However, low concentration of analytes in sweat is a significant limitation, requiring high-performing sensors for this application. Electrochemical sensors, due to their high sensitivity, low cost, and miniaturization, play a crucial role in realizing the potential of sweat as a key sensing medium. MXenes, recently developed anisotropic two-dimensional atomic-layered nanomaterials composed of early transition metal carbides or nitrides, are currently being explored as a material of choice for electrochemical sensors. Their large surface area, tunable electrical properties, excellent mechanical strength, good dispersibility, and biocompatibility make them attractive for bio-electrochemical sensing platforms. This review presents the recent progress made in MXene-based bio-electrochemical sensors such as wearable, implantable, and microfluidic sensors and their applications in disease diagnosis and developing point-of-care sensing platforms. Finally, the paper discusses the challenges and limitations of MXenes as a material of choice in bio-electrochemical sensors and future perspectives on this exciting material for sweat-sensing applications.

Preparation of chitin/MXene/poly(L-arginine) composite aerogel spheres for specific adsorption of bilirubin

Currently, hemoperfusion is clinically the most rapid and effective treatment for removing toxins from the blood. The core of hemoperfusion is the sorbent inside the hemoperfusion device. Due to the complex composition of the blood, adsorbents tend to adsorb substances such as proteins in the blood (non-specific adsorption) while adsorbing toxins. Hyperbilirubinemia is caused by excessive levels of bilirubin in the human blood, causing irreversible damage to the patient's brain and nervous system, and even leading to death. High adsorption and high biocompatibility adsorbents with specific bilirubin adsorption are urgently needed to treat hyperbilirubinemia. Herein, poly(L-arginine) (PLA) which can specifically adsorb bilirubin, was introduced into chitin/MXene (Ch/MX) composite aerogel spheres. Ch/MX/PLA prepared by supercritical CO₂ technology had higher mechanical properties than Ch/MX and can withstand 50,000 times its own weight. The in vitro simulated hemoperfusion test showed that the adsorption capacity of Ch/MX/PLA was as high as 596.31 mg/g, which was 15.38 % higher than that of Ch/MX. Binary and ternary competitive adsorption tests showed that Ch/MX/PLA also had good adsorption capacity in the presence of a variety of interfering molecules. In addition, hemolysis rate testing and CCK-8 testing confirmed that Ch/MX/PLA had better biocompatibility and hemocompatibility. Ch/MX/PLA can meet the required properties of clinical hemoperfusion sorbents and has the ability to produce mass production. It has good application potential in the clinical treatment of hyperbilirubinemia.

MXenes Antibacterial Properties and Applications: A Review and Perspective

The mutations of bacteria due to the excessive use of antibiotics, and generation of antibiotic-resistant bacteria have made the development of new antibacterial compounds a necessity. MXenes have emerged as biocompatible transition metal carbide structures with extensive biomedical applications. This is related to the MXenes' unique combination of properties, including multifarious elemental compositions, 2D-layered structure, large surface area, abundant surface terminations, and excellent photothermal and photoelectronic properties. The focus of this review is the antibacterial application of MXenes, which has attracted the attention of researchers since 2016. A quick overview of the synthesis strategies of MXenes is provided and then summarizes the effect of various factors (including structural properties, optical properties, surface charges, flake size, and dispersibility) on the biocidal activity of MXenes. The main mechanisms for deactivating bacteria by MXenes are discussed in detail including rupturing of the bacterial membrane by sharp edges of MXenes nanoflakes, generating the reactive oxygen species (ROS), and photothermal deactivating of bacteria. Hybridization of MXenes with other organic and inorganic materials can result in materials with improved biocidal activities for different applications such as wound dressings and water purification. Finally, the challenges and perspectives of MXene nanomaterials as biocidal agents are presented.