Key characteristics of MXene-based nanobiosensors for detection of cancer biomarkers.
This chapter provides information about basic properties of MXenes (2D nanomaterials) that are attractive for a design of various types of nanobiosensors. The second part of the chapter discusses MXene synthesis and various protocols for modification of MXene making it a suitable matrix for immobilization of bioreceptors such as antibodies, DNA aptamers or DNA molecules. The final part of the chapter summarizes examples of MXene-based nanobiosensors developed using optical, electrochemical and nanomechanical transducing schemes. Operational characteristics of such devices such as sensitivity, limit of detection, assay time, assay reproducibility and potential for multiplexing are provided. In particular MXene-based nanobiosensors for detection of a number of cancer biomarkers are shown here.
1.1 MXenes: their precursors, characterization, unique properties and applications
Nanomaterials of the 2D kind are in the research spotlight due to their superior properties like ultrathin structure and intriguing physico-chemical properties [1, 2, 3]. Graphene has made researchers believing in extracting single layer transition metal dichalcogenides, which in turn has led to extensive research dedicated towards 2D nanomaterials [4, 5]. Since their inception, 2D nanomaterials have been characterized to have exceptional electronic, mechanical, and optical properties. These outstanding characteristics have driven research to use them in almost all fields of materials science and nanotechnology [6, 7, 8]. Rather recently in 2011 and 2012, Gogotsi, Barsoum, and colleagues have successfully prepared a new kind of 2D nanomaterial - MXenes, composed of a large group of transition metal carbides and carbonitrides [9, 10, 11, 12, 13]. These 2D nanomaterials are found to possess many striking properties and boost attraction in applications such as energy storage [14, 15, 16], electromagnetic shielding [17, 18], water treatment [19, 20], disease treatment  and (bio)sensing [22, 23], MXenes are made up of atomic layers of different materials like transition metal carbides, nitrides, or carbonitrides. All MAX phases consist of two-dimensional slabs of close-packed alternating layers of
The selective chemical etching of “A” in “MAX” phases have led to successful synthesis of MXenes. MAX phases are found to have elusive properties like stiff elasticity, good thermal and electrical conductivity, as well as relatively low thermal expansion coefficients and resistance towards chemical attack. There is a general formula for MXene synthesis where, “MAX” phases have a formula of M
2D MXenes are candidates for energy storage  (Li-ion batteries, supercapacitors) and electromagnetic interference shielding applications [31, 32, 33, 34, 35] and in the form of composites become ever more useful for sensing as
2. MXene synthesis
Generally, top-down selective etching process is used for the synthesis of MXenes . Strong etching solutions containing a fluoride ion (F−) such as hydrofluoric acid (HF), ammonium difluoride (NH4HF2), and a mixture of hydrochloric acid (HCl) and lithium fluoride (LiF) are used for production of MXene in such processes . Since typically, the etching process results in replacement of the M-A bond by M-O, M-OH, M-H, and M-F bonds on the surface of MXenes, the structure of MXenes can be expressed as M
A single and/or few layers of MXene can be synthetized by exfoliation or delamination of a multilayer structure of a MAX phase. The composition and electrochemical properties of MXene strongly depend on the conditions used during etching procedure . As an example, application of LiF/HCl as etchant led to production of MXene with interlayers intercalated with Li+ ions. Exfoliation can be done by a simple shaking or by sonication and prolonged sonication time results in production of MXene with small size of nanosheets and high density of defects . An alternative to use of highly corrosive and harmful HF is to employ small organic molecules or ions such as urea , dimethyl sulphoxide (DMSO)  (only for Ti3C2Tx MXene) or isopropylamine as etchants . MAX phase containing Si can be also exfoliated using tetrabutylammonium hydroxide (TBAOH) and tetramethylammonium hydroxide (TMAOH) .
3. MXene characterization
Since introduction of nanolayered and machinable MXenes in 2011 by Gogotsi and co-workers through wet-etching process with HF to obtain multilayered flakes of Ti3C2T
The electrochemical behavior employing methods like cyclic voltammetry (CV), chronoamperometry (CA), differential pulse voltammetry (DPV) and electrochemical impedance spectroscopy (EIS) revealed significant findings related to the electrochemical activity of MXene. The electrochemical investigation of Ti3C2Tx MXene to detect significant analytes (O2, H2O2 and NADH) was performed by applying cyclic voltammetry and chronoamperometry techniques, whereas Ti3C2TX demonstrated electrocatalytic activity towards H2O2 reduction with LOD at nanomolar level . Unfortunately, formation of TiO2 layer or domains with subsequent TiO2 dissolution caused by F− ions was observed during oxidation process at anodic potential window in a plain phosphate buffer electrolyte pH 7.0 leading to the decrease in electrochemical activity of Ti3C2Tx MXene.
The improvement of stability and redox behavior was achieved by further modification of MXene with nanoparticles of platinum (Ti3C2Tx/Pt) [69, 72]. The electrocatalytically active sensor based on Ti3C2Tx/Pt nanocomposite successfully determined H2O2 by CA, and moreover small organic molecules (acetaminophen, dopamine, ascorbic acid, uric acid) were selectively determined by DPV .
In addition electrochemical study confirmed significant differences in a negative charge density on the MXene surface as well electrocatalytic activity depending on the etchant (HF, LiF/HCl) used during MXene synthesis with preference towards utilization of LiF/HCl .
Aryldiazonium salts were utilized in modification of Ti3C2Tx MXene either spontaneously by free electrons or electrochemically. Electrochemical modification of Ti3C2Tx MXene by aryldiazonium-based grafting with derivatives bearing a SB- or CB- betaine pendant moiety was performed by cyclic voltammetry in a potential window from 0 V to −1 V with a sweep rate of 0.25 V s−1 and 48 cycles. The electrochemical grafting resulted in denser CB or SB layer on MXene interface, lower interfacial resistance and an electrochemically active surface area for SB layer in comparison to CB layer .
In the following years the exponential increase in the number of affinity-based MXene biosensors can be expected, though it is necessary to develop advanced strategies for modification of MXene interfaces with an effort to eliminate non-specific binding of proteins, bring in anti-fouling behavior and immobilize target biomolecules. Electrochemical methods can be employed as a useful tools for interfacial patterning, characterization of MXene-based biosensors and furthermore ultrasensitive detection of cancer related biomarkers .
4. MXene functionalization
4.1 Covalent modification of Ti3C2 MXenes with biomolecules
Functionalization and various methods for synthesis of MXenes can result in production of the nanomaterial with a diverse range of properties. This is why, it is very important to describe synthesis of MXenes in full details. Another point to focus on is to properly describe delamination conditions since the flake size and density of defects governs MXene’s surface properties and stability. It is important to know the molecular structure of MXenes in order to decide the best application of such nanomaterial for catalysis, (bio)sensing or for chemical adsorption of various compounds.
Due to presence of -OH groups on surface, functionalization of MXene employing silylation reagents was developed by a simple reaction with triethoxysilane derivatives [74, 75, 76]. Such modification led to production of nanosheets of Ti3C2-MXene uniformly patterned by aminosilane moieties allowing NHS/EDC-based amine coupling for covalent immobilization of bioreceptors such as anti- carcinoembryonic antigen (CEA) antibodies .
Another viable surface modification of MXenes can be done by applying zwitterions. It was observed that spontaneous grafting of sulfobetaine (SB) and carboxybetaine (CB) derivatives onto Ti3C2Tx MXene is feasible . The approach is similar to spontaneous grafting of diazonium salt modified zwitterions to gold nanoshell modified particles by consuming surface plasmons (free electron cloud) present within Au nanoshells . Even though spontaneous grafting of diazonium salt modified zwitterions to MXene was feasible, electrochemically triggered grafting of diazonium salts bearing zwitterionic pendants was more effective (Figure 1) . Electrochemical characterization tools confirmed a much quicker spontaneous SB grafting compared to spontaneous CB grafting. Zwitterionic modification is considered as a benchmark to design antibiofouling interfaces with such modification offering to reduce dramatically non-specific protein binding compared to an unmodified MXene interface . It is worth mentioning that grafting of a mixed layer composed of CB and SB can be applied to tune density of carboxylic groups and by amine coupling chemistry it is possible to finely tune density of immobilized bioreceptors for effective and efficient recognition of an analyte
Besides application of APTES there are other strategies for modification of MXene such as self-initiated photo-grafting and photopolymerization not requiring an anchor layer, self-assembled monolayer (SAM) and initiator, applying a nature polymer, soy phospholipid (SP) improving permeability, stable cycling, and retention and PEGylation of MXene improving the water dispersibility of MXene by electrostatic adsorption .
Recently, a novel MXene modification approach was developed by substitution or elimination reactions in molten inorganic salts. Such modification allowed to synthetize MXenes containing = O, -NH, =S, -Cl, -Se, -Br, and -Te surface terminations .
4.2 Preparation of hybrid nanoparticles based on MXene
The hydrothermal method run in a Teflon-lined stainless steel autoclave (150°C, 5 h; aqueous solution of vitamin C and Fe3+ salt) allowed preparation of composite of MXene with small magnetic Fe3O4 nanoparticles with an average size of ∼4.9 nm (TiO2/Ti3C2Tx/Fe3O4). These hybrid magnetic nanoparticles show a great promise for selective enrichment of various biomolecules/antigens based on affinity interactions .
Other promising nanocomposite option is represented by MXene sheets combined with metallic NPs [84, 85, 86, 87], which can be further effectively modified by crosslinkers due to their high affinity towards MXene or by other biomolecules for final detection of target molecules/biomarkers. MXene/metallic nanoparticles (NPs) based nanocomposites can be prepared by spontaneous reduction of salts of precious metals or by applying an external reducing agent such as NaBH4. A simple spontaneous reduction of metallic salts to form Ag, Au, and Pd nanoparticles onto the Ti3C2Tx MXene sheets was applied for formation of particles exhibiting surface–enhanced Raman spectroscopy (SERS) phenomenon . Moreover, an AuNP/MXene composite boosts sensitivity of detection of oncomarker such as microRNA . Similarly, the composite consisting of Ti3C2Tx MXene and PtNPs was prepared by means of
Graphite oxide as another 2D material was used to form composite together with MXene and such a composite led to a stable and efficient electrochemical detection of H2O2 and maintained hemoglobin biological activity even after ink jet printing applied for a sensor-based application .
4.3 Electrostatic and other interactions
MXene surface can be patterned
DNA aptamer activated through EDC/NHS chemistry was covalently immobilized onto MXene electrostatically modified with polyethyleneimine (PEI) .
Any conductive interface can be patterned by MXene by a simple casting of a MXene dispersion on untreated electrodes with formation of MXene layer after drying . Alternatively, the electrodes can be pretreated in order to make them more adhesive for formation of MXene layer. To make surface of screen-printed electrodes (SPEs) hydrophilic for subsequent deposition of MXene, SPEs were electrochemically activated in 0.1 M NaOH by CV in a potential range from −0.6 V to 1.3 V . SPEs patterned with delaminated MXene suspension as signal enhancer were applied for quantifying acetaminophen (ACOP) and isoniazid (INZ) in blood serum samples . The presence of abundant highly active surface sites due to functional groups (=O, -F and -OH) offers additional opportunity for MXene to interact with various positively charged functional groups of molecules.
Besides electrostatic modification of MXene by a modifier applied as glue for subsequent attachment of bioreceptors, electrostatic interactions could be applied also to modify MXene by redox molecules. Methylene blue as a redox probe due to its positive charge can be electrostatically deposited on MXene layer with a final immobilization of the enzyme urease on the surface using glutaraldehyde . Moreover electrostatic interaction was utilized for deposition of positively (CTA+) charged cetyltrimethylammonium chloride (CTAC) on the negatively (OH−) charged Nb2C nanosheets resulting in CTAC-anchored Nb2C nanosheets and subsequently
Rich surface chemistry of MXenes can be also applied for interaction with a number of molecules. High applicability of exfoliated MXene (e-MXene) has been investigated as a matrix due to its high laser energy absorption, electrical conductivity and photothermal conversion for laser desorption/ionization time-of-flight mass spectrometry (LDI-MS) analysis of various analytes (saccharides - glucose, sorbitol, sucrose, and mannitol, amino acids - Arg, Phe, His, and Pro, peptide - leu-enkephalin and antibiotics - sulfamerazine and norfloxacin, benzylpyridinium salt (BP), environmental pollutants). Before LDI-MS measurement 1 μL of each small molecule solution was spotted on a target plate, mixed with 1 μL of e-MXene suspension and dried under ambient condition. The e-MXenes exhibiting a high resolution and salt-tolerance demonstrated a strong potential for the development of an efficient analytical platform based on LDI-MS analysis . In addition, Ti3C2 MXene assisted LDI-LIFT-TOF/TOF was utilized for differentiation and relative quantitative analysis of three types of glycan isomers resulting in higher sensitivity, better homogeneity and stable relative peak intensity for glycan analysis. Moreover nine disaccharides, two trisaccharides, three heptasaccharides and ten natural product extractions were resolved by applying MXene with LDI-LIFT-MS/MS. The enhanced sensitivity and background-free nature of the fragment profile obtained by LDI-LIFT-TOF/TOF opens up a new realm for nanomaterial assisted glycan structural analysis and/or enrichment either through MXenes themselves or in combination with other functionalized magnetic nanoparticles .
5. Advanced 2D MXenes-based nanobiosensors as ultrasensitive detection tools
The link between progressive detection and daily/routine tests is fostered by (bio)sensing platforms employing nanomaterials/nanostructures with outstanding electronic, electrocatalytic, magnetic, mechanical, and optical properties. Novel multifunctional nanometer-sized structures combine advantageous large surface-to-volume ratio, controlled morphology and structure that would allow immobilizing bioreceptors with preserved biocompatibility, biostability and biodistribution . Compared to other 2D materials (graphene, graphitic carbon nitride, MoS2), MXenes nanomaterials carry a unique combination of excellent electrical conductivity, complete metal atomic layers, ease of functionalization, high stability, hydrophilicity, large surface area, ultrathin 2D sheet-like morphology, excellent mechanical properties and good bio-compatibility [49, 77].
Bioreceptor‘s intrinsic characteristics including its affinity towards the analyte, structural stability during biosensor’s operation and a methodology deployed for bioreceptor immobilization onto the transducing surface can significantly affect sensitivity, selectivity and robustness (reproducibility, stability
Both enhanced biocompatibility and increase of the transducing surface area of the (bio)sensors related to enhanced catalytic activity drive a design of 2D MXene nanomaterial-based biosensors utilizing aptamers, antibodies, enzymes and protein molecules [23, 60, 68]. Ultrathin 2D sheet-like morphology with potential for high density incorporation of a number of functional groups as well as excellent ion intercalation behavior also show up as promising features for (bio)sensing applications . On the other hand the implementation of MXenes as next-generation detection devices will require a substantial improvement of the stability of MXenes towards oxidation.
“Detect-to-protect” biosensors are compact analytical devices converting the biochemical reaction into an analytical and measureable signal. Due to their high specificity which is directly dependent on the receptor used (biomolecules or synthetic compounds), their sensitivity, compact size and simple operation, biosensors are the tool of choice for detection of chemical and biological components. Principally, biosensors are formed by two components, a biorecognition part consisting of a biological or synthetic receptor (enzymes, antibodies, nucleic acids, organelles, plant and animal tissue, whole organism, or organs) that utilizes a specific biochemical or chemical reaction mechanism with an analyte and a transducer where the interaction between a bioreceptor and an analyte is transformed into a measurable signal. There are two major obstacles in biosensor development; incorporation/immobilization of (bio)receptors in suitable matrix and monitoring/quantifying the interactions between the analytes and these receptors .
In order to allow for a rapid screening of analytes/antigens from human samples a real-time analysis is the preferred approach. The corresponding biosensor should be cheap, small, portable and user-friendly.
The key part of a biosensor is the transducer, which screens a physical change accompanying the bioaffinity reaction (
The morphology of ultrathin 2D Ti3C2 MXene single or few layered nanosheets with high density of functional groups offers improved biomolecule loading and rapid access to the analyte. The covalent immobilization of biorecognition elements (DNAs, enzymes, proteins,
5.1 State-of-the-art approaches of MXenes-based nanobiosensors for cancer biomarkers detection
Cancer is one of the deadliest diseases worldwide, and acquiring cancer-specific data by quantitative analysis of cancer-associated biomarkers is crucial to monitor cancer progression and for the early treatment . As reported by the World Health Organization, the year of 2030 should be marked by approximately 12 million cancer related deaths, making cancer a major public health problem and one of the most prominent death-causing factors worldwide. The number of new cases of cancer (cancer incidence) is presently around 439
A tumor/cancer marker is a substance produced by a tumor or by the host in response to a cancer cell that can be objectively measured and evaluated as an indicator of cancerous processes within the body. The term tumor marker was firstly coined in 1847 and presently there are more than 100 known different tumor markers . Biomarkers have a great potential for screening and diagnostics because they are present in blood and provide information about the health condition . In healthy individuals, the tumor marker concentration is comparatively low level or even absent, while increased values can reveal development and/or progression of a disease . Serum biomarkers providing key information about the disease are important for management of cancer patients since blood aspiration is only a moderately invasive procedure. There is clear need for early-stage cancer diagnostics, efficient treatment and posttreatment monitoring to avoid progress of the disease into advanced stages. Therefore there is an enormous demand for efficient less-invasive investigation
The unique physico-chemical properties of MXenes make them a significant tool that can be employed in the cancer therapy (photothermal therapy, photodynamic therapy, radiation therapy, chemotherapy), cancer imaging (CT/MRI/PA imaging) as well as cancer theranostic applications .
5.2 MXene-based electrochemical nanobiosensors
Electrochemical biosensors are prospective tool of choice for an early-stage diagnostics of cancer diseases . Electrochemical methods such as CV, CA, DPV, EIS, square wave voltammetry (SWV) provide a number of advantages. They are reliable, easy-to-use, affordable and highly sensitive and reliable [107, 115, 116]. Lab-on-chip biosensors are compact and portable miniaturized devices that can be employed in cancer biomarkers research leading to potential clinical applications. Biosensors employing surface nanoarchitectures with this type of detection offer attractive features including straightforward miniaturization, excellent LODs, robustness, small analyte volumes and the ability to be applied in turbid biofluids with optically absorbing and fluorescing compounds.
Single/few-layered MXene (Ti3C2) nanosheets were functionalized with (3-aminopropyl)triethoxysilane (APTES) to enable covalent attachment of bio-receptor onto
Due to their excellent electrical conductivity and large specific surface area with a large number of potential attachment binding sites, 2D MXenes are also applied as a conductive support for immobilization of aptamer probes. Wang
The label-free strategy for the ultrasensitive detection of miRNA-182 was based on glassy carbon electrode (GCE) modified step-by-step by van der Waals forces and electrostatic interactions with MoS2/Ti3C2, AuNPs, ssRNA . BSA was used to block unbound gold particles surface and avoid nonspecific adsorption. The biosensor was able to determine miRNA-182 with LOD of 0.43 fM (a linear range of 1 fM - 0.1 nM) by DPV method . The recovery was 105%, 95.3% and 93.0% for the concentration of 10−10 M, 10−12 M and 10−14 M of the analyte respectively, manifesting its effective detection of miRNA-182 in real sample .
Duan with co-workers  developed an impedimetric aptasensing strategy based on a novel zero dimensional (0D)/2D nanohybrid of Ti3C2Tx nanosheets decorated with FePc QDs (denoted as Ti3C2Tx@using iron phthalocyanine quantum dots (FePcQDs)) for miRNA-155 detection. The miRNA-155 was established by applying impedimetric aptasensor with LOD of 4.3 aM (S/N = 3, a linear concentration range from 0.01 fM to 10 pM). The observed relative standard deviation (RSD) of the five aptasensors for detection of miRNA-155 was as low as 2.98
Multiple (miRNA-21 and miRNA-141) and rapid (80 min) analysis of onco microRNAs in total plasma was carried out with combination of AuNPs (5 nm) decorated MXene as an electrode interface and a duplex-specific nuclease (DSN) as an amplification system applied onto home-made screen-printed gold electrode (SPGE) . As the initial step functionalization of two magnetic particles (MPs) with two different single-stranded DNAs (ssDNAs) was performed through labeling with methylene blue (MB) and ferrocene (Fc) that were partially complementary to the target miRNA. After the invasion of targets and amplification cycle, the released uncleaved DNA sequences harboring redox labels were hybridized with the electrochemical sensor platforms for subsequent measurements. To enhance the electrochemical signal, the SPGE was modified with the synthesized MXene-Ti3C2Tx and patterned with AuNPs and further loaded with abundant ssDNAs (base) to provide a significantly higher electrochemical signal compared to the AuNP/Au electrodes (almost 4 orders of magnitude increase). The LODs of the biosensor exhibiting multiplex ability, antifouling activity and single mutation recognition for microRNA-21 and microRNA-141 detection reaching low LOD levels down to 204 aM and 138 aM (a wide linear range up to 50 nM), respectively. The synergic effect of combining MXene based electrochemical amplification and DSN target recycling, resulted in a short assay time of 80 min, a good assay reproducibility (RSD ≈ 4.7%) and stability of 95.2% and 97.1% of its initial signal values assigned to MB and Fc, respectively, after 4 weeks of storage .
In addition, PSA was sensitively detected with capacitance-based enzyme immunosensor  based on enzymatic biocatalytic precipitation of precipitate on interdigitated micro-comb electrode (IDE). AuNPs heavily functionalized with HRP and detection antibodies (HRP-Au-Ab2) were utilized as the signal generating probe. Firstly, MXene dispersion in 1.0 wt % Nafion ethanol solution was dropped onto IDE to modify it. Next anti-PSA capture antibodies (Ab1) were physically adsorbed onto the nanosheets. Subsequently PSA, HRP-Au-Ab2 conjugates, H2O2 and HRP-tyramine conjugates were incubated step-by-step with the immunosensor at room temperature. The target PSA was determined with LOD of 0.031 ng mL−1 in a linear range up to 50 ng mL−1 with RSD of 10.7%, indicating good reproducibility .
An impedimetric aptasensor based on the nanostructured multicomponent hybrid of Ti3C2Tx nanosheets and phosphomolybdic acid (PMo12) nanoparticles integrated by embedding within the polypyrrole (PPy) matrix (PPy@Ti3C2Tx/PMo12) was utilized for detection of osteopontin (OPN) . The PPy@Ti3C2Tx/PMo12-based aptasensor estimated OPN with LOD of 0.98 fg mL−1 in a linear range of 0.05–10,000 pg. mL−1. The biosensor exhibited low RSD of the assays of around 1.7% and during the biosensor offered also good operational stability .
5.3 MXene-based optical nanobiosensors
Surface plasmon resonance (SPR) is a principal technique for
Due to its absorption, a few-layer Ti3C2Tx MXene can contribute to the improved sensitivity of SPR biosensors. Enhanced sensitivity by 16.8%, 28.4%, 46.3% and 33.6% was achieved for the proposed SPR biosensors based on Au with 4 layers, Ag with 7 layers, Al with 12 layers and Cu with 9 layers of Ti3C2Tx, respectively .
The platform based on prism/gold layer/MXene/WS2/black phosphorus using monolayer of each nanomaterial was proved as a novel SPR sensing material with enhanced sensitivity of 15.6% compared to a bare metal film . MXene-based composite, g-C3N4/MXene-AgNPs, including g-C3N4 as a photocatalyst, MXene as a co-catalyst and AgNPs as an electron mediator offered enhanced photocatalytic activity. The increased optical absorption and reduced band-gap energy due to the SPR effect of AgNPs deposited on such nanocomposite modified interface was observed .
Wu with co-workers  took advantage of hydrophilic and biocompatible Ti3C2 surface as a platform for making a nanohybrid consisting of multi-walled carbon nanotubes (MWCNTs)-polydopamine (PDA)-Ag nanoparticles (AgNPs) as a signal probe to develop SPR biosensor, that is easy to prepare, convenient to operate, and provides high sensitivity and selectivity. In order to obtain good orientation and immobilization of monoclonal anti-CEA antibody (Ab1), synthesized Ti3C2/AuNPs composite was firstly decorated with staphylococcal protein A (SPA) to which Ab1 was captured by affinity interaction through its Fc region. Polyclonal anti-CEA antibodies (Ab2) were conjugated with a nanohybrid through Schiff-base reaction between amino residues and quinone groups of PDA. By introducing a MWPAg-Ab2 conjugate to form a sandwich format, LOD of 0.07 fM was achieved for CEA detection (a dynamic range of 2 × 10−16 - 2 × 10−8 M). However, there are some limitations of such biosensing platform including time-consuming fabrication of the interfacial layer
Among various investigation methods for detection of cancer biomarkers, fluorescence analysis methods, especially fluorescent nanoprobes based on “turn on” mechanism, are regarded as sensitive and reliable analytical tools for cancer diagnosis. The nanoprobes can be ideally stabilized in both extracellular and intracellular microenvironment and respond to multi-biomarkers with different spatial distributions to achieve multilayer information of diverse biomarkers range from cell membrane to the cytoplasm at a cellular level . Wang with colleagues  investigated fluorescence quenching capacity of Ti3C2 MXenes for biosensing of dual biomarkers in single (MCF-7) living cells. A chimeric DNA-functionalized Ti3C2 probe was employed for real-time and multilayer simultaneous fluorescent imaging of plasma membrane glycoprotein MUC1 and cytoplasmic microRNA-21 at nM concentration
A strand displacement dual amplification (SDDA) strategy was developed by Chen
PSA was the both qualitatively and quantitatively examined through a sandwich-type immunoreaction and a photothermal measurement by applying Ti3C2 MXene quantum dots (QDs)-encapsulated liposome with a high photothermal efficiency . Ti3C2 MXene QDs as the innovative photothermal signal beacons were entrapped in the liposome for the labeling of the secondary antibody on the surface. The sandwich-type assay was carried out by coupling a low-cost microplate with a homemade 3D printed device. Under NIR-laser irradiation of 808 nm, Ti3C2 MXene QDs converted the light energy into heat, and the shift in the temperature correlating with the analyte concentration. LOD of 0.4 ng mL−1 for PSA was obtained by a near-infrared (NIR) photothermal immunoassay (a linear range of 1.0 ng mL−1 - 50 ng mL−1). The portable equipment employing a portable NIR imaging camera was able to collect the visual thermal data for semi-quantitative analysis of target PSA within 3 min .
5.4 Detection of exosomes as a source of cancer biomarkers by applying 2D MXenes
Exosomes as type of endosome-derived cell-secreted vesicles with the structure of a lipid bilayer membrane are responsible for signal transduction in intercellular communication and extracellular matrix remodeling. In addition exosomes can also carry cargo affecting neighboring cells and they can form pre-metastatic niches . Thus, exosomes are behind localized tumor development, progression and induction of distant tumors forming metastasis. The fact, that a substantially higher cellular activity of tumor cells results in the production of a greater number of exosomes than in normal/healthy cells, makes them hot candidates for cancer diagnostics in itself .
Electrochemiluminescence (ECL) as an upcoming technique joining the benefits of both electrochemistry and chemiluminescence, has been widely applied for biomarker analysis thanks to its high sensitivity, short response time and low background signal . A biosensor based on the application of MXene and ECL was developed for sensitive detection of exosomes . First, MXene (
Another MXene-based biosensor for the detection of exosomes was prepared by Fang
The novel 2D nanomaterial MXene has a potential to significantly influence the field of biosensing including affinity-based biosensors with expected exponential increase in related works to be published in the years to come. MXene-based biosensors offer adequate sensitivity required for detection of cancer biomarkers present in blood down to ng mL−1 level or better (Table 1). However a great deal of effort needs to be invested into finding proper decorating strategies for MXene to simultaneously allow immobilization of biomolecules, but at the same time providing resistance towards non-specific protein binding. Matching this criteria, affinity MXene-based biosensors can be applied for analysis of complex samples such as blood serum or plasma . Point-of-care tests (POC) employing MXene/based devices represent promising candidates with benefits such as adaptability in different/adverse environment, automation of tests, reduced cost, miniaturization, interference-free detection,
|Target biomarker||Biosensor architecture||Detection method||LOD||Linear range||Reference|
|CEA||BSA/anti-CEA/f-Ti3C2-MXene/GC||Electrochemical/ CV||0.000018 ng mL−1||0.0001–2000 ng mL−1|||
|CEA||Ti3C2MXene/AuNPs/SPA/Ab1 and MWCNTs-PDA-AgNPs/Ab2||SPR||0.07 fM||2 × 10−16 - 2 × 10−8 M|||
|CEA||Ab2-conjugated SPA/HGNPs/N-Ti3C2-MXene||SPR||0.15 fM||0.001–1000 pM|||
|MUC1||cDNA-Fc/MXene/Apt/Au/ GCE||Electrochemical/ DPV||0.33 pM||1.0 pM - 10 mM|||
|miRNA-182||BSA/ssRNA/ AuNPs/ MoS2/Ti3C2/GCE||Electrochemical/ DPV||0.43 fM||1 fM - 0.1 nM|||
|miRNA-155||cDNA/Ti3C2Tx@ FePcQDs/AE||Electrochemical/ EIS||4.3 aM||0.01 fM - 10 pM|||
|miRNA-21 and miRNA-141||ssDNAs/AuNP@ MXene/SPGE||Electrochemical/ DPV||204 aM (miRNA-21) and 138 aM (miRNA-141)||500 aM - 50 nM||[ 88]|
|PSA||BSA/anti-PSA/AuNPs-M-NTO-PEDOT/GCE||Electrochemical/ DPV||0.03 pg. L−1||0.0001–20 ng mL−1||[ 120]|
|PSA||HRP-Au-Ab2-PSA-Ab1-MXene/IDE||Electrochemical/ EIS, CV||0.031 ng mL−1||0.1–50 ng mL−1|||
|VEGF165||MB/DNA/HT/HP1/AuNPs/Ti3C2/BiVO4/GCE||Photoelectro-chemical||3.3 fM||10 fM - 100 nM||[ 122]|
|OPN||Apt/PPy@Ti3C2Tx/ PMo12/AE||Electrochemical/ EIS||0.98 fg mL−1||0.05–10,000 pg. mL−1||[ 53]|
|Exosomes||ECL probe - (MXenesBPQDs@Ru(dcbpy)32+-PEI-AbCD63 exosomes/Apt/ILs/SiO2 NUs/GCE||ECL||37.0 particles μL−1||1.1 × 102–1.1 × 107 particles μL−1||[ 135]|
|Exosomes||MXenes-Apt2/exosomes/Apt1/PNIPAMAuNPs/ GCE||ECL||125 particles μL−1||5 × 102–5 × 106 particles μL−1|||
|Exosomes||Cy3 labeled CD63 aptamer (Cy3-CD63 aptamer)/Ti3C2 MXenes||Ratiometric fluorescence resonance||1.4 × 103 particles mL−1||104–109 particles mL−1|||
|miRNA-21 and miRNA-10b||DNA-NaYF4:Yb,Tm/Er UCNPs and Ti3C2 nanosheets||Fluorescence - fluorescence resonance energy transfer (FRET) assay||0.62 fM (miRNA-21) and 0.85 fM (miRNA-10b)||5 fM - 100 pM|||
The authors would like to acknowledge financial support from the Slovak Research and Development Agency APVV 17–0300 and from projects granted by the Ministry of Health of the Slovak Republic No. 2018/23-SAV-1 and 2019/68-CHÚSAV-1. This book chapter was supported by Qatar University Grant IRCC-2020-004. The statements made herein are solely the responsibility of the authors.
|Ab1||monoclonal anti-CEA antibody, monoclonal mouse anti-human PSA capture antibody|
|Ab2||polyclonal anti-CEA antibody, polyclonal rabbit anti-human PSA detection antibody|
|AE||bare Au electrode|
|anti-CEA||carcinoembryonic antibody monoclonal antibody|
|Au, AuNPs||Au nanoparticles|
|BPQDs||black phosphorous quantum dots|
|BSA||bovine serum albumin|
|cDNA||complementary deoxyribonucleic acid|
|cDNA-Fc||ferrocene-labeled complementary deoxyribonucleic acid|
|DPV||differential pulse voltammetry|
|EIS||electrochemical impedance spectroscopy|
|f-Ti3C2-MXene||MXene functionalized with aminosilane|
|FePcQDs||phthalocyanine quantum dots|
|GC, GCE||glassy carbon electrode|
|HGNPs||hollow gold nanoparticles|
|IDE||interdigitated microcomb electrode|
|IL||ionic liquid (1-carboxymehyl-3-methylimidazolium chloride)|
|M-NTO||3D sodium titanate nanoribbons|
|MWCNTs||multi-walled carbon nanotubes|
|N − Ti3C2-MXene||amino-functionalized Ti3C2-MXene|
|PNIPAM||Poly (N-isopropylacrylamide), carboxylic acid|
|PSA||prostate specific antigen|
|SiO2 NUs||SiO2 nanourchin|
|SPA||staphylococcal protein A|
|SPGE||screen-printed gold electrode|
|SPR||surface plasmon resonance|
|VEGF165||vascular endothelial growth factor 165|
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