# Recent Advances of MXene-Based Electrochemical Immunosensors
## 2 Composition, Structure and Property of MXenes  
MXenes are a family of transition metal carbides, nitrides or carbonitrides discovered by [PERSON]'s group in 2011. MXenes are generally etched from the parent MAX phase with a hexagonal crystal structure and P6\({}_{3}\)/mmc symmetry [23]. The MAX phase can be represented by a general formula: M\({}_{n+1}\)AX\({}_{n}\), where M is an early transition metal element (Ti, V, Zr, Nb, Mo, Hf, Sc, etc.), A is usually a group IIIA or IVA element (Al, Si, In, Pb, etc.), X is carbon or nitrogen element and \(n\) stands for 1, 2, 3, or 4 [24; 25; 26]. The M-X bond is stronger than the M-A bond [27; 28]; thus, the A layer can be selectively removed from the MAX phase by an etchant to obtain MXene. The transition metal atoms (M) in the resulting MXene crystal are interlaced with carbon and/or nitrogen atoms (X), and the overall crystal maintains a hexagonal close-packed structure [29]. MXenes can be expressed with the general formula M\({}_{n+1}\)X\({}_{n}\)T\({}_{\rm x}\), where T\({}_{\rm x}\) is the surface functional group such as -OH, -O, -F, and -Cl. To date, more than 100 MXenes have been reported, of which more than 30 MXenes have been synthesized experimentally, and about 80 MXenes have been predicted by theoretical calculations [26]. Furthermore, all the above MXenes can be classified into four types in molecular structure: M\({}_{2}\)XT\({}_{x}\), M\({}_{3}\)X\({}_{2}\)T\({}_{\rm x}\), M\({}_{4}\)X\({}_{3}\)T\({}_{\rm x}\) and M\({}_{5}\)C\({}_{4}\)T\({}_{\rm x}\)[30].  
Similar to other 2D materials, MXenes have a large specific surface area, which is beneficial for immobilizing biomolecules or adsorbing signal probes [31]. However, MXenes show several unique advantages. For example, MXenes have both excellent electrical conductivity [32] and tunable electronic structure [22]; thus, they can not only be used as sensing materials to accelerate electron transport, but also for fabricating special electronic devices. The abundant hydrophilic groups on the surface of MXenes endow them with a strong potential to combine with other active materials or biomolecules [33; 34], which is beneficial for the construction and optimization of biosensors. MXenes are generally non-toxic and biocompatible with biological organisms, making them promising alternative biomaterials for versatile biomedical applications [23]. In addition, MXenes have good stretchability and can be used as conductive coatings for the fabrication of wearable electronics and biometric sensors [35]. Furthermore, MXenes can also be used as inks for printing, exhibiting the potential to realize miniaturization, integration and industrialization of MXenes-based devices [36]. In view of this, MXenes show promising prospects in the fabrication of high-performance biosensors. However, MXenes suffer from the inevitable issue of restacking and aggregation due to strong van der Waals forces between the layers [37; 38], which affects their electrochemical performance. To address this issue, researchers tend to modify MXene nanosheets by introducing other active materials, which can act as not only spacers to prevent aggregation of MXene, but also promoters to enhance the electrochemical performance through synergistic effects. For example, researchers have introduced metal nanoparticles [39; 40], carbon nanotubes [41; 42], graphene [43; 44], MoS\({}_{2}\)[45], conductive polymers [46] and ionic liquids [47] into MXenes to prepare interface materials with excellent electron transport properties, thereby further obtaining electrochemical sensors with good analytical performance.