Synthesis and Structures of MXenes: Comparison
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Among various two−dimensional (2D) materials, MXenes have attracted widespread interest due to their unique surface properties, as well as mechanical, optical, electrical and biocompatible properties, and have been applied in various fields, particularly in the preparation of biosensors, which play a critical role. 

  • two-dimensional material
  • MXenes
  • biosensor
  • electrochemistry

1. Introduction

The main two−dimensional (2D) material is a solid crystal consisting of a single or several atomic layers, a sheet thickness of 1–10 Å, and a lateral size ranging from 100 nm to several μm [1].  Two−dimensional materials with properties such as large specific surface area and unique electronics are focuses of research in many research fields [2]. Since 2004, Novoselov et al. performed exfoliation to obtain graphene nanostructures; since then, the two−dimensional material has attracted much attention [3]. In 2011, Gogotsi et al. prepared a two−dimensional Ti3C2 nanosheet named MXenes [4]. MXenes are typically a few μm laterally and 1 nm thick or less. It shows superior physicochemical properties compared to other two−dimensional nanomaterials [5]. It shows superior physicochemical properties compared to other two−dimensional nanomaterials [6].

The precursor of MXenes is the MAX phase. MAX consists of Mn+1Xn units and an alternately stacked “A” element single atomic plane, expressed as Mn+1AXn. The unique crystal structure of the MAX phase combines the excellent properties of ceramics and metals [7]. Etching the “A” element of the MAX phase yields two−dimensional nanomaterial MXenes with a structural formula of Mn+1XnTx [8]. MXenes can be expressed as M2XF2, M2X(OH)2, M2XO2, etc. M is a transition metal; “A” is an element of Groups 13 and 14 of the periodic table; X is boron, carbon, or nitrogen; n includes integers from 1 to 3; Tx denotes surface groups [9]. A list of important synthesis and processes in the field of MXenes research over the last decade, as well as the development of novel MXenes core components and surface group control technologies. Compared to the precursor MAX phase, derivative MXenes retain metallic and electrical conductivity benefits of MAX but also offer smaller lateral dimensions and thicknesses, as well as unique physical and chemical properties [10].
The central area of current advanced biosensing research studies is developing biosensors for detecting biological and chemical molecules that affect disease or are damaging to the human body. The most advanced biosensors can accurately and rapidly detect the target, predict the onset of the disease in time, and receive immediate medical attention [11].  Hence, high sensitivity and selectivity are significant for the design of biosensors. Due to its unique mechanical, hydrophilic, biocompatibility, and other excellent properties, MXenes are frequently used as a new biosensing platform. Electrochemical biosensors are essential for biological, environmental, and pharmaceutical fields. It offers high sensitivity, long−term reliability and high accuracy, rapidity, low cost, and easy miniaturisation [12].  In addition, electrochemical biosensors offer a further path for creating next−generation point−of−care testing devices [13]. With advancing nanotechnology with respect to MXene−based optical biosensors, unprecedented progress has been made in optical analysis. Optical analysis has advantages of high sensitivity, high selectivity, fast analysis, and good reproducibility. It has been widely used in biochemistry and biomedical and environmental analysis and has received increasing attention [14].

2.  Synthesis and Structures of MXenes

2.1. Synthesis of MXenes

There are two methods for the synthesis of MXenes. The top−down method is the most commonly used, which can be used to exfoliate multilayer materials into a few−layer or single−layer MXenes sheet. The second method is a bottom−up approach, which focuses on the growth of Mxenes from atoms or molecules [15].

2.1.1.  Top−Down Method

Selective etching disintegrates the strong covalent bonds between the MX and the A layers in the MAX phase. The primary method is etching with hydrofluoric acid (HF), molten salts, etc. In this process, oxygen (O), hydroxyl (OH), and fluorine (F) replace the M−A strong metal bond [16]. There are two main steps to gain 2D MXenes by HF: etching and exfoliation. Although the direct use of HF is straightforward and practical, it causes environmental pollution and damages to the human body [4]. In situ HF can be obtained by reacting a fluorinated salt with mild acid, which is less toxic to MXenes surfaces [17]. Researchers explored new synthetic methods. The typical chemical reaction equation for the synthesis of MXenes in the MAX phase is as follows [9].
M n + 1 AX n + 3 HF → AF 3 + M n + 1 X n + 3 2 H 2
M n + 1 X n + 2 H 2 O → M n + 1 X n ( OH ) 2 + H 2
M n + 1 X n + 2 HF → M n + 1 X n F 2 + H 2
MXenes must undergo an exfoliation process to obtain nanosheet structures: The surface groups of MXenes result in the layers being linked by hydrogen and Van der Waals forces [3].  Exfoliation enhances the interlayer spacing by weakening interactions between layers using various molecular and ionic processes [18]. The molten salt method uses fluorinated molten salts, Lewis salts [19].he synthesis does not involve fluoride, reducing the risk of synthesis [20]. The mechanism of MXenes formation in molten salts is similar to that of conventional HF methods: ZnCl2 and CuCl2 high−temperature molten salts strip a more comprehensive range of MAX phase materials [8]. n the molten salt of Lewis acids, Zn2+, Cu2+, and Cl− are consistent with acting H+ and F− in HF. Minimally intensive layer delamination (MILD) and electrochemical etching can also be used for MAX etching, producing high−quality, non−toxic MXenes [21].

2.1.2. Bottom−Up Method

 Bottom−up synthesis methods have been reported, such as chemical vapor deposition (CVD) [22]，template [23], and plasma−enhanced pulsed laser deposition (PE−PLD) [24]. MXenes produced by this method possess good crystalline quality and controllable structure and size [15]. Xu et al. used CVD to synthesize high−quality Mo2C crystals [22]. The synthesis of Mo2C MXene/graphene heterostructures and Mo2C MXene−graphene hybrid films by this method has been reported [25]. Compared to CVD, the template method has a relatively high yield of MXenes. Two−dimensional MXenes are mainly obtained by carbonizing or nitriding two−dimensional transition metal oxide (TMO) nanosheet templates. Xia et al. prepared hexagonal−structured 2D h−MoN nanosheets using precursor MoO2 nanosheets [23]. PE−PLD is a successful method for preparing large−area ultra−thin face−centered cubic (FCC) Mo2C MXene [24]. The stability of MXenes is an important property and limits its application to a certain extent. Researchers have tried to improve its stability. High concentrations of HF accelerate the degradation of MXenes and affect its structure, so relatively mild reaction conditions are necessary [26]. Organic solvents mitigate the oxidation of MXenes. Contact with water should be avoided as much as possible to prevent oxidation [27]. The oxidation of MXenes is quicker in liquid media than in solid media, and this degradation process is exacerbated by photocatalysis and thermocatalysis [28]. he storage of MXenes in Ar−sealed vials at 4℃ exhibits high stability at room temperatures [29].

2.2. Strustures of Mxenes

The crystal structure within a 2D material can affect its properties [15]. There are six types of MXenes structures: (1) single transition metal MXenes (Ti3C2 and Nb4C3); (2) solid solution MXenes ((Ti, V)3C2 and (Cr, V)3C2); (3) sequential planar internal and external bimetal MXenes with one transition metal occupying the outer layer (Cr and Mo); the central metal is another metal (Nb and Ta) [30]; (4) ordered double−transition metals MXenes ((Cr2V) C2); (5) orrderly double vacancy MXenes (Mo1.33CTx) [31]; (6) random empty space MXenes (Nb1.33CTx) [32].

[Computational simulation studies have been reported to identify novel stable MXenes structures, contributing to exploratory studies [33]. The properties and applications of these materials can be adapted by various parameters for composition, surface modification by heat treatment or chemical pathways, and structural adjustments [34]. MXenes have two−dimensional structures, one−dimensional structures, three−dimensional structures, and zero−dimensional structures [35][36].

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