# Recent Advances of MXene-Based Electrochemical Immunosensors
## 1 Introduction  
Electrochemical immunosensors are the largest class of affinity biosensing devices for protein detection. Similar to the other biosensors, the electrochemical immunosensor consists of a biorecognition element, a transducer and a signal readout system. In a classical electrochemical immunosensor, the analyte (antigen) is specifically recognized through the capture antibody (Ab\({}_{1}\)) coated on the electrode surface in the biorecognition element. Then, a recognition signal is generated on the electrode surface and converted into an electrical signal (current, voltage, resistance, etc.) through the transducer. Finally, a readable signal can be displayed on the readout system, so as to realize the purpose of analyte detecting. Based on the types of electrical signal converted by the transducer, electrochemical immunosensors can be classified into amperometric, potentiometric, conductometric, impedimetric and capacitive sensors [1; 2; 3; 4]. Based on the structural design, electrochemical immunosensors can be generally divided into non-labeled (label-free) and labeled (sandwich-type) immunosensors [5; 6; 7]. The non-labeled electrochemical immunosensor can detect the analyte through the change in the electrochemical impedance signal caused by antigen-antibody binding. The labeled electrochemical immunosensor usually requires a suitable electroactive label (enzyme, nanoparticle, etc. ), which is conjugated to the detection antibody (Ab\({}_{2}\)). The immuno-complex can be finally detected with the electroactive label via enzymatic or catalytic reaction, thus reflecting the level of analyte. Combing electrochemical analysis with antigen-antibody reaction, the electrochemical immunosensor has several distinct advantages, such as high specificity, excellent sensitivity, operational simplicity and low cost [8; 9; 10; 11], and has received extensive attention in several fields such as the early diagnosis of disease, clinical analysis, food safety and environmental monitoring [12; 13; 14]. The performance of electrochemical biosensors largely depends on the sensing materials. The introduction of nanomaterials in the sensing system has promoted the development of electrochemical biosensors. To date, a variety of nanomaterials have beensynthesized and used to fabricate electrochemical biosensors. Among these materials, two-dimensional (2D) layered materials have attracted significant interest because of their large specific surface area, tunable electronic structure, excellent flexibility and mechanical properties [15; 16; 17]. The 2D layered materials, including graphene and transition metal dichalcogenides (TMDs) represented by MoS\({}_{2}\), have been widely applied in electrochemical biosensors. However, these 2D materials suffer from the problems of low electrical conductivity (MoS\({}_{2}\)), high hydrophobicity (MoS\({}_{2}\), graphene) and difficulty in surface functionalization [18; 19], which limit their further applications in electrochemical biosensors. With the development of nanomaterials, MXenes--as emerging graphene-like 2D layered functional nanomaterials--show the potential to overcome the problems above and become promising candidates for electrochemical biosensors. To date, a certain amount of experimental works on MXenes-based electrochemical immunosensors have been reported. Furthermore, some of these works have been discussed in several review articles of MXene-based electrochemical sensors. For example, [PERSON] et al. [18] reviewed MXenes-based electrochemical sensors for the detection of biomarkers, drugs, and environmental contaminants, which detailed an immunosensor using MXene as an electrode modifier for carcinoembryonic antigen detection. [PERSON] and [PERSON] [20] presented the application progress of Ti\({}_{3}\)C\({}_{2}\)T\({}_{\rm x}\) MXene in electrochemical biosensors and described the effect of MXene nanocomposites as electrode modifiers on the sensing performance of immunosensors. [PERSON] et al.