# Recent Advances of MXene-Based Electrochemical Immunosensors
## 3 Electrochemical Immunosensors Based on MXenes
### MXenes as Detection Antibody Label in Electrochemical Immunosensors  
Early diagnosis of disease is an important goal of electrochemical immunosensors. The level of biomarkers in body fluids is often very low in the early stage of disease [7]; thus, a label-free immunosensor that increases the electrochemical response merely by modifying electrodes would be not sufficient for meeting detection needs. To achieve high-sensitivity detection of analytes, it is necessary to construct a sandwich-type immunosensor by labeling detection antibody with various signaling molecules to further increase the electrochemical response. MXenes have intrinsic good electrocatalytic activity and can be directly used as signaling molecules to label detection antibodies [59; 65]. In addition, attributed to the large specific surface area and powerful ability to bind other signaling molecules, MXenes have been also used as excellent carriers for other signaling molecules. [PERSON] et al. [59] designed a sandwich-type immunosensor for PSA detection (Figure 5). In their work, Ti\({}_{3}\)C\({}_{2}\)T\({}_{\rm X}\) MXene@AuNPs, with good electrocatalytic activity for H\({}_{2}\)O\({}_{2}\) redox reaction, was used as a signal molecule to label detection antibody for signal amplification. With AuNPs-ATPGO (gold nanoparticles/p-aminothiophenal functionalized graphene oxide) composite modified GCE as a sensing platform, the immunosensor obtained a LOD of 3.0 fg mL\({}^{-1}\). [PERSON] and co-worker [60] fabricated a sandwiched im  
Figure 4: Schematic illustration of the fabrication steps of the thread-based electrochemical immunosensor for cortisol detection. Reprinted with permission from Ref. [56]. 2022, Elsevier. munosensor for the quantitative detection of CEA. In their work, Ti\({}_{3}\)C\({}_{2}\) MXenes were introduced into CuAu-LDH to improve the conductivity and facilitate the electron transfer between Cu\({}^{2+}\) and Cu\({}^{+}\). After that, the Ti\({}_{3}\)C\({}_{2}\) MXenes-anchored CuAu-LDH 2D materials were used to label detection antibody. SWV and amperometry (i-t) dual-mode analysis were employed to reduce systematic errors and improve detection accuracy. The immunosensor finally achieved a low LOD (SWV: 33.6 fg mL\({}^{-1}\) and i-t: 45.4 fg mL\({}^{-1}\)). For the early diagnosis of cardiovascular disease, a sandwich-type immunosensor was designed for cTnI detection [61]. CuPtRh \(\text{CNBs}/\text{NH}_{2}\text{-Ti}_{3}\text{C}_{2}\), composed of trimetallic hollow CuPtRh cubic nanoboxes and few-layer ultrathin ammoniated MXene, was applied for detection antibody labels (Figure 6a). The embedded CuPtRh CNBs acted as not only a spacer to prevent the \(\text{NH}_{2}\text{-Ti}_{3}\text{C}_{2}\) layer from restacking, but also a connector to immobilize more \(\text{Ab}_{2}\) through stable Pt-N and Rh-N bonds. In addition, the CuPtRh CNBs provided more active sites for catalytic H\({}_{2}\)O\({}_{2}\) reduction, thereby effectively amplifying the current signal of the immunosensor. The designed immunosensor exhibited acceptable reproducibility, stability and specificity for cTnI detection, with a LOD of 8.3 fg mL\({}^{-1}\). [PERSON] et al. [62] developed a sandwich-type immunosensor for the detection of h-FABP, another important biomarker in the diagnosis of AMI. To prepare the immunosensor, Cd\({}_{0.5}\)Zn\({}_{0.5}\)S/Ti\({}_{3}\)C\({}_{2}\)T\({}_{\text{x}}\) MXene nanocomposite was used to label detection antibody to amplify the signal. The Cd\({}_{0.5}\)Zn\({}_{0.5}\)S/Ti\({}_{3}\)C\({}_{2}\)T\({}_{\text{x}}\) MXene nanocomposite with large specific surface area not only facilitated the \(\text{Ab}_{2}\) immobilization, but also promoted electron transfer, resulting from the remarkable synergistic effect between Cd\({}_{0.5}\)Zn\({}_{0.5}\)S and Ti\({}_{3}\)C\({}_{2}\)T\({}_{\text{x}}\) MXene.