# Recent Advances of MXene-Based Electrochemical Immunosensors
## 3 Electrochemical Immunosensors Based on MXenes
### MXenes as Electrode-Modified Nanomaterials in Electrochemical Immunosensors
To overcome the restacking and agglomeration of MXenes, [PERSON] and co-worker [50] synthesized 3D M-NTO by oxidizing and alkalizing Ti\({}_{3}\)C\({}_{2}\) MXene nanosheets. After anchoring PEDOT and further electrodeposition AuNPs on the surface, the AuNPs/M-NTO-PEDOT composite was obtained and applied for EMN. Then, a label-free immunosensor was fabricated based on this composite for the quantitative determination of PSA. Since the large specific surface area of AuNPs/M-NTO-PEDOT composite can load a large number of capture antibodies, and its excellent electrical conductivity can promote charge transfer, the immunosensor exhibited excellent sensitivity, with a low LOD of 0.03 pg L\({}^{-1}\). Recently, a sandwich-type immunosensor for quantitative PSA screening was fabricated by integrating MXene (Ti\({}_{3}\)C\({}_{2}\))\(-\)AuNPs as EMN and dopamine-loaded liposomes for signal amplification. The immunosensor also showed good analytical performance [51]. The efficient trace detection of CYFRA21-1 is critical for early diagnosis and prognosis of non-small cell lung cancer (NSCLC). [PERSON] et al. developed a sandwiched immunosensor for ultrasensitive detection of CYFRA21-1 [52]. Combing AuNPs decorated Ti\({}_{3}\)C\({}_{2}\)T\({}_{x}\)-MXene (AuNP-Ti\({}_{3}\)C\({}_{2}\)T\({}_{x}\)) as EMN to load numerous capture antibodies and accelerate the electron transfer rate, and toluidine blue (TB) modified AuNPs-doped covalent organic framework (COF) polymer as a detection antibody tag for amplifying the signal, the immunosensor showed excellent sensitivity for CYFRA21-1 detection and can be applied to real serum samples analysis. Multiplex detection of tumor biomarkers is more reliable than single-antigen detection. [PERSON] group [53] reported a label-free immunosensor for multiplex detection of bladder cancer markers (Figure 3). Two-dimensional MXNSs were first controllably deposited onto a gold dual interdigitated microelectrode (DIDuE) via electroplating to enhance the active interface. A task-specific ionic liquid AFBPB, with excellent ionic conductivity, was subsequently modified onto MXNS surfaces via \(\pi\)-\(\pi\) and electrostatic adsorption to further covalently bind antibodies. Attributed to the synergistic electrochemical effect of MXNSs  
Figure 2: Schematic diagram of the mechanism for electrochemical CEA detection. Adapted with permission from Ref. [48]. 2018, Elsevier. and FBPB, the redox current of the MXNSs-AFBPB modified DID\(\upmu\)E was seven times higher than that of the bare electrode. Using Apo-A1 and NMP 22 as model analytes, the resulting MXNSs-AFBPB modified DID\(\upmu\)E immunosensor exhibited a wide linear response range (0.1 pg mL\({}^{-1}\) to 50 ng mL\({}^{-1}\)) with LOD values as low as 0.3 and 0.7 pg mL\({}^{-1}\), respectively, and acceptable practicability in the detection of human urine samples. MXene-based immunosensors for detecting biomarkers of cardiovascular disease have been reported. By modifying _f_-Ti\({}_{3}\)C\({}_{2}\)-MXene on the screen-printed carbon electrodes (SPCEs), [PERSON] and co-worker [54] developed a label-free immunosensor for the detection of cTnI, one of the most specific biomarkers for acute myocardial infarction (AMI). Since the excellent conductivity and large surface area of _f_-Ti\({}_{3}\)C\({}_{2}\)-MXene facilitated both the electron transfer and antibody immobilization, the immunosensor achieved rapid and sensitive detection of cTnI, exhibiting great potential in clinical monitoring of AMI. Recently, another label-free immunosensor for the analysis of human cTnT was reported [9]. To overcome the problems of restacking and anodic oxidation of MXenes, 2D Ti\({}_{3}\)C\({}_{2}\)Tx nanosheets were covalently functionalized with first-generation Poly(amidoamine) (PAMAM) dendrimers to form MXene@PAMAM composite. The synergistic effect of MXene and PAMAM endowed the composite with good electrical conductivity, large specific surface areas and impressive electrochemical stability.