The unique structure, composition and physicochemical properties of the two-dimensional (2D) MXenes nanomaterials have made it another "star material" in the field of two-dimensional materials research besides graphene in the past decade. MXenes is currently expanding its applications from mechanical, optical, electronic, and energy storage to biomedical and environmental protection. This is mainly due to its large specific surface area, abundant termination groups, biocompatibility properties, and easy surface functionalization using various polymers or nanoparticles, making it ideal for precise biosensing and bioimaging, nano platform for monitoring toxic gases and liquid contaminants. At present, MXenes materials research in the field of sensors mainly focuses on current bioelectrochemical sensing, bio/gas resistance sensing and piezoresistive sensing. For example, MXenes materials are used as immobilized matrix for proteins, biological enzymes, and bioluminescent materials in bioelectrochemical sensing. The immobilized matrix is used to improve the transferring efficiency and rate of electron mass by utilizing its large specific surface area and high conductivity, so as to improve the sensing sensitivity and reduce the detection limit. Bio/gas resistance sensing is based on the sensitivity of MXenes materials to the conductivity of externally adsorbed molecules (biomolecules or gas molecules), while the adsorption of foreign biomolecules or gas molecules is based on the interaction of the termination functional group of MXenes and the molecules. The research on piezoresistive sensing is mainly focused on portable or a wearable piezoresistive sensor. MXenes is subjected to stress and its layer spacing changes, resulting in changes in its electrical conductivity and electrical signals. It can be seen that in the application of sensors, the large specific surface area and conductivity of the MXenes material and the surface termination functional group are important. However, the conductivity of MXenes depends on the surface termination group. The existence of functional groups will to some extent reduce the conductivity of MXenes, and even some groups make it into a semiconductor, which is not conducive to the high conductivity of the sensor. Therefore, the exist of termination group and the high conductivity are a pair of contradictions, and the research work needs to find a balance between the two. In addition, different functional groups also have different effects on the conductivity of MXenes materials of different element types. Therefore, researchers are actively seeking MXenes materials of different element types that are more suitable for sensing while studying the use of further functionalized modified electrodes (such as modifying precious metal nanoparticles, carbon nanotubes, etc. ) to overcome the problem of electrical conductivity. This paper briefly summarizes the research progress in the preparation, structure and properties of MXenes materials, and focuses on the construction of MXenes sensors designed for biomedical and environmental protection applications and their latest research progress, including current-type biosensing, wearable biosensing, MXenes reduction electrochemical sensing, bioelectrical resistance sensing, gas resistance sensing, strain/resistive sensing. The article also discusses the difficulties and challenges of MXenes in the application of sensing. We hope this article can provide useful guidance for researchers in the development and application of MXenes sensors. © 2021, Materials Review Magazine. All right reserved.
