MXenes and Their Applications in Wearable Sensors

Abstract

MXenes, a kind of two-dimensional material of early transition metal carbides and carbonitrides, have emerged as a unique class of layered-structured metallic materials with attractive features, as good conductivity comparable to metals, enhanced ionic conductivity, hydrophilic property derived from their hydroxyl or oxygen-terminated surfaces, and mechanical flexibility. With tunable etching methods, the morphology of MXenes can be effectively controlled to form nanoparticles, single layer, or multi-layer nanosheets, which exhibit large specific surface areas and is favorable for enhancing the sensing performance of MXenes based sensors. Moreover, MXenes are available to form composites with other materials facilely. With structure design, MXenes or its composite show enhanced mechanical flexibility and stretchability, which enabled its wide application in the fields of wearable sensors, energy storage, and electromagnetic shielding. In this review, recent progress in MXenes is summarized, focusing on its application in wearable sensors including pressure/strain sensing, biochemical sensing, temperature, and gas sensing. Furthermore, the main challenges and future research are also discussed.

Keywords: MXenes; biosensor; gas sensor; pressure sensor; strain sensor.

Figure 1

An overview of the applications of MXenes in the wearable devices field. Reproduced from Cai et al. (2018) with permission from American Chemical Society; reproduced from Liu et al. (2018) with permission from WILEY-VCH Verlag GmbH & Co; reproduced from Li X.-P. et al. (2019) with permission from Elsevier Inc; reproduced from An et al. (2018); reproduced from Wang et al. (2019); reproduced from Li M. et al. (2019) with permission from Elsevier B.V; reproduced from Kim et al. (2018) with permission from American Chemical Society; reproduced from Lee et al. (2017) with permission from American Chemical Society.

Figure 2

(A) Schematic for the exfoliation process of MAX phases and formation of MXenes; (B) The image of the multi-layer of MXenes; (C) Structure of MAX phases and the corresponding MXenes; reproduced from Naguib et al. (2013) with permission from WILEY-VCH Verlag GmbH & Co. (D) The image of the single-layer of MXenes. (A,B,D) are reproduced from Naguib et al. (2012) with permission from American Chemical Society.

Figure 3

MXenes for strain sensor. (A) Fabrication process of a sandwich-like Ti₃C₂T_x MXene/CNT layer; (B) SEM images of sandwich-like Ti₃C₂T_x MXene/CNT layers. (C) Relative resistance responses of the sensor in phonation and motion; (A–C) are reproduced from Cai et al. (2018) with permission from American Chemical Society. (D) Schematic illustration of the Ti₃C₂T_x-AgNW-PDA/Ni²⁺ sensor based on the “brick-and-mortar” architecture; reproduced from Shi et al. (2019) with permission from American Chemical Society. (E) Schematic illustration of the mechanism of the electromechanical responses of M-hydrogel; (F) Schematic for vocal sensing and (G) Resistance change in response to similarly sounding letters “B.” (E–G) are reproduced from Zhang et al. (2018).

Figure 4

MXenes for pressure sensor. (A) Working micromechanism and SEM image of MXenes-material for piezoresistive sensor; reproduced from Ma et al. (2017). (B) Schematic illustration and application of MX/rGO aerogel sensor; reproduced from Ma Y. et al. (2018) with permission from American Chemical Society. (C) Schematic elasticity mechanisms and application of C-MX/CNC; reproduced from Zhuo et al. (2019) with permission from The Royal Society of Chemistry. (D) Schematic of interlocking structure of simulated human skin and the application of pulse measurement; reproduced from Wang et al. (2019) with permission from American Chemical Society.

Figure 5

MXenes for biosensor. (A) Schematic illustrations for preparing the Ti₃C₂ QDs by using a liquid exfoliation and solvothermal treatment approach; reproduced from Chen et al. (2018) with permission from The Royal Society of Chemistry. (B) Schematic showing the synthesis process of Au/MXene nanocomposites; (C) GOx/Au/MXene/Nafion/GCE at a constant voltage of−0.402 V; reproduced from Rakhi et al. (2016). (D) Application of MXenes for detection of NH₃. (D) is reproduced from Yu et al. (2015) with permission from American Chemical Society. (E) Schematic of the adsorption process of water molecules at the Ti₃C₂/TiO₂ composite film. Reproduced from Li N. et al. (2019) with permission from American Chemical Society.