4. Materials and Methods

Materials. 3-Hydroxytyramine hydrochloride (dopamine) was purchased from Adamas. Lithium fluoride (LiF, 99.9%) was purchased from Adamas. Ti3AlC2 MAX phase (400 mesh) was purchased from 11 Technology Co., Ltd. Hydrochloric acid (HCl, 37%) was purchased from Adamas.
Synthesis of MXene nanosheets . To synthesize Ti3C2Tx nanosheets from Ti3AlC2 MAX phase, the etching solution is LiF/HCl.[55] First, a certain amount of LiF and HCl (9 M) was stirred for 30 min to ensure that LiF is dissolved. Then the solution was heated to 35° C and the precursor MAX phase was slowly etched at a certain speed for 24 h to etch the Al layer. The resulting solution was repeatedly washed with deionized water until the pH value was higher than 6 to obtain a few layers of Ti3C2Tx nanosheets.
MXene/PDA biocomposite film preparation. First of all, the concentration of MXene solution was calculated by vacuum filtration. Subsequently, dopamine of different masses was used to control the mass ratios to 1:0; 1:1/3; 1:1 and 1:3. We stirred the solution magnetically for 30 minutes at a certain speed, then adjusted the pH of the solution to about 8.5, and continued to stir continuously for 12 hours under certain conditions. Subsequently, the solvent was removed and a composite membrane was formed by vacuum filtration using a cellulose acetate membrane as a substrate. At last, it was named as MXene/PDA film and dried overnight at room temperature.
Fabrication of flexible pressure sensor. Copper films were first drawn from both ends of the MXene/PDA flexible film as electrodes, and then the two flexible PVC films were cleaned with deionized water, ethanol and acetone, respectively, and covered at both ends of the pressure-sensitive material to prevent material contamination and ensured the stability of the equipment during the test.
Characterization . The crystalline structures of MXene film and MXene/PDA were characterized by X-ray diffraction (XRD, D8-Advance, Germany) with 2θ in the ranges of 5~60° at room temperature. The morphology of the Ti3C2Tx MXene film and MXene/PDA biocomposite film were observed with field emission scanning electron microscopy (FE-SEM, JEOL JSM-7001F, Japan) and transmission electron microscopy (TEM, Tecnai G220S-Twin, USA). The thickness and surface topography of sensitive materials were characterized by atomic force microscopy (AFM, Cypher, USA). The surface compositions and chemical states were examined by X-ray photoelectron spectroscopy (XPS, VG ESCALAB 210, USA). The sensing performances are measured by the flexible device analysis system (AES-4SD, SINO AGGTECH, China).