Nacre-inspired cellulose nanofiber/MXene flexible composite film with mechanical robustness for humidity sensing

Inspired by nacre-layered nanostructure, 1D 2,2,6,6-tetramethyl-1-piperidinyloxy (TEMPO)-oxidized cellulose nanofibers (TOCNFs) were used as the template to assemble 2D MXene nanosheets into a layered TOCNF/MXene nanocomposite film by a vacuum-assisted filtration strategy. The synergistic effect of the MXene "brick" and TOCNFs "mortar" endowed the composite film with excellent flexibility and a tensile strength of 128.13 MPa, which were attributed to interactions between the interconnected three-dimensional network and multiple hydrogen bonds between TOCNFs and MXene. The humidity-sensing mechanism of the sensor involved the swelling/contraction of channels between MXene interlayers induced by adsorbed H₂O and the swelling of TOCNFs. The TOCNF/MXene sensor showed a maximum response (-ΔI/I₀) of 90 % under 97 % RH, outstanding bending and folding durability (up to 50 cycles), and long-term stability. Lastly, the sensor could dynamically monitor human respiration, skin, and fingertip humidity, suggesting its promising applications in smart wearable electronics.