Highly Conductive, Stable, and Self-Healing MXene-Based Hydrogel Sensor via a Controlled Assembly of Polydopamine and Cellulose Nanocrystal

MXene is a promising conductive nanofiller for hydrogels due to its excellent electricity conductivity and water dispersibility. However, MXene is prone to oxidize in the presence of air and water, resulting in a significant loss of conductivity. Polydopamine (PDA) has been coated on MXene to enhance its antioxidation stability via the physical barrier and chemical reducing ability of PDA, which unavoidably causes severe aggregation and a significant decrease in conductivity due to the crosslinking and insulation of PDA. Herein, we propose a facile strategy to construct a highly conductive, stable, and self-healing MXene-based polyvinyl alcohol (PVA) hydrogel by a controlled assembly of PDA and cellulose nanocrystal (CNC). PDA is first formed by oxidation self-polymerization in PVA solution without the presence of CNC and MXene, which can effectively reduce the content of aggregation-inducing groups and avoid the formation of an insulating PDA layer on the surface of MXene. The addition of CNCs results in the easy dispersion of a high content of MXene via hydrogen bonding and electrostatic interactions. The PVA-PDA hydrogel with MXene and CNC as conductive and reinforcing nanofillers (PP-CM) is cross-linked by dynamic borax covalent bonds and shows a conductivity of 7.14 S m⁻¹. The introduction of PDA effectively protects MXene and results in only a 14% decrease in conductivity after 7 days, significantly improving antioxidant stability. This hydrogel also possesses rapid self-healing capabilities, achieving 90.5% self-healing efficiency within 10 min. This versatile approach opens new avenues for the preparation and application of MXene-based conductive hydrogels.