Flame-retardant liquid metal in cellulose/MXene fire warning sensors

Developing fast, durable, and multifunctional fire-warning sensors remains challenging, as existing materials struggle to combine rapid response, cyclic stability, and robust mechanical and flame-retardant performance. Here, we fabricated Cellulose/MXene-PDA@LM (CMPL) films via sequential layer-by-layer assembly of MXene/cellulose (CM) gel and PDA@LM layers. This work for the first time introduction of liquid metal (LM) into fire-warning sensors, revealing unexpected flame-retardant functionality. LM significantly reduced the peak heat release rate of CMPL by 65.1% compared to CM film. When integrated into a 12 V circuit with a 30 W warning lamp, the CMPL film responds to thermal stimulus from an alcohol lamp with ultrafast response and recovery times of ∼4 s and ∼ 2 s, respectively. Its resistance responds almost instantaneously to thermal stimulus and changes over 30 orders of magnitude within the fire-warning response period, while maintaining stable cyclic performance for over 1 h. Mechanistic studies reveal that cellulose pyrolysis, PDA carbonization, and LM exothermic oxidation synergistically promote MXene oxidation, forming a dense C–LM_xO_y-doped TiO₂ network that enables sensitive and reversible resistance transitions for rapid fire detection. This work provides a new strategy for designing green, cyclically reusable fire-warning sensors, with strong implications for next-generation fire safety systems.