Wafer‐Scale Synthesis of Molecularly Engineered 2D Covalent Organic Framework Films for Highly‐Sensitive and Rapid‐Response Humidity Sensing

ABSTRACT Covalent organic frameworks (COFs) hold exceptional potential for humidity sensing due to their tunable chemical structure and high porosity, yet their uncontrolled relationships between molecular design, nanoscale architecture, and sensing performance have hindered their practical applications. Here, we report the rational molecular engineering of wafer‐scale, ultrathin 2D imine‐linked COF films via interfacial polymerization, illustrating precise control over electronic band structure, nanoscale porosity, and hygroscopicity for exceptional humidity sensing, thus unveiling the underlying structural‐property correlations. By simultaneously incorporating triazine and multi‐hydroxyl groups, the humidity sensor based on 2D COF TPT‐THTA exhibits a high sensitivity (66 124% per %RH), fast response/recovery times (0.12/0.40 s), and minimal hysteresis (ΔRH ≈ 1.0%), due to a synergistic effect of high structural polarity, excellent hydrophilicity, and the nanoscale confined crystalline pore framework. The nanometer‐scale thickness and ultrasmooth surface facilitate efficient charge transport and water adsorption kinetics. Leveraging these properties, we further demonstrate a prototypical wearable sensor for real‐time respiratory monitoring, which is capable of accurately tracking physiological states and detecting pathological patterns (asthma, apnea). This work establishes a fundamental molecular engineering paradigm for 2D COF film‐based sensors, bridging the gap between programmable materials and next‐generation high‐performance health diagnostics.