Flow-Based In Situ Synthesis of Covalent Organic Framework Thin Films and Liquid-Phase Quartz Crystal Microbalance with Dissipation Monitoring (QCM-D) Analysis of Adsorption Kinetics

Covalent organic frameworks (COFs) constitute a class of functional porous materials with high stability and tunable porosity, rendering them attractive for adsorption-based applications. To study the interaction between COFs and target molecules, the quartz crystal microbalance with dissipation monitoring (QCM-D), which offers high sensitivity to mass changes with a sampling rate of ∼50 ms, is a well-suited tool for analyzing adsorption processes in real time. However, current QCM-D-based adsorption studies have remained largely qualitative, primarily relying on comparison of the magnitudes of frequency shifts. This occurs because the intrinsic mass of the film coated onto QCM-D sensors is often not determined, which prevents the quantification of the adsorption capacity. To address this challenge, a flow-based strategy to directly grow homogeneous COF thin films on sensor surfaces under QCM-D monitoring was established. This approach facilitates reliable mass determination, robust film attachment, and reproducible film preparation with precise control over film growth. To account for the contribution of the embedded solvent during film deposition, deuterated solvent exchange was performed. Through the use of methylene blue as a model adsorbate, liquid-phase QCM-D measurements revealed adsorption capacities of ∼31 mg·g^-1 for triformylphloroglucinol-phenylenediamine (TpPa) COF and ∼259 mg·g^-1 for TpPa-SO₃H within 60 min. It was found that a single kinetic model cannot describe the entire process. Instead, the pseudo-second-order (PSO) model best described the initial surface-controlled phase, whereas subsequent uptake was dominated by intraparticle diffusion (IPD). A hybrid kinetic model with a time-weighted function successfully captured the transition between these two stages and provided insights into the difference in mass transportation in TpPa and TpPa-SO₃H. This methodology provides a platform for high-speed, quantitative and real-time liquid-phase analysis of COF-guest interactions with high potential for extension to other porous materials.