239: Unraveling the size-selectivity paradox in MOF-based QCM sensors: Enhanced response to large alcohols despite diffusion restrictions

Abstract: This study challenges the conventional molecular sieving model by demonstrating that metal-organic framework (MOF)-based quartz crystal microbalance (QCM) sensors exhibit significantly enhanced responses to alcohol molecules exceeding the nominal pore size of the framework. Using MIL-125-NH2 as a model sensing layer and a homologous series of unsaturated alcohols, we reveal that the adsorption time constant increases linearly with molecular diameter, while the integral response grows nearly exponentially with molecular weight. Although the effective diffusion coefficient decreases sharply with size, the normalized diffusion hindrance ratio decreases logarithmically. These findings indicate that stronger host–guest interactions for larger molecules outweigh their kinetic penalties, leading to a counterintuitive “size-selectivity paradox.” We propose a multi-mechanism model incorporating conformational flexibility, framework adaptability, and cumulative non-covalent interactions to explain this behavior. This work provides a quantitative framework for designing high-performance microporous sensors through synergistic control of diffusion and adsorption dynamics.