Pore-Size-Dependent Catalytic Activity of Porphyrinic MOFs Guided by Isothermal Titration Calorimetry for Electrochemical Biosensing

The modulation of the porosity of metal-organic frameworks (MOFs) is pivotal for tailoring their catalytic performance and related sensing applications. Nevertheless, regulating the relationship between the MOF pore structure and catalytic activity remains highly challenging. Herein, we present the first systematic research on the effect of pore size and topology on electrochemical catalysis, employing three porphyrinic MOFs: PCN-222(Fe), PCN-223(Fe), and PCN-224(Fe). Using methylene blue (MB) as a model redox substrate, the three MOFs exhibit pore-size-dependent catalytic performance. Interestingly, PCN-224(Fe), with a matched pore size and active site spacing, demonstrates the highest electrochemical activity. Leveraging isothermal titration calorimetry (ITC), the strong host-guest interactions between PCN-224(Fe) and MB are confirmed by the largest positive-binding association constants and the smallest entropic contribution. Moreover, density functional theory (DFT) calculations further explain the synergistic effect of d-π coupling and π-π stacking interactions related to the catalysis mechanism, as well as the electron transfer behavior of PCN-224(Fe) during the electrochemical catalytic process. This study provides thermodynamic and theoretical insights to investigate the MOF pore structure-property relationships in electrochemical biosensors.