Preparation and Application of Fluorescent Bioconjugates in Probing the pH and Solvent Relaxation Dynamics in ZIF-8 Cavities

In this study, we prepared two fluorescent bioconjugates, BSA-FITC and BSA-coumarin 343, and employed them as a molecular probe to study the microenvironment in ZIF-8 cavities. By comparing the excitation spectra of BSA- FITC and BSA-FITC encapsulated in ZIF-8, we found that the local pH in ZIF-8 cavities is similar to that in bulk water. For BSA-coumarin 343, we observed clear spectral relaxation dynamics in a nanosecond timescale. The steady-state and time-resolved emission spectra of BSA-coumarin 343 suggested that encapsulating the protein in ZIF-8 significantly reduced the number of free water molecules but had only minor effects on the solvation dynamics of the water bound to the protein surface. Our studies demonstrated the feasibility of using bioconjugates to investigate the properties of water molecules in the ZIF-8 matrix, which is important for regulating the activities of the encapsulated enzyme.