Molecular Accessibility and Diffusion of Resorufin in Zeolite Crystals

• Published in: Chemistry : a European journal 2024-01, Vol.30 (1), p.e202302553-n/a
• Main Authors: Erik Maris, J. J., Parker, Luke A., Stanciakova, Katarina, Nikolopoulos, Nikolaos, Berendsen, Koen M. H., Blaaderen, Alfons, Meirer, Florian, Rabouw, Freddy T., Weckhuysen, Bert M.
• Format: Article
• Language: English
• Subjects: Accessibility; aggregation; Aqueous solutions; Chemistry; Confocal microscopy; Crystals; Diffusion coefficient; Dyes; Fluorescence; fluorescence probes; fluorescence spectroscopy; host–guest chemistry; Materials science; Pores; Resorufin; Scanning microscopy; Zeolites
• ISSN: 0947-6539; 1521-3765
• DOI: 10.1002/chem.202302553

We have used confocal laser scanning microscopy on the small, fluorescent resorufin dye molecule to visualize molecular accessibility and diffusion in the hierarchical, anisotropic pore structure of large (~10 μm‐sized) zeolite‐β crystals. The resorufin dye is widely used in life and materials science, but only in its deprotonated form because the protonated molecule is barely fluorescent in aqueous solution. In this work, we show that protonated resorufin is in fact strongly fluorescent when confined within zeolite micropores, thus enabling fluorescence microimaging experiments. We find that J‐aggregation guest–guest interactions lead to a decrease in the measured fluorescence intensity that can be prevented by using non‐fluorescent spacer molecules. We characterized the pore space by introducing resorufin from the outside solution and following its diffusion into zeolite‐β crystals. The eventual homogeneous distribution of resorufin molecules throughout the zeolite indicates a fully accessible pore network. This enables the quantification of the diffusion coefficient in the straight pores of zeolite‐β without the need for complex analysis, and we found a value of 3×10−15 m2 s−1. Furthermore, we saw that diffusion through the straight pores of zeolite‐β is impeded when crossing the boundaries between zeolite subunits. Resorufin has been investigated as a fluorescent probe for 3D microimaging in microporous materials. Understanding the processes, such as aggregation, that affect the measured fluorescence intensity enables the probe to be used to measure real‐time diffusion, microporosity, and accessibility in zeolite‐β crystals.