HR MAS NMR, powder XRD and Raman spectroscopy study of inclusion phenomena in βCD nanosponges

Three different samples of beta cyclodextrin nanosponges (CDNS) are prepared from beta cyclodextrin (beta CD) and pyromellitic dianhydride (PMA). CDNS are cross-linked, nanoporous materials whose pore size can be modulated by suitable choice of the CD/PMA molar ratio. In the presence of aqueous solutions they can swell giving rise to gel-like behavior. The Raman spectra of dry and water treated CDNS are described, with emphasis on the group vibration modes in the low frequency part of spectrum, sensitive to molecular environment and cross-linking degree, and on O-H/C-H vibration modes of dry/swollen CDNS, in turn providing information on the hydration dynamics. Powder X-ray diffraction data indicate low crystallinity and the presence of bulk water within the 3D polymer network. High resolution magic angle spinning (HR MAS) NMR spectroscopy is successfully used for investigation of swollen CDNS. The NMR signals of bulk and "bound" water indicate two different states of water molecules inside the gel. Probe solute fluorescein is used to spot on the diffusion properties inside the gel. In one case the diffusion coefficient of fluorescein measured in CDNS results one order of magnitude higher than that in D2O. The acceleration effect uncovered indicates that the motion of fluorescein inside the porous gel is driven by both hydrodynamic and electrostatic factors.