NMR-Restricted Diffusion and Relaxation Analysis of the β-Cyclodextrin Nanosponge in the Presence of Heavy Metals

In the recent past, NMR methods such as diffusometry, relaxometry, and cryoporometry have gained attention as some of the most promising noninvasive methods of choice to describe porous solids of topical interest in terms of their structure and physical properties relevant for optimization of their design. The present study aims to employ such NMR methods to explore deposition or adsorption of heavy metal ions onto a specific beta-cyclodextrin nanosponge (β-CDNS) surface, modifying the textural features of the porous material. Here, we employed ¹H NMR-restricted diffusion and relaxation measurements of water as a probe liquid to characterize the pore geometry such as surface-to-volume ratio (S/V), pore size, tortuosity, and surface relaxivity (ρ) of the β-CDNS synthesized following a literature-reported method. NMR-restricted diffusion measurements performed on a water-adsorbed β-CDNS sample allowed extraction of a surface-to-volume ratio of 0.127 μm^-1, a uniform pore size of 47.24 μm, and a tortuosity of 1.816. Two heavy metal ions, viz., Mn²⁺ and Cu²⁺, are chosen to investigate the effect of metal adsorption on the nanosponge surface in terms of surface relaxivity (ρ) by employing NMR transverse relaxation time measurement of water in the absence and presence of the metal ions. The drastic increase of water proton surface relaxivity is observed with the deposition of metal ions indicating a trend of β-CDNS-Cu ≫ β-CDNS-Mn > β-CDNS. It is anticipated that such detailed NMR analysis of β-CD-based nanosponges will allow gradation of the porous materials as per their effectiveness in various applications such as environmental remediation, gas adsorption, and drug delivery, to name a few.