TRAPPING OF Gd(III) IONS BY KEPLERATE POLYANIONIC NANOCAPSULES IN WATER: A 1H FAST FIELD-CYCLING NMR RELAXOMETRY STUDY

Polyoxomolybdates of the type [{MoVIMoVI5O21(H2O)6}12{MoV2O4(ligandn-)}30](12+30n)-, also known as Keplerates (Kps), are polyanionic nanocapsules (diameter ? 3 nm) bearing 20 {Mo9O9} pores connected to a central cavity by 20 channels. In solution they can entrap cations in different sites of their porous structure in equilibrium with cations free in solution. Hydrophilic colloids were prepared through the self-assembly of KpOAc or KpHPO4 (i.e. Kp with ligand = acetate or hydrogen phosphate) macroanions and Gd(III) cations in water, further stabilized by F-127 Pluronics. The strongly enhanced water proton relaxivity observed at 20 MHz was postulated to derive from the trapping of Gd(III) aqua ions in the nanocapsules [1,2]. 1H FFC NMR relaxometry allowed this hypothesis to be verified on aqueous suspensions containing either KpOAc or KpHPO4 and Gd(III) in different proportions (Fig. 1). The analysis of 1H longitudinal relaxivity vs the Larmor frequency on the basis of the theory for paramagnetic relaxation enhancement gave a detailed description of the state of [Gd(H2O)8]3+ ions in dependence of the different capsule charge and Gd to Kp molar ratios [3]. [Gd(H2O)8]3+ ions were found to be trapped in the KpOAc capsules, most probably located in up to 11 pores, with no evidence for free ions. On the other hand, equilibria between trapped and free [Gd(H2O)8]3+ ions established in the suspensions containing KpHPO4, which depended on the Gd to Kp molar ratios, and a maximum of 3-4 Gd per capsule was found. A major role of the NH4 + counter-ion was invoked to rationalize the different behavior of KpOAc and KpHPO4. References [1] J. Elistratova, B. Akhmadeev, A. Gubaidullin, V. Korenev, M. Sokolov, I. Nizameev, A. Stepanov, I. Ismaev, M. Kadirov, A. Voloshina, A. Mustafina, New J. Chem., 2017, 41, 5271-5275. [2] J. Elistratova, B. Akhmadeev, V. Korenev, M. Sokolov, I. Nizameev, A. Gubaidullin, A. Voloshina, A. Mustafina, Soft Matter, 2018, 14, 7916-7925. [3] S. Pizzanelli, R. Zairov, M. Sokolov, M. C. Mascherpa, A. Mustafina, L. Calucci, submitted manuscript