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

Polyoxomolybdates known as Keplerates (Kps) are polyanionic nanocapsules (diameter 9 nm) bearing 20 Mo9O9 pores connected to a central cavity by 20 channels. In solution they can entrap cations in dierent 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 by the nanocapsules [1,2]. 1H FFC NMR relaxometry allowed this hypothesis to be veried on aqueous suspensions containing either KpOAc or KpHPO4 and Gd(III) in dierent proportions. The analysis of the 1H longitudinal relaxivity, r1, vs the Larmor frequency curves (Fig. 1) 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 dierent 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 dierent behavior of KpOAc and KpHPO4. References: [1] J. Elistratova, et al., New J. Chem. 41, 5271-5275, (2017). [2] J. Elistratova et al., Soft Matter 14, 7916-7925, (2018). [3] S. Pizzanelli et al., J. Phys. Chem. C, accepted manuscript, (2019).