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(L)}30](12+30n)-, also referred to as keplerates (Kps), are polyanionic nanocapsules 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 the internal cavity, in the pores, or on the capsule surface) in equilibrium with cations free in solution. The porosity of Kps was previously exploited to prepare nanostructured potential contrast agents through the self-assembly of KpOAc or KpHPO4 (i.e., Kp with L = acetate or hydrogen phosphate) macroanions and Gd(III) cations in water, accompanied by stabilization with 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. In the present work, 1H fast field cycling NMR relaxometry allowed this hypothesis to be verified on aqueous suspensions containing either KpOAc or KpHPO4 and Gd(III) in different proportions. The analysis of 1H longitudinal relaxivity versus 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 these systems in dependence on the different capsule charge and Gd to Kp molar ratios. [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 and a maximum of 3 to 4 ions per capsule were found. A major role of the NH4+ counterion was considered to rationalize the different behavior of the studied Kps.