Aqueous solutions of triblock copolymers used as the media affecting the magnetic relaxation properties of gadolinium ions trapped by metal-oxide nanostructures

Abstract

The present work introduces the optimization of the oxidative degradation, complex formation and aggregation of the water-soluble metal oxide nanostructures (the so-called keplerates) [{W6O21(H2O)5(OAc)0.5}12{Mo2O4(OAc)}30]48− (W72Mo60), [{Mo6O21(H2O)3(SO4)}12(VO)30(H2O)20]26− (Mo72V30), and [{Mo6O21}12Fe30(OAc)12{Mo2O7(H2O)}2{H2Mo2O8(H2O)}(H2O)91] (Mo72Fe30) in aqueous solutions containing Gd3+ ions and triblock copolymers, which can be used as a tool to enhance magnetic relaxivity of the gadolinium-bound nanostructures. The particularly significant enhancement in the magnetic relaxivity of the Gd3+ ions bound with W72Mo60 versus Mo72Fe30/V30 is due to the more convenient size of the external pores in the keplerate framework, which are the binding sites for Gd3+ aqua ions. The structure of the W72Mo60 keplerate is also responsible for the high stabilities of its gadolinium complexes against oxidative degradation in the solutions. The aggregation of the gadolinium-bound W72Mo60 keplerates controlled by the Gd:keplerate molar ratio is a factor in addition to the structure affecting the relaxivity. The assembly of the gadolinium-bound keplerates with the triblock copolymers enhances the relaxivity values by limiting the size of the aggregates, while the relaxivity of the molecular complexes is insignificantly affected by the assembly with the triblock copolymers. The gadolinium complexes exhibiting competitively high magnetic relaxivity values, coupled with their high chemical stability and low cytotoxicity, are introduced herein as the result of the structural optimization, both on molecular and supramolecular levels.