The NMR relaxivity of gadolinium (III) solutions as the function of oxidation level and flake size of graphene oxide

Chelating by graphene oxide (GO) results in dramatic shortening of the proton relaxation times of Gd3+ aqueous solutions. However, the factors, leading to the increase of relaxivity values, and the mechanisms behind the observed phenomena remain elusive. In this work, we investigated how the relaxation times of the GO/Gd3+ solutions depend on the oxidation level and the flake size of GO. The values of spin-lattice (r1) and spin-spin (r2) relaxivities increase with the oxidation degree of the used GO samples. The relaxivity also increases with decreasing the particle size of GO flakes by sonication. The observed phenomena are explained by the increase in the number of the edge functional groups, generated under the influence of both parameters. The content of these groups, their formation, and chelation of Gd3+ ions is discussed in the manuscript. Viscosity of solutions does not directly affect the relaxation times. With the use of high oxidation level and small particle size GO samples we have registered the record high relaxivity values: r2 = 114.7 mM−1 s−1, and r1 = 97.0 mM−1 s−1. This opens straight routes for developing the GO/Gd3+ based MRI contrast agents.