Binding modes of Fe(III) with graphene oxide in aqueous solutions. Competition with Sr²⁺, Cs⁺, Na⁺ ions and Fe(III) chelators

© 2020 Elsevier B.V. The work is devoted to the analysis of the state of the Fe(III) ions in aqueous solutions of graphene oxide (GO) using NMR-relaxation method. The presence of graphene oxide in solution suppresses the hydrolysis of Fe(III) in a wide pH range, and provides high values of the spin-spin relaxivity of the Fe(III)-GO system. Experiments with varying concentrations of Fe (III) and GO demonstrate the presence of the two types of the sites on GO surface: the A-type sides that strongly bind cations (the source of high relaxivity), and the B-type sites, weakly binding cations (providing lower relaxivity). Addition of the single charged (Cs+, Na+) or double charged (Sr2+) cations to the Fe(III)-GO solutions leads to a decrease in relaxivity due to the displacement of the Fe(III) ions from A to B-type sites with the Na+ < Cs+ < Sr2+ increasing efficiency. The possibility of releasing bound Fe(III) from GO by using two different chelators (ethylenediaminetetraacetic acid (EDTA) and 3,4-dihydroxybenzoic acid (3.4-DHB)) was tested in a wide range of pH