No abstract

The sorption capacity of graphene oxide (GO) toward different metal cations has been the subject of several recent studies. However, the reported quantitative data are controversial, and the mechanism of chemical bonding between GO and metal cations is poorly understood. Clarifying these questions can eventually help to reveal the fine chemical structure of GO that remains ambiguous. In this work, we study the binding of Gd and Mn by GO in the presence of several competing metal cations by the H NMR relaxation method. As a general trend, the efficiency of the metal cations to bind to GO increases with ionic charge, and depends on their ability to form coordinate-covalent bonds with GO oxygen groups. The efficiency of the competing metal cations to "replace" Gd and Mn increases in the order Na < Cs < Ca < Sr < Ga < Lu. GO contains two different types of binding sites, bonding to which results in either high or low NMR relaxivity of the resulting Gd-GO and Mn-GO solutions. Gd and Mn, being replaced from the high-relaxivity sites by the large excess of competing cations, are not released into the bulk solution, but only migrate to the low-relaxivity sites, remaining covalently bonded to GO. The absolute majority of the existing carboxyl groups in GO are located at tiny few-carbon-atom-vacancy defects on the major planes. The density of these vacancy defects is estimated as one per every 200 carbon atoms.

Graphene oxide (GO) aqueous solutions are known to form liquid crystals that can switch in electric fields. Magnetic fields as external stimuli are inefficient toward GO because of its diamagnetic properties, and GO is known to be insoluble in most of the organic solvents. In this study, composites of GO with oleate-protected magnetite nanoparticles were prepared as stable colloid solutions in the mixed isopropanol–chloroform solvents. The structure of the composite particles and the optical properties of their solutions can be controlled by the ratio of the mixing parent components. The as-prepared solutions are highly responsive to external magnetic field. As the consequence, the optical transmission and the direction of light scattering can be efficiently manipulated. These systems pave the way for fabricating functional materials, such as magneto-optical switches, density-gradient materials, and micromotors. Solubility in nonpolar organic solvents broadens the scope of their potential applications.

In this study, we investigate chemical interactions of Mn2+ ions with graphene oxides, prepared by Hummers (HGO) and Brodie (BGO) methods in aqueous solutions by means of the NMR relaxation....