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.

No abstract

The formation of iron oxide nanoparticles obtained by the thermal decomposition of iron–oleate complexes via a “heating-up” process was monitored by the NMR relaxation method, which allows us to track the average diameter of iron oxide nanoparticles. The analysis of dependencies of the T 1/T 2 ratio and 1/T 2 values on time at the heating of the reaction mixture demonstrated that the nucleation and nanoparticle growth processes could proceed in two ways depending on the presence of oleic acid in the solution: continuously without separation of the nucleation stage or discretely with the separation of the nucleation and growth of nanoparticles. In the case of excess of oleic acid, the nucleation process follows the well-known LaMer model, characterized by a burst of nucleation and separation of nucleation and growth under continuous monomer supply. In the absence of oleic acid in the system, the nucleation and growth of nanoparticles are not separated and proceed continuously. The presence of a sufficient amount of oleic acid is the reason why the process of nucleation and growth of nanoparticles by heating up follows the path of “burst nucleation,” which is similar to the “hot injection” method. That is, oleic acid acts as a “fuse” for the explosive “burst nucleation.” This new approach shows that the “hot injection” method can be carried out not only mechanically (by introducing a precursor into the reaction system) but also chemically, with the help of a reagent that has been in the system from the very beginning.