Salting-out as a scalable, in-series purification method of graphene oxides from microsheets to quantum dots

Chemically modified graphenes (CMGs) are used to reinforce composites and as platforms for sensing, electrodes in batteries, conductive films for displays, and others. However, performance of CMG materials is often limited by their physicochemical heterogeneity. Here, we report that simple addition of ammonium sulfate to heterogeneous CMG mixtures, results in effective, scalable, and in-series purification, by salting-out precipitation. The mechanism is based on differences in the dispersibility of the individual graphene oxide (GO). At relatively low ammonium sulfate concentrations (∼10 mM), large, GOs with predominant hydrophobic domains are precipitated. As the salt concentration increases, smaller and less GOs with predominant hydrophobic domains are purified, and eventually graphene quantum dots (GQDs) are isolated. The method described herein was inspired by a well-known protein purification process called ‘salting-out,’ in which proteins of different sizes and surface charge, successively become indispersible over a narrow range of salt concentrations. The salting-out process for GOs is simple and scalable, and can be used to achieve large-scale purification.