Synthesis of GO dispersions

GO was synthesized using modified Hummers' method⁶¹. SP-1 grade 325 mesh graphite powder (Bay Carbon, Inc.), sulfuric acid, potassium persulfate, phosphorus pentoxide and potassium permanganate (Sigma-Aldrich), were used for the synthesis. The synthesized GO was exfoliated by sonication (UP-100 Ultrasonic processor) in RO water for 1 h, followed by centrifugation to remove the un-exfoliated GO. The average lateral size of the GO sheets was determined using a SEM (FEI Nova NanoSEM 450 FEGSEM (2012)) and was estimated to be ∼0.9±0.4 μm (90 sheets were measured to calculate the average sheet size). An Ocean Optics USB4000 ultraviolet–visible spectrometer was used to determine the GO concentrations by measuring the absorbance at 230 nm (using a quartz cuvette, Starna Cells Pty. Ltd, Australia). Various concentrations of GO dispersions were prepared using a superabsorbent polymer (cross-linked polyacrylate copolymer based hydrogel beads, Demi Co., Ltd, China). For example, within ∼1 h, a 10 ml GO dispersion with a concentration of 20 mg ml⁻¹ was obtained from a 1 l suspension of 0.25 mg ml⁻¹ GO using 10 g of the hydrogel beads. To avoid possible concentration polarization around the beads and to speed up the absorbent process the container was mildly agitated by a magnetic stirrer. After the hydrogel beads were saturated with water, they were removed from the concentrated solution. The GO deposits adhering to the surface of the saturated beads were removed by washing them with RO water. The saturated hydrogel beads could be reused after drying them at 50 °C overnight.