Graphite flakes (Nacional de Grafite Ltda, Brazil) were used as the starting material to prepare graphene oxide by the modified Hummers' method as previously described . While endotoxin-free water (water for injections) was acquired from Fresenius Kabi Ltd., UK, all other reagents were purchased from Merck-Sigma Aldrich, UK. All glassware and instruments used for the reaction were previously washed with 35% nitric acid (HNO3) and left to dry overnight the oven at 180ºC, as a depyrogenation step. All steps were performed in a non-pyrogenic environment, which involved the minimisation of potential contamination by using depyrogenated glassware and water for injections, and handling the GO dispersions only under sterile laminar flow hoods.