GO was prepared after oxidation of graphite flakes using a modified version of the Hummers' method, under endotoxin-free conditions, as described previously by our group and summarised in Figure 1 . In order to maintain endotoxin-free conditions throughout the process, we took various precautions which included the depyrogenation of all used glassware, the use of disposable sterilised consumables and non-pyrogenic water for the chemical reaction steps, and the handling of GO dispersion under a sterile laminar flow, as described in . This procedure in its totality has been described in the Experimental section. The absence of endotoxin was confirmed in all three GO types by a cell-based assay described in , which indicated a contamination lower than 0.01 EU/mL (Figure , Supporting Information).