Influence of surface oxygen clusters upon molecular stacking of paclitaxel over graphene oxide sheets

Aqueous dispersibility of graphene oxide (GO) sheets can be improved through enrichment of oxygen content (oxidation). However, the resulting 'oxygen clusters' are accompanied with numerous point defects, depletive to pi-pi stacking behavior of sheets. In this work, we have investigated the properties of sheets oxidized with 2, 4, 6, 8, 10 and 12 weight equivalents of KMnO4. Sheets were characterized by spectroscopic, X-ray diffraction (XRD) and electron microscopic techniques. Paclitaxel (PLX) was used as a model drug to analyze the changes in the loading efficiency at different levels of oxidation. Density of oxygen clusters increased as we shifted across increasing proportion of KMnO4. This was accompanied with increased inter-layer spacing, disruption of pure graphitic domains and changes in hybridization of carbon atoms (sp(2) to sp(3)). However, these defects exhibited the tendency of saturation at a finite proportion of KMnO4. Oxygen groups negatively affected the loading efficiency of sheets for PLX. Improvement in the loading efficiency of reduced GO (rGO) sheets clarified the predominant role of pi-pi interactions at the surface. Our study reveals that high oxidation may offset the loading potential of highly aqueous dispersible GO sheets. Therefore, the extent of oxidation must be tuned taking into account the availability of H-bond forming groups in the drug molecule.