Oxidation degree or sheet size: What really matters for the photothermal effect and ecotoxicity of graphene oxide?

Abstract A set of thirteen GO samples obtained following different methodologies and described by eight structural parameters - hydrodynamic diameter (658 nm - 142 nm), size polydispersity index, optical absorption coefficient, lambda maximum, D/G peak intensity ratio, crystallite size, distance between defects, and pH - was submitted to NIR 808 nm irradiation (experiment 1) and fish embryo toxicity (FET) test performed with zebrafish embryos (experiment 2). Principal component analysis (PCA) applied to the obtained data shows that the optical absorption coefficient and lambda maximum, which are strongly dependent on the GO oxidation degree and presence of carboxylic acid groups, are the most relevant features for the photothermal effect. Conversely, size is the predominant parameter for the ecotoxicity. Only samples exhibiting the smallest sheet size (below 200 nm) do not interfere either on the hatching time or the rate of alive hatched individuals. GO samples composed of larger sheets, regardless their oxidation and carboxylation degrees, impart negative effects to the embryo’s development stages, including hatching delay, death of embryos and abnormalities on hatched individuals. It is therefore concluded that the photothermal effect is more dependent on the oxidation degree, whereas ecotoxicity is more dependent on the size of GO sheets.