Dissimilar adsorption of higher-order aggregates compared with monomers and dimers of methylene blue on graphene oxide: An optical spectroscopic perspective

Abstract

Graphene oxide (GO) is widely used as an adsorbent for organic dyes because of its high surface area and dispersibility. Several studies have investigated the interaction of GO with methylene blue (MB), a model organic dye used in assessing adsorptive efficacy of materials. Such studies establish that MB molecules actively adsorb onto GO nanosheets through a suite of forces, primarily comprising of electrostatic forces, van der Waals forces and π-π interactions. Furthermore, it is largely presumed that all molecular forms of MB—monomers, dimers, and higher-order aggregates, adsorb on GO in an analogous manner, i.e., these studies presume that the relative intensities of these forces are similar for all species. In this study, we show that, although monomers and dimers evince similar adsorptive characteristics with GO, the adsorptive behaviour of higher-order aggregates differs from them. Using spectroscopic insights, we demonstrate a preferential adsorption of monomers and dimers at certain GO concentrations, and vice-versa. We found that this preferential adsorption also depends on the lateral dimensions of GO nanosheets. We posit that this difference is because of an inherent distinction in the ways these molecular species interact with GO—while the monomers and dimers interact through London dispersion and electrostatic forces, the higher-order aggregates largely interact through π-π interactions. These physical insights supplement the present understanding on the fundamental interfacial science between GO and organic dyes.