Fluorescence probing of binding sites on graphene oxide nanosheets with Oxazine 1 dye

Abstract

The adsorption of the fluorescent dye, Oxazine 1 (OX) was studied on graphene oxide (GO), in the range of low (⩽10−7 M) OX concentrations, exploiting the high sensitivity of fluorescence spectroscopy. Dry GO was characterized using XRD, TEM, XPS, Raman and IR spectroscopy, while its chemistry in aqueous suspension was examined with potentiometric titration. Adsorption isotherms were measured at pH 6.4 and 10.2 at temperatures 20 and 35 °C. All the isotherms were well described by the Langmuir equation. The saturation capacities nm were higher at pH 10.2 than at 6.4 at the respective temperature, in accordance with the presence of additional dissociated acidic groups on the GO surface. The nm values corresponded to a few percent occupancy of the dissociated acidic groups of GO. The equilibrium constant KL for OX was substantially higher than the values reported for the adsorption of other cationic dyes from more concentrated solutions, indicating that our results refer to binding to the most active sites. The large negative adsorption enthalpy and the positive adsorption entropy are also indicative of strong binding interactions. In this case, probably the Coulomb interactions between the opposite charges on the adsorbent and adsorbate are dominant in the binding interactions.