Effect of pH on fluorescence quenching of organic dyes by graphene oxide

Graphene oxide (GO) has been recently used in several fluorescence sensing systems as a fluorescence quencher. The interactions between GO sheet and fluorophore directly affect the fluorescence quenching efficiency of GO. To understand these interactions, methylene blue (MB), rhodamine B (RhB) and sulforhodamine B (SRhB) were used as model fluorophores at various pH values in aqueous buffer solutions. The synthesized GO was characterized by UV-vis spectroscopy, infrared spectroscopy, thermogravimetric analysis, zeta potential measurement and particle size measurement. The interactions between GO and dyes were studied by fluorescence spectroscopy. The fluorescence quenching efficiency of GO to each dye is differently affected by the pH attributed to electrostatic interaction, pi-pi stacking and hydrogen bonding. While electrostatic interaction is the most important interaction for GO-MB and GO-RhB, the pi-pi stacking also plays an important role for GO-SRhB system. The temperature dependence of the quenching efficiency suggests a static quenching mechanism and the decrease of the quenching efficiency with the reducing GO size is probably due to a lower number of sp(2) domains in the GO sheets.