Fluorescence quenching of α-tocopherol by graphene dispersed in aqueous surfactant solutions

The fluorescence of α-tocopherol in different solutions of zwitterionic (lauryl sulfobetaine, LSB), non-ionic (polyoxyethylene-23-lauryl ether, Brij L23) and cationic (hexadecyltrimethylammonium bromide, CTAB) surfactants, as well as in graphene (G) dispersions in these surfactants at different weight ratios has been comparatively investigated. A quenching phenomenon of the vitamin fluorescence in the micellar dispersions of G has been found, the effect being more pronounced for dispersions in Brij L23. This surfactant causes the highest degree of G exfoliation and is the most effective in stabilizing G dispersions. The fluorescence intensity ratio, calculated in the absence and in the presence of G, increases linearly with G concentration up to 30 mg L⁻¹ for dispersions in 5 mM Brij L23 and in 5 mM CTAB with and without NaCl. A more prominent fluorescence quenching effect is found at low surfactant concentrations, ascribed to a stronger interaction between α-tocopherol and G when the nanomaterial is less covered by the surfactant. Time-resolved fluorescence measurements indicate a static quenching mechanism between α-tocopherol and G. The fluorescence intensity and α-tocopherol concentration in 5 mM Brij L23 aqueous solutions and in G (2.0 wt%) dispersions in this surfactant follow a linear relationship, as well as the differences of the intensities measured in each case.