Silica nanodisks as platforms for fluorescence lifetime-based sensing of pH

Core-shell conjugates of silica nanodisks and fluorescent dyes have been prepared. Rhodamine B, the reference, has been attached to the core, by surface functionalization of the pristine SNDs. Then, a layer of silica has been deposited on the composite nanodisks. Finally, the surface has been functionalized with fluorescein in one case and protoporphyrin IX in the other. These dyes exhibit pH-dependent fluorescence properties. The nanoconjugates are found to sense the pH of the medium, through systematic variation of the fluorescence intensity ratios of the reporter dye at the surface and the reference dye at the core. Moreover, the fluorescence lifetimes and corresponding amplitudes of the reporter dyes have been found to be reliable parameters for assessing the pH of the medium, even though the variation in lifetimes of fluorescein is rather small. In case of protoporphyrin, however, this variation is significantly large. Besides, the change in amplitudes is prominent in acidic as well as alkaline solutions. The temporal parameters can thus be used to ascertain the pH of the medium, when used in conjunction with each other.