A device and a method for measuring viscosity that includes attaching molecular rotors to a solid surface, exposing the solid surface to a fluid having a viscosity to be measured, and taking optical measurements to determine viscosity. The solid surface is preferably quartz, polystyrene or silicate glass, such as a fiber optic probe or a glass cuvette. The molecular rotors are of the type that includes an electron-donor group and electron-acceptor group that are linked by a single bond so that the groups may rotate with respect to one another, and that exhibit a fluorescence emission when rotation is hindered.