Synthesis and use of an in-solution ratiometric fluorescent viscosity sensor

A procedure for the synthesis of a ratiometric viscosity fluorescent sensor is described in this protocol. The essential requirement for the design of this sensor is the attachment of a primary fluorophore that has both a viscosity-independent fluorescence emission (coumarin dye shown in blue) and an emission from a fluorophore that exhibits viscosity-dependent fluorescent quantum yield (p-amino cinnamonitrile dye shown in red). The use of sensor 1 in viscosity measurements involves solubilization in a liquid of interest and excitation of the primary fluorophore at lambda(ex) = 360 nm. The secondary fluorophore is simultaneously excited via resonance energy transfer. The ratio of the fluorescent emission of the secondary over the primary fluorophore provides a fast and precise measurement of the viscosity of the solvent. The synthesis of compound 1 using commercially available materials can be completed within 5 d.