A molecular rotor as viscosity sensor in aqueous colloid solutions

Background: Molecular rotors exhibit viscosity-dependent quantum yield, allowing non-mechanical determination of fluid viscosity. We analyzed fluorescence in the presence of viscosity-modulating macromolecules several orders of magnitude larger than the rotor molecule. Method of approach: Fluorescence of aqueous starch solutions with a molecular rotor in solution was related to viscosity obtained in a cone-and-plate viscometer. Results: In dextran solutions, emission intensity was found to follow a power-law relationship with viscosity. Fluorescence in hydroxyethylstarch solutions showed biexponential behavior with different exponents at viscosities above and below 1.5 mPa s. Quantum yield was generally higher in hydroxyethylstarch than in dextran solutions. The power-law relationship was used to backcalculate viscosity from intensity with an average precision of 2.2% (range of -5.5% to 5.1%). Conclusions: This study indicates that hydrophilic molecular rotors are suitable as colloid solution viscosity probes after colloid-dependent calibration.