Extending the reach of immunoassays to optically dense specimens by using two-photon excited fluorescence polarization.

Fluorescence anisotropy/polarization measurements represent a powerful tool for quantifying biomolecule/ligand complexation. These types of measurements are also at the heart of a wide variety of commercial homogeneous fluoroimmunoassays. In this note, we demonstrate the power of two-photon excited fluorescence anisotropy (2-PEFA) measurements as a tool for quantifying hapten/antibody association in the presence of a strongly absorbing, nonfluorescent dye. The results of these experiments show that 2-PEFA measurements are intrinsically more sensitive when compared to traditional one-photon excited fluorescence anisotropy (1-PEFA) strategies and 2-PEFA-based measurements allow one to perform accurate hapten/antibody binding measurements in strongly absorbing samples directly under conditions where 1-PEFA measurements fail completely. Overall, the 2-PEFA approach offers significant advantages when compared to traditional 1-PEFA methods especially in strongly absorbing samples. 2-PEFA also opens the door to perform more rapid and reliable polarization/anisotropy-based measurements with minimal sample preparation.