Spectrally Resolved Single-Molecule Orientation Imaging Reveals a Direct Correspondence between the Polarity and Microviscosity Experienced by Nile Red in Supported Lipid Bilayer Membranes

Molecular-level interactions among lipids, cholesterol, and water dictate the nanoscale membrane organization of lipid bilayers into liquid-ordered (Lo) and liquid-disordered (Ld) phases, characterized by different polarities and orders. Generally, solvatochromic dyes easily discriminate polarity difference between Lo and Ld phases, whereas molecular flippers and rotors show distinct photophysics depending on the membrane order. Despite progress in single-molecule spectral imaging and single-molecule orientation mapping, direct experimental proof linking polarity with microviscosity sensed by the same probe eludes us. Here, we demonstrate spectrally resolved single-molecule orientation localization microscopy to connect nanoscopic localization of a probe on a bilayer membrane with its emission spectra, three-dimensional dipole orientation, and rotational constraint offered by the local microenvironment and highlight the excellent correspondence between the polarity and order experienced by the same probe. This technique has the potential to address nanoscale heterogeneity and dynamics, especially in biology and material sciences.