Expanding the Scope of Reporting Nanoparticles: Sensing of Lipid Phase Transitions and Nanoviscosities in Lipid Membranes.

ABSTRACT

Biological membrane fluidity and thus the local viscosity in lipid membranes are of vital importance for many life processes and implicated in various diseases. Here, we introduce a novel viscosity sensor design for lipid membranes based on a reporting nanoparticle, a sulfated dendritic polyglycerol (dPGS), conjugated to a fluorescent molecular rotor, indocarbocyanine (ICC). We show that dPGS-ICC provides high affinity to lipid bilayers, enabling viscosity sensing in the lipid tail region. The systematic characterization of viscosity- and temperature-dependent photoisomerization properties of ICC and dPGS-ICC allowed us to determine membrane viscosities in different model systems and in living cells using fluorescence lifetime imaging (FLIM). dPGS-ICC distinguishes between ordered lipids and the onset of membrane defects in small unilamellar single lipid vesicles and is highly sensitive in the fluid phase to small changes in viscosity introduced by cholesterol. In microscopy-based viscosity measurements of large multilamellar vesicles, we observed an order of magnitude more viscous environments by dPGS-ICC, lending support to the hypothesis of heterogeneous nanoviscosity environments even in single lipid bilayers. The existence of such complex viscosity structures could explain the large variation in the apparent membrane viscosity values found in the literature, depending on technique and probe, both for model membranes and live cells. In HeLa cells, a tumor-derived cell line, our nanoparticle-based viscosity sensor detects a membrane viscosity of ∼190 cP and is able to discriminate between cell membrane and intracellular vesicle localization. Thus, our results show the versatility of the dPGS-ICC nano-conjugate in physicochemical and biomedical applications by adding a new analytical functionality to its medical properties.