Optical scattering and microscopic imaging of cellular exo- and endocytosis

We present a solution to Maxwell's equations for core-shell nanoparticle scattering near an isotropic substrate covered with an anisotropic thin film, based on an extension of the Bobbert-Vlieger solution for particle... [ view full abstract ]

We present a solution to Maxwell's equations for core-shell nanoparticle scattering near an isotropic substrate covered with an anisotropic thin film, based on an extension of the Bobbert-Vlieger solution for particle scattering near a substrate, delivering an exact solution in the near-field as well as far-field. We successfully apply the developed scattering model to the calculation of light scattering on an optical model representing a lipid vesicle near a lipid bilayer, whereby the lipids are characterized through a biaxial optical model (Figure 1). Hereby, we pave the path for understanding quantitatively how light scatters during a cellular exo- or endocytosis event during microscopic observation taking into account lipid induced anisotropy. Through the application of ellipsometric angles it is effectively demonstrated that realistically small optical anisotropy values significantly alter far-field optical scattering in respect to an equivalent optical model for cellular endocytosis consisting of isotropic components only (Figure 2 & 3). We hereby predict a significant impact of lipid-induced optical anisotropy on the experimental observation of exo- or endocytic microscopic imaging with e.g. Differential Interference Contrast (DIC) microscopy. Furthermore, we integrate this extended Bobbert-Vlieger scattering solution into a rigorous model of Differential Interference Contrast (DIC) image formation which allows for characterizing DIC, through simulation, as a tool for imaging of exo- or endocytosis events. We compare theoretical predictions with experimental high numerical aperture DIC imaging of dielectric oxide nanoparticles with organic shell.

Figure 1: optical model for light scattering off a liposome above a lipid bilayer.

Figure 2: Ellipsometry angles Ψ (left) and Δ (right) for wavelength λ (top – angle-of-incidence 70°) and angle-of-incidence θ (bottom – wavelength 488 nm) resolved ellipsometry for liposomes with radii of 50 nm, 150 nm and 250 nm on top of a lipid bilayer (δ=R). We consider light scattering off a structure as depicted in Figure 1 (n_∥,sh = n_∥,lb = 1.45; n_⊥,sh= n_⊥,lb=1.46; n_i=n_s=1.33; d_1,2= 5 nm).

Figure 3: Ellipsometry angles Ψ and Δ for a liposome above a lipid bilayer for varying distance between the liposome and lipid bilayer (δ-R). Model parameters are identical to model considered in Figure 2.