Intake of silica nanoparticles by giant lipid vesicles: influence of particle size and thermodynamic membrane state

Abstract

The uptake of nanoparticles into cells often involves their engulfment by the plasma membrane and a fission of the latter. Understanding the physical mechanisms underlying these uptake processes may be achieved by the investigation of simple model systems that can be compared to theoretical models. Here, we present experiments on a massive uptake of silica nanoparticles by giant unilamellar lipid vesicles (GUVs). We find that this uptake process depends on the size of the particles as well as on the thermodynamic state of the lipid membrane. Our findings are discussed in the light of several theoretical models and indicate that these models have to be extended in order to capture the interaction between nanomaterials and biological membranes correctly.

Keywords: cells; endocytosis; engulfment; fission; gel phase; giant unilamellar lipid vesicles (GUV); lipid membranes; liquid phase; nanoparticle; phosphocholines; uptake; vesicles; wrapping.

Figure 1

An endocytosis-like uptake of particles involves three major steps: adhesion (1), engulfment (2), and fission (3). During the last process a membrane defect is induced, which will heal over time.

Figure 2

Phase diagram describing the interaction of particles with a spherical vesicle with initially zero tension. The parameters are here defined as follows: R₀ denotes the initial vesicle radius; a corresponds to the particle radius r; ; ζ = g_ad/g_ten. In short, very small particles will not be wrapped at all due to the bending resistance (white). The wrapping of big particles is rather limited by membrane tension (light grey). Only for low vesicle tension and sufficiently big particles full wrapping is possible (dark grey). Reprinted with permission from [21]. Copyright 2002 American Chemical Society.

Figure 3

1,2-dioleoyl-sn-glycero-3-phosphocholine (DOPC) vesicle (green), 1 min (left) and 10 min (right) after the incubation with nanoparticles (r = 42 nm, magenta). Obviously, vesicle membrane is consumed while particles are internalized.

Figure 4

Expected van der Waals (solid line) and double layer (dashed line) binding energies as a function of the particle–membrane distance. The double layer interaction dominates the system for the relevant separation distances. Additionally the solution of the Hogg–Healy–Fuerstenau-equation for a cationic particle with ζ = +30 mV and a membrane with ζ = −30 mV at l = 0.77 nm (dotted line) is given.

Figure 5

Time series of two DOPC vesicles in close contact. Upon the uptake of particles, initially a sudden, strong adhesion is induced, followed by a successive return to spherical shape. At 15 s the maximum adhesion area is achieved.

Figure 6

Size dependence for fluid phase vesicles. Left: A DOPC vesicle (green) after incubation with 123 nm particles (magenta) for 15 min. There are no signs of particle uptake. Right: The vesicle from Figure 3 after 10 min incubation with 42 nm particles. The contrast between these two systems is obvious.

Figure 7

Gel phase vesicles (green) after incubation with particles (magenta). Left: 42 nm particles. The uptake behavior here is comparable to the fluid phase situation. Right: 123 nm particles. In contrast to fluid phase vesicles (see Figure 6), the particles adhere strongly to the membrane and some are internalized. However most particles remain bound to the membrane.

Figure 8

Determination of the threshold number of particles N_thr for an uptake without volume loss. The different curves represent different initial vesicle radii R. g_ad = −0.5 mJ/m² and g_ten = 200 mN/m were chosen as before.

Figure 9

Nanoparticles are added to the medium. After equilibration a vesicle is observed at constant focal height. The evolution of the vesicle size is monitored by means of the observed cross section area S(t).