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Direct proof of spontaneous translocation of lipid-covered hydrophobic nanoparticles through a phospholipid bilayer

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Science Advances  02 Nov 2016:
Vol. 2, no. 11, e1600261
DOI: 10.1126/sciadv.1600261
  • Fig. 1 Suggested translocation mechanism of hydrophobic NPs through a lipid bilayer.

    Dodecanethiol-capped AuNPs (6 nm) get wrapped by a lipid layer. The lipid-covered AuNPs destabilize the bilayer by generating pores and pass through the bilayer by taking away nonfluorescent and fluorescent lipids. The lipid exchange mechanism is demonstrated experimentally by starting with NPs having a nonfluorescent lipid coating, which then pick up fluorescent lipids from the bilayer upon translocation.

  • Fig. 2 Interaction of hydrophobic NPs with lipid bilayers obtained from SCMF theory.

    (A) Density profiles of lipid heads and tails around embedded NPs. The NP diameter varies between 2 nm ≤ d ≤ 10 nm, and the interaction parameter varies between −5 kT ≥ ε ≥ −7 kT. (B) Free energy difference ΔF as a function of the NP diameter for different interaction parameters ε. (C) Free energy difference ΔF as a function of the distance between the bilayer center and the core of an NP, ε = −7 kT.

  • Fig. 3 Interaction regimes of trapping and translocation for hydrophobic NPs with ε = −7.0 kT.

    The interaction regimes are given by three solutions of SCMF equations for 2 nm (A) and 4 nm (B) NPs showing trapping of NPs in the bilayer and for 6-nm NPs (C) showing translocation of NPs. Insertion (purple): Lipid-coated NP touches the upper leaflet without structural rearrangement. Embedding (yellow): The NP fuses with the upper leaflet and exchanges its lipid coating with the bilayer. Escape (blue): The NP is wrapped by lipids, forms a pore in the bilayer, and is thus free to leave.

  • Fig. 4 Microfluidic setup.

    (A) Two aqueous fingers surrounded by squalene-lipid solution form a bilayer at their contact area. NPs can be added to the aqueous phase of either of the aqueous fingers. (B) Visualization of the most probable conformations of lipids around a pore formed by large hydrophobic NPs, d = 6.0 nm and ε = −7 kT, and modeled by SCMF theory. (C) Optical fluorescence microscopy time series demonstrating a single NP translocating through a lipid bilayer. AuNPs were added to the aqueous finger at the right. The NP leaving the bilayer (bright spot) to the initially particle-free side of the bilayer is indicated by a dashed circle. Another NP leaving the membrane to the side of the bilayer, which initially contains NPs, is visible from t = 30 ms.

  • Fig. 5 Experimental interaction of NPs with the bilayer.

    Measurements were performed for lipid-coated AuNPs with diameters of 2 nm (pink), 4 nm (orange), and 6 nm (blue) at a concentration of c ≈ 0.1 μg/ml. (A) Capacitance measurements as a function of time of a pure DMPC bilayer (green) and of a DMPC bilayer in the presence of NPs dispersed in the aqueous phase. (B to D) Size distribution of NPs that crossed the bilayer [2 nm (B), 4 nm (C), and 6 nm (D)], as analyzed from their Brownian motion. For the sake of clarity, the histograms are plotted as a function of the particle core diameter without the 1-dodecanethiol and DMPC coating.

  • Fig. 6 Kinetic translocation pathway.

    (A) Conductance measurement during a single translocation event from a 6-nm AuNP recorded at low particle concentration of c ≈ 0.01 ng/ml. The insets represent the different stages of the translocation pathway, as found by numerical simulation. (B) Translocation times measured from several individual translocation events.

Supplementary Materials

  • Supplementary material for this article is available at http://advances.sciencemag.org/cgi/content/full/2/11/e1600261/DC1

    Proofs of lipid exchange after NP insertion

    fig. S1. Fluorescence micrograph showing that lipids from the NP coating go into the bilayer upon translocation.

    fig. S2. Fluorescence microscopy time series.

    fig. S3. The three water droplets, described in this section, are observed under epifluorescence microscopy.

    fig. S4. Hydrodynamic diameter measurements from dynamic light scattering technique.

    fig. S5. Schematic view of 6-nm dodecanethiol-capped AuNPs.

  • Supplementary Materials

    This PDF file includes:

    • fig. S1. Fluorescence micrograph showing that lipids from the NP coating go into the bilayer upon translocation.
    • fig. S2. Fluorescence microscopy time series.
    • fig. S3. The three water droplets, described in this section, are observed under epifluorescence microscopy.
    • fig. S4. Hydrodynamic diameter measurements from dynamic light scattering technique.
    • fig. S5.Schematic view of 6-nm dodecanethiol-capped AuNPs.

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