BALM: Watching the Formation of Tethered Bilayer Lipid Membranes with Submicron Lateral Resolution

Achieving High Visibility of Monolayer MoS2 Using Backside-Illuminated Thin Films

Interference reflection microscopy (IRM) and backside absorbing layer microscopy (BALM) have emerged as powerful optical microscopy methods for the study of nanomaterials and biological samples. These techniques consist in using an inverted optical microscope in reflection mode to observe objects deposited either on glass (IRM) or on a nanometric absorbing metallic film (BALM). The thickness of the BALM absorbing layer and of optional additional transparent layers, as well as the choice of incident wavelength and top medium, act as powerful levers for maximizing the resultant contrast of a given sample. However, the use of BALM to study samples with high absorption coefficient has been limited so far in the literature. Furthermore, the complex refractive index (ñ = n + iκ) of layers in a specific BALM optical stack have so far not been measured, and thus experimentally informed simulations are nonexistent. In this work, we use variable angle spectroscopic ellipsometry (VASE) to measure ñ(λ) of each layer in BALM (and IRM) stacks consisting of different combinations of glass, Cr, Au, and AlOx, with the two-dimensional (2D) transition metal dichalcogenide (TMD) MoS2 as the sample under study. Using the measured values, we simulate contrast spectra and compare them against data. We manipulate the film thicknesses and top media to engineer favorable contrast conditions both in the blue and red regimes of the visible spectrum, with peak contrasts of ≈80%. Finally, we demonstrate how the 2-layer BALM stack can double as both an optically sensitive system and at the same time a metal-oxide-semiconductor structure allowing for electrostatic gating. We use this structure to do in situ local charge density imaging of a 2D MoS2 capacitor, which showcases the versatility of these types of BALM stacks.

Curved Membrane Mimics for Quantitative Probing of Protein-Membrane Interactions by Surface Plasmon Resonance

A majority of biomimetic membranes used for current biophysical studies rely on planar structures such as supported lipid bilayer (SLB) and self-assembled monolayers (SAMs). While they have facilitated key information collection, the lack of curvature makes these models less effective for the investigation of curvature-dependent protein binding. Here, we report the development and characterization of curved membrane mimics on a solid substrate with tunable curvature and ease in incorporation of cellular membrane components for the study of protein-membrane interactions. The curved membranes were generated with an underlayer lipid membrane composed of DGS-Ni-NTA and POPC lipids on the substrate, followed by the attachment of histidine-tagged cholera toxin (his-CT) as a capture layer. Lipid vesicles containing different compositions of gangliosides, including GA1, GM1, GT1b, and GQ1b, were anchored to the capture layer, providing fixation of the curved membranes with intact structures. Characterization of the curved membrane was accomplished with surface plasmon resonance (SPR), fluorescence recovery after photobleaching (FRAP), and nano-tracking analysis (NTA). Further optimization of the interface was achieved through principal component analysis (PCA) to understand the effect of ganglioside type, percentage, and vesicle dimensions on their interactions with proteins. In addition, Monte Carlo simulations were employed to predict the distribution of the gangliosides and interaction patterns with single point and multipoint binding models. This work provides a reliable approach to generate robust, component-tuning, and curved membranes for investigating protein interactions more pertinently than what a traditional planar membrane offers.

Mechanisms of Influenza Virus HA2 Peptide Interaction with Liposomes Studied by Dual-Wavelength MP-SPR

A phospholipid-based liposome layer was used as an effective biomimetic membrane model to study the binding of the pH-dependent fusogenic peptide (E4-GGYC) from the influenza virus hemagglutinin HA2 subunit. To this end, a multiparameter surface plasmon resonance approach (MP-SPR) was used for monitoring peptide-liposome interactions at two pH values (4.5 and 8) by means of recording sensorgrams in real time without the need for labeling. Biotinylated liposomes were first immobilized as a monolayer onto the surface of an SPR gold chip coated with a streptavidin layer. Multiple sets of sensorgrams with different HA2 peptide concentrations were generated at both pHs. Dual-wavelength Fresnel layer modeling was applied to calculate the thickness (d) and the refractive index (n) of the liposome layer to monitor the change in its optical parameters upon interaction with the peptide. At acidic pH, the peptide, in its α helix form, entered the lipid bilayer of liposomes, inducing vesicle swelling and increasing membrane robustness. Conversely, a contraction of liposomes was observed at pH 8, associated with noninsertion of the peptide in the double layer of phospholipids. The equilibrium dissociation constant KD = 4.7 × 10-7 M of the peptide/liposome interaction at pH 4.5 was determined by fitting the "OneToOne" model to the experimental sensorgrams using Trace Drawer software. Our experimental approach showed that the HA2 peptide at a concentration up to 100 μM produced no disruption of liposomes at pH 4.5.