A cell free biomembrane platform for multimodal study of influenza virus hemagglutinin and for evaluation of entry-inhibitors against hemagglutinin

Seasonal periodic pandemics and epidemics caused by Influenza A viruses (IAVs) are associated with high morbidity and mortality worldwide. They are frequent and unpredictable in severity so there is a need for biophysical platforms that can be used to provide both mechanistic insights into influenza virulence and its potential treatment by anti-IAV agents. Host membrane viral association through the glycoprotein hemagglutinin (HA) of IAVs is one of the primary steps in infection. HA is thus a potential target for drug discovery and development against influenza. Deconvolution of the multivalent interactions of HA at the interfaces of the host cell membrane can help unravel therapeutic targets. In this contribution, we reported the effect of a multivalent HA glycoprotein association on various glycosphingolipid receptors (GD1a, GM3, GM1) doped asymmetrically into an artificial host membrane spanned across an aqueous filled microcavity array. The extent of HA association and its impact on membrane resistance, capacitance, and diffusivity was measured using highly sensitive electrochemical impedance spectroscopy (EIS) and fluorescence lifetime correlation spectroscopy (FLCS). Furthermore, we investigated the inhibition of the influenza HA glycoprotein association with the host mimetic surface by natural and synthetic sialic acid-based inhibitors (sialic acid, Siaα2,3-GalOMe, FB127, 3-sialyl lactose) using electrochemical impedance spectroscopy and observe that while all inhibit, they do not prevent host binding. Overall, the work demonstrates the platform provides a label-free screening platform for the biophysical evaluation of new inhibitors in the development of potential therapeutics for IAV infection prevention and treatment.

Nano-substructured plasmonic pore arrays: a robust, low cost route to reproducible hierarchical structures extended across macroscopic dimensions

Plasmonic nanostructures are important across diverse applications from sensing to renewable energy. Periodic porous array structures are particularly attractive because such topography offers a means to encapsulate or capture solution phase species and combines both propagating and localised plasmonic modes offering versatile addressability. However, in analytical spectroscopic applications, periodic pore arrays have typically reported weaker plasmonic signal enhancement compared to particulate structures. This may be addressed by introducing additional nano-structuring into the array to promote plasmonic coupling that promotes electric field-enhancement, whilst retaining pore structure. Introducing nanoparticle structures into the pores is a useful means to promote such coupling. However, current approaches rely on either expensive top-down methods or on bottom-up methods that yield random particle placement and distribution. This report describes a low cost, top-down technique for preparation of nano-sub-structured plasmonic pore arrays in a highly reproducible manner that can be applied to build arrays extending over macroscopic areas of mm² to cm². The method exploits oxygen plasma etching, under controlled conditions, of the cavity encapsulated templating polystyrene (PS) spheres used to create the periodic array. Subsequent metal deposition leads to reproducible nano-structuring within the wells of the pore array, coined in-cavity nanoparticles (icNPs). This approach was demonstrated across periodic arrays with pore/sphere diameters ranging from 500 nm to 3 μm and reliably improved the plasmonic properties of the substrate across all array dimensions compared to analogous periodic arrays without the nano-structuring. The enhancement factors achieved for metal enhanced emission and surface enhanced Raman spectroscopy depended on the substrate dimensions, with the best performance achieved for nanostructured 2 μm diameter pore arrays, where a more than 10⁴ improvement over Surface Enhanced Raman Spectroscopy (SERS) and 200-fold improvement over Metal Enhanced Fluorescence (MEF) were observed for these substrates compared with analogous unmodified pore arrays. The experiments were supported by Finite-Difference Time-Domain (FDTD) calculations used to simulate the electric field distribution as a function of pore nano-structuring.

Fibrinogen Motif Discriminates Platelet and Cell Capture in Peptide-Modified Gold Micropore Arrays

Human blood platelets and SK-N-AS neuroblastoma cancer-cell capture at spontaneously adsorbed monolayers of fibrinogen-binding motifs, GRGDS (generic integrin adhesion), HHLGGAKQAGDV (exclusive to platelet integrin α_IIbβ₃), or octanethiol (adhesion inhibitor) at planar gold and ordered 1.6 μm diameter spherical cap gold cavity arrays were compared. In all cases, arginine/glycine/aspartic acid (RGD) promoted capture, whereas alkanethiol monolayers inhibited adhesion. Conversely only platelets adhered to alanine/glycine/aspartic acid (AGD)-modified surfaces, indicating that the AGD motif is recognized preferentially by the platelet-specific integrin, α_IIbβ₃. Microstructuring of the surface effectively eliminated nonspecific platelet/cell adsorption and dramatically enhanced capture compared to RGD/AGD-modified planar surfaces. In all cases, adhesion was reversible. Platelets and cells underwent morphological change on capture, the extent of which depended on the topography of the underlying substrate. This work demonstrates that both the nature of the modified interface and its underlying topography influence the capture of cancer cells and platelets. These insights may be useful in developing cell-based cancer diagnostics as well as in identifying strategies for the disruption of platelet cloaks around circulating tumor cells.

Electrochemically Triggered Release of Reagent to the Proximal Leaflet of a Microcavity Supported Lipid Bilayer

A novel and versatile approach to electrichemically triggering the release of a reagent, β-cyclodextrin (β-CD), selectively to the proximal leaflet of a supported lipid bilayer is described. Selective delivery is achieved by creating a spanning lipid bilayer across a microcavity array and exploiting the irreversible redox disassembly of the host-guest complex formed between thiolated ferrocene (Fc) and β-cyclodextrin (β-CD) in the presence of chloride. Self-assembled monolayers of the ferrocene-alkanethiols were formed regioselectively on the interior surface of highly ordered 2.8 μm cavities while the exterior top surface of the array was blocked with a monolayer of mercaptoethanol. The Fc monolayers were complexed with β-CD or β-CD-conjugated to streptavidin (β-CD-SA). Phospholipid bilayers were then assembled across the array via combined Langmuir-Blodgett/vesicle fusion leading to a spanning bilayer suspended across the aqueous filled microcavities. Upon application of a positive potential, ferrocene is oxidized to ferrocinium cation, disrupting the inclusion complex and leading to the release of the β-CD into the microcavity solution where it diffuses to the lower leaflet of the suspended bilayer. Disassembly of the supramolecular complex within the cavities and binding of the β-CD-SA to a biotinylated bilayer was followed by voltammetry and impedance spectroscopy where it caused a large increase in membrane resistance. For unmodified β-CD, the extraction of cholesterol from a cholesterol containing bilayer was evident in a decrease in the bilayer resistance. For the first time, this direct approach to targeted delivery of a reagent to the proximal layer of a lipid bilayer offers the potential to build models of bidirectional signaling (inside-out vs outside-in) in cell membrane model systems.

Peptide-Mediated Platelet Capture at Gold Micropore Arrays

Ordered spherical cap gold cavity arrays with 5.4, 1.6, and 0.98 μm diameter apertures were explored as capture surfaces for human blood platelets to investigate the impact of surface geometry and chemical modification on platelet capture efficiency and their potential as platforms for surface enhanced Raman spectroscopy of single platelets. The substrates were chemically modified with single-constituent self-assembled monolayers (SAM) or mixed SAMs comprised of thiol-functionalized arginine-glycine-aspartic acid (RGD, a platelet integrin target) with or without 1-octanethiol (adhesion inhibitor). As expected, platelet adhesion was promoted and inhibited at RGD and alkanethiol modified surfaces, respectively. Platelet adhesion was reversible, and binding efficiency at the peptide modified substrates correlated inversely with pore diameter. Captured platelets underwent morphological change on capture, the extent of which depended on the topology of the underlying substrate. Regioselective capture of the platelets enabled study for the first time of the surface enhanced Raman spectroscopy of single blood platelets, yielding high quality Raman spectroscopy of individual platelets at 1.6 μm diameter pore arrays. Given the medical importance of blood platelets across a range of diseases from cancer to psychiatric illness, such approaches to platelet capture may provide a useful route to Raman spectroscopy for platelet related diagnostics.

Colloidally templated two-dimensional conducting polymer arrays and SAMs: binary composition patterning and chemistry

A facile approach and strategy toward binary-composition, two-dimensional (2D) patterned surfaces of conducting polymer periodic arrays, together with thiol self-assembled monolayers (SAMs) is described. The method involved a Langmuir-Blodgett (LB)-like deposition of latex microsphere particles, electropolymerization via cyclic voltammetric (CV) techniques, and self-assembly of an amphiphile. The LB-like technique enabled the monolayer deposition of different sizes of polystyrene (PS) particles in hexagonal packing arrangement on planar substrates. Combining the LB-like method with CV electropolymerization is advantageous because it provides deposition control of a polymer interconnected network, controlled composition ratio of polymer and SAMs, and control of 2D size and spacing of the spherical void pattern. Electrochemical-quartz crystal microbalance (EC-QCM) in situ monitoring of the film deposition quantified a constant and linear growth rate, with varying viscoelastic behavior of the conducting polymer adsorption on planar and PS-templated substrates. The dual-patterned surface provided a good imaging contrast as observed by atomic force microscopy (AFM). Complementary analyses such as X-ray photoelectron spectroscopy (XPS), attenuated total internal reflection infrared (ATR IR) spectroscopy, ultraviolet-visible (UV-vis) spectroscopy, and static contact angle measurements were used to characterize the formation of the patterned surface. The versatility of the method enables the potential for making various types of quantitative binary compositions and patterned surfaces using different combinations of conducting polymer or functional SAMs, which can be extended in the future to polymer brushes and layer-by-layer assembly of various materials.