Hybrid organic–inorganic compounds are receiving increasing attention for photoelectrochemical (PEC) devices due to their high electron transport efficiency and facile synthesis. Biosynthesis is a potentially low-cost and eco-friendly method to precipitate transition-metal-based semiconductor nanoparticles (NPs) in an organic matrix. In this work, we examine the structure and composition of bacterially precipitated (BAC) cadmium sulfide (CdS) NPs using electron microscopy, and we determine their PEC properties and the energy band structure by electrochemical measurements. In addition, by taking advantage of the organic matrix, which is residual from the biosynthesis process, we fabricate a prototype photocharged capacitor electrode by incorporating the bacterially precipitated CdS with a reduced graphene oxide (RGO) sheet. Our results show that the hydrophilic groups associated with the organic matrix make BAC CdS NPs a potentially useful component of PEC devices with applications for energy conversion and storage.

AbstractWe present terminal deoxynucleotidyl transferase-catalyzed enzymatic polymerization (TcEP) for the template-free synthesis of high-molecular-weight, single-stranded DNA (ssDNA) and demonstrate that it proceeds by a living chain-growth polycondensation mechanism. We show that the molecular weight of the reaction products is nearly monodisperse, and can be manipulated by the feed ratio of nucleotide (monomer) to oligonucleotide (initiator), as typically observed for living polymerization reactions. Understanding the synthesis mechanism and the reaction kinetics enables the rational, template-free synthesis of ssDNA that can be used for a range of biomedical and nanotechnology applications.

AbstractWe present terminal deoxynucleotidyl transferase-catalyzed enzymatic polymerization (TcEP) for the template-free synthesis of high-molecular-weight, single-stranded DNA (ssDNA) and demonstrate that it proceeds by a living chain-growth polycondensation mechanism. We show that the molecular weight of the reaction products is nearly monodisperse, and can be manipulated by the feed ratio of nucleotide (monomer) to oligonucleotide (initiator), as typically observed for living polymerization reactions. Understanding the synthesis mechanism and the reaction kinetics enables the rational, template-free synthesis of ssDNA that can be used for a range of biomedical and nanotechnology applications.

We present a new method to fabricate semiconducting, transition metal nanoparticles (NPs) with tunable bandgap energies using engineered Escherichia coli. These bacteria overexpress the Treponema denticola cysteine desulfhydrase gene to facilitate precipitation of cadmium sulphide (CdS) NPs. Analysis with transmission electron microscopy, X-ray diffraction, and X-ray photoelectron spectroscopy reveal that the bacterially precipitated NPs are agglomerates of mostly quantum dots, with diameters that can range from 3 to 15 nm, embedded in a carbon-rich matrix. Additionally, conditions for bacterial CdS precipitation can be tuned to produce NPs with bandgap energies that range from quantum-confined to bulk CdS. Furthermore, inducing precipitation at different stages of bacterial growth allows for control over whether the precipitation occurs intra- or extracellularly. This control can be critically important in utilizing bacterial precipitation for the environmentally-friendly fabrication of functional, electronic and catalytic materials. Notably, the measured photoelectrochemical current generated by these NPs is comparable to values reported in the literature and higher than that of synthesized chemical bath deposited CdS NPs. This suggests that bacterially precipitated CdS NPs have potential for applications ranging from photovoltaics to photocatalysis in hydrogen evolution.

Interfaces and subsurface layers are critical for the performance of devices made of 2D materials and heterostructures. Facile, nondestructive, and quantitative ways to characterize the structure of atomically thin, layered materials are thus essential to ensure control of the resultant properties. Here, we show that contact-resonance atomic force microscopy—which is exquisitely sensitive to stiffness changes that arise from even a single atomic layer of a van der Waals-adhered material—is a powerful experimental tool to address this challenge. A combined density functional theory and continuum modeling approach is introduced that yields sub-surface-sensitive, nanomechanical fingerprints associated with specific, well-defined structure models of individual surface domains. Where such models are known, this information can be correlated with experimentally obtained contact-resonance frequency maps to reveal the (sub)surface structure of different domains on the sample.Keywords: 2D materials and heterostructures; ab initio calculations; contact-resonance atomic force microscopy; elastic properties; surfaces and interfaces

•Synthesis of a new, copper-free Clickable azobenzene.•The copper-free Click reaction between hydrophilic ssDNA and hydrophobic azobenzene-DBCO is successful with high efficiency.•Photoresponsive DNA micelles can undergo controlled self-assembly.We demonstrate the reversible micellar aggregation of a DNA-azobenzene conjugate in aqueous conditions, in which the photoisomerization of the initially apolar trans-azobenzene moiety to the polar cis isomer causes disassembly of the aggregates. The molecular basis for this phenomena is a change in the hydrophobic/hydrophilic balance of the conjugate as the more polar cis azobenzene isomer is formed upon exposure to 365 nm irradiation. The conjugates were prepared by copper-free Click chemistry between an azide-modified, 53-base ssDNA and a cyclooctyne derivative of azobenzene. The photocontrolled aggregation of the conjugate was studied by dynamic light scattering and atomic force microscopy. The reversible micellar aggregation for a DNA-azobenzene conjugate has not been previously reported and holds promise for photocontrolled drug delivery applications.

This review focuses on surface-grafted DNA, and its use as a molecular building block that exploits its unique properties as a directional (poly)anion that exhibits molecular recognition. The selected examples highlight innovative applications of DNA at surfaces and interfaces ranging from molecular diagnostics and sequencing to biosensing.

Ferroelectric surfaces can have very high surface charge densities that can be harnessed for manipulation of charged colloidal particles and soft matter in aqueous environments. Here, we report on the electrical double layer (EDL) formed by polarized ultrasmooth lead zirconium titanate (US-PZT) thin films in dilute electrolyte solutions. Using colloidal probe force microscopy (CPFM) measurements, we show that the ion distribution within the double layer can be changed by reversing the ferroelectric polarization state of US-PZT. The interaction force in dilute 1:1 electrolyte solution between the negatively charged probe and a positive surface charge (upward polarized) US-PZT thin film is attractive, while the interaction force is repulsive for a negative surface charge (downward polarized) film. We modeled these interactions with a constant-potential EDL model between dissimilar surfaces with the inclusion of a Stern layer. We report the surface potentials at the inner and outer-Helmholtz planes both for polarization states and for a range of ionic strength solutions. Effects of free-charge carriers, limitations of the analytical model, and effects of surface roughness are discussed.Keywords: colloidal probe force microscopy; electrolyte solution; interfacial forces; lead zirconium titante; PZT; surface charge;

•Formation of complex, biomimetic SLBs using simple vesicle fusion techniques.•Optimization of experimental conditions forms complex SLBs via vesicle fusion.•α-Helical peptides act as catalyst to induce vesicle rupture and SLB formation.•Bilayer-edge induced vesicle fusion uses micro-fluidics to form native-derived SLB.Vesicle fusion has long provided an easy and reliable method to form supported lipid bilayers (SLBs) from simple, zwitterionic vesicles on siliceous substrates. However, for complex compositions, such as vesicles with high cholesterol content and multiple lipid types, the energy barrier for the vesicle-to-bilayer transition is increased or the required vesicle–vesicle and vesicle–substrate interactions are insufficient for vesicle fusion. Thus, for vesicle compositions that more accurately mimic native membranes, vesicle fusion often fails to form SLBs. In this paper, we review three approaches to overcome these barriers to form complex, biomimetic SLBs via vesicle fusion: (i) optimization of experimental conditions (e.g., temperature, buffer ionic strength, osmotic stress, cation valency, and buffer pH), (ii) α-helical (AH) peptide-induced vesicle fusion, and (iii) bilayer edge-induced vesicle fusion. AH peptide-induced vesicle fusion can form complex SLBs on multiple substrate types without the use of additional equipment. Bilayer edge-induced vesicle fusion uses microfluidics to form SLBs from vesicles with complex composition, including vesicles derived from native cell membranes. Collectively, this review introduces vesicle fusion techniques that can be generalized for many biomimetic vesicle compositions and many substrate types, and thus will aid efforts to reliably create complex SLB platforms on a range of substrates.Figure optionsDownload full-size imageDownload high-quality image (55 K)Download as PowerPoint slide

In this study, we present a technique to create complex, high cholesterol-containing supported lipid bilayers (SLBs) using α-helical (AH) peptide-induced vesicle fusion. Vesicles consisting of POPC:POPE:POPS:SM:Chol (9.35:19.25:8.25:18.15:45.00) were used to form a SLB that models the native composition of the human immunodeficiency virus-1 (HIV-1) lipid envelope. In the absence of AH peptides, these biomimetic vesicles fail to form a complete SLB. We verified and characterized AH peptide-induced vesicle fusion by a quartz crystal microbalance with dissipation monitoring, neutron reflectivity, and atomic force microscopy. Successful SLB formation entailed a characteristic frequency shift of −35.4 ± 0.8 Hz and a change in dissipation energy of 1.91 ± 0.23 × 10−6. Neutron reflectivity measurements determined the SLB thickness to be 49.9 +1.9−1.5 Å, and showed the SLB to be 100 +0.0−0.1% complete and void of residual AH peptide after washing. Atomic force microscopy imaging confirmed complete SLB formation and revealed three distinct domains with no visible defects. This vesicle fusion technique gives researchers access to a complex SLB composition with high cholesterol content and thus the ability to better recapitulate the native HIV-1 lipid membrane.

Self-assembled microsphere monolayers (SMMs) hold significant promise for micro- and nanopatterning. Here we exploit, for the first time, SMMs as stamps for microcontact printing (μCP) and demonstrate this to fabricate patterned initiator templates that can subsequently be amplified into polymer brushes by surface initiated atom transfer radical polymerization (SI-ATRP). SMM stamps avoid the need for expensive and sophisticated instrumentation in pattern generation, and provide a broad range of accessible surface chemistries and pitch size control.

As a fast developing soft lithographic technique, the development of micro-contact printing (μCP) has exceeded the original aim of replicating poly(dimethylsiloxane) (PDMS) stamp patterns. Here we exploited several extended μCP strategies with various printing conditions (over-force or swelling induced physical deformation, and UV-Ozone treated chemical surface modification to a PDMS stamp), combining with surface initiated atom transfer radical polymerization (SI-ATRP), to pattern complex poly(N-isopropylacrylamide) (PNIPAAM) brush microstructures. These series of μCP strategies avoid the need for expensive and sophisticated instrumentation in patterning complex polymer brush microstructures that do not exist on the original PDMS stamp.

Stimulus-responsive polymer brushes (SRPBs) exhibit a change in conformation and structure, often accompanied by a noticeable change in surface energy, due to an external stimulus such as a change in solvent composition, temperature, pH, ionic strength, light, or mechanical stress. SRPBs offer exciting and new possibilities to fabricate adaptive or responsive smart materials. This review summarizes selected, recent progress in SRPB applications in the field of surface wettability switching, mechanical actuation, and environmental sensing. Furthermore, we review selected papers from an emerging area in which SRPBs are used for nano- and microfabrication.

Glucose responsive polymer brushes were synthesized on gold substrates and microcantilever arrays. The response properties of these brushes were evaluated by exposing them to different glucose concentrations for a range of pH values. This work demonstrates the potential for polymer brush-functionalized micromechanical cantilevers as glucose detectors. Furthermore, the work demonstrates that stimulus-responsive polymer brushes on micromechanical cantilevers have a significantly larger bending response due to glucose binding compared with self-assembled monolayers.

Mucins have long been recognized as instrumental to biolubrication but the molecular details of their lubrication mechanisms have only been explored relatively recently. The glycoprotein PRG4, also known as lubricin, shares many features with mucins and appears to lubricate through similar mechanisms. A number of studies have contributed to a more in-depth understanding of mucin adsorption and layer formation on surfaces and the mechanisms by which these layers lubricate. Although mucinous glycoproteins differ in their aggregation properties, their adsorption behaviors on surfaces, and in their ability to reduce friction, they share important similarities favorable for lubrication. They are highly hydrated, they adsorb strongly to a broad range of surfaces, and the layers they form are both sterically and electrostatically repulsive, all attributes thought to contribute to boundary lubrication. They also hydrophilize hydrophobic surfaces, promoting the formation of aqueous fluid films that can lower friction at already relatively low sliding speeds. In this paper we briefly review current knowledge of mucin adsorption and lubrication, with a focus on recent advances.Research Highlights► Mucins and mucinous glycoproteins share important similarities that are favorable for lubrication. ► Mucins are highly hydrated, they adsorb strongly to a broad range of surfaces, and the layers they form are sterically repulsive, all attributes thought to contribute to boundary lubrication. ► Viscous boundary lubrication by a stratified mucin layer has been observed; a novel finding with potentially broad implications for aqueous boundary lubrication. ► Mucin coatings promote the formation of aqueous fluid films that can lower friction at already relatively low sliding speeds. ► More work is needed to investigate the role of hydration and the associated effects of pH and ionic strength on mucin lubrication.

Lubricin and hyaluronic acid (HA), molecular constituents of synovial fluid, have long been theorized to play a role in joint lubrication and wear protection. While lubricin has been shown to function as a boundary lubricant, conflicting evidence exists as to the boundary lubricating ability of hyaluronic acid. Here, we use colloidal force microscopy to explore the friction behavior of these two molecules on the microscale between chemically uniform hydrophilic (hydroxyl-terminated) and hydrophobic (methyl-terminated) surfaces in physiological buffer solution. Behaviors on both surfaces are physiologically relevant since the heterogeneous articular cartilage surface contains both hydrophilic and hydrophobic elements. Friction between hydrophobic surfaces was initially high (µ = 1.1, at 100 nN of applied normal load) and was significantly reduced by lubricin addition while friction between hydrophilic surfaces was initially low (µ = 0.1) and was slightly increased by lubricin addition. At lubricin concentrations above 200 µg/ml, friction behavior on the two surfaces was similar (µ = 0.2) indicating that nearly all interaction between the two surfaces was between adsorbed lubricin molecules rather than between the surfaces themselves. In contrast, addition of HA did not appreciably alter the frictional behavior between the model surfaces. No synergistic effect on friction behavior was seen in a physiological mixture of lubricin and HA. Lubricin can equally mediate the frictional response between both hydrophilic and hydrophobic surfaces, likely fully preventing direct surface-to-surface contact at sufficient concentrations, whereas HA provides considerably less boundary lubrication.

A polymer brush consisting of 70% poly(N-isopropylacrylamide) (PNIPAAM) and 30% polymethacrylic acid (PMAA) was synthesized from gold substrates with a “grafting from” AIBN-type free-radical initiator. Fractionation of two peptides, bradykinin and buccalin, was accomplished in less than 120 s by placing a 30 pM (pH ∼6.2) droplet onto the polymer brush substrate. The eluant containing the anionic buccalin is pipetted away for MALDI analysis while the cationic bradykinin adsorbed to the swollen anionic brush and was subsequently released by adding a droplet of formic acid to the substrate. This caused the brush to collapse and release the bradykinin, much like squeezing a sponge; these nanosponge substrates exhibited very high loading capacity (>2.0 mg/mL) compared to plasma-polymer-modified MALDI substrates. Ellipsometric measurements showed that complementary peptides adsorb rapidly while those of the same charge do not, and MALDI-MS analysis of the two fractions showed separation of both peptides. The adsorption of bradykinin was monitored over time, and 85% of the peptide had been adsorbed to the nanosponge in 1 min from a 0.5 mg/mL aqueous solution.

A significant scientific and engineering challenge of recent years has been the fabrication of patterned polymeric and biomacromolecular brush nanostructures on surfaces. These structures provide researchers with a rich platform on which to exploit and observe nanoscale phenomena. In this review we present an overview of the field and highlight, through selected examples, recent advances in the nanostructuring of polymer and biomacromolecular brushes. This includes a brief overview of polymer brush synthesis techniques and how these are integrated with nanolithographic and templating approaches. We discuss the characterization of polymeric nanostructures and its associated difficulties, and we provide some perspective of how we see the future direction of the field evolving.

Stimulus-responsive macromolecules have attracted significant interest due to their potential applications in molecular motors, drug delivery, sensors, and actuation devices. Poly(N-isopropylacrylamide) (pNIPAAM) alone or as a copolymer is a stimulus-responsive polymer that undergoes an inverse phase transition triggered by changes in the solvent quality, such as temperature, ionic strength, pH, or co-solvent concentration. Associated with this phase transition is a significant conformational change. We show that micro-cantilevers, decorated on one side with a pNIPAAM brush or poly(N-isopropylacrylamide-co-N-vinylimidazole) (pNIPAAM-VI) (7:3) brush, can be used to detect and transduce this phase transition behavior. Changes in the conformational state of the brush, induced by the phase transition or changes in osmotic pressure, cause significant changes in the surface stress in the brush that leads to detectable changes in cantilever deflection. We show that the use of pNIPAAM and its copolymers is exciting for cantilever actuation and sensing because commonly available micro-fabricated cantilever springs offer a simple and non-intrusive way to detect changes in solvent type, temperature, and pH, promising great potential for sensing applications in micro-fluidic devices.

Microcantilevers have been used over the last decade to detect biomolecules from solution. Specific binding events on one surface of the microcantilever create a differential stress, resulting in measurable deflection. Here we use this principle to detect human immunodeficiency virus type 1 (HIV-1) envelope glycoprotein (Env) gp120 from solution. We observed deflections approximately twice that of the baseline (in PBS) upon specific binding of gp120 to cantilevers decorated on one side with monoclonal antibodies (mAbs) A32 or T8. Subsequent incubation with mAb 17b (known to bind an A32-induced epitope on gp120) further increased deflection of A32- but not T8-presenting cantilevers. This work shows the capability of microcantilever deflection sensors to detect an induced-fit interaction at test concentrations of 8 μg/mL gp120 and 0.17 mg/mL 17b. Further development of this technique could lead to a portable, low-cost device for the effective detection of HIV-1.

ObjectiveOsteoarthritis (OA) is a degenerative joint disease characterized by the progressive loss of articular cartilage. While macroscale degradation of the cartilage extracellular matrix (ECM) has been extensively studied, microscale changes in the chondrocyte pericellular matrix (PCM) and immediate microenvironment with OA are not fully understood. The objective of this study was to quantify osteoarthritic changes in the micromechanical properties of the ECM and PCM of human articular cartilage in situ using atomic force microscopy (AFM).MethodAFM elastic mapping was performed on cryosections of human cartilage harvested from both condyles of macroscopically normal and osteoarthritic knee joints. This method was used to test the hypotheses that both ECM and PCM regions exhibit a loss of mechanical properties with OA and that the size of the PCM is enlarged in OA cartilage as compared to normal tissue.ResultsSignificant decreases were observed in both ECM and PCM moduli of 45% and 30%, respectively, on the medial condyle of OA knee joints as compared to cartilage from macroscopically normal joints. Enlargement of the PCM, as measured biomechanically, was also observed in medial condyle OA cartilage, reflecting the underlying distribution of type VI collagen in the region. No significant differences were observed in elastic moduli or their spatial distribution on the lateral condyle between normal and OA joints.ConclusionOur findings provide new evidence of significant site-specific degenerative changes in the chondrocyte micromechanical environment with OA.

One of the major constituents of the synovial fluid that is thought to be responsible for chondroprotection and boundary lubrication is the glycoprotein lubricin (PRG4); however, the molecular mechanisms by which lubricin carries out its critical functions still remain largely unknown. We hypothesized that the interaction of lubricin with type II collagen, the main component of the cartilage extracellular matrix, results in enhanced tribological and wear properties. In this study, we examined: (i) the molecular details by which lubricin interacts with type II collagen and how binding is related to boundary lubrication and adhesive interactions; and (ii) whether collagen structure can affect lubricin adsorption and its chondroprotective properties. We found that lubricin adsorbs strongly onto denatured, amorphous, and fibrillar collagen surfaces. Furthermore, we found large repulsive interactions between the collagen surfaces in presence of lubricin, which increased with increasing lubricin concentration. Lubricin attenuated the large friction and also the long-range adhesion between fibrillar collagen surfaces. Interestingly, lubricin adsorbed onto and mediated the frictional response between the denatured and native amorphous collagen surfaces equally and showed no preference on the supramolecular architecture of collagen. However, the coefficient of friction was lowest on fibrillar collagen in the presence of lubricin. We speculate that an important role of lubricin in mediating interactions at the cartilage surface is to attach to the cartilage surface and provide a protective coating that maintains the contacting surfaces in a sterically repulsive state.

Our work is motivated by the observation that rare, broadly neutralizing antibodies (NAbs), 4E10 and 2F5, associate with HIV-1 lipids as part of a required first step in neutralization before binding to membrane-proximal antigens. Subsequently, induction of these types of NAbs may be limited by immunologic tolerance due to autoreactivity with host cell membranes. Despite the significance of this lipid reactivity there is little experimental evidence detailing NAb–membrane interactions. Simple and efficient screening assays are needed to select antibodies that have similar lipid reactivity as known NAbs. To this end we have developed a surface plasmon resonance (SPR) spectroscopy based assay that monitors antibody binding to thiol self-assembled monolayers (SAMs) that replicate salient lipid surface chemistries and NAb binding to lipid surfaces. Specifically, we probed the relative importance of charge and hydrophobicity on antibody–surface interactions. We found that NAb binding to hydrophobic thiol surfaces was significantly greater than that of control monoclonal antibodies (mAbs). Furthermore, we confirmed the importance of charge-mediated antibody surface interactions, originally suggested by results from mAb interactions with conventional lipid vesicle/bilayer surfaces. Our approach, using self-assembled thiol monolayers that replicate the binding behavior of NAbs on lipid surfaces, thus provides an efficient and useful tool to screen interactions of mAbs and lipid-reactive NAbs.Highlights► Surface plasmon resonance assay that monitors antibody binding to thiol monolayers. ► HIV-1 neutralizing antibodies (NAbs) bind to hydrophobic thiols. ► Confirmed charge mediated antibody surface interactions. ► Suggest NAbs can embed into the hydrophobic lipid membrane core.

Articular cartilage provides a low-friction, wear-resistant surface for the motion of diarthrodial joints. The objective of this study was to develop a method for in situ friction measurement of murine cartilage using a colloidal probe attached to the cantilever of an atomic force microscope. Sliding friction was measured between a chemically functionalized microsphere and the cartilage of the murine femoral head. Friction was measured at normal loads ranging incrementally from 20 to 100 nN with a sliding speed of 40 μm/s and sliding distance of 64 μm. Under these test conditions, hydrostatic pressurization and biphasic load support in the cartilage were minimized, providing frictional measurements that predominantly reflect boundary lubrication properties. Friction coefficients measured on murine tissue (0.25±0.11) were similar to those measured on porcine tissue (0.23±0.09) and were in general agreement with measurements of boundary friction on cartilage by other researchers. Using the colloidal probe as an indenter, the elastic mechanical properties and surface roughness were measured in the same configuration. Interfacial shear was found to be the principal mechanism of friction generation, with little to no friction resulting from plowing forces, collision forces, or energy losses due to normal deformation. This measurement technique can be applied to future studies of cartilage friction and mechanical properties on genetically altered mice or other small animals.

This review focuses on surface-grafted DNA, and its use as a molecular building block that exploits its unique properties as a directional (poly)anion that exhibits molecular recognition. The selected examples highlight innovative applications of DNA at surfaces and interfaces ranging from molecular diagnostics and sequencing to biosensing.

Glucose responsive polymer brushes were synthesized on gold substrates and microcantilever arrays. The response properties of these brushes were evaluated by exposing them to different glucose concentrations for a range of pH values. This work demonstrates the potential for polymer brush-functionalized micromechanical cantilevers as glucose detectors. Furthermore, the work demonstrates that stimulus-responsive polymer brushes on micromechanical cantilevers have a significantly larger bending response due to glucose binding compared with self-assembled monolayers.

In this study, we present a technique to create complex, high cholesterol-containing supported lipid bilayers (SLBs) using α-helical (AH) peptide-induced vesicle fusion. Vesicles consisting of POPC:POPE:POPS:SM:Chol (9.35:19.25:8.25:18.15:45.00) were used to form a SLB that models the native composition of the human immunodeficiency virus-1 (HIV-1) lipid envelope. In the absence of AH peptides, these biomimetic vesicles fail to form a complete SLB. We verified and characterized AH peptide-induced vesicle fusion by a quartz crystal microbalance with dissipation monitoring, neutron reflectivity, and atomic force microscopy. Successful SLB formation entailed a characteristic frequency shift of −35.4 ± 0.8 Hz and a change in dissipation energy of 1.91 ± 0.23 × 10−6. Neutron reflectivity measurements determined the SLB thickness to be 49.9 +1.9−1.5 Å, and showed the SLB to be 100 +0.0−0.1% complete and void of residual AH peptide after washing. Atomic force microscopy imaging confirmed complete SLB formation and revealed three distinct domains with no visible defects. This vesicle fusion technique gives researchers access to a complex SLB composition with high cholesterol content and thus the ability to better recapitulate the native HIV-1 lipid membrane.