Molecular fingerprinting of biological nanoparticles with a label-free optofluidic platform

Label-free detection of multiple analytes in a high-throughput fashion has been one of the long-sought goals in biosensing applications. Yet, for all-optical approaches, interfacing state-of-the-art label-free techniques with microfluidics tools that can process small volumes of sample with high throughput, and with surface chemistry that grants analyte specificity, poses a critical challenge to date. Here, we introduce an optofluidic platform that brings together state-of-the-art digital holography with PDMS microfluidics by using supported lipid bilayers as a surface chemistry building block to integrate both technologies. Specifically, this platform fingerprints heterogeneous biological nanoparticle populations via a multiplexed label-free immunoaffinity assay with single particle sensitivity. First, we characterise the robustness and performance of the platform, and then apply it to profile four distinct ovarian cell-derived extracellular vesicle populations over a panel of surface protein biomarkers, thus developing a unique biomarker fingerprint for each cell line. We foresee that our approach will find many applications where routine and multiplexed characterisation of biological nanoparticles are required.

Interactions of Bacterial Quorum Sensing Signals with Model Lipid Membranes: Influence of Membrane Composition on Membrane Remodeling

We report the influence of membrane composition on the multiscale remodeling of multicomponent lipid bilayers initiated by contact with the amphiphilic bacterial quorum sensing signal N-(3-oxo)-dodecanoyl-l-homoserine lactone (3-oxo-C12-AHL) and its anionic headgroup hydrolysis product, 3-oxo-C12-HS. We used fluorescence microscopy and quartz crystal microbalance with dissipation (QCM-D) to characterize membrane reformation that occurs when these amphiphiles are placed in contact with supported lipid bilayers (SLBs) composed of (i) 1,2-dioleoyl-sn-glycero-3-phosphocholine (DOPC) containing varying amounts of cholesterol or (ii) mixtures of DOPC and either 1,2-dioleoyl-sn-glycero-3-phosphoethanolamine (DOPE, a conical zwitterionic lipid) or 1,2-dioleoyl-sn-glycero-3-phospho-l-serine (DOPS, a model anionic lipid). In general, we observe these mixed-lipid membranes to undergo remodeling events, including the formation and subsequent collapse of long tubules and the formation of hemispherical caps, upon introduction to biologically relevant concentrations of 3-oxo-C12-AHL and 3-oxo-C12-HS in ways that differ substantially from those observed in single-component DOPC membranes. These differences in bilayer reformation and their associated dynamics can be understood in terms of the influence of membrane composition on the time scales of molecular flip-flop, lipid packing defects, and lipid phase segregation in these materials. The lipid components investigated here are representative of classes of lipids that comprise both naturally occurring cell membranes and many useful synthetic soft materials. These studies thus represent a first step toward understanding the ways in which membrane composition can impact interactions with this important class of bacterial signaling molecules.

Interactions of Bacterial Quorum Sensing Signals with Model Lipid Membranes: Influence of Acyl Tail Structure on Multiscale Response

Many common bacteria use amphiphilic N-acyl-L-homoserine lactones (AHLs) as signaling molecules to coordinate group behaviors at high cell densities. Past studies demonstrate that AHLs can adsorb to and promote the remodeling of lipid membranes in ways that could underpin cell-cell or host-cell interactions. Here, we report that changes in AHL acyl tail group length and oxidation state (e.g., the presence or absence of a 3-oxo group) can lead to differences in the interactions of eight naturally occurring AHLs in solution and in contact with model lipid membranes. Our results reveal that the presence of a 3-oxo group impacts remodeling when AHLs are placed in contact with supported lipid bilayers (SLBs) of the phospholipid 1,2-dioleoyl-sn-glycero-3-phosphocholine (DOPC). Whereas AHLs that have 3-oxo groups generally promote the formation of microtubules, AHLs that lack 3-oxo groups generally form hemispherical caps on the surfaces of SLBs. These results are interpreted in terms of the time scales on which AHLs translocate across bilayers to relieve asymmetrical bilayer stress. Quartz crystal microbalance with dissipation measurements also reveal that 3-oxo AHLs associate with DOPC bilayers to a greater extent than their non-3-oxo analogues. In contrast, we observed no monotonic relationship between AHL tail length and bilayer reformation. Finally, we observed that 3-oxo AHLs facilitate greater transport or leakage of molecular cargo across the membranes of DOPC vesicles relative to AHLs without 3-oxo groups, also suggesting increased bilayer disruption and destabilization. These fundamental studies hint at interactions and associated multiscale phenomena that may inform current interpretations of the behaviors of AHLs in biological contexts. These results could also provide guidance useful for the design of new classes of synthetic materials (e.g., sensor elements or drug delivery vehicles) that interact with or respond selectively to communities of bacteria that use 3-oxo AHLs for cell-cell communication.

Bacterial Quorum Sensing Signals Promote Large-Scale Remodeling of Lipid Membranes

We report that N-acyl-l-homoserine lactones (AHLs), a class of nonionic amphiphiles that common bacteria use as signals to coordinate group behaviors, can promote large-scale remodeling in model lipid membranes. Characterization of supported lipid bilayers (SLBs) of the phospholipid 1,2-dioleoyl-sn-glycero-3-phosphocholine (DOPC) by fluorescence microscopy and quartz crystal microbalance with dissipation (QCM-D) reveals the well-studied AHL signal 3-oxo-C12-AHL and its anionic head group hydrolysis product (3-oxo-C12-HS) to promote the formation of long microtubules that can retract into hemispherical caps on the surface of the bilayer. These transformations are dynamic, reversible, and dependent upon the head group structure. Additional experiments demonstrate that 3-oxo-C12-AHL can promote remodeling to form microtubules in lipid vesicles and promote molecular transport across bilayers. Molecular dynamics (MD) simulations predict differences in thermodynamic barriers to translocation of these amphiphiles across a bilayer that are reflected in both the type and extent of reformation and associated dynamics. Our experimental observations can thus be interpreted in terms of accumulation and relief of asymmetric stresses in the inner and outer leaflets of a bilayer upon intercalation and translocation of these amphiphiles. Finally, experiments on Pseudomonas aeruginosa, a pathogen that uses 3-oxo-C12-AHL for cell-to-cell signaling, demonstrate that 3-oxo-C12-AHL and 3-oxo-C12-HS can promote membrane remodeling at biologically relevant concentrations and in the absence of other biosurfactants, such as rhamnolipids, that are produced at high population densities. Overall, these results have implications for the roles that 3-oxo-C12-AHL and its hydrolysis product may play in not only mediating intraspecies bacterial communication but also processes such as interspecies signaling and bacterial control of host-cell response. Our findings also provide guidance that could prove useful for the design of synthetic self-assembled materials that respond to bacteria in ways that are useful in the context of sensing, drug delivery, and in other fundamental and applied areas.

Expanding the Potential of the Solvent-Assisted Method to Create Bio-Interfaces from Amphiphilic Block Copolymers

Artificial membranes, as materials with biomimetic properties, can be applied in various fields, such as drug screening or bio-sensing. The solvent-assisted method (SA) represents a straightforward method to prepare lipid solid-supported membranes. It overcomes the main limitations of established membrane preparation methods, such as Langmuir-Blodgett (LB) or vesicle fusion. However, it has not yet been applied to create artificial membranes based on amphiphilic block copolymers, despite their enhanced mechanical stability compared to lipid-based membranes and bio-compatible properties. Here, we applied the SA method on different amphiphilic di- and triblock poly(dimethylsiloxane)-block-poly(2-methyl-2-oxazoline) (PDMS-b-PMOXA) copolymers and optimized the conditions to prepare artificial membranes on a solid support. The real-time membrane formation, the morphology, and the mechanical properties have been evaluated by a combination of atomic force microscopy and quartz crystal microbalance. Then, selected biomolecules including complementary DNA strands and an artificial deallylase metalloenzyme (ADAse) were incorporated into these membranes relying on the biotin-streptavidin technology. DNA strands served to establish the capability of these synthetic membranes to interact with biomolecules by preserving their correct conformation. The catalytic activity of the ADAse following its membrane anchoring induced the functionality of the biomimetic platform. Polymer membranes on solid support as prepared by the SA method open new opportunities for the creation of artificial membranes with tailored biomimetic properties and functionality.

From spherical compartments to polymer films: exploiting vesicle fusion to generate solid supported thin polymer membranes

Solid supported polymer membranes as scaffold for the insertion of functional biomolecules provide the basis for mimicking natural membranes. They also provide the means for unraveling biomolecule-membrane interactions and engineering platforms for biosensing. Vesicle fusion is an established procedure to obtain solid supported lipid bilayers but the more robust polymer vesicles tend to resist fusion and planar membranes rarely form. Here, we build on vesicle fusion to develop a refined and efficient way to produce solid supported membranes based on poly(dimethylsiloxane)-poly(2-methyl-2-oxazoline) (PMOXA-b-PDMS-b-PMOXA) amphiphilic triblock copolymers. We first create thiol-bearing polymer vesicles (polymersomes) and anchor them on a gold substrate. An osmotic shock then provokes polymersome rupture and drives planar film formation. Prerequisite for a uniform amphiphilic planar membrane is the proper combination of immobilized polymersomes and osmotic shock conditions. Thus, we explored the impact of the hydrophobic PDMS block length of the polymersome on the formation and the characteristics of the resulting solid supported polymer assemblies by quarz crystal microbalance with dissipation monitoring (QCM-D), atomic force microscopy (AFM) and spectroscopic ellipsometry (SE). When the PDMS block is short enough, attached polymersomes restructure in response to osmotic shock, resulting in a uniform planar membrane. Our approach to rapidly form planar polymer membranes by vesicle fusion brings many advantages to the development of synthetic planar membranes for bio-sensing and biotechnological applications.

Supported Lipid Bilayer Formation: Beyond Vesicle Fusion

Supported lipid bilayers (SLBs) are cell-membrane-mimicking platforms that can be formed on solid surfaces and integrated with a wide range of surface-sensitive measurement techniques. SLBs are useful for unravelling details of fundamental membrane biology and biophysics as well as for various medical, biotechnology, and environmental science applications. Thus, there is high interest in developing simple and robust methods to fabricate SLBs. Currently, vesicle fusion is a popular method to form SLBs and involves the adsorption and spontaneous rupture of lipid vesicles on a solid surface. However, successful vesicle fusion depends on high-quality vesicle preparation, and it typically works with a narrow range of material supports and lipid compositions. In this Feature Article, we summarize current progress in developing two new SLB fabrication techniques termed the solvent-assisted lipid bilayer (SALB) and bicelle methods, which have compelling advantages such as simple sample preparation and compatibility with a wide range of material supports and lipid compositions. The molecular self-assembly principles underpinning the two strategies and important experimental parameters are critically discussed, and recent application examples are presented. Looking forward, we envision that these emerging SLB fabrication strategies can be widely adopted by specialists and nonspecialists alike, paving the way to enriching our understanding of lipid membrane properties and realizing new application possibilities.

Influence of NaCl Concentration on Bicelle-Mediated SLB Formation

The deposition of two-dimensional bicellar disks on hydrophilic surfaces is an emerging approach to fabricate supported lipid bilayers (SLBs) that requires minimal sample preparation, works at low lipid concentrations, and yields high-quality SLBs. While basic operating steps in the fabrication protocol mimic aspects of the conventional vesicle fusion method, lipid bicelles and vesicles have distinct architectural properties, and understanding how experimental parameters affect the efficiency of bicelle-mediated SLB formation remains to be investigated. Herein, using the quartz crystal microbalance-dissipation and localized surface plasmon resonance techniques, we investigated the effect of bulk NaCl concentration on bicelle-mediated SLB formation on silicon dioxide surfaces. For comparison, similar experiments were conducted with vesicles as well. In both cases, SLB formation was observed to occur rapidly provided that the NaCl concentration was sufficiently high (>50 mM). Under such conditions, the effect of NaCl concentration on SLB formation was minor in the case of bicelles and significant in the case of vesicles where it is expected to be related primarily to osmotic pressure. At lower NaCl concentrations, bicelles also formed SLBs but slowly, whereas adsorbed vesicles remained intact. These findings were complemented by time-lapsed fluorescence microscopy imaging and fluorescence recovery after photobleaching measurements that corroborated bicelle-mediated SLB formation across the range of tested NaCl concentrations. The results are discussed by comparing the architectural properties of bicelles and vesicles along with theoretical analysis of the corresponding adsorption kinetics.

Biomolecules Turn Self-Assembling Amphiphilic Block Co-polymer Platforms Into Biomimetic Interfaces

Biological membranes constitute an interface between cells and their surroundings and form distinct compartments within the cell. They also host a variety of biomolecules that carry out vital functions including selective transport, signal transduction and cell-cell communication. Due to the vast complexity and versatility of the different membranes, there is a critical need for simplified and specific model membrane platforms to explore the behaviors of individual biomolecules while preserving their intrinsic function. Information obtained from model membrane platforms should make invaluable contributions to current and emerging technologies in biotechnology, nanotechnology and medicine. Amphiphilic block co-polymers are ideal building blocks to create model membrane platforms with enhanced stability and robustness. They form various supramolecular assemblies, ranging from three-dimensional structures (e.g., micelles, nanoparticles, or vesicles) in aqueous solution to planar polymer membranes on solid supports (e.g., polymer cushioned/tethered membranes,) and membrane-like polymer brushes. Furthermore, polymer micelles and polymersomes can also be immobilized on solid supports to take advantage of a wide range of surface sensitive analytical tools. In this review article, we focus on self-assembled amphiphilic block copolymer platforms that are hosting biomolecules. We present different strategies for harnessing polymer platforms with biomolecules either by integrating proteins or peptides into assemblies or by attaching proteins or DNA to their surface. We will discuss how to obtain synthetic structures on solid supports and their characterization using different surface sensitive analytical tools. Finally, we highlight present and future perspectives of polymer micelles and polymersomes for biomedical applications and those of solid-supported polymer membranes for biosensing.

Optimizing the Formation of Supported Lipid Bilayers from Bicellar Mixtures

Supported lipid bilayers (SLBs) are widely studied model membrane platforms that are compatible with various surface-sensitive measurement techniques. SLBs are typically formed on silica-based materials, and there are numerous possible fabrication routes involving either bottom-up molecular self-assembly or vesicle adsorption and rupture. In between these two classes of fabrication strategies lies an emerging approach based on depositing quasi-two-dimensional lamellar, bicellar disks composed of a mixture of long-chain and short-chain phospholipids to promote the formation of SLBs. This approach takes advantage of the thermodynamic preference of long-chain phospholipids to form planar SLBs, whereas short-chain phospholipids have brief residence times. Although a few studies have shown that SLBs can be formed on silica-based materials from bicellar mixtures, outstanding questions remain about the self-assembly mechanism as well as the influence of the total phospholipid concentration, ratio of the two phospholipids (termed the "q-ratio"), and process of sample preparation. Herein, we address these questions through comprehensive quartz crystal microbalance-dissipation, fluorescence microscopy, and fluorescence recovery after photobleaching experiments. Our findings identify that optimal SLB formation occurs at lower total concentrations of phospholipids than previously used as short-chain phospholipids behave like membrane-destabilizing detergents at higher concentrations. Using lower phospholipid concentrations, we also discovered that the formation of SLBs proceeds through a two-step mechanism involving a critical coverage of bicellar disks akin to vesicle fusion. In addition, the results indicate that at least one cycle of freeze-thaw-vortexing is useful during the sample preparation process to produce SLBs. Taken together, the findings in this work identify optimal routes for fabricating SLBs from bicellar mixtures and reveal mechanistic details about the bicelle-mediated SLB formation process, which will aid further exploration of bicellar mixtures as tools for model membrane fabrication.

Solid surface vs. liquid surface: nanoarchitectonics, molecular machines, and DNA origami

Comparisons of science and technology between these solid and liquid surfaces would be a good navigation for current-to-future developments.

Relationship between vesicle size and steric hindrance influences vesicle rupture on solid supports

Although it is thermodynamically favorable for adsorbed vesicles to rupture with increasing vesicle size, this study demonstrates that steric hindrance acts as a kinetic barrier to impede large vesicles from rupturing.