Streptavidin Coverage on Biotinylated Surfaces

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.

Regulatory T cell-derived exosome mediated macrophages polarization for osteogenic differentiation in fracture repair

Refractory fracture presents an intractable challenge in trauma treatment. Selective polarization of macrophages as well as the recruitment of osteogenic precursor cells play key roles in osteogenic differentiation during fracture healing. Here we constructed regulatory T cell (Treg)-derived exosomes (Treg-Exo) for the treatment of fracture. The obtained exosomes displayed a spheroid shape with a hydrated particle size of approximately 130 nm. With further purification using CD39 and CD73 antibody-modified microfluidic chips, CD39 and CD73 specifically expressing exosomes were obtained. This kind of Treg-Exo utilized the ectonucleotidases of CD39 and CD73 to catalyze the high level of ATP in the fracture area into adenosine. The generated adenosine further promoted the selective polarization of macrophages. When interacting with mesenchymal stem cells (MSCs, osteogenic precursor cells), both Treg-Exo and Treg-Exo primed macrophages facilitated the proliferation and differentiation of MSCs. After administration in vivo, Treg-Exo effectively promoted fracture healing compared with conventional T cell-derived exosome. To further improve the delivery efficacy of exosomes and integrate multiple biological processes of fracture healing, an injectable hydrogel was fabricated to co-deliver Treg-Exo and stromal cell-derived factor 1 alpha (SDF-1α). With the dual effect of Treg-Exo for macrophage polarization and SDF-1α for MSC recruitment, the multifunctional hydrogel exerted a synergistic effect on fracture repair acceleration. This study provided a promising therapeutic candidate and synergistic strategy for the clinical treatment of fracture.

Protein sensing using deep subwavelength-engineered photonic crystals

We demonstrate a higher sensitivity detection of proteins in a photonic crystal platform by including a deep subwavelength feature in the unit cell that locally increases the energy density of light. Through both simulations and experiments, the sensing capability of a deep subwavelength-engineered silicon antislot photonic crystal nanobeam (PhCNB) cavity is compared to that of a traditional PhCNB cavity. The redistribution and local enhancement of the energy density by the 50 nm antislot enable stronger light-molecule interaction at the surface of the antislot and lead to a larger resonance shift upon protein binding. This surface-based energy enhancement is confirmed by experiments demonstrating a nearly 50% larger resonance shift upon attachment of streptavidin molecules to biotin-functionalized antislot PhCNB cavities.

Competitive Specific Anchorage of Molecules onto Surfaces: Quantitative Control of Grafting Densities and Contamination by Free Anchors

The formation of surfaces decorated with biomacromolecules such as proteins, glycans, or nucleic acids with well-controlled orientations and densities is of critical importance for the design of in vitro models, e.g., synthetic cell membranes and interaction assays. To this effect, ligand molecules are often functionalized with an anchor that specifically binds to a surface with a high density of binding sites, providing control over the presentation of the molecules. Here, we present a method to robustly and quantitatively control the surface density of one or several types of anchor-bearing molecules by tuning the relative concentrations of target molecules and free anchors in the incubation solution. We provide a theoretical background that relates incubation concentrations to the final surface density of the molecules of interest and present effective guidelines toward optimizing incubation conditions for the quantitative control of surface densities. Focusing on the biotin anchor, a commonly used anchor for interaction studies, as a salient example, we experimentally demonstrate surface density control over a wide range of densities and target molecule sizes. Conversely, we show how the method can be adapted to quality control the purity of end-grafted biopolymers such as biotinylated glycosaminoglycans by quantifying the amount of residual free biotin reactant in the sample solution.

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

Label-free detecting 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. Herein, we first thoroughly 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 is required.

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

Label-free detecting 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. Herein, we first thoroughly 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 is required.

Protein nanoarrays using the annexin A5 two-dimensional crystal on supported lipid bilayers

Protein nanoarrays are regularly ordered patterns of proteins fixed on a solid surface with a periodicity on the order of nanometers. They have significant potential applications as highly sensitive bioassays and biosensors. While several researchers have demonstrated the fabrication of protein nanoarrays with lithographic techniques and programmed DNA nanostructures, it has been difficult to fabricate a protein nanoarray containing a massive number of proteins on the surface. We now report the fabrication of nanoarrays of streptavidin molecules using a two-dimensional (2D) crystal of annexin A5 as a template on supported lipid bilayers that are widely used as cell membranes. The 2D crystal of annexin A5 has a six-fold symmetry with a period of about 18 nm. There is a hollow of a diameter of about 10 nm in the unit cell, surrounded by six trimers of annexin A5. We found that a hollow accommodates up to three streptavidin molecules with their orientation controlled, and confirmed that the molecules in the hollow maintain their specific binding capability to biotinylated molecules, which demonstrates that the fabricated nanoarray serves as an effective biosensing platform. This methodology can be directly applied to the fabrication of nanoarrays containing a massive number of any other protein molecules.

Recruitment of Receptors and Ligands in a Weakly Multivalent System with Omnipresent Signatures of Superselective Binding

Recruitment of receptors at membrane interfaces is essential in biological recognition and uptake processes. The interactions that induce recruitment are typically weak at the level of individual interaction pairs, but are strong and selective at the level of recruited ensembles. Here, a model system is demonstrated, based on the supported lipid bilayer (SLB) that mimics the recruitment process induced by weakly multivalent interactions. The weak (mm range) histidine-nickel-nitrilotriacetate (His₂ -NiNTA) pair is employed owing to its ease of implementation in both synthetic and biological systems. The recruitment of receptors (and ligands) induced by the binding of His₂ -functionalized vesicles on NiNTA-terminated SLBs is investigated to identify the ligand densities necessary to achieve vesicle binding and receptor recruitment. Threshold values of ligand densities appear to occur in many binding characteristics: density of bound vesicles, size and receptor density of the contact area, and vesicle deformation. Such thresholds contrast the binding of strongly multivalent systems and constitute a clear signature of the superselective binding behavior predicted for weakly multivalent interactions. This model system provides quantitative insight into the binding valency and effects of competing energetic forces, such as deformation, depletion, and entropy cost of recruitment at different length scales.

QCM-D Investigations on Cholesterol-DNA Tethering of Liposomes to Microbubbles for Therapy

Lipid-shelled microbubbles (MBs) offer potential as theranostic agents, capable of providing both contrast enhancement in ultrasound imaging as well as a route for triggered drug release and improved localized drug delivery. A common motif in the design of such therapeutic vehicles is the attachment of the drug carrier, often in the form of liposomes, to the microbubble. Traditionally, such attachments have been based around biotin-streptavidin and maleimide-PDP chemistries. Comparatively, the use of DNA-lipid tethers offers potential advantage. First, their specificity permits the construction of more complex architectures that might include bespoke combinations of different drug-loaded liposomes and/or targeting groups, such as affimers or antibodies. Second, the use of dual-lipid tether strategies should increase the strength of the individual tethers tethering the liposomes to the bubbles. The ability of cholesterol-DNA (cDNA) tethers for conjugation of liposomes to supported lipid bilayers has previously been demonstrated. For in vivo applications, bubbles and liposomes often contain a proportion of polyethylene glycol (PEG) to promote stealth-like properties and increase lifetimes. However, the associated steric effects may hinder tethering of the drug payload. We show that while the presence of PEG reduced the tethering affinity, cDNA can still be used for the attachment of liposomes to a supported lipid bilayer (SLB) as measured via QCM-D. Importantly, we show, for the first time, that QCM-D can be used to study the tethering of microbubbles to SLBs using cDNA, signified by a decrease in the magnitude of the frequency shift compared to liposomes alone due to the reduced density of the MBs. We then replicate this tethering interaction in the bulk and observe attachment of liposomes to the shell of a central MB and hence formation of a model therapeutic microbubble.

A disordered tether to iLID improves photoswitchable protein patterning on model membranes

Reversible protein patterning on model membranes is important to reproduce spatiotemporal protein dynamics in vitro. An engineered version of iLID, disiLID, with a disordered domain as a membrane tether improves the recruitment of Nano under blue light and the reversibility in the dark, which enables protein patterning on membranes with higher spatiotemporal precision.

Kinetic Analysis of Prostate-Specific Antigen Interaction with Monoclonal Antibodies for Development of a Magnetic Immunoassay Based on Nontransparent Fiber Structures

Prostate cancer is the second most common cancer diagnosed in men worldwide. Measuring the prostate-specific antigen (PSA) is regarded as essential during prostate cancer screening. Early diagnosis of this disease relapse after radical prostatectomy requires extremely sensitive methods. This research presents an approach to development of an ultrasensitive magnetic sandwich immunoassay, which demonstrates the limit of PSA detection in human serum of 19 pg/mL at a dynamic range exceeding 3.5 orders of concentration. Such attractive performance stems, inter alia, from the kinetic analysis of monoclonal antibodies (mAbs) against free PSA to select the mAbs exhibiting best kinetic characteristics and specificity. The analysis is carried out with a label-free multiplex spectral-correlation interferometry compatible with inexpensive single-use glass sensor chips. The high sensitivity of developed PSA immunoassay is due to electronic quantification of magnetic nanolabels functionalized by the selected mAbs and three-dimension porous filters used as an extended solid phase. The assay is promising for PSA monitoring after radical prostatectomy. The proposed versatile approach can be applied for the rational design of highly sensitive tests for detection of other analytes in many fields, including in vitro diagnostics, veterinary, food safety, etc.

Unraveling pH Effects on Ultrafiltration Membrane Fouling by Extracellular Polymeric Substances: Adsorption and Conformation Analyzed with Localized Surface Plasmon Resonance

Extracellular polymeric substances (EPSs) can conform and orient on the surface according to the applied aquatic conditions. While pH elevation usually removes EPSs from membranes, small changes in pH can change the adsorbed EPS conformation and orientation, resulting in a decrease in membrane permeability. Accordingly, EPS layers were tested with localized surface plasmon resonance (LSPR) sensing and quartz crystal microbalance with dissipation monitoring (QCM-D) using a hybrid sensor. A novel membrane-mimetic hybrid QCM-D-LSPR sensor was designed to indicate both "dry" mass and mechanical load ("wet" mass) of the adsorbed EPS. The effect of pH on the EPS layer's viscoelastic properties and hydrated thickness analyzed by QCM-D corroborates with the shift in EPS areal concentration, Γ_S, and the associated EPS conformation, analyzed by LSPR. As pH elevates, the processes of (i) elevation in EPS layer's thickness (QCM-D) and (ii) decrease in the EPS areal density, Γ_S (LSPR), provide a clear indication for changes in EPS conformation, which decrease the effective ultrafiltration (UF) membrane pore diameter. This decrease in the pore diameter together with the increase in surface hydrophobicity elevates UF membrane hydraulic resistance.

Density Control over MBD2 Receptor-Coated Surfaces Provides Superselective Binding of Hypermethylated DNA

Using the biomarker hypermethylated DNA (hmDNA) for cancer detection requires a pretreatment to isolate or concentrate hmDNA from nonmethylated DNA. Affinity chromatography using a methyl binding domain-2 (MBD2) protein can be used, but the relatively low enrichment selectivity of MBD2 limits its clinical applicability. Here, we developed a superselective, multivalent, MBD2-coated platform to improve the selectivity of hmDNA enrichment. The multivalent platform employs control over the MBD2 surface receptor density, which is shown to strongly affect the binding of DNA with varying degrees of methylation, improving both the selectivity and the affinity of DNAs with higher numbers of methylation sites. Histidine-10-tagged MBD2 was immobilized on gold surfaces with receptor density control by tuning the amount of nickel nitrilotriacetic acid (NiNTA)-functionalized thiols in a thiol-based self-assembled monolayer. The required MBD2 surface receptor densities for DNA surface binding decreases for DNA with higher degrees of methylation. Both higher degrees of superselectivity and surface coverages were observed upon DNA binding at increasing methylation levels. Adopting the findings of this study into hmDNA enrichment of clinical samples has the potential to become more selective and sensitive than current MBD2-based methods and, therefore, to improve cancer diagnostics.

Distinct Binding Properties of Neutravidin and Streptavidin Proteins to Biotinylated Supported Lipid Bilayers: Implications for Sensor Functionalization

The exceptional strength and stability of noncovalent avidin-biotin binding is widely utilized as an effective bioconjugation strategy in various biosensing applications, and neutravidin and streptavidin proteins are two commonly used avidin analogues. It is often regarded that the biotin-binding abilities of neutravidin and streptavidin are similar, and hence their use is interchangeable; however, a deeper examination of how these two proteins attach to sensor surfaces is needed to develop reliable surface functionalization options. Herein, we conducted quartz crystal microbalance-dissipation (QCM-D) biosensing experiments to investigate neutravidin and streptavidin binding to biotinylated supported lipid bilayers (SLBs) in different pH conditions. While streptavidin binding to biotinylated lipid receptors was stable and robust across the tested pH conditions, neutravidin binding strongly depended on the solution pH and was greater with increasingly acidic pH conditions. These findings led us to propose a two-step mechanistic model, whereby streptavidin and neutravidin binding to biotinylated sensing interfaces first involves nonspecific protein adsorption that is mainly influenced by electrostatic interactions, followed by structural rearrangement of adsorbed proteins to specifically bind to biotin functional groups. Practically, our findings demonstrate that streptavidin is preferable to neutravidin for constructing SLB-based sensing platforms and can improve sensing performance for detecting antibody–antigen interactions.