Optimization of the hybrid bilayer membrane method for immobilization of avidin on quartz crystal microbalance

Hybrid bilayer membranes as platforms for biomimicry and catalysis

Hybrid bilayer membrane (HBM) platforms represent an emerging nanoscale bio-inspired interface that has broad implications in energy catalysis and smart molecular devices. An HBM contains multiple modular components that include an underlying inorganic surface with a biological layer appended on top. The inorganic interface serves as a support with robust mechanical properties that can also be decorated with functional moieties, sensing units and catalytic active sites. The biological layer contains lipids and membrane-bound entities that facilitate or alter the activity and selectivity of the embedded functional motifs. With their structural complexity and functional flexibility, HBMs have been demonstrated to enhance catalytic turnover frequency and regulate product selectivity of the O₂ and CO₂ reduction reactions, which have applications in fuel cells and electrolysers. HBMs can also steer the mechanistic pathways of proton-coupled electron transfer (PCET) reactions of quinones and metal complexes by tuning electron and proton delivery rates. Beyond energy catalysis, HBMs have been equipped with enzyme mimics and membrane-bound redox agents to recapitulate natural energy transport chains. With channels and carriers incorporated, HBM sensors can quantify transmembrane events. This Review serves to summarize the major accomplishments achieved using HBMs in the past decade.

Enhanced avidin binding to lipid bilayers using PDP-PE lipids with PEG-biotin linkers

Two of the most important aspects of lipid bilayers that have increased their popularity in the field of nanotechnology and biosensors are their fluid nature, which is highly beneficial in ensuring the spatial organization of attached molecules, and the relative ease in which they can be manipulated to change the surface chemistry. Here we have used two different types of functionalized lipids to study the interaction of avidin, which is a common approach to attach further ligands for study. We have tested the commonly used Biotinyl-Cap-PE lipids at different molar percentages and reveal that avidin is not evenly distributed, but forms what looks like clusters even at low percentage occupancy which hampers the level of avidin that can be associated with the surface. We have then successfully employed the novel strategy of using PDP-PE lipids which contain a reducible disulphide to which we added maleamide-PEG-biotin spacers of different lengths. There is a more even distribution of avidin on these layers and thereby increasing the amount and efficiency of avidin association. The reduced levels of avidin that was being associated with the Biotinyl-Cap-PE layers as compared to the PDP-PE lipids could be analysed with QCM-D and interferometry approaches, but it was only with SEEC microscopy that the reason for the reduced occupancy was resolved.

Formation of Alkanethiol Supported Hybrid Membranes Revisited

A phospholipid monolayer supported on an alkanethiol self-assembled monolayer (SAM) constitutes a supported hybrid membrane, a model of biological membranes optimized for electronic access through the underlying metal support surface. It is believed that phospholipids, when deposited from aqueous liposome suspension, spontaneously cover the alkanethiol-modified surface, owing to the reduction of surface free energy of the hydrophobic alkane surface exposed to the solution. However, the formation of the hybrid layer has to overcome significant energy barriers in rupturing the vesicle and "unzipping" the membrane leaflets; hence drivers of the spontaneous hybrid membrane formation are unclear. In this work, the authors studied the efficiency of the liposome deposition method to form hybrid membranes on octanethiol and hexadecanethiol SAMs in aqueous environment. Using quartz crystal microbalance to monitor the deposition process it was found that the hybrid membrane did not form spontaneously; the deposit was dominated by hemi-fused liposomes that can only be removed by applying osmotic stress. However, osmotic stress yielded a reproducible layer characterized by ≈-5Hz frequency change that is also confirmed by fluorescence microscopy imaging, irrespective of lipid concentration and the chain length of the SAMs. The frequency change is ≈20% of the frequency change expected for a tightly bound bilayer membrane, or 40% of a single leaflet, suggesting that the lipid layer is in a different conformation compared to a bilayer membrane: the acyl chains are most likely parallel to the SAM surface, likely due to strong hydrophobic interaction. Comparing these results to the literature it appears that the initial formation of hybrid membranes is inhibited by the ionic environment, while osmotic stress leads to the observed unique layer conformation.

Orientation and density control of proteins on solid matters by outer membrane coating: Analytical and diagnostic applications

Autodisplay is an expression system for the display of recombinant proteins on the outer membrane (OM) of gram negative bacteria and has been developed for translocation studies, whole cell biocatalysis, bioremediation, inhibitor screening, and enzyme refolding. Recently, affinity proteins such as IgG-binding Z-domains and biotin-binding streptavidin have been autodisplayed on the OM of Escherichia coli for analytical and biomedical applications. The secretion mechanism of the autodisplay system was used and orientation and density control of these affinity proteins were determined. Affinity protein-autodisplaying E. coli cells have been used to coat solid supports in immunoassays. For this purpose, the OM of autodisplayed E. coli cells was separated and isolated by the aid of detergents. The structure of the resulting OM liposomes as well as their physico-chemical parameters, were analyzed. OM liposomes were used subsequently for coating various solid matters including microplates and biosensor transducer surfaces and the formation of OM layers were monitored. OM layer formation on solid matters was shown to increase the sensitivity of immunoassays and biosensors. In this review, analytical and diagnostic applications are described in particular concerning orientation and density control of autodisplayed affinity proteins.

Reversible biofunctionalization of surfaces with a switchable mutant of avidin

Label-free biosensors detect binding of prey molecules (″analytes″) to immobile bait molecules on the sensing surface. Numerous methods are available for immobilization of bait molecules. A convenient option is binding of biotinylated bait molecules to streptavidin-functionalized surfaces, or to biotinylated surfaces via biotin-avidin-biotin bridges. The goal of this study was to find a rapid method for reversible immobilization of biotinylated bait molecules on biotinylated sensor chips. The task was to establish a biotin-avidin-biotin bridge which was easily cleaved when desired, yet perfectly stable under a wide range of measurement conditions. The problem was solved with the avidin mutant M96H which contains extra histidine residues at the subunit-subunit interfaces. This mutant was bound to a mixed self-assembled monolayer (SAM) containing biotin residues on 20% of the oligo(ethylene glycol)-terminated SAM components. Various biotinylated bait molecules were bound on top of the immobilized avidin mutant. The biotin-avidin-biotin bridge was stable at pH ≥3, and it was insensitive to sodium dodecyl sulfate (SDS) at neutral pH. Only the combination of citric acid (2.5%, pH 2) and SDS (0.25%) caused instantaneous cleavage of the biotin-avidin-biotin bridge. As a consequence, the biotinylated bait molecules could be immobilized and removed as often as desired, the only limit being the time span for reproducible chip function when kept in buffer (2-3 weeks at 25 °C). As expected, the high isolectric pH (pI) of the avidin mutant caused nonspecific adsorption of proteins. This problem was solved by acetylation of avidin (to pI < 5), or by optimization of SAM formation and passivation with biotin-BSA and BSA.

Functionalization of quartz crystal microbalances with liposomes containing the N-hydroxysuccinimide ester of palmitic acid

We developed a fast and simple method to functionalize a quartz crystal microbalance (QCM) with liposomes composed of phosphatidylcholine lipid and the N-hydroxysuccinimide (NHS) ester of palmitic acid. The liposome was applied directly to a bare gold surface of a QCM to prepare the lipid bilayer presenting NHS groups on the surface. The whole functionalization process was completed within 1 h using stored lipid films. Streptavidin immobilization efficiency of the method was comparable to the 1-ethyl-3-(3-dimethylaminopropyl)carbodiimide (EDC)/NHS chemical method, and the activity of the biotinylated antibody immobilized through the streptavidin was stably retained in repeated surface regeneration with the dissociation buffer.

Application of paper-supported printed gold electrodes for impedimetric immunosensor development

In this article, we report on the formation and mode-of-operation of an affinity biosensor, where alternate layers of biotin/streptavidin/biotinylated-CRP-antigen/anti-CRP antibody are grown on printed gold electrodes on disposable paper-substrates. We have successfully demonstrated and detected the formation of consecutive layers of supra-molecular protein assembly using an electrical (impedimetric) technique. The formation process is also supplemented and verified using conventional surface plasmon resonance (SPR) measurements and surface sensitive characterization techniques, such as X-ray photoelectron spectroscopy (XPS) and atomic force microscopy (AFM). The article provides a possible biosensor development scheme, where-(1) fabrication of paper substrate (2) synthesis of gold nanoparticle inks (3) inkjet printing of gold electrodes on paper (4) formation of the biorecognition layers on the gold electrodes and (5) electrical (impedimetric) analysis of growth-all are coupled together to form a test-structure for a recyclable and inexpensive point-of-care diagnostic platform.

Detection of Edwardsiella tarda by fluorometric or biosensor methods using a peptide ligand

In this study, we identified a peptide ligand for Edwardsiella tarda from a phage peptide library and tested two approaches for sensitive detection of the bacteria with the peptide labeled with fluorescein or biotin. At first, the fluorescent peptide was proved to be advantageous in the fluorescence polarization (FP) assay because sensitivity of the assay is maximized when a fluorophore is linked to a small molecule. The FP assay using the fluorescent peptide enabled detection of E. tarda in a range from 5.2×10(3) to 2.1×10(5) cells. Second, we devised a new assay method using a quartz crystal microbalance (QCM) biosensor connected to a filter module. When a mixture of E. tarda and the biotinylated peptide was injected into the filter module, the E. tarda-peptide complex was separated from the unbound peptide by a filter and detected with a streptavidin-coated QCM sensor chip. On injection of samples containing the biotinylated peptide and E. tarda, concentration-dependent frequency change was observed in a range from 8×10(2) to 8×10(6) cells. The two approaches are expected to facilitate development of assay methods using other bacteria-binding peptides.