Electrochemical impedance spectroscopy and atomic force microscopic studies of electrical and mechanical properties of nano-black lipid membranes and size dependence

Dry Two-Step Self-Assembly of Stable Supported Lipid Bilayers on Silicon Substrates

Artificial membranes are models for biological systems and are important for applications. We introduce a dry two-step self-assembly method consisting of the high-vacuum evaporation of phospholipid molecules over silicon, followed by a subsequent annealing step in air. We evaporate dipalmitoylphosphatidylcholine (DPPC) molecules over bare silicon without the use of polymer cushions or solvents. High-resolution ellipsometry and AFM temperature-dependent measurements are performed in air to detect the characteristic phase transitions of DPPC bilayers. Complementary AFM force-spectroscopy breakthrough events are induced to detect single- and multi-bilayer formation. These combined experimental methods confirm the formation of stable non-hydrated supported lipid bilayers with phase transitions gel to ripple at 311.5 ± 0.9 K, ripple to liquid crystalline at 323.8 ± 2.5 K and liquid crystalline to fluid disordered at 330.4 ± 0.9 K, consistent with such structures reported in wet environments. We find that the AFM tip induces a restructuring or intercalation of the bilayer that is strongly related to the applied tip-force. These dry supported lipid bilayers show long-term stability. These findings are relevant for the development of functional biointerfaces, specifically for fabrication of biosensors and membrane protein platforms. The observed stability is relevant in the context of lifetimes of systems protected by bilayers in dry environments.

Electrical characterization of ZnO-coated nanospring ensemble by impedance spectroscopy: probing the effect of thermal annealing

The effects of thermal annealing on the electrical properties of randomly oriented ZnO-coated nanospring ensembles were extensively investigated through AC impedance spectroscopy. Annealing the nanospring mats for an hour at 873 K in air showed significant change in ZnO morphology, reduced electrical conductivity due to the presence of grain boundaries, decreased apparent donor concentration, and faster decay of sub-band gap photocurrent. The role of the nanospring-nanospring junctions in the conduction of carriers in the ensemble was also examined, as well as evaluation of their responsiveness to thermal and optical stimulations. This work identifies the effects of heat treatment in the presence of air on the electrical properties of the nanospring ensembles, which are related to the mesoscopic morphology and interconnect within the ensemble and the properties of the ZnO coating.

Bacterial Concentration Detection using a PCB-based Contactless Conductivity Sensor

Capacitively coupled contactless conductivity detection (C⁴D) is an improved approach to avoid the problems of labor-intensive, time-consuming and insufficient accuracy of plate count as well as the high-cost apparatus of flow cytometry (FCM) in bacterial counting. This article describes a novel electrode-integrated printed-circuit-board (PCB)-based C⁴D device, which supports the simple and safe exchange of capillaries and improves the sensitivity and repeatability of the contactless detection. Furthermore, no syringe pump is needed in the detection, it reduces the system size, and, more importantly, avoids the effect on the bacteria due to high pressure. The recovered bacteria after C⁴D detection at excitation of 25 Vpp and 60⁻120 kHz were analyzed by flow cytometry, and a survival rate higher than 96% was given. It was verified that C⁴D detection did not influence the bacterial viability. Moreover, bacteria concentrations from 10⁶ cells/mL to 10⁸ cells/mL were measured in a linear range, and relative standard deviation (RSD) is below 0.2%. In addition, the effects on bacteria and C⁴D from background solutions were discussed. In contrast to common methods used in most laboratories, this method may provide a simple solution to in situ detection of bacterial cultures.

Electrophysiology of Epithelial Sodium Channel (ENaC) Embedded in Supported Lipid Bilayer Using a Single Nanopore Chip

Nanopore-based technologies are highly adaptable supports for developing label-free sensor chips to characterize lipid bilayers, membrane proteins, and nucleotides. We utilized a single nanopore chip to study the electrophysiology of the epithelial Na⁺ channel (ENaC) incorporated in supported lipid membrane (SLM). An isolated nanopore was developed inside the silicon cavity followed by fusing large unilamellar vesicles (LUVs) of DPPS (1,2-dipalmitoyl-sn-glycero-3-phosphoserine) and DPPE (1,2-dipalmitoyl-sn-glycero-3-phosphoethanolamine) to produce a solvent-free SLM with giga-ohm (GΩ) sealed impedance. The presence and thickness of SLM on the nanopore chip were confirmed using atomic force spectroscopy. The functionality of SLM with and without ENaC was verified in terms of electrical impedance and capacitance by sweeping the frequency from 0.01 Hz to 100 kHz using electrochemical impedance spectroscopy. The nanopore chip exhibits long-term stability for the lipid bilayer before (144 h) and after (16 h) incorporation of ENaC. Amiloride, an inhibitor of ENaC, was utilized at different concentrations to test the integrity of fused ENaC in the lipid bilayer supported on a single nanopore chip. The developed model presents excellent electrical properties and improved mechanical stability of SLM, making this technology a reliable platform to study ion channel electrophysiology.

Lipid Bilayer Membrane in a Silicon Based Micron Sized Cavity Accessed by Atomic Force Microscopy and Electrochemical Impedance Spectroscopy

Supported lipid bilayers (SLBs) are widely used in biophysical research to probe the functionality of biological membranes and to provide diagnoses in high throughput drug screening. Formation of SLBs at below phase transition temperature (Tm) has applications in nano-medicine research where low temperature profiles are required. Herein, we report the successful production of SLBs at above-as well as below-the Tm of the lipids in an anisotropically etched, silicon-based micro-cavity. The Si-based cavity walls exhibit controlled temperature which assist in the quick and stable formation of lipid bilayer membranes. Fusion of large unilamellar vesicles was monitored in real time in an aqueous environment inside the Si cavity using atomic force microscopy (AFM), and the lateral organization of the lipid molecules was characterized until the formation of the SLBs. The stability of SLBs produced was also characterized by recording the electrical resistance and the capacitance using electrochemical impedance spectroscopy (EIS). Analysis was done in the frequency regime of 10-2-10⁵ Hz at a signal voltage of 100 mV and giga-ohm sealed impedance was obtained continuously over four days. Finally, the cantilever tip in AFM was utilized to estimate the bilayer thickness and to calculate the rupture force at the interface of the tip and the SLB. We anticipate that a silicon-based, micron-sized cavity has the potential to produce highly-stable SLBs below their Tm. The membranes inside the Si cavity could last for several days and allow robust characterization using AFM or EIS. This could be an excellent platform for nanomedicine experiments that require low operating temperatures.

Confinement of a β-barrel protein in nanoperforated free-standing nanomembranes for ion transport

Bioinspired free-standing nanomembranes (FSNMs) for selective ion transport have been tailored by immobilizing the Omp2a β-barrel membrane protein inside nanoperforations created in flexible poly(lactic acid) (PLA) nanomembranes. Perforated PLA FSNMs have been prepared by spin-coating a 99 : 1 PLA : poly(vinyl alcohol) mixture, and through a phase segregation process nanofeatures with dimensions similar to the entire nanomembrane thickness (∼110 nm) were induced. These nanofeatures have subsequently been transformed into nanoperforations (diameter: ∼51 nm) by selective solvent etching. The protein confined inside the nanopores of PLA FSNMs preserves the β-barrel structure and organizes in ovoid aggregates. The transport properties of Na⁺, K⁺, and Ca²⁺ across non-perforated PLA, nanoperforated PLA, and Omp2a-filled nanoperforated PLA have been monitored by measuring the nanomembrane resistance with electrochemical impedance spectroscopy (EIS). The incorporation of nanoperforations enhances the transport of ions across PLA nanomembranes, whereas the functionality of immobilized Omp2a is essential to exhibit effects similar to those observed in biological nanomembranes. Indeed, Omp2a-filled nanoperforated PLA nanomembranes exhibit stronger affinity towards Na⁺ and Ca²⁺ ions than towards K⁺. In summary, this work provides a novel bioinspired strategy to develop mechanically stable and flexible FSNMs with channels for ion transport, which are precisely located inside artificial nanoperforations, thus holding great potential for applications in biofiltration and biosensing.

Formation of lipid bilayer membrane in a poly(dimethylsiloxane) microchip integrated with a stacked polycarbonate membrane support and an on-site nanoinjector

This paper describes a new and facile approach for the formation of pore-spanning bilayer lipid membranes (BLMs) within a poly(dimethylsiloxane) (PDMS) microfluidic device. Commercially, readily available polycarbonate (PC) membranes are employed for the support of BLMs. PC sheets with 5 μm, 2 μm, and 0.4 μm pore diameters, respectively, are thermally bonded into a multilayer-stack, reducing the pore density of 0.4 μm-pore PC by a factor of 200. The BLMs on this support are considerably stable (a mean lifetime: 17 h). This multilayer-stack PC (MSPC) membrane is integrated into the PDMS chip by an epoxy bonding method developed to secure durable bonding under the use of organic solvents. The microchip has a special channel for guiding a micropipette in the proximity of the MSPC support. With this on-site injection technique, tens to hundreds of nanoliters of solutions can be directly dispensed to the support. Incorporating gramicidin ion channels into BLMs on the MSPC support has confirmed the formation of single BLMs, which is based on the observation from current signals of 20 pS conductance that is typical to single channel opening. Based on the bilayer capacitance (1.4 pF), about 15% of through pores across the MSPC membrane are estimated to be covered with BLMs.

Shaped apertures in photoresist films enhance the lifetime and mechanical stability of suspended lipid bilayers

Planar lipid bilayers suspended in apertures provide a controlled environment for ion channel studies. However, short lifetimes and poor mechanical stability of suspended bilayers limit the experimental throughput of bilayer electrophysiology experiments. Although bilayers are more stable in smaller apertures, ion channel incorporation through vesicle fusion with the suspended bilayer becomes increasingly difficult. In an alternative bilayer stabilization approach, we have developed shaped apertures in SU8 photoresist that have tapered sidewalls and a minimum diameter between 60 and 100 μm. Bilayers formed at the thin tip of these shaped apertures, either with the painting or the folding method, display drastically increased lifetimes, typically >20 h, and mechanical stability, being able to withstand extensive perturbation of the buffer solution. Single-channel electrical recordings of the peptide alamethicin and of the proteoliposome-delivered potassium channel KcsA demonstrate channel conductance with low noise, made possible by the small capacitance of the 50 μm thick SU8 septum, which is only thinned around the aperture, and unimpeded proteoliposome fusion, enabled by the large aperture diameter. We anticipate that these shaped apertures with micrometer edge thickness can substantially enhance the throughput of channel characterization by bilayer lipid membrane electrophysiology, especially in combination with automated parallel bilayer platforms.

Engineering lipid bilayer membranes for protein studies

Lipid membranes regulate the flow of nutrients and communication signaling between cells and protect the sub-cellular structures. Recent attempts to fabricate artificial systems using nanostructures that mimic the physiological properties of natural lipid bilayer membranes (LBM) fused with transmembrane proteins have helped demonstrate the importance of temperature, pH, ionic strength, adsorption behavior, conformational reorientation and surface density in cellular membranes which all affect the incorporation of proteins on solid surfaces. Much of this work is performed on artificial templates made of polymer sponges or porous materials based on alumina, mica, and porous silicon (PSi) surfaces. For example, porous silicon materials have high biocompatibility, biodegradability, and photoluminescence, which allow them to be used both as a support structure for lipid bilayers or a template to measure the electrochemical functionality of living cells grown over the surface as in vivo. The variety of these media, coupled with the complex physiological conditions present in living systems, warrant a summary and prospectus detailing which artificial systems provide the most promise for different biological conditions. This study summarizes the use of electrochemical impedance spectroscopy (EIS) data on artificial biological membranes that are closely matched with previously published biological systems using both black lipid membrane and patch clamp techniques.

Nanoparticle size controls aggregation in lamellar nonionic surfactant mesophase

We show that the size of silica nanoparticles influences the nature of their aggregation in an aqueous solution of a relatively hydrophobic nonionic surfactant, C12E4. We present results for dispersions of silica nanoparticles with sizes varying from 8 to 26 nm, in a 75: 25 C12E4/water system, that forms a lamellar phase, Lα, at room temperature. Addition of silica particles does not affect the formation of the Lα phase. Nanoparticles smaller than about 11 nm aggregate irreversibly in the C12E4/water system. However, nanoparticles larger than about 15 nm aggregate in the Lα phase, but are dispersed at temperatures above the Lα order-disorder temperature. Thus, in contrast to the smaller particles, aggregation of silica nanoparticles larger than about 15 nm is reversible with temperature. We use small-angle neutron scattering (SANS) to demonstrate that these results can be explained by the size-dependent wrapping of nanoparticles by surfactant bilayers. Larger particles, above 15 nm in size, are sterically stabilized by the formation of an adsorbed surfactant bilayer. The cost of bilayer bending inhibits adsorption onto the highly curved surfaces of smaller particles, and these "bare" particles aggregate irreversibly.