Artificial cells: unique insights into exocytosis using liposomes and lipid nanotubes

High-throughput optical quality control of lipid multilayers fabricated by dip-pen nanolithography

Surface supported phospholipid multilayers are promising materials for nanotechnology because of their tendency to self-organize, their innate biocompatibility, the possibility to encapsulate other materials within the multilayers, and the ability to control the multilayer thickness between ∼ 2 and 100 nm during fabrication. Dip-pen nanolithography (DPN) is an atomic force microscopy (AFM) based fabrication method that allows high-throughput fabrication and integration of a variety of micro- and nanostructured materials including lipid multilayers, with areal throughputs on the scale of cm(2) min(-1). Although multilayer thickness is a critical feature that determines the functionality of the lipid multilayer structures (for instance as carriers for other materials as well as optical scattering properties), reliable height characterization by AFM is slow (on the order of µm(2) min(-1)) and a bottleneck in the lithographic process. Here we describe a novel optical method to reliably measure the height of fluorescent multilayers with thicknesses above 10 nm, and widths above the optical diffraction limit based on calibrating the fluorescence intensity using one-time AFM height measurements. This allows large surface areas to be rapidly and quantitatively characterized using a standard fluorescence microscope. Importantly, different pattern dimensions (0D dots, 1D lines or 2D squares) require different calibration parameters, indicating that shape influences the optical properties of the structured lipid multilayers. This method has general implications in the systematic and high-throughput optical characterization of nanostructure-function relationships.

Temporal resolution in electrochemical imaging on single PC12 cells using amperometry and voltammetry at microelectrode arrays

Carbon-fiber-microelectrode arrays (MEAs) have been utilized to electrochemically image neurochemical secretion from individual pheochromocytoma (PC12) cells. Dopamine release events were electrochemically monitored from seven different locations on single PC12 cells using alternately constant-potential amperometry and fast-scan cyclic voltammetry (FSCV). Cyclic voltammetry, when compared to amperometry, can provide excellent chemical resolution; however, spatial and temporal resolution are both compromised. The spatial and temporal resolution of these two methods have been quantitatively compared and the differences explained using models of molecular diffusion at the nanogap between the electrode and the cell. A numerical simulation of the molecular flux reveals that the diffusion of dopamine molecules and electrochemical reactions both play important roles in the temporal resolution of electrochemical imaging. The simulation also reveals that the diffusion and electrode potential cause the differences in signal crosstalk between electrodes when comparing amperometry and FSCV.

Electrochemistry in nanoscopic volumes

Exploration of electrochemical properties in ultrasmall volumes is still an emerging area. It is not only of great importance for the fundamental research, but also endowed with practical significance in the area of bioanalysis and medicine. Microelectrodes with superior electrochemical characteristics and versatile configurations are suitable tools for the investigation in confined geometries, and remarkable progress involving both preparation methods and theoretical interpretation has been made during the last few decades. Despite this success, electrochemical studies in nanoscopic volumes are still highly challenging due to the less predictable situations in very limited spatial and temporal domains, as well as difficulty in micromanipulation at the nanoscale. In this mini-review, we will summarize the main strategies for this topic, briefly look through the recent advances, and specifically introduce the design and application of a new kind of on-chip ultrasmall electrochemical cells based on micro- and nanogap electrodes, which are prepared by photolithographic method with volume ranging from femtolitre to attolitre. Finally, the limits of current systems and the future perspectives of this field are discussed.

Release monitoring of single cells on a microfluidic device coupled with fluorescence microscopy and electrochemistry

A method for monitoring the biological exocytotic phenomena on a microfluidic system was proposed. A microfluidic device coupled with functionalities of fluorescence imaging and amperometric detection has been developed to enable the real-time monitoring of the exocytotic events. Exocytotic release of single SH-SY5Y neuroblastoma cells was studied. By staining the cells located on integrated microelectrodes with naphthalene-2,3-dicarboxaldehyde, punctuate fluorescence consistent with localization of neurotransmitters stored in vesicles was obtained. The stimulated exocytotic release was successfully observed at the surface of SH-SY5Y cells without refitting the commercial inverted fluorescence microscope. Spatially and temporally resolved exocytotic events from single cells on a microfluidic device were visualized in real time using fluorescence microscopy and were amperometrically recorded by the electrochemical system simultaneously. This coupled technique is simple and is hoped to provide new insights into the mechanisms responsible for the kinetics of exocytosis.

Liposome and lipid bilayer arrays towards biosensing applications

Sensitive and selective biosensors for high-throughput screening are having an increasing impact in modern medical care. The establishment of robust protein biosensing platforms however remains challenging, especially when membrane proteins are involved. Although this type of proteins is of enormous relevance since they are considered in >60% of the pharmaceutical drug targets, their fragile nature (i.e., the requirement to preserve their natural lipid environment to avoid denaturation and loss of function) puts strong additional prerequisites onto a successful biochip. In this review, the leading approaches to create lipid membrane-based arrays towards the creation of membrane protein biosensing platforms are described. Liposomes assembled in micro- and nanoarrays and the successful set-ups containing functional membrane proteins, as well as the use of liposomes in networks, are discussed in the first part. Then, the complementary approaches to create cell-mimicking supported membrane patches on a substrate in an array format will be addressed. Finally, the progress in assembling free-standing (functional) lipid bilayers over nanopore arrays for ion channel sensing will be reported. This review illustrates the rapid pace by which advances are being made towards the creation of a heterogeneous biochip for the high-throughput screening of membrane proteins for diagnostics, drug screening, or drug discovery purposes.

Neutrophil secretion induced by an intracellular Ca2+ rise and followed by whole-cell patch-clamp recordings occurs without any selective mobilization of different granule populations

We have investigated calcium-induced secretion in human neutrophils, using a whole-cell patch-clamp technique. Mobilization of subcellular granules to the cell membrane was followed as the change in membrane capacitance (ΔC_m). Both the magnitude and the kinetics of the response differed between low and high concentrations of Ca²⁺. A sustained secretion following a short lag phase was induced by high concentrations of Ca²⁺ (100 μM and higher). A stable plateau was reached after 5–7 minutes at ΔC_m values corresponding to values expected after all specific as well as azurophil granules have been mobilized. Capacitance values of the same magnitude could be obtained also at lower Ca²⁺ concentrations, but typically no stable plateau was reached within the measuring time. In contrast to previous studies, we were unable to detect any pattern of secretion corresponding to a distinct submaximal response or selective mobilization of granule subsets specified by their Ca²⁺-sensitivity.

Steady-state electrochemical determination of lipidic nanotube diameter utilizing an artificial cell model

By exploiting the capabilities of steady-state electrochemical measurements, we have measured the inner diameter of a lipid nanotube using Fick's first law of diffusion in conjunction with an imposed linear concentration gradient of electroactive molecules over the length of the nanotube. Fick's law has been used in this way to provide a direct relationship between the nanotube diameter and the measurable experimental parameters Deltai (change in current) and nanotube length. Catechol was used to determine the Deltai attributed to its flux out of the nanotube. Comparing the nanotube diameter as a function of nanotube length revealed that membrane elastic energy was playing an important role in determining the size of the nanotube and was different when the tube was connected to either end of two vesicles or to a vesicle on one end and a pipet tip on the other. We assume that repulsive interaction between neck regions can be used to explain the trends observed. This theoretical approach based on elastic energy considerations provides a qualitative description consistent with experimental data.

Reconstruction of aperture functions during full fusion in vesicular exocytosis of neurotransmitters

Individual vesicular exocytosis of adrenaline by dense core vesicles in chromaffin cells is considered here as a paradigm of many situations encountered in biology, nanosciences and drug delivery in which a spherical container releases in the external environment through gradual uncovering of its surface. A procedure for extracting the aperture (opening) function of a biological vesicle fusing with a cell membrane from the released molecular flux of neurotransmitter as monitored by amperometry has been devised based on semi-analytical expressions derived in a former work [C. Amatore, A. I. Oleinick, I. Svir, ChemPhysChem 2009, 10, DOI: 10.1002/cphc.200900646]. This precise analysis shows that in the absence of direct information about the radius of the vesicle or about the concentration of the adrenaline cation stored by the vesicle matrix, current spikes do not contain enough information to determine the maximum aperture angle. Yet, a statistical analysis establishes that this maximum aperture angle is most probably less than a few tens of degrees, which suggests that full fusion is a very improbable event.

Thermodynamics and dynamics of the formation of spherical lipid vesicles

We propose a free energy expression accounting for the formation of spherical vesicles from planar lipid membranes and derive a Fokker-Planck equation for the probability distribution describing the dynamics of vesicle formation. We find that formation may occur as an activated process for small membranes and as a transport process for sufficiently large membranes. We give explicit expressions for the transition rates and the characteristic time of vesicle formation in terms of the relevant physical parameters.

Hybrid capillary-microfluidic device for the separation, lysis, and electrochemical detection of vesicles

The primary method for neuronal communication involves the extracellular release of small molecules that are packaged in secretory vesicles. We have developed a platform to separate, lyse, and electrochemically measure the contents of single vesicles using a hybrid capillary-microfluidic device. This device incorporates a sheath-flow design at the outlet of the capillary for chemical lysis of vesicles and subsequent electrochemical detection. The effect of sheath-flow on analyte dispersion was characterized using confocal fluorescence microscopy and electrochemical detection. At increased flow rates, dispersion was minimized, leading to higher separation efficiencies but lower detected amounts. Large unilamellar vesicles (diameter approximately 200 nm), a model for secretory vesicles, were prepared by extrusion and loaded with an electroactive molecule. They were then separated and detected using the hybrid capillary-microfluidic device. Determination of size from internalized analyte concentration provides a method to characterize the liposomal suspension. These results were compared to an orthogonal size measurement using dynamic light scattering to validate the detection platform.

Monitoring of vesicular exocytosis from single cells using micrometer and nanometer-sized electrochemical sensors

Communication between cells by release of specific chemical messengers via exocytosis plays crucial roles in biological process. Electrochemical detection based on ultramicroelectrodes (UMEs) has become one of the most powerful techniques in real-time monitoring of an extremely small number of released molecules during very short time scales, owing to its intrinsic advantages such as fast response, excellent sensitivity, and high spatiotemporal resolution. Great successes have been achieved in the use of UME methods to obtain quantitative and kinetic information about released chemical messengers and to reveal the molecular mechanism in vesicular exocytosis. In this paper, we review recent developments in monitoring exocytosis by use of UMEs-electrochemical-based techniques including electrochemical detection using micrometer and nanometer-sized sensors, scanning electrochemical microscopy (SECM), and UMEs implemented in lab-on-a-chip (LOC) microsystems. These advances are of great significance in obtaining a better understanding of vesicular exocytosis and chemical communications between cells, and will facilitate developments in many fields, including analytical chemistry, biological science, and medicine. Furthermore, future developments in electrochemical probing of exocytosis are also proposed.

Macromolecular crowding improves polymer encapsulation within giant lipid vesicles

We report the effect of macromolecular crowding on encapsulation efficiency of fluorescently labeled poly(ethylene glycol) (PEG) and dextran polymers within individual giant lipid vesicles (GVs). Low concentrations of the fluorescently labeled polymers (82 nM to 186 pM) were mixed with varying concentrations of nonfluorescent polymers that served as crowding agents during vesicle formation by gentle hydration. Encapsulation efficiency of the fluorescently labeled polymers in individual GVs (EEind) was determined via confocal fluorescence microscopy. EEind for high molecular weight polymers (e.g., fluorescein isothiocyanate (FITC)-dextran 500 and 2000 kDa) increased substantially in the presence of several weight percent unlabeled PEG or dextran. For example, when 0.24 microM FITC dextran 500 kDa was encapsulated, addition of 3% PEG 8 kDa improved the mean concentration in the GVs from 0.14 microM (+/-50%) to 0.24 microM (+/-12%). Light scattering data indicate reduced hydrodynamic radii for polymers as a function of increasing polymer concentration, suggesting that the improvements in EEind result from polymer condensation due to macromolecular crowding. Polymeric cosolutes did not significantly impact EEind for lower molecular weight polymers (e.g., Alexa Fluor 488-PEG 20 kDa), which already encapsulated efficiently (EEind to approximately 1). However, for both the higher and lower molecular weight labeled polymers, cosolutes led to improved uniformity in EEind for vesicles within a batch. Methods for improving the value and homogeneity of EEind for polymeric solutes in lipid vesicles are important in a variety of applications, including the use of vesicles as microreactors and as vehicles for drug delivery.

Controllable biomolecule release from self-assembled organic nanotubes with asymmetric surfaces: pH and temperature dependence

The release behavior of fluorescent dyes, oligo DNAs and spherical proteins from self-assembled organic nanotubes having 7-9 nm inner diameters has been studied in terms of novel nanocontainers with high-axial ratios. Both much smaller inner diameters and asymmetric inner and outer surfaces are characteristic of the nanotubes. The acid-dissociation constant (pKa) of the amino groups located at the inner surface and the thermal phase transition temperature (Tg-l) of the nanotube were evaluated based on the pH titration and variable-temperature circular dichroism (CD) spectroscopic experiments, respectively. Each guest was slowly released from both open ends of the nanotube under weak alkaline conditions (pH 8.5), as a result of the decrease in electrostatic attraction between the inner surface and the guests. Elevated temperatures above the obtained Tg-l converted the monolayer membrane of the nanotube from a solid state to a fluid one, promoting the remarkably fast release of the guests. The unique release properties of the nanotube as a nanocontainer with two terminal open ends were compared with those of liposomes that posses a closed hollow space covered with fluid bilayer membranes.

Shape optimization in lipid nanotube networks

Starting from a high surface free-energy state, lipid nanotube networks are capable to self-organize into tree-like structures with particular geometrical features. In this work we analyze the process of self-organization in such networks, and report a strong similarity to the Euclidian Steiner Tree Problem (ESTP). ESTP is a well-known NP-hard optimization problem of finding a network connecting a given set of terminal points on a plane, allowing addition of auxiliary points, with the overall objective to minimize the total network length. The present study shows that aggregate lipid structures self-organize into geometries that correspond to locally optimal solutions to such problems.

Short-chain alcohols promote accelerated membrane distention in a dynamic liposome model of exocytosis

We have used amperometric measurements in a model system consisting of two liposomes connected with a membrane nanotube to monitor catechol release during artificial exocytosis and thereby to elucidate the effect of small-chain alcohols on this dynamic membrane process. To determine the rate of membrane shape change, catechol release during membrane distention was monitored amperometrically, and the presence of alcohols in the buffer was shown to accelerate the membrane distention process in a concentration-dependent manner. Compression isotherms for the same lipid composition in the absence and presence of ethanol and 1-propanol were measured to determine how these short-chain alcohols affect the lipid packing in monolayers. The isotherms show a marked decrease in lipid packing density that is dependent on the particular alcohol and its concentration. Comparison of the electrochemical and isotherm results suggests a correlation between decreasing lipid packing density and increasing rates of membrane shape change.

The PC12 cell as model for neurosecretion

This review attempts to touch on the history and application of amperometry at PC12 cells for fundamental investigation into the exocytosis process. PC12 cells have been widely used as a model for neural differentiation and as such they have been used to examine the effects of differentiation on exocytotic release and specifically release at varicosities. In addition, dexamethasone-differentiated cells have been shown to have an increased number of releasable vesicles with increased quantal size, thereby allowing for an even broader range of applications including neuropharmacological and neurotoxicological studies. PC12 cells exhibiting large numbers of events have two distinct pools of vesicles, one about twice the quantal size of the other and each about half the total releasable vesicles. As will be outlined in this review, these cells have served as an extremely useful model of exocytosis in the study of the latency of stimulation-release coupling, the role of exocytotic proteins in regulation of release, effect of drugs on quantal size, autoreceptors, fusion pore biophysics, environmental factors, health and disease. As PC12 cells have some advantages over other models for neurosecretion, including chromaffin cells, it is more than likely that in the following decade PC12 cells will continue to serve as a model to study exocytosis.

Nanoelectrochemistry of mammalian cells

There is a significant current interest in development of new techniques for direct characterization of the intracellular redox state and high-resolution imaging of living cells. We used nanometer-sized amperometric probes in combination with the scanning electrochemical microscope (SECM) to carry out spatially resolved electrochemical experiments in cultured human breast cells. With the tip radius approximately 1,000 times smaller than that of a cell, an electrochemical probe can penetrate a cell and travel inside it without apparent damage to the membrane. The data demonstrate the possibility of measuring the rate of transmembrane charge transport and membrane potential and probing redox properties at the subcellular level. The same experimental setup was used for nanoscale electrochemical imaging of the cell surface.

Liposomes: technologies and analytical applications

Liposomes are structurally and functionally some of the most versatile supramolecular assemblies in existence. Since the beginning of active research on lipid vesicles in 1965, the field has progressed enormously and applications are well established in several areas, such as drug and gene delivery. In the analytical sciences, liposomes serve a dual purpose: Either they are analytes, typically in quality-assessment procedures of liposome preparations, or they are functional components in a variety of new analytical systems. Liposome immunoassays, for example, benefit greatly from the amplification provided by encapsulated markers, and nanotube-interconnected liposome networks have emerged as ultrasmall-scale analytical devices. This review provides information about new developments in some of the most actively researched liposome-related topics.

Actin is not required for nanotubular protrusions of primary astrocytes grown on metal nano-lawn

We used sub-micron metal rod decorated surfaces, 'nano-lawn' structures, as a substrate to study cell-to-cell and cell-to-surface interactions of primary murine astrocytes. These cells form thin membranous tubes with diameters of less than 100 nm and a length of several microns, which make contact to neighboring cells and the substrate during differentiation. While membrane protrusions grow on top of the nano-lawn pillars, nuclei sink to the bottom of the substrate. We observed gondola-like structures along those tubes, suggestive of their function as transport vehicles. Elements of the cytoskeleton such as actin fibers are commonly believed to be essential for triggering the onset and growth of tubular membrane protrusions. A rope-pulling mechanism along actin fibers has recently been proposed to account for the transport or exchange of cellular material between cells. We present evidence for a complementary mechanism that promotes growth and stabilization of the observed tubular protrusions of cell membranes. This mechanism does not require active involvement of actin fibers as the formation of membrane protrusions could not be prevented by suppressing polymerization of actin by latrunculin B. Also theoretically, actin fibers are not essential for the growing and stability of nanotubes since curvature-driven self-assembly of interacting anisotropic raft elements is sufficient for the spontaneous formation of thin nano-tubular membrane protrusions.

The nature and efficiency of neurotransmitter exocytosis also depend on physicochemical parameters

Exocytosis is an important biological process used by cells to deliver messengers or effectors to target cells with high spatial, quantitative, and kinetic precision. This process occurs by interaction and fusion of vesicles containing the (bio)chemical information with the cell membrane to release their contents into the surrounding medium. Because of its importance for life, this mechanism underlies many biological controlling factors, including different families of proteins and enzymes. Tremendous efforts have been made over the last decade toward their determination. However, in parallel, many studies have also shown that the physical and chemical characteristics of the exocytosis actors (vesicle, membrane, and extracellular medium) could directly affect the quantitative or kinetic features of secretion. The major pieces evidence for this influence, which have been reported in the literature, are reviewed herein. It demonstrates undoubtedly that pure biological aspects cannot be segregated from the physicochemical context of living mechanisms.

Temperature-dependent differences between readily releasable and reserve pool vesicles in chromaffin cells

Statistical differences between amperometric traces recorded from chromaffin cells using K(+) and Ba(2+) secretagogues support the assertion that readily releasable pool (RRP) and reserve pool (RP) vesicles can be probed with pool-specific secretagogues. Release from the RRP was evoked by K(+) while release from the RP was evoked by Ba(2+). Similar temperature-dependent changes in spike area and half-width for both pools suggest that the content of RRP and RP vesicles is similar and packaged in the same way. Differences between the vesicle pools were revealed in the temperature dependence of spike frequency. While the burst spike frequency of the RRP, which is comprised of pre-docked and primed vesicles, increased 2.8% per degrees C, the RP spike frequency increased 12% per degrees C. This difference is attributed to a temperature-dependent mobilization of the RP. Furthermore, the RP exhibited more foot events at room temperature than the RRP but this difference was not apparent at 37 degrees C. This trend suggests that RP vesicle membranes have a compromised surface tension compared to RRP vesicles. Collectively, the changes of release characteristics with temperature reveal distinctions between the RRP and the RP.

Dynamic microcompartmentalization of giant unilamellar vesicles by sol-gel transition and temperature induced shrinking/swelling of poly(N-isopropyl acrylamide)

Giant unilamellar vesicles (GUVs) were microinjected with aqueous solutions of poly(-isopropyl acrylamide) (PNIPAAm). Temperature-dependent sol-gel phase transitions of the solutions, followed by shrinking and swelling of the resulting hydrogel, were studied in the presence of a variety of co-solutes within the GUV. Reversible formation of a dense, spherical hydrogel structure (compartment) was observed in all cases with defined shrinking/swelling behaviour at temperatures above the lower critical solution temperatures (LCSTs). Nanotube-mediated merging of two vesicles with thus formed compartments resulted in a single GUV with two internalized hydrogel structures. As an application example, we demonstrate how fluorescent nanoparticles can be immobilized in such gel structures.

Nanostructure-mediated drug delivery

Nanotechnology is expected to have an impact on all industries including semiconductors, manufacturing, and biotechnology. Tools that provide the capability to characterize and manipulate materials at the nanoscale level further elucidate nanoscale phenomena and equip researchers and developers with the ability to fabricate novel materials and structures. One of the most promising societal impacts of nanotechnology is in the area of nanomedicine. Personalized health care, rational drug design, and targeted drug delivery are some of the benefits of a nanomedicine-based approach to therapy. This review will focus on the development of nanoscale drug delivery mechanisms. Nanostructured drug carriers allow for the delivery of not only small-molecule drugs but also the delivery of nucleic acids and proteins. Delivery of these molecules to specific areas within the body can be achieved, which will reduce systemic side effects and allow for more efficient use of the drug.

Solid supported lipid bilayers: From biophysical studies to sensor design

The lipid bilayer is one of the most eloquent and important self-assembled structures in nature. It not only provides a protective container for cells and sub-cellular compartments, but also hosts much of the machinery for cellular communication and transport across the cell membrane. Solid supported lipid bilayers provide an excellent model system for studying the surface chemistry of the cell. Moreover, they are accessible to a wide variety of surface-specific analytical techniques. This makes it possible to investigate processes such as cell signaling, ligand-receptor interactions, enzymatic reactions occurring at the cell surface, as well as pathogen attack. In this review, the following membrane systems are discussed: black lipid membranes, solid supported lipid bilayers, hybrid lipid bilayers, and polymer cushioned lipid bilayers. Examples of how supported lipid membrane technology is interfaced with array based systems by photolithographic patterning, spatial addressing, microcontact printing, and microfluidic patterning are explored. Also, the use of supported lipid bilayers in microfluidic devices for the development of lab-on-a-chip based platforms is examined. Finally, the utility of lipid bilayers in nanotechnology and future directions in this area are discussed.

Probing Cellular Chemistry in Biological Systems with Microelectrodes

Over the past 20 years, the technological impediments to fabricating electrodes of micrometer dimensions have been largely overcome. These small electrodes can be readily applied to probe chemical events at the surface of tissues or individual biological cells; they can even be used to monitor concentration changes within intact animals. These measurements can be made on rapid time scales and with minimal perturbation of the system under study. Several recent applications have provided important insights into chemical processes at cells and in tissues. Examples include molecular flux measurements at the surface of single cells and through skin-which can offer insights into oxidative stress, exocytosis, and drug delivery-and real-time brain neurotransmitter monitoring in living rats, which reveals correlations between behavior and molecular events in the brain. Such findings can promote interdisciplinary collaborations and may lead to a broader understanding of the chemical aspects of biology.

Coupling bending and shear effects on liposome deformation

Cell membrane deformation induced by external mechanical stimuli has been studied extensively over the past three decades. The present study focuses on the coupling of in-plane shear H and out-of-plane bending B of liposome membrane and its influences on the deformation of a single vesicle subjected to (i) external compressive load via two parallel platens and (ii) contact forces caused by a rigid substrate. Our results show that the increase of membrane resultant stress in both loading configurations causes the liposome to become more rigid and the degree of vesicle deformation decreases when the in-plane shearing effect is dominant. A theoretical approach is developed to facilitate cell membrane characterization under different biomechanical stimuli.

Single-file electrophoretic transport and counting of individual DNA molecules in surfactant nanotubes

We demonstrate a complete nanotube electrophoresis system (nanotube radii in the range of 50 to 150 nm) based on lipid membranes, comprising DNA injection, single-molecule transport, and single-molecule detection. Using gel-capped electrodes, electrophoretic single-file transport of fluorescently labeled dsDNA molecules is observed inside nanotubes. The strong confinement to a channel of molecular dimensions ensures a detection efficiency close to unity and identification of DNA size from its linear relation to the integrated peak intensity. In addition to constituting a nanotechnological device for identification and quantification of single macromolecules or biopolymers, this system provides a method to study their conformational dynamics, reaction kinetics, and transport in cell-like environments.

Monitoring Dopamine Release from Single Living Vesicles with Nanoelectrodes

Carbon fiber nanoelectrodes (tip diameter = ca. 100 nm) have been first used to monitor real-time dopamine release from single living vesicles of single rat pheochromocytoma (PC12) cells. The experiments show that active and inactive release sites exist on the surface of cells, and the spatial distributions have been differentiated even in the same active release zone. It is first demonstrated that multiple vesicles can sequentially release dopamine at the same site of the cell surface, which possibly plays the main role in the dopamine release from PC12 cells.

Characterization of interaction between cationic lipid-oligonucleotide complexes and cellular membrane lipids using confocal imaging and fluorescence correlation spectroscopy

Complexes formed by cationic liposomes and single-strand oligodeoxynucleotides (CL-ODN) are promising delivery systems for antisense therapy. ODN release from the complexes is an essential step for inhibiting activity of antisense drugs. We applied fluorescence correlation spectroscopy and confocal laser scanning microscopy to monitor CL-ODN complex interaction with membrane lipids leading to ODN release. To model cellular membranes we used giant unilamellar vesicles and investigated the transport of Cy-5-labeled ODNs across DiO-labeled membranes. For the first time, we directly observed that ODN molecules are transferred across the lipid bilayers and are kept inside the giant unilamellar vesicles after release from the carriers. ODN dissociation from the carrier was assessed by comparing diffusion constants of CL-ODN complexes and ODNs before complexation and after release. Freely diffusing Cy-5-labeled ODN (16-nt) has diffusion constant D(ODN) = 1.3 +/- 0.1 x 10(-6) cm2/s. Fluorescence correlation spectroscopy curves for CL-ODN complexes were fitted with two components, which both have significantly slower diffusion in the range of D(CL-ODN) = approximately 1.5 x 10(-8) cm2/s. Released ODN has the mean diffusion constant D = 1.1 +/- 0.2 x 10(-6) cm2/s, which signifies that ODN is dissociated from cationic lipids. In contrast to earlier studies, we report that phosphatidylethanolamine can trigger ODN release from the carrier in the full absence of anionic phosphatidylserine in the target membrane and that phosphatidylethanolamine-mediated release is as extensive as in the case of phosphatidylserine. The presented methodology provides an effective tool for probing a delivery potential of newly created lipid formulations of CL-ODN complexes for optimal design of carriers.

BIOMIMETIC NANOSCALE REACTORS AND NETWORKS

Methods based on self-assembly, self-organization, and forced shape transformations to form synthetic or semisynthetic enclosed lipid bilayer structures with several properties similar to biological nanocompartments are reviewed. The procedures offer unconventional micro- and nanofabrication routes to yield complex soft-matter devices for a variety of applications for example, in physical chemistry and nanotechnology. In particular, we describe novel micromanipulation methods for producing fluid-state lipid bilayer networks of nanotubes and surface-immobilized vesicles with controlled geometry, topology, membrane composition, and interior contents. Mass transport in nanotubes and materials exchange, for example, between conjugated containers, can be controlled by creating a surface tension gradient that gives rise to a moving boundary or by induced shape transformations. The network devices can operate with extremely small volume elements and low mass, to the limit of single molecules and particles at a length scale where a continuum mechanics approximation may break down. Thus, we also describe some concepts of anomalous fluctuation-dominated kinetics and anomalous diffusive behaviours, including hindered transport, as they might become important in studying chemistry and transport phenomena in these confined systems. The networks are suitable for initiating and controlling chemical reactions in confined biomimetic compartments for rationalizing, for example, enzyme behaviors, as well as for applications in nanofluidics, bioanalytical devices, and to construct computational and complex sensor systems with operations building on chemical kinetics, coupled reactions and controlled mass transport.

Amperometric Detection of Exocytosis in an Artificial Synapse

A liposome model of exocytosis has been used to examine the nanometer environment of the space between the electrode and the membrane. First, this has been used to test models of coulometric efficiency for different size vesicles with 5- and 33-microm electrodes. The resulting model has a best fit that suggests that the liquid space is approximately 300 nm across the gap. Given this dimension, the volume of the electrode-membrane space is not large enough to accommodate the volume of larger vesicles in cells such as the mast cells of the beige mouse. Second, the model suggests that flow of solution from the exocytosis event is more important than diffusion. Flow from the finite vesicle volume past the electrode leads to less charge passed. Third, and finally, this system can be used to model transport in the synapse and so it is possible to examine the idea that transmitter flows in addition to diffusing from the synapse. This model should be useful in understanding and quantifying the escape of transmitter from synapses in vivo.

A nanofluidic switching device

We present a nanofluidic switching function which allows directed transport in lipid bilayer nanotubes ( approximately 100 nm in diameter) between any two or several nanotube-connected vesicle containers (5-10 mum radius) in a large network. Liquid is transported in the form of small vesicular structures (10-15-10-18 L), and the transport direction is controlled by a two-point perturbation technique, where the surface membrane tension is decreased in one container while it is increased in another container. When reaching a target container, the transported vesicular structures could also inject their material into it.