Resistive-pulse detection of multilamellar liposomes

Simultaneous Multipass Resistive-Pulse Sensing and Fluorescence Imaging of Liposomes

Simultaneous multipass resistive-pulse sensing and fluorescence imaging have been used to correlate the size and fluorescence intensity of individual E. coli lipid liposomes composed of E. coli polar lipid extracts labeled with membrane-bound 3,3-dioctadecyloxacarbocyanine (DiO) fluorescent molecules. Here, a nanopipet serves as a waveguide to direct excitation light to the resistive-pulse sensing zone at the end of the nanopipet tip. Individual DiO-labeled liposomes (>50 nm radius) were multipassed back and forth through the orifices of glass nanopipets' 110-150 nm radius via potential switching to obtain subnanometer sizing precision, while recording the fluorescence intensity of the membrane-bound DiO molecules. Fluorescence was measured as a function of liposome radius and found to be approximately proportional to the total membrane surface area. The observed relationship between liposome size and fluorescence intensity suggests that multivesicle liposomes emit greater fluorescence compared to unilamellar liposomes, consistent with all lipid membranes of the multivesicle liposomes containing DiO. Fluorescent and nonfluorescent liposomes are readily distinguished from each other in the same solution using simultaneous multipass resistive-pulse sensing and fluorescence imaging. A fluorescence "dead zone" of ∼1 μm thickness just outside of the nanopipet orifice was observed during resistive-pulse sensing, resulting in "on/off" fluorescent behavior during liposome multipassing.

Elucidating the Shape of Current Transients in Electrochemical Resistive-Pulse Sensing of Single Liposomes

Electrochemical resistive-pulse (ERP) sensing with conductive carbon nanopipettes (CNPs) has recently been developed and employed for the detection of single liposomes and biological vesicles, and for the analysis of redox molecules contained in such vesicles. However, the origins of different shapes of current transients produced by the translocation of single vesicles through the CNP remain poorly understood. Herein, we report extensive finite-element simulations of both portions of an ERP transient, the current blockage by a vesicle approaching and passing through the pipet orifice and the faradaic current spike due to oxidation/reduction of redox species released from a vesicle on the carbon surface, for different values of parameters defining the geometry and dynamics of the vesicle/CNP system. The effects of the pipet geometry, surface charge, transport, vesicle trajectory, and collision location on the shape of current transients are investigated. The possibility of quantitative analysis of experimental ERP transients produced by translocations of liposomes and extracellular vesicles by fitting them to simulated curves is demonstrated. The developed theory can enable a more reliable interpretation of complicated ERP signals and characterization of the size and contents of single biological and artificial vesicles.

Combining dynamic Monte Carlo with machine learning to study nanoparticle translocation

Resistive pulse sensing (RPS) measurements of nanoparticle translocation have the ability to provide information on single-particle level characteristics, such as diameter or mobility, as well as ensemble averages. However, interpreting these measurements is complex and requires an understanding of nanoparticle dynamics in confined spaces as well as the ways in which nanoparticles disrupt ion transport while inside a nanopore. Here, we combine Dynamic Monte Carlo (DMC) simulations with Machine Learning (ML) and Poisson-Nernst-Planck calculations to simultaneously simulate nanoparticle dynamics and ion transport during hundreds of independent particle translocations as a function of nanoparticle size, electrophoretic mobility, and nanopore length. The use of DMC simulations allowed us to explicitly investigate the effects of Brownian motion and nanoparticle/nanopore characteristics on the amplitude and duration of translocation signals. Simulation results were verified with experimental RPS measurements and found to be in quantitative agreement.

Electrochemistry under confinement

Although the term 'confinement' regularly appears in electrochemical literature, elevated by continuous progression in the research of nanomaterials and nanostructures, up until today the various aspects of confinement considered in electrochemistry are rather scattered individual contributions outside the established disciplines in this field. Thanks to a number of highly original publications and the growing appreciation of confinement as an overarching link between different exciting new research strategies, 'electrochemistry under confinement' is the process of forming a research discipline of its own. To aid the development a coherent terminology and joint basic concepts, as crucial factors for this transformation, this review provides an overview on the different effects on electrochemical processes known to date that can be caused by confinement. It also suggests where boundaries to other effects, such as nano-effects could be drawn. To conceptualize the vast amount of research activities revolving around the main concepts of confinement, we define six types of confinement and select two of them to discuss the state of the art and anticipated future developments in more detail. The first type concerns nanochannel environments and their applications for electrodeposition and for electrochemical sensing. The second type covers the rather newly emerging field of colloidal single entity confinement in electrochemistry. In these contexts, we will for instance address the influence of confinement on the mass transport and electric field distributions and will link the associated changes in local species concentration or in the local driving force to altered reaction kinetics and product selectivity. Highlighting pioneering works and exciting recent developments, this educational review does not only aim at surveying and categorizing the state-of-the-art, but seeks to specifically point out future perspectives in the field of confinement-controlled electrochemistry.

Development of an optical flow through detector for bubbles, crystals and particles in oils

The characterisation of bubbles or particles in an oil poses some unique challenges. In contrast to water solutions, the use of electrochemical detection approaches is more difficult in an oil. However, optical sensing systems have considerable potential in this area. Here we use a flow through channel approach and monitor the light propagation through this structure in an optical transmission sensor arrangement (OTS). This simple approach is demonstrated to be useful at detecting bubbles produced in the oil as a result of cavitation induced by high intensity ultrasound (HIU). The optical technique is shown to have an analytical basis. Bubble detection from an operating HIU source is shown to depend on position of the sensor with respect to the source. Critically, the bubble population can be followed for extended time periods after the ultrasonic source has been terminated. The detection of crystals is also demonstrated. Hence, this technique is ideal for the study of the effects of HIU on oils as they crystallise over extended time periods.

Recent Progress in Quantitatively Monitoring Vesicular Neurotransmitter Release and Storage With Micro/Nanoelectrodes

Exocytosis is one of the essential steps for chemical signal transmission between neurons. In this process, vesicles dock and fuse with the plasma membrane and release the stored neurotransmitters through fusion pores into the extracellular space, and all of these steps are governed with various molecules, such as proteins, ions, and even lipids. Quantitatively monitoring vesicular neurotransmitter release in exocytosis and initial neurotransmitter storage in individual vesicles is significant for the study of chemical signal transmission of the central nervous system (CNS) and neurological diseases. Electrochemistry with micro/nanoelectrodes exhibits great spatial-temporal resolution and high sensitivity. It can be used to examine the exocytotic kinetics from the aspect of neurotransmitters and quantify the neurotransmitter storage in individual vesicles. In this review, we first introduce the recent advances of single-cell amperometry (SCA) and the nanoscale interface between two immiscible electrolyte solutions (nanoITIES), which can monitor the quantity and release the kinetics of electrochemically and non-electrochemically active neurotransmitters, respectively. Then, the development and application of the vesicle impact electrochemical cytometry (VIEC) and intracellular vesicle impact electrochemical cytometry (IVIEC) and their combination with other advanced techniques can further explain the mechanism of neurotransmitter storage in vesicles before exocytosis. It has been proved that these electrochemical techniques have great potential in the field of neuroscience.

Machine Learning to Improve the Sensing of Biomolecules by Conical Track-Etched Nanopore

Single nanopore is a powerful platform to detect, discriminate and identify biomacromolecules. Among the different devices, the conical nanopores obtained by the track-etched technique on a polymer film are stable and easy to functionalize. However, these advantages are hampered by their high aspect ratio that avoids the discrimination of similar samples. Using machine learning, we demonstrate an improved resolution so that it can identify short single- and double-stranded DNA (10- and 40-mers). We have characterized each current blockade event by the relative intensity, dwell time, surface area and both the right and left slope. We show an overlap of the relative current blockade amplitudes and dwell time distributions that prevents their identification. We define the different parameters that characterize the events as features and the type of DNA sample as the target. By applying support-vector machines to discriminate each sample, we show accuracy between 50% and 72% by using two features that distinctly classify the data points. Finally, we achieved an increased accuracy (up to 82%) when five features were implemented.

Use of microaspiration to study the mechanical properties of polymer gel microparticles

The mechanical properties of polyacrylamide (PA) and polydimethylsiloxane (PDMS) microparticle populations have been measured using microaspiration, a recently developed experimental technique. Microaspiration is an augmented version of micropipette aspiration, in which optical microscopy data are obtained as individual soft particles pass through the tip of a micropipette. During microaspiration, the ion current passing through the pipette tip is also measured, and the synchronised optical and current data streams are used to study and quantify mechanical properties. Ion current signatures for the poroelastic PA particles were qualitatively different from those of the viscoelastic PDMS particles. For PA particles the current gradually reduced during each aspiration event, whereas for PDMS particles the current trace resembled a negative top hat function. For PA particles it was found that the maximum change in current during aspiration (ΔIh) increased with particle size. By considering the initial elastic response, a mean effective shear modulus (G') of 6.6 ± 0.2 kPa was found for aspiration of 115 PA particles of ∼10-20 μm diameter. Using a viscoelastic model to describe flow into the pipette, a mean initial effective elastic modulus (E0') of 3.5 ± 1.7 MPa was found for aspiration of 17 PDMS particles of ∼ 9-11 μm diameter. These moduli are consistent with previously reported literature values, providing initial validation of the microaspiration method.

An Analytical Differential Resistance Pulse System Relying on a Time Shift Signal Analysis-Applications in Coulter Counting

Improving the sensitivity and ultimately the range of particle sizes that can be detected with a single pore extends the versatility of the Coulter counting technique. Here, to enable a pore to have greater sensitivity, we have developed and tested a novel differential resistive pulse sensing (DiS) system for sizing particles. To do this, the response was generated through a time shift approach utilizing a "self-servoing regime" to enable the final signal to operate with a zero background in the absence of particle translocation. The detection and characterization of a series of polystyrene particles, forced to translocate through a cylindrical glass microchannel (GMC) by a suitable static pressure difference using this approach, is demonstrated. An analytical response, which scales with the size of the particles employed, was verified. Parasitic capacitive effects are discussed; however, translocations on the millisecond time scale can be detected with high sensitivity and accuracy using the approach described.

Particle detection on microfluidic chips by differential resistive pulse sensing (RPS) method

The resistive pulse sensing (RPS) method has been widely used for characterization of particles, cells, and biomolecules due to its merits of high sensitivity and resolution. This paper investigates working parameters involved in detecting submicron and micron-sized particles by the differential RPS method on microfluidic chips. Effects of particle-to-sensor size ratio, ionic concentration and pH of the electrolyte solution, and applied electric field are studied systematically by using polystyrene particles with a size range from 140 nm to 5 µm. The results show that both the amplitude and the signal-to-noise ratio (SNR) of the RPS signals increase with the particle-to-sensor size ratio as well as the ionic concentration of the electrolyte media. The amplitude of the RPS signals also increases with increasing applied voltage, while the SNR experiences an upslope at low voltages and a decline under the condition of high voltages. pH has little effect on the background noise of the differential RPS signals but reduces the amplitude of the RPS signals at high pH. Grouping of RPS signals is considered to be caused by interactions between the sensor walls and the particles. Nanoparticle detection by the differential RPS method can be enhanced by optimizing these working parameters.

Collision and Coalescence of Single Attoliter Oil Droplets on a Pipet Nanopore

We describe the use of a quartz pipet nanopore to study the collision and coalescence of individual emulsion oil droplets and their subsequent nanopore translocation. Collision and coalescence of single toluene droplets at a nanopore orifice are driven primarily by electroosmosis and electrophoresis and lead to the fast growth of a trapped oil droplet. This results in a stepwise current response due to the coalesced oil droplet increasing its volume and its ability to partially block the nanopore's ionic current, allowing us to use the resistive-pulse method to resolve single droplet collisions. Further growth of the trapped oil droplet leads to a complete blockage of the nanopore and a nearly 100% current decay. The trapped oil droplet shows enormous mechanical stability at lower voltages and stays in its trapped status for hundreds of seconds. An increased voltage can be used to drive the trapped droplet into the pipet pore within several milliseconds. Simultaneous fluorescence imaging and amperometry were performed to examine droplet collision, coalescence, and translocation, further confirming the proposed mechanism of droplet-nanopore interaction. Moreover, we demonstrate the unique ability to perform fast voltammetric measurements on a nanopore-supported attoliter oil droplet and study its voltage-driven ion transfer processes.

Detection and sizing of nanoparticles and DNA on PDMS nanofluidic chips based on differential resistive pulse sensing

The RPS (Resistive Pulse Sensing) technique is a popular tool for the label-free detection of particles. This paper describes a simple, cost-effective PDMS nanofluidic chip for the detection and characterization of nanoparticles based on the differential RPS technique with high resolution and sensitivity. The chip is composed of two layers of PDMS slabs. Microchannel systems fabricated by the photolithography method on the top layer are used for sample loading and differential signal acquisition, and a straight nanochannel on the bottom layer fabricated by an unconventional approach bridging the gap between the microchannels works as an RPS sensing gate. A single-stage differential amplifier is used to amplify the RPS signals when particles or DNA pass through the sensing gate. It was demonstrated that this nanofluidic RPS chip can detect nanoparticles as small as 23 nm with a high SNR (Signal-to-Noise Ratio). The experimental results also show that the device is able to distinguish nanoparticles of smaller size differences such as 60 nm and 83 nm with high resolution, showing superior performance in comparison with the results obtained from DLS (Dynamic Light Scattering). This differential nano-RPS chip was also applied to detect the translocation of dsDNA molecules.

Resistive-pulse and rectification sensing with glass and carbon nanopipettes

Along with more prevalent solid-state nanopores, glass or quartz nanopipettes have found applications in resistive-pulse and rectification sensing. Their advantages include the ease of fabrication, small physical size and needle-like geometry, rendering them useful for local measurements in small spaces and delivery of nanoparticles/biomolecules. Carbon nanopipettes fabricated by depositing a thin carbon layer on the inner wall of a quartz pipette provide additional means for detecting electroactive species and fine-tuning the current rectification properties. In this paper, we discuss the fundamentals of resistive-pulse sensing with nanopipettes and our recent studies of current rectification in carbon pipettes.

Nanoparticle mechanics: deformation detection via nanopore resistive pulse sensing

Solid-state nanopores have been widely used in the past for single-particle analysis of nanoparticles, liposomes, exosomes and viruses. The shape of soft particles, particularly liposomes with a bilayer membrane, can greatly differ inside the nanopore compared to bulk solution as the electric field inside the nanopores can cause liposome electrodeformation. Such deformations can compromise size measurement and characterization of particles, but are often neglected in nanopore resistive pulse sensing. In this paper, we investigated the deformation of various liposomes inside nanopores. We observed a significant difference in resistive pulse characteristics between soft liposomes and rigid polystyrene nanoparticles especially at higher applied voltages. We used theoretical simulations to demonstrate that the difference can be explained by shape deformation of liposomes as they translocate through the nanopores. Comparing our results with the findings from electrodeformation experiments, we demonstrated that the rigidity of liposomes can be qualitatively compared using resistive pulse characteristics. This application of nanopores can provide new opportunities to study the mechanics at the nanoscale, to investigate properties of great value in fundamental biophysics and cellular mechanobiology, such as virus deformability and fusogenicity, and in applied sciences for designing novel drug/gene delivery systems.

High-Speed Multipass Coulter Counter with Ultrahigh Resolution

Coulter counters measure the size of particles in solution by passing them through an orifice and measuring a resistive pulse, i.e., a drop in the ionic current flowing between two electrodes placed on either side of the orifice. The magnitude of the pulse gives information on the size of the particle; however, resolution is limited by variability in the path of the translocation, due to the Brownian motion of the particle. We present a simple yet powerful modified Coulter counter that uses programmable data acquisition hardware to switch the voltage after sensing the resistive pulse of a nanoparticle passing through the orifice of a nanopipet. Switching the voltage reverses the direction of the driving force on the particle and, when this detect-switch cycle is repeated, allows us to pass an individual nanoparticle through the orifice thousands of times. By measuring individual particles more than 100 times per second we rapidly determine the distribution of the resistive pulses for each particle, which allows us to accurately determine the mean pulse amplitude and deliver considerably improved size resolution over a conventional Coulter counter. We show that single polystyrene nanoparticles can be shuttled back and forth and monitored for minutes, leading to a precisely determined mean blocking current equating to sub-angstrom size resolution.

Co-ordinated detection of microparticles using tunable resistive pulse sensing and fluorescence spectroscopy

Tunable resistive pulse sensing (TRPS) has emerged as a useful tool for particle-by-particle detection and analysis of microparticles and nanoparticles as they pass through a pore in a thin stretchable membrane. We have adapted a TRPS device in order to conduct simultaneous optical measurements of particles passing through the pore. High-resolution fluorescence emission spectra have been recorded for individual 1.9 μm diameter particles at a sampling period of 4.3 ms. These spectra are time-correlated with RPS pulses in a current trace sampled every 20 μs. The flow rate through the pore, controlled by altering the hydrostatic pressure, determines the rate of particle detection. At pressures below 1 kPa, more than 90% of fluorescence and RPS events were matching. At higher pressures, some peaks were missed by the fluorescence technique due to the difference in sampling rates. This technique enhances the particle-by-particle specificity of conventional RPS measurements and could be useful for a range of particle characterization and bioanalysis applications.

Use of solid-state nanopores for sensing co-translocational deformation of nano-liposomes

This works reports detection of electric field and hydrodynamic stress induced deformation of sub-100 nm liposomes during translocation through solid-state nanopore.

Applications of tunable resistive pulse sensing

This Review focusses on the recent surge in applied research using tunable resistive pulse sensing, a technique used to analyse submicron colloids in aqueous solutions on a particle-by-particle basis.

Counting and dynamic studies of the small unilamellar phospholipid vesicle translocation with single conical glass nanopores

Phospholipid vesicles are ubiquitous cellular organelles that perform vital functions including materials transport and information transmission and have found promising biomedical applications. Although the transmembrane translocation (via nanopores) of phospholipid vesicles, especially small unilamellar phospholipid vesicles (SUVs), is recognized to be very important for these processes and applications, the details and dynamics remain not very clear. Herein, we use single conical glass nanopores as a model platform to systematically investigate the translocation dynamics of SUVs (∼50-60 nm in diameter) through small nanopores with orifice diameters ranging from ∼14 to 72 nm. Dynamic translocation of individual SUVs one by one through the nanopores was clearly observed and was analyzed by the occurrence of periodic oscillation in ionic current blockage signal under a negatively applied voltage. Translocation behaviors of the SUVs, in terms of magnitude and duration of ionic current blockage signal, varied and can be modulated by changing nanopore size, solution pH, vesicle concentration, applied voltage, and inner surface charge properties of the nanopores. The translocation rate of the SUVs through an ∼72 nm nanopore is typically on a time scale of a few seconds (per SUV translocation event) and found nonlinearly proportional to the concentration of the SUVs. Moreover, the electrophoretic force has been verified as a main force to drive the SUVs through the nanopore since there is a nearly linear relationship between the current blockage frequency of SUVs translocation and the applied bias potentials ranging from -0.6 to -1 V. The findings provide fundamental insights into the translocation and interactions of SUVs with nanopores, and the reported nanopore platform may find potential useful bioapplications in single-cell and single-vesicle studies.

Resistive-pulse analysis of nanoparticles

The development of nanopore fabrication methods during the past decade has led to the resurgence of resistive-pulse analysis of nanoparticles. The newly developed resistive-pulse methods enable researchers to simultaneously study properties of a single nanoparticle and statistics of a large ensemble of nanoparticles. This review covers the basic theory and recent advances in applying resistive-pulse analysis and extends to more complex transport motion (e.g., stochastic thermal motion of a single nanoparticle) and unusual electrical responses (e.g., resistive-pulse response sensitive to surface charge), followed by a brief summary of numerical simulations performed in this field. We emphasize the forces within a nanopore governing translocation of low-aspect-ratio, nondeformable particles but conclude by also considering soft materials such as liposomes and microgels.

Lipid-coated nanocapillaries for DNA sensing

We report a simple and efficient way to accomplish the chemical modification of glass nanopores by means of lipid self-assembly. Lipid coating improves the success rate of these glass nanopores as biosensors to detect λ-DNA.

Tunable negative differential electrolyte resistance in a conical nanopore in glass

Liquid-phase negative differential resistance (NDR) is observed in the i-V behavior of a conical nanopore (~300 nm orifice radius) in a glass membrane that separates an external low-conductivity 5 mM KCl solution of dimethylsulfoxide (DMSO)/water (v/v 3:1) from an internal high-conductivity 5 mM KCl aqueous solution. NDR appears in the i-V curve of the negatively charged nanopore as the voltage-dependent electro-osmotic force opposes an externally applied pressure force, continuously moving the location of the interfacial zone between the two miscible solutions to a position just inside the nanopore orifice. An ~80% decrease in the ionic current occurs over less that a ~10 mV increase in applied voltage. The NDR turn-on voltage was found to be tunable over a ~1 V window by adjusting the applied external pressure from 0 to 50 mmHg. Finite-element simulations based on solution of Navier-Stokes, Poisson, and convective Nernst-Planck equations for mixed solvent electrolytes within a negatively charged nanopore yield predictions of the NDR behavior that are in qualitative agreement with the experimental observations. Applications in chemical sensing of a tunable, solution-based electrical switch based on the NDR effect are discussed.