Separation and characterization of sub-microm- and microm-sized particles by capillary zone electrophoresis

Electrophoresis of a weakly charged dielectric fluid droplet in a cylindrical pore

Electrophoresis of a weakly charged dielectric droplet with constant surface charge density in a chargeless cylindrical pore is investigated theoretically in this study, focusing on the boundary confinement effect of the double layer, which in turn determines the ultimate motion of the droplet. A patched pseudo-spectral method based on the Chebyshev polynomial is adopted to solve the resulting governing fundamental electrokinetic equations. Mobility reversal, among other interesting phenomena, is observed when the droplet is in a narrow cylindrical pore. No such observation was made in the corresponding motion of a rigid particle. The droplet with a thick double layer may even move against the prediction based on the Coulomb electrostatic law, for instance, a positively charged droplet may move against the electric field. The significant enhancement of the motion-deterring double layer polarization due to the severe steric boundary confinement within a narrow cylindrical pore is found to be responsible for this seemingly peculiar phenomenon. Moreover, smaller droplets may move in the opposite direction of the larger ones. The results are useful in capillary electrophoresis involving droplets in particular and migration of droplets through narrow channels in general.

Optimized hybrid dielectrophoretic microchip for separation of bioparticles

Point-of-care diagnostics requires a smart separation of particles and/or cells. In this work, the multiorifice fluid fractionation as a passive method and dielectrophoresis-based actuator as an active tool are combined to offer a new device for size-based particle separation. The main objective of the combination of these two well-established techniques is to improve the performance of the multiorifice fluid fractionation by taking advantage of dielectrophoresis-based actuator for separating particles. Initially, by using numerical simulations, the effect of using dielectrophoresis-based actuator in multiorifice fluid fractionation on the separation of particles was investigated, and the size of the device was optimized by 25% compared to a device without dielectrophoresis-based actuator. Also, adding dielectrophoresis-based actuator to multiorifice fluid fractionation can extend the range of flow rates needed for separation. In the absence of dielectrophoresis-based actuator, the separation took place only when the flow rate is 100 μL/min, in the presence of dielectrophoresis-based actuator (20 Vp-p), the separation happened in flow rates ranging from 70 to 120 μL/min.

Promising application of probiotic microorganisms as Pickering emulsions stabilizers

The purpose of this work was to study the ability of nineteen food-grade microorganisms as Pickering emulsion (PE) stabilizers. Medium-chain triacylglycerol (MCT) oil-in-water (50:50) PEs were fabricated by 10 wt% or 15 wt% of thermally-inactivated yeast, cocci, Bacillus spp. and lactobacilli cells. The characteristics of microorganisms related to "Pickering stabilization" including morphology, surface charge, interfacial tension, and "contact angle" were firstly studied. After that, the cells-stabilized PEs were characterized from both kinetic and thermodynamic viewpoints, microstructure and rheological properties. The interfacial tension and "contact angle" values of various microorganisms ranged from 16.33 to 38.31 mN/m, and from 15° to 106°, respectively. The mean droplet size of PEs ranged from 11.51 to 57.69 µm. Generally, the physical stability of cell-stabilized PEs followed this order: lactobacilli > Bacillus spp. > cocci > yeast. These variations were attributed to the morphology and cell wall composition. Increasing the microorganism concentration significantly increased the physical stability of PEs from a maximum of 12 days at 10 wt% to 35 days at 15 wt% as a result of better interface coverage. Shear-thinning and dominant elastic behaviors were observed in PEs. Physical stability was affected by the free energy of detachment. Therefore, food-grade microorganisms are suggested for stabilizing PEs.

Particle Manipulation with External Field; From Recent Advancement to Perspectives

Physical forces, such as dielectric, magnetic, electric, optical, and acoustic force, provide useful principles for the manipulation of particles, which are impossible or difficult with other approaches. Microparticles, including polymer particles, liquid droplets, and biological cells, can be trapped at a particular position and are also transported to arbitrary locations in an appropriate external physical field. Since the force can be externally controlled by the field strength, we can evaluate physicochemical properties of particles from the shift of the particle location. Most of the manipulation studies are conducted for particles of sub-micrometer or larger dimensions, because the force exerted on nanomaterials or molecules is so weak that their direct manipulation is generally difficult. However, the behavior, interactions, and reactions of such small substances can be indirectly evaluated by observing microparticles, on which the targets are tethered, in a physical field. We review the recent advancements in the manipulation of particles using a physical force and discuss its potentials, advantages, and limitations from fundamental and practical perspectives.

A simple method for the analysis of extracellular vesicles enriched for exosomes from human serum by capillary electrophoresis with ultraviolet diode array detection

Extracellular vesicles (EVs) are membrane enclosed vesicles (<1 µm), such as exosomes (30-150 nm), involved in cell communication, which have important biological implications. In this study, EV preparations were enriched for exosomes from human serum by polyethylene glycol (PEG) precipitation. Different variables of the PEG precipitation method (i.e. concentration of PEG, filtration and centrifugation of the resuspended pellets) were evaluated by measuring the size of the isolated particles by dynamic light scattering (DLS) and nanoparticle tracking analysis (NTA). In addition, a novel capillary electrophoresis-ultraviolet diode array (CE-UV-DAD) method was developed to obtain characteristic multiwavelength electrophoretic profiles of the EV preparations. Using EV preparations precipitated with 10% m/v of PEG, a background electrolyte (BGE) of 0.1 M Tris and 0.25 M boric acid at pH 7.9 with 0.5% m/v of hydroxypropyl cellulose (HPC) allowed reducing the adsorption of the EVs to the inner wall of the fused silica separation capillary. Sodium dodecyl sulfate (SDS) at 0.1% m/v was also necessary to enhance dispersibility, while homogenizing the charge of the particles to improve the size-dependent separation induced by HPC. Under these optimized conditions, a characteristic electrophoretic multiwavelength profile of the EV preparation and a standard of exosomes was obtained, and separation showed excellent reproducibility and appropriate analysis times. The obtained electrophoretic fingerprints are a simple, effective and complementary tool for the quality control of EV preparations.

Low-electric-potential-assisted diffusiophoresis for continuous separation of nanoparticles on a chip

Nanoparticle separation techniques are of significant importance in nanoscience and nanotechnological applications and different concentration gradients, electric/dielectric forces, flow/pressure fields, and acoustic waves have been intensively investigated. However, precise separation of nanoparticles has many technical challenges in terms of sizes, shapes, and material properties, limiting the separation resolution, capability, applicability, throughput and so on. In this study, we present a microfluidic device for continuous separation of nanoparticles by combining diffusiophoresis (DP) and electrophoresis (EP) to achieve high separation performance. Concentration gradients formed from sodium chloride (NaCl) and potassium acetate (K-acetate) passively drive the diffusiophoretic migration of nanoparticles. Simultaneously, a low electric potential is additionally applied to impose a synergistic effect on nanoparticle migration by size and surface charge, which is called low-electric-potential-assisted DP (LEPDP). Using a LEPDP-based separation device, we demonstrate the separation of nanoparticles having different sizes (diameters of 500, 200, and 50 nm) and under different surface-charge conditions (carboxylated polystyrene, silica, and polylactide). The resulting separation performance exceeded 95%, in terms of size uniformity, which is about two times better than that obtained using DP alone. We also emphasize that the enhancement of separation performance only needs a small voltage (<1 V), thereby demonstrating that our multiphysical approach could be utilized for high-resolution and portable nanoparticle separation on a chip without the side effects associated with high electric fields. Lastly, we ensure that rapid and precise bio/chemical sensing and analysis of various nanosized particles would be envisioned by strategically combining two nonlinear but synergistic migration effects.

Modern Approach to Medical Diagnostics - the Use of Separation Techniques in Microorganisms Detection

Background:

Analytical chemistry and biotechnology as an interdisciplinary fields of science have been developed during many years and are experiencing significant growth, to cover a wide range of microorganisms separation techniques and methods, utilized for medical therapeutic and diagnostic purposes. Currently scientific reports contribute by introducing electrophoretical and immunological methods and formation of devices applied in food protection (avoiding epidemiological diseases) and healthcare (safety ensuring in hospitals).

Methods:

Electrophoretic as well as nucleic-acid-based or specific immunological methods have contributed tremendously to the advance of analyses in recent three decades, particularly in relation to bacteria, viruses and fungi identifications, especially in medical in vitro diagnostics, as well as in environmental or food protection.

Results:

The paper presents the pathogen detection competitiveness of these methods against conventional ones, which are still too time consuming and also labor intensive. The review is presented in several parts following the current trends in improved pathogens separation and detection methods and their subsequent use in medical diagnosis.

Discussion:

Part one, consists of elemental knowledge about microorganisms as an introduction to their characterization: descriptions of divisions, sizes, membranes (cells) components. Second section includes the development, new technological and practical solution descriptions used in electrophoretical procedures during microbes analyses, with special attention paid to bio-samples analyses like blood, urine, lymph or wastewater. Third part covers biomolecular areas that have created a basis needed to identify the progress, limitations and challenges of nucleic-acid-based and immunological techniques discussed to emphasize the advantages of new separative techniques in selective fractionating of microorganisms.

Physicochemical Characterization of Phthalocyanine-Functionalized Quantum Dots by Capillary Electrophoresis Coupled to a LED Fluorescence Detector

Capillary zone electrophoresis (CZE) complemented with Taylor Dispersion Analysis-CE (TDA-CE) was developed to physicochemically characterize phthalocyanine-capped core/shell/shell quantum dots (QDs) at various pH and ionic strengths. An LED-induced fluorescence detector was used to specifically detect the QDs. The electropherograms and taylorgrams allowed calculating the phthalocyanine-QDs (Pc-QDs) ζ-potential and size, respectively, and determining the experimental conditions for colloidal stability. This methodology allowed evidencing either a colloidal stability or an aggregation state according to the background electrolytes nature. The calculated ζ-potential values of Pc-QDs decreased when ionic strength increased, being well correlated with the aggregation of the nanoconjugates at elevated salt concentrations. For the same reason, the hydrodynamic diameter of Pc-QDs increased with increasing background electrolyte ionic strength. The use of electrokinetic methodologies has provided insights into the colloidal stability of the photosensitizer-functionalized QDs in physiologically relevant solutions and, thereby, its usefulness for improving their design and applications for photodynamic therapy.

Membraneless water filtration using CO2

Water purification technologies such as microfiltration/ultrafiltration and reverse osmosis utilize porous membranes to remove suspended particles and solutes. These membranes, however, cause many drawbacks such as a high pumping cost and a need for periodic replacement due to fouling. Here we show an alternative membraneless method for separating suspended particles by exposing the colloidal suspension to CO₂. Dissolution of CO₂ into the suspension creates solute gradients that drive phoretic motion of particles. Due to the large diffusion potential generated by the dissociation of carbonic acid, colloidal particles move either away from or towards the gas–liquid interface depending on their surface charge. Using the directed motion of particles induced by exposure to CO₂, we demonstrate a scalable, continuous flow, membraneless particle filtration process that exhibits low energy consumption, three orders of magnitude lower than conventional microfiltration/ultrafiltration processes, and is essentially free from fouling.

Water treatment processes mostly rely on the use of membranes and filters, which have high pumping costs and require periodic replacement. Here, the authors describe an efficient membraneless method that induces directed motion of suspended colloidal particles by exposing the suspension to CO₂.

Using capillary electrophoresis to characterize polymeric particles

Capillary electrophoresis (CE) was used for the characterization of a variety of polymeric micron and sub-micron particles based on size, surface functionality, and binding properties. First, a robust capillary zone electrophoresis (CZE) method was developed for the baseline separation and quantitation of commercially available polystyrene particles with various surface modifications (including amino, carboxylate, and sulfate functional groups) and various sizes (0.2, 0.5, 1.0, and 3.0μm). The separation of DNA-templated polyacrylamide particles from untemplated particles (as used for the Ion Torrent Personal Genome Machine) was demonstrated. Finally, using the 29-base thrombin aptamer and thrombin protein as a model system, a study was undertaken to determine dissociation constants for the aptamer and protein in free solution and when the aptamer was conjugated to a particle, with the goal of better understanding how the use of solid substrates, like particles, affects selection and binding processes. Dissociation constants were determined and were found to be approximately 5-fold higher for the aptamer conjugated to a particle relative to that in free solution.

A microfluidics-based on-chip impinger for airborne particle collection

Capturing airborne particles from air into a liquid is a critical process for the development of many sensors and analytical systems. A miniaturized airborne particle sampling device (microimpinger) has been developed in this research. The microimpinger relies on a controlled bubble generation process produced by driving air through microchannel arrays. The particles confined in the microscale bubbles are captured in the sampling liquid while the bubbles form, are released and travel in a millimetre-scale sealed liquid reservoir. The microchannel arrays in the impinger are fabricated using a soft-lithography method with polydimethylsiloxane (PDMS) as the structural material. To prevent air leakage at the connections, a PDMS-only sealing technique is successfully developed. The hydrophobicity of the microchannel surface is found to be critical for generating continuous and stable bubbles in the bubbling process. A Teflon layer is coated on the walls of a microchannel array by vapor deposition which effectively increases the hydrophobicity of the PDMS. The collection efficiency of the microimpinger is measured by counting different sizes of fluorescent polystyrene latex particles on polycarbonate membrane filters. Collection efficiencies above 90% are achieved. Furthermore, the particle capturing mechanisms during the injection, formation and rise of a single microbubble are investigated by a computational fluid dynamics (CFD) model. The Navier-Stokes equations are solved along with the use of the volume-of-fluid (VOF) method to capture the bubble deformations and the particles are tracked using a Lagrangian equation of motion. The model is also employed to study the effect of bubble size on the collection efficiency of the microimpinger.

Determination of the mobility of amine- and carboxy-terminated fluospheres and quantum dots by capillary electrophoresis

The pharmacokinetics of nanoparticle (NP) theranostics can, in principle, be predicted based on NP size and zeta potential. Zeta potentials are typically measured using bench top zetasizer instruments, which calculate zeta potential based on mobility data collected from solutions in a small sample cell. However, correlations between zeta potentials measured by zetasizer instruments and those calculated from mobilities determined by instruments designed for capillary electrophoresis may not be direct. To that end, mobilities of a variety of NPs were determined by a capillary electrophoresis and used to calculate zeta potentials based on Henry’s equation. The calculated zeta potentials were then compared to zeta potentials measured directly from a zetasizer. It was found that absolute values of the two methods differed, but the relative zeta potential trends per particle type were similar. These trends were demonstrated by data that showed that the zeta potentials measured using a zetasizer correlated highly with zeta potentials determined by capillary electrophoresis.

Analysis of polyvinyl alcohol microbubbles in human blood plasma using capillary electrophoresis

Recently, a new type of ultrasound contrast agent that consists of air-filled microbubbles stabilized with a shell of polyvinyl alcohol was developed. When superparamagnetic nanoparticles of iron oxide are incorporated in the polymer shell, a multimodal contrast agent can be obtained. The biodistribution and elimination pathways of the polyvinyl alcohol microbubbles are essential to investigate, which is limited with today's techniques. The aim of the present study was, therefore, to develop a method for qualitative and quantitative analysis of microbubbles in biological samples using capillary electrophoresis with ultraviolet detection. The analysis parameters were optimized to a wavelength at 260 nm and pH of the background electrolyte ranging between 11.9 and 12. Studies with high-intensity ultrasonication degraded microbubbles in water showed that degraded products and intact microbubbles could be distinguished, thus it was possible to quantify the intact microbubbles solely. Analysis of human blood plasma spiked with either plain microbubbles or microbubbles with nanoparticles demonstrated that it is possible to separate them from biological components like proteins in these kinds of samples.

Fluidic Grooves on Doped-Ice Surface as Size-Tunable Channels

We propose a new principle for fabrication of size-tunable fluidic nano- and microchannels with a ubiquitous green material, water. Grooves filled with a solution are spontaneously formed on the surface of ice when an appropriate dopant is incorporated. Sucrose doping allows the development of grooves with lengths of 300 μm along the boundaries of ice crystal grains. This paper focuses on controlling the size of the liquid-filled groove and reveals its applicability to size-selective differentiation of nano- and micromaterials. The width of this groove can be varied in a range of 200 nm to 4 μm by adjusting the working temperature of the frozen platform. The channel dimension is reproducible as long as the same frozen condition is employed. We demonstrate the size-selective entrapment of particles as well as the state evaluation of DNA by controlling the physical interference of the ice wall with the electrophoretic migration of particles.

Simultaneous measurement of individual mitochondrial membrane potential and electrophoretic mobility by capillary electrophoresis

Mitochondrial membrane potential varies, depending on energy demand, subcellular location, and morphology and is commonly used as an indicator of mitochondrial functional status. Electrophoretic mobility is a heterogeneous surface property reflective of mitochondrial surface composition and morphology, which could be used as a basis for separation of mitochondrial subpopulations. Since these properties are heterogeneous, methods for their characterization in individual mitochondria are needed to better design and understand electrophoretic separations of subpopulations of mitochondria. Here we report on the first method for simultaneous determination of individual mitochondrial membrane potential and electrophoretic mobility by capillary electrophoresis with laser-induced fluorescence detection (CE-LIF). Mitochondria were isolated from cultured cells, mouse muscle, or liver, and then polarized, labeled with JC-1 (a ratiometric fluorescent probe, which indicates changes in membrane potential), and separated with CE-LIF. Red/green fluorescence intensity ratios from individual mitochondria were used as an indicator of mitochondrial membrane potential. Reproducible distributions of individual mitochondrial membrane potential and electrophoretic mobility were observed. Analysis of polarized and depolarized regions of interest defined using red/green ratios and runs of depolarized controls allowed for the determination of membrane potential and comparison of electrophoretic mobility distributions in preparations containing depolarized mitochondria. Through comparison of these regions of interest, we observed dependence of electrophoretic mobility on membrane potential, with polarized regions of interest displaying decreased electrophoretic mobility. This method could be applied to investigate mitochondrial heterogeneity in aging or disease models where membrane potential is an important factor.

Continuous size-based separation of microparticles in a microchannel with symmetric sharp corner structures

A new microchannel with a series of symmetric sharp corner structures is reported for passive size-dependent particle separation. Micro particles of different sizes can be completely separated based on the combination of the inertial lift force and the centrifugal force induced by the sharp corner structures in the microchannel. At appropriate flow rate and Reynolds number, the centrifugal force effect on large particles, induced by the sharp corner structures, is stronger than that on small particles; hence after passing a series of symmetric sharp corner structures, large particles are focused to the center of the microchannel, while small particles are focused at two particle streams near the two side walls of the microchannel. Particles of different sizes can then be completely separated. Particle separation with this device was demonstrated using 7.32 μm and 15.5 μm micro particles. Experiments show that in comparison with the prior multi-orifice flow fractionation microchannel and multistage-multiorifice flow fractionation microchannel, this device can completely separate two-size particles with narrower particle stream band and larger separation distance between particle streams. In addition, it requires no sheath flow and complex multi-stage separation structures, avoiding the dilution of analyte sample and complex operations. The device has potentials to be used for continuous, complete particle separation in a variety of lab-on-a-chip and biomedical applications.

Electrokinetic characterization of superparamagnetic nanoparticle-aptamer conjugates: design of new highly specific probes for miniaturized molecular diagnostics

With the view of designing new nanoparticle (NP)-aptamer conjugates and proving their suitability as biorecognition tools for miniaturized molecular diagnostics, new maghemite-silica core-shell NP-aptamer conjugates were characterized for the first time in terms of grafting rate and colloidal stability under electrophoretic conditions using capillary electrophoresis. After the grafting rate (on the order of six to 50) of the lysozyme-binding aptamer had been estimated, the electrophoretic stability and peak dispersion of the resulting oligonucleotide-NP conjugates were estimated so as to determine the optimal separation conditions in terms of buffer pH, ionic strength and nature, as well as temperature and electric field strength. The effective surface charge density of the NPs was close to zero for pH lower than 5, which led to some aggregation. The NPs were stable in the pH range from 5 to 9, and an increase in electrophoretic mobility was evidenced with increasing pH. Colloidal stability was preserved at physiological pH for both non-grafted NPs and grafted NPs in the 10-100 mM ionic strength range and in the 15-60 °C temperature range. A strong influence of the nature of the buffer counterion on NP electrophoretic mobility and peak dispersion was evidenced, thus indicating some interactions between buffer components and NP-aptamer conjugates. Whereas an electric field effect (50-900 V cm(-1)) on NP electrophoretic mobility was evidenced, probably linked to counterion dissociation, temperature seems to have an appreciable effect on the zeta potential and aptamer configuration as well. This information is crucial for estimating the potentialities of such biorecognition tools in electrophoretic systems.

Characterization of CdSe nanocrystals coated with amphiphiles. A capillary electrophoresis study

We have synthesized CdSe nanocrystals (NCs) possessing a trioctylphosphine surface passivation layer and modified with amphiphilic molecules to form a surface bilayer. The NCs covered with single amphiphiles are not stable in aqueous solution, but a mixed amphiphilic system is shown to provide stability in solution over several months. The solutions of the modified NCs were characterized by UV-Vis absorbance, photoluminescence, and transmission electron microscopy. An electrophoretic study revealed two operational modes. The first relies on the enrichment of NCs using a micellar plug as a tool. The accumulation of NCs at the plug-electrolyte buffer interface results in a sharp peak. By controlling the electrophoretic conditions, nanocrystals were forced to exit a micellar plug into an electrolyte buffer. We conclude that a system consisting of modified nanocrystals and a micellar plug can act as a mixed pseudomicellar system, where modified nanocrystals play the role of pseudomicelles.

Figure

Characterization of CdSe quantum dots with bidentate ligands by capillary electrophoresis

The CdSe quantum dots (QDs) with bidentate ligands: a-diimine (NN) and dihydrolipoic acid (DHLA) were synthesized and characterized by UV-Vis, particle size and capillary electrophoretic techniques. Two systems were analyzed: CdSe with one ligand (CdSe/ligand) and CdSe with two different ligands (CdSe//ligand1/ligand2), where ligand = α-diimine or DHLA. Hydrodynamic features of functionalized QDs were characterized by zone capillary electrophoretic (CZE), and particle size techniques and these methods were consistent. It was established that CZE, micellar (MEKC) and microemulsion (MEEKC) modes were suitable for separating charged CdSe QDs and that no peaks were obtained for QDs passivated with electrically neutral ligands. For CdSe QDs with neutral (NN) ligands, a preconcentration method with the use of a micellar plug was introduced for visualizing these QDs. A sharp peak representing neutral QDs was obtained within the zone of micellar plug of a non-ionic surfactant, Here, a ligand character used for CdSe modification and the type of the electrophoretic method applied were the determining factors for the QDs peak visualization. Moreover, examples of visualization of charged and neutral QDs on the same run were presented, and for this purpose, dual mechanism (separation/preconcentration) was proposed.

Electrophoretic Mobility of Lipoprotein Nanoparticle Mimics

Lipoprotein particles (LPPs) are biological nanoparticles whose physiological roles are greatly influenced by their sizes. The four major classes of LP are: very low density lipoprotein, intermediate density lipoprotein, low density lipoprotein (LDL) and high density lipoprotein. Since the predominance of small, dense LDLs is associated with increased risk of coronary artery disease (CAD) and diabetes mellitus, LPP profiling can be used to predict metabolic risk factors. Highly tunable LPP mimics can be synthesized using nanoparticles to carefully control for size, lipid composition and surface charge to facilitate the study LPPs in CAD. Here, we engineered LPP mimics using gold nanoparticles between 10-50 nm in diameters. We measured the mobility and zeta potential of these LPP mimics and showed that each mimics have distinct electrokinetic properties and are electrostatically stable.