Nonaqueous Electrophoresis Research Articles

Conductive coatings based on concentrated silver organosols stabilized with Tergitol NP4/Aerosol OT mixture

Concentrated organosols of silver nanoparticles stabilized by a mixture of anionic AOT and nonionic Tergitol NP4 surfactants were obtained using high-performance emulsion synthesis and non-aqueous electrophoresis. The nanoparticles can be dispersed both in nonpolar organic solvents and in water with the preservation of its size and stability. The substitution of the dispersion medium from nonpolar to polar is accompanied by a significant increase in the electrokinetic potential of nanoparticles and a change in the sign of the surface charge from positive to negative. The minimum thermolysis temperature necessary for the formation of a continuous conductive film can be varied by changing in the composition of the surfactant mixture. In combination with the full wettability of both hydrophobic and hydrophilic substrates, such set of properties makes the obtained concentrates promising precursors for use in functional printing.

Electrophoretic concentration and production of conductive coatings from silver nanoparticles stabilized with non-ionic surfactant Span 80

Spherical silver nanoparticles with a diameter of 9.4 nm and stabilized by a non-ionic surfactant Span 80 were obtained by a microemulsion synthesis in nonpolar media. Nanoparticles have a thick adsorption layer of about 30 nm in the initial organosol and they are coated with a surfactant monolayer at low concentrations of Span 80. It was shown that nanoparticles have a high value of electrokinetic potential (more than 30 mV). High electrokinetic potential made it possible to obtain concentrated silver organosols by non-aqueous electrophoresis. The metal content in the separated concentrates reached 2.35 M. Mirror-like films of various thicknesses (from 700 nm to 8 µm) and high conductivity (up to 0.1 μOhm/□) were produced by heat treatment of concentrates at temperatures of 200 and 250 °C. The silver content in the films reached 96% mass. • Charged silver nanoparticles stabilized with a nonionic surfactant Span 80 were synthesized. • Ag/Span 80 composites with a metal content >40%wt. were obtained by non-aqueous electrophoresis. • Mirror-like coatings with a resistance <0.1 μOhm/□ were obtained by thermolysis of such composites.

Non-aqueous electrophoresis integrated with electrospray ionization mass spectrometry on a thiol-ene polymer-based microchip device.

Non-aqueous capillary electrophoresis (NACE) on microfluidic chips is still a comparatively little explored area, despite the inherent advantages of this technique and its application potential for, in particular, lipophilic compounds. A main reason is probably the fact that implementation of NACE on microchips largely precluded the use of polymeric substrate materials. Here, we report non-aqueous electrophoresis on a thiol-ene-based microfluidic chip coupled to mass spectrometry via an on-chip ESI interface. Microchips with an integrated ESI emitter were fabricated using a double-molding approach. The durability of thiol-ene, when exposed to different organic solvents, was investigated with respect to swelling and decomposition of the polymer. Thiol-ene exhibited good stability against organic solvents such as methanol, ethanol, N-methylformamide, and formamide, which allows for a wide range of background electrolyte compositions. The integrated ESI emitter provided a stable spray with RSD% of the ESI signal ≤8%. Separation efficiency of the developed microchip electrophoresis system in different non-aqueous buffer solutions was tested with a mixture of several drugs of abuse. Ethanol- and methanol-based buffers provided comparable high theoretical plate numbers (≈ 6.6 × 104-1.6 × 105m-1) with ethanol exhibiting the best separation efficiency. Direct coupling of non-aqueous electrophoresis to mass spectrometry allowed for fast analysis of hydrophobic compounds in the range of 0.1-5μgmL-1 and 0.2-10μgmL-1 and very good sensitivities (LOD ≈ 0.06-0.28μgmL-1; LOQ ≈ 0.20-0.90μgmL-1). The novel combination of non-aqueous CE on a microfluidic thiol-ene device and ESI-MS provides a mass-producible and highly versatile system for the analysis of, in particular, lipophilic compounds in a wide range of organic solvents. This offers promising potential for future applications in forensic, clinical, and environmental analysis. Graphical abstract.

Online sample preconcentration techniques in nonaqueous capillary and microchip electrophoresis

One of the major drawbacks of electrophoresis in both capillary and microchip is the unsatisfactory sensitivity. Online sample preconcentration techniques can be regarded as the most common and powerful approaches commonly applied to enhance overall detection sensitivity. While the advances of various online preconcentration strategies in capillary and microchip employing aqueous background electrolytes are well-reviewed, there has been limited discussion of the feasible preconcentration techniques specifically developed for capillary and microchip using nonaqueous background electrolytes. This review provides the first consolidated overview of various online preconcentration techniques in nonaqueous capillary and microchip electrophoresis, covering the period of the last two decades. It covers developments in the field of sample stacking, isotachophoresis, and micellar-based stacking. Attention is also given to multi-stacking strategies that have been used for nonaqueous electrophoresis.

Varying the Surface Charges of Gold Nanoparticles in Span 80, AOT, and Span 80 + AOT Micellar Systems in n-Decane

Gold nanoparticles with different diameters of the metal core are obtained by reducing HAuCl4 with hydrazine in reverse emulsions stabilized by Span 80 (9.4 ± 0.4 nm), AOT (6.1 ± 0.2 nm), and an equimolar Span 80 + AOT mixture (10.0 ± 0.2 nm). After synthesis, only the nanoparticles in the AOT solutions (ζ potential 16.8 ± 0.9 mV) display electrophoretic mobility. Nanoparticles in the form of a small-volume liquid phase concentrate are also separated from excess reagents and by-products by concentration (non-aqueous electrophoresis, or centrifugation) and diluted with micellar solutions (Span 80, AOT, and Span 80 + AOT mixture) or pure solvents (n-decane and chloroform). As a result, 24 organosols are obtained, including ones with positive (11), negative (8), and uncharged (5) nanoparticles. Of particular potential interest are organosols that have the same macrostructure in terms of surfactants, but with oppositely charged gold nanoparticles (7 pairs of organosols are obtained).

Preparation of Conductive Silver Films from Electrophoretic Concentrates Stabilized with Sorbitan Monooleate and Sodium Bis(2-Ethylhexyl)Sulfosuccinate in n-Decane

A high-productivity method has been proposed for the synthesis of silver nanoparticles with sizes of 11.2 ± 0.3 nm in water-in-oil emulsions stabilized with sorbitan monooleate (Span 80). It has been shown that, in the course of the synthesis, initial reagents are transferred from emulsion droplets into Span 80 micelles. As a result, nanoparticles are formed due to the intermicellar exchange by the mechanism of micellar (microemulsion) synthesis. The addition of an anionic surfactant, sodium bis(2-ethylhexyl)sulfosuccinate (AOT) provides the nanoparticle surface with a negative charge (ζ-potential is –50.0 mV). This has made it possible to employ nonaqueous electrophoresis for obtaining a liquid concentrate, the evaporation of which has yielded a dry Ag/Span 80/AOT composite. Silver contents in the concentrate and composite are 30 and 70%, respectively. X-ray diffraction and thermal analysis methods have been used to study the composite thermolysis, which results in the formation of conductive silver films (0.43 Ω/sq).

Synthesis and Characterization of Silver Nanoparticles in Reverse Micelles of Nonionic Surfactants and in Their Mixed Micelles with AOT

Stable silver nanoparticles have been obtained in reverse micelles of known and available nonionic surfactants, sorbitan monooleate (Span 80) and tetraethoxylated dodecanol and p-nonylphenol (Brij 30 and Tergitol NP-4, respectively), in n-decane. The obtained nanoparticles have been characterized using a number of physicochemical methods. The stability, hydrodynamic diameter, and electrokinetic potential of the nanoparticles have been studied as depending on the amounts of added water; chloroform; and an anionic surfactant, sodium bis(2-ethylhexyl)sulfosuccinate (AOT). It has been shown that, for different surfactants, AOT should be introduced in different ways to provide the nanoparticles with the electrokinetic potential values sufficient for enhancing their stability and ensuring the possibility of concentrating them by nonaqueous electrophoresis. In the case of Brij 30, it is preferable to introduce AOT after the synthesis as a “charging additive,” while, in the case of Tergitol NP-4, it should be added as a cosurfactant at the stage of the synthesis. The nanoparticles synthesized in Span 80 micelles have a rather high electrokinetic potential even in the absence of the additives. In this case, the use of AOT as a cosurfactant decreases the electrokinetic potential of the nanoparticles, while, being added after the synthesis, it increases the potential.

Nonaqueous electrophoresis on microchips: A review.

The use of organic solvents as electrolytic medium in electrophoresis has become an important alternative for the analysis of compounds that exhibit low or no solubility in water. In recent years, nonaqueous electrophoresis has been extensively explored in conventional capillary systems for different applications. On the other hand, this separation strategy is still not as popular as free solution electrophoresis on chip-based platforms due to the effects of solvent in the background electrolyte on the sample injection, detection performance, and microfluidic platform compatibility. In this way, this review summarizes the main achievements on nonaqueous microchip electrophoresis (NAME). To the best of our knowledge, this is the first review dedicated to discuss exclusively nonaqueous electrophoresis on chip-based systems. For this purpose, some important theoretical aspects involved when separations are performed in organic medium, such as equilibrium, interactions and electrophoretic considerations, are included in the review. In addition, the main challenges, advantages and influences of nonaqueous media on the sample injection, detection as well as the choice of the substrate to fabricate chip-based electrophoresis devices are highlighted. Last, examples showing the feasibility of nonaqueous microchip electrophoresis for applications exploiting different methodologies, operational, and instrumental conditions are summarized and discussed. We hope this review can be useful to spread the huge potential of nonaqueous electrophoresis on microfluidic platforms.

Emulsion Synthesis and Electrophoretic Concentration of Gold Nanoparticles in Sodium Bis(2-Ethylhexyl) Sulfosuccinate Solution in n-Decane

Organosols of gold nanoparticles are obtained by hydrazine reduction of a chloroauric acid solution in a dynamic emulsion of water in n-decane stabilized with a surfactant, sodium bis(2-ethylhexyl) sulfosuccinate. The gold nanoparticles are characterized by phase analysis light scattering, photon correlation spectroscopy, transmission and scanning electron microscopy, X-ray phase analysis, spectrophotometry, thermal analysis, and elemental analyses. The hydrodynamic diameter (≈8 nm), electrophoretic mobility (3.9 × 10–10 m2/(V s)), and electrokinetic potential (30 mV) of the synthesized particles are determined. A liquid-phase concentrate of nanoparticles is separated from the organosol by nonaqueous electrophoresis. Hydrophobic electrically conducting films with a contact angle of 115° and a specific resistance of 0.41 Ω/mm2 are obtained by the thermal treatment of the concentrate.

Synthesis and Electrophoretic Concentration of Silver Nanoparticles in Water-in-Oil Emulsions of Sodium Bis(2-Ethylhexyl) Sulfosuccinate and Preparation of Conductive from Them Coatings by Selective Laser Sintering

Stable silver organosols with a particle diameter of 7 nm containing up to 0.1 M metal have been prepared in water-in-oil dynamic emulsions stabilized with a surfactant, sodium bis(2-ethylhexyl) sulfosuccinate. Liquid-phase concentrates with silver concentrations up to 8 M have been obtained from them by nonaqueous electrophoresis. Thin films and powdered redispersible composites with silver contents up to 75 wt % have been prepared by solvent evaporation. Conductve lines with a width of nearly 70 μm have been formed by selective laser sintering of the thin films of these composites. The specific electrical conductivity of the lines is about 20% of the conductivity of bulk silver. It has been shown that the unsintered composite may be used repeatedly.

Silver nanoparticles stabilized by AOT and Tergitol NP-4 mixture: Influence of composition on electrophoretic concentration, properties of concentrated organosols and conductivity of films

A comparative analysis of the main physico-chemical characteristics of silver nanoparticles produced in mixed reverse micelles of nonionic surfactant Tergitol NP-4 with addition of anionic AOT was carried out. The nanoparticles are shown to have a positive electrokinetic potential. The only exception is nanoparticles synthesized in Tergitol NP-4 reverse micelles without addition of AOT, which have a zero electrokinetic potential. An increase in AOT concentration in the mixture leads to a decrease in the size of particles produced and an increase in the electrokineticpotential. It was shown that silver nanoparticles with close to quantitative extraction rates can be concentrated by non-aqueous electrophoresis with AOT content in mixture equaled 5 mol% and more. For mixture with content of AOT more than 50 mol%, no concentration is observed for surfactants. In the systems with high content of Tergitol NP-4 an increase in the concentration of surfactants is observed compared to the starting organosol. A decrease in AOT concentration leads to a decrease in the stability of the nanoparticles that results in a reversible change in the surface plasmon resonance spectra of nanoparticles under drying and irreversible agglomeration of nanoparticles with a decrease in the surfactant concentration. The presence of reversible agglomeration of the nanoparticles is likely due to the strong negative effect of Tergitol NP-4 on the stability as well as the conductivity and adhesion of coatings produced by thermolysis of the nanoparticle concentrate.

Aqueous and non-aqueous electrophoresis and micellar electrokinetic capillary chromatography of a mixture of quinoline-2-thione and 8-mercaptoquinoline hydrochloride

In this paper three variants of the electrophoretic method for the determination of thioamides, quinoline derivatives are given: aqueous capillary electrophoresis, micellar electrokinetic capillary chromatography and non-aqueous electrophoresis.

Separating excess surfactant from silver and gold nanoparticles in micellar concentrates by means of nonaqueous electrophoresis

It is shown by physicochemical means (IR Fourier spectroscopy, CHN-analysis with preliminary sorption of surfactant on SiO2) that the content of sodium bis(2-ethylhexyl)sulfosuccinate (AOT) in an electrophoretic concentrate remains unchanged during the electrophoretic concentration of silver and gold nanoparticles. Diluting the concentrate and carrying out the second stage of electrophoresis reduces the concentration of surfactant from 0.25 to 0.015 M while maintaining the mass concentration of nanoparticles. Nanoparticles in organosols before and after electrophoresis are characterized by means of photon correlation spectroscopy, phase analysis light scattering, and spectrophotometry. Conducting films on glass substrates are obtained from concentrates with a different contents of surfactant via water–alcohol treatment and thermolysis.

SiO2 and TiO2 nanoparticles in Aerosol OT reverse microemulsions: Synthesis and characterization

Stable SiO2 and TiO2 organosols were prepared by hydrolyzing tetraethyl orthosilicate (TEOS) in the presence of 6–12 M NH3 and titanium(IV) isopropylate (TTIP) in reverse microemulsions of 0.12–0.25 M bis(2-ethylhexyl) sulfosuccinate (Aerosol OT, AOT) in n-decane with the aqueous pseudophase content of 2–3 vol %, 0.018–0.090 M TEOS, and 0.15–0.55 vol %, 0.003–0.025 M TTIP. The degree of hydrolysis was monitored by IR spectroscopy (for TEOS) and spectrophotometry (for TTIP). Oxide nanoparticles were characterized by photon-correlation spectroscopy (PCS) (D h = 8–100 nm) and laser electrophoresis (ζ-potential = 7.4–11.6 mV). The occurrence of surface potential made it possible to separate the oxides from the excess of surfactant by nonaqueous electrophoresis and to determine particle sizes (7–40 nm) by means of transmission electron microscopy (TEM).

Synthesis, characterization, and electrophoretic concentration of titanium dioxide nanoparticles in AOT microemulsions.

Stable organosols of TiO2 nanoparticles were prepared by hydrolysis of titanium tetraisopropoxide (TTIP) in microemulsions of sodium bis(2-ethylhexyl)sulfoxynate (АОТ) in n-decane with increasing the content of aqueous pseudophase from 0.15 to 0.85vol.%. As the water content increased, the hydrodynamic diameter of nanoparticles grew from 10 to 225nm, and the ζ-potential, from -6 to 18mV (the surface of TiO2 nanoparticles was recharged when the water content was 0.45vol.%). Nonaqueous electrophoresis in a capacitor-type cell made it possible to concentrate nanoparticles with a diameter of 60 to 225nm (concentration factor was 10), separate 20nm and 225nm particles, and decrease the content of АОТ in organosol by an order of magnitude. Preparation of a concentrate of nanoparticles with a low content (0.015M) of AOT included the following stages: (i) electrophoresis after synthesis; (ii) sampling of the concentrate and its twenty-fold dilution with pure n-decane; and (iii) repeated electrophoresis. In situ laser and spectrophotometric scanning of the interelectrode space showed the formation of a sharp boundary between the raffinate and the layer of moving nanoparticles during electrophoresis.

High performance separation of quaternary amines using microchip non-aqueous electrophoresis coupled with contactless conductivity detection

This study describes the development of an analytical methodology for the separation of quaternary amines using non-aqueous microchip electrophoresis (NAME) coupled with capacitively coupled contactless conductivity detection (C4D). All experiments were performed using a commercial microchip electrophoresis system consisting of a C4D detector, a high-voltage sequencer and a microfluidic platform to assemble a glass microchip with integrated sensing electrodes. The detection parameters were optimized and the best response was reached applying a 700-kHz sinusoidal wave with 14Vpp excitation voltage. The running electrolyte composition was optimized aiming to achieve the best analytical performance. The mixture containing methanol and acetonitrile at the proportion of 90:10 (v:v) as well as sodium deoxycholate provided separations of ten quaternary amines with high efficiency and baseline resolution. The separation efficiencies ranged from 8.7×104 to 3.0×105 plates/m. The proposed methodology provided linear response in the concentration range between 50 and 1000μmol/L and limits of detection between 2 and 27μmol/L. The analytical feasibility of the proposed methodology was tested in the determination of quaternary amines in corrosion inhibitor samples often used for coating oil pipelines. Five quaternary amines (dodecyltrimethylammonium chloride, tetradecyltrimetylammonium bromide, cetyltrimethylammonium bromide, tetraoctylammonium bromide and tetradodecylammonium bromide) were successfully detected at concentration levels from 0.07 to 6.45mol/L. The accuracy of the developed methodology was investigated and the achieved recovery values varied from 85 to 122%. Based on the reported data, NAME-C4D devices exhibited great potential to provide high performance separations of hydrophobic compounds. The developed methodology can be useful for the analysis of species that usually present strong adsorption on the channel inner walls.

Aqueous and non-aqueous microchip electrophoresis with on-chip electrospray ionization mass spectrometry on replica-molded thiol-ene microfluidic devices

This work describes aqueous and non-aqueous capillary electrophoresis on thiol-ene-based microfluidic separation devices that feature fully integrated and sharp electrospray ionization (ESI) emitters. The chip fabrication is based on simple and low-cost replica-molding of thiol-ene polymers under standard laboratory conditions. The mechanical rigidity and the stability of the materials against organic solvents, acids and bases could be tuned by adjusting the respective stoichiometric ratio of the thiol and allyl (“ene”) monomers, which allowed us to carry out electrophoresis separation in both aqueous and non-aqueous (methanol- and ethanol-based) background electrolytes. The stability of the ESI signal was generally ≤10% RSD for all emitters. The respective migration time repeatabilities in aqueous and non-aqueous background electrolytes were below 3 and 14% RSD (n=4-6, with internal standard). The analytical performance of the developed thiol-ene microdevices was shown in mass spectrometry (MS) based analysis of peptides, proteins, and small molecules. The theoretical plate numbers were the highest (1.2–2.4×104m−1) in ethanol-based background electrolytes. The ionization efficiency also increased under non-aqueous conditions compared to aqueous background electrolytes. The results show that replica-molding of thiol-enes is a feasible approach for producing ESI microdevices that perform in a stable manner in both aqueous and non-aqueous electrophoresis.

Obtaining electrostatically bound CdS-SiO2 aggregates from electrophoretic concentrates of CdS nanoparticles

Nonaqueous electrophoresis reveals that the electrokinetic potential of CdS nanoparticles increases slightly (85–120 mV) along with the concentration (0–5 × 10−3 M) of sodium bis(2-ethylhexyl) sulfosuccinate (AOT) in n-decane, while negatively charged SiO2 particles acquire positive charge (switching from −75 up to +135 mV). The energies of interparticle interactions in CdS-CdS and CdS-SiO2 systems are calculated from these parameters and the literature values of the Hamaker constants according to the Deryaguin-Landau-Verwey-Overbeek (DLVO) theory. It is concluded that the presence of a minimum (2.5 kBT) on the potential dependences of the CdS-SiO2 system indicates the formation of CdS-SiO2 aggregates electrostatically bound by heterocoagulation at low concentrations of AOT. The luminescent properties of the obtained ultrafine CdS-SiO2 powders depend on the CdS content.

Structure of adsorption layer of silver nanoparticles in sodium bis(2-ethylhexyl) sulfosuccinate solutions in n-decane as observed by photon-correlation spectroscopy and nonaqueous electrophoresis.

Photon correlation spectroscopy, nonaqueous electrophoresis, and transmission electron microscopy were used to study the structure of silver nanoparticles (NPs) in n-decane, as a dependence of the concentration of sodium bis(2-ethylhexyl) sulfosuccinate (AOT) and temperature. If the concentration of AOT is lower than the critical micelle concentration (CMC), a silver NP is covered with a monolayer of AOT and reveals no electrophoretic mobility. At average concentrations (from CMC to 0.1 M) the hydrodynamic diameter of a NP does not change, but the ζ-potential increases from 0 to 110 mV. When the concentration of AOT increases from 0.1 to 1 M, ζ potential drops to 13 mV, and the hydrodynamic diameter increases to 90 nm. An increase in temperature to 70 °C leads to a reversible decrease in diameter to 40 nm. The hypothesis of clustering (polylayer adsorption) of "empty" micelles on silver NPs is proposed for the qualitative interpretation of the experimental data.

Properties of conducting films of electrophoretic concentrates of silver and gold nanoparticles in AOT surfactant

Liquid concentrates of silver and gold nanoparticles with 1–2 M metal concentrations were isolated by electrophoresis in a capacitor-type cell from AOT reverse micellar solutions in n-decane. The electrophoretic concentrates and the starting micellar solutions were characterized by nonaqueous electrophoresis, transmission electron microscopy, photon correlation spectroscopy (dynamic light scattering), and spectrophotometry. The hydrodynamic diameter of silver and gold nanoparticles was 13.2 and 8.6 nm, respectively; the ζ-potential was 70 and 13 mV. The drying of the concentrates on glass and silicon substrates and subsequent treatment with a 30% solution of water in ethanol gave mirror conducting Ag, Au, Ag-Au, and Au/Ag films containing on the average 80% metal and 20 wt % AOT. The film structure, morphology, and composition were studied by scanning electron microscopy (SEM) and energy dispersion analysis (EDX).