Adding sodium dodecylsulfate to the running electrolyte enhances the separation of gold nanoparticles by capillary electrophoresis

This paper describes employing capillary electrophoresis (CE) for the separation of gold colloids in nanometer-size regimes. Adding sodium dodecylsulfate (SDS) surfactant to the running buffer enhances the capability of CE to separate gold nanoparticles. We found that the optimized separation conditions involved SDS (70 mM), 3-cyclohexylamoniuopropanesulfonic acid (CAPS) buffer (10 mM), pH 10.0, and an applied voltage of 20 kV. We propose that the charged surfactants associate onto the surface of the gold nanoparticles and cause a change in the charge-to-size ratio of gold nanoparticle, which is a function of the surface area of nanoparticle and the surfactant concentration of running electrolyte. At high concentrations of the surfactant in the running electrolyte—i.e., when the surface of the gold nanoparticles is fully occupied with SDS—a linear relationship exists between the electrophoretic mobility and nanoparticles having diameters ranging from 5.3 to 38 nm. Based on the results of separating the 5.3 and 19 nm nanoparticles, we estimate that the size resolution (R_s=1.0) is 5.0 nm. The relative standard deviations of the electrophoretic mobilities of the 5.3 and 19 nm gold nanoparticles are 0.97 and 0.54%, respectively.     

Size‐dependent electrophoretic migration and separation of water‐soluble gold nanoclusters by capillary electrophoresis

Recently, water‐soluble gold nanoclusters (AuNCs) have attracted more and more attention due to their unique properties. In this study, penicillamine‐protected gold nanoclusters (Pen‐AuNCs) were synthesized and initially fractionated by sequential size‐selective precipitation (SSSP). The crude Pen‐AuNCs and SSSP fractions were separated by capillary zone electrophoresis (CZE) with a diode array detector. The effects of key parameters, including the concentration of phosphate buffer, pH value and the ethanol content were systematically investigated. The separation of water‐soluble poly‐disperse AuNCs were well achieved at 30 mM phosphate buffer with 7.5% EtOH, pH 12.0, and applied voltage of 15 kV. The linear correlation between AuNCs diameter and mobility was observed. This finding provides an important reference for CE separation and product purification of water‐soluble AuNCs or other nanomaterials.     

Selective extraction of melamine using 11-mercaptoundecanoic acid-capped gold nanoparticles followed by capillary electrophoresis

This study describes the use of 11-mercaptoundecanoic acid-capped gold nanoparticles (MUA-AuNPs) for selective extraction of melamine prior to analysis by capillary electrophoresis with UV detection. The highest degree of melamine-induced aggregation of MUA-AuNPs was found to occur at pH 5.0, indicating that the NP aggregation is mainly because of hydrogen bonding between the carboxylate groups of MUA and the amine groups of melamine. Moreover, the degree of melamine-induced NP aggregation gradually increased when the chain length of the mercaptoalkanoic acid was increased from two to 12 carbon atoms. At pH 5.0, the extraction efficiency of melamine was highly dependent on the concentration of MUA-AuNPs, the concentration of dithiothreitol (DTT), the extraction time between MUA-AuNPs and melamine, and the incubation time between melamine-adsorbed AuNPs and DTT. The separation of the extracted melamine and DTT (releasing agent) was accomplished using a solution of 10 mM phosphate (pH 6.0) containing 1.6% (v/v) poly(diallyldimethylammonium chloride). Under the optimum extraction and separation conditions, the limit of detection at a signal-to-noise ratio of 3 was estimated to be 77 pM for melamine, with linear range of 1-1000 nM. The proposed method was successfully applied to the determination of melamine in tap water and in milk.     

Disposable twin gold electrodes for amperometric detection in capillary electrophoresis

We describe a simple and easy way to construct gold microelectrodes for amperometric detection in capillary electrophoresis (CE). The gold microelectrodes, in single or twin sets, were obtained from recordable compact discs (gold-sputtered type), which present highly reproducible surface characteristics. The performance of these electrodes was evaluated by using a home-made CE equipment. The basic steps for the electrode construction are: drawing on a microcomputer; laser printing of the design on wax paper; heat-transfer of the toner onto the gold surface of a peeled recordable compact disc (CD-R); etching of the gold layer from unprinted regions; removal of the toner with a solvent; sealing of unused electrode areas with varnish. One electrode at a time was connected to a potentiostat (or two, to a bipotentiostat) and operated in a wall jet configuration relative to the CE capillary outlet. The amperometric signals were integrated for quantification purposes. Repetitive injections (n = 10) of a mixture containing iodide, ascorbic acid, dipyrone, and acetaminophen (20, 200, 500, and 100 microM), presented relative standard deviations of 2.9, 4.5, 6.1, and 4.0%, respectively. For these analytes, the detection limits (S/N = 3, 30 s of 100 mm hydrodynamic injection) were 0.1, 0.5, 3.1, and 1.1 microM, respectively.     

Highly efficient size separation of CdTe quantum dots by capillary gel electrophoresis using polymer solution as sieving medium

In this paper, we present a new method for highly efficient size separation of water-soluble CdTe quantum dots (QDs) based on CGE using polymer solution as sieving medium. CdTe QDs were synthesized in aqueous phase by a chemical route with mercaptopropionic acid as a ligand. In the alkaline solution, CdTe QDs possess negative charges and migrate to the anode in the electric field. In linear polyacrylamide sieving medium, the migration time of CdTe QDs was increased with the size of CdTe QDs. The effects of some factors, such as types, concentrations, and pH of sieving media, on the separation of CdTe QDs were investigated systematically. Highly efficient separation of CdTe QDs was obtained in linear polyacrylamide sieving medium, and collection of fractions was automatically accomplished by CGE technique. Our preliminary results show that CGE technique is an efficient tool for characterization and size-dependent separation of water-soluble nanoparticles. In addition, the fraction collection in CGE may be useful in certain special applications such as fabrication of nanodevices in the future.     

Studying the size/shape separation and optical properties of silver nanoparticles by capillary electrophoresis

This paper describes the feasibility of employing capillary electrophoresis (CE) to separate silver particles in nanometer regimes. We have found that the addition of an anionic surfactant, sodium dodecyl sulphate (SDS), to the running electrolyte prevents coalescence of the silver particles during the process, which improves the separation performance; the concentration of SDS required for optimal silver nanoparticle separation is ca. 20 mM. By monitoring the electropherograms using a diode-array detection (DAD) system, we have also investigated the separation of suspended silver nanorods with respect to their shapes. Our results demonstrate that the combination of CE and DAD is a powerful one for the separation and characterization of various silver nanoparticles.     

Use of capillary electrophoresis for characterisation of vinyl‐terminated Au nanoprisms and nanooctahedra

It is described a simple, rapid and efficient methodology to characterise and separate gold nanoprisms and nanooctahedra by capillary electrophoresis. This technique is suitable to distinguish between morphologies and it can be used as a powerful separation tool after a customised synthesis of both structures. This synthesis was carried out by amending two parameters, temperature and pH, and a sharp decrease was found in nanotriangles when temperature was increased from 70 up to 95°C. However, when the synthesis was performed at a given temperature, an increase in pH did not promote an important change in isolation of any structure until pH = 9.5, critical in the final morphology of the nanoparticle. Gold nanoprisms and nanooctahedra were successfully separated by capillary electrophoresis according to differences in charge‐to‐mass ratio of the morphologies. Final particle morphology was confirmed by transmission electron microscopy analysis. Under optimal working conditions, a mixture containing both shapes of gold nanoparticles was initially injected and two major peaks were obtained for each structure. Capillary electrophoresis allowed to study pH and temperature influence on both morphologies. It was inferred that the ratio between triangles and octahedra decreased to a great extent when increasing both temperature and pH.     

Recent advances in nanomaterial‐based capillary electrophoresis

This review gives a summary of applications of different nanomateials, such as gold nanoparticles (AuNPs), carbon‐based nanoparticles, magnetic nanoparticles (MNPs), and nano‐sized metal organic frameworks (MOFs), in electrophoretic separations. This review also emphasizes the recent works in which nanoparticles (NPs) are used as pseudostationary phase (PSP) or immobilized on the capillary surface for enhancement of separation in CE, CEC, and microchips electrophoresis.     

Investigation of iron oxide nanoparticles by capillary zone electrophoresis

The electrophoretic behavior of γ-Fe₂O₃ nanoparticles was studied in aqueous solutions of Na₂SO₄-NaOH (pH 10.8) and of Na₂SO₄-Na₃cit (pH 7.1) as running electrolytes. Two electrophoretic zones (smooth and with spikes) due to colloidal and suspended particles of approximately the same size range were formed during the runs. The suspension stability and size distribution were shown to depend on the composition of electrolyte used for dispersing the solids. The effects of electric field strength, injection time, injection pressure as well as sodium citrate concentration were studied and particle electrophoretic mobilities were calculated. Electron micrographs of particles studied were obtained. Preparation of reference samples based on the colloidal γ-Fe₂O₃ has been discussed.     

Selective enrichment of aminothiols using polysorbate 20-capped gold nanoparticles followed by capillary electrophoresis with laser-induced fluorescence

In this article, we report a simple method for selective enrichment of aminothiols using Tween 20-capped gold nanoparticles (AuNPs) prior to capillary electrophoresis coupled with laser-induced fluorescence (CE-LIF). Compared to citrate-capped AuNPs, Tween 20-capped AuNPs exhibit the ability to disperse in a highly saline solution and selectively extract aminothiols through the formation of Au–S bonds. After extraction and centrifugation, 1 mM thioglycollic acid (TGA) was utilized to remove aminothiols that attached to the NP surfaces. After a solution of 8.0 mL aminothiols were extracted using 2× AuNPs (200 μL), the extracted aminothiols derivatized with o-phthalaldehyde at pH 12.0 were detected by CE-LIF. As a result, the limits of detection at a signal-to-noise ratio of 3 for homocysteine (HCys), glutathione (GSH), and γ-glutamycysteine (Glu-cys) are 4013.2, 79.8, and 382.8 pM, respectively. The use of this probe provided approximately 11-, 282-, and 21-fold sensitivity improvements for HCys, GSH, and Glu-cys, respectively. A practical analysis of HCys, GSH, and Glu-cys in human urine sample has been accomplished by this present method.     

A highly sensitive capillary electrophoresis immunoassay strategy based on dual‐labeled gold nanoparticles enhancing chemiluminescence for the detection of prostate‐specific antigen

An enzyme and antibody dual labeled gold nanoparticles enhancing chemiluminescence strategy was developed for highly sensitive CE immunoassay (IA) of prostate‐specific antigen (PSA). In this work, gold nanoparticles were labeled with horseradish peroxidase and antiprostate specific antigen‐antibody, and used as the marker (Ab^*). After PSA (antigen, Ag) was added into the system, a noncompetitive immune reaction was happen between Ab^* and Ag to form an immune complex (Ag–Ab^*). Subsequently, the obtained Ag–Ab^* and unreacted Ab^* were separated by CE, and the chemiluminescence intensity of Ag‐Ab^* was used to estimate PSA concentration. The calibration curve showed a good linearity in the range of 0.25–10 ng/mL. Based on a S/N of 3, the detection limit for PAS was estimated to be 0.092 ng/mL. Proposed CE method was applied for PSA quantification in human serum samples from healthy volunteers and patients with prostate cancer. The obtained results demonstrated that the proposed CE method may serve as an alternative tool for clinical analysis of PSA.     

Ultrasensitive detection of indoleamines by combination of nanoparticle-based extraction with capillary electrophoresis/laser-induced native fluorescence

For the first time, citrate-capped gold nanoparticles (citrate-AuNPs) have been used for the selective extraction of indoleamines – 5-hydroxytryptophan (5-HTP), tryptophan (Trp), tryptamine (TA), 5-hydroxytryptamine (5-HT), and 5-hydroxyindoleacetic acid (5-HIAA) – prior to their analysis by capillary electrophoresis/laser-induced native fluorescence (CE/LINF). The extinction spectra obtained for the citrate-AuNPs in the presence of indoleamines revealed that 5-HTP, 5-HT, and 5-HIAA were extracted mainly because of van der Waals interactions between the indole ring and the citrate-AuNPs (hydrophobic surface), while 5-HT and TA were extracted by electrostatic attractions between the amine group of the indoleamines and the citrate ligands adsorbed on the AuNP surface. The extracted indoleamines could be liberated from the AuNP surface by the addition of high concentrations of 2-mercaptoethanol (2-ME), which binds strongly to the AuNPs. The sensitivity of this method to indoleamines could be significantly enhanced by increasing the AuNP concentration, incubation time, and sample volume. Under optimal extraction and separation conditions, the combination of NP-based extraction and CE-LINF provided 48-, 4077-, 985-, 920-, and 4030-fold improvements in the limits of detection (signal-to-noise ratio of 3) for 5-HTP, Trp, TA, 5-HT, and 5-HIAA as compared to the analysis of five indoleamines by CE-LINF. In addition, this proposed method was successfully used for the determination of TA and 5-HT in urine.     

Size separation of silica nanospheres by means of capillary zone electrophoresis

The electrophoretic mobility of silica nanospheres was shown to be a function of separation conditions such as pH and phosphate concentration of a carrier electrolyte. The separation selectivity can be controlled by the separation conditions and optimised depending on the sample composition. The effects of pH and phosphate concentration of buffer solutions on the nanosphere electrophoretic mobility are explained using the Overbeek-Booth electrokinetic theory taking into account both electrophoretic retardation and the relaxation effect.     

A technique for capillary joining in capillary electrophoresis

Summary Aspects of cracking and joining capillaries have been investigated. Capillary coupling was achieved using various methods. The most successful used hydrofluoric acid-etched capillaries to form male and female ends which were then joined together. This type of joint was used to connect sections of capillary of similar and different internal diameters with minimal loss in resolution, peak width and number of theoretical plates. (Uridine and hypoxanthine was used as a test mixture). For hypoxanthine on a 50 m/50 m etched joined capillary 10 cm from the detector window the number of theoretical plates was 96.6% of that for hypoxanthine on an unbroken capillary. Following the relative success of capillary joining, a coupled capillary flowcell (50 m/200 m) was produced and evaluated.     

Protein separation by capillary gel electrophoresis: A review

Capillary gel electrophoresis (CGE) has been used for protein separation for more than two decades. Due to the technology advancement, current CGE methods are becoming more and more robust and reliable for protein analysis, and some of the methods have been routinely used for the analysis of protein-based pharmaceuticals and quality controls. In light of this progress, we survey 147 papers related to CGE separations of proteins and present an overview of this technology. We first introduce briefly the early development of CGE. We then review the methodology, in which we specifically describe the matrices, coatings, and detection strategies used in CGE. CGE using microfabricated channels and incorporation of CGE with two-dimensional protein separations are also discussed in this section. We finally present a few representative applications of CGE for separating proteins in real-world samples.     

Nanoparticle-filled capillary electrophoresis for the separation of long DNA molecules in the presence of hydrodynamic and electrokinetic forces

We report the analysis of long DNA molecules by nanoparticle-filled capillary electrophoresis (NFCE) under the influences of hydrodynamic and electrokinetic forces. The gold nanoparticle (GNP)/polymer composites (GNPPs) prepared from GNPs and poly(ethylene oxide) were filled in a capillary to act as separation matrices for DNA separation. The separations of lambda-DNA (0.12-23.1 kbp) and high-molecular-weight DNA markers (8.27-48.5 kbp) by NFCE, under an electric field of -140 V/cm and a hydrodynamic flow velocity of 554 microm/s, were accomplished within 5 min. To further investigate the separation mechanism, the migration of lambda-DNA was monitored in real time using a charge-coupled device (CCD) imaging system. The GNPPs provide greater retardation than do conventional polymer media when they are encountered during the electrophoretic process. The presence of interactions between the GNPPs and the DNA molecules is further supported by the fluorescence quenching of prelabeled lambda-DNA, which occurs through an energy transfer mechanism. Based on the results presented in this study, we suggest that the electric field, hydrodynamic flow, and GNPP concentration are the three main determinants of DNA separation in NFCE.     

Pesticide analysis by capillary electrophoresis

In this work, a critical and updated revision of the current situation of the analysis of pesticides by Capillary Electrophoresis (CE) is presented. The review has been written in two main sections. The first one presents a thorough revision of the various offline and on-line sample preconcentration procedures that have been used in conjunction with CE to analyze these compounds. The second part reviews the various detection strategies (i.e., UV, LIF, MS, and electrochemical) and CE modes that have been applied to the analysis of pesticides. Future trends that can be expected from this hot research area are also discussed.     

Analysis of silica nanoparticles by capillary electrophoresis coupled to an evaporative light scattering detector

A simple and rapid methodology has been developed to identify and separate silica nanoparticles (SiO₂NPs) of different sizes in aqueous solution by capillary zone electrophoresis coupled to an evaporative light scattering detector (CE-ELSD). SiO₂NPs were separated using 3 mM ammonium acetate buffer, containing 1% methanol at pH 6.9. SiO₂NPs of 20, 50 and 100 nm were successfully separated under the optimum experimental conditions. CE coupled to ELSD has been proven to be an effective separation technique to determine particles with such small sizes, although the peaks are very close to each other, and it is a promising technique that may allow the separation of other types of nanoparticles. Confirmation by TEM and quantification of the SiO₂ content was also carried out by inductively coupled plasma-mass spectrometry (ICP-MS). The new method was applied to the analysis of real samples, in order to assess its ability to avoid matrix effects in the determination of SiO₂NPs in these kinds of samples.     

Amphiphilic silica nanoparticles as pseudostationary phase for capillary electrophoresis separation

Amphiphilic silica nanoparticles surface-functionalized by 3-aminopropyltriethoxysilane (APTES) and octyltriethoxylsilane (OTES) were successfully prepared and characterized using scanning electron microscopy (SEM), transmission electron microscopy (TEM), Fourier transform infrared spectrometry (FT-IR) and thermogravimetry (TG) techniques. The potential use of these bifunctionalized nanoparticles as pseudostationary phases (PSPs) in capillary electrophoresis (CE) for the separation of charged and neutral compounds was evaluated in terms of their suitability. As expected, fast separation of representative aromatic acids was fulfilled with high separation efficiency, because they migrate in the same direction with the electroosmotic flow (EOF) under optimum experimental conditions. Using a buffer solution of 30mmol/L phosphate (pH 3.0) in the presence of 0.5mg/mL of the synthesized bifunctionalized nanoparticles, the investigated basic compounds were baseline-resolved with symmetrical peaks. Due to the existence of amino groups on the surface of nanoparticles, "silanol effect" that occurs between positively charged basic analytes and the silanols on the inner surface of capillary was greatly suppressed. Furthermore, the separation systems also exhibited reversed-phase (RP) behavior when neutral analytes were tested.     

Aptamer‐based detection and quantitative analysis of human immunoglobulin E in capillary electrophoresis with chemiluminescence detection

A novel aptamer‐based CE with chemiluminescence (CL) assay was developed for highly sensitive detection of human immunoglobulin E (IgE). The IgE aptamer was conjugated with gold nanoparticles (AuNPs) to form AuNPs‐aptamer that could specifically recognize the IgE to produce an AuNPs‐aptamer‐IgE complex. The mixture of the AuNPs‐aptamer‐IgE complex and the unbounded AuNPs‐aptamer could be effectively separated by CE and sensitively detected with luminol‐H₂O₂ CL system. By taking the advantage of the excellent catalytic behavior of AuNPs on luminol‐H₂O₂ CL system, the ultrasensitive detection of IgE was achieved. The detection limit of IgE is 7.6 fM (S/N = 3) with a linear range from 0.025 to 250 pM. Successful detection of IgE in human serum samples was demonstrated and the recoveries of 94.9–103.2% were obtained. The excellent assay features of the developed approach are its specificity, sensitivity, adaptability, and very small sample consumption. Our design provides a methodology model for determination of rare proteins in biological samples.