Electrophoretic methods for separation of nanoparticles

This article reviews progress in the application of electrophoretic techniques for the separation of nanoparticles. Numerous types of nanoparticles have recently been synthesised and integrated into different products and procedures. Consequently, analytical methods for the efficient characterisation of nanoparticles are now required. Several studies have revealed that gel electrophoresis can readily be used for separating nanoparticles according to their size or shape. However, many other studies focused on separation of nanoparticles by CE. In some cases nanoparticles could be separated by CZE, simply using pure buffer as the BGE. In other studies, buffer additives (most often SDS) were used, enabling fast separations of metallic nanoparticles by size. Other CE methods also allowed for separation of nanoparticle conjugates with biomolecules. Dielectrophoresis is yet another electrophoretic technique useful in separation and characterisation of nanoparticles; particularly nanotubes. Detection methods often used after electrophoretic separation include UV/Vis absorption and fluorescence spectroscopy. Examples of recent and relevant older reports are presented here. The authors conclude that electrophoretic methods for nanoanalysis can provide inexpensive and efficient tools for quality assurance and safety control; and as a consequence, they can augment transfer of nanotechnologies from research to industry.     

A rapid and simple separation and direct detection of glutathione by gold nanoparticles and graphene‐based MALDI–TOF‐MS

In this study, we present a rapid and simple method for the separation and direct detection of glutathione by combining gold nanoparticles and MALDI–TOF‐MS with graphene as matrix. Gold nanoparticles enable the selective capture of thiol‐containing compounds. Gold nanoparticles bound with analytes can be mixed with graphene matrix for direct analysis by MALDI–TOF‐MS, which can avoid sample loss and contamination during transfer process. Compared with a conventional matrix, α‐cyano‐4‐hydroxycinnamic acid, graphene exhibits an excellent desorption/ionization efficiency, thermal and mechanical properties. The use of graphene as matrix avoids the fragmentation of analytes. Stable analysis was achieved with less background interference even at the concentration of 0.625 ng/μL. To further confirm its efficiency, the optimized approach was applied to the separation and detection of glutathione in mouse liver extraction. This result showed the great potential of detection of biologically important thiols in biochemical and biomedical research.     

Rapid purification and size separation of gold nanoparticles via diafiltration

Purification and size-based separation of nanoparticles remain significant challenges in the preparation of well-defined materials for fundamental studies and applications. Diafiltration shows considerable potential for the efficient and convenient purification and size separation of water-soluble nanoparticles, allowing for the removal of small-molecule impurities and for the isolation of small nanoparticles from larger nanostructures in a single process. Herein, we report studies aimed at assessing the suitability of diafiltration for (i) the purification of water-soluble thiol-stabilized 3-nm gold nanoparticles, (ii) the separation of a bimodal distribution of nanoparticles into the corresponding fractions, and (iii) the separation of a polydisperse sample into fractions of differing mean core diameter. NMR, thermogravimetric analysis (TGA), and X-ray photoelectron spectroscopy (XPS) measurements demonstrate that diafiltration produces nanoparticles with a much higher degree of purity than is possible by dialysis or a combination of solvent washes, chromatography, and ultracentrifugation. UV-visible spectroscopic and transmission electron microscopic (TEM) analyses show that diafiltration offers the ability to separate nanoparticles of disparate core size. These results demonstrate the applicability of diafiltration for the rapid and green preparation of high-purity gold nanoparticle samples and the size separation of heterogeneous nanoparticle samples. They also suggest the development of novel diafiltration membranes specifically suited to high-resolution nanoparticle size separation.     

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.     

Analysis and applications of nanoparticles in the separation sciences: A case of gold nanoparticles

Nanometer-sized gold particles—gold nanoparticles (Au NPs)—are attracting a great deal of attention for their use in various technologies, including catalysis, optical and electronic devices, and separation science. In the emerging field of nanomaterials, the design, synthesis, and characterization of nanostructures are critical features because the manipulation of these structures has a direct effect on their resulting macroscopic properties. Nanostructures fabricated in layers on surfaces—for example, through self-assembly processes—have several potential applications in separation science. This review provides an introduction to the characterizations of Au NPs using size exclusion chromatography, high performance liquid chromatography (HPLC), and electrophoresis, and their self-assembly onto solid supports for analyses based on HPLC, gas chromatography, and capillary electrophoresis. In addition, sample concentration strategies involving the use of self-assembly approaches for surface modification of Au NPs are also discussed.     

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.     

Preparation of β‐cyclodextrin‐gold nanoparticles modified open tubular column for capillary electrochromatographic separation of chiral drugs

In this paper, β‐cyclodextrin (β‐CD) modified gold nanoparticles (AuNPs) coated open tubular column (OT column) was prepared for capillary electrochromatography. The open tubular column was constructed through self‐assembly of gold nanoparticles on 3‐mercaptopropyl‐trimethoxysilane (MPTMS) prederivatized capillary and subsequent modification of thiols β‐cyclodextrin (SH‐β‐CD). Scanning electron microscopy (SEM), transmission electron microscopy (TEM) and ultraviolet visible spectroscopy were carried out to characterize the prepared open tubular column and synthesized gold nanoparticles. By comparing different coating times of gold nanoparticles and thiols β‐cyclodextrin, we got the optimal conditions for preparing the open tubular column. Also, the separation parameters were optimized including buffer pH, buffer concentration and applied voltage. Separation effectiveness of open tubular column was verified by the separation of four pairs of drug enantiomers including bifonazole, fexofenadine, omeprazole and lansoprazole, and satisfactory separation results were achieved for these analytes studied. In addition, the column showed good stability and repeatability. The relative standard deviation values less than 5% were obtained through intra‐day, inter‐day, and column‐to‐column investigations.     

Biodistribution of colloidal gold nanoparticles after intravenous administration: effect of particle size

Purpose of the present research work was to evaluate the biological distribution of differently size gold nanoparticles (NP) up on intravenous administration in mice. Another objective was to study effect of particle size on biological distribution of gold NP to enable their diverse applications in nanotechnology. Gold NP of different particle sizes, mainly 15, 50, 100 and 200nm, were synthesized by modifying citrate ion concentration. Synthesized gold nanoparticles were characterized by SEM and their size distribution was studied by particle size analyzer. Gold NP was suspended in sodium alginate solution (0.5%, w/v) and administered to mice (1g/kg, intravenously) [n=3]. After 24h of administration of gold NP, blood was collected under light ether anesthesia, mice were sacrificed by cervical dislocation and various tissues/organs were removed. The tissues were then washed with saline, homogenized and lysed with aqua regia. The determination of gold in samples was carried out quantitatively by inductively coupled plasma mass spectrometry (ICP-MS). SEM study revealed spherical morphology of gold NP with narrow particle size distribution. Biodistribution study revealed gold NPs of all sizes were mainly accumulated in organs like liver, lung and spleen. The accumulation of gold NP in various tissues was found to be depending on particle size. 15nm gold NP revealed higher amount of gold and number of particles in all the tissues including blood, liver, lung, spleen, kidney, brain, heart, stomach. Interestingly, 15 and 50nm gold NP were able to pass blood-brain barrier as evident from gold concentration in brain. Two-hundred nanometers gold NP showed very minute presence in organs including blood, brain, stomach and pancreas. The results revealed that tissue distribution of gold nanoparticles is size-dependent with the smallest 15nm nanoparticles showing the most widespread organ distribution.     

Self-assembling gold nanoparticles on thiol-functionalized poly(styrene-co-acrylic acid) nanospheres for fabrication of a mediatorless biosensor

A novel strategy to construct a sensitive mediatorless sensor of H₂O₂ was described. At first, a cleaned gold electrode was immersed in thiol-functionalized poly(styrene-co-acrylic acid) (St-co-AA) nanosphere latex prepared by emulsifier-free emulsion polymerization St with AA and function with dithioglycol to assemble the nanospheres, then gold nanoparticles were chemisorbed onto the thiol groups and formed monolayers on the surface of poly(St-co-AA) nanospheres. Finally, horseradish peroxidase (HRP) was immobilized on the surface of the gold nanoparticles. The sensor displayed an excellent electrocatalytical response to reduction of H₂O₂ without the aid of an electron mediator. The biosensor showed a linear range of 8.0 μmol L⁻¹–7.0 mmol L⁻¹ with a detection limit of 4.0 μmol L⁻¹. The biosensor retained more than 97.8% of its original activity after 60 days’ storage. Moreover, the studied biosensor exhibited good current reproducibility and good fabrication reproducibility.     

Formulation of gold nanoparticles with hibiscus and curcumin extracts induced anti-cancer activity

Gold nanoparticles (AuNPs) have shown a potential for biological applications due to their biocompatibility and high efficiency in drug delivery. Most of the times, the chemical routs are being used to synthesize the AuNPs products. In this paper, eco-friendly non-chemical rout was used to prepare AuNPs by utilizing hibiscus and curcumin extracts as reducing and stabilizing agents, and subsequently their anticancer activities were investigated. The synthesized AuNPs were characterized by using ultraviolet–visible spectroscopy (UV–Vis spectroscopy), Fourier transform infrared (FTIR) spectroscopy, scanning electron microscopy (SEM), and transmission electron microscopy (TEM). UV–Vis spectroscopy analysis confirmed the characteristics absorption peak of gold, and FTIR findings were highlighted the characteristics boding. SEM and TEM analyses showed that the particles were predominantly spherical in shape. The particles were well dispersed when they were prepared under Hibiscus extracts with average size ～ 13 nm. An interesting morphology was observed when AuNPs were prepared with curcumin, where particles displayed an interconnected morphology (average size ～ 18 nm). The anticancer cell activity of AuNPs was studied against human colorectal carcinoma cells (HCT-116) and breast cancer cells (Michigan Cancer Foundation-7 (MCF-7)). The results of anticancer study showed that the treatment of cancer cells with AuNPs decreased the number of cells significantly as compared to control cells. The AuNPs -Hibiscus specimen showed a better inhibiting property than AuNPs -Curcumin, which is attributed to their uniform dispersion and small size.     

Improved separation and size characterization of gold nanoparticles through a novel capillary zone electrophoresis method using poly(sodium4‐styrenesulfonate) as stabiliser and a stepwise field strength gradient

A novel capillary zone electrophoresis (CZE) method was developed for an improved separation and size characterization of pristine gold nanoparticles (AuNP) using uncoated fused‐silica capillaries with UV‐Vis detection at 520 nm. To avoid colloid aggregation and/or adsorption during runs, poly(sodium 4‐styrenesulfonate) (PSS) was added (1%, w/v) in the running buffer (CAPS 10 mM, pH 11). This polyelectrolyte conferred an enhanced stabilization to AuNP, both steric and electrostatic, exalting at the same time their differences in electrophoretic mobility. Resolution was further and successfully improved through a stepwise field strength gradient by the application of 25 kV for the first 5 min and then 10 kV. Migration times varied linearly with particles diameters showing relative standard deviations better than 1% for daily experiments and 3% for interday experiments. A comparison with the size distribution obtained by transmission electron microscopy (TEM) allowed assessing that the electrophoretic profile can reasonably be considered as representative of the effective size heterogeneity of each colloid. Finally, the practical utility of the proposed method was demonstrated by measuring the core diameter of a gold colloid sample produced by chemical synthesis which was in good agreement with the value obtained by TEM measurements.     

Use of cyclodextrin‐modified gold nanoparticles for enantioseparations of drugs and amino acids based on pseudostationary phase‐capillary electrochromatography

The application of chemical‐modified gold nanoparticles (GNPs) as chiral selector for the enantioseparation based on pseudostationary phase‐CEC (PSP‐CEC) is presented. GNPs modified by thiolated β‐CD were characterized by NMR and FT‐IR. The nanoparticle size was determined to be of 9.5 nm (+2.5 nm) by Transmission Electron Microscopy (TEM) and UV spectra. Four pairs of dinitrophenyl‐labeled amino acid enantiomers (DL‐Val, Leu, Glu and Asp) and three pairs of drug enantiomers (RS‐chlorpheniramine, zopiclone and carvedilol) were analyzed by using modified GNPs as the chiral selector in PSP‐CEC. Good theoretical plate number (up to 2.4×10⁵ per meter) and separation resolution (up to 4.7) were obtained even with low concentration of modified GNPs (0.8–1.4 mg/mL). The corresponding concentration of β‐CD in the buffer was only 0.30?0.53 mM, which was much lower than the optimum concentration of 15 mM if pure β‐CD was used as chiral selector. Our results showed that thiolated β‐CD modified GNPs have more sufficient interaction with the analytes, resulting in significant enhancement of enantioseparation. The study shed light on potential usage of chemical modified GNPs as chiral selector for enantioseparation based on PSP‐CEC.     

Analytical performance of molecular beacons on surface immobilized gold nanoparticles of varying size and density

The high quenching efficiency of metal nanoparticles has facilitated its use as quenchers in molecular beacons. To optimize this system, a good understanding of the many factors that influence molecular beacon performance is required. In this study, molecular beacon performance was evaluated as a function of gold nanoparticle size and its immobilization characteristics. Gold nanoparticles of 4 nm, 15 nm and 87 nm diameter, were immobilized onto glass slides. Each size regime offered distinctive optical properties for fluorescence quenching of molecular dyes that were conjugated to oligonucleotides that were immobilized to the gold nanoparticles. Rigid double stranded DNA was used as a model to place fluorophores at different distances from the gold nanoparticles. The effect of particle size and also the immobilization density of nanoparticles was evaluated. The 4 nm and 87 nm gold nanoparticles offered the highest sensitivity in terms of the change in fluorescence intensity as a function of distance (3-fold improvement for Cy5). The optical properties of the molecular fluorophore was of significance, with Cy5 offering higher contrast ratios than Cy3 due to the red-shifted emission spectrum relative to the plasmon peak. A high density of gold nanoparticles reduced contrast ratios, indicating preference for a monolayer of immobilized nanoparticles when considering analytical performance. Molecular beacon probes were then used in place of the double stranded oligonucleotides. There was a strong dependence of molecular beacon performance on the length of a linker used for attachment to the nanoparticle surface. The optimal optical performance was obtained with 4 nm gold nanoparticles that were immobilized as monolayers of low density (5.7 × 10¹¹ particles cm⁻²) on glass surfaces. These nanoparticle surfaces offered a 2-fold improvement in analytical performance of the molecular beacons when compared to other nanoparticle sizes investigated. The principles developed in this study would assist in the design of solid phase molecular beacons using gold nanoparticles.     

Effect of size and protein environment on electrochemical properties of gold nanoparticles on carbon electrodes

We studied the electrochemical properties of gold nanoparticles (GNPs) and their complexes with proteins using square-wave voltammetry. Effect of the nanoparticle size and detection procedure was explored upon the oxidation of GNPs on a glassy carbon electrode (GCE). For pre-characterized GNPs of 13, 35 and 78 nm diameter, the oxidation peak potential was + 0.98, + 1.03 and + 1.06 V vs. Ag/AgCl, respectively. The conjugation of GNPs with four different proteins was verified by UV–Vis spectroscopy and atomic force microscopy indicated the formation of protein shells around GNPs. This process hampered the oxidation of GNPs on bare GCE causing pronounced decrease in the current response by an average factor of 72. GCE modification with carbon nanotubes weakly influenced the sensitivity of GNP detection but resulted in a 14.5-fold signal increase averaged for all GNP–protein complexes. The acidic dissolution and electrodeposition of GNPs or their complexes adsorbed on GCE allowed superior signal amplification directly proportional to nanoparticle size. The results are useful for the optimization of voltammetric analysis of GNP–protein complexes and can be extended to the characterization of other metal nanostructures and their complexes with biological components.     

Green synthesis of gold nanoparticles from Fritillaria cirrhosa and its anti-diabetic activity on Streptozotocin induced rats

To evaluate the anti-diabetic effect of Fritillaria cirrhosa gold nanoparticles on Streptozotocin (STZ) stimulated diabetic preclinical models. The albino rats of either sex were equally distributed to five different groups. Group-I represented as Control; Group-II represented as diabetic control (STZ alone); Group-III represented as 10 mg/kg body weight of Fritillaria cirrhosa gold nanoparticles + diabetes; Group-IV represented as 20 mg/kg body weight of Fritillaria cirrhosa gold nanoparticles + diabetes; Group-V represented as 0.1 mg/kg body weight of glibenclamide + diabetes. The animals were killed after the experimental period. The blood and organs samples were gathered and stored for the additional investigations. The gold nanoparticles were inspected via the UV spectrophotometer, HR-TEM, XRD and FT-IR techniques. The bodyweight, kidney and liver weight were tabulated. The food and water consumption were monitored in all the experimental rats. The serum markers, hepatic markers and renal markers were quantified in both normal and investigational rats. The lipid peroxidation and antioxidant status were quantified in the control and experimental rats. The histopathological alterations were also studied in all the experimental animals. The standard drug glibenclamide was used to compare the synthesized AuNPs. The study revealed that the AuNPs treatment restores the serum, hepatic and renal marker in the STZ-challenged diabetic rats. The AuNPs treatment modulates the antioxidants level and decreased the lipid peroxidation by its antioxidant properties. The pathology results revealed that the AuNPs treatment induces the regeneration of islets cells of pancreas in the experimental rats. The research study proved that the Fritillaria cirrhosa AuNPs exerts anti-diabetic properties.     

Nonionic surfactant‐capped gold nanoparticles for selective enrichment of aminothiols prior to CE with UV absorption detection

In this study, we describe the use of Tween 20‐capped gold nanoparticles (AuNPs) as selective probes for the extraction of aminothiols from an aqueous solution. Tween 20 molecules noncovalently attached to the surface of AuNPs to form Tween 20–AuNPs were used for the selective extraction of aminothiols through the formation of Au–S bonds. After extraction and centrifugation, the aminothiols were detached from the surface of the AuNPs by adding DTT in a high concentration. We used this probe in combination with CE and UV absorption detection. On‐line concentration and separation of the released aminothiols were performed by using 1.6% v/v poly(diallyldimethylammonium chloride) as an additive in CE. Under optimal extraction and stacking conditions, the LOD at a S/N of 3 were 28, 554, and 456 nM for glutathione (GSH), cysteine (Cys), and homocysteine (HCys), respectively. In comparison with the normal injection without the extraction procedure, approximately 2280‐, 998‐, and 904‐fold improvements in the sensitivity were observed for GSH, Cys, and HCys, respectively. We have validated the application of our method on the basis of the analysis of GSH and HCys in human urine samples. It is believed that this approach has significant potential to be extended to clinical diagnosis.     

Mechanisms and applications of plasmon-induced charge separation at TiO2 films loaded with gold nanoparticles

Plasmon-induced photoelectrochemistry in the visible region was studied at gold nanoparticle-nanoporous TiO(2) composites (Au-TiO(2)) prepared by photocatalytic deposition of gold in a porous TiO(2) film. Photoaction spectra for both the open-circuit potential and short-circuit current were in good agreement with the absorption spectrum of the gold nanoparticles in the TiO(2) film. The gold nanoparticles are photoexcited due to plasmon resonance, and charge separation is accomplished by the transfer of photoexcited electrons from the gold particle to the TiO(2) conduction band and the simultaneous transfer of compensative electrons from a donor in the solution to the gold particle. Besides its low-cost and facile preparation, a photovoltaic cell with the optimized electron mediator (Fe(2+/3+)) exhibits an optimum incident photon to current conversion efficiency (IPCE) of 26%. The Au-TiO(2) can photocatalytically oxidize ethanol and methanol at the expense of oxygen reduction under visible light; it is potentially applicable to a new class of photocatalysts and photovoltaic fuel cells.     

Direct application of gold nanoparticles to one-pot electrochemical biosensors

Gold nanoparticles (AuNPs) have been widely employed for the fabrication of electrochemical biosensors. In most cases, AuNPs are immobilized on the surface of an electrode, so they are difficult to be regenerated, making the use of the biosensor unfriendly. In this work, by adopting AuNPs directly as the electrolytes, we have developed a novel AuNPs-based electrochemical detection system. In brief, AuNPs-catalyzed oxidation of glucose is combined with a HRP-catalyzed reaction as well as an electrocatalytic reaction to compose cascade reactions in the electrolyte. Thus, the intensity of the electrocatalytic signals has quantitative relation with the concentration of glucose, and favors the sensitive detection of glucose. Furthermore, because the catalysis of AuNPs may be blocked under the interaction with single-stranded DNA and unblocked in the presence of a complementary sequence, detection of DNA and even single-nucleotide polymorphism can thereby been achieved. This one-pot detection system can be operated and regenerated very easily, since all the components are integrated in the electrolytes of AuNPs, and the unmodified electrode can be reused after being rinsed. This concept by integrating the advantages of sensitive electrochemical detection with the easy-to-operate nanocolloidal system may also promote the development of other kinds of electrochemical biosensors.