Potential role of gold nanoparticles for improved analytical methods: an introduction to characterizations and applications.

Nanoparticle-based material is a revolutionary scientific and engineering venture that will invariably impact the existing analytical separation and preconcentration for a variety of analytes. Nanoparticles can be regarded as a hybrid between small molecule and bulk material. A material on the nanoscale produces considerable changes on various properties, making them size- and shape-dependent. Gold nanoparticles (Au NPs), one of the wide variety of core materials available, coupled with tunable surface properties in the form of inorganic or inorganic-organic hybrid have been reported as an excellent platform for a broad range of analytical methods. This review aims to introduce the basic principles, examples, and descriptions of methods for the characterization of Au NPs by using chromatography, electrophoresis, and self-assembly strategies for separation science. Some of the latest important applications of using Au NPs as stationary phases toward open-tubular capillary electrochromatography, gas chromatography, and liquid chromatography as well as roles of run buffer additive to enhance separation and preconcentration in the field of chromatographic, electrophoretic and in chip-based systems are reviewed. Additionally, we review Au NPs-assisted state-of-the-art techniques involving the use of micellar electrokinetic chromatography, an online diode array detector, solid-phase extraction, and mass spectrometry for the preconcentration of some chemical compounds and biomolecules.

Surface cleaning of the nanowire field-effect transistor for gene detection.

The efficiency of DNA immobilization by using various surface cleaning methods is studied in this work. The degree of surface cleaning is evaluated by the surface tension measurement to reveal the contribution from the polar and apolar terms. The observation from the fluorescent microscope images indicates the effectiveness of surface clean by the acetone and ethanol mixtures, as well as the sulphuric acid and hydrogen peroxide mixtures. We also fabricate a series of back-gated, 60-nm nanowired (NW) field-effect transistor (FET) sensors for mutation gene detection by following the developed acetone and ethanol mixtures. Electrical properties of the NWFET sensor demonstrate the n-channel depletion characteristics. The current of the sensor is reduced once the attachment of negative charge molecules. The single-stranded capture DNA is chemically immobilized onto the surface of silicon NWFET by three-step reactions. The sensor surface demonstrates the great performance of current shift after the suitable cleaning. The NWFET sensor is successfully applied to detect the BRAF(V599E) mutation genes from the hybridized processes. The sensing behaviour estimated from the electrical signal reaches the femtomolar level.

Preconcentration and separation of neutral steroid analytes using a combination of sweeping micellar electrokinetic chromatography and a Au nanoparticle-coated solid phase extraction sorbent.

This paper describes the preconcentration of three neutral steroids (testosterone, progesterone, and testosterone propionate) through a combination of off-line preconcentration using a gold nanoparticle (Au NP)-coated silica gel solid phase extraction (SPE) sorbent prior to on-line preconcentration using sweeping micellar electrokinetic chromatography (MEKC). In the initial phase of this study, the sweeping-MEKC parameters for the capillary electrophoresis (CE) separation of the steroid analytes were optimized. The sweeping-MEKC effect was greatest when the sample matrix contained ca. 18% acetonitrile and 100mM sodium dihydrogen phosphate (pH 7.0). Next, under the optimized sweeping-MEKC operating conditions, a commercial C(18)-bonded silica gel and a Au NP-coated silica gel were tested for their use as SPE sorbents for the SPE-sweeping-MEKC preconcentration of steroid-spiked urine samples. Of these two sorbents, the Au NP-coated SPE sorbent displayed a superior clean-up efficiency toward the sample matrix. Size exclusion chromatography (SEC) was used to characterize the interactions between urinary proteins and the Au NPs. The results indicated that the removal of the interfering signals from the urinary proteins was probably due to their interactions with residual Au metal surfaces of the Au NP-coated SPE sorbent. When combining Au NP-coated silica gel SPE with sweeping-MEKC for the CE analysis of steroid-spiked urine samples, the limits of detection (at a signal-to-noise ratio of N=3) for testosterone, progesterone, and testosterone propionate were ca. 1.59, 1.20, and 1.15mug/L, respectively; in addition, the detection sensitivities (based on peak heights) of these steroids improved by ca. 700-, 1090-, and 1100-fold, respectively, relative to those of the analytes that had not been subjected to preconcentration.

A high-efficiency capillary electrophoresis-based method for characterizing the sizes of Au nanoparticles.

In this paper, we present a highly efficient capillary electrophoresis (CE)-based method for characterizing the sizes of Au nanoparticles (NPs). We used the reversed electrode polarity stacking mode (REPSM) of CE for on-line enhancement of the detection and separation of Au NPs. Under the on-line enhancement separation conditions [buffer: SDS (70 mM) and 3-cyclohexylamino-1-propanesulfonic acid (10 mM) at pH 10.0; applied voltage: 20 kV; using the REPSM for sample concentration], the detection sensitivity toward the Au NPs improved by up to 260-fold. A linear relationship (R(2)=0.994) existed between the electrophoretic mobilities and the sizes of the Au NPs within the range of diameters from 5.3 to 59.9 nm; the relative standard deviations of the electrophoretic mobilities for these NPs were below 0.6%. When using these conditions to analyze Au NPs produced through seed-assisted synthesis, a good correlation existed between the sizes obtained using CE and those provided by transmission electron microscopy. Furthermore, the analyses performed using CE were rapid (<4 min) and required very low sample volumes (several nanoliters), yet they proceeded with high sensitivity. These findings confirm that CE is an efficient tool for characterizing the sizes of NPs.

On-line enhancement and separation of nanoparticles using capillary electrophoresis.

We describe a rapid, simple, and highly efficient capillary electrophoresis (CE)-based method for the analysis of nanoparticles (NPs). In this study, we used the reversed electrode polarity stacking mode (REPSM) of CE to assess the feasibility of enhancing the detection of Au NPs and Au/Ag NPs, optimizing parameters such as the length of time for which the REPSM was applied, the concentrations of the buffer and the sodium dodecylsulfate (SDS) surfactant, and the pH. Under the optimized on-line enhancement conditions [buffer: SDS (40 mM) and 3-cyclohexylamino-1-propanesulfonic acid (CAPS; 10 mM) at pH 10.0; applied voltage: 20 kV; REPSM applied for 24s], the detection limits of the Au NPs and Au/Ag NPs increased by ca. 30- and 140-fold, respectively. In addition, when the NPs were subjected to on-line enhancement and separation by CE using diode array detection (DAD), this approach allowed chemical characterization of the NP species. Our results suggest that such CE analyses will be useful for accelerating the rates of fabrication and characterization of future nanomaterials.

Analytical separation of Au/Ag core/shell nanoparticles by capillary electrophoresis.

This paper demonstrates that capillary electrophoresis (CE) can be employed for characterizing the sizes of a series of Au/Ag core/shell nanoparticles (NPs). We effected the CE separation of Au/Ag core/shell NPs using a mixed buffer of sodium dodecyl sulphate (SDS) (40 mM) and 3-(cyclohexylamino)propanesulfonic acid (10 mM) at pH 9.7 and an applied voltage of 20 kV. A linear relationship (R(2)>0.99) existed between the electrophoretic mobilities and the sizes of the Au/Ag core/shell NPs within the diameter range from 25 to 90 nm; the relative standard deviations of these electrophoretic mobilities were <0.9%. From the good correlation between the results obtained by CE and those provided by scanning electron microscopy, we confirmed that this CE method is a valid one for characterizing the sizes of Au/Ag core/shell NP samples. In addition, when the Au/Ag core/shell NPs were separated through CE and detected using an on-line photodiode array detector, this approach allowed the chemical characterization of the NP species. This CE approach should allow the rapid and cost-effective characterization of a number of future nanomaterials.

Open tubular capillary electrochromatography using capillaries coated with films of alkanethiol-self-assembled gold nanoparticle layers.

We have used alkanethiol self-assembly and dithiol layer-by-layer (LBL) self-assembly processes to prepare an Au nanoparticle (NP)-coated open tubular capillary electrochromatography (OTCEC) column for the separation of three neutral steroid drugs (testosterone, progesterone, and testosterone propionate). The CEC column was fabricated through LBL self-assembly of Au NPs on a 3-aminopropyltrimethoxysilane (APTMS)-modified fused-silica capillary and subsequent surface functionalization of the Au NPs through self-assembly of alkanethiols. We investigated the electrochromatographic properties of the resulting Au NP-coated CEC column using a "reversed phase" test mixture of three steroid drugs. We found that the key factors affecting the separation performance were the number of Au NP layers, the length of the carbon-atom chain of the alkanethiol self-assembled on the Au NPs, the percentage of organic modifier, and the pH of the running electrolyte. This study reveals that the self-assembly of alkanethiols and dithiols onto Au NPs provides stationary phases for CEC separation that are easy to prepare and whose retention behavior is highly controllable and reproducible. We believe that our findings will contribute to further studies of the application of nanotechnology to separation science.

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.

Extremely highly efficient on-line concentration and separation of gold nanoparticles using the reversed electrode polarity stacking mode and surfactant-modified capillary electrophoresis.

In this study, gold nanoparticles (Au NPs) were separated using the reversed electrode polarity stacking mode (REPSM) of a capillary electrophoresis (CE) system for on-line enhancement prior to performing surfactant-modified CE separation. Under optimized conditions [running electrolyte buffer, sodium dodecyl sulfate (70 mM) and 3-cyclohexylamino-1-propanesulfonic acid (10 mM) at pH 10.0; applied voltage, 20 kV; operating temperature, 25°C; REPSM strategy for sample on-line concentration; REPSM applied prior to initializing separation], two parameters were varied to further enhance the concentration and separation of the Au NPs: (i) the rate of polarity switching (from -20 to +20 kV) between the REPSM and surfactant-modified CE separation modes and (ii) the length of the capillary column. At a polarity switching rate of 1333 kV min(-1) and a column length of ca. 83.5 cm, the resolution of the separation of a mixture of 5.3- and 40.1-nm Au NPs was greater than 19; in addition, the numbers of theoretical plates for the 5.3- and 40.1-nm-diameter Au NPs were greater than 15,000 and up to 1.15×10(7), respectively-the latter being extremely high. Thus, this CE-based method for separating Au NPs provided high performance in terms of separation resolution and the number of theoretical plates, both of which were improved by greater than fivefold relative to those published previously. Notably, the sensitivity enhancement factors for the 5.3- and 40.1-nm-diameter Au NPs were improved (by ca. 20- and 500-fold, respectively) relative to those obtained using conventional surfactant-modified CE separation.

Monitoring the on-line concentration and separation of gold nanoparticles using the reversed electrode polarity stacking mode and micellar electrokinetic chromatography.

In this study, I separated gold nanoparticles (Au NPs) using the reversed electrode polarity stacking mode (REPSM) of a capillary electrophoresis system for on-line enhancement prior to performing micellar electrokinetic chromatography (MEKC). In the first part of this paper, I discuss the effects of several new and important parameters, the nature of the added salt, the pH of the sample matrix, and the size of the injection buffer plug that appears after introducing the samples, on the systems' on-line concentration and separation performance for a variety of sample compositions. In the second part of this paper, I describe my use of the optimized conditions to monitor the migration of the Au NPs during the concentration and separation processes. The results of these studies suggest that combining REPSM with MEKC is a very good strategy for the on-line concentration and separation of Au NPs at trace concentrations.

Using reversed-phase liquid chromatography to monitor the sizes of Au/Pt core/shell nanoparticles.

This paper describes the use of reversed-phase liquid chromatography (RPLC) to rapidly characterize Au/Pt core/shell nanoparticles (NPs) produced through seed-assisted synthesis. We monitored the sizes of Au/Pt core/shell NPs by using a porous silica-based RPLC column (pore size: ca. 100 nm) and 30 mM sodium dodecyl sulfate in deionized water as the mobile phase; the plot of the retention time with respect to the logarithm of the size of the Au NPs was linear (R(2)=0.997) for diameters falling in the range from 5.3 to 40.1 nm; from five consecutive runs, the relative standard deviations of these retention times were less than 0.4%. We used the optimal separation conditions of the RPLC system to study the effects that the rate of addition of the reducing agent and the volumes of the seed, shell precursor metal ion, and reducing agent solutions had on the sizes of the Au/Pt core/shell NPs. A good correlation existed between the sizes of the Au/Pt core/shell NPs determined through RPLC and those determined using transmission electron microscopy. RPLC appears to be a useful technique for monitoring the sizes of NPs and nanomaterials in general.

Using micellar electrokinetic chromatography for the highly efficient preconcentration and separation of gold nanoparticles.

In this paper, we report the use of micellar electrokinetic chromatography (MEKC) for the highly efficient preconcentration and separation of gold nanoparticles (Au NPs). We used the reversed electrode polarity stacking mode (REPSM) of the MEKC system for the on-line enhancement and separation of the Au NPs. Several parameters had dramatic effects on the systems' performance, including the concentration of sodium dodecylsulfate (SDS) surfactant, the presence of salts in the NP solution, the pH of the running electrolyte, and the temperature of the capillary. Under the optimized conditions [buffer: SDS (70 mM) and 3-cyclohexylamino-1-propanesulfonic acid (CAPS; 10 mM) at pH 10.0; applied voltage: 20 kV; operating temperature: 25 degrees C; additive: sodium dihydrogenphosphate (NaH(2)PO(4), 10 mM); REPSM strategy for sample preconcentration], the number of theoretical plates for the 5.3- and 40.1-nm-diameter Au NPs were 3000 and (an ultrahigh) 2.1 x 10(6), respectively; in addition, the detection sensitivities toward the Au NPs were enhanced ca. 20- and 380-fold, respectively, relative to those obtained using standard MEKC analysis conditions. Furthermore, monitoring the electropherograms using diode-array detection allowed us to identify and characterize the sizes of the separated NPs from their UV-vis spectra. Our findings suggest that MEKC is a highly efficient tool for both the preconcentration and separation of NPs.

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.

Combining optical lithography with rapid microwave heating for the selective growth of Au/Ag bimetallic core/shell structures on patterned silicon wafers.

We demonstrate a novel approach for the production of patterned films of nanometer-sized Au/Ag bimetallic core/shell nanoparticles (NPs) on silicon wafers. In this approach, we first self-assembled monodisperse Au NPs, through specific Au...NH(2) interactions, onto a silicon substrate whose surface had been modified with a pattern of 3-aminopropyltrimethoxysilane (APTMS) groups to form a sandwich structure having the form Au NPs/APTMS/SiO(2). These Au NPs then served as seeds for growing the Au/Ag bimetallic core/shell NPs: we reduced silver ions to Ag metal on the surface of Au seeds under rapid microwave heating in the presence of sodium citrate. Energy-dispersive X-ray analysis confirmed that the Au/Ag bimetallic core/shell NPs grew selectively on the regions of the surface of the silicon wafer that had been patterned with the Au seeds. Scanning electron microscopy images revealed that we could synthesize well-scattered, high-density (>82%) thin films of Au/Ag bimetallic core/shell NPs through the use of this novel strategy. The patterned structures that can be formed are simple to produce, easily controllable, and highly reproducible; we believe that this approach will be useful for further studies of nanodevices and their properties.