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Electrostatic interactions play an essential role in many analytical applications including molecular sensing and transport studies using nanopores and separation of charged species. Here, we report the voltammetric quantification of electrostatic ion enrichment in a 5-20 nm thin electrochemical cell. A simple lithographic micro/nanofabrication process was used to create ultrathin-layer cells (UTLCs) with a critical dimension (i.e., cell thickness) as small as 5 nm. The voltammetric response of a UTLC was found to be largely dominated by the electrostatic interaction between charges on the cell walls and the redox species. We show that the ultrasmall cell dimension yielded a 100-300-fold enrichment for cationic redox species. An interesting surface adsorption effect was also demonstrated. [ABSTRACT FROM AUTHOR]

We report direct observation of electrochemical and thermal dealloying processes of individual metal alloy nanoparticles (NPs). Electrochemical dealloying of single Ag-Hg alloy NPs was achieved in a basic solution (e.g., pH 13) by oxidizing Hg under controlled potentials. Ag can also be oxidized during single-particle collision. However, it requires elevated potentials. The strong basic environment promoted the formation of metal oxides during collision leading to a unique core-shell type nanostructure which was further confirmed by transmission electron microscopy (TEM). In thermal dealloying, Hg was evaporated due to the use of a high-energy electron beam and the process was imaged in situ inside a TEM. Both electrochemical and thermal dealloying processes resulted in the transformation of an amorphous NP to a more stable Ag-Hg alloy nanocrystal. This work demonstrates that NP collision can be a useful tool to study dealloying processes of various nanomaterials at a single-particle level. [ABSTRACT FROM AUTHOR]

Among human body fluids, serum plays a key role for diagnostic tests and, increasingly, for metabolomics analysis. However, the high protein content of serum poses significant challenges for nuclear magnetic resonance (NMR)-based metabolomics studies because it can strongly interfere with metabolite signal detection and quantitation. Although several methods for protein removal have been proposed, including ultrafiltration and organic-solvent-induced protein precipitation, there is currently no standard operating procedure for the elimination of protein from human serum samples. Here, we introduce novel procedures for the removal of protein from serum by the addition of nanoparticles. It is demonstrated how serum protein can be efficiently, cost-effectively, and environmentally friendly removed at physiological pH (pH 7.4) through attractive interactions with silica nanoparticles. It is further shown how serum can be processed with nanoparticles prior to ultrafiltration or organic-solvent-induced protein precipitation for optimal protein removal. After examination of all of the procedures, the combination of nanoparticle treatment and ultrafiltration is found to have a minimal effect on the metabolite content, leading to remarkably clean homo- and heteronuclear NMR spectra of the serum metabolome that compare favorably to other methods for protein removal. [ABSTRACT FROM AUTHOR]

Here, we report a nanopipette-based electrochemical approach to prepare metal nanoelectrodes with excellent control over electrode size, shape, and thickness of the insulation wall. Nanoelectrodes are prepared by electrochemical plating in a laserpulled quartz nanopipette tip immersed in a liquid gallium/indium alloy electrode, which not only protects the ultrasmall quartz tip but also starts electrodeposition from the tip orifice. This versatile approach enables reproducible fabrication of electrodes of several different metals, including gold, platinum, silver, and copper. Moreover, nanoelectrodes with varying sizes can be easily prepared by focused ion-beam milling. A unique aspect of this method is the control over the thickness of quartz insulation walls relative to the size of the electroactive surface enabling control of the RG (defined as the radius of the insulating sheath over the radius of the active metal electrode). As such, these nanoelectrodes may be especially attractive as useful nanoprobes in high-resolution imaging applications, such as scanning electrochemical microscopy. [ABSTRACT FROM AUTHOR]

Metabolomics aims at a complete characterization of all metabolite's in biological samples in terms of both their identities and concentrations. Because changes of metabolite's and their concentrations are a direct reflection of cellular activity, it allows for a better understanding of cellular processes and function to be obtained. Although NMR spectroscopy is routinely applied to complex biological mixtures without purification, overlapping NMR peaks often pose a challenge for the comprehensive and accurate identification of the underlying metabolite's. To address this problem, we present a novel nanoparticle-based strategy that differentiates between metabolite's based on their electric charge. By adding electrically charged silica nanoparticles to the solution NMR sample, metabolite's of opposite charge bind to the nanoparticles and their NMR signals are weakened or entirely suppressed due to peak broadening caused by the slow rotational tumbling of the nanometer-sized nanoparticles. Comparison of the edited with the original spectrum significantly facilitates analysis and reduces ambiguities in the identification of metabolite's. This method makes NMR directly sensitive to the detection of molecular charges at constant pH, as demonstrated here both for model mixtures and human urine. The simplicity of the approach should make it useful for a wide range of metabolomics applications. [ABSTRACT FROM AUTHOR]

Fluorescence-enabled electrochemical microscopy (FEEM) is demonstrated as a new technique to image o transient concentration profiles of redox species generated on ultramicroelectrodes (UMEs). FEEM converts an electrical signal into an optical signal by electrically coupling a conventional redox reaction to a fluorogenic reporter reaction on a closed bipolar electrode. We describe the implementation of FEEM for diffusion layer imaging and use an array of thousands of parallel bipolar electrodes to image the diffusion layers of UMEs in two and three dimensions. This new technique provides a way to image an entire 2-dimensional lateral cross section of a dynamic diffusion layer in a single experiment By taking several of these lateral cross sections at different axial positions in the diffusion layer, a 3-dimensional image of the diffusion layer can be built. We image the diffusion layer of a 10 μm diameter carbon fiber electrode over the course of a cyclic voltammetry experiment and compare the FEEM-generated images to concentration profiles generated from numerical simulation. We also image the diffusion layer of a two electrode array consisting of two 10 pm diameter carbon fibers over the course of a potential step experiment. [ABSTRACT FROM AUTHOR]

Recently, we introduced an online multijunction capillary isoelectric focusing (OMJ-CIEF) fractionator to fractionate proteins and peptides in electrospray-friendly solution. In this follow-up study, the original configuration of the fractionator was modified to improve the resolving power and reproducibility of separation. The major improvements include stabilization of the electrical current through the device using a voltage divider and stepwise elution of peptide zones in conjunction with the repeated refocusing of remaining peptides. Also, a novel algorithm was developed to calculate more accurately the pi values of peptides identified from experimental data. The standard deviation of calculated pi values for unmodified peptides from the theoretically predicted pi values was on average 0.21 pH units, which is more accurate than in standard-resolution gel-based methods. In order to characterize the analytical performance of the improved device, it was applied for the pi fractionation of yeast proteome digest into 18 fractions, with the collected fractions being analyzed by reverse-phase liquid chromatography coupled with tandem mass spectrometry. Approximately 37% of 20047 identified peptides were detected in only one fraction and 27% - in two fractions. On average, every peptide was found in 2.4 fractions. These results strongly indicate the suitability of the improved device as a first dimension of separation in multidimensional shotgun proteomics analysis, with a potential for fully automated workflow. [ABSTRACT FROM AUTHOR]

Continuous plasma separation will be greatly helpful for dynamic metabolite monitoring in kinetics research and drug development. In this work, we proposed a continuous on-chip plasma separation method based on the natural aggregating and sedimentation behavior of red blood cells at low shear rate. In this approach, a glass capillary was first used to realize quick and obvious delamination of blood cells from plasma. A novel "dual-elbow" connector was designed to change the direction of delamination. The blood was finally separated by laminar flow and bifurcation on the microchip. Results demonstrated that the present device can efficiently and stably separate plasma from blood in a continuous means, e.g., in a 4 h separation we did not observe clogging or a trend of clogging. In addition, the present approach can avoid the damage to cells which usually occurs in separation with high shear rate in a microchannel and possible contaminants to plasma. The proposed microchip device is robust, simple, and inexpensive for long time plasma separation with high plasma recovery and less sample consumption. The present work provides an effective tool for metabolite monitoring in pharmacokinetics research and drug development. [ABSTRACT FROM AUTHOR]

Carbon-fiber-microelectrode arrays (MEAs) have been utilized to electrochenilcally image neurochemical secretion from individual pheochromocytoma (PC12) cells. Dopamine release events were elecirochemicaily monitored from seven different locations on single Pci 2 cells using alternately constant-potential amperomehy and fast-scan cyclic voltammetzy (FSCV). Cyclic voltammetry, when compared to amperometry, can provide excellent chemical resolution; however, spatial and temporal resolution are both compromised. The spatial and temporal resolution of these two methods have been quantitatively compared and the differences explained using models of molecular diffusion at the nanogap between the electrode and the cell. A nuinencal simulation of the molecular flux reveals that the diffusion of dopamine molecules and elecirochemical reactions both play important roles in the temporal resolution of elecirochemical imaging. The simulation also reveals that the diffusion and electrode potential cause the differences in signal crosstalk between electrodes when comparing amperometxy and FSCV. [ABSTRACT FROM AUTHOR]

We present the preparation, characterization, and analytical application of silica nanochannels in the size range of 5-100 nm. These cylindrical-shaped nanochannels are prepared using a simple laser-assisted mechanical pulling process, followed by partial enclosure into a glass mi- cropipet. The nanochannels are characterized using a combination of optical microscopy, scanning electron microscopy (SEM), and resistance measurements in an electrolyte solution. The SEM results show that the nanochannel has circular geometry at the orifice. Ohmic response has been obtained from current-voltage measurements in KCI solutions using a silica nanochannel as small as 9 nm in diameter. These nanochannels have been utilized to sense single 40 nm polystyrene nanoparticles. A linear response has been observed between the detec- tion rate and the concentration of nanoparlicles in the range of 0-25 nM. The silica nanochannels have also been applied to the study of molecular transport of double-stranded DNA. Electroosmosis-driven molecular translocation has been observed for genomic-length )-DNA through a 9 mu nanochannel in a 3 M KCl solution. [ABSTRACT FROM AUTHOR]

We report the fabrication and characterization of carbon microelectrode arrays (MEAs) and their application to spatially and temporally resolve neurotransmitter release from single pheochromocytoma (PC12) cells. The carbon MEAs are composed of individually addressable 2.5-µm-radius microdisks embedded in glass. The fabrication involves pulling a multibarrel glass capillary containing a single carbon fiber in each barrel into a sharp tip, followed by beveling the electrode tip to form an array (10-20 µm) of carbon microdisks. This simple fabrication procedure eliminates the need for complicated wiring of the independent electrodes, thus allowing preparation of high- density individually addressable microelectrodes. The carbon MEAs have been characterized using scanning electron microscopy, steady-state and fast-scan voltammetry, and numerical simulations. Amperometric results show that subcellular heterogeneity in single-cell exocytosis can be electrochemically detected with MEAs. These ultrasmall electrochemical probes are suitable for detecting fast chemical events in tight spaces, as well as for developing multifunctional electrochemical microsensors. [ABSTRACT FROM AUTHOR]

Large perfusive silica beads (particle size 110 μm, throughpore ∼2 μm) held in place by the keystone effect were used as single-particle flits for the manufacture of particulate packed capillary columns. High-quality capillary electrochromatographic separations of a standard test mixture of alkylbenzenes were obtained over the full voltage range of 5-30 kV, with no requirement for pressurization. Excellent robustness was demonstrated by the reproducibility of migration times, peak efficiencies, and resolution during 100 consecutive runs at the highest voltage (30 kV) without thermostating and pressurization. Superior performance relative to traditional sinter-fritted columns is ascribed to the heat-free flitting process and short flit length of ∼110 pm. [ABSTRACT FROM AUTHOR]

The steady-state voltammetric response of the truncated conical-shaped glass nanopore electrode is presented. Analytical theory, finite-element simulations, and experimental measurement of the diffusive flux of a redox molecule through the pore orifice demonstrate that the steady-state current decreases rapidly as the pore depth increases and then asymptotically approaches a constant value when the pore depth is ∼50× larger than the pore orifice. The asymptotic limit of the steady-sate current is only a function of the pore orifice radius and the cone angle of the pore and has a finite value for all cone angles greater than zero. Experimental confirmation of the predicted dependence on pore depth is obtained using nanopore electrodes with 100-1000 nm orifice radii, by measuring the steady-state voltammetric current corresponding to the oxidation of ferrocene in acetonitrile solutions containing an excess of supporting electrolyte. [ABSTRACT FROM AUTHOR]

The interstitial spaces between spherical particles in a packed column can act as a sieve that passes microorganisms below a certain size. If the bed is a perfusion-type material (containing a binary distribution of large and small pores), colloidal-size microorganisms are subject only to pore exclusion, while all molecules are subject to size exclusion among the various pores. Thus, microorganisms elute first, followed by macromolecules, and then small molecules. Coupling this separation method to an ICP magnetic sector mass spectrometer provides a sensitive, direct means to study the microbial uptake of heavy metals (i.e., uranium) from theft surrounding environments. Multiple metal ions can be monitored in the microorganism and in the surrounding solution. In this way, definitive information can be provided for the remediation of radioactive waste sites. The effect of uranium on microbial growth is also discussed. [ABSTRACT FROM AUTHOR]

Evaluates the chiral stationary phases of high performance liquid chromatography. Classification of chiral selectors; Measurement of amino acid enantiomeric impurity; Guidelines for the selection of proper chiral stationary phase columns.