Biomedical subjects

Joseph Wang

Publications and source records attributed to Joseph Wang.

At least 19 recordsLinked to original sources

PubMed2003

On-chip integration of enzyme and immunoassays: simultaneous measurements of insulin and glucose.

A microfluidic device coupling immunological and enzymatic assays within a single microchannel has been developed for simultaneous measurements of insulin and glucose. Such a dual-mode (enzyme/immuno) protocol involves precolumn reactions of insulin and glucose with the enzyme-labeled anti-human insulin and glucose-dehydrogenase/NAD+, respectively, followed by the electrophoretic separation of the free antibody, antibody-antigen complex, and the NADH product of the enzymatic reaction. The separation is followed by a postcolumn reaction of the alkaline-phosphatase tracer with the p-NPP substrate and a downstream amperometric detection of the p-nitrophenol and NADH products. Despite the huge concentration difference [millimolar (glucose) and nanomolar (insulin)] and the use of different assay principles, the new biochip responds independently to the corresponding target analytes, with linear dynamic ranges over their clinically relevant ranges. Complete assays, carried out within less than 4 min, lead to good precision (RSD 0.36%) for the insulin/glucose ratio. The resulting biochip allows simultaneous testing for insulin and glucose to be performed more rapidly, easily, and economically, and hence it holds great promise for improved management of diabetes.

Antibodies↗

PubMed2003

Carbon nanotube/teflon composite electrochemical sensors and biosensors.

The fabrication and attractive performance of carbon nanotube (CNT)/Teflon composite electrodes, based on the dispersion of CNT within a Teflon binder, are described. The resulting CNT/Teflon material brings new capabilities for electrochemical devices by combining the advantages of CNT and "bulk" composite electrodes. The electrocatalytic properties of CNT are not impaired by their association with the Teflon binder. The marked electrocatalytic activity toward hydrogen peroxide and NADH permits effective low-potential amperometric biosensing of glucose and ethanol, respectively, in connection with the incorporation of glucose oxidase and alcohol dehydrogenase/NAD(+) within the three-dimensional CNT/Teflon matrix. The accelerated electron transfer is coupled with minimization of surface fouling and surface renewability. These advantages of CNT-based composite devices are illustrated from comparison to their graphite/Teflon counterparts. The influence of the CNT loading upon the amperometric and voltammetric data, as well as the electrode resistance, is examined. SEM images offer insights into the nature of the CNT/Teflon surface. The preparation of CNT/Teflon composites overcomes a major obstacle for creating CNT-based biosensing devices and expands the scope of CNT-based electrochemical devices.

Biosensing Techniques↗

PubMed2003

Microscale solid-phase extraction system for explosives.

A simple, semi-automated, microcolumn solid-phase extraction (SPE) system is optimized for the extraction, preconcentration and HPLC analysis of seven different explosives and explosive derivatives contaminating seawater, river water and well water samples. The microcolumns were constructed from 1/16 in. O.D. PTFE tubing (1 in.=2.54 cm) packed with 0.5-1.5 mg of SPE material. LiChrolut EN or Porapak R. The extraction system consisted of two syringe pumps and several solenoid valves. Optimal detection limits were realized when the sample water flow-rate was maximally increased within the limits of the pump, 5-10 ml/min (despite exceeding the breakthrough threshold of the SPE microcolumn), and when the eluate volume collected from the column was minimized, <5 microl (despite very low recovery percentages).

Automation↗

PubMed2003

Electrochemical coding technology for simultaneous detection of multiple DNA targets.

Nucleic-acid hybridization assays based on the use of different inorganic-colloid (quantum dots) nanocrystal tracers for the simultaneous electrochemical measurements of multiple DNA targets are described. Three encoding nanoparticles (zinc sulfide, cadmium sulfide, and lead sulfide) are used to differentiate the signals of three DNA targets in connection to stripping-voltammetric measurements of the heavy metal dissolution products. These products yield well-defined and resolved stripping peaks at -1.12 V (Zn), -0.68 V (Cd), and -0.53 V (Pb) at the mercury-coated glassy-carbon electrode (vs Ag/AgCl reference). The position and size of these peaks reflect the identity and level of the corresponding DNA target. The multi-target detection capability is coupled to the amplification feature of stripping voltammetry (to yield femtomole detection limits) and with an efficient magnetic removal of nonhybridized nucleic acids to offer high sensitivity and selectivity. The protocol is illustrated for the simultaneous detection of three DNA sequences related to the BCRA1 breast-cancer gene in a single sample in connection to magnetic beads bearing the corresponding oligonucleotide probes. The new electrochemical coding is expected to bring new capabilities for DNA diagnostics, and for bioanalysis, in general.

Breast Neoplasms↗

PubMed2003

Solubilization of carbon nanotubes by Nafion toward the preparation of amperometric biosensors.

The ability to solubilize single-wall and multiwall carbon nanotubes (CNT) in the presence of the perfluorinated polymer Nafion is described. Such use of Nafion as a solubilizing agent for CNT overcomes a major obstacle for creating CNT-based biosensing devices. Their association with Nafion does not impair the electrocatalytic properties of CNT. The resulting CNT/Nafion modified glassy-carbon electrodes exhibit a strong and stable electrocatalytic response toward hydrogen peroxide. The marked acceleration of the hydrogen peroxide redox process is very attractive for the operation of oxidase-based amperometric biosensors, as illustrated for the highly selective low-potential (-0.05 V vs Ag/AgCl) biosensing of glucose. These findings open the door for using CNT in a wide range of chemical sensors and nanoscale electronic devices.

Biosensing Techniques↗

PubMed2003

Microchip capillary electrophoresis coupled with a boron-doped diamond electrode-based electrochemical detector.

The attractive behavior and advantages of a diamond electrode detector for a micromachined capillary electrophoresis (CE) system are discussed. A chemically vapor-deposited boron-doped diamond (BDD) film band (0.3 x 6.0 mm) electrode is used for end-column amperomettic detection. The favorable performance of the diamond electrode microchip detector is indicated from comparison to a commonly used thick-film carbon detector. The diamond electrode offers enhanced sensitivity, lower noise levels, and sharper peaks for several groups of important anaytes (nitroaromatic explosives, organophosphate nerve agents, phenols). The favorable signal-to-background characteristics of the BDD-based CE detector are coupled with a greatly improved resistance to surface fouling and greater isolation from high separation voltages. The enhanced stability is indicated from a RSD of 0.8% for 60 repetitive measurements of 5 ppm 2,4,6-trinitrotoluene (vs RSD of 10.8% at the thick-film carbon electrode). A highly linear response is obtained for the explosives 1,3-dinitrobenzene and 2,4-dinitrotoluene over the 200-1,400 ppb range, with detection limits of 70 and 110 ppb, respectively. Factors influencing the performance of the BDD detector are assessed and optimized. The attractive properties of BDD make it very promising material for electrochemical detection in CE microchip systems and other micromachined flow analyzers.

Air Pollutants, Occupational↗

PubMed2003

Application of diamond microelectrodes for end-column electrochemical detection in capillary electrophoresis.

Highly boron-doped diamond microelectrodes were employed in an end-column electrochemical detector for capillary electrophoresis (CE). The diamond microline electrodes were fabricated from conducting diamond thin films (exposed surface area, 300 x 50 microm), and their analytical performance as CE detectors was evaluated in a laboratory-made CE installation. The CE-ED system exhibited high separation efficiency for the detection of several catecholamines, including dopamine (DA), norepinephrine (NE), and epinephrine (E), with excellent analytical performance, for example, 155,000 theoretical plates for DA. The diamond-based electrochemical detection system also displayed low detection limits (approximately 20 nM for E at S/N = 3) and a highly reproducible current response with 10 repetitive injections of mixed analytes containing DA, NE, and E (each 50 microM), with relative standard deviations (RSD) of approximately 5%. The performance of the diamond detector in CE was also evaluated in the detection of chlorinated phenols (CP). When compared to the carbon fiber microelectrode, the diamond electrode exhibited lower detection limits in an end-column CE detection resulting from very low noise levels and highly reproducible analyses without electrode polishing due to analyte fouling, which makes it possible to perform easier and more stable CE analysis.

Journal Article↗

PubMed2003

Microfabricated electrophoresis chip for bioassay of renal markers.

A novel capillary electrophoresis chip-based detection system for simultaneous measurements of the renal markers creatine, creatinine, p-aminohippuric acid, and uric acid is described. Fluid control is used for mixing the sample with the enzymes creatininase (CA), creatinase (CI), and sarcosine oxidase (SOx) and for separating the neutral hydrogen peroxide end product from the anionic p-aminohippuric and urate species. The 'total' (creatinine and creatine) signal was measured with the running buffer containing all three enzymes, while the creatine signal alone was recorded by mixing the sample with the CI-SOx solution. Creatinine concentrations are measured by comparing the response in the presence and absence of CA. The peroxide product and the oxidizable p-aminohippuric and uric acids are detected electrochemically at a downstream gold-coated thick-film amperometric detector. The four renal markers are readily measured within 5 min, while creatinine/creatine within less than 2 min. Factors influencing the performance, including the level of three enzymes, separation voltage, and detection potential, are optimized. Applicability to urine samples is demonstrated. Such a multianalyte microchip detection device would allow renal function testing to be performed more rapidly, easily, and economically in the point-of-care setting.

Biomarkers↗

PubMed2003

Nonaqueous electrophoresis microchip separations: conductivity detection in UV-absorbing solvents.

The use of organic solvents in microfabricated capillary electrophoresis (CE) devices is demonstrated in connection with the separation of aliphatic amines in pure dimethylformamide, dimethylacetamide, dimethyl sulfoxide, or propylene carbonate media. Contactless conductivity detection is employed for monitoring the separated solutes in these UV-absorbing solvents. The effect of the physicochemical properties of the organic solvent upon the migration behavior is investigated. The apparent mobility increases nearly linearly with the reciprocal of the solvent viscosity, while the electroosmotic mobility increases in a linear fashion with the dielectric constant/ viscosity ratio. Some deviation from theoretical predictions is observed using propylene carbonate. The nonaqueous CE microchip offers high separation efficiency, reflected in plate numbers ranging from 93,680 to 127,680, using a separation voltage of +3,000 V, a dimethylformamide medium, and a contactless conductivity detection. Experimental parameters affecting the analytical performance of the nonaqueous CE/conductivity microchip are examined. Calibration and precision experiments indicate a linear and reproducible response. Such use of organic solvents can benefit microchip separations through extended scope (toward nonpolar solutes) and tunable selectivity.

Journal Article↗

PubMed2003

Flow injection amperometric detection of OP nerve agents based on an organophosphorus-hydrolase biosensor detector.

A flow-injection system with an organophosphorus-hydrolase (OPH)-biosensor detector has been developed and characterized for the rapid detection of organophosphorus (OP) nerve agents. The enzyme was immobilized onto a thin-film gold detector through a cystamine-glutaraldehyde coupling. Factors influencing the performance were optimized. The resulting flow system offered a fast, sensitive, selective, and stable response. The peak current increased linearly with the concentration of paraoxon and methyl parathion over the 1-10 microM range (sensitivity, 2.29 and 1.04 nA/microM, respectively). The OPH-biosensor flow injection systems offered low detection limits (e.g. 0.1 microM paraoxon), along with a good precision (R.S.D. of 3.6% for 20 successive injections of a 1.0 microM paraoxon solution). The OPH-biosensor flow detector offers great promise for rapid field screening of OP pesticides and nerve agents.

Aryldialkylphosphatase↗

PubMed2003

Carbon-nanotube-modified glassy carbon electrodes for amplified label-free electrochemical detection of DNA hybridization.

The preparation and attractive performance of carbon-nanotube modified glassy-carbon (CNT/GC) electrodes for improved detection of purines, nucleic acids, and DNA hybridization are described. The surface-confined multiwall carbon-nanotube (MWCNT) facilitates the adsorptive accumulation of the guanine nucleobase and greatly enhances its oxidation signal. The advantages of CNT/GC electrodes are illustrated from comparison to the common unmodified glassy carbon, carbon paste and graphite pencil electrodes. The dramatic amplification of the guanine signal has been combined with a label-free electrical detection of DNA hybridization. Factors influencing the enhancement of the guanine signal are assessed and optimized. The performance characteristics of the amplified label-free electrochemical detection of DNA hybridization are reported in connection to measurements of nucleic-acid segments related to the breast-cancer BRCA1 gene.

Animals↗

PubMed2002

Dual conductivity/amperometric detection system for microchip capillary electrophoresis.

The performance characteristics and advantages of a new dual electrochemical microchip detection system based on simultaneous conductivity and amperometric measurements are described. The system relies on the combination of a contactless conductivity detector with an end-column thick-film amperometric detector. Such coupling of the conductivity and amperometric detection modes in a single separation channel greatly enhances the sample characterization to offer simultaneous measurements of both ionic and electroactive species, improved reproducibility, and confirmation of peak identity. The simultaneous measurement of nitroaromatic and ionic explosives is used for demonstrating the ability to detect both electroactive and ionic species. Major improvements are also observed for analytes responding at both detectors. For example, the generation of dual response ratios can be used to improve the reproducibility and confirm the peak identity/integrity. Such dual response ratios reflect the distinct redox and conductivity properties of the individual analytes. The independence of the two detectors is reflected in the absence of "cross-talk" effects. The behavior of the dual detector is comparable with those of the individual detectors. Such a dual electrochemical detection system is easy to implement and requires inherently portable low-cost instrumentation.

Aminophenols↗

PubMed2002

Measurements of chemical warfare agent degradation products using an electrophoresis microchip with contactless conductivity detector.

This paper reports on a microfluidic device for the screening of organophosphonate nerve agent degradation products. The miniaturized system relies on an efficient chip-based separation of alkyl methylphosphonic acids (breakdown products of Sarin, Soman, and VX nerve agents) followed by their sensitive contactless conductivity detection. Experimental parameters relevant to the separation and detection processes have been optimized to yield high sensitivity (with 48-86 microg L(-1) detection limits), fast response (50 s for a three alkyl methylphosphonic acid mixture), high precision (RSD = 3.8-5.0%), and good linearity (over the 0.3-100 mg L(-1) range). Applicability to natural (river) water samples is demonstrated. The new microsystem offers promise for monitoring degradation products of chemical warfare agents, with advantages of speed/warning, efficiency, portability, sample size, and cost compared to conventional ion chromatography or capillary electrophoresis systems.

Chemical Warfare Agents↗

PubMed2002

Contactless conductivity detector for microchip capillary electrophoresis.

A microfabricated electrophoresis chip with an integrated contactless conductivity detection system is described. The new contactless conductivity microchip detector is based on placing two planar sensing aluminum film electrodes on the outer side of a poly(methyl methacrylate) (PMMA) microchip (without contacting the solution) and measuring the impedance of the solution in the separation channel. The contactless route obviates problems (e.g., fouling, unwanted reactions) associated with the electrode-solution contact, offers isolation of the detection system from high separation fields, does not compromise the separation efficiency, and greatly simplifies the detector fabrication. Relevant experimental variables, such as the frequency and amplitude of the applied ac voltage or the separation voltage, were examined and optimized. The detector performance was illustrated by the separation of potassium, sodium, barium, and lithium cations and the chloride, sulfate, fluoride, acetate, and phosphate anions. The response was linear (over the 20 microM-7 mM range) and reproducible (RSD = 3.4-4.9%; n = 10), with detection limits of 2.8 and 6.4 microM (for potassium and chloride, respectively). The advantages associated with the contactless conductivity detection, along with the low cost of the integrated PMMA chip/detection system, should enhance the power and scope of microfluidic analytical devices.

Anions↗

PubMed2002

Magnetically-induced solid-state electrochemical detection of DNA hybridization.

A magnetic triggering of a solid-state electrical transduction of DNA hybridization is described. Positioning of an external magnet below the thick-film electrode attracts the DNA/particle network and enables the solid-state electrochemical stripping detection of the silver tracer. TEM imaging indicates that the hybridization event results in a three-dimensional aggregate structure in which duplex segments link the metal nanoparticles and magnetic spheres, and that most of this assembly is covered with the silver precipitate. This leads to a direct contact of the metal tag with the surface (in connection to the magnetic collection) and enables the solid-state electrochemical transduction (without prior dissolution and subsequent electrodeposition of the metal), using oxidative dissolution of the silver tracer. No such aggregates (and hence magnetic "collection") are observed in the presence of noncomplementary DNA, that is, without the linking hybrid. The new method couples high sensitivity of silver-amplified assays with effective discrimination against excess of closely related nucleotide sequences (including single-base imperfections). Such direct electrical detection of DNA/metal-particle assemblies can bring new capabilities to the detection of DNA hybridization, and could be applied to other bioaffinity assays.

DNA↗

PubMed2002

Capillary electrophoresis-electrochemistry microfluidic system for the determination of organic peroxides.

A microfluidic analytical system for the separation and detection of organic peroxides, based on a microchip capillary electrophoresis device with an integrated amperometric detector, was developed. The new microsystem relies on the reductive detection of both organic acid peroxides and hydroperoxides at -700 mV (vs. Ag wire/AgCl). Factors influencing the separation and detection processes were examined and optimized. The integrated microsystem offers rapid measurements (within 130 s) of these organic-peroxide compounds, down to micromolar levels. A highly stable response for repetitive injections (RSD 0.35-3.12%; n = 12) reflects the negligible electrode passivation. Such a "lab-on-a-chip" device should be attractive for on-site analysis of organic peroxides, as desired for environmental screening and industrial monitoring.

Electrochemistry↗

PubMed2002

Optimal environment for glucose oxidase in perfluorosulfonated ionomer membranes: improvement of first-generation biosensors.

An optimal environment for glucose oxidase (GOx) in Nafion membranes is achieved using an advanced immobilization protocol based on a nonaqueous immobilization route. Exposure of glucose oxidase to water-organic mixtures with a high (85-95%) content of the organic solvent resulted in stabilization of the enzyme by a membrane-forming polyelectrolyte. Such an optimal environment leads to the highest enzyme specific activity in the resulting membrane, as desired for optimal use of the expensive oxidases. Casting solution containing glucose oxidase and Nafion is completely stable over 5 days in a refrigerator, providing almost absolute reproducibility of GOx-Nafion membranes. A glucose biosensor was prepared by casting the GOx-Nafion membranes over Prussian Blue-modified glassy carbon disk electrodes. The biosensor operated in the FIA mode allows the detection of glucose down to the 0.1 microM level, along with high sensitivity (0.05 A M(-1) cm(-2)), which is only 10 times lower than the sensitivity of the hydrogen peroxide transducer used. A comparison with the recently reported enzyme electrodes based on similar H2O2 transducers (transition metal hexacyanoferrates) shows that the proposed approach displays a dramatic (100-fold) improvement in sensitivity of the resulting biosensor. Combined with the attractive performance of a Prussian Blue-based hydrogen peroxide transducer, the proposed immobilization protocol provides a superior performance for first-generation glucose biosensors in term of sensitivity and detection limits.

Biosensing Techniques↗

PubMed2002

Single-channel microchip for fast screening and detailed identification of nitroaromatic explosives or organophosphate nerve agents.

A single-channel chip-based analytical microsystem that allows rapid flow injection measurements of the total content of organic explosive or nerve agent compounds, as well as detailed micellar chromatographic identification of the individual ones, is described. The protocol involves repetitive rapid flow injection (screening) assays--to provide a timely warning and alarm--and switching to the separation (fingerprint identification) mode only when harmful compounds are detected. While micellar electrokinetic chromatography, in the presence of sodium dodecyl sulfate (SDS), is used for separating the neutral nitroaromatic explosive and nerve agent compounds, an operation without SDS leads to high-speed measurements of the "total" explosives or nerve agent content. Switching between the "flow injection" and "separation" modes is accomplished by rapidly exchanging the SDS-free and SDS-containing buffers in the separation channel. Amperometric detection was used for monitoring the separation. Key factors influencing the sample throughput, resolution, and sensitivity have been assessed and optimized. Assays rates of about 360 and 30/h can thus be realized for the "total" screening and "individual" measurements, respectively. Ultimately, such development will lead to the creation of a field-deployable microanalyzer and will enable transporting the forensic laboratory to the sample source.

Chemical Warfare Agents↗