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Biomedical subjects

G S Cha

Publications and source records attributed to G S Cha.

16 recordsLinked to original sources
PubMed2001

A planar amperometric creatinine biosensor employing an insoluble oxidizing agent for removing redox-active interferences.

A planar microchip-based creatinine biosensor employing an oxidizing layer (e.g., a PbO2 film), where interfering redox-active substances are broken (i.e., oxidized) to redox-inactive products, was developed to facilitate the microfabrication of the sensor and to provide improved, reliable determination of creatinine in physiological samples. The feasibility of using hydrophilic polyurethanes in permselective barrier membranes for creatinine biosensors and the effect of adding a silanizing agent (adhesion promoter) on the sensor performance (e.g., sensitivity, stability, and lifetime) are described. The proposed creatinine microsensor with a three-layer configuration, i.e., enzyme, protecting, and oxidizing layers, exhibits good electrochemical performance in terms of response time (t95% = 98 s at 100-->200 microM creatinine change), linearity (1-1000 microM, r = 0.9997), detection limit (0.8 microM), and lifetime (approximately 35 days). The creatinine biosensor devised in a differential sensing arrangement that compensates the erroneous results from creatine is considered to be suitable for assay of serum specimens.

Biosensing Techniques↗
PubMed2001

Effect of pre-treatment on the surface and electrochemical properties of screen-printed carbon paste electrodes.

The effect of various electrochemical pre-treatment methods on the surface and electrochemical properties of screen-printed carbon paste electrodes (SPCE) prepared with three different commercial products was examined. It was observed that a positively charged redox couple, e.g., hexaammineruthenium(III), exhibited quasi-reversible behavior at the untreated SPCE. However, the cyclic voltammograms (CVs) of the SPCE prepared with general-purpose carbon inks did not exhibit clear redox peaks to other representative redox couples [e.g., hexacyanoferrate(III), hexachloroiridate(IV), dopamine, and hydroquinone] without activation. Electrochemical pre-treatment methods were sought in four different aqueous solutions, i.e., sulfuric acid, potassium chloride, sodium hydrogencarbonate, and sodium carbonate, applying various activation potentials. It was found that the pre-treatment procedure in saturated Na2CO3 solution at 1.2 V provides a mild and effective condition for activating the SPCE. By measuring the water contact angles at the SPCE surfaces and recording their SEM images, it was confirmed that the electrochemical pre-treatment effectively removes the organic binders from the surface carbon particles. A prolonged period of activation (> 5 min) or the use of high potentials (> 1.2 V) increased the capacitance of the electrode over 20 microF cm(-2). The pre-treated SPCE behaved like a random array microelectrode, exhibiting a sigmoidal-shaped CV at a slow scan rate. The short pre-anodization method in Na2CO3 solution was generally applicable to most SPCE prepared with general-purpose carbon inks.

Journal Article↗
PubMed2001

Effect of dissolved CO2 on the potential stability of all-solid-state ion-selective electrodes.

The influence of dissolved CO2 on the potentiometric responses of all-solid-state ion-selective electrodes (ISEs) was systematically examined with four different types of electrodes fabricated by pairing pH-sensitive and pH-insensitive metal electrodes (Pt and Ag/AgCl, respectively) with pH-sensitive and pH-insensitive ion-selective membranes (H+-selective membrane based on tridodecylamine and Na+-selective membrane based on tetraethyl calix[4]arenetetraacetate, respectively). The experimental results clearly showed that the carbonic acid formed by the diffused CO2 and water vapor at the membrane/metal electrode interface varies the phase boundary potentials both at the inner side of the H+-selective membrane (deltaE(in)mem) and at the metal electrode surface (deltaEelec). The potential changes, deltaE(in)mem and deltaEelec, occurring at the facing boundaries, are opposite in their sign and result in a canceling effect if both the membrane and metal surface are pH-sensitive. Consequently, the H+-selective membrane coated on a pH-sensitive electrode (Pt) tends to exhibit a smaller CO2 interference than that on a pH-insensitive electrode (Ag/AgCl). When the all-solid-state Na+ and K+ ISEs were fabricated with both pH-insensitive metal electrode and ion-selective membrane, they did not suffer from CO2 interference. It was also confirmed that plasticization of the PVC leads to increased CO2 permeation. Various types of intermediate layers were examined to reduce the CO2 interference problem in the fabrication of H+-selective all-solid-state ISEs. The results indicated that the H+-selective electrode needs an intermediate layer that maintains a constant pH unless the carbonic acid formation at the interfacial area is effectively quenched.

Journal Article↗
PubMed2000

Ion chromatography detector based on solid-state ion-selective electrode array.

A variety of neutral carrier type ionophores for monovalent cations were employed to prepare solid-state cation-selective electrodes (SSEs) for use as a detector in single-column ion chromatography (IC). The polyurethane-based pseudoreference electrode made it possible to assemble an array type SSE detector for IC. An SSE-based detector provides not only the overall chromatogram for the separated ion species (monensin methyl ester-nonactin-based membrane), but also the enhanced chromatogram for specified ions of interest (valinomycin as K+ and nonactin for NH4+). This feature makes it possible to perform highly quantitative analysis with low detection limits even if the separation efficiency of the ion-exchange is not sufficient. Since SSE-based IC detectors are easily miniaturized and replaceable at low cost, they are an ideal component of a portable IC system.

Cations, Monovalent↗
PubMed2000

A planar pCO2 sensor with enhanced electrochemical properties.

To develop planar microchemical pCO2 sensing devices with improved electrochemical properties, we combined two advanced technologies. One is a differential sensor arrangement to simplify the microfabrication procedure by employing pH-sensitive gas-permeable membranes, and the other is the use of an enzyme (carbonic anhydrase) to shorten total measurement time by accelerating the rate of CO2 hydration. The adhesion of the polyurethane-matrix gas-permeable membrane is enhanced significantly by incorporating a silanizing reagent (silicon tetrachloride), improving the stability and extending sensor lifetime. The proposed differential pCO2 microelectrodes exhibited significantly improved performance in their preconditioning period, response and recovery times, stability, response slope, and lifetime.

Biosensing Techniques↗
PubMed2000

A disposable amperometric sensor screen printed on a nitrocellulose strip: a glucose biosensor employing lead oxide as an interference-removing agent.

A new type of disposable amperometric sensor is devised by screen printing thick-film electrodes directly on a porous nitrocellulose (NC) strip. The chromatographic NC strip is then utilized to introduce various sample pretreatment layers. As a preliminary application, a glucose biosensor based on hydrogen peroxide detection is constructed by immobilizing glucose oxidase (GOx) on the NC electrode strip and by formulating a strong oxidation layer (i.e., PbO2) at the sample loading area, placed below the GOx reaction band. The screen-printed PbO2 paste serves as a sample pretreatment layer that removes interference by its strong oxidizing ability. Samples applied are carried chromatographically, via the PbO2 paste, to the GOx layer, and glucose is catalyzed to liberate hydrogen peroxide, which is then detected at the electrode surface. The proposed NC/PbO2 strip sensor is shown to be virtually insusceptible to interfering species such as acetaminophen and ascorbic and uric acids and to exhibit good performance, in terms of the sensor-to-sensor reproducibility (standard deviation, +/-0.026 - +/-0.086 microA), the sensitivity (slope, -0.183 microA/mM), and the linearity (correlation coefficient, 0.994 in the range of 0-10 mM).

Biosensing Techniques↗
PubMed2000

Conductimetric membrane strip immunosensor with polyaniline-bound gold colloids as signal generator.

For point-of-care examination, an immuno-chromatographic assay system based on conductimetric detection was investigated by utilizing, as signal generator, colloidal gold with polyaniline bound on the metal surface. Although the gold is a widely used label for antibodies to produce colorimetric signals, the tracer does not lend itself for a suitable electric conduction along the gold particles due to the presence of protein barriers (e.g. immunoglobulin and blocking agent) against electron transfer. To overcome this problem, we introduced a conducting polymer, for instance, polyaniline, as a conductivity-modulating agent on the gold surface after immobilizing an antibody specific to human albumin used as model analyte. This novel signal generator amplified the conductimetric signal 4.7 times compared with the plain gold, and the signal was also maximum 2.3-fold higher than that from the photometric system under the same analytical conditions. The latter effect resulted from an exponential pattern in the dose-response curve of the electric signal that was different from the conventional sigmoidal shape.

Aniline Compounds↗
PubMed1999

Fabrication of butyrylcholinesterase sensor using polyurethane-based ion-selective membranes.

A simple method of enzyme immobilization was investigated which is useful for fabrication of enzyme sensors based on polymeric ion-selective membranes. The enzyme membrane was built by coating a thin hydrophilic polyurethane (HPU) film directly mixed with an enzyme over an underlying polyurethane (PU)-based ion-selective membrane. This highly simple method of enzyme immobilization was applied to the fabrication of a potentiometric butyrylcholinesterase-based biosensor for the determination of organophosphorus pesticides. The enzyme was well entrapped within the HPU film and the intrinsic potentiometric response of the underlying ion-selective PU membrane was not influenced significantly by the outer HPU/enzyme membrane. The enzyme electrode was optimized by changing systematically the composition of the enzyme membrane to evaluate the effect of the changes on sensor response. The sensor was successfully applied to the analysis of paraoxon, an organophosphorus pesticide.

Biosensing Techniques↗
PubMed1998

Microtiter plate-format optode.

Microtiter plate-format optodes could be assembled by casting bulk-response membranes into the standard 96-well polypropylene-based plate or by screen printing them on an optically transparent substrate with 96-well pattern. The compositions of thick optode membranes, especially the ratios of poly(vinyl chloride) (PVC) to plasticizer [bis-(2-ethylhexyl) sebacate (DOS)], were carefully optimized to provide reproducible and rapid response. Adjusting the ratio of PVC to DOS by 1:6, bulk-response membranes containing neutral carrier (4-tert-butyl calix[4]arene tetraacetic acid tetraethyl ester for sodium-selective membrane or valinomycin for potassium-selective membrane) and lipophilic pH indicator (ETH 5294) could exhibit equilibrium response in 5 min. The practical utility of microtiter plate-format optodes has been examined by determining clinically relevant electrolytes in serum samples. It was demonstrated that microtiter plate-format optodes can provide high sample throughput (approximately 100 samples in less than 5 min), analytical performance comparable to that of a potentiometric clinical analyzer, and additional information on electrolytes using the same samples prepared for other colorimetric measurements.

Animals↗
PubMed1995

Homogeneous assays for riboflavin mediated by the interaction between enzyme-biotin and avidin-riboflavin conjugates.

Homogeneous-type enzyme-linked competitive binding assays utilizing the synthetic enzyme-biotin and avidin-riboflavin conjugates are developed for the detection of riboflavin as well as its binder protein. The activity of the enzyme-biotin conjugate is inhibited in the presence of the avidin-riboflavin conjugate, and the observed inhibition is reversed in an amount dependent on the concentration of riboflavin binding protein (RBP) added. Upon additions of free riboflavin to the mixture, activity is reinhibited in an amount proportional to the riboflavin concentration. Three different enzymes are examined as the labels: glucose-6-phosphate dehydrogenase, adenosine deaminase, and alkaline phosphatase. The catalytic activity of these enzymes, when conjugated with biotin, is shown to be inhibited to a significant degree (> 90%) by the binding of the avidin-riboflavin conjugate, and reversed upon additions of RBP.

Adenosine Deaminase↗
PubMed1991

Electrochemical performance, biocompatibility, and adhesion of new polymer matrices for solid-state ion sensors.

Ammonium and potassium ion-selective membranes formulated with PVC/hydroxylated PVC, polyurethane/hydroxylated PVC, and moisture-curable silicone rubber matrices are studied in an effort to extend the lifetime of solid-state ion sensors through improved membrane adhesion. The PVC/membranes exhibit electrochemical performance equivalent to that of conventional PVC membranes in terms of slope, detection limit, and selectivity. The polyurethane- and silicone-rubber-based membranes have better adhesion to silicon nitride than do PVC or hydroxylated PVC matrices. Incorporating a silanizing reagent (silicon tetrachloride) significantly improves the adhesion of the polyurethane matrix. The use of silicon tetrachloride in membrane matrices also enhances the electrochemical stability of the interfacial potential between ion-selective polymer-matrix membranes and silver epoxy inner reference electrodes of solid-state sensors. The biocompatibility of the polymer matrices is examined via radiotracer protein adsorption studies and whole blood clotting time measurements. The polyurethane- and silicone-rubber-based membranes exhibit less overall nonspecific protein adsorption than the PVC or hydroxylated PVC matrices.

Adhesiveness↗
PubMed1990

Homogeneous enzyme-linked binding assay for studying the interaction of lectins with carbohydrates and glycoproteins.

A simple and rapid homogeneous enzyme-linked binding assay method for studying lectin-carbohydrate interactions is described. The method is based on the homogeneous inhibition of appropriate enzyme-saccharide conjugates by specific carbohydrate-binding lectins. In the presence of carbohydrate structures recognized by the lectins, enzyme activity is regained in an amount of proportional to the concentration of carbohydrate. The new method can be used to rapidly assess the relative carbohydrate specificity of the various lectins and for the selective analytical detection of simple saccharides and complex glycoproteins. Indeed, when Jacalin lectin is used in conjunction with a malate dehydrogenase-galactose conjugate, selective measurement of human IgA (immunoglobulin A) at microgram per milliliter levels in less than 10 min is possible. The potential for using this analytical methodology for determining changes in the carbohydrate structure of intact recombinant glycoproteins is also discussed.

Carbohydrates↗
PubMed1990

Use of ionomer membranes to enhance the selectivity of electrode-based biosensors in flow-injection analysis.

The use of ionomer membranes to enhance the selectivity of potentiometric enzyme electrodes in flow-injection measurement arrangements is examined. The ionomer membranes employed are permeable to analyte substrates but relatively impermeable to detectable ions that would normally interfere with the measurement of the substrates if the enzyme electrodes were in direct contact with the sample. As a model system, the selectivity of enzyme electrodes prepared with nonactin-based ammonium-sensitive polymeric membranes is evaluated. In the preferred configuration, a thin hydrophilic anion-exchange membrane is incorporated within a flow-through dialysis unit upstream from the enzyme-electrode detector. As the sample passes through the dialysis unit, neutral or anionic analyte molecules (urea or glutamine) move through the membrane while the permeation of endogenous ammonium ions and other cations in the sample is retarded. A flowing recipient buffer on the other side of the membrane carries the analyte substrate to the enzyme-electrode detector. Enhancements in selectivity for analyte substrates over endogenous ammonium and potassium ions are greater than or equal to 9-fold when compared to enzyme-electrode flow-injection analysis (FIA) systems assembled without the ionomer membrane unit. The analytical utility of the proposed system is demonstrated by the accurate measurements of urea in blood serum and L-glutamine in hybridoma bioreactor media.

Biosensing Techniques↗
PubMed1988

Solid phase enzyme-linked competitive binding assay for riboflavin.

A new solid-phase enzyme-linked assay for riboflavin (vitamin B2) is described. The assay is based on the competition between analyte vitamin molecules and a glucose-6-phosphate dehydrogenase-3-carboxymethylriboflavin conjugate for a limited number of riboflavin-binding protein sites immobilized on Sepharose particles. Significant improvements in conjugate catalytic activity and thus detectability are achieved by optimizing the reaction conditions used to covalently link 3-carboxymethylriboflavin to the enzyme. Optimization experiments include studying the effects of reaction pH and organic solvent composition. Final assay detection limits and the sensitivity of the dose-response curves are dependent on the ratio of conjugate to binding protein sites utilized in an equilibrium assay protocol. Selectivity of the method correlates well with that predicted based on the known association constants of riboflavin-binding protein with flavin analogs. The assay is shown to offer adequate detection limits and selectivity for direct measurement of riboflavin in urine, infant formula, and vitamin capsules.

Binding Sites↗