The morphology and evolution of bipyramidal gold nanoparticles

We examine the growth and evolution with time of bipyramidal gold nanoparticles grown by a seed-mediated process. The nanoparticles are characterized both by their physical dimensions determined by transmission electron microscopy and by the wavelength position of their localized surface plasmon resonance. Each growth's physical dimensions correspond to particular initial conditions, and we observe two distinct modes of temporal evolution during growth. The effects of varying silver nitrate concentration and growth time are also explored. We observe a linear relationship between the tip radius of curvature and the wavelength of the longitudinal localized surface plasmon resonance (LSPR) peak. Critical parameters for synthesizing bipyramidal nanoparticles with sharp tips and correct length to width ratio are determined.

Fluorogenic assay for beta-glucuronidase using microchip-based capillary electrophoresis

Microchip capillary electrophoresis (CE) was used with a model enzyme assay to demonstrate its potential application to combinatorial drug screening. Hydrolysis with beta-glucuronidase of the conjugated glucuronide, fluorescein mono-beta-D-glucuronide (FMG), liberated the fluorescent product, fluorescein. FMG and fluorescein were detected by fluorescence, with excitation and emission at 480 and 520 nm, respectively. Microchip CE was used to separate FMG and fluorescein. Fluorescein production was monitored to assess beta-glucuronidase activity. Michaelis-Menten enzyme kinetics analysis yielded the Km value. The results were compared with those from experiments done by conventional CE. The Km value for beta-glucuronidase with FMG is being reported for the first time as 18 microM. The inhibition of beta-glucuronidase by the competitive inhibitor D-saccharic acid-1,4-lactone (SL) was also determined using microchip CE. Reactions were done with various concentrations of inhibitor and constant beta-glucuronidase and FMG concentrations. A dose-response plot was acquired and the IC50 value for SL was determined to be 3 microM.

Small volume bead assay for ovalbumin with electrochemical detection

A bead based sandwich enzyme immunoassay coupled to electrochemical detection for ovalbumin has been developed. The enzyme label alkaline phosphatase was used to convert the substrate 4-aminophenyl phosphate to electroactive product 4-aminophenol. The detection was done in a microdrop by continuously monitoring the enzyme turnover with a rotating disk electrode. This reduces dilution of the enzyme product, a key to achieving low detection limits. The assay developed has a detection limit of 0.1 ng ml-1. Assay sensitivity in complex matrices such as food and serum was compared.

Spectroelectrochemical sensing based on multimode selectivity simultaneously achievable in a single device. 9. Incorporation of planar waveguide technology

Incorporation of planar waveguide technology into a spectroelectrochemical sensor is described. In this sensor design, a potassium ion-exchanged BK7 glass waveguide was over-coated with a thin film of indium tin oxide (ITO) that served as an optically transparent electrode. A chemically selective film was spin-coated on top of the ITO film. The sensor supported five optical modes at 442 nm and three at 633 nm. Investigations on the impact of the ITO film on the optical properties of the waveguide and on the spectroelectrochemical performance of the sensor are reported. Sensing was based on the change in attenuation of light propagated through the waveguide resulting from an optically absorbing analyte. By applying either a triangular or square wave excitation potential waveform, electromodulation of the optical signal has been demonstrated with Fe(CN)6(3-/4-) as a model electroactive couple that partitions into a PDMDAAC-SiO2 film [where PDMDAAC = poly(dimethyldiallylammonium chloride)] and absorbs at 442 nm.

Spectroelectrochemical sensing based on multimode selectivity simultaneously achievable in a single device. 5. Simulation of sensor response for different excitation potential waveforms

The simulation of the optical response in spectroelectrochemical sensing has been investigated. The sensor consists of a sensing film coated on an optically transparent electrode (OTE). The mode of detection is attenuated total reflection. Only species that partition into the sensing film, undergo electrochemistry at the potentials applied to the OTE, and have changes in their absorbance at the wavelength of light propagated within the glass substrate of the OTE can be sensed. A fundamental question arises regarding the excitation potential waveforms employed to initiate the electrochemical changes observed. Historically, selection has been based solely upon the effectiveness of the waveform to quickly electrolyze any analyte observable by the optical detection method employed. In this report, additional requirements by which the waveform should be selected for use in a remote sensing configuration are discussed. The effectiveness of explicit finite difference simulation as a tool for investigating the applicability of three different excitation potential waveforms (square, triangle, sinusoid) is demonstrated. The simulated response is compared to experimental results obtained from a prototype sensing platform consisting of an indium tin oxide OTE coated with a cation-selective, sol-gel-derived Nafion composite film designed for the detection of a model analyte, tris(2,2'-bipyridyl)ruthenium(II) chloride. Using a diffusion coefficient determined from experimental data (5.8 x 10(-11) cm2 s for 5 x 10(-6) M Ru(bipy)3(2+)), the simulator program was able to accurately predict the magnitude of the absorbance change for each potential waveform (0.497 for square, 0.403 for triangular, and 0.421 for sinusoid), but underestimated the number of cycles required to approach steady state. The simulator program predicted 2 (square), 3 (triangle), and 5 cycles (sinusoid), while 5 (square), 15 (triangle), and 10 (sinusoid) cycles were observed experimentally.

Spatially addressed deposition and imaging of biochemically active bead microstructures by scanning electrochemical microscopy

A new procedure is described to deposit paramagnetic beads on surfaces to form microscopic agglomerates. By using surface-modified beads, microscopic structures with defined biochemical activity are formed. The shape and size of agglomerates were characterized by scanning electron microscopy (SEM), and the biochemical activity was mapped with scanning electrochemical microscopy (SECM). This approach is demonstrated using beads modified with anti-mouse antibodies (Ab). After allowing them to react with a conjugate of mouse IgG and alkaline phosphatase (ALP), the beads were deposited as agglomerates of well-defined size and shape. The biochemical activity was recorded in the generation-collection SECM mode by oxidizing 4-aminophenol formed in the ALP-catalyzed hydrolysis of 4-aminophenyl phosphate at the surface of the beads. The signal height correlated with both the amount of beads present in one agglomerate and the proportion of Ab binding sites saturated with the ALP mouse IgG conjugate. The feedback mode of the SECM was used to image streptavidin-coated beads after reaction with biotinylated glucose oxidase.

Feasibility studies of simultaneous multianalyte amperometric immunoassay based on spatial resolution

A multianalyte immunoassay concept based on the geometric separation of different analyte-specific antibodies has been demonstrated. The assay and amperometric detection are done in a cell with two working electrodes controlled at the same potential, and the amperometric signal at each electrode is monitored. The distance between any two adjacent electrodes in this prototype is 2.5 mm, and during the course of amperometric measurement, the product formed at one electrode does not reach the other working electrode within 20 min after the addition of enzyme substrate. Thus, the method relies on the spatial resolution between the different antibodies being such that measurements are taken before cross-interference due to diffusion can occur. Identical enzyme labels (alkaline phosphatase, ALP) and substrates (p-aminophenyl phosphate, PAPP) are used for all analytes. Alkaline phosphatase-conjugated rat anti-mouse IgG was immobilized by passive adsorption. Our studies showed that this concept is feasible and can be applied to the simultaneous measurement of multiple analytes.

Capillary electrochemical enzyme immunoassay (CEEI) for phenobarbital in serum

A competitive heterogeneous capillary enzyme immunoassay with electrochemical detection has been developed for phenobarbital in serum. The oxidized primary antibody was attached covalently to the modified interior surface of a microcapillary (22 microl). The competition between analyte phenobarbital and alkaline phosphatase labeled phenobarbital for a limited number of antibody binding sites was complete in 1.5 h. The enzymatic product (p-aminophenol) from the catalytic conversion of the substrate (p-aminophenyl phosphate) was detected by amperometric flow injection analysis. The calibration curve for phenobarbital had a detection limit of 30 microg l(-1) (2.8 pmoles or 0.65 ng) and a range of 30-3000 microg l(-1). The assay could be used to determine the phenobarbital serum concentration in a 4 microl clinical serum sample without pretreatment.

Spectroscopic and electrochemical evaluation of a perfluorosulfonated ionomer and its gel as preconcentrating media for [ReI(DMPE)3]+, where DMPE = 1,2-bis(dimethylphosphino)ethane

The interaction of [ReI(DMPE)3]+, where DMPE = 1,2-bis(dimethylphosphino)ethane, a nonradioactive analogue of a heart imaging agent, with Nafion gel, which is Nafion plasticized with tri-n-butyl phosphate, has been evaluated spectroscopically and electrochemically. Thin-layer spectroelectrochemistry on the rhenium compound yields a linear Nernst plot with an n value of 0.99 and E degree' of 0.049 V vs Ag/AgCl. The electrochemistry is consistent with a reversible one-electron transfer between the mono- and dicationic forms of the complex. The UV-visible spectrum of electrogenerated [ReII(DMPE)3]2+ is identical to that obtained by air oxidation of [ReI(DMPE)3]+. Thin, free-standing films of Nafion gel and Nafion that were sufficiently clear to record visible spectra were cast. Spectroscopic measurement of the partitioning of [ReI-(DMPE)3]+ from aqueous solution into these films shows a more rapid uptake of the complex by the Nafion gel. Preconcentration factors into Nafion gel and Nafion were 350 and 50, respectively, after 4 h of soaking. Cyclic voltammetry of 1.0 x 10(-4)-1.0 x 10(-7) M (ReI(DMPE)3]+ in 0.15 M supporting electrolyte aqueous solution at bare gold and spectroscopic graphite electrodes suggests that the complex adsorbs to these electrodes. By comparison, the well-defined cyclic voltammograms at Nafion gel-modified electrodes exhibit diffusion-controlled behavior. The formal reduction potential at Nafion gel-modified electrodes is shifted positively compared to bare electrodes. A current enhancement of approximately 4 was observed at Nafion gel-modified spectroscopic graphite over a bare electrode. A calibration plot of peak current for differential pulse voltammetry vs concentration at Nafion gel-modified spectroscopic graphite was linear in the 10(-7)-10(-5) M concentration range, with a detectable signal down into the 10(-9) M range.

Separation of aromatic acids, DOPA, and methyl-DOPA by capillary electrophoresis with dendrimers as buffer additives

Polyamidoamine starburst dendrimers with terminal carboxylate groups are used as a pseudo-stationary phase in electrokinetic chromatography for separating the selected aromatic amino acids phenylalanine, phenylglycine, homophenylalanine, and tyrosine and the catecholamines 3-(3,4-dihydroxyphenyl)alanine and 3-(3,4-dihydroxyphenyl)-2-methylalanine at different pH levels. A significant difference in analyte selectivity is observed between dendrimers of different generations and at different pH levels. At pH 7.0, the utility of the dendrimers for separating these analytes is limited by relatively low selectivity and a noisy baseline. However, good separation and selectivity are obtained with low generation dendrimers (G0.5 and G1.5) at low pH. Strong association of the solute with dendrimers is observed for high generations (G2.5, G3.5, and G5.5).

The analysis of fountain pen inks by capillary electrophoresis with ultraviolet/visible absorbance and laser-induced fluorescence detection

Information on the identity of inks adds to the circumstantial evidence in legal cases involving fraudulent documents. In combination with optical methods, multiple thin-layer chromatography (TLC) is currently the analytical tool used by forensic chemists to separate, compare and distinguish inks based on their dye composition. In our studies, capillary electrophoresis (CE) was used for the analysis of water-soluble fountain pen inks. Inks are complex mixtures of synthetic organic and inorganic dyes, surfactants, resins and other components. The investigations included the development of an electrophoretic separation method, the optimization of an extraction procedure for inks from paper as well as the evaluation of ultraviolet/visible (UV/VIS) absorbance and laser-induced fluorescence (LIF) detection for the analysis of inks by CE. Good results for the separation of 17 blue and black inks of different manufacturers and countries of origin were obtained with 100 mM borate buffer, pH 8.0, containing 20% methanol. The electropherograms of the inks and their extracts from paper showed patterns that were in most cases distinctly different from each other. Ultraviolet/visible scans can be used to compare spectra of the separated main and trace components of inks. Fluorescence detection at different excitation and emission wavelengths was more sensitive, but added to the complexity of the electropherograms due to the excitation of coextracted fluorescing paper components. The resolution power of CE combined with the information content provided by the detection modes investigated prove CE to be a powerful tool for the identification of water-soluble inks used on paper documents.

Electrochemical behavior of [ReI(DMPE)3]+, where DMPE = 1,2-bis(dimethylphosphino)ethane, at perfluorosulfonated ionomer-modified electrodes

The perfluorosulfonated ionomer Nafion shows potential utility as a polymer film to enhance the electrochemical detection of [ReI(DMPE)3]+, where DMPE = 1,2-bis-(dimethylphosphino)ethane. [ReI(DMPE)3]+, a nonradioactive radiopharmaceutical analog for heart imaging, partitions strongly into Nafion films on glassy carbon. Well-defined, chemically reversible cyclic voltammograms are obtained for the [ReI(DMPE)3]+/[ReII(DMPE)3]2+ couple with Eo' shifted positively by 60 mV relative to its value on bare glassy carbon. [ReI(DMPE)3]+ partitions into Nafion more strongly than the oxidized form, [ReII-(DMPE)3]2+. The detection limit for [ReI(DMPE)3]+ by cyclic voltammetry was improved by 2-3 orders of magnitude by the Nafion film. Differential pulse voltammetry for oxidation of [ReI(DMPE)3]+ at the Nafion-modified electrode has a detection limit of 2.5 x 10(-9) M compared to 1.0 x 10(-7) M at the bare electrode. A preconcentration factor of 1 x 10(6) for partitioning of [ReI(DMPE)3]+ from 0.05 M NaCl into Nafion on a glassy carbon electrode was measured.

Zeptomole-detecting biosensor for alkaline phosphatase in an electrochemical immunoassay for 2,4-dichlorophenoxyacetic acid

A bienzyme substrate-recycling biosensor in a flow injection analysis system is described for the sensitive measurement of alkaline phosphatase (ALP) and applied to the fast readout of a competitive immunoassay for the widely used pesticide 2,4-dichlorophenoxyacetic acid (2,4-D). The phenol-indicating biosensor consists of a Clark-type electrode covered by a membrane with coentrapped tyrosinase and quinoprotein glucose dehydrogenase. ALP dephosphorylates phenyl phosphate to phenol (K(m) = 36 microM) outside the flow system. Phenol is oxidized in the sensor membrane by the oxygen-consuming tyrosinase via catechol to o-quinone. The quinone is reconverted to catechol by glucose dehydrogenase. This substrate cycling results in a 350-fold amplified sensor response to phenol. The oxygen consumption of the enzyme couple in the presence of phenol is monitored as a decrease in current. A total of 3.2 fM ALP (320 zmol/ 100 microL) has been detected after a 57.5 min incubation with phenyl phosphate. All involved reagents are stable over the time of measurement. The sensor does not produce any measurable blank signals. The immunoassay detects 0.1 microgram/L 2,4-D, the maximum concentration for pesticides allowed in drinking water by European Community regulations. The applicability of this biosensor for fast immunoassay readout is demonstrated by a 2 min incubation. By comparison, a standard photometric method (p-nitrophenyl phosphate) requires overnight incubation.

Imaging of immobilized antibody layers with scanning electrochemical microscopy

Visualization of immobilized antibodies can be achieved with scanning electrochemical microscopy (SECM) by saturation of the antigen binding sites with an alkaline phosphatase-antigen conjugate, which catalyzes hydrolysis of the redox-inactive 4-aminophenyl phosphate to the redox-active 4-aminophenol (PAP). PAP was detected in the collection mode at an amperometric SECM tip. The tip current reflects the density of active binding sites in the immobilized antibody layer. The application of this approach for immunosensing research has been demonstrated with the optimization of a covalent immobilization procedure of antibodies on glass. The special advantages and present limitations of the procedures are discussed.

Electrochemical dehydrogenase-based homogeneous assays in whole blood

An electrochemical method has been developed for determining NADH in whole blood for dehydrogenase-based assays by flow-injection analysis. NADH generated by dehydrogenase is oxidized by an electron-transfer coupling reagent, 2,6-dichloroindophenol (DCIP). The reduced form of DCIP (DCIPH2) is measured amperometrically by flow-injection analysis. Endogenous interferents were inhibited by p-hydroxymercuribenzoate. Electrode fouling by proteins was not observed under assay conditions. The Emit theophylline enzyme immunoassay and the hexokinase glucose assay were used as models. For the glucose assay, the intraassay CVs were 15% at 0.31 g/L and 3.5% at 1.82 g/L. Recoveries of glucose from whole blood (compared with that for aqueous standards) were 109%, 97.9%, and 101% at 0.050, 2.00, and 5.00 g/L glucose, respectively, and 104%, 101%, and 102% for theophylline at concentrations of 5.0 (low), 16.4 (medium), and 30.2 (high) mg/L, respectively, with corresponding precisions of 12%, 9.5%, and 8.8%. Both assays correlated well with results by reference methods. These studies demonstrate that this method can measure NADH in whole blood without prior separation and that it is potentially applicable to other dehydrogenase-based assays in whole blood.

Electrochemical dehydrogenase-based homogeneous assays in whole blood

An electrochemical method has been developed for determining NADH in whole blood for dehydrogenase-based assays by flow-injection analysis. NADH generated by dehydrogenase is oxidized by an electron-transfer coupling reagent, 2,6-dichloroindophenol (DCIP). The reduced form of DCIP (DCIPH2) is measured amperometrically by flow-injection analysis. Endogenous interferents were inhibited by p-hydroxymercuribenzoate. Electrode fouling by proteins was not observed under assay conditions. The Emit theophylline enzyme immunoassay and the hexokinase glucose assay were used as models. For the glucose assay, the intraassay CVs were 15% at 0.31 g/L and 3.5% at 1.82 g/L. Recoveries of glucose from whole blood (compared with that for aqueous standards) were 109%, 97.9%, and 101% at 0.050, 2.00, and 5.00 g/L glucose, respectively, and 104%, 101%, and 102% for theophylline at concentrations of 5.0 (low), 16.4 (medium), and 30.2 (high) mg/L, respectively, with corresponding precisions of 12%, 9.5%, and 8.8%. Both assays correlated well with results by reference methods. These studies demonstrate that this method can measure NADH in whole blood without prior separation and that it is potentially applicable to other dehydrogenase-based assays in whole blood.

gamma-irradiation-induced grafting of poly (styrenesulfonate) to poly(tetrafluoroethylene) shielded microelectrodes

A new method for the fabrication and polymer modification of microelectrodes is described. These electrodes are constructed by heat sealing the electroactive material in dual shrink/melt poly(tetrafluoroethylene) (PTFE) under vacuum. The PTFE shield may be activated to provide a support upon which polymers of interest may be grafted. gamma-Irradiation was used to graft polymerize styrene to the surface. The poly-(styrene) was subsequently sulfonated with chlorosulfonic acid to form poly(styrenesulfonate). Scanning electron microscopy and Raman microspectroscopy provide evidence that the poly-(styrenesulfonate) film has been formed and extends over the electrode material. Voltammetry indicates that hexaammine-ruthenium(III) cation is preconcentrated and stabilized via an association with the polymer film.

Fiber-optic-graded-index-lens absorbance sensor with wavelength-scanning capability

A single optical-fiber absorbance sensor that contains a graded-index lens is described. The sensor can be tailored for a desired broad wavelength region, path length, and size. Laboratory evaluations of sensors of varying sizes and path lengths are presented. The sensors, even without the added expense of optical coatings, report true absorbance spectra for the 420-850-nm wavelength region with a linear response over a wide absorbance range. Direct comparison with several other sensor configurations shows that the graded-index-lens-based sensor has a high optical efficiency. Potential applications of the sensor include absorbance measurements at hazardous or remote sites and in vivo medical applications.

Simultaneous immunoassay using electrochemical detection of metal ion labels

The concept of a simultaneous dual analyte immunoassay based on two different metal ion labels is demonstrated. The model system consists of two proteins, human serum albumin (HSA) and immunoglobulin G (IgG). Bismuth and indium ions have been coupled to these proteins through the bifunctional chelating agent diethylenetriamine-pentaacetic acid (DTPA). A maximum molar labeling ratio of 6:1 and 10:1 was obtained for HSA and IgG, respectively. Following a competitive equilibrium between unlabeled and labeled protein for a limited amount of specific antibody immobilized on polystyrene, the bound metal ion labels were released by acidification and detected by differential pulse anodic stripping voltammetry (ASV). Limits of detection for HSA and IgG are 1.8 and 0.6 microgram/mL, respectively. Application of the dual immunoassay to human serum samples gave results that were comparable to those obtained by nephelometry.

Electrochemical homogeneous enzyme immunoassay of theophylline in hemolyzed, icteric, and lipemic samples

We demonstrate here an electrochemical homogeneous enzyme immunoassay for theophylline, which can be performed in hemolyzed, lipemic, and icteric samples. The assay used an unmodified Syva EMIT theophylline kit. One of the enzymatic reaction products, NADH, reacted with 2,6-dichloroindophenol (DCIP) to reduce DCIP to DCIPH2, which was detected electrochemically with flow-injection analysis. The inter- and intraassay coefficients of variation of this manual technique were < 9% at theophylline concentrations of 14 to 34 mg/L. The CVs were 9-15% at low concentrations (6.3 mg/L), which is below the therapeutic range. Analytical recoveries were 91-97% for normal serum and 92-111% for hemolyzed, icteric, or lipemic sera. The measured concentrations (y) were compared with those obtained by the fluorescence polarization immunoassay (x); a scatter plot of the results showed a linear relationship of y = 1.00 x - 0.57 mg/L (r = 0.966, Sy/x = 1.51). This alternative way to measure the serum concentration of theophylline overcomes the shortcomings of spectrophotometric methods, by which it is difficult to measure theophylline in severely hemolyzed, icteric, or lipemic sera.

Electrochemical homogeneous enzyme immunoassay of theophylline in hemolyzed, icteric, and lipemic samples

We demonstrate here an electrochemical homogeneous enzyme immunoassay for theophylline, which can be performed in hemolyzed, lipemic, and icteric samples. The assay used an unmodified Syva EMIT theophylline kit. One of the enzymatic reaction products, NADH, reacted with 2,6-dichloroindophenol (DCIP) to reduce DCIP to DCIPH2, which was detected electrochemically with flow-injection analysis. The inter- and intraassay coefficients of variation of this manual technique were < 9% at theophylline concentrations of 14 to 34 mg/L. The CVs were 9-15% at low concentrations (6.3 mg/L), which is below the therapeutic range. Analytical recoveries were 91-97% for normal serum and 92-111% for hemolyzed, icteric, or lipemic sera. The measured concentrations (y) were compared with those obtained by the fluorescence polarization immunoassay (x); a scatter plot of the results showed a linear relationship of y = 1.00 x - 0.57 mg/L (r = 0.966, Sy/x = 1.51). This alternative way to measure the serum concentration of theophylline overcomes the shortcomings of spectrophotometric methods, by which it is difficult to measure theophylline in severely hemolyzed, icteric, or lipemic sera.

Small-volume voltammetric detection of 4-aminophenol with interdigitated array electrodes and its application to electrochemical enzyme immunoassay

A small-volume voltammetric detection of 4-aminophenol (PAP) has been developed using an interdigitated array (IDA) microelectrode cell in order to apply the IDA to electrochemical enzyme immunoassay. The signal of PAP at the IDA was steady state, and its magnitude was amplified compared with that of the usual single electrode due to redox cycling of PAP between the two finger sets of the IDA. A linear relationship between PAP concentration and cathodic limiting current was obtained from 1 to 1000 microM, reproducibly. The minimum sample volume in the measurement was reduced to 800 nL. High sample throughput of less than 1-min detection time per sample was achieved on 2-10-microL PAP samples. This IDA cell was applied to the electrochemical enzyme immunoassay of mouse IgG. Alkaline phosphatase was used as the enzyme label. The mouse IgG concentration was evaluated by detecting the concentration of PAP, which is the product of enzymatic reaction of the substrate, 4-aminophenyl phosphate (PAPP). Anti-mouse IgG was covalently immobilized on the glass surface of the small-volume immunowells by carbodiimide coupling. The assay range was 10-1000 ng/mL using 10-microL sample and 20-microL substrate solutions.

Biosensors based on polymer networks formed by gamma irradiation crosslinking

Water-soluble polymers immobilized by gamma irradiation have been investigated as a means of developing electrochemical sensors. Enzyme-based sensors for glucose and lactate have been made by immobilizing glucose oxidase and lactate oxidase, respectively, on platinized graphite electrodes. The enzyme is entrapped in a polymeric network of poly(vinyl alcohol) that is formed by gamma radiation crosslinking. Electrodes coated with poly (N-vinylpyrrolidone) and its corresponding monomer and then crosslinked with gamma radiation show an extraction of catecholamines into the polymer film that enhances the analytical signal for their detection by electrochemical oxidation. Poly(dimethyldiallylammonium chloride) spin-coated on a screen-printed electrochemical cell provides sufficient ionic conductivity for the cell to function as a gas sensor for oxygen, which is detected by reduction at a platinum working electrode.

Electrochemical determination of azidothymidine in human whole blood

An electrochemical method based on differential pulse voltammetry is presented for the determination of AZT in whole blood of fasted subjects. A protein-free supernatant of whole blood is prepared using HClO4 precipitation followed by neutralization with phosphate buffer. The AZT is reduced at a hanging mercury drop electrode. The linear dynamic range of standards in buffer is from the detection limit of 4.1 nM to 206.5 microM (1.1 to 55,200 ng/ml). However, in spiked blood samples the linear dynamic range is from 0.029 to 0.29 microM (7.75 to 77.5 ng/ml). The whole blood assay yields a recovery of 92.30 +/- 5.92% compared to the standard solution assay. After a 30-min preparation time, each sample can be analyzed in 10 min by a manual procedure.

Determination of metabolically derived nitroprocainamide in the urine of procainamide-dosed humans and rats by liquid chromatography with electrochemical detection

The N-oxidized metabolites of the antiarrhythmic procainamide have previously been implicated as inciting agents in the autoimmune condition drug-related lupus. Although much data have been collected with respect to the in vitro behavior of these metabolites, relatively little has been accomplished in vivo because of their extreme reactivity. The determination of nitroprocainamide (NPA), a stable decomposition product of the reactive hydroxylamine and nitroso species, in the urine of rats dosed with procainamide is reported here using the sensitive and selective method of HPLC with electrochemical detection. For orally and i.v.-dosed animals, up to microgram amounts of NPA were excreted over 24 hr from an initial dose of 66-100 mg procainamide/kg body weight. Also, the apparent elimination of microgram quantities of NPA in the urine specimens of 9 of 11 patients undergoing treatment with procainamide was observed. This suggests that N-oxidation of the aromatic ring of procainamide is occurring at sufficient levels to result in the formation of significant amounts of the reactive hydroxylamine and nitroso metabolites in vivo, and may have direct implications in the diverse and widespread symptomatology associated with procainamide-induced drug-related lupus.

Electrochemical enzyme immunoassay for phenytoin by flow-injection analysis incorporating a redox coupling agent

Using phenytoin as a model analyte, we demonstrate an electrochemical enzyme immunoassay based on flow-injection analysis and incorporating 2,6-dichloroindophenol (DCIP) as a redox coupling agent. DCIP reacts with NADH to form NAD+ and DCIPH2, the reduced form of the coupling agent. The production of DCIPH2 is monitored at +250 mV vs Ag/AgCl. This low applied potential improves selectivity in the biological matrix, differentiating against components that are oxidizable at the more-positive potentials required for direct electrochemical detection of NADH. The kinetics-based assay also eliminates other common interferences, mainly from ascorbic acid and glutathione. This system does not require precolumns or analytical columns for isolation of the NADH response. Good agreement with a routine clinical laboratory procedure for phenytoin is obtained for clinical samples (r = 0.95), illustrating the feasibility of such an approach.

Flow-injection analysis with electrochemical detection of reduced nicotinamide adenine dinucleotide using 2,6-dichloroindophenol as a redox coupling agent

The determination of reduced nicotinamide adenine dinucleotide (NADH) by electrochemical oxidation requires a more positive potential than is predicted by the formal reduction potential for the NAD+/NADH couple. This problem is alleviated by use of 2,6-dichloroindophenol (DCIP) as a redox coupling agent for NADH. The electrochemical characteristics of DCIP at the glassy carbon electrode are examined by cyclic voltammetry and hydrodynamic voltammetry. NADH is determined by reaction with DCIP to form NAD+ and DCIPH2. DCIPH2 is then quantitated by flow-injection analysis with electrochemical detection by oxidation at a detector potential of +0.25 V at pH 7. NADH is determined over a linear range of 0.5 to 200 microM and with a detection limit of 0.38 microM. The lower detection potential for DCIPH2 compared to NADH helps to minimize interference from oxidizable components in serum samples.

Comparison of methods for following alkaline phosphatase catalysis: spectrophotometric versus amperometric detection

An amperometric method for alkaline phosphatase is described and compared to the most widely used spectrophotometric method. Catalytic hydrogenation of 4-nitrophenylphosphate (the substrate in the spectrophotometric method) gives 4-aminophenylphosphate (the substrate in the amperometric method). The latter substrate has the formula C6H6NO4PNa2.5H2O and a Mr of 323. The Michaelis constant for 4-aminophenylphosphate in 0.10 M, pH 9.0. Tris buffer is 56 microM, while it is 82 microM for 4-nitrophenyl phosphate. The amperometric method has a detection limit of 7 nM for the product of the enzyme reaction, which is almost 20 times better than the spectrophotometric method. Similarly, with a 15-min reaction at room temperature and in a reaction volume of 1.1 ml, 0.05 microgram/l alkaline phosphatase can be detected by electrochemistry, almost an order of magnitude better than by absorption spectrophotometry. Amperometric detection is ideally suited for small-volume and trace immunoassay.

Heterogeneous enzyme immunoassay of alpha-fetoprotein in maternal serum by flow-injection amperometric detection of 4-aminophenol

A sandwich-type heterogeneous enzyme immunoassay with flow-injection analysis for alpha-fetoprotein (AFP) in human serum has been developed. 4-Aminophenol, the product of enzymatic reaction, is detected amperometrically. The interassay CV for this electrochemical enzyme immunoassay was less than 8.2%, with a minimum detection limit for AFP of 0.163 micrograms/L. The calibration curve had a linear range of 0.316-100 micrograms/L. Studies with 48 human maternal serum samples, comparing results by this method with those by a commercial kit, showed a good correlation (r = 0.961). This procedure provides an alternative method for determining low concentrations of AFP in human maternal serum.

Heterogeneous enzyme immunoassay of alpha-fetoprotein in maternal serum by flow-injection amperometric detection of 4-aminophenol

A sandwich-type heterogeneous enzyme immunoassay with flow-injection analysis for alpha-fetoprotein (AFP) in human serum has been developed. 4-Aminophenol, the product of enzymatic reaction, is detected amperometrically. The interassay CV for this electrochemical enzyme immunoassay was less than 8.2%, with a minimum detection limit for AFP of 0.163 micrograms/L. The calibration curve had a linear range of 0.316-100 micrograms/L. Studies with 48 human maternal serum samples, comparing results by this method with those by a commercial kit, showed a good correlation (r = 0.961). This procedure provides an alternative method for determining low concentrations of AFP in human maternal serum.

Electrochemical immunoassay: an ultrasensitive method

Hydrodynamic electrochemical techniques such as liquid chromatography and flow injection analysis with electrochemical detection are very effective for the rapid determination of the enzyme-generated product in enzyme immunoassays. The authors have used this detection method in various assay formats using both alkaline phosphatase and glucose-6-phosphate dehydrogenase as labels. Assays for digoxin will be used illustratively. Recently, the authors have used 70 mL microcapillary hematocrit tubes as the immunoassay reaction vessel and alkaline phosphatase as the labeling enzyme. The assay, complete in 30 min, had a detection limit of 5,6 x 10 -20 moles of IgG in serum. The linear range was four orders of magnitude. This low detection limit is due to a combination of the favorable geometry of the reaction vessel and the suppression of nonspecific adsorption by the addition of ion-pairing blocking agents. Even lower detectable amounts should be achievable with smaller reaction vessels. The capability for detecting such small amounts of analyte is potentially useful for the analysis of extremely small samples such as single cells and blood samples from premature infants.

Influence of bovine immunoglobulin G on faradaic reactions at electrodes

The effect of bovine immunoglobulin G (IgG), adsorbed on different electrode materials, on faradaic reactions of hexacyanoferrate(III)-hexacyanoferrate(II), hydroquinone-benzoquinone, nicotinamide adenine dinucleotide, reduced from (NADH), and phenol is described and discussed. The reactions are partially inhibited by IgG, but the current and peak potentials reach protein-independent values for higher concentrations of IgG. The hydroquinone-benzoquinone couple can be used for detection purposes at IgG-covered electrodes in electrochemical immunoassays.

Voltammetric behaviour of immunoglobulin G at stationary mercury electrodes

A.c. voltammetric measurements have shown that bovine and human immunoglobulin G are adsorbed at a mercury electrode over a broad potential range and at open circuit. A current signal at ca. -0.55 V was identified as being due to an interfacial process connected with reversible protein re-orientations in the adsorption layer and, possibly, with fast faradaic reactions of both adsorbed redox states of the immunoglobulin.

Competitive heterogeneous enzyme immunoassay for theophylline by flow-injection analysis with electrochemical detection of p-aminophenol

A competitive enzyme-linked immunoabsorbent assay based on the flow-injection amperometric detection of p-aminophenol has been investigated with use of the materials and general procedure of a commercial kit for the determination of theophylline in human serum. The antibody is immobilized on glass beads, and the enzyme label is alkaline phosphatase (EC 3.1.3.1). The high currents generated during the electrochemical detection allowed a rapid (35 min) and simple determination of theophylline throughout its therapeutic range (10-20 mg/L) and also in the subtherapeutic range (detection limit of about 80 micrograms/L).

Competitive heterogeneous enzyme immunoassay for theophylline by flow-injection analysis with electrochemical detection of p-aminophenol

A competitive enzyme-linked immunoabsorbent assay based on the flow-injection amperometric detection of p-aminophenol has been investigated with use of the materials and general procedure of a commercial kit for the determination of theophylline in human serum. The antibody is immobilized on glass beads, and the enzyme label is alkaline phosphatase (EC 3.1.3.1). The high currents generated during the electrochemical detection allowed a rapid (35 min) and simple determination of theophylline throughout its therapeutic range (10-20 mg/L) and also in the subtherapeutic range (detection limit of about 80 micrograms/L).

Electrochemical investigations of immunologically reactive procainamide metabolites

As a result of the implication of N-oxidized procainamide metabolites in drug-related lupus (DRL), the electrochemical behaviour of these compounds was investigated and a coulometric synthesis of the nitroso derivative developed using a previously described carbon packed bed bulk electrolysis flow cell. The electrochemical characterization of the parent p-substituted aromatic amine and the N-oxidized derivatives was achieved through systematic comparison with previously well described aromatic amine and nitro systems using cyclic voltammetry and liquid chromatography with electrochemical detection (LC-EC). Chromatographically assisted hydrodynamic voltammetry indicated current limiting plateau potentials of 0.45 and -0.2 V versus Ag/AgCl, respectively, for synthetically prepared procainamide hydroxylamine and electrolytically prepared nitrosoprocainamide. Reaction characterization and binding behaviour is described for each of the procainamide metabolites following in vitro incubations with cysteine, glutathione, ascorbic acid and mouse haemoglobin.

Technetium-99m complexes of dimethylaminomethylene diphosphonate (DMAD)--I. Anion exchange HPLC characterization of 99mTc(NaBH4)-DMAD mixtures

Radiopharmaceutical analogs prepared by sodium borohydride reduction of 99mTcO4- in the presence of DMAD have been characterized by anion exchange HPLC (high performance liquid chromatography) separation of the resulting mixture into component 99mTc-DMAD complexes. The distribution of complexes within a radiopharmaceutical formulation can be manipulated by controlling technetium concentration, pH, presence or absence of air, and time post reaction.

Technetium-99m complexes of dimethylaminomethylene diphosphonate (DMAD)--II. Biological distributions of 99mTc-DMAD components isolated by anion exchange HPLC

HPLC isolated components of 99mTc(NaBH4)-DMAD mixtures exhibit significantly different biological properties than do analogous components of 99mTc(NaBH4)-MDP and 99mTc(NaBH4)-HEDP mixtures. Most importantly, 99mTc-DMAD components desorb from normal bone whereas analogous 99mTc-MDP and 99mTc-HEDP components do not. However, within an osteogenic rat model, an HPLC isolated 99mTc-DMAD component does not exhibit an unusually high abnormal/normal tibia uptake ratio.

Solid-phase electrochemical enzyme immunoassay with attomole detection limit by flow injection analysis

A sandwich electrochemical enzyme immunoassay with flow injection analysis for the model antigen mouse IgG has been developed with alkaline phosphatase as the enzyme label. The enzyme substrate, 4-aminophenyl phosphate and its enzymatic reaction product, 4-aminophenol have been studied by cyclic and hydrodynamic voltammetry. The determination of 4-aminophenol by flow injection analysis with electrochemical detection (FIAEC) has a linear range of 5.0 x 10(-8) to 1.0 x 10(-5) M, a detection limit of 2.4 x 10(-8) M, and a sample throughput of 72 samples/h. The detection limit is set by a background capacitance response, which depends on the ionic strength difference between the sample and the mobile phase. The sandwich immunoassay has been characterized with respect to substrate concentration for the enzymatic reaction, detection limit, dynamic range and sources of error. Mouse IgG can be determined with a detection limit of 0.81 pg ml-1 by a 30-min substrate incubation time and a six orders of magnitude linear dynamic range.

Electrochemical determination of N-oxidized procainamide metabolites and functional assessment of effects on murine cells in vitro

Because of the implication of N-oxidized metabolites of procainamide in the induction of drug-related lupus, we have studied the electrochemical behavior of these metabolites and developed an electrochemical synthesis of nitrosoprocainamide. This synthesis was developed using procainamide hydroxylamine as the starting material which was oxidized to the nitroso species at an applied potential of 700 mV vs Ag/AgCl using a carbon packed bed bulk electrolysis flow cell. Conversion efficiencies of greater than 95% were achieved with this method. Subsequent studies with a chemically diverse series of biocompounds were used to investigate possible reactions between the procainamide hydroxylamine and nitroso species and these selected molecules. Only antioxidants such as cysteine, glutathione and ascorbic acid were found to react with the nitroso compound as determined by electrochemical methods, and this reaction was characterized as primarily a simple redox reaction at physiological pH. Animal studies conducted with murine spleen cells incubated with mitogens and various procainamide compounds demonstrated that the N-oxidized metabolites are the active immunopharmacologic agents.

Sequestration electrochemistry: the interaction of chlorpromazine and human orosomucoid

A simple and rapid method is presented for determination of the association constants and stoichiometries describing ligand macromolecule interactions. Based on flow injection analysis and electrochemical detection by amperometry, the only requirements for direct measurements are that the ligand have redox properties and that these properties change upon binding to the macromolecule. Bound ligand may then be measured in the presence of free ligand. Detection limits are of the order of 2 pmol of ligand or less, a level that should provide access to previously unmeasurable systems. For the exemplary system, chlorpromazine and human orosomucoid, K0ass was determined as 0.39 X 10(6) M-1 with 0.76 chlorpromazine binding sites of this affinity per orosomucoid molecule.

Extending the detection limit of solid-phase electrochemical enzyme immunoassay to the attomole level

Electrochemical enzyme immunoassay methodology has been developed to take advantage of the selectivity of antibody reactions, the amplification feature of an enzyme-based assay, and the ease with which small amounts of the enzyme-generated product can be detected electrochemically. A heterogeneous sandwich enzyme immunoassay was used in this work as the model assay. In this type of assay, the antigen is sandwiched between the enzyme conjugate and a primary antibody that is adsorbed to the solid phase. Alkaline phosphatase is a suitable enzyme for electrochemical assays since it catalyzes the conversion of electroinactive phenyl phosphate to electroactive phenol. The product, phenol, is then quantitated by liquid chromatography with electrochemical detection in a thin-layer flow cell with a carbon paste electrode at 0.895 V vs Ag/AgCl. The current produced by the oxidation of phenol is directly proportional to the analyte (antigen) concentration. The problem associated with these types of solid-phase immunoassays is that the adsorption of the primary antibody is desired while the adsorption of other assay proteins is not. The detection limits are generally defined by the ability to control this nonspecific adsorption. The detection limit of a previous electrochemical assay for rabbit IgG was 100 pg/ml and was limited by a large background current observed in the absence of antigen. In the present study, each step of the assay was examined in order to determine the sources of this background current, and it was found that the major contribution was from the nonspecific adsorption of the enzyme conjugate. Using combinations of Tween 20 and bovine serum albumin as blocking agents, the level of nonspecific adsorption was reduced by 96%.(ABSTRACT TRUNCATED AT 250 WORDS)