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.

Immunoassay by differential pulse polarography.

An immunological method based on labeling an antigen with an electroactive group detectable by differential pulse polarography has been demonstrated. Estriol labeled with mercuric acetate is electroactive, giving a reduction wave at -300 millivolts versus a standard calomel electrode. Addition of estriol antibody to 4-mercuric acetate estriol diminishes the peak current as a result of the antigen-antibody binding reaction. Separation of free-labeled estriol from antibody-bound-labeled antigen is unnecessary. The method is potentially useful as an analytical immunological technique.

Immobilization of lactate oxidase in a poly(vinyl alcohol) matrix on platinized graphite electrodes by chemical cross-linking with isocyanate.

A new method for development of an electrochemical sensor based on lactate oxidase is dedbed. Platinized spectroscopic-grade graphite electrodes were modified by chemically cross-linking l-lactate oxidase from Pediococcus species into a poly(vinyl alcohol) network through reaction with a tri-isocyanate. The immobilized enzyme exhibits high activity and long-term stability. The sensor provides a linear response to l-lactate over a concentration range of 2 x 10(-5)-4 x 10(-3)M and a sensitivity of 1.71 muA.1. mmole(-1). The response time of the sensor is 10-45 sec and the detection limit is 10muM. Stable response to the substrate was obtained over a period of 3 months. The new sensor was also used for the analysis of some dairy products without any special pretreatment.

Dual-analyte spectroscopic sensing in sol-gel derived polyelectrolyte-silica composite thin films.

Ferrozine (3-(2-pyridyl)-5,6-diphenyl-1,2,4-triazine-p,p'-disulfonic acid, monosodium salt hydrate), an iron indicator, and HTPS (8-hydroxyl-1,3,6-pyrenetrisulfonic acid, trisodium salt), a pH indicator, were immobilized in sol-gel derived PDMDAAC-SiO(2) (where PDMDAAC stands for poly(dimethyldiallylammonium chloride), composite thin films via ion-exchange. The two indicators were immobilized in two adjacent sections of the same PDMDAAC-SiO(2) film which was supported on a glass optical substrate. The spectroscopic response of the film to both Fe(2+) and H(+) in solutions was investigated by attenuated total reflection (ATR) spectrometry at two well-separated wavelengths, 562 nm for Fe(2+) and 460 nm for H(+). The Ferrozine/HPTS immobilized PDMDAAC-SiO(2) films had the following characteristics: linear range, 2.5x10(-6)-5.0x10(-5) M for Fe(2+), pH 4.1-6.8 for H(+); sensitivity, 2.2x10(4) DeltaA/M for Fe(2+), 0.583 DeltaA/pH for H(+).

Chromatographic characterization of electrochemically generated technetium-HEDP skeletal imaging agents.

Reduction of pertechnetate [(99m)TcO(-)(4)] by controlled-potential coulometry in the presence of 1-hydroxy-1,1-ethanediphosphonate (HEDP) leads to the formation of (99m) Tc complexes which are suitable for bone imaging. The complex radiopharmaceutical mixtures can be separated into their respective components by anion-exchange high-performance liquid chromatography. The distribution of the complexes in the mixtures is dependent on the cell potential, pH, technetium concentration, and presence or absence of air. A single component formed in high yield and isolated from the mixture by HPLC was investigated as a potential bone-imaging agent. This particular complex is produced only in low yields when prepared by chemical reduction of (99m) TcO(-)(4). Thus electrochemistry shows promise in aiding in the development of a more efficacious bone-imaging agent by allowing selective generation of individual (99m) Tc-HEDP complexes.

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.

Characterization of technetium radiopharmaceuticals by thin-layer spectroelectrochemistry.

Optically transparent thin-layer electrode (OTTLE) techniques have been used to characterize technetium complexes of biomedical importance. Different Tc oxidation states have been generated at the electrodes and absorption spectra determined. Small cell volumes allow the study of materials available in small quantities, and a flow cell configuration has also been used. Heart and bone imaging agents have been studied.

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.

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.

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.

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.

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).

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.

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.

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.

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.

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.

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.