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At least 577 records · Page 32Linked to original sources
PubMed2002

Chemical microcharacterization of ultrathin iodide conversion layers and adsorbed thiocyanate surface layers on silver halide microcrystals with time-of-flight SIMS.

The technique of imaging time-of-flight secondary ion mass spectrometry (TOF-SIMS) and dual beam depth profiling has been used to study the composition of the surface of tabular silver halide microcrystals. Analysis of individual microcrystals with a size well below 1 microm from a given emulsion is possible. The method is successfully applied for the characterization of silver halide microcrystals with subpercent global iodide concentrations confined in surface layers with a thickness below 5 nm. The developed TOF-SIMS analytical procedure is explicitly demonstrated for the molecular imaging of adsorbed thiocyanate layers (SCN) at crystal surfaces of individual crystals and for the differentiation of iodide conversion layers synthesized with KI and with AgI micrates (nanocrystals with a size between 10 and 50 nm). It can be concluded that TOF-SIMS as a microanalytical, surface-sensitive technique has some unique properties over other analytical techniques for the study of complex structured surface layers of silver halide microcrystals. This offers valuable information to support the synthesis of future photographic emulsions.

Bromides↗
PubMed1994

Prevention of the rapid degradation of subcutaneously implanted Ag/AgCl reference electrodes using polymer coatings.

To assess the effect of the biological response to implanted Ag/AgCl reference electrodes on the electrode stability, uncoated and polymer-coated Ag/AgCl electrodes were implanted subcutaneously in rats. After 1 week of implantation, uncoated Ag/AgCl electrode potentials, measured in 0.1 M KCl, shifted by about -180 mV, and both voltammetry and electron microscopy showed that all the AgCl was removed. The electrodes could be significantly protected by coating with polyurethane or a perfluorinated ionomer (Nafion) cured at 120 degrees C for 1 h. Electron micrographs showed the 120 degrees C cured Nafion and polyurethane coatings remained intact over 2 weeks of implantation. Following 2 weeks of implantation the cured, Nafion-coated electrodes' potentials were shifted by -15 +/- 7 mV relative to the initial values. Voltammetry showed that they were still not polarizable. The current densities obtained with the coated reference electrodes are sufficient for their use as counter/pseudoreference electrodes with implantable two-electrode glucose sensor systems. The tissue response to coated electrodes was minimal in comparison to the response to uncoated reference electrodes.

Animals↗
PubMed1985

Determination of methane in ambient air by multiplex gas chromatography.

A multiplex gas chromatographic technique for the determination of methane in ambient air over extended periods is reported. A modest gas chromatograph which uses air as the carrier gas was modified by adding a silver oxide sample modulator for multiplex operation. The modulator selectively catalyzes the decomposition of methane in air. The resulting analytical systems requires no consumables beyond power. A profile of the methane concentration in this laboratory was obtained for an 8-day period. During this period, methane concentration varied with an approximately daily period from a low of 1.53 +/- 0.60 ppm to a high of 4.63 +/- 0.59 ppm over the entire 8 days. Some of the measured concentrations are higher than those reported elsewhere indicating the presence of some local source or sources for methane. This work has demonstrated the utility of a relatively simple multiplex gas chromatograph for the analysis of environmental samples. The technique should be applicable to other trace components in air through use of other selective modulators.

Air↗
PubMed2003

Silver-induced enhancement of thiochrome-based peroxide measurements.

Thiamine is presently one of the most attractive substrates used for sensitive fluorometric measurements of peroxides. Thiochrome (TC), a highly fluorescent product, is formed in enzyme-mediated oxidations. It is assumed that H(2)O(2) is nearly quantitatively converted to TC. The reaction cannot differentiate H(2)O(2) from many other peroxides such as methylhydroperoxide (MHP); to perform differential measurements, H(2)O(2) can first be selectively destroyed by a suitable catalyst such as MnO(2). In substituting Ag(2)O for MnO(2) to accomplish the selective destruction of H(2)O(2), we achieved the stated objective but were puzzled by a 3-fold increase in the MHP response in the presence of Ag(2)O. It was soon discovered that traces of dissolved Ag(+) and Hg(2+) can dramatically increase the yield of TC in this reaction from either H(2)O(2) or MHP; the normal yield in fact is only 20%. We present here a reaction scheme and kinetic model that adequately describes this behavior and should provide a path to substantially increase the sensitivity of this important assay method.

Flow Injection Analysis↗
PubMed2005

Gram-scale synthesis and biofunctionalization of silica-coated silver nanoparticles for fast colorimetric DNA detection.

A direct silica-coating method has been developed for the gram-scale synthesis of well-dispersed Ag@SiO(2) nanoparticles. Subsequent surface functionalization via the well-established silica surface chemistry provided arching points for straightforward bioconjugation with amino-terminated oligonucleotides. Fast hybridization kinetics of the resulting robust oligo-modified Ag@SiO(2) nanoprobes with complementary target oligonucleotides render themselves very useful for the fast colorimetric DNA detection based on the sequence-specific hybridization properties of DNA. Additionally, the reliable protocols developed in this study for preparing and functionalizing Ag@SiO(2) nanoparticles can be readily extended to other silica-coated nanoparticles, which can also provide a specific platform for the covalent attachment of biomolecules such as amino-rich proteins, enzymes, or amino-terminated oligonucleotides for diverse bioapplications.

Colorimetry↗
PubMed2004

Surface-enhanced Raman spectroscopy for bacterial discrimination utilizing a scanning electron microscope with a Raman spectroscopy interface.

Surface-enhanced Raman scattering (SERS) utilizing colloidal silver has already been shown to provide a rapid means of generating "whole-organism fingerprints" for use in bacterial identification and discrimination. However, one of the main drawbacks of the technique for the analysis of microbiological samples with optical Raman microspectroscopy has been the inability to acquire pre-emptively a region of the sample matrix where both the SERS substrate and biomass are both present. In this study, we introduce a Raman interface for scanning electron microscopy (SEM) and demonstrate the application of this technology to the reproducible and targeted collection of bacterial SERS spectra. In secondary electron mode, the SEM images clearly reveal regions of the sample matrix where the sodium borohydride-reduced silver colloidal particles are present, Stokes spectra collected from these regions are rich in vibrational bands, whereas spectra taken from other areas of the sample elicit a strong fluorescence response. Replicate SERS spectra were collected from two bacterial strains and show excellent reproducibility both by visual inspection and as demonstrated by principal components analysis on the whole SERS spectra.

Bacillus subtilis↗
PubMed2005

Glucose sensing using near-infrared surface-enhanced Raman spectroscopy: gold surfaces, 10-day stability, and improved accuracy.

This research presents the achievement of significant milestones toward the development of a minimally invasive, continuously monitoring, glucose-sensing platform based on the optical quantitation of glucose in interstitial fluid. We expand our initial successes in the measurement of glucose by surface-enhanced Raman scattering (SERS), demonstrating substantial improvements not only in the quality and optical properties of the substrate system itself but also in the robustness of the measurement methodology and the amenability of the technique to compact, diode laser-based instrumentation. Herein, we compare the long-term stability of gold to silver film over nanosphere (AuFON, AgFON) substrates functionalized with a partitioning self-assembled monolayer (SAM) using both electrochemical and SERS measurements. AuFONs were found to be stable for a period of at least 11 days. The switch to AuFONs not only provides a more stable surface for SAM formation but also yields better chemometric results, with improved calibration and validation over a range of 0.5-44 mM (10-800 mg/dL). Measured values for glucose concentrations in phosphate-buffered saline (pH approximately 7.4) based on 160 independent SERS measurements on AuFONs have a root-mean-square error of prediction of 2.7 mM (49.5 mg/dL), with 91% of the values falling within an extended A-B range on an expanded Clarke error grid. Furthermore, AuFONs exhibit surface plasmon resonances at longer wavelengths than similar AgFONs, which make them more efficient for SERS at near-infrared wavelengths, enabling the use of low-power diode lasers in future devices.

Glucose↗
PubMed2006

Fabrication of a planar-form screen-printed solid electrolyte modified Ag/AgCl reference electrode for application in a potentiometric biosensor.

This study features the fabrication of a planar-form, solid electrolyte modified, (PSEM) Ag/AgCl reference electrode using a screen-printing method. The PSEM Ag/AgCl reference electrode uses agar gel as the inner electrolyte and chloroprene rubber for the liquid junction and insulator. These common low-cost materials and the simple fabrication processes involved render the proposed reference electrode an ideal candidate for cost-efficient mass production. It is shown that the developed reference electrode is insensitive to most of the physiologically important ionic species, including Na+, K+, Li+, Ca2+, NH4+, and Cl-, under continuous measurement conditions. Moreover, as with conventional commercial reference electrodes, the proposed reference electrode exhibits a reversible response, which is maintained until the agar gel dries out. The PSEM Ag/AgCl reference electrode is integrated with an iridium oxide modified Pt-based pH indicator electrode to form a chip-type pH biosensor. The performance of this biosensor is consistent with that obtained from a pH meter based on a macroscopic commercial Ag/AgCl reference electrode. The experimental results confirm that the proposed biosensor is capable of providing precise pH measurements of various real samples. Accordingly, the PSEM Ag/AgCl reference electrode presented in this study provides a viable alternative to the macroscopic Ag/AgCl reference electrode used in many conventional chip-based pH sensors.

Biosensing Techniques↗
PubMed2006

Electrochemical multianalyte immunoassays using an array-based sensor.

A novel amperometric biosensor for performing simultaneous electrochemical multianalyte immunoassays is described. The sensor consisted of eight iridium oxide sensing electrodes (0.78 mm(2) each), an iridium counter electrode, and a Ag/AgCl reference electrode patterned on a glass substrate. Four different capture antibodies were immobilized on the sensing electrodes via adsorption. Quantification of proteins was achieved using an ELISA in which the electrochemical oxidation of enzyme-generated hydroquinone was measured. The spatial separation of the electrodes enabled simultaneous electrochemical immunoassays for multiple proteins to be conducted in a single assay without amperometric cross-talk between the electrodes. The simultaneous detection of goat IgG, mouse IgG, human IgG, and chicken IgY was demonstrated. The detection limit was 3 ng/mL for all analytes. The sensor had excellent precision (1.9-8.2% interassay CV) and was comparable in performance to commercial single-analyte ELISAs. We anticipate that chip-based sensors, as described herein, will be suitable for the mass production of economical, miniaturized, multianalyte assay devices.

Animals↗
PubMed2006

Picomolar peroxide detection using a chemically activated redox mediator and square wave voltammetry.

A method for low-level, low-potential electrochemical detection of hydrogen peroxide using a chemically activated redox mediator is presented. This method is unique in that it utilizes a mediator, Amplex Red, which is only redox-active when chemically oxidized by H2O2 in the presence of the enzyme horseradish peroxidase (HRP). When employed in concert with microelectrode square wave voltammetry to optimize sensing at ultralow concentrations (<1 microM), this method exhibits marked improvements in analytical sensitivity and detection limits (limit of detection as low as 8 pM) over existing protocols. Sensing schemes incorporating both freely diffusing and immobilized HRP are evaluated, and the resulting analytical sensitivities are 1.22 +/- 0.04 and (2.1 +/- 0.6) x 10(-1) microA/(microM mm2), respectively, for peroxide concentrations in the high picomolar to low micromolar range. A second linear region exists for lower peroxide concentrations. Furthermore, quantitative enzyme kinetics analysis using Michaelis-Menten parameters is possible through interpretation of data collected in this scheme. Km values for soluble and immobilized HRP were 84 +/- 13 and 504 +/- 19 microM, respectively. This method is amenable to any biological detection scheme that generates hydrogen peroxide as a reactive product.

Biosensing Techniques↗
PubMed1998

Silver-plated vitamins: a method of detecting tocopherols and carotenoids in LC/ESI-MS coupling.

In LC/MS, nonpolar substances in the majority of cases cannot be ionized by standard electrospray ionization (ESI) because they obviously lack a site for protonation or deprotonation. The ionization of carotenoids and tocopherols can be greatly enhanced by the addition of silver ions. The Ag(+)-carotenoid and Ag(+)-tocopherol adducts thus formed render these substances amenable to MS. alpha-, beta-, gamma-, and delta-tocopherol, alpha-tocopherol acetate, and the various isomers of lycopene and beta-carotene were separated by C30 RP-HPLC and could be identified by online ESI-MS. A mixture of six different carotenoids was analyzed by scanning the mass range from m/z 500 to 800. The mass spectra of the peaks revealed that all carotenoids and most tocopherols were partially oxidized to radical cations. The detection limit for canthaxanthin was approximately 500 fmol while that of beta-carotene was below 300 fmol. An increase in sensitivity in the MRM mode can be attained by monitoring ions formed by loss of elemental silver from the adducts in the CID cell. Dichloromethane extracts of tomato, carrot, and vegetable juices, a vitamin drink, and a commercial infant food product were analyzed by LC/MS. After postcolumn argentation, from the mass-selective extracts of the TIC, the carotenoids and tocopherols present could be identified by their masses and their retention times. For all studies, a silver perchlorate solution with an overall concentration of 50 micrograms/mL was used.

Antioxidants↗
PubMed1999

Adjusting the distance of electrochemical microsensors from secreting cell monolayers on the micrometer scale using impedance.

There are emerging applications of electrochemical microsensors where the distance of the sensor from an insulating plane needs to be adjusted and/or accurately known. The plane may be merely an obstruction or the source of a species whose release rate needs to be measured. An example is in cell secretion studies where a monolayer of cells is stimulated to secrete ions and/or other biochemical species which then diffuse away from the cells while being measured by a microsensor. Sensor response will thus depend on both the rate of release and the distance of the sensor from the cells. To obtain accurate release rates, the precision of the scheme to control electrode distance from the monolayer needs to be on the micrometer scale for species with ionic diffusivities. Optical (stereomicroscope and microruler) and mechanical (precalibrated micrometer screw) methods to precisely position the electrode are difficult to use under realistic circumstances (due to opaqueness of the chamber, and/or the medium, or irreproducible chamber depth). In this work we propose to correlate electrochemical cell impedance with sensor distance. This scheme has been used to adjust the distance of a chloride (tip diameter approximately 250 microns) and a potassium (tip diameter approximately 1000 microns) ion-selective microelectrode in the 0-250- and 0-2500-micron range, respectively, from a planar obstruction as well as from a monolayer of cells with a best precision of +/- 5 microns (n = 6) for the chloride and about +/- 20 microns for the potassium sensor. Larger electrodes have a broader range of distances over which they are sensitive, albeit with a poorer spatial resolution. This was verified by using Ag disk electrodes of 250 and 500 microns in diameters in AgNO3 solution.

Buffers↗
PubMed2006

Periplasmic metal-resistance protein CusF exhibits high affinity and specificity for both CuI and AgI.

The periplasmic protein CusF, as a part of the CusCFBA efflux complex, plays a role in resistance to elevated levels of copper and silver in Escherichia coli. Although homologues have been identified in other Gram-negative bacteria, the substrate of CusF and its precise role in metal resistance have not been described. Here, isothermal titration calorimetry (ITC) was used to demonstrate that CusF binds with high affinity to both Cu(I) and Ag(I) but not Cu(II). The affinity of CusF for Ag(I) was higher than that for Cu(I), which could reflect more efficient detoxification of Ag(I) given the lack of a cellular need for Ag(I). The chemical shifts in the nuclear magnetic resonance (NMR) spectra of CusF-Ag(I) as compared to apo-CusF show that the region of CusF most affected by Ag(I) binding encompasses three absolutely conserved residues: H36, M47, and M49. This suggests that these residues may play a role in Ag(I) coordination. The NMR spectra of CusF in the presence of Cu(II) do not indicate specific binding, which is in agreement with the ITC data. We conclude that Cu(I) and Ag(I) are the likely physiological substrates.

Binding Sites↗
PubMed2001

Structures and related properties of AgX bearing 3,3'-thiobispyridine (X- = NO3-, BF4-, CLO4-, and PF6-.

Infinite molecular helices [Ag(3,3'-Py2S)]X (3,3'-Py2S = 3,3'-thiobispyridine; X- = BF4-, ClO4-, and PF6-) have been rationally constructed or induced. Crystallographic characterization (X- = BF-, monoclinic P2(1)/n, a = 8.946(3) A, b = 14.130(2) A, c = 10.124(2) A, beta = 107.83(2) degrees, V = 1218.3(5) A3, Z = 4, R = 0.0351; X- = ClO4-, monoclinic P2(1)/n, a = 8.884(1) A, b = 14.305(3) A, c = 10.110(1) A, beta = 106.78(1) degrees, V = 1230.1(3) A3, Z = 4, R = 0.0417; X- = PF6-, monoclinic P2(1)/c, a = 10.959(2) A, b = 9.808(2) A, c = 14.065(3) A, beta = 112.03(2) degrees, V = 1401.4(5) A3, Z = 4, R = 0.0442) reveals that the skeletal structure is an oblong cylindrical cationic helix consisting of alternating Ag(I) and 3,3'-Py2S species and that its counteranions are pinched in two columns inside each helix. The formation of the helical coordination polymer appears to be primarily associated with a suitable combination of the skewed conformer of 3,3'-Py2S and the potential linear geometry of the N-Ag(I)-N bond. However, the framework of the nitrate analogue [Ag(3,3'-Py2S)NO3] (monoclinic P2(1)/c, a = 8.177(2) A, b = 10.291(1) A, c = 14.771(2) A, beta = 102.19(1) degrees, V = 1214.9(4) A3, Z = 4, R = 0.0300) is a two-dimensional network consisting of an 18-membered ring unit, where each 3,3'-Py2S acts as a N,N',S- tridentate ligand connecting three tetrahedral silver(I) ions with the monodentate nitrate weakly bonded to the silver (Ag.O = 2.65(1) A) rather than acting as a counteranion. The anion exchange of [Ag(3,3'-Py2S)NO3] with BF4-, ClO4-, or PF6- has been accomplished in aqueous media. The two-dimensional networks are easily converted into the helices via the anion exchange, but the reverse anion exchange proceeds slightly. Thermal analyses indicate a relationship between the thermal stabilities and the structural properties.

Anions↗
PubMed2004

Sulfur bridging interactions of cis-planar NiII-S2N2 coordination units with nickel(II), copper(I,II), zinc(II), and mercury(II): a library of bridging modes, including NiII(micro2-SR)2MI,II rhombs.

Sulfur bridging interactions between three cis-planar NiII-S2N2 complexes and NiII, CuI,II, ZnII, and HgII reactants were investigated by synthesis and X-ray crystal structures of some 24 complexes. This work was stimulated by recent crystallographic structures of the A-cluster of carbon monoxide dehydrogenase/acetylcoenzyme A synthase. This bridged biological assembly has the minimal formulation [Fe4S4]-(micro2-SCys)-[M((micro2-SCys)2Gly)Ni] with M = NiII, CuI, and ZnII at sites distal and proximal, respectively, to the iron-sulfur cluster. Bridges supported by representations of the distal nickel site were sought by reactions of the complexes [NiII(LH-S2N2)]2- and [NiII(LR-S2N2)], with 5-5-5 chelate ring patterns. Reaction products implicate the bridges Ni-(micro2-S)1,2-M in a variety of molecular structures, some with previously unknown connectivities of bridge atoms. The most frequently encountered bridge units are the nonplanar rhombs Ni(2-S)2M involving both sulfur atoms of a given complex. Those with M = NiII are biologically relevant inasmuch as the catalytic metal at the proximal site is nickel. The complex [Ni(L-655)]2-, containing the 6-5-5 ring pattern and coordination sphere of the distal nickel site, was prepared and structurally characterized. It was shown to sustain Ni2(micro2-S)2 rhombic interactions in the form of trinuclear [[Ni(L-655)]2Ni]2- and [[Ni(L-655)]Ni(R2PCH2CH2PR2)] (R = Et, Ph) in which the second NiII simulates the proximal site. Bridging interactions of NiII-S2N2 complexes are summarized, and geometrical features of Ni2(2-S)2 rhombs in these complexes, as dependent on ring patterns, are considered (LH-S2N2 = N,N'-ethylenebis(2-mercaptoisobutyramide)(4-); LR-S2N2 = trans-rac-N,N'-bis(2-mercapto-2-methylprop-1-yl)-1,2-cyclohexanediamine(2-); L-655 = N-(2-mercaptopropyl)-N'-(2'-mercaptoethyl)glycinamide(4-)).

Bridged-Ring Compounds↗
PubMed2003

Protein-conjugated nanoparticles from rapid expansion of supercritical fluid solution into aqueous solution.

The method of rapid expansion of a supercritical solution into a liquid solvent (RESOLV) was applied to the preparation of bovine serum albumin protein-conjugated silver sulfide nanoparticles. The conjugate samples were characterized by using a series of instrumental techniques. The results show that the monodispersed nanoparticles in the conjugates are well-coated directly with the protein. Because the protein undergoes solution pH-dependent association and dissociation, the protein-nanoparticle conjugates also assemble and disassemble with changes in solution pH in a reversible fashion.

Hydrogen-Ion Concentration↗
PubMed2004

AgBF4-impregnated poly(vinyl phenyl ketone): an ethylene sensing film.

Incorporation of silver tetrafluoroborate (AgBF4) into poly(vinyl phenyl ketone) (PVPK) renders the photoluminescent polymer responsive to ethylene. Polymer films prepared with a 2:1 ratio of Ag+ ions to polymer acetophenone groups responded with a quench of photoluminescence. Conditioned films showed a luminescence quench that was proportional to ethylene concentration before saturation occurred. Stern-Volmer analysis of the photoluminescence response suggested the presence of sites that were accessible and sites that were inaccessible to ethylene. Perturbations in polymer-metal interactions were monitored with infrared spectroscopy, revealing changes upon Ag+ incorporation, polymer film conditioning, and exposure to ethylene.

Borates↗