The response of halide ion-selective electrodes to redox systems : A comparison between silver/silver halide electrodes and silver sulphide- based halide ion-selective electrodes

Silver/silver chloride and bromide electrodes, prepared by anodizing ordinary silver electrodes, and the corresponding ion-selective electrodes based on silver sulphide, were tested for their susceptibility towards redox systems. It proved that the latter type of electrode responded significantly to strong oxidants. In contrast, the silver/silver halide types were highly resistant to redox interference provided that the silver halide layer was free from open pores. This could be achieved by generation of sufficiently thick layers and by selection of suitable current densities during electrodeposition (<20 mA cm^-2). The interrelation between the conditions of silver chloride film generation and redox resistance of the resulting electrodes is described in detail.     

Nucleation-growth kinetics of the oxidation of silver nanocrystals to silver halide crystals

The electrochemical oxidation of silver nanocrystals to silver halide crystals proceeds by a process of nucleation and growth. The mechanism is confirmed by analyzing chronopotentiograms using a new extension of nucleation theory. The theory makes it possible to derive plots of nucleation-growth currents vs potential, and growth rates vs potential, directly from experimental data. Such plots yield powerful insights into the reaction kinetics. In situ AFM imaging reveals that a few thousand of silver nanocrystals are oxidized to only a few tens of silver halide crystals, without pronounced loss of active material. The mechanism of this remarkable process is described in this paper. In particular, it is shown that the decrease in crystal population proceeds via an oversaturated silver solution, i.e., a process that is mediated by “driven” Ostwald ripening across the electrode surface. At the same time, the low solubility of silver species in bulk solution means that few silver ions escape from the surface. This combination of features explains why the transformation from silver to silver halide is near-stoichiometric yet highly reconstructive.Dedicated to our friend and colleague Professor Dr. Alan Bond on the occasion of his 60th birthday     

A silver halide recording material with luminescence reading

Methods for the conversion of image silver nanoparticles into luminescent products were developed; this considerably increased the signal-to-noise ratio of luminescence reading. The principles of the design of a silver halide multilayer nonreversible CD were considered. A conclusion was drawn that classical silver halide photography can be used in the process of manufacturing multilayer optical disks (CD LROM).     

Preparation of dendritic nanostructures of silver and their characterization for electroreduction

Silver nanostructures of different morphologies including well-defined dendrites were synthesized on an Au substrate by a simple surfactant-free method without using any template. The morphology of the material was investigated by field-emission transmission electron microscopy and scanning electron microscopy. The crystal nature of the dendritic nanostructure was revealed from their X-ray diffraction and electron diffraction patterns. Effects of applied potential, electrolysis time, and the solution concentration were studied. The possible formation mechanism of the dendritic morphology was discussed from the aspects of kinetics and thermodynamics based on the experiment results. The H(2)O(2) electroreduction ability of the dendritic materials was characterized. Use of silver dendrite-modified electrode as H(2)O(2) sensor was also demonstrated.     

Synthesis of mixed silver halide nanocrystals in reversed micelles

The specifics of the synthesis of silver halide nanocrystals of mixed composition and the core-shell structures in reversed micelles were experimentally studied. It was shown that homogeneous AgBr _x I_{1 ? x} nanocrystals of ～5 nm in size with the iodide concentration up to 70%, as well as the core-shell structures AgI/AgBr and AgBr/AgI, can be synthesized by the micellar synthesis. It was found that the relation of the crystalline structures of the core and shell materials plays an important role in the shell formation. The shell of γ-AgI alone is formed on the AgBr nanocrystals with a close lattice type, whereas β-AgI with the hexagonal lattice forms an individual phase of nanoparticles, rather than the shell.     

Interactions of silver halide sols with hydrolyzed hafnium species

Summary Coagulation and reversal of charge effects of freshly prepared and heated solutions of hafnium tetrachloride have been studied as a function of the p_H using aged silver halide sols and solsin statu nascendi. It was shown that the critical coagulation concentration and the critical stabilization concentration (due to charge reversal) increased with increasing p_H. These observations have been related to the hydrolysis of the hafnium ion. At p_H values above 4 essentially the entire amount of hafnium is present in the form of the neutral soluble, species Hf(OH)₄. This accounts for the inability of the hafnium solutions to reverse the charge of the sols at higher p_H values. The adsorption measurements carried out with the aid of the radioactive isotope¹⁸¹Hf showed that the neutral hydrolyzed species are strongly adsorbed on negatively charged silver iodide particles. The adsorbed amounts of hafnium on a AgI sol are considerably larger than in the case of charged hydrolyzed ions (such as hydrolyzed thorium ions) on a similar sol. This is explained by the ability of the neutral hafnium species, Hf(OH)₄. to form a close-packed adsorbed layer. The results confirm previous findings that the enhanced adsorption of hydrolyzed ions is caused by the presence of the hydroxyl group, whereas the ionic charge plays a negligible role in this case.

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Zusammenfassung Die Koagulations- und Umladungserscheinungen von frisch dargestellten und durch Erwärmung gealterten Lösungen von Hafniumchlorid wurden an Silberhalogenid-Solen in der Abhängigkeit vom p_H untersucht. Es wurde festgestellt, daß die kritische Koagulationskonzentration und die kritische Stabilisationskonzentration (die Umladungsgrenze) mit steigendem p_H höher werden. Diese Beobachtungen wurden durch die Hydrolyse des Hafnium-Ions erklärt. Wenn p_H>4 ist, bestcht nahezu die ganze Menge von Hafnium als neutrale, gelöste Hf(OH)₄ Moleküle. Damit wird es erklärt, daß die Silberhalogenid-Teilchen bei höheren p_H-Werten durch Hafniumsalze nicht umgeladen werden können. Die mittels des radioaktiven Isotopen¹⁸¹Hf durchgeführten Messungen zeigten, daß die neutralen Hf(OH)₄ — Moleküle stark an den negativ geladenen Silberjodid-Teilchen adsorbiert sind. Die adsorbierte Menge per Mol von AgI ist beträchtlich größer als die Menge der geladenen, hydrolysierten Ionen (wie z. B. ThOH³⁺), die an dem gleichen Sol bestimmt wurde. Dieser Effekt ist leicht erklärt, da man mit den neutralen adsorbierten Molekülen eine fest gepackte Adsorptionsschicht bilden kann. Diese Resultate bestätigen die früheren Befindungen, daß die höhere Adsorptionsfähigkeit der hydrolysierten Ionen durch die Hydroxylgruppe verursacht wird, während die Ionenladung eine zu vernachlässigende Rolle spielt.

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Supported by the U.S. Army Research Office (Durham), Grant No. DA-ARO (D)-31-124-G 656.

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Part of a PhD Thesis byL. J. Stryker, supported by a NASA Traineeship.     

Bio-analytical applications of mid-infrared spectroscopy using silver halide fiber-optic probes

Infrared-spectroscopy has proved to be a powerful method for the study of various biomedical samples, in particular for in-vitro analysis in the clinical laboratory and for non-invasive diagnostics. In general, the analysis of biofluids such as whole blood, urine, microdialysates and bioreactor broth media takes advantage of the fact that a multitude of analytes can be quantified simultaneously and rapidly without the need for reagents. Progress in the quality of infrared silver halide fibers enabled us to construct several flexible fiber-optic probes of different geometries, which are particularly suitable for the measurement of small biosamples. Recent trends show that dry film measurements by mid-infrared spectroscopy could revolutionize analytical tools in the clinical chemistry laboratory, and an example is given. Infrared diagnostic tools show a promising potential for patients, and minimal-invasive blood glucose assays or skin tissue pathology in particular cannot be left out using mid-infrared fiber-based probes. Other applications include the measurement of skin samples including penetration studies of vitamins and constituents of cosmetic cream formulations. A further field is the micro-domain analysis of biopsy samples from bog mummified corpses, and recent results on the chemistry of dermis and hair samples are reported. Another field of application, for which results are reported, is food analysis and bio-reactor monitoring.     

Electroreduction of cerium ions on silver electrode in halide melts at 973 K

The mechanism of electroreduction of cerium ions in equimolar KCl-NaCl melt is explored at 973 K. The effect of the anionic composition of the melt on the electroreduction of cerium ions is studied. It is shown that the electrodeposition of metal cerium from halide melts on a silver electrode is the primary electrochemical process that occurs at potentials more positive than those corresponding to the supporting-electrolyte decomposition. The electroreduction of chloride complexes of cerium on a silver electrode in the melt in both steady-and non-steady-state polarization modes at rates below V ≤ 0.5 V/s is limited by the diffusion delivery; at higher polarization rates, the charge-transfer stage predominates.     

Specifics of optical sensitization of silver halide nanocrystals under intense exposures

The results of experiments that revealed the specifics of action of pulse radiation on solutions of optical sensitizers and the feasibility of enhancement of the holographic recording efficiency in layers based on silver halide nanocrystals by doping with additional sensitizers are reported.     

Esterification and etherification by silver oxide-organic halide reaction gas chromatography.

In an attempt to separate a number of methylated nucleoside constituents reportedly present in various mammalian RNA preparations, a comparison of the migration of selected nucleoside standards was assessed via thin-layer chromatographic techniques with respect to three solvent systems and an adsorbent of varying proportions of silica gel and microcrystalline cellulose. The results of these studies revealed that a mixture of seventeen methylated constituents could be separated following two-dimensional development on an adsorbent containing 60% cellulose and 40% silica gel. Constituents separated included the 2'-O-methylated derivatives of adenosine, guanosine, cytidine and uridine, 1-, N6- and N6,N6-methyladenosines, 1-methylinosine, 1-, 7-, N2- and N2,N2-methylguanosines, 3-, 4- and 5-methylcytidines and 3- and 5-methyluridines. In addition, all of the above constituents were separated from their respective non-methylated nucleosides and bases.     

Electrocatalytic potentialities of silver as a cathode for organic halide reductions

The peculiar halide affinity for silver results in an extraordinary electrocatalytic activity for the reduction of halides (either glycosyl halides or, more generally, aryl and alkyl halides). The most striking features are: (a) a reduction potential shift in the positive direction of about 1000 mV with respect to glassy carbon and 500 mV with respect to mercury; (b) a cage effect, evidenced in previous synthetic work concerning bromosugars, promoted by the halide acting as a bridge between the electrode surface and the reacting substrate, which mainly results in dimerization and/or addition products. The above electrocatalytic effect is here investigated by means of a systematic reactivity study on Ag, Hg and glassy carbon cathodes, with a variety of substrates. The effect of the supporting electrolyte is also analysed in detail, providing a first inspection on specific halide/silver interactions in acetonitrile media.     

Synthesis of silver nanowires with different aspect ratios as alcohol-tolerant catalysts for oxygen electroreduction

The aqueous-phase synthesis of crystalline silver nanowires with different aspect ratios has been achieved by altering seeds. Cyclic voltammetry and tafel results showed that silver nanowires with high aspect ratio exhibited better activity and alcohol-tolerant stability for oxygen electroreduction in alkaline media than nanowires with low aspect ratio.     

Thermochemical study of silver halide nanoparticles formation conditions in AOT inverted micelles

AgCl and AgBr nanoparticles formation conditions were studied by a thermochemical method in AOT (sodium bis(2-ethylhexyl)sulfosuccinate) inverted micellar systems, in AOT—dioctyl sulfide (DOS) mixed micelles, and (for comparison) in aqueous solutions. The heats of formation of AgCl and AgBr nanoparticles in AOT micelles in exchange reactions with potassium halides are, respectively, −55.5 × (1 ± 0.07) and −68.6 × (1 ± 0.07) kJ/mol, that is, smaller in magnitude than the values obtained for aqueous solutions (−68 × (1 ± 0.07) and −88 × (1 ± 0.07) kJ/mol). This difference arises from the existence of particle interactions causing the formation of coagulation contacts between halide particles followed by precipitation in an aqueous phase and the absence of such interactions in a micellar medium. DOS interacts with AOT (to form mixed micelles) and with silver ions (in long-term contact), thus reducing the heats of reactions.     

Fabrication of high-precision silver halide layers for nanoscale linear-and angular-translation measurement systems

A silver halide emulsion casting technique was designed and studied for the fabrication of thin holographic layers of 3 ± 0.5 μm thickness free of phase distortions and with the required resolving power depending on the optical-range wavelength in the holographic recording of metrological gratings.     

Novel infrared optical probes for process monitoring and analysis based on next-generation silver halide fibers

Mid-infrared spectroscopy has proved to be a powerful method for the study of various samples and chemical media as found in different industrial processes. In general, the analysis of such samples takes advantage of the fact that multiple analytes can be quantified simultaneously and rapidly without the need for additional reagents. When compared to near-infrared spectroscopy, for which quartz fiber probes can be successfully applied, the application of previously used mid-infrared fiber materials was restricted due to deficiencies with regard to their optical transmission and mechanical properties. Progress in the quality of infrared transparent silver halide fibers and their extrusion with different cross-sections enabled us to construct several flexible fiber-optic probes of different geometries which are particularly suitable and inert for process monitoring. Transmission and attenuated total reflection measurement techniques have mainly been employed for the analysis of liquid and gaseous media. One larger field, for which results are reported, is chemical reactor monitoring. Other applications are concerned with bio-reactor monitoring, or quasi-continuous measurements for the food industry. Infrared spectroscopic cosmetic assays for determining the chemical composition of skin-care formulations are a further promising field of application, for which an example is given.     

Organic functionalization of metal catalysts: Enhanced activity towards electroreduction of carbon dioxide

Electrochemical reduction of carbon dioxide has been attracting extensive interest due to its fundamental significance both in environmental protection and in energy storage. In this review, recent progress in the manipulation of the catalytic activity and selectivity of various transition metals towards CO₂ reduction reaction (CO₂RR) is summarized within the context of deliberate surface functionalization by select organic ligands. This is primarily manifested in three effects, interfacial charge transfer, suppression of hydrogen evolution, and stabilization of key reaction intermediates. The review is concluded with a perspective of the challenges and promises in the structural engineering of metal catalysts for enhanced CO₂RR performance.     

Thermodynamics and phase equilibrium studies of silver halide-cobalt(II) halide systems

Phase diagrams for the systems AgCl-CoCl₂ and AgBr-CoBr₂ were determined by differential scanning calorimetry. The systems are of the eutectic type. Eutectic points are at 271±2 K, 19.5 mol% CoCl₂ and 653±2 K, 17.0 mol% CoBr₂, respectively. The solid solubility does not exceed 2 and 4 mol% in the systems AgCl-CoCl₂ and AgBr-CoBr₂, respectively. Thermodynamic activities of components in molten mixtures and molar free enthalpies of mixing were determined for both systems on the basis of liquidus curves. Deviations from ideality were not found to be considerable.     

Synthesis of silver nanowires and their applications in the electrochemical detection of halide

A silver nanowires modified platinum (Ag NWs/Pt) electrode was developed for simultaneous and selective determination of chloride, bromide and iodide ions by cyclic voltammetry in aqueous solutions. Silver nanowires were synthesized by an l-cysteine-assisted poly (vinyl pyrrolidone) (PVP)-mediated polyol route. X-ray diffraction (XRD) and scanning electron microscopy (SEM), transmission electron microscopy (TEM) and energy dispersive X-ray spectroscopy (EDS) were employed to investigate the prepared nanowires. The intrinsic high surface area and the fast electron transfer rate ascribed from the nanowire structure could further improve halide detection performance. The determination was based on measurement of the well-separated oxidation peak currents of respective silver halides formed on the surface of silver during an anodic potential sweep. The concentration range was linear from 50 μM to 20.2 mM for bromide and iodide and 200 μM to 20.2 mM for chloride, and the sensitivity was 0.059 μA/mM, 0.042 μA/mM and 0.032 μA/mM for chloride, bromide and iodide, respectively. The correlation coefficient was 0.999 in each case. The Ag NWs/Pt electrode offered a useful platform for the development of a highly sensitive halide sensor.