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The electrochemical oxidation of aliphatic carboxylic acids, hexanoic, heptanoic, and lauric acid, under biphasic conditions is studied as a model system for ultrasound enhanced Kolbe electrosynthesis processes. Power ultrasound is used to generate an in situ emulsified medium and to remove reaction products continuously from the electrode surface. A clean and highly efficient process at platinum electrodes with formation of 'one electron' products only occurs in marked contrast to processes in monophasic media. For hexanoic acid the Kolbe dimer product R-R is formed in up to 75% yield with 45% current efficiency at 0.18 A cm-2 current density and in the presence of 190 W cm-2 ultrasound. The mechanism is explained in terms of a dynamically modified electrode surface, at which hydrophobic products are immediately 'trapped' via partitioning into a non-polar organic phase and transported away into the emulsion system. Kolbe electrosynthesis is undertaken both at platinum electrodes and at free-standing polycrystalline boron-doped diamond electrodes, in order to minimize the surface erosion effect induced by power ultrasound. The type and yield of products obtained from the biphasic Kolbe electrolysis process at diamond electrodes are essentially identical to those observed at platinum and based on this observation, the presence of a biphasic reaction layer at the electrode surface is postulated to govern the process. © 2001 Elsevier Science B.V.

Solutions containing hen egg white lysozyme (HEWL) and nitrite were exposed to ultrasonic irradiation in order to study the possible sonochemical modifications. This is the first demonstration of the nitration of tyrosine residues in a protein (lysozyme) by the use of an ultrasonic field alone. Sonochemically nitrated lysozyme was detected using the immunochemical techniques dot blot immunodetection and enzyme-linked immunosorbent assay (ELISA). The sonically oxidised and nitrated protein solutions were analysed by Fourier transform ion cyclotron resonance (FT-ICR) mass spectrometry. Hydroxylated species were found in the absence of nitrite, whereas nitration was the major modification in the presence of nitrating agent, implying a competing mechanism between hydroxyl radicals and nitrite. Circular dichroism (CD) indicated that the ultrasonic experimental conditions chosen in this study had little effect on the tertiary and secondary structures of HEWL. Whilst enzymatic assay showed that the presence of nitrite provided a protective effect on the inactivation of the protein under ultrasonic irradiation, nevertheless partially purified, sonically nitrated lysozyme showed a dramatic decrease in lytic activity.

In this paper we report the successful use of a non-metallic electrode material, boron-doped diamond (BDD), for the anodic electro-oxidative modification of hen egg white lysozyme (HEWL). Platinum electrodes can give rise to loss of activity of HEWL in electrosynthetic studies, whereas activity is retained on boron-doped diamond which is proposed as an effective substitute material for this purpose. We also compare literature methods of electrode pre-treatment to determine the most effective in electrosynthesis. Our findings show a decrease in total nitroprotein yield with decreasing nitrite concentration and an increase with increasing solution pH, confirming that, at a BDD electrode, the controlling factor remains the concentration of tyrosine phenolate anion. Purification of mono- and bis-nitrated HEWL and assay of enzymic activity showed better retention of activity at BDD electrode surfaces when compared to platinum. The products from electro-oxidation of HEWL at BDD were confirmed by electrospray ionization Fourier transform ion cyclotron resonance (ESI-FT-ICR) mass spectrometry, which revealed unique mass increases of +45 and +90Da for the mono- and bis-nitrated lysozyme, respectively, corresponding to nitration at tyrosine residues. The nitration sites were confirmed as Tyr23 and Tyr20.

A thin film comprising highly sterically hindered iron phthalocyanine derivatives incorporated into sol–gel networks shows effective sensitivity to carbon monoxide gas, when monitored by UV–visible absorption spectroscopy at 670 nm. The absorbance of a sol–gel thin film incorporating octaphenyl-di-tert-butylphthalocyanine iron(II) was particularly sensitive to carbon monoxide gas and showed a change in absorption after exposure to 1000 ppm CO gas for 1 min, while repetitive exposures produced only a limited loss of response. The optical response of the thin film gradually decreases with increasing temperature, and the binding energy for this sensing phenomenon was calculated to be 0.401 eV. The spectrum recovers at room temperature some 5 min after exposure to the gas.

The electrochemical reduction of silver thiosulphate was studied potentiostatically on platinum electrodes in the absence and presence of ultrasound (20 kHz). This system is irreversible and the reaction is both diffusion and kinetically controlled. The slowest step is the kinetic reaction especially the chemisorption of ions at the electrode surface. Ultrasound greatly improves the mass transport, which can be explained by changing from diffusion to mainly convection. This paper reports the effect of ultrasound upon electrode kinetic and mass-transport parameters at various RDE rotation speeds and ultrasonic intensities. It was found that the heterogeneous rate constant (kf) is improved in the presence of ultrasound due to the increase in the formal or standard heterogeneous rate constant (k0) (approximately by 10-fold under sonication).

Thiophene-S-oxides (thiophene monoxides) are relatively new compounds, less stable than the better-known thiophene-S-dioxides. They are useful as synthons for a range of applications, including in the production of pharmaceuticals. They have interesting photochemical properties, but in this presentation we contrast the electro-oxidative voltammetry of differently substituted derivatives. We also compare carbocyclic compounds such as tetracyclone, the electro-oxidation of which at relatively high potentials has never been reported in silent or insonated conditions.

This paper describes recent studies in organic sonoelectrochemistry at Coventry University, including the oxidation of thiophene monoxides, degradation of dye pollutants, formation of conducting polymers and electrosynthetic modification of proteins.

The electrochemistry of microdroplets, shown to be nearly monodisperse, of N,N,N‘,N‘-tetraalkyl-para-phenylenediamine oils (TRPD, R = n-butyl, n-hexyl, n-heptyl, and n-nonyl) immobilized on a basal plane pyrolytic graphite electrode and immersed into aqueous electrolyte solution is studied using cyclic voltammetry. Upon oxidation of the TRPD droplet to the cation radical TRPD+•, anion uptake from, or cation loss into the aqueous solution takes place, so as to maintain electroneutrality within the oily deposit. The former process is shown to produce an ionic liquid, with the anion insertion taking place at the triple phase boundary of electrode |TRPD oil| aqueous electrolyte; the latter process, in contrast, takes place at the interface between the two immiscible liquids, and with two-thirds-order kinetics. The possibility of a chemical reaction taking place between the electrogenerated and inserted ions at the three-phase junction, viz. redox-catalysis or otherwise, is illustrated via reference to two sys...

A new SECM approach for studying the lateral diffusion of redox-active amphiphiles in Langmuir monolayers at an air/water (A/W) interface is described. To apply this technique practically, a triple potential step transient measurement is utilized at a submarine ultramicroelectrode (UME) placed in the water phase close (1−2 μm) to the monolayer. In the first potential step, an electroactive species is generated at the UME by diffusion-controlled electrolysis of a precursor. This species diffuses to, and reacts with, the redox-active amphiphile at the A/W interface resulting in the formation of the initial solution precursor, which undergoes diffusional feedback to the UME. In this first step, the rate constant for electron transfer between the solution mediator and the surface-confined species can be measured from the UME current−time transient. In the second period, the potential step is reversed to convert the electrogenerated species to its initial form. Lateral diffusion of electroactive amphiphile int...

Solutions of both basic and acidic dyes were subject to sonolysis, electrolysis, and sonoelectrolysis. Only basic dyes were decolorized by ultrasound alone. Removal of the acidic dye Sandolan Yellow required the use of an electrooxidation process. The rate of electrochemical decolorization in the absence of ultrasound was dependent on the type of electrolyte, the electrolyte concentration, the reaction temperature, and the current density. The sonoelectrooxidation of Sandalon Yellow needed to be performed in a sealed cell to minimize the effects of ultrasonic degassing.

Insonation of cyclohexanoate electrooxidation at platinum in methanol produces a similar switch in product distribution from one-electron per-molecule dimers towards products from the two-electron route via the carbocation, irrespective of whether the ultrasonic bath employs 38 or 850 kHz frequencies. However, 850 kHz produces a cleaner reaction and higher yields, with more than two-fold greater yield of total extracted products over the silent system, and one-and-a-half times yield enhancement over insonation at 38 kHz. Keywords: Sonoelectrochemistry, Carboxylate electrooxidation, Ultrasound, Frequency dependence

We report improved techniques for the fabrication of biomimetic membranes protectively sandwiched between two slabs of agarose gel and the evaluation of their characteristics relevant for use in a biosensor based on gated channel proteins. The gel protects the membrane from mechanical contact, from low-energy solvents and from drying out. The new methods allow the preparation in high yield of sensor cells with a front gel thickness less than 100 μm. During fabrication, gel-protected membranes of DOPC (dioleoyl phosphatidylcholine) readily incorporate valinomycin in functional form. DOPC membranes respond as expected and within a few minutes to aqueous solutions of gramicidin applied to the front gel layer. In long-term studies of protected membranes of the synthetic lipid PSPC, no failures were observed over runs of up to 3 weeks and the membrane leakage decreased, reaching within 7 days values comparable to those reported for unsupported bilayer lipid membranes. Initial attempts to incorporate functional nicotinic acetylcholine receptors have shown promise.

This paper describes the effect of ultrasound on electrochemical parameters important in the removal of silver from photographic processing solutions. Decomposition voltages (obtained galvanostatically) and discharge potentials (obtained potentiostatically) for the reduction of the silver complex, Ag(S2O3) 2 3- was studied in dilute aqueous Na2S2O3/NaHSO3 solution on stainless steel and carbon disc electrodes in both the absence and presence of ultrasound. Under silent conditions, silver deposition is the main reaction occurring at the electrode. Under sonication at 20 kHz and 500 kHz, the reduction wave shifts anodically with increasing ultrasonic intensity. Similarly, the discharge of hydrogen and oxygen shifts anodically and cathodically respectively with increasing ultrasonic power. The decrease in decomposition voltage in the presence of ultrasound is due to the combined effect of a decrease in anodic overpotential and an increase in cathodic overpotential.

Preparative electrooxidation of lysozyme at copper electrodes held at potentials around 1.2 V vs. a saturated calomel reference electrode induces the formation of a yellow chromophore with a concomitant decrease in the pI of the protein. Ion-exchange high-performance liquid chromatography revealed two new lysozyme species with pI values of 10·8 and 10·7 (lysozyme-11.0) which bear the chromophore. Sequence analysis of these two species showed that protein with lower pI was modified at both Tyr 23 and Tyr 20 and the other exclusively at Tyr 23. Ribonuclease A, subtilisin BPN', and BSA were also found to produce the same chromophore using similar electrochemical reaction schemes. Characterization of the chromophore by a variety of techniques revealed that it is apparently 3-nitrotyrosine.

This paper describes the previously unreported electrogenerated chemiluminescence (ECL) from the electro-oxidation of several aryl carboxylates at a platinum flag electrode in aqueous, methanolic and acetonitrile solutions using tetramethylammonium hydroxides as a supporting electrolyte. In most cases electrochemiluminescence was markedly enhanced by simultaneous irradiation with ultrasound during electrolysis. Increase in electrochemiluminescence intensity with electrolysis current and carboxylate concentration was observed. Addition of a radical scavenger or purging of the solution with oxygen lowers ECL emission intensity. In contrast the presence of oxygen enhances the intensity of sonoluminescence, which is a concurrent but weaker process occurring under ultrasonic irradiation alone. Ring-substituted phenylacetates almost always produced less electrochemiluminescence than the unsubstituted parent molecule, though within the substituents studied chloro-substituted phenylacetates produced more ECL intensity than either methoxy- or nitro-substituted derivatives.

International audience; Sonochemistry is a technique that offers promise for pollutant degradation, but earlier studies on various chlorinated substrates do not give a definitive view of the effectiveness of this methodology. We now report a thorough study of ultrasonic operational variables upon perchloroethylene (PCE) degradation in water (variables include ultrasonic frequency, power and system geometry as well as substrate concentration) and we attempt to close the mass balance where feasible. We obtained fractional conversions of >97% showing very effective loss of pollutant starting material, and give mechanistic proposals for the reaction pathway based on cavitational phenomena inducing pyrolytic and free radical processes. We note major products of Cl− and CO2/CO, and also trichloroethylene (TCE) and dichloroethylene (DCE) at ppm concentrations as reported earlier. The formation at very low (ppb) concentration of small halocompounds (CHCl3, CCl4) and also of higher-mass species, such as pentachloropropene, hexachloroethane, is noteworthy. But of particular importance in our work is the discovery of significant quantities of chloroacetate derivatives at ppm concentrations. Although these compounds have been described as by-products with other techniques such as radiolysis or photochemistry, this is the first time that these products have been identified in the sonochemical treatment of PCE; this allows a much more effective account of the mass balance and may explain earlier inconsistencies. This reaction system is now better identified, but a corollary is that, because these haloacetates are themselves species of some toxicity, the use of ultrasound here may not sufficiently diminish wastewater toxicity.

The iodination of proteins remains a useful tool in biochemistry for radiolabelling. However, chemical or enzymatic iodination is difficult to control and can give deleterious polyiodination. Previously, we have shown that electrooxidation with nitrite is a rapid method for the selective nitration of tyrosine residues in proteins. In principle, it should be possible to substitute a number of electrooxidisable anions into the tyrosine phenol ring.Electrochemical iodination is more difficult to control than nitration because the rapid anodic oxidation of I− leads to persistent formation of the iodinating triiodide anion. However, application of pulsed electrooxidation and reduction cycles is shown to be an effective procedure for the selective mono and double-iodination of myoglobin, which may have general application to other proteins in controlling of the level of iodination.Mono- and double-iodination of myoglobin by this method was confirmed by electrospray FT-ICR mass spectrometry. Infrared multiphoton dissociation (IRMPD) enabled localization of the site of mono-iodination to be restricted to either His97 or Tyr103. More extensive sequence coverage was obtained with electron capture dissociation (ECD), allowing unambiguous assignment of the site of iodination to Tyr103.

The effect of ultrasound on the voltammetry of copper in alkaline solution is reported. At pH 7 the electrode surface remains active after scanning to ca. +1.0 V (vs. SCE) and the effects of ultrasound show the expected substantial enhancement in limiting current due to improved mass transport under ultrasound. However at pH 9, whereas the silent scan is only slightly altered in gross detail from that obtained at pH 7, the sonicated scan is significantly different. This shows the expected current increase only up until ca. +0.6 V (vs. SCE), where there is a substantial loss of current showing a passivation phenomenon that is enhanced by ultrasound. In addition, during the reverse (reduction) scan under ultrasound an anodic peak appears. This suggests reactivation of the electrode during the cathodic sweep, possibly by reductive removal of a transient species from the electrode/(hydr)oxide interface at a potential where oxidation still occurs. Increasing the pH to 11 further shifts the cathodic peaks in the silent voltammogram.

The electro-oxidation of electrolytically unsupported ensembles of N,N-diethyl-N‘,N‘-dialkyl-para-phenylenediamine (DEDRPD, R = n-butyl, n-hexyl, and n-heptyl) redox liquid femtoliter volume droplets immobilized on a basal plane pyrolytic graphite electrode is reported in the presence of aqueous electrolytes. Electron transfer at these redox liquid modified electrodes is initiated at the microdroplet−electrode−electrolyte three-phase boundary. Dependent on both the lipophilicity of the redox oil and that of the aqueous electrolyte, ion uptake into or expulsion from the organic deposits is induced electrolytically. In the case of hydrophobic electrolytes, redox-active ionic liquids are synthesized, which are shown to catalyze the oxidation of l-ascorbic acid over the surface of the droplets. In contrast, the photoelectrochemical reduction of the anaesthetic reagent halothane proceeds within the droplet deposits and is mediated by the ionic liquid precursor (the DEDRPD oil).

Ultrasound was found to increase the oxidation peak current and hence the decomposition rate of thiosulphate 50-fold compared to silent conditions. The effects of the ultrasonic frequency (20 and 38 kHz) and power upon the electrochemical oxidation of thiosulphate in aqueous KCl (1 mol dm-3) at stationary stainless steel and platinum electrodes were studied chronoamperometrically and potentiostatically (at various scan rates). No sigmoidal-shaped voltammograms were observed for the redox couple S4O6(2-)/S2O3(2-) in the presence of ultrasound. However, application of ultrasound to this redox couple provided an increase in the oxidation peak current at the frequencies employed, the magnitude of which varied with concentration, scan rate and ultrasonic power. Under sonication at 20 and 38 kHz, the oxidation peak potential shifted anodically with increasing ultrasonic power. This anodic shift in potential may be due to the formation of hydroxyl radicals, changes in electrode surface composition and complex adsorption phenomena. The large increase in oxidation peak currents and the rates of decomposition of thiosulphate, in the presence of ultrasound, are explained in terms of enhanced mass transfer at the electrode due to cavitation and acoustic streaming together with microstreaming coupled with adsorption phenomena. It is also shown that changes in macroscopic temperature throughout the experiment are insufficient to cause the observed enhanced diffusion.

The voltammetry of the water-insoluble redox liquids para-N,N,N‘,N‘-tetraalkylphenylenediamines (TAPDs; A = butyl, hexyl, heptyl, octyl, and nonyl) at the TAPD|electrode|electrolyte interface is studied. The ion-transfer processes between the aqueous and the organic phaseconsequent of the necessity to maintain charge neutrality during electrochemical oxidation and reduction reactions in the organic phaseare elucidated as a function of the Gibbs free energy of transfer of the electrolyte anions, of the concentration of TAPD in the organic phase, and of the length of the alkyl chain of the phenylenediamines. Two conditions are considered: the voltammetry of microdroplets of the pure redox liquids and the voltammetry of the TAPDs dissolved in nitrobenzene, either deposited as microdroplets at an electrode surface. The oxidation of the neutral redox liquids, accompanied by the uptake of anions from the adjacent aqueous solution leads to the formation of ionic liquid phases, TAPD+X- and TAPD2+X-2 (X- = monova...

Application of simultaneous ultrasound to representative solution-phase reversible voltammetric couples produce a step-shaped voltammogram at platinum electrodes of both 'macro' and 'micro' dimensions. The limiting current increases with ultrasonic power, but is not markedly affected by ultrasonic frequency in the 20-800kHz region. In contrast the complex voltammetry of a platinized platinum electrode surface within the hydrogen adsorption regime in aqueous acid medium is very little affected by sonication. Factors affecting the reproducibility of sonoelectrochemical experiments when employing ultrasonic sources are discussed. © 1995 Chapman and Hall.

Halogenated thiophenes have been cycloadded oxidatively to maleimides to give halogenated thiabicyclo[2.2.1]heptene S-oxides which have been subjected to an electrochemical extrusion of SO to give various halogeno-substituted phthalimides.

Azo-benzene chromophores with an oxygen donor and five different end-groups have been attached to octadecyl chains or to polysiloxane backbones via hexadecyl spacer groups. These were deposited by the Langmuir-Blodgett (LB) technique using our own design of diamond-barrier trough. The LB deposition properties of the polymers were superior to those of the low molecular mass materials, which did not show second harmonic generation (SHG) as deposited monolayers. The strength of SHG from the polymers correlated with acceptor strength, but the maximum signal was only about 10% of that of hemicyanine. Only one of the polymers gave thick films of very high quality; this had a hydroxyl end-group and showed no SHG except when deposited from an acid subphase. This was presumed to result from protonation of the azo group to create an acceptor. The quality of the deposited films may correlate with their phase behaviour; the best material showed only a presumed soft crystalline phase. The film-forming properties of that film were remarkable; loss figures of around 2.5 dB cm−1 have been recorded at 633 nm in films of 1.3 Μm thickness. Future work will seek to isolate and independently control the factors involved in good film formation and in high SHG.

The voltammetry of various well-defined systems in acetonitrile solution has been studied using both micro and macroelectrodes in the presence of power ultrasound. A simple model is established which quantifies the mass transport observed under these conditions; this assumes that the effect of ultrasound is to promote mixing within the bulk of the solution up to within a certain distance of the electrode surface. Thus the ultrasound serves to thin the diffusion layer which would exist at the corresponding electrode under silent conditions. The relative enhancement of transport limited currents by ultrasound is dependent on the size of the electrode; for micrometre-sized electrodes the steady state limiting current observed tends to that predicted under silent conditions whereas for large electrodes a thin, steady-state diffusion layer is seen with a thickness which is power dependent. In addition to steady-state experiments, a.c. impedance measurements and potentials steps are used to verify the model proposed. © 1996 Chapman and Hall.