Sonoelectroanalytical detection of ultra-trace arsenic

The electroanalytical detection of arsenic on a gold electrode is investigated in the presence and absence of ultrasound. It is found that under quiescent conditions a detection limit of 1.8×10-7 is achievable using a 120 seconds accumulation period. Applying optimised ultrasound during the accumulation period was found to reduce the limit of detection to 1 × 10-8 M from using a deposition period of 60 s at -0.5 V, while increasing the sensitivity by a factor of 15. The methodology was tested on a river sample containing significant copper contamination and electrode passivating organic materials. This technique provides promise for 'in the field' measurements due to the electrode-depassivating effects of ultrasound. © 2005 WILEY-VCH Verlag GmbH and Co. KGaA, Weinheim

Sonically assisted electroanalytical detection of ultratrace arsenic.

A simple portable handheld electrochemical sensor with an integrated sound source for the detection of ultratrace quantities of arsenic using square wave anodic stripping voltammetry is described. The sensor uses low-frequency sound (250 Hz) during the arsenic deposition step to enhance the sensitivity of the arsenic stripping response. It is found that under quiescent (silent) conditions a detection limit of 2.1 x 10(-7) M with a sensitivity of 0.51 M(-1) A is achievable using a 120-s accumulation period, while applying low-frequency sound using a "sonotrode" reduced this detection limit to 3.7 x 10(-9) M with an increased sensitivity of 27.2 M(-1) A. Thus, the low-frequency sonotrode is shown to increase the sensitivity by ca. 50 times while reducing the limit of detection by 2 orders of magnitude. A study of the effect of copper contamination is carried out as well as analysis in real samples; it is found that although as expected copper detrimentally effects the arsenic limit of detection, it does not rise significantly above 10(-8) M levels

The electrochemical detection of arsenic(III) at a silver electrode

A study of three electrode substrates namely gold, platinum and silver, for arsenic detection via anodic stripping voltammetry is reported. Hitherto it has been accepted that gold is the most suitable metallic surface for use in this context, as suggested by Forsberg and co-workers (Forsberg, G.; O'Laughlin, J. W.; Megargle, R. G. Anal. Chem. 1975, 47, 1586.). We revisit these experiments and find that by switching from hydrochloric acid to nitric acid the oxidation of silver that had previously masked the arsenic stripping signal at this surface is shifted considerably enough to allow a clear, analytically reliable As(III) stripping signal to be detected. In contrast to silver and gold platinum is found to have poor performance as an electrode substrate for arsenic detection. Using ASV a LOD of 6.3 × 10-7 M is found for As(III) detection at a silver electrode, similar to that which we have previously reported at a gold electrode (A. O. Simm, C. E. Banks and R. G. Compton. Electroanalysis, 2005, 17, 335.) The use of ultrasound was then investigated to further reduce the LOD, which was found to be 1.4 × 10-8 M. Apart from reduced cost of silver it also has an added advantage over gold in that it has a higher hydrogen reduction overvoltage enabling a 100 mV more negative deposition potential to be used before the onset of hydrogen evolution when compared to a gold electrode. © 2005 Wiley-VCH Verlag GmbH and Co. KGaA

Oxidation of electrodeposited copper on Boron Doped Diamond in acidic solution: Manipulating the size of copper nanoparticles using voltammetry

The in situ deposition of copper, in acidic solution onto a Boron Doped Diamond electrode, using cyclic voltammetry is explored and produced surfaces are imaged using Atomic Force Microscopy. A uniformly dense covering of copper nanoparticles is produced when the potential of a freshly polished BDD electrode is swept from 0 V in a negative direction. For example, in 1 M H 2SO4 with a Cu(II) concentration of 1 mM, nanoparticles of height 10.1 nm, diameter 74.6 nm and a density of 16.1 particles per μm 2 are created when the potential is swept to -0.35 V. The higher the concentration of Cu(II) in solution or the larger the magnitude of the end potential the larger the nanoparticles are and the more densely they are spread. When the direction of the scan is reversed and a positive potential sweep carried out evidence from the observed cyclic voltammograms and AFM images shows that copper is being incompletely stripped from the electrode surface. If the potential is then cycled continuously ten times, as would happen when the process is used for electroanalytical purposes, then an irregular and irreproducible deposit is observed. One can infer from this evidence that the incompletely stripped copper is electrochemically active and therefore adversely affecting subsequent deposition processes. Comparison to existing literature shows that the discrete application of particular deposition and stripping potentials is a much better way to produce a deposit of copper nanoparticles than application of potential through cyclic voltammetry

Silver nanoparticle assemblies supported on glassy-carbon electrodes for the electro-analytical detection of hydrogen peroxide.

Electrochemical detection of hydrogen peroxide using an edge-plane pyrolytic-graphite electrode (EPPG), a glassy carbon (GC) electrode, and a silver nanoparticle-modified GC electrode is reported. It is shown, in phosphate buffer (0.05 mol L(-1), pH 7.4), that hydrogen peroxide cannot be detected directly on either the EPPG or GC electrodes. However, reduction can be facilitated by modification of the glassy-carbon surface with nanosized silver assemblies. The optimum conditions for modification of the GC electrode with silver nanoparticles were found to be deposition for 1 min at -0.5 V vs. Ag from 5 mmol L(-1) AgNO3/0.1 mol L(-1) TBAP/MeCN, followed by stripping for 2 min at +0.5 V vs. Ag in the same solution. A wave, due to the reduction of hydrogen peroxide on the silver nanoparticles is observed at -0.68 V vs. SCE. The limit of detection for this modified nanosilver electrode was 2.0 x 10(-6) mol L(-1) for hydrogen peroxide in phosphate buffer (0.05 mol L(-1), pH 7.4) with a sensitivity which is five times higher than that observed at a silver macro-electrode. Also observed is a shoulder on the voltammetric wave corresponding to the reduction of oxygen, which is produced by silver-catalysed chemical decomposition of hydrogen peroxide to water and oxygen then oxygen reduction at the surface of the glassy-carbon electrode

The electrochemical reaction mechanism of arsenic deposition on an Au(111) electrode

The cyclic voltammograms and linear sweep voltammograms of arsenic deposited on an Au(1 1 1) electrode were measured in a phosphate buffer (pH 1.0) containing 0.1 mM NaAsO2 under IR compensation mode in order to gain an insight into the mechanism of arsenic deposition and stripping at an Au(1 1 1) surface. The amount of arsenic deposit is determined to be approximately one monolayer by calculating the charge of anodic stripping peak of the linear sweep voltammograms. The rate of deposition decreases as the amount of the deposit increases. The analyses reveal that the electrodeposition of arsenic is a totally irreversible electrode reaction and the exchange current density is 6.3 × 10-7 A cm-2. The Tafel plot analyses indicate that the transfer of the first accepted electron is the rate-determining step. © 2005 Elsevier B.V. All rights reserved

Novel methods for the production of silver microelectrode-arrays: their characterisation by atomic force microscopy and application to the electro-reduction of halothane.

A new method is proposed for the simple preparation of random silver micro and nano-electrode arrays. This employs acoustic streaming directed at a glassy carbon surface to "mechanically" attach particles from a suspension of metal colloidal or other small particles. The particles tend to adhere to the substrate at points of imperfection such as scratches, crevices etc. These arrays are compared with arrays formed by the electro-deposition of silver at a glassy carbon substrate, with the silver being partially stripped off, leaving a stable micro and nanoparticle array on the surface. Both surfaces are characterised using optical and atomic force microscopy. The two types of electrodes are evaluated to their analytical utility via the electrochemical reduction of halothane and their performance compared with that of a silver macroelectrode. A notable increase in sensitivity and peak current is observed

Acoustically fabricated random microelectrode assemblies.

We report the insonation of bismuth, silver, copper and tungsten metal particles suspended in octane in the vicinity of a glassy carbon electrode. AFM and voltammetry reveal that metal particles are immobilised onto the electrode substrate. In the case of bismuth, silver and copper, the possible melting of the metal particles due to the high sonochemical conditions cannot unambiguously be ruled out. However, it is likely that the immobilisation of the metal particles occurs predominantly through mechanical attachment due to the high rates of mass transport, evidenced from the fact that tungsten can be immobilised at a glassy carbon surface which has a melting point (mp 3410 degrees C) outside the likely sonochemical conditions. The immobilised particles are found to be in electrical contact with the glassy carbon electrodes which can then act as random assemblies of microelectrodes. Proof-of-concept for use in electro-analysis is examined for the possible detection of arsenic and cadmium at a silver and bismuth random microelectrode assemblies, respectively. This approach suggests a simple generic methodology for the construction of microelectrode assemblies via abrasive attachment induced by insonation with power ultrasound