Shelley J. Wilkins

Electrochemical detection of nitrate and nitrite at a copper modified electrode

The development of a reagentless electrode system for the determination of nitrate is presented. The approach is based upon the deposition of a macroporous copper deposit which shows marked selectivity for nitrate ion under mildly acidic conditions (pH 3) with a linear range extending from 10 to 200 μM nitrate. Fabrication of the layer is both inexpensive and simple and is relatively substrate independent (Cu, Au, glassy carbon) as the response is attributed to the electrodeposited support. The influence of potential interferents such as chloride, nitrite and complexing agents such as citric acid were examined. In particular, the reduction of nitrite was found to occur at potentials significantly less negative than those required for nitrate reduction and as a result clear resolution of both voltammetric peaks is obtainable. The lack of direct interference between the two ions combined with wide peak separation (200 mV) provides a system that may offer some potential in speciation studies. The analytical applicability of the proposed system was demonstrated through the standard addition determination of nitrate in a number of authentic samples with the nitrate concentrations independently corroborated using established Griess assay spectroscopic protocols.

Ionic liquid modified electrodes. Unusual partitioning and diffusion effects of Fe(CN)64−/3− in droplet and thin layer deposits of 1-methyl-3-(2,6-(S)-dimethylocten-2-yl)-imidazolium tetrafluoroborate

Room temperature ionic liquids exhibit unusual and, for electrochemical applications, promising properties such as ionic conductivity and non-volatility. Microdroplet and thin film deposits of an ionic liquid, 1-methyl-3-(2,6-(S)-dimethylocten-2-yl)-imidazolium tetrafluoroborate (MDIM+BF4−) on electrode surfaces are studied in order to assess the ability of aqueous ions to partition into the ionic liquid. Two complementary methodologies, deposition of ionic liquid onto a 4.9 mm diameter basal plane pyrolytic graphite electrode followed by voltammetric analysis and casting of an ionic liquid film onto a random array of 7 μm diameter carbon microelectrodes (RAMTM electrode) followed by chronoamperometry, are employed. The ability of hydrophilic ions to partition from the water phase into the ionic liquid phase is unexpected and is shown to depend strongly on (i) the type of ion, (ii) specific interaction of the ion with the ionic liquid, and (iii) the concentration of the supporting electrolyte. Voltammetric responses obtained for the reduction of Fe(CN)63− partitioned into microdroplet and thin film deposits of MDIM+BF4− indicate selective uptake of ferricyanide into the ionic liquid phase. Chronoamperometric experiments at RAMTM electrodes are used to quantify both the concentration of Fe(CN)63− in the ionic liquid and the diffusion coefficient.

Simultaneous Electrochemical Detection and Determination of Lead and Copper at Boron‐Doped Diamond Film Electrodes

Boron-doped diamond (BDD) was used as solid electrode material for the simultaneous detection of trace lead and copper in solution using anodic stripping voltammetry (ASV). Peaks due to Cu and Pb were identified and the conditions under which independent measurements of each can be undertaken are delineated. Voltammetry and AFM imaging is used to show that while both metals nucleate as their pure phases on BDD, once formed the copper nuclei act as favorable sites for the subsequent nucleation and growth of lead; the latter act inhibiting hydrogen evolution on the copper surface. However under stripping conditions, as the lead is preferentially oxidized, an extra peak due to hydrogen evolution at freshly exposed copper appears in SWV experiments. Nevertheless, ASV at BDD electrodes provides the basis for a method of independent detection of Cu and Pb via conventional standard addition procedures.

Electroanalytical exploitation of quinone–thiol interactions: application to the selective determination of cysteine

Square wave voltammetry was applied to the detection of cysteine through the use of an indirect assay that exploits the reaction of the thiol with a quinone indicator. Voltammetric discrimination between unreacted quinone and the corresponding quinone-cysteine adduct is possible with clear resolution of the latter peak providing a linear response from 5 to 47 microM. The selectivity of the approach was assessed with no interference from cystine, lysine, paracetamol or 4-aminophenol. The response recorded in the presence of a massive excess of ascorbic acid was also investigated and the integrity of the approach confirmed. The effects of other sulfhydryl thiols, homocysteine and glutathione, were also assessed and found to present no appreciable change in the voltammetric profile. The practical utility of the approach was investigated through examining the response to cysteine in urine.

Anodic stripping voltammetry of sulphide at a nickel film: towards the development of a reagentless sensor

The determination of sulphide at an electrochemically generated nickel oxide layer at glassy carbon and screen-printed electrodes in acidic media has been examined and appraised. The NiO layer was found to produce a stripping-like signal to sulphide and gave a linear peak current response from 20 to 90 muM. The response was further enhanced by repetitive cycling allowing accumulation of nickel sulphide at the electrode surface such that lower micromolar levels of sulphide (i.e. 5 muM) can be determined. The response at the NiO layer to sulphide is shown to be reproducible over a period of 24 h, thereby offering the development of a disposable amperometric sensor for sulphide.

Electrochemical Detection of Nitrate at a Copper Modified Electrode Under the Influence of Ultrasound

Ultrasound has been applied to the detection of nitrate using a copper modified electrode. The electrode systems employs a macroporous copper deposit whose response to nitrate ion has been previously characterized. This report details the influence ultrasound (20 kHz) exerts on the structure of the deposit and on the response to nitrate. The deposit was substantially removed by the acoustic streaming and cavitational processes common to 20 kHz ultrasound even at relatively mild power outputs and large horn electrode separations. However, bombardment with 20 kHz ultrasound does not remove the deposit entirely with a compact copper layer remaining. This portion of the deposit was found to retain significant nitrate sensitivity compared with the bare, unmodified electrode. The retention of the compact catalytic layer under the influence of ultrasound enabled the use of sonolinear sweep voltammetry which was found to provide signal enhancement significantly greater than the response observed under silent conditions. The analytical utility of the technique was assessed through the determination of nitrate in treated sewage outfall water. The silent and sonochemical results were found to be within 5% of the value obtained from an independent spectroscopic analysis.

Towards greener disposal of waste cathode ray tubes via ultrasonically enhanced lead leaching

The disposal of waste cathode ray tubes (CRT) from old televisions and discarded computer monitors is rapidly becoming an issue of growing environmental concern due to the leaching of lead and other toxins by groundwater from landfill sites. Imminent legislation in both the EU and the USA is likely to demand much stricter control on the disposal of such hazardous electronic equipment, with a drive towards lead removal prior to landfill and/or recycling. This paper describes the use of power ultrasound to facilitate the removal of lead from the heavily-leaded CRT glass via an accelerated leaching protocol, with the aim of producing a lead-free product for greener disposal or more ideally for glass recycling purposes. The paper also describes the use of cathodic stripping voltammetry at a boron-doped diamond sensor, as a quick, easy and cost-effective technique for monitoring the rate of lead released from the CRT glass into a mildly acidic leachate.

Electrochemical detection of glutathione: An electrochemically initiated reaction pathway

ABSTRACT The electrochemically initiated reaction of glutathione with catechol in aqueous phosphate buffer has been examined as a facile route to the selective electrochemical determination of the former in the presence of other potential interferents (including cysteine, homocysteine). The results provide a linear detection range of 1–80 µM for glutathione with a corresponding limit of detection of 0.94 µM. The efficacy of the approach has been shown by the 104% (RSD = 7.3%, N = 3) recovery of 10 µM glutathione in a growth tissue media containing a number of important biomarkers.

Lead Dioxide Deposition and Electrocatalysis at Highly Boron-Doped Diamond Electrodes in the Presence of Ultrasound

Boron-doped diamond (BDD) is u versatile and novel electrode material which, due to its mechanical and chemical robustness, wide potential window, low background interference, und ease of chemical modification, is becoming an interesting alternative to conventional electrodes for a wide range of electrochemical applications. It is shown in this study that BDD is a good substrate for the ultrasound-enhanced electrodeposition of lead dioxide, producing strongly adhered electrically conducting deposits. Power ultrasound is used to enhance both the efficiency of the PbO 2 deposition procedure and the rate of the electrocatalytic ethylene glycol oxidation process at the PbO 2 -modified BDO electrode. The presence of high levels of aqueous organic material is shown to interfere with the lead dioxide deposition process. Under optimized insonation conditions the PbO 2 deposit, quantified by using cathodic stripping voltammetry, is shown to be mechanically stable. When used in conjunction with power ultrasound to perform the electrocatalytic oxidation of ethylene glycol two distinct types of oxidation processes at PbO 2 , chemically rate limited and electrochemically rate limited, are observed.

Channel flow cell studies on the evaluation of surface pretreatments using phosphoric acid or polymaleic acid for calcite stone protection.

The dissolution kinetics of surface-pretreated and weathered calcite was investigated in dilute acid using a channel flow cell with microdisk detection. Two pretreatments were studied, polymaleic acid and phosphoric acid. Treatment with polymaleic acid was shown to significantly passivate calcite but to a lesser extent than the phosphoric acid and the former coating was found to be less effective for protection of calcite from acid attack. However, treatment of calcite with phosphoric acid resulted in the passivation of calcite from acid attack which strongly inhibited dissolution, an effect that was enhanced even further after exposure to the environment.