Stripping voltammetry of copper and lead using gold electrodes modified with self-assembled monolayers

One problem associated with using bare solid metal electrodes, such as gold and platinum, in stripping analysis to determine heavy metal ions such as lead and copper ions in dilute solutions is that underpotential deposition (UPD) gives multiple stripping peaks in the analysis of mixtures. These peaks are often overlapped and cannot be conveniently used for analytical purposes. Bifunctional alkylthiols, such as 3-mercaptopropionic acid, with an ionizable group on the other terminal end of the thiol can form self-assembled monolayers (SAMs) on the surface of the gold electrode. It is shown that such an SAM-modified gold electrode minimizes the UPD effects for the stripping analysis of lead and copper. The anodic peak potential shifts and the peak shape changes, indicating that the SAM changes the deposition and stripping steps of these heavy metal ions. Thus, the sensitivity levels for both single species and mixtures can be significantly improved for the conventional solid electrodes. The mechanism of the deposition reaction at the SAM-modified gold electrodes is discussed. Received: 29 May 1997 / Accepted: 24 June 1997     

Detection of metal ions without interference from dissolved oxygen by reverse pulse amperometry in flow-injection systems and liquid chromatography

Reverse pulse amperometry (r.p.a.) is used to monitor fluid streams for the presence of electroactive metal ions using a mercury electrode without the requirement of deaeration. The technique is based on the application of an unsymmetrical square wave with a large negative initial potential for deposition of the metal ion followed by a positive potential pulse for anodic stripping of the metal. The analytical signal is measured during the stripping process at a potential where dissolved oxygen is not reduced. R.p.a. is applied at a polarographic detector in flow-injection systems and ion-exchange chromatography for detection of several ions of transition and heavy metals in aqueous solutions.     

Automated two-dimensional separation flow system with electrochemical preconcentration, stripping, capillary electrophoresis and contactless conductivity detection for trace metal ion analysis

This paper describes the automation of a fully electrochemical system for preconcentration, cleanup, separation and detection, comprising the hyphenation of a thin layer electrochemical flow cell with CE coupled with contactless conductivity detection (CE-C?D). Traces of heavy metal ions were extracted from the pulsed-flowing sample and accumulated on a glassy carbon working electrode by electroreduction for some minutes. Anodic stripping of the accumulated metals was synchronized with hydrodynamic injection into the capillary. The effect of the angle of the slant polished tip of the CE capillary and its orientation against the working electrode in the electrochemical preconcentration (EPC) flow cell and of the accumulation time were studied, aiming at maximum CE-C?D signal enhancement. After 6 min of EPC, enhancement factors close to 50 times were obtained for thallium, lead, cadmium and copper ions, and about 16 for zinc ions. Limits of detection below 25 nmol/L were estimated for all target analytes but zinc. A second separation dimension was added to the CE separation capabilities by staircase scanning of the potentiostatic deposition and/or stripping potentials of metal ions, as implemented with the EPC-CE-C?D flow system. A matrix exchange between the deposition and stripping steps, highly valuable for sample cleanup, can be straightforwardly programmed with the multi-pumping flow management system. The automated simultaneous determination of the traces of five accumulable heavy metals together with four non-accumulated alkaline and alkaline earth metals in a single run was demonstrated, to highlight the potentiality of the system.     

Mercury Films on Commercial Carbon Screen‐Printed Devices for the Analysis of Heavy Metal Ions: a Critical Evaluation

The suitability of mercury films on commercial screen‐printed electrodes for the analysis of heavy metal ions is critically tested for the particular case of Pb(II)‐ions. Although determination is possible by anodic stripping voltammetry with a reasonable detection limit (8.9 µg L^?1), important drawbacks are noticed as a consequence of the heterogeneous deposition of mercury on the rough surface of screen‐printed devices.     

Underpotential deposition and anodic stripping voltammetry at mesoporous microelectrodes

Using the technique of liquid crystal templating a series of high surface area mesoporous platinum microelectrodes was fabricated. The underpotential deposition of metal ions at such electrodes was found to be similar to that at conventional platinum electrodes. The phenomena of underpotential deposition, in combination with the intrinsic properties of mesoporous microelectrodes (i.e. a high surface area and efficient mass transport) was exploited for the purpose of anodic stripping voltammetry. In particular the underpotential deposition of Ag(+), Pb(2+) and Cu(2+) ions was investigated and it was found that mesoporous microelectrodes were able to quantify the concentration of ions in solution down to the ppb range. The overall behaviour of the mesoporous electrodes was found to be superior to that of conventional microelectrodes and the effects of interference by surfactants were minimal.     

Electrochemical Impedance Characterization of Nafion‐Coated Carbon Film Resistor Electrodes for Electroanalysis

Carbon film disk electrodes with Nafion coatings have been characterized by electrochemical impedance spectroscopy (EIS) with a view to a better understanding of their advantages and limitations in electroanalysis, particularly in anodic stripping voltammetry of metal ions. After initial examination by cyclic voltammetry, spectra were recorded over the full potential range in acetate buffer solution at the bare electrodes, electrodes electrochemically pretreated in acid solution, and Nafion‐coated pretreated electrodes in the presence and absence of dissolved oxygen. EIS equivalent circuit analysis clearly demonstrated the changes between these electrode assemblies. In order to simulate anodic stripping voltammetry conditions, spectra were also obtained in the presence of cadmium and lead ions in solution at Nafion‐coated electrodes, both after metal ion deposition and following re‐oxidation. Permanent changes to the structure of the Nafion film occurred, which has implications for use of these electrode assemblies in anodic stripping voltammetry at relatively high trace metal ion concentrations.     

Simultaneous determination of trace amounts of zinc, lead and copper in rum by anodic stripping voltammetry

The determination of traces of Zn, Pb and Cu in rum samples by anodic stripping voltammetry without previous treatment or addition of a supporting electrolyte, using a hanging mercury drop electrode, is described. The choice of an appropriate stripping voltammetric method and deposition potential minimizes the influence of the organic content and ensures good reproductibility of the measurements. The reliability of the method was tested by comparing the results with those given by absorption spectrometry, the differences being about 10%. The method allows heavy metal ions to be determined in the mug l(-1) concentration range.     

Contribution of migration current to the voltammetric deposition and stripping of lead with and without added supporting electrolyte at a mercury-free carbon fibre microdisc electrode

In order to assess the contribution and analytical significance of migration, electrochemical studies on the deposition and stripping of lead at a carbon fibre microelectrode (diameter of 10 μm) have been undertaken in aqueous solutions containing 1 mM lead ions with variable KNO₃ supporting electrolyte concentrations (10⁻¹ to 10⁻⁵ M), as well as in the total absence of deliberately added supporting electrolyte. The methodology involved the application of cyclic voltammetry to characterise the Pb²⁺ (solution)+2e⁻Pb (metal) process in both the reduction (Pb deposition) and stripping (Pb dissolution) directions. The use of a mercury-free carbon surfaces means that the lead stripping does not occur from the amalgam state, as is commonly the case in anodic stripping voltammetry. In the deposition step, the current rises sharply with potential in response to a lead nucleation-growth process and then reaches an almost potential independent limiting value. The stripping step, obtained on the reverse scan, exhibited oxidation peak currents resulting from the redissolution or stripping of the metal from the electrode surface. The influence of the electrolyte concentration and hence migration current at −0.8 V versus Ag/AgCl for the deposition process, as well on the redissolution peak current and the dependence of the voltammograms on scan rate (10–1000 mV) are discussed. Interestingly, neither deposition limiting nor stripping peak currents vary in a simple manner with added supporting electrolyte concentrations, with maximum values being observed at 10⁻⁵ M rather than zero concentration of added KNO₃. An important implication for the voltammetric determination of lead in low ionic strength media by the very sensitive technique of anodic stripping voltammetry is that use of the method of standard additions commonly employed to minimise unknown matrix problems, is prone to error when contributions to the process from migration current are important.     

Simultaneous determination of lead,copper, and mercury at a modified carbon paste eletrode containing humic acid

The modified carbon paste electrode (CPE) responding simultaneously to lead(II), copper(II), and mercury(II) ions has been constructed by incorporating humic acid (HA) into the graphite powder with Nujol oil. Simple immerging of the electrode into the measuring solution containing these metal ions led to the chemical deposition of the ions onto the electrode through the complexation of the ions with HA. Cyclic and differential pulse voltammetry (DPV) characterized the modified electrode's surfaces. Several cyclings of the potential regenerated the electrode (from more positive than the stripping potential of reduced Hg to more negative than the reduction of Pb(II)ion), which was then used for another deposition. After five deposition/measurement/regeneration cycles, the peak current of voltammograns of the analyte decreased slightly. The response reproduced with a 5.1% relative standard deviation. We also applied ihe differential pulse technique to the previously mentioned system. Here, the detection limit tor Pb(II), Cu(II), and Hg(II) ions were 5.0 × 10⁻⁹ M 8.0 × 10⁻⁹ M, and 8.0 × 10⁻⁹ M, respectively, for 20 minutes of deposition time. After pretreatment of silver(I) ion with KC1, we could not observe any interference by other metal ions on the determination of the test ions in aqueous solution. Satisfactory results were acquired for the determination of the test metal ions in certified standard urine reference material SRM's 2670 (trace elements in urine).     

Interaction between co-deposited metals during stripping voltammetry at boron-doped diamond electrodes

Electrochemical processes, which underlie the use of conductive diamond electrodes for the simultaneous detection of two or more metal ions in solution by anodic stripping voltammetry (ASV), have been investigated. The model analyte system studied contains the two metal species, Ag+(aq) and Pb2+(aq), and the experimental techniques employed include cyclic and square wave voltammetries, along with X-ray photoelectron spectroscopy and secondary electron microscopy. Although the bulk metallic forms of Ag and Pb are immiscible, several interactions in the system between the two metal species present are observed, which significantly influence the electrodeposition and electrodissolution processes which underlie ASV. The subsequent nucleation and growth of a given metal on the electrode surface is enhanced by the presence of the second metal on the surface. The encapsulation of one metal by the other, within the metal particulates that form on the electrode surface, significantly reduces the stripping yield at the potentials characteristic of the individual metals. The stripping potentials are also influenced by bonding interactions between deposited Ag and Pb, which broaden the characteristic stripping peaks in cyclic voltammetry, as well as producing underpotential deposition and stripping. Given these interactions, the extent to which ASV at diamond electrodes can be used to determine the solution concentrations of Ag+(aq) and Pb2+(aq) is considered.     

Ex Situ Scanning Electrochemical Microscopy (SECM) Investigation of Bismuth‐ and Bismuth/Lead Alloy Film‐Modified Gold Electrodes in Alkaline Medium

Scanning electrochemical microscopy (SECM) in feedback mode was employed to characterise the reactivity and microscopic peculiarities of bismuth and bismuth/lead alloys plated onto gold disk substrates in 0.1 mol L^?1 NaOH solutions. Methyl viologen was used as redox mediator, while a platinum microelectrode was employed as the SECM tip. The metal films were electrodeposited ex situ from NaOH solutions containing either bismuth ions only or both bismuth and lead ions. Approach curves and SECM images indicated that the metal films were conductive and locally reactive with oxygen to provide Bi³⁺ and Pb²⁺ ions. The occurrence of the latter chemical reactions was verified by local anodic stripping voltammetry (ASV) at the substrate solution interface by using a mercury‐coated platinum SECM tip. The latter types of measurements allowed also verifying that lead was not uniformly distributed onto the bismuth film electrode substrate. These findings were confirmed by scanning electron microscopy images. The surface heterogeneity produced during the metal deposition process, however, did not affect the analytical performance of the bismuth coated gold electrode in anodic stripping voltammetry for the determination of lead in alkaline media, even in aerated aqueous solutions. Under the latter conditions, stripping peak currents proportional to lead concentration with a satisfactory reproducibility (within 5 % RSD) were obtained.     

Non-conductive nanomaterial enhanced electrochemical response in stripping voltammetry: The use of nanostructured magnesium silicate hollow spheres for heavy metal ions detection

Nanostructured magnesium silicate hollow spheres, one kind of non-conductive nanomaterials, were used in heavy metal ions (HMIs) detection with enhanced performance for the first time. The detailed study of the enhancing electrochemical response in stripping voltammetry for simultaneous detection of ultratrace Cd²⁺, Pb²⁺, Cu²⁺ and Hg²⁺ was described. Electrochemical properties of modified electrodes were characterized by cyclic voltammetry (CV) and electrochemical impedance spectroscopy (EIS). The operational parameters which have influence on the deposition and stripping of metal ions, such as supporting electrolytes, pH value, and deposition time were carefully studied. The anodic stripping voltammetric performance toward HMIs was evaluated using square wave anodic stripping voltammetry (SWASV) analysis. The detection limits achieved (0.186 nM, 0.247 nM, 0.169 nM and 0.375 nM for Cd²⁺, Pb²⁺, Cu²⁺ and Hg²⁺) are much lower than the guideline values in drinking water given by the World Health Organization (WHO). In addition, the interference and stability of the modified electrode were also investigated under the optimized conditions. An interesting phenomenon of mutual interference between different metal ions was observed. Most importantly, the sensitivity of Pb²⁺ increased in the presence of certain concentrations of other metal ions, such as Cd²⁺, Cu²⁺ and Hg²⁺ both individually and simultaneously. The proposed electrochemical sensing method is thus expected to open new opportunities to broaden the use of SWASV in analysis for detecting HMIs in the environment.     

Use of binary metal deposits on a cylindrical carbon-fiber microelectrode in the voltammetric determination of metal ions

The effect of binary metal deposits on a cylindrical carbon-fiber microelectrode on the determination of metals by direct and stripping voltammetry was studied. The electrolytic deposition of a binary system of copper and thallium, cadmium, lead, or mercury on the electrode in an alkaline solution resulted in the disappearance of the electroreduction peak of dissolved oxygen in the potential range from -0.8 to -1.4 V and in a decrease in the background current. Under the conditions of limited diffusion, the peak currents of Ni(II), Co(II), and Zn(II) in differential pulse voltammograms were 3–7 times higher than those calculated for a reversible electrode process under the conditions of semi-infinite diffusion. Because of this, the determination limit for metal ions in direct voltammetry was lowered to 1 X 10^-6 M. With a binary copper-thallium system, the peak current of zinc(II) reduction can be be detected in the presence of 5000-fold molar amounts of copper(II). The deposition of binary copper-lead and copper-thallium systems under the conditions of limited diffusion reduced the effect of negative interaction between the components of these systems and made possible the determination of lead(II) and thallium(I) by stripping voltammetry using additional peaks.     

Anodic Stripping Voltammetry of Heavy Metals on a Metal Catalyst Free Carbon Nanotube Electrode

Anodic stripping voltammetry (ASV) determination of Pb²⁺, Cd²⁺, and Zn²⁺ was done using metal catalyst free carbon nanotube (MCFCN) electrodes. Osteryoung square wave stripping voltammetry (OSWSV) was selected for detection. The MCFCNTs are synthesized via Carbo Thermal Carbide Conversion method which leads to residual transition metal free in the CNT structure. The new material shows very good results in detecting heavy metal ions, such as Pb²⁺, Cd²⁺, and Zn²⁺. The calculated limits of detection were 13 nM, 32 nM and 50 nM for Pb²⁺, Cd²⁺ and Zn²⁺, respectively with a deposition time of 150 s.     

Semipermeable ion-exchange membranes as a preconcentration matrix for trace analysis by electrochemical and neutron-activation techniques

Ion-exchange membranes were used in conjunction with neutron-activation analysis and anodic stripping voltammetry for the determination of certain trace metal ions. The various parameters which govern the applicability, limitations and sensitivity of the methods were investigated. A new membrane "barrier" electrode assembly was introduced and found to be useful for the anodic stripping determination of trace metal ions in the presence of surface active materials.     

Trace element speciation by anodic stripping voltammetry.

The effects of a range of complexing agents and a detergent on the stripping process in anodic stripping voltammetry (ASV) were compared by using both differential-pulse and d.c. techniques. The detergent and most of the ligands, added after deposition was complete, seriously affected the height of the differential-pulse waves, and fulvic acid even decreased the area of the d.c. stripping peak. In order to obtain a correlation between ASV-labile metal measurements and bioassays, it is important that complexing agents in the sample solution affect only the deposition stage, and not the stripping stage in ASV. In order to achieve this result, the double acidification technique is recommended, where a voltammogram is recorded with deposition at the natural pH of the sample, but with stripping carried out at pH less than 2, then a second voltammogram run on the same solution, but with both deposition and stripping effected at pH less than 2. Excellent agreement was obtained between ASV-labile metal measured by the double acidification method and algal assays for a variety of natural and synthetic waters.     

An electrochemical DNA hybridization detection assay based on a silver nanoparticle label

A novel, sensitive electrochemical DNA hybridization detection assay, using silver nanoparticles as the oligonucleotide labeling tag, is described. The assay relies on the hybridization of the target DNA with the silver nanoparticle-oligonucleotide DNA probe, followed by the release of the silver metal atoms anchored on the hybrids by oxidative metal dissolution and the indirect determination of the solubilized Ag(I) ions by anodic stripping voltammetry (ASV) at a carbon fiber ultramicroelectrode. The influence of the relevant experimental variables, including the surface coverage of the target oligonucleotide, the duration of the silver dissolution steps and the parameters of the electrochemical stripping measurement of the silver(I) ions, is examined and optimized. The combination of the remarkable sensitivity of the stripping metal analysis at the microelectrode with the large number of silver(I) ions released from each DNA hybrid allows detection at levels as low as 0.5 pmol L(-1) of the target oligonucleotides.     

Sonoelectrochemical investigation of silver analysis at a highly boron-doped diamond electrode

A study into the sonoelectroanalysis of silver at a highly boron-doped diamond electrode is presented, exploring the benefits of the introduction of power ultrasound and new electrode materials into classical electrochemical techniques. Both cathodic and anodic stripping voltammetry have been investigated in terms of their analytical suitability towards silver detection. Cathodic stripping voltammetry, via electrodeposited silver oxide, was affected by the unusual chemistry of the highly oxidising Ag(2+) species and the characterisation of this system is discussed in detail. Anodic stripping, via deposition of metallic silver on the bare boron-doped diamond electrode surface under ultrasound, coupled with square-wave voltammetry, was successfully employed in the development of a sensitive technique for the analysis of trace silver ions. A detection limit for Ag(+) of 10(-9) M for a 300-s deposition, with a linear range of at least two orders of magnitude, and the beneficial effects of controlling the speciation of Ag(+) via complexation with chloride ions are reported.     

Characterization of graphite–polyurethane composite electrodes modified with organofunctionalized SBA-15 nanostructured silica in the presence of heavy metal ions. Application to anodic stripping voltammetry

A graphite–polyurethane composite electrode with Santa Barbara Amorphous 15, SBA-15, mesoporous silica organofunctionalized with 2-benzothiazolethiol (BTSBA) as bulk modifier has been characterized electrochemically by voltammetry and electrochemical impedance spectroscopy (EIS) in the presence of cadmium ions, as an example of a toxic trace heavy metal, as well as by solid-state ¹³C-NMR and by scanning electron microscopy. EIS measurements performed on the modified composite electrodes to investigate the influence of BTSBA on the deposition of cadmium during square wave anodic stripping voltammetry showed that organofunctionalization of the SBA-15 bulk modifier in the composite electrode facilitates heavy metal deposition. Experiments were also carried out with a mixture of submicromolar cadmium, lead, copper and mercury ions and led to similar conclusions.     

Anodic stripping voltammetric determination of Zn, Pb and Cu traces in whisky samples

The simultaneous “in natura” determination of trace Zn, Pb and Cu in whisky samples by anodic stripping voltammetry (ASV), using a hanging mercury drop electrode, without previous treatment or addition of supporting electrolyte is described. The choice of an appropriate stripping voltammetric method and deposition potential minimizes the influence of the organic content and ensures a good reproducibility of the measurements. The reliability of the method was tested comparing the results with those of atomic absorption spectroscopy (AAS), with differences of about 10%. The method allows the determination of heavy metal ions in the μg L^–1 range.