Factors influencing redox magnetohydrodynamic-induced convection for enhancement of stripping analysis

Factors affecting the use of redox magnetohydrodynamics (MHD) to enhance the stripping analysis response to heavy metals have been investigated. The analytes were Pb2+, Cd2+, Cu2+, and Tl+ at concentrations ranging from 5 nM to 2 microM. Co-deposition of analytes with Hg2+ (to form a thin Hg film electrode) occurs along with reduction of a high concentration of Fe3+. The Fe3+ provides the high cathodic current necessary to produce a significant Lorentz force, and therefore enhanced convection and larger stripping signals and sensitivities, when the analysis is performed in the presence of an external magnetic field. The effects of varying Fe3+ concentration (1-100 mM), working electrode size (10 microm-3 mm), and magnetic field strengths (0-1.77 T) generated with electromagnets and NdFeB permanent magnets were investigated. Using 100 mM Fe3+ as the MHD-generating redox species at a 3-mm working electrode and in a magnetic field of 1.77 T, peak areas from linear sweep voltammetry were increased by as much as 159 +/- 5%, compared to the signal obtained in the absence of a magnetic field. Experimental detection limits as low as 5 nM were achieved with only a 1-min preconcentration time. A field strength as low as 0.12 T offers some signal enhancement with 100 mM Fe3+. While linear scan anodic stripping voltammetry was used primarily to obtain the signals after the deposition step, potentiometric stripping analysis was also investigated. Redox MHD is an attractive alternative convection method for applications involving sample volumes too small for mechanical stirring or for in-field applications using portable devices that cannot be complicated by the instrumentation required for mechanical stirring.     

Dynamics and performance of fast linear scan anodic stripping voltammetry of cd, pb, and cu using in situ-generated ultrathin mercury films

The dynamics of fast linear scan (LS) ASV for the simultaneous detection of Cd, Pb, and Cu was investigated at various scan rates (0.5-10 V/s) and at different metal ion concentrations (50-800 nM) utilizing ultrathin mercury films (9 nm) at a conventional size (d(0) = 1 mm) electrode. Results of the investigation show that when the thin films were utilized, diffusion of metals through the mercury film was not the rate-limiting step of the stripping process at moderate to fast scan rates (0.5-10 V/s). A fairly linear relationship between the peak height and scan rate was observed at scan rates (0.5-10 V/s) beyond the upper limit of the theoretical model for the behavior of LS-ASV. In addition, peak width at half-height (b(1/2)) as low as 33 mV was achieved at 0.5 V/s. The behavior of LS-ASV in terms of peak width at these scan rates is thus different from what the theoretical model of LS-ASV would have predicted. For the ultrathin mercury films, at least two additional factors, kinetics and concentration, have significant effects on practical LS-ASV. Experimental results show that the stripping process of Cu was primarily kinetic-controlled for fast scans, while those for Cd and Pb were dependent on both scan rates and concentrations. The ultrathin mercury film resulted in a significant enhancement of the ratio of signal-to-baseline slope (i(p)/Δi(b), a ratio used to measure the effectiveness of discrimination of the peak signal against the steep sloping baseline in LS-ASV) for Cd and Pb stripping peaks, but only a slight enhancement for Cu stripping peaks. The optimal performance of LS-ASV in terms of sensitivity, peak width, and enhancement of the i(p)/Δi(b) ratio for the three metals was achieved at 2 V/s. Because of the high reproducibility of the background currents of the stable in situ MTFs, background subtraction was carried out at 2 V/s with little hysteresis. This feature, combined with the enhancement of the i(p)/Δi(b) ratio at the fast scan rate of 2 V/s, allowed for the detection of sub-ppb levels of Cd, Pb, and Cu at a deposition time of 2 min.     

Anodic stripping voltammetry coupled on-line with inductively coupled plasma mass spectrometry: optimization of a thin-layer flow cell system for analyte signal enhancement

Parameters affecting analyte signal enhancement in anodic stripping voltammetry-inductively coupled plasma mass spectrometry (ASV-ICP-MS), using a thin-layer ASV cell and microconcentric nebulization (MCN), have been examined. Silver was used as a test analyte and was deposited at a glassy carbon working electrode. The MCN allowed use of solution flow rates that were beneficial to optimum electrolytic performance of the thin-layer cell. High analyte deposition efficiencies obtained with the thin-layer cell, combined with minimal sample consumption of the MCN, allowed substantial signal enhancement (>400 times higher than continuous nebulization level) to be obtained with 2-3 mL of sample and deposition times of less than 30 min. Signal enhancement was strongly influenced by the opposing effect of flow rate on the electrolytic deposition efficiency (deposition efficiency decreases with increasing flow rate) and on the quantity of analyte delivered to the cell (analyte mass throughput increases with increasing flow rate). Excellent linearity for stripping peak heights was demonstrated for a wide range of analyte deposition times and for peak heights and peak areas (r > 0.999) over a wide concentration range (25 ng/L-20 μg/L). Precision was good (RSD typically <3% for n = 3-6) except for a high Ag blank contributed to by corrosion of the counter electrode and by Ag diffusion from the reference electrode into the cell. Details of the flow manifold and ASV cells are discussed, along with relevant performance characteristics of the MCN.     

Imaging of metal ion dissolution and electrodeposition by anodic stripping voltammetry-scanning electrochemical microscopy

We have developed a new imaging method for scanning electrochemical microscopy (SECM) employing fast-scan anodic stripping voltammetry (ASV) to provide sensitive and selective imaging of multiple chemical species at interfaces immersed in solution. A rapid cyclic voltammetry scan (100 V/s) is used along with a short preconcentration time (300-750 ms) to allow images to be acquired in a normal SECM time frame. A Hg-Pt film electrode is developed having an equivalent Hg thickness of 40 nm that has good sensitivity at short preconcentration times and also retains thin-film behavior with high-speed voltammetric stripping. Fast-scan anodic stripping currents are shown to be linear for 1-100 microM of Pb (2+) and Cd (2+) solutions using a preconcentration time of 300 ms. SECM images showing the presence of Pb (2+) and Cd (2+) at concentrations as low as 1 microM are presented. In addition, a single ASV-SECM image is shown to produce unique concentration maps indicating Cd (2+) and Pb (2+), generated in situ from a corroding sample, while simultaneously detecting the depletion of O 2 at this sample. The transient voltammetric response at the film electrode is simulated and shows good agreement with the experimental behavior. We discuss the behavior of images and concentration profiles obtained with different imaging conditions and show that mass-transport limitations in the tip-substrate gap can induce dissolution. ASV-SECM can thus be used to detect and study induced dissolution not only at bulk metal surfaces but also on underpotential deposition layers, in this case Cd and Pb on Pt. In addition, we discuss how surface diffusion phenomena may relate to the observed ASV-SECM behavior.     

Ultramicroband array electrode. 1. Analysis of mercury in contaminated soils and flue gas exposed samples using a gold-plated iridium portable system by anodic stripping voltammetry

The applicability of a gold-plated iridium Nano-Band array ultramicroelectrode (6 microm by 0.2 microm, 64-microm interspacing, 100 electrode bands) in the analysis of mercury using a portable system is demonstrated by anodic stripping voltammetry in real-life samples. Optimized measurement parameters, 0.1 M HCl electrolyte, plating potential of 0 mV, and staircase scan mode were identified. The dynamic linear range is 10-180 ppb at 5-s deposition time with 1.5 microC of gold plated. The experimental detection limit for Hg2+ in 0.1 M HCl was 0.5 ppb at a deposition time of 4 min and a scan rate of 10 V/s. Real-life samples, such as flue gas exposed samples from flue gas simulators could be analyzed within 5 min using the method of standard additions. We identified a field-portable extraction procedure for soil samples using 1:1 concentrated HNO3/30% H2O2 mixture, compatible with ASV and the iridium electrode. The detection limit for soils is 1 ppm. The results obtained using ASV are in good agreement with reference values using cold vapor atomic absorption for the sample matrixes studied here. To our knowledge, this is the first mercury application using a reusable iridium array ultramicroelectrode. The portable potentiostat is less than 500 g, and together with the portable digestion method, makes the Nano-Band Explorer system field applicable.     

Stripping analyses of mercury using gold electrodes: irreversible adsorption of mercury

The electrochemical deposition and stripping of mercury on gold surfaces was investigated to assess whether gold electrodes would return to mercury-free states after stripping analyses. X-ray photoelectron spectroscopy studies demonstrate the presence of mercury on gold foil electrodes that have undergone controlled-potential deposition procedures in Hg(2+) solutions (10 nM-0.1 mM) followed by stripping and cleaning in mercury-free electrolyte. Results show that mercury is not completely removed electrochemically from the gold electrodes, even when the oxidizing potential is +2.5 V vs Ag/AgCl. Bulk electrolyses deposition and stripping procedures coupled with cold vapor atomic absorption spectroscopic analyses of solutions after deposition and stripping are also reported. Results suggest that the nature of the gold electrode is fundamentally altered by irreversible adsorption of mercury; that is, mercury is adsorbed during deposition and some of the mercury is retained even after stripping and cleaning. The implications and strategies for using stripping analysis and gold electrodes for the measurement of mercury under the experimental conditions employed in this study are discussed.     

Preconcentration and Voltammetric Determination of Mercury(II) at a Chemically Modified Glassy Carbon Electrode

In this work, the organic compound 2-mercaptobenzimidazole was covalently bound on the surface of a glassy carbon rod, via silanization, yielding a material capable of selectively complexing Hg(2+) ions. This material was applied as an electrode for voltammetric determination of mercury(II) following its nonelectrolytic preconcentration. After exchanging the medium, the voltammetric measurements were carried out by anodic stripping in the differential pulse mode (pulse amplitude, 50 mV; scan rate, 1.25 mV s(-)(1)) using 10(-)(2) mol L(-)(1) NaSCN solution as supporting electrolyte. An anodic stripping peak was obtained at 0.06 V (vs SCE) by scanning the potential from -0.3 to +0.3 V. After a 5 min preconcentration period in a pH 4.0 Hg(2+) solution, this electrode shows increasing voltammetric response in the range 0.1-2.2 μg mL(-)(1), with a relative standard deviation of 5% and a practical detection limit of 0.1 μg mL(-)(1) (5.0 × 10(-)(7) mol dm(-)(3)). Compared with the conventional stripping approach, this chemically modified glassy carbon electrode procedure presented good discrimination against interference from Cu(II) in up to 10-fold molar excess.     

Factors controlling stripping voltammetry of lead at polycrystalline boron doped diamond electrodes: new insights from high-resolution microscopy

We report wide-ranging studies to elucidate the factors and issues controlling stripping voltammetry of metal ions on solid electrodes using the well-known Pb/Pb(2+) couple on polycrystalline boron doped diamond (pBDD) as an exemplar system. Notably, high-resolution microscopy techniques have revealed new insights into the features observed in differential pulse anodic stripping voltammetry (DPV-ASV) which provide a deeper understanding of how best to utilize this technique. DPV-ASV was employed in an impinging wall-jet configuration to detect Pb(2+) in the nanomolar to micromolar concentration range at a pBDD macrodisk electrode. The deposition process was driven to produce a grain-independent homogeneous distribution of Pb nanoparticles (NPs) on the electrode surface; this resulted in the observation of narrow stripping peaks. Lower calibration gradients of current or charge versus concentration were found for the low concentrations, correlating with a lower than expected (from consideration of the simple convective-diffusive nature of the deposition process) amount of Pb deposited on the surface. This was attributed to the complex nature of nucleation and growth at solid surfaces in this concentration regime, complicating mass transport. Furthermore, a clear shift negative in the stripping peak potential with decreasing concentration was seen correlating with a change in the size of the deposited NP, suggesting an NP size-dependent redox potential for the Pb/Pb(2+) couple. At high concentrations a nonlinear response was observed, with less Pb detected than expected, in addition to the observation of a second stripping peak. Atomic force microscopy (AFM) and field emission scanning electron microscopy revealed the second peak to be due to a change in deposition morphology from isolated NPs to grain-independent heterogeneous structures comprising both thin films and NPs; the second peak is associated with stripping from the thin-film structures. AFM also revealed a substantial amount of Pb remaining on the surface after stripping at high concentration, explaining the nonlinear relationship between stripping peak current (or charge) and concentration. Finally, the use of an in situ cleaning procedure between each measurement was advocated to ensure a clean Pb-free surface (verified by AFM and X-ray photoelectron spectroscopy analysis) between each run. The studies herein highlight important and complex physicochemical processes involved in the electroanalysis of heavy metals at solid electrodes, such as pBDD, that need to be accounted for when using stripping voltammetry methods.     

Sequencing Batch Reactor (SBR) for the removal of Hg2+ and Cd2+ from synthetic petrochemical factory wastewater

Petrochemical factories which manufacture vinyl chloride monomer and poly vinyl chloride (PVC) are among the largest industries which produce wastewater contains mercury and cadmium. The objective of this research is to evaluate the performance of a lab-scale Sequencing Batch Reactor (SBR) to treat a synthetic petrochemical wastewater containing mercury and cadmium. After acclimatization of the system which lasted 60 days, the SBR was introduced to mercury and cadmium in low concentrations which then was increased gradually to 9.03±0.02 mg/L Hg and 15.52±0.02 mg/L Cd until day 110. The SBR performance was assessed by measuring Chemical Oxygen Demand, Total and Volatile Suspended Solids as well as Sludge Volume Index. At maximum concentrations of the heavy metals, the SBR was able to remove 76-90% of Hg(2+) and 96-98% of Cd(2+). The COD removal efficiency and MLVSS (microorganism population) in the SBR was affected by mercury and cadmium concentrations in influent. Different species of microorganisms such as Rhodospirilium-like bacteria, Gomphonema-like algae, and sulfate reducing-like bacteria were identified in the system. While COD removal efficiency and MLVSS concentration declined during addition of heavy metals, the appreciable performance of SBR in removal of Hg(2+) and Cd(2+) implies that the removal in SBR was not only a biological process, but also by the biosorption process of the sludge.     

Antimony film electrode for electrochemical stripping analysis

In this work, an antimony film electrode (SbFE) is reported for the first time as a possible alternative for electrochemical stripping analysis of trace heavy metals. The SbFE was prepared in situ on a glassy carbon substrate electrode and employed in combination with either anodic stripping voltammetry or stripping chronopotentiometry in nondeaerated solutions of 0.01 M hydrochloric acid (pH 2). Several key operational parameters influencing the electroanalytical response of SbFE were examined and optimized, such as deposition potential, deposition time, and composition of the measurement solution. The SbFE exhibited well-defined and separated stripping signals for both model metal ions, Cd(II) and Pb(II), surrounded with low background contribution and a relatively large negative potential range. The electrode revealed good linear behavior in the examined concentration range from 20 to 140 microg L-1 for both test metal ions, with a limit of detection (3sigma) of 0.7 microg L-1 for Cd(II) and 0.9 microg L-1 for Pb(II) obtained after a 120 s deposition step, and good reproducibility, with a relative standard deviation (RSD) of +/-3.6% for Cd(II) and +/-6.2% for Pb(II) (60 microg L-1, n = 12). When comparing the SbFE with the commonly used mercury film electrode and recently introduced bismuth film electrode, the newly proposed electrode offers a remarkable performance in more acidic solutions (pH < or = 2), which can be advantageous in electrochemical analysis of trace heavy metals, hence contributing to the wider applicability of electrochemical stripping techniques in connection with "mercury-free" electrodes.     

Hg(0) oxidative absorption by K(2)S(2)O(8) solution catalyzed by Ag(+) and Cu(2+)

The aqueous phase oxidation of gaseous elemental mercury (Hg(0)) by potassium persulfate (K(2)S(2)O(8), KPS) catalyzed by Ag(+) and Cu(2+) was investigated using a glass bubble column reactor. Concentrations of gaseous Hg(0) and aqueous Hg(2+) were measured by cold vapor generation atomic absorption spectrometry (CVAAS). The effects of several experimental parameters on the oxidation were studied; these include different types of catalysts, pHs and concentrations of potassium persulfate, temperatures, Hg(0) inlet concentrations and tertiary butanol (TBA). The results showed that the removal efficiency of Hg(0) increased with increasing concentration of potassium persulfate and catalysts Ag(+), Cu(2+) and Ag(+) provided better catalytic effect than Cu(2+). For example, in the presence of 5.0mmoll(-1) KPS, the mercury removal efficiency could reach 75.4 and 97.0% for an Ag(+) concentration of 0.1 and 0.3mmoll(-1), respectively, and 69.8 and 81.9% for 0.1 and 0.3mmoll(-1) Cu(2+). On the other hand, high temperature and the introduction of TBA negatively affect the oxidation. Furthermore, the removal efficiency of Hg(0) was much greater in neutral solution than in either acidic or alkaline solution. But the influence of pH was almost eliminated upon the addition of Ag(+) and Cu(2+), and high Hg(0) inlet concentration also has positive impact on the removal efficiency of Hg(0). The possible catalytic oxidation mechanism of gaseous mercury by KPS was also proposed.     

Voltammetric detection of mercury and copper in seawater using a gold microwire electrode

A procedure is presented by which mercury and copper are determined simultaneously in seawater and dilute acid (0.01 M HCl) by anodic stripping voltammetry using gold microwire electrodes. It was found that anion (halide) adsorption is the cause for a gradual decrease in the height and potential of the mercury peak. The effect is eliminated by including an anion desorption step in the analysis at -0.8 V prior to each scan. This step was found to greatly improve the stability of the scans and enabled the use of background subtraction. Advantages of the microwire electrodes were a low roughness of the surface, without a need for pretreatment, and a very small diffusion layer (2 microm with stirring). Under the optimized voltammetric conditions, the detection limits were 6 pM mercury and 25 pM copper using 300-s deposition. These values are well below those reported previously for other electrodes including rotating disk electrodes. Measurements of the influence of the major anions I-, Br-, Cl-, SO4(2-), F-, HCO3-, and B(OH)4 on the response for mercury showed that bromide and chloride are predominantly responsible for the underpotential deposition mechanism of mercury in seawater. The method was applied to coastal water samples from Liverpool Bay.     

Magnetic bead-based chemiluminescent metal immunoassay with a colloidal gold label

A novel, sensitive chemiluminescent (CL) immunoassay has been developed by taking advantage of a magnetic separation/mixing process and the amplification feature of colloidal gold label. First, the sandwich-type complex is formed in this protocol by the primary antibody immobilized on the surface of magnetic beads, the antigen in the sample, and the second antibody labeled with colloidal gold. Second, a large number of Au3+ ions from each gold particle anchored on the surface of magnetic beads are released after oxidative gold metal dissolution and then quantitatively determined by a simple and sensitive Au3+-catalyzed luminol CL reaction. Third, this protocol is evaluated for a noncompetitive immunoassay of a human immunoglobulin G, and a concentration as low as 3.1 x 10(-12) M is determined, which is competitive with colloidal gold-based anodic stripping voltammetry (ASV), colorimetric ELISA, or immunoassays based on fluorescent europium chelate labels. The high performance of this protocol is related to the sensitive CL determination of Au3+ ion (detection limit of 2 x 10(-10) M), which is 25 times higher than that by ASV at a single-use carbon-based screen-printed electrode. From the analytical chemistry point of view, this protocol will be quite promising for numerous applications in immunoassay and DNA hybridization.     

A new Schiff's base ligand immobilized agarose membrane optical sensor for selective monitoring of mercury ion

A highly selective optical sensor was developed for the Hg(2+) determination by chemical immobilization of 2-[(2-sulfanylphenyl)ethanimidoyl]phenol (L), on an agarose membrane. Spectrophotometric studies of complex formation between the Schiff's base ligand L and Hg(2+), Sr(2+), Mn(2+), Cu(2+), Al(3+), Cd(2+), Zn(2+), Co(2+) and Ag(+) metal ions in methanol solution indicated a substantially larger stability constant for the mercury ion complex. Consequently, the Schiff's base L was used as an appropriate ionophore for the preparation of a selective Hg(2+) optical sensor, by its immobilization on a transparent agarose film. A distinct color change, from yellow to green-blue, was observed by contacting the sensing membrane with Hg(2+) ions at pH 4.5. The effects of pH, ionophore concentration, ionic strength and reaction time on the immobilization of L were studied. A linear relationship was observed between the membrane absorbance at 650 nm and Hg(2+) concentrations in a range from 1×10(-2) to 1×10(-5) mol L(-1) with a detection limit (3σ) of 1×10(-6) mol L(-1). No significant interference from 100 times concentrations of a number of potentially interfering ions was detected for the mercury ion determination. The optical sensor was successfully applied to the determination of mercury in amalgam alloy and spiked water samples.     

Anodic stripping voltammetry combined on-line with inductively coupled plasma-MS via a direct-injection high-efficiency nebulizer

A direct-injection high-efficiency nebulizer (DIHEN) is used to couple a thin-layer electrochemical flow cell on-line with an ICP-mass spectrometer to perform anodic stripping voltammetry (ASV) at a thin mercury film followed by subsequent ICPMS measurements for the stripped metal analytes. The resultant hyphenated technique (ASV-DIHEN-ICPMS) is capable of analyzing select heavy metals present at ultratrace levels (down to low-ppt to sub-ppt levels) that are lower than the detection limits obtained by conventional ICPMS. In addition to its good analytical performance, the technique offers other attractive features such as the ability to eliminate detrimental matrix effects that can compromise ICPMS analyses and the possibility of probing electrode reactions involving trace amounts metal species with ICPMS. For conducting ASV on-line with ICPMS, the DIHEN was found to be more advantageous than the microconcentric nebulizer in terms of minimizing memory effects and potential artifacts caused by the erosion of the Hg film into the flowing solution stream. Compared to a direct injection nebulizer (DIN), the DIHEN was easier to operate. Moreover, its simpler design and the lack of back pressure from the DIHEN capillary made it more compatible with coupling to the thin-layer electrochemical cell than a DIN system.     

On-site analysis of arsenic in groundwater using a microfabricated gold ultramicroelectrode array

Rapid on-site analysis of arsenic in groundwater was achieved with a small battery-powered unit in conjunction with a microfabricated gold ultramicroelectrode array (Au-UMEA). The sensor, consisting of 564 UME disks with a unique gold surface created by electron beam evaporation, was demonstrated to be highly sensitive to low-ppb As3+ using square wave anodic stripping voltammetry. The influence of the square wave frequency, pulse amplitude, and deposition potential on the arsenic peak stripping current was investigated. Varying those theoretical parameters yielded results surprisingly similar to those for the thin Hg film case. The performance of the Au-UMEA was evaluated for reproducibility and reliability. Three stability tests showed an average relative standard deviation of 2.5% for 15 consecutive runs. Limits of detection were investigated, and 0.05 ppb As3+ could be measured while maintaining a S/N of 3:1. Interference studies were performed in the presence of 50-500 ppb of Cu2+, Hg2+, and Pb2+. On-site analysis of groundwater containing arsenic was performed with a small battery-powered potentiostat. Quantification was done through standard additions, and these results were compared to the standard EPA methodology.     

Detection of heavy metal ions in water by high-resolution surface plasmon resonance spectroscopy combined with anodic stripping voltammetry

High-resolution differential surface plasmon resonance (SPR) with anodic stripping voltammetry (ASV) capability has been demonstrated for detecting heavy metal ions in water. Metal ions are electroplated onto the gold SPR sensing surface and are quantitatively detected by stripping voltammetry. Both the SPR angular shift and electrochemical current signal are recorded to identify the type and amount of the metal ions in water. The performance of the combined approach is further enhanced by a differential detection approach. The gold sensor surface is divided into a reference and a sensing area, and the difference in the SPR angles from the two areas is detected with a quadrant cell photodetector as a differential signal. Our system demonstrated quantitative detection of copper, lead, and mercury ions in water from part-per-million to sub-part-per-billion levels with good linearity.     

Immobilization of aqueous Hg(II) by mackinawite (FeS)

As one of the major constituents of acid volatile sulfide (AVS) in anoxic sediments, mackinawite (FeS) is known for its ability to scavenge trace metals. The interaction between aqueous Hg(II) (added as HgCl(2)) and synthetic FeS was studied via batch sorption experiments conducted under anaerobic conditions. Due to the release of H(+) during formation of hydrolyzed Hg(II) species which is more reactive than Hg(2+) in surface adsorption, the equilibrium pH decreased with the increase in Hg(II)/FeS molar ratio. Counteracting the loss of FeS solids at lower pH, the maximum capacity for FeS to remove aqueous Hg(II) was approximately 0.75 mol Hg(II) (mol FeS)(-1). The comparison of X-ray power diffraction (XRPD) patterns of synthetic FeS sorbent before and after sorption showed that the major products formed from the interaction between FeS and the aqueous Hg(II) were metacinnabar, cinnabar, and mercury iron sulfides. With the addition of FeS at 0.4 g L(-1) to a 1 mM Hg(II) solution with an initial pH of 5.6, Fe(2+) release was approximately 0.77 mol Fe(2+) per mol Hg(II) removed, suggesting that 77% of Hg(II) was removed via precipitation reaction under these conditions, with 23% of Hg(II) removed by adsorption. Aeration does not cause significant release of Hg(II) into the water phase.     

Sub-femtomolar electrochemical detection of DNA hybridization based on latex/gold nanoparticle-assisted signal amplification

We report a relatively simple electrostatic method for modifying submicrometer-size latex spheres with gold nanoparticles (AuNPs) based on layer-by-layer modification of the latex by polyelectrolytes. The AuNP coverages for 343- and 501-nm-diameter spheres were 4.0 x 10 (10) +/- 1.3 x 10 (10) and 8.2 x 10 (10) +/- 2.7 x 10 (10) particles cm (-2), respectively, which is an increase of 1 order of magnitude on the previously reported coverage at latex-AuNPs using streptavidin-biotin binding (Kawde, A.N.; Wang, J. Electroanalysis 2004, 16, 101-107). Due to the fact that the AuNPs used here are also of a larger size (mean diameter 15.5 +/- 1.6 nm, cf. 5 nm), this represents an increase of 2 orders of magnitude in the number of Au atoms delivered per sphere. The spheres were attached to DNA probes specific to E. coli and used to detect probe hybridization by dissolution of the AuNPs, followed by measurement of Au (3+) ions by anodic stripping voltammetry (ASV). Use of differential pulse voltammetry for the stripping step, along with optimization of the ASV conditions, enabled a detection limit of 0.5 fM, which is, to the best of our knowledge, equal or lower than previous voltammetric nanoparticle methods for detection of DNA hybridization.     

Determination of cardiac troponin I for the auxiliary diagnosis of acute myocardial infarction by anodic stripping voltammetry at a carbon paste electrode

An electrochemical immunoassay for cardiac troponin I (cTnI) combining the concepts of the dual monoclonal antibody "sandwich" principle, the silver enhancement on the nano-gold particle, and the anodic stripping voltammetry is described. Four main steps were carried out to obtain the analytical signal, i.e., electrode preparation, immunoreaction, silver enhancement, and anodic stripping voltammetric detection. A linear relationship between the anodic stripping peak current and concentration of cTnl from 1 to 20 ng/ml and a limit of detection of 0.8 ng/ml were obtained. The established method was tested by determining cTnI in acute myocardial infarction (AMI) samples using enzyme-linked immunoadsorbent assay (ELISA) for comparison analysis, and good results were obtained.