Prevention of contamination and other accuracy risks in voltammetric trace metal analysis of natural waters

A water-soluble BODIPY based 'OFF/ON' fluorescent probe for the detection of Cd2+ ions with high selectivity and sensitivity

A water-soluble dilithium salt BODIPY derivative (LiBDP) with appended dicarboxylate pseudo-crown ether [NO4] coordinating sites has been designed, synthesized and characterized successfully for the selective and sensitive recognition of Cd2+ in aqueous media. The chemosensor exhibits a remarkable increase in fluorescence intensity as well as a distinct color change upon the addition of Cd2+ over other environmentally and biologically relevant metal ions in H2O. The fluorometric response of LiBDP is attributed to the metal chelation-enhanced fluorescence (MCHEF) effect which has been confirmed by a strong association constant of 2.57 ± 1.06 × 105 M-1 and Job's plot, indicating 1 : 1 binding stoichiometry between LiBDP and Cd2+. Frontier molecular orbital analysis (obtained from DFT studies) also illustrates the turn-on fluorescence of the probe by blocking photoinduced electron transfer (PET) after coordination to Cd2+. The probe can detect Cd2+ in a competitive environment up to a submicromolar level in a biologically significant pH range. The sensor is proved to be reversible and reusable by the alternative addition of Cd2+ followed by S2-. The OFF/ON/OFF sensing behavior is utilized to construct an INHIBIT molecular logic gate based on the two inputs of Cd2+ and S2- and a fluorescence intensity at 512 nm as an output. The test paper experiment demonstrates the practical utility of LiBDP to monitor Cd2+ in an aqueous sample. Finally, the sensing probe was utilized to monitor Cd2+ in living cells.

SWASV speciation of Cd, Pb and Cu for the determination of seawater contamination in the area of the Nicole shipwreck (Ancona coast, Central Adriatic Sea)

The study reports for the first time on the heavy metal contamination of the waters surrounding a shipwreck lying on the sea floor. Square wave anodic stripping voltammetry has been used for a survey of the total and dissolved Cd, Pb and Cu contents of the seawater at the site of the sinking of the Nicole M/V (Coastal Adriatic Sea, Italy). Results show that the hulk has a considerable impact as regards all three metals in the bottom water, especially for the particulate fraction concentrations, which increased by factors of ≈ 9 (Cd), ≈ 3 (Pb) and ≈ 5 (Cu). The contaminated plume extended downstream for about 2 miles. Much lower contamination was observed for dissolved bottom concentrations; nevertheless Pb (0.56 ± 0.03 nmol/L) is higher than the Italian legal limits established for 2015 and Cd (0.23 ± 0.03 nmol/L) is very close the limit of Cd will be exceeded if the hulk is not removed.

Detection of lead ions in picomolar concentration range using underpotential deposition on silver nanoparticles-deposited glassy carbon electrodes

The efficacy of silver-deposited glassy carbon electrode for the determination of lead ions at the sub-nanomolar concentration ranges is investigated. The silver nanoparticles are electrodeposited on glassy carbon electrode using chronoamperometry and the electrode surface is characterized using SEM. Lead ions are detected in the region of underpotential deposition. The analysis is performed in square wave mode in the stripping voltammetry without the removal of oxygen. The detection limit of 10 pM has been obtained with a constant potential of -0.7 V during the electrodeposition step for a period of 50s. The interference of surfactants in the detection of lead ions is also studied.

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.

Anodic stripping voltammetry enhancement by redox magnetohydrodynamics

The effect of an external magnetic field on linear scan anodic stripping voltammetry (ASV) in solutions of 10(-6)-10(-7) M concentrations of lead, cadmium, and copper at mercury films on glassy carbon electrodes has been investigated. A high concentration of Hg(2+) was added to the analyte solution to induce a large cathodic current during the deposition step. Therefore, a large Lorentz force from the net flux of charge through the magnetic field resulted in convection due to magnetohydrodynamics. The faster delivery of analytes to the mercury film electrode during deposition caused an increase in the anodic stripping peaks. The effect of varying Hg(2+) concentrations (0-60 mM) and magnetic field strengths (0-1.77 T) on the enhancement of the stripping peaks was investigated. Enhancements as large as 129% for peak currents and 167% for peak areas were observed. An enhancement of approximately 100% was observed when 60 mM Fe(3+) replaced high concentrations of Hg(2+). This method of convection exhibits promise for small-volume ASV analysis with possible improved limits of detection and decreased preconcentration times.

Metal concentrations in mussel Crenomytilus grayanus and oyster Crassostrea gigas in relation to contamination of ambient sediments

The relationship between Zn, Cu, Cd, Pb, and Ni concentrations in soft tissues of mussels and oysters and those in ambient sediments was examined. The study area was the coastal habitats of the northwestern Sea of Japan, which has a broad range of contamination due to urban sewage. Significant increases of all metals, except Ni, were observed in mussel Crenomytilus grayanus at concentrations of easily leachable metals in ambient sediments higher than 2, 100, and 800 microg/g for Cd, Cu, and Zn, respectively. Pb was accumulated by the mussels with no distinct threshold in Pb contamination of sediments. After 2 months, C. grayanus transplanted from a pristine to a contaminated locality increased in Pb up to 12%, and Cu up to 68% of concentrations of these metals in the contaminated local mussels, but Zn and Cd concentrations increased only in the excretory tissue, not in the somatic tissue, or in the soft tissue as a whole. Such regulation of metal accumulation puts limitations on the use of C. grayanus for monitoring of slightly and moderately contaminated localities. On the other hand, the oyster Crassostrea gigas showed definite accumulation of all metals, except Ni, at moderate contamination of ambient sediments. As contamination of sediments increased, increases of Pb, Zn and Cu concentrations in C. gigas slowed, possibly due to physiological control at very high metal concentrations in oyster soft tissue. Thus, the mussel C. grayanus should be used mainly for the monitoring of heavy contaminated localities, and the oyster C. gigas is more suitable as an indicator of low and moderate contamination.

Benthic fluxes of cadmium, lead, copper and nitrogen species in the northern Adriatic Sea in front of the River Po outflow, Italy

Trace heavy metal (Cd, Pb and Cu) and nitrogen species (N-NO3, N-NO2 and N-NH4) fluxes between sediment and water were examined for approximately 4 days, in a coastal marine station located in the northern Adriatic Sea in front of the River Po outflow. An in situ benthic chamber, equipped with electronic devices for monitoring and adjustment of oxygen and pH and with a temperature detector, was used. The benthic chamber experiment enabled study of the temporal trend of metals and nutrients when oxygen concentration varied in a controlled environment. Although particular care was devoted to chamber deposition and parameter control, sediment resuspension occurred at the beginning of the experiment and O2 fluctuations were observed during the course of the experiment. Pb concentration was affected by both resuspension and oxic conditions in bottom water, which prevented determination of any reasonable Pb flux value. Cd and Cu, not influenced by oxygen fluctuations, reached an equilibrium phase in a short period with initial positive fluxes from sediment of 0.68 (S.D. = 0.07) and 6.9 (S.D. = 5.6) pmol cm(-2) h(-1), respectively. With regard to nitrogen species, the highest positive flux was that of N-NH4 (10.5, S.D. = 2.4, nmol cm(-2) h(-1)) whose concentration increased in the chamber, while nitrate concentration (initial flux of -5.7, S.D. = 1.5, nmol cm(-2) h(-1)) immediately decreased after the beginning of the experiment. Nitrite concentration was almost constant throughout the experiment and its flux was generally low (initial flux 0.1, S.D. = 0.9, nmol cm(-2) h(-1)).