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- Stripping chronopotentiometry at scanned deposition potential (SSCP). Part 6: features of irreversible complex systemsPublication . Town, R. M.; Pinheiro, J. P.; Domingos, R. F.; Van Leeuwen, H. P.The features of SSCP waves for complex species involving a nonreversible electron transfer process are described. For quasireversible systems, with electron transfer rate constants, k0, between O(10 4) to O(10 6) m s 1, the shape of the SSCP wave is dependent on the value of k0, the deposition time and the electrode size. Under these conditions, the stability of a metal complex can be determined from the shift in half-wave deposition potential, Ed,1/2, as compared to the metal-only case. This is true even when the system is not fully labile, so long as the shape of the SSCP wave is not altered in the presence of the complexant, i.e., k0 remains the same. The experimental parameters, notably deposition time and electrode size, can be optimised to achieve this condition. For irreversible systems, k0 < O(10 7) m s 1, the SSCP wave shape is independent of k0, deposition time, and electrode size. Further decreasing k0 only leads to additional shift in the wave from E0. In such cases the contribution to the Ed,1/2 from the stability constant cannot be a priori deconvoluted from that due to k0 if its value changes due to the presence of ligand. In irreversible systems, direct reduction of electroactive complexes is not uncommon, nor is its appearance at potentials more positive, i.e., more reversibly, than that for the metal itself.
- Scanned stripping chronopotentiometry of metal complexes: lability diagnosis and stability computationPublication . Pinheiro, J. P.; Van Leeuwen, H. P.A method is presented for analyzing the dynamic speciation features of metal complexes based on stripping chronopotentiometry at a scanned deposition potential (SSCP). The shift in the SSCP half-wave deposition potential, DEd;1=2, is straightforwardly related to the complex stability, K, irrespective of the degree of lability; the limiting wave height, s , quantifies the metal species accumulated in the electrode and thus depends on both the lability and mass transport properties of the metal complex species in solution. For complexes with a lower diffusion coefficient than the free metal ion, K calculated from DEd;1=2 will be the same as that derived from the relative s values so long as the system is fully labile. Discrepancies between DEd;1=2-derived and s -derived K values indicate loss of lability. We show that this approach is a sensitive indicator of lability, as illustrated by analysis of cadmium and lead binding by carboxylated nanospheres.
- Metal speciation dynamics in colloidal ligand dispersions. Part 2: electrochemical labilityPublication . Pinheiro, J. P.; Minor, M.; Van Leeuwen, H. P.We investigate the dynamic nature of metal speciation in colloidal dispersions using a recently proposed theory [J.P. Pinheiro, M. Minor, H.P. Van Leeuwen, Langmuir, 21 (2005) 8635] for complexing ligands that are situated on the surface of the particles. The new approach effectively modifies the finite rates of association/dissociation of the colloidal metal complexes, thus invoking consideration of the two basic dynamic criteria: the association/dissociation kinetics of the volume complexation reaction (the ‘‘dynamic’’ criterion), and the interfacial flux of free metal to a macroscopic surface due to dissociation of complex species (the ‘‘lability’’ criterion). We demonstrate that the conventional approach for homogeneous systems that assume a smeared-out ligand distribution, overestimates both the dynamics and the lability of metal complexes when applied to colloidal ligands. It is also shown that the increase of lability with increasing particle radius, as expected for a homogeneous solution, is moderated for spherical microelectrodes and practically eliminated for planar electrodes.
- Ion-exchange voltammetry with nafion/poly(sodium 4-styrenesulfonate) mixed coatings on mercury film electrodes: Characterization studies and application to the determination of trace metalsPublication . Rocha, L. S.; Pinheiro, J. P.; Carapuça, H. M.This work aimed to produce improved polymer coatings for the modification of thin mercury film electrodes (TMFEs). The goal is to obtain sensitive, reproducible, mechanically stable and antifouling devices suitable for the determination of trace metal cations in complex media. Therefore, novel mixed coatings of two sulfonated cationexchange polymers of dissimilar characteristicssNafion (NA) and poly(sodium 4-styrenesulfonate) (PSS)swere produced by solvent evaporation onto glassy carbon electrodes. The effect of the mass ratio (NA:PSS) on the film morphology was studied by scanning electron microscopy, revealing the formation of biphasic polymer systems, where PSS bead-shaped clusters appeared randomly dispersed into a uniform and compactNAenvironment. The permselectivity/ ion-exchange features of the mixed films onto glassy carbon were evaluated using cathecol, urate, and dopamine. To allow trace metal analysis, thin mercury films were plated through the NA/PSS coatings, being the reproducibility and ion-exchange features of the mixed coatings-TMFE evaluated using lead ions. The best NA/PSS coating was found for the mass ratio of 5.3. Analytical performance of the NA/PSS-TMFE yielded a detection limit of 5.5 nM (3ó), and the application of this modified electrode to an untreated polluted estuarine water sample produced significant improvements in the quality of the signal compared with that for a bare TMFE.
- Modeling of Cd uptake and efflux kinetics in metal-resistant bacterium cupriavidus metalliduransPublication . Hajdu, R.; Pinheiro, J. P.; Galceran, J.; Slaveykova, V. I.The Model of Uptake with Instantaneous Adsorption and Efflux, MUIAE, describing and predicting the overall Cd uptake by the metal-resistant bacterium Cupriavidus metallidurans CH34, is presented.MUIAEtakes into account different processes at the bacteria-medium interface with specific emphasis on the uptake and efflux kinetics and the decrease in bulk metal concentration. A single set of eight parameters provides a reasonable description of experimentally determined adsorbed and internalized Cd, as well as the evolution of dissolved Cd concentrations with time, for an initial Cd concentration between 10-8 and 10-4 M, covering the situation of contaminated environments and heavily polluted effluents. The same set of parameters allowed successful prediction of the internalized and adsorbed Cd as a function of the measured free Cd ion concentration in the presence of natural and anthropogenic ligands. The findings of the present study reveal the key role of Cd efflux and bulk depletion on the overall Cd uptake by C. metallidurans, and the need to account for these processes to understand and improve the efficiency of the metal removal from the contaminated environment.
- Effect of humic acid on Cd(II), Cu(II), and Pb(II) uptake by freshwater algae: kinetic and cell wall speciation considerationsPublication . Lamelas, C.; Pinheiro, J. P.; Slaveykova, V. I.The present study examines the effect of humic acid on the uptake kinetics of Cd(II), Cu(II), and Pb(II) by the freshwater alga Chlorella kesslerii. The results demonstrated that the relative proportion of Pb in the cell wall layer is greater than that of the internalized Pb, while internalized Cd and Cu were comparable or greater than the adsorbed metal concentration. In the presence of 10 mg L-1 humic acid (HA), Cd and Cu uptake kinetics were consistent with that predicted by measured free metal concentrations. For Pb, the uptake flux and amount of lead bound to internalization and adsorption sites were an order of magnitude higher than those found at the same free lead ion concentrations in the presence of citric acid. Chemodynamic modeling suggested that the enhancement of the Pb uptake flux in the presence of HA originates from an increasing amount of Pb bound to the internalization sites through a ternary complex formation between lead-humic acid complex and internalization sites. Cell wall speciation calculations indicated that the lead-humic acid complex is the predominant species in the cell wall layer, while for Cu(II) and Cd(II) metal bound to the internalization (Cu) and adsorption (Cd) sites significantly dominated over the M-HA complex. The findings of the work show the relevance of the cell wall layer concentration and speciation and its key role in defining the local equilibrium conditions between metal and internalizations sites. The results of the present kinetic study have important consequences for improvement of the mechanistic understanding of the role of dissolved organic matter in metal uptake in phytoplankton and biogeochemical cycling of metals in the surface waters.
- Ligand size polydispersity effect on SSCP signal interpretationPublication . Rocha, Luciana S.; Botero, Wander G.; Alves, Nuno G.; Moreira, José; Costa, Ana M. Rosa da; Pinheiro, J. P.The present study aims to establish unambiguously the conditions required for the validity of the average diffusion (D) approximation in fully labile systems with significant ligand size polydispersity. The average diffusion coefficient is a key parameter in mass transfer that affects signal interpretation in dynamic electroanalytical techniques. To achieve this goal, the binding of Cd(II) and Pb(II) to binary and ternary mixtures containing chemically homogenous (PSS)n-COOH polymers (ligand excess conditions were required) of different sizes (4, 10 and 30 KDa) was evaluated. It was experimentally evidenced that the average diffusion coefficient (D), can indeed be computed as the weighted average of several metal-polymer complexes of diverse sizes.
- Metal speciation dynamics in colloidal ligand dispersionsPublication . Pinheiro, J. P.; Minor, M.; Van Leeuwen, H. P.In this work we propose a dynamic metal speciation theory for colloidal systems in which the complexing ligands are localized on the surface of the particles; i.e., there is spatial heterogeneity of binding sites within the sample volume. The differences between the complex formation and dissociation rate constants of complexes in colloidal dispersions and those in homogeneous solutions originate from the differences in kinetic and mass transport conditions. In colloidal systems, when the effective rate of dissociation of the surface complexes becomes fully diffusion controlled, its value is defined via the geometrical parameters of the particle. We assess the extent to which the conventional approach of assuming a homogeneously smeared-out ligand distribution overestimates the lability of surface complexes in colloidal ligand dispersions. The validity of the theory is illustrated by application to binding of lead and cadmium by carboxyl modified latex particles: our approach correctly predicts the formation/dissociation rate constants, which differ by several orders of magnitude from their homogeneous solution counterparts.
- Electrochemical DNA sensor for detection of single nucleotide polymorphismsPublication . Marques, L. P. J.; Cavaco, Isabel Maria Palma Antunes; Pinheiro, J. P.; Ribeiro, Vera; Ferreira, GuilhermeIn recent years there has been an increased interest in using biosensors for the recognition and monitoring of molecule interactions. DNA sensors and gene chips are particularly relevant for directly applying the information gathered from the genome projects. In this work electrochemical techniques are used to develop methodologies to detect DNA polymorphisms in human genes using cytochrome P450 3A4 (CYP3A4) as a model gene. CYP3A4*1B oligonucleotides were immobilized on the surface of a gold electrode and hybridized with fully complementary oligonucleotide sequences as well as with mismatched sequences corresponding to the CYP3A4*1A reference sequence. The methodology developed is based on double-stranded DNA’s ability to transport charge along nucleotide stacking. The perturbation of the double helix pi-stack introduced by a mismatched nucleotide reduces electron flow and can be detected by measuring the attenuation of the charge transfer. The methodology developed could identify CYP3A4*1A homozygotes by the 5 μC charge attenuation observed when compared with DNA samples containing at least one CYP3A4*1B allele.
- Metal specification dynamics and bioavailability: bulk depletion effectsPublication . Pinheiro, J. P.; Galceran, J.; Van Leeuwen, H. P.Under conditions of bulk depletion, the speciation and bioavailability of trace metals must be considered at two different time scales: (i) the time scale of the biouptake flux, as determined by diffusion of the bioactive free metal, dissociation of the bioinactive complex species, and the internalization rate; and (ii) the time scale of depletion of the bulk medium. The implications of these two time scales for the speciation dynamics are discussed in terms of experimental conditions. The geometry of the system is taken into account via a spherical cellular model. It considers a spherical organism depleting a spherical volume in a nonstirred medium and assumes linear adsorption of the metal at the biointerface and first-order internalization kinetics. In cases where the rate of biouptake is fully controlled by the internalization step, concentration gradients in the medium are insignificant. Then the biouptake becomes independent of the geometry of the system, and the model has a much simpler solution. Examples of trace metal uptake by microorganisms are analyzed: (i) cobalt uptake by Prochlorococcus in the presence of NTA, under conditions where bulk depletion is the controlling process due to the large number of organisms and high internalization rates, (ii) silver uptake by Chlamydomonas reinhardtii with significant effects of bulk depletion, due to the high internalization rate; (iii) lead uptake by Chlorella vulgaris with pratically negligible bulk depletion due to the low internalization rate of the metal; and (iv) lead uptake by intestinal Caco-2 cells, illustrating the simplification of the bulk depletion model for a system with different geometry where internalization is the rate-controlling step.