José Manuel Díaz-Cruz

Evaluation of a highly sensitive amperometric biosensor with low cholinesterase charge immobilized on a chemically modified carbon paste electrode for trace determination of carbamates in fruit, vegetable and water samples

A highly sensitive amperometric biosensor for determination of carbamate pesticides directly in water, fruit and vegetable samples has been evaluated, electrochemically characterized and optimized. The biosensor strip was fabricated in screen printed technique on a ceramic support using silver-based paste for reference electrode, and platinum-based paste for working and auxiliary electrodes. The working electrode was modified by a layer of carbon paste mixed with cobalt(II) phthalocyanine and acetylcellulose. Cholinesterase (ChE) enzymes with low enzymatic charge were immobilized on this layer. The operational simplicity of the biosensor consists in that a small drop (∼50 μl) of substrate or sample is deposited on a horizontally positioned biosensor strip representing the microelectrochemical cell. The working potential of the biosensor was 370 mV versus Ag/AgI on a strip reference electrode preventing the interference of electroactive species which are oxidable at more positive potentials. The biosensor was applied to investigate the degradation of two reference ChE inhibitors in freeze dried water under different storage conditions and for direct determination of some N-methylcarbamates (NMCs) in fruit and vegetable samples at ppb concentration levels without any sample pretreatment. A comparison of the obtained results for the total carbamate concentration was done against those obtained using HPLC measurements.

Antimony film screen-printed carbon electrode for stripping analysis of Cd(II), Pb(II), and Cu(II) in natural samples.

An in-situ antimony film screen-printed carbon electrode (in-situ SbSPCE) was successfully used for the determination of Cu(II) simultaneously with Cd(II) and Pb(II) ions, by means of differential pulse anodic stripping voltammetry (DPASV), in a certified reference groundwater sample with a very high reproducibility and good trueness. This electrode is proposed as a valuable alternative to in-situ bismuth film electrodes, since no competition between the electrodeposited copper and antimony for surface sites was noticed. In-situ SbSPCE was microscopically characterized and experimental parameters such as deposition potential, accumulation time and pH were optimized. The best voltammetric response for the simultaneous determination of Cd(II), Pb(II) and Cu(II) ions was achieved when deposition potential was -1.2 V, accumulation time 120 s and pH 4.5. The detection and quantification limits at levels of μg L(-1) suggest that the in-situ SbSPCE could be fully suitable for the determination of Cd(II), Pb(II) and Cu(II) ions in natural samples.

Differential pulse voltammetric study of the complexation of Cd(II) by the phytochelatin (γ-GluCys)2Gly assisted by multivariate curve resolution

Abstract A multivariate curve resolution method by alternating least-squares (MCR-ALS) is applied to differential pulse voltammograms measured on the Cd(II)+(γ-GluCys) 2 Gly system as a model of metal–phytochelatin interactions at concentrations of both components in the range 10 −7 –10 −5 mol l −1 . The course of complexation is different when peptide is titrated with metal from that when metal is titrated with peptide. The combined analysis of both matrices from titrations of peptide with metal and of metal with peptide allowed the resolution of the system. The analysis of the resulting pure voltammograms and concentration profiles of the resolved components suggested the presence of four different types of bound Cd(II) and made possible the formulation of a complexation model.

Induced reactant adsorption in metal—polyelectrolyte systems: pulse polarographic study

Abstract A theoretical model is developed for describing the reduction, by normal-pulse polarography, of a metal ion in the presence of a macromolecular ligand, including both the ligand adsorption and the induced adsorption of the metal ion. The following basic assumptions are made: reversible charge transfer at a stationary planar electrode (static mecury drop electrode); labile complex; large excess of ligand compared with the total metal concentration; Langmuirian adsorption for both the ligand and complex species; and diffusion coefficients for ligand and complex species very different from that of the free metal. The mathematical approach is based on transforming the system of differential equations with their boundary conditions into an integral equation which has been solved numerically. A simple procedure to obtain stability constants and adsorption parameters simultaneously from the experimental data is described. This model has been used to reproduce some experimental data from the Cd(II)—polymethacrylate system.

Heavy Metal Binding by Tannic Acid: A Voltammetric Study

The interactions between tannic acid (TA) and the heavy metal ions Cu(II), Pb(II), Cd(II), and Zn(II), have been studied by several voltammetric techniques as a model for the metal binding of tannin substances and also to evaluate the possible effects of such compounds on the voltammetric analysis of natural samples. The study has shown quite a different behavior of the systems considered: Cd(II) is practically not complexed by TA, whereas Pb(II) and Zn(II) produce labile complexes and Cu(II) seems to form an inert complex with TA. In the cases of the Zn(II)+TA and Pb(II)+TA systems, it was possible to give a rough estimation of the stability constants and the relative diffusion coefficients of the complexes. From these values, it appears that the affinity for TA increases in the order Cd(II)<Zn(II)<Pb(II). In contrast, the possible stabilization of Cu(I) species during the voltammetric measurement hindered a quantitative study of the complexation of Cu(II) by TA.

Thermodynamics of Cd2+ and Zn2+ binding by the phytochelatin (γ-Glu-Cys)4-Gly and its precursor glutathione

Isothermal titration calorimetry (ITC) was used to study the binding of Cd(2+) and Zn(2+) by glutathione (GSH) and phytochelatins (PC(n)), the metal sequestering compounds in plants and algae. The results are compared with those obtained by differential pulse polarography (DPP) assisted by multivariate curve resolution with alternating least squares (MCR-ALS) and by electrospray ionization mass spectrometry (ESI-MS). ITC allows one to determine (i) the stoichiometries of the different complexes (and confirms those found by DPP/MCR-ALS and ESI-MS) and (ii) their binding and thermodynamic parameters. For Cd-PC(4), the sequential binding sites model with two identical sites yields the best fitting of ITC curves and confirms the presence of CdPC(4) and Cd(2)PC(4) complexes. For Zn-PC(4), exothermic formation of ZnPC(4) is reported. Conditional stability and formation constants for Cd-GSH and Zn-GSH are determined from the fitting of the proper model to experimental ITC curves. The effect of different buffers in the complexation processes shows the key role of the choice of the buffer in calorimetric study.

Sputtered bismuth screen-printed electrode: a promising alternative to other bismuth modifications in the voltammetric determination of Cd(II) and Pb(II) ions in groundwater.

A commercially available sputtered bismuth screen-printed electrode (BispSPE) has been pioneeringly applied for the simultaneous determination of Cd(II) and Pb(II) ions in a certified groundwater sample by means of differential pulse anodic stripping voltammetry (DPASV) as an alternative to more conventional bismuth screen-printed carbon electrodes (BiSPCEs). BispSPEs can be used for a large set of measurements without any previous plating or activation. The obtained detection and quantification limits suggest that BispSPEs produce a better analytical performance as compared to In-situ BiSPCE for Pb(II) and Cd(II) determination, but also to Ex-situ BiSPCE for Cd(II) determination. The results confirm the applicability of these devices for the determination of low level concentrations of these metal ions in natural samples with very high reproducibility (0.7% and 2.5% for Pb(II) and Cd(II) respectively), and good trueness (0.3% and 2.4% for Pb(II) and Cd(II) respectively).

Differential pulse polarographic study of the Pb(II) complexation by glutathione

Differential pulse polarography (DPP) is applied to the study of the complexes of Pb(II) with glutathione (GSH). As occurred in previous studies on Cd(II)– and Zn(II)–GSH complexes, the system presented overlapping of the peaks of free metal and metal complexes but, additionally, the interference of the anodic signals of mercury in the presence of GSH. This necessitated, even more than in previous work, the use of a multivariate curve resolution method (MCR-ALS) to extract quantitative information from the system. Despite the important interference of anodic signals, the combined use of DPP and MCR-ALS suggested the presence of two kinds of Pb(II)–GSH bonds and allowed the prediction of stoichiometries for the complexes formed.

Protolytic control in stripping voltammetric titrations of metal polyacid complexes

A systematic study was made of the influence of variations of pH and of other protolytic parameters on the voltammetric response of metal—polyacid systems. The complexation of metal ions at the 10−6 mol 1−1 concentration level was measured by different procedures, including control of pH and of the degree of dissociation (αd) of the polyacid. The results show that, in order to obtain realistic values of the formation constant of the metal—polyacid complexes, voltammetric titrations must be done under conditions of constant αd. A procedure involving the adjustment of the pH of the metal solution to the same value as that of the polyacid solution was found to yield a practically useful method for maintaining constant complexation conditions (constant αd). Experimental evidence was obtained by means of different-pulse anodic stripping voltammetry for the model system Zn(II)-polymethacrylic acid.

Glutathione modified screen-printed carbon nanofiber electrode for the voltammetric determination of metal ions in natural samples.

This work reports the development of a glutathione modified electrode via electrografting on a screen-printed carbon nanofiber substrate (GSH-SPCNFE). GSH-SPCNFE was compared to a classical screen-printed carbon electrode modified with glutathione (GSH-SPCE) for the simultaneous voltammetric determination of Cd(II) and Pb(II). Their electrochemical characterization and analytical performance suggest that SPCNFE could be a much better support for GSH immobilization. The applicability of GSH-SPCNFE for the determination of low concentration levels of Pb(II) and Cd(II) ions in environmental samples was successfully tested in a certified wastewater reference material by means of stripping voltammetry with a very high reproducibility and good trueness.