Héctor Fernández

Solvent effects on the kinetics of heterogeneous electron transfer processes. The TMPD/TMPD·+ redox couple

Abstract Solvent effects on the heterogeneous electron exchange between N , N , N ′, N ′-tetramethyl- p -phenylendiamine (TMPD) and its monocation radical (TMPD ·+ ) have been studied by chronocoulometry at a platinum electrode in perchlorate solutions in a wide range of aprotic and hydrogen-bonded solvents. Formal potentials, diffusion coefficients, anodic transfer coefficients and formal rate constants have been evaluated. The variation of the formal potential with the solvent can be interpreted by determining the solvent—solvent—solute interactions using Taft theory. Correlations between predicted and experimental formal potentials for the Taft model were an improvement over those obtained using the Gutmann donor number approach. A weak correlation between the anodic transfer coefficient and the Taft β parameter was also found. The variations in the rate constants were analyzed on the basis of current electron-transfer models using a statistical method to separate the effects of the longitudinal relaxation time τ L from those of the solvent permittivity parameter γ. It has been shown that the dynamics of solvent relaxation affect the heterogeneous electron-transfer rate in this case, but the rate constants depend on τ −0.53 instead of τ −1 , as theory predicts. The degree of reaction adiabaticity and the relative sizes of the inner- and outer-sphere components of the Gibbs energy of activation were considered.

Synthetic antioxidants determination in lard and vegetable oils by the use of voltammetric methods on disk ultramicroelectrodes

Abstract Voltammetry on ultramicroelectrodes was used to determine antioxidants tert-butylhydroxyanisole (BHA), tert-butylhydroxytoluene (BHT) and propyl gallate (PG) in vegetable oils or lard. The present work demonstrates the advantages of using not only the conventional three electrode configuration but also the non-conventional two electrode configuration cell system, with respect to the determination of antioxidants. Different ways of performing the determinations are compared either through step by step extractive procedures or through direct measurements using standard addition. Detection limits in the range of (0.20–1.25)×10−5 mol dm−3, (1.00–1.25)×10−5 mol dm−3 and (0.60–1.00)×10−5 mol were obtained for BHA and BHT in vegatable oil and PG in lard determinations, respectively, depending upon the medium composition. Relative standard deviations for antioxidants concentration were about 6–7% for most of media studied and can be compared with those reported for HPLC from collaborative studies, i.e. 4.54, 6.61 and 3.18% for BHT, BHA and PG, respectively.

Development of a highly sensitive noncompetitive electrochemical immunosensor for the detection of atrazine by phage anti-immunocomplex assay.

The development of immunosensors for the detection of small molecules is of great interest because of their simplicity, high sensitivity and extended analytical range. Due to their size, small compounds cannot be simultaneously recognized by two antibodies impeding their detection by noncompetitive two-site immunoassays, which are superior to competitive ones in terms of sensitivity, kinetics, and working range. In this work, we combine the advantages of magneto-electrochemical immunosensors with the improved sensitivity and direct proportional signal of noncompetitive immunoassays to develop a new Phage Anti-Immunocomplex Electrochemical Immunosensor (PhAIEI) for the detection of the herbicide atrazine. The noncompetitive assay is based on the use of recombinant M13 phage particles bearing a peptide that specifically recognizes the immunocomplex of atrazine with an anti-atrazine monoclonal antibody. The PhAIEI performed with a limit of detection (LOD) of 0.2 pg mL(-1), which is 200-fold better than the LOD obtained using the same antibody in an optimized conventional competitive ELISA, with a large increase in working range. The developed PhAIEI was successfully used to assay undiluted river water samples with no pretreatment and excellent recoveries. Apart from the first demonstration of the benefits of integrating phage anti-immunocomplex particles into electrochemical immunosensors, the extremely low and environmentally relevant detection limits of atrazine attained with the PhAIEIS may have direct applicability to fast and sensitive detection of this herbicide in the environment.

Handling time misalignment and rank deficiency in liquid chromatography by multivariate curve resolution: Quantitation of five biogenic amines in fish

Biogenic amines (BAs) are used for identifying spoilage in food. The most common are tryptamine (TRY), 2-phenylethylamine (PHE), putrescine (PUT), cadaverine (CAD) and histamine (HIS). Due to lack of chromophores, chemical derivatization with dansyl was employed to analyze these BAs using high performance liquid chromatography with a diode array detector (HPLC-DAD). However, the derivatization reaction occurs with any primary or secondary amine, leading to co-elution of analytes and interferents with identical spectral profiles, and thus causing rank deficiency. When the spectral profile is the same and peak misalignment is present on the chromatographic runs, it is not possible to handle the data only with Multivariate Curve Resolution and Alternative Least Square (MCR-ALS), by augmenting the time, or the spectral mode. A way to circumvent this drawback is to receive information from another detector that leads to a selective profile for the analyte. To overcome both problems, (tri-linearity break in time, and spectral mode), this paper proposes a new analytical methodology for fast quantitation of these BAs in fish with HPLC-DAD by using the icoshift algorithm for temporal misalignment correction before MCR-ALS spectral mode augmented treatment. Limits of detection, relative errors of prediction (REP) and average recoveries, ranging from 0.14 to 0.50 µg mL(-1), 3.5-8.8% and 88.08%-99.68%, respectively. These are outstanding results obtained, reaching quantification limits for the five BAs much lower than those established by the Food and Agriculture Organization of the United Nations and World Health Organization (FAO/WHO), and the European Food Safety Authority (EFSA), all without any pre-concentration steps. The concentrations of BAs in fish samples ranged from 7.82 to 29.41 µg g(-1), 8.68-25.95 µg g(-1), 4.76-28.54 µg g(-1), 5.18-39.95 µg g(-1) and 1.45-52.62 µg g(-1) for TRY, PHE, PUT, CAD, and HIS, respectively. In addition, the proposed method spends less than 4 min in an isocratic run, consuming less solvent in accordance with the principles of green analytical chemistry.

A study of the bielectronic electro-reduction of cercosporin phytotoxin in highly acidic non-aqueous medium

The kinetics of the electro-reduction of the cercosporin phytotoxin in 1 M HClO4+ACN is analyzed on the basis of the theory presented for the nine-member square scheme when protonations are assumed to be at equilibrium. Experimental results obtained fit fairly well the theoretical model proposed by Laviron for 2e−, 2H+ reactions. The formal heterogeneous rate constant, the voltammetric half wave potential and the cathodic transfer coefficient for the overall electrode process were determined from a fitting procedure of experimental square wave voltammograms by employing the COOL algorithm. Average values of 0.184 V, 0.45 and 0.019 cm s−1 were calculated for the half wave potential, the cathodic transfer coefficient and the formal heterogeneous rate constant, respectively. The convolution analysis of cyclic voltammograms has been used to obtain the individual heterogeneous rate constants of the separate one-electron processes. Average values calculated were 0.013 and 0.008 cm s−1, respectively. However, a complete description of the redox behaviour of cercosporin through the nine-member square scheme could not be achieved due to the lack of some thermodynamic parameters. An average value of 8×10−6 cm2 s−1 was obtained for the diffusion coefficient of cercosporin from both chronocoulometry and convolution voltammetry measurements. Square wave voltammetry was also used to generate Ip versus c*cer calibration curves for this fungal metabolite. Detection limits of 5.8×10−7 and 2.8×10−7 M could be determined theoretically from calibration curves performed at 40 and 100 Hz, respectively, while minimal concentrations in the range of 1.9 to 3.8×10−6 M could be measured experimentally by the same technique at those frequencies.

Electrochemical and spectroscopic studies of the TMPD/TMPD.+ redox couple in non-aqueous binary solvent mixtures by using ultramicroelectrodes. Role of the preferential solvation phenomenon

Abstract Thermodynamic and diffusion parameters for the electro-oxidation of the TMPD/ TMPD .+ redox couple (TMPD: N , N , N ′, N ′-tetramethyl p -phenylenediamine) on Pt disk ultramicroelectrodes were determined in mixtures of acetonitrile with different aprotic and protic solvents: dimethylsulfoxide, dimethylformamide, toluene, methanol, ethanol, propan-2-ol and n -butanol. The dependence of half-wave potentials and diffusion coefficients of the electroactive species on the mixture composition was analyzed. A preferential solvation degree was detected and quantified from the variation of the frequency of the maximum of the uv -visible absorption bands of TMPD and TMPD .+ as well as from the solvodynamic radius of TMPD with the composition of the solvent mixture. Viscosities of the mixtures were measured and half-wave potentials and maximum uv -band frequencies in pure solvents were also analyzed for comparison. The behaviour of the different systems has been explained in terms of both non-specific dipolar and specific interactions, as the most important ones to be considered among the components of the redox couple and the solvents of the mixtures.

Electrochemical magneto immunosensor based on endogenous β-galactosidase enzyme to determine enterotoxicogenic Escherichia coli F4 (K88) in swine feces using square wave voltammetry

Diseases caused by enterotoxicogenic Escherichia coli F4 (K88) (ETEC F4) are a problem in swine production establishments. Due to the high rate of mortality and morbidity of E. coli infections, a rapid and accurate diagnosis is important in order to choose an appropriate treatment to reduce the economic impact. Therefore, an electrochemical magneto-immunosensor (EMI) was developed to detect and quantify ETEC F4 in swine feces samples through a direct non-competitive immunoassay. ETEC F4 was selectively captured by immunomagnetic separation. The detection principle was based on the activity of β-galactosidase endogenous enzyme (β-gal), which hydrolyses the p-aminophenyl-β-D-galactopyranoside (p-APG) producing p-aminophenol (p-AP), which was oxidized on a carbon screen printed electrode (CSPE) using square wave voltammetry (SWV). All parameters related to construction and electrochemical responses were optimized. The total analysis time to quantify ETEC F4 using the EMI was less than 2h and the limit of detection (LOD) was 33CFUmL-1. The perceptual relative error (%Er) was 20%. The magneto-immunosensor was validated versus conventional method of culture and plate count, obtaining a very good agreement. The EMI is simple, fast and economical to detect and quantify ETEC F4 in swine feces samples, being thus a valuable tool in swine production.

The electrochemical behaviour of the altenuene mycotoxin and its acidic properties

The electrooxidation of altenuene (ALT), one of the mycotoxins of the Alternaria alternata genus, on a glassy carbon disk electrode is studied for the first time by using cyclic and square wave voltammetry. From the electrochemical responses, a complex reaction mechanism could be inferred. Values of 1.06 × 10 − 5 cm 2 s − 1 , 1.116 V and 2 were determined for the diffusion coefficient, the apparent formal potential and the electron number, respectively, for the overall electrode process by convolution analysis of linear scan voltammograms. Square wave voltammetry was used to generate Ip versus c ALT calibration curves for this fungal metabolite. A detection limit of 4.0 ×10 − 7 M was determined for a 2:1 signal to noise ratio. The acid dissociation constant for ALT was determined from conventional UV–vis spectrophotometric measurements. Experimental variations of absorbance as a function of pH at a given wavelength were fitted by using the exact equation that describes the system. Good agreement between the experimental absorbance versus pH plots and the curves generated by the fitting process was found.

Determination of kinetic parameters of the enzymatic reaction between soybean peroxidase and natural antioxidants using chemometric tools

The oxidation of eugenol, isoeugenol and vanillin natural antioxidants catalyzed by the soybean peroxidase enzyme was studied using uv-vis spectroscopy. An experimental design was used to optimize the different variables. The multivariate curve resolution method was used to obtain the profiles of antioxidant absorbances as a function of time due to uv-vis absorption bands of both antioxidants and the enzymatic reaction product/s show a strong overlap. From these results, apparent Michaelis-Menten constants as well as the kinetic parameters k1 and k3 involved in the catalytic cycle of peroxidases were calculated. The antioxidant apparent acidity constants were also determined at different pHs from uv-vis spectrophotometric measurements. Values of k1 were (0.6 ± 0.1) × 105 M-1 s-1, (2.0 ± 0.2) × 105 M-1 s-1 and (7.0 ± 0.5) × 106 M-1 s-1 and k3 (4.0 ± 0.2) × 105 M-1 s-1, (6.0 ± 0.6) × 105 M-1 s-1 and (6.0 ± 0.9) × 106 M-1 s-1 for eugenol, isoeugenol and vanillin, respectively.

Development of an electrochemical biosensor for the determination of triglycerides in serum samples based on a lipase/magnetite-chitosan/copper oxide nanoparticles/multiwalled carbon nanotubes/pectin composite

A very sensitive electrochemical biosensor to determine totals triglycerides (TGs) in serum samples has been developed. It is based on the electrochemical oxidation of glycerol at glassy carbon electrodes modified with magnetic nanoparticles bonded to lipase enzyme and copper oxide nanoparticles, both supported on a multiwalled carbon nanotubes/pectin dispersion. Glycerol is produced by enzymatic reaction between the TGs present in samples and the lipase immobilized. The quantification of triglycerides was performed by amperometric measurements. The proposed electrochemical biosensor improves the performance of others methods developed for the TGs quantification. The determination of TGs does not need a pretreatment of serum samples. The PLS-1 algorithm was used for the quantification of TGs. According to this algorithm, the of detection and quantification limits were from 3.2 × 10-3 g L-1 to 3.6 × 10-3 g L-1, and from 9.6 × 10-3 to 1.1 × 10-2 g L-1, respectively. The sensitivity was 1.64 × 10-6 A L g-1. The proposed electrochemical biosensor exhibited a very good performance, a stability of 20 days, very good reproducibility and repeatability, and it is presented as a very good alternative for the determination of TGs in human serum clinical samples.