Ana M. Baruzzi

Amperometric biosensor for direct blood lactate detection.

An amperometric sensor for lactate quantification is presented. The developed biosensor requires only 0.2 U of lactate oxidase, which is immobilized in a mucin/albumin hydrogel matrix. By protecting the platinum surface with a Nafion membrane, typical interference related to negatively charged species such as ascorbic acid has been minimized to practically undetectable levels. Electrochemical properties associated with the Nafion membrane are assessed as a function of Nafion concentration. In a phosphate buffer solution of pH 7.0, linear dependence of the catalytic current upon lactate bulk concentration was obtained between 2 and approximately 1000 microM. A detection limit of 0.8 microM can be calculated considering 3 times the standard deviation of the blank signal divided by the sensitivity of the sensor. The lactate biosensor presents remarkable operational stability and sensitivity (0.537 +/- 0.007) mA.M(-1), where the error is the standard deviation of the slope calculated from the linear regression of the calibration curve of a fresh biosensor. In this regard, the sensor keeps practically the same sensitivity for 5 months, while the linear range decreases until an upper value of 0.8 mM is reached. Assays performed with whole blood samples spiked with 100 microM lactate gave (89 +/- 6)% of recovery.

Interfacial phase transfer of amino-complexed Ni(II) cations: A model for solvent extraction of transition metals

Abstract The Gibbs energy of interfacial transfer of Ni(II) complexed with some selected substituted ethylenediamines which differ in hydrophobicity, is measured voltammetrically and compared with the corresponding Gibbs energy of transfer of Ni(II) complexed with aromatic diazines. In the case of the ethylenediamino and alkyl ethylenediamino complexes, there exists clear evidence that the fast formation of the tris complexes in the reaction layer of both phases enhances the extraction of Ni(II) into the organic phase. The transfer of the protonated and unprotonated diamines is also studied. The collected thermodynamic data allow the calculation of the stability constants of the complexes in the organic phase which are higher by five orders of magnitude than in the aqueous phase. This kinetic and thermodynamic effect would strongly facilitate the solvent extraction of transition metals under technical conditions.

Competitive transfer of H+ and Li+ ions through the water-1,2-dichloroethane interface mediated by 1,10-phenanthroline

Abstract The transfer of H + and Li + ions through the water-1,2-dichloroethane interface in the presence of 1,10-phenanthroline is studied by cyclic voltammetry. These ions compete for phenanthroline in the organic phase; the pH and the concentration of the ligand relative to that of the cation determine which predominates in the transfer. It is concluded that the transfer of H + mediated by phenanthroline occurs via a direct mechanism, whereas “facilitated” transfer is operative in the case of Li + .

Interfacial phenomena involving Fe(III)–ofloxacin complexes at the water|1,2-dichloroethane interface

Abstract When the interface formed between an aqueous solution containing ofloxacin, a fluorquinolone antibiotic complexed with Fe(III), and an organic solution of a supporting electrolyte in 1,2-dichloroethane is potentiodynamically polarized, interesting electroadsorption phenomena are observed. The narrow negative voltammetric peak observed at much lower potentials than those corresponding to the diffusional transfer of a triply charged species, the linear response of I p with v , the difference in the potentials of the positive and negative processes and the shift of this peak towards more negative potentials with sweep rate as coverage is increased, provide clear evidence of an irreversible adsorption–desorption process displaying attractive interaction between the adsorbate molecules at high coverage. These results are reinforced by the appearance of an important charge transfer resistance value in ac measurements.

Quantitative determination of erythromycin and its hydrolysis products by cyclic voltammetry at the interface between water and 1,2-dichloroethane

Abstract An electrochemical method for quantifying erythromycin (Er) salts and their hydrolysis products is described. Cyclic voltammetry at the water/1,2-dichloroethane interface in a four-electrode system was used. A linear relationship between Ip and the erythromycin concentration was obtained up to the solubility limit of the antibiotic. The various hydrolysis products which can be obtained in acid or basic solutions were characterized by their respective transfer potentials. Similar hydrolysis products were observed for aged solutions of Er lactobionate. It is concluded that the proposed method is suitable for the quantification of Er and for distinguishing it from its hydrolysis products.

Complex kinetic behaviour of “reduced” CO2 electro-oxidation at Pt electrodes

The electro-oxidation of “reduced” CO2 intermediate adsorbed at different adsorption times and potentials was studied by means of potentiodynamic oxidation profiles and current transients at constant potential. At potentials >0.75 V a Langmuir isotherm is obeyed, while a ·er isotherm is found at potentials <0.75 V. The kinetic behaviour of (CO2)r electro-oxidation can be explained through the existence of two intermediate species of similar chemical composition but different energy content. These species are related through an exchange equilibrium, assisted by the catalyst surface.

Electrochemical study of the interaction of alkali and alkaline-earth cations with a dibehenoyl phosphatidylcholine monolayer at the water/1,2-dichloroethane interface

Abstract The transfer of alkali and alkaline-earth cations across a dibehenoyl phosphatidyl choline (DBPC) monolayer is analysed by cyclic voltammetry and capacitance measurements. A cation adsorption process at the polar head of the phospholipid was evidenced. After the adsorption an enhancement of the current or a blocking of the transfer process was produced, depending on the nature and cation concentration. This blocking arises as a consequence of the packing induced by Li + or alkaline-earth cations adsorption at high concentrations which, in addition, is responsible for the destruction of the monolayer at high positive potentials. Capacitance measurements reinforce this statement. This paper also illustrates how electrochemical measurements at ITIES are a useful methodology for studying cation interactions with lipidic membranes and the transfer processes which occur across them.

Comparative analysis of the transfer of alkaline and alkaline-earth cations across the water/1,2-dichloroethane interface

The transfer of alkaline and alkaline-earth cations across the water/1,2-dichloroethane interface in presence of 1,10-phenanthroline (phen) in the organic phase was studied using cyclic voltammetry in a concentration range wide enough to cover the following experimental conditions: CM2+(w) ⪢ Cphen(o) and Cphen(o) ⪢ CM2+(w). The results obtained (when CM2+(w) ⪢ Cphen(o)) were compared with the theoretical voltammetric criteria reported by Homolka1l] for facilitated transfer. It appears that the transfer of all these cations occurs via a facilitated mechanism. The stoichiometry of all complexes formed in the organic phase is also reported. The stability constants for Ca2+-phen and Sr2+-phen complexes were estimated and compared with the tendency observed in the transfer potentials of these cations.

The influence of solution composition on the kinetics of “reduced” CO2 electrooxidation at polycrystalline platinum

Abstract The electrooxidationof “reduced” CO 2 intermediate adsorbed at polycrystalline Pt was studied for comparison in 0.5 M H 2 SO 4 , 0.05 M HClO 4 and 1 M KOH. The voltammograms showed a current peak multicity in 0.05 M HClO 4 and in 1 M KOH. Current transients in 0.05 M HClO 4 showed a complex decay at E o , between 0.6 and 0.8 V. In 1 M KOH these transients showed a minimum and a maximum which shift in the time scale with E ad . The kinetic interpretation of the results was attempted considering a Temkin isotherm at low positive potentials. Tafel slopes obtained from the dependence of E p vs. log υ are close to RT/F at high positive potentials. This value accounts for a chemical reaction as rds. Anion Adsorption at the polycrystalline Pt electrode would change the surface energy for Pt(OH) electroformation and hence it should influence the electrooxidation processes indirectly.

Adsorption of phenosafranin at the water ∣ 1,2 DCE interface: a voltammetric approach☆

Abstract In the present paper the interfacial behavior of a cationic dye, phenosafranin (PhS+), is voltammetrically analyzed. A sharp and narrow peak during the negative sweep was observed. The dependence of this peak on concentration, sweep rate and switch potential was a clear indication of an adsorption–desorption process similar to those observed on solid electrodes. It was determined that the adsorbed species results from the ion pairing between dicarbollylcobaltate (DCC−) and PhS+ at the interface as the process is observed only when the interface is polarized at the potential transfer of DCC−. The process fitted with a Frumkin isotherm and indicated a strong attractive interaction (g=−2.3) between the adsorbed species. This interaction was also analyzed spectrophotometrically.