Fernando Garay

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.

Square-wave voltammetry of quasi-reversible electrode processes with coupled homogeneous chemical reactions

Abstract The electrochemical behaviour of systems complicated by electrode kinetic and quasi-reversible preceding or following homogeneous chemical reactions under square-wave voltammetry (SWV) conditions is analysed theoretically. The results are discussed in detail, considering the influence of rate and equilibrium constants, together with experimentally controlled parameters such as f and E sw . Both kinetic stages act synergistically for the case of a CE mechanism, but the EC reaction scheme exhibits more complex behaviour, especially for reversible and quasi-reversible electrochemical reactions, which present a minimum response of current for the quasi-reversible chemical reactions. These curves are characteristic for each system, providing not only the bases for their distinction but also for the extraction of kinetic and thermodynamic information.

Square wave stripping voltammetry of Cd–oxine complexes; surface redox reactions

The effect of the reactant adsorption on square wave voltammetry of a 2-electron quasi-reversible redox reaction is analysed experimentally and theoretically. Current potential profiles and the characteristics of the peak curve and the peak potential were analysed at different pH and adsorption potentials for Cd–oxine complexes. The rate constants ks=6, 1.3 and 0.4 s−1 were obtained at pH 6.4, 7.8 and 11, respectively. Analytical applications were also considered.

Cathodic stripping square wave voltammetry of Cu(II)–oxine complexes. A mechanistic study

The reaction mechanism for the reduction of Cu(II)–oxine complexes on mercury electrodes is studied by cyclic voltammetry (CV) and square wave voltammetry (SWV). The quasi-reversible two-electron transfer is analysed from a theoretical point of view according to the EED and EDE mechanistic schemes based on an analysis of the Butler–Volmer formalism. The effects of SW parameters and solution composition are analysed. Fitting experimental data with theoretical curves allowed the evaluation of charge transfer rate constants, as well as discrimination between adsorbed or soluble species. In buffered solution at pH 7.7, a ks value of 0.8 s−1 was determined, for a one-step process involving adsorbed reactants and soluble products. In unbuffered solutions, a split in the redox wave is observed with ks values of 60 and 4 s−1 for each process, assigned to the reduction of CuOx+ and Cu(Ox)2, respectively.

Toward feedback-controlled anesthesia: Voltammetric measurement of propofol (2,6-diisopropylphenol) in serum-like electrolyte solutions

Propofol is a widely used, potent intravenous anesthetic for ambulatory anesthesia and long-term sedation. The target steady state concentration of propofol in blood is 0.25-10 μg/mL (1-60 μM). Although propofol can be oxidized electrochemically, monitoring its concentration in biological matrixes is very challenging due to (i) low therapeutic concentration, (ii) high concentrations of easily oxidizable interfering compounds in the sample, and (iii) fouling of the working electrode. In this work we report the performance characteristics of an organic film coated glassy carbon (GC) electrode for continuous monitoring of propofol. The organic film (a plasticized PVC membrane) improved the detection limit and the selectivity of the voltammetric sensor due to the large difference in hydrophobicity between the analyte (propofol) and interfering compounds of the sample, e.g., ascorbic acid (AA) or p-acetamidophenol (APAP). Furthermore, the membrane coating prevented electrode fouling and served as a protective barrier against electrode passivation by proteins. Studies revealed that sensitivity and selectivity of the voltammetric method is greatly influenced by the composition of the PVC membrane. The detection limit of the membrane-coated sensor for propofol in PBS is reported as 0.03 ± 0.01 μM. In serum-like electrolyte solutions containing physiologically relevant levels of albumin (5%) and 3 mM AA and 1 mM APAP as interfering agents, the detection limit was 0.5 ± 0.4 μM. Both values are below the target concentrations used clinically during anesthesia or sedation.

Quasi-reversible EC reactions at spherical microelectrodes analysed by square-wave voltammetry

Abstract A mathematical model for a quasi-reversible EC mechanism at the surface of spherical macro- and micro-electrodes is presented for the case of square-wave voltammetry. The analysis considers the influence of the chemical and the electrochemical kinetics as well as the contribution of sphericity to the voltammetric responses. The apparent electrode sphericity and the global apparent reversibility are jointly affected by the variation of frequency (f). If the electrode radius (ro) is smaller than 30 μm, but the sphericity contribution is neglected, it is not possible to obtain good kinetic information. For Esw>0.2 V the system is shifted far from the steady-state and the kchem value can be analysed regardless of the ks value. However, for Esw=50 mV, it is not possible to evaluate kchem if ks≤10−3 or K>1. Simple equations for the dependence of ΔΨp on f, ro and Esw have been provided. Nevertheless, the analysis of such complex electrochemical systems should require not only a study of these dependences, but also fitting of the theoretical voltammograms with experimental results to provide some useful mechanistic information.

Adsorptive square wave voltammetry of metal complexes. Effect of ligand concentration: Part II. Experimental applications☆

Abstract Theoretical models developed in Part I of this work for the square wave voltammetric response of inert metal complexes are applied to Cd(II)– and Cu(II)–8-OH-quinoline species adsorbed on mercury at pH 6.7. A wide range of experimental conditions were considered, taking into account the effect of ligand concentration. Cyclic voltammetry experiments were also performed for the purpose of comparison. Fitting between experimental and theoretical curves was excellent, assuming that the predominant species in bulk solution were CdOx + and Cu(Ox) 2 and that the electrochemical reaction mechanism involved ligand desorption after the reduction of both complexes. From the simulation procedure the following parameters were estimated: electrochemical rate constants k s,CdOx + = (7±1) s −1 and k s,Cu(Ox) 2 = (1.5±0.1) s −1 ; charge transfer coefficients α CdOx + =0.51±0.01 and α Cu(Ox) 2 =0.48±0.01; adsorption constants for the electroactive species K ad,CdOx =(4.5±0 .5) × 10 −3 cm and K ad,CuOx =(8±2)×10 −3 cm . Surface concentrations Γ M were also evaluated, observing that, for the ligand to complex ratios analysed, Γ CdOx increased steadily with oxine concentration whereas Γ CuOx remained constant. Local ligand concentrations close to the electrode were also evaluated.

Adsorptive square wave voltammetry of metal complexes. Effect of ligand concentration: Part I. Theory

Abstract The electrochemical behaviour of non-labile metallic complexes under square wave voltammetry (SWV) conditions is analysed theoretically, considering the influence of ligand adsorption–desorption processes as well as the ligand concentration on the quasi-reversible redox reaction mechanism. The dependence of current and peak potentials on the transfer reaction rate and on complex stoichiometry is considered. Voltammetric responses for processes in which the ligand is desorbed or remains adsorbed after the electroreduction process are compared. Diagnosis criteria for the selection of optimum SWV parameters in view of analytical applications are presented for each case.

Adsorptive square-wave voltammetry applied to study the reduction mechanism of Cu–sulfoxine and Cu–ferron complexes

The reduction mechanisms of Cu(II) in solutions with the quinoline derivatives sulfoxine (Sox) and ferron (Fer) are studied by the simulation of their experimental square-wave voltammetric curves. Sox and Fer form very stable complexes with Cu(II) that can be accumulated on mercury electrodes by adsorption. Cathodic linear scan stripping voltammetry was performed for comparison purposes. The reaction schemes consider the reduction of an adsorbed oxidized complex with a ligand/metal stoichiometric ratio higher than that of the product. The remaining ligand may remain adsorbed or be released in solution. The fits between experimental and theoretical curves reveal the prevalence of the adsorbed species Cu(Sox)22− and Cu(Fer)22− at pH 6.4 and 5.3, respectively, which are the same as predicted by the distribution species diagram for the bulk of the solution. At these pH values, Cu(Sox)22− and Cu(Fer)22− complexes present essentially the same Ep and electrochemical rate constants ks=(1.3±0.1) s−1, and thus the same effective stability constants. The dependence of differential peak currents on the logarithm of the frequency describes the so-called quasi-reversible maximum (fmax). The frequency of this maximum is lower for solutions with high pH, indicating that the charge transfer rate is decreased when the stability of the complexes is enlarged. The fmax value is related to the ligand/complex concentration ratio and it is applied to the estimation of ks. Changes at the surface concentrations were evaluated considering the ligand to complex ratios at the electrode surface and at the solution bulk.

Adsorptive square wave voltammetry of metal complexes. Effect of ligand concentration.: Part IV. Experimental applications on Hg–ferron complexes

Abstract The theoretical models developed in Part III of this work are employed for the study of the oxidative adsorption of ferron (7-iodo-8-hydroxyquinolin-5-sulphonic acid) on mercury under square wave voltammetric conditions. The fits between experimental and theoretical results indicated that the electrochemical reaction involves the direct transfer of two electrons in solutions with pH