A. González-Cortés

Electrochemical detection of phenolic estrogenic compounds at carbon nanotube-modified electrodes

The use of a carbon nanotube-modified glassy carbon electrode (CNT-GCE) for the LC-EC detection of phenolic compounds with estrogenic activity is reported. Cyclic voltammograms for phenolic endocrine disruptors and estrogenic hormones showed, in general, an enhancement of their electrochemical oxidation responses at CNT-GCE attributable to the electrocatalytic effect caused by CNTs. Hydrodynamic voltammograms obtained under flow injection conditions lead to the selection of +700mV as the potential value to be applied for the amperometric detection of the phenolic estrogenic compounds, this value being remarkably less positive than those reported in the literature using other electrode materials. Successive injections of these compounds demonstrated that no electrode surface fouling occurred. A mobile phase consisting of a 50:50 (v/v) acetonitrile:0.05moll(-1) phosphate buffer of pH 7.0 was selected for the chromatographic separation of mixtures of these compounds, with detection limits ranging between 98 and 340nmoll(-1). Good recoveries were obtained in the analysis of underground well water and tap water samples spiked with some phenolic estrogenic compounds at a 14nmoll(-1) concentration level.

An electrochemical immunosensor for testosterone using functionalized magnetic beads and screen-printed carbon electrodes

A disposable electrochemical immunosensor using screen-printed carbon electrodes (SPCEs) and protein A-functionalized magnetic beads (MBs) was developed for the determination of testosterone. Anti-testosterone was immobilized onto MBs and a direct competitive immunoassay involving testosterone labeled with peroxidase (HRP) was performed. The resulting conjugate was trapped on the SPCE with a small magnet. Testosterone determination was carried out by amperometry at -0.2V upon H2O2 additions using hydroquinone (HQ) as the redox mediator. The experimental variables involved in the immunosensor response to testosterone were evaluated. Under the optimized conditions, a calibration plot for testosterone was obtained with a linear range between 5.0×10(-3) and 50 ng/mL (r=0.995). The detection limit was 1.7 pg/mL and the EC50 was 0.25±0.04 ng/mL. These characteristics are notably better than those achieved with other reported immunosensors. Furthermore, anti-testosterone/MBs conjugates were shown to be stable for at least 25 days. A good selectivity was also found against other steroid hormones. The usefulness of the immunosensor was demonstrated by analyzing human serum spiked with 1 and 10 ng/mL testosterone.

Ultrasensitive detection of adrenocorticotropin hormone (ACTH) using disposable phenylboronic-modified electrochemical immunosensors.

This work reports for the first time an electrochemical immunosensor for the determination of adrenocorticotropin hormone (ACTH). The immunoelectrode design involves the use of amino phenylboronic acid for the oriented immobilization of anti-ACTH antibodies onto screen-printed carbon modified electrode surfaces. A competitive immunoassay between the antigen and the biotinylated hormone for the binding sites of the immobilized antibody was performed. The electroanalytical response was generated by using alkaline phosphatase-labelled streptavidin and 1-naphtyl phosphate as the enzyme substrate. The electrochemical oxidation of the enzyme reaction product, 1-naphtol, measured by differential pulse voltammetry was employed to monitor the affinity reaction. Under the optimized working conditions, an extremely low detection limit of 18 pg/L was obtained. Cross-reactivity was evaluated against other hormones (cortisol, estradiol, testosterone, progesterone, hGH and prolactin) and the obtained results demonstrated an excellent selectivity. The developed immunosensor was applied to a human serum sample containing a certified amount of ACTH with good results.

Electrochemical immunosensor for rapid and sensitive determination of estradiol

This work describes the preparation of an electrochemical immunosensor for estradiol based on the surface modification of a screen printed carbon electrode with grafted p-aminobenzoic acid followed by covalent binding of streptavidin (Strept) and immobilization of biotinylated anti-estradiol (anti-estradiol-Biotin). The hormone determination was performed by applying a competitive immunoassay with peroxidase-labelled estradiol (HRP-estradiol) and measurement of the amperometric response at -200 mV using hydroquinone (HQ) as redox mediator. The calibration curve for estradiol exhibited a linear range between 1 and 250 pg mL(-1) (r=0.990) and a detection limit of 0.77 pg mL(-1) was achieved. Cross-reactivity studies with other hormones related with estradiol at physiological concentration levels revealed the practical specificity of the developed method for estradiol. A good reproducibility, with RSD=5.9% (n=8) was also observed. The operating stability of a single bioelectrode modified with anti-estradiol-Biotin-Strept was nine days when it was stored at 8°C under humid conditions between measurements. The developed immunosensor was applied to the analysis of certified serum and spiked urine samples with good results.

Synthesis, electrochemistry and photophysical properties of phenylenevinylene fullerodendrimers

Abstract The synthesis, spectroscopic characterization, electrochemistry and photophysical measurements of new phenylenevinylene fullerodendrimers are described. Cyclic and Osteryoung square wave voltammetry studies indicate that prepared fullerodendrimers show improved electron affinity over the parent C 60 . Photophysical investigations reveal a strong interaction between the dendrimers and C 60 .

Microcylinder Polymer Modified Electrodes as Amperometric Detectors for Liquid Chromatographic Analysis of Catecholamines

The use of cylindrical carbon fiber microelectrodes (CFMEs) of various mm in length modified with poly(3-methylthiophene) (P3MT) films as amperometric microsensors for the LC-EC detection of catecholamines is reported. Cyclic voltammetry at the modified microelectrodes of dopamine, epinephrine, catechol, serotonin and ascorbic acid showed electrocatalytic ability of the modified surface. This allowed the application of low potentials (a0.35 V) for the amperometric detection of the above-mentioned compounds. Flow injection measurements carried out using a 2:98 v=v methanol:0.025 mol L ˇ1 H2PO4 ˇ =HPO4 2ˇ solution of pH 7.0, showed no evidence of fouling of the modified microelectrode surface. No cleaning or regeneration treatment was needed when working with the same polymer modified microelectrode during the whole working day. Moreover, RSD values for ip lower than 5 % were obtained when amperometric measurements were carried out with three different P3MT-coated CFMEs. These modified microelectrodes have also shown to be suitable for amperometric detection in HPLC. A mobile phase composed of 2:98 % methanol:phosphate buffer solution of pH 7.0 allowed a good separation of mixtures of ascorbic acid, epinephrine, dopamine, serotonin and catechol with detection limits ranging between 14 and 48 pmol. Furthermore, as an application, the determination of those compounds in spiked serum samples, was accomplished with good results.

Analytical performance of cylindrical carbon fiber microelectrodes in low-permitivity organic solvents: determination of vanillin in ethyl acetate

The electrochemical behavior of cylindrical carbon fiber microelectrodes (CFMEs), 8 mm in length, has been tested in lowpermitivity organic solvents such as toluene and ethyl acetate. Near-to-ideal steady-state responses were obtained by cyclic voltammetry for ferrocene solutions in these media, using low potential scan rates and a relatively high concentration of background electrolyte. A good adjustment of the voltammetric responses to that predicted by the theory was found in both solvents. Well-defined oxidation peaks were also obtained by differential-pulse and square wave voltammetry. Square-wave voltammetry (SWV) in ethyl acetate has been used to develop a method for the determination of the food additive vanillin. A linear calibration graph was obtained in the 1.010 ˇ5 to 7.010 ˇ4 mol l ˇ1 concentration range with a slope of (7.10.1)10 3 mA l mol ˇ1 . A limit of detection of 4.210 ˇ6 mol l ˇ1 vanillin, and a relative standard deviation of 2.0% for a concentration level of 5.010 ˇ4 mol l ˇ1 (na10) were obtained. The SWV method was applied to the determination of vanillin in dehydrated pudding powder. This determination was performed directly in the samples extract in ethyl acetate. The obtained results were statistically compared with those provided by a reference spectrophotometric method, and no significant differences between both methods were found. # 1999 Elsevier Science B.V. All rights reserved.

A disposable electrochemical immunosensor for prolactin involving affinity reaction on streptavidin-functionalized magnetic particles

A novel electrochemical immunosensor was developed for the determination of the hormone prolactin. The design involved the use of screen-printed carbon electrodes and streptavidin-functionalized magnetic particles. Biotinylated anti-prolactin antibodies were immobilized onto the functionalized magnetic particles and a sandwich-type immunoassay involving prolactin and anti-prolactin antibody labelled with alkaline phosphatase was employed. The resulting bio-conjugate was trapped on the surface of the screen-printed electrode with a small magnet and prolactin quantification was accomplished by differential pulse voltammetry of 1-naphtol formed in the enzyme reaction using 1-naphtyl phosphate as alkaline phosphatase substrate. All variables involved in the preparation of the immunosensor and in the electrochemical detection step were optimized. The calibration plot for prolactin exhibited a linear range between 10 and 2000 ng mL(-1) with a slope value of 7.0 nA mL ng(-1). The limit of detection was 3.74 ng mL(-1). Furthermore, the modified magnetic beads-antiprolactin conjugates showed an excellent stability. The immunosensor exhibited also a high selectivity with respect to other hormones. The analytical usefulness of the immnunosensor was demonstrated by analyzing human sera spiked with prolactin at three different concentration levels.

Analytical application of self assembled monolayers on gold electrodes: critical importance of surface pretreatment

Polycrystalline bare gold electrodes have been used as substrate for the preparation of self assembled monolayers (SAMs) of alkanethiols. Chemical cleaning and several mechanical polishing procedures of the electrode have been carefully tested in order to prepare monolayers of octadecyl-, mercaptans. By analyzing the cyclic voltammetric curves of hexacyanoferrate III and chlorpromazine and the electrochemical desorption of the coated monolayer, it appeared that the response at the bare electrodes is related to the cleaning procedure and that the structure of SAM coated gold electrodes is influenced by the polishing material used, i.e. diamond or alumina slurry. Electrochemical results have been confirmed by Auger analysis of the electrode surface.

Carbon Nanohorns as a Scaffold for the Construction of Disposable Electrochemical Immunosensing Platforms. Application to the Determination of Fibrinogen in Human Plasma and Urine

We describe in this work a novel electrochemical immunosensor design making use of carbon nanohorns (CNHs) as a scaffold for the preparation of disposable immunosensing platforms for the determination of fibrinogen (Fib). The approach involved the immobilization of Fib onto activated CNHs deposited on screen-printed carbon electrodes (SPCEs) and the implementation of an indirect competitive assay using anti-Fib labeled with horseradish peroxidase (HRP) and hydroquinone (HQ) as the redox mediator. Both CNHs and the Fib-CNHs covalent assembly were characterized by microscopic and electrochemical techniques. The different variables affecting the analytical performance of the amperometric immunosensing strategy were optimized. The calibration plot for Fib allowed a range of linearity between 0.1 and 100 μg/mL (r(2) = 0.994) and a detection limit of 58 ng/mL to be achieved. The Fib-CNHs/SPCEs exhibited an excellent storage stability of at least 42 days. The developed immunosensor provides, in general, an analytical performance better than that reported for other Fib immunosensors and commercial ELISA kits. This simple and relatively low cost immunosensor configuration permitted the sensitive and selective determination of Fib in human plasma and urine.