Antonio Zapardiel

Selective detection of dopamine in the presence of ascorbic acid using carbon nanotube modified screen-printed electrodes

This work reports on the performance of carbon nanotube modified screen-printed electrodes (SPE-MWCNT) for the selective determination of dopamine (DA) in the presence of ascorbic acid (AA) by adsorptive stripping voltammetry (AdSV). Several operating conditions and parameters were examined including the electrochemical pre-treatment and the previous AA interaction and DA accumulation in the presence AA at physiological conditions. Under the chosen conditions, DA peak current of differential pulse voltammograms increases linearly with DA concentration in the range of 5.0 x 10(-8) to 1.0 x 10(-6) mol L(-1) with a limit of detection of 1.5 x 10(-8) mol L(-1) in connection with 600s accumulation time. The sensitivity obtained for DA was independent from the presence or absence of AA; therefore, the proposed method can be readily applied to detect DA in real samples. The proposed methodology was successfully used for the quantification of DA in urine samples.

Determination of camazepam and bromazepam in human serum by adsorptive stripping voltammetry

Adsorptive stripping voltammetry was used for the determination of trace amounts of the benzodiazepines camazepam and bromazepam. This very sensitive method is based on controlled adsorptive pre-concentration of the drugs on the hanging mercury drop electrode. Measurements were taken by differential-pulse voltammetry after determination of the optimum accumulation conditions. The response was linear in the range 3 × 10–9–9 × 10–9M(270 s and-0.60 V for the pre-concentration) for camazepam and 1 × 10-8–8 × 10–8M(90 s and-0.40 V) for bromazepam. In the concentration ranges investigated, the relative standard deviation was lower than 6%. The method is applicable to the determination of the drugs in human serum, with detection limits of 20 ng ml-1 of serum (30 s and-0.62 V for the pre-concentration) for camazepam and 200 ng ml–1 of serum (10 s and-0.41 V) for bromazepam.

Application of matrix solid-phase dispersion to the propham and maleic hydrazide determination in potatoes by differential pulse voltammetry and HPLC

The application of the matrix solid-phase dispersion (MSPD) process as sample treatment in connection with the electrochemical detection is studied for the first time. For this purpose, a novel methodology is introduced for the extraction of propham and maleic hydrazide herbicides from potatoes samples based in the MSPD process prior to their electrochemical detection. Potato samples disruption was done by blending them with C(8) bonded-phase and selective herbicide extraction was achieved by successive treatment of the blended with 50mM phosphate buffer pH 7.4 (for maleic hydrazide) and methanol (for propham). The extraction procedure efficiency was estimated using differential pulse voltammetry in potato samples spiked with the herbicides yielding recovery values of 98% and 68% for propham and maleic hydrazide, respectively. No significant adverse effect of the MSPD process was observed on the herbicides electrochemical signals. For comparison, recovery studies using HPLC with UV detection were carried out and a good correlation in the results obtained by using both techniques was observed.

Measurement of nanomolar levels of psychoactive drugs in urine by adsorptive stripping voltammetry.

Adsorptive stripping voltammetry was used to determine nanomolar levels of the benzodiazepines pinazepam, camazepam, bromazepam and thienodiazepine (BrTDO) in urine. Measurements were made by differential pulse voltammetry at a hanging mercury drop electrode. The influences of various operational conditions on the stripping response were examined. The optimum accumulation potentials and accumulation times were -0.40 V and up to 60 sec for pinazepam, -0.60 V and up to 40 sec for camazepam, -0.40 V and up to 30 sec for bromazepam and -0.60 V and up to 60 sec for BrTDO, respectively. The effects of various urine components on the voltammetric response were also studied, and preliminary separation of the drugs was found necessary because of interference by creatinine and uric acid. The proposed method is appropriate for the determination of the four drugs in urine up to the 1000 ng/ml level with short accumulation periods (10-60 sec). The relative standard deviation for the 500 ng/ml level of the drugs in urine (30-sec accumulation) was less than 3%.

Comparative study of multi walled carbon nanotubes-based electrodes in micellar media and their application to micellar electrokinetic capillary chromatography.

This work reports on a comparative study of the electrochemical performance of carbon nanotubes-based electrodes in micellar media and their application for amperometric detection in micellar electrokinetic capillary chromatography (MEKC) separations. These electrodes were prepared in two different ways: immobilization of a layer of carbon nanotubes dispersed in polyethylenimine (PEI), ethanol or Nafion onto glassy carbon electrodes or preparation of paste electrodes using mineral oil as binder. Scanning electron microscopy (SEM) was employed for surface morphology characterization while cyclic voltammetry of background electrolyte was used for capacitance estimation. The amperometric responses to hydrogen peroxide, amitrol, diuron and 2,3-dichlorophenol (2,3CP) in the presence and in the absence of sodium dodecylsulphate (SDS) were studied by flow injection analysis (FIA), demonstrating that the electrocatalytic activity, background current and electroanalytical performance were strongly dependent on the electrodes preparation procedure. Glassy carbon electrodes modified with carbon nanotubes dispersed in PEI (GC/(CNT/PEI)) displayed the most adequate performance in micellar media, maintaining good electrocatalytic properties combined with acceptable background currents and resistance to passivation. The advantages of using GC/(CNT/PEI) as detectors in capillary electrophoresis were illustrated for the MEKC separations of phenolic pollutants (phenol, 3-chlorophenol, 2,3-dichlorophenol and 4-nitrophenol) and herbicides (amitrol, asulam, diuron, fenuron, monuron and chlortoluron).

Determination of 3-amino-1,2,4-triazole (amitrole) in environmental waters by capillary electrophoresis

3-Amino-1,2,4-triazole (amitrole) is a widely used pesticide, with many difficulties to be analyzed at the regulatory level in drinking water, because its high solubility in water. This paper describes a simple and fast method for the simultaneous determination of amitrole and atrazin-2-hydroxy, principal degradation product of s-triazines, by capillary zone electrophoresis. Separation and determination of these herbicides in water samples was performed in 0.02 mol l(-1) phosphate buffer at pH 3.2. The method allows determination of the amitrole and atrazin-2-hydroxy in water samples in concentration lower than 100 mug l(-1). The detection limits using a previous preconcentration step of amitrole in Alberche River (Comunidad Autónoma de Madrid, Spain) and drinking water spiked samples was of 4 mug l(-1).

Analysis of total polyphenols in wines by FIA with highly stable amperometric detection using carbon nanotube-modified electrodes

The use of glassy carbon electrodes (GCEs) modified with multi-walled carbon nanotube (CNT) films for the continuous monitoring of polyphenols in flow systems has been examined. The performance of these modified electrodes was evaluated and compared to bare GCE by cyclic voltammetry experiments and by flow injection analysis (FIA) with amperometric detection monitoring the response of gallic, caffeic, ferulic and p-coumaric acids in 0.050 M acetate buffer pH 4.5 containing 100 mM NaCl. The GCE modified with CNT dispersions in polyethyleneimine (PEI) provided lower overpotentials, higher sensitivity and much higher signal stability under a dynamic regime than bare GCEs. These properties allowed the estimation of the total polyphenol content in red and white wines with a remarkable long-term stability in the measurements despite the presence of potential fouling substances in the wine matrix. In addition, the versatility of the electrochemical methodology allowed the selective estimation of the easily oxidisable polyphenol fraction as well as the total polyphenol content just by tuning the detection potential at +0.30 or 0.70 V, respectively. The significance of the electrochemical results was demonstrated through correlation studies with the results obtained with conventional spectrophotometric assays for polyphenols (Folin-Ciocalteu, absorbance at 280 nm index and colour intensity index).

Analysis of polyphenols in white wine by CZE with amperometric detection using carbon nanotube-modified electrodes†

A method for the simultaneous detection of five polyphenols (caffeic, chlorogenic, ferulic and gallic acids and (+)‐catechin) by CZE with electrochemical detection was developed. Separation of these polyphenols was performed in a 100 mM borate buffer (pH 9.2) within 15 min. Under optimized separation conditions, the performance of glassy carbon (GC) electrodes modified with multiwalled carbon nanotube layer obtained from different dispersions was examined. GC electrode modified with a dispersion of multi‐walled carbon nanotubes (CNT) in polyethylenimine has proven to be the most suitable CNT‐based electrode for its application as amperometric detector for the CZE separation of the studied compounds. The excellent electrochemical properties of this electrode allowed the detection of the selected polyphenols at +200 mV and improved the efficiency and the resolution of their CZE separation. Limits of detection below 3.1 μM were obtained with linear ranges covering the 10−5 to 10−4 M range. The proposed method has been successfully applied for the detection (ferulic, caffeic and gallic acids and (+)‐catechin) and the quantification (gallic acid and (+)‐catechin) of polyphenols in two different white wines without any preconcentration step. A remarkable signal stability was observed on the electrode performance despite the presence of potential fouling substances in wine.

Design and adaptation of miniaturized electrochemical devices integrating carbon nanotube‐based sensors to commercial CE equipment

The design of new electrochemical devices integrating carbon nanotube sensors and their adaptation to commercial CE equipments are described. One of these designs was made for using commercial screen‐printed electrodes, whereas the second was projected for coupling commercial glassy carbon electrodes. The electrochemical characterization of these devices revealed that their hydrodynamic behaviour is strongly influenced by the electrode modification with multi‐wall carbon nanotubes that provided faster and/or more sensitive signals. The analytical applicability of these devices was illustrated for the CZE separation of chlorinated phenols and the MEKC separation of endocrine disruptors, where the use of carbon nanotube sensors has proved to be advantageous when compared with unmodified ones, with good electrocatalytic properties combined with acceptable background currents and a remarkable resistance to passivation.

Screening and confirmatory methods for the analysis of macrocyclic lactone mycotoxins by CE with amperometric detection

A simple analytical scheme for the screening and quantification of zearalenone and its metabolites, α‐zearalenol and β‐zearalenol, is reported. Extracts from maize flour samples were collected by supercritical fluid extraction and afterwards, they were analyzed by CE with amperometric detection. This scheme allowed a rapid and reliable identification of contaminated flour samples according to the reference value established for zearalenone by directive 2005/38/EC (200 μg/kg). The sample screening method was carried out by CZE using 25 mM borate separation buffer at pH 9.2 and 25.0 kV as separation voltage, monitoring the amperometric signal at +700 mV with a carbon paste electrode. In this way, total amount of mycotoxins was determined and samples were processed in 4 min with a detection limit of 12 μg/L, enough to discriminate between positive (more than 200 μg/L total mycotoxins) and negative samples (less than 200 μg/L total mycotoxins). Positive samples were then subjected to CZE separation and quantification of each analyte was done with 50 mM borate running buffer modified with 30% methanol at pH 9.7 and 17.5 kV as separation voltage. Under these conditions, separation was achieved in 15 min with detection limits from 20 to 35 μg/L for each analyte.