L. Agüí

Role of carbon nanotubes in electroanalytical chemistry: A review

This review covers recent advances in the development of new designs of electrochemical sensors and biosensors that make use of electrode surfaces modification with carbon nanotubes. Applications based on carbon nanotubes-driven electrocatalytic effects, and the construction and analytical usefulness of new hybrid materials with polymers or other nanomaterials will be treated. Moreover, electrochemical detection using carbon nanotubes-modified electrodes as detecting systems in separation techniques such as high performance liquid chromatography (HPLC) or capillary electrophoresis (CE) will be also considered. Finally, the preparation of electrochemical biosensors, including enzyme electrodes, immunosensors and DNA biosensors, in which carbon nanotubes play a significant role in their sensing performance will be separately considered.

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.

Electrochemical determination of homocysteine at a gold nanoparticle-modified electrode.

The construction of a colloidal gold-cysteamine-carbon paste electrode, Au(coll)-Cyst-CPE, for the electrochemical determination of homocysteine is reported. The improved voltammetric behaviour of homocysteine at Au(coll)-Cyst-CPE with respect to that observed at a gold disk electrode is attributed to an enhanced electron transfer kinetics as a consequence of the array distribution of gold nanoparticles immobilized onto the Cyst SAM. Cyclic voltammetry of homocysteine showed an adsorption-controlled current for scan rates between 500 and 5000 mV s(-1). The hydrodynamic voltammogram constructed for homocysteine allowed the selection of a potential value of +600 mV, where the background current is negligible, for the amperometric detection of the analyte at the Au(coll)-Cyst-CPE. Using a flow rate of 0.8 ml min(-1), the R.S.D. value for i(p) after 25 repetitive injections of homocysteine was of 4.3%, and one single electrode could be used for more than 15 days without any treatment or regeneration procedure of the modified electrode surface. An HPLC method for the separation and quantification of homocysteine and related thiols, using amperometric detection at the modified electrode has been developed. A mobile phase consisting of 2:98% (v/v) acetonitrile:0.05 mol l(-1) buffer solution of pH 2.0, and a detection potential of +0.80 V were selected. Separation with baseline resolution and retention times of 3.00, 3.60, 4.52, 5.71 and 7.79 min were obtained for cysteine, homocysteine, glutathione, penicillamine and N-acetyl-cysteine, respectively. Calibration graphs were constructed for all the separated compounds. Detection limits ranged between 20 nM for cysteine and 120 nM for penicillamine, with a value for homocysteine of 30 nM. These values compare advantageously with those achieved with previously reported HPLC methods using electrochemical, UV, fluorescence and MS detection modes. The developed method was applied to the determination of cysteine and homocysteine serum samples with good results.

Graphite-Teflon-Peroxidase Composite Electrochemical Biosensors. A Tool for the Wide Detection of Phenolic Compounds

Graphite-Teflon-peroxidase composite electrodes, constructed by simple physical inclusion of the enzyme into the bulk of the electrode matrix, have been used for the amperometric detection of 18 phenolic compounds. Monitoring of the enzyme reaction was carried out by the electrode reduction of the generated phenoxy radicals, in the presence of a constant concentration of 5.0×10−4 mol L−1 H2O2, at an applied potential of 0.00 V. Different aspects regarding the stability of the composite bioelectrode showed the robustness of the biosensor design. The kinetic constants of the enzyme reactions for the phenolic compounds tested were calculated. The limits of detection obtained were better than those found in the literature for peroxidase biosensors using other composite matrices, and the differences in sensitivity observed for the different phenolic substrates are discussed in terms of their pKa values and substituents. The analytical signals obtained with the graphite-Teflon-HRP electrodes have been compared with those yielded by a graphite-Teflon-tyrosinase composite electrode constructed in a similar way. With the aim of monitoring the largest possible number of phenolic compounds with the highest possible sensitivity, coimmobilization of peroxidase and tyrosinase into the composite electrode matrix was carried out.

Biosensors in forensic analysis. A review

Forensic analysis is an important branch of modern Analytical Chemistry with many legal and socially relevant implications. Biosensors can play an important role as efficient tools in this field considering their well known advantages of sensitivity, selectivity, easy functioning, affordability and capability of miniaturization and automation. This article reviews the latest advances in the use of biosensors for forensic analysis. The different methodologies for the transduction of the produced biological events are considered and the applications to forensic toxicological analysis, classified by the nature of the target analytes, as well as those related with chemical and biological weapons critically commented. The article provides several Tables where the more relevant analytical characteristics of the selected reported methods are gathered.

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.

Electrochemical immunosensor for the determination of insulin-like growth factor-1 using electrodes modified with carbon nanotubes-poly(pyrrole propionic acid) hybrids.

An amperometric immunosensor for the determination of the hormone insulin-like growth factor 1 (IGF1) is reported for the first time in this work. As electrochemical transducer, a multiwalled carbon nanotubes-modified glassy carbon electrode on which poly(pyrrole propionic acid) was electropolymerized was prepared. This approach provided a high content of surface confined carboxyl groups suitable for direct covalent binding of anti-IGF1 monoclonal antibody. A sandwich-type immunoassay using a polyclonal antibody labeled with peroxidase, hydrogen peroxide as the enzyme substrate and catechol as redox mediator was employed to monitor the affinity reaction. All the variables involved in the preparation of the modified electrode were optimized and the electrodes were characterized by electrochemical impedance spectroscopy and cyclic voltammetry. Moreover, the different experimental variables affecting the amperometric response of the immunosensor were also optimized. The calibration graph for IGF1 showed a range of linearity extending from 0.5 to 1000 pg/mL, with a detection limit, 0.25 pg/mL, more than 100 times lower than the lowest values reported for the ELISA immunoassays available for IGF1 (30 pg/mL, approximately). Excellent reproducibility for the measurements carried out with different immunosensors and selectivity against other hormones were also evidenced. A commercial human serum spiked with IGF1 at different levels between 0.01 and 10.0 ng/mL was analyzed with good results.

Flow injection and HPLC determination of furosemide using pulsed amperometric detection at microelectrodes

The flow-injection and HPLC determination of the diuretic drug furosemide using pulsed amperometric detection (PAD) at cylindrical carbon fibre microelectrodes (CFMEs) is reported. Experimental conditions such as pH (6.5) and buffer concentration (0.05 mol l(-1) HPO4(2-)/H2PO4(-)) were optimized using square-wave voltammetry (SWV). Repetitive flow-injection amperometric measurements at +1.25 V for furosemide showed a continuous decrease in the peak current, probably as a consequence of the microelectrode surface fouling. However, a suitable amperometric detection of furosemide was achieved using a PAD program consisting of a two-step potential waveform with alternating anodic and cathodic polarization. The anodic (detection) potential was +1.25 V (time of application 0.1 s), and the cathodic (cleaning) potential was -0.20 V (t=0.2 s). A linear calibration graph was obtained for furosemide in the 5.0 x 10(-7)-1.0 x 10(-4) mol l(-1) concentration range, with a limit of detection of 1.7 x 10(-7) mol l(-1). HPLC-PAD at carbon fibre microelectrodes was used for the determination of furosemide in the presence of several thiouracil drugs and oxytetracycline (OTC). The mobile phase selected was a 25:75 acetonitrile:5.0 x 10(-3) mol l(-1) NaH2PO4 (pH 5.0) mixture. A linear calibration graph was obtained for furosemide in the 1-100 microM range, with a limit of detection of 0.55 microM. The usefulness of this method for the determination of furosemide in real samples was evaluated by performing the analysis of commercial milk samples spiked with furosemide at a concentration level of 4.5 x 10(-7) mol l(-1) (150 ng ml(-1)), as well as with other thiouracil drugs and OTC. A mean recovery of 95+/-5% furosemide was obtained.

Analytical voltammetry in low-permitivity organic solvents using disk and cylindrical microelectrodes. Determination of thiram in ethyl acetate

Abstract The electrochemical behavior of the N , N -dialkyldithiocarbamate pesticide thiram at a Au microdisk electrode (10 μm φ) and at cylindrical carbon fiber microelectrodes (CFMEs) (8 μm φ, 8 mm length) in low-permitivity organic solvents such as toluene and ethyl acetate is reported. The obtained cyclic voltammograms were very different in shape depending on the microelectrode and the solvent used, as well as on thiram concentration. Analytically useless responses were obtained with the gold microelectrode. However, a good fitting of the voltammetric signals at cylindrical CFMEs with the theoretical model proposed by Aoki was found. The influence of variables such as the concentration of thiram and of the background electrolyte, the length of the CFMEs and the potential scan rate was tested by CV. The oxidation responses obtained by square-wave voltammetry at carbon fiber microelectrodes in ethyl acetate were found to be analytically suitable to develop a method for the determination of thiram at low concentration levels. A linear calibration graph in the 1.0×10 −6 –6.0×10 −4 mol l −1 concentration range was obtained, with a slope of (1.2±0.1)×10 4 μA l mol −1 . The limit of detection was 4.3×10 −7 mol l −1 thiram, and the relative standard deviation at a 5.0×10 −5 mol l −1 concentration level ( n =10) was of 1.6%. A good reproducibility of the electroanalytical measurements was found with no need of cleaning or pretreatment procedures of the CFMEs surface. The SWV method was applied with good results to the determination of thiram in spiked grapes, by measuring directly the net current in the analyte extract in ethyl acetate. Commercial fungicides and pharmaceutical preparations containing thiram were also analysed.