Agustín Costa-García

Manufacture and evaluation of carbon nanotube modified screen-printed electrodes as electrochemical tools

Carboxylated multiwalled carbon nanotubes (MWCNT-COOH) dissolved in a mixture of DMF:water were used to modify the surfaces of commercially available screen-printed electrodes (SPEs). The morphology of the MWCNT-COOH and the modified SPEs was characterized by transmission electron microscopy (TEM) and scanning electron microscopy (SEM), respectively. SEM analysis showed a porous structure formed by a film of disordered nanotubes on the surface of the working electrode. The modification procedure with MWCNT-COOH was optimised and it was applied to unify the electrochemical behaviour of different gold and carbon SPEs by using p-aminophenol as the benchmark redox system. The analytical advantages of the MWCNT-COOH-modified SPEs as voltammetric and amperometric detectors as well as their catalytic properties were discussed through the analysis, for instance, of dopamine and hydrogen peroxide. Experimental results show that the electrochemical active area of the nanotube-modified electrode increased around 50%. The repeatability of the modification methodology is around 6% (R.S.D.) and the stability of MWCNT-COOH-modified SPEs is ensured for, at least, 2 months.

Colloidal gold as an electrochemical label of streptavidin–biotin interaction

A new electrochemical method to monitor biotin-streptavidin interaction, based on the use of colloidal gold as an electrochemical label, is investigated. Biotinylated albumin is adsorbed on the pretreated surface of a carbon paste electrode (CPE). This modified electrode is immersed in colloidal gold-streptavidin labelled solutions. Adsorptive voltammetry is used to monitor colloidal gold bound to streptavidin, obtaining a good reproducibility of the analytical signal (R.S.D. = 3.3%). A linear relationship between peak current and streptavidin concentration from 2.5 x 10(-9) to 2.5 x 10(-5) M is obtained when a sequential competitive assay between streptavidin and colloidal gold-labelled streptavidin is carried out. On the other hand, the adsorption of streptavidin on the electrode surface was performed, followed by the reaction with biotinylated albumin labelled with colloidal gold. In this way, a linear relationship between peak current and colloidal gold labelled biotinylated albumin concentration is achieved with a limit of detection of 7.3 x 10(9) gold particles per ml (5.29 x 10(-9) M in biotin).

Simultaneous detection of free and total prostate specific antigen on a screen-printed electrochemical dual sensor

Voltammetric enzyme dual sensors for simultaneous determination of free and total prostate specific antigen (fPSA and tPSA) are described. Alkaline Phosphatase (AP) and a mixture solution of 3-indoxyl phosphate and silver ions were used as the enzymatic label and substrate, respectively. 8A6 or 5G6 antibodies specific for free and total PSA, respectively, were immobilized on different screen-printed electrodes (SPEs)--screen-printed carbon electrodes, screen-printed gold electrodes and screen-printed carbon electrodes modified with nanogold--in order to be able to select one of the surfaces as the most adequate one to develop the dual sensor. Screen-printed carbon electrodes modified with nanogold were the SPEs with the best analytical characteristics and lead to the most repeatable bioelectrodes, so they were selected for the development of the dual sensor. On Dualsensor-nAu electrodes, 8A6 antibody was immobilized on one working electrode and 5G6 antibody was immobilized on the other one by deposition of a drop of solution of each antibody and left overnight at 4 degrees C. Biotinylated anti-PSA antibody and streptavidin-AP conjugate were used as detection reagents, giving rise, to our knowledge, to the first simultaneous electrochemical biosensor for free and total PSA. The PSA dual sensor was used to monitor PSA production from three different cultures of human androgen-sensitive prostate tumor cells.

Electrochemical study and flow injection analysis of paracetamol in pharmaceutical formulations based on screen-printed electrodes and carbon nanotubes

Acetaminophenol or paracetamol is one of the most commonly used analgesics in pharmaceutical formulations. Acetaminophen is electroactive and voltammetric mechanistic studies for the electrode processes of the acetaminophenol/N-acetyl-p-quinoneimine redox system are presented. Carbon nanotubes modified screen-printed electrodes with enhanced electron transfer properties are used for the study of the electrochemical-chemical oxidation mechanism of paracetamol at pH 2.0. Quantitative analysis of paracetamol by using its oxidation process (in a Britton-Robinson buffer solution pH 10.0) at +0.20 V (vs. an Ag pseudoreference electrode) on an untreated screen-printed carbon electrode (SPCE) was carried out. Thus, a cyclic voltammetric based reproducible determination of acetaminophen (R.S.D., 2.2%) in the range 2.5x10(-6) M to 1x10(-3) M, was obtained. However, when SPCEs are used as amperometric detectors coupled to a flow injection analysis (FIA) system, the detection limit achieved for paracetamol was 1x10(-7) M, one order of magnitude lower than that obtained by voltammetric analysis. The repeatability of the amperometric detection with the same SPCE is 2% for 15 successive injections of 10(-5) M acetaminophen and do not present any memory effect. Finally, the applicability of using screen-printed carbon electrodes for the electrochemical detection of paracetamol (i.e. for quality control analysis) was demonstrated by using two commercial pharmaceutical products.

Electrochemical determination of mercury: a review.

Mercury is a metal that has been extensively studied, in large part due to its high toxicity. Therefore, mercury levels must be monitored in different sample types using analytical methods. This review summarizes the electrochemical methods that have been used for mercury analysis in a variety of samples. A critical evaluation of the methods and electrode materials employed for mercury analysis is presented according to the following classifications: bare electrodes, chemically modified electrodes and nanostructured electrodes. The advantages and disadvantages of each type of electrode material regarding mercury analysis are also presented.

AC voltammetric carbon paste-based enzyme immunosensors

Carbon paste electrodes, previously anodised in a basic media, are the basis for the development of a new voltammetric immunosensor device. Passive adsorption of the appropriate immunochemical reagent was performed onto the electrode surface. Alkaline Phosphatase labelled immunoglobulin was the tracer used in this work, 3-indoxyl phosphate being a very suitable enzymatic substrate for the electrochemical detection of the corresponding affinity reaction. The hydrolysis of this molecule generates indigo dimmer. This product was detected by alternating current voltammetry taking advantage of the adsorptive and inherent electrodic properties that it exhibits. The same electrochemical anodisation was used at the end of one assay to remove the entire protein layer attached to the carbon paste surface, allowing the formation of a new sensing phase and the use of the same support in several consecutive experiments. The methodology was applied to the design of two different immunoassays for the determination of human IgG. Good reproducibility of the electrodic signal and a limit of detection around 10(-10) M were achieved.

Development of an immunosensor for the determination of rabbit IgG using streptavidin modified screen-printed carbon electrodes.

Voltammetric enzyme immunosensors based on the employment of streptavidin modified screen-printed carbon electrodes (SPCEs) for the detection of rabbit IgG, as a model analyte, were described. Alkaline phosphatase (AP) and 3-indoxyl phosphate (3-IP) were used as the enzymatic label and substrate, respectively. The adsorption of streptavidin was performed by deposition of a drop of a streptavidin solution overnight at 4 degrees C on the pre-oxidized surface of the SPCEs. The analytical characteristics of these sensors were evaluated using biotin conjugated to AP. The immunosensor devices were based on a specific reaction of rabbit IgG with its biotinylated antibodies, which were immobilised on the modified screen-printed carbon electrodes through the streptavidin:biotin reaction. The immunosensors were used for a direct determination of AP labelled rabbit IgG, and for free rabbit IgG detection using a sequential competitive immunoassay. A calibration curve in the range of 5 x 10(-11) to 1 x 10(-9)M of rabbit IgG was obtained with a estimated detection limit of 5 x 10(-11)M (7.0ng/ml). These immunosensors were stable for 5 months if they were stored at 4 degrees C.

Multiwalled carbon nanotube modified screen-printed electrodes for the detection of p-aminophenol: Optimisation and application in alkaline phosphatase-based assays

Carboxylated multiwalled carbon nanotubes (MWCNT-COOH) were used to modify the working electrode surface of different screen-printed electrodes. The effect of this modification on the electrodic characteristics (double layer capacitance, electroactive area and heterogeneous rate constants for the electron transfer) was evaluated and optimized for the cyclic voltammetric determination of p-aminophenol. The enzymatic hydrolysis of p-aminophenylphosphate was employed for the quantification of alkaline phosphatase, one of the most important label enzymes in immunoassays. Finally, ELISA assays were carried out to quantify pneumolysin using this enzymatic system. Results obtained indicated that low superficial densities of MWCNT-COOH (0.03-0.06 microg mm(-2)) yielded the same electrodic improvements but with better analytical properties.

Adsorption of immunoglobulin G on carbon paste electrodes as a basis for the development of immunoelectrochemical devices.

The attachment of Immunoglobulin G (IgG) to a carbon paste electrode is investigated in this work. Studies of the immobilization of the immunoglobulin on this electrode were carried out. Alkaline phosphatase (AP) has been used as an enzyme label, linked to the antibody, for the determination of the adsorbed immunoglobulin. Various substrates for this enzyme have been tested and alpha-naphthyl phosphate was found to be the best because of the lower oxidation potential of the enzymatic product, alpha-naphthol, and greater sensitivity under the same experimental conditions. An electrodic pretreatment based on an anodic oxidation of the electrode surface in a phosphate media pH 9 was carried out prior to the adsorption step. This activation is also suitable for the removal of the IgG layer and the regeneration of the electrodic surface after each determination, with the intention of using the same electrode in the subsequent assays. A good reproducibility of the signal was achieved in this way (Relative Standard Deviation, R.S.D. = 3.7%). Using cyclic voltammetry, IgG labelled with AP has been quantified in a range from 2 x 10(-12) to 7 x 10(-11) M and a detection limit of 2.3 x 10(-12) M (signal-to-noise ratio = 3) was found. Finally, human IgG was quantified under non-optimized conditions on the electrodic surface through the reaction with AP labelled IgG using two different immunological designs.

Celiac disease detection using a transglutaminase electrochemical immunosensor fabricated on nanohybrid screen-printed carbon electrodes

Celiac disease is a gluten-induced autoimmune enteropathy characterized by the presence of tissue tranglutaminase (tTG) autoantibodies. A disposable electrochemical immunosensor (EI) for the detection of IgA and IgG type anti-tTG autoantibodies in real patients samples is presented. Screen-printed carbon electrodes (SPCE) nanostructurized with carbon nanotubes and gold nanoparticles were used as the transducer surface. This transducer exhibits the excellent characteristics of carbon-metal nanoparticle hybrid conjugation and led to the amplification of the immunological interaction. The immunosensing strategy consisted of the immobilization of tTG on the nanostructured electrode surface followed by the electrochemical detection of the autoantibodies present in the samples using an alkaline phosphatase (AP) labelled anti-human IgA or IgG antibody. The analytical signal was based on the anodic redissolution of enzymatically generated silver by cyclic voltammetry. The results obtained were corroborated with a commercial ELISA kit indicating that the electrochemical immunosensor is a trustful analytical screening tool.