A Highly Sensitive Modified Glassy Carbon Electrode with a Carboxylated Multi-walled Carbon Nanotubes/Nafion Nano Composite for Voltammetric Sensing of Dianabol in Biological Fluid.

The extraordinary prerequisite for the analysis of an anabolic steroid, namely dianabol (DB), has inspired towards the development of a cost-effective and high-performance sensing probe. Thus, a simple and robust electrochemical sensor (c-MWCNTs-Nafion®lGCE) for dianabol (DB), a widely used steroid, was developed using a glassy carbon electrode (GCE) modified with functionalized carboxylated multi-walled carbon nanotubes (c-MWCNT) and Nafion®. At pH 7 - 8, differential pulse-cathodic stripping voltammetry (DP-CSV) displayed two cathodic peaks at -0.85 and -1.35 V that varied linearly over a wide range (9.0 × 10-9 (2.7 µg L-1) - 9.0 × 10-6 (2.7 × 103 µg L-1) mol L-1) and 2.9 × 10-6 (8.7 × 102 µg L-1) - 8.0 × 10-5 (2.4 × 104 µg L-1) mol L-1) of DB concentrations, respectively. The low limits of detection and quantification at peak I (-0.85 V) were 2.7 × 10-9 (8.1 × 10-1 ng mL-1) and 9.0 × 10-9 (2.7 ng mL-1) mol L-1, respectively. The repeatability and reproducibility displayed relative standard deviations lower than 5%. The method was applied for DB analysis in human urine and subsequently compared with the standard HPLC method. Interference of common metabolites in biological fluids samples to DB sensing was insignificant. This method has distinctive advantages e.g. precise, short analytical time, sensitive, economical, reproducible and miniaturized sample preparation for DB analysis in biological samples of human origin.

Effect of GO nanosheets on spectrophotometric determination of tyrosine in urine and serum using nitrosonaphthol.

Here, we aimed to use graphene oxide to improve the selectivity and sensitivity of Tyr determination via the reaction with 1-nitroso-2-naphthol as a selective reagent of Tyr. The reaction between Tyr and 1-nitroso-2-naphthol in absence and presence of GO was studied spectrophotometrically. Different parameters such as concentrations, temperature, incubation time were optimized. The obtained data showed that the maximum absorbance was achieved by using 2 mL of 0.03% 1-nitroso-2-naphthol at temperature 60 °C for 10 min. On the basis of calibration curve of various concentrations of Tyr in the presence of 20 µg mL-1 GO, the limit of detection was 6.4 × 10-6 M (1.15 µg mL-1), where in absence of GO was 1.1 × 10-5 M (19.9 µg mL-1). The selectivity of Tyr in presence of other amino acids and phenols was studied with and without GO. The data obtained revealed that the selectivity of Tyr in presence of GO with respect to some amino acids and phenols was improved. The proposed method has been applied for the determination of Tyr in urine and serum samples. Therefore, GO is a powerful catalytic surface for the sensitive and selective determination of Try in biological fluids.

Uptake of Tyrosine Amino Acid on Nano-Graphene Oxide.

Graphene oxide (GO) is emerging as a promising nanomaterial with potential application in the detection and analysis of amino acids, DNA, enzymes, and proteins in biological fluid samples. So, the reaction of GO with amino acids should be characterized and determined before using it in biosensing methods and devices. In this study, the reaction of tyrosine amino acid (Tyr) with GO was characterized using FT-IR, UV-vis spectrophotometry, and scanning electron microscopy (SEM) before its use. The optimum conditions for GO's interaction with Tyr amino acid have been studied under variable conditions. The optimum conditions of pH, temperature, shaking time, and GO and tyrosine concentrations for the uptaking of tyrosine amino acid onto the GO's surface from aqueous solution were determined. The SEM analysis showed that the GO supplied was in a particle size range between 5.4 and 8.1 nm. A pH of 8.4-9.4 at 25 °C and 5 min of shaking time were the optimum conditions for a maximum uptake of 1.4 µg/mL of tyrosine amino acid onto 0.2 mg/mL of GO.

Preparation of L-tryptophan imprinted microspheres based on carboxylic acid functionalized polystyrene.

In this work, polymerizable (4-vinylbenzoyl)-L-tryptophan (VBLT) was synthesized and characterized utilizing, elemental analysis, mass spectra, FTIR, (1)H and (13)CNMR. VBLT was then copolymerized with styrene and divinylbenzene cross-linker using potassium persulfate free radical initiator. The template L-Trp molecules were then leached out from the cross-linked network leaving selective recognition cavities, which are able to selectively rebind with L-Trp than D-Trp. The obtained molecularly imprinted LT-CPS resin was examined using various instrumental techniques such as SEM and FTIR to be then employed in a series of adsorption experiments to evaluate the essential parameters for efficient selective extraction of L-Trp. The kinetics of adsorption displayed the best fit with pseudo-second-order kinetic model, suggesting chemical sorption as the rate determining step. Additionally, the most effective interpretation of the adsorption isotherm data was acquired by Langmuir isotherm, and the maximum adsorption capacities were 155±2 and 82±1 mg/g for L- and D-Trp, respectively. Moreover, chiral resolution of L, D-Trp racemic mixture was performed using column of LT-CPS.

Amperometric immunosensor for detection of celiac disease toxic gliadin based on Fab fragments.

Immunosensor sensitivity is strongly dependent on the density of free active epitopes per surface area, which could be achieved via well-oriented immobilization of antibody fragments as bioreceptor molecules. Here, we report on the development of an electrochemical gliadin immunosensor based on the spontaneous self-assembly of antigliadin Fab fragments (CDC5-Fab) on Au surfaces. The analytical performance of this immunosensor is compared with a similar containing whole CDC5 antibodies previously modified with thiol groups (CDC5-SH) as the recognition element. Fab fragments were generated by reduction of the disulfide bond of F(ab)(2) fragments obtained by bromelain digestion of CDC5 antibody. Surface plasmon resonance (SPR) was used to evaluate the degree of immobilization and recognition ability of immobilized CDC5-Fab and CDC5-SH on gold surfaces. The studied surface chemistries were evaluated in terms of time required for SAM formation, stability, susceptibility to nonspecific interactions, and sensitivity using surface plasmon resonance, electrochemical impedance spectroscopy (EIS), and amperometry. CDC5-Fab formed a stable monolayer on gold after 15 min and retained >90% of antigen recognition ability after 2 months of storage at 4 degrees C. Detection of gliadin of Fab modified electrodes was evaluated by impedance and amperometry. Labeless impedimetric detection achieved a LOD of 0.42 microg/mL while the amperometric immunosensor based on Fab fragments showed a highly sensitive response with an LOD of 3.29 ng/mL. The Fab based immunosensor offers the advantages of being highly sensitive, easy, and rapid to prepare, with a low assay time.

Amperometric sensing of ascorbic acid using a disposable screen-printed electrode modified with electrografted o-aminophenol film.

Electrode modification by electrochemical reduction of diazonium salts of different aryl derivatives is useful for catalytic, analytical and biotechnological applications. A simple and sensitive method for the electrocatalytic detection of ascorbic acid using disposable screen-printed carbon electrodes modified with an electrografted o-aminophenol film, via the electrochemical reduction of its in situ prepared diazonium salts in aqueous solution, is presented. The performance of two commercial SPEs as substrates for grafting of diazonium films has been compared and the grafting process optimized with respect to deposition time and diazonium salt concentration, with the modified surfaces being characterised using cyclic voltammetry. The functionalised screen-printed electrodes demonstrated an excellent electrocatalytic activity towards the oxidation of ascorbic acid shifting the overpotential from 298 and 544 mV to 160 and 244 mV, respectively vs. Ag/AgCl. DC amperometric measurements were carried out for the quantitative determination of ascorbic acid using the modified electrodes. The catalytic oxidation peak current was linearly dependent on the ascorbic acid concentration in the range of 2-20 microM, with a correlation coefficient 0.998, and a limit of detection of 0.86 microM was obtained with an excellent reproducibility (RSD% = 1.98, n = 8). The functionalised screen-printed electrodes exhibited notable surface stability, and were used as a simple and precise disposable sensor for the selective determination of ascorbic acid.

Amperometric determination of ascorbic acid in real samples using a disposable screen-printed electrode modified with electrografted o-aminophenol film.

Ascorbic acid (AA) is an antioxidant considered to play a crucial role in human health. Therefore, diverse methods for the determination of AA in foods have been developed, most of them time-consuming and requiring costly instrumentation. A simple and sensitive method for the quantification of AA in fresh fruits and vegetables and commercial juices using an amperometric sensor is presented on the basis of disposable screen-printed carbon electrodes (SPEs) modified with an o-aminophenol (o-AP) film selective for the detection of AA. The sensor exhibited a linear response for AA from 2-20 microM, with a correlation coefficient r2 = 0.998 and a limit of detection of 0.86 microM. Common possible interferents of the sample matrices were tested, and results showed high selectivity of the o-AP SPEs toward AA. The sensor exhibited an excellent reproducibility (RSD% = 1.98, n = 8) and surface stability. The method was validated by a comparison to a reference method, and excellent correlation is obtained.

Electrochemical immunosensor for detection of celiac disease toxic gliadin in foodstuff.

Celiac disease is a gluten-sensitive enteropathy that affects as much as 1% of the population. Patients with celiac disease should maintain a lifelong gluten-free diet, in order to avoid serious complications and consequences. It is essential to have methods of analysis to reliably control the contents of gluten-free foods, and there is a definitive need for an assay that is easy to use, and can be used on site, to facilitate the rapid testing of incoming raw materials or monitoring for gluten contamination, by industries generating gluten-free foods. Here, we report on the development of an electrochemical immunosensor exploiting an antibody raised against the putative immunodominant celiac disease epitope, for the measurement of gliadin content and potential celiac toxicity of a foodstuff. To develop the gliadin immunosensor, we explored the use of two surface chemistries, based on the use of dithiols, 22-(3,5-bis((6-mercaptohexyl)oxy)phenyl)-3,6,9,12,15,18,21-heptaoxadocosanoic acid (1) and 1,2-dithiolane-3-pentanoic acid (thioctic acid) (2), for anchoring of the capture antibody. The different surface chemistries were evaluated in terms of time required for formation of self-assembled monolayers, stability, susceptibility to nonspecific binding, reproducibility, and sensitivity. The thioctic acid self-assembled monolayer took more than 100 h to attain a stable surface and rapidly destabilized following functionalization with capture antibody, while the heptaoxadocosanoic acid surface rapidlyformed (less than 3 h) and was stable for at least 5 days, stored at room temperature, following antibody immobilization. Both surface chemistries gave rise to highly sensitive immunosensors, with detection limits of 5.5 and 11.6 ng/mL being obtained for 1 and 2, respectively, with nonspecific binding of just 2.7% of the specific signal attained. The immunosensors were extremely reproducible, with RSD of 5.2 and 6.75% obtained for 1 and 2 (n = 5, 30 ng/mL), respectively. Finally, the immunosensor was applied to the analysis of commercial gluten-free and gluten-containing raw and processed foodstuffs, and excellent correlation achieved when its performance compared to that of an ELISA.

Simultaneous detection of ascorbate and uric acid using a selectively catalytic surface.

The direct and selective detection of ascorbate at conventional carbon or metal electrodes is difficult due to its large overpotential and fouling by oxidation products. Electrode modification by electrochemical reduction of diazonium salts of different aryl derivatives is useful for catalytic, analytical and biotechnological applications. A monolayer of o-aminophenol (o-AP) was grafted on a glassy carbon electrode (GCE) via the electrochemical reduction of its in situ prepared diazonium salts in aqueous solution. The o-aminophenol confined surface was characterized by cyclic voltammetry. The grafted film demonstrated an excellent electrocatalytic activity towards the oxidation of ascorbate in phosphate buffer of pH 7.0 shifting the overpotential from +462 to +263 mV versus Ag/AgCl. Cyclic voltammetry and d.c. amperometric measurements were carried out for the quantitative determination of ascorbate and uric acid. The catalytic oxidation peak current was linearly dependent on the ascorbate concentration and a linear calibration curve was obtained using d.c. amperometry in the range of 2-20 microM of ascorbate with a correlation coefficient 0.9998, and limit of detection 0.3 microM. The effect of H2O2 on the electrocatalytic oxidation of ascorbate at o-aminophenol modified GC electrode has been studied, the half-life time and rate constant was estimated as 270 s, and 2.57x10(-3) s(-1), respectively. The catalytically selective electrode was applied to the simultaneous detection of ascorbate and uric acid, and used for their determination in real urine samples. This o-AP/GCE showed high stability with time, and was used as a simple and precise amperometric sensor for the selective determination of ascorbate.