Amperometric detection for capillary electrophoresis at a sol–gel carbon composite electrode

Thiazole-Based Silver Ion Sensor for Sequential Colorimetric Visualization of Epinephrine in the Brain Tissues of an Alzheimer's Disease Model of Mouse

A thiazole-based probe, N'-((2-aminothiazol-5-yl)methylene)benzohydrazide (TBH), has been efficiently synthesized and characterized for the selective and sensitive detection of the neurotransmitter epinephrine (EP). The sensing strategy is based on the use of TBH for sequential colorimetric sensing of Ag+ and EP via in situ formation of Ag nanoparticles (Ag NPs) from the TBH-Ag+ complex. The generated Ag NPs lead to a bathochromic shift in absorption maximum and a change in color of the solution from light brown to reddish brown. TBH-Ag+ shows remarkable selectivity toward EP versus other drugs, common cations, anions, and some biomolecules. Moreover, TBH-Ag+ has a low detection limit for EP at 1.2 nM. The coordination of TBH-Ag+ has been proposed based on Job's plot, Fourier transform infrared spectroscopy (FT-IR), high-resolution mass spectrometry (HRMS), 1H NMR titration, X-ray photoelectron spectroscopy (XPS), energy-dispersive X-ray analysis (EDAX), and density functional theory (DFT) studies. The composition and morphology of the generated Ag NPs have been analyzed by XPS, scanning electron microscopy (SEM), transmission electron microscopy (TEM), and dynamic light scattering (DLS). The proposed sensing mechanism for EP has been supported by XPS of Ag after the reaction. Further, the sensitivity of TBH-Ag+ toward EP in brain tissues of an Alzheimer's disease model of mouse has been evaluated. A thorough comparison was done for evaluation of the proposed method.

Novel Modifications to Carbon-Based Electrodes to Improve the Electrochemical Detection of Dopamine

In this work, we describe three simple modifications to carbon electrodes that were found to improve the detection of an exemplar neurotransmitter (dopamine) in the presence of physiological interferents (ascorbic acid and/or uric acid). First, the electro-oxidation of ascorbic acid, as a pretreatment, at boron-doped diamond electrode (BDE) interfaces is studied. This treatment did suppress the detection of ascorbic acid oxidation signal, but only in a manner suitable for single-use detection of high concentrations of dopamine (i.e., > 1 μM). Second, the hydrogenation of BDE by electrochemical cathodic treatment and plasma hydrogenation was investigated. Large cathodic, applied potentials (i.e., > - 5 V) and hydrogen plasma pretreatment of BDE lead to the partial and complete oxidization of ascorbic acid before dopamine, respectively. The consequence at hydrogen-plasma treated BDE is the complete electrochemical separation of these two species without any typical catalytic reactions between the analytes. Third, the modification of glassy carbon electrodes with carbon black nanoparticles is explored. This modification enables the simultaneous detection of ascorbic acid, dopamine and uric acid, significantly enhancing the sensitivity of dopamine. Dopamine was best detected using the unconventional route of detecting 5,6-dihydroxyindole, which is made possible by use of carbon-black nanoparticles. The potential of all three studied modifications to be of electroanalytical use is highlighted throughout this work.

Simultaneous determination of sugars and ascorbic acid by capillary zone electrophoresis with amperometric detection at a carbon paste electrode modified with polyethylene glycol and Cu2O

In this paper, a kind of novel carbon paste electrode modified with double modifiers-polyethylene glycol (PEG) and Cu(2)O (PEG/Cu(2)O CPME) in capillary zone electrophoresis with amperometric detection (CZE-AD) was applied to simultaneously determine three sugars: glucose, sucrose, fructose and ascorbic acid (AA). The catalytic electrochemical properties of PEG/Cu(2)O CPME could enhance sensitivity obviously compared with carbon paste electrode modified with only PEG or Cu(2)O at a relatively lower detection potential (+0.3 V versus SCE). Especially, the electrochemical detection response of AA increased obviously to the same level of saccharides by adding PEG into the Cu(2)O carbon paste modified electrode. The four analytes could be perfectly separated within 22 min, the linear ranges were from 1.0 x 10 (-6) to 5.0 x 10(-5) mol L(-1) and the detection limits were at 10(-7) mol L(-1) magnitude (S/N=3). The present working electrode was successfully employed to analyse beverage samples with recoveries in the range 93-107% and RSDs less than 4%. Above results demonstrated that capillary zone electrophoresis coupled with the PEG/Cu(2)O carbon paste modified electrode was of convenient preparation, high sensitivity, good repeatability and could be used in the rapid determination of practical samples.

Temperature sensitive dopamine-imprinted (N,N-methylene-bis-acrylamide cross-linked) polymer and its potential application to the selective extraction of adrenergic drugs from urine

A temperature sensitive dopamine-imprinted polymer was prepared in 80% aqueous methanol solution by free-radical cross-linking co-polymerisation of methacrylic acid and acrylamide at 60 degrees C in the presence of N,N-methylene-bis-acrylamide as the cross-linker and dopamine hydrochloride as template molecule. The resulting molecularly imprinted polymer (MIP) formed temperature responsive materials, which could be used for the selective separation of appropriate dopamine and adrenergic compounds from a liquid matrix at ambient temperatures. The thermoresponsive MIP exhibited a swelling-deswelling transition in 80% aqueous methanol solution at about 35 degrees C. The capacity of the thermoresponsive MIP to recognise the template molecule when present in aqueous methanol solution changed with temperature, with the highest selectivity found at 35 degrees C. Additionally, binding parameters obtained from Scatchard analyses indicate that increasing temperature resulted in an increased affinity and binding capacity of specific binding sites, but had less effect on non-selective binding sites. Subsequently, the thermoresponsive MIP was tested for its application as a sorbent material, utilisable in the selective solid-phase extraction (SPE) of dopamine and other adrenergic compounds (epinephrine, isoproterenol, salbutamol and serotonin) from urine samples. It was shown that the compounds that were structurally related to dopamine could be removed by elution, while dopamine and serotonin, the analytes of interest, remained strongly adsorbed to the adsorbent during SPE applications. The thermoresponsive MIP displayed different efficiency in clean-up and enrichments using the SPE protocol at different temperatures.

Carbon nanotube aqueous sol-gel composites: enzyme-friendly platforms for the development of stable biosensors

A new type of composite material based on carbon nanotubes and an aqueous sol-gel process has been developed. The electrochemical characteristics of these composites were investigated and compared to composites made with an alkoxy silane sol-gel process. The use of carbon nanotubes, as the conductive part of the composite, facilitated fast electron transfer rates. The feasibility of this type of composite for the development of biosensors was demonstrated using l-amino acid oxidase. The stability of the enzyme was increased when it was encapsulated in the aqueous sol-gel, and the sensor retained more that 50% of its response after 1 month of testing.

Disposable Amperometric Sensor for Neurotransmitters Based on Screen-Printed Electrodes Modified with a Thin Iridium Oxide Film

Potential cycling in the range from -0.2 to +1.2 V is used for the electrodeposition of hydrous iridium oxide films onto a screen-printed electrode from a saturated solution of alkaline iridium(III) solution. The iridium oxide redox couple shows a stable and obvious reversible redox, with the formal potential being pH dependent in the range 1-14. The properties, stability and electrochemical properties of iridium oxide films were investigated by cyclic voltammetry. A modified electrode showed excellent catalytic activity toward the oxidation of neurotransmitters (catecholamines) over a wide pH range (2-8). The electrocatalytic behavior is further exploited as a sensitive detection scheme for adrenaline and dopamine by hydrodynamic amperometry. Under the optimized conditions, the calibration curves are linear in the concentration range 0.1-70 and 0.1-15 microM for dopamine and adrenaline determination, respectively. The detection limit and sensitivity are 30 nM and 30 nA/microM for adrenaline and 15 nM and 80 nA/microM for dopamine. Finally, the analytical performance of the modified electrode was demonstrated for the elimination of interference by uric acid in catecholamines determination when present in a 1000-fold concentration excess.

Electrochemical detector based on sol-gel-derived carbon composite material for capillary electrophoresis microchips

An on-chip disk electrode based on sol-gel-derived carbon composite material could be easily and reproducibly fabricated. Unlike other carbon-based electrodes reported previously, this detector is rigid, convenient to fabricate, and amenable to chemical modifications. Based on the stable and reproducible characters of this detector, a copper particle-modified detector was developed for the detection of carbohydrates which extends the application of the carbon-based electrode. In our experiments, the performance of the new integrated detector for rapid on-chip measurement of epinephrine and glucose was illustrated. Experimental procedures including the fabrication of this detector, the configuration of separation channel outlet and electrode verge, and the performance characteristics of this new electrochemical detector were investigated.

Evaluation of a sol-gel derived carbon composite electrode as an amperometric detector for capillary electrophoresis

In this paper, we report the construction and application of a sol-gel derived carbon composite electrode (CCE) as an amperometric detector for capillary electrophoresis. The electrochemical properties were characterized and compared with those of conventional carbon fiber and carbon paste electrode (CPE). Experimental results show that peak-to-peak noise of CCE was about 20% of CPE and electrode capacitance was comparatively low. When applied to the detection of dopamine and epinephrine, the optimal detection potential for CCE was 0.1 V lower than CPE under the same separation conditions; CCE with diameter of 75 and 100 microm could achieve a low detection limit of 3 x 10(-8) and 6 x 10(-8) M for the detection of epinephrine, which approaching that of the 33-microm diameter carbon fiber electrode. Also, the linearity for epinephrine at CCE was more than two orders of magnitude, which was slightly wider than that of carbon fiber electrode. Applications to real sample analysis were tested by the determination of betahistine dihydrochloride in tablets and human urine. Using CCE with diameter < or = 100 microm as an amperometric detector after capillary electrophoresis separation, a low detection limit and a wide linear range combined with excellent reproducibility were obtained. This CCE possesses of many advantages, namely, convenience, ease of fabrication, low cost and high stability.

Amperometric and voltammetric detection for capillary electrophoresis

The focus of this article is amperometric and voltammetric detection coupled with capillary electrophoresis. Fundamental concepts and progress in the field of capillary electrophoresis with electrochemical detection (CEEC) that have occurred within the past three years, including new methodologies and unique applications, are highlighted. This review contains 95 references.

Advances in the voltammetric analysis of small biologically relevant compounds

The problems associated with attempting to apply voltammetric techniques to the analysis of biologically relevant organics within complex media are identified and, through reviewing the very recent literature (1999-mid-2001), possible solutions are described. The boundaries of the search were limited to research targeted at the resolution of specific problems, associated with quantitative determinations. Various strategies have emerged to counter problems of poor sensitivity and selectivity and these have been summarized and critically appraised. Where possible, the characteristics of each approach have been distilled into a table format to ease comparison. Emphasis has been placed on the collation of information that will improve the intrinsic electrode response and as such should be of value to those interested in pursuing electroanalytical methodologies regardless of context.

Oxidation of adrenaline and noradrenaline by solved molecular oxygen in a FIA assembly

A simple and effective procedure is proposed for the study and simultaneous determination of adrenaline and noradrenaline. The fluorimetric determination of both substances is performed in a flow injection assembly and by oxidation of both drugs with the solved molecular oxygen. The influence of different parameters is empirically studied and the interpretation of the reaction mechanism is also added. The determination of adrenaline is monitored at 450 nm and the outputs at 520 nm correspond to the adrenaline and noradrenaline global amount; for both lectures lambda(exc) 329 nm. The influence of temperature is relevant and analytical determination occurred at 55 degrees C by immersing the sample loop in a water bath. The linear range for adrenaline is over 0.5-20 microg ml(-1), limit of detection for both compounds is 0.2 microg ml(-1): the influence of foreign compounds as potential interferents is also tested; and, finally the procedure is applied to determination of both chatecolamines in synthetic samples.

Separation of kaempferols in Impatients balsamina flowers by capillary electrophoresis with electrochemical detection

Capillary electrophoresis with wall-jet amperometric detection was used to detect kaempferol and its derivatives kaempferol-3-glucoside, kaempferol-3-glucosylrhamnoside and kaempferol-3-(p-coumaroyl)glucoside. The influence of buffer pH on separation was investigated and optimized. With a phosphate buffer at pH 7.5, nearly complete separation of the four kaempferols was achieved according to their different electrophoretic mobilities. The detection potential was also evaluated and optimized. At detection potential of +0.80 V vs. saturated calomel electrode, an amperometric response with high sensitivity and stability was obtained for these four compounds. Detection limit estimated for all the kaempferols examined was less than 1.4 fmol, based on S/N=3. The use of this method for the separation and detection of these compounds present in balsam flowers (Impatiens balsamina) is reported.

Capillary electrophoresis with an integrated on-capillary tubular detector based on a carbon sol-gel-derived platform

An integrated on-capillary tubular electrochemical detector for capillary electrophoresis systems has been fabricated based on sol-gel technique. It consists of a sol-gel carbon composite tubular electrode attached permanently onto the outlet of the separation capillary. The device greatly eases the setting up of capillary electrophoresis with electrochemical detection (CEEC) as it makes possible electrode/capillary alignment without the aid of a micromanipulator since this integrated unit can be simply immersed in the CE separation buffer in an ordinary three-electrode stationary cell. To improve analytical performance of the integrated unit, the external wall of the exit capillary was etched with HF after the polyimide coating of the capillary had been removed. Influences of the working electrode length and the wall thickness at the outlet of capillary on the separation efficiency and amperometric sensitivity were assessed and optimized. The practical applicability of this configuration is demonstrated with the detection of both catecholamines and carbohydrates. The advantages, namely, versatility, convenience, ease of operation, and low-cost, of the new design combined with an excellent performance lead to high stability and low detection limits.