Improved voltammetric peak separation and sensitivity of uric acid and ascorbic acid at nanoplatelets of graphitic oxide

In this study, much improved voltammetric peak separation and sensitivity of uric acid and ascorbic acid was observed at a screen-printed carbon electrode modified with nanoplatelets of graphitic oxide (GO). Electrochemical sensing of uric acid and ascorbic acid was further used to explore the role of oxygen functionalities and edge plane sites on electrocatalysis at GO. We successfully apply a microwave-assisted hydrothermal elimination method to remove the oxygen-containing functional groups from the GO surface. The edge plane on GO can be retained and the density of oxygen-containing functional groups can be easily controlled by varying the microwave treatment temperature. Using this platform, such discrimination to peak separation is attributed to the very distinct behavior of uric acid and ascorbic acid to form hydrogen bonds with oxo-surface groups (especially COOH group) at GO.     

Electrochemical detection of propofol at the preanodized carbon electrode

Preanodized screen printed carbon electrode (SPCE) has been utilized for the detection of propofol. Here the preanodized SPCE possess the specific functional groups which help the detection and determination of propofol. The proposed SPCE shows a clear oxidation peak for the detection of propofol in pH 7.0 phosphate buffer solutions. Interestingly, it shows a well-defined individual oxidation peak for the detection of propofol in the presence interferences (mixture of ascorbic acid, dopamine, and uric acid). This type of pretreated SPCE successfully enhances the electrooxidation current and overcomes the interference effects and clearly exhibits the signals for the propofol detection using cyclic voltammetry and flow injection analysis techniques. The preanodized SPCE shows the electrooxidation signals for the propofol detection in the linear range of 0.09 to 0.90 μM, respectively. Further, the sensitivity of the proposed electrode for the propofol detection is found to be 3.6 μA μM⁻¹.     

Simultaneous determination of ascorbic acid, uric acid and neurotransmitters with a carbon ceramic electrode prepared by sol-gel technique

A sol-gel carbon composite electrode (CCE) has been prepared by mixing a sol-gel precursor (e.g. methyltrimethoxysilane) and carbon powder without adding any electron transfer mediator or specific reagents. It was demonstrated that this sensor can be used for simultaneous determination ascorbic acid, neurotransmitters (dopamine and adrenaline) and uric acid. Direct electrochemical oxidation of ascorbic acid, uric acid and catecholamines at a carbon composite electrode was investigated. The experimental results were compared with other common carbon based electrodes, specifically, boron doped diamond, glassy carbon, graphite and carbon paste electrodes. It was found that the CCE shows a significantly higher of reversibility for dopamine. In addition, in comparison to the other electrodes used, for CCE the oxidation peaks of uric acid, ascorbic acid and catecholamines in cyclic and square wave voltammetry were well resolved at the low positive potential with good sensitivity. The advantages of this sensor were high sensitivity, inherent stability and simplicity and ability for simultaneous determination of uric acid, catecholamines and ascorbic acid without using any chromatography or separation systems. The analytical performance of this sensor has been evaluated for detection of biological molecules in urine and serum as real samples.     

Facile preparation of a cellulose microfibers–exfoliated graphite composite: a robust sensor for determining dopamine in biological samples

A simple and robust dopamine (DA) sensor was developed using a cellulose microfibers (CMF)–exfoliated graphite composite-modified screen-printed carbon electrode (SPCE) for the first time. The graphite-CMF composite was prepared by sonication of pristine graphite in CMF solution and was characterized by high-resolution scanning electron microscopy, Fourier transform, infrared, and Raman spectroscopy. The cyclic voltammetry results reveal that the graphite-CMF composite modified SPCE has superior electrocatalytic activity against oxidation of dopamine than SPCE modified with pristine graphite and CMF. The presence of large edge plane defects on exfoliated graphite and abundant oxygen functional groups of CMF enhance electrocatalytic activity and decrease potential to oxidize DA. Differential pulse voltammetry was used to quantify DA using the graphite-CMF composite-modified SPCE and demonstrated a linear response for DA detection in the range of 0.06–134.5 µM. The sensor shows a detection limit at 10 nM with an appropriate sensitivity and displays appropriate recovery of DA in human serum samples with good repeatability. Sensor selectivity is demonstrated in the presence of 50-fold concentrations of potentially active interfering compounds including ascorbic acid, uric acid, and dihydroxybenzene isomers.     

A Sensitive Electrochemical Approach for Melamine Detection Using a Disposable Screen Printed Carbon Electrode

We report here a simple and easy electrochemical approach for sensitive detection of non‐electroactive melamine using a disposable screen printed carbon electrode (SPCE) with uric acid as the recognition element. It is based on the competitive adsorptive behavior of melamine at the preanodized SPCE causing suppression in the oxidation current of uric acid. A linear range up to 126 ppb with a detection limit of 1.6 ppb (S/N=3) is achieved at the preanodized SPCE by differential pulse voltammetry. The electrochemical method is successfully applied to detect the melamine content in tainted milk powder and dog food.     

Determination of catechol derivatives on pretreatment and copolymer coated glassy carbon electrode

A glassy carbon electrode was pretreated electrochemically and was coated with a copolymer of maleic acid anhydride attached with Eastman-AQ55D (MA/AQ). The voltammetric behavior of a series of biologically important compounds, such as dopamine, L-DOPA, DOPAC, ascorbic acid and uric acid were examined at both pretreated and coated electrodes. Electrochemical pretreatment increased peak current of dopamine and L-DOPA while decreased that of ascorbic acid, uric acid and DOPAC. The copolymer coating caused a decrease of peak currents, but effectively hindered the anionic species (ascorbic acid, uric acid and DOPAC) access to the electrode surface. In flow injection and liquid chromatographic analysis. The dopamine and L-DOPA yielded the better selectivity response at MA/AQ electrode than at bare and AQ electrodes.     

Simultaneous detection of epinephrine,uric acid,and ascorbic acid with graphene-modified electrode

A graphene-modified glassy carbon electrode was obtained via drop-casting method and applied to the simultaneous detection of epinephrine, uric acid, and ascorbic acid by cyclic voltammetry in a phosphate buffer solution (pH 3.0). The oxidation potentials of epinephrine, uric acid, and ascorbic acid were 0.484, 0.650, and 0.184 V at the graphene-modified glassy carbon electrode, respectively. The peak separation between epinephrine Pand uric acid, epinephrine and ascorbic acid, and uric acid and ascorbic acid was about 166, 300, and 466 mV, respectively. So, this graphene-modified electrode can be used for simultaneous determination of each component in a mixture.     

Biomolecule-free,selective detection of clenbuterol based on disposable screen-printed carbon electrode

We report here the development of a selective clenbuterol sensor made of disposable screen-printed carbon electrode (SPCE) without the need of adding any biorecognition element. Good analytical performance was achieved through the proper function of both the oxygen functionalities and edge plane sites on the “preanodized” SPCE (SPCE*). It is the amino group of clenbuterol to effectively form hydrogen bond with the SPCE* to induce the adsorption of clenbuterol. The edge plane sites enhance the electron transfer process and further help the dimer formation of clenbuterol to generate electroactivity for analysis. Square wave voltammetry was applied to increase the detection sensitivity with a linear response in the range of 7–1000 ppb and a detection limit of 0.51 ppb (S/N = 3). In the real sample analysis, results observed were satisfactory with meat, human blood, and human urine. High reproducibility in sensor fabrication further favors the disposable purpose of applications.     

Treated Screen Printed Electrodes Based on Electrochemically Reduced Graphene Nanoribbons for the Sensitive Voltammetric Determination of Dopamine in the Presence of Uric Acid

In this research, the graphene oxide nanoribbons (GONRs) were substantially synthesized by the oxidative longitudinal unzipping of the multi‐walled carbon nanotubes (MWCNTs). Then, a direct electrochemical technique was employed for reducing GONRs adsorbed on the screen printed carbon electrode (SPCE). Electrochemical reduction effectively eliminated the oxygen‐containing groups in the GONRs and produced the electrochemically reduced graphene nanoribbons (ERGNRs). Field emission scanning electron microscopy (FE‐SEM), transmission electron microscopy (TEM), and X‐ray diffraction (XRD) were employed to characterize the materials. The modified SPCE with ERGNRs (ERGNRs/SPCE) displayed acceptable electrocatalytic characteristics towards the oxidation of dopamine (DA) and uric acid (UA) and applied to the simultaneous determination of these two analytes. ERGNRs/SPCE has a peak potential difference of 245 mV between DA and UA. The anodic peak currents of DA and UA were linear within the concentration ranges between 0.5 and 300.0 μM and 1.0 to 400.0 μM in phosphate buffer (pH=7.0) respectively. The detection limit of the technique for DA is 0.15 μM (S/N=3) and for UA is 0.3 μM (S/N=3). The proposed approach has been applied to the determination of DA and UA in real samples and generated acceptable outputs.     

Application of PEDOT‐CNT Microelectrodes for Neurotransmitter Sensing

In this work, composite microelectrodes from poly(3,4‐ethylenedioxythiophene) (PEDOT) and carbon nanotubes (CNT) are characterized as electrochemical sensing material for neurotransmitters. Dopamine can be detected using square wave voltammetry at these microelectrodes. The CNTs improve the sensitivity by a factor of two. In addition, the selectivity towards dopamine in the presence of ascorbic acid and uric acid was examined. While both electrodes, PEDOT and PEDOT‐CNT are able to detect all measured concentrations of dopamine in the presence of uric acid, small concentrations of dopamine and ascorbic acid are only distinguishable at PEDOT‐CNT electrodes. Changing the pH has a strong influence on the selectivity. Moreover, it is possible to detect concentrations as low as 1 µM dopamine in complex cell culture medium. Finally, other catecholamines like serotonin, epinephrine, norepinephrine and L ‐dopa are also electrochemically detectable at PEDOT‐CNT microelectrodes.     

An electrochemically preanodized screen-printed carbon electrode for achieving direct electron transfer to glucose oxidase

Here we report the unique property of a preanodized screen-printed carbon electrode (SPCE1) that can allow direct electron transfer (DET) reaction of glucose oxidase (GOx). The GOx can be immobilized in the composite of oxygen functionalities and edge plane sites generated during preanodization without additional cross-linking agents. The electron transfer rate of GOx is greatly enhanced to 4.38 s⁻¹ as a result of the conformational change of GOx in the microenvironment enabling the accessibility of active site for GOx to the electrode. The analytical versatility is further improved with the aid of Nafion film. As a consequence, the as-prepared electrode can be used as a glucose biosensor and the number of potential foreign species is then restricted by molecular size, permeation and/or (bio)chemical reaction. Most importantly, the disposable nature of the proposed electrode is expected to promote the DET-related researches.     

A highly selective and sensitive dopamine and uric acid biosensor fabricated with functionalized ordered mesoporous carbon and hydrophobic ionic liquid

An ordered mesoporous carbon material functionalized with carboxylic acid groups was synthesized. It was characterized by powder X-ray diffraction, transmission electron microscopy, Fourier transform IR spectroscopy and N₂ adsorption/desorption. Furthermore, this material was used to modify an electrode surface combined with a hydrophobic ionic liquid. The functionalized ordered mesoporous carbon/ionic liquid gel modified electrode shows excellent electrocatalytic performances for the oxidation of dopamine, uric acid and ascorbic acid. The presence of the ionic liquid promotes the electron transfer. Linear responses for dopamine and uric acid were obtained in the ranges of 0.1 to 500 μM and from 0.1 to 100 μM with detection limits of 4.1 and 2.5 nM (signal-to-noise ratio of 3), respectively, under optimum conditions. A quick and sensitive biosensor based on functionalized ordered mesoporous carbon and an ionic liquid has been developed for the first time for the detection of dopamine and uric acid in the presence of a large amount of ascorbic acid.     

Facile low-temperature growth of carbon nanosheets toward simultaneous determination of dopamine, ascorbic acid and uric acid

The bare Pt electrode modified directly and simply by carbon nanosheets, which were synthesized by microwave plasma enhanced chemical vapor deposition at relatively low temperature, can be used to simultaneously and effectively detect dopamine, ascorbic acid and uric acid.     

Activity of SiDbCl in the Electrooxidation of Ascorbic Acid,Dopamine, and Uric Acid

Selective electroanalytical responses for ascorbic acid, dopamine and uric acid at a carbon modified electrode based on 3‐n‐propyl‐1‐azonia‐4‐azabicyclo[2.2.2]octane silsesquioxane chloride (SiDbCl) is reported. The overlapped peaks observed at an unmodified electrode are resolved into three well defined voltammetric peaks allowing the simultaneous determination of the three species. Detection limits of 37, 0.3 and 0.1 μmo L⁻¹ of ascorbic acid, dopamine and uric acid, respectively, were calculated from calibration curves based on differential pulse voltammetric experiments performed in Britton ‐ Robinson buffer solution at pH 7.04.     

Simultaneous Determination of Ascorbic Acid,Dopamine and Uric Acid at Pt Nanoparticles Decorated Multiwall Carbon Nanotubes Modified GCE

A modified electrode was fabricated by electrochemically deposition of Pt nanoparticles on the multiwall carbon nanotube covered glassy carbon electrode (Pt nanoparticles decorated MWCNT/GCE). A higher catalytic activity was obtained to electrocatalytic oxidation of ascorbic acid, dopamine, and uric acid due to the enhanced peak current and well‐defined peak separations compared with both, bare and MWCNT/GCE. The electrode surfaces were characterized by scanning electron microscopy (SEM), X‐ray diffraction (XRD) and electrochemical impedance spectroscopy (EIS). Individual and simultaneous determination of AA, DA, and UA were studied by differential pulse voltammetry. The detection limits were individually calculated for ascorbic acid, dopamine, and uric acid as being 1.9×10^?5 M, 2.78×10^?8 M, and 3.2×10^?8 M, respectively. In simultaneous determination, LODs were calculated for AA, DA, and UA, as of 2×10^?5 M, 4.83×10^?8 M, and 3.5×10^?7 M, respectively.     

Simultaneous Sensitive Determination of Dopamine and Uric Acid in the Presence of Excess Ascorbic Acid with a Magnetic Chitosan Microsphere/Thionine Modified Electrode

Magnetic chitosan microspheres (MCMS) and thionine were incorporated in a modified electrode for the simultaneous sensitive determination of dopamine (DA) and uric acid (UA). Due to the unique properties of the MCMS and the electron mediation of thionine, this modified electrode showed excellent electrocatalytic oxidation toward dopamine and uric acid with a large separation of peak potentials and a significant enhancement of peak currents. However, the electrochemical behavior of ascorbic acid may be depressed at the modified electrode. Differential pulse voltammetry was used for the simultaneous sensitive determination of dopamine and uric acid in the presence of excess ascorbic acid at this modified electrode. The current responses showed excellent linear relationships in the range of 2–30 µM and 9–100 µM for dopamine and uric acid, respectively. The detection limits were estimated to be 0.5 µM and 2.3 µM for dopamine and uric acid, respectively. In addition, this modified electrode showed excellent repeatability, good stability, and satisfactory reliability, thus indicating potential for the practical applications.     

Voltammetric determination of dopamine in the presence of ascorbic and uric acids using partial least squares regression: determination of dopamine in human urine and plasma

A new differential pulse voltammetric method for dopamine determination at a bare glassy carbon electrode has been developed. Dopamine, ascorbic acid (AA) and uric acid (UA) usually coexist in physiological samples. Because AA and UA can be oxidized at potentials close to that of DA it is difficult to determine dopamine electrochemically, although resolution can be achieved using modified electrodes. Additionally, oxidized dopamine mediates AA oxidation and the electrode surface can be easily fouled by the AA oxidation product. In this work a chemometrics strategy, partial least squares (PLS) regression, has been applied to determine dopamine in the presence of AA and UA without electrode modification. The method is based on the electrooxidation of dopamine at a glassy carbon electrode in pH 7 phosphate buffer. The dopamine calibration curve was linear over the range of 1–313 μM and the limit of detection was 0.25 μM. The relative standard error (RSE %) was 5.28%. The method has been successfully applied to the measurement of dopamine in human plasma and urine.      

Highly selective dopamine quantification using a glassy carbon electrode modified with a melanin-type polymer

A highly selective dopamine quantification at a new polymer-modified electrode in the presence of large excess of ascorbic acid and 3,4-dihydroxyphenyl acetic acid (dopac) is described. The electrochemical detection was performed at a glassy carbon electrode modified with a melanin-type polymer obtained by polymerization of 3.0×10⁻³ M -dopa in 0.050 M phosphate buffer solution pH 7.40 by applying 1.00 V for 60 min. The polymeric film exhibits attractive permselectivity excluding anionic species such as potassium ferricyanide, ascorbic acid, dopac and uric acid. Cationic species such as epinephrine, norepinephrine and dopamine and neutral ones such as catechol and hydrogen peroxide can be oxidized at the polymer-modified electrode. The use of ascorbic acid in the measurement solution allows the amplification of dopamine oxidation signal due to the reduction of the electrochemically generated dopaminequinone. By using 1.0×10⁻³ M ascorbic acid, the detection limit for dopamine is 5.0 nM. The interference for the maximum physiological concentrations of ascorbic acid and dopac in nervous centers, i.e. 500 μM ascorbic acid and 50 μM dopac is 8.1 and 1.4%, respectively.     

Individual and simultaneous determination of uric acid and ascorbic acid by flow injection analysis

Flow injection methods for the individual and simultaneous determination of ascorbic acid and uric acid are proposed. A spectrophotometer and a miniamperometric detector are connected in sequence. The calibration graphs for uric acid obtained by measuring its absorbance at 293 nm and its current at +0.6 V are linear up to at least 80 and 70 mug/ml, respectively, with an rsd (n = 10) of 1 % for both methods at mid-range concentrations. The calibration graph for ascorbic acid with amperometric detection is linear up to 80 mg/l. with an rsd (n = 10) of 0.8% at 30 mg/l. The simultaneous determination of uric acid and ascorbic acid is based on measurement of the absorbance of uric acid at 393 nm and amperometric determination of both analytes at +0.6 V. The average relative errors of the analysis of binary mixtures of uric acid and ascorbic acid are 2.2 and 4.2%, respectively.     

Electrochemical oxidation of catecholamines and catechols at carbon nanotube electrodes

The differences in the electrochemical oxidation of two commonly known catecholamines, dopamine and norepinephrine, and one catechol, dihydroxyphenylacetic acid (DOPAC), at three different types of carbon based electrodes comprising conventionally polished glassy carbon (GC), nitrogen-doped carbon nanotubes (N-CNTs), and non-doped CNTs were assessed. Raman microscopy and X-ray photoelectron spectroscopy (XPS) were employed to evaluate structural and compositional properties. Raman measurements indicate that N-CNT electrodes have ca. 2.4 times more edge plane sites over non-doped CNTs. XPS data show no evidence of oxygen functionalities at the surface of either CNT type. N-CNTs possess 4.0 at. % nitrogen as pyridinic, pyrrolic, and quaternary nitrogen functionalities that result in positively charged carbon surfaces in neutral and acidic solutions. The electrochemical behavior of the various carbon electrodes were investigated by cyclic voltammetry conducted in pH 5.8 acetate buffer. Semiintegral analysis of the voltammograms reveals a significant adsorptive character of dopamine and norepinephrine oxidation at N-CNT electrodes. Larger peak splittings, DeltaE(p), for the cyclic voltammograms of both catecholamines and a smaller DeltaE(p) for the cyclic voltammogram for DOPAC at N-CNT electrodes suggest that electrostatic interactions hinder oxidation of cationic dopamine and norepinephrine, but facilitate anionic DOPAC oxidation. These observations were supported by titrimetry of solid suspensions to determine the pH of point of zero charge (pH(pzc)) and estimate the number of basic sites for both CNT varieties. This study demonstrates that carbon purity, the presence of exposed edge plane sites, surface charge, and basicity of CNTs are important factors for influencing adsorption and enhancing the electrochemical oxidation of catecholamines and catechols.