Selective Voltammetric Detection of Uric Acid in the Presence of Ascorbic Acid at Well‐Aligned Carbon Nanotube Electrode

The voltammetric behaviors of uric acid (UA) and L ‐ascorbic acid (L ‐AA) were studied at well‐aligned carbon nanotube electrode. Compared to glassy carbon, carbon nanotube electrode catalyzes oxidation of UA and L ‐AA, reducing the overpotentials by about 0.028 V and 0.416 V, respectively. Based on its differential catalytic function toward the oxidation of UA and L ‐AA, the carbon nanotube electrode resolved the overlapping voltammetric response of UA and L ‐AA into two well‐defined voltammetric peaks in applying both cyclic voltammetry (CV) and differential pulse voltammetry (DPV), which can be used for a selective determination of UA in the presence of L ‐AA. The peak current obtained from DPV was linearly dependent on the UA concentration in the range of 0.2 μM to 80 μM with a correlation coefficient of 0.997. The detection limit (3δ) for UA was found to be 0.1 μM. Finally, the carbon nanotube electrode was successfully demonstrated as a electrochemical sensor to the determination of UA in human urine samples by simple dilution without further pretreatment.     

Flow injection amperometric sensing of uric acid and ascorbic acid using the self-assembly of heterocyclic thiol on Au electrode

Au electrode modified with the self-assembled monolayer of a heterocyclic thiol, mercaptotriazole (MTz), is used for the electroanalysis of uric acid (UA) and ascorbic acid (AA). MTz forms a less compact self-assembly on Au electrode. The self-assembly of MTz on Au electrode favors the oxidation of UA and AA at less positive potential. Significant decrease (∼400 mV) in the overpotential and enhancement in the peak current for the oxidation of interfering AA with respect to the unmodified electrode is observed. The negative shift in the oxidation peak potential of AA favors electrochemical sensing of UA without any interference. Two well-separated voltammetric peaks for AA and UA are observed in their coexistence. The large separation between the two voltammetric peaks allows the simultaneous or selective sensing of the analytes without compromising the sensitivity. Linear response is obtained for a wide concentration range. This electrode could sense as low as 1 μM of UA in the presence of 10-fold excess of interfering AA. No change in the sensitivity (0.012 μA/μM) of the electrode toward UA in the presence and absence of AA is observed. Reproducible and stable amperometric flow injection response was obtained upon repetitive injection.     

Modified platinum electrode with phytic acid and single-walled carbon nanotube: Application to the selective determination of dopamine in the presence of ascorbic and uric acids

A platinum (Pt) electrode modified by single-walled carbon nanotubes (SWNTs) and phytic acid (PA) was investigated by voltammetric methods in buffer solution. The PA-SWNTs/Pt-modified electrode demonstrated substantial enhancements in electrochemical sensitivity and selectivity towards dopamine (DA) in the presence of L-ascorbic acid (AA) and uric acid (UA). The PA-SWNTs films promoted the electron transfer reaction of DA, while the PA film, acting as a negatively charged linker, combined with the positively charged DA to induced DA accumulation in the film at pH under 7.4. However, the PA film restrained the electrochemical response of the negatively charged AA due to the electrostatic repulsion. The anodic peak potentials of DA, AA and UA could be separated by electrochemical techniques, and the interferences from AA and UA were effectively eliminated in the DA determination. Linear calibration plots were obtained in the DA concentration range of 0.2-10 μM and the detection limit of the DA oxidation current was determined to be 0.08 μM at a signal-to-noise ratio of 3. The results indicated that the modified electrode can be used to determine DA without interference from AA and UA, while ensuring good sensitivity, selectivity, and reproducibility.     

Gold Electrode Modified with Self‐Assembled Monolayer of Cysteamine‐Functionalized MWCNT and Its Application in Simultaneous Determination of Dopamine and Uric Acid

The voltammetric behavior of dopamine (DA) and uric acid (UA) on a gold electrode modified with self‐assembled monolayer (SAM) of cysteamine (CA) conjugated with functionalized multiwalled carbon nanotubes (MWCNTs) was investigated. The film modifier of functionalized SAM was characterized by means of scanning electron microscopy (SEM) and also, electrochemical impedance spectroscopy (EIS) using para‐hydroquinone (PHQ) as a redox probe. For the binary mixture of DA and UA, the voltammetric signals of these two compounds can be well separated from each other, allowing simultaneous determination of DA and UA. The effect of various experimental parameters on the voltammetric responses of DA and UA was investigated. The detection limit in differential pulse voltammetric determinations was obtained as 0.02 µM and 0.1 µM for DA and UA, respectively. The prepared modified electrode indicated a stable behavior and the presence of surface COOH groups of the functionalized MWCNT avoided the passivation of the electrode surface during the electrode processes. The proposed method was successfully applied for the determination of DA and UA in urine samples with satisfactory results. The response of the gold electrode modified with MWCNT‐functionalized SAM method toward DA, UA, and ascorbic acid (AA) oxidation was compared with the response of the modified electrode prepared by the direct casting of MWCNT.     

One step fabrication of nanoelectrode ensembles formed via amphiphilic block copolymers self-assembly and selective voltammetric detection of uric acid in the presence of high ascorbic acid content

A novel one-step approach to glassy carbon nanoelectrode ensembles (NEEs) with the pores of 20-120 nm in radii has been developed using an amphiphilic block copolymer [polystyrene-block-poly (acrylic acid)] self-assembly. This procedure is simple and fast, and requires only conventional, inexpensive electrochemical instrumentation. Electrochemical methods were used to characterize the NEEs prepared using this new procedure. The NEEs drastically suppressed the response of ascorbic acid (AA) and resolved the overlapping voltammetric response of uric acid (UA) and AA into two well-defined peaks with a large anodic peak difference (ΔE_pa) of about 310 mV. The peak current obtained from differential pulse voltammetry (DPV) was linearly dependent on the UA concentration in the range of 0.25-50 μM at neutral pH (PBS, pH 6.86) with a correlation coefficient of 0.999, and the detection limit was 0.04 μM (S/N = 3). The NEEs has also been demonstrated to be applicable in the detection of UA in serum and urine samples with excellent sensitivity and selectivity. The NEEs will hopefully be of good application for further sensor development.     

A quercetin-modified biosensor for amperometric determination of uric acid in the presence of ascorbic acid

The present work reports a quercetin-modified wax-impregnated graphite electrode (Qu/WGE) prepared through an electrochemical oxidation procedure in quercetin-containing phosphate buffer solution (PBS), for the purpose of detecting uric acid (UA) in the presence of ascorbic acid (AA). During modification quercetin was oxidized to the corresponding quinonic structure, and in the blank buffer solution the electrodeposited film exhibits a voltammetric response anticipated for the surface-immobilized quercetin. Retarding effect of the film towards the reaction of anionic species was found; therefore the pH of sample solutions was selected to ensure the analyte in molecular form. At suitable pHs the Qu/WGE shows excellent electrocatalytic effect towards the oxidation of both AA and UA, and separates the voltammetric signal of UA from AA by about 280 mV, allowing simultaneous detection of these two species. A linear relation between the peak current and concentration was obtained for UA in the range of 1-50 μM in the presence of 0.5 mM AA, with a detection limit 1.0 μM (S/N = 3). This sensor was stable, reproducible and outstanding for long-term use.     

Electropolymerized film of functionalized thiadiazole on glassy carbon electrode for the simultaneous determination of ascorbic acid, dopamine and uric acid

The present study reports the simultaneous determination of ascorbic acid (AA), dopamine (DA) and uric acid (UA) in 0.20 M phosphate buffer solution (pH 5.0) using electropolymerized ultrathin film of 5-amino-2-mercapto-1,3,4-thiadiazole (AMT) on glassy carbon (GC) electrode. The bare GC electrode does not separate the voltammetric signals of AA, DA and UA. However, electropolymerized AMT (p-AMT) modified GC electrode not only resolved the voltammetric signals of AA, DA and UA but also dramatically enhanced their oxidation peak currents when compared to bare GC electrode. The enhanced oxidation currents for AA, DA and UA at p-AMT modified electrode are due to the electrostatic interactions between them and the polymer film. Using amperometric method, we achieved the lowest detection of 75 nM AA, 40 nM DA and 60 nM UA at p-AMT modified electrode. The amperometric current was linearly increased from 200 nM to 0.80 mM for each AA, DA and UA and the lowest detection limit was found to be 0.92, 0.07 and 0.57 nM, respectively (S/N = 3). The practical application of the modified electrode was demonstrated by the determination of DA in dopamine hydrochloride injection.     

Modification of carbon paste with congo red supported on multi-walled carbon nanotube for voltammetric determination of uric acid in the presence of ascorbic acid

A chemically modified carbon-paste electrode (CPE) is prepared by incorporating congo red (CR) immobilized on multi-walled carbon nanotube (MWCNT). The results show that CR is effectively immobilized on the surface of MWCNT under the ultrasonic agitation in aqueous solution and further incorporating the nafion. The prepared electrode, due to the electrostatic repulsions between the CR and ascorbate anion, is capable to mask the response of the ascorbic acid (AA) completely and provide an effective method for the detection of minor amounts of uric acid (UA) in the presence of high concentrations of AA. On the other hand, an increase in the microscopic area of the electrode by addition of MWCNT together with the electrocatalytic activity caused to a significant enhancement in the voltammetric response to UA. Optimization of the amounts of composite modifier in the matrix of CPE is performed by cyclic and differential pulse voltammetric measurements. The modified electrode shows a linear response to UA in the range of 1.0 × 10⁻⁷–1.0 × 10⁻⁴ M with a detection limit of 1.0 × 10⁻⁸ M. The electrode exhibits excellent accuracies for the determination of UA in the presence of high concentrations of AA (a recovery of 97.6%). The response of the electrode toward sulfhydryl compounds such as cysteine, penicillamine, and glutathione is not considerable. This reveals a good selectivity for the voltammetric response toward UA. The effective electrocatalytic property, ability for masking the voltammetric responses of the other biologically reducing agents, ease of preparation, and surface regeneration by simple polishing together with high reproducibility and stability of the responses make the modified electrode suitable for the selective and sensitive voltammetric detection of sub-micromolar amounts of UA in clinical and pharmaceutical preparations.     

Quantification of methyldopa in pharmaceuticals using a glassy carbon electrode modified with carbon nanotubes

The quantification of methyldopa in pharmaceuticals has been carried out using a glassy carbon electrode(GCE) modified with multi-walled carbon nanotubes(MWCNTs). Methyldopa exhibited a quasi-reversible response with a peak potential separation of 473 m V on a bare GCE. However, the cyclic voltammetric behaviour of methyldopa was improved with the increase of the amount of MWCNTs. It was also shown that the electrocatalytic activity of the electrode towards the response of methyldopa was higher with larger amount of film on the surface. The results showed that the peak current was proportional to the concentration of methyldopa with a linear dynamic range of 0.005–0.388 mmol/L and a detection limit of 1.0 nmol/L was obtained using square wave voltammetry. The experimental data showed that the detection limit of methyldopa and peak separation from interfering compounds such as ascorbic acid(AA) and uric acid(UA) were improved using the proposed procedure. The method was successfully applied for the determination of methyldopa in pharmaceuticals.     

A Selective Voltammetric Method for Uric Acid Detection at a Glassy Carbon Electrode Modified with Electrodeposited Film Containing DNA and Pt‐Fe(III) Nanocomposites

A novel biosensor by electrochemical codeposited Pt‐Fe(III) nanocomposites and DNA film was constructed and applied to the detection of uric acid (UA) in the presence of high concentration of ascorbic acid (AA). Based on its strong catalytic activity toward the oxidation of UA and AA, the modified electrode resolved the overlapping voltammetric response of UA and AA into two well‐defined peaks with a large anodic peak difference (ΔE_pa) of about 380mV. The catalytic peak current obtained from differential pulse voltammetry (DPV) was linearly dependent on the UA concentration from 3.8×10^?6 to 1.6×10^?4 M (r=0.9967) with coexistence of 5.0×10^?4 M AA. The detection limit was 1.8×10^?6 M (S/N=3) and the presence of 20 times higher concentration of AA did not interfere with the determination. The modified electrode shows good sensitivity, selectivity and stability.     

Electrochemical behavior of isoproterenol in the presence of uric acid and folic acid at a carbon paste electrode modified with 2,7-bis(ferrocenyl ethyl)fluoren-9-one and carbon nanotubes

This paper reports the selective determination of isoproterenol (IP) in the presence of uric acid (UA) and folic acid (FA) using 2,7-bis(ferrocenyl ethyl)fluoren-9-one modified carbon nanotube paste electrode (2,7-BFCNPE) in 0.1 M phosphate buffer solution (PBS) (pH 7.0). The bare carbon paste electrode does not separate the voltammetric signals of IP, UA, and FA. However, 2,7-BFCNPE not only resolved the voltammetric signals of IP, UA, and FA with potential differences of 150, 325, and 475 mV between IP–UA, UA–FA, and IP–FA, respectively, but also dramatically enhanced the oxidation peak currents of them when compared to bare carbon paste electrode. In PBS of pH 7.0, the oxidation current increased linearly with two concentration intervals of IP, one is 0.08 to 17.5 μM and the other is 17.50 to 700.0 μM. The detection limit (3σ) obtained by DPV was 26.0 ± 2 nM. The practical application of the modified electrode was demonstrated by determining IP in IP injection, urine, and human blood serum.     

Fabrication of poly(orthanilic acid)–multiwalled carbon nanotubes composite film-modified glassy carbon electrode and its use for the simultaneous determination of uric acid and dopamine in the presence of ascorbic acid

A multiwalled carbon nanotubes (MWNT) modified glassy carbon electrode (GCE) coated with poly(orthanilic acid) (PABS) film (PABS–MWNT/GCE) has been fabricated and used for simultaneous determination of dopamine (DA) and uric acid (UA) in the presence of ascorbic acid (AA) by differential pulse voltammetry (DPV). Scanning electron microscopy, Fourier transform infrared spectra, and electrochemical techniques have been used to characterize the surface morphology of the PABS–MWNT composite film and the polymerization of ABS on electrode surface. In comparison with the bare GCE and the MWNT-modified GCE, the PABS–MWNT composite film-modified GCE, which combines the advantages of MWNT and the self-doped PABS, exhibits good selectivity and sensitivity for the simultaneous and selective determination of UA and DA in the presence of AA. Due to the different electrochemical responses of AA, DA, and UA, PABS–MWNT/GCE can resolve the overlapped oxidation peak of DA and UA into two well-defined voltammetric peaks with enhanced current responses using both cyclic voltammetry (CV) and DPV. The peak potential separations between DA and UA are 170 mV using CV and 160 mV using DPV, respectively, which are large enough for the selective and simultaneous determination of these species. In the presence of 0.5 mM AA, the DPV peak currents are linearly dependent on the concentration of UA and DA in the range of 6–55 and 9–48 μM with correlation coefficients of 0.997 and 0.993, respectively. The detection limits (S/N = 3) for detecting UA and DA are 0.44 and 0.21 μM, respectively. The PABS–MWNT/GCE shows good reproducibility and stability and has been used for the simultaneous determination of DA and UA in the presence of AA in samples with satisfactory results.     

Electrodeposition of gold nanoclusters on overoxidized polypyrrole film modified glassy carbon electrode and its application for the simultaneous determination of epinephrine and uric acid under coexistence of ascorbic acid

A novel biosensor was fabricated by electrochemical deposition of gold nanoclusters on ultrathin overoxidized polypyrrole (PPyox) film, formed a nano-Au/PPyox composite on glassy carbon electrode (nano-Au/PPyox/GCE). The properties of the nanocomposite have been characterized by field emission scanning electron microscope (FE-SEM), X-ray photoelectron spectroscopy (XPS), powder X-ray diffraction (XRD) and electrochemical investigations. The nano-Au/PPyox/GCE had strongly catalytic activity toward the oxidation of epinephrine (EP), uric acid (UA) and ascorbic acid (AA), and resolved the overlapping voltammetric response of EP, UA and AA into three well-defined peaks with a large anodic peak difference. The catalytic peak currents obtained from differential pulse voltammetry increased linearly with increasing EP and UA concentrations in the range of 3.0 × 10⁻⁷ to 2.1 × 10⁻⁵ M and 5.0 × 10⁻⁸ to 2.8 × 10⁻⁵ M with a detection limit of 3.0 × 10⁻⁸ and 1.2 × 10⁻⁸ M (s/n = 3), respectively. The results showed that the modified electrode can selectively determine EP and UA in the coexistence of a large amount of AA. In addition, the sensor exhibited excellent sensitivity, selectivity and stability. The nano-Au/PPyox/GCE has been applied to determination of EP in epinephrine hydrochloride injection and UA in urine samples with satisfactory results.     

Simultaneous determination of epinephrine, uric acid and xanthine in the presence of ascorbic acid using an ultrathin polymer film of 5-amino-1,3,4-thiadiazole-2-thiol modified electrode

This paper describes the simultaneous determination of epinephrine (EP), uric acid (UA) and xanthine (XN) in the presence of ascorbic acid (AA) using electropolymerized ultrathin film of 5-amino-1,3,4-thiadiazole-2-thiol (p-ATT) modified glassy carbon (GC) electrode in 0.2 M phosphate buffer solution (pH 5). Although bare GC electrode resolves the voltammetric signals of AA and XN, it fails to resolve the voltammetric signals of EP and UA in a mixture. However, the p-ATT modified electrode not only separates the voltammetric signals of AA, EP, UA and XN with potential difference of 150, 120 and 400 mV between AA-EP, EP-UA and UA-XN, respectively but also shows higher oxidation current for these molecules. The p-ATT modified electrode exhibits excellent selectivity towards the oxidation of EP, UA and XN in the presence of 40-fold higher concentration of AA. Further, the p-ATT modified electrode was also used for the selective determination of EP in the presence of 40-fold higher concentrations of AA, UA and XN. Using amperometric method, we achieved the lowest detection of 40 nM EP and 60 nM each UA and XN. The amperometric current response was increased linearly with increasing EP concentration in the range of 4.0 × 10⁻⁸ to 4.0 × 10⁻⁵ M and the detection limit was found to be 27 × 10⁻¹¹ M (S/N = 3). The practical application of the present modified electrode was demonstrated by determining the concentration of EP in epinephrine tartrate injection and XN in human urine samples.     

Simultaneous Voltammetric Detection of Salsolinol and Uric Acid in the Presence of High Concentration of Ascorbic Acid with Gold Nanoparticles/Functionalized Multiwalled Carbon Nanotubes Composite Film Modified Electrode

This work demonstrates gold nanoparticles (AuNPs)/functionalized multiwalled carbon nanotubes (f‐MWCNT) composite film modified gold electrode via covalent‐bonding interaction self‐assembly technique for simultaneous determination of salsolinol (Sal) and uric Acid (UA) in the presence of high concentration of ascorbic acid (AA). In pH 7.0 PBS, the composite film modified electrode exhibits excellent voltammetric response for Sal and UA, while AA shows no voltammetric response. The oxidation peak current is linearly increased with concentrations of Sal from 0.24–11.76 μmol L^?1 and of UA from 3.36–96.36 μmol L^?1, respectively. The detection limits of Sal and UA is 3.2×10^?8 mol L^?1 and 1.7×10^?7 mol L^?1 , respectively.     

Selective determination of uric acid in the presence of ascorbic acid using a penicillamine self-assembled gold electrode

The electrochemical behaviors of uric acid (UA) at the penicillamine (Pen) self-assembled monolayers modified gold electrode (Pen/Au) have been studied. The Pen/Au electrode is demonstrated to promote the electrochemical response of UA by cyclic voltammetry (CV). The diffusion coefficient D of UA is 6.97 × 10⁻⁶ cm² s⁻¹. In differential pulse voltammetric (DPV) measurements, the Pen/Au electrode can separate the UA and ascorbic acid (AA) oxidation potentials by about 120 mV and can be used for the selective determination of UA in the presence of AA. The detection limit was 1 × 10⁻⁶ mol L⁻¹. The modified electrode shows excellent sensitivity, good selectivity and antifouling properties.     

Simultaneous determination of ascorbic acid, dopamine, uric acid and xanthine using a nanostructured polymer film modified electrode

This paper describes the simultaneous determination of ascorbic acid (AA), dopamine (DA), uric acid (UA) and xanthine (XN) using an ultrathin electropolymerized film of 2-amino-1,3,4-thiadiazole (p-ATD) modified glassy carbon (GC) electrode in 0.20 M phosphate buffer solution (pH 5.0). Bare GC electrode failed to resolve the voltammetric signals of AA, DA, UA and XN in a mixture. On the other hand, the p-ATD modified electrode separated the voltammetric signals of AA, DA, UA and XN with potential differences of 110, 152 and 392 mV between AA-DA, DA-UA and UA-XN, respectively and also enhanced their oxidation peak currents. The modified electrode could sense 5 μM DA and 10 μM each UA and XN even in the presence of 200 μM AA. The oxidation currents were increased from 30 to 300 μM for AA, 5 to 50 μM for DA and 10 to 100 μM for each UA and XN, and the lowest detection limit was found to be 2.01, 0.33, 0.19 and 0.59 μM for AA, DA, UA and XN, respectively (S/N = 3). The practical application of the present modified electrode was demonstrated by the determination of AA, UA and XN in human urine samples.     

Electrochemical Determination of Tyrosine in the Presence of Dopamine and Uric Acid at the Surface of Gold Nanoparticles Modified Carbon Paste Electrode

A gold nanoparticles modified carbon paste electrode (GN‐CPE) was used as a highly sensitive electrochemical sensor for determination of tyrosine (Tyr), dopamine (DA) and uric acid (UA) in phosphate buffer solution (PBS). The study and measurements were carried out by using cyclic voltammetry (CV), differential pulse voltammetry (DPV) and chronoamperometry methods. In DPV, the GN‐CPE could separate the oxidation peak potentials of DA and UA present in the same solution, though at the unmodified CPE the peak potentials were indistinguishable. The prepared electrode showed voltammetric responses with high sensitivity and selectivity for Tyr, DA and UA in optimal conditions, which makes it very suitable for simultaneous determination of these compounds. The calibration curves for Try, DA and UA were linear for the concentrations of each species. The proposed voltammetric approach was also applied to the determination of Tyr concentration in human serum as a real sample.     

Simultaneous voltammetric detection of dopamine and uric acid in the presence of high concentration of ascorbic acid using multi-walled carbon nanotubes with methylene blue composite film-modified electrode

An electrochemically functional nanocomposite through the adsorption of methylene blue onto the multi-walled nanotubes (MB-MWNTs) was prepared, and a sensitive voltammetric sensor was fabricated. The modified electrode showed excellent electrocatalytic activity toward dopamine (DA) and uric acid (UA) in 0.1 M phosphate solution medium (pH 3.0). Compared to the bare electrode, the MB-MWNTs film-modified electrode not only remarkably enhanced the anodic peak currents of DA and UA, i.e., shifted the anodic peak potential of DA negatively, but also avoided the overlapping of the anodic peaks of DA and UA. The interference of ascorbic acid (AA) was eliminated. Under the optimized conditions, the peak separation between AA and DA and between DA and UA was 219 and 174 mV, respectively. In the presence of 1.0 mM AA and 10.0 μM UA, the anodic peak current was linear to the concentration of DA in the range of 0.4–10.0 μM with a detection limit of 0.2 μM DA. The anodic peak current of UA was linear to the concentration in the range of 2.0–20.0 and 20.0–200.0 μM with a lowest detection limit of 1.0 μM in the presence of 1.0 mM AA and 1.0 μM DA.     

Selective Voltammetric Determination of Uric Acid in the Presence of Ascorbic Acid at Ordered Mesoporous Carbon Modified Electrodes

A novel chemically modified electrode was fabricated by immobilizing ordered mesoporous carbon (OMC) onto a glassy carbon (GC) electrode. The electrocatalytic behavior of the OMC modified electrode towards the oxidation of uric acid (UA) and ascorbic acid (AA) was studied. Compared to a glassy carbon electrode, the OMC modified electrode showed a faster electron transfer rate and reduced the overpotentials greatly. Furthermore, the OMC modified electrode resolved the overlapping voltammetric responses of UA and AA into two well‐defined voltammetric peaks with peak separation of ca. 0.38 V. All results show that the OMC modified electrode has a good electrocatalytic ability to UA and AA, and has an excellent response towards UA even in the presence of high concentration AA.