Silver Doped Poly(L‐valine) Modified Glassy Carbon Electrode for the Simultaneous Determination of Uric Acid,Ascorbic Acid and Dopamine

In this paper, a silver doped poly(L ‐valine) (Ag‐PLV) modified glassy carbon electrode (GCE) was fabricated through electrochemical immobilization and was used to electrochemically detect uric acid (UA), dopamine (DA) and ascorbic acid (AA) by linear sweep voltammetry. In pH 4.0 PBS, at a scan rate of 100 mV/s, the modified electrode gave three separated oxidation peaks at 591 mV, 399 mV and 161 mV for UA, DA and AA, respectively. The peak potential differences were 238 mV and 192 mV. The electrochemical behaviors of them at the modified electrode were explored in detail with cyclic voltammetry. Under the optimum conditions, the linear ranges were 3.0×10^?7 to 1.0×10^?5 M for UA, 5.0×10^?7 to 1.0×10^?5 M for DA and 1.0×10^?5 to 1.0×10^?3 M for AA, respectively. The method was successfully applied for simultaneous determination of UA, DA and AA in human urine samples.     

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.     

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.     

Simultaneous Voltammetric Determination of Uric Acid and Ascorbic Acid Using a Carbon‐Paste Electrode Modified with Multi‐Walled Carbon Nanotubes/Nafion and Cobalt(II)nitrosalophen

A composition of multiwalled carbon nanotube (MWCNT), Nafion and cobalt(II)‐5‐nitrosalophen (CoNSal) is applied for the modification of carbon‐paste electrode (CPE). The pretreated MWCNT is well dispersed in the alcoholic solution of Nafion under the ultrasonic agitation, and the resulted suspension is used as modifier (with 10% w/w) in the matrix of the paste electrode. The prepared electrode further modified by addition of 3 wt% of CoNSal. The resulted modified electrode is used as a sensitive voltammetric sensor for simultaneous determination of uric acid (UA) and ascorbic acid (AA). The electrode showed efficient electrocatalytic activity in lowering the anodic overpotentials and enhancement of the anodic currents. This electrode is able to completely resolve the voltammetric response of UA and AA. The effects of potential sweep rate and pH of the buffer solution on the response of the electrode, toward UA and AA, and the peak resolution is thoroughly investigated by cyclic and differential pulse voltammetry (CV and DPV). The best peak resolution for these compounds using the modified electrode is obtained in solutions with pH 4. The ΔE_p for UA and AA in these methods is about 315 mV, which is considerably better than previous reports for these compounds. A linear dynamic range of 1×10^?7 to 1×10^?4 M with a detection limit of 6×10^?8 M is resulted for UA in buffered solutions with pH 4.0. The voltammetric response characteristics for AA are obtained as, the linear range of 5×10^?7 to 1×10^?4 M with the detection limit of 1×10^?7 M. The voltammetric detection system was very stable and the reproducibility of the electrode response, based on the six measurements during one month, was less than 3.5% for the slope of the calibration curves of UA and AA. The prepared modified electrode is successfully applied for the determination of AA and UA in mixture samples and reasonable accuracies are resulted.     

Dopamine and Glucose Sensors Based on Glassy Carbon Electrodes Modified with Melanic Polymers

This work deals with the study of polymers electrogenerated from different catechols at glassy carbon electrodes and the analytical applications of the resulting modified electrodes for dopamine quantification and glucose biosensing. The electropolymerization was performed from a 3.0×10^?3 M catechol solution (catechol, dopamine, norepinephrine, epinephrine or L ‐dopa in a 0.050 M phosphate buffer pH 7.40) by applying 1.00 V for 60 min. The properties of the polymers are very dependent on the nature of the catechol, L ‐dopa being the best. Glassy carbon electrodes modified with melanic polymers electrogenerated from L ‐dopa and norepinephrine were found to be suitable for dopamine determinations in flow systems, although the behavior was highly dependent on the nature of the monomer. Detection limits of 5.0 nM dopamine and interferences of 9.0 and 2.6% for 5.0×10^?4 M ascorbic acid and 5.0×10^?5 M dopac, respectively, were obtained at the glassy carbon electrode modified with a melanin‐type polymer generated from L ‐dopa (using 1.0×10^?3 M AA in the measurement solution). The advantages of using a melanin‐type polymer generated from dopamine to improve the selectivity of glucose biosensors based on carbon paste electrodes containing Pt and glucose oxidase (GOx) are also discussed. The resulting bioelectrodes combines the high sensitivity of metallized electrodes with the selectivity given by the polymeric layer. They exhibit excellent performance for glucose with a rapid response (around 10 seconds per sample), a wide linear range (up to 2.5×10^?2 M glucose), low detection limits (143 μM) and a highly reproducible response (R.S.D of 4.9%). The bioelectrodes are highly stable and almost free from the interference of large excess of easily oxidizable compounds found in biological fluids, such as ascorbic acid (AA), uric acid (UA) and acetaminophen.     

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.     

Voltammetric Determination of Uric Acid at a Fullerene‐C60‐Modified Glassy Carbon Electrode

Glassy carbon electrode modified by microcrystals of fullerene‐C₆₀ mediates the voltammetric determination of uric acid (UA) in the presence of ascorbic acid (AA). Interference of AA was overcome owing to the ability of pretreated fullerene‐C₆₀‐modified glassy carbon electrode. Based on its strong catalytic function towards the oxidation of UA and AA, the overlapping voltammetric response of uric acid and ascorbic acid is resolved into two well‐defined voltammetric peaks with lowered oxidation potential and enhanced oxidation currents under conditions of both linear sweep voltammetry (LSV) and Osteryoung square‐wave voltammetry (OSWV). At pH 7.2, a linear calibration graph is obtained for UA in linear sweep voltammetry over the range from 0.5 μM to 700 μM with a correlation coefficient of 0.9904 and a sensitivity of 0.0215 μA μM^?1 . The detection limit (3σ) is 0.2 μM for standard solution. AA in less than four fold excess does not interfere. The sensitivity and detection limit in OSWV were found as 0.0255 μA μM^?1 and 0.12 μM, for standard solution respectively. The presence of physiologically common interferents (i.e. adenine, hypoxanthine and xanthine) negligibly affects the response of UA. The fullerene‐C₆₀‐modified electrode exhibited a stable, selective and sensitive response to uric acid in the presence of interferents.     

Selective Detection of Uric Acid in the Presence of Ascorbic Acid and Dopamine Using Polymerized Luminol Film Modified Glassy Carbon Electrode

Electrochemically polymerized luminol film on a glassy carbon electrode (GCE) surface has been used as a sensor for selective detection of uric acid (UA) in the presence of ascorbic acid (AA) and dopamine (DA). Cyclic voltammetry was used to evaluate the electrochemical properties of the poly(luminol) film modified electrode. Scanning electron microscopy (SEM) and atomic force microscopy (AFM) have been used for surface characterizations. The bare GCE failed to distinguish the oxidation peaks of AA, DA and UA in phosphate buffer solution (pH 7.0), while the poly(luminol) modified electrode could separate them efficiently. In differential pulse voltammetric (DPV) measurements, the modified GCE could separate AA and DA signals from UA, allowing the selective determination of UA. Using DPV, the linear range (3.0×10^?5 to 1.0×10^?3 M) and the detection limit (2.0×10^?6 M) were estimated for measurement of UA in physiological condition. The applicability of the prepared electrode was demonstrated by measuring UA in human urine samples.     

Voltammetric Determination of Uric Acid with a Glassy Carbon Electrode Coated by Paste of Multiwalled Carbon Nanotubes and Ionic Liquid

The voltammetric behavior of uric acid (UA) has been studied at a multiwalled carbon nanotube‐ionic liquid (i.e., 1‐butyl‐3‐methylimidazolium hexafluorophosphate, BMIMPF₆) paste coated glassy carbon electrode (MWNTs‐BMIMPF₆/GC). It is found that UA can effectively accumulate at this electrode and cause a sensitive anodic peak at about 0.49 V (vs. SCE) in pH 4.0 phosphate buffer solutions. Experimental parameters influencing the response of the electrode, such as solution pH and accumulation time, are optimized for uric acid determination. Under the optimum conditions, the anodic peak current is linear to UA concentration in the range of 1.0×10^?8 M to 1.0×10^?6 M and 2.0×10^?6 M to 2.0×10^?5 M. The detection limit is 5.0×10^?9 M for 180 s accumulation on open circuit. The electrode can be regenerated by successively cycling in a blank solution for about 3 min and exhibits good reproducibility. A 1.0×10^?6 M UA solution is measured for eight times using the same electrode regenerated after every determination, and the relative standard deviation (RSD) of the peak current is 3.2%. As for different electrodes fabricated by the same way the RSD (i.e., the electrode to electrode deviation) is 4.2%(n=9). This method has been applied to the determination of UA in human urine samples, and the recoveries are 99%–100.6%. In addition, comparison is made between MWNTs‐BMIMPF₆/GC and MWNTs/GC. Results show that the MWNTs‐BMIMPF₆/GC exhibits higher sensitivity, selectivity and ratio of peak current to background current.     

Simultaneous Determination of Dopamine and Ascorbic Acid Using the Nano‐Gold Self‐Assembled Glassy Carbon Electrode

Electrochemical behavior of dopamine (DA) was investigated at the gold nanoparticles self‐assembled glassy carbon electrode (GNP/LC/GCE), which was fabricated by self‐assembling gold nanoparticles on the surface of L ‐cysteine (LC) modified glassy carbon electrode (GCE) via successive cyclic voltammetry (CV). A pair of well‐defined redox peaks of DA on the GNP/LC/GCE was obtained at E_pa=0.197 V and E_pc=0.146 V, respectively. And the peak separation between DA and AA is about 0.2 V, which is enough for simultaneous determination of DA and AA. The peak currents of DA and AA were proportional with their concentrations in the range of 6.0×10^?8–8.5×10^?5 mol L^?1 and 1.0×10^?6–2.5×10^?3 mol L^?1, with the detection limit of 2.0×10^?8 mol L^?1 and 3.0×10^?7 mol L^?1 (S/N=3), respectively. The modified electrode exhibits an excellent reproducibility, sensibility and stability for simultaneous determination of DA and AA in human serum with satisfactory result.     

Improved Voltammetric Response of L‐Tyrosine on Multiwalled Carbon Nanotubes‐Ionic Liquid Composite Coated Glassy Electrodes in the Presence of Cupric Ion

L ‐Tyrosine can exhibit a small anodic peak on multiwalled carbon nanotubes (MWCNTs) coated glassy carbon electrodes (GCE). At pH 5.5 its peak potential is 0.70 V (vs. SCE). When an ionic liquid (i.e., 1‐octyl‐3‐methylimidazolium hexafluorophosphate, [omim][PF₆]) is introduced on the MWCNT coat, the peak becomes bigger. Furthermore, in the presence of Cu²⁺ ion the anodic peak of L ‐tyrosine increases further due to the formation of Cu²⁺‐L ‐tyrosine complex, while the peak potential keeps unchanged. Therefore, a sensitive voltammetry based on the oxidation of Cu²⁺‐L ‐tyrosine complex on MWCNTs‐[omim][PF₆] composite coated electrode is developed for L ‐tyrosine. Under the optimized conditions, the anodic peak current is linear to L ‐tyrosine concentration in the range of 1×10^?8–5×10^?6 M, and the detection limit is 8×10^?9 M. The modified electrode shows good reproducibility and stability. In addition, the voltammetric behavior of other amino acids is explored. It is found that among them tryptophan (Trp) and histidine (His) can also produce sensitive anodic peak under same experimental conditions, and their detection limits are 4×10^?9 M and 4×10^?6 M, respectively.     

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.     

Selective Electrochemical Determination of Dopamine with Molecularly Imprinted Poly(Carbazole‐co‐Aniline) Electrode Decorated with Gold Nanoparticles

Dopamine (DA) is a significant neurotransmitter in the central nervous system, coexisting with uric acid (UA) and ascorbic acid (AA). UA and AA are easily oxidizable compounds having potentials close to that of DA for electrochemical analysis, resulting in overlapping voltammetric response. In this work, a novel molecularly imprinted (MI) electrochemical sensor was proposed for selective determination of DA (in the presence of up to 80‐fold excess of UA and AA), relying on gold nanoparticles (Au_nano)‐decorated glassy carbon (GC) electrode coated with poly(carbazole (Cz)‐co‐aniline (ANI)) copolymer film incorporating DA as template (DA imprinted‐GC/P(Cz‐co‐ANI)‐Au_nano electrode, DA‐MIP‐Au_nano electrode). The DA recognizing sensor electrode showed great electroactivity for analyte oxidation in 0.2 mol L^?1 pH 7 phosphate buffer. Square wave voltammetry (SWV) was performed within 10^?4–10^?5 mol L^?1 of DA, of which the oxidation peak potential was observed at 0.16 V. The limit of detection (LOD) and limit of quantification (LOQ) were 2.0×10^?6 and 6.7×10^?6 mol L^?1, respectively. Binary and ternary synthetic mixtures of DA‐UA, DA‐AA and DA‐UA‐AA yielded excellent recoveries for DA. Additionally, DA was quantitatively recovered from a real sample of bovine serum spiked with DA, and determined in concentrated dopamine injection solution. The developed SWV method was statistically validated against a literature potentiodynamic method using a caffeic acid modified‐GC electrode.     

Electrocatalytic Oxidation and Voltammetric Determination of L‐Cysteic Acid at the Surface of p‐Bromanil Modified Carbon Paste Electrode

The electrochemical properties of L ‐cysteic acid studied at the surface of p‐bromanil (tetrabromo‐p‐benzoquinone) modified carbon paste electrode (BMCPE) in aqueous media by cyclic voltammetry (CV) and double step potential chronoamperometry. It has been found that under optimum condition (pH 7.00) in cyclic voltammetry, the oxidation of L ‐cysteic acid at the surface of BMCPE occurs at a half‐wave potential of p‐bromanil redox system (e.g., 100 mV vs. Ag|AgCl|KCl_sat), whereas, L ‐cysteic acid was electroinactive in the testing potential ranges at the surface of bare carbon paste electrode. The apparent diffusion coefficient of spiked p‐bromanil in paraffin oil was also determined by using the Cottrell equation. The electrocatalytic oxidation peak current of L ‐cysteic acid exhibits a linear dependency to its concentration in the ranges of 8.00×10^?6 M–6.00×10^?3 M and 5.2×10^?7 M–1.0×10^?5 M using CV and differential pulse voltammetry (DPV) methods, respectively. The detection limits (2σ) were determined as 5.00×10^?6 M and 4.00×10^?7 M by CV and DPV methods. This method was used as a new, selective, rapid, simple, precise and suitable voltammetric method for determination of L ‐cysteic acid in serum of patient's blood with migraine disease.     

Simultaneous Detection of Dopamine and Uric Acid under Coexistence of Ascorbic Acid with DNA/Pt Nanocluster Modified Electrode

A novel biosensor by electrochemically codeposited Pt nanoclusters and DNA film was constructed and applied to detection of dopamine (DA) and uric acid (UA) in the presence of high concentration ascorbic acid (AA). Scanning electron microscopy and X‐ray photoelectron spectroscopy were used for characterization. This electrode was successfully used to resolve the overlapping voltammetric response of DA, UA and AA into three well‐defined peaks with a large anodic peak difference (ΔE_pa) of about 184 mV for DA and 324 mV for UA. The catalytic peak current obtained from differential pulse voltammetry was linearly dependent on the DA concentration from 1.1× 10^?7 to 3.8×10^?5 mol·L^?1 with a detection limit of 3.6×10^?8 mol·L^?1 (S/N=3) and on the UA concentration from 3.0×10^?7 to 5.7×10^?5 mol·L^?1 with a detection limit of 1.0×10^?7 mol·L^?1 with coexistence of 1.0×10^?3 mol·L^?1 AA. The modified electrode shows good sensitivity and selectivity.     

Electrocatalytic Oxidation of Dopamine by Ferrocene in Lipid Film Cast on a Glassy Carbon Electrode

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The Influence of Gold Nanoparticles on Simultaneous Determination of Uric Acid and Ascorbic Acid

A three-dimensional L-cysteine (L-cys) monolayer assembled on gold nanoparticles (GNP) providing simultaneous detection of uric acid (UA) and ascorbic acid (AA) was studied in this work. The cyclic voltammetry demonstrated that, at a bare glassy carbon electrode (GCE) or planar gold electrode, the mixture of UA and AA showed one overlapped oxidation peak; whereas when the electrode was modified with GNP, the oxidation peaks for UA and AA were separated. While a GNP modified electrode was further modified with L-cys monolayer (L-cys/GNP/GCE), namely, three-dimensional L-cys monolayer, a better separation for UA and AA response was obtained. Interestingly, the L-cys monolayer-modified planar gold electrode presented a block effect on the oxidation of AA, which was facilitated by the three-dimensional L-cys monolayer attributed to its distinct structure. The pH of solution presented a noticeable effect on the separation of UA and AA at GNP modified electrodes with or without L-cys monolayer. Wide concentration ranges from 2 × 10^?6?1 × 10^?3 M to UA and 2 × 10^?6?8 × 10^?4 M to AA could be obtained at L-cys/GNP/GCE.     

Electrocatalytic Oxidation and Determination of Dopamine in the Presence of Ascorbic Acid and Uric Acid at a Poly (4‐(2‐Pyridylazo)‐Resorcinol) Modified Glassy Carbon Electrode

A sensitive and selective electrochemical method for the determination of dopamine (DA) was developed using a 4‐(2‐Pyridylazo)‐Resorcinol (PAR) polymer film modified glassy carbon electrode (GCE). The PAR polymer film modified electrode shows excellent electrocatalytic activity toward the oxidation of DA in a phosphate buffer solution (PBS) (pH 4.0). The linear range of 5.0×10^?6–3.0×10^?5 M and detection limit of 2.0×10^?7 M were observed. Simultaneous detection of AA, DA and UA has also been demonstrated on the modified electrode. This work provides a simple and easy approach to selective detection of DA in the presence of AA and UA.     

Differential Pulse Voltammetric Determination of Uric Acid on Carbon‐Coated Iron Nanoparticle Modified Glassy Carbon Electrodes

A carbon‐coated iron nanoparticles (CIN, a new style fullerence related nanomaterial) modified glassy carbon electrode (CIN/GCE) has been developed for the determination of uric acid (UA). Electrochemical behaviors of UA on CIN/GCE were explored by cyclic voltammetry (CV) and differential pulse voltammetry (DPV). It was found that the voltammetric response of UA on CIN/GC was enhanced dramatically because of the strong accumulation effect of CIN and the large working area of the CIN/GC electrode. The parameters including the pH of supporting electrolyte, accumulation potential and time, that govern the analytical performance of UA have been studied and optimized. The DPV signal of UA on CIN/GCE increased linearly with its concentration in the range from 5.0×10^?7 to 2.0×10^?5 M, with a detection limit of 1.5×10^?7 M (S/N=3). The CIN/GCE was used for the determination of UA in samples with satisfactory results. The proposed CIN/GCE electrochemical sensing platform holds great promise for simple, rapid, and accurate detection of UA.     

Fabrication of Nanocomposite Containing Naphthoquinone and Nanogold Supported on Poly(2,6‐pyridinedicarboxylic acid) Film for Voltammetric Determination of N‐Acetyl‐L‐Cysteine

A modified electrode was fabricated by grafting of poly (2,6‐pyridinedicarboxylic acid) film (PDC) by electropolymerization of 2,6‐pyridinedicarboxylic acid on the glassy carbon electrode (GCE). Then, gold nanoparticles (NG) and 1,2‐naphthoquinone‐4‐sulfonic acid sodium (Nq) were immobilized on the PDC/GCE to prepare Nq/NG/PDC/GCE by immersing electrode into NG and Nq solution, respectively. The Nq species on NG/PDC/GCE could catalyze electrooxidation of N‐acetyl‐L ‐cysteine (NAC) with lowering the over potential by about 600 mV. This method used for detection of NAC in dynamic range from 4.0×10^?6 M to 1.30×10^?4 M with a detection of limit (2σ) 8.0×10^?7 M.