Amperometric determination of ascorbic acid at a novel ‘self-doped’ polyaniline modified microelectrode

A ‘self-doped’ polyaniline modified microelectrode, prepared by cyclic potential sweep on a microdisk gold electrode from –0.2 to 0.85 V in 0.5 mol/L sulfuric acid containing aniline and o-aminobenzoic acid, has been developed. The copolymerized process and the resulting polymer characteristics were investigated in detail. This composite film indicated a good electrochemical activity in a wide pH range even in basic solution. Meanwhile, the redox couple exhibited an excellent electrocatalytic activity for the oxidation of ascorbic acid. The oxidation overpotential of ascorbic acid was decreased over 200 mV at this modified electrode compared with a bare gold one. Moreover, the effects of film thickness and pH on the catalytic efficiency were further studied. The dependence of catalytic currents on the concentration of ascorbic acid was linear in the range of 1.2 × 10^–5～ 2.4 × 10^–3 mol/L with a correlation coefficient of 0.996. Also, the determination of ascorbic acid in actual samples was evaluated and the results are satisfactory.     

Electrochemical synthesis of polyaniline in surface-attached poly(acrylic acid) network, and its application to the electrocatalytic oxidation of ascorbic acid

Acrylic acid was first electropolymerized on the surface of a gold electrode. Then, polyaniline (PANI) was electrodeposited on the poly(acrylic acid) (PAA) network to give a PANI–PAA composite film. Scanning electron microscopy and electrochemical studies confirmed the formation of PANI–PAA composite which exhibited excellent electroactivity over a wide pH range. The electro-oxidation of ascorbic acid (AA) was studied in detail. The modified electrode exhibits significantly reduced oxidation overpotential. The response towards AA is linear in the range 1.0 μM to 9.3 mM (R?=?0.9997, n?=?33) at a potential of 0.1 V (vs. SCE). The sensitivity is 207 μA mM^-1 cm^-2, and the detection limit is 1.0 μM (S/N?=?3). Interferences by uric acid and dopamine are negligible. The electrode thus enables sensitive and selective determination of AA, with a performance superior to many other PANI–based ascorbate sensors.     

An Electrochemical Biosensor with Uricase Immobilized on Functionalized Gold Coated Copper Wire Electrode for Urinary Uric Acid Assay

A new second generation uricase electrode for urinary uric acid determination has been developed by chemically binding both uricase and redox mediator to inexpensive copper wire through simple electrodeposition of gold on copper surface and subsequent functionalization of the gold with L‐methionine. During a 209‐day testing period, the overall electrode performance exhibits in average a low oxidation potential of 0.33 V, a response time of 5 s, a widest linear calibration concentration range (0–2.38 mM, r²>0.9952), a sensitivity of 50 μA mM^?1, and a detection limit of 2.4 μM. The measurement accuracy and precision for the determination of uric acid in human urine specimens were 85.6–95.5 % and 0.3–2.4 %, respectively. The developed uricase electrode is potential for clinical applications.     

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.     

Single step modification of copper electrode for the highly sensitive and selective non-enzymatic determination of glucose

A non-enzymatic sensor was developed for the determination of glucose in alkaline medium by anodisation of copper in sodium potassium tartrate solution. The morphology of the modified copper electrode was studied by scanning electron microscopy, and its electrochemical behavior by cyclic voltammetry and electrochemical impedance spectroscopy. The electrode enables direct electrocatalytic oxidation of glucose on a CuO/Cu electrode at 0.7 V in 0.1 M sodium hydroxide. At this potential, the sensor is highly selective to glucose even in the presence of ascorbic acid, uric acid, or dopamine which are common interfering species. The sensor displays a sensitivity of 761.9 μA mM^?1 cm^?2, a linear detection range from 2 μM to 20 mM, a response time of <1 s, and a detection limit of 1 μM (S/N = 3). It was tested for determination of glucose level in blood serum.     

Nonenzymatic Glucose Detection with Good Selectivity Against Ascorbic Acid on a Highly Porous Gold Electrode Subjected to Amalgamation Treatment

A nonenzymatic glucose sensor with good selectivity for the ascorbic acid oxidation is presented. After the gold polycrystalline electrode was subjected to amalgamation treatment, two advantageous effects were observed. One is the enhancement of the surface roughness and the other is an increase in the catalytic current in the glucose oxidation. Besides the known first effect, the latter provided another advantageous effect in a fabrication of nonenzymatic glucose sensor. Using a gold electrode subjected to amalgamation treatment for 60 s, two calibration curves for glucose oxidation at two different potentials of ?0.1 V and 0.25 V were obtained and compared. At the potential of ?0.1 V, at which no ascorbic acid was oxidized and no interference effect was observed, a current sensitivity of 16 μA cm^?2 mM^?1 from zero to 10 mM glucose concentration range was obtained. At the other potential of 0.25 V, at which ascorbic acid was easily oxidized, a satisfactory calibration curve with negligible ascorbic acid interference was also obtained together with a more enhanced current sensitivity of 32 μA cm^?2 mM^?1.     

Amperometric determination of ascorbic acid at a novel ‘self-doped’ polyaniline modified microelectrode

A ‘self-doped’ polyaniline modified microelectrode, prepared by cyclic potential sweep on a microdisk gold electrode from –0.2 to 0.85 V in 0.5 mol/L sulfuric acid containing aniline and o-aminobenzoic acid, has been developed. The copolymerized process and the resulting polymer characteristics were investigated in detail. This composite film indicated a good electrochemical activity in a wide pH range even in basic solution. Meanwhile, the redox couple exhibited an excellent electrocatalytic activity for the oxidation of ascorbic acid. The oxidation overpotential of ascorbic acid was decreased over 200 mV at this modified electrode compared with a bare gold one. Moreover, the effects of film thickness and pH on the catalytic efficiency were further studied. The dependence of catalytic currents on the concentration of ascorbic acid was linear in the range of 1.2 × 10^–5∼ 2.4 × 10^–3 mol/L with a correlation coefficient of 0.996. Also, the determination of ascorbic acid in actual samples was evaluated and the results are satisfactory. Received: 7 October 1997 / Revised: 3 February 1998 / Accepted: 7 February 1998     

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.     

Voltammetric determination of dopamine in the presence of ascorbic acid at a chemically modified electrode

A Nafion/ruthenium oxide pyrochlore chemically modified electrode (CME) was used for the selective determination of dopamine (DA) in the presence of a high concentration of ascorbic acid by square-wave voltammetry. Compared to a bare glassy carbon electrode, the CME exhibits an apparent shift of the oxidation potentials in cathodic direction and a marked enhancement of the current response. The selective sensing of DA is achieved by combining the electrocatalytic function of the ruthenium oxide pyrochlore catalyst with the charge-exclusion and preconcentration features of Nation. With a preconcentration time of 60 s at a potential of −0.3 V (vs. Ag/AgCl), linear calibration plots are obtained for dopamine in 0.1 M, phosphate buffer (pH 7.4) over 0–20 μM with a detection limit (3σ) of 0.1 μM.     

Electrochemical Sensor for the Determination of Dopamine in Presence of High Concentration of Ascorbic Acid Using a Fullerene‐C60 Coated Gold Electrode

A fullerene‐C₆₀‐modified gold electrode is employed for the determination of dopamine in the excess of ascorbic acid using square‐wave voltammetry. Based on its strong catalytic function towards the oxidation of dopamine and ascorbic acid, the overlapping voltammetric response of both the biomolecules at the bare electrode is resolved into two well‐defined voltammetric peaks with lowered oxidation potential and enhanced oxidation currents. Linear calibration curves for dopamine are obtained using square‐wave voltammetry over the concentration range 1 nM–5.0 μM in 0.1 M phosphate buffer solution at pH 7.2 with a correlation coefficient of 0.9931 and the detection limit (3σ) is estimated to be 0.26×10^?9 M. The interference studies showed that the presence of physiologically common interferents (i.e. uric acid, citric acid, tartaric acid, glucose and sodium chloride) negligibly affects the response of dopamine. The practical analytical utility of the method is illustrated by quantitative determination of dopamine in commercially available pharmaceutical formulation and human body fluids, viz. urine and blood plasma, without any preliminary treatment.     

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.     

Efficient Electrocatalytic Detection of Epinephrine at Gold Electrodes Modified with Self‐Assembled Metallo‐Octacarboxyphthalocyanine Complexes

Electrocatalysis of epinephrine at gold electrode pre‐modified with the self‐assembled monolayer of cysteamine and subsequently integrated with novel metallo‐octacarboxyphthalocyanine (MOCPc where M=Fe, Co and Mn) complexes (Au‐Cys‐MOCPc) was investigated. The electrodes showed response to the presence of epinephrine. The oxidation peak potential (E_p/V vs. Ag|AgCl, sat'd KCl) and charge transfer resistance (R_ct (kΩ)) in epinephrine solution depend markedly on the central metal of the phthalocyanine cores: Au‐Cys‐FeOCPck_ch=4.1×10⁷ M^?1 s^?1) which is higher than that of the Au‐cys‐CoOCPc or Au‐cys‐MnOCPc electrode. Mechanism, recognizing the mediation of the electrocatalytic process by the central M(II)/M(III) redox processes was proposed. Epinephrine electro‐oxidation at the Au‐cys‐FeOCPc electrode was studied in more details for the response characteristics. The diffusion coefficient of epinephrine was evaluated as (2.62±0.23)×10^?9 cm² s^?1. It was established that Au‐Cys‐FeOCPc is suitable for sensitive determination of epinephrine in physiological pH (7.40) conditions showing linear concentration range of up to 300 nM, with excellent sensitivity (0.53±0.01 nA nM^?1), and very low limits of detection (13.8 nM) and quantification (45.8 nM). The peak separation between ascorbic acid and epinephrine is large enough (190 mV) to permit simultaneous determination of both epinephrine and ascorbic acid in physiological pH 7.4 conditions using the Au‐cys‐FeOCPc electrode. Au‐cys‐FeOCPc electrode was successfully used for the determination of epinephrine in epinephrine hydrochloric acid injection with recovery of ca. 98.4%.     

Simultaneous Determination of Dopamine and Ascorbic Acid at an In‐Site Functionalized Self‐Assembled Monolayer on Gold Electrode

The in‐site functionalization of 4‐aminothiophenol (4‐ATP) self‐assembled monolayer on gold electrode at physiological pH yields a redox active monolayer of 4′‐mercapto‐N‐phenylquinone diimine (MNPD). The functionalized electrode exhibits excellent electrocatalytic responses towards dopamine (DA) and ascorbic acid (AA), reducing the overpotentials by about 0.22 V and 0.34 V, respectively, with greatly enhanced current responses. Due to its different catalytic activities toward DA and AA, the modified electrode resolves the overlapping voltammetric responses of DA and AA into two well‐defined voltammetric peaks by differential pulse voltammetry (DPV), which can be used for the simultaneous determination of these species in a mixture. The catalytic peak current obtained from DPV was linearly related to DA and AA concentration in the ranges of 5.0×10^?6?1.25×10^?4 M and 8.0×10^?6?1.3×10^?4 M with correlation coefficient of 0.999 and 0.998, respectively. The detective limits (3σ) for DA and AA were found to be 1.2×10^?6 M and 2.4×10^?6 M, respectively. The modified electrode shows good sensitivity, selectivity and stability, and has been applied to the determination of DA and AA simultaneously in samples with satisfactory results.     

Chitosan Incorporating Cetyltrimethylammonium Bromide Modified Glassy Carbon Electrode for Simultaneous Determination of Ascorbic Acid and Dopamine

Simultaneous determination of a neurotransmitter, dopamine (DA), and ascorbic acid (AA) is achieved at neutral pH on a chitosan incorporating cetyltrimethylammonium bromide (CTAB) modified glassy carbon (GC) electrode. Differential pulse voltammetry (DPV) technique was used to investigate the electrochemical response of DA and AA at a glassy carbon electrode modified with chitosan incorporating CTAB. An optimum 6.0 mmol L^?1 of CTAB together with 0.5 wt% of chitosan was used to improve the resolution and the determination sensitivity. In 0.1 mol L^?1 aqueous phosphate buffer solution of pH 6.8, the chitosan‐CTAB modified electrode showed a good electrocatalytic response towards DA and AA. The anodic peak potential of DA shifted positively, while that of AA shifted negatively. Thus, the difference of the anodic peaks of DA and AA reached 0.23 V, which was enough to separate the two anodic peaks very well. The presented method herein could be applied to the direct simultaneous determination of DA and AA without prior treatment. The anodic peak currents (I_pa) of DPV are proportional to DA in the concentration range of 8 μM to 1000 μM, to that of AA 10 μM to 2000 μM, with correlation coefficients of 0.9930 and 0.9945, respectively. The linear range is much wider than previously reported.     

Highly Sensitive and Selective Determination of Dopamine in the Presence of Ascorbic Acid Using Pt@Au/MWNTs Modified Electrode

Nanoparticles containing platinum and gold supported on MWNTs (Pt@Au/MWNTs) were successfully prepared by successive reduction. A novel sensor was fabricated with Pt@Au/MWNTs nanocomposite modified glassy carbon electrode coating with nafion. Compared with the previous reports, the electrode exhibited greatly improved performance towards the oxidation and determination of dopamine in the presence of ascorbic acid. A higher sensitivity of 1.16 mA cm^?2 mM^?1 with a wider linear concentration range of dopamine up to 120 μM and a low detection limit of 8.0×10^?8 M were obtained in neutral solution. Moreover, the modified electrode presented high stability and reproducibility. The interference of ascorbic acid can be eliminated effectively. The present work provides a simple and practical approach to selective detection of dopamine in the presence of ascorbic acid.     

Fabrication and characterization of poly 2-napthol orange film modified electrode and its application to selective detection of dopamine

The present work is based on the use of a redox mediator containing an azo group for the selective determination of dopamine in the presence of uric acid and ascorbic acid by electrochemical method. A modified electrode was prepared by electrochemical polymerization of the poly 2-napthol orange film (P2NO) on the paraffin wax-impregnated graphite electrode (PIGE) by applying potential between ?0.6 and 0.8 V at scan rate of 50 mV s^?1 for 30 segments. The modified P2NO film electrode was characterized by ATR-IR spectroscopy, FE-SEM, cyclic voltammetry (CV), electrochemical impedance spectroscopy (EIS), differential pulse voltammetry (DPV), hydrodynamic voltammetry (HDV), and chronoamperometry (CA). The P2NO film modified electrode exhibited selective determination of dopamine in the presence of uric acid and ascorbic acid, and the electrocatalytic activity for oxidation of dopamine was excellent. The linear range for the determination of dopamine was 0.6 to 250 μM with a limit of detection of 0.13 μM. The modified P2NO electrode showed good stability and reproducibility. The modified electrode was used for real sample analysis such as human blood serum, rat blood serum, and pharmaceutical samples (dopamine hydrochloride injection). The results obtained were found to be satisfactory.     

Electroanalysis of Dopamine at RuO2 Modified Vertically Aligned Carbon Nanotube Electrode

Electrochemical behavior of dopamine at the RuO₂‐modified vertically aligned carbon nanotubes electrode was investigated by cyclic voltammetry, differential pulse voltammetry and chronoamperometry. The RuO₂‐modified carbon nanotube electrode showed higher electrocatalytic activity towards the oxidation of dopamine than the MWNTs electrode in 0.10 M phosphate buffer solution. At an applied potential of +0.4 V, the RuO₂/MWNTs electrode exhibited a wide detection range up to 3.6×10^?3 M with detection limit of 6.0×10^?8 M (signal/noise=3) for dopamine determination. Meanwhile, the optimized sensor for dopamine displayed a sensitivity of 83.8 μA mM^?1 and response time of 5 s with addition of 0.20 mM dopamine. In addition, DPV experiment revealed that interfering species such as ascorbic acid and uric acid could be effectively avoided. The RuO₂/MWNTs electrode presents stable, highly sensitive, favorable selectivity and fast amperometric response of dopamine.     

A poly(3-acetylthiophene) modified glassy carbon electrode for selective voltammetric measurement of uric acid in urine sample

A reliable and reproducible method for the determination of uric acid in urine samples has been developed. The method is based on the modification of a glassy carbon electrode by 3-acetylthiophene using cyclic voltammetry. The poly(3-acetylthiophene) modified glassy carbon electrode showed an excellent electrocatalytic effect towards the oxidation of uric acid in 0.1 m phosphate buffer solution (PBS) at pH 7.2. Compared with a bare glassy carbon electrode (GCE), an obvious shift of the oxidation peak potential in the cathodic direction and a marked enhancement of the anodic current response for uric acid were observed. The poly(3-acetylthiophene)/GCE was used for the determination of uric acid using square wave voltammetry. The peak current increased linearly with the concentration of uric acid in the range of 1.25 x 10(-5)-1.75 x 10(-4) M. The detection limit was 5.27 x 10(-7) M by square wave voltammetry. The poly(3-acetylthiophene)/GCE was also effective to determine uric acid and ascorbic acid in a mixture and resolved the overlapping anodic peaks of these two species into two well-defined voltammetric peaks in cyclic voltammetry at 0.030 V and 0.320 V (vs. Ag/AgCl) for ascorbic acid and uric acid, respectively. The modified electrode exhibited stable and sensitive current responses toward uric acid and ascorbic acid. The method has successfully been applied for determination of uric acid in urine samples.     

Determination of Norepinephrine Alone and in the Presence of Ascorbic and Uric Acids Using a Gold Electrode Modified with Gold Nanoparticles and Self‐Assembled Layers of meso‐2,3‐Dimercaptosuccinic Acid

This paper reports on the modification of gold electrodes with self‐assembled layers (SAMs) composed of meso‐2,3‐dimercaptosuccinic acid, cysteamine and gold nanoparticles, respectively and their application to quantitative determination of norepinephrine alone and in the presence of ascorbic and uric acids in solution at pH 7. The modification was carried out on two kinds of templates: a bare gold electrode (2D electrode) and a gold electrode coated in the first step with gold nanoparticles (3D electrode). Cyclic voltammograms reveal an enhancement of the norepinephrine electrooxidation in comparison to a bare, (non‐modified) gold electrode. The oxidation peaks for norepinephrine, ascorbic acid and uric acid have a peak‐to‐peak separation that enables their selective determination even in a complex mixture.     

Electroanalysis of Norepinephrine at Bare Gold Electrode Pure and Modified with Gold Nanoparticles and S‐Functionalized Self‐Assembled Layers in Aqueous Solution

Gold nanoparticles (Au‐NPs), cystamine (CA) and 3,3′‐dithiodipropionic acid (DTDPA) modified gold bare electrodes were applied in voltammetric sensors for simultaneous detection of norepinephrine (NEP), ascorbic (AA) and uric (UA) acids. A linear relationship between norepinephrine concentration and current response was obtained in the range of 0.1 μM to 600 μM M with the detection limit ≤0.091 μM for the electrodes modified at 2D template and in the range of 0.1 μM to 700 μM M with the detection limit ≤0.087 μM for the electrodes modified at 3D template The results have shown that using modified electrodes it is possible to perform electrochemical analysis of norepinephrine without interference of ascorbic and uric acids, whose presence is the major limitation in norepinephrine determination at a bare gold electrode. The modified SAMs electrodes show good selectivity, sensitivity, reproducibility and high stability.