Selective detection of uric acid in the presence of ascorbic acid at physiological pH by using a beta-cyclodextrin modified copolymer of sulfanilic acid and N-acetylaniline

A beta-cyclodextrin (CD) modified copolymer membrane of sulfanilic acid (p-ASA) and N-acetylaniline (SPNAANI) on glassy carbon electrode (GCE) was prepared and used to determine uric acid (UA) in the presence of a large excess of ascorbic acid (AA) by differential pulse voltammetry (DPV). The properties of the copolymer were characterized by X-ray photoelectron spectra (XPS) and Raman spectroscopy. The oxidation peaks of AA and UA were well separated at the composite membrane modified electrode in phosphate buffer solution (PBS, pH 7.4). A linear relationship between the peak current and the concentration of UA was obtained in the range from 1.0 x 10(-5) to 3.5 x 10(-4)mol L(-1), and the detection limit was 2.7 x 10(-6)mol L(-1) at a signal-to-noise ratio of 3. Two hundred and fifty-fold excess of AA did not interfere with the determination of UA. The application of the prepared electrode was demonstrated by measuring UA in human serum samples without any pretreatment, and the results were comparatively in agreement with the spectrometric clinical assay method.     

Simultaneous determination of dopamine, ascorbic acid, and uric acid using carbon ionic liquid electrode

A recently constructed carbon composite electrode using room temperature ionic liquid as pasting binder was employed as a novel electrode for sensitive, simultaneous determination of dopamine (DA), ascorbic acid (AA), and uric acid (UA). The apparent reversibility and kinetics of the electrochemical reaction for DA, AA, and UA found were improved significantly compared to those obtained using a conventional carbon paste electrode. The results show that carbon ionic liquid electrode (CILE) reduces the overpotential of DA, AA, and UA oxidation, without showing any fouling effect due to the deposition of their oxidized products. In the case of DA, the oxidation and reduction peak potentials appear at 210 and 135mV (vs Ag/AgCl, KCl, 3.0M), respectively, and the CILE shows a significantly better reversibility for dopamine. The oxidation peak due to the oxidation of AA occurs at about 60mV. For UA, a sharp oxidation peak at 340mV and a small reduction peak at 250mV are obtained at CILE. Differential pulse voltammetry was used for the simultaneous determination of ternary mixtures of DA, AA, and UA. Relative standard deviation for DA, AA, and UA determinations were less than 3.0% and DA, AA, and UA can be determined in the ranges of 2.0x10(-6)-1.5x10(-3), 5.0x10(-5)-7.4x10(-3), and 2.0x10(-6)-2.2x10(-4)M, respectively. The method was applied to the determination of DA, AA, and UA in human blood serum and urine samples.     

Novel 2,2'-[1,2-ethanediylbis(nitriloethylidyne)]-bis-hydroquinone double-wall carbon nanotube paste electrode for simultaneous determination of epinephrine, uric acid and folic acid

The electro-oxidation of epinephrine (EP), uric acid (UA), folic acid (FA), and their mixture has been studied by modified carbon nanotube paste electrode of 2,2'-[1,2-ethanediylbis(nitriloethylidyne)]-bis-hydroquinone using cyclic voltammetry, chronoamperometry and differential pulse voltammetry. This modified electrode exhibited potent and persistent electron mediating behavior followed by well-separated oxidation peaks towards EP, UA and FA with activation overpotential. For the ternary mixture containing EP, UA and FA the three compounds can be well separated from each other at the scan rate of 20mVs(-1). The obtained catalytic peak current, was linearly dependent on the EP, UA and FA concentrations in the range of 0.7-1200muM, 25-750muM and 15-800muM and the detection limits for EP, UA and FA were 0.216+/-0.004, 8.8+/-0.2 and 11.0+/-0.3muM, respectively. The diffusion coefficient (D), and the kinetic parameters such as electron transfer coefficient, (alpha) and heterogeneous rate constant, (k') for EP were also determined using electrochemical approaches. The modified electrode showed good sensitivity, selectivity and stability, and was employed for the determination of EP, UA and FA in the real samples.     

Simultaneous voltammetric determination of norepinephrine, ascorbic acid and uric acid on polycalconcarboxylic acid modified glassy carbon electrode

A novel polycalconcarboxylic acid (CCA) modified glassy carbon electrode (GCE) was fabricated by electropolymerization and then successfully used to simultaneously determine ascorbic acid (AA), norepinephrine (NE) and uric acid (UA). The characterization of electrochemically synthesized Poly-CCA film was investigated by atomic force microscopy (AFM), electrochemical impedance spectroscopy (EIS) and voltammetric methods. It was found that the electrochemical behavior of the polymer-modified electrode depended on film thickness, i.e., the electropylmyerization time. Based on the electrochemical data, the charge transfer coefficient (alpha) and the surface coverage (Gamma) were calculated. This poly-CCA modified GCE could reduce the overpotential of ascorbic acid (AA), norepinephrine (NE) and uric acid (UA) oxidation in phosphate buffer solution (pH 6.0), while it increases the peak current significantly. The current peak separations of AA/NE, NE/UA and AA/UA on this modified electrode are 91mV, 256mV and 390mV in CV at 100mVs(-1), respectively. Therefore, the voltammetric responses of these three compounds can be well resolved on the polymer-modified electrode, and simultaneously determination of these three compounds can be achieved. In addition, this modified electrode can be successfully applied to determine AA and NE in injection and UA in urine samples without interferences.     

Electrochemical sensor based on nitrogen doped graphene: simultaneous determination of ascorbic acid, dopamine and uric acid

Nitrogen doped graphene (NG) was prepared by thermally annealing graphite oxide and melamine mixture. After characterization by atomic force microscopy and X-ray photoelectron spectroscopy etc., the electrochemical sensor based on NG was constructed to simultaneously determine small biomolecules such as ascorbic acid (AA), dopamine (DA) and uric acid (UA). Due to its unique structure and properties originating from nitrogen doping, NG shows highly electrocatalytic activity towards the oxidation of AA, DA and UA. The electrochemical sensor shows a wide linear response for AA, DA and UA in the concentration range of 5.0×10(-6) to 1.3×10(-3)M, 5.0×10(-7) to 1.7×10(-4)M and 1.0×10(-7) to 2.0×10(-5)M with detection limit of 2.2×10(-6)M, 2.5×10(-7)M and 4.5×10(-8)M at S/N=3, respectively. These results demonstrate that NG is a promising candidate of advanced electrode material in electrochemical sensing and other electrocatalytic applications.     

Simultaneous determination of catecholamines, uric acid and ascorbic acid at physiological levels using poly(N-methylpyrrole)/Pd-nanoclusters sensor

An interesting electrochemical sensor has been constructed by the electrodeposition of palladium nanoclusters (Pd_nano) on poly(N-methylpyrrole) (PMPy) film-coated platinum (Pt) electrode. Cyclic voltammetry, electrochemical impedance spectroscopy (EIS), and scanning electron microscopy were used to characterize the properties of the modified electrode. It was demonstrated that the electroactivity of the modified electrode depends strongly on the electrosynthesis conditions of the PMPy film and Pd_nano. Moreover, the modified electrode exhibits strong electrocatalytic activity toward the oxidation of a mixture of dopamine (DA), ascorbic acid (AA), and uric acid (UA) with obvious reduction of overpotentials. The simultaneous analysis of this mixture at conventional (Pt, gold [Au], and glassy carbon) electrodes usually struggles. However, three well-resolved oxidation peaks for AA, DA, and UA with large peak separations allow this modified electrode to individually or simultaneously analyze AA, DA, and UA by using differential pulse voltammetry (DPV) with good stability, sensitivity, and selectivity. This sensor is also ideal for the simultaneous analysis of AA, UA and either of epinephrine (E), norepinephrine (NE) or l-DOPA. Additionally, the sensor shows strong electrocatalytic activity towards acetaminophen (ACOP) and other organic compounds. The calibration curves for AA, DA, and UA were obtained in the ranges of 0.05 to 1 mM, 0.1 to 10 μM, and 0.5 to 20 μM, respectively. The detection limits (signal/noise [S/N] = 3) were 7 μM, 12 nM, and 27 nM for AA, DA, and UA, respectively. The practical application of the modified electrode was demonstrated by measuring the concentrations of AA, DA, and UA in injection sample, human serum, and human urine samples, respectively, with satisfactory results. The reliability and stability of the modified electrode gave a good possibility for applying the technique to routine analysis of AA, DA, and UA in clinical tests.     

Electrochemical synthesis of polyaniline nano-networks on p-aminobenzene sulfonic acid functionalized glassy carbon electrode Its use for the simultaneous determination of ascorbic acid and uric acid

A composite film of polyaniline (PAN) nano-networks/p-aminobenzene sulfonic acid (ABSA) modified glassy carbon electrode (GCE) has been fabricated via an electrochemical oxidation procedure and applied to the electro-catalytic oxidation of uric acid (UA) and ascorbic acid (AA). The ABSA monolayer at GCE surface has been characterized by X-ray photo-electron spectroscopy (XPS) and electrochemical techniques. Atomic force microscopy (AFM), field emission scanning electron microscope (SEM), electrochemical impedance spectroscopy (EIS), UV-visible absorption spectra (UV-vis) and cyclic voltammetry (CV) have been used to investigate the PAN-ABSA composite film, which demonstrates the formation of the composite film and the maintenance of the electroactivity of PAN in neutral and even in alkaline media. Due to its different catalytic effects towards the electro-oxidation of UA and AA, the modified GCE can resolve the overlapped voltammetric response of UA and AA into two well-defined voltammetric peaks with both CV and differential pulse voltammetry (DPV), which can be used for the selective and simultaneous determination of these species in a mixture. The catalytic peak currents are linearly dependent on the concentrations of UA and AA in the range of 50-250 and 35-175mumoll(-1) with correlation coefficients of 0.997 and 0.998, respectively. The detection limits for UA and AA are 12 and 7.5mumoll(-1), respectively. Besides the good stability and reproducibility of PAN-ABSA/GCE due to the covalent attachment of ABSA at GCE surface, the modified electrode also exhibits good sensitivity and selectivity.     

Determination of ascorbic acid in individual rat hepatocyte by capillary electrophoresis with electrochemical detection

A method for the direct determination of ascorbic acid (AA) in individual rat hepatocyte based on capillary electrophoresis (CE) coupled with electrochemical detection (ECD) using a new kind of homemade carbon fiber micro-disk bundle electrode has been described. Individual rat hepatocytes were injected into a fused-silica capillary with an inner diameter of 25 microm, and lysed by 0.1% sodium dodecylsulfate (SDS) as cell lysis solution. The following conditions were suitable for the determination of AA: running buffer, 1.83 x 10(-2) mol/l Na2HPO4-1.70 x 10(-3) mol/l NaH2PO4 (pH 7.8); separation voltage, 20.0 kV; detection potential, 0.80 V (vs. saturated calomel electrode (SCE)). The concentration limit of detection (LOD) of the method was 1.7 x 10(-6) mol/l at a signal-to-noise (S/N) ratio of 3, and the mass LOD was 3.0 fmol. The linear dynamic range was from 5.0 x 10(-6) to 5.0 x 10(-4) mol/l with a correlation coefficient of 0.9962 for the injection voltage of 5.0 kV and injection time of 10s. The relative standard deviation (R.S.D.) was 0.85% for the migration time and 1.8% for the peak current. This method was successfully applied to AA determination in rat hepatocyte. The recovery was between 91% and 97%, and the amount of AA in single rat hepatocyte ranged from 28 to 63 fmol.     

Simultaneous electrochemical determination of dopamine, uric acid and ascorbic acid using palladium nanoparticle-loaded carbon nanofibers modified electrode

Palladium nanoparticle-loaded carbon nanofibers (Pd/CNFs) were prepared by electrospinning and subsequent thermal treatment processes. Pd/CNFs modified carbon paste electrode (Pd/CNF-CPE) displayed excellent electrochemical catalytic activities towards dopamine (DA), uric acid (UA) and ascorbic acid (AA). The oxidation overpotentials of DA, UA and AA were decreased significantly compared with those obtained at the bare CPE. Differential pulse voltammetry was used for the simultaneous determination of DA, UA and AA in their ternary mixture. The peak separation between UA and DA, DA and AA was 148 mV and 244 mV, respectively. The calibration curves for DA, UA and AA were obtained in the range of 0.5-160 microM, 2-200 microM, and 0.05-4mM, respectively. The lowest detection limits (S/N=3) were 0.2 microM, 0.7 microM and 15 microM for DA, UA and AA, respectively. With good selectively and sensitivity, the present method was applied to the determination of DA in injectable medicine and UA in urine sample.     

The electrochemical modification of clenbuterol for biosensors of dopamine, norepinephrine, adrenalin, ascorbic acid and uric acid at paraffin-impregnated graphite electrode

The electrochemical modification of clenbuterol (CLB) was studied at paraffin-impregnated graphite electrode (WGE) in two potential ranges of 0.0-1.6V and -1.2 to 1.2V. Various methods including X-ray photoelectron spectroscopy (XPS), UV-spectroelectrochemistry, infrared (IR) spectra and electrochemical techniques have been used to characterizing the modification. Clenbuterol can be modified at the electrode surface by carbon-nitrogen linkage or carbon-carbon linkage in 0.0-1.6V or -1.2 to 1.2V, respectively. The electrochemical behaviors of dopamine (DA), norepinephrine (NE), adrenalin (EP), ascorbic acid (AA) and uric acid (UA) were studied at clenbuterol-modified paraffin-impregnated graphite electrode (CLB/WGE), and it was found that all these compounds could be detected successfully.     

Hollow nitrogen-doped carbon microspheres pyrolyzed from self-polymerized dopamine and its application in simultaneous electrochemical determination of uric acid, ascorbic acid and dopamine

Hollow nitrogen-doped carbon microspheres (HNCMS) as a novel carbon material have been prepared and the catalytic activities of HNCMS-modified glassy carbon (GC) electrode towards the electro-oxidation of uric acid (UA), ascorbic acid (AA) and dopamine (DA) have also been investigated. Comparing with the bare GC and carbon nanotubes (CNTs) modified GC (CNTs/GC) electrodes, the HNCMS modified GC (HNCMS/GC) electrode has higher catalytic activities towards the oxidation of UA, AA and DA. Moreover, the peak separations between AA and DA, and DA and UA at the HNCMS/GC electrode are up to 212 and 136 mV, respectively, which are superior to those at the CNTs/GC electrode (168 and 114 mV). Thus the simultaneous determination of UA, AA and DA was carried out successfully. In the co-existence system of UA, AA and DA, the linear response range for UA, AA and DA are 5-30 μM, 100-1000 μM and 3-75 μM, respectively and the detection limits (S/N = 3) are 0.04 μM, 0.91 μM and 0.02 μM, respectively. Meanwhile, the HNCMS/GC electrode can be applied to measure uric acid in human urine, and may be useful for measuring abnormally high concentration of AA or DA. The attractive features of HNCMS provide potential applications in the simultaneous determination of UA, AA and DA.     

Simultaneous and sensitive determination of a quaternary mixture of AA, DA, UA and Trp using a modified GCE by iron ion-doped natrolite zeolite-multiwall carbon nanotube

The evaluation of a novel modified glassy carbon electrode modified with iron ion-doped natrolite zeolite-multiwalled carbon nanotube for the simultaneous and sensitive determination of ascorbic acid (AA), dopamine (DA), uric acid (UA) and tryptophan (Trp) has been described. The measurements were carried out using cyclic voltammetry in buffer solution with pH 1. This modified electrode exhibits potent and persistent electroxidation behavior followed by well-separated oxidation peaks towards AA, DA, UA and Trp with increasing of the oxidation current. For the quaternary mixture containing AA, DA, UA and Trp, the 4 compounds can well separate from each other at the scan rate of 100 mVs(-1) with a potential difference of 270 mV, 150 mV and 260 mV for the oxidation peak potentials of AA-DA, DA-UA and UA-Trp, respectively, which was large enough to simultaneous determine AA, DA, UA and Trp. The catalytic peak current obtained, was linearly dependent on the AA, DA, UA and Trp concentrations in the range of 7.77-833 μM, 7.35-833 μM, 0.23-83.3 μM and 0.074-34.5 μM and the detection limits for AA, DA, UA and Trp were 1.11, 1.05, 0.033 and 0.011 μM, respectively. The analytical performance of this sensor has been evaluated for simultaneous detection of AA, DA, UA and Trp in human serum and urine samples.     

Simultaneous determination of ascorbic acid, dopamine and uric acid using high-performance screen-printed graphene electrode

A disposable and sensitive screen-printed electrode using an ink containing graphene was developed. This electrode combined the advantages of graphene and the disposable characteristic of electrode, which possessed wide potential window, low background current and fast electron transfer kinetics. Compared with the electrodes made from other inks, screen-printed graphene electrode (SPGNE) showed excellent electrocatalytic activity for the oxidation of ascorbic acid (AA), dopamine (DA), and uric acid (UA). Three well-defined sharp and fully resolved anodic peaks were found at the developed electrode. Differential pulse voltammetry was used to simultaneous determination of AA, DA, and UA in their ternary mixture. In the co-existence system of these three species, the linear response ranges for the determination of AA, DA, and UA were 4.0-4500 μM, 0.5-2000 μM, and 0.8-2500 μM, respectively. The detection limits (S/N=3) were found to be 0.95 μM, 0.12 μM, and 0.20 μM for the determination of AA, DA, and UA, respectively. Furthermore, the SPGNE displayed high reproducibility and stability for these species determination. The feasibility of the developed electrode for real sample analysis was investigated. Results showed that the SPGNE could be used as a sensitive and selective sensor for simultaneous determination of AA, DA, and UA in biological samples, which may provide a promising alternative in routine sensing applications.     

Voltammetric determination of mefenamic acid at lanthanum hydroxide nanowires modified carbon paste electrodes

Lanthanum hydroxide nanowires modified carbon paste electrode (LNW/CPE) exhibiting an electrocatalytic response toward the oxidation of mefenamic acid (MFA) is described. The catalytic action of the LNW/CPE on the oxidation of MFA via one-electron and one-proton transfer is attributed to the formation of the porous construction and the increase of efficient surface of the electrode due to the adulteration of LNW with carbon powders. Using the LNW/CPE, a linear sweep voltammetric method for the determination of MFA and other drugs with diphenylamine parent is proposed. A linear range of 2.0 x 10(-11) to 4.0 x 10(-9)mol L(-1) is obtained along with a detection limit of 6.0 x 10(-12)mol L(-1).     

Selective electrochemical sensor for folic acid at physiological pH using ultrathin electropolymerized film of functionalized thiadiazole modified glassy carbon electrode

This paper demonstrated the selective determination of folic acid (FA) in the presence of important physiological interferents, ascorbic acid (AA) and uric acid (UA) at physiological pH using electropolymerized film of 5-amino-2-mercapto-1,3,4-thiadiazole (p-AMT) modified glassy carbon (GC) electrode. Bare GC electrode fails to determine the concentration of FA in the presence of AA and UA due to the surface fouling caused by the oxidized products of AA and FA. However, the p-AMT film modified electrode not only separates the voltammetric signals of AA, UA and FA with potential differences of 170 and 410 mV between AA–UA and UA–FA, respectively but also shows higher oxidation current for these analytes. The p-AMT film modified electrode displays an excellent selectivity towards the determination of FA even in the presence of 200-fold AA and 100-fold UA. Using amperometric method, we achieved the lowest detection of 75 nM UA and 100 nM each AA and FA. The amperometric current response was increased linearly with increasing FA concentration in the range of 1.0 × 10⁻⁷–8.0 × 10⁻⁴ M and the detection limit was found to be 2.3 × 10⁻¹⁰ M (S/N = 3). The practical application of the present modified electrode was successfully demonstrated by determining the concentration of FA in human blood serum samples.     

Real-time investigation of the interaction between primaquine phosphate and bovine serum albumin (BSA) by piezoelectric quartz crystal impedance analysis

Real-time investigation of the interaction between primaquine phosphate and bovine serum albumin by the piezoelectric quartz crystal impedance (PQCI) analysis was carried out for the first time. Three kinds of electrodes were investigated. Compared with bare gold (Au) electrode, the gold electrode self-assembled of nanogold colloids exhibits maintained biocompatibility, increased capacity and more bioactivity. Additionally, on the basis of the multi-dimensional information provided by the PQCI analysis, the real-time interaction information and the kinetics of the binding process was investigated and a response model was deduced. At 37 degrees C, the binding rate (k1), dissociation rate (k(-1)) and equilibrium constants (Ka) were 4.19x10(2) (mol l(-1))(-1) s(-1), 1.01x10(-3) s(-1) and 4.15x10(5) (mol l(-1))(-1) for the electrode modified by nanogold particles; 3.83x10(2) (mol l(-1))(-1) s(-1), 9.70x10(-4) s(-1) and 3.95x10(5) (mol l(-1))(-1) for the bare gold electrode, respectively.     

A sensitive determination of estrogens with a Pt nano-clusters/multi-walled carbon nanotubes modified glassy carbon electrode

On the top of a multi-walled carbon nanotubes (MWNTs) modified glassy carbon electrode (MWNTs/GCE), Pt nanoclusters were electrochemically deposited, fabricating a Pt/MWNTs composite modified electrode, Pt/MWNTs/GCE. X-ray photoelectron spectroscopy, powder X-ray diffraction and field emission scanning electron microscope were used for the surface characterization of the electrode, and demonstrated the formation and distribution of Pt clusters of Pt nanoparticles of 8.4 nm in averaged size in the MWNTs matrix. The preliminary study found that this composite modified electrode has strong electrocatalytic activity toward the oxidation of estrogens involving estradiol, estrone and estriol. The voltammetric behavior of estrogens on this electrode was investigated by cyclic voltammetry, linear sweep voltammetry and square-wave voltammetry. In comparison with the MWNTs/GCE or a Pt nanoparticles modified GCE prepared in the similar way, this composite modified electrode exhibited much higher current sensitivity and catalytic activity. This electrode is also stable. The linear range of square-wave voltammetric determination was 5.0 x 10(-7)-1.5 x 10(-5)mol/L for estradiol, 2.0 x 10(-6)-5.0 x 10(-5)mol/L for estrone, and 1.0 x 10(-6)-7.5 x 10(-5)mol/L for estriol. Under an assumption that the concentration ratio of estradiol:estrone:estriol is 2:2:1, the real sample of blood serums was tested for the determination using this electrode. Satisfactory result was obtained with averaged recovery of 105%.     

Liquid chromatography with amperometric detection using functionalized multi-wall carbon nanotube modified electrode for the determination of monoamine neurotransmitters and their metabolites

The fabrication and application of a novel electrochemical detection (ED) method with the functionalized multi-wall carbon nanotubes (MWNTs) chemically modified electrode (CME) for liquid chromatography (LC) were described. The electrochemical behaviors of dopamine (DA) and other monoamine neurotransmitters at the CME were investigated by cyclic voltammetry (CV) and differential pulse voltammetry (DPV). The results indicated that the CME exhibited efficient electrocatalytic effects on the current responses of monoamine neurotransmitters and their metabolites with high sensitivity, high stability and long-life activity. In LC-ED, DA, norepinephrine (NE), 3-methoxy-4-hydroxyphenylglycol (MHPG), 3,4-dihydroxyphenylacetic acid (DOPAC), 5-hydroxytryptamine (5-HT), 5-hydroxyindoleacetic acid (5-HIAA) and homovanillic acid (HVA) had good and stable current responses at the CME. The linear ranges of seven analytes were over four orders of magnitude and the detection limits were 2.5 x 10(-10) mol/l for DA, 2.5 x 10(-10) mol/l for NE, 5.0 x 10(-10) mol/l for MHPG, 3.0 x 10(-10) mol/l for DOPAC, 3.5 x 10(-10) mol/l for 5-HT, 6.0 x 10(-10) mol/l for 5-HIAA, 1.25 x 10(-9) mol/l for HVA. The application of this method coupled with microdialysis sampling for the determination of monoamine neurotransmitters and their metabolites in Parkinsonian patients' cerebrospinal fluid was satisfactory.     

Uric acid determination in the presence of ascorbic acid using self-assembled submonolayer of dimercaptothiadiazole-modified gold electrodes

This article reports the determination of uric acid (UA) in the presence of ascorbic acid (AA) using a self-assembled submonolayer of heteroaromatic dithiol, 2,5-dimercapto-1,3,4-thiadiazole (DMcT), on gold (Au) electrode. Submonolayer to multilayers of DMcT can be prepared on Au electrode by varying the soaking time of Au electrode in 1mM aqueous solution of DMcT. The formation of submonolayer, monolayer, and multilayers of DMcT on Au electrode was confirmed from its reductive desorption measurements and electrochemical blocking behavior toward ferricyanide. Interestingly, submonolayer of DMcT separates the voltammetric signal of UA from AA by 210 mV, whereas monolayer and multilayers of DMcT fail to separate them. The voltammetric signals of AA and UA are highly stable and reproducible at submonolayer of DMcT. Fast electron transfer, weak hydrogen bonding interactions with AA and UA, and prevention of fouling effect caused by oxidized product of AA can be achieved at submonolayer of DMcT, and thus it successfully separates the voltammetric signals of AA and UA. The practical application of the current system is demonstrated by measuring the concentration of UA in human urine samples without any treatment.     

Enhanced electron-transfer reactivity of horseradish peroxidase in phosphatidylcholine films and its catalysis to nitric oxide

Direct electrochemistry of horseradish peroxidase (HRP) embedded in film of phosphatidylcholine (PC) is investigated at a pyrolytic graphite electrode by voltammetric methods. The electron-transfer reactivity between incorporated HRP and the electrode is found to be greatly enhanced by phosphatidylcholine film. Cyclic voltammetry (CV) of this incorporated peroxidase shows a pair of well-defined and nearly reversible peaks, and the cathodic and anodic peak potentials are located at about -0.261 and -0.180 V, respectively versus saturated calomel electrode at pH 5.5. Ultraviolet-visible absorption spectra indicate that the heme microenvironment of HRP in phosphatidylcholine film is similar to that of its native status. It is also observed that HRP modified electrode is able to catalyze the electrochemical reduction of nitric oxide. Experimental results reveal that the peak current related to nitric oxide reduction is linearly proportional to its concentration in the ranges of 2.0 x 10(-7) -5.0 x 10(-6) mol (-1) and 2.0 x 10(-5) -1.0 x 10(-4) mol(-1), based on which an unmediated biosensor for nitric oxide is developed.