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 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.     

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 simultaneous electrochemical detection of ascorbic acid, dopamine, and uric acid using graphene/size-selected Pt nanocomposites

In this study, a graphene/Pt-modified glassy carbon (GC) electrode was created to simultaneously characterize ascorbic acid (AA), dopamine (DA), and uric acid (UA) levels via cyclic voltammetry (CV) and differential pulse voltammetry (DPV). During the preparation of the nanocomposite, size-selected Pt nanoparticles with a mean diameter of 1.7 nm were self-assembled onto the graphene surface. In the simultaneous detection of the three aforementioned analytes using CV, the electrochemical potential differences among the three detected peaks were 185 mV (AA to DA), 144 mV (DA to UA), and 329 mV (AA and UA), respectively. In comparison to the CV results of bare GC and graphene-modified GC electrodes, the large electrochemical potential difference that is achieved via the use of the graphene/Pt nanocomposites is essential to the distinguishing of these three analytes. An optimized adsorption of size-selected Pt colloidal nanoparticles onto the graphene surface results in a graphene/Pt nanocomposite that can provide a good platform for the routine analysis of AA, DA, and UA.     

Electrochemical detection of dopamine using porphyrin-functionalized graphene

A new type of porphyrin-functionalized graphene was synthesized and used for highly selective and sensitive detection of dopamine (DA). The aromatic π-π stacking and electrostatic attraction between positively-charged dopamine and negatively-charged porphyrin-modified graphene can accelerate the electron transfer whereas weakening ascorbic acid (AA) and uric acid (UA) oxidation on the porphyrin-functionalized graphene-modified electrode. Differential pulse voltammetry was used for electrochemical detection, the separation of the oxidation peak potentials for AA-DA, DA-UA and UA-AA is about 188 mV, 144 mV and 332 mV, which allows selectively determining DA. The detection limit of DA can be as low as 0.01 μM. More importantly, the sensor we presented can detect DA in the presence of large excess of ascorbic acid and uric acid. With good sensitivity and selectivity, the present method was applied to the determination of DA in real hydrochloride injection sample, human urine and serum samples, respectively, and the results was satisfactory.     

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 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 voltammetric detection of dopamine and uric acid at their physiological level in the presence of ascorbic acid using poly(acrylic acid)-multiwalled carbon-nanotube composite-covered glassy-carbon electrode

The use of poly(acrylic acid) (PAA)-multiwalled carbon-nanotubes (MWNTs) composite-coated glassy-carbon disk electrode (GCE) (PAA-MWNTs/GCE) for the simultaneous determination of physiological level dopamine (DA) and uric acid (UA) in the presence of an excess of ascorbic acid (AA) in a pH 7.4 phosphate-buffered solution was proposed. PAA-MWNTs composite was prepared by mixing of MWNTs powder into 1 mg/ml PAA aqueous solution under sonication. GCE surface was modified with PAA-MWNTs film by casting. AA demonstrates no voltammetric peak at PAA-MWNTs/GCE. The PAA-MWNTs composite is of a high surface area and of affinity for DA and UA adsorption. DA exhibits greatly improved electron-transfer rate and is electro-catalyzed at PAA-MWNTs/GCE. Moreover, the electro-catalytic oxidation of UA at PAA-MWNTs/GCE is observed, which makes it possible to detect lower level UA. Therefore, the enhanced electrocatalytic currents for DA and UA were observed. The anodic peak currents at approximately 0.18 V and 0.35 V increase with the increasing concentrations of DA and UA, respectively, which correspond to the voltammetric peaks of DA and UA, respectively. The linear ranges are 40 nM to 3 microM DA and 0.3 microM to 10 microM UA in the presence of 0.3 mM AA. The lowest detection limits (S/N=3) were 20 nM DA and 110 nM UA.     

Au-nanoclusters incorporated 3-amino-5-mercapto-1,2,4-triazole film modified electrode for the simultaneous determination of ascorbic acid, dopamine, uric acid and nitrite

A novel biosensor has been constructed by the electrodeposition of Au-nanoclusters (nano-Au) on poly(3-amino-5-mercapto-1,2,4-triazole) (p-TA) film modified glassy carbon electrode (GCE) and employed for the simultaneous determination of dopamine (DA), ascorbic acid (AA), uric acid (UA) and nitrite (NO₂⁻). NH₂ and SH groups exposed to the p-TA layer are helpful for the electrodeposition of nano-Au. The combination of nano-Au and p-TA endow the biosensor with large surface area, good biological compatibility, electricity and stability, high selectivity and sensitivity and flexible and controllable electrodeposition process. In the fourfold co-existence system, the linear calibration plots for AA, DA, UA and NO₂⁻ were obtained over the range of 2.1–50.1 μM, 0.6–340.0 μM, 1.6–110.0 μM and 15.9–277.0 μM with detection limits of 1.1 × 10⁻⁶ M, 5.0 × 10⁻⁸ M, 8.0 × 10⁻⁸ M and 8.9 × 10⁻⁷ M, respectively. In addition, the modified biosensor was applied to the determination of AA, DA, UA and NO₂⁻ in urine and serum samples by using standard adding method with satisfactory results.     

Flower-like ZnO decorated polyaniline/reduced graphene oxide nanocomposites for simultaneous determination of dopamine and uric acid

A novel sensor was fabricated by electrochemical deposition of ZnO flower-like/polyaniline nanofiber/reduced graphene oxide nanocomposite (ZnO/PANI/RGO) on glassy carbon electrode (GCE) for direct detection of dopamine (DA) and uric acid (UA) in the presence of fixed concentration of ascorbic acid (AA). Surface morphology and characterization of the modified electrodes were confirmed by field emission scanning microscopy (FE-SEM), X-ray diffraction (XRD), Raman and FT-IR spectroscopies. For individual detection, the linear responses were in the two concentration ranges of 0.001–1 μM and 1–1000 μM with detection limit 0.8 nM (S/N = 3) for DA, and also 0.1–100 μM and 100–1000 μM with detection limit 0.042 μM (S/N = 3) for UA. Simultaneous determination of these species in their mixture solution showed the linear responses in the two concentration ranges of 0.1–90 μM and 90–1000 μM with detection limit 0.017 μM (S/N = 3) for DA and also showed two linear range of 0.5–90 μM and 100–1000 μM with detection limit 0.12 μM (S/N = 3) for UA, with coexistence of 1000 μM AA. The applicability of sensor for the analysis of DA, and UA in dopamine injection solution, human serum and human urine samples was successfully demonstrated.     

Chemometric-assisted kinetic-spectrophotometric method for simultaneous determination of ascorbic acid, uric acid, and dopamine

A chemometric-assisted kinetic spectrophotometric method has been developed for simultaneous determination of ascorbic acid (AA), uric acid (UA), and dopamine (DA). This method relies on the difference in the kinetic rates of the reactions of analytes with a common oxidizing agent, tris(1,10-phenanthroline) and iron(III) complex (ferritin, [Fe(phen)₃]³⁺) at pH 4.4. The changes in absorbance were monitored spectrophotometrically. The data obtained from the experiments were processed by chemometric methods of artificial neural network (ANN) and partial least squares (PLS). Acceptable techniques of prediction set, randomization t test, cross-validation, and Y randomization were applied for the selection of the best chemometric method. The results showed that feedforward artificial neural network (FFANN) is more efficient than the other chemometric methods. The parameters affecting the experimental conditions were optimized, and it was found that under optimal conditions Beer’s law is followed in the concentration ranges of 4.3–74.1, 4.3–78.3, and 2.0–33.0 μM for AA, UA, and DA, respectively. The proposed method was successfully applied to the determination of analytes in serum and urine samples.     

Electrochemical determination of uric acid at ordered mesoporous carbon functionalized with ferrocenecarboxylic acid-modified electrode

Ordered mesoporous carbon (OMC) functionalized with ferrocenecarboxylic acid (Fc) was used to modify the glassy carbon (GC) electrode. The characterization of OMC–Fc shows that, after anchoring ferrocene on the mesoporous, ordered mesostructure of the material (OMC–Fc) remains intact and Fc is electrochemically accessible. The obtained OMC–Fc-modified electrode was used to investigate the electrochemical behavior of uric acid (UA). UA oxidation is catalyzed by this electrode in aqueous buffer solution (pH 7.3) with a decrease of 200 mV in overpotential compared to GC electrode. The detection and determination of UA in the presence of ascorbic acid (AA), the main interferent, were achieved. The voltammetric signals due to UA and AA were well separated with a potential difference of 308 mV, a separation that can allow the simultaneous determination of UA and AA. With amperometric method, at a constant potential of 375 mV, the catalytic current of UA versus its concentration shows a good linearity in the range 60–390 μM (R = 0.998) with a detection limit of 1.8 μM (S/N = 3). These results are not influenced by the presence of AA in the sample solution. With good stability and reproducibility, the present OMC–Fc-modified electrode was applied in the determination of UA content in urine sample and satisfactory results were obtained.     

ZnO–CuxO/polypyrrole nanocomposite modified electrode for simultaneous determination of ascorbic acid,dopamine, and uric acid

Novel zinc oxide (ZnO) nanosheets and copper oxide (Cu_xO, CuO, and Cu₂O) decorated polypyrrole (PPy) nanofibers (ZnO–Cu_xO–PPy) have been successfully fabricated for the simultaneous determination of ascorbic acid (AA), dopamine (DA), and uric acid (UA). The morphology and structure of ZnO–Cu_xO–PPy nanocomposites were characterized by scanning electron microscopy (SEM), X-ray diffraction (XRD), and Raman spectroscopy. Compared with the bare glassy carbon electrode (GCE), PPy/GCE, Cu_xO–PPy/GCE, and ZnO–PPy/GCE, ZnO–Cu_xO–PPy/GCE exhibits much higher electrocatalytic activities toward the oxidation of AA, DA, and UA with increasing peak currents and decreasing oxidation overpotentials. Cyclic voltammetry (CV) results show that AA, DA, and UA could be detected selectively and sensitively at ZnO–Cu_xO–PPy/GCE with peak-to-peak separation of 150 and 154 mV for AA–DA and DA–UA, respectively. The calibration curves for AA, DA, and UA were obtained in the ranges of 0.2 to 1.0 mM, 0.1 to 130.0 μM, and 0.5 to 70.0 μM, respectively. The lowest detection limits (signal/noise = 3) were 25.0, 0.04, and 0.2 μM for AA, DA, and UA, respectively. With good selectivity and sensitivity, the current method was applied to the determination of DA in injectable medicine and UA in urine samples.     

Highly selective and sensitive determination of dopamine using a Nafion/carbon nanotubes coated poly(3-methylthiophene) modified electrode

A poly(3-methylthiophene) modified glassy carbon electrode coated with Nafion/single-walled carbon nanotubes film was fabricated and used for highly selective and sensitive determination of dopamine. The hybrid film surface of the modified electrode was characterized by scanning electrochemical microscopy (SECM) and the results indicated that the carbon nanotubes were dispersed uniformly on the conductive polymer. The experimental results suggest that the hybrid film modified electrode combining the advantages of poly(3-methylthiophene), carbon nanotubes with Nafion exhibits dramatic electrocatalytic effect on the oxidation of dopamine (DA) and results in a marked enhancement of the current response. In 0.1M phosphate buffer solution (PBS) of pH 7.0, the differential pulse voltammetric (DPV) peak heights are linear with DA concentration in three intervals, viz. 0.020-0.10 microM, 0.10-1.0 microM and 1.0-6.0 microM, with correlation coefficients of 0.9993, 0.9996 and 0.9993, respectively. The detection limit of 5.0 nM DA could be estimated (S/N=3). Moreover, the interferences of ascorbic acid (AA) and uric acid (UC) are effectively diminished. This hybrid film modified electrode can be applied to the determination of DA contents in dopamine hydrochloride injection and human serum. These attractive features provide a potential application for either in vitro measurement of DA in the presence of excess AA and UA or as detectors in flow injection analysis (FIA) and high performance liquid chromatography (HPLC).     

Differential pulse voltammetric simultaneous determination of acetaminophen and ascorbic acid using single-walled carbon nanotube-modified carbon-ceramic electrode

Single-walled carbon nanotube-modified carbon–ceramic electrode (SWCNT/CCE) was employed for the simultaneous determination of acetaminophen (APAP) and ascorbic acid (AA). The SWCNT/CCE displayed excellent electrochemical catalytic activities toward APAP and AA oxidation compared with bare CCE. In the differential pulse voltammetry technique, both AA and APAP gave sensitive oxidation peaks at −62 and 302 mV versus saturated calomel electrode, respectively. Under the optimized experimental conditions, APAP and AA gave linear responses over ranges of 0.2 to 150.0 μM (R² = 0.998) and 5.0 to 700.0 μM (R² = 0.992), respectively. The lower detection limits were found to be 0.12 μM for APAP and 3.0 μM for AA. The investigated method showed good stability, reproducibility, and repeatability as well as high recovery in pharmaceutical and biological samples.     

Simultaneous voltammetric measurement of ascorbic acid, epinephrine and uric acid at a glassy carbon electrode modified with caffeic acid

A stable electroactive thin film of poly(caffeic acid) has been deposited on the surface of a glassy carbon electrode by potentiostatic technique in an aqueous solution containing caffeic acid. Poly(caffeic acid) was used as a modified electrode for the detection of ascorbic acid (AA), epinephrine (EP), uric acid (UA) and their mixture by cyclic voltammetry. This modified electrode exhibits potent and persistent electron-mediating behavior followed by well-separated oxidation peaks towards AA, EP and UA with activation overpotential. For the ternary mixture containing AA, EP and UA, the three compounds can well separate from each other at the scan rate of 20 mVs(-1) with a potential difference of 156, 132 and 288 mV between AA and EP, EP and UA and AA and UA, respectively, which was large enough to determine AA, EP and UA individually and simultaneously. The catalytic peak current obtained, was linearly dependent on the AA, EP and UA concentrations in the range of 2.0 x 10(-5) to 1.0 x 10(-3) mol l(-1), 2.0 x 10(-6) to 8.0 x 10(-5) mol l(-1) and 5.0 x 10(-6) to 3.0 x 10(-4) mol l(-1), and the detection limits for AA, EP and UA were 7.0 x 10(-6), 2.0 x 10(-7) and 6.0 x 10(-7) mol l(-1), respectively. The modified electrode shows good sensitivity, selectivity and stability, and has been applied to the determination of EP in practical injection samples and that of EP, UA and AA simultaneously with satisfactory results.     

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.     

An amperometric uric acid biosensor based on modified Ir-C electrode

The level of uric acid (UA) has a high relationship with gout, hyperuricemia and Lesch-Nyan syndrome. The determination of UA is an important indicator for clinics and diagnoses of kidney failure. An amperometric UA biosensor based on an Ir-modified carbon (Ir-C) working electrode with immobilizing uricase (EC 1.7.3.3) was developed by thick film screen printing technique. This is the first time to report the utilization of an uricase/Ir-C electrode for the determination of UA by using chronoamperometric (CA) method. The high selectivity of UA biosensor was achieved due to the reduction of H(2)O(2) oxidation potential based on Ir-C electrode. Using uricase/Ir-C as the sensing electrode, the interference from the electroactive biological species, such as ascorbic acid (AA) and UA (might be directly oxidized on the sensing electrode) was slight at the sensing potential of 0.25 V (versus Ag/AgCl). UA was detected amperometrically based on uricase/Ir-C electrode with a sensitivity of 16.60 microAmM(-1) over the concentration range of 0.1-0.8 mMUA, which was within the normal range in blood. The detection limit of UA biosensor was 0.01 mM (S/N=6.18) in pH 7 phosphate buffer solution (PBS) at 37 degrees C. The effects of pH, temperature, and enzymatic loading on the sensing characteristics of the UA biosensor were also investigated in this study.     

Tellurium-nanowire-coated glassy carbon electrodes for selective and sensitive detection of dopamine

Tellurium-nanowire-coated glassy carbon electrodes (TNGCEs) have been fabricated and employed for selective and sensitive detection of dopamine (DA). TNGCEs were prepared by direct deposition of tellurium nanowires, 600 ± 150 nm in length and 16 ± 3 nm in diameter, onto glassy carbon electrodes, which were further coated with Nafion to improve their selectivity and stability. Compared to the GCE, the TNGCE is more electroactive (by approximately 1.9-fold) for DA, and its selectivity toward DA over ascorbic acid (AA) and uric acid (UA) is also greater. By applying differential pulse voltammetry, at a signal-to-noise ratio of 3, the TNGCE provides a limit of detection of 1 nM for DA in the presence of 0.5mM AA and UA. Linearity (R(2)=0.9955) of the oxidation current at 0.19 V against the concentration of DA is found over the range 5 nM-1 μM. TNGCEs have been applied to determine the concentration of dopamine to be 0.59 ± 0.07 μM in PC12 cells.