Simultaneous determination of 3,4-dihydroxyphenylacetic acid, uric acid and ascorbic acid by poly(L-arginine)/multi-walled carbon nanotubes composite film

The poly(L-Arginine)(PArg)-multiwalled carbon nanotubes (MWCNTs) composite film was used to modify glassy carbon electrode (GCE) to fabricate PArg/MWCNTs/GCE through electropolymerization of L-Arginine on MWCNTs/GCE. The PArg/MWCNTs/GCE exhibited high electro-catalytic activities towards the oxidation of 3,4-dihydroxyphenylacetic acid (DOPAC), uric acid (UA), and ascorbic acid (AA), and could be sensitively used for simultaneous determination of DOPAC, AA, and UA in pH 7.4 phosphate-buffered solution (PBS).The linear ranges were 7 microM to 2.7 mM for DOPAC, 3 microM to 1.2 mM for UA, and 70 microM to 1.4 mM for AA. The detection limits were 1.3 microM for DOPAC, 0.7 microM for UA and 20 microM for AA.     

Electrochemical detection of thiols with a coenzyme pyrroloquinoline quinone modified electrode

A modified electrode sensor for the detection of thiols is described. The sensor was constructed by incorporation of the coenzyme pyrroloquinoline quinone (PQQ) into a polypyrrole (PPy) film on a glassy carbon electrode substrate by the electropolymerization of pyrrole in the presence of PQQ. The electrochemical properties of entrapped PQQ in the PPy film were influenced by the applied potential during electropolymerization and by film thickness, both of which were optimized to yield a stable and reproducible response for entrapped PQQ. The PQQ/ PPy sensor was utilized for the amperometric detection of cysteine, homocysteine, penicillamine, N-acetylcysteine, and glutathione. The response for each thiol in pH 8.42 borate buffer was found to be linear with detection limits (S/N = 3) ranging from 13.2 microM for glutathione to 63.7 nM for cysteine with sensitivities of 0.023 nA/microM and 4.71 nA/microM, respectively. The response and detection limits were found to be sensitive to the nature of the thiol and the solution pH. Furthermore, in the presence of dopamine, ascorbic acid, or uric acid, the pH-dependent redox potential of the PQQ catalyst allows tuning of the detection potential to enhance the selectivity for thiols over these potential electroactive interferences.     

Electrochemical sensing using quantum-sized gold nanoparticles

This paper describes the electrocatalytic activity of quantum-sized thiolate protected Au(25) nanoparticles and their use in electrochemical sensing. The Au(25) film modified electrode exhibited excellent mediated electrocatalytic activity that was utilized for amperometric sensing of biologically relevant analytes, namely, ascorbic acid and uric acid. The electron transfer dynamics in the Au(25) film was examined as a function of Au(25) concentration, which manifested the dual role of Au(25) as an electronic conductor as well as a redox mediator. The electron transfer study has further revealed the correlation between the electronic conductivity of the Au(25) film and the sensing sensitivity.     

Determination of serotonin on a glassy carbon electrode modified by electropolymerization of meso-tetrakis(2-aminophenyl)porphyrin and single walled carbon nanotubes

A chemically modified electrode [poly(TAPP)-SWNT/GCE] was prepared by electropolymerization of meso-tetrakis(2-aminophenyl)porphyrin (TAPP)-single walled carbon nanotubes (SWNT) on the surface of a glassy carbon electrode (GCE). This modified electrode was employed as an electrochemical biosensor for the determination of serotonin concentration and exhibited a typical enhance effect on the current response of serotonin and lower oxidation overpotential. The biosensor was very effective to determined 5-HT in a mixture. The linear response was in the range 2.0 x 10(-7) to 1.0 x 10(-5) M, with a correlation coefficient of 0.999 [i(p)(microA) = 3.406 C (microM)+0.132] on the anodic current, with a detection limit of 1 x 10(-9) M. Due to the relatively low currents and different potentials in the electrochemical responses to ascorbic acid and dopamine, the modified electrode is a useful and effective sensing device for the selective and sensitive serotonin determination in the presence of ascorbic acid and dopamine.     

Sensitive determination of uric acid by using graphene quantum dots as a new substrate for immobilisation of uric oxidase

A novel strategy for highly sensitive electrochemical detection of uric acid (UA) was proposed based on graphene quantum dots (GQDs), GQDs were introduced as a suitable substrate for enzyme immobilisation. Uric oxidase (UOx) was immobilised on GQDs modified glassy carbon electrode (GCE). Transmission electron microscope, scanning electron microscopy, cyclic voltammetry and electrochemical impedance spectroscopy techniques were used for characterising the electrochemical biosensor. The developed biosensor responds efficiently to UA presence over the concentration linear range 1–800 μM with the detection limit 0.3 μM. This novel biosensing platform based on UOx/GQDs electrode responded even more sensitively than that based on GCE modified by UOx alone. The inexpensive, reliable and sensitive sensing platform based on UOx/GQDs electrode provides wide potential applications in clinical.Inspec keywords: organic compounds, graphene devices, quantum dots, enzymes, biosensors, biochemistry, electrochemical electrodes, electrochemical sensors, transmission electron microscopy, scanning electron microscopy, voltammetry (chemical analysis), electrochemical impedance spectroscopy, nanomedicine, molecular biophysicsOther keywords: sensitive uric acid determination, graphene quantum dots, uric oxidase immobilisation, electrochemical detection, GQD, enzyme immobilisation, glassy carbon electrode, GCE, transmission electron microscope, scanning electron microscopy, cyclic voltammetry, electrochemical impedance spectroscopy, electrochemical biosensor, C     

Investigation of the electrochemical and electrocatalytic behavior of single-wall carbon nanotube film on a glassy carbon electrode

The electrochemical behavior of a film of single-wall carbon nanotubes (SWNTs) functionalized with carboxylic acid groups was studied extensively on a glassy carbon (GC) electrode. One stable couple corresponding to the redox of the carboxylic acid group, which was supported by XPS and IR experiments, was observed. The electrode process involved four electrons, while the rate-determining step was a one-electron reduction. The SWNT film-modified electrode showed favorable electrocatalytic behavior toward the oxidation of biomolecules such as dopamine, epinephrine, and ascorbic acid.     

Electroanalytical performance of carbon films with near-atomic flatness

Physicochemical and electrochemical characterization of carbon films obtained by pyrolyzing a commercially available photoresist has been performed. Photoresist spin-coated on to a silicon wafer was pyrolyzed at 1,000 degrees C in a reducing atmosphere (95% nitrogen and 5% hydrogen) to produce conducting carbon films. The pyrolyzed photoresist films (PPF) show unusual surface properties compared to other carbon electrodes. The surfaces are nearly atomically smooth with a root-mean-square roughness of <0.5 nm. PPF have a very low background current and oxygen/carbon atomic ratio compared to conventional glassy carbon and show relatively weak adsorption of methylene blue and anthraquinone-2,6-disulfonate. The low oxygen/carbon ratio and the relative stability of PPF indicate that surfaces may be partially hydrogen terminated. The pyrolyzed films were compared to glassy carbon (GC) heat treated under the same conditions as pyrolysis to evaluate the electroanalytical utility of PPF. Heterogeneous electron-transfer kinetics of various redox systems were evaluated. For Ru(NH3)6(3+/2+), Fe(CN)6(3-/4-), and chlorpromazine, fresh PPF surfaces show electron-transfer rates similar to those on GC, but for redox systems such as Fe3+/2+, ascorbic acid, dopamine, and oxygen, the kinetics on PPF are slower. Very weak interactions between the PPF surface and these redox systems lead to their slow electron-transfer kinetics. Electrochemical anodization results in a simultaneous increase in background current, adsorption, and electron-transfer kinetics. The PPF surfaces can be chemically modified via diazonium ion reduction to yield a covalently attached monolayer. Such a modification could help in the preparation of low-cost, high-volume analyte-specific electrodes for diverse electroanalytical applications. Overall, pyrolysis of the photoresist yields an electrode surface with properties similar to a very smooth version of glassy carbon, with some important differences in surface chemistry.     

Carbon-Pt nanoparticles modified TiO2 nanotubes for simultaneous detection of dopamine and uric acid

The present work describes sensing application of modified TiO2 nanotubes having carbon-Pt nanoparticles for simultaneous detection of dopamine and uric acid. The TiO2 nanotubes electrode was prepared using anodizing method, followed by electrodeposition of Pt nanoparticles onto the tubes. Carbon was deposited by decomposition of polyethylene glycol in a tube furnace to improve the conductivity. The C-Pt-TiO2 nanotubes modified electrode was characterized by cyclic voltammetry and differential pulse voltammetry methods. The modified electrode displayed high sensitivity towards the oxidation of dopamine and uric acid in a phosphate buffer solution (pH 7.00). The electro-oxidation currents of dopamine and uric acid were linearly related to the concentration over a wide range of 3.5 x 10(-8) M to 1 x 10(-5) M and 1 x 10(-7) M to 3 x 10(-5) M respectively. The limit of detection was determined as 2 x 10(-10) M for dopamine at signal-to-noise ratio of 3. The interference of uric acid was also investigated. Electro-oxidation currents of dopamine in the presence of fix amount of uric acid represented a linear behaviour towards successive addition of dopamine in range of 1 x 10(-7) M to 1 x 10(-5) M. Furthermore, in a solution containing dopamine, uric acid and ascorbic acid the overlapped oxidation peaks of dopamine and ascorbic acid could be easily separated by using C-Pt-TiO2 nanotubes modified electrode.     

Light-assisted synthesis of Pt-Zn porphyrin nanocomposites and their use for electrochemical detection of organohalides

Pt-Zn porphyrin nanocomposites have been synthesized using zinc porphyrin and dihydrogen hexachloroplatinate in the presence of light and ascorbic acid. TEM and AFM imaging revealed that Pt nanoparticles with an average diameter of approximately 3.5 nm were embedded within the Zn porphyrin matrix. The glassy carbon electrode was modified with Nafion-stabilized Pt-Zn porphyrin nanocomposites and used for dehalogenation of carbon tetrachloride, chloroform, pentachlorophenol, chlorobenzene, and hexachlorobenzene as five test models. The Pt-Zn porphyrin nanocomposite-modified electrode exhibited catalytic activity for the reduction of organohalides at -1.0 V versus Ag/AgCl. Raman signatures confirmed the dehalogenation of chlorobenzene by the nanocomposite-modified electrode. The above two aliphatic and three aromatic organohalides had detection limits of 0.5 microM with linearity up to 8 microM. The modified electrode was good for at least 80 repeated measurements of 4 microM chlorobenzene with a storage stability of 1 month at room temperature. The deactivation of the electrode activity was associated with the loss of platinum nanoparticles from the nanocomposite structure.     

Facile synthesis of gold nanoparticles using carbon dots for electrochemical detection of neurotransmitter,dopamine in human serum and as a chemocatalyst for nitroaromatic reduction

Herein, the authors reported a carbon dots mediated synthesis of gold nanoparticles (AuNPs) at room temperature. Transmission electron microscopy revealed that the AuNPs are spherical in shape with a size of 10 nm. As‐prepared AuNPs was immobilised on carbon paste electrode and subjected to electrochemical sensing of an important neurotransmitter dopamine. Differential pulse voltammetry studies revealed sensitive and selective determination of dopamine in the presence of commonly interfering ascorbic acid and uric acid. The linear detection range was 10–600 μM and the limit of detection was 0.7 ± 0.18 μM. The practical application was demonstrated by measuring dopamine in human blood serum and urine samples. The catalytic activity of AuNPs was evaluated by sodium borohydride mediated reduction of nitroaromatic compounds. The reduction kinetics was found to be pseudo‐first‐order kinetics. All the tested nitroaromatics reduced to corresponding amines in <10 min.Inspec keywords: voltammetry (chemical analysis), electrochemical sensors, biosensors, nanosensors, reduction (chemical), organic compounds, nanofabrication, gold, catalysis, transmission electron microscopy, electrochemical electrodes, blood, nanoparticles, carbon, chemical variables measurement, catalysts, particle sizeOther keywords: nitroaromatic compounds, reduction kinetics, gold nanoparticles, chemocatalyst, nitroaromatic reduction, carbon dots mediated synthesis, room temperature, transmission electron microscopy, carbon paste electrode, electrochemical sensing, ascorbic acid, uric acid, linear detection range, human blood serum, urine samples, sodium borohydride mediated reduction, neurotransmitter dopamine, differential pulse voltammetry, catalytic activity, pseudofirst‐order kinetics, amines, temperature 293 K to 298 K, C‐Au     

Fabrication and application of poly(alizarin red S)-carbon nanotubes composite film based nitrite sensor

In this work, a novel electrochemical nitrite sensor for sensitive determination of nitrite based on poly(alizarin red S)-multi-wall carbon nanotubes (PARS-MWNTs) composite film on the glassy carbon electrode was described. The surface morphologies of different electrodes were characterized by scanning electron microscopy. Cyclic voltammetry, chronocoulometry and linear sweep voltammetry were used to investigate the electrochemical response and oxidation mechanism of nitrite at the PARS-MWNTs composite film based sensor. The experimental parameters were optimized, such as electropolymerization pH value, film thickness and detection pH value et al. Under optimal working conditions, the oxidation peak current of nitrite linearly increased with its concentration in the range of 30 nM to 1.1 mM with a low detection limit of 2 nM. The PARS-MWNTs composite film based nitrite sensor was applied to the determination of nitrite in sausage, and the good recovery indicated that it may have practical applications in nitrite monitoring system.     

Selective permeation of hydrogen peroxide through polyelectrolyte multilayer films and its use for amperometric biosensors.

A platinum electrode was coated with polyelectrolyte multilayer (PEM) films to prepare an amperometric hydrogen peroxide sensor which can be used in the presence of possible interferences such as ascorbic acid, uric acid, and acetaminophen. The PEM films were prepared on the surface of a Pt disk electrode by an alternate deposition of polycation and polyanion from the aqueous solutions through electrostatic force of attraction. The Pt electrodes coated with a poly(allylamine)/poly(vinyl sulfate) or poly(allylamine)/poly(styrenesulfonate) film were used successfully for detecting H2O2 selectively in the presence of the possible interfering agents. It was suggested that H2O2 can diffuse into the PEM film smoothly while the ascorbic acid, uric acid, and acetaminophen cannot penetrate the film by a size exclusion mechanism. On the other hand, the electrodes coated with PEM films containing poly(ethyleneimine) or poly(diallyldimethylammonium chloride) were not useful for the selective determination of H2O2. The results were rationalized based on the different permeability of the films due to the different molecular density or packing in the PEM films. The PEM film-coated electrode was useful for constructing glucose biosensors by coupling with glucose oxidase.     

Iridium-based electrocatalytic systems for the determination of insulin

Two electrochemical catalytic systems for the determination of insulin were developed. The homogeneous system was based on the oxidation of insulin by chloro complexes of iridium(IV). Kinetic studies revealed that the aquation of iridium complexes activated them toward the oxidation of insulin in acidic solutions; e.g., the rate constant was equal to 25, 900, and 8,400 L mol(-1) s(-1) for the oxidation of insulin by the IrCl62-, Ir(H2O)CI5-, and Ir(H2O)2Cl4 complexes, respectively. The inertness of the iridium complexes argued for the outer-sphere mechanism of the homogeneous oxidation reaction. Electroplating of aquated iridium complexes on the glassy carbon electrode resulted in the formation of the iridium oxide (IrOx) surface film, which was used in the heterogeneous detection system for insulin. The catalytic activity of the IrOx film toward insulin oxidation was ascribed to a combination of electron-transfer mediation and oxygen transfer which was related to the acid/base chemistry of the film. The IrOx film electrode was used as an amperometric detector for flow injection analysis of insulin in pH 7.40 phosphate buffer. Linear least-squares calibration curves over the range 0.05-0.50 microM (five points) had slopes of 35.2 +/- 0.4 nA microM(-1) and correlation coefficients of 0.999. The detection limit for insulin was 20 nM using the criterion of a signal of 3 times the peak-to-peak noise. The advantageous properties of the detector based on the IrOx film are its inherent stability at physiological pH, high catalytic activity toward insulin oxidation, and simplicity of preparation.     

Electrochemical sensor to detect neurotransmitter using gold nano-island coated ITO electrode

Parkinson disease is a chronic neurodegenerative disorder characterized by the loss of dopamine, which is a neurotransmitter in the substantia nigra. In this study, a simple, rapid and inexpensive method to fabricate gold nano-island film (GNIF) coated ITO electrode has been developed based on electrochemical deposition of Au onto ITO substrate. The nanostructured film surface was characterized by scanning electron microscopy (SEM). Cyclic voltammetry (CV) and differential pulse voltammetry (DPV) were used to evaluate the electrochemical behavior of induvidul dopamine and uric acid solution were studied. Moreover, GNIF/ITO electrode was applied to detecte DA in the presence of Bovine Serum Albumin (50 microM) as an interference. These results demonstrate that, interfering component has no effect on the determination of DA at GNIF electrode, hence this GNIF electrode is suitable for the determination of DA with high sensitivity and selectivity. Then, GNIF coated ITO electrode was applied to monitor the electrochemical simultaneous detection of dopamine and uric acid mixtures based on CV and DPV with high sensitivity. GNIF-modified ITO electrode showed a linear range for the determination of dopamine concentration from 0.1 microM to 40 microM in the presence of 50 microM of uric acid. Based on these results, the proposed technique can be a promising method to construct a highly sensitive biosensor as well as highly efficient protein chip.     

纳米Fe3O4-还原氧化石墨烯复合修饰玻碳电极的制备及电化学检测多巴胺

通过电化学还原法制备纳米Fe₃O₄-还原氧化石墨烯复合修饰玻碳（Fe₃O₄-rGO/GCE）电极,用于多巴胺（DA）的检测。采用SEM、TEM和循环伏安对纳米Fe₃O₄-rGO复合材料进行表征。在pH为7.0的磷酸盐缓冲液（PBS）中,采用循环伏安法研究了DA在纳米Fe₃O₄-rGO/GC上的电化学行为。实验结果表明,较裸GC电极和rGO修饰（rGO/GC）电极,由于纳米Fe₃O₄与rGO的协同作用,纳米Fe₃O₄-rGO/GC显著增大了Fe₃O₄-rGO/GC复合材料电极电化学活性面积和氧化峰电流强度i_pa。DA的浓度在6.0×10^-8~2.0×10^-6 mol/L和2.0×10^-6~8.0×10^-5 mol/L范围内,与氧化峰电流强度i_pa呈良好的线性关系,检出限达4.0×10^-9 mol/L（信噪比S/N=3）。抗坏血酸和尿酸共存物几乎不干扰DA的测定,选择性高。Fe₃O₄-rGO/GC修饰电极用于盐酸DA注射液中的DA含量测定,获得结果较好,回收率为97.1%~103.9%。     

Fabrication of TiO2 nanoparticles/surfactant polymer complex film on glassy carbon electrode and its application to sensing trace dopamine

A novel method for the fabrication of a TiO₂/Nafion nano-film on glassy carbon electrode (NTGCE) is described. In the presence of dispersant, TiO₂ nanoparticles were dispersed into water to give a homogeneous and stable suspension. After the solvent evaporation, a porous and uniform TiO₂ nano-film was obtained on the GCE surface. Further coated with Nafion, the complex film possesses remarkable stability in aqueous solution. This nano-film was characterized by scanning electron microscopy (SEM) and atomic force microscopy (AFM). The prepared electrode showed excellent electrocatalytic behavior of dopamine and high concentration of ascorbic acid does not interfere with the dopamine detection. Based on this, an electrochemical method is developed for the determination of dopamine with simplicity and high sensitivity.     

Preparation of ferrocene nanocrystals by the ultrasonic-solvent-substitution method and their electrochemical properties

Ferrocene nanocrystals are successfully prepared by the ultrasonic-solvent-substitution method. Transmission electron microscopy images show that the average diameter of the nanocrystals is about (40+/-5) nm and X-ray diffraction analysis indicated that the products obtained have a high purity. The optical properties of the products are studied by IR and UV/Vis spectroscopy. UV/Vis spectra indicate that, compared with ferrocene bulk material, the nanospheres show an obvious blue shift. Cyclic voltammetry is used to test the electrocatalytic oxidation properties of the ferrocene nanospheres towards ascorbic acid on the glassy carbon electrode. The results show that the ferrocene-nanosphere-modified glassy carbon electrode exhibits a better electrocatalytic effect than that modified by the ferrocene bulk material in phosphate buffer (pH 5.00). The important factors in the preparation of and a possible mechanism for the formation of ferrocene nanospheres is also discussed. The method could potentially be applied to the preparation of nanomaterials of other organometallic compounds.     

Electrochemical oxidation of oxalic acid at highly boron-doped diamond electrodes

Electrochemical oxidation of oxalic acid has been investigated at bare, highly boron-doped diamond electrodes. Cyclic voltammetry and flow injection analysis with amperometric detection were used to study the electrochemical reaction. Hydrogen-terminated diamonds exhibited well-defined peaks of oxalic acid oxidation in a wide pH range. A good linear response was observed for a concentration range from 50 nM to 10 microM, with an estimated detection limit of approximately 0.5 nM (S/N = 3). In contrast, oxygen-terminated diamonds showed no response for oxalic acid oxidation inside the potential window, indicating that surface termination contributed highly to the control of the oxidation reaction. An investigation with glassy carbon electrodes was conducted to confirm the surface termination effect on oxalic acid oxidation. Although a hydrogen-terminated glassy carbon electrode showed an enhancement of signal-to-background ratio in comparison with untreated glassy carbon, less stability of the current responses was observed than that at hydrogen-terminated diamond.     

Multiwalled Carbon Nanotubes Modified Electrode as a Sensor for Adsorptive Stripping Voltammetric Determination of Hydrochlorothiazide

The study of electrochemical behavior and determination of hydrochlorothiazide, a thiazide diuretic and antihypertensive drug, on multiwalled carbon nanotubes (MCNTs) modified glassy carbon electrode (GCE) is described by adsorptive stripping voltammetry in open circuit potential. The cyclic voltammetric results indicate that MCNTs remarkably enhance the oxidation of hydrochlorothiazide in wide pH range of 2.0–9.5, which is leading to considerable improvement of anodic peak current for hydrochlorothiazide, and allow the development of a highly sensitive voltammetric sensor for determination of hydrochlorothiazide in pharmaceutical and urine samples. Electrochemical studies by alpha-Fe$_{2}$O $_{3}$ nanoparticles modified carbon paste electrode show these oxides have no electrocatalytic effect for hydrochlorothiazide oxidation and support iron oxide impurities in the MCNTs are not active sites in sensing of hydrochlorothiazide. Under optimized conditions, the oxidation peak have two linear dynamic range of 2.0–20.0 nM and 0.2–100.0 $mu$M with experimental detection limit of 0.8 nM and a precision of $≪hbox{4}hbox{%}$ (RSD for 8 analysis).      

Enhanced oxidation of diclofenac sodium at a nano-structured electrochemical sensing film constructed by multi-wall carbon nanotubes–surfactant composite

A multi-walled carbon nanotubes (MWNTs)–dihexadecyl hydrogen phosphate (DHP) film-coated glassy carbon electrode (GCE) was fabricated, and the voltammetric determination method of diclofenac sodium was investigated on this modified electrode by using different kinds of electrochemical techniques. The results showed that this nano-structured film electrode exhibits excellent enhancement effects on the electrochemical oxidation of diclofenac sodium. The oxidation peak current of diclofenac sodium at this film-modified electrode increased significantly compared with that at a bare glassy carbon electrode. Based on the experiment outcomes a possible mechanism was proposed and discussed. The proposed method was demonstrated by using diclofenac sodium tablets and the result was satisfying.