Factors affecting in vivo electrochemistry: electrode-tissue interaction and the ascorbate amplification effect

While in vivo electrochemistry has been shown to be useful for discovering new neurophysiological phenomena, there is still considerable controversy about the identity of the compounds being measured and the concentration of those compounds in extracellular fluid in brain. We have found that carbon paste electrodes undergo changes in sensitivity and specificity for dopamine and other compounds after being implanted in brain. We have also examined the effect of ascorbate on the selective enhancement of catecholamine peaks to provide an explanation for the apparently very high concentrations of dopamine measured in the extracellular fluid space. After temporary brain implantation (20 min), carbon paste electrodes tested in vitro showed increased sensitivity and lower oxidation potentials for dopamine, norepinephrine and serotonin. These brain-treated electrodes also detected 3,4-dihydroxyphenylacetic acid (DOPAC) as a distinct peak at +0.16 V, although the electrode sensitivity for DOPAC was some 25 times lower than that for dopamine. Brain treatment did not alter electrode sensitivity or oxidation potential for 5-HIAA. The oxidation current for ascorbic acid when processed as the semiderivative showed no distinct peak in the potential range -0.2 to +0.4V for either untreated or brain-treated electrodes. However ascorbic acid amplified the electrochemical peaks of catechols in direct proportion to the ratio of the concentration of ascorbate to the concentration of the catechol. In the physiologic concentration range of 300 microM ascorbate, the electrochemical signal for 1 microM dopamine was amplified 4250%. While ascorbate amplification improves detectability of dopamine and norepinephrine, it also introduces ambiguity since changing catechol concentrations cannot be distinguished from changing ascorbate concentrations.     

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.     

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 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.     

SiO2/SnO2/Sb2O5 microporous ceramic material for immobilization of Meldola's blue: application as an electrochemical sensor for NADH

The mixed oxide SiO(2)/SnO(2), containing 25 wt% of SnO(2), determined by X-ray fluorescence, was prepared by the sol-gel method and the porous matrix obtained was then grafted with Sb (V), resulting the solid designated as (SiSnSb). XPS indicated 0.7% of Sb atoms on the surface. Sb grafted on the surface contains Br?nsted acid centers (SbOH groups) that can immobilize Meldola's blue (MB(+)) cationic dye onto the surface by an ion exchange reaction, resulting the solid designated as (SiSnSb/MB). In the present case a surface concentration of MB(+)=2.5×10(-11) mol cm(2) on the surface was obtained. A homogeneous mixture of the SiSnSb/MB with ultra pure graphite (99.99%) was pressed in disk format and used to fabricate a working electrode that displayed an excellent specific electrocatalytic response to NADH oxidation, with a formal potential of -0.05 V at pH 7.3. The electrochemical properties of the resulting electrode were investigated thoroughly with cyclic voltammetric and chronoamperometry techniques. The proposed sensor showed a good linear response range for NADH concentrations between 8×10(-5) and 9.0×10(-4) mol L(-1), with a detection limit of 1.5×10(-7) mol L(-1). The presence of dopamine and ascorbic acid did not show any interference in the detection of NADH on this modified electrode surface.     

Mediatorless voltammetric oxidation of NADH and sensing of ethanol

A simple, selective and sensitive method for the detection of NADH and ethanol is presented. Self-assembled monolayers (SAMs) of mercaptopyrimidine (MPM) and their derivatives, thiocytosine (TC) and 4,6-diamino-2-mercaptopyrimidine (DMP) on gold (Au) electrode are used for the voltammetric detection of NADH and ethanol in neutral aqueous solution. A decrease of 200-300 mV in the overpotential associated with an observable increase in the peak current was obtained for the oxidation of NADH on MPM and TC monolayer-modified electrodes without any redox mediator. The facilitated electron transfer for the oxidation of NADH at the TC monolayer is ascribed to the existence of stable cationic p-quinonoid form of TC. The electrode modified with DMP monolayer could not exhibit stable response for NADH owing to the fouling of electrode surface. The MPM and TC monolayer-modified electrodes show high selectivity and excellent sensitivity (MPM: 0.633+/-0.005 microA cm(-2) microM(-1); TC: 0.658+/-0.008 microA cm(-2) microM(-1)) towards NADH with detection limit (3sigma) of 2.5 and 0.5 microM, respectively. Presence of large excess of ascorbate (AA) does not interfere the detection of NADH and the monolayer-modified electrode shows individual voltammetric peaks for AA and NADH. Voltammetric sensing of ethanol using alcohol dehydrogenase on MPM and TC monolayer-modified electrode is successfully demonstrated and these electrode can detect as low as 0.5 mM ethanol in neutral pH. The sensitivity of the MPM and TC monolayer-modified electrodes toward ethanol was found to be 3.24+/-0.03 and 3.435+/-0.04 microA cm(-2) mM(-1), respectively.     

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.     

Simultaneous determination of dopamine and serotonin on a glassy carbon electrode coated with a film of carbon nanotubes

A chemically modified electrode based on the carbon nanotube film-coated glassy carbon electrode (GCE) is described for the simultaneous determination of dopamine (DA) and serotonin (5-HT). The multiwall carbon nanotube (MWNT) film-coated GCE exhibits a marked enhancement effect on the current response of DA and 5-HT and lowers oxidation overpotentials. The responses of DA and 5-HT merge into a large peak at a bare GCE, but they yield two well-defined oxidation peaks at the MWNT film-coated GCE. The experimental parameters were optimized, and a direct electrochemical method for the simultaneous determination of DA and 5-HT was proposed. The interference of ascorbic acid (AA) was investigated, and the results showed that a large excess of AA did not interfere with the voltammetric responses of DA and 5-HT. The modified electrode has been successfully applied for the assay of 5-HT and DA in human blood serum.     

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.     

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).     

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.     

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 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.     

Film electrode prepared from oppositely charged silicate submicroparticles and carbon nanoparticles for selective dopamine sensing

Film electrodes prepared from oppositely charged silicate submicroparticles and carbon nanoparticles was applied for selective dopamine sensing. Mesoporous silicate submicroparticles with tetraalkylammonium functionalities were prepared by sol-gel method. They were immobilised on an indium tin oxide film surface together with phenylsulphonated carbon nanoparticles by layer-by-layer method: alternative immersion into their suspensions. As it is shown by scanning electron microscopy the obtained film is composed of silicate submicroparticles covered by carbon nanoparticles. The nanoparticulate film is stable and its electroactive surface is significantly larger than substrate. Accumulation of redox active cations indicates that only fraction charged functionalities of carbon nanoparticles are employed in film formation. The obtained electrode exhibits catalytic properties towards dopamine oxidation and its interferences as ascorbic acid, uric acid and acetaminophen. This allows for selective determination of tenth micromolar concentration of dopamine in the presence of these interferences at milimolar level. The detection limit and linear range were determined to 0.1 × 10?? mol dm?3 and 0.3-18 × 10?? mol dm?3 respectively.     

Iron(II) phthalocyanine-modified carbon-paste electrode for potentiometric detection of ascorbic acid

A chemically modified electrode constructed by incorporating iron(II) phthalocyanine [Fe(II)Pc] into carbon-paste matrix was used as a sensitive potentiometric sensor for detection of ascorbic acid. The resulting electrode exhibits catalytic properties for the electrooxidation of ascorbic acid, and lowers the overpotential for the oxidation of this compound. The faster rate of electron transfer results in a near-Nernstian behavior of the modified electrode, and makes it a suitable potentiometric sensor for detection of ascorbic acid. A linear response in concentration range from 10(-6) to 10(-2) M (0.18--1800 microg ml(-1)) was obtained with a detection limit of 5 x 10(-7) M for the potentiometric detection of ascorbic acid. The modified electrode was used for the determination of ascorbic acid in vitamin preparations. The recovery was 97.2--102.4% for the vitamin added to the preparations with a relative standard deviation of less than 5%. The modified electrode exhibited a fast response time (<10 s),had good stability, and had an extended lifetime.     

A novel sensor based on electrochemical polymerization of diglycolic acid for determination of acetaminophen

Diglycolic acid (DA) polymer was coated on glassy carbon (GC) electrode by cyclic voltammetry (CV) technique for the first time. The electrochemical performances of the modified electrode were investigated by CV and electrochemical impedance (EIS). The obtained electrode showed an excellent electrocatalytic activity for the oxidation of acetaminophen (ACOP). A couple of well-defined reversible electrochemical redox peaks were observed on the ploy(DA)/GC electrode in ACOP solution. Compared with bare GC electrode, the oxidation peak potential of ACOP on ploy(DA)/GC electrode moved from 0.289 V to 0.220 V. Meanwhile, the oxidation peak current was much higher on the modified electrode than that on the bare GC electrode, indicating DA polymer modified electrode possessed excellent performance for the oxidation of ACOP. This kind of capability of the modified electrode can be enlisted for the highly sensitive and selective determination of ACOP. Under the optimized conditions, a wide linear range from 2 × 10(-8) to 5.0 × 10(-4)M with a correlation coefficient 0.9995 was obtained. The detection limit was 6.7 × 10(-9)M (at the ratio of signal to noise, S/N=3:1). The modified electrode also exhibited very good stability and reproducibility for the detection of ACOP. The established method was applied to the determination of ACOP in samples. An average recovery of 100.1% was achieved. These results indicated that this method was reliable for determining ACOP.     

Application of CdS quantum dots modified carbon paste electrode for monitoring the process of acetaminophen preparation

In this research article, a novel, selective, and sensitive modified carbon paste electrode (CPE) using CdS quantum dots (QDs) is presented. The highly stable CdS QDs were successfully synthesized in an in situ process using Na₂S₂O₃ as a precursor and thioglycolic acid as a catalyst and capping agent. The synthesis of CdS QDs was studied using X-ray diffraction (XRD) and transmission electron microscopy (TEM) techniques. The synthesized CdS QDs were used for preparation of a modified carbon paste electrode (CdS/CPE). The electrochemical behavior of the electrode toward p-aminophenol (PAP) and acetaminophen (Ac) was studied, and the results demonstrated that the CdS/CPE exhibited good electrocatalytic performance toward PAP and Ac oxidation. The oxidation peak potential of each analyte in the mixture was well separated. As a result, a selective and reliable method was developed for the determination of PAP and Ac simultaneously without any chemical separations. Application of the fabricated electrode for monitoring the process of Ac preparation from PAP was investigated. The obtained results show that CdS/CPE has satisfactory analytical performance; it could be a kind of attractive and promising nanomaterial-based sensor for process monitoring via the electrochemical approach.     

Hydroxyurea and p-aminophenol are the suicide inhibitors of ascorbate peroxidase

Guaiacol peroxidase from spinach catalyzes the oxidation of p-aminophenol to produce the aminophenoxy radical as the primary product which is converted further into a stable oxidation product with an absorption peak at 470 nm. The p-aminophenol radicals oxidize ascorbate (AsA) to produce monodehydroascorbate radicals. Kinetic analysis indicates that p-aminophenol radicals also oxidize monodehydroascorbate to dehydroascorbate. Incubation of AsA peroxidase from tea leaves and hydrogen peroxide with p-aminophenol, p-cresol, hydroxyurea, or hydroxylamine results in the inactivation of the enzyme. No inactivation of the enzyme was found upon incubation of the enzyme with these compounds either in the absence of hydrogen peroxide or with the stable oxidized products of these compounds. The enzyme was protected from inactivation by the inclusion of AsA in the incubation mixture. The radicals of p-aminophenol and hydroxyurea were produced by AsA peroxidase as detected by their ESR signals. These signals disappeared upon the addition of AsA, and the signal characteristic of monodehydroascorbate was found. Thus, AsA peroxidase is inactivated by the radicals of p-aminophenol, p-cresol, hydroxyurea, and hydroxylamine which are produced by the peroxidase reaction, and it is protected from inactivation by AsA via the scavenging of the radicals. Thus, these compounds are the suicide inhibitors for AsA peroxidase. Isozyme II of AsA peroxidase, which is localized in chloroplasts, is more sensitive to these compounds than isozyme I. In contrast to AsA peroxidase, guaiacol peroxidase was not affected by these various compounds, even though each was oxidized by it and the corresponding radicals were produced.     

Pyruvate as a fluorescence quencher: a new spectroscopic assay for pyruvate reactions

Optimal conditions for the extraction from brain tissue and the simultaneous quantification of catechol and indole derivatives were determined after a systematic degradation study in water and perchloric acid. The roles of three parameters, namely temperature, presence of antioxidant agents, and time, were considered. Adrenaline, noradrenaline, dopamine, homovanillic acid, 5-hydroxytryptophan, 5-hydroxyindole acetic acid, serotonin, and epinephrine were separated by HPLC and detected electrochemically. The results indicated a great instability of the indole derivatives at an ambient temperature, in an acid medium, and in the absence of a protective agent. Therefore, when perchloric acid has to be used for deproteinization, the lowest concentration (0.1 M) is preferable. The samples have to be kept on ice, in darkness, and protected by ascorbic acid and sodium ethylenediamine tetracetate.     

Brain dialysis in conscious rats confirms in vivo electrochemical evidence that dopaminergic stimulation releases ascorbate

The first faradiac peak in the voltammogram at +0.12 volts vs an Ag/AgCl reference recorded using a carbon paste working electrode results primarily from the oxidation of extracellular ascorbic acid (AA) with lesser contributions from dopamine (DA) and 3,4-dihydroxyphenylacetic acid (DOPAC). The rise in this peak following DA agonist treatment cannot be explained by changes in DA or DOPAC levels since these would be expected to decrease. We carried out brain dialysis and in vivo voltammetry studies in parallel to determine the identity of the substances released into the striatal extracellular fluid by the DA agonist, pergolide, and the dopamine releaser, d-amphetamine.