Carbon nanotube-modified microelectrodes for simultaneous detection of dopamine and serotonin in vivo

Dopamine and serotonin are important neurotransmitters that interact in the brain. While dopamine is easily detected with electrochemical sensors, the detection of serotonin is more difficult because reactive species formed after oxidation can adsorb to the electrode, reducing sensitivity. Carbon nanotube treatments of electrodes have been used to increase the sensitivity, promote electron transfer, and reduce fouling. Most methods have focused on nanotube coatings of large electrodes and slower electrochemical techniques that are not conducive to measurements in vivo. In this study, we investigated carbon-fiber microelectrodes modified with single-walled carbon nanotubes for the co-detection of dopamine and serotonin in vivo. Using fast-scan cyclic voltammetry, S/N ratios for the neurotransmitters increased after nanotube coating. Electrocatalytic effects of nanotubes were not apparent at fast scan rates but faster kinetics were observed with slower scanning. Nanotube-modified microelectrodes showed significantly less fouling after exposure to serotonin than bare electrodes. The nanotube-modified electrodes were used to monitor stimulated dopamine and serotonin changes simultaneously in the striatum of anesthetized rat after administration of a serotonin synthetic precursor. These studies show that nanotube-coated microelectrodes can be used with fast scanning techniques and are advantageous for in vivo measurements of neurotransmitters because of their greater sensitivity and resistance to fouling.     

Functional groups modulate the sensitivity and electron transfer kinetics of neurochemicals at carbon nanotube modified microelectrodes

The surface properties of carbon-based electrodes are critically important for the detection of biomolecules and can modulate electrostatic interactions, adsorption and electrocatalysis. Carbon nanotube (CNT) modified electrodes have previously been shown to have increased oxidative sensitivity and reduced overpotential for catecholamine neurotransmitters, but the effect of surface functionalities on these properties has not been characterized. In this study, we modified carbon-fiber microelectrodes (CFMEs) with three differently functionalized single-wall carbon nanotubes and measured their response to serotonin, dopamine, and ascorbic acid using fast-scan cyclic voltammetry. Both carboxylic acid functionalized and amide functionalized CNTs increased the oxidative current of CFMEs by approximately 2-6 fold for the cationic neurotransmitters serotonin and dopamine, but octadecylamine functionalized CNTs resulted in no significant signal change. Similarly, electron transfer was faster for both amide and carboxylic acid functionalized CNT modified electrodes but slower for octadecylamine CNT modified electrodes. Oxidation of ascorbic acid was only increased with carboxylic acid functionalized CNTs although all CNT-modified electrodes showed a trend towards increased reversibility for ascorbic acid. Carboxylic acid-CNT modified disk electrodes were then tested for detection of serotonin in the ventral nerve cord of a Drosophila melanogaster larva, and the increase in sensitivity was maintained in biological tissue. The functional groups of CNTs therefore modulate the electrochemical properties, and the increase in sensitivity from CNT modification facilitates measurements in biological samples.     

Charge-selective recognition at fibroin-modified electrodes for analytical application

A novel fibroin-modified electrode with charge recognition is reported. The characteristics of silk fibroin membranes have been exploited for analytical applications. The membrane, with an isoelectric point of pH 4.5, was applied to graphite and carbon-fiber electrodes. The modified electrode was negatively charged in solutions of pH>4.5, and so rejected anions and attracted cations. In solutions of pH<4.5 the electrode was positively charged, and so rejected cations and attracted anions. The pH-responsive charge recognition of the modified electrode was investigated for some neurocompounds. A fibroin carbon-fiber electrode was used for in-vivo determination of the concentration of the cationic neurotransmitter dopamine (DA).This revised version replaces the article published online on April 2005.     

Nafion-CNT coated carbon-fiber microelectrodes for enhanced detection of adenosine

Adenosine is a neuromodulator that regulates neurotransmission. Adenosine can be monitored using fast-scan cyclic voltammetry at carbon-fiber microelectrodes and ATP is a possible interferent in vivo because the electroactive moiety, adenine, is the same for both molecules. In this study, we investigated carbon-fiber microelectrodes coated with Nafion and carbon nanotubes (CNTs) to enhance the sensitivity of adenosine and decrease interference by ATP. Electrodes coated in 0.05 mg mL(-1) CNTs in Nafion had a 4.2 ± 0.2 fold increase in current for adenosine, twice as large as for Nafion alone. Nafion-CNT electrodes were 6 times more sensitive to adenosine than ATP. The Nafion-CNT coating did not slow the temporal response of the electrode. Comparing different purine bases shows that the presence of an amine group enhances sensitivity and that purines with carbonyl groups, such as guanine and hypoxanthine, do not have as great an enhancement after Nafion-CNT coating. The ribose group provides additional sensitivity enhancement for adenosine over adenine. The Nafion-CNT modified electrodes exhibited significantly more current for adenosine than ATP in brain slices. Therefore, Nafion-CNT modified electrodes are useful for sensitive, selective detection of adenosine in biological samples.     

Overoxidation of carbon-fiber microelectrodes enhances dopamine adsorption and increases sensitivity

The voltammetric responses of carbon-fiber microelectrodes with a 1.0 V and a 1.4 V anodic limit were compared in this study. Fast-scan cyclic voltammetry was used to characterize the response to dopamine and several other neurochemicals. An increase in the adsorption properties of the carbon fiber leads to an increase in sensitivity of 9 fold in vivo. However the temporal response of the sensor is slower with the more positive anodic limit. Increased electron transfer kinetics also causes a decrease in the relative sensitivity for dopamine vs. other neurochemicals, and a change in their cyclic voltammograms. Stimulated release in the caudate-putamen was pharmacologically characterized in vivo using Ro-04-1284 and pargyline, and was consistent with that expected for dopamine.     

Application of PEDOT‐CNT Microelectrodes for Neurotransmitter Sensing

In this work, composite microelectrodes from poly(3,4‐ethylenedioxythiophene) (PEDOT) and carbon nanotubes (CNT) are characterized as electrochemical sensing material for neurotransmitters. Dopamine can be detected using square wave voltammetry at these microelectrodes. The CNTs improve the sensitivity by a factor of two. In addition, the selectivity towards dopamine in the presence of ascorbic acid and uric acid was examined. While both electrodes, PEDOT and PEDOT‐CNT are able to detect all measured concentrations of dopamine in the presence of uric acid, small concentrations of dopamine and ascorbic acid are only distinguishable at PEDOT‐CNT electrodes. Changing the pH has a strong influence on the selectivity. Moreover, it is possible to detect concentrations as low as 1 µM dopamine in complex cell culture medium. Finally, other catecholamines like serotonin, epinephrine, norepinephrine and L ‐dopa are also electrochemically detectable at PEDOT‐CNT microelectrodes.     

Surface states of titanium dioxide nanoparticles modified with enediol ligands

Control of surface states of titanium dioxide nanoparticles using 2-(3,4-dihydroxyphenyl)ethylamine (dopamine) and 3,4-dihydrophenylacetic acid, which act as ligands to the undercoordinated surface sites (carrier traps), is demonstrated by electrochemical techniques. The deepest traps were found to be most reactive and are selectively removed by the addition of the ligands which enhances the kinetics of electron accumulation in the film. Furthermore, a shift in the Fermi level to more positive potentials was detected for electrodes modified with the negatively charged ligand (3,4-dihydrophenylacetic acid) compared to that of electrodes modified with the positively charged ligand (dopamine). The presence of the negative charge on the ligand also contributed to the underpotential of hydrogen evolution on 3,4-dihydrophenylacetic acid-modified electrodes.     

Single‐Walled Carbon Nanotubes on Tungsten Wires: A New Class of Microelectrochemical Sensors

This paper reports the fabrication and the outstanding performance characteristics of novel microelectrodes consisting of tungsten (W) wires coated with homogeneous layers of single‐walled C nanotubes (SWNT). A series of studies using cyclic voltammetry indicate that the SWNT‐modified W electrodes possess interesting electrochemical features. In fact, they are able to catalyse electron transfer reactions involving a series of inorganic and biological molecules. These electrodes are characterized by a fast electron transfer, a wide working potential window, and a low background current. Moreover they demonstrate excellent reproducibility, good stability in various chemical media, and very high sensitivity towards a series of inorganic and organic compounds. The SWNT modified microelectrodes have been tested for the capacity to electrochemically detect ferrocene monocarboxylic acid and potassium hexacyanoferrate as well of a series of interesting biological molecules which include catechol, caffeic acid, DOPAC, ascorbic acid, L ‐tyrosine, acetaminophen, guanine, uric acid, and the neurotransmitters dopamine, epinephrine, and serotonin (5‐HT) hydrochloride. The advantages of the SWNT‐modified W electrodes are illustrated by comparing their analytical performance with that of conventional electrodes.     

Nanohybrid Materials Consisting of Poly[(3‐aminobenzoic acid)‐co‐(3‐aminobenzenesulfonic acid)‐co‐aniline] and Multiwalled Carbon Nanotubes for Immobilization of Redox Active Cytochrome c

The development of a new surface architecture for the efficient direct electron transfer of positively charged redox proteins is presented. For this reason different kinds of polyaniline terpolymers consisting of aminobenzoic acid (AB), aminobenzenesulfonic acid (ABS) and aniline (A) with different monomer ratios were synthesized. The P(AB‐ABS‐A) were grafted to the surface of multiwalled carbon nanotubes (MWCNTs). FTIR measurements prove the covalent binding to the carboxylic groups of the MWCNTs while conductivity tests show an increase in the conductivity of the nanohybrid in comparison to the polymers. The [MWCNT‐P(AB‐ABS‐A)] nanohybrids were used for the immobilization of redox active cytochrome c (cyt.c). The positively charged protein can electrostatically interact with the negatively charged nanohybrid. Cyclic voltammetry (CV) shows an increase in the protein loading on [MWCNT‐P(AB‐ABS‐A)] coupled to cysteamine modified gold electrodes in comparison to non‐grafted MWCNTs. A further increase in the sulfonation degree of P(AB‐ABS‐A) leads to an enhanced current output of the modified electrodes. The redox activity of the polymer decreases after the immobilization of the cyt.c on the nanohybrid. For the first time polymers covalently grafted to the surface of MWCNTs are used in a biosensor.     

Modified microelectrodes and multivariate calibration for flow injection amperometric simultaneous determination of ascorbic acid, dopamine, epinephrine and dipyrone

Flow injection amperometric quantification of ascorbic acid (AA), dopamine (DA), epinephrine (EP) and dipyrone (DI) in mixtures (in the microgram g-1 range) was successfully performed by using an array of microelectrodes with units modified by the electrodeposition of different noble metals, together with multivariate calibration analysis. The four groups of microelectrodes utilized included a pure gold electrode and electrodes modified by electrodeposition of platinum, palladium or a mixture of platinum + palladium. The array of microelectrodes was inserted in a flow cell and the amperometric data acquisition was performed with a four-channel potentiostat. The analysis of the resulting signals was carried out by a multivariate calibration method, using a group of 16 standard mixtures selected by a two-level factorial design. The analysis of synthetic samples and pharmaceutical compounds containing AA and DI led to very similar values to those obtained by the classical iodimetric analysis. The average absolute errors (in microgram g-1) calculated for each analyte were 0.3, 0.2, 0.4 and 0.4 for AA, DA, EP and DI, respectively.     

Modified carbon-containing electrodes in stripping voltammetry of metals. Part II. Composite and microelectrodes

The second part of the review, which covers modified carbon-containing electrodes, describes composite and microelectrodes. Electrodes made of commercial and laboratory carbon-containing composite materials are discussed. Impregnated and thick-film electrodes and microelectrodes made of carbon fibers form a separate group. Various modifiers and methods of electrode modification are presented. Prospects for the future development of solid-state modified electrodes are considered.     

Comparison of electrode materials for the detection of rapid hydrogen peroxide fluctuations using background-subtracted fast scan cyclic voltammetry

Hydrogen peroxide (H(2)O(2)) is a critically important signaling molecule. Endogenous H(2)O(2) mediates diverse physiological processes both intra- and intercellularly; and enzymatically generated H(2)O(2) is a widely used reporter molecule at biosensors that rely on enzymes to detect non-electroactive species. However, the development and application of electroanalytical methods for the direct detection of this molecule has been challenging because the electron transfer kinetics for the irreversible oxidation of H(2)O(2) are slow. We comparatively characterize the electrochemical oxidation of H(2)O(2) on bare and Nafion(?)-coated platinum and carbon-fiber microdisc electrodes using fast-scan cyclic voltammetry (FSCV). Using a waveform ranging from +0.2 to +1.3 V at 400 V s(-1), the electrocatalytic properties of the platinum surface were not readily apparent, and the carbon-fiber microelectrode demonstrated greater sensitivity and selectivity toward H(2)O(2). Nafion(?)-coating further enhanced detection on carbon electrodes. These results confirm that platinum electrodes, with or without Nafion(?), will not work acceptably with this approach, and confirm the value of carbon-fiber microelectrodes relative to more traditionally used platinum electrodes in the direct detection of rapid H(2)O(2) fluctuations using FSCV.     

Ring-modified carbon fiber microelectrodes and multi-microelectrode devices

Two new types of modified microelectrodes were used alone and in multi-microelectrode devices. Carbon fibers with diameters of 7.2 μm were modified either by electropolymerization, or by thermal polymerization of a mixture of monomers to give a thick coating of modifier around the cylindrical fiber. The modified fibers were then coated with an insulating layer. The tips of the electrodes were polished perpendicular to the axis of the fiber to give ring-modified disk electrodes. Copolymers of poly(vinylferrocene) and poly(vinylpyridine) with crosslinked polystyrene were used, and demonstrated behavior similar to that for surface modified electrodes, except that the electrodes could be polished to renew the surface. Multi-microelectrode devices were prepared. For example, a ring-modified working electrode, a silver/silver chloride coated practical reference electrode, and a platinum auxiliary electrode were used in a molded epoxy probe in electrolyte solution to give cyclic voltammograms that were similar to those expected for a ring-modified working electrode using traditional reference and auxiliary electrodes.     

Characterization of amperometry for in vivo measurement of dopamine dynamics in the rat brain

Constant potential amperometry with Nafion-coated carbon-fiber electrodes has been evaluated as a technique for in vivo detection of the neurotransmitter dopamine. The results of this technique have been compared to results obtained with fast-scan cyclic voltammetry at the same electrode during release of dopamine into the extracellular space of the brain during electrical stimulation of neurons. The data indicate that constant potential amperometry is a viable technique for detecting low concentrations of dopamine. Dopamine permeates the film more quickly with constant-potential amperometry than with repeated fast-scan cyclic voltammetry as predicted by diffusion equations. For the case of cyclic voltammetry, it is demonstrated that the temporal delay caused by diffusion through Nafion film can be removed by deconvolution procedures. Despite the suitability of constant potential amperometry as an in vivo monitoring technique, it does have several disadvantages when compared to fast-scan cyclic voltammetry. The diffusion layer extends outside of the Nafion film making determination of concentration based on in vitro calibrations more difficult to interpret. The reported concentrations are larger than obtained by cyclic voltammetry, a technique with the diffusion layer restricted to the Nafion film, and this result is likely an underestimation of the effect of the catalytic reaction between the o-quinone of dopamine and ascorbate. Amperometry was found to provide only slightly improved signal-to-noise ratios than cyclic voltammetry despite the use of greater filtering. This was because the advantage of dopamine accumulation in the film was lost. In addition, the small magnitude of the amperometric signal makes it more susceptible to electrical interference.     

Modification of glassy carbon electrode by the electrochemical oxidation of 3‐aminophenylcalix[4]pyrrole in nonaqueous media

3‐Aminophenylcalix[4]pyrrole (3APCP) was grafted to a glassy carbon (GC) surface during the electrochemical oxidation process in 0.1 M tetrabutylammoniumtetra‐fluoroborate (TBATFB) containing acetonitrile solution. The presence of a surface film was confirmed electrochemically by comparing voltammograms of dopamine and ferricyanide redox probes at the bare and modified electrodes. Reflection‐absorption infrared spectroscopy (RAIRS), XPS, atomic force microscopy (AFM), ellipsometry and the contact angle measurements were also employed to characterize 3APCP film on GC electrode. RAIRS analysis revealed that calix[4]pyrrole was bonded to the glassy carbon surface via the etheric linkage. The stability of the modified GC electrode was also studied. Copyright © 2011 John Wiley & Sons, Ltd.     

Facile Protection of Lithium Metal for All-Solid-State Batteries

A nanolayer of reactive propyl acrylate silane groups was deposited on a lithium surface by using a simple dipping method. The polymerization of cross-linkable silane groups with a layer of ally-ether-ramified polyethylene oxide was induced by UV light. SEM analysis revealed a good dispersion of silane groups grafted on the lithium surface and a layer of polymer of about 4 μm was obtained after casting and reticulation. The electrochemical performance for the unmodified and modified lithium electrodes were compared in symmetrical Li/LLZO/Li cells. Stable plating/stripping and low interfacial resistance were obtained when the modified lithium was utilized, indicating that the combination of silane and polymer deposition is promising to increase Li-metal/garnet contact.     

双柱碳纤维微电极的研究及其在测定多巴胺中的应用

报道了双柱微电极的制作方法,提出了用双柱碳纤维微电极在抗坏血酸存在下选择性地测定多巴胺.探讨了电极反应机理.多巴胺的浓度在5.0×10^-4～5.0×10^-6mol/L范围内与收集电流成正比.抗坏血酸浓度<5.0×10^-4mol/L时对测定结果无影响.     

Monolithic integration of three-material microelectrodes for electrochemical detection on PMMA substrates

Monolithic integration of three-material microelectrodes for electrochemical detection on poly (methyl methacrylate) (PMMA) substrates is presented. Au–Ag–Pt three-material electrodes were all fabricated based on polymer compatible photolithography processes, and the fabrication sequence of the electrodes was optimized. The C–Ag–Pt three-electrode system was also demonstrated. To reduce the electrical resistance, the carbon electrode was made on a silver intermediate layer which was simultaneously fabricated with Ag electrodes. A PMMA/poly(dimethylsiloxane) electrochemical sensing microchip with the Au–Ag–Pt three-electrode systems was constructed. The reproducibility of the three-electrode system from single and different microchips was characterized. The performance of the microchip was evaluated by two kinds of electrochemical probes (Ru(bpy)₃Cl₂ and dopamine).     

Electrochemical Properties of a Boron‐Doped Diamond Electrode Modified with Gold/Polyelectrolyte Hollow Spheres

Oppositely charged polyelectrolyte (poly(allyamine hydrochloride) (PAH) and poly(sodium 4‐styrene‐sulfonate) (PSS)), and negatively charged gold nanoparticles (Au) were assembled alternately on polystyrene (PS) spheres via layer‐by‐layer technique, and the different PAH/(PSS/PAH)_n/(Au/PAH)_m/Au composite hollow spheres were derived by dissolving PS core. These hollow spheres were used to modify boron‐doped diamond (BDD) electrodes for electrochemical sensors. The cyclic voltammetric results for dopamine (DA) detection demonstrated that hollow‐sphere‐modified BDD exhibited better electrocatalytic activity than did bare BDD. Influence of the wall thickness and composition of hollow spheres on electrochemical properties were investigated. The results showed that the oxidative peak potential of DA and the peak current varied with different PSS/PAH and Au/PAH layers. The optimized wall structure of hollows spheres was PAH/(PSS/PAH)₇/(Au/PAH)₅/Au.     

Investigation of the electrochemical and electrocatalytic behavior of positively charged gold nanoparticle and L-cysteine film on an Au electrode

Positively charged gold nanoparticle (positively charged nano-Au), which was prepared, characterized by ξ-potential and transmission electron microscopy (TEM) was used in combination with l-cysteine to fabricate a modified electrode for electrocatalytic reaction of biomolecules. Compared with electrodes modified by negatively charged gold nanoparticle/l-cysteine, or l-cysteine alone, the electrode modified by the positively charged gold nanoparticle/l-cysteine exhibited excellent electrochemical behavior toward the oxidation of biomolecules such as ascorbic acid, dopamine and hydrogen peroxide. Moreover, the proposed mechanism for electrocatalytic response of positively charged gold nanoparticle was discussed. The immunosensor showed a specific to ascorbic acid in the range 5.1 × 10⁻⁷-6.7 × 10⁻⁴ M and a low detection limit of 1.5 × 10⁻⁷ M. The experimental results demonstrate that positively charged gold nanoparticle have more efficient electrocatalytic reaction than negatively charged gold nanoparticle, which opens up new approach for fabricating sensor.