Electrochemical pretreatment of carbon fibers for in vivo electrochemistry: effects on sensitivity and response time

Interpreting Dynamic Interfacial Changes at Carbon Fiber Microelectrodes Using Electrochemical Impedance Spectroscopy

Carbon-fiber microelectrodes are instrumental tools in neuroscience used for the electroanalysis of neurochemical dynamics and recordings of neural activity. However, performance is variable and dependent on fabrication strategies, the biological response to implantation, and the physical and chemical composition of the recording environment. This presents an analytical challenge, as electrode performance is difficult to quantitatively assess in situ, especially when electrodes are permanently implanted or cemented in place. We previously reported that electrode impedance directly impacts electrochemical performance for molecular sensing. In this work, we investigate the impacts of individual components of the electrochemical system on impedance. Equivalent circuit models for glass- and silica-insulated carbon-fiber microelectrodes were determined using electrochemical impedance spectroscopy (EIS). The models were validated based on the ability to assign individual circuit elements to physical properties of the electrochemical system. Investigations were performed to evaluate the utility of the models in providing feedback on how changes in ionic strength and carbon fiber material alter impedance properties. Finally, EIS measurements were used to investigate the electrode/solution interface prior to, during, and following implantation in live brain tissue. A significant increase in impedance and decrease in capacitance occur during tissue exposure and persist following implantation. Electrochemical conditioning, which occurs continually during fast-scan cyclic voltammetry recordings, etches and renews the carbon surface, mitigating these effects. Overall, the results establish EIS as a powerful method for characterization of carbon-fiber microelectrodes, providing unprecedented insight into how real-world factors affect the electrode/solution interface.

Spectroelectrochemical Characterization of the Dynamic Carbon-Fiber Surface in Response to Electrochemical Conditioning

The effects of electrochemical preconditioning of P-55 pitch-based carbon-fiber microelectrodes were quantitatively examined in this study. Microstructural characterization of the electrode surface was done using Raman spectroscopy and scanning electron microscopy. Electrochemical performance was evaluated using cyclic voltammetry. The data show that application of positive potentials provides beneficial structural modifications to the electrode surface. Electrodes that were preconditioned using a static potential of +1.0 V exhibited enhanced sensitivity and electron transfer properties when compared to electrodes conditioned for the same amount of time with dynamic (triangular) waveforms reaching +1.0 V. Conditioning elicited microstructural changes to the electrode surface that were dependent on the amount of time spent at potentials greater than ∼1.0 V. Importantly, the data demonstrate that the carbon-fiber microstructure is dynamic. It is able to quickly and continuously undergo rapid structural reorganization as potential is applied, repeatedly alternating between a relatively ordered state and one that exhibits greater disorder in response to applied electrochemical potentials that span the range commonly used in voltammetric experiments.

Socially induced serotonergic fluctuations in the male auditory midbrain correlate with female behavior during courtship

Cues from social partners trigger the activation of socially responsive neuromodulatory systems, priming brain regions including sensory systems to process these cues appropriately. The fidelity with which neuromodulators reflect the qualities of ongoing social interactions in sensory regions is unclear. We addressed this issue by using voltammetry to monitor serotonergic fluctuations in an auditory midbrain nucleus, the inferior colliculus (IC), of male mice (Mus musculus) paired with females, and by concurrently measuring behaviors of both social partners. Serotonergic activity strongly increased in male mice as they courted females, relative to serotonergic activity in the same males during trials with no social partners. Across individual males, average changes in serotonergic activity were negatively correlated with behaviors exhibited by female partners, including broadband squeaks, which relate to rejection of males. In contrast, serotonergic activity did not correlate with male behaviors, including ultrasonic vocalizations. These findings suggest that during courtship, the level of serotonergic activity in the IC of males reflects the valence of the social interaction from the perspective of the male (i.e., whether the female rejects the male or not). As a result, our findings are consistent with the hypothesis that neuromodulatory effects on neural responses in the IC may reflect the reception, rather than the production, of vocal signals.

Domain-dependent effects of DAT inhibition in the rat dorsal striatum

The rat dorsal striatum exhibits domain-dependent kinetics of dopamine release and clearance. The present report describes the domain-dependent actions of nomifensine (20 mg/kg i.p.), a competitive dopamine uptake inhibitor, on evoked dopamine responses recorded by voltammetry during electrical stimulation of the medial forebrain bundle. In slow domains, nomifensine increases the initial rate of evoked overflow, increases response overshoot, does not affect the slope of the linear segment of the dopamine clearance profile, and slows the non-linear segment of the clearance profile. In fast domains, nomifensine does not affect the initial rate of overflow, increases the end-of-stimulus overshoot, and decreases the slope of the linear segment of the dopamine clearance profile. Collectively, these findings do not concur with existing models of evoked dopamine release that describe the effect of nomifensine as an increase in the effective KM of dopamine uptake. These findings suggest that dopamine clearance after evoked release is affected by both dopamine uptake and a restricted extracellular diffusion process.

Electrochemically pretreated carbon microfiber electrodes as sensitive HPLC-EC detectors

The paper focuses on the analysis and detection of electroactive compounds using high-performance liquid chromatography (HPLC) combined with electrochemical detection (EC). The fabrication and utilization of electrochemically treated carbon fiber microelectrodes (CFMs) as highly sensitive amperometric detectors in HPLC are described. The applied pretreatment procedure is beneficial for analytical characteristics of the sensor as demonstrated by analysis of the model set of phenolic acids. The combination of CFM with separation power of HPLC technique allows for improved detection limits due to unique electrochemical properties of carbon fibers. The CFM proved to be a promising tool for amperometric detection in liquid chromatography.

Microdialysis probes alter presynaptic regulation of dopamine terminals in rat striatum

The insertion of microdialysis probes into the rat striatum disrupts dopaminergic activity near the probe track. The present study suggests that a substantial fraction of DA terminals near the probe track (200 μm) survive the probe implantation itself but that the surviving terminals experience altered presynaptic inhibition. We found that probe implantation did not just alter the amplitude of evoked dopamine responses recorded by voltammetry, but also changed their temporal profile in a fashion similar to that previously observed by quinpirole, an agonist of dopamine D2 autoreceptors. Altered presynaptic inhibition is supported by a hypersensitivity of evoked dopamine responses recorded near to microdialysis probes to raclopride, a D2 antagonist. Further, we found that evoked dopamine release was also hypersensitive to a final dose of the dopamine transporter inhibitor, nomifensine.

Direct measurements of xanthine in 2000-fold diluted xanthinuric urine with a nanoporous carbon fiber sensor

High selectivity and sensitivity is reported in the measurements of xanthine in urine by fast scan cyclic voltammetry (FSV) with a nanostructured carbon fiber sensor of 3.5 +/- 0.4 mum radius. Fabrication of the sensors for the measurements is described. Fabrication of the nanostructure at the carbon fiber sensor surface exposes surface pores. SEM images confirm the formation of the nanostructure. The results indicate that the nanostructure improves the sensitivity and limit of detection (LOD) in the measurements of xanthine and uric acid. The sensors allow rapid direct measurements of xanthine in 2000-fold diluted xanthinuric urine and of uric acid in 2000-fold diluted normal urine. The sensitivity and the LOD of xanthine is 0.40 +/- 0.02 nA microM(-1) (0.995) and 1 microM, respectively, and 0.99 +/- 0.01 nA microM(-1) (0.998) and 500 nM for uric acid. The concentration of xanthine in 2000-fold diluted xanthinuric urine is 1.6 +/- 0.2 muM from FSV and from HPLC. The concentration of xanthine and uric acid in urine can be determined by pre- or post-calibration of the sensor in buffer or by the method of standard addition.

One-step formation of ultra-thin chemically functionalized redox-active Langmuir-Schaefer Nafion films

A novel "one-step procedure" to incorporate different cationic redox-active species within ultra-thin Nafion films is reported. Ultra-thin films of Nafion containing trimethylammonioferrocene (FA), tris(2,2'-bipyridyl)ruthenium(), Ru(bpy), and hexaaminoruthenium(), [Ru(NH)], were prepared using the Langmuir-Schaefer (LS) technique. The morphology and thickness of the films were evaluated with atomic force microscopy (AFM). Film thicknesses were 1.9 ± 0.7, 1.8 ± 0.9, 1.5 ± 0.9 nm per layer for Nafion-FA, Nafion-Ru(bpy)and Nafion-[Ru(NH)], respectively. Electrochemical data confirmed the successful incorporation of the mediators into the films and proved that the redox activity was maintained during extensive voltammetric cycling. Key parameters such as surface coverage, concentration of the mediator within the films and apparent diffusion coefficients were extracted using cyclic voltammetry. To demonstrate the utility of the redox-active films for sensing applications, films with Ru(bpy) incorporated were utilized for the electrocatalysis of oxalate in aqueous solution.

Nitric oxide sensor based on carbon fiber covered with nickel porphyrin layer deposited using optimized electropolymerization procedure

Electropolymerization regime of meso-tetrakis(3-methoxy-4-hydroxyphenyl) porphyrin is optimized to yield films possessing both electrocatalytical and permselective properties towards nitric oxide oxidation. The sensor composed of electrochemically oxidized carbon fiber, covered solely with nickel porphyrin derivative layer electropolymerized using our method, is characterized by high selectivity towards nitrite (1:600), ascorbate (1:8000) and dopamine (>1:80), determined by constant potential amperometry at 830 mV (vs. Ag/AgCl). Selectivity for ascorbate and dopamine as well as detection limit for NO (1.5 nM at S/N=3) is 5-10 times better than parameters usually reported for Nafion coated porphyrinic sensors. Nafion coating can further enhance selectivity properties as well as aids to the stability of the sensors' responses.

Carbon nanotube-modified carbon fiber microelectrodes for in vivo voltammetric measurement of ascorbic acid in rat brain

This study demonstrates a new electrochemical method for in vivo measurements of ascorbic acid (AA) in rat brain with multiwalled carbon nanotube (MWNT)-modified carbon fiber microelectrodes (CFMEs) based on the electrochemical property of MWNTs for facilitating AA oxidation. Cyclic voltammetry results indicate that the prepared MWNT-modified CFMEs possess a marked electrocatalytic activity toward AA oxidation and can be used for its selective measurement in the presence of other kinds of electroactive species coexisting in rat brain, such as 3,4-dihydroxyphenylacetic acid, uric acid, and 5-hydroxytryptamine. The selectivity of the MWNT-modified CFMEs toward AA measurement is further studied in vivo by exogenously infusing ascorbate oxidase into the brain, and the results confirm that the prepared electrodes are selective and can thus be used for reliable in vivo measurements of AA in rat brain, combined with their good stability during in vivo measurements. The basal level of striatum AA is determined to be 0.20 +/- 0.05 mM (n = 3). The application of the voltammetric method with the MWNT-modified CFMEs is preliminarily demonstrated for in vivo observation of homeostatic regulation of striatum AA with exogenous infusion of AA into the brain.

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.

Sequential injection system for the enzymatic determination of ethanol in wine

A sequential injection system was developed for the enzymatic determination of ethanol in wine. The spectrophotometric determination is based on the enzymatic reaction catalyzed by alcohol dehydrogenase in the presence of NAD+. The system was applied to the determination of ethanol in a range of 0.008-0.024% (v/v) with good repeatability; RSD(n=10) < 2.3%. The results obtained with the developed system showed good agreement with those obtained by using the reference method. The determination rate was 25 h(-1); 1 micromol of NAD+, 1.1 units of enzyme, and 50 microL of sample were consumed per determination; and the waste produced was 2.2 mL per assay.

Electrochemical behavior of a covalently modified glassy carbon electrode with aspartic acid and its use for voltammetric differentiation of dopamine and ascorbic acid

Aspartic acid was covalently grafted on to a glassy carbon electrode (GCE) by amine cation radical formation in the electrooxidation of the amino-containing compound. X-ray photoelectron spectroscopic (XPS) measurement and cyclic voltammetric experiments proved the aspartic acid was immobilized as a monolayer on the GCE. Electron transfer to Fe(CN)6(4-) in solution of different pH was studied by cyclic voltammetry. Changes in solution pH resulted in the variation of the charge state of the terminal group; surface pK(a) values were estimated on the basis of these results. Because of electrostatic interactions between the negatively charged groups on the electrode surface and dopamine (DA) and ascorbic acid (AA), the modified electrode was used for electrochemical differentiation between DA and AA. The peak current for DA at the modified electrode was greatly enhanced and that for AA was significantly reduced, which enabled determination of DA in the presence of AA. The differential pulse voltammetric (DPV) peak current was linearly dependent on DA concentration over the range 1.8 x 10(-6)-4.6 x 10(-4) mol L(-1) with slope (nA micromol(-1) L) and intercept (nA) of 47.6 and 49.2, respectively. The detection limit (3delta) was 1.2 x 10(-6) mol L(-1). The high selectivity and sensitivity for dopamine was attributed to charge discrimination and analyte accumulation. The modified electrode has been used for determination of DA in samples, in the presence of AA, with satisfactory results.

Detecting subsecond dopamine release with fast-scan cyclic voltammetry in vivo

BACKGROUND

Dopamine is a potent neuromodulator in the brain, influencing a variety of motivated behaviors and involved in several neurologic diseases. Measurements of extracellular dopamine in the brains of experimental animals have traditionally focused on a tonic timescale (minutes to hours). However, dopamine concentrations are now known to fluctuate on a phasic timescale (subseconds to seconds).

APPROACH

Fast-scan cyclic voltammetry provides analytical chemical measurements of phasic dopamine signals in the rat brain.

CONTENT

Procedural aspects of the technique are discussed, with regard to appropriate use and in comparison with other methods. Finally, examples of data collected using fast-scan cyclic voltammetry are summarized, including naturally occurring dopamine transients and signals arising from electrical stimulation of dopamine neurons.

SUMMARY

Fast-scan cyclic voltammetry offers real-time measurements of changes in extracellular dopamine concentrations in vivo. With its subsecond time resolution, micrometer-dimension spatial resolution, and chemical selectivity, it is the most suitable technique currently available to measure transient concentration changes of dopamine.

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.

Electroanalytical applications of cationic self-assembled monolayers: square-wave voltammetric determination of dopamine and ascorbate

Gold electrodes modified with cationic self-assembled monolayers (SAMs) of 2,2'-dithiobisethaneamine (CYST) and 6,6'-dithiobishexaneamine (DTH) were used for the simultaneous determination of dopamine (DA) and ascorbate (AA). The cationic SAM modified electrodes have several advantages over the bare electrode for the oxidation of AA. A very large (approximately 450 mV) decrease in the overpotential for the oxidation of AA when compared with the bare electrode has been observed at the cationic monolayer-modified electrode. The electrostatic interaction of negatively charged AA with the monolayer shift the oxidation peak potential of AA to less positive potential and enhances the peak current. On the other hand, the positively charged DA is repelled from the monolayer and the oxidation potential shifts to more positive potential when compared to the bare electrode. The electrochemical oxidation of AA at the mixed monolayer of CYST and diethyl disulfide (DEDS) supports the influence of cationic terminal group of the monolayer on the oxidation of AA. Since the oxidation of AA occurs well before the oxidation potential of DA is reached, the homogeneous catalytic oxidation of AA by the oxidized DA has been advantageously eliminated at the monolayer-modified electrode. The cationic self-assembled monolayers successfully detect DA in the presence of high concentration of AA. The sensitivity of the electrode modified with CYST monolayer was found to be 0.036 and 0.021 microA/microM towards AA and DA, respectively.

Changes in the kinetics of dopamine release and uptake have differential effects on the spatial distribution of extracellular dopamine concentration in rat striatum

The objective of this study was to examine whether the limited diffusion distance of dopamine in rat striatum produces spatial heterogeneity in the extracellular dopamine concentration on a dimensional scale of a few micrometers. Such heterogeneity would be significant because it would imply that the concentration of dopamine at a given receptor depends on the receptor's ultrastructural location. Spatially resolved measurements of extracellular dopamine were performed in the striatum of chloral hydrate-anesthetized rats with carbon fiber microdisk electrodes. Dopamine was monitored during electrical stimulation of the nigrostriatal pathway before and after administration of drugs that selectively affect the kinetics of evoked dopamine release and dopamine uptake. The effects of nomifensine (20 mg/kg), L-DOPA (250 mg/kg), and alpha-methyl-p-tyrosine (250 mg/kg) on the amplitude of the stimulation responses were examined. The outcome of these experiments was compared with predictions derived from a mathematical model that combines diffusion with the kinetics of release and uptake. The results demonstrate that the extracellular dopamine concentration is spatially heterogeneous on a micrometer scale and that changing the kinetics of dopamine release and uptake has different effects on this spatial distribution. The impact of these results on brain neurochemistry is considered.

Preservation of NADH voltammetry for enzyme-modified electrodes based on dehydrogenase

Minimizing overpotential and generating high faradaic currents are critical issues for fast-scan voltammetry of beta-nicotinamide adenine dinucleotide (NADH) for the sensitivity of enzyme-modified electrodes based on dehydrogenases. Although NADH voltammetry exhibits high overpotential and poor voltammetric peak shape at solid electrode surfaces, modification of the electrode surface can improve the electrochemical response at carbon fibers. However, these improvements are severely degraded upon the covalent attachment of enzyme. The creation of improved electron-transfer properties and the retention of these properties throughout the enzyme attachment process is the focus of this study. A novel polishing and electrochemical pretreatment method was developed which generated a decreased overpotential and a high faradaic current at carbon-fiber electrodes for NADH. Factors that lead to a degradation of voltammetric response during the enzyme fabrication were investigated, and both the aging and the covalent modification of the pretreated surface contributed to this degradation. Attachment processes that minimized the preparation time, in turn, maximized the retention of the facile electron-transfer properties. These attachment processes included varying the surface attachment reactions for the enzyme. Preparation time reduction techniques included modeling existing techniques and then improving kinetic and mass transport issues where possible. Alternate covalent attachment methods included a direct electrochemical amine reaction and an electrochemically reductive hydrazide reaction. The surface attachment and retention of electron-transfer properties of these probes were confirmed by fluorescence and electrochemical studies.

In vivo nitric oxide sensor using non-conducting polymer-modified carbon fiber

Nitric oxide (NO) is emerging as a very important and ubiquitous gaseous messenger in the body. The response characteristics of NO sensors made of non-conducting polymer modified carbon fiber electrodes are investigated to determine their selectivity, sensitivity, and stability for in vivo use. A composite polymer, comprising Nafion, m-phenylenediamine, and resorcinol, showed the best selectivity and stability to amperometric NO detection. The non-conducting, self-limiting polymer film protects the electrode from interference and fouling by other biochemicals. Although the relative sensitivity to NO of the modified sensor is lower than that of the unmodified carbon fiber electrodes (less than 6%), the composite polymer electrode showed high selectivity against ascorbic acid (> 2000:1), nitrite (> 600:1), and dopamine (> 200:1). The stability of the NO sensor was maintained for at least 1 week. The NO sensitivity after in vivo experiments (n = 8) is 88.1 +/- 5.6% of initial sensitivity data obtained before in vivo experiments. Preliminary in vivo experiments done with this electrode are shown to capture elevated NO levels in brain following an ischemic injury.

Evaluation of stearate-graphite paste electrodes for chronic measurement of extracellular dopamine concentrations in the mammalian brain

Chronoamperometric procedures, in combination with pharmacological treatments, were used to verify whether stearate-modified graphite paste recording electrodes (SGEs) could measure basal extracellular dopamine (DA) concentrations in the striatum of awake rats over a 3-week period of implantation. Baseline chronoamperometric signals were unaffected by systemic injections of ascorbate (AA) or the monoamine oxidase inhibitor pargyline, or by intraventricular infusions of the AA degrading enzyme AA-oxidase. In contrast, systemic injections of d-amphetamine or nomifensine increased, and gamma-butyrolactone decreased, the signal in a reproducible fashion over a similar test period. In addition, 6-hydroxydopamine lesions of the nigrostriatal DA pathway attenuated the ability of d-amphetamine to increase, and gamma-butyrolactone to decrease, the chronoamperometric signal. In separate studies, reverse microdialysis, performed with dialysis probes implanted directly adjacent to SGEs in the striatum, permitted the assessment of electrode selectivity, sensitivity, response linearity, and detection limits to DA. Perfusion of the probe with normal Ringer solution (5 microliters/min) decreased the baseline chronoamperometric signal by 10 nA. Comparable decreases in the baseline signal were observed after systemic injections of gamma-butyrolactone or medial forebrain bundle infusions of tetrodotoxin, suggesting these decreases reflected depletion of extracellular DA to levels below the electrode's detection limit. Reverse dialysis with high concentrations of AA, DOPAC, 5-HT, or 5-HIAA, failed to reverse the decrease in the chronoamperometric signal induced by dialysis. In contrast, reverse dialysis with a physiologically relevant range of DA concentrations, in rats pretreated with the DA uptake blocker nomifensine, increased the chronoamperometric signal in a linear fashion with a detection threshold of < 20 nM. Combined, these results indicate that the baseline chronoamperometric signals recorded at +0.20 V in the striatum with SGEs do not reflect changes in extracellular concentrations of AA, DA metabolites, or indoles, but rather represent neuronally mediated nanomolar changes in extracellular DA concentrations, even after extended periods of implantation in brain tissue.

Improved electrochemical properties of stearate-graphite paste electrodes after albumin and phospholipid treatments

Stearate-graphite paste electrodes (SGEs) exhibit enhanced dopamine sensitivity and insensivity to asorbic acid electrocatalytic effects in vitro following exposure to unidentified contituents of rat brain tissue homogenates. The present study utilized voltammetry and chronamperometry to compare the electrochemical characteristics of brain-treated SGEs to those treated with potential brain constituent candidates (albumin proteins and phospholipids). Albumin treatments markedly attenuated interference from ascorbate catalytic effects whereas lipids enhanced both electrode capacitance and sensitivity to dopamine. Combined treatments resulted in electrochemical properties that were similar to brain-treated SGEs. Potential mechanisms by which albumin may attenuate ascorbate electrocatalysis of dopamine were investigated using high performance liquid chromatography, with electrochemical detection. The reduction in ascorbate electrocatalytic effects at albumin-treated SGEs may be due to nucleophilic binding of dopamine oxidation products to albumin attached to the electrode surface. Therefore, the unambiguous detection of dopamine by SGEs in vivo may be related to interactions with factors in brain having similar surface-modifying properties.