Probing Cellular Chemistry in Biological Systems with Microelectrodes

In vivo solid-phase microextraction for tissue bioanalysis

Conventional in vitro or ex vivo bioanalytical quantitative sample preparation methods for the determination of compounds in biological tissues are often coupled with challenges in obtaining an assay representative of the system of interest. The rising interest in in vivo microsampling bioanalytical methods is due to the unique advantages they offer over their in vitro counterparts. In vivo solid-phase microextraction (SPME), a diffusion-based microsampling tool, has been successfully applied in recent studies to various biological systems. This review presents recent trends in tissue bioanalysis using in vivo SPME as a sample preparation tool. Efforts were made to discuss the various bioapplications of the method while highlighting possible strategies for improved sensitivity where needed. In vivo SPME devices currently employed for the various applications have also been described. In addition, we highlight selectivity of a new class of biocompatible coatings that can potentially improve the coverage of metabolites for untargeted metabolomics.

Multichannel bipotentiostat integrated with a microfluidic platform for electrochemical real-time monitoring of cell cultures

An electrochemical detection system specifically designed for multi-parameter real-time monitoring of stem cell culturing/differentiation in a microfluidic system is presented. It is composed of a very compact 24-channel electronic board, compatible with arrays of microelectrodes and coupled to a microfluidic cell culture system. A versatile data acquisition software enables performing amperometry, cyclic voltammetry and impedance spectroscopy in each of the 12 independent chambers over a 100 kHz bandwidth with current resolution down to 5 pA for 100 ms measuring time. The design of the platform, its realization and experimental characterization are reported, with emphasis on the analysis of impact of input capacitance (i.e., microelectrode size) and microfluidic pump operation on current noise. Programmable sequences of successive injections of analytes (ferricyanide and dopamine) and rinsing buffer solution as well as the impedimetric continuous tracking for seven days of the proliferation of a colony of PC12 cells are successfully demonstrated.

Investigations into Potential Extrasynaptic Communication between the Dopaminergic and Nitrergic Systems

Nitric oxide is unconstrained by cell membranes and can therefore act along a broad distance as a volume transmitter. Spillover of nitric oxide between neurons may have a major impact on central nervous system diseases and particularly on neurodegeneration. There is evidence whereby communication between nitrergic and dopaminergic systems plays an essential role in the control of the nigrostriatal pathway. However, there is sparse information for either the coexistence or overlap of nitric oxide and dopaminergic structures. The dual localization of immunoreactivity for nitric oxide synthase (NOS) and tyrosine hydroxylase, enzymes responsible for the synthesis of nitric oxide and dopamine, respectively, was examined in neurons of the nigrostriatal pathway in the rat brain by means of a double-immunohistochemical method and confocal laser scanning microscopy, acquired at the resolution limit. After perfusional fixation, the brains were cut and double-immunostained. A proximity analysis of tyrosine hydroxylase and NOS structures was done using binary masks generated from the respective maximum projections, using confocal laser microscopy. Unrevealed regions were determined somatodendritic positive for both NOS and tyrosine hydroxylase, within an image limit resolution at 2 μm-wide margin. The described interconnected localization of nNOS(+) and TH(+) containing neuronal fibers and cells bodies in the nigrostriatal pathway propose a close anatomical link between the two neurotransmitters.

Characterization of a 32 μm diameter carbon fiber electrode for in vivo fast-scan cyclic voltammetry

Carbon fiber electrodes (CFE) are commonly used for in vivo detection of catecholamines due to their excellent electrochemical properties and biocompatibility. Fast-scan cyclic voltammetry (FSCV) combined with CFEs permits the detection of catecholamines such as dopamine (DA) with high specificity and reliability. However, advances in neuroscience constantly demand sensors with greater sensitivities and selectivities. This study investigated an untreated CFE of 32 μm diameter and 300 μm exposed length for detection of DA using FSCV. Despite the larger area of the working electrode, we observed only a small increase in the background current in comparison with the commonly used CFE of 7 μm diameter and 100 μm exposed length. The sensitivity of the 32 μm CFE was 9 times greater than that of the 7 μm CFE. These larger electrodes exhibited good linearity and a 6 fold higher signal-to-noise ratio than 7 μm CFEs in vitro. The 32 μm CFE showed significantly better selectivity for DA in preference to 3,4-dihydroxyphenylacetic acid than the 7 μm CFE and similar selectivity to the 7 μm CFE for ascorbic and homovanillic acid. The electrodes displayed good temporal resolution and electrochemical stability in both in vitro and in vivo tests.

Rapid, sensitive detection of neurotransmitters at microelectrodes modified with self-assembled SWCNT forests

Carbon nanotube (CNT) modification of microelectrodes can result in increased sensitivity without compromising time response. However, dip coating CNTs is not very reproducible and the CNTs tend to lay flat on the electrode surface which limits access to the electroactive sites on the ends. In this study, aligned CNT forests were formed using a chemical self-assembly method, which resulted in more exposed CNT ends to the analyte. Shortened, carboxylic acid functionalized single-walled CNTs were assembled from a dimethylformamide (DMF) suspension onto a carbon-fiber disk microelectrode modified with a thin iron hydroxide-decorated Nafion film. The modified electrodes were highly sensitive, with 36-fold higher oxidation currents for dopamine using fast-scan cyclic voltammetry than bare electrodes and 34-fold more current than electrodes dipped in CNTs. The limit of detection (LOD) for dopamine was 17 ± 3 nM at a 10 Hz repetition rate and 65 ± 7 nM at 90 Hz. The LOD at 90 Hz was the same as a bare electrode at 10 Hz, allowing a 9-fold increase in temporal resolution without a decrease in sensitivity. Similar increases were observed for other cationic catecholamine neurotransmitters, and the increases in current were greater than for anionic interferents such as ascorbic acid and 3,4-dihydroxyphenylacetic acid (DOPAC). The CNT forest electrodes had high sensitivity at 90 Hz repetition rate when stimulated dopamine release was measured in Drosophila . The sensitivity, temporal resolution, and spatial resolution of these CNT forest modified disk electrodes facilitate enhanced electrochemical measurements of neurotransmitter release in vivo.

Sodium dodecyl sulfate-modified electrochemical paper-based analytical device for determination of dopamine levels in biological samples

We report the development of an electrochemical paper-based analytical device (ePAD) for the selective determination of dopamine (DA) in model serum sample. The ePAD device consists of three layers. In the top layer, SU-8 photoresist defines a hydrophilic sample application spot on the filter paper. The middle layer was made from transparency film and contained two holes, one for sample preconcentration and the other for the surfactant to allow transfer to the third layer. A screen-printed carbon electrode formed the bottom layer and was used for electrochemical measurements. In the absence of the anionic surfactant, sodium dodecyl sulfate (SDS), the oxidation peaks of DA, ascorbic acid (AA) and uric acid (UA) overlapped. With the addition of SDS, the DA oxidation peak shifted to more negative values and was clearly distinguishable from AA and UA. The oxidation potential shift was presumably due to preferential electrostatic interactions between the cationic DA and the anionic SDS. Indeed, whilst the SDS-modified paper improved the DA current five-fold, the non-ionic Tween-20 and cationic tetradecyltrimethylammonium bromide surfactants had no effect or reduced the current, respectively. Furthermore, only the SDS-modified paper showed the selective shift in oxidation potential for DA. DA determination was carried out using square-wave voltammetry between -0.2 and 0.8 V vs. Ag/AgCl, and this ePAD was able to detect DA over a linear range of 1-100 μM with a detection limit (S/N=3) of 0.37 μM. The ePAD seems suitable as a low cost, easy-to-use, portable device for the selective quantitation of DA in human serum samples.

Real-time stochastic detection of multiple neurotransmitters with a protein nanopore

The detection of several different neurotransmitters with the same sensor in real-time would be a powerful asset to the field of neurochemistry. We have developed a detector for a broad range of neurotransmitters including amino acids, catecholamines, and nucleotides, which relies on the reversible binding of the analytes to a copper(II) complex within an engineered protein nanopore.

Nanoelectrodes: recent advances and new directions

This article reviews recent work involving the development and application of nanoelectrodes in electrochemistry and related areas. We first discuss common analytical methods for characterizing the size, shape, and quality of nanoelectrodes, including electron microscopy, steady-state cyclic voltammetry, scanning electrochemical microscopy, and surface modification. We then emphasize recent developments in fabrication techniques that have led to structurally well-defined nanoelectrodes. We highlight recent advances in the application of nanoelectrodes in important analytical chemistry areas, such as single-molecule studies, single-nanoparticle electrochemistry, and measurements of neurotransmitters from single neuronal cells.

Catecholamines in the bed nucleus of the stria terminalis reciprocally respond to reward and aversion

BACKGROUND

Traditionally, norepinephrine has been associated with stress responses, whereas dopamine has been associated with reward. Both of these catecholamines are found within the bed nucleus of the stria terminalis (BNST), a brain relay nucleus in the extended amygdala between cortical/limbic centers, and the hypothalamic-pituitary-adrenal axis. Despite this colocalization, little is known about subsecond catecholamine signaling in subregions of the BNST in response to salient stimuli.

METHODS

Changes in extracellular catecholamine concentration in subregions of the BNST in response to salient stimuli were measured within the rat BNST with fast-scan cyclic voltammetry at carbon-fiber microelectrodes.

RESULTS

A discrete subregional distribution of release events was observed for different catecholamines in this nucleus. In addition, rewarding and aversive tastants evoked inverse patterns of norepinephrine and dopamine release in the BNST. An aversive stimulus, quinine, activated noradrenergic signaling but inhibited dopaminergic signaling, whereas a palatable stimulus, sucrose, inhibited norepinephrine while causing dopamine release.

CONCLUSIONS

This reciprocal relationship, coupled with their different time courses, can provide integration of opposing hedonic states to influence response outputs appropriate for survival.

Simultaneous impedimetric and amperometric interrogation of renal cells exposed to a calculus-forming salt

The complexity of the cellular response, induced even by the simplest experimental stimulus, requires an increased number of cellular parameters to be simultaneously monitored. An all electrochemical system allowing the simultaneous and real-time monitoring of both cell adherence and superoxide release into the extracellular space was developed to address this challenge. Cell adherence (to neighboring cells and to substrate) was monitored using non-faradaic impedance spectroscopy while the superoxide release was monitored using a cytochrome c-based amperometric biosensor. The system was used to observe for the first time how these two cellular parameters are changing in real-time for renal cells exposed to calcium oxalate, a calculus-forming salt. It was discovered that calcium oxalate crystals decrease cell adherence and in the same time induce oxidative stress by an overproduction of superoxide. Subconfluent cells, without fully developed tight junctions, appear to be more vulnerable than confluent cells with tight junctions indicating the important protective role of these junctions.

Dopamine release in the basal ganglia

Dopamine (DA) is a key transmitter in the basal ganglia, yet DA transmission does not conform to several aspects of the classic synaptic doctrine. Axonal DA release occurs through vesicular exocytosis and is action potential- and Ca²⁺-dependent. However, in addition to axonal release, DA neurons in midbrain exhibit somatodendritic release by an incompletely understood, but apparently exocytotic, mechanism. Even in striatum, axonal release sites are controversial, with evidence for DA varicosities that lack postsynaptic specialization, and largely extrasynaptic DA receptors and transporters. Moreover, DA release is often assumed to reflect a global response to a population of activities in midbrain DA neurons, whether tonic or phasic, with precise timing and specificity of action governed by other basal ganglia circuits. This view has been reinforced by anatomical evidence showing dense axonal DA arbors throughout striatum, and a lattice network formed by DA axons and glutamatergic input from cortex and thalamus. Nonetheless, localized DA transients are seen in vivo using voltammetric methods with high spatial and temporal resolution. Mechanistic studies using similar methods in vitro have revealed local regulation of DA release by other transmitters and modulators, as well as by proteins known to be disrupted in Parkinson's disease and other movement disorders. Notably, the actions of most other striatal transmitters on DA release also do not conform to the synaptic doctrine, with the absence of direct synaptic contacts for glutamate, GABA, and acetylcholine (ACh) on striatal DA axons. Overall, the findings reviewed here indicate that DA signaling in the basal ganglia is sculpted by cooperation between the timing and pattern of DA input and those of local regulatory factors.

Design, synthesis, and characterization of novel nanowire structures for photovoltaics and intracellular probes

Semiconductor nanowires (NWs) represent a unique system for exploring phenomena at the nanoscale and are expected to play a critical role in future electronic, optoelectronic, and miniaturized biomedical devices. Modulation of the composition and geometry of nanostructures during growth could encode information or function, and realize novel applications beyond the conventional lithographical limits. This review focuses on the fundamental science aspects of the bottom-up paradigm, which are synthesis and physical property characterization of semiconductor NWs and NW heterostructures, as well as proof-of-concept device concept demonstrations, including solar energy conversion and intracellular probes. A new NW materials synthesis is discussed and, in particular, a new "nanotectonic" approach is introduced that provides iterative control over the NW nucleation and growth for constructing 2D kinked NW superstructures. The use of radial and axial p-type/intrinsic/n-type (p-i-n) silicon NW (Si-NW) building blocks for solar cells and nanoscale power source applications is then discussed. The critical benefits of such structures and recent results are described and critically analyzed, together with some of the diverse challenges and opportunities in the near future. Finally, results are presented on several new directions, which have recently been exploited in interfacing biological systems with NW devices.

Electroanalytical eavesdropping on single cell communication

This article reviews measurement of single cell exocytosis with microelectrodes, covering history, basic instrumentation, cell types investigated, and fundamental insight gained.

Electrochemical biosensors for on-chip detection of oxidative stress from immune cells

Seamless integration of biological components with electrochemical sensors is critical in the development of microdevices for cell analysis. The present paper describes the integration miniature Au electrodes next to immune cells (macrophages) in order to detect cell-secreted hydrogen peroxide (H(2)O(2)). Photopatterning of poly(ethylene glycol) (PEG) hydrogels was used to both immobilize horseradish peroxidase molecules onto electrodes and to define regions for cell attachment in the vicinity of sensing electrodes. Electrodes micropatterned in such a manner were enclosed inside poly(dimethylsiloxane) fluid conduits and incubated with macrophages. The cells attached onto the exposed glass regions in the vicinity of the electrodes and nowhere else on the non-fouling PEG hydrogel surface. A microfluidic device was converted into an electrochemical cell by placing flow-through Ag∕AgCl reference and Pt wire counter electrodes at the outlet and inlet, respectively. This microdevice with integrated H(2)O(2)-sensing electrodes had sensitivity of 27 μA∕cm(2) mM with a limit of detection of 2 μM. Importantly, this microdevice allowed controllable seeding of macrophages next to electrodes, activation of these cells and on-chip monitoring of H(2)O(2) release in real time. In the future, this biosensor platform may be utilized for monitoring of macrophage responses to pathogens or for the study of inflammatory signaling in micropatterned cell cultures.

Head-to-head comparisons of carbon fiber microelectrode coatings for sensitive and selective neurotransmitter detection by voltammetry

Voltammetry is widely used to investigate neurotransmission and other biological processes but is limited by poor chemical selectivity and fouling of commonly used carbon fiber microelectrodes (CFMs). We performed direct comparisons of three key coating materials purported to impart selectivity and fouling resistance to electrodes: Nafion, base-hydrolyzed cellulose acetate (BCA), and fibronectin. We systematically evaluated the impact on a range of electrode parameters. Fouling due to exposure to brain tissue was investigated using an approach that minimizes the use of animals while enabling evaluation of statistically significant populations of electrodes. We find that BCA is relatively fouling-resistant. Moreover, detection at BCA-coated CFMs can be tuned by altering hydrolysis times to minimize the impact on sensitivity losses while maintaining fouling resistance. Fibronectin coating is associated with moderate losses in sensitivity after coating and fouling. Nafion imparts increased sensitivity for dopamine and norepinephrine but not serotonin, as well as the anticipated selectivity for cationic neurotransmitters over anionic metabolites. Although Nafion has been suggested to resist fouling, both dip-coating and electrodeposition of Nafion are associated with substantial fouling, similar to levels observed at bare electrodes after exposure to brain tissue. Direct comparisons of these coatings identified unique electroanalytical properties of each that can be used to guide selection tailored to the goals and environment of specific studies.

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.

Electroanalytical approaches to study signaling mechanisms in the gastrointestinal tract

Electroanalytical techniques over the past few years have been applied to study real-time release of various signaling molecules in the GI tract. These approaches have become highly attractive as they provide dynamic spatial information on the amount of signaling molecules released. Although these approaches are relatively new to the field, the studies to date have provided useful insights into the alterations in signaling mechanisms during maturation, obesity and in a model of colitis. New methods and techniques have also allowed the possibility to obtain information on the signaling process and future developments will provide a wide diverse array of information that will be of benefit to all researchers in the field of gastroenterology. This review focuses on the types of techniques utilized, the information they can provide, their potential advantages and disadvantages in monitoring signaling processes in the gastrointestinal tract, the existing scientific studies that have utilized electroanalytical methods to date and the future potential impact of such approaches.

Cocaine cues drive opposing context-dependent shifts in reward processing and emotional state

BACKGROUND

Prominent neurobiological theories of addiction posit a central role for aberrant mesolimbic dopamine release but disagree as to whether repeated drug experience blunts or enhances this system. Although drug withdrawal diminishes dopamine release, drug sensitization augments mesolimbic function, and both processes have been linked to drug seeking. One possibility is that the dopamine system can rapidly switch from dampened to enhanced release depending on the specific drug-predictive environment. To test this, we examined dopamine release when cues signaled delayed cocaine delivery versus imminent cocaine self-administration.

METHODS

Fast-scan cyclic voltammetry was used to examine real-time dopamine release while simultaneously monitoring behavioral indexes of aversion as rats experienced a sweet taste cue that predicted delayed cocaine availability and during self-administration. Furthermore, the impact of cues signaling delayed drug availability on intracranial self-stimulation, a broad measure of reward function, was assessed.

RESULTS

We observed decreased mesolimbic dopamine concentrations, decreased reward sensitivity, and negative affect in response to the cocaine-predictive taste cue that signaled delayed cocaine availability. Importantly, dopamine concentration rapidly switched to elevated levels to cues signaling imminent cocaine delivery in the subsequent self-administration session.

CONCLUSIONS

These findings show rapid, bivalent contextual control over brain reward processing, affect, and motivated behavior and have implications for mechanisms mediating substance abuse.

Nanocavity redox cycling sensors for the detection of dopamine fluctuations in microfluidic gradients

Electrochemical mapping of neurotransmitter concentrations on a chip promises to be an interesting technique for investigating synaptic release in cellular networks. In here, we present a novel chip-based device for the detection of neurotransmitter fluctuations in real-time. The chip features an array of plane-parallel nanocavity sensors, which strongly amplify the electrochemical signal. This amplification is based on efficient redox cycling via confined diffusion between two electrodes inside the nanocavity sensors. We demonstrate the capability of resolving concentration fluctuations of redox-active species in a microfluidic mixing gradient on the chip. The results are explained by a simulated concentration profile that was calculated on the basis of the coupled Navier-Stokes and convection-diffusion equations using a finite element approach.

Fabrication and characterization of laser pulled platinum microelectrodes with controlled geometry

The development of a reproducible procedure for the fabrication of Pt disk-shaped microelectrodes with characteristic dimensions ranging from 50 nm to 1 μm in diameter was carried out using a laser pulling technique. The governing physical phenomena involved in their fabrication are discussed, and the importance of adding a critical quartz thinning step in the general procedure is demonstrated. The preparation of the microelectrodes involves sealing a platinum wire inside a quartz tubing using a pipet puller, thinning the composite material (platinum/quartz assembly), and laser pulling it to obtain two microelectrodes. The resulting microelectrodes display reproducible well-controlled geometry, which is important to downstream quantitative scanning electrochemical studies and imaging. Mechanical polishing of the microelectrode is required and remains the critical step in the fabrication of nanometer size electrodes. Following production, the microelectrodes are characterized by electron microscopy, scanning electrochemical microscopy, and cyclic voltammetry. Development of these microelectrodes is motivated by their subsequent application to electrocatalysis and their potential in theoretical study because of their well-defined geometry.

Electrochemistry in nanoscopic volumes

Exploration of electrochemical properties in ultrasmall volumes is still an emerging area. It is not only of great importance for the fundamental research, but also endowed with practical significance in the area of bioanalysis and medicine. Microelectrodes with superior electrochemical characteristics and versatile configurations are suitable tools for the investigation in confined geometries, and remarkable progress involving both preparation methods and theoretical interpretation has been made during the last few decades. Despite this success, electrochemical studies in nanoscopic volumes are still highly challenging due to the less predictable situations in very limited spatial and temporal domains, as well as difficulty in micromanipulation at the nanoscale. In this mini-review, we will summarize the main strategies for this topic, briefly look through the recent advances, and specifically introduce the design and application of a new kind of on-chip ultrasmall electrochemical cells based on micro- and nanogap electrodes, which are prepared by photolithographic method with volume ranging from femtolitre to attolitre. Finally, the limits of current systems and the future perspectives of this field are discussed.

Efficient water oxidation at carbon nanotube-polyoxometalate electrocatalytic interfaces

Water is the renewable, bulk chemical that nature uses to enable carbohydrate production from carbon dioxide. The dream goal of energy research is to transpose this incredibly efficient process and make an artificial device whereby the catalytic splitting of water is finalized to give a continuous production of oxygen and hydrogen. Success in this task would guarantee the generation of hydrogen as a carbon-free fuel to satisfy our energy demands at no environmental cost. Here we show that very efficient and stable nanostructured, oxygen-evolving anodes are obtained by the assembly of an oxygen-evolving polyoxometalate cluster (a totally inorganic ruthenium catalyst) with a conducting bed of multiwalled carbon nanotubes. Our bioinspired electrode addresses the one major challenge of artificial photosynthesis, namely efficient water oxidation, which brings us closer to being able to power the planet with carbon-free fuels.

Real-time monitoring of cell viability by its nanoscale height change with oxygen as endogenous indicator

A method for real-time evaluation of cell viability was developed by using oxygen as an endogenous indicator in scanning electrochemical microscopy to monitor the nanoscale height change of a single cell in a physiological environment with a novel Pt nanodisk electrode and a newly designed step-approaching strategy.

Three-dimensional, flexible nanoscale field-effect transistors as localized bioprobes

Nanoelectronic devices offer substantial potential for interrogating biological systems, although nearly all work has focused on planar device designs. We have overcome this limitation through synthetic integration of a nanoscale field-effect transistor (nanoFET) device at the tip of an acute-angle kinked silicon nanowire, where nanoscale connections are made by the arms of the kinked nanostructure, and remote multilayer interconnects allow three-dimensional (3D) probe presentation. The acute-angle probe geometry was designed and synthesized by controlling cis versus trans crystal conformations between adjacent kinks, and the nanoFET was localized through modulation doping. 3D nanoFET probes exhibited conductance and sensitivity in aqueous solution, independent of large mechanical deflections, and demonstrated high pH sensitivity. Additionally, 3D nanoprobes modified with phospholipid bilayers can enter single cells to allow robust recording of intracellular potentials.

Microfluidic cell arrays for metabolic monitoring of stimulated cardiomyocytes

An array of PDMS microchambers was aligned to an array of sensor electrodes and stimulating microelectrodes, which was used for the electrochemical monitoring of the metabolic activity of single isolated adult ventricular myocytes inside the chamber array, stimulated within a transient electric field. The effect of the accumulation of metabolic byproducts in the limited extracellular volume of the picolitre chambers was demonstrated by measuring single muscle cell contraction optically, while concomitant changes in intracellular calcium transients and pH were recorded independently using fluorescent indicator dyes. Both the amplitude of the cell shortening and the magnitude of the intracellular calcium transients decreased over time and both nearly ceased after 20 min of continuous stimulation in the limited extracellullar volume. The intracellular pH decreased gradually during 20 min of continuous stimulation after which a dramatic pH drop was observed, indicating the breakdown of the intracellular buffering capacity. After continuous stimulation, intracellular lactate was released into the microchamber through cell electroporation and was detected electrochemically at a lactate microbiosensor, within the chamber. A mitochondrial uncoupler was used to mimic ischaemia and thus to enhance the cellular content of lactate. Under these circumstances, intracellular lactate concentrations were found to have risen to approximately 15 mM. This array system has the potential of simultaneous electrochemical and optical monitoring of extracellular and intracellular metabolites from single beating heart cells at a controlled metabolic state.

Solutions of the coupled reaction and diffusion equations within polymer-modified ultramicroelectrodes

In this work, we have theoretically analyzed the non-steady-state current response witnessed at conducting polymer-modified ultramicroelectrodes when the electrode is operating under batch amperometric conditions. Furthermore, the effect of substrate diffusion in the solution adjacent to the polymer film on both the concentration profiles and current response was examined. Analytical expressions pertaining to substrate concentration, mediator concentration, and current response pertaining to all values of diffusion parameters are presented. In addition, we have employed "reduction of order method" to evaluate the approximate analytical solutions of reaction diffusion equations.

Understanding changes in uptake and release of serotonin from gastrointestinal tissue using a novel electroanalytical approach

Serotonin (5-HT) is well known to be a key neurotransmitter within the gastrointestinal (GI) tract, where it is responsible for influencing motility. Obtaining dynamic information about the neurotransmission process (specifically the release and reuptake of 5-HT) requires the development of new approaches to measure the extracellular 5-HT concentration profile. In this work constant-potential amperometry has been utilised at +650 mV vs. Ag|AgCl to measure in vitro the overflow of 5-HT. Steady-state levels of 5-HT have been observed, due to continuous mechanical stimulation of the tissue from the experimental protocol. Measurements are conducted at varying tissue-electrode distances in the range of 5 to 1100 microm. The difference in the current from the bulk media and that from each tissue-electrode distance is obtained, and the natural log of this current is plotted versus the tissue-electrode distance. The linear fit to the log of the current is derived, and its intercept, I(0), with the vertical axis and its slope are calculated. The reciprocal of the slope, indicated as slope(-1), is used as a marker of reuptake. The ratio between intercept, I(0), and the reciprocal of the slope, I(0)/slope(-1), is a measure of the flux at the tissue surface and it can be used as a marker for the 5-HT release rate. Current measurements for ileum and colon tissue indicated a significantly higher reuptake rate in the colon, showed by a lower slope(-1). In addition, the ratio, I(0)/slope(-1), indicated that the colon has a higher 5-HT flux compared to the ileum. Following the application of the serotonin selective reuptake inhibitor (SSRI), fluoxetine, both tissues showed a higher value of slope(-1), as the reuptake process is blocked preventing clearance of 5-HT. No differences were observed in the ratio, I(0)/slope(-1), in the ileum, but a decrease was observed in the colon. These results indicate that ileum and colon are characterised by different reuptake and release processes. The new approach we propose provides pivotal information on the variations in the signalling mechanism, where steady state levels are observed and can be a vital tool to study differences between normal and diseased tissue and also the efficacy of pharmacological agents.