Striving for new solar fuels, the water oxidation reaction currently is considered to be a bottleneck, hampering progress in the development of applicable technologies for the conversion of light into storable fuels. This review compares and unifies viewpoints on water oxidation from various fields of catalysis research. The first part deals with the thermodynamic efficiency and mechanisms of electrochemical water splitting by metal oxides on electrode surfaces, explaining the recent concept of the potential-determining step. Subsequently, novel cobalt oxide-based catalysts for heterogeneous (electro)catalysis are discussed. These may share structural and functional properties with surface oxides, multinuclear molecular catalysts and the catalytic manganese–calcium complex of photosynthetic water oxidation. Recent developments in homogeneous water-oxidation catalysis are outlined with a focus on the discovery of mononuclear ruthenium (and non-ruthenium) complexes that efficiently mediate O2 evolution from water. Water oxidation in photosynthesis is the subject of a concise presentation of structure and function of the natural paragon—the manganese–calcium complex in photosystem II—for which ideas concerning redox-potential leveling, proton removal, and OO bond formation mechanisms are discussed. The last part highlights common themes and unifying concepts.

The Mechanism of Water Oxidation: From Electrolysis via Homogeneous to Biological Catalysis

Review: Carbon nanotube based electrochemical sensors for biomolecules.

Neurotransmitters are chemicals that are secreted by neurons and relay messages to target cells. The goal of in vivo electrochemistry is to provide a real-time view of neurotransmitters in the extracellular space of the brain. This may be done in brain slices or the intact brain of anesthetized animals to probe the basic functions that regulate neurotransmitter levels. In other experiments, the measurements need to be made in the brain of behaving animals so that correlations of neurotransmitter fluctuations and specific behaviors can be made. For the neurotransmitter dopamine this can be accomplished today by chemical sensing of this neurotransmitter with fast scan cyclic voltammetry (FSCV) at carbon-fiber microelectrodes. Dopamine is an important target because it is a central player in the brain ‘reward’ system, although its precise function is not understood. Proposed roles for dopamine in reward have included the mediation of hedonia (pleasure)1, a messenger of incentive salience (wanting)2, or an error signal that promotes the learning associated with goal-directed behavior3,4. These multiple interpretations of dopaminergic function have arisen because, until recently, a real-time view of dopamine and its actions in an awake, behaving animal was unavailable. At the same time, new electrochemical technologies are being developed to measure other neurotransmitter actions. Electrochemical approaches are well suited for this application because they allow a neurotransmitter to be measured with high time resolution, enabling its precise role in the execution of behavioral tasks to be investigated.

In this review we will describe current methods to detect neurotransmitters and monitor their concentration dynamics within neural tissue. The requirements for these methods are quite stringent. They need to be sufficiently selective so that the measured responses are unequivocally due to a specific molecule. They need to be sufficiently sensitive that they can detect these substances in the physiological range. The best established methodologies are for dopamine, so the majority of the applications of the methods described herein will involve this neurotransmitter. As will be seen, the goals in measuring neurotransmitter functions are diverse. On one hand, investigators are unraveling the mechanisms that control neurotransmitter concentrations. These studies range from examining biochemical synthesis to metabolism. On the other hand, investigators are questioning how the neurotransmitter interacts with its receptors and what message it conveys. Yet a third major interest is the role of a neurotransmitter in specific behaviors. To obtain a complete view of neurotransmission and information processing, chemical sensors need to be combined with traditional neurochemical tools. We will illustrate this approach with some specific examples.

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Monitoring Rapid Chemical Communication in the Brain

Heteroatom modification represents one of the largest studied areas of research related to nanostructured carbon materials, with integrated applications stretching from energy production and storage to sustainability and medical uses. While a wide variety of dopants (boron, phosphorus, iodine, fluorine, etc.) have been studied, doping carbon structures with nitrogen ad-atoms has arguably experienced the greatest progress and brought the most attention over the last several years. Research in this field has conclusively demonstrated that nitrogen doping is an effective way to tailor the properties of carbon and tune the material for various applications of interest. This review provides a comprehensive overview of advances in the last half decade on state-of-the-art carbon modification with nitrogen heteroatoms. Improvements in well-established fabrication/modification processes are discussed as well as novel strategies. Additionally, recent theoretical and experimental findings related to the benefits and effects of nitrogen modification for specific applications in the energy and environmental fields are reviewed.

Recent progress on nitrogen/carbon structures designed for use in energy and sustainability applications

Metallophthalocyanine-based molecular materials as catalysts for electrochemical reactions

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.

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

CuO nanoparticle sensor for the electrochemical determination of dopamine

Adenosine modulates blood flow and neurotransmission and may be protective during pathological conditions such as ischemia and stroke. A real-time sensor of adenosine concentrations is needed to understand its physiological actions and the extent of receptor activation. Microelectrodes are advantageous for in vivo measurements because they are small and can make fast measurements. The goal of this study was to characterize detection of physiological adenosine concentration changes at carbon-fiber microelectrodes with subsecond temporal resolution. The oxidation potential of adenosine is +1.3 V, so fast-scan cyclic voltammetry (FSCV) was performed with an applied potential from -0.4 to 1.5 V and back at 400 V/s every 100 ms. Two oxidation peaks were detected for adenosine with T-650 carbon fibers. The second oxidation peak at 1.0 V occurs after the initial oxidation at 1.5 V and is due to a sequential oxidation step. Adsorption was maximized to obtain detection limits of 15 nM, lower than basal adenosine concentrations in the brain. The electrode was insensitive to the metabolite inosine and seven times more sensitive to adenosine than ATP. The enzymatic degradation of adenosine was monitored with FSCV. This microelectrode sensor will be valuable for biological monitoring of adenosine.

B.E. Kumara Swamy

Papers

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

CuO nanoparticle sensor for the electrochemical determination of dopamine

Subsecond detection of physiological adenosine concentrations using fast-scan cyclic voltammetry.

Voltammetric resolution of dopamine in the presence of ascorbic acid and uric acid at poly (calmagite) film coated carbon paste electrode

Electrochemical sensor for the detection of dopamine in real samples using polyaniline/NiO, ZnO, and Fe3O4 nanocomposites on glassy carbon electrode