12 research outputs found

    Improved Electrochemical Microsensor for the Real-Time Simultaneous Analysis of Endogenous Nitric Oxide and Carbon Monoxide Generation

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    An amperometric dual NO/CO microsensor was developed on the basis of a working electrode incorporating dual Pt microdisks (each diameter, 76 μm) and a Ag/AgCl reference electrode covered with a gas permeable membrane. One of the Pt disks was sequentially electrodeposited with Pt and Sn; the other Pt disk was deposited with Pt–Fe­(III) oxide nanocomposites. The first showed activity for the oxidation of both NO and CO; the second showed activity only for NO oxidation. In the copresence of NO and CO, the currents measured at each electrode, respectively, represented the concentrations of CO and NO. The sensor showed high stability during the monitoring of organ tissue for at least 2.5 h and high selectivity to NO over CO at the Pt–Fe­(III) oxide working electrode. Real-time coupled dynamic changes of NO and CO generated by a living C57 mouse kidney were monitored simultaneously and quantitatively in response to a NO synthase inhibitor (N<sup>G</sup>-nitro-l-arginine methyl ester), for the first time. CO was found to increase and NO decreased upon addition of the inhibitor, suggesting a possible reciprocal interaction between these endogenous gases

    Nanotubular Iridium–Cobalt Mixed Oxide Crystalline Architectures Inherited from Cobalt Oxide for Highly Efficient Oxygen Evolution Reaction Catalysis

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    Here, we report the unique transformation of one-dimensional tubular mixed oxide nanocomposites of iridium (Ir) and cobalt (Co) denoted as Ir<sub><i>x</i></sub>Co<sub>1–<i>x</i></sub>O<sub><i>y</i></sub>, where <i>x</i> is the relative Ir atomic content to the overall metal content. The formation of a variety of Ir<sub><i>x</i></sub>Co<sub>1–<i>x</i></sub>O<sub><i>y</i></sub> (0 ≤ <i>x</i> ≤ 1) crystalline tubular nanocomposites was readily achieved by electrospinning and subsequent calcination process. Structural characterization clearly confirmed that Ir<sub><i>x</i></sub>Co<sub>1–<i>x</i></sub>O<sub><i>y</i></sub> polycrystalline nanocomposites had a tubular morphology consisting of Ir/IrO<sub>2</sub> and Co<sub>3</sub>O<sub>4</sub>, where Ir, Co, and O were homogeneously distributed throughout the entire nanostructures. The facile formation of Ir<sub><i>x</i></sub>Co<sub>1–<i>x</i></sub>O<sub><i>y</i></sub> nanotubes was mainly ascribed to the inclination of Co<sub>3</sub>O<sub>4</sub> to form the nanotubes during the calcination process, which could play a critical role in providing a template of tubular structure and facilitating the formation of IrO<sub>2</sub> by being incorporated with Ir precursors. Furthermore, the electroactivity of obtained Ir<sub><i>x</i></sub>Co<sub>1–<i>x</i></sub>O<sub><i>y</i></sub> nanotubes was characterized for oxygen evolution reaction (OER) with rotating disk electrode voltammetry in 1 M NaOH aqueous solution. Among diverse Ir<sub><i>x</i></sub>Co<sub>1–<i>x</i></sub>O<sub><i>y</i></sub>, Ir<sub>0.46</sub>Co<sub>0.54</sub>O<sub><i>y</i></sub> nanotubes showed the best OER activity (the least-positive onset potential, greatest current density, and low Tafel slope), which was even better than that of commercial Ir/C. The Ir<sub>0.46</sub>Co<sub>0.54</sub>O<sub><i>y</i></sub> nanotubes also exhibited a high stability in alkaline electrolyte. Expensive Ir mixed with cheap Co at an optimum ratio showed a greater OER catalytic activity than pure Ir oxide, one of the most efficient OER catalysts

    Insertable Fast-Response Amperometric NO/CO Dual Microsensor: Study of Neurovascular Coupling During Acutely Induced Seizures of Rat Brain Cortex

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    This paper reports the fabrication of an insertable amperometric dual microsensor and its application for the simultaneous and fast sensing of NO and CO during acutely induced seizures of living rat brain cortex. NO and CO are important signaling mediators, controlling cerebrovascular tone. The dual NO/CO sensor is prepared based on a dual microelectrode having Au-deposited Pt microdisk (WE1, 76 μm diameter) and Pt black-deposited Pt disk (WE2, 50 μm diameter). The different deposited metals for WE1 and WE2 allow the selective anodic detection of CO at WE1 (+0.2 V vs Ag/AgCl) and that of NO at WE2 (+0.75 V vs Ag/AgCl) with sufficient sensitivity. Fluorinated xerogel coating on this dual electrode provides exclusive selectivity over common biological interferents, along with fast response time. The miniaturized size (end plane diameter < 300 μm) and tapered needle-like sensor geometry make the sensor become insertable into biological tissues. The sensor is applied to simultaneously monitor dynamic changes of NO and CO levels in a living rat brain under acute seizure condition induced by 4-aminopyridine in cortical tissue near the area of seizure induction. In-tissue measurement shows clearly defined patterns of NO/CO changes, directly correlated with observed LFP signal. Current study verifies the feasibility of a newly developed NO/CO dual sensor for real-time fast monitoring of intimately connected NO and CO dynamics

    Highly Efficient Silver–Cobalt Composite Nanotube Electrocatalysts for Favorable Oxygen Reduction Reaction

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    This paper reports the synthesis and characterization of silver–cobalt (AgCo) bimetallic composite nanotubes. Cobalt oxide (Co<sub>3</sub>O<sub>4</sub>) nanotubes were fabricated by electrospinning and subsequent calcination in air and then reduced to cobalt (Co) metal nanotubes via further calcination under a H<sub>2</sub>/Ar atmosphere. As-prepared Co nanotubes were then employed as templates for the following galvanic replacement reaction (GRR) with silver (Ag) precursor (AgNO<sub>3</sub>), which produced AgCo composite nanotubes. Various AgCo nanotubes were readily synthesized with applying different reaction times for the reduction of Co<sub>3</sub>O<sub>4</sub> nanotubes and GRR. One hour reduction was sufficiently long to convert Co<sub>3</sub>O<sub>4</sub> to Co metal, and 3 h GRR was enough to deposit Ag layer on Co nanotubes. The tube morphology and copresence of Ag and Co in AgCo composite nanotubes were confirmed with SEM, HRTEM, XPS, and XRD analyses. Electroactivity of as-prepared AgCo composite nanotubes was characterized for ORR with rotating disk electrode (RDE) voltammetry. Among differently synthesized AgCo composite nanotubes, the one synthesized via 1 h reduction and 3 h GRR showed the best ORR activity (the most positive onset potential, greatest limiting current density, and highest number of electrons transferred). Furthermore, the ORR performance of the optimized AgCo composite nanotubes was superior compared to pure Co nanotubes, pure Ag nanowires, and bare platinum (Pt). High ethanol tolerance of AgCo composite nanotubes was also compared with the commercial Pt/C and then verified its excellent resistance to ethanol contamination

    Spongelike Nanoporous Pd and Pd/Au Structures: Facile Synthesis and Enhanced Electrocatalytic Activity

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    This paper reports the facile synthesis and characterization of spongelike nanoporous Pd (snPd) and Pd/Au (snPd/Au) prepared by a tailored galvanic replacement reaction (GRR). Initially, a large amount of Co particles as sacrificial templates was electrodeposited onto the glassy carbon surface using a cyclic voltammetric method. This is the key step to the subsequent fabrication of the snPd/Au (or snPd) architectures by a surface replacement reaction. Using Co films as sacrificial templates, snPd/Au catalysts were prepared through a two-step GRR technique. In the first step, the Pd metal precursor (at different concentrations), K<sub>2</sub>PdCl<sub>4</sub>, reacted spontaneously to the formed Co frames through the GRR, resulting in a snPd series. snPd/Au was then prepared via the second GRR between snPd (prepared with 27.5 mM Pd precursor) and Au precursor (10 mM HAuCl<sub>4</sub>). The morphology and surface area of the prepared snPd series and snPd/Au were characterized using spectroscopic and electrochemical methods. Rotating disk electrode (RDE) experiments for oxygen reduction in 0.1 M NaOH showed that the snPd/Au has higher catalytic activity than snPd and the commercial Pd-20/C and Pt-20/C catalysts. Rotating ring-disk electrode (RRDE) experiments reconfirmed that four electrons were involved in the electrocatalytic reduction of oxygen at the snPd/Au. Furthermore, RDE voltammetry for the H<sub>2</sub>O<sub>2</sub> oxidation/reduction was used to monitor the catalytic activity of snPd/Au. The amperometric <i>i</i>–<i>t</i> curves of the snPd/Au catalyst for a H<sub>2</sub>O<sub>2</sub> electrochemical reaction revealed the possibility of applications as a H<sub>2</sub>O<sub>2</sub> oxidation/reduction sensor with high sensitivity (0.98 mA mM<sup>–1</sup> cm<sup>–2</sup> (<i>r</i> = 0.9997) for H<sub>2</sub>O<sub>2</sub> oxidation and −0.95 mA mM<sup>–1</sup> cm<sup>–2</sup> (<i>r</i> = 0.9997) for H<sub>2</sub>O<sub>2</sub> reduction), low detection limit (1.0 μM), and a rapid response (<∼1.5 s)

    Fundamental Study of Facile and Stable Hydrogen Evolution Reaction at Electrospun Ir and Ru Mixed Oxide Nanofibers

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    Electrochemical hydrogen evolution reaction (HER) has been an interesting research topic in terms of the increasing need of renewable and alternative energy conversion devices. In this article, Ir<sub><i>x</i></sub>Ru<sub>1–<i>x</i></sub>O<sub><i>y</i></sub> (<i>y</i> = 0 or 2) nanofibers with diverse compositions of Ir/IrO<sub>2</sub> and RuO<sub>2</sub> are synthesized by electrospinning and calcination procedures. Their HER activities are measured in 1.0 M NaOH. Interestingly, the HER activities of Ir<sub><i>x</i></sub>Ru<sub>1–<i>x</i></sub>O<sub><i>y</i></sub> nanofibers improve gradually during repetitive cathodic potential scans for HER, and then eventually reach the steady-state consistencies. This cathodic activation is attributed to the transformation of the nanofiber surface oxides to the metallic alloy. Among a series of Ir<sub><i>x</i></sub>Ru<sub>1–<i>x</i></sub>O<sub><i>y</i></sub> nanofibers, the cathodically activated Ir<sub>0.80</sub>Ru<sub>0.20</sub>O<sub><i>y</i></sub> shows the best HER activity and stability even compared with IrO<sub><i>y</i></sub> and RuO<sub><i>y</i></sub>, commercial Pt and commercial Ir (20 wt % each metal loading on Vulcan carbon), where a superior stability is possibly ascribed to the instant generation of active Ir and Ru metals on the catalyst surface upon HER. Density functional theory calculation results for hydrogen adsorption show that the energy and adsorbate–catalyst distance at metallic Ir<sub>0.80</sub>Ru<sub>0.20</sub> are close to those at Pt. This suggests that mixed metallic Ir and Ru are significant contributors to the improved HER activity of Ir<sub>0.80</sub>Ru<sub>0.20</sub>O<sub><i>y</i></sub> after the cathodic activation. The present findings clearly demonstrate that the mixed oxide of Ir and Ru is a very effective electrocatalytic system for HER

    Dual Electrochemical Microsensor for Real-Time Simultaneous Monitoring of Nitric Oxide and Potassium Ion Changes in a Rat Brain during Spontaneous Neocortical Epileptic Seizure

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    In this work, we developed a dual amperometric/potentiometric microsensor for sensing nitric oxide (NO) and potassium ion (K<sup>+</sup>). The dual NO/K<sup>+</sup> sensor was prepared based on a dual recessed electrode possessing Pt (diameter, 50 μm) and Ag (diameter, 76.2 μm) microdisks. The Pt disk surface (WE1) was modified with electroplatinization and the following coating with fluorinated xerogel; and the Ag disk surface (WE2) was oxidized to AgCl on which K<sup>+</sup> ion selective membrane was loaded subsequent to the silanization. WE1 and WE2 of a dual microsensor were used for amperometric sensing of NO (106 ± 28 pA μM<sup>–1</sup>, <i>n</i> = 10, at +0.85 V applied vs Ag/AgCl) and for potentiometric sensing of K<sup>+</sup> (51.6 ± 1.9 mV pK<sup>–1</sup>, <i>n</i> = 10), respectively, with high sensitivity. In addition, the sensor showed good selectivity over common biological interferents, sufficiently fast response time and relevant stability (within 6 h in vivo experiment). The sensor had a small dimension (end plane diameter, 428 ± 97 μm, <i>n</i> = 20) and needle-like sharp geometry which allowed the sensor to be inserted in biological tissues. Taking advantage of this insertability, the sensor was applied for the simultaneous monitoring of NO and K<sup>+</sup> changes in a living rat brain cortex at a depth of 1.19 ± 0.039 mm and near the spontaneous epileptic seizure focus. The seizures were induced with 4-aminopyridine injection onto the rat brain cortex. NO and K<sup>+</sup> levels were dynamically changed in clear correlation with the electrophysiological recording of seizures. This indicates that the dual NO/K<sup>+</sup> sensor’s measurements well reflect membrane potential changes of neurons and associated cellular components of neurovascular coupling. The newly developed NO/K<sup>+</sup> dual microsensor showed the feasibility of real-time fast monitoring of dynamic changes of closely linked NO and K<sup>+</sup> in vivo

    Hierarchically Driven IrO<sub>2</sub> Nanowire Electrocatalysts for Direct Sensing of Biomolecules

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    Applying nanoscale device fabrications toward biomolecules, ultra sensitive, selective, robust, and reliable chemical or biological microsensors have been one of the most fascinating research directions in our life science. Here we introduce hierarchically driven iridium dioxide (IrO<sub>2</sub>) nanowires directly on a platinum (Pt) microwire, which allows a simple fabrication of the amperometric sensor and shows a favorable electronic property desired for sensing of hydrogen peroxide (H<sub>2</sub>O<sub>2</sub>) and dihydronicotinamide adenine dinucleotide (NADH) without the aid of enzymes. This rational engineering of a nanoscale architecture based on the direct formation of the hierarchical 1-dimensional (1-D) nanostructures on an electrode can offer a useful platform for high-performance electrochemical biosensors, enabling the efficient, ultrasensitive detection of biologically important molecules

    Growth of Highly Single Crystalline IrO<sub>2</sub> Nanowires and Their Electrochemical Applications

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    We present the facile growth of highly single crystalline iridium dioxide (IrO<sub>2</sub>) nanowires on SiO<sub>2</sub>/Si and Au substrates via a simple vapor phase transport process under atmospheric pressure without any catalyst. Particularly, high-density needle-like IrO<sub>2</sub> nanowires were readily obtained on a single Au microwire, suggesting that the melted surface layer of Au might effectively enhance the nucleation of gaseous IrO<sub>3</sub> precursors at the growth temperature. In addition, all the electrochemical observations of the directly grown IrO<sub>2</sub> nanowires on a single Au microwire support favorable electron-transfer kinetics of [Fe­(CN<sub>6</sub>)]<sup>4–/3–</sup> as well as Ru­(NH<sub>3</sub>)<sub>6</sub><sup>3+/2+</sup> at the highly oriented crystalline IrO<sub>2</sub> nanowire surface. Furthermore, stable pH response is shown, revealing potential for use as a miniaturized pH sensor, confirmed by the calibration curve exhibiting super-Nernstian behavior with a slope of 71.6 mV pH<sup>–1</sup>

    Surface Design of Eu-Doped Iron Oxide Nanoparticles for Tuning the Magnetic Relaxivity

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    Relaxivity tuning of nanomaterials with the intrinsic <i>T</i><sub>1</sub>–<i>T</i><sub>2</sub> dual-contrast ability has great potential for MRI applications. Until now, the relaxivity tuning of T<sub>1</sub> and T<sub>2</sub> dual-modal MRI nanoprobes has been accomplished through the dopant, size, and morphology of the nanoprobes, leaving room for bioapplications. However, a surface engineering method for the relaxivity tuning was seldom reported. Here, we report the novel relaxivity tuning method based on the surface engineering of dual-mode <i>T</i><sub>1</sub>–<i>T</i><sub>2</sub> MRI nanoprobes (DMNPs), along with protein interaction monitoring with the DMNPs as a potential biosensor application. Core nanoparticles (NPs) of europium-doped iron oxide (EuIO) are prepared by a thermal decomposition method. As surface materials, citrate (Cit), alendronate (Ale), and poly­(maleic anhydride-<i>alt</i>-1-octadecene)/poly­(ethylene glycol) (PP) are employed for the relaxivity tuning of the NPs based on surface engineering, resulting in EuIO-Cit, EuIO-Ale, and EuIO-PP, respectively. The key achievement of the current study is that the surface materials of the DMNP have significant impacts on the <i>r</i><sub>1</sub> and <i>r</i><sub>2</sub> relaxivities. The correlation between the hydrophobicity of the surface material and longitudinal relaxivity (<i>r</i><sub>1</sub>) of EuIO NPs presents an exponential decay feature. The <i>r</i><sub>1</sub> relaxivity of EuIO-Cit is 13.2-fold higher than that of EuIO-PP. EuIO can act as <i>T</i><sub>1</sub>–<i>T</i><sub>2</sub> dual-modal (EuIO-Cit) or <i>T</i><sub>2</sub>-dominated MRI contrast agents (EuIO-PP) depending on the surface engineering. The feasibility of using the resulting nanosystem as a sensor for environmental changes, such as albumin interaction, was also explored. The albumin interaction on the DMNP shows both <i>T</i><sub>1</sub> and <i>T</i><sub>2</sub> relaxation time changes as mutually confirmative information. The relaxivity tuning approach based on the surface engineering may provide an insightful strategy for bioapplications of DMNPs and give a fresh impetus for the development of novel stimuli-responsive MRI nanoplatforms with <i>T</i><sub>1</sub> and <i>T</i><sub>2</sub> dual-modality for various biomedical applications
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