12 research outputs found
Improved Electrochemical Microsensor for the Real-Time Simultaneous Analysis of Endogenous Nitric Oxide and Carbon Monoxide Generation
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–FeÂ(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–FeÂ(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
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
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
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
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
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
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
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
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 [FeÂ(CN<sub>6</sub>)]<sup>4–/3–</sup> as well as RuÂ(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
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 polyÂ(maleic anhydride-<i>alt</i>-1-octadecene)/polyÂ(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