6 research outputs found
Capacitance Performance of Sub‑2 nm Graphene Nanochannels in Aqueous Electrolyte
Molecular
dynamics simulations were used to explain the origin
and properties of electrical double-layer capacitance in short graphene
nanochannels with width below 2 nm. The results explain the previously
reported experimental result on the nonmonotonic dependence of the
capacitance with the channel width. The mechanism for the anomalous
increase of the capacitance in sub-1 nm in pore diameter is attributed
here to the width-dependent radial location of counterions in the
nanochannels and the restricted number of co-ions. Decrease of the
channel width lowers the number of co-ions and positions the counterions
closer to the channel walls. For nanochannels with width ranging from
1 to 2 nm, co-ions are allowed to enter the nanochannel, and both
types of ions assume alternating layered distributions leading to
the decrease of the capacitance. Voltage is another control parameter
which allows understanding capacitance in graphene nanochannels. As
the voltage increases, due to limited space near the charged surface,
more counterions need to be located in the center of the nanochannel,
resulting in further capacitance decrease
Viscosity and Conductivity Tunable Diode-like Behavior for Meso- and Micropores
Rectifying pores, which transport
ions mainly in one direction
blocking the ionic flow in the other, were shown to be important in
the preparation of chemical sensors, components of ionic circuits,
and mimics of biological channels. Ionic rectification has been shown
with various engineered systems, but pores with similar opening diameters
often rectify to a various uncontrolled extent. In this Letter we
present a system of single meso-pores, whose current–voltage
curves and rectification can be tuned with great precision via viscosity
and conductivity gradients of solutions placed on both sides of the
membrane. The mechanism of rectification is based on electroosmotically
induced flow, which fills the entire volume of the pore with a single
solution from either side of the membrane. The highly predictable
rectifying system can find various applications, including measuring
viscosity of unknown media and tuning electrokinetic passage of particles
Direction Dependence of Resistive-Pulse Amplitude in Conically Shaped Mesopores
Conically
shaped pores such as glass pipets as well as asymmetric
pores in polymers became an important analytics tool used for the
detection of molecules, viruses, and particles. Electrokinetic or
pressure driven passage of single particles through a single pore
causes a transient change of the transmembrane current, called a resistive-pulse,
whose amplitude is the measure of the particle volume. The shape of
the pulse reflects the pore topography, and in a conical pore, resistive
pulses have a shape of a tick point. Passage of particles in both
directions was reported to produce pulses of the same amplitude and
shapes that are mirror images of each other. In this manuscript we
identify conditions at which the amplitude of resistive-pulses in
a conical mesopore is direction dependent. Neutral particles entering
the pore from the larger entrance of a conical pore, called the base,
block the current to a larger extent than the particles traveling
in the opposite direction. Negatively charged particles on the other
hand size larger when being transported in the direction from tip
to base. The findings are explained via voltage-regulated ionic concentrations
in the pore such that for one voltage polarity a weak depletion zone
is formed, which increases the current blockage caused by a particle.
For the opposite polarity, an enhancement of ionic concentrations
was predicted. The findings reported here are of crucial importance
for the resistive-pulse technique, which relates the current blockage
with the size of the passing object
Anomalous Mobility of Highly Charged Particles in Pores
Single micropores in resistive-pulse
technique were used to understand
a complex dependence of particle mobility on its surface charge density.
We show that the mobility of highly charged carboxylated particles
decreases with the increase of the solution pH due to an interplay
of three effects: (i) ion condensation, (ii) formation of an asymmetric
electrical double layer around the particle, and (iii) electroosmotic
flow induced by the charges on the pore walls and the particle surfaces.
The results are important for applying resistive-pulse technique to
determine surface charge density and zeta potential of the particles.
The experiments also indicate the presence of condensed ions, which
contribute to the measured current if a sufficiently high electric
field is applied across the pore
Interfacial Synthesis of Highly Stable CsPbX<sub>3</sub>/Oxide Janus Nanoparticles
The poor stability
of CsPbX<sub>3</sub> (X = Cl, Br, I) nanocrystals
(NCs) has severely impeded their practical applications. Although
there are some successful examples on encapsulating multiple CsPbX<sub>3</sub> NCs into an oxide or polymer matrix, it has remained a serious
challenge for the surface modification/encapsulation using oxides
or polymers at a single particle level. In this work, monodisperse
CsPbX<sub>3</sub>/SiO<sub>2</sub> and CsPbBr<sub>3</sub>/Ta<sub>2</sub>O<sub>5</sub> Janus nanoparticles were successfully prepared by combining
a water-triggered transformation process and a sol–gel method.
The CsPbBr<sub>3</sub>/SiO<sub>2</sub> NCs exhibited a photoluminescence
quantum yield of 80% and a lifetime of 19.8 ns. The product showed
dramatically improved stability against destruction by air, water,
and light irradiation. Upon continuous irradiation by intense UV light
for 10 h, a film of the CsPbBr<sub>3</sub>/SiO<sub>2</sub> Janus NCs
showed only a slight drop (2%) in the PL intensity, while a control
sample of unmodified CsPbBr<sub>3</sub> NCs displayed a 35% drop.
We further highlighted the advantageous features of the CsPbBr<sub>3</sub>/SiO<sub>2</sub> NCs in practical applications by using them
as the green light source for the fabrication of a prototype white
light emitting diode, and demonstrated a wide color gamut covering
up to 138% of the National Television System Committee standard. This
work not only provides a novel approach for the surface modification
of individual CsPbX<sub>3</sub> NCs but also helps to address the
challenging stability issue; therefore, it has an important implication
toward their practical applications
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Highly Charged Particles Cause a Larger Current Blockage in Micropores Compared to Neutral Particles
Single pores in the
resistive-pulse technique are used as an analytics
tool to detect, size, and characterize physical as well as chemical
properties of individual objects such as molecules and particles.
Each object passing through a pore causes a transient change of the
transmembrane current called a resistive pulse. In high salt concentrations
when the pore diameter is significantly larger than the screening
Debye length, it is assumed that the particle size and surface charge
can be determined independently from the same experiment. In this
article we challenge this assumption and show that highly charged
hard spheres can cause a significant increase of the resistive-pulse
amplitude compared to neutral particles of a similar diameter. As
a result, resistive pulses overestimate the size of charged particles
by even 20%. The observation is explained by the effect of concentration
polarization created across particles in a pore, revealed by numerical
modeling of ionic concentrations, ion current, and local electric
fields. It is notable that in resistive-pulse experiments with cylindrical
pores, concentration polarization was previously shown to influence
ionic concentrations only at pore entrances; consequently, additional
and transient modulation of resistive pulses was observed when a particle
entered or left the pore. Here we postulate that concentration polarization
can occur across transported particles at any particle position along
the pore axis and affect the magnitude of the entire resistive pulse.
Consequently, the recorded resistive pulses of highly charged particles
reflect not only the particles’ volume but also the size of
the depletion zone created in front of the moving particle. Moreover,
the modeling identified that the effective surface charge density
of particles depended not only on the density of functional groups
on the particle but also on the capacitance of the Stern layer. The
findings are of crucial importance for sizing particles and characterizing
their surface charge properties