17 research outputs found
The design and characterization of multifunctional aptamer nanopore sensors
Aptamer-modified nanomaterials provide a
simple, yet powerful sensing platform when combined with resistive pulse sensing technologies. Aptamers adopt a more stable tertiary structure in the presence of a target analyte, which results in a change in charge density and velocity of the carrier particle. In practice the tertiary structure is specific for each aptamer and target, and the strength of the signal varies with different applications and experimental conditions. Resistive pulse sensors (RPS) have single
particle resolution, allowing for the detailed characterization of the sample. Measuring the velocity of aptamer-modified nanomaterials as they traverse the RPS provides information on their charge state and densities. To help understand how the aptamer structure and charge density effects the sensitivity of aptamer-RPS assays, here we study two metal binding
aptamers. This creates a sensor for mercury and lead ions that is capable of being run in a range of electrolyte concentrations, equivalent to river to seawater conditions. The observed results are in excellent agreement with our
proposed model. Building on this we combine two aptamers together in an attempt to form a dual sensing strand of DNA for the simultaneous detection of two metal ions. We show experimental and theoretical responses for the aptamer which creates layers of differing charge densities around the nanomaterial. The density and diameter of these zones effects both the viability and sensitivity of the assay. While this approach allows the interrogation of the DNA structure, the data also highlight the limitations and considerations for future assays
Diffusion and Trapping of Single Particles in Pores with Combined Pressure and Dynamic Voltage
In this article we
report resistive-pulse experiments with polystyrene particles whose
transport through pores is controlled by modulating the driving voltage
during the process of translocation. Balancing electric and hydrostatic
forces acting on the particles allowed us to observe a random walk
of single particles in a pore for tens of seconds and to quantify
their diffusion coefficient using two methods. The first approach
is based on the mean square displacement and requires passage of multiple
particles for a range of diffusion times. The diffusion coefficient
of individual particles was determined based on the variance of their
local diffusion velocities. The developed methods for measuring the
diffusion coefficient in pores are applicable to particles of different
sizes, do not require fluorescence labeling, and are entirely based
on ion current recordings. In addition, application of a modulating
voltage signal together with rising edge triggers enabled transporting
the same particle back and forth in the pore without letting the particle
leave the pore
Concentration-Polarization-Induced Precipitation and Ionic Current Oscillations with Tunable Frequency
At
the nanoscale, charges present at the surfaces of liquid–solid
interfaces greatly influence the properties of ions and molecules
present in the solution, and can lead to nanoscale effects such as
ion selectivity, ion current rectification, and modulation of local
ionic concentrations. Concentration polarization is another nanoscale
phenomenon whereby ion concentrations are enriched at one opening
of an ion-selective nanopore and depleted at the other. We show that
when a nanopore is in contact with a weakly soluble salt present at
a concentration below its solubility product, concentration polarization
can lead to locally enhanced ionic concentrations and precipitation
of the salt. Formed precipitates partially or fully occlude the nanopore’s
opening as indicated by a measured transient decrease of the nanopore’s
conductance. We have identified experimental conditions at which the
locally created precipitate is either pushed through or dissolved,
clearing the pore entrance and allowing the precipitation reaction
to occur again. The dynamic process of precipitate formation and dissolution
is observed as ion current fluctuations and oscillations with frequencies
reaching 200 Hz. The frequency of the system operation exceeds other
nanopore-based oscillators by 2 orders of magnitude, which we believe
stems from the 30 nm length of the pores examined here, versus ∼10
μm long pores reported before
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
Voltage-Induced Modulation of Ionic Concentrations and Ion Current Rectification in Mesopores with Highly Charged Pore Walls
It
is believed that ion current rectification (ICR), a property
that assures preferential ionic transport in one direction, can only
be observed in nanopores when the pore size is comparable to the thickness
of the electric double layer (EDL). Rectifying nanopores became the
basis of biological sensors and components of ionic circuits. Here
we report that appreciable ICR can also occur in highly charged conical,
polymer mesopores whose tip diameters are as large as 400 nm, thus
over 100-fold larger than the EDL thickness. A rigorous model taking
into account the surface equilibrium reaction of functional carboxyl
groups on the pore wall and electroosmotic flow is employed to explain
that unexpected phenomenon. Results show that the pore rectification
results from the high density of surface charges as well as the presence
of highly mobile hydroxide ions, whose concentration is enhanced for
one voltage polarity. This work provides evidence that highly charged
surfaces can extend the ICR of pores to the submicron scale, suggesting
the potential use of highly charged large pores for energy and sensing
applications. Our results also provide insight into how a mixture
of ions with different mobilities can influence current–voltage
curves and rectification
DNA-Modified Polymer Pores Allow pH- and Voltage-Gated Control of Channel Flux
Biological channels embedded in cell
membranes regulate ionic transport
by responding to external stimuli such as pH, voltage, and molecular
binding. Mimicking the gating properties of these biological structures
would be instrumental in the preparation of smart membranes used in
biosensing, drug delivery, and ionic circuit construction. Here we
present a new concept for building synthetic nanopores that can simultaneously
respond to pH and transmembrane potential changes. DNA oligomers containing
protonatable A and C bases are attached at the narrow opening of an
asymmetric nanopore. Lowering the pH to 5.5 causes the positively
charged DNA molecules to bind to other strands with negative backbones,
thereby creating an electrostatic mesh that closes the pore to unprecedentedly
high resistances of several tens of gigaohms. At neutral pH values,
voltage switching causes the isolated DNA strands to undergo nanomechanical
movement, as seen by a reversible current modulation. We provide evidence
that the pH-dependent reversible closing mechanism is robust and applicable
for nanopores with opening diameters of up to 14 nm. The concept of
creating an electrostatic mesh may also be applied to different organic
polymers
Probing Porous Structure of Single Manganese Oxide Mesorods with Ionic Current
Characterization
of materials in confined spaces, rather than attempting
to extrapolate from bulk material behavior, requires the development
of new measurement techniques. In particular, measurements of individual
meso- or nanoscale objects can provide information about their structure
which is unavailable by other means. In this report, we perform measurements
of ion currents through a few hundred nanometer long MnO<sub>2</sub> rods deposited in single polymer pores. The recorded current confirms
an existence of a meshlike character of the MnO<sub>2</sub> structure
and probes the effective size of the mesh voids and the polarity of
surface charges. The recorded ion current through deposited MnO<sub>2</sub> structure also suggests that the signal is mostly due to
metal cations and not to protons. This is the first time that ionic
current measurements have been used to characterize mesoporous structure
of this important electrode material
Polarization of Gold in Nanopores Leads to Ion Current Rectification
Biomimetic
nanopores with rectifying properties are relevant components
of ionic switches, ionic circuits, and biological sensors. Rectification
indicates that currents for voltages of one polarity are higher than
currents for voltages of the opposite polarity. Ion current rectification
requires the presence of surface charges on the pore walls, achieved
either by the attachment of charged groups or in multielectrode systems
by applying voltage to integrated gate electrodes. Here we present
a simpler concept for introducing surface charges via polarization
of a thin layer of Au present at one entrance of a silicon nitride
nanopore. In an electric field applied by two electrodes placed in
bulk solution on both sides of the membrane, the Au layer polarizes
such that excess positive charge locally concentrates at one end and
negative charge concentrates at the other end. Consequently, a junction
is formed between zones with enhanced anion and cation concentrations
in the solution adjacent to the Au layer. This bipolar double layer
together with enhanced cation concentration in a negatively charged
silicon nitride nanopore leads to voltage-controlled surface-charge
patterns and ion current rectification. The experimental findings
are supported by numerical modeling that confirm modulation of ionic
concentrations by the Au layer and ion current rectification even
in low-aspect ratio nanopores. Our findings enable a new strategy
for creating ionic circuits with diodes and transistors
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