We
study the effect of electrolyte concentration on the shape of
ion current pulses in resistive-pulse sensing. We show that electrokinetic
passage of several hundred nanometers in diameter charged polystyrene
particles through a micropore leads to formation of current increase
when the particles exit the pore. The particle entrance, as reported
before, causes formation of the current decrease, which is a measure
of the particle size. Formation of the double peak, i.e., current
decrease followed by a current increase, is especially pronounced
if the resistive-pulse experiments are carried out in KCl concentrations
below 200 mM. In order to explain the pulse shape, experiments were
designed in which the particles passed through the pore only by either
electroosmosis or electrophoresis. The presented experiments and modeling
indicate that while both electroosmosis and electrophoresis affect
the ion current pulse, formation of the positive peak is mainly determined
by the latter effect and the charged state of the particle. The importance
of the findings for resistive-pulse analysis is discussed.
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 multi-electrode 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 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.
A unique tool to investigate volume effects on the structure and metabolism of living cells is pressure perturbation. We present measurements of volume
Cytosine (C) modifications such as 5-methylcytosine (mC) and 5-hydroxymethylcytosine (hmC) are important epigenetic markers associated with gene expression 1 and tumorigenesis 2, 3. However, bisulfite conversion, the gold standard methodology for mC mapping can not distinguish mC and hmC bases. Recent studies have demonstrated hmC detection via peptide recognizing 4, enzymes 5, fluorescence 6 and hmC-specific antibodies 7, nevertheless, a method for directly discriminating C, mC and hmC bases without labeling, modification and amplification is still missing. Here we show that the DNA duplex containing single cytosine-cytosine (C-C), cytosine-methylcytosine (C-mC) and cytosine-hydroxymethylcytosine (C-hmC) mismatches can be discriminated by their interactions with Ag þ inside the alpha-hemolysin nanopore. Molecular dynamics simulations reveal that the paring of a CC mismatch through hydrogen bond results in a binding site for cations, such as K þ and Agþ. Cytosine modifications such as mC and hmC disrupted both the hydrogen bonds, which subsequently disrupts Ag þ binding. As a result, these modifications can be distinguished in the nanopore by differences in the stability of DNA-Ag þ complexes.
scite is a Brooklyn-based organization that helps researchers better discover and understand research articles through Smart Citations–citations that display the context of the citation and describe whether the article provides supporting or contrasting evidence. scite is used by students and researchers from around the world and is funded in part by the National Science Foundation and the National Institute on Drug Abuse of the National Institutes of Health.