Modulation of Charge Density and Charge Polarity of Nanopore Wall by Salt Gradient and Voltage

Abstract

Surface charge plays a very important role in biological processes including ionic and molecular transport across a cell membrane. Placement of charges and charge patterns on walls of polymer and solid-state nanopores allowed preparation of ion-selective systems as well as ionic diodes and transistors to be applied in building biological sensors and ionic circuits. In this article, we show that the surface charge of a 10 nm diameter silicon nitride nanopore placed in contact with a salt gradient is not a constant value, but rather it depends on applied voltage and magnitude of the salt gradient. We found that even when a nanopore was in contact with solutions of pH equivalent to the isoelectric point of the pore surface, the pore walls became charged with voltage-dependent charge density. Implications of the charge gating for detection of proteins passing through a nanopore were considered, as well. Experiments performed with single 30 nm long silicon nitride nanopores were described by continuum modeling, which took into account the surface reactions on the nanopore walls and local modulation of the solution pH in the pore and at the pore entrances. The results revealed that manipulation of surface charge can occur without changing pH of the background electrolyte, which is especially important for applications where maintaining pH at a constant and physiological level is necessary. The system presented also offers a possibility to modulate polarity and magnitude of surface charges in a two-electrode setup, which previously was accomplished in more complex multielectrode systems.

Rational ion transport management mediated through membrane structures

“…Under the effects of the asymmetric shape, [83,84] charge, [85] and the components [67] of nanochannels or asymmetric external stimuli (e.g., pH, [86,87] pressure, [88] concentration, [89,90] and temperature gradients [91] ), specific ions show preferential transport in one direction at a bias, whereas they are blocked in the opposite direction at an inverse bias. For example, neg-atively charged conical nanochannels exhibit ion rectification owing to their asymmetric structure.…”

Electrochemical Quantitative Evaluation of the Surface Charge of a Poly(1‐Vinylimidazole) Multilayer Film and Application to Nanopore pH Sensor

“…Nanopore pH sensing is based on the ionic current changes associated with the acid-base dissociation equilibrium of the polyelectrolytes around the nanopores. The ionic current is affected by various state changes near the nanopore, such as the film thickness and film density [40][41][42][43][44][45]. Additionally, the ion current is strongly affected by the electrolyte concentration (ion strength) in solution [23].…”

Surface Potential/Charge Sensing Techniques and Applications

“…Nanopores take advantage of the electrostatic effects inside nano-confined space in the presence of surface charges, which gives them high sensitivity and new sensing mechanisms [82]. The change in surface charge regulates the ionic conductance of nanopores through pH [83], divalent cations and anions [84], ionic concentrations [85], temperature [86], surface chemistry and biomolecules [87], gas [88], salt gradient and voltage polarity [13], and can be characterized by the current-voltage (I-V) curve due to the conductance change that is related to the inner wall surface charge regulation, or the instant current change related to the passing of charged particles through nanopores. The non-uniform distribution of surface charge inside the nanopore greatly affects the current rectification ratio.…”

“…Surface potential φ o has a linear relationship with surface charge density as σ = C dl φ o , where C dl is the double layer capacitance that is determined by the ionic strength and ionic valence. Surface potential can be modulated by adsorption of target molecules, e.g., chemical concentrations (pH [10][11][12], metal ion concentration [13,14], etc. ), biological species (DNA [15][16][17], protein [18], etc.…”