Group Behavior of Nanoparticles Translocating Multiple Nanopores

Modulation of Ionic Current Rectification in Short Bipolar Nanopores

Bipolar nanopores, with asymmetric charge distributions, can induce significant ionic current rectification (ICR) at ultrashort lengths, finding potential applications in nanofluidic devices, energy conversion, and other related fields. Here, with simulations, we investigated the characteristics of ion transport and modulation of the ICR inside bipolar nanopores. With bipolar nanopores of half-positive and half-negative surfaces, the most significant ICR phenomenon appears at various concentrations. In these cases, the ICR ratios are independent of electrolyte types. In other cases where nanopores have oppositely charged surfaces of different lengths, ICR ratios are related to the mobility of anions and cations. The pore length and surface charge density can enhance ICR. As the pore length increases, ICR ratios first increase and then approach their saturation, which is determined by the surface charge density. External surface charges of nanopores can promote the ICR phenomenon mainly due to the enhancement of ion enrichment inside the nanopores by external surface conductance. The effective width of exterior charged surfaces under various conditions is also explored, which is inversely proportional to the pore length and salt concentration and linearly related to the pore diameter, surface charge density, and applied voltage. Our results may provide guidance for the design of bipolar porous membranes.

Next-Generation Nanopore Sensors Based on Conductive Pulse Sensing for Enhanced Detection of Nanoparticles

Nanopore sensing has been successfully used to characterize biological molecules with single-molecule resolution based on the resistive pulse sensing approach. However, its use in nanoparticle characterization has been constrained by the need to tailor the nanopore aperture size to the size of the analyte, precluding the analysis of heterogeneous samples. Additionally, nanopore sensors often require the use of high salt concentrations to improve the signal-to-noise ratio, which further limits their ability to study a wide range of nanoparticles that are unstable at high ionic strength. Here, a new paradigm in nanopore research that takes advantage of a polymer electrolyte system to comprise a conductive pulse sensing approach is presented. A finite element model is developed to explain the conductive pulse signals observed and compare these results with experiments. This system enables the analytical characterization of heterogeneous nanoparticle mixtures at low ionic strength . Furthermore, the wide applicability of the method is demonstrated by characterizing metallic nanospheres of varied sizes, plasmonic nanostars with various degrees of branching, and protein-based spherical nucleic acids with different oligonucleotide loadings. This system will complement the toolbox of nanomaterials characterization techniques to enable real-time optimization workflow for engineering a wide range of nanomaterials.

Modulation mechanism of ionic transport through short nanopores by charged exterior surfaces

Short nanopores have various applications in biosensing, desalination, and energy conversion. Here, the modulation of ionic transport by charged exterior surfaces is investigated through simulations with sub-200 nm long nanopores under applied voltages. Detailed analysis of the ionic current, electric field strength, and fluid flow inside and outside nanopores reveals that charged exterior surfaces can increase ionic conductance by increasing both the concentration and migration speed of charge carriers. The electric double layers near charged exterior surfaces provide an ion pool and an additional passageway for counterions, which lead to enhanced exterior surface conductance and ionic concentrations at pore entrances and inside the nanopores. We also report that charges on the membrane surfaces increase the electric field strength inside nanopores. The effective width of a ring with surface charges placed at pore entrances (Lcs) is considered as well by studying the dependence of the current on Lcs. We find a linear relationship between the effective Lcs and the surface charge density and voltage, and an inverse relationship between the geometrical pore length and salt concentration. Our results elucidate the modulation mechanism of ionic transport through short nanopores by charged exterior surfaces, which is important for the design and fabrication of porous membranes.

Ion Transport in pH-Regulated Double-Barreled Nanopores

Single-molecule detection and characterization with nanopores is a powerful technique that does not require labeling. Multinanopore systems, especially double nanopores, have attracted wide attention and have been applied in many fields. However, theoretical studies of electrokinetic ion transport in nanopores mainly focus on single nanopores. In this paper, for the first time, a theoretical study of pH-regulated double-barreled nanopores is conducted using three-dimensional Poisson-Nernst-Planck equations and Navier-Stokes equations. Four ionic species and the surface chemistry on the walls of the nanopores are included. The results demonstrate that the properties of the bulk salt solution significantly affect nanopore conductivity and ion transport phenomena in nanopores. There are two ion-enriched zones and two ion-depleted zones in double-barreled nanopores. Due to the symmetry of the double-barreled nanopore structure and surface charge density, there is no ionic rectification effect in double-barreled nanopores. The ion selectivity is similar to that of conventional single pH-regulated nanopores.

Deep Learning of Nanopore Sensing Signals Using a Bi-Path Network

Temporal changes in electrical resistance of a nanopore sensor caused by translocating target analytes are recorded as a sequence of pulses on current traces. Prevalent algorithms for feature extraction in pulse-like signals lack objectivity because empirical amplitude thresholds are user-defined to single out the pulses from the noisy background. Here, we use deep learning for feature extraction based on a bi-path network (B-Net). After training, the B-Net acquires the prototypical pulses and the ability of both pulse recognition and feature extraction without a priori assigned parameters. The B-Net is evaluated on simulated data sets and further applied to experimental data of DNA and protein translocation. The B-Net results are characterized by small relative errors and stable trends. The B-Net is further shown capable of processing data with a signal-to-noise ratio equal to 1, an impossibility for threshold-based algorithms. The B-Net presents a generic architecture applicable to pulse-like signals beyond nanopore currents.

Mechanism and Kinetics of Lipid Bilayer Formation in Solid-State Nanopores

Solid-state nanopores provide a highly versatile platform for rapid electrical detection and analysis of single molecules. Lipid bilayer coating of the nanopores can reduce nonspecific analyte adsorption to the nanopore sidewalls and increase the sensing selectivity by providing possibilities for tethering specific ligands in a cell-membrane mimicking environment. However, the mechanism and kinetics of lipid bilayer formation from vesicles remain unclear in the presence of nanopores. In this work, we used a silicon-based, truncated pyramidal nanopore array as the support for lipid bilayer formation. Lipid bilayer formation in the nanopores was monitored in real time by the change in ionic current through the nanopores. Statistical analysis revealed that a lipid bilayer is formed from the instantaneous rupture of individual vesicle upon adsorption in the nanopores, differing from the generally agreed mechanism that lipid bilayer forms at a high vesicle surface coverage on a planar support. The dependence of the lipid bilayer formation process on the applied bias, vesicle size, and concentration was systematically studied. In addition, the nonfouling properties of the lipid bilayer coated nanopores were demonstrated during long single-stranded DNA translocation through the nanopore array. The findings indicate that the lipid bilayer formation process can be modulated by introducing nanocavities intentionally on the planar surface to create active sites or changing the vesicle size and concentration.

A pumpless solution exchange system for nanopore sensors

This paper proposes a nanopore-based sensor exploiting the solution exchange of a droplet-based lipid bilayer driven by a superabsorbent polymer. Biological nanopores are candidates for use in portable sensors because of their potential to recognize and detect single molecules. One of the current challenges in the development of portable nanopore sensors is the inability to achieve continuous detection. To achieve continuous detection, we have exploited the suction force of a superabsorbent polymer to drive the continuous microfluidic flow required to wash the analyte out of the droplet. The superabsorbent polymer drives the microfluidic flow without electricity, and the developed solution exchange system remains compact. To demonstrate solution exchange in the droplet containing the lipid bilayer, the concentration of heptakis(6-O-sulfo)-β-cyclodextrin was monitored in a time-dependent manner using α-hemolysin nanopores. A reduction in the concentration, attributable to solution exchange, was successfully observed. We believe that the proposed system will increase the portability and usability of nanopore sensors.