Effects of off-axis translocation through nanopores on the determination of shape and volume estimates for individual particles

Ionic current blockade in a nanopore due to an ellipsoidal particle

Nanopores in solid-state membranes have been used to detect, identify, filter, and characterize nanoparticles and biological molecules. In this work, we simulate an ionic flow through a nanopore while an ellipsoidal nanoparticle translocates through a pore. We numerically solve the Poisson-Nernst-Planck equations to obtain the ionic current values for different aspect ratios, sizes, and orientations of a translocating particle. By extending the existing theoretical model for the ionic current in the nanopore to the particles of ellipsoidal shape, we propose semiempirical fitting formulas which describe our computed data within 5% accuracy. We also demonstrate how the derived formulas can be used to identify the dimensions of nanoparticles from the available experimental data which may have useful applications in bionanotechnology.

Improved Conductance Blockage Modeling of Cylindrical Nanopores, from 2D to Thick Membranes

The ionic current blockage from a nanopore sensor is a fundamental metric for characterizing its dimensions and identifying molecules translocating through it. Yet, most analytical models predicting the conductance of a nanopore in both open and obstructed states remain inaccurate. Here, using an oblate spheroidal coordinate framework to study the electrical response of nanopore access regions, we reveal that the widely used model from Kowalczyk et al. significantly overestimates access region contributions when blocked by a cylindrical object, like DNA. To address this, we present an improved analytical model for the obstructed access resistance, which we establish as highly accurate through finite-element simulations, especially for ultrathin membranes and long narrow channels. Equipped with an improved nanopore conductance model, this work provides tools for more accurate calculation of the pore size and for the expected blockade from DNA, of high practical value for many biosensing applications.

Orientational Pathways during Protein Translocation through Polymer-Modified Nanopores

Protein translocation through nanopores holds significant promise for applications in biotechnology, biomolecular analysis, and medicine. However, the interpretation of signals generated by the translocation of the protein remains challenging. In this way, it is crucial to gain a comprehensive understanding on how macromolecules translocate through a nanopore and to identify what are the critical parameters that govern the process. In this study, we investigate the interplay between protein charge regulation, orientation, and nanopore surface modifications using a theoretical framework that allows us to explicitly take into account the acid-base reactions of the titrable amino acids in the proteins and in the polyelectrolytes grafted to the nanopore surface. Our goal is to thoroughly characterize the translocation process of different proteins (GFP, β-lactoglobulin, lysozyme, and RNase) through nanopores modified with weak polyacids. Our calculations show that the charge regulation mechanism exerts a profound effect on the translocation process. The pH-dependent interactions between proteins and charged polymers within the nanopore lead to diverse free energy landscapes with barriers, wells, and flat regions dictating translocation efficiency. Comparison of different proteins allows us to identify the significance of protein isoelectric point, size, and morphology in the translocation behavior. Taking advantage of these insights, we propose pH-responsive nanopores that can load proteins at one pH and release them at another, offering opportunities for controlled protein delivery, separation, and sensing applications.

A Universal Approximation for Conductance Blockade in Thin Nanopore Membranes

Nanopore-based sensing platforms have transformed single-molecule detection and analysis. The foundation of nanopore translocation experiments lies in conductance measurements, yet existing models, which are largely phenomenological, are inaccurate in critical experimental conditions such as thin and tightly fitting pores. Of the two components of the conductance blockade, channel and access resistance, the access resistance is poorly modeled. We present a comprehensive investigation of the access resistance and associated conductance blockade in thin nanopore membranes. By combining a first-principles approach, multiscale modeling, and experimental validation, we propose a unified theoretical modeling framework. The analytical model derived as a result surpasses current approaches across a broad parameter range. Beyond advancing our theoretical understanding, our framework's versatility enables analyte size inference and predictive insights into conductance blockade behavior. Our results will facilitate the design and optimization of nanopore devices for diverse applications, including nanopore base calling and data storage.

Multivariate analysis of nanoparticle translocation through a nanopore to improve the accuracy of resistive pulse sensing

The advent of nanopore-based sensors based on resistive pulse sensing gave rise to a remarkable breakthrough in the detection and characterization of nanoscale species. Some strong correlations have been reported between the resistive pulse characteristics and the particle's geometrical and physical properties. These correlations are commonly used to obtain information about the particles in commercial devices and research setups. The correlations, however, do not consider the simultaneous effect of influential factors such as particle shape and off-axis translocation, which complicates the extraction of accurate information from the resistive pulses. In this paper, we numerically studied the impact of the shape and position of particles on pulse characteristics in order to estimate the errors that arise from neglecting the influence of multiple factors on resistive pulses. We considered the sphere, oblate, and prolate particles to investigate the nanoparticle shape effect. Moreover, the trajectory dependency was examined by considering the translocation of nanoparticles away from the nanopore axis. Meanwhile, the shape effect was studied for different trajectories. We observed that the simultaneous effects of influential parameters could lead to significant errors in estimating particle properties if the coupled effects are neglected. Based on the results, we introduce the "pulse waveshape" as a novel characteristic of the resistive pulse that can be utilized as a decoupling parameter in the analysis of resistive pulses.

High-precision rapid testing of omicron SARS-CoV-2 variants in clinical samples using AI-nanopore

A digital platform that can rapidly and accurately diagnose pathogenic viral variants, including SARS-CoV-2, will minimize pandemics, public anxiety, and economic losses. We recently reported an artificial intelligence (AI)-nanopore platform that enables testing for Wuhan SARS-CoV-2 with high sensitivity and specificity within five minutes. However, which parts of the virus are recognized by the platform are unknown. Similarly, whether the platform can detect SARS-CoV-2 variants or the presence of the virus in clinical samples needs further study. Here, we demonstrated the platform can distinguish SARS-CoV-2 variants. Further, it identified mutated Wuhan SARS-CoV-2 expressing spike proteins of the delta and omicron variants, indicating it discriminates spike proteins. Finally, we used the platform to identify omicron variants with a sensitivity and specificity of 100% and 94%, respectively, in saliva specimens from COVID-19 patients. Thus, our results demonstrate the AI-nanopore platform is an effective diagnostic tool for SARS-CoV-2 variants.

Aβ42 fibril and non-fibril oligomers characterization using a nanopipette

The Aβ42 aggregates with different structures and morphology was investigated through a single molecule label-free technique. To this end, the quartz nanopipettes were functionalized with polyethylene glycol. The set of Aβ42- epigallocatechin-3-gallate fibrils with length (from 85 nm to 250 nm) obtained by sonication was detected. The comparison of experimental and computed value of the amplitude of relative current blockade using a geometrical model show that for fibrils longer than 80 nm, the discriminating parameter is their diameter. Then, non-fibril oligomers obtain from Aβ42(Osaka) aggregation at different time seed was investigated. The analysis of the amplitude of relative current blockade shows that detected oligomers are smaller than 30 nm regardless the aggregation time. In addition, the wide distributions of the dwell time suggests the polymorph character of the sample.