The Study Of Particle Transport In Micropores Based On Resistive-Pulse Technique

Since developed in 1950s,the resistive-pulse technique has been widely used to detect biomolecules,viruses,particles and cells,based on its numerous advantages,such as easy conduction,ultrahigh flux,high precision and repeatable detection.The dimensions of measured objects can range from nanoscale to microscale.So,it has many potential applications in DNA sequencing,virus detection,disease prevention and treatment.In order to simulate the detection process of viruses and cells using micropores,a series of synthetic particles measurement have been conducted through resistive-pulse technique using track-etched polymer micropores.Based on our resistive-pulse detection of particles,we can simplify the translocation of bio-particles through pores and find the generally physical pictures.We think our results are of great importance for the bio-particle detection.Through resistive-pulse detection,the size and surface charge property of a particle can be predicted based on the obtained blockade current and duration time when it translocates through the pore.The properties of the particle and pore,which serve as important components in experiments,have great influence on the particle translocation.Focusing on how to further improve the detection accuracy of the particle size and surface charge density,and extend the application of this technique,in this dissertation we have considered the influences of the particle shape,the particle surface charge density and the pore shape on the characteristics of pulse signals caused by particles.We have obtained 5 main findings:1.The influence of the particle shape on its blockade current.Using polymer micropores with longitudinal irregularities,spherical particles and two kinds of rod-shaped particles with different lengths have been detected using resistive-pulse technique.The ion current modulations within resulting resistive pulses carried information on the length of passing objects.Based on the comparison between experimental results and theoretical predictions,the short rods are found to rotate while translocating,and displace an effective volume that is larger than their geometrical volume.So,we have achieved the successful distinguishment between objects of similar volume but different shapes.2.The influence of the particle surface charge density on the duration time of its translocation.Two kinds of particles with different surface properties have been detected in KC1 solutions.The particle speed is found to have a complex dependence on its surface charge density.The mobility of the highly charged 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.Our experiments also indicate the presence of condensed ions on the particle surfaces.3.The influence of the particle surface charge density on its blockade current.From the detection using three kinds of particles with different surface charge properties in pH8 and pH10 solutions,highly charged hard spheres can cause a significant increase of the resistive-pulse amplitude compared to neutral particles of the same diameter.The observation is explained by the concentration polarization created across the passing particle,revealed by numerical modeling of ionic concentrations.Thus,the recorded resistive pulses of highly charged particles reflect not only the particle volume but also the size of the depletion zone created in front of the moving particle.In the solution with a high concentration,the blockade current of highly charged particles shares the same value as the neutral particles.4.The influence of the pore shape on the duration time of its translocation.In cylinder polymer pores,the passage time of～400 nm diameter particles becomes direction dependent,that is,the particle translocation times from left to right and right to left are different.These pores are characterized by an undulating inner diameter such that at least one wider zone called a cavity separates two narrower regions of different lengths.We propose that the observed direction dependence of the translocation velocity is caused by an asymmetric efficiency of particle focusing toward the pore axis,which leads to a direction dependent set of particle trajectories.5.The influence of the pore shape on its blockade current.The amplitude of resistive-pulses in a conical micropore is found to be 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 is predicted.