Highly Charged Particles Cause a Larger Current Blockage in Micropores Compared to Neutral Particles

Abstract

Single pores in the resistive-pulse technique are used as an analytics tool to detect, size, and characterize physical as well as chemical properties of individual objects such as molecules and particles. Each object passing through a pore causes a transient change of the transmembrane current called a resistive pulse. In high salt concentrations when the pore diameter is significantly larger than the screening Debye length, it is assumed that the particle size and surface charge can be determined independently from the same experiment. In this article we challenge this assumption and show that highly charged hard spheres can cause a significant increase of the resistive-pulse amplitude compared to neutral particles of a similar diameter. As a result, resistive pulses overestimate the size of charged particles by even 20%. The observation is explained by the effect of concentration polarization created across particles in a pore, revealed by numerical modeling of ionic concentrations, ion current, and local electric fields. It is notable that in resistive-pulse experiments with cylindrical pores, concentration polarization was previously shown to influence ionic concentrations only at pore entrances; consequently, additional and transient modulation of resistive pulses was observed when a particle entered or left the pore. Here we postulate that concentration polarization can occur across transported particles at any particle position along the pore axis and affect the magnitude of the entire resistive pulse. Consequently, the recorded resistive pulses of highly charged particles reflect not only the particles' volume but also the size of the depletion zone created in front of the moving particle. Moreover, the modeling identified that the effective surface charge density of particles depended not only on the density of functional groups on the particle but also on the capacitance of the Stern layer. The findings are of crucial importance for sizing particles and characterizing their surface charge properties.

Nanoscale Investigation of Generation 1 PAMAM Dendrimers Interaction with a Protein Nanopore

“…The reader should remember however, that the applicability of this formula is not straightforward. One notable difficulty in applying it to faithfully evaluate the analyte volume from the Δ I block , lies in that for a nanopore shape departed from the cylinder geometry, the current blockade varies with the molecule position inside the nanopore, becoming more prevalent as the cross-sectional domains of the nanopore get smaller 69, 70 , and the net charge on a particle can contribute to the current blockade 71–74 .…”

“…5), at pH = 7. As above, we draw attention on the difficulty to apply the formula above to un-equivocally evaluate the analyte’s volume from the ∆I block 71–74 .…”

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“…found that highly charged particles can cause a larger current blockage when carboxylated polystyrene particles translocated through PET and PC nanopores with diameters of around 1000 nm. [ 239 ] Both experimental and numerical results showed that in the zone in front of the passing particles, an ion depletion occurs contributing to current blockage. So, the current decrease amplitude can be determined by both particle size and the ion depletion zone size.…”

The Design and Characterization of Multifunctional Aptamer Nanopore Sensors

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