We study the effect of electrolyte concentration on the shape of ion current pulses in resistive-pulse sensing. We show that electrokinetic passage of several hundred nanometers in diameter charged polystyrene particles through a micropore leads to formation of current increase when the particles exit the pore. The particle entrance, as reported before, causes formation of the current decrease, which is a measure of the particle size. Formation of the double peak, i.e., current decrease followed by a current increase, is especially pronounced if the resistive-pulse experiments are carried out in KCl concentrations below 200 mM. In order to explain the pulse shape, experiments were designed in which the particles passed through the pore only by either electroosmosis or electrophoresis. The presented experiments and modeling indicate that while both electroosmosis and electrophoresis affect the ion current pulse, formation of the positive peak is mainly determined by the latter effect and the charged state of the particle. The importance of the findings for resistive-pulse analysis is discussed.

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Summary Intrinsically photosensitive retinal ganglion cells (ipRGCs) combine directly photosensitivivity through melanopsin with synaptically-mediated drive from classical photoreceptors through bipolar-cell input. Here, we sought to provide a fuller description of the least understood ipRGC type, the M5 cell, and discovered a distinctive functional characteristic — chromatic opponency (ultraviolet excitatory, green inhibitory). Serial electron microscopic reconstructions revealed that M5 cells receive selective UV-opsin drive from Type 9 cone bipolar cells, but also mixed cone signals from bipolar Types 6, 7 and 8. Recordings suggest that both excitation and inhibition are driven by the ON channel, and that chromatic opponency results from M-cone-driven surround inhibition mediated by wide-field spiking GABAergic amacrine cells. We show that M5 cells send axons to the dLGN, and are thus positioned to provide chromatic signals to visual cortex. These findings underscore that melanopsin’s influence extends beyond unconscious reflex functions to encompass cortical vision, perhaps including the perception of color.

In this article, we report detection of deformable, hydrogel particles by the resistive-pulse technique using single pores in a polymer film. The hydrogels pass through the pores by electroosmosis and cause formation of a characteristic shape of resistive pulses indicating the particles underwent dehydration and deformation. These effects were explained via a non-homogeneous pressure distribution along the pore axis modeled by the coupled Poisson-Nernst-Planck and Navier-Stokes equations. The local pressure drops are induced by the electroosmotic fluid flow. Our experiments also revealed the importance of concentration polarization in the detection of hydrogels. Due to the negative charges as well as branched, low-density structure of the hydrogel particles, the concentration of ions in the particles is significantly higher than in the bulk. As a result, when an electric field is applied across the membrane, a depletion zone can be created in the vicinity of the particle observed as a transient drop of the current. Our experiments using pores with openings between 200 and 1600 nm indicated the concentration polarization dominated the hydrogels' detection of pores wider than 450 nm. The results are of importance for all studies that involve transport of molecules, particles, and cells through pores with charged walls. The developed inhomogeneous pressure distribution can potentially influence the shape of the transported species. The concentration polarization changes the interpretation of the resistive pulses; the observed current change does not necessarily reflect only the particle size but also the size of the depletion zone that is formed in the particle vicinity.

The resistive-pulse technique has been used to detect and size objects which pass through a single pore. The amplitude of the ion current change observed when a particle is in the pore is correlated with the particle volume. Up to date, however, the resistive-pulse approach has not been able to distinguish between objects of similar volume but different shapes. In this manuscript, we propose using pores with longitudinal irregularities as a sensitive tool capable of distinguishing spherical and rod-shaped particles with different lengths. The ion current modulations within resulting resistive pulses carry information on the length of passing objects. The performed experiments also indicate the rods rotate while translocating, and displace an effective volume that is larger than their geometrical volume, and which also depends on the pore diameter.

Pores with undulating opening diameters have emerged as an analytical tool enhancing the speed of resistive-pulse experiments, with a potential to simultaneously characterize size and mechanical properties of translocating objects. In this work, we present a detailed study of the characteristics of resistive-pulses of charged and uncharged polymer particles in pores with different aspect ratios and pore topography. Although no external pressure difference was applied, our experiments and modeling indicated the existence of local pressure drops, which modified axial and radial velocities of the solution. As a consequence of the complex velocity profiles, pores with undulating pore diameter and low-aspect ratio exhibited large dispersion of the translocation times. Distribution of the pulse amplitude, which is a measure of the object size, was not significantly affected by the pore topography. The importance of tuning pore geometry for the application in resistive-sensing and multipronged characterization of physical properties of translocating objects is discussed.

In this article we report resistive-pulse experiments with polystyrene particles whose transport through pores is controlled by modulating the driving voltage during the process of translocation. Balancing electric and hydrostatic forces acting on the particles allowed us to observe a random walk of single particles in a pore for tens of seconds and to quantify their diffusion coefficient using two methods. The first approach is based on the mean square displacement and requires passage of multiple particles for a range of diffusion times. The diffusion coefficient of individual particles was determined based on the variance of their local diffusion velocities. The developed methods for measuring the diffusion coefficient in pores are applicable to particles of different sizes, do not require fluorescence labeling, and are entirely based on ion current recordings. In addition, application of a modulating voltage signal together with rising edge triggers enabled transporting the same particle back and forth in the pore without letting the particle leave the pore.

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