A hybrid resistive pulse-optical detection platform for microfluidic experiments.

Resistive-pulse sensing is a label-free method for characterizing individual particles as they pass through ion-conducting channels or pores. During a resistive pulse experiment, the ionic current through a conducting channel is monitored as particles suspended in the solution translocate through the channel. The amplitude of the current decrease during a translocation, or 'pulse', depends not only on the ratio of the particle and channel sizes, but also on the particle position, which is difficult to resolve with the resistive pulse signal alone. We present experiments of simultaneous electrical and optical detection of particles passing through microfluidic channels to resolve the positional dependencies of the resistive pulses. Particles were tracked simultaneously in the two signals to create a mapping of the particle position to resistive pulse amplitude at the same instant in time. The hybrid approach will improve the accuracy of object characterization and will pave the way for observing dynamic changes of the objects such as deformation or change in orientation. This combined approach of optical detection and resistive pulse sensing will join with other attempts at hybridizing high-throughput detection techniques such as imaging flow cytometry.

Polarization of Gold in Nanopores Leads to Ion Current Rectification.

Biomimetic nanopores with rectifying properties are relevant components of ionic switches, ionic circuits, and biological sensors. Rectification indicates that currents for voltages of one polarity are higher than currents for voltages of the opposite polarity. Ion current rectification requires the presence of surface charges on the pore walls, achieved either by the attachment of charged groups or in multielectrode systems by applying voltage to integrated gate electrodes. Here we present a simpler concept for introducing surface charges via polarization of a thin layer of Au present at one entrance of a silicon nitride nanopore. In an electric field applied by two electrodes placed in bulk solution on both sides of the membrane, the Au layer polarizes such that excess positive charge locally concentrates at one end and negative charge concentrates at the other end. Consequently, a junction is formed between zones with enhanced anion and cation concentrations in the solution adjacent to the Au layer. This bipolar double layer together with enhanced cation concentration in a negatively charged silicon nitride nanopore leads to voltage-controlled surface-charge patterns and ion current rectification. The experimental findings are supported by numerical modeling that confirm modulation of ionic concentrations by the Au layer and ion current rectification even in low-aspect ratio nanopores. Our findings enable a new strategy for creating ionic circuits with diodes and transistors.

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

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.

Role of Particle Focusing in Resistive-Pulse Technique: Direction-Dependent Velocity in Micropores.

Passage time through single micropores is an important parameter used to quantify the surface charge and zeta potential of particles. In the resistive-pulse technique, the measured time of pressure- or electric-field-induced translocation is assumed to be direction independent. This assumption is supported by the low velocities of the particles and the supporting fluid such that the transport reversibility known for Stokes flow is expected to apply. In this article, we present examples of micropores in which passage time of ∼400 nm diameter particles becomes direction-dependent; that is, the particles' 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. The reported pores present the simplest system in which time-broken symmetry has been observed. The results are of importance for sensing of particles and molecules by the resistive-pulse technique since pores used for detection are often characterized by finite roughness or noncylindrical shape. This article also points to the role of particle focusing in the magnitude and distribution of the translocation times.

Anomalous mobility of highly charged particles in pores.

Single micropores in resistive-pulse technique were used to understand a complex dependence of particle mobility on its surface charge density. We show that the mobility of highly charged carboxylated 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. The results are important for applying resistive-pulse technique to determine surface charge density and zeta potential of the particles. The experiments also indicate the presence of condensed ions, which contribute to the measured current if a sufficiently high electric field is applied across the pore.

Pores with longitudinal irregularities distinguish objects by shape.

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.