Electroosmosis-Driven Nanofluidic Diodes

Nanofluidic diodes based on asymmetric bio-inspired surface coatings in straight glass nanochannels

In this study, we present nanofluidic diodes fabricated from straight glass nanochannels and functionalized using bio-inspired polydopamine (PDA) and poly-L-lysine (PLL) coatings. The resulting PDA coatings are shown to be asymmetric due to a combination of transport considerations which can be leveraged to provide a measure of control over the effective channel geometry. By subsequently introducing a layer of amine-bearing PLL chains covalently bound to the PDA, we enhance heterogeneities in the charge and ion distributions within the channel and enable significant current rectification between forward-bias and reverse-bias modes; our PDA-PLL-coated channels yielded a rectification ratio greater than 1000 in a 100 nm channel filled with 0.01× phosphate-buffered saline solution (PBS). We further demonstrated that at higher ionic strength conditions, reducing the solution pH increased the number of protonated amines within the PLL layer, amplifying the charge disparities along the channel and leading to greater rectification. As nanofluidic diodes with bipolar surface charge distributions tend to provide superior performance compared to those with a single wall charge polarity, we imposed a more bipolar charge distribution in our devices by partially coating our PDA-PLL-coated channels with negatively charged polyacrylic acid (PAA). These enhanced bipolar channels exhibited greater current rectification than the PDA-PLL-coated channels, reaching rectification ratios in excess of 100 even in more physiologically-relevant 1× PBS solutions. Our fabrication approach and the results herein provide a promising platform from which the scientific community can build upon in the relentless endeavor for improved sensitivity in biosensors and other analytical devices.

Regulating Nonlinear Ion Transport through a Solid-State Pore by Partial Surface Coatings

Using functional nanofluidic devices to manipulate ion transport allows us to explore the nanoscale development of blue energy harvesters and iontronic building blocks. Herein, we report on a method to alter the nonlinear ionic current through a pore by partial dielectric coatings. A variety of dielectric materials are examined on both the inner and outer surfaces of the channel with four different patterns of coated or uncoated surfaces. Through controlling the specific part of the surface charge, the pore can behave like a resistor, diode, and bipolar junction transistor. We use numerical simulations to find out the reason for the asymmetric ion transport in the pore and illustrate the relationship between specifically charged surfaces and electroosmotic flow. These findings help understand the role of the corresponding surface composition in ion transport, which provides a direct approach to modify the electroosmotic-flow-driven ionic current rectification in the channel-based device via dielectric coatings.

Reversal of Electroosmotic Flow in Charged Nanopores with Multivalent Electrolyte

We study the reversal of electroosmotic flow in charged cylindrical nanopores containing multivalent electrolyte. Dissipative particle dynamics is used to simulate the hydrodynamics of the electroosmotic flow. The electrostatic interactions are treated using 3D Ewald summation, corrected for a pseudo-one-dimensional geometry of the pore. We observe that, for sufficiently large surface charge density, condensation of multivalent counterions leads to the reversal of the pore's surface charge. This results in the reversal of electroosmotic flow. Our simulations show that the Smoluchowski equation is able to quantitatively account for the electroosmotic flow through the nanopore, if the shear plane is shifted from the position of the Stern contact surface.

Ionic heat dissipation in solid-state pores

Energy dissipation in solid-state nanopores is an important issue for their use as a sensor for detecting and analyzing individual objects in electrolyte solution by ionic current measurements. Here, we report on evaluations of heating via diffusive ion transport in the nanoscale conduits using thermocouple-embedded SiN_x pores. We found a linear rise in the nanopore temperature with the input electrical power suggestive of steady-state ionic heat dissipation in the confined nanospace. Meanwhile, the heating efficiency was elucidated to become higher in a smaller pore due to a rapid decrease in the through-water thermal conduction for cooling the fluidic channel. The scaling law suggested nonnegligible influence of the heating to raise the temperature of single-nanometer two-dimensional nanopores by a few kelvins under the standard cross-membrane voltage and ionic strength conditions. The present findings may be useful in advancing our understanding of ion and mass transport phenomena in nanopores.

Salt Gradient Control of Translocation Dynamics in a Solid-State Nanopore

Tuning capture rates and translocation time of analytes in solid-state nanopores are one of the major challenges for their use in detecting and analyzing individual nanoscale objects via ionic current measurements. Here, we report on the use of salt gradient for the fine control of capture-to-translocation dynamics in 300 nm sized SiN_x nanopores. We demonstrated a decrease up to a factor of 3 in the electrophoretic speed of nanoparticles at the pore exit along with an over 3-fold increase in particle detection efficiency by subjecting a 5-fold ion concentration difference across the dielectric membrane. The improvement in the sensor performance was elucidated to be a result of the salt-gradient-mediated electric field and electroosmotic flow asymmetry at nanochannel orifices. The present findings can be used to enhance nanopore sensing capability for detecting biomolecules such as amyloids and proteins.

Electroosmotic Flow in Polarizable Charged Cylindrical Nanopores

We present a simulation method to study electroosmotic flow in charged nanopores with dielectric contrast between their interior and the surrounding medium. To perform simulations, we separate the electrostatic energy into the direct Coulomb and the polarization contributions. The polarization part is obtained using periodic Green functions and can be expressed as a sum of fast converging modified Bessel functions. On the other hand, the direct Coulomb part of the electrostatic energy is calculated using fast converging three-dimensional (3D) Ewald summation method, corrected for a pseudo one-dimensional (1D) geometry. The effects of polarization are found to be particularly important for systems with multivalent counterions and narrow nanopores. Depending on the surface charge density, polarization can increase the volumetric flow rate by 200%. For systems with 3:1 electrolyte, we observe that there is a saturation of the volumetric flow rate. In this case, for polarizable pores, the flow rate is 100% higher than for nonpolarizable pores.

Electroosmotic Flow Hysteresis for Fluids with Dissimilar pH and Ionic Species

Electroosmotic flow (EOF) involving displacement of multiple fluids is employed in micro-/nanofluidic applications. There are existing investigations on EOF hysteresis, i.e., flow direction-dependent behavior. However, none so far have studied the solution pair system of dissimilar ionic species with substantial pH difference. They exhibit complicated hysteretic phenomena. In this study, we investigate the EOF of sodium bicarbonate (NaHCO₃, alkaline) and sodium chloride (NaCl, slightly acidic) solution pair via current monitoring technique. A developed slip velocity model with a modified wall condition is implemented with finite element simulations. Quantitative agreements between experimental and simulation results are obtained. Concentration evolutions of NaHCO₃-NaCl follow the dissimilar anion species system. When NaCl displaces NaHCO₃, EOF reduces due to the displacement of NaHCO₃ with high pH (high absolute zeta potential). Consequently, NaCl is not fully displaced into the microchannel. When NaHCO₃ displaces NaCl, NaHCO₃ cannot displace into the microchannel as NaCl with low pH (low absolute zeta potential) produces slow EOF. These behaviors are independent of the applied electric field. However, complete displacement tends to be achieved by lowering the NaCl concentration, i.e., increasing its zeta potential. In contrast, the NaHCO₃ concentration has little impact on the displacement process. These findings enhance the understanding of EOF involving solutions with dissimilar pH and ion species.