Polarization of disk electrodes in high-conductivity electrolyte solutions.

We investigate the polarization of disk electrodes immersed in an electrolyte solution and subjected to a small external AC voltage over a wide range of frequencies. A mathematical model is developed based on the Debye-Falkenhagen approximation to the coupled Poisson-Nernst-Planck equations. Analytical techniques are used for predicting the spatial distribution of the electric potential and the complex impedance of the system. Scales for impedance and frequency are identified, which lead to a self-similar behavior for a range of frequencies. Experiments are conducted with gold electrodes of sizes in the range 100-350 µm immersed in a high-conductivity KCl solution over five orders of magnitude in frequency. A collapse of data on impedance magnitude and phase angle onto universal curves is observed with scalings motivated by the mathematical model. A direct comparison with the approximate analytical formula for impedance is made without any fitting parameters, and a good agreement is found for the range of frequencies where the analytical model is valid.

Nanoscale Meniscus Dynamics in Evaporating Thin Films: Insights from Molecular Dynamics Simulations.

Evaporation studies are focused on unraveling heat transfer and flow dynamics near the solid-liquid-vapor contact line, particularly focusing on the meniscus, which encompasses the nonevaporating adsorbed layer, thin-film, and bulk meniscus regions. Continuum models assume that there are no evaporating adsorbed layers due to the strong intermolecular forces. However, recent molecular dynamics (MD) simulations have unveiled the significant role of adsorbed layers in thin-film evaporation. Leveraging a recently published energy-based interface detection method, the current study presents nonequilibrium MD simulation results for thin-film evaporation from a phase-change-driven nanopump using liquid argon confined between parallel platinum plates. Notably, unlike the transient simulations often encountered in the literature, the simulation system achieves a statistically steady transport. In this context, we showcase the shapes of the evaporating menisci for two distinct channel heights, 8 and 16 nm, and elucidate the underlying flow physics through velocity vectors and temperature contours. This comprehensive investigation advances our understanding of thin-film evaporation and its mechanisms, offering insights that span from nanoscale phenomena to broader thermal management applications.

Changes in Electrical Capacitance of Cell Membrane Reflect Drug Partitioning-Induced Alterations in Lipid Bilayer.

The plasma membrane is a lipid bilayer that establishes the outer boundary of a living cell. The composition of the lipid bilayer influences the membrane's biophysical properties, including fluidity, thickness, permeability, phase behavior, charge, elasticity, and formation of flat sheet or curved structures. Changes in the biophysical properties of the membrane can be occasioned when new entities, such as drug molecules, are partitioned in the bilayer. Therefore, assessing drugs for their effect on the biophysical properties of the lipid bilayer of a cell membrane is critical to understanding specific and non-specific drug action. Previously, we reported a non-invasive technique for real-time characterization of cellular dielectric properties, such as membrane capacitance and cytoplasmic conductivity. In this study, we discuss the potential application of the technique in assessing the biophysical properties of the cell membrane in response to interaction with amiodarone compared to aspirin/acetylsalicylic acid and glucose. Amiodarone is a potent drug used to treat cardiac arrhythmia, but it also exerts various non-specific effects. Compared to aspirin and glucose, we measured a rapid and higher magnitude increase in membrane capacitance on cells under amiodarone treatment. Increased membrane capacitance induced by aspirin and glucose quickly returned to baseline in 15 s, while amiodarone-induced increased capacitance sustained and decreased slowly, approaching baseline or another asymptotic limit in ~2.5 h. Because amiodarone has a strong lipid partitioning property, we reason that drug partitioning alters the lipid bilayer context and subsequently reduces bilayer thickness, leading to an increase in the electrical capacitance of the cell membrane. The presented microfluidic system promises a new approach to assess drug-membrane interactions and delineate specific and non-specific actions of the drug on cells.

Impedance-Based Neutralizing Antibody Detection Biosensor with Application in SARS-CoV-2 Infection.

Although safe and efficacious coronavirus disease-2019 (COVID-19) vaccines are available, real protective immunity is revealed by the serum COVID-19 neutralizing antibody (NAb) concentration. NAbs deactivate the virus by attaching to the viral receptor-binding domain (RBD), which interacts with angiotensin-converting enzyme 2 (ACE2) on the human cell. This paper introduces inexpensive, rapid, sensitive, and quantifiable impedance-based immunosensors to evaluate the NAb. The sensor limit of detection is experimentally determined in different buffer dilutions using bovine IgG-anti-bovine IgG interaction. The dominance of AC electrokinetic transport and molecular diffusion in the sensor is investigated using scaling analysis and numerical simulations. The results demonstrated that the sensor detection mechanism is mainly based on the diffusion of the biomolecules onto the electrode surface. After evaluating the sensor working principles, viral RBD buffers, including different NAb concentrations, are applied to the sensor, immobilized with the human ACE2 (hACE2). Results demonstrate that the sensor is capable of NAb detection in the analytical measuring interval between 45 ng/mL and 185 ng/mL. Since the present sensor provides fast test results with lower costs, it can be used to assess the NAb in people's blood serum before receiving further COVID vaccine doses.

DC-electrokinetic motion of colloidal cylinder(s) in the vicinity of a conducting wall.

The boundary effects on DC-electrokinetic behavior of colloidal cylinder(s) in the vicinity of a conducting wall is investigated through a computational model. The contribution of the hydrodynamic drag, gravity, electrokinetic (i.e., electrophoretic and dielectrophoretic), and colloidal forces (i.e., forces due to the electrical double layer and van der Waals interactions) are incorporated in the model. The contribution of electrokinetic and colloidal forces are included by introducing the resulting forces as an external force acting on the particle(s). The colloidal forces are implemented with the prescribed expressions from the literature, and the electrokinetic force is obtained by integrating the corresponding Maxwell stress tensor over the particles' surfaces. The electrokinetic slip-velocity together with the thin electrical double layer assumption is applied on the surfaces. The position and velocity of the particles and the resulting electric and flow fields are obtained and the physical insight for the behavior of the colloidal cylinders are discussed in conjunction with the experimental observations in the literature.

A Microfluidic Dielectric Spectroscopy System for Characterization of Biological Cells in Physiological Media.

Dielectric spectroscopy (DS) is a promising cell screening method that can be used for diagnostic and drug discovery purposes. The primary challenge of using DS in physiological buffers is the electrode polarization (EP) that overwhelms the impedance signal within a large frequency range. These effects further amplify with the miniaturization of the measurement electrodes. In this study, we present a microfluidic system and the associated equivalent circuit models for real-time measurements of cell membrane capacitance and cytoplasm resistance in physiological buffers with 10 s increments. The current device captures several hundreds of biological cells in individual microwells through gravitational settling and measures the system's impedance using microelectrodes covered with dendritic gold nanostructures. Using PC-3 cells (a highly metastatic prostate cancer cell line) suspended in cell growth media (CGM), we demonstrate stable measurements of cell membrane capacitance and cytoplasm resistance in the device for over 15 min. We also describe a consistent application of the equivalent circuit model, starting from the reference measurements used to determine the system parameters. The circuit model is tested using devices with varying dimensions, and the obtained cell parameters between different devices are nearly identical. Further analyses of the impedance data have shown that accurate cell membrane capacitance and cytoplasm resistance can be extracted using a limited number of measurements in the 5 MHz to 10 MHz range. This will potentially reduce the timescale required for real-time DS measurements below 1 s. Overall, the new microfluidic device can be used for the dielectric characterization of biological cells in physiological buffers for various cell screening applications.

Energy-Based Interface Detection for Phase Change Processes of Monatomic Fluids in Nanoconfinements.

An energy-based liquid-vapor interface detection method is presented using molecular dynamics simulations of liquid menisci confined between two parallel plates under equilibrium and evaporation/condensation conditions. This method defines the liquid-vapor interface at the location where the average kinetic energy of atoms first exceeds the average potential energy imposed by all neighboring molecules. This definition naturally adapts to the location of the menisci relative to the walls and can properly model the behavior of the liquid adsorbed layers. Unlike the density cutoff methods frequently used in the literature that suffer from density layering effects, this new method gives smooth and continuous liquid-vapor interfaces in nanoconfinements. Surface tension values calculated from the equilibrium MD simulations match the Young-Laplace equation better when using the radius of curvatures calculated from this method. Overall, this energy-based liquid-vapor interface detection method can be used in studies of nanoscale phase change processes and other relevant applications.

Rapid and Sensitive Detection of Nanomolecules by an AC Electrothermal Flow Facilitated Impedance Immunosensor.

Conventional immunosensors typically rely on passive diffusion dominated transport of analytes for binding reaction and hence, it is limited by low sensitivity and long detection times. We report a simple and efficient impedance sensing method that can be utilized to overcome both sensitivity and diffusion limitations of immunosensors. This method incorporates the structural advantage of nanorod-covered interdigitated electrodes and the microstirring effect of AC electrothermal flow (ACET) with impedance spectroscopy. ACET flow induced by a biased AC electric field can rapidly convect the analyte onto nanorod structured electrodes within a few seconds and enriches the number of binding molecules because of the excessive effective surface area. We performed numerical simulations to investigate the effect of ACET flow on the biosensor performance. The results indicated that AC bias to the side electrodes could induce fast convective flow, which facilitates the transport of the target molecules to the binding region located in the middle as a floating electrode. The temperature rise due to the Joule heating effect was measured using a thermoreflectance imaging method to find the optimum device operation conditions. The change of impedance caused by the receptors-target molecules binding at the sample/electrode interface was experimentally measured and quantified in real-time using the impedance spectroscopy technique. We observed that the impedance sensing method exhibited extremely fast response compared with those under no bias conditions. The measured impedance change can reach saturation in a minute. Compared to the conventional incubation method, the ACET flow enhanced method is faster in its reaction time, and the detection limit can be reduced to 1 ng/mL. In this work, we demonstrate that this sensor technology is promising and reliable for rapid, sensitive, and real-time monitoring of biomolecules in biologically relevant media such as blood, urine, and saliva.

The role of water models on the prediction of slip length of water in graphene nanochannels.

Slip lengths reported from molecular dynamics (MD) simulations of water flow in graphene nanochannels show significant scatter in the literature. These discrepancies are in part due to the used water models. We demonstrate self-consistent comparisons of slip characteristics between the SPC, SPC/E, SPC/Fw, TIP3P, TIP4P, and TIP4P/2005 water models. The slip lengths are inferred using an analytical model that employs the shear viscosity of water and channel average velocities obtained from nonequilibrium MD simulations. First, viscosities for each water model are quantified using MD simulations of counterflowing, force-driven flows in periodic domains in the absence of physical walls. While the TIP4P/2005 model predicts water viscosity at the specified thermodynamic state with 1.7% error, the predictions of SPC/Fw and SPC/E models exhibit 13.9% and 23.1% deviations, respectively. Water viscosities obtained from SPC, TIP4P, and TIP3P models show larger deviations. Next, force-driven water flows in rigid (cold) and thermally vibrating (thermal) graphene nanochannels are simulated, resulting in pluglike velocity profiles. Large differences in the flow velocities are observed depending on the used water model and to a lesser extent on the choice of rigid vs thermal walls. Depending on the water model, the slip length of water on cold graphene walls varied between 34.2 nm and 62.9 nm, while the slip lengths of water on thermal graphene walls varied in the range of 38.1 nm-84.3 nm.

Characterization of Temperature Rise in Alternating Current Electrothermal Flow Using Thermoreflectance Method.

Alternating current electrothermal flow (ACET) induced by Joule heating is utilized to transport biologically relevant liquids in microchannels using simple electrode designs. However, Joule heating may cause significant temperature rises, which can degrade biological species, and hence, ACET may become impractical for biomicrofluidic sensors and other possible applications. In this study, the temperature rise at the electrode/electrolyte interface during ACET flow is measured using a high-resolution, noninvasive, thermoreflectance imaging method, which is generally utilized in microelectronics thermal imaging applications. The experimental findings reveal that Joule heating could result in an excessive temperature rise, exceeding 50 °C at higher voltage levels (20 Vpp). The measured data are compared with the results of the enhanced ACET theoretical model, which predicts the temperature rise accurately, even at high levels of applied voltages. Overall, our study provides a temperature measurement technique that is used for the first time for electrode/electrolyte systems. The reported results are critical in designing biomicrofluidic systems with significant energy dissipation in conductive fluids.

Self-Similar Response of Electrode Polarization for Binary Electrolytes in Parallel Plate Capacitor Systems.

Classical electrochemistry problem of polarization of an electrode immersed in a symmetric binary electrolyte and subjected to a small external ac voltage is revisited. The Nernst-Planck equations are simplified to the Debye-Falkenhagen equation, which is solved together with the Poisson equation, leading to analytical formulas for the space charge density and impedance of the system for two parallel plate electrodes. We then define a limit of thin electrical double layer and illustrate the emergence of the characteristic time scale, τc = λDL/D, a function of the Debye length, λD, the electrode separation distance, L, and the ionic diffusion coefficient D. Normalizing the impedance magnitude with the solution resistance and making the frequency dimensionless with the τc, we show that all analytical, numerical, and experimental data for different solution conductivities and electrode separation distances collapse onto a single curve. To account for the Stern layer effects, we conducted numerical simulations based on the modified Poisson-Nernst-Planck model and showed that the results agree with our analytical solution for a range of concentrations, with small discrepancies observed only above 0.1 M. Based on the proposed model, experimental impedance spectroscopy results at ac potentials can be used to obtain detailed knowledge of the corresponding surface (and space) charge densities on the electrodes.

Charged nanoporous graphene membranes for water desalination.

Water desalination using positively and negatively charged single-layer nanoporous graphene membranes are investigated using molecular dynamics (MD) simulations. Pressure-driven flows are induced by the motion of specular reflection boundaries with a constant speed, resulting in a prescribed volumetric flow rate. Simulations are performed for 14.40 Å hydraulic pore diameter membrane with four different electric charges distributed on the pore edges. Salt rejection efficiencies and the resulting pressure drops are compared with the previously obtained base-line case of 9.9 Å diameter pristine nanoporous graphene membrane, which exhibits 100% salt rejection with 35.02 MPa pressure drop at the same flow rate. Among the positively charged cases, q = 9e shows 100% and 98% rejection for Na+ and Cl- ions respectively, with 35% lower pressure drop than the reference. For negatively charged pores, optimum rejection efficiencies of 94% and 93% are obtained for Na+ and Cl- ions for the q = -6e case, which requires 60.6% less pressure drop than the reference. The results indicate the high potential of using charged nanoporous graphene membranes in reverse osmosis (RO) desalination systems with enhanced performance.

Atomic Scale Interfacial Transport at an Extended Evaporating Meniscus.

Recent developments in fabrication techniques have enabled the production of nano- and Ångström-scale conduits. While scientists are able to conduct experimental studies to demonstrate extreme evaporation rates from these capillaries, theoretical modeling of evaporation from a few nanometers or sub-nanometer meniscus interfaces, where the adsorbed film, the transition film, and the intrinsic region are intertwined, is absent in the literature. Using the computational setup constructed, we first identified the detailed profile of a nanoscale evaporating interface and then discovered the existence of lateral momentum transport within and associated net evaporation from adsorbed liquid layers, which are long believed to be at the equilibrium established between equal rates of evaporation and condensation. Contribution of evaporation from the adsorbed layer increases the effective evaporation area, reducing the excessively estimated evaporation flux values. This work takes the first step toward a comprehensive understanding of atomic/molecular scale interfacial transport at extended evaporating menisci. The modeling strategy used in this study opens an opportunity for computational experimentation of steady-state evaporation and condensation at liquid-vapor interfaces located in capillary nanoconduits.

Quantification of Cell Death Using an Impedance-Based Microfluidic Device.

Dielectric spectroscopy is a nondestructive method to characterize dielectric properties by measuring impedance data over a frequency spectrum. This method has been widely used for various applications such as counting, sizing, and monitoring biological cells and particles. Recently, utilization of this method has been suggested in various stages of the drug discovery process due to low sample consumption and fast analysis time. In this study, we used a previously developed microfluidic system to confine single PC-3 cells in microwells using dielectrophoretic forces and perform the impedance measurements. PC-3 cells are treated with 100 µM Enzalutamide drug, and their impedance response is recorded until the cells are totally dead as predicted with viability tests. Four different approaches are used to analyze the impedance spectrum. Equivalent circuit modeling is used to extract the cell electrical properties as a function of time. Principal component analysis (PCA) is used to quantify cellular response to drug as a function of time. Single frequency measurements are conducted to observe how the cells respond over time. Finally, opacity ratio is defined as an additional quantification method. This device is capable of quantitatively measuring drug effects on biological cells and detecting cell death. The results show that the proposed microfluidic system has the potential to be used in early stages of the drug discovery process.

Gold nanostructure microelectrode arrays for in vitro recording and stimulation from neuronal networks.

An ideal microelectrode array (MEA) design should include materials and structures which exhibit biocompatibility, low electrode polarization, low impedance/noise, and structural durability. Here, the fabrication of MEAs with indium tin oxide (ITO) electrodes deposited with self-similar gold nanostructures (GNS) is described. We show that fern leaf fractal-like GNS deposited on ITO electrodes are conducive for neural cell attachment and viability while reducing the interfacial impedance more than two orders of magnitude at low frequencies (100-1000 Hz) versus bare ITO. GNS MEAs, with low interfacial impedance, allowed the detection of extracellular action potentials with excellent signal-to-noise ratios (SNR, 20.26 ± 2.14). Additionally, the modified electrodes demonstrated electrochemical and mechanical stability over 29 d in vitro.

Self-Similar Interfacial Impedance of Electrodes in High Conductivity Media: II. Disk Electrodes.

Electrode polarization effects were investigated using impedance spectroscopy measurements for planar and nanorod-structured gold disk electrodes at 100 Hz to 1 MHz frequency range and in 0.25 S/m to 1.5 S/m conductivity KCl solutions. Diameters of planar electrodes were varied from 50 µm to 2 mm to examine the effect of electrode size on impedance spectra. Normalizing the impedance magnitude with the spreading resistance and frequency with the characteristic time scale, all experimental data collapsed onto a universal curve, proving self-similarity. Experimental impedance results were compared well with that obtained from the numerical solution of Poisson-Nernst-Planck equations in axisymmetric domain. The influence of surface morphology was also investigated by generating cylindrical nanorods on a planar electrode. The 500 µm diameter electrode surface was covered with cylindrical nanorods with known height, diameter, and separation distance, which were characterized using scanning electron microscopy. The characteristic time scale for the nanorod-structured electrode increased by the surface enlargement factor obtained by cyclic voltammetry measurements. Self-similar interfacial impedance of electrodes was modeled using a constant phase element model. Current findings describe the coupled effects of electrode diameter, electrolyte conductivity, and electrode surface morphology on the impedance spectra of electrode/electrolyte system when the electric double layer between the nanorods does not overlap.

Saltwater transport through pristine and positively charged graphene membranes.

Transport of saltwater through pristine and positively charged single-layer graphene nanoporous membranes is investigated using molecular dynamics simulations. Pressure-driven flows are induced by motion of specular reflecting boundaries at feed and permeate sides with constant speed. Unlike previous studies in the literature, this method induces a desired flow rate and calculates the resulting pressure difference in the reservoirs. Due to the hexagonal structure of graphene, the hydraulic diameters of nano-pores are used to correlate flow rate and pressure drop data. Simulations are performed for three different pore sizes and flow rates for the pristine and charged membrane cases. In order to create better statistical averages for salt rejection rates, ten different initial conditions of Na+ and Cl- distribution in the feed side are used for each simulation case. Using data from 180 distinct simulation cases and utilizing the Buckingham Pi theorem, we develop a functional relationship between the volumetric flow rate, pressure drop, pore diameter, and the dynamic viscosity of saltwater. A linear relationship between the volumetric flow rate and pressure drop is observed. For the same flow rate and pore size, charged membranes exhibit larger pressure drops. Graphene membranes with 9.90 Å pore diameter results in 100% salt rejection with 163.2 l/h cm2 water flux, requiring a pressure drop of 35.02 MPa.

Electrical Impedance Measurements of Biological Cells in Response to External Stimuli.

Dielectric spectroscopy (DS) is a noninvasive technique for real-time measurements of the impedance spectra of biological cells. DS enables characterization of cellular dielectric properties such as membrane capacitance and cytoplasmic conductivity. We have developed a lab-on-a-chip device that uses an electro-activated microwells array for capturing, DS measurements, and unloading of biological cells. Impedance measurements were conducted at 0.2 V in the 10 kHz to 40 MHz range with 6 s time resolution. An equivalent circuit model was developed to extract the cell membrane capacitance and cell cytoplasmic conductivity from the impedance spectra. A human prostate cancer cell line, PC-3, was used to evaluate the device performance. Suspension of PC-3 cells in low conductivity buffers (LCB) enhanced their dielectrophoretic trapping and impedance response. We report the time course of the variations in dielectric properties of PC-3 cells suspended in LCB and their response to sudden pH change from a pH of 7.3 to a pH of 5.8. Importantly, we demonstrated that our device enabled real-time measurements of dielectric properties of live cancer cells and allowed the assessment of the cellular response to variations in buffer conductivity and pH. These data support further development of this device toward single cell measurements.

Electric field controlled transport of water in graphene nano-channels.

Motivated by electrowetting-based flow control in nano-systems, water transport in graphene nano-channels is investigated as a function of the applied electric field. Molecular dynamics simulations are performed for deionized water confined in graphene nano-channels subjected to opposing surface charges, creating an electric field across the channel. Water molecules respond to the electric field by reorientation of their dipoles. Oxygen and hydrogen atoms in water face the anode and cathode, respectively, and hydrogen atoms get closer to the cathode compared to the oxygen atoms near the anode. These effects create asymmetric density distributions that increase with the applied electric field. Force-driven water flows under electric fields exhibit asymmetric velocity profiles and unequal slip lengths. Apparent viscosity of water increases and the slip length decreases with increased electric field, reducing the flow rate. Increasing the electric field above a threshold value freezes water at room temperature.

Self-Similar Interfacial Impedance of Electrodes in High Conductivity Media.

Electrode polarization (EP) happening due to accumulation of ions at the electrode/electrolyte interface is an inevitable phenomenon while measuring impedance spectrum in high conductivity buffers and at low RF spectrum. Well-characterized time scales elucidating the EP effect are important for the rational design of microfluidic devices and impedance sensors. In this Article, interfacial impedance at the electrode/electrolyte interface is investigated considering channel height and Debye length effects on characteristic time scale in a binary electrolyte solution using parallel plate electrode configuration. Experimental results reveal self-similarity of normalized electrical impedance as a function of the normalized frequency. The experimental results also match with numerical solutions obtained by finite element simulation of unsteady fully coupled Poisson-Nernst-Planck (PNP) equations. Furthermore, fractal shaped gold nanostructured electrodes are examined, and it has been proven that electric double layer (EDL) formed on porous electrode surfaces acts as a thick EDL and modifications to the characteristic time scale is necessary for porous electrodes. Finally, a constant phase element (CPE) model is proposed to account for the self-similar impedance spectrum, which can be used for different channel heights and solution conductivities.