Compensating for Electrode Polarization in Dielectric Spectroscopy Studies of Colloidal Suspensions: Theoretical Assessment of Existing Methods

Impedance of nanocapacitors from molecular simulations to understand the dynamics of confined electrolytes

Nanoelectrochemical devices have become a promising candidate technology across various applications, including sensing and energy storage, and provide new platforms for studying fundamental properties of electrode/electrolyte interfaces. In this work, we employ constant-potential molecular dynamics simulations to investigate the impedance of gold-aqueous electrolyte nanocapacitors, exploiting a recently introduced fluctuation-dissipation relation. In particular, we relate the frequency-dependent impedance of these nanocapacitors to the complex conductivity of the bulk electrolyte in different regimes, and use this connection to design simple but accurate equivalent circuit models. We show that the electrode/electrolyte interfacial contribution is essentially capacitive and that the electrolyte response is bulk-like even when the interelectrode distance is only a few nanometers, provided that the latter is sufficiently large compared to the Debye screening length. We extensively compare our simulation results with spectroscopy experiments and predictions from analytical theories. In contrast to experiments, direct access in simulations to the ionic and solvent contributions to the polarization allows us to highlight their significant and persistent anticorrelation and to investigate the microscopic origin of the timescales observed in the impedance spectrum. This work opens avenues for the molecular interpretation of impedance measurements, and offers valuable contributions for future developments of accurate coarse-grained representations of confined electrolytes.

Development of a high-frequency dielectric spectrometer using a portable vector network analyzer

A simple and novel setup for high-frequency dielectric spectroscopy of materials has been developed using a portable vector network analyzer. The measurement principle is based on radio frequency reflectometry, and both its capabilities and limitations are discussed. The results obtained on a typical liquid crystal prove that the device can provide reliable spectra between 107 and 109 Hz, thus extending the capabilities of conventional impedance analyzers.

On the low-frequency dispersion observed in dielectrophoresis spectra

The foundation of dielectrophoresis (DEP) as a tool for biological investigation is the use of the Clausius-Mossotti (C-M) factor to model the observed behaviour of cells experiencing DEP across a frequency range. Nevertheless, it is also the case that at lower frequencies, the DEP spectrum deviates from predictions; there exists a rise in DEP polarisability, which varies in frequency and magnitude with different cell types and medium conductivities. In order to evaluate the origin of this effect, we have studied DEP spectra from five cell types (erythrocytes, platelets, neurons, HeLa cancer cells and monocytes) in several conditions including medium conductivity and cell treatment. Our results suggest the effect manifests as a low-pass dispersion whose cut-off frequency varies with membrane conductance and capacitance as determined using the DEP spectrum; the effect also varies as a logarithm of medium conductivity and Debye length. These together suggest that the values of membrane capacitance and conductance depend not only on the impedance of the membrane itself, but also of the surrounding double layer. The amplitude of the effect in different cell types compared to the C-M factor was found to correlate with the depolarisation factors for the cells' shapes, suggesting that this ratio may be useful as an indicator of cell shape for DEP modelling.

Collective and non-collective molecular dynamics in a ferroelectric nematic liquid crystal studied by broadband dielectric spectroscopy

A great deal of effort has been recently devoted to the study of dielectric relaxation processes in ferroelectric nematic liquid crystals, yet their interpretation remains unclear. In this work, we present the results of broadband dielectric spectroscopy experiments of a prototypical ferroelectric nematogen in the frequency range 10 Hz-110 MHz at different electrode separations and under the application of DC bias fields. The results evidence a complex behavior in all phases due to the magnitude of polar correlations in these systems. The observed modes have been assigned to different relaxation mechanisms based on existing theoretical frameworks.

Comparison of Thin-Film Capacitor Geometries for the Detection of Volatile Organic Compounds Using a ZIF-8 Affinity Layer

Their chemical diversity, uniform pore sizes, and large internal surface areas make metal-organic frameworks (MOFs) highly suitable for volatile organic compound (VOC) adsorption. This work compares two geometries of capacitive VOC sensors that use the MOF material ZIF-8 as an affinity layer. When using a permeable top electrode (thickness < 25 nm), the metal-insulator-metal (MIM) sandwich configuration exhibits superior sensitivity, an improved detection limit, and a smaller footprint than the conventional interdigitated electrode layout. Moreover, the transduction of VOC adsorption in ZIF-8 via MIM capacitors is more sensitive to polar VOCs and provides better selectivity at high loadings than gravimetric and optical transductions.

Analysis of electromagnetic response of cells and lipid membranes using a model-free method

Electromagnetic radiation (EMR) is omnipresent on earth and may interact with the biological systems in diverse manners. But the scope and nature of such interactions remain poorly understood. In this study, we have measured the permittivity of cells and lipid membranes over the EMR frequency range of 20 Hz to 4.35 × 10¹⁰ Hz. To identify EMR frequencies that display physically intuitive permittivity features, we have developed a model-free method that relies on a potassium chloride reference solution of direct-current (DC) conductivity equal to that of the target sample. The dielectric constant, which reflects the capacity to store energy, displays a characteristic peak at 10⁵-10⁶ Hz. The dielectric loss factor, which represents EMR absorption, is markedly enhanced at 10⁷-10⁹ Hz. The fine characteristic features are influenced by the size and composition of these membraned structures. Mechanical disruption results in abrogation of these characteristic features. Enhanced energy storage at 10⁵-10⁶ Hz and energy absorption at 10⁷-10⁹ Hz may affect certain membrane activity relevant to cellular function.

A disposable impedance-based sensor for in-line cell growth monitoring in CAR-T cell manufacturing

This paper presents the development of low-cost, disposable impedance-based sensors for real-time, in-line monitoring of suspension cell culture. The sensors consist of electrical discharge machining (EDM) cut aluminum electrodes and polydimethylsiloxane (PDMS) spacers, both of which are low-cost materials that can be safely disposed of. Our research demonstrates the capability of these low-cost sensors for in-line, non-invasive monitoring of suspension cell growth in cell manufacturing. We use a hybrid equivalent circuit model to extract key features/parameters from intertwined impedance signals, which are then fed to a novel physics-inspired (gray-box) model designed for α-relaxation. This model determines viable cell count (VCC), a critical quality attribute (CQA) in cell manufacturing. Predicted VCC trends are then compared with image-based cell count data to verify their accuracy.

Frequency and field-dependent response of confined electrolytes from Brownian dynamics simulations

Using Brownian dynamics simulations, we investigate the effects of confinement, adsorption on surfaces, and ion-ion interactions on the response of confined electrolyte solutions to oscillating electric fields in the direction perpendicular to the confining walls. Nonequilibrium simulations allows to characterize the transitions between linear and nonlinear regimes when varying the magnitude and frequency of the applied field, but the linear response, characterized by the frequency-dependent conductivity, is more efficiently predicted from the equilibrium current fluctuations. To that end, we (rederive and) use the Green-Kubo relation appropriate for overdamped dynamics, which differs from the standard one for Newtonian or underdamped Langevin dynamics. This expression highlights the contributions of the underlying Brownian fluctuations and of the interactions of the particles between them and with external potentials. Although already known in the literature, this relation has rarely been used to date, beyond the static limit to determine the effective diffusion coefficient or the DC conductivity. The frequency-dependent conductivity always decays from a bulk-like behavior at high frequency to a vanishing conductivity at low frequency due to the confinement of the charge carriers by the walls. We discuss the characteristic features of the crossover between the two regimes, most importantly how the crossover frequency depends on the confining distance and the salt concentration, and the fact that adsorption on the walls may lead to significant changes both at high and low frequencies. Conversely, our results illustrate the possibility to obtain information on diffusion between walls, charge relaxation, and adsorption by analyzing the frequency-dependent conductivity.

3D printable phantom for mimicking electrical properties of dermal tissue

Skin cancer is one of the most ubiquitous forms of cancer that is often overdiagnosed or missed by traditional diagnostic techniques. Bioimpedance spectroscopy (BIS) is a technology that aims to take advantage of the variations in electrical properties of tissue to identify ectopic formations. It is difficult to develop BIS technologies without obtaining tumor tissue samples. One solution is to use a "tissue phantom," a synthetic structure that mimics the properties of tissue. Current solutions using natural biomaterials, such as gelatin, have not been able to create complex tissue geometries that are vital to honing BIS diagnostics. However, semi-synthetic polymers, such has gelatin methacrylate (GelMA), offer the benefits of possessing similar electrical properties to their respective source biomaterial while being 3D printable. In this work, we first measured the impedance of porcine dermal tissue. We then applied these impedance measurements to create an electrically accurate tissue phantom using a photocurable hydrogel, GelMA, and varying concentrations of NaCl, aluminum powder, and titanium dioxide powder.

Split electrodes for electrical-conductivity-based tissue discrimination

This work presents a method to minimize the inadvertent cutting of tissues in surgeries involving bone drilling. We present electrical impedance measurements as an assistive technology to image-guided surgery to achieve online guidance. Proposed concept is to identify and localize the landmarks via impedance measurements and then use this information to superimpose the estimated drilling trajectory on the offline maps obtained by pre-operative imaging. To this end., we propose an asymmetric electrode geometry., split electrodes., capable of distinguishing impedance variations as a function of rotation angle. The feasibility of the proposed approach is verified with numerical analysis. A probe with stainless steel electrodes has been fabricated and tested with a technical phantom. Although the results are impacted by a non-ideality in the phantom., we could show that the variation of impedance as a function of rotation angle can be used to localize the regions with different impedivities. Clinical Relevance- Presented approach may be used to minimize the inadvertent cutting of tissues in surgeries involving bone drilling.

Electrokinetic detection of the salt-free condition in colloids. Application to polystyrene latexes

Because of their singular phenomenology, the so-called salt-free colloids constitute a special family of dispersed systems. Their main characteristic is that the dispersion medium ideally contains only the solvent and the ions compensating exactly the surface charge of the particles. These ions (often called released counterions) come into the solution when the surface groups responsible for the particles charge get ionized. An increasing effort is nowadays dedicated to rigorously compare theoretical model predictions for ideal salt-free suspensions, where only the released counterions are supposed to be present in solution, with appropriately devised experiments dealing with colloids as close as possible to the ideal salt-free ones. Of course, if the supporting solution is aqueous, the presence of atmospheric contamination and any other charged species different from the released counterions in the solution must be avoided. Because this is not an easy task, the presence of dissolved atmospheric CO₂ and of H⁺ and OH^- from water dissociation cannot be fully discarded in aqueous salt-free solutions (often denominated realistic in such case). Ultimately, at some point, the role of the released counterions will be comparable or even larger in highly charged concentrated colloids than that of added salts. These topics are covered in the present contribution. The model results are compared with experimental data on the dynamic mobility and dielectric dispersion of polystyrene spheres of various charges and sizes. As a rule, it is found that the model correctly predicts the significance of alpha and Maxwell-Wagner-O'Konski relaxations. Positions and amplitudes of such relaxations are well predicted, although it is necessary to assume that the released counterions are potassium or sodium instead of protons, otherwise the frequency spectra of experimental mobility and permittivity differ very significantly from those theoretically calculated. The proposed electrokinetic evaluation is an ideal tool for detecting in situ the possible contamination (or incomplete ion exchange of the latexes). A satisfactory agreement is found when potassium counterions are assumed to be in solution, mostly if one considers that the comparison is carried out without using any adjustable parameters.

Zwitterions for impedance spectroscopy: The new buffers in town

Studying the role of buffers in impedance spectroscopy is a relatively unexplored area. We demonstrate a special class of biologically relevant buffers known as Good's zwitterionic buffers that show improved performance over standard electrolyte buffers (e.g. PBS) currently widely used in impedance spectroscopy measurements of bacterial suspensions. Our theoretical and experimental comparisons of conductivity of classical and zwitterionic buffers at various different concentrations show that ion-ion interaction effects are significantly higher in zwitterionic buffers as compared to classical buffers at the concentrations at which they are used. This and the fact that zwitterions have larger sizes leads to the lowering of their conductivity which significantly improves their impedance sensing ability. We illustrate through an example of heat-induced ionic release in model S. typhi and S. aureus bacteria that having a low conductivity buffer is indeed beneficial for biological impedance measurements. In fact, the best buffer for impedance studies can be chosen solely based on their electrical properties as long as they are also biologically compatible. This gives Good's zwitterionic buffers an edge over conventional media as they satisfy both these criteria.

AC Electrokinetics of Salt-Free Multilayered Polymer-Grafted Particles

Interest in the electrical properties of the interface between soft (or polymer-grafted) nanoparticles and solutions is considerable. Of particular significance is the case of polyelectrolyte-coated particles, mainly taking into account that the layer-by-layer procedure allows the control of the thickness and permeability of the layer, and the overall charge of the coated particle. Like in simpler systems, electrokinetic determinations in AC fields (including dielectric dispersion in the 1 kHz-1 MHz frequency range and dynamic electrophoresis by electroacoustic methods in the 1-18 MHz range) provide a large amount of information about the physics of the interface. Different models have dealt with the electrokinetics of particles coated by a single polymer layer, but studies regarding multi-layered particles are far scarcer. This is even more significant in the case of so-called salt-free systems; ideally, the only charges existing in this case consist of the charge in the layer(s) and the core particle itself, and their corresponding countercharges, with no other ions added. The aims of this paper are as follows: (i) the elaboration of a model for the evaluation of the electrokinetics of multi-grafted polymer particles in the presence of alternating electric fields, in dispersion media where no salts are added; (ii) to carry out an experimental evaluation of the frequency dependence of the dynamic (or AC) electrophoretic mobility and the dielectric permittivity of suspensions of polystyrene latex spherical particles coated with successive layers of cationic, anionic, and neutral polymers; and (iii) finally, to perform a comparison between predictions and experimental results, so that it can be demonstrated that the electrokinetic analysis is a useful tool for the in situ characterization of multilayered particles.

Impedance Spectroscopy as a Tool for Monitoring Performance in 3D Models of Epithelial Tissues

In contrast to traditional 2D cell cultures, both 3D models and organ-on-a-chip devices allow the study of the physiological responses of human cells. These models reconstruct human tissues in conditions closely resembling the body. Translation of these techniques into practice is hindered by associated labor costs, a need which may be remedied by automation. Impedance spectroscopy (IS) is a promising, automation-compatible label-free technology allowing to carry out a wide range of measurements both in real-time and as endpoints. IS has been applied to both the barrier cultures and the 3D constructs. Here we provide an overview of the impedance-based analysis in different setups and discuss its utility for organ-on-a-chip devices. Most attractive features of impedance-based assays are their compatibility with high-throughput format and supports for the measurements in real time with high temporal resolution, which allow tracing of the kinetics. As of now, IS-based techniques are not free of limitations, including imperfect understanding of the parameters that have their effects on the impedance, especially in 3D cell models, and relatively high cost of the consumables. Moreover, as the theory of IS stems from electromagnetic theory and is quite complex, work on popularization and explanation of the method for experimental biologists is required. It is expected that overcoming these limitations will lead to eventual establishing IS based systems as a standard for automated management of cell-based experiments in both academic and industry environments.

AC Conductivity Measurements of Ultradilute Colloidal Suspensions in HEPES Buffer

Impedance spectroscopy was used to probe the AC conductivity of extremely dilute colloidal suspensions (2.5 × 10^-5 ≤ Φ_w/v ≤ 4.0 × 10^-2) comprising of polystyrene microspheres (PS; κa ≫ 1 and ζ = -65 mV), gold nanoparticles (Au NPs; κa > 1 and ζ = -26 mV), and Au-coated PS metallodielectric particles (Au-PS) in HEPES buffer. When AC electric fields of strength 10 mV and 1 MHz were applied via 100 μm gap interdigitated microelectrodes across 10 μL samples, a highly resistive (θ_capacitive < 1°) and nonmonotonic response was obtained with particle concentrations at steady state. While the suspensions were less resistive (than the buffer) below a critical concentration, they became more resistive above it. More interestingly, particle-particle interactions took place in suspensions with concentrations as low as 0.005% w/v. We believe this unique behavior is linked to the ion size asymmetry in the HEPES molecule that provides an ideal microenvironment for counterionic polarization around the particles. The exact mechanism of polarization in HEPES, however, still remains elusive as the current theoretical models for simple electrolytes fail to explain our data.

A compact design of a characterization station for far UV photodetectors

A newly fabricated characterization station is presented. It is a compact, cost-effective, and easily adjustable apparatus. Each part including 4-pin probe, manipulators, operating temperature, and applied bias can be independently controlled. The station can provide highly reliable, reproducible, and economical methods to quickly conduct and complete the characterizations of a large amount of sensing materials within a short period of time. It is particularly suitable for studies of various nanostructured materials and their related thermal effect, polarization effect, sensitivity, and electrical and electronic properties.

Oral naringenin nanocarriers: Fabrication, optimization, pharmacokinetic and chemotherapeutic efficacy assessments

AIM

To enhance oral bioavailability and chemotherapeutic efficacy of naringenin (NG) by fabricating the NG-encapsulated Soluthin-maltodextrin-based nanocarrier (NC) system.

MATERIALS & METHODS

NG-encapsulated nanocarriers (NG/NCs) were developed, and in vitro physicochemically characterized. Furthermore, Wistar rats were used to evaluate the pharmacokinetic profile. Furthermore, in vitro and in vivo colorectal cancer efficacy was evaluated in BALB/c mice-bearing colon-26 cells.

RESULTS

The NG/NCs demonstrated favorable mean particle size (176 ± 2.35 nm) and percent entrapment efficiency (70.83 ± 4.55%), respectively. The oral bioavailability was found to be approximately 116-fold higher and in vitro cytotoxicity exhibited approximately 21-fold reduction as compared with pure NG. Moreover, optimized NG/NCs demonstrated significant tumor suppression compared with pure NG in vivo.

CONCLUSION

The NG/NCs would be an efficient formulation for enhancing oral bioavailability and chemotherapeutic efficacy of NG.

Dynamic electrophoretic mobility and electric permittivity of concentrated suspensions of plate-like gibbsite particles

In this paper we present experimental results on the electrokinetic behavior of planar gibbsite particles in concentrated suspensions. The dc electrophoretic mobility measurements are in this case of little significance, as they are scarcely informative. In the present investigation, we show that the dielectric dispersion and dynamic electrophoresis can in contrast provide such information. The complicating factors are of course the non-spherical shape and the finite particle concentration, as no complete theory of these phenomena exists for such systems. We propose to use first of all a model of dynamic electrophoresis of spheroids in which the effect of volume fraction is considered by means of an approximate theory previously obtained for spheres, based on the evaluation of electrical and hydrodynamic interactions between particles. In addition, the role of volume fraction on the high frequency inertial relaxation is also ascertained and used to obtain a volume fraction-independent radius of the gibbsite spheroids. A similar approach is used for the evaluation of dielectric dispersion data. Both the dynamic mobility and dielectric constant dependencies on frequency were obtained for gibbsite suspensions of different volume fractions in 0.5mMKCl. The theoretical treatments elaborated were applied to these data, and a coherent picture of the geometrical and electrical characteristics of the particles was obtained.