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.

Development of Ni-Sr(V,Ti)O3-δ Fuel Electrodes for Solid Oxide Fuel Cells

A series of strontium titanates-vanadates (STVN) with nominal cation composition Sr1-xTi1-y-zVyNizO3-δ (x = 0–0.04, y = 0.20–0.40 and z = 0.02–0.12) were prepared by a solid-state reaction route in 10% H2–N2 atmosphere and characterized under reducing conditions as potential fuel electrode materials for solid oxide fuel cells. Detailed phase evolution studies using XRD and SEM/EDS demonstrated that firing at temperatures as high as 1200 °C is required to eliminate undesirable secondary phases. Under such conditions, nickel tends to segregate as a metallic phase and is unlikely to incorporate into the perovskite lattice. Ceramic samples sintered at 1500 °C exhibited temperature-activated electrical conductivity that showed a weak p(O2) dependence and increased with vanadium content, reaching a maximum of ~17 S/cm at 1000 °C. STVN ceramics showed moderate thermal expansion coefficients (12.5–14.3 ppm/K at 25–1100 °C) compatible with that of yttria-stabilized zirconia (8YSZ). Porous STVN electrodes on 8YSZ solid electrolytes were fabricated at 1100 °C and studied using electrochemical impedance spectroscopy at 700–900 °C in an atmosphere of diluted humidified H2 under zero DC conditions. As-prepared STVN electrodes demonstrated comparatively poor electrochemical performance, which was attributed to insufficient intrinsic electrocatalytic activity and agglomeration of metallic nickel during the high-temperature synthetic procedure. Incorporation of an oxygen-ion-conducting Ce0.9Gd0.1O2-δ phase (20–30 wt.%) and nano-sized Ni as electrocatalyst (≥1 wt.%) into the porous electrode structure via infiltration resulted in a substantial improvement in electrochemical activity and reduction of electrode polarization resistance by 6–8 times at 900 °C and ≥ one order of magnitude at 800 °C.

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 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 10s 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 minutes. 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 1s. Overall the new microfluidic device can be used for dielectric characterization of biological cells in physiological buffers for various cell screening applications.

The Role of Sintering Temperature and Dual Metal Substitutions (Al3+, Ti4+) in the Development of NASICON-Structured Electrolyte

The aim of this study is to synthesize Li1+xAlxTixSn2−2x(PO4) sodium super ion conductor (NASICON) -based ceramic solid electrolyte and to study the effect of dual metal substitution on the electrical and structural properties of the electrolyte. The performance of the electrolyte is analyzed based on the sintering temperature (550 to 950 °C) as well as the composition. The trend of XRD results reveals the presence of impurities in the sample, and from Rietveld Refinement, the purest sample is achieved at a sintering temperature of 950 °C and when x = 0.6. The electrolytes obey Vegard′s Law as the addition of Al3+ and Ti4+ provide linear relation with cell volume, which signifies a random distribution. The different composition has a different optimum sintering temperature at which the highest conductivity is achieved when the sample is sintered at 650 °C and x = 0.4. Field emission scanning electron microscope (FESEM) analysis showed that higher sintering temperature promotes the increment of grain boundaries and size. Based on energy dispersive X-ray spectroscopy (EDX) analysis, x = 0.4 produced the closest atomic percentage ratio to the theoretical value. Electrode polarization is found to be at maximum when x = 0.4, which is determined from dielectric analysis. The electrolytes follow non-Debye behavior as it shows a variety of relaxation times.

Ionic Conductivity of Hybrid Composite Solid Polymer Electrolytes of PEOnLiClO4-Cubic Li7La3Zr2O12 Films

Ionic conductivity of the polyethylene oxide-LiClO4 (PEOnLiClO4) solid polymer electrolyte (SPE) films with an EO:Li ratio (n) of 10, 12, 15, as well as the hybrid composite solid polymer electrolyte (CSPE) films of PEOnLiClO4 containing 50 wt% of cubic-Li7La3Zr2O12 (LLZO) sub-micron sized particles, have been studied by varying Li-salt content in the films. The complex AC dielectric permittivity and conductivity data obtained from electrical impedance measurements were fitted using a generalized power-law, including the effects of electrode polarization applied at low AC frequencies to obtain various relaxation times. In addition to increased mechanical and thermal robustness, the CSPE films show higher values of ionic conductivity, >10−4 S/cm at room temperature compared to those of SPE films with n = 12 and 15. On the contrary, the ionic conductivity of CSPE with n = 10 decreases by a factor of three compared to the corresponding SPE film due to increased polymer structural reorientation and Li-ion pairing effects. The Vogel–Tammann–Fulcher behavior of the temperature-dependent conductivity data indicates a close correlation between the ionic conductivity and polymer segmental relaxation. The PEO12LiClO4-LLZO film shows the lowest activation energy of ~0.05 eV.

The Low Frequency Electrical Properties of Sea Ice

<p>This thesis summarises an experimental and theoretical study of the low frequency electrical properties of sea ice. The aim of the research was to first demonstrate, and then gain a physical understanding of, the microstructural dependence of a sea ice impedance measurement. In particular, we sought to realise how the effective electrical properties of the medium depended on the volume fraction, orientation, dimensions, and connectivity of the dispersed brine phase. The experimental portion of the project was performed on laboratory grown, artificial sea ice. We monitored the variation with time, and temperature, of the broadband sea ice impedance using four-electrode measurement cells embedded within the ice. The four-electrode measurement allowed us to realise and eliminate the contribution of electrode polarization to the measured impedance. By representing the electrical response of sea ice as a complex conductivity, we formulated a broadband physical model to describe the medium. The model distinguished bulk conduction, bulk polarization, and interfacial polarization. A complex non-linear least squares fitting procedure revealed the individual contribution of these physical processes and we studied their variation with temperature. We found that the bulk material underwent a dielectric relaxation with activation energy Ea = 0.20 + and - 0.04eV. We linked the bulk material properties with a two phase microstructural model, with realistic input parameters.</p>

The Low Frequency Electrical Properties of Sea Ice

<p>This thesis summarises an experimental and theoretical study of the low frequency electrical properties of sea ice. The aim of the research was to first demonstrate, and then gain a physical understanding of, the microstructural dependence of a sea ice impedance measurement. In particular, we sought to realise how the effective electrical properties of the medium depended on the volume fraction, orientation, dimensions, and connectivity of the dispersed brine phase. The experimental portion of the project was performed on laboratory grown, artificial sea ice. We monitored the variation with time, and temperature, of the broadband sea ice impedance using four-electrode measurement cells embedded within the ice. The four-electrode measurement allowed us to realise and eliminate the contribution of electrode polarization to the measured impedance. By representing the electrical response of sea ice as a complex conductivity, we formulated a broadband physical model to describe the medium. The model distinguished bulk conduction, bulk polarization, and interfacial polarization. A complex non-linear least squares fitting procedure revealed the individual contribution of these physical processes and we studied their variation with temperature. We found that the bulk material underwent a dielectric relaxation with activation energy Ea = 0.20 + and - 0.04eV. We linked the bulk material properties with a two phase microstructural model, with realistic input parameters.</p>

Effect on Current Density on Zero Charge Corrosion Protection of Pure Mg in 3.5% NaCl Solution

The cathodic protection uses two-electrode polarization, which requires large currents and substantial voltages. Efforts are being made to identify possibilities for improvements by developing zero-charge corrosion protection techniques. Studies were performed to determine the zero-charges potential effect by analyzing corrosion signs on reactive metal samples such as pure Mg. Mg samples were fed by current/ voltage pulses for 120 hours, with specified pulse parameters and varied Ecorr- offsets, ranging from +2 to -218 mV. The volumetric hydrogen gas collection technique is used to determine the hydrogen evolution rate. Surface observation is carried out by stereomicroscope to determine the presence of corrosion signs on the sample surface. Overall, all current densities and hydrogen evolution rates had very low readings on the studied Ecor offset parameters. Mg samples fed with pulses at -1800 and -1900 mV vs SCE revealed zero charge potential effects since their surface was clean and showed no indications of corrosion even after being exposed to the corrosive solution for 120 hours. Thus, corrosion protection is successfully done and meets the Epzc condition.