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PubMed2006

Simulation studies of ionic liquids: orientational correlations and static dielectric properties.

The ionic liquids BMIM+I-, BMIM+BF4-, and BMIM+PF6- were simulated by means of the molecular dynamics method over a time period of more than 100 ns. Besides the common structural analysis, e.g., radial distribution functions and three dimensional occupancy plots, a more sophisticated orientational analysis was performed. The angular correlation functions g(00)110(r) and g(00)101(r) are the first distance dependent coefficients of the pairwise orientational distribution function g(rij,Omega1,Omega2,Omega12). These functions help to interpret the three dimensional plot and reveal interesting insights into the local structure of the analyzed ionic liquids. Furthermore, the collective network of ionic liquids can be characterized by the Kirkwood factor Gkappa(r) [J. Chem. Phys. 7, 911 (1939)]. The short-range behavior (r<10 A) of this factor may be suitable to predict the water miscibility of the ionic liquid. The long-range limit of Gkinfinity is below 1 which demonstrates the strongly coupled nature of the ionic liquid networks. In addition, this factor relates the orientational structure and the dielectric properties of the ionic liquids. The static dielectric constant epsilon(omega=0) for the simulated system is 8.9-9.5. Since in ionic liquids the very same molecule contributes to the total dipole moment as well as carries a net charge, a small, but significant contribution of the cross term between the total dipole moment and the electric current to epsilon(omega=0) is observed.

Computer Simulation↗
PubMed2001

Glucose sensing based on interdigitated array microelectrode.

A micro glucose sensor consisting of an interdigitated array gold microelectrode was developed. The interdigitated array structure, which has 10 microns band width and 10 microns band gap, was fabricated in a small region (2.5 x 5 mm2) on a quartz substrate. Glucose oxidase was chemically fixed onto the electrode surface through self-assembled monolayer of 11-mercaptoundecanoic acid; ferroceneacetic acid was used as electron mediator. Electrochemical properties of the glucose oxidase-immobilized microelectrode were investigated by cyclic voltammogram measurements. Results confirmed that the reductive ferroceneacetic acid generated at counter electrode diffuses through a narrow band gap (10 microns) and can reach the working electrode surface.

Acetic Acid↗
PubMed2003

Presynaptic capacitance measurements and Ca2+ uncaging reveal submillisecond exocytosis kinetics and characterize the Ca2+ sensitivity of vesicle pool depletion at a fast CNS synapse.

The intracellular Ca2+ sensitivity of synaptic vesicle fusion is an important determinant of transmitter release probability, but it is unknown for most CNS synapses. We combined whole-cell membrane capacitance measurements and Ca2+ uncaging at the large calyx of Held nerve terminals to determine the Ca2+ sensitivity of synaptic vesicle fusion at a glutamatergic CNS synapse, independent of recording EPSCs. Capacitance increases measured 30-50 msec after elevating the intracellular Ca2+ concentration ([Ca2+]i) by Ca2+ uncaging were half-maximal at approximately 5 microm [Ca2+]i. At 10 microm [Ca2+]i, capacitance increases reached maximal values (256 +/- 125 fF; mean +/- SD), indicating the depletion of an average pool of approximately 4000 readily releasable vesicles. Vesicle pool depletion was confirmed in cross-depletion experiments, in which capacitance responses were measured after Ca2+ uncaging, or after combined stimuli of prolonged presynaptic depolarizations and Ca2+ uncaging. To analyze the Ca2+-dependent rates of vesicle pool depletion, the capacitance rise after Ca2+ uncaging was fitted with single- or double-exponential functions. The fast time constants of double-exponential fits, and the time constants of single-exponential fits were 2-3 msec at 10-15 microm [Ca2+]i and reached submillisecond values at 30 microm [Ca2+]i. These results suggest that three to five readily releasable vesicles can fuse within <1 msec at each active zone of a calyx of Held, given that [Ca2+]i rises sufficiently high. Submillisecond kinetics of exocytosis are reached at significantly lower [Ca2+]i than at ribbon-type sensory synapses previously investigated by capacitance measurements.

Acetates↗
PubMed2005

Self-assembled multilayers of gold nanoparticles: nitrate-induced rectification of quantized capacitance charging and effects of alkaline (earth) ions in aqueous solutions.

Gold nanoparticle multilayers were self-assembled onto an electrode surface by using a dipping method. The particle assemblies exhibited quantized capacitance charging characteristics in aqueous media that were rectified by hydrophobic anions such as PF6-, BF4- and ClO4-, similar to the behavior with the monolayer counterparts. More interestingly, even in the presence of less hydrophobic anions such as NO3-, very well-defined single electron transfers were observed voltammetrically with these particle multilayers, a response unseen previously with particle monolayers. This was ascribed, in part, to the enhanced interactions between the particle multilayers and the electrolyte anions as well as the minimization of the structural defects within the particle thin films as compared to the monolayer counterparts. Further studies showed that with particles functionalized with oligo(ethylene oxide) moieties, the particle charge transfer properties were also found to be affected by electrolyte cations, reflected by the variation of the particle molecular capacitance and formal potentials with the nature of the alkaline (earth) metal ions.

Electric Capacitance↗
PubMed1992

Low-amplitude, low-frequency electric field-stimulated bone cell proliferation may in part be mediated by increased IGF-II release.

We have developed an in vitro model incorporating a low-amplitude (10(-7) V/cm), low frequency (f less than 100 Hz), capacitively coupled electric field in order to study the mechanism through which an electric field may increase bone cell proliferation. Utilizing this model we have previously shown that electric field-stimulated bone cell proliferation was dependent on release of mitogen activity into the culture medium from exposed cells. The current studies were intended to characterize this mitogen activity. In these studies we found that electric field-stimulated human bone cell proliferation was associated with increased IGF-II mRNA accumulation and IGF-II secretion suggesting that IGF-II may in part mediate the increase in bone cell proliferation following electric field exposure.

Bone and Bones↗
PubMed2004

A plasma-polymerized film for capacitance immunosensing.

A capacitance immunosensor based on a plasma-polymerized ethylenediamine film (PPEF) has been developed. The resulting PPEF is studied with scanning electrode micrograph (SEM), IR reflection spectrum and cyclic voltammetry. SEM and IR reflection spectrum showed that the plasma-polymerized film (PPF) formed on the gold electrode surface is quite homogeneous, flat, nonporous and contains plenty of free-reacted -NH2. Moreover, cyclic voltammetry showed that the hexacyanoferrate redox reactions were blocked well by the formed PPF, that is to say, the formed PPF has excellent insulating characteristics. To investigate its applicability for capacitive immunosensing, goat-anti-human IgG antibody (IgGAb) was coupled to the PPF-coated gold electrode surface via glutaraldehyde (GA) to form an immunoglobulin G (IgG) probe. Alternating current (ac) impedance and capacitance measurement were used in the immunoassay. The experiment results show that the PPEF is applicable to form insulating layer of capacitive immunosensors.

Biosensing Techniques↗
PubMed1994

Dielectric spectroscopy of human blood.

Impedance measurement is an established technique for studying the passive electric properties of cell membranes. Dispersions can be detected by studying the electric properties (capacitance and conductance) in the radio-frequency range (kHz-mHz). The theoretical interpretation is based on the Maxwell-Wagner effect at the interface between the cytoplasm and the cell membrane. The specific electric variables of the membrane, the cytoplasm, as well as the surrounding plasma (medium) are estimated by non-linear regression fitting and appropriate equations. Using a four-electrode technique, we have measured the impedance with a commercial instrument working in the frequency range of 0.2-10 mHz interfaced to a computer. Differences were found in conductivity and capacitance of blood from 1) persons exposed to organic solvents, 2) patients with metal exposure, and 3) patients with cardiovascular disease. The effects of plasma components and haematocrit are crucial when undertaking measurements on whole blood. The results are difficult to interpret but we consider perturbations in the erythrocyte membrane to be involved. Potential clinical applications will be promoted by the development of the software.

Aspirin↗
PubMed1998

Biochemical pathway mediating the response of bone cells to capacitive coupling.

Rat calvarial bone cells or mouse MC3T3-E1 bone cells subjected to a capacitively coupled electric field of 20 mV/cm consistently showed significant increases in cellular proliferation as determined by deoxyribonucleic acid content. Verapamil, a membrane calcium channel blocker; W-7, a calmodulin antagonist; indocin, a prostaglandin synthesis inhibitor; or bromophenacyl bromide, a phospholipase A2 inhibitor, each at a concentration that did not interfere with cell proliferation in control cultures, inhibited proliferation in those cultures subjected to the electric field. In contrast, neomycin, an inhibitor of the inositol phosphate cascade, did not inhibit this electrically induced cellular proliferation. Prostaglandin E2 production also was increased significantly with electrical stimulation, and this increase was inhibited by verapamil or indocin but not by neomycin. Thus, the data suggest that the signal transduction mediating the proliferative response of cultured bone cells to a capacitively coupled field involved transmembrane calcium translocation via voltage gated calcium channels, activation of phospholipase A2, and a subsequent increase in prostaglandin E2. Increases in cytosolic calcium and activated calmodulin are implied. The inositol phosphate pathway, unlike its dominant role in signal transduction in mechanically stimulated bone cells, does not appear to play a role in signal transduction in the proliferative response of bone cells to electrical stimulation.

Animals↗
PubMed1978

Electronic monitoring of events within dynamic particulate beds: conductance and capacitance measurements.

Methods for monitoring mechanical events occurring within particulate solids systems in a dynamic state are described. The electrical conductance and capacitance characteristics of such systems, as they relate to the degree of bed dilation or expansion, extent of interparticulate contact, and intensity of particle motion, were studied and are discussed. To establish the potential of this approach, harmonically vibrated beds of monodispersed conducting spheres were used. A technique, based on the frequency modulation of standard FM broadcast frequency carrier signals, was developed to measure low and high frequency fluctuations in bed capacitance. The electrical conductance of these systems also was determined by both voltage drop and current flow methods. The experimental techniques developed are broadly applicable to various materials and modes of agitation or flow. They permit the evaluation of the time courses of both bed dilation and particle motion which, in turn, are known to determine of modify critically powder flow and mixing behavior.

Chemistry, Pharmaceutical↗
PubMed2005

Electrochemical determination of reversible redox species at interdigitated array micro/nanoelectrodes using charge injection method.

In this work, the interdigitated array microelectrodes/nanoelectrodes (approximately 0.2 mm2 surface area) have been fabricated and characterized using the charge injection method for the electrochemical determination of reversible redox species. Using p-aminophenol as the redox species, approximately 4 x 10(-7) M and 6 x 10(-9) M detection limits on the species concentration ar respectively achieved with the microelectrodes and the nanoelectrodes.

Aminophenols↗
PubMed2006

Determination of the capacitance of solid-state potentiometric sensors: An electrochemical time-of-flight method.

A dual microelectrode electrochemical time-of-flight technique in which diffusion flux of Ag+, Cl-, or H+ ions electrochemically produced at a generator electrode is measured by recording potential-time transients with Ag, Ag/AgCl, or iridium oxide potentiometric microsensors, respectively, is developed. The generator and microsensor electrodes are typically spaced by 50-100 microm and are incorporated in the lithographically fabricated thin-layer-type devices. Under conditions of moderate rates of the ion electrogeneration, the potential-time (E-t) transients recorded with the three microsensors show excellent agreement with theory involving linear diffusion equations and the experimentally determined Nernstian slopes of the microsensors. However, when the generator current, or the initial concentration of the primary ion of interest is low, appreciable delays in the recorded E-t transients are observed due to the finite capacitance of the micropotentiometric sensors. The recorded delay in the E-t transients can be quantitatively accounted for by including the sensor capacitance (C) in the theoretical description of the transients. Direct comparison between the theoretical and the experimental E-t transients yields the sensor's capacitance. This capability of our new technique is unique in that it allows determination of the capacitance of a potentiometric sensor at open circuit. In the cases of silver electrodes, this method results in C = 31 +/- 2 microF/cm2, a value that is in agreement with those obtained by other methods. The results for silver chloride sensors yield a C in the range of 100-140 +/- 10 microF/cm2. The specific values depend on sensor preparation and the resulting roughness of the Ag/AgCl interface. Iridium oxide sensors show a capacitance that linearly depends on the thickness of the film. Specific capacitance of these microporous films was determined to be 59 +/- 6 F/cm3.

Electric Capacitance↗
PubMed1989

Frequency dependence of increased cell proliferation, in vitro, in exposures to a low-amplitude, low-frequency electric field: evidence for dependence on increased mitogen activity released into culture medium.

In order to investigate the mechanism(s) through which an electric field can increase bone cell proliferation, we have developed an in vitro model incorporating a low-amplitude (estimated 10(-7) V/cm in the serum-free culture medium), low-frequency, capacitively coupled electric field. In previous studies with this model, we have shown that electric field exposure can increase bone cell proliferation both in chick tibiae organ cultures and in calvaria-derived monolayer cell cultures. The current in vitro studies demonstrate that skeletal tissue responses to a 30 min electric field exposure are characterized by a) a frequency window for both increased cell proliferation and increased release of mitogen activity into the cell-conditioned medium, with a peak near 16 Hz; b) a dependence on conditioned medium from exposed cells for increased cell proliferation; and c) a correlation between the alkaline phosphatase content of the bone cell cultures and effects of electric field exposure on both cell proliferation and release of mitogen activity into the conditioned medium.

Alkaline Phosphatase↗
PubMed2004

Dynamic regulation of mechanosensitive channels: capacitance used to monitor patch tension in real time.

All cells, from bacteria to human, are mechanically sensitive. The most rapid of these membrane protein transducers are mechanosensitive ion channels, ionic pores in the membrane that open and close in response to membrane tension. In specific sensory organs, these channels serve the senses of touch and hearing, and inform the central nervous system about the filling of hollow organs such as the bladder. Non-specialized cells use these channels to report on changes in cell volume and local strain. To preserve dynamic sensitivity, sensory receptors adapt to steady-state stimuli. Here we show that in rat astrocytes, the most abundant cells in the brain, this apparent adaptation to the stimulus is actually an inactivation. We have been able to track the time course of local strain by measuring attofarad changes in membrane capacitance and show that it is not correlated with loss of channel activity. The reduction in current with time is caused by an increased occupancy of low conductance states, and a reduction in the probability of opening, not a relaxation of local stress. The occupancy of these substates depends on the integrity of the cell's cytoplasm. However, while disruption of the cytoskeleton leads to a loss of inactivation, it leaves activation unaffected. The activation process is voltage-insensitive, closely correlated with changes in capacitance, and seems to arise solely from stress in the bilayer. The inactivation rate decreases with depolarization, and kinetic analysis suggests that the process involves multiple cytoplasmic ligands. Surprisingly, multivalent ions such as Gd(+3) and Ca(+2) that bind to the lipids and affect channel gating, do not affect the strain-induced increase in membrane capacitance; contrary to expectations, membrane elasticity is unchanged.

Animals↗
PubMed1997

Quantitative study of calcium uptake by tumorigenic bone (TE-85) and neuroblastoma x glioma (NG108-15) cells exposed to extremely-low-frequency (ELF) electric fields.

To verify the effect of cell culture state on frequency dependent increase in proliferation as well as Ca2+ flux across the plasma membrane, tumorigenic bone (TE-85) and neuroblastoma x glioma (NG108-15) cells cultured in the presence of fetal bovine serum (FBS) were exposed to capacitively coupled electric (CCEF) fields in the extremely low frequency (ELF) range of 10 to 18 Hz. [3H]Thymidine incorporation and 45Ca2+ uptake were used as endpoints. TE-85 cells cultured in the presence of 10% FBS did not exhibit a frequency dependent increase in proliferation in contrast to previous studies under growth arrested culture conditions, in which the cells were deprived of FBS. However, both TE-85 and NG108-15 cells had an increase in 45Ca2+ uptake in response to a 16 Hz 18.3 mV/cm CCEF. Fura-2 digital imaging microscopy was used to verify addition of 0.5 mM La3+ and 0.5 mM ionomycin as negative and positive controls, respectively. Imaging microscopy data was combined with 45Ca2+ incorporation results to quantify free intracellular calcium ([Ca2+]i) increase in response to CCEF exposure. TE-85 [Ca2+]i increased from 140 to 189-210 nM where as NG108-15 [Ca2+]i increased from 67 to 189-210 nM. These results suggested that serum deprivation may be a requirement for a frequency dependent increase in proliferation in TE-85 cells but is not necessary for the electric field induced increase in 45Ca2+ uptake in both TE-85 and NG108 cells. The present study also represents the first demonstration of increased 45Ca2+ uptake by neuroblastoma and/or glioma cells in response to an electric field exposure.

Animals↗
PubMed2006

Capacitance imaging of the skin surface.

BACKGROUND/AIMS: A new device allowing recording capacitance images of the skin surface was recently presented. Parameters, extracted from the gray-level histogram of the images, are tested for a new approach of skin surface hydration measurement in comparison with the classical capacitance method. Illustration of the interest of having both images and parameters for studying the homogeneity and the level of skin surface hydration are presented. METHODS: Software for selecting a region of interest from an image and measuring the parameters derived from its gray-level histogram was used to characterize skin hydration. RESULTS: There is a very close correlation between a Corneometer and the parameters extracted from the SkinChip measurements. The importance of having capacitance images of skin is demonstrated in case of non-homogeneity of the skin hydration, either because of photoaging or following an inflammation process. CONCLUSION: Capacitance imaging is a necessary tool for both completely describing and quantifying skin surface hydration.

Dermatitis↗