This work describes the determination of gibberellic acid in growth promoters formulations used in agriculture by sequential injection analysis and potentiometric detection. The potentiometric detectors with improved characteristics used a PVC membrane prepared with Mn(III)tetraphenylporphyrin-Cl as electroactive specie. Different membranes formulated with several ionic additives were compared in order to select the most suitable one concerning slope, response time, reproducibility and selectivity. A membrane containing 6 wt% of manganese tetraphenylporphyrin-Cl [(Mn(TPP)Cl], 27 wt% of PVC, 66 wt% of dibutylphthalate (DBP), as mediator solvent, and 1 wt% of sodium tetraphenylborate, as anionic additive, was chosen for analytical applications. When coupled to a SIA system the tubular electrode presented a linear range between 5×10-4 mol L-1 and 8×10-3 mol L-1, a slope of -64.5 ± 1.6 mV/dec and lower potentiometric selectivity coefficients (log Kpot) than those obtained with the conventional electrodes. Standard deviation of 0.01 and 0.4 (n= 4) and a sampling rate of 30 samples/hour were obtained in the analysis of real samples.
Light-driven electron transfer reactions cause the active accumulation of protons inside thylakoids, yet at steady state the electrical potential difference across the thylakoid membrane is very small; therefore, there must be a flux of other ions to balance the charge that would otherwise be built up by the net movement of H+. This paper presents direct measurements of ion movements through channels in the thylakoid membrane. These were made possible by fusing thylakoid vesicles from spinach (Spinacia oleracea L.) into planar lipid bilayers, using techniques developed originally to study sarcoplasmic reticulum. No Mg2+ current was found, but voltage-dependent channels have been characterized, these being somewhat selective for K+ over Cl−. The data are consistent with a role for these channels in charge balance during light-driven H+ movements.
The use of Ca2+-selective microelectrodes is difficult because of some basic problems: (a) electrodes with submicron tips may display non-Nernstian slopes; (b) liquid membrane microelectrodes respond only slowly (within seconds) to changes in ion activity; (c) turgid plant cells with tough walls damage the sensitive tip. This article describes concisely recent advances in fabricating Ca2+-selective single and double-barreled microelectrodes and their intracellular applications to different plant cell materials. Beveling the electrodes, mixing the sensor components with polyvinylchloride, insulation of the hydrated glass, and stabilization of the tips with inert materials are considered the basic concepts to circumvent most difficulties. It is concluded that the Ca2+-electrode can be a useful tool in plant physiology, but in spite of recent progress this technique remains experimentally demanding.
We report here on an investigation of net nitrate and proton fluxes in root cells of maize (Zea mays L.) seedlings grown without (noninduced) and with (induced) 0.1 millimolar nitrate. A microelectrode system described previously (IA Newman, LV Kochian, MA Grusak, WJ Lucas  Plant Physiol 84: 1177-1184) was utilized to quantify net ionic fluxes from the measurement of electrochemical potential gradients for NO3− and H+ within the unstirred layer at the root surface. The nitrate-inducibility, pH dependence, and concentration dependence of net NO3− uptake correlated quite closely with the electrical response of maize roots to nitrate under the same experimental conditions (as described in PR McClure, LV Kochian, RM Spanswick, JE Shaff  Plant Physiol 93: 281-289). Additionally, it was found that potential inhibitors of the plasmalemma H+-ATPase (vandate, diethylstilbestrol), which were shown to abolish the electrical response to NO3− (in PR McClure, LV Kochian, RM Spanswick, JE Shaff  Plant Physiol 93: 281-289), dramatically inhibited NO3− absorption. These results strongly indicate that the NO3− electrical response is due to the operation of a NO3− transport system in the plasmalemma of maize root cells. Furthermore...
Short-term ion uptake into roots of Limnobium stoloniferum was followed extracellularly with ion selective macroelectrodes. Cytosolic or vacuolar pH, together with the electrical membrane potential, was recorded with microelectrodes both located in the same young root hair. At the onset of chloride, phosphate, and nitrate uptake the membrane potential transiently decreased by 50 to 100 millivolts. During Cl− and H2PO4− uptake cytosolic pH decreased by 0.2 to 0.3 pH units. Nitrate induced cytosolic alkalinization by 0.19 pH units, indicating rapid reduction. The extracellular medium alkalinized when anion uptake exceeded K+ uptake. During fusicoccin-dependent plasmalemma hyperpolarization, extracellular and cytosolic pH remained rather constant. Upon K+ absorption, FC intensified extracellular acidification and intracellular alkalinization (from 0.31 to 0.4 pH units). In the presence of Cl− FC induced intracellular acidification. Since H+ fluxes per se do not change the pH, recorded pH changes only result from fluxes of the stronger ions. The extra- and intracellular pH changes, together with membrane depolarization, exclude mechanisms as K+/A− symport or HCO3−/A− antiport for anion uptake. Though not suitable to reveal the actual H+/A− stoichiometry...
White light (25 watts per square meter) induced an increase in plasma membrane K+-channel activity and a 30- to 70-millivolt transient membrane depolarization (completed in 2-3 minutes) in Arabidopsis thaliana leaf mesophyll cells. Transport characteristics of three types of ion channels in the plasma membrane were determined using inside-out patches. With 220 millimolar K+ on the cytoplasmic side of the patch and 50 millimolar K+ in the pipette, (220/50 K), the open-channel current-voltage curves of these channels were sigmoidal and consistent with an enzyme kinetic model. Two channel types were selective for K+ over Na+ and Cl−. One (named PKC1) had a maximum conductance (Gmax) of 44 picosiemens at a membrane voltage (Vm) of −65 mV in (220/50 K) and is stimulated by light. The other (PKC2) had Gmax = 66 picosiemens at Vm = 60 millivolts in (220/50 K). The third channel type (PCC1) transported K+ and Na+ about equally well but not Cl−. It had Gmax = 109 picosiemens at Vm = 55 millivolts in (250/50 K) with 10 millimolar Ca2+ on the cytoplasmic side. Reducing Ca2+ to 0.1 millimolar increased PCC1 open-channel currents by approximately 50% in a voltage-independent manner. Averaged over time, PKC2 and PCC1 currents strongly outward rectified and PKC1 currents did so weakly. Reductants (1 millimolar dithiothreitol or 10 millimolar β-mercaptoethanol) added to the cytoplasmic side of an excised patch increased the open probability of all three channel types.
Ammonium ion and proton concentration profiles near the surface of a planar bilayer lipid membrane (BLM) generated by an ammonium ion gradient across the BLM are studied by means of microelectrodes. If the concentration of the weak base is small compared with the buffer capacity of the medium, the experimental results are well described by the standard physiological model in which the transmembrane transport is assumed to be limited by diffusion across unstirred layers (USLs) adjacent to the membrane at basic pH values (pH > pKa) and by the permeation across the membrane itself at acidic pH values. In a poorly buffered medium, however, these predictions are not fulfilled. A pH gradient that develops within the USL must be taken into account under these conditions. From the concentration distribution of ammonium ions recorded at both sides of the BLM, the membrane permeability for ammonia is determined for BLMs of different lipid composition (48 x 10(-3) cm/s in the case of diphytanoyl phosphatidylcholine). A theoretical model of weak electrolyte transport that is based on the knowledge of reaction and diffusion rates is found to describe well the experimental profiles under any conditions. The microelectrode technique can be applied for the study of the membrane permeability of other weak acids or bases...
Gigaohm seals made between patch pipettes and hydrophobic substrates have a finite conductance which are cation-selective and capable of producing quantized gating indistinguishable from the gating of biological ion channels. The selectivity sequence and streaming potentials of these seals suggests the existence of a pore of similar dimensions to the nicotinic acetylcholine channel. The ionic selectivity of these seals appears similar to the seal selectivity observed with membrane patches (Fischmeister, R., R. K. Ayer, and R. L. DeHann. 1986. Pfluegers Arch. 406:73-82) and the possibility of discrete gating within the seal region suggests caution when interpreting patch clamp data from unfamiliar preparations. The data suggests that the permeation pathway is the narrow space between the hydrophobic substrate and the pipette. Since this space has one hydrophobic wall, a hydrophilic channel lining may not be essential for channel permeation and gating.
The open channel characteristics of the bacterial porin Omp32 from Comamonas acidovorans were investigated by means of conductance measurements in planar lipid bilayers of the Montal-Mueller type. Particularly at low salt conditions (< or = 30 mM KCl) Omp32 exhibited some unusual asymmetric and nonlinear functional properties. Current-voltage relationship measurements showed that conductance depends on the orientation of porin molecules and is a nonlinear function of the applied membrane potential. Conductance also depends on the salt concentration in a manner not common to porins and the salt concentration modulates the nonlinearity of conductance-voltage relationships. Omp32 is strongly anion-selective. The nonlinear and asymmetric conductance of the open channel is a new observation in porins.
Ion concentration polarization is the fundamental transport phenomenon that occurs near ion-selective membranes, but this important membrane phenomenon has been poorly understood due to theoretical and experimental challenges. Here, we report the first direct measurements of detailed flow and electric potential profiles within and near the depletion region. This work is an important step towards a full characterization of this coupled transport problem. Using microfabricated electrodes integrated with the microfluidic device, we measured and confirmed that the electric field inside an ion depletion region is amplified more than 30 fold compared to outside of the depletion zone due to the highly non-uniform ion concentration distribution along the microchannel. As a result, the electrokinetic motion of both fluid (electroosmosis) and particle (electrophoresis) was significantly amplified. The detailed flow profile within the depletion zone was also measured for the first time by optically tracking photobleached neutral dye molecules. We further showed that the amplified electrokinetic flows generated in this device may be used as a field-controlled, microfluidic fluid pump and switch.
Ten Ag+-selective ionophores have been characterized in terms of their potentiometric selectivities and complex formation constants in solvent polymeric membranes. The compounds with π-coordination show much weaker interactions than those with thioether or thiocarbamate groups as the coordinating sites. Long-term studies with the best ionophores show that the lower detection limit of the best Ag+ sensors can be maintained in the subnanomolar range for at least one month. The best ionophores have also been characterized in fluorescent microspheres. The so far best lower detection limits of 3× 10−11M (potentiometrically) and 2 × 10−11M Ag+ (optically) are found with bridged thiacalixarenes.
Dynamic control of chemical microenvironments is essential for continued development in numerous fields of life sciences. Such control could be achieved with active chemical circuits for delivery of ions and biomolecules. As the basis for such circuitry, we report a solid-state ion bipolar junction transistor (IBJT) based on conducting polymers and thin films of anion- and cation-selective membranes. The IBJT is the ionic analogue to the conventional semiconductor BJT and is manufactured using standard microfabrication techniques. Transistor characteristics along with a model describing the principle of operation, in which an anionic base current amplifies a cationic collector current, are presented. By employing the IBJT as a bioelectronic circuit element for delivery of the neurotransmitter acetylcholine, its efficacy in modulating neuronal cell signaling is demonstrated.
All-solid-state sensors with polyvinyl chloride (PVC)-based membranes using off-the-shelf N-hydroxysuccinimide (NHS) and succinimide (Succ) ionophores were prepared using DOP (dioctyl phthalate) and NPOE (ortho-nitrophenyloctyl ether) as plasticizers. Good responses were obtained when NHS was used. The potentiometric response of the proposed electrode is independent of pH over the range 2–6. The electrode shows a fast response time of 0.25 s. The electrode exhibits a Super-Nernstian response, with 37.5 mV/decade, with a potentiometric detection limit of 4.4 μM. The proposed sensor revealed good selectivity towards a group of transition metal ions.
The cost of materials is one of the biggest barriers for wastewater driven microbial fuel cells (MFCs). Many studies use expensive materials with idealistic wastes. Realistically the choice of an ion selective membrane or nonspecific separators must be made in the context of the cost and performance of materials available. Fourteen membranes and separators were characterized for durability, oxygen diffusion and ionic resistance to enable informed membrane selection for reactor tests. Subsequently MFCs were operated in a cost efficient reactor design using Nafion, ethylene tetrafluoroethylene (ETFE) or polyvinylidene fluoride (PVDF) membranes, a nonspecific separator (Rhinohide), and a no-membrane design with a carbon-paper internal gas diffusion cathode. Peak power densities during polarisation, from MFCs using no-membrane, Nafion and ETFE, reached 67, 61 and 59 mWm-2, and coulombic efficiencies of 68±11%, 71±12% and 92±6%, respectively. Under 1000Ω, Nafion and ETFE achieved an average power density of 29 mWm-2 compared to 24 mWm-2 for the membrane-less reactors. Over a hypothetical lifetime of 10 years the generated energy (1 to 2.5 kWhm-2) would not be sufficient to offset the costs of any membrane and separator tested.
In developing bean (Phaseolus vulgaris) seeds, phloem-imported nutrients move in the symplast from sieve elements to the ground parenchyma cells where they are transported across the plasma membrane into the seed apoplast. To study the mechanisms underlying this transport, channel currents in ground parenchyma protoplasts were characterized using patch clamp. A fast-activating outward current was found in all protoplasts, whereas a slowly activating outward current was observed in approximately 25% of protoplasts. The two currents had low selectivity for univalent cations, but the slow current was more selective for K(+) over Cl(-) (P(K):P(Cl) = 3.6-4.2) than the fast current (P(K):P(Cl) = 1.8-2.5) and also displayed Ca(2+) selectivity. The slow current was blocked by Ba(2+), whereas both currents were blocked by Gd(3+) and La(3+). Efflux of K(+) from seed coat halves was inhibited 25% by Gd(3+) and La(3+) but was stimulated by Ba(2+) and Cs(+), suggesting that only the fast current may be a component in the pathway for K(+) release. An "instantaneous" inward current observed in all protoplasts exhibited similar pharmacology and permeability for univalent cations to the fast outward current. In outside-out patches, two classes of depolarization-activated cation-selective channels were observed: one slowly activating of low conductance (determined from nonstationary noise to be 2.4 pS) and another with conductances 10-fold higher. Both channels occurred at high density. The higher conductance channel in 10 mM KCl had P(K):P(Cl) = 2.8. Such nonselective channels in the seed coat ground parenchyma cell could function to allow some of the efflux of phloem-imported univalent ions into the seed apoplast.; Wen-Hao Zhang...
examined in Choanephora cucurbita rum during the early stages
of infection by Piptocephalis virginiana » There was a small
but consistent increase in the leakage of electrolytes, amino
acids and sugars as a result of infection. These low levels
of differential leakage in infected tissues are explained on
the basis of the nature of this obligate, biotrophic, mycoparasitic
system. Quantitative analysis of the twenty six
amino acids and amino compounds detected in the leacheates —
showed similar profiles in infected and control host and no
new species of amino acids or amino compounds were detected
in either infected or control host leacheates. Comparatively
high amounts of aspartic acid, glutamic acid and alanine were
found in the leacheates of host and infected host . Analyses
of the sugars comprising the leacheates of infected and
control host showed the presence of eight sugars, among which
glucose was found in significant amounts (50-53%) ' The
nutritional implication of this preferential leakage is
discussed. No significant difference was observed in the
leacheates of infected host sugar profiles compared with that
of the control host. Profiles of the internal pool sugars of
infected and control host did not reflect that obtained from
the leacheate data...
Intracellular micro-electrode techniques were used to measure the electrical resistances of the cell membranes and the shunt pathway and intracellular ionic activities in trout urinary bladder when the tissue was incubated in Ringer solution and in the presence of the polyene antibiotic ionophore amphotericin B. In control conditions the transepithelial potential was zero and the intracellular potential was -56 mV. The intracellular ionic activities measured with single- and double-barrel ion-sensitive micro-electrodes for the first time in a fish bladder (aiNa = 16 mM, aiK = 87 mM, and aiCl = 21 mM) indicate an active accumulation of K and Cl ions and an active extrusion of Na ions by the cell. The maintenance of intracellular Cl activity above its equilibrium value depended on the presence of Na ions in the mucosal medium, but was independent of the presence of K ions. Flat cable analysis yielded values for transepithelial, apical, basolateral and shunt resistances of 197, 2790, 1986 and 205 omega cm-2 respectively. Equivalent circuit analysis using amphotericin B yielded similar values for shunt resistance. The paracellular pathway accounts for 96% of transepithelial current flow and this epithelium may be classified as 'leaky'. The cells are electrically coupled with a space constant of 354 micron. Amphotericin B when added to the mucosal solution induced an immediate serosa positive transepithelial potential of about 9 mV and a short-circuit current of 64 microA cm-2. The Vt was ouabain sensitive and dependent on mucosal Na concentration. The origin of the antibiotic induced transepithelial potential was an increase in the sum of the cell membrane electromotive forces. The apical membrane potential depolarized to -7 mV and its resistance fell to 433 omega cm-2. During the first 10 min of exposure aiNa increased to 80 mM and aiK decreased to 7 mM with only a small change in aiCl. The changes in cellular Na+ and K+ activities were in accordance with their passive redistribution down their electrochemical gradients.
Artículo de publicación ISI; A new coumarin-based ‘turn-off’ fluorescent probe, 7-(diethylamino)-N-(1,3-dihydroxy-2-(hydroxymethyl)
propan-2-yl)-2-oxo-2H-chromene-3-carboxamide (AGD) was synthesized. This compound is
highly selective for ferrous ions (Fe2þ) and can reversibly detect them in aqueous medium. The probe
localizes to the cell membrane in living cells, where it can detect changes in Fe2þ concentration. Molecular
dynamics (MD) simulations indicate that AGD interacts with the lipid bilayer at the level of the
Three polyvinylchloride (PVC) membrane sensors for the determination of moexipril hydrochloride were prepared and characterized. The sensors are based on the use of the ion association complexes of moexipril cation with either ammonium reineckate (sensor 1) or tetraphenyl borate (sensor 2) or phosphotungistic acid (sensor 3) counter anions as ion exchange sites in the PVC matrix. The performance characteristics of these sensors were evaluated according to IUPAC recommendations, which reveal a fast, stable and linear response for moexipril over the concentration range of 10-6 to 10-2 M for the three sensors with cationic slopes of 29.1, 30.1 and 30.2 mV per concentration decade for the three sensors, respectively. The direct potentiometric determination of moexipril hydrochloride using the proposed sensors gave recoveries % of 99.64 ± 0.34, 99.34 ± 0.56 and 99.68 ± 0.42 for the three sensors, respectively. The sensors were used for determination of moexipril hydrochloride in pharmaceutical formulations and in plasma. Validation of the method shows suitability of the proposed sensors for use in quality control assessment of moexipril hydrochloride. The obtained results were in a good agreement with those obtained using the reported spectrophotometric method.