pH Fronts Research Articles

The Role of Ph Fronts in Tissue Electroporation Based Treatments

Treatments based on electroporation (EP) induce the formation of pores in cell membranes due to the application of pulsed electric fields. We present experimental evidence of the existence of pH fronts emerging from both electrodes during treatments based on tissue EP, for conditions found in many studies, and that these fronts are immediate and substantial. pH fronts are indirectly measured through the evanescence time (ET), defined as the time required for the tissue buffer to neutralize them. The ET was measured through a pH indicator imaged at a series of time intervals using a four-cluster hard fuzzy-c-means algorithm to segment pixels corresponding to the pH indicator at every frame. The ET was calculated as the time during which the number of pixels was 10% of those in the initial frame. While in EP-based treatments such as reversible (ECT) and irreversible electroporation (IRE) the ET is very short (though enough to cause minor injuries) due to electric pulse characteristics and biological buffers present in the tissue, in gene electrotransfer (GET), ET is much longer, enough to denaturate plasmids and produce cell damage. When any of the electric pulse parameters is doubled or tripled the ET grows and, remarkably, when any of the pulse parameters in GET is halved, the ET drops significantly. Reducing pH fronts has relevant implications for GET treatment efficiency, due to a substantial reduction of plasmid damage and cell loss.

Inhibition of pH fronts in corrosion cells due to the formation of cerium hydroxide

The effect of cerium-based corrosion inhibitors on the pH front between the alkaline cathode and acidic anode in corrosion cells has been studied. The cerium component of these inhibitors can affect the pH front since it precipitates in an alkaline environment as cerium hydroxide, which is important since the corrosion inhibition mechanism of the cerium component is a result of its deposition as a highly electrical resistive (passivation) layer on the cathode. It is studied whether the cerium can reach the cathode when fed into the corrosion cell from an external source after the onset of corrosion. To this end a simulation model was set up that includes the Poisson–Nernst–Planck theory to describe ion transport and the Frumkin–Butler–Volmer equation to describe charge transfer at the electrodes. In this model both the self-dissociation of water and the formation of cerium hydroxide are taken into account. To support our findings experimentally a corrosion cell consisting of an aluminum and copper electrode was used, in which the pH fronts were visualized using a pH-indicator. Two types of inhibitors were used; namely, highly soluble CeCl3 and sparsely soluble cerium dibutylphosphate, Ce(dbp)3. The results show that CeCl3 can reduce the size of the alkaline region and reach the cathode to form a passivation layer, whereas the solubility in case of Ce(dbp)3 is too low to supply sufficient amounts of trivalent cerium cations to penetrate the alkaline region. This behavior can be explained by the simulation results, which reveal a threshold for the corrosion inhibitor solubility below which no passivation of the cathode occurs.

Geochemical impact of a low-pH cement liner on the near field of a repository for spent fuel and high-level radioactive waste

In Switzerland the geological storage in the Opalinus Clay formation is the preferred option for the disposal of spent fuel (SF) and high-level radioactive waste (HLW). The waste will be encapsulated in steel canisters and emplaced into long tunnels that are backfilled with bentonite. Due to uncertainties in the depth of the repository and the associated stress state, a concrete liner might be used for support of emplacement tunnels.Numerical reactive transport calculations are presented that investigate the influence of a concrete liner on the adjacent barrier materials, namely bentonite and Opalinus Clay. The geochemical setup was tailored to the specific materials foreseen in the Swiss repository concept, namely MX-80 bentonite, low-pH concrete (ESDRED) and Opalinus Clay. The heart of the bentonite model is a new conceptual approach for representing thermodynamic properties of montmorillonite which is formulated as a multi-component solid solution comprised of several end-members.The presented calculations provide information on the extent of pH fronts, on the sequence and extent of mineral phase transformations, and on porosity changes on cement–clay interfaces. It was found that the thickness of the zone containing significant mineralogical alterations is at most a few tens of centimeters thick in both the bentonite and the Opalinus Clay adjacent to the liner. Near both interfaces, bentonite–concrete liner and concrete liner–Opalinus Clay, the precipitation of minerals causes a reduction in the porosity. The effect is more pronounced and faster at the concrete liner–Opalinus Clay interface. The simulations reveal that significant pH-changes (i.e. pH>9) in bentonite and Opalinus Clay are limited to small zones, less than 10cm thick at the end of the simulations. It is not to be expected that the zone of elevated pH will extend much further at longer times.

The Role of pH Fronts in Reversible Electroporation

We present experimental measurements and theoretical predictions of ion transport in agar gels during reversible electroporation (ECT) for conditions typical to many clinical studies found in the literature, revealing the presence of pH fronts emerging from both electrodes. These results suggest that pH fronts are immediate and substantial. Since they might give rise to tissue necrosis, an unwanted condition in clinical applications of ECT as well as in irreversible electroporation (IRE) and in electrogenetherapy (EGT), it is important to quantify their extent and evolution. Here, a tracking technique is used to follow the space-time evolution of these pH fronts. It is found that they scale in time as , characteristic of a predominantly diffusive process. Comparing ECT pH fronts with those arising in electrotherapy (EChT), another treatment applying constant electric fields whose main goal is tissue necrosis, a striking result is observed: anodic acidification is larger in ECT than in EChT, suggesting that tissue necrosis could also be greater. Ways to minimize these adverse effects in ECT are suggested.

Electrochemical treatment of tumors using a one-probe two-electrode device

We have recently shown that in the classical electrochemical treatment (EChT) of tumors, i.e., the passage of a direct electric current through two electrodes inserted locally in the tumor tissue, from an initial uniform condition two pH fronts evolve, expanding towards each other, inducing extreme pH changes and tumor destruction, mainly by necrosis. Here we extend these results introducing a one-probe two-electrode device (OPTED) containing the cathode and the anode very close to each other (1 mm). Experiments show that upon application of the OPTED-EChT, two half-spherical pH fronts, one basic and the other acid (from cathode and anode, respectively), expand towards the periphery configuring a distorted full sphere. Tracking of the pH front advance reveals a time scaling close to t 1/2, signature of a diffusion-controlled process. An analytic model presented allows the estimation of the time needed for total tumor destruction with a minimum compromise of healthy tissue. Main advantages of the OPTED are the insertion of one applicator rather than two or more (thus minimizing tissue intrusion, for instance, in the nervous system), the ability to reach tumors beyond capabilities of conventional surgery and the minimization of electric current circulation through the treated organ. We propose here this new design which could have significant implications in EChT optimal operative conditions, in particular, in the way in which the evolving pH spherical fronts can cover and destroy a cancer cell spherical casket.

Front propagation in the bromate–sulfite–ferrocyanide–aluminum (III) system: Autocatalytic front in a buffer system

We study the propagation of pH fronts in a narrow, pseudo-one-dimensional strip of gel containing bromate, sulfite, ferrocyanide and aluminum ions. The front propagation speed v decreases with [ Al 3+] as 1 / v = 1 / v 0 + c [ Al 3 + ] , where c is a constant. We determine the diffusion coefficients of protons and ferricyanide ions and find that Al(OH) 3 loaded in agarose or acrylamide gels binds protons reversibly, thereby slowing the propagating fronts without significantly affecting the diffusion of ferricyanide ions. We present a model that qualitatively reproduces the experimental behavior, and we suggest some general principles governing autocatalytic fronts in buffered systems.

PH front tracking in the electrochemical treatment (EChT) of tumors: Experiments and simulations

The extreme cathodic and anodic pH fronts induced in the electrochemical treatment of tumors (EChT) are the main cause of tumor necrosis. Here, we study pH fronts interaction in a tissue under EChT through in vitro and in silico modeling. The in vitro model considers the tumor tissue as a buffered gel with saline (NaCl) content and the cathodic treated area as the area of phenolphtalein virage. The in silico model solves the 1D Nernst–Planck equations for ion transport in a four-ion electrolyte. In silico modeling predicts an initial neutral pH profile evolving into extreme cathodic alkaline and anodic acidic fronts moving towards each other and colliding, thus excluding the existence of a biological pH region between them. Moreover, the model predicts that as electric current increases, pH front scaling grows as t 1 , unveiling a transition from a diffusion to a migration governed regime. Theory and simulations have a strong correlation with experimental measurements. Since necrotic areas correlate well with those covered by alkaline and acid fronts advance, pH front tracking can be used to predict the extent of tumor destruction and thus, the assessment of EChT effectiveness.

PH-Wertpufferung im Abstrom von Reaktionswänden

Sequenced column experiments were conducted in order to assess the impact of elevated pH caused by two possible reactive media combinations for permeable reactive barriers (Fe degrees/Oxygen Release Compound (ORC) and Fe degrees/activated carbon) on the groundwater quality in a lignite sand. In the experiments, pH values > 9,5 downgradient of Fe degrees and ORC reactors were buffered to lower values by the lignite sand. In the lignite sand, pH fronts formed, propagating downgradient at a velocity that was 220-1,100 times slower than the groundwater flow velocity (0.71 and 0.83 m/d, respectively). The amount of base equivalents buffered in both experiments (52 and 7.3 meq/kg sediment, respectively) depended mainly on the different amounts of base equivalents present in the inflowing solutions. The majority of the base neutralization can be explained by the de-protonation of functional groups on mineral surfaces and organic carbon. The extent of other buffering mechanisms, such as mineral dissolution and pyrite oxidation, was limited in time. For the lignite sand examined here, high pH values downgradient of permeable reactive barriers will be limited to a narrow zone, even in the long term, and high pH values will not limit the possible implementation of a permeable reactive barrier.

Detection of advected concentration and pH fronts from self‐potential measurements

In this paper, we report on a series of experiments aimed at testing the possibility of detecting concentration or pH fronts advected through a well‐controlled, laboratory‐scale, artificial aquifer system, using self‐potential monitoring. A rectangular Plexiglas tank was filled up with water‐saturated sand, inside of which an array of unpolarizable electrodes was installed. After a nearly uniform, steady state flow was established by tilting the tank and controlling the water level at both ends, we abruptly changed the salt concentration or pH of the upstream fluid. During the subsequent motion of the concentration or pH front thus generated, the electrical potential differences between each electrode and a reference were recorded. We observed systematic, sharp variations successively occurring at increasing distances in the flow direction. These signals precisely corresponded to the calculated arrival times of the front at the various electrodes. Additional experiments were performed, in which the lateral motion of a transverse concentration or pH front was monitored. In the case of a NaCl front, the self‐potential signal can be modeled as the sum of two terms, (1) a junction potential term produced by the difference in ionic mobility of Na+ and Cl− and (2) an electrokinetic term corresponding to the variations of the electrokinetic coupling coefficient caused by changes in concentration. In the case of a pH front, only the electrokinetic term appears to be present.

A transition from propagating fronts to target patterns in the hydrogen peroxide–sulfite–thiosulfate medium

In this study we explored nonlinear spatial–temporal behavior in the oxidation of sulfite by hydrogen peroxide, in which thiosulfate was added to implement a negative feedback, which consequently led to a transition from propagating fronts to target patterns. Our investigation in the homogeneous system showed that increasing the concentration of hydrogen peroxide or decreasing the concentration of sulfite accelerated both the positive and negative feedbacks and therefore could facilitate the transition from front waves to target patterns. On the other hand, increasing the initial concentration of hydrogen ions or decreasing the concentration of thiosulfate mainly accelerated the autocatalytic production of hydrogen ions. The experimental observations were qualitatively reproduced with a model developed recently by Rabai and Hanazaki. In reaction–diffusion media, spontaneous formation of propagating pH fronts was obtained within a narrow range of sulfite and thiosulfate concentrations. The velocity of these traveling fronts was found to decrease with respect to increasing concentrations of thiosulfate and sulfite or decreasing concentrations of hydrogen ion and hydrogen peroxide.

Purification of recombinant green fluorescent protein using chromatofocusing with a pH gradient composed of multiple stepwise fronts.

Green fluorescent protein (GFP), which fluoresces in the green region of the visible spectrum and is widely used as a reporter for gene expression and regulation, was overexpressed in the JM105 strain of Escherichia coli transformed with pBAD-GFP. A two-step chromatofocusing procedure was used to purify GFP starting from cell lysate, with each step employing a pH gradient extending from pH 5.5 to 4.0. The first chromatofocusing step was performed using a low-pressure column in which a retained stepwise pH front formed by adsorbed buffering species was used to capture GFP directly from clarified cell lysate and selectively focus it into a chromatographic band. The second step utilized a high-performance column under mass overloaded conditions where a similar pH front acted as a protein displacer and led to the formation of a highly concentrated rectangular band of GFP. The overall procedure yielded a 50-fold increase in purity, a 20-fold volume reduction, and a recovery and purity for GFP of 60% and 80%, respectively. Because the method employs a strong-base ion-exchange column packing and low-cost buffers formed with formic and acetic acids instead of the proprietary column packings and polyampholyte elution buffers more generally used for chromatofocusing, it appears to be a practical alternative for the preparative ion-exchange chromatography of GFP in particular and for the recovery of recombinant proteins from cell lysate in general. A discussion is also given concerning the choice of appropriate buffers for the rational design of pH gradients involving retained, stepwise pH fronts that span a given pH range and of the use of the fluorescence properties of GFP for flow visualization and chromatographic process development.

Neutralising processes of municipal solid waste incineration bottom ash in a flow-through system

The neutralising processes of MSWI bottom ash in a flow-through system were investigated by pH titration in a serial batch system. A one-dimensional coupled transport (advection) and geochemical reaction model was used to simulate the neutralising reactions. It has been found that the formation and movement of pH fronts are essential features of neutralising processes in flow-through systems. This characteristic and the washing-out effect makes the neutralising capacity obtained from batch experiment difficult to apply directly to a flow-through system. The neutralising ability of the MSWI bottom ash was significantly less in a flow-through system than in a batch system. It was found that the available neutralising ability of the MSWI bottom ash in the flow-through system is only a third of the neutralising capacity in a batch system. Carbonation reactions have a large influence on the neutralising processes of the MSWI bottom ash. The carbonation transformation plays an important role in the formation of controlling minerals for the MSWI bottom ash in the neutralising processes. Geochemical simulations combined with experiments provided valuable information about the neutralising processes in the flow-through system. This approach provides needed insights into reaction mechanisms and helps to check the basic assumptions for a geochemical model.

Dynamic On-Column pH Monitoring in Capillary Electrophoresis: Application to Volume-Limited Outlet Vials

With capillary electrophoresis, buffer pH must be constant to achieve consistent migration times. Irreproducible separations have been attributed to pH changes due to water hydrolysis in the inlet/outlet vials. A method of measuring the pH of the electrolyte on-column is described that uses wavelength-resolved fluorescence detection. C.SNARF-1 is a fluorescent pH indicator that has a large change in fluorescence emission profile depending on pH. When it is incorporated into the running buffer, monitoring the pH-dependent emission spectra of the C.SNARF-1 allows column pH to be calculated. With reduced-volume outlet buffer vials in the nanoliter to low microliter range, significant changes in pH and column conductivity are measured during a single electrophoretic run, with pH fronts greater than 3 units passing a fixed point on the capillary over a several second period. These changes appear to be caused by reverse-migrating OH- produced at the capillary outlet by the hydrolysis of water.