Fabrication and Characterization of Iridium Oxide pH Microelectrodes Based on Sputter Deposition Method

pH value plays an important role in many fields such as chemistry and biology; therefore, rapid and accurate pH measurement is very important. Because of its advantages in preparation, wide test range, rapid response, and good biocompatibility, iridium oxide material has received more and more attention. In this paper, we present a method for preparing iridium oxide pH microelectrodes based on the sputter deposition method. The sputtering parameters of iridium oxide are also studied and optimized. Open-circuit potential tests show that microelectrodes exhibit near-Nernstian pH response with good linearity (about 60 mV/pH), fast response, high stability (a slight periodic fluctuation of potential change <2.5 mV in 24 h), and good reversibility in the pH range of 1.00–13.00.

Membrane Permeabilities of Ascorbic Acid and Ascorbate

Vitamin C (VC)—a collective term for the different oxidation and protonation forms of ascorbic acid (AscH)—is an essential micronutrient that serves as (i) a potent antioxidant and (ii) a cofactor of a manifold of enzymatic processes. Its role in health is related to redox balance maintenance, which is altered in diseases such as obesity, cancer, neurodegenerative diseases, hypertension, and autoimmune diseases. Despite its importance, VC uptake has been poorly investigated. Available literature values for the passive membrane permeability P of lipid bilayers for AscH scatter by about 10 orders of magnitude. Here, we show by voltage clamp that P − of AscH’s anionic form (ascorbate Asc − ) is negligible. To cross the membrane, Asc − picks up a proton in the membrane vicinity and releases it on the other side of the membrane. This leads to a near-membrane pH drop that was visualized by scanning pH microelectrodes. The AscH concentration dependent pH profiles indicated P = 1.1 ± 0.1 × 10 − 8 cm / s . Thus, AscH’s P is comparable to that of sorbitol and much lower than that of other weak acids like acetic acid or salicylic acid. The observation suggests that the capacity of the passive transcellular transport pathway across the lipid matrix does not suffice to ensure the required VC intake from the gastrointestinal tract.

The gut microenvironment of helicid snails (Gastropoda: Pulmonata): in-situ profiles of pH, oxygen, and hydrogen determined by microsensors

In-situ profiles of pH, oxygen, and hydrogen were measured in isolated guts of starved terrestrial gastropods belonging to four species, Cornu aspersum (syn. Helix aspersa), Elona quimperiana, Helix pomatia, and Helix lucorum (excepted pH), using Clark-type oxygen and hydrogen microsensors and liquid-ion-exchanger pH microelectrodes. The pH profiles in the two phyllophagous species, H. pomatia and C. aspersum, increased by 0.9 and 1.4 from the crop to the distal intestine (pH 6.4 and 7.4, respectively). In the saprophagous E. quimperiana, as in H. pomatia, the pH along the gut axis remained acidic (5.1–6.6), suggesting saprophagous habits in the latter. In all four species, no oxygen was detected in the gut lumen. Nevertheless, steep oxygen gradients around the gut epithelium indicated high oxygen-uptake rates. The estimated respiratory activity of the intestine ranged between 6.5 (E. quimperiana) and 13.1 (H. lucorum) μmol O2·g fresh mass–1·h–1. Hydrogen accumulated in the intestine and digestive gland of all snails tested, with the highest values in E. quimperiana and H. pomatia (58 and 78 μM, respectively). These results provide the basis for a better understanding of the microbial and biochemical processes involved in digestion.

Modulation of Spreading Depression by Changes in Extracellular pH

Spreading depression (SD) and related phenomena have been implicated in hypoxic-ischemic injury. In such settings, SD occurs in the presence of marked extracellular acidosis. SD itself can also generate changes in extracellular pH (pHo), including a pronounced early alkaline shift. In a hippocampal slice model, we investigated the effect of interstitial acidosis on the generation and propagation of SD in the CA1 stratum radiatum. In addition, a carbonic anhydrase inhibitor (benzolamide) was used to decrease buffering of the alkaline shift to investigate its role in the modulation of SD. pHo was lowered by a decrease in saline HCO3 − (from 26 to 13 to 6.5 mM at 5% CO2), or by an increase in the CO2 content (from 5 to 15% in 26 mM HCO3 −). Recordings with pH microelectrodes revealed respective pHo values of 7.23 ± 0.13, 6.95 ± 0.10, 6.67 ± 0.09, and 6.97 ± 0.12. The overall effect of acidosis was an increase in the threshold for SD induction, a decrease in velocity, and a shortened SD duration. This inhibition was most pronounced at the lowest pHo(in 6.5 mM HCO3 −) where SD was often blocked. The effects of acidosis were reversible on return to control saline. Benzolamide (10 μM) caused an approximate doubling of the early alkaline shift to an amplitude of 0.3–0.4 U pH. The amplified alkalosis was associated with an increased duration and/or increased velocity of the wave. These effects were most pronounced in acidic media (13 mM HCO3 −/5% CO2) where benzolamide increased the SD duration by 55 ± 32%. The initial velocity (including time for induction) and propagation velocity (measured between distal electrodes) were enhanced by 35 ± 25 and 26 ± 16%, respectively. Measurements of [Ca2+]o demonstrated an increase in duration of the Ca2+ transient when the alkaline shift was amplified by benzolamide. The augmentation of SD caused by benzolamide was blocked in media containing the N-methyl-d-aspartate (NMDA) receptor antagonistdl−2-amino-5-phosphonovaleric acid. These data indicate that the induction and propagation of SD is inhibited by a fall in baseline pH characteristic of ischemic conditions and that the early alkaline shift can remove this inhibition by relieving the proton block on NMDA receptors. Under ischemic conditions, the intrinsic alkalosis may therefore enable SD and thereby contribute to NMDA receptor-mediated injury.

Stratification of microbial metabolic processes and redox potential change in an aerobic biofilm studied using microelectrodes

In this study we used oxygen, sulfide, redox potential and pH microelectrodes to examine the stratification of microbial metabolic processes and the change of redox potential within an aerobic biofilm used to treat azo dye containing wastewater. These microelectrodes have tip diameters of 3 to 20 μm and a high spatial resolution. They were used to measure the profiles of oxygen, total dissolved sulfide, redox potential and pH as a function of depth in the biofilm. These profiles demonstrated that oxygen was depleted at 550 μm from the surface and the deeper section of the biofilm was actually anaerobic. While aerobic oxidation took place only in a shallow layer near the surface, sulfate reduction occurred in the deeper anaerobic zone, even with a low concentration of sulfate (6.75 mg/l as SO2−) in the bulk solution. We discovered a sharp decrease of redox potential (271 mV) from a positive potential to a negative potential within a very narrow band of 50 μm near the interface between the aerobic zone and the sulfate reduction zone. The new experimental findings support the concept of stratification of the microbial metabolic processes in biofilms.