PH-sensing

The glass pH electrode has been the most widely used tool for measurement of pH. Optical pH sensing is one of the most well established methods of pH determinations, which is based on measurements of the absorption spectmm of an indicator, either dissolved in the test solution or immobilized on a substrate. 

Proton-sensing GPCRs. Table 1 pH-sensing receptors... 

TDAG8 appears restricted to the immune system. As tissue inflammation is usually followed by local hypoxia and acidosis, the pH-sensing property of TDAG8 appeared of particular interest in this context. Unexpectedly, however, the phenotype of mice... 

Li, X.-P., A. M. Gilmore, S. Caffarri et al. (2004). Regulation of photosynthetic light harvesting involves intrathylakoid lumen pH sensing by the PsbS protein. J Biol Chem 279(22) 22866-22874. 

Spectacular differences in absorption/excitation spectra are often observed for the dyes that exist in protonation-deprotonation equilibria. Their straightforward application is for pH sensing and also for designing the reporters, in which the shifting of such equilibrium by external proton donor or acceptor group is involved in sensing event. 

Delmarre D., Meallet-Renault R., Bied-Charreton C., Pasternack R., Incorporation of water-soluble porphyrins in sol-gel matrices and application to pH sensing, Anal. Chim. Acta 1999 401 125-128. 

Blue R., Stewart G., Fibre-optic evanescent wave ph sensing with dye doped sol-gel films, Intern. J. Optoelectron. 1995 10 211-222. 

It is clear from the Nemst equation that the temperature of the solution affects the response slope (2.303A7//0 of the calibration curve. The electrode voltage changes linearly in relationship to changes in temperature at a given pH therefore, the pH of any solution is a function of its temperature. For example, the electrode response slope increases from 59.2mV/pH at 25°C to 61.5 mV/pH at a body temperature of 37°C. For modem pH sensing systems, a temperature probe is normally combined with the pH electrode. The pH meter with an automatic temperature compensation (ATC) function automatically corrects the pH value based on the temperature of the solution detected with the temperature probe. 

The pH sensing glass consists of mainly Si02 with some non-silicon components. The most widely known example of pH sensitive glass is Coming 015, which contains... 

Polymer-based pH sensors are not suitable for continuous in-vivo measurements due to the poor biocompatibility of plasticizers used in the polymer membrane. To minimize such a problem, surface treatment or using a reduced amount of plasticizers has been proposed [71]. In order to improve stability and adhesion, polyurethane has been used as an alternative to PVC membranes in the construction of pH sensing membranes [72, 73],... 

Another example of an Si-based pH sensing device is an electrolyte-insulator-semiconductor (EIS) structure. In a typical EIS sensor, a voltage applied to the semiconductor back gate (Vgate) attracts ions in the electrolyte to the oxide surface. The... 

Although the redox reaction mechanisms of iridium oxide are still not clear, most researchers believe that the proton exchange associated with oxidation states of metal oxides is one of the possible pH sensing mechanisms [41, 87, 100, 105], During electrochemical reactions, oxidation state changes in the hydrated iridium oxide layer are... 

Considering the H+ dependent redox reaction between two oxidation states of the iridium oxide as the basis of the pH sensing mechanism, the electrode potential changes to the hydrogen ion concentration are expressed by Nemstian equation ... 

Such electrodes should be sufficient as a reference electrode for short-term usage or as a disposable electrode. However, the requirement of a pre-hydration time may limit its applications for fast measurements, such as POCT (the point-of-care testing), due to its slow response time. In fact, the lack of long-term stable microreference electrodes will continue to hamper the development of integrated pH sensing systems. 

FIGURE 10.8 Two-dimensional pH distribution measured by an iridium oxide-based planar micro-pH electrode array after stimulation for only one minute. The electrode site in dark color is the Pt stimulating electrode. All other 15 electrodes are iridium oxide pH sensing electrodes. (Reproduced from [19], with permission from the Electrochemical Society, Inc.)... 

Metal/metal oxides are the materials of choice for construction of all-solid-state pH microelectrodes. A further understanding of pH sensing mechanisms for metal/metal oxide electrodes will have a significant impact on sensor development. This will help in understanding which factors control Nemstian responses and how to reduce interference of the potentiometric detection of pH by redox reactions at the metal-metal oxide interface. While glass pH electrodes will remain as a gold standard for many applications, all-solid-state pH sensors, especially those that are metal/metal oxide-based microelectrodes, will continue to make potentiometric in-vivo pH determination an attractive analytical method in the future. 

U. H. Manyam, Application of dye-doped sol gel for pH sensing, PhD thesis, Rutgers University, 2006. 

A combination pH electrode is the most commonly used electrode to determine soil pH. It is illustrated in Figure 9.5 and shown in Figure 9.6 (D), which shows the pH sensing bulb and reference side. As an alternative to the... 

Thus as in the titration curves in the solution, the response range of the pH indicators for a signal protonation process is about 2-3 pH units for absorbance-based measurements. Long-range pH sensing has to be achieved by using mixed dyes doped in solid support. While fluorescence sensors have a wider linearity because it is only related to one form of the indicator. 

M. J.P. Leiner and P. Hartmann, Theory and practice in optical pH sensing, Sensor Actuat. B-Chem., 11(1-3) (1993) 281-289. 

Sun H, Scharff-Poulsen AM, Gu H, Almdal K (2006) Synthesis and characterization of ratiometric pH sensing nanoparticles with covalently attached fluorescent dyes. Chem Mater... 

Allard E, Larpent C (2008) Core-shell type dually fluorescent polymer nanoparticles for ratiometric pH-Sensing. J Polym Sci Part A Polym Chem 46 6206-6213... 

Funfak A, Cao J, Wolfbeis OS, Martin K, Kohler JM (2009) Monitoring cell cultivation in microfluidic segments by optical pH sensing with a micro flow-through fluorometer using dye-doped polymer particles. Microchim Acta 164 279-286... 

---