PH sensing mechanisms

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 ... 

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. 

The principle of pH electrode sensing mechanisms which are based on glass or polymer membranes is well investigated and understood. Common to all potentiometric ion selective sensors, a pH sensitive membrane is the key component for a sensing mechanism. When the pH sensitive membrane separates the internal standard solution with a constant pH from the test solution, the potential difference E across the membrane is determined by the Nemst equation ... 

Other pH-sensing transducers used in biosensors are metal oxide electrodes. Beside the common antimony oxide electrode, palladium oxide and iridium oxide probes have been coupled with immobilized enzymes. These sensors may be miniaturized by using chemical vapor deposition technology. Moreover, they are mechanically more stable than glass electrodes. Unfortunately the measuring signal of metal oxide electrodes is affected by redox active substances. 

To detect CO2 in ambient air, the sensor is utilized as a novel design Severinghaus sensor. A sputtered thin film of iridium oxide is used as the pH sensing electrode. Iridium oxide, as well as several other metal oxides, has been shown to be pH sensitive [26], although the mechanism of this effect is not understood in detail. It is believed that the pH dependence is related to one of the redox couples of iridium oxide, such as... 

As a supplement for widely used glass electrodes, ISFET sensors provide some unique features. One of them is the capability of dry storage, which helps to avoid the problem of shelf-life and the hydration requirement time for glass electrodes. Fast response is another feature of an ISFET pH sensor. In a comparison study of commercial pH sensors reported by Smit et al. [84], the ISFET sensor showed about a ten times faster response than that of glass electrodes, and a response similar to that of an iridium oxide-based electrode [100, 104], This fast response time of ISFET sensors was attributed to its sensing mechanism, which is based on the electrostatic interaction of H+ ions with surface charge at the gate surface [66]. 

In this section, the application of the sensing mechanism described in Section 3.1 to an in vitro environmental model containing interfering cell culture medium is described. In this experiment the cell culture medium was used in place of deionized water, that is, 0.5 ml DMEM F-12 culmre media containing 1% fetal bovine serum (FBS) was mixed with 5.5 ml AuNPs stock solution to obtain a final AuNPs concentration of 2.25 X 10 NPs ml with pH 6.85. Additionally, to adjust the ion concentration, as was done previously, 100 pi of 10 mM FeCl3 at pH 2.90 was added into the mixture. Finally, 1 ml of 15 mM GSH at pH 3.44 was added to mediate the AuNPs. All optical characterizations were performed within 6 h of solution preparations. 

Maruyama, H., Matsumoto, H., Fukuda,T. and Aral, F, Functionalized hydrogel surface patterned in a chip for local pH sensing, Micro Electro Mechanical Systems, 2008. MEMS 2008. IEEE 21st International Conference on, pp. 224-227. 

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