POTENTIOMETRIC SENSING

Oyama and Hirokawa [804] described a potentiometric pH sensor that used a film of poly(l-amino pyrene) (P(lAPyre)) in which no internal standard solution, as is required in conventional glass pH electrodes, was used. The pH detection was based on the sensor functioning as an ion-sensitive field effect transistor (ISFET). The sensor had high ion selectivity with respect to Na, and Ca, and insensitivity to O2, much like a conventional pH glass-membrane electrode. Response time was in the 30 to 60 sec region, with excellent linearity of response in the pH 4.0 to 10.0 region. 

An interesting P(ANi)-based sensor for the detection of HCN gas was described by Langmaier and Janata [805] Into a P(ANi) film electropolymerized on a Pt substrate were incorporated Hg and Ag salts, which formed P(ANi)-Hg and P(ANi)-Ag clusters due to the spontaneous reduction of the metals. These transition metal/CP clusters then adsorbed HCN, causing a change in the open circuit potential of the P(ANi) film. The response time was however very slow, of the order of minutes. Fig. 17-17 shows some results from this work. As with the conductometric sensor work cited earlier, this work appeared to show that CP sensors are not particularly amenable to detection of gases. 

Typical response of PANI Hg layer to concentration step change of HCN. Concentration steps are (a) 0 to 1.50 3.77 8.36 and 19.3 ppm HCN. For (b) the step is 0 to 19.3 ppm HCN. After Reference [806], reproduced with permission. 

A conductometric P(ANi)/glucose-oxidase sensor was described in Sec. 17.2 above. The same sensor construction could in principle also be used for potentiometric detection of glucose, since the variation of open circuit potential with pH is substantial such work has been described recently [191, 807]. The sensing range is claimed to be higher, with an acceptable calibration curve claimed to be obtained over three orders of magnitude [191]. 

---

Potentiometric sensing based on type 3 derivatives of a-, P-, and y-cyclodextrins has been reported.We have been interested in the type 1 cyclodextrin derivatives developed by Tagaki, e.g., host 41. This type of cyclodextrin derivative is characteristic in that the primary hydroxyl groups at the C-6 positions are exhaustively substituted with long alkyl chains. At the surface of an oiganic membrane, this structural feature allows the secondary hydroxyl side (wider open end of the cavity) to face the aqueous solution to accommodate a guest molecule. The interfacial receptor functions of these cyclodextrin hosts have been confirmed mainly by rt-A isotherm studies. 

Molecular Mechanism of Potentiometric Sensing at the Surface of Ion-Selective Liquid Membranes... 

Reference to Table 4.1 indicates that olefins can be determined by the electrochemical generation in situ of halogens. Bromine is effective for both olefins and sulfur compounds and is the basis for an automatic coulometric titrator for continuous analysis of petroleum streams.17 The basic principle of this instrument is a potentiometric sensing system that monitors bromine concentration in a continuously introduced sample stream. The bromine in the solution reacts with the sample components and causes a decrease in the concentration of bromine. When this decrease is sensed by the potentiometric detection electrodes, the electrolysis current producing bromine adjusts itself to maintain the bromine concentration. Because the sample is introduced at a constant rate, the electrolysis current becomes directly proportional to the concentration of the sample component. Thus, the instrument records the electrolysis current as concentration of sample component and provides a continuous monitor for olefins or sulfur in petroleum streams. 

Zhou YX, Yu B, Levon K (2003) Potentiometric sensing of chiral amino acids. Chem Mater 15(14) 2774-2779... 

Zhou Y, Yu B, Shiu E, Levon K (2004) Potentiometric sensing of chemical warfare agents surface imprinted polymer integrated with an indium tin oxide electrode. Anal Chem 76(10) 2689-2693... 

Potentiometric sensors operate at thermodynamic equihbrium conditions. Thus, in practical potentiometric sensing, the potential measurement needs to be made under zero-current conditions. Consequently, a high-input impedance electrometer is often used for measurements. Also, the response time for a potentiometric sensor to reach equilibrium conditions in order to obtain a meaningful reading can be quite long. These considerations are essential in the design and selection of potentiometric sensors for biomedical apphcations. 

Two possible architectures for TFT-addressed biosensor arrays are illustrated in Figs. 6.10 and 6.11. For potentiometric sensing, the biosensing pad is connected to the gate of the TFT (see Fig. 6.10), which acts as the transducer. A dummy transistor, where no biomolecular interaction occurs, can be used for differential measurements [45]. For current detection, the sensing pad needs to be connected to the source or the drain of the TFT as shown in Fig. 6.11. 

The potentiometric sensing of urea was also demonstrated by entrapping the enzyme urease within a polypyrrole membrane electrosynthesized on a platinum electrode [75]. The bioactive sensing membrane was synthesized by potentiostatic electropolymerization of pyrrole monomer in a solution containing the urease enzyme and a nucleophilic electrolyte such as NaOH, NaCOs, or NaHCOs. This biosensor exhibited a good Nemstian response to urea, with a slope of 31.8 mV/decade over an mea concentration range of 1.0x10 mo]/dm -0.3 mol/dm. ... 

In the potentiometric sensing of H2, (ii) is useful for H2 in inert gases while (i) is especially suited for H2 in air. On the other hand, only (iii) is applicable to the amperometric sensor and the four-probe type sensor. 

Shim JH et al (2012) A nanoporous ruthenium oxide framework for amperometric sensing of glucose and potentiometric sensing of pH. Microchimica Acta 177(1-2) 211-219... 

Slowinska K, Feldberg SW, MajdaM (2003) An electrochemical time-of-flight technique with galvanostatic generation and potentiometric sensing. J Electroantil Chem 554-555 61-69... 

As stated previously, the observed potential from the glass electrode varies in a known manner, the Nernst equation. This equation shows the relationship between any potentiometric sensing electrode and the ion to which the electrode is sensitive. When applied to the pH glass electrode and the hydrogen ion activity, it... 

H.B. Sadeghi, S.A. Ebrahimi, A. Tamaddon, F. Bozorgvar, H. Afifinia, N. Almasian and S. Mollaei, Potentiometric sensing of lamotrigine based on molecularly imprinted polymers, Electroanalysis, 23 (11) 2716-2723, 2011. 

---