PH-ISFET

FIGURE 10.4 Microphotograph of the pH ISFET with an on-chip pH meter. The whole device was coated with SU-8 photoresist using standard UV photolithography, leaving a 500 pm square well as the ISFET opening. (Reproduced from [85], with permission from Elsevier.)... 

M.L. Pourciel-Gouzy, W. Sant, I. Humenyuk, L. Malaquin, X. Dollat, and P. Temple-Boyer, Development of pH-ISFET sensors for the detection of bacterial activity. Sens. Actuators B. 103, 247-251 (2004). 

Alegret et al. devised a pH ISFET based on a flow-through cell designed by themselves and an FI manifold including a gas-diffusion module for the on-line separation of gaseous analytes with acid-base properties. In this way, they obtained a linear determination range of 1 x 10 -1 x 10 M for ammonia and 7 x 10" -4 x 10 M for sulphur dioxide, with an RSD of 1% and 0.5%, respectively [153]. 

The above conceptual and operational pH definitions for solutions in non-aqueous and mixed solvents are very similar to those for aqueous solutions [16]. At present, pH values are available for the RVS and some primary standards in the mixtures between water and eight organic solvents (see 5 in Section 6.2) [17]. If a reliable pH standard is available for the solvent under study, the pH can be determined with a pH meter and a glass electrode, just as in aqueous solutions. However, in order to apply the IUPAC method to the solutions in neat organic solvents or water-poor mixed solvents, there are still some problems to be solved. One of them is that it is difficult to get the RVS in such solvents, because (i) the solubility of KHPh is not enough and (ii) the buffer action of KHPh is too low in solutions of an aprotic nature [18].8) Another problem is that the response of the glass electrode is often very slow in non-aqueous solvents,9 although this has been considerably improved by the use of pH-ISFETs [19]. Practical pH measurements in non-aqueous solutions and their applications are discussed in Chapter 6. 

The glass electrode responds especially slowly in protophilic aprotic solvents like DMSO and DMF, sometimes taking 1 h to reach a steady potential. In such cases, the use of Si3N4- and Ta205-type pH-ISFETs is very promising, because they almost respond instantaneously and with Nernstian or near-Nemstian slopes [19]. 

Acid-base, redox, precipitation and chelometric titrations are usually dealt with in textbooks on analytical chemistry. The titration curves in these titrations can be obtained potentiometrically by use of appropriate indicator electrodes, i.e. a pH-glass electrode or pH-ISFET for acid-base titrations, a platinum electrode for redox titrations, a silver electrode or ISEs for precipitation titrations, and ISEs for... 

Redox potential pH Ionic activities Inert redox electrodes (Pt, Au, glassy carbon, etc.) pH-glass electrode pH-ISFET iridium oxide pH-sensor Electrodes of the first land and M" /M(Hg) electrodes) univalent cation-sensitive glass electrode (alkali metal ions, NHJ) solid membrane ion-selective electrodes (F, halide ions, heavy metal ions) polymer membrane electrodes (F, CN", alkali metal ions, alkaline earth metal ions)... 

Fig. 6.5 Titration curves of 5 mM picric acid in AN with 1 M BU4NON (in MeOH), recorded simultaneously with four pH sensors, but at different titration speeds for (a) to (c). In (a), curve 1 is for Si3N4-ISFET, 2 for Ta205-ISFET, 3 for Ir02 pH-sensor, and 4 for glass electrode. The pH-ISFETs were obtained from Shindengen Indus. Co. and the Ir02 pH-sensor from TOA Electronics Ltd [17c].

Recently, Ta2Os- and Si3N4-type pH-ISFETs have been used in non-aqueous systems, by preparing them to be solvent-resistant [17]. In various polar non-aqueous solvents, they responded with Nernstian or near-Nernstian slopes and much faster than the glass electrode. The titration curves in Fig. 6.5 demonstrate the fast (almost instantaneous) response of the Si3N4-ISFET and the slow response of the glass electrode. Some applications of pH-ISFETs are discussed in Section 6.3.1. 

The pH window is very wide in solvents that are weak both in acidity and basicity. The widths of the pH window are well over 30 in such solvents, compared to about 14 in water (Table 6.6). The usefulness of these expanded pH regions is discussed in Section 3.2.2. In particular, potentiometric acid-base titrations in such solvents are highly useful in practical chemical analyses as well as physicochemical studies [22]. Acid-base titrations in lion-aqueous solvents were popular until the 1980s, but now most have been replaced by chromatographic methods. However, the pH-ISFETs are promising to realize simple, rapid and miniature-scale acid-base titrations in lion-aqueous solvents. For example, by use of an Si3N4-type pH-ISFET, we can get an almost complete titration curve in less than 20 s in a solution containing several different acids [17d]. 

Ion solvation has been studied extensively by potentiometry [28, 31]. Among the potentiometric indicator electrodes used as sensors for ion solvation are metal and metal amalgam electrodes for the relevant metal ions, pH glass electrodes and pH-ISFETs for H+ (see Fig. 6.8), univalent cation-sensitive glass electrodes for alkali metal ions, a CuS solid-membrane electrode for Cu2+, an LaF3-based fluoride electrode for l , and some other ISEs. So far, method (2) has been employed most often. The advantage of potentiometry is that the number and the variety of target ions increase by the use of ISEs. 

In our earlier work we have studied the realization of an amperometric transducer using thin film and photolithography techniques (7) as well as that of an AI2O3 type pH-ISFET (2). The use of the former in an enzymatic glucose electrode (3) and of the latter in for example a K+ sensitive device (4) has been reported. In both cases the control of membrane thicknesses and adhesion was the most critical parameter of the sensor realization. In this paper we will focus on different on-wafer and on-chip... 

Most ISEs are based on purely physicochemical and non-catalytic recognition elements solid membranes with fixed ionic sites (e.g. the glass pH electrode), ion-exchange polymer membranes or plasticised hydrogel membranes incorporating ionophores [9], Silicon oxide or metal oxides act as the recognition element in pH-ISFETs, gas-sensitive FETs, solid-state electrolyte, solid-state semiconductor and many conductometric gas sensors. 

The results provided in the 1st PT campaign for pH, conductivity, Ca, K, Na and chloride were obtained by in-house fabricated microelectrodes (Chapter 4.1.4 of this book). The sensor used for pH determination is a pH-ISFET with silicone nitride membrane, the sensors for Na+, K+, Ca2+ and Cl- are ISFETs with ion-selective membranes, while the sensors for conductivity is a 4-bar platinum electrode. All the circuits used for measuring with the sensors (ISFET meter and conductivity meter) were also developed in-house. The results for the ions (Na+, K+, Ca2+ and Cl-) were received being expressed in activity, therefore could not be compared with the other PT results. 

Niu MN, Ding XF, Tong QY (1996) Effect of two types of surface sites on the characteristics of Si3N -gate pH-ISFETs. Sens Actuators B 37 13-17... 

A nonglass ISFET pH electrode is shown in Figure 13.17. The reference electrode and temperature sensor are incorporated in the small probe. For information on pH ISFETS, see www.phmeters.com (IQ Scientific Instruments, Inc.), www. sentronph.com (Sentron, Inc.), and www.servonics.com/Jenco.htm (Jenco Instruments, Inc.). 

Arnoux, C., Buser, R., Decroux, M., Van den Vlekkert, H. H., De Rooij, N. F. Analysis of the Stracture and Drift of AljOj Layers Used as a pH-Sensitive Material For pH-ISFETs , Proc. 4th International Cottference on Solid-State Sensors and Actuators fThmsducers 87), Tokyo, Japan, June 2-S, 1987. pp. 751-754. 

Direct combination of biochemical and electronic signal amplification has been achieved by immobilizing the enzyme couple Lacc / GDH on the gate of a pH-ISFET. 

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