Microelectrodes for in-vivo determination of pH - Electrochemical Sensors, Biosensors and Their Biomedical Applications - page 269

reported a PVC membrane electrode with a wide pH linear range of 1.7 to 13.2 based
on 4,4 -bis[(N,N-dialyl-amino)methyl]azobenzene. The long alkyl chains in the terti-

ary amino group of the ionophore are sensitive in the basic range; while the azo group
is sensitive in the acidic range. In addition to pH ionophore/polymer membranes, con-
ductive polymers have been used in the fabrication of pH sensors [69].
Among all the polymers used in preparing ion-selective membranes, poly(vinylchloride)
(PVC) is the most widely used matrix due to its simplicity of membrane preparation [32,
70]. In order to ensure the mobility of the trapped ionophore, a large amount of plasticizer
(approximately 66%) is used to modify the PVC membrane matrix (approximately 33%).
Such a membrane is quite similar to the liquid phase, because diffusion coeffi cients for dis-
solved low molecular weight ionophores are high, on the order of 10 7 -10 8 cm 2 /s [59].
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 plasticiz-
ers 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].
Some pH sensitive ionophores have been incorporated into liquid membranes in
the construction of capillary glass micro pH sensors. Apart from ETH 1907 trido-
decylamine [74], and ETH 2418 [4

-(dipropylamino)-2-azobenzene-carboxylic acid
octadecylester] [75], the Fluka 95291 hydrogen ion sensitive resin cocktail A, a well-
known commercial pH sensitive ionophore, has been used in many studies [76].

10.3.3 Silicon-based pH microelectrodes
The fabrication techniques for IC circuits and MEMs have been adapted in the devel-
opment of silicon (Si) wafer-based pH sensors. The ion sensitive fi eld-effect transistor
(ISFET) for pH measurements is developed on the basis of the metal oxide semicon-
ductor fi eld-effect transistor (MOSFET). The fi rst pH sensitive ISFET reported in
1970 by Bergveld was developed for electrophysiological measurements on neural
responses. He used a FET without a metal gate and exposed the bare gate oxide layer
to a saline solution [56]. While ISFET fi nds continuing applications in electrophysi-
ological measurements, such as bacterial activity detection [77] and determination of
the pH gradient across a cell membrane [78], it has also become increasingly useful for
in-line process monitoring applications, especially for food, pharmaceutical, cosmetic,
and water purifi cation industries [52, 65].
A schematic diagram of an ISFET is shown in Fig. 10.3. In the ISFET, the MOSFET's
metallic gate is replaced by an insulator layer that is pH sensitive. The ISFET uses a dif-
ferent pH sensing mechanism than that of conventional glass electrodes [66]. When the
gate is exposed to an electrolyte solution, there is a pH dependent charge at the double-
layer gate insulator-solution interface. Under a constant gate voltage, the induced electric
fi eld from the pH dependent surface charge electrostatically infl uences the current ( i s-d )
fl ow between the source and drain. In this way, the current change between the source and