Microelectrodes for in-vivo determination of pH - Electrochemical Sensors, Biosensors and Their Biomedical Applications - page 268

SiO 2 , Na 2 O, and CaO for pH up to 9 [60]. Improvements have been made in the compo-
sition of the glass such that measurements can extend to more alkaline ranges with the
aid of LiO 2 , Cs 2 O, BaO, and La 2 O 3 [61], and stability can be increased and impedance
can be lowered using components such as LiO 2 , BaO, Ti 2 O, and La 2 O 3 [62]. Damage
of pH sensitive glass in terms of an increase in inner resistance, a decrease in electrode
sensitivity, and a shift of the zero point due to high temperature steam sterilization, was
overcome by the optimization of glass compositions using a glass consisting of SiO 2 ,
Li 2 O, La 2 O 3 , Nd 2 O 3 , and Nb 2 O 5 [48].
The resistance of glass electrodes is very high, typically between 10 and 100 M Ω ,

with some as high as 4 G m) with

suffi cient mechanical strength is often used, in order to minimize resistance as much
as possible. In addition, shielding of the glass electrode is necessary to reduce electro-
magnetic noise. A pH meter with high input impedance of 10 12

Ω [60]. Because of this, a thinner glass bulb (50-200 µ

or more, which is
much higher than electrode internal resistance, is commonly used to minimize electrode
polarization for accurate measurements.
The glass pH electrodes show ideal Nernstian response independent of redox inter-
ferences and have a long lifetime; however, they do not measure correct values when
hydrogen ion concentration is either high or low. This is known as acid error or alka-
line error [63]. While acid error starts approximately at pH

Ω

2 for most pH glasses,

the alkaline error starts at pH 8 to 11 depending on the sources and composition of

the glass membrane. At these pH values the electrode is sensitive to sodium and potas-
sium ions [54]. However, some researchers attributed the deviation to two non-linear
electrode response slopes, one for H and one for OH [64]. The temperature effect
for glass electrodes is reduced or compensated by using a stand-alone or built-in ther-
mistor, or a thermocouple as a temperature probe and an automatic temperature correc-
tion (ATC) function, found in most modern pH meters.

10.3.2 Polymer membrane-based pH microelectrodes
Although glass pH electrodes are, in general, simple to use and available at a reason-
able cost, they are limited by the potential problems of glass breakage [65] and min-
iaturization diffi culties [60, 66]. One of the alternative approaches to preparation of
non-glass pH sensors is to use polymer-based pH sensitive membranes to replace solid
glass membranes.
Unlike the glass membrane, which contains fi xed binding sites, polymer mem-
branes contain incorporated mobile ion exchangers or ionophores (ion carriers) in the
membrane matrix. The ionophore is usually present in small amounts (approximately
1% or 10 2 M), which is relatively low when compared to the glass electrode. Such ion
carriers are able to complex with ions reversibly and transfer them through a polymer
membrane by carrier translocation [67]. A variety of ion carriers (ion exchanger, neu-
tral or charged carrier) have been used in the preparation of pH sensitive microelec-
trodes showing Nerstian potentiometric pH response [63].
The dynamic pH response range of a membrane-based pH electrode can be tailored
by the incorporation of different functional groups in the ion carriers. Yuan et al . [68]