Microelectrodes for in-vivo determination of pH - Electrochemical Sensors, Biosensors and Their Biomedical Applications - page 260

ischemia [29], in human atherosclerotic plaques during myocardial infarction [30], and
in the intestines during hemorrhagic shock [31].
The electrochemical detection of pH can be carried out by voltammetry (amper-
ometry) or potentiometry. Voltammetry is the measurement of the current potential
relationship in an electrochemical cell. In voltammetry, the potential is applied to the
electrochemical cell to force electrochemical reactions at the electrode-electrolyte
interface. In potentiometry, the potential is measured between a pH electrode and a
reference electrode of an electrochemical cell in response to the activity of an electro-
lyte in a solution under the condition of zero current. Since no current passes through
the cell while the potential is measured, potentiometry is an equilibrium method.
Although a few amperometric pH sensors are reported [32], most pH electrodes are
potentiometric sensors. Among various potentiometric pH sensors, conventional glass
pH electrodes are widely used and the pH value measured using a glass electrode is
often considered as a gold standard in the development and calibration of other novel
pH sensors in vivo and in vitro [33]. Other pH electrodes, such as metal/metal oxide
and ISFETs have received more and more attention in recent years due to their robust-
ness, fast response, all-solid format and capability for miniaturization. Potentiometric
microelectrodes for pH measurements will be the focus of this chapter.
10.1.3 Advantages of microelectrodes for the determination of pH
In general, optical-based pH measurement techniques require relatively expensive and
cumbersome instruments, and their sophisticated method cannot be easily carried out for
routine assay. Interfering contact and reactions of the dye molecules, particularly consid-
ering in-vivo measurements, cannot be excluded [34]. Some other possible factors, such
as a weaker signal at shorter response times, complications in microfabrication, and dif-
fi culties in attaching the chemical or biological agents to the small fi ber tip, are potential
limitations for the application of these optical sensors to in-vivo measurements in micro
environments [35].
Microelectrode pH sensors offer advantages over other techniques, specifi cally in
biology and medicine, as they allow sensitive detection in spatially and temporally
resolved measurements in small sample volumes. Furthermore, those that are poten-
tiometric particularly have the advantage over other sensor types, being simpler in
both sensor design and measurement device. Generally speaking, microelectrode pH
sensors offer higher sensitivity and selectivity for fi ne variation of pH measurement
in vivo than optical pH sensors. They are very reliable, provide a wide dynamic pH
response, and are convenient to use.
10.2 CHARACTERIZATION OF pH MICROELECTRODES
10.2.1 pH and pH measurements
pH is the measurement of the acidity or alkalinity of a solution. Concepts of acids
and bases in analytical chemistry are commonly based on the theories developed by