Microelectrodes for in-vivo determination of pH - Electrochemical Sensors, Biosensors and Their Biomedical Applications - page 277

10.4 ADVANCED MICROELECTRODE SYSTEMS FOR
pH DETERMINATION

Advances in both microfabrication technology and electronics have accelerated the
development of advanced microelectrode systems for pH determination. The microfab-
rication technology, taking advantage of the thin fi lm and thick fi lm techniques devel-
oped originally for the semiconductor industry, are increasingly applied for making
all-solid-state sensors and complete analytical microsystems, either in a lab-on-a-chip
format for a micro-total analysis systems ( µ TAS) or in a multisensing array format.

10.4.1 All-solid-state pH microelectrodes
The miniaturized pH sensors are of great importance in biomedical and clinical appli-
cations. By taking advantage of all-solid-state confi gurations, the sensor size can be
greatly reduced and the sensors can be mass produced with improved reproducibil-
ity. An all-solid-state pH sensor uses direct electrical contact between the pH sensitive
membrane and the inner metal contact. Among pH sensitive membranes used in the
fabrication of all-solid-state pH sensors, metal and metal oxides show clear advantages
over other membranes, such as glass membranes [60] and polymer membranes [112],
due to their well-defi ned metal/metal oxide interface and compatibility with micro-
fabrication techniques. As an all-solid-state pH sensor, ISFET has an insulator/metal
interface.
Poor adhesion of membrane to metal is the leading cause of failure in solid-state
potentiometric sensors [116]. For glass membranes, the mismatch of thermal coef-
fi cients of expansion between thin glass membrane and metal (mostly Pt) has been
attributed to premature failure due to hairline crack formations in the glass layer [60].
For polymer-based membranes, water vapor penetration was reported to compromise
the membrane-metal interface, therefore affecting the sensor's performance.
Yoon et al. [112] reported an all-solid-state sensor for blood analysis. The sensor
consists of a set of ion-selective membranes for the measurement of H , K , Na ,
Ca 2 , and Cl . The metal electrodes were patterned on a ceramic substrate and cov-
ered with a layer of solvent-processible polyurethane (PU) membrane. However, the
pH measurement was reported to suffer severe unstable drift due to the permeation
of water vapor and carbon dioxide through the membrane to the membrane-electrode
interface. For conducting polymer-modifi ed electrodes, the adhesion of conducting
polymer to the membrane has been improved by introducing an adhesion layer. For
example, polypyrrole (PPy) to membrane adhesion is improved by using an adhesion
layer, such as Nafi on [60] or a composite of PPy and Nafi on [117].
Another problem that is common for all membrane-based solid-state sensors is the
ill-defi ned membrane-metal interface. A large exchange current density is required to
produce a reversible interface for a stable potentiometric sensor response. One approach
to improving this interface is to use conducting polymers. Conducting polymers are
electroactive π -conjugated polymers with mixed ionic and electronic conductivity. They