Microelectrodes for in-vivo determination of pH - Electrochemical Sensors, Biosensors and Their Biomedical Applications - page 278

are able to transduce an ionic signal into an electrical signal. Such polymers provide an
interface between pH selective membranes and metallic transducers, which replaces the
internal electrolyte of a conventional pH sensor. Research efforts have been made to
utilize conducting polymers in a number of designs of all-solid-state miniaturized sen-
sors [118, 119, 120]. Electroactive

-conjugated polymers, like polyaniline (PANI) and
polypyrrole (PPy), are most commonly used in sensor fabrications.

π

10.4.2 pH Microelectrode for a lab-on-a-chip
Multisensing devices such as lab-on-a-chip sensing systems are becoming an important
area of sensor research and development because of the advantages they hold over con-
ventional single analyte sensors [121]. The technology now exists to enable fabrication
of miniature microfl uidic systems capable of switching, regulating, mixing, and sepa-
rating samples. Integrating sample preparation and separation with sensing elements
in a single miniaturized device has led to the concept of

TAS.
A good example of such planar multisensor systems arranged in a lab-on-a-chip
format was described by Vonau et al. [91] and shown in Fig. 10.6. The system con-
tains planar sensors on ceramic substrates which determine pH, dissolved oxygen, and
electrical conductivity in biological liquids. The sensors were combined in a microfl u-
idic system to characterize metabolic processes in biological cells. A four-electrode
impedance sensor operated at 300 Hz was used to evaluate metabolism of growth and
cell adhesion. A thick fi lm RuO 2 electrode was used for pH determination. A layer of
20

µ

m thick Pt seed layer, and then
sintered at about 900ºC to form the pH sensitive layer. RuO 2 electrodes show signifi -
cantly lower internal resistances of 1-5 M

µ m ruthenium paste was screen-printed onto the 5 µ

at 25ºC and at 1 MHz compared to thick
fi lm glass electrodes with values greater than 800 M

Ω

. The pH sensitivity determined
from calibration curves was about 52 mV/pH between pH 4 and 9.2. The integrated
oxygen sensor consists of two Pt electrodes and an Ag/AgCl reference electrode in a
three-electrode measuring system with the Pt cathode less than 100

Ω

m.
The integrated planar silver chloride electrode uses a thin layer of 150

µ

m polymer
that consists of a heat curing epoxy resin poly-hydroxy-ethylmethacrylate (PHEMA) to
immobilize the KCl electrolyte. The potential drift of the reference electrode reduced
to 59

µ

V/h after a conditioning phase of several hours. However, this reference elec-
trode was only used for PO 2 measurement, while an external reference electrode was
used for pH measurement.
Challenges remain in the development of lab-on-a-chip sensing systems. The
overall lifetime of a sensor chip is always determined by the sensor with the shortest
lifetime, which in most cases is the depletion of reference electrolytes. Measures to
minimize cross-talking among sensors, especially when biosensors are integrated in
the system, also should be implemented [122]. The development of compatible deposi-
tion methods of various polymeric membranes on the same chip is another key step in
the realization of multisensing devices.
By incorporating on-chip electronics or using external analyzers with advanced control
and signal processing functions, the lab-on-a-chip or

µ

µ TAS can act as “smart sensors”,