Microelectrodes for in-vivo determination of pH - Electrochemical Sensors, Biosensors and Their Biomedical Applications - page 292

10.6 CONCLUSIONS AND OUTLOOK

Potentiometric microelectrodes have been widely used for pH measurements in many
in-vivo applications. This chapter has provided an overview of these electrodes and a
discussion of a number of practical challenges related to sensor designs, fabrication
methods, and applications.
Many approaches have been employed to improve the reliability and biocompat-
ibility of pH sensors for in-vivo pH measurements. Examples of these approaches are
the development of new pH sensitive materials, optimization of sensor designs, modi-
fi cation of existing microfabrication techniques, and development of new technologies.
While searching for and developing advanced pH sensitive electrode materials will con-
tinue to be an exciting area of research, efforts to improve existing electrode materi-
als through the modifi cation of structure, physical dimension, composition, and surface
morphology will remain a practical and effective way to enhance sensor performance.
Microfabrication of sensing devices using thin fi lm and thick fi lm technology has
led to miniaturized sensors and certain integration of various sensors and microfl uidic
devices to form so-called

TAS or lab-on-a-chip systems. Such microfabricated sen-
sors and microsystems will play an important role in the future of pH measurements. In
addition to adapting standard microfabrication technologies to fi t the needs of chemical
sensors, new techniques are also under development, e.g. precision liquid handling and
dispensing technologies, and electropolymerization of conducting polymer membranes
on electrode surfaces.
Microelectrodes with all-solid-state designs will receive more and more atten-
tion. However, ill-defi ned pH sensitive membrane-metal interfaces will limit conven-
tional membrane based all-solid-state sensors in applications requiring high accuracy
and reproducibility. Lack of reliable solid-state reference electrodes will continue to
restrain the development of all-solid-state

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TAS or lab-on-a-chip systems. It has been
envisioned that with the advancement in digital instrumentation technology, hand-held
analyzers together with such

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TAS systems will provide fast and reliable point-of-care
testing (POCT) for the determination of various clinical parameters including pH.
Some progress in implantable pH sensors has been made; however, reliability and
biocompatibility remain as main challenges for the development of long-term implanta-
ble pH sensors. Sensor technology is also progressing towards using nano-scaled sensor
devices in analytical applications. Such progress in nanotechnologies and device pack-
aging technologies show high potential for the development of implantable sensors.
Metal/metal oxides are the materials of choice for construction of all-solid-state pH
microelectrodes. A further understanding of pH sensing mechanisms for metal/metal
oxide electrodes will have a signifi cant impact on sensor development. This will help
in understanding which factors control Nernstian responses and how to reduce inter-
ference of the potentiometric detection of pH by redox reactions at the metal-metal
oxide interface. While glass pH electrodes will remain as a gold standard for many
applications, all-solid-state pH sensors, especially those that are metal/metal oxide-
based microelectrodes, will continue to make potentiometric in-vivo pH determination
an attractive analytical method in the future.

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