Biomedical Engineering Reference
In-Depth Information
internal elements can alternatively be housed in a silicone tube that functions
both as a container and as a gas-permeable membrane [92,93]. If the impe-
dance of the indicator electrode is low, signal amplification with an ISFET
is not necessary. An iridium oxide indicator electrode can be used instead of
the ISFET in the structure shown in Fig. 4.31d [94].
As a representative micro ion sensor, the ISFET has been the subject of
hundreds of papers [95]. Its major function is to transduce the membrane
impedance to make the sensor unsusceptible to noise. Some groups of ion-
selective electrodes can be successfully miniaturized without transducing the
signal using an ISFET. Planar ion-selective electrodes can be realized using
the structure shown in Fig. 4.31b [96-99]. An internal reference electrode
pattern (usually Ag/AgCl) is formed on a substrate and an electrolyte layer
and the ion-selective membrane are subsequently formed.
In order to improve mechanical instability, the coated-“film” electrode
has been used [100-102]. The coated-film electrode has a membrane-metal
interface between which the interfacial potential is not well-defined, and os-
motic transport of water to the interface causes noise and potential drift. In
practice, if the electrode is calibrated just prior to use, satisfactory accuracy
seems to be obtained [100]. In applying the microfabricated ion-selective el-
ectrodes to in vivo use, nonspecific protein adsorption and cell adhesion pose
a problem. In order to minimize biofouling, physicochemical or biological
modification has been tried, using materials such as polyHEMA [103].
Although miniaturization of the indicator electrode including the ISFET
has advanced dramatically over the past two decades, the miniaturization
and batch-fabrication of a reliable reference electrode has not advanced until
very recently. In realizing a miniaturized reference electrode, three lines of
approach have been proposed [104]. The first is the so-called pseudo-reference
electrode which is usually composed of a metal pattern such as Ag/AgCl that
is in direct contact with a sample solution. In potentiometric sensors, it ser-
ves as a potential standard in combination with an indicator microelectrode
or used as the third electrode in an ISFET-REFET (Reference Field Effect
Transistor) configuration [105,106]. Needless to say, the potential of this type
of reference electrode varies depending upon the activity of the primary ions
determining the electrode potential. Therefore, care must be taken to main-
tain the activity of the primary ion constant.
The second type is the solid-state reference electrode without a liquid
junction. This includes the application of an ion-insensitive membrane of-
ten found in differential ISFETs [107]. A perm-selective membrane such as
Nafion
TM
has also been used to suppress the effusion of chloride ions con-
tained in the internal electrolyte solution of a Ag/AgCl reference electrode
[108]. However, the interface between the former membrane and the solutions
might become polarized by blocking the exchange current. Also, the latter
membrane might generate additional potential due to the perm-selectivity
