Biomedical Engineering Reference
In-Depth Information
itself. In both cases, the situations in which a reliable reference electrode
potential can be obtained would be limited.
The third approach is to miniaturize a conventional liquid-junction el-
ectrode, which would be most practical considering the accuracy required
for potentimetric detection. In some miniature liquid-junction reference el-
ectrodes, a silicon microcontainer fabricated by anisotropic etching has been
used. A liquid junction was formed on one portion of the container using a
restraining material such as porous silicon [109,110], porous glass [111], and
polyHEMA [110]. In relation to this, it is necessary to store KCl electrolyte as
much as possible to achieve a long lifetime. With a screen-printed electrolyte
layer containing KCl in a solid powdered form, a lifetime exceeding 100 h has
been achieved [112]. It might be considered that a long lifetime can be achie-
ved by restraining the effusion of internal KCl. However, this is closely related
to the liquid-junction potential. If the effusion is restrained too much, it im-
mediately results in a liquid-junction potential that exceeds 10 mV or more.
There is a tradeoff between the lifetime and the liquid-junction potential.
4.6.4
Microfabricated Biosensors
Amperometric biosensors have been very actively studied, and blood glucose
testing using planar-type glucose sensors has advanced dramatically [113].
Some models have already been commercialized and created a big market. In
fabricating amperometric microbiosensors, the working, reference, and auxi-
liary electrode patterns or cathode and anode pair are integrated. An enzyme
is immobilized on the working electrode area. Hydrogen peroxide produced
by an enzymatic reaction is often measured with a platinum thin-film or
thick-film working electrode.
In immobilizing an enzyme, a method very frequently used for microbio-
sensors is to immobilize the enzyme in bovine serum albumin with a bifun-
ctional reagent such as glutaraldehyde [114]. In fabricating microbiosensors,
the enzyme-immobilized membrane must be formed as a pattern on a neces-
sary portion of the sensitive area of a transducer. Representative techniques
for selective pattern formation are summarized in Table 4.4. As seen in the
table, a couple of techniques can be combined.
Microbiosensors fabricated by just immobilizing an enzyme are sometimes
so sensitive that the linear range of the calibration curves cannot cover the
higher range of concentration required in real analyses. The linear range can
be expanded using a diffusion-limiting membrane. Representative materials
for the membrane include polyurethane, silicone, and polyHEMA.
Electrochemical biosensors whose working electrode is in direct contact
with the sample solution are influenced by interfering compounds such as
L-ascorbic acid, uric acid, and acetaminophen. In order to minimize the in-
terference, a perm-selective membrane is often used. A couple of examples
include a cation exchange polymer such as Nafion
TM
and cellulose acetate.
Another technique often seen is to use a pair of working electrodes with the
