Biomedical Engineering Reference
In-Depth Information
At least four major obstacles are in the way of the applications of glucose
ISFETs based biosensor: (a) the dramatic inl uence of the buf er capac-
ity of a sample on the sensor response; (b) the non-linear dependence of
the enzyme kinetics and buf er capacity of a pH sample; (c) inl uence on
the response ionic strength; (d) co-substrate limitation of the enzymatic
reaction. Regarding the last point, the glucose concentration in human
blood is normally about 5 mM, reaching 20 mM and more for diabetics.
However, the concentration of oxygen dissolved in a sample, as a rule, does
not exceed 0.5 mM. So because of the unfavorable ratio of glucose and
oxygen in real blood, the dynamic range of the biosensor is usually limited
by oxygen. To avoid this situation with the “oxygen dei cit” the analyzed
samples were preliminary diluted.
Sensor design, creation of biomembrane and measurements were made
similar to that as it was described above in case of urea analysis. In addi-
tional to biomembrane was prepared with the help of polyacrylamide gel.
Both membranes showed good results at the biosensor application. h e
substrate sensitivity of this biosensor reached a maximum of ~10
-5
M of
glucose in unbuf ered medium and depends dramatically on the buf er
capacity of the sample. h is is due to the presence in the sample of a mobile
pH-buf er that augments the l ux of H
+
ions produced by the enzymatic
reaction out of the membrane by means of the 'carrier-mediated' transport
mechanism (“facilitated dif usion”) [48].
h e dependence of the biosensor response on the ionic strength of the
sample is characterized by the next data. h e curve levels of at [NaCl] >150
mM and does not change when using MgCl
2
, or KC1 instead of NaCl. h is
dependence tends to linearize when plotted against the logarithm of the
ionic strength, thus implying the mainly electrostatic nature of the ef ect.
It is also shown that there is no substantial ef ect of the protein content of
a sample on the biosensor response. Since the solution buf er capacity is
strongly inl uenced by proteins, it follows that only those components of
a buf er that are sui ciently small and mobile to penetrate inside the bio-
membrane can af ect the sensor response.
h e obtained results allowed us to recommend the following protocol
for the glucose determination in blood by the ISFETs-based biosensor: (I)
to overcome the problem of 'oxygen dei cit' and to adjust the glucose con-
centration in a sample to the sensor quasi-linear operating range (0.1-2.0
mM), blood should be diluted in 10-20 times: (2) since the concentration
of buf er in blood does not exceed 30 mM (mainly bicarbonates) there-
fore diluting the sample with, e.g., 2-5 mM Na-phospste buf er (Na-PhB)
(pH 7.3-7.5) should be done (in this case the buf er capacity of the result-
ing solution will be mainly determined by the diluent); (3) to stabilize the
