Biomedical Engineering Reference
In-Depth Information
well-known advantages: miniaturisation, high sensitivity, low cost and
multi-detection potential (this is especially important for the creation of
multi-biosensors). Such biosensors were adapted for dif erent purposes
including glucose and urea determinations [40-44], with the idea of fur-
ther use in clinical analyses. It can be very important to have such bio-
sensors in many clinical cases dealing with diabetes or kidney and liver
diseases, when fast and very sensitive glucose or urea measurement is
essential. Unfortunately, the silicon nitrate ISFETs shown not higher sta-
bility and now we propose to change them by that based on the cerium
oxide. h e last were applied already by us for the immune biosensors
at the control of
S. thyphymurium
level [45]. Since the principle of the
application, obtained results are similar in case of work with both types
of ISFETs we will pay attention now such type of biosensors intended for
glucose and urea control.
13.3.1
Analysis of the Urea Level in Blood [46]
h e immobilized urease stipulates pH changes around the gate surface
according the reaction: (NH
2
)
2
CO + 2H
2
0 + H+ urease 2NH
+
4
+ HCO
3-
Sensor chips with two ISFETs were fabricated by N-channel LOCOS-
technology. h e dimensions of a
p
-silicon wafer were 3 mm wide, 10 mm
long and 0.3 mm thick. Sensor sensitivity was linear from pH 3 to pH 10
with a slope of 40-45 mV/pH unit. h e procedures for making ISFETs
and their properties were reported previously [39-43]. Urease was immo-
bilized from solutions with BSA at a concentration of 50 mg ml
-I
in in
20 mM PBS, pH 7.4 and 2.5% glycerol mixed in equal volumes. A drop of
this mixture (0.1 μl) was deposited on the sensitive area of the sensor. A
similar membrane prepared only with BSA in buf er solution was placed
onto the reference ISFETs. To complete a polymerization of the biomem-
brane the sensor chips were transferred to a saturated vapor of GA for
half an hour. All the experiments were performed at 23
C in a 2.5 ml
measuring cell under constant stirring. Sensor was immersed in the work-
ing buf er for 0.5-1 h before use. Between the experiments sensors were
stored at 4
°
C in 20 mM phosphate buf er pH 7.4 with 1 mM ethylenedi-
aminetetraacetic acid (EDTA). h e dif erential measurement method was
used to eliminate the non-specii c inl uence of the external conditions
(temperature, light and pH l uctuations) on the biosensor signal. A typical
time-response curve of the urea biosensor had an exponential character.
h e maximum signal was achieved at er 1-3 min. h is time depends on
the membrane thickness. h e output of the urease sensor in 1 mM urea
solution had a maximum at pH 7.0, which is within the range for optimal
°
