Biomedical Engineering Reference
In-Depth Information
urease activity (pH 7.0-7.5) and close to the human blood (pH 7.4). h is
makes possible the measurement of blood samples without further adjust-
ment of pH value. h e biosensor response for urea also depends on the
ambient buf er capacity, the signal being sharply reduced by an increase
in the buf er concentration from 1 to 10 mM. However, the linear part of
the calibration curve is extended for higher buf er concentrations, making
readings for 0.25-1.50 mM urea possible. Because the buf er capacity of
blood is relatively high (due to the presence of proteins and buf er salts)
reductions of biosensor output can occur during the addition of blood
samples to the measuring cell. To avoid the ef ects of blood buf er capacity,
the concentration of working buf er in the measurement cell should there-
fore be not less than 5 mM. h e main salt component of blood is sodium
chloride, with a concentration of 150 mM. h is conditions strongly af ect
on the ei ciency of the urea biosensor. h e biosensor output falls by 50%
in the presence of 200 mM NaCI and stays constant during further addi-
tions up to 500 mM NaCI. h us to eliminate the inl uence of the salt
concentration of blood samples, a working buf er should contain at least
200 mM NaCI. Using these selected parameters from model solutions, the
urea biosensor has been tested on blood samples from laboratory animals.
In such experiments a 25-fold dilution of serum samples has been found
to be optimal for obtaining a reliable response. h e presence of serum
in the working buf er changes the slope of the calibration curve, which
is probably due to the buf er capacity of blood proteins. It was created
the calibration curve to estimate the urea concentration in animal blood
samples and the results were compared with those obtained by traditional
chemical way. We have obtained a very high correlation for both methods.
Such biosensor can be used 20 times without a decrease of the response
value. Deviations between individual response values are about 10%. At er
130 measurements a 30% decrease in response value was observed.
h us, this urea biosensor seems to be suitable for further testing in med-
ical practice and especially if the silicon nitrate in ISFETs will be changed
on cerium oxide as it was mentioned above.
13.3.2
Determination of the Glucose Level in Blood [47]
Now there are a several dozen papers dealing with enzymatic ISFETs based
biosensors. h ey are based on the following reaction catalysed by β-glucose
oxidase (GOD) immobilized on the gate dielectric of the ISFETs: β-D-
glucose + O
2
β-glucose oxidase D-glucono-δ-lactone + H
2
O
2
. Subsequent
hydrolysis of gluconolactone giving gluconic acid (pK~3.8) changes the
solution pH value near the sensor surface.
