Figure 12 shows the general measurement set-up that is used for the pH-static enzyme
sensor. The system uses two pH-sensitive ISFETs with an integrated actuator
electrode, one for the actual enzyme sensor and a second as a reference that measures
the background pH of the sample solution. The second chip is identical with the first,
except that its membrane does not contain the enzyme. Both ISFETs measure with
respect to a common (quasi-) reference electrode which is actually the platinum
electrode on the reference ISFET. The ISFETs are operated with a constant sourcedrain
voltage and a constant drain current. Their differential signal reflects the
substrate-dependent pH change induced in the membrane of the first ISFET.
Figure 13 compares the output of a classical ISFET-based enzyme sensor with that of
the pH-static sensor. The lines with solid symbols show the measured pH change as a
function of the urea concentration at pH 7 in phosphate buffers of 50, 10 and 2 mM.
The lines with open symbols show the control current used by a pH-static sensor as
measured simultaneously with the first three curves. As can be seen, the sensitivity
and dynamic range of the “normal” enzyme sensor depends strongly on the buffer
capacity. In a weak buffer, the response saturates quickly as the pH in the membrane
approaches 9. In a strong buffer, the maximum concentration that can be measured is
much larger but the sensitivity is reduced.