Nanotechnology-Enabled Sensors

7.4 Proteins in Nanotechnology Enabled Sensors 425
Fig. 7.46 A schematic of a working electrode for the early type of glucose electrochemical
sensors.
In the working electrode, the outer layer is polycarbonate. The surface
of polycarbonate is hydrophilic and permeable to glucose but not to the
other constituents of blood (such as proteins). In the sensing process, a
drop of the patient’s blood is placed on this surface. The middle layer of
the working electrode consists of an enzyme, glucose oxidase, which catalyses
the reaction of glucose with O2 forming gluconolactone (a gluconic
acid) and hydrogen peroxide (H2O2). The bottom layer is made of cellulose
acetate which is permeable to H2O2. The hydrogen peroxide is oxidized at
the working electrode which is held at a positive voltage (approximately
+0.6 V) versus the Ag/AgCl electrode. This produces a current according
to the redox half-equation:
H2O2 → O2 + 2H + + 2e – (7.2)
The resulting current is proportional to the glucose concentration of the
sample, and so the blood sugar levels of diabetics can be monitored.
7.4.8 Enzyme Nanopraticle Hybrid based Sensors
As described in the previous section, redox enzymes can be efficiently
utilized in the fabrication of sensors. However, such enzymes generally
lack the capability of proper alignment on electrodes, have difficulties in
directly communicating with electrodes, and the electron transfer rate via