Electrochemical glucose biosensors - Electrochemical Sensors, Biosensors and Their Biomedical Applications - page 60

3.3.2 Oxygen dependence
Since oxidase-based devices rely on the use of oxygen as the physiological electron
acceptor, they are subject to errors accrued from fl uctuations in the oxygen tension
and the stoichiometric limitation of oxygen. These include fl uctuations in the sensor
response and a reduced upper limit of linearity. Such limitation (known as the “oxygen
defi cit”) refl ects the fact that normal oxygen concentrations are about an order of mag-
nitude lower than the physiological level of glucose.
Several avenues have been proposed for addressing this oxygen limitation. One
approach relies on the use of mass-transport limiting fi lms (such as polyurethane or
polycarbonate) for tailoring the fl ux of glucose and oxygen, i.e. increasing the oxygen/
glucose permeability ratio [1, 25]. A two-dimensional cylindrical electrode, designed
by Gough's group [25, 26], has been particularly attractive for addressing the oxygen
defi cit by allowing oxygen to diffuse into the enzyme region of the sensor from both
directions and glucose diffusion only from one direction. We addressed the oxygen
limitation of glucose biosensors by designing an oxygen-rich carbon paste enzyme
electrode [27]. Such a biosensor is based on a fl uorocarbon (Kel-F oil) pasting liquid,
which has very high oxygen solubility, allowing it to act as an internal source of oxy-
gen. The internal fl ux of oxygen can thus support the enzymatic reaction even in oxy-
gen-free glucose solutions. It is possible also to circumvent the oxygen demand issue
by replacing the GOx with glucose dehydrogenase (GDH) that does not require an oxy-
gen cofactor [28].

3.4 SECOND-GENERATION GLUCOSE BIOSENSORS

3.4.1 Electron transfer between GOx and electrode surfaces
Further improvements (and attention to the above errors) can be achieved by replac-
ing the oxygen with a non-physiological (synthetic) electron acceptor, which is able to
shuttle electrons from the redox center of the enzyme to the surface of the electrode.
Glucose oxidase does not directly transfer electrons to conventional electrodes because
a thick protein layer surrounds its fl avin redox center. Such a thick protein shell intro-
duces a spatial separation of the electron donor-acceptor pair, and hence an intrinsic
barrier to direct electron transfer, in accordance with the distance dependence of the
electron transfer rate [29]:

10 13 e 0.91( d 3) e [ ( ∆ G λ)/4 RT λ] K et (3)

where correspond to the free and reorganization energies accompanying the

electron transfer, respectively, and d the actual electron transfer distance. The minimi-
zation of the electron transfer distance (between the immobilized GOx and the elec-
trode surface) is thus crucial for ensuring optimal performance. Accordingly, different
innovative strategies have been suggested for establishing and tailoring the electrical
contact between the redox center of GOx and electrode surfaces.

∆ G and λ