Obviously, this type of sensor is based on either
oxygen depletion (cathodic) or the electrochemical
oxidation of H2O2 (anodic). However, direct oxidation of H2O2 requires a highly positive overpotential, and hence this type of sensor suffers
from interferences due to the presence of easily
oxidizable species in the biological fluids. Although the cathodic measurement of molecular
oxygen concentration is perfectly suitable for glucose monitoring in solution, these devices also
suffer potential difficulties when operating in vivo
or in undiluted biological samples where the response is limited by physiological amounts of oxygen available. To solve the oxygen deficit problem,
attempts have been made in glucose detection to
use an additional catalase in recycling O2 from
H2O2,45 or to develop approaches that allow O2 to
be delivered externally to the electrode surface,
thereby producing an excess level of O2 (compared
with that of glucose) at the electrode surface.46
This excess amount of O2 will allow for better
regulation of the reaction environment and,
hence, oxygen (instead of glucose) does not become the limiting reagent in the reaction. However, incorporation of catalase is undesirable for
in vivo use of microelectrodes because a variation
in the O2 level in the surrounding medium may
cause fluctuation in electrode response and thus a
second electrode would be needed to measure the
actual O2 level so that the accurate concentration
of the substrate can be determined. The main concern associated with the use of an external O2
delivery system, on the other hand, is the difficulty in regulating the oxygen generation or supply, leading to possible introduction of an inadequate amount of oxygen to the electrode membrane.46