Biomedical Engineering Reference
In-Depth Information
9.4.1.2 Clark's Oxygen Electrode
Clark's dissolved oxygen sensor is the most well-known amperometric biosensor
of this type. Monitoring oxygen consumption is important during cell culture and
microbial development. Among the various tools to determine oxygen, the Clark's
(named after Leland C. Clark, Jr.) oxygen sensor is the most widely used and has
been applied in clinical analysis, fermentation monitoring, and biosensor develop-
ment. The Clark electrode consists of a working electrode (cathode) maintained
at a negative external potential relative to a reference electrode (anode) and the
electrolyte [Figure 9.9(b)]. The cathode is made from noble metals, such as Pt or
Au, so that the electrode surface does not participate in the chemical reactions. The
current is produced by the chemical reduction of oxygen on the cathode surface,
expressed by:
−
−
O2 2HO HO2OH
HO
++
e
↔
+
2
2
2
2
+↔
2
e
−
2OH
−
22
In many samples, there often exist other electroactive species reducible at the
same potential and/or surface-active species adsorbable on the cathode surface that
could interfere with the reduction of oxygen. Hence, the electrode compartment
is isolated from the reaction chamber by a thin polymeric (Teflon, polypropylene)
membrane, which allows oxygen diffusion to reach the cathode. As a result, dis-
solved oxygen (DO) in the sample has to diffuse through the membrane and the
electrolyte between the membrane and the cathode.
Once a negative external potential is applied to the working electrode, the
surface will provide electrons to the oxygen molecule. The reduction allows a cur-
rent to flow. At low potentials, the electrode current is governed by the exchange
current density and the overpotential of the reactions above. At higher potentials,
the oxygen concentration (activity) and the surface become negligible. The specific
value of the polarization potential varies with the material used for the cathode. At
the polarization potential the reaction rate of oxygen reduction is fast and there is
little or no oxygen accumulation on the cathode surface. Therefore, the reaction
rate is limited only by the oxygen diffusion rate from the sample to the cathode
surface. Consequently, the limiting current is linearly proportional to the oxygen
partial pressure (or activity) in contact with the external surface of the membrane.
The current flowing is proportional to the activity of oxygen provided the solu-
tion is stirred constantly to minimize the formation of an unstirred layer next to
the membrane. Under these conditions, the current becomes independent of the
voltage. Clark-type electrodes have undergone significant development and mini-
aturization since their first development. Using the principle of Clark-type oxygen-
sensing electrodes, various biosensors have been developed by immobilizing en-
zymes, antibodies, or micro-organisms which catalyze the oxidation of biochemical
organic compounds.
