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Bio potential Electrodes and Chemical Sensors
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Bio potential Electrodes and Chemical Sensors
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III. Biopotential Electrodes and Chemical SensorsElectrodes Electrolyte Interface, Half-Cell Potential, Polarization, Polarizable and Non
Polarizable, Electrodes, Reference Electrode, Hydrogen Electrode, Electrode Skin-Interface
and Motion Artifact. Surface Electrodes. Oxygen electrodes, CO2 electrodes, enzyme
electrode, construction, ISFET for glucose, urea etc. fiber optic sensors.
3.1 Biopotential Electrodes
• Electrodes that are capable of picking up the electrical signals of the body are called
as biopotential electrodes.
• Signals are developed due to chemical activity in the cells/ biological system.
• Chemical activity is brought about by Na+, K+ and Cl- ions concentration gradient and
unbalanced conditions lead to chemical activity in the human body.
• Current flows in the measuring circuit for at least a fraction of the period of time over
which the measurement is made.
• Bioelectric potential generated in the body are ionic potential.
• Electrode carries out a transducing function, because current is
• carried in the body by ions, whereas it is carried in the electrode
• and its lead wire by electrons.
• A transducer that convert the body ionic current in the body into the traditional
electronic current flowing in the electrode.
• Able to conduct small current across the interface between the body and the electronic
measuring circuit.
3.2 Electrodes-Electrolyte interface
Figure 3.1 Electrodes – Electrolyte interface
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,A net current (I) that crosses the interface passing from the electrode to electrolyte consists of
1. e- moving in opposite to current in electrode
2. Cations c+ moving in same direction of current
3. Anions A- moving in opposite to current in electrolyte
Electrode consists metallic atomC . Electrolyte consists cations C+ & anions A- .
• The electrode is made up of some atoms of the same material as the cations and that
this material in the electrode at the interface can become oxidized to form a cation and
one or more free electrons.
• The cation is discharged into the electrolyte; the electron remains as a charge carrier in
the electrode.
• These ions are reduced when the process occurs in the reverse direction
• an anion coming to the electrode–electrolyte interface can be oxidized to a neutral atom,
giving off one or more free electrons to the electrode.
• Oxidation reaction causes atom to lose electron
• Reduction reaction causes atom to gain electron
• Oxidation is dominant when current flow from electrode to electrolyte and reduction
dominate when the current flow is in the opposite.
3.3 Half cell potential
• Voltage developed at electrode-electrolyte interface is called half cell potential or
electrode potential.
• In the case of a metal solution interface, an electrode potential results from the
difference in rates between two opposing forces
• the passage of ions from the metal into the solution.
• The combination of metallic ions in solution with electrons in the metal to form atoms
of the metal.
• So, when a metal electrode comes into contact with an electrolyte (body fluid), there is
a tendency for the electrode to discharge ions into solution and for ions in the electrolyte
to combine with the electrode.
• The net result is the creation of a charge gradient, the spatial arrangement of which is
called the electrical double layer.
• Electrodes in which no net transfer of charge occurs across the metal electrolyte
interface is called as perfectly polarised electrodes.
• Electrodes in which unhindered exchange of charge is possible across the metal
electrolyte interface are called nonpolarizable electrodes.
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, 3.3 Polarizable and non-polarizable electrodes
Perfectly polarizable electrodes
• Electrodes in which no net transfer of charge occurs across the metal electrolyte
interface when a current is applied is called as perfectly polarised electrodes. Example:
Platinum Electrode
• The electrode behaves like a capacitor and overpotential is due to concentration.
Non polarizable electrodes
• Electrodes in which current passes freely across the electrode- electrolyte interface are
called nonpolarizable electrodes.
• Electrodes in which unhindered exchange of charge is possible across the metal
electrolyte interface are called nonpolarizable electrodes.
• Here current flows freely across the interface and energy is not required for it. Example:
Ag/AgCl electrode.
• Thus, for perfectly non-polarizable electrodes there are no over-potentials.
• Electrode interface impedance is represented as a resistor.
3.4 Polarization
Half cell potential is altered when there is current flowing in the electrode due to electrode
polarization. Overpotential is the difference between the observed half-cell potential with
current flow and the equilibrium zero-current half-cell potential.
Mechanism Contributed to overpotential –
Ohmic overpotential: voltage drop along the path of the current, and current changes resistance
of electrolyte and thus, a voltage drop does not follow ohm's law.
Concentration overpotential: Current changes the distribution of ions at the electrode-
electrolyte interface
Activation overpotential: current changes the rate of oxidation and reduction. Since the
activation energy barriers for oxidation and reduction are different, the net activation energy
depends on the direction of current and this difference appear as voltage.
Vp =VR +Vc +VA
These three mechanisms of polarization are additive.
Thus the net over-potential of an electrode is given by
Vp = E° +VR +Vc +VA
where Vp = total potential, or polarization potential, of the electrode
E° = half-cell potential
VR = ohmic overpotential
Vc = concentration overpotential
VA = activation overpotential
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