Nanotechnology-Enabled Sensors

102 Chapter 3: Transduction Platforms
-0.2 -0.1
Fig. 3.25 The Fe 3+ /Fe 2+ redox system voltammograms when the reduction rate
constant is changing at a constant applied voltage change rate.
Decreasing the rate constant decreases the concentrations of ions at the
electrode surface and slows down the reaction kinetics. In this case, the
equilibrium is not established rapidly. As a result, the position of the current
peak shifts to the higher voltages upon the reduction of the rate
constant. However, decreasing the rate constant makes the system less
reversible.
Generally charge transfer reactions are reversible if k 0 > 0.1 – 1 cm/s and
irreversible for k 0 < 10 –4 – 10 –5 cm/s. They are referred as quasi-reversible
for values that fall between reversible and irreversible.
If the target analyte concentration, during the sensing measurements or
the surface of electrodes, alters, it results in a non reversible electrochemical
interaction. Many disposable electrochemical sensors for medical applications
are based on irreversible interactions.
Capacitive current
Current
0.1 0.2
Voltage
In addition to the diffusion current, capacitive currents also exist in electrochemical
processes. The double layer forms a capacitive dielectric region
between the bulk analyte area and the surface of the electrode. This
capacitance is known as the double layer capacitance and its value is proportional
to the surface area of plates. The capacitive current is essentially
unwanted in most of the sensing applications as the sensing information is
generally extracted from the diffusion current curves.
Ep
Decreasing rate
constant