Nanotechnology-Enabled Sensors

-0.2 -0.1
Current
0.1 0.2
Voltage
Fig. 3.24 Changing the scan rate in a Fe 3+ /Fe 2+ redox system.
Ep
3.4 Electrical Transducers
Increasing
the scan rate
101
The value of the peak currents are proportional to the square root of the
scan rate according to Eq. (3.72), ip ∝ u t . In slow voltage scans, the diffusion
layer grows further from the electrode surface than in fast scans.
Consequently, the ion flux to the electrode surface becomes smaller decreasing
the current magnitude.
For the Fe 3+ /Fe 2+ redox high concentration system, the electron transfer
kinetics is fast. A rapid system is generally a reversible electron transfer
system. Conversely, for a slow electron transfer system, the I-V characteristics
depict quasi-reversible or irreversible electron transfer systems.
Reversible systems are often encountered in sensing measurements
when the concentration of the target analyte is high in the environment in
comparison of the concentration of the molecules interacting on the surface
of the electrodes. In such cases, a repeating cycle gives the same response
and reversing the voltage sweep produces a mirrored I-V curve.
Sensors based on reversible reactions can be reused as their surface dose
not change.
In Fig. 3.25 the IV curves for the Fe 3+ /Fe 2+ redox system voltamogramms
are recorded as the reduction rate constant (k 0 ) is changing at a
constant applied voltage change rate.