Nanotechnology-Enabled Sensors

84 Chapter 3: Transduction Platforms
concentrations. It will be seen that ΔG 0 is a function of voltage produced
by the electrochemical interactions. As a result, the concentrations of target
analytes can be obtained using voltage or current measurements.
3.4.3 Nernst Equation
In this section, the fundamental basis about the electrochemical sensors
will be presented. Electrochemistry deals with the transfer of charge from
an electrode to its surrounding environment. Electrochemistry uses electrical
measurements for analytical applications. 8 During an electrochemical
process, chemical changes take place at the electrodes and charges transfer
through the media. In fact, the largest and oldest group of chemical sensors
are electrochemical sensors. Sensors as diverse as enzyme electrodes, high
temperature metal oxide gas sensors in automobiles, fuel cells, etc. are included
in this category.
Electrochemistry is primarily concerned with redox reactions. A redox
reaction involves transfer of electron from one species to another. A species
is oxidized when it loses electrons, conversly is reduced when it gains
electrons. An oxidizing agent, which is also called an oxidant, receives
electrons from another substance and is reduced in the process. A reducing
agent, which is also called a reductant donates electrons to another substance
and oxidizes.
For understanding the performance of electrochemical sensors, we first
need to become familiar with some basic electrochemical concepts such
as: Galvanic cells, reference electrodes, salt bridges, and standard reduction
potentials.
A Galvanic (or voltaic) cell uses spontaneous chemical reactions to generate
electricity. 9 In such a cell, one reagent oxidizes and another reduces
and a voltage difference is produced as a result of these reactions. If an
electrode is placed in an electrolyte solution (an electrolyte solution is a
substance that dissociates into free ions when dissolved producing an electrically
conductive medium), it generates a potential. However, this potential
cannot be measured directly. Always a combination of two of such an
electrode-electrolyte system is needed. Each of the electrode-electrolyte
system is called a half-cell. 10
The two half-cells must be connected by means of an electrically conductive
membrane or bridge. For many interactions the net reaction is
spontaneous but little current flows through the circuit as aqueous ions
may react at the other electrode surface. This generates no flow of current
through the external circuit. For instance in: