Nanotechnology-Enabled Sensors

Cd(s) + 2Ag + (aq) Cd 2+ (aq) + 2Ag(s), (3.46)
aqueous Ag + ions can interact directly at the Cd(s) surface, which generates
no net current. This means that such interactions cannot be monitored
in a sensing measurement. In order to avoid this problem, we can separate
the reactant into two half-cells by connecting the two half-cells with a salt
bridge.
A standard reduction potential (shown by E°) can be used to predict the
generated voltage when different half-cells are connected to each other.
The term standard means the activities of all species are unity. 9 A hydrogen
electrode is generally used for the standard. The electrode consists of a
Pt surface in contact with an acidic solution for which AH+=1 mol. A
stream of H2 gas (1 bar pressure and 25°C) is purged through the electrode
to saturate the surface of electrode with aqueous H2. The reaction is:
85
H + (aq) ½ H2. (3.47)
A potential of zero is assigned to this standard hydrogen electrode
(SHE).
The generated voltage is the difference between electrode potentials of
the two half-cells. The magnitude of potential depends on: (a) the nature of
electrodes (b) the nature and concentrations of solutions (c) the liquid junction
potential at the membrane (or the salt bridge). An example of a conventional
Galvanic cell set-up, with zinc and copper electrodes, is shown
in Fig. 3.13. If concentrations of electrolytes are 1 mole then the potential
is measured to be equal to 1.1 V as:
Zn(s) → Zn 2+ + 2e –
3.4 Electrical Transducers
+0.763 V (3.48)
Cu 2+ + 2e – → Cu(s) –0.337 V (3.49)
Zn(s) + Cu 2+ → Zn 2+ + Cu(s) +1.100 V (3.50)
The Gibbs free energy for this reaction is negative (the reaction proceeds
spontaneously at room temperature). This cell can be employed as a
practical battery.
In Fig. 3.13 the salt bridge consists of a tube filled with a gel containing
high concentration of KNO3, which does not affect the cells reactions. The
ends of the bridge are covered with porous glass disks that allow ions to
diffuse but minimize the mixing of solutions inside and outside the bridge.
In this case, K + from the bridge migrates into the cathode compartment and
a small amount of NO3 – migrates from the cathode into the bridge. Ion
immigration offsets the charge build up that would occur. The migration of