Nanotechnology-Enabled Sensors

3.4 Electrochemical Transducers 79
The sensitivities for the SP resonance sensors may also be calculated
similarly in similar manner to that of optical waveguide based sensors. For
2
metals such as gold and silver where ( −ε M′
>> n′ F ) the sensitivities can be
described as: 3
∂N
∂d
F′
2π
⎛ 1
≈ ⎜
λ ⎜
⎝ n
2
C
1
−
n′
2
F
⎞
⎟
N
⎠
4
n
( −ε
′
C
M
)
1/
2
, (3.26)
where λ is the optical wavelength, nC is the refractive index of the sample
media, n′F is the refractive index of the metal, and ε′M is the real part of the
dielectric constant of the metal. As can be seen, the sensitivity is inversely
proportional wavelength and to (–ε′M) 1/2 .
Eq. (3.26) describes sensitivity, with reference to a layer added on the
surface of the sensor. In addition, the change of the effective refractive index
is also a function of the change of refractive index of the sample medium.
Using (3.26) it can be shown that the sensitivities of SPR sensors are
generally 5-10 times larger than those of optical waveguides. Furthermore,
by varying the type of metal thin film, and SPR wavelength, the sensitivity
may also be enhanced. For example, an SPR based sensor utilizing a gold
surface, and having optical wavelength of 632.8 nm, has a sensitivity that
is 1.4 times larger than that of a silver layer device, and it is almost double
that of silver at 780 nm. 3
3.4 Electrochemical Transducers
Electrochemical transducers generate signals that result from the presence
and interaction of chemical species. They make use of various chemical
effects to monitor concentrations of such species. The two main effects
that are utilized in electrochemical sensors are the Volta effect (voltammetry);
in which two dissimilar metals are brought into intimate contact resulting
in the formation of a contact potential, and the Galvanic effect
(amperometry); in which a potential difference is formed when different
conducting materials are placed in an electrolyte solution. Usage of nanomaterials
can enhance the performance for both sensor types. Nanostructured
thin film can increase the surface area to volume ratio at the sensitive
regions of the electrodes, enhance and tailor the electrochemical, optical
and mechanical properties of the sensor and sensitive layer, etc. As will be
demonstrated, utilizing these effects in various ways also utilized to investigate
and quantify the concentration of target analytes and monitoring of
associated chemical reactions.