Nanotechnology-Enabled Sensors

382 Chapter 7: Organic Nanotechnology Enabled Sensors
Although the concepts of self-assembly are applicable to any material,
currently the most promising avenues for self-assembly are those concerned
with organic components. Such self-assembled materials are used
in controlling the growth of nanomaterials, fabricating electrical insulators,
making sensitive and selective layers, developing superlattices of organic
compounds with accurate thicknesses, etc. There are also several other advantages
of employing self-assembly in nanotechnology: 13 it can be utilized
to directly incorporate biological structures as components in the final
systems as well as producing structures that are relatively defect-free and
flawless since it requires that the target structures are thermodynamically
stable.
Despite all the advances that have been made, the mechanisms responsible
for self-assembly have not yet been fully understood. In addition, it is
still not possible to mimic many of the processes known to happen in biological
systems. As a result, self-assembling processes cannot, in general,
be designed and carried out on demand. Many of the ideas that are essential
to the development of this area are simply not yet under control such as
the molecular conformation, the entropy and enthalpy relationships, and
the nature of the non-covalent forces that connect the particles.
Self-assembled monolayers (SAMs) are surfaces that consist of a single
(mono) layer of molecules on a surface. SAMs are becoming increasingly
useful in different technologies, especially in developing sensors and fabricating
accurate micro/nano devices. This is due to the fact that, they allow
the film thickness and the composition to be precisely controlled at the
scale of ~ 0.1 nm.
Amphiphilic molecules, which are molecules that contain both hydrophilic
and hydrophobic groups, are widely used in self-assembly processes.
Due to their hydrophobic-hydrophilic nature, amphiphilic molecules find
a plethora of applications in our daily lives such as paint dispersants,
cosmetic ingredients, detergents, soaps. SAMs can use these molecules to
form surfaces suitable for molecular immobilization on transducers. Such
molecules should consist of two different head and tail functional groups
where one end sticks to the sensor’s surface and the other end interacts
with the analyte biomolecules. Hence, they make a strong link between the
biomolecules and sensor.
For the formation of SAMs, the substrate is generally immersed into a
dilute solution of the self-assembling molecules and a monolayer film
gradually forms (Fig. 7.13). The monolayer formation rate can range from
just a few minutes to several hours.