Nanotechnology-Enabled Sensors

424 Chapter 7: Organic Nanotechnology Enabled Sensors
A protein can also inactivate itself by the hydrolysis of its internal
guanosine triphosphate (GTP) to guanosine diphosphate (GDP) without
the aid of a catalyst. The GTP-binding protein is active and by releasing a
phosphate and turning into GDP it changes into an inactive state.
There are a large number of related GTP-binding proteins that function
as molecular switches in a cell. The dissociation of GDP and its replacement
by GTP can turn a switch on. The GTP-binding proteins often bind to
other proteins in order to control their enzymatic activities and they play
an important role in the intercellular signaling pathways.
A large conformational change can be produced in response to nucleotide
hydrolysis. Elongation factor Tu (EF-Tu), a GTP-binding protein,
elongates a polypeptide chain during the cellular synthesis processes. The
hydrolysis of bound-GTP in EF-Tu causes only a small change in the position
of the amino acid at the nucleotide binding site of approximately
0.1 nm. The release of the phosphate group changes the intramolecular
bond similarly to a latch and creates a major change in the shape, which in
turn releases the tRNA molecule to allow the synthesis process to be conducted.
One of the most important enzyme types in sensing applications are the
redox enzymes groups. In redox reactions inside the body of a living organism
a substrate becomes oxidized or reduced. This action is uses energy and
support life within the body of the living organism. Redox enzymes accelerate
the redox reactions to render them biologically useful. Redox enzymes
also control the highly reactive intermediates. As a result, in such enzymes
generally only a single substrate is employed and a single specified product
is produced.
Redox enzymes are used in many sensing applications such as the
glucose sensors. However, these redox enzymes usually lack direct communications
with electrodes. As a result, they have to be used along with
materials which mediate the diffusion of electrons into electrodes.
Example: Blood Glucose Sensors
One of the most popular electrochemical sensors in the market is the
blood glucose monitor. 118 Patients with diabetes require the monitoring the
glucose level in their blood. This monitoring has to be conducted several
times a day to allow them to control their disease through insulin injections.
Early types of home glucose sensors consisted of a three layer working
electrode (Fig. 7.46) and an Ag/AgCl reference electrode (Chap. 3).