Nanotechnology-Enabled Sensors

7.4 Proteins in Nanotechnology Enabled Sensors 421
Fig. 7.44 A schematic of an ELISA. (1) The first antibody is immobilized on the
substrate; (2) the antigen is added and interacts with the antibody, (3) the detecting
antibody (second antibody) is added which binds to the other side of antigen. The
secondary antibody is enzyme-linked (4) the substrate is added. It is the substrateenzyme
interaction which produces a detectable signal.
Inside a cell, most proteins do not work continuously; instead their activities
are regulated. As a result, the proteins in a cell are usually in a state
of equilibrium. The activities of cellular proteins are controlled in an integrated
manner. For instance, the catalytic activities of enzymes are regulated
by other molecules. The system can be very elaborate. Many enzymes
may operate at the same time in a cell. Many of them may compete for the
same substrate at the same time. Their factions should be coordinated.
The regulation of enzymes can occur at different levels. At one level,
the cell controls how many molecules of each enzyme are made. At the
second level, the cell can control the enzymatic activities by confining
them to sub-cellular compartments. The third level, which is the most rapid
control system, is where the cell adjusts the reaction rate of each enzyme.
The third level of controlling enzymes finds applications in nanotechnology
as it can be programmed by the user outside a cell. The control
functions as follow: molecules other than substrates, when added to the
environment, may become attached to the binding sites and thus hinder the