Nanotechnology-Enabled Sensors

432 Chapter 7: Organic Nanotechnology Enabled Sensors
of ions such as Na + , K + , Cl – and Ca 2+ . In order to do this, they have to
show incredible selectivity. Narrow channels do not allow large ions.
Channels with negatively charged interiors deter negative ions. However,
channels should be more selective letting only one type of ions to pass
through and only in one direction. For example, when ions pass through
these channels they make transient contact with the walls. This contact allows
the protein to recognize them and only allow the right ions to pass.
This contact also works as a counter that generates a passage rate count for
a cell.
These ion channels are not continuously open but instead are gated. A
specific stimulus triggers them to switch between an opened and closed
state by a change in their conformation.
Membrane transporters are useful in the development of accurate and
selective sensors. They are extremely selective and can respond to even a
single molecule or ion. The ion channels described above are also called
nanopores. Many examples of their applications as selective membranes
can be found in the literature. There are many practical ways of using
transporters. In the case of ion channels, it is possible to measure the current
which is generated by the passage of single ions through these channels.
124,125 Single pores are used in stochastic sensing where individual
interactions between the pores and analyte molecules are detected. 126
One of the procedures for conducting such measurements is patchclamp
recording. In a patch-clamp recording, a glass tube with an open tip,
of a diameter of only a few micrometers is used (Fig. 7.52). The glass electrode
is filled with an aqueous conductive solution. This tip is pressed
against the wall of a cell membrane. With gentle suction a small part of
this membrane can be removed. Neher and Sakmann developed the patch
clamp in the late 1970s and further perfected in the early 1980s. 127,128 They
received the Nobel Prize in Physiology or Medicine in 1991 for this work.
A metal wire is inserted at the other end of the tube. Finally, this microtube
electrode can be used in a solution of ions along with other metal electrodes
with accurate circuits connected to measure the generated currents
(Fig. 7.52).