Nano-Bio Integration - Bionanoelectronics: Bioinquiring and Bioinspired Devices - page 239

The mechanosensitive channel protein, extracted from E. coli , allows passage of
molecules with diameters smaller than 3 nm. Its conformation varies in response
to the tension in the lipid membrane and forms a pore under severe hypoosmotic
stress conditions induced by light and/or pH. Controlled opening of nanocontainers
activated by adding a positively charged compound to the bulk solution could
be monitored by the fluorescence of dyes introduced in the nanocontainers. This
nanocontainer, filled with active molecules, could be used for smart drug delivery.

9.3 Nanobioelectronics and Optoelectronics

The integration of biological features and nanoelectronic devices is not an easy task
since biomolecules are often soft materials, which operate in aqueous solutions,
whereas nanotechnological processes require more robust and stiff materials. DNA
is a versatile material for nanobioelectronics since it can not only influence the
electrical conductivity of inorganic systems with which it interacts, but its own
conduction depends on doping conditions. More precisely, the canonical Watson-
Crick base pairs can be substituted by metal complexes, which influence the
conductive or even magnetic properties of the metal-base pair molecules ( Clever
et al. 2007 ). In addition, DNA networks can be patterned on the Si=SiO 2 surface,
which makes this biological molecule compatible with standard semiconductor
nanotechnologies.
Nanobioelectronics implies ultimately the integration of biological systems with
nanoelectronic devices. A successful integration implies that both components
have comparable dimensions, which is only possible when networks or arrays
of nanostructures are used. Such an integration of a purple cell membrane from
Halobacterium salinarum and a nanotube network field-effect transistor, which
consists of a dense network of randomly oriented carbon nanotubes contacted by
electrodes, with the gate voltage assured by the buried substrate, is reported in
Bradley et al. ( 2005 ). The purple cell membrane contains the bacteriorhodopsin
protein, which is light-sensitive and has a permanent electric dipole moment that
induces an electric field directed from the extracellular membrane side to the
cytoplasmic side. Patches of the 5-nm-thick purple membranes were deposited on
nanotube network field-effect transistors oriented with the cytoplasmic side or the
extracellular side toward the nanotubes, as well as with a mixture of these orien-
tations and the transfer characteristics (current vs. gate voltage) were measured.
The reference devices, not covered with biological membranes, showed threshold
voltages and significant hysteresis loops at left-moving and right-moving sweeps
of the gate voltage, which narrowed considerably when purple membranes were
present, irrespective of their orientations. The change of threshold voltages when
the biological systems were deposited, however, depended on their orientations:
shifts of 2.2 V and 0.4 V were measured, respectively, for the cytoplasmic and
extracellular orientations, while for mixed orientations, the corresponding shift