Bioinspired Devices - Bionanoelectronics: Bioinquiring and Bioinspired Devices - page 222

8.3 Bioinspired Technological Processes

Not only innovative devices and materials mimic the biological world, but efficient
catalytic reactions ( Que and Tolman 2008 ) and technological processes for nano- or
micrometer-size structures that could take place at nonextremal temperature, pres-
sure, and/or chemical conditions are sometimes inspired from nature. Present-day
nanotechnology relies heavily on silicon or silica nanostructures of various shapes
and sizes ( Losic et al. 2009 ). Biosilica is also found in nature, porous silica shells,
for example, being encountered in the frustules of microscopic unicellular algae
called diatoms. The frustules consist of two valves connected by girdle bands, each
valve being composed of silica plates that separate stacked hexagonal chambers.
There is a huge variety of shapes in the diatom frustules determined by the structural
diversity of the groups of nanosized pores in the plates, each diatom from the
100,000 existing species being characterized by a distinctive frustule morphology
( Losic et al. 2009 ). Since biosilica in diatoms forms in specialized compartments, in
mild acidic environment, and at moderate temperatures and pressures, bioinspired
silica production could have advantages over common technologies.
For example, a controlled bioinspired growth of silica nanoparticles has been
reported in Bauer et al. ( 2007 ). It occurs in confined environments containing
monosilicic acids and spherical reverse micelles in the presence of a branched
polyamine, which is structurally similar to polyamines isolated from unicellular
marine algae Cylindrotheca fusiformis and Stephanopyxis turris , various morpholo-
gies and particle sizes being obtained by changing the ratio of water and reverse
micelles in the container. In a similar manner, silica structures synthesized from
a mixture of chitosan solution previously incubated for different times and in the
presence of prehydrolyzed tetraethylorthosilicate showed a spherical shape with a
diameter of 40 nm for a chitosan solution incubation time of 56 h and appeared as
sheets of carambola-like silica particles with diameters of 20-40 nm for incubation
times of 150 h; after 280 h incubation times, the chitosan solution precipitates ( Leng
et al. 2008 ). For instance, solid particles with a trimodal size distribution, having
diameters ranging from 56 to 500 nm, are obtained for smaller water-to-micelles
ratios, while hollow and robust micrometer-sized silica shells are produced for larger
values of this ratio. More complex structures, consisting of hollow silica capsules,
which are reinforced with carbon nanotubes, have been inspired by sea anemones
( Sanles-Sobrido et al. 2008 ). These rigid structures can be assembled by an applied
magnetic field if the capsules are loaded with Fe 3 O 4 magnetic nanoparticles.
Besides silica, sulfide semiconductor nanocrystals such as CuS, CdS, PbS,
or Ag 2 S can be fabricated using bioinspired techniques, which produce them in
aqueous solution without toxic ligands ( Pejoux et al. 2010 ). The inspiration comes
this time from mollusks, which synthesize complex calcium carbonate crystals in
mild conditions on nucleating sheets of proteins from precursors with low solubility
in water. In a similar manner, core/shell enzyme/Ag 2 S sulfide semiconductors are
grown in mild conditions on an enzyme nucleating template in the presence of
a controlled concentration gradient of S 2

ions. Crystalline nanoparticles with a