Bioinspired Devices - Bionanoelectronics: Bioinquiring and Bioinspired Devices - page 223

mean diameter of 35 nm are obtained if the synthesized enzyme urease, which is
a powerful catalyst, is moderately inhibited by the concentration of the Ag C ions
so that the S 2 concentration gradient determined by the catalytic activity of the
enzyme reaches the supersaturation regime close to the enzyme surface. On the
contrary, when the enzyme is only slightly inhibited, the S 2 gradient is higher
and the supersaturation regime is attained in a larger volume around the enzyme,
the result being amorphous particles with 142-nm average diameter because the
enzyme cannot control the crystallization process that takes place far away from
its surface. A fully inhibited enzyme is not able to produce S 2 ion precursors and
thus to template the insoluble Ag 2 S particles. The enzyme is moderately, slightly,
and fully inhibited for Ag C ion concentrations of 10 13 M;10 7 M, and 10 4 M,
respectively.
Useful light manipulating two- or three-dimensional photonic crystals are diffi-
cult to fabricate. Biology can help construct such structures in a more direct way
than through inspiration. In fact, wings of the Papilio blumei butterfly, for example,
which are covered by millions of scales with a length of about 100m that consist
of arrays of concavities with a diameter of 5-10m and contain nanometer scale air
chambers separated by chitin laminae, can be used for producing artificial photonic
structures ( Gaillot et al. 2008 ). More precisely, up to 50-nm-thick TiO 2 layers
can be deposited on parts of butterfly wings by the low-temperature atomic layer
deposition technique either as a skin that coats only the outer part of the wing or
both inner and outer parts, if penetrating through microcracks on the surface. In
the first case, the dominant reflection peak of the wing, at 524 nm, is preserved,
and additional peaks at 652, 742, 850, and 1,054 appear with an intensity that
increases with the coating thickness due to the Fabry-Perot-type behavior of the
coated wing. In the second case, the dominant color changes from green (524 nm)
to red (648 nm) as the coating thickness increases to 10 nm and the intensity of the
dominant reflection peak decreases in comparison to the intensities of the Fabry-
Perot peaks mentioned before because the air trapped in the wing concavities is
partially replaced with TiO 2 with a higher refractive index. So, the chromaticity and
intensity of the reflected light of this periodic hybrid inorganic-organic structure is
controlled by the thickness of the coating layer.
Recently, a mass fabrication technique for replicating insect corneas, with the
aim of reflection reduction for obliquely illuminated solar cells, has been reported
in Pulsifer et al. ( 2010 ). It has the advantage of sacrificing only a single biotemplate,
in this case, containing several corneas of blowflies, to produce a master negative of
nickel, which can be used subsequently either as a stamping die or as a casting mold
for multiple replicas. In order to capture the fine surface features of the corneas, a
250-nm-thick Ni film is first deposited on an array of corneas using the conformal-
evaporated-film-by-rotation method, followed by electroforming of a 60-m-thick
nickel layer, which strengthens the conformal coating, the processing ending, finally,
with the removal of the blowfly corneas. Features down to 200 nm in size, including
the 20-m-diameter individual lenslets of the compound eye, are captured with
high fidelity.