Biomolecular Architecture for Nanotechnology - Bionanoelectronics: Bioinquiring and Bioinspired Devices - page 165

bacteria to bridge a distance of 12 m between Au electrodes deposited on an
oxidized Si wafer, it is essentially that the bacteria are alive, at least during a part
of the deposition process ( Berry et al. 2004 ). Lysine-coated gold nanoparticles with
diameters of 30 nm in a solution of pH D 7 can be deposited on bacteria, which have
diameters and lengths of 0.9 and 5m, respectively, due to the high specific affinity
of live bacteria to lysine; this affinity degrades significantly for dead bacteria. The
span between the gold electrodes can be bridged by two bacteria covered with
gold nanoparticles. A single gold monolayer forms because electrostatic repulsion
between positively charged nanoparticles prevents the deposition of following
layers. After deposition of the gold nanoparticles, an electric field rearranges the
morphology of the metal monolayer such that its electrical conductivity is enhanced
more than ten times. The increase in conductivity can be explained by an increase
in the number of branching/parallel current pathways in the percolating network.

5.4 Nonperiodic Biological Scaffolds for Inorganic Structures

In many cases, inorganic structures, in particular inorganic nanoparticles, use
biological molecules as templates, even if the latter show no specific pattern.
Individual biomolecular scaffolds can be seen as zero-dimensional templates. For
example, biological scaffolds for colloidal quantum dot synthesis can determine
and tune their optical properties. In this case,Pb 2C ions seed a nucleotide ligand,
which hinders the growth of a bulk semiconductor if S 2 ions are added afterwards
to the solution ( Hinds et al. 2006 ). In the growth process, the nucleotide binds the
precursor metal cation, caps the nanoparticles when the reagents are consumed,
passivates the eventual defect states on the surface of the nanoparticle, and favors
the formation of stable dispersed nanoparticles in solution. The process takes
place for a molecular ratio nucleotide:Pb 2C W S 2 of 3:1:1. The spherical PbS
nanocrystals obtained using the GTP nucleotide have a diameter of 4 nm and show
photoluminescence with a quantum efficiency of 1-2%.
CdS semiconductor nanocrystals can be also fabricated using transfer RNA
(tRNA) polynucleotides from E. coli with a cloverleaf secondary structure as
template and nanocrystal ligands ( Ma et al. 2006 ). In this case, no additional ligand
or functionalization is needed for the aqueous synthesis of CdS. The spherical
CdS particles synthesized with this tRNS from ions of Cd 2C and S 2 have mean
diameters of 4:4 ˙ 0:4 nm, whereas semiconductor nanoparticle diameters obtained
with a mutant tRNA had larger values, of 5:5 ˙ 1:0 nm.
Biomolecules can also act as active templates for the growth of inorganic
architectures. For example, enzymes modified with gold nanoparticles catalyze
the formation of metallic nanowires and nanoparticles with different morphologies
(triangular, spherical, prismatic, etc.) and allow the orthogonal growth of nanowires
from different metals if immobilized on Si surfaces ( Willner et al. 2006 ).
Although plasmonics usually requires linear chains of metallic nanoparticles to
guide surface plasmon resonances along the regions with enhanced electromagnetic