Biomolecular Architecture for Nanotechnology - Bionanoelectronics: Bioinquiring and Bioinspired Devices - page 157

of the probe molecule. The length of the gap can vary between few nanometers
to few micrometers and is defined with single-base accuracy, i.e., an accuracy of
about 0.3 nm.
This sequence-specific lithography can also locate molecular objects and metallic
clusters on specific locations along the DNA substrate or can construct DNA
junctions, in which a RecA polymerized short molecule undergoes homologous
reaction with one end of a long molecule followed by branch migration ( Keren
et al. 2002 ). The fabrication of a room-temperature carbon nanotube (CNT) field-
effect transistor using DNA templating and homologous genetic recombination
was presented in Keren et al. ( 2003 ). After the homologous recombination and
before incubation in the AgNO 3 solution in the technological sequence in Fig. 5.4 ,a
streptavidin-coated single-walled CNT (SWCNT) is bound to the RecA-containing
strand by first exposing this strand to a primary antibody to RecA and then to a
biotin-conjugated secondary antibody with a high affinity to the primary antibodies.
In this way, SWCNTs were selectively positioned on RecA via biotin-streptavidin
binding. Then, the DNA-CNT complex was stretched on a passivated and oxidized
Si wafer so that the DNA and the stiff CNT become aligned. The CNT-FET
fabrication ends with the electroless metallization process on the silver clusters
nucleation centers, in which RecA acts as resist. The SWCNT, which is longer
than the gap determined by RecA, makes good contact with the metallic Au wires
because metal is deposited also on the ends of the SWCNT. The technological
processes following those in Fig. 5.4 a are represented schematically in Fig. 5.5 .
CNT FETs fabricated by this method, with a p-type conduction channel and lengths
of 130-200 nm, work at room temperature and use as gate electrode a p C Si
substrate.
Another method of sequence-selective metallization has been described in Burley
et al. ( 2006 ). In this case, an enzymatic approach was used for selective aldehyde
labeling, in which acetylene reporter groups, introduced by polymerase into selected
genes, reacted with aldehyde azides to directly incorporate aldehyde functions into
the genes. In the following step, Ag deposition was directed to aldehyde-modified
DNA only.

a b

biotin

anti RecA

+ AgNO 3

Ag clusters

+

SWCNT

streptavidin

c

Au

SWCNT

Fig. 5.5 Fabrication steps of a CNT FET: ( a ) binding a SWCNT on the homologous sequence in
Fig. 5.4 a, ( b ) formation of Ag clusters by incubation of the resulting structure in AgNO 3 ,( c )the
final CNT FET