BIOMEDICAL APPLICATIONS IV: CARBON NANOTUBE-NUCLEIC ACID COMPLEXES FOR BIOSENSORS, GENE DELIVERY AND SELECTIVE CANCER THERAPY - Carbon Nanotubes: From Bench Chemistry to Promising Biomedical Applications - page 116

Interestingly, the attachment of QDs did not necessarily reduce the water
solubility of the DNA-wrapped SWCNT conjugates, because of suficient
remaining surface COOH groups on QDs even after conjugation (Fig. 5.13).
Another advantage of these well-dispersed conjugates is that such samples
can be optically excited to give rise to sharp peaks of dispersed SWCNTs in the
UV/vis/NIR spectrum. This allowed the investigation of the influence of DNA
molecules on the spectral shift of QDs. The emission spectra of small droplets
of sample solutions of QDs alone, DNA-QD conjugates with no SWCNTs, and
DNA-SWCNT or DNA-QD conjugates were compared and reported (Fig. 5.14).
The QD-DNA exhibited a negligible blue shift, suggesting that DNA had
little effect on QDs' spectral properties, while further conjugation with
SWCNTs resulted in a noticeable blue shift of at least 4 nm, which conirmed
that carboxyl QDs were conjugated onto DNA-SWCNTs. Taken together,
these results promote the fabrication of QD-CNT hybrids for advanced
nanoelectronics and nanosensors. At the same time, it is one of the very few
cases in which the free amino group in guanine is covalently bonded to the
QD, although the DNA chain is only physically wrapped around the tubes.
On the contrary, Bianco et al. demonstrated that it is possible to synthesise
SWCNT-adenine complexes by amidation reactions between acid/amine-
functionalised ( f -)SWCNTs and amine/acid-functionalised adenine derivatives
( Fig. 5.15) . 42 These procedures offer the possibility of generating controlled
horizontal alignment of the nanotubes on highly oriented pyrolytic graphite
(HOPG) surfaces and of complexing them with metal ions, thus giving rise
to a highly patterned supramolecular assembly of SWCNTs. Nucleobases can
assemble into organised structures via hydrogen-bonding interactions, 43-52
the mechanism of which involves adsorption followed by rearrangement of
molecules on the surface to generate hierarchial assemblies. 53-55
The tri-ethylene glycol (TEG) monomethyl ether chains seemed to be
responsible for the parallel orientation of the complex f -SWCNT 2b (Figs.
5.15 and 5.16), through an interdigitation process that keeps the nanotubes
apart, while adenine residues at the tips undergo self-hydrogen bonding
interactions. 48,49 This behaviour can explain the elongated shape of the
nanotubes under AFM (Fig. 5.16a) , while SWCNT 2a, being without TEG
chains, forms fibrils of tubes (Fig. 5.16b).
As regards the ability of adenine (attached to the CNT surface) to coordinate
the formation of Ag(I) nanoparticle-CNT conjugates, f -SWCNT 2a derivatives
were used to avoid interferences from oxygen and nitrogen atoms deriving
from the TEG chain of f -SWCNT 2b. Beadlike Ag(I) nanoparticles present over
the nanotubes were observed, probably because of the interaction of adenine
with Ag(I) ions (Fig. 5.17) . Interestingly, the same procedures with AgNO 3
solution and SWCNT-COOH 1a showed only physical adsorption of heavy,
non-aligned clusters of Ag(I) on the surface of nanotubes.