OVERVIEW ON THE MAJOR RESEARCH ACTIVITIES ON CARBON NANOTUBES BEING DONE IN AMERICA, EUROPE AND ASIA - Carbon Nanotubes: From Bench Chemistry to Promising Biomedical Applications - page 255

to be dispersed in an aqueous medium and become biocompatible. To that
purpose, the authors found better results with tubes previously sonicated
in sodium cholate, which was subsequently replaced by the polymeric PL-
PEG complex; although this step was not necessary for the functionalisation,
it proved particularly useful at providing less damage to the nanotubes, a
better quantum yield and sharp E 11 absorption peaks (1,000-1,300 nm) in
the UV-vis-NIR spectra in comparison with SWCNTs directly suspended in
PL-PEG. In fact, despite the fact that the exchange SWCNTs were injected
at a dose (17 mg/L) 15 times lower, they still showed a stronger in vivo
NIR photoluminescence signal (red) than the direct-SWCNT-treated mice.
This fact, in combination with the good tissue penetration and the ultra-
low background autoluorescence in the emission range, envisages the use
of biocompatible nanotube luorophores as promising biological imaging
agents.
Another non-covalent functionalisation consists of the recently published
conjugation of dextran and a phospholipid 1,2-distearoyl- sn -glycero-3-
phosphoethanolamine (dextran-DSPE), which was subsequently adsorbed
onto nanomaterials (SWCNTs, NPs and NRs). 58 Dextran is a naturally occurring
polymer made up of α (1 → 6)-linked glucose monomers with 5% branching
at the 4 position. The hyperbranched structure and packed interior result
in a globular shape, thus rendering it similar to dendrimers and allowing a
coating more resistant to protein adhesion than a linear polymer. Moreover, its
polyhydroxylated structure makes dextran a very hydrophilic polymer, while
maintaining several hydroxyl groups, which may be further functionalised
with PEG units to originate hyperbranched polymeric structures with
longer circulation in vivo . The authors proposed a phospholipid-dextran
conjugate, in which the DSPE was bound at a single point to the reducing
end of the dextran (Fig. 9.9) . This was possible through the anomeric end of
dextran, which differs in reactivity from the hydroxyl groups on the rest of
the polymer. 59 The complex was subsequently adsorbed onto nanomaterials,
showing improved photophysical properties in comparison with commonly
used PEGylated structures.
The authors also demonstrated that such conjugation maintained the
key requirements necessary for an optimal coating in view of biomedical
applications, such as (i) a stable coating even under high dilutions, as would
be expected in vivo ; (ii) avoidance of non-speciic binding of proteins that
might determine low blood circulation time; (iii) present functional groups
for a selective targeting; (iv) absence of aggregation of nanomaterials under
various conditions and preservation of the intrinsic physical properties of the
material under investigation.