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 275

increases, photoemission from SWCNTs reaches a saturation value, suggesting
an upper limit on the exciton density for each nanotube species. Kono and co-
workers not only demonstrated this phenomenon, but they also developed a
model based on diffusion-limited exciton-exciton annihilation, which allowed
estimating exciton densities in the saturation regime. 123 It is also possible, by
applying a magnetic ield ( B ) along the tube axis, to enhance the brightness
of the dark state of SWCNTs, leading to a new bright peak with increasing
intensity till a dominant role at B > 3T. 124 This behaviour could be considered
universal, since it was observed for more than 50 different tubes, showing
values of 1-4 meV for tube diameters of 1.0-1.3 nm. This emphasised how
the surrounding environment could inluence the excitonic ine structure of
the tubes.
In addition to these observations associated with SWCNTs of speciic
chirality, the inner shells of DWCNTs have also shown some NIR emission, 125
although such luorescence is weaker than SWCNTs' by a factor of at least
10,000. Better proiles have been obtained with self-assembling peptides,
which show the ability to suspend the tubes in solution while preserving
strong NIR luminescence. 126 Until now, a limiting aspect of CNTs has been
their highly hydrophobic character and their tendency to aggregate into
nonemissive bundles. Therefore, CNTs need to be functionalised in order to
become dispersible and biocompatible. Covalent functionalisation, although
providing a stable coating against displacement or disruption, tends to
quench the NIR nanotube emission. Previous attempts to suspend SWCNTs
included pluronics (synthetic nonionic surfactants), 127 single-stranded DNA 128
and bovine serum albumin (BSA). 129 In addition, the ionic surfactant sodium
dodecylbenzene sulphonate (SDBS, which is not biocompatible) allowed by
far the strongest NIR emission. In fact, it was used in another experiment
to suspend SWCNTs in aqueous solution and enabled the calculation of the
product of absorption cross section and luorescence quantum yield for 12
( n , m ) SWCNTs, 130 thus deriving the empirical calibration factors needed to
deduce quantitative ( n , m ) distributions from bulk luorimetric intensities.
In comparison with this and other suspending agents, peptides show the
advantage of a simple and versatile chemical assembly of biocompatible amino
acids into a variety of engineered structures with tailored functionalities.
These peptides are organised with an A-B-A block motif, in which the
peripheral A domain contains electrostatically charged amino acids while
the interior B domain consists of alternating hydrophilic and hydrophobic
amino acids, which cause the peptide to assume a β-sheet conformation.
Interestingly, these peptides are not acting as simple amphiphilic surfactants.
Instead, they interact with the nanotubes in a more speciic fashion, in which
their tendency to self-assemble is reinforced by the presence of the SWCNTs.
As a proof of that, it was observed that the peptides that allowed the brightest
SWCNT emission were not those that displayed the strongest tendency to