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 278

devices is the ability to impart selective analyte binding without disrupting
optical and electronic properties. A recent example towards that goal is the
synthesis of a 3,4-diaminophenyl-functionalised dextran (DAP-dex) wrapping
around SWCNTs in such a way as to provide a rapid and selective luorescence
detection of nitric oxide (NO), which is a vital biological messenger. 135 The
advantage of this functionalisation relies on the immediate bleaching of the
NIR luorescence of SWCNT-DAP-dex complex by NO, but without affecting
other reactive nitrogen or oxygen species. This indicates that the resulting
optical sensor is able to detect the NO molecules deriving from NO synthase
in macrophages in real time and it could be eventually extended for in vivo
detection in animal models.
During some investigations on SWCNTs sensors, it was noticed that, once
photogenerated excitons in SWCNTs undergo one-dimensional quantum
coninement, the signals deriving even from an individual molecule could
be ampliied tremendously, also in case of living cells and tissues. In view
of this observation, the authors have prepared a collagen ilm, containing
SWCNTs able to sense single-molecule adsorption of quenching species.
More precisely, H 2 O 2 , H + and Fe(CN) 6 3- were investigated as quenching
molecules. 136 H 2 O 2 diplayed the highest quenching equilibrium constant
of 1.59 at 20 μM. Interestingly, reverse (unquenching) rate constants were
concentration-independent while the forward (quenching) rate varied along
with concentration and the redox potential of the quencher.
Taking advantage of SWCNTs presenting a 1D electronic structure sensitive
to molecular adsorption, the research group has identiied at least four modes
in SWCNTs that can be modulated to uniquely ingerprint agents by the degree
to which they alter either the emission band intensity or wavelength. 137 This
is particularly useful for those species (e.g., reactive oxygen species [ROS])
that present half-lives between a nanosecond and a millisecond and form
adducts with DNA, since it acts as a label-free sensor able to convert chemical
information into an NIR signal. Therefore, exposing the SWCNT-DNA complex
to several analytes simultaneously has been demonstrated to achieve signal
multiplexing.
As reported in Fig. 9.22, in the irst reaction the d(GT) 15 oligonucleotide-
bound nanotube (SWCNT-DNA) 128 is exposed to a chemotherapeutic alkylating
agent, which reacts with the guanine nucleobase and will lead to eventual
strand breakage, resulting in apoptosis of cancer cells. The interaction of
such agent with the SWCNT-oligonucleotide complex results in a uniform
red shift in the photoluminescence bands of both (6, 5) and (7, 5) nanotubes
(Fig. 9.22b). The second reaction involves direct adsorption of H 2 O 2 by the
nanotube, which results in attenuation of both the nanotubes' emission and a
slight concomitant energy shift (Fig. 9.22c). Singlet oxygen, generated in the