BIOMEDICAL APPLICATIONS VI: CARBON NANOTUBES AS BIOSENSING AND BIO-INTERFACIAL MATERIALS - Carbon Nanotubes: From Bench Chemistry to Promising Biomedical Applications - page 201

Strano et al. 62 reported that when the pH was between 6 and 2.5, the
photoluminescence of dispersed SWNTs quenched as long as the nanotubes
were previously adsorbed with oxygen at the sidewall. The quench was
reversible and luorescence recovered when pre-adsorbed oxygen was
diminished. In some other cases, the disturbance on the luorescence caused
by chemicals was not reversible. Usrey and coworkers 63 found that reaction
with 4-chlorobenzene diazonium caused a prompt decrease in SWNTs'
photoluminescence in a few minutes, proportionally to the addition of the
reactant. The quenching of the luorescence was caused by both non-covalent
and covalent defects on the nanotubes and could be predicted as a linear
combination of the covalent and non-covalent behaviours.
The mechanism of the luorescence quench has been extensively
studied. The reversible quenching of SWNTs' luorescence by means of
oxidative charge-transfer reactions with small redox-active organic dye
molecules indicated that the chiral selective reactivity of compounds with
CNTs is not only diameter dependent but also charge transfer dependent. 64
The quenc hing of luorescence for SWNTs, which experience a transition
between (6, 5) and (7, 5), can be used to detect the existence of 4-amino-
1,1-azobenzene-3,4-disulphonic acid (AB), since AB can act as an electron
acceptor and shows a band gap of ~1.2 eV. On the contrary, the effectiveness
of bleaching occurs at a band gap of ~1.1 eV when mordant yellow 10 is the
acceptor. Therefore the tubes with transitions between (10, 2) and (9, 4) can
show sensitive luorescence quenching upon the addition of mordant yellow
10. So SWNTs with a different chirality can be used as selective sensors for
chemical analysis.
The NIR from semiconducting CNTs is a very sensitive tool and can even
be used to detect a single molecule. Cognet and collaborators 65 reported that
the excitation luorescence of CNTs could be quenched by single-molecule
reactions. They immobilised dispersed single nanotubes in agarose gel.
Reactants, including sulphuric acid, approached the nanotube molecule
from one edge of the gel. The luorescence from the individual CNTs
showed stepwise quenching by acid, rather than a continuous decrease.
These discrete luorescence intensity steps were attributed to individual
protonation reactions at the nanotube surface. Moreover, upon addition of
NaOH solution to the quenched nanotubes, the NIR luorescence intensity
recovered and showed a stepwise increase in intensity, suggesting individual
un-protonation reaction on the SWNT sidewalls. However, although 4-
chlorobenzenediazonium tetraluoroborate, instead of sulphuric acid, could
quench SWNTs, the luorescence could not be re-established because of the
irreversible covalent reaction. These research results indicated that SWNTs
may be highly sensitive to local chemical and physical perturbations.