Biomedical Engineering Reference
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was removed from the solution. On the other hand, degassing the SWCNT/
surfactant solution increased its luorescence intensity, especially when the
temperature was raised to 80 ° C, after which it started to decay. The collected
spectroscopic data suggested that the photoreaction was driven by absorption
of UV light by SWCNTs. Although the mechanism of this photoreaction is not
yet understood, it is thought that, similarly to the previous example, radicals
are generated during the irradiation process. This assumption was conirmed
by the suppression of the reaction when irradiations were performed in air
rather than degassed solutions: O 2 in fact is a well-known radical scavenger,
so it quenched the radical reactive intermediates.
Overall, these results indicated that the reaction was initiated by the
SWCNT photon absorption, leading to the generation of an excited state,
which thereafter activated small molecules, likely water, in their close
surroundings. Semiconducting SWCNTs seem good candidates as their
excited-state lifetimes, reported in the literature, are in the picosecond range,
while metallic tubes have shorter lifetimes (femtosecond range). 115 Such
difference provided semiconducting tubes enough time to react with the
molecules in their surroundings and give selective reactions, while metallic
SWCNTs remained essentially unreactive toward photohydroxylation. This
work represented the irst selective reaction in the liquid phase that takes
advantage of an intrinsic property of the tubes to develop both separation
processes and selective functionalisation chemistries.
In a different experiment, it was observed that the addition of diazonium
SWCNTs/surfactant suspensions quenched the intrinsic NIR luorescence
of semiconducting SWCNTs through sidewall reactions. 116 Fluorescence
spectroscopy was employed to quantify the structure-dependent relative
reactivities of these reactions. In previous studies, it had been suggested
that the mechanism behind this reaction involved injection of electrons
from the metallic SWCNTs into the diazonium salt ( Fig. 9.19a) , implying that
the semiconducting SWCNT species with smaller band gaps would react
more readily. However, the observed results showed the opposite trend.
Alternatively, the electron-deicient diazonium salt becomes an electron-rich
diazotate that reacts with another molecule of diazonium salt to yield the
diazoanhydride (Fig. 9.19b). This electron-rich anhydride could preferentially
form charge transfer complexes with the larger-band-gap, electron-poor
SWCNTs, implying that greater reaction selectivity should occur for diazonium
salts in which the R group more strongly promotes diazoanhydride formation.
Since this hypothesis was not conirmed, the authors concluded that it seems
likely that different reaction mechanisms are dominant in the reactions of
diazonium salts with metallic versus semiconducting SWCNTs.
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