Biomedical Engineering Reference
In-Depth Information
esteriication or the zwitterionic COO
-
NH
3
+
formation.
117
This oxidising
procedure is usually known as “defect functionalisation”, since it takes
place at the ends or at speciic positions of pre-existing defects of CNTs. If
not exaggerated, it preserves the macroscopic features of CNTs so that their
electronic and mechanical properties are not lost.
118
-
120
In addition, it has been
employed to link biological molecules to CNTs via stable covalent bonds. For
example, bovine serum albumin (BSA) has been incorporated on
f
-CNTs (both
single- and multi-walled) via diimide-activated amidation, demonstrating
that the protein, once bound to nanotubes, retains its activity.
82
Similarly,
streptavidin (a protein with potential clinical applications in anticancer
therapy)
121
and DNA have been also bound to CNTs via amide linkage.
83,84
In conclusion, this process permits the introduction of carboxylic groups
and carboxylated fractions that enable further manipulation of both the
activity of incorporated biomolecules and the spectroscopic properties of
f
-CNTs. However, it can also introduce an excess of defects or generate ultra-
short
f
-CNTs.
122
2.4 COVALENT FUNCTIONALISATION ON THE EXTERNAL
SIDEWALLS
Among the most powerful methodologies aimed to functionalise CNTs, the
use of 1,3-dipolar cycloaddition represents a fascinating example of covalent
bonding because it is extremely versatile and easy to execute. It requires only
an
α
-amino acid (or a correspondent ester) reacting with an aldehyde or a
ketone, to generate
in situ
azomethine ylides, which are very reactive species,
and affording the formation of pyrrolidine rings on the sidewalls of CNTs
(Scheme 2.4).
NHBoc
O
O
N
Boc-NHCH
2
CH
2
OCH
2
CH
2
OCH
2
CH
2
NHCH
2
COOH
(CH
2
O)
n
, DMF, 130 °C
NH-Boc
NH
3
+
Cl
-
O
O
O
O
N
N
HCl 4 M dioxane
Scheme 2.4
1,3-Dipolar cycloaddition of azomethine ylides.
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