BIOMEDICAL AND BIOMEDICINE APPLICATIONS OF CNTs - Bio-Inspired and Nanoscale Integrated Computing - page 487

through modification of their hydrophobic chains, while also providing versatility
of the functionality that they impart through the modification of their head
groups.

18.2.1.6. DNA. DNA, as a naturally occurring polymer, can also associate
with carbon nanotubes through p stacking interactions formed by the aromatic
nucleotide bases on the DNA molecule. DNA has proved effective in allowing for
the aqueous suspension of SWCNTs; by adjusting the DNA sequence, its binding
properties towards CNTs can be modified.

18.2.1.7. Proteins. Proteins have been shown to associate with CNTs
predominantly through hydrophobic interactions, as well as through charge
transfer interactions. This property has been exploited in the design of specific
peptides for controlled interactions with CNTs.

18.2.1.8. Endohedral Functionalization. Endohedral functionalization re-
fers to modification of the inside of CNTs in order to generate hybrid materials
and utilize CNTs as ''nano test-tubes'' incorporating biomolecules. Endohedral
growth of metallic, ionic, and semiconducting nanocrystals, as well as the
incorporation of fullerenes, has been shown, and the incorporation of proteins
demonstrates potential in sensing applications.

18.2.2. Covalent Functionalization
There are two main strategies for covalently functionalizing nanotubes: end and
defect modification and sidewall modification. These covalent modifications arise
from the difference in reactivity at the nanotube ends and sidewalls (as well as at
structurally perturbed areas) and, therefore, each type of functionalization

requires distinct chemical approaches.

18.2.2.1. Ends and Defects. As discussed in the chemical reactivity of
CNTs, the ends are more reactive than the sidewalls. Treatment of CNTs with
oxidative agents results in introducing oxygenated groups, such as carboxylic acid,
ketone, alcohol, and ester, at the nanotube ends and defect sites, thereby leaving
the ends open and possibly cutting and shortening CNTs. Such treatments remove
amorphous carbon and metal catalyst particles and can remove smaller diameter
(more reactive) tubes. Techniques have been developed to probe the degree of
oxidation and the type of oxygenated groups formed. Oxidation also results in
hole-doping of the CNTs causing perturbation of its electronic structure, though
the perturbations are less pronounced than those produced in the case of sidewall
modification as discussed below.
Through the oxidation of CNTs, various chemical species can be introduced
via the covalently incorporated oxygenated species. Coordination of cadmium to
oxidized multiwalled carbon nanotubes (MWCNTs) was used to grow quantum
dots on the tube surfaces. Other modifications of oxidized CNTs by noncovalent