BIOMEDICAL AND BIOMEDICINE APPLICATIONS OF CNTs - Bio-Inspired and Nanoscale Integrated Computing - page 486

Noncovalent methods preserve the pristine CNT structure while covalent modifica-
tion introduces structural perturbations. Details of the various functionalization
techniques have been previously reviewed. Here, we summarize the breadth of
techniques that have been employed in functionalizing CNTs [1, 3, 5].
18.2.1. Noncovalent Functionalization
Various noncovalent interactions such as p stacking, hydrophobic, and van der
Waals interactions have allowed for the functionalization of CNTs with a wide
range of molecules.
18.2.1.1. Surfactants. Surfactants have been found to associate with CNTs
via van der Waals interactions through their hydrophobic chains, rendering the
CNTs hydrophilic, and enabling them to disperse in aqueous environments. With
this method, SWCNTs can be isolated from aggregated bundles, allowing for
spectroscopic probing of individual SWCNTs. Surfactants have been used to
prevent nonspecific interactions between SWCNTs and proteins. Surfactants with
modified head groups have also been used to link SWCNTs to specific molecules.
18.2.1.2. Aromatic Organic Molecules. Aromatic organic molecules can
associate with CNTs via p stacking interactions. In particular, modified pyrene
molecules have been used to modify SWCNTs in various ways. This strategy has
been used to modify SWCNT surface properties and the linking of SWCNTs to
various moieties, imparting diverse functionality of the CNT complexes. For
example, functionalization through modified pyrene molecules has been used to
link the following: CNTs to proteins, opening the door for biological applications;
gold nanoparticles, resulting in enhanced Raman signals of the CNTs; metallo-
porphyrins, in order to exploit the electron acceptor/donor properties of
SWCNTs; and initiation sites for polymerization, allowing the ability to coat
SWCNTs with polymers.
18.2.1.3. Fluorophores. Fluorophores have been shown to associate with
CNTs via hydrophobic interactions, allowing for the visualization of CNTs with
fluorescence microscopy. Other noncovalent means of introducing fluorescence
capabilities include the addition of quantum dots (QDs) via streptavidin-
conjugated QDs, where the streptavidin is absorbed on the CNT surface. Finally,
fluorescent polymer wrapping also allows for the fluorescent detection of CNTs.
18.2.1.4. Polymers. Polymers interact with CNTs through several non-
covalent interactions and offer a wide-range functionality, as demonstrated
through the rich history of polymer science and advances in their use for drug
delivery.
18.2.1.5. Lipids. Lipids comprise a class of molecules that interact with
CNTs similar to surfactants. Lipids offer control of their interactions with CNTs