Biology Reference
In-Depth Information
are strongly dependent on physical properties such as chirality,
diameter, and length. For example, CNTs are either metallic
conductors or semiconductors based on chirality of the structure
[79], whereas diamond is insulating and graphite is semimetallic.
Historically, MWCNTs were the first to be observed in 1991 by
Dr. Sumio Iijima [80] and shortly thereafter SWCNTs were syn-
thesized by arc discharge [81]. Now CNTs are synthesized by
arc discharge of graphite, laser vaporization, and chemical vapor
deposition methods. To date, the use of CNTs for electrochemical
biosensing has been summarized in several excellent reviews [82],
with recent reviews specifically on DNA functionalization of CNTs
[83]. Presented here are recent innovations in electrochemical
DNA detection using CNTs followed by a description of their
implementation into sensing devices. Before expanding on these
areas, a brief overview of key methods used for functionalization
of CNTs is provided since it is a prerequisite to immobilize
biomoleculeson CNTs in a reliable manner.
14.4.1
Functionalization of Carbon Nanotubes with DNA
The potential use of CNTs as electrochemical DNA sensors depends
greatly on their solubility in aqueous media as well as routine
assembly into integrated devices. DNA and other biomolecules
have been successfully immobilized on CNTs by various covalent
and noncovalent binding methods [84]. For covalent attachment,
CNTs are typically activated by chemical oxidation in strong acids,
resulting in the formation of various oxygenated functional groups,
the most prevalent being carboxylic acid groups at the reactive
open ends of the tube, or defect sites at the side walls. This not
only increases their solubility but also presents opportunity for
further modification of the nanotubes. Esterification or amidation
reactions can then be carried out on the oxidized CNTs using
either acid chlorides as intermediates, or carbodiimide coupling
agents [85]. The modified CNTs then react directly with DNA
either targeting an amine site or a thiol site introduced at the
5' end of the DNA molecule. This approach is primarily used to
functionalize the ends of the nanotubes, and although it is fairly
simple, it is not specific. In comparison, sidewall functionalization
