BIOMEDICAL AND BIOMEDICINE APPLICATIONS OF CNTs - Bio-Inspired and Nanoscale Integrated Computing - page 501

a drop of bodily fluid on a small, disposable chip, patients may be able to identify
the early onset of disease. The market potential for health care devices with these
characteristics is undoubtedly tremendous.
The capability of specifically attaching (and preventing the attachment of)
biomolecules to CNTs has provided the possibility of using nanotubes as the
biosensors for future diagnostic products. There are two main types of CNT
sensors: electronic sensors and optical sensors.
18.4.4.1. Electronic Sensors. The introduction of electronic biosensors
would provide several advantages over existing biosensors, which are primarily
based on different optical techniques. Electronic sensors do not require complex
reagents and expensive equipment, as in the case of optical-based detection
systems, and have the added advantage of versatility of implementation due to
being an electronic device. Additionally, electronic detectors could be more
sensitive and accurate than optical systems. There are two different electronic
sensors that are being developed using nanotubes.
Field effect transistors (FETs) are constructed by connecting the source and
drain of the FET with a single semiconducting SWCNT that forms the conducting
channel [57-59]. The device detects changes in the electrical characteristics of the
FET resulting from molecules interacting with the nanotube. As chemical sensors,
these devices were initially shown to be capable of single molecule detection both
in air and in liquid environments. Through the ability of CNTs to interact with
biomolecules and by taking advantage of functionalization schemes, these devices
have also been employed in biosensing. Carbon nanotube FETs (NTFETs) exhibit
unparalleled sensitivities as compared to conventional FET devices while having
advantages over optical techniques with comparable sensitivity. NTFETS have
since been used to probe the mechanism of CNT-protein surface interactions, in
the detection of receptor-ligand interactions through the use of biomolecule-
funtionalized CNTs, in probing enzymatic reactions, and in the detection of
viruses. Another class of NTFET devices employs a network of SWCNTs forming
a CNT film as the conducting channel. The network architecture allows for better
reproducibility and eased manufacturability, but is less sensitive than a single
nanotube device. CNT network FETs have been integrated with cell membranes.
Further research will focus on sensing capabilities in physiological serum [59]. The
low limit of detection of NTFETs is characteristic of CNTs responding to the
binding of any molecule. Applications in complex mediums such as serum will
require stringent prevention of nonspecific binding through robust functionaliza-
tion schemes.
Amperometric sensors relate the current induced in a particular reaction to
the concentration of the species involved in the reaction. CNT-based sensors
employing dense, vertically aligned, and biofunctionalized SWCNTs have been
developed for probing biomolecule-induced charge transfer [64]. These devices
take advantage of the directional conduction pathway of CNTs by attaching the
base of the tubes to an electrode with analyte interactions occurring at their
functionalized tips. Like field effect transistor sensors, further advances for