BIOMEDICAL AND BIOMEDICINE APPLICATIONS OF CNTs - Bio-Inspired and Nanoscale Integrated Computing - page 502

amperometric sensors are needed to prevent nonspecific binding while maintaining
high specificity at the molecular level.
18.4.4.2. Optical Sensors. NIR optical sensors based on CNTs take ad-
vantage of perturbations in the nanotube's optical properties due to their surface
modification. For example, a group [65] developed a CNT-based probe to detect
DNA hybridization. Single-stranded DNA was noncovalently absorbed onto
SWCNTs, and the hybridization with its complementary DNA on the nanotube
surface was detected through monitoring alterations in the NIR band gap
fluorescence of the tubes. This same group has also developed an in vivo optical
sensor for monitoring glucose levels using the optical properties of CNTs [66] by
noncovalently modifying SWCNTs affecting the optical properties of the CNT.
In this way, the NIR fluorescence can be coupled to glucose concentration. The
SWCNT complex was encapsulated into a 200- m m-diameter dialysis capillary to
retain the complex while allowing the analyte to diffuse through the dialysis
membrane. This device was then imaged through a human epidermal tissue
sample and exhibited a limit of detection of 34.7 m M and sensitivities within the
range of blood glucose regulations of diabetic patients.
18.5. RESEARCH TOOLS FOR DISCOVERING NEW DRUGS
The costs of bringing a new drug to market using existing methods such as
combinatorial chemistry are substantial. Current techniques in drug discovery
require considerable quantities of proteins of interest in order to meet the limits of
detection of the probing devices employed. Additionally, existing tools do not
provide sensitive enough data about binding events between proteins and other
biological molecules. Beyond the diagnostic applications outlined above, CNT-
based biosensors could also be widely employed in drug discovery. CNT electrical
and optical detection systems might be used to characterize induced molecular
interactions. Perhaps the most powerful new tool for drug discovery will emerge
from CNTs incorporated onto the tips of atomic force microscopes (AFM).
Nanotube-based AFM tips could enable researchers to prove an array of target
proteins for specific interactions at the single-molecule level.
CNTs have been incorporated onto the tips of conventional probes for atomic
force microscopes, greatly enhancing this already powerful tool [67]. The high
aspect ratio and small radius of CNTs offers enhanced resolution. The flexibility of
nanotubes makes these tips robust and sensitive to fragile biological samples.
Additionally, beyond topographical imaging, the rich functionalization chemistry
of CNTs offers the potential for probing a variety of different biological molecules.
End oxidized MWCNTs, with diameters ranging from 15 to 50 nm, were shown to
achieve less than 3 nm resolution due to their functionalization. Chemical modifica-
tion of CNT tips has allowed for applications such as conducting measurements as
a function of pH, imaging based on specific molecular interactions, and measuring
single-molecule protein-protein interactions. An extension of this technology