  Carbon nanotubes exhibit many unique intrinsic physical and chemical
properties and have been intensively explored for biological and biomedical
applications. In this review, we summarize the main results of our and other
groups in this field and clarify that surface functionalization is critical to
the behaviors of carbon nanotubes in biological systems. Ultra-sensitive
detection of biological species with carbon nanotubes can be realized after
surface passivation to inhibit the non-specific binding of bio-molecules on the
hydrophobic nanotube surface. Electrical nanosensors based on nanotubes provide
a label-free approach to biological detections. Surface enhanced Raman
spectroscopy of CNT opens up a method of protein microarray with down to 1 fM
detection sensitivity. In vitro and in vivo toxicity studies reveal that well
water soluble and serum stable nanotubes are biocompatible, non-toxic and
potentially useful for biomedical applications. In vivo biodistributions vary
with the functionalization and possibly also sizes of nanotubes, with a
tendency of accumulation in the reticuloendothelial systems, including the
liver and spleen, after intravenous administration. If well functionalized,
nanotubes may be excreted mainly through the biliary pathway in feces. Carbon
nanotube-based drug delivery has shown promises in various in vitro and in vivo
experiments including delivery pf small interfering RNA, paclitaxel and
doxorubicin. Moreover, SWNTs with various interesting intrinsic optical
properties have been used as novel photoluminance, Raman and photoacoustic
contrast agents for imaging of cells and animals. Further multidisciplinary
explorations in this field are promising and may bring new opportunities to the
realm of biomedicine.
