Surface functionalized nanoparticles are extensively used as
nanoscaffolds for various biological applications ranging from
bioanalysis and bioimaging to diagnosis and therapy. This
review article mainly focuses on the chemical and bioconjugate
reactions on the surfaces of nanomaterials such as silica
nanoparticles, gold nanoparticles and quantum clusters, semiconductor quantum dots, carbon nanotubes, fullerene and
graphene. The chemical and bioconjugate reactions discussed
in this article formulate these nanomaterials for various biological applications such as biosensing, bioimaging, drug and
gene delivery, and therapy. The large surface to volume ratio is
the common property among these nanomaterials, which is
useful for the conjugation of various sensors, targeting molecules, contrast agents, drugs and genes at high local concentrations. Furthermore, the fundamental optical properties of these
nanomaterials show certain common features that are valuable
for multiplexed biosensing and bioimaging, and phototherapy.
For example, the shape- and size-dependent tunable plasmon of
gold nanorods is comparable with the size-dependent tunable
photoluminescence of semiconductor quantum dots and gold
quantum clusters. Other common properties of nanomaterials are
the photothermal responses of silica nanoparticles, gold nanorods,
carbon nanotubes, fullerene and graphene, visible to NIR photoluminescence of gold quantum clusters, semiconductor quantum
dots, carbon nanotubes and graphene, photoacoustic response
of silica nanoparticles, gold nanorods, carbon nanotubes and
graphene, and the photosensitized singlet oxygen production
by gold quantum clusters, semiconductor quantum dots and
C60. Furthermore, many functional groups and chemical and
bioconjugate reactions are common to the formulation of these
nanomaterials into biosensors, imaging probes, drug and gene
delivery systems, and nanomedicine. Although multimodal and
multifunctional nanoscaffolds constructed by the conjugation
of various targeting molecules, drugs, genes and contrast agents
to nanomaterials are extensively investigated in the imaging and
treatment of cancer cells and tumors, controversial reports about
toxicity and pharmacokinetics hamper the clinical applications of
nanomaterial formulations. Therefore, a number of issues such as
biocompatibility, toxicity, in vivo and in vitro targeting efficiency,
bioavailability, renal and hepatobileary clearance, and long
term pharmacological and toxicological issues of functionalized nanomaterials are under rigorous investigation. The
current scenario of research in the formulation and testing of
bioengineered nanomaterials as sensors, contrast agents,
drugs, and drug and gene delivery systems suggest that the
complete transformation of conventional medicine into nanomedicine cannot be too far away.