Biomedical Engineering Reference
In-Depth Information
just as effectively as unlabeled cells; there was no obvious difference in their ability
to form tumors in mice after 40 days and QDs had no adverse effects on the
physiology of the host animal or labeled cells [
17
].
Quantum dots are nanoparticulate clusters of semiconductor material (smaller
than the Bohr exciton radius) that show quantum confinement effects. The quantum
confinement effect means that the optical properties of these nanoparticles are
controlled by their size, rather than their composition, which makes them useful
optical imaging agents. The size of the band gap of these materials dictates the
energy of the photon emitted and, according to Plancks equation (where energy is
inversely proportional to the wavelength), also the wavelength of emitted light.
QDs have been the focus of great interest recently because of their biological
imaging capabilities [
18
], via their bright fluorescence, photostability, and their
narrow and size-tunable emission spectrum [
19
].
3 Surface Modification
Nanoparticle surface modification is of tremendous importance to prevent nanopar-
ticle aggregation prior to injection, decrease the toxicity, and increase the solubility
and the biocompatibility in a living system [
20
]. Imaging studies in mice clearly
show that QD surface coatings alter the disposition and pharmacokinetic properties
of the nanoparticles. The key factors in surface modifications include the use of
proper solvents and chemicals or biomolecules used for the attachment of the drug,
targeting ligands, proteins, peptides, nucleic acids etc. for their site-specific bio-
medical applications. The functionalized or capped nanoparticles should be prefer-
ably dispersible in aqueous media.
Surface modification is necessary in nanoparticles for various reasons: (1) to
make them biocompatible and non-immunogenic for biomedical applications,
(2) to make them dispersible in aqueous media for most biomedical applications,
(3) to stabilize the nanoparticles in water for long period, (4) to prevent agglomera-
tion of nanoparticles by use of some capping agents and surfactant molecules, (5) to
render specificity towards their target cell or tissue, and (6) to render sterically
accessible functional groups for bioconjugation etc.
There are some important points to be remembered during the choice of
materials for surface modifications: (1) Most in vivo biomedical applications
need the particles to be well dispersed and stable in water. (2) Most of the synthesis
methods that produce highly monodisperse, homogeneous nanoparticles use
organic solvents. (3) Capping agent, surfactant, and the surface moieties to be
attached on the surface of nanoparticles should be chosen carefully. (4) Biocom-
patible capping agents should be used that do not show any adverse effects like
platelet aggregation upon administration into the blood or thrombosis, stenosis etc.
Some studies show that carbon nanotubes can aggregate blood platelets [
21
].
(5) Capping agents should overcome the RES uptake by macrophages and other
cells. Conventional surface non-modified nanoparticles are usually caught in the
