NANOPARTICLES FOR LABELING
Consider luminescent CdSe nanorods as an example of nanoparticles used for labeling (Fig. 7.9). These nanorods can be used as fluorescent labels for molecular biology because they absorb light from the UV to around 550 nm and emit strongly at 590 nm. They were made—appropriately enough—in the lab of Thomas Nann, in Freiberg, Germany.
These nanorods measure approximately 3 nm in width by 10 to 20 nm in length. A core of luminous cadmium selenide (CdSe) is surrounded by a shell of ZnS (zinc sulfide, wurtzite) to protect the core against oxidation. Outside this is a layer of silica, which allows coupling of phosphonates or amines to the exterior of the nanorod. These hydrophilic groups make the nanorods water soluble. These outer chemical groups also allow attachment of the nanorods to proteins.
The scaffold inside eukaryotic cells is built from cylindrical protein structures known as microtubules. These are often disassembled into monomers (known as tubulin) and reassembled in different locations. Nanorods can be used to follow this remodeling by attaching them to the tubulin monomers. On addition of guanosine triphosphate (GTP), assembly of microtubules is stimulated and the fluorescent nanorods can be seen aggregating into linear structures.
Why use a complex multilayered nanostructure instead of a simple fluorescent dye?
(a) Although nanocrystals have narrow emission peaks, they have broad absorption peaks (rather than narrow ones like typical dyes). Consequently they do not bleach during excitation and can therefore be used for continuous long-term irradiation and monitoring.
(b) Nanocrystals have high brightness—the product of molar absorptivity and quantum yield. (Molar absorptivity is the absorbance of a one molar solution of pure solute at a given wavelength; the higher it is, the more light is absorbed. The quantum yield is the ratio of photons absorbed to photons emitted during fluorescence.)
(c) The emission maximum of a nanocrystal depends on the size and so can be set to any desired wavelength by making crystals of the appropriate size (see later discussion).
Nanoparticles can also be targeted to specific tissues, such as cancer cells, by adding appropriate antibodies or receptor proteins to the nanoparticle surface. Fluorescent nanoparticles are often known as quantum dots and are now commercially available for a wide range of biological labeling. Although fluorescent dyes can be attached to other molecules, nanoparticles are more versatile in this regard. Quantum dots can be used to label DNA molecules as well as proteins. Thus labeling of PCR primers with quantum dots results in fluorescently labeled PCR products—a variant referred to as quantum dot PCR.
A variety of materials have been used to give better contrast enhancement in MRI. Nanoparticles containing a variety of materials are beginning to see increasing use in this area. For example, superparamagnetic iron oxide (SPIO) nanoparticles act as good MRI contrast agents. Their magnetic properties vary with particle size. Larger particles, of greater than 300 nm, are used for bowel, liver, and spleen. Smaller particles, of 20 to 40 nm, have shown higher diagnostic accuracy for detecting early tumors in lymph nodes than conventional materials.