colorless through yellow and red to brown for CdSe, and from colorless through
yellow and red to a deep brown for CdTe). Nanocrystal formation is rapid, but
lowering the temperature allows the nanocrystals to slowly anneal after formation.
The reaction can be monitored by observing the shift in the photoluminescence
peak towards longer wavelengths. All manipulations during synthesis must be
carried out under an inert atmosphere. Once the QDs have been obtained, subsequent manipulations can be carried out in air, if necessary, without any adverse
effects on the optical properties of the nanocrystals. Washing is typically accomplished by dissolving the reaction mixture in chloroform followed by the addition
of methanol to precipitate the TOPO-capped QDs out of the colloidal solution.
Centrifugation, removal of the supernatant, and redissolving in chloroform can be
repeated several times to obtain deeply colored colloidal solutions of TOPO-capped
CdE QDs. Size-selective precipitation is unnecessary because the synthesis yields
nearly monodisperse populations (ca. 5% RSD). The nanocrystal size can be controlled by varying any one or a combination of parameters: the initial monomer
concentration (typically 2.5–50 mM), the initial injection temperature or growth
temperature, and the growth time (ranging from minutes to hours, depending on
the desired particle size and particular synthetic methods). Periodic injection of
additional precursor solutions into the reaction mixture during the growth period
can also be used to grow larger nanocrystals.
To increase the photostability and luminescence quantum yields, the nanocrystals can be capped with MS (where M ¼ Zn or Cd) [27, 28]. One route to accomplishing this begins with the CdE cores obtained from the previously described
synthesis and, after an annealing period, a solution of MS in TOP is added dropwise (to prevent MS nucleation and promote epitaxial growth) to the reaction mixture. The optimal capping layer thickness is obtained when the luminescence
reaches a maximum, as determined during the slow addition process. This step
yields high quality and nearly monodisperse TOPO-capped (CdE)MS (core)shell
QDs that are soluble in a variety of organic solvents. In order to make these nanocrystals soluble in aqueous media, the QD surface must be linked to molecules
with hydrophilic groups, as discussed in the following section.
12.4
Surface Modification and Bioconjugation
Two key issues in utilizing QDs as fluorescent biological labels are (i) making high
quality nanocrystals soluble in aqueous media and (ii) choosing an appropriate
surface modification technique that enables the nanocrystals to be conjugated to
biological macromolecules while avoiding deleterious effects on the optical properties. Several studies, using nuclear magnetic resonance (NMR) spectroscopy and
X-ray photoemission spectroscopy (XPS) [33, 34], have probed the nanocrystal surface in order to characterize the QD surface. While the general aspects of the
nanocrystal surface properties are held in consensus, details of the exact surface
chemistries at play are still under debate. The nanocrystal interior has been well
12 Core–Shell Semiconductor Nanocrystals for Biological Labeling
410
yellow and red to a deep brown for CdTe). Nanocrystal formation is rapid, but
lowering the temperature allows the nanocrystals to slowly anneal after formation.
The reaction can be monitored by observing the shift in the photoluminescence
peak towards longer wavelengths. All manipulations during synthesis must be
carried out under an inert atmosphere. Once the QDs have been obtained, subsequent manipulations can be carried out in air, if necessary, without any adverse
effects on the optical properties of the nanocrystals. Washing is typically accomplished by dissolving the reaction mixture in chloroform followed by the addition
of methanol to precipitate the TOPO-capped QDs out of the colloidal solution.
Centrifugation, removal of the supernatant, and redissolving in chloroform can be
repeated several times to obtain deeply colored colloidal solutions of TOPO-capped
CdE QDs. Size-selective precipitation is unnecessary because the synthesis yields
nearly monodisperse populations (ca. 5% RSD). The nanocrystal size can be controlled by varying any one or a combination of parameters: the initial monomer
concentration (typically 2.5–50 mM), the initial injection temperature or growth
temperature, and the growth time (ranging from minutes to hours, depending on
the desired particle size and particular synthetic methods). Periodic injection of
additional precursor solutions into the reaction mixture during the growth period
can also be used to grow larger nanocrystals.
To increase the photostability and luminescence quantum yields, the nanocrystals can be capped with MS (where M ¼ Zn or Cd) [27, 28]. One route to accomplishing this begins with the CdE cores obtained from the previously described
synthesis and, after an annealing period, a solution of MS in TOP is added dropwise (to prevent MS nucleation and promote epitaxial growth) to the reaction mixture. The optimal capping layer thickness is obtained when the luminescence
reaches a maximum, as determined during the slow addition process. This step
yields high quality and nearly monodisperse TOPO-capped (CdE)MS (core)shell
QDs that are soluble in a variety of organic solvents. In order to make these nanocrystals soluble in aqueous media, the QD surface must be linked to molecules
with hydrophilic groups, as discussed in the following section.
12.4
Surface Modification and Bioconjugation
Two key issues in utilizing QDs as fluorescent biological labels are (i) making high
quality nanocrystals soluble in aqueous media and (ii) choosing an appropriate
surface modification technique that enables the nanocrystals to be conjugated to
biological macromolecules while avoiding deleterious effects on the optical properties. Several studies, using nuclear magnetic resonance (NMR) spectroscopy and
X-ray photoemission spectroscopy (XPS) [33, 34], have probed the nanocrystal surface in order to characterize the QD surface. While the general aspects of the
nanocrystal surface properties are held in consensus, details of the exact surface
chemistries at play are still under debate. The nanocrystal interior has been well
12 Core–Shell Semiconductor Nanocrystals for Biological Labeling
410