dimers and higher oligomers [61]. Alternatively, small bi-functional molecules like
bis(acyl hydrazide) [62] or 1,6-hexanedithiol [60] have been used to generate
discrete QD oligomers, which could be purified up to a level of ~70% by sizeselective precipitation. Xu et al. [58] have recently shown that density gradient
ultracentrifugation is an efficient tool for separating directly coupled or dye-bridged
QD dimers and trimers from QD monomers and higher oligomers.
Colloidal QDs dispersed in organic solvents are capped by hydrophobic ligands
such as long-chain amines or oleic acid. By adding a bad solvent (methanol) to a
solution of QDs in a good solvent (toluene) the QDs will precipitate, and can be
separated from the solvent mixture by standard centrifugation. By this procedure,
ligands may detach from the QD surface, leading to the onset of aggregation.
Consequently, by repeating the precipitation/dissolution cycles many times and
depending on the binding strength of the ligands, a larger and larger fraction of the
material may aggregate and finally becomes insoluble in the good solvent. It has
been shown that the degree of aggregation induced by the precipitation/dissolution
cycles could be controlled, yielding a distribution of directly coupled QD oligomers
in the good solvent [58]. Subsequently, dimers and trimers were separated
and enriched by density gradient ultracentrifugation. CdSe/CdS/ZnS core–shells
particles were used in this study.
The density gradient in the ultracentrifugation tube was built with cyclohexane/
CCl 4 mixtures, with the gradient from bottom to top adjusted from 90–40% with
respect to the CCl 4 volume ratio. On top of the gradient solution, 0.1–0.2 mL of the
QD solutions were applied. A typical ultracentrifugation took ~10 min at
56,000 rpm and, under the right conditions [58], led to the appearance of several
bands. After ultracentrifugation, material was removed from the various bands by a
pipette and after further dilution their composition examined by transmission
electron microscopy (TEM). By this approach, monomer, dimer, and trimer
fractions could be separated from higher oligomers residing at the bottom of the
tube. The purity of the various fractions could be increased by a second round of
ultracentrifugation, yielding purities of up to 90%.
Besides directly coupled QD oligomers, dye-bridged variants could be
assembled by employing the bi-functional TDI spacer 38. This type of QD oligomer
was prepared by mixing QDs dissolved in toluene and 38 dissolved in methanol to
yield a QD-to-dye ratio of typically 1:3. After ultracentrifugation, several bands
appeared along the tube and TEM images of the various fractions showed that again
monomers, dimers, and trimers could be separated. Spectroscopic investigations in
bulk solution indicated that EET from the QDs to the dye “bridge” occurred [58].
In Fig. 6, high resolution TEM images of a directly coupled QD dimer and a QD
dimer crosslinked by TDI are shown. The two QDs are in close contact in the first
case, but there is a clear interparticle distance in the second case caused by the dye
bridge. By analyzing approximately 20 dimers for each case, it was found that the
QDs touched each other in ~95% of the directly coupled dimers. In contrast, for the
38 cross-linked dimers, a clear separation between the QDs was found in ~70% of
the images. The observation of close contact in directly coupled dimers gave further
support to the assumption that in this case loss of ligands is responsible for the
formation of oligomers.
Optical Properties of Assemblies of Molecules and Nanoparticles
79
bis(acyl hydrazide) [62] or 1,6-hexanedithiol [60] have been used to generate
discrete QD oligomers, which could be purified up to a level of ~70% by sizeselective precipitation. Xu et al. [58] have recently shown that density gradient
ultracentrifugation is an efficient tool for separating directly coupled or dye-bridged
QD dimers and trimers from QD monomers and higher oligomers.
Colloidal QDs dispersed in organic solvents are capped by hydrophobic ligands
such as long-chain amines or oleic acid. By adding a bad solvent (methanol) to a
solution of QDs in a good solvent (toluene) the QDs will precipitate, and can be
separated from the solvent mixture by standard centrifugation. By this procedure,
ligands may detach from the QD surface, leading to the onset of aggregation.
Consequently, by repeating the precipitation/dissolution cycles many times and
depending on the binding strength of the ligands, a larger and larger fraction of the
material may aggregate and finally becomes insoluble in the good solvent. It has
been shown that the degree of aggregation induced by the precipitation/dissolution
cycles could be controlled, yielding a distribution of directly coupled QD oligomers
in the good solvent [58]. Subsequently, dimers and trimers were separated
and enriched by density gradient ultracentrifugation. CdSe/CdS/ZnS core–shells
particles were used in this study.
The density gradient in the ultracentrifugation tube was built with cyclohexane/
CCl 4 mixtures, with the gradient from bottom to top adjusted from 90–40% with
respect to the CCl 4 volume ratio. On top of the gradient solution, 0.1–0.2 mL of the
QD solutions were applied. A typical ultracentrifugation took ~10 min at
56,000 rpm and, under the right conditions [58], led to the appearance of several
bands. After ultracentrifugation, material was removed from the various bands by a
pipette and after further dilution their composition examined by transmission
electron microscopy (TEM). By this approach, monomer, dimer, and trimer
fractions could be separated from higher oligomers residing at the bottom of the
tube. The purity of the various fractions could be increased by a second round of
ultracentrifugation, yielding purities of up to 90%.
Besides directly coupled QD oligomers, dye-bridged variants could be
assembled by employing the bi-functional TDI spacer 38. This type of QD oligomer
was prepared by mixing QDs dissolved in toluene and 38 dissolved in methanol to
yield a QD-to-dye ratio of typically 1:3. After ultracentrifugation, several bands
appeared along the tube and TEM images of the various fractions showed that again
monomers, dimers, and trimers could be separated. Spectroscopic investigations in
bulk solution indicated that EET from the QDs to the dye “bridge” occurred [58].
In Fig. 6, high resolution TEM images of a directly coupled QD dimer and a QD
dimer crosslinked by TDI are shown. The two QDs are in close contact in the first
case, but there is a clear interparticle distance in the second case caused by the dye
bridge. By analyzing approximately 20 dimers for each case, it was found that the
QDs touched each other in ~95% of the directly coupled dimers. In contrast, for the
38 cross-linked dimers, a clear separation between the QDs was found in ~70% of
the images. The observation of close contact in directly coupled dimers gave further
support to the assumption that in this case loss of ligands is responsible for the
formation of oligomers.
Optical Properties of Assemblies of Molecules and Nanoparticles
79