Ligand Chemistry - Semiconductor Quantum Dots: Organometallic and Inorganic Synthesis - page 181

passivation of the alloy ZnCdS shell (materials with a ZnS shell had quantum
yields of only 13%). The cysteine-capped core/shell QDs had an inorganic
core of 3.6 nm as determined by transmission electron microscopy (TEM),
and a hydrodynamic diameter of only 5. 9 nm, notably small for a QD biol-
abel. 138 The preparation of such cysteine-capped QDs has provided particles
small enough to be cleared through the renal system, due to the small size
(ideally <5.5 nm) and zwitterionic charge which prevented serum protein
adsorption. 139 Interestingly, the cysteine-capped particles required the
addition of dithiothreitol (DTT) to prevent the dimerisation of the capping
agent; DTT has itself been used as a capping agent 140 and its role as an
additive in this case was not discussed. Cysteine-capped QDs have also been
further conjugated with dye molecules and small-molecule targeting ligands
for tumour imaging while maintaining a small enough hydrodynamic
diameter to be cleared. This required the coordination of only 5

d n 1 y 4 n g | 6

- 10 targeting

molecules per QD. 141
Alivisatos also reported the use of thiols in the preparations of water-
soluble QDs for labelling applications. In this work, (3-mercaptopropyl)tri-
methoxysilane was coordinated to the particle surface as an anchor point for
the growth of a silica shell. 142 The silica shell was added in a long procedure
based on Mulvaney s seminal work on silica shell deposition on various

nanoparticles, notably gold. 143 In this case, the thiol surfactant can be
considered as a precursor for the

'

nal shell of SiO 2 , which could be further
decorated with various functional groups. 144,145 The emission of silica-
passivated QDs was partially quenched, although quantum yields of up to
18% were reported. The use of silica-capped CdSe/ZnS particle was one of the
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.

rst nanomaterials used for biolabelling applications although this has
generally been surpassed by simpler, more reproducible methods. Related
routes to siloxane-capped QDs have, however, been developed for use in
electroluminescent devices. 146
Thiolated PEG 147 has become a notable ligand used in nanoparticle biol-
abelling, as attachment of the long-chain PEG molecule results in excellent
water solubility. Functionalised PEG groups can also be used as precursors
for further structures: a notable study includes the use of nitriloacetic acid-
based ligands which incorporated a nickel complex. The nickel complex was
found to bind to histidine, and used to visualise 5HT2C serotonin receptors.
In this case, the particles were initially capped with PEG with terminal amine
groups, which were then cross-linked to the nitriloacetic acid ligand. 148
In most cases, the monodentate thiol ligands impart only a temporal
stability to the nanoparticle dispersion, in some cases lasting only a matter of
hours or days. 72,119 Dithiol and multidentate-based ligands extend the
stability in water to, in some cases, years. 149 Notably, one of the earliest
examples using dithiols was the phase transfer of CdSe/ZnS QDs using
dithiothreitol (DTT), which was followed by linking oligonucleotides to the
ligands which were then used in uorescent in situ hybridization (FISH)

experiments. 140 The DTT-capped QDs were reportedly stable for 3

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4 weeks,
substantially longer than monodentate thiol-capped QDs. This simple

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