characterized using transmission electron microscopy and X-ray diffraction, which
clearly identifies the crystalline phase and gives strong evidence that the QD surface is faceted rather than smooth [4, 10]. Given the high radius of curvature of the
QD surface, the steric hindrance associated with the packing of molecules on the
surface is somewhat relaxed, allowing a slightly higher number of surface groups
to be attached per unit surface area on the nanocrystalline material as compared to
bulk substrates.
Quantum dots synthesized using techniques outlined in Section 12.3 have a
capping layer of TOPO molecules strongly coordinated to the surface metal atoms.
While TOPO is an ideal capping molecule for maintaining QD solubility and optical properties in organic solvents [36], the extremely hydrophobic nature of this
surface requires that they be replaced with molecules containing polar groups that
allow the QDs to be transferred into aqueous media. With this purpose in mind,
several methods have been designed to prepare water-soluble QDs by displacing
the TOPO molecules with a more appropriate layer (Fig. 12.3).
One method involved the use of a siloxane coating for creating water-soluble
(CdSe)ZnS [15, 37]. In this case, mercaptopropyltrimethoxysilane (MPS) displaces
the TOPO molecules, and a siloxane shell is formed via the hydrolysis of the silanol groups under basic conditions. Following the initial shell formation, a mixture
of trihydroxysilylpropylmethylphosphonate monosodium salt (TMP) and MPS are
added to give the QD surface hydrophilic character and provide surface thiol
groups for subsequent bioconjugation. The resulting surface is negatively charged
due to the surface phosphonate groups. Alternatively, TMP can be replaced during
the second step with poly(ethylene glycol) (PEG)-silane or a mixture of PEG-silane
and trimethoxysilypropyl trimethylammonium chloride in order to obtain a neutral
or slightly positively charged surface [38]. Subsequently, the thiol groups can be
converted to amine or carboxy groups using standard procedures. These surface
groups can be conjugated to biological molecules using an appropriate bifunctional
linker and any one of the many well established techniques [39]. These QDs are
highly stable against aggregation and show no reduction in the quality of the optical properties. However, it is difficult to scale up this complex procedure which
yields only milligram quantities per batch.
A different procedure was reported by Chan and Nie, which involves the adsorption of bifunctional ligands such as mercaptoacetic acid, mercaptosuccinic
acid, dithiothreitol, glutathione or histidine directly onto the QD surface [16, 40].
Others have shown that a host of mercapto compounds such as mercaptobenzoic
acid, mercaptopropionic acid, mercaptoundecanoic acid, mercaptohexadecanoic
acid are also amenable to the same procedure [19, 41]. Essentially, the desired surface molecule is added to TOPO-capped QDs dissolved in chloroform in conjunction with an organic base. The driving force behind this scheme is the high affinity
of the thiol or amino groups towards the surface metal atoms which causes the
displacement of TOPO molecules on the surface. Within minutes of the addition
(carried out at room temperature) the QDs precipitate out of the organic solution
and can be isolated and readily redissolved in basic aqueous solution. Although
gram quantities of water-soluble QDs can be obtained using this technique, slow
12.4 Surface Modification and Bioconjugation 411
clearly identifies the crystalline phase and gives strong evidence that the QD surface is faceted rather than smooth [4, 10]. Given the high radius of curvature of the
QD surface, the steric hindrance associated with the packing of molecules on the
surface is somewhat relaxed, allowing a slightly higher number of surface groups
to be attached per unit surface area on the nanocrystalline material as compared to
bulk substrates.
Quantum dots synthesized using techniques outlined in Section 12.3 have a
capping layer of TOPO molecules strongly coordinated to the surface metal atoms.
While TOPO is an ideal capping molecule for maintaining QD solubility and optical properties in organic solvents [36], the extremely hydrophobic nature of this
surface requires that they be replaced with molecules containing polar groups that
allow the QDs to be transferred into aqueous media. With this purpose in mind,
several methods have been designed to prepare water-soluble QDs by displacing
the TOPO molecules with a more appropriate layer (Fig. 12.3).
One method involved the use of a siloxane coating for creating water-soluble
(CdSe)ZnS [15, 37]. In this case, mercaptopropyltrimethoxysilane (MPS) displaces
the TOPO molecules, and a siloxane shell is formed via the hydrolysis of the silanol groups under basic conditions. Following the initial shell formation, a mixture
of trihydroxysilylpropylmethylphosphonate monosodium salt (TMP) and MPS are
added to give the QD surface hydrophilic character and provide surface thiol
groups for subsequent bioconjugation. The resulting surface is negatively charged
due to the surface phosphonate groups. Alternatively, TMP can be replaced during
the second step with poly(ethylene glycol) (PEG)-silane or a mixture of PEG-silane
and trimethoxysilypropyl trimethylammonium chloride in order to obtain a neutral
or slightly positively charged surface [38]. Subsequently, the thiol groups can be
converted to amine or carboxy groups using standard procedures. These surface
groups can be conjugated to biological molecules using an appropriate bifunctional
linker and any one of the many well established techniques [39]. These QDs are
highly stable against aggregation and show no reduction in the quality of the optical properties. However, it is difficult to scale up this complex procedure which
yields only milligram quantities per batch.
A different procedure was reported by Chan and Nie, which involves the adsorption of bifunctional ligands such as mercaptoacetic acid, mercaptosuccinic
acid, dithiothreitol, glutathione or histidine directly onto the QD surface [16, 40].
Others have shown that a host of mercapto compounds such as mercaptobenzoic
acid, mercaptopropionic acid, mercaptoundecanoic acid, mercaptohexadecanoic
acid are also amenable to the same procedure [19, 41]. Essentially, the desired surface molecule is added to TOPO-capped QDs dissolved in chloroform in conjunction with an organic base. The driving force behind this scheme is the high affinity
of the thiol or amino groups towards the surface metal atoms which causes the
displacement of TOPO molecules on the surface. Within minutes of the addition
(carried out at room temperature) the QDs precipitate out of the organic solution
and can be isolated and readily redissolved in basic aqueous solution. Although
gram quantities of water-soluble QDs can be obtained using this technique, slow
12.4 Surface Modification and Bioconjugation 411