Chemistry Reference
In-Depth Information
in FI even if their routine application is still controversial (owing to
the toxicity of Cd
2+
ions) [46, 47]. Their relatively narrow emission
bands, whose position is controlled either by the particle size or, for
a given size, by tuning the composition of alloyed semiconducting QD
(CdSeTe), together with their broad absorption spectra, offer a large
palette of distinct color enabling multilabeling [48]. Since several
synthetic routes have been developed for the preparation of water-
soluble QD with narrow size distributions and high quantum yields,
the literature is overflowing with examples of biological materials
labeled by QD [49, 50]. Indeed the most common synthetic routes
of QD lead to hydrophobic fluorescent nanoparticles since they
are coated by trioctylphosphine/trioctylphosphine oxide (TOP/
TOPO). The biological application of QD requires, therefore, their
functionalization by capping hydrophilic molecules. Besides water
dispersity, the functionalization plays two supplementary important
roles: capping ligands can (i) serve as anchoring sites for grafting
molecules and (ii) ensure, by passivating the QD surface, their
protection and consequently the preservation of optical properties.
A representative list of caps and the QD dispersal strategies they use
is provided in a review written by the research group of Mattoussi
[5]. These strategies can be grouped into three major routes (Scheme
4.7). The first one consists in the exchange of hydrophobic monolayer
of TOP/TOPO by hydrophilic thiol or phosphine terminated ligands
(mercaptocarboxylic acid, alkylthiol-terminated DNA, thioalkylated
oligo-ethyleneglycols). This functionalization is driven by mass action
and is possible because sulphur or phosphorus atom can establish
strong dative bond with the chalcogenide species present at the
surface of QD. However, it must be pointed out that the colloidal and
chemical stability is considerably improved when hydrophilic ligands
used for replacing TOP/TOPO carry at least two thiol or phosphine
functions (dihydrolipoic acid derivatives, oligomeric phosphines).
The second strategy described in the literature is based on the
encapsulation of QD in a silica shell. As aforementioned (see above,
Section 4.2.2), silica shells afford multiple ways for further surface
modification. The first step of the formation of silica shell lies in
the replacement of TOP/TOPO by mercaptopropyltrimethoxysilane
(MPTMS) whose thiol end allows the immobilization onto the QD.
The growth of the silica shell is, therefore, generated by hydrolysis-
condensation of polysiloxane precursors mixture (TEOS, APTES,
MPTMS, etc.) from the alkoxysilyl groups of MPTMS bound to the
