Chemistry Reference
In-Depth Information
These reactions allowed simple conjugation of, for example, further lumi-
nescent peptides onto the QD for FRET studies and enzymatic assays.
Polyhistidine-based peptides have also been used to attach dopamine to QDs,
forming QD
peptide bioconjugates that acted as pH sensors.
317
Related polypeptides, polymers of amino acids, which can formmicelle-like
structures that encapsulate a number of nanoparticles to forma large cluster of
nanoparticles have also been used in phase-transfer procedures. A block
copolypeptide such as poly(EG
2
-lys)
100
-
b
-poly(asp)
30
linked to organically
soluble Fe
2
O
3
particles
via
the aspartic acid residue resulted in a cluster of 20
particles, exhibiting enhanced stability inphysiological conditions.
318
In related
work, a peptide/polymer hybrid material consisting of a peptide backbone
with PEG side groups and thiol/amine groups for coordination, produced to
averyspeci
-
dopamine
-
d
n
1
y
4
n
g
|
6
c accuracy with regard to chain length and functional groups, has
been used to phase-transfer CdSe/ZnCdS core/shell QDs, with a drop in
quantum yield from 48% to 16%.
319
Cysteine-functionalised polyaspartic acid
has also been used to cap QDs,
320
although the hydrophobic particles required
phase transfer into a micelle prior to polymer capping. The polyaspartic acid-
capped QDs could thembe functionalised with antibodies, and used in protein
detection studies and cell labelling. Upon phase transfer, no emission
quenching was observed and the materials appeared stable for months.
Cysteine-rich
-peptides (phytochelatins) have also been used in the phase
transfer of QDs and labelling studies. The peptides were engineered to
consist of 20 amino acids, 10 of which formed a cysteine-rich hydrophobic
domain that coordinated to the QDs, while the other 10 formed a polar,
negatively charged hydrophilic linking domain that could be engineered to
include functional groups, such as biotin or binding sequences.
321
Interest-
ingly, the adhesive cysteine-rich domain required an alanine group between
every two cysteine groups, as a cysteine-only chain would not stabilise the
particle (although it was hypothesised that the alanine group, in the form of
bulky 3-cyclohexylalanine, could also have been involved in the bonding
process). Here, the cysteine groups were converted to cysteine thiolates that
coordinated to the zinc sul
a
.
de shell by addition of a base. The resulting
materials showed excellent stability in bu
er solution, with a minimal size
increase consistent with the attachment of a single layer of peptide, while
displaying no obvious change in the position of absorption or emission,
although a reduction in quantum yield was observed (the emission
quenching in bu
er solutions could reportedly be addressed by UV irradia-
tion
322
). Incubation of biotin-functionalised particles with HeLa cells that
expressed GPI-anchored avidin-CD14 chimeric proteins resulted in targeted
cell labelling with little non-speci
c labelling observed.
In this chapter, we have shown how the synthesis of the inorganic core is
only part of the inherent chemistry required for the preparation of useful
QDs. Functionality can be induced by using di
ering surface ligands and
a range of applications can be realised by the exploitation of engineered
surfactants.
Search WWH ::
Custom Search