Chemistry Reference
In-Depth Information
Other biological molecules are notable for their ability to act as a linking
molecule in the same manner as simple mercaptoacids in early studies.
127
Glutathione, a tripeptide, has also been successfully used as a thiol-based
surfactant which has a further two carboxylic acid groups and an amine
group available for cross-linking once on the nanoparticle surface.
310
Gluta-
thione has been used to phase-transfer CdSeTe/CdS QDs into water. In this
case, a macrocycle, 1,4,7,10-tetraazacyclododecane-1,4,7,10-tetra-acetic acid
(DOTA) was linked to one of the available amine groups which then trapped
a gadolinium ion while maintaining the QD luminescence. This was then
demonstrated to be MRI active, and hence the particles could potentially be
used as a bimodal imaging agent.
311
A related ligand, glutathione tetra-
methyl-ammonium salt, has also been used to phase-transfer metal nano-
particles and QDs into water.
312
In this case, a methanolic solution of the
glutathione was necessary because of the lack of mutual solubility. Addition
of the ligand initially caused the particles to precipitate, although they dis-
solved back into solution as the polarity of the solvent increased, and
d
n
1
y
4
n
g
|
6
nally,
rapid phase transfer was a
ected by addition of water. The phase transfer
was reversible upon further addition of hexadecyltrimethylammonium
bromide, which coordinated to the positive charge of the glutathione ligand,
resulting in long alkyl chains exposed to solution. The hexadecyl-
trimethylammonium bromide could then be removed by addition of tetra-
methylammonium decanoate, allowing the particles to return to the aqueous
phase.
Ai
et al.
have examined the role of a combination of amino acids in capping
CdSe/ZnS QDs, and found that surface exchange of TOPO for a combination
of histidine and
N
-acetyl-cysteine (NAC) at pH 8 red-shi
.
ed the absorption
spectra and increased the photoluminescence quantum yield.
313
Interest-
ingly, the addition of the amino acids on their own was found to be inef-
fective. The use of histidine takes advantage of the well-known interactions
between the imidazole group and zinc (the increase in photoluminescence
from amine species has already been discussed earlier), while the use of the
thiol-containing NAC introduces thiolates that have been known to increase
the emission intensity at low concentration (again, discussed earlier). The
use of histidine-rich peptides as capping agents for QDs has also been
explored by Sapsford
et al.
who described the binding a
nity of such
materials to QD surfaces in assays, with peptides with more than four
monomer units binding better, notably HIS
6
.
314
(Similarly, histidine-rich
genetically modi
ed proteins have been used to assemble QDs.
315
)
The presence of the native ligands on the QD surface also a
ected the
ability of the peptide to attach to the surface; peptides with a small molecular
mass could easily attach to DHLA and DHLA
PEG passivated particles,
but larger proteins rich in histidine were not able to penetrate the ligand
shell. These peptides could also be engineered to provide spacing units
and pendant functional groups such as a 4-formylbenzoyl, which were then
available for further reactions such as aniline-catalysed hydrazone ligation.
316
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