Driving oxygen coordinated ligand exchange at nanocrystal surfaces using trialkylsilylated chalcogenides

Article abstract of DOI:10.1039/c0cc02220a

A general, efficient method is demonstrated for exchanging native oxyanionic ligands on inorganic nanocrystals with functional trimethylsilylated (TMS) chalcogenido ligands. In addition, newly synthesized TMS mixed chalcogenides leverage preferential reactivity of TMS-S bonds over TMS-O bonds, enabling efficient transfer of luminescent nanocrystals into aqueous media with retention of their optical properties.

Full text of DOI:10.1039/c0cc02220a

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Driving oxygen coordinated ligand exchange at nanocrystal surfaces
using trialkylsilylated chalcogenideswz
Marissa A. Caldwell,b Aaron E. Albers,a Seth C. Levy,a Teresa E. Pick,a Bruce E. Cohen,a
Brett A. Helms*a and Delia J. Milliron*a
Received 30th June 2010, Accepted 11th November 2010
DOI: 10.1039/c0cc02220a
A general, efficient method is demonstrated for exchanging
native oxyanionic ligands on inorganic nanocrystals with
functional trimethylsilylated (TMS) chalcogenido ligands. In
addition, newly synthesized TMS mixed chalcogenides leverage
preferential reactivity of TMS–S bonds over TMS–O bonds,
enabling efficient transfer of luminescent nanocrystals into
aqueous media with retention of their optical properties.
binding sites at cadmium chalcogenide NC surfaces.9 In their
scheme, native oxyanionic ligands at NC surfaces were
chemically transformed by various TMS reagents to non-
coordinating TMS phosphonate esters (and related species),
with the reactive partner from the TMS-reagent replacing the
oxyanionic ligand at the surface. We have investigated the
extension of this chemoselective strategy to a diverse selection
of NC compositions and common oxyanionic ligands
(Scheme 1). We also report three novel TMS reagents derived
from mercaptoalkanoic acids used for ligand exchange and
quantitative transfer of quantum dots to aqueous buffers. We
show conclusively that reactive ligand exchange by TMS-
chalcogenides provides a general platform for efficient replace-
ment of common native ligands on NCs of diverse inorganic
composition with specifically designed functional ligands.
Proton Nuclear Magnetic Resonance (1H NMR) spectro-
scopy was used to evaluate the efficacy of oxygen coordinated
ligand exchange using TMS reagents. For this purpose,
Advances in chemical synthesis have afforded high quality
nanocrystals (NCs) of varying compositions with exceptional
control over size and morphology.1 A central strategy used
to achieve such control is the passivation of NC surfaces
by hydrophobic ligands, such as oleic acid or octadecyl-
phosphonic acid.2 However, for many applications it is
necessary to replace the generic hydrophobic ligands with
specific functional ligands, e.g., in order to facilitate aqueous
dispersion,3 enable specific targeting4 or add complementary
optoelectronic functionality.5 It remains a significant challenge
to find general and efficient mechanisms for such ligand
exchange reactions.
trimethylsilylated
1-mercapto-3,6,9,12-tetraoxotridecane
(TMS-S-TEG) was selected as the exchange ligand because
of its distinct 1H NMR resonances between d 3.2–4.0 ppm and
was synthesized according to previously published procedures.10
Conventionally, ligand exchange is carried out by mass
action; excesses of several orders of magnitude of the new
ligand are used to drive reactions. These reactions can none-
theless proceed slowly and the conditions can vary widely for
specific ligand and inorganic core chemical compositions. For
example, the commonly used exchange protocol for pyridine
surface replacement involves hours of heating at elevated
temperatures in neat pyridine,6 while amines or thiols are
typically exchanged in concentrated solution at room
temperature.7 Still other protocols involve phase transfer over
hours or days.8
1
By comparing the integration of H NMR resonances unique
to the native and exchange ligands, respectively, we can
approximate the exchange efficiency. In some cases native,
unbound ligands persisted in the analyzed product, so the
calculated exchange percentages are lower limits. These signals
are readily distinguished in the post-exchange spectra where
broad peaks sharpen, indicating a transition from a bound to
an unbound state (Fig. 1).
To investigate generality of our reaction scheme with respect
to both the native ligand functional head group and the
inorganic core composition, several NCs were prepared.
We hypothesized that broad applicability and greater
efficiency might be achievable using a chemically-driven
approach in which chemoselective reactions at the head groups
of native ligands could render these ligands non-coordinating.
Along this line, the Alivisatos group recently reported the use
of various trimethylsilylated compounds (e.g., TMS-Cl or
TMS-S-alkyl) to investigate the chemical nature of ligand
a The Molecular Foundry, Lawrence Berkeley National Laboratory,
1 Cyclotron Road, Berkeley, CA 94720, USA.
E-mail: bahelms@lbl.gov, dmilliron@lbl.gov;
Fax: +1 510 486 6166; Tel: +1 510 486 6723
b Department of Chemistry, Stanford University, Stanford, CA 94305,
USA
w This article is part of the ‘Emerging Investigators’ themed issue for
ChemComm.
z Electronic supplementary information (ESI) available: Synthetic and
experimental details, including ligand syntheses. See DOI: 10.1039/
c0cc02220a
Scheme 1 Ligand exchange using trimethylsilylated ligands.
c
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Table 1 Hydrodynamic diameter (d), PL quantum yield (Fl) and zeta
potential (z) of hydrophobic and ligand exchanged, aqueous QDs.
Measurements were performed in CHCl3 (Qdot 545, 605 ITK) or
Millipore H2O (DHLA-, MPA-, MUA-QDs) at 25 1C
Sample
d/nm
FI
x/mV
Qdot 545 ITK
DHLA-QD545
MPA-QD545
MUA-QD545
Qdot 605 ITK
DHLA-QD605
MPA-QD605
MUA-QD605
9
10
8
13
9
13
11
20
0.60
0.07
0.30
0.52
0.65
0.03
0.32
0.35
NA
ꢁ34
ꢁ22
ꢁ40
NA
ꢁ23
ꢁ18
ꢁ56
Fig. 1 1H NMR spectra of phosphonate-capped CdSe NCs before (a)
and after (b) ligand exchange and oleate-capped CdSe NCs before (c)
and after (d) ligand exchange with TMS-S-TEG. (*) Denotes residual
native ligand. (+) Denotes solvent impurity.
Alkylphosphonate- and alkylcarboxylate-capped CdSe and
alkylcarboxylate-capped CdTe NCs were synthesized using
an automated NC synthesis robot (the Workstation for
Automated Nanomaterials Discovery and Analysis, or
WANDA1a), and isolated using previously reported
procedures.9,11 Alkylcarboxylate-capped ZnO NCs were also
synthesized and purified according to previously reported
procedures.12 Reactive ligand exchange was performed by
adding TMS-S-TEG (2–7 ꢀ 104 equivalents per NC, or
approximately 8–100 per surface metal atom [ESIz]) to a
dispersion of NCs in chloroform and stirring for several hours
at 25 1C. The TEG-S-bound NCs were isolated by several
rounds of precipitation into hexanes from chloroform and
subjected to NMR spectroscopic analysis in toluene-d8 to
determine the extent of ligand exchange.
reagent (5 ꢀ 105 equivalents per QD) to a dispersion of QDs in
chloroform and stirring for several hours at 25 1C. An equal
volume of sodium tetraborate buffer (50 mM, pH 10) was
added, and the samples were allowed to stand for 1 h at 25 1C.
Quantitative transfer of QDs to the aqueous phase was
evidenced by the lack of detectable luminescence in the chloro-
form layer, with all green or orange luminescence observed in
the aqueous phase.
To determine the hydrodynamic diameters of these aqueous
QDs, dynamic light scattering (DLS) was performed before
and after surface remodeling (Table 1). MPA-, DHLA-, and
MUA-capped QDs followed a predictable size progression,
with MPA producing the smallest QDs in each series. No
evidence of aggregation was observed for any of the exchanged
samples, in contrast to non-chemically driven exchange
reactions under analogous conditions which resulted in severe
aggregation [ESIz]. In previous reports, longer reaction times
and/or greater excesses were used to effect mass action
exchange with this family of ligands.13,14
The impact of coordinating head groups on exchange
efficiencies was evaluated by comparing alkylphosphonate
and alkylcarboxylate-capped CdSe NCs. Despite differences
in the chemistry of the native ligands, exchange occurred to
approximately the same extent in both systems, affording 63%
and 60% substitution, respectively. The composition of the
inorganic core, however, was found to have a significant effect
on ligand exchange efficiency. Alkylcarboxylate-capped CdTe
NCs exhibited a higher extent of substitution (91%) compared
to the analogous CdSe system, whereas similarly coordinated
ZnO NCs showed markedly lower exchange (38%). This trend
suggests that TMS-driven reactive ligand exchange is more
effective on semiconducting NC compounds exhibiting
more covalent character, and less effective on those which
are more ionic (ionicity increases as CdTe o CdSe o ZnO),
implying that the nanocrystal surface still exerts influence over
the ligand exchange.
NC optical absorption spectra were similar before and after
exchange, indicating that these mild reaction conditions do not
induce significant defects or etching of the NCs (Fig. 2). Before
reaction, green-emitting QDs exhibit an absorbance maximum
at 531 nm (Fig. 2a). Ligand exchange with bisTMS-DHLA,
bisTMS-MPA, and bisTMS-MUA on these NCs proceeds
with only slight variations in the resulting spectra, with the
first exciton peak clearly resolved in all cases (Fig. 2a). Similar
results were observed for orange-emitting NCs (Fig. 2b).
Significantly enhanced absorbance below 450 nm, far from
Encouraged by the general applicability of this reactive
ligand exchange approach, we proceeded with novel trimethyl-
silylated reagents to transfer hydrophobic, highly luminescent
semiconducting NCs to aqueous dispersions and evaluated the
impact on NC optical characteristics. Ligand exchanges with
hydrophilic ligands dihydrolipoic acid (DHLA), mercapto-
propanoic acid (MPA), and mercaptoundecanoic acid (MUA)
under mass action conditions have previously been reported as
effective for preparing aqueous dispersions.13 We therefore
synthesized TMS versions of these reagents (ESIz) to evaluate
the efficacy reactive ligand exchange for aqueous transfer.
Reactive ligand exchange was carried out on commercially
available green-emitting (Qdot 545 ITK) or orange-emitting
(Qdot 605 ITK) CdSe/ZnS core/shell hydrophobic quantum
dots (Invitrogen Corporation) by adding the desired TMS
Fig. 2 Absorption and emission spectra of variously capped (a) Qdot
545 NCs and (b) Qdot 605 NCs. Absorbance spectra, shown as dotted
lines, are normalized at the first exciton peak. Emission spectra
(solid lines) are normalized to reflect PL quantum yields. Spectra were
acquired on 10 nM dispersions in CHCl3 or sodium tetraborate
buffer (10 mM, pH 10) with excess exchange ligand at 1 mM at 25 1C
(lex = 400 nm).
c
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the excitonic features, was observed for MPA-capped QDs of
both colors, though its origin is not yet known.
leveraged to introduce designer ligands of interest for a wide
variety of biological, electronic, and energy applications.
The authors gratefully acknowledge Dr Emory Chan for
synthesis of CdSe and CdTe NCs using WANDA and Tracy
Mattox for synthesis of ZnO NCs. Research was completed
entirely at The Molecular Foundry, Lawrence Berkeley
National Laboratory, Berkeley, CA, USA and was supported
by the Office of Science, Office of Basic Energy Sciences of the
US Department of Energy under contract no. DE-AC02-
05CH11231. M.A.C. is partially supported by the IBM
Graduate Student Fellowship and the Stanford Non-Volatile
Memory Technology Research Initiative (NMTRI) and its
member companies. A.E.A. is partially supported by an Ernest
Orlando Lawrence Postdoctoral Fellowship. S.C.L. was
supported by the Office of Science, Department of Energy’s
Science Undergraduate Laboratory Internship (SULI) program.
Striking differences were observed in luminescence quantum
yields before and after reaction with the three bisTMS mixed
chalcogenides, though the spectral characteristics remained
nearly unchanged in all cases (Fig. 2). Trends in PL quantum
yields for both green- and orange-emitting QDs are similar; for
both, reaction with bisTMS-DHLA proceeded with cata-
strophic losses in emission intensity (Fig. 2 and Table 1). In
sharp contrast, bisTMS-MPA and bisTMS-MUA were much
more effective in maintaining bright QDs in buffered aqueous
environments. In the best case, the quantum yield of the
MUA-capped QDs decreases by only 13% (7% absolute)
following ligand exchange (Table 1). Similar decreases in
quantum yield were also observed in MUA exchanged CdTe
and CdSe cores (ESIz).
Differences in ligand coverage, binding geometries, electronic
passivation efficacy, and lability are known determinants of
QD PL efficiency.15 While the contributions of these factors to
the reported variations of quantum yield are challenging to
disentangle, zeta potential measurements offer some insight
(Table 1). All the QDs have negative zeta values, consistent
with thiolate head group coordination to QD surfaces and
carboxylate tail group presentation toward the aqueous
environment. MPA consistently produced QDs with substan-
tially smaller magnitude zeta values than MUA. Since these
ligands each contain one thiolate and one carboxylate group
per molecule, this trend likely reflects a higher coverage with
MUA than with MPA. Furthermore, this zeta potential
difference correlates with the differences in PL quantum yield
between MPA- and MUA-capped QDs, suggesting that ligand
coverage may be a major factor, particularly for the green
QDs. However, the zeta potentials of DHLA-capped QDs lie
between those of comparable MPA- and MUA-capped QDs, a
result which does not correlate with their lowest PL quantum
yield. In this case, however, zeta potential differences cannot
be directly equated with differences in ligand coverage since
DHLA contains an additional thiolate group per molecule.
While further analyses will be required to fully unravel the
combination of factors responsible, the empirical observations
reported here are already useful in guiding ligand selection.
Specifically, among ligands capable of facilitating aqueous
dispersion of QDs, DHLA is particularly ill-suited for
maintaining high PL quantum yield, while MUA introduced
through our mild reactive exchange process is highly effective
for smaller (green) QDs. MUA is also moderately effective for
larger QDs, though the quantum yield of the orange QDs
dropped from 65 to 35% following ligand exchange.
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oxyanionic ligand exchange at NC surfaces using TMS
chalcogenido reagents. We have found that ligand exchange
is largely independent of the native coordinating head group,
but varies significantly with the inorganic core composition,
correlating with the covalent character of the material. Among
aqueous QD dispersions prepared using novel bisTMS
ligands, MUA-capped QDs exhibit significantly higher
quantum yield compared to QDs capped with DHLA or
MPA. We anticipate that the mild reaction conditions and
predictable functional group tolerance of this method may be
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558 Chem. Commun., 2011, 47, 556–558
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