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Colloidal CdSe Nanocrystals Passivated by a Dye-Labeled Multidentate Polymer Quantitative Analysis by Size-Exclusion Chromatography.

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Angewandte
Chemie
Nanocrystals
DOI: 10.1002/ange.200502538
Colloidal CdSe Nanocrystals Passivated by a DyeLabeled Multidentate Polymer: Quantitative
Analysis by Size-Exclusion Chromatography**
Mingfeng Wang, Tieneke E. Dykstra, Xudong Lou,
Mayrose R. Salvador, Gregory D. Scholes,* and
Mitchell A. Winnik*
Quantum dots are a class of colloidal semiconductor nanocrystals (NCs) that show unique size-dependent optical
[*] M. Wang, T. E. Dykstra, X. Lou, M. R. Salvador, G. D. Scholes,
M. A. Winnik
Department of Chemistry
University of Toronto
80 St. George Street, Toronto, M5S 3H6 Ontario (Canada)
Fax: (+ 1) 416-978-0541
E-mail: gscholes@chem.utoronto.ca
mwinnik@chem.utoronto.ca
[**] The authors thank NSERC Canada for their support of this research.
G.D.S. acknowledges the A. P. Sloan Foundation. We thank Dr.
M. A. Hines for the kind gift of ZnSe samples. We acknowledge
helpful discussions with Dr. X.-S. Wang and Dr. Z. Yin, as well as the
help of Mr. Y. Wang, Dr. N. Coombs, and Dr. M. Mamak for the TEM
measurements.
Supporting information for this article is available on the WWW
under http://www.angewandte.org or from the author.
Angew. Chem. 2006, 118, 2279 –2282
properties.[1] These materials are of great interest for applications in light-emitting diodes,[2] photovoltaic devices,[3] and
biological labels.[4] High-quality NCs of II–VI semiconductor
materials such as CdSe and CdTe can be obtained with
controlled sizes and shapes through an organometallic
approach.[5] These as-prepared NCs, which are chemically
passivated by a layer of organic ligands such as tri-noctylphosphine oxide (TOPO), are dispersible only in nonpolar organic solvents and are typically unstable in solution in
the absence of excess free ligands.[1b, 6] These factors limit their
use for biological and material applications. As a consequence, ligand-based functionalization of the NC surface is an
essential step to control their physical properties as well as to
provide the necessary chemical accessibility and biological
functionality.
For this purpose, various polymeric substances such as
organic dendrons,[7] linear[8–9] or hyperbranched[10] polymers,
chemically-modified proteins,[11] and amphiphilic polymers[12]
have been employed to manipulate the surface properties of
NCs through a ligand-exchange or micellar encapsulation
process. Analytical methods such as dynamic light scattering,[9c, 10] gel electrophoresis,[7, 9b, 12c] fluorescence resonance
energy transfer,[9e, 11] thermogravimetric analysis,[12b] and
atomic force microscopy[13] have been used to characterize
the polymer binding on NCs. However, the quantitative
analysis of polymeric ligands on NCs still remains a considerable challenge. There are two distinct problems that need to
be solved. First, one needs a methodology to separate the NC
from free polymer. The polymer is normally added in excess
to ensure that the NC surfaces are saturated or encapsulated,
and this excess polymer is very difficult to remove from the
sample. This problem can in principle be solved by centrifugation or dialysis,[12d] but then one encounters the second
problem. One needs analytical methods to determine if any
free polymer remains in the solution and to quantify the
amount of polymer bound to the NC. This paper presents a
quantitative analytical method based upon size-exclusion
chromatography (SEC) that addresses this need.
To facilitate the detection of polymeric ligands on NCs by
spectroscopic methods, we synthesized a low-molecularweight sample of poly(dimethylaminoethyl methacrylate)
labeled at one end with pyrene as a fluorescent tracer (PyrPDMAEMA, Mn = 6700) by atom-transfer radical polymerization (ATRP; for details of the synthesis and characterization, see the Supporting Information). Pyr-PDMAEMA
can bind to CdSe NCs as a multidentate ligand, as there is a
dimethylamino function on each pendant group (Scheme 1).
The CdSe NCs were prepared by an established organometallic synthesis with TOPO as ligands.[5] Two samples were
used in this study. The first (CdSe520 nm) had a diameter of
3.4 0.3 nm as determined by transmission electron microscopy (TEM) and an emission maximum at lem = 520 nm. The
particle diameter of the second sample (CdSe550 nm) was 4.0 0.4 nm and the emission maximum was at lem = 550 nm. Both
samples were purified by precipitation in methanol and
dispersed in tetrahydrofuran (THF) before use. The mixture
of CdSe–TOPO NCs and Pyr-PDMAEMA was stirred in
THF overnight at room temperature, and a homogenous
transparent solution was obtained.
2006 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
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Scheme 1. Ligand exchange between TOPO-capped CdSe NCs and the
multidentate homopolymer (Pyr-PDMAEMA) terminated by a pyrene
group.
Figure 1. SEC traces of signal intensity versus elution time of TOPOcapped CdSe520 nm NCs (c), Pyr-PDMAEMA (b), and a mixture of both
components (a). The first elution peak in (a) at 11.5 min is attributed
to CdSe NCs, while the peak at 15.7 min is derived from the excess
free polymer in the mixture. Eluant: NMP; flow rate: 0.6 mL min1.
SEC is a commonly used method to determine the
molecular weight and size distribution of organic polymers,
but its application to inorganic colloidal particles is rare.[14]
band at 346 nm, attributed to pyrene absorption, is evident in
When a CdSe NC sample mixed with excess Pyr-PDMAEMA
the absorption spectrum of the NCs separated by the SEC
was injected into the SEC column, two peaks were observed.
column. More-striking evidence for the presence of pyrene is
The first peak (at 11.5 min, Figure 1 a) is attributed to the
seen in the photoluminescence (PL) spectrum presented in
CdSe520 nm NCs and the second peak (at 15.7 min) is due to the
Figure 2 b, in which the characteristic emission band of the
excess free polymer in the mixture. At 343 nm, the UV/Vis
pyrene chromophore from 370 to 430 nm is well resolved
detector monitors both the S0 !S2 absorption of pyrene as
well as the continuum absorption
below
510 nm
of
the
CdSe520 nm NCs. It is evident from
the SEC traces of CdSe520 nm itself
(Figure 1 b) and of pure PyrPDMAEMA (Figure 1 c) that the
NCs elute much faster than the free
polymer. The low molecular weight
and relatively low polydispersity
(PDI) of the polymer facilitates the
separation of the components. Our
results are consistent with those
reported by Wilcoxon et al.,[14d]
who found that dense Au colloids
in water eluted at the size cutoff of
their aqueous SEC column, thus
giving a sharper peak than with
linear, commercial polymer standards such as polyethylene glycol.
Although the chromatogram in
Figure 1 a indicates clear separation of the NCs and free polymer, it
provides no information about the
nature of the surface groups on the
Figure 2. Absorption (a, c) and emission (b, d) spectra of CdSe520 nm/Pyr-PDMAEMA (a, b) and ZnSe/
CdSe particles. To obtain this vital
Pyr-PDMAEMA (c, d) complexes after separation by the SEC column. For comparison, the
information, we collected the
absorbance and photoluminescence spectra of TOPO-capped CdSe and ZnSe NCs as well as Pyrmaterial corresponding to the
PDMAEMA are also shown. The inset in (a) highlights the absorption difference around 346 nm
band at t = 10 to 12 min in Figbetween CdSe–TOPO and CdSe/Pyr-PDMAEMA. The pyrene groups on the ZnSe/Pyr-PDMAEMA
ure 1 a and analyzed the sample by
complex show both monomer emission (370 to 420 nm) and a longer-wavelength emission that may
UV/Vis and fluorescence spectrosbe due to an excimer, but which needs further investigation. ZnSe–TOPO, as shown in (d), is
copy. Figure 2 a shows that a new
nonfluorescent.
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www.angewandte.de
2006 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
Angew. Chem. 2006, 118, 2279 –2282
Angewandte
Chemie
from the quantum-dot emission. The strong emission at
longer wavelength indicates that the photoluminescence of
CdSe NCs is preserved after ligand exchange with the
polymer and passage through the SEC column.
As polymers with many functional binding sites bind to
surfaces cooperatively, desorption of the polymer from the
surface is anticipated to be very slow. We tested the
reversibility of the adsorption by re-injecting into the SEC
column samples of the CdSe/Pyr-PDMAEMA NCs that had
been collected from the column. Reanalysis by SEC of the
samples that were allowed to stand in N-methyl-2-pyrrolidinone solution for up to one week showed only the NC peak at
12 min and no detectable trace of free Pyr-PDMAEMA (an
example is provided in the Supporting Information). This
result suggests that the binding of the polymer on the NCs is
strong, and no dissociation can be detected.
To test the scope of our methodology, we repeated these
experiments with a sample of TOPO-stabilized ZnSe NCs
(synthesized according to the method of Hines[15]). The results
(see Figure 2 c, d and Supporting Information) demonstrate
that Pyr-PDMAEMA binds to the ZnSe surface without
dissociation. This result, coupled with our earlier finding that
PDMAEMA homopolymer adsorbs strongly to the surface of
ZnS-overcoated CdSe NCs, suggests that this polymer is an
effective multidentate ligand for a wide variety of NCs.
The dye at the end of the polymer chain makes it possible
to determine the mean number of polymers bound to each
NC particle. This information could in principle be obtained
from Figure 2 a, c by assuming additivity of the absorbance of
the dye and NC at 346 nm. This analysis is complicated not
only by the background contribution of the NC itself,[11] but
also by band broadening and possible dipole-strength redistribution of the pyrene in the vicinity of the NC. Evidence for
these effects is seen in Figure 2 as enhanced absorbance at
wavelengths at which the expected contribution of free
pyrene groups is small and as the long wavelength tail in the
pyrene emission for dyes bound to the NC.[16] The origin of
this effect is not understood, but points to an interaction of
some of the pyrene groups with the NC surface.
As a consequence, we developed a different approach
based upon monitoring the amount of free polymer removed
from a calibrated polymer solution by the addition of TOPOcovered NCs. Two samples with the same concentration of
Pyr-PDMAEMA but different concentrations of CdSe520 nm
were prepared and examined by SEC. Figure 3 shows that, for
injections of identical volume, the intensity of the peak
corresponding to excess polymer in the chromatogram of
CdSe520 nm/Pyr-PDMAEMA decreases with the increase in
NC concentration. This experiment can be calibrated by
injecting identical volumes (50 mL) of a series of solutions
with varying polymer concentration (Figure 4). As an example, a sample was prepared by mixing 0.5 mL of 1.68 D 105 m
TOPO-capped CdSe550 nm NCs with 0.5 mL of a 3.3 mg mL1
(4.93 D 104 m) solution of Pyr-PDMAEMA. From the SEC
results for this mixture, we deduce that the concentration of
free polymer chains in the solution decreased from 2.47 D 104
to 1.42(0.16) D 104 m. From three separate measurements,
we calculate that on average 12.5 1.9 polymer molecules
were bound to each CdSe550 nm particle. Analogous experiAngew. Chem. 2006, 118, 2279 –2282
Figure 3. SEC traces of Pyr-PDMAEMA mixed with different concentrations of TOPO-capped CdSe520 nm NCs: a) 1.2 D 105 m CdSe–TOPO and
2.5 D 102 m Pyr-PDMAEMA; b) 2.4 D 106 m CdSe–TOPO and
2.5 D 102 m Pyr-PDMAEMA. Injection volume: 50 mL ; eluant: NMP;
flow rate: 0.6 mL min1.
Figure 4. a) SEC traces of Pyr-PDMAEMA for a series of solutions of
varying concentration in THF. The injection volume (50 mL) was
identical for all samples. b) A calibration curve showing that the
integrated areas (Aint) of the peaks corresponding to the polymer in
the SEC chromatograms in (a) increase linearly with polymer concentration. Eluant: NMP; flow rate: 0.5 mL min1.
ments with the smaller CdSe520 nm NC led to the conclusion
that on average 4.6 0.6 polymers were bound to each
particle.
In summary, SEC analysis combined with spectroscopic
characterization has provided unambiguous evidence that the
pyrene-labeled polymer Pyr-PDMAEMA becomes tightly
2006 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
www.angewandte.de
2281
Zuschriften
bound to CdSe NCs. The NCs are eluted much faster through
the chromatographic column than the pure polymer. As a
result, the excess free polymer is separated effectively from
the polymer-bound NCs by the SEC column. Moreover, the
number of the polymeric ligands bound to each NC was
quantified from the SEC chromatogram by comparing the
amount of the polymer chains in solution before and after
addition of CdSe–TOPO NCs. This approach can be applied
to any system in which one can synthesize a derivative of a
multidentate ligand bearing a single dye substituent, and is
thus expected to be of great utility for the characterization of
other NC systems in which a protein, dendron, or polymer is
bound to the surface.
Experimental Section
The SEC measurements were performed by using an AM Gel Linear/
5 exclusion column (American Polymer Standards Corporation) and
a Viscotek VE-1121 GPC solvent pump. The flow rate was
0.6 mL min1 and the injected volume was 50 mL unless indicated
specifically. N-Methyl-2-pyrrolidinone (NMP; HPLC grade) was used
as the mobile phase. Two detectors were used: a Viscotek VE-3210
UV/Vis detector (l = 343 nm) and a Waters 410 differential refractometer (RI).
The extinction coefficient of the CdSe NCs was determined by
the empirical equation e = 5857 D (d)2.65 (d is the diameter of the
CdSe NCs, as determined by TEM).[17] From the absorbance (A) of
first peak around 510 nm, the concentration (c) of NCs in THF was
calculated through the Beer–Lambert law (c = A/(e l)). Optical
absorption spectra were collected at room temperature on a
Perkin–Elmer Lambda 25 spectrometer using 1.00-cm quartz cuvettes. Photoluminescence spectra were measured with a SPEX
Fluorolog-3 spectrofluorometer (Jobin Yvon/SPEX, Edison, New
Jersey).
Received: July 20, 2005
Revised: December 21, 2005
Published online: March 3, 2006
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Keywords: cadmium selenide · nanocrystals · polymers ·
quantum dots · size-exclusion chromatography
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colloidal, labeled, exclusion, dye, nanocrystals, quantitative, polymer, multidentate, cdse, passivated, size, chromatography, analysis
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