Luminescent CdSe/CdS core/shell nanocrystals in dendron boxes: Superior chemical, photochemical and thermal stability

Article abstract of DOI:10.1021/ja028469c

The surface ligands, generation-3 (G3) dendrons, on each semiconductor nanocrystal were globally cross-linked through ring-closing metathesis (RCM). The global cross-linking of the dendron ligands sealed each nanocrystal in a dendron box, which yielded bo

Full text of DOI:10.1021/ja028469c

Published on Web 03/05/2003
Luminescent CdSe/CdS Core/Shell Nanocrystals in Dendron
Boxes: Superior Chemical, Photochemical and Thermal
Stability
Wenzhuo Guo, J. Jack Li, Y. Andrew Wang, and Xiaogang Peng*
Contribution from the Department of Chemistry and Biochemistry, UniVersity of Arkansas,
FayetteVille, Arkansas 72701
Received September 9, 2002 ; E-mail: xpeng@comp.uark.edu
Abstract: The surface ligands, generation-3 (G3) dendrons, on each semiconductor nanocrystal were
globally cross-linked through ring-closing metathesis (RCM). The global cross-linking of the dendron ligands
sealed each nanocrystal in a dendron box, which yielded box-nanocrystals. Although the dendron ligands
coated CdSe nanocrystals (CdSe dendron-nanocrystals) were already quite stable, the stability of CdSe
box-nanocrystals against chemical, photochemical, and thermal treatments were dramatically improved in
comparison to that of the original dendron-nanocrystals. Furthermore, the box structure of the ligands
monolayer coupled with the stable inorganic CdSe/CdS core/shell nanocrystals resulted in a class of
extremely stable nanocrystal/ligands complexes. The band edge photoluminescence of the core/shell
dendron-nanocrystals and box-nanocrystals were partially remained, and could be further brightened through
controlled chemical oxidation or photooxidation. Practically, the stability of the box-nanocrystals is sufficient
for most fundamental studies and technical applications. The box-nanocrystals may represent a general
solution for the commonly encountered instability for many types of colloidal nanocrystals. The size
distribution of the empty dendron boxes formed by the dissolution of the inorganic nanocrystals in
concentrated HCl was very narrow. The empty boxes as new types of polymer capsules are soluble in
solution, mesoporous, and with a very thin but stable peripheral. Those nanometer-sized cavities should
be of interest for many purposes in the field of solution host-guest chemistry.
including semiconductor,16-23 metal,24-26 and oxide27-31 sys-
tems, has put the processing chemistry on an even more urgent
Introduction
Colloidal nanocrystals are of great interest for both funda-
mental studies1-4 and technical applications5-11 due to their
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and reliable processing chemistry,14,15 especially the latter. The
recent rapid progress on synthesis of high quality nanocrystals,
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structure of the surface ligands monolayer, the interface between
the inorganic core and the organic ligands monolayer, and the
structure of the surface of the inorganic nanocrystals. Unfor-
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J. AM. CHEM. SOC. 2003, 125, 3901-3909
3901

A R T I C L E S
Guo et al.
However, it is evidenced that the technical key challenge of
the processing chemistry of colloidal nanocrystals is to obtain
stable nanocrystal/ligands complexes. It is not difficult to find
evidences in the literature which support this statement. The
lifetime of the light emitting diodes (LEDs) based on semicon-
ductor nanocrystals was short,5 which is likely a result of the
dissociation of the organic ligands from the nanocrystals due
to the thermal effects of the devices in operation. The trouble-
some conjugation chemistry related to the promising biological
labeling using semiconductor nanocrystals has been also found
to be associated with the detachment of the organic ligands from
the nanocrystals.14,15 The long-pursued enhancement effect of
magnetic nanocrystals for magnetic resonance imaging is still
in its infancy because of the instability of the ligands on the
surface of the nanocrystals.32,33 It would be very interesting to
study the chemical reactivity of colloidal nanocrystals in solution
if the ligands can stabilize the initial nanocrystals, the intermedi-
ates and the resulting nanocrystals.
on the surface of each nanocrystal through RCM36-38 to form
a dendron box around each nanocrystal (box-nanocrystal). The
formation of the dendron boxes was confirmed by NMR and
MS. The chemical, photochemical and thermal stability of the
resulting box-nanocrystals were quantitatively or semiquanti-
tatively examined. High quality CdSe/CdS core/shell box-
nanocrystals demonstrated superior stability against HCl etching,
chemical oxidation with H2O2, photochemical oxidation, and
thermal sintering. The resulting stable semiconductor nano-
crystals should be of great interest for many fundamental studies,
such as measurement of the sized dependent melting point of
nanocrystals and studies on the size-specified reactivity of
nanocrystals. Furthermore, those luminescent and stable core/
shell nanocrystals should greatly benefit several types of
technical applications, such as light emitting diodes (LEDs),5,6
solid-state lasers,11 and biological labeling,8,10,11 and so forth.
The empty dendron boxes formed by the dissolution of the
inorganic nanocrystals in concentrated HCl represent a new class
of nanometer sized polymer capsules, which are nearly mono-
disperse, soluble, stable, and with a very thin peripheral.
Two types of stability issues related to nanocrystal/ligands
complexes have been identified. Type I, the organic ligands
dissociate from the inorganic core. In solution, this results in
uncontrollable chemical properties of the outer surface of the
ligands monolayer and the detachment of the desired chemical/
biochemical functions from the inorganic core.15 In many cases,
this also causes the precipitation of the nanocrystals. In both
solid state and solution phase, the dissociation of the organic
ligands and the inorganic core often causes undesired variations
of the properties of nanocrystals, such as decrease of either
photoluminescence22 or electroluminescence5 brightness. Type
II, the inorganic core can be oxidized, etched, and even
completely dissolved, which normally defeats the function of
the original nanocrystal/ligands complexes.
Experimental Section
Chemicals. Triphenylmethanol, 2-aminoethanethiol hydrochloride,
bromoactyl bromide, dially amine, triethylsilane, trifluoroacetic acid
(TFA), tetramethylammonium hydroxide, trioctylphosphine oxide
(TOPO, technical grade, 90%), trioctylphosphine (TOP, technical grade,
90%) cadmium acetate hydrate (99.99+%), selenium powder (100
mesh, 95%), anhydrous toluene (99.8%), diatomaceous earth, Grubbs’
catalyst (second generation) were purchased from Aldrich. Anhydrous
ethyl ether, benzene, chloroform, N,N-dimethylformamide (DMF),
acetone, ethyl acetate, methanol, triethylamine, and K2CO3, were
purchased from EM science. All chemicals were used directly without
further purification.
Synthesis of Dendron Ligands. G3 dendron ligand 6 was synthe-
sized as shown in Scheme 1. The focal point of the dendron ligands,
a thiol group, was the binding site to surface cadmium atoms of the
nanocrystals. In the synthesis, triphenylmethyl group was applied to
protect thiol group. With this protection, dendron precursors were stable
under both acidic and basic conditions. Generally, compound 2,
2-tritysulfanyl-ethylamine was obtained upon the treatment of triph-
enylmethanol with 2-aminoethanethiol hydrochloride in TFA, followed
by neutralization with aqueous NaOH. The acetonitrile solution of 2
was refluxed with N-(tert-Butoxycarbonyl)aziridine to give tert-
butoxycarbony (BOC) protected first generation of dendron. Compound
3 was obtained as yellow oil after the deprotection of BOC groups
with TFA. Reaction of bromoactyl bromide with dially amine at 0 °C
afforded compound 4, N, N-diallyl-2-bromo-acetamide. The triphenyl-
methyl protected G3 dendron ligand was readily prepared by N-
alkylation of compound 3 with active R-bromide 4 in the presence of
aqueous K2CO3. Dendron ligand 6 was finally obtained by treatment 5
with TFA and triethylsilane.
This work intended to stabilize nanocrystal/ligands complexes
by improving the stability of both inorganic core and the organic
ligands monolayer, with an emphasis on the latter. Semiconduc-
tor nanocrystals were chosen because of their general interest,
easy detection and relatively better established synthetic and
ligands chemistry. The stability of CdSe nanocrystal cores was
enhanced by the epitaxial growth of a thin layer of CdS on their
surface.34,35 It was confirmed that such core/shell structures can
improve the photochemical stability of CdSe nanocrystals by
the confinement of the photogenerated charges inside the core
material.34,35 The stability enhancement of the ligands monolayer
was achieved by the cross-linking of all surface ligands of each
nanocrystal through the standard ring-closing metathesis (RCM).
To reach the desired global cross-linking, a generation 3 (G3)
dendron with thiol as the anchoring group and eight carbon-
carbon double bond as the terminal groups was employed as
the surface ligands. The relatively good chemical stability of
the dendron ligands coated nanocrystals (dendron-nanocrystals)14
enabled the nanocrystal/ligands complexes to readily survive
the cross-linking reaction and related purification procedures.
The multiple double bonds of each dendron ligand made it
possible to obtain global cross-linking of all the dendron ligands
2-Tritysulfanyl-ethylamine (2). Triphenylmethanol (22.9 g, 88
mmol) was added to a solution of 2-aminoethanethiol hydrochloride
(10 g, 88 mmol) in trifluroacetic acid (TFA, 40 mL). The result solution
was stirred at room temperature for about 40 min. TFA was removed
under reduced pressure. The residue was triturated with ethyl ether.
The white precipitate formed was filtered, partitioned with aqueous
NaOH (40 mL, 1 N) and extracted with ethyl acetate (3 × 50 mL).
The combined organic layers were dried over Na2SO4. The product
(32) Wunderbaldinger, P.; Bogdanov, A., Jr.; Weissleder, R. Eu. J. Radiology
2000, 34, 156.
(33) Weissleder, R. Radiology 1999, 212, 609-614.
(34) Peng, X.; Schlamp, M. C.; Kadavanich, A. V.; Alivisatos, A. P. J. Am.
Chem. Soc. 1997, 119, 7019-7029.
(36) Wendland, M. S.; Zimmerman, S. C. J. Am. Chem. Soc. 1999, 121, 1389-
1390.
(37) Hecht, S.; Frechet, M. J. Angew. Chem. Int. Ed. 2001, 40, 74.
(38) Zimmerman, S. C.; Wendland, M. S.; Rakow, N. A.; Zharov, I.; Suslick,
K. S. Nature 2002, 418, 399-403.
(35) Li, J.; Wang, Y. A.; Peng, X.; Guo, W.; Mishima, T.; Johnson, M.,
submitted.
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Luminescent CdSe/CdS Core/Shell Nanocrystals
A R T I C L E S
Scheme 1. Synthesis of Dendron Ligand. Reagent and
Condition: A, 2-aminoethanethiol Hydrochloride, TFA, 25 °C, 30
min, 85%; B, 1 M NaOH, 96%; C,
N-(tert-Butoxycarbonyl)Aziridine, CH3CN, Reflux 3 days, 67%; D,
TFA, 30 Mins, 92%; E, 1 M NaOH, 98%; F, THF, K2CO3, Reflux,
12 Hours, 62%; G, TFA, Triethylsilane, 95%; H, Ethyl Acetate,
72%
(85/15) to obtain light yellow product (0.7 g, yield: 73%). ESI-MS
1
(m/z): 954 (M + H+). H NMR δD (CDCl3): 7.21∼7.33 (m, 15H,
phenyl), 5.68 (m, 8H, CH), 5.07 (m, 16H, CHCH2), 3.44∼3.88 (m,
24H, COCH2, CH2CHCH2), 2.81 (m, 2H, SCH2), 2.31 (m, 10H, NCH2),
13C NMR δ: 170.02, 144.98, 133.21, 133.14, 129.67, 127.98, 126.63,
117.46, 116.57, 66.58, 55.31, 53.91, 48.70, 47.82, 30.24. Anal. Calcd
for C57H75N7O4S: C, 71.74; H, 7.92; N, 10.27. Found: C, 70.96; H,
7.37; N, 10.41.
N,N-Dially-2-({2-[[2-bis-diallylcarbamoylmethyl-amino)-
ethyl]-(2-mercapto-ethyl)-amino}-ethyl}-diallylcarbamoyl-
methyl-amino)-acetamide (6). Triethylsilane (0.1 mL) was added to
a solution of 5 (0.5 g, 0.5 mmol) in trifluroacetic acid (10 mL). A
white precipitate formed upon the addition was removed by filtration
through diatomaceous earth. TFA was removed under reduced pressure
to give product as light yellow oil (0.34 g, 95%). ESI-MS (m/z): 712
1
(M + H+). H NMR δD (CDCl3): 5.68 (m, 8H, CH), 5.07 (m, 16H,
CHCH2), 3.44∼3.88 (m, 24H, COCH2, CH2CHCH2), 2.75 (m, 2H,
SCH2), 2.31 (m, 10H, NCH2).13C NMR δ: 170.21, 133.05, 132.90,
117.48, 116.57, 68.52, 58, 13, 55.47, 48.57, 47.92, 28.71. Anal. Calcd
for C38H61N7O4S: C, 64.10; H, 8.64; N, 13.77. Found: C, 63.25; H,
8.49; N, 13.44.
Synthesis of CdSe and CdSe/CdS Nanocrystals. TOPO-capped
or amine-caped CdSe nanocrystals were synthesized using the standard
“greener” methods reported previously.15,21,22 The typical size of the
nanocrystals used in this work was about 3.3 nm with the first excitonic
absorption peak at 550 nm. The CdSe/CdS core/shell nanocrystals were
synthesized through the newly developed solution atomic layer epitaxy
(SALE) method35 which will be reported later. The core size of the
core/shell nanocrystals was about 3.5 nm and the shell thickness was
about 1.5 monolayers of CdS.
Surface Modification of Nanocrystal with Dendron Ligands.
CdSe and CdSe/CdS core/shell nanocrystals were modified with
dendron ligand 6 through a procedure modified from a previously
reported one.14,15 Typically, 20 mg of TOPO-capped CdSe nanocrystals
in CHCl3 was added to a solution of 150 mg ligand in 15 mL of 1:1
ratio MeOH/CHCl3, and the pH value of such solution was adjusted to
10.3 by the addition of tetramethylammonium hydroxide. The reaction
was carried out under dark at room temperature or elevated temperatures
overnight. In case of primary amine-coated CdSe/CdS core/shell
nanocrystals, the basic solution of ligand and nanocrystals was first
set to reflux under nitrogen for 4 h, and then the solution was cooled
to room temperature and stirred for 12 h. The resulting dendron-
nanocrystals were precipitated with minimum amount of ethyl ether,
separated by centrifugation and decantation.
Formation of Box-Nanocrystals. The terminal allyl groups of the
dendron-nanocrystals were cross-linked to form dendron-nanocrystals
using Grubbs’ ruthenium alkylidene catalyst (Scheme 2). Typically,
the second generation of Grubbs’ catalyst36,38 (2 mol % per alkene
group) was added to a 1:1 ratio CH2Cl2/benzene solution (10-5 M, based
on alkene group) of the dendron-nanocrystals. The amount of ligands
on the surface of nanocrystals was estimated by assuming a close
packing of ligands on the surface of nanocrystals. The reaction was
carried out at room temperature for 48 h under dark. DMSO (20 µl)
and silica gel (0.5 g) were added to the reaction solution to remove the
catalyst by a standard protocol. The box-nanocrystals were then
precipitated out with minimum amount of ethyl ether, separated by
centrifugation and decantation.
Characterization of Box-Nanocrystals and Empty Dendron
Boxes. For the NMR studies, 20 mg of nanocrystals before and after
cross-linking were dissolved /precipitated in the suitable solvents
system, separated by centrifugation and decantation at least three times.
After drying under high vacuum, the resulting samples were dissolved
in DMSO (d6) and measured with a JEOL 270 MHz NMR spectro-
photometer.
was obtained as white solid after solvent evaporation (23.8 g, yield:
85%). ESI-MS (m/z): 320 (M + H+). 1H NMR δD (CDCl3): 7.21∼7.43
(m, 15H, pheny), 2.58 (t, 2H, SCH2), 2.32 (t, 2H, CH2N).13C NMR δ:
144.98, 129.68, 127.94, 126.73, 66.63, 41.15, 36.38. Anal. Calcd for
C21H21NS: C, 78.95; H, 6.63; N, 4.38. Found: C, 77.90; H, 6.57; N,
4.35.
Bis-(2-amino-ethyl)-(2-tritylsulfanyl)-amine (3). The CH3CN (135
mL) solution of compound 2 (9.3 g, 29 mmol) and N-(tert-butoxycar-
bonyl)aziridine (12.3 g, 87 mmol) was refluxed for 3 day. The solvent
was removed under reduced pressure. The residue oil was purified by
silica gel column chromatography with CHCl3 as eluent to obtain 12.3
g of BOC protected G1 dendron as light yellow solid which was treated
with TFA, dried under reduced pressure, partitioned with aqueous
NaOH to give compound 3 as yellow oil (7.9 g, yield: 68%). ESI-
MS (m/z): 406 (M + H+). 1H NMR δD (CDCl3): 7.23∼7.44 (m, 15H,
phenyl), 2.78 (t, 2H, SCH2), 2.60∼2.70 (m, 4H, CH2NH2), 2.27∼2.35
(m, 6H, CH2N). 13C NMR δ: 145.00, 129.68, 127.92, 126.67, 66.80,
56.61, 53.32, 39.62, 30.23. Anal. Calcd for C25H31N3S: C, 74.03; H,
7.70; N, 10.36. Found: C, 73.56; H, 7.32; N, 9.78.
N,N-Diallyl-2-bromo-acetamide (4). Bromoacetyl bromide (45 g,
0.22 mol) in ethyl acetate (25 mL) was added dropwise to a solution
of dially amine (9.6 g, 0.1 mol), Et3N (15 mL), in ethyl acetate (50
mL) at 0 °C. The reaction was then warmed to ambient temperature,
and the stirring was continued for 3 h. The resulting mixture was washed
with saturated NaHCO3 solution (3 × 100 mL), dried over Na2SO4.
The evaporation of the solvent give 16.2 g product as light yellow oil
1
(yield: 75%). ESI-MS (m/z): 218 (M + H+). H NMR δD (CDCl3):
5.75 (m, 2H, CH), 5.19 (m, 4H, CH2), 3.95 (d, 4H, NCH2), 3.81 (2,
2H, BrCH2).13C NMR δ: 166.89, 132.64, 132.21, 117.66, 117.21, 50.10,
48.23, 26.24. Anal. Calcd for C8H12BrNO: C, 44.06; H, 5.55; N, 6.42.
Found: C, 43.33; H, 5.65; N, 6.26.
N,N-Dially-2-({2-[[2-bis-diallylcarbamoylmethyl-amino)-ethyl]-
(2-trutylsulfanyl-ethyl)-amino]-ethyl}-daillylcarbamoylmethyl-amino)-
acetamide (5). compound 4 (0.9 g, 4.1 mmol) was added to a solution
of 3 (0.4 g, 1 mmol), K2CO3 (0.84 g, 6 mmol, in 3 mL H2O) in THF
(15 mL). The reaction mixture was heated at 65 °C overnight. The
solvent was removed under reduced pressure. The residue was purified
by silica gel column chromatography eluted with CHCl3/MoOH
For the mass spectroscopy, the dendron-nanocrystals and the box-
nanocrystals were digested by HCl. To do so, the nanocrystals were
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A R T I C L E S
Guo et al.
Scheme 2. (1) Surface Ligands Exchange to Form
Dendron-nanocrystals, (2) Formation of Box-nanocrystal, and
(3)“Evacuation” of the Box
Figure 1. UV-vis and PL spectra of CdSe core and CdSe/CdSe core/
shell box-nanocrystals.
dissolved in DMF, then concentrated HCl was added. A colorless solu-
tion was obtained after 3 days. The clear solution was extracted with
benzene, concentrated and detected with MALDI-TOF mass spectros-
copy (Bruker Reflex III) using dihydrobenzoic acid as the matrix.
Chemical, Thermal and Photochemical Stability of Box-Nano-
crystals. Two types of chemical stabilities of nanocrystals were
investigated, i.e., HCl etching and H2O2 oxidation. In a typical HCl
etching experiment, HCl (0.4 mL, 0.2 M) was added to a cuvette
containing 0.6 mL nanocrystals DMF solution (the initial optical density
(OD) at the first absorption peak was adjusted to ∼0.5). The UV-vis
spectra were recorded at a certain time interval until the solution became
turbid. The H2O2 oxidation experiments were carried out similarly to
the HCl etching, but with H2O2 (0.1 mL, 3 wt. % in water) as oxidant.
The thermal stability was examined by sintering the nano-
crystals. The nanocrystal samples were dissolved and precipitated with
suitable solvent system at least three times. The samples were then
placed in a vacuum oven and dried for 2 h prior to heating. The oven
temperature was raised to 100 °C and kept that temperature for about
60 min. After the oven was cooled to ambient temperature, the solubility
of the resulting fine powder was tested, and the UV-vis absorption
spectra of the solutions were recorded.
The photochemical stability of the nanocrystals was studied following
the previously reported procedure.14,15 The nanocrystals were dissolved
in DMF, and the optical density (OD) was adjusted to ∼0.3 at the first
absorbance peak. The small vials containing the samples were placed
∼4 cm under the ultraviolet lamp (UVP model UVGL-25 multiband).
The lamp was set to 254 nm (short wavelength). The UV-vis
absorption was monitored at certain time interval. The normalized OD
at the original excitation absorption peak was used as the indicator of
the photooxidation of the inorganic nanocrystals.14,15
Figure 2. NMR spectra before and after the cross-linking reaction.
the original ligands by the thiol-based dendron ligands. The
photoluminescence (PL) of the core nanocrystals was completely
quenched.15 On the contrast, the PL of the core/shell nanocrystals
was partially remained as observed previously.8 The optical
spectra of the nanocrystals after the surface replacement were
the same as the ones shown in Figure 1, which are the ones for
the corresponding box-nanocrystals. H1NMR results revealed
the complete replacement of the original ligands by the thiol
dendrons.
Formation of a Dendron Box around each Nanocrystal:
Box-Nanocrystals. The typical RCM worked well for the cross-
linking of the double bonds at the outer surface of the dendron
ligands monolayer.1H NMR spectra (Figure 2) revealed that the
original double bonds of the dendron ligands were out of the
detection limit and the formation of the new double bonds
through the cross-linking was readily observed.
Results
The CdSe/CdS core/shell nanocrystals were synthesized
through the newly developed solution atomic layer epitaxy
(S-ALE) using cadmium oleate and elemental sulfur as the
precursors.35 The surface of the as-prepared core/shell nano-
crystals was coated with primary amines. The CdSe core
nanocrystals used in this study were synthesized using standard
methods and coated with TOPO and/or primary amines as the
surface ligands.15,21,22
The absorption spectra of both plain core and core/shell
nanocrystals did not change upon the surface replacement of
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Luminescent CdSe/CdS Core/Shell Nanocrystals
A R T I C L E S
The cross-linking process did not affect either PL or absorp-
tion properties of both types of nanocrystals (Figure 1).
However, the solubility of the nanocrystals after the cross-
linking varied significantly from that of the original dendron-
nanocrystals. The original dendron-nanocrystals were soluble
in typical nonpolar organic solvents, but the cross-linked
nanocrystals were only soluble in aromatic solvents or polar
organic solvents, such as benzene, DMSO, and DMF. The
solubility of the box-nanocrystals was considered to be consis-
tent with the resulting structures of the cross-ring metathesis.
Because each G3 dendron ligand has eight double bonds as
its terminal groups (Schemes 1 and 2), it was expected that the
complete cross-linking proven by the NMR measurements
should form a “global” network of all ligands on the surface of
each nanocrystal. In fact, typical dendrimer boxes reported in
the literature were generally based on G3 dendrimers.36,38,39 To
prove this hypothesis, the nanocrystals after the cross-linking
reaction were digested by concentrated HCl solution, which
completely dissolved the CdSe nanocrystals. The clear and
colorless solution was examined with H1NMR and only broad
peaks similar to the ones shown in Figure 2 (bottom) were
detected. This result indicates that the ligands shell was still
remain as large species in the solution after the removal of the
center inorganic core.
Figure 3. Mass spectra of the solutions obtained by the digestion of CdSe
dendron-nanocrystals before (bottom) and after (top) the cross-linking
reaction.
linking between particles was insignificant. This result is not
surprising, given the close packing nature of the double bonds
at the outer surface of the ligands monolayer on each nanocrystal
and the relatively low molar concentration of the nanocrystals.
After cross-linking, the TEM pictures of nanocrystals were
almost identical to the ones before the cross-linking, with the
majority of the box-nanocrystals far away from each other and
a small fraction of the particles only separated by a distance
approximately equal to the thickness of two monolayers of
ligands. This implies that interparticle cross-linking cannot be
completely ruled out although it is unlikely.
The strong evidence of the expected “global” cross-linking
of all dendron ligands of a given nanocrystal was given by the
Mass spectroscopy measurements with the clear solution
obtained by the HC1 digestion. As shown in Figure 3. Before
the cross-linking, only the dendron ligands in the digestion
solution were detected by Mass spectroscope. However, after
the cross-linking, a distinguishable peak with a very high
molecular mass dominated the spectrum. Typically, the related
mass of the peak observed after the cross-linking corresponded
to about 15 to 50 dendron units with a full width at half-
maximum as 2 to 5 dendron units. This number was compared
to the values measured by the gravimetric method before
digestion (see next paragraph) and a good agreement between
these two methods was obtained.
This gravimetric method was also used for the determination
of the number of dendron ligands on the surface of the dendron-
nanocrystals before the cross-linking. The number of dendrons
on the surface of the nanocrystals seemed to be related to the
reaction temperature in the ligands replacement step although
we have not found an optimal condition to control it yet. After
the cross-linking reaction, the number of the dendron units on
each box-nanocrystal was found typically 10-30% less than
that of the dendron ligands of the original dendron-nanocrystals.
At this point, it is not clear why this happened.
The average number of the dendron units on the surface of
each box-nanocrystal before digestion were obtained as follows.
Nanocrystals after the cross-linking were carefully purified and
1
the purity was examined by H NMR. Typically, if any free
small molecules existed in the solution, very sharp and intense
NMR signals would be detected. Certain amount of the purified
nanocrystals in the form of fine powder was weighed to
determine the total mass of the nanocrystal/ligands complexes,
and then, those nanocrystals were dispersed in a known-volume
solvent. The total number of inorganic nanocrystals in the
solution was determined by the absorbance of the solution and
the absorption extinction coefficient of the nanocrystals using
Beer’s Law,40 and the size of the CdSe nanocrystal core was
determined by the first peak of the absorption spectrum. With
the above information, the average number of the dendron units
per nanocrystal could be easily calculated.
Upon the Exposure to a Strong Acid. HCl (pH ) 1.5), the
CdSe dendron-nanocrystals decomposed immediately and the
inorganic nanocrystals precipitated out of the solution (Figure
4, top-left). The first absorption peak of the solid residual was
drastically shifted to blue, indicating a significant shrinkage of
the inorganic nanocrystals. However, it took about 2 h for the
corresponding CdSe box-nanocrystals to precipitate. As shown
in Figure 4 bottom-left, only a few nanometers blue-shift of
the first absorption peak was detected within twenty minutes
of the acid etching reaction.
In comparison, the core/shell nanocrystals were somewhat
more stable for both dendron-nanocrystals and box-nanocrystals,
at least for the relatively short time exposures. The precipitation
of the CdSe/CdS core/shell dendron nanocrystals occurred after
a few minutes of the exposure to HCl. The CdSe/CdS core/
shell box-nanocrystals did not show any noticeable blue-shift
within 20 min of the exposure.
The agreement between the gravimetric measurements and
the Mass measurements on the number of dendron units before
and after the HCl digestion seems to suggest that the cross-
(39) Schultz, L. G.; Zhao, Y.; Zimmerman, S. C. Angew. Chem., Int. Ed. Engl.
2001, 40, 1962-1966.
(40) Yu, W. W.; Peng, X., to be submitted.
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A R T I C L E S
Guo et al.
The size of the inorganic core at the precipitation point of
the plain CdSe nanocrystals in both cases was about 2.5 nm.
The size of the original nanocrystals was about 3.3 nm. This
means that the precipitation occurred when about one monolayer
of CdSe was removed from the inorganic nanocrystals. For the
core/shell box-nanocrystals, the precipitation also occurred after
about one monolayer of CdS was etched away by HCl.
Strong Oxidant. H2O2 (0.15 mol/L), was found to etch both
CdSe and CdSe/CdS nanocrystals. The dendron-nanocrystals
were much less stable than the corresponding box-nanocrystals
when they were exposed to H2O2. Figure 5 illustrates the
stability of the nanocrystals by plotting the change of the
absorbance at the first absorption peak of the original samples.
For plain CdSe nanocrystals, oxidation of the nanocrystals was
immediately detected for the dendron-nanocrystals and the
inorganic nanocrystals precipitated after about 5 min in the H2O2
solution. The CdSe box-nanocrystals were significantly more
stable than the corresponding dendron-nanocrystals although the
oxidation of the inorganic core was also observed. Precipitation
of the nanocrystals occurred after the reaction proceeded for
about 35 min in the case of the CdSe box-nanocrystals.
The CdSe/CdS core/shell nanocrystals were generally more
stable than the corresponding plain core nanocrystals. Especially,
the core/shell box-nanocrystals did not show any detectable
change upon the exposure to H2O2 at this concentration for at
least several hours.
Figure 5. Temporal evolution of the optical density (OD) at the first
absorption peak of the original nanocrystals in H2O2 solution.
Thermal Stability. The thermal stability of the nanocrystals
was examined by sintering the purified nanocrystal powder at
100 °C for about 1 h under vacuum. Except the sintering
process, the purification, storage and other processes were all
handled in air under ambient conditions. It is known that
The size of the nanocrystals at the precipitation point in this
test was found to be about one monolayer smaller than the
original nanocrystals, which is similar to the case of the HCl
etching mentioned above.
Figure 4. Temporal evolution of the UV-vis spectra of the nanocrystals with different inorganic and/or ligands structures etched by HCl solution.
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3906 J. AM. CHEM. SOC. VOL. 125, NO. 13, 2003

Luminescent CdSe/CdS Core/Shell Nanocrystals
A R T I C L E S
Figure 6. UV-vis spectra of the solutions of nanocrystals after and before
sintering at 100 °C under vacuum.
Figure 8. Intensity increase and blue shift of the PL spectra of the
nanocrystals upon photooxidation.
Evidently, the dendron-nanocrystals of the current ligand
system were moderately stable against photo-oxidation. The
plain CdSe dendron-nanocrystals were gradually photo-oxidized
(Figure 7), and precipitated out of the solution in about 35 h.
The CdSe/CdS core/shell dendron-nanocrystals were signifi-
cantly more stable than the plain core dendron-nanocrystals,
and its precipitation occurred after about 100 hours inside the
photo-oxidation chamber.
The box-nanocrystals of both plain core and core/shell
nanocrystals were much more stable than the corresponding
dendron-nanocrystals. For the plain core box-nanocrystals, there
was no observable oxidation of the inorganic core detected after
the sample was in the photo-oxidation chamber for about 125
h although the precipitation was observed quite rapidly. In the
case of the core/shell box-nanocrystals, the photo-oxidation was
not detected for 27 d (650 h). After 27 d exposure in the photo-
oxidation chamber, the precipitation of the nanocrystals occurred
in a rapid fashion which was very similar to that of the CdSe
box-nanocrystals shown in Figure 7. The precipitation point of
the core/shell box-nanocrystals did not show in Figure 7 because
of its different time scale.
PL. The PL of the original core/shell nanocrystals was
partially remained, about 20% of the original value, after the
replacement of the surface amine ligands by the thiol-based
dendrons as shown in Figure 1. The cross-linking and the
thermal sintering under vacuum did not change the emission
properties of the core/shell dendron-nanocrystals and the box-
nanocrystals. However, the PL properties of the core/shell
nanocrystals were varied in an interesting fashion by the
chemical oxidation and photo-oxidation. The PL brightness
decreased slightly at the beginning. After this initial stage, the
PL brightness often increased significantly (Figure 8), at least
with a 2-folds of increase of the relative intensity of the PL
peak. The increase of the PL brightness was found always
associated with a slight blue shift of the absorption spectra
(Figure 8). A similar PL brightening was also observed if the
core/shell dendron-nanocrystals were stored in air with room
light after several weeks.
Figure 7. Photochemical stability (under UV radiation) of the nanocrystals
in air.
nanocrystals coated by typical commercial ligands can barely
survive the purification process, including repeated dissolution,
precipitation and drying, in air. We previously reported that
dendron-nanocrystals can survive those routing treatments
without significantly change the solubility of the nanocrystals.
The plain core CdSe dendron-nanocrystals did not survive
the harsh heating treatment although it could be dried in a
vacuum at room temperature. After the sintering, CdSe dendron-
nanocrystals became insoluble in typical solvents (Figure 6).
In comparison, the solubility of the CdSe box-nanocrystals, the
core/shell dendron-nanocrystals and the core/shell box-nano-
crystal was all well maintained, and no any change of the
absorption and PL of the nanocrystals was detected for all three
samples.
Photochemical Stability. The photochemical stability of the
nanocrystals in air under UV radiation was studied following
the established procedure.14,15 Figure 7 illustrates the results for
four different samples, CdSe dendron-nanocrystals, CdSe box-
nanocrystals, CdSe/CdS core/shell dendron-nanocrystals, and
CdSe/CdS core/shell box-nanocrystals.
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A R T I C L E S
Guo et al.
The PL properties of the core/shell box-nanocrystal treated
under the above chemical oxidation conditions, and the photo-
oxidation conditions within approximately 10 days did not show
very significant change although some fluctuation was observed,
within 20-30% relative intensity variation. However, if the core/
shell box-nanocrystal was treated by highly concentrated H2O2
solution, about 100% increase of the PL intensity of the
nanocrystals was observed along with a few nanometer blue-
shift of the absorption spectrum. It should be pointed out that,
with the same concentrated H2O2 solution, the core/shell
dendron-nanocrystals were precipitated out of the solution
almost instantaneously. Prolonged photo-oxidation (more than
about 10 days under the given conditions) was also able to
slowly enhance the PL of the core/shell box-nanocrystals (Figure
8). The photobrightening of the PL of core/shell box-nano-
crystals under ambient conditions was not observed, likely
because it would be extremely slow.
In general, the PL brightening of core/shell nanocrystals
through photooxidation was easier to be controlled than that
by chemical oxidation, and it was easier to achieve the PL
brightening for the core/shell dendron-nanocrystals than for the
core/shell box-nanocrystals (Figure 8). It was found that the
relative PL efficiency after the brightening could be even higher
than the original core/shell nanocrystals coated by amines in
organic solvent. If the oxidation process was stopped, the
brightened nanocrystals would maintain their PL efficiency at
the stop point. In this way, the PL quantum yield of the
brightened core/shell dendron-nanocrystals could be stabled at
about 20% to 30%. This brightening phenomenon is in further
study and the details will be published later.
5-10% standard deviation in size. This distribution matched
quite well with the size distribution of the nanocrystals sealed
inside the boxes. From this information, it should be safe to
conclude that the surface coverage of the dendron ligands among
a batch of nanocrystals was very uniform although they might
vary from batch to batch and depend on the reaction conditions
as mentioned above. The cross-linking reactions also did not
affect this uniformity though 10-30% loss of the dendrons was
observed in the cross-linking process.
It is well documented that relatively large species can diffuse
in to and out of a dendrimer box. In fact, this permeability is
needed for both drug delivery and molecular imprinting. For
example, a recent report revealed that the original porphyrin
core of dendrimer-boxes could easily be removed by simply
cutting the chemical bonds between the core and its peripheral.38
Furthermore, a slightly smaller but rigid porphyrin molecule
could readily squeeze into the empty box. This phenomenon
was considered as a new route toward molecular imprinting. In
the current report, it was observed that the inorganic nanocrystals
could escape the dendron boxes after the chemical bonds
between the inorganic nanocrystals and the ligands boxes were
destroyed. It would be interesting to see if the empty dendron-
boxes generated using nanocrystals as the core could be used
for molecular imprinting. To our knowledge, they are likely
the largest empty dendron boxes with a narrow size distribution.
In the contrast to the large cavity of the box, the wall of the
box is quite thin, which is based on a G3 dendron. All of these
structural features may be of interest for molecular imprinting
and other types of applications in the field of host-guest
chemistry. Prior to this work, Wu et al.42 attempted to obtain
nanometer sized polymer capsules through RCM, using Au
nanocrystals as the templates and a thiol molecule with three
double bonds as the ligands. They noticed that the NMR signals
became very sharp after the inorganic cores were removed,
which is different from our observations mentioned above. The
Au nanocrystals used in their case were about 5 nm in size. If
a global cross-linking occurred, the resulting empty capsules
should possess a significantly higher molecular mass than that
observed in this work, i.e., between 10 000 and 50 000 Dalton.
Typically, the peaks of the NMR spectra of such large sized
species should be very broad at room temperature, instead of
sharp ones observed in their case. Likely, only three double
bonds per ligand in their case might not be enough to globally
cross-link all ligands of a given nanocrystal. Unfortunately, they
only reported some AFM measurements and NMR results of
the capsules. It is difficult to judge in what degree they obtained
the desired capsules.
The plain core nanocrystals, including the dendron-nano-
crystals and the box-nanocrystals, did not emit as soon as the
original surface ligands were replaced by the thiol based dendron
ligands (Figure 1). This was true even if those highly lumines-
cent CdSe nanocrystals isolated at the “bright point” in a growth
reaction22 were employed for the surface replacement. In
addition, all following treatments, including cross-linking,
thermal sintering, chemical oxidation and photochemical oxida-
tion, did not bring back noticeable PL for the nanocrystals.
Discussions
RCM has been widely used for the cross-linking of the
branches of dendrimers and the resulting structures are called
dendrimer boxes.36-38 In some cases, the core of the dendrimer
box was chemically removed to form an empty box.36,38 Those
dendrimer boxes are being tested as molecular imprinting
structures38 and drug delivery carriers.37 In the case of semi-
conductor nanocrystals, Grubbs’ catalysts were also employed
to build up a thick ligands layer around each semiconductor
nanocrystal by the extension of the chain length of each ligand.41
The results shown in this work revealed that the versatile feature
of Grubbs’ catalysts can also been exploited to form a thin but
globally cross-linked ligand monolayer on the surface of each
semiconductor nanocrystal.
Despite the decrease of the thickness of the ligand monolayer,
the stability of the box-nanocrystals against harsh chemical,
thermal and photochemical treatments was proven dramatically
better than that of the corresponding dendron-nanocrystals.
These results are consistent with the mechanism of the photo-
oxidation of semiconductor nanocrystals, which suggested that
the diffusion of active oxygen species is the rate determining
step.14 On the basis of the data obtained in the studies of
chemical stabilities of the dendron-nanocrystals and box-
nanocrystals, it is reasonable to conclude that diffusion of the
The size distribution of those empty dendron boxes revealed
by mass spectroscopy (Figure 3) was quite narrow. Typically,
the full width at the half-maximum (fwhm) of the mass peak
was about 2-4 dendron units, which corresponded to about
(41) Skaff, H.; Ilker, M. F.; Coughlin, E. B.; Emrick, T. J. Am. Chem. Soc.
2002, 124, 5729-5733.
(42) Wu, M.; O’Neill, S. A.; Brousseau, L. C.; McConnell, W. P.; Shultz, D.
A.; Linderman, R. J.; Feldheim, D. L. Chem. Commu. 2000, 775-776.
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Luminescent CdSe/CdS Core/Shell Nanocrystals
A R T I C L E S
small reactants into the nanocrystal-ligands interface was also
the rate determining step for those related reactions.
oxidation or photo-oxidation of the shell would not affect the
peak position of the absorption spectra because of the very high
energy band gap of ZnS in comparison to that of either CdS or
CdSe. As documented in literature, the absorption peak position
of CdSe/ZnS core/shell nanocrystals with one monolayer to
seven monolayers of ZnS did not show detectable difference.47
In our case, because CdS was the shell semiconductor, the slight
oxidation of shell could be readily detected by the blue-shift of
the absorption spectrum of the core/shell nanocrystals. A more
detailed discussion of brightening phenomena will be in another
publication.35
The superior stability of CdSe/CdS core/shell box-nanocrys-
tals against both photo-oxidation and chemical oxidation was
expected. The electronic confinement of both photogenerated
hole and electrons inside the core materialsCdSe in this cases
should efficiently suppress photo-oxidation of the CdS shell as
reported previously.34 In terms of chemical oxidation, the
oxidation of Se2- should be much easier than that of S2- based
on common chemical knowledge.43 Consequently, CdSe/CdS
core/shell nanocrystals should be more stable against chemical
oxidation than CdSe plain core nanocrystals.
The understanding of the better stability of core/shell nano-
crystals against thermal sintering in solid state and acid etching
in solution is somewhat complicated. Our experimental results
revealed that, in solution, the bonding strength between thiol
ligands and either CdSe or CdS nanocrystals is very similar.44
If the bonding situation in solid state at elevated temperature
did not change, then the related stability of the two types of
nanocrystals should be identical if their ligands layer was the
same. Evidently, this is not consistent with the experimental
results. Thermodynamically, the stability against HCl etching
of the nanocrystals should be related to the precipitation product
of the inorganic crystals exposed to the solution, given the
similar bonding strength of the thiol ligands with either CdSe
or CdS nanocrystals. If thermodynamic factor played a dominat-
ing role, then the etching of CdSe/CdS core/shell nanocrystals
should be faster than that of the CdSe core nanocrystals, which
is not consistent with the experimental results.
The excellent thermal stability of the box-nanocrystals
indicates that the irreversible aggregation in solid state of
colloidal nanocrystals can be very efficiently stopped by
“sealing” each nanocrystal inside a dendron box. In this way,
nanocrystals are completely isolated from each other, and it is
very difficult for a nanocrystal to escape from its box. This
means that the poor purification and storage properties of
colloidal nanocrystals could be improved if those nanocrystals
were sealed inside desired dendron boxes. More importantly,
the performance of the solid state devices made of nanocrystals,
such as LEDs and solid state lasers, should be significantly
enhanced.5 In addition, the thin thickness of the ligands
monolayer of the box-nanocrystals should also be ideal for
efficient charge injection for the solid-state optoelectronic
devices. For biological labeling in aqueous solution, some
additional development is needed to couple bio-functional
groups onto the surface of the box-nanocrystals.
Conclusions
The globally cross-linking of the dendron ligands on the
surface of the nanocrystals practically sealed each nanocrystal
inside a dendron box, which resulted in very stable box-
nanocrystals. When the inorganic nanocrystals inside the boxes
were CdSe/CdS core/shell nanocrystals, the box-nanocrystals
possessed superior stability against chemical, photochemical,
and thermal treatments. The luminescence brightness of the core/
shell nanocrystals could be partially remained after they were
embedded inside the boxes. Furthermore, the luminescence
efficiency could be further enhanced by controlled chemical or
photochemical oxidation of the nanocrystals. The luminescent
core/shell nanocrystals described in this work should be
sufficient for the development of many important applications
based on semiconductor nanocrystals. The extremely stable box-
nanocrystals should be ideal species for studying size dependent
chemical and physical properties of high quality semiconductor
nanocrystals. The stabilization strategy presented here should
be possible to be extended to other colloidal nanocrystal systems,
provided that the chemistry involved is reasonably simple and
standardized in the literature. The cavity at the center of an
empty dendron box formed by the dissolution of the inorganic
nanocrystal core in concentrated strong acid is nanometer sized
and with a very thin shell. Those nanometer sized capsules ion
represent a new class of mesoporous structures, which are
soluble in certain solvents and have a very narrow size
distribution. Those mesoporous capsules could be of great
importance for guest-host chemistry, such as for drug delivery
and molecular imprinting.
Acknowledgment. Financial support from the NSF and the
University of Arkansas is acknowledged. J.J.L. is grateful for
the Research Assistant Fellowship provided by Center for
Sensing Technology And Research (C-STAR) at the University
of Arkansas. Y.A.W. thanks for the support from the “seeds”
program of the UA-OU Joint MRSEC.
The PL brightening of CdSe/CdS core/shell nanocrystals upon
photoradiation can be ascribed to the oxidation of the shell
material because it was always associated with a noticeable blue-
shift of the absorption spectrum (Figure 8). This conclusion is
further supported by the related brightening by the chemical
oxidation. In literature, PL brightening phenomena of CdSe/
ZnS core/shell nanocrystals were repeatedly reported.45,46 It is
worth to notice that, if ZnS was the shell semiconductor,
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(45) Mattoussi, H.; Mauro, J. M.; Goldman, E. R.; Anderson, G. P.; Sundar, V.
C.; Mikulec, F. V.; Bawendi, M. G. J. Am. Chem. Soc. 2000, 122, 12 142.
(46) Manna, L.; Scher, E. C.; Li, L.-S.; Alivisatos, A. P. J. Am. Chem. Soc.
2002, 124, 7136-7145.
(47) Dabbousi, B. O.; RodriguezViejo, J.; Mikulec, F. V.; Heine, J. R.; Mattoussi,
H.; Ober, R.; Jensen, K. F.; Bawendi, M. G. J. Phys. Chem. B 1997, 101,
9463-9475.
(43) Cotton, A. M. AdV. Inor. Chem. 1997.
(44) Aldana, J.; Peng, X., in preparation.
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