reversible phase transfer of quantum dots and metal nanoparticles
TRANSCRIPT
Reversible phase transfer of quantum dots and metal nanoparticlesw
Yifeng Wei, Jun Yang and Jackie Y. Ying*
Received (in Cambridge, UK) 11th December 2009, Accepted 16th February 2010
First published as an Advance Article on the web 15th March 2010
DOI: 10.1039/b926194j
A general, reversible phase transfer protocol was demonstrated
for quantum dots and metal nanoparticles. The protocol involves
ligand exchange based transfer of nanoparticles from organic
medium to aqueous phase, followed by electrostatic interaction
based reversible transfer of nanoparticles between aqueous and
organic phases.
Nanoparticles of different morphologies and sizes can be
derived with solution chemistry in polar solvents (e.g. water)
and non-polar environments. Each method has its own unique
advantages and disadvantages.1 The specific applications often
require the transfer of newly formed nanoparticles from a
polar environment to a non-polar environment, or vice versa,
in order to maximize the respective advantages of these
environments based on processing considerations. This
makes phase transfer an important aspect in the synthesis,
functionalization and application of nanostructured materials.
Several strategies have been recently described for reversibly
transferring nanoparticles between aqueous and organic
solvents. These include using a thermosensitive ligand,2 using
a pH-sensitive surfactant,3 and using a ligand that can undergo
reversible host–guest complexation.4 However, these require
unusual temperatures, pH or specialized ligands, which may
adversely affect the optical properties of the nanocrystals.
Furthermore, none of these strategies have been shown to be
robust enough for multiple cycles of phase transfer. Herein we
report a general phase transfer protocol, which can transfer
QDs and metal nanoparticles reversibly between organic and
aqueous phases. This method works at room temperature and
neutral pH, and can be applied to nanoparticles with any
anionic ligands. The robust reversibility of the method has been
demonstrated by repeating the phase transfer for 10 cycles.
In this work, organic-soluble luminescent core–shell
CdSe@CdZnS nanocrystals synthesized using oleic acid
(OA) as a capping agent were used as an example to
demonstrate our reversible phase transfer protocol (see
Electronic Supplementary Information (ESI) for detailsw).The transfer of CdSe@CdZnS nanocrystals from organic to
aqueous phase was conducted using glutathione tetramethyl-
ammonium salt (GTMA) (see ESI Fig. S1w) as the transfer
agent (see Fig. 1). The strategy was based on ‘capping agent
exchange’,5 and involved the substitution of the native
OA with bifunctional ligands (GTMA), which possesses a
surface-anchoring moiety to bind to the inorganic QD surface
and an opposing hydrophilic end group (e.g. carboxylate
group) to achieve water solubility.
The direct transfer of OA-stabilized QDs from organic to
water by mixing the QD organosol with an aqueous solution
of GTMA was not successful. The particles were aggregated
at the interface between chloroform and water, instead of
transferring into the aqueous phase. As the exchange between
OA and GTMA could only occur at the interface of
chloroform and water, we postulated that the failure to
transfer the particles was the result of poor contact between
the two phases due to their lack of mutual solubility.
With this in mind, methanol, which is miscible with
chloroform and a good solvent for GTMA, was selected in
place of water to increase the interfacial contact between
OA-stabilized QDs and GTMA. Dropwise addition of
methanolic GTMA caused the QDs (or metal nanoparticles)
to precipitate, indicating that the OA has been displaced by
GTMA. With the further addition of methanolic GTMA
solution, the precipitates re-dissolved due to an increase in
polarity of the solvent, enabling the ligand exchange to be
completed in a homogeneous solution. Upon the addition of
water, the GTMA-coated nanoparticles were transferred to
the aqueous phase (see ESI Fig. S2(2,6)w). This phase transferprocess took place rapidly after the mixing of reagents.
To calculate the yield of this process, the particles were
Fig. 1 Schematic showing the functionalization of QDs and metal
nanoparticles. (1) OA shell on the particles renders the particles
hydrophobic and soluble in organic solvents. (2) After replacement
by GTMA, the negatively charged carboxylate groups render the
particles hydrophilic, allowing for phase transfer from organic to
aqueous phase. (3) Upon electrostatic interaction with hexadecyl-
trimethylammonium bromide (CTAB), the ion pairs between R4N+
and surface-bound anions provide for phase transfer back to organic
solvents. (4) Formation of more hydrophobic compounds upon the
addition of tetramethylammonium decanoate (TMAD) enables the
transfer of the particles back to the aqueous phase.
Institute of Bioengineering and Nanotechnology, 31 Biopolis Way,The Nanos, Singapore 138669, Singapore.E-mail: [email protected]; Fax: (+65) 6478-9020w Electronic supplementary information (ESI) available: Experimentaldetails of particle synthesis and phase transfer, images of the phasetransfer of particles, FTIR spectra, PL spectra of QDs, UV-vis spectraof Au nanoparticles. See DOI: 10.1039/b926194j
This journal is �c The Royal Society of Chemistry 2010 Chem. Commun., 2010, 46, 3179–3181 | 3179
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precipitated from the aqueous phase with the addition of
acetone, and then dried at room temperature in vacuum.
The yield of the particles after transfer was estimated to be
>90%. The losses were likely caused by centrifugation, and
nanoparticle attachment to the walls of the container.
Fig. 2a and b show the transmission electron microscopy
(TEM) images of CdSe@CdZnS in chloroform and in water
respectively. The slight increase in the particle size and change
in the particle morphology upon phase transfer were most
likely caused by particle agglomeration, which usually
occurred during the stabilizer exchange,6 whereby OA was
progressively displaced by GTMA to form GTMA-stabilized
QDs. The replacement of OA by GTMA was supported by the
disappearance of the Fourier-transform infrared (FTIR) peaks
at 2850 cm�1 and 2919 cm�1 (ESI Fig. S3b), which were
attributed to the symmetric and asymmetric stretches of
methylene groups.
It should be noted that the OA-stabilized QDs could not be
re-dispersed in toluene or chloroform after several rounds of
washing and centrifugation. This could be easily explained by
the progressive loss of the stabilizer (OA) as a fresh solvent
(toluene or chloroform) was used in each re-dispersion
attempt. The need to re-establish equilibrium between free
and adsorbed stabilizers would slowly but eventually deplete
the adsorbed stabilizer to a level that was inadequate to
maintain the particles in suspension. The process of displacing
OA from the QD surface by GTMA could be depicted by the
scheme presented in a previous report,7 which involved
the adsorption of OA in equilibrium with the surrounding,
and the progressive displacement of OA by GTMA. It was
assumed that the binding of GTMA to the QD surface was
more irreversible than the adsorptive interaction between OA
and the QD surface.
The failure to directly transfer OA-stabilized QDs from the
organosol to the aqueous solution of GTMA could also be
understood from simple adsorption principles. GTMA
dissolved in water was unable to exchange sufficiently with
OA due to inadequate contact between these molecules and
the QDs. In addition, the exchange between OA adsorbed on
QD surface and those in the surroundings or the insufficient
exchange of OA and GTMA at the interface of organic and
aqueous phases would sometimes lead to the partial exposure
of QD surface, establishing the anchor points for particle
agglomeration. As a result, the GTMA layers on the larger
QD particles would provide hydrophilic forces that were not
strong enough to pull the QDs into the aqueous phase
efficiently. Hence, prolonged stirring of the mixture of QD
organosol and aqueous GTMA solution would give rise to the
aggregation of QDs at the interface between organic and
aqueous phases, but no particle transfer from organic to
aqueous phase would occur.
With our methanol-mediated method, a wide variety of QDs
and metal nanoparticles with different sizes or morphologies
could be effectively transferred from organic phase to aqueous
phase. In addition to the CdSe@CdZnS QDs mentioned
earlier, Au, Ag, CdS rods and PbS were successfully
transferred from the organic phase to water (Fig. 3). The
QD or metal hydrosols thus obtained were very stable, and no
agglomeration was observed after several months of storage in
air. It should be noted that PbS was a special case; after phase
transfer, the GTMA-PbS hydrosol was only stable for B15 h.
The detailed agglomeration mechanism for this case has yet to
be understood.
The carboxylate groups on GTMA imparted a negative
charge to the QDs or metal nanoparticles, allowing us to
manipulate the aqueous or organic solubility of the nano-
particles using electrostatic interactions. Our approach to the
reversible phase transfer of nanoparticles between aqueous
and organic phases was based on an understanding of
the principle behind the use of hydrophobic salts, such as
tetraoctylammonium bromide (TOAB), as reagents for the
phase transfer of AuCl4� from water into non-polar organic
Fig. 2 TEM images of CdSe@CdZnS QDs: (a) as prepared
(d = 7.2 nm), (b) transferred from chloroform to water using GTMA
as transfer agent (d = 8.1 nm), (c) transferred to toluene upon
addition of CTAB (d = 8.1 nm), and (d) transferred back to water
upon the addition of TMAD (d = 8.1 nm).
Fig. 3 TEM image of (a) Au (13 nm), (b) Ag (11 nm), (c) CdS rods
(50 nm), and (d) PbS (12 nm) transferred from chloroform to water
using GTMA as the transfer agent.
3180 | Chem. Commun., 2010, 46, 3179–3181 This journal is �c The Royal Society of Chemistry 2010
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solvents in the well-established two-phase liquid/liquid
synthesis of thiol-derivatized Au nanoparticles.8 In this study,
TOAB was mixed with HAuCl4 in a water–toluene biphasic
system, whereby the more hydrophobic salt, (TOA)+(AuCl4)�
partitioned into toluene, while the more hydrophilic
HBr partitioned into water. A similar scheme involving the
exchange of counterions was used here for nanoparticles.
The addition of CTAB (see Fig. 1) resulted in the transfer of
QDs and metal nanoparticles from the aqueous phase into
chloroform (ESI, Fig. S2(3,7)w). The anionic QDs or metal
nanoparticles were more hydrophobic per unit charge than
Br�. Hence, the anionic nanoparticles and the cationic
(CTA)+ partitioned into the chloroform phase in the form
of a complex held together by electrostatic interactions.
The more hydrophilic (TMA)+Br� partitioned into the
aqueous phase. Compared with CdSe@CdZnS-GTMA
(ESI Fig. S3bw), QDs upon transfer back to chloroform
showed two FTIR peaks at 2850 cm�1 and 2919 cm�1
(ESI Fig. S3cw), which were attributed to the symmetric and
asymmetric stretches of the methylene groups of CTAB.
The subsequent addition of TMAD (see Fig. 1) resulted in
the transfer of QDs and metal nanoparticles from chloroform
back into the aqueous phase (ESI Fig. S2(4,8)w). This time, the
anionic QDs or metal nanoparticles were less hydrophobic per
unit charge than the decanoate anion. Hence, the anionic
nanoparticles and the cationic (TMA)+ partitioned into the
aqueous phase in the form of a hydrophilic salt. The more
hydrophobic (CTA)+D� partitioned into chloroform.
The resulting FTIR spectrum (ESI Fig. S3dw) was quite
similar to that of GTMA-stabilized QDs (ESI Fig. S3bw),indicating the dissociation of the (QD)�(CTA)+ complex.
The above phase transfer processes between the organic and
aqueous phases could be performed repeatedly, indicating that
the transfer of QDs and metal nanoparticles between organic
and aqueous phases was completely reversible. Fig. 2c and d
show the TEM images of QDs transferred between aqueous
and organic solvents based on electrostatic interactions. No
changes in particle size or morphology were observed since no
ligand exchange was involved. In all cases, methanol was used
as a mediating solvent to improve the interfacial contact
between the surface of QDs or metal nanoparticles and the
organic- or aqueous-insoluble ligands. Our methanol-
mediated method could overcome the 10 nm upper limit on
particle size for the phase transfer from aqueous to organic
phase that was observed by Cheng and Wang on Au
nanoparticles,9 whereby tetraoctylammonium cations were
used as the phase transfer agent and methanol was not
employed as a mediating solvent.
The optical properties of QDs and metal nanoparticles
before and after the reversible phase transfer were
characterized by photoluminescence (PL) and ultraviolet-
visible (UV-vis) spectroscopies. ESI Fig. S4w shows that the
UV and PL spectra of CdSe@CdZnS nanocrystals before and
after transfer were similar in band positions. The PL intensity
of the QDs decreased after phase transfer from toluene to
water, most likely induced by an increase in trapping sites
during the phase transfer process.10 ESI Fig. S5w shows the
UV-vis absorption spectra of Au nanoparticles as-prepared
and after up to 10 cycles of transfer between organic and
aqueous phases. Compared with the original oleylamine-
stabilized Au nanoparticles, a red shift of B9 nm was
observed, indicating a slight growth in Au particle size during
the ligand exchange between oleylamine and GTMA.
In summary, a completely reversible phase transfer
protocol, which could repeatedly transfer QDs and metal
nanoparticles between the organic medium and aqueous
phase, has been developed. This approach involved the ligand
exchange based transfer of QDs or metal nanoparticles from
organic medium to aqueous phase. Subsequently, reversible
transfer of QDs or metal nanoparticles between the aqueous
and organic phases was achieved via electrostatic interaction.
Methanol was employed as an effective mediating solvent for
all transfer processes to improve the interfacial contact
between the QDs or metal nanoparticles and the organic- or
aqueous-insoluble ligands.
We thank Dr Yuangang Zheng for his assistance in
optimizing the QD synthesis. This work was supported by
the Institute of Bioengineering and Nanotechnology
(Biomedical Research Council, Agency for Science, Technology
and Research, Singapore).
Notes and references
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2 B. Qin, Z. Zhao, R. Song, S. Shanbhag and Z. Tang,Angew. Chem., Int. Ed., 2008, 47, 9875.
3 H. Jiang and J. Jia, J. Mater. Chem., 2008, 18, 344.4 D. Dorokhin, N. Tomczak, M. Han, D. N. Reinhoudt,A. H. Velders and G. J. Vancso, ACS Nano, 2009, 3, 661.
5 H. T. Uyeda, I. L. Medintz, J. K. Jaiswal, S. M. Simon andH. Mattoussi, J. Am. Chem. Soc., 2005, 127, 3870; K. Susumu,H. T. Uyeda, I. L. Medintz, T. Pons, J. B. Delehanty andH. Mattoussi, J. Am. Chem. Soc., 2007, 129, 13987.
6 L. O. Brown and J. E. Hutchison, J. Am. Chem. Soc., 1997, 119,12384; L. O. Brown and J. E. Hutchison, J. Am. Chem. Soc., 1999,121, 882.
7 J. Yang, J. Y. Lee, T. C. Deivaraj and H. P. Too, J. ColloidInterface Sci., 2004, 277, 95.
8 M. Brust, M. Walker, D. Bethell, D. J. Schiffrin and R. Whyman,J. Chem. Soc., Chem. Commun., 1994, 801.
9 W. Cheng and E. Wang, J. Phys. Chem. B, 2004, 108, 24.10 J. Aldana, Y. A. Wang and X. Peng, J. Am. Chem. Soc., 2001, 123,
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