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: jyying@ibn.a-star.edu.sg; 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

COMMUNICATION www.rsc.org/chemcomm | ChemComm

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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

1 M. Sastry, Curr. Sci., 2003, 85, 1735; J. Yang, E. H. Sargent,S. O. Kelley and J. Y. Ying, Nat. Mater., 2009, 8, 683.

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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