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2009, 113, 4748–4750 Published on Web 02/27/2009
Synthesis of Amphiphilic CdTe Nanocrystals Aliaksei Dubavik, Vladimir Lesnyak, Wladimir Thiessen, Nikolai Gaponik, Thomas Wolff, and Alexander Eychmu¨ller* Physikalische Chemie, TU Dresden, Bergstrasse 66b, D-01062 Dresden, Germany ReceiVed: January 7, 2009; ReVised Manuscript ReceiVed: February 10, 2009
The synthesis of CdTe nanocrystals capped with thiolated methoxypolyethylene glycols can be performed both in water and in toluene. The evolving nanocrystals are compatible with a variety of different solvents. A phase transfer of nanocrystals initially prepared in toluene to water and subsequently to chloroform occurs spontaneously without any adjustments to the composition or to the properties of the pure solvents used. By this amphiphilic behavior, the thus-synthesized nanoparticles might be of superior interest in biological applications because they should have the ability to overcome barriers of varying polarity such as cell membranes. Introduction Photochemical stability, continuous absorption spectra with size-dependent onsets, size- and composition-tunable photoluminescence with high quantum yields, and compatibility with various solvents together with the corresponding processability contribute to the ongoing interest in semiconductor nanocrystals1 with regard to various applications in photonics, optoelectronics, bioimaging, and biolabeling. The phase transfer of nanocrystals between solvents of different chemical nature (e.g., polar (water) and nonpolar) has a significant influence on many of these applications because it constitutes a large degree of control of their compatibility with technological processes or with bioenvironments such as water, various buffers, and cell media, as well as their assembly and self-assembly capabilities. The interest in water-compatible nanocrystals is driven by their great promise in biological applications2 and has resulted in a variety of approaches allowing their direct synthesis in water3 or efficient phase transfer to water from nonpolar organic solvents. Nanocrystals synthesized via so-called hot-injection routes in organic solvents,4,5 such as CdSe-based core-shell nanoparticles, were transferred to water with the aid of short-chain thiols,6 or silica shells,7 or amphiphilic molecules.8 However, once transferred, the nanocrystals normally lose their compatibility with the initial solvent. The successful synthesis of Au colloids modified with amphiphilic ligands showing biphase compatibility has sparked additional interest.9 However, not every procedure adopted for noble metal nanoparticles is also applicable for semiconductor particles. It is well known that even slight changes in the surface composition may lead to complete quenching of the photoluminescence of semiconductor nanocrystals. This may explain why only very recently some first examples of semiconductor nanocrystal colloids possessing biphase compatibility were reported. The reversible phase transfer of thioglycolic acid (TGA)-capped CdTe nanocrystals between water and a chloroform phase was mediated by the addition of 1-hexadecylamine * Corresponding author. E-mail: alexander.eychmueller@ chemie.tu-dresden.de. Fax: +49(0)35146337164. Tel: +49(0)35146339843.
10.1021/jp900140y CCC: $40.75
and the control of the pH of the aqueous phase.10 As a more advanced approach, the one-pot synthesis of CdTe11 and several other12 nanocrystals was performed in the presence of an amphiphilic multidentate ligand in low-molecular-weight polyethyleneglycole (PEG) as a noncoordinating solvent. The amphiphilicity of the resulting nanocrystals is based on a readily forming secondary ligand layer on the surface of the nanocrystals that allows the switching from the originally hydrophobic to hydrophilic surface properties. This switching may, however, be reversible only in the presence of some excess ligand in solution. By this, the amphiphilicity of the nanocrystals in both cases mentioned is not inherent to them but rather a collective property of several components constituting the colloidal solution. In this letter, we report the synthesis of CdTe nanocrystals stabilized with thiol-modified PEG oligomers. The resulting amphiphilicity of the nanocrystals is an inherent property of their surface, and it is preserved after carefully washing out of solution any excess amphiphilic ligand. The nanocrystals reversibly transfer between different phases spontaneously (i.e., without any adjustment of ionic strength, pH, or composition of the phases). These light-emitting nanocrystals may find applications in the visualization of cell transport because they should be able to penetrate through the hydrophobichydrophilic barriers of the cell membranes. Experimental Section All chemicals used were analytical grade or the highest purity available. Milli-Q water (Millipore) was used as a solvent in the water-based syntheses. Al2Te3 lumps (Cerac Inc.) were used as a source for the generation of H2Te gas. For the syntheses of mPEG-TGA and mPEG-SH, see Supporting Information. Synthesis of CdTe/mPEG-TGA Nanocrystals. The nanoparticles were synthesized according to ref 3. Typically for toluene-based synthesis, 0.158 g (0.685 mmol) of Cd(CH3COO)2 was dissolved in 30 mL of toluene, and 0.756 g (1.78 mmol) of mPEG-TGA was added under stirring. The resulting transparent solution was carefully deaerated via bubbling Ar for 20-30 2009 American Chemical Society
Letters min. Then H2Te gas generated by the reaction of 0.05 g (0.114 mmol) of Al2Te3 with excess 0.5 M H2SO4 was passed through the solution with a slow Ar flow. The molar ratio of Cd2+/Te2-/ mPEG-TGA was 1/0.5/1.3. The further nucleation and growth of the nanocrystals proceeded by refluxing at 110 °C under openair conditions. The colloid obtained was purified by reprecipitation using diethyl ether as a nonsolvent. Synthesis of CdTe/mPEG-SH Nanocrystals. The same procedure and molar amounts were employed in toluene- and water-based syntheses of CdTe/mPEG-SH nanocrystals. In the case of the water-based approach, toluene as a solvent was replaced by water, and Cd(ClO4)2 · 6H2O was used as the Cd source. Additionally, before H2Te gas injection the pH of the precursor solution was adjusted to 12 with 1 M NaOH. The toluene-based nanocrystal colloids were purified by reprecipitation employing hexane as a nonsolvent. The aqueous colloids were purified using dialysis against Milli-Q water. Phase Transfer. The purified CdTe/mPEG-TGA or CdTe/ mPEG-SH colloid in toluene was added to water. After about 1 h, the nanoparticles transfer spontaneously from the organic phase into water. In the same manner, the transfer of CdTe/ mPEG-SH nanocrystals takes 10-12 h from the initial aqueous solution into chloroform. In both cases, the transfer can be strongly accelerated by vigorous shaking. Similarly, the triphase spontaneous transport of toluene-based CdTe/mPEG-SH nanoparticles from toluene through water into chloroform was carried out. Characterization of Nanocrystals. UV-vis absorption spectra were collected with a Cary 50 spectrophotometer (Varian). Fluorescence spectra were measured at room temperature using a FluoroMax-2 spectrofluorimeter (Instruments SA). The photoluminescence quantum yields (PL QYs) of the nanocrystal solutions were estimated by comparison with Rhodamine 6G in ethanol assuming its PL QY to be 95%. FTIR spectra were recorded on a Thermo Nicolet Avatar 360 spectrometer. Powder X-ray diffraction (XRD) measurements were carried out with a D5000 diffractometer (Siemens, Cu KR radiation). Samples for XRD were made by placing finely dispersed powders of nanocrystals on a standard Si wafer. Results and Discussion In the following text, we describe in a four-step process how the amphiphility of as-prepared colloidal CdTe nanocrystals has been achieved. (The main results are also summarized in Table SI1, Supporting Information.) The well-developed synthesis of thiol-capped CdTe nanocrystals13 was chosen as a model starting system for the purpose of the present work. Because TGA is known to be one of the best ligands for these nanocrystals, the first amphiphilic ligand chosen here was the ester of TGA and methoxypolyethylene glycol (mPEG) (Figure 1a). This mPEGTGA ester is readily soluble both in water and in nonpolar organics (toluene and chloroform). The addition of this ligand to an aqueous colloidal solution of CdTe nanocrystals stabilized with a short-chain thiol (TGA or mercaptopropionic acid) resulted in the spontaneous phase transfer of these nanocrystals into the organic phase (chloroform), thus confirming the ability of this ligand to serve as a phase-transfer mediator. In the second step, the synthesis of the nanoparticles was performed in water in the presence of this mPEG-TGA ligand only. The properties of the evolving well-emitting CdTe nanocrystals were comparable to those prepared with TGA only. However, no amphiphility was observed. A detailed FTIR investigation (Supporting Information, Figure SI1) showed that the ester bond is not stable and breaks during boiling in water
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Figure 1. Chemical structures of mPEG-TGA (a) and mPEG-SH (b).
Figure 2. Absorbance and photoluminescence spectra of CdTe/mPEGSH nanocrystals synthesized during 27 h in toluene (a) and 29 h in aqueous solutions (b). λex ) 450 nm.
at a relatively high pH. Most probably, the growing nanocrystals were stabilized by TGA being formed during the hydrolysis of the ester. To avoid this hydrolysis, in the third step the synthesis with mPEG-TGA as the stabilizer molecule was performed in water-free toluene. The resulting nanocrystals are small (below ca. 2 nm)13 and possess moderate band gap emission in the green spectral region (PL QY of ca. 13%), together with a pronounced trap-related emission appearing as a shoulder on the low-energy side of the band gap emission. The powder XRD (Supporting Information, Figure SI2) reveals the main features of very small crystallites exhibiting the cubic phase of CdTe. The reflexes are slightly shifted to larger angles, indicating the formation of an additional CdS crystal phase, which is characteristic of various syntheses of CdTe stabilized with thiols.13 This synthesis was relatively slow, and larger particles were not achieved even under prolonged reflux. After separating from the remaining excess ligand and reaction byproduct (multiple precipitations with diethyl ether, centrifugation, and dissolving in pure toluene), the nanocrystals were also found to be spontaneously soluble in water as well as in toluene and chloroform. However, back transfer from water into nonpolar organics (toluene and chloroform) is possible only by the addition of excess mPEGTGA. Thus, in the fourth step we avoided the use of the ester in the aqueous synthesis. Instead, yet another closely related ligand was synthesized by the direct thiolation of mPEG. These mPEGSH (Figure 1b) molecules show solubility in water and in nonpolar solvents and may be used as phase-transfer mediators that are similar to mPEG-TGA. The nanoparticle synthesis in the presence of mPEG-SH was successfully performed both in water and in toluene. In both cases, we still were not able to grow the nanocrystals larger than about 2.2 nm. However, the optical quality (Figure 2) of mPEG-SH-stabilized nanocrystals (PL QY of ca. 24%, pronounced absorption maxima, and almost no trap emission) is doubtlessly superior to that of the mPEGTGA stabilized nanoparticles. The thus-prepared nanocrystals are mixable in any proportion with most of the common polar and nonpolar solvents. It was literally impossible to find any “bad solvent” with which to perform the common size-selective precipitation procedures for
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Figure 3. Photographs of vials demonstrating the spontaneous triphase transfer of CdTe/mPEG-SH nanocrystals from toluene through water into chloroform with time under daylight.
the cleaning of the nanoparticles synthesized with mPEG-SH in water. These nanocrystals were cleaned solely by dialysis. Fortunately enough, mPEG-SH-stabilized CdTe nanoparticles synthesized in toluene may be precipitated by hexane or diethyl ether. Size-selected particles and cleaned precipitates of these were easily redissolved in pure water, toluene, and chloroform. As shown in Figure 3 and Figure SI3, these nanocrystals that were redissolved in toluene transferred spontaneously into the water phase over approximately 1 h (or only over several minutes when the transfer was accelerated by shaking). Moreover, in contrast to all previous findings discussed above, the nanocrystals now being completely transferred into pure water were equally found to transfer spontaneously and completely into the next nonpolar phase, namely, into chloroform, which is known to be a better solvent than toluene for many organic substances. It should be noted that the toluene-water transfer is much faster than that from water to chloroform. This may demonstrate a somewhat stronger affinity of the nanoparticles to hydrophilic rather than hydrophobic (toluene) media. However, because the interactions on the phase boundary between water and chloroform resulted in the spontaneous formation of some intermediate emulsion-like layer that is not observed at the toluene-water interface (Figure 3), we can rationalize the observation as follows. In toluene, the nanocrystals surrounded by mPEG are dissolved mainly via nonpolar (van der Waals) interactions between solvent and hydrocarbon moieties of mPEG and by some dipole-induced dipole interactions involving the aromatic ring of toluene. In water, much stronger hydrogen bond interactions between water and the PEG ether oxygens exist. During transfer to chloroform, these hydrogen bonds have to be broken and replaced by dipole-dipole interactions between chloroform and PEG oxygens (whereby water molecules are set free, producing entropy). This transfer process can be expected to require more time (because of activation energy) than that from toluene to water. In summary, we have demonstrated that with the proper design and the fine tuning of their structure ligands such as thiolated polyethylene glycols enable the synthesis of semiconductor nanocrystals in water as well as in a variety of organic
Letters solvents. This newly developed stabilizer facilitates the reversible and repeatable transfer between phases while maintaining the emission properties of the CdTe nanocrystals. No further adjustments of the solvent content such as the addition of other surface-active species are needed. The approach developed seems to be flexible and may be adjusted to other nanocrystalline materials and solvent combinations by sensibly varying the composition of the thiolated polyethylene glycol. We believe that apart from the appealing and virtually unlimited technological processability these inherently amphiphilic light-emitting nanocrystals may also find applications in the visualization of biological transport phenomena. This is because they should be capable of leaving their aqueous biological surroundings, permeating nonpolar cell membranes, and leaving those again when heading toward the aqueous cell interior. Acknowledgment. Financial support from the DFG under contract WO 376/18-1 is gratefully acknowledged. This is dedicated to Dr. Karl-Heinz Grellmann on the occasion of his 80th birthday. Supporting Information Available: Summary of main results of the work; FTIR spectra of mPEG, the mPEG-TGA ester, and the product of mPEG-TGA hydrolysis; an XRD pattern of CdTe nanocrystals; photographs of vials under UV light; and the syntheses of mPEG-TGA and mPEG-SH. This material is available free of charge via the Internet at http:// pubs.acs.org. References and Notes (1) Alivisatos, A. P. Science 1996, 271, 933. (2) Michalet, X.; Pinaud, F. F.; Bentolila, L. A.; Tsay, J. M.; Doose, S.; Li, J. J.; Sundaresan, G.; Wu, A. M.; Gambhir, S. S.; Weiss, S. Science 2005, 307, 538. (3) Gaponik, N.; Talapin, D. V.; Rogach, A. L.; Hoppe, K.; Shevchenko, E. V.; Kornowski, A.; Eychmu¨ller, A.; Weller, H. J. Phys. Chem. B 2002, 106, 7177. (4) Murray, C. B.; Norris, D. J.; Bawendi, M. G. J. Am. Chem. Soc. 1993, 115, 8706. (5) de Mello Donega, C.; Liljeroth, P.; Vanmaekelbergh, D. Small 2005, 1, 1152. (6) Chan, W. C. W.; Nie, S. Science 1998, 281, 2016. (7) Gerion, D.; Pinaud, F.; Williams, S. C.; Parak, W. J.; Zanchet, D.; Weiss, S.; Alivisatos, A. P. J. Phys. Chem. B 2001, 105, 8861. (8) Pellegrino, T.; Manna, L.; Kudera, S.; Liedl, T.; Koktysh, D.; Rogach, A. L.; Keller, S.; Raedler, J.; Natile, G.; Parak, W. J. Nano Lett. 2004, 4, 703. (9) Edwards, E. W.; Chanana, M.; Wang, D.; Mo¨hwald, H. Angew. Chem., Int. Ed. 2008, 47, 320. (10) Jiang, H.; Jia, J. J. Mater. Chem. 2008, 18, 344. (11) Kairdolf, B. A.; Smith, A. M.; Nie, S. J. Am. Chem. Soc. 2008, 130, 12866. (12) Smith, A. M.; Nie, S. Angew. Chem., Int. Ed. 2008, 47, 9916. (13) Rogach, A. L.; Franzl, T.; Klar, T. A.; Feldmann, J.; Gaponik, N.; Lesnyak, V.; Shavel, A.; Eychmu¨ller, A.; Rakovich, Y. P.; Donegan, J. F. J. Phys. Chem. C 2007, 111, 14628.
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