One-Phase Synthesis of Gold Nanoparticles with Varied Solubility

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One-Phase Synthesis of Gold Nanoparticles with Varied Solubility Aliaksei Dubavik, Vladimir Lesnyak,* Nikolai Gaponik, and Alexander Eychm€uller Physical Chemistry, TU Dresden, Bergstr. 66b, 01062 Dresden, Germany

bS Supporting Information ABSTRACT: We developed a straightforward synthesis of gold nanoparticles with diameters in the range 2.1 7.0 nm which display solubility in both aqueous and nonpolar (toluene, chloroform) media. This versatile solubility of the nanoparticles is achieved by the use of a thiolated PEG capping agent. Their plasmon resonance band is virtually unaltered in different media.

’ INTRODUCTION Nanoparticles (NPs) including metal, semiconductor, and insulator materials are of a great interest due to their promising application in a wide range of fields: optics, electronics, catalysis, biomedicine, etc. Particularly, the syntheses of noble metal NPs (Au, Ag, Pt, Pd) have a long tradition, and fundamental synthetic strategies have therefore been known for some decades or even centuries.1a At present colloidal gold attracts special attention as a unique material for nanoplasmonics and nonlinear optics. However, for the assembling of desired functional structures and/or the compatibility of the NPs with processing media proper control of particle size, size distribution, shape, and especially the surface composition are essential.1b d Therefore, the development of novel preparation procedures enabling the controllable and desirable handling of gold NPs is still in the focus of modern nanochemistry.1 One of the most important parameters to be controlled in all colloidal synthetic approaches is that of the particles surface coverage. This outermost shell protects the particles from the surrounding environment and prevents their aggregation. The surface agents determine the solubility of the colloidal particles and their compatibility with the dispersion medium, which is important for their processing, assembly, etc. In this respect, an ideal stabilizer would provide solubility and compatibility of the particles with various media ranging from polar to nonpolar. Usually, the NPs capped by a certain stabilizer are compatible with compounds of similar nature (via polar polar or nonpolar nonpolar interactions) which limits the range of their applications. In order to adopt the NPs to different media, various stabilizer exchange techniques are employed for their phase transfer. Up to date ligand exchange methods have been reported for transfer of colloidal nanocrystals of Au, Ag, Pt, and Pd from organics to aqueous media.2 Many other examples of this kind of interphase processing include the transfer of semiconductor nanocrystals (CdTe) synthesized in aqueous solution into organics3 and in the opposite direction,4 CdSe5 and PbSe6 from organics r 2011 American Chemical Society

into water, as well as of iron oxide NPs by ligand exchange7 and by polymer stabilizer adjustment.8 Recently, more advanced approaches for the synthesis of Au, Pd, Ag, CdTe, PbSe, and Fe3O4 nanocrystals possessing various solubility were introduced.9 These NPs being capped by amphiphilic polymeric stabilizers are capable of a reversible phase transfer by adjusting the ionic strength of the water phase (stimuliresponsive behavior)9a or by adding some excess ligand to the solution.9b The coverage with amphiphilic stabilizers demonstrated to favor the self-assembly of CdSe/CdS nanocrystals with subsequent micelle and vesicle formation,10 to enhance the solubility and stability of Au NPs,11 and to resist their nonspecific bindings with proteins, DNA, and RNA.12 All these properties are particularly important for biological applications. Recently, we have reported on the synthesis of CdTe nanocrystals which are compatible with a variety of solvents and capable of spontaneous transfer between the different phases.13 In the present work we have extended this synthetic approach and developed a facile synthesis of gold NPs stabilized by thiolmodified methoxypoly(ethylene glycol) (mPEG-SH) oligomers. We demonstrate the interoperability of these NPs with media of different polarity as well as their direct and complete transfer across phase boundaries without stirring and/or agitation or any additional treatments or medium adjustments. The method developed allows the preparation of stable and highly concentrated gold colloids. Poly(ethylene glycol) as a surface coating has advantages owing to its biocompatibility, reduced toxicity, and its amphiphilic nature. Shimmin et al. have reported a systematic study of the effects of thiolated polymers on the size of Au NPs.14 Foos et al. investigated gold nanoclusters stabilized by thiols after a ligand exchange reaction.15 The synthesis of di-, tri-, and Received: May 4, 2011 Revised: July 13, 2011 Published: July 14, 2011 10224

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tetra(ethylene glycol) protected Au nanocrystals was optimized by employing methanol/water mixtures as media for the reduction of the gold precursors.16 Although the synthesis of PEG stabilized gold NPs has already been quite well investigated, an amphiphilic behavior of these colloids was not addressed thus far.

’ EXPERIMENTAL SECTION Synthesis of Stabilizers. Stabilizers of three different lengths were synthesized according to the method described in ref 13 (see also Supporting Information). Their structure was proved by NMR and IR analysis. Synthesis of Gold Nanoparticles. For a synthesis of Au/mPEGSH NPs at a molar ratio of gold/stabilizer =1/1, 10 mL of water solution containing 0.11 g (0.3 mmol) of mPEG350-SH (or the corresponding amount of mPEG500-SH or mPEG750-SH) was vigorously mixed with 0.118 g (0.3 mmol) of HAuCl4  3H2O dissolved in 30 mL of H2O for 10 15 min at room temperature. Thereafter, 30 mL of the freshly prepared NaBH4 (0.113 g, 3 mmol) aqueous solution was added to the mixture and left stirring overnight. The thus-prepared gold colloid was filtered in order to separate large aggregates of NPs, whose content typically was less than 10 wt % relative to the initial Au amount. The Au NPs were isolated from the supernatant by evaporation of water on a rotary evaporator. The nanocrystals synthesized are readily soluble in both polar and nonpolar solvents. Phase Transfer of Gold Nanoparticles. Prior to phase transfer test, the Au NPs being dispersed in toluene were precipitated by the addition of hexane in order to remove unreacted species, especially the rest of the unreacted stabilizer which could exert influence on the solubility of particles. Preparation of Gold Polymer Composites. Composites of polymers and Au NCs were prepared by mixing gold colloids having ∼1 wt % concentration both in water and in toluene with equal volumes of 10 wt % solution of poly(vinyl alcohol) (PVA) in water and 20 wt % of polystyrene (PSt) in toluene, respectively, with subsequent drop-casting of the obtained mixtures onto glass plates and drying in an oven at 60 °C. Resulted loading of NCs is estimated as 10 wt % of gold for PVA composite and 5 wt % for PSt film. Characterization of Gold Nanoparticles. UV vis absorption spectra were collected with a Varian Cary 50 spectrophotometer at room temperature. Samples for transmission electron microscopy (TEM) were prepared by dropping diluted NP colloids onto copper grids coated with thin Formvar carbon film and subsequently evaporating the solvent. TEM images were obtained on a Tecnai T20 microscope, operating at 200 kV (FEI).

’ RESULTS AND DISCUSSION For a synthesis of Au NPs we modified the well-known twophase Brust approach.17 Attempting to use the classical protocol including a mixed toluene/water solvent and tetraoctylammonium bromide (TOAB) as a phase-transfer agent, we found that at the end of the reaction the Au NPs were collected in the aqueous phase in contrast to the originally dodecanethiol stabilized particles which were soluble in toluene. Therefore, we simplified this method excluding toluene and TOAB, and by this we developed a facile onephase aqueous synthesis providing stable gold NP colloids by employing gold precursor, stabilizer, and reducing agent. In order to achieve proper amphiphilicity of the particles (favorable balance between hydrophilic and hydrophobic properties), we applied three mPEG-SH stabilizers with different chain lengths: (H3C (O CH2 CH2)n SH), n ≈ 7 for mPEG350-SH, n ≈ 11 for mPEG500-SH, and n ≈ 16 for mPEG750-SH. The synthesis was monitored by UV vis

Figure 1. Absorption spectra of Au NPs stabilized by mPEG-SH of different molar weights in water (a) and Au/mPEG350-SH NPs dispersed in different media (b). The inset is a sketch of a gold particle capped by mPEG-SH.

spectroscopy. Absorption spectra of the gold NPs capped with different thiolated mPEGs are presented in Figure 1a. As is seen from the figure, the longer the mPEG chain, the less pronounced is the gold plasmon band which is suggestive of a decrease in nanoparticles size with increasing polymer chain length.18 TEM imaging confirms this assumption: the average diameters of the Au/mPEG350-SH, Au/mPEG500-SH, and Au/mPEG750-SH NPs obtained at a molar ratio of Au/stabilizer = 1/1 were found to be 4.8 ( 1.8, 2.5 ( 1.0, and 2.2 ( 0.8 nm, respectively. A similar tendency was observed by Shimmin et al. and was explained by an acceleration of the particle nucleation rate at very early stages of the reduction through a possible coordination of ethylene glycol units with gold atoms.14 At the same time, the larger polar polymers which consist of a larger number of monomer units enhance this effect by acting just as bigger nets. Therefore, larger particles are formed in the presence of the shorter polymer mPEG350-SH. These NPs display a quite pronounced surface plasmon band at around 520 nm, which retains its position in different solvents (see Figure SI1). This independence of the plasmon position from the surrounding media contrasts with Drude model and with the experimental results reported.19 This observation suggests that the local proximal surrounding of the gold particles remains essentially the same in different solvents, namely the partly hydrated poly(ethylene glycol) shell. Since the NPs were originally synthesized in the aqueous phase, a complete removal of the water molecules using common drying techniques is hardly possible due to their strong affinity to polar compounds. This hypothesis is verified by dynamic light scattering (DLS) measurements: the mean average solvodynamic diameter of the Au/mPEG350-SH nanocrystals was determined in toluene to be 77 nm, in water 41 nm, and in chloroform 24 nm (see Figure SI2). On the other hand, estimated from TEM images and appended by the fully extended mPEG-S- chains, the sizes of the particles are 9.7, 11.2, and 14.6 nm for Au/mPEG350-SH, Au/mPEG500-SH, and Au/ mPEG750-SH, respectively. Thus, the results of the DLS suggest the formation of small aggregates of particles due to interactions 10225

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Figure 2. TEM images of Au NPs synthesized in the presence of mPEG350-SH at different Au/mPEG350-SH molar ratios: 1/1 (a), 1/2 (b), and 1/3 (c). The insets in (b) show the Au NPs after heat treatment for 1 and 2 h. The scale bar is the same for all images: 50 nm. Histograms below display corresponding size distributions of the particles.

between the stabilizer molecules which, however, do not affect the overall stability of the gold colloids. The Au NPs obtained form very stable colloids. It was difficult to precipitate the as prepared particles from water solution due to their inherent diverse solubility. The particles were easily soluble in polar and nonpolar media: acetone, water, alcohols, toluene, mesitylene, chloroform, etc. Owing to their high colloidal stability, significant concentrations of colloids (up to several weight percent of gold) are gained in all these solvents. This versatile solubility of Au NPs facilitates their processing. For example, these particles easily form composites with both water-soluble polymers like poly(vinyl alcohol) and with polymers soluble in organics like, e.g., polystyrene. Figure 1b shows absorption spectra of the Au/mPEG350-SH NPs in different surroundings. The shift of the plasmon band from 520 in liquid media to 534 nm in solid PVA and PSt is presumably caused by the change of the refractive index of the medium, by the increase of its dielectric constant due to the high concentrations of the NPs in the polymer matrix, and by their dipole dipole interactions in contrast to the diluted solutions.20 The variation of the initial ratio of gold/stabilizer allows for the control of the particle sizes. This synthesis obeys a general rule: the higher the content of the stabilizer, the smaller are the NPs formed. TEM observations provide the following mean diameters of the Au/mPEG350-SH nanocrystals: 4.8 ( 1.8, 2.9 ( 1.0, and 2.1 ( 0.4 nm for the gold/stabilizer ratios of 1/1, 1/2, and 1/3, respectively (see Figure 2). As is seen from the TEM

Figure 3. Photographs demonstrating the spontaneous triphase transfer of Au/mPEG350-SH NPs from toluene into water and subsequently into chloroform in the course of time.

images, the small NPs tend to aggregate on the grids after solvent evaporation due to the quite strong interactions between the stabilizer molecules. Further growth of the Au NPs via Ostwald ripening was realized by heating at 150 160 °C in mesitylene, similar to the approach reported by Shimizu et al.21 As an example, TEM images of the Au/mPEG350-SH particles treated for 1 and 2 h are shown in Figure 2b (insets). Heating leads to the NP growth from 2.9 ( 1.0 to 6.5 ( 1.1 nm after 1 h and to 7.0 ( 1.4 nm after 2 h of treatment. It also results in the focusing of nanocrystals size distribution: from quite broad initial (34% size deviation down to (17% and (20% for 1 and 2 h of heating, respectively. We note that the particles are still soluble in different media even after heat treatment. We have investigated the amphiphilic behavior of the prepared Au/mPEG-SH particles in different solvents in the form of phase 10226

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Langmuir transfers. The purified gold NPs were dissolved in toluene and placed as the third component above the already prepared twophase liquid consisting of water and chloroform (see Figure 3). After the addition of the toluene solution the Au/mPEG350-SH NPs start to transfer from the organic layer to the water layer within 1 h. After 24 h they have completely moved into water. The process of further full migration to the chloroform phase needed ∼1 week. The Au NPs stabilized by the longer chain stabilizers (mPEG500-SH and mPEG750-SH) also showed similar amphiphilic behavior. However, in these cases the transfer rate from toluene to the aqueous phase was slower and faster from the aqueous to the chloroform than for Au/mPEG350-SH. This fact implies that increasing the molar mass of mPEG-SH enhances the hydrophobic properties of the NPs.

’ CONCLUSIONS In this work we have demonstrated a facile one-phase aqueous synthesis of gold NPs stabilized by thiolated methoxy poly(ethylene glycol)s of different molar masses, which possess varied solubility. This approach allows obtaining stable highly concentrated gold colloids. The Au/mPEG-SH nanocrystals are compatible with media of different polarity. They maintain the characteristic plasmon band in various solvents. Variations of the synthetic parameters enable control of NP sizes. The gold colloids undergo a spontaneous triphase transfer owing to their inherent compatibility with media of different polarity. We assume that the synthetic method developed is applicable for the synthesis of other noble metal particles like palladium, platinum, or silver. ’ ASSOCIATED CONTENT

bS

Supporting Information. Absorption spectra of Au NPs stabilized by mPEG-SH of different molar weights in water and after their redissolving in toluene and chloroform; description of the mPEG-SH synthesis; results of DLS measurements of Au/mPEG350-SH NPs in different solvents. This material is available free of charge via the Internet at http://pubs.acs.org.

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’ AUTHOR INFORMATION Corresponding Author

*E-mail: [email protected].

’ ACKNOWLEDGMENT We gratefully acknowledge Christine Mickel (IFW Dresden e.V.) for assistance in the TEM imaging. This work was supported by the EU FP7 projects INNOVASOL and the NoE Nanophotonics4Energy. ’ REFERENCES (1) (a) Burda, C.; Chen, X.; Narayanan, R.; El-Sayed, M. A. Chem. Rev. 2005, 105, 1025–1102. (b) Daniel, M. C.; Astruc, D. Chem. Rev. 2004, 104, 293–346. (c) Murphy, C. J.; Sau, T. K.; Gole, A. M.; Orendorff, C. J.; Gao, J.; Gou, L.; Hunyadi, S. E.; Li, T. J. Phys. Chem. B 2005, 109, 13857–13870. (d) Grzelczak, M.; Perez-Juste, J.; Mulvaney, P.; Liz-Marzan, L. M. Chem. Soc. Rev. 2008, 37, 1783–1791. (e) Sau, T. K.; Rogach, A. L. Adv. Mater. 2010, 22, 1781–1804. (2) (a) Gittins, D. I.; Caruso, F. Angew. Chem., Int. Ed. 2001, 40, 3001–3004. (b) Gittins, D. I.; Caruso, F. ChemPhysChem 2002, 3, 110–113. 10227

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