Facile Phase Transfer of Large, Water-Soluble Metal Nanoparticles to

Article pubs.acs.org/Langmuir

Facile Phase Transfer of Large, Water-Soluble Metal Nanoparticles to Nonpolar Solvents Paul J. G. Goulet, Gilles R. Bourret, and R. Bruce Lennox* Department of Chemistry and Centre for Self-Assembled Chemical Structures (CSACS), McGill University, 801 Sherbrooke Street West, Montreal, Quebec, Canada H3A 2K6 S Supporting Information *

ABSTRACT: The facile phase-transfer of large, water-soluble metal nanoparticles to nonpolar solvent is reported here. Thiol-terminated polystyrene (PS-SH) is ligand-exchanged onto water-soluble metal nanoparticles in single-phase acetone/water mixtures, generating a precipitate. The solvent is then removed and the particles are redissolved in nonpolar solvent. This approach is demonstrated for nanoparticles of different metal (Au and Ag), size (3 to >100 nm), shape (spheres, rods, and wires, etc.), and leaving ligand (citrate, cetyltrimethylammonium bromide, poly(vinylpyrrolidone), and 4dimethylaminopyridine. The resulting PS-SH-stabilized nanoparticles maintain their initial size and shape, and are highly stable. They are soluble in various organic solvents (toluene, benzene, chloroform, dichloromethane, and tetrahydrofuran), and can be readily dried, purified, and re-dissolved. This method makes possible the utilization of a full range of existing nanoparticle cores in nonpolar solvents with a single ligand. It provides access to numerous nanomaterials that cannot be obtained through direct synthesis in nonpolar solvent, and is expected to be of significant value in a number of applications.

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INTRODUCTION Metallic nanoparticles of various core and ligand shell compositions, sizes, and shapes are now synthetically accessible.1−4 They are synthesized in both polar2,5−7 and nonpolar media8−12 and are employed toward an increasing number of applications in sensing, catalysis, therapeutics, labeling, diagnostics, and controlled release.3,4,13−17 Their application, however, is often hindered by a lack of particles with a particular combination of desired properties (i.e., chemical functionality, size, shape, and solubility, etc.). General strategies that permit the facile modification of particular nanoparticle properties are thus highly desirable. Ligand exchange is perhaps the most powerful of these, allowing alteration of the chemical functionality and solubility of nanoparticles, often without changes in their size and shape.18−20 Applications commonly require postsynthetic phase-transfer of nanoparticles between polar and nonpolar solvents, and this can often be accomplished through ligand substitution.21,22 Significant work has been directed toward the transfer of nanoparticles from nonpolar solvents to water for applications in biology and medicine.21,22 In particular, the development of strategies for the phase-transfer of semiconductor nanocrystals (quantum dots) into water has been crucial to their widespread use.23−29 Less work has been done, however, on the phasetransfer of nanoparticles from water to nonpolar solvents. Phase-transfer in this direction offers many significant advantages including: (1) numerous methods have been developed for the synthesis of water-soluble nanoparticles of © 2012 American Chemical Society

various sizes, shapes, and compositions that cannot be obtained through direct synthesis in nonpolar solvent; (2) the great majority of widely available and inexpensive metal precursors and reductants are water-soluble; (3) nanoparticle syntheses conducted in water do not rely on the use of phase-transfer agents (i.e., tetraoctylammonium bromide) that can cause significant complications in both syntheses and workup; (4) solution processing of nanoparticles in nonpolar solvent is typically much easier; (5) effective steric stabilization of nanoparticles in a nonpolar solvent often allows samples to be dried, purified, and redissolved, and used at very high concentrations in solution. Drying of samples also generally permits their long-term storage without significant degradation. The majority of methods that have been developed for phase-transfer of nanoparticles from water to nonpolar solvents have been included in two detailed reviews on this topic.21,22 Unfortunately, due to the poor extent of steric and electrostatic stabilization of large metallic nanoparticles by small molecule ligands in nonpolar solvent, most of these methods are applicable only to particles with diameters less than ca. 10 nm.20,22,30,31 Other methods are limited to nanoparticles with specific leaving ligands,20,22,31,32 or involve complex procedures that yield products with limited stability.33 Several groups have recently reported the phase-transfer of cetyltrimethylammonium bromide stabilized (CTAB-stabilized) Au nanorods to Received: October 3, 2011 Revised: January 9, 2012 Published: January 27, 2012 2909

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by adding NaBH4 (600 μL, 0.01 M) to a solution prepared by mixing HAuCl4 (250 μL, 0.01 M) and CTAB (9.75 mL, 0.1 M), and stirring for 2 min. A growth solution was prepared by mixing solutions of CTAB (9.5 mL, 0.1 M), AgNO3 (75 μL, 0.01 M), HAuCl4 (500 μL, 0.01 M), and ascorbic acid (55 μL, 0.1 M). The seed solution was allowed to sit for 2 h after the NaBH4 addition, and then 12 μL was added to the growth solution. This mixture was capped, inverted twice gently, and left at 25 °C overnight without stirring. Citrate-stabilized Ag nanoparticles were synthesized using the widely employed method of Lee and Meisel.46 An aqueous solution of AgNO3 (0.090 g in 500 mL) was brought to a boil, and 10 mL of a sodium citrate solution (1%) was added. This solution was refluxed for 1 h, and then allowed to cool. PVP-stabilized Ag nanoparticles were prepared following the method of Sun and Xia.47 Anhydrous ethylene glycol (15 mL) was heated at 160 °C for 1 h, and ethylene glycol solutions of AgNO3 (0.383 g in 9 mL) and PVP (Mw 55kD, 0.375 g in 9 mL) were added to it simultaneously at a rate of 0.375 mL/min. The reaction was maintained at 160 °C for 45 min and allowed to cool. This solution was then diluted with water to a total volume of 60 mL and centrifuged (6000 rpm, 15 min.). The pellet was redispersed in 60 mL of water. Phase-transfer of nanoparticles was accomplished by quickly adding 30 mL of an as-prepared aqueous metal nanoparticle solution to 100 mL of a stirring (ca. 600 rpm) acetone solution (5 mg/mL) of thiol-terminated polystyrene (PS-SH). Upon stirring of the single-phase mixture, ligand-exchange occurred and a sticky precipitate (close in color to the original nanoparticle solution) formed and was deposited on the magnetic stir bar and the walls of the round-bottom flask. When the color of the solution phase was completely lost, the acetone/water mix was decanted off and the PS-SH-stabilized nanoparticle precipitate was washed with acetone and water and readily redissolved in toluene. This toluene solution was washed further with water and dried under reduced pressure for storage. Further purification of PS-SH-stabilized nanoparticle samples was accomplished through repeated precipitationcentrifugation steps (6000 rpm, 15 min) in 30 mL dichloromethane (10%) and acetone (90%) mixtures. The supernatant was checked for free PS-SH by adding methanol and looking for signs of precipitation. Transmission electron microscopy (TEM) imaging was performed using a Philips CM200 transmission electron microscope. Samples were cast from solution onto 400 mesh carbon-coated Cu grids. Localized surface plasmon resonance (LSPR) spectra were collected using a Cary 5000 UV−visible− near-IR spectrophotometer. All 1H NMR spectra were obtained using a 400 MHz Varian Mercury solution NMR spectrometer. Samples were dissolved in CDCl3, except trisodium citrate which was dissolved in deuterium oxide. Infrared absorption spectra were collected using a Pike MIRacle ATR accessory interfaced with a PerkinElmer Spectrum BX FT-IR spectrometer. Raman scattering spectra were collected with a Horiba Jobin Yvon LabRAM HR Raman spectrometer using 633 nm laser excitation. Spectra were recorded from samples in the solid state.

organic solvents.34−37 Our group has extensively explored the properties and use of small (100 nm), shape (spheres, rods, and wires, etc.), and leaving ligand (citrate, CTAB, poly(vinylpyrrolidone) (PVP), and 4-dimethylaminopyridine (DMAP)). The resulting PS-SHstabilized nanoparticles preserve their initial size and shape, and are highly stable. They are soluble in various organic solvents, and can be easily dried, purified, and re-dissolved. This method permits the use of a wide range of existing nanoparticle cores in nonpolar solvents with a single ligand. It provides access to numerous nanomaterials that cannot be obtained through direct synthesis in nonpolar solvent. It is expected to be of great value for a variety of applications (i.e., catalysis, surfaceenhanced Raman scattering (SERS), and nanocomposites, etc.). In particular, there is strong potential for PS-SH to be employed in mixed ligand shells, or chemically tailored for added nanoparticle functionality.

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EXPERIMENTAL SECTION Unless otherwise noted, reagents were obtained from SigmaAldrich and used as received. Thiol-terminated polystyrene (155 units, Mn 16 200, PI 1.1) was prepared according to a previously published method.38,39 Citrate-stabilized Au nanoparticles were prepared following the method of Frens.6 Briefly, 50 mL of a 0.01% (by weight) aqueous HAuCl4 solution was brought to boiling and 300 μL of a 1% trisodium citrate solution was added. The solution was refluxed for 1 h, and then allowed to cool. DMAP-stabilized Au nanoparticles were prepared using the method of Gittins and Caruso,42 as described by Gandubert and Lennox.43 A toluene solution of tetraoctylammonium bromide (3.06 g in 100 mL) was stirred with an aqueous solution of hydrogen tetrachloroaurate (0.500 g in 40 mL) until the phase-transfer of the AuCl4− anion was complete. This was indicated by the aqueous phase becoming colorless and the toluene phase becoming orange. A freshly prepared aqueous solution of NaBH4 (0.525 g in 30 mL) was then added dropwise, generating a deep red color in the toluene phase. The solution was stirred for ca. 12 h, and the organic phase was extracted and diluted to a total volume of 250 mL. This solution was then stirred with an aqueous solution of 4-(dimethylamino)pyridine (3.05 g in 250 mL) until phase-transfer of the nanoparticles to the aqueous phase was complete. CTAB-stabilized Au nanorods were prepared according to the Ag(I)-assisted growth method, as described by Smith et al.44,45 A gold seed solution was prepared

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RESULTS AND DISCUSSION Scheme 1 depicts the general phase-transfer strategy employed in this work. As-prepared aqueous metal nanoparticle solution was added quickly to a stirring acetone solution of thiolterminated polystyrene (PS-SH). Upon stirring of the single2910

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Scheme 1. Phase-Transfer of Water-Soluble Metal Nanoparticles to Nonpolar Solvent via Ligand Exchange with Thiol-Terminated Polystyrene in Acetone/Water

phase mixture, ligand-exchange occurred and a sticky precipitate (close in color to the original nanoparticle solution) quickly formed and was deposited on the magnetic stir bar and the walls of the round-bottom flask. The acetone/water mix was then decanted off and the PS-SH-stabilized nanoparticle precipitate was washed with acetone and water and readily redissolved in toluene. No signs of irreversible particle aggregation were observed for this process. When this approach was attempted with dodecanethiol substituted for PS-SH, however, irreversible particle aggregation (yielding insoluble precipitates) was observed. We attribute this to the inferior steric stabilization provided by the alkanethiol relative to the much larger PS-SH polymer.30 Also, when two-phase (toluene/ water) phase transfer is attempted with this ligand, a stable emulsion is formed and amphiphilic nanoparticles are generated through incomplete ligand exchange.41 These amphiphilic particles are not soluble in nonpolar solvents. The phase-transfer procedure used here was demonstrated for several representative nanoparticle samples of different metal, size, shape, and leaving ligand (Figure 1). These were prepared according to widely employed literature syntheses and include: citrate-stabilized Au (spheres);6 DMAP-stabilized Au (spheres);42 CTAB-stabilized Au (nanorods);44,45 citratestabilized Ag (multiple shapes);46 and PVP-stabilized Ag (multiple shapes).47 Once transferred to toluene, PS-SHstabilized nanoparticle samples demonstrated remarkable solution stability (typically far greater than the original particles in water), and could be purified by repeated precipitationcentrifugation steps. Purity and ligand exchange were confirmed by 1H NMR (Figure S1 of the Supporting Information), infrared absorption (Figure S2 of the Supporting Information), and Raman scattering (Figure S3 of the Supporting Information) spectra. With all three techniques, the PS-SH-stabilized nanoparticle samples show spectral features consistent with PS-SH, and show no sign of the presence of the outgoing ligands. Raman scattering from these plasmonic nanoparticle samples is more properly termed surface-enhanced Raman scattering (SERS), and as expected, the small Au particles from DMAP (ca. 6 nm) provide very weak electromagnetic enhancement relative to the larger nanoparticle samples.15 The assumed metal−sulfur stretches could not be conclusively assigned for these samples. The intense, broad peaks at 234 cm−1 in the spectra collected from the Ag nanoparticle samples can be attributed to Ag−O stretches that are commonly observed in SERS.48 PS-SH-

Figure 1. Images of (a) citrate-stabilized Au, (b) DMAP-stabilized Au, (c) CTAB-stabilized Au, (d) citrate-stabilized Ag, and (e) PVPstabilized Ag nanoparticle samples in water (left vial in all pictures) and corresponding PS-SH-stabilized nanoparticle samples in toluene (right vial in all pictures).

stabilized nanoparticle samples could also be dried completely under reduced pressure and redissolved, allowing them to be stored indefinitely without noticeable change. All samples demonstrated excellent solubility in toluene, benzene, chloroform, dichloromethane, and THF (Figures S4−S8 of the Supporting Information). Figure 2 shows representative TEM images of each of the nanoparticle samples before and after phase-transfer (i.e., in water and in toluene). TEM image analysis reveals that the initial size and shape of the nanoparticles is maintained throughout the phase-transfer process. The citrate- and DMAPstabilized Au spheres had average diameters of ca. 30 and 6 nm, respectively. The CTAB-stabilized Au nanorods had an average aspect ratio of ca. 4 with a mean length of ca. 40 nm and a mean width of ca. 10 nm. The citrate- and PVP-stabilized Ag nanoparticles had broad size and shape distributions. Spheres, wires, cubes, rods, and plates were all observed in these samples, with sizes ranging from 10s of nm to micrometers. While other approaches to phase transfer are limited to particles of very small size,21,22,31 it is clear that nanostructures of virtually any size or shape can be stabilized by PS-SH using this single-phase method. Figure 3 shows localized surface plasmon resonance spectra, in water and in toluene, for each of the five metal nanoparticle cores studied here. In each case, the LSPR maxima are redshifted in toluene due to its greater refractive index. LSPR shifts with changes in the refractive index of the surrounding medium are well-known and have been documented in several publications.21,34,49 The citrate- and DMAP-stabilized Au spheres show single LSPR peaks at 525 and 523 nm, respectively, in water. These peaks are shifted to 540 and 530 nm, respectively, in toluene. The CTAB-stabilized Au nanorods in water have transverse and longitudinal LSPR modes at 525 and 759 nm, respectively. These peaks are shifted to 537 and 801 nm upon phase transfer to toluene. The citrate- and PVPstabilized Ag nanoparticle samples showed very broad LSPR extinction throughout the visible due to their polydispersity. The citrate-stabilized sample showed maxima at 347, 383, 427, 2911

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Figure 3. Localized surface-plasmon resonance (LSPR) spectra of (a) citrate-stabilized Au, (b) DMAP-stabilized Au, (c) CTAB-stabilized Au, (d) citrate-stabilized Ag, and (e) PVP-stabilized Ag nanoparticle samples in water (top/blue spectra) and corresponding PS-SHstabilized nanoparticle samples in toluene (bottom/red spectra). Spectra are offset for clarity.

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CONCLUSIONS A general strategy for the facile phase-transfer of large, watersoluble metal nanoparticles to nonpolar solvent was presented here. Thiol-terminated polystyrene (PS-SH) was ligandexchanged onto water-soluble metal nanoparticles in singlephase acetone/water mixtures, generating a hydrophobic precipitate. This precipitate could then be redissolved in nonpolar solvent. This approach was demonstrated for nanoparticles of different metal, size, shape, and leaving ligand. The resulting PS-SH-stabilized nanoparticles maintained their initial size and shape, and were highly stable. They could be dissolved in various organic solvents and readily dried, purified, and redissolved. This method will allow a full range of interesting nanoparticle cores to be utilized in nonpolar solvents.

Figure 2. TEM images of (a) citrate-stabilized Au, (b) DMAPstabilized Au, (c) CTAB-stabilized Au, (d) citrate-stabilized Ag, and (e) PVP-stabilized Ag nanoparticle samples from water (images on left) and corresponding PS-SH-stabilized nanoparticle samples from toluene (images on right).

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ASSOCIATED CONTENT

S Supporting Information *

Figures showing 1H NMR, infrared absorption, Raman scattering, and UV−visible absorption data. This material is available free of charge via the Internet at http://pubs.acs.org.

and 509 nm in water that shift to 355, 401, 461, and 574 nm, respectively, in toluene. The PVP-stabilized sample had maxima at 350 and 437 nm in water that shifted to 365 and 459 nm upon phase transfer of the nanoparticles to toluene. Small increases in the full width at half-maximum (fwhm) of the LSPR peaks were observed upon phase transfer of the nanoparticles to toluene. However, the overall band shape was maintained, and no tell-tale signs of irreversible aggregation were observed (in either extinction or TEM images). These minimal changes can therefore be attributed to increased LSPR damping arising from the strong thiol adsorption of PS-SH.20,50

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AUTHOR INFORMATION

Corresponding Author

*E-mail: [email protected].

ACKNOWLEDGMENTS We thank Dr. David Liu for TEM imaging and Dr. Muriel Corbierre for synthesis of thiol-terminated polystyrene. The Natural Sciences and Engineering Research Council of Canada (NSERC) and the Centre for Self-Assembled Chemical 2912

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Structures (CSACS) are gratefully acknowledged for financial support.

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