Charged Gold Nanoparticles in Non-Polar Solvents: 10-min

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Charged Gold Nanoparticles in Non-Polar Solvents: 10-min Synthesis and 2D Self-Assembly Matthew N. Martin, James I. Basham,† Paul Chando, and Sang-Kee Eah* Department of Physics, Applied Physics, and Astronomy, Rensselaer Polytechnic Institute, Troy, New York 12180, USA. †Present address: Department of Electrical Engineering, The Pennsylvania State University, University Park, PA 16802. Received June 14, 2009. Revised Manuscript Received March 31, 2010 We report a fast and highly reproducible chemical synthesis method for colloidal gold nanoparticles which are negatively charged in nonpolar solvents and coated with hydrophobic organic molecules. If a hexane droplet containing charged gold nanoparticles is mixed with a larger toluene droplet, nanoparticles immediately float to the air-toluene interface and form a close-packed monolayer film. After evaporation of the solvent molecules, the monolayer film of nanoparticles can be deposited to any substrate without any limit in size. The synthesis does not require a postsynthesis cleaning step, since the two immiscible liquid phases separate the reaction byproducts from gold nanoparticles and a minimal amount of coating molecules is used.

Introduction Organic molecule-coatings make nanoparticles stable and dispersible in nonaqueous media via steric repulsion. Unlike the irreversible aggregation of electrostatically stabilized nanoparticles in water, alkanethiolate-coated gold nanoparticles are so stable that they can be dried into powder and redispersed into solvents without change in size.1 This high stability enables postsynthesis size filtering for obtaining monodisperse nanoparticles from polydisperse samples in order to study their sizedependent properties.2,3 Uniform thin films of close-packed nanoparticles with organic ligand molecules can be fabricated for applications such as nanoelectronics,4 light-emitting diodes,5 solar cells,6 and magnetic data storage.7 Langmuir-Blodgett (LB) deposition is typically used for two-dimensional (2D) selfassembly of nanoparticles coated with hydrophobic organic ligands into a close-packed monolayer film at the air-water interface.8,9 By using a Teflon disk with a hole, lateral compression in the LB method can be avoided for faster monolayer formation.10,11 Spin coating is another method for flat substrates, but proves difficult to achieve monolayer films with, usually generating multilayer films of nanoparticles.7 Recently, a very simple method was reported for 2D self-assembly of alkanethiolatecoated gold nanoparticles at the air-toluene interface of an *Corresponding author. E-mail: [email protected]. (1) Brust, M.; Walker, M.; Bethell, D.; Schiffrin, D. J.; Whyman, R. J. Chem. Soc., Chem. Commun. 1994, 801–802. (2) Murray, C. B.; Norris, D. J.; Bawendi, M. G. J. Am. Chem. Soc. 1993, 115, 8706–8715. (3) Hicks, J. F.; Templeton, A. C.; Chen, S. W.; Sheran, K. M.; Jasti, R.; Murray, R. W.; Debord, J.; Schaaf, T. G.; Whetten, R. L. Anal. Chem. 1999, 71, 3703–3711. (4) Tseng, R. J.; Huang, J.; Ouyang, J.; Kaner, R. B.; Yang, Y. Nano Lett. 2005, 5, 1077–1080. (5) Coe, S.; Woo, W.-K.; Bawendi, M.; Bulovic, V. Nature 2002, 420, 800–803. (6) Oregan, B.; Gratzel, M. Nature 1991, 353, 737–740. (7) Sun, S. H.; Murray, C. B.; Weller, D.; Folks, L.; Moser, A. Science 2000, 287, 1989–1992. (8) Heath, J. R.; Knobler, C. M.; Leff, D. V. J. Phys. Chem. B 1997, 101, 189–197. (9) Schultz, D. G.; Lin, X. M.; Li, D. X.; Gebhardt, J.; Meron, M.; Viccaro, P. J.; Lin, B. H. J. Phys. Chem. B 2006, 110, 24522–24529. (10) Santhanam, V.; Liu, J.; Agarwal, R.; Andres, R. P. Langmuir 2003, 19, 7881–7887. (11) Santhanam, V.; Andres, R. P. Nano Lett. 2004, 4, 41–44.

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evaporating toluene droplet, generating a high quality monolayer film.12-14 After evaporation of toluene, a monolayer film of closepacked nanoparticles can be deposited to any substrate without limit in size, since a toluene droplet can be very large, even covering the whole surface of 3 in. silicon wafer, or very small such as ∼10 μm: the size of droplets used by an inkjet printer. In addition to its simplicity, this toluene method enables the fabrication of monolayer films of closed-packed nanoparticles which are spatially uniform at any length scale without complex patterns at the micrometer scale.15,16 Excess 1-dodecanethiol (DDT) was proposed as the key property for the toluene droplet method,14 though its detailed 2D self-assembly mechanism is not yet fully understood. Simply adding excess ligands does not make coated nanoparticles of other materials, such as semiconducting and magnetic, float to the air-liquid interface of a toluene droplet. Even for alkanethiolate-coated gold nanoparticles, the detailed synthesis steps12 modified from the Brust method1 must be followed strictly to ensure the 2D self-assembly property. We hypothesize that controlling properties of nanoparticles during synthesis is more essential for 2D self-assembly at the air-toluene interface, rather than adjusting postsynthesis parameters such as the amount of excess ligands, the concentration of nanoparticles, and the evaporation rate of the solvent. The Brust synthesis method and its many variations for alkanethiolate-coated gold nanoparticles use the surfactant, tetraoctylammonium bromide (TOAB) to phase-transfer gold ions from water to toluene, where they are reduced to neutral atoms and form nanoparticles coated with hydrophobic alkanethiol molecules.1,3,8,17,18 The complete nanoparticle formation in toluene is slow, taking a few hours (12) Lin, X. M.; Jaeger, H. M.; Sorensen, C. M.; Klabunde, K. J. J. Phys. Chem. B 2001, 105, 3353–3357. (13) Narayanan, S.; Wang, J.; Lin, X. M. Phys. Rev. Lett. 2004, 93, 135503. (14) Bigioni, T. P.; Lin, X. M.; Nguyen, T. T.; Corwin, E. I.; Witten, T. A.; Jaeger, H. M. Nat. Mater. 2006, 5, 265–270. (15) Tang, J.; Ge, G. L.; Brus, L. E. J. Phys. Chem. B 2002, 106, 5653–5658. (16) Rabani, E.; Reichman, D. R.; Geissler, P. L.; Brus, L. E. Nature 2003, 426, 271–274. (17) Lin, X. M.; Sorensen, C. M.; Klabunde, K. J. J. Nanopart. Res. 2000, 2, 157–164. (18) Shimizu, T.; Teranishi, T.; Hasegawa, S.; Miyake, M. J. Phys. Chem. B 2003, 107, 2719–2724.

Published on Web 04/14/2010

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Figure 1. Gold nanoparticles are (a) made in water, (b) mixed with acetone, and (c) phase-transferred to hexane with DDT molecules simply by shaking the vial. The whole synthesis takes less than 10 min and does not need a cleaning step, since all reaction byproducts remain in the water-acetone phase.

even with a strong reducing chemical like borohydride. In addition, TOAB and other reaction byproducts must be cleaned and removed after synthesis. TOAB turned out to be exceedingly difficult to remove completely.19 In order to exploit the 2D self-assembly property in a toluene droplet, we have developed a fast synthesis recipe that can be repeated many times, in order to reproducibly and precisely control all of the parameters. While gold ions and large gold nanoparticles in water are difficult to be phase-transferred to a nonpolar organic solvent, it is known that small gold nanoparticles can move across the solvent boundary after being coated with hydrophobic organic ligands.20,21 Therefore, we have studied the synthesis of gold nanoparticles using the same set of chemicals in the Brust method except TOAB, by making gold nanoparticles in water and then phase-transferring them to nonpolar organic solvents, instead of phase-transferring gold ions. In addition, we aimed to synthesize gold nanoparticles that can float to the air-liquid interface of a toluene droplet for 2D self-assembly. Other existing phasetransfer methods20-23 for coating nanoparticles in the water phase and extracting them to an organic solvent immiscible with water, do not provide nanoparticles with the subtle difference in solubility between toluene and hexane, and therefore a toluene droplet method cannot be used for their 2D self-assembly. Here we report a TOAB-free synthesis method for dodecanethiolate-protected gold nanoparticles, which takes less than 10 min and does not require a cleaning step. Gold nanoparticles are made in water by borohydride reduction and phase-transferred to hexane (Figure 1). The phase transfer is done with the help of acetone instead of a surfactant simply by vigorous handshaking for 30 s, during which gold nanoparticles are coated with hydrophobic organic ligands. We discovered a “sweet zone” for making nearly monodisperse gold nanoparticles and precisely controlling their diameter from 3.2 to 5.2 nm in water, without stabilizing molecules, which turned out to be critical for the phasetransfer of nanoparticles. These gold nanoparticles are dispersible in hexane, but indispersible in toluene, and self-assemble into a monolayer film of closed-packed nanoparticles at the air-liquid interface of a toluene droplet. We found out that being negatively charged in nonpolar solvents is the key property required for gold nanoparticles to float to the air-toluene interface.

Experimental Section Materials. The following chemicals were used as obtained: HAuCl4 3 3H2O, NaBH4 granules, 1.0 M HCl solution, 1.0 M (19) Waters, C. A.; Mills, A. J.; Johnson, K. A.; Schiffrin, D. J. Chem Commun. 2003, 4, 540–541. (20) Jana, N. R.; Gearheart, L.; Murphy, C. J. Langmuir 2001, 17, 6782–6786. (21) Motte, L.; Pileni, M. P. J. Phys. Chem. B 1998, 102, 4104–4109. (22) Meldrum, F. C.; Kotov, N. A.; Fendler, J. H. Langmuir 1994, 10, 2035–2040. (23) Underwood, S.; Mulvaney, P. Langmuir, 1994, 10, 3427-3430.

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NaOH solution, and 1-dodecanethiol from Sigma-Aldrich; acetone, hexane, and toluene from VWR; 50 nm thick SiN membrane TEM apertures from Ted Pella. Deionized (DI) water with a resistivity of 18.2 MΩcm was used. 40 mL KG-33 borosilicate glass vials from Kimble-Kontes were used after cleaning with DI water, acetone, hexane, and being heated at 540 °C. No rigorous cleaning material such as aqua regia or a hydrofluoric/nitric acid mixture was used for glass wares. Synthesis of Nanoparticles in Water. An aqueous stock solution of 50 mM gold chloride anions (AuCl4-) in a glass vial was made by dissolving HAuCl4 3 3H2O with the same molar amount of HCl, ensuring stability for more than several months. An aqueous stock solution of 50 mM borohydride anions (BH4-) in a glass beaker was made by dissolving NaBH4 granules with the same molar amount of NaOH, guaranteeing stability for several hours at room temperature. For the smallest nanoparticles of 3.2 nm in diameter, we added 100 μL of the AuCl4-/Hþ solution to a glass vial with water and later injected 300 μL of the BH4-/ OH- solution all at once, while stirring on a mechanical shaker for uniform mixing. The total weight of the aqueous solution was controlled to be 10 g so that the concentration of gold ions is 0.50 mM. The solution changed color from light yellow to orange immediately, and then to red while the vial was stirred for 1 min to release hydrogen gas molecules. For nanoparticles of other sizes, the amount of the BH4-/OH- solution was increased from 300 to 650 μL followed by heating for 2-3 min at the boiling temperature of water on a hot plate. The average diameter of gold nanoparticles was precisely controlled from 3.2 to 5.2 nm. The amount of the BH4-/OH- solution was changed from 200 to 1300 μL during the search for the “sweet zone” before heating. Phase Transfer of Nanoparticles to Hexane. 5.0 g acetone was added to the 10 g aqueous solution of gold nanoparticles and mixed by hand for 1 s, immediately after which 5.0 g of hexane with 1.0 or 0.5 μmol of DDT was added, and then the vial was shaken vigorously by hand for 30 s. The water-acetone phase became colorless, and the hexane phase turned dark red due to the phase-transfer of gold nanoparticles. No postsynthesis cleaning step was employed, since all reaction byproducts remain in the water-acetone phase. We verified that gold nanoparticles in hexane are stably dispersed for longer than several months. 2D Self-Assembly of Nanoparticles. A hexane droplet containing gold nanoparticles was deposited to a larger toluene droplet on a piece of Teflon tape. Hexane evaporates ∼4 times faster than toluene. With a 5 mm diameter toluene droplet, the toluene evaporation and deposition of a monolayer film of nanoparticles took ∼5 min in a fume hood. The interior of the hexanetoluene droplet became colorless immediately and the surface became purple, if the volume ratio of toluene to hexane was larger than 3. If the volume ratio of toluene to hexane is smaller, the floating of nanoparticles to the air-liquid interface takes slightly longer. If a hexane droplet containing gold nanoparticles was directly put on a piece of Teflon tape, its color remained red throughout the evaporation, indicating that gold nanoparticles stayed within the shrinking hexane droplet. Physical Measurements. Ultraviolet-visible (UV-vis) extinction spectra were obtained on gold nanoparticles in water in a 5 mm thick cuvette utilizing a Shimadzu double-beam UV-vis spectrophotometer. No software normalization was used. Instead the same concentration of gold ions, 0.50 mM was used for normalization. A spectrum with only DI water was used for baseline correction. A water droplet, a toluene droplet and a hexane droplet with gold nanoparticles were put on TEM apertures to fabricate monolayer films of close-packed nanoparticles, which were examined in the transmission mode of a field-emission scanning electron microscope (FE-SEM) at the acceleration voltage of 30 kV. For a toluene droplet with gold nanoparticles at the air-liquid interface between two copper plates, an electric field of 100 V/mm was applied. Movies were taken with a consumer digital camera (Canon). DOI: 10.1021/la100591h

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Results and Discussion Stabilizer-Free Gold Nanoparticles in Water. Neutral gold atoms reduced from gold ions in water meet each other and form nanoparticles that are stable via electrostatic repulsion due to adsorbed anions. In nanoparticle synthesis, it is challenging to simultaneously tune the size and guarantee a monodisperse size distribution. Gold nanoparticles made by using citric acid as both a weak (slow reaction for 30-60 min while boiling the solution) reducer and a stabilizer, were first reported by Turkevich et al.24 and refined by Frens25 at a diameter range of 20-40 nm, which is far too large for phase-transfer to nonpolar solvents. For fast formation of gold nanoparticles at room temperature within a few seconds, strong reducers are used: alkaline tetrakis(hydroxymethyl)phosphonium chloride (THPC)26 for diameters 50 nm with the solution’s color varying among brown, orange, red, and purple.20,28-32 Solutions of borohydride freshly prepared and/or cooled down in an ice bath are used to slow down the reaction rate of borohydride with water molecules and therefore increase the reproducibility. The much smaller size of borohydride compared with other molecules might be thought to be responsible for poor stability of gold nanoparticles in water. We found out that adding the same molar amount of NaOH to the aqueous solution of borohydride keeps the reducing solution’s reactivity steady for more than several hours at room temperature: an important point for high reproducibility. Similarly we added the same molar amount of HCl to the gold chloride stock solution, ensuring stability for several months. Since each borohydride anion has four reactive hydride arms and the gold ion’s charge state is þ3, three borohydride anions may reduce four gold ions to neutral gold atoms. However, the actual stoichiometry is not that simple due to the reactions of borohydride anions and gold chloride ions with protons and hydroxide anions,33 which we added to the two aqueous stock solutions for stability and reproducibility. In addition, excess boron-based anions are required as stabilizing anions for gold nanoparticles. Experimentally, we discovered a “sweet zone” to obtain nearly monodisperse gold nanoparticles using only borohydride for reducing gold ions and stabilizing nanoparticles. We found out that the ratio of BH4-/OH- ions to AuCl4-/Hþ ions can be adjusted carefully to make nearly monodisperse gold nanoparticles in water as shown in Figure 2. Below the low boundary of 300% and above the high boundary of 1200%, the UV-vis spectra of gold nanoparticles in water have significant extinction around 700 nm, which indicates the existence of large gold nanoparticles, and therefore the size distribution is polydisperse. Within the “sweet zone”, between 300 and 1200%, extinction below ∼450 nm and above ∼600 nm is almost identical, (24) Turkevich, J.; Hillier, J.; Stevenson, P. C. Discuss. Faraday Soc. 1951, 11, 55–75. (25) Frens, G. Nature (London), Phys. Sci 1973, 241, 20–22. (26) Duff, D. G.; Baiker, A.; Edwards, P. P. Langmuir 1993, 9, 2301–2309. (27) Andreescu, D.; Sau, T. K.; Goia, D. V. J. Colloid Interface Sci. 2006, 298, 742–751. (28) Creighton, J. A.; Blatchford, C. G.; Albrecht, M. G. J. Chem. Soc., Faraday Trans. 2 1979, 75, 790–798. (29) Tschopp, T.; Podack, E. R.; M€uller-Eberhard, H. J. Proc. Natl. Acad. Sci. U.S.A. 1982, 79, 7474–7478. (30) Birrell, G. B.; Hedberg, K. K.; Griffith, O. H. J. Histochem. Cytochem. 1987, 35, 843–853. (31) Handley, D. A. In Colloidal Gold: Principles, Methods, and Applications; Hayat, M. A., Ed.; Academic Press: San Diego, CA, 1989; Chapter 2. (32) Patil, V.; Malvankar, R. B.; Sastry, M. Langmuir 1999, 15, 8197–8206. (33) Goia, D. V.; Matijevic, E. Colloids Surf. A 1999, 146, 139–152.

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Figure 2. Nearly monodisperse gold nanoparticles in water can be obtained by adjusting the ratio of BH4-/OH- ions to AuCl4-/Hþ ions in the range from 300 to 1200%.

indicating that nearly monodisperse gold nanoparticles are made. If the extinction at ∼400 nm is verified to be linearly proportional to the concentration of gold atoms, it can be used to normalize UV-vis spectra for solutions with different concentrations of gold atoms. Note that we did not perform any software data normalization; instead, the concentration of gold ions was fixed to 0.50 mM during the synthesis. Gold nanoparticles in water, without any stabilizing chemicals, are stabilized by physisorbed boron-based anions, whose electrostatic repulsion energy decreases as the concentration of the screening ions increases. This partially explains why the size of nanoparticles and the plasmon peak around ∼510 nm become larger with more BH4-/OH- ions in the “sweet zone”. We suspect that the “sweet zone” may be different at other concentrations of gold ions. As a strong reducing chemical, borohydride can reduce all gold ions to neutral gold atoms in less than 1 s at room temperature. The immediate color change from light yellow to orange indicates that all the gold ions are reduced to neutral gold atoms virtually simultaneously (fast nucleation). Small gold nanoparticles grow into larger ones by sintering (slow growth), changing the solution’s color to red. Very quick reduction of all the gold ions may be responsible for the nearly monodisperse size distribution. Since gold nanoparticles initially grow rapidly and hydrogen gas bubbles change light extinction significantly, we kept shaking the glass vial for 1 min to reach the slow growth stage and release hydrogen molecules. This 1 min step has significantly increased the reproducibility of the UV-vis spectra and therefore the size of nanoparticles. One minute shaking while exposed to aerial oxygen molecules might be important for oxidation of borohydride anions catalyzed by small gold nanoparticles34 which slows down as gold nanoparticles become larger than 3 nm in diameter.35 The discovery of the “sweet zone” and the stabilization of stock solutions with HCl and NaOH are both very important for reproducibility and optimizing the synthesis parameters. We note that stabilizer-free gold nanoparticles in water produced by this method are stable for more than 1 year. Size-tuning of nearly monodisperse gold nanoparticles. After establishing a fast and reproducible synthesis method for nearly monodisperse nanoparticles in water, we were able to tune the size of gold nanoparticles by changing the amount of the BH4-/OH- solution. To speed up the slow growth stage, we heated aqueous solutions of gold nanoparticles for 2-3 min at the boiling temperature of water. Figure 3 shows UV-vis extinction spectra for gold nanoparticles in water with average diameters (34) Brown, H. C.; Brown, C. A. J. Am. Chem. Soc. 1962, 84, 1493–1494. (35) Bond, G. C.; Louis, C.; Thompson, D. T. Catalysis by Gold; Imperial College Press: London, 2006.

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Figure 3. UV-vis extinction spectra of 5 aqueous solutions of gold nanoparticles with average diameters of 3.2, 4.0, 4.8, 4.2, and 5.2 nm. The numbers of total gold ions are the same without data normalization.

between 3.2 and 5.2 nm. For gold nanoparticles of 4.0 nm average diameter, gold atoms were reduced by 300 μL of the BH4-/OHsolution. After 1 min shaking, the vial was heated for 100 s. For gold nanoparticles of 4.8, 4.2, and 5.2 nm average diameters, 425, 575, and 650 μL of the BH4-/OH- solution were used, respectively, and heated for 180 s. The heated vials were cooled before taking UV-vis extinction spectra and the phase-transfer to hexane. From one batch to another, we could obtain the same set of UV-vis spectra reproducibly just by slightly adjusting the amount of reducing solution to compensate for aging and concentration fluctuation of the BH4-/OH- solution. UV-vis spectroscopy is a rapid and nondestructive method to measure and control the size of nanoparticles precisely and reproducibly, and is necessary, since the color change of nanoparticles in solutions cannot be noticed by eye at this size range (see Supporting Information). Recently it was reported that the diameter of gold nanoparticles made with citric acid can be controlled between 20 and 40 nm by controlling the pH from 6.5 to 7.4.36 In this paper the range of pH is much smaller, and the size of gold nanoparticles is controlled by three factors: the amount of BH4-, the amount of OH-, and the heating time. We note that the concentration of gold ions was fixed to 0.50 mM in this study. At other concentrations, all the parameters are expected to change, including the “sweet zone”. Therefore, the reduction stoichiometry of gold ions and the formation of gold nanoparticles in water is quite complex, for which a quantitative understanding is under progress in a separate study. Figure 4 shows micrographs of gold nanoparticles coated with DDT molecules after phase-transfer, on 50 nm thick SiN membranes by FE-SEM using a transmission mode detector. The gold nanoparticles of 3.2, 4.0, and 4.8 nm average diameters are more uniform in size than those of 4.2 and 5.2 nm average diameters (see Supporting Information for size analysis and whole micrographs) perhaps due to the sintering growth mechanism of small gold nanoparticles into larger ones in water. Higher monodispersity of nanoparticles can be obtained by a size-selective filtering step, which is time-consuming.2,3 We note that the size uniformity of these gold nanoparticles is similar to that of nearly monodisperse gold nanoparticles37 and ones after size-filtering with supersaturated CO.38 More spatially uniform mixing of the

(36) Ji, X.; Song, X.; Li, J.; Bai, Y.; Yang, W.; Peng, X. J. Am. Chem. Soc. 2007, 129, 13939–13948. (37) Jana, N. R.; Peng, X. G. J. Am. Chem. Soc. 2003, 125, 14280–14281. (38) McLeod, M. C.; Anand, M.; Kitchens, C. L.; Roberts, C. B. Nano Lett. 2005, 5, 461–465.

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Figure 4. FE-SEM micrographs of gold nanoparticles coated with DDT molecules with average diameters of (A) 3.2, (B) 4.0, (C) 4.8, (D) 4.2, and (E) 5.2 nm.

two aqueous solutions like in a microfluidic device may generate more monodisperse gold nanoparticles.39,40 Phase Transfer of Gold Nanoparticles from Water to Hexane with Acetone. The phase transfer of gold nanoparticles from water to hexane strongly depends on the size of nanoparticles.20,21 After the phase transfer, the water-acetone phase becomes colorless, indicating that all of the gold nanoparticles are removed. Without acetone, a majority of gold nanoparticles either remain in the water phase or are trapped at the hexanewater interface. Even with acetone, more gold nanoparticles are trapped at the interface between hexane and water-acetone as the size increases, perhaps due to partial coating of DDT molecules. While both methanol and ethanol destabilize gold nanoparticles in water, changing the color immediately, destabilization by acetone is much slower, confirmed by little change in the UV-vis extinction spectrum over several minutes. Previously it was reported that acetone helps the phase transfer of nanoparticles from water to neat DDT by improving the mixing of the liquids.41,42 In contrast, we used very small amounts of DDT molecules for the phase transfer so that the amount of free excess DDT molecules, not on the surface of gold nanoparticles, can be minimized without a cleaning step. The molar ratio of total DDT molecules to gold atoms was only 20% for the 3.2 nm gold nanoparticles and 10% for others. FE-SEM in both transmission and reflection modes is very sensitive to the amount of nonconducting organic molecules which hinder gold nanoparticles at this size range from being clearly resolved due to charging. In the more sensitive reflection mode we found out that it is extremely challenging to resolve these small gold nanoparticles, if we use 100% DDT. Also, we observed that more DDT molecules did not increase (decrease) the amount of gold nanoparticles dispersible in the hexane phase (trapped at the liquid-liquid interface and the wall of the glass vial). Another big difference from previous reports on the phase transfer of nanoparticles is the very brief shaking (30 s) and separation (a few minutes), while it takes several hours in some cases.20-22 Efficient phase-transfer of nanoparticles must depend on many parameters such as the size of nanoparticles, the amounts of the three liquids, the concentrations of the ions in (39) Wagner, J.; K€ohler, J. M. Nano Lett. 2005, 5, 685–691. (40) Tsunoyama, H.; Ichikuni, N.; Tsukuda, T. Langmuir 2008, 24, 11327– 11330. (41) Gaponik, N.; Talapin, D. V.; Rogach, A. L.; Eychm€uller, A.; Weller, H. Nano Lett. 2002, 2, 803–806. (42) Qian, H.; Zhu, M.; Andersen, U. N.; Jin, R. J. Phys. Chem. A 2009, 113, 4281–4284.

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Figure 5. If a hexane droplet containing charged gold nanoparticles is mixed with a larger toluene droplet, nanoparticles immediately float to the air-liquid interface and form a monolayer film. After evaporation of the solvent molecules, the monolayer of closepacked nanoparticles can be deposited to any substrate. Charged gold nanoparticles in a hexane droplet do not form a monolayer film.

water, the chain length of alkanethiols, temperature, and mechanical shaking strength, whose optimization is a complex task and not fully achieved yet. We speculate that not using any stabilizers except borohydride and hydroxide might be responsible for such efficient phase transfer in addition to the precise size control of nanoparticles. Fast phase transfer of noble metal nanoparticles made by THPC from water to toluene was also achieved by adding acid during shaking.43 We used hexane instead of toluene, because charged gold nanoparticles are indispersible in toluene. Similarly, we used acetone instead of acid to facilitate phase transfer, since gold nanoparticles must retain some negative charges for 2D self-assembly at the air-liquid interface of a toluene droplet. 2D Self-Assembly of Nanoparticles in a Toluene Droplet. As Figure 5 shows, we observed that gold nanoparticles are stably dispersed in hexane but indispersible in toluene. While gold nanoparticles in a hexane droplet do not form a monolayer film, gold nanoparticles in a hexane-toluene droplet float to the air-liquid interface immediately and self-assemble into a 2D monolayer film (see Movie S1). If a hexane droplet is mixed with a smaller toluene droplet, the gold nanoparticles slowly float to the air-liquid interface until there are more toluene molecules than hexane molecules, while hexane evaporates ∼4 times faster than toluene. At the air-toluene interface, the gold nanoparticles move almost freely and are easily pushed by air molecules (see Movie S2). The color of gold nanoparticles strongly depends on the interparticle distance. Inside a hexane droplet, the interparticle distance is larger than the diameter of the nanoparticles and the color is red. At the air-toluene interface, the color becomes purple due to close-packing. After evaporation of the solvent molecules, the monolayer film of gold nanoparticles can be deposited to any substrate with no limit in size and without any sophisticated instruments. Unlike the LB method we do not even need a lateral compressor, since the shrinking toluene droplet automatically forces the nanoparticles to form a close-packed monolayer film. We even fabricated a spatially uniform monolayer film of gold nanoparticles covering the whole surface of a 3 in. silicon wafer. (43) Sarathy, K. V.; Raina, G.; Yadav, R. T.; Kulkarni, G. U.; Rao, C. N. R. J. Phys. Chem. B 1997, 101, 9876–9880.

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Figure 6. Photographs of gold nanoparticles floating at the airtoluene interface (top) and forming 2D monolayer films on a solid substrate (bottom). Smaller gold nanoparticles cannot keep the monolayer formation after evaporation of toluene molecules.

Like the size-dependent phase transfer of nanoparticles, the 2D self-assembly property of the gold nanoparticles strongly depends on their size. As shown in Figure 6, smaller gold nanoparticles of 3.2 and 4.0 nm average diameter form a monolayer film at the air-toluene interface. However, their monolayer films collapse on a solid substrate after toluene evaporates. Larger nanoparticles keep their monolayer film both at the air-toluene interface and on a solid substrate. The color change of the gold nanoparticles at the air-toluene interface of a toluene droplet according to different sizes can be identified by eye due to the enhanced plasmon coupling between close-packed nanoparticles,44 while no color change can be noticed by eye in solution. We transferred the monolayer films of gold nanoparticles at the air-toluene interface to the air-water interface by adding a water droplet to the mixture droplet of hexane and toluene. In this way we could deposit monolayer films of nanoparticles of all sizes on SiN membranes for FE-SEM micrographs. The chemicals used in this synthesis method are identical to those in the popular Brust method1 except we do not use TOAB. The physical configuration of gold nanoparticles protected by a monolayer of DDT molecules is also identical. However, gold nanoparticles by the Brust method do not show the 2D selfassembly property of floating at the air-toluene interface. Hexane, a linear chain of 6 carbon atoms, is similar to the linear chain of 12 carbon atoms in DDT, while toluene is ring-shaped. We speculate that hexane molecules more easily fit in the space between gold nanoparticles coated with DDT than toluene molecules. However, this is insufficient to explain the unexpected difference of the gold nanoparticles’ dispersion between hexane and toluene. Charged Nanoparticles in Nonpolar Solvents. Figure 7 shows gold nanoparticles floating at the air-toluene interface of a toluene droplet between two metal plane electrodes. When there is no applied electric field, gold nanoparticles are pushed randomly by air molecules flowing nearby. Nanoparticles are pushed to the opposite direction of the 100 V/mm applied electric field, overcoming random fluctuations (Movie S3). From this experiment we conclude that these gold nanoparticles are negatively charged in nonpolar solvents.45 Therefore, we propose that being negatively charged is the key property for 2D self-assembly at the air-toluene interface. Toluene is special in that the solubility of these gold nanoparticles is low enough for them to (44) Ung, T.; Liz-Marzan, L. M.; Mulvaney, P. J. Phys. Chem. B 2001, 105, 3441–3452. (45) Hsu, M. F.; Dufresne, E. R.; Weitz, D. A. Langmuir 2005, 21, 4881–4887.

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droplet. Also we note that it took several hours to remove very slowly evaporating excess DDT molecules in the case of Bigioni et al., while it takes only ∼5 min for toluene’s evaporation with our clean gold nanoparticles. During the synthesis of gold nanoparticles in water, all the cations (Au3þ and 2Hþ) except Naþ are reduced by excess anions. Therefore, the counter cations must be Naþ. The fast reactions initially decrease the amount of OH- ions from n to n - 2.25 0.75(2 - R) - R

Figure 7. Negatively charged gold nanoparticles at the airtoluene interface are pushed to the opposite direction of an applied electric field.

float to the air-liquid interface but high enough so that they move around freely at the 2D interface. They float to the airliquid interface of a methanol-hexane mixture droplet but a multilayer film is formed, if the methanol portion is much larger than hexane. For charge neutrality in nonpolar solvents, counter cations must stay together with gold nanoparticles possessing negatively charged cores. The electric force to an electric dipole is proportional to the spatial gradient of the applied electric field. Therefore, electric dipoles must move symmetrically at the air-liquid interface of a toluene droplet between the two metal plates. Instead, the negatively charged gold nanoparticles of purple color are pushed to the opposite direction of the applied electric field and the invisible counter cations to the other side, implying spatial separation of charges. From this experiment, we know only the negative sign of gold nanoparticles, since we do not understand all the complex fluid phenomena including the river-delta shaped distribution and how the electric force overcomes the random force. As a control experiment, we synthesized DDT-protected gold nanoparticles in toluene using the “standard” Brust method,1 which are completely dispersible in toluene and stay inside a toluene droplet. Therefore, we could not observe any movement by the applied electric field. There are significant differences between the Brust method and ours. In our case, gold ions are reduced to form nanoparticles in water and then phase-transferred to hexane using acetone as opposed to phase-transferring gold ions to toluene using a surfactant, TOAB before reducing them. In addition, the presence of excess OH- ions might be the most critical difference. For another control experiment, we replaced DDT with 4-methylbenzenethiol (MBT) for coating gold nanoparticles. We confirmed that shaking the vial removes all gold nanoparticles from the water phase. However, MBTcoated gold nanoparticles are trapped at the liquid-liquid interface instead of being stably dispersed in hexane. We also tried toluene instead of hexane with the same result. Therefore, we believe that 2D self-assembly of DDT-protected gold nanoparticles in a toluene droplet is a very delicate phenomenon with two major control parameters: DDT’s shape difference from toluene and negative charges in nonpolar solvents. Bigioni et al. proposed that adding excess DDT molecules is the key for gold nanoparticles to float to the air-toluene interface.14 However, we found out that charged gold nanoparticles’ floating to the air-toluene interface does not depend on the amount of free excess DDT molecules. We speculate that the charge number of their gold nanoparticles is smaller than that of our nanoparticles resulting in higher solubility in toluene and slower floating speed, in which case excess DDT is needed for 2D self-assembly to compensate for the lower charge number. We observed that our charged gold nanoparticles are also indispersible in a neat DDT Langmuir 2010, 26(10), 7410–7417

3 9 3 3 BH4 - þ Au3þ þ OH- f BðOHÞ3 þ Au þ H2 4 4 4 2

ð1Þ

  1 3 1 ð2 - RÞ BH4 - þ Hþ þ OH- f BðOHÞ3 þ H2 4 4 4

ð2Þ

RfHþ þ OH- f H2 Og

ð3Þ

where 0 < R < 2. The slow reaction, hastened by heating, increases the amount of OH- ions to n - 2.25 - 0.75(2 - R) - R þ n - 0.75 - 0.25(2 - R) = 2n - 5 which is independent of R 

 3 2-R fBH4 - þ 4H2 O f BðOHÞ3 þ OH- þ 4H2 g n- 4 4 ð4Þ

where n is the molar ratio of BH4- and OH- ions to AuCl4- ions. We checked that gold nanoparticles in water become aggregated by adding (2n - 5) molar ratio of Hþ and Cl- ions to gold atoms for neutralizing OH- in the cases of n = 5, 6, and 7. To be sure that the aggregation is not due to screening of the electrostatic repulsion by increased concentration of ions, we added 2(2n - 5) molar ratio of Naþ and Cl- ions and found out that gold nanoparticles stay stably dispersed. From this experiment, we can exclude Cl- ions for anions adsorbed on the surface of gold nanoparticles in water and hexane. One or both of the other anions, OH- and B(OH)4-, might remain on the surface of gold nanoparticles during the phase-transfer and be responsible for the negative charges in hexane. Another possibility for the negative charge of gold nanoparticles in hexane is that the thiol (R-S-H) group of some DDT molecules loses Hþ instead of neutral H during bonding to gold, providing extra electrons to the gold core. During the phasetransfer in the mixture of water, acetone, and hexane, the thiol group of some DDT molecules becomes negative thiolate (R-S-) due to reaction of Hþ from the thiol group with OH- in the water phase. The molar ratio of DDT molecules to gold atoms was only 0.1 or 0.2, while the molar ratio of OH- was 2n - 5 (3 < n < 6.75). The S-Au bond turned out to be more complex than the previous standard model, forming the “staple” structure of R-S-Au-S-R in the case of Au102 clusters protected by aromatic thiolates.46 However, some thiols retain the H atom47 instead of becoming thiolates or are slow to lose it48 at the surface of a gold nanoparticle. Therefore, the fate of H in the thiol group during bonding to gold still remains unclear. Moreover, it is not clear whether H is lost as Hþ or H2. Both events might take place on a gold nanoparticle that stays at the liquid-liquid interface (46) Jadzinsky, P. D.; Calero, G.; Ackerson, C. J.; Bushnell, D. A.; Kornberg, R. D. Science 2007, 318, 430–433. (47) Hasan, M.; Bethell, D.; Brust, M. J. Am. Chem. Soc. 2002, 124, 1132–1133. (48) Petroski, J.; Chou, M.; Creutz, C. J. Organomet. Chem. 2009, 694, 1138– 1143.

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Figure 8. Negatively charged gold nanoparticles in hexane are oxidized by Au3þ ions and lose the 2D self-assembly property (left), while the same nanoparticles are not changed by Hþ and Cl- ions (right).

during the phase-transfer with excess OH- ions in the water phase, whose delicate balance could lead to quantitative control of trapped charges of gold nanoparticles in nonpolar solvents. To complement the physical experiment above, we performed a chemical oxidation49-52 experiment to decrease the amount of negative charges on gold nanoparticles in hexane using Au3þ ions. Figure 8 shows the experimental procedure and the 2D selfassembly results of oxidized nanoparticles on the left side and unchanged ones on the right side. First, 2 g samples of the hexane solutions containing gold nanoparticles were mixed with 2 g of acetone, and their colors immediately changed from red to purple due to aggregation of gold nanoparticles. Then, 0.1 g of water containing 5.0 μmol of HAuCl4 and 5.0 μmol of HCl was added to the left vial changing its color back to red, while adding 0.1 g of water containing 5.0 μmol of HCl did not change the color in the right vial. DI water, 4.0 g, was added and the vials were shaken for 30 s to better mix gold nanoparticles in hexane and ions in water. The complete separation took 80 min in the left vial, while it was immediate in the right. After separation of the two liquid phases, the hexane phase with gold nanoparticles came back to red in both vials. The water-acetone phase of the left vial was light yellow due to Au3þ ions, just a tiny portion of which were reduced for oxidation of negatively charged gold nanoparticles. We also tried a strong oxidizing ion, Ce4þ in cerium sulfate, Ce(SO4)2 and ammonium cerium nitrate, Ce(NH4)2(NO3)6. With Ce4þ, gold nanoparticles are extracted from the hexane phase to the liquid-liquid interface and therefore we could not observe the 2D self-assembly phenomenon. Hexane droplets with gold nanoparticles from the two vials were mixed with larger toluene droplets. Less charged gold nanoparticles due to oxidation from the left vial, floated to the air-liquid interface of a toluene-hexane mixture droplet slowly in more than 2 min and the monolayer of nanoparticles collapsed at the end, as shown in the left side of the bottom micrographs of Figure 8. Unchanged gold nanoparticles, from the right vial, floated immediately and a monolayer of close-packed nanoparticles was formed after evaporation of solvent molecules. This chemical oxidation experiment, especially the change in the floating speed, qualitatively supports our claim that the 2D self-assembly property of our gold nanoparticles is strongly dependent on their charge. Quantitative understanding is very challenging so far, and we are currently trying to develop a new measurement method for the charge number distribution in a very nonpolar solvent, hexane. We note that studying the charge of (49) Wuelfing, W. P.; Green, S. J.; Pietron, J. J.; Cliffel, D. E.; Murray, R. W. J. Am. Chem. Soc. 2000, 122, 11465–11472. (50) Choi, J.-P.; Murray, R. W. J. Am. Chem. Soc. 2006, 128, 10496–10502. (51) Negishi, Y.; Chaki, N. K.; Shichibu, Y.; Whetten, R. L.; Tsukuda, T. J. Am. Chem. Soc. 2007, 129, 11322–11323. (52) Zhu, M. Z.; Eckenhoff, W. T.; Pintauer, T.; Jin, R. J. Phys. Chem. C 2008, 112, 14221–14224.

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nanoparticles is not possible in other solvents such as toluene, chloroform, and acetone, where nanoparticles cannot be stably dispersed. It was previously reported that storing charges in the dried form of gold nanoparticles coated with insulating DDT molecules is possible.53 It was recently demonstrated, via electrospray ionization mass spectroscopy, that the core of Au144 clusters is charged, while the coating organic ligands are neutral.51 We speculate that our gold nanoparticles have more than one or two extra electrons, in which case the 2D floating was not reported.54 We expect that this synthesis method for nanoparticles can be extended to other metals such as silver and platinum in order to exploit the 2D self-assembly property at the air-toluene interface. Also, it might be possible to make semiconducting and magnetic nanoparticles coated with insulating organic molecules have the same 2D self-assembly property by injecting enough electrons. Currently, we are trying to measure and control the charge number of gold nanoparticles experimentally, in addition to theoretically understanding the underlying mechanisms of 2D self-assembly.

Conclusion We present a chemical synthesis method for alkanethiolatecoated gold nanoparticles that is simple, fast, highly reproducible, and size-tunable from 3.2 to 5.2 nm diameter. Gold nanoparticles are synthesized in water and then phase-transferred to hexane by coating them with insulating DDT molecules, in less than 10 min without postsynthesis cleaning, using minimal amounts of chemicals. Being negatively charged in nonpolar solvents is proposed to be the key property of gold nanoparticles coated with hydrophobic DDT molecules for 2D self-assembly at the air-liquid interface of a toluene droplet, which is also strongly dependent on the size of nanoparticles. A monolayer film of close-packed gold nanoparticles at the air-liquid interface of a toluene droplet can be deposited to any substrate with no limit in size, providing a very simple and effective fabrication method for 2D monolayer films of close-packed nanoparticles. We believe that controlling the charge number of nanoparticles in nonpolar solvents together with precise size-tuning is a new research direction with many open questions, especially for exploiting the 2D self-assembly property in a toluene droplet. Acknowledgment. We thank Peter Persans for allowing us to use his laboratory and instruments. We are also grateful to Toshiharu Teranishi for allowing us to take the infrared spectra (53) Pietron, J. J.; Hicks, J. F.; Murray, R. W. J. Am. Chem. Soc. 1999, 121, 5565–5570. (54) Shevchenko, E. V.; Talapin, D. V.; Kotov, N. A.; O’Brien, S.; Murray, C. B. Nature 2006, 439, 55–59.

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in his laboratory, during which M.N.M. was supported by the JSPS summer program/NSF-EAPSI under award number OISE0914144. This work was supported by Rensselaer Polytechnic Institute’s (RPI) start-up fund. J.I.B. and P.C. were supported by the National Science Foundation’s REU program and RPISURP respectively. We also thank the reviewers for comments and revision suggestions.

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Article

Supporting Information Available: Text giving experimental details and movie captions, figures showing UV-vis extinction spectra, photographs of gold nanoparticles, SEM micrographs, FE-SEM micrographs, and infrared spectra, and movies showing movement of gold nanoparticles under various conditions. This material is available free of charge via the Internet at http://pubs.acs.org.

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