Aqueous Synthesis of Alkanethiolate-Protected Ag Nanoparticles

These nanoparticles were characterized using 1H NMR, UV−vis ... Langmuir 2015 31 (46), 12742-12752 ... Zachary Farrell , Cameron Shelton , Caroline ...
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Aqueous Synthesis of Alkanethiolate-Protected Ag Nanoparticles Using Bunte Salts Young-Seok Shon* and Erin Cutler Department of Chemistry, Western Kentucky University, Bowling Green, Kentucky 42101 Received March 5, 2004. In Final Form: June 1, 2004 The one-pot synthesis of monolayer-protected metal nanoparticles derived from sodium S-dodecylthiosulfate (Bunte salt) in aqueous solution is described. Silver nanoparticles, which were produced by the borohydride reduction of silver nitrate in H2O, were stabilized by the adsorption of S-dodecylthiosulfate followed by the removal of the SO3- moiety. Temporary stabilization of silver sols by the adsorption of borohydride and borate prevented aggregation of silver nanoparticles in H2O. The syntheses of other metal nanoparticles, including gold, copper, and palladium particles in H2O, were less successful. Gold and copper particles were completely aggregated and precipitated out immediately after the addition of NaBH4, yielding only insoluble clusters. Stable and soluble palladium nanoparticle could be prepared, but the presence of Pd-thiolate complex was also observed. These nanoparticles were characterized using 1H NMR, UV-vis spectroscopy, FT-IR spectroscopy, and transmission electron microscopy.

Introduction Nanoparticles with a diameter of less than 5 nm have generated intense interest over the past decade due to their potential applications in nanoscale electronics,1 catalysis,2 and optics.3 Various particle cores including Au,4-6 Ag,7-10 Cu,11 and Pd12,13 have been investigated. Recently, research efforts have been intensified for the synthesis of Ag nanoparticles. This is due to important roles played by Ag particles in antimicrobiral application, the substrate for surface-enhanced Raman spectroscopy, and catalysis. Alkanethiols have been the choice of capping reagents for Ag nanoparticles because of the advantage for stability over other capping reagents, such as alkylamines14 and unsaturated carboxylates,15 that are weakly anchored to Ag particle cores. The alkanethiolate monolayers prevent aggregation of particles, stabilize them from * To whom correspondence may be addressed. E-mail: [email protected]. (1) Gittins, D. I.; Bethell, D.; Schiffrin, D. J.; Nichols, R. J. Nature 2000, 408, 67. (2) Crooks, R. M.; Zhao, M.; Sun, L.; Chechik, V.; Yeung, L. K. Acc. Chem. Res. 2001, 34, 181. (3) Kelly, K. L.; Coronado, E.; Zhao, L. L.; Schatz, G. C. J. Phys. Chem. B 2003, 107, 668. (4) Templeton, A. C.; Wuelfing, W. P.; Murray, R. W. Acc. Chem. Res. 2000, 33, 27. (5) (a) Brust, M.; Walker, M.; Bethell, D.; Schiffrin, D. J.; Whyman, R. J. Chem. Soc. Chem. Commun. 1994, 801. (b) Brust, M.; Fink, J.; Bethell, D.; Schiffrin, D. J.; Kiely, C. J. J. Chem. Soc., Chem. Commun. 1995, 1655. (6) Schaaff, T. G.; Shafigullin, M. N.; Khoury, J. T.; Vezmar, I.; Whetten, R. L. J. Phys. Chem. B 2001, 105, 8785. (7) (a) Van Hyning, D. L.; Klemperer, W. G.; Zukoski, C. F. Langmuir 2001, 17, 3120. (b) Van Hyning, D. L.; Zukoski, C. F. Langmuir 1998, 14, 7034. (8) Kim, H. S.; Ryu, J. H.; Jose, B.; Lee, B. G.; Ahn, B. S.; Kang, Y. S. Langmuir 2001, 17, 5817. (9) Sun, Y.-P.; Atorngitjawat, P.; Meziani, M. J. Langmuir 2001, 17, 5707. (10) Wang, T. C.; Rubner, M. F.; Cohen, R. E. Langmuir 2002, 18, 3370. (11) Chen, S.; Sommers, J. M.; J. Phys. Chem. B 2001, 105, 8816. (12) Kim, S.-W.; Park, J.; Jang, Y.; Chung, Y.; Hwang, S.; Hyeon, T.; Kim, Y. W. Nano Lett. 2003, 3, 1289. (13) Quiros, I.; Yamada, M.; Kubo, K.; Mizutani, J.; Kurihara, M.; Nishihara, H. Langmuir 2002, 18, 1413. (14) Manna, A.; Imae, T.; Iida, M.; Hisamatsu, N. Langmuir 2001, 17, 6000. (15) Wang, W.; Chen, X.; Efrima, S. J. Phys. Chem. B 1999, 103, 7238.

harsh reaction conditions, and enhance their solubility in organic solvents.4 Other sulfur-containing organic compounds, such as dialkyl disulfide,16 dialkyl sulfide,17,18 tetradentate thioether,19 methylthiirane,20 thiosalicylic acid,21 and xanthate,22 have been used to generate monolayer-protected clusters (MPCs). The objective of this research was to extend the range of successfully reactive sulfur containing species useful for the synthesis of stable, soluble, and isolable coinage metal nanoparticles. Alkyl thiosulfates have been used for the generation of gold MPCs23 and self-assembled monolayers on gold and copper.24-26 Alkyl thiosulfates have a far less repulsive odor than alkanethiols and are soluble in both aqueous and organic solutions, whereas long-chain normal alkanethiols are mostly insoluble in aqueous solutions. Stable alkanethiolate-protected nanoparticles are mostly synthesized in a two-phase system using H2O and organic solvents such as toluene.4,5a There are some examples of using water-soluble ligands (tiopronin or glutation) to stabilize metal nanoparticles, which results in watersoluble but organic-solvent-insoluble particles.27 Traditional metal colloid syntheses use aqueous solution with reducing agents such as alcohols, citrates, and alkyl sulfates.28 However, the stability of these colloids has been (16) Porter, L. A., Jr.; Ji, D.; Westcott, S. L.; Graupe, M.; Czernuszewicz, R. S.; Halas, N. J.; Lee, T. R. Langmuir 1998, 14, 7378. (17) Hasan, M.; Bethell, D.; Brust, M. J. Am. Chem. Soc. 2002, 124, 1132. (18) Shelley, E. J.; Ryan, D.; Johnson, S. R.; Couillard, M.; Fitzmaurice, D.; Nellist, P. D.; Chen, Y.; Palmer, R. E.; Preece, J. A. Langmuir 2002, 18, 1791. (19) Maye, M. M.; Chun, S. C.; Han, L.; Rabinovich, D.; Zhong, C.-J. J. Am. Chem. Soc. 2002, 124, 4958 (20) Suzuki, M.; Miyazaki, T.; Hisamitsu, H.; Kadoma, Y.; Morioka, Y. Langmuir 1999, 15, 7409. (21) Tan, Y.; Wang, Y.; Jiang, L.; Zhu, D. J. Colloid Interface Sci. 2002, 249, 336. (22) Tzhayik, O.; Sawant, P.; Efrima, S.; Kovalev, E.; Klug, J. T. Langmuir 2002, 18, 3364. (23) Shon, Y.-S.; Gross, S. M.; Dawson, B.; Porter, M.; Murray, R. W. Langmuir 2000, 16, 6555. (24) Lukkari, J.; Meretoja, M.; Kartio, I.; Laajalehto, K.; Rajama¨ki, M.; Lindstro¨m, M.; Kankare, J. Langmuir 1999, 15, 3529. (25) Hsueh, C.-C.; Lee, M.-T.; Freund, M. S.; Ferguson, G. S. Angew. Chem., Int. Ed. 2000, 39, 1227. (26) Lusk, A. T.; Jennings, G. K. Langmuir 2001, 17, 7830. (27) (a) Templeton, A. C.; Chen, S.; Gross, S. M.; Murray, R. W. Langmuir 1999, 15, 66. (b) Schaaff, T. G.; Knight, G.; Shafigullin, M. N.; Borkman, R. F.; Whetten, R. L. J. Phys. Chem. B 1998, 102, 10643.

10.1021/la049417z CCC: $27.50 © 2004 American Chemical Society Published on Web 07/10/2004

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problematic, as they aggregate and precipitate out over time in aqueous solution.4,28 Since alkyl thiosulfates are readily soluble in H2O, water would be the ideal solvent of choice because of the inexpensive and nontoxic properties that are more beneficial for environmentally benign “greener” and cost-effective chemical processing.29

Langmuir, Vol. 20, No. 16, 2004 6627 Scheme 1. A Schematic Diagram of the Synthesis of Ag MPCs Using Sodium S-Dodecylthiosulfate in H2O

Experimental Section Materials. The following materials were purchased from the indicated suppliers and used as received: silver nitrate (AgNO3‚ 3H2O), hydrogen tetrachloroaurate (HAuCl4‚3H2O), copper dichromate (Cu(ClO4)2), potassium tetrachloropalladate (K2PdCl4), sodium borohydride (NaBH4), 1-dodecanethiol, 1-bromododecane, sodium thiosulfate, and acetonitrile from Acros; ethyl alcohol, toluene, iodine crystals, acetone, and dichloromethane from Fisher. Water was purified by a Millipore Simplicity Nanopure Ultrapure water system. Synthesis of Sodium S-Dodecylthiosulfate. Sodium Sdodecylthiosulfates were prepared according to the reported procedure.23 Into a 300-mL round-bottom flask equipped with a reflux condenser were placed 6.23 g (25 mmol) of 1-bromododecane in 50 mL of ethanol and 6.21 g (25 mmol) of Na2S2O3‚5H2O in 50 mL of Nanopure water. The reaction mixture was refluxed for 3 h and the solvents were removed under vacuum. The crude product was dissolved in hot ethanol and then recrystallized. 1H NMR (270 MHz, CD3OD): δ 3.03 (t, 2H, SCH2), 1.73 (quin, 2H, SCH2CH2), 1.45-1.20 (m, 18H), 0.88 (t, 3H, CH3). Synthesis of MPCs from Bunte Salts in H2O. The synthesis of nanoparticles from sodium S-dodecylthiosulfate leads to dodecanethiolate-protected nanoparticles. The reaction involves the elimination of the SO3- moiety upon interaction of Sdodecylthiosulfate with the particle surface.23-26 The following specific reaction conditions were systematically varied: (1) metal complexes (AgNO3, HAuCl4, Cu(ClO4)2, K2PdCl4); (2) the mole ratio of S-dodecylthiosulfate to metal compound; (3) the reaction temperature. A 0.068 g (0.4 mmol) sample of AgNO3‚3H2O in 20 mL of Nanopure water was placed in a 200 mL Erlenmeyer flask. A 0.122 g (0.4 mmol) portion of sodium S-dodecylthiosulfate was added with vigorous stirring to the reaction mixture and heated at a low temperature (∼40 °C) for approximately 5 min to ensure the complete dissolution of the S-dodecylthiosulfate. The reaction flask was removed from heat and stirred at room temperature for 10 min before adding 0.152 g (4.0 mmol) of NaBH4 in 5 mL of Nanopure water over a period of 5 s. The reaction mixture quickly darkened upon the reductant addition. After being stirred for 1 h, the reaction mixture was placed on a glass filtration frit leaving a black solid. The product was exhaustively washed with water, ethanol, acetonitrile, and acetone. Synthesis of Ag MPCs from Bunte Salts in Toluene/H2O. The synthesis of Ag MPCs in toluene/H2O from sodium Sdodecylthiosulfate was analogous to the synthesis of Au MPCs from sodium S-dodecylthiosulfate.23 A 0.068 g (0.4 mmol) of AgNO3‚3H2O in 25 mL of water was placed in the reaction flask. A 2-fold molar excess (0.25 g, 0.8 mmol) of sodium S-dodecylthiosulfate in 20 mL (3:1 water/methanol) and 1.09 g (2.0 mmol) of tetraoctylammonium bromide were added to the reaction mixture. The reaction mixture was stirred for ca. 10 min at room temperature, before adding 0.15 g (4.0 mmol) of NaBH4 in 10 mL of Nanopure water over a period of ca. 5 s. The solution quickly darkens during borohydride addition. After being stirred for 3 h, the water phase was discarded and the toluene was removed under vacuum, leaving a black solid. The black precipitate was suspended in 50 mL of ethanol and placed on a glass filtration frit. The product was exhaustively washed with ethanol, acetonitrile, and acetone. Synthesis of Ag MPCs from Dodecanethiols in Toluene/ H2O. A 0.68 g (0.4 mmol) portion of AgNO3‚3H2O in 25 mL of Nanopure water was placed in the reaction flask. A 0.54 g (1.0 mmol) portion of tetraoctylammonium bromide and a 0.08 g (0.4 mmol) portion of dodecanethiol were added to the reaction mixture. The reaction mixture was stirred for ca. 10 min at room

Synthesis and General Characteristics of MPCs. The results show that the synthesis of silver nanoparticles using the aforementioned reaction conditions yield dodecanethiolate-protected nanoparticles comparable to those synthesized from alkanethiols in organic solvents (Scheme 1).16,30 When sodium borohydride is added to aqueous silver nitrate solution, silver sols (small clusters of reduced atomic silver) are initially produced by the borohydride reduction of silver nitrate.7 It is known that the borohydride undergoes a side reaction with water, producing borate anions. The borohydride and borate can temporarily stabilize silver sols by adsorption onto the surfaces and provide a substantial electrostatic barrier to aggregation.7 It is also reported that silver sols can store electrons in a strongly reducing environment, providing an enhanced electrostatic repulsion between the nanoparticles in the presence of borohydride. Due to this stabilization, S-dodecylthiosulfate, which can be dissolved in water, now can slowly replace borohydride and borate

(28) Frens, G. Nature (London), Phys. Sci. 1973, 241, 20-22. (29) Raveendran, P.; Fu, J.; Wallen, S. L. J. Am. Chem. Soc. 2003, 125, 13940.

(30) Sandhyarani, N.; Resmi, M. R.; Unnikrishnan, R.; Vidyasagar, K.; Ma, S.; Antony, M. P.; Selvam, G. P.; Visalakshi, V.; Chandrakumar, N.; Pandian, K.; Tao, Y.-T.; Pradeep, T. Chem. Mater. 2000, 12, 104.

temperature, before adding 0.15 g (4.0 mmol) of NaBH4 in 10 mL of Nanopure water over a period of ca. 5 s. The solution quickly darkens during borohydride addition. After the mixture was stirred for 1 h, the water phase was discarded, and the toluene was removed under vacuum, leaving a black solid. The black precipitate was suspended in 50 mL of ethanol and placed on a glass filtration frit. The product was exhaustively washed with ethanol, acetonitrile, and acetone. Measurements. Proton NMR spectra were recorded on a JEOL CPX FT-NMR spectrometer operating at 270 MHz in CDCl3 solutions and internally referenced to δ 7.26 ppm. Infrared spectra were obtained, using a Perkin-Elmer 1600 FT-IR spectrometer, of films of MPCs pressed into a KBr plate. The spectra were recorded from 4500 to 450 cm-1. UV-vis spectra of dichloromethane solutions in quartz cells were acquired on a Shimadzu UV-2101 PC spectrophotometer. Transmission electron microscopy (TEM) images of nanoparticles were obtained with a JEOL 120CX scanning/transmission electron microscope operating at 120 keV. Samples were prepared for TEM by casting a single drop of a ∼1 mg/mL hexane solution onto standard carbon-coated (80-100 Å) Formvar film on copper grids (600 mesh) and drying in air for at least 30 min. Several regions were imaged at 100000×. Size distributions of the metal cores were obtained from digitized photographic enlargements with Scion Image Beta Release 2.

Results and Discussion

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and adsorb onto the surface of silver particles. Eventually, the SO3- moiety (or some sulfur species derived from it) is eliminated from the surface of silver particles after the surface-induced S-SO3- bond cleavage of S-dodecylthiosulfate. This results in the formation of dodecanethiolate-protected Ag nanoparticles. The elimination of SO3moiety from S-alkylthiosulfate while reacting with a metal surface has been reported several times.23-26 The produced dodecanethiolate-protected Ag nanoparticles are initially dispersed in H2O without complete precipitation. This is probably due to the presence of unreacted S-dodecylthiosulfate, resulting in the formation of emulsion, as do surfactants (e.g., sodium dodecyl sulfate) in soap solutions. The characteristics of Ag MPCs generated from Sdodecylthiosulfate in water were compared to those of Ag MPCs synthesized from the original two-phase reaction using both dodecanethiol and S-dodecylthiosulfate. Synthesis of Ag MPCs from dodecanethiol in water was not successful, yielding only insoluble aggregated materials. The mole ratio of S-dodecylthiosulfate to AgNO3 was varied from 3:1 (3×) to 1:1 (1×) to 1:3 (1/3×), because the previous reports have shown that the size or stability of nanoparticles is largely governed by the relative concentration of organic moieties to metal salts.4 Reaction temperature was also varied to see the effects of reaction condition in nanoparticle formation. Results of these comparisons are provided in the later section. The synthesis of Au, Cu, and Pd nanoparticles from S-dodecylthiosulfate in water was also attempted. When AuCl4- and Cu(ClO4)2 are reduced by NaBH4 in the presence of S-dodecylthiosulfate in water, only insoluble aggregated materials are identified as reaction products. The soluble and isolable monolayer-protected Pd nanoparticles could be synthesized from K2PdCl4 with Sdodecylthiosulfate in water. However, the products were identified as the mixture of unreacted Pd-thiolate complexes and Pd MPCs. The Ag and Pd MPCs synthesized from sodium Sdodecylthiosulfate in water were readily soluble in organic solvents such as dichloromethane, hexane, and toluene. These solubility properties clearly suggest that the monolayers of these Ag and Pd MPCs expose hydrophobic CH3 or CH2 groups and not hydrophilic S2O3- groups. Properties of Silver MPCs. UV-vis Spectroscopy. The surface plasmon (SP) band energy of nanoparticles is sensitive to the electronic and optical properties of the particle surface and of the protecting monolayer.4 Ag MPCs synthesized from S-dodecylthiosulfate in water have almost identical surface plasmon band at ca. 425 nm comparable to Ag MPCs generated from alkanethiols (Figure 1).31,32 Unprotected Ag particles are known to exhibit an absorption band with a maximum at ca. 390 nm.7a,31 This shift of the SP band by capping of Ag particles can be attributed to the bond formation of the Ag clusters and the ligands.22,31 The appearance of the SP band at ca. 425 nm provides clear evidence for the adsorption of S-dodecylthiosulfate. Ag MPCs synthesized from S-dodecylthiosulfate (1×) at room temperature show an extra band at ca. 350 nm, which is most likely associated with a coordinate compound of silver thiolates.33 The presence of this band indicates some oxidation of Ag MPCs during the particle synthesis. When Ag MPCs were prepared with a higher concentration (3×) of S-dodecylthiosulfate, more substantial oxidation (31) He, S.; Yao, J.; Jiang, P.; Shi, D.; Zhang, H.; Xie, S.; Pang, S.; Gao, H. Langmuir 2001, 17, 1571. (32) Yonezawa, T.; Onoue, S.; Kimizuka, N. Langmuir 2000, 16, 5218. (33) Fijolek, H. G.; Gonzalez-Duarte, P.; Park, S. H.; Suib, S. L.; Natan, M. J. Inorg. Chem. 1997, 36, 5299.

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Figure 1. UV-vis spectra of CH2Cl2 solution of (a) Ag MPCs generated from S-dodecylthiosulfate in H2O and (b) Ag MPCs generated from S-dodecylthiosulfate and dodecanethiol in toluene/H2O.

was observed. This was clearly evidenced by the relatively large absorption at ca. 350 nm. The UV-vis data suggest that the S-dodecylthiosulfate/Ag molar ratio of ca. 1.0 is optimal in terms of stability of the Ag particle during the reaction. When Ag MPCs are synthesized from S-dodecylthiosulfate (1×) at the increased reaction temperature of 50 °C, the absorption band at 350 nm almost disappeared, which indicates better protection of particles at higher temperature. Ag MPCs synthesized from dodecanethiol or S-dodecylthiosulfate in toluene/H2O did not show the presence of this absorption band at 350 nm (Figure 1b). This is probably due to better protection by ligands (especially, dodecanethiol) and less oxidation of Ag particles in organic solvents. 1 H NMR Spectroscopy. A proton NMR spectrum of Ag MPCs generated from S-dodecylthiosulfate (1×, 50 °C) was obtained. The result showed only two broad resonances at δ 0.8-0.9 and 1.2-1.5 for methyl and methylene (Figure 2a). This spectrum was nearly identical to those of dodecanethiolate-protected Au and Ag nanoparticles generated from dodecanethiols using the Schiffrin protocol.5a There were no additional peaks corresponding to free dodecanethiol, dodecyl disulfide, or S-dodecylthiosulfate, which would occur as sharp resonances. These results suggest that the monolayers of hydrocarbon derived from S-dodecylthiosulfate are attached to the nanoparticle as a dodecanethiolate after elimination of the SO3- moiety. The results also showed the same peakbroadening effect, as do monolayers of alkanethiolateprotected nanoparticles generated from alkanethiols.4 Alkanethiolate-protected nanoparticles were decomposed by reaction with iodine crystals.34 This decomposition yields the ligands as dialkyl disulfide, which has a sharp triplet resonance at δ 2.7 for CH2-S in addition to two resonances at δ 0.8-0.9 and 1.2-1.5 in proton NMR (34) Templeton, A. C.; Hostetler, M. J.; Warmoth, E. K.; Chen, S.; Hartshorn, C. M.; Krishnamurthy, V. M.; Forbes, M. D. E.; Murray, R. W. J. Am. Chem. Soc. 1998, 120, 4845.

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Langmuir, Vol. 20, No. 16, 2004 6629 Table 1. Average Core Size and IR Data of MPCs Generated from Sodium S-Dodecylthiosulfate or Dodecanethiol MPCsa

TEMb (diameter, nm)

FT-IR (νa(CH2), cm-1)

Ag MPCs, Bunte Ag MPCs, Bunte, 50 °C Ag MPCs, thiol, Tol/H2O Ag MPCs, Bunte, Tol/H2O Pd MPCs, Bunte

3.3 ( 0.8 3.5 ( 1.0 3.4 ( 1.1 4.3 ( 1.0 2.0 ( 0.7

2918 2916 2916 2919 2923

a The ligand/metal precursor molar ratio used in the reaction is 1:1. b TEM results, average metal core size from analysis of histogram of TEM images.

Figure 4. Transmission electron micrograph of Ag MPCs generated from (a) S-dodecylthiosulfate (1×, 50 °C) in H2O and (b) dodecanethiol in toluene/H2O. Figure 2. 1H NMR spectra (CDCl3) of (a) Ag MPCs (1×, 50 °C) generated from S-dodecylthiosulfate in H2O and (b) I2decomposed Ag MPCs (1×, 50 °C) generated from S-dodecylthiosulfate. The sharp resonance at 1.54 ppm is due to H2O impurity in the CDCl3 solution.

Figure 3. IR spectra of Ag MPCs (1×, 50 °C) generated from S-dodecylthiosulfate in H2O.

spectra. Ag MPCs synthesized from S-dodecylthiosulfate also yield dodecyl disulfides by decomposition with iodine, confirmed by analysis of the product by 1H NMR spectroscopy (Figure 2b). This NMR result is more support for the hypothesis that the MPCs have alkanethiolate rather than S-alkylthiosulfate ligands on the nanoparticle surface. Fourier Transform Infrared (FTIR) Spectroscopy. Infrared spectroscopy provides structural and conformational information regarding monolayers on metal clusters. The presence of CH3 and CH2 stretching bands from the Ag MPCs generated from S-dodecylthiosulfate is observed (Figure 3). The frequency and bandwidth of CH2 and CH3 bands are the indicative of crystalline-order of alkyl chains in these monolayers on the nanoparticle surface.35,36 The broadening and increase in energy (higher wavenumber)

of these stretches indicate poorly ordered alkyl chains and a high population of gauche defects, whereas sharp bands at lower energies indicate ordered alkyl chains and trans-zigzag conformation. FT-IR frequencies of Ag MPCs generated from S-dodecylthiosulfate and dodecanethiol are listed in Table 1. The asymmetric stretching bend (νaCH2) of Ag MPCs generated from sodium S-dodecylthiosulfate (1×, 50 °C) appears at ∼2916 cm-1 with a degree of crystallinity similar to that of MPCs generated from dodecanethiol. These results suggest that Ag MPCs derived from both S-dodecylthiosulfate in water at 50 °C and dodecanethiol in toluene/H2O have monolayers with similar degree of packing and monolayer ordering. Ag MPCs derived from S-dodecylthiosulfate at room temperature in H2O and in toluene/H2O showed νa(CH2) at 2918 and 2919 cm-1, respectively. It is noted that the small difference in the wavenumber shown here is within the instrumental error. Transmission Electron Microscopy (TEM). Ag MPCs generated from S-dodecylthiosulfate appear to be roughly spherical, which is very much comparable to the previous reports on Au and Ag MPCs generated from alkanethiols (Figure 4).31,32 TEM results in Table 1 show that MPCs generated from S-dodecylthiosulfate both at room temperature and at 50 °C have similar average core size and dispersity. This suggests that a nucleation-growth-passivation process of Ag particles is not affected by the variation in the reaction temperature. The average core size of these Ag MPCs generated from S-dodecylthiosulfate in water was very similar to Ag MPCs generated from dodecanethiol in toluene/H2O. However, the average core size of Ag MPCs generated from S-dodecylthiosulfate in toluene/H2O was somewhat larger than that of other Ag MPCs. The result indicates the passivation of Ag particles (35) Choo, H.; Cutler, E.; Shon, Y.-S. Langmuir 2003, 19, 8555. (36) Maye, M. M.; Zheng, W.; Leibowitz, F. L.; Ly, N. K.; Zhong, C.-J. Langmuir 2000, 16, 490.

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image were much smaller than that of Ag MPCs as shown in Table 1. However, UV-vis (Figure 5b) and 1H NMR (Figure 5c) results of the isolated Pd particles showed the presence of subsequent amount of Pd-thiolate complex as an impurity. Unlike other metal (Au, Ag, and Cu) nanoparticles, it is known that the Pd nanoparticles do not exhibit a well-defined SP band, but feature only the Mie scattering.37-39 In contrast, the UV-vis spectrum of Pd MPCs generated from S-dodecylthiosulfate in water showed two absorption bands at ca. 305 and 400 nm. The peak at ca. 400 nm has been observed for Pd(II)-thiolate complexes.38 The origin of the absorption band at ca. 305 nm is not certain. The absorption peak at ∼320 nm has been ascribed to unreacted Pd complexes.37 The 1H NMR spectra of Pd MPCs generated from S-dodecylthiosulfate in H2O showed two broad resonances at δ 0.8-0.9 and 1.2-1.5 for CH3 and CH2s, respectively, with additional peaks at δ 2.4-2.5 and 1.7-1.8, which corresponds to R-CH2 and β-CH2 protons to the S-Pd interface (Figure 5c). The presence of these additional resonances indicated that the oxidation of Pd particles to Pd-thiolate complexes could occur during the synthesis of Pd MPCs using S-dodecylthiosulfate. Previous studies have described that initially formed Pd nanoparticles could decompose back into extremely fine particles or oxidized ionic complexes.37,38 The FT-IR frequency of Pd MPCs generated from S-dodecylthiosulfate is also listed in Table 1. The νa(CH2) appeared at much higher energy (∼2923 cm-1) for the Pd MPCs compared to that of Ag MPCs (∼2916 cm-1). The difference in band frequency of Pd MPCs suggested that the monolayers on Pd MPCs derived from sodium Sdodecylthiosulfates are probably less densely packed and less ordered. However, the higher energy of νa(CH2) might be a result of the presence of Pd-thiolate complex.

Figure 5. Spectroscopic results of Pd MPCs generated from S-dodecylthiosulfate: (a) TEM image, (b) UV-vis spectrum in CH2Cl2, and (b) 1H NMR spectrum of Pd MPCs.

by S-dodecylthiosulfate in a two-phase system (toluene/ H2O) is probably slightly slower than that in a one-phase system (H2O). Synthesis of Other Metal Particles. The synthesis of Au, Cu, and Pd MPCs from S-dodecylthiosulfate in water has also been attempted with HAuCl4, Cu(ClO4)2, and K2PdCl4 as their respected metal core sources. Unlike Ag particles, Au and Cu particles quickly aggregated in aqueous solution after NaBH4 addition. It seems that formation of the electrostatic barrier by borohydride and borate anion did not occur for Au and Cu metal particles. The reason for instability of these systems is not completely understood at this moment. When K2PdCl4 was reduced by NaBH4 in the presence of S-dodecylthiosulfate in water, soluble and isolable monolayer-protected Pd nanoparticles were produced. TEM image of Pd MPCs confirmed formation of Pd nanoparticles (Figure 5a). These Pd MPCs also appear to be roughly spherical, comparable to Ag MPCs. The average core dimensions obtained from this

Conclusions A cost effective and more environmentally benign synthesis of stable and isolable alkanethiolate-protected silver and palladium nanoparticles is presented. Silver nanoparticles were stabilized by the adsorption of Bunte salts followed by the removal of the SO3- moiety. Temporary stabilization of silver sols by the adsorption of borohydride and borate played a key role in this reaction process. The synthesis of gold and copper nanoparticles from Bunte salts was not successful. Alkanethiolateprotected Pd nanoparticles were synthesized using Bunte salts with moderate success, due to the oxidation of Pd particles to Pd-thiolate complexes. Acknowledgment. This research was supported by grants from Research Corporation (Cottrell College Science Award), Kentucky Science and Engineering Foundation, and Western Kentucky University. The authors thank Dr. John Andersland for the assistance on TEM experiments. LA049417Z (37) Chen, S.; Huang, K.; Stearns, J. A. Chem. Mater. 2000, 12, 540. (38) Zamborini, F. P.; Gross, S. M.; Murray, R. W. Langmuir 2001, 17, 481. (39) Quiros, I.; Yamada, M.; Kubo, K.; Mizutani, J.; Kurihara, M.; Nishihara, H. Langmuir 2002, 18, 1413.