Critical Role of Water and the Structure of Inverse Micelles in the Brust

LETTER pubs.acs.org/Langmuir

Critical Role of Water and the Structure of Inverse Micelles in the Brust
Schiffrin Synthesis of Metal Nanoparticles Ying Li, Oksana Zaluzhna, and YuYe J. Tong* Department of Chemistry, Georgetown University, 37th & O Streets NW, Washington, DC 20057, United States

bS Supporting Information ABSTRACT: Although Brust
Schiffrin two-phase synthesis is a popular method for synthesizing ligand-protected metal nanoparticles with an average size of less than 5 nm, the details on how the reactions can be controlled from a mechanistic point of view are still unclear, therefore hindering efforts to synthesize monodisperse metal nanoparticles. It was recently discovered that this method is basically an inverse-micelle-based synthesis (Li, Y.; Zaluzhna, O.; Xu, B.; Gao, Y.; Modest, J. M.; Tong, Y. Y. J. J. Am. Chem. Soc. 2011, 133, 2092). In this letter, the critical role of water and the structure of inverse micelles in typical synthesis of gold nanoparticles were further investigated. We found that (1) water encapsulated in the inverse micelles of [TOA]þ that also hosted metal ions formed a hydrophilic microenvironment that acted as a reaction-enabling proton-accepting medium for the thiol protons (RS
H) and (2) not only the presence but also the amount of water in the reaction medium has a profound effect on the Au(I) precursor species (a pure [TOA][AuX2] complex or a mixture of a [TOA][AuX2] complex and polymeric [AuISR]n species), the reduction of Au(III) by thiols, and the formation of uniform small metal nanoparticles. A quantitative analysis of the 1H NMR of the encapsulated water enabled an estimation of the size and composition of the involved inverse micelles.

he Brust
Schiffrin (B
S) two-phase method1 is a popular synthetic route to prepare chalcogenate-protected metal nanoparticles (NPs),2
10 in particular thiolate-protected Au NPs.11
21 This is due to their easy-to-follow synthesis procedures, good shelf stability of the products, and their redissolvability in organic solvent.11,14 Although mechanistic details of the synthesis are still unclear, it has generally been accepted that a typical B
S two-phase synthesis consists of three steps: (i) the phase transfer of Au(III) ions from an aqueous layer to an organic layer (i.e., toluene or benzene) with the help of phase-transfer agent TOAB (tetrabutylammonium bromide), (ii) the reduction of Au(III) to Au(I) by the added thiols, and (iii) the reduction of Au(I) to Au(0) by NaBH4 in the presence of thiols and/or other sulfur-containing ligands that leads to the formation of Au NPs. A long-held belief concerning the metal precursor in the B
S synthesis of metal NPs, probably due to earlier papers by Schaaff et al.16 and Alvarez et al.,22 had been until recently23,24 that the [M(I)SR]n polymeric metal
thiolate species is the metal ion precursor of metal NPs. However, a recent thought-provoking paper by Goulet and Lennox24 has made a different claim, providing evidence that the metal
TOAB complex, [TOA][M(I)X2], is the correct metal ion precursor, which was confirmed by our recent work.23 There are still some unanswered, fundamentally important questions in the B
S two-phase synthesis: What is the structure of the inverse micelles that have been identified recently23 as a critical ingredient in the B
S synthesis? What is the fate of the thiol proton? What is the role of water? Herein, we report a study that addresses all of these questions by which a much better

T

r 2011 American Chemical Society

mechanistic understanding of the B
S two-phase synthesis can be achieved. Two key advances in our general understanding of the B
S two-phase synthesis of metal NPs achieved recently form the basis and motivation of the current study. The first is the recent discovery24 that the phase transfer in the B
S synthesis led to the formation of the [TOA][M(III)X4] complex and the subsequent reduction by thiols produced the [TOA][M(I)X2] complex instead of the long-believed polymeric [M(I)SR]n species. The second is the realization23 that the B
S synthesis of monolayer-protected metal NPs was basically an inverse micelle (formed during the phase transfer of Au(III) by TOAB)-based synthesis and that the metal
sulfur bond did not form until the formation of metal nuclear centers. These two advances have put us on the track of a detailed mechanistic investigation of how different ingredients in the reaction medium influence the final formation of metal NPs and guided us in formulating specific experimental measurements that combined principal Raman and 1H NMR spectroscopy with controlled synthetic steps. This careful analytical study of the role of water and micelles helps to establish an optimal synthesis procedure for synthesizing homogenously distributed metal NPs. The existence of water makes the synthesized Au NPs smaller and more monodisperse. Received: March 29, 2011 Revised: May 12, 2011 Published: May 20, 2011 7366

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State of the Metal Ion Precursors. To facilitate ensuing discussions, we name the mixture (aqueous plus organic layers) right after the phase transfer as Sol-0, the separated organic phase of Sol-0 with added thiols as Sol-I, and Sol-0 (no removal of aqueous layer) with added thiols as Sol-II. Raman spectra (Figure S1 in the Supporting Information) confirmed the formation of the [TOA][AuX4] complex in the organic phase of Sol-0. Once a proper amount of thiols (e.g., S/Au = ∼3) was added to the solutions under stirring, the initial wine-red Au(III) turned colorless within 30 min, indicating a reduction of Au(III) to Au(I). For Sol-I, it remained colorless and crystal clear even after being stirred for >1 h, but for Sol-II, the solution became cloudy white (Figure S2a) and formed three well-separated layers after being still for a while (Figure S2b). The top (Sol-II-T) and bottom (Sol-II-B) layers were clear and colorless, and the middle layer (Sol-II-M) was opaque white. Notice that no middle opaque-white layer was formed if the thiol/Au ratio was less than 2. The Raman spectra of Sol-I, Sol-II-T, and Sol-II-M together with the relevant reference materials are shown in Figure S3. For Sol-I and Sol-II-T, the disappearance of the RS
H vibration25 at 2568 cm
1 is accompanied by the appearance of the RS
SR vibration26 at 525 cm
1. The appearance of a peak at 209 cm
1 due to the Au
Br vibration,27 the absence of a peak at 327 cm
1 from the Au
SR bond,23,26 and the identical spectral features to those of [TOA][AuBr2] indicate that only the [TOA][AuBr2] complex was formed as the Au precursor species in Sol-I or Sol-II-T. Sol-II-M shows the same spectral features as the polymeric [AuSR]n species, including Au
SR vibrational band whose exact frequency depends slightly on the alkyl chain length (Figure S4). The appearance of this peak at 327 cm
1 is strong evidence that Sol-II-M contained the polymeric [AuSR]n species in which Au
SR bonds were expected. (See SI for the details.) Similar phenomena were observed when Ag(I) was used as the metal source in the B
S two-phase synthesis. Although no redox reaction occurred between thiols and Ag(I),24 the Ag
S vibrational band (at ∼320 cm
1) was still observed in the Raman spectra (Figure S5) when the mixture of [TOA][AgBr2] and 3 equiv of dodecanethiol was stirred overnight or longer in the presence of water (such as Sol-II above). Altogether, the above observations strongly suggest that the amount of water plays a major differentiating role in the formation of the metal(I) polymeric species. Thus, the reactions that took place upon the addition of thiols in the B
S two-phase synthesis were as follows. If the reaction solution is Sol-II (i.e, no removal of the aqueous layer), then reaction 1 occurs:

½TOA½AuX4  þ RSH f ½TOA½AuX2  þ ½AuSRn þ RSSR þ HX

ð1Þ

The appearance of polymeric species is in agreement with Schaaff et al.16 and Alvarez et al.’s observations,22 but if the reaction solution is Sol-I (i.e., with the aqueous phase removed), then reaction 2 occurs, which is in agreement with the observations by Goulet and Lennox:24 ½TOA½AuX4  þ RSH f ½TOA½AuX2  þ RSSR þ HX ð2Þ Therefore, we have discovered that the chemical state of the Au(I) precursor species in the B
S two-phase synthesis depends on the specific reaction medium (i.e., with or without an aqueous

phase) before the addition of thiols and the ratio of thiol to Au. If the aqueous layer was removed after the phase transfer of Au(III) (i.e., AuCl4
) with TOAB from the aqueous to the organic phase and only to the latter thiols were added (Sol-I), then the precursor species was the [TOA][AuX2] complex. However, if thiols were added to the mixture of aqueous and organic phases (Sol-II) and the thiol/Au ratio was larger than 2, then the precursor species was a mixture of the [TOA][AuX2] complex and the polymeric [AuSR]n species. When the ratio of thiol to Au was less than 2, regardless of whether the aqueous layer was removed, the precursor species was always the [TOA][AuXm] (m = 2 or 4) complex. The effect of the different Au(I) precursors on the final formation of Au NPs was studied by adding 1 mL of a NaBH4 aqueous solution to 10 mL toluene solutions of Sol-I (i.e., the [TOA][AuX2] complex), Sol-II (i.e., a mixture of the [TOA][AuX2] complex and the polymeric [AuSR]n species), and synthesized polymeric [AuSR]n with 1 equiv of TOAB (the addition of TOAB ensured that all three reaction media contained the same amount of surfactant). The respective Au NPs so obtained are shown in Figure S6, where Sol-I produced the smallest (1.16 ( 0.17 nm) Au NPs, polymeric [AuSR] with TOAB produced the largest (2.26 ( 0.39 nm) Au NPs, and Sol-II produced in between size (1.48 ( 0.23 nm) Au NPs, with the breadth of the size distribution being the same order of magnitude. Thus, polymeric [AuSR]n is in general not a good metal precursor species for forming homogeneous ultrasmall Au NPs, and the presence of polymeric species increases the polydispersity of the synthesized Au NPs. Structure of the Inverse Micelles. It is well known that inverse micelles can be formed when surfactants are dissolved in organic solvents with or without water.28 Some water can be solubilized in the polar core together with the polar head of surfactants to form a “water pool” or encapsulated water (enH2O). The formation of such inverse micelles indeed happened when an organic solution of TOAB was mixed with water or HAuCl4 aqueous solution as shown in our recent study.23 A detailed illustration is shown in panel A of Figure 1, where the 1H NMR spectra of the organic layer separated after mixing (with 1 h of stirring) 16.4 mg (0.03 mmol) of TOAB dissolved in 0.8 mL of C6D6 with different amounts of water (the separated benzene phase was used for NMR measurements): (a) 0.210 mL; (b) 0.105 mL; (c) 0.070 mL; (d) 0.0425 mL; and (e) 0 mL. For reference, Figure 1f is the 1H NMR spectrum of 0.8 mL of C6D6 mixed with 0.070 mL of H2O whose peak position at 0.44 ppm is very close to the tabulated value (0.4 ppm).29 The appearance of a peak at ∼2.44 ppm in Figure 1a
e indicates the formation of en-H2O by surfactant TOAB.30 This becomes clearer by comparing spectra c and f in Figure 1. Both have the same initial C6D6/H2O volumetric ratio (80:7) for mixing and stirring, but the former has an additional 16.4 mg of TOAB that can form inverse micelles in C6D6 in the presence of water but not the latter that was TOAB-free. The accompanying appearance of a peak at 2.43 ppm and the disappearance of the water peak in C6D6 at 0.44 ppm provide direct experimental evidence of the formation of en-H2O in the inverse micelles of TOAB, with the peak position determined by the fast exchange between water dissolved in C6D6 and encapsulated in inverse micelles (vide infra). Notice that there exists a trace amount of water even in the pure TOAB C6D6 solution whose peak shows up at 2.33 ppm (Figure 1e). Also notice that both peak positions of the R-protons (i.e., H2C
Nþ at ∼3.34 ppm) 7367

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Figure 1. (A) 1H NMR spectra of the organic layer separated after mixing (with 1 h of stirring) 16.4 mg (0.03 mmol) of TOAB dissolved in 0.8 mL of C6D6 with different amounts of water: (a) 0.21, (b) 0.105, (c) 0.07, (d) 0.0425, and (e) 0 mL for (f) a solution that is the same as in c but without TOAB. (B) 1H NMR spectra of the organic layer separated after mixing 16.4 mg (0.03 mmol) of TOAB in 0.8 mL of C6D6 with different amounts of HAuCl4 aqueous solution (0.1421 M): (g) 0.21, (h) 0.105, (i) 0.07, and (j) 0.0425; (k) [TOA][AuBr4] (0.0125 mmol) in 0.8 mL of C6D6 mixed with 0.09 mL of H2O; and (l) [TOA][AuBr4] (0.025 mmol) in 0.8 mL of C6D6. (C) 1H NMR spectra (m
p) of the samples prepared by adding dodecanethiol (2S/1Au) to samples g
j; (q) for the sample prepared by adding dodecanethiol (3S/1Au) to sample k; and for [TOA][AuBr2] (0.025 mmol) mixed with (r) didodecyl disulfide and (s) dodecanethiol (3S/1Au).

and (fast-exchanging) H2O are independent of the initial amount of water during mixing. This indicates that the amount of water taken up by the benzene solution of TOAB was rather constant, implying a constant formation of the inverse micelles. When the Au salt was added, it became quite a different story, which is illustrated by the 1H NMR spectra collected in panel B of Figure 1 where the 1H NMR spectra of 16.4 mg TOAB in 0.8 mL of C6D6 mixed with different volumes (the same values as in Figure 1a
d, respectively) of an aqueous solution of HAuCl4 (0.1421 M) are shown (the separated benzene phase was used for NMR measurements): (g) 0.210 mL; (h) 0.105 mL; (i) 0.070 mL; and (j) 0.0425 mL. The corresponding TOAB/Au ratios were 1:1, 2:1, 3:1, and 5:1. For comparison, the 1H NMR spectra of the synthesized [TOA][AuBr4] complex (0.0125 mmol) in 0.8 mL of C6D6 with (0.09 mL) and without H2O mixing are shown in Figure 1k, l. Contrary to the constant peak positions as seen in Figure 1a
e, the water and R-proton peaks became dependent on the amount of mixed water (i.e., of the Au(III) ions). The pair (H2O/R-protons) peak values are now 0.70/2.97, 2.34/ 3.18, 2.63/3.22, and 2.69/3.26 ppm for TOAB/Au ratios of 1:1, 2:1, 3:1, and 5:1, respectively. That the peak position of the R-protons in the TOAB/Au = 1:1 case (Figure 1g) is the same as that of the synthesized [TOA][AuBr4] complex (Figure 1k, l) indicates strongly that one [TOA]þ cation was associated with one [AuX4]
anion where X could be either Br
or/and Cl
(Figure S1d). As the TOAB/Au ratio increases, more and more [TOA]þ cations can be associated with halogen anions (e.g., Br
or Cl
) other than the Au(III) complex. However, that fact that only one R-proton peak was observed in all cases is a sign of fast exchange between the Au(III) complex unit and the halogen anion in the inverse micelles. Now taking 2.97 ppm (Figure 1g, k, l) and 3.34 ppm (Figure 1a
d) as the peak positions of the R-protons in two extremities (all Au(III) units vs all halogen anions), we calculated the expected peak position using the fast-exchange model δR-proton = y  2.97 þ (1
y)  3.34 (in ppm) where y is the fraction of the Au(III) units among the total anions (i.e., [AuX4]
and X
). We found 3.16, 3.22, and 3.27 ppm for

TOAB/Au ratios of 2:1, 3:1, and 5:1, respectively. These should be compared with 3.18, 3.22, and 3.26 ppm, the experimentally observed values (Figure 1h
j). That the calculated values are in excellent agreement with the experimentally observed ones indicates strongly that the partitioning of the Au(III) units and the halogen anions in a given inverse micelle follows the nominal stoichiometry. The H2O peak position is also no longer constant and neither is its amplitude. In particular, the presence of the [TOA][AuX4] complex seems to suppress the amount of en-H2O significantly, as evidenced by comparing parts g and a of Figure 1. To be more quantitative, we integrated the relevant proton peaks and normalized them by the peak integral (set to be 8 for the eight protons in four R-CH2 groups per [TOA]þ cation (pTc)) of R-CH2 because the same amount of TOAB (16.4 mg) was used in all samples (Tables S1
S3). The numbers so obtained are the numbers of protons pTc. As can be calculated from the data in Tables S1
S3, the average values for the
CH3 group and the residual proton in C6D6 (0.8 mL) were 11.4 ( 0.4 and 7.2 ( 0.4, respectively. The closeness of the former to the expected value of 12 and the excellent reproducibility of both values (i.e., very small standard deviations) well justify our choice of the R-CH2 peak as the internal reference and the accuracy of the measurements. As expected, the amount of H2O in Figure 1a
d was a constant: 5.1 ( 0.2 (∼2.5 H2O pTc, Table S1). So is the saturated free water in C6D6 shown in Figure 1f, k (1.68 ( 0.04, or ∼0.8 H2O pTc, Table S1). It is reasonable to assume that there existed only two types of water in the benzene solution: the H2O dissolved in benzene (resonant at 0.44 ppm) and the en-H2O in inverse micelles. Because a discernible water layer was always formed after the mixing in all experiments, the amount of water used should be high enough to saturate the C6D6 solvent and micelles. Thus, the absence of the peak at 0.44 ppm indicates that these two types of water must experience a fast exchange on the NMR timescale. By subtracting the amount of water dissolved in C6D6, one can find that the amount of en-H2O is ∼1.74 H2O pTc. Now assuming (ad hoc) a stoichiometry of 1:1 between [TOA]þ of a given inverse micelle and surface water molecules of en-H2O, a dispersion (number of surface H2Os over 7368

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Langmuir the total en-H2O) of 0.57 can be calculated. Using this dispersion and the molecular volume of room-temperature bulk H2O (∼0.03 nm3), we estimate that the size of a spherical volume of the inverse micelle has a diameter of ∼2.5 nm and each micelle consists of ∼156 [TOA]þ that encapsulate ∼273 H2O molecules (details in the SI). In contrast, the amounts of H2O in Figure 1g
j were 1.32, 3.75, 3.90, and 4.91, respectively, (Table S2) as the amount of Au(III) decreased. At a TOAB/Au(III) ratio of 1:1, the H2O peak position was at 0.7 ppm, which is very close but not equal to that of H2O dissolved in C6D6 (0.44 ppm). Additionally, the amount of water (1.32) was even lower than that of saturated water dissolved in C6D6 (1.68). Therefore, we believe that there was only a very tiny amount of en-H2O in this sample. As the TOAB/Au ratio increased from 1:1 to 2:1, 3:1, and 5:1, an increasing number of [TOA]þ were free from being associated with the Au(III) complex. Consequently, the amount of en-H2O increased, from almost zero to 1.04, 1.11, and 1.62 pTc. Notice that the last value is quite close to the value (1.74 pTc) of the Au(III)-free micelles. Assuming (ad hoc again) that the size of the micelles did not change in the presence of the Au(III) complex, the numbers of Au(III) cations in a given micelle would thus be ∼156, 78, 52, and 31 for TOAB/Au(III) ratios of 1:1, 2:1, 3:1, and 5:1, respectively. However, mixing water with the synthesized [TOA][AuBr4] complex did not produce enH2O, as evidenced by comparing parts k and f of Figure 1, of which the latter did not contain TOAB. The same peak position and amount of water were observed in both samples. Fate of the Thiol Protons. Once the organic (benzene or toluene) phase is separated in a B
S two-phase synthesis, the next step is to add thiols to the separated organic layer, and reaction 2 is expected to happen. Because the reaction happens in an organic (hydrophobic) medium, understanding the processes involving hosting the reaction-generated hydrophilic proton is fundamentally important. For instance, we have observed that the reduction of the synthesized [TOA][AuBr4] complex to [TOA][AuBr2] by thiols in an anhydrous organic (toluene or benzene) solvent was extremely slow but happened almost instantaneously in the presence of H2O. To determine what happens to
H after the S
H bond of the thiol is broken during the reduction of Au(III), 1H NMR spectra of the samples prepared by adding dodecanethiols (2S/1Au) to the solutions in Figure 1g
j were recorded. These spectra are shown in panel C of Figure 1, together with those of relevant reference materials. The appearance of the disulfide peak and the residual thiol peak in Figure 1m
p, as indicated by the arrows, clearly demonstrates that the reduction of Au(III) by thiols did occur. Quantitative reaction stoichiometry can be calculated from the data in Table S3. The calculated S (i.e., thiol and disulfide)/Au ratios are 1.9:1, 2.0:1, 2.1:1, and 2.3:1 for the four samples (Figure 1m
p), respectively. These values are very close to the nominal ratio of 2:1, indicating the reliability of this quantitative analysis. The most striking difference between Figure 1m
p and their respective parent spectra (Figure 1g
j) is the large, positive shift of the water peak from 0.7 ppm in Figure 1g to 4.16 ppm in Figure 1m, 2.34 ppm in Figure 1h to 5.56 ppm in Figure 1n, 2.63 ppm in Figure 1i to 4.69 ppm in Figure 1o, and 2.69 ppm in Figure 1j to 3.96 ppm in Figure 1p. We attribute such positive shifts to the acidification of the original water by the reduction-produced proton.31 In other words, the existing inversemicelle-encapsulated water or/and organic-solvent-dissolved

LETTER

Figure 2. TEM images with size distribution histograms and UV
visible spectra of dodecanethiolate-protected Au NPs synthesized from the toluene solution: (a) the [TOA][AuBr4] complex, (b) the [TOA][AuBr4] complex with water dissolved in 1.6 mL of toluene, and (c) the [TOA][AuBr4] complex with 0.175 mL of H2O. (Note that 0.025 mmol of Au(III), 10 mL of toluene, and 1 mL of NaBH4 aqueous solution were used in all cases. Au/[TOA]þ/thiol/NaBH4 = 1:1:3:10.)

water provided a receiving medium for the reaction-generated protons, enabling Reaction 2 to proceed forward readily. Although H2O saturated in C6D6 made the reduction of Au(III) happen when thiols were added to the organic solution of [TOA][AuBr4] (Figure 1k), no acidified en-H2O (i.e., H3Oþ) was observed (Figure 1q) because the generated protons were not encapsulated in the inverse micelles and could not be observed by NMR. However, no reduction was observed when disulfide (Figure 1r) or thiol (Figure 1s) was added to the organic solution of [TOA][AuBr2]. Role of Water in Forming Smaller NPs. The effect of water on the formation of NPs in the B
S two-phase synthesis was studied by using the synthesized [TOA][AuBr4] complex as the Au source for the NP synthesis because the starting material (the complex) can be made anhydrous. When the anhydrous [TOA][AuBr4] complex was dissolved in 10 mL of toluene and then thiol was added, it was in a stage as Sol-I but without any en-H2O. By directly adding 1 mL of NaBH4 to this solution, the Au NPs shown in Figure 2 were synthesized. As can be seen, the Au NPs shown in Figure 2a had a larger average NP size and a much wider size distribution (1.79 ( 0.49 nm) than those in Figure S6a (1.16 ( 0.17 nm). However, the only difference between the two syntheses was that the latter contained en-H2O (Figure 1m), highlighting the important role that water plays in the B
S synthesis. It turns that not only the presence of water but also the amount of water is equally important. To illustrate the latter, we compared two syntheses that were identical except for the amount of water involved. For synthesis A (syn-A), 0.175 mL of water and 1.6 mL of [TOA][AuBr4] toluene solution was first mixed and stirred for 30 min. Then the organic phase of the mixture was separated out and diluted to 10 mL with toluene. Although the water solubility in toluene is about four times smaller than in benzene,32 a tiny amount of water was still expected to exist in the separated organic phase. For synthesis B (syn-B), no separation of the organic phase was carried out. The remaining synthetic steps (adding thiols and NaBH4) were the same, so the only difference between syn-A and syn-B was the amount of water (much more water in syn-B). However, the resulting Au NPs 7369

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Langmuir Scheme 1. Possible Precursor Species of Au Ions in a Brust
Schiffrin Two-Phase Synthesis

were very different. The syn-A-produced Au NPs are shown in Figure 2b (2.11 ( 0.31 nm), syn-B-produced Au NPs are shown in Figure 2c (1.27 ( 0.18 nm). Clearly, Au NPs made with syn-B (i.e., with more water present) had a smaller average size and a better size distribution. In summary, we have clarified that the Au(I) precursor in a B
S two-phase synthesis is either the [TOA][AuX2] complex or a mixture of the [TOA][AuX2] complex and polymeric [AuISR]n species, which depends on the reaction condition (i.e., with or without an aqueous layer) before the addition of thiols, as illustrated by Scheme 1. We have shown (by 1H NMR spectra) where the thiol proton (RS
H) goes when disulfide is formed after the redox reaction between Au(III) and thiol. More importantly, our data have clearly demonstrated that the inverse micelles of [TOA]þ encapsulating metal ions M(I) together with water are a reaction microenvironment before the addition of NaBH4 aqueous solution in a typical B
S two-phase synthesis. Not only the presence but also the amount of water in the reaction medium has a profound effect on the reduction of Au(III) by thiols and on the formation of uniform small NPs. The presence of polymeric [AuSR]n species formed by not removing the aqueous layer before adding thiol may increase the polydispersity of the synthesized NPs. Additionally, the extremely small size (∼2.5 nm) of the inverse micelles is probably the main reason that the B
S synthesis does not work very well for synthesizing >5 nm metal NPs. It is expected that these new insights will help to achieve better size and size distribution control in metal NPs synthesis.

’ ASSOCIATED CONTENT

bS

Supporting Information. Experimental and synthesis details, tables of proton numbers of intermediates per [TOA]þ in Figure 1, Raman spectra, photographs of Sol-II, and TEM images of the Au NPs synthesized from Sol-I, Sol-II, and polymeric [AuSR]n. This material is available free of charge via the Internet at http://pubs.acs.org.

’ AUTHOR INFORMATION Corresponding Author

*E-mail: [email protected].

’ ACKNOWLEDGMENT This work was supported by grants from the National Science Foundation (CHE 04-56848 and CHE 07-02859). We thank Mr. Yangwei Liu for assistance with some of the reported syntheses, Dr. Bolian Xu for discussing the Raman spectra, and the UMD NISP Laboratory for allowing us to use their TEM facility.

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