Metal nanoparticles (NPs) made of tens, hundreds, or thousands of atoms can have tunable chemical and physical properties as a function of NP size (number of atoms), elemental composition, and/or chemical environment (ligand-stabilized, matrix-embedded, or structurally-encaged). These NPs are artificial atoms^1–7  and novel building blocks for new materials that hold novel physicochemical properties as compared to the existing (atomic/molecular) materials. It is expected that these novel materials will enable widespread technological breakthroughs in the not too distant future, for instance in molecular and/or nano-electronics and clean energy generation.^8,9  Within this broad context, organoligands, particularly organothiolate-stabilized metal (mainly Au) NPs, have been subjected to intensive research over the last two decades due to their potential applications in nano-optics,¹⁰  nano-electronics,¹¹  (bio)sensing¹²  and medicinal science (theranostics).¹³ 

The first step towards any practical applications of metal NPs is the synthesis of these metal NPs, preferably air-stable and of homogeneous size distribution and known chemical composition. Among many synthetic methods, the Brust–Schiffrin two-phase method (BSM) synthesis worked out by Brust, Schiffrin, and company in 1994,¹⁴  including its late variants, is definitively the most widely employed synthetic approach to make <5 nm organo-ligand-stabilized metal NPs. Briefly, a typical BSM consists of three steps: Step 1, metal ions are phase transferred (PT-ed) from an aqueous to an organic phase (usually toluene or benzene) with a PT reagent (usually tetraoctylammonium bromide (TOAB), i.e. R₄NBr, R=C₈H₁₇). Step 2, organochalcogen-containing ligand (usually RSH) is added to the separated organic phase during which Au^III cations can be reduced to Au^I cations. Step 3, metal ions residing in the separated organic phase are reduced into M⁰ by a reducing reagent like NaBH₄ during which organochalcogenolate-protected metal NPs are formed.

Despite the prevailing use of the BSM in the synthesis of sub-5 nm metal (mainly Au) NPs (according to Thomson Reuters' Web of Knowledge, the original paper¹⁴  has accumulated a current number of citations as high as 3755, and counting), mechanistic details of the BSM synthesis have been sketchy until very recently.^15–18  A long-held belief concerning the metal precursor in the synthesis of metal NPs, probably due to earlier papers by Whetten et al.,^19,20  has been that the metal-thiolate polymer, [Au^ISR]_n, is the metal ion precursor of metal NPs. However, a recent paper by Goulet and Lennox¹⁸  has shown that the metal–TOA⁺ complex, [TOA][Au^IBr₂], can also be the major metal ion precursor. Our ensuing studies have not only confirmed the results of Goulet and Lennox, but also proposed that the BSM synthesis is an inverse micelle based approach based on their proton NMR results and showed via Raman spectroscopic study that the Au–S bond does not form until the formation of Au NPs.¹⁵  In this chapter, we will review and discuss in various degrees of detail the relevant chemistry involved, particularly the role of encapsulated water, in the BSM synthesis of alkylchalcogenolate-stabilized metal NPs unravelled after the paper of Goulet and Lennox¹⁸  and highlight the similarity and difference when ligands containing different chalcogen elements (S, Se, or Te) are used as the starting source of the NP-stabilizing agents.

The experimental evidence of possible encapsulated water by TOAB in an organic phase came first from the observation of a large down-field shift (∼2 ppm, due largely to the appearance of hydrogen bonding among the water molecules that strongly suggests the formation of water aggregates) of the water proton peak in C₆D₆ containing dissolved TOAB (0.03 mmol of TOAB in 0.8 mL C₆D₆) as compared to that of pure C₆D₆ after both being mixed with 0.105 mL Milli-Q water (18.2 MΩ) and the extra water layer being then removed, as shown in Figure 1.1. The water peak at 2.43 ppm was also reported in the Goulet and Lennox paper.¹⁸ 

More convincing and detailed evidence of the inverse micelle formation is shown in Figure 1.2, proton NMR spectra of a series of samples prepared by mixing various amounts of TOAB with 0.8 mL C₆D₆ and 0.210 mL Milli-Q water and then separating the undissolved water. The two clearly distinguishable regimes enable the critical micelle concentration (CMC) to be determined:²¹  the intersection of the two dashed lines gives the CMC of TOAB in C₆D₆=7.5 mM, which is about 4 to 5 times smaller than the TOAB concentrations generally used in a typical BSM synthesis of metal NPs. That is, under the normal condition of the BSM synthesis, inverse micelles enclosed by TOAB are formed.

Unlike other well-known inverse micelle systems, such as sodium 2-ethylhexylsulfosuccinate (AOT), whose size can be readily varied by changing water/surfactant ratio,²²  our proton NMR data show that the inverse micelles of TOAB can be saturated even with a very small amount of water, as shown by the proton NMR spectra in Figure 1.3A and the corresponding normalized peak integrals in Figure 1.3B.¹⁶  The samples for the spectra in Figure 1.3A were prepared with the same amount of TOAB (0.03 mmol) in 0.8 mL C₆D₆ but mixed with different amounts of water before the organic phase being separated for the NMR measurements. Interestingly, the amount of encapsulated water remained constant even if the water amount varied from 0.0425 mL to 0.21 mL, so did the peak positions of the encapsulated water and –CH₂N⁺.

With the information in Figure 1.3, we can estimate the average size of the inverse micelles. According to Figure 1.3B, the average number of protons from the H₂O in Samples (a–d) is 5.11 per TOAB. After subtracting the number of protons from the H₂O saturated in C₆D₆ (i.e. 1.63 per TOAB (Sample (f)), the number of protons from the H₂O encapsulated in the inverse micelles formed by TOAB is 3.48, which means that the ratio of the encapsulated H₂O to TOAB is 1.74. Assuming that one molecule of TOAB is surrounded by one molecule of H₂O, 57% of the encapsulated H₂O might be located in the outer layer of the inverse micelle core. Together with the fact that the H₂O volume per molecule is 0.03 nm³ at 20 °C (density of H₂O: 0.998 g mL⁻¹), the diameter of the inverse micelle core, i.e. the part of the H₂O encapsulated in the inverse micelle of TOAB, is ∼2.5 nm, which is at the small end of known inverse micelles.²² 

The first step of the BSM synthesis is the phase transfer (PT) of metal ions from the initial aqueous solution to the organic phase using TOAB as PT agent. The proton spectra in Figure 1.4A were obtained after the PT of different amounts of Au ions into the organic phase.¹⁶  The corresponding normalized proton peak integrals and sample preparation parameters are collected in Figure 1.4B. In addition to constant C₆D₆ volume (0.8 mL) and amount of TOAB (0.03 mmol), the volumes of the aqueous solutions were kept the same as used for Figure 1.3. Thus, the corresponding TOAB:Au ratios were 1:1, 2:1, 3:1, and 5:1 for spectra (g) to (j) respectively. 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 spectra (k) and (l) respectively.

As can be seen in Figure 1.4A, the introduction of metal species not only caused clear variation of peak positions of the encapsulated water and α-proton in –CH₂N⁺, but also the amount of encapsulated water. The pair (H₂O/–CH₂N⁺) peak values are now down-field shifted as the Au content decreases: 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. Notice that the α-¹H peak position of –CH₂N⁺ in the TOAB : Au=1:1 case [Figure 1.4(g)] is the same as that of the synthesized [TOA][AuBr₄] complex [Figure 1.4(k), (l)]. This indicates strongly that one [TOA]⁺ cation was associated with one [Au^IIIX₄]⁻ anion with X=Br⁻ or/and Cl⁻. As the TOAB : Au ratio increases, which corresponds to a decrease of Au content because a constant TOAB amount was used, more and more [TOA]⁺ cations can be associated with halogen anions (e.g., Br⁻ or Cl⁻) other than the Au^III complex. However, the fact that only one α-¹H peak of –CH₂N⁺ was observed in all cases is a sign of fast exchange between the Au^III complex units and the halogen anions in the inverse micelles.

Now taking 2.97 ppm [Figure 1.4(g), (k), (l)] and 3.34 ppm [Figure 1.3(a) to (d)] as the α-¹H peak positions of the –CH₂N⁺ in two extremities, i.e., all Au^III units vs. all halogen anions, we calculated the expected peak position using the fast-exchange model δ(α-¹H)=2.97y+3.34(1−y) (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 the experimentally observed values: 3.18, 3.22, and 3.26 ppm [Figure 1.4(h) to (j)]. That these two sets of values are in excellent agreement indicates strongly that the partitioning of the Au^III units and the halogen anions in a given inverse micelle follows the nominal stoichiometry.

Figure 1.5 presents the ¹H NMR spectra of samples containing Ag ions in the organic phase after the PT. The samples were prepared with 0.8 mL C₆D₆, 0.03 mmol of TOAB, 0.4 mL aqueous solution of AgNO₃ of different concentrations. After the PT of Ag ions, the organic phase was collected and used for NMR measurements. As can be seen from the figure, the peak positions of water are very similar to those in Figure 1.4A, indicating the formation of inverse micelles. Moreover, the same trend in down-field shift and increase of water amount were observed as the Ag content decreases in the order TOAB : Ag=3:1, 3.5:1, 4:1, and 5:1. The peak integrals for the encapsulated water (normalized by setting that of α-¹H of –CH₂N⁺ to 8) are 2.7, 2.9, 3.4, and 3.7 respectively, which are also close to those found in the Au case (see Figure 1.4B).

We show in Figure 1.6 the ¹H NMR spectra of the organic phase after PT of Pd^II (from PdCl₂) or Pt^VI (from H₂PtCl₆). The samples were again prepared with constant C₆D₆ volume (0.8 mL) and constant TOAB amount (0.03 mmol) but various metal content: TOAB : Pd or Pt=1:1, 2:1, 3:1, and 5:1 for spectra (a) to (d) respectively. Interestingly, as the metal content decreases, we observe again the down-field shift and amplitude increase of the water peak for both Pd^II and Pt^VI samples, as those observed for the Au and Ag samples in Figures 1.4A and 1.5, respectively, also suggesting the formation of inverse micelles. However, the trend of variation for the α-¹H peak of –CH₂N⁺ in the Pd^II case (left panel in Figure 1.6) is opposite to those of the three other metals, for which the chemical reason is still unclear but probably has to do with the difference in the type of complex structure formed with TOA⁺. Notwithstanding such difference, the water aggregates behave in a remarkably similar fashion that is related directly to the formation of inverse micelles, as clearly alluded to by the results shown in Figure 1.6.

The second step in a typical BSM synthesis is to add ligand to the organic phase that contains PT-ed metal ions. This is the step where divergence exists in the literature as to what precursor species become the PT-ed Au^III ions. Earlier work by Whetten asserted that the Au^III ions were reduced to Au^I and formed [Au^IRS]_n polymeric species.^19,20  This assertion has been widely accepted, cited, or/and assumed as the Au precursor species in the BSM synthesis until the Goulet and Lennox paper¹⁸  in which [TOA][Au^IX₄] and [TOA][Au^IX₂] complexes were shown to be the relevant metal ion precursors, which also applies to Ag and Cu. Our work confirms this important discovery by showing that no metal–sulfur bond is formed during this step, as presented in Figure 1.7.¹⁵ 

It is expected that a Au–S bond will form when RSH self-assembles on a Au surface or polymeric [AuSR]_n species are formed. This is indeed what we have observed, as shown in Figure 1.7 by the presence of a Au–SR vibrational band at 327 cm⁻¹ in the Raman spectra (c) and (d) for C₁₂SH self-assembling on a rough Au surface and for the synthesized [AuSR]_n-like polymer respectively. However, no such vibrational band was observed when C₁₂SH was added to an organic phase of TOAB that contained PT-ed Au ions obtained in the first step of a BSM synthesis (see spectrum (e) in Figure 1.7), indicating no formation of the Au–S bond that is expected for the presence of polymeric [AuSR]_n species. The appearance of a Au–Br band at 209 cm⁻¹ in spectrum (e) is direct experimental evidence confirming the exchange of Br⁻ from TOAB with Cl⁻ in the original (AuCl₄)⁻.

Figure 1.8 shows the Raman spectra of species formed in the 2^nd step of the BSM synthesis of Ag (A) and Cu (B) NPs and of some reference materials.¹⁵  Again, no Ag–S or Cu–S vibrational band was observed among the species formed in the 2^nd step of the BSM synthesis: spectrum (c) in Figure 1.8A and B. No RS–SR but RS–H observed in the Ag case indicates that no reduction of Ag^I took place. On the other hand, a strong RS–SR band was observed in the Cu case, illustrating that the added C₁₂SH reduced Cu^II to Cu^I without forming a Cu–S bond.

Figure 1.9A presents the proton NMR spectra of the organic phase obtained in the 2^nd step of the BSM synthesis for samples (g) to (k) shown in Figure 1.4A, together with those of reference disulfide (spectrum (r)) and thiol (spectrum (s)).¹⁶  The formation of disulfide (peak at 2.58 ppm) in all samples indicates that the added thiol reduced Au^III to Au^I ions, which was also observed by Goulet and Lennox previously.¹⁸  The appearance of even more down-field shifted water peaks evidences the acidification of the encapsulated water by the reaction

in which acidic protons were generated. That is, the existing inverse micelle encapsulated water or/and organic-solvent-dissolved water provided a hydrophilic receiving medium for the reaction-generated protons, enabling Reaction (1.1) to proceed forward readily.

Figure 1.10 compares the proton NMR spectra of the reaction intermediate C₆D₆ solutions in the 2^nd step of the BSM synthesis of Pd (a) and Pt (b) NPs. As can be seen, the added C₁₂SH did not react with PT-ed Pd^II ions but reduced PT-ed Pt^IV ions. As in the case of Au, the latter led to the formation of disulfide:

The large down-field shift of the encapsulated water peak (5 ppm) was the result of Reaction (1.2).

When using Se- and Te- (vide infra) containing organo-ligands as metal NP-stabilizers, one has to use air-stable diselenides or ditellurides rather than air-instable selenols or tellurols. Earlier work by Ulman et al.²³  showed that Au NPs synthesized with one-phase (1p) BSM had better quality than those synthesized by two-phase BSM. Our research has shown that this is highly likely related to the fact that only one dominant single metal precursor species exists for the 1p BSM synthesis while multiple metal precursor species co-exist in the 2p BSM synthesis.

Figure 1.11 presents the ¹H and ¹³C NMR spectra of the reaction intermediate solution (a)/(c) and the starting dioctyl-diselenide (C₈Se)₂ (b)/(d) of the 1p BSM synthesis in which the THF solution of (C₈Se)₂ and aqueous solution of HAuCl₄ were directly mixed and no PT agent was used.²⁴  The ¹H peak of ¹H₂CSe– at 2.75 ppm in the starting ligand [Figure 1.11(b)] disappeared in the 1p intermediate [Figure 1.11(a)], which indicates that the Se–Se bond of the starting ligand was broken, i.e., the reaction between (C₈Se)₂ and HAuCl₄ occurred. This also corroborates with the large ¹³C shift for C₁/C₂ of the reaction intermediate solution [Figure 1.11(c)] vs. those of the starting ligand [Figure 1.11(d)]. Most importantly, the eight ¹³C peaks in Figure 1.11(c) imply that the 1p intermediate consists dominantly of a single type of metal complex.

Figure 1.12 shows the corresponding Raman spectra of the starting ligand (a), the reaction intermediate solution (b), and aqueous solution of HAuCl₄.²⁴  The RSe–SeR vibrational band at 287 cm⁻¹ observed in the starting ligand [spectrum (a)] largely disappeared in spectrum (b) of the reaction intermediate solution, indicating the majority of Se–Se bonds were broken, in agreement with the NMR results discussed above. This is further corroborated by the disappearance of the characteristic bands of starting HAuCl₄. Moreover, aided by DFT calculations,²⁴  the band 142 cm⁻¹ was assigned to Au–Se vibration. Additionally, XPS measurements showed that the oxidation state of Se in the reaction intermediate solution was 2+ instead of the starting −1 and that of Au was −2. Thus, by combining the above NMR, Raman, and XPS results altogether, it was concluded that the reaction between HAuCl and (C₈Se) did occur and produced dominantly one type of metal complex, highly likely in the form of Cl₂AuSe(C₈)Cl₂.

¹H NMR (A) and Raman (B) spectra of the reaction intermediate solutions obtained in the 2^nd step of the 2p BSM synthesis with different TOAB : Au ratios but the same Au : Se (1:1) ratio are presented in Figure 1.13.²⁴  Both ¹H NMR and Raman of the simply mixed C₆D₆ solution of TOAB and (C₈Se)₂ (spectrum (a) in Figure 1.13A and B) proved that no reaction took place between the two species, although the slight blue-shift of the RSe–SeR band²⁵  (from 287 to 292 cm⁻¹) does indicate a certain degree of interaction between them. However, reaction occurred when the Au^III ions were added, but was not as fast as observed in the 1p BSM synthesis, as indicated by the persisting proton peak of ^_C¹H₂N⁺ at 2.73 ppm in Figure 1.13A and the RSe–SeR vibrational band at 292 cm⁻¹ in Figure 1.13B. As TOAB : Au ratio increased, spectral features (2.73 ppm peak in (A) and 287 cm⁻¹ band in (B)) related to the RSe–SeR bond decreased and eventually disappeared completely (from (a) to (e)), implying that the majority of RSe–SeR bonds were broken.

The observation of the hydronium ions (H₃O⁺), as indicated in Figure 1.13A, is intriguing because a simple phase transfer of Au^III ions does not generate observable hydronium ions. Although the subsequent addition of thiol does, where the hydrogen in the –SH group acts like a reductant, (C₈Se)₂ does not have such an obvious proton source. Thus, hydrolysis of encapsulated water, such as Se–Se+2H₂O→2SeOH+H⁺, might have happened. We speculate that the band at 660 cm⁻¹ is related to a hydrogen-bonding-like Se–H₂O interaction²⁶  before the Se–Se bond breaking and the band at 642 cm⁻¹ to a Se–OH like bond after the Se–Se bond breaking. On the other hand, the appearance of the further down-field shifted water whose amount increases as the TOAB : Au ratio decreases suggests the existence of inverse micelles and that the degree of the hydrolysis reaction depended on the amount of encapsulated water: the larger the latter is, the higher the former will be. The available XPS data for the 3TOAB:1Au (e) and 2TOAB:1 Au (d) samples indicate that the oxidation states of Au in these two 2p samples (84.2 eV and 84.3 eV respectively) were basically the same as that of Au in the 1p synthesis (84.25 eV) but those of the Se (57.98 eV and 58.05 eV) were different from the 1p synthesis (58.4 eV).²⁷  That is, the smaller binding energies in the former indicate that the charge screening of Se was higher than that in the latter, which is consistent with the proposed Se–OH bonding.

Both the ¹H NMR (Figure 1.13A) and Raman (Figure 1.13B) data strongly suggest that more than one species exist as the 2p intermediates. The appearance of encapsulated H₃O⁺ as shown in Figure 1.13A suggests that hydrolysis of water had happened and likely led to species such as X₂AuSe(C8)(OH)X (X=Br or Cl) as one of the intermediates whose exact compositions still need to be further studied. One important difference between the 1p and 2p BSM syntheses, however, is that the results shown in Figures 1.11 and 1.12 were highly reproducible but not those in Figure 1.13 which appeared to be highly dependent on many hard-to-control factors.

Among organochalcogenolate-stabilized metal NPs, organotellurolate-metal NPs are the least investigated system. Figure 1.14 presents the proton NMR spectra of (a) the intermediate reaction C₆D₆ solution obtained in the 2^nd step of the BSM synthesis of Au NPs using (C₁₂Te)_2, (b) that obtained in the 1^st step, (c) pure (C₁₂Te)₂ solution, and (d) pure C₁₂SH solution. The first observation is that, in great contrast to using thiol (d), there was no acidic H⁺ formation when (C₁₂Te)₂ was used (a): the peak position of the encapsulated water remained almost unchanged as compared to the organic phase right after PT (b), although broadened significantly. Instead, a peak assignable to RTeX₃ appeared (a). Also, little unreacted (C₁₂Te)₂ is left.²⁸ 

Raman measurements²⁸  corroborate well with the NMR ones. Figure 1.15 compares the Raman spectrum (a) of the sample (a) in Figure 1.14 with that of (b) pure (C₁₂Te)₂, (c) the synthesized [TOA][Au^IBr₂] complex, and (d) TOAB. Several observations can be made here. First, disappearance of the RTe–TeR band at 194 cm⁻¹ in the reaction intermediate indicates that the reaction between the added (C₁₂Te)₂ and the PT-ed [TOA][Au^IIIX₄] complex broke the Te–Te bond. Second, the appearance of the 209 cm⁻¹ band^15,29  in (a) that can be reasonably assigned to the Au–Br₂ band in the synthesized [TOA][Au^IBr₂] complex (c) suggests that the initial Au^III ions were reduced to Au^I ions. Third, that no Au–Te vibrational band (∼190 cm⁻¹) was observed implies that no Au–Te bond was formed during the reaction. Fourth, assisted by the DFT calculations, the 280 cm⁻¹ band can be assigned to RTe–X₃.

Moreover, the available XPS data showed that the oxidation state of Au in the reaction intermediate [sample a in Figure 1.14(a) and 1.15(a)] was +1 and that of Te was +2. Therefore, summarizing the above discussion, we propose the following stoichiometric reaction for the reduction of Au^III to Au^I by the ditelluride, whose oxidation state changes accordingly from −1 to +2:

On the other hand, the ditelluride does not react with PT-ed Ag complex if added to the separated Ag-containing organic phase.

As alluded to in the previous discussion, the inverse micelle encapsulated water offers a hydrophilic micro-environment as a proton accepting reaction medium, enabling Reaction (1) to proceed. Such an important role was further confirmed by using the synthesized, anhydrous metal complex as the starting material. 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.¹⁶ 

It has been observed that if the inverse micelle encapsulated water is the main form of water existing in the BSM synthesis, then addition of thiol does not lead to the formation of Au–S bonds, i.e., formation of polymeric [AuSR]_n species. However, if more water beyond saturating the inverse micelles exists, then addition of thiol can lead to the formation of polymeric [AuSR]_n species providing that the thiol : Au ratio is larger than 2. For instance, if the water layer of the original aqueous solution of Au salt was not discarded after the PT of Au ions to the organic phase and to which thiol was added, then stirring would lead to the formation of a white cloudy material that sits between the aqueous and organic layers. Such a white cloudy material gave solution Raman spectrum (c) in Figure 1.16 in which those of the self-assembled thiol on a rough Au surface (a) and the synthesized polymeric [AuSR]_n species (b) are also presented for comparison.¹⁶ 

That Figure 1.16(c) is almost identical to Figure 1.16(b) suggests strongly that the white cloudy material was the polymeric [AuSR]_n species. On the other hand, stirring the organic phase obtained in the 2^nd step of a BSM synthesis in the air long enough can also lead to the formation of a small amount of polymeric [AuSR]_n species in addition to forming disulfide, as indicated by Figure 1.16d. Summarily, Figure 1.17 presents a general metal NP formation mechanism¹⁵  in a BSM synthesis (A) and reaction conditions¹⁶  that lead to different, complex vs. polymer, reaction routes.

It has also been discovered that the encapsulated water is also essential for S–S or Se–Se bond breaking in a BSM synthesis if the former is used as the source of stabilizing ligand. This is illustrated by the proton spectra of the reaction intermediate solutions with and without encapsulated water presented in Figure 1.18.³⁰  For both (C₁₂S)₂ and (C₁₂Se)₂, the pre-existence of encapsulated water enabled S–S and Se–Se bond breaking to happen, as indicated by the appearance of the acidified water peaks in Figure 1.18(a) and (b) for disulfide and in Figure 1.18(d) and (e) for diselenide with TOAB : Au ratio=3:1 and 2:1 respectively. On the other hand, the presence of remaining disulfide as suggested by the residual peak at 2.57 ppm suggests that diselenide was more reactive than disulfide. Without the presence of inverse micelle encapsulated water, no S–S and Se–Se bond breaking took place, as confirmed by spectra in Figure 1.18(c) and (f) where no acidified water peaks but those of disulfide (2.57 ppm) and diselenide (2.78 ppm) are observed.

Since no viable proton sources other than water existed in the reaction solutions and no vibrational evidence of Au–S^15,31  and Au–Se²⁴  bond formation was observed, the appearance of acidic protons as described above strongly implies that the overall reaction may involve hydrolysis, most likely in the form:

where X stands for either S or Se. The presence of paramagnetic Au^II ions was alluded to, although not definitively, by the preliminary electrochemistry and UV characterizations that at least ascertained that the species in the reaction solutions were neither Au^III nor Au^I.

The last step of a normal BSM synthesis is to reduce the metal ions to the zero valence state, usually by NaBH₄, and form metal NPs.¹⁴  The exact chemical state of the metal ion precursors and the composition of the reaction intermediate solutions before this final stage are critically important in determining how much control, and therefore how good the quality of the metal NPs, can be achieved. Figure 1.19 compares the TEM images of three Au NP samples made with the same starting (0.025 mmol) [TOA][Au^IBr₂] complex in a mixture of 10 mL toluene and 0.21 mL water but slightly different ligand compositions before the addition of aqueous solution of NaBH₄: (a) 3C₁₂SH, (b) a mixture of 1C₁₂SH and 1(C₁₂S)₂, and (c) 1.5(C₁₂S)₂.¹⁷  Since Au^I was the starting metal ion, the addition of thiol would not lead to Reaction (1) and neither would it form thiolate polymer. As can be seen, a mixture of different ligands, which is usually the case in a normal BSM synthesis of thiolate-stabilized Au NPs, degrades the quality of the Au NPs formed. If a single type of ligand is used, thiol would be a better choice than disulfide.

In Section 1.3.2, we discussed the chemical state of the species existing in the reaction intermediate solutions of the 1p vs. 2p BSM synthesis of organoselenolate-stabilized Au NPs. It was concluded that a single dominant metal precursor species existed in the reaction intermediate solution of the 1p BSM synthesis but multiple species co-existed in that of the 2p BSM synthesis. The TEM images of the final Au NPs made by the 1p (a) and 2p (b) BSM synthesis are presented in Figure 1.20.²⁴  As can be clearly seen, the former generated much better quality Au NPs than the latter did.

In Section 1.4, we discussed the enabling effect of the inverse micelle encapsulated water to break the S–S or Se–Se bond that led to different chemical states of the reaction solutions before the addition of NaBH₄. Figure 1.21,³⁰  presents the TEM images of the Au NPs formed after adding NaBH₄ to reaction solutions that gave the proton NMR spectra shown in Figure 1.18 (a), (c), (d) and (g). The contrast in quality of Au NPs made with and without encapsulated water is quite impressive: from 1.9±0.3 nm to 3.6±1.6 nm for disulfide and from 2.4±0.8 nm to 5.8±2.1 nm for diselenide.

In addition to the identification of [TOA][Au^IIIBr₄] and [TOA][Au^IBr₂] complexes as the major intermediate species in the BSM synthesis by Goulet and Lennox,¹⁸  we have demonstrated, as discussed in the previous sections, that the Au–S bond, and metal–ligand bond in general, does not form until the metal ions are reduced to zero valence atoms and forming nascent metal clusters, as illustrated in Figure 1.17A. This discovery led us to experiment reversing the order of adding stabilizing ligand and reductant NaBH₄ to the reaction solution,¹⁵  i.e., adding NaBH₄ first then followed by adding the ligand. Indeed, narrowly distributed thiolate-stabilized Au NPs could be made by using the reversed BSM synthesis and the size could be controlled by the time interval between the addition of the reductant and the ligand even with the same S : Au ratio, as shown in Figure 1.22.¹⁵  Notice that in the normal BSM synthesis, S : Au ratio is used to control the NP size. Thus, the same S : Au ratio would lead to the same NP size.

One unique feature of the reversed BSM synthesis is that it enables sub-2 nm selenolate- or tellurolate-stabilized Au NPs to be synthesized as demonstrated by Figure 1.23,¹⁵  which had never been achieved previously. In the syntheses that led to the formation of those Au NPs, the starting source for Au ions was the synthesized [TOA][Au^IBr₂] complex and dioctyl diselenide or didodecyl ditelluride as stabilizing ligand. 10 s lapsed between adding the reductant and the ligand. It is also worth noting that the reversed BSM applies equally well to synthesizing small Ag and Cu NPs.

In this chapter, we have discussed in rather detailed fashion metal NP formation chemistry, particularly the chemical state of the intermediate species leading to the final formation of metal NPs, in light of the recent advanced mechanistic understanding of the popular BSM synthesis. The most salient point of the latter is that the BSM synthesis is basically an inverse micelle process in which encapsulated water plays a critical role in determining the details of metal NP formation chemistry. For instance, it provides a hydrophilic proton accepting reaction-facilitating medium if thiol is used as the source of stabilizing ligand (Reaction (1.1) and (1.2)). In the case of disulfide or diselenide, it also acts as a bond breaking enabler (Reaction (1.4)). On the other hand, what critical role the encapsulated water would play in the case of ditellurite (Reaction (1.3)) is still not clear.

It is important to point out that the inverse micelles formed by TOAB may behave differently from the traditionally well-studied inverse micelles such as AOT in that the wall of TOAB inverse micelles may not be as closely packed, therefore as rigid, as that of AOT-like inverse micelles because of the four equal-length, therefore space-occupying branches, particularly for shorter ones. Moreover, it has been observed that the TOAB inverse micelles do not have an easily measurable size range that can be varied because they only need a tiny amount of water to get saturated. Still, their physicochemical properties warrant further investigations due to the prevailing use of the BSM synthesis in making sub-5 nm metal NPs. Overall, it is expected that the chemistry detailed in this chapter will be helpful to any further attempts at improving synthesis of metal NPs of better quality and with better control of properties.

The research in the Tong Lab at Georgetown University is financially supported by NSF (CHE-0923910 and CHE-0456848) and by DOE-BES (DE-FG02–07ER15895).