Mechanistic Insights into the Brust–Schiffrin Synthesis of Organochalcogenolate-Stabilized Metal Nanoparticles

YUAN GAO, YANGWEI LIU, YING LI, OKSANA ZALUZHNA AND YUYE J. TONG

Department of Chemistry, Georgetown University, 37th & O Streets, NW, Washington, DC 20057, USA

1.1 Introduction

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 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. Within this broad context, organoligands, particularly organothio-late-stabilized metal (mainly Au) NPs, have been subjected to intensive research over the last two decades due to their potential applications in nano-optics, nano-electronics, (bio)sensing and medicinal science (theranostics).

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, 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 AuIII cations can be reduced to AuI 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 paper 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. A long-held belief concerning the metal precursor in the synthesis of metal NPs, probably due to earlier papers by Whetten et al., has been that the metal-thiolate polymer, [AuISR]n, is the metal ion precursor of metal NPs. However, a recent paper by Goulet and Lennox has shown that the metal–TOA+ complex, [TOA][AuIBr2], 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. 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 alkyl-chalcogenolate-stabilized metal NPs unravelled after the paper of Goulet and Lennox 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.

1.2 Phase Transfer of Metal Ions: Formation of Inverse Micelle Encapsulated Water

1.2.1 Proton NMR Evidence of Encapsulated Water

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.

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: 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...