New Insight into Intermediate Precursors of Brust-Schiffrin Gold Nanoparticles Synthesis Lili Zhu, † Chen Zhang, † Chengchen Guo, † Xiaoliang Wang, † Pingchuan Sun, ‡ Dongshan Zhou, † Wei Chen, † and Gi Xue* ,† † Department of Polymer Science and Engineering, School of Chemistry and Chemical Engineering, The State Key Laboratory of Coordination Chemistry, Nanjing University, 210093, People’s Republic of China ‡ Key Laboratory of Functional Polymer Materials, Ministry of Education, Nankai University, Tianjin 300071, People’s Republic of China * S Supporting Information ABSTRACT: There is an ongoing intensive debate on the mechanism of gold nanoparticles formation regarding the intermediate precursors prior to the addition of reducing agent. A new detailed view of the widely used Brust-Schiffrin two-phase method to prepare gold nanoparticles is presented here. Precursor species of these reactions have been identified and quantified by NMR, UV-visible, Fourier-transform Raman spec- troscopy, etc. We demonstrate that tetraalkylammonium gold complexes ([TOA][AuX 2 ]) and soluble gold thiolate ([TOA][AuSRX] and [TOA][Au(SR) 2 ]) were detectable as the precursors by NMR spectros- copy. Their relative contents depend on the concentration of reactants. Higher concentration of the reactants is favorable for the formation of soluble thiolate. Polymeric gold thiolate [Au(I)SR] n could eventually precipitate from the solution under specific conditions. The clear mechanism presented here is of great significance to tailor the size and properties of the final products. ■ INTRODUCTION The outstanding behaviors of gold nanoparticles (Au NPs) arise from their fascinating optical, electronic, catalytic, and chemical properties, which are intimately correlated with their size and the chemical nature of their core and surface species. 1-7 Wet chemistry techniques based on chemical reactions in solution are widely used to yield Au NPs with a wide range of sizes, shapes, and dielectric environments. Among them, the Brust-Schiffrin two-phase method is the most popular route to prepare organic ligands (mostly thiolate) stabilized Au NPs. 1 Since it was introduced, it has inspired a number of related approaches 7,8 and been successfully applied to synthesize other metal nanoparticles. 9,10 The kinetics for the NPs formation is very critical to generate uniform NPs with size control. 11-14 However, there remain significant questions regarding the detailed mechanism. 15-22 In particular, there is an ongoing debate regarding the precursor species prior to reduction. Brust-Schiffrin two-phase synthesis generally consists of three steps: (i) hydrogen tetrachloroaurate (HAuCl 4 ) in water was transferred into the organic layer (i.e., toluene, chloroform, or benzene) by the phase transfer agent tetraoctylammonium bromide (TOAB); (ii) thiols were added to the organic phase and Au(III) was reduced to Au(I); and (iii) finally, 1-3 nm monodispersed Au NPs was synthesized by addition of the reducing agent sodium borohydride (NaBH 4 ). The early accepted assumption about this method by Schaaff, Murray et al. has been that polymeric Au(I) thiolate [Au(I)SR] n can be generated as intermediate precursors. 12-16 However, these proposed precursor species have been confirmed to be insoluble in common solvents and are very difficult to investigate in solution. Yet recently, research from Lennox 3 and Tong 4,23 has shown otherwise and draws the mechanistic studies into question. It was shown that [Au(I)SR] n was not a measurable (by 1 H NMR) precursor in reaction solution and tetraalkylammonium metal complex [TOA][AuX 2 ] was the relevant intermediate Au species. In spite of these reports, a limited number of studies have examined the mechanism. In general, the synthesis mechanism remains a major challenge and deserves further exploration. Investigating the composition of precursor species could also lead to clues on how to control the properties of resulting NPs, thus a particular focus is placed on the precursor of metal ions prior to the addition of reductant. In the present study, we used 1-octadecanethiol (C 18 SH) to undergo the Brust-Schiffrin two-phase reaction in toluene. We focused our investigations on the composition of the intermediate precursors after addition of thiols into Au 3+ solution. Presursors before adding the reductant were in situ monitored by NMR spectroscopy (1D 1 H NMR, Homonuclear J-resolved 2D NMR, 1 H- 1 H COSY). Tetraalkylammonium gold complexes ([TOA][AuX 2 ]) and soluble gold thiolate Received: March 1, 2013 Revised: May 3, 2013 Published: May 6, 2013 Article pubs.acs.org/JPCC © 2013 American Chemical Society 11399 dx.doi.org/10.1021/jp402116x | J. Phys. Chem. C 2013, 117, 11399-11404