Comparative study of alkylthiols and alkylamines for the phase transfer of gold nanoparticles from an aqueous phase to n-hexane
Graphical abstract
Highlights
► A ligand-assisted phase transfer was used to transfer Au-NPs from an aqueous solution to n-hexane. ► 1-Dodecanethiol (DDT) was the most efficient ligand with the highest transfer efficiency (>96%). ► DDT-assisted phase transfer can preserve the shape and size of Au-NPs during the phase transfer. ► The transfer efficiency is dependent on the molar ratio of ligand to Au-NPs. ► The transfer index is a parameter to evaluate the transfer efficiency.
Introduction
Noble metal nanoparticles (NMNPs) have been extensively studied over the last decade due to their unique physical and chemical properties as compared to their bulk metal equivalents [1], [2]. They have multifarious applications in the areas of opto-electronics, catalysis, film growth seeding, etc. [3]. In many cases, this is attributed to the surface plasmon resonance (SPR) phenomenon of these metal NPs [4], [5]. For example, gold nanoparticles (Au-NPs) show this phenomenon that manifests itself as an intense light absorption at 520 nm [4], [6]. Furthermore, the intensity of this light absorption is significantly dependent on the size of the Au-NPs [7]. Normally, Au-NPs (d > 5 nm) possess the most intense SPR absorption and are therefore the most utilized particles [4], [6], [8].
With modern preparation methods, good yields of Au-NPs can be readily prepared in aqueous phases, resulting in stable dispersions. For example, Au-NPs with particle size in the range of 9 and 120 nm can be prepared in water by the reduction in HAuCl4 with sodium citrates [9]. However, these Au-NPs prepared in aqueous phase are insoluble in organic solvents and flocculate slowly. Therefore, numerous protocols should be developed for preparing Au-NPs in organic solvents, because NPs in organic solvents can be synthesized at relatively high concentrations (up to 1 mol L−1 of reactant) with improved monodispersity compared to those prepared in aqueous solutions. However, preparation of metal NPs in organic solvents requires organic-solvent-soluble metal salts which are comparatively expensive and difficult to obtain. In contrast, water-based synthesis of metal NPs can use more reasonably priced water-soluble metal salts as starting materials. Evidently, compared with that in organic solvents, synthesis in aqueous solution is more prevalent because it is rapid, simple, and more environmentally friendly.
In light of the above, the phase transfer of metal NPs prepared in aqueous solution to an organic solvent is a crucial step dealing with the before-mentioned problems. To our knowledge, metal NPs can be transferred from aqueous solutions to organic solvents because of varying their surfaces from hydrophilicity to hydrophobicity using surfactant modification [10], [11], [12]. So far, many types of metal NPs have been transferred from an aqueous phase to an organic phase with the assistance of alkylamine or alkylthiol compounds [5], [13], [14], [15], [16], [17], [18], [19]. For example, Yang et al. [13] prepared alkylamine-stabilized noble metal NPs (<7 nm) by transferring the NPs from an aqueous layer to toluene. McMahon and Emory [20] developed a method to transfer large Au-NPs (d > 45 nm) from aqueous solutions to chloroform with dicyclohexylamine (DCHA). In addition, Sekiguchi et al. [21] also developed an octanethiol-assisted method for transferring small Au-NPs (5 nm) from an aqueous phase to chloroform. However, in the existing reports, most of the phase transfer methods are only applicable to small (d < 5 nm) and/or large (d > 20 nm) Au-NPs [13], [20], [21], [22], [23], and very few studies have focused on whether alkylthiols and/or alkylamines can transfer Au-NPs (d: 5–20 nm) from an aqueous to an organic phase. Moreover, there have been no studies reported on the comparison between alkylthiols and alkylamines for phase transfer of Au-NPs, although some recent studies have investigated the effect of ligands on phase transfer of Au-NPs [5], [17], [18], [19], [20], [21], [22], [23].
We herein report a simple method, namely ligand-assisted phase transfer, which can be used to transfer citrate-stabilized Au-NPs with particles size in the range of 5 and 25 nm from an aqueous solution to an organic phase (n-hexane) with high efficiency. The whole transfer process can be completed within only 1 min. In order to optimize the ligand-assisted phase transfer method, four different ligands, namely 1-dodecanethiol (DDT), 1-octadecanethiol (ODT), dodecylamine (DDA), and octadecylamine (ODA) were investigated, and the transfer efficiency (TE) as a function of ligand concentration in the n-hexane (or the molar ratio of ligand to Au-NPs) was determined. Moreover, the transfer index (TI) was calculated to confirm the TE. Finally, the transferred Au-NPs were characterized by UV–vis spectroscopy and transmission electron microscopy (TEM) to evaluate their shape and size after the phase transfer. The results of this study are expected to help better choose a ligand to efficiently transfer the Au-NPs from water to an organic solvent.
Section snippets
Materials
All reagents were of analytical grade and used as received without further purification. Sodium borohydride (NaBH4, 99%) and tetrachloroaurate (HAuCl4, 99.99%) from Merck (Darmstadt, Germany) were used as a reducing agent and a precursor, respectively. 1-Dodecanethiol (>98%), 1-octadecanethiol (>98%), dodecylamine (>98%), and octadecylamine (>98%) used in this study were purchased from VWR International (Leuven, Belgium). Polyvinylpyrrolidone 10,000 (PVP10) and polyvinylpyrrolidone 40,000
Effect of ligand on the phase transfer of Au-NPs from aqueous solution to n-hexane
To our knowledge, the transfer of NPs strongly depends on both the chemical affinity between the ligand and the NPs and the solubility of the ligand-NPs-complex compounds in the solvent. The four ligands investigated in this study have a strong binding affinity to Au-NPs due to their thiol or amine groups [26], [27], [28], [29]. Moreover, they are long-chained hydrophobic molecules which are well dissolved in solvents like n-hexane. However, it was found that they have different capacities of
Conclusions
On the basis of a comparative study, we developed an efficient ligand-assisted phase transfer method which can transfer the Au-NPs (d: 5–25 nm) from an aqueous solution to n-hexane. The phase transfer of Au-NPs with 5–25 nm was ignored in the previous studies [13], [20], [21], [22], [23]. In this study, the molar ratio of DDT to Au-NPs is a critical factor affecting the transfer efficiency, and 270–310 is found to be the optimum range, under which the transfer efficiency of this method is >96%.
Acknowledgments
This research project was financed by the Bavarian State Ministry for the Environment and Public Health. L. Li thanks the China Scholarship Council for a scholarship.
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