C60-mediated self-assembly of gold nanoparticles at the liquid/liquid interface

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Abstract

C60-mediated self-assembly of gold nanoparticles at the liquid/liquid interface in the form of a stable nanocomposite film is repoted. The metallic luster of the interfacial film results from the electronic coupling of Au nanoparticles, suggesting the formation of closely packed nanoparticle thin films. The interfacial film of nanoparticles could be transferred to mica substrates and carbon-coated transmission electron microscopy (TEM) grids, and then studied by UV–vis, Raman spectroscopy and TEM.

Introduction

The intense research activity in the field of nanoparticles is motivated by the search for new materials in order to further miniaturize electronic devices, as well as by the fundamental question of how molecular electronic properties evolve with increasing size in this intermediate region between molecular and solid-state physics [1]. In this respect, molecularly bridged nanoparticle aggregates [2], [3], [4], [5] have been attracting growing interest as a result of their collective electronic, optical, and magnetic properties being distinctly different from a corresponding collection of individual nanoparticles or the extended solid. The 2D/3D control over the spatial arrangement of nanoparticles is primarily based on the thiolamphilic nature [2] of metal nanoparticles, hydrogen-bonding interactions [3], the highly specific recognition interaction of antigens/antibodies [4], and specific base-pairing interactions between DNA and its complementary strand [5].

The liquid/liquid interface has also served as a fertile medium for nanoparticle assembly. Efrima and co-worker first demonstrated that metal liquid-like films of silver could, under stringent conditions, be synthesized at the interface between an organic solvent such as dichloromethane and water [6]. Recently, Kumar et al. have observed that aromatic molecules such as benzene and anthracene present in the organic phase bind strongly with aqueous gold nanoparticles [7]. This process leads to the immobilization of the gold nanoparticles in the form of a highly localized film at the interface. Most recently, Reincke et al. reported that the introduction of ethanol can pull hydrophilic citrate-stabilized Au nanoparticles into the water/heptane interface, leading to a closely packed monolayer [8]. Meanwhile, Duan et al. directed the assembly of hydrophobic and hydrophilic nanoparticles at water/oil interfaces by capping the nanoparticles with appropriate ligands [9]. These previous reports demonstrate a promising way to create a 2D or 3D arrangement of hydrophobic or hydrophilic nanoparticles at water/oil interfaces.

Herein, we present our noticeable finding that C60 can mediate self-assembly of gold nanoparticles at the liquid/liquid interface in the form of a stable nanocomposite film. The fullerenes have been attracting tremendous research interest covering a wide range of fundamental and applied aspects [10], [11]. The role of fullerenes as building blocks for nanomaterials has recently been challenged by monolayer protected gold nanocrystals. For instance, it has been shown that fullerene molecules induce the aggregation of tetraoctylammonium bromide stabilized gold particles in toluene solution with the fullerene relatively weakly bound to the surface of the gold particles [12].

Section snippets

Experimental

The gold sol was prepared by following the literature with difference only in the molar ratio of HAuCl4 to sodium citrate [13]. Namely, 40 mg of HAuCl4 (Aldrich) was initially dissolved in 90 mL of water, and the solution was heated to boiling. 10.2 mL of aqueous solution of sodium citrate (40 mM) was then added to the HAuCl4 solution under vigorous stirring, and boiling was continued for ca. 15 min. The concentration of gold nanoparticles was calculated to be about 6.3 nM by assuming an average 19.6 

Results and discussion

Aqueous suspensions of gold nanoparticles with average diameter of 19.6 ± 3.7 nm were prepared by chemical reduction method as described above. Five milliliters of the freshly prepared gold hydrosol was taken in a vial along with 4.7 mL of diethyl ether resulting in a biphasic mixture with the colorless organic part on top and wine-red colored hydrosol below (Fig. 1, left). Upon addition of 0.3 mL of a 0.1 mM solution of C60 in toluene to this mixture, a thin film of golden reflectance and blue

Conclusions

We have shown that C60 mediate the transfer of gold nanoparticles in water solution to diethyl ether/water interface, results in directing the self-assembly of nanoparticles in the form of a novel nanocomposite film. The interfacial film of nanoparticles could be transferred to different solid substrates and were analyzed by various spectroscopy techniques. The experimental results indicate the formation of closely packed nanoparticles monolayers at water/oil interfaces with the fullerene bound

Acknowledgements

This work was supported by a grant from the MOST/KOSEF to the Environmental Biotechnology National Core Research Center (grant #: Rl5-2003-012-01003-0). The authors are grateful to Joeoong Hahn at Seoul National University for providing a sample of purified C60. The authors also thank Dr. Sun Tach Lim at the Central Laboratory of Gyeongsang National University for TEM measurements.

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