Luminescent Ag7 and Ag8 Clusters by Interfacial Synthesis

Abstract

Molecular quantum clusters of noble metals are a fascinating area of contemporary interest in nanomaterials. While Au11, [1] Au13, [2] and Au55 [3] have been known for a few decades, several new clusters were discovered recently. These include Au8, [4] Au18, [5] Au25, [6] Au38, [7] and so on. Au11 has also been the subject of recent research. In view of their luminescence, several of these clusters are expected to be important in biolabeling and fluorescence resonance energy transfer as well as for creating luminescent patterns. There are many examples of template-assisted synthesis of water-soluble luminescent silver clusters with cores ranging from Ag2 to Ag8, having characteristic electronic transitions between 400–600 nm. However, unlike the case of gold, there are only limited examples of monolayer-protected silver analogues. Silver clusters protected with aryl, aliphatic, and chiral thiols have been reported, some of which have characteristic optical and mass spectrometric signatures. There is also a family of well-characterized metal-rich silver chalcogenide clusters. Besides single-crystal diffraction, mass spectrometry has also been used for detailed understanding of these clusters. Ag clusters with and without luminescence have also been reported. Herein we present gram-scale syntheses of two luminescent silver clusters, protected by small molecules containing thiol groups, with well-defined molecular formulas, by interfacial synthesis. This new synthetic approach has become promising in several other areas including semiconductor nanoparticles, two-dimensional superlattices, and 3D structures. A crude mixture of redand blue-green-emitting clusters Ag8(H2MSA)8 and Ag7(H2MSA)7 (H2MSA: mercaptosuccinic acid), respectively, was synthesized in gram quantities by an interfacial etching reaction conducted at an aqueous/ organic interface starting from H2MSA-protected silver nanoparticles (Ag@H2MSA) [19] as precursor (for details see the Experimental Section and Figure S1 in the Supporting Information). During the reaction, the optical absorption spectrum of the aqueous phase showed gradual disappearance of the surface plasmon resonance at 400 nm (Figure 1A) of metallic silver nanoparticles. The color of the aqueous phase gradually changed from brown to yellow and finally to orange. The particles of Ag@MSA are polydisperse (Figure 1Ca) and form smaller clusters in the aqueous phase upon etching (Figure 1Cb) with complete disappearance of the nanoparticles. The unetched particles move to the junction of the two phases and form a self-assembled film of monodisperse nanoparticles, resembling two-dimensional superlattices (Figure 1Cc), which appears blue in color. The smaller clusters formed in the reaction upon longer electron-beam irradiation coalesce to form nanoparticles (Figure S2). It is known that such clusters are unstable to high-energy electrons. The peak at 600 nm, which appears at shorter reaction time (60 min) and may be due to interplasmon coupling, disappears slowly, and a new feature is seen at 550 nm after 48 h of reaction (Figure 1A). In accordance with previous studies on silver clusters, we assign this peak to interband Figure 1. A) Time-dependent UV/Vis spectra of the clusters synthesized during interfacial etching at room temperature. B) UV/Vis absorption spectra of the clusters obtained from the two bands in PAGE. The inset shows a photograph of the wet gel after electrophoresis in UV light at room temperature, and the inset to the inset an image of the first band at 273 K. C) HRTEM images of a) assynthesized Ag@(H2MSA), b) the product obtained after interfacial etching, and c) particles in the blue layer at the interface. Individual clusters are not observable by TEM, but aggregates are seen faintly (b, shown in circles). Insets of (a) and (b) are photographs of Ag@MSA and crude cluster samples. d) Photographs of aqueous of cluster solutions of first (cluster 1) and second (cluster 2) PAGE bands at 273 K and room temperature, respectively. D) Luminescence emission of cluster 1 and cluster 2 in water, excited at 550 and 350 nm, respectively.