Enantioselective silver nanoclusters: Preparation, characterization and photoluminescence spectroscopy

Abstract

Herein, we report a new wet-synthesis method to separate some water-soluble chiral silver nanoclusters with high yield. The cluster material was obtained by the reduction of silver nitrate with NaBH4 in the presence of three ligands l-penicillamine (l-pen), d-penicillamine (d-pen) and racemic mixture of penicillamine (rac-pen), functioning as capping ligand. For characterizing all silver cluster samples, the particle size was assessed by transmission electron microscopy (TEM) and powder X-ray diffraction (XRD) and their average chemical formula was determined from thermogravimetric analysis (TGA) and elemental analysis (EA). The particles sizes of all three clusters are 2.1 ± 0.2 nm. The optical properties of the samples were studied by four different methods: UV-vis spectroscopy, Fourier transform infrared spectroscopy (FTIR), photoluminescence spectroscopy (PL) and circular dichroism (CD) spectroscopy. The spectra are dominated by the typical and intense plasmon peak at 486 nm accompanied by a small shoulder at 540 nm. Infrared spectroscopy was measured for the free ligand and protected silver nanoclusters, where the disappearance of the S-H vibrational band (2535–2570 cm−1) in the silver nanoclusters confirmed anchoring of ligand to the cluster surface through the sulfur atom. PL studies yielded the fluorescent properties of the samples. The main focus of this work, however, lies in the chirality of the particles. For all silver clusters CD spectra were recorded. While for clusters capped with one of the two enantiomers (D- or L-form) typical CD spectra were observed, no significant signals were detected for a racemic ligand mixture. Furthermore, silver clusters show quite large asymmetry factors (up to 3 × 10−4) in comparison to most other ligand protected clusters. These large factors and bands in the visible range of the spectrum suggest a strong chiral induction from the ligand to the metal core. Textural features of the prepared silver nanoclusters were investigated using nitrogen adsorption-desorption at −196 °C. Specific surface area SBET, pore volume and average pore diameter were calculated.