Novel Strategies for Preparing Highly Luminescent Gold Nanoclusters

Abstract

Quantum-sized gold nanoclusters with high photo-luminescence properties have been one of the briskly studied materials for their applications in light emitting display, biomedical imaging, optical sensors and more. Also atomically precise gold nanoclusters with well-defined core shell structures have bright prospects to achieve high luminescence quantum efficiency. In this presentation, we present the novel strategies for enhancing the luminescence of water soluble Au22SG18 via ion-pairing bulk tetraoctylammonium (TOA) cations and covalent attachment of small aromatic chromophores to glutathione. TOA was ion-paired to glutathione by phase transferring the water soluble Au22SG18 to the organic solvent under pH 9. TOA ion-paired Au22 showed a massive increase in the luminescence (Q.Y. ~60 %) due to the enhanced rigidity of the gold-ligand shell, which was also shown in the PL lifetime and steady state luminescence under the solvent freezing point. Furthermore, the small aromatic chromophores, such as pyrene, was covalently attached by EDC/NHS coupling reaction which enhanced the luminescence of water soluble Au22 by more than 8 times. Compared to the result from TOA-Au22, the enhancement was on account to the increased rigidity of the gold-ligand shell from the functionalized aromatics as well as the fluorescence resonance energy transfer (FRET) phenomenon from pyrene to Au22. Steady-state and femtosecond time-resolved fluorescence measurements of each Au22 and pyrene functionalized Au22 (Au22-Py) was taken, for the calculation of energy transfer efficiency, which was 20 %. However, the ultrafast transient absorption result suggested the possibility of charge transfer occurring from Au22 to pyrene hindering the energy transfer phenomenon. In order to enhance the energy transfer contribution instead of the charge transfer, TOA was ion-paired to the gold-ligand shell on Au22-Py followed by a 21 fold luminescence enhancement. These strategies for adjusting the gold-ligand shell of the nanoclusters, opens up a new way for preparing highly luminescent gold nanoclusters in water that can be used for luminescent display applications and biological imaging.