Enhanced Photoluminescence from Chromophore-Functionalized Gold Clusters: Mechanism and Dynamics
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Water soluble highly luminescent optical materials have great potential for applications in biological imaging and sensing. Among different materials, semiconductor quantum dots have shown sharp luminescence spectra and size-dependent photoluminescence in the visible region. However, they are insoluble in water and toxic for biological systems both for their chemical composition as well as for their larger size. In this context, quantum-sized metal clusters with sizes less than 2 nm were found to be safer alternatives to well-studied semiconductor quantum dots. However, metal clusters suffer from inherently low photoluminescence quantum yields and research efforts are progressing to enhance the PL quantum efficiencies. Here, we have investigated the photoluminescence properties of chromophore-functionalized Au22(SG)18 clusters to enhance the PL quantum yields as well as understand the mechanism for this enhancement. The studies were carried out on three chromophores-functionalized Au22 clusters, including: Pyrene amine, amine-terminated Cyanine5.5 and 5-amino fluorescein. Optical absorption, steady-state fluorescence, and fluorescence anisotropy measurements were carried out on different pyrene loaded Au22 clusters and pyrene-Au22 functionalized with ToA in toluene. The studies have shown strong quenching of pyrene fluorescence with evidence of excimer emission in Pyrene-Au22 samples in water with photoluminescence enhancement of Au22. The measurements suggested facile aggregation of pyrene in water samples and evidence for energy transfer from pyrene to Au22. In contrast, pyrene aggregation is minimized in Au22-ToA sample in toluene and strong energy transfer from pyrene to Au22 was observed in steady-state PL and anisotropy data. The investigations on Cy5.5-Au22 samples have also shown aggregation of Cy5.5 on the surface of Au22 in water and quenching of Au22 PL was observed that can be either due to energy transfer from Au22 to Cy5.5 or electron transfer from Au22. In Au22-ToA samples, the aggregation of Cy5.5 was minimized and evidence for energy transfer from Au22 to Cy5.5 was observed.