Enhanced Photoluminescence of the Bi-icosahedral Au25 Nanocluster Using an Anthracene-based Fluorophore

Fluorescent molecules have enabled single-molecule detection of toxins, biomarkers, and pollutants under controlled conditions. Unfortunately, these fluorophores are typically organic molecules that degrade or become photobleached when applied to non-ideal systems. Noble metal nanoclusters, in particular gold nanoclusters (AuNCs), pose a solution to the problems of degradation and photobleaching. Despite the low fluorescence quantum yield of AuNCs without thiolated ligands, labeling these clusters with fluorescent ligands allows enhancement of the fluorescence intensity and manipulation of the emission wavelength. In this work, we explore the labeling of the bi-icosahedron Au₂₅ (bi-Au₂₅) nanocluster. The bi-Au₂₅ nanocluster has unique stability and electrochemical properties making it an attractive, yet poorly studied, candidate for fluorescent labeling. In order to demonstrate its potential as a near-IR emitting nanocluster we synthesized and labeled the bi-Au₂₅ with the novel fluorophore 6-(9-Anthryl)-5-hexyne-1-thiol, or simply anthracenethiol. Two common ligands used in the synthesis of bi-Au₂₅ are hexanethiol and phenylethanethiol, and we spectroscopically verify the ability for the straight-chain alkane hexanethiol to block the labeling of bi-Au₂₅, while the aromatic phenylethanethiol enables the labeling of the complex. These products are characterized with square wave voltammetry, UV-vis and fluorescence spectrophotometry, and NMR spectrometry. The fluorescently labeled bi-Au₂₅ nanocluster demonstrates a 25x increase in NIR photoluminescence at ~ 810 nm when originally capped with phenylethanethiol, and not the long-chain alkanethiol. The quantum yield of this cluster has been improved from 0.0786% in the unlabeled cluster to 1.97% in the labeled product.