Fluorescence Dynamics in BSA-Protected Au₂₅Nanoclusters

Abstract

Fluorescent Au 25 nanoclusters recently have drawn considerable research interest due to their unique properties and potential applications. Despite significant advances in their synthesis methods and application development, the origin of the fluorescence and underlying mechanism still remain unclear. In this work we investigate the fluorescence dynamics in BSA-protected Au 25 nanoclusters by time-resolved photoluminescence and transient absorption techniques covering picosecond to microsecond time scales. We demonstrate here that the red fluorescence consists of both prompt fluorescence and thermally activated delayed fluorescence, and the latter is more dominant. A small energy gap of 30 meV between the triplet and the singlet states was determined from our temperature-dependent time-resolved fluorescence measurement. Moreover, we elucidate that the absorption band at 2.34 eV corresponds to the HOMO− LUMO transition in this system due to the interaction between Au 25 NCs and BSA. We also show that an effective relaxation pathway exists from the higher excited state to the LUMO.

The excited state dynamics of protein-encapsulated Au nanoclusters

“…The different ligands in Au-MCP and Au-proteins may be responsible for the difference in peak positions. For the red emission, there are three PLE bands at 325, 380 and 500 nm for the Au nanoclusters, which are slightly different from the results of previous studies on BSA-Au nanoclusters that reported excitation peak at 480 nm [10], and two excitation peaks at 400 nm and around 500 nm [14], respectively. The small difference in PLE spectra (around 20 nm) may be due to different instruments or measurement conditions used, e.g.…”

“…This assignment is consistent with a previous report on BSA-Au nanoclusters, in which two excitation bands at 500 and 400 nm were attributed to transitions from the ground state to the first and a higher excited state, respectively [14]. The 325 nm excitation band could be a vibronic feature of the second excited electronic state.…”

Intense fluorescence of Au20

“…32 15 Xu et al found fluorescence for polydisperse Au nanoclusters on BSA (bovine serum albumin) from 650 to 700 nm with 100 nm linewidth. 14 Wen et al reported fluorescence of Au 25 /BSA at 690 nm with 120 nm linewidth, 34 and finally, Liu et al presented fluorescence on human insulin Au nanodots at about 700 nm with 150 nm linewidth. 10 The important and yet open question is whether the fluorescence in these ligand-protected Au nanoclusters is intrinsic to the metal core or to the cluster ligand system.…”

Fluorescence depletion properties of insulin–gold nanoclusters

“…Although this is still a reasonable number, the simple two-level model cannot explain the existence of the undepleted background. According to a previous study of protein-protected metallic nanoclusters, there is a triplet state in gold nanoclusters [12]. When a gold nanocluster is excited, the excited electrons exhibit a substantial probability of crossing into the triplet state, known as intersystem crossing (ISC).…”

“…The ideal label for STED should exhibit not only outstanding photostability, but also bright emission to compensate for fluorescence loss due to depletion [9,10]. Both requirements can be met by protein-protected metallic nanoclusters, such as bovine-serum-albumin-protected Au 25 nanoclusters and insulin-gold nanoclusters (IGNCs) [11][12][13]. An IGNC is essentially fluorescent insulin with a 1-nm Au core at its center.…”