The aromatic peptide protected gold nanoclusters with significant Stokes shift: Ligand-mediated two-step FRET

During the last decade, a great variety of ligand protected gold nanoclusters (AuNCs) have been synthesized, and their broad applications have been intensively reported. Although the spectroscopic properties of AuNCs have been comprehensively explored, the mechanism of the significant Stokes shift (> 200 nm) and the specific role played by surface ligands have not been clearly explained yet. In this study, a series of fluorescent AuNCs with huge Stokes shift (up to 530 nm) were successfully prepared by employing the rationally designed tri-peptides as the protecting ligands, and their spectroscopic properties were systematically investigated. The detailed measurements on the example product, YCY-AuNCs (Tyr-Cys-Tyr liganded AuNCs), showed that the energy absorbed by the tyrosine (~ 250 nm) can be effectively transferred through the ligand-mediated two-step Förster resonance energy transfer (FRET) process and released as fluorescence emission in the near-infrared fluorescence (NIR) range (~ 780 nm), which resulted in the significant apparent Stokes shift. The YCY ligands play a critical role by offering the tyrosine groups (donor of the first FRET pair), generating the dityrosine-like structure on the AuNCs surface (acceptor of the first FRET pair and donor of the second FRET pair), and protecting the cores (acceptor of the second acceptor). The additional ligand exchange experiments and the investigation on the other AuNCs further demonstrated that the sufficient high density of the aromatic groups is also essential to mediate the two-step FRET and achieve the remarkable Stokes shift. We believe that the aromatic ligand-mediated FRET mechanism not only offers a new theoretical explanation for the huge Stokes shift exhibited in AuNCs, but also provides a general strategy for the construction of new materials with large Stokes shift.