Excited-State Dynamics of Photoluminescent Gold Nanoclusters and Their Assemblies with Quantum Dot Donors

Abstract

Gold nanoclusters (AuNCs) are a recently developed class of photoluminescent nanomaterial with potential for detection and imaging applications in both chemistry and biology. The AuNC small size (Au104NC) in combination with their long-lived, near-infrared (NIR) emission suggests that they could be applied as a biological label directly and also as a potential acceptor for energy transfer-based biosensing. Previous studies have shown that functionalized AuNCs can act as energy-transfer acceptors; however, the underlying mechanism appears to be complex and does not necessarily follow a classical Förster process with a (1/R)6 distance dependence. Here, we report a femtosecond and nanosecond time-resolved spectroscopy study to examine the energy dissipation and transfer processes of photoluminescent AuNC acceptors displaying different functionalized end groups and coupled with 625 nm emitting semiconductor quantum dot (QD) donors. Examination of AuNCs alone on the femtosecond timescale show coherent vibrational oscillations within the first 15 ps after excitation in the visible and NIR region. Fourier transform of the kinetic data reveal ∼0.18 and 0.40 THz low-frequency vibrations in aqueous AuNC solutions, with the latter attributed to coherent vibrations following hot electronic relaxation. A biphasic decay of the AuNC excited state on the microsecond timescales confirm their exceptionally long-lived lifetimes and that the photophysics is largely unaffected by the functional end group on the ligand. Finally, ultrafast studies of AuNCs coupled to the 625 QD donors reveal excited-state quenching and wavelength shifts of the QD’s ground-state bleach and the appearance of the excited state of the AuNC resulting from energy transfer occurring on the ∼200 ps timescale.