Unveiling the photoluminescence dynamics of gold nanoclusters with fluorescence correlation spectroscopy

Abstract

Gold nanoclusters (AuNCs) have captured significant interest for their photoluminescent properties; however, their rapid photodynamics remain elusive while probed by ensemble-averaging spectroscopy techniques. To address this challenge, we use fluorescence correlation spectroscopy (FCS) to uncover the photoluminescence dynamics of colloidal Au₁₈(SG)₁₄ nanoclusters. Our FCS analysis reveals the photoluminescence (PL) brightness per nanocluster, elucidating the impact of photoexcitation saturation and ligand interactions. Unlike DNA-encapsulated silver nanoclusters, their gold counterparts notably exhibit minimal blinking, with moderate amplitudes and 200 μs characteristic times. Our data also clearly reveal the occurrence of photon antibunching in the PL emission, showcasing the quantum nature of the PL process, with each AuNC acting as an individual quantum source. Using zero-mode waveguide nanoapertures, we achieve a 16-fold enhancement of the PL brightness of individual AuNCs. This constitutes an important enabling proof-of-concept for tailoring emission properties through nanophotonics. Overall, our study bridges the gap between ensemble-averaged techniques and single-molecule spectroscopy, offering new insights into AuNC photodynamics for biosensing and imaging applications.

Fig. 1. (a) Vial containing Au₁₈(SG)₁₄ used for the measurements and schematic representation of the AuNC (orange: gold atoms and light yellow: sulfur, instead of glutathione, only simplified functional groups –SCH₃ are represented). (b) Absorption, excitation, and emission spectra of Au₁₈(SG)₁₄. We used an excitation laser of 557 nm for further experiments and APDs collecting photons in the range 650–750 nm. (c) Scheme of the FCS setup used for our measurements. (d) FCS correlation curve of Au₁₈(SG)₁₄ illustrating various photophysical processes occurring at different time scales. (e) Comparison of the photoluminescence (PL) intensity and fluorescence decay trace of Au₁₈(SG)₁₄ dispersed in H₂O and D₂O. (f) Comparison of the PL lifetime decays in D₂O and H₂O.

Fig. 2. (a) Normalized FCS correlation and numerical fits of Au₁₈(SG)₁₄ in D₂O at two different powers (6 μW and 30 μW). (b) Comparison of the numerical fits of the normalized FCS correlation of Au₁₈(SG)₁₄ in D₂O with increasing the laser power from 3 μW to 80 μW. The trend in the numerical fits with increasing laser power is denoted by the arrow. (c) Comparison of the normalized FCS correlations and fits of Au₁₈(SG)₁₄ in the presence and absence of PDA at 20 μW. The measurements of Au₁₈(SG)₁₄ without PDA are performed in 98% v/v of D₂O and the measurements of Au₁₈(SG)₁₄ with PDA are performed in a PBS buffer of pH 11. The excitation laser used is 557 nm.

Fig. 3. Comparison of different photophysical parameters of Au₁₈(SG)₁₄ obtained from the FCS data with increasing excitation laser power and in the presence or absence of PDA. (a) Fluorescence brightness per Au₁₈(SG)₁₄ with and without PDA. (b) Dark state amplitude T_DS. (c) Dark state blinking time τ_DS. (d) Antibunching time τ_A.

Fig. 4. Enhancement of AuNC PL using zero-mode waveguide nanoapertures. (a) Scanning electron microscope (SEM) image of an aluminium ZMW aperture of 110 nm dimeter and its schematic representation at the microscope focus. (b) FCS correlation and numerical fits of Au₁₈(SG)₁₄ in ZMW at two different powers (1.5 μW and 15 μW) of 557 nm excitation. (c) Comparison of the numerical fits of the FCS correlation of Au₁₈(SG)₁₄ in a ZMW with increasing the laser power from 1.5 μW to 20 μW. The trends with increasing laser powers are denoted by the arrows. (d) PL lifetime decay traces of Au₁₈(SG)₁₄ in confocal (diffraction-limited) and ZMW nanoapertures. (e) Comparison of Au₁₈(SG)₁₄ brightness per nanocluster in the confocal and ZMW setup. (f) Comparison of diffusion times of Au₁₈(SG)₁₄ in the confocal and in the ZMW. (g–i) Evolution of different photophysical parameters of Au₁₈(SG)₁₄ in the ZMW obtained from the FCS data with increasing excitation laser power: (g) dark state amplitude T_DS, (h) dark state blinking time τ_DS and (i) antibunching time τ_A.