  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, the gold counterparts notably
exhibit minimal blinking, with moderate amplitudes and 0.2 ms 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.
