Atomically precise N-heterocyclic carbene (NHC)-stabilized gold nanoclusters (Au NCs) have attracted great attention among a variety of gold nanoclusters. Herein, a series of three NHC-stabilized Au13 NCs have been synthesized featuring ligands with different steric hindrances. Their electrochemical and photophysical properties were investigated. The bidentate NHC ligands with pendant pyrenes rendered the most sterically hindered Au13 NC, [Au13(bis-NHCPyr)5Cl2]3+, highly luminescent (quantum yield, 56%) and long-lived (3.4 μs, dichloromethane) among the three Au13 NCs. The excellent photoluminescence properties were attributed to the suppression of excited-state vibrational relaxation arising from the cluster structural rigidity. Further transient spectroscopic investigation of the Au NC excited state also showed favorable electron transfer to methyl viologen with the highest quantum yield from [Au13(bis-NHCPyr)5Cl2]3+ (Φe–T, 12%). Understanding the relationship between the excited-state behavior and the sterically hindered structure of NHC-ligated Au NCs will assist in better designing NHC-stabilized Au NCs as photosensitizers for applications in organic photoredox catalysis, sensors, and solar energy conversion.

Many synthetic nanomaterials known as nanozymes can catalyze biologically relevant molecular transformations just as natural enzyme does. “Photonanozyme” utilizes light as a spatial and temporal control for the regulation of nanozyme activities. Here we report a glutathione-modified gold nanocluster as a photonanozyme that catalyzes the reduction of nitrobenzene under light. Aniline was found as the sole photoreductive product. The photocatalytic reactions at variable light fluences were found to follow the classical Michaelis–Menten enzyme kinetics from which kinetic rate constants were quantified. Intracellular reduction of a nitro-group-containing fluorescent probe demonstrates the viability of gold nanoclusters as biocompatible photonanozymes performing catalysis in a mammalian cell environment. This study reinvents gold nanoclusters as photonanozymes that mimic naturally occurring nitroreductase for potential prodrug activation.

Atomically precise gold nanoclusters (Au NCs) have high specific surface area and abundant unsaturated active sites. Traditionally, Au NCs are employed as thermocatalysts for multielectron transfer redox catalysis. Meanwhile, Au NCs also exhibit discrete energy levels, tunable photophysical and electrochemical properties, including visible to near infrared absorption, microsecond long-lived excited-state lifetime, and redox chemistry. In recent years, Au NCs are increasingly employed as visible to near infrared photocatalysts for their high photocatalytic activity and unique selectivity. This review focuses on the photophysical properties of a variety of Au NCs and their employment as photocatalysts in photocatalytic reactions and related applications including solar energy conversion and photodynamic therapies.

Photocatalytic activation of prodrugs for killing cancer cells is an attractive alternative phototherapy to photodynamic therapy that typically relies on the supply of oxygen. Although prodrugs are widely developed, few other than Pt(IV) complexes are studied for photocatalytic activation. Herein, we report the photocatalytic reduction of nitrobenzene to aniline, an important chemical conversion for a large class of nitroaromatic prodrugs in cancer therapy that was previously limited to only enzyme-catalyzed activation. The carefully designed photocatalyst is a photosensitizer (PS)-functionalized gold nanocluster (Au NC) (abbreviated as Au-PS) in which a ruthenium coordination compound as the PS is covalently linked to the glutathione-ligated Au NC surface. Visible light excitation of the photocatalyst reduces nitrobenzene to aniline with 100% selectivity. The remarkably high selectivity is attributed to the specific catalytic nature of the reduced Au NC from photoinduced charge separation within the Au-PS analyzed by time-resolved absorption spectroscopy. In vitro experiments show that the nitroaromatic prodrug 5-(aziridin-1-yl)-2,4-dinitrobenzamide (CB1954) induces significant cytotoxicity in the presence of the Au-PS and light under hypoxia. The photocatalytic nitroaromatic prodrug activation by the Au-PS provides an alternative approach in the category of photochemotherapy to confront hypoxic cancer cells.