Research is turning toward nanotechnology for solutions to current limitations in biomedical imaging and analytical detection applications. New to fluorescent nanomaterials that could help advance such applications are protein-stabilized gold nanoclusters. They are potential candidates for imaging agents and sensitive fluorescence sensors because of their biocompatibility and intense photoluminescence. This review discusses the strategy for synthesizing fluorescent protein-gold nanoclusters and the characterization methods employed to study these systems. Optical properties and relevant light-emitting applications are reported to present the versatility of protein-gold nanoclusters. These new bio-nano hybrids are an exciting new system that remains to be explored in many aspects, especially regarding the determination of gold nanocluster local structure and the enhancement of quantum yields. Understanding how to finely tune the optical properties will be pivotal for improving fluorescence imaging and other nanocluster applications. There is a promising future for fluorescent protein-gold nanoclusters as long as research continues to uncover fundamental structure-property relationships.

We report element-specific X-ray spectroscopy results on the structure and bonding of Au24Pt, a thiolate-protected bimetallic nanocluster. Platinum L3-edge extended X-ray absorption fine-structure (EXAFS) data, in association with X-ray photoelectron spectroscopy (XPS) compositional analysis, was used to identify the location of the Pt dopant to be in the center of the icosahedron Au13 core. Comparison of Au24Pt with the structure of Au25 by gold L3-edge EXAFS clearly shows contraction of both metal–thiolate and metal–metal bond distances, caused by Pt doping. The doping effect on the electronic properties of Au24Pt was further evaluated by high-resolution Au 4f core-level XPS and ab initio calculations, which elucidate the importance of bimetallic (Pt–Au) bonding and bond contraction effects on the properties of Pt-doped thiolate-gold nanoclusters.

Findings on the structure and formation of luminescent protein-stabilized gold clusters reveal interlocked gold-thiolate rings and a unique bio-assembled pathway.

Because of the small size and large surface area of thiolate-protected Au nanoclusters (NCs), the protecting ligands are expected to play a substantial role in modulating the structure and properties, particularly in the solution phase. However, little is known on how thiolate ligands explicitly modulate the structural properties of the NCs at atomic level, even though this information is critical for predicting the performance of Au NCs in application settings including as a catalyst interacting with small molecules and as a sensor interacting with biomolecular systems. Here, we report a combined experimental and theoretical study, using synchrotron X-ray spectroscopy and quantum mechanics/molecular mechanics simulations, that investigates how the protecting ligands impact the structure and properties of small Au18(SR)14 NCs. Two representative ligand types, smaller aliphatic cyclohexanethiolate and larger hydrophilic glutathione, are selected, and their structures are followed experimentally in both solid and solution phases. It was found that cyclohexanethiolate ligands are significantly perturbed by toluene solvent molecules, resulting in structural changes that cause disorder on the surface of Au18(SR)14 NCs. In particular, large surface cavities in the ligand shell are created by interactions between toluene and cyclohexanethiolate. The appearance of these small molecule-accessible sites on the  NC surface demonstrates the ability of Au NCs to act as a catalyst for organic phase reactions. In contrast, glutathione ligands encapsulate the Au NC core via intermolecular interactions, minimizing structural changes caused by interactions with water molecules. The much better protection from glutathione ligands imparts a rigidified surface and ligand structure, making the NCs desirable for biomedical applications due to the high stability and also offering a structural-based explanation for the enhanced photoluminescence often reported for glutathione-protected Au NCs.

The recent discovery on the total structure of Au36(SR)24, which was converted from biicosahedral Au38(SR)24, represents a surprising finding of a face-centered cubic (FCC)-like core structure in small gold-thiolate nanoclusters. Prior to this finding, the FCC feature was only expected for larger (nano)crystalline gold. Herein, we report results on the unique bonding properties of Au36(SR)24 that are associated with its FCC-like core structure. Temperature-dependent X-ray absorption spectroscopy (XAS) measurements at the Au L3-edge, in association with ab initio calculations, show that the local structure and electronic behavior of Au36(SR)24 are of more molecule-like nature, whereas its icosahedral counterparts such as Au38(SR)24 and Au25(SR)18 are more metal-like. Moreover, site-specific S K-edge XAS studies indicate that the bridging motif for Au36(SR)24 has different bonding behavior from the staple motif from Au38(SR)24. Our findings highlight the important role of "pseudo"-Au4 units within the FCC-like Au28 core in interpreting the bonding properties of Au36(SR)24 and suggest that FCC-like structure in gold thiolate nanoclusters should be treated differently from its bulk counterpart.