Atomically Precise Colloidal Metal Nanoclusters and Nanoparticles: Fundamentals and Opportunities.

Colloidal nanoparticles are emerging as a novel class of materials and have received intense research interest. With the advances of chemical synthetic methods, scientists are pushing the envelope of fine-tuning the quality of colloidalnanoparticles. However, their crystal structures still remain mysterious due to the inevitable polydispersity. It is thus desirable to control the quality of colloidal nanoparticles with atomic precision. Recently, thiolate-protected gold nanoclusters [Aun(SR)m, where n represents the number of gold atoms and m represents the number of thiolate protecting ligands, respectively] have successfully achieved atomic level precision and their crystal structures have been solved. This novel class of material offers uniqueopportunities for fundamental investigations and practical applications in catalysis, sensing, optics and so on. Due to the ultrasmall size regime (1-3 nm), gold nanoclusters show molecular-like properties such as single electron HOMOLUMO transition due to the strong quantum confinement effect. Furthermore, the crystal structures of gold nanoclusters can provide detailed information on thebonding between gold and sulfur atoms, which can significantly benefit the understanding of surface catalytic processes. These unprecedented advantages make gold nanoclusters a well-defined platform to investigate the electrocatalytic applications and probe the structure-property correlations. This thesis explores the catalytic applications of atomically precise gold nanoclusters, especially in the field of electrocatalysis. Specifically, goldnanoclusters are employed as effective catalysts for the reduction of 4-nitrophenol (chapter 2) and electroreduction of CO2 to CO (chapter 3). Mechanistic understandings are achieved owing to the solved crystal structures of gold nanoclusters. In chapters 4 and 5, the strategies of constructing hybrid nanocomposites with gold nanoclusters are proposed and demonstrated in catalyst design. Gold nanoclusters are loaded onto two dimensional CoSe2 andMoS2 nanosheets. The interface between gold nanoclusters and CoSe2 (or MoS2) nanosheets is found to be the active sites for electrocatalytic water splitting. In chapter 6, the focus of this thesis moves from gold to palladium.Ultrasmall palladium nanoclusters are successfully synthesized with excellent performance for electrocatalytic oxygen reduction reaction. The exciting opportunities of gold nanoclusters in electrocatalysis lie not only in their extraordinary performance but also in the atomic-level monodispersity. It is because of the well-defined nature of gold nanoclusters compared to conventional plasmonic nanoparticles that probing mechanistic understanding inelectrocatalysis and understanding structure-property correlations both become possible. Therefore, gold nanoclusters hold high promise for electrocatalysis, andfuture endeavors are expected for further explorations in this field.

Controlling nanoparticles with atomic precision has long been a major dream of nanochemists. Breakthroughs have been made in the case of gold nanoparticles, at least for nanoparticles smaller than ∼3 nm in diameter. Such ultrasmall gold nanoparticles indeed exhibit fundamentally different properties from those of the plasmonic counterparts owing to the quantum size effects as well as the extremely high surface-to-volume ratio. These unique nanoparticles are often called nanoclusters to distinguish them from conventional plasmonic nanoparticles. Intense work carried out in the last few years has generated a library of stable sizes (or stable stoichiometries) of atomically precise gold nanoclusters, which are opening up new exciting opportunities for both fundamental research and technological applications. In this review, we have summarized the recent progress in the research of thiolate (SR)-protected gold nanoclusters with a focus on the reported stable sizes and their optical absorption spectra. The crystallization of nanoclusters still remains challenging; nevertheless, a few more structures have been achieved since the earlier successes in Au102(SR)44, Au25(SR)18 and Au38(SR)24 nanoclusters, and the newly reported structures include Au20(SR)16, Au24(SR)20, Au28(SR)20, Au30S(SR)18, and Au36(SR)24. Phosphine-protected gold and thiolate-protected silver nanoclusters are also briefly discussed in this review. The reported gold nanocluster sizes serve as the basis for investigating their size dependent properties as well as the development of applications in catalysis, sensing, biological labelling, optics, etc. Future efforts will continue to address what stable sizes are existent, and more importantly, what factors determine their stability. Structural determination and theoretical simulations will help to gain deep insight into the structure-property relationships.

Atomically precise, ligand-protected metal nanoclusters are of great interest for their well-defined structures, intriguing physicochemical properties, and potential applications in catalysis, biology, and nanotechnology. Their structure precision provides many opportunities to correlate their geometries, stability, electronic properties, and catalytic activities by closely integrating theory and experiment. In this Account, we highlight recent theoretical advances from our efforts to understand the metal-ligand interfaces, the energy landscape, the electronic structure and optical absorption, and the catalytic applications of atomically precise metal nanoclusters. We mainly focus on gold nanoclusters. The bonding motifs and energetics at the gold-ligand interfaces are two main interests from a computational perspective. For the gold-thiolate interface, the -RS-Au-SR- staple motif is not always preferred; in fact, the bridging motif (-SR-) is preferred at the more open facets such as Au(100) and Au(110). This finding helps understand the diversity of the gold-thiolate motifs for different core geometries and sizes. A great similarity is demonstrated between gold-thiolate and gold-alkynyl interfaces, regarding formation of the staple-type motifs with PhC≡C- as an example. In addition, N-heterocyclic carbenes (NHCs) without bulky groups also form the staple-type motif. Alkynyls and bulky NHCs have the strongest binding with the gold surface from comparing 27 ligands of six types, suggesting a potential to synthesize NHC-protected gold clusters. The energy landscape of nanosystems is usually complex, but experimental progress in synthesizing clusters of the same Au-S composition with different R groups and isomers of the same Au n(SR) m formula have made detailed theoretical analyses of energetic contributions possible. Ligand-ligand interactions turn out to play an important role in the cluster stability, while metastable isomers can be obtained via kinetic control. Although the superatom-complex theory is the starting point to understand the electronic structure of atomically precise gold clusters, other factors also greatly affect the orbital levels that manifest themselves in the experimental optical absorption spectra. For example, spin-orbit coupling needs to be included to reproduce the splitting of the HOMO-LUMO transition observed experimentally for Au25(SR)18-, the poster child of the family. In addition, doping can lead to structural changes and charge states that do not follow the superatomic electron count. Atomically precise metal nanoclusters are an ideal system for understanding nanocatalysis due to their well-defined structures. Active sites and catalytic mechanisms are explored for selective hydrogenation and hydrogen evolution on thiolate-protected gold nanoclusters with and without dopants. The behavior of H in nanogold is analyzed in detail, and the most promising site to attract H is found to be coordinately unsaturated Au atoms. Many insights have been gained from first-principles studies of atomically precise, ligand-protected gold nanoclusters. Interesting and important questions remaining to be addressed are pointed out in the end.

Quantum-sized, thiolate-protected gold nanoclusters (NCs) with atomic precision have attracted substantial research attention over the past decades due to their interesting optical, electronic properties and unusually high catalytic activities. However, despite the remarkable success has been made in the synthesis and characterization of gold nanoclusters, most synthetic approaches suffer from the production of a mixture of different cluster sizes and often a quite low yield of specific sized clusters. Therefore, the products have to be separated on the basis of various complicated processes. The difficulty in isolating and purifying nanoclusters has become a major obstacle to the practical applications of metal nanocluster materials. On the other hand, intensive studies have shown that the optical, electronic, and catalytic properties of gold nanoclusters are strongly dependent on the core size, composition, and structure. Thus, it is highly desirable to develop facile protocols that permit the synthesis, isolation, purification, and characterization of monodispersed, atomically precise gold nanoclusters with control over size in order to fully understand their size-dependent properties. This chapter describes the recent progress in the synthesis, characterization, and study of monodispersed gold nanoclusters.KeywordsCharacterizationGoldNanoclustersNanoparticlesSynthesis

Thiolate protected gold nanoclusters (TPNCs) are a unique class of nanomaterials finding applications in various fields, such as biomedicine, optics, and catalysis. The atomic precision of their structure, characterized through single crystal x-ray diffraction, enables the accurate investigation of their physicochemical properties through electronic structure calculations. Recent experimental efforts have led to the successful heterometal doping of TPNCs, potentially unlocking a large domain of bimetallic TPNCs for targeted applications. However, how TPNC size, bimetallic composition, and location of dopants influence electronic structure is unknown. To this end, we introduce novel structure-property relationships (SPRs) that predict electronic properties such as ionization potential (IP) and electron affinity (EA) of AgAu TPNCs based on physically relevant descriptors. The models are constructed by first generating a hypothetical AgAu TPNC dataset of 368 structures with sizes varying from 36 to 279 metal atoms. Using our dataset calculated with density functional theory (DFT), we employed systematic analyses to unravel size, composition, and, importantly, core-shell effects on TPNC EA and IP behavior. We develop generalized SPRs that are able to predict electronic properties across the AgAu TPNC materials space. The models leverage the same three fundamental descriptors (i.e., size, composition, and core-shell makeup) that do not require DFT calculations and rely only on simple atom counting, opening avenues for high throughput bimetallic TPNC screening for targeted applications. This work is a first step toward finely controlling TPNC electronic properties through heterometal doping using high throughput computational means.

We present novel insights into the structural mechanisms of four crystallized thiolate-protected gold nanoclusters, namely, Au16(SR)12, Au21(SR)15, Au29(SR)19, and Au30(SR)18. These nanoclusters exhibit one or two surface gold core atoms that lack thiolate group protection, termed "naked gold core atoms". The existence of "naked gold core atoms" can be explained by the presence of an individual surface gold core atom that cannot establish a bond with the staple motif (-[SR(AuSR)n]-, where n = 0, 1, 2, ...) since each staple motif requires two surface gold core atoms for bonding. Alternatively, it can be attributed to the situation in which the two surface gold atoms are too widely separated to be linked by a staple motif. Further structural analyses of the coordination environment surrounding the "naked gold core atoms" reveal the presence of a nearby Au5 ring. The average bond length between the "naked gold core atom" and the gold atoms in the Au5 ring is determined to be 3.0 Å, indicating robust aurophilic interactions among them. Hence, the Au5 ring can be regarded as a protective motif that shields the "naked gold core atom" from requiring an additional SR motif for binding. This is also the rationale behind the use of quotation marks for the term "naked gold core atoms". After comprehending the structural mechanism of these "naked gold core atoms", we can design the structures of highly stable thiolate- and phosphine-protected gold nanoclusters by introducing phosphine ligands onto the "naked gold core atoms". Our work not only contributes to the understanding of the intricate interfacial interactions in thiolate-protected gold nanoclusters but also provides valuable insights for synthesizing a greater variety of hybrid ligand-protected gold nanoclusters experimentally.

Over the last two decades, ligand-protected gold nanoclusters (Au NCs) have experienced continued interest because of their controlled synthesis with atomic precision and unique, tunable properties...

Colloidal zinc metallic nanoparticles are synthesized using pulsed laser ablation of metal plate in an aqueous solution of suitable surfactant to prevent aggregation. UV-visible absorption, TEM, small angle X-ray diffraction and wide-angle X-ray diffraction are used for the characterization of colloidal zinc metallic nanoparticles. Colloidal nanoparticles are found highly stable for a long time.

Achieving atomic precision in nanostructured materials is essential for comprehending formation mechanisms and elucidating structure-property relationships. Within the realm of nanoscience and technology, atomically precise ligand-protected noble metal nanoclusters (NCs) have emerged as a rapidly expanding area of interest. These clusters manifest quantum confinement-induced optoelectronic, photophysical, and chemical properties, along with remarkable catalytic capabilities. Among coinage metals, silver distinguishes itself for the fabrication of stable nanoclusters, primarily due to its cost-effectiveness compared to gold. This minireview provides an overview of recent advancements since 2020 in synthetic methodologies and ligand selections toward attaining NCs boasting a minimum of two free valence electrons. Additionally, it explores strategies for fine-tuning optical properties. The discussion extends to surface reactivity, elucidating how exposure to ligands, heat, and light induces transformations in size and structure. Of paramount significance are the applications of silver NCs in catalytic reactions for energy and chemical conversion, supplemented by in-depth mechanistic insights. Furthermore, the review delineates challenges and outlines future directions in the NC field, with an eye toward the design of new functional materials and prospective applications in diverse technologies, including optoelectronics, energy conversion, and fine chemical synthesis.

The rational design and synthesis of materials with tailored properties remains a long-standing goal in advanced materials science. Metal nanoclusters (MNCs), distinguished by their atomic precision and molecule-like properties-including discrete energy levels, strong photoluminescence, and high property tunability-represent promising platforms for applications spanning catalysis to biomedicine. This perspective presents a comprehensive synthesis planning framework comprising three critical stages, i.e., target design, route development, and condition optimization, systematically addressing MNC rational design and synthesis with special emphasis on thiolate-protected gold nanoclusters as exemplary systems. We first discuss design considerations for core and ligand shell engineering based on their profound influence on overall material properties. Subsequently, we examine methods and synthetic mechanisms for atomic-level tailoring of core and ligand shells to achieve target MNC synthesis. We then elucidate condition parameter tuning considerations based on their deterministic roles in reaction outcomes. While this structured approach provides a systematic methodology for MNC development, significant challenges persist owing to the high structural and synthetic complexity of MNCs. We then discuss the opportunities brought by recent advances in machine learning and high-throughput experimentation, which have demonstrated potential in addressing these challenges based on their superior computational and data analytical capabilities. We advocate for systematic adoption of this synthesis planning approach enhanced by data-driven methods, addressing inherent limitations in future development to better exploit these integrated approaches for accelerating rational MNC design and synthesis.

In this work, we integrate atomically precise noble metal nanoclusters (NCs) on gold nanorods (AuNRs) to create hybrid plasmonic-luminescent nanomaterials. Initially, we assemble luminescent Ag29(LA)12 NC (LA = lipoic acid) to silica shell-encapsulated AuNRs. The resulting nanostructure shows plasmon-enhanced luminescence in aqueous medium as well as in the solid state. Atomic precision of the fluorophores used in this case allows detailed characterization of individual nanocomposites by diverse techniques, including transmission electron microscopy (TEM) and 3D electron tomographic reconstruction. We extend this strategy to prepare similar structures with gold NC protected with bovine serum albumin (Au30BSA). These two examples demonstrate the generic nature of the present strategy in preparing plasmonic-luminescent hybrid nanostructures using atomically precise NCs.

The floating memory properties of thiolate-protected gold (Au:SR) nanoclusters, Au25(SR)18, Au38(SR)24, Au144(SR)60, and Au333(SR)79 (R = C12H25), and Au colloids were investigated using four-probe measurements in vacuum. Uniform and monolayer films of Au:SR nanoclusters or Au colloids were formed as floating memory layers on p-type Si substrates by Langmuir–Blodgett method. The fluoropolymer (CYTOP, ∼15 nm thick) was spin-coated on top to form a hydrophobic insulating layer. Using Au dot (∼40 nm thick) as the anode electrode, the capacitance–voltage (C–V) measurements were performed using the Au and Cu plate electrodes as contact points for the two probes. Clockwise hysteresis curves originating from the Au:SR nanoclusters or Au colloids were observed, and the hysteresis width was dependent on the size of the Au:SR nanoclusters and the sample temperature. In particular, for the Au38(SR)24 nanocluster, multiple phases were observed in the C–V curve, implying their application in multivalued memory devices.

As nanotechnology continues to push the boundaries across disciplines, there is an increasing need for engineering nanomaterials with atomic-level precision for self-assembly across length scales, i.e., from the nanoscale to the macroscale. Although molecular self-assembly allows atomic precision, extending it beyond certain length scales presents a challenge. Therefore, the attention has turned to size and shape-controlled metal nanoparticles as building blocks for multifunctional colloidal self-assemblies. However, traditionally, metal nanoparticles suffer from polydispersity, uncontrolled aggregation, and inhomogeneous ligand distribution, resulting in heterogeneous end products. In this feature article, I will discuss how virus capsids provide clues for designing subunit-based, precise, efficient, and error-free self-assembly of colloidal molecules. The atomically precise nanoscale proteinic subunits of capsids display rigidity (conformational and structural) and patchy distribution of interacting sites. Recent experimental evidence suggests that atomically precise noble metal nanoclusters display an anisotropic distribution of ligands and patchy ligand bundles. This enables symmetry breaking, consequently offering a facile route for two-dimensional colloidal crystals, bilayers, and elastic monolayer membranes. Furthermore, inter-nanocluster interactions mediated via the ligand functional groups are versatile, offering routes for discrete supracolloidal capsids, composite cages, toroids, and macroscopic hierarchically porous frameworks. Therefore, engineered nanoparticles with atomically precise structures have the potential to overcome the limitations of molecular self-assembly and large colloidal particles. Self-assembly allows the emergence of new optical properties, mechanical strength, photothermal stability, catalytic efficiency, quantum yield, and biological properties. The self-assembled structures allow reproducible optoelectronic properties, mechanical performance, and accurate sensing. More importantly, the intrinsic properties of individual nanoclusters are retained across length scales. The atomically precise nanoparticles offer enormous potential for next-generation functional materials, optoelectronics, precision sensors, and photonic devices.

Atomically precise, ligand-protected gold nanoclusters (AuNCs) attract considerable attention as contrast agents in the biosensing field. However, the control of their optical properties and functionalization of surface ligands remain challenging. Here we report a strategy to tailor AuNCs for the precise detection of protein carbonylation—a causal biomarker of ageing. We produce Au15SG13 (SG for glutathione) with atomic precision and functionalize it with a thiolated aminooxy moiety to impart protein carbonyl-binding properties. Mass spectrometry and molecular modelling reveal the key structural features of Au15SG12-Aminooxy and its reactivity towards carbonyls. Finally, we demonstrate that Au15SG12-Aminooxy detects protein carbonylation in gel-based 1D electrophoresis by one- and two-photon excited fluorescence. Importantly, to our knowledge, this is the first application of an AuNC that detects a post-translational modification as a nonlinear optical probe. The significance of post-translational modifications in life sciences may open avenues for the use of Au15SG13 and other nanoclusters as contrast agents with tailored surface functionalization and optical properties.

Triangular Au3 and tetrahedral Au4 are key structural units in the face-centered cubic gold core of thiolate-protected gold nanoclusters. Understanding their stacking arrangements is essential for elucidating the growth mechanisms of these gold cores. In this study, we design two new isomers of Au29(SR)19 nanoclusters via deliberately adjusting the stacking pattern of Au3 and Au4 based on the grand unified model and ring model to show preferable packing arrangements. Among the experimental isomer and the two newly constructed isomers of Au29(SR)19, the Au3 and Au4 units display three distinct stacking arrangements: completely independent Au3; Au3 sharing a vertex with one Au4; and Au3 sharing vertices with two Au4. Density functional theory calculations reveal that the isomer with a completely independent Au3 unit owns the lower relative energy, which suggests that forming independent Au3 in thiolate-protected gold nanoclusters is relatively straightforward, while achieving alternative stacking configurations with Au3 and Au4 is more challenging, aligning with experimental observations. This study provides a theoretical foundation for understanding the stacking arrangements between triangular Au3 and tetrahedral Au4 units, offering valuable insights for designing ligand-protected gold nanoclusters featuring triangular Au3 units.