Thiolate-protected Metal Nanoclusters Research Articles

Ligand effects on thiols protected iron nanoclusters towards photoluminescence and antibacterial studies

Over the past few decade, thiolate-protected metal nanoclusters have gained great attention due to their high surface-to-volume ratio and greater surface area which finds applications in electronic, optical, magnetic, and biomedical fields. Herein, we report the tailoring of surface ligands protected over the ultra-small iron nanoclusters using three different thiols namely glutathione (GSH), 3-mercaptopropionic acid (MPA) and 3-mercaptopropane sulfonate (MPS). To understand the role of surface ligands on as prepared FeNCs (namely FeGSH, FeMPA and FeMPS NCs) was characterized by photophysical and bioimaging studies. The HRTEM and AFM results confirmed that these FeGSH, FeMPA and FeMPS NCs were in the average size range of 1–6 nm. The varying functionality and robustness of GSH, MPA and MPS protected over these individual FeNCs resulted in tunable quantum yields of 5.6 %, 5.8 % and 2.1 % for FeGSH, FeMPA and FeMPS NCs respectively. Further, these three FeNCs were investigated for cell viability and fluorescent microscopic analysis with the MCF-7 breast cancer cell line. The cell viability of FeGSH, FeMPA and FeMPS NCs with cancer cell line were found to be 96.62 %, 98.31 % and 93.29 % respectively for a fixed concentration range of 0 to 100 nM. The Phase contrast and fluorescent microscopy results confirm FeNCs induction which demonstrates that the cells were noncytotoxic and biocompatible for various applications, and this was further validated by the MTT test. The in vitro results imply that the ligand shell thickness and functionality here resulted in varying bioimaging results and antibacterial properties.

Selective formation of [Au23(SPh t Bu)17]0, [Au26Pd(SPh t Bu)20]0 and [Au24Pt(SC2H4Ph)7(SPh t Bu)11]0 by controlling ligand-exchange reaction.

To use atomically precise metal nanoclusters (NCs) in various application fields, it is essential to establish size-selective synthesis methods for the metal NCs. Studies on thiolate (SR)-protected gold NCs (Aun(SR)m NCs) revealed that the atomically precise Aun(SR)m NC, which has a different chemical composition from the precursor, can be synthesized size-selectively by inducing transformation in the framework structure of the metal NCs by a ligand-exchange reaction. In this study, we selected the reaction of [Au25(SC2H4Ph)18]− (SC2H4Ph = 2-phenylethanethiolate) with 4-tert-butylbenzenethiol (tBuPhSH) as a model ligand-exchange reaction and attempted to obtain new metal NCs by changing the amount of thiol, the central atom of the precursor NCs, or the reaction time from previous studies. The results demonstrated that [Au23(SPhtBu)17]0, [Au26Pd(SPhtBu)20]0 (Pd = palladium) and [Au24Pt(SC2H4Ph)7(SPhtBu)11]0 (Pt = platinum) were successfully synthesized in a high proportion. To best of our knowledge, no report exists on the selective synthesis of these three metal NCs. The results of this study show that a larger variety of metal NCs could be synthesized size-selectively than at present if the ligand-exchange reaction is conducted while changing the reaction conditions and/or the central atoms of the precursor metal NCs from previous studies.

Predicting ligand removal energetics in thiolate-protected nanoclusters from molecular complexes.

Thiolate-protected metal nanoclusters (TPNCs) have attracted great interest in the last few decades due to their high stability, atomically precise structure, and compelling physicochemical properties. Among their various applications, TPNCs exhibit excellent catalytic activity for numerous reactions; however, recent work revealed that these systems must undergo partial ligand removal in order to generate active sites. Despite the importance of ligand removal in both catalysis and stability of TPNCs, the role of ligands and metal type in the process is not well understood. Herein, we utilize Density Functional Theory to understand the energetic interplay between metal-sulfur and sulfur-ligand bond dissociation in metal-thiolate systems. We first probe 66 metal-thiolate molecular complexes across combinations of M = Ag, Au, and Cu with twenty-two different ligands (R). Our results reveal that the energetics to break the metal-sulfur and sulfur-ligand bonds are strongly correlated and can be connected across all complexes through metal atomic ionization potentials. We then extend our work to the experimentally relevant [M25(SR)18]- TPNC, revealing the same correlations at the nanocluster level. Importantly, we unify our work by introducing a simple methodology to predict TPNC ligand removal energetics solely from calculations performed on metal-ligand molecular complexes. Finally, a computational mechanistic study was performed to investigate the hydrogenation pathways for SCH3-based complexes. The energy barriers for these systems revealed, in addition to thermodynamics, that kinetics favor the break of S-R over the M-S bond in the case of the Au complex. Our computational results rationalize several experimental observations pertinent to ligand effects on TPNCs. Overall, our introduced model provides an accelerated path to predict TPNC ligand removal energies, thus aiding towards targeted design of TPNC catalysts.

Thiolate-Protected Bimetallic Nanoclusters: Understanding the Relationship between Electronic and Catalytic Properties

Thiolate-protected metal nanoclusters, which are smaller than 2 nm and have a specific number of metal atoms, have been greatly investigated in areas such as catalysis, sensing, and energy conversion because of their unique chemical, optical, structural, and electronic properties. Doping monometallic nanoclusters with another metal offers the opportunity to enhance these properties even further. The atomic structure of thiolate-protected bimetallic nanoclusters has been thoroughly studied using various X-ray methods, but the electronic structures of these complexes are often under-discussed. This Perspective summarizes works examining the electronic properties (charge states and energy levels) of these materials using density functional theory, square-wave voltammetry, UV-vis spectroscopy, and X-ray photoelectron spectroscopy. This information is then related to the catalytic activities of these complexes in various representative reactions (e.g., carbon-carbon coupling, hydrogenation, and oxidation). The significance of the structure-property relationship between the electronic properties and the catalytic performance of thiolate-protected bimetallic nanoclusters is demonstrated.

Ligand and support effects on the reactivity and stability of Au38(SR)24 catalysts in oxidation reactions

Thiolate protected metal nanoclusters are emerging materials for the preparation of atomically defined heterogeneous catalysts. Recently it was revealed that the ligands migrated to the support upon cluster deposition, which influences the catalytic behaviour. Here we examined the role of the protecting thiolate ligands on the cyclohexane oxidation for Au38(SR)24 supported on CeO2 and Al2O3. Sulfur containing products were detected. XANES S K-edge measurements revealed SOx species on the support during the reaction. The results indicate (i) an active and complex role of the thiolate ligand and (ii) changes of cluster (surface) structure, depending on support material and reaction conditions.

Catenane Structures of Homoleptic Thioglycolic Acid-Protected Gold Nanoclusters Evidenced by Ion Mobility-Mass Spectrometry and DFT Calculations.

Thiolate-protected metal nanoclusters have highly size- and structure-dependent physicochemical properties and are a promising class of nanomaterials. As a consequence, for the rationalization of their synthesis and for the design of new clusters with tailored properties, a precise characterization of their composition and structure at the atomic level is required. We report a combined ion mobility-mass spectrometry approach with density functional theory (DFT) calculations for determination of the structural and optical properties of ultra-small gold nanoclusters protected by thioglycolic acid (TGA) as ligand molecules, Au10(TGA)10. Collision cross-section (CCS) measurements are reported for two charge states. DFT optimized geometrical structures are used to compute CCSs. The comparison of the experimentally- and theoretically-determined CCSs allows concluding that such nanoclusters have catenane structures.

Toward Total Synthesis of Thiolate-Protected Metal Nanoclusters.

Total synthesis, where desired organic- and/or biomolecules could be produced from simple precursors at atomic precision and with known step-by-step reactions, has prompted centuries-lasting bloom of organic chemistry since its conceptualization in 1828 (Wöhler synthesis of urea). Such expressive science is also highly desirable in nanoscience, since it represents a decisive step toward atom-by-atom customization of nanomaterials for basic and applied research. Although total synthesis chemistry is less established in nanoscience, recent years have witnessed seminal advances and increasing research efforts devoted into this field. In this Account, we discuss recent progress on introducing and developing total synthesis routes and mechanisms for atomically precise metal nanoclusters (NCs). Due to their molecular-like formula and properties (e.g., HOMO-LUMO transition, strong luminescence and stereochemical activity), atomically precise metal NCs could be regarded as "molecular metals", holding potential applications in various practical sectors such as biomedicine, energy, catalysis, and many others. More importantly, the molecular-like properties of metal NCs are sensitively dictated by their size and composition, suggesting total synthesis of them as an indispensable basis for reliably realizing their practical applications. Atomically precise thiolate-protected Au, Ag and their alloy NCs are employed as model NCs to exemplify design strategies and governing principles in total synthesis of inorganic nanoparticles. This Account starts with a brief summary of total synthesis methodologies of atomically precise metal NCs. Following the methodological summary is a detailed discussion on the mechanisms governing these synthetic strategies, which is the main focus of this Account. Based on unprecedented precision (at atomic resolution) and ease (ensured by size-dependent properties) of tracking clusters' size/structure changes, mechanisms driving growth (e.g., reduction growth and seeded growth) and functionalization (e.g., alloying reaction and ligand exchange) of metal NCs have been explored at molecular level. With definitive step-by-step reaction routes, two-electron (2 e-) reduction (driving the growth reactions) and surface motif exchange (SME, prompting alloying and ligand exchange reactions) are discussed in depth and details. In addition to those sub- and/or individual-cluster level understandings, the self-assembly chemistry delivering high orderliness and enhanced materials performance in NC assemblies/supercrystals is also deciphered. This Account is then concluded with our perspectives toward potential development of cluster chemistry. Advances in total synthesis chemistry of metal NCs could not only serve as guidelines for future synthetic practice of NCs, but also provide molecular-level clues for many pending fundamental puzzles in nanochemistry, including nucleation growth, alloying chemistry, surface engineering and evolution of metamaterials.

Thermodynamic stability of ligand-protected metal nanoclusters

Despite the great advances in synthesis and structural determination of atomically precise, thiolate-protected metal nanoclusters, our understanding of the driving forces for their colloidal stabilization is very limited. Currently there is a lack of models able to describe the thermodynamic stability of these ‘magic-number’ colloidal nanoclusters as a function of their atomic-level structural characteristics. Herein, we introduce the thermodynamic stability theory, derived from first principles, which is able to address stability of thiolate-protected metal nanoclusters as a function of the number of metal core atoms and thiolates on the nanocluster shell. Surprisingly, we reveal a fine energy balance between the core cohesive energy and the shell-to-core binding energy that appears to drive nanocluster stabilization. Our theory applies to both charged and neutral systems and captures a large number of experimental observations. Importantly, it opens new avenues for accelerating the discovery of stable, atomically precise, colloidal metal nanoclusters.

The Innermost Three Gold Atoms Are Indispensable To Maintain the Structure of the Au18(SR)14 Cluster

The physical and chemical properties of thiolate-protected metal nanoclusters are highly dependent on their cluster composition. Such physicochemical properties could be further enriched by incorporating two noble metals inside one cluster. Here we report an efficient synthesis of thiolate-protected (AuAg)18 clusters (or Au18-xAgx(SR)14 clusters) by adopting a simple CO-reduction method. The composition and electronic/optical properties of the Au18-xAgx(SR)14 clusters are rationally tuned by adjusting the feeding ratio of Ag-to-Au. The experimental data also suggest that the incoming Ag atoms would first replace the Au atoms in the core and a maximum of six Ag atoms can be incorporated into the structure of Au18(SR)14. More interestingly, doping more than six Ag atoms leads to the collapse of the Au9 core, and the incoming Ag atoms would only go into the motif sites. A detailed investigation on the compositional and optical properties of the as-doped AuAg clusters finally highlights the crucial role of th...

Luminescent Metal Nanoclusters with Aggregation-Induced Emission.

Thiolate-protected metal nanoclusters (or thiolated metal NCs) have recently emerged as a promising class of functional materials because of their well-defined molecular structures and intriguing molecular-like properties. Recent developments in the NC field have aimed at exploring metal NCs as novel luminescent materials in the biomedical field because of their inherent biocompatibility and good photoluminescence (PL) properties. From the fundamental perspective, recent advances in the field have also aimed at addressing the fundamental aspects of PL properties of metal NCs, shedding some light on developing efficient strategies to prepare highly luminescent metal NCs. In this Perspective, we discuss the physical chemistry of a recently discovered aggregation-induced emission (AIE) phenomenon and show the significance of AIE in understanding the PL properties of thiolated metal NCs. We then explore the unique physicochemical properties of thiolated metal NCs with AIE characteristics and highlight some recent developments in synthesizing the AIE-type luminescent metal NCs. We finally discuss perspectives and directions for future development of the AIE-type luminescent metal NCs.

Precursor engineering and controlled conversion for the synthesis of monodisperse thiolate-protected metal nanoclusters

In very recent years, thiolate-protected metal nanoclusters (or thiolated MNCs) with core sizes smaller than 2 nm have emerged as a new direction in nanoparticle research due to their discrete and size dependent electronic structures and molecular-like properties, such as HOMO-LUMO transitions in optical absorptions, quantized charging, and strong luminescence. Synthesis of monodisperse thiolated MNCs in sufficiently large quantities (up to several hundred micrograms) is necessary for establishing reliable size-property relationships and exploring potential applications. This Feature Article reviews recent progress in the development of synthetic strategies for the production of monodisperse thiolated MNCs. The preparation of monodisperse thiolated MNCs is viewed as an engineerable process where both the precursors (input) and their conversion chemistry (processing) may be rationally designed to achieve the desired outcome - monodisperse thiolated MNCs (output). Several strategies for tailoring the precursor and the conversion process are analyzed to arrive at a unifying understanding of the processes involved.