Probing the Structure and Charge State of Glutathione-Capped Au25(SG)18 Clusters by NMR and Mass Spectrometry

Molecular Interactions in Atomically Precise Metal Nanoclusters

For nanochemistry, precise manipulation of nanoscale structures and the accompanying chemical properties at atomic precision is one of the greatest challenges today. The scientific community strives to develop and design customized nanomaterials, while molecular interactions often serve as key tools or probes for this atomically precise undertaking. In this Perspective, metal nanoclusters, especially gold nanoclusters, serve as a good platform for understanding such nanoscale interactions. These nanoclusters often have a core size of about 2 nm, a defined number of core metal atoms, and protecting ligands with known crystal structure. The atomically precise structure of metal nanoclusters allows us to discuss how the molecular interactions facilitate the systematic modification and functionalization of nanoclusters from their inner core, through the ligand shell, to the external assembly. Interestingly, the atomic packing structure of the nanocluster core can be affected by forces on the surface. After discussing the core structure, we examine various atomic-level strategies to enhance their photoluminescent quantum yield and improve nanoclusters' catalytic performance. Beyond the single cluster level, various attractive or repulsive molecular interactions have been employed to engineer the self-assembly behavior and thus packing morphology of metal nanoclusters. The methodological and fundamental insights systemized in this review should be useful for customizing the cluster structure and assembly patterns at the atomic level.

Synthesis of atomically precise Ag16 nanoclusters and investigating solvent-dependent ultrafast relaxation dynamics

In this article, the main focus is to employ a new synthetic strategy to prepare atomically precise Ag nanoclusters (NCs) and unveil the critical role played by the solvents in the excited state dynamics of Ag NCs. The compositional analysis confirms the formula of the nanoclusters as Ag16(PDT)8(PPh3)4 (Ag-PDT NCs). These NCs showed a sharp absorption band at 525 nm and a comparatively broad absorption band at 633 nm. The emission maximum was 630 nm with a quantum yield (QY) of 0.23%. Three-component relaxation dynamics was retrieved from global analysis and described as core relaxation (664 fs), core-to-surface state relaxation (500 ps), and ground state relaxation (>1 ns) for Ag NCs in the DCM solvent. The time constants are slightly higher at 1.25 ps, 624.25 ps, and >1 ns for Ag NCs in the DMF solvent because of the less effective charge separation. The high QY in DMF follows this low charge separation (0.23% vs. 0.63%). The straight-chain dithiol capping agent (with lower electron density than an electron-rich aromatic ring) is mainly responsible for this less effective charge separation. Finding the pivotal role of the solvent in NC chemistry will help to characterize it thoroughly and produce a strategy for precise applications in various fields.

Conversion of Ultrasmall Glutathione-Coated Silver Nanoparticles during Dispersion in Water into Ultrasmall Silver Sulfide Nanoparticles

Ultrasmall silver nanoparticles (2 nm) were prepared by reduction with sodium borohydride (NaBH4) and stabilized by the ligand glutathione (a tripeptide: glycine-cysteine-glutamic acid). NMR spectroscopy and optical spectroscopy (UV and fluorescence) revealed that these particles initially consist of silver nanoparticles and fluorescing silver nanoclusters, both stabilized by glutathione. Over time, the silver nanoclusters disappear and only the silver nanoparticles remain. Furthermore, the capping ligand glutathione eliminates hydrogen sulfide (H2S) from the central cysteine and is released from the nanoparticle surface as tripeptide glycine-dehydroalanine-glutamic acid. Hydrogen sulfide reacts with the silver core to form silver sulfide. After four weeks in dispersion at 4 °C, this process is completed. These processes cannot be detected by transmission electron microscopy (TEM), small-angle X-ray scattering (SAXS), or differential centrifugal sedimentation (DCS) as these methods cannot resolve the mixture of nanoparticles and nanoclusters or the nature of the nanoparticle core. X-ray photoelectron spectroscopy showed the mostly oxidized state of the silver nanoparticle core, Ag(+I), both in freshly prepared and in aged silver nanoparticles. These results demonstrate that ultrasmall nanoparticles can undergo unnoticed changes that considerably affect their chemical, physical, and biological properties. In particular, freshly prepared ultrasmall silver nanoparticles are much more toxic against cells and bacteria than aged particles because of the presence of the silver clusters.

Possibilities and limitations of solution-state NMR spectroscopy to analyze the ligand shell of ultrasmall metal nanoparticles

Ultrasmall nanoparticles have a diameter between 1 and 3 nm at the border between nanoparticles and large molecules. Usually, their core consists of a metal, and the shell of a capping ligand with sulfur or phosphorus as binding atoms. While the core structure can be probed by electron microscopy, electron and powder diffraction, and single-crystal structure analysis for atom-sharp clusters, it is more difficult to analyze the ligand shell. In contrast to larger nanoparticles, ultrasmall nanoparticles cause only a moderate distortion of the NMR signal, making NMR spectroscopy a qualitative as well as a quantitative probe to assess the nature of the ligand shell. The application of isotope-labelled ligands and of two-dimensional NMR techniques can give deeper insight into ligand-nanoparticle interactions. Applications of one- and two-dimensional NMR spectroscopy to analyze ultrasmall nanoparticles are presented with suitable examples, including a critical discussion of the limitations of NMR spectroscopy on nanoparticles.

Peptide-Directed Synthesis of Aggregation-Induced Emission Enhancement-Active Gold Nanoclusters for Single- and Two-Photon Imaging of Lysosome and Expressed αvβ3 Integrin Receptors

This study explores the synthesis and characterization of aggregation-induced emission enhancement (AIEE)-active gold nanoclusters (AuNCs), focusing on their near-infrared luminescence properties and potential applications in biological imaging. These AIEE-active AuNCs were synthesized via the NaBH4-mediated reduction of HAuCl4 in the presence of peptides. We systematically investigated the influence of the peptide sequence on the optical features of the AuNCs, highlighting the role of glutamic acid in enhancing their quantum yield (QY). Among the synthesized peptide-stabilized AuNCs, EECEE-stabilized AuNCs exhibited the maximum QY and a pronounced AIEE effect at pH 5.0, making them suitable for the luminescence imaging of intracellular lysosomes. The AIEE characteristic of the EECEE-stabilized AuNCs was demonstrated through examinations using transmission electron microscopy, dynamic light scattering, zeta potential analysis, and single-particle imaging. The formation of the EECEE-stabilized AuNCs was confirmed by size-exclusion chromatography and mass spectrometry. Spectroscopic and electrochemical examinations uncover the formation process of EECEE-stabilized AuNCs, comprising EECEE-mediated reduction, NaBH4-induced nucleation, complex aggregation, and subsequent cluster growth. Furthermore, we demonstrated the utility of these AuNCs as luminescent probes for intracellular lysosomal imaging, leveraging their pH-responsive AIEE behavior. Additionally, cyclic arginylglycylaspartic acid (RGD)-modified AIEE dots, derived from cyclic RGD-linked peptide-induced aggregation of EECEE-stabilized AuNCs, were developed for single- and two-photon luminescence imaging of αvβ3 integrin receptor-positive cancer cells.

Mass spectrometers as cryoEM grid preparation instruments

Structure determination by single-particle cryoEM has matured into a core structural biology technique. Despite many methodological advancements, most cryoEM grids are still prepared using the plunge-freezing method developed ∼40 years ago. Embedding samples in thin films and exposing them to the air-water interface often leads to sample damage and preferential orientation of the particles. Using native mass spectrometry to create cryoEM samples, potentially avoids these problems and allows the use of mass spectrometry sample isolation techniques during EM grid creation. We review the recent publications that have demonstrated protein complexes can be ionized, flown through the mass spectrometer, gently landed onto EM grids, imaged, and reconstructed in 3D. Although many uncertainties and challenges remain, the combination of cryoEM and MS has great potential.

-SR removal or -R removal? A mechanistic revisit on the puzzle of ligand etching of Au25(SR)18 nanoclusters during electrocatalysis

Accurate identification of active sites is highly desirable for elucidation of the reaction mechanism and development of efficient catalysts. Despite the promising catalytic performance of thiolated metal nanoclusters (NCs), their actual catalytic sites remain elusive. Traditional first-principles calculations and experimental observations suggested dealkylated S and dethiolated metal, respectively, to be the active centers. However, the real kinetic origin of thiolate etching during the electrocatalysis of NCs is still puzzling. Herein, we conducted advanced first-principles calculations and electrochemical/spectroscopic experiments to unravel the electrochemical etching kinetics of thiolate ligands in prototype Au25(SCH3)18 NC. The electrochemical processes are revealed to be spontaneously facilitated by dethiolation (i.e., desorption of -SCH3), forming the free HSCH3 molecule after explicitly including the solvent effect and electrode potential. Thus, exposed under-coordinated Au atoms, rather than the S atoms, serve as the real catalytic sites. The thermodynamically preferred Au-S bond cleavage arises from the selective attack of H from proton/H2O on the S atom under suitable electrochemical bias due to the spatial accessibility and the presence of S lone pair electrons. Decrease of reduction potential promotes the proton attack on S and significantly accelerates the kinetics of Au-S bond breakage irrespective of the pH of the medium. Our theoretical results are further verified by the experimental electrochemical and spectroscopic data. At more negative electrode potentials, the number of -SR ligands decreased with concomitant increase of the vibrational intensity of S-H bonds. These findings together clarify the atomic-level activation mechanism on the surface of Au25(SR)18 NCs.

Metal nanoclusters: from fundamental aspects to electronic properties and optical applications

Monolayer-protected noble metal clusters, also called nanoclusters, can be produced with the atomic precision and in large-scale quantity and are playing an increasingly important role in the field of nanoscience. To outline the origin and the perspectives of this new field, we overview the main results obtained on free metal clusters produced in gas phase including mainly electronic properties, the giant atom concept, the optical properties, briefly the role of the metal atom (alkali, divalent, noble metal) and finally the atomic structure of clusters. We also discuss the limitations of the free clusters. Then, we describe the field of monolayer-protected metal clusters, the main results, the new offered perspectives, the added complexity, and the role of the ligand beyond the superatom concept.

Detection and imaging of bacterial biofilms with glutathione-stabilized gold nanoclusters

Bacterial biofilms colonize chronic wounds and surfaces of medical devices, thus making the development of reliable methods for imaging and detection of biofilms crucial. Although fluorescent identification of bacteria is sensitive and non-destructive, the lack of biofilm-specific fluorescent dyes limits the application of this technique to biofilm detection. Here, we demonstrate, for the first time, that fluorescent glutathione-stabilized gold nanoclusters (GSH-AuNCs) without targeting ligands can specifically interact with extracellular matrix components of Gram-negative and Gram-positive bacterial biofilms resulting in fluorescent staining of bacterial biofilms. By contrast, fluorescent bovine serum albumin-stabilized gold nanoclusters and 11-mercaptoundecanoic acid - stabilized gold nanoclusters do not stain the extracellular matrix of biofilms. According to molecular docking studies, GSH-AuNCs show affinity to several targets in extracellular matrix, including amyloid-anchoring proteins, matrix proteins and polysaccharides. Some experimental evidence was obtained for the interaction of GSH-AuNCs with the lipopolysaccharide (LPS) that was isolated from the matrix of Azospirillum baldaniorum biofilms. Based on GSH-AuNCs properties, we propose a new fluorescent method for the measurement of biofilm biomass with a limit of detection 1.7 × 10⁵ CFU/mL. The sensitivity of the method is 10-fold higher than the standard biofilm quantification with the crystal violet assay. There is a good linear relationship between the fluorescence intensity from the biofilms and the number of CFU from the biofilms in the range from 2.6 × 10⁵ to 6.7 × 10⁷ CFU/mL. The developed nanocluster-mediated method of biofilm staining was successfully applied for quantitative detection of biofilm formation on urinary catheter surface. The presented data suggest that fluorescent GSH-AuNCs can be used to diagnose medical device-associated infections.

The first solid-state route to luminescent Au(I)-glutathionate and its pH-controlled transformation into ultrasmall oligomeric Au10-12(SG)10-12 nanoclusters for application in cancer radiotheraphy

There is still a need for synthetic approaches that are much faster, easier to scale up, more robust and efficient for generating gold(I)-thiolates that can be easily converted into gold-thiolate nanoclusters. Mechanochemical methods can offer significantly reduced reaction times, increased yields and straightforward recovery of the product, compared to the solution-based reactions. For the first time, a new simple, rapid and efficient mechanochemical redox method in a ball-mill was developed to produce the highly luminescent, pH-responsive Au(I)-glutathionate, [Au(SG)]n. The efficient productivity of the mechanochemical redox reaction afforded orange luminescent [Au(SG)]n in isolable amounts (mg scale), usually not achieved by more conventional methods in solution. Then, ultrasmall oligomeric Au10-12(SG)10-12 nanoclusters were prepared by pH-triggered dissociation of [Au(SG)]n. The pH-stimulated dissociation of the Au(I)-glutathionate complex provides a time-efficient synthesis of oligomeric Au10-12(SG)10-12 nanoclusters, it avoids high-temperature heating or the addition of harmful reducing agent (e.g., carbon monoxide). Therefore, we present herein a new and eco-friendly methodology to access oligomeric glutathione-based gold nanoclusters, already finding applications in biomedical field as efficient radiosensitizers in cancer radiotherapy.

A Revealing Insight into Gold Cluster Photocatalysts: Visible versus (Vacuum) Ultraviolet Light

[Au₂₅(PPh₃)₁₀(SC₂H₄Ph)₅Cl₂]²⁺ (Au25) supported on TiO₂ (P25) exhibited distinct photocatalytic behaviors in the oxidation of amines using visible or ultraviolet light. The activity under visible light (455 nm) was superior to that under ultraviolet light. To gain insight into the origin of this difference, we investigated the photoreaction pathways of Au25 isolated in the gas phase upon irradiation with a pulsed laser with wavelengths of 455, 193, and 154 nm. High-resolution mass spectrometry revealed photon energy-dependent pathways for Au25: dissociation of the PPh₃ ligands and PPh₃AuCl units at 455 nm, dissociation into small [Au_nS_m]⁺ ions (n = 3-20; m = 0-4) at 193 nm, and ionization affording the triply charged state at 154 nm. These results were substantiated by density functional theory simulations. On the basis of these results, we proposed that the inferior photocatalytic activity of Au25/P25 under ultraviolet light is mainly due to the poor photostability of Au25.

The fate of organic species upon sintering of thiol-stabilised gold nanoparticles under different atmospheric conditions

Understanding and controlling the sintering behavior of gold nanoparticles is important for applications such as printed electronics, catalysis and sensing that utilise these materials. Here we examine the processes by which thiol-protected gold nanoparticles thermally sinter under a variety of atmospheres. We find that upon sintering, the surface-bound thiyl ligands exclusively form the corresponding disulfide species when released from the gold surface. Experiments conducted using air, hydrogen, nitrogen, or argon atmospheres revealed no significant differences between the temperatures of the sintering event nor on the composition of released organic species. When conducted under high vacuum, the sintering event occurred at lower temperatures compared to ambient pressures in cases where the resulting disulfide had relatively high volatility (dibutyl disulfide). Hexadecylthiol-stabilized particles exhibited no significant differences in the temperatures of the sintering event under ambient pressures compared to high vacuum conditions. We attribute this to the relatively low volatility of the resultant dihexadecyl disulfide product.

Chiral Inversion and Recovery of Supramolecular Luminescent Copper Nanocluster Hydrogels Triggered by Polyethyleneimine and Polyoxometalates

Construction of controllable chiroptical supramolecular luminescence systems is of great significance for developing intelligent chiral luminescence materials with precise and effective regulation and understanding chirality-switching phenomena in biological systems, which has attracted extensive attention. Because chiral metal nanoclusters (NCs) can provide facilities for the study of nanoscale chiral effects, in this study, we select chiral glutathione-stabilized copper NCs (G-SH-Cu NCs) to construct a supramolecular luminescent hydrogel with achiral branched polyethyleneimine (PEI) and polyoxometalates [Na₉(EuW₁₀O₃₆)·32H₂O, denoted as EuW₁₀]. Thus, a chiral property precise controlled system was constructed by self-assembly. Interestingly, the addition of PEI to G-SH-Cu NC solution induced the formation of luminescent hydrogels with chiral inversion, while further addition of EuW₁₀ not only enhanced the luminescence of the hydrogel but also recovered the chiroptical properties. The chiral inversion behavior is possibly ascribed to the hydrogen bond interaction/electrostatic interaction between G-SH-Cu NCs and PEI in the chiral inversion process, while the competition of hydrogen bonding interaction (between G-SH-Cu NCs and PEI) and electrostatic interaction (between PEI and EuW₁₀) was accountable for the chiral recovery process. Manipulation of chirality inversion in the metal NC-containing coassemblies is rare, while this work establishes a feasible strategy to modulate the chiral inversion behavior of Cu NCs, which not only produces new physicochemical properties of metal NCs through synergistic behavior but also offers a feasible way to realize the potential application of chiroptical materials.

An ultrastable thiolate/diglyme ligated cluster: Au20(PET)15(DG)2

The synthesis and characterization of an Au₂₀(PET)₁₅(DG)₂ (PET = phenylethane thiol; DG = diglyme) cluster is reported. Mass spectrometry reveals this as the first diglyme ligated cluster where diglyme ligands survive ionization into the gas phase. Thermal analysis shows the cluster degrades at 156 °C, whereas the similar Au₂₀(PET)₁₆ cluster degrades at 125 °C, representing markedly increased thermal stability. A combination of NMR spectroscopy and computational modeling suggests that the diglyme molecules bind in a tridentate manner for this cluster, resulting in a binding energy of 35.2 kcal mol^-1 for diglyme, which is comparable to the value of ∼40 kcal mol^-1 for thiolates. IR and optical spectroscopies show no evidence of assembly of this cluster, in contrast to Au₂₀(PET)₁₅(DG), which readily assembles into dimeric species, which is consistent with a tridentate binding motif. Evidence for stacking among Au-bound and non-bound diglyme molecules is inferred from thermal and mass analysis.

Metadynamics molecular dynamics and isothermal Brownian-type molecular dynamics simulations for the chiral cluster Au18

In an effort to gain insight into enantiomeric transitions, their transition mechanism, time span of transitions and distribution of time spans etc, we performed molecular dynamics (MD) simulations on chiral clusters Au₁₀, Au₁₅and Au₁₈, and found that viable reaction coordinates can be deduced from simulation data for enlightening the enantiomeric dynamics for Au₁₀and Au₁₅, but not so for Au₁₈. The failure in translating the Au₁₈-L ⇌ Au₁₈-R transitions by MD simulations has been chalked up to the thermal energyk_BTat 300 K being much lower than energy barriers separating the enantiomers of Au₁₈. Two simulation strategies were taken to resolve this simulation impediment. The first one uses the well-tempered metadynamics MD (MMD) simulation, and the second one adeptly applies first a somewhat crude MMD simulation to locate a highly symmetrical isomer Au_18Sand subsequently employed it as initial configuration in the MD simulation. In both strategies, we work in collective variable space of lower dimensionality. The well-tempered MMD simulation tactic was carried out aiming to offer a direct verification of Au₁₈enantiomers, while the tactic to conduct MMD/MD simulations in two consecutive simulation steps was intended to provide an indirect evidence of the existence of enantiomers of Au₁₈given that energy barriers separating them are much higher than ca.k_BTat 300 K. This second tactic, in addition to confirming indirectly Au₁₈-L and Au₁₈-R starting from the symmetrical cluster Au_18S, the simulation results shed light also on the mechanism akin to associative/nonassociative reaction transitions.

How different are the surfaces of semiconductor Ag2Se quantum dots with various sizes?

The surface of nanocrystals plays a dominant role in many of their physical and chemical properties. However, controllability and tunability of nanocrystal surfaces remain unsolved. Herein, we report that the surface chemistry of nanocrystals, such as near-infrared Ag₂Se quantum dots (QDs), is size-dependent and composition-tunable. The Ag₂Se QDs tend to form a stable metal complex on the surface to minimize the surface energy, and therefore the surface chemistry can be varied with particle size. Meanwhile, changes in surface inorganic composition lead to reorganization of the surface ligands, and the surface chemistry also varies with composition. Therefore, the surface chemistry of Ag₂Se QDs, responsible for the photoluminescence (PL) quantum yield and photostability, can be tuned by changing their size or composition. Accordingly, we demonstrate that the PL intensity of the Ag₂Se QDs can be tuned reversely by adjusting the degree of surface Ag⁺ enrichment via light irradiation or the addition of AgNO₃. This work provides insight into the control of QD surface for desired PL properties.

Milk protein-shelled gold nanoparticles with gastrointestinally active absorption for aurotherapy to brain tumor

Orally absorbable gold nanoparticles (AuNP) having cancer ablation therapy is strongly demanded to treat glioblastoma multiforme (GBM) for patients with its highest incidence rate. Here, we develop a milk protein lactoferrin-conjugated AuNP for its oral absorption and targeting to the GBM through the interaction between lactoferrin (Lf) and lactoferrin receptor (LfR) that is highly expressed in the intestine, blood-brain barrier and GBM. For stability and long circulation of AuNP, glutathione and polyethylene glycol (PEG) is introduced, which is called to Lf-PEG-AuNP. When Lf-PEG-AuNP are orally administered to orthotopic GBM-bearing mice, 11-fold and 8-fold higher concentrations of AuNP are measured in bloodstreams and GBM in the brain, respectively, compared with unconjugated-AuNP. Therefore, orally administered Lf-PEG-AuNP exhibit an outstanding temperature rise in GBM by irradiating laser and significantly reduce tumor volume. Collectively, we suggest that the Lf-PEG-AuNP can fundamentally target GBM in the brain through oral absorption, and that its efficient photothermal therapy is possible.

Size-Controlled Synthesis of Modifiable Glycine-Terminated Au Nanoclusters as a Platform for Further Functionalization

An improved and simple synthetic method for producing stable narrow-sized glycine-cystamine (Gly-CSA)-functionalized Au nanoclusters (NCs) from protected Fmoc-glycine-cystamine (Fmoc-Gly-CSA)-functionalized Au NCs is demonstrated in this study. The NC size and size distribution can be controlled directly as a function of reducing agent concentration with the formation of smaller NC core diameters at higher concentrations of NaBH₄. Furthermore, when using 0.30 M NaBH₄, three UV-vis absorption peaks at 690, 440, and 390 nm were seen, which are consistent with the formation of Fmoc-Gly-CSA-functionalized Au₂₅L₁₈ NCs. After deprotection of the Gly-CSA-functionalized Au NCs, the reactivity of the primary amine groups was investigated. Methyl acrylate-glycine-cystamine (MA-Gly-CSA)-functionalized Au NCs with terminal acetyl groups were formed via the Michael addition reaction of terminal amine groups with methyl acrylate. This reaction resulted in the formation of ester-terminated Au NCs including atom-precise MA-Gly-CSA Au₂₅(SR)₁₈ NCs. The functionalization of the ligand was confirmed by ¹H NMR and UV-vis spectra, and TEM images of MA-Gly-CSA- and Gly-CSA-functionalized Au NCs showed that the size of the NCs remained unchanged after the reaction. With controllable NC size and facile functionalization of the Gly-CSA-functionalized Au NCs, these clusters have promising potential as scaffolds for biomedical applications.

Nanotechnology in Tumor Biomarker Detection: The Potential of Liganded Nanoclusters as Nonlinear Optical Contrast Agents for Molecular Diagnostics of Cancer

Cancer is one of the leading causes of premature death, and, as such, it can be prevented by developing strategies for early and accurate diagnosis. Cancer diagnostics has evolved from the macroscopic detection of malignant tissues to the fine analysis of tumor biomarkers using personalized medicine approaches. Recently, various nanomaterials have been introduced into the molecular diagnostics of cancer. This has resulted in a number of tumor biomarkers that have been detected in vitro and in vivo using nanodevices and corresponding imaging techniques. Atomically precise ligand-protected noble metal quantum nanoclusters represent an interesting class of nanomaterials with a great potential for the detection of tumor biomarkers. They are characterized by high biocompatibility, low toxicity, and suitability for controlled functionalization with moieties specifically recognizing tumor biomarkers. Their non-linear optical properties are of particular importance as they enable the visualization of nanocluster-labeled tumor biomarkers using non-linear optical techniques such as two-photon-excited fluorescence and second harmonic generation. This article reviews liganded nanoclusters among the different nanomaterials used for molecular cancer diagnosis and the relevance of this new class of nanomaterials as non-linear optical probe and contrast agents.

Glutathione stabilized green-emission gold nanoclusters for selective detection of cobalt ion

A glutathione stabilized Au nanoclusters (GSH-Au NCs) was synthesized here and used to selective detection of cobalt ion. The as-prepared GSH-Au NCs had strong green light emission around 500 nm, and the features of the NCs have been systematically characterized by UV-vis absorption, X-ray photoelectronic spectroscopic, Fourier transform infrared spectroscopy and transmission electron microscope characterization. The interactions between the GSH-Au NCs and metal ions was studied, and the results indicated that the fluorescence of the GSH-Au NCs could be quenched in the presence of Co²⁺ ion at pH of 6.0. The quenching ratio was linear with the concentration of Co²⁺ ions, and the calibration curve was I₀/I = 0.1187c_co + 0.6085 in the Co²⁺ concentration ranges from 2.0 to 50.0 μM with correlation coefficient (R²) of 0.9950 and the limit of detection (LOD, 3σ) of 0.124 μM. In addition, we collected environmental water samples to test the reliability of the method and demonstrated this method is simple, rapid, and selective.

Origins of Visible and Near-Infrared Emissions in [Au25(SR)18]- Nanoclusters

The origin of photoluminescence in [Au₂₅(SR)₁₈]^- nanoclusters remains elusive, and there is not a universal model that can well explain the experimental result. Here, we design Au₂₅ nanoclusters protected by four different types of ligands for investigation of the photoluminescence mechanism by looking into the visible to near-infrared emissions. On the basis of time-resolved emission and nanosecond transient absorption spectroscopy analyses, we propose a model that can well explain the emission bands of Au₂₅ nanoclusters. The visible and near-infrared emissions have different lifetimes and are found to arise from the core-shell charge transfer state and the Au₁₃ core state, respectively. The obtained insight will help to understand how the excited state deactivates and to further engineer the photoluminescence of metal nanoclusters.

Post-Synthesis Modification of Photoluminescent and Electrochemiluminescent Au Nanoclusters with Dopamine

Here, we report a post-synthesis functionalization of the shell of Au nanoclusters (NCs) synthesized using glutathione as a thiolate ligand. The as-synthesized Au NCs are subjected to the post-synthesis functionalization via amidic coupling of dopamine on the cluster shell to tailor photoluminescence (PL) and electrochemiluminescence (ECL) features of the Au NCs. Because the NCs' PL at ca. 610 nm is primarily ascribed to the Au(I)-thiolate (SG) motifs on the cluster shell of the NCs, the post-synthesis functionalization of the cluster shell enhanced the PL intensity of the Au NCs via rigidification of the cluster shell. In contrast to the PL enhancement, the post-synthesis modification of the cluster shell does not enhance the near-infrared (NIR) ECL of the NCs because the NIR ECL at ca. 800 nm is ascribed to the Au(0)-SG motifs in the metallic core of the NCs.

Cell density-dependent accumulation of low polarity gold nanocluster in cultured vascular endothelial cells

We have previously reported the cytotoxicity and various biological responses of organic-inorganic hybrid molecules. However, because all the molecules used were electrophilic, the effect of the hybrid molecule without electrophilicity remains unclear. The glutathione-protected gold nanocluster, Au₂₅(SG)₁₈, is an organic-inorganic hybrid molecule that shows a low intramolecular polarity and high stability. In this study, we examined the cytotoxicity and intracellular accumulation of Au₂₅(SG)₁₈ in cultured vascular endothelial cells and compared these characteristics with those of negatively charged gold nanoparticles (AuNPs). Both Au₂₅(SG)₁₈ and AuNPs accumulated in vascular endothelial cells in a dose-dependent manner without cytotoxicity and more accumulation was observed at low cell densities. However, Au₂₅(SG)₁₈ accumulated significantly less than AuNPs in the cells. These results suggest that the intramolecular polarity of organic-inorganic hybrid molecules could regulate intracellular accumulation.

Chirality and Surface Bonding Correlation in Atomically Precise Metal Nanoclusters

Chirality is ubiquitous in nature and occurs at all length scales. The development of applications for chiral nanostructures is rising rapidly. With the recent achievements of atomically precise nanochemistry, total structures of ligand-protected Au and other metal nanoclusters (NCs) are successfully obtained, and the origins of chirality are discovered to be associated with different parts of the cluster, including the surface ligands (e.g., swirl patterns), the organic-inorganic interface (e.g., helical stripes), and the kernel. Herein, a unified picture of metal-ligand surface bonding-induced chirality for the nanoclusters is proposed. The different bonding modes of M-X (where M = metal and X = the binding atom of ligand) lead to different surface structures on nanoclusters, which in turn give rise to various characteristic features of chirality. A comparison of Au-thiolate NCs with Au-phosphine ones further reveals the important roles of surface bonding. Compared to the Au-thiolate NCs, the Ag/Cu/Cd-thiolate systems exhibit different coordination modes between the metal and the thiolate. Other than thiolate and phosphine ligands, alkynyls are also briefly discussed. Several methods of obtaining chiroptically active nanoclusters are introduced, such as enantioseparation by high-performance liquid chromatography and enantioselective synthesis. Future perspectives on chiral NCs are also proposed.

In Vivo Neuroelectrophysiological Monitoring of Atomically Precise Au25 Clusters at an Ultrahigh Injected Dose

Atomically precise Au₂₅(SG)₁₈ clusters have shown great promise in near-infrared II cerebrovascular imaging, X-ray imaging, and cancer radiotherapy due to their high atomic number, unique molecular-like electronic structure, and renal clearable properties. Therefore, it is important to study the in vivo toxicity of Au₂₅ clusters. Unfortunately, previous toxicological investigations focused on low injected doses (<100 mg kg^-1) and routine research methods, such as blood chemistry and biochemistry, which cannot reflect neurotoxicity or tiny changes in neural activity. In this work, in vivo neuroelectrophysiology of Au₂₅ clusters at ultrahigh injected doses (200, 300, and 500 mg kg^-1) was investigated. Local field potential showed that the Au₂₅-treated mice showed a spike in delta rhythm and moved to lower frequency over time. The power spectrum showed a 38.3% reduction in the peak value at 10 h post-injection of Au₂₅ clusters compared with 3 h post-injection, which gradually became close to the normal level, indicating no permanent damage to the nervous system. Moreover, no significant structural changes were found in both neurons and glial cells at the histological level. These results of in vivo neuroelectrophysiology will encourage scientists to make more exciting discoveries on nervous system diseases by employing Au₂₅ clusters even at ultrahigh injected doses.

Enhancement of near-infrared photoluminescence in Mg:Er:LiNbO3 containing Au nanoparticles synthesized by direct ion implantation

Embedded gold (Au) nanoparticles (NPs) are formed in Mg:Er:LiNbO₃ single crystals by Au ion implantation and subsequent thermal annealing. Absorption of the Mg:Er:LiNbO₃ crystals with Au NPs is found to be enhanced significantly in the visible light wavelength band owing to the localized surface plasmon resonance (LSPR) effect. The calculated LSPR effect by Mie theory shows good agreement with the absorption spectra. A significantly enhanced Er related photoluminescence (PL) at 1.54 μm for crystals with Au NPs is also observed compared with samples without Au NPs. Energy transfer between Au NPs and Er is found to be responsible for the PL enhancement in the as-implanted samples while local field enhancement induced by LSPR is considered the dominant factor in the annealed samples. The dependence of PL enhancement on NP size makes it possible to tailor intensity by varying the annealing temperature. An avenue to enhance and modulate the PL of dielectrics with embedding Au NPs synthesized by ion implantation is presented in this study.

In Situ Ligand-Directed Growth of Gold Nanoparticles in Biological Tissues

Fundamental understandings and precise control of nanoparticle growth in the complex biological environment are crucial to broadening their potential applications in tissue imaging. Herein, we report that glutathione (GSH), a widely used capping ligand for precise control of the size of gold nanoparticle (AuNP) down to single-atom level in test tubes, can also be used to direct the selective growth of the AuNPs in the mitochondria of renal tubule cells as well as hippocampus cells in the tissues. Precise control of this growth process can lead to the formation of both ultrasmall AuNPs with near-infrared luminescence and large plasmonic AuNPs. The observed selective growth of the AuNPs is likely due to unique GSH storage function of the mitochondria. Using a different ligand, β-glucose thiol, we also found that the brush border of the intestine for glucose absorption became the major site for the growth of luminescent AuNPs. These findings suggest that selective growth of AuNPs in the biological tissues can indeed be directed with specific ligands, opening up a new avenue to tissue labeling and future development of artificial bionano hybrid systems.

Development of a facile and sensitive method for detecting alkaline phosphatase activity in serum with fluorescent gold nanoclusters based on the inner filter effect

In this work, a simple and sensitive method based on the inner filter effect (IFE) of p-nitrophenol (PNP) on the fluorescence of gold nanoclusters (AuNCs) has been developed for detecting alkaline phosphatase (ALP) activity.

Light activated synthesis of the atomically precise fluorescent silver cluster Ag18(Capt)14

Metal clusters of gold and silver with highly tunable optical and electronic properties are attractive candidates for next generation medical imaging and therapy. Of these two most commonly studied metals, silver clusters often exhibit superior optical properties (i.e. stronger absorbance and higher emission quantum yield). The atomically precise synthesis of these clusters is essential before their use in biological applications can be realized. However, most cluster synthetic routes result in complex mixtures, where isolation and/or characterization can become incredibly challenging. Using photochemistry, we demonstrate a synthetic route for silver thiolate clusters resulting in the isolation of a pure eighteen-atom silver cluster capped by fourteen captopril ligands, Ag₁₈(Capt)₁₄. The facile control over the reduction of Ag(i) salt that this photochemical route affords can be readily applied as a general synthesis for isolating other new, atomically precise clusters.

Controlling magnetism of Au133(TBBT)52 nanoclusters at single electron level and implication for nonmetal to metal transition

The [Au₁₃₃(SR)₅₂]^q nanocluster is discovered to possess one spin per particle when q = 0, but no unpaired electron when q = +1.

The transition from the discrete, excitonic state to the continuous, metallic state in thiolate-protected gold nanoclusters is of fundamental interest and has attracted significant efforts in recent research. Compared with optical and electronic transition behavior, the transition in magnetism from the atomic gold paramagnetism (Au 6s¹) to the band behavior is less studied. In this work, the magnetic properties of 1.7 nm [Au₁₃₃(TBBT)₅₂]⁰ nanoclusters (where TBBT = 4-tert-butylbenzenethiolate) with 81 nominal “valence electrons” are investigated by electron paramagnetic resonance (EPR) spectroscopy. Quantitative EPR analysis shows that each cluster possesses one unpaired electron (spin), indicating that the electrons fill into discrete orbitals instead of a continuous band, for that one electron in the band would give a much smaller magnetic moment. Therefore, [Au₁₃₃(TBBT)₅₂]⁰ possesses a nonmetallic electronic structure. Furthermore, we demonstrate that the unpaired spin can be removed by oxidizing [Au₁₃₃(TBBT)₅₂]⁰ to [Au₁₃₃(TBBT)₅₂]⁺ and the nanocluster transforms from paramagnetism to diamagnetism accordingly. The UV-vis absorption spectra remain the same in the process of single-electron loss or addition. Nuclear magnetic resonance (NMR) is applied to probe the charge and magnetic states of Au₁₃₃(TBBT)₅₂, and the chemical shifts of 52 surface TBBT ligands are found to be affected by the spin in the gold core. The NMR spectrum of Au₁₃₃(TBBT)₅₂ shows a 13-fold splitting with 4-fold degeneracy of 52 TBBT ligands, which are correlated to the quasi-D₂ symmetry of the ligand shell. Overall, this work provides important insights into the electronic structure of Au₁₃₃(TBBT)₅₂ by combining EPR, optical and NMR studies, which will pave the way for further understanding of the transition behavior in metal nanoclusters.

Targeted Gold Nanocluster-Enhanced Radiotherapy of Prostate Cancer

For over a hundred years, X-rays have been a main component of the radiotherapeutic approaches to treat cancer. Yet, to date, no radiosensitizer has been developed to selectively target prostate cancer. Gold has excellent X-ray absorptivity and is used as a radiotherapy enhancing material. In this work, ultrasmall Au25 nanoclusters (NCs) are developed for selective prostate cancer targeting, radiotherapy enhancement, and rapid clearance from the body. Targeted-Au25 NCs are rapidly and selectively taken up by prostate cancer in vitro and in vivo and also have fast renal clearance. When combined with X-ray irradiation of the targeted cancer tissues, radiotherapy is significantly enhanced. The selective targeting and rapid clearance of the nanoclusters may allow reductions in radiation dose, decreasing exposure to healthy tissue and making them highly attractive for clinical translation.

Gold Clusters Prevent Inflammation-Induced Bone Erosion through Inhibiting the Activation of NF-κB Pathway

Inflammation-induced bone erosion is a major pathological factor in several chronic inflammatory diseases that often cause severe outcomes, such as rheumatoid arthritis and periodontitis. Plenty of evidences indicated that the inflammatory bone destruction was attributed to an increase in the number of bone-resorbing osteoclasts. However, anti-resorptive therapy alone failed to prevent bone loss in an inflammatory condition. Conventional anti-inflammation treatments are usually intended to suppress inflammation only, but ignore debilitating the subsequent bone destruction. Therefore, inhibition of proinflammatory activation of osteoclastogenesis could be an important strategy for the development of drugs aimed at preventing inflammatory bone destruction. Methods: In this study, we synthesized a peptide coated gold cluster to evaluate its effects on inflammatory osteoclastogenesis in vitro and inflammation-induced bone destruction in vivo. The in vitro anti-inflammation and anti-osteoclastogenesis effects of the cluster were evaluated in LPS-stimulated and receptor activator of nuclear factor κB ligand (RANKL) stimulated macrophages, respectively. The LPS-induced expression of crucial pro-inflammation cytokines and RANKL-induced osteoclastogenesis as well as the activation of NF-κB pathway in both situations were detected. The inflammation-induced RANKL expression and subsequent inflammatory bone destruction in vivo were determined in collagen-immunized mice. Results: The gold cluster strongly suppresses RANKL-induced osteoclast formation via inhibiting the activation of NF-κB pathway in vitro. Moreover, treatment with the clusters at a dose of 5 mg Au/kg.bw significantly reduces the severity of inflammation-induced bone and cartilage destruction in vivo without any significant toxicity effects. Conclusion: Therefore, the gold clusters may offer a novel potent therapeutic stratagem for inhibiting chronic inflammation associated bone destruction.

Statistically correlating NMR spectra and LC-MS data to facilitate the identification of individual metabolites in metabolomics mixtures

NMR and LC-MS are two powerful techniques for metabolomics studies. In NMR spectra and LC-MS data collected on a series of metabolite mixtures, signals of the same individual metabolite are quantitatively correlated, based on the fact that NMR and LC-MS signals are derived from the same metabolite covary. Deconvoluting NMR spectra and LC-MS data of the mixtures through this kind of statistical correlation, NMR and LC-MS spectra of individual metabolites can be obtained as if the specific metabolite is virtually isolated from the mixture. Integrating NMR and LC-MS spectra, more abundant and orthogonal information on the same compound can significantly facilitate the identification of individual metabolites in the mixture. This strategy was demonstrated by deconvoluting 1D ¹³C, DEPT, HSQC, TOCSY, and LC-MS spectra acquired on 10 mixtures consisting of 6 typical metabolites with varying concentration. Based on statistical correlation analysis, NMR and LC-MS signals of individual metabolites in the mixtures can be extracted as if their spectra are acquired on the purified metabolite, which notably facilitates structure identification. Statistically correlating NMR spectra and LC-MS data (CoNaM) may represent a novel approach to identification of individual compounds in a mixture. The success of this strategy on the synthetic metabolite mixtures encourages application of the proposed strategy of CoNaM to biological samples (such as serum and cell extracts) in metabolomics studies to facilitate identification of potential biomarkers.

Nuclear and Electron Magnetic Resonance Spectroscopies of Atomically Precise Gold Nanoclusters

Atomically precise gold nanoclusters display properties that are unseen in larger nanoparticles. When the number of gold atoms is sufficiently small, the clusters exhibit molecular properties. Their study requires extensive use of classic molecular physical chemistry and, thus, methods such as vibrational spectroscopies, electrochemistry, density functional theory and molecular dynamics calculations, and of course nuclear magnetic resonance (NMR) and electron paramagnetic resonance (EPR) spectroscopies. NMR and EPR studies have been mostly carried out on the benchmark, stable molecules Au₂₅(SR)₁₈, Au₃₈(SR)₂₄, Au₁₀₂(SR)₄₄, and Au₁₄₄(SR)₆₀ (where SR = thiolate). In this Account, we showcase examples primarily taken from our previous and ongoing NMR and EPR studies, which we hope will trigger further interest in the use of these sensitive, though often underutilized, techniques. Indeed, 1D and 2D NMR spectra of pure, atomically precise clusters can be very detailed and informative. Molecular clusters are molecules and, thus, have discrete energy levels and undergo stepwise oxidation or reduction. The effect of the charge state on the chemical shifts and line shapes is a function of the ligand type (ligands differ due to specific bonds with different Au atom types) and the position of the chemical group along the ligand backbone: for groups near the Au core, they can be very dramatic. Ligand-protected gold clusters are hard-soft molecules where a hard metal core is surrounded by a dynamic molecular layer. The latter provides a nanoenvironment that interfaces the cluster core with the surrounding environment and can be permeated by molecules and ions. NMR spectroscopy is especially useful to assess its structure. For example, the data show that whereas long alkanethiolates form bundles, shorter chains exhibit more conformational freedom and are quite folded. NMR spectroscopy allows studying diastereotopic effects and provides information on possible hydrogen bonds of ligands with sulfur or surface gold atoms. EPR spectroscopy is a very precise technique to check and characterize the magnetic state of gold clusters or clusters doped with foreign-metal atoms. Electron nuclear double resonance (ENDOR) provides a powerful tool to assess the interaction of an unpaired electron with nuclei, as we showed for ¹⁹⁷Au and ¹H. It can be used as a sensitive probe of the spin-density distribution in nanoclusters: for example, it showed that the singly occupied molecular orbital may span outside the Au core by nearly 6 Å. Solid-state EPR spectroscopy has provided compelling evidence that the specific ligands and the crystallinity degree are very important factors in determining the interactions between clusters in the solid state. Depending on the condition, paramagnetic, superparamagnetic, ferromagnetic, or antiferromagnetic behavior can be observed. Time-resolved EPR was successfully tested to determine the efficiency of singlet-oxygen generation via sensitization of Au₂₅ clusters. This Account thus demonstrates some of the remarkable insights that can be gained into the properties of atomically precise clusters through detailed NMR and EPR studies.

Characterization of chemically modified gold and silver clusters in gas phase

Atomically precise Au and Ag clusters protected by organic ligands can be viewed as chemically modified Au/Ag superatoms and have attracted interest as promising building units of functional materials and ideal platforms for studying the size-dependent evolution of structures and properties. Their structures, stability, and physicochemical properties have been characterized in solution and solid (or crystalline) phases by various methods conventionally used in materials science. However, novel and complementary information on their intrinsic stability and structures can be obtained by applying a variety of gas-phase methods, including mass spectrometry, ion mobility mass spectrometry, collision- or surface-induced dissociation mass spectrometry, photoelectron spectroscopy, and photodissociation mass spectrometry, to the chemically modified Au/Ag superatoms isolated in the gas phase. This perspective describes our recent efforts in the gas-phase studies on chemically synthesized Au/Ag superatoms.

Aprotinin Encapsulated Gold Nanoclusters: A Fluorescent Bioprobe with Dynamic Nuclear Targeting and Selective Detection of Trypsin and Heavy Metal

Fluorescence imaging has currently emerged as one of the most frequently used noninvasive imaging technologies to selectively monitor biological processes in living systems. In past decades, gold nanoclusters (Au NCs) has received increasing attraction because of their intrinsic fluorescence and their inherent biocompatibility. As a stabilizing and reducing agent, an abundant, sustainable, and widely used polypeptide derived drug molecule, aprotinin (Ap), is selected for the synthesis of Au nanoclusters (Ap-Au NCs) due to characteristic bioactivity, excellent biocompatibility, biodegradability, and non-allergenic character. Herein, Ap encapsulated Au NCs with desirable red fluorescence was facilely produced for the first time, which were subsequently used for cell imaging and detection of various analytes. Much interestingly, dynamically subcellular targeting  from the cytoplasm to the nucleus in HeLa cells was observed. Besides, it has shown that, the selective and quantitative detection of trypsin has been established by using Ap-Au NCs. Finally, Ap-Au NCs were readily used for quantitative detection of mercury and copper. The photoluminescence of the Ap-Au NCs was quenched with the addition of the aforementioned analytes. This study not only  discusses a multifunctional nanomaterial  for cell imaging, dynamically nuclear targeting and biosensing, but also opens crucial insights on the integration of funtional biomolecule with metal nanoclusters intended for extensively biomedical applications.

Characterizing gold nanoparticles by NMR spectroscopy

Gold nanoparticles have attracted considerable attention in recent research because of their wide applications in various fields such as material science, electrical engineering, physical science, and biomedical engineering. Researchers have developed many methods for synthesizing different kinds of gold nanoparticles, where the sizes and surface chemistry of the nanoparticles are considered to be the two key factors. Traditionally, the sizes of nanoparticles are determined by electron microscopy whereas the surface chemistry is characterized by optical spectroscopies such as infrared spectroscopy and Raman spectroscopy. Compared with that, nuclear magnetic resonance (NMR) spectroscopy provides a more advanced and convenient way for size determination and surface chemistry investigations by combining one- and multiple-dimensional NMR spectroscopy and diffusion-order NMR spectroscopy. Here, we show a thorough study that NMR spectroscopy can be applied to characterize small thiol-protected gold nanoparticles, including size determination, surface chemistry investigation, and structural study. The results show that the nanoparticles' sizes determined by NMR agree well with transmission electron microscopy results. Furthermore, the ligand densities of nanoparticles were determined by quantitative NMR spectroscopy, and the structures of ligands capped on the surfaces were studied thoroughly by one- and multiple-dimensional NMR spectroscopy. In this work, we establish a general method for researchers to characterize nanostructures by using NMR spectroscopy.

Synthesis and characterization of size-controlled atomically precise gold clusters

In this article, synthetic strategies and characterization methodologies of atomically precise gold clusters have been summarized. The typical and effective synthetic strategies including a systematic “size-focusing” methodology has been developed for attaining atomically precise gold clusters with size control. Another universal synthetic methodology is ligand exchange-induced size/structure transformation (LEIST) based on from one stable size to another. These two methodologies have largely expanded the “universe” of atomically precise gold clusters. Elite of typical synthetic case studies of ligand protected gold clusters are presented. Important characterization techniques of these atomically precise gold clusters also are included. The identification and characterization of gold clusters have been achieved in terms of nuclearity (size), molecular formulation, and geometrical structures by the combination of these techniques. The determination of gold cluster structure based on single crystals is of paramount importance in understanding the relationship of structure–property. The criterion and selection of these typical gold clusters are all “strictly” atomically precise that all have been determined ubiquitously by single crystal diffraction. These related crystallographic data are retrieved from Cambridge Crystallographic Data Centre (CCDC) up to 30th November 2017. Meanwhile, the cutting edge and other important characterization methodologies including electron diffraction (ED), extended X-ray absorption fine structure (EXFAS), and synchrotron sources are briefly reviewed. The new techniques hold the promise of pushing the limits of crystallization of gold clusters. This article is not just an exhaustive and up to date review, generally summarized synthetic strategies, but also a practical guide regarding gold cluster synthesis. We called it a “Cookbook” of ligand protected gold clusters, including synthetic recipes and characterization details.

Graphical Abstract: