Fluorescent Enhancement of [AgS4] Microplates by Mechanical Force Induced Crystallinity Breaking.

Mechanofluorochromic materials are a type of "smart" material because of their adjustable fluorescent properties under external mechanical force, making them significant members of the materials family. However, as the fluorescent characteristics of these materials highly depend on their microstructures, the still insufficiently in-depth research linking molecular structures to light emission motivates researchers to explore the fluorescent properties of these materials under external stimuli. In this work, based on synthetic [AgS4] microplates, we explore a fascinating mechanical-induced photoluminescent enhancement phenomenon. By applying mechanical force to solid-state [AgS4] to damage the surface morphology, a significant enhancement in photoluminescence is observed. Moreover, the emitted intensity increases with the extent of damage, which can be attributed to alterations in crystallinity. This work provides valuable insights into the relationship among photoluminescence, crystallinity, and mechanical force, offering new strategies for designing luminescent devices.

Enzyme-activatable charge transfer in gold nanoclusters.

Surface-protecting ligands, as a major component of metal nanoclusters (MNCs), can dominate molecular characteristics, performance behaviors, and biological properties of MNCs, which brings diversity and flexibility to the nanoclusters and largely promotes their applications in optics, electricity, magnetism, catalysis, biology, and other fields. We report herein the design of a new kind of water-soluble luminescent gold nanoclusters (AuNCs) for enzyme-activatable charge transfer (CT) based on the ligand engineering of AuNCs with 6-mercaptopurine ribonucleoside (MPR). This elaborately designed cluster, Au5(MPR)2, can form a stable intramolecular CT state after light excitation, and exhibits long-lived color-tunable phosphorescence. After the cleavage by purine nucleoside phosphorylase (PNP), the CT triplet state can be easily directed to a low-lying energy level, leading to a bathochromic shift of the emission band accompanied by weaker and shorter-lived luminescence. Remarkably, these ligand-engineered AuNCs show high affinity towards PNP as well as decent performance for analyzing and visualizing enzyme activity and related drugs. The work of this paper provides a good example for diversifying physicochemical properties and application scenarios of MNCs by rational ligand engineering, which will facilitate future interest and new strategies to precisely engineer solution-based nanocluster materials.

White-Emitting Gold Nanocluster Assembly with Dynamic Color Tuning.

We report that constructed Au nanoclusters (NCs) can afford amazing white emission synergistically dictated by the Au(0)-dominated core-state fluorescence and Au(I)-governed surface-state phosphorescence, with record-high absolute quantum yields of 42.1% and 53.6% in the aqueous solution and powder state, respectively. Moreover, the dynamic color tuning is achieved in a wide warm-to-cold white-light range (with the correlated color temperature varied from 3426 to 24 973 K) by elaborately manipulating the ratio of Au(0) to Au(I) species and thus the electron transfer rate from staple motif to metal kernel. This study not only exemplifies the successful integration of multiple luminescent centers into metal NCs to accomplish efficient white-light emission but also inspires a feasible pathway toward customizing the optical properties of metal NCs by regulating electron transfer kinetics.

Enzyme-mimic catalytic activities and biomedical applications of noble metal nanoclusters.

Noble metal (e.g., Au and Ag) nanoclusters (NCs), which exhibit structural complexity and hierarchy comparable to those of natural proteins, have been increasingly pursued in artificial enzyme research. The protein-like structure of metal NCs not only ensures enzyme-mimic catalytic activity, including peroxidase-, catalase-, and superoxide dismutase-mimic activities, but also affords an unprecedented opportunity to correlate the catalytic performance with the cluster structure at the molecular or atomic levels. In this review, we aim to summarize the recent progress in programming and demystify the enzyme-mimic catalytic activity of metal NCs, presenting the state-of-the-art understandings of the structure-property relationship of metal NC-based artificial enzymes. By leveraging on a concise anatomy of the hierarchical structure of noble metal NCs, we manage to unravel the structural origin of the catalytic performance of metal NCs. Noteworthily, it has been proven that the surface ligands and metal-ligand interface of metal NCs are instrumental in influencing enzyme-mimic catalytic activities. In addition to the structure-property correlation, we also discuss the synthetic methodologies feasible to tailoring the cluster structure at the atomic level. Prior to the closure of this review with our perspectives in noble metal NC-based artificial enzymes, we also exemplify the biomedical applications based on the enzyme-mimic catalysis of metal NCs with the theranostics of kidney injury, brain inflammation, and tumors. The fundamental and methodological advancements delineated in this review would be conducive to further development of metal NCs as an alternative family of artificial enzymes.

Enzyme-Inspired Ligand Engineering of Gold Nanoclusters for Electrocatalytic Microenvironment Manipulation.

Natural enzymes intricately regulate substrate accessibility through specific amino acid sequences and folded structures at their active sites. Achieving such precise control over the microenvironment has proven to be challenging in nanocatalysis, especially in the realm of ligand-stabilized metal nanoparticles. Here, we use atomically precise metal nanoclusters (NCs) as model catalysts to demonstrate an effective ligand engineering strategy to control the local concentration of CO2 on the surface of gold (Au) NCs during electrocatalytic CO2 reduction reactions (CO2RR). The precise incorporation of two 2-thiouracil-5-carboxylic acid (TCA) ligands within the pocket-like cavity of [Au25(pMBA)18]- NCs (pMBA = para-mercaptobenzoic acid) leads to a substantial acceleration in the reaction kinetics of CO2RR. This enhancement is attributed to a more favorable microenvironment in proximity to the active site for CO2, facilitated by supramolecular interactions between the nucleophilic Nδ- of the pyrimidine ring of the TCA ligand and the electrophilic Cδ+ of CO2. A comprehensive investigation employing absorption spectroscopy, mass spectrometry, isotopic labeling measurements, electrochemical analyses, and quantum chemical computation highlights the pivotal role of local CO2 enrichment in enhancing the activity and selectivity of TCA-modified Au25 NCs for CO2RR. Notably, a high Faradaic efficiency of 98.6% toward CO has been achieved. The surface engineering approach and catalytic fundamentals elucidated in this study provide a systematic foundation for the molecular-level design of metal-based electrocatalysts.

Photoluminescent Characterization of Metal Nanoclusters: Basic Parameters, Methods, and Applications.

Metal nanoclusters (MNCs) can be synthesized with atomically precise structures and molecule formulae due to the rapid development of nanocluster science in recent decades. The ultrasmall size range (normally < 2 nm) endows MNCs with plenty of molecular-like properties, among which photoluminescent properties have aroused extensive attention. Tracing the research and development processes of luminescent nanoclusters, various photoluminescent analysis and characterization methods play a significant role in elucidating luminescent mechanism and analyzing luminescent properties. In this review, it is aimed to systematically summarize the normally used photoluminescent characterizations in MNCs including basic parameters and methods, such as excitation/emission wavelength, quantum yield, and lifetime. For each key parameter, first its definition and meaning is introduced and then the relevant characterization methods including measuring principles and the revelation of luminescent properties from the collected data are discussed. Then, it is discussed in details how to explore the luminescent mechanism of MNCs and construct NC-based applications based on the measured data. By means of these characterization strategies, the luminescent properties of MNCs and NC-based designs can be explained quantitatively and qualitatively. Hence, this review is expected to provide clear guidance for researchers to characterize luminescent MNCs and better understand the luminescent mechanism from the measured results.

Unraveling a Concerted Proton-Coupled Electron Transfer Pathway in Atomically Precise Gold Nanoclusters.

Metal nanoclusters (MNCs) represent a promising class of materials for catalytic carbon dioxide and proton reduction as well as dihydrogen oxidation. In such reactions, multiple proton-coupled electron transfer (PCET) processes are typically involved, and the current understanding of PCET mechanisms in MNCs has primarily focused on the sequential transfer mode. However, a concerted transfer pathway, i.e., concerted electron-proton transfer (CEPT), despite its potential for a higher catalytic rate and lower reaction barrier, still lacks comprehensive elucidation. Herein, we introduce an experimental paradigm to test the feasibility of the CEPT process in MNCs, by employing Au18(SR)14 (SR denotes thiolate ligand), Au22(SR)18, and Au25(SR)18- as model clusters. Detailed investigations indicate that the photoinduced PCET reactions in the designed system proceed via an CEPT pathway. Furthermore, the rate constants of gold nanoclusters (AuNCs) have been found to be correlated with both the size of the cluster and the flexibility of the Au-S framework. This newly identified PCET behavior in AuNCs is prominently different from that observed in semiconductor quantum dots and plasmonic metal nanoparticles. Our findings are of crucial importance for unveiling the catalytic mechanisms of quantum-confined metal nanomaterials and for the future rational design of more efficient catalysts.

Engineering Au44 Nanoclusters for NIR-II Luminescence Imaging-Guided Photoactivatable Cancer Immunotherapy.

Immunotherapy is an advanced therapeutic strategy of cancer treatment but suffers from the issues of off-target adverse effects, lack of real-time monitoring techniques, and unsustainable response. Herein, an ultrasmall Au nanocluster (NC)-based theranostic probe is designed for second near-infrared window (NIR-II) photoluminescence (PL) imaging-guided phototherapies and photoactivatable cancer immunotherapy. The probe (Au44MBA26-NLG for short) is composed of atomically precise and NIR-II emitting Au44MBA26 NCs (here MBA denotes water-soluble 4-mercaptobenzoic acid) conjugated with immune checkpoint inhibitor 1-cyclohexyl-2-(5H-imidazo[5,1-a]isoindol-5-yl)ethanol (NLG919) via a singlet oxygen (1O2)-cleavable linker. Upon NIR photoirradiation, the Au44MBA26-NLG not only enables NIR-II PL imaging of tumors in deep tissues for guiding tumor therapy but also allows the leverage of photothermal property for cancer photothermal therapy (PTT) and the photogenerated 1O2 for photodynamic therapy (PDT) and releasing NLG919 for cancer immunotherapy. Such a multiple effect modulated by Au44MBA26-NLG prompts the proliferation and activation of effector T cells, upshifts systemic antitumor T-lymphocyte (T cell) immunity, and finally suppresses the growth of both primary and distant tumors in living mice. Overall, this study may provide a promising theranostic nanoplatform toward NIR-II PL imaging-guided phototherapies and photoactivatable cancer immunotherapy.

Ligand effect on switching the rate-determining step of water oxidation in atomically precise metal nanoclusters.

The ligand effects of atomically precise metal nanoclusters on electrocatalysis kinetics have been rarely revealed. Herein, we employ atomically precise Au25 nanoclusters with different ligands (i.e., para-mercaptobenzoic acid, 6-mercaptohexanoic acid, and homocysteine) as paradigm electrocatalysts to demonstrate oxygen evolution reaction rate-determining step switching through ligand engineering. Au25 nanoclusters capped by para-mercaptobenzoic acid exhibit a better performance with nearly 4 times higher than that of Au25 NCs capped by other two ligands. We deduce that para-mercaptobenzoic acid with a stronger electron-withdrawing ability establishes more partial positive charges on Au(I) (i.e., active sites) for facilitating feasible adsorption of OH- in alkaline media. X-ray photo-electron spectroscopy and theoretical study indicate a profound electron transfer from Au(I) to para-mercaptobenzoic acid. The Tafel slope and in situ Raman spectroscopy suggest different ligands trigger different rate-determining step for these Au25 nanoclusters. The mechanistic insights reported here can add to the acceptance of atomically precise metal nanoclusters as effective electrocatalysts.

Ligand engineering of Au44 nanoclusters for NIR-II luminescent and photoacoustic imaging-guided cancer photothermal therapy.

Developing a high-performance noninvasive probe for precise cancer theranostics is very challenging but urgently required. Herein, a novel Au nanoclusters (NCs)-based probe was designed for cancer theranostics via ligand engineering by conjugating photoluminescent (PL) Au44 NCs in the second near-infrared window (NIR-II, 1000-1700 nm) with aromatic photoacoustic (PA)/photothermal molecules through click chemistry. This design bypasses the incompatibility dilemma between photoluminescence (PL) attributes and PA/photothermal properties because the rigidity of the PA/photothermal molecules can lead to aggregation-induced emission (AIE) of the Au(i)-ligand shell of the Au NCs by constraining their nonradiative relaxation. Benefiting from strong NIR-II PL with emissions at 1080 and 1240 nm, high photothermal conversion efficiency (65.12%), low cytotoxicity, appropriate renal clearance, and enhanced permeability and retention (EPR) effect, the as-designed Au NC-based theranostic probe achieves ultradeep NIR-II PL/PA imaging-guided cancer photothermal therapy (PTT). Remarkably, 16 days after photothermal treatment guided by NIR-II PL/PA imaging, mice were all healed without tumor recurrence, while the average life span of the mice in the control groups was only 17-21 days. This study is interesting because it provides a paradigm for designing a metal NC-based theranostics probe, and it may add fundamentally and methodologically to noninvasive imaging-guided disease therapy.

Multi-Targeted Peptide-Modified Gold Nanoclusters for Treating Solid Tumors in the Liver.

Apoptosis and autophagy determine the fate of cancer cells. However, simply promoting apoptosis of tumor cells is limited in the treatment of unresectable solid liver tumors. Generally, autophagy is considered the anti-apoptotic "guardian". But the pro-apoptotic effects of autophagy can be activated by excessive endoplasmic reticulum (ER) stress. Here, amphiphilic peptide-modified glutathione (GSH)-gold nanocluster aggregates (AP1 P2 -PEG NCs) were designed with the enrichment of solid liver tumors and the prolonged stress in the ER, which can achieve the mutual promotion of autophagy and apoptosis in liver tumor cells. In this study, orthotopic and subcutaneous liver tumor models show the anti-tumor effectiveness of AP1 P2 -PEG NCs, with a better antitumor effect than sorafenib, biosafety (Lethal Dose, 50% (LD50 ) of 827.3 mg kg-1 ), wide therapeutic window (non-toxic in 20 times of therapeutic concentration) and high stability (blood half-life of 4 h). These findings identify an effective strategy to develop peptide-modified gold nanocluster aggregates with low toxicity, high potency, and selectivity for solid liver tumors treatment.

Suppression of kernel vibrations by layer-by-layer ligand engineering boosts photoluminescence efficiency of gold nanoclusters.

The restriction of structural vibration has assumed great importance in attaining bright emission of luminescent metal nanoclusters (NCs), where tremendous efforts are devoted to manipulating the surface landscape yet remain challenges for modulation of the structural vibration of the metal kernel. Here, we report efficient suppression of kernel vibration achieving enhancement in emission intensity, by rigidifying the surface of metal NCs and propagating as-developed strains into the metal core. Specifically, a layer-by-layer triple-ligands surface engineering is deployed to allow the solution-phase Au NCs with strong metal core-dictated fluorescence, up to the high absolute quantum yields of 90.3 ± 3.5%. The as-rigidified surface imposed by synergistic supramolecular interactions greatly influences the low-frequency acoustic vibration of the metal kernel, resulting in a subtle change in vibration frequency but a reduction in amplitude of oscillation. This scenario therewith impedes the non-radiative relaxation of electron dynamics, rendering the Au NCs with strong emission. The presented study exemplifies the linkage between surface chemistry and core-state emission of metal NCs, and proposes a strategy for brighter emitting metal NCs by regulating their interior metal core-involved motion.

Emerging ultrasmall luminescent nanoprobes for in vivo bioimaging.

Photoluminescence (PL) imaging has become a fundamental tool in disease diagnosis, therapeutic evaluation, and surgical navigation applications. However, it remains a big challenge to engineer nanoprobes for high-efficiency in vivo imaging and clinical translation. Recent years have witnessed increasing research efforts devoted into engineering sub-10 nm ultrasmall nanoprobes for in vivo PL imaging, which offer the advantages of efficient body clearance, desired clinical translation potential, and high imaging signal-to-noise ratio. In this review, we present a comprehensive summary and contrastive discussion of emerging ultrasmall luminescent nanoprobes towards in vivo PL bioimaging of diseases. We first summarize size-dependent nano-bio interactions and imaging features, illustrating the unique attributes and advantages/disadvantages of ultrasmall nanoprobes differentiating them from molecular and large-sized probes. We also discuss general design methodologies and PL properties of emerging ultrasmall luminescent nanoprobes, which are established based on quantum dots, metal nanoclusters, lanthanide-doped nanoparticles, and silicon nanoparticles. Then, recent advances of ultrasmall luminescent nanoprobes are highlighted by surveying their latest in vivo PL imaging applications. Finally, we discuss existing challenges in this exciting field and propose some strategies to improve in vivo PL bioimaging and further propel their clinical applications.

Modulating Catalytic Activity and Stability of Atomically Precise Gold Nanoclusters as Peroxidase Mimics via Ligand Engineering.

Metal nanoclusters (NCs), composed of a metal core and protecting ligands, show promising potentials as enzyme mimics for producing fuels, pharmaceuticals, and valuable chemicals, etc. Herein, we explore the critical role of ligands in modulating the peroxidase mimic activity and stability of Au NCs. A series of Au15(SR)13 NCs with various thiolate ligands [SR = N-acetyl-l-cysteine (NAC), 3-mercaptopropionic acid (MPA), or 3-mercapto-2-methylpropanoic acid (MMPA)] are utilized as model catalysts. It is found that Au15(NAC)13 shows higher structural stability than Au15(MMPA)13 and Au15(MPA)13 against external stimuli (e.g., pH, oxidants, and temperature) because of the intramolecular hydrogen bonds. More importantly, detailed enzymatic kinetics data show that the catalytic activity of Au15(NAC)13 is about 4.3 and 2.7 times higher than the catalytic activity of Au15(MMPA)13 and Au15(MPA)13, respectively. Density functional theory (DFT) calculations reveal that the Au atoms on the motif of Au NCs should be the active centers, whereas the superior peroxidase mimic activity of Au15(NAC)13 should originate from the emptier orbitals of Au atoms because of the electron-withdrawing effect of acetyl amino group in NAC. This work demonstrates the ligand-engineered electronic structure and functionality of atomically precise metal NCs, which afford molecular and atomic level insights for artificial enzyme design.

Supercrystal engineering of atomically precise gold nanoparticles promoted by surface dynamics.

The controllable packing of functional nanoparticles (NPs) into crystalline lattices is of interest in the development of NP-based materials. Here we demonstrate that the size, morphology and symmetry of such supercrystals can be tailored by adjusting the surface dynamics of their constituent NPs. In the presence of excess tetraethylammonium cations, atomically precise [Au25(SR)18]- NPs (where SR is a thiolate ligand) can be crystallized into micrometre-sized hexagonal rod-like supercrystals, rather than as face-centred-cubic superlattices otherwise. Experimental characterization supported by theoretical modelling shows that the rod-like crystals consist of polymeric chains in which Au25 NPs are held together by a linear SR-[Au(I)-SR]4 interparticle linker. This linker is formed by conjugation of two dynamically detached SR-[Au(I)-SR]2 protecting motifs from adjacent Au25 particles, and is stabilized by a combination of CH⋯π and ion-pairing interactions between tetraethylammonium cations and SR ligands. The symmetry, morphology and size of the resulting supercrystals can be systematically tuned by changing the concentration and type of the tetraalkylammonium cations.

Gold nanocluster with AIE: A novel photodynamic antibacterial and deodorant molecule.

Designing long-lasting yet high-efficiency antimicrobial and deodorant agents is an everlasting goal for environmental and public health. Here we present the design of AIE-featured Au nanoclusters (NCs) for visible-light-driven antibacterial and deodorant applications. Owing to the intriguing AIE traits, the good harvest of visible-light, and rich surface chemistry, the AIE-featured Au NCs unprecedentedly exhibit excellent visible-light-driven antibacterial activities against gram-positive (≥98.5%) and gram-negative bacteria (≥99.94%), which is resulted from their photodynamic producibility of abundant reactive oxygen species including O2•-, •OH and H2O2 via O2 reduction and subsequent H2O2 oxidation. In addition, the Au NCs are demonstrated to be biocompatible, and easy to be deployed for downstream antibacterial and deodorant applications. For example, the Au NCs-modified domestic materials (e.g., latex, ceramic glaze, organic fiber, and clothings) achieve long-lasting antibacterial efficiency of 99% and deodorant efficiency of >97.9% under visible-light irradiation. This work may shed light on designing novel AIE-featured metal NCs with photodynamic antibacterial and deodorant functions, enabling metal NCs and corresponding downstream materials to step into the photodynamic antibacterial and deodorant era.

Aggregation-Induced Emission of Gold Nanoclusters by Ionic Liquids for White Light-Emitting Diode and Multiple-Ion Probe Applications.

White light-emitting diodes (WLEDs) require stable luminophores with bright emission, a large Stokes shift, and good processability. Herein, we fabricate WLEDs on the basis of synergetic aggregation-induced emission (AIE) of Au nanoclusters (NCs) induced by ionic liquids. Au NCs with orange emission at 615 nm are electrostatically bonded to 1-ethyl-3-methyl-imidazolium acetate (EmimOAc). The electrostatic bonding not only introduces complementary blue emission (from EmimOAc) into Au NCs but also enhances cluster emission via an intercluster AIE mechanism. Overall white emission of Au NC-EmimOAc composites can be further improved by their encapsulation into nanogels templated by cationic chitosan. Color conversion films constructed from these nanogels manifest ultrabright white emission on a commercial GaN ultraviolet LED chip, featuring a CIE 1931 color coordinate of (0.366, 0.368) and greatly enhanced mechanical strength. By properly selecting ionic liquids, we are also able to achieve multiple-ion detection of Cu2+, Sn2+, and Fe3+.

Cation polymer-induced aggregation of water-soluble Au(I)-thiolate complexes and their photoluminescence properties.

Au(I)-thiolate complexes are a new class of aggregation-induced emission (AIE) material. Here we demonstrate a new aggregation strategy of water-soluble Au(I)-thiolate complexes induced by cationic polymers at optimized pH values. The generated AIE shows longer wavelengths than the emission induced by other methods.

Bis-Schiff base linkage-triggered highly bright luminescence of gold nanoclusters in aqueous solution at the single-cluster level.

Metal nanoclusters (NCs) have been developed as a new class of luminescent nanomaterials with potential applications in various fields. However, for most of the metal NCs reported so far, the relatively low photoluminescence quantum yield (QY) in aqueous solution hinders their applications. Here, we describe the utilization of bis-Schiff base linkages to restrict intramolecular motion of surface motifs at the single-cluster level. Based on Au22(SG)18 (SG: glutathione) NCs, an intracluster cross-linking system was constructed with 2,6-pyridinedicarboxaldehyde (PDA), and water-soluble gold NCs with luminescence QY up to 48% were obtained. The proposed approach for achieving high emission efficiency can be extended to other luminescent gold NCs with core-shell structure. Our results also show that the content of surface-bound Au(I)-SG complexes has a significant impact on the PDA-induced luminescence enhancement, and a high ratio of Au(I)-SG will be beneficial to increasing the photoluminescence intensity of gold NCs.