In Situ Depletion-Guided Engineering of Nanoshell-like Gold Nanocluster Assemblies with Enhanced Peroxidase-like Nanozyme Activity.

Functional superstructures constructed from metal nanoclusters (MNCs) hold great promise in providing highly tunable photoluminescence (PL), catalytic activity, photothermal stability, and biological functionality. However, their controlled synthesis with well-defined size, structure, and properties remains a significant challenge. Herein, we introduce a novel approach that combines depletion attraction and thermal activation to induce the in situ formation of spherical superclusters (AuSCs) from Au(I)-thiolate complexes within the assembly. Extensive characterization and electron tomographic reconstruction reveal that Au(I)-thiolate complexes can be sequentially transitioned into metallic Au0, resulting in hollow nanoshell-like structures with consistent size (∼110 nm) and diverse shell configurations. Our results demonstrate that AuSCs with thinner shells, containing a high concentration of Au(I)-thiolate complexes, exhibit the highest PL, while AuSCs with thicker shells, containing high concentrations of metallic gold atoms and low ligand density, show remarkable peroxidase-like nanozyme activity in the 3,3',5,5'-tetramethylbenzidine (TMB) oxidation reaction.

Switchable catalysis and CO2 sensing by reduction resistant, luminescent copper-thiolate complexes.

Metal-thiolate complexes have been the focus of research for several years because of their unique photophysical properties and their use as a precursor for synthesizing various well-defined metal nanoclusters. A rational understanding of their structure-property relationship is necessary to realize their full potential in practical applications. Herein, we demonstrate the synthesis of a unique copper-thiolate complex with reversibly switchable catalytic and photoluminescence (PL) properties. The as-synthesized complex at basic pH (Complex B) showed cyan PL with a strong peak at ∼488 nm (cyan) and a small shoulder peak at ∼528 nm (green). When the pH of the complex was changed to acidic (Complex A), the PL was switched to light green. Such pH-responsive PL properties were demonstrated to be useful for pH and CO2 sensing. The switchable properties originate from their two distinct structural states at two different pHs. We found that Complex A was resistant to high concentrations of a strong reducing agent, and had an intermediate oxidation state of copper (Cu+) with good thermodynamic stability. Furthermore, the switchable catalytic property was investigated with a 4-nitrophenol reduction and 3,3',5,5'-tetramethylbenzidine (TMB) oxidation reaction. The reduction kinetics followed pseudo-first-order, where the catalytic activity was enhanced by more than 103 times when Complex B was switched to Complex A. A similar trend was also observed for TMB oxidation. Our design strategy demonstrates that redox switchable metal-thiolate complexes could be a powerful candidate for a plethora of applications.

Depletion Driven Assembly of Ultrasmall Metal Nanoclusters: From Kinetically Arrested Assemblies to Thermodynamically Stable, Spherical Superclusters.

Nanoscale assembly of ultrasmall metal nanoclusters (MNCs) by means of molecular forces has proven to be a powerful strategy to engineer their molecule-like properties in multiscale dimensions. By leveraging depletion attraction as the guiding force, herein, we demonstrate the formation of kinetically trapped NCs assemblies with enhanced photoluminescence (PL) and excited state lifetimes and extend the principle to cluster impregnated cationic nanogels, nonluminescent Au(I)-thiolate complexes, and weakly luminescent CuNCs. We further demonstrate a thermal energy driven kinetic barrier breaking process to isolate these assemblies. These isolated assemblies are thermodynamically stable, built from a strong network among several discrete, ultrasmall AuNCs and exhibit several unusual properties such as high stability in various pH, strong PL, microsecond lifetimes, large Stocks shifts, and higher accumulation in the lysosome of cancer cells. We anticipate our strategy may find wider use in creating a large variety of MNC-based assemblies with many unforeseen arrangements, properties, and applications.

Engineering Au Nanoclusters for Relay Luminescence Enhancement with Aggregation-Induced Emission.

The research of aggregation-induced emission (AIE) has been growing rapidly for the design of highly luminescent materials, as exemplified by the library of AIE-active materials (or AIEgens) fabricated and explored for diverse applications in different fields. Herein, we reported a relay luminescence enhancement of luminescent Au nanoclusters (Au NCs) through AIE. In addition, we demonstrated the emergence of reduced aggregation-caused luminescence by adjusting the temperature of the Au NC solution. The key to induce this effect is to attach a thermosensitive polymer poly(N-isopropylacrylamide) (PNIPAAm) on the surface of Au NCs, which will shrink at high temperature. More interestingly, the as-synthesized Au NCs-PNIPAAm can self-assemble into vesicles, resulting in an obvious decrease in the luminescence intensity in aqueous solution. The combination of relay luminescence enhancement (by AIE) and luminescence decrease (induced by thermosensitive polymers) will be beneficial to the understanding and manipulation of the optical properties of Au NCs, paving the way for their practical applications.

Polycationic Silver Nanoclusters Comprising Nanoreservoirs of Ag+ Ions with High Antimicrobial and Antibiofilm Activity.

Silver-based nano-antibiotics are rapidly developing as promising alternatives to conventional antibiotics. Ideally, to remain potent against a wide range of drug-resistant and anaerobic bacteria, silver-based nano-antibiotics should easily penetrate through the bacterial cell walls and actively release silver ions. In this study, highly monodispersed, ultrasmall (<3 nm), polycationic silver nanoclusters (pAgNCs) are designed and synthesized for the elimination of a range of common Gram-negative and Gram-positive pathogens and their corresponding established and matured biofilms, including those composed of multiple species. The pAgNCs also show greatly enhanced antibacterial efficacy against anaerobic bacteria such as Fusobacterium nucleatum and Streptococcus sanguinis. These results demonstrate that the cationic nature facilitates better penetration to the bacterial cell membrane while the presence of a high percentage (>50%) of silver ions (i.e., Ag+ nanoreservoirs) on the cluster surface maintains their efficiency in both aerobic and anaerobic conditions. Significantly, the pAgNCs showed a strong capacity to significantly delay the development of bacterial resistance when compared to similar-sized negatively charged silver nanoparticles or conventional antibiotics. This study demonstrates a novel design strategy that can lay the foundation for the development of future highly potent nano-antibiotics effective against a broad spectrum of pathogens and biofilms needed in many everyday life applications and industries.

Development and characterization of an oral microbiome transplant among Australians for the treatment of dental caries and periodontal disease: A study protocol.

BACKGROUND: Oral microbiome transplantation (OMT) is a novel concept of introducing health-associated oral microbiota into the oral cavity of a diseased patient. The premise is to reverse the state of oral dysbiosis, and restore the ecological balance to maintain a stable homeostasis with the host immune system. This study will assess the effectiveness, feasibility, and safety of OMT using an interdisciplinary approach. METHODS/DESIGN: To find donors suitable for microbial transplantation, supragingival plaque samples will be collected from 600 healthy participants. Each sample (200µL) will subsequently be examined in two ways: 1) 100µL of the sample will undergo high-throughput 16S rRNA gene amplicon sequencing and shotgun sequencing to identify the composition and characterisation of a healthy supragingival microbiome, 2) the remaining 100µL of the plaque sample will be mixed with 25% artificial saliva medium and inoculated into a specialised in-vitro flow cell model containing a hydroxyapatite disk. To obtain sufficient donor plaque, the samples would be grown for 14 days and further analysed microscopically and sequenced to examine and confirm the growth and survival of the microbiota. Samples with the healthiest microbiota would then be incorporated in a hydrogel delivery vehicle to enable transplantation of the donor oral microbiota. The third step would be to test the effectiveness of OMT in caries and periodontitis animal models for efficacy and safety for the treatment of oral diseases. DISCUSSION: If OMTs are found to be successful, it can form a new treatment method for common oral diseases such as dental caries and periodontitis. OMTs may have the potential to modulate the oral microbiota and shift the ecological imbalances to a healthier state.

Ultra-small gold nanoclusters assembled on plasma polymer-modified zeolites: a multifunctional nanohybrid with anti-haemorrhagic and anti-inflammatory properties.

Hemostatic agents are pivotal for managing clinical and traumatic bleeding during emergency and domestic circumstances. Herein, a novel functional hybrid nanocomposite material consisting of plasma polymer-modified zeolite 13X and ultra-small gold nanoclusters (AuNCs) was fabricated as an efficient hemostatic agent. The surface of zeolite 13X was functionalised with amine groups which served as binding sites for carboxylate terminated AuNCs. Protein corona studies revealed the enhanced adsorption of two proteins, namely, coagulation factors and plasminogen as a result of AuNCs immobilization on the zeolite surface. The immune response studies showed that the hybrid nanocomposites are effective in reducing inflammation, which combined with a greater attachment of vitronectin, may promote wound healing. The hemostatic potential of the nanocomposite could be directly correlated with their immunomodulatory and anti-haemorrhagic properties. Together, the hybrid nanoengineered material developed in this work could provide a new avenue to tackle life-threatening injuries in civilian and other emergencies.

Driving Forces and Routes for Aggregation-Induced Emission-Based Highly Luminescent Metal Nanocluster Assembly.

The development of ultrasmall, luminescent metal nanoclusters (MNCs) with aggregation-induced emission (AIE) characteristics is a relatively new research area that has gained significant attention in various multidisciplinary applications such as optoelectronics, sensing, imaging, and therapy. The numerous scientific breakthroughs in the AIE field provide many tools that, if incorporated into MNCs design strategies, could help realize various new and exciting MNC-based avenues that maximize the utilization of the AIE phenomenon. Indeed, leveraging the aggregation strategies from the AIE community with the judicious use of various covalent and noncovalent interactions has been demonstrated to be effective for constructing several MNC-based hybrid assemblies with enhanced AIE characteristics. In this Perspective, we summarize the key driving forces and routes of MNC assembly together with their impact on deciphering the working mechanism behind the AIE process. These strategies can inspire the design of highly luminescent MNC-based hierarchical functional materials across multiple length scales.

Traceable Nanocluster-Prodrug Conjugate for Chemo-photodynamic Combinatorial Therapy of Non-small Cell Lung Cancer.

In cancer treatment, image-guided combinatorial therapy is usually a more promising approach than conventional therapy because it may overcome the drawbacks of conventional cancer treatment, such as tumor recurrence and multidrug resistance. To achieve a high therapeutic effect in image-guided combinatorial therapy, the therapeutic material should be traceable, biocompatible, and yet highly effective in eradicating tumors. For this purpose, we developed a traceable nanocarrier consisting of atomically precise gold nanoclusters (Au NCs, Au22(SG)18, abbreviated as Au22 NCs, where SG stands for glutathione) and a biopolymer (i.e., chitosan). This traceable nanocarrier (Chito-Au22) was then combined with dual prodrugs (i.e., chemotherapeutic platinum (Pt(IV)) prodrug and photodynamic aminolevulinic acid (ALA) prodrug) through a bioconjugation method. It was found that the final nanocomposite (abbreviated as Pt(IV)-ALA-Chito-Au22) has a pH-responsive drug release behavior, and the cumulative drug release can exceed 50% within 12 h at an acidic pH of 5.0. After 15 min of white light irradiation, the nanocomposite showed a synergistic killing effect on the A549 non-small cell lung carcinoma cell line. The Pt(IV)-ALA-Chito-Au22 nanocomposite also showed a high cellular uptake capacity and reactive oxygen species (ROS) generation capability, resulting in a significant killing effect on three-dimensional (3D) multicellular A549 spheroids. In the presence of light, the volume of the multicellular spheroids treated by our nanocomposites was reduced more than two times compared with those treated by a single prodrug/component. The nanocomposite also showed good cell viability on normal lung cell lines. The multifunctional nanocomposites developed in this study have broad prospects in both therapeutic and diagnostic applications.

Ultrasmall AgNP-Impregnated Biocompatible Hydrogel with Highly Effective Biofilm Elimination Properties.

Ultrasmall silver nanoparticles (AgNPs; size < 3 nm) have attracted a great deal of interest as an alternative to commercially available antibiotics due to their ability to eliminate a wide range of microbial pathogens. However, most of these ultrasmall AgNPs are highly reactive and unstable, as well as susceptible to fast oxidation. Therefore, both the stability and toxicity remain major shortcomings for their clinical application and uptake. To circumvent these problems, we present a novel strategy to impregnate ultrasmall AgNPs into a biocompatible thermosensitive hydrogel that enables controlled release of silver alongside long-term storage stability and highly potent antibacterial activity. The advantage of this strategy lies in the combination of a homogenous dispersion of AgNPs in a hydrogel network, which serves as a sustained-release reservoir, and the unique feature of ultrasmall AgNP size, which provides an improved biofilm eradication capacity. The superior biofilm dispersion properties of the AgNP hydrogel is demonstrated in both single-species and multispecies biofilms, eradicating ∼80% of established biofilms compared to untreated controls. Notably, the effective antibacterial concentration of the formulation shows minimal toxicity to human fibroblasts and keratinocytes. These findings present a promising novel strategy for the development of AgNP hydrogels as an efficient antibacterial platform to combat resistant bacterial biofilms associated with wound infections.

The Impact of Engineered Silver Nanomaterials on the Immune System.

Over the last decades there has been a tremendous volume of research efforts focused on engineering silver-based (nano)materials. The interest in silver has been mostly driven by the element capacity to kill pathogenic bacteria. In this context, the main area of application has been medical devices that are at significant risk of becoming colonized by bacteria and subsequently infected. However, silver nanomaterials have been incorporated in a number of other commercial products which may or may not benefit from antibacterial protection. The rapid expansion of such products raises important questions about a possible adverse influence on human health. This review focuses on examining currently available literature and summarizing the current state of knowledge of the impact of silver (nano)materials on the immune system. The review also looks at various surface modification strategies used to generate silver-based nanomaterials and the immunomodulatory potential of these materials. It also highlights the immune response triggered by various silver-coated implantable devices and provides guidance and perspective towards engineering silver nanomaterials for modulating immunological consequences.

Biocompatible functionalisation of nanoclays for improved environmental remediation.

Among the wide range of materials used for remediating environmental contaminants, modified and functionalised nanoclays show particular promise as advanced sorbents, improved dispersants, or biodegradation enhancers. However, many chemically modified nanoclay materials are incompatible with living organisms when they are used in natural systems with detrimental implications for ecosystem recovery. Here we critically review the pros and cons of functionalised nanoclays and provide new perspectives on the synthesis of environmentally friendly varieties. Particular focus is given to finding alternatives to conventional surfactants used in modified nanoclay products, and to exploring strategies in synthesising nanoclay-supported metal and metal oxide nanoparticles. A large number of promising nanoclay-based sorbents are yet to satisfy environmental biocompatibility in situ but opportunities are there to tailor them to produce "biocompatible" or regenerative/reusable materials.

Core-in-cage structure regulated properties of ultra-small gold nanoparticles.

Understanding the structure-property relationships of novel materials is pivotal for the advances in science and technology. Thiolate ligand protected ultra-small gold nanoparticles (AuNPs; diameter below 3 nm) constitute an emerging class of nanomaterials with molecule-like properties that make them distinct from their larger counterparts. Here we provide new insights into the structure-property relationships of these nanomaterials by developing a series of ultra-small AuNPs, having comparable size and surface functionalities, but with different core-in-cage structures. We identified the density of metallic core and cage containing Au(i)-thiolate motifs, as well as cage rigidity as crucial factors that can significantly modulate the optical and biological properties of these AuNPs. In particular, AuNPs having a longer motif with a more rigid cage structure exhibited stronger luminescence while those containing a high percentage of loosely bound oligomeric Au(i)-thiolate motifs in the cage (semi-rigid structure) had better antibacterial activity. We also studied for the first time the inflammatory response to these NPs and revealed the importance of cage structure. We envisage that the finding reported in this paper can be applied not only to ultra-small AuNPs but also to other nanomaterials to develop new pathways to exciting future applications in electronics, sensing, imaging and medicine.

The interplay between size and valence state on the antibacterial activity of sub-10 nm silver nanoparticles.

Silver nanoparticles (AgNPs) have attracted enormous interest because of their excellent antibacterial properties, low cytotoxicity and limited evidence for resistance. As a general trend, smaller nanoparticles are considered to have stronger antibacterial activity. In this work we investigate whether this trend is valid for the sub-10 nm region by designing and synthesising three types of sub-10 nm AgNPs (∼1.87, ∼2.93 and ∼6.53 nm) to reveal the influence of size, valence state and structure on the antibacterial potency of AgNPs. We found that NPs with a size of ∼2.93 nm having a high concentration of silver in the first valence state presented the highest bacterial killing potency as well as low cytotoxicity to mammalian cells. The new insights presented in this study open future avenues for the engineering of highly potent silver nanoantibiotics that can be incorporated into future advanced medical devices and therapies capable of protecting patients from infections.

Unraveling the molecular mechanism of photosynthetic toxicity of highly fluorescent silver nanoclusters to Scenedesmus obliquus.

While the discovery of numerous attractive properties of silver at the nanoscale has increased their demand in many sectors including medicine, optics, sensing, painting and cosmetics, it has also raised wide public concerns about their effect on living organisms in aquatic environment. Despite the continuous effort to understand the various aspects of the toxicity of silver nanomaterials, the molecular level understanding on their cytotoxicity mechanism to biological organisms has remained unclear. Herein, we demonstrated the underlying mechanism of the photosynthetic toxicity against green algae namely, Scenedesmus obliquus by using an emerging silver nanomaterial, called silver nanoclusters (defined as r-Ag NCs). By exploiting the unique fluorescence properties of r-Ag NCs along with various other analytical/biological tools, we proposed that the photosynthetic toxicity of r-Ag NCs was largely attributed to the "joint-toxicity" effect of particulate form of r-Ag NCs and its released Ag+, which resulted in the disruption of the electron transport chain of light reaction and affected the content of key enzymes (RuBP carboxylase/ oxygenase) of Calvin cycle of algae cells. We believe that the present study can also be applied to the assessment of the ecological risk derived from other metal nanoparticles.

Probing the Microporous Structure of Silica Shell Via Aggregation-Induced Emission in Au(I)-Thiolate@SiO2 Nanoparticle.

An efficient method to investigate the window size of the silica shell generated via the classical Stöber method is reported by making use of the unique aggregation-induced emission property of Au(I)-thiolate complexes, which can precisely probe the porosity of the silica shell in Au(I)-thiolate@SiO2 nanoparticles.

Functionalization of metal nanoclusters for biomedical applications.

Metal nanoclusters (NCs) are emerging as a new class of functional nanomaterials in the area of biological sensing, labelling, imaging and therapy due to their unique physical and chemical properties, such as ultrasmall size, HOMO-LUMO transition, strong luminescence together with good photostability and biocompatibility. A recent surge of interest in this field is the surface functionalization of these metal NCs through which one can tailor their physicochemical properties, such as stability in solution, and strong luminescence, as well as their biodistribution and toxicity in biological systems, which in turn can empower these functionalized NCs with desirable targeting, imaging, and therapeutic ability for biomedical applications. In this review, we first introduce the functionalization strategies for the metal NCs developed in the past few years, followed by highlighting some biomedical applications of these functionalized metal NCs. We then discuss the difference of in vitro and in vivo fate as well as toxicity between various functionalized metal NCs. Finally, we present a short discussion on the current challenges and provide an outlook of the future developments of these functional metal NCs.

Uptake and effect of highly fluorescent silver nanoclusters on Scenedesmus obliquus.

The release of silver nanoparticles (Ag NPs) in aquatic environment has caused wide public concern about their effects on living organisms (e.g., algae). However, how these small NPs exert cytotoxicity in the living organisms has always been under heated debate. In this study, the uptake and toxicity effects of strongly red-emitting fluorescent silver nanoclusters (r-Ag NCs) exposed to the green algae Scenedesmus obliquus was investigated. Upon exposure to pure r-Ag NCs and r-Ag NCs containing l-cysteine, the algae growth inhibition test showed that Ag(+) ions released from r-Ag NCs played an important role in the toxicity of r-Ag NCs along with the toxicity of intact r-Ag NCs. Furthermore, no signals of intracellular reactive oxygen species (ROS) were observed indicating that r-Ag NCs or released Ag(+) ions - mediated growth inhibition of algae cells was independent of ROS production. Transmission electron microscopy (TEM) and laser scanning confocal microscopy (LSCM) were employed to study cellular uptake and cytotoxicity. Furthermore, analysis of differential expressed gene demonstrated that r-Ag NCs as well as the released Ag(+) ions can simultaneously exist inside the algae cells, and inhibit the transcriptomic process of genes by their "joint-toxicity" mechanism. Taken together, our study provides a new insight into the molecular mechanisms of r-Ag NCs and Ag(+) ions exposure to the aquatic organism and can be applied to early diagnosis of ecologic risk mediated by others metal-based NPs.

Insights into the effect of surface ligands on the optical properties of thiolated Au25 nanoclusters.

An abnormal optical absorption at about 780 and 980 nm was observed for Au25 nanoclusters (NCs) protected by negatively charged thiolate ligands collocating with positively or neutrally charged thiolate ligands, largely due to the resultant surface charge anisotropy on the NC surface which could induce structural distortions of the thiolated Au25 NCs.

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.