Design and mechanistic study of a novel gold nanocluster-based drug delivery system

Photofunctional Gold Nanocluster Composites for Bioapplications

Gold nanoclusters (AuNCs), with customized structures and diverse optical properties, are promising optical materials. Constructing composite systems by the assembly and incorporation of AuNCs can utilize their optical properties to achieve diagnostic and therapeutic applications in the biological field. Therefore, the exploration of the assembly behaviors of AuNCs and the enhancement of their performance has attracted widespread interest. In this review, we introduce multiple interactions and assembly modes that are prevalent in nanocomposites and microcomposites based on AuNCs. Then, the functions of AuNC composites for bioapplications are demonstrated in detail. These composite systems have inherited and enhanced the inherent optical performances of the AuNCs to meet diverse requirements for biological sensing and optical treatments. Finally, we discuss the prospects of AuNC composites and highlight the challenges and opportunities in biomedical applications.

The roles of templates consisting of amino acids in the synthesis and application of gold nanoclusters

Gold nanoclusters (AuNCs) with low toxicity, high photostability, and facile synthesis have attracted great attention. The ligand is of great significance in stabilizing AuNCs and regulating their properties. Ligands consisting of amino acids (proteins and peptides) are an ideal template for synthesizing applicative AuNCs due to their inherent bioactivity, biocompatibility, and accessibility. In this review, we summarize the correlation of the template consisting of amino acids with the properties of AuNCs by analyzing different peptide sequences. The selection of amino acids can regulate the fluorescence excitation/emission and intensity, size, cell uptake, and light absorption. By analyzing the role played by AuNCs stabilized by proteins and peptides in the application, universal rules and detailed performances of sensors, antibacterial agents, therapeutic reagents, and light absorbers are reviewed. This review can guide the template design and application of AuNCs when selecting proteins and peptides as ligands.

Ultrasmall Coinage Metal Nanoclusters as Promising Theranostic Probes for Biomedical Applications

Ultrasmall coinage metal nanoclusters (NCs, <3 nm) have emerged as a novel class of theranostic probes due to their atomically precise size and engineered physicochemical properties. The rapid advances in the design and applications of metal NC-based theranostic probes are made possible by the atomic-level engineering of metal NCs. This Perspective article examines (i) how the functions of metal NCs are engineered for theranostic applications, (ii) how a metal NC-based theranostic probe is designed and how its physicochemical properties affect the theranostic performance, and (iii) how metal NCs are used to diagnose and treat various diseases. We first summarize the tailored properties of metal NCs for theranostic applications in terms of biocompatibility and tumor targeting. We focus our discussion on the theranostic applications of metal NCs in bioimaging-directed disease diagnosis, photoinduced disease therapy, nanomedicine, drug delivery, and optical urinalysis. Lastly, an outlook on the challenges and opportunities in the future development of metal NCs for theranostic applications is provided.

Luminescent Gold Nanoclusters for Bioimaging: Increasing the Ligand Complexity

Fluorescence, and more in general, photoluminescence (PL), presents important advantages for imaging with respect to other diagnostic techniques. In particular, detection methodologies exploiting fluorescence imaging are fast and versatile; make use of low-cost and simple instrumentations; and are taking advantage of newly developed powerful, low-cost, light-based electronic devices, such as light sources and cameras, used in huge market applications, such as civil illumination, computers, and cellular phones. Besides the aforementioned simplicity, fluorescence imaging offers a spatial and temporal resolution that can hardly be achieved with alternative methods. However, the two main limitations of fluorescence imaging for bio-application are still (i) the biological tissue transparency and autofluorescence and (ii) the biocompatibility of the contrast agents. Luminescent gold nanoclusters (AuNCs), if properly designed, combine high biocompatibility with PL in the near-infrared region (NIR), where the biological tissues exhibit higher transparency and negligible autofluorescence. However, the stabilization of these AuNCs requires the use of specific ligands that also affect their PL properties. The nature of the ligand plays a fundamental role in the development and sequential application of PL AuNCs as probes for bioimaging. Considering the importance of this, in this review, the most relevant and recent papers on AuNCs-based bioimaging are presented and discussed highlighting the different functionalities achieved by increasing the complexity of the ligand structure.

One-step synthesized antimicrobial peptide-functionalized gold nanoclusters for selective imaging and killing of pathogenic bacteria

The development of multifunctional nanomaterials with bacterial imaging and killing activities is of great importance for the rapid diagnosis and timely treatment of bacterial infections. Herein, peptide-functionalized gold nanoclusters (CWR11-AuNCs) with high-intensity red fluorescence were successfully synthesized via a one-step method using CWR11 as a template and by optimizing the ratio of CWR11 to HAuCl4, reaction time, pH, and temperature. The CWR11-AuNCs bound to bacteria and exhibited selective fluorescence microscopy imaging properties, which is expected to provide a feasible method for locating and imaging bacteria in complex in vivo environments. In addition, CWR11-AuNCs not only retained the antibacterial and bactericidal activities of CWR11 but also exhibited certain inhibitory or killing effects on gram-negative and gram-positive bacteria and biofilms. The MICs of CWR11-AuNCs against Escherichia coli and Staphylococcus aureus were 178 and 89 μg/ml, respectively. Surprisingly, cell viability in the CWR11-AuNC-treated group was greater than that in the CWR11-treated group, and the low cytotoxicity exhibited by the CWR11-AuNCs make them more promising for clinical applications.

Antibacterial metal nanoclusters

Combating bacterial infections is one of the most important applications of nanomedicine. In the past two decades, significant efforts have been committed to tune physicochemical properties of nanomaterials for the development of various novel nanoantibiotics. Among which, metal nanoclusters (NCs) with well-defined ultrasmall size and adjustable surface chemistry are emerging as the next-generation high performance nanoantibiotics. Metal NCs can penetrate bacterial cell envelope more easily than conventional nanomaterials due to their ultrasmall size. Meanwhile, the abundant active sites of the metal NCs help to catalyze the bacterial intracellular biochemical processes, resulting in enhanced antibacterial properties. In this review, we discuss the recent developments in metal NCs as a new generation of antimicrobial agents. Based on a brief introduction to the characteristics of metal NCs, we highlight the general working mechanisms by which metal NCs combating the bacterial infections. We also emphasize central roles of core size, element composition, oxidation state, and surface chemistry of metal NCs in their antimicrobial efficacy. Finally, we present a perspective on the remaining challenges and future developments of metal NCs for antibacterial therapeutics.

Levonorgestrel-protected Au8 and Au10 clusters with different antimicrobial abilities

Gold nanoclusters exhibit significant potential in antimicrobial applications due to their good stability and desirable biocompatibility in the mammalian cell model. However, most of the previously reported gold nanocluster antimicrobial agents do not have an atomic-precise structure, causing difficulties in understanding the structure-property correlation. In this study, structurally defined gold-levonorgestrel clusters, named Au₈(C₂₁H₂₇O₂)₈ (Au₈NCs) and Au₁₀(C₂₁H₂₇O₂)₁₀ (Au₁₀NCs), with the same ligand-to-metal ratio but different inner cores were prepared for antibacterial activity investigations, demonstrating that Au₈NCs exhibited a stronger antibacterial activity owing to the more significant damage it causes on the bacteria wall and membrane, and a stronger inhibition of glutathione reductase activity in bacteria. The leakage of the intracellular components and enzyme inhibition caused an imbalance of the intracellular antioxidant defence system, and consequently killed bacteria. These results indicated that the structure of gold nanoclusters has an important effect on their biological activity, indicating that it as a key factor to consider in the future design of antimicrobial agents.

Gold Nanoclusters as Nanoantibiotic Auranofin Analogues

Auranofin, a gold(I)-complex with tetraacetylated thioglucose (Ac4 GlcSH) and triethylphosphine ligands, is an FDA-approved drug used as an anti-inflammatory aid in the treatment of rheumatoid arthritis. In repurposing auranofin for other diseases, it was found that the drug showed significant activity against Gram-positive but was inactive against Gram-negative bacteria. Herein, the design and synthesis of gold nanoclusters (AuNCs) based on the structural motif of auranofin are reported. Phosphine-capped AuNCs are synthesized and glycosylated, yielding auranofin analogues with mixed triphenylphosphine monosulfonate (TPPMS)/Ac4 GlcSH ligand shells. These AuNCs are active against both Gram-negative and Gram-positive bacteria, including multidrug-resistant pathogens. Notably, an auranofin analogue, a mixed-ligand 1.6 nm AuNC 4b, is more active than auranofin against Pseudomonas aeruginosa, while exhibiting lower toxicity against human A549 cells. The enhanced antibacterial activity of these AuNCs is characterized by a greater uptake of Au by the bacteria compared to AuI complexes. Additional factors include increased oxidative stress, moderate inhibition of thioredoxin reductase (TrxR), and DNA damage. Most intriguingly, the uptake of AuNCs are not affected by the bacterial outer membrane (OM) barrier or by binding with the extracellular proteins. This contrasts with AuI complexes like auranofin that are susceptible to protein binding and hindered by the OM barrier.

Development of S4A-BSA-Au NPs for enhanced anti-tumor therapy of canine breast cancer

S4A ((1R,2R,3S)-1,2-propanediol acetal-zeylenone) is one of the derivatives of zeylenone and exhibits superior cytotoxicity against the canine breast cancer cell line CIPp. However, its poor aqueous solubility and toxicity to normal tissue limit its clinical application. Therefore, in order to enhance the anticancer effect of S4A, in this article, BSA/BSA-Au-nanocluster-aggregated core/shell nanoparticles (B-BANC-NPs) were prepared by using bovine serum albumin (BSA) and HAuCl4, and then we further synthesized S4A-BSA-Au NPs which were spherical, with a diameter of about 60 nm. In vitro cytotoxicity assessed by using CCK-8 assay demonstrated that the IC50 value of the S4A-BSA-Au NPs was 10.39 μg mL-1, which was not significantly different from that of S4A (10.45 μg mL-1). In vitro apoptosis assay showed that the apoptosis rate of cells treated with S4A-BSA-Au NPs was 20.12%, which was significantly higher than that of the control group treated with S4A (11.3%). Notably, S4A-BSA-Au NPs were shown to effectively accumulate at tumor sites with fluorescence tracing. Besides, the effect of S4A-BSA-Au NPs on SPARC expression was determined by western blotting, and the result showed that 24 h after applying S4A-BSA-Au NPs, SPARC expression in low, middle and high dosage groups was lower than that of the control group, and the tendency showed dose dependence. The results revealed that S4A-BSA-Au NPs could effectively improve the anti-tumor activity of S4A on canine breast cancer, which may be associated with their abilities to effectively accumulate within tumor and to reduce the expression of SPARC.

Nanoantibiotics to fight multidrug resistant infections by Gram-positive bacteria: hope or reality?

The spread of antimicrobial resistance in Gram-positive pathogens represents a threat to human health. To counteract the current lack of novel antibiotics, alternative antibacterial treatments have been increasingly investigated. This review covers the last decade's developments in using nanoparticles as carriers for the two classes of frontline antibiotics active on multidrug-resistant Gram-positive pathogens, i.e., glycopeptide antibiotics and daptomycin. Most of the reviewed papers deal with vancomycin nanoformulations, being teicoplanin- and daptomycin-carrying nanosystems much less investigated. Special attention is addressed to nanoantibiotics used for contrasting biofilm-associated infections. The status of the art related to nanoantibiotic toxicity is critically reviewed.

Insights into the Impact of Gold Nanoclusters Au10SG10 on Human Microglia

The purpose of the current study is to uncover the impact of small liganded gold nanoclusters with 10 gold atoms and 10 glutathione ligands (Au₁₀SG₁₀) on several biomarkers in human microglia. We established the links connecting the atomically precise structure of Au₁₀SG₁₀ with their properties and changes in several biomolecules under oxidative stress. Au₁₀SG₁₀ caused the loss of mitochondrial metabolic activity, increased lipid peroxidation and translocation of an alarmin molecule, high mobility group box 1 (HMGB1), from the nucleus to the cytosol. Molecular modeling provided an insight into the location of amino acid interaction sites with Au₁₀SG₁₀ and the nature of bonds participating in these interactions. We show that Au₁₀SG₁₀ can bind directly to the defined sites of reduced, oxidized, and acetylated HMGB1. Further studies with similar complementary approaches merging live-cell analyses, determination of biomarkers, and cell functions could lead to optimized gold nanoclusters best suited for diagnostic and bioimaging purposes in neuroscience.

Novel dual-emissive fluorescent immunoassay for synchronous monitoring of okadaic acid and saxitoxin in shellfish

In this work, we present a novel dual-emissive fluoroimmunoassay for synchronous monitoring of okadaic acid (OA) and saxitoxin (STX) using multicolor fluorescent labels composed of sulfur, phosphorous co-doped graphene quantum dots (S, P-GQDs), and ovalbumin (OVA)-coated gold nanoparticles (OVA-AuNPs). The novel OVA-AuNPs were prepared by the reduction of chloroauric acid under alkaline conditions using OVA as a reducing agent. Both S, P-GQDs and OVA-AuNPs exhibit bright fluorescence, more importantly, a large emission wavelength difference (Δλ = 156 nm) under an excitation of 400 nm and relatively independent fluorescence behavior, which are essential to realizing the dual-signal marks in a directly mixing system. Using a competitive fluorescence-linked immunosorbent assay (cFLISA) format, the dual-emissive cFLISA was successfully utilized to measure OA and STX contents in Alectryonella plicatula (commonly named as fingerprint oyster) and the detection results were in good agreement with the commercial enzyme-linked immunosorbent assay (ELISA) kits.

Ultrasmall Luminescent Metal Nanoparticles: Surface Engineering Strategies for Biological Targeting and Imaging

In the past decade, ultrasmall luminescent metal nanoparticles (ULMNPs, d < 3 nm) have achieved rapid progress in addressing many challenges in the healthcare field because of their excellent physicochemical properties and biological behaviors. With the sharp shrinking size of large plasmonic metal nanoparticles (PMNPs), the contributions from the surface characteristics increase significantly, which brings both opportunities and challenges in the application-driven surface engineering of ULMNPs toward advanced biological applications. Here, the systematic advancements in the biological applications of ULMNPs from bioimaging to theranostics are summarized with emphasis on the versatile surface engineering strategies in the regulation of biological targeting and imaging performance. The efforts in the surface functionalization strategies of ULMNPs for enhanced disease targeting abilities are first discussed. Thereafter, self-assembly strategies of ULMNPs for fabricating multifunctional nanostructures for multimodal imaging and nanomedicine are discussed. Further, surface engineering strategies of ratiometric ULMNPs to enhance the imaging stability to address the imaging challenges in complicated bioenvironments are summarized. Finally, the phototoxicity of ULMNPs and future perspectives are also reviewed, which are expected to provide a fundamental understanding of the physicochemical properties and biological behaviors of ULMNPs to accelerate their future clinical applications in healthcare.

Synthesis and solution isomerization of water-soluble Au9 nanoclusters prepared by nuclearity conversion of [Au11(PPh3)8Cl2]Cl

Water-soluble gold nanoclusters (AuNCs) are popular in biomedical applications such as bioimaging, labelling, drug delivery, and biosensing. Despite their widespread applications, the synthesis of water-soluble phosphine-capped AuNCs is not as straightforward as their organic-soluble equivalents. Organic soluble phosphine-passivated [Au₉(L)₈]³⁺ are 6-electron closed-shell AuNCs that are generally prepared via the reduction of a phosphine-Au(I) complex by NaBH₄. A similar approach attempted for the water-soluble ligand triphenylphosphine monosulfonate (TPPMS) using [AuTPPMS]Cl resulted in a mixture of cluster sizes that required gel electrophoresis or fractional precipitation to isolate the Au₉ product. In this work, we report the synthesis of water-soluble [Au₉(L)₈]³⁺ nanoclusters in high yield through the biphasic ligand exchange of [Au₁₁(PPh₃)₈Cl₂]Cl with water-soluble phosphines such as TPPMS and 4-(diphenylphosphino)benzoic acid (DPPBA). The small molecule byproducts can be completely removed by size-based separation methods, like size exclusion chromatography or dialysis, as confirmed by ³¹P and ¹H nuclear magnetic resonance (NMR) as well as diffusion ordered spectroscopy (DOSY). Furthermore, [Au₉(DPPBA)₈]Cl₃ underwent a visible pH- and temperature-induced isomerization in ethanol between the 'crown' and 'butterfly' isomers of [Au₉(L)₈]³⁺ which has not been previously reported. Cytotoxicity evaluation of these water-soluble nanoclusters gave CC₅₀ values of 36 μg mL^-1 and 70 μg mL^-1 against A549 human alveolar epithelial cells, and 30 μg mL^-1 and 40 μg mL^-1 against NIH/3T3 mouse fibroblast cells for [Au₉(TPPMS)₈]Cl₃ and [Au₉(DPPBA)₈]Cl₃, respectively. For comparison, auranofin, an FDA-approved gold drug, is more than an order of magnitude more toxic with a CC₅₀ value of 7.7 μg mL^-1 against A549 cells.

High Stability Au NPs: From Design to Application in Nanomedicine

In recent years, Au-based nanomaterials are widely used in nanomedicine and biosensors due to their excellent physical and chemical properties. However, these applications require Au NPs to have excellent stability in different environments, such as extreme pH, high temperature, high concentration ions, and various biomatrix. To meet the requirement of multiple applications, many synthetic substances and natural products are used to prepare highly stable Au NPs. Because of this, we aim at offering an update comprehensive summary of preparation high stability Au NPs. In addition, we discuss its application in nanomedicine. The contents of this review are based on a balanced combination of our studies and selected research studies done by worldwide academic groups. First, we address some critical methods for preparing highly stable Au NPs using polymers, including heterocyclic substances, polyethylene glycols, amines, and thiol, then pay attention to natural product progress Au NPs. Then, we sum up the stability of various Au NPs in different stored times, ions solution, pH, temperature, and biomatrix. Finally, the application of Au NPs in nanomedicine, such as drug delivery, bioimaging, photothermal therapy (PTT), clinical diagnosis, nanozyme, and radiotherapy (RT), was addressed concentratedly.

Gold Nanoparticles Dissolve Extracellularly in the Presence of Human Macrophages

INTRODUCTION

Gold nanoparticles (AuNPs) have the potential to be used in various biomedical applications, partly due to the inertness and stability of gold. Upon intravenous injection, the NPs interact with the mononuclear phagocyte system, first with monocytes in the blood and then with macrophages in tissue. The NP-macrophage interaction will likely affect the stability of the AuNPs, but this is seldom analyzed. This study aimed to elucidate the role of macrophages in the biodissolution of AuNPs and underlying mechanisms.

METHODS

With an in vitro dissolution assay, we used inductively coupled plasma mass spectrometry to quantitatively compare the dissolution of 5 and 20 nm AuNPs coated with citrate or PEG in cell medium alone or in the presence of THP1-derived macrophages at 24 hours. In addition, we analyzed the cell dose, compared extra- and intracellular dissolution, and explored the possible role of reactive nitrogen species.

RESULTS

The results showed a higher cellular dose of the citrate-coated AuNPs, but dissolution was mainly evident for those sized 5 nm, irrespective of coating. The macrophages clearly assisted the dissolution, which was approximately fivefold higher in the presence of macrophages. The dissolution, however, appeared to take place mainly extracellularly. Acellular experiments demonstrated that peroxynitrite can initiate oxidation of gold, but a ligand is required to keep the gold ions in solution.

CONCLUSION

This study suggests extracellular dissolution of AuNPs in the presence of macrophages, likely with the contribution of the release of reactive nitrogen species, and provides new insight into the fate of AuNPs in the body.

State of the Art and Perspectives on the Biofunctionalization of Fluorescent Metal Nanoclusters and Carbon Quantum Dots for Targeted Imaging and Drug Delivery

The interface of nanobio science and cancer nanomedicine is one of the most important current frontiers in research, being full of opportunities and challenges. Ultrasmall fluorescent metal nanoclusters (MNCs) and carbon quantum dots (CQDs) have emerged as promising fluorescent nanomaterials due to their unique physicochemical and optical properties, facile surface functionalization, good photostability, biocompatibility, and aqueous dispersity. These characteristics make them advantageous over conventional fluorophores such as organic dye molecules and semiconductor quantum dots (QDs) for the detection, diagnosis, and treatment of various diseases including cancer. Recently, researchers have focused on the biofunctionalization strategy of the MNCs and CQDs which can tailor their physicochemical and biological properties and, in turn, can empower these biofunctionalized nanoprobes for diverse applications including imaging, drug delivery, theranostics, and other biomedical applications. In this invited feature article, we first discuss some fundamental structural and physicochemical characteristics of the fluorescent biocompatible quantum-sized nanomaterials which have some outstanding features for the development of multiplexed imaging probes, delivery vehicles, and cancer nanomedicine. We then demonstrate the diverse surface engineering of these fluorescent nanomaterials with reactive target specific functional groups which can help to construct multifunctional nanoprobes with improved targeting capabilities having minimal toxicity. The promising future of the biofunctionalized fluorescent quantum-sized nanomaterials in the field of bioanalytical and biomedical research is elaborately demonstrated, showing selected recent works with relevant applications. This invited feature article finally ends with a short discussion of the current challenges and future prospects of the development of these bioconjugated/biofunctionalized nanomaterials to provide insight into this burgeoning field of MNC- and CQD-based diagnostics and therapeutic applications.

Enhanced Ultrasound Contrast of Renal-Clearable Luminescent Gold Nanoparticles

Renal-clearable nanoparticles are typically fast eliminated through the free glomerular filtration, which show weak interaction with the renal compartments and negligible ultrasound signals, raising challenges in direct imaging of kidney diseases. Here, we report the ultrasmall renal-clearable luminescent gold nanoparticles (AuNPs) with both pH-induced charge reversal and aggregation properties, and discover that enhanced ultrasound contrast could be facilely acquired through the increased tubular reabsorption and in situ aggregation of AuNPs in renal tubule cells in injured kidneys. The tuning elimination pathway of the renal-clearable luminescent AuNPs is further demonstrated to provide a synergistical fluorescence and ultrasound imaging strategy for diagnosing early kidney injury with precise anatomical information.

Gold Cluster Capped with a BCL-2 Antagonistic Peptide Exerts Synergistic Antitumor Activity in Chronic Lymphocytic Leukemia Cells

Chronic lymphocytic leukemia (CLL) is still incurable by conventional chemotherapy due to the resistance to apoptosis. We have previously found that a peptide-capped gold cluster (Au₂₅Sv₉) can target on the aberrant oxidative stress in CLL cells to specially inhibit thioredoxin reductase (TrxR) activity, resulting in significant apoptosis. However, the required doses of the gold cluster for inducing apoptosis are high, restricting its potential for further applications. Notably, the most recent studies suggested that CLL cells overexpressed antiapoptotic BCL-2 protein to prevent chemotherapy-induced apoptosis, indicating that BCL-2 could be a promising target for CLL therapy. Regrettably, the nonmitochondrial-targeted Au₂₅Sv₉ has little effect on BCL-2. In this study, we successfully screened a modified BADBH3 peptide (B1P) that could antagonize BCL-2 protein in CLL cells. We found that B1P could effectively sensitize MEC-1 cells to a subliminal dose of Au₂₅Sv₉. To simplify the treatment regimen, we directly fabricated a gold cluster capped with the B1P peptides by one-step synthesis to integrate the BCL-2 antagonistic activity into the gold the cluster, named BGC. We already found that low doses of BGC could significantly induce more apoptosis in MEC-1 cells than equivalent doses of the Au₂₅Sv₉ cluster or B1P peptide alone. Mechanistically, in addition to the inherent inhibitory effect of gold clusters on TrxR activity, BGC could bind to BCL-2 on mitochondria and activate the BCL-2 family-mediated mitochondrial apoptosis cascade more effectively. These results demonstrated that antagonizing the overexpressed BCL-2 in CLL cells, together with inhibiting TrxR simultaneously by a single gold cluster, is a promising strategy for the treatment of CLL cells. This study will provide a paradigm and reference for the development of functionalized gold clusters with rationally designed peptides, and opens up a new opportunity for the treatment of CLL in clinical settings.

Role of Small Moiety of a Large Ligand: Tyrosine Templated Copper Nanoclusters

To explore the underlying formation mechanism of luminescent metal nanoclusters (NCs) using a small moiety such as amino acids (outside the milieu of a protein environment) as templates, herein we report blue-emitting copper nanoclusters (CuNCs) using l-tyrosine (l-Tyr) as a capping agent as well as a reducing agent. We also demonstrate the effect of an in situ fibrillation of Tyr on the luminescence and structural properties of NCs. Fluorescence studies along with microscopic imaging revealed the rapid formation of a dityrosine (di-Tyr) moiety in an alkaline medium followed by an aggregated "Tamarix dioica leaf"-like fibrillar pattern along with CuNCs. Our present investigation delineates the role played by π-π interactions in the formation of the fibrillar structures. We substantiated the fundamentals of using a small molecule of a large ligand that can serve as a template and also show how these NCs once formed destroy the fibrils of di-Tyr as a function of time.

Insights and Perspectives Regarding Nanostructured Fluorescent Materials toward Tackling COVID-19 and Future Pandemics

The COVID-19 outbreak has exposed the world's preparation to fight against unknown/unexplored infectious and life-threatening pathogens. The unavailability of vaccines, slow or sometimes unreliable real-time virus/bacteria detection techniques, insufficient personal protective equipment (PPE), and a shortage of ventilators and many other transportation equipments have further raised serious concerns. Material research has been playing a pivotal role in developing antimicrobial agents for water treatment and photodynamic therapy, fast and ultrasensitive biosensors for virus/biomarkers detection, as well as for relevant biomedical and environmental applications. It has been noticed that these research efforts nowadays primarily focus on the nanomaterials-based platforms owing to their simplicity, reliability, and feasibility. In particular, nanostructured fluorescent materials have shown key potential due to their fascinating optical and unique properties at the nanoscale to combat against a COVID-19 kind of pandemic. Keeping these points in mind, this review attempts to give a perspective on the four key fluorescent materials of different families, including carbon dots, metal nanoclusters, aggregation-induced-emission luminogens, and MXenes, which possess great potential for the development of ultrasensitive biosensors and infective antimicrobial agents to fight against various infections/diseases. Particular emphasis has been given to the biomedical and environmental applications that are linked directly or indirectly to the efforts in combating COVID-19 pandemics. This review also aims to raise the awareness of researchers and scientists across the world to utilize such powerful materials in tackling similar pandemics in future.

A review on recent advances in amino acid and peptide-based fluorescence and its potential applications

Fluorescence is widely used to detect functional groups and ions, and peptides are used in various fields due to their excellent biological activity.

Development of coinage metal nanoclusters as antimicrobials to combat bacterial infections

Infections from antibiotic-resistant bacteria have caused huge economic loss and numerous deaths over the past decades. Researchers are exploring multiple strategies to combat these bacterial infections. Metal nanomaterials have been explored as therapeutics against these infections owing to their relatively low toxicity, broad-spectrum activity, and low bacterial resistance development. Some coinage metal nanoclusters, such as gold, silver, and copper nanoclusters, can be readily synthesized. These nanoclusters can feature multiple useful properties, including ultra-small size, high catalytic activity, unique photoluminescent properties, and photothermal effect. Coinage metal nanoclusters have been investigated as antimicrobials, but more research is required to tap their full potential. In this review, we discuss multiple advantages and the prospect of using gold/silver/copper nanoclusters as antimicrobials.

Facile Preparation of Homogeneous Copper Nanoclusters Exhibiting Excellent Tetraenzyme Mimetic Activities for Colorimetric Glutathione Sensing and Fluorimetric Ascorbic Acid Sensing

Nanozymes are artificial enzymes, which can substitute traditional biological enzymes for multifield applications. However, to date, it remains challenging to search novel mimic enzymes or multienzyme mimics. Herein, a facile and green method for preparing monodisperse, homogeneous copper nanoclusters (Cu NCs) with smaller size was developed, which used cysteamine as a template and hydrazine hydrate as a reductant to reduce Cu²⁺. The as-prepared Cu NCs exhibited excellent tetraenzyme-like activities, including peroxidase (POD)-, catalase (CAT)-, superoxide dismutase (SOD)-, and ascorbic acid oxidase (AAO)-mimic activities. The mechanisms, kinetics, and catalytic performances of Cu NCs were systematically studied. Moreover, based on the POD-like activity of Cu NCs, sensitive and simple colorimetric sensing glutathione (GSH) was explored, with the low limit of detection of 0.89 μM GSH (S/N = 3). Additionally, a novel fluorimetric ascorbic acid (AA) sensor was developed with the linear range of 0.5-30 μM and limit of detection (LOD) of 0.144 μM, on the basis of the principle that AA is oxidized to dehydroascorbic acid (DHAA) specifically catalyzed by the AAO-like activity of Cu NCs, while DHAA can further react with o-phenylenediamine (OPDA) to generate a highly fluorescent quinoxaline (DFQ) derivative. The as-proposed colorimetric GSH sensor and the fluorimetric AA sensor were capable of detecting GSH and AA, respectively, in real samples accurately and reproducibly. Thus, the Cu NCs-based multienzyme mimic is a promising candidate for biocatalysis and biosensing.

Novel synthesis of orange-red emitting copper nanoclusters stabilized by methionine as a fluorescent probe for norfloxacin sensing

In the present work, we report a novel chemical approach for the synthesis of orange-red emitting copper nanoclusters (Cu NCs) using L-methionine as stabilizing agent at room temperature for the first time. The synthetic route is facile, economical and viable. The methionine stabilized copper nanoclusters (Cu NCs/Met) were thoroughly characterized by TEM, FT-IR, XPS, UV-Vis, steady state and transient fluorescence spectroscopy. The results show the synthesized Cu NCs/Met with a fluorescence quantum yield of 4.37% possessed high stability and excellent optical features such as large Stokes shift and long fluorescence lifetime (8.3 μs). Significantly, the fluorescence intensity of Cu NCs/Met could be efficiently quenched by norfloxacin (NOR) pharmaceutical. A fast and cost-effective NOR sensor was proposed employing Cu NCs/Met as the fluorescent nanoprobe, and the quenching mechanisms were attributed to inner filter effect and agglomeration-induced quenching. The developed sensor exhibited a high sensitivity and selectivity towards NOR in a wide linear range from 0.05 to 250 μM with a detection limit as low as 17 nM. Moreover, the practicability of the developed NOR sensor for real sample assay was validated with satisfactory recoveries, indicating this sensing platform with great potential for label-free pharmaceutical detection in complex systems.

Atomic-level separation of thiolate-protected metal clusters

Fine metal clusters have attracted much attention from the viewpoints of both basic and applied science for many years because of their unique physical/chemical properties and functions, which differ from those of bulk metals. Among these materials, thiolate (SR)-protected gold clusters (Au_n(SR)_m clusters) have been the most studied metal clusters since 2000 because of their ease of synthesis and handling. However, in the early 2000s, it was not easy to isolate these metal clusters. Therefore, high-resolution separation methods were explored, and several atomic-level separation methods, including polyacrylamide gel electrophoresis (PAGE), high-performance liquid chromatography (HPLC), and thin-layer chromatography (TLC), were successively established. These techniques have made it possible to isolate a series of Au_n(SR)_m clusters, and much knowledge has been obtained on the correlation between the chemical composition and fundamental properties such as the stability, electronic structure, and physical properties of Au_n(SR)_m clusters. In addition, these high-resolution separation techniques are now also frequently used to evaluate the distribution of the product and to track the reaction process. In this way, high-resolution separation techniques have played an essential role in the study of Au_n(SR)_m clusters. However, only a few reviews have focused on this work. This review focuses on PAGE, HPLC, and TLC separation techniques, which offer high resolution and repeatability, and summarizes previous studies on the high-resolution separation of Au_n(SR)_m and related clusters with the purpose of promoting a better understanding of the features and the utility of these techniques.

Tumor microenvironment-responsive multifunctional peptide coated ultrasmall gold nanoparticles and their application in cancer radiotherapy

Two important features are required for promising radiosensitizers: one is selective tumor cell targeting to enhance the therapeutic outcome via lethal DNA damage and the other is rapid clearance to enable excellent biocompatibility for potential clinical application. Herein, ultrasmall gold nanoparticles (Au NPs) with diameter smaller than 5 nm were prepared and covered with a multifunctional peptide to endow them with selective tumor cell uptake capability. Combined with X-ray irradiation, the responsive Au NPs demonstrated superior radio-sensitizing toxicity and rapid renal clearance in vivo. Methods: A responsive peptide (Tat-R-EK) consists of three build blocks were used: a cell and even nuclear penetrating block derived from human immunodeficiency virus-1 transactivator of transcription protein (Tat), an cathepsin B cleavable linker, and a zwitterionic antifouling block. Ultrasmall Au NPs were prepared and then covered by the peptide via the Au-S bonds between gold and thiol groups from cysteine. The morphology, colloidal stability and the responsiveness of obtained Au@Tat-R-EK NPs were studied using transmittance electron microscopy and dynamic laser scattering. The selective cancer cell uptake and accumulation of Au@Tat-R-EK NPs in cancer tissue were studied via ICP-MS in vitro and in vivo, respectively. The cytotoxicity of Au@Tat-R-EK NPs on HepG2 cancer cells was evaluated in terms of cell viability, DNA damage, intracellular reactive oxygen species generation, and apoptosis analysis. Finally, the biocompatibility and tumor destruction ability against orthotopic LM3 liver cancers were verified in vivo. Results: Multifunctional peptide modified ultrasmall Au NPs were successfully prepared. The Au NPs exhibited enough colloidal stability and cathepsin B-responsive surface change, leading to selectively uptake by cancer cells in vitro and accumulation to tumor sites in vivo. Combined with X-ray irradiation, the responsive Au NPs demonstrated superior radio-sensitizing cytotoxicity in vitro and therapeutic outcome on mouse liver cancer in vivo. The ultrasmall size enables rapid clearance of the Au NPs, guarantees the biocompatibility in vivo for potential clinical applications. Conclusion: Some obstacles faced by the Au NPs-based radiotherapy, such as short circulation half-life, non-specific distribution, slow clearance and low radio-sensitizing effect, were effective solved through rational design of the smart nanomedicine. This work provides new insight in designing tumor microenvironment-responsive nanomedicine in cancer radiotherapy.

Gold Nanoclusters for Bacterial Detection and Infection Therapy

Infections caused by antibiotic-resistant bacteria have become one of the most serious global public health crises. Early detection and effective treatment can effectively prevent deterioration and further spreading of the bacterial infections. Therefore, there is an urgent need for time-saving diagnosis as well as therapeutically potent therapy approaches. Development of nanomedicine has provided more choices for detection and therapy of bacterial infections. Ultrasmall gold nanoclusters (Au NCs) are emerging as potential antibacterial agents and have drawn intense attention in the biomedical fields owing to their excellent biocompatibility and unusual physicochemical properties. Recent significant efforts have shown that these versatile Au NCs also have great application potential in the selective detection of bacteria and infection treatment. In this review, we will provide an overview of research progress on the development of versatile Au NCs for bacterial detection and infection treatment, and the mechanisms of action of designed diagnostic and therapeutic agents will be highlighted. Based on these cases, we have briefly discussed the current issues and perspective of Au NCs for bacterial detection and infection treatment applications.

Doping Copper Atoms into the Nanocluster Kernel: Total Structure Determination of [Cu30Ag61(SAdm)38S3](BPh4)

Doping active metal (i.e., Cu) into the kernel of noble metal nanoclusters (i.e., Au/Ag nanocluster) remains challenging in the synthesis of alloy nanoclusters. Herein, we report the synthesis and the total structure determination of a bimetallic [Ag₆₁Cu₃₀(SAdm)₃₈S₃]BPh₄ (Ag₆₁Cu₃₀) nanocluster. The Ag₆₁Cu₃₀ nanocluster is composed of an Ag₁₃@Cu₃₀ kernel which is further capped by a peripheral Ag₄₈(SAdm)₃₈S₃ shell. The icosidodecahedron Cu₃₀ middle layer connects the innermost icosahedral Ag₁₃ core and Ag atoms at the outermost Ag₄₈(SR)₃₈S₃ shell, demonstrating that the Cu atoms in the Cu₃₀ layer are in a metallic state.

Activation of atom-precise clusters for catalysis

The use of atom-precise, ligand-protected metal clusters has exceptional promise towards the fabrication of model supported-nanoparticle heterogeneous catalysts which have controlled sizes and compositions. One major challenge in the field involves the ease at which metallic clusters sinter upon removal of protected ligands, thus destroying the structural integrity of the model system. This review focuses on methods used to activate atom-precise thiolate-stabilized clusters for heterogeneous catalysis, and strategies that can be used to mitigate sintering. Thermal activation is the most commonly employed approach to activate atom-precise metal clusters, though a variety of chemical and photochemical activation strategies have also been reported. Material chemistry methods that can mitigate sintering are also explored, which include overcoating of clusters with metal oxide supports fabricated by sol-gel chemistry or atomic layer deposition of thin oxide films or encapsulating clusters within porous supports. In addition to focusing on the preservation of the size and morphology of deprotected metal clusters, the fate of the removed ligands is also explored, because detached and/or oxidized ligands can also greatly influence the overall properties of the catalyst systems. We also show that modern characterization techniques such as X-ray absorption spectroscopy and high-resolution electron microscopy have the capacity to enable careful monitoring of particle sintering upon activation of metal clusters.

Benzeneselenol-modified gold nanoclusters for cancer therapy

We report that benzeneselenol-modified gold nanoclusters (Se_Au NCs) can induce autophagy and interfere with actin expressions in cancer cells.

A New Lamellar Gold Thiolate Coordination Polymer, [Au(m-SPhCO2H)]n, for the Formation of Luminescent Polymer Composites

The photoluminescence of gold thiolate clusters brings about many potential applications, but its origin is still elusive because of its complexity. A strategy in understanding the structure–properties relationship is to study closely related neutral gold thiolate coordination polymers (CPs). Here, a new CP is reported, [Au(m-SPhCO₂H)]_n. Its structure is lamellar with an inorganic layer made of Au–S–Au–S helical chains, similar to the [Au(p-SPhCO₂H)]_n analog. An in-depth study of its photophysical properties revealed that it is a bright yellow phosphorescent emitter with a band centered at 615 nm and a quantum yield (QY) of 19% at room temperature and in a solid state. More importantly, a comparison to the para-analog, which has a weak emission, displayed a strong effect of the position of the electron withdrawing group (EWG) on the luminescent properties. In addition, [Au(m-SPhCO₂H)]_n CPs were mixed with organic polymers to generate transparent and flexible luminescent thin films. The ability to tune the emission position with the appropriate contents makes these nontoxic polymer composites promising materials for lighting devices.

Facile in situ synthesis of ultrasmall near-infrared-emitting gold glyconanoparticles with enhanced cellular uptake and tumor targeting

The simultaneous possession of high tumor-targeting efficiency, long blood circulation, and low normal-tissue retention is critical for future clinically translatable nanomedicines. Herein, we reported a facile in situ glycoconjugation strategy for the synthesis of near-infrared (NIR)-emitting gold glyconanoparticles (AuGNPs, ∼2.4 nm) using 1-thio-β-d-glucose as both the surface ligand and the reducing agent in the presence of a gold precursor. The ultrasmall AuGNPs showed similar low healthy organ retention to that of the renal-clearable ultrasmall nonglyconanoparticles, but ∼10 and 2.5 times higher in vitro and in vivo tumor-targeting efficiencies, respectively, were observed. This facile glycoconjugation strategy of ultrasmall AuGNPs was found to show activity towards glucose transporters in the cancer cells and prolonged blood circulation with both renal and hepatobiliary clearance pathways, which synergistically enhanced the tumor targeting of the ultrasmall AuGNPs. This discovery provides a smart strategy for the improvement in tumor targeting by ultrasmall NPs and further strengthens our understanding of glycoconjugation in designing future clinically translatable nanomedicines.

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.

Tailoring the stability, photocatalysis and photoluminescence properties of Au11 nanoclusters via doping engineering

Dopants in gold nanoclusters have been proved to mediate the intrinsic electronic properties of homo-clusters. In this work, we report the precise synthesis of atomically precise Au₈Ag₃(PPh₃)₇Cl₃ alloy nanoclusters with multiple Ag dopants for the first time. Their structure was resolved by single-crystal X-ray crystallography. Au₈Ag₃(PPh₃)₇Cl₃ nanoclusters possessed a similar structure topology to the well-known Au₁₁(PPh₃)₇Cl₃ nanoclusters. It is observed that the three Ag atoms were fixed at the cluster surface and bound selectively with the chlorine ligands in a C₃-axis manner. The alloy nanoclusters exhibited a closed-shell electronic structure (i.e., 8(Au 6s¹) + 3(Ag 5s¹) - 3(Cl) = 8e), as evidenced by electrospray ionization-mass spectrometry (ESI-MS). The photothermodynamic stability of alloy clusters was remarkably improved (e.g., full decomposition after 7 days under sunlight irradiation vs. 3 days for Au₁₁(PPh₃)₇Cl₃ clusters). DFT calculations indicated that the Ag dopants in a C₃-axis manner could obviously delocalize the electrons of Au to the orbitals of P atoms and then mediate the electronic property of the clusters. Shrinkage of the HOMO-LUMO gap to 1.67 eV of Au₈Ag₃(PPh₃)₇Cl₃ was observed as compared with that of homo-nanoclusters of Au₁₁(PPh₃)₇Cl₃ (2.06 eV). The electrochemical gap of Au₈Ag₃(PPh₃)₇Cl₃ alloy nanoclusters was 1.272 V, which was higher than that of Au₁₁(PPh₃)₇Cl₃ nanoclusters, which indicated higher electrochemical stability, as evidenced by the differential pulse voltammetry (DPV) method. Au₈Ag₃(PPh₃)₇Cl₃ clusters exhibited three specific photoluminescence peaks at 405, 434 and 454 nm. AuAg alloy clusters exhibited twofold greater activity than homo gold clusters in the photooxidation of benzylamine, which was mainly due to the unique electronic properties of the alloy clusters. Controllable heteroatom doping engineering is a powerful method to tune the electronic properties of clusters, and then improve their photothermodynamic and electrochemical stability simultaneously for potential photocatalytic applications.

The Ligand-Exchange Reactions of Rod-Like Au25-n Mn (M=Au, Ag, Cu, Pd, Pt) Nanoclusters with Cysteine - A Density Functional Theory Study

The atomic precision of ultrasmall noble-metal nanoclusters (NMNs) is fundamental for elucidating structure-property relationships and probing their practical applications. So far, the atomic structure of NMNs protected by organic ligands has been widely elucidated, whereas the precise atomic structure of NMNs protected by water-soluble ligands (such as peptides and nucleic acid), has been rarely reported. With the concept of "precision to precision", density functional theory (DFT) calculations were performed to probe the thermodynamic plausibility and inherent determinants for synthesizing atomically precise, water-soluble NMNs via the framework-maintained two-phase ligand-exchange method. A series of rod-like Au_25-n M_n (M=Au, Ag, Cu, Pd, Pt) NMNs with the same framework but varied ligands and metal compositions was chosen as the modeling reactants, and cysteine was used as the modeling water-soluble ligand. It was found that the acidity of the reaction remarkably affects the thermodynamic facility of the ligand exchange reactions. Ligand effects (structural distortion and acidity) dominate the overall thermodynamic facility of the ligand-exchange reaction, while the number and type of doped metal atom(s) has little influence.

Antimicrobial Gold Nanoclusters: Recent Developments and Future Perspectives

Bacterial infections have caused serious threats to public health due to the antimicrobial resistance in bacteria. Recently, gold nanoclusters (AuNCs) have been extensively investigated for biomedical applications because of their superior structural and optical properties. Great efforts have demonstrated that AuNCs conjugated with various surface ligands are promising antimicrobial agents owing to their high biocompatibility, polyvalent effect, easy modification and photothermal stability. In this review, we have highlighted the recent achievements for the utilizations of AuNCs as the antimicrobial agents. We have classified the antimicrobial AuNCs by their surface ligands including small molecules (< 900 Daltons) and macromolecules (> 900 Daltons). Moreover, the antimicrobial activities and mechanisms of AuNCs have been introduced into two main categories of small molecules and macromolecules, respectively. In accordance with the advancements of antimicrobial AuNCs, we further provided conclusions of current challenges and recommendations of future perspectives of antimicrobial AuNCs for fundamental researches and clinical applications.

A label-free fluorescent probe for the detection of adenosine 5'‑triphosphate via inhibiting the aggregation-induced emission enhancement of glutathione modified silver nanoclusters triggered by zinc ion

It is important to establish sensitive and simple analysis methods for adenosine 5'‑triphosphate (ATP). A label-free fluorescent probe for the determination of ATP was constructed based on glutathione modified silver nanoclusters (AgNCs/GSH). AgNCs/GSH showed aggregation-induced emission enhancement (AIEE) property in the organic solvent. The effects of metal ions on the fluorescence of AgNCs/GSH were also studied. Only zinc ion enhanced the fluorescence of AgNCs/GSH obviously. This was because Zn²⁺ coordinated with AgNCs/GSH to cause the aggregation of AgNCs/GSH, which was sufficiently proved by TEM. With the addition of ATP, Zn²⁺ bound with ATP through ZnOP bond and the binding between Zn²⁺ and AgNCs/GSH was inhibited. Hence the fluorescence of AgNCs/GSH was decreased with increasing the ATP concentration. The fluorescence response was linear in the ATP concentration range of 1-110 μM, and the detection limit was 0.8 μM. Then this method was successfully applied for determining ATP in the samples of human urine and rat serum, the recoveries were in the range of 97.6%-103%.

Distinct photophysical properties in atom-precise silver and copper nanocluster analogues

The synthesis of atom-precise analogues of homometallic nanoclusters remains a great challenge. Herein we report the first pair of atom-precise copper/silver-thiolate halide cluster analogues, namely [Cu17/Ag17I3S(C2B10H10S2)6(CH3CN)11] (Cu17 and Ag17), obtained by bottom-up self-assembly and complete-metal-exchange-induced cluster-to-cluster transformation, respectively. The differences in optical absorption and emission of these analogues were fully elucidated by experimental and theoretical methods.

Tailoring the photoluminescence of atomically precise nanoclusters

Due to their atomically precise structures and intriguing chemical/physical properties, metal nanoclusters are an emerging class of modular nanomaterials. Photo-luminescence (PL) is one of their most fascinating properties, due to the plethora of promising PL-based applications, such as chemical sensing, bio-imaging, cell labeling, phototherapy, drug delivery, and so on. However, the PL of most current nanoclusters is still unsatisfactory-the PL quantum yield (QY) is relatively low (generally lower than 20%), the emission lifetimes are generally in the nanosecond range, and the emitted color is always red (emission wavelengths of above 630 nm). To address these shortcomings, several strategies have been adopted, and are reviewed herein: capped-ligand engineering, metallic kernel alloying, aggregation-induced emission, self-assembly of nanocluster building blocks into cluster-based networks, and adjustments on external environment factors. We further review promising applications of these fluorescent nanoclusters, with particular focus on their potential to impact the fields of chemical sensing, bio-imaging, and bio-labeling. Finally, scope for improvements and future perspectives of these novel nanomaterials are highlighted as well. Our intended audience is the broader scientific community interested in the fluorescence of metal nanoclusters, and our review hopefully opens up new horizons for these scientists to manipulate PL properties of nanoclusters. This review is based on publications available up to December 2018.