Overcoming bacterial physical defenses with molecule-like ultrasmall antimicrobial gold nanoclusters

Enhanced Antimicrobial Efficiency of Gold Nanoclusters via Improved Sonodynamic Activity and Out-Membrane Crossing Capacity

Antimicrobial sonodynamic therapy (SDT) holds great promise in clinical practice regarding its noninvasiveness, high safety profile, and absence of resistance concern. However, exploring high-efficiency sonodynamic sensitizers is slow-moving and remains a big challenge. We, herein, employed gold nanoclusters (Au NCs) as a novel class of sonodynamic sensitizers, demonstrating notable antimicrobial efficacy in treating infected wounds. Specifically, l-arginine (Arg) and 6-azido-2-thiothymidine (ATT) cocapped Au NCs featured enhanced structural rigidity, suppressing nonradiative relaxation of excited electrons and achieving a reactive oxygen species (ROS) yield exceeding 45%. Moreover, the modification of ATT-Au NCs by Arg imparted amino acid-like properties to the Au NCs, while the ultrasound (US) up-regulates the expression of OmpF porins in E. coli. This synergy resulted in a burst of ROS production within the bacterial cells, ultimately leading to a four-order-of-magnitude reduction in microbial viability.

Enhancing antibacterial photodynamic therapy with NIR‐activated gold nanoclusters: Atomic‐precision size effect on reducing bacterial biofilm formation and virulence

Persistent biofilm infections pose a critical health threat with their relentless presence and amplified antibiotic resistance. Traditional antibacterial photodynamic therapy can inhibit bacteria extracellularly but struggles to control biofilm formation and virulence. Thus, there is an urgent need to develop photosensitizers, such as ultra‐small gold nanoclusters (AuNCs), that can penetrate biofilms and internalize into bacteria. However, AuNCs still face the challenge of insufficient reactive oxygen species (ROS) production and limited near‐infrared light absorption. This study develops a model of indocyanine green (ICG)‐sensitized AuNCs with atomic‐precision size effect. This approach achieved near‐infrared light absorption while inhibiting radiation transitions, thereby regulating the generation of ROS. Notably, different‐sized AuNCs (Au10NCs, Au15NCs, Au25NCs) yielded varied ROS types, resulting from different energy level distributions and electron transfer rates. ICG‐Au15NCs achieved a treatment efficacy of 99.94% against Staphylococcus aureus infections in vitro and significantly accelerated wound healing in vivo. Moreover, this study highlights the unique role of ICG‐AuNCs in suppressing quorum sensing, virulence, and ABC transporters compared to their larger counterparts. This strategy demonstrates that atomic‐precision size effect of AuNCs paves the way for innovative approaches in antibacterial photodynamic therapy for infection control.

Progress in antibacterial applications of nanozymes

Bacterial infections are a growing problem, and antibiotic drugs can be widely used to fight bacterial infections. However, the overuse of antibiotics and the evolution of bacteria have led to the emergence of drug-resistant bacteria, severely reducing the effectiveness of treatment. Therefore, it is very important to develop new effective antibacterial strategies to fight multi-drug resistant bacteria. Nanozyme is a kind of enzyme-like catalytic nanomaterials with unique physical and chemical properties, high stability, structural diversity, adjustable catalytic activity, low cost, easy storage and so on. In addition, nanozymes also have excellent broad-spectrum antibacterial properties and good biocompatibility, showing broad application prospects in the field of antibacterial. In this paper, we reviewed the research progress of antibacterial application of nanozymes. At first, the antibacterial mechanism of nanozymes was summarized, and then the application of nanozymes in antibacterial was introduced. Finally, the challenges of the application of antibacterial nanozymes were discussed, and the development prospect of antibacterial nanozymes was clarified.

Metal-polyphenol coordination nanosheets with synergistic peroxidase-like and photothermal properties for efficient antibacterial treatment

Bacterial infections pose a serious threat to human health and socioeconomics worldwide. In the post-antibiotic era, the development of novel antimicrobial agents remains a challenge. Polyphenols are natural compounds with a variety of biological activities such as intrinsic antimicrobial activity and antioxidant properties. Metal-polyphenol obtained by chelation of polyphenol ligands with metal ions not only possesses efficient antimicrobial activity but also excellent biocompatibility, which has great potential for application in biomedical and food packaging fields. Herein, we developed metal-polyphenol coordination nanosheets named copper oxidized tannic acid quinone (CuTAQ) possessing efficient antibacterial and anti-biofilm effects, which was synthesized by a facile one-pot method. The synthesis was achieved by chelation of partially oxidized tannic acid (TA) with Cu2+ under mild conditions, which supports low-cost and large-scale production. It was demonstrated that CuTAQ exhibited high antibacterial activity via disrupting the integrity of bacterial cell membranes, inducing oxidative stress, and interfering with metabolism. In addition, CuTAQ exhibits excellent peroxidase catalytic activity and photothermal conversion properties, which play a significant role in enhancing its bactericidal and biofilm scavenging abilities. This study provides insights for rational design of innovative metal-polyphenol nanomaterials with efficient antimicrobial properties.

Carbon Dots' Potential in Wound Healing: Inducing M2 Macrophage Polarization and Demonstrating Antibacterial Properties for Accelerated Recovery

Bacterial infections and persistent inflammation can impede the intrinsic healing process of wounds. To combat this issue, researchers have delved into the potential use of carbon dots (CDs) in the regulation of inflammation and counteract infections. These CDs were synthesized using a microwave-assisted hydrothermal process and have demonstrated outstanding antibacterial and antibiofilm properties against Gram-positive and Gram-negative bacteria. Additionally, CDs displayed biocompatibility at therapeutic concentrations and the ability to specifically target mitochondria. CD treatment effectively nullified lipopolysaccharide-triggered reactive oxygen species production by macrophages, while simultaneously promoting macrophage polarization toward an anti-inflammatory phenotype (M2), leading to a reduction in inflammation and an acceleration in wound healing. In vitro scratch assays also revealed that CDs facilitated the tissue-repairing process by stimulating epithelial cell migration during reepithelialization. In vivo studies using CDs topically applied to lipopolysaccharide (LPS)-stimulated wounds in C57/BL6 mice demonstrated significant improvements in wound healing due to enhanced fibroblast proliferation, angiogenesis, and collagen deposition. Crucially, histological investigations showed no indications of systemic toxicity in vital organs. Collectively, the application of CDs has shown immense potential in speeding up the wound-healing process by regulating inflammation, preventing bacterial infections, and promoting tissue repair. These results suggest that further clinical translation of CDs should be considered.

Size and charge effects of metal nanoclusters on antibacterial mechanisms

Nanomaterials, specifically metal nanoclusters (NCs), are gaining attention as a promising class of antibacterial agents. Metal NCs exhibit antibacterial properties due to their ultrasmall size, extensive surface area, and well-controlled surface ligands. The antibacterial mechanisms of metal NCs are influenced by two primary factors: size and surface charge. In this review, we summarize the impacts of size and surface charge of metal NCs on the antibacterial mechanisms, their interactions with bacteria, and the factors that influence their antibacterial effects against both gram-negative and gram-positive bacteria. Additionally, we highlight the mechanisms that occur when NCs are negatively or positively charged, and provide examples of their applications as antibacterial agents. A better understanding of relationships between antibacterial activity and the properties of metal NCs will aid in the design and synthesis of nanomaterials for the development of effective antibacterial agents against bacterial infections. Based on the remarkable achievements in the design of metal NCs, this review also presents conclusions on current challenges and future perspectives of metal NCs for both fundamental investigations and practical antibacterial applications.

Green Synthesis of Antimicrobial Peptide-Protected Silver Nanoclusters with Regulated Antibacterial Behavior

The antibacterial system based on the silver element has been a very promising antibacterial material. However, the antibacterial activity of silver nanomaterials largely depends on their chemical composition and physical properties. Herein, we prepared ultrasmall silver nanoclusters by directly reducing silver ions with antimicrobial peptide in a green way. The positively charged peptide ligands drove the silver nanoclusters binding to bacteria by electrostatic attraction. Contrary to the large-sized silver nanomaterials, the ultrasmall silver nanoclusters were able to rapidly penetrate bacteria membranes via strong hydrophobic association, significantly promoting the generation of reactive oxygen species, and the subsequent high oxidative stress led to bacterial death. Moreover, the silver nanoclusters with antibacterial peptide ligand exhibited good stability, low cytotoxicity, and long-term antibacterial activity. Besides, synergistic enhancement of silver nanoclusters on antibacterial activity was observed. Therefore, the silver nanoclusters conjugated with the antimicrobial peptide can act as a synergistic antibacterial agent, in which their bio-interactions with bacteria have been regulated to achieve a rapid, long-lasting, and broad-spectrum antibacterial effect.

Gold nanocluster-based fluorescent sensors for in vitro and in vivo ratiometric imaging of biomolecules

Gold nanoclusters (AuNCs) are promising nanomaterials for ratiometric fluorescent probes due to their tunable fluorescence wavelengths dependent on size and structure, as well as their biocompatibility and resistance to photobleaching. By incorporating an additional fluorescence spectral peak, dual-emission AuNC-based fluorescent probes have been developed to enhance the signal output reproducibility. These probes can be fabricated by integrating various luminescent nanomaterials with AuNCs. This review focuses on the preparation methods and applications of ratiometric fluorescent probes derived from AuNCs and other fluorescent nanomaterials or fluorescent dyes for both in vitro and in vivo bioimaging of target analytes. Additionally, the review delves into the sensing mechanisms of AuNC-based ratiometric probes, their synthetic strategies, and the challenges encountered when using AuNCs for ratiometric bioimaging. Moreover, we explore the application of protein-stabilized AuNCs and thiolate-capped AuNC-based ratiometric fluorescent probes for biosensing and bioimaging. Two primary methods for assembling AuNCs and fluorophores into ratiometric fluorescent probes are discussed: triggered assembly and self-assembly. Finally, we address the challenges and issues associated with ratiometric bioimaging using AuNCs and propose future directions for further advancing AuNCs as ratiometric imaging agents.

Eradication of Bacterial Persister Cells By Leveraging Their Low Metabolic Activity Using Adenosine Triphosphate Coated Gold Nanoclusters

Bacteria first develop tolerance after antibiotic exposure; later genetic resistance emerges through the population of tolerant bacteria. Bacterial persister cells are the multidrug-tolerant subpopulation within an isogenic bacteria culture that maintains genetic susceptibility to antibiotics. Because of this link between antibiotic tolerance and resistance and the rise of antibiotic resistance, there is a pressing need to develop treatments to eradicate persister cells. Current anti persister cell strategies are based on the paradigm of "awakening" them from their low metabolic state before attempting eradication with traditional antibiotics. Herein, we demonstrate that the low metabolic activity of persister cells can be exploited for eradication over their metabolically active counterparts. We engineered gold nanoclusters coated with adenosine triphosphate (AuNC@ATP) as a benchmark nanocluster that kills persister cells over exponential growth bacterial cells and prove the feasibility of this new concept. Finally, using AuNC@ATP as a new research tool, we demonstrated that it is possible to prevent the emergence of antibiotic-resistant superbugs with an anti-persister compound. Eradicating persister cells with AuNC@ATP in an isogenic culture of bacteria stops the emergence of superbug bacteria mediated by the sub-lethal dose of conventional antibiotics. Our findings lay the groundwork for developing novel nano-antibiotics targeting persister cells, which promise to prevent the emergence of superbugs and prolong the lifespan of currently available antibiotics.

Current perspectives of metal-based nanomaterials as photocatalytic antimicrobial agents and their therapeutic modes of action: A review

Efforts to restrict the emergence and progression of multidrug-resistant bacterial strains should heavily involve the scientific community, including government bodies, researchers, and industries, in developing new and effective photocatalytic antimicrobial agents. Such changes warrant the modernization and upscaling of materials synthesis laboratories to support and expedite the mass production of materials at the industrial scale for the benefit of humankind and the environment. Despite the massive volume of publications reporting the potential usage of different types of metal-based nanomaterials as antimicrobial agents, reviews uncovering the similarities and differences among the various products remain lacking. This review details the basic and unique properties of metal-based nanoparticles, their use as photocatalytic antimicrobial agents, and their therapeutic modes of action. It shall be noted that compared to traditional antibiotics, the mode of action of photocatalytic metal-based nanomaterials for killing microorganisms are completely different, despite displaying promising performance against antibiotic-resistant bacteria. Besides, this review uncovers the differences in the mode of actions of metal oxide nanoparticles against different types of bacteria, as well as towards viruses. Last but not least, this review comprehensively describes previous published clinical trials and medical usages involving contemporary photocatalytic antimicrobial agents.

Gold-Based Nanostructures for Antibacterial Application

Bacterial infections have become a fatal threat because of the abuse of antibiotics in the world. Various gold (Au)-based nanostructures have been extensively explored as antibacterial agents to combat bacterial infections based on their remarkable chemical and physical characteristics. Many Au-based nanostructures have been designed and their antibacterial activities and mechanisms have been further examined and demonstrated. In this review, we collected and summarized current developments of antibacterial agents of Au-based nanostructures, including Au nanoparticles (AuNPs), Au nanoclusters (AuNCs), Au nanorods (AuNRs), Au nanobipyramids (AuNBPs), and Au nanostars (AuNSs) according to their shapes, sizes, and surface modifications. The rational designs and antibacterial mechanisms of these Au-based nanostructures are further discussed. With the developments of Au-based nanostructures as novel antibacterial agents, we also provide perspectives, challenges, and opportunities for future practical clinical applications.

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.

Ultra-Small Silver Nanoparticles: A Sustainable Green Synthesis Approach for Antibacterial Activity

The present study centers on the synthesis of ultra-small silver nanoparticles (AgNPs) with antibacterial properties using citrus peel residues (orange, lemon, and grapefruit) as reducing and stabilizing agents, and on assessing their antibacterial activity against multidrug-resistant clinical Staphylococcus aureus. The synthesized AgNPs were analyzed by various techniques, including UV-Vis spectroscopy, SAED, TEM, XRD, FTIR, and Raman. The results demonstrate the formation of ultra-small, monodisperse, quasi-spherical AgNPs with an average particle size of 2.42 nm for AgNPs produced with mixed extracts. XRD analysis indicated that the AgNPs have a crystal size of 9.71 to 16.23 nm. The AgNPs exhibited potent inhibitory activity against resistant S. aureus, with a minimum inhibitory concentration (MIC) of 15.625 to 62.50 ppm. The findings suggest that the ultra-small nanometer size of the AgNPs could be attributed to the synthesis method that employs ambient conditions and the presence of polyphenolic compounds from citrus peel. Consequently, AgNPs obtained through sustainable green synthesis hold significant potential in combating clinical multi-resistant bacterial strains that are challenging to treat and eradicate. This approach also contributes to the revaluation of citrus residues in the region, which is an ongoing environmental issue today.

Cu(I) complexes with aggregation-induced emission for enhanced photodynamic antibacterial application

This communication reports the design of aggregation-induced emission (AIE)-featured PEG-condensed Cu(I)-p-MBA aggregates (PCuA). Benefiting from the AIE trait and intrinsic antibacterial property of Cu species, the as-developed PCuA exhibits enhanced photodynamic antibacterial activities against broad-spectrum bacteria, providing a paradigm in the design of novel antibacterial agents.

Turning on the Antimicrobial Activity of Gold Nanoclusters Against Multidrug-Resistant Bacteria

In this work, we show that the addition of thiourea (TU) initiated broad-spectrum antimicrobial activity of otherwise inactive D-maltose-capped gold nanoclusters (AuNC-Mal). For example, AuNC-Mal/TU was effective against multidrug-resistant Pseudomonas aeruginosa with a minimum inhibitory concentration (MIC) of 1 μg mL-1 (2.5 μM [Au]) while having 30-60 times lower in vitro cytotoxicity against mammalian cells. The reaction of AuNC-Mal and TU generated the antimicrobial species of [Au(TU)2 ]+ and smaller AuNCs. TU increased the accumulation of Au in bacteria and helped maintain the oxidation state as AuI (vs. AuIII ). The modes of action included the inhibition of thioredoxin reductase, interference with the CuI regulation and depletion of ATP. Moreover, the antimicrobial activity did not change in the presence of colistin or carbonyl cyanide 3-chlorophenylhydrazone, suggesting that AuNC-Mal/TU was indifferent to the outer membrane barrier and to bacterial efflux pumps.

Elucidating the mechanisms of action of antibiotic-like ionic gold and biogenic gold nanoparticles against bacteria

The antimicrobial action of gold depends on different factors including its oxidation state in the intra- and extracellular medium, the redox potential, its ability to produce reactive oxygen species (ROS), the medium components, the properties of the targeted bacteria wall, its penetration in the bacterial cytosol, the cell membrane potential, and its interaction with intracellular components. We demonstrate that different gold species are able to induce bacterial wall damage as a result of their electrostatic interaction with the cell membrane, the promotion of ROS generation, and the consequent DNA damage. In-depth genomic and proteomic studies on Escherichia coli confirmed the superior toxicity of Au (III) vs Au (I) based on the different molecular mechanisms analyzed including oxidative stress, bacterial energetic metabolism, biosynthetic processes, and cell transport. At equivalent bactericidal doses of Au (III) and Au (I) eukaryotic cells were not as affected as bacteria did, maintaining unaffected cell viability, morphology, and focal adhesions; however, increased ROS generation and disruption in the mitochondrial membrane potential were also observed. Herein, we shed light on the antimicrobial mechanisms of ionic and biogenic gold nanoparticles against bacteria. Under selected conditions antibiotic-like ionic gold can exert a strong antimicrobial activity while being harmless to human cells.

In Situ Structure Transformation of a Sprayed Gel for pH-Ultrasensitive Nano-Catalytic Antibacterial Therapy

Nano-catalytic bacterial killing provides new opportunities to address ever-increasing antibiotic resistance. However, the intrinsic catalytic activity usually depends on a much lower pH conditions (pH = 2-5) than that in the weakly acidic bacterial microenvironments (pH = 6-7) for reactive oxygen species production by Fenton reactions. Herein, a MnSiO₃ -based pH-ultrasensitive "in situ structure transformation" is first reported to significantly promote the adhesion between material and bacteria, and shorten the diffusion distance (<20 nm) to compensate ultra-short life (<200 ns) of ·OH generated by Mn²⁺ -mediated Fenton-like reaction, finally enhancing its nano-catalytic antibacterial performance in weakly acidic conditions. A separated spray bottle is further designed to achieve in situ gelation at the wound site, which demonstrates excellent shape adaptability to complicated and rough surfaces of wounds, allowing for long-term nano-catalyst release. As a result, bacterial-infected wound healing is efficiently promoted. Herein, the in situ sprayed nano-catalytic antibacterial gel presents a promising paradigm for bacterial infection treatment.

Gold nanoclusters-loaded hydrogel formed by dimeric hydrogen bonds crosslinking: A novel strategy for multidrug-resistant bacteria-infected wound healing

Restoring skin integrity after wound infection remains a tougher health challenge due to the uncontrolled antibiotic-resistant pathogens caused by antibiotic abuse. Herein, an injectable hydrogel with dual antibacterial and anti-inflammatory activities composed of gold nanoclusters (GNCs) and carbomer (CBM) is developed for wound dressing to overcome multidrug-resistant infection. Firstly, both experimental investigations and molecular dynamics simulation validate the protonation state of 6-mercaptohexanoic acid (MHA) ligands play an important role in its antibacterial action of GNCs. The self-organizing GNCs-CBM composite hydrogel is then spontaneously cross-linked by the dimeric hydrogen bonds (H-bonds) between the MHA ligands and the acrylic acid groups of CBM. Benefitting from the dimeric H-bonds, the hydrogel becomes thickening enough as an ideal wound dressing and the GNCs exist in the hydrogel with a high protonation level that contributes to the enhanced bactericidal function. In all, by combining bactericidal and immunomodulatory actions, the GNCs-CBM hydrogel demonstrated excellent synergy in accelerating wound healing in animal infection models. Hence, the dimeric H-bonds strengthening strategy makes the GNCs-CBM hydrogel hold great potential as a safe and effective dressing for treating infected wounds.

Antibiofilm activity of ultra-small gold nanoclusters against Fusobacterium nucleatum in dental plaque biofilms

Pathogenic dental plaque biofilms are universal and harmful, which can result in oral infections and systemic diseases. Many conventional therapeutic methods have proven insufficient or ineffective against plaque biofilms. Therefore, new strategies are urgently needed. Fusobacterium nucleatum (F. nucleatum), a periodontal pathogen associated with a variety of oral and systemic diseases, is thought to be central to the development and structure of dental plaques. Here, ultra-small gold nanoclusters (AuNCs) were prepared. They exhibited potent antibacterial activity against F. nucleatum through enhanced destruction of bacterial membranes and generation of reactive oxygen species. Furthermore, due to their excellent penetration, the AuNCs could inhibit biofilm formation and destroy mature biofilms in vitro. Their antibiofilm efficacy was further confirmed in a mouse model, where they reduced biofilm accumulation and ameliorated inflammation. Meanwhile, the disruption of oral and gut microbiota caused by colonization of oral F. nucleatum could be partially restored through AuNCs treatment. Therefore, AuNCs could be considered as promising antibiofilm agents and have great potential in the clinical treatment of dental plaque.

Lethal Interactions of Atomically Precise Gold Nanoclusters and Pseudomonas aeruginosa and Staphylococcus aureus Bacterial Cells

Ultrasmall metal nanoclusters (NCs) are employed in an array of diagnostic and therapeutic applications due to their tunable photoluminescence, high biocompatibility, polyvalent effect, ease of modification, and photothermal stability. However, gold nanoclusters' (AuNCs') intrinsically antimicrobial properties remain poorly explored and are not well understood. Here, we share an insight into the antimicrobial action of atomically precise AuNCs based on their ability to passively translocate across the bacterial membrane. Functionalized by a hydrophilic modified-bidentate sulfobetaine zwitterionic molecule (AuNC-ZwBuEt) or a more hydrophobic monodentate-thiolate, mercaptohexanoic acid (AuNC-MHA) molecule, 2 nm AuNCs were lethal to both Gram-negative Pseudomonas aeruginosa and Gram-positive Staphylococcus aureus bacteria. The bactericidal efficiency was found to be bacterial strain-, time-, and concentration-dependent. The direct visualizations of the translocation of AuNCs and AuNC-cell and subcellular interactions were investigated using cryo-soft X-ray nano-tomography, transmission electron microscopy (TEM), and scanning TEM energy-dispersive spectroscopy analyses. AuNC-MHA were identified in the bacterial cytoplasm within 30 min, without evidence of the loss of membrane integrity. It is proposed that the bactericidal effect of AuNCs is attributed to their size, which allows for efficient energy-independent translocation across the cell membrane. The internalization of both AuNCs caused massive internal damage to the cells, including collapsed subcellular structures and altered cell morphology, leading to the eventual loss of cellular integrity.

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.

Conjugating AIE-featured AuAg nanoclusters with highly luminescent carbon dots for improved visible-light-driven antibacterial activity

Metal nanoclusters (NCs) have emerged as novel antibacterial agents featuring broad-spectrum antibacterial activity without drug resistance for bacteria, but suffer from fast antibacterial invalidation due to their consumption by bacteria. Herein we report the design of a visible-light-driven photodynamic antibacterial agent based on conjugating aggregation-induced emission (AIE)-featured AuAg NCs with highly luminescent carbon dots (CDs). The conjugation of CDs with AuAg NCs could not only enhance the visible-light harvest, but also promote charge carrier generation/separation via charge/energy transfer, leading to the production of abundant reactive oxygen species (ROS) for bacterial killing under visible-light irradiation. Consequently, the as-obtained CDs@AuAg NCs display excellent photodynamic antibacterial activity against both Gram-positive and Gram-negative bacteria with 4-5 orders of magnitude reduction in colony forming units, which is different from the conventional metal NC-based analogue relying on self-consumption for bacterial killing. In addition, the CDs@AuAg NCs are found to be free of cytotoxicity; the ROS capture experiments indicate that the photoproduced H₂O₂ by CDs@AuAg NCs is the main active species for bacterial killing, accounting for nearly 48% of the total antibacterial efficacy. This study provides a paradigm for the design of metal NC-based photodynamic antibacterial agents for diverse bactericidal applications.

Multifunctional fluorescent gold nanoclusters with enhanced aggregation-induced emissions (AIEs) and excellent antibacterial effect for bacterial imaging and wound healing

To explore new alternatives to combat increasing risk of bacterial infection, in this work, a cationic antimicrobial peptide (HHC10) and glutathione (GSH) co-ligand protected ultra-small gold nanoclusters (Au NCs) was constructed by a simple one-pot method. The intrinsic luminescent property of GSH-protected Au NCs (Au_xGSH) endowed enhanced aggregation-induced emissions (AIEs) of co-ligand-protected Au NCs (Au_xGSH-HHC10), which exhibited a very strong orange luminescence. Based on the AIE effect, for one thing, Au_xGSH could be applied to rapidly and selectively detect Gram-positive bacteria. For another, Au_xGSH-HHC10 exhibited potential for multicolor imaging of both Gram-negative and Gram-positive bacteria. Besides, as-synthesized Au_xGSH-HHC10 could act as potent nanoantibiotics against both Gram-negative and Gram-positive bacteria, which could not only avoid drug tolerance but also be effective toward drug-resistance bacteria. The antibacterial mechanism indicated that the synergetic effect of the generation of reactive oxygen species (ROS), binding with DNA, and broad-spectrum antibacterial activity of HHC10 led to the membrane damage, depolarization, and interference of biological function, thus enhancing the antibacterial effect. More importantly, such an Au NCs could realize excellent therapeutic outcomes for wound healing in vivo, and showed good biocompatibility and biosafety toward health tissues. The results will provide a great potential for the application of Au NCs for imaging-guided antibacterial platform.

Luminescent gold-peptide spheric aggregates: selective and effective cellular targeting

Although the restriction of intramolecular motion has been well recognized as the fundamental of aggregation induced emission enhancement (AIEE), the regulation mechanism of gold nanoclusters (AuNCs) based AIEE system are still unclear. In this paper, we have investigated the Zn²⁺-induced AIEE process of thiolate ligands (i.e., cysteine, glutathione and an 8-mer peptide) protected AuNCs, which shows a pH-dependent evolution from single AuNCs to spheric aggregates to irregular network. Using photoluminescent enhancement ratio as an index, the concept of "mid-pH" is proposed to indicate the optimal pH for the formation of spheric AuNCs aggregates. Importantly, the surface ligands allow the formation of spheric AuNCs aggregates at tunable mid-pH between 5.7 and 7.5. Owing to the appropriate size and surface peptide targetability, the spheric AuNCs aggregates can be successfully screened for targeted tumor cell uptake and imaging at physiological pH. The cell uptake mechanism study showed that AuNCs aggregates was specifically recognized by arginine-glycine-aspartic acid (RGD) sequence on the ligand and integrin αvβ3 on the cell surface, thus mainly through clathrin-mediated endocytosis. This work provides new sight to artificially regulate the construction of efficient cellular imaging probes.

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.

Preparation of Photocatalytic and Antibacterial MOF Nanozyme Used for Infected Diabetic Wound Healing

Bacterial infection has been a considerable obstacle for diabetic wound healing. A multifunctional nanoplatform used as nanozyme for bacterial infected diabetic wound is extremely attractive. Therefore, gold nanoclusters modified zirconium-based porphyrin metal-organic frameworks (Au NCs@PCN) were constructed by an in situ growth method. Through SEM, TEM, and EDS mapping, the surface of ellipsoid-shaped particles around 190 nm was observed to be evenly interspersed with 5-8 nm gold nanoclusters. Notably, Au NCs@PCN exhibits excellent performance in exciting ROS generation and photothermal effects. Under near-infrared (NIR) laser irradiation, Au NCs@PCN can be heated to 56.2 °C and produce ROS, showing an effective killing effect on bacteria. Antibacterial studies showed that Au NCs@PCN inhibited MRSA and Amp^r E. coli by destroying membrane structure and inducing protein leakage up to 95.3% and 90.6%, respectively. Animal experiments showed that Au NCs@PCN treated diabetic rats had reduced wound coverage to 2.7% within 21 days. The immunoblot analysis showed that proangiogenic and proepithelial cell proliferation factors were expressed significantly up-regulated. These results prove that Au NCs@PCN with photocatalytic and nanozyme activity has a broad application prospect for promoting diabetic infected wound healing.

Covalent Attachment of Aggregation-Induced Emission Molecules to the Surface of Ultrasmall Gold Nanoparticles to Enhance Cell Penetration

Three different alkyne-terminated aggregation-induced emission molecules based on a para-substituted di-thioether were attached to the surface of ultrasmall gold nanoparticles (2 nm) by copper-catalyzed azide–alkyne cycloaddition (click chemistry). They showed a strong fluorescence and were well water-dispersible, in contrast to the dissolved AIE molecules. The AIE-loaded nanoparticles were not cytotoxic and easily penetrated the membrane of HeLa cells, paving the way for an intracellular application of AIE molecules, e.g., for imaging.

Controlling the Interaction between Fluorescent Gold Nanoclusters and Biointerfaces for Rapid Discrimination of Fungal Pathogens

Nondestructive detection and discrimination of fungal pathogens is essential for rapid and precise treatment, which further effectively prevents antifungal resistance from overused drugs. In this work, fluorescent gold nanoclusters served as the basis for discriminating Candida species. Varied on surface ligands, these gold nanoclusters demonstrated different optical properties as a result of the perturbation effects of ligands. The biointerface interaction between the surface ligands of gold nanoclusters and the cell walls of Candida species can be constructed, and their restriction on ligands perturbation effect produced enhanced fluorescence signals. Owing to the variation of the cell wall composition, cells of different Candida species demonstrated different degrees of association with the gold nanoclusters, leading to discriminable amounts of fluorescence enhancements. The reverse signal response from these gold nanoclusters gives rise to a synergistic and effective assay that allows identification of Candida species.

Ligand-protected nanoclusters and their role in agriculture, sensing and allied applications

Nano biotechnology, when coupled with green chemistry, can revolutionize human life because of the vast opportunities and benefits it can offer to the quality of human life. Luminescent metal nanoclusters (NCs) have recently developed as a potential research area with applications in different areas like medical, imaging, sensing etc. Recently these new candidates have proved to be beneficial in the food supply chain enabling controlled release of nutrients, pesticides and as nanosensors for the detection of contaminants and play roles in healthy food storage and maintaining food quality. An assortment of nanomaterials has been employed for these applications and reviews have been published on the use of nanotechnology in agriculture. Ligand-protected metal nanoclusters are a distinctive class of small organic-inorganic nanostructures that garnered immense research interest in recent years owing to their stability at specific "magic size" compositions along with tunable properties that make them promising candidates for a wide range of nanotechnology-based applications. This review tries to consolidate the recent developments in the area of ligand-protected nanoclusters in connection with the detection of pesticides, food contaminants, heavy metal ions and plant growth monitoring for healthy agricultural practices. Its antimicrobial activity to manage the microbial contamination is highlighted. The review also throws light on the various perspectives by which food production and allied areas will be transformed in future.

Correlation between Imaging Features and Pathological Stages of Primary Lung Tumors Based on Nanocontrast Agents

As one of the conventional methods of lung cancer detection, computed tomography (CT) usually requires the use of contrast agents to enhance the imaging effect. Conventional iodine contrast agents have poor signal-to-noise ratio and are prone to adverse reactions. It is necessary to find more effective and safe contrast agents for CT scans. The gold nanoparticles with secondary electron effect and photoelectric absorption effect can prolong the display time of the patient's blood circulation after being injected into the patient's body, which makes the nanocontrast agent a research hotspot in the field of CT imaging. In this study, ultrasmall gold nanoclusters with a diameter of about 5 nm were used as the contrast agent in CT scans. It was found that CT scans based on nanocontrast agents can obtain high-quality lung cancer imaging images, and the patient has no obvious adverse reactions. When observing the CT image, it was found that the stage of lung cancer patients can be clearly distinguished through the CT scan image. When analyzing the consistency of CT imaging and pathological classification, the Kappa value was 0.810, indicating that the two have a high degree of consistency. Therefore, this study believes that the imaging characteristics of primary lung tumors based on nanocontrast agents are highly correlated with their pathological types.

Prospects and applications of synergistic noble metal nanoparticle-bacterial hybrid systems

Hybrid systems composed of living cells and nanomaterials have been attracting great interest in various fields of research ranging from materials science to biomedicine. In particular, the interfacing of noble metal nanoparticles and bacterial cells in a single architecture aims to generate hybrid systems that combine the unique physicochemical properties of the metals and biological attributes of the microbial cells. While the bacterial cells provide effector and scaffolding functions, the metallic component endows the hybrid system with multifunctional capabilities. This synergistic effort seeks to fabricate living materials with improved functions and new properties that surpass their individual components. Herein, we provide an overview of this research field and the strategies for obtaining hybrid systems, and we summarize recent biological applications, challenges and current prospects in this exciting new arena.

Bimetallic palladium@copper nanoparticles: Lethal effect on the gram-negative bacterium Pseudomonas aeruginosa

Infections by the gram-negative bacterium Pseudomonas aeruginosa are on the rise, and its antibiotic resistance is a tough challenge for clinical therapeutics worldwide. Therefore, it is an urgent to find alternative antibiotics that possess preferable bactericidal efficiency and are safer than silver (Ag) nanoparticles (Ag NPs). Here, we synthesized small palladium@copper (Pd_1.9Cu) alloy NPs with preferable antibacterial functions. We also used a bacteria-infected skin wound mouse model to confirm the sterilization effect of Pd_1.9Cu NPs. Pd_1.9Cu NPs killed P. aeruginosa at a low concentration, displaying a more powerful bactericidal effect than Ag NPs in vitro. In addition, Pd_1.9Cu NPs broke through the bacterial membrane, leading to DNA fragmentation and leakage of genomic DNA and proteins. The underlying mechanism was to trigger the burst of intracellular reactive oxygen species generation and accelerated ion release (Cu and Pd). Pd_1.9Cu NPs were also more capable of disinfection than Ag NPs and ceftazidime in vivo, promoting speedy wound recovery. Simultaneously, the biocompatibility of Pd_1.9Cu NPs was satisfactory both in vitro and in vivo. These results show that Pd_1.9Cu NPs are a promising nanomedicine to treat P. aeruginosa infection.

Induction of apoptosis on ovarian adenocarcinoma cells, A2780 by tricyclohexylphosphanegold (I) mercaptobenzoate derivatives via intrinsic and extrinsic pathways

Tricyclohexylphosphanegold(I) n-mercaptobenzoate (n = 2, 3, 4) labelled as 1-3 were previously reported to significantly suppress thioredoxin reductase (TrxR) activities towards ovarian cancer cells, A2780, in vitro. Herein, we explored the role of 1-3 for their apoptosis inducing ability against A2780 cells. 1-3 exhibited IC₅₀ values at 1.19 ± 0.03 µM, 2.28 ± 0.04 μM and 0.78 ± 0.01 μM, respectively, compared to cisplatin at 26.8 ± 0.15 µM. The compounds induced A2780 apoptosis via a caspase-dependent mitochondrion pathway as evidenced by ROS production, cytochrome c release, caspases-3/7, -8, -9 and -10 activation, APAF1 and BAX upregulation as well as BCL2A1 and BCL2 genes' downregulation. In addition, the death mode of 1-3 was also mediated via death receptor extrinsic pathway manifested by FAS, FASL, FADD, and TNFR1 genes' upregulation via Human Rt PCR analysis. In addition, 1-3 significantly caused A2780 arrest at S phase, which was associated with the upregulation of TP53, E2F1, RB1 and CDKN1A upregulation and downregulation of CDK1, CDK4, CDC25A and CDC25C genes. Based on these promising results, these phosphanegold(I) thiolate derivatives could act as feasible candidates for further advanced in vivo ovarian cancer studies to develop novel chemotherapeutic agents derived from metal-based agents.

Gold nanoclusters treat intracellular bacterial infections: Eliminating phagocytic pathogens and regulating cellular immune response

Intracellular bacterial infection is underlying many serious human disorders, leading to high morbidity and mortality. The development of safe and efficient therapeutic agents is the most effective solutions to combat intracellular bacterial infections. Recently, ultrasmall gold nanoclusters (AuNCs) have emerged as an innovative nanoantibiotics against multidrug-resistant bacterial infections due to their inherent antibacterial activity. However, the therapeutic effects of AuNCs on intracellular bacterial infections and their effects on host cells still remain unvisited. Here, we demonstrate the therapeutic potential of 4,6-diamino-2-mercaptopyrimidine-functionalized AuNCs (AuDAMP) for intracellular multidrug-resistant infections in a co-culture model of macrophages and methicillin-resistant Staphylococcus aureus (MRSA). The AuNCs were found to show a superior intracellular antibacterial capability, which can eliminate most of the MRSA phagocytosed by macrophages, and without exhibiting obvious cytotoxicity on host RAW 264.7 macrophages at tested concentrations. More importantly, treatment of AuDAMP exerts critical roles on enhancing the innate immune response to defend against pathogens invading inside the host cells and alleviating the bacterial infection-induced inflammatory response to avoid pyroptosis by up-regulating significantly xenophagy level in macrophages. Taken together, our results suggest that AuNCs hold great potential for the treatment of intracellular bacterial infections.

Charge transfer channels of silver @ cuprous oxide heterostructure core-shell nanoparticles strengthen high photocatalytic antibacterial activity

Marine biological fouling has always been a hot research topic. In this study, silver @ cuprous oxide (Ag@Cu₂O) core-shell nanoparticles were synthesized via in-situ synthesis method and developed an outstanding antibacterial activity. The bacteriostasis efficiency of Ag@Cu₂O reached to 99% and 98% against Staphylococcus aureus and Pseudomonas aeruginosa, respectively. The minimum inhibitory concentration of Ag@Cu₂O decreased from 113.6 μg/mL to 56.8 μg/mL compared with Cu₂O. Ag@Cu₂O had better antibacterial activity than Cu₂O with lower content of Cu₂O and was more environment friendly. The heterostructure formed at the interface between Ag and Cu₂O promoted the separation and diffusion of photogenerated electron-hole pairs through the charge transfer channel and promoted the generation of reactive oxygen species. The outstanding antibacterial activity of Ag@Cu₂O was strongly depended on the generation of the reactive oxygen species. Density functional theory and finite element method calculations demonstrated that the structure of core-shell improved photocatalytic efficiency. Additionally, synergetic effect of released Ag⁺ and Cu²⁺ also enhanced the bacteriostasis rate and the long-term antifouling performance in 60 days. Hence, the synthesized core-shell Ag@Cu₂O can be applied as novel antifoulants in the marine field.

Reversing Bacterial Resistance to Gold Nanoparticles by Size Modulation

One major frustration in developing antibiotics is that bacteria can quickly develop resistance that would require an entirely new cycle of research and clinical testing to overcome. Although plenty of bactericidal nanomaterials have been developed against increasingly severe superbugs, few reports have studied the resistance to these nanomaterials. Herein, we show that antibacterial 4,6-diamino-2-pyrimidine thiol (DAPT)-capped gold nanoparticles (AuDAPTs) can induce a 16-fold increased minimum inhibitory concentration (MIC) of E. coli only after very long term exposure (183 days), without developing cross-resistance to commercialized antibiotics. Strikingly, we recovered the bactericidal activities of AuDAPTs to the resistant strain by tuning the sizes of AuDAPTs without employing new chemicals. Such slow, easy-to-handle resistance induced by AuDAPTs is unprecedented compared to traditional antibiotics or other nanomaterials. In addition to the novel antibacterial activities and biocompatibilities, our approach will accelerate the development of gold nanomaterial-based therapeutics against multi-drug-resistant (MDR) bacterial infections.

Modulating catalytic activity of human topoisomerase II α enzyme by fluorescent gold nanoclusters

Precise monitoring of the enzyme activity by a suitable modulator is one of the very fundamental aspects of drug designing that provides the opportunity to overcome the challenges of several diseases. Herein, inhibition of human Topoisomerase IIα enzyme which serves as a potential target site for several anti-cancer drugs is demonstrated by using ultra-small size gold nanoclusters (Au NCs) with the dimension comparable with size of the active site of the enzyme. Molecular dynamics simulation results demonstrate that the Au NCs strongly interact with the human Topo IIα enzyme at its active site or allosteric site depending on forms of enzyme. Additionally, binding energy and interaction profile provides the molecular basis of understanding of interactions of ultra-small size Au NCs and human Topo IIα enzyme. Enthalpy change (ΔH) and binding constant (K) are measured based on a sequential binding model of the Au NCs with the enzyme as demonstrated by the ITC study. Moreover, the in-vitro inhibition study of the catalytic activity of the enzyme and gel electrophoresis indicates that the ultra-small size Au NCs may be used as a potent inhibitor of human Topo IIα enzyme.