Target-Oriented Synthesis of Triphenylphosphine Functionalized Carbon Dots with Negative Charge for ROS Scavenging and Mitochondrial Targeting

Triphenylphosphine functionalized carbon dots (TPP-CDs) showcase robust mitochondria targeting capacity owing to their positive electrical properties. However, TPP-CDs typically involve complicated synthesis steps and time-consuming postmodification procedures. Especially, the one-step target-oriented synthesis of TPP-CDs and the regulation of TPP linkage modes remain challenges. Herein, we propose a free-radical-initiated random copolymerization in combination with hydrothermal carbonation to regulate the TPP backbone linkage for target-oriented synthesis of triphenylphosphine copolymerization carbon dots (TPPcopoly-CDs). The linkage mechanism of random copolymerization reactions is directional, straightforward, and controllable. The TPP content and IC50 of hydroxyl radicals scavenging ability of TPPcopoly-CDs are 53 wt % and 0.52 mg/mL, respectively. TPP serves as a charge control agent to elevate the negatively charged CDs by 20 mV. TPPcopoly-CDs with negative charge can target mitochondria, and in the corresponding mechanism the TPP moiety plays a crucial role in targeting mitochondria. This discovery provides a new perspective on the controlled synthesis, TPP linkage modes, and mitochondrial targeting design of TPP-CDs.

Enhancing Fenton-like Degradation of Organic Pollutants at Neutral pH by Multivalent Cu NCs/HAp Nanocatalysts

Heterogeneous Fenton-like catalysis is a widely used method for the degradation of organic pollutants. However, it still has some limitations such as low activity in the neutral condition, low conversion rates of metals with different valence states, and potential secondary metal pollution. In this study, a Fenton-like nanocatalyst was first created by generating ultrasmall copper nanoclusters (Cu NCs) on the surface of hydroxyapatite (HAp) through a process of doping followed by modification. This resulted in the formation of a composite nanocatalyst known as Cu NCs/HAp. With the help of hydrogen peroxide (H2O2), Cu NCs/HAp exhibits an outstanding Fenton-like catalytic performance by efficiently degrading organic dyes such as methylene blue under mild neutral conditions. The removal rate can reach over 83% within just 30 min, demonstrating ideal catalytic universality and stability. The improved Fenton-like catalytic performance of Cu NCs/HAp can be ascribed to the synergistic effect of the multivalent Cu species through two simultaneous reaction pathways. During route I, the embedded Cu NCs with a core-shell Cu0/Cu+ structure can undergo sequential oxidation to form Cu2+, which continuously activates H2O2 to generate hydroxyl radicals (•OH) and singlet oxygen (1O2). In route II, Cu2+ produced from route I and initially adsorbed on the surface of HAp can be reduced by H2O2, thus regenerating Cu+ species for route I and achieving a closed-loop reaction. This work has confirmed that Cu NCs loaded on HAp may be an alternative Fenton-like catalyst for degradation of organic pollutants and environmental remediation, opening up new avenues for potential applications of other Cu NCs in future water pollution control.

Biological Interaction and Imaging of Ultrasmall Gold Nanoparticles

The ultrasmall gold nanoparticles (AuNPs), serving as a bridge between small molecules and traditional inorganic nanoparticles, create significant opportunities to address many challenges in the health field. This review discusses the recent advances in the biological interactions and imaging of ultrasmall AuNPs. The challenges and the future development directions of the ultrasmall AuNPs are presented.

Molecularly or atomically precise nanostructures for bio-applications: how far have we come?

A huge variety of nanostructures are promising for biomedical applications, but only a few have been practically applied. Among the various reasons, the limited structural preciseness is a critical one, as it increases the difficulty in product quality control, accurate dosing, and ensuring the repeatability of material performance. Constructing nanoparticles with molecule-like preciseness is becoming a new research field. In this review, we focus on the artificial nanomaterials that can currently be molecularly or atomically precise, including DNA nanostructures, some metallic nanoclusters, dendrimer nanoparticles and carbon nanostructures, describing their syntheses, bio-applications and limitations, in view of up-to-date studies. A perspective on their potential for clinical translation is also given. This review is expected to provide a particular rationale for the future design of nanomedicines.

Water-Soluble Au25 Clusters with Single-Crystal Structure for Mitochondria-Targeting Radioimmunotherapy

Atomically precise gold clusters play an important role in the development of high-Z-element-based radiosensitizers, due to their intriguing structural diversity and advantages in correlating structures and properties. However, the synthesis of gold clusters with both water-solubility and single-crystal structure remains a challenge. In this study, atomically precise Au₂₅(S-TPP)₁₈ clusters (TPP-SNa = sodium 3-(triphenylphosphonio)propane-1-thiolate bromide) showing both mitochondria-targeting ability and water-solubility were obtained via ligand design for enhanced radioimmunotherapy. Compared with Au₂₅(SG)₁₈ clusters (SG = glutathione), Au₂₅(S-TPP)₁₈ exhibited higher radiosensitization efficiency due to its mitochondria-targeting ability, higher ROS production capacity, and obvious inhibition upon thioredoxin reductase (TrxR). In addition, the enhanced radiotherapy-triggered abscopal effect in combination with checkpoint blockade displayed effective growth inhibition of distant tumors. This work reveals the ligand-regulated organelle targeting ability of metal clusters by which feasible strategies to promote their application in precise theranostics could be realized.

Crown Ether-Capped Gold Nanoclusters as a Multimodal Platform for Bioimaging

The distinct polarity of biomolecule surfaces plays a pivotal role in their biochemistry and functions as it is involved in numerous processes, such as folding, aggregation, or denaturation. Therefore, there is a need to image both hydrophilic and hydrophobic bio-interfaces with markers of distinct responses to hydrophobic and hydrophilic environments. In this work, we present a synthesis, characterization, and application of ultrasmall gold nanoclusters capped with a 12-crown-4 ligand. The nanoclusters present an amphiphilic character and can be successfully transferred between aqueous and organic solvents and have their physicochemical integrity retained. They can serve as probes for multimodal bioimaging with light (as they emit near-infrared luminescence) and electron microscopy (due to the high electron density of gold). In this work, we used protein superstructures, namely, amyloid spherulites, as a hydrophobic surface model and individual amyloid fibrils with a mixed hydrophobicity profile. Our nanoclusters spontaneously stained densely packed amyloid spherulites as observed under fluorescence microscopy, which is limited for hydrophilic markers. Moreover, our clusters revealed structural features of individual amyloid fibrils at a nanoscale as observed under a transmission electron microscope. We show the potential of crown ether-capped gold nanoclusters in multimodal structural characterization of bio-interfaces where the amphiphilic character of the supramolecular ligand is required.

In-situ fabrication of novel Au nanoclusters-Cu2+@sodium alginate/hyaluronic acid nanohybrid gels for cuproptosis enhanced photothermal/photodynamic/chemodynamic therapy via tumor microenvironment regulation

Multimodal combined therapy (MCT) is an emerging avenue to eliminate tumor cells by the synergistic effect of various therapeutic methods. However, the complex tumor microenvironment (TME) is becoming the key barrier to the therapeutic effect of MCT due to the excessive existence of H⁺ ions, H₂O₂, and glutathione (GSH), the lack of O₂, and the relaxation of ferroptosis. To overcome these limitations, smart nanohybrid gels with excellent biocompatibility, stability and targeting function were prepared by using gold nanoclusters as cores and an in situ cross-linking composite gel of sodium alginate (SA)/hyaluronic acid (HA) as the shell. The obtained Au NCs-Cu²⁺@SA-HA core-shell nanohybrid gels possessed near-infrared light response synergistically benefitting photothermal imaging guided photothermal therapy (PTT) and photodynamic therapy (PDT). Meanwhile, the H⁺-triggered release of Cu²⁺ ions from the nanohybrid gels not only induces cuproptosis to avoid the relaxation of ferroptosis, but also catalyzes H₂O₂ in the TME to generate O₂ to simultaneously improve the hypoxic microenvironment and PDT effect. Furthermore, the released Cu²⁺ ions could consume the excessive GSH to form Cu⁺ ions effectively, which caused the formation of hydroxyl free radicals (·OH) to kill tumor cells, synergistically realizing GSH consumption-enhanced PDT and chemodynamic therapy (CDT). Hence, the novel design in our work provides another research avenue for cuproptosis-enhanced PTT/PDT/CDT via TME modulation.

Light-related activities of metal-based nanoparticles and their implications on dermatological treatment

Metal-based nanoparticles (MNPs) represent an emerging class of materials that have attracted enormous attention in many fields. By comparison with other biomaterials, MNPs own unique optical properties which make them a potential alternative to conventional therapeutic agents in medical applications. Especially, owing to the easy access to the skin, the use of MNPs based on their optical properties has gained importance for the treatment of a variety of skin diseases. This review provides an insight into the different optical properties of MNPs, including photoprotection, photocatalysis, and photothermal, and highlights their implications in treating skin disorders, with a special emphasis on their use in infection control. Finally, a perspective on the safety concern of MNPs for dermatological use is discussed and analyzed. The information gathered and presented in this review will help the readers have a comprehensive understanding of utilizing the photo-triggered activity of MNPs for the treatment of skin diseases.

Plasmonic Enhancement of Two-Photon Excited Luminescence of Gold Nanoclusters

Plasmonic-enhanced luminescence of single molecules enables imaging and detection of low quantities of fluorophores, down to individual molecules. In this work, we present two-photon excited luminescence of single gold nanoclusters, Au₁₈(SG)₁₄, in close proximity to bare gold nanorods (AuNRs). We observed 25-times enhanced emission of gold nanoclusters (AuNCs) in near infrared region, which was mainly attributed to the resonant excitation of localized surface plasmon resonance (LSPR) of AuNRs and spectral overlap of LSPR band with photoluminescence of AuNCs. This work is an initial step in application of combined nanoparticles: gold nanorods and ultrasmall nanoclusters in a wide range of multiphoton imaging and biosensing applications.

Multifunctional Host Polymers Assist Au Nanoclusters Achieve High Quantum Yield and Mitochondrial Imaging

The high biocompatibility and excellent photostability of Au nanoclusters (AuNCs) make them stand out in the bioimaging of nanoparticles. However, the low quantum yield and inferior targeting ability of water-soluble AuNCs greatly limit their biological applications. In this study, we designed and synthesized multifunctional host polymers PolySC₄AP and FGGC@AuNCs to fabricate PolySC₄AP/FGGC@AuNC assemblies via a host-guest interaction based on SC₄ (sulfonatocalix[4]arene) and positively charged FGGC ligands (phenylalanine-glycine-glycine-cysteine). Owing to the host-guest assembly strategy and rigid polymer matrix, the quantum yield of FGGC@AuNCs was significantly promoted from 7.0 to 35.3%, accompanied by considerable morphological changes of FGGC@AuNCs. Moreover, PolySC₄AP/FGGC@AuNCs could monitor the location of mitochondria along with R (Pearson's correlation coefficients) value for the co-localization as high as 0.9605, which provided a novel strategy for targeted bioimaging with luminophore.

Effects of protecting groups on luminescent metal nanoclusters: spectroscopic signatures and applications

Luminescent metal nanoclusters (NCs) have been established as next-generation fluorophores. Their biocompatible and non-toxic nature, along with excellent chemical- and photo-stability, enables them to find applications in multi-disciplinary areas. However, preparing NCs which are stable is always challenging, primarily owing to their small size and propensity to self-aggregate. In this review, we highlight a holistic approach as to how ligands and templates can monitor the stability of NCs, tune their spectroscopic signatures, and alter their applications. The role of small molecules of a large ligand in the preparation of NCs and their associated limitations are also discussed. We have summarized how these NCs can be utilized in sensing several metal ions, pH, viscosity and temperature of many systems which have biological relevance. Additionally, these luminescent metal NCs find usage in cell-imaging, discriminating between cancerous and non-cancerous cell lines and also targeting specific organelles within the cellular environment.

Lighting up Individual Organelles With Fluorescent Carbon Dots

Cell organelles play crucial roles in the normal functioning of an organism, therefore the disruption of their operation is associated with diseases and in some cases death. Thus, the detection and monitoring of the activities within these organelles are of great importance. Several probes based on graphene oxide, small molecules, and other nanomaterials have been developed for targeting specific organelles. Among these materials, organelle-targeted fluorescent probes based on carbon dots have attracted substantial attention in recent years owing to their superior characteristics, which include facile synthesis, good photostability, low cytotoxicity, and high selectivity. The ability of these probes to target specific organelles enables researchers to obtain valuable information for understanding the processes involved in their functions and/or malfunctions and may also aid in effective targeted drug delivery. This review highlights recently reported organelle-specific fluorescent probes based on carbon dots. The precursors of these carbon dots are also discussed because studies have shown that many of the intrinsic properties of these probes originate from the precursor used. An overview of the functions of the discussed organelles, the types of probes used, and their advantages and limitations are also provided. Organelles such as the mitochondria, nucleus, lysosomes, and endoplasmic reticulum have been the central focus of research to date, whereas the Golgi body, centrosome, vesicles, and others have received comparatively little attention. It is therefore the hope of the authors that further studies will be conducted in an effort to design probes with the ability to localize within these less studied organelles so as to fully elucidate the mechanisms underlying their function.

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

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

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.

Atomically Precise Gold Nanoclusters: Towards an Optimal Biocompatible System from a Theoretical-Experimental Strategy

Potential biomedical applications of gold nanoparticles have increasingly been reported with great promise for diagnosis and therapy of several diseases. However, for such a versatile nanomaterial, the advantages and potential health risks need to be addressed carefully, as the available information about their toxicity is limited and inconsistent. Atomically precise gold nanoclusters (AuNCs) have emerged to overcome this challenge due to their unique features, such as superior stability, excellent biocompatibility, and efficient renal clearance. Remarkably, the elucidation of their structural and physicochemical properties provided by theory-experiment investigations offers exciting opportunities for site-specific biofunctionalization of the nanoparticle surface, which remains a significant concern for most of the materials in the biomedical field. This concept highlights the advantages conferred by atomically precise AuNCs for biomedical applications and the powerful strategy combining computational and experimental studies towards finding an optimal biocompatible AuNCs-based nanosystem.

An effective NIR laser/tumor-microenvironment co-responsive cancer theranostic nanoplatform with multi-modal imaging and therapies

Cancer is still a major threat to human health at present. Developing new types of integrated nanoplatforms for the accurate diagnosis and effective treatment of cancer is very significant. Herein, an intelligent dual-stage core-shell cancer theranostic nanoplatform (Fe3+@Au1Ag24@PbP) with NIR laser/tumor-microenvironment (TME) co-responsiveness and multi-modal imaging-therapy was successfully prepared, which was composed of the precisely structured oil-soluble Au1Ag24 nanoclusters (NCs) and Fe3+ ions easily assembled within the oil and aqueous phases of the polyethylene glycol (PEG) block grafted polyketal (PK) copolymer (PK-b-PEG, PbP) vesicles, respectively. In this system, we were delighted to find that the prepared Au1Ag24 NCs possess multi-photoresponsive properties, endowing the nanoplatform with photoacoustic (PA)/photothermal (PT) imaging and synergetic photothermal therapy (PTT)/photodynamic therapy (PDT) for cancer under near-infrared (NIR) laser irradiation. On the other hand, Fe3+ ions exhibit multi-TME response and regulation behaviors, including as catalysts for the decomposition of endogenous hydrogen peroxide (H2O2) in the solid tumor to produce O2 and as the oxidizing agent for the consumption of the intracellular GSH to avoid the reduction of the generated 1O2; therefore, the synchronously formed Fe2+ ions from the redox of Fe3+ with GSH could further react with H2O2 to produce hydroxyl radical (˙OH), which induced ferroptosis-based cancer treatment. The PbP shell possesses TME/pH sensitivity for controlled drug release and passive targeting, causing a large increase in Au1Ag24/Fe3+ accumulation within the weakly acidic tumor region and reducing the side effects on normal tissues. Both in vitro and in vivo experiments demonstrate that the Fe3+@Au1Ag24@PbP nanoplatform presented excellent PA/PT imaging-guided synergetic PTT/PDT/ferroptosis effects toward tumor cells and tumors. This integrating multi-responsive and multi-modal theranostic nanoplatform paves a new way for effective cancer therapy.

Bidirectional Regulation of Singlet Oxygen Generation from Luminescent Gold Nanoparticles through Surface Manipulation

Singlet oxygen (¹ O₂ ) generation has been observed from ultrasmall luminescent gold nanoparticles (AuNPs), but regulation of ¹ O₂ generation ability from the nanosized noble metals has remained challenging. Herein, the ¹ O₂ generation ability of ultrasmall AuNPs (d ≈ 1.8 nm) is reported to be highly correlated to the surface factors including the amount of Au(I) species and surface charge. By taking the advantages of facile in situ PEGylation, it is discovered that a high amount of Au(I) species and surface charge results in strong ability in generation of ¹ O₂ , whereas a relative low amount of Au(I) species and surface charge leads to weak ability in ¹ O₂ production. A feasible general strategy is then developed to controllably regulate the ¹ O₂ generation efficiency of the AuNPs through facile ligand exchange with positively-charged or negatively-charged thiolated ligands. The AuNPs as nanophotosensitizer for ¹ O₂ generation in the cellular level is also demonstrated to be highly controllable through surface ligand exchange with synergistical effects of ¹ O₂ generation ability and subcellular distribution to lysosome or mitochondria. The strategy in the bidirectional regulation of ¹ O₂ generation from ultrasmall AuNPs provides guidance for future design of nanosized metal nanomedicine toward specific disease diagnosis and treatment.

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.

Fe3O4@MnO2@PPy nanocomposites overcome hypoxia: magnetic-targeting-assisted controlled chemotherapy and enhanced photodynamic/photothermal therapy

Here, a multifunctional nanocomposite composed by a core of iron oxide (Fe₃O₄) and two shells of manganese dioxide (MnO₂) and polypyrrole (PPy) was successfully designed and synthesized for drug delivery and magnetic targeting assisted synergistic effects of chemotherapy and improved PDT/PTT.