Recent advances in templated synthesis of metal nanoclusters and their applications in biosensing, bioimaging and theranostics

As a kind of promising nanomaterials, metal nanoclusters (MNCs) generally composed of several to hundreds of metal atoms have received increasing interest owing to their unique properties, such as ultrasmall size (<2 nm), fascinating physical and chemical properties, and so on. Recently, template-assisted synthesis of MNCs (e.g., Au, Ag, Cu, Pt and Cd) has attracted extensive attention in biological fields. Up to now, various templates (e.g., dendrimers, polymers, DNAs, proteins and peptides) with different configurations and spaces have been applied to prepare MNCs with the advantages of facile preparation, controllable size, good water-solubility and biocompatibility. Herein, we focus on the recent advances in the template-assisted synthesis of MNCs, including the templates used to synthesize MNCs, and their applications in biosensing, bioimaging, and disease theranostics. Finally, the challenges and future perspectives of template-assisted synthesized MNCs are highlighted. We believe that this review could not only arouse more interest in MNCs but also promote their further development and applications by presenting the recent advances in this area to researchers from various fields, such as chemistry, material science, physiology, biomedicine, and so on.

Developing fluorescent copper nanoclusters: Synthesis, properties, and applications

Metal nanoclusters exhibit strong fluorescence emission, providing immense potential for developments in biological labeling and imaging. Copper nanoclusters in particular, due to their unique optical properties such as molecular-like absorption and strong luminescence, represent a novel fluorescent nanomaterial for sensing and bioimaging applications. This review describes research progress on Cu nanoclusters in recent years, investigating the synthesis techniques, their properties, and their promising applications. A concluding summary provides an outlook on the future research challenges for Cu nanoclusters and their corresponding synthesis techniques.

Current status and future perspectives of gold nanoparticle vectors for siRNA delivery

Discovering the vast therapeutic potential of siRNA opened up new clinical research areas focussing on a number of diseases and applications; however significant problems with siRNA stability and delivery have hindered its clinical applicability. As a result, interest in the development of practical siRNA delivery systems has grown in recent years. Of the numerous siRNA delivery strategies currently on offer, gold nanoparticles (AuNPs) stand out thanks to their biocompatibility and capacity to protect siRNA against degradation; not to mention the versatility offered by their tuneable shape, size and optical properties. Herein this review provides a complete summary of the methodologies for functionalizing AuNPs with siRNA, paying singular attention to the AuNP shape, size and surface coating, since these key factors heavily influence cellular interaction, internalization and, ultimately, the efficacy of the hybrid particle. The most noteworthy hybridization strategies have been highlighted along with the most innovative and outstanding in vivo studies with a view to increasing clinical interest in the use of AuNPs as siRNA nanocarriers.

Subnanometer Gold Clusters Adhere to Lipid A for Protection against Endotoxin-Induced Sepsis

Endotoxicity originating from a dangerous debris (i.e., lipopolysaccharide, LPS) of Gram-negative bacteria is a challenging clinical problem, but no drugs or therapeutic strategies that can successfully address this issue have been identified yet. In this study, we report a subnanometer gold cluster that can efficiently block endotoxin activity to protect against sepsis. The endotoxin blocker consists of a gold nanocluster that serves as a flakelike substrate and a coating of short alkyl motifs that act as an adhesive to dock with LPS by compacting the intramolecular hydrocarbon chain-chain distance ( d-spacing) of lipid A, an endotoxicity active site that can cause overwhelming cytokine induction resulting in sepsis progression. Direct evidence showed the d-spacing values of lipid A to be decreased from 4.19 Å to either 3.85 or 3.54 Å, indicating more dense packing densities in the presence of subnanometer gold clusters. In terms of biological relevance, the concentrations of key pro-inflammatory NF-κB-dependent cytokines, including plasma TNF-α, IL-6, and IL-1β, and CXC chemokines, in LPS-challenged mice showed a noticeable decrease. More importantly, we demonstrated that the treatment of antiendotoxin gold nanoclusters significantly prolonged the survival time in LPS-induced septic mice. The ultrasmall gold nanoclusters could target lipid A of LPS to deactivate endotoxicity by compacting its packing density, which might constitute a potential therapeutic strategy for the early prevention of sepsis caused by Gram-negative bacterial infection.

Highly selective and sensitive visualization and identification of glycoproteins using multi-functionalized soluble dendrimer

Glycoproteins are the most important and complex group of posttranslational modifications known in proteins. Many clinical biomarkers and therapeutic targets in cancer are glycoproteins. However, the isolation of glyco-specific antibodies and their poor stability remains a significant challenge in analytical method and diagnostic development. In this work, for the first time, we present a technology for highly efficient and selective glycosylation analysis on membrane without the use of glyco-specific antibodies. This approach, termed Nanopoly-BAV, which uses polyamidoamine dendrimers multifunctionalized with boronic acid for specific binding to glycoproteins and with biotin groups for glycoproteins visualization. The Nanopoly-BAV confers femtomolar sensitivity, exceptional glycoprotein specificity and selectivity with as high as 100000 folds for glycoproteins over nonglycoproteins. This synthetic, robust and highly selective Nanopoly-BAV has a great potential to measure cell signaling events by clearly distinguishing actual glycosylation signals from protein expression changes with superior stability. This technique may provide a powerful tool to monitor cellular signaling pathways and discovering new signaling events.

Recent advances in biomedical applications of fluorescent gold nanoclusters

Fluorescent gold nanoclusters (AuNCs) are emerging as novel fluorescent materials and have attracted more and more attention in the field of biolabeling, biosensing, bioimaging and targeted cancer treatment because of their unusual physicochemical properties, such as long fluorescence lifetime, ultrasmall size, large Stokes shift, strong photoluminescence, as well as excellent biocompatibility and photostability. Recently, significant efforts have been committed to the preparation, functionalization and biomedical application studies of fluorescent AuNCs. In this review, we have summarized the strategies for preparation and surface functionalization of fluorescent AuNCs in the past several years, and highlighted recent advances in the biomedical applications of the relevant fluorescent AuNCs. Based on these observations, we also give a discussion on the current problems and future developments of the fluorescent AuNCs for biomedical applications.

Tailoring Enzyme-Like Activities of Gold Nanoclusters by Polymeric Tertiary Amines for Protecting Neurons Against Oxidative Stress

The cytotoxicity of nanozymes has drawn much attention recently because their peroxidase-like activity can decompose hydrogen peroxide (H2 O2 ) to produce highly toxic hydroxyl radicals (•OH) under acidic conditions. Although catalytic activities of nanozymes are highly associated with their surface properties, little is known about the mechanism underlying the surface coating-mediated enzyme-like activities. Herein, it is reported for the first time that amine-terminated PAMAM dendrimer-entrapped gold nanoclusters (AuNCs-NH2 ) unexpectedly lose their peroxidase-like activity while still retaining their catalase-like activity in physiological conditions. Surprisingly, the methylated form of AuNCs-NH2 (i.e., MAuNCs-N(+) R3 , where R = H or CH3 ) results in a dramatic recovery of the intrinsic peroxidase-like activity while blocking most primary and tertiary amines (1°- and 3°-amines) of dendrimers to form quaternary ammonium ions (4°-amines). However, the hidden peroxidase-like activity is also found in hydroxyl-terminated dendrimer-encapsulated AuNCs (AuNCs-OH, inside backbone with 3°-amines), indicating that 3°-amines are dominant in mediating the peroxidase-like activity. The possible mechanism is further confirmed that the enrichment of polymeric 3°-amines on the surface of dendrimer-encapsulated AuNCs provides sufficient suppression of the critical mediator •OH for the peroxidase-like activity. Finally, it is demonstrated that AuNCs-NH2 with diminished cytotoxicity have great potential for use in primary neuronal protection against oxidative damage.

Endotoxin Nanovesicles: Hydrophilic Gold Nanodots Control Supramolecular Lipopolysaccharide Assembly for Modulating Immunological Responses

In this study, we sought to control the assembly of an endotoxin known as the biologically supramolecular lipopolysaccharide (LPS, which consists of three portions: an O antigen, a core carbohydrate, and a lipid A molecule) in order to modulate immunological responses in a manner that has the potential for utilization in vaccine development. Changing the structures of LPS aggregates from lamellas to specific nonlamellas (i.e., cubosomes and hexosomes) can dramatically enhance the strength of LPS in causing inflammatory responses, leading to highly active responses. In order to control the formation of cubosome-free and hexosome-free nonlamellas, we designed a simple strategy based on the use of hydrophilic gold nanodots (AuNDs) to control LPS assembly to facilitate the formation of stable endotoxin nanovesicles, which are stable precursors of cubosomes and hexosomes with specific immunological effects. Structurally, the wall thicknesses of these nanovesicles are exactly twice the lengths of a single LPS molecule, indicating that the LPS molecules adopt a tail-to-tail arrangement (with the lipid A portions acting as the tail domain). The involvement of the hydrophilic AuNDs to laterally link polar domains of LPS can result in the progressive extension of an endotoxically active zone of lipid A assembly, leading to the eventual formation of large-size nanovesicles. Our results showed that endotoxin nanovesicles with such dense lipid A units can elicit the stronger inflammatory gene expressions, including interleukin 6 (IL-6), IL-1A, TNF-α, C-X-C chemokine ligand (CXCL) 1, 2, and 11, which have characteristics of T-helper 1 adjuvants. These findings provide evidence that the concept of manipulating the surface hydrophilicity of AuNDs to control LPS assembly in order to avoid the formation of highly active cubosomes and hexosomes, and thereby modulate immunological responses appropriately, could prove useful in vaccine development.

Interactions of nitroxide radicals with dendrimer-entrapped Au8-clusters: a fluorescent nanosensor for intracellular imaging of ascorbic acid

When gold nanoparticles (AuNPs) become extremely small (<2 nm in diameter) as gold nanoclusters (AuNCs), an intriguing issue is whether the interactions of free radicals with AuNCs would be essentially different at sufficiently small size. Herein, we report for the first time that the fluorescence of a polyamidoamine (PAMAM) dendrimer-entrapped Au₈-cluster is quenched by the paramagnetic nitroxide radical. Based on an upward curving Stern-Volmer plot, the system shows complex fluorescence quenching with a combination of static and dynamic quenching processes. The quenching mechanism associated with the interactions between Au₈-clusters and nitroxide radicals was explored by combined fluorescence and electron paramagnetic resonance (EPR) studies. The controlled quenching of the fluorescent Au₈-cluster can be developed as a turn-on fluorescence probe for sensing ascorbic acid (AA) in living cells.