From synthesis to applications of biomolecule-protected luminescent gold nanoclusters

Gold nanoclusters (AuNCs) are a class of novel luminescent nanomaterials that exhibit unique properties of ultra-small size, featuring strong anti-photo-bleaching ability, substantial Stokes shift, good biocompatibility, and low toxicity. Various biomolecules have been developed as templates or ligands to protect AuNCs with enhanced stability and luminescent properties for biomedical applications. In this review, the synthesis of AuNCs based on biomolecules including amino acids, peptides, proteins and DNA are summarized. Owing to the advantages of biomolecule-protected AuNCs, they have been employed extensively for diverse applications. The biological applications, particularly in bioimaging, biosensing, disease therapy and biocatalysis have been described in detail herein. Finally, current challenges and future potential prospects of bio-templated AuNCs in biological research are briefly discussed.

Interaction of Au(III) with amino acids: a vade mecum for medicinal chemistry and nanotechnology

Au(III) is highly reactive. At odds with its reduced counterpart, Au(I), it is hardly present in structural databases. And yet, it is the starting reactant to form gold nanoclusters (AuNCs) and the constitutive component of a new class of drugs. Its reactivity is a world apart from that of the iso-electronic Pt(II) species. Rather than DNA, it targets proteins. Its interaction with amino acid residues is manifold. It can strongly interact with the residue backbones, amino acid side chains and protein ends, it can form appropriate complexes whose stabilization energy reaches up to more than 40 kcal mol-1, it can affect the pKa of amino acid residues, and it can promote charge transfer from the residues to the amount that it is reduced. Here, quantum chemical calculations provide quantitative information on all the processes where Au(III) can be involved. A myriad of structural arrangements are examined in order to determine the strongest interactions and quantify the amount of charge transfer between protonated and deprotonated residues and Au(III). The calculated interaction energies of the amino acid side chains with Au(III) quantitatively reproduce the experimental tendency of Au(III) to interact with selenocysteine, cysteine and histidine and negatively charged amino acids such as Glu and Asp. Also, aromatic residues such as tyrosine and tryptophan strongly interact with Au(III). In proteins, basic pH plays a role in the deprotonation of cysteine, lysine and tyrosine and strongly increases the binding affinity of Au(III) toward these amino acids. The amino acid residues in the protein can also trigger the reduction of Au(III) ions. Sulfur-containing amino acids (cysteine and methionine) and selenocysteine provide almost one electron to Au(III) upon binding. Tyrosine also shows a considerable tendency to act as a reductant. Other amino acids, commonly identified in Au-protein adducts, such as Ser, Trp, Thr, Gln, Glu, Asn, Asp, Lys, Arg and His, possess a notable reducing power toward Au(III). These results and their discussion form a vade mecum that can find application in medicinal chemistry and nanotech applications of Au(III).

Visible to NIR-II Photoluminescence of Atomically Precise Gold Nanoclusters

Atomically precise gold nanoclusters (NCs) have emerged as a new class of precision materials and attracted wide interest in recent years. One of the unique properties of such nanoclusters pertains to their photoluminescence (PL), for it can widely span visible to near-infrared-I and -II wavelengths (NIR-I/II), and even beyond 1700 nm by manipulating the size, structure, and composition. The current research efforts focus on the structure-PL correlation and the development of strategies for raising the PL quantum yields, which is nontrivial when moving from the visible to the near-infrared wavelengths, especially in the NIR-II regions. This review summarizes the recent progress in the field, including i) the types of PL observed in gold NCs such as fluorescence, phosphorescence, and thermally activated delayed fluorescence, as well as dual emission; ii) some effective strategies that are devised to improve the PL quantum yield (QY) of gold NCs, such as heterometal doping, surface rigidification, and core phonon engineering, with double-digit QYs for the NIR PL on the horizons; and iii) the applications of luminescent gold NCs in bioimaging, photosensitization, and optoelectronics. Finally, the remaining challenges and opportunities for future research are highlighted.

Hemoglobin-stabilized gold nanoclusters displaying oxygen transport ability, self-antioxidation, auto-fluorescence properties and long-term storage potential

The development of hemoglobin (Hb)-based oxygen carriers (HBOCs) holds a lot of potential to overcome important drawbacks of donor blood such as a short shelf life or the potential risk of infection. However, a crucial limitation of current HBOCs is the autoxidation of Hb into methemoglobin (metHb), which lacks oxygen-carrying capacity. Herein, we address this challenge by fabricating a Hb and gold nanoclusters (AuNCs) composite (Hb@AuNCs) which preserves the exceptional features of both systems. Specifically, the Hb@AuNCs retain the oxygen-transporting properties of Hb, while the AuNCs provide antioxidant functionality as shown by their ability to catalytically deplete harmful reactive oxygen species (ROS). Importantly, these ROS-scavenging properties translate into antioxidant protection by minimizing the autoxidation of Hb into non-functional metHb. Furthermore, the AuNCs render Hb@AuNCs with auto-fluorescence properties which could potentially allow them to be monitored once administered into the body. Last but not least, these three features (i.e., oxygen transport, antioxidant and fluorescence properties) are well maintained following storage as a freeze-dried product. Thus, overall, the as-prepared Hb@AuNCs hold the potential to be used as a multifunctional blood surrogate in the near future.

Engineering Liganded Gold Nanoclusters as Efficient Theranostic Agents for Cancer Applications

Luminescent gold nanoclusters are rapidly gaining attention as efficient theranostic targets for imaging and therapeutics. Indeed, their ease of synthesis, their tunable optical properties and tumor targeting make them potential candidates for sensitive diagnosis and efficacious therapeutic applications. This concept highlights the key components for designing gold nanoclusters as efficient theranostics focusing on application in the field of oncology.

Nanoparticle-based single molecule fluorescent probes

Single molecule fluorescent probes have attracted considerable attention duet to their ultimate sensitivity, fast response, low sample consumption, and high signal-to-noise ratio. Nanoparticles with outstanding optical properties make them perfect candidates for probes in application of single molecule detection. In this review, we focus on various kinds of nanoparticles acting as single molecule fluorescent probes, including quantum dots, upconverting fluorescent nanoparticles, carbon dots, single-wall carbon nanotubes, fluorescent nanodiamonds, polymeric nanoparticles, nanoclusters, and metallic nanoparticles. Optical properties of various nanoparticles and their recent application in single molecule fluorescent probes are explored. How nanoparticles boost the sensitivity of detection is emphasized in combination with different sensing strategies. Future trends of nanoparticles in single molecule detection are also discussed. We hope this review can provide practical guidance for researchers who work on nanoparticle-based single molecule fluorescent probes.

Interactions of Metal Nanoclusters with Light: Fundamentals and Applications

The interactions of materials with light determine their applications in various fields. In the past decade, ultrasmall metal nanoclusters (NCs) have emerged as a promising class of optical materials due to their unique molecular-like properties. Herein, the basic principles of optical absorption and photoluminescence of metal NCs, their interactions with polarized light, and light-induced chemical reactions, are discussed, highlighting the roles of the core and protecting ligands/motifs of metal NCs in their interactions with light. The metal core and protecting ligands/motifs determine the electronic structures of metal NCs, which are closely related to their optical properties. In addition, the protecting ligands/motifs of metal NCs contribute to their photoluminescence and chiral origin, further promoting the interactions of metal NCs with light through various pathways. The fundamentals of light-NC interactions provide guidance for the design of metal NCs in optical applications, which are discussed in the second part. In the last section, some strategies are proposed to further understand light-NC interactions, highlighting the challenges and opportunities. It is hoped that this work will stimulate more research on the optical properties of metal NCs and their applications in various fields.

The endocytosis of nano-Pt into non-small cell lung cancer H1299 cells and intravital therapeutic effect in vivo

Lung cancer remains the most common fatal malignant disease, and the 5-year survival rate of patients with metastasis is merely 6%. In this research, the platinum nanocluster (short for nano-Pt) was used for optical imaging without the help of other fluorescent probes and possess targeted antitumor activity as well as low systemic toxicity. The endocytic pathway and distribution of nano-Pt in non-small cell lung cancer NSCLC H1299 cells was explored by the means of quantitative and qualitative tests. Furthermore, the targeting capability and antitumor efficiency of nano-Pt was detected by intravital imaging experiment and antitumor experiment. The research implies that nano-Pt entered H1299 cells dominatingly through macropinocytosis and clathrin-dependent endocytosis pathway, and has significant antitumor efficiency, targeting properties and reliable safety for mouse tumor, indicating this nano-Pt has great potential for clinical diagnosis and therapy of NSCLC H1299 cells.

Photoluminescent nanocluster-based probes for bioimaging applications

In the continuous search for versatile and better performing probes for optical bioimaging and biosensing applications, many research efforts have focused on the design and optimization of photoluminescent metal nanoclusters. They consist of a metal core composed by a small number of atoms (diameter < 2-3 nm), usually coated by a shell of stabilizing ligands of different nature, and are characterized by molecule-like quantization of electronic states, resulting in discrete and tunable optical transitions in the UV-Vis and NIR spectral regions. Recent advances in their size-selective synthesis and tailored surface functionalization have allowed the effective combination of nanoclusters and biologically relevant molecules into hybrid platforms, that hold a large potential for bioimaging purposes, as well as for the detection and tracking of specific markers of biological processes or diseases. Here, we will present an overview of the latest combined imaging or sensing nanocluster-based systems reported in the literature, classified according to the different families of coating ligands (namely, peptides, proteins, nucleic acids, and biocompatible polymers), highlighting for each of them the possible applications in the biomedical field.

The effects of surface functionality and size of gold nanoparticles on neuronal toxicity, apoptosis, ROS production and cellular/suborgan biodistribution

Gold nanoparticles are emerging as promising nanomaterials to create nanoscale therapeutic delivery systems. The aim of the study was to synthesis of highly monodisperse and stable gold nanoparticles functionalized with polyethyleneimine (PEI) and polyethylene glycol (PEG), multiparametric investigation of their neuronal toxicological effects and evaluation of the cellular/suborgan biodistribution. Gold nanoparticles (AuNP₂₀ and AuNP₅₀) were synthesized and their surfaces were electrostatically modified by PEI and PEG. Dorsal root ganglion (DRG) sensory neurones were isolated from BALB/c mice. Cell viability, apoptosis and ROS production were evaluated in vitro. Cellular and suborgan biodisribution of the AuNPs were investigated using inductively coupled plasma mass spectrometry (ICP-MS) technique. PEI and PEG surface coating increased both biocompatibility and biodistribution of the AuNPs. ICP-MS measurements showed the presence of gold in liver, spleen, kidney, heart, blood and brain within a 30 days period. The size and surface chemistry of the AuNPs are important parameters for potential nanoteranostic applications in the future studies.

Ciprofloxacin conjugated gold nanorods with pH induced surface charge transformable activities to combat drug resistant bacteria and their biofilms

The ever-growing threat of drug-resistant pathogens and their biofilms based persistent, chronic infections has created an urgent call for new strategies to deal with multidrug resistant bacteria (MDR). Near-infrared (NIR) laser-induced photothermal treatment (PTT) of gold nanorods (AuNRs) disinfects microbes by local heating with low possibility to develop resistant. However, PTT disinfection strategy of AuNRs alone shows less efficiency in killing multidrug resistant strains (i.e. Methicillin-resistant Staphylococcus aureus, MRSA) and their matured biofilms. Herein, a novel synergistic chemo-photothermal integrated antimicrobial platform (P(Cip-b-CB)-AuNRs) was fabricated which show enhanced killing efficiency against MRSA in both planktonic and biofilm phenotypes. Polymethacrylate copolymers with pendant ciprofloxacin (Cip) and the carboxyl betaine groups (P(Cip-b-CB)) were synthesized using reversible addition-fragmentation chain transfer (RAFT) polymerization. P(Cip-b-CB) was decorated onto AuNRs via gold-thiol bond which resulted in AuNRs with acidic-induced surface charge-switchable activities and lipase triggered Cip release properties (P(Cip-b-CB)-AuNRs). The lower pH value and overexpress of lipase are characteristics for microenvironment of microbial infections and their biofilms, which ensure the targeting on, penetration into and on-demand release of Cip from the nanocomposites in bacterial infection sites and their biofilms. The bacterial cell membrane was disrupt by photothermal therapy which could improve its permeability and sensitivity to antibiotics, meanwhile lipase-triggered release of Cip ensures a high concentration of antibiotics at the site of bacterial infection. Besides their NIR induced PTT disinfection activities, the increased local temperature generated by NIR light irradiation accelerated Cip release which further enhanced the antibacterial efficiency, leading to synergistic antibacterial activities of chemo-photothermal therapy. Taken together, the designed synergistic chemo-photothermal integrated antimicrobial platform is a promising antibacterial agent for fighting MDR bacterial infections and their biofilms.

Silver-enhanced fluorescence of bimetallic Au/Ag nanoclusters as ultrasensitive sensing probe for the detection of folic acid

Folic acid (FA) is the natural form of water-soluble vitamins widely found in most plants and animal products and its deficiency leads to several human body abnormalities. The advancements of metal nanoclusters are highly increasing due to their molecule-like optical properties and attractive applications. Because of increasingly demand of noble metal nanoclusters as sensing templates, different synthesis methods have been developed for facile synthesis of noble metal nanoclusters. Herein, red-emitting fluorescent bovine serum albumin (BSA)-capped Au-Ag bimetallic NCs are facilely synthesized through green one-pot synthetic approach. The effect of silver on the fluorescence properties of Au NCs was investigated and it was found that introduction of silver can enhance the fluorescence intensity. The fluorescence intensity of the as-prepared Au-Ag nanoclusters gets quenched in the presence of folic acid in an aqueous medium and it was used as ultrasensitive sensing probe for FA detection. The developed Au-Ag NCs-based sensing probe shows linear response in the wide range of 0-100 μM and the detection limit is as low as 0.47 nM. Its applicability has also been confirmed successfully in real human serum, urine and FA tablet samples. Due to the high stability, sensitivity and selectivity, the developed bimetallic cluster sensing system is highly promising to be applied in the pharmaceutical and clinical laboratories.

Synergistic integration of metal nanoclusters and biomolecules as hybrid systems for therapeutic applications

Therapeutic nanoparticles are designed to enhance efficacy, real-time monitoring, targeting accuracy, biocompatibility, biodegradability, safety, and the synergy of diagnosis and treatment of diseases by leveraging the unique physicochemical and biological properties of well-developed bio-nanomaterials. Recently, bio-inspired metal nanoclusters (NCs) consisting of several to roughly dozens of atoms (<2 nm) have attracted increasing research interest, owing to their ultrafine size, tunable fluorescent capability, good biocompatibility, variable metallic composition, and extensive surface bio-functionalization. Hybrid core-shell nanostructures that effectively incorporate unique fluorescent inorganic moieties with various biomolecules, such as proteins (enzymes, antigens, and antibodies), DNA, and specific cells, create fluorescently visualized molecular nanoparticle. The resultant nanoparticles possess combinatorial properties and synergistic efficacy, such as simplicity, active bio-responsiveness, improved applicability, and low cost, for combination therapy, such as accurate targeting, bioimaging, and enhanced therapeutic and biocatalytic effects. In contrast to larger nanoparticles, bio-inspired metal NCs allow rapid renal clearance and better pharmacokinetics in biological systems. Notably, advances in nanoscience, interfacial chemistry, and biotechnologies have further spurred researchers to explore bio-inspired metal NCs for therapeutic purposes. The current review presents a comprehensive and timely overview of various metal NCs for various therapeutic applications, with a special emphasis on the design rationale behind the use of biomolecules/cells as the main scaffolds. In the different hybrid platform, we summarize the current challenges and emerging perspectives, which are expected to offer in-depth insight into the rational design of bio-inspired metal NCs for personalized treatment and clinical translation.

Supramolecular Paradigm for Capture and Co-Precipitation of Gold(III) Coordination Complexes

A new supramolecular paradigm is presented for reliable capture and co-precipitation of haloauric acids (HAuX₄ ) from organic solvents or water. Two classes of acyclic organic compounds act as complementary receptors (tectons) by forming two sets of directional non-covalent interactions, (a) hydrogen bonding between amide (or amidinium) NH residues and the electronegative X ligands on the AuX₄ ^- , and (b) electrostatic stacking of the electron deficient Au center against the face of an aromatic surface. X-ray diffraction analysis of four co-crystal structures reveals the additional common feature of proton bridged carbonyls as a new and predictable supramolecular design element that creates one-dimensional polymers linked by very short hydrogen bonds (CO⋅⋅⋅OC distance <2.5 Å). Two other co-crystal structures show that the amidinium-π⋅⋅⋅XAu interaction will reliably engage AuX₄ ^- with high directionality. These acyclic compounds are very attractive as co-precipitation agents within new "green" gold recovery processes. They also have high potential as tectons for controlled self-assembly or co-crystal engineering of haloaurate composites. More generally, the supramolecular paradigm will facilitate the design of next-generation receptors or tectons with high affinity for precious metal square planar coordination complexes for use in advanced materials, nanotechnology, or medicine.

Ultrasmall Au and Ag Nanoclusters for Biomedical Applications: A Review

Noble metal (e.g., Au, Ag, Pt, Pd, and their alloys) nanoclusters (NCs) have emerged as a new type of functional nanomaterial in nanoscience and nanotechnology. Owing to their unique properties, such as their ultrasmall dimension, enhanced photoluminescence, low toxicity, and excellent biocompatibility, noble metal NCs-especially Au and Ag NCs-have found various applications in biomedical regimes. This review summarizes the recent advances made in employing ultrasmall Au and Ag NCs for biomedical applications, with particular emphasis on bioimaging and biosensing, anti-microbial applications, and tumor targeting and cancer treatment. Challenges, including the shared and specific challenges for Au and Ag NC toward biomedical applications, and future directions are briefly discussed at the end.