Synthesis and spectroscopic observation of dendrimer-encapsulated gold nanoclusters

Encapsulation of gold nanoclusters: stabilization and more

Gold nanoparticles with only a few atoms, known as gold nanoclusters (AuNCs), have dimensions below 2 nm and feature singular properties such as size dependent luminescence. AuNCs are also highly photostable and have catalytic activity, low toxicity and good biocompatibility. With these properties, they are extremely promising candidates for application in bioimaging, sensing and catalysis. However, when stabilized only with small capping ligands, their use is hindered by lack of colloidal stability. Encapsulation of the AuNCs can contribute to provide a more robust protection and even to improve their properties. Here, we review the encapsulation of AuNCs in polymers, silica and metal organic frameworks (MOFs) for applications in bioimaging, sensing and catalysis.

Gold nanoclusters: An ultrasmall platform for multifaceted applications

Gold nanoclusters (Au NCs) with a core size below 2 nm form an exciting class of functional nano-materials with characteristic physical and chemical properties. The properties of Au NCs are more prominent and extremely different from their bulk counterparts. The synthesis of Au NCs is generally assisted by template or ligand, which impart excellent cluster stability and high quantum yield. The tunable and sensitive physicochemical properties of Au NCs open horizons for their advanced applications in various interdisciplinary fields. In this review, we briefly summarize the solution phase synthesis and origin of the characteristic properties of Au NCs. A vast review of recent research work introducing biosensors based on Au NCs has been presented along with their specifications and detection limits. This review also highlights recent progress in the use of Au NCs as bio-imaging probe, enzyme mimic, temperature sensing probe and catalysts. A speculation on present challenges and certain future prospects have also been provided to enlighten the path for advancement of multifaceted applications of Au NCs.

Highly fluorescent gold nanoclusters stabilized by food proteins: From preparation to application in detection of food contaminants and bioactive nutrients

Applications of nanotechnology in food have rapidly increased in the past decades. Ultra-small gold nanoclusters (Au NCs), composed of several to roughly a hundred atoms, represent a kind of novel nanomaterials. The Au NCs directed by food proteins have drawn considerable research attention due to their environment-friendly preparation, strong fluorescence, excellent photo-stability, and favorable biocompatibility. These interesting protein-Au hybrids have opened up a new area at the nano-bio-food interface, not only did they provide the missing link between single metal atoms and plasmonic metal nanoparticles, but also developed the hybrid system between biomacromolecule and inorganic ions. In this review, we highlighted the synthesis strategies and optical properties of the Au NCs stabilized by typical food proteins as well as their applications in detection of food contaminants or bioactive nutrients. In addition, we discussed current challenges and future development in food proteins- directed gold nanoclusters for size-controlled synthesis and multifunctional applications.

Fluorescent metal quantum clusters: an updated overview of the synthesis, properties, and biological applications

A brief history of metal quantum clusters, their synthesis methods, physical properties, and an updated overview of their applications is provided.

Facile Fabrication of a Gold Nanocluster-Based Membrane for the Detection of Hydrogen Peroxide

In this work, we present a simple and rapid method to synthesize red luminescent gold nanoclusters (AuNCs) with high quantum yield (QY, ~16%), excellent photostability and biocompatibility. Next, we fabricated a solid membrane by loading the as-prepared AuNCs in an agar matrix. Different from nanomaterials dispersed in solution, the AuNCs-based solid membrane has distinct advantages including convenience of transportation, while still maintaining strong red luminescence, and relatively long duration storage without aggregation. Taking hydrogen peroxide (H₂O₂) as a typical example, we then employed the AuNCs as a luminescent probe and investigated their sensing performance, either in solution phase or on a solid substrate. The detection of H₂O₂ could be achieved in wide concentration ranges over 805 nM–1.61 mM and 161 μM–19.32 mM in solution and on a solid membrane, respectively, with limits of detection (LOD) of 80 nM and 20 μM. Moreover, the AuNCs-based membrane could also be used for visual detection of H₂O₂ in the range of 0–3.22 mM. In view of the convenient synthesis route and attractive luminescent properties, the AuNCs-based membrane presented in this work is quite promising for applications such as optical sensing, fluorescent imaging, and photovoltaics.

Synthesis of catalytically active gold clusters on the surface of Fe3O4@SiO2 nanoparticles

This work proposes a novel method to obtain catalytically active gold clusters by using the water-soluble 5,10,15,20-Tetrakis(4-trimethyl-ammonio-phenyl)porphyrin under mild conditions instead of using strong reducing agents.

Controlling an electrostatic repulsion by oppositely charged surfactants towards positively charged fluorescent gold nanoclusters

An anionic surfactant enabled the first accomplish of cationic-thiolate protected fluorescent gold nanoclusters by chemical reduction.

Cytotoxicity of BSA-Stabilized Gold Nanoclusters: In Vitro and In Vivo Study

Gold nanoclusters (Au NCs) are one of the most promising fluorescent nanomaterials for bioimaging, targeting, and cancer therapy due to their tunable optical properties, yet their biocompatibility still remains unclear. Herein, the cytotoxicity of bovine serum albumin (BSA)-stabilized Au NCs is studied by using three tumor cell lines and two normal cell lines. The results indicate that Au NCs induce the decline of cell viabilities of different cell lines to varying degrees in a dose- and time-dependent manner, and umbilical vein endothelial cells which had a higher intake of Au NCs than melanoma cells show more toxicity. Addition of free BSA to BSA-Au NCs solutions can relieve the cytotoxicity, implying that BSA can prevent cell damage. Moreover, Au NCs increase intracellular reactive oxygen species (ROS) production, further causing cell apoptosis. Furthermore, N-acetylcysteine, a ROS scavenger, partially reverses Au NCs-induced cell apoptosis and cytotoxicity, indicating that ROS might be one of the primary reasons for the toxicity of BSA-Au NCs. Surprisingly, Au NCs with concentrations of 5 and 20 nM significantly inhibit tumor growth in the xenograft mice model of human liver cancer, which might provide a new avenue for the design of anti-cancer drug delivery vehicles.

Preparation of multi-coloured different sized fluorescent gold clusters from blue to NIR, structural analysis of the blue emitting Au7 cluster, and cell-imaging by the NIR gold cluster

Blue, green, orange-red, red and NIR emitting gold quantum clusters have been prepared in aqueous media by using a bioactive peptide glutathione (reduced) at physiological pH. Matrix-assisted laser desorption ionization-time-of-flight mass spectrometry (MALDI-TOF MS) analyses show that the core structure sizes of the five different gold clusters are Au7 (blue), Au16 (green), Au19 (orange-red), Au21 (red) and Au22 (NIR). The photo-stability and pH-stability of these quantum clusters have been measured, and these are photo-stable against continuous UV irradiation for a few hours. They also exhibit moderate to good pH-stability within the pH range of 5-12.5. A computational study reveals the organisation of gold atoms in the thiolate-protected blue quantum cluster and its several structural parameters, including the mode of interaction of ligand molecules with Au atoms in the Au7 cluster. Interestingly, it has been found that NIR emitting gold quantum cluster can easily be internalized into the adenocarcinomic human alveolar basal epithelial cell line (A549 cell line). Moreover, a MTT assay indicates that our NIR emitting gold quantum cluster show very low cytotoxicy to A549 cancer cells.

Optical reading of contaminants in aqueous media based on gold nanoparticles

With increasing trends of global population growth, urbanization, pollution over-exploitation, and climate change, the safe water supply has become a global issue and is threatening our society in terms of sustainable development. Therefore, there is a growing need for a water-monitoring platform with the capability of rapidness, specificity, low-cost, and robustness. This review summarizes the recent developments in the design and application of gold nanoparticles (AuNPs) based optical assays to detect contaminants in aqueous media with a high performance. First, a brief discussion on the correlation between the optical reading strategy and the optical properties of AuNPs is presented. Then, we summarize the principle behind AuNP-based optical assays to detect different contaminants, such as toxic metal ion, anion, and pesticides, according to different optical reading strategies: colorimetry, scattering, and fluorescence. Finally, the comparison of these assays and the outlook of AuNP-based optical detection are discussed.

Rapid synthesis of highly luminescent and stable Au20 nanoclusters for active tumor-targeted imaging in vitro and in vivo

Rapid synthesis of protein-stabilized Au20 nanoclusters (Au20NCs) with high fluorescence quantum yield (QY) up to ∼15% is successfully achieved by manipulating the reaction kinetics. The as-obtained Au20NCs, identified by mass spectrometry, have an average size of 2.6 nm, with strong fluorescence emission at 620 nm (2.00 eV) upon excitation at either 370 nm (3.35 eV) or 470 nm (2.64 eV). The advantages of the as-obtained Au20NCs, including small sizes, high fluorescence QY, excellent photostability, non-toxicity, and good stability in biological media, make them ideal candidates as good luminescent probes for optical imaging in vitro and in vivo. Our results demonstrate that the uptake of Au20NCs by both cancer cells and tumor-bearing nude mice can be improved by receptor-mediated internalization, compared with that by passive targeting. Because of their selective accumulation at the tumor sites, the Au20NC probes can be used as potential indicators for cancer diagnosis. This work not only provides a new understanding of the rapid synthesis of highly luminescent Au20NCs but also demonstrates that the functionalized-Au20NCs are excellent probes for active tumor-targeted imaging in vitro and in vivo.

Synthesis of gold nanoparticles and nanoclusters in a supramolecular gel and their applications in catalytic reduction of p-nitrophenol to p-aminophenol and Hg(ii) sensing

Seven gelator molecules giving supramolecular gels produced Au-nanoparticles and fluorescent, small Au-nanoclusters. Such Au-nanoparticle containing gels catalyzed the reduction of p-nitrophenol to p-aminophenol without NaBH₄. The fluorescent Au-nanoclusters acted as a Hg(ii) sensor.

Expression at a 20L scale and purification of the extracellular domain of the Schistosoma mansoni TSP-2 recombinant protein: a vaccine candidate for human intestinal schistosomiasis

A novel recombinant protein vaccine for human schistosomiasis caused by Schistosoma mansoni is under development. The Sm-TSP-2 schistosomiasis vaccine is comprised of a 9 kDa recombinant protein corresponding to the extracellular domain of a unique S. mansoni tetraspanin. Here, we describe the cloning and the expression of the external loop of Sm-TSP-2 recombinant protein secreted by Pichia Pink the process development at 20L scale fermentation, and the two-steps purification, which resulted in a protein recovery yield of 31% and a protein purity of 97%. The developed processes are suitable for the production of purified protein for subsequent formulation and Phase 1 clinical studies.

Luminescent nanoparticles and their applications in the life sciences

Nanoparticles have recently emerged as an important group of materials used in numerous disciplines within the life sciences, ranging from basic biophysical research to clinical therapeutics. Luminescent nanoparticles make excellent optical bioprobes significantly extending the capabilities of alternative fluorophores such as organic dyes and genetically engineered fluorescent proteins. Their advantages include excellent photostability, tunable and narrow spectra, controllable size, resilience to environmental conditions such as pH and temperature, combined with a large surface for anchoring targeting biomolecules. Some types of nanoparticles provide enhanced detection contrast due to their long emission lifetime and/or luminescence wavelength blue-shift (anti-Stokes) due to energy upconversion. This topical review focuses on four key types of luminescent nanoparticles whose emission is governed by different photophysics. We discuss the origin and characteristics of optical absorption and emission in these nanoparticles and give a brief account of synthesis and surface modification procedures. We also introduce some of their applications with opportunities for further development, which could be appreciated by the physics-trained readership.

Facile synthesis and optical properties of magic-number Au13 clusters

Synthesis of molecular gold clusters through a post-synthetic scheme involving HCl-promoted nuclearity convergence was examined with various phosphine ligands. Systematic studies with a series of bis(diphenylphosphino) ligands (Ph(2)P-(CH(2))(m)-PPh(2)) using electrospray ionization mass spectrometry (ESI-MS) and electronic absorption spectroscopy demonstrated that the use of dppp (m = 3), dppb (m = 4) and dpppe (m = 5) as the ligands resulted in the formation of [Au(13)P(8)Cl(4)](+) type clusters, whereas the [Au(13)P(10)Cl(2)](3+) type cluster was formed with dppe (m = 2). The cluster species did not survive the HCl treatment step when monophosphines PPh(3), PMe(2)Ph, and POct(3) were employed, but [Au(13)(POct(3))(8)Cl(4)](+) was isolated as a minor product in the NaBH(4) reduction of Au(POct(3))Cl in aqueous THF. Electronic absorption and photoluminescence studies of a series of Au(13) clusters revealed that their optical properties are highly dependent on the phosphine/chloride composition ratio, but are far less so on the phosphine structure.

Template synthesis of subnanometer gold clusters in interfacially cross-linked reverse micelles mediated by confined counterions

A cationic surfactant with a triallylammonium headgroup was cross-linked photochemically in the presence of a hydrophilic dithiol in the reverse micelle (RM) configuration. The interfacially cross-linked reverse micelles (ICRMs) are unusual templates for nanomaterials synthesis. Our previous work indicated that the ICRMs could extract anionic metal salts such as tetracholoroaurate into the hydrophilic interior, and the entrapped aurate was reduced without externally added reducing agent to form subnanometer luminescent gold clusters [Zhang, S.; Zhao, Y. ACS Nano 2011, 5, 2637-2646]. In this work, the bromide counterions were established as the reducing agent in the template synthesis. The reduction of tetrachloroaurate was proposed to happen through ligand exchange on the aurate by the bromide ions, reductive elimination of halogen, and disproportionation of the Au(I) intermediate. The size of the gold clusters could be tuned rationally by the water-to-surfactant ratio (W(0)) and the reducing agent. Monodisperse Au(4) and Au(9-10) clusters as well as larger Au(18) and Au(23) clusters were obtained from the ICRM templates. The as-prepared, metastable gold clusters were subject to reconstruction triggered by ligand exchange on the surface but could be stabilized through proper surface protection using a chelating dithiol.

Facile preparation of organic nanoparticles by interfacial cross-linking of reverse micelles and template synthesis of subnanometer Au-Pt nanoparticles

A single- and a double-tailed cationic surfactant with the triallylammonium headgroup formed reverse micelles (RMs) in heptane/chloroform containing a small amount of water. The reverse micelles were cross-linked at the interface upon UV irradiation in the presence of a water-soluble dithiol cross-linker and a photoinitiator. The resulting interfacially cross-linked reverse micelles (ICRMs) of the single-tailed surfactant aggregated in a solvent-dependent fashion, whereas those of the double-tailed were identical in size as the corresponding RMs. The ICRMs could extract anionic metal salts, such as AuCl(4)(-) and PtCl(6)(2-), from water into the organic phase. Au and Pt metal nanoparticles were produced upon reduction of metal salts. The covalent nature of the ICRMs made the template synthesis highly predictable, with the size of the metal particles controlled by the amount of the metal salt and the method of reduction. Nanoalloys were obtained by combining two metal precursors in the same reaction. Reduction of the ICRM-entrapped aurate also occurred without any external reducing agents, and the gold nanoparticles differed dramatically from those obtained through sodium borohydride reduction. The same template allowed the preparation of luminescent Au(4), Au(8), and Au(13)-Au(23) clusters, as well as gold nanoparticles several nanometers in size, simply by using different amounts of gold precursor and reducing conditions.

Synthesis and characterization of NiSn dendrimer-encapsulated nanoparticles

We report the synthesis and characterization of NiSn dendrimer-encapsulated nanoparticles (DENs) with sizes in the range of approximately 1.2 nm. These types of materials have potential applications in energy storage, and particles in the 1-3 nm size range are particularly attractive for this use. The NiSn DENs described here contain an average of 147 atoms and are encapsulated within hydrophobic, sixth-generation poly(amidoamine) dendrimers. DENs prepared using four different Ni/Sn ratios, along with monometallic Ni and Sn DENs, are described. To prevent oxidation, the synthesis was carried out under dry conditions in toluene. These bimetallic DENs were characterized by UV-vis spectroscopy, transmission electron microscopy, energy-dispersive X-ray spectroscopy, and X-ray photoelectron spectroscopy. The results indicate that the compositions of the nanoparticles reflect the ratio of Ni(2+) and Sn(2+) used in the first step of the synthesis; the NiSn nanoparticles remain encapsulated within the dendrimers, and when dry they have a degree of stability even after a short exposure to air.

Protein-directed synthesis of highly fluorescent gold nanoclusters

A simple, one-pot, "green" synthetic route, based on the "biomineralization" capability of a common commercially available protein, bovine serum albumin (BSA), has been developed for the preparation of highly stable Au nanocrystals (NCs) with red emission and high quantum yield.

Synthesis of fluorescent carbohydrate-protected Au nanodots for detection of Concanavalin A and Escherichia coli

This study describes a novel, simple, and convenient method for the preparation of water-soluble biofunctional Au nanodots (Au NDs) for the detection of Concanavalin A (Con A) and Escherichia coli (E. coli). First, 2.9 nm Au nanoparticles (Au NPs) were prepared through reduction of HAuCl(4).3H(2)O with tetrakis(hydroxymethyl)phosphonium chloride (THPC), which acts as both a reducing and capping agent. Addition of 11-mercapto-3,6,9-trioxaundecyl-alpha-D-mannopyranoside (Man-SH) onto the surfaces of the as-prepared Au NPs yielded the fluorescent mannose-protected Au nanodots (Man-Au NDs) with the size and quantum yield (QY) of 1.8 (+/-0.3) nm and 8.6%, respectively. This QY is higher than those of the best currently available water-soluble, alkanethiol-protected Au nanoclusters. Our fluorescent Man-Au NDs are easily purified and by multivalent interactions are capable of sensing, under optimal conditions, Con A with high sensitivity (LOD = 75 pM) and remarkable selectivity over other proteins and lectins. To the best of our knowledge, this approach provided the lowest LOD value for Con A when compared to the other nanomaterials-based detecting method. Furthermore, we have also developed a new method for fluorescence detection of E. coli using these water-soluble Man-Au NDs. Incubation with E. coli revealed that the Man-Au NDs bind to the bacteria, yielding brightly fluorescent cell clusters. The relationship between the fluorescence signal and the E. coli concentration was linear from 1.00 x 10(6) to 5.00 x 10(7) cells/mL (R(2) = 0.96), with the LOD of E. coli being 7.20 x 10(5) cells/mL.

Highly fluorescent noble-metal quantum dots

Highly fluorescent, water-soluble, few-atom noble-metal quantum dots have been created that behave as multielectron artificial atoms with discrete, size-tunable electronic transitions throughout the visible and near infrared. These molecular metals exhibit highly polarizable transitions and scale in size according to the simple relation E(Fermi)/N(1/3), predicted by the free-electron model of metallic behavior. This simple scaling indicates that fluorescence arises from intraband transitions of free electrons, and these conduction-electron transitions are the low-number limit of the plasmon-the collective dipole oscillations occurring when a continuous density of states is reached. Providing the missing link between atomic and nanoparticle behavior in noble metals, these emissive, water-soluble Au nanoclusters open new opportunities for biological labels, energy-transfer pairs, and light-emitting sources in nanoscale optoelectronics.