Protected Gold Nanoclusters Research Articles

Structural characterization of site-modified nanocapsid with monodispersed gold clusters

Hepatitis E Virus-like particles self-assemble in to noninfectious nanocapsids that are resistant to proteolytic/acidic mucosal delivery conditions. Previously, the nanocapsid was engineered to specifically bind and enter breast cancer cells, where successful tumor targeting was demonstrated in animal models. In the present study, the nanocapsid surface was modified with a solvent-exposed cysteine to conjugate monolayer protected gold nanoclusters (AuNC). Unlike commercially available gold nanoparticles, AuNCs monodisperse in water and are composed of a discrete number of gold atoms, forming a crystalline gold core. Au102pMBA44 (Au102) was an ideal conjugate given its small 2.5 nm size and detectability in cryoEM. Au102 was bound directly to nanocapsid surface cysteines via direct ligand exchange. In addition, Au102 was functionalized with a maleimide linker (Au102_C6MI) for maleimide-thiol conjugation to nanocapsid cysteines. The AuNC-bound nanocapsid constructs were conjugated in various conditions. We found Au102_C6MI to bind nanocapsid more efficiently, while Au102 remained more soluble over time. Nanocapsids conjugated to Au102_C6MI were imaged in cryoEM for single particle reconstruction to localize AuNC position on the nanocapsid surface. We resolved five unique high intensity volumes that formed a ring-shaped density at the 5-fold symmetry center. This finding was further supported by independent rigid modeling.

Inositol directed facile “green” synthesis of fluorescent gold nanoclusters as selective and sensitive detecting probes of ferric ions

In this paper, we developed one-pot facile synthetic approach for preparing fluorescent gold nanoclusters (AuNCs) by utilizing reducing-cum-stabilizing inositol (INOS). The as-prepared INOS protected gold nanoclusters (INOS@AuNCs) have excellent monodispersity with uniform size <2.0nm and exhibit strong inherent fluorescence at 470nm with a decay time of 12.01ns. Interestingly, ferric ions (Fe3+) can significantly quench the fluorescence of the synthesized INOS@AuNCs leading to the efficient Fe3+ sensing. On the basis of results obtained from high-resolution transmission electron microscopy (HRTEM), an aggregation-induced fluorescence quenching mechanism was proposed. Based on these fascinating findings, a facile, fast, “green” and highly sensitive AuNCs-dependant fluorescent sensor probe was developed to detect Fe3+ ions in water resources and Over-The-Counter (OTC) medicines. Stern-Volmer plot showed a good linear calibration curve for Fe3+ assay in the range of 1.0–1000μM (R2=0.996) and good precision response to 100.0μM Fe3+ with RSD of 2.94% (n=3) was also achieved. The obtained limit of detection of 0.54μM for the determination of Fe3+ ions is 10 times lower than the limit value (≈5.5μM) allowed by the U.S. Environmental Protection Agency in drinkable water, indicating the sensitivity of the fluorescent probe of Fe3+ sensing. The excellent selectivity obtained among 16 types of metal ions underpins INOS@AuNCs as a promising sensor for real time detection of Fe3+ in environmental water samples and pharmaceutical tablets.

One-pot green synthesis of supramolecular β-cyclodextrin functionalized gold nanoclusters and their application for highly selective and sensitive fluorescent detection of dopamine

We report here one-pot synthetic method for preparing gold nanoclusters (AuNCs) by employing pristine β-cyclodextrin (β-CD) as reducing-cum-stabilizing agent for the first time. The developed AuNCs were used as a selective and sensitive fluorescent probe for sensing dopamine (DA) based on β-CD supramolecular interaction and quenching effect. The as-synthesized β-CD protected gold nanoclusters (β-CD@AuNCs) showed good monodispersity with ultra-small size < 2.0nm and exhibited strong fluorescence intensity at 450nm β-CD@AuNCs have been systematically characterized by UV–vis absorption spectroscopy, photoluminescence spectroscopy, X-ray photoelectron spectroscopy, Fourier transform infrared spectroscopy, transmission electron microscopy and dynamic light scattering. DA can trigger significant fluorescence quenching of β-CD@AuNCs on the basis of photo-induced electron transfer (PET) mechanism. Fluorescence lifetimes characterization of β-CD@AuNCs and DA/β-CD@AuNCs conjugate were performed to confirm the proposed PET process. A good linearity range (100.0nM to 80.0μM) for DA detection was obtained by applying the Stern-Volmer equation with lower LOD (3σ/s) of 20.0nM, revealing the high sensitivity of the method. This fluorescent AuNCs sensing probe for DA demonstrates excellent selectivity among 24 biological interferents owing to the specificity of steric host-guest interactions between DA and the nanocavity of β-CD capped on AuNCs. The developed nanosensor was successfully applied in the detection of DA in human serum plasma and injection dosage forms, indicating its good practicability for DA detection in biological and pharmaceutical research.

Melamine dependent fluorescence of glutathione protected gold nanoclusters and ratiometric quantification of melamine in commercial cow milk and infant formula

Companies processing the milk for the further production of powdered infant formulation normally check the protein level through a test measuring nitrogen content. The addition of melamine which is a nitrogen-rich organic chemical in milk increases the nitrogen content and therefore enhances its apparent protein content. However, the melamine causes kidney failure and death owing to the formation of kidney stone. Thus the determination of melamine in humans and milk products have gained great significance in recent years. The gold nanoclusters (AuNCs) have attracting features due to its unique electronic and optical properties like fluorescence nature. Therefore one can use AuNCs in the field of biosensor, bio-imaging, nanobiotechnology, drug delivery, diagnosis etc. We report, a new ratiometric nanosensor established for the selective and sensitive detection of melamine based optical sensing using glutathione stabilized AuNCs. The AuNCs were characterized by high-resolution transmission electron microscopy (HR-TEM), UV–visible and Photoluminescence (PL) spectroscopic techniques. In the presence of melamine, the PL intensity at 430nm increases owing to the (turn-on) enhancement in fluorescence, whereas PL intensity at 610nm decreases due to the melamine-induced aggregation and subsequent aggregation-enhanced emission quenching. The observed changes were ascribed to the hydrogen bonding interaction between melamine and AuNCs, which led to the aggregation of the nanoclusters. This was confirmed by dynamic light scattering and HR-TEM measurements. The present probe showed an extreme selectivity towards the determination of 28.2μM melamine in the presence of 100-fold excess of common interfering molecules such as Alanine, Glycine, Glucose, Cystine etc. The proposed method was successfully applied to determine melamine in cow milk.

New insights into the stability and structural evolution of some gold nanoclusters.

Revealing the stability and structural patterns is important for precisely synthesizing or assembling ligand protected nanoclusters, and even their applications as functional nanomaterials. Investigations on structural evolutional patterns and structural stability are very challenging, because structures change with the nanocluster size and the structural stability depends on both the electron structures of cores and ligand type. Herein, we propose a hybrid superatom network (hSAN) model to understand the stability of some gold nanoclusters with different kinds of ligands. In this model, 4c-2e superatom Au4 can form conjugated superatom networks by vertex sharing, and ligands further connect the conjugated superatom networks together to form a bigger complex network, i.e. a hSAN. The stability of the clusters, including [Au24(C[triple bond, length as m-dash]CPh)14(PPh3)4]2+, Au28(S-c-C6H11)20, Au36(SCH2Ph-tBu)8Cl20, Au40(O-MBT)24 and Au52(TBBT)32 can be explained uniformly by the hSAN model. Beyond that, a new heuristic structural pattern named the Au13 topological rule is proposed. In the light of this heuristic rule, every Au7 bi-tetrahedral kernel is included in an Au13 structure with quasi-Oh symmetry, i.e. as long as the Au7 bi-tetrahedral kernel is formed, it will be surrounded by six Au atoms to form an Au13 structure topologically. According to this understanding, a new nanocluster [Au44(C[triple bond, length as m-dash]CH3)26(PCH3)4]2+ and a new nanowire with the structural evolutional formula [Au(20n+4)(C[triple bond, length as m-dash]CH3)(12n+2)(PCH3)4]2+ (n = 1, 2, 3, 4, …) are predicted. Both the understanding of the stability and the structure rule are free from the type of ligand, and will be useful for the structural predictions and determinations of ligand protected gold nanoclusters.

A novel dual-functional biosensor for fluorometric detection of inorganic pyrophosphate and pyrophosphatase activity based on globulin stabilized gold nanoclusters

A novel ultrasensitive dual-functional biosensor for highly sensitive detection of inorganic pyrophosphate (PPi) and pyrophosphatase (PPase) activity was developed based on the fluorescent variation of globulin protected gold nanoclusters (Glo@Au NCs) with the assistance of Cu2+. Glo@Au NCs and PPi were used as the fluorescent indicator and substrate for PPase activity evaluation, respectively. In the presence of Cu2+, the fluorescence of the Glo@Au NCs will be quenched owing to the formation of Cu2+-Glo@Au NCs complex, while PPi can restore the fluorescence of the Cu2+-Glo@Au NCs complex because of its higher binding affinity with Cu2+. As PPase can catalyze the hydrolysis of PPi, it will lead to the release of Cu2+ and re-quench the fluorescence of the Glo@Au NCs. Based on this mechanism, quantitative evaluation of the PPi and PPase activity can be achieved ranging from 0.05 μM to 218.125 μM for PPi and from 0.1 to 8 mU for PPase, with detection limits of 0.02 μM and 0.04 mU, respectively, which is much lower than that of other PPi and PPase assay methods. More importantly, this ultrasensitive dual-functional biosensor can also be successfully applied to evaluate the PPase activity in human serum, showing great promise for practical diagnostic applications.

Covalently linked multimers of gold nanoclusters Au102(p-MBA)44and Au∼250(p-MBA)n

We present the synthesis, separation, and characterization of covalently-bound multimers of para-mercaptobenzoic acid (p-MBA) protected gold nanoclusters. The multimers were synthesized by performing a ligand-exchange reaction of a pre-characterized Au102(p-MBA)44 nanocluster with biphenyl-4,4'-dithiol (BPDT). The reaction products were separated using gel electrophoresis yielding several distinct bands. The bands were analyzed by transmission electron microscopy (TEM) revealing monomer, dimer, and trimer fractions of the nanocluster. TEM analysis of dimers in combination with molecular dynamics simulations suggest that the nanoclusters are covalently bound via a disulfide bridge between BPDT molecules. The linking chemistry is not specific to Au102(p-MBA)44. The same approach yields multimers also for a larger monodisperse p-MBA-protected cluster of approximately 250 gold atoms, Au∼250(p-MBA)n. While the Au102(p-MBA)44 is not plasmonic, the Au∼250(p-MBA)n nanocluster supports localized surface plasmon resonance (LSPR) at 530 nm. Multimers of the Au∼250(p-MBA)n exhibit additional transitions in their UV-vis spectrum at 630 nm and 810 nm, indicating the presence of hybridized LSPR modes. Well-defined structures and relatively small sizes make these systems excellent candidates for connecting ab initio theoretical studies and experimental quantum plasmonics. Moreover, our work opens new possibilities in the controlled synthesis of advanced monodisperse nanocluster superstructures.

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

A novel positively charged fluorescent gold nanocluster was successfully synthesized using the shortest cationic thiol, thiocholine. Effective control of electrostatic repulsion by the introduction of an anionic surfactant afforded a nanocluster that showed blue fluorescence emission.

Fluorescence enhancement of DHLA protected gold nanoclusters in the presence of salt

The mechanism for the fluorescence enhancement of DHLA protected gold nanoclusters has been investigated based on the adsorption of the thiolate ligand.

Novel gold nanocluster electrochemiluminescence immunosensors based on nanoporous NiGd-Ni2O3-Gd2O3 alloys.

Herein, three-dimensional nanoporous NiGd alloy (NP-NiGd) was prepared by selectively dealloy Al from NiGdAl alloy in mild alkaline solution, then Ni2O3 and Gd2O3 grew further on the surface of NP-NiGd to obtain the NP-NiGd-Ni2O3-Gd2O3. On this basis, NP-NiGd-Ni2O3-Gd2O3 was further functionalized with gold nanoparticles (NP-NiGd-Ni2O3-Gd2O3@Au) and acted as sensor platform to fabricate a novel electrochemiluminescence (ECL) immunosensor. Bovine serum albumin protected gold nanoclusters (AuNCs@BSA) were prepared and acted as illuminant. AuNCs@BSA modified graphene oxide (GO/AuNCs@BSA) were used as labels of second antibody. In order to characterize the performance of the ECL immunosensor, carcino embryonie antigen (CEA) was used as the model to complete the experiments. Due to the good performances of NP-NiGd-Ni2O3-Gd2O3@Au (high surface area, excellent electron conductivity) and AuNCs@BSA (low toxicity, biocompatibility, easy preparation and good water solubility), the ECL immunosensor exhibited a wide range from 10(-4) to 5ng/mL with a detection limit of 0.03pg/mL (S/N=3). The immunosensor with excellent stability, acceptable repeatability and selectivity provided a promising method to detect CEA in human serum sample sensitively.

A New Crystal Structure of Au36 with a Au14 Kernel Cocapped by Thiolate and Chloride.

This study presents a new crystal structure of a gold nanocluster coprotected by thiolate and chloride, with the formula of Au36(SCH2Ph-(t)Bu)8Cl20. This nanocluster is composed of a Au14 core with two Cl atoms, a pair of pentameric Au5(SCl5) staple motifs, and a pair of hexameric Au6(S3Cl4) motifs. It is noteworthy that the "Au-Cl-Au" staple motifs are observed for the first time in thiolate protected gold nanoclusters. More importantly, the formation of the Cl-Au3 motifs is found to be mainly responsible for the configuration of the gold nanocluster. This work will offer a new perspective to understand how the ligands affect the crystal structure of gold nanocluster.

Effect of quencher, denaturants, temperature and pH on the fluorescent properties of BSA protected gold nanoclusters

In this paper, we have synthesized BSA protected gold nanoclusters (BSA Au nanocluster) and studied the effect of quencher, protein denaturant, pH and temperature on the fluorescence properties of the tryptophan molecule of the BSA Au nanocluster and native BSA. We have also studied their effect on the peak emission of BSA Au nanoclusters (650nm). The photophysical characterization of a newly developed fluorophore in different environments is absolutely necessary to futher develop their biomedical and analytical applications. It was observed from our experiments that the tryptophan in BSA Au nanoclusters is better shielded from the polar environment. Tryptophan in native BSA showed a red shift in its peak emission wavelength position. Tryptophan is a highly polarity sensitive dye and a minimal change in its microenvironment can be easily observed in its photophysical properties.

Fluorescence switching method for cascade detection of salicylaldehyde and zinc(II) ion using protein protected gold nanoclusters.

A new fluorescence switching sensor for cascade detection of salicylaldehyde (SA) and Zinc(II) ion was developed based on bovine serum albumin protected gold nanoclusters (BSA-AuNCs). In the detection, SA interacted with amino groups of BSA-AuNCs, inducing simultaneous formation of fluorescent Schiff base and fluorescence quenching of AuNCs. Zn(II) could further strongly coordinate with the Schiff base ligands, leading to blue-shift and increase of the fluorescence from Schiff base-metal coordination complexes and simultaneous recovery of fluorescence from AuNCs. The new fluorescence switching sensor for Zn(2+) detection has advantages of simplicity, rapidity, naked-eye detection, high sensitivity and selectivity. The linear range of the method for Zn(2+) detection is from 0.1 μM to 100 μM with the limit of detection (LOD) of 29.28 nM. In practical samples, the recoveries of the samples ranged from 99.63% to 100.58%.

Ratiometric fluorescence detection of tyrosinase activity and dopamine using thiolate-protected gold nanoclusters.

In this work, a sensitive and selective ratiometric fluorescence sensing platform was built for the detection of tyrosinase (TYR) activity and dopamine (DA) using glutathione (GSH) protected gold nanoclusters (Au NCs) as probes. Upon excitation at 350 nm, Au NCs displayed an intense red emission, which could be effectively quenched by quinones. TYR, a typical polyphenol oxidase, can catalyze the oxidization of DA to o-quinone and therefore quenched the fluorescence of Au NCs. Moreover, the reaction of TYR and DA gave rise to an emission band at 400 nm, which increased in a TYR/DA-concentration-dependent manner. The ratiometric signal variations were utilized for facile, sensitive, and selective detection of TYR activity and DA. A linear range was obtained from 0.006-3.6 unit mL(-1) of TYR activity, while the linear range for detection of DA was 1.0 nM to 1.0 mM. Additionally, it constructed a useful platform for TYR inhibitor screening in biomedical research.

Fluorescence sensitization of gold-glutathione nanoclusters by aqueous solutions of sodium and potassium ions

Gold nanoclusters (Au-NCs) are powerful alternatives to commonly used dyes for use in fluorescent assays and in imaging. This is due to their high brightness and biocompatibility. Glutathione protected gold nanoclusters (Au-NCs@GSH) were used to investigate the effect of sodium and potassium ions (at 0.1 to 1 mM concentrations) on their fluorescence. It was found that addition of these alkali ions does not modify the emission wavelength of AuNCs@GSH, but fluorescence intensity undergoes a gradual increase (by ~130 %) until a plateau is reached. Fluorescence is particularly enhanced by KSCN. The effect is interpreted in terms of a decrease in the band gap of the AuNCs@GSH that depends on the nature of the anion. The results illustrate the role of anions and monovalent cations commonly used in buffers with respect to the tuning of the fluorescence of AuNCs@GSH. The findings should be taken into account when using AuNCs@GSH in assays based on fluorescence enhancement or quenching.

Construction of OVA-stabilized fluorescent gold nanoclusters for sensing glucose

Protein protected gold nanoclusters have outstanding physical and chemical properties that make them excellent scaffolds for the construction of novel chemical and biological probes. In this study, a simple one-pot synthesis method was proposed for the preparation of fluorescent probes based on ovalbumin-stabilized gold nanoclusters. This strategy allowed the generation of water-soluble gold nanoclusters within 5 min. The as-prepared fluorescent probe exhibited a red fluorescence emission at 625 nm, and good thermostability. The fluorescent probe was applied to measure glucose concentrations based on the hydrogen peroxide-induced fluorescence quenching principle, and showed favorable biocompatibility, high sensitivity and good selectivity. As a result of the advantageous properties and performance of this fluorescent probe, the present assay allowed for the selective determination of glucose in the range of 5.0x10(-6) to 10.0x10(-3) mol/L with a detection limit of 1.0x10(-6) mol/L. Moreover, the glucose content in urinary samples was analyzed using the constructed fluorescent probe: this indicated the potential of the fluorescent gold nanoclusters for applications in biological and clinical diagnosis and therapy.

Atomically precise metal nanoclusters: stable sizes and optical properties.

Controlling nanoparticles with atomic precision has long been a major dream of nanochemists. Breakthroughs have been made in the case of gold nanoparticles, at least for nanoparticles smaller than ∼3 nm in diameter. Such ultrasmall gold nanoparticles indeed exhibit fundamentally different properties from those of the plasmonic counterparts owing to the quantum size effects as well as the extremely high surface-to-volume ratio. These unique nanoparticles are often called nanoclusters to distinguish them from conventional plasmonic nanoparticles. Intense work carried out in the last few years has generated a library of stable sizes (or stable stoichiometries) of atomically precise gold nanoclusters, which are opening up new exciting opportunities for both fundamental research and technological applications. In this review, we have summarized the recent progress in the research of thiolate (SR)-protected gold nanoclusters with a focus on the reported stable sizes and their optical absorption spectra. The crystallization of nanoclusters still remains challenging; nevertheless, a few more structures have been achieved since the earlier successes in Au102(SR)44, Au25(SR)18 and Au38(SR)24 nanoclusters, and the newly reported structures include Au20(SR)16, Au24(SR)20, Au28(SR)20, Au30S(SR)18, and Au36(SR)24. Phosphine-protected gold and thiolate-protected silver nanoclusters are also briefly discussed in this review. The reported gold nanocluster sizes serve as the basis for investigating their size dependent properties as well as the development of applications in catalysis, sensing, biological labelling, optics, etc. Future efforts will continue to address what stable sizes are existent, and more importantly, what factors determine their stability. Structural determination and theoretical simulations will help to gain deep insight into the structure-property relationships.

A visual sensor array for pattern recognition analysis of proteins using novel blue-emitting fluorescent gold nanoclusters.

This paper describes a visual sensor array for pattern recognition analysis of proteins based on two different optical signal changes: colorimetric and fluorometric, by using two types of novel blue-emitting collagen protected gold nanoclusters and macerozyme R-10 protected gold nanoclusters with lower synthetic demands. Eight proteins have been well-discriminated by this visual sensor array, and protein mixtures after one-dimensional polyacrylamide gel electrophoresis also could be well-discriminated. The possible mechanism of this sensor array was illustrated and validated by fluorescence spectra, X-ray photoelectron spectroscopy (XPS), fluorescence lifetime, isothermal titration calorimetry (ITC), and matrix-assisted laser desorption/ionization-time-of-flight mass spectrometry (MALDI-TOF MS) experiments. It was attributed to that the adsorption of proteins onto the surface of gold nanoclusters (Au NCs), forming the protein-Au NCs complex. Furthermore, serums from normal and hepatoma patients were also effectively discriminated by this visual sensor array, showing feasible potential for diagnostic applications.

Evidence of energy transfer from tryptophan to BSA/HSA protected gold nanoclusters

This work reports on the chromophores interactions within protein-protected gold nanoclusters. We conducted spectroscopic studies of fluorescence emissions originated from gold nanoclusters and intrinsic tryptophan (Trp) in BSA or HSA proteins. Both steady state fluorescence and lifetime measurements showed a significant Forster Resonance Energy Transfer (FRET) from Trp to the gold nanocluster. Tryptophan lifetimes in the case of protein-protected gold nanoclusters are 2.6 ns and 2.3 ns for BSA and HSA Au clusters while 5.8 ns for native BSA and 5.6 for native HSA. The apparent distances from Trp to gold nanocluster emission center, we estimated as 24.75 Å for BSA and 23.80 Å for HSA. We also studied a potassium iodide (KI) quenching of protein-protected gold nanoclusters and compared with the quenching of BSA and HSA alone. The rates of Trp quenching were smaller in BSA-Au and HSA-Au nanoclusters than in the case of free proteins, which is consistent with shorter lifetime of quenched Trp(s) and lower accessibility for KI. While Trp residues were quenched by KI, the emissions originated from nanoclusters were practically unquenched. In summary, for BSA and HSA Au clusters, we found 55% and 59% energy transfer efficiency respectively from tryoptophan to gold clusters. We believe this interaction can be used to our advantage in terms of developing resonance energy transfer based sensing applications.

Enhancing near IR luminescence of thiolate Au nanoclusters by thermo treatments and heterogeneous subcellular distributions.

A five-to-ten fold enhancement, up to ca. 5-10% quantum efficiency, of near IR luminescence from monothiolate protected gold nanoclusters was achieved by heating in the presence of excess ligand thiols. An emission maximum in the 700-900 nm range makes these Au nanoclusters superior for bioimaging applications over other emissions centered below 650 nm due to reduced background interference, albeit visible emissions could have higher quantum efficiency. The heating procedure is shown to be effective to improve the luminescence of Au nanoclusters synthesized under a variety of conditions using two types of monothiols: mercaptosuccinic acid and tiopronin. Therefore, this heating method is believed to be a generalizable approach to improve the near IR luminescence of aqueous soluble Au nanoclusters, which enables better bioimaging applications. The high quantum yield is found relatively stable over a wide pH range. PEGylation of the Au nanoclusters reduces their quantum efficiency but improves their permeation into the cytoplasm. Interestingly, z-stack confocal analysis clearly reveals the presence of Au nanoclusters inside the cell nucleus in single cell imaging. The finding addresses controversial literature reports and demonstrates the internalization and heterogeneous subcellular distributions, particularly inside the nucleus. The high luminescence intensity, small overall dimension, cell and nuclear distribution, chemical stability and low-to-non toxicity make these Au nanoclusters promising probes for broad cell dynamics and imaging applications.