Encapsulated Gold Nanoclusters Research Articles

X-Ray-Triggered CO-Release from Gold Nanocluster: All-in-One Nanoplatforms for Cancer Targeted Gas and Radio Synergistic Therapy.

Glycolysis-dominant metabolic pathway in cancer cells can promote their therapeutic resistance against radiotherapy (RT). Carbon monoxide (CO) as a glycolysis inhibitor can enhance the efficiency of RT. Herein, an X-ray responsive CO-releasing nanocomposite (HA@AuNC@CO) based on strong host-guest interactions between the radiosensitizer and CO donor for enhanced RT is developed. The encapsulated gold nanoclusters (CD-AuNCs) can effectively generate cytotoxic reactive oxygen species (ROS) under X-ray radiation, which not only directly inactivate cancer cells but also induce in situ CO gas generation from adamantane modified metal carbonyl (Ada-CO) for glycolysis inhibition. Both in vitro and in vivo results demonstrate that HA@AuNC@CO exhibits active targeting toward CD44 overexpressed cancer cells, along with excellent inhibition of glycolysis and efficient RT against cancer. This study offers a new strategy for the combination of gas therapy and RT in tumor treatment.

Schottky-functionalized Z-scheme heterojunction: Improved photoelectric conversion efficiency and immunosensing

Designing photovoltaic materials with good photoelectric activity is the crucial to boost the sensitivity of photoelectrochemical (PEC) biosensors. To meet this concern, a Schottky-functionalized direct Z-scheme heterojunction photovoltaic material was proposed by electrodeposition of gold nanoparticles on two kinds of bismuth oxyhalide composites surface (bismuth oxybromide and bismuth oxyiodide with different but matched band gaps) (depAu/BiOI/BiOBr). Specifically, synergistic effect was achieved through the direct Z-scheme heterojunction formed by BiOBr and BiOI as well as the gold Schottky junction, resulting in the enhanced light harvest and photoelectric conversion efficiency. Meanwhile, combined with sandwich immunotechnology, a “signal-off” PEC biosensor was fabricated for highly sensitive detection of carcinoembryonic antigen (CEA). In which, using depAu/BiOI/BiOBr modified glassy carbon electrodes both as the photoactive sensing interface and capture antibody loading matrix, polyethyleneimine copper complex encapsulated gold nanoclusters labeled detection antibody (Ab2-Au@PEI-Cu) as the quencher, the photocurrent decreased with the increasing target CEA introduced by sandwich immune reaction. The proposed smart PEC immunoassay platform exhibited a wide detection range (1.0 fg/mL-2.0 ng/mL) and a detection limit as low as 0.11 fg/mL with favorable selectivity and stability. In addition, this PEC sensing strategy can be easily extended for other tumor marker analysis, which offers a new perspective of using multiple bismuth oxyhalide as photoactive materials for early diseases diagnosis.

Green Chemical Synthesis of N-Cholyl-L-Cysteine Encapsulated Gold Nanoclusters for Fluorometric Detection of Mercury Ions.

Herein we report a simple, single-step, cost-effective, environmentally friendly, and biocompatible approach using sodium salt of N-cholyl-L-cysteine (NaCysC) capped gold nanoclusters (AuNCs) with green emission properties at above the CMC in aqueous medium under UV-light irradiation. The primary and secondary CMC of NaCysC was found to be 4.6 and 10.7mM respectively using pyrene as fluorescent probe. The synthesized AuNCs exhibit strong emission maxima at 520nm upon excitation at 375nm with a large Stokes shift of 145nm. The surface functionality and morphology of NCs are studied by fourier transform infrared spectroscopy, dymanic light scattering studies and transmission electron microscopy. The formation of AuNCs was completed within 5h and exhibit high stability for more than 6months. The NaCysC templated AuNCs selectively quenches the Hg2+ ions with higher sensitivity in aqueous solution over the other metal ions. The fluorescence analysis of Hg2+ showed a wide linear range from 15 to 120µM and a detection limit was found to be 15nM.

PH-Responsive metal-organic framework encapsulated gold nanoclusters with modulated release to enhance photodynamic therapy/chemotherapy in breast cancer.

Gold nanoclusters (AuNCs) with an ultra-small size, as new inorganic photosensitizers, have been shown to be promising in photodynamic therapy (PDT), but their application has been restricted due to short blood circulation. It is therefore important to develop stimuli-responsive AuNC-based nanoprobes to achieve highly efficient PDT. Here, metal-organic framework (MOF, ZIF-8) encapsulated AuNCs (AuNCs@MOF) were synthesized, and then they were loaded with doxorubicin (DOX) to obtain pH-responsive nanoprobes (AuNCs@MOF-DOX) with modulated release for enhanced PDT/chemotherapy. In an acidic tumor microenvironment, the structure of ZIF-8 collapsed, accelerating the release of the AuNCs and DOX in the tumor cells, and enhancing the performance of PDT/chemotherapy. Under irradiation with a 670 nm laser, a large amount of singlet oxygen was generated, and the release rate of DOX increased to 77.1% at a pH value of 5.5. By single PDT and single chemotherapy, the tumors were only partially inhibited, but they completely disappeared using the combination of PDT and chemotherapy. The prepared pH-responsive AuNCs@MOF-DOX nanoprobes with modulated release showed excellent PDT/chemotherapy performance, and will be important bi-functional nanoprobes for synergistic therapy.

Supramolecules‐Guided Synthesis of Brightly Near‐Infrared Au22 Nanoclusters with Aggregation‐Induced Emission for Bioimaging

AbstractProtein‐protected gold nanoclusters with emission in the near‐infrared wavelength range have been widely considered for applications in biomedical fields. However, their quantum yield (QY) remains low, thus limiting their practical applications. Herein, a novel strategy to synthesize bovine serum albumin–encapsulated gold nanoclusters (BSA‐AuNCs) with QY of 23% is developed. Assembled coordination polymers (supramolecules) of Au(I)‐BSA complexes are initially formed because of the intermolecular forces between BSA ligands. The forces are easily controlled by pH level during the reaction, leading to significant change in the photoluminescence of BSA‐AuNCs. By regulating the pH and reaction temperature, Au(0)@Au(I) core‐shell structured BSA‐AuNCs are fabricated in 2 h. Importantly, such AuNCs are in a rigidified state with high Au(I) content in the shell, offering an explanation for their high luminescence character. Further increasing QY to 29% is achieved by confining BSA‐AuNCs into a cationic polymer, poly(allylamine) hydrochloride (AuNCs@PAH). Enhanced cellular uptake and improved sensitivity of AuNCs@PAH to glutathione compared to BSA‐AuNCs is demonstrated. These findings may give insights into the synergistic effect of pH level and reaction temperature on the properties of protein‐encapsulated AuNCs and provide a possible way for up‐scaled fabrication of brighter AuNCs protected by other protein templates.

Lysozyme encapsulated gold nanoclusters for probing the early stage of lysozyme aggregation under acidic conditions

Protein aggregation can lead to several incurable amyloidosis diseases. The full aggregation pathway is not fully understood, creating the need for new methods of studying this important biological phenomenon. Lysozyme is an amyloidogenic protein which is often used as a model protein for studying amyloidosis. This work explores the potential of employing Lysozyme encapsulated gold nanoclusters (Ly-AuNCs) to study the protein's aggregation. The fluorescence emission properties of Ly-AuNCs were studied in the presence of increasing concentrations of native lysozyme and as a function of pH, of relevance in macromolecular crowding and inflammation-triggered aggregation. AuNC fluorescence was observed to both redshift and increase in intensity as pH is increased or when native lysozyme is added to a solution of Ly-AuNCs at pH 3. The long (μs) fluorescence lifetime component of AuNC emission was observed to decrease under both conditions. Interestingly it was found via Time-Resolved Emission Spectra (TRES) that both AuNC fluorescence components increase in intensity and redshift with increasing pH while only the long lifetime component of AuNC was observed to change when adding native lysozyme to solution; indicating that the underlying mechanisms for the changes observed are fundamentally different for each case. It is possible that the sensitivity of Ly-AuNCs to native lysozyme concentration could be utilized to study early-stage aggregation.

A fluorescence turn-on biosensor based on transferrin encapsulated gold nanoclusters for 5-hydroxytryptamine detection

In the past few decades, protein-templated fluorescent gold nanoclusters have attracted much attention in the field of biosensing as their excellent optical properties and good biocompatibility. In this work, a fluorescence turn-on biosensor was fabricated for the sensitive detection of 5-hydroxytryptamine (5-HT) with transferrin encapsulated gold nanoclusters (Tf-Au NCs). The Tf-Au NCs were facilely prepared with the assistance of microwave radiation in several minutes and exhibited good monodispersity as well as strong red-emission. It was demonstrated that an aggregation-enhanced emission of Tf-Au NCs was triggered by 5-HT due to the high affinity between 5-HT and the sialic acid (SA) residues of transferrin. Based on these findings, a highly selective turn-on fluorescent sensor was fabricated for the quantitative determination of 5-HT in the range of 0.2–50 μM (R2 = 0.994) with a detection limit of 0.049 μM (S/N = 3). The developed sensor was also successfully applied for the 5-HT detection in human serum with satisfactory recoveries of 96.20–108.6%.

Study of Glucose Binding Protein Encapsulated Gold Nanoclusters by Molecular Dynamic Simulation

Protein encapsulated gold nanoclusters has attracted great attention for their excellent fluorescent properties and potential biomedical applications. Glucose Binding Protein (GBP) has a high sensitivity and selectivity to glucose binding that makes them ideal for biosensor development. It is anticipated that GBP encapsulated gold nanoclusters could be a promising glucose sensor. Here we investigated the growth of gold nanoclusters in GBP using Molecular Dynamics (MD) simulation. To facilitation the nucleation of gold nanoclusters at specific sites, cysteine mutations were introduced in GBP. It is found that the nucleation site of gold nanoclusters inside mutant GBP are different from those in native GBP. Gold nanoclusters were formed near the mutated cysteine and tyrosine residues. Glucose remained in the binding site of a mutant GBP with gold nanoclusters although no conformational change was observed in MD simulation, similar to a native GBP. This work suggests the possibility of growing gold nanoclusters in the designed site within GBP and a new glucose sensor based on mutated GBP protected gold nanoclusters.

Aprotinin Encapsulated Gold Nanoclusters: A Fluorescent Bioprobe with Dynamic Nuclear Targeting and Selective Detection of Trypsin and Heavy Metal.

Fluorescence imaging has currently emerged as one of the most frequently used noninvasive imaging technologies to selectively monitor biological processes in living systems. In past decades, gold nanoclusters (Au NCs) has received increasing attraction because of their intrinsic fluorescence and their inherent biocompatibility. As a stabilizing and reducing agent, an abundant, sustainable, and widely used polypeptide derived drug molecule, aprotinin (Ap), is selected for the synthesis of Au nanoclusters (Ap-Au NCs) due to characteristic bioactivity, excellent biocompatibility, biodegradability, and non-allergenic character. Herein, Ap encapsulated Au NCs with desirable red fluorescence was facilely produced for the first time, which were subsequently used for cell imaging and detection of various analytes. Much interestingly, dynamically subcellular targeting from the cytoplasm to the nucleus in HeLa cells was observed. Besides, it has shown that, the selective and quantitative detection of trypsin has been established by usingAp-Au NCs.Finally, Ap-Au NCs were readily used for quantitative detection of mercury and copper. The photoluminescence of the Ap-Au NCs was quenched with the addition of theaforementioned analytes. This study not onlydiscusses a multifunctional nanomaterial for cell imaging, dynamicallynuclear targeting and biosensing, but also opens crucial insights on the integration of funtional biomolecule with metal nanoclusters intendedfor extensively biomedical applications.

Polyallylamine hydrochloride coating enhances the fluorescence emission of Human Serum Albumin encapsulated gold nanoclusters

Protein encapsulated gold nanoclusters have received much attention due to the possibility of using them as a non-toxic fluorescent probe or marker for biomedical applications, however one major disadvantage currently is their low brightness and quantum yield in comparison to currently used fluorescent markers. A method of increasing the fluorescence emission of Human Serum Albumin (HSA) encapsulated gold nanoclusters (AuNCs) via a Polyallylamide hydrochloride (PAH) coating is described. PAH molecules with a molecular weight of ~17,500 Da were found to enhance the fluorescence emission of HSA-AuNCs by 3-fold when the protein/polymer concentration ratio is 2:1 in solution. Interestingly, the fluorescence lifetime of the AuNCs was found to decrease while the native tryptophan (TRP) fluorescence lifetime also decreased during the fluorescence emission intensity enhancement caused by the PAH binding. Coinciding with the decrease in fluorescence lifetime, the zeta potential of the system was observed to be zero during maximum fluorescence intensity enhancement, causing the formation of large aggregates. These results suggest that PAH binds to the HSA-AuNCs acting as a linker; causing aggregation and rigidification, which results in a decrease in separation between native TRP of HSA and AuNCs; improving Förster Resonance Energy Transfer (FRET) and increasing the fluorescence emission intensity. These findings are critical to the development of brighter protein encapsulated AuNCs.

Lysozyme encapsulated gold nanoclusters: effects of cluster synthesis on natural protein characteristics.

The study of gold nanoclusters (AuNCs) has seen much interest in recent history due to their unique fluorescence properties and environmentally friendly synthesis method using proteins as a growth scaffold. The differences in the physicochemical properties of lysozyme encapsulated AuNCs in comparison to natural lysozyme are characterised in order to determine the effects AuNCs have on natural protein behaviour. The hydrodynamic radius (dynamic light scattering), light absorbance (UV-Vis), electrophoretic mobility, relative density, dynamic viscosity, adsorption (quartz crystal microbalance) and circular dichroism (CD) characteristics of the molecules were studied. It was found that lysozyme forms small dimer/trimer aggregates upon the synthesis of AuNCs within the protein. The diameter of Ly-AuNCs was found to be 8.0 nm across a pH range of 2-11 indicating dimer formation, but larger aggregates with diameters >20 nm were formed between pH 3 and 6. The formation of larger aggregates limits the use of Ly-AuNCs as a fluorescent probe in this pH range. A large shift in the protein's isoelectric point was also observed, shifting from 11.0 to 4.0 upon AuNC synthesis. This resulted in major changes to the adsorption characteristics of lysozyme, observed using a QCM. A monolayer of 8 nm was seen for Ly-AuNCs at pH 4, offering further evidence that the proteins form small aggregates, unlike the natural monomer form of lysozyme. The adsorption of Ly-AuNCs was seen to decrease as pH was increased; this is in major contrast to the lysozyme adsorption behaviour. A decrease in the α-helix content was observed from 25% in natural lysozyme to 1% in Ly-AuNCs. This coincided with an increase in the β-sheet content after AuNC synthesis indicating that the natural structure of lysozyme was lost. The formation of protein dimers, the change in the protein surface charge from positive to negative, and secondary structure alteration caused by the AuNC synthesis must be considered before attempting to utilise Ly-AuNCs as in vivo probes.

Detection of ricin by using gold nanoclusters functionalized with chicken egg white proteins as sensing probes

Ricin produced from the castor oil plant, Ricinus communis, is a well-known toxin. The toxin comprises A and B chains. Ricin A chain can cause toxicity by inhibiting protein synthesis, and ricin B can bind to the galactose ligand on the cell membrane of host cells. Inhalation or ingestion of ricin may even lead to death. Therefore, rapid and convenient sensing methods for detecting ricin in suspicious samples must be developed. In this study, we generated protein encapsulated gold nanoclusters (AuNCs@ew) with bright photoluminescence by using chicken egg white proteins as starting materials to react with aqueous tetrachloroaurate. The generated nanoclusters, which were mainly composed of chicken ovalbumin-encapsulated AuNCs, can recognize ricin B because of the presence of Galβ(1→4)GlcNAc ligands on chicken ovalbumin. The generated conjugates of AuNCs@ew and ricin B were heavy and readily settled down under centrifugation (13,000rpm, 60min). Thus, bright spots resulting from the conjugates at the bottom of the sample vials were easily visualized by the naked eye under ultraviolet light illumination. The limit of detection (LOD) was ~4.6µM. The LOD was reduced to ~400nM when fluorescence spectroscopy was used as the detection tool, while the LOD can be further improved to ~7.8nM when using matrix-assisted laser desorption/ionization mass spectrometry as the detection method. We also demonstrated the feasibility of using the proposed approach to selectively detect ricin B chain in complex samples.

Human serum albumin encapsulated gold nanoclusters: effects of cluster synthesis on natural protein characteristics.

The differences in the physiochemical properties between native Human Serum Albumin (HSA) and HSA encapsulated gold nanoclusters (HSA-AuNCs) are characterised. The light absorbance (UV-Vis), electrophoretic mobility, dynamic viscosity, density, hydrodynamic radius (DLS), absorption (QCM) and chemical bonding (XPS) characteristics of the molecules were studied. The UV-Vis and DLS data show the formation of large aggregates for HSA-AuNCs between pH 4-6 which is not observed for native HSA. This observation was further supported by QCM measurements showing a large increase in mass adsorbed at pH 6 between HSA and HSA-AuNCs. The DLS data also revealed a hydrodynamic diameter of 12 nm for HSA-AuNCs, nearly double that of 7 nm for native HSA at pH higher than 6, suggesting the formation of compact HSA-AuNCs dimers. The electrophoretic mobility data for both HSA-AuNCs and HSA were converted to zeta potentials. The zeta potential of HSA-AuNCs was seen to be more negative between pH 6-12, suggesting that the protein surface interacts with unreacted gold salt anions. Measurements of density and viscosity were also found to be in agreement with previous data suggesting that HSA-AuNCs form aggregates. XPS data also suggest that not all reactants are used up during the HSA-AuNC synthesis and positive side chains play a part in the initial synthesis stages. It was concluded that HSA-AuNCs most likely form dimers at natural and high pH. Between pH 4-6 HSA-AuNCs form very large aggregates limiting their use as fluorescent probes in this pH range. It was also found that the native characteristics of HSA are altered upon HSA-AuNC synthesis which needs to be taken into consideration when applying HSA-AuNCs as fluorescent probes in all fluorescent imaging and sensing.

Hidden Dityrosine Residues in Protein-Protected Gold Nanoclusters

The protein ligand shells of fluorescent protein-protected gold nanoclusters play an important role in the physiochemical properties and sensing applications of the nanoclusters. Recently, more and more attention has been paid to the investigation of the changes in the protein structure elements induced by the introduction of the nanoclusters in the proteins. In this work, the strategy of removal of the encapsulated gold nanoclusters from the protein ligand cages has been proposed, producing the “hollow” (or possibly “imprinted”) proteins for investigations for the first time. Nontoxic cysteamine was used as the etchant of the gold nanoclusters. With bovine serum albumin, lysozyme, and ovalbumin as model proteins, it was found that the luminescent dityrosine cross-links exist in the protein-protected gold nanoclusters, however, inner filter effect caused by the gold nanoclusters hide them.

Locating the nucleation sites for protein encapsulated gold nanoclusters: a molecular dynamics and fluorescence study.

Fluorescent gold nanoclusters encapsulated by proteins have attracted considerable attention in recent years for their unique properties as new fluorescence probes for biological sensing and imaging. However, fundamental questions, such as the nucleation sites of gold nanoclusters within proteins and the fluorescence mechanism remain unsolved. Here we present a study of the location of gold nanoclusters within bovine serum albumin (BSA) combining both fully atomistic molecular dynamic (MD) simulations and fluorescence spectroscopic studies. The MD simulations show gold clusters growing close to a number of cysteine sites across all three domains of BSA, although just two major sites in domains IIB and IA were found to accommodate large clusters comprising more than 12 atoms. The dependence of the fluorescence on pH is found to be compatible with possible nucleation sites in domains IIB and IA. Furthermore, the energy transfer between tryptophan and gold nanoclusters reveals a separation of 29.7 Å, further indicating that gold nanoclusters were most likely located in the major nucleation site in domain IIB. The disclosure of the precise location of the gold nanoclusters and their surrounding amino acid residues should help better understanding of their fluorescence mechanism and aid their optimization as fluorescent nanoprobes.

Preparation of polymer incarcerated gold nanocluster catalysts (PI-Au) and their application to aerobic oxidation reactions of boronic acids, alcohols, and silyl enol ethers

Heterogeneous gold nanocluster catalysts immobilized by the method known as polymer incarceration were prepared. Polystyrene-derived polymers with epoxide and alcohol moieties, which could be cross-linked under heating conditions, were employed as supports for their preparation. Cationic gold salts were reduced in a solution of NaBH4 and the polymers. Poor solvents for the polymers were added, and the polymers were precipitated and encapsulated gold nanoclusters with weak but multiple interactions between a gold nanocluster surface and the π electrons of benzene rings. The polymer capsules were heated under neat conditions to afford heterogeneous gold nanocluster catalysts; namely, polymer-incarcerated gold nanoclusters. The catalysts thus prepared could be applied to the aerobic oxidation of phenyl boronic acids, alcohols, and silyl enol ethers. We found that the choice of polymers, good and poor solvents for the polymers, metal loadings, heating conditions for cross-linking, and final activation were all crucial for obtaining high-activity catalysts.

IRGD-coupled responsive fluorescent nanogel for targeted drug delivery

In this investigation, we have designed and synthesized a multifunctional nanogel for anti-tumor drug delivery. Thermo- and pH-responsive poly (N-isopropyl acrylamide-co-acrylic acid) nanogels (NGs) were synthesized by free radical precipitation polymerization. Positive charged chemotherapeutic drug doxorubicin (DOX) was introduced into the negatively charged swollen NGs by electrostatic adsorption at pH 7.4. Fluorescent bovine serum albumin (BSA) encapsulated gold nanoclusters (AuNCs) were conjugated onto the surface of NGs, followed by functionalization of tumor targeting peptide iRGD onto the BSA for tumor targeting. Interestingly, the DOX-encapsulated iRGD-decorated NGs maintain both thermo- and pH-responsive properties, which are favorable for achieving a controlled drug release in tumor tissues. Stable red fluorescent emission, derived from AuNCs, was used to detect and track the NGs in vitro. As expected, the iRGD motif mediated specific targeting to tumor and endothelial cells and enhanced cellular uptake of the NGs, as demonstrated by flow cytometry and confocal microscopy assays. In vitro cytotoxicity studies proved that the presence of iRGD enhanced the cytotoxic efficiency of DOX to the targeted cells. All together, our current study indicates that the NGs drug-carriers can deliver chemotherapeutic drug specifically to the tumor and endothelial cells with enhanced anti-tumor efficacy and controlled drug release.

Synthesis and Characterization of Water Soluble Gold Nanoclusters of Varied Core Size

A series of water soluble gold nanoclusters of variable core size were prepared and characterized. The clusters were synthesized using a ligand substitution reaction between CH3(OCH2CH2)3SH and hexanethiol encapsulated gold nanoclusters, and characterized using NMR, FTIR, and UV/Vis spectroscopy, as well as TGA, DSC, and TEM. Conductivity values were obtained for the clusters and increased as expected along with the gold core size. The substitution reaction used in the synthesis was followed by 1H NMR and found to produce only partial ligand substitution after 24 h, consistent with solubility observations and necessitating use of a two-step preparation scheme.