Water-soluble Clusters Research Articles

Microwave-assisted one-step synthesis of water-soluble manganese-carbon nanodot clusters

Using metal coordination to assemble carbon nanodots (CND) into clusters can enhance their photophysical properties for applications in sensing and biomedicine. Water-soluble clusters of CNDs are prepared by one-step microwave synthesis starting from ethylenediaminetetraacetic acid, ethylenediamine and MnCl2·4H2O as precursors. Transmission electron microscopy and powder X-Ray diffraction techniques indicate that the resulting clusters form spherical particles of 150 nm constituted by amorphous CNDs joined together with Mn ions in a laminar crystalline structure. The nanomaterial assemblies show remarkable fluorescence quantum yields (0.17–0.20) and magnetic resonance imaging capability (r1 = 2.3-3.8 mM–1.s–1). In addition, they can be stabilized in aqueous solutions by phosphate ligands, providing a promising dual imaging platform for use in biological systems.

(Digital Presentation) Fabrication of Monolayer-Protected Gold Cluster Thin Films: Electrochemical and Optical Properties

Quantum-sized monolayer-protected gold clusters have captivated the imagination of scientists over the last two decades and show exciting optical, electrochemical and catalytic properties when compared to larger-sized plasmonic gold nanoparticles, making them useful for myriad new applications in photonics, catalysis and sensors. One interesting property is the photoluminescence (PL) of gold clusters making them suitable as biological imaging labels. However, the low PL Quantum Yields (QY) are a concern for these clusters and research is focused on improving the PL QYs. One strategy, our research group has worked on is the shell-gold rigidification via the use of surfactants and proteins. Shell ligand rigidification was achieved by ion pairing with bulky tertiary alkyl ammonium groups and aromatic chromophores. Another strategy to reduce the non-radiative deactivation pathways is by making the gold cluster thin films. In a recent study, it was shown that solid state films of clusters not only have manifold increase in PL quantum yield but also their luminescent lifetimes and two photon absorption cross-sections are greatly enhanced in comparison to their solution analogues. Researchers have worked on various strategies to make gold cluster thin films for device applications. One problem with thin films of gold clusters is that the clusters lose their interesting optical and electrochemical properties that were observed in solution phase. For making the clusters useful by enhancing PL for photonic device applications, it is important for the clusters to retain their properties in solid state, which is the main objective of the study carried out here. One problem is that drying intrinsic gold clusters results in crystallization whose optical properties are difficult to characterize. In this study, we have used Nafion and polyvinylpyrrolidone (PVP) to make thin films of water-soluble glutathione-protected gold clusters. Water soluble cluster Au22(SG)18 with reported PLQY of 8% were synthesized and characterized by UV-VIS absorption and PL measurements and thin films of these clusters were made using water soluble PVP and Nafion polymers via drop casting. Enhanced PL and electrochemical properties were observed for clusters in solid state. Similarly, non-polar gold cluster films are fabricated with Polystyrene and optical absorption and PL properties were retained in the solid state.Although drop casting with polymers was found to retain the optical properties, the uniformity of films is a problem. The uniformity of the film is dependent on solvent’s surface tension thus diminishing the quality of the film. Therefore, there is a need of depositing films of gold clusters in a controlled manner which governs their thickness and uniformity. Electropolymerization technique is known to make films during the polymerization of monomers possessing functional groups whose oxidation or reduction via anti-bonding or non-boding electrons on a heteroatom. In this study, we have used electropolymerization strategy to fabricate thin films of gold clusters on conducting substrates. The film is deposited at the working electrode during cyclic voltammetric (CV) sweep when passed through the required reduction or oxidation potential of the monomer under suitable conditions. This technique makes a uniform film throughout the exposed area of the working electrode active surface and the thickness of the film is often proportional to the number of sweep cycles employed. Therefore, introduction of gold clusters in this process with monomers which are previously known to bond with each other will incorporate clusters in the film during the polymerization process and will ease their film formation with uniform thickness. Amongst various size gold clusters, the one which shows good stability as a result of following the magic number series is Au25(SR)18 - . The stability of clustersis one of the important parameter required for practical applications. Aniline and other amine based monomers are used as precursors to make thin films of Au25 on conducting substrate such as Indium tin oxide. By solubilizing the gold clusters with the monomers they are deposited along with the polymers.

Synthesis, characterization, and electrochemical properties of new water-soluble Mn12O12(O2CR)16(H2O)4 clusters

We report our attempts to produce water-soluble Mn clusters of the type [Mn12O12(O2CR)16(H2O)4] and the synthesis, spectroscopic, structural, and electrochemical characterization of the three new compounds that were obtained. Clusters 2, 3, and 4 were prepared via substitution of the acetate ligands in [Mn12O12(O2CMe)16(H2O)4] (1) with either 3,4-diaminobenzoic acid, L-proline or L-ascorbic acid, respectively, which are all inexpensive and readily available. Clusters 2, 3, and 4 were characterized by elemental analysis, UV-Vis, and FTIR spectroscopies, XPS, MS, and XRD analysis, suggesting that the clusters retain their structure during the substitution reaction, albeit 4 was obtained partially substituted and reduced. Electrochemical measurements in acetate buffer at pH 6, including continuous cyclic voltammetry scans of the free ligands and of the clusters, imply that 4 is stable to the oxidation process, while in 2 the primary amine ligands are oxidized rapidly, leading to precipitation of the cluster. Overall, the voltammetric measurements support the spectroscopic-based proposed structures.

Water-soluble single source precursors for homo- and hetero-metallic nanoparticle catalysts supported on nanocarbons

Homo- and hetero-metallic catalysts were prepared in water from a single metal source and in a single step, and were tested in cinnamaldehyde selective hydrogenation. Two new water-soluble clusters, namely [Ru5C(CO)14(TPPTS)1] and [Ru5PtC(CO)15(TPPTS)1], were synthesized by the addition of phosphine TPPTS to starting homo- and hetero-metallic clusters soluble in organic solvents. They were then impregnated onto nanocarbons (CNF and CNT) in water at different pH values. Cluster deposition occurred via two adsorption modes and was the most successful when attractive electrostatic interactions took place. Thermal activation of these samples led to heterogeneous catalysts with a small mean particle size between 2 and 4nm. The home-made catalysts showed increased conversions and selectivities (up to 73%) towards the thermodynamically unfavored product cinnamyl alcohol compared to a commercial catalyst. The selectivity was in general higher with CNF support, with less oxygenated function and with bigger particles. The selectivity increased slightly with the addition of platinum to ruthenium but the activity was enhanced tremendously (up to 57% at t=30min).

Facile synthesis of water soluble fluorescent metal (Pt, Au, Ag and Cu) quantum clusters for the selective detection of Fe3+ions as both fluorescent and colorimetric probes

A facile and green synthetic route is developed to prepare water soluble, green-fluorescent Pt, Au, Ag and Cu quantum clusters (QCs), employing innoxious L-histidine as a capping agent. These metal QCs give out emissions at 455–495 nm, with maximum excitation wavelengths ranging between 350 and 401 nm. Interestingly, these histidine-stabilized metal quantum clusters also possess non-linear optical properties like two-photon luminescence, which is detected when they are exposed to a femtosecond laser at 812 nm. Luminescence quantum yields of Pt (2.93%), Au (13.10%), Ag (6.97%) and Cu (8.29%) clusters are found to be distinctly different, and lifetime measurement results suggest that the emissions of metal clusters (4.59–7.39 ns) stem from singlet transitions among different electron energy levels rather than metal–ligand electron transfer behaviors. The Pt, Au, Ag and Cu quantum clusters have average sizes of 1.3–1.9 nm, and are mainly composed of 11, 9, 4 and 7 metal atoms, respectively, according to the electrospray ionization mass spectrometry results. Moreover, the as-prepared metal clusters are sensitive to aqueous ferric ions, both as highly selective fluorescent and colorimetric probes in a low and wide concentration range, and are promising in many biological applications.

Oxygen’s Role in Aqueous Gold Cluster Synthesis

The presence of oxygen in thiolate-protected gold nanoparticle synthesis influences product distribution. Oxygen’s diradical nature underlies this effect, and oxygen can be replaced with radical initiators in the synthesis of organosoluble gold nanoclusters. The role of O2 in the synthesis of water-soluble clusters such as Au102(p-MBA)44, Au25(SR)18, as well as the thiol etching of water-soluble colloidal gold particles is not yet established. Herein it is shown that radicals, either from O2 or from radical initiators such as 4-hydroxy-TEMPO, are necessary components for synthesis of water-soluble thiolate-protected gold nanoclusters, as well as the etching of aqueous colloidal gold by thiols. Furthermore, air-free synthetic routes to water-soluble gold nanoclusters Au102(SR)44 and Au25(SR)18 are described. Overall, the understanding of the role of radicals in the synthesis of water-soluble gold clusters will allow standardization of often difficult to reproduce syntheses that attract increasing attention...

Molecule-like photodynamics of Au102(pMBA)44 nanocluster.

Photophysical properties of a water-soluble cluster Au102(pMBA)44 (pMBA = para-mercaptobenzoic acid) are studied by ultrafast time-resolved mid-IR spectroscopy and density functional theory calculations in order to distinguish between molecular and metallic behavior. In the mid-IR transient absorption studies, visible or near-infrared light is used to electronically excite the sample, and the subsequent relaxation is monitored by studying the transient absorption of a vibrational mode in the ligands. Based on these studies, a complete picture of energy relaxation dynamics is obtained: (1) 0.5-1.5 ps electronic relaxation, (2) 6.8 ps vibrational cooling, (3) intersystem crossing from the lowest triplet state to the ground state with a time constant 84 ps, and (4) internal conversion to the ground state with a time constant of ∼3.5 ns. A remarkable finding based on this work is that a large cluster containing 102 metal atoms behaves like a small molecule in a striking contrast to a previously studied slightly larger Au144(SC2H4Ph)60 cluster, which shows relaxation typical for metallic particles. These results therefore establish that the transition between molecular and metallic behavior occurs between Au102 and Au144 species.

Synthesis of β-cyclodextrin and poly(amido amine) dendron click cluster and its synergistic binding property

In this study, we have successfully synthesized the click cluster of β-cyclodextrin (β-CD) and NH2-terminated poly(amido amine) (PAMAM) dendrons via efficient Cu(I)-catalyzed azide–alkyne cycloaddition (CuAAC). The highly water-soluble click cluster, namely CDG2H, was carefully characterized by various analytical tools to confirm the structure and product purity. 1H NMR further resolved the degree of protonation at different pH, suggesting that a tetracationic peripheral attached to the primary face of β-CD at physiological condition. This polycationic character allows CDG2H to effectively bind with anionic targets through a synergistic effect combining host–guest and electrostatic interactions. The UV–visible absorption and fluorescence spectroscopic analyses both indicated that CDG2H has relatively higher binding affinity toward anionic binaphthyl model compounds than unmodified β-CD. In addition, because of the optically active cyclodextrin core, CDG2H still revealed significant chiral discrimination for enantiomeric binaphthyl pairs.

Electrochemical Characterization of Water-Soluble Au25 Nanoclusters Enabled by Phase-Transfer Reaction.

We report the synthesis and electrochemical characterization of a new water-soluble Au25 cluster protected with (3-mercaptopropyl)sulfonate. The presence of sulfonate terminal groups on the surface of the cluster enabled facile phase transfer of the water-soluble cluster to organic phase by ion-pairing with hydrophobic counterions. The phase-transferred form of the water-soluble Au25 cluster was found to retain its integrity and allowed investigation of its electrochemical properties in organic media. The voltammetric investigation of the phase-transferred Au25 cluster in mixtures of CH2Cl2 and toluene has revealed that the cluster exhibits the characteristic Au25 peak pattern, but the electrochemical HOMO-LUMO energy gap of the cluster varies from 1.39 to 1.66 V depending on the solvent polarity. The origin of the solvent dependence is explained by the electrostatic field effect of the sulfonate anion on the redox potentials of the Au25 cluster.

A water soluble and air-stable tripalladium cluster

Complexation of 2,7-dipyridin-2-ylnaphthyridine (bpnp) with a mixture of (PhCN)(2)PdCl(2) and Pd(2)(dba)(3) resulted in the formation of a water soluble and air-stable tripalladium cluster [Pd(3)(bpnp)(2)Cl(2)]Cl(2), which was characterized by NMR spectroscopic and X-ray crystallographic analyses.

Characterizing individual Au25(SG)18 clusters within a nanopore detector

ABSTRACTMetallic quantum clusters are stable structures that can exhibit many useful properties. Clusters can be ligand stabilized in aqueous environments to expand their usefulness as biosensors. There are some limitations in characterizing the physical and chemical properties of individual water soluble clusters. This report describes initial results of a new approach for detecting and characterizing individual gold nanoclusters (Au25(SG)18) in an aqueous solution with nanopore-based resistive pulse sensing. Here the nanopore is a single alpha hemolysin from Staphylococcus aureus. Clusters that enter through the cis side of the pore (large vestibule) usually create shallow current blockades with a mean residence time of several milliseconds. Clusters that enter through the trans side of the pore (narrow lumen) create deeper blockades that are either very short (∼200 μs), long lived (∼50 ms) or trapped (>10s). The short and long lived blockades yield sufficient statistics to help characterize the clusters and the trapped state events may allow for additional analysis and controlled delivery of individual clusters. We demonstrate the possibility of this additional analysis by performing I-V measurements on individually trapped clusters. These show an optimal voltage for confining a cluster within the pore.

Stabilities of cubane type [Fe 4S 4(SR) 4] 2− clusters in partially aqueous media

The stability of cubane-type [Fe 4S 4(SR) 4] 2− clusters in mixed organic/aqueous solvents was examined as an initial step in the development of stable water-soluble cluster compounds possibly suitable for reconstitution of scaffold proteins in protein biosynthesis. The research involves primarily spectrophotometric assessment of stability in 20–80% Me 2SO/aqueous media (v/v), from which it was found that conventional clusters tend to be stable for up to 12 h in 60% Me 2SO but are much less stable at higher aqueous content. α-Cyclodextrin mono- and dithioesters and thiols were prepared as ligand precursors for cluster binding, which was demonstrated by spectroscopic methods. A potentially bidentate cyclodextrin dithiolate was found to be relatively effective for cluster stabilization in 40% Me 2SO, suggesting (together with earlier results) that other exceptionally large thiolate ligands may promote cluster stability in aqueous media.

Luminescent quantum clusters of gold in transferrin family protein, lactoferrin exhibiting FRET

We report the synthesis of highly luminescent, water soluble quantum clusters (QCs) of gold, which are stabilized by an iron binding transferrin family protein, lactoferrin (Lf). The synthesized AuQC@Lf clusters were characterized using UV-Visible spectroscopy, X-ray photoelectron spectroscopy (XPS), transmission electron microscopy (TEM), photoluminescence (PL), matrix assisted laser desorption ionization mass spectrometry (MALDI-MS), FTIR spectroscopy and circular dichroism (CD) spectroscopy along with picosecond-resolved lifetime measurements. Detailed investigations with FTIR and CD spectroscopy have revealed changes in the secondary structure of the protein in the cluster. We have also studied Förster resonance energy transfer (FRET) occurring between the protein and the cluster. The ability of the clusters to sense cupric ions selectively at ppm concentrations was tested. The stability of clusters in widely varying pH conditions and their continued luminescence make it feasible for them to be used for intracellular imaging and molecular delivery, particularly in view of Lf protection.

Functionalized Au22 Clusters: Synthesis, Characterization, and Patterning

We synthesized fluorescent, porphyrin-anchored, Au(22) clusters in a single step, starting from well-characterized Au(25) clusters protected with glutathione (-SG) by a combined core reduction/ligand exchange protocol, at a liquid-liquid interface. The prepared cluster was characterized by UV/vis, photoluminescence, X-ray photoelectron spectroscopy (XPS), scanning electron microscopy, elemental analysis, and matrix-assisted laser desorption ionization mass spectrometry. The absence of a 672 nm intraband transition of Au(25) and the simultaneous emergence of new characteristic peaks at 520 and 635 nm indicate the formation of the Au(22) core. An increase in the binding energy of 0.4 eV in Au 4f core-level peaks confirmed the presence of a reduced core size. Quantitative XPS confirmed the Au/S ratio. The presence of a free base, tetraphenylporphyrin (H(2)TPPOAS-), on the Au(22) core was confirmed by fluorimetric titrations with Cu(2+) and Zn(2+) ions. From all of these, the composition of the cluster was determined to be Au(22)[(-SG)(15)(-SAOPPTH(2))(2)], which was supported by mass spectrometry and elemental analysis. We utilized the fluorescence nature of these water-soluble clusters for the fabrication of fluorescent patterns by soft lithography. The patterns were studied using tapping-mode atomic force microscopy and confocal fluorescence imaging.

Universal scattering behavior of coassembled nanoparticle-polymer clusters

Water-soluble clusters made from 7-nm inorganic nanoparticles have been investigated by small-angle neutron scattering. The internal structure factor of the clusters was derived and exhibited a universal behavior as evidenced by a correlation hole at intermediate wave vectors. Reverse Monte Carlo calculations were performed to adjust the data and provided an accurate description of the clusters in terms of interparticle distance and volume fraction. Additional parameters influencing the microstructure were also investigated, including the nature and thickness of the nanoparticle adlayer.

Colloidal silver nanoparticles stabilized by a water-soluble triosmium cluster

Silver nanoparticles stabilized by the water soluble triosmium cluster Os 3(μ-H)(CO) 10S(CH 2) 10COO]Na were prepared by both photochemical and chemical reduction of silver nitrate. The silver nanoparticles were characterized by UV–Vis spectroscopy and high resolution TEM. The particles obtained by chemical reduction showed remarkable stability.

The water-soluble cluster cation [H3Ru3(C6H6)(C6Me6)2(O)]+: Improved synthesis, aerobic oxidation, electrochemical properties and ligand exchange studies

The synthesis of the trinuclear cluster cation [H3RU3(C6H6)(C6Me6)(2)(O)](+) (1) has been considerably improved by changes in the NaBH4 addition step and by introducing chromatographic methods; in addition, the redox and ligand exchange properties of 1 have been studied. Although exposure of ail aqueous solution of 1 to air yields the oxidised cluster [H2RU3(C6H6)(C6Me6)(2)(O)(OH)](+) (2), cyclic voltammetry of [1][BF4] in acetonitrile reveals a first reversible oxidation step that does not involve 2. Bulk electrolysis of I and 2 in the same medium affords only decomposition products. Ligand exchange in I takes place only at higher temperatures: by heating a mixture of toluene with ail aqueous solution of [1][BF4] (1000: 1) to 110 degrees C for 2 h, the formation of the toluene derivative [H3RU3(C6H5Me)(C6Me6)(2)(O)](+) (3) is observed in small quantities. H/D exchange of 1 with D2O does not occur up to 90 degrees C however, in the presence of D-2, H/D exchange with 1 is observed to give the deuterated derivative [D3RU3(C6H6)(C6Me6)(2)(O)](+) (1a). The results provide ail improved synthesis of 1, as well as information about its redox and ligand-exchange reactions, results necessary to understand and develop the chemistry of 1.

Fluorescent silver oligomeric clusters and colloidal particles

Highly luminescent single colloidal silver nanoparticles under blue excitation were detected. The intense luminescence observed from the single particles is attributed to different emissive centers. Picosecond lifetimes of those single nano-objects have been measured. Comparison with colloidal particles emission formed in aqueous solution, using polyacrylate polyanion as stabilizer is drawn. The optical properties of silver colloidal particles in solution are observed while their surface is changed by deposition of silver ion. A large visible emission under UV or surface plasmon region excitations is observed for Ag n @Ag +, while no fluorescence emission was found for Ag n or Ag + systems. The intense visible light emission changes should primarily come from chemical surface active sites. It was observed that the oligomeric clusters, composed of only few atoms Ag 4 + x x + and Ag 7 3 + , have a substantial emission in the same visible region. A model which ascribes the photoemission of the chemically modified silver colloidal particle to oligomeric clusters on the Ag particle surface is proposed. Those emissive water soluble silver clusters have potential for biological labelling, and light emitting sources in nanoscale optoelectronics applications.

Solution properties, electrochemical behavior and protein interactions of water soluble triosmium carbonyl clusters

The synthesis and solution chemistry of the water soluble clusters [Os 3(CO) 9(μ-η 2-Bz)(μ-H)L +] (HBz=quinoxaline, L +=[P(OCH 2CH 2NMe 3) 3I 3], 1) along with its negatively charged analog [Os 3(CO) 9(μ-η 2-Bz)(μ-H)L −] (L −=[P(C 6H 4SO 3) 3Na 3], 2) are reported. In addition, we have examined the reduction potentials of the complexes [Os 3(CO) 9(μ-η 2-Bz)(μ-H)L] (HBz=phenanthridine, L=L + ( 3); HBz=5,6 benzoquinoline, L=L + ( 4); HBz=3-amino quinoline, L=L + ( 5); HBz=3-amino quinoline, L=L − ( 6). The neutral analog of 1 and 2 [Os 3(CO) 9(μ-η 2-Bz)(μ-H) PPh 3] (Bz=quinoxaline, 7) was also examined for comparison. Both compounds 1 and 2 show pH dependent NMR spectra that are interpreted in terms of the extent of protonation of the uncoordinated quinoxaline nitrogen which impacts the degree of aggregation of the clusters in aqueous solution. Compound 1 undergoes a reversible 1e − reduction in water while 2 undergoes a quasi-reversible 1e − reduction at more negative potentials as expected from the difference in charge on the phosphine ligand. Compound 7 undergoes a marginally reversible CV in methylene chloride at a potential intermediate between the positively and negatively charged clusters. The overall stability of the radical anions of 1, 2 and 7 is somewhat less than the corresponding decacarbonyl [Os 3(CO) 10(μ-η 2-Bz)(μ-H)] (HBz=quinoxaline). While complexes 1 and 2 show reversible 1e − reductions, all the other complexes examined show 1e − and/or two 1e − irreversible reductions in aqueous and non-aqueous solvents. The potentials for these complexes follow expected trends relating to the charge on the phosphine and the pH of the aqueous solutions. The ligand dependent trends are compared with those of the previously reported corresponding decacarbonyls. The interactions of the positively and negatively charged clusters with albumin have been investigated using the transverse and longitudinal relaxation times of the hydride resonances as probes of binding to the protein. Evidence of binding is observed for both the positive and negative clusters but the positive and negative clusters exhibit distinctly different rotational correlation times. Two additional complexes [Os 3(CO) 9(μ-η 2-Bz)(μ-H)L] (HBz=2-methylbenzimidazole, L=L + ( 8); L=L − ( 10) and HBz=quinoline-4-carboxaldehyde, L=L + ( 9); L=L − ( 11)) are reported in connection with these studies.

Growth of aligned SWNT arrays from water-soluble molecular clusters for nanotube device fabrication

A simple method to fabricate parallel and/or crossed networks of ultra-long single-walled carbon nanotube (SWNT) arrays based on carbon monoxide-chemical vapor deposition (CO-CVD) by a “fast-heating” growth process is reported. The catalysts which are water-soluble inorganic molecular clusters containing Fe and Mo atoms can be easily “drawn” on the substrate by an ink pen using the cluster solution as “ink” or transferred by micro-contact printing (μCP) using poly (dimethylsiloxane) (PDMS) elastomer stamp onto substrate. The as-grown SWNTs are millimetres in length with diameters ranging from 0.7 to 2.0 nm and with controllable location and direction. Multi-dimensional crossed-networks of SWNTs can be easily fabricated by multi-step growth processes. Patterning catalysts using water-soluble clusters can be compatible with ink-jet printing techniques for generating large-area well-oriented and precisely-located SWNTs. We have also demonstrated that the ease of patterning and growing aligned nanotube arrays provides a reliable way to fabricate nanotube electronic devices.