Self-assembled Clusters Research Articles

Rhenium(I)–copper(I) carbonyl cyanide clusters

Tetranuclear cyanide rhenium(I)–copper(I) coordination compounds were synthesized in the course of simple one-pot reactions in mild conditions. A new class of mixed-metal compounds based on rhenium(I) clusters that are comprised of the pseudo-rhombic-shaped {Re2Cu2(μ-CN)4} unit were found. Two kinds of geometries adopted by the self-assembled cyclic clusters are observed, both of which can be described as approximately block-like overall geometries with different coordination modes. The unique polynuclear structure {ReCu(CH3CN)(μ-CN)2(CO)2(PPh3)2}n can be formed under the reaction conditions. Presented compounds were characterised by single crystal X-ray diffraction and spectroscopic studies.

Elucidating optical field directed hierarchical self-assembly of homogenous versus heterogeneous nanoclusters with femtosecond optical tweezers.

Insights into the morphology of nanoclusters would facilitate the design of nano-devices with improved optical, electrical, and magnetic responses. We have utilized optical gradient forces for the directed self-assembly of colloidal clusters using high-repetition-rate femtosecond laser pulses to delineate their structure and dynamics. We have ratified our experiments with theoretical models derived from the Langevin equation and defined the valid ranges of applicability. Our femtosecond optical tweezer-based technique characterizes the in-situ formation of hierarchical self-assembled clusters of homomers as well as heteromers by analyzing the back focal plane displacement signal. This technique is able to efficiently distinguish between nano-particles in heterogeneous clusters and is in accordance with our theory. Herein, we report results from our technique, and also develop a model to describe the mechanism of such processes where corner frequency changes. We show how the corner frequency changes enables us to recognize the structure and dynamics of the coagulation of colloidal homogeneous and heterogeneous clusters in condensed media over a broad range of nanoparticle sizes. The methods described here are advantageous, as the backscatter position-sensitive detection probes the in-situ self-assembly process while other light scattering approaches are leveraged for the characterization of isolated clusters.

Self-Assembled Magnetic Pt Nanocomposites for the Catalytic Reduction of Nitrophenol

Self-assembled nanotechnology has been used widely in various well-defined microstructures fabrication. Herein, this synthetic protocol was successfully utilized to in situ or hierarchically prepare magnetic Pt nanocomposites Fe3O4@Pt via a distinguishing two-step process. The distinction of these two pathways is due to the different synthetic processes. In the case of in situ preparation (Path I), it involves a supramolecular self-assembly of boron clusters with γ-CD on Fe3O4 substrate and then followed by in situ formation of Pt nanoparticles (NPs). But in the case of hierarchical preparation (Path II), the first step is preparing Pt NPs colloids reduced by boron clusters Cs2[closo-B12H12], and subsequent step is immobilizing the as-prepared [closo-B12H12]2–-capped Pt NPs on the Fe3O4@γ-CD. The success of these two fabrication strategies lies in the use of trifunctional boron clusters (reductant, stabilizer, and superchaotropic anchor). Both of the as-prepared Fe3O4@Pt nanocomposites showed good catalytic performance in the selective nitro-group reduction of nitroaromatics. A control test further indicated that the in situ prepared Fe3O4@Pt nanocomposites, with smaller size and higher loading of Pt NPs, exhibited a better catalytic performance and recyclability.

Mechanistically Driven Control over Cubane Oxo Cluster Catalysts.

Predictive and mechanistically driven access to polynuclear oxo clusters and related materials remains a grand challenge of inorganic chemistry. We here introduce a novel strategy for synthetic control over highly sought-after transition metal {M4O4} cubanes. They attract interest as molecular water oxidation catalysts that combine features of both heterogeneous oxide catalysts and nature's cuboidal {CaMn4O5} center of photosystem II. For the first time, we demonstrate the outstanding structure-directing effect of straightforward inorganic counteranions in solution on the self-assembly of oxo clusters. We introduce a selective counteranion toolbox for the controlled assembly of di(2-pyridyl) ketone (dpk) with M(OAc)2 (M = Co, Ni) precursors into different cubane types. Perchlorate anions provide selective access to type 2 cubanes with the characteristic {H2O-M2(OR)2-OH2} edge-site, such as [Co4(dpy-C{OH}O)4(OAc)2(H2O)2](ClO4)2. Type 1 cubanes with separated polar faces [Co4(dpy-C{OH}O)4(L2)4] n+ (L2 = OAc, Cl, or OAc and H2O) can be tuned with a wide range of other counteranions. The combination of these counteranion sets with Ni(OAc)2 as precursor selectively produces type 2 Co/Ni-mixed or {Ni4O4} cubanes. Systematic mechanistic experiments in combination with computational studies provide strong evidence for type 2 cubane formation through reaction of the key dimeric building block [M2(dpy-C{OH}O)2(H2O)4]2+ with monomers, such as [Co(dpy-C{OH}O)(OAc)(H2O)3]. Furthermore, both experiments and DFT calculations support an energetically favorable type 1 cubane formation pathway via direct head-to-head combination of two [Co2(dpy-C{OH}O)2(OAc)2(H2O)2] dimers. Finally, the visible-light-driven water oxidation activity of type 1 and 2 cubanes with tuned ligand environments was assessed. We pave the way to efficient design concepts in coordination chemistry through ionic control over cluster assembly pathways. Our comprehensive strategy demonstrates how retrosynthetic analyses can be implemented with readily available assembly directing counteranions to provide rapid access to tuned molecular materials.

Structure, Dynamics, and Mechanical Properties of Polyimide-Grafted Silica Nanocomposites

A continuing challenge in polymer nanocomposites (PNCs) is to control nanoparticle (NP) dispersion and understand its role in property enhancements. Previous studies have been focused on the flexible polymer chain systems. In this study, we report the structure, glass transition behavior, segmental dynamics, and mechanical properties of polyimide (PI) nanocomposites that consisted of either bare or PI-grafted silica NPs to discriminate the role of grafted rigid chains in polymer–NP interactions and dynamic response of such hybrids. Silica NPs are well dispersed in the poly(amic acid) (the precursor of PI) nanocomposites. After thermal imidization, the aggregate structure and self-assembled small clusters of NPs are observed in ungrafted and grafted silica NP-filled PI composites, respectively. The glass transition temperature (Tg) shifts to high temperature by the addition of silica NPs resulting from strong polymer–NP interactions and steric hindrance, and Tg deviation is more visible with increasing the...

Self-Assembly of Polynuclear Complexes Based on Spacer- Armed Pyridylazoles

The paper reviews literature data on the structural features of coordination compounds based on spacer-armed pyridylazoles (pyrazoles, 1,2,4- and 1,2,3-triazoles, tetrazoles). It is shown that the realization of the “maximum site occupancy” (MSO) principle for the metal:ligand ratio of 2:3 leads to the self-assembly of polynuclear clusters, their metal cores having highly symmetric architectures of tetrahedron, trigonal prism, cube, truncated tetrahedron, and cuboctahedron.

Theoretical Elucidation of β-O-4 Bond Cleavage of Lignin Model Compound Promoted by Sulfonic Acid-Functionalized Ionic Liquid.

While the depolymerization of lignin to chemicals catalyzed by ionic liquids has attracted significant attention, the relevant molecular mechanism, especially the cleavage of specific bonds related to efficient depolymerization, still needs to be deeply understood for the complexity of this natural aromatic polymer. This work presents a detailed understanding of the cleavage of the most abundant β-O-4 bond in the model system, guaiacylglycerol β-guaiacyl ether, by a Brønsted acidic IL (1-methyl-3-(propyl-3-sulfonate) imidazolium bisulfate ([C3SO3Hmim][HSO4]) using density functional theory calculation and molecular dynamics simulation. It has been found that [C3SO3Hmim][HSO4] generates zwitterion/H2SO4 via proton transfer with an energy barrier of 0.38 kcal/mol, which plays a dominant role in the lignin depolymerization process. Subsequently, the reaction can be carried out via three potential pathways, including (1) the dehydration of α-C-OH, (2) dehydration of γ-C-OH, and (3) the protonation of β-O. The electrophilic attack of H2SO4 and the hydrogen-bonding interaction between GG and zwitterion are the two most important factors to promote the depolymerization reaction. In all steps, the dehydration of α-C-OH route is computed to be favored for the experiment. The relatively higher energy barrier for β-O-4 bond dissociation among these reaction steps is attributed to the hindrance of the self-assembled clusters of GG in the mixed system. Further, the dense distribution of H13([C3SO3Hmim]) surrounding O21(GG), indicated by sharp peaks in RDFs, reveals that -SO3H in cations plays a substantial role in solvating lignin. Hopefully, this work will demonstrate new insights into lignin depolymerization by functionalized ILs in biomass conversion chemistry.

Self-assembled micellar clusters based on Triton-X-family surfactants for enhanced solubilization, encapsulation, proteins permeability control, and anticancer drug delivery

Non-ionic surfactants have raised a considerable interest for solubilization, encapsulation, permeabilization and controlled release of various compounds due to their unique physicochemical properties. Nevertheless, it is still challenging to create convenient self-assembled multifunctional materials with high solubilization and encapsulation capacities by preserving their advanced capabilities to protect loaded cargos without altering their characteristics. In this work, we present an extended concept of micellar clusters (MCs) formation based on partial entrapment and stabilization of chelate ligands by hydrophobic forces found on the non-ionic surfactant micelle interface of the Triton-X family (TX-100/TX-114), followed by subsequent complexation of the preformed structures either by metal ions or a supporting chelator. The formation aspects, inner structure and the role of external factors such as the addition of competitive ligands have been extensively studied. MCs loaded by hydrophobic fluorescent compounds with high encapsulation efficiency demonstrate an excellent optical response in aqueous media without crystallization as well as sufficient increase in solubility of toxic hydrophobic compounds such as bilirubin (>50 times compared to pure surfactants). Furthermore, Triton-X-based MCs provide a unique feature of selective permeability to hydrophilic ligand-switching proteins such as UnaG and BSA demonstrating bright “turn-on” fluorescence signal either inside the cluster or on its interface via complexation. The proposed strategies allowed us to successfully encapsulate and visualize a newly synthesized, highly hydrophobic anticancer PTR-58-CLB-CAMP peptide drug, while MCs loaded by the drug exhibit a considerable antitumor activity against HeLa cells.

Self-assembled thixotropic silver cluster hydrogel for anticancer drug release

A biocompatible thixotropic hydrogel via photoreduction and self-assembly of silver clusters is described. This reaction avoids use of hydrogelators, crosslinkers or any exogenous toxic reducing agents, making it a facile and green method. Moreover, the formed ‘smart’ thixotropic hydrogel is pH-responsive and can encapsulate the hydrophobic anticancer drug doxorubicin hydrochloride (DOX). The thixotropic property allows it to be sol state, which can be injected directly into the site of breast tumors in mice; after removal of the shear stress, the sol reverts back to hydrogel, where it remains for delivery of DOX into cancerous cells. The DOX-loaded hydrogel presents remarkable inhibition efficacy towards tumor growth, in addition to reducing the organ toxicity.

Preparation of colloidal molecules with temperature-tunable interactions from oppositely charged microgel spheres.

The self-assembly of small colloidal clusters, so-called colloidal molecules, into crystalline materials has proven extremely challenging, the outcome often being glassy, amorphous states where positions and orientations are locked. In this paper, a new type of colloidal molecule is therefore prepared, assembled from poly(N-isopropylacrylamide) (PNIPAM)-based microgels that due to their well documented softness and temperature-response allow for greater defect tolerance compared to hard spheres and for convenient in situ tuning of size, volume fraction and inter-particle interactions with temperature. The microgels (B) are assembled by electrostatic adsorption onto oppositely charged, smaller-sized microgels (A), where the relative size of the two determines the valency (n) of the resulting core-satellite ABn-type colloidal molecules. Following assembly, a microfluidic deterministic lateral displacement (DLD) device is used to effectively isolate AB4-type colloidal molecules of tetrahedral geometry that possess a repulsive-to-attractive transition on crossing the microgels' volume phase transition temperature (VPTT). These soft, temperature-responsive colloidal molecules constitute highly promising building blocks for the preparation of new materials with emergent properties, and their optical wavelength-size makes them especially interesting for optical applications.

Самоорганизация полиядерных комплексов на основе спейсерированных пиридилазолов

Проанализированы литературные данные о структурных особенностях координационных соединений на основе спейсерированных пиридилазолов (пиразолы, 1,2,4- и 1,2,3-триазолы, тетразолы). Показано, что реализация принципа MSO — "maximum site occupancy" при соотношении металл:лиганд = 2:3 приводит к самоорганизации полиядерных кластеров с геометрией металлоостова в виде высокосимметричных архитектур, таких как тетраэдр, тригональная призма, куб, усеченный тетраэдр и кубооктаэдр.

Nanosheets and 2D-nanonetworks by mutually assisted self-assembly of fullerene clusters and DNA three-way junctions.

Programmable construction of two dimensional (2D) nanoarchitectures using short DNA strands is of utmost interest in the context of DNA nanotechnology. Previously, we have demonstrated fullerene-cluster assisted self-assembly of short oligonucleotide duplexes into micrometer long, semiconducting nanowires. This report demonstrates the construction of micrometer-sized nanosheets and 2D-nanonetworks from the mutual self-assembly of fullerene nanoclusters with three way junction DNA (3WJ-DNA) and 3WJ-DNA with a 12-mer overhang (3WJ-OH), respectively. The interaction of unique sized fullerene clusters prepared from an aniline appended fullerene derivative, F-An, with two 3WJ-DNAs, namely, 3WJ-20 and 3WJ-30, having 20 and 30 nucleobases, respectively at each strand was characterized using UV-visible absorption, circular dichroism and fluorescence techniques. The morphological characterization of nanosheets embedded with F-An clusters was performed via AFM, TEM and DLS analyses. The programmability and structural tunability of the resultant nanostructures were further demonstrated using 3WJ-OH containing a cytosine rich, single stranded DNA 12-mer overhang, which forms entangled 2D-nanonetwork structures instead of nanosheets due to the differential interaction of F-An nanoclusters with single and duplex strands of 3WJ-OH. Moreover, the selective modification of the cytosine rich sequence present in 3WJ-OH with silver nanoclusters (AgNCs) resulted in significant enhancement in silver nanocluster fluorescence (∼40%) compared to 3WJ-OH/AgNCs owing to the additional stability of AgNCs embedded in 2D nanostructures. This unique strategy of constructing DNA based 2D nanomaterials and their utilization in the integration of functional motifs could find application in the area of DNA nanotechnology and bio-molecular sensing.

Photoinduced photoluminescence enhancement in self-assembled clusters of formamidinium lead bromide perovskite nanocrystals.

Nanocrystals of formamidinium lead bromide perovskite (FAPbBr3) self-assemble into clusters in powder and film samples and provide a prolonged photoluminescence lifetime, which is attributed to the diffusion of charge carriers through interparticle states formed among nanocrystals. Interestingly, the photoluminescence lifetime decreases and the emission intensity increases for the clusters, which is with the increase in the intensity of excitation light. By doping the nanocrystal clusters with C60, we successfully harvested the photogenerated charge carriers. Nonetheless, at high intensities of excitation, the rate of radiative recombination becomes comparable to that of the electron transfer to C60. Thus, the optimum rate of electron transfer to C60 is accomplished by minimally exciting the self-assembled nanocrystals.

Synthesis, particle shape characterization, magnetic properties and surface modification of superparamagnetic iron oxide nanochains

We report monodisperse, chain-like particles (nanochains) consisted of silica-coated maghemite (γ-Fe2O3) nanoparticle clusters prepared by colloidal chemistry and magnetic field-induced self-assembly of nanoparticle clusters. In order to quantify the shapes of chain-like particles, we have used the measure for shape convexity which is also called solidity. We functionalize the surface of the nanochains with amino (NH2) and carboxyl groups (COOH) in order to modify surface charge. These surfaces of nanochains provide better colloidal stability and their potential for practical applications in biomedicine. The enhanced colloidal stability of the surface modified nanochains is confirmed by Zeta potential (ζ-potential) analysis. Magnetic properties of the nanochains show superparamagnetic state at room temperature since the nanochains are composed of tiny nanoparticles as their building blocks. The measured M(H) data at room temperature have been successfully fitted by the Langevin function and magnetic moment μp = 20,526 μB for sphere-like nanoparticle clusters and μp = 20,767 μB for nanochains are determined. The determined magnetic parameters have revealed that the nanochains show a magnetic moment of the nanoparticles higher than the one of individual nanoparticle clusters. These differences can be attributed to the collective magnetic properties of superparamagnetic iron oxide nanoparticles (SPION) assembled in different morphologies (isotropic and anisotropic morphology).

Fundamental and Practical Limits of Achieving Artificial Magnetism and Effective Optical Medium by Using Self-Assembly of Metallic Colloidal Clusters

The self-assembly of metallic colloidal clusters (so called plasmonic metamolecules) has been viewed as a versatile, but highly effective approach for the materialization of the metamaterials exhibiting artificial magnetism at optical frequencies (including visible and near infrared (NIR) regimes). Indeed, several proofs of concepts of plasmonic metamolecules have been successfully demonstrated in both theoretical and experimental ways. Nevertheless, this self-assembly strategy has barely been used and still remains an underutilized method. For example, the self-assembly and optical utilization of the plasmonic metamolecules have been limited to the discrete unit of the structure; the materialization of effective optical medium made of plasmonic metamolecules is highly challenging. In this work, we theoretically exploited the practical limits of self-assembly technology for the fabrication of optical magnetic metamaterials.

Controlling Fragment Competition on Pathways to Addressable Self-Assembly.

Addressable self-assembly is the formation of a target structure from a set of unique molecular or colloidal building blocks, each of which occupies a defined location in the target. The requirement that each type of building block appears exactly once in each copy of the target introduces severe restrictions on the combinations of particles and on the pathways that lead to successful self-assembly. These restrictions can limit the efficiency of self-assembly and the final yield of the product. In particular, partially formed fragments may compete with each other if their compositions overlap, since they cannot be combined. Here, we introduce a "completability" algorithm to quantify competition between self-assembling fragments and use it to deduce general principles for suppressing the effects of fragment incompatibility in the self-assembly of small addressable clusters. Competition originates from loops in the bonding network of the target structure, but loops may be needed to provide structural rigidity and thermodynamic stability. An optimal compromise can be achieved by careful choice of bonding networks and by promoting semihierarchical pathways that rule out competition between early fragments. These concepts are illustrated in simulations of self-assembly in two contrasting addressable targets of 20 unique components each.

In Situ Atomic-Level Studies of Gd Atom Release and Migration on Graphene from a Metallofullerene Precursor

We show how gadolinium (Gd)-based metallofullerene (Gd3N@C80) molecules can be used to create single adatoms and nanoclusters on a graphene surface. An in situ heating holder within an aberration-corrected scanning transmission electron microscope is used to track the adhesion of endohedral metallofullerenes (MFs) to the surface of graphene, followed by Gd metal ejection and diffusion across the surface. Heating to 900 °C is used to promote adatom migration and metal nanocluster formation, enabling direct imaging of the assembly of nanoclusters of Gd. We show that hydrogen can be used to reduce the temperature of MF fragmentation and metal ejection, enabling Gd nanocluster formation on graphene surfaces at temperatures as low as 300 °C. The process of MF fragmentation and metal ejection is captured in situ and reveals that after metal release, the C80 cage opens further and fuses with the surface monolayer carbon glass on graphene, creating a highly stable carbon layer for further Gd adatom adhesion. Small voids and defects (∼1 nm) in the surface carbon glass act as trapping sites for Gd atoms, leading to atomic self-assembly of 2D monolayer Gd clusters. These results show that MFs can adhere to graphene surfaces at temperatures well above their bulk sublimation point, indicating that the surface bound MFs have strong adhesion to dangling bonds on graphene surfaces. The ability to create dispersed single Gd adatoms and Gd nanoclusters on graphene may have impact in spintronics and magnetism.

Controlling Anisotropic Growth of Colloidal ZnSe Nanostructures.

Semiconductor nanocrystals serve as outstanding model systems for studying quantum confined size and shape effects. Shape control is an important knob for controlling their properties but so far it has been well developed mainly for heavy-metal containing semiconductor nanocrystals, limiting their further widespread utilization. Herein, we report a synthesis of heavy-metal free ZnSe nanocrystals with shape and size control through utilization of well-defined molecular clusters. In this approach, ZnSe nanowires are synthesized and their length and shape control is achieved by introduction of controlled amounts of molecular clusters. As a result of [Zn4(SPh)10](Me4N)2 clusters (Zn4 clusters) addition, short ZnSe nanorods or ZnSe nanodots can be obtained through tuning the ratio of Zn4 clusters to ZnSe. A study using transmission electron microscopy revealed the formation of a hybrid inorganic-organic nanowire, whereby the ligands form a template for self-assembly of ZnSe magic size clusters. The hybrid nanowire template becomes shorter and eventually disappears upon increasing amount of Zn4 clusters in the reaction. The generality of the method is demonstrated by using isostructural [Cu4(SPh)6](Me4N)2 clusters, which presented a new approach to Cu doped ZnSe nanocrystals and provided also a unique opportunity to employ X-ray absorption fine structure spectroscopy for deciphering the changes in the local atomic-scale environment of the clusters and explaining their role in the process of the nanorods formation. Overall, the introduction of molecular clusters presented here opens a path for growth of colloidal semiconductor nanorods, expanding the palette of materials selection with obvious implications for optoelectronic and biomedical applications.

Self-Assembled Growth of Tail-Like Cluster Composed of Flower-Shaped ZnO Microwires by Chemical Vapor Deposition Method

A self-assembled ZnO tail-like cluster (TC) had been successfully synthesized by a simple chemical vapor deposition method. Scanning electron microscopy observations show that ZnO TC is composed of bushy ZnO microwires with flower-shaped cross sections. Long and narrow furrows can be clearly observed on the surface of the ZnO TC. A possible growth model is proposed to discuss the formation mechanism. The analytical result indicates that the flower-shaped ZnO microwires are formed by the lateral coalescence of ZnO wires at high temperature. The room temperature PL spectrum shows a prominent UV emission band around 380 nm, and no green emission is found, implying that the unique flower-shaped ZnO microwires have high optical quality. This controlled growth of ZnO TC may have implication for potential applications in novel optoelectronic micro/nanodevices in the near future.

Resistive Switching Phenomena of Metal-Organic Framework Nanosheets Detected By Peakforce Tunneling Atomic Force Microscopy

Graphene[1], an atom-thin sheet of carbon discovered in 2004, has created a brand new era in two-dimensional (2D) materials owing to its immense potential in both basic and applied research. In recent years, numerous new 2D nanomaterials possessing atomically thin structures have been investigated.[2] Metal-organic Framework (MOF) nanosheets, as a new category of 2D materials, exhibit diverse structural and physicochemical properties as a result of the self-assembly of metal ions or clusters and polydentate bridging ligands.[3] Arising from their unique structural features combining MOF and 2D nano-structures, MOF nanosheets exhibit unprecedented physical, electronic, chemical, and optical properties and offer implementation in application field. Resistive random-access memory is a predominant candidate for future non-volatile memories. It relies on the formation/annihilation of highly conductive paths, leading to the creation of two stable and reversible resistive states.[4] To achieve the desirable memory characteristics, new materials and a better understanding of the physics triggering the resistive switching (RS) in this system are required. MOF nanosheets with ultrathin nanostructures, high-density active sites and rapid ion/electron transport can offer best opportunities for fundamental and technological research in the RS applications. However, the progress in fundamental understanding has been hampered by the extremely small sizes (of the region) involved in the RS transition region presenting a severe challenge for its physical characterization using conventional metrology methods. PeakForce tunneling (PFTUNA) atomic force microscopy[5] provides direct, precise force which makes it shows high spatial resolution and routine high current sensitivity. Therefore, the MOF nanosheets combined with PFTUNA technology can present the best tradeoff between existing process technology and RS performances. In this work, we intend to synthesize Ag-S nanosheets with distorted hexagonal topology (Figure 1a-b). By mean of PFTUNA technology of AFM, MOF nanosheet based memory device is constructed and characterized (Figure 1c). A conductive probe to apply a voltage to the top electrode of the single device thereby setting the memory device into its state ON, i.e., its low-resistive state (LRS), or OFF state, i.e., its high-resistive state (HRS) (Figure 1d). Figure 1