Spherical Nanoclusters Research Articles

Modeling the processes of electrophoretic transfer for spherical nanoclusters of iron-molybdenum polyoxometalates

Processes of electrophoretic transfer of spherical nanocluster polyoxometalates containing molybdenum and iron through a membrane are modeled on the basis of experimental data. Using the thermodynamic approach, the data allow us to predict the depth of polyoxometalate penetration into the membrane and the probability of its accumulation over time.

Interaction between nanocluster polyoxometallates and low-molecular-weight organic compounds

The equilibrium sorption of methanol and benzene vapors by spherical porous nanocluster polyoxometallates with keplerate structures of different compositions (Mo132 and Mo72Fe30) and polyoxometallate (Mo138) with a toroidal structure and patterns in the kinetics of these processes are studied. Changes in the chemical potentials of the sorbates and polyoxometallates, specific Gibbs energies of the interaction of the polyoxometallates with the indicated substances, and the partial molar entropy of the interaction of Mo132 with methanol are calculated. The stability of the polyoxometallate structure in the sorbate vapors is studied via IR and Raman spectroscopy.

Nanostar Clustering Improves the Sensitivity of Plasmonic Assays.

Star-shaped Au nanoparticles (Au nanostars, AuNS) have been developed to improve the plasmonic sensitivity, but their application has largely been limited to single-particle probes. We herein describe a AuNS clustering assay based on nanoscale self-assembly of multiple AuNS and which further increases detection sensitivity. We show that each cluster contains multiple nanogaps to concentrate electric fields, thereby amplifying the signal via plasmon coupling. Numerical simulation indicated that AuNS clusters assume up to 460-fold higher field density than Au nanosphere clusters of similar mass. The results were validated in model assays of protein biomarker detection. The AuNS clustering assay showed higher sensitivity than Au nanosphere. Minimizing the size of affinity ligand was found important to tightly confine electric fields and improve the sensitivity. The resulting assay is simple and fast and can be readily applied to point-of-care molecular detection schemes.

Size-Tunable Fe3O4 Spherical Nanoclusters Through a One-Pot Hydrothermal Synthesis.

A new method is presented for the synthesis of monodisperse, size-tunable Fe3O4 spherical nanocluster particles through a simple, one-step hydrothermal reaction, according to a kinetics-controlled self-assembly process of smaller nanocrystals into hierarchical mesoporous aggregates. The mean diameter of the particles can be controlled over a broad range up to ~230 nm by simply varying the concentration of the precipitating reagent (urea or ammonia). The particles can be easily dispersed in water with excellent colloidal stability, exhibit a high surface area of ~ 60 m2 g(-1), and demonstrate size-dependent magnetic separation kinetics, where the larger nanoclusters exhibit rapid magnetophoresis, and the smaller nanoclusters remain inseparable. Thus particle size control is essential for improving magnetic separation processes.

Ion acceleration by intense, few-cycle laser pulses with nanodroplets

The energy distribution of electrons and ions emerging from the interaction of a few-cycle Gaussian laser pulse with spherical nanoclusters is investigated with the aim of determining prospects for accelerating ions in this regime. It is found that the direct conversion of laser energy into dense attosecond electron nanobunches results in rapid charge separation and early onset of Coulomb-explosion-dominated ion dynamics. The ion core of the cluster starts to expand soon after the laser has crossed the droplet, the fastest ions attaining 10 s of MeV at relativistic intensities. The current investigation should serve as a guide for contemporary experiments, i.e., using state-of-the-art few-cycle ultraintense lasers and nanoclusters of solid density.

Facile and Scalable Synthesis of Novel Spherical Au Nanocluster Assemblies@Polyacrylic Acid/Calcium Phosphate Nanoparticles for Dual-Modal Imaging-Guided Cancer Chemotherapy.

Engineering novel theranostic agents with both imaging and therapeutic functions have profound impact on molecular diagnostics, imaging, and therapeutics. In this paper, we develop for the first time a simple, scalable, and reproducible route to synthesize novel multifunctional spherical Au nanoclusters assemblies encapsulated by a polyacylic acid (PAA)/calcium phosphate (CaP) shell with aggregation enhanced fluorescence property (designated as AuNCs-A@PAA/CaP). Furthermore, the resulting AuNCs-A@PAA/CaP nanoparticles (NPs) possess a high payload of doxorubicin as synergetic pH-sensitive drug delivery vehicles to employ for dual-modal computed tomography (CT) and fluorescence imaging-guided liver cancer chemotherapy in vivo. The results reveal that AuNCs-A@PAA/CaP NPs not only provide excellent bimodal CT and fluorescence contrast imaging but also present efficient tumor ablation under the guidance of CT and fluorescence imaging, to achieve excellent chemotherapeutic efficacy to the hepatocarcinoma cell line (H-22) bearing mice through intravenous injection. Comprehensive blood tests and careful histological examinations reveal no apparent toxicity of AuNCs-A@PAA/CaP NPs. Our work highlights the great promise of AuNCs-A@PAA/CaP NPs for guiding and monitoring the chemotherapeutic process using simultaneous dual-modality CT and fluorescence imaging through a single theranostic agent.

Interaction of keplerate-type molybdenum-based porous spherical nanoclusters with polymer macromolecules

Characteristics of aqueous solutions of keplerate-type nanocluster polyoxomolybdates (POMs) and water-soluble nonionogenic polymers (polyvinyl alcohol, polyvinylpyrrolidone), such as dynamic viscosity, optical density, and aggregate particle size, have been compared. It has been shown that physicochemical properties of solutions in which POM associates with polymer macromolecules are formed are a nonlinear function of the component ratio. Oscillations of the above parameters are associated with the non-random character of interaction between the solution components and its change in the course of successive increase in the relative concentration of sorbate polymer molecules.

Multifunctional spherical gold nanocluster aggregate@polyacrylic acid@mesoporous silica nanoparticles for combined cancer dual-modal imaging and chemo-therapy.

Multifunctional spherical aggregated gold nanoclusters encapsulated by polyacrylic acid/mesoporous silica shell nanoparticles (A-AuNC@PAA/mSiO2 NPs) with aggregation enhanced fluorescence (AEF) properties were fabricated by a facile and reproducible synthetic strategy. The as-prepared NPs were employed as novel theranostic agents for synergistic fluorescence/X-ray computed tomography imaging and chemo-therapy of liver cancer in vitro and in vivo.

Application of Fano resonance effects in optical antennas formed by regular clusters of nanospheres

This paper describes an analytical model developed to study the Fano resonance effect in clusters of spherical plasmonic nanoparticles under local excitation. The model depicted the case of a parallel single dipole emitter that was near-field coupled to a pentamer or heptamer cluster of nanospheres. Spatial polarization and field distributions of the optical states and resonance spectra for these cluster configurations were calculated. It was discovered that polarization interference between the nanoparticles triggered the formation of a second peak in the directivity spectra at 690 nm, and this in turn provided a mechanism for the occurrence of subradiant mode effects. The directivity calculation was analyzed in order to qualify the redirection of emission. Performances of various nanoantennae were investigated and fully characterized in terms of spatial geometric differences and the Fano resonance effect on plasmonic nanoparticles in the optical domain. Light radiation patterns were found to be significantly affected by nanosphere sizes and positioning of nanospheres with respect to the dipole. The analytical treatment of these modeled nanoantennae yielded results that are applicable to physical design and utilization considerations for pentamer and heptamer clusters in nanoantennae mechanisms.

Highly stable polymer coated nano-clustered silver plates: a multimodal optical contrast agent for biomedical imaging

Here, we present a new optical contrast agent based on silver nanoplate clusters embedded inside of a polymer nano matrix. Unlike nanosphere clusters, which have been well studied, nanoplate clusters have unique properties due to the different possible orientations of interaction between the individual plates, resulting in a significant broadening of the absorption spectra. These nanoclusters were immobilized inside of a polymer cladding so as to maintain their stability and optical properties under in vivo conditions. The polymer-coated silver nanoplate clusters show a lower toxicity compared to the uncoated nanoparticles. At high nanoparticle concentrations, cell death occurs mostly due to apoptosis. These nanoparticles were used for targeted fluorescence imaging in a rat glioma cell line by incorporating a fluorescent dye into the matrix, followed by conjugation of a tumor targeting an F3 peptide. We further used these nanoparticles as photoacoustic contrast agents in vivo to enhance the contrast of the vasculature structures in a rat ear model. We observed a contrast enhancement of over 90% following the nanoparticle injection. It is also shown that these NPs can serve as efficient contrast agents, with specific targeting abilities for broadband multimodal imaging that are usable for diagnostic applications and that extend into use as therapeutic agents as well.

Multimodal Magneto‐Plasmonic Nanoclusters for Biomedical Applications

Multimodal nanostructures can help solve many problems in the biomedical field including sensitive molecular imaging, highly specific therapy, and early cancer detection. However, the synthesis of densely packed, multicomponent nanostructures with multimodal functionality represents a significant challenge. Here, a new type of hybrid magneto‐plasmonic nanoparticles is developed using an oil‐in‐water microemulsion method. The nanostructures are synthetized by self‐assembly of primary 6 nm iron oxide core‐gold shell particles resulting into densely packed spherical nanoclusters. The dense packing of primary particles does not change their superparamagnetic behavior; however, the close proximity of the constituent particles in the nanocluster leads to strong near‐infrared (NIR) plasmon resonances. The synthesis is optimized to eliminate nanocluster cytotoxicity. Immunotargeted nanoclusters are also developed using directional conjugation chemistry through the Fc antibody moiety, leaving the Fab antigen recognizing region available for targeting. Cancer cells labeled with immunotargeted nanoclusters produce a strong photoacoustic signal in the NIR that is optimum for tissue imaging. Furthermore, the labeled cells can be efficiently captured using an external magnetic field. The biocompatible magneto‐plasmonic nanoparticles can make a significant impact in development of point‐of‐care assays for detection of circulating tumor cells, as well as in cell therapy with magnetic cell guidance and imaging monitoring.

Light scattering from coupled plasmonic nanospheres on a substrate: erratum

An efficient numerical method based on half-space Green function and spherical harmonics expansion is used to study the light scattering from coupled multiple nanospheres on a substrate. The ellipsometric spectra for various geometries of coupled Au nanospheres are calculated and analyzed to realize the effects of plasmonic coupling of closely-spaced nanospheres. With only a few parameters to describe the distribution of various coupled nanosphere clusters embedded in a random distribution of nanospheres, the calculated ellipsometric spectra can fit the experimental data very well. This illustrates that our realistic model calculations can be used for determination of the distribution of nanospheres on a substrate or embedded in multilayer structures such as biological samples.

Morphology dependent photoinduced electron transfer from N,N-dimethylaniline to semiconductor cadmium sulfide

Photoluminescence of large (150–200 nm) colloidal CdS nanoparticles in isopropanol was efficiently quenched by the electron donor molecule N,N-dimethylaniline (DMA). The quenching rates were found to be accelerated with improvement of the crystallinity of the CdS substrates. Electron-transfer from DMA to spherical CdS nanoclusters with 2–4 nm polycrystalline domains inside exhibited the slowest quenching rate (kq ∼ 0.65 × 1010 M−1 S−1). However the quenching rate was enhanced by ∼2 times (kq ∼ 1.2 × 1010 M−1 S−1) when crystal plane alignments were moderately improved. Further improvement of the quenching rate (kq ∼ 3.5 × 1010 M−1 S−1) was successfully achieved when the morphology of the CdS substrates was changed to nanorods. This enhancement is attributed to the single crystalline structure i.e. highly ordered crystal planes along the axis of the nanorod.

Experimental Observation of Giant Chiroptical Amplification of Small Chiral Molecules by Gold Nanosphere Clusters

We have experimentally observed around 2 orders of magnitude circular dichroism (CD) enhancement in the visible region for cysteine molecules located in the hotspots of gold nanosphere clusters. The observed plasmon-induced CD responses show a significant correlation with the chiral nature of molecules at the hotspots. These results provide a concrete experimental demonstration on the predicted chiroptical transfer and amplification effect that arises from hotspot-mediated exciton–plasmon interactions in a strongly coupled metallic nanostructure, even though the exciton–plasmon coupling works at a far off-resonant regime. Our findings suggest here that plasmonic hotspot-based CD amplifier may provide a new strategy for ultrasensitive detection and quantification of molecular chirality—a key aspect for various bioscience and biomedicine applications.

Giant circular dichroism enhancement and chiroptical illusion in hybrid molecule-plasmonic nanostructures

Recently, there are great interest in studying the interaction between chiral molecules and plasmonic particles, because a weak circular dichroism (CD) signal in the ultraviolet (UV) region from chiral molecules can be both enhanced and transferred to the visible wavelength range by using plasmonic particles. Thus, ultrasensitive probe of tiny amounts of chiral substance by CD are worth waiting for. Here we present another way to strongly enhance CD of chiral molecules by using plasmonic particle cluster, which need not transfer to the visible wavelength. The method to calculate CD of chiral molecules in nanosphere clusters has been developed by means of multiple scattering of electromagnetic multipole fields. Our calculated results show that 2 orders of magnitude CD enhancement in the UV region for chiral molecules can be realized. Such a CD enhancement is very sensitive to the cluster structure. The cluster structure can cause chiroptical illusion in which a mirror symmetry in the CD spectra of opposite enantiomeric molecules is broken. The correction of quantum size effect on the phenomenon has also been considered. Our findings open up an alternative avenue for the ultrasensitive detection and illusion of chiral information.

Plasmon‐Enhanced Second‐ and Third‐Order Harmonic Generation in Spherical Free‐Electron Nanoclusters with Diffuse Surface

AbstractThe nonlinear scattering of a laser pulse off spherical nanoclusters with free electrons and with a diffuse surface is examined in the collisionless hydrodynamics approximation in the framework of perturbation theory with respect to the laser pulse intensity, as well as of the steady‐state approximation. In a previous publication [S.V. Fomichev and W. Becker, Phys. Rev. A 81, 063201 (2010)] we reported the full nonlinear hydrodynamic model of forced collective electron motion confined to a cluster with diffuse surface and introduced two different perturbation theories corresponding to different laser intensity regimes. In the current paper, in the framework of this hydrodynamic model we focus on the properties of plasmon resonance‐enhanced third‐harmonic generation in a spherical cluster and its dependence on the shape of its diffuse surface whose role increases for nonlinear processes. At the same time, the quadrupole second‐harmonic generation in a spherical cluster is also inspected as a necessary intermediate step. Both cold metal clusters in vacuum or in a dielectric surrounding and hot laser‐heated and laser‐ionized clusters are considered within the same approach for a wide range of the fundamental laser frequency. Nonlinear laser excitation of the dipole plasmon Mie resonance in spherical clusters, as well as of other respective multipole plasmon resonances is investigated analytically and numerically in detail (position, width, and strength) versus the cluster‐surface diffuseness, the outer ionization degree in charged clusters, the electron‐density diffuseness, and their interplay. Under certain conditions, depending on the various cluster parameters, different secondary nonlinear resonances are found. (© 2013 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)

Permanent Porosity Derived From the Self‐Assembly of Highly Luminescent Molecular Zinc Carbonate Nanoclusters

Hungry hungry micropores: Reaction of [Zn4(μ3-OH)2(L)4(tBu)2] (LH=8-hydroxyquinoline) with CO2 forms a 3D microporous material with spherical zinc carbonate nanoclusters packed in a diamondoid manner in the solid state. This compound exhibits one of the highest Brunauer—Emmett—Teller surface areas for a noncovalent porous material, with high H2, CO2, and CH4 uptake.

DFT model cluster studies of O2 adsorption on hydrogenated titania sub-nanoparticles

In the present paper, we examine the general applicability of different TiO2 model clusters to study of local chemical events on TiO2 sub-nanoparticles. Our previous DFT study of TiO2 activation through H adsorption and following deactivation by O2 adsorption using small amorphous Ti8O16 cluster were complemented by examination of rutile-type and spherical Ti15O30 nanoclusters. The obtained results were thoroughly compared with experimental data and results of related computational studies using other TiO2 models including periodic structures. It turned out that all considered model TiO2 model systems provide qualitatively similar results. It was shown that atomic hydrogen is adsorbed with negligible activation energy on surface O atoms, which is accompanied by the appearance of reduced Ti(3+) species and corresponding localized band gap 3d-Ti states. Oxygen molecule is adsorbed on Ti(3+) sites spontaneously forming molecular O2 (-) species by capturing an extra electron of Ti(3+) ion, which results in disappearance of Ti(3+) species and corresponding band gap states. Calculated g-tensor values of Ti(3+) and O2 (-) species agree well with the results of EPR studies and do not depend on the used TiO2 model cluster. Additionally, it was shown that the various cluster calculations provide results comparable with the calculations of periodic structures with respect to the modeling of chemical processes under study. As a whole, the present study approves the validity of molecular cluster approach to study of local chemical events on TiO2 sub-nanoparticles.

Light scattering from coupled plasmonic nanospheres on a substrate

An efficient numerical method based on half-space Green’s function and spherical harmonics expansion is used to study the light scattering from coupled multiple nanospheres on a substrate. The ellipsometric spectra for various geometries of coupled Au nanospheres are calculated and analyzed to realize the effects of plasmonic coupling of closely spaced nanospheres. With only a few parameters to describe the distribution of various coupled nanosphere clusters embedded in a random distribution of nanospheres, the calculated ellipsometric spectra can fit the experimental data very well. This illustrates that our realistic model calculations can be used for determination of the distribution of nanospheres on a substrate or embedded in multilayer structures, such as biological samples.

Magnetic-Responsive Hybrids of Fe3O4 Nanoparticles with β-Lactoglobulin Amyloid Fibrils and Nanoclusters

We report on the synthesis and magnetic-responsive behavior of hybrids formed by dispersing negatively charged iron oxide (Fe3O4) magnetic nanoparticles in positively charged β-lactoglobulin protein solutions at acidic pH, followed by heating at high temperatures. Depending on the pH used, different hybrid aggregates can be obtained, such as nanoparticle-modified amyloid fibrils (pH 3) and spherical nanoclusters (pH 4.5). We investigate the effect of magnetic fields of varying strengths (0-5 T) on the alignment of these Fe3O4-modified amyloid fibrils and spherical nanoclusters using a combination of scattering, birefringence and microscopic techniques and we find a strong alignment of the hybrids upon increasing the intensity of the magnetic field, which we quantify via 2D and 3D order parameters. We also demonstrate the possibility of controlling magnetically the sol-gel behavior of these hybrids: addition of salt (NaCl, 150 mM) to a solution containing nanoparticles modified with β-lactoglobulin amyloid fibrils (2 wt % fibrils modified with 0.6 wt % Fe3O4 nanoparticles) induces first the formation of a reversible gel, which can then be converted back to solution upon application of a moderate magnetic field of 1.1 T. These hybrids offer a new appealing functional colloidal system in which the aggregation, orientational order and rheological behavior can be efficiently controlled in a purely noninvasive way by external magnetic fields of weak intensity.