Silica Shell Thickness Research Articles

Tribromide immobilized on surface of magnetic nanoparticles modified tris(triazine-triamine): A versatile and highly active catalyst for oxidation of sulfides and oxidative coupling of thiols

In this article, we show that tribromide heterogenized on the surface of silica-coated magnetic nanoparticles modified with N2,N4,N6-tris(aminomethyl)-1,3,5-triazine-2,4,6-triamine [Fe3O4@SiO2-tris(triazine-triamine)-Br3] is a novel and efficient reusable nanocatalyst for the oxidation of sulfides to sulfoxides and oxidative coupling of thiols to disulfides using hydrogen peroxide as green oxidant. The structure of the as-fabricated Fe3O4@SiO2-tris(triazine-triamine)-Br3 was characterized by a series of spectroscopic techniques including FT-IR spectroscopy, SEM, TEM, EDX, VSM, XRD and TGA techniques. TEM and SEM analysis confirmed that the typical silica shell thickness was assessed to be around 20 nm. Recycling studies revealed that the Fe3O4@SiO2-tris(triazine-triamine)-Br3 could be easily recovered by a simple magnetic separation and recycled at least 6 times without deterioration in catalytic activity.

Efficient and stable SiO2-encapsulated NiPt/HY catalyst for catalytic cracking of β-O-4 linkage compound

Development of highly active and stable catalyst for C–O bonds cracking is important in biomass conversion, in this work, we report a SiO2-encapsulated NiPt/HY core-shell catalyst for phenethoxybenzene (PEB) catalytic cracking. The as-grown silica shell with defined pores serves as a ‘storage’ to enhance local concentration of PEB and intermediate isomers and promotes catalytic cracking on NiPt/HY core. With the increase of silica shell thickness from 0 to 40 nm, PEB reaction rate has increased more than five times, and the major cracking product selectivity is raised to above 90%, which is attributed to the continuous conversion of PEB isomers within the shell. This designed structure endows the NiPt/HY@SiO2 with a high activity and no obvious deactivation in PEB cracking to corresponding phenol and aromatics. The outstanding performance benefit from the enrichment effect on the unique core-shell structure, as well as the moderate acidity originated from silica regulation. This work can be extended to the modulation of microenvironment to promote the catalytic behavior of core-shell catalysts.

Tribromide Immobilized on Amino-Functionalized Magnetic Nanoparticles: A Active Magnetically Recoverable Catalyst for the Synthesis of Heterocycles

A novel tribromide nanocatalyst was successfully fabricated via the heterogenization of tribromide ion on the surface of silica-coated magnetic nanoparticles modified with N2,N4,N6-tris(aminomethyl)-1,3,5-triazine-2,4,6-triamin [Fe3O4@SiO2-tris(triazine-triamine)-Br3] and was well characterized by Fourier-transform infrared spectroscopy (FTIR), Scanning Electron Microscopy (SEM), Transmission Electron Microscopy (TEM), Energy Dispersive X-Ray Analysis (EDX), vibrating-sample magnetometer (VSM), X-ray diffraction analysis (XRD) and Thermogravimetric analysis (TGA) spectroscopic techniques. TEM and SEM analysis confirmed that the typical silica shell thickness was assessed to be around 20 nm. The as-fabricated Fe3O4@SiO2-tris(triazine-triamine)-Br3 nanomaterial was shown high catalytic activity in the multicomponent synthesis of highly substituted piperidines and polyhydroquinolines. Recycling experiments revealed that the Fe3O4@SiO2-tris(triazine-triamine)-Br3 could be easily recovered by a simple magnetic separation and recycled at least 7 times without deterioration in catalytic activity.

Synthesis of highly monodisperse superparamagnetic iron oxide core@mesoporous silica shell particles with independently tunable size, and porosity

Superparamagnetic iron oxide nanoparticles, SPIONs, especially in the form of nanoscopic aggregates thereof, find broad applications as magnetic resonance imaging contrast agents, as biological tools for straightforward separation of biomolecules and cells, as magnetically tunable colorimetric sensors exhibiting structural colors, etc. Furthermore, if the SPIONs are covered with another oxide, typically silica, the size and refractive index of the particles can be modified. In addition, if the silica layer is mesoporous additional functionalities can be introduced. Such particles could serve as, for example, theranostic nanoparticles capable of serving both as drug carriers and contrast agents, as easily separable high-capacity adsorbers, and if the particles are monodisperse, colorimetric sensors. Many reports cover the synthesis of such SPION core-silica shell particles, but to date the structural tunability and reproducibility aspect has not received deserved attention. Here we report highly reproducible syntheses leading to SPION cores carrying silica shells with independently tunable shell thicknesses and porosities. The products are highly monodisperse in all cases, as evidenced by the fact that pellets of the particles show structural colors. The shell formation process is followed in detail, and is related to the applied synthesis parameters, which allows for rational further fine-tuning of the structural characteristics of the core-shell particles for a given application. • We report results related to the synthesis of highly monodisperse, superparamgnetic aggregates of iron oxide nanoparticles of various sizes with different silica shells of various thicknesses, and porosities. • The silica shell porosity can be adjusted independently of the total particle size and the silica shell thickness. • The possibility for independent tuning of total particle size, shell thickness, and shell porosity makes it possible to rationally optimize these core-shell particles for a given application.

Nanoparticle core size and spacer coating thickness-dependence on metal-enhanced luminescence in optical oxygen sensors

Metal-enhanced luminescence (MEL) originated from near field interactions of luminescence with the surface plasmon resonance (SPR) of nearby metallic nanoparticles (NPs) is an effective strategy to increase luminescence detection sensitivity in oxygen sensors. Once the excitation light induces SPR, the generated enhanced local electromagnetic filed will result in an enhanced excitation efficiency and an increased radiative decay rates of luminescence in close proximity. Meanwhile, the nonradioactive energy transfer from the dyes to the metal NPs, leading to emission quenching, can also be affected by their separation. The extent of the intensity enhancement depends critically on the particle size, shape and the separation distance between the dye and the metal surface. Here, we prepared core-shell Ag@SiO2 with different core sizes (35 nm, 58 nm and 95 nm) and shell thickness (5–25 nm) to investigate the size and separation dependence on the emission enhancement in oxygen sensors at 0–21% oxygen concentration. Intensity enhancement factors of 4–9 were observed with a silver core size of 95 nm and silica shell thickness of 5 nm at 0–21% O2. In addition, the intensity enhancement factor increases with increasing core size and decreasing shell thickness in the Ag@SiO2-based oxygen sensors. Using Ag@SiO2 NPs result in brighter emission throughout the 0–21% oxygen concentration. Our fundamental understanding of MEP in the oxygen sensors provides us the opportunity to design and control efficient luminescence enhancement in oxygen and other sensors. .

Magnetic Nanoparticles Supported Copper Nanocomposite: A Highly Active Nanocatalyst for Synthesis of Benzothiazoles and Polyhydroquinolines

A novel magnetically recoverable copper nanocatalyst was fabricated via the immobilization of a copper complex on the surface of silica-coated magnetic nanoparticles modified with a novel Schiff base ligand [Fe3O4@SiO2-bis(aminopyridine)-Cu]. FT-IR spectroscopy, SEM, TEM, EDX, VSM, XRD, XPS, TGA and AAS, ICP-OES techniques were applied to characterize the structure of the as-fabricated [Fe3O4@SiO2-bis(aminopyridine)-Cu(II)] nanocomposite. TEM and SEM analysis confirmed that the typical silica shell thickness was assessed to be around 20 nm. Catalytic studies revealed that the Fe3O4@SiO2-bis(aminopyridine)-Cu(II) nanocomposite is a highly efficient and green nanocatalyst for the synthesis of benzothiazoles and polyhydroquinolines. The [Fe3O4@SiO2-bis(aminopyridine)-Cu(II)] nanocatalyst could be easily recovered by a simple magnetic separation and recycled at least 6 times without deterioration in catalytic activity.

Silica‐Coated Gold Nanorod Supraparticles: A Tunable Platform for Surface Enhanced Raman Spectroscopy

AbstractPlasmonic nanoparticle assemblies are promising functional materials for surface‐enhanced Raman spectroscopy (SERS). Gold nanorod (AuNR) assemblies are of particular interest due to the large, shape‐induced local field enhancement and the tunable surface plasmon resonance of the AuNRs. Designing the optimal assembly structure for SERS, however, is challenging and requires a delicate balance between the interparticle distance, porosity, and wetting of the assembly. Here, a new type of functional assemblies–called supraparticles–fabricated through the solvent‐evaporation driven assembly of silica‐coated gold nanorods into spherical ensembles, in which the plasmonic coupling and the mass transport is tuned through the thickness and porosity of the silica shells are introduced. Etching of the AuNRs allowed fine‐tuning of the plasmonic response to the laser excitation wavelength. Using a correlative SERS‐electron microscopy approach, it is shown that all supraparticles successfully amplified the Raman signal of the crystal violet probe molecules, and that the Raman signal strongly increased when decreasing the silica shell thickness from 35 to 3 nm, provided that the supraparticles have a sufficiently high porosity. The supraparticles introduced in this study present a novel class of materials for sensing, and open up a wide parameter space to optimize their performance.

The influence of shell thickness on coke resistance of core-shell catalyst in CO2 catalytic reforming of biomass tar

The development of catalysts with resistance to sintering and coke deposition is the key to tar cracking during biomass gasification technology. In this work, the core-shell catalysts with mesoporous and microporous silica-coated nickel nanoparticles were prepared for the CO2 reforming of toluene as a model compound for biomass tar. The influence of thickness of SiO2 shell layer on catalyst activity, coke and sintering resistance of the catalysts in the CO2 reforming of toluene was explored. Appropriate increasing in the thickness of the silica shell can significantly increase the specific surface area, pore volume and the interaction between core and shell of the catalysts, which can further improve the reactivity and coke resistance ability. However, excessive increase in shell thickness can lead to a drastic decrease in the specific surface area and pore volume of the catalyst, resulting in significant coke deposition. The [email protected]2-4 catalyst showed the highest catalytic activity of toluene conversion of around 50% within 300 min, stability H2/CO ratio of 0.25∼0.3 and durability of 26 h lifetime in the CO2 reforming of toluene. Overall, the optimization of the silica shell thickness can improve the reactivity and coke resistance ability of [email protected]2.

Stability, dissolution, and cytotoxicity of NaYF4-upconversion nanoparticles with different coatings

Upconversion nanoparticles (UCNPs) have attracted considerable attention owing to their unique photophysical properties. Their utilization in biomedical applications depends on the understanding of their transformations under physiological conditions and their potential toxicity. In this study, NaYF4:Yb,Er UCNPs, widely used for luminescence and photophysical studies, were modified with a set of four different coordinatively bound surface ligands, i.e., citrate, alendronate (AA), ethylendiamine tetra(methylene phosphonate) (EDTMP), and poly(maleic anhydride-alt-1-octadecene) (PMAO), as well as silica coatings with two different thicknesses. Subsequently, the aging-induced release of fluoride ions in water and cell culture media and their cytotoxic profile to human keratinocytes were assessed in parallel to the cytotoxic evaluation of the ligands, sodium fluoride and the lanthanide ions. The cytotoxicity studies of UCNPs with different surface modifications demonstrated the good biocompatibility of EDTMP-UCNPs and PMAO-UCNPs, which is in line with the low amount of fluoride ions released from these samples. An efficient prevention of UCNP dissolution and release of cytotoxic ions, as well as low cytotoxicity was also observed for UCNPs with a sufficiently thick silica shell. Overall, our results provide new insights into the understanding of the contribution of surface chemistry to the stability, dissolution behavior, and cytotoxicity of UCNPs. Altogether, the results obtained are highly important for future applications of UCNPs in the life sciences and bioimaging studies.

Back Cover

In the Chinese legend, the third eye of “Er Lang Shen (二郎神)” has super magic power, which can not only act as a detector to see the essence of matter, but also emit a strong and versatile light beam just like laser. Adopting the special functions of Er Lang's third eye, this cover picture displays the photo‐stability of SPASER nanoparticles based on Au@SiO2 core‐shell nanostructure under the illumination of pulsed laser. In this paper, the effect of physical size and structure of SPASER nanoparticles on their photothermal stability is studied theoretically and experimentally. The results showed that SPASER nanoparticles with smaller gold core and thicker silica shell have a better photothermal stability. More details are discussed in the article by Xu et al. on page 507—514. image

Precise control over the silica shell thickness and finding the optimal thickness for the peak heat diffusion property of AuNR@SiO2.

Silica-coated gold nanorods (AuNRs) exhibit significantly enhanced photothermal effects and photoacoustic (PA) signal intensities, which is beneficial for various nanophotonic applications in materials science. However, the silica shell thickness for optimum enhancement is not fully understood and is even controversial depending on the physical state of the silica shell. This is because of the lack of systematic investigations of the nanoscale silica shell thickness and the photothermal effect. This study provides a robust synthetic method to control the silica shell thickness at the nanoscale and the physical state-dependent heat diffusion property. The selected base and solvent system enabled the production of silica-coated AuNRs (AuNR@SiO2) with silica shell thicknesses of 5, 10, 15, 20, 25, 30, 35, and 40 nm. AuNRs with a 20 nm silica shell showed the highest photothermal effect with a 1.45-times higher photothermal efficiency than that of AuNRs without a silica shell. The low density of the silica shell on the AuNRs showed a low photothermal effect and photostability. It was found that the disruption of cetyltrimethyl ammonium bromide (CTAB) layers on the AuNRs was responsible for the low photostability of the AuNRs. The simulation study for the heat diffusion property showed facilitated heat diffusion in the presence of a 20 nm silica shell. In a cell-based study, AuNRs with a 20 nm silica shell showed the most sensitive photothermal effect for cell death. The results of this robust study can provide conclusive conditions for the optimal silica shell thickness to obtain the highest photothermal effect, which will be useful for the future design of nanomaterials in various fields of application.

Improvement of monodispersity and shape symmetry of silica shell for PbS quantum dots by introducing surface silanization and adjusting reverse micelle size

Semiconductor quantum dots (QDs) are one of the promising constituent materials of quantum information processing devices. The technique of silica coating of colloidal QDs using the reverse micelle method has been realized for improving position controllability, but the problem is that monodispersity and shape symmetry deteriorate when increasing the coating thickness. In this paper, we report three effective ways to achieve large silica-coated QDs with solving this problem, i.e. proper silanization on the QD surface, control of reverse micelle size by adjusting the amount of QD solvent, and two-step formation of silica shell. Proper substitution of ligands on the QD surface in the early stages of silica shell formation was important for uniform coating reaction. An amount of toluene as QD solvent determined the size of reverse micelles during the silica shell formation. There was an optimum combination of inverse micelle size and silica shell size to obtain silica-coated QDs with good monodispersity and high shape symmetry. We succeeded in growing the thick silica shell with expanding reverse micelle size by additionally supplying toluene with the raw material using the optimum silica-coated QDs as a growth nucleus.

Low threshold lasing from silicon Mie resonators

The unique electromagnetic response of high-index dielectric resonators with Mie resonances facilitates the selective enhancement and confinement of light, holding great promise for nanoscale lasing. However, up to now, nanolasing exploiting Mie resonances of individual Si nanowires has been limited to theoretical proposals. Here, we demonstrate a strategy to realize nanolasing with Si nanowires in the SOI platform. It is found that, by carefully tailoring the sizes of the Si nanowires, the Mie resonances of Si nanowires enhance the pumping/emission fields, leading to continuous-wave lasing at room temperature with low threshold in the visible range. The lasing of a dye at the wavelength of 523 nm assisted by the Mie resonances of Si nanowire is demonstrated experimentally. It is shown that the lasing wavelength changes with the width of nanowire due to the different scattering peak from Mie theory. In addition, the quality factor Q of lasing modes can be affected by the thickness of silica shell which determines the interaction volume of the pumping light and dyes. The proposed strategy paves the way towards practical integrated nanolasers for single-photon sources and micro-display, which have potential applications of on-chip optoelectrical devices.

The Southern Ocean diatom Pseudo-nitzschia subcurvata flourished better under simulated glacial than interglacial ocean conditions: Combined effects of CO2 and iron.

The 'Iron Hypothesis' suggests a fertilization of the Southern Ocean by increased dust deposition in glacial times. This promoted high primary productivity and contributed to lower atmospheric pCO2. In this study, the diatom Pseudo-nitzschia subcurvata, known to form prominent blooms in the Southern Ocean, was grown under simulated glacial and interglacial climatic conditions to understand how iron (Fe) availability (no Fe or Fe addition) in conjunction with different pCO2 levels (190 and 290 μatm) influences growth, particulate organic carbon (POC) production and photophysiology. Under both glacial and interglacial conditions, the diatom grew with similar rates. In comparison, glacial conditions (190 μatm pCO2 and Fe input) favored POC production by P. subcurvata while under interglacial conditions (290 μatm pCO2 and Fe deficiency) POC production was reduced, indicating a negative effect caused by higher pCO2 and low Fe availability. Under interglacial conditions, the diatom had, however, thicker silica shells. Overall, our results show that the combination of higher Fe availability with low pCO2, present during the glacial ocean, was beneficial for the diatom P. subcurvata, thus contributing more to primary production during glacial compared to interglacial times. Under the interglacial ocean conditions, on the other hand, the diatom could have contributed to higher carbon export due to its higher degree of silicification.

Colloidal Polymers of Iron Oxide Cubes Prepared by Dipolar-Driven Assembly and In Situ Welding with Silica

The assembly of colloids under directional interactions is a powerful way to create 1D chain-like structures (colloidal polymers). However, these colloidal polymers usually disassociate into separate colloids when the surrounding conditions are changed, and additional solidification treatments are required. Herein, we report an in situ welding of colloidal polymers simultaneous with assembly process in solution. When dispersed in a solution containing trace amounts of water, iron oxide dipolar cubes self-associate into chains, and the simultaneous in situ hydrolysis-condensation of tetraethylorthosilicate welds these chains into permanent colloidal polymers. With low water contents, the 1D colloidal assembly is directed by self-induced dipolar interactions via the step-growth mechanism, and the water content has no significant effect on the assembly process but changes the silica shell thickness on the colloidal polymers. With higher water contents, 2D and 3D structures are formed due to the nondirectional attraction of the water on the colloids.

Tailoring Plasmonics of Au@Ag Nanoparticles by Silica Encapsulation

AbstractHybrid metallic nanoparticles (NPs) encapsulated in oxide shells are currently intensely studied for plasmonic applications in sensing, medicine, catalysis, and photovoltaics. Here, a method for the synthesis of Au@Ag@SiO2 cubes with a uniform silica shell of variable and adjustable thickness in the nanometer range is introduced and their excellent, highly reproducible, and tunable optical response is demonstrated. Varying the silica shell thickness, the excitation energies of the single NP plasmon modes can be tuned in a broad spectral range between 2.55 and 3.25 eV. Most importantly, a strong coherent coupling of the surface plasmons is revealed at the silver–silica interface with Mie resonances at the silica–vacuum interface leading to a significant field enhancement at the encapsulated NP surface in the range of 100% at shell thicknesses t ≃ 20 nm. Consequently, the synthesis method and the field enhancement open pathways to a widespread use of silver NPs in plasmonic applications including photonic crystals and may be transferred to other non‐precious metals.

Surface charge control of hierarchical ceria/silica hybrid shells for enhanced dispersion stability

Although ceria particles are currently receiving great interest with their unique catalytic and optical properties, the problem of agglomeration is yet to be overcome, which is caused by their high density and surface energy, consequently hampering the practical applications. Herein, a novel method of preparing hierarchical shells of ceria (CeO2) and silica (SiO2) on a polystyrene (PS) core is introduced to control the shape and surface charge of the particle, which give rise to different dispersion conditions. The PS/CeO2/SiO2 hybrid core–shell microspheres provide a wide spectrum of surface charge via adjustment of the thickness of the outermost silica shell from 20 to 40 nm, which sets the isoelectric point (IEP) in the range of 5.1–8.6. Subsequently, the silica shell significantly extends the product’s stability and utilization windows due to the controlled surface charge and affinity between the particle surface and the dispersion medium. The developed multilayered core–shell microspheres and the method of synthesis have great potential for various applications that require sophisticated control of surface properties.

Lanthanopolyoxometalate‐Silica Core/Shell Nanoparticles as Potential MRI Contrast Agents

AbstractThe NMR relaxivities of the decatungstolanthanoate core‐shell nanoparticles, prepared by encapsulating [Ln(W5O18)2]9− polyoxometalates (LnPOM) within amorphous silica shells (K9[Ln(W5O18)2]@SiO2), were studied along the Ln series. The relaxivity of GdPOM is slightly higher than for Gd‐DTPA due to second‐sphere relaxation effects, but the values for the other paramagnetic LnPOMs are much smaller due to the short T1e values of their Ln3+‐ions. The NPs have core‐shell spherical structures, with LnPOM‐containing cores with 9.5–28 nm diameters, and 4.0–11.0 nm thick amorphous silica shells. In water suspensions, the NPs have negative zeta potentials (−32.5 to −40.0 mV) and time‐dependent hydrodynamic diameters (31–195 nm) reflecting the formation of aggregates. The relaxivities of GdPOM@SiO2 NPs suspensions (r1=10.97 (mM Gd)−1 s−1, r2=12.02 (mM Gd)−1 s−1, 0.47 T, 25 °C) are considerably larger than for the GdPOM solutions, indicating that their silica shell is significantly porous to water. This increase is limited by the agglomeration of the complexes in the NPs core, limiting their access to water to those at the core surface. Replacing half of the Gd3+ ions by Eu3+ decreases the NPs r1 and r2 relaxivities at 0.47 T to 20 % and 35 % of their initial values, which are still considerable, but does not affect the efficient luminescence properties of the Eu3+ centers. This indicates that the mixed NPs have potential as dual modality MRI/optical imaging contrast agents.

Effect of Silica Encapsulation on Cobalt-Based Catalysts for Fischer–Tropsch Synthesis under Different Reaction Conditions

Cobalt nanoparticles prepared by a solvothermal treatment were encapsulated in silica with different shell thicknesses by the Stöber method. The application of these materials as catalysts in Fischer–Tropsch synthesis was investigated under different reaction conditions, and their behaviors were compared to a silica-supported cobalt catalyst. The protective effect displayed by the silica shell avoided sintering of the active phase and influenced the reaction selectivity and activity in comparison with the supported catalyst. The silica shell thickness also played an important role in catalytic performance. A decrease in shell thickness caused an enhancement in selectivity toward C19–C24 hydrocarbons, indicating the occurrence of a confinement effect by silica pores over hydrocarbon growth. Furthermore, a high production of methane might be related to the occurrence of CO diffusion limitations due to the thicker silica shell. The activity and selectivity remained similar at different reaction temperatures. However, the use of a lower temperature caused an increase in stability.

A Silica Coating Approach to Enhance Bioconjugation on Metal-Encoded Polystyrene Microbeads for Bead-Based Assays in Mass Cytometry.

Bead-based assays in flow cytometry are multiplexed analytical techniques that allow rapid and simultaneous detection and quantification of a large number of analytes from small volumes of samples. The development of corresponding bead-based assays in mass cytometry (MC) is highly desirable since it could increase the number of analytes detected in a single assay. The microbeads for these assays have to be labeled with metal isotopes for MC detection. One must also be able to functionalize the bead surface with affinity reagents to capture the analytes. Metal-encoded polystyrene microbeads prepared by multi-stage dispersion polymerization can produce effective isotopic signals in MC with relatively small bead-to-bead variations. However, functionalizing this microbead surface with bioaffinity agents remains challenging, possibly due to the interference of the steric-stabilizing PVP corona on the microbead surface. Here, we report a systematic investigation of a silica coating approach to coat Eu-encoded microbeads with thin silica shells, to functionalize the surface with amino groups, and to introduce bioaffinity agents. We examine the effect of silica shell roughness on the bioconjugation capacity and the effect of silica shell thickness on signal quality in MC measurements. To limit non-specific binding, we converted the amino groups on the microbead surface to carboxylic acid groups. Antibodies were effectively attached to microbead by first conjugating NeutrAvidin to the carboxyl-modified bead surface and then attaching biotinylated antibodies to the NeutrAvidin-modified bead surface. The antibody-modified microbeads can specifically capture antigens, which were marked with isotopic labels, and generate strong signals in MC. These are promising results for the development of bead-based assays in MC.