Uniform Silica Shell Research Articles

Preparation and Characterization of Uniform and Controlled Silica Encapsulating on Lithium Yttrium Fluoride-Based Upconversion Nanoparticles.

In this work, we present an advancement in the encapsulation of lithium yttrium fluoride-based (YLiF4:Yb,Er) upconversion nanocrystals (UCNPs) with silica (SiO2) shells through a reverse microemulsion technique, achieving UCNPs@SiO2 core/shell structures. Key parameters of this approach were optimized to eliminate the occurrence of core-free silica particles and ensure a controlled silica shell thickness growth on the UCNPs. The optimal conditions for this method were using 6 mg of UCNPs, 1.5 mL of Igepal CO-520, 0.25 mL of ammonia, and 50 μL of tetraethyl orthosilicate (TEOS), resulting in a uniform silica shell around UCNPs with a thickness of 8 nm. The optical characteristics of the silica-encased UCNPs were examined, confirming the retention of their intrinsic upconversion luminescence (UC). Furthermore, we developed a reliable strategy to avoid the coencapsulation of multiple UCNPs within a single silica shell. This approach led to a tenfold increase in the UC luminescence of the annealed particles compared to their nonannealed counterparts, under identical silica shell thickness and excitation conditions. This significant improvement addresses a critical challenge and amplifies the applicability of the resulting UCNPs@SiO2 core/shell structures in various fields.

Ultra-Fine Control of Silica Shell Thickness on Silver Nanoparticle-Assembled Structures.

To study the distance-dependent electromagnetic field effects related to the enhancement and quenching mechanism of surface-enhanced Raman scattering (SERS) or fluorescence, it is essential to precisely control the distance from the surface of the metal nanoparticle (NP) to the target molecule by using a dielectric layer (e.g., SiO2, TiO2, and Al2O3). However, precisely controlling the thickness of this dielectric layer is challenging. Herein, we present a facile approach to control the thickness of the silica shell on silver nanoparticle-assembled silica nanocomposites, SiO2@Ag NPs, by controlling the number of reacting SiO2@Ag NPs and the silica precursor. Uniform silica shells with thicknesses in the range 5–40 nm were successfully fabricated. The proposed method for creating a homogeneous, precise, and fine silica coating on nanocomposites can potentially contribute to a comprehensive understanding of the distance-dependent electromagnetic field effects and optical properties of metal NPs.

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.

A method for the growth of uniform silica shells on different size and morphology upconversion nanoparticles.

Lanthanide-doped upconversion nanoparticles have emerged as attractive candidates for biomedical applications. This is due to their excitation and emission wavelengths, which lay the foundation for deeper penetration depth into biological tissue, higher resolution due to reduced scattering and improved imaging contrast as a result of a decrease in autofluorescence background. Usually, their encapsulation within a biocompatible silica shell is a requirement for their dispersion within complex media or for further functionalization of the upconversion nanoparticle surface. However, the creation of a silica shell around upconversion nanoparticles can be often challenging, many times resulting in partial silica coating or nanoparticle aggregation, as well as the production of a large number of silica particles as a side product. In this work we demonstrate a method to accurately predict the experimental conditions required to form a high yield of silica-coated upconversion nanoparticles, regardless of their shape and size.

Direct silica coating of drug crystals for ultra-high loading.

To push the limit of synthetic control, we create a thin layer (5 nm) of silica on the surface of drug nanocrystals, achieving a loading content (88%) that approaches the theoretical limit. The uniform silica shell provides a tailored diffusion barrier for controlled drug release. The method can be generally applied to 11 organic crystals, including 4 drugs.

Ultrasonic-Assisted Sol–Gel Synthesis of Core–Shell Silica Particles for High-Performance Liquid Chromatography

Core–shell particles (CSPs), also known as superficially porous particles or fused-core particles, consisting of a solid core and a porous shell, have emerged as an advanced technology in high-performance liquid chromatography (HPLC). We report here an ultrasonic-assisted sol–gel method for the fabrication of CSPs with uniform mesoporous silica shell using tetraethoxysilane as the silica source and dodecylamine as a catalyst, template and porogen agent in a methanol–water solution. Instead of stirring, sonication was adopted to guarantee the particles monodispersity and uniformity. The shell thickness between 300 and 700 nm could be obtained by adjusting the sonication time and the volume ratio of methanol–water. Pore sizes from 3.4 to 8.5 nm were tuned by hydrothermal treatment. The C18-bonded 2.7 μm CSPs with a shell thickness of ~ 500 nm showed some advantages over a commercial 1.7 μm BEH column, and successfully applied for the analyses of a Chinese medicine and peptides. The CSPs could be a promising candidate as HPLC packing materials.

Amine-Functionalized Fe2O3–SiO2 Core–Shell Nanoparticles With Tunable Sizes

Iron oxide magnetic nanoparticles (MNPs) coated with uniform silica shell were synthesized through thermal decomposition and inverse microemulsion methods. The iron oxide MNPs were further coated with silica shells through hydrolysis reaction. For the resulting core–shell MNPs, size regulation of both the magnetic core and the porous shell were achieved, enabling the modulation of their magnetic properties and magnetic interactions. Core–shell MNPs were finally functionalized with amine groups, and immobilized on gold surface due to charge-neutral amine/gold interactions. It was observed that the immobilization process was enhanced under external magnetic field.

Synthesis of optically active bromophenol blue encapsulated mesoporous silica–titania nanomatrix: structural and sensing characteristics

Mesostructured silica-titania (ST) and bromophenol blue encapsulated silica–titania (BPB-ST) nanomatrices are synthesized by sol–gel method at low temperature. FESEM analysis shows uniform crack-free porous surface of ST matrix. EDX mapping analysis shows that the BPB species are uniformly distributed/immobilized throughout the ST matrix lattices. AFM images reveal 45.68 nm thick film with low average surface roughness ~1.27 nm, after encapsulation of BPB dye in ST matrix. TEM analysis exhibits uniform interior titania core and silica shell network, with average particle size 2.4 ± 0.4 nm for ST sample which reduced to 0.8 ± 0.3 nm after BPB encapsulation. Mesoporous high surface area 488 m2/g and pore volume of 0.55 cm3/g encapsulated matrix is found to be beneficial for sensing analysis. Transparent (67%) mesoporous nanoparticles with low refractive index 1.41 is obtained for encapsulated nanomatrix. The synthesized sensing material is fast responsive <1 sec and found to be linear for pH 3–12. High pKa value of 7.57 at 580 nm for ST-BPB leads to the broader sensitivity range, repetitive, easy-to use, and color indication system. The ability to change color in response to external stimulus makes ST-BPB a potential sensing material in food.

Fabrication of Monodispersed Au@SiO2 Nanoparticles with Highly Stable Silica Layers by Ultrasound-Assisted Stöber Method

Metal@dielectric composite nanostructures are of high demand for their vast technological applications. The Stober method, either in its original form or by modification, has been utilized for the fabrication of silica shells over metal, semiconductor, or even dielectric nanostructures, with the aim to protect them from degradation, enhance their biocompatibility, or use them for molecular anchoring. However, the stability of silica shells and the dispersion of core–shell nanostructures remain the main limitations for their efficient applications. Here we demonstrate that utilization of ultrasound during hydrolysis and condensation of the metal–organic silicon precursor in Stober process can produce stable and uniform silica shell layers around gold nanoparticles, enhancing both their stability and dispersion. Through transmission electron microscopy and infrared spectroscopy techniques, we demonstrate that the Au@SiO2 nanoparticles fabricated with ultrasound treatment during silica shell growth contain a...

Fabrication of hydrazine sensor based on silica-coated Fe2O3 magnetic nanoparticles prepared by a rapid microwave irradiation method

A facile, efficient and rapid method for fabrication of silica-coated Fe2O3 magnetic nanoparticles (NPs) by a microwave (MW) irradiation method is reported. The homogeneous heating produced by the MW irradiation is the key to attaining the effective and uniform coating on Fe2O3 magnetic nanoparticles (NPs) in water-ethanol solution. HRTEM images confirmed the successful formation of uniform silica shells around the Fe2O3 NPs surface whereas FT-IR data show the structural differences between non-coated and silica-coated Fe2O3 NPs. These uniformly coated Fe2O3 NPs showed superior dispersibility than the bare Fe2O3 NPs as confirmed by the zeta potential measurements. For potential chemical sensor development, silica-coated Fe2O3 magnetic NPs were deposited onto a flat glassy carbon electrode (GCE, surface area, 0.0316 cm2) to give a sensor with a fast response against selective hydrazine in phosphate buffer phase. Hydrazine sensor also exhibits a good sensitivity with long-term stability and enhanced electrochemical performances. The calibration plot is linear (r2: 0.9911) over the 0.2 nM to 2.0 mM hydrazine concentration ranges. The sensitivity and detection limit are ∼12.658 μAmM−1cm−2 and 76.0 pM (signal-to-noise ratio, at a SNR of 3) respectively. It is also commenced a promising future sensitive sensor development using silica-coated Fe2O3 magnetic NPs by I-V method for the important applications of hazardous and carcinogenic compounds in environmental fields.

Core-shell nanoparticles by silica coating of metal oxides in a dual-stage hydrothermal flow reactor.

γ-Fe2O3@SiO2, TiO2@SiO2, and α-Fe2O3@SiO2 core-shell nanoparticles were synthesized by a surfactant-free hydrothermal method in a continuous flow dual-stage reactor. Uniform silica shells of 2 nm thickness were obtained when grown on γ-Fe2O3 and TiO2 nanoparticles while thicker and more irregular shells were deposited on α-Fe2O3 due to low pH.

Magnetic La1−x Sr x MnO3 nanoparticles as contrast agents for MRI: the parameters affecting 1H transverse relaxation

Magnetic nanoparticles of the La1−xSrxMnO3 perovskite phase (x = 0.20–0.45) were synthesized by a sol–gel method followed by thermal and mechanical treatments. The particles were coated with a uniform silica shell, and differential centrifugation yielded a product with high colloidal stability in water. X-ray powder diffraction (XRD) data showed that the mechanical processing did not affect the lattice parameters of the magnetic cores but only reduced their mean size dXRD. The magnetic properties of the bare particles were mainly controlled by the chemical composition and were also affected by the size of the particles. Subsequent silica coating led to an effective decrease in magnetization. Relaxometry measurements were focused primarily on colloids using magnetic cores of the same size (dXRD ≈ 20 nm) and different compositions, and coated with a shell measuring approximately 20 nm in thickness. The exceedingly high transverse relaxivities [r2(20 °C) = 290–430 s−1 mmol−1 L at B0 = 0.5 T] of the samples exhibited pronounced temperature dependence and correlated very well with the magnetic data. Additional samples differing in the size of the cores and silica shell thickness were prepared as well to analyze the effect of the particles on 1H transverse relaxation. The results suggest that the dominant regime of transverse relaxation is the static dephasing regime.

Magnetic properties of novel superparamagnetic iron oxide nanoclusters and their peculiarity under annealing treatment

The aim of this work is to present the magnetic properties of novel superparamagnetic iNANOvative™|silica nanoparticle clusters. A TEM analysis showed that these nanoparticle clusters, approximately 80nm in size, contained an assembly of maghemite nanoparticles in the core and an amorphous silica shell. The maghemite nanoparticles in the core were approximately 10nm in size, whereas the uniform silica shell was approximately 15-nm thick. The number of magnetic nanoparticles that were densely packed in the core of the single nanocluster was estimated to be approximately 67, resulting in a high magnetic moment for the single nanocluster of mnc∼1.2×106μB. This magnetic property of the nanoparticle cluster is advantageous for its easy manipulation using an external magnetic field, for example, in biomedical applications, such as drug delivery, or for magnetic separation in biotechnology. The magnetic properties of the iNANOvative™|silica nanoparticle clusters were systematically studied, with a special focus on the influence of the magnetic interactions between the nanoparticles in the core. For comparison, the nanoparticle clusters were annealed for 3h at 300°C in air. The annealing had no influence on the nanoparticles’ size and phase; however, it had a unique effect on the magnetic properties, i.e., a decrease of the blocking temperature and a weakening of the inter-particle interactions. We believe that this surprising observation is related to the thermal decomposition of the organic surfactant on the surfaces of the nanoparticles’ at the high annealing temperatures, which resulted in the formation of amorphous carbon inside the nanocluster.

Laser-Induced Nanowelding of Linearly Assembled and Silica-Coated Gold Nanorods to Fabricate Au@SiO2 Core–Shell Nanowires

Au@SiO2 core–shell nanowires have been successfully fabricated by irradiating infrared nanosecond pulses to linearly self-assembled and silica-coated gold nanorods. Gold nanorods have been self-assembled linearly by adding isopropanol in the absence of any assembling agents to form gold nanochains. The end-to-end assembled gold nanorods then have been directly encapsulated with uniform silica shells having tunable thickness to produce silica-coated gold nanochains. The irradiation of the nanostructures loaded on a transmission electron microscopy (TEM) grid for 5 min with nanosecond laser pulses of 1064 nm at fluence of 0.38 mJ/cm2 has produced Au@SiO2 core–shell nanowires. This is a novel and straightforward synthetic approach for the production of silica-coated gold nanowires having strong near-infrared absorption, high stability, and easily functionalizable surface.

Method To Incorporate Anisotropic Semiconductor Nanocrystals of All Shapes in an Ultrathin and Uniform Silica Shell

In this work, we present a method for the incorporation of anisotropic colloidal nanocrystals of many different shapes in silica in a highly controlled way. This method yields a uniform silica shell, with thickness tunable from 3 to 17 nm. The silica shell perfectly adapts to the shape of the nanocrystals, preserving their anisotropy, a crucial requisite for shape-dependent applications. Our method is based on an adaptation of the reverse microemulsion method. High control over the nucleation and growth of the shell is obtained by slowing down the hydrolysis and condensation rates of the silica precursor by lowering the ammonia concentration. This is shown to be essential for the formation of a uniform silica shell in the case of CdSe/CdS core/shell nanorods. Additionally, the general applicability of this method is demonstrated by coating different anisotropic semiconductor nanocrystals such as nanostars and 2D nanoplatelets. These results thus represent a crucial step toward the fabrication of highly processable and functionalized anisotropic nanoparticles.

Visible Mie Scattering from Hollow Silica Particles with Particulate Shells

A series of colloidal nanocomposite dispersions are synthesized by alcoholic dispersion polymerization of styrene in the presence of an ultrafine silica sol. The original core/shell polystyrene/silica nanocomposite particles have mean diameters ranging from 321 to 471 nm, as determined by dynamic light scattering. Upon calcination of the polystyrene cores, some shrinkage occurs but intact hollow silica shells are observed by transmission electron microscopy. On visual inspection, these silica residues display remarkable colors that vary depending on the particle diameter. When examined in transmittance mode (i.e., with an illuminated background) the silica powders appear yellow to red in color, but when viewed in reflectance (i.e., with a dark background) relatively intense blue/green colors are observed. The latter phenomenon has been analyzed by visible reflectance spectroscopy and the reflectance maximum depends on the dimensions of the silica shell, which are in turn dictated by the initial nanocomposite particle diameter. Small-angle X-ray scattering is used to determine the packing density of the silica nanoparticles, both in the original polystyrene/silica nanocomposite particles and in the calcined silica shells. Combined with geometrical considerations, this allows the equivalent uniform silica shell thickness to be calculated for a particulate silica shell and this parameter is then related to the theoretical predictions made by Retsch et al. for hollow particles comprising uniform silica shells (see Retsch, M.; Schmelzeisen, M.; Butt, H. J.; Thomas, E. L. Nano Lett., 2011, 11, 1389).

A general and versatile procedure for coating of hydrophobic nanocrystals with a thin silica layer enabling facile biofunctionalization and dye incorporation.

Here we report a method for coating of hydrophobic nanocrystals (NCs) with a thin silica layer. By using this approach 11 nm NaGdF4 NCs were coated with a mesoporous silica shell thinner than 20 nm resulting in highly monodisperse core-shell nanocomposite particles smaller than 50 nm. The synthetic protocol used here is based on the two-phase system that ensures high repeatability of the nanoparticle morphology and can be conveniently adapted for large-scale production. The use of an organic solvent as a diluent for tetraethoxysilane allows us to control not only the thickness, but also the porosity of the uniform silica shell. This procedure can be applied for coating NCs of various sizes and can be generalized to encapsulate other nanocrystals, e.g. semi-conductor quantum dots, into silica spheres. Furthermore, it allows facile surface modification with -NH2 groups that can be then used to conjugate biomolecules to the nanoparticle surface, as well as straightforward incorporation of various organic molecules (e.g. dyes or drugs) into the silica shell during its growth. To demonstrate potential applications up-converting NaGd0.80Yb0.18Er0.02F4 NCs were used as cores and zinc phthalocyanine was incorporated into the silica shell as a photosensitizer. Under irradiation with a 980 nm laser diode efficient generation of singlet oxygen was observed indicating that such nanocomposite particles have the capability to be used for photodynamic therapy.

Synthesis and up-conversion photoluminescence properties of NaYF4:Yb3+, Er3+@sSiO2@mSiO2 nanoparticles

NaYF4:Yb3+, Er3+ nanoparticles were synthesized through co-precipitation using EDTA as the chelator. The monodisperse core–shell structure NaYF4:Yb3+, Er3+@sSiO2 were synthesized by sol–gel method using TEOS as silica source. The core–shell–shell structure NaYF4:Yb3+, Er3+@sSiO2@mSiO2 were prepared by coating mesoporous silica shell onto NaYF4:Yb3+, Er3+@sSiO2 through hydrolysis of TEOS using CTAB as mesoporous template. Effects of CTAB concentration, TEOS concentration and different addition ways of CTAB were studied. The results show that the NaYF4:Yb3+, Er3+@sSiO2@mSiO2 nanoparticles with uniform amorphous silica and mesoporous silica shell were successfully synthesized. Up-conversion spectra of these nanoparticles were recorded with 980nm laser excitation under room temperature. There were no changes of the emission centers of nanoparticles before or after silica coating, but the emission intensities of nanoparticles after silica coating were significantly enhanced. Furthermore, the relative intensity ratio between red emission and green emission was decreased after silica coating.

A facile and green strategy for large-scale synthesis of silica nanotubes using ZnO nanorods as templates

Hollow silica nanostructures exhibit important applications in catalysis, sensing, and gene delivery due to their increased surface areas, good biocompatibility, and unique optical features. Here we report a simple and green approach to synthesize silica nanotubes using environment—friendly ZnO nanorods as templates. The ZnO nanorods are first coated with a layer of uniform silica shell by a sol–gel method. Then silica nanotubes are derived from the ZnO@SiO2 nanohybrids after removal of the ZnO nanorod cores in diluted hydrochloric acid solution. The samples at different stages were characterized by X-ray diffraction, X-ray photoelectron spectroscopy, Fourier transform infrared spectroscopy, Energy-dispersive X-ray spectroscopy, and Transmission electron microscopy. By controlling the structure of the ZnO@SiO2 nanohybrids, such as shell thickness, and the diameter and length of ZnO nanorods, size controllable SiO2 nanotubes can be expected.

Mesoporous Silica Coating on Carbon Nanotubes: Layer-by-Layer Method

It is of great interest to develop a simple, general, and easy-handling procedure for mesoporous silica coating. A facile, single-step method to coat iron oxide nanoparticles has been reported by Hyeon's group. However, up to present, this method only successfully applied to those zero-dimensional nanostructures heavily capped by cetyltrimethyl ammonium bromide (CTAB); no others are reported. It is unknown how this simple method is feasible in coating those nanostructures not capped by CTAB. Herein, using carbon nanotubes (CNTs) as the model, through an analogous layer-by-layer assembly method, many more CTAB molecules were found to anchor to CNTs, on which uniform mesoporous silica shells can successfully be formed by Hyeon's coating method. We believe that this contribution will pave the way for advancing the single-step method to become a general protocol in the mesoporous silica coating field.