Synthesis of porous hollow silica nanostructures using hydroxyapatite nanoparticle templates

Hollow Silica Nano and Micro Spheres with Polystyrene Templating: A Mini-Review

Synthesis of monodisperse hollow silica nanospheres, especially using a hard template route, has been shown to be successful, but a high yield is needed for this strategy to be used on an industrial scale. On the other hand, there is a research gap in the synthesis of hollow silica microspheres due to the popularity and easiness of the synthesis of silica nanospheres despite the larger spheres being beneficial in some fields. In this review, current trends in producing hollow silica nanospheres using hard templates, especially polystyrene, are briefly presented. Soft templates have also been used to make highly polydisperse hollow silica spheres, and complex designs have improved polydispersity. The effect of the main parameters on the coating is presented here to provide a basic understanding of the interactions between the silica and template surface in the absence or presence of surfactants. Surface charge, surface modification, parameters in the sol-gel method and interaction between the silica and templates need to be further improved to have a uniform coating and better control over the size, dispersity, wall thickness and porosity. As larger organic templates will have lower surface energy, the efficiency of the micro sphere synthesis needs to be improved. Control over the physical structure of hollow silica spheres will open up many opportunities for them to be extensively used in fields ranging from waste removal to energy storage.

A six-fold difference in structure results in a six-order difference in conductivity: silica shell nanoarchitectonics on carbon black particles

Carbon black (Ketchen Black with a particle size of several tens of nm) was coated with silica with a varied thickness of 2 and 12 nm. Carbon/silica core-shell particles were grafted with the γ-methacryloxypropylsilyl group to be homogeneously dispersed into a poly(methyl methacrylate) film. The electrical conductivity of the poly(methyl methacrylate) films containing carbon/silica particles was successfully controlled by the thickness of the silica layer; silica coating with 2 nm thickness gave a conducting film, while that with 12 nm thickness gave a less conducting film with a remarkable difference on the order of 10⁶ (in volume conductivity).

Hollow Silica Particles: A Novel Strategy for Cost Reduction

Thermal insulation materials are highly sought after for applications such as building envelopes, refrigerators, cryogenic fuel storage chambers, and water supply piping. However, current insulation materials either do not provide sufficient insulation or are costly. A new class of insulation materials, hollow silica particles, has attracted tremendous attention due to its potential to provide a very high degree of thermal insulation. However, current synthesis strategies provide hollow silica particles at very low yields and at high cost, thus, making the particles unsuitable for real-world applications. In the present work, a synthesis process that produces hollow silica particles at very high yields and at a lower cost is presented. The effect of an infrared heat absorber, carbon black, on the thermal conductivity of hollow silica particles is also investigated and it is inferred that a carbon black-hollow silica particle mixture can be a better insulating material than hollow silica particles alone.

Cellulose-based dispersants and flocculants

Natural dispersants and flocculants, often referred to as dispersion stabilizers and liquid-solid separators, respectively, have secured a promising role in the bioprocessing community. They have various applications, including in biomedicine and in environmental remediation. A large fraction of existing dispersants and flocculants are synthesized from non-safe chemical compounds such as polyacrylamide and surfactants. Despite numerous advantages of synthetic dispersants and flocculants, issues such as renewability, sustainability, biocompatibility, and cost efficiency have shifted attention towards natural homologues, in particular, cellulose-based ones. Within the past decade, cellulose derivatives, obtained via chemical and mechanical treatments of cellulose fibrils, have successfully been used for these purposes. In this review article, by dividing the functional cellulosic compounds into "polymeric" and "nanoscale" categories, we provide insight into the engineering pathways, the structural frameworks, and surface chemistry of these "green" types of dispersants and flocculants. A summary of their efficiency and the controlling parameters is also accompanied by recent advances in their applications in each section. We are confident that the emergence of cellulose-based dispersing and flocculating agents will extend the boundaries of sustainable green technology.

Hollow Silica Particles: Recent Progress and Future Perspectives

Hollow silica particles (or mesoporous hollow silica particles) are sought after for applications across several fields, including drug delivery, battery anodes, catalysis, thermal insulation, and functional coatings. Significant progress has been made in hollow silica particle synthesis and several new methods are being explored to use these particles in real-world applications. This review article presents a brief and critical discussion of synthesis strategies, characterization techniques, and current and possible future applications of these particles.

Removal of Cr(VI) from Aqueous Solution by Polypyrrole/Hollow Mesoporous Silica Particles

The removal of Cr(VI) in wastewater plays an important role in human health and environment. In this work, polypyrrole/hollow mesoporous silica particle (PPy/HMSNs) adsorbents have been newly synthesized by in-situ polymerization, which prevent the aggregation of pyrrole in the process of polymerization and exhibit highly selective and powerful adsorption ability for Cr(VI). The adsorption process was in good agreement with the quasi-second-order kinetic model and the Langmuir isotherm model. And the maximum adsorption capacity of Cr(VI) was 322 mg/g at 25 °C. Moreover, the removal rate of Cr(VI) by PPy/HMSNs was ~100% in a number of binary systems, such as Cl⁻/Cr(VI), NO₃⁻/Cr(VI), SO₄²⁻/Cr(VI), Zn²⁺/Cr(VI), Fe³⁺/Cr(VI), Sn⁴⁺/Cr(VI), and Cu²⁺/Cr(VI). Thus, the PPy/HMSNs adsorbents have great potential for the removal of Cr(VI) in wastewater.

Biomimetic 1-Aminocyclopropane-1-Carboxylic Acid Oxidase Ethylene Production by MIL-100(Fe)-Based Materials

A novel core@shell hybrid material based on biocompatible hydroxyapatite nanoparticles (HA) and the well-known MIL-100(Fe) (Fe₃O(H₂O)₂F(BTC)₂·nH₂O, BTC: 1,3,5-benzenetricarboxylate) has been prepared following a layer-by-layer strategy. The core@shell nature of the studied system has been confirmed by infrared, X-ray powder diffraction, N₂ adsorption, transmission electron microscopy imaging, and EDS analyses revealing the homogeneous deposition of MIL-100(Fe) on HA, leading to HA@MIL-100(Fe) rod-shaped nanoparticles with a 7 nm shell thickness. Moreover, both MIL-100(Fe) and HA@MIL-100(Fe) have demonstrated to act as efficient heterogeneous catalysts toward the biomimetic oxidation of 1-aminocyclopropane-1-carboxylic acid into ethylene gas, a stimulator that regulates fruit ripening. Indeed, the hybrid material maintains the catalytic properties of pristine MIL-100(Fe) reaching 40% of conversion after only 20 min. Finally, the chemical stability of the catalyst in water has also been monitored for 21 days by inductively coupled plasma-mass spectrometry confirming that only ca. 3% of Ca is leached.

Biodegradable Silica-Based Nanoparticles: Dissolution Kinetics and Selective Bond Cleavage

Silica-based nanomaterials are extensively used in industrial applications and academic biomedical research, thus properly assessing their toxicity and biodegradability is essential for their safe and effective formulation and use. Unfortunately, there is often a lot of confusion in the literature with respect to the toxicity and biodegradability of silica since various studies have yielded contradictory results. In this contribution, we first endeavor to underscore that the simplistic model of silica should be discarded in favor of a more realistic model recognizing that all silicas are not created equal and should thus be considered in the plural as silicas and silica hybrids, which indeed hold various biocompatibility and biodegradability profiles. We then demonstrated that all silicas are-as displayed in Nature-degradable in water by dissolution, as governed by the laws of kinetics. Lastly, we explore the vast potential of tuning the degradability of silica by materials design using various silica hybrids for redox-, pH-, enzymatic-, and biochelation-mediated lysis mechanisms.

Deposition of a titania layer on spherical porous silica particles and their nanostructure-induced vapor sensing properties

A titania-stearic acid hybrid layer was deposited onto well-defined silica-hexadecyltrimethylammonium hybrid spherical particles with 854 nm size to obtain nanoporous particles with a useful hierarchical core-shell structure. The deposition of a 35 nm-thick titania layer was confirmed by transmission electron microscopy. The core-shell particles were washed with acidic ethanol (solvent extraction) and calcined at 550 °C for 5 h to remove the template, resulting in the formation of nanoporous titania coated nanoporous silica spherical particles, which have a bimodal pore size distribution attributed to the hierarchical porous core and porous shell structure. The nanoporous titania coated particles exhibited an unusual crystal phase transition; only anatase was present even after the calcination at 1000 °C for 1 h. This would be due to the interfacial bonding between the core silica and the shell titania, preventing the crystal phase transition from anatase to rutile. On the other hand, the direct calcination of the titania-stearic acid coated particles without solvent extraction led to a shell composed of both anatase and rutile. The transformation to rutile could be caused by the strong exothermic reaction during the oxidative decomposition of the occluded stearic acid. Furthermore, the intense exothermic reaction induced the formation of a yolk-shell structure, which played a role in the sensitive/selective sensing properties for acetic acid when the yolk-shell particles were coated onto a nanomechanical Membrane-type Surface stress Sensor (MSS).

Fabrication of Hollow Silica Microspheres with Orderly Hemispherical Protrusions and Capability for Heat-Induced Controlled Cracking

Hollow silica microspheres with orderly protrusions on their outer and inner surfaces were fabricated in three simple steps: (1) suspension polymerization of a polymerizable monomer containing silica nanoparticles to obtain polymeric microspheres with a layered shell of silica particles; (2) sol-gel reaction of tetraethoxysilane (TEOS) on the surface of the microspheres to connect the silica nanoparticles; (3) removal of polymer core by calcination. The shell composed of silica-connected silica nanoparticles remained spherical even after calcination, and the characteristic surface morphology with protrusions were obtained on both inner and outer surfaces. Measurements of the mechanical strength revealed that the compression modulus of the hollow microspheres increased with increasing thickness of the silica layer, which could be controlled by changing the concentration of TEOS in the sol-gel reaction. Rapid heating of the hollow silica microspheres with the thin silica-connected layer led to silica shell cracking, and the cracks were mostly observed in the connecting layer between the silica nanoparticles. The stress was probably concentrated in the connecting layer because of its lower thickness than the nanoparticles. Such characteristic of the hollow microspheres is useful for a capsule with capability for heat-induced controlled cracking caused by internal pressure changes.

Continuous, size and shape-control synthesis of hollow silica nanoparticles enabled by a microreactor-assisted rapid mixing process

Hollow silica nanoparticles (HSNPs) were synthesized using a microreactor-assisted system with a hydrodynamic focusing micromixer. Due to the fast mixing of each precursor in the system, the poly(acrylic acid) (PAA) thermodynamic-locked (TML) conformations were protected from their random aggregations by the immediately initiated growth of silica shells. When altering the mixing time through varying flow rates and flow rate ratios, the different degrees of the aggregation of PAA TML conformations were observed. The globular and necklace-like TML conformations were successfully captured by modifying the PAA concentration at the optimized mixing condition. Uniform HSNPs with an average diameter ∼30 nm were produced from this system. COMSOL numerical models was established to investigate the flow and concentration profiles, and their effects on the formation of PAA templates. Finally, the quality and utility of these uniform HSNPs were demonstrated by the fabrication of antireflective thin films on monocrystalline photovoltaic cells which showed a 3.8% increase in power conversion efficiency.

Degradability and Clearance of Silicon, Organosilica, Silsesquioxane, Silica Mixed Oxide, and Mesoporous Silica Nanoparticles

The biorelated degradability and clearance of siliceous nanomaterials have been questioned worldwide, since they are crucial prerequisites for the successful translation in clinics. Typically, the degradability and biocompatibility of mesoporous silica nanoparticles (MSNs) have been an ongoing discussion in research circles. The reason for such a concern is that approved pharmaceutical products must not accumulate in the human body, to prevent severe and unpredictable side-effects. Here, the biorelated degradability and clearance of silicon and silica nanoparticles (NPs) are comprehensively summarized. The influence of the size, morphology, surface area, pore size, and surface functional groups, to name a few, on the degradability of silicon and silica NPs is described. The noncovalent organic doping of silica and the covalent incorporation of either hydrolytically stable or redox- and enzymatically cleavable silsesquioxanes is then described for organosilica, bridged silsesquioxane (BS), and periodic mesoporous organosilica (PMO) NPs. Inorganically doped silica particles such as calcium-, iron-, manganese-, and zirconium-doped NPs, also have radically different hydrolytic stabilities. To conclude, the degradability and clearance timelines of various siliceous nanomaterials are compared and it is highlighted that researchers can select a specific nanomaterial in this large family according to the targeted applications and the required clearance kinetics.

Mesoporous silica coated silica-titania spherical particles: from impregnation to core-shell formation

The coating of solid surfaces with inorganic materials is a promising approach not only to impart various functionalities but also to modify physicochemical properties that are affected by the geometry/structure of the coating. In this study, a silica-hexadecyltrimethylammonium (silica-CTA) hybrid layer was deposited on monodispersed spherical particles composed of titania and octadecylamine (titania-ODA) by a sol-gel reaction of tetraethoxysilane in aqueous CTA/ammonia/methanol solution. The formation of the coating was confirmed by SEM and TEM observations. The coating thickness varied from a few nm to 100 nm depending on the Si/Ti ratio. We found that Si/Ti = 0.68 resulted in the formation of microporous silica-titania particles with the pore size of 0.7 nm as revealed by nitrogen adsorption/desorption measurements. Because the titania-ODA particles can be converted to mesoporous titania particles after removing ODA by acid/base treatment, the silica species can be impregnated into the titania particles and replace ODA under basic conditions. By increasing the Si/Ti molar ratio up to 1.4, silica-titania particles with non-porous structures were obtained. An amorphous to anatase transition occurred at around 800 °C, indicating the complete impregnation of silica inside the titania particles. Further increases of the Si/Ti molar ratio (to 3.4 and 6.8) led to the formation of the silica-CTA shell on the core particles, and the shell was converted to mesoporous silica layers with a pore size of 2 nm after calcination at 550 °C for 5 h. Non-linear control of the pore size/structure is presented for the first time; this will be useful for the precise design of diverse hybrid materials for optical, catalytic and biomedical applications.

Interfacial engineering for silica nanocapsules

Silica nanocapsules have attracted significant interest due to their core-shell hierarchical structure. The core domain allows the encapsulation of various functional components such as drugs, fluorescent and magnetic nanoparticles for applications in drug delivery, imaging and sensing, and the silica shell with its unique properties including biocompatibility, chemical and physical stability, and surface-chemistry tailorability provides a protection layer for the encapsulated cargo. Therefore, significant effort has been directed to synthesize silica nanocapsules with engineered properties, including size, composition and surface functionality, for various applications. This review provides a comprehensive overview of emerging methods for the manufacture of silica nanocapsules, with a special emphasis on different interfacial engineering strategies. The review starts with an introduction of various manufacturing approaches of silica nanocapsules highlighting surface engineering of the core template nanomaterials (solid nanoparticles, liquid droplets, and gas bubbles) using chemicals or biomolecules which are able to direct nucleation and growth of silica at the boundary of two-phase interfaces (solid-liquid, liquid-liquid, and gas-liquid). Next, surface functionalization of silica nanocapsules is presented. Furthermore, strategies and challenges of encapsulating active molecules (pre-loading and post-loading approaches) in these capsular systems are critically discussed. Finally, applications of silica nanocapsules in controlled release, imaging, and theranostics are reviewed.

Formation of hollow silica nanospheres by reverse microemulsion

Uniform hollow silica nanospheres (HSNs) synthesized with reverse microemulsion have great application potential as nanoreactors because enzymes or nanocatalysts can be easily encapsulated de novo in synthesis. Water-in-oil (w/o) reverse microemulsions comprising the polymeric surfactant polyoxyethylene (5) isooctylphenyl ether (Igepal CA-520), ammonia and water in a continuous oil phase (alkanes) coalesce into size-tunable silica nanoparticles via diffusion aggregation after the introduction of silica precursors. Here, we elucidate in detail the growth mechanism for silica nanoparticles via nucleation of ammonium-catalyzed silica oligomers from tetraethylorthosilicate (TEOS) and nanoporous aminopropyltrimethoxy silane (APTS) in the reverse microemulsion system. The formation pathway was studied in situ with small-angle X-ray scattering (SAXS). We find a four-stage process showing a sigmoidal growth behavior in time with a crossover from the induction period, early nucleation stage, coalescence growth and a final slowing down of growth. Various characterizations (TEM, N2 isotherm, dynamic light scattering, zeta potential, NMR, elemental analysis) reveal the diameters, scattering length density (SLD), mesoporosity, surface potentials and chemical compositions of the HSNs. Oil phases of alkanes with different alkyl chains are systematically employed to tune the sizes of HSNs by varying oil molar volumes, co-solvent amounts or surfactant mixture ratios. Silica condensation is incomplete in the core region, with the silica source of TEOS and APTS leading to the hollow silica nanosphere after etching with warm water.

One-step synthesis of amine-functionalized hollow mesoporous silica nanoparticles as efficient antibacterial and anticancer materials

In this study, amine-functionalized hollow mesoporous silica nanoparticles with an average diameter of ∼100 nm and shell thickness of ∼20 nm were prepared by an one-step process. This new nanoparticulate system exhibited excellent killing efficiency against mycobacterial (M. smegmatis strain mc(2) 651) and cancer cells (A549).

Designed synthesis of hydroxyapatite nanostructures: bullet-like single crystal and whiskered hollow ellipsoid

Hydroxyapatite (HAp) nanostructures of whiskered hollow ellipsoid and bullet-like single crystal were synthesized under mild reaction conditions by using a template-free "one pot" synthetic method. Immersing calcium carbonate precursor into ammonium phosphate solution resulted in the HAp phase. Formed HAp crystals were characterized by X-ray diffraction and transmission electron microscopy techniques. The stability and phase composition of calcium carbonate influenced the morphology and crystallinity of HAp. The transformation of the most stable calcite precursor yielded the bullet-like HAp single crystal of 300-600 nm in length, ~40 nm in tip diameter and ~80 to ~100 nm in bottom diameter. The metastable vaterite precursor showed the formation of the whiskered hollow ellipsoid nanostructures composed of HAp nanorods of ~10 nm in diameter. The driving force for the whole transformation process was the difference in solubility of calcium carbonate and HAp. At the same time, Kirkendall effect and Ostwald ripening played important roles in the formation of the different HAp nanostructures.

Hollow-structured mesoporous materials: chemical synthesis, functionalization and applications

Hollow-structured mesoporous materials (HMMs), as a kind of mesoporous material with unique morphology, have been of great interest in the past decade because of the subtle combination of the hollow architecture with the mesoporous nanostructure. Benefitting from the merits of low density, large void space, large specific surface area, and, especially, the good biocompatibility, HMMs present promising application prospects in various fields, such as adsorption and storage, confined catalysis when catalytically active species are incorporated in the core and/or shell, controlled drug release, targeted drug delivery, and simultaneous diagnosis and therapy of cancers when the surface and/or core of the HMMs are functionalized with functional ligands and/or nanoparticles, and so on. In this review, recent progress in the design, synthesis, functionalization, and applications of hollow mesoporous materials are discussed. Two main synthetic strategies, soft-templating and hard-templating routes, are broadly sorted and described in detail. Progress in the main application aspects of HMMs, such as adsorption and storage, catalysis, and biomedicine, are also discussed in detail in this article, in terms of the unique features of the combined large void space in the core and the mesoporous network in the shell. Functionalization of the core and pore/outer surfaces with functional organic groups and/or nanoparticles, and their performance, are summarized in this article. Finally, an outlook of their prospects and challenges in terms of their controlled synthesis and scaled application is presented.

Synthesis of hierarchical hollow silica microspheres containing surface nanoparticles employing the quasi-hard template of poly(4-vinylpyridine) microspheres

A facile method of preparing hierarchical hollow silica microspheres containing surface silica nanoparticles (HHSMs) through the sol-gel process of tetraethylorthosilicate employing a quasi-hard template of non-cross-linking poly(4-vinylpyridine) microspheres is proposed. The quasi-hard template contains the inherent catalyst of the basic pyridine group, and a few of the polymer chains can escape from the template matrix into the aqueous phase, which initiates the sol-gel process spontaneously both on the surface of the template used to prepare the hollow silica shell and in the aqueous phase to produce the surface silica nanoparticles. By tuning the weight ratio of the silica precursor to the quasi-hard template, HHSMs with a size of about 180 nm and a shell thickness ranging from 14 to 32 nm and surface silica nanoparticles ranging from 17 to 36 nm are produced initially through the deposition of surface silica nanoparticles onto the silica shell, followed by template removal either by calcination or solvent extraction. The synthesized HHSMs are characterized, and a possible mechanism for the synthesis of HHSMs is proposed.