Robust Amino-Functionalized Mesoporous Silica Hollow Spheres Templated by CO2 Bubbles

Hollow-structured mesoporous silica has wide applications in catalysis and drug delivery due to its high surface area, large hollow space, and short diffusion mesochannels. However, the synthesis of hollow structures usually requires sacrificial templates, leading to increased production costs and environmental problems. Here, for the first time, amino-functionalized mesoporous silica hollow spheres were synthesized by using CO₂ gaseous bubbles as templates. The assembly of anionic surfactants, co-structure directing agents, and inorganic silica precursors around CO₂ bubbles formed the mesoporous silica shells. The hollow silica spheres, 200–400 nm in size with 20–30 nm spherical shell thickness, had abundant amine groups on the surface of the mesopores, indicating excellent applications for CO₂ capture, Knoevenagel condensation reaction, and the controlled release of Drugs.

Interfacial Assembly and Applications of Functional Mesoporous Materials

Functional mesoporous materials have gained tremendous attention due to their distinctive properties and potential applications. In recent decades, the self-assembly of micelles and framework precursors into mesostructures on the liquid-solid, liquid-liquid, and gas-liquid interface has been explored in the construction of functional mesoporous materials with diverse compositions, morphologies, mesostructures, and pore sizes. Compared with the one-phase solution synthetic approach, the introduction of a two-phase interface in the synthetic system changes self-assembly behaviors between micelles and framework species, leading to the possibility for the on-demand fabrication of unique mesoporous architectures. In addition, controlling the interfacial tension is critical to manipulate the self-assembly process for precise synthesis. In particular, recent breakthroughs based on the concept of the "monomicelles" assembly mechanism are very promising and interesting for the synthesis of functional mesoporous materials with the precise control. In this review, we highlight the synthetic strategies, principles, and interface engineering at the macroscale, microscale, and nanoscale for oriented interfacial assembly of functional mesoporous materials over the past 10 years. The potential applications in various fields, including adsorption, separation, sensors, catalysis, energy storage, solar cells, and biomedicine, are discussed. Finally, we also propose the remaining challenges, possible directions, and opportunities in this field for the future outlook.

The use of amino-based functional molecules for the controllable synthesis of noble-metal nanocrystals: a minireview

Controlling the morphologies and structures of noble-metal nanocrystals has always been a frontier field in electrocatalysis. Functional molecules such as capping agents, surfactants and additives are indispensable in shape-control synthesis. Amino-based functional molecules have strong coordination abilities with metal ions, and they are widely used in the morphology control of nanocrystals. In this minireview, we pay close attention to recent advances in the use of amino-based functional molecules for the controllable synthesis of noble-metal nanocrystals. The effects of various amino-based molecules on differently shaped noble-metal nanocrystals, including zero-, one-, two-, and three-dimensional nanocrystals, are reviewed and summarized. The roles and mechanisms of amino-based small molecules and long-chain ammonium salts relating to the morphology-control synthesis of noble-metal nanocrystals are highlighted. Relationships between shape and electrocatalytic properties are also described. Finally, some key prospects and challenges relating to the controllable synthesis of noble-metal nanocrystals and their electrocatalytic applications are proposed.

Toward the Formulation of Stable Micro and Nano Double Emulsions through a Silica Coating on Internal Water Droplets

Delivery systems able to coencapsulate both hydrophilic and hydrophobic species are of great interest in both fundamental research and industrial applications. Water-in-oil-in-water (w₁/O/W₂) emulsions are interesting systems for this purpose, but they suffer from limited stability. In this study, we propose an innovative approach to stabilize double emulsions by the synthesis of a silica membrane at the water/oil interface of the primary emulsion (i.e., inner w₁/O emulsion). This approach allows the formulation of stable double emulsions through a two-step process, enabling high encapsulation efficiencies of model hydrophilic dyes encapsulated in the internal droplets. This approach also decreases the scale of the double droplets up to the nanoscale, which is not possible without silica stabilization. Different formulation and processing parameters were explored in order to optimize the methodology. Physicochemical characterization was performed by dynamic light scattering, encapsulation efficiency measurements, release profiles, and optical and transmission electron microscopies.

Nanostructured silver decorated hollow silica and their application in the treatment of microbial contaminated water at room temperature

Effect of silver decoration on hollow silica and its antimicrobial properties.

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.

Silicon hollow sphere anode with enhanced cycling stability by a template-free method

Silicon is a promising alternative anode material since it has a ten times higher theoretical specific capacity than that of a traditional graphite anode. However, the poor cycling stability due to the huge volume change of Si during charge/discharge processes has seriously hampered its widespread application. To address this challenge, we design a silicon hollow sphere nanostructure by selective etching and a subsequent magnesiothermic reduction. The Si hollow spheres exhibit enhanced electrochemical properties compared to the commercial Si nanoparticles. The initial discharge and charge capacities of the Si hollow sphere anode are 2215.8 mAh g^-1 and 1615.1 mAh g^-1 with a high initial coulombic efficiency (72%) at a current density of 200 mA g^-1, respectively. In particular, the reversible capacity is 1534.5 mAh g^-1 with a remarkable 88% capacity retention against the second cycle after 100 cycles, over four times the theoretical capacity of the traditional graphite electrode. Therefore, our work demonstrates the considerable potential of silicon structures for displacing commercial graphite, and might open up new opportunities to rationally design various nanostructured materials for lithium ion batteries.

Nanoscale Polydopamine (PDA) Meets π-π Interactions: An Interface-Directed Coassembly Approach for Mesoporous Nanoparticles

Well known for the adhesive property, mussel-inspired polydopamine (PDA) has been shown to enhance performance in a wide range of adsorption-based applications. However, imparting porous nanostructures to PDA materials for enhanced loading capacities has not been demonstrated even when surfactants were present in the synthesis. Herein, we report on the preparation of mesoporous PDA particles (MPDA) based on the assembly of primary PDA particles and Pluronic F127 stabilized emulsion droplets on water/1,3,5-trimethylbenzene (TMB) interfaces. The key to the formation of this new type of the MPDA structure is the full utilization of the π-π stacking interactions between PDA structures and the π-electron-rich TMB molecules. Remarkably, this method presents a facile approach for MPDA particles with an average diameter of ∼90 nm, slit-like pores with a peak size of ∼5.0 nm as well as hollow cavities. When used as the adsorbent for a model dye RhB, the MPDA particles achieved an ultrahigh RhB adsorption capacity of 1100 μg mg^-1, which is significantly higher than that for the PDA-reactive dyes with Eschenmoser structure. Moreover, it was demonstrated that the cavity space in MPDA can facilitate high volumetric uptake in a capillary filling/stacking manner via the π-π interactions. These developments pave a new avenue on the mechanism and the designed synthesis of functional PDA materials by organic-organic composite assembly for advanced adsorption applications.

One-pot synthesis of mesoporous silica hollow spheres with Mn-N-C integrated into the framework for ethylbenzene oxidation

Mesoporous silica spheres with Mn-N-C materials integrated into the framework are synthesized via the surfactant (CTAB) template-assisted one-pot approach. A manganese porphyrin is used as the precursor of the Mn-N-C structure. The as-prepared catalyst exhibits remarkable activity and stability in heterogeneous catalytic systems for ethylbenzene oxidation.

Radical polymerization of miniemulsions induced by compressed gases

Pressurization of a macroemulsion comprising a vinyl monomer/water/surfactant can result in formation of a transparent miniemulsion without use of high energy mixing, suitable for synthesis of polymeric nanoparticlesviaminiemulsion polymerization.

Controllable synthesis of hollow mesoporous silica particles by a facile one-pot sol–gel method

A simple and facile one-pot sol–gel method is proposed for the fabrication of hollow mesoporous silica particles. Both the particle size and the shell thickness can be well controlled.

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.

Supercritical or compressed CO2 as a stimulus for tuning surfactant aggregations

Surfactant assemblies have a wide range of applications in areas such as the chemical industry, material science, biology, and enhanced oil recovery. From both theoretical and practical perspectives, researchers have focused on tuning the aggregation behaviors of surfactants. Researchers commonly use solid and liquid compounds such as cosurfactants, acids, salts, and alcohols as stimuli for tuning the aggregation behaviors. However, these additives can present economic and environmental costs and can contaminate or modify the product. Therefore researchers would like to develop effective methods for tuning surfactant aggregation with easily removable, economical, and environmentally benign stimuli. Supercritical or compressed CO(2) is abundant, nontoxic, and nonflammable and can be recycled easily after use. Compressed CO(2) is quite soluble in many liquids, and the solubility depends on pressure and temperature. Therefore researchers can continuously influence the properties of liquid solvents by controlling the pressure or temperature of CO(2). In this Account, we briefly review our recent studies on tuning the aggregation behaviors of surfactants in different media using supercritical or compressed CO(2). Supercritical or compressed CO(2) serves as a versatile regulator of a variety of properties of surfactant assemblies. Using CO(2), we can switch the micellization of surfactants in water, adjust the properties of reverse micelles, enhance the stability of vesicles, and modify the switching transition between different surfactant assemblies. We can also tune the properties of emulsions, induce the formation of nanoemulsions, and construct novel microemulsions. With these CO(2)-responsive surfactant assemblies, we have synthesized functional materials, optimized chemical reaction conditions, and enhanced extraction and separation efficiencies. Compared with the conventional solid or liquid additives, CO(2) shows some obvious advantages as an agent for modifying surfactant aggregation. We can adjust the aggregation behaviors continuously by pressure and can easily remove CO(2) without contaminating the product, and the method is environmentally benign. We can explain the mechanisms for these effects on surfactant aggregation in terms of molecular interactions. These studies expand the areas of colloid and interface science, supercritical fluid science and technology, and chemical thermodynamics. We hope that the work will influence other fundamental and applied research in these areas.

Spherical silica micro/nanomaterials with hierarchical structures: synthesis and applications

This paper reviews the progress made recently in synthesis and applications of spherical silica micro/nanomaterials with multilevel (hierarchical) structures. The spherical silica micro/nanomaterials with hierarchical structures are classified into four main structural categories that include (1) hollow mesoporous spheres, (2) core-in-(hollow porous shell) spheres, (3) hollow spheres with multiple porous shells and (4) hierarchically porous spheres. Due to the complex structures and being focused on spherical silica micro/nanomaterials, some novel methods based on the combination of two routine methods or two surfactants, and some special synthetic strategies are proposed to produce the spherical silica micro/nanomaterials with hierarchical structures. Compared with the same-sized solid, porous or hollow silica spheres, these fantastic spherical silica micro/nanomaterials with hierarchical structures exhibit enhanced properties which may enable them to be used in broad and promising applications as ideal scaffolds (carriers) for biological, medical, and catalytic applications.

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.

Switching micellization of pluronics in water by CO2

The micellization of amphiphilic molecules is an interesting topic from both theoretical and practical points of view. Herein we have studied the effects of compressed CO(2) on the micellization of Pluronics in water by means of fluorescence, UV/Vis spectra, and small-angle X-ray scattering. It was found that CO(2) can induce the micellization of Pluronics in water, and the micelle can return to the initial state of molecular dispersion after depressurization. Therefore, the micellization of Pluronics in water can be switched through the easy control of pressure. Different from the common micelles with hydrophobic cores, interestingly, this CO(2)-induced micelle has an amphiphilic core, in which hydrophobic and hydrophilic domains coexist. On account of the ability to dissolve both polar and nonpolar components in the micellar core, the CO(2)-induced micelles can improve the reagent compatibilities frequently encountered in various applications. In an attempt to address this advantage, this micelle was utilized as template to the one-step synthesis of Au/silica core-shell composite nanoparticles. Furthermore, the underlying mechanism for the CO(2)-induced micellization of Pluronics in water was investigated by a series of experiments.

Reversible Addition–Fragmentation Chain Transfer (RAFT) Polymerization in Miniemulsion Based on In Situ Surfactant Generation

Reversible addition–fragmentation chain transfer (RAFT) polymerization of styrene has been implemented in aqueous miniemulsion based on the in situ surfactant generation approach using oleic acid and potassium hydroxide in the absence of high energy mixing. The best results were obtained using the RAFT agent 3-benzylsulfanyl thiocarbonyl sufanylpropionic acid (BSPAC), most likely as a result of the presence of a carboxylic acid functionality in the RAFT agent that renders it surface active and thus imparts increased colloidal stability. Stable final miniemulsions were obtained with no coagulum with particle diameters less than 200 nm. The results demonstrate that the RAFT miniemulsion polymerization of styrene employing the low energy in situ surfactant method is challenging, but that a system that proceeds predominantly by a miniemulsion mechanism can be achieved under carefully selected conditions.

Synthesis of monodisperse mesoporous silica hollow microcapsules and their release of loaded materials

Monodisperse mesoporous silica hollow capsules (MSHCs) with sizes ranging from 570 to 75 nm were synthesized using the sol-gel method combined with the template-assisted method. Monodisperse polystyrene (PS) particles were used as templates for the hollow structure of the MSHCs, and cylindrical micelles of cationic surfactant were used to create mesopores across the shell of the MSHC. To obtain MSHCs with a degree of polydispersity in diameter comparable to that of the PS templates and spherical in shape with uniform shell thicknesses, the conditions for the synthesis were systematically examined. It was found that the ranges of the reaction conditions to obtain such MSHCs had to be narrow because (1) the colloidal stability of the particles must be maintained before and after the sol-gel reaction and (2) the rate of the silica formation during the reaction must be regulated to attain sufficient shell thickness to retain the hollow structure and to achieve smooth surfaces. The sustained release of dye molecules loaded in the MSHCs was confirmed, indicating that our MSHC is a candidate for use as a drug carrier in drug delivery systems or as a container for microreactors.

Preparation of hierarchical architectures of silica particles with hollow structure and nanoparticle shells: a material for the high reflectivity of UV and visible light

Silica microcapsules (silica hollow particles) are readily prepared by a single step of the interfacial reaction method, where a W/O/W emulsion is employed effectively. This is a simple (one-step process), inexpensive approach (silica source is sodium silicate) to producing hollow silicas. The addition of NaCl to the sodium silicate solution as the inner water phase of the W/O/W emulsion plainly influenced the shell structure of the silica hollow particles. The increase of the addition of NaCl expanded the size of the mesopores in their silica shell, which reached to macropores (>50 nm). The nanoparticles in the shells of some silica hollow particles attained approximately 200-400 nm in size, which is comparable to the wavelengths of UV and visible light. According to the diffuse reflection spectra of the silica hollow particles in powder form, these particles showed the high reflection of UV and visible light, which increased with added NaCl in the preparation process of the interfacial reaction method. The reflectance of a silica hollow particle from 300 to 800 nm in wavelength was over 90%, which was significantly higher than a common solid (not hollow) silica gel. In addition, even the reflectance of UV light shorter than 300 nm in wavelength was greater than 60%. These characteristic reflections in a wide range of wavelengths were caused by both nanoparticle shells whose sizes are comparable with the wavelength of light and the hollow structures of the main micrometer-sized particles.