Review on Synthesis of Colloidal Hollow Particles and Their Applications

Fabrication of polymeric microspheres for biomedical applications

Polymeric microspheres (PMs) have attracted great attention in the field of biomedicine in the last several decades due to their small particle size, special functionalities shown on the surface and high surface-to-volume ratio. However, how to fabricate PMs which can meet the clinical needs and transform laboratory achievements to industrial scale-up still remains a challenge. Therefore, advanced fabrication technologies are pursued. In this review, we summarize the technologies used to fabricate PMs, including emulsion-based methods, microfluidics, spray drying, coacervation, supercritical fluid and superhydrophobic surface-mediated method and their advantages and disadvantages. We also review the different structures, properties and functions of the PMs and their applications in the fields of drug delivery, cell encapsulation and expansion, scaffolds in tissue engineering, transcatheter arterial embolization and artificial cells. Moreover, we discuss existing challenges and future perspectives for advancing fabrication technologies and biomedical applications of PMs.

Shape-Tunable Hollow Polysiloxane Nanoparticles Based on a Surfactant-Free Soft Templating Method and Their Application as a Drug Carrier

A surfactant-free soft-templating method has been used to prepare polysiloxane hollow nanoparticles with a controllable shape. This method is simple and has the potential for large-scale preparation. For the first time, we successfully obtained hollow polysiloxane nanoparticles with different shapes, including eccentric hollow polysiloxane microspheres (EHPM), apple-like hollow polysiloxane microparticles (AHPM), and bowl-like hollow polysiloxane microparticles (BHPM), by simply changing the solvent. In this method, the hydrolyzed methyltriethoxysilane (MTES) not only stabilizes the system as a surfactant but also acts as a reactant for subsequent reactions, so no additional surfactant is needed. In addition, the formation mechanism of hollow polysiloxane microparticles with different shapes is also proposed: that is, MTES hydrolyzed under acidic conditions to form a surfactant, which changes the system from suspension to a stable oil-in-water emulsion. Then, under alkaline conditions, the hydrolyzed MTES polycondenses and nucleates at the oil-water interface. At the same time, with the process of polycondensation, the hydrolyzed MTES will migrate to the nucleation site driven by surface tension, thus forming an eccentric core/shell (solvent/polysiloxane) structure. Due to the different forces between hydrolyzed MTES and different solvents, the deviation degree of hollow in microspheres is different, thus forming particles with various morphologies. This synthesis method provides a new idea for the preparation of shapeable anisotropic hollow structures. Finally, we use AHPM to study the application of the drug load. The results show that the prepared hollow polysiloxane particles have a good drug loading capacity and release performance. It can be predicted that the shape-tunable hollow polysiloxane particles prepared by this method have broad application prospects in the field of drug delivery.

BaSO4-Epoxy Resin Composite Film for Efficient Daytime Radiative Cooling

Conventional cooling methods are based on active cooling technology by air conditioning, which consumes a large amount of energy and emits greenhouse gases. Radiative cooling is a novel promising passive cooling technology that uses external space as the cooling source and requires no additional energy consumption. Herein, we propose an approach to prepare highly dispersed BaSO4 nanoparticles (NPs) using a direct precipitation method combined with the in situ surface modification technology. The as-prepared PVP-modified BaSO4 NPs with an average size of 20 nm can be stably dispersed in ethanol for more than 6 months and then were used as building blocks to prepare spherical BaSO4 clusters with an average size of 0.9 μm using a scalable spray drying technique. The BaSO4 NPs/clusters (mass ratio 1:1) were used for preparing radiative cooling epoxy resin film, showing a high solar reflectance of 71% and a high sky window emissivity of 0.94. More importantly, this composite film displays superior radiative cooling performance, which can reduce the ambient temperature by 13.5 °C for the indoor test and 7 °C for the outdoor test. Compared with the commercial BaSO4 filled film, our BaSO4-epoxy resin composite film offers advantages not only in radiative cooling but also in mechanical properties with a 16.6% increase of tensile strength and 40.1% increase of elongation at break, demonstrating its great application potential in the field of building air conditioning.

Surface Tension-Induced Eccentric Hollow Polysiloxane Microspheres in a Surfactant-Free System and Their Applications as a Nanoreactor and Nanomotor

Eccentric hollow polysiloxane microspheres (EHPMs) have attracted significant attention due to their potential in energy storage, drug delivery, and heterogeneous catalysis applications. However, their preparation pathways are often particularly complex. Therefore, it is critical to find a simple method for preparing EHPMs. In this study, a surfactant-free emulsification method is proposed to prepare EHPM. Under acidic conditions, methyl triethoxysilane (MTES) is hydrolyzed at the oil-water interface, with the hydrolyzed MTES demonstrating amphiphilic properties, and it could be anchored on the xylene surface to form an oil-in-water emulsion. The solution, when adjusted to alkaline, nucleated from a point at the oil-water interface. Driven by the surface tension, the hydrolyzed MTES migrated to the nucleation site with decreasing hydrophilicity. As a result, an EHPM formed. This process provides a simple, low cost, and environmentally friendly strategy for the preparation of EHPM, which demonstrated potential in catalytic and nanomotor applications.

Diffusion-mediated carving of interior topologies of all-natural protein nanoparticles to tailor sustained drug release for effective breast cancer therapy

Proteins are promising base materials for developing drug carriers with efficient blood circulation due to low possibilities of clearance by macrophages. However, such natural biopolymers have highly sophisticated molecular structures, preventing them from being assembled into nano-platforms with manipulable payload release profiles. Here, we report the self-assembly of two natural proteins (milk casein and rice protein) into protein nanoparticles (NPs, ∼150 nm) with tailorable release profiles. Diffusion of plant-derived paclitaxel (PTX)-containing eugenol into the hydrophobic cores of the NPs and subsequent dialysis to remove eugenol from the cores lead to the carving of the NP interiors. With the increase in the mass ratios of casein and rice protein, this process generates all-natural NPs with PTX loaded in their full cavities, semi-full cavities, or solid cores. These NPs can be efficiently uptaken by breast cancer cells and could kill the cancer cells efficiently. PTX in these NPs demonstrates increasingly sustained in vivo release profiles from full cavities, semi-full cavities, to solid cores, gradually extending its pharmacokinetic profiles in blood plasma to favor drug accumulation in breast tumor models. Consequently, the NPs with solid cores completely inhibit tumor growth in vivo, more effectively than those with full and semi-full cavities. Our work opens up a new avenue to the use of diffusion-mediated nanoscale carving in producing biomaterials with controllable interior topologies relevant to drug release profiles.

The enhanced removal of arsenite from water by double-shell CuOx@MnOy hollow spheres (DCMHS): behavior and mechanisms

To facilitate removing As(III) from water through an "oxidation-adsorption" process, the double-shell CuO_x@MnO_y hollow spheres (DCMHS) have been fabricated via a two-step co-precipitation route combined with the soft-template method. The surface characterization results showed that Mn oxides were formed without segregation and uniformly distributed on the surface of CuO_x hollow spheres. DCMHS could achieve outstanding performance to remove As(III) with an As maximum adsorption capacity of 32.15 mg/g. Meanwhile, the kinetics results illustrated that the oxidative activity of DCMHS was strengthened due to its specific structure, and part of As(III) was converted to As(V) during the adsorption process. Also, air aeration could further enhance As(III) oxidation and thus improving As removal. The As(III) removal performance could be maintained under neutral and weak alkaline conditions. Phosphate, silicate, and carbonate anions could depress the removal performance, while chloride ions and sulfate anions barely influenced As removal. Moreover, DCMHS could be regenerated using NaOH and KMnO₄ solutions without breaking the hollow sphere structure. Based on the spectroscopic analysis results, As(III) molecules were converted to As(V) via two pathways, including the oxidation by Mn oxides or superoxide radicals. The Cu-Mn synergistic effect could not only enhance the oxidative activity of Mn oxides but also produce superoxide radicals via the activation of surface-adsorbed oxygen molecules. Afterwards, the newly formed As(V) could be attached to the hydroxyl groups through surface complexation. Therefore, this work has provided insights into the morphology design of Mn-oxide-containing adsorbents and supplemented the interface reaction mechanisms for enhancing As(III) removal.

Complex Polymeric Microstructures with Programmable Architecture via Pickering Emulsion-Templated In Situ Polymerization

Aside from smooth and spherical microcapsules, the concept of tailoring complex polymeric microstructures is being taken a step ahead due to their great demand in various applications and fundamental studies in the subjects of microfluidics and nanotechnology. Size, shape, and morphology are of paramount importance for their functional performance and various applications. However, simple, inexpensive, versatile, and high-throughput techniques for fabricating microcapsules with controlled morphology remain a bottleneck for discoveries in the subject of polymer colloids. In this paper, we directly fulfill this need by reporting a novel approach of Pickering emulsion-templated in situ polymerization for tailoring complex polymeric microstructures comprised of a composite shell of titanium dioxide nanoparticle (TiO₂ NP)-embedded poly(melamine-urea-formaldehyde) (polyMUF) and a core of hexadecane (HD, soft template). At first, we hydrophobize TiO₂ NPs by chemisorbing long-chain biobased myristic acid via a bidentate chelating complex and precisely tune their wettability by varying the grafting density of myristic acid to obtain highly stable oil-in-water (O/W) Pickering emulsion. Thereafter, we employ the optimized TiO₂ NPs in the intended encapsulation strategy that enables various microstructures and morphologies with the particle diameter ranging from 5 to 20 μm. Careful manipulation of reaction parameters and copolymer components leads to novel complex microstructures: smooth, raspberry-like, partially budded, hollow, filled, single-holed, and closed-cell-like microstructures. Particle properties such as morphology, size, shell thickness, and core content are governed by the TiO₂ NP content, core-to-shell ratio, copolymer component, conversion, and pH value. Based on the results of a series of control experiments, novel mechanisms for the formation of various such microstructures are proposed.

Inorganic reinforced poly(ionic liquid) microcapsules: confined cooling-assisted phase separation self-assembly and enhanced electro-responsive property

We report a simple preparation of inorganic reinforced poly(ionic liquid) (PIL) microcapsules by combining dispersion polymerization and confined cooling-assisted phase separation self-assembly. Silane coupling agent-modified PIL microbeads were first prepared by dispersion polymerization. Then, the microbeads were dissolved in a mixed solvent composed of good solvent and non-solvent to form hollow SiO_x microcapsules at a relatively high temperature. Finally, the solution was cooled to induce the nucleation and growth of dissolved PIL chains on the inner and outer surface of hollow SiO_x microcapsules to form inorganic reinforced microcapsules with asymmetric PIL/SiO_x /PIL sandwich-like shell. The morphology of microcapsules can be controlled by adjusting PIL concentration and cooling rate. The inorganic reinforced microcapsules show enhanced suspended stability and electro-responsive characteristic when used as the dispersed phase of smart suspensions. This article is protected by copyright. All rights reserved.

A surprisingly gentle approach to cavity containing spherocylindrical microparticles from ordinary polymer dispersions in flow

Herein, we demonstrate a facile approach to fully transform spherical polymeric microparticles to elongated spherocylinders containing an internal cavity under ambient and mild stirring conditions. Critical to the process is to deform the amorphous and non-crosslinked particles under glassy conditions for an unusually long time; 120 hours for the poly(styrene-co-glycidyl methacrylate) microparticles discussed in greatest detail. Larger particles in the 5 micron and greater range were markedly more susceptible to the shear imposed by stirring the aqueous dispersion. The resulting morphology is robust and kinetically frozen yet reverts to the original spherical shape if annealed above the glass transition temperature with suitable temperature or plasticizer. The volume fraction of the internal void can be modulated by particle composition and process conditions and is irregular in shape we believe as a result of a cavitation event during plastic deformation.

Preparation of Salt-Responsive Hollow Hydrophilic Polymer Particles by Inverse Suspension Polymerization

Hydrophilic polymer particles with a hollow structure have potential applications such as carriers for hydrophilic drugs. However, there are few reports on preparation and morphology control of such particles via a simple method. In this study, hollow hydrophilic polymer particles were prepared by inverse suspension polymerization for water droplets containing 2-acrylamido-2-methyl-1-propanesulfonic acid (AMPS) anions, 1-vinylimidazole (VIm) cations, oligo(ethylene glycol) diacrylate (OEGDA), dextran, and an initiator via the self-assembling phase-separated polymer (SaPSeP) method developed in our lab. The inner morphology of the particle could be controlled (as single- or multi-hollow structures) by changing the concentrations of the OEGDA and the dextran. The obtained hollow particles could encapsulate a hydrophilic fluorescent substance in their hollow region when the substance was added to the primary droplets before polymerization. In addition, the poly(AMPS-co-VIm-co-OEGDA) shell of the particles exhibited an ionic cross-linked structure, which could be stimulated by salt. The poly(AMPS-co-VIm-co-OEGDA) hollow particles with the encapsulated substance released the substance when salt was added to the dispersion. These results indicated that the applicability of the SaPSeP method can be broadened for morphology control of the hydrophilic polymer particles encapsulating water-soluble materials.

Molecular Bottom-Up Approaches for the Synthesis of Inorganic and Hybrid Nanostructures

Chemical routes for the synthesis of nanostructures are fundamental in nanoscience. Among the different strategies for the production of nanostructures, this article reviews the fundamentals of the bottom-up approaches, focusing on wet chemistry synthesis. It offers a general view on the synthesis of different inorganic and hybrid organic–inorganic nanostructures such as ceramics, metal, and semiconductor nanoparticles, mesoporous structures, and metal–organic frameworks. This review article is especially written for a wide audience demanding a text focused on the basic concepts and ideas of the synthesis of inorganic and hybrid nanostructures. It is styled for both early researchers who are starting to work on this topic and also non-specialist readers with a basic background on chemistry. Updated references and texts that provide a deeper discussion and describing the different synthesis strategies in detail are given, as well as a section on the current perspectives and possible future evolution.

Unimodal sized silica nanocapsules produced through water-in-oil emulsions prepared by sequential irradiation of kilo- and submega-hertz ultrasounds

This study investigates the regulation of the size of 100 nm hollow-sphere silica particles using surfactant-free water-in-oil (W/O) emulsion. First, water droplets were dispersed in soybean oil via sequential ultrasound irradiation (28 kHz → 200 kHz → 950 kHz). A precursor of hollow silica particles was prepared using hydrolysis and polymerization of methylsilyl trichloride into a stable W/O emulsion. The final structure/morphology of the silica particles was influenced by the volume ratio of water/soybean oil, the cycle number of the sequential ultrasound irradiation, and the amount of organosilane added to the emulsion. The emulsion was stabilized by Ostwald ripening, as the size distribution at 5/10³ (water/oil = v/v) was a bimodal split between a water droplet size of a few μm and some with a size of a few tens of nm. The most appropriate cycle number was 3 in this system. Further cycling to 5 resulted in a broad and bimodal size distribution of the final particles due to rapid coalescence of water droplets. Subsequent hydrolysis of methylsilyl trichloride consumed water with diminishing large droplets, forming fine and unimodal (0.12 ± 0.02 μm) hollow silica particles. Very fine and uniform-sized hollow particles (0.08 ± 0.01 μm) were successfully produced by decreasing the volume ratio to 1/10³ (water/oil) because of a transparent stable emulsion as a homogeneous template of the hollow structures.

Silica nanocapsules were prepared using water droplets dispersed in soybean oil via sequential ultrasound irradiation (28 kHz → 200 kHz → 950 kHz).

Surface Permeability of Membrane and Catalytic Performance Based on Redox-Responsive of Hybrid Hollow Polymeric Microcapsules

"Smart" polymeric microcapsules with excellent permeability of membranes have drawn considerable attention in scientific and industrial research such as drug delivery carriers, microreactors, and artificial organelles. In this work, hybrid hollow polymeric microcapsules (HPs) containing redox-active gold-sulfide bond were prepared with bovine serum albumin, inorganic metal cluster (AuNCs), and poly(N-isopropylacrylamide) conjugates by using Pickering emulsion method. HPs were transferred from water-in-oil to water-in-water by adding PEGbis(N-succinimidylsuccinate). To achieve redox-responsive membrane, the Au-S bond units incorporated into the microcapsules' membranes, allowed us to explore the effects of a new stimuli, that is, the redox Au-S bond breaking on the microcapsules' membranes. The permeability of these hybrid hollow polymeric microcapsules could be sensitively tuned via adding environment-friendly hydrogen peroxide (H2O2), resulting from a fast fracture of Au-S bond. Meanwhile, AuNCs and conjugates could depart from the microcapsules, and enhance the permeability of the membrane. Based on the excellent permeability of the membrane, phosphatase was encapsuled into HPs and p-nitrophenyl phosphate as a substrate. After adding 1 × 10-2 and 1 × 10-4 M H2O2, the catalytic efficiency was nearly 4.06 and 2.22 times higher than that of HPs in the absence of H2O2, respectively. Hence, the unique redox-responsive HPs have potential applications in biocatalytic reaction, drug delivery, and materials as well as in bioscience.

A novel self-templating strategy for facile fabrication of monodisperse polymeric microporous capsules with a tunable hollow structure

The self-templating fabrication of monodisperse polymeric microporous capsules with a tunable hollow structure for gas storage, efficient iodine capture and heterogeneous catalysis.

Performance and mechanisms of sulfadiazine removal using persulfate activated by Fe3O4@CuOx hollow spheres

A Fe-Cu bimetal catalyst (FCHS) was synthesized by depositing Fe₃O₄ on the shell of CuO_x hollow spheres, which were prepared via a soft template method. Several characterization methods, including XRD, SEM-EDS&mapping, TEM, FTIR, and XPS, were used to reveal the morphology and surface properties of FCHS. The characterization results demonstrated that the double-shell hollow structure is formed with a dense coating of Fe₃O₄ nanoparticles on the surface of CuO_x hollow spheres. FCHS can exhibit excellent catalytic activity to degrade sulfadiazine (SDZ) with the oxidant of persulfate (PS). The optimal SDZ removal performance was explored by adjusting reaction parameters, including catalyst dosage, oxidant dosage, and solution pH. The SDZ removal efficiency in the FCHS + PS system could reach 95% at the optimal reaction condition ([catalyst]₀ = 0.2 g/L, [PS]₀ = 2 mM, pH = 7.0) with 5 mg/L of SDZ. Meanwhile, the degradation efficiency decreased with the coexistence of phosphate or carbonate anions. According to the results of radicals scavenging experiments and the electron paramagnetic resonance analysis, the radicals of SO₄·^-, O₂·^- and ·OH generated in the FCHS + PS system contribute to the degradation of SDZ. Moreover, the results of XPS revealed that the solid-state charge-transfer redox couple of Fe(III)/Fe(II) and Cu(I)/Cu(II) can promote the activation of PS. It means that the cooperation effect between Cu oxides and Fe oxides in the double-shell structure is beneficial to the catalytic degradation of SDZ. Furthermore, four possible pathways for SDZ degradation were proposed according to the analysis of intermediate products detected by the LCMS-IT-TOF.

Hollow polymer nanocapsules with a ferrocenyl copolymer shell

Hollow polymer nanocapsules consisting of ferrocenyl shell have been developed by crosslinking the polymer chains grafted over silica nanoparticles synthesized via one pot surface-initiated RAFT polymerization.

Incorporation Behavior of Nonionic Emulsifiers inside Particles and Secondary Particle Nucleation during Emulsion Polymerization of Styrene

In previous studies, it was found that a large amount of the polyoxyethylene nonylphenyl ether nonionic emulsifier, Emulgen 911 (E911), was incorporated inside polymer particles obtained at the completion of a conventional emulsion polymerization with potassium persulfate as a typical water-soluble initiator. In this study, to understand the incorporation phenomenon in more detail, the incorporation behavior during a batch emulsion polymerization of styrene was investigated. The percentage of E911 incorporated inside polystyrene (PS) particles relative to the total weight of E911 used in the polymerization increased to 18% until 50% conversion and then decreased and levelled off to 13% at 70-80% conversion. A similar incorporation behavior was observed in a seeded emulsion polymerization of styrene, in which E911 was added to an emulsifier-free PS seed emulsion to set the same E911 concentration as the batch emulsion polymerization before swelling of the PS seed particles with styrene at 70 °C for 24 h. These indicate that E911 molecules absorbed once into styrene-swollen PS particles partially exited therefrom into the aqueous medium during the polymerizations. When the E911 concentration in the aqueous medium is kept above the critical micelle concentration for a long period by the repartitioning process of E911 from styrene droplets into styrene-swollen particles via aqueous medium and/or exiting from the styrene-swollen particles into the aqueous medium, secondary particle nucleation continues during the polymerization, resulting in PS particles having a broad size distribution.

Water-assisted synthesis of mesoporous calcium carbonate with a controlled specific surface area and its potential to ferulic acid release

A carbonation process to control the specific surface area of mesoporous calcium carbonate and the dissolution profile of ferulic acid on mesoporous carbonate particles are presented. The effects of water content on the physicochemical properties, specific surface area, pore size, crystallinity, and morphology are evaluated. Mesoporous calcium carbonate particles are synthesised with well-controlled specific surface areas of 38.8 to 234 m² g⁻¹. Each of the submicron-size secondary particles consists of a primary particle of nano-size. During secondary particle formation, primary particle growth is curbed in the case with less water content. By contrast, growth is promoted via dissolution and recrystallisation in the presence of water. The release rates of ferulic acid are gradually enhanced with increasing specific surface area of the mesoporous calcium carbonate, that reflects crystallinity of ferulic acid.

A carbonation process to control the specific surface area of mesoporous calcium carbonate and the dissolution profile of ferulic acid on mesoporous carbonate particles are presented.

Template Regeneration in Galvanic Replacement: A Route to Highly Diverse Hollow Nanostructures

The ability to produce a diverse spectrum of hollow nanostructures is central to the advances in many current and emerging areas of technology. Herein, we report a general method to craft hollow nanostructures with highly tunable physical and chemical parameters. The key strategy is to regenerate the nanoscale sacrificial templates in a galvanic replacement reaction through site-selective overgrowth. As examples, we demonstrate the syntheses of nanocages and nanotubes made of silver, gold, palladium, and/or platinum with well-controlled wall thicknesses and elemental distributions. Using the nanocages of silver and gold as models, we demonstrate they possess intriguing plasmonic properties and offer superior performance in biosensing applications. This study provides a powerful platform to customize hollow nanostructures with desired properties and therefore is expected to enable a variety of fundamental studies and technologically important applications.