Zhihai Cao

Recent advances in the preparation of hybrid nanoparticles in miniemulsions.

In this review, we summarize recent advances in the synthesis of hybrid nanoparticles in miniemulsions since 2009. These hybrid nanoparticles include organic-inorganic, polymeric, and natural macromolecule/synthetic polymer hybrid nanoparticles. They may be prepared through encapsulation of inorganic components or natural macromolecules by miniemulsion (co)polymerization, simultaneous polymerization of vinyl monomers and vinyl-containing inorganic precursors, precipitation of preformed polymers in the presence of inorganic constituents through solvent displacement techniques, and grafting polymerization onto, from or through natural macromolecules. Characterization, properties, and applications of hybrid nanoparticles are also discussed.

Preparation of Raspberry-like Nanocapsules by the Combination of Pickering Emulsification and Solvent Displacement Technique

Well-defined raspberry-like nanocapsules were prepared by the combination of Pickering emulsification and solvent displacement technique by using silica particles as stabilizer and hexadecane (HD) as soft template. The formation of the capsule morphology is caused by the phase separation of poly(styrene-co-4-vinyl pyridine) (poly(St-co-4-VP)) in the droplets due to the diffusion of good solvent for the (co)polymer to the aqueous continuous phase. The size of capsules was successfully reduced from tens of micrometers in the dispersion by simply stirring to the nanorange by the employment of sonication and Ostwald ripening. The formation of silica-particles-armored nanocapsules was confirmed by transmission electron microscopy (TEM), high-resolution scanning electron microscopy (HRSEM), dynamic light scattering (DLS), and zeta potential measurement. The colloidal stability and particle properties, including size and morphology, depend on the amount of HD, and copolymers, the sonication time, the dispersion pH value, the type of solvent, and the copolymer composition.

Preparation of Mesoporous Submicrometer Silica Capsules via an Interfacial Sol–Gel Process in Inverse Miniemulsion

Mesoporous silica capsules with submicrometer sizes were successfully prepared via the interfacial hydrolysis and condensation reactions of tetraethoxysilane (TEOS) in inverse miniemulsion by using hydrophilic liquid droplets as template. The inverse miniemulsions containing pH-controlled hydrophilic droplets were first prepared via sonication by using poly(ethylene-co-butylene)-b-poly(ethylene oxide) (P(E/B)-PEO) or SPAN 80 as surfactant. TEOS was directly introduced to the continuous phase of an inverse miniemulsion. The silica shell was formed by the deposition of silica on the surface of droplets. The formation of capsule morphology was confirmed by transmission electron microscopy (TEM) and field emission scanning electron microscopy (FESEM). The mesoporous structure was verified by nitrogen sorption measurements. The specific surface area could be tuned by the variation of the amount of cetyltrimethylammonium bromide (CTAB) and TEOS, and the pore size by the amount of CTAB. The influences of synthetic parameters on the particle size and morphology were investigated in terms of the amount of CTAB, pH value in the droplets, TEOS amount, surfactant amount, and type of solvent with low polarity. A formation mechanism of silica capsules was proposed.

Narrowly size-distributed cobalt salt containing poly(2-hydroxyethyl methacrylate) particles by inverse miniemulsion.

Cobalt-containing hybrid particles have been prepared through the encapsulation of cobalt tetrafluoroborate hexahydrate (CoTFB) via inverse miniemulsion polymerization of 2-hydroxyethyl methacrylate (HEMA). We systematically varied the amount and type of cosolvent (water, methanol, ethanol, ethylene glycol), apolar continuous phase (cyclohexane, isooctane, isopar M, hexadecane), amount of cobalt salt, and molecular weight of the polymeric surfactant. The influence of those parameters on the particle size, size distribution, and particle morphology were investigated. Narrowly size-distributed hybrid particles with good colloidal stability could be obtained in a wide range of cobalt content between 5.7 and 22.6 wt % salt relative to the monomer. The addition of a cosolvent such as water not only promotes the loading of metal salt but also has a positive influence on narrowing the particle size distribution. We assume that generally narrowly size-distributed particles can be obtained for a large variety of combinations of polar/apolar phase by adjusting the balance between osmotic and Laplace pressure via the solubility of the metal salt in the continuous phase and lowering the interfacial tension by adjusting the hydrophilic-lipophilic balance (HLB) value of the surfactant. The results show a significant advantage of the inverse miniemulsion over the direct system with respect to the variability and total amount of metal salt without losing the narrow particle size distribution and colloidal stability.

Inverse Pickering Emulsions with Droplet Sizes below 500 nm

Inverse Pickering emulsions with droplet diameters between 180 and 450 nm, a narrow droplet size distribution, and an outstanding stability were prepared using a miniemulsion technique. Commercially available hydrophilic silica nanoparticles were used to stabilize the emulsions. They were hydrophobized in situ by the adsorption of various neutral polymeric surfactants. The influence of different parameters, such as kind and amount of surfactant as hydrophobizing agent, size and charge of the silica particles, and amount of water in the dispersed phase, as well as the kind of osmotic agent (sodium chloride and phosphate-buffered saline), on the emulsion characteristics was investigated. The systems were characterized by dynamic light scattering, transmission electron microscopy, cryo-scanning electron microscopy (cryo-SEM), thermogravimetric analysis, and semiquantitative attenuated total reflection infrared spectroscopy. Cryo-SEM shows that some silica particles are obviously rendered hydrophilic and form a three-dimensional network inside the droplets.

Synthesis of Silver/Poly(2-hydroxyethyl methacrylate) Particles via a Combination of Inverse Miniemulsion and Silver Ion Reduction in a “Nanoreactor”

Silver salt/poly(2-hydroxyethyl methacrylate) (poly(HEMA)) hybrid particles were first prepared by inverse miniemulsion polymerization of 2-hydroxyethyl methacrylate (HEMA) with silver tetrafluoroborate (AgBF(4)) as a lipophobe. High silver salt loads of up to 13% with respect to the disperse phase were achieved. The silver/poly(HEMA) hybrid particles were subsequently formed via a gas-phase in situ reduction of AgBF(4) by hydrazine on the surfaces of silver salt/poly(HEMA) particles. The formation of silver nanoparticles was confirmed by UV-vis spectroscopy and X-ray diffraction. The morphology of the hybrid particles of silver salt/poly(HEMA) and silver/poly(HEMA) was fully characterized by transmission electron microscopy (TEM), atomic force microscopy (AFM), and dynamic light scattering (DLS). The influence of the reaction parameters including the type and amount of cosolvent, salt content, and type of surfactant on the particle properties and colloidal stability during the reduction process was thoroughly investigated.

Transition-Metal Salt-Containing Silica Nanocapsules Elaborated via Salt-Induced Interfacial Deposition in Inverse Miniemulsions as Precursor to Functional Hollow Silica Particles

Aqueous core-silica shell nanocapsules were successfully prepared using liquid droplets containing transition-metal salt as templates in inverse miniemulsions. The formation of the silica shell was attributed to the interfacial deposition of silica species induced by the presence of the transition-metal salt. In addition to the control of the particle morphology, the incorporated transition-metal salts could be used to derivatize the particles and confer additional functionalities to the hollow silica particles. To demonstrate the derivatization, the magnetic hollow silica particles were prepared by converting iron salts to magnetic iron oxides by heat treatment. The particle morphology, size, and size distribution were characterized by transmission electron microscopy and scanning electron microscopy. The results show that the particle properties strongly depend on the type and the amount of salts, the amount of tetraethoxysilane (TEOS), the pH of the droplets, and the ratios of 2-hydroxyethyl methacrylate to aqueous HCl solution. The specific surface area and pore properties were characterized by N2 sorption measurements. The pore properties and specific surface area could be tuned by varying the amount of salt. Levels of elements and of iron oxides in the magnetic hollow particles were measured by energy-dispersive X-ray spectroscopy. Iron was distributed homogenously with silicon and oxygen in the sample. The magnetization measured by a magnetic property measurement system confirmed the successful conversion of the iron salts to magnetic iron oxides.

Fabrication of nanogel core-silica shell and hollow silica nanoparticles via an interfacial sol-gel process triggered by transition-metal salt in inverse systems

Nanogel (hydrophilic polymer nanoparticles) core-silica shell nanoparticles were successfully fabricated via hydrolysis and condensation reaction of tetraethoxysilane (TEOS). Transition-metal tetrafluoroborate-containing nanogels were used as templates for fabrication in inverse systems (cyclohexane as continuous phase). Magnetic, hollow silica particles were subsequently formed by removing the polymer core and converting iron salts to iron oxides via heat treatment. We propose that the formation of the core-shell morphology is induced by the promoted precipitation of silica species at the surface of nanogels due to the interaction between silica species and transition-metal tetrafluoroborate. The influence of the synthesis parameters (type and amount of salts, pH of the nanogels, and amount of TEOS) on the particle morphology was systematically investigated. The pore properties and specific surface area of the hollow silica particles could be modified by the varying the amount of salt.

Synthesis of narrowly size-distributed metal salt/poly(HEMA) hybrid particles in inverse miniemulsion: versatility and mechanism.

Hybrid particles containing different hydrophilic metal salts such as tetrafluoroborates of iron(II), cobalt(II), nickel(II), copper(II), and zinc(II), and nitrates of cobalt(II), nickel(II), copper(II), zinc(II), and iron(III), and cobalt(II) chloride were synthesized via inverse miniemulsion polymerization of 2-hydroxyethyl methacrylate (HEMA). All salts delivered narrowly size-distributed hybrid particles with the exception of iron(III), where only the nitrate salt could be successfully employed. The size and size distribution of the hybrid particles were characterized by dynamic light scattering (DLS) and transmission electron microscopy (TEM). The particle morphology and the distribution of salt in the dried particles were observed by TEM. The influences of the type of metal salts and salt content on the particle size distribution were extensively investigated.

Preparation of Janus Pd/SiO2 Nanocomposite Particles in Inverse Miniemulsions

Janus Pd/SiO2 nanocomposite particles (NCPs) were successfully synthesized through a combination of the sol-gel process of tetramethoxysilane in inverse miniemulsions and in situ reduction of Pd salts via a gas diffusion process of hydrazine. The formation of Pd nanoparticles (NPs) was verified by X-ray diffraction. The Janus morphology of the Pd/SiO2 NCPs was confirmed by microscopic observation. The Pd/SiO2 NCPs displayed a mesoporous structure. The content of Pd NPs in the NCPs could be conveniently adjusted by the K2PdCl4 loading. A formation mechanism of the Janus Pd/SiO2 NCPs was proposed. The mesoporous Janus Pd/SiO2 NCPs show good catalytic activity toward the reduction of p-nitrophenol with NaBH4.