Growth Kinetics of Nanosize Silica in a Nonionic Water-in-Oil Microemulsion: A Reverse Micellar Pseudophase Reaction Model

In Situ and Real-Time Monitoring of Nanoparticle Formation in Microemulsion by Means of Dielectric Spectroscopy

Dielectric spectroscopy was employed, for the first time, to monitor the formation process of silica nanoparticles in a nonionic surfactant-based microemulsion in situ and in real time. Two dominant relaxations were observed in the frequency range of 1 MHz-3 GHz during this process. The relaxation at the lower frequency range was confirmed to be mainly ascribed to interfacial polarization, whose relaxation parameters, together with the electrical property of the synthesis system, were used to characterize the evolution of this dynamic formation process. Four evolution stages are distinctively revealed, including an induction stage, a nucleation dominant stage, an early particle growth stage, and a late growth stage. The dynamic features at each evolution stages were discussed in terms of the dielectric characteristics of the system. It is strongly suggested that dielectric spectroscopy is an effective tool for the in situ mechanistic study of nanoparticle formation in microemulsion.

Coating of upconversion nanoparticles with silica nanoshells of 5-250 nm thickness

A concept for the growth of silica shells with a thickness of 5-250 nm onto oleate-coated NaYF₄:Yb³⁺/Er³⁺ upconversion nanoparticles (UCNP) is presented. The concept enables the precise adjustment of shell thicknesses for the preparation of thick-shelled nanoparticles for applications in plasmonics and sensing. First, an initial 5-11 nm thick shell is grown onto the UCNPs in a reverse microemulsion. This is followed by a stepwise growth of these particles without a purification step, where in each step equal volumes of tetraethyl orthosilicate and ammonia water are added, while the volumes of cyclohexane and the surfactant Igepal^® CO-520 are increased so that the ammonia water and surfactant concentrations remain constant. Hence, the number of micelles stays constant, and their size is increased to accommodate the growing core-shell particles. Consequently, the formation of core-free silica particles is suppressed. When the negative zeta potential of the particles, which continuously decreased during the stepwise growth, falls below -40 mV, the particles can be dispersed in an ammoniacal ethanol solution and grown further by the continuous addition of tetraethyl orthosilicate to a diameter larger than 500 nm. Due to the high colloidal stability, a coalescence of the particles can be suppressed, and single-core particles are obtained. This strategy can be easily transferred to other nanomaterials for the design of plasmonic nanoconstructs and sensor systems.

Interface formation during silica encapsulation of colloidal CdSe/CdS quantum dots observed by in situ Raman spectroscopy

We investigate the encapsulation of CdSe/CdS quantum dots (QDs) in a silica shell by in situ Raman spectroscopy and find a distinct shift of the CdS Raman signal during the first hours of the synthesis. This shift does not depend on the final silica shell thickness but on the properties of the initial core-shell QD. We find a correlation between the Raman shift rate and the speed of the silica formation and attribute this to the changing configuration of the outermost layers of the QD shell, where an interface to the newly formed silica is created. This dependence of Raman shift rate on the speed of silica formation process will give rise to many possible studies concerning the growth mechanism in the water-in-oil microemulsion, rendering in situ Raman a valuable instrument in monitoring this type of reaction.

One-Pot Synthesis of Size- and Composition-Controlled Ni-Rich NiPt Alloy Nanoparticles in a Reverse Microemulsion System and Their Application

Bimetallic nanoparticles have been the subject of numerous research studies in the nanotechnology field, in particular for catalytic applications. Control of the size, morphology, and composition has become a key challenge due to the relationship between these parameters and the catalytic behavior of the particles in terms of activity, selectivity, and stability. Here, we present a one-pot air synthesis of 2 nm Ni₉Pt₁ nanoparticles with a narrow size distribution. Control of the size and composition of the alloy particles is achieved at ambient temperature, in the aqueous phase, by the simultaneous reduction of nickel and platinum precursors with hydrazine, using a reverse microemulsion system. After deposition on an alumina support, this Ni-rich nanoalloy exhibits unprecedented stability under the harsh conditions of methane dry reforming.

Construction of Silica-Based Micro/Nanoplatforms for Ultrasound Theranostic Biomedicine

Ultrasound (US)-based biomedicine has been extensively explored for its applications in both diagnostic imaging and disease therapy. The fast development of theranostic nanomedicine significantly promotes the development of US-based biomedicine. This progress report summarizes and discusses the recent developments of rational design and fabrication of silica-based micro/nanoparticles for versatile US-based biomedical applications. The synthetic strategies and surface-engineering approaches of silica-based micro/nanoparticles are initially discussed, followed by detailed introduction on their US-based theranostic applications. They have been extensively explored in contrast-enhanced US imaging, US-based multi-modality imaging, synergistic high-intensity focused US (HIFU) ablation, sonosensitizer-enhanced sonodynamic therapy (SDT), as well as US-triggered chemotherapy. Their biological effects and biosafety have been briefly discussed to guarantee further clinical translation. Based on the high biocompatibility, versatile composition/structure and high performance in US-based theranostic biomedicine, these silica-based theranostic agents are expected to pave a new way for achieving efficient US-based theranostics of disease by taking the specific advantages of material science, nanotechnology and US-based biomedicine.

Improved Synthesis of Nanosized Silica in Water-in-Oil Microemulsions

Present contribution describes modified Stöber synthesis of silica nanoparticles in oil-in-water microemulsion, formulated using heptane, 2-ethylhexanol, Tween® 85 nonionic surfactant, and tetraethyl orthosilicate (TEOS). After some specified incubation time, ammonium hydroxide was added and the reaction mixture was stirred for 24 hours at room temperature. Prior to synthesis, pseudoternary diagram was created for oil-rich area and Winsor IV region was identified. These microemulsions were used for synthesis of silica particles. Resulting particles were characterized by dynamic light scattering, electrokinetic measurements, specific surface area measurements, and powder diffraction. Particles’ diameter was ranging between ca. 130 and 500 nm; usually monodisperse distribution was obtained. The specific surface area of nanoparticles was ranging between 250 and 300 m2/g. Notably, productivity per unit volume of solution was 3 to 5 times higher than for previously reported procedures. Our method can be extended, because polymeric materials can be added to dispersed aqueous phase. In our studies, β-cyclodextrin and hydroxyethylcellulose have been used, giving particles between 170 and 422 nm, with the surface area larger than 300 m2/g.

Facile preparation of doxorubicin-loaded upconversion@polydopamine nanoplatforms for simultaneous in vivo multimodality imaging and chemophotothermal synergistic therapy

The development of biosafe nanoplatforms with diagnostic and therapeutic multifunction is extremely demanded for designing cancer theranostic medicines. Here, a facile methodology is developed to construct a multifunctional nanotheranostic that gathers five functions, upconversion luminescence (UCL) imaging, T1-weighted magnetic resonance imaging (MRI), X-ray computed tomography (CT) imaging, photothermal therapy (PTT), and chemotherapy, into one single nanoprobe (named as UCNP@PDA5-PEG-DOX). For generating the UCNP@PDA5-PEG-DOX, a near-infrared light (NIR)-absorbing polydopamine (PDA) shell is directly coated on oleic-acid-capped β-NaGdF4:Yb(3+),Er(3+)@β-NaGdF4 upconverting nanoparticle (UCNP) core for the first time to form monodisperse, ultrastable, and noncytotoxic core-shell-structured nanosphere via water-in-oil microemulsion approach. When combined with 808 nm NIR laser irradiation, the UCNP@PDA5-PEG-DOX shows great synergistic interaction between PTT and the enhanced chemotherapy, resulting in completely eradicated mouse-bearing SW620 tumor without regrowth. In addition, leakage study, hemolysis assay, histology analysis, and blood biochemistry assay unambiguously reveal that the UCNP@PDA5-PEG has inappreciable cytotoxicity and negligible organ toxicity. The results provide explicit strategy for fabricating multifunctional nanoplatforms from the integration of UCNP with NIR-absorbing polymers, important for developing multi-mode nanoprobes for biomedical applications.

Silica nanoparticles for cell imaging and intracellular sensing

There is increasing interest in the use of nanoparticles (NPs) for biomedical applications. In particular, nanobiophotonic approaches using fluorescence offers the potential of high sensitivity and selectivity in applications such as cell imaging and intracellular sensing. In this review, we focus primarily on the use of fluorescent silica NPs for these applications and, in so doing, aim to enhance and complement the key recent review articles on these topics. We summarize the main synthetic approaches, namely the Stöber and microemulsion processes, and, in this context, we deal with issues in relation to both covalent and physical incorporation of different types of dyes in the particles. The important issue of NP functionalization for conjugation to biomolecules is discussed and strategies published in the recent literature are highlighted and evaluated. We cite recent examples of the use of fluorescent silica NPs for cell imaging in the areas of cancer, stem cell and infectious disease research, and we review the current literature on the use of silica NPs for intracellular sensing of oxygen, pH and ionic species. We include a short final section which seeks to identify the main challenges and obstacles in relation to the potential widespread use of these particles for in vivo diagnostics and therapeutics.

Nonporous Silica Nanoparticles for Nanomedicine Application

Nanomedicine, the use of nanotechnology for biomedical applications, has potential to change the landscape of the diagnosis and therapy of many diseases. In the past several decades, the advancement in nanotechnology and material science has resulted in a large number of organic and inorganic nanomedicine platforms. Silica nanoparticles (NPs), which exhibit many unique properties, offer a promising drug delivery platform to realize the potential of nanomedicine. Mesoporous silica NPs have been extensively reviewed previously. Here we review the current state of the development and application of nonporous silica NPs for drug delivery and molecular imaging.

High-relaxivity and luminescent silica nanoparticles as multimodal agents for molecular imaging

The design and synthesis of a new bimodal contrast agent for magnetic resonance imaging and optical imaging is reported. Tunable-sized silica nanoparticles were synthesized by a microemulsion-mediated pathway and used as carriers for paramagnetic and luminescent probes. The near-infrared luminescent agent was a ruthenium complex that was directly entrapped in the silica shell to provide photoluminescence enhancement and to make it highly photostable as it was protected from the surrounding environment. The paramagnetic activity came from a Gd-DTPA derivative that was grafted on the silica surface. NMRD profiles showed a strong relaxivity enhancement (increase of 432% in the r1 value at 20 MHz) when the paramagnetic complex was grafted at the nanoparticle surface, because of a reduction of its mobility. Polyethylene glycol was also grafted at the nanoparticle surface to enhance the nanoparticle residence time in the bloodstream. A thorough characterization of the material confirmed its potential as a very effective bimodal contrast agent.

Efficient desulfurization by polymer-inorganic nanocomposite membranes fabricated in reverse microemulsion

The sulfur in gasoline will convert to SO(2) after combustion under high temperature, which adversely affects human health and the environment. Membrane technique in particular pervaporation offers a number of potential advantages over conventional FCC gasoline desulfurization processes. The present study focuses on the performance enhancement of PDMS membrane by incorporating silica nanoparticles. Specifically, silica nanoparticles formed by the catalysis and templating of protamine in w/o reverse microemulsion are in situ embedded into PDMS bulk matrix, endowing the resultant oleophilic nanocomposite membranes with appropriate free volume properties and superior separation performance. Through the rational manipulation of biomimetic mineralization at water-oil interface, silica particles with uniform size are acquired. Following this protocol, by introducing organic PDMS oligomers into the oil phase, PDMS-SiO(2) nanocomposite membranes are prepared in a facile way. The resultant nanocomposite membranes display superior permeability and permselectivity in the pervaporative desulfurization using thiophene/n-octane binary mixture as model gasoline, for example, under the condition of 500 ppm sulfur in feed (40 L/h) at 30°C, an enrichment factor of 4.83-5.82 with a normalized permeation rate of 6.61-10.76 × 10(-5)kgm/m(2)h is acquired.

Synthesis and characterization of A4[Re6Q8L6]@SiO2 red-emitting silica nanoparticles based on Re6 metal atom clusters (A = Cs or K, Q = S or Se, and L = OH or CN)

Metal atom clusters constitute very promising candidates as luminophores for applications in biotechnology because they are nanosized entities offering robust luminescence in the near-infrared field (NIR). However, they cannot be used as prepared for biological applications because of potential toxic effects and quenching of the clusters' luminescence in aqueous media, and they therefore need to be dispersed in a biocompatible matrix. We describe herein the encapsulation of octahedral rhenium clusters, denoted as A(4)[Re(6)Q(8)L(6)] (A = Cs or K, Q = S or Se, and L = OH or CN), in silica nanoparticles by a water-in-oil microemulsion process, paying particular attention to the clusters' stability. The obtained A(4)[Re(6)Q(8)L(6)]@SiO(2) nanoparticles are 30 nm in size with good monodispersity and a perfectly spherical shape, as shown by scanning electron microscopy (SEM). The presence of cluster units inside the silica matrix was evidenced by scanning transmission electron microscopy in annular dark-field mode (ADF-STEM). From the point of view of their optical properties, the A(4)[Re(6)Q(8)L(6)]@SiO(2) nanoparticles show red and NIR emission under UV excitation, even when dispersed in water. The evolution of the structural and luminescence properties of clusters before and after encapsulation was followed by Raman and photoluminescence spectroscopy.

Time-resolved in situ small-angle X-ray scattering study of silica particle formation in nonionic water-in-oil microemulsions

The formation of silica particles by the ammonia-catalyzed hydrolysis of tetraethyl orthosilicate (TEOS) in the polyoxyethylene (5) nonylphenyl ether (NP-5)/cyclohexane/water microemulsion system was investigated by time-resolved small-angle X-ray scattering (SAXS). The SAXS data could be modeled as a combination of two species where one describes the silica-particle containing microemulsion droplets and the other the reverse droplets. The analysis allowed the determination of the evolution of the system of particles of silica and reverse droplets. A model of nucleation and growth of the silica particles is confirmed and the volume fraction versus time data for the silica particles is in agreement with first order kinetics with respect to TEOS concentration. Moreover to describe the long time evolution of the system, a correlation among the silica particles has been taken into account by introducing a structure factor with a local silica volume fraction eta = 0.1. This high local density is 2 orders of magnitude larger than the global silica fraction and can be explained in terms of depleting interaction.

Formation of silica nanoparticles in microemulsions

Silica nanoparticles for controlled release applications have been produced by the reaction of tetramethylorthosilicate (TMOS) inside the water droplets of a water-in-oil microemulsion, under both acidic (pH 1.05) and basic (pH 10.85) conditions. In-situ FTIR measurements show that the addition of TMOS to the microemulsion results in the formation of silica as TMOS, preferentially located in the oil phase, diffuses into the water droplets. Once in the hydrophilic domain, hydrolysis occurs rapidly as a result of the high local concentration of water. Varying the pH of the water droplets from 1.05 to 10.85, however, considerably slows the hydrolysis reaction of TMOS. The formation of a dense silica network occurs rapidly under basic conditions, with IR indicating the slower formation of more disordered silica in acid. SAXS analysis of the evolving particles shows that approximately 11 nm spheres are formed under basic conditions; these are stabilized by a water/surfactant layer on the particle surface during formation. Under acidic conditions, highly uniform approximately 5 nm spheres are formed, which appear to be retained within the water droplets (approximately 6 nm diameter) and form an ordered micelle nanoparticle structure that exhibits sufficient longer-range order to generate a peak in the scattering at q approximately equal to 0.05 A-1. Nitrogen adsorption analysis reveals that high surface area (510 m2/g) particles with an average pore size of 1 nm are formed at pH 1.05. In contrast, base synthesis results in low surface area particles with negligible internal porosity.

Kinetic Studies on the Synthesis of Hydroxyapatite Nanowires by Solvothermal Methods

Hydroxyapatite nanowires with a high axial ratio have been synthesized in reverse micelle solutions that consist of cetyltrimethylammonium bromide (CTAB), n-pentanol, cyclohexane, and the reactant solution by solvothermal methods. This paper focusses on the kinetic studies of the solvothermal reaction and the linear growth of hydroxyapatite nanowires. When the reaction was carried out at low temperatures (65°C), the experimental results showed that the reaction rate was of zero order since the whole reaction was diffusion controlled with constant diffusion coefficients. In the middle to high temperature range (130–200°C), the kinetics were characterized by second order reaction kinetics. Since the controlling factor was activation energy and the apparent activation energy was large, the reaction rate was more sensitive to the temperature. Therefore, the exponent of the reaction rate constant increased by two when the temperature was increased from 130 to 200°C. By calculating the yields of products and the specific surface areas at different times, the linear and overall growth rate equations of the hydroxyapatite nanowires could be obtained. The experimental effective growth order of the crystals was 11. The larger growth order indicated that the crystal could grow more effectively in one direction because of the induction of the surfactant in the experiment system.

Synthesis and Magnetic Properties of Silica-Coated FePt Nanocrystals

Colloidal FePt nanocrystals, 6 nm in diameter, were synthesized and then coated with silica (SiO2) shells. The silica shell thickness could be varied from 10 to 25 nm. As-made FePt@SiO2 nanocrystals have low magnetocrystalline anisotropy due to a compositionally disordered FePt core. When films of FePt@SiO2 particles are annealed under hydrogen at 650 degrees C or above, the FePt core transforms to the compositionally ordered L1(0) phase, and superparamagnetic blocking temperatures exceeding room temperature are obtained. The SiO2 shell prevents FePt coalescence at annealing temperatures up to approximately 850 degrees C. Annealing under air or nitrogen does not induce the FePt phase transition. The silica shell limits magnetic dipole coupling between the FePt nanocrystals; however, low temperature (5 K) and room temperature magnetization scans show slightly constricted hysteresis loops with coercivities that decrease systematically with decreased shell thickness, possibly resulting from differences in magnetic dipole coupling between particles.

Influence of microwave radiation on the growth of gold nanoparticles and microporous zincophosphates in a reverse micellar system

The water core of reverse micelles has been extensively used as the site for synthesis of a variety of materials. However, water-in-oil reverse micelles have a limited range of temperatures over which they are stable as a single phase. Directing heat to the water cores, the usual site of synthesis without heating the bulk provides added opportunities for synthesis. Microwave radiation is a method for superheating the water cores. In this study, we use an H2O-sodium bis(2-ethylhexyl) sulfosuccinate (AOT)-heptane reverse micelle system for the synthesis of Au particles by hydrazine reduction of HAuCl4 in the presence and absence of microwave radiation. The duration of the microwave radiation was limited to a 2-min duration at a power of 300 W, thereby ensuring that the reverse micelle phase is maintained during the synthesis. At all hydrazine concentrations studied (0.5-2 M), the presence of microwave radiation led to an increase in the particle size of Au. The second system examined was the growth of microporous zincophosphate-X (ZnPO-X, an analogue of the faujasite structure) synthesized from H2O-dioctyldimethylammonium chloride (DODMAC)-heptane reverse micelle system. Microwave radiation was applied for 1 min at 150 W at various stages of the nucleation and growth process, and did not disrupt the reverse micelle system. Product analysis after 48 h of reaction showed that the 1-min microwave pulse, if applied during the nucleation stage (the first 4 h), promoted the formation of NaZnPO4.H2O over ZnPO-X. The effect of the microwave pulse at the growth stage was to promote the formation of ZnPO-X. Absorption of the microwave radiation by the water core and surrounding polar surfactant molecules leads to a rapid rise in local temperature (predicted to be approximately 150 degrees C/min for the AOT system), increasing the rates of intramicellar reactions.

Dispersion of SiO2-Based Nanocomposites with High Performance Liquid Chromatography

Core-shell structured Ag/SiO2 nanocomposite has been synthesized by a cyclohexane/Igepal/water reverse micelle system. The spherical nanocomposite particles were washed and concentrated with high performance liquid chromatography (HPLC) to remove the surfactant added during synthesis. Spherical SiO2 micrometer-scale particles were packed in the HPLC column as a stationary phase for the washing and dispersing of Ag/SiO2 nanocomposite particles. Surface modification of Ag/SiO2 nanocomposite particles and SiO2 microspheres with silane coupling agent enhanced the surface charge of the particles and improved the efficiency of washing with HPLC. Well-dispersed Ag/SiO2 stable suspensions were successfully attained in ethanol/water mixed solvents after HPLC washing. The state of dispersion for the Ag/SiO2 nanocomposite suspension was systematically assessed using dynamic light scattering (DLS) and transmission electron microscope (TEM) and spin coat/atomic force microscope (AFM) analyses. The mechanism of the enabling HPLC washing protocol for SiO2-based nanoparticles is discussed.

Silver nanoparticle formation in microemulsions acting both as template and reducing agent

A novel method of making silver nanoparticles in water-in-oil microemulsions using the surfactants as both the reducing agent and as the structure-directing agent is presented. Since no external strong reducing agent is used the kinetics of the formation is slow, which makes it possible to study the silver nanoparticle formation in situ. The microemulsions used were based on either the nonionic surfactant Brij30 (C12E4), which reduces the silver ion to metallic silver and is thereby partly oxidized, or mixtures of Brij30 and AOT (sodium bis(2-ethylhexyl) sulfosuccinate, where the latter does not reduce the silver ions. The influences of silver ion and nonionic surfactant concentrations on the formation kinetics of the nanoparticles were followed in situ using UV-vis spectroscopy, and both parameters were found to have a big influence. The microemulsion droplet's size, size distribution, and shape were examined by small-angle X-ray scattering (SAXS), and the formed silver nanoparticles were studied using both transmission electron microscopy and SAXS. The SAXS measurements showed that the presence of silver nitrate does not affect the microemulsion systems noticeably and that the droplet's size and shape are retained during the particle formation. It is shown that the size and morphology of the particles do not directly follow the shape and size of the microemulsion droplets even though there is a relation between the droplet size and the radii of the formed particles.

Polymerization of w/o microemulsions for the preparation of transparent SiO2/PMMA nanocomposites

Reverse w/o microemulsions composed of methyl methacrylate (MMA) forming the oil phase, nonionic surfactants, and water are used for the synthesis of transparent SiO2/PMMA nanocomposites. An inorganic precursor, tetraethoxysilane (Si(OEt)(4), TEOS), is hydrolyzed in the reverse micelles containing aqueous ammonia. During the hydrolysis of TEOS, polymerization of the continuous MMA phase is initiated using AIBN (azobisisobutyronitrile), and after thermal polymerization at 333 K for 12 h, solid blocks of PMMA are obtained in which nanometer-sized silica particles are trapped in the solid polymer matrix. According to small-angle X-ray and dynamic light scattering experiments, the water droplets in MMA microemulsions are 12 nm (R(W) = 13) in diameter, whereas after polymerization of the microemulsion, the SiO2 particles in the transparent SiO2/PMMA composites are 26 nm in diameter. Transmission electron micrographs demonstrate a low degree of agglomeration in the composites. In comparison with materials generated from micelle-free solutions, the particle size distribution is narrow. The reverse micelle-mediated approach produces composites of high transparency comparable with that of pure PMMA.

Surfactant-templated nanomaterials synthesis

The ability of amphiphilic compounds to self-assemble into well-defined structures can be taken advantage of for synthesis of inorganic materials with nanometer dimensions. The principle, which is biomimetic in character, is discussed with special attention put on three areas: microemulsion-based synthesis of nanoparticles, preparation of mesoporous materials from surfactant templates, and surfactant-mediated crystallization.