Nanoparticles In Microemulsions Research Articles

The Interplay between Nucleation and the Rates of Chemical Reduction in the Synthesis of Bimetallic Nanoparticles in Microemulsions: A Computer Study

Monte Carlo simulations were conducted to investigate the interaction between nucleation and the rates of chemical reduction in Au/Ag, Au/Pt and Au/Pd nanoparticles prepared in microemulsions using a one-pot method. The impact of nucleation on final nanostructure depends on the critical nucleus size value: at a high critical nucleus size, nucleation becomes the main factor in determining the final nanostructure, even with a very large difference in reduction rates, as seen in the Au/Pd pair. However, when the critical nucleus size is small, the difference in reduction rates of the two metals becomes the key parameter determining the final nanostructure. Furthermore, the relevance of heteroatomic nucleation on the mechanism of nanoparticle formation depends on the difference between the reduction rates of the two metals. Smaller differences, such as in the Au/Ag or Au/Pt pairs, result in a greater impact of heteroatomic nucleation on the final nanostructure. In contrast, in the Au/Pd pair, heteroatomic nucleation becomes less important due to the low availability of Pd until late stages of synthesis. This study provides deeper insight into the complex mechanisms that govern reactions in microemulsions.

Production of Pd nanoparticles in microemulsions. Effect of reaction rates on the particle size.

In the synthesis of metallic nanoparticles in microemulsions, we hypothesized that the particle size is controlled by the reaction rate and not by the microemulsion size. Thus, the changes observed in the particle sizes as reaction conditions, such as concentrations, temperatures, the type of surfactant used, etc., are varied which should not be correlated directly to the modification of these conditions but indirectly to the changes they produce in the reaction rates. In this work, the microemulsions were formulated with benzene and water as continuous and dispersed phases, respectively, using n-dodecyltrimethylammonium bromide (DTAB) and n-octanol as the surfactant and cosurfactant. Using time-resolved UV-vis spectroscopy, we measured the reaction rates in the production of palladium (Pd) nanoparticles inside the microemulsions at different reactant concentrations and temperatures, keeping all the other parameters constant. The measured reaction rates were then correlated with the particle sizes measured by transmission electron microscopy (TEM). We found that the nanoparticle size increases linearly as the reaction rate increases, independently of the actual reactant concentration or temperature. We proposed a simple model for the observed kinetics where the reaction rate is controlled mainly by the diffusion of the reducing agent. With this model, we predicted that the particle size should depend indirectly, via the reaction kinetics, on the micelle radius, the water volume and the total microemulsion volume. Some of these predictions were indeed observed and reported in the literature.

Formation of Gold Nanoparticles in Water-in-Oil Microemulsions: Experiment, Mechanism, and Simulation

Self-assembled water-in-oil (W/O) microemulsions have been reported as a suitable route for synthesis of size-controlled nanoparticles. However, the mechanism of formation of nanoparticles in microemulsions is still not completely understood. In this work, gold nanoparticles (GNPs) were synthesized via the W/O microemulsion route. As the molar ratio of water and dioctyl sodium sulphosuccinate (AOT) (R) increased from 2.5 to 5.0 to 7.5, the corresponding water drop diameter increased from 2.7 to 5.0 to 7.3 nm. In parallel, the mean hydrodynamic diameter of GNPs increased from 6.5 to 11.3 to 15.6 nm for corresponding R values of 2.5, 5.0, and 7.5. Therefore, although there is a monotonically increasing trend of the mean diameter of GNPs with the initial drop diameter, for all values of R, the mean diameter of GNPs was significantly higher than the initial drop diameter. Consequently, previously known simulation vastly underpredicts the experimental GNP diameter. However, only on redefining the particle-particle coagulation event (during coalescence of microemulsion drops containing particles) does the current kinetic Monte Carlo (kMC) simulation agree well with the experimental results. In addition, we also find that the coagulation efficiency of solid nanoparticles (βp) increases with R, and βp is lesser than the coalescence efficiency of liquid drops (βd) over the range of R values concerned. Hence, a combined simulation and experimental study enumerates the dynamics of size evolution of nanoparticles and the events involved in their formation in a W/O microemulsion system.

Synthesis of Pt/M (M = Au, Rh) Nanoparticles in Microemulsions: Controlling the Metal Distribution in Pt/M Catalysts

The synthesis of Au/Pt and Pt/Rh nanoparticles in microemulsions by a one-pot method has been studied by computer simulation using stoichiometric and in-excess reducing agent. Our study demonstrates that Pt is reduced faster in the presence of Rh than in the presence of Au, and this acceleration is attributed to the higher availability of reducing agent in the Pt/Rh than in Au/Pt synthesis. Final bimetallic nanoparticles show a better metal segregation in Pt/Rh when compared with Au/Pt nanoparticles. In Au/Pt synthesis, Pt reaction rate depends on the ease with which reactants can be exchanged between micelles and the availability of reducing agent. When Pt/Rh nanoparticles are synthesized using a rigid film, the slow intermicellar exchange runs together with the slow reduction rates of the metals. It results in a very slow consumption of reducing agent, so the amount of reducing agent does not affect Pt reaction rate.

Slowing Down Kinetics in Microemulsions for Nanosegregation Control: A Simulation Study

The synthesis of bimetallic nanoparticles in microemulsions has been studied by simulation. The aim of this study was to understand the principles underlying kinetics in colloidal systems, which allowed us to further investigate the interaction between the intermicellar exchange rate and chemical reduction and ultimately to discover how these nanostructures may become. We present here results for three couples of metals with different reduction rates, the reduction rate of the faster metal being 10 times faster than the rate of the slower one in all cases. Under these conditions, the impact of slowing down chemical reductions on the final nanoarrangement can be studied. Our results show that a bimetallic nanostructure can be tuned by changing the microemulsion composition only when both metal reduction rates are quite fast in relation to the intermicellar exchange rate. If this is not the case, the compartmentalization of the reaction medium does not play any role because of chemically controlled kinetics...

Controllable synthesis of polydopamine nanoparticles in microemulsions with pH-activatable properties for cancer detection and treatment.

Polydopamine nanoparticles (PDA NPs) which combine diagnostic and therapeutic functions are potentially useful in biomedicine. However, it is difficult to synthesize PDA NPs of a relatively small size (≤50 nm in diameter) using the traditional polymerization of dopamine monomers in an alkaline water-ethanol solution at room temperature. Herein, PDA NPs with average diameters ranging from 25 nm to 43 nm are prepared in a way which is similar to the silica-like reverse microemulsion process. The size of the PDA NPs can be modulated by changing the amount of dopamine monomers in the microemulsion. After conjugation with ferric ions (Fe3+), the poly(ethylene glycol) modified Fe-PDA NPs (termed as PEG-Fe-PDA NPs) exhibited pH-activatable magnetic resonance imaging (MRI) contrast and high photothermal performance. The combination of a small dimension and the pH-activatable MRI contrast can greatly facilitate tumor accumulation and increase the tumor imaging sensitivity against animal models in vivo. Completely inhibited tumor growth was achieved by the PEG-Fe-PDA NPs mediated by photothermal therapy with MRI guidance.

The impact of the confinement of reactants on the metal distribution in bimetallic nanoparticles synthesized in reverse micelles.

A kinetic study on the formation of bimetallic nanoparticles in microemulsions was carried out by computer simulation. A comprehensive analysis of the resulting nanostructures was performed regarding the influence of intermicellar exchange on reactivity. The objects of this study were metals having a difference in standard reduction potential of about 0.2–0.3 V. Relatively flexible microemulsions were employed and the concentration of the reactants was kept constant, while the reaction rate of each metal was monitored as a function of time using different reactant proportions. It was demonstrated that the reaction rates depend not only on the chemical reduction rate, but also on the intermicellar exchange rate. Furthermore, intermicellar exchange causes the accumulation of slower precursors inside the micelles, which favors chemical reduction. As a consequence, slower reduction rates strongly correlate with the number of reactants in this confined media. On the contrary, faster reduction rates are limited by the intermicellar exchange rate and not the number of reactants inside the micelles. As a result, different precursor proportions lead to different sequences of metal reduction, and thus the arrangement of the two metals in the nanostructure can be manipulated.

Facile preparation of Pd–Au bimetallic nanoparticles via in-situ self-assembly in reverse microemulsion and their electrocatalytic properties

Pd–Au bimetallic nanoparticles were facilely prepared on Ni foil substrate via in-situ self-assembly in reverse microemulsion of water/Triton X-100/n-hexanol/n-hexane. The Pd–Au nanomaterials were characterized by scanning electron microscopy (SEM), transmission electron microscopy (TEM) and X-ray diffraction (XRD). A homogeneous alloy structure of Pd–Au in the bimetallic nanoparticles is indicated by XRD analysis and TEM. The SEM results show that the resultant bimetallic nanoparticles disperse uniformly on Ni foil with a narrow size distribution. The apparent size of bimetallic nanoparticles deposited on Ni depends on the reaction time of self-assembly. The TEM results reveal that the Pd–Au bimetallic nanoparticles consist of smaller homogeneous alloy nanoparticles with an average diameter of 5.1nm, size of which depends on the size of micro water droplets. The growth mechanism of the Pd–Au nanoparticles in microemulsion was also explored. The electrocatalytic performance of the Pd–Au NPs/Ni electrode was studied by cyclic voltammetric and chronoamperometric measurements. Electrochemical experiments show that the Pd–Au nanoparticles have good catalytic activity and good stability for ethanol oxidation in alkaline media.

Preparation of Size Controllable BaTiO<sub>3</sub> Nanoparticles in Microemulsion at Low Temperature

Size controllable Barium titanate nanoparticles were synthesized in microemulsion consisting of water, OP-10, hexanol and cyclohexane under atmospheric pressure and low temperature, with Ba (OH)2·8H2O and tetrabutyl titanate used as starting reactants. Products were characterized by powder X-ray diffraction (XRD), transmission electron microscopy (TEM) and energy-dispersive X-ray spectroscopy (EDX). The results show that 65°C is the proper temperature for the synthesis. The particle size can be controlled by varying the reactants’ concentration, ω value (molar ratios of water to surfactant) and aging time.

Formation of dispersed palladium–nickel bimetallic nanoparticles in microemulsions: synthesis, characterization, and their use as efficient heterogeneous recyclable catalysts for the amination reactions of aryl chlorides under mild conditions

The amination cross-coupling reaction by Pd–Ni (1 : 1) bimetallic nanoparticles in water solvent.

An in situ X-ray absorption spectroscopy study of copper nanoparticles in microemulsion

The present study investigated the formation of Cu0 metal nanoparticle clusters in reverse micelles of pseudo-ternary systems of cetyltrimethylammonium bromide (CTAB) surfactant, butanol (cosurfactant), heptane as the oil phase and aqueous solutions of CuSO4·5H2O and NaBH4 using in situ X-ray absorption spectroscopy (XAS). Microemulsions were prepared with a fixed percentage (60%) of the oil phase and a variable proportion of water and surfactant/cosurfactant. The reduction of Cu2+ and the formation of Cu0 metal nanoparticle clusters were monitored using sodium borohydride as a reducing agent. X-ray absorption spectroscopy in the X-ray absorption near edge structure (XANES) and extended X-ray absorption fine structure (EXAFS) regions allows the reduction of Cu2+ and the formation of Cu0 clusters to be analysed. A decrease in the water concentration from 15% to 13% resulted in an increase in Cu0 production.

Preparation of Size and Morphology Controlled Gold Nanoparticles in Micro-Emulsion and in Aqueous

In this study, we prepared gold nanoparticles in micro-emulsion and found the optimum ratio of micro-emulsion for preparing gold nanoparticles. We reported a new stabilizer for preparing gold nanoparticles in aqueous and found the optimum mass ratio between the stabilizer and chlorauric acid. All gold nanoparticles prepared by these two methods were characterized by transmission electron microscopy, ultraviolet-visible spectrophotometer and synchronous light-scattering spectrophotometer. The results of transmission electron microscopy showed that the diameter range of gold nanoparticles prepared in micro-emulsion was 5~10 nm, and that in aqueous was 10~15 nm. They all had homogeneous distribution and dense structures.

Preparation of NiO nanoparticles in microemulsion and its gas sensing performance

Nano-sized nickel oxides have been synthesized in a water-in-oil microemulsion. The as-synthesized samples were characterized by powder X-ray diffraction (XRD), transmission electronic microscopy (TEM) and nitrogen adsorption/desorption. The particle size of nickel oxide can be controlled from 11.5 to 31.5nm by varying the proportion of water, surfactant and oil in the microemulsion, mixing method, and calcining temperature. Gas sensors based on as-synthesized nickel oxide are fabricated and investigated. They exhibit much higher sensitivities to hydrogen sulfide, ethanol and nitrogen dioxide, compared to those based on the conventional bulk NiO. Furthermore, the response of as-synthesized materials to various kinds of target gases increases with the decreasing of the particle size of gas sensors. It is noted that the NiO sensor with particle size of 11.5nm displays high degree of selectivity, coupled with high response value, making it particularly interesting for H2S-monitoring applications.

Micro-emulsion under ultrasound facilitates the fast synthesis of quantum dots of CdS at low temperature

Semiconductor nano-particles of CdS (about 2 nm) with a hexagonal phase have been prepared at a relatively low temperature (60 °C) and short time in micro-emulsion (O/W) under ultrasound. This study presents the effects of ultrasonic irradiation on the formation of CdS nano-particles in micro-emulsion and compares the results with samples prepared without sonication. The nano-particles have been characterized by high-resolution transmission electron microscopy (HRTEM), X-ray diffraction (XRD), energy dispersed analysis of X-ray (EDAX), UV–visible spectroscopy, and surface area measurements (BET). The effect of some important factors such as sonication time, intensity of ultrasound, temperature, and oil fraction was studied on the prepared nano-particles. The particle size can be controlled by applying the ultrasonic waves on the micro-emulsion under proper conditions. It has been postulated that bubble collapse generates high temperature and many nucleation sites which lead to the uniform spherical particles with small size and fast transition phase. Further evidence was obtained by UV–visible absorption and a photoluminescence spectrum. The study of optical properties reveals that the band-gap of the CdS nano-particle decreases with increase of sonication time and reach an approximate constant value. It was also observed that nano-particles grew faster at the beginning of sonication and then reached an almost constant value. The growth of nano-particles at different intervals during sonication was followed by UV–visible spectroscopy.

Highly monodisperse and sub-nano silver particles synthesis via microemulsion technique

Highly monodisperse silver nanoparticles were synthesized by chemical reduction of silver nitrate dissolved in water core of one microemulsion and sodium borohydride (reducing agent) dissolved in water core of another microemulsion. The surfactant used in the study was di-octyl sodium sulfosuccinate or aerosol OT (AOT), which is an effective surfactant for synthesizing silver nanoparticles in microemulsion. In the present work, influence of water-to-surfactant mole ratio ( ω), molar ratio of reducing agent-to-silver nitrate ( R′) and stirring effect on silver nanoparticles size and size distribution were studied. In the present work cyclohexane was chosen as continuous oil phase. Dynamic light scattering (DLS) and transmission electron microscopy (TEM) were used for elucidating the structure, size and size distribution of the synthesized silver nanoparticles. TEM micrographs show that the synthesized nanoparticles were having average diameter of 5.5 nm, which increased ω. Isooctane was also used as a continuous oil phase to study the influence of water-to-surfactant ratio. The results showed a highly monodisperse and sub-nano size silver particles formation. The surface area of the synthesized particles was calculated to be 1.46 × 10 10 mm 2/g. An increase in R′ from 0.66 to 1 resulted in slightly smaller nanoparticles production, but on further increase of R′ led to bigger particles formation.

Sensitization of TiO 2 nanoparticles in micro-emulsion by photo-excited dye molecules: A femtosecond transient absorption study

Photo-induced electron transfer reaction in controlled sized TiO 2 particles sensitized by catechol and quinizarin 6-sulfonate (Qz-6S) has been investigated using femtosecond transient absorption spectroscopy. Size-controlled TiO 2 nanoparticles were synthesized in reverse micelles using di-octyl sulfosuccinate (Aerosol-OT, AOT) as the surfactant stabilizer. The particle sizes of the nanoparticles were controlled by varying the water-to-surfactant molar ratio, w 0 = [H 2O]/[AOT]. Optical absorption measurements confirm that only strong coupling between the nanoparticles and the sensitizers can ensure sensitization the nanoparticles in the micro-emulsion media. Electron transfer process has been determined by monitoring electron in the conduction band in nanoparticle at 900 nm and parent cation in the visible region of the sensitizer molecules. Electron injection has been measured from the appearance signal of injected electron at 900 nm and it is found to be pulse-width (<50 fs) in both the systems. We have also carried out electron transfer studies of both the dye-nanoparticle systems in aqueous solutions where the particles have been prepared by sol–gel process and compared the data studied in micro-emulsion to see the effect of micro-environment.

Preparation and Antiwear Properties of Silver Sulfide Nanoparticles in Microemulsion

Ag_2S nanoparticles were prepared in two different W/O microemulsions,the structure and morphologies of as-prepared Ag_2S nanoparticles were characterized by means of X-ray powder diffraction (XRD),transmission electron microscope(TEM).The tribological propcrties of liquid paraffin containing Ag_2S nanoparticles were evaluated on a four-ball test machine,the worn steel surface morphology were analyzed by using of scanning electron microscope(SEM).Results show that Ag_2S nanoparticles synthesized in two microemulsions,i.e.isooctane/sodium oleate/n-pentanol/water solution(sampleⅠ) and liquid paraffin/Span 80-Tween 80/n-butanol/water solution(sampleⅡ)both possess monocline crystal structure,and exhibit an even grain distribution.They have average particle size of 18nm and 5nm,respectively.The antiwear tests show that Ag_2S nanoparticles dispersed in liquid paraffin processed from liquid paraffin/Span 80-Tween 80/n-butanol/water solution microemulsion exhibit good antiwear properties which can improve load carrying capacity of the base oil remarkably.

Thermostable Biocatalytic Films of Enzymes and Polylysine on Electrodes and Nanoparticles in Microemulsions

Microemulsions of oil, water and surfactant were evaluated as media for biocatalysis at high temperatures employing films of polylysine (PLL) and the enzymes horseradish peroxidase (HRP), soybean peroxidase (SBP) and the protein myoglobin (Mb). PLL was covalently linked to oxidized pyrolytic graphite electrodes or carboxylated 500 nm diameter silica nanoparticles, then cross-linked by amidization to HRP, SBP and Mb. The resulting film systems were stable at 90 degrees C for >12 h in microemulsions. Characterization of the microemulsions by conductivity, viscosity and probe diffusion coefficients suggested that these media have bicontinuous microstructures from 25 to 90 degrees C. UV circular dichroism and visible spectroscopy confirmed that the enzymes retained near-native conformation in the films at temperatures as high as 90 degrees C. Oxidation of o-methoxyphenol to 3,3'-dimethoxy-4,4'-biphenoquinone by enzyme-PLL films on silica nanoparticles gave yields 3-5-fold larger in microemulsions at 90 degrees C compared to the same reaction at 25 degrees C. The best yields were in CTAB microemulsions and were 3-fold larger than in buffers at 90 degrees C.

In situ fabrication of mesoporous CdS nanoparticles in microemulsion by gamma ray irradiation

A facile and efficient method is described for the in situ synthesis of mesoporous CdS semiconductor nanoparticles by gamma ray irradiation in oil-in-water microemulsion. The prepared mesoporous CdS nanoparticles were characterized by high-resolution transmission electron microscopy (HRTEM), electron diffraction (ED), scanning electron microscope (SEM), X-ray diffraction (XRD) and Brunauer–Emmett–Teller (BET) gas sorptometry. The mesoporous structure of CdS nanoparticles is well-characterized by TEM and XRD. The obtained mesoporous CdS nanoparticles possessed a high surface area of 82.6 m2/g and a number of pores with the pore size distribution centered at about 9.4 nm. A possible growth mechanism for the mesoporous structure has been proposed.

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.