Microemulsion Methods

Over the past few years, a large number of experimental approaches have been successfully used as routes to synthesize nanorods or nanowires based on titania, such as combining sol-gel processing with electrophoretic deposition,152 spin-on process,153 sol-gel template method,154-157 metalorganic chemical vapor deposition,158-159 anodic oxidative hydrolysis,160 sonochemical synthesis,161 inverse microemulsion method,162 molten salt-assisted and pyrolysis routes163 and hydrothermal synthesis.163-171 We will discuss more in detail the latter preparation, because the advantage of this technique is that nanorods can be obtained in relatively large amounts. 

Generally, two common methods, the Stober method and the reverse microemulsion method are used for synthesis of silica nanoparticles. As derivatives of a sol-gel process, both methods involve hydrolysis of a silicon alkoxide precursor to form a hydroxysilicate followed by polycondensation of the hydroxysilicate to form a silica nanoparticle [44]. 

The reverse microemulsion method can be used to manipulate the size of silica nanoparticles [25]. It was found that the concentration of alkoxide (TEOS) slightly affects the size of silica nanoparticles. The majority of excess TEOS remained unhydrolyzed, and did not participate in the polycondensation. The amount of basic catalyst, ammonia, is an important factor for controlling the size of nanoparticles. When the concentration of ammonium hydroxide increased from 0.5 (wt%) to 2.0%, the size of silica nanoparticles decreased from 82 to 50 nm. Most importantly, in a reverse microemulsion, the formation of silica nanoparticles is limited by the size of micelles. The sizes of micelles are related to the water to surfactant molar ratio. Therefore, this ratio plays an important role for manipulation of the size of nanoparticles. In a Triton X-100/n-hexanol/cyclohexane/water microemulsion, the sizes of obtained silica nanoparticles increased from 69 to 178 nm, as the water to Triton X-100 molar ratio decreased from 15 to 5. The cosurfactant, n-hexanol, slightly influences the curvature of the radius of the water droplets in the micelles, and the molar ratio of the cosurfactant to surfactant faintly affects the size of nanoparticles as well. 

In summary, a suitable association between dye molecules and the silica matrix is necessary for synthesis of DDSNs. Without the presence of chemical bonds or electronic interactions, the dye molecules will leak out from silica nanoparticles through the silica pores [22], Such DDSNs will provide unstable florescence signals and cannot be used as a labeling agent in bioanalysis. Meanwhile, water solubility is critical for a dye molecule when using a reverse microemulsion method to make the DDSNs. 

A. Martinez-Arias, M. Femandez-Garcia, V. Ballesteros, L.N. Salamanca, J.C. Conesa, C. Otero and J. Soria, Characterization of high surface area Zr-Ce (1 1) mixed oxide prepared by a microemulsion method, Langmuir 15,4796-4802 (1999). 

Pto.45Pdo.45Bio.n/C in 0.2 M NaOH + 0.1 M EC, at various potentials (each 100 mV) from 130 to 1030 mV/RHE (1) and chromatograms of the anodic outlet of a SAMEC working with a Pto.45 Pdo.45Bio.i/C anode without fuel recirculation (2). Electrocatalysts were prepared according to the water-in-oil microemulsion method. 

Figure1.22 Polarization curvesfortheoxidation ofO.Ol M NaBH4 in a 1 M NaOH solution recorded on (1) Pt (50 wt%)/C and (2) Au (50wt%)/C i = 20mVs Q.= 1000rpm, T=20°C. Catalysts were prepared according to the water in oil microemulsion method.

Demarconnay et al. used a method derived from that of the so-called water-in-oil microemulsion method to prepare well dispersed Ag/C catalysts [116]. The onset of the oxygen reduction wave is only shifted by 50 mV towards lower potentials on an Ag/C catalyst compared with that obtained on a Pt/C catalyst and the limiting current... 

Figure 1.23 j(E) polarization curves recorded at a rotation rate 2 = 2500 rpm in an 02-saturated 0.1 M NaOH electrolyte (T=20°C, v = 5 mVs ) for (dashed line) 20wt%Ag/C and (solid line) 20wt% Pt/C. Ag/C catalyst was prepared according to the water-in-oil microemulsion method, whereas Pt/C catalyst was prepared using the Bonnemann method. 

W/o microemulsion method has been successful for preparing QDs and other semiconductor nanoparticulates. The preparation and properties of these particles have been extensively reviewed [114,186]. 

Bagwe RP, Yang CY, Hilliard LR, Tan WH (2004) Optimization of dye-doped silica nanoparticles prepared using a reverse microemulsion method. Langmuir 20 8336-8342... 

Konno et al. prepared magnetite particles by using microemulsion method... 

Ritschel, W.A., et al. 1990. Improvement of peroral absorption of cyclosporine A by microemulsions. Methods Find Exp Clin Pharmacol 12 127. 

The aim of the present investigation is to synthesize nano-sized particles from CaCC>3 and BaCC>3 by the microemulsion method. 

W. Wang, X. Fu, J. Tang, L. Jiang, Preparation of submicron spherical particles of silica by the water-in-oil microemulsion method, Colloids Surf. A Physicochem. Eng. Aspects 81 (1993) 177-180. 

Typical emission and excitation spectra of ZnS Eu3+ nanociystals, prepared by using the microemulsion method are shown in fig. 17 (Bol et al., 2002). In the emission spectrum,... 

Ishizumi and Kanemitsu (2005) have studied PL properties of Eu3+ doped ZnO nanorods fabricated by a microemulsion method. The PL of bound exciton recombination and ZnO defects was observed near 370 and 650 nm under 325-nm light excitation, but no emission of Eu3+ occurred. On the other hand, the sharp PL peaks due to the intra-4f transitions of Eu3+ ions appeared under nonresonant excitation below the band-gap energy of ZnO (454 and 457.9 nm) in addition to direct excitation to 5D2 (465.8 nm). Therefore the authors concluded that the energy transfer occurs from the ZnO nanorods to Eu3+ ions through ZnO-defect states. This energy transfer mechanism seems very different from the previous one and more spectroscopic evidence is required to confirm it. 

The sensitivity of fluorescence-based assays can hence be greatly improved by the use of dye-doped silica nanoparticles and this approach has been pioneered and subsequently deeply investigated by Tan and coworkers.15 Their luminophore of choice was the water-soluble, positively charged tris(2,2 -bipyridyl)dichlororuthenium(II) [Ru(bpy)3]2+ hydrochloride that can be easily incorporated into silica nanoparticles prepared using the reverse microemulsion method. The charge complementarity between the dye and the silica matrix prevents leaching from the particles.15... 

Among the different possibilities, the water-in-oil microemulsion method [63-73] can be a good approach. This method was derived from that developed by Boutonnet et al. [64], Two different microemulsions are prepared, one containing the metal salts (e.g., 99.9% II2PtCI6, SnCl2,... 

The double inverse microemulsion method was also used to synthesize per-ovskite-type mixed metal oxides [ 155]. One microemulsion solution contained nitrate salts of either Ba(N03)2/Pb(N03)2, La(N03)3/Cu(N03)2 or La(N03)3/ Ni(N03)2, and the other microemulsion contained ammonium oxalate or oxalic acid as the precipitant. These metal oxalate particles of about 20 nm were readily calcined into single phase perovskite-type BaPb03, La2Cu04 and LaNi03. The calcinations required for the microemulsion-derived mixed oxalates were 100-250 °C below the temperatures used for the metal oxalates prepared by a conventional aqueous solution precipitation method. 

A great number of works on the aqueous synthesis of ceria NPs have been reported from very early years, since the ultrafine ceria powders are re-examined from fhe view point of nanotechnology for example, the simple precipitation methods (Zhang et al., 2002a), the microemulsion methods (Masui et al., 1997 Zarur and Ying, 2000), urea assisted hydro-thermal methods (Hirano and Inagaki, 2000 Hirano and Kato, 1999), hydrothermal methods with supercritical conditions (Adschiri et al., 2001). 

The microemulsion method utilizes a water/oil/surfactant system to construct a micro reactor, in which NCs could be s)mthesized. The microemulsions have a wide range of applications from oil recovery fo fhe s)mfhesis of nanoparticles. Microemulsion is a system of water, oil, and surfactant, and it is an optically isotropic and thermod3mamically stable solution. At molecular scale, the microemulsion is heterogeneous with an internal structure either of nanospherical monosized droplefs (micelles or reverse micelles) or a bicontinuous phase, depending on the given temperature as well as the ratio of its constituents (Eriksson et al., 2004). The small droplets could be utilized as microreactors in order to s)mthesize the fine NCs in a controllable way. 

Shi et al. prepared CeF3 Lu " NPs from the quaternary reverse micelle system, which contained CTAB, n-butanol, -octane, and water. The characteristics emission of Lu + was observed (Lian et al., 2004). Similarly, Qin et al. obtained Yb -Tm + codoped YF3 nanobundles through a microemulsion method in another quaternary reverse microemulsion system with water/CTAB/cyclohexane/l-pentanol (Wang et al., 2008c). 

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