Reverse microemulsions method

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. 

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... 

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... 

CH4 oxidation has been experienced for ceria supported on a barium hexaaluminate, an heat resistant support. Preparation by a new reverse microemulsion method leads to ceria nanoparticles deposited on support and having a BET area close to 100 mVg after calcination at 1000 0 [72]. Such ultrahigh disperse nanoparticles show exceptional thermal resistance the authors mentioned that ceria particles prepared with a size of 6 nm sinters only to 18 nm after a calcination at 1IOO°C under a water containing atmosphere. Of course excellent activity in methane combustion has been observed. According to their experimental conditions calculated specific activity expressed as mol(CH4).h. m was estimated to 6.4x10 at 500°C whereas Bozo [44J reported a value of 1.5x1 O at the same temperature both values look similar. Thus the difference in methane conversion may be related to BET area only which is spectacularly preserved using the reverse micro-emulsion method for synthesis. 

Figure 13.5. Catalytic methane combustion of methane on a) aA),20i9 issued from sol-gel process, b) BaAlijOt issued from reverse microemulsion method and c) CeOj-BaAliPty composite issued from microemulsion method (Reprinted from Letters to Nature. Ref. 72).

Zhan, Z., Song, W. and Jiang, D. (2004) Preparation of nanometer sized ln203 particles by a reverse microemulsion Method. /. Colloid Interface Sci., 271, 366-371. 

The effect of several important preparation parameters on the size and stability of Ti02 particles prepared by hydrolysis of tetraisopropyltitanate has been observed by Moran et al.. A dramatic increase in the rate of particle aggregation as the water to surfactant mole ratio increases was reported. Furthermore, the preparation of titania from tetrabutyltitanate by the reverse microemulsion method has been reported by Ju and co-workers. Results from TEM analysis reveal an increase in particle size when co increased from 1L9 to 55.5 and then a particle size decrease when co increased from 55.5 to 110.9. Ti02 has been prepared from the same precursor by Wu et al. ° and Fu and Qutubuddin. ... 

In another reverse microemulsion method [262], an oil phase/Triton X-35 / H2O mixture was added to an oil phase/Triton X-35/Ti-tetrapropoxide combination to obtain gel particles. Note here that the Triton X series of products can be described as alkylaryl polyether alcohols, the numbers indicating the average number of ethylene oxide units (3 in case of Triton X-35) see also Table 2.2. The oil phase was cyclohexane or decane. Yet another two-microemulsion method used non-ionic surfactants NP-5 and NP-9 (wt ratio 1 1) and cyclohexane as the oil phase [263]. As aqueous phase, the authors used a TiCl4 solution and an ammonia solution. The microemulsions were prepared and mixed at 13 C to obtain titania particles. The suspension was poured into acetone, the precipitate centrifuged, washed with acetone and vacuum dried for 2 h. The particle size was about 5 nm. 

The reverse microemulsion method is based on the controlled hydrolysis of tetra-ethoxysilane (TEOS) molecules and their ammonia catalysed condensation like the Stober method, but the reaction milieu is in this case a stable and macroscopically isotropic dispersion of a surfactant and water in a hydrocarbon. In this system the hydrolysis is confined inside the aqueous nuclei where precursors condense to form the nanoparticles. Optimized synthetic protocols and experimental conditions allow one to obtain nanoparticle samples in the dimensional range of about 15-200 nm [70, 76] (Fig. 11). 

Fig. 11 Schematic representation of the reverse microemulsion method for the synthesis of dye doped silica nanoparticles...

The reverse microemulsion method along with the sol-gel method has been successfully used in the preparation of the so-called core-shell stmctures. One example is titania-coated silica particles (Eu and Qutubuddin, 2001). The protocol had two main steps (a) microemulsion synthesis of silica particles from TEOS, and (b) deposition of TiOa on silica particles by addition of a snrfactant solntion of titanium n-butoxide. The particle size was about 30-40 nm. 

The utilization of water-in-oil, or reverse, microemulsion method for NMOFs synthesis has been reported by Rieter et al. [73], who prepared a NMOFGd(BDC)i 5(H20>2 (BDC = 1,4-benzenedicarboxylate) by reacting GdCb and bis(methylammonium)ben-zene-l,4-dicarboxylate in the cationic cetyltrimethylammonium bromide (CTAB)/... 

A reverse microemulsion method is applied for the preparation of composite nanoparticles [96]. By controlling the amount of surfactant and water, fabrication of particles in water-in-oil microemulsions (reverse micelles) affords great control over the size and shape of the particles [15]. This procedure takes advantage of two selforganizing processes. First, the reverse micelles are used to synthesize metallic... 

Figure 7.1 Transmission electron microscopy image of silica nanoparticles synthesized via the reverse microemulsion method.

In this work, nanoparticulates of compounds precursors of LDHs for preparing Mn, Mn-Cu and Mn-Co mixed oxides were successfully synthesized by the reverse microemulsion method. It was observed that the precursor obtained from the above method had similar characteristics for preparing mixed oxide catalysts used in the oxidation of ethanol. This method was compared with the conventional co-precipitation method. 

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