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Due to limitations of the existing preparative methods of hollow nanoparticles by either heating at high temperature (>600 °C) or by using strong acid, alkali, or an organic solvent, it was not possible up till now to encapsulate any sensitive organic molecule like enzyme or others inside the cavity of hollow nanoparticles. We have demonstrated a much softer method of preparing hollow silica nanoparticles with horseradish peroxidase (HRP) inside the cavity by synthesizing HRP-doped core-shell silica-coated silver chloride nanoparticles and finally leaching out silver chloride with dilute ammonia at low temperatures. TEM pictures showed the hollow cavity inside the nanoparticles. The enzyme entrapped in these particles was active. The turnover number of HRP entrapped into these hollow particles and dispersed in aqueous buffer (pH 7.2) ( k cat = 2.56 × 10 6 s −1 ) was found to be less than that of free enzyme in aqueous buffer ( k cat = 6.133 × 10 7 s −1 ) but higher than that of HRP entrapped in solid-core silica nanoparticles and dispersed in aqueous buffer ( k cat = 1.05 × 10 5 s −1 ). The result showed that hollow nanoparticles could be prepared using soft chemical methods and sensitive chemicals like active enzyme could be entrapped in the cavities and it retains its activity.

Organically modified silanes (ORMOSIL) such as vinyl triethoxysilane readily aggregate in the aqueous cores of reverse micelles where the triethoxysilane moieties are hydrolyzed to form a hydrated silica network and the vinyl groups protruded out from the surface of the nanoparticles toward the hydrophobic side of the micellar interface. These particles are spherical and the size distribution of the particles is relatively narrow, with an average diameter of 87 nm. Surface vinyl silica nanoparticles so formed have been oxidized to surface carboxylic silica nanoparticles, followed by chemical conjugation with polyethyleneglycol amine (PEG amine) through the ethyl-3-(3-dimethylaminopropyl) (EDCI) carbodiimide reaction. The characteristic surface groups have been identified by Fourier transform infrared spectroscopy, while the size and the morphology of the particles have been studied by dynamic light scattering and transmission electron microscopy. It has been found that about 80-85% of the carboxylic groups are PEGylated during the EDCI reaction.

Titanium dioxide nanoparticles prepared in water-in-oil microemulsion droplets by controlled hydrolysis of TiCl(4)-generated crystalline nanoparticles of sizes from 115 nm down to 6 nm diameter depending on the size of the aqueous core of the micellar droplets. Powder X-ray diffraction of the vacuum-dried product (without sintering) indicated the presence of an unusual type of orthorhombic crystal structure nearly similar to titanium dioxide crystals prepared at high pressure. On gradual heating up to 900 degrees C these metastable crystals are converted into relatively more stable nanorods perhaps through making and breaking of the Ti-O-Ti bonds. It has been concluded that chemical pressure generated within the constrained volume of aqueous core of the reverse micellar droplets is responsible for the unusual crystal structure of TiO(2) nanoparticles.

Ormosil (organically modified silane) such as n-octyl triethoxy silane has been found to aggregate in the form of normal micelles as well as reverse micelles in which the triethoxy silane moeities are hydrolyzed to form a hydrated silica network while the n-octyl groups are held together through hydrophobic interaction. These nanoparticles are spherical in shape and are nearly monodispersed with an average diameter of below 100 nm. The nanoparticles originating from the micellar aggregate have an hydrophobic core with a layer of the hydrated silica network at the surface. The hydrophobic core can host hydrophobic molecules such as tetraphenyl porphyrin, which is leached out of the particles extremely slowly compared to that in Triton X-100 micelles. The nanoparticles originating from the reverse micelles have a hydrated silica network in the core surrounded by the hydrophobic n-octyl chains on the particle surface. The hydrophilic silica cores of these nanoparticles have been used to encapsulate horseradish peroxidase (HRP) and the enzyme shows its activity and follows Michaelis-Menten kinetics.

Polyacrylamide nanoparticles of different size (100 nm) were prepared in reverse micelles under various dynamic conditions of micellar systems. The nanoparticles were always larger than the aqueous core in which they were formed; however, the size of the nanoparticles could be controlled if the interdroplet interaction and coalescence rate in reverse micelles were regulated. Factors such as interfacial rigidity of the droplets, size of the aqueous core, temperature, and concentrations of acrylamide (monomer) and N ,N '-methylenebisacrylamide (crosslinking agent) have a profound effect on the size of nanoparticles. By adjustment of these parameters in reverse micelles, nanoparticles ranging from about 10 to more than 100 nm have been prepared.

Platinum nanoparticles prepared in reverse micelles have been used as catalysts for the electron transfer reaction between hexacyanoferrate(III) and thiosulfate ions. Nanoparticles of average diameter ranging between 10 and 80 nm have been used as catalysts. The kinetic study of the catalytic reaction showed that for a fixed mass of catalyst the catalytic rate did not increase proportionately to the decrease in particle size over the whole range from 10 to 80 nm. The maximum reaction rate has been observed for average particle diameter of about 38 nm. Particles below diameter 38 nm exhibit a trend of decreasing reaction rate with the decrease in particle size, while those above diameter 38 nm show a steady decline of reaction rate with increasing size. It has been postulated that in the case of particles of average size less than 38 nm diameter, a downward shift of Fermi level with a consequent increase of band gap energy takes place. As a result, the particles require more energy to pump electrons to the adsorbed ions for the electron transfer reaction. This leads to a reduced reaction rate catalyzed by smaller particles. On the other hand, for nanoparticles above diameter 38 nm, the change of Fermi level is not appreciable. These particles exhibit less surface area for adsorption as the particle size is increased. As a result, the catalytic efficiency of the particles is also decreased with increased particle size. The activation energies for the reaction catalyzed by platinum nanoparticles of diameters 12 and 30 nm are about 18 and 4.8 kJ/mol, respectively, indicating that the catalytic efficiency of 12-nm-diameter platinum particles is less than that of particles of diameter 30 nm. Extremely slow reaction rate of uncatalyzed reaction has been manifested through a larger activation energy of about 40 kJ/mol for the reaction.

Injectable hydrogel polymeric nanoparticles of polyvinylpyrrolidone cross-linked with N,N'-methylene bis-acrylamide and encapsulating water-soluble macromolecules such as FITC-dextran (FITC-Dx) have been prepared in the aqueous cores of reverse micellar droplets. These particles are 100 nm and below in diameter with a narrow size distribution. When dispersed in aqueous buffer these particles appear to be transparent and give an optically clear solution. Lyophilized powder of these nanoparticles is redispersable in aqueous buffer without any change in the size and morphology of the particles. The efficiency of FITC-Dx entrapment by these nanoparticles is high (>70%) and depends on the amount of cross-linking agent present in the polymeric material. The release of the entrapped molecules from these nanoparticles depends on the degree of cross-linking of the polymer, particle size, pH of the medium, and extent of loading, as well as temperature.

Yeast alcohol dehydrogenase (YADH) solubilized in reverse micelles of aerosol OT (i.e., AOT or sodium bis (2-ethyl hexyl) sulfosuccinate) in isooctane has been shown to be catalytically more active than that in aqueous buffer under optimum conditions of pH, temperature, and water content in reverse micelles. Studies of the secondary structure conformational changes of the enzyme in reverse micelles have been made from circular dichroism spectroscopy. It has been seen that the conformation of YADH in reverse micelles is extremely sensitive to pH, temperature, and water content. A comparison has been made between the catalytic activity of the enzyme and the alpha-helix content in the conformation and it has been observed that the enzyme is most active at the maximum alpha-helix content. While the beta-sheet content in the conformation of the entrapped enzyme was found to be dependent on the enzyme-micelle interface interaction, the alpha-helix and random coil conformations are governed by the degree of entrapment and the extent of rigidity provided by the micelle core to the enzyme structure. Copyright 2000 Academic Press.

pH- and temperature-sensitive hydrogel nanoparticles of copolymers of vinylpyrrolidone (VP) and acrylic acid (AA) cross-linked with NN' methylene bis acrylamide (MBA) of sizes up to 50 nm diameter loaded with marker compound FITC-dextran (mol wt. 19.3 kD) were prepared in the aqueous core of reverse micellar droplets and were dispersed in aqueous buffer. These particles have high entrapment efficiency, and the lyophilized powder can be redissolved in buffer without any significant agglomeration. The release of FITC-dextran from these particles was found to be pH- and temperature-dependent. The release was slow in acid solution, but it increased considerably as the pH of the medium was increased. The release rate was also increased with the increase of temperature. Copyright 1998 Academic Press.

The effect of nonamphiphilic organic compounds like benzene and nitrobenzene, both nonclassical “cosurfactants”, on the ternary AOT/water/isooctane system was studied by 1H-NMR spectroscopy and viscosity measurements. From the latter it is concluded that an apparent desolvation of the aqueous microphases occurs, which increases the packing density of the surfactants in the monolayer. According to a molecular model of Mitchell and Ninham, this process should enhance the maximum water uptake, a prediction in agreement with experiment. 1H-NMR measurements demonstrate that the solubilized water gradually adopts the properties of bulk water as the additive concentration is increased within the “cosurfactant” region. In the “trans-cosurfactant” region, progressive solvation of the surfactants by the additive within the interfacial domain reduces their amphiphilicity with a concomitant decrease of their solubilizing power.