Silica nanoparticles find applications in many fields such as catalysis, chromatography, ceramics, and as binding agents. Recent studies have also reported their use in biotechnology for bio-sensing and drug delivery. Silica particles are suitable for application in chemo-mechanical polishing (CMP) because silica can be directly precipitated as mono dispersed spheres with narrow size distribution.
Silica nanoparticle is an example of multi-functional nanoparticle specially developed for drug delivery applications. On the surface of a metal nanoparticle template, a silica shell is grown. The size and shape are governed by the geometry of the metal template particle, while the thickness of the shell is controlled by manipulating the stoichiometry of the silica growth. The silica shell incorporates amine molecules that are utilized to covalently bind other bio molecules to the surface. Different fluorophores can be incorporated in the shell to allow for identification and tracking in-vivo. The porosity of the shell can be tuned to control the rate of diffusive delivery of drugs from the interior of the shell into the body. Such particles have immediate application for non-invasive methods of targeted drug delivery.
Synthesis
Colloidal silica particles and agglomerates are synthesised from hydrolysis of metal alkoxide, followed by condensation and polymerisation reactions. Other methods of preparation of silica nanoparticles include Thermal decomposition of TEOS, Decomposition of rice husk, Combustion-flame CVC and Wet chemical synthesis.
The following are few procedures for the synthesis of silica nanoparticles from literature.
(i) Silica-impregnated nickel nanoparticles : In a typical procedure, tetra ethyl ortho silicate (TEOS) is used for synthesis of NSP as raw silica source. TEOS (0.016 mol) is dissolved slowly in acidified co-block polymer solution under constant mixing environment to get uniform solution to promote hydration of TEOS and decrease of residual ethoxy groups affecting the assembly of the particles. Subsequently, NiSO4 in 1:4 molar ratio of silica to nickel is added and stirred for 24 h. The resulting reactant is neutralized with 25% ammonia solution to get effective formation of ordered nanoparticles through self-assembly of silicate and nonionic co-block polymer as influenced by the decrease in polarity with the assembly of the species. Co-block polymer nonionic surfactant surround the silica composite and thus act as a grain growth inhibitor and stabilize the ordered nanoparticles. Finally, the solution is aged for 24 h at room temperature, centrifuged, washed with de-ionized water, rinsed with distilled acetone, and oven dried in order to get silica-impregnated nickel nanoparticles (NSP), which were further calcined at 500°C under N2 atmosphere.
(ii) Silica nanoparticles have been prepared using water in oil (W/O) emulsion system at room temperature that employs a water-soluble amine as catalyst and tetraethylorthosilicate (TEOS) as the silica source. The pH value of the aqueous phase and the water: surfactant ratio is the key factors contributing to the formation and final size of stable and regular spherical silica particles. When the pH value of the aqueous phase is controlled between 8 and 9, silica particles could be synthesized. The shell thickness of the hollow particles as and when prepared increases with the length of the hydrocarbon tail of the amine catalyst. The viscosity of the external oil phase determines the shape regularity of the spherical silica hollow particles.
(iii) Aqueous ammonia (3.14 ml, 28 – 30 %) is added to a solution containing 74 ml of ethanol (99%) and 10 ml of deionised water, TEOS (6 ml, 98 %) is added to the above prepared mixture at 298 K with vigorous stirring for one hour to yield uniform silica spheres. A mixture containing 5 ml of TEOS and 2 ml octadecyltrimethoxy silane (90%) is added to the colloidal solution containing the silica sphere and further reacted for one hour. The resulting octadecyl group incorporated silica nanocomposite is centrifuged, calcined at 823 K for 6 hours under air to produce solid shell silica material.
(iv) Tetraethylorthosilicate (TEOS), NH4 OH aqueous solution and ethanol (EtOH) are used and the water used for the sample preparation is purified by both ion-exchange and distillation. Reagents are mixed into the two starting time solutions of ethanol: (A) TEOS/ EtOH; and (B) NH4 OH/ H2 O/ EtOH. The contents of the solutions (A) and (B) are adjusted so that the concentrations of TEOS, H2 O, and NH4 OH would be at the prescribed concentrations. The solutions are prepared in a glove box at room temperature under dry air. The humidity in the glove box is kept below a few percent. The solutions (A) and (B) are mixed with each other at 298 K, and the mixture is stirred vigorously by hand for approximately 6s. The glycerol is added directly to the water/ammonia/ethanol mixture prior to the addition of TEOS. Depending on different molar ratio of reagents, the condensation reaction begins after various times. This could be easily observed, because, after the invisible hydrolysis reaction forming silicic acid, the condensation of the supersaturated silicic acid is indicated by an increasing opalescence of the mixture starting 2-10 minutes after adding the TEOS. After this transformation, a turbid white suspension is formed after a few minutes more.
(v) Mesoporous silica nanoparticles: These are synthesized by reacting tetraethyl orthosilicate with a template made of micellar rods. This results in a collection of nano-sized spheres or rods that are filled with a regular arrangement of pores. The template can then be removed by washing with a solvent adjusted to the proper pH. In another technique, the mesoporous particle could be synthesized using a simple sol-gel method or a spray drying method. Tetraethyl orthosilicate is also used with an additional polymer monomer (as a template).
(vi) Solid silica gel can be produced by drop-wise addition (~ 2 drops/s) of diluted sodium silicate to 28 ml of 2.5% HCl with stirring for 250 rpm at 60°C until a cloudy, viscous gel is formed. The volume of sodium silicate is noted down to be 10 mL. It is then thoroughly washed with distilled water until free from Cl ions. Diluted silver nitrate solution is added to the filtrate for testing the removal of Cl ions. Its presence is indicated by the formation of a white precipitate. This test is repeated until no white precipitate is formed on addition of silver nitrate. The product is then dried in oven at 100°C for more than 24 hours, and calcined in air at 1000°C for 1 hour.
(vii) The silicon nanoparticle uses a controlled method to synthesize doped silicon nanoparticles and to develop an understanding of ink formulation, incorporating these silicon nanoparticles. These nanocomposite inks find application in printed solar cells. Printed electronic devices fall in the broader category of flexible electronics, an exciting emerging technology that attracts great interest internationally. Using hot-wire chemical vapour deposition (CVD) and an undisclosed technique, doped silicon nanoparticles of controlled size, morphology, composition and structure are synthesized. The synthesis system consists of a multi-functional stainless reaction chamber with vacuum pumps, gas handling system, as well as electronic measurement and control.
(viii) Thermal catalytic pyrolysis (HWCVD) can produce mostly amorphous -Si:H nanoparticles.
(ix) Commercially available SiO2 nano powders synthesized by flame aerosol processes consist to a major part of large aggregates, formed by collision and sintering of primary particles during the process. These aggregates impair the physical and chemical benefits, which could be achieved from the properties of the primary, non-aggregated nanoparticles. Silica nanoparticles are produced by high-temperature oxidation of hexamethyldisiloxane (HMDSO) vapour in an aerosol co-flow diffusion flame reactor. Oxygen and methane are used as oxidant and fuel gas, respectively. Particle size and morphology was controlled by flame configuration, O2 flow rate, CH4 and HMDSO concentration.
(x) Silicon nanoparticles can be synthesized inside a stainless steel cylindrical chamber with inner diameter of 150 mm and a volume of 2.65 l. A water cooled carousel supported 6 substrates at a distance of 50 mm from the filament and allowed selective deposition per substrate. Filaments are constructed from a 30 mm long 0.5 mm diameter tungsten wire of 99.95% purity wound into a 6 mm diameter coil. The gas feed pipe ends 30 mm above the centre of the filament. The temperature of the stainless steel substrates, in the carousel, did not exceed 70 °C. Silica nanoparticles were produced with silane as precursor and hydrogen dilutions ranging from 0 to 80% and at operating pressures of 0.2 to 48 mbar. For each hydrogen dilution ratio, 5 substrates were loaded into the carousel and particles were produced with the same filament, but with the pressure being increased in 8 min intervals. Upon completion of the synthesis, the system was flushed with nitrogen for 4 min and samples were stored at atmosphere.
(xi) Facile synthesis of silica nanoparticles with narrow particle size distribution in aqueous L-Lysine solutions derives from the simplicity of the synthesis via hydrolysis of tetraethylorthosilicate (TEOS) in an aqueous solution of L-Lysine and the identification of a range of handles (e.g., pH, silica content, hydrolysis and hydrothermal ageing temperature) for tuning particle diameter from less than 5 nm to more than 20 nm. The initial stage of nanoparticle formation mimics that for tetrapropylammonium- and other alkylamonium-silica nanoparticles in that the nanoparticles rapidly form upon exceeding the silica solubility limit. The as-prepared powder varies in colour from yellowish to dark brown and gets deposited on all surfaces inside the reaction chamber
Different molar ratios of reagents have effect on the structure and morphology of silica particles at room temperature. When the reaction is conducted at 60º C, silica nanoparticles are also obtained. This temperature is based upon a limitation of the boiling point of the reagents. Spherical and agglomerated silica nanoparticles, which are obtained using different molar ratios of reagents; the molar ratio of the solvent is also important. With a lower molar ratio of solvent (ethanol), agglomerated silica particles are obtained.
When a narrow size distortion is required, a small molar ratio of ethanol should be employed. The optimum conditions for synthesizing silica nanoparticles are considered to be with the same molar ratio of TEOS and ammonia and a higher molar ratio of ethanol giving rise to smaller silica nanoparticles with a broad distribution of particle sizes. Using different solvents such as methanol, ethanol, propanol, butanol and ethanol-glycerol, different structures are obtained. From methanol and ethanol-glycerol, a stable sol could be obtained, but when butanol and ethanol are used, precipitation could be easily observed. Different experiments show that the presence of glycerol during synthesis affects the precipitation.
Realization of silica nanoparticle-crystals and controlled assembly of large-area nanoparticle films has implications spanning technologies of coatings and colloidal lithography to chemical sensing and biology (i.e., cell encapsulation and anti-immuno rejection coatings of implants). Challenges to device fabrication in each of these areas, however, derive at least in part from the lack of a simple means for synthesizing stable and mono disperse silica nanoparticles, and the limited capabilities for rationally tuning particle size, porosity, order, and monolayer continuity.