Silica-Based Nanoparticles: Design and Properties - Advanced Fluorescence Reporters in Chemistry and Biology - page 232

to fluorescence, DDSNs can also be designed with other functions. An amorphous
silica matrix is a carrier of various materials. By encapsulating other materials
into the DDSNs, fluorescent nanoparticles with multiple functionalities can be
developed.
2.1 Synthesis of Silica Nanoparticles
2.1.1 St¨ber Method
The St¨ber method is also known as a sol-gel method [ 44 , 45 ]. It was named after
St¨ber who first reported the sol-gel synthesis of colloid silica particles in 1968 [ 45 ].
In a typical St¨ber method, silicon alkoxide precursors such as tetramethylorthosili-
cate (TMOS) and tetraethylorthosilicate (TEOS), are hydrolyzed in a mixture of
water and ethanol. This hydrolysis can be catalyzed by either an acid or a base. In
sol-gel processes, an acidic catalyst is preferred to prepare gel structure and a basic
catalyst is widely used to synthesize discrete silica nanoparticles. Usually ammonium
hydroxide is used as the catalyst in a St¨ber synthesis. With vigorous stirring,
condensation of hydrolyzed monomers is carried out for a certain reaction time
period. The resultant silica particles have a nanometer to micrometer size range.
The St¨ber method can be used to form core-shell silica nanoparticles when a
presynthesized core is suspended in a water-alcohol mixture. The core can be a
silica nanoparticle or other types of nanomaterials [ 46 , 47 ]. If the core is a silica
nanoparticle, before adding silicon alkoxide precursors, the hydroxysilicates hydro-
lyzed from precursors condense by the hydroxide groups on the surface of the silica
cores to form additional layers. If the core is a colloid, surface modification of the
core might be necessary. For example, a gold colloid core was modified by poly
(vinylpyrrolidone) prior to a silica layer coating [ 46 ].
Besides the traditional St ¨ ber method, a number of modified St ¨ ber methods
have been reported for preparation of various silica nanoparticles. An important
example is the synthesis of mesoporous silica nanoparticles by modifying the
St ¨ ber method [ 41 , 48 - 53 ]. Mesoporous silica nanomaterials have a wide variety
of applications including drug delivery and developments of catalysts [ 50 , 51 ].
The porosity of the silica nanoparticles produced by the traditional St¨ber method
is smaller than that of the mesoporous materials. To prepare mesoporous silica
nanoparticles, cetyltrimethyl ammonium bromide (CTAB) [ 49 , 50 , 54 ], cetyltri-
methyl ammonium chloride (CTAC) [ 40 , 42 , 48 ] or octadecyltrimethoxysilane
was added to the synthetic solution as a template to support the “large” pores in
the silica matrix. After the silica nanoparticles are made, the templates are
removed by washing or heating the nanoparticles. Finally, the mesoporous silica
nanoparticles can be obtained. The St¨ber method is fast and simple. The major
challenges of this method are how to improve the uniformity of the silica
nanoparticles and how to efficiently dope dye molecules within the matrix.