Silica-Based Nanoparticles: Design and Properties - Advanced Fluorescence Reporters in Chemistry and Biology - page 234

hydrolytic reaction time [ 44 , 45 ]. As mentioned above, the size of produced silica
particles is distributed in a relatively broad range.
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 nanopar-
ticles. 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 nano-
particles. 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 influ-
ences 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.
The size of the silica nanoparticles can also be manipulated by changing organic
solvents in the reverse microemulsion. A recent work systematically reported the
correlation between the organic solvents and size of silica nanoparticles [ 56 ]. This
investigation revealed that in a reverse microemulsion, as the length of the organic
solvent alkane chain increased the size of the silica nanoparticles became larger
(Fig. 2 ). A cyclohexane/ n -hexahecane binary solvent mixture was used to prepare
tunable silica nanoparticles. By reducing the percentage of n -hexadecane in the
mixture, the size of nanoparticles continuously decreased from 100 to 50 nm. When
a ternary solvent reverse microemulsion was used, the tunable size range could be
extended down to 20 nm. A model explaining the effect of organic solvents on silica
nanoparticles size was proposed. This size effect was attributed to the different
micelle sizes and interdroplet percolation efficiency in various solvents.
2.2 Methods of Doping Dye Molecules into Silica Nanoparticles
2.2.1 Covalent Binding
The most reliable doping method is to chemically link dye molecules onto the silica
matrix via covalent bonds [ 5 , 58 ]. In this method, the dye molecules should be
modified by linking a functional group of alkoxide. A common strategy to make an
alkoxide modified dye derivative is to covalently attach a succinimide [ 12 , 59 ] ester
or isothiocyanate [ 5 , 13 ] modified dye to a coupling reagent, 3-aminopropyl-
trimethoxysilane (APTS). The dye-APTS conjugate can hydrolyze and condense
with the silicate during the formation of silica nanoparticles. As a result, dye
molecules are covalently encapsulated into silica nanoparticles. Since the covalent
bonds are stable at various conditions, this method prevents dye molecules from