Biomedical Engineering Reference
In-Depth Information
Synthetic methods
Pyrolysis
Sol-gel process
Fumed or pyrogenic silica
Stöber method
Template method
Advantages
Advantages
Advantages
Surfactant free
Ease of synthesis
Scale of production
Size control
Surface modification
Expense
Dispersibility
Composites
Size control
Surface modification
Dispersibility
Composites
Shape control
Porosity control
Disadvantages
Size control
Surface modification
Dispersibility
Shape control
Porosity control
Disadvantages
Disadvantages
Possible surfactant
Shape control
Porosity control
Possible surfactant
Expense
FIGURE 4.1
Schematic of various synthesis methods for silica-based nanoparticles. The most commonly used silica
nanoparticles are synthesized by either pyrolysis (fumed silica) or the sol
gel process (engineered
nanoparticles). Each method has advantages and disadvantages for biomedical applications including size
control, scale of synthesis, surface modification, and incorporation of other compounds.
4.3.1
Methods
Fumed silica is synthesized by the pyrolysis method in which silicon tetrachloride reacts with
oxygen in a flame, and the SiO
2
seed grows in size or aggregates
[8]
. The sol
gel process
[9,10]
is
performed in a liquid phase and silica nanoparticles are synthesized with an acidic or basic catalyst
and alcoholic solvent in the presence of silicon alkoxide. When the reaction occurs under alkaline
conditions, this is referred to as the St¨ber method
[11]
(
Figure 4.2
). In this method, the concentra-
tion of precursor and catalyst can be used to control the size and results in a sphere, the most
stable shape in nature. Using the template method, nanomaterials can be synthesized similar to a
confined nanosized cavity that can be generated by surfactants including polymers. The surfactant,
which has a hydrophilic head (or part) and hydrophobic tail (or part) in a single molecule (or poly-
mer), forms a micelle in water or an inverse micelle (or reverse micelle) in organic solvent as
shown in
Figure 4.3
. This micelle size is dependent on the water-to-surfactant molar ratio
(W
0
5
[H
2
O]/[surfactant]). In this case, both basic and acidic catalyst can be used
[12,13]
.
Mesoporous silica nanoparticles (MSNs) are essentially silica particles with pores of varying
sizes. The pores allow the resulting particles to be used as carriers for therapeutics or biological
active compounds
[14]
. Pore width, which can be measured and analyzed by N
2
adsorption/desorp-
tion
[15]
, is classified by the International Union of Pure and Applied Chemistry (IUPAC) as
macropores—exceeding 50 nm, mesopores—2
50 nm, and micropores—not exceeding 2 nm
[16]
.
MSNs can also be synthesized by the sol
gel process although this process tends to aggregate
MSNs that are large in size (typically greater than a micrometer). More recently MSNs in the range
of 20
100 nm have been achieved using a double surfactant system
[17]
.
