Biomedical Engineering Reference
In-Depth Information
the particles, known as the sol. The solubility of the precursors in the solutions depends on the Gibbs
free energy of mixing and solubility limit of the reactants. Two main groups of precursors are most
common—metal alkoxides M(OR)
n
and metallic salts M
m
X
n
where X is an anionic group. For mixed
sols judicious choice of solvents is critical. The most common nanoparticles for dental materials
are oxides; water is therefore always present as a major reactant in order to transform the precursor.
Precursors undergo hydrolysis and condensation reactions to generate metal-oxo or metal-hydroxy
polymers. Thus, the sol evolves towards the formation of a gel-like diphasic system containing both a
liquid phase and a solid phase that comprise morphologies ranging from discrete particles to continu-
ous polymeric networks. Since the sol-gel method generates materials from atomic levels, it is often
referred to as a “bottom-up” process. By controlling the purity of the precursor solution(s) it is pos-
sible to control the chemistry of the end-product.
The basic steps involved in sol-gel synthesis of oxide nanoparticles consists of the hydrolysis and
condensation of metal alkoxide M(OR)
n
, where M is a metal atom, and R is an organic group, accord-
ing to the following scheme:
Hydrolysis
M(OR)
H O
→
M(OH)
4
ROH
4
2
4
Condensation
M(OH)
M(OH)
→ (OH) M O M(OH)
- -
H O
4
4
3
3
2
The kinetics of hydrolysis and condensation reactions is governed mainly by the ratio of molar
concentrations of water and alkoxide. In general, larger
R
values (groups containing
3 carbon
atoms) generate powder particles whereas lower
R
values are more suitable for depositing thin films
or fibers
[6]
.
There are three regimes in the generation of monodispersed particles in the nanometers range. The
first is the induction period during which the reaction slowly generates chemical complexes which serve
as the growth units. The second is the nucleation period during which the concentration of the solution
builds up until a critical supersaturation is reached and nucleation occurs
[7]
. This is followed by the
growth period. The shape of the oxide particles by the sol-gel process has been modelled by Pierre
[8]
and is dependent on the growth mechanisms of the particles. The oxide particles grow by mononuclear
or polynuclear mechanisms. During the initial stages, the growth occurs by a mononuclear mechanism
whereby successive layers are deposited on the nucleated particle surface. The resultant particles form
in well-defined shapes governed by the particular crystal structure of the oxide. As the particles grow
the area of the layers increases and growth mechanism progressively changes to a polynuclear growth
mechanism. The nucleation of a new layer takes place before the completion of the deposition of the
previous layer. The particle surface becomes less smooth at the molecular scale but less faceted at the
macroscopic scale. The particular crystallographic shape is lost and the particle becomes spherical. As
the particle size of the polynuclear species increases at a certain point the supply of chemical complexes
becomes insufficient because they are consumed by the growing particles. At this point, the transport of
chemical complexes by diffusion becomes rate controlling. Conditions such as sol concentration, rate of
addition, mixing, pH, and nature of the counter ion of metallic salts are some of the factors that deter-
mine the size and shape of the final particles. An extensive treatment is provided in Ref.
[8]
.
Control of the structure of the nanoparticles is a special challenge since as the size of nanopar-
ticles increases, diffusion becomes limited, and agglomeration of the particles takes place. Most often
