Synthesis and Characterization of Hydroxyapatite Nanocoatings by Sol–Gel Method for Clinical Applications - Biological and Biomedical Coatings: Processing and Characterization - page 46

important in engineering, biomedical, and optical applications such as in the field of anti-
reflective coatings (Pettie et al. 1988) and on medical porous membranes such as artificial
liver or retinal implants.
Precursors
Although it is loosely used, sol-gel process methods can be categorized as alcohol- or aqueous-
based. As the name suggest, alcohol-based systems generally exclude water buildup until
the hydrolysis stage, whereas aqueous-based systems are carried out in the presence of
water. However, there are also nonhydrolytic sol-gel processes that do not require the
presence of solvents at all. Similarly, sol-gel precursors can be classified as either alkox-
ides or nonalkoxides. While alkoxides are the preferred precursors for sol-gel production
owing to their volatility, other compounds, such as metal salts, can also be used. This is
often the case for Group I and II elements whose alkoxides are solid, nonvolatile, and in
many cases have a low solubility (Percy et al. 2000).
The preparation of sol solutions involves the use of solvents. These solvents are usually
organic alcohols. While the main objective of the solvent is to dissolve solid precursors,
they are also used to dilute liquid precursors and minimize the effect of concentration gra-
dients. The particular solvent employed can influence factors such as particle morphology
(Harris et al. 1988) and crystallization temperatures (de Kambilly and Klein 1989).
Alkoxide Precursors
Metal alkoxides are probably the best starting materials in sol-gel preparations (Schroder
1966). All metals are capable of forming metal alkoxides of the form M(OR) x , where M is a
metal, R is an alkyl group, and x is the valence state of the metal. Another type of alkoxide
that exists is the double alkoxide, which has the general formula M x ′M y (OR) z , where M′
and M are metals, R is an alkyl group, and x , y , and z are the valence states of the met-
als. Double alkoxides have the added advantage of retaining their stoichiometry (Thomas
1988).
While the organic group influences the stability of the alkoxide, precursors should also
be selected so they have adequate volatility to enable a clean break of the M-OR and MO-R
bonds to produce materials free from organics (Mazdiyasni 1982; Eichorst and Payne 1988).
Most metal alkoxides are sensitive to heat, light, and moisture. Sensitivity to moisture can
be attributed to the electronegative alkoxy groups, which renders the metal atom prone to
nucleophilic attack (Bradley et al. 1978).
While alkoxides are the most favorable sol-gel precursor, there are two exceptions to
this rule. These are the alkoxides of silicon and phosphorus. Silicon alkoxides require an
acid or base catalyst for hydrolysis, and even with such an addition, hydrolysis is very
slow. In the same way, trialkylphosphates are very stable and difficult to hydrolyze, and as
a result they are generally not used widely as phosphorus precursors in sol-gel production
(Thomas 1988).
Nonalkoxide Precursors
Metal salts provide an ideal alternative to alkoxides if they can be transformed readily into
their respective oxide by thermal or oxidative decomposition. It is also preferable for such
salts to be soluble in organic solvents. Candidates are salts produced from organic acids,
such as acetates, formats, and citrates. Nitrates are the most suitable inorganic salts, as