History and Classification of Non-Siliceous Hybrid Materials - Mesoporous Organic-Inorganic Non-Siliceous Hybrid Materials - page 18

alkoxide with a phosphonic acid and then the formation of the M-O-M bonds of
the metal oxide network by hydrolysis/condensation of the remaining alkoxide
group. The burdensome work to remove the residual organic solvent was not
needed.
Organic “capping” agents containing terminal carboxylic groups have been
widely used to prepare quantum dots or nanocrystals [ 78 - 81 ]. The capping agent
limits the size of the nanoparticles by preventing further particle growth and
agglomeration during synthesis and can also be useful in controlling the particle
reactivity, imparting solubility or packing characteristics, and protecting both nan-
oparticle and its environment from destructive interactions. If different functional
groups are anchored on the carboxylate linkages, variation of tremendous phys-
icochemical properties can be realized. To date, reports regarding carboxylated
mesoporous material are rare because the relatively low stability of C-O-M may
prohibit their practical applications.
Anchoring of the functional groups is driven by condensation or by complexa-
tion. Correspondingly, grafting can be covalent (practically irreversible) or coordi-
native (partially reversible). Stronger grafting groups are needed for sensing and
catalysis, while more labile functions are meaningful in the quest for controlled
delivery or reversible signaling as well. The grafting strength will be also impor-
tant for the even incorporation of the R function along the pore systems. Indeed,
there are two key factors that control the homogeneous incorporation of organic
functions: the accessibility of the pore systems; and the reactivity of the organic
functional molecules toward the pore surface. The first factor will essentially
depend on the possibility of pore interconnection, and the symmetry and orien-
tation of the pore mesostructure. Noticeably, pore blocking can occur during the
post-functionalization process.
2.3.2 Direct Synthesis
An alternative method to synthesize organically functionalized mesoporous hybrid
is the cocondensation method (one-pot synthesis). It is possible to prepare meso-
structured hybrid phases by the cocondensation of metallic precursors and organic
functional groups in the presence or absence of structure-directing agents, leading
to materials with organic residues anchored covalently and homogeneously in the
pore walls (Fig. 2.8 ) [ 64 , 65 , 82 ]. By using structure-directing agents known from
the synthesis of pure mesoporous silica phases (e.g., MCM or SBA silica phases),
organically modified silicas can be prepared in such a way that the organic func-
tionalities project into the pores. These will be elaborately discussed in Chap. 3 .
Since the organic functionalities are direct components of the mesostructured
hybrid matrixes, pore blocking is not a problem in the cocondensation method.
Furthermore, the organic linkers are generally more homogeneously distributed
than in materials synthesized involving the grafting process. The tendency toward
homocondensation reactions, which is caused by the different hydrolysis and