Chemistry Reference
In-Depth Information
us with access to a range of extent of interactions between the inorganic and organic
components. The organic component, which could provide us with both the size
and flexible reactivity that we desired, was a simple, partially-hydrolysed polysac-
charide, soluble starch.
The widely used commercial product known as soluble starch is typically a corn
starch which has been processed to reduce its molecular weight to enable it to be
more readily dissolved in aqueous solution. Although not extensively soluble in wa-
ter, solutions of 1 to 3 % by weight can be prepared. At the higher end of these con-
centrations, the starch solutions show a significant increase in viscosity relative to
that of water and a resulting reduction in processability for forming hybrid materials
using sol-gel techniques. However, materials based on a solutions containing less
than 3 % soluble starch demonstrated a significant range of interesting properties.
Using soluble starch as the organic component in our biohybrid materials pro-
vided us with the ability to explore different degrees of network interaction between
the inorganic and organic components. The conceptually simplest situation is one
in which an interpenetrating but not covalently interacting network is formed. The
interpenetrating networks (IPN) are well described in the formation of various types
of hybrid materials. In general, one polymeric network is dissolved or swelled to
allow for the formation of the second network
in situ.
For example, a review by
Mauritz et al. from 2007 describes research using the organic component as a scaf-
fold for inorganic sol-gel polymerization with Nafion as the scaffold and a range
of silane and other metal alkoxides as the inorganic precursors.[
18
] Similarly, use
of the readily water soluble tetrakis(2-hydroxyethyl) orthosilicate (THEOS) silane
has facilitated the development of interpenetrating hybrid materials based on wa-
ter soluble biopolymers. The use of THEOS in combination with polysaccharides
was reviewed in 2005 [
19
]describing the potential that these developments have for
improving the range of possible applications for biohybrids. An example of such
an application was described by Wang et al. where chitosan has been used as the
organic component, with THEOS undergoing
in-situ
hydrolysis and condensation
to form an interpenetrating biosilicate hybrid for use in an amperometric biosensor
for hydrogen peroxide.[
20
]
The availability and wide range of pre-existing biopolymers such as chitosan,
starch and carrageenan along with the diversity of monomeric silanes tends to en-
sure that typically the biopolymer is used as the high molecular weight species
while the silane precursor undergoes condensation to develop the inorganic network
in the presence of the biopolymer. In our own research we have instead developed
a process where the silicate network is also preformed which allows us to use the
resulting silicate nanoparticles as material building blocks regulating the degree of
interaction with the biopolymer component through the use of organic crosslinking
interactions such as epoxy-amine crosslinking.
We have found that by working with both a preformed biopolymer and pre-
formed silicate nanoparticle, the material properties of the resulting hybrid can be
more systematically evaluated as the concentration, presence or absence of either
of the two components does not alter the chemical conditions of the formation of
the individual components. Rather, the two components are prepared separately in a
