Chemistry Reference
In-Depth Information
15.4.2. Thin Films and Membranes
Thin films or membranes are traditionally better suited for sensing applications
because of their more uniform morphology and better optical properties (Schneider
et al. 2005). However, the preparation of MIP thin films or membranes is challenging.
Polymerization of the traditional MIP formulation yields irregular cracked
films because of the inability of the highly cross-linked matrix to accommodate the
contraction of the polymer matrix induced by polymerization and porogen
evaporation. Thus, new polymerization methods had to be developed to prepare
MIP thin films. These methods include surface initiated polymerizations, polymer
grafting,
polymerizations
using
nonvolatile
porogens,
and
electrochemical
polymerizations.
Surface Initiated Polymerizations.
Thin MIP coatings have been prepared on a
variety of surfaces via the covalent anchoring of initiator or monomer units to the
surface (Quaglia et al. 2001; Ruckert et al. 2002; Piacham et al. 2005). The most
common are on silica surfaces that can be functionalized with organo orthosilicates.
For example, a propranolol imprinted polymer coating was synthesized in a
fused silica capillary column by surface initiation (Schweitz 2002). This 0.15-2 mm
thick MIP coating was demonstrated to be enantioselective for the template
(S-propranolol). The method of surface initiated atom transfer radical polymer-
ization was utilized for the preparation of an MIP nanotube membrane using a
porous anodic alumina oxide (Wang et al. 2006). The b-estradiol imprinted polymer
nanotube membrane has high affinity and selectivity for the template molecule.
Polymer Grafting.
Surface grafted MIPs are prepared by attaching preformed
imprinted polymers to a surface or reinitiating polymerization under imprinting con-
ditions from a polymer surface (Sellergren et al. 2002; Fairhurst et al. 2004; Hattori
et al. 2004; Yang et al. 2005; Titirici and Sellergren 2006). For example, the grafting
of an epinephrine imprinted polymer film onto the polystyrene surface of a microplate
was realized through oxidation of 3-aminophenylboronic acid by ammonium persul-
fate (Piletsky, Piletska, et al. 2000). This MIP-coated microplate displayed excellent
stability and reproducibility. Polypropylene membranes were imprinted with desme-
tryn by photografting with 2-acrylamido-2-methylpropanesulfonic acid (functional
monomer) and MBA (cross-linker) in water with benzophenone as the photoinitiator
(Piletsky, Matuschewski, et al. 2000). These MIP membranes possess good
selectivity for triazine herbicides in water. The surface grafting approach has
several advantages: 1) it has the possibility of modifying an inert surface, 2) the
synthesis and immobilization of the MIP membrane can be performed in one step,
3) the grafted polymers usually have fast response times because they are very
thin, and 4) the thickness of the polymer membrane can be regulated by the polymeri-
zation time or initiator efficiency.
Polymerizations Using Nonvolatile Porogens.
Most recently, MIP mem-
branes and thin films have been prepared under conditions that limit the evaporation
