Novel Chemical Sensors Based on Swellable Molecularly Imprinted Polymer Microspheres in a Hydrogel 

1. The selected template molecule is in the first step mixed with functional monomers.
2. Complex formation of the template molecule with the functional monomers
3. Polymerization of the resulting printing complex with an excess of cross-linking agent in an inert solvent to form a rigid polymer.
4. Removal of the template molecule by extraction.

        The resulting MIPs have a permanent memory of the original template in term of complementary size, geometry, and orientation. These specific binding sites can selectively recognize the target molecule, even in a complex solution.

Advantages of MIPs

1. MIPs are very stable even in severe conditions, which natural molecules cannot tolerate, such as organic solvents, acidic/basic solutions, high temperature, etc.

2. They have high selectivity similar to that of natural systems, therefore, MIPs could be called “artificial antibodies or proteins”.

3. MIPs are easily obtained by compolyerization of suitable functional monomers and cross-linkers in the presence of the print molecule.

4. The cost is considerably lower than  natural antibodies or proteins.

These unique properties make them suitable for sensor technology and have potential applications

Swellable MIPs

        Recent study of swellable MIPs has been reported in the literature [1]. Copolymer gels consisting N-isopropylacrylamide and acrylic acid have been prepared in the presence and absence of template molecule. These MIPs can undergo swelling changes and at the same time have their molecular specificity. The swelling and shrinking is based on the thermal properties of poly NIPA, which is swollen at low temperature in water and undergoes a reversible phase transition at high temperature.

Chemical Sensor Based on Polymer Swelling

        Our group has been developing optically sensitive membranes that consist of polymer microspheres. Following is the principle for the pH chemical sensor based on polymer swelling and shrinking.

        The microspheres has been designed to swell and shrink as a function of the changes of their environment, such as temperature, pH, and other analyte, etc. The interaction of analyte and polymer causes polymer to swell or shrink, resulting in a change in the optical properties, such as refractive index (n). Either transmission or reflection measurements can be used to determine analyte concentration.  

Sensor Design

        All of these encourage us to develop a new sensor. First, we design the reaction to prepare swellable poly NIPA MIP microspheres selectively sensing to template, then embed them a hydrogel membrane. Microspheres swell at room temperature,  shrink at high temperature. The shrinking extent is different in different concentration of template. Less shrinking for high concentration of template due to more  interaction of template with polymer, which results in low refractive index. So the turbidity changes of the hydrogel membrane could be used to measure the concentration of template.

Synthesis of poly NIPA-MAA MIP Microspheres

        Following is the procedure to make theophylline (THO) templated MIPs by dispersion polymerization. THO interacts with MAA via hydrogen bond and electrostatic force. After polymerization, THO can be removed from MIPs by extraction with acetic acid/methanol solution. The resulting imprinted cavity can selectively response to THO. The microspheres are very uniform with the size of ca. 1.0 micron. 

Selectivity

        To exam the molecular specificity, caffeine was selected as another template molecule with the structure different from THO only by a  methyl group. There is no response to caffeine with the concentration as high as 1.0x10^-4 M.                                                                                                                 

Sensitivity

         1x10^-8 M THO

Advisor: Dr. W. Rudolf Seitz, Department of Chemistry, University of New Hampshire, Durham, NH 03824