Date of Award
Program or Major
Doctor of Philosophy
W R Seitz
Polymer swelling can be achieved by derivatizing a functional group onto the polymer that selectively binds a particular analyte. For example, at low pH, the particles in a hydrogel membrane swell in response to the increased hydrogen ion concentration and the membrane reflects less light. When the refractive index of the membrane is less than that of the particles, swelling decreases the refractive index of the particles. This brings it closer to the refractive index of the membrane, and decreases the scattering of light making the membrane more transparent.
On the other hand if the refractive index of the membrane is higher than that of the microspheres, swelling of the microspheres increases the difference between the refractive indices of the two media. This makes the membrane less transparent. By coupling selective polymer swelling with a transduction method that responds to the changes in polymer size, it is possible to make a chemical sensor.
Polymer microspheres that change size as a function of pH and metal ion concentration have been prepared by suspension and dispersion polymerization respectively. For the pH sensor, poly (vinylbenzylchloride) was prepared by suspension polymerization using the Shirazu porous glass (SPG) method. The particles were lightly crosslinked with divinylbenzene followed by amination with NNN'-trimethylethylenediamine.
Poly (hydroxyethylmethacrylate)(polyHEMA) microspheres were prepared by dispersion polymerization and then derivatized with diethylenetriamine. These aminated polymer microspheres suspended in PVA and polyurethane hydrogels respond to Cu2+ ions in solution. The complexation ratio of Cu2+: N was determined to be 1:2. The response time is ∼minutes, which is much longer than was observed with pH response. This was attributed to the bigger size of Cu2+, which restricts their movement into the polymer microspheres.
A new design of a molecularly imprinted polymer was developed. It was based on theophylline templated, uncrosslinked poly N-n-propylacrylamide (polyNNPAM) (principal monomer), co-polymerized with 4-(2-acroyloyloxyethylamino)-7-nitro-2, 1,3-benzoxadiazole (NBD-AE2) (fluorophore) and methacrylic acid (MAA) (functional monomer). This polymer was prepared by dispersion polymerization in acetonitrile solvent. Since the co-polymer remains in solution rather than as a separate phase (because of the absence of crosslinks), dialysis was the method chosen to extract the template, with many changes of the external solution. The sensor responds by increase in fluorescence with increased concentrations of theophylline. It was shown to have a high sensitivity (1 x 10-7M) and was highly selective because there was no change in fluorescence for increasing caffeine concentration up to 1 x 10-3M.
Mwangi, George K., "Chemical sensors based on swelling polymer microspheres and uncrosslinked fluorescently-labelled molecularly imprinted polymers" (2005). Doctoral Dissertations. 270.