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Title of Talk:
Compound Specific Imprinted Microspheres for Optical Sensing
Abstract of Talk:
The use of molecularly imprinted polymers is being investigated as the basis of a sensitive and selective sensing method for the detection of pharmaceutical and other emerging organic contaminants, which are at parts per billion levels, in aquatic environments. Moderately crosslinked molecularly imprinted polymeric microspheres (ca. 1 micron in diameter), which are designed to swell and shrink as a function of analyte concentration in aqueous media, have been prepared. These microspheres are incorporated into hydrogel membranes. Chemical sensing is based on changes in the refractive index of the membrane that accompany swelling of the molecularly imprinted microspheres. These changes are measured by surface plasmon resonance spectroscopy (SPR). For SPR, the polymer microspheres are directly applied to a gold surface where they are held in place by electrostatic attraction. Encapsulation of the polymeric microspheres is achieved by micropipetting the membrane formulation onto the surface of the SPR substrate where it is distributed across the gold surface by spatula prior to polymerization.
The prototype SPR sensor is both sensitive and specific. The addition of as little as 1.0x10-7 M theophylline is sufficient to cause a change in the refractive index of the membrane, which we were able to detect by SPR. Higher concentrations of theophylline produced a larger change in refractive index. In contrast, the same membrane showed no response to 1.0x10-4 M caffeine. (Caffeine and theophylline differ by only a single methyl group.) This result, we believe, is significant for two reasons. First, selectivity has been introduced into SPR analyses using these membranes. Studies where biological receptors have been used to functionalize Au or Ag surfaces with analyte specific receptors for pollutant monitoring have often been unsuccessful due to problems associated with antigen stability and cross reactivity. Second, the likelihood is high that parts per billion detection limits for theophylline and other so-called emerging organic contaminants can be achieved with this approach to chemical sensing once the polymeric formulation used to develop the imprinted polymer and hydrogel membrane are optimized. Currently, we are using a polymer formulation developed from N-isopropylacrylamide or N-N-propylacrylamide (transduction monomer), metharcylic acid (recognition monomer), and moderate concentrations of methylenebisacrylamide (crosslinker), and template to prepare molecularly imprinted polymers that swell in the presence of the targeted analyte. However, the concentration of the recognition monomer is probably too high. Furthermore, the thickness of our membrane is approximately 75mm, and the size of the microspheres is approximately 800nm. The membrane needs to be thinner to minimize diffusion distances, ensuring facile mass transfer. Smaller microspheres (approximately 200nm) will mean that a larger number of polymer particles can be immobilized on the Au surface and the entire particle will lie within the region of the evanescent wave.
The proposed technology has several important advantages for chemical sensing. The hydrogel membrane can serve as a "filter" to block out larger molecules, e.g. humic acid that might otherwise foul the microspheres. Another advantage of this approach to sensing is that it can be implemented at any wavelength. The microspheres are stable. They are not subject to photo-degradation and can undergo multiple swelling and shrinking cycles without degrading. Furthermore, swelling and shrinking of the microspheres has a minimal effect on the size of the hydrogel, and does not generate enough force to affect adhesion of the hydrogel to a substrate. By comparison, previously reported approaches to chemical sensing that involve polymer swelling share the common feature that swelling introduces stress causing the polymer to crack and tear as well as delaminate.