We are developing optically sensitive membranes that consist of lightly crosslinked, derivatized polystyrene microspheres embedded in a hydrogel. The microspheres are designed to swell and shrink as a function of analyte concentration. The optical properties of the membrane are affected by both the change in size of the microsphere and the change in refractive index that accompanies swelling. This can be measured either as a change in membrane turbidity or reflectance. This effect avoids the use of photosensitive indicators and makes it possible to remotely measure optical changes in the near infrared using technology developed for fiber optic communications. We have already developed membranes that respond reversibly and sensitively to pH. In addition, we have demonstrated the feasibility of ion selective detection using poly(4-hydroxy,3- nitrostyrene) microspheres treated to contain an ionophore. Ion binding by the ionophore is accompanied by deprotonation of the hydroxy group. We also have acquired preliminary data demonstrating the feasibility of remotely measuring changes in pH through several hundred meters of fiber. A pulse of light is introduced into two different lengths of optical fiber using a 2X2 fiber optic coupler. The return pulse is monitored vs. time by a photodiode. The signals from the two fibers are distinguished based on differences in the transit time through the fiber. Research to further develop this remote measurement technology is proposed in five areas: l. Develop procedures for simultaneously and independently controlling microsphere porosity and hydrophilicity using two-step seeded polymerization: The amount of porogenic solvent added in the second polymerization step controls porosity. Hydrophilicity will be controlled by preparing copolymers of styrene and 2,4,5- trichlorophenylacrylate (TCPA) followed by conversion of the TCPA to a more hydrophilic functional group such as an amide. 2. Investigate the effect of reducing the density of derivatized functional groups on the polymer: Based on preliminary observations, we expect to be able to substantially improve our response times by reducing the density of derivatized functional groups on the polymer while retaining excellent sensitivity. 3. Develop ion selective sensors: We will expand upon our demonstration that ion selective detection is feasible by exploring the effects of formulation and ionophore structure on the rate and magnitude of response to cations. We also determine whether the selectivity is as expected based on the ionophore's affinity for different ions. 4. Sensors for two analytes: Turbidity spectra vary with microsphere size. We propose to demonstrate the feasibility of making sensors that simultaneously respond to two analytes by preparing membranes that contain two populations of microspheres that differ in size. For example, we propose to develop a membrane that simultaneously responds to pH and ionic strength by making a membrane that contains large, i.e. ca. 1.5 micrometers in diameter, pH sensitive microspheres and small, i.e. ca. 0.5 to 0.7 micrometers in diameter, ionic strength sensitive microspheres. 5. Remote Measurements through Fiber Optics: We will improve our current system for remote measurements of membrane optical properties by making the measurement at longer wavelengths where Rayleigh scattering is less and by developing new technology for preparing thin membranes on fibers in which the core has been etched to form a small cavity.
