## Doctoral Dissertations

Spring 1993

Dissertation

Chemistry

#### Degree Name

Doctor of Philosophy

W Rudolf Seitz

#### Abstract

Fiber optic chemical sensors based on the polymer bead swelling and shrinking have been evaluated and improved. The sensing element is a crosslinked polymer bead which swells or shrinks as a function of analyte. The change in bead size is coupled to the movement of a small mirror in front of two optical fibers. One fiber conducts light from a LED source to the mirror, the other fiber collects reflected light and carries it to a photomultiplier tube which serves as the detector. Changes in mirror position change the amount of light reflected into the collection fiber.

The variation in intensity with mirror position was calculated for seven different optical fiber arrangements using two theoretical models and compared to experimental measurements.

Dowex SBR-P beads (8% crosslinked, 16-40 mesh) have been used to sense changes in salt concentrations between 0.10 M to 1.50 M. NaCl, KCl, NaF, Na acetate, $\rm CaCl\sb2 Na\sb2SO\sb4, Na\sb2MoO\sb4$ and $\rm Na\sb3PO\sb4$ have been tested with the sensor. Intensity increases with concentration for all salts but the magnitude of the response varies for different salts. If the concentration of Cl$\sp-$ is same, the sensor has approximately same response regardless cation Na$\sp+$ K$\sp+$, or Ca$\sp{2+}$ even though the ionic strengths of solutions differ. The sensor is reversible, reaching a constant value after three minutes for all salts over a wide concentration range. The sensor can distinguish 0.01 M difference in concentration between 0.10 to 0.20 M NaCl.

The Dowex bead has also been used to determine the degree of water saturation in several organic solvents. Both water miscible solvents (acetone, ethanol) and water immiscible solvents (MEK, butanol, 3-heptanone, 3-pentanone and 4-methyl-2-pentanone) have been tested on the sensor. The lowest water concentration the sensor can detect is 0.48% (v/v) water in 3-heptanone. The sensor is reversible, and the response time is approximately 3 minutes for all the solvents. As many as 30 cycles have been recorded with a single bead in acetone and water before the bead falls apart.

Poly(vinylbenzyl chloride) beads have been derivatized with iminodiacetic acid, iminodiacetonitrile and malonate. The polymer bead with the malonate functional group has been used to demonstrate the feasibility of sensors for metal ions based on polymer swelling and shrinking. At pH 7.0, the bead shrinks when malonate forms a neutral complex with Ca$\sp{2+}$ and swells when the complex dissociates.

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