New approaches to fiber optic chemical sensors for ions

Two problems of current fiber optic chemical sensors (FOCS) were addressed in this research. First, an immobilized ligand was characterized as a possible reagent for detecting anions selectively. Second, a new approach to fiber optic chemical sensing based on polymer swelling was demonstrated. The immobilized reagent explored for selective detection of anions is marketed as tris(carboxymethyl) ethylenediaminediacetic acid (TED). Characterization of this reagent showed that it is actually a mixture of ligands with ethylenediaminediacetic acid (EDDA) being the major species. Conclusive evidence was found in the EPR spectra. Several model ligands including a sample of TED from a private source were compared to the commercial product. EPR spectrum of the commercial reagent most resembled the EDDA spectrum. Theoretical and experimental capacities and conditional complexation constant of immobilized ligand for Cu(II) were measured and found to be consistent with this interpretation. A new type of sensor based on polymer swelling coupled to optical displacement was demonstrated. Two commercially available bead polymers, Dowex 50W and SP Sephadex served as reagent phases for sensing ions. When the sensor is placed into an ionic solution the bead contracts. This brings a reflector in contact with the bead closer to the end of the fiber or fibers such that the intensity of the reflected light reaching the detector changes. Response is both rapid, on the order of seconds, and reversible. Both reagents detect a change in concentration of ions in solution, but the sensor based on SP Sephadex is more sensitive. Total change in the normalized reflected intensity at 400 nm for Dowex 50W is 48% and for SP Sephadex is 93%. Dynamic range was 0.100 to 1.0 M for Dowex 50W and 0.001 to 0.1 for SP Sephadex. A CM Sephadex bead polymer serves as a sensor for pH. Response is both reversible and rapid. Relative intensity changes are on the order of 10% for a pH change of 1.0 unit at 0.01 M ionic strength. An ionic strength of 0.1 reduces the response due to the change in pH. Parameters which affect the performance of the sensors include number of fibers employed, degree of crosslinking of the bead, bead diameter and salt type