Development of multianalyte optical sensor arrays for ionic analysis of small volumes of tear fluid

This work summarizes the development of an optical analytical platform for the ionic analysis of small volumes of aqueous fluids, particularly human tear fluid. The analytical platform is comprised of a glass capillary tube. The inner wall of the glass capillary is coated by spatially separated bands of optical sensors for the assessment of pH, the sodium and potassium concentrations (sensor array). The capillary serves as a 50 microliter volume sampling unit for tear fluid and as an optical cuvette, at the same time.;In this work first, the plasticized PVC membrane-based ion selective optical sensor (optode) membranes are characterized for the analysis of potassium and sodium ions in aqueous samples. It is documented that the performance characteristics of the K+ and Na+ selective optodes are adequate for the analysis of human tear fluid without dilution, i.e., they had linear response in the anticipated concentration ranges of the two alkali metal ions in raw tear samples.;Next, the development and characterization of a pH sensitive optode is presented that it adequate to measure the pH of tear fluid inside a capillary tube. The active component of the pH sensitive optode is bromophenol blue, which is embedded in a thin sol-gel coating on the inner wall of the capillary. Formulations to produce dye-entrapped gels and procedures for gel curing were optimized to obtain optical pH sensors with acceptable precision and accuracy in the physiological pH range of human tear fluid.;Finally, the optical sensor arrays were applied for the simultaneous determinations of pH and potassium ion concentrations or the pH and sodium ion concentrations in commercially available artificial tear drops and in pooled human tear fluid samples. The results provided by the optical sensors were compared to results gained in determining the K+ and Na + concentrations in the same samples by atomic absorption spectroscopy. In summary, we found that the K+ and Na+ optodes are adequate for the analysis of human tear fluids, however certain preservative, e.g., benzalkonium chloride, in particular artificial tear fluids, e.g., Visine, interfere with the optical measurements.