cause of the hydration profile (Figure 4), adds complexity to the questions related to the shape of the space charge region. The variation is ex pected to affect the response function of any technique that is sensitive to the shape of the space-charge region, such as the optical or the ac imped ance measurement, but it would not affect the equilibrium potentiometric measurement. Although there appears to be great motivation for developing ion optodes, they cannot circumvent the problems inherent in the measure m e n t of s i n g l e - i o n a c t i v i t i e s — particularly without a reference of some kind. Moreover, ion optodes are certain to be affected by changes in the activities of electrically neutral species, namely water, in the mem brane. As such, the sensors would be vulnerable to changes in sample os molality. However, under welldefined and constant conditions, ion optodes may provide a m e a n s for measuring ion concentration. These conditions may even be complex, as in the case of simulated whole blood. If the requirements of a specific sens ing situation warrant the use of ion optodes, a calibration done under identical conditions must be incorpo rated into the application protocol. The critique of optical sensing pre sented in this article applies only to the sensing of ions. Optical sensors for electrically neutral species are likely to be affected to a far lesser de gree by variations in the sensing ma trix. It is these optical sensors that offer genuine advantages over other types of chemical sensors.
(10) Seiler, K.; Morf, W. E.; Rusterholz, Lee, J.; Robinson, G. W. Chem. Phys. Lett. B.; Simon, W. Anal. Set. 1989, 5, 557-61. 1989, 161, 444-48. (11) Morf, W. E.; Seiler, K.; Rusterholz, (25) Schneider, H. In Solute-Solvent Inter B.; Simon, W. Anal. Chem. 1990, 62, actions; Coetzee, J. F.; Ritchie, C. D., 738-42. Eds.; Marcel Dekker: New York, 1969; (12) Wang, K.; Seiler, K.; Morf, W. E; Vol. 1, p. 313. Spichiger, U. E.; Simon, W.; Lindner, E.; (26) Li, X.; Harrison, D.J. Anal. Chem. Pungor, E. Anal. Sci. 1990, 6, 715-20. 1991, 63, 2168-74. (13) Morf, W. E.; Seiler, K.; Lehmann, B.; (27) Li, X.; Petrovic, S.; Harrison, D. J. Behringer, C; Tan, S.S.S.; Hartman, K.; Sens. Actuators 1990, 81, 275-80. Sorensen, P. R.; Simon, W. In Ion-Selec (28) Verpoorte, E.M.J.; Harrison, D.J. tive Electrodes; Pungor, E., Ed.; Pergamon / Electroanal. Chem. 1992, 325, 153-66. Press: New York, 1988; Vol. 5, pp. 11531. (14) Hauser, P. C; Perriset, P.M.J.; Tan, S.S.S.; Simon, W. Anal. Chem. 1990, 62, 1919—23 (15) Urbano, E.; Offenbacher, H.; Wolfbeis, O. S. Anal. Chem. 1984, 56, 427-29. (16) Wolfbeis, O. S.; Schaffar, B.P.H. Anal. Chim. Acta 1987, 198, 1-12. (17) Opitz, N.; Lubbers, D. W. Talanta 1988 35 123—27 (18) Schaffar, B.P.H.; Wolfbeis, O. S. Anal. Chim. Acta 1989, 217, 1-9. (19) Schaffar, B.P.H.; Wolfbeis, O.; LeitJin Janata received his Ph.D. in analyti ner, A. Analyst 1988, 113, 693-97. cal chemistry from Charles University in (20) Kawabata, Y.; Kamichika, T.; Prague in 1965. After postdoctoral studies Imasaka, T.; Ishibashi, N. Anal. Chem. 1990, 62, 1528-31, 2054-55. at the University of Michigan, he spent six (21) Dickert, F. L.; Lehmann, Ε. Η.; years at the ICI Corporate Laboratory in Schreiner, S. K.; Kimmel, H.; Mages, England, and in 1976 he joined the fac G. R. Anal. Chem. 1988, 60, 1377-80. (22) Morf, W. E. The Principles of Ion Selec ulty of the University of Utah. In 1992 he tive Electrodes and of Membrane Transport; became an associate director of the Molec Elsevier: Amsterdam, 1981. ular Science Research Center at the Pa (23) Boyd, R. H. In Solute-Solvent Interac cific Northwest Laboratory. His research tions; Coetzee, J. F.; Ritchie, C. D., Eds.; interests include chemical sensors, electro Marcel Dekker: New York, 1969; Vol. 1. (24) Fillingim, T. G.; Zhu, S-B.; Yao, S.; chemistry, and microfabrication.
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References
Stability for Low Noise
(1) Seitz, W. R. CRC Crit. Rev. Anal. Chem. 1988, 19, 135-73. (2) Biosensors with Fiberoptics; Wise, D. L.; Wingard, L. B., Jr., Eds.; Humana Press: Clifton, NJ, 1991. (3) Janata, J. Anal. Chem. 1987,59, 135156. (4) Zhujun, Z.; Seitz, W. R. Anal. Chim. Acta 1984, 100, 47-55. (5) Chau, L-K.; Porter, M. D. Anal. Chem. 1990, 62, 1964-71. (6) Morf, W. E.; Seiler, K; Lehmann, B.; Behringer, C; Hartman, K.; Simon, W. PureAppl. Chem. 1989, 61, 1613-18. (7) Perisset, P.M.J.; Hauser, P. C; Tan, S.S.S.; Seiler, K.; Morf, W. E.; Simon, W. Chimia 1989, 43, 10-11. (8) Morf, W. E.; Seiler, K.; Sorensen, P. R.; Simon, W. In 5th Symposium on IonSelective Electrodes; Pungor, E., Ed.; Pergamon Press: New York, 1989; pp. 14151. (9) Tan, S.S.S.; Hauser, P. C; Chaniotakis, Ν. Α.; Suter, G.; Simon, W. Chimia 1989, 43, 257-61.
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