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CALIXARENE-BASED MATERIALS FOR CHEMICAL SENSORS 133 pyrroles, calix[4]uracils etc.) by various authors. This discussion, however, is confined to calixarenes as defined by Gutsche et al. (1981). 11. CALIXARENE-BASED MATERIALS FOR RECOGNITION OF ALKALI AND ALKALINE EARTH METAL IONS Because they are mostly insoluble, parent calixarenes have not been very useful as sensor materials. However, their derivatives (especially their phenoxy esters) have found applications as molecular recep- tors for alkali and alkaline earth cations. This is especially true for analysis of blood as well as for the separation of radionuclides for commercial and military operations. For example, 11a and 11b in Figure 3.15 have been determined to be excellent sensors for sodium ions in blood (Teltingdiaz et al. 1991; Diamond 1994). More recently, Miller and co-workers described a microelectrode based on calixarene ester 11b for measuring Na + in plant cells without interference from K + in the 0.1­100 mM activity range (Carden et al. 2001). Chawla and Srinivas (1993) synthesized calix[4]arene­coumarin conjugates to discriminate Na + and K + through fluorescence measurements. It was determined that the fluorescence intensity of 12a­c decreases upon interaction with Na + , but the intensity increases markedly when Na + is replaced by K + . This observation proved useful for translation into molecular diagnostics for monitor- ing Na + /K + balance in biofluids. Recently, Ramanjaneyulu et al. (2009) examined compound 13 in Figure 3.15 as a sensor mate- rial for an ion-selective electrode (ISE) to measure cesium in simulated high-concentration radioactive wastes. The authors examined the effects of various parameters relevant to membrane preparation for the polyvinyl chloride (PVC)­based ISE and determined that the ISE based on this calixarene gives a linear response to cesium concentrations as low as 10 -5 M. The Nernstian slope of the response was measured to be 56 mV per decade for cesium with a response time of less than 20 s, and without any interference from rubidium, potassium, sodium, ammonium, strontium, barium, calcium, magnesium, lead, zinc, nickel, copper, and cerium over a large pH range of 3­11 (Ramanjaneyulu et al. 2009). Me Me Me Me CH CH OH O O HO O CH 2 O O O R 11a; R= Me 11b; R=Et 4 R O O R" OO R" O CH 2 6 O 14 O OC 2 H 5 R" R"= CH 2 CO 2 Et R' 12a R=R'=H 12b, R=NO 2 , R'=H, O 12c, R=H, R'=NO 2 O 13 N N Me NO 2 Figure 3.15. Compounds 11­14.