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A new efficient and highly selective fluorescent chemosensor for determination of Pb2+ has been obtained by covalent attachment of two pendent proton-ionizable dansylcarboxamide groups to the calix[4]arene preorganized in the partial cone conformation. This geometry of the calixarene moiety was chosen on the basis of the prior ¹H NMR study of conformations adopted by the flexible dansyl-containing prototype upon complexation with lead ion. In acidic MeCN-H20 (1:1 v/v) solutions, the partial cone fluoroionophore allowed for detection of Pb2+ at the levels as low as 2.5 ppb, which is totally compatible with the regulations of the U.S. Environmental Protection Agency and the World Health Organization on the limiting content of this hazardous pollutant in drinking water. [ABSTRACT FROM AUTHOR]