Dual-host combinations of cation and anion sensors have unique potential for selective detection of ion pairs, such as NH4NO3, via solvent extraction. Selective sensors for NH4+ and NO3- were synthesized and used together for ion-pair sensing of ammonium nitrate both in organic solvents (using Bu4N+NO3 - and NH4+PF6-) and in extraction of NH4NO3 from water into dichloromethane. A fluorescent sensor for NH4+ based on 1,3,5-triethylbenzene shows remarkable binding and sensing selectivity for NH4+ vs. K+. Fluorescence and 1H-NMR titrations reveal surprising differences in sensing properties and binding constants for the tris-(3,5-dimethyl)pyrazole vs. the tris(3,5-diphenyl)pyrazole. The role of ion pairing and solvation is revealed by X-ray and theoretical DFT studies. We have also demonstrated a unique dual-host extraction-based ion-pair sensing paradigm using Förster Resonance Energy Transfer (FRET), showing selectivity for NH4NO3. The fluorescence emission of the NH4+ sensor tris-(3,5-dimethyl)pyrazole (305-340 nm), is compatible with the excitation wavelength of the dansyl fluorophore of the nitrate sensor 1,3,5-Tris-(5-dimethylamino-1-naphthalenesulfonamido)methyl]-2,4,6-triethylbenzene, thus resulting in FRET emission upon combined use of these two sensors for the NH4NO3 ion pair. Contact of dichloromethane solutions containing the two hosts with aqueous solutions of NH4NO3 (1 x 10-5 M to 1 x 10-4 M ), resulted in FRET fluorescence enhancements at 510 nm, with increasing concentrations of NH4NO3, while NaNO3, KNO3, NaCl and KCl showed only minimal fluorescence responses, under identical conditions. The ability of the tris-pyrazole to bind cations, such as NH4+, was also exploited in a detailed fluorescence and 1H-NMR Ln(III), binding study using tris-pyrazoles with varying substitution patterns. The dependence of fluorescence responses on pyrazole substitution that had been observed for NH4+ was also observed for different Ln(III), indicating the significant role of ion pairing for Ln(III) binding and fluorescence sensing. Likewise, the tris-dansyl nitrate receptor, in its deprotonated form, was also found to be an efficient Hg(II) fluorescent sensor. An X-ray crystal structure showed the ability of the trianionic version of this receptor to bind three Hg(II) atoms, also containing three CH3COO- counteranions. The X-ray crystal structure of the same receptor with HgCl2 gave a 2:1 complexation pattern, with one Hg atom complexed by two bis-deprotonated receptor molecules