21-(9-Anthrylmethyl)-4,17,13,16-tetraoxa-1,10,21-triazabicycle[8.8.5]tricosane-19,23-dione (I) was synthesized and characterized as a fluoroionophore for the selective, optical detection of lithium ions. The compound was prepared in a conventional four step synthesis. I is based on a bridged diazacrown structure, which provides a semi-rigid binding framework. Binding takes place by electrostatic interactions between the oxygen atoms of the crown and the cation and is transduced to fluorescence emission from an attached anthracene fluorophore. In a 3:1 dichloromethane/tetrahydrofuran solvent mixture, I acts as an intramolecular electron transfer 'off-on' fluorescence switch, exhibiting a greater than 190-fold enhancement in fluorescence emission intensity in the presence of lithium ions. The relative selectivity of I for lithium ions over sodium, potassium and ammonium ions expressed as log K (where more negative values indicate higher selectivity) were log K Li+,Na+ = - 3.36, log K Li+,K+ = - 1.77 and log K Li+,NH4+ = - 2.78. 21-(16-Mercaptohexadecan-1-oyl)-4,7,13,16-tetraoxa-1,10,21-triazabicyclo-[8.8.5]trico sine-19,23-dione (II) was synthesized in six steps using conventionalmethods. 11-Mercapto-N-(4-(9,15,18-triisopropyl-6,12-dimethyl-2,5,8,11,14,17-hexaoxo-1,7,13-trioxa-4,10,16-triazacyclooctadecan-3-yl)butyl) undecan-amide (III) was synthesized in fifteen steps with an orthogonal protecting group strategy. These molecules were prepared as a self-assembled monolayer (SAM) on gold. Characterization of a gold surface modified with II and III was carried out by sessile drop contact angle goniometry, ellipsometry, and grazing angle FT-IR spectroscopy. The cation recognition properties of the SAM were studied by impedance spectroscopy. The films of II show selectivity for lithium ions in solution over potassium and sodium ions, with selectivity values: log K Li+,Na+ = -1.30 and log K Li+,K+ = -0.92. The films of III showed selectivity for ammonium ions in aqueous solution over potassium and sodium ions, with selectivity values calculated to be log K NH4+,Na+ = -1.23 and log K NH4+,K+ = -1.17. To the best of our knowledge, these are the first demonstrations of lithium and ammonium sensors fabricated using self-assembled monolayer technology. Compounds IV and V are macrobicyclic compounds designed and synthesized as ammonium selective ionophores for use in ion selective electrode (ISE) applications. The structures of IV and V are based on bicyclic depsipeptide motifs which are expected to provide binding sites for ammonium ions due to the tetrahedral arrangement of carbonyl groups that participate in hydrogen bonding interactions. The synthesis of compound IV, involving 22 steps, was unsuccessful, potentially due to steric effects on the final (second) cyclization. The alternative molecule (V) was successfully synthesized in six steps using solution phase and solid phase peptide synthesis. Ionophore V incorporated into an ISE format provided selectivity for ammonium ions over sodium ions (log K NH4+, Na+ ~ - 0.08 to -0.47), however it did not show selectivity for ammonium ions over potassium ions (log K K+,NH4+ ~ -0.47 to - 0.61).