Micrometer-sized lithium ion-selective microoptodes based on a “tailed” neutral ionophore and a fluorescent anionic dye Original Research Article

The preparation and response features of a micrometer-sized lithium ion-selective optode based on a liquid membrane were examined. The optode membrane was a plasticized poly(vinyl chloride) (PVC)-based copolymer incorporating a lipophilic 14-crown-4 derivative as the neutral lithium ionophore and a dibromofluorescein derivative as the fluorescent anionic dye. The detection mode was a fluorescence change based on the ion-pair extraction/ion exchange principle caused by the fluorescent anionic dye and the lithium ionophore. The 5-μm-sized microoptode was prepared by the micropipette fabrication method and characterized by measuring the optical responses to Li+ concentrations with the time-resolved photon counting method. The microoptode responded to lithium ion concentrations of ∼0.5 to ∼500 mM. The micrometer-sized lithium ion-selective microoptode was successfully obtained when a tailed ionophore was used. The anchor effect of the tailed ionophore was useful for the lithium ion-selective microoptode to resolve leaching of the ionophore, which is a significant problem in a microoptode based on a liquid membrane. This demonstration indicates that other ion-selective microoptodes can be obtained simply by replacing the tailed ionophore.