Micrometer-sized sodium ion-selective optodes based on a 'tailed' neutral ionophore

The preparation and response features of a micrometer-sized sodium ion- selective fiber optode based on a liquid membrane were described. The sensing membrane is a plasticized poly(vinyl chloride)-based copolymer with a neutral sodium ionophore and an anionic dye. To fabricate a micrometer-sized fiber optode, a 'micropipet fabrication method' was proposed to fix a liquid membrane-based optode on the small tip of an optical fiber probe, in which the optode size can be varied from ~1 μm to more than 10 μm. Our first optode incorporated a 16-crown-5 derivative as the neutral sodium ionophore and a dibromofluorescein derivative as the single-emission anionic dye. The sensor response was monitored by measuring the fluorescence intensity under a time-resolved photon-counting method. At the first stage of the investigation, it was found that the ionophore including a sodium ion in its cavity leached from the membrane phase. However, we have discovered that the problem can be resolved by using a 'tailed' ionophore, which is an ionophore possessing a lipophilic long alkyl chain. The 'tail' of the ionophore functions as an 'anchor' that prevents leaching of the ionophore from the membrane phase into the water phase. The anchor effect of the tailed ionophore was clearly demonstrated with 6-μm-sized sodium ion-selective optodes. Using a 3-μm-sized optode, the sensor response was examined and successfully explained by the response theory. The size limitation of the optode was also examined using the response features of a 1.5-μm-sized fiber optode. The second optode incorporated a tailed 16-crown-5 derivative as the neutral sodium ionophore and a coumarin derivative as the dual-emission anionic dye. The second optode is more practical than the first optode because the problem of fluorescence distortion due to photobleaching and solvent effect is resolved by ratiometric calibration where the sensor response is monitored by the spectral shift of the dual-emission fluorescence. The sensor response of an 8-μm-sized fiber optode having ratiometric calibration was examined and successfully explained by the response theory.