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 ionophore and an anionic dye. In order to fabricate a micrometer-sized fiber optode, a "micropipette fabrication method" was newly proposed to fix a liquid membrane-based optode on the small tip of an optical fiber probe. 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", which prevents leaching of die 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. In addition, the problem of fluorescence distortion due to photobleaching and solvent effect was resolved by a ratiometric calibration in which 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 response theory. Our method gives a general preparation for ultrasmall ion-selective fiber optodes because other ion-selective optodes can be obtained simply by replacing the tailed ionophore.
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