Evanescent-Wave Fiber Optic Sensing of the Anionic Dye Uranine Based on Ion Association Extraction

Abstract

Herein, we propose an evanescent-wave fiber optic sensing technique for the anionic dye uranine based on ion association extraction. The sensor was prepared by removing a section of the cladding from a multimode fiber and hydrophobization of the exposed core surface. Uranine was extracted in association along with hexadecyltrimethylammonium (CTA) ion onto the fiber surface and detected via absorption of the evanescent wave generated on the surface of the exposed fiber core. The effect of CTA⁺ concentration added for ion association was investigated, revealing that the absorbance of uranine increased with increasing CTA⁺ concentration. A change in the sensor response as a function of the added uranine concentration was clearly observed. The extraction data were analyzed using a distribution equilibrium model and a Freundlich isotherm. The uranine concentration in the evanescent field of the fiber optic was up to 54 times higher than that in the bulk solution, and the limit of detection (3σ) for uranine was found to be 1.3 nM.

Keywords: evanescent-wave fiber optic sensor; ion association; uranine.

Figure 1

(a) Absorbance spectra of various concentrations of hexadecyltrimethylammonium bromide (CTAB) and 1 µM uranine obtained 300 s after mixing by the fiber optic sensor prepared in this study. (b) Absorbance at 500 nm as a function of CTAB concentration.

Figure 2

Plot of absorbance/[uranine] vs. the equilibrium concentration of CTA⁺.

Figure 3

Plot of log absorbance versus log equilibrium concentration of CTA⁺.

Figure 4

Absorbance spectrum of 1 µM uranine/0.25 mM CTAB obtained using the fiber optic probe and that after washing with water.

Figure 5

(a) Absorbance spectra obtained by the fiber optic sensor 300 s after mixing uranine at various concentrations and 0.25 mM CTAB. (b) Absorbance at 500 nm as a function of uranine concentration. (c) Absorbance at 500 nm as a function of time.