Large dynamic range dual-mode pH sensors via dye-doped ionic liquid fiber optofluidic lasers

Abstract

We report a fiber optofluidic laser (FOFL) using an RhB-doped ionic liquid (BmimPF₆) as the gain medium and explore its application for large dynamic range highly sensitive pH sensing. Due to the high Q-factor of the FOFL and the unique merits of BmimPF₆, lasing emission presents a threshold of only 0.61 μJ mm⁻¹. Particularly, lasing emission behaviors are strongly dependent on the pH value of the gain medium, i.e., in the pH range 4.28–6.37, the lasing central wavelength blue-shifts monotonically with a sensitivity as high as 5.02 nm per pH unit, which we attribute to the conversion of the cationic form of RhB to the zwitterionic form caused by the deprotonation of the COOH group. Under alkaline conditions (pH 7.20–11.17), the lasing emission intensity exhibits a significant decrease and the corresponding lasing central wavelength is also blue-shifted due to the solvent effect. The sensitivity based on the wavelength shift is 3.03 nm per pH unit, which is 4-fold higher than that of fluorescence-based sensing, while the sensitivity based on the variation of the lasing emission intensity is almost three orders of magnitude higher than that of fluorescence-based sensing. Our work presents a novel dual sensing paradigm in response to different pH conditions, which can greatly improve the reliability and discrimination of pH sensing.