Fig. 1: Concept of a dielectric fiber-optic sensor for sensing localized ion mobility.

a Schematic diagram of the optical fiber sensing area. (Inset: photograph of the dielectric fiber-optic sensor). b Z-direction Poynting vectors of the bent silica microfiber at 1550 nm with a diameter of 8.8 μm calculated via numerical mode simulation software (HE₁₁ and HE₁₂ modes). For bending factor, the curvature radius (r) of the microfiber was 1.4 mm. c Transverse electric field amplitude distributions of the HE₁₂ mode of the silica microfiber calculated via numerical mode simulation software. d Schematic diagram of the transmission spectrum utilized in localized proton concentration sensing. e Schematic diagram of the sensing setup. (BBS: broadband source; OSA: optical spectrum analyzer; EW: electrochemical workstation; CE: counter electrode; RE: reference electrode; WE: working electrode. The pulse signal in Fig. 1e is a schematic. During electrolysis, protons and hydroxyl ions were produced at the two electrodes and diffused towards the middle of the electrolyte). f pH sensitivity of the optical microfiber sensor without an interface. Ion interference: extra ions introduced during the pH regulation process. These ions caused changes in the refractive index of the solution, thereby affecting the measurement results of the sensor’s pH responses. Apparent pH sensitivity: the pH sensitivity of the sensor when sodium hydroxide or hydrochloric acid was added to the solution, which included refractive index interference caused by the extra ions. Actual pH sensitivity: the pH sensitivity subtracted the ‘ion interference’ from the ‘apparent pH sensitivity with ion interference’