The detection of the refractive index has important practical significance and application value in national defense, aerospace, industry and agriculture, food safety, and other key fields. The optical-fiber lossy mode resonance (LMR) sensors have been widely used in the design and development of refractive index sensors due to their label-free measurement and high detection sensitivity. In addition, the optical-fiber LMR is different from surface plasma resonance (SPR), which is mainly manifested in the following aspects. The excitation of LMR does not depend on the polarization of light, and the film material is widely available and inexpensive. In addition, the resonance wavelength and sensing sensitivity of LMR can be adjusted by changing the thickness of the sensing film. However, most LMR sensors based on multi-mode optical fiber usually have low detection sensitivity. As a kind of semiconductor metal oxide, TiO₂ not only has the basic performance of a semiconductor but also shows the characteristics of a large specific surface area, loose porosity, strong adhesion, and stable chemical properties. Therefore, in this paper, based on the excitation of optical-fiber LMR refractive index sensing with ITO, TiO₂ nanoparticles are electrostatically assembled on the ITO film to improve refractive index sensitivity. The promising application of metal oxide nanoparticles in LMR refractive index sensing is further validated.

The investigation is based on the theory of optical-fiber LMR sensors. The attenuated total reflectance method of the Kretschmann configuration is used to calculate the optical-fiber LMR spectrum. According to the theoretical model of the optical-fiber LMR refractive index sensor, the relationship between ITO thickness and resonance wavelength of LMR sensor is analyzed by numerical calculations. Besides, the theoretical simulations illustrate its feasibility as a refractive index sensor. The ITO film and TiO₂ nanoparticles are prepared by magnetron sputtering and electrostatic self-assembly method, respectively. At first, the performance of the optical-fiber LMR refractive index sensor with a single ITO film structure is investigated, and the change in LMR resonance wavelength is observed by dipping the sensor into a glycerol solution with different concentrations. Next, the refractive index sensing performance of the optical-fiber ITO-LMR sensor based on the assisted enhancement of TiO₂ nanoparticles is elaborately investigated. By comparing the refractive index sensing performance of the two sensors, the conclusion is drawn.

The designed optical fiber LMR sensor with ITO film and TiO₂ nanoparticles exhibits deserved refractive index detection performance. Fig. 6 shows the LMR resonance spectrum of the structure with magnetron-sputtered ITO film on the side wall of the optical fiber. The resonance wavelength appears to be red-shifted as the refractive index of the external analyte to be measured increases. Specifically, as the external refractive index changes from 1.3333 to 1.3840, the sensitivity of the optical-fiber LMR sensor is 407.062 nm/RIU with a fitting coefficient of 0.995. The TiO₂/PSS bilayer film is electrostatically self-assembled based on the ITO-LMR sensing probe. The LMR refractive index sensing performance with ITO film and TiO₂ nanoparticles is shown in Fig. 7. With the increase in the solution refractive index, its resonance wavelength shifts significantly toward the long wavelength direction. The refractive index sensitivity of the sensor reaches up to 1651.659 nm/RIU. Due to the advantage of the large specific surface area of TiO₂ nanoparticles, the sensitivity is improved by a factor of 3.058 compared with the LMR sensor coated with only a single ITO film. The detection resolution of the TiO₂-ITO-LMR refractive index sensor is higher than 8.89×10^-4 RIU.

In this paper, an LMR refractive index sensor based on the assisted enhancement of TiO₂ nanoparticles is designed. The sensor activates the LMR effect based on the phase matching of the lossy mode wave and the evanescent wave of the multimode fiber to carry out refractive index sensing. The effect of TiO₂ nanoparticles on the optical-fiber ITO-LMR sensor is experimentally investigated. The ITO film and TiO₂ nanoparticles are plated by magnetron sputtering and electrostatic assembly. The results of the refractive index sensing experiments show that in the refractive index variation range of 1.3333-1.3840, the sensitivity of the TiO₂-ITO-LMR sensor can reach 1651.659 nm/RIU due to the advantage of the large specific surface area of TiO₂ nanoparticles. For the ITO-LMR refractive index sensor, the sensitivity is improved by 3.058 times. In addition, the TiO₂-ITO-LMR sensor has a resolution of more than 8.89×10^-4 RIU for refractive index detection.