Fig. 1. Schematic diagram of micro-nano fiber sensor

Fig. 2. Schematic diagram of optical fiber tapering system

Fig. 3. Schematic diagram of fiber surface functionalization. (a) Hydroxyl group is produced by KOH solution treatment; (b) treatment with silanizing reagents to produce carboxyl groups; (c) immobilization of the EDTA on surface of optical fiber sensor; (d) functionalized fiber optic sensor

Fig. 4. Schematic diagram of the experimental setup

Fig. 5. Refractive index sensitivity diagram of the sensor. (a) Spectra of the sensor S₁ near refractive index 1.33; (b) relationship between wavelength shift and refractive index of sensor S₁, S₂, and S₃

Fig. 6. Temperature characteristics. (a) Spectral response of the sensor in the temperature range of 30-50 ℃; (b) linear relationship between sensor wavelength change and temperature change

Fig. 7. Experimental results. (a) Spectral response of sensors in deionized water over time; (b) drift of sensors in deionized water for 15 min; (c) relationship curve between wavelength shift and Pb²⁺ concentration; (d) spectral response over time when the sensor was immersed in Pb²⁺(100 ng/mL) solution; (e) spectral drift of sensors immersed in different Pb²⁺ solution concentrations over time