Fig. 1. Electric field distribution of HE₁₁ and HE₁₂ modes

Fig. 2. Structure and spectrum of the fabricated MNF. (a) Structure diagram and microscopic view of the MNF; (b) spectra of the MNF in the air

Fig. 3. The process showing modifying MNF with CaAlg hydrogen

Fig. 4. Schematic diagram of the experimental setup

Fig. 5. Response of MNF spectra to ambient refractive index. (a) Spectral response; (b) fitted results

Fig. 6. Detection results of the sensor for aqueous solutions with different pH values. (a) Spectra; (b) fitted results

Fig. 7. Spectral evolution and resonant wavelength shift of the sensors coated with different concentrations of CaAlg hydrogels. (a) (b) 2% NaAlg hydrogel+3% CaCl₂ solution; (c) (d) 3% NaAlg hydrogel+5% CaCl₂ solution

Fig. 8. Repeatability tests of the sweat sensor. (a) The first test; (b) the second test; (c) the third test; (d) data fitting

Fig. 9. Response time test for the sensor

Fig. 10. Consistency tests for three sensors. (a)‒(c) Experiment with increasing pH; (d)‒(f) experiment with decreasing pH

Fig. 11. Detection stability of the sensor within 100 min. (a) The change in resonant peak wavelength; (b) the fluctuation of resonant peak wavelength