[7] [7] ALDABA A L, LEZ-VILA G, DEBLIQUY M, et al. Polyaniline-coated tilted fiber Bragg gratings for pH sensing[J]. Sensors and Actuators B: Chemical, 2018, 254: 1087-1093.

[8] [8] PABBISETTI V N K, MADHUVARASU S S. Hydrogel-coated fiber Bragg grating sensor for pH monitoring[J]. Optical Engineering, 2016, 55(6): 066112-1-066112-7.

[9] [9] CHENG X, BONEFACINO J, GUAN B O, et al. All-polymer fiber-optic pH sensor[J]. Optics Express, 2018, 26(11): 14610-14616.

[10] [10] JANTING J, PEDERSEN J, WOYESSA G, et al. Small and robust all-polymer fiber Bragg grating based pH sensor[J]. Journal of Lightwave Technology, 2019, 37(18): 4480-4486.

[11] [11] SHIVANANJU B N, PRASAD V, ARAVIND C G, et al. Nanomaterials coated multiplexed fiber Bragg grating for multiparameter sensing[C]//SPIE. Proceedings of International Society for Optics and Photonics. Washington: SPIE, 2015: 937036-1-937036-7.

[12] [12] LIN Y, CHEN S, WANG M, et al. Fiber-optic fast response pH sensor in fiber Bragg gating using intelligent hydrogel coatings[J]. Optical Engineering, 2015, 54(5): 057107-1-057107-4.

[13] [13] LEI M, ZHANG Y N, HAN B, et al. In-line Mach-Zehnder interferometer and FBG with smart hydrogel for simultaneous pH and temperature detection[J]. IEEE Sensors Journal, 2018, 18: 7499-7504.

[14] [14] HARTINGS M R, CASTRO N J, GILL K, et al. A photonic pH sensor based on photothermal spectroscopy[J]. Sensors and Actuators B: Chemical, 2019, 301: 127076-1-127076-4.

[15] [15] DE S K, ALURU N R, JOHNSON B, et al. Equilibrium swelling and kinetics of pH-responsive hydrogels: models, experiments, and simulations[J]. Journal of Microelectromechanical Systems, 2002, 11(5): 544-555.

[16] [16] JOHNSON B D, BEEBE D J, CRONE W C. Effects of swelling on the mechanical properties of a pH-sensitive hydrogel for use in microfluidic devices[J]. Materials Science & Engineering C Biomimetic & Supramolecular Systems, 2004, 24(4): 575-581.

[20] [20] LIANG X, CHEN S, HUANG H, et al. Study on sensitivity improving of fiber Bragg grating based pH sensor[J]. Photonic Sensors, 2014, 4(1): 28-33.