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• Optical fiber sensor for pH measurement as low-cost solution. • pH sensitive material as polyaniline using synthetic oxidative polymerization on optical fiber surface. • Transition between protonated and deprotonated polyaniline increasing RI of the layer as the pH increases. • FBGs inscribed by UV laser in tapering fiber and D-shaped fiber as optical solution. • Sensors with good reproducibility and repeatability with excelent sensitivities for tapers and D-shape fibers. Optical fiber sensors show potential for monitoring multiple parameters essential to the growth and health of fish, particularly the pH value of the aquaculture medium. Two types of optical fiber sensors, using tapered silica single mode fibers with a Bragg grating inscribed and D-shaped plastic optical fibers (home-made and reused) were produced. A pH-sensitive layer made of polyaniline (PANI) was deposited on the optical fibers' surface. The sensors demonstrated reproducibility and repeatability with a sensitivity of −0.22 ± 0.02 dB/pH, −472 ± 9 Counts/pH, and −(26 ± 2) × 102 Counts/pH for tapers, D-shape home-made, and reused, respectively, but only for increasing pH. Stabilization time tests indicated that a minimum of 60 s is required for a stable response, with no significant cross-sensitivities regarding temperature and ionic strength. [ABSTRACT FROM AUTHOR]

Direct laser scribing (DLS) technology is applied to commercial polyimide (PI) coated optical fibers producing laser-induced graphene (LIG) coated optical fibers. The PI-coated optical fiber is first inscribed with off-center Bragg gratings via point-by-point method using a femtosecond laser system. Then, the off-center Bragg grating is irradiated with a CO 2 laser and the PI coating is photothermally converted into LIG without affecting the grating. Scanning electron microscopy (SEM) and Raman spectroscopy prove the presence of the laser-induced graphene layer on the optical fiber, and Fourier transform infrared (FTIR) spectroscopy confirms that all the PI coating was converted into LIG. The resulting LIG-coated off-center Bragg grating (FBG) sensors respond to variations of the surrounding refractive index (RI) with a sensitivity of 38.08 ± 1.57 nm per RI units (RIU). In addition, after removal of the PI coating, obtaining a bare-FBG, the sensitivity decreases to 25.63 ± 0.66 nm R I U − 1 . Therefore, this strategy, here reported for the first time, paves the way to produce promising novel optical transducers aiming to develop bio- and chem-sensing devices. • Direct laser scribing technology applied to commercial polyimide optical fibers. • Laser-induced graphene produced directly on optical fiber for sensing. • The resulting LIG-coated off-center FBG sensors respond to variations of the surrounding RI. • High sensitivity compared with after removal of the PI coating. [ABSTRACT FROM AUTHOR]