Abstract
Biogenic amines, such as putrescine are potential indicators of food storage condition and deterioration. The real time measurement of their concentration in food may become an important method of food control. It was found that putrescine diffuses through a thin layer made from a solution of Poly(ethylene-co-vinyl acetate) (PEVA) and maleic anhydride. Poly(ethylene-co-vinyl acetate) is a common non-chlorinated vinyl capable to adsorb specific analytes as putrescine which upon diffusion, reversibly binds to the maleic anhydride causing the polymer swelling resulting in spectral changes from the optical point of view. Long Period Fiber Gratings coated with 30 nm titanium dioxide, a high refractive index material used to increase the intrinsic sensitivity to the external refractive index, were overcoated with a thin layer of maleic anhydride doped Poly(ethylene-co-vinyl acetate). When exposed to solutions containing small concentrations of putrescine the resonant band corresponding to the LP1,6 cladding mode was found to move to shorter wavelengths. The observed blue shift corresponds to the increasing concentration of putrescine in the fiber sensor structure. Further work is being carried out to improve the sensitivity and the limit of detection of the sensing system as well as to increase range of operation, which is presently limited to 0.3 to 0.5 M.

Abstract
In this work, a long-period fiber grating (LPG) based sensor was evaluated as a sensing device for micro-force measurement, in the order of micro Newtons. It was used an LPG fabricated by arc-inducted technique in a SMF-28 standard optical fiber. The optical fiber was fixed between two clamps with a separation of 150 mm with the middle of the LPG located at the center. Characterizations were performed in terms of temperature, curvature and strain. The grating was then used as a micro-force sensor by means of both curvature and strain, induced by a hung mass in a stretched fiber. Furthermore, the evaluation of a precurvature LPG was performed to assess if an increase of sensitivity is achieved. Micro-force sensitivity achieved with the stretched LPG was 1.41 nm/mN and it was demonstrated that its sensitivity can be enhanced to 5.14 nm/mN with a pre-curvature of 2.2 m–1 applied to the LPG, achieving a spectral resolution of at least 15.6 µN.

Abstract
The study of sensing materials to the detection of carbon dioxide (CO2) was achieved using p-nitrophenol (pNPh) as a colorimetric indicator. The sensing material was polymerized (NPLn), functionalized with 3-triethoxysilyl propyl isocyanate (IPTES) which sensitivity was tested in the form of a membrane as is and encapsulated in hollow silica nanoparticles. The sensing membranes were tested in a closed gas system comprising very precise flow controllers to deliver different concentrations of CO2 (vs. N2). The combination of the sensing membranes with multimode optical fibers and a dual-wavelength diode (LED) allows the measurement of the CO2 through the analysis of the induced absorbance changes with a self-referenced ratiometric scheme. The analysis of the sensing materials have shown significant changes in their chemical and physical properties and the results attest these materials with a strong potential for assessing CO2 dynamics in environmental, medical, and industrial applications.

Abstract
The fabrication of Turn Around Point Long Period Fiber Grating in standard SMF-28e fibers through femtosecond laser direct writing is demonstrated and its sensing sensitivity is improved by coating the fiber with titanium dioxide through physical vapor deposition. © 2021 The Author(s).

Abstract
The precise production of 4.5 µm off-axis first order FBG in an SMF-28e fiber through femtosecond laser direct writing is demonstrated. The off-axis FBG potential as sensing head for torsion sensing was successfully explored. © 2021 The Author(s).

Abstract
Optical fiber Mach-Zehnder interferometers were fabricated by combining two TiO2 coated LPFGs fabricated through femtosecond laser direct writing. Results of its refractometric characterization are presented and compared with the single LPFGs sensors. Wavelength sensitivity of 1175 nm/RIU at 1.38 and spectral resolution of 2.2×10-5 were achieved. © 2021 The Author(s).