Abstract
Continuous monitoring of hydrogen (H2) concentration is critical for safer use, which can be done using optical sensors. Palladium (Pd) is the most commonly used transducer material for this monitoring. This material absorbs H2 leading to an isotropic expansion. This process is reversible but is affected by the interaction with interferents, and the lifetime of Pd thin films is a recurring issue. Fiber Bragg Grating (FBG) sensors are used to follow the strain induced by H2 on Pd thin films. In this work, it is studied the stability of Pd-coated FBGs, protected with a thin Polytetrafluoroethylene (PTFE) layer, 10 years after their deposition to assess their viability to be used as H2 sensors for long periods of time. It was found that Pd coatings that were PTFE-protected after deposition had a longer lifetime than unprotected films, with the same sensitivities that they had immediately after their deposition, namely 23 and 10 pm/vol% for the sensors with 150 and 100 nm of Pd, respectively, and a saturation point around 2 kPa. Furthermore, the Pd expansion was analyzed in the presence of H2, nitrogen (N2), carbon dioxide (CO2), methane (CH4) and water vapor (H2O), finding that H2O is the main interferent. Finally, an exhaustive test for 90 h is also done to analyze the long-term stability of Pd films in dry and humid environments, with only the protected sensor maintaining the long-term response. As a result, this study emphasizes the importance of using protective polymeric layers in Pd films to achieve the five-year lifetime required for a real H2 monitoring application.

Abstract
An optical fiber pH sensor based on a long-period fiber grating (LPFG) is reported. Two oppositely charged polymers, polyethylenimine (PEI) and polyacrylic acid (PAA), were alternately deposited on the sensing structure through a layer-by-layer (LbL) electrostatic self-assembly technique. Since the polymers are pH sensitive, their refractive index (RI) varies when the pH of the solution changes due to swelling/deswelling phenomena. The fabricated multilayer coating retained a similar property, enabling its use in pH-sensing applications. The pH of the PAA dipping solution was tuned so that a coated LPFG achieved a pH sensitivity of (6.3 +/- 0.2) nm/pH in the 5.92-9.23 pH range. Only two bilayers of PEI/PAA were used as an overlay, which reduces the fabrication time and increases the reproducibility of the sensor, and its reversibility and repeatability were demonstrated by tracking the resonance band position throughout multiple cycles between different pH solutions. With simulation work and experimental results from a low-finesse Fabry-Perot (FP) cavity on a fiber tip, the coating properties were estimated. When saturated at low pH, it has a thickness of 200 nm and 1.53 +/- 0.01 RI, expanding up to 310 nm with a 1.35 +/- 0.01 RI at higher pH values, mostly due to the structural changes in the PAA.

Abstract
The excitation of two different electromagnetic surface waves-surface plasmon polaritons (SPPs) and Bloch surface waves (BSWs)-is demonstrated in a 1-D metal-dielectric photonic crystal with numerical and experimental studies. The discussed structure consists of an Ag-TiO2 thin-film stack forming a metal-insulator-metal-insulator device. The thickness of the TiO2 layer placed between the metals is tested for two different values (50 and 300 nm), which also allows the excitation of guided-mode resonances. It is observed that BSWs in this metal-dielectric structure behave similar to the case of all-dielectric photonic crystals, whereas the SPP modes display similar properties to those excited in metal-insulator-metal cavities. The sensitivity of these surface states to variations in the refractive index (RI) of the external dielectric is characterized. For the case of the plasmonic modes, a maximum sensitivity of (7.2 +/- 0.3) x 10(3) nm/RIU was measured, while for the BSW the maximum sensitivity was (1.20 +/- 0.05) x 10(2) nm/RIU. Due to the large field enhancement and penetration on external media, these surface states display exceptional properties for application in optical sensors, and the presented results provide interesting possibilities in the design of novel sensing structures with a flexible selection of surface states for interrogation.

Abstract
The sensitivity of one-dimensional Bloch surface wave (BSW) sensors to external refractive index variations using Kretschmann's configuration is calculated analytically by employing first-order perturbation theory for both TE and TM modes. This approach is then validated by com- parison with both transfer matrix method simulations and experimental results for a chosen photonic crystal structure. Experimental sensitivities of (8.4 +/- 0.2)x102 and (8.4 +/- 0.4)x102 nm/RIU were obtained for the TE and TM BSW modes, corresponding to errors of 0.02% and 4%, respectively, when comparing with the perturbation the- ory approach. These results provide interesting insights into photonic crystal design for Bloch surface wave sensing by casting light into the important parameters related with sen- sor performance.(c) 2023 Optica Publishing Group

Abstract
Hydrogen peroxide is usually added to products to delay the development of microorganisms mainly in milk, hence increasing its stability over time, however the side effects can become devastating to human health.A technique is presented consisting of detecting hydrogen peroxide as an adulterant in milk through a sensor where pretreatment of the sample is not necessary, using a single use membrane. The detection of hydrogen peroxide in fresh-raw, whole, semi-skimmed and skimmed milk was performed using a luminol chem-iluminescence reaction.For hydrogen peroxide water solutions, a linear response was attained from 1.0 x 10-4 to 9.0 x 10-3 %w/w and an LOD (limit of detection) of 3.0 x 10-5 %w/w was determined. An R-squared value of 0.97 and a relative standard deviation lower than 10%, were achieved.Hydrogen peroxide concentration as low as 1.0 x 10-3 %w/w was measured for fresh-raw, skim and whole milk and for semi-skimmed milk, as low as 2.0 x 10-3 %w/w.The methodology presented, as long as our knowledge, is original, rapid, ecological and inexpensive. In regard of the sensitivity obtained, the methodology has great possibility to be applied in the detection of hydrogen peroxide in several areas. It is envisaged monitoring of food quality, agriculture systems and environment pollution.