## 2 Materials and Methods  
Sol-gel pH-sensitive coatings upon common soda line glass slides were prepared following a previous patent by some of the present authors [25]. The silica matrix was provided from tetrathoxysilane (Si(OC2H5)4, TEOS), absolute ethanol (EtOH), and hydrochloric acid (37 wt. % HCl). All reagents used were of analysis grade purity. The pH-sensitive phase added to the prehydrolized silica matrix was chlorophenol red (C19H12 Cl2O5S, 3'-3'-dichlorophenolsulfonaphthalene, CR), an organic dye that changes its optical absorption depending on pH: yellow below pH 4.8, violet above pH 6.7, pink around neutral pH. Details on sol synthesis are provided in [19]. Coatings upon glass slides were obtained by dipping at room temperature and at a drawing rate of 1.35 mm \(\times\) s\({}^{-1}\). Thermal partial densification of coatings was achieved at 60 \({}^{\circ}\)C for 3 days in a forced air stove. After that, the coated area of the glass slide was cut from the uncoated one, and the glass edges were then polished. Figure 1 shows the aspect of one of the sensors obtained. Sensors were calibrated one by one using pH-buffered solutions (Buffer Hydrion Salt, Sigma Aldrich, St. Louis, MO, USA) in which they were immersed for 10 min. After that immersion time, a visible spectrum was recorded within the 380-750 nm wavelength range. An Ocean Optics model HR 4000 CG spectrophotometer was used. Sensor calibration was mainly carried out at pH 5, 6, 7 and 8. These pH values were selected because the most important pH range to be evaluated is around neutrality. Values of the absorption band intensity at 575 nm (characteristic band of CR) for each pH nominal calibration were used to set up the calibration curve. Figure 2 shows the calibration curve for one of the sensors synthesized and the error bars corresponding to standard deviation. On the basis of individual calibration curves for all sensors synthesized, estimated sensor accuracy is \(\pm 0.1\) units of the pH scale. Once the sensors were calibrated, they were installed in different positions of exhibition spaces of the MNCN, according to criteria of specimens' importance and/or potential risk of urban pollution impact as well as of visitors' impact. Table 1 summarizes sensor positions during the evaluation carried out in February 2019. Figure 1: Aspect of a pH sensor obtained from the glass slide, coated, cut, polished and finally protected by a polyethylene bag and provided with a gripper. Sensors were sensitized for 24 h, and then their optical response was recorded in situ using the patented portable measure unit [25], which was connected to a laptop. The computer was provided with the specific software for handling the records [26]. Methodology for recordings consisted of taking the sensor from its sensitization position and, immediately, introducing it in the measure unit. Then, the pH values detected appear in the laptop screen. This routine was repeated for all sensors installed, and, after that, all sensors were regenerated one by one at the laboratory by using pH-buffered solutions and repeating the calibration procedure explained above. Following this procedure, the sensors could be reused many times in the same or in different positions, according to needs and criteria of the museum collection curators. In general, sensors were installed once again in the same position to check potential pH changes due to meteorological seasons and/or to visitors, but in other cases in which pH showed noticeable stability near neutrality, the corresponding sensors were installed in other positions to obtain additional data on environmental pH. Recordings were taken after the first 24 h of sensor exposition and then at both 48 and 72 h. These later recordings were taken to confirm the results of the first recordings since 24 h is enough for sensors to be fully sensitized.