Acetic acid

Employing the bottom-up sol-gel procedure, the synthesis of the TiO₂ nanoparticles, chemically modified with silver, was realized. Thus, 25 mL of titanium isopropoxide (97%, Sigma-Aldrich, Darmstadt, Germany) was added to acetic acid (48 mL) (99–100%, Chem-Lab NV, Zedelgem, Belgium) under stirring. Deionized water (150 mL), including 5 mol of 99.8% silver nitrate (Fisher Scientific Chemical, Loughborough, UK), was then added to the solution, drop-wise under vigorous stirring. After 5 h of stirring, the solution was a clear sol (titanium isopropoxide, acetic acid and water were used according to 1:10:100 molar ratio). The new solution was then heated until the solvent had been completely evaporated. Afterward, the prepared gel was consequently calcinated at 600 °C for 2 h. The resulting nanopowder was triturated, then purified via rinsing and centrifuged in order to remove any remaining impurities. In the end, it had a gray color (Figure 2).

Chitosan was obtained from Seikagaku Corporation (Tokyo, Japan). Inorganic salts (reagent grade) used for adjusting the relative humidity in chambers employed to determine the water sorption behavior of the Chitosan-based films were from Merck KGaA (Darmstadt, Germany). Folin–Ciocalteu was from Fluka (Steinheim, Germany). Gallic acid, glycerol (>99%), and choline cloride were from Sigma Chemical Co (St. Louis, MO, USA). Quercetin (quercetin dehydrate, 97%) was from Alfa Aesar GmbH & Co KG, whereas acetic acid (glacial 99–100% a.r.) was from Chem-Lab NV. All aqueous solutions were made using distilled water of HPLC quality.

The cotton and polyester jersey knits were provided by A. Ferreira & Filhos. One knit was composed of 100% polyester with a thickness of 0.68 mm and a mass per unit area of 207 g/m² and the other of 100% cotton with a thickness of 0.89 mm and a mass per unit area of 187 g/m². The pH indicators used for the development of the sensors were methyl red (C₁₅H₁₅N₃O₂), methyl red sodium salt (C₁₅H₁₅N₃O₂Na), methyl orange (C₁₄H₁₄N₃NaO₃S), bromocresol purple (C₂₁H₁₆Br₂O₅S), bromothymol blue (C₂₇H₂₈Br₂O₅S) and were purchased from Sigma-Aldrich (St. Louis, Mo., EUA). The chemicals used in the evaluation of samples sensitivity were the following: acetic acid acquired from Chem-lab (Zedelgem, Belgium); sodium hydroxide purchased from Normax (Marinha Grande, Portugal); formic acid obtained from Fisher Scientific (Hampton, NH., EUA); ammonia purchased from VWR Chemicals (Radnor, PA, EUA); and citric acid acquired from Scharlau (Barcelona, Spain). The polyurethane polymeric base employed in the preparation of pH-sensitive formulations before application on fibrous substrates, Edolan SN and Thickener A02 were purchased from ADI Center Portugal, S.A (Santo Tirso, Portugal). Glycerol was obtained from Scharlau. The filter paper, used as substrate in one part of this work, was purchased from Normax.