Anhydrous citric acid

The gelatin from fish skin (Pangasius hypopthalmus), with 250 bloom and 11% moisture content, was purchased from Vinh Hoan Collagen’s Factory (Dong Thap province, Vietnam). Agar, with 12% moisture content and 4.63 g SO₄²⁻/kg, was provided by Hai Long Robika Factory (Dong Thap province, Vietnam). Glycerol, with a purity of 99.01%, and anhydrous citric acid were obtained from Panreac (Barcelona, Spain). All chemicals were of food grade and they were used as received without further purification.

The following reagents were used: tiamulin, standard substance (Dr. Ehrenstorfer GmbH—Augsburg, Germany), disodium salt of ethylenediaminetetraacetic acid (Na₂EDTA dihydrate; Sigma—Livonia, MI, USA), anhydrous citric acid p.a. (Panreac—Barcelona, Spain), pure trichloroacetic acid, 99%, (Fluka -France), disodium phosphate anhydrous (Na₂HPO₄) p.a. (Panreac), acetonitrile, methanol, formic acid, 98–100% (Riedel-de Haen—Nottingham, UK), trichloroacetic acid, 20% (m/v).
The extraction solution is McIlvaine–EDTA buffer 0.1 M at pH 4, stable for a week. Standard stock solutions (1000 µg/mL) in methanol were prepared, stable for 6 months at −20 °C. On the day of use, 5 standard calibration solutions (0.05; 0.1; 0.25; 0.5; 1.0 μg/mL) were prepared in control sample extract (matrix) corresponding to 25–500 μg/kg.

Ultrapure water obtained using a Nanopure II system (Barnstead, NH, USA) was employed for the preparation and dilution of all the solutions. PDMS membranes were synthesized by using Sylgard^® 184 Silicone Elastomer Kit (base and curing agent) obtained by Dow Corning (Midland, MI, USA). Tetraethyl orthosilicate (TEOS ≥ 99.0%), silicon dioxide nanoparticles (SiO₂NPs, 99.5%, 5–15 nm particle size), N-(1-Naphthyl) ethylenediamine dihydrochloride (NEDD), 1-methyl-3-octylimidazolium hexafluorophosphate (OMIM PF₆), Zinc nanopowder (ZnNPs, 99%, 40–60 nm particle size), hexadecyltrimethylammonium bromide (CTAB) were provided by Sigma-Aldrich (St. Louis, MO, USA). Sulfanilamide (SA) was obtained from Guinama (Valencia, Spain). Sodium Dodecyl Sulphate (SDS), potassium nitrate and anhydrous citric acid were provided by Panreac (Barcelona, Spain). Zn powder and sodium nitrite were purchased from Probus (Badalona, Spain). Silver nitrate was obtained from Scharlab (Barcelona, Spain).