Calcium chloride

Anthraquinone dyes used in this paper, C.I. Acid Blue 225 (AB225) and C.I. Acid Violet 109 (AV109), were obtained from Lanaset (Lanaset Violet B, Lanaset Blue 2R, Huntsman International LL, The Woodlands, TX, USA). HRP peroxidase (EC 1.11.1.7; donor: H₂O₂ oxidoreductase) with a specific activity of 250 purpurogallin units per mg, pyrogallol, 2-mercaptoethanol, and hydrochloric acid were obtained from Sigma-Aldrich (St. Louis, MO, USA). Tris(hydroxymethyl)-methylamine and sodium alginate of low viscosity were obtained from Thermo Fisher Scientific (Waltham, MA, USA). Iron (III) chloride hexahydrate was obtained from Analytika (Prague, Czech Republic). Hydrogen peroxide (H₂O₂) 3% (0.97 M) was purchased from Galafarm (Belgrade, Serbia). Calcium chloride and iron (II) sulfate heptahydrate were obtained from Lach-Ner (Neratovice, Czech Republic). Sodium-dihydrogen phosphate, disodium-hydrogen phosphate and acetic acid were obtained from Zorka Pharma (Šabac, Serbia). 1-ethyl-3-(3-dimethylaminopropyl) carbodiimide (EDAC) was obtained from Thermo Fisher Scientific (Waltham, MA, USA). Phenolphthalein indicator was obtained from Molar Chemicals KFT (Budapest, Hungary). Ammonium hydroxide was obtained from Centrohem (Stara Pazova, Serbia). Other chemicals used in this work were of commercial analytical grade.

Hydrogels with distinct gelation mechanisms (physical, ionic, chemical crosslinking) [20 (link)] were studied. As an example of a physically crosslinked matrix, the linear thermoreversible polysaccharide agarose (Agarose E, Condalab, Madrid, Spain) at 1 wt.%, was used [21 (link)]. As an example of an ionically crosslinked matrix, sodium alginate (Sigma-Aldrich, Prague, Czech Republic) at 2 wt.% crosslinked by calcium chloride (Lach-Ner, Neratovice, Czech Republic) at a two to one weight ratio was chosen [22 (link)]. For chemically crosslinked hydrogels, poly(vinyl alcohol) (Sigma-Aldrich, Prague, Czech Republic) mixed with chitosan (low molecular weight, Sigma-Aldrich, Prague, Czech Republic) and crosslinked by epichlorohydrin (Sigma-Aldrich, Prague, Czech Republic) was employed [23 (link)]. L-α-Phosphatidylcholine (lecithin) was incorporated into all hydrogel samples before gelation at three different weight percentage concentrations (Sigma-Aldrich, Czech Republic, Prague).
The materials and their concentrations and ratios were selected on the basis of data previously reported [20 (link),21 (link),22 (link),23 (link),24 (link)] and can be seen in the table below (Table 10).