Fecl3

In a typical reaction procedure, 20 mL of 0.5 M calcium chloride (CaCl₂, Sigma, USA) was mixed with 20 mL of 0.2 M trisodium citrate (Na₃C₆H₅O₇, Fisher Scientific, India) and 0.1 M FeCl₃ (Sigma, USA). Volume of 0.1 M FeCl₃ added was varied as per the required percentage of doping. 5 mL of 0.3 M diammonium hydrogen phosphate ((NH₄)₂HPO₄, S.D Fine Chemicals, India) mixed with 0.2 mL of 3 N ammonium hydroxide (NH₄OH, Fisher Scientific, India) was added drop wise to the above mixture of CaCl₂, Na₃C₆H₅O₇ and FeCl₃ under constant stirring to obtain nCP:Fe. The precipitate was washed 4 times in hot distilled water by centrifugation at 8500 rpm for 15 minutes and redispersed in PBS for further studies.

The interactions of iron, phosphate and cyanide were examined in vitro by: (i) sequestering phosphate with iron, using a mixture of KH₂PO₄ and FeCl₃ (both Sigma, USA), (ii) sequestering Fe³⁺ ions with KCN (Alfa Aesar, USA), and subsequently adding PO4³⁻ (KH₂PO₄) to the mixture and (iii) first mixing FeCl₃ and KH₂PO₄ (to prepare iron-phosphate complexes) and subsequently adding KCN to the mixture to sequester iron and indirectly release phosphate from the complexes. Different ratios of FeCl₃:KH₂PO₄ (1:1, 5:1, 10:1, 50:1, 100:1) and KCN:FeCl₃ (1:1 and 10:1) at different concentrations of FeCl₃ (10, 100 μM and 1 mM) and KH₂PO₄ (10, 100 μM) were examined. The mixture of the first two components was incubated at 20°C for 1 h at 250 rpm on a rotary shaker. Afterwards the third component was added, and the mixture again incubated under the same conditions. The pH of the mixtures was determined using the inoLab pH 730 pH meter (WTW, Germany) and the Sentix® Mic pH electrode (WTW, Germany). For each combination of the final mixture liquid samples were collected to determine the concentration of free phosphate (colorimetric measurement described above).

For coloration with Prussian blue, the PDA-coated nanoPE was first immersed in 0.16 M FeCl₃ (97%, Alfa Aesar) solution (15 ml) for 20 min. The functional groups in PDA can reduce Fe³⁺ to Fe²⁺, and bind the reduced Fe²⁺ ions on the nanoPE surface. Next, 0.12 M K₃Fe(CN)₆ (99%, ACROS Organics) with the same volume was added dropwise into the above solution. The [Fe(CN)₆]³⁻ ions then react with the Fe²⁺ ions on the nanoPE surface to form Prussian blue (Fe^III₄[Fe^II(CN)₆]₃·xH₂O), resulting in blue color. For coloration with permanent maker dye, a permanent maker pen (Sharpie) in red color was used to directly paint on the textile surface.