*2.1. Preparation of Poly(o-anisidine)*

Using a chemical oxidative approach, the *o*-anisidine was polymerized into poly(*o*anisidine) (Figure 1). Two solutions were prepared and named solution A and solution B using the conventional process. By combining 3 g of monomer with 10 mL of 1 M HCl, solution A was created. Then, with magnetic stirring, solution A was added to an ice bath. The next step was to create solution B, which was made by combining 4 g of ammonium persulfate with 10 mL of 1 M HCl and keeping it in an ice bath. Solution B was added dropwise in solution A, which was already constantly stirred. The reaction contents were continuously stirred for 2 h. After that, it was left undisturbed overnight. The bluishgreen suspension was prepared, which was subjected to centrifugation for isolation and purification. The material was isolated and washed with deionized water three times for better purification. Finally, the material was dried for 6 h at 80 ◦C in an electric oven and we proceeded with the characterization [17].

#### *2.2. Preparation of Barium-Sulfate Nanoparticles*

BaSO4 nanoparticles were prepared by using the method already reported by Ahmad et al. [44]. The 20 mL of the BaCl2 (0.5 M) solution, along with 20 mL absolute ethanol, was taken in a conical flask. Then, the (NH4)2SO4 solution (0.1 M) was prepared in distilled water. Then, the ammonium-sulfate solution was added dropwise to the conical flask containing the barium-chloride solution for 20 min with constant stirring at 25 ◦C. Afterwards, the gelatinous white barium-sulfate nanoparticles were formed, which were centrifuged to separate them from each other. Distilled water and ethanol were used to wash the item multiple times. Finally, the material was dried in an electric oven at 120 ◦C for 12 h. The dried material was collected to obtain the barium-sulfate nanoparticles.

**Figure 1.** Proposed scheme for polymerization of *o*-anisidine.
