Potential Zn2+ release from the PTU/ZnO composites was investigated by a photometric method, which is based on the reaction of zincon (2-carboxy-2′-hydroxy-5′-sulfoformazylbenzene). This chemical complex binds Zn2+ and forms a blue complex at pH 8.5–9.5. Prior to the Zn2+ concentration measurements, PTU/ZnO composites were washed with 2-propanol to remove potential superficial contaminations. To simulate a short-time exposure of those materials to an aqueous environment, the composite materials were immersed in deionized water for one day; for long-time exposure for 27 days. To investigate the potential Zn2+ release from PTU/ZnO composites, the topside and backside of the cast specimens with 1 wt.% and 5 wt.%, s- and t-ZnO were tested individually.

The 0.1 wt.% zincon solution contained 13 mg zincon in 0.2 mL 1 M NaOH solution and 10 mL deionized water. 0.1 mL of the aqueous test solution (incubated with the PTU/ZnO composites) was mixed with 0.3 mL zincon solution. The pH was adjusted by borax buffer (Na2B4O7x10H2O (pH 9)). Deionized water was added to obtain a total volume of 5 mL. The intensity of the blue color was measured photometrically, by an Epoch 2 Microplate Spectrophometer (BioTek Instruments Inc., Winooski, USA) at a wavelength of 620 nm. A calibration curve was constructed to assess the concentration of Zn2+ in tested samples. The amount of Zn2+ detected is presented in Table S2 in the Supplementary Materials section.

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