*2.1. Materials*

Commercial PBSA was purchased from Mitsubishi Chemical Co. (Tokyo, Japan), under the trade name BioPBS™ FD92PM. Zinc nitrate hexahydrate (Zn(NO3) <sup>2</sup>·6H2O), titanium tetrachloride (TiCl4), urea (CH4N2O), sodium hydroxide (NaOH), and stearic acid (SA, C18H38O2) were acquired from Sigma-Aldrich (St. Louis, MO, USA). All chemicals were used without further purification.
