2.2 Preparation of material
The material was prepared with a modified method from literatures [31,32]. Typically, 200 mL of methanol solution contained 10.50 g of BTC was dropped into 250 ml of FeCl3 solution (0.3 mol·L-1) at a condition of stirring vigorously. The saffron sediment was washed several times with ultrapure water (18.25 MΩ·cm-1) and ethanol following stirring 12 h at room temperature. Then, the saffron product was dried at 60 ℃ in oven.