Quantification of Carboxy Group. The carboxy group of the PP NWFs was quantified via the TBO assay with a modification of a previously reported method. TBO is a cationic dye that forms a complex with a carboxy group at a 1:1 ratio via electrostatic interaction. First, non-treated, photo-oxidized (Ox), and plasma-treated (Plasma) PP NWFs (10-15 mg; 2 × 2 cm 2 ) were incubated in 4 mL of the TBO solution (0.1% TBO, 1 mM NaOH) (15 min, 40 °C, shaken at 1300 rpm). The samples were washed with 4 mL of 1 mM NaOH (5 min, 40 °C, shaken at 1300 rpm). The washing process was repeated for a minimum of five times until the supernatant was clear. Finally, TBO was desorbed by incubation in 4 mL of 20% SDS solution (30 min, 40 °C, shaken at 1300 rpm). The concentration of TBO in the SDS supernatants was calculated from the absorption at 630 nm using a UV-visible spectrophotometer V-750 (Jasco, Japan). The results were calculated as the average of three repetitions.