Surface Modification of PP NWFs. First, PP NWFs were oxidized by photo-activated ClO2 • gas at 80 °C for 20 min, based on a previously reported method. After photo-oxidation, new peaks in the IR spectra appeared at approximately 3500 and 1720 cm -1 , which were ascribed to the stretching of the hydroxy (-OH) and carbonyl (C=O) groups, respectively (Figure ). In contrast, no significant change in the peak was observed for the Plasma. These results indicated that the oxidation proceeded successfully. The C=O functional groups, such as carboxy groups, were introduced on the PP NWF surface through photo-activated ClO2 • oxidation, similar to the previously reported PP plate. It is important to note that the oxidation reaction proceeded even in NWFs with complex threedimensional structures.