# Repetitive Bacterial Disinfection of Respiratory by Polydopamine Coating
## 3 Results and Discussion  
The inner condition of the mask (30-35 \({}^{\circ}\)C, RH 90%) [2] is an ideal environment for the growth of deadly pathogens; thus, long-term usage of masks results in unexpected risks from pathogens (e.g., bacteria). Figure 1 shows a schematic of the developed method of conferring antibacterial properties to conventional N95 masks via polydopamine (PDA) coating. Two sequential dip-coatings in dopamine hydrochloride solution (8 and 0.8 mg/mL, respectively) developed thick and stable PDA layers on the main filtering part (melt-blown polypropylene), allowing a large amount of H2O2 production while breathing [26]. The detailed principle of the antibacterial effects is as follows: when the catechol in the PDA-coated polypropylene filter interacts with oxygen from a breath, catechol can be instantly oxidized and transformed into quinone; reactive oxygen species (ROS), including H2O2, are simultaneously produced as by-products [22; 23]. The produced H2O2 can be released from the human body as non-toxic water. In contrast, the unmanaged H2O2 in bacteria can be further transformed into hydroxyl radicals (OH-) [24], resulting in rupture of the lipid membrane in bacteria [25]. Figure 1: Experimental scheme for producing polydopamine-coated antibacterial mask and disinfection mechanism. XPS was used to demonstrate the successful introduction of PDA into the PP mesh filter. The PDA-coated PP (PP-PDA1, the number indicates the coating cycle) prepared using the two sequential coatings, illustrated in Figure 1, was used for the analysis. Four pieces of evidence of successful PDA coating were observed as follows: (1) A strong N1s peak appeared in the XPS profile of PP-PDA1, whereas a nitrogen peak was observed in that of bare PP (Figure 2a,b). (2) The atomic ratio of the C1s peak decreased significantly (from 98.2% to 75.7%) after PDA coating, which should be the consequence of screening of bare PP. (3) The atomic ratio of C1s, N1s, and O1s in the PP-PDA1 sample (10:1:2) matched well with the theoretical atomic ratio of PDA (8:1:2). (4) From the high-resolution C1s peak, bare PP was composed of 98.7% C-C and 1.3% C-O, whereas the composition of the oxygenated/nitrogenated functional groups notably increased to 30.1% (-C-O/C-N), 16.0% (-C=O), and 15.9% (-COOH) (Figure 2c,d and Table S1). As a result of the successful PDA coating, a clear color change was observed at the surface. The black color is a distinctive signal of melanin-like PDA owing to its inherent molecular structure that absorbs various ranges of visible light [27]. SEM was used to visually identify the polydopamine-coated layer of PP-PDA1. The polydopamine layer itself has a thickness of tens of nanometers and is difficult to distinguish using SEM. However, the presence of polydopamine can be indirectly verified through the adhesion of the polydopamine nanoparticles spontaneously generated during polydopamine synthesis [28]. Nanoparticles were observed in the polydopamine-coated PP filter (Figure 2f). These results confirmed that PP was successfully coated with PDA. Figure 2: (**a,b**) XPS survey profiles, (**c,d**) high-resolution C1s spectra, and (**e,f**) SEM images of bare polypropylene and polydopamine-coated polypropylene, respectively. Mimicking the practical conditions in masks is essential for achieving reliable antibacterial activity of the PDA-coated PP filter. According to a previous report [2], we performed all relevant experiments, including bacterial inoculation and H2O2 production experiments, in a specialized chamber, maintaining a humidity of 90% at a constant temperature of 35 \({}^{\circ}\)C (Figure 3a). For the bacterial growth experiment, PP-PDA1 was dipped into _E. coli_ solution with a concentration of 3-8 \(\times\) 10\({}^{9}\) colony-forming units (CFU)/mL. The concentration of _E. coli_ was calculated after 9 h of incubation (see Supplementary Materials for the detailed procedures).