## 3 Results and Discussion
### Insight into Formation of PDA-Au by Plasma-Assisted Method  
We then investigated the reaction mechanism for the preparation of PDA-Au via a plasma-assisted approach. The growth pathway of colloidal AuNPs in this method is generally attributed to the reduction of Au\({}^{3+}\) ions by plasma-induced reactive species, such as electrons and H\({}_{2}\)O\({}_{2}\)[35; 40; 42]. In order to elucidate the key components involved in the formation of AuNPs, Na\({}_{2}\)SO\({}_{4}\) and isopropanol were added to the precursor solution as scavengers for electrons (e\({}^{-}\)) and \(\bullet\)OH, respectively [43]. Under the presence of an e\({}^{-}\) scavenger, the number of AuNPs on PDA reduced significantly (Figure 5a). In contrast, with the presence of an \(\bullet\)OH scavenger, the amount of AuNPs only decreased slightly (Figure 5b). Since H\({}_{2}\)O\({}_{2}\) is produced through the recombination of \(\bullet\)OH radicals, the presence of isopropanol could hinder the formation of H\({}_{2}\)O\({}_{2}\) in plasma-treated aqueous solutions [44]. This observation indicates that plasma-induced electrons are the main contributors for the decoration of AuNPs on PDA NSs. Figure 4: Scanning electron microscope images of plasma-synthesized PDA-Au with different HAuCl\({}_{4}\) concentrations of (**a**,**b**) 0.2 mM, (**c**,**d**) 0.5 mM, and (**e**,**f**) 1 mM. Size distribution histogram of PDA-Au with HAuCl\({}_{4}\) of (**g**) 0.2 mM, (**h**) 0.5 mM, and (**i**) 1 mM. We then performed the plasma treatment of HAuCl\({}_{4}\) without the presence of PDA NSs. After 10 min of treatment, the yellowish precursor turned into a dark red wine color, indicating the formation of AuNPs. This result indicated that the plasma-liquid interaction is an effective approach for the synthesis of AuNPs. The UV-Vis spectrum of the plasma-synthesized AuNPs shows a surface plasmon resonance peak at around 546 nm, suggesting the presence of large-sized particles (Figure S3). SEM imaging further confirms that the AuNPs range in size from approximately 70 to 110 nm (Figure S3). We evidenced that the size of free AuNPs in the solution is significantly larger than the size of AuNPs formed on the surface of PDA NSs. A previous study by [PERSON] et al. demonstrated that anchored Au\({}^{3+}\) ions yield smaller particle sizes after plasma treatment compared to unanchored ones [41]. Accordingly, the difference in AuNP size observed in the presence and absence of PDA NSs can be explained by the interaction of PDA and the Au\({}^{3+}\) precursor. Upon mixing, Au\({}^{3+}\) ions can form complexes with the catechol moieties of PDA. These Au\({}^{3+}\)-catechol complexes on the PDA NS surface act as nucleation sites for AuNP formation. The Au\({}^{3+}\)-catechol complex can be reduced to Au\({}^{0}\) by PDA. However, due to the mild reducing potential of catechol (E\({}^{0}\) = \(-\)0.699 V), the in situ reduction may take longer time to finish, possibly leading to the formation of irregular and larger particles. By applying plasma treatment, the reduction of the Au\({}^{3+}\)-catechol complex can be significantly accelerated. This is because solvated electrons, generated during plasma exposure, are among the strongest reducing species, with a high reduction potential of E\({}^{0}\) = \(-\)2.9 V [45]. Additionally, H\({}_{2}\)O\({}_{2}\) generated during plasma treatment may contribute to the reduction of Au\({}^{3+}\), as confirmed by previous studies [42]. Our previous studies have shown that the Ar plasma jet can produce H\({}_{2}\)O\({}_{2}\) with the concentration of 9.4 mM within 10 min [43]. The reduction of the Au\({}^{3+}\) complex can be described by the following reactions (1-2):  
\[\mathrm{Au}^{3+}+3\mathrm{e}^{-}\rightarrow\mathrm{Au}^{0} \tag{1}\]  
\[\mathrm{Au}^{3+}+\mathrm{H}_{2}\mathrm{O}_{2}\rightarrow\mathrm{Au}^{0} \tag{2}\]  
Based on this, we proposed a plausible reaction mechanism for the rapid decoration of AuNPs on PDA NSs by plasma-liquid interactions (Scheme 2).