## 3 Results and Discussion
### Catalytic Activity for Organic Dye Removal  
The catalytic performance of PDA-Au was evaluated using the reduction of Rhodamine B (RhB) with NaBH\({}_{4}\) as the reducing agent. UV-Vis absorption spectroscopy was used to examine the changes in the main absorption peak of RhB at 554 nm. For comparison, we also used PDA NSs as catalysts with the same reaction conditions. Figure 6a shows the temporal evolution of the RhB absorption spectrum using PDA-Au as the catalyst. After 3 min, approximately 98% of RhB was removed, indicating the high efficiency of PDA-Au. On the other hand, the degradation of RhB by PDA nanospheres took more than 35 min to complete. Interestingly, in the case of PDA-Au, a new absorption peak appeared at 420 nm, which was not observed with PDA NSs. This new peak can be attributed to intermediates formed during the decomposition process, which are subsequently degraded [24]. The degradation rates are demonstrated in Figure 6c. The kinetic rate constant for the PDA-Au catalyst in the reduction of RhB is 1.405 min\({}^{-1}\), which is significantly higher than that of PDA NSs (0.051 min\({}^{-1}\)) (Figure 6d). Compared to several previously reported catalysts for RhB degradation, the plasma-synthesized PDA-Au catalyst demonstrates relatively high catalytic activity (Table 1). Additionally, we also investigated the catalytic degradation of MB by PDA-Au in the presence of NaBH\({}_{4}\) (Figure S4). In this case, the complete decoloration of MB was achieved within 7 min using the PDA-Au catalyst. In contrast, the decomposition process using PDA NSs required 60 min to be completed. The rate constant for MB decomposition with PDA-Au is approximately 0.560 min\({}^{-1}\), while PDA NSs exhibit a significantly lower rate of 0.066 min\({}^{-1}\) (Figure S5). These results further highlight the effective catalytic activity of the plasma-synthesized PDA-Au catalyst in the degradation of organic dyes.  
Another important factor for evaluating a catalyst is its reusability over time. The recyclability of the PDA-Au catalyst was assessed through five successive RhB degradation cycles. After each catalytic cycle, PDA-Au was recollected by centrifugation, washed, and resuspended under identical conditions. As shown in Figure 7, PDA-Au retained over 82% of its catalytic performance after five cycles, highlighting the reusability of this plasma-synthesized catalyst. The loss of catalytic performance is attributed to the loss of catalysts after the recollection and redesign processes.  
\begin{table}
\begin{tabular}{c c c c c c c} \hline \hline
**Catalyst** & **RhB Concentration** & **Treatment Volume** & **Degradation Time** & **Degradation Efficiency** &
\begin{tabular}{c} **Pseudo-First-** \\ **Order Rate** \\ **Constant** \\ \end{tabular} & **Ref.** \\ \hline AuNPs & 2.5 \(\times\) 10\({}^{-5}\) M & 3 mL & 7 min & - & 0.762 min\({}^{-1}\)\({}^{*}\) & [46] \\ Br-AuNPs & 0.1 mM & 3 mL & 12 min & - & 0.364 min\({}^{-1}\)\({}^{*}\) & [47] \\ BR-AgNPs & 0.1 mM & 3 mL & 10 min & - & 0.424 min\({}^{-1}\)\({}^{*}\) & [47] \\ TA-CuAu NPs & 0.1 mM & 4 mL & 10 min & - & 0.262 min\({}^{-1}\)\({}^{*}\) & [48] \\ PtRh ANMPs & 0.3 mM & 2.36 mL & 12 min & 97.22\% & 0.354 min\({}^{-1}\) & [49] \\ Pt black & 0.3 mM & 2.36 mL & 40 min & 47.29\% & 0.056 min\({}^{-1}\) & [49] \\ AgNPs@PAN/Go-SH & 10 mg L\({}^{-1}\) & 50 mL & 14 min & - & 0.266 min\({}^{-1}\) & [50] \\ Ag-AgCl NPs & 50 mg/100 mL & 5 mL & 5 min & 96\% & 0.748 min\({}^{-1}\) & [51] \\ AgNPs & 0.05 mM & 3.5 mL & 10 min & - & 0.600 min\({}^{-1}\) & [52] \\ PDA-Au & 10 ppm & 3.5 mL & 7 min & >98\% & 1.405 min\({}^{-1}\) & This work \\ \hline \hline \end{tabular}  
* Converted from reported rate constant value in s\({}^{-1}\).  
\end{table}
Table 1: Comparison of reported catalysts for degradation of RhB with NaBH\({}_{4}\).  
Figure 6: Catalytic activity of **(a)** PDA-Au and **(b)** PDA NSs for removal of 10 ppm RhB. **(c)** Degradation rate of RhB under catalysis of PDA-Au and PDA NSs. **(d)** Pseudo-first-order kinetic for removal of RhB of PDA-Au and PDA NSs.