## 3 Results and Discussion
### Morphological and Structural Characterizations  
Figure 2a,b display SEM images of PDA nanospheres prepared through the chemical polymerization procedure using dopamine hydrochloride as a monomer. SEM images were analyzed using ImageJ 1.54g software to estimate the size distribution of the PDA nanospheres, which was found to be 205.0 \(\pm\) 20.9 nm. The PDA nanospheres exhibited a spherical shape with a smooth surface (Figure S1). In contrast, the SEM images of PDA-Au (Figure 2c,d) reveal small AuNPs anchored on the surface of the PDA nanospheres, confirming the success of the plasma-assisted decoration process. The average size of AuNPs in the plasma-assisted PDA-Au (PDA-Au) was estimated to be 10.3 \(\pm\) 2.0 nm. For comparison, AuNPs decorated on PDA using an in situ chemical synthesis approach (cPDA-Au) were also prepared. For cPDA-Au, the same PDA-HAuCl\({}_{4}\) precursor solution was stirred for 24 h without plasma treatment. SEM images of cPDA-Au (Figure 2e,f) show that the average diameter of the AuNPs was approximately 35.3 \(\pm\) 11.5 nm, significantly larger than those on plasma-synthesized PDA-Au. The histograms of size distribution for both conditions are presented in Figure S2. These results provide evidence that the plasma-assisted method yields smaller and more homogeneously distributed AuNPs compared to the conventional in situ chemical reduction approach, suggesting that plasma treatment promotes a more uniform nucleation of AuNPs on the PDA NS surface. Figure 1: The plasma jet system for the preparation of PDA-Au. (**a**) An illustration of the plasma jet setup. (**b**) The current–voltage waveform of the plasma jet recorded by an oscilloscope. (**c**) The OES spectrum of the plasma effluent obtained from the spectrometer. Figure 3a demonstrates the XRD patterns of the prepared PDA NSs and PDA-Au for comparison. The XRD pattern of PDA exhibits no distinct diffraction peaks, indicating the amorphous nature of PDA. In contrast, the XRD pattern of PDA-Au displays characteristic diffraction peaks at 38.3\({}^{\circ}\), 44.4\({}^{\circ}\), 64.6\({}^{\circ}\), and 77.5\({}^{\circ}\), corresponding to the (111), (200), (220), and (311) planes of standard gold structure, as referenced by JCPDS card number 00-004-0784. We noted that the XRD of cPDA-Au prepared via chemical synthesis by [PERSON] et al. also exhibited similar characteristic peaks [16]. Thus, the obtained pattern further verifies the formation of crystalline metallic Au on the PDA structure by plasma treatment. Moreover, the presence of sharp and intense peaks indicates the good crystallinity of the Au nanoparticles formed on the PDA surface. The strong peak at 20 value of 38.3\({}^{\circ}\) suggests that the (111) plane is the dominant orientation of the gold nanocrystals, a characteristic commonly associated with plasma-induced AuNPs [35; 37]. The crystallite size of small AuNPs on PDA structures was calculated using [PERSON]'s equation:  
\[D=\frac{K\lambda}{\beta cos\theta}\]  
where D is the crystallite size, K is the shape factor (\(\sim\)0.9 for spherical-like nanoparticles), \(\lambda\) is the X-ray wavelength (0.154 nm), \(\theta\) is the Bragg angle and \(\beta\) is the full width at half maximum at 20 [38]. From the obtained XRD pattern, the average crystallite size was calculated to be about 7.4 nm, which is smaller than the mean nanoparticle size of 10.3 nm. FTIR analysis was carried out to identify the functional groups present in the prepared samples. Figure 3b shows the FTIR spectra of PDA (orange line) and Au-PDA (blue line). In the PDA spectrum, the broad absorption peak in the 3200-3500 cm\({}^{-1}\) range can be attributed to the O-H stretching vibration of the catechol group and the N-H stretching vibration of the amine group in the PDA molecular structure [39]. The peaks at 1630 cm\({}^{-1}\) and 1380 cm\({}^{-1}\) correspond to the vibrational modes of the unsaturated C=C stretching and saturated C-C stretching of the aromatic ring, respectively.