## 2 Results and Discussion
### In-Solution Characterization of PDA-Coated Adsorbent Materials: UV-Vis Spectra  
The formation of single (+) and double (++) PDA coating layers both on DE and PVP-co-S was first assessed in solution by UV-Vis spectroscopy. After the PDA functionalization, a broad absorption peak appeared over 285 nm both in DE/PDA and PVP-co-S/PDA spectra, likely ascribable to the presence of the polymer on both supports (Figure 2). This broadened peak is typical of polyaromatic groups that are produced during the spontaneous oxidative polymerization of dopamine monomers in solution, and a general increased trend (in absorbance profiles) could be observed passing from one single layer to a double layer. PDA partially covered the intrinsic UV-Vis bands of PVP-co-S, whose values were set around 295, 390, 505 and 689 nm. Solid State Characterization of PDA-Coated Adsorbent Materials: Raman Spectroscopy, AFM and Spectroscopic Ellipsometry  
For a solid state characterization of the materials, all six solutions were spotted from hydroalcoholic solutions and dried on the specific support and subjected to Raman, SEM, AFM, and ellipsometric analyses. Figure 3 shows Raman spectra of (a) bare DE and DE/PDA (single (+) PDA layer and double (++) PDA layer) and (b) pure PVP-co-S, PVP-co-S/PDA (single (+) PDA layer and double (++) PDA layer) recorded in the range 300-2000 cm\({}^{-1}\). The Raman spectrum of DE showed the Si-O modes, while the spectrum of PVP-co-S showed vibrational bands typical of C-O, C-C, C-N and C-H bonds. Raman spectra of both DE/PDA (+ and ++) and PVP-co-S/PDA (+ and ++) showed a strong fluorescence masking most of vibrational signals and some weak CH, CC, CO and CN signals. No significant difference was highlighted by the Raman technique between the supports with one or two PDA layers. All six solutions were spotted on a silicon surface and characterized by scanning electron microscopy (SEM) and atomic force microscopy (AFM). Bare DE and DE/PDA (single (+) and double (++) PDA layer) were firstly investigated, and the resulting SEM images are shown in Figure 4.The bare DE sample was characterized by the co-presence of silica debris derived from pinnate and centrate diatom shells, commonly present in fossil biosilica. In the case of DE/PDA+ and DE/PDA++, the biosilica nanostructure appeared more compact and smoother as an effect of the PDA coverage. The different superficial organization and coverage of PDA on DE were highlighted by atomic force microscopy through a combination of topographical and phase measurements. Figure 4b shows 30 \(\upmu\)m \(\times\) 30 \(\upmu\)m topographical and phase images of DE, DE/PDA+, and DE/PDA++. Topography of DE films showed a very rough surface, with a roughness that decreased from 421 nm to 282 nm with the increasing number of PDA layers, consistently with a decrease in DE porosity, as already revealed by SEM. Figure 2: In-solution absorption spectra of (**a**) bare DE and DE/PDA (single (+) PDA layer and double (++) PDA layer); (**b**) pure PVP-co-S, PVP-co-S/PDA (single (+) PDA layer and double (++) PDA layer). A zoomed 5 \(\upmu\)m \(\times\) 5 \(\upmu\)m phase image of the DE/PDA++ highlighted how the PDA was distributed on even smaller areas. Morphological characterization by AFM and SEM of DE samples showed that, even though the PDA did not seem to negatively affect the integrity of the structures both on large and smaller areas, increasing the number of PDA layers decreased the DE porosity, with a consequent increase in its clogging state, suggesting a lower capacity for entrapment of chemicals or pollutants. Moreover, PDA formed aggregates on DE. Both effects negatively affected the DE, whose required property as a material for pollutant removal is a marked nano- and microporosity conferred by a large surface area and free nanopores. In Figure 5, SEM images of pure PVP-co-S and PVP-co-S/PDA (single (+) and double (++) PDA layer) are shown: the pure PVP-co-S appeared as a uniform, fused, smooth material on the silicon surface.