*3.7. Atomic Force Microscopy (AFM) analysis*

Surface analyses of the coated Polyetheretherketone films were evaluated by AFM. Figure 10 shows AFM images of the systems with the first coating layer. The images confirm the homogeneity of the nanoparticles deposited in the PEEK substrate, just as the SEM imaging previously revealed. Likewise, the height of the images is consistent with the TEM analysis, where a coating of the nanometric order with excellent particle distribution on the surface of the polymer was obtained. Figure 10b shows a collapse caused by the piezoelectric tip of the equipment, because the PEEK film and the silver coating are very thin.

**Figure 10.** Atomic force microscopy (AFM) images of the (**a**) PEEK/Ag0.04, (**b**) PEEK/Ag0.08 and (**c**) PEEK/Ag0.12 systems with one layer.

The AFM images of the PEEK/Ag systems obtained with two layers of silver are shown in Figure 11. The images represent the remarkable increase in the thickness of each sample obtained with two layers of silver. The PEEK films with two layers of silver deposited on their surface at concentrations of 0.04 and 0.08 mol/L shown in Figure 11a,b exhibit a small amount of aggregate particles. Figure 11c shows the excellent homogenization in the second stage of coating of the polymer, generating small agglomerations because of the high concentration of silver.

**Figure 11.** AFM images of the (**a**) PEEK/Ag0.04, (**b**) PEEK/Ag0.08 and (**c**) PEEK/Ag0.12 systems with two layers.

The average thickness of the three samples after the second coating process is in the range between 200 and 300 nm, and increased proportionally with the increase in Ag concentration. In general, the AFM analyses confirm the results by SEM regarding the homogeneous distribution of the silver nanoparticles deposited in the PEEK substrate.

The analysis of the roughness of PEEK materials coated with silver was evaluated with the root mean square (RMS) parameter shown in Table 2. The RMS value is related to the amount of silver deposited in the polymer. When increasing the amount of silver, the average roughness of the material increases forming several nanometric-sized valleys. The PEEK samples coated with a single layer of silver have small surface roughness values, due to the thin layer of silver deposited in the chemical synthesis method. In the same way, samples coated with two layers of silver have higher RMS values according to the quantity of silver nanoparticles deposited on the surface of the polymer [55]. The amount of nanoparticles deposited on both sides of the polymer has an inference in the antibacterial properties. This occurs because more Ag<sup>+</sup> species are generated which are responsible for inhibiting bacterial growth as explained in the following section and as reported by Liu et al. [55]. The surface roughness value also provides information on the adhesion of the silver nanoparticles on the polymer, corroborating the TEM analysis, since as a nanometric-sized coating, electrostatic interactions between the metallic silver and the polymer result [56]. In addition, the surface contact of the nanometer probe with the silver deposited in a single layer does not drag but leaves a groove upon movement, showing the effective adhesion with the polymer as indicated by Wenfei Li et al. [57]. On the other hand, in the samples with two layers the adhesion improves since the entire surface of the polymer is coated with AgNPs increasing the electrostatic interactions.


**Table 2.** Surface roughness values of the different PEEK samples coated with silver.
