## 3 Results  
Samples with a high phage concentration (10\({}^{12}\) TU/100 \(\mu\)L) were prepared for the SEM analysis, depositing microdrops on a thin Cr metallization film to avoid charge accumulation during the microscope observation. It was possible to observe the assembled M13 phages with Au NPs, deposited on polished silicon substrates, at different magnifications. In the acquired images, it is possible to see, mainly, the solution salts and micrometric agglomerations with no perception of the phage filamentation. Au NPs are observed at the highest magnification of 70,000\(\times\). The image quality was improved, employing an aluminum polished substrate. Additionally, in this case, it was not possible to observe the single nanoparticles (of the order of 10 nm) but only near-spherical aggregated particles (of the order of 200 nm or larger), as shown in Figure 2a, and the global characteristic spectrum of the elements present in the observed zone (Figure 2b). The EDX analysis performed on the area confirms the presence of Au NPs by the gold X-ray characteristic M lines, as it is possible to observe from the shown spectrum. It is even possible to observe the characteristic peaks of C, O, Na, and Si, typical elements of both the phage solution and the used substrate. These nanoparticle aggregations may be originated by their addition to the chemical biological molecular environment in the M13 solution or by the sample preparation technique adopted for the sample observation at the electron microscope. The SEM preparation, in fact, needs a thin metallic cover film to make the samples conductive, while, instead, for the TEM observation, it is needed to drip the solution onto a transparent thin polymer film covering a thin metallic grid. AFM topographic images allowed to observe the filamentous phage structures. In particular, it was possible observe the Au nanoparticles as white spots, and the phages as whitish traces. In the acquired images, the M13 phages are detected as small peaks (X-ray profile), while the Au NPs are larger and intense peaks in the orthogonal Z dimension. The phages usually include several small nanoparticles along their length. The nanoparticles have a distribution ranging mainly between 10 nm and 100 nm, and the M13 filamentous phages, a length ranging mainly between 0.5 microns and 2 microns and a thickness lower than 10 nm. As it is possible to observe, generally, in each termination of a single phage, 2-8 Au NPs are attached, on average. The Au NPs adhesion to the phages occurs through the protein external layers. Studies of the adhesion of Au NPs with proteins are reported in the literature [22]. In facts, their exposition to biological fluids induces proteins and other biomolecules adsorption onto the surface to form a protein coating sphere around the Au  
Figure 2: SEM images of the M13 phages with Au NPs compounds (**a**) and relative EDX spectrum of the drops deposited on silicon substrate (**b**). NPs [23]. The literature reports about the protein interaction with the Au NPs, proposing different biological effects. For example, the protein may be altered by this adhesion to the Au NP and its physiological functions can be modified [24; 25]. A series of M13 phages with Au NPs observed by AFM is shown in Figure 3. The first figure (Figure 3a) shows the Au NPs as brilliant nanospheres distributed on an aluminum surface. The average size of the Au NPs is about 10-20 nm ranging between 2 nm and 50 nm. Sometimes the nanoparticles appear joined to each other, especially near the phages, whose proteins may induce nanoparticle coalescence. Far from the phages, the average distance between the nanoparticles is of about 500 nm but near the phages, the average distance decreases and some nanoparticles are in contact. Observing many TEM images, the nanoparticles show thickening when very near the phages. It seems that phages act as attractors of the gold nanoparticles.