## 3 Results
From two to eight nanoparticles are bonded together with the phage, as it is observable in the detailed contrasted image of Figure 3b (corresponding to the image of phase contrast generally used to highlight the different nature of the materials, thus putting in evidence the metallic nanoparticles with respect to the biological structures) relative to the morphology observation of Figure 3a, which enhances the Au NPs image reducing the contrast of the light elements present in the biological structures. In the background, it is barely possible to observe the phage filamentation, not well resolved and focused, showing a significant density of NPs attached to phages. In this figure, it is possible to observe a phage with about 8 Au NPs (bottom), a second phage with about 10 Au NPs or more (top) and a third phage, to the far left, with more than 5 Au NPs. Some nanoparticles appear larger due to the accumulation of nanoparticles. Generally, the spherical Au NPs are aligned along the phage filamentation, and they are always present in the head and tail of the phage. The AFM images show that the phage filamentation has a length of about 1 micron and a thickness less than 10 nm. Figure 3: AFM image of Au NPs distribution with respect to phage filamentations (**a**), same image in phase contrast to enhance the Au NPs (**b**), and of single phages showing high number of nanoparticles adherent to them (**c**,**d**). Figure 3c shows a detailed image of a phage, where the Au NPs near the head are about 20-30 nm distant from one another. The tail contains also small Au NPs but, being out of focus, it is not well resolved. In this case, the phage length is about 2 \(\upmu\)m, and 12 nanoparticles are adherent to the phage with a linear density of 12 Au NPs/1 \(\upmu\)m. Figure 3d shows another detailed phage of the solution distributed on the Al substrate. It is about 2 m in length and contains Au NPs from 5 nm up to 50 nm in diameter. The nanoparticle linear density on the phage observed in this case is 18 NPs/2 \(\upmu\)m.  
TEM imaging has allowed to obtain detailed information about the size of Au NPs and M13 phages. It is possible the identification of two main sizes of Au NPs, with dimensions of 10 nm and 50 nm. Moreover, it is observed that, generally, the NPs are attached at the two extremities of the phages, in clusters ranging from 1 to 3 nanoparticles of 10-20 nm each. TEM indicates that the average size of the nanoparticle is equal to 20 nm, while the average phages filamentation length is 1 micron. TEM images are well defined and contrasted by the presence of the heavy metal nanoparticles bonded to the phages surface. Figure 4 shows four TEM images of single phages and their bonded nanoparticles. The marker length is 10\({}^{2}\) nm. Figure 4a shows a phage with one single Au NP centered with respect to the filamentation and a couple of Au NPs, one bonded to the phage and the other bonded to the first nanoparticle. The phage length and thickness are about 500 nm and 10 nm, respectively, while the Au NP diameter is about 40 nm. The NPs are bonded almost in the center of the filamentation. Figure 4b shows a phage with a NP bonded to the tail and another bonded near the head. In the phage head position is bonded a very little nanoparticle with a dimension comparable with the phage thickness of 10 nm. The small Au NP is less brilliant than the other, due to its lower size. Figure 4c shows a phage at which a single Au-NP with a diameter of about 40 nm is bonded. In turn, this nanoparticle is linked to others, forming a row of more than 4 nanoparticles. Figure 4d shows a phage with a length of about 1 micron at which one nanoparticle is bonding to the head. Observing the filamentation seems that very little NPs, lower than 10 nm, evaluated at about 2 nm, are centered with the filamentation and arranged in alignment with it and equally spaced. A big nanoparticle of about 40 nm is not bonded to the phage, but it may be that the preparation of the sample took place before such adherence occurred.