## 1 Introduction  
M13 phage, 880 nm long and 6.6 nm wide, is a virus consisting of -2700 copies of the major coat protein (pVIII), surrounding a single-stranded DNA [1, 2]. Its properties, such as high sensitivity to ultrasonication and stability under different conditions, such as organic solvents [1], allow it to be employed for different applications in virotronics, as well as in electronics [3], for the development of optical biosensors [4, 5] and for medical devices [6]. Specifically, the use of phages in human applications have increased researchers' interest due to their absence of tropism for mammalian cells and innate ability to cross biological membranes, such as vascular endothelium and other mammalian cell barriers [7, 8]. It has been demonstrated that phages rapidly translocate throughout the body upon administration, with a distribution that depends on the route of administration and initial phage titer [7, 8]. The distribution in a specific tissue can be increased by the phage display technology that allows to display peptide able to specifically bind eukaryotic cells [9, 10]. Moreover, phages can be easily internalized by receptor-mediated endocytosis if they display an internalizing peptide [7].  
M13 phages can be used to promote the nucleation of specific inorganic materials with a high level of selectivity and control [11] or for templating inorganic nanoparticles. Basedon the covalent chemical bonds, following a self-assembly approach, gold nanoparticles (Au NPs) prepared by laser ablation, following the so-called top-down method, can be embedded into M13 phages [12]. The literature reports that the surface-enhanced Raman scattering (SERS) nanoprobe based on the M13 phages exhibits bactericidal capabilities and can be employed for the selective detection and inactivation of _Staphylococcus aureus_[13]. Moreover, M13 phages fitted together with metallic nanoparticles of Ag or Au, can be employed to modify the cytotoxicity in healthy and tumor cells generating conditions for cell apoptosis [12; 14]. Such studies highlight how the use of M13 phages with heavy metallic nanoparticles can open new and interesting avenues in tumor targeting for its treatment.  
In this work, it is investigated the affinity of M13 phages to Au NPs by means of different microscopy techniques, such as SEM (scanning electron microscopy), AFM (atomic force microscopy) and TEM (transmission electron microscopy) and by complementary analyses, such as EDX (energy dispersive X-ray spectroscopy), UV-VIS absorption and Raman spectroscopies. The goal of the study consists of the preparation of phage cultures to transport metallic nanoparticles toward specific tumoral tissues and to use them as a contrast medium, increasing the imaging contrast for the radiotherapy diagnostic and biological studies.  
Gold is a biologically compatible element because it is an inert noble metal and is thermally and chemically stable [15]. Due to the increasing of the effective atomic number of the organs and tissues in which the Au NPs will be located, a significant increment of the radiotherapy efficiency and of hyperthermia and imaging improvement are expected [16]. Nano-and microstructures bonded to Au NPs can be better highlighted, thanks to the fluorescence emission and optical spectroscopy of heavy nanoparticles [17]. Moreover, the rising of light absorption at specific wavelengths due to the presence of Au NPs, known as surface plasmon resonance (SPR) [18], represents an important effect in hyperthermia, destroying the cancer cells inducing controllable heating.