A great interest in gold nanorods is being motivated by their potential applications in various technologies including optical filtering, subwavelength imaging, data storage, and sensor devices [1, 2]. One efficient approach in preparing large quantities of gold nanorods is via seed-mediated synthesis in the presence of a surfactant . Nonetheless, polydispersity and byproducts in the form of nanospheres and nanoplates may be limiting for certain optical applications. Recent progress in purification  and synthesis of gold-nanorods  shows great promise. Moreover, interest in nanorods of various metals had arisen as well. For example, copper oxide nanorods had been produced by Liu et al. and Cheng , who were motivated by potential applications of copper oxide nanorods in solar and electrochemical cells. While their methods have potential for the production of copper oxide nanorods on surfaces, scalable solution-based methods that produce monodisperse and well-dispersed copper (Cu) nanorods is still a topic of significant importance in fundamental studies and commercial applications.
Our strategy for the synthesis of Cu-nanorods is biotemplating. Biotemplating is an attractive method to synthesize nanosize inorganic materials because it takes advantage of the well-defined size and shape of the biological structures and the surface functional groups that can interact with metal atoms leading to nucleation and growth of nanoparticles. It can potentially produce a wide variety of materials for applications in electronics, sensing, optics, and cancer therapy [8–10].
Biomolecules such as DNA, amino acids, peptides, protein cages, and viruses have been used as templates and scaffolds for the synthesis of inorganic nanomaterials including metals and semiconductors . DNA  and fiber-like protein structures like microtubules  have been used as biotemplates for the synthesis of Cu-nanowires. Still the utilization of DNA and microtubules for biotemplating face some challenges. For example, DNA requires specialized techniques for the production of straight nanowires and the aspect ratio of Cu-nanowires from microtubules is difficult to control due to the polydisperse nature of the microtubules.
Rod-like viruses provide the following advantages for the synthesis of nanorods: (1) well-define shape and dimensions in the nanoscale, (2) stability at broad pH ranges, (3) easy to purify in large scale, (4) mechanically robust, which allows the utilization of ultracentrifugation and sonication techniques during sample processing, and (3) virus particles are intrinsically monodisperse. The rod-like plant virus tobacco mosaic virus (TMV) and bacteriophages like fd and M13 are ideal templates for producing high aspect ratio materials such as nanorods. These viruses also share common favourable characteristics for biotemplates including stability over a wide pH range, and a net negative charge at neutral pH. TMV is a 300 nm long cylindrical rod with an outer diameter of 18 nm and a 4 nm central cavity. Approximately, 2130 identical coat protein (CP) subunits form a right-handed helix around the viral single stranded RNA [14, 15]. Filamentous bacteriophage fd and wild type M13 are structurally identical. They are 880 nm in length and 6.6 nm in diameter . Each phage consists of approximately 2700 CP (pVIII) subunits wrapped around a circular loop of single stranded DNA. fd and M13 differ by one amino acid per CP, which results in a net 30% more negative charge in fd[17, 18]. The M13 phage is a widely-used cloning system as a phage display for expression of small peptides  used to identify amino acid sequences that are specific towards metals, metal oxides , and semiconductor surfaces .
High aspect ratio viral protein structures have been explored to fabricate metallic nanorods. Even though a variety of metals have been deposited on TMV [22–35] and M13 [36, 37], continuous coating have been reported only for Pd , Pt , Co , and Ni [32, 33] on TMV and Ag  and Au/Ag alloy  on M13. Meanwhile, there are not reports to date on the metallization of the fd bacteriophage.
Among the various metals, Cu offers the advantages of high electric conductivity and low cost. If high quality biotemplated Cu-nanorods and Cu-nanowires can be fabricated in large quantities, they may be of utility as interconnects in future nanoscale electronics . Previous strategies for copper incorporation into TMV include photochemical reduction of Cu2+ TMV , direct chemical reduction  of CuCl2, and copper reduction inside the TMV channel . Major issues reported in the literature include sparse and uneven Cu coverage , product aggregation , poor yield and difficulties in controlling the length of the resulting Cu-nanorods.
In the current work, we report the synthesis of straight, continuous and dispersed Cu-nanorods and Cu-nanowires by electroless deposition of Cu on Pd-activated virus outer surfaces. Our solution-based method is performed in aqueous solution and at room temperature, making it amenable for large scale production. The Cu-TMV nanorods were characterized using transmission electron microscopy (TEM), scanning electron microscopy (SEM), X-ray photoelectron spectroscopy (XPS), and a nanoparticle size determination system. Furthermore, we produced Cu-fd and Cu-M13 nanorods and PANI-Cu-TMV nanowires to demonstrate the versatility of this metallization procedure for other biotemplates. The current work opens the possibility of generating a variety of nanorods and nanowires of different lengths ranging from 300 nm to micron sizes.