3.1 Metal Nanowires
Metal nanowires are used for a multitude of applications from electronics and sensors to lasers due to their improved conductivity [49]. Industrial-scale production of metal nanowires is most commonly achieved via polyol synthesis. However, it is not environmentally-friendly or sustainable as it uses various toxic polyhydric reducing agents such as ethylene glycol and propylene glycol in an energy-intensive reaction process that occurs at high temperatures [8, 50–52]. In contrast, highly conductive and uniform nanowires may be synthesized without toxic reagents or high temperatures via templating on TMV or its VLP [53]. To enable creation of palladium, platinum or gold nanowires, additional lysine, cysteine, and/or tyrosine side chains are added to the surface of the viral template to provide sufficient reducing power for nanoparticle synthesis and potential nucleation sites for metal binding. The introduction of this reduction power can decrease or completely remove the need for additional reducing agents in nanowire synthesis, thereby lowering costs [29, 54].
For thinner nanowires, metal may be deposited in the narrow viral channel of TMV, rather than on the surface [55, 56]. Buffering agents are used to alter the ionic state of amino acid side chains, changing its electrostatic interactions to decrease metal deposition on the surface. The inner channel, whose side chains are distinct from the surface, remain charged and electrostatically interact with metal solutions to adsorb and reduce synthesized metal nanoparticles. These techniques may be directly applied to platinum or palladium nanowires; however, nanowires composed of other metals such as copper, cobalt, and/or nickel require a core of Pd and Pt, which then serves as catalytic sites for reduction of other metals forming thin alloy nanowires such as CoFe, CoNi, FeNi, and CoFeNi [44, 55]. Bimetallic ferromagnetic alloys with Pt such as CoPt and FePt3 may also be synthesized directly by mixing precursor metal salt solutions with platinum or palladium salts in the same deposition bath thus simplifying the two-step process [43].