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].