The viruses, which have little to no enzymatic functions outside a host cell (at least in the wild state, AFAIK), likely isn’t producing the TiO2 nanoparticles as much as _enabling_ production. Organics binds well to TiO2 and can be crystalline seeds as well as adhesive layer.

@ Anatomy Student:

It is very economical compared to, say, vacuum processes if you can produce electronic (or other) films by water soluble materials. Substrates can be spun, i.e. poured on and spun out to required thickness set by rotation speed among other factors, having the excess leave the substrate.

Or otherwise used to produce a thin coat rapidly, dipping is certainly fast if not so reproducible. Later processes are used to fixate the film.

Here I would presume what I wrote above, the virus-tube layer is used as TiO2 seed (preferably in some subsequent liquid chemical growth process if possible). At some or several stages resulting layers are likely fixated somehow, though the protein/Tio2 bond certainly helps anyway. Heated/sintered maybe, at the end probably glass covered or perhaps coated with a thin layer of transparent MnO2 depending on the application.

One reason alone to supply an over-layer is that you can increase optical efficiency, say by successively adapt refraction index between the air/film interface to couple more light into the device. This may at a guess be an economical tipping point for solar cell applications that aren’t all that efficient at the basics. Glass usually goes the wrong way though (high refraction index).