Preparation and magnetoviscosity of nanotube ferrofluids by viral scaffolding and ALD on porous templates

Current models for magnetoviscosity suggest that replacing the spherical nanoparticles of a conventional ferrofluid with magnetic nanotubes would lead to a stronger field‐induced viscosity enhancement and a much‐improved stability against shear thinning – two important parameters for technological exploitation of the magnetoviscous effect. We report the development of positive and negative templating strategies for the synthesis of magnetic nanotubes out of a variety of materials. Our positive template is Tobacco mosaic virus (TMV) – in natural form or genetically engineered to express specific surface chemistries and lengths – which we exploit as a template for the electroless deposition (ELD) of nanosized clusters of nickel and as a scaffold for magnetic particles in a conventional ferrofluid. Our negative templating strategy employs porous anodic aluminum oxide (AAO) as a substrate for the atomic layer deposition (ALD) of a conformal coating of iron oxide, offering precise control over the length and wall thickness of the resulting nanotubes. Both strategies were scaled up to produce the mass quantities of uniform‐aspect‐ratio nanotubes that are needed for macroscopic ferrofluid volumes. The magnetoviscosity of these “nanotube ferrofluid” samples was studied as a function of applied magnetic field and shear frequency, and a particularly strong effect was found to be induced by viral scaffolding.