Manipulation of protein crystals using a magnetic field by assembling Fe_xO_y nanoparticles

Manipulation of protein crystals using an external field is a topic of growing interest in several fields, such as X-ray crystallography and crystal processing. The aim of this study was to develop a method for manipulating crystals using a magnetic field by assembling iron oxide nanoparticles inside a lysozyme crystal. Poly(vinylpyrrolidone)-stabilised iron oxide nanoparticles, prepared through pulsed laser ablation in a solution, were preferentially incorporated in the {101} sectors rather than in the {110} sectors of the tetragonal lysozyme crystal, similar to the gold (Au) and platinum (Pt) nanoparticles studied previously. To keep the crystals intact in solutions, the outer surface of the nanoparticle-assembled crystal was coated with a pure lysozyme crystal and the coated crystals were introduced into a solution containing glycerol. The pure lysozyme crystal at the surface of the nanoparticle-assembled crystal is less likely to dissolve compared with the nanoparticle-assembled crystal itself. Additionally, glycerol has a delaying effect on the dissolution of crystals owing to its high viscosity. The authors successfully demonstrated the handling of protein crystals by commercially available needle magnets in solution. This method requires a simple device with a low cost, without any requirement for control conditions and energy, thus facilitating easy and inexpensive handling of the crystal.