Biological Crystallization of Self-Aligned Iron Oxide Nanoparticles

Crystal growth and magnetic behavior of iron oxide nanoparticles assembled with biomolecules have been investigated. The nanoparticles assembled with trypsin molecules exhibit superparamagnetism at room temperature with blocking temperature (
$sim$
80 K) significantly lower than those without trypsin (
$sim$
140 K). This is attributed to reduced magnetostatic couplings between particles due to increased distance between particles separated by trypsin molecules. Moreover, the synthesized nanoparticle–biomolecule assemblies consist of a unique one-dimensional self-assembled arrays of nanoparticles found by structural analysis using transmission electron microscopy. The moirÉ fringes observed from the particle arrays indicate that the particles are aligned with slight misorientation of their crystallographic axes. Such an unusual formation of nanoparticle arrays may be relevant to specific ligand sites in trypsin molecules and the magnetostatic interparticle couplings.