  Diatoms are characterised by silica cell walls (frustules), which have highly
ordered micro-/nano-structures. As the synthesis of such structures remains
challenging, diatom frustules offer a promissing alternative to conventional
porous particles in micro-/nano-engineering. In particular, for applications in
drag deliverly systems, biosensors, and filters, an understanding of particle
motion inside diatoms is of great impotance. In this study, we investigated
nanoparticle (NP) motions inside diatom frustules using the single particle
tracking (SPT) method. For these measruements, the diameter of the NP was about
one-tenth smaller than that of the frustule. Inside the frustule, the diffusion
motions of the NPs were suppressed, but this suppression was weakened near the
exit of the frustule. Moreover, diffusion anisotropy between the axial and
radial directions of the frustule was observed. This anisotropy is difficult to
detect with ensemble methods; thus, the SPT method is a powerful approach for
investigating NP motions in frustules.
