The
adsorption/desorption mechanisms of biomolecules in porous
materials have attracted significant attention because of their applications
in many fields, including environmental, medical, and industrial sciences.
Here, we employ confocal fluorescence microspectroscopy to reveal
the diffusion behavior of zinc myoglobin (ZnMb, 4.4 nm × 4.4
nm × 2.5 nm) as a spherical protein in a single mesoporous silica
particle (pore size of 15 nm). The measurement of the time course
of the fluorescence depth profile of the particle reveals that intraparticle
diffusion is the rate-limiting process of ZnMb in the particle. The
diffusion coefficients of ZnMb in the particle for the distribution
(Ddis) and release (Dre) processes are determined from the rate constants,
e.g., Ddis = 1.65 × 10–10 cm2 s–1 and Dre = 3.68 × 10–10 cm2 s–1, for a 10 mM buffer solution. The obtained D values for various buffer concentrations are analyzed
using the pore and surface diffusion model. Although surface diffusion
is the main distribution process, the release process involves pore
and surface diffusion, which have not been observed with small organic
molecules; the mechanism of transfer of small molecules is pore diffusion
alone. We demonstrate that the mass transfer kinetics of ZnMb in the
silica particle can be explained well on the basis of pore and surface
diffusion.