Kinetic Analysis of the Mass Transfer of Zinc Myoglobin in a Single Mesoporous Silica Particle by Confocal Fluorescence Microspectroscopy

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 (D_dis) and release (D_re) processes are determined from the rate constants, e.g., D_dis = 1.65 × 10^–10 cm² s^–1 and D_re = 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.