Diffusion behavior of rhodamine 6G in single octadecylsilyl-functionalized silica particle revealed by fluorescence correlation spectroscopy

We investigated the diffusion behavior of rhodamine 6G (Rh6G) within single octadecylsilyl-functionalized (ODS) silica particle in an acetonitrile (ACN)/water system using fluorescence correlation spectroscopy (FCS). FCS measurements were conducted at the center of the particle to exclusively determine the intraparticle diffusion coefficient (D). The obtained D values were analyzed based on a pore and surface diffusion model, the results of which indicate that surface diffusion primarily governs the intraparticle diffusion of Rh6G. Furthermore, an increase in the concentration of ACN (C_ACN) resulted in a corresponding increase in the surface diffusion coefficient (D_s), whereas the addition of NaCl did not significantly affect the D_s values. We attributed this dependence of D_s to the dielectric constant change in the interfacial liquid phase formed on the ODS layer. Specially, D_s values of (4.0 ± 0.5) × 10^–7, (7.7 ± 1.1) × 10^–7, (1.0 ± 0.3) × 10^–6, and (1.1 ± 0.2) × 10^–6 cm² s⁻¹ were obtained for C_ACN = 20, 30, 40, and 50 vol%, respectively. We anticipate that this approach will contribute to advancing research on intraparticle mass transfer mechanisms.

Graphical abstract