Intraparticle Diffusion Behavior of Rhodamine 6G in Single Silica Particle Revealed by Fluorescence Correlation Spectroscopy

Understanding the intraparticle diffusion mechanism within single particles is crucial for processes such as chromatographic separation, drug delivery, and solid extraction. However, when the time required to reach distribution equilibrium is extremely short (on the order of several seconds), conventional kinetic detection methods pose significant challenges in observing intraparticle diffusion behavior. In this study, we employed fluorescence correlation spectroscopy (FCS)─a technique capable of detecting diffusion behavior at equilibrium─to investigate the intraparticle diffusion of rhodamine 6G (Rh6G) within single porous silica particles of varying pore sizes. The autocorrelation coefficients of Rh6G were fitted using two-component analysis, revealing faster and slower diffusion components associated with the pore diffusion, without and with adsorption/desorption, respectively-behaviors that were not observed by the kinetic method described by our previous study. Further analysis of the slower diffusion component was conducted using pore and surface diffusion models. Our findings indicate that pore diffusion is the primary diffusion mechanism for Rh6G within silica particles. Thus, we demonstrated that the intraparticle diffusion mechanism of Rh6G in silica particles can be elucidated using FCS measurements.