Probing Ion Diffusion and Ion Sieving through Hollow Porous Silica Shells by Imaging the Mobility of Colloids Inside the Shells

Ionic transport through porous membranes and porous materials has received enormous attention due to its importance to many applications. An innovative methodology is proposed to study ion diffusion and ion sieving through mesoporous silica membranes (shells). The mobility of fluorescently labeled core particles within a hollow porous shell, filled with an index-matched electrolyte solution, is observed using confocal laser scanning microscopy. The core motion range, i.e., the area explored by the core within the hollow compartment, sensitively changed depending on the local ionic concentration. Monitoring transitions in the core motion range is a practical way to detect which ions can migrate through the shells and on what timescale. For instance, lithium and chloride ions easily diffused through the porous silica shells, resulting in a core motion range that changed relatively quickly upon change of the ion concentrations outside of the shell. However, the motion range changed significantly slower upon changing to a bigger cation (tetraoctylammonium ion). This proof of principle experiment can be explained by the Gibbs-Donnan effect, revealing that the detection of core motion ranges is a good probe to measure both ion diffusion and ion sieving through porous membranes.