Exploring inner surface of hollow silica nanoparticles by the time-domain nuclear magnetic resonance (TD-NMR)

• TD-NMR independently evaluated inner and outer surfaces of hollow silica nanoparticles. • Relaxation curves showed short and long components from inner and outer surfaces, respectively. • Relative surface areas correlated with particle size and relaxation times. Hollow silica nanoparticles possess unique properties derived from their nanoscale hollow interior enclosed by a solid silica shell. To fully exploit these features, independent control and evaluation of the inner and outer shell surfaces are essential. In this study, we apply time-domain nuclear magnetic resonance (TD-NMR) to separately characterize these two surfaces for the first time. A quantitative framework was established to distinguish outer and inner surface contributions based on particle concentration and cavity size. The T 2 relaxation curves of hollow particle–ethanol suspensions were resolved into long and short components. The long T 2 decreased with increasing particle concentration, similar to dense particles, indicating ethanol molecules bound to the outer surface in exchange with bulk liquid. In contrast, the short T 2 remained nearly constant regardless of concentration, suggesting liquid confined within the hollow interior where the surface-to-liquid ratio is fixed. A critical minimum cavity size required to detect inner-surface relaxation was identified. These results demonstrate that TD-NMR enables rapid characterization of occluded liquids and shell microstructure, offering a practical tool for porous material evaluation and routine quality control.