Holographic Perfusion Porosimetry of Individual Colloidal Particles*

The in-line hologram of a micrometer-scale colloidal sphere can be analyzed with the Lorenz-Mie theory of light scattering to obtain precise measurements of the sphere's diameter and refractive index. The same technique also can be used to characterize porous and irregularly shaped colloidal particles provided that the extracted parameters are interpreted with effective-medium theory to represent the properties of an equivalent effective sphere. We demonstrate through experiments on mesoporous silica spheres, protein aggregates and nanoparticle agglomerates that the effective-sphere model consistently accounts for the influence of the medium on the particle's measured effective refractive index.This dependence yields information on the particles' structure and composition that cannot be obtained in other ways, including their porosity, the polydispersity of their porosity, and the size distribution and connectivity of their pores.