The role of the medium in the effective-sphere interpretation of holographic particle characterization data.

Mary Ann Odete,Fook Chiong Cheong,David G Grier,Jesse J Elliott,Annemarie Winters,Laura A Philips

doi:10.1039/c9sm01916b

Abstract

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. Here, we demonstrate that the effective-sphere model consistently accounts for changes in the refractive index of the medium as it fills the pores of porous particles and therefore yields quantitative information about such particles' structure and composition. In addition to the sample-averaged porosity, holographic perfusion porosimetry gauges the polydispersity of the porosity. We demonstrate these capabilities through measurements on mesoporous spheres, fractal protein aggregates and irregular nanoparticle agglomerates, all of which are noteworthy for their industrial significance.

Highlights

Holographic particle characterization uses in-line holographic video microscopy to measure the diameters and refractive indexes of colloidal particles in their native media while simultaneously tracking their three-dimensional motions.[1]
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
We demonstrate that the effective-sphere model consistently accounts for changes in the refractive index of the medium as it fills the pores of porous particles and yields quantitative information about such particles’ structure and composition

Summary

Introduction

These previous studies all focused on the relationship between the particle’s internal structure and the measured effective-sphere characteristics in a medium of fixed refractive index. The present study provides a complementary experimental test of the medium’s role in determining the effective-sphere characteristics of porous particles, mesoporous silica spheres, protein aggregates with branched fractal structure and nanoparticle agglomerates dispersed in chemical–mechanical planarization (CMP) slurries. These model systems were chosen for their relevance to drug delivery and catalysis,[15] biopharmaceutical development and manufacturing,[16] and semiconductor processing,[17] respectively. Holographic perfusion porosimetry provides precise measurements of both the population-averaged porosity and the polydispersity of the porosity

Holographic particle characterization

Effective sphere model

Effective-sphere characterization of mesoporous silica spheres

Effective sphere analysis of protein aggregates and nanoparticle agglomerates

Polydispersity of porosity

Conclusions