Live-Cell Imaging by Confocal Raman and Fluorescence Microscopy Recognizes the Crystal Structure of Calcium Carbonate Particles in HeLa Cells

Abstract

Porous calcium carbonate (CaCO₃ ) vaterite particles are very attractive templates for the encapsulation of pharmaceuticals and for the construction of hollow polyelectrolyte capsules, sensors, and enzyme-catalyzed reactors. Although CaCO₃ is biocompatible and biodegradable, little is known about the intercellular behavior and properties of vaterite particles in the cytoplasm of cells. In this work, the authors combine confocal Raman and fluorescent microscopy for the imaging of porous CaCO₃ vaterite particles in HeLa cells to study the uptake and status of the particles inside the cells in real time. Analysis of the fluorescence images shows that the particles penetrated the plasma membrane 3 h after being added to the cell culture and that the internalization of the particles continued up to 48 h. The crystal structure of individual vaterite particles in the cytoplasm of HeLa cells did not obviously change for 144 h. For clusters of particles, however, the authors identify Raman spectroscopic signatures of the stable calcite phase after 72 h of incubation, confirming an ion-exchange mechanism of vaterite transformation to calcite. The results indicate that these imaging approach to examining inorganic particles in living cells may have theranostic applications.

Keywords: HeLa cells; Raman imaging; calcium carbonate; fluorescence microscopy; vaterite.