Size and phase preservation of amorphous calcium carbonate nanoparticles in aqueous media using different types of lignin for contrast-enhanced ultrasound imaging

Abstract

Hypothesis: Calcium carbonate (CaCO₃) nanoparticles could have great potential for contrast-enhanced ultrasound imaging (CEUS) due to their gas-generating properties and sensitivity to physiological conditions. However, the use of nano CaCO₃ for biomedical applications requires the assistance of stabilizers to control the size and avoid the fast dissolution/recrystallization of the particles when exposed to aqueous conditions.

Experiments: Herein, we report the stabilization of nano CaCO₃ using lignin, and synthesized core-shell amorphous CaCO₃-lignin nanoparticles (LigCC NPs) with a diameter below 100 nm. We have then investigated the echogenicity of the LigCC NPs by monitoring the consequent generation of contrast in vitro for 90 min in linear and non-linear B-mode imaging.

Findings: This research explores how lignin type and structure affect stabilization efficiency, lignin structuration around CaCO₃ cores, and particle echogenicity. Interestingly, by employing lignin as the stabilizer, it becomes possible to maintain the echogenic properties of CaCO₃, whereas the use of lipid coatings prevents the production of signal generation in ultrasound imaging. This work opens new avenue for CEUS imaging of the vascular and extravascular space using CaCO₃, as it highlights the potential to generate contrast for extended durations at physiological pH by utilizing the amorphous phase of CaCO₃.

Keywords: Calcium carbonate nanoparticles; Contrast-enhanced ultrasound imaging; Core–shell particles; Lignin.