Citric Acid-modified Hydroxyapatite Nanoparticles as an Antibiotic Protein Carrier*

Hydroxyapatite nanoparticles (HAp NPs) are widely used as protein and drug delivery systems since HAP has excellent biocompatibility. During the protein loading process, HAp NPs may change the secondary structures of the proteins, which may affect their biological functions. Using citric acid (CA) to functionalize HAp NPs is a good approach to increase the loading capacity of positively-charged proteins, such as lysozyme, which is an antimicrobial. However, the influence of CA concentration and aging time on HAP NP morphology, surface charge, lysozyme loading capacity, and lysozyme conformation are still not fully understood. Herein, CA-functionalized HAp NPs (CA-HAp NPs) were synthesized, characterized, and loaded with antimicrobial protein, lysozyme. We observed that a higher concentration of CA leads to a greater negative surface charge of HAp NPs, which increases lysozyme loading capacity while maintaining its biological function during the adsorption/release process. The lysozyme-loaded CA HAp NPs developed here have potential applications as antimicrobial fillers in dental composites used for restoratives and wider protein delivery applications for various other biomedical applications.