A new core-shell nanostructure consisting of inorganic hydroxyapatite (HAP) nanoparticles as the core and organic alginate as the shell (denoted as HAP@Alg) was successfully synthesized by a pre-gel method and applied to pH-responsive drug delivery systems (DDS). HAP@Alg nanoparticles have the advantages of hydroxyapatite and alginate, where hydroxyapatite provides pH-responsive degradability, and alginate provides excellent biocompatibility and COOH functionality. Through the subsequent addition of CaCl(2) and phosphate solutions to the alginate solution, HAP@Alg nanoparticles with controllable particle sizes (ranging from 160 to 650 nm) were obtained, and their core-shell structure was confirmed through transmission electron microscopy (TEM) observation. Rhodamine 6G (R6G), a positively charged dye, was selected as a model drug for pH-sensitive DDS. R6G was encapsulated in the HAP/Alg nanoparticles upon synthesis, and its loading efficiency could reach up to approximately 63.0%. The in vitro release behavior of the loaded R6G at different pH values was systematically studied, and the results indicated that more R6G molecules were released at lower pH conditions. For example, after releasing for 8 h, the release amount of R6G at pH 2.0 was 2.53-fold the amount at pH 7.4. We attributed this pH-sensitive release behavior to the dissolution of the HAP core in acidic conditions. The results of the MTT assay and confocal laser scanning microscopy indicated that the HAP@Alg were successfully uptaken by liver cancer cells (HepG2) without apparent cytotoxicity. The synthesized HAP@Alg nanoparticles show great potential as drug nanovehicles with high biocompatibility, enhanced drug loading, and pH-responsive features for future intracellular DDS.

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