Multifunctional hydroxyapatite nanoparticles for drug delivery and multimodal molecular imaging

Hydroxyapatite (HAp) is the most important constituent of biological tissues such as bone and teeth and exhibits several characteristic features. HAp nanoparticles (NPs) are good host materials and can be functionalized with various kinds of dopants and substrates. By endowing HAp NPs with desired properties in order to render them suitable for biomedical applications including cellular imaging, non-invasive and quantitative visualisation of molecular process occurring at cellular and subcellular levels becomes possible. Depending on their functional properties, HAp based nanoprobes can be divided into three classes, i.e., luminescent HAp NPs (for both downconversion and upconversion luminescence), magnetic HAp NPs, and luminomagnetic HAp NPs. Luminomagnetic HAp NPs are particularly attractive in terms of bimodal imaging and even multimodal imaging by virtue of their luminescence and magnetism. Functionalised HAp NPs are potential candidates for targeted drug delivery applications. This review (with 166 references) spotlights the cellular imaging applications of three types of HAp NPs. Specific sections cover aspects of molecular imaging and the various imaging modes, a comparison of the common types of nanoprobes for bioimaging, synthetic methods for making the various kinds of HAp NPs, followed by overviews on fluorescent NPs for bioimaging (such as quantum dots, gold nanoclusters, lanthanide-doped or fluorophore-doped NPs), magnetic HAp NPs for use in magnetic resonance imaging (MRI), luminomagnetic HAp NPs for bimodal imaging, and sections on drug delivery as well as cellular imaging applications of HAp based nanoprobes (including targeted imaging).