Europium Doped Calcium Deficient Hydroxyapatite as Theranostic Nanoplatforms: Effect of Structure and Aspect Ratio

We present the development of theranostic nanoplatforms (NPs) based on a europium (Eu³⁺) doped calcium deficient hydroxyapatite (CDHA) core functionalized with cyclodextrin (β-CD) and cucurbitural (CB[7]). The composition, crystalline structure, aspect ratio, surface area, morphology, and luminescence property of the NPs were investigated by means of X-ray diffraction (XRD), Fourier transform infrared spectroscopy (FTIR), energy dispersive X-ray analysis (EDAX), the Brunauer-Emmett-Teller (BET) method, transmission electron microscopy (TEM), and fluorescence spectroscopy. The perceivable effects of Eu³⁺ doping appear in the minor peak shift to larger angles attributed to lower crystallite size and smaller aspect ratios coupled with greater structural strain in the rod shaped theranostic NPs and a shift in their zeta potential toward less negative values. Cell parameter calculations suggest that the doping of Eu³⁺ would cause the a-axis parameter to decrease slightly as the ionic radius of Eu³⁺ is smaller than that of Ca²⁺. Moreover drug release profiles employing 5-fluorouracil (5FU) suggest that these luminescent NPs depict controlled and sustained release profiles. Further the emissive intensities of the NPs in the carrier systems increase with cumulative released amounts of 5FU, suggesting that release of the drug can be monitored by changes in luminescent intensity. In addition, native NPs manifest commendable cytocompatibility as demonstrated by MTT and live/dead protocols, whereas the 5FU loaded NPs demonstrated over 80% HeLa cell death, signifying their therapeutic potential. We envision that these NPs can serve as effective and practical multifunctional probes for theranostic applications.