Magnetic hyperthermia treatment using calcium phosphate nanoparticles is an evolutionary choice because of its excellent biocompatibility. In the present work, Fe3+ is incorporated into HAp nanoparticles by thermal treatment at various temperatures. Induction heating was examined within the threshold Hf value of 4.58 × 106 kA m–1 s–1 (H is the strength of alternating magnetic field and f is the operating frequency) and sample concentration of 10 mg/mL. The temperature-dependent structural modifications are well correlated with the morphological, surface charge, and magnetic properties. Surface charge changes from +10 mV to −11 mV upon sintering because of the diffusion of iron in the HAp lattice. The saturation magnetization has been achieved by sintering the nanoparticles at 400 and 600 °C, which has led to the specific absorption rate of 12.2 and 37.2 W/g, respectively. Achievement of the hyperthermia temperature (42 °C) within 4 min is significant when compared with the existing magnetic calcium phosphate nanoparticles. The systematic investigation reveals that the HAp nanoparticles partially stabilized with FeOOH and biocompatible α-Fe2O3 exhibit excellent induction heating. In vitro tests confirmed the samples are highly hemocompatible. The importance of the present work lies in HAp nanoparticles exhibiting induction heating without compromising the factors such as Hf value, low sample concentration, and reduced duration of applied field.

Multifunctional nanosized particles are very beneficial in the field of biomedicine. Bioactive and highly biocompatible calcium phosphate (CaP) nanoparticles (∼50 nm) exhibiting both superparamagnetic and fluorescence properties were synthesized by incorporating dual ions (Fe3+ and Sr2+) in HAp (hydroxyapatite) [Ca10(PO4)6(OH)2]. Insertion of Fe3+ creates oxygen vacancies at the PO4 3– site, thereby destabilizing the structure. Thus, in order to maintain the structural stability, Sr2+ has been incorporated. This incorporation of Sr2+ leads to an intense emission at 550 nm. HAp nanoparticles when subjected to thermal treatment (800 °C) transform to β-TCP, exhibiting emission at 710 nm due to the emergence of an intermediate band. Moreover, these nanoparticles exhibit fluorescence in visible light when compared to the other UV and IR fluorescence excitation sources which could damage the tissues. The synthesis involving the combination of ultrasound and microwave techniques resulted in the distribution of Fe3+ in the interstitial sites of CaP, which is responsible for the excellent fluorescent properties. Moreover, thermally treated CaP becomes superparamagnetic, without affecting the desired optical properties. The bioactive, biocompatible, magnetic, and fluorescent properties of this resorbable CaP which is free from toxic heavy metals (Eu, Gd, etc.) could help in overcoming the long-term cytotoxicity. This could also be useful in tracking the location of the nanoparticles during drug delivery and magnetic hyperthermia. The bioactive fluorescent CaP nanoparticle helps in monitoring the bone growth and in addition, it could be employed in cell imaging applications. The in vitro MCF-7 imaging using the nanoparticles after 24 h of uptake at 465 nm evidences the bioimaging capability of the prepared nanoparticles. The reproducibility of the defect level is essential for the defect-induced emission properties. The synthesis of nontoxic fluorescent CaP is highly reproducible with the present synthesis method. Hence, it could be safely employed in various biomedical applications.

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