Increased photoluminescence and photodynamic therapy efficiency of hydroxyapatite-β-cyclodextrin-methylene blue@carbon powders with the favor of hydrogen bonding effect.

To meet the requirements of theranostics with diagnosis and treatment, photodynamic-based therapy is simultaneously enabled with the incorporation of methylene blue (MB) as imaging agent and photosensitizer in core-shell structured drug vehicles. Citrate-modified hydroxyapatite (HAp) powders are first grafted with β-cyclodextrin (CD), then combined with MB molecules through electrostatic interactions, and finally encapsulated with carbon shells through hydro-thermal carbonization of glucose to prepare HAp-CD-MB@C powders. Processing parameters of carbonization temperature, glucose addition, reaction time and CD addition are varied to prepare drug carriers with modulated crystallite degrees and photo-physical properties. Increased crystallite sizes of HAp are accompanied with the formation of C=O, C=C and C-OH groups in carbon shell, endowing sustainable release behaviors of MB through carbonous structures. High photoluminescence intensities are fairly related with red-shifted vibration peaks of groups in tightly combined MB molecules through hydrogen bonds. This hydrogen bonding effect is significantly increased for HAp-CD-MB@C140 with the splitting of CH-involved vibration peaks in infrared spectra, which causes increase in photoluminescence intensity and four-fold increase in generation ratio of singlet oxygen. The present studies shed light on preparation of core-shell structured drug carriers, modulation of aggregate states of MB molecules, enhancement of photo-physical properties and improvement of generation ratio of singlet oxygen during photodynamic-based therapy.