Incorporation of 5-methoxyisophthalate ions into octacalcium phosphate for red-shifted fluorescence properties

Aromatic dicarboxylate ion (DCI)-incorporated octacalcium phosphate (OCP) exhibits fluorescence through its aromatic groups. Because OCP is an established bone substitute, aromatic DCI incorporation may extend its use to theranostic applications. However, fluorescence in reported OCP with incorporated aromatic DCIs has been limited to the ultraviolet–visible (UV–Vis) region; therefore, further extension toward longer wavelengths is crucial for bioimaging applications. Previous studies have shown that electron-donating methoxy groups can cause red shifts in absorption and fluorescence spectra. To explore the impact of methoxy-containing aromatic DCIs on the optical properties of OCP, OCP incorporating 5-methoxyisophthalate ions (Met-IP-OCP) was synthesized using 15 mmol of Met-IP and a 6 h reaction period. The successful formation of Met-IP-OCP was confirmed by X-ray diffraction and Fourier-transform infrared spectroscopy. Compared with IP-OCP, Met-IP-OCP exhibited modified thermal behavior and altered crystal morphology. UV–Vis absorption analysis revealed a red shift in the absorption onset of Met-IP-OCP compared with IP-OCP. Fluorescence analysis also showed red shifts in both excitation and emission wavelengths for Met-IP-OCP relative to IP-OCP. These results suggest that incorporating aromatic DCIs with electron-donating substituents can be used to tune the optical properties of OCP toward longer wavelengths, providing valuable insights into the molecular design of fluorescent calcium phosphate materials for potential bioimaging applications.