Effective Thermal Diffusion of Eu(III) and F Ions into Hydroxyapatite Nanoparticles by Citric Acid Coordinative Mediation

Abstract

We synthesized citric acid (Cit)-coordinated Eu3+ and F– codoped hydroxyapatite (Cit/Eu:HA-F) nanoparticles (NPs) and investigated their ion diffusion processes based on photoluminescence (PL) properties at the near-surfaces with thermal treatment. At 85 °C, the Cit coordination layer effectively suppressed the nonradiative deactivation of HA NP surfaces in the PL process, and the quantum yield (ηint) of Eu3+ ions reached a maximum value of ca. 33%, indicating the highest PL efficiency in the HA system. At 250 °C, Cit was decomposed into aconitic acid with a high optical absorption coefficient and reducing ability, so that the ηint of the Eu ions in the NPs were minimized and the partial Eu3+ ions were resultantly reduced to Eu2+ ions. At around 350 °C, the Eu ions were partially diffused from Ca­(I) into Ca­(II) sites. At 550 °C, the proportion of Eu ions in Ca­(II) sites dramatically reached up to ca. 31%, and the F– ions simultaneously diffused into the HA structures by substituting for the OH sites. Therefore, it was elucidated that the Cit molecules coordinated to the Eu and F– codoped HA surfaces achieved higher ηint and promoted the diffusion of various ions into the HA structures with thermal treatment and resultantly controlled the existence ratio of the Eu3+ and Eu2+ ions at the near surfaces, which could be expected to control the PL color by the Cit coordinative mediation technique and realize the creation of cell-labeling luminescent nanomaterials in biomedical fields.

Color-Tunable Photoluminescence of Hydroxyapatite Particles by Thermochemical Reactions of Organic Acids

“…45 However, it is difficult to achieve both wider PL range and higher chroma in these systems; the technology is still a challenge. According to our study, 54 the red-color-emitting Eu 3+ ions could be reduced to be the blue-color-emitting Eu 2+ ions by the thermochemical behavior of the organic acid ions coordinated to the Eu:HA-F particles. 55−57 However, the PL intensity of the Eu 2+ ions was much lower than that of Eu 3+ ions, so it was difficult to widen the PL wavelength range.…”

“…Considering that intermolecular and intramolecular dehydrations are difficult among the Tris molecules, the intermolecular dehydration reactions between Cit and Tris were more likely to occur. Therefore, it is possible that the intermolecular dehydration reactions between Cit−Cit and Cit−Tris also occurred in addition to the intramolecular dehydration reactions of Cit during thermal treatment, 54 and novel carbon compounds would be formed (Figure 1d).…”

“…Here, the PL wavelengths at 400−500 nm for the Eu 2+ ions and 400−570 nm for the carbon compounds overlapped, and it is difficult to separately identify these two PL centers. According to our previous study, 54 Cit was changed to AA under thermal treatment at 175 °C, which reduced the Eu 3+ ions to Eu 2+ ions. Thus, we speculated that the blue-color PL derived from both the Eu 2+ ions and the carbon compounds.…”

“…For the N-CC/Eu:HA-F particles, the n−π* transition of C�O bonds in the nitrogen-containing carbon compounds, 83,89 the 4f 6 5d 1 → 4f 7 ( 8 S 7/2 ) transition of the Eu 2+ ion 54 (Figure 4d), and the f−f transition of the Eu 3+ ion 90 (Figure 4e) were observed under excitation at 365 and 395 nm. Furthermore, the PL peaks derived from the n−π* transition were red-shifted with increasing thermal treatment time.…”

“…Then, the PL intensity of the carbon compounds decreased by the desorption of the carbon compounds from the particles with the thermal treatment time of 6−10 h. On the other hand, the PL intensity of the Eu 3+ ions decreased, and the rate of decrease was 80%. It was considered that the Eu 3+ ions diffused from the particle surface into the Ca(I) sites, some of the Eu 3+ ions were reduced to the Eu 2+ ions, 54 and the coordination environment of some Eu 3+ ions with Cit was also changed with the thermal treatment. In addition, when the particles were excited at 365 nm, which was the maximum excitation wavelength of the carbon compounds (Figure 7b), the PL intensity of the carbon compound increased with the thermal treatment (rate of increase: 590%), and the rate of decrease in the PL intensity of the Eu 3+ ions was 48%, which was clearly suppressed as compared with the case in the excitation at 395 nm.…”