Water-Mediated Photoluminescence Color Tuning of Carbon–Hydroxyapatite Hybrid Nanoparticles

Biological tissues contain various endogenous chromophores that absorb light at specific wavelengths, which reduce the contrast and visibility in macroscopic observation. Therefore, it is necessary to develop color-tunable nanoparticles for labeling biological tissues. In this study, Eu3+/F– codoped hydroxyapatite nanoparticles coordinated with biocompatible organic acid ions (citric acid and l-cysteine) were synthesized in a wettable state, and their photoluminescence (PL) properties were evaluated after thermal treatment at 85 °C in either Teflon or glass Petri dishes. Teflon dishes with low surface free energy enhanced the intermolecular cohesion of water molecules induced nanoparticle aggregation, resulting in the precipitation of blue-emitting carbon compounds (CCs) on the nanoparticle surfaces within 20 h and yielding an internal quantum yield (ηint) of 28.1%. Prolonged thermal treatment, however, decreased the ηint due to the increased carbonization reactions among and within CCs. In contrast, glass Petri dishes with higher surface free energy promoted water dispersion and evaporation, thereby strengthening the nanoparticle interactions and facilitating the formation of CC for up to 30 h. Consequently, the PL intensity was enhanced, yielding a maximum ηint of 28.4% with blue-color-emitting. These results indicated that the aggregation or dispersion state of water molecules in nanoparticle liquids effectively controls the precipitation of CCs on the nanoparticles. Based on the optimal synthesis conditions (85 °C for 30 h in glass Petri dishes), the nanoparticles were exposed to the air to adsorb water molecules, and their PL properties were further investigated. The precipitated CCs possessed hydrophilic functional groups that adsorbed water molecules and formed hydrogen bonds, lowering the energy levels of the electron-excited states of the surface functional groups. Consequently, the PL peak exhibited a red-shift and the PL color was changed from blue to green. Notably, after 14 h of exposure, green-color-emitting with an ηint of 21.8% was obtained. These findings demonstrated that the thermal treatment container, duration, and adsorbed water can regulate the nanoparticle interactions, the carbonization degree of CCs, and the energy levels of surface functional groups, enabling the effective tuning of PL color. This study established a strategy for developing photoluminescent apatite-based nanomaterials.