Water‑Soluble α‑NaGdF₄/β‑NaYF₄:Yb,Er Core–Shell Nanoparticles: Controlled Synthesis and Superior Upconversion Luminescence

Abstract

Hexagonal NaREF₄ (RE = Y, Gd, Lu) fluorides are the benchmark hosts for lanthanide‑doped upconversion (UC) nanomaterials, offering crystal lattices that promote intense, photostable emission. Yet, the cubic (α) phase normally dominates at low temperatures, hindering the production of small, hexagonal (β) NaREF₄ crystals via conventional solvothermal routes. In this study, we employ a heterogeneous core‑induced strategy to grow β‑NaYF₄: Yb,Er shells on α‑NaGdF₄ cores, achieving water‑soluble, sub‑50 nm particles. Structural analysis via XRD, TEM, and EDX confirms the core–shell architecture and elucidates the role of cation exchange in driving the hexagonal shell growth. Observation of Gd³⁺ UC emission further verifies the formation of a hetero interface. This approach offers a scalable, low‑temperature pathway to bright, biocompatible UC probes.

Background

Lanthanide‑doped UC nanoparticles are prized for their sharp emission peaks, photostability, and near‑infrared excitation, making them ideal for bio‑imaging and sensing. To maximize sensitivity, UC particles must emit strongly under 980‑nm excitation, be sub‑50 nm, and possess hydrophilic surfaces for functionalization. Among UC hosts, hexagonal NaREF₄ (RE = Y, Gd, Lu) fluoride crystals stand out due to their superior luminescence. Numerous synthesis routes—solvothermal, thermal decomposition, and dopant‑engineering—have been explored, yet each faces limitations: solvothermal methods often yield cubic phases; thermal decomposition requires harsh, oxygen‑free conditions and yields hydrophobic particles; dopant‑induced phase control demands high dopant loadings that risk host lattice distortion.

In 2014, our group pioneered a heterogeneous core–shell approach, using small cubic cores to nucleate hexagonal shells. Building on that, we selected α‑NaGdF₄ cores, which possess a larger Gd³⁺ ionic radius (0.938 Å) compared to Y³⁺ (0.9 Å), facilitating Y³⁺ substitution and interfacial lattice distortion that promotes β‑NaYF₄ growth. This strategy also offers reduced reaction times and yields water‑soluble, small particles without the need for post‑surface modification.

Methods

Materials

High‑purity rare‑earth chlorides (GdCl₃, YCl₃, YbCl₃, ErCl₃), NaCl, KF, ethylene glycol, and PVP (K‑30) were used without further purification.

NaGdF₄ Core Synthesis

Reagents were dissolved in ethylene glycol, mixed with 0.5 g PVP, and heated at 180 °C for 30 min in a PTFE autoclave. After cooling, the cores were purified by centrifugation and dispersed in 10 mL EG for further use.

Core/Shell Synthesis

α‑NaGdF₄ cores were combined with YCl₃, YbCl₃, ErCl₃, NaCl, and KF under stirring, then subjected to solvothermal treatment at 180 °C for 2–24 h. The resulting core/shell particles were isolated, washed, and either vacuum‑dried for XRD/TEM or re‑dispersed in water for optical tests.

Characterization

XRD (Rigaku RU‑200b), SEM (JEOL JEM‑7500F), TEM/HRTEM (FEI Tenai F‑20), EDX, and UC emission (Hitachi F‑4500, 980‑nm laser) were employed. Lifetime measurements used a 953.6‑nm pulsed laser (10 ns, 10 Hz).

Results and Discussion

We first confirmed α‑NaGdF₄ as the core via XRD and TEM. Subsequent solvothermal growth in the presence of Y³⁺, Yb³⁺, Er³⁺, and KF yielded β‑NaYF₄ shells, as evidenced by the appearance of β‑NaYF₄ peaks (JCPDS 16‑334) and the disappearance of cubic peaks after 24 h. TEM images show the evolution from ~23 nm cubic cores to ~115 nm × 125 nm hexagonal prisms as shell thickness increases. HRTEM reveals lattice fringes corresponding to the (110) plane of hexagonal NaYF₄ and the (111) plane of cubic NaGdF₄, confirming the core/shell architecture.

EDX and line‑scan mapping illustrate Gd³⁺ confined to the core, while Y³⁺ and Yb³⁺ uniformly coat the outer shell, further confirming the heterostructure. Importantly, Gd³⁺ UC emission peaks at 312–317 nm and 277 nm appear only in core/shell samples, confirming energy transfer across the hetero interface and validating the hexagonal shell growth mechanism.

UC spectra under 980‑nm excitation (16 W cm⁻²) show dominant green emission (515–560 nm, ²H₁₁/₂ → ⁴I₁₅/₂) and weaker blue/red components. Emission intensity increases steadily with shell growth time, while spectral ratios remain constant, indicating uniform dopant distribution. Lifetime studies of Yb³⁺ ²F₅/₂ and Er³⁺ ⁴F₉/₂, ²H₉/₂ levels exhibit monotonic increases from 2 to 24 h, correlating with thicker shells and enhanced luminescence.

Conclusions

We demonstrated a facile, low‑temperature, heterogeneous core–shell strategy that transforms α‑NaGdF₄ cores into β‑NaYF₄:Yb,Er shells, yielding water‑soluble UC nanoparticles with tunable size and shape. XRD, TEM, EDX, and UC data collectively confirm the core/shell architecture and the critical role of cation‑exchange‑induced lattice distortion in driving hexagonal shell growth. The resulting particles exhibit bright, stable upconversion emission suitable for biomedical imaging and sensing applications.