Hydrothermal Synthesis of Well-Defined Red-Emitting Eu-Doped GdPO₄ Nanophosphors and Investigation of Their Morphology and Optical Properties

Abstract

Rare-earth-doped GdPO₄ nanoparticles have recently attracted much scientific interest due to the simultaneous optical and magnetic properties of these materials and their possible application in bio-imaging. Herein, we report the hydrothermal synthesis of GdPO₄:Eu³⁺ nanoparticles by varying different synthesis parameters: pH, <Gd>:<P> molar ratio, and Eu³⁺ concentration. It turned out that the Eu³⁺ content in the synthesized nanoparticles had little effect on particle shape and morphology. The synthesis media pH, however, has showed a pronounced impact on particle size and distribution, i.e., the nanoparticle length can be adjusted from hundreds to tens of nanometers by changing the pH from 2 to 11, respectively. Increasing the <Gd>:<P> molar ratio resulted in a decrease in nanoparticle length and an increase in its width. The temperature-dependent measurements in the 77–500 K range revealed that the GdPO₄:50%Eu³⁺ sample maintains half of its emission intensity, even at room temperature (TQ_1/2 = 291 ± 19 K).

1. Introduction

Inorganic nanosized materials, doped with various lanthanide ions, have distinctive chemical and optical properties; thus, they are applied in multiple fields, including catalysis [1,2], temperature sensing [3], magnetic resonance imaging [4,5], biomedicine [6,7], anti-counterfeiting [8,9], dye-sensitized solar cells [10], etc. Such luminescent nanomaterials should possess the following characteristics if they are to be applied practically, i.e., particles should be non-toxic to humans and the environment [11], stable in colloidal suspensions [12], possess the desired morphology and narrow particle size distribution (PSD) [13], possess strong absorption and efficient emission, etc. [14,15]. The inorganic nanosized phosphors are usually synthesized via sol–gel, precipitation, or other wet chemistry routes [16]. Unfortunately, many of these synthesis methods often yield agglomerated or even bulk materials (especially if post-synthesis annealing is performed); therefore, additional milling or crashing is required to obtain nanosized particles. The thermal decomposition or hot-injection synthesis methods are more suitable for nanoparticle preparation, ensuring the reproducibility and narrow PSD of various inorganic nanophosphors [17]. However, such synthesis approaches ignore the principles of green chemistry since they depend on environmentally hazardous precursors and solvents. These methods also lead to the formation of hydrophobic nanoparticles, narrowing down their potential fields of application (e.g., biomedicine) [18,19]. In order to apply such nanoparticles in the biomedical field, additional surface modification is essential to transfer such hydrophobic nanoparticles into the aqueous colloids. There are numerous approaches reporting stabilization of optically active nanoparticles in aqueous dispersions: utilization of surfactants, such as TWEEN or SPAN [20], exchange of ligands using citric acid [21], usage of low molecular weight functional phosphates [22], usage of phosphoryl-PEG derivatives [23], or commercially available polymers, e.g., PVP [24]. Meanwhile, the hydrothermal synthesis method allows researchers to obtaining nanophoshors with various morphologies (both nano- and microrods [25], wires [26], prisms [13], cubes [27], spheres [28], etc.) by altering the synthesis parameters. This method meets the fundamentals of green chemistry since no volatile or toxic materials are released into the environment during and after the synthesis process. Moreover, the luminescent phosphors, synthesized via the hydrothermal method, are hydrophilic, indicating that the solid form of these phosphors can be effortlessly redispersed into an aqueous media. This, in turn, heightens the odds of these materials finding practical use in the biomedical field [12]. Therefore, the hydrothermal synthesis method is considered as a high-ranking chemical engineering tool for synthesizing novel luminescent, electronic, magnetic, and catalytic materials [29]. Recently, the REPO₄ nanoparticles, possessing a rhabdophane crystal structure, have gained much scientific interest, especially in the field of luminescent materials [30,31]. Among all rare-earth orthophosphates, GdPO₄ shows the most exceptional properties. Gd³⁺ stands out from other trivalent lanthanide ions by having seven unpaired electrons in the 4f orbital ([Xe]4f⁷ electronic configuration) and demonstrating magnetic properties. Materials containing trivalent gadolinium ions are widely used as MRI agents, host lattices for fluorescent lamp phosphors, X-ray intensifying screens, scintillators for X-ray tomography, etc. [32,33,34,35]. In the meantime, luminophores doped with trivalent europium are utilized in red fluorescent lamps, LEDs, and as bio-imaging or anti-counterfeit pigments [36,37,38]. Eu³⁺ possesses six electrons in 4f orbital (adopts [Xe]4f⁶ electronic configuration). When excited, europium(III) ions emit red to reddish-orange light caused by ⁵D₀ → ⁷F_0–4 optical transitions [36]. Moreover, due to the spin-forbidden nature of emission transitions, Eu³⁺ possesses longer photoluminescence (PL) lifetimes (in the order from several to a few hundred milliseconds [36,39]) if compared to other rare-earth ions emitting in the red spectral region (for instance, Er³⁺ (⁴F_9/2 → ⁴I_15/2 transition at about 650 nm) τ ≈ 550 μs; Tm³⁺ (³H₄ → ³H₆ transition at about 790 nm) τ ≈ 700 μs; Ho³⁺ (⁵F₅ → ⁵I₈ transition at about 660 nm) τ ≈ 1 ms [40]). This is an exceptionally advantageous feature, since long lifetimes allow researchers to avoid an undesirable protein autofluorescence (of a short lifetime) by employing time-resolved detection methods. Thus, Eu³⁺-doped luminescent nanoparticles could be easily detected in biological tissues. Ergo, doping the GdPO₄ host lattice with Eu³⁺ could extend the application fields of such unique materials. The combination of both trivalent Gd and Eu ions empowers the creation of dual-modal opto-magnetic inorganic nanoprobes for theranostics. Several interesting papers were published recently regarding the hydrothermal synthesis of GdPO₄ nanoparticles; however, most of them yielded long (more than 500 nm) nanorods or nanowires. Such materials are unsuitable for biological applications, requiring sizes < 100 nm [41]. Zhang et al., for instance, reported the hydrothermal synthesis of GdPO₄ nanowires 20–200 nm in width and 1−3 μm in length [42]. Interestingly, the monoclinic (P2₁/n, #14) phase was obtained even after performing the synthesis in water, which was assigned to a high synthesis temperature (240 °C). Hernandes et al. [43], on the other hand, performed the hydrothermal synthesis of GdPO₄:Eu³⁺ nanowires at 160 °C using glycerol as a co-solvent. In this case, nanowires possessing trigonal structure were obtained with width in tens of nanometers and length in hundreds of nanometers (reaction media pH = 1.6). The authors also showed that increasing pH to 12 yields irregularly shaped nanoparticles which were tens of nanometers in diameter. Yan et al., in turn, demonstrated that the GdPO₄:Eu³⁺ nanoparticle size and shape could be controlled by selecting the appropriate solvent or mixture of solvents [44]. They showed that 50−500 nm and 20−50 nm spheres of GdPO₄:Eu³⁺ can be obtained using dimethylaniline (DMA) and N-methyl-2pyrrolidone as a solvent. On the other hand, rod-like particles were obtained when water was used as a solvent. However, the synthesis duration was three days, which is not in favor of practical applications. It is also interesting to note that the GdPO₄ nanowires, obtained by the hydrothermal synthesis method, can be converted into magnetic GdPO₄ aerogel, as recently reported by Janulevius et al. [26]. There are only a few techniques that were developed to yield smaller lanthanide orthophosphate nanoparticles in the form of nanospheres [13], nanorods [13,45], or less uniform elongated nanoparticles (length in the range from 100 to 200 nm) [46,47], hexagons (ca. 15 nm) [48], and nanocubes (ca. 75 nm) [49]. Besides the hydrothermal synthesis, the GdPO₄ nanoparticles can also be prepared by co-precipitation or sol–gel methods. For instance, Di et al. reported the synthesis of GdPO₄ by the aqueous co-precipitation method (80 °C for 12 h), yielding nanowires from 30 to 100 nm in diameter and from several hundred nanometers to several micrometers in length [50]. Unfortunately, such particles are too large for bio-applications. Huang et al., in turn, reported the co-precipitation synthesis of GdPO₄ nanorods employing water/alcohol as synthesis media [51]. Surprisingly, the urchin-like structures were obtained at the beginning, consisting of ca. 120 nm needle-shaped particles radiating from the center. However, after several minutes, the urchin-like structure started to collapse, and GdPO₄·H₂O hydrogel was formed in the end. The synthesis of urchin-like GdPO₄:Eu³⁺ hollow spheres was also reported by Xu et al. The two-step procedure first involved the co-precipitation synthesis of Gd(OH)CO₃:Eu³⁺ colloidal spheres followed by hydrothermal synthesis at 180 °C for 24 h, yielding relatively large (ca. 250 nm in diameter) hollow spheres [52]. In 2018, Rosas Camacho et al. reported the sol–gel synthesis of GdPO₄ phosphors doped with lanthanide ions [53]. Unfortunately, in order to obtain single-phase materials and eliminate the organic reagents, the annealing step at 1000 °C was performed, resulting in highly agglomerated particles. It should also be noted that Kumar et al. reported the Pechini-type sol–gel synthesis of GdPO₄ nanowires (20−50 nm in diameter and from several hundreds of nanometers to several micrometers in length) [54]. However, a mixture of hexagonal and monoclinic phases was obtained. Subsequently, the monoclinic phase was obtained after annealing at 1000 °C for two hours, but the particle size increased even further.

In this study, the manipulation of GdPO₄ nanoparticle morphology via the different synthesis parameters, such as the pH of the reaction media, and the initial molar ratio of gadolinium to phosphorus (<Gd>:<P>) were reported. The evolution of the emission and excitation spectra, as well as the average PL lifetime values as a function of Eu³⁺ concentration in the GdPO₄ host lattice, was investigated in detail. The temperature-dependent photoluminescence properties of the sample exhibiting the highest PL intensity (GdPO₄:50%Eu³⁺) were also examined and presented in this study.

2. Materials and Methods

Materials used were as follows: Gd₂O₃ (99.99%, Tailorlux, Münster, Germany), Eu₂O₃ (99.99%, Tailorlux, Münster, Germany), NH₄H₂PO₄ (≥99%, Carl Roth, Karlsruhe, Germany), tartaric acid (99.99%, Eurochemicals, Vilnius, Lithuania), nitric acid (70%, Eurochemicals, Vilnius, Lithuania), and ammonium hydroxide (30%, Chempur, Karlsruhe, Germany). Ln(NO₃)₃ was prepared by dissolving Ln₂O₃ in diluted nitric acid.

All samples were prepared via the hydrothermal synthesis method manipulating only two parameters, i.e., <Gd>:<P> molar ratio and the pH of the reaction mixture. Firstly, two series of undoped GdPO₄ samples were produced as follows:

• nine samples were synthesized under a neutral reaction media (pH = 7) using different <Gd>:<P> molar ratios (1:7.5, 1:10, 1:12.5, 1:15, 1:17.5, 1:20, 1:25, 1:30, 1:50);
• nine samples were synthesized under a molar ratio of <Gd>:<P> = 1:10 and different pH of the reaction mixture (2, 3, 4, 5, 6, 8, 9, 10, 11).

The detailed synthesis procedure of GdPO₄ samples, doped with Eu³⁺ in alkaline media (pH = 10) at a molar ratio <Gd>:<P> = 1:10, is presented below [13]. Overall, a set of ten GdPO₄:Eu³⁺ nanoparticles was prepared where the Eu³⁺ concentration was 0.5, 1, 2.5, 5, 7.5, 10, 20, 50, 75, and 100%.

The synthesis procedure starts with the formation of tartaric acid–Ln³⁺ complex, which was induced by mixing stoichiometric amounts of Ln(NO₃)₃ (0.4 M) and tartaric acid (30 mL 0.3 M) aqueous solutions. The obtained mixture was left under magnetic stirring conditions for 30 min at room temperature. Afterward, the pH of the produced solution was adjusted to 10 by adding an NH₄OH solution. Subsequently, 20 mL of freshly prepared aqueous NH₄H₂PO₄ solution was poured at once, instantly turning the transparent reaction mixture into the turbid one. The morphology of the GdPO₄ nanoparticles depends on the <Gd>:<P> molar ratio; therefore, a different concentration of NH₄H₂PO₄ solution was prepared each time since the volume of the solution was kept constant, i.e., 20 mL. Furthermore, the pH of the obtained reaction mixture was again adjusted to 10 using NH₄OH solution and then diluted to 80 mL by adding DI water, followed by adjusting the pH value once again, if required. Consequently, the produced solution was left under magnetic stirring conditions for 30 min at room temperature. Finally, the reaction mixture was poured into a Teflon liner and placed inside the stainless-steel autoclave. The hydrothermal reaction took place at a 160 °C for 24 h. The synthesized particles were centrifuged four times at 10,000 rpm for ten minutes. In between centrifugation cycles, particles were washed under ultrasound conditions using deionized (DI) water. The obtained powders were either dried at 70 °C for 24 h or stored in an aqueous media.

The phase purity of prepared GdPO₄ or Eu-doped GdPO₄ samples was examined by the X-ray diffraction (XRD) technique. XRD patterns were recorded using a Rigaku MiniFlexII diffractometer operating in Bragg–Brentano geometry in a 5° ≤ 2θ ≤ 80° range under a Ni-filtered Cu K_α radiation. The scanning step width was 0.02°, and the scanning speed was 5°/min. A zero-diffraction plate made from Si crystal (MTI Corporation, Richmond, CA, USA) was used as a sample holder.

To determine the morphology and size of the synthesized phosphate particles, scanning electron microscope (SEM) images were taken on a field-emission Hitachi SU-70 electron microscope. The electron acceleration voltage was 5 kV. Particle size and particle size distribution (PSD) were evaluated manually (by taking 50 random particles per sample) using ImageJ (v1.8.0) software.

The Eu³⁺/Gd³⁺ ratio in the GdPO₄:Eu³⁺ samples was determined by inductively coupled plasma–optical emission spectroscopy (ICP−OES), using Perkin-Elmer Optima 7000DV spectrometer. The samples were dissolved in nitric acid (Rotipuran^® Supra 69%, Carl Roth) and diluted to the required volume with DI water. The calibration solutions were prepared by the appropriate dilution of the stock standard solutions (single-element ICP standards, 1000 mg/L, Carl Roth).

Excitation and emission spectra were recorded with an Edinburgh Instruments FLS980 spectrometer (double grating Czerny-Turner excitation and emission monochromators, 450 W Xe arc lamp, single-photon counting photomultiplier Hamamatsu R928P). When measuring excitation spectra, λ_em was set to 587.5 nm (excitation and emission slits being 0.50 and 3.50 nm, respectively). Analogously, when measuring emission spectra, λ_ex was set to 393 nm (excitation and emission slits being 3.50 and 0.50 nm, respectively). Each spectrum was recorded with 0.5 nm step width and 0.2 s dwell (integration) time. Emission spectra were corrected for instrument response using the correction file provided by Edinburgh Instruments. Excitation spectra were corrected by a reference detector.

Color coordinates (in CIE 1931 color space) of the synthesized samples were calculated using F980 Spectrometer Software (v.1.3.1) from Edinburgh Instruments.

PL decay curves were recorded with Edinburgh Instruments FLS980 spectrometer using a μ-flash lamp (μF2) as an excitation source. The pulse repetition rate was 25 Hz; λ_ex and λ_em were set to 393 and 587.5 nm, respectively.

The temperature-dependent excitation and emission spectra and PL decay curves were also recorded with Edinburgh Instruments FLS980 spectrometer, employing cryostat “MicrostatN” from Oxford Instruments (cooling agent–liquid nitrogen) for the temperature control. All measurements were conducted at 77 K and in the range from 100 to 500 K in 50 K intervals (stabilization time was 120 s, and temperature tolerance was set to ±5 K).

TQ_1/2 (the temperature at which a luminescent material loses half of its emission intensity) and E_a (activation energy–the amount of energy that must be given to induce thermal quenching) values for the synthesized samples were calculated using the following equations:

$$I(T)=\frac{I_0}{1+B{e}^{-\frac{E_a}{k_{B}T}}}$$

(1)

$$T{Q}_{1/2}=-\frac{E_a}{k_B\ln \frac{1}{B}}$$

(2)

where I(T)—normalized integrated emission value at a certain temperature (T); I₀—the highest normalized integrated emission value (in this case equal to 1); B—quenching frequency factor; k_B—Boltzmann constant, equal to 8.6173 · 10⁻⁵ eV/K [55].

3. Results and Discussion

XRD patterns of the produced undoped GdPO₄ samples match well with the reference pattern at any chosen <Gd>:<P> molar ratio or pH of the reaction mixture (Figure 1). This indicates that phase pure particles with the trigonal crystal structure (space group P3₁21 (#152)) were obtained.

Figure 2a presents the SEM images of GdPO₄ particles, synthesized at neutral reaction media (pH = 7), changing only the <Gd>:<P> molar ratio. These images show that wider nanorods are obtained with decreasing <Gd>:<P> molar ratio (i.e., increasing <P> concentration). The results obtained from the SEM images are in good agreement with the XRD patterns (see Figure 1a). Clearly, the peaks in the XRD pattern of the smallest particles are broader if compared to the XRD patterns of the larger particles. SEM images depicted in Figure 2b reveal that the length of GdPO₄ rods tends to decrease from sub-micro to nano-dimensions, with pH values changing from acidic to alkaline. Thus, the pH of the reaction media has a substantially greater effect on the size of the synthesized phosphates than the effect of the <Gd>:<P> molar ratio.

Figure 1 also shows that the ratio of (200) (ca. 30°) and (102) (ca. 32°) peak intensity is sensitive to changes in <Gd>:<P> (please refer to Figure 1a) and synthesis media pH (please refer to Figure 1b). These changes can be explained by analyzing the particle size and shape. For instance, the relative intensity of (200) peak increases with increasing <P> content in the reaction media. The relevant SEM images also show that the particles get wider with increasing <P> concentration. Keeping in mind that the particles grow along the c-axis direction [56], the relative (200) peak intensity must increase since there are more facets on the particle surface related to this lattice plane. Furthermore, Figure 2b shows that the relative intensity of the (200) peak increases with decreasing reaction media pH. It was already discussed that the length of the particles increases with decreasing media pH. At the same time, the width of the nanoparticles barely changes. Therefore, more and more facets related to the (200) lattice plane are on the particle surface, resulting in the increase in the (200) peak intensity. One should also keep in mind that the rod-shaped particles are subject to the preferred orientation, which is also in favor of (200) intensity. The double unit cell (along the c-axis) of the GdPO₄ crystal structure, together with (200) and (102) planes, is shown in Figure S1 for better visualization.

SEM images of the synthesized GdPO₄ samples were also used to calculate the average size of the produced particles (please refer to Figures S2 and S3 in Electronic Supplementary Information (ESI)). The obtained results are depicted graphically as a function of <Gd>:<P> molar ratio (please refer to Figure 2c) and as a function of the reaction mixture pH (please refer to Figure 2d). With increasing <Gd>:<P> molar ratio, the average particle length and dispersion slightly decreased. On the contrary, the average particle width and PSD tend to increase with increasing <Gd>:<P> molar ratio (please refer to Table S1). As for increasing the pH value of the reaction media, both the length and the width of the phosphate particles and their PSD tend to decrease (please refer to Table S2). This behavior relies on the complexation ability of different phosphate anion species. It is established that the stronger ability of PO₄³⁻ than H₂PO₄⁻ and HPO₄²⁻ to coordinate with RE³⁺ leads to the preferential formation of REPO₄ nuclei. In alkaline media, the dominant phosphate anion species are HPO₄²⁻ and PO₄³⁻. Thus, under such conditions, significantly faster formation of REPO₄ nuclei occurs compared to nuclei formation under acidic media. This behavior was also observed by Wang et al. [57].

The smallest and the most monodisperse GdPO₄ nanorods were obtained under a molar ratio <Gd>:<P> = 1:10 (pH = 10). Their average length is equal to ca. 81 nm, and their average width is ca. 17 nm. Therefore, these conditions were selected for the synthesis of GdPO₄ nanoparticles doped with Eu³⁺. Moreover, such particles are small enough for biomedical applications since studies show that even larger nanoparticles are successfully accumulated in the cells [19,41].

Figure 3a is the XRD patterns of three out of ten synthesized GdPO₄:Eu³⁺ samples (the Eu³⁺ concentration in eight samples was 0.5, 1, 2.5, 5, 7.5, 10, 20, 50, 75, and 100%). The given XRD patterns match well with the reference pattern, indicating that produced materials are characterized by trigonal crystal structure with no impurity phases present (the VIII coordinated Gd³⁺ (R = 1.053 Å) are replaced by VIII coordinated Eu³⁺ (R = 1.066 Å), which is only 1.23% larger [58] than Gd³⁺; therefore, such an ionic radii difference falls within the limits of the solid solution formation range determined by Vegard’s law [59]).

SEM images of GdPO₄ nanorods containing 1, 10, 20, and 50% Eu³⁺ are shown in Figure 3b–e, respectively. The SEM images of GdPO₄ nanorods, doped with other Eu³⁺ concentrations, are provided in Figure S4. The calculated average particle sizes are plotted in Figure 3f and tabulated in Table S3. The average length and the width of orthophosphate nanorods vary between ca. 74 to 93 nm and between ca. 16 to 23 nm, respectively. Nevertheless, particle length and thickness variation fall within the standard deviation limits. Therefore, the incorporation of Eu³⁺ into GdPO₄ NPs does not cause significant changes in the size of the obtained nanorods. To confirm that the actual Eu³⁺ concentration in the GdPO₄:Eu³⁺ samples is the same as the nominal one, the Eu³⁺ and Gd³⁺ concentrations were determined by ICP−OES. The nominal and measured values of Eu³⁺ and Gd³⁺ concentrations are given in Table S4 and match well with each other. Therefore, we can conclude that Eu³⁺ easily replaces Gd³⁺ in the GdPO₄ structure.

The excitation spectra (λ_em = 587.5 nm) of GdPO₄:Eu³⁺ samples doped with 1%, 10%, and 50% Eu³⁺ are given in Figure 4a. All spectra contain the typical sets of Eu³⁺ excitation lines originating from the intraconfigurational [Xe]4f⁶ ↔ [Xe]4f⁶ transitions: ca. 295 nm (⁷F₀ → ⁵F_J), ca. 317 nm (⁷F₀ → ⁵H_J), ca. 360 nm (⁷F₀ → ⁵D₄), ca. 370–390 nm (⁷F_0,1 → ⁵L_7,8;⁵G_J), ca. 395 nm (⁷F₀ → ⁵L₆) (the strongest transition), ca. 415 nm (⁷F₁ → ⁵D₃), ca. 465 nm (⁷F₀ → ⁵D₂), ca. 525 nm (⁷F₀ → ⁵D₁), and ca. 532 nm (⁷F₁ → ⁵D₁) [36]. The broad excitation band in the range of 250–280 nm is associated with the ligand-to-metal charge transfer (CT) band (O²⁻ → Eu³⁺) [60]. Moreover, the optical transitions of Gd³⁺ (ca. 272 nm ⁸S → ⁶I_J and ca. 309 nm ⁸S → ⁶P_J) are also observed in the excitation spectra of GdPO₄:Eu³⁺ samples when monitoring Eu³⁺ emission. Therefore, it can be concluded that Gd³⁺ → Eu³⁺ energy transfer occurs in these phosphors. This can be confirmed by analyzing the intensity of Gd³⁺ lines in the excitation spectra. The intensity of Gd³⁺ excitation lines (ca. 272 nm) is the highest when Eu³⁺ concentration is the lowest, i.e., 1%. Furthermore, the intensity of Gd³⁺ excitation lines gradually decreases with increasing Eu³⁺ concentration. At the same time, the concentration of Gd³⁺ decreases; thus, there is less Gd³⁺ that could transfer the energy to Eu³⁺, resulting in a decline of Gd³⁺ excitation line intensity. The intensity of Eu³⁺ excitation lines increases with increasing Eu³⁺ concentration and reaches the maximum in 50% Eu³⁺ sample.

The emission spectra (λ_ex = 393 nm) of GdPO₄:Eu³⁺ samples doped with 1%, 10%, and 50% Eu³⁺ are given in Figure 4b (for the emission spectra of all Eu³⁺-doped samples, please refer to Figure S5). All the spectra contain the typical sets of Eu³⁺ emission lines at ca. 578 nm (⁵D₀ → ⁷F₀), ca. 590 nm (⁵D₀ → ⁷F₁), ca. 615 nm (⁵D₀ → ⁷F₂), ca. 650 nm (⁵D₀ → ⁷F₃), and ca. 695 nm (⁵D₀ → ⁷F₄). Typically, the strongest Eu³⁺ emission transitions are ⁵D₀ → ⁷F₁ (magnetic dipole (MD)) and ⁵D₀ → ⁷F₂ (electric dipole (ED)). However, in rare-earth orthophosphates, garnets, and some europium complexes (for instance, Eu(Tp)₃ (Tp = hydrotris(pyrazol-1-yl)borate), [Eu(4-picoline-N-oxide)₈](PF₆)₃, etc.), the strongest intensity is observed for the ⁵D₀ → ⁷F₄ transition. The high intensity of ⁵D₀ → ⁷F₄ transitions in these materials is attributed to the specific symmetry (like D_4d) of the compounds or the optical basicity of these materials [36]. This was also the case in our study. Similar to the excitation spectra, the emission line intensity in emission spectra increased with increasing Eu³⁺ concentration and reached a maximum for the 50% Eu³⁺-doped sample. This is also the case with the overall emission intensity, which gradually increased, following the same trend (please refer to Figure 4d). Since the excitation spectra of GdPO₄:Eu³⁺ samples (please refer to Figure 4a) also contained the Gd³⁺ lines, we have also measured the Eu³⁺ emission spectra upon Gd³⁺ excitation (λ_ex = 273 nm). The recorded spectra are given in Figure S6. The Eu³⁺ emission intensity increases up to 10% Eu³⁺ concentration and then abruptly decreases with a further Eu³⁺ concentration increase. It is worth mentioning that samples doped with 0.5 and 50% Eu³⁺ possess virtually the same emission intensity. Since Gd³⁺ concentration decreases with increasing Eu³⁺ concentration, the decrease in Eu³⁺ emission intensity at higher Eu³⁺ concentrations is caused by lower Gd³⁺ concentration, leading to a less efficient Gd³⁺ → Eu³⁺ energy transfer.

The PL decay curves (λ_ex = 393 nm, λ_em = 587.5 nm) of GdPO₄:Eu³⁺ samples are shown in Figure 4c. The PL lifetime values were calculated using the following equation [61]:

$${\tau }_{1/e}=\frac{{\displaystyle {\int }_0^{\infty }I(t)tdt}}{{\displaystyle {\int }_0^{\infty }I(t)dt}}$$

(3)

Here, I(t) stands for PL intensity at time t. The change in average τ_1/e values as a function of Eu³⁺ concentration is plotted in Figure 4d, whereas the exact calculated τ_1/e values are summarized in Table S7. The PL decay curves get steeper with increasing Eu³⁺ concentration, indicating that the PL lifetime values decrease. The average PL lifetime values of Eu³⁺ emission at 587.5 nm decrease from ca. 3043 μs to ca. 173 μs as the concentration of Eu³⁺ in the NPs increase from 0.5 to 100% (please refer to Figure 4d and Table S5).

The color coordinates in CIE 1931 color space were calculated for each sample doped with Eu³⁺. The obtained color coordinates are located directly on the edge of the CIE 1931 color space diagram (please refer to Figure 4e), indicating that the red emission of the produced NPs would be perceived as a rich and monochromatic light by the human eye. In addition, with increasing Eu³⁺ concentration in the samples, a slight red shift of color coordinates is observed. However, this shift is relatively insignificant (especially in heavier Eu³⁺-doped samples), and color coordinates can be considered stable regardless of the amount of Eu³⁺ (please refer to Table S5 for the precise calculated color coordinate values).

As was discussed above, the GdPO₄:50%Eu³⁺ phosphor exhibited the highest emission intensity and, therefore, was selected for temperature-dependent measurements. Figure 5a,b shows that the intensity of both excitation and emission spectra of the GdPO₄:50%Eu³⁺ sample drops dramatically when the temperature increases from 77 to 500 K. It is also interesting to note that the excitation spectrum recorded at 77 K does not contain any excitation lines originating from the ⁷F₁ level, indicating that thermal population of this level is significantly suppressed at such low temperature. The normalized integrated emission intensity as a function of temperature is presented in the inset of Figure 5b. These data were used to calculate TQ_1/2 and E_a, and the obtained values are equal to 291 ± 19 K and 0.049 ± 0.007 eV, respectively. The TQ_1/2 value (in the 77 to 500 K range) showed that this phosphor maintained half of its emission intensity even at room temperature.

The PL decay curves (please refer to Figure 5c) of the GdPO₄:50%Eu³⁺ sample got steeper with increasing temperature, indicating the decreasing average PL lifetime values of Eu³⁺. This indeed was confirmed after calculating the average PL lifetime values, which are plotted in Figure 5d, and their exact values are given in Table S6. It turned out that the average PL lifetime values of GdPO₄:50%Eu³⁺ phosphor decreased from ca. 871 μs to ca. 257 μs with the temperature increase from 77 to 500 K, which can be related to the decreasing internal efficiency of the phosphor.

The temperature-dependent emission spectra of GdPO₄:50%Eu³⁺ phosphor were also used to calculate the temperature-dependent color coordinates, which are plotted in Figure 5e. The exact calculated values of color coordinates are summarized in Table S6. A slight red shift of calculated color coordinates is observed with the increasing temperature; however, this shift is relatively insignificant, and color coordinates can be considered temperature-stable, especially at higher temperatures.

4. Conclusions

In summary, we have demonstrated that the aqueous hydrothermal synthesis method is highly suitable for preparing GdPO₄:Eu³⁺ nanoparticles possessing hydrophilic surfaces. The nanoparticle size and morphology can be controlled by selecting the appropriate pH and <Gd>:<P> molar ratio. The width of GdPO₄ nanoparticles decreased from 93 to 23 nm, and the length decreased from 635 to 99 nm when the pH of reaction media was increased from 4 to 11. Furthermore, the width of GdPO₄ nanoparticles increased from 19 to 60 nm, and the length decreased from 154 to 128 nm when the <Gd>:<P> molar ratio was changed from 1:7.5 to 1:50. The nanoparticle size and particle size distribution of the Eu³⁺-doped samples, on the other hand, remained virtually the same, regardless of the Eu³⁺ concentration. The PL measurements showed that the emission intensity of GdPO₄:Eu³⁺ samples increased with increasing Eu³⁺ concentration and reached the maximum for the GdPO₄:50%Eu³⁺ sample. The temperature-dependent measurements in a 77–500 K range revealed that the GdPO₄:50%Eu³⁺ sample possesses relatively high luminescence thermal stability. This sample maintained half of its emission intensity, even at room temperature (TQ_1/2 = 291 ± 19 K). The determined thermal optical stability of the Eu³⁺-doped samples is sufficient for various applications, including luminescent security inks, bio-imaging probes, etc. Recent studies also showed the magnetic properties of GdPO₄ nanoparticles. Therefore, the combination of Gd³⁺ magnetic properties and Eu³⁺ distinctive luminescence properties extends the possible application field of these nanomaterials even further. Such unique opto-magnetic nanoparticles could be applied in biomedicine as selective bio-imaging probes or MRI contrast materials.