Layered Gadolinium-Europium-Terbium Hydroxides Sensitised with 4-Sulfobenzoate as All Solid-State Luminescent Thermometers

Abstract

Ternary layered gadolinium-europium-terbium basic chlorides were synthesised using a facile hydrothermal-microwave technique. A continuous series of solid solutions was obtained in a full range of rare earth concentrations. To sensitise the luminescence of Eu³⁺ and Tb³⁺, a 4-sulfobenzoate anion was intercalated in the ternary layered rare earth hydroxides using one of two methods—a high-temperature ion exchange or a single-stage synthesis. The luminescent colour of the materials was governed by the gadolinium content: at low and medium gadolinium concentrations (0–70%), layered Gd-Eu-Tb basic sulfobenzoate exhibited a bright red europium luminescence; at high gadolinium content (70–90%), a bright green terbium luminescence was observed. The colour coordinates of layered Gd-Eu-Tb basic sulfobenzoate luminescence depended on the temperature in the physiological range (20–50 °C). The relative thermal sensitivity of the obtained materials was up to 2.9%·K⁻¹.

1. Introduction

Recently discovered layered rare earth hydroxides (LRHs) are a new class of pillared inorganic materials that possess excellent anion exchange properties [1,2,3,4]. These materials are considered to be structural analogues of well-known layered double hydroxides (LDHs); they have the general formula Ln₂(OH)_6–x(A^m–)_x/m·nH₂O (Ln = RE, A = anion, x = 1–2, n = 0–2). LRHs are currently the focus of attention due to their unique properties; they combine the features of both host rare earth hydroxide matrices and guest intercalated anions. LRHs are widely used as phosphors [5,6,7,8,9,10,11], sensors [12,13,14], sorbents [15,16], catalysts [17], and containers for drug delivery [18,19]. A unique feature of this class of compounds is the possibility to tune their anionic and cationic composition while preserving the main structural motif, which enables the precise setting of the properties of functional materials based on such compounds [20,21]. This is one of the key differences between LRH-based materials and materials based on metal–organic frameworks and rare earth coordination compounds. In the latter, ligand replacement can lead to dramatic changes in the crystal structure.

Luminescent materials based on LRHs are currently highly promising functional materials [22]. Layered terbium and europium hydroxides are of the greatest interest, since the corresponding cations show intense luminescence in the visible range [23]. In addition to the design of phosphors with a high quantum yield [21,24], materials with tunable luminescence properties can be obtained based on terbium and europium compounds. Primarily, such materials include sensors providing a negative [25,26] or positive [27,28] luminescence response to the detected molecules. Thus, a biosensor for the detection and measurement of a range of protein biomarkers in human serum was created on the basis of an Eu³⁺-complex, with a detection limit of <100 fM [29]. A terbium(III) complex with 1,10-phenanthroline was demonstrated to be capable of detecting ascorbic acid with a detection limit of 7.4 × 10⁻⁵ mol L⁻¹ [30]. Materials with a temperature-dependent luminescent property [31,32,33,34] are of particular interest since they can be used for contactless and local temperature measurements in biological systems, including single cells [35].

Currently, temperature sensors based on terbium and europium coordination compounds are considered to be most efficient [36]; their relative temperature sensitivity exceeds 1%·K⁻¹. Such compounds can, however, be toxic to living beings because of their relatively high solubility and low chemical stability. Therefore, researchers are searching for new poorly soluble and biocompatible matrices for the immobilisation of complexes of europium(III) and terbium(III) with luminescent properties that are not inferior to coordination compounds of rare earth elements (REE) [37,38]. One of the possible matrices for the immobilisation of complexes and cations of terbium and europium are LRHs [22], whose solubility is several orders of magnitude lower than that of typical rare earth coordination compounds; furthermore, their solubility product can be as low as 10⁻²⁰ [22]. Recently, Zhu et al. demonstrated a luminescent thermometer based on a Y/Eu binary LRH intercalated with neutral terbium(III) complexes [39]. The LRH obtained was a chameleon luminophore that exhibited colour emissions from green to pink with an increase in temperature from 77 to 450 K [39], but the temperature sensitivity of the resulting system was less than 1%·K⁻¹. A significant gap in the design of luminescent systems based on LRHs is the paucity of studies of the temperature-dependent luminescence of LRHs containing Eu³⁺ and Tb³⁺ cations directly in the metal hydroxide layers. Note that such systems are much easier to synthesise than LRHs intercalated with rare earth element complexes.

The luminescence of terbium and europium cations in LRHs should be sensitised, since water and hydroxyl groups present in LRHs quench the luminescence of rare earth elements. Antenna ligands sensitising REE luminescence are typically aromatic carboxylates, such as benzoate [40], terephthalate [20,21,23,24], salicylate [21], 1,3,5-phenyltricarboxylate [24,26,27], 1,2,4,5-phenyltetracarboxylate [24,27], 4-biphenylcarboxylate [41], and 2-naphthoate [20] anions. Sulfobenzoates are close analogues of benzene dicarboxylates; the presence of the sulfo-group makes the anion exchange reactions with LRHs easier and enables the formation of phases that are well crystallised, compared with benzene dicarboxylates. For example, the structure of a new LRH compound, layered yttrium basic 4-sulfobenzoate with the composition Y₃(OH)₇(C₇H₄O₅S)·H₂O, was recently described [42]. It was shown that the 4-sulfobenzoate anion had similar optical properties to those of the terephthalate anion and that it was capable of sensitising Eu³⁺ luminescence.

For sensor materials based on LRHs, the analytical signal can be luminescence intensity [8,14,39,43] or lifetime [7,43], although the most convenient signal is the colour (colour coordinates) of luminescence [7,39,43,44,45]. For layered terbium and europium hydroxides, colour tuning can be achieved by changing the molar ratio between terbium and europium cations in the mixed compound [46] or by mixing isostructural compounds of terbium and europium [47]. For systems co-doped with terbium and europium cations, the phenomenon of Tb³⁺ → Eu³⁺ excitation energy transfer is observed. The control of this transfer is the most promising direction in the design of luminescent temperature sensors [39]. The efficiency of Tb³⁺ → Eu³⁺ energy transfer, according to most known mechanisms, is proportional to the distance between optical centres. In layered rare earth hydroxides, this distance can be controlled simply by changing the concentrations of Tb³⁺ and Eu³⁺ in the matrix of yttrium or gadolinium hydroxide. The use of gadolinium is preferable, since it has additional capabilities in terms of the luminescence sensitisation of Tb³⁺ and Eu³⁺ cations and also has a cation size (105 pm) that is closer to that of Tb³⁺ (104 pm) or Eu³⁺ (107 pm) than that of Y³⁺ (102 pm).

To date, only binary and ternary systems of layered gadolinium-europium-terbium hydroxides with Eu_xGd_1−x [11], Tb_1−xEu_x [48], or Gd_0.5Tb_0.5−xEu_x [46] cationic composition have been described; the latter system is the most promising for layered rare earth hydroxides with tunable luminescence. In the current study, an emphasis was placed on synthesising ternary layered Gd-Eu-Tb hydroxides in the full range of cationic composition to obtain materials with tunable luminescence. To sensitise luminescence in the ternary layered Gd-Eu-Tb hydroxides, 4-sulfobenzoate anions were intercalated in their interlayer space. High-temperature anion exchange and single-stage microwave-assisted synthesis were used for this intercalation. The latter approach provided the rapid synthesis of well-crystallised materials with bright composition-dependent and temperature-dependent luminescence.

2. Results and Discussion

2.1. Synthesis of Ternary Layered Rare Earth Basic Chlorides

To confirm the uniform distribution of terbium and europium cations in the matrix of layered gadolinium hydroxide, ternary layered basic chlorides were synthesised in a wide range of compositions (ESI, Figure S1 and Table S1). The refined lattice parameters of the basic chlorides were obtained, and their cationic composition according to energy-dispersive X-ray analysis (EDX) data are presented in the form of ternary heat maps in which colours indicate the values of the (Gd_1−x−yEu_xTb_y)₂(OH)₅Cl·nH₂O unit cell parameters (Figure 1). In most cases, the colours change uniformly as a function of composition, indicating the formation of solid solutions.

A convenient variable for describing the composition of substitutional solid solutions is the average radius over the cation [49,50,51,52]. The approach that plots the dependences of the unit cell parameters on the average radius makes it possible to simplify the presentation of data and to confirm the formation of solid solutions with linear sections on two-dimensional plots. In the case of layered basic Gd-Eu-Tb chlorides, the average radius of rare earth cations (the sum of the radii of gadolinium, europium, and terbium, normalised to their content), is an integral characteristic of the deformation of the metal hydroxide core along the ab layer plane. Figure 1 shows the unit cell parameters for (Gd_1−x−yEu_xTb_y)₂(OH)₅Cl·nH₂O as a function of the average radius of rare earth cations. The linear dependence of the a (12.80–12.95 Å) and b (7.25–7.35 Å) parameters on the average radius indicates a uniform deformation of the basic rare earth chloride metal-hydroxide lattice in the layers. At the same time, the c parameter varied non-linearly with the average radius, although these changes were very small (8.42–8.46 Å). Such differences between the behaviour of a/b and c parameters were due to the anisotropy of the structure of layered hydroxides. The parameter c correlated with the interlayer distance of the layered basic rare earth chlorides, and its value primarily determines the content of water or impurity anions, such as carbonate anions [22]. Accordingly, Figure 1c,g indicates that solid solutions of layered basic Gd-Eu-Tb chlorides were characterised by a higher degree of hydration and/or the presence of more impurity anions in comparison to individual layered basic Gd, Eu, or Tb.

Thus, even under conditions of rapid hydrothermal microwave synthesis [53,54,55], crystalline ternary solid solutions of layered basic chlorides (Eu, Gd and Tb) were formed. For the first time, ternary layered basic rare earth chlorides of the (Gd_1−x−yEu_xTb_y)₂(OH)₅Cl·nH₂O composition were obtained in such a wide range of compositions (x, y = 0, 0.05, 0.15, 0.25, 0.35, 0.45, 0.5, and 1). These data greatly expand the known range of existing LRH solid solutions previously described for layered basic Gd-Eu-Tb chlorides, nitrates, and dodecylsulfates [21,27,46,56].

2.2. Anion Exchange with Potassium 4-Sulfobenzoate

To sensitise the luminescence of terbium and europium cations in the ternary layered Gd-Eu-Tb hydroxides, the 4-sulfobenzoate anion was intercalated in these hydroxides. This anion is an analogue of the terephthalate anion capable of sensitising the luminescence of both terbium(III) and europium(III) cations [57]. Due to the presence of the sulfo group in its composition, the anion exchange reactions do not lead to the formation of impurity phases, in contrast to the terephthalate anion [58]. Furthermore, the greatest efficiency of Eu³⁺ luminescence sensitisation was recently demonstrated with the 4-sulfobenzoate anion, as compared with other benzene dicarboxylates and sulfobenzoates [42].

Figure 2 shows X-ray diffraction patterns for the reaction products of layered Gd-Eu-Tb basic chlorides and potassium 4-sulfobenzoate at 120 °C after 24 h. In all cases, the appearance of reflections of the Ln₃(OH)₇(C₇H₄O₅S)·H₂O phase is observed. However, the presence of a halo and low lattice symmetry (P2₁/c space group) of the obtained compounds prevented refinement of the crystal lattice parameters with sufficient accuracy, based on the Ln₃(OH)₇(C₇H₄O₅S)·H₂O structure previously solved for Ln = Y [42].

Table S2 (ESI) shows the Gd:Eu:Tb:Cl:S ratios according to EDX data for layered Gd-Eu-Tb basic chlorides and products of their high-temperature ion exchange with an aqueous solution of potassium 4-sulfobenzoate under hydrothermal conditions (120 °C). In the ion exchange products, the initial ratios of rare earth cations were retained, which indicates the preservation of the cationic composition of the metal hydroxide lattice during ion exchange. As a result of ion exchange, the Cl:Ln ratio in the products of interaction with 4-sulfobenzoate significantly decreased and tended toward zero (0–0.015). This result confirms the completeness of the ion exchange reaction.

The S:Ln ratio in the products of interaction with 4-sulfobenzoate varied in the range 0.18–0.28. For the stoichiometric compounds of the LREH-III class, including Ln₃(OH)₇C₇H₄O₅S·nH₂O, this ratio should be equal to 0.33. The lower S:Ln values for the products of the interaction of rare earth basic chlorides with 4-sulfobenzoate anions can be explained by the release of hydroxyl ions into the interlayer space of LRHs, which hindered the entry of 4-sulfobenzoate anions. The presumable scheme of the anion exchange reaction can be represented as follows:

Ln₂(μ-OH)₅Cl·nH₂O + A²⁻ → Ln₃(μ-OH)₇(C₇H₄O₅S)_1−x(OH)_2x·mH₂O + Cl⁻ + (n−m)H₂O

(1)

Ln₃(μ-OH)₇(C₇H₄O₅S)_1−x(OH)_2x·mH₂O + A²⁻ → Ln₃(μ-OH)₇A·mH₂O + OH⁻

(2)

where A²⁻ is the 4-sulfobenzoate anion, μ-OH are bridging hydroxyl groups binding rare-earth cations within the metal-hydroxide layers, and OH are free hydroxyl groups in the interlayer space. Reaction 2 is presumably slower than 1, hindering the formation of the stoichiometric compound.

According to SEM data (Figure 3), rare earth basic chlorides formed spheroidal (~2 µm) aggregates of lamellar particles (~100 nm thick). As a result of anion exchange reactions between rare earth basic chlorides and potassium 4-sulfobenzoate, spheroidal aggregates were destroyed with the formation of isolated lamellar particles with a lateral size of about 5 μm. An increase in particle size as a result of an anion exchange reaction may indicate that the Ostwald ripening of layered Gd-Eu-Tb basic sulfobenzoate particles takes place.

A study of the products of layered Gd-Eu-Tb basic chlorides reaction with potassium 4-sulfobenzoate suggested that, at the first stage of the interaction, chloride anion was replaced by 4-sulfobenzoate, while the morphology of the layered hydroxide particles was preserved. Then, the 4-sulfobenzoate anion replaced hydroxyl groups in the rare earth coordination polyhedra, accompanied by LRH recrystallisation with the formation of the Ln₃(OH)₇C₇H₄O₅S·nH₂O phase, which had a different particle morphology. The recrystallisation proceeded in a non-quantitative manner, and the presence of an amorphised product of anion exchange follows from the halos at 5–15° 2θ in the corresponding XRD patterns (Figure 2e–h). The most important result of anion exchange reactions between potassium 4-sulfobenzoate and layered Gd-Eu-Tb basic chlorides was the preservation of the cationic composition of the layered hydroxide.

2.3. Single-Stage Synthesis of Ternary Layered Rare Earth Gd-Eu-Tb Basic Sulfobenzoates

It is well known that the Y₃(OH)₇C₇H₄O₅S·H₂O phase can be obtained, not only by anion exchange reactions, but also directly as a result of single-stage synthesis under hydrothermal microwave conditions [42]. This method significantly reduces the duration of synthesis (from several days to 30 min) and enables the formation of well crystallised products.

Figure 4 shows the comparison of the IR spectra of layered basic Gd-Eu-Tb sulfobenzoates obtained by different methods, namely high-temperature anion exchange reaction and single-stage synthesis. Both spectra show intense narrow vibration bands of COO⁻ (1350–1570 cm⁻¹), SO₃⁻ (970–1270 cm⁻¹), and Ln–O (400–700 cm⁻¹). At the same time, the ratio of the intensities of the OH^- vibration bands (3200–3800 cm⁻¹) to COO⁻, SO₃⁻, and Ln–O for a series of layered hydroxides obtained by ion exchange is noticeably higher than in a single-stage synthesis, which confirms the higher content of hydroxyl ions in products of ion exchange reactions.

Figure 5 shows data on rare earth element content in the samples obtained, according to EDX data. The S:Ln ratio varied in the range 0.309:1–0.337:1, confirming that these compounds belong to the LREH-III class of layered hydroxides (S:Ln = 0.33). The composition of the samples corresponded to the formula (Gd_1−xTb_0.9xEu_0.1x)₃(OH)₇(C₇H₄O₅S)·nH₂O.

For the products of one-step synthesis, the unit cell parameters were refined using structural data for the layered yttrium basic sulfobenzoate, Y₃(OH)₇(C₇H₄O₅S)·H₂O [42]. The refined unit cell parameters are shown in Figure 5. X-ray patterns of the samples obtained are shown in Figure S2 (ESI). Up to 60% gadolinium content, a linear change in the unit cell parameters was observed, which indicates a uniform deformation of the crystal structure of ternary layered Gd-Eu-Tb basic sulfobenzoates with a change in composition. At a higher content of gadolinium, a sharp increase in all unit cell parameters was observed, indicating significant changes in the crystal structure of basic sulfobenzoates. Such anomalous behaviour (deviation from a Vegard-like trend) is a fairly common phenomenon in multicomponent systems [59,60]. Another reason for a significant change in the lattice parameters may be the clustering of terbium or europium cations with a decrease in their concentration [61].

2.4. Luminescent Properties

The intercalation of the 4-sulfobenzoate anion led to luminescence sensitisation of both Eu³⁺ and Tb³⁺ in individually layered europium and terbium hydroxides, respectively. Figure 6 shows the luminescence spectra of the anion exchange reaction products between individual layered europium and terbium hydroxides with aqueous solutions of potassium 4-sulfobenzoate. There is an intense band in the excitation spectra of the anion exchange products (Figure 6a,b), corresponding to the transitions between the ground and excited states of the 4-sulfobenzoate anion (Figure 6e). The position of the edge of the excitation band was not dependent on the cationic composition (~280 nm).

To establish the composition ranges of ternary layered Gd-Eu-Tb basic sulfobenzoates (Gd_1−x−yTb_xEu_y)₃(OH)₇(C₇H₄O₅S)·nH₂O in which the Tb³⁺→Eu³⁺ energy transfer (Figure 6e) [46,62,63] takes place, the absolute intensity ratio µ = I_Eu/(I_Eu + I_Tb) was calculated, where I_Eu and I_Tb are the intensities of the most intense bands of europium (615 nm, ⁵D₀–⁷F₂ transition) [64,65] and terbium (544 nm, ⁵D₄–⁷F₅ transition) [66], respectively. The value µ = 1 corresponds to the luminescence of europium only; at µ = 0, only the luminescence of terbium is observed. The values obtained were plotted on a ternary diagram of the cationic composition of layered hydroxides (Figure 7b). For a wide range of compositions (µ ≈ 1), Eu³⁺ predominantly luminesces, thus indicating the efficient Tb³⁺→Eu³⁺ energy transfer. The colour coordinates of the luminescence of the powders obtained are plotted in Figure 7a. It can be seen that most of the coordinates are in the red part of the spectrum, corresponding to the luminescence of Eu³⁺ optical centres. The ⁵D₀ → ⁷F₂ Eu³⁺ luminescence lifetime for the sample with the highest terbium content, (Gd_0.24Tb_0.70Eu_0.06)₃(OH)₇(C₇H₄O₅S)·nH₂O was 0.265 ms, being in line with the previously reported terbium-europium co-doped layered rare earth hydroxides [67].

A luminescence other than red is observed only in the range of compositions (Gd_1−x−yTb_xEu_y)₃(OH)₇(C₇H₄O₅S)·nH₂O with a low europium content (y < 0.2) (Figure 7). As the content of gadolinium decreases, this region becomes narrower (y < 0.05). Accordingly, by fixing a low Eu/Tb ratio and varying the gadolinium content, the (Gd_1−xTb_0.9xEu_0.1x)₃(OH)₇(C₇H₄O₅S)·nH₂O compositions were obtained, with the luminescence colour changing from red (x = 1, gadolinium content 0%) to green (x = 0.11, gadolinium content 89%). The high content of terbium ensured the efficient Tb³⁺ → Eu³⁺ transfer, and the variation of the gadolinium content made it possible to quantitatively determine how this transfer was affected by the distance between the Tb³⁺ and Eu³⁺ cations. Figure 8 shows the dependences of the intensity ratio of the most intense terbium to europium luminescence bands on the composition of the solid solutions. It can be seen that the luminescence of terbium started from a critical value of gadolinium content (50%). To draw further conclusions from this observation, it is necessary to understand the nature of the distribution of rare earth cations in the layered basic sulfobenzoates. Unfortunately, the nature of this distribution for mixed LRHs remains unclear at present [56]. Thus, in the structure of layered basic rare earth chlorides, there are two REE positions with different CNs (8 and 9). Accordingly, REE with a smaller radius will tend to occupy positions with CN 8, which will lead to a nonrandom distribution of REE in mixed basic chlorides [56]. However, in mixed layered rare earth basic sulfobenzoates, all REE positions have CN = 8 [42], which makes it highly probable for there to be a random distribution of REE in metal hydroxide layers.

The estimated calculation of average distances between terbium and europium atoms, assuming their random distribution in layered rare earth basic sulfobenzoates, was performed as follows (ESI, Figure S3). Gadolinium atoms were designated as A₁, and terbium and europium atoms were designated as A₂. Accordingly, the average distance between europium and terbium atoms as a function of gadolinium content was considered to be the average distance between A₂ atoms at different ratios of the number of A₁ atoms to the number of A₂ atoms. The fraction of gadolinium atoms in the unit cell was denoted as ν_A1 and the sum of the fractions of terbium and europium atoms in the unit cell as ν_A2, so that ν_A1 + ν_A2 = 1. To simplify calculations, for all mixed rare earth basic sulfobenzoates, the unit cell parameters were considered to be equal to the corresponding parameters of Y₃(OH)₇(C₇H₄O₅S)·H₂O (a = 13.964 Å, b = 10.539 Å, c = 12.351 Å, β = 109.96°) [42]. The simplest structural motif of the Y₃(OH)₇(C₇H₄O₅S)·H₂O metal hydroxide layer consists of 12 polyhedra and occupies an area equal to c·b, where c and b are the unit cell parameters for Y₃(OH)₇(C₇H₄O₅S)·H₂O. The area $S_{A2}=\left(c·b\right)/\left(12·{\nu }_{A2}\right)$ was calculated in order to estimate the average distance $\langle R_{intra}\rangle$ between A₂ atoms inside the layer. The average distance $\langle R_{intra}\rangle$ was calculated as the diameter of a circle with area S_A2, i.e., $\langle R_{intra}\rangle$=$2\sqrt{\left(c·b\right)/\left(12\pi ·{\nu }_{A2}\right)}$. To estimate the average distance $\langle R_{inter}\rangle$ between A₂ atoms in adjacent metal hydroxide layers of layered rare earth basic sulfobenzoates, the generating line of a cone with a circle of diameter S_A2 at the base was calculated, with the height being the distance between adjacent metal hydroxide layers $h=a·\cos \left(\beta -90°\right)$, where a and β are the unit cell parameters for Y₃(OH)₇(C₇H₄O₅S)·H₂O. Accordingly, the average distance was $\langle R_{inter}\rangle =2\sqrt{\left(c·b\right)/\left(12\pi ·{\nu }_{A2}\right)+a^2·{\cos }^2\left(\beta -90°\right)}$.

Thus, a calculation was made of the average distance between europium and terbium ions, at which the blocking of the Tb³⁺→Eu³⁺ transfer begins. Based on the graph shown in Figure 8, such blocking began to appear in layered Gd-Eu-Tb basic sulfobenzoates at a gadolinium mole fraction of 0.5. Substituting this value into the above formulas provided an average distance of 5.3 Å between the terbium and europium cations inside the layer and 13.8 Å between the layers. According to previously published data [56], the Tb³⁺→Eu³⁺ energy transfer occurs between the layers, since here the rare earth cations are connected by bridging hydroxyl groups, quenching the transferred energy. Based on this, 13.8 Å is the average distance, above which energy transfer between terbium and europium cations becomes impossible. Such a large distance (>1 Å) between optical centres suggests that the excitation transfer between them occurs via Coulomb interaction (Förster mechanism) [68]. Unlike the exchange mechanism (Dexter mechanism), Coulomb interaction does not require the overlap of electron clouds and can transfer energy over distances of up to hundreds of Å. In the case of energy transfer between rare earth cations, the Coulomb interaction occurs via the dipole–quadrupole interaction [69]. Note that the critical distance obtained (13.8 Å) is twice that of the critical transfer distance (6–7 Å) calculated for Tb³⁺→Eu³⁺ energy transfer in calcium phosphate glasses [69], which may be due to the difference in the refractive indices of the matrices used.

2.5. Temperature Dependence of Luminescence

From Figure 8, it can be seen that the dependences I(Tb ⁵D₄–⁷F₅)/I(Eu ⁵D₀–⁷F₂) at 100 K and 300 K diverge with increasing gadolinium content in the mixed basic sulfobenzoates. Such an effect can be associated with the effects of concentration quenching and different temperature dependences of the population of the radiative levels of the Tb³⁺ and Eu³⁺ cations [70]. At the same time, the observed changes in luminescence with temperature were insignificant and were localised within the region of relatively low temperatures, the measurement of which is of no particular practical interest.

A much more interesting area for temperature measurement is the area of physiological temperatures. The use of luminescent particles makes it possible to measure the temperature of individual cells or even their organelles, which is in demand in cell research and cancer therapy [70]. For all layered Gd-Eu-Tb basic sulfobenzoate compositions, in the luminescence spectrum of which both terbium and europium bands were observed, luminescence spectra were recorded in the temperature range of 20–90 °C (ESI, Figure S4). The fine structure of the Eu ⁵D₀–⁷F₂ band changed significantly as the temperature increased.

There was a hypsochromic shift and an increase in the intensity of one of the components of the Eu ⁵D₀–⁷F₂ band, which was hypersensitive to the local environment of Eu³⁺. At the same time, the remaining bands, both terbium and europium, in most cases, only slightly changed with temperature and can be used as a reference (Figure 9a). Changing the intensity ratio (Figure 9b) of the terbium and europium bands also led to significant changes in the colour coordinates of the luminescence (Figure 9c and ESI, Figure S4). The most noticeable shift in the luminescence coordinates can be observed for the (Gd_0.65Tb_0.33Eu_0.02)₃(OH)₇(C₇H₄O₅S)·nH₂O sample obtained by a one-stage method (Figure 9). With an increase in temperature (18 → 90 °C), the intensity of the Eu^{3+ 5}D₀–⁷F₂ band increased several times with respect to the Tb^{3+ 5}D₄–⁷F₆ band (Figure 9b). The linear nature of the dependence of the ratio of the integral intensities of the Δ = Eu^{3+ 5}D₀–⁷F₂/Tb^{3+ 5}D₄–⁷F₆ bands in the temperature range t = 20–50 °C enabled an estimate to be made of the maximum relative temperature sensitivity of the resulting system $\frac{1}{\mathsf{\Delta }}\left(\frac{d\mathsf{\Delta }}{dt}\right)$ as 2.9%·K⁻¹ (at 30 °C), which is comparable to, and even exceeds, the corresponding values for complexes [71] and organometallic polymers of terbium-europium [70]. In terms of sensitivity, the compound obtained was also better than the only known thermometer based on LRHs [39] in the temperature range from −196 to 177 °C. In addition, the thermometer described in the literature contained a neutral terbium complex, whose chemical stability is lower, and whose toxicity is presumably higher, than the similar characteristics of the metal hydroxide backbone containing terbium and europium [72].

Observed changes in europium luminescence can be associated with a decrease in the local symmetry of the Eu³⁺ environment [73]. In the case of layered Gd-Eu-Tb basic sulfobenzoates, such a change in the symmetry of the environment of the rare earth cations may occur due to the removal of residual water from crystallisation, which is indirectly confirmed by thermal analysis data (ESI, Figure S5). The thermal cycling (ESI, Figure S6) of the samples at 20–90 °C showed the reversibility of the luminescent property, but the signal value decreased presumably due to irreversible loss of water molecules in the layered hydroxide structure (see thermal analysis data, ESI, Figure S5).

3. Materials and Methods

3.1. Materials

The following compounds were used at the start: EuCl₃·6H₂O (99.99%, Lanhit, Moscow, Russia), TbCl₃·6H₂O (99.99%, Lanhit, Moscow, Russia), Gd₂O₃ (99.9%, Lanhit, Moscow, Russia), potassium 4-sulfobenzoate (95%, Sigma Aldrich, Burlington, VT, USA), NaCl (99%, Komponent-Reaktiv, Moscow, Russia), and hexamethylenetetramine (99+%, Alfa Aezar, Ward Hill, Haverhill, MA, USA). A gadolinium chloride solution was prepared by dissolving Gd₂O₃ (99.99%, Lanhit, Moscow, Russia) in hydrochloric acid (98%, Khimmed, Moscow, Russia) at 80 °C.

Layered rare earth basic chlorides were synthesised using the previously developed technique of homogeneous precipitation under hydrothermal-microwave (HTMW) treatment conditions, which has been successfully employed for the synthesis of layered yttrium [74], gadolinium [75], europium [76] basic nitrates, layered yttrium basic closo-dodecaborate [77] and basic 4-sulphobenzoate [42], as well as a number of other compounds [78,79,80,81]. 10 mL of 1 M aqueous solution of NaCl and 10 mL of a 1.4 M aqueous solution of hexamethylenetetramine (HMT) were added to 10 mL of an aqueous solution containing a mixture of EuCl₃·6H₂O, TbCl₃·6H₂O, and GdCl₃·6H₂O in set ratios (Figure 10). The resulting reaction mixture was placed in an autoclave and subjected to HTMW treatment at 140 °C for 30 min. After the synthesis, the autoclaves were cooled in air and the precipitate formed was filtered off on a glass filter, washed several times with distilled water, and then dried at 60 °C with a relative humidity of ~75%.

3.2. Anion Exchange Reactions

The anion exchange reactions were carried out as follows: 50 mg of previously obtained layered basic rare earth chloride powder was dispersed in 30 mL of a 0.0036 M solution of potassium 4-sulfobenzoate (4-sulfobenzoate:REE molar ratio = 1:1). The suspension was placed in a glass autoclave (30% filling degree) and subjected to heat treatment at 120 °C for 24 h. The resulting product was separated on a glass filter, washed several times with distilled water, and then dried overnight at 60 °C.

3.3. Single-Stage Synthesis of (Gd_1−xTb_0.9xEu_0.1x)₃(OH)₇(C₇H₄O₅S)·nH₂O

10 mL of aqueous solution of potassium 4-sulfobenzoate and 10 mL of 0.14 M aqueous solution of hexamethylenetetramine were added to 10 mL of aqueous solution of LnCl₃∙nH₂O (Ln = Y, Eu, Tb) containing these elements in a set molar ratio (see Figure 10) and which had a total REE concentration of 0.1 M. In all cases, a 5-fold molar excess of potassium 4-sulfobenzoate to LRH was taken. The resulting reaction mixture was placed in a 100 mL Teflon autoclave (30% filling degree) and subjected to hydrothermal microwave treatment (500 W) in a Berghof Speedwave MWS-4 oven at 200 °C for 30 min. After completion of the synthesis, the autoclaves were cooled in air. The precipitate formed was separated by centrifugation, washed several times with distilled water, and then dried at 60 °C for 24 h.

3.4. Methods

Powder X-ray diffraction (XRD) patterns of the samples were collected on a Bruker D8 Advance diffractometer (Bragg–Brentano geometry) with CuKα radiation. The structure was refined by the Le Bail method, using TOPAS software v. 4.2. The parameters for the layered europium hydroxide structure obtained by Geng et al. were used as initial values for refinement [82]. The microstructure of LRH samples was investigated using high-resolution scanning electron microscopy (Carl Zeiss NVision 40 equipped with an Oxford Instruments X-Max EDX detector, Abingdon, England). The IR spectra of the resulting compounds were recorded on an ALPHA FTIR spectrometer (Bruker, Billerica, USA) in the range of 400–4000 cm⁻¹, with a resolution of 0.5 cm⁻¹ in the attenuated total reflection mode. No additional sample preparation was performed. Weight loss during thermolysis was determined using a TG 209 F1 NETZSCH thermobalance in a dry air flow at a flow rate of 20 mL/min and in a temperature range of 35–650 °C at a heating rate of 10 °C/min. The data obtained were processed using the NETZSCH Proteus Thermal Analysis software package v. 6.1.0. The content of carbon, hydrogen, sulfur, and nitrogen in the samples was determined using a EuroVector EA 3000 CHN analyser (Pavia, Italy) over a temperature range up to 900 °C. Excitation and luminescence spectra of the powders were recorded using a PerkinElmer LS-55 luminescence spectrophotometer (Waltham, MA, USA) with 0.5 nm resolution at 100 K and 300 K. Luminescence spectra in the temperature range 15–90 °C were recorded using an Ocean Optics modular optical system (Orlando, FL, USA) equipped with a Peltier element (Kryotherm, Saint-Petersburg, Russia). The sample was placed on a cooled Peltier element (50 W, Kryotherm, Saint-Petersburg, Russia) connected to a current source (Figure 11). Using an Ocean Optics TP300-UV-VIS probe (Orlando, FL, USA), excitation radiation (280 nm) was applied to the sample from a MonoScan2000 monochromator (Orlando, FL, USA) and recorded with a detector, which transferred the signal to the computer. The spectra were processed using SpectraSuite software v.1.0. Lifetimes were measured using a R928P photon counter (Hamamatsu Photonics, Hamamatsu City, Shizuoka, Japan) with a xenon pulse lamp as the excitation source. To measure the Eu³⁺ luminescence lifetimes, the europium emission wavelength of 612 nm and the excitation wavelength of 275 nm were fixed.

4. Conclusions

For the first time, a series of solid solutions of layered rare earth basic chlorides of the composition (Gd_1−x−yEu_xTb_y)₂(OH)₅Cl·nH₂O (x, y = 0, 0.1, 0.3, 0.7, 0.9, 1) was obtained via hydrothermal-microwave treatment. For the series of solid solutions obtained, there was a linear change in the volume and parameters a and b of the unit cell with the average REE radius. Two methods for preparing ternary layered Gd-Eu-Tb basic sulfobenzoates were implemented: high-temperature anion exchange reactions and one-step synthesis. In both cases, the Ln₃(OH)₇C₇H₄O₅S·nH₂O phases were formed. As a result of anion exchange reactions, the cationic composition of solid solutions was preserved. The one-stage method made it possible to significantly reduce (by up to 30 min) the synthesis duration of solid solutions of layered gadolinium-europium-terbium hydroxides.

The intercalation of the 4-sulfobenzoate anion into the interlayer space of LRHs led to the sensitisation of Tb³⁺ and Eu³⁺ luminescence. The colour coordinates of the luminescence of layered Eu-Gd-Tb hydroxides depended on gadolinium content: the emission spectra of (Gd_1−x−yTb_xEu_y)₃(OH)₇(C₇H₄O₅S)·nH₂O (x + y > 0.7) mostly contained europium luminescence bands, while (Gd_1−x−yTb_xEu_y)₃(OH)₇(C₇H₄O₅S)·nH₂O (x + y < 0.7) mostly contained terbium luminescence bands. The critical distance between the terbium and europium cations, which resulted in the blocking of the Tb³⁺−Eu³⁺ transfer and a change in the luminescence colour in (Gd_1−x−yTb_xEu_y)₃(OH)₇(C₇H₄O₅S)·nH₂O, was 13.8 Å. In addition to composition-dependent dual centre luminescence for (Gd_1−x−yTb_xEu_y)₃(OH)₇(C₇H₄O₅S)·nH₂O (x + y < 0.6), temperature-dependent luminescence was observed in the physiological temperature range of 18–90 °C. The maximum relative temperature sensitivity of 2.9%·K⁻¹ was obtained for the rare earth composition (Gd_0.65Tb_0.33Eu_0.02)₃(OH)₇(C₇H₄O₅S)·nH₂O.