Novel light-emitting clays with structural Tb³⁺ and Eu³⁺ for chromate anion detection

Abstract

Tb³⁺ and Eu³⁺ ions were encapsulated for the first time in the inorganic layers of a synthetic saponite clay following a one-pot synthetic approach. The co-presence of the two metal ions led to tuneable light-emitting properties, promoted by an efficient Tb³⁺ → Eu³⁺ energy transfer and enhanced Stokes shift character. To our knowledge, the so-prepared luminescent material was tested for the first time as an optical sensor for the detection of chromate anions in water.

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Over the past years, synthetic smectite clays, and in particular saponite, have attracted great interest in the scientific community, due to their peculiar physico-chemical properties, such as controlled and tuneable chemical composition and specific surface area, excellent chemical versatility, high thermal stability and robustness, adjustable surface acidity and relatively simple and low cost synthesis.^1–4

Saponite materials have been used in several applications, spanning from polymer science to heterogeneous catalysis.^5–9 Examples of their applications in the agricultural and construction fields, for environmental purposes (i.e. recovering of heavy metals from soils and waters) and in optical/optoelectronic devices are also reported.^10–15

Recently, a flourishing research branch on interesting layered and porous materials (including silicas, silicates, clays, MOFs…)^16,17 has been related to their combination with rare-earth ions. Lanthanides represent key elements for many technologies used in our society (i.e. in energy devices, for biomedical uses…) owing to their unique electronic, magnetic, optical and catalytic properties.^18–20 Preliminary studies on the preparation of lanthanide-containing layered and/or porous materials with specific functionalities have been made, with newfound applications as sensors for targets of interests (i.e. biothiols, surfactants, fingerprint detection…), as luminescent thermometers,²¹ or in bio-imaging fields (i.e. as oral MRI contrast agents).^22–30 In clay materials, for example, the lanthanides (i.e. Eu³⁺, Tb³⁺, Gd³⁺…) were normally incorporated as complexes or free ions into the clay interlayer space by using post-synthesis intercalation procedures.^24,30 However, these synthetic protocols are quite expensive and require long preparation times. Moreover, depending on the complexes' stability, metal leaching processes can occur, thus leading to a potential environmental contamination and to toxic effects in different organisms. These disadvantages could be overcome by design synthetic strategies based on the direct insertion of the lanthanide ions in the clay synthesis gel, as previously performed with vanadium and niobium metals.^5,31,32

Based on these considerations, in this study, two luminescent lanthanides, Tb³⁺ and Eu³⁺, were simultaneously incorporated in the structure of a synthetic saponite clay (hereafter named Na-TbEuSAP), using a single-step hydrothermal approach. A solid with only Tb³⁺ ions was also prepared as a reference (hereafter named Na-TbSAP). A multi-technique characterization approach was employed to investigate the physico-chemical properties of the solids. These novel light-emitting saponites were also tested as potential optical sensors for inorganic anions in water. In this respect, it was proved that the lanthanide complexes, when incorporated in different hybrid solids or gels (i.e. metal–organic frameworks, polymers…), can be employed as efficient sensors for different kinds of substrates (ions and organic molecules) by exploiting the so-called “antenna effect” phenomena.^33,34 To the best of our knowledge, the lanthanide-based clays studied in the literature has never been investigated as sensors for the detection of anionic species.

Among the anions, the chromate (CrO₄²⁻), a common source of the hexavalent chromium (Cr^(VI)) which is regarded as the most toxic and dangerous form of this element, was selected for the sensor tests.^35–38 The CrO₄²⁻ is an environmentally-hazardous inorganic pollutant commonly present in soils and waters, and it has been found in several public drinking waters worldwide as well near particular industrial plants.^35–38 The amount of chromium in water and soil is strictly regulated by international and national policies: i.e. the limit in drinking waters for EPA (United Sates Environmental Protection Agency) is of 100 ppb, while in Italy is even lower (50 ppb).^39,40 Thus, the development of efficient sensors for Cr^(VI) compounds, especially in water media, is therefore of great importance in terms of public health and safety.

In detail, Na-TbEuSAP and Na-TbSAP samples were prepared by a modified one-pot hydrothermal synthesis protocol, followed by sodium-exchange process (Scheme S1 in the ESI†).^5,31,32 Eu³⁺ and Tb³⁺ were introduced in the form of chloride salts during the synthesis gel along with the silicon, aluminium and magnesium precursors. The final materials were then submitted to ionic exchange procedure to replace the different cations located in the saponite gallery (i.e. Al³⁺, Mg²⁺, H₃O⁺) with Na⁺ ions.

The lanthanides loading in the final materials was determined by inductively coupled plasma mass spectrometry (ICP-MS). The Tb³⁺ amount was found to be 0.021 and 0.031 mmol g⁻¹ for TbSAP and TbEuSAP samples, respectively. For both solids, the Na⁺-exchange procedure did not alter the amount of Tb³⁺ species. For TbEuSAP sample, the Eu³⁺ content resulted to be 0.023 mmol g⁻¹, without modification after the ion-exchange procedure. The differences in the Tb³⁺ concentrations observed in the two materials could be ascribed to a certain variability in the chemical composition of the samples, as testified by the different cationic exchange capacity of TbSAP and TbEuSAP (see below). In fact, numerous parameters (i.e. the acidity of the gel during the condensation phase, the amount of water, etc…) including the co-presence of two different metal ions in competition with similar structural sites can positively or negatively affect the final loading of the metal ions. In general, however, the results suggest that both metals are mainly located in the framework of the saponite clay, as schematically proposed in Fig. 1B.

Fig. 1 (A) X-ray diffraction pattern of Na-SAP (a), Na-TbSAP (b) and Na-TbEuSAP (c) samples. (B) Schematic representation of the Na-TbEuSAP clay.

The inclusion of the lanthanides ions during the synthesis did not alter the layered structure of the saponite,^{1–7,12,22,23} as indicated by the X-ray powder diffraction (XRPD) analyses (Fig. 1A). In particular, the X-ray patterns of Na-TbSAP and Na-TbEuSAP showed the typical reflections of the saponite clay. Nevertheless, the signal associated to the basal plane (001) in both samples, with d-spacing value in the 7.4–8.0° 2θ range, is less defined respect to the parent Na-SAP sample, thus indicating a greater structural disorder of the lamellae packing.¹

The cation-exchange capacity (CEC) of the clays, evaluated by UV-Visible spectroscopy analysis^30,31 (Fig. S1 in the ESI†), was estimated to be 41.6 ± 4.8 meq./100 g and 61.2 ± 5.2 meq./100 g for Na-TbSAP and Na-TbEuSAP, respectively. These values are in line with the results obtained for other metal-containing saponites.^5,31,32

The solids showed high thermal stability, as indicated by the thermogravimetric analyses (TGA). The TGA (Fig. S2A in the ESI†) and the derivative curves (DTG) (Fig. S2B in the ESI†) of Na-TbSAP (curve a) and Na-TbEuSAP (curve b) show a first weight loss around 100 °C related to the evaporation of physisorbed water, followed by a gradual release of the interlamellar water (with an initial dehydroxylation of the layered structure) in the 150–750 °C range. A third weight loss around 825 °C is correlated to the collapse of the saponite framework, with formation of the relative metal oxides.^1–4

The photophysical properties of the luminescent saponites were thoroughly investigated by photoluminescence (PL) spectroscopy. The excitation spectra at solid state of Na-TbSAP and Na-TbEuSAP (Fig. 2A), collected at the most intense emission line of Tb³⁺ at 545 nm, showed a common broad band with a maximum at 270 nm associated to the ⁷F₆-5d electronic transition of Tb³⁺.^41–43 Both spectra are also composed by another characteristic peak of Tb³⁺, associated to its intra-4f⁸ electronic transitions (⁷F₆–⁵D_J, J = 2,3).^41–43 When monitored at the most intense emission line of Eu³⁺ at 615 nm (Fig. 2Ac), the spectrum of Na-TbEuSAP presented the same bands, previously observed, along with additional peaks ascribed to the intra-4f⁶ transitions of Eu³⁺ (⁷F_0,1–⁵H_J, ⁵D_J, ⁵L_J and ⁵G_J), with a λ_max at 395 nm (⁷F₀–⁵L₆).^44–47 The presence of the Tb³⁺ transitions in the spectrum of Na-TbEuSAP analysed at the emission line of Eu³⁺ is a clear indication of the occurrence of a Tb³⁺ → Eu³⁺ metal-to-metal energy transfer (MMET) process.^48–51 This hypothesis was confirmed by analysing the emission spectra at the solid state of Na-TbEuSAP under irradiation at λ_max of Eu³⁺ (395 nm) and Tb³⁺ (270 nm) (Fig. 2B).

Fig. 2 (A) Excitation spectra at solid state of Na-TbSAP (a, λ_em = 545 nm) and Na-TbEuSAP (b, λ_em = 545 nm, c, λ_em = 615 nm). (B) Emission spectra at solid state of Na-TbSAP (a, λ_exc = 270 nm) and Na-TbEuSAP (b, λ_exc = 395 nm, c, λ_exc = 270 nm); the intensity of Na-TbSAP spectrum was normalized (divided by five) for a better comparison. (C) CIE 1931 xy chromaticity diagrams derived from emission spectra in frame B. The colour of each emission spectrum in frame B is associated to its corresponding xy coordinates (frame C and Table S2†).

Indeed, the emission spectrum collected at 270 nm showed emission peaks of both Tb³⁺ (⁵D₄–⁷F_J, J = 6–3)^41–43 and Eu³⁺ (⁵D₀–⁷F_J, J = 0–4).^45–49 The Tb³⁺ → Eu³⁺ energy transfer photo-enhanced the luminescence of europium, as indicated by the increased intensity of the ⁵D₀–⁷F₂ emission band of Eu³⁺ at 615 nm (compared to the same band obtained after direct excitation at 395 nm) of ca. 90%. This also suggests that Tb³⁺ and Eu³⁺ are probably located in close proximity, thus ensuring an efficient optical communication between them.^48–51

Additional information on the local chemical environment surrounding the lanthanides were extrapolated from the collected emission spectra. An heterogeneous distribution of the Eu³⁺ sites in the Na-TbEuSAP is derived by the presence of two weak peaks for the ⁵D₀–⁷F₀ transition at ca. 560 nm (Fig. 2Bc).⁵² The multiplicity of this band is commonly associated with the number of distinct chemical domains around the europium, since both its initial and final states are non-degenerated.⁵² Moreover, the Eu³⁺ and Tb³⁺ centres in both clays are placed in a low symmetry environment, as suggested by the high values of their asymmetry factor (R > 2) (Table S1 in the ESI†). The R factor was calculated from the intensity ratio of electric-dipole/ED ⁵D₀ → ⁷F₂ on magnetic-dipole/MD ⁵D₀ → ⁷F₁ for Eu³⁺,^{44–47,53,54} and of ED ⁵D₄ → ⁷F₅/MD ⁵D₄ → ⁷F₆ for Tb³⁺.⁵⁵

PL spectra allowed to extrapolate the colorimetric features of the samples, calculating their chromaticity coordinates (xy) and related parameters (RGB and Hex) according to CIE 1931 colour spaces (Fig. 2C and Table S2 in the ESI†).⁵⁶ A modulation of the colour emission was observed for the bi-functional saponite. Indeed, while the direct excitation of Eu³⁺ at 395 nm in Na-TbEuSAP led to an emission in the visible red region, the sample after excitation at 270 nm showed a blue shift towards a final yellow emission. As a comparison, the emission colour of Na-TbSAP is green.

The parameters that describe the Tb³⁺–Eu³⁺ energy transfer process were derived by considering the experimental lifetimes of donor (D, Na-TbSAP) and donor–acceptor (DA, Na-TbEuSAP) systems, measured by time-resolved fluorescence spectroscopy.^{48–51,57–59} The intensity decay curves of ⁵D₄ excited state of Tb³⁺ at λ_em = 545 nm (⁵D₄–⁷F₆) were collected at solid state, under excitation at 370 nm (Fig. S3 in the ESI†). The curves were fitted with a bi-exponential function, from which the average τ values were calculated. The energy transfer rate (k_EnT) and efficiency (E_EnT) parameters were calculated from eqn (1) and (2), using the lifetimes of the donor (Tb³⁺) in the presence (τ_DA) or absence (τ_D) of the acceptor (Eu³⁺), measured at 545 nm:^57–59

The results, reported in Table 1, showed a high energy transfer efficiency of ca. 81%, with a rate constant of 74 100 s⁻¹. Surprisingly, the lifetime decay for Na-TbSAP is very low (5.66 × 10⁻⁵ s). A possible explanation is related to the fact that the first coordination sphere of the metal contains coordinated water molecules, adsorbed or intercalated in the solid material, which essentially determine the site lifetimes.

Table 1 Experimental lifetimes (τ) of Na-TbSAP (D) and Na-TbEuSAP (DA), collected by monitoring the ⁵D₄–⁷F₅ emission transition of Tb³⁺ at 545 nm under irradiation at 370 nm and the Tb³⁺ → Eu³⁺ energy transfer rate (k_EnT) and efficiency (E_EnT)

τD [s]	τDA [s]	kEnT [s^−1]	EEnT [%]
5.66 × 10^−5	1.09 × 10^−5	7.41 × 10^4	80.74

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Finally, the luminescence at solid state of Na-TbSAP and Na-TbEuSAP proved to be stable even after 1 h of irradiation at 270 nm, as demonstrated in photobleaching tests reported in Fig. S4.†

Considering their interesting photophysical properties, the luminescent saponite samples were preliminarily tested for the optical detection of the chromate anion (CrO₄²⁻) in water. For the sensing test, the samples were dispersed in water and then put in contact with different concentrations of CrO₄²⁻, from 0.0001 to 1 mM. Subsequently, for each experimental point, an emission spectrum was collected. The PL spectra were obtained at the two main λ_exc of Tb³⁺ and Eu³⁺ (270 and 395 nm, respectively), monitoring their emission bands at 545 and 615 nm (see in the ESI† for an accurate description of the procedure). The analyses were performed on the aqueous suspensions of Na-TbSAP and Na-TbEuSAP samples under continuous stirring, at room temperature. Prior the sensing tests, the suspensions stability of the saponites dispersed in water were carefully assessed by dynamic light scattering (DLS) analysis, performed at 25 °C (Fig. S5 in the ESI†). The suspensions remained qualitatively homogeneous and stable, with a hydrodynamic diameter of the particles of 16 and 35 nm (polydispersity index (PDI) of 0.71 and 0.65) for Na-TbSAP and Na-TbEuSAP, respectively. Nevertheless, the PDI index calculated for these samples must be carefully considered for two reasons: (i) the light emission properties of these samples can interfere with the laser source used for the analysis; (ii) the samples are composed by lamellae with different aspect ratio. Such particles morphology can influence the quality of the data analysis.

The emission spectra of the aqueous suspensions of Na-TbSAP and Na-TbEuSAP treated with progressive CrO₄²⁻ concentrations are reported in Fig. S6 in the ESI.† The suspensions of the samples analysed were stirred continuously at room temperature during the sensing tests, to maintain a good stability over time. First, the spectra collected before the addition of CrO₄²⁻ anions (black curves (a) in Fig. S6†) showed the typical emission peaks of Tb³⁺ and Eu³⁺,^41–47 as observed in the solid-state analyses reported in Fig. 2B. The spectra of pristine samples in water showed no intensity reduction over time, thus confirming the high suspensions stability in the experimental conditions adopted, without any effect of particles aggregation or deposition on the final optical properties.

Afterwards, the samples were put in contact with the CrO₄²⁻ anion. In the presence of increasing amount of CrO₄²⁻ (from red (b) to blue (c) curves in Fig. S6†), the intensity of the peaks at 545 and 615 nm decreased for both solids, thus suggesting that both metals are probably involved in the interaction with the anion.

The evolution of the normalized PL intensity of the band at 545 nm (λ_exc = 270 nm) as a function of the CrO₄²⁻ concentration is reported in Fig. 3A. For both Na-TbSAP and Na-TbEuSAP samples, an exponential decay of the intensity of the band at 545 nm is observed. In particular, a marked photoluminescence decrease after contacting the samples with a CrO₄²⁻ concentration of 1 μM (= 52 ppb of Cr^(VI), lower than the EPA value of 100 ppb) is visible.^35–38 Indeed, the fluorescence is almost quenched for both samples with a CrO₄²⁻ concentration of 500 μM. In these experimental conditions, the Na-TbEuSAP displayed a PL intensity attenuation at 1 μM of 63% compared to the 15% of Na-TbSAP, implying a more rapid CrO₄²⁻ detection response which could be ascribed to differences in the chemical composition of the samples (i.e. loadings and localization of the metal sites,…).

Fig. 3 (A) Normalized PL intensity of the band at 545 nm in the presence of different CrO₄²⁻ concentrations for Na-TbSAP (a) and Na-TbEuSAP (b). (B) Normalized PL intensity of the 615 nm band as a function of the different CrO₄²⁻ concentrations for Na-TbEuSAP at λ_exc = 270 (a) and 395 nm (b).

Similar results have been obtained by monitoring the normalized PL intensity of the band at 615 nm of Na-TbEuSAP, under excitation at 270 nm and 395 nm (Fig. 3B). The peak intensity quickly decreases with a similar trend under both excitations. Under direct excitation at 395 nm, however, a slightly more rapid decline of the PL intensity was detected.

These preliminary tests demonstrated high sensing capabilities of Tb³⁺/Eu³⁺-saponite clays, with a rapid recognition of CrO₄²⁻ in water at low concentrations due to a marked quenching process, especially for Na-TbEuSAP sample. The sensing performance also proved to be highly modular: indeed, it is possible to easily monitor two distinct emission bands (545 and 615 nm) under two separate irradiations (270 and 395 nm).

The quenching mechanism is probably governed by diffusional phenomena of the CrO₄²⁻ anions between the clay particles. As commonly known, the saponite clays are very efficient cationic exchanger solids.¹ The diffusion of anions inside the interlayer space of clay is unlikely to happen. For these reasons, our hypothesis is that the interaction between the CrO₄²⁻ and the Tb³⁺/Eu³⁺ sites may occur on the galleries entrance or on the particle's surfaces, between the clay tactoids, in accessible framework tetrahedral sites.⁶⁰ The interaction metal/substrate could also be favoured by the swelling of the saponite lamellae in water, which results in more exposed lanthanides centres to surrounding CrO₄²⁻ anions.⁶¹

The quenching efficiency was clarified in more detail with the aid of the Stern–Volmer relationship, which describes the kinetics of photophysical deactivation processes between a quencher molecule (Q) and an excited fluorophore (A*).^62–65 In our case, Q is the CrO₄²⁻ and A* is Eu³⁺ and Tb³⁺ ions. In general, two quenching processes are usually encountered: (1) static or SQE (formation of a Q–A ‘complex’ in the ground-state) and (2) dynamic or DQE (collisional). SQE and DQE are governed by Stern–Volmer equations (eqn (S1) and (S2)), reported in the ESI.†^62–65 By plotting the evolution of F₀/F vs. [Q], where F₀ and F are the fluorescence intensities of the 545 or 615 nm band without and in the presence of the quencher at different concentrations, it was possible to extrapolate the type of quenching process and calculate the relative quenching constants.^62–65 The Stern–Volmer plots reported in Fig. S7 in the ESI† exhibited an upward curvature in all the cases analysed, which is indicative of a quadratic relationship between fluorescence intensity and the anion concentrations, as reported in the literature.⁶³ This non-linear dependence, even at low concentrations, suggest that both static and dynamic quenching are occurring with the same fluorophore, in both clays. Similar behaviour was previously documented in the literature by Lin et al. to explain complex quenching mechanisms occurred for MnO₂ nanosheets towards Au nanocluster fluorescence.⁶³ The phenomenon is governed by a modified Stern–Volmer equation (eqn (S3) in the ESI†), from which (K_D × K_S) and (K_D + K_S) parameters can be derived.⁶³ The combined quenching dynamic (K_D) and static (K_S) constants, obtained by fitting the data in Fig. S7† (Table S3 in the ESI†), indicate that the quenching efficiency is dependent on the excitation wavelength, the monitored emission signal and the presence of both lanthanides. In particular, the quenching seems to be more pronounced for Na-TbEuSAP.

The performance demonstrated for both samples in these preliminary tests are extremely encouraging, and are comparable to those of other luminescent probes based on MOFs mostly containing transitions metals and Eu³⁺,⁶⁶ Tb³⁺-complexes,⁶⁷ coordinated polymers with Zn²⁺ and Tb³⁺,⁶⁸ and carbon quantum dots.⁶⁹

Conclusions

In this study, for the first time two luminescent lanthanides ions (Tb³⁺ and Eu³⁺) were successfully introduced in the framework of synthetic saponite clays through a simple one-pot hydrothermal method. The obtained samples, containing in the structure Tb³⁺ or a simultaneous combination of Tb³⁺ and Eu³⁺ ions, possessed good cation-exchange capacity, layered structure and high thermal stability. The luminescent clay showed interesting photoluminescence features. In particular, the co-presence of both metal centres in the framework gives rise to an efficient Tb³⁺ → Eu³⁺ energy transfer, that greatly enhances the europium luminescence. Moreover, the clay demonstrated a high stoke shift and tuneable emission colours. These excellent photophysical properties were exploited for the optical recognition of chromate anion in water, which is a common environmental pollutant and a source of hexavalent chromium. In the tests, all the luminescent solids demonstrated high photoluminescence sensing capabilities, directly exploiting the intrinsic optical properties of the two lanthanides, with a rapid detection response at very low concentrations (at μM level) due to a complex mixed static/dynamic quenching mechanism involved in the process.

The simple one-pot synthetic method and the attractive physico-chemical properties of these luminescent clays are extremely encouraging for the future development of new metal-containing clay materials with one or more functionalities. The materials prepared in this study could find application in the rational design of effective fluorescent sensors for targets of interest.

The use of photoluminescence spectroscopy for these purposes is expanding and has some important advantages respect to traditional elemental analyses techniques: (i) the sample does not require specific treatments before analysis, (ii) the measurement is fast and easy to perform, (iii) can be applied to different types of matrices. One of the main disadvantages is related to the poor selectivity for the identification of multiple species. This specific topic along with a more comprehensive overview of the sensing properties of the lanthanide-clays (i.e. detection limits and reusability) are currently under investigation.

Conflicts of interest

There are no conflicts to declare.

Acknowledgements

The authors are fully grateful to Mr Marco Barbetta for his help in the experiments and to Dr Elena Perin (DiSIT, Università del Piemonte Orientale, Alessandria, Italy) for ICP-MS analyses. Financial support from the Università del Piemonte Orientale (FAR-2019) is also acknowledged.

Notes and references

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Footnote

† Electronic supplementary information (ESI) available. See DOI: 10.1039/d0ra05693f

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