Ultrasound-Assisted Synthesis of Luminescent Micro- and Nanocrystalline Eu-Based MOFs as Luminescent Probes for Heavy Metal Ions

Abstract

The luminescent coarse-, micro- and nanocrystalline europium(III) terephthalate tetrahydrate (Eu₂bdc₃·4H₂O) metal-organic frameworks were synthesized by the ultrasound-assisted wet-chemical method. Electron micrographs show that the europium(III) terephthalate microparticles are 7 μm long leaf-like plates. According to the dynamic light scattering technique, the average size of the Eu₂bdc₃·4H₂O nanoparticles is equal to about 8 ± 2 nm. Thereby, the reported Eu₂bdc₃·4H₂O nanoparticles are the smallest nanosized rare-earth-based MOF crystals, to the best of our knowledge. The synthesized materials demonstrate red emission due to the ⁵D₀–⁷F_J transitions of Eu³⁺ upon 250 nm excitation into ¹ππ* state of the terephthalate ion. Size reduction results in broadened emission bands, an increase in the non-radiative rate constants and a decrease in both the quantum efficiency of the ⁵D₀ level and Eu³⁺ and the luminescence quantum yields. Cu²⁺, Cr³⁺, and Fe³⁺ ions efficiently and selectively quench the luminescence of nanocrystalline europium(III) terephthalate, which makes it a prospective material for luminescent probes to monitor these ions in waste and drinking water.

1. Introduction

Rare-earth-based metal-organic frameworks (MOFs) are actively used in various fields of science and technology as luminescent sensors [1,2,3,4,5,6,7,8,9,10,11], LED components [12], luminescent probes for bioimaging [13,14], and luminescent thermometers [15,16]. Small-sized crystals of the rare-earth-based MOFs are especially interesting due to their unique properties. Such materials have a large specific surface area, and as a result, they can effectively adsorb other ions and molecules, which is necessary for the development of sensitive luminescent sensors [17,18,19]. The presence of heavy metals in drinking water can cause numerous disorders and diseases of humans and animals [20,21]. Therefore, one must develop new sensors for such pollutants. MOFs are actively used as luminescent and electrochemical sensors for heavy metal ion detection in drinking and wastewater [1,2,3,6,7,10]. Nanosized luminescent MOFs are able to penetrate the cell membrane and are therefore used in bioimaging as luminescent probes [13,14]. The nano-sized rare-earth-based MOFs can be synthesized by several synthetic routes [13,14,22,23,24,25] such as solvothermal, reverse microemulsion, surfactant-assisted, microwave, and ultrasonic methods. The resulting small-sized particles usually have sizes from 40 to 5000 nm.

In our current study, we report the room-temperature ultrasonic-assisted wet chemical method of the synthesis of the small-sized luminescent Eu₂bdc₃·4H₂O MOFs including 8 nm nanoparticles—the smallest nanosized rare-earth-based MOF crystals, to the best of our knowledge. The luminescent properties of the coarse-, micro- and nanocrystalline europium(III) terephthalate are studied. In addition, the selective luminescence quenching by heavy metal ions is also reported.

2. Materials and Methods

2.1. Reagents

Europium chloride hexahydrate was purchased from Chemcraft (Russia). Benzene-1,4-dicarboxylic (terephthalic, H₂bdc) acid (>98%) sodium hydroxide (>99%), polyethylene glycol 6000 (PEG-6000, for synthesis), iron(III) chloride hexahydrate (>99%), iron(II) sulphate heptahydrate (>99%), chromium(III) chloride hexahydrate (>99%), magnesium chloride hexahydrate (>99%), nickel(II) chloride hexahydrate (>99%), lead(II) nitrate (>99%), cobalt(II) chloride hexahydrate (>99%), anhydrous zinc chloride (>98%), cadmium chloride hydrate (>98%), barium chloride dihydrate (>99%), copper(II) chloride dihydrate (>99%), and EDTA disodium salt (0.1M aqueous solution) were purchased from Sigma-Aldrich Pty Ltd. (Germany) and used without additional purification. The 0.2 M solutions of the above-mentioned salts were prepared and standardized by complexometric titration with EDTA. An amount of 0.3 moles of the terephthalic acid and 0.6 moles of the sodium hydroxide were dissolved in the distilled water to obtain 1 L of 0.3 M solution of the disodium terephthalate (Na₂bdc).

2.2. Synthesis

The europium(III) terephthalate was obtained by mixing the EuCl₃ and Na₂bdc solutions. Sample 1 was synthesized by a slow mixing of equal volumes of the 2 mM Na₂bdc and 1 mM EuCl₃ solutions accompanied by vigorous stirring (

Table 1. Overview of the synthesis of europium(III) terephthalates 1–3.

Sample	C(EuCl3)	C(Na2bdc)	PEG-6000	Ultrasonication	Stirring
1	1 mM	2 mM	-	-	+
2	1 mM	2 mM	20%	+	+
3	0.5 mM	1 mM	-	+	+

). Sample 2 was synthesized by a slow mixing of the equal volumes of the 2 mM Na₂bdc solution and the solution containing 1 mM EuCl₃ and 20% PEG-6000, accompanied by ultrasonication (40 kHz, 60 W) and vigorous stirring. The white precipitates of europium(III) terephthalate (Samples 1 and 2) were separated from the reaction mixture by centrifugation (4000× g) and washed with deionized water 5 times. Sample 3 was synthesized by a slow mixing of equal volumes of 1 mM Na₂bdc and 0.5 mM EuCl₃ accompanied by ultrasonication (40 kHz, 60 W) and vigorous stirring. The obtained clear solution was centrifugated at 7500× g; however, no solid was precipitated. The addition of both polar (methanol and acetone) and non-polar solvents (ethanol–dichloromethane mixture) did not result in the salting-out of any solid. Therefore, we used the solution of Sample 3 in the further experiments. All experiments were performed at the temperature of 25 °C.

2.3. Characterization

The morphologies of the microstructures of the synthesized Samples 1 and 2 were characterized using scanning electron microscopy (SEM) with a Zeiss Merlin electron microscope (Zeiss, Germany) equipped with the energy-dispersive X-ray spectroscopy (EDX) module (Oxford Instruments INCAx-act, UK). X-ray powder diffraction (XRD) measurements were performed on a D2 Phaser (Bruker, USA) X-ray diffractometer using Cu Kα radiation (λ = 1.54056 Å). The particle size distribution of the aqueous solution of Sample 3 was revealed by the dynamic light scattering technique with an SZ-100 Series Nanoparticle Analyzer (Horiba Jobin Yvon, Japan) The luminescence spectra were recorded with a Fluorolog-3 fluorescence spectrometer (Horiba Jobin Yvon, Japan). Lifetime measurements were performed with the same spectrometer using a pulsed Xe lamp (pulse duration 3 µs). The absolute values of the photoluminescence quantum yields were recorded using a Fluorolog 3 Quanta-phi device. All measurements were performed at the temperature of 25 °C.

3. Results and Discussion

3.1. Morphology

A scanning electron microscope was used to observe the shape and the size of the particles in the synthesized materials. Sample 1, which was synthesized by a slow mixing of equal volumes of sodium terephthalate (2 mM) and europium chloride (1 mM) aqueous solutions, precipitated in the form of a polycrystalline solid with the average particle size of 120 ± 30 µm (Figure 1). The observed species consisted of smaller particles stacked together forming dendrimer-like microparticle assemblies. The addition of the non-ionic surfactant (PEG-6000) to the reaction mixture and ultrasonication without a change in the Eu³⁺ and bdc²⁻ concentrations (Sample 2) prevented the aggregation of the microparticles and resulted in the formation of individual microparticles (Figure 2a–c). The particles had a leaf-like shape with ratio length:width:height of about 13:5:1. The particles size was obtained from SEM images, the particle size distribution is shown in Figure 2d,e. The average length and width were calculated from these distributions and are equal to 7.1 ± 1.6 and 2.8 ± 0.8 µm, respectively. We found that under ultrasonication the solution remained clear to the eye when the concentration of Eu³⁺ and bdc²⁻ was decreased twofold (1 mM Na₂bdc and 0.5 mM EuCl₃) both in the absence and the presence of the surfactant (PEG-6000). We could not precipitate the solid from the reaction mixture using high-speed centrifugation or by salting-out using organic solvents. Therefore, the formation of the nano-sized particles of europium(III) terephthalate was supposed. In order to exclude the contribution of the PEG micelles to the experimental data, in further experiments we carefully studied the aqueous suspension of Sample 3 obtained by a slow mixing of equal volumes of the 1 mM Na₂bdc and 0.5 mM EuCl₃ accompanied by ultrasonication and vigorous stirring without a PEG-6000 addition. The particle size distribution was revealed by a dynamic light scattering technique, resulting in the average particle size equal to about 8 ± 2 nm (Figure 3). The SEM-EDX study of 3 aggregates formed by drying the reaction mixture on the silicon plate revealed the presence of Eu in the sample but did not determine the particle size due to the insufficient spatial resolution of the used SEM microscope. The direct observation of the species using TEM was also problematic because the high-energy electron radiation (>100 kV) burned out the sample due to the decomposition of an organic linker (terephthalate ion).

In our study, we have found that ultrasonication and PEG-6000 addition significantly decreases the particle size and prevents aggregation. The increase in particle size can be achieved via continuous growth of a particle or gradual aggregation of various particles or seed crystals. The contact of particles can reduce the total surface area in the aggregation process resulting in overall energy reduction. Ultrasonication can encourage surface tension between the species caused by the acoustic radiation force on a compressible particle [26]. The effect of PEG addition on the particle size can be explained by the well-known properties of surfactants including polyethylene glycol to be adsorbed on the particles or seed crystals that decrease their surface energy and prevents aggregation [27,28,29]. Surprisingly, we revealed that the twofold decrease in the reagents’ concentration leads to size reduction for several orders. A recent kinetic study of zinc-2-methylimidazole MOF ZIF-8 [30] reported that nucleation and crystal growth rates non-monotonously depend on the concentration of the reagents. During the low concentrations of the metal ions and the organic linker, the 1:1 M:L complex dominates. This state is called the “pre-equilibrium”. Further nucleation is associative and fast because the central atom has several weakly coordinated solvent molecules and can easily react with other 1:1 complexes resulting in the formation of oligomeric secondary building units (SBUs) [31]. Increasing the concentration of the metal ions and the organic linker leads to the domination of 1:2 and 1:3 M:L complexes. The aggregation of 1:2 and 1:3 M:L complexes into SBUs is slower than 1:1 complexes, which results in slower nucleation. Therefore, nucleation is faster than the growth process in solutions containing low concentrations of the metal ions and the organic linker, which explains the formation of the smaller particle size of the MOFs crystallizing at low concentrations.

3.2. Crystal Structure

The X-ray powder diffraction (XRD) patterns were measured (Figure 4) for Samples 1 and 2 to discover the crystalline phase of the obtained materials. We could not precipitate Sample 3 from the solution; therefore, the XRD pattern of Sample 3 was not measured. Analysis of XRD patterns demonstrated that synthesized materials 1 and 2 are isostructural with the Tb₂bdc₃·4H₂O [32], the typical crystalline phase of lanthanide terephthalates [22], which indicated that materials 1 and 2 were obtained in a form of Eu₂bdc₃·4H₂O. This structure is a three-dimensional metal-organic framework (MOF), where octacoordinated Eu³⁺-ions are bound to the two water molecules and six terephthalate ions through the oxygen atoms (Figure 4). XRD peaks of Eu₂bdc₃·4H₂O in Samples 1 and 2 slightly diverge from their counterparts measured for Tb₂bdc₃·4H₂O reported previously [32]. To compare the structures of Eu₂bdc₃·4H₂O and Tb₂bdc₃·4H₂O materials, the refinement of unit cell parameters was performed for the Eu₂bdc₃·4H₂O samples (

Table 2. Unit cell parameters for Tb₂bdc₃·4H₂O [32] and Eu₂bdc₃·4H₂O (Samples 1 and 2).

Sample	a, Å	b, Å	c, Å	α, deg.	β, deg.	γ, deg.	V, Å3
Tb2bdc3·4H2O	6.14	10.07	10.10	102.25	91.12	101.52	596.63
Eu2bdc3·4H2O (1)	6.20	9.85	10.29	102.15	89.75	105.10	592.91
Eu2bdc3·4H2O (2)	6.16	9.8	10.22	101.84	90.27	104.86	582.19

). One can observe that the structure of coarse-crystalline Eu₂bdc₃·4H₂O (1) is slightly different from that of Tb₂bdc₃·4H₂O. The ionic radius of the octacoordinated Eu³⁺ ion (1.066 Å) is slightly larger than that of the octacoordinated Tb³⁺ ion (1.040 Å) [33], which most likely results in minor differences between Eu₂bdc₃·4H₂O and Tb₂bdc₃·4H₂O structures. The unit cell parameters of microcrystalline Eu₂bdc₃·4H₂O (2) are somewhat different, both from that of coarse-crystalline Eu₂bdc₃·4H₂O (1) and Tb₂bdc₃·4H₂O [32], which is likely caused by the surface defects due to the relatively small particle size of several micrometers.

3.3. Luminescent Properties

Terephthalate ions are known to intensively absorb ultraviolet light, promoting them into the ¹ππ* singlet electronic excited state [32,34,35]. In europium(III) terephthalate, the ¹ππ* state efficiently undergoes the ³ππ* triplet electronic excited state by intersystem crossing due to the heavy atom effect [35] followed by an energy transfer to ⁵D₁ level of the Eu³⁺ ion, due to relatively close energy values of the lowest energy ³ππ* excited state of terephthalate ion [35] (≈20,000 cm⁻¹) and ⁵D₁ level of Eu³⁺ ion [36] (≈19,000 cm⁻¹). ⁵D₁ level of the Eu³⁺ ion [36] then undergoes internal conversion followed by emission corresponding to ⁵D₀–⁷F_J (J = 0–5) transitions. Figure 5a presents the emission spectra of the europium(III) terephthalate series (1–3) upon 250 nm excitation into the ¹ππ* singlet electronic excited state of the terephthalate ion. The emission spectra include narrow lines corresponding to the transitions from excited ⁵D₀ to lower ⁷F_J levels: ⁵D₀–⁷F₀ (578 nm),⁵D₀-–⁷F₁ (590 nm), ⁵D₀–⁷F₂ (615 nm), ⁵D₀–⁷F₃ (649 nm), and ⁵D₀–⁷F₄ (697 nm). The emission spectrum of nanocrystalline 3 also contains spectrally broad band peaking at about 420 nm, which corresponds to the terephthalate phosphorescence [35]. The most prominent transitions in the emission spectra are magnetic dipole ⁵D₀–⁷F₁ and forced electric dipole ⁵D₀–⁷F₂ and ⁵D₀–⁷F₄ transitions. The excitation spectrum (λ_em = 615 nm) of nanocrystalline 3 resembles its UV–Vis absorption spectrum (Figure 5b) consisting of a 250 nm band as well as a 280 nm shoulder corresponding to the transitions into ¹ππ* singlet electronic excited states of the terephthalate ion. One can notice that emission bands corresponding to the f–f transitions of the Eu³⁺ ion significantly broaden with the particle size reduction. Thus, the ⁵D₀–⁷F₂ band of coarse-crystalline 1, microcrystalline 2, and nanocrystalline 3 have full width at half maximum (fwhm) equal to 48, 66, and 238 cm⁻¹, respectively. The smaller particles have larger surface-to-volume ratio and the number of structural defects, which results in a larger dispersion of energies of electronic levels of Eu³⁺ ions caused by the larger non-uniformity of the local environment of europium ions [37,38]. The luminescence decay curves of europium(III) terephthalate (Figure 5c) are fitted by single-exponential functions:

$$I_{lum}\left(t\right)=I_o{e}^{-\frac{t}{{\tau }_f}}$$

(1)

where time constant τ_f corresponds to the observed lifetime of ⁵D₀ level. The observed lifetime of ⁵D₀ level of coarse-crystalline 1, microcrystalline 2, and nanocrystalline 3 were found to be equal to 393 ± 3, 371 ± 4, and 115 ± 2 μs, respectively.

Luminescence decay is affected by the combination of radiative and nonradiative processes. Radiative decay rate is determined by dipole transition strength and local-field correction. Nonradiative processes include multi-phonon relaxation, quenching on impurities (e.g., O-H group of water molecules) and cooperative processes (cross-relaxation, energy migration). Detailed descriptions of these processes were provided in our earlier papers [39,40]. The radiative and nonradiative decay rates of Eu³⁺-doped phosphors can be calculated from the emission spectrum using 4f–4f intensity theory [41]. Magnetic dipole ⁵D₀–⁷F₁ transition probability A_0-1 = A_MD,0∙n₀³ = 14.65∙1.5³ = 49 s⁻¹. A_MD,0 is the spontaneous emission probability of the magnetic dipole ⁵D₀–⁷F₁, 14.65 s⁻¹, and n₀ is the refractive index, 1.5 [34]. Radiative decay rates A_0–λ (λ = 2, 4) of the ⁵D₀–⁷F_λ emission transition can be obtained from this formula:

$$A_{0-\mathsf{\lambda }}=A_{0-1}\frac{{\mathsf{\nu }}_{0-1}}{{\mathsf{\nu }}_{0-\mathsf{\lambda }}}\frac{{\mathrm{I}}_{0-\mathsf{\lambda }}}{{\mathrm{I}}_{0-1}},$$

(2)

where I_0–λ and ν_0–λ are the integral intensity and frequency of the ⁵D₀–⁷F_λ emission transition. The total radiative decay rate, A_r, could be calculated by summing all the A_0–λ radiative decay rates (λ = 1, 2, 4). The total decay rate is reciprocal to the observed lifetime of ⁵D₀ level, shown in Figure 5c, $A_{total}=\frac{1}{{\tau }_f}$, whereas the nonradiative probability can be calculated as: $A_{nr}=A_{total}-A_r$. Quantum efficiency of ⁵D₀ level is $\eta =\frac{A_r}{A_{total}}$. Decay rates and quantum efficiencies of the ⁵D₀ level of europium(III) terephthalates 1–3 are summarized in

Table 3. Radiative (A_r), nonradiative (A_nr) and total (A_total) decay rates, quantum efficiencies (η) of the ⁵D₀ level of europium(III) and Eu³⁺ luminescence quantum yields (Φ) upon excitation into ¹ππ* singlet electronic excited state of terephthalate ion terephthalates 1–3.

Sample	Ar (s−1)	Anr (s−1)	Atotal (s−1)	η (%)	Φ (%)
1	371	2193	2564	14.5	10 ± 1
2	290	2405	2695	10.8	5 ± 1
3	150	8545	8695	1.7	1.5 ± 0.5

.

Analyzing Table 2, one can see the quantum efficiencies of the ⁵D₀ level and Eu³⁺ luminescence quantum yields decrease in series 1–3 simultaneously with particle size, whereas nonradiative decay rate constants increase upon the size reduction. Smaller particles have larger surface-to-volume ratios, resulting in more efficient quenching of the ⁵D₀ level and Eu³⁺ by the water molecules in an aqueous solution [42]. Comparing the quantum efficiencies of the ⁵D₀ level and Eu³⁺ luminescence quantum yields values, one can notice that the η/Φ ratio is equal to 0.5–0.9, which indicates a very efficient energy transfer from initially excited terephthalate chromophore to the ⁵D₀ level of Eu³⁺ ion.

3.4. Sensing Transition Metal Cations

Previous studies demonstrated that the presence of impurities such as ions of transition metals (Fe³⁺, Cu²⁺, Pb²⁺, MnO₄⁻, Cr₂O₇²⁻) [43], and organic compounds (aromatic, nitroaromatic, carbonyl compounds) can significantly quench the luminescence of the Eu-based metal-organic frameworks [1,2,3,4,5,6,7,8,9] making them prospective for the design of luminescent sensors for various pollutants and explosives. To reveal the selectivity of the europium(III) terephthalate MOF luminescence quenching to the various metal cations, 80 μL of aqueous suspensions of coarse-crystalline 1 (C(Eu³⁺) = 8 mM) was mixed with the 100 μL of metal salt solutions (C(Mⁿ⁺) = 100 mM; Mⁿ⁺ = Fe³⁺, Ca²⁺, Ba²⁺, Cr³⁺, Fe²⁺, Mg²⁺, Ni²⁺, Pb²⁺, Cu²⁺, Co²⁺, Zn²⁺, Cd²⁺) or distilled water. After 30 min, the photographs of these solutions under 254 nm illumination were recorded (Figure 6a,b). It was found that the Eu-based red emission faded only in the presence of Fe³⁺, Cr³⁺ and Cu²⁺ ions (Figure 6a) starting from metal ion concentration 10–50 mM (Figure 6b). The emission spectra of aqueous solutions of nanocrystalline 3 (C(Eu³⁺) = 5 μM) in the absence and in the presence of various concentrations of Cu²⁺, Cr³⁺, and Fe³⁺ ions (λ_exc = 250 nm) indicate the quenching of Eu^{3+ 5}D₀–⁷F_λ luminescence by the above-mentioned metal ions (Figure 6c–e). The dependence of the 615 nm emission band intensity on the Cu²⁺, Cr³⁺, and Fe³⁺ concentration is given in Figure 6f. The concentration dependence resembles the step-function, where luminescence intensity sharply falls starting from the certain concentration of metal ion: 1 μM of Cu²⁺ and 30 μM of Cr³⁺ or Fe³⁺. Surprisingly, we revealed that the addition of Fe³⁺ ions resulted in simultaneous quenching for the Eu^{3+ 5}D₀–⁷F_λ luminescence (591, 615, and 697 nm bands) and the terephthalate phosphorescence (420 nm), whereas the addition of Cu²⁺ and Cr³⁺ ions almost failed to reduce the intensity of the terephthalate phosphorescence band at 420 nm (Figures S1–S3, Supplementary Materials). This observation indicates a different quenching mechanism of Eu^{3+ 5}D₀–⁷F_λ luminescence by the above-mentioned metal ions. Most likely, Cu²⁺, Cr³⁺, and Fe³⁺ ions somehow coordinate with the oxygens of terephthalate ligands, but Fe³⁺ ions quench the ³ππ* triplet electronic excited state of terephthalate ion, whereas Cu²⁺ and Cr³⁺ ions quench the ⁵D₀ level of Eu³⁺. To reveal the complete quenching mechanism, one must study the excited-state dynamics of singlet and triplet electronic states of terephthalate ion, as well as the ⁵D₀ level of Eu³⁺, depending on the heavy metal ion concentration by time-resolved transient absorption and luminescence spectroscopy methods. We have found that nanocrystalline europium(III) terephthalate MOF 3 demonstrates significantly lower limits of detection on Cu²⁺, Cr³⁺, and Fe³⁺ ions than coarse-crystalline 1 (10–50 mM for coarse-crystalline 1 vs 1–30 μM for nanocrystalline 3, Figure 6b,f). This observation is explained by a larger surface-to-volume ratio of nanoparticles relatively to the bulk material, resulting in a higher luminescence quenching efficiency of the later materials due to a greater number of coordination sites. The sensitivity of our materials to Cu²⁺, Cr ³⁺ and Fe³⁺ ions is comparable with the best reported luminescent MOF-based sensors reported previously (

Table 4. Limits of detection (LOD) of nanocrystalline europium(III) terephthalate tetrahydrate 3 and previously reported materials for Cu²⁺, Cr ³⁺ and Fe³⁺ ions.

Sensing Material	Method	Target Contaminant	LOD	Ref.
Eu2(bdc)3·4H2O	luminescent	Cu2+	1 μM	Current work
Tb(BTC)(H2O)	luminescent	Cu2+	10 μM	[2]
CDs@Eu-DPA MOFs	luminescent	Cu2+	26.3 nM	[3,43]
[Eu(PDC)1.5(DMF)]·(DMF)0.5(H2O)0.5	luminescent	Cu2+	10 mM	[7,44]
Eu2(FMA)2(OX)(H2O)4·4H2O	luminescent	Cu2+	100 μM	[7,45]
[Eu4(BPT)4(DMF)2(H2O)8]	luminescent	Cu2+	10 μM	[7,46]
[Tb3(L)2(HCOO)(H2O)5]·DMF·4H2O	luminescent	Cu2+	100 μM	[7,47]
[Eu(ox)2(H2O)](Me2NH2)(H2O)3	luminescent	Cu2+	10 μM	[10,48]
Zr6(O)8(OH2)8(tpdc)4	luminescent	Cu2+	1 μM	[10,49]
PCN-222-Pd(II)	luminescent	Cu2+	50 nM	[10,50]
Me2NH2@MOF-1	electrochemical	Cu2+	10 pM	[3,51]
Eu2(bdc)3 nanoparticles	luminescent	Fe3+	30 μM	Current work
[Me2NH2][In(abtc)]·solvents	luminescent	Fe3+	34.5 μM	[52]
[LnK(BPDSDC)(DMF)(H2O)]·x(solvent)	luminescent	Fe3+	10 μM	[10,53]
[Eu(BTPCA)(H2O)]·2DMF·3H2O	luminescent	Fe3+	10 μM	[7,54]
[Eu(HL)(H2O)2]n·2H2O	luminescent	Fe3+	1 μM	[7,55]
EuL	luminescent	Fe3+	100 μM	[7,56]
[H2NMe2]3[Tb(DPA)3]	luminescent	Fe3+	10 μM	[7,57]
Eu (4′-(4-carboxyphenyl)-2,2′: 6′,2″-terpyridine)3	luminescent	Fe3+	100 μM	[7,58]
[H(H2O)8][DyZn4(imdc)4(im)4]	luminescent	Fe3+	1 mM	[7,59]
Eu3+@Ga2(OH)4(C9O6H4)	luminescent	Fe3+	0.28 μM	[7,60]
nTbL	luminescent	Fe3+	10 μM	[7,61]
[Eu(atpt)1.5(phen)(H2O)]n	luminescent	Fe3+	500 μM	[7,62]
[(CH3)2NH2] ·[Tb(bptc)]·xS	luminescent	Fe3+	10 μM	[7,63]
Tb-BTB	luminescent	Fe3+	10 μM	[7,64]
[Eu3(BDC)4.5(H2O)(DMF)2]	luminescent	Fe3+	1 μM	[7,65]
[Cd(L)(BPDC)]·2H2O	luminescent	Fe3+	2 μM	[8,66]
[Cd(L)(SDBA)(H2O)]∙0.5H2O	luminescent	Fe3+	2 μM	[8,66]
[Zn5(hfipbb)4(trz)2(H2O)2]	luminescent	Fe3+	10 μM	[8,67]
[Eu(Hpzbc)2(NO3)]·H2O	luminescent	Fe3+	10 μM	[8,68]
[Eu(L)(H2O)2]·NMP·H2O	luminescent	Fe3+	100 nM	[8,69]
[Tb(L1)1.5(H2O)]·3H2O	luminescent	Fe3+	10 μM	[10,70]
Bisdiene macrocycle	luminescent	Fe3+	0.58 μM	[71]
2-(cyclohexylamino)-3-phenyl-4Hfuro [3,2-c]chromen-4-one	luminescent	Fe3+	1.73 μM	[72]
[Me2NH2][In(abtc)]·solvents	luminescent	Fe3+	34.5 μM	[53]
PPCOT/NiFe2O4/C-SWCNT	electrochemical	Fe3+	100 pM	[73]
Eu2(bdc)3 nanoparticles	luminescent	Cr3+	30 μM	Current work
Tb(BTC)(H2O)	luminescent	Cr3+	10 μM	[2]
[TbK(BPDSDC)(DMF)(H2O)2]			10 μM	[8,74]
[Eu2L3(DMF)3]·2DMF·5H2O	luminescent	Cr3+	75.2 nM	[75]
ATNA deriviative	electrochemical	Cr3+	130 pM	[76]

). Despite the higher sensitivity of electrochemical MOF-based sensors (Table 4), the luminescent sensors can be used for the design of relatively inexpensive express tests on heavy metal ions.

4. Conclusions

In summary, we reported the ultrasound-assisted wet-chemical synthesis and characterization of luminescent coarse-, micro-, and nano-crystalline Eu₂bdc₃·4H₂O MOFs. The particles of coarse-crystalline Eu₂bdc₃·4H₂O, which are synthesized by the mixing of sodium terephthalate and europium chloride aqueous solutions without ultrasound, are dendrimer-like microparticle assemblies with the average particle size of 120 ± 30 µm. The microcrystalline MOFs were prepared by mixing sodium terephthalate and europium chloride aqueous solutions with the addition of PEG-6000 in the presence of ultrasonication. The microparticles have the shape of leaf-like plates and an average size of 7.1 × 2.8 µm. The average size of Eu₂bdc₃·4H₂O nanoparticles, synthesized by the mixing of low-concentration sodium terephthalate and europium chloride aqueous solutions in the presence of ultrasonication, is equal to about 8 ± 2 nm. Thus, the reported Eu₂bdc₃·4H₂O nanoparticles are the smallest nanosized rare-earth-based MOF crystals, to the best of our knowledge. The emission spectra of synthesized materials exhibit narrow lines corresponding to transitions from excited ⁵D₀ to lower ⁷F_J levels of Eu³⁺ ion: ⁵D₀–⁷F₀ (578 nm),⁵D₀–⁷F₁ (590 nm), ⁵D₀–⁷F₂ (615 nm), ⁵D₀–⁷F₃ (649 nm), and ⁵D₀–⁷F₄ (697 nm). Size reduction resulted in a broadening of the emission bands. The Eu³⁺ luminescence quantum yields, upon excitation into ¹ππ* singlet electronic excited state of terephthalate ion, were found to be of 10 ± 1%, 5 ± 1% and 1.5 ± 0.5% for coarse-, micro- and nanocrystalline Eu₂bdc₃·4H₂O MOFs, respectively. The nonradiative decay rate of nanocrystalline europium(III) terephthalate was significantly larger that the corresponding values of Eu₂bdc₃·4H₂O MOFs, which resulted from more efficient quenching of the ⁵D₀ level and Eu³⁺ by the water molecules in aqueous solution due to greater surface-to-volume ratio of nanocrystalline MOF. The Cu²⁺, Cr³⁺, and Fe³⁺ ions efficiently and selectively quench the Eu^{3+ 5}D₀−⁷F_λ luminescence of nanocrystalline Eu₂bdc₃·4H₂O MOFs starting from the relatively low concentrations of metal ion: 1 μM of Cu²⁺ and 30 μM of Cr³⁺ or Fe³⁺. The reported nanocrystalline europium(III) terephthalateis one of the most sensitive luminescent MOF-based sensorfor Cu²⁺, Cr ³⁺ and Fe³⁺ ions (Table 4). Therefore, synthesized nanocrystalline Eu₂bdc₃·4H₂O MOFs can be considered promising luminescent probes for heavy metal ions in waste and drinking water.