Topological control of metal–organic frameworks toward highly sensitive and selective detection of chromate and dichromate

Abstract

Luminescent metal–organic frameworks (LMOFs) have been extensively used as sensitive and selective sensors for carcinogenic chromium(VI) oxyanions. However, the correlation between the MOF structure and the Cr(VI) sensing efficacy remains underexplored, hindering the rational design of MOFs for Cr(VI) sensing. Herein, we overcome this challenge by judiciously selecting the rarely touched thorium cations and a tetraphenylethene-based ligand for building two novel thorium-based MOFs, Th-BCTPE-1 and Th-BCTPE-2. Despite being built from identical precursors, Th-BCTPE-1 shows more sensitive luminescence quenching responses to CrO₄²⁻ and Cr₂O₇²⁻ than Th-BCTPE-2, producing the second highest quenching constants of CrO₄²⁻ among all LMOF-based Cr(VI) sensors. We further decoupled the influences of different structural variables and the corresponding physiochemical properties, including porosity, BET surface area, photoluminescence quantum yield, and adsorption capacity, showing that the sensing efficacy of Cr(VI) oxyanions is more relevant to the adsorption capacity and the degree of the inner filter effect than the rest of the variables in this system.

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Introduction

Chromates and dichromates have been extensively used to improve the corrosion resistance of active metals via the formation of stable chromate conversion coating.¹ Moreover, Cr(VI) oxyanions are essential in advanced nuclear cycles since chromate and dichromate have been used as corrosion inhibitors in cooling water for nuclear reactors.^2,3 Cooling water with an approximate Cr(VI) concentration of 700 μg kg⁻¹ is discharged from the reactors to natural water bodies after cooling in retention basins.⁴ As a consequence, high-level Cr(VI) contamination (ranging up to nearly 300 mg kg⁻¹) has been identified in the groundwater at the Hanford site and the nearby Columbia river, which imposes a potential hazard to human health due to its high carcinogenicity.^4,5 Furthermore, the presence of chromate in high-level waste (HLW) leads to the formation of undesirable spinel crystals during the vitrification of nuclear waste, which increases the anticipated cost and risk of HLW vitrification.⁶ Hence, facile detection and the subsequent effective sequestration of chromate and dichromate from contaminated water and HLW are highly indispensable.^7–11 The concentration of Cr(VI) solution can be determined colorimetrically by the diphenylcarbazide method.⁴ However, this technology suffers from low sensitivity (limit of detection of 0.5 mg kg⁻¹). Instrumental techniques including inductively coupled plasma atomic emission spectroscopy (ICP-AES) and inductively coupled plasma mass spectrometry (ICP-MS) are advantageous in terms of limit of detection (LOD)¹² but they are unable to distinguish the toxic Cr(VI) from the benign Cr(III) ion, the latter of which is the dominant natural background species of chromium in sediments.

As a class of porous inorganic–organic hybrid materials, metal–organic frameworks (MOFs) are scientifically compelling and functionally evolving with considerable advances made in various fields including separation, sensing, and catalysis.^13–18 Incorporating luminescent metal nodes and/or organic linkers within MOFs engenders the sensing capacities of MOFs in a turn-off, turn-on, or fluorochromic manner, making them suitable for chromate/dichromate sensing.^19–22 Furthermore, the highly tunable compositions, structures, and pore functionalities of MOFs give rise to a series of chromate/dichromate sensors with high sensitivity and excellent selectivity.^23–28 Although significant progress in the development of MOFs for Cr(VI) detection has been achieved, understanding the relationship between the MOF structure and the sensing efficacy remains in its infancy, which hinders the rational design of MOFs for Cr(VI) sensing. The study by Sun and coworkers indicated that modifying the pore shape and volume of MOFs via altering the linker shape have a notable influence on the detection sensitivity of dichromate.²⁵ Moreover, a mixed-ligand strategy gave rise to three cadmium coordination polymers, of which the one-dimensional polymer exhibited the best sensing performance.²⁹ However, both the organic linkers and the topologies of MOFs are changed in these systems, rendering the decoupling of the influence of each variable on the luminescence properties rather difficult. Therefore, an outstanding challenge in obtaining the structure–property correlation is to alter the underlying topologies of MOFs without introducing additional variables.

We have recently undertaken a study on the control of the polymorphism of MOFs by utilizing thorium-based metal–organic frameworks (Th-MOFs) as a platform.³⁰ One of the key merits of Th-MOFs is that the appropriate “hardness” of Th⁴⁺ can afford different secondary building units (SBUs) with a wide range of nuclearity and correspondingly diverse topologies.^31–35 Besides, the Th⁴⁺ cation typically adopts a nine-coordinate capped square antiprism geometry, which provides an additional coordinating site on its capping addenda for creating new structures.^36–39 This can be exemplified by a series of Th-MOFs that feature five unique topologies even though they are built from the same 2′-methyl-[1,1′:4′,1′′-terphenyl]-4,4′′-dicarboxylate (MeTPDC²) ligand.³⁸ Such topological polymorphism of Th-MOFs facilitates a better understanding of how the structure of MOF, in isolation, influences the sensing efficacy of Cr(VI) oxyanions.

Herein, we showcase a synthetic modulation approach to obtain two topologically distinct Th-MOFs, [Th₆(OH)₄(O)₄(H₂O)₆](BCTPE)₆·(DMF)₁₈·(H₂O)₉ (Th-BCTPE-1) and [Th₆(OH)₄(O)₄(H₂O)₆](BCTPE)₅(HCOO)₂·(DMF)₂₆·(H₂O)₃₂ (Th-BCTPE-2), built from the same (E)-4,4′-(1,2-diphenylethene-1,2-diyl)dibenzoate linker (BCTPE²⁻). As a π-electron-rich dicarboxylate ligand, H₂BCTPE was used as a fluorophore to engender the luminometric sensing capacities of Th-BCTPE-1 and Th-BCTPE-2 for chromate and dichromate. Notably, the UiO-type MOF Th-BCTPE-1 features greater quenching constants (K_SV) and a lower limit of detection (LOD) than Th-BCTPE-2 for both chromate and dichromate. The polymorphism of these Th-MOFs and the corresponding different sensing efficacies show the profound effect of structure on Cr(VI) sensing and shed light on the future rational design of highly efficient Cr(VI) sensors.

Results and discussion

By modifying the stoichiometry, modulator, and reaction temperature, solvothermal reactions between the identical precursors of Th(NO₃)₄·6H₂O and (E)-4,4′-(1,2-diphenylethene-1,2-diyl)dibenzoic acid (H₂BCTPE) allow for the isolation of two different Th-MOFs, Th-BCTPE-1 and Th-BCTPE-2. When trifluoroacetic acid was selected as a modulator, a reaction at 80 °C gave rise to octahedral single-crystals of Th-BCTPE-1 (Fig. S1a†). Incorporating nitric acid in the synthesis at 120 °C resulted in the crystallization of prismatic crystals of Th-BCTPE-2 (Fig. S1b†).

Single-crystal X-ray diffraction (SCXRD) study revealed that Th-BCTPE-1 adopts a typical UiO type structure with the cubic Fm3̄m space group and fcu topology (Table S1†). The asymmetric unit of its framework is composed of one crystallographically unique 1/8 Th⁴⁺, 1/6 μ₃-OH/μ₃-O, 1/8 coordinating H₂O (1/48 [Th₆(μ₃-OH)₄(μ₃-O)₄(H₂O)₆]¹²⁺), and 1/8 BCTPE²⁻. As shown in Fig. 1a, the secondary building unit (SBU) of Th-BCTPE-1 is built from six Th cations interconnected by four μ₃-OH and four μ₃-O groups and further decorated by six H₂O molecules. The afforded [Th₆(μ₃-OH)₄(μ₃-O)₄(H₂O)₆]¹²⁺ cluster is bridged by twelve BCTPE²⁻ dicarboxylate linkers with the neighbouring clusters, generating a 12-connected SBU. Two types of voids, namely an octahedral (9.0 Å) and a tetrahedral (6.2 Å) void, were identified. These cavities are occupied by disordered DMF and H₂O guest species, accounting for a total volume of 34.1% as estimated by PLATON.⁴⁰

Fig. 1 (a) The 12-connected Th₆(μ₃-OH)₄(μ₃-O)₄(H₂O)₆(R-COO)₁₂ SBU, 3D structure, and simplified topology of Th-BCTPE-1. (b) The 10-connected Th₆(μ₃-OH)₄(μ₃-O)₄(H₂O)₆(HCOO)₂(R-COO)₁₀ SBU, 3D structure, and simplified topology of Th-BCTPE-2. Colour code: Th atoms are shown in blue, O atoms are in red, C atoms from the BCTPE²⁻ ligands are in grey, and O atoms from the HCOO⁻ ligand are in light yellow.

Th-BCTPE-2 crystallizes in the P4₂/mmc space group and its asymmetric unit is assembled from one 1/8 Th₆(μ₃-OH)₄(μ₃-O)₄(H₂O)₆(HCOO)₂ cluster, one 1/2 and one 1/8 BCTPE²⁻ anion. In sharp contrast to Th-BCTPE-1, the SBU of Th-BCTPE-2 is assembled from a [Th₆(μ₃-OH)₄(μ₃-O)₄(H₂O)₆]¹²⁺ core chelated with ten BCTPE²⁻ ligands and two HCOO⁻ anions (Fig. 1b). It is worth noting that HCOO⁻ anions originated from the hydrolyzation of DMF molecules in the presence of nitric acid with extended reaction time. Unlike the bitopic BCTPE²⁻ ligands that interconnect the neighbouring Th₆ SBUs, the HCOO⁻ groups in Th-BCTPE-2 terminate the bridging of SBUs. Thus, each Th₆ SBU is connected to ten neighbouring ones, giving a final three-dimensional porous network with the bcu topology. Such a structure renders open channels of approximately 8.1 Å × 8.1 Å, that result in a much higher solvent-accessible volume of 64.7% than that of Th-BCTPE-1.

Th-BCTPE-1 and Th-BCTPE-2 can be obtained as pure phases, as confirmed by the similar powder X-ray diffraction (PXRD) patterns of the as-synthesized samples and the simulated ones (Fig. S2†). The permanent porosities of these Th-MOFs were examined by N₂ adsorption/desorption isotherms measured at 77 K. Both Th-BCTPE-1 and Th-BCTPE-2 exhibited typical type I isotherms with saturated uptakes of 69.8 and 115.4 cm³ g⁻¹, respectively, corresponding to Brunauer–Emmett–Teller (BET) surface areas of 188 and 316 m² g⁻¹ (Fig. 2). This trend matches well with the tendency of void volumes obtained from PLATON. The desorption branches of both materials feature slight hysteresis loops, which are characteristic of moderate interactions between N₂ and the frameworks. Thermogravimetric analysis (TGA) study revealed that Th-BCTPE-1 and Th-BCTPE-2 are stable up to 490 and 510 °C, respectively (Fig. S3†). Both Th-MOFs undergo three stages of weight loss, and the first and second stages correspond to the departure of guest species including DMF and H₂O. Th-BCTPE-2 features a greater amount of solvent loss (w% = 39.9%) due to its larger void space than that of Th-BCTPE-1.

Fig. 2 N₂ adsorption/desorption isotherms of Th-BCTPE-1 and Th-BCTPE-2 at 77 K.

Despite the tremendous effort focused on the aggregation-induced emission (AIE) of tetraphenylethene, there has been only one MOF report to date integrating the BCTPE²⁻ ligand, a dicarboxylate derivative of TPE.⁴¹ The luminescence properties of Th-BCTPE-1 and Th-BCTPE-2 were initially investigated by collecting their emission spectra. Upon 365 nm UV excitation, Th-BCTPE-1 and Th-BCTPE-2 exhibit green luminescence with emission bands centered at 495 and 500 nm, respectively, which are red-shifted in comparison with the blue emission (λ_max = 467 nm) of the pure H₂BCTPE ligand (Fig. S4†). The luminescence of these Th-MOFs can be ascribed to the matrix coordination-induced emission (MCIE), whereas that of H₂BCTPE could result from the AIE effect.⁴² Moreover, the photoluminescence quantum yields (PLQYs) under ambient conditions were measured to be 50.53% and 53.07% for Th-BCTPE-1 and Th-BCTPE-2, respectively, both of which are higher than 39.19% for H₂BCTPE. This observation indicates that the ordered and spatial anchoring of fluorescent BCTPE²⁻ moieties inside rigid Th-MOFs can efficiently eliminate possible dissipative processes and turn on fluorescence.^43–45 In addition, Th-BCTPE-1, Th-BCTPE-2 and H₂BCTPE show bi-exponential luminescence decays with average lifetimes of 3.21, 2.45, and 2.87 ns, respectively (Fig. S5†).

Given the intense emissions and high PLQYs of both materials, we sought to investigate the potential utility of Th-BCTPE-1 and Th-BCTPE-2 for chromate and dichromate sensing. Moreover, their variety and multiplicity in terms of structures, void spaces, and surface areas make them ideal candidates for elucidating the correlations of the MOF structure with the Cr(VI) sensing efficacy. In this regard, equivalent amounts (0.16 μmol) of Th-BCTPE-1 or Th-BCTPE-2 were dispersed in 2 mL K₂CrO₄ or K₂Cr₂O₇ solution with increasing concentrations. Both Th-BCTPE-1 and Th-BCTPE-2 were finely ground to make the materials homogeneously dispersed in Cr(VI) solutions and to eliminate the effect of morphology difference. As shown in Fig. 3, the emissions of both Th-MOFs were gradually quenched upon increasing the addition of CrO₄²⁻ or Cr₂O₇²⁻ and differences in the sensing efficacy of CrO₄²⁻ can be resolved between the two Th-MOFs. Specifically, the addition of 50 μM CrO₄²⁻ resulted in 88.1% and 58.0% decreases in the luminescence intensities of Th-BCTPE-1 and Th-BCTPE-2, respectively, suggesting that Th-BCTPE-2 is more robust against quenching than Th-BCTPE-1 (Fig. 4a). To gain deeper insight into the quenching behaviours, the quenching constants (K_SV) in the lower concentration range (0.1–1 μM) were derived from the Stern–Volmer (S–V) plots, I₀/I = 1 + K_SV[Q] − 1, where [Q] is the concentration of the quencher, and I₀ and I are emission intensities before and after adding quenchers, respectively.⁴⁶ As shown in Fig. 4b, the S–V plots for Th-BCTPE-1 and Th-BCTPE-2 are linear in the range of 0.1–1 μM and downward deviations from the linear dependence occur at higher concentrations for both materials. The K_SV values were calculated to be 2.4(1) × 10⁵ and 1.30(7) × 10⁵ M⁻¹ for Th-BCTPE-1 and Th-BCTPE-2, respectively, which is indicative of the higher sensing efficacy of Th-BCTPE-1 than that of Th-BCTPE-2. Notably, the K_SV of Th-BCTPE-1 for CrO₄²⁻ is the second highest among all MOF-based chemosensors (Table S2†).⁴⁷ The limits of detection (LODs) were calculated to be 9.0 and 159 nM (3σ/k) for Th-BCTPE-1 and Th-BCTPE-2, respectively, which agree well with that of K_SV (Table S3†).⁴⁸

Fig. 3 Normalized luminescence spectra of (a) Th-BCTPE-1 as a function of increasing CrO₄²⁻ concentration, (b) Th-BCTPE-2 as a function of increasing CrO₄²⁻ concentration, (c) Th-BCTPE-1 as a function of increasing Cr₂O₇²⁻ concentration, and (d) Th-BCTPE-2 as a function of increasing Cr₂O₇²⁻ concentration.

Fig. 4 (a) The quenching rates of Th-BCTPE-1 and Th-BCTPE-2 as a function of CrO₄²⁻ concentration. (b) Stern–Volmer plots of luminescence quenching of Th-BCTPE-1 and Th-BCTPE-2 in CrO₄²⁻ solution. (c) The quenching rates of Th-BCTPE-1 and Th-BCTPE-2 as a function of Cr₂O₇²⁻ concentration. (d) Stern–Volmer plots of luminescence quenching of Th-BCTPE-1 and Th-BCTPE-2 in Cr₂O₇²⁻ solution.

Aligned with the CrO₄²⁻ sensing, the sensing capacities of Th-BCTPE-1 and Th-BCTPE-2 for Cr₂O₇²⁻ were also investigated, showing that their difference in Cr₂O₇²⁻ sensing efficacy is relatively trivial in the concentration range of 0–50 μM (Fig. 4c). However, the quenching rate of Th-BCTPE-1 is much faster than that of Th-BCTPE-2 in a lower concentration range as shown in (Fig. S6†), suggesting that Th-BCTPE-2 is more quenching-resistant to Cr₂O₇²⁻ than Th-BCTPE-1. In a similar vein, the quenching constants for Cr₂O₇²⁻ were acquired from the S–V plots, showing a higher K_SV (4.6(3) × 10⁵ M⁻¹) of Th-BCTPE-1 than 2.222(9) × 10⁵ M⁻¹ of Th-BCTPE-2. Th-BCTPE-1 again exhibited a higher sensing efficacy for Cr₂O₇²⁻ than Th-BCTPE-2 (Fig. 4d). Moreover, the LODs for Cr₂O₇²⁻ were derived from the linear regression analysis and calculated to be 4.6 and 94 nM for Th-BCTPE-1 and Th-BCTPE-2, respectively, whose trend agrees well with those for CrO₄²⁻.

Besides the quenching efficacies, the adsorption capacities of Th-BCTPE-1 and Th-BCTPE-2 for Cr(VI) oxyanions were also comparatively investigated. Scanning electron microscopy and energy dispersive X-ray spectroscopy (SEM-EDS) revealed that Cr was homogeneously dispersed on the surface of CrO₄²⁻- and Cr₂O₇²⁻-soaked crystals (Fig. S7†). Moreover, the sorption isotherms indicated that both Th-BCTPE-1 and Th-BCTPE-2 can capture CrO₄²⁻ and Cr₂O₇²⁻ anions. The uptake of anionic CrO₄²⁻ and Cr₂O₇²⁻ by neutral Th-BCTPE-1 and Th-BCTPE-2 could be largely attributed to the electrostatic interactions between cationic SBUs and anions.^38,49,50 The sorption isotherms can be better fitted with the Langmuir model than the Freundlich model (Table S4†). Notably, Th-BCTPE-1 featured higher maximum uptake capacities of CrO₄²⁻ and Cr₂O₇²⁻ (1.58 and 1.05 mol mol⁻¹) than those of Th-BCTPE-2 (1.22 and 0.93 mol mol⁻¹) (Fig. 5a and b). This observation appears to be counterintuitive at first glance since Th-BCTPE-2 has a greater void volume and a larger BET surface area than those of Th-BCTPE-1. However, a recent study by us revealed that the high porosity of MOFs cannot guarantee high adsorption capacity.⁵¹ Since Th-BCTPE-1 incorporates fewer solvent species, its open void is more accessible than Th-BCTPE-2, which renders Th-BCTPE-1 a more efficient absorbent for both CrO₄²⁻ and Cr₂O₇²⁻.

Fig. 5 Adsorption isotherms of Th-BCTPE-1 and Th-BCTPE-2 for (a) CrO₄²⁻ and (b) Cr₂O₇²⁻ fitted with the Langmuir model (solid line) and the Freundlich model (dashed line).

Elucidating the correlation between the structure of MOF and the sensing efficacy of Cr(VI) oxyanions is essential to the rational design of new chemosensors.^52–54 Therefore, structural and photophysical parameters, including void volumes, BET surface areas, PLQYs, K_SV values, and LODs, as well as the maximum adsorption capacities, are summarized in Table 1. As seen, the porosity, surface area, and PLQY are not necessarily relevant to the sensing efficacy. In contrast, the trend of maximum adsorption capacity is in line with that of K_SV, implying that the sensing efficacy could be more strongly associated with Cr(VI) adsorption. Indeed, additional insights into the correlation between the sensing efficacy and sorption capacity can be derived from the S–V plots extending to higher a concentration range (Fig. S8†). For both CrO₄²⁻ and Cr₂O₇²⁻ sensing, Th-BCTPE-1 showed saturation plateaux at higher concentrations than Th-BCTPE-2. These observations conjointly suggest that the luminophores of Th-BCTPE-1 are more accessible to CrO₄²⁻ and Cr₂O₇²⁻ than those of Th-BCTPE-2, which could be largely ascribed to the higher adsorption capacities of Th-BCTPE-1 for Cr(VI) oxyanions. In addition, a UV-vis absorption spectroscopy study revealed that the spectra of K₂CrO₄ and K₂Cr₂O₇ exhibit two intense absorption peaks at 230–310 nm and 310–450 nm, both of which overlap with the excitation spectra of Th-BCTPE-1 and Th-BCTPE-2 (Fig. S9†). Consequently, the emission intensity reduction and eventual luminescence quenching upon titration of chromium oxyanions can be attributed to a strong inner filter effect.^55–57 More importantly, the overlap efficacy between the excitation spectrum of Th-BCTPE-1 and the absorption spectrum of Cr(VI) is greater than that of Th-BCTPE-2, which makes Th-BCTPE-1 as a more sensitive sensor for Cr(VI) detections.

Table 1 The void volumes, BET surface areas, photoluminescence quantum yields, K_SV values, LODs, and maximum adsorption capacities of Th-BCTPE-1 and Th-BCTPE-2

Products	Void (%)	BET surface area (m^2 g^−1)	PLQY (%)	CrO4^2−			Cr2O7^2−
				K SV (M^−1)	LOD (nM)	Q max (mol mol^−1)	K SV (M^−1)	LOD (nM)	Q max (mol mol^−1)
Th-BCTPE-1	34.1	188	50.53	2.4(1) × 10^5	9.0	1.58	4.6(3) × 10^5	4.6	1.05
Th-BCTPE-2	64.7	316	53.07	1.30(7) × 10^5	159	1.22	2.222(9) × 10^5	94	0.93

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The high sensing efficacy, low LOD, and excellent stability of Th-BCTPE-1 make it well-suited for the practical application of Cr(VI) sensing (Fig. S10†). To evaluate its potential applicability to real-world conditions, the interfering effects from the environmentally relevant cations and anions were examined by dispersing finely ground Th-BCTPE-1 in M(NO₃)_x (M = Al³⁺, Cr³⁺, Mg²⁺, Ca²⁺, Sr²⁺, Cu²⁺, Zn²⁺, Mn²⁺, Ni²⁺, or K⁺, 2 mM) and Na_xL solutions (L = F⁻, Cl⁻, Br⁻, I⁻, IO₃⁻, NO₃⁻, HCO₃⁻, SO₄²⁻, BO₃³⁻, Cr₂O₇²⁻, and CrO₄²⁻, 2 mM) (Fig. 6a). As shown in Fig. 6b, the luminescence of Th-BCTPE-1 was completely quenched by CrO₄²⁻ and Cr₂O₇²⁻ with a ratio of 99.84% and 99.91%, respectively. In contrast, minor to moderate emission reductions (−3.52% to 23.62%) occurred upon immersing Th-BCTPE-1 in interfering cation or anion solutions. These results are testaments to the excellent sensing selectivity of Th-BCTPE-1 over the environmentally relevant cationic and anionic species.⁵⁸ To achieve the facile and on-site sensing of the Cr(VI) oxyanion, we further fabricated the MOF-based polyvinylidene fluoride (PVDF) membrane by dispersing finely ground Th-BCTPE-1 in PVDF.^59,60 As shown in Fig. 6c, the as-prepared Th-BCTPE-1@PVDF membrane shows green luminescence under a 365 nm UV lamp.

Fig. 6 (a) Photographs of Th-BCTPE-1 immersed in different cation and anion solutions under UV irradiation. (b) Photoluminescence spectra of Th-BCTPE-1 immersed in different cation and anion solutions. (c) Photographs of a Th-BCTPE-1@PVDF membrane showing its excellent reusability.

Upon applying CrO₄²⁻ or Cr₂O₇²⁻ solution, a dark spot was observed and its darkness increased with the increasing Cr(VI) concentration. More strikingly, the quenched luminescence of Th-BCTPE-1@PVDF can be recovered by simply rinsing the membrane with DI water, indicating the excellent reusability of Th-BCTPE-1@PVDF.

Conclusions

In summary, LMOFs have been considered a promising material platform for sensing carcinogenic chromate and dichromate. Therefore, unravelling the correlation between the MOF structure and the sensing efficacy is essential to the rational design of high-performance MOF-based sensors. In this work, a simple synthetic modulation approach has given rise to two luminescent Th-MOFs, Th-BCTPE-1 and Th-BCTPE-2, which are built from similar hexanuclear SBUs and identical BCTPE²⁻ linkers. Such polymorphism of Th-MOFs in terms of topology allows for elucidating how the structure of MOF, in isolation, influences the sensing efficacy of Cr(VI) oxyanions. Indeed, we find that the UiO-type MOF Th-BCTPE-1 has greater K_SV and lower LOD for both CrO₄²⁻ and Cr₂O₇²⁻ than the more porous Th-BCTPE-2. Th-BCTPE-1 features the second highest K_SV of CrO₄²⁻ (2.84(4) × 10⁵) among all MOF-based Cr(VI) sensors. Moreover, the sensing efficacies of CrO₄²⁻ and Cr₂O₇²⁻ have proved to be positively correlated with the adsorption capacity and degree of the inner filter effect; the higher ones for Th-BCTPE-1 render its luminophores more accessible to Cr(VI) oxyanions. In contrast, other physiochemical properties, including porosity, BET surface area, and photoluminescence quantum yield, appear to play a much less significant role in determining the sensing efficacy. In addition, Th-BCTPE-1 exhibits excellent sensing selectivity for CrO₄²⁻ and Cr₂O₇²⁻, which is a key merit for the deployment of LMOFs in environmentally related conditions.

Experimental section

Synthesis

Th-BCTPE-1 . A mixture of Th(NO₃)₄·6H₂O (4.7 mg, 0.008 mmol), 4,4′-(1,2-diphenylethene-1,2-diyl)dibenzoic acid (H₂BCTPE, 1.7 mg, 0.004 mmol), DMF (0.308 mL), MeOH (0.2 mL), and trifluoroacetic acid (0.04 mL) in a capped vial was heated at 80 °C for 48 h. Colourless octahedral crystals were obtained, filtered, washed with MeOH and Et₂O, and dried at room temperature. The pH values before and after crystallization of Th-BCTPE-1 were 0.91 and 5.71, respectively. Yield: 47% based on H₂BCTPE. Anal. calcd for [Th₆(μ₃-OH)₄(μ₃-O)₄(BCTPE)₆(H₂O)₆]·(C₃H₇NO)₁₈·(H₂O)₉, C₂₂₂H₂₆₈N₁₈Th₆O₆₅, C, 47.44%; H, 4.81%; N, 4.49%. Found: C, 47.08%; H, 4.48%; N, 4.77%.

Th-BCTPE-2 . A mixture of Th(NO₃)₄·6H₂O (4.7 mg, 0.008 mmol), 4,4′-(1,2-diphenylethene-1,2-diyl)dibenzoic acid (H₂BCTPE, 0.9 mg, 0.002 mmol), DMF (0.879 mL), and concentrated nitric acid (0.025 mL) in a capped vial was heated at 120 °C for 96 h. Colourless needle crystals were obtained, filtered, washed with MeOH and Et₂O, and dried at room temperature. The pH values before and after the crystallization of Th-BCTPE-2 were 1.12 and 5.33, respectively. Yield: 44% based on H₂BCTPE. Anal. calcd for [Th₆(μ₃-OH)₄(μ₃-O)₄(BCTPE)₅(HCOO)₂(H₂O)₆]·(C₃H₇NO)₂₆·(H₂O)₃₂, C₂₂₀H₃₅₄N₂₆Th₆O₉₆, C, 42.00%; H, 5.67%; N, 5.79%. Found: C, 41.51%; H, 5.33%; N, 5.99%.

Characterization

Single-crystal X-ray diffraction data were collected on a Bruker D8-Venture single-crystal X-ray diffractometer equipped with an IμS 3.0 microfocus X-ray source (Mo-Kα radiation, λ = 0.71073 Å) and a CMOS detector. The data frames were collected using the APEX3 program and processed using the SAINT routine. The empirical absorption correction was applied using the SADABS program.⁶¹ The structure was solved by intrinsic phasing with ShelXT and refined with ShelXL using OLEX2.^62–64 All the non-H atoms were subjected to anisotropic refinement by a full-matrix program. Contributions to scattering due to these highly disordered solvent molecules were removed using the SQUEEZE routine of PLATON.⁴⁰ Structures were then refined again using the data generated. Crystal data and details of the data collection are given in Table S1.† Powder X-ray diffraction (PXRD) data were collected from 2 to 40° with a step of 0.02° on a Bruker D8 Advance diffractometer with Cu Kα radiation (λ = 1.54178 Å). The calculated PXRD pattern was produced from the CIFs using the Mercury 1.4.2 program. The N₂ adsorption isotherms were obtained at 77 K by using a Micromeritics ASAP 2020. The freshly prepared Th-MOFs were soaked in 20 mL of DCM three times over 1 h (20 min each) and subsequently immersed in 20 mL n-hexane three times over 1 h (20 min each). The excitation and luminescence spectra were collected on an Edinburgh Instruments FS5 fluorescence spectrometer. The photoluminescence lifetimes and quantum yields were obtained on an Edinburgh Instruments FLS 980 spectrometer. Scanning electron microscopy (SEM) images and energy-dispersive spectroscopy (EDS) data were collected on a Zeiss Merlin Compact LEO 1530 VP scanning electron microscope with the energy of the electron beam being 15 kV. Thermalgravimetric analysis (TGA) was carried out on a NETZSCH STA 449 F3 Jupiter instrument in the range of 30–800 °C under a nitrogen flow at a heating rate of 10 °C min⁻¹.

Author contributions

J. L. conceived and designed the research. Z. J. L., Y. J., and X. L. synthesized the materials and performed the luminescence study. Z. J. L. and Y. L. solved the crystal structures. X. L. W. performed the BET study. J. L., Z. J. L., J. Q., J. Q. W., Z. H. Z., M. Y. H., and L. Z. analysed the data and wrote the manuscript. All authors have given approval to the manuscript.

Conflicts of interest

The authors declare no competing financial interests.

Acknowledgements

This work was supported by the National Natural Science Foundation of China (22076196, U22B20139, 21906163, and 21876182), the Strategic Priority Research Program of the Chinese Academy of Sciences (XDA21000000), and the K. C. Wong Education Foundation (GJTD-2018-10).

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Footnote

† Electronic supplementary information (ESI) available: PXRD, TGA, excitation and PL spectra, photoluminescence lifetimes, quenching rates, SEM-EDS, and crystallographic data. CCDC 2213561 and 2213562. For ESI and crystallographic data in CIF or other electronic format see DOI: https://doi.org/10.1039/d2qi02631g

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