Water-Robust Zinc-Organic Framework with Mixed Nodes and Its Handy Mixed-Matrix Membrane for Highly Effective Luminescent Detection of Fe3+, CrO42-, and Cr2O72-in Aqueous Solution

Bowen Qin, Xiaoying Zhang,* Jingjing Qiu, Godefroid Gahungu, Haiyan Yuan,* and Jingping Zhang*

Article

Water-Robust Zinc−Organic Framework with Mixed Nodes and Its

Handy Mixed-Matrix Membrane for Highly Eﬀective Luminescent

Detection of Fe³⁺, CrO₄, and Cr₂O₇in Aqueous Solution

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ABSTRACT: Metal−organic frameworks (MOFs) and MOF-

based composites as luminescent sensors with excellent economic

practicability and handy operability have attracted much attention.

Herein, we designed and fabricated a porous Zn-based MOF,

[Zn(OBA)₂(L₁)·2DMA]_n[1; H₂OBA = 4,4′-oxybis(benzoic acid),

L₁= 2,4,6-tris(4-pyridyl)pyridine, and DMA = N,N-dimethylace-

tamide], with mixed nodes under solvothermal conditions, and the

pore size of 5.9 Å was calculated from N₂adsorption isotherms by

using a density functional theory model. The as-synthesized

compound 1 is stable in diﬀerent boiling organic solvents and

water solutions with a wide pH range of 2−12 and exhibits intense

luminescence emission at 360 nm under excitation of 290 nm.

Signiﬁcantly, compound 1 shows high selective detection of Fe³⁺, CrO₄²⁻, and Cr₂O₇in aqueous solution even under the

interference of other ions. Compound 1 can quickly sense these ions in a short time and has a striking sensitivity toward Fe³⁺with an

ultralow limit of detection (LOD) of 1.06 μM. The relatively low LODs for CrO₄²⁻and Cr₂O₇²⁻are 3.87 and 2.37 μM, respectively,

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compared to the reported works. Meanwhile, compound 1 can be reused to detect Fe³⁺, CrO₄²⁻, and Cr₂O₇six times by simple

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regeneration. Considering the practicability, a mixed-matrix membrane (MMM) incorporated compound 1 and poly(methyl

methacrylate) has been constructed. This MMM displays quick detection of Fe³⁺, CrO₄²⁻, and Cr₂O₇²⁻and prompt regeneration by

lifting from the analyte. This useful MMM shows a comparable LOD below 4.35 μM for these ions. This work presents a cost-

eﬀective Zn-based MOF as a functional platform for simple but useful sensing of Fe³⁺, CrO₄²⁻, and Cr₂O₇in aqueous solution.

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imperative and challenging to design and synthesize stable

MOFs retaining their integrities in diﬀerent pH aqueous

solutions.²⁵Up to now, several traditional detection methods

such as atomic absorption spectrometry, inductively coupled

plasma atomic emission spectrometry (ICP-AES), and ion-

mobility spectrometry have been available for probing these

ions but are time-consuming and of high cost.^24,26For MOF-

based sensors, complex synthesis methods and expensive raw

materials including organic linkers and metal sources also limit

their development and application. Herein, the abundant

transition metals with d¹⁰conﬁgurations and the accessible

combinations of carboxylic acid and nitrogen heterocyclic

ligands have been proven to be very helpful to constructing

cost-eﬀective MOFs with versatility.^22,27−29For example, Sun

and co-workers synthesized MOF membranes with improved

1. INTRODUCTION

The detection of heavy-metal ions is necessary because of their

toxicity and bioaccumulation in a living body.¹⁻⁵For instance,

Fe³⁺plays indispensable roles in the processes of hemoglobin

formation, oxygen uptake, and the syntheses of DNA and

RNA.⁶Both deﬁciency and excess of Fe³⁺can lead to serious

biological disorders.⁷Similarly, hexavalent chromium existing

as the soluble anion CrO₄²⁻/Cr₂O₇is highly carcinogenic

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and can destroy DNA and the protein and enzyme systems of

the human body even at low concentration.^8,9Hence, the

selective detection of Fe³⁺, CrO₄²⁻, and Cr₂O₇²⁻is urgent and

has attracted increasing interest.

Luminescent metal−organic frameworks (MOFs) with novel

structures and permanent channels have been widely explored

for their sensing applications in ions, pH values, explosives, and

antibiotics.¹⁰⁻¹³However, the luminescent MOFs detecting

both Fe³⁺and chromate ions (CrO₄and Cr₂O₇²⁻) are

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Received: October 29, 2020

Published: January 20, 2021

limited.¹⁴⁻¹⁷Meanwhile, most of the luminescence sensing on

the basis of MOFs is performed in nonaqueous media because

of the poor water stability of these chemosensors.¹⁸⁻²¹This

unsatisfactory stability also hinders the further reuse of

luminescent MOFs for sensing ions.²²⁻²⁴Therefore, it is

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employing a MAPADA P7 double-beam UV−vis spectrophotometer

with the range of 200−800 nm at room temperature.

gas separation by using imidazole and 3,3′,5,5′-azobenzenete-

tracarboxylic acid ligands.³⁰Furthermore, the general sensing

detection of MOF materials was performed in the dispersed

phases of sensors, deviating from the handy operability and

repeatability, which are also signiﬁcant indexes for the practical

sensing of luminescent MOFs. To solve this obstacle, it is

feasible to design and prepare a mixed-matrix membrane

(MMM) incorporating a luminescent MOF and a polymer as a

test device.³¹⁻³⁴Meanwhile, most of MOFs utilize uniﬁed

nodes to construct porous frameworks.^30,35,36In this regard,

two kinds of metal nodes incorporated into a skeleton can

further improve the diversity of the skeleton and optimize the

structural stability.^25,37−41

2.2. Synthesis of Ligand L₁. In a round-bottom ﬂask (250 mL), a

mixture of 4-pyridinecarboxaldehyde (2.14 g, 20 mmol) and 4-

acetylpyridine (4.85 g, 40 mmol) was dispersed in CH₃CH₂OH (100

mL). A KOH aqueous solution (2.8 g, 50 mmol, 85%) was then

added, and the solution was stirred for 2 h at 0 °C. Subsequently, an

ammonia solution (70 mL, 25%) was charged into this mixture. Then,

the reaction mixture was heated and reﬂuxed for 12 h. After complete

reaction, the mixture was cooled at 0 °C for 1 h, and the light-purple

L₁ligand (2.98 g, 9.61 mmol, yield 48%) was collected by ﬁltration

and washed with ice-cold CH₃CH₂OH (100 mL). ¹H NMR (CDCl₃,

500 MHz): δ 8.84 (s, 4H), 8.82 (d, 2H), 8.10 (d, 4H), 8.06 (s, 2H),

7.66 (d, 2H).

2.3. Synthesis of [Zn(OBA)₂(L₁)·2DMA]_n(1). A mixture of

H₂OBA (0.065 g, 0.25 mmol), L₁ligand (0.045 g, 0.15 mmol), and

Zn(CH₃COO)₂·2H₂O (0.11 g, 0.5 mmol) in 8 mL of DMA was

placed in a 23 mL Teﬂon-lined stainless steel autoclave. The mixture

was heated in a 120 °C oven for 72 h (Scheme S1). After cooling to

room temperature, the colorless crystals of compound 1 were

harvested (yield: 85% based on H₂OBA). Elem anal. Calcd for

C₅₆H₄₈N₆O₁₂Zn₂(1127.74): C, 59.64; H, 4.29; N, 7.45. Found: C,

59.58; H, 4.32; N, 7.43.

2.4. X-ray Structure Determination. A Bruker D8 VENTURE

diﬀractometer equipped with graphite-monochromated Mo Kα

radiation (λ = 0.71073 Å) was employed to collect diﬀraction data

of compound 1 under the ϕ and ω scan techniques. The crystal

structure was solved by direct methods, and all of the non-hydrogen

atoms were reﬁned anisotropically on F²by full-matrix least squares

with the SHELXL package.⁴³Crystallographic data and structure

reﬁnement are exhibited in Table S1. We also listed the selected bond

lengths and angles in Table S2. Full crystallographic data are stored in

CCDC 2040578.

On the basis of the above considerations, a 3D luminescent

Zn-based MOF, [Zn(OBA)₂(L₁)·2DMA]_n(1; H₂OBA = 4,4′-

oxybis(benzoic acid), L₁= 2,4,6-tris(4-pyridyl)pyridine, and

DMA = N,N-dimethylacetamide; CCDC 2040578), with a

pore size of 5.9 Å has been assembled by employing the low-

cost V-shaped H₂OBA, the rigid nitrogen heterocyclic L₁with

a large π-conjugated system, and Zn(CH₃COO)₂·2H₂O under

a solvothermal method. The porous compound 1 shows

satisfactory chemical stability in a H₂O solution (pH = 2−12)

and diﬀerent boiling organic solvents and thermal stability

under 300 °C in air, inspiring the experimental exploration

under hard conditions. In a series of sensing tests, we found

that compound 1 can selectively detect Fe³⁺, CrO₄²⁻, and

2−

Cr₂O₇in a short response time in aqueous solution.

Moreover, these quenching detections could not be interfered

with even in the presence of other cations and anions.

Meanwhile, compound 1 shows low limits of detection

2.5. Activation of Compound 1. The fresh synthesized

compound 1 was washed with DMA and soaked for 3 days in

CH₃OH, during which the solvent was poured out and fresh solvent

was charged every 12 h. Then the solid samples were dried in a

vacuum oven at 150 °C. The structure of activated compound 1 was

maintained, as conﬁrmed by the PXRD pattern (Figure S1).

2.6. Luminescence Sensing Experiments. All sensing experi-

ments were performed at ambient temperature. A total of 75 mg of

ground compound 1 was added to a 250 mL volumetric ﬂask, and

H₂O was charged to form a suspension (0.3 mg mL⁻¹) by

ultrasonication for 10 min. For ion-sensing experiments, 9 mL of an

aqueous suspension of compound 1 and 1 mL of the corresponding

M(NO₃)_x(M = Zn²⁺, Co²⁺, Cd²⁺, Mg²⁺, K⁺, Ni²⁺, Mn²⁺, Na⁺, Fe³⁺,

(LODs) toward Fe³⁺, CrO₄²⁻, and Cr₂O₇of 1.06, 3.87,

2−

and 2.37 μM, conﬁrming excellent sensitivity of this

compound. In addition, compound 1 can be easily and quickly

recovered and then reused six times, conﬁrming the good

recyclability. To further improve the potential for practical

applications, we designed and prepared a MMM incorporating

compound 1 and poly(methyl methacrylate) (PMMA),

showing blue emission. This MMM can quickly detect Fe³⁺,

2−

CrO₄²⁻, and Cr₂O₇and then be regenerated by lifting from

the analytical solutions, demonstrating its quick response and

good selectivity. The experiments further indicate that this

MMM possesses a comparable LOD below 4.35 μM for these

three kinds of ions. Therefore, compound 1 is an excellent

chemosensor.

−

Cr³⁺, Pb²⁺, Cu²⁺, and Al³⁺) or NaX (X = PO₄³⁻, CO₃²⁻, NO₃, ClO₄,

Cl⁻, Br⁻, SO₄²⁻, NO₂, CrO₄²⁻, and Cr₂O₇²⁻) aqueous solution (10

−

mM) were well mixed. The antijamming experiments were carried out

by mixing 9 mL of an aqueous suspension of compound 1, 0.5 mL of

a Fe³⁺, CrO₄²⁻, and Cr₂O₇²⁻aqueous solution (20 mM), and 0.5 mL

of diﬀerent interfering ion aqueous solutions (20 mM). The

luminescence titration experiments of cations and anions were

2. EXPERIMENTAL SECTION

2.1. Materials and Methods. All chemicals used in this work

were purchased commercially and employed without further

puriﬁcation. The ligand L₁was prepared according to the previous

method with some modiﬁcation.⁴²The ¹H NMR spectrum was

measured with a Bruker Avance Neo 500 MHz spectrometer. Powder

X-ray diﬀraction (PXRD) was performed with a SmartLab X-ray

diﬀractometer equipped with Cu Kα (λ = 1.54184 Å) radiation and a

graphite monochromator in the range of 5−40°. The data of

thermogravimetric analysis (TGA) were collected by using a Rigaku

PTC-10ATG-DTA analyzer under a N₂atmosphere with a heating

speed of 10 °C min⁻¹from room temperature to 800 °C. An

Elementar Vario EL analyzer was employed to perform elemental

analyses (C, H, and N). The Micromeritics ASAP 2020 M gas

adsorption analyzer was used to test gas adsorption. The

luminescence spectra were recorded by utilizing a FLSP920

Edinburgh ﬂuorescence spectrometer with a slit width of 1.0 nm. A

Leeman Laboratories Prodigy ICP-AES system was used to conduct

ICP analyses. The UV−vis absorption spectra were recorded by

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performed by adding step by step a Fe³⁺, CrO₄²⁻, and Cr₂O₇

aqueous solution (10 mM) to 10 mL of a compound 1 suspension,

respectively.

2.7. Preparation of MMMs Incorporating Compound 1.

Compound 1 (50 mg) with adequate grinding was added to a

PMMA/chloroform solution (50 mL, 20 mg mL⁻¹), and then the

dispersed suspensions were ultrasonicated for 0.5 h. The luminescent

MMMs with a mass loading of 5% were formed by pouring and tiling

the above mixed slurry on culture ware, followed by heating in an

oven at 40 °C overnight. The free-standing luminescent MMMs with

uniform size (1.0 cm × 3.6 cm) were obtained by peeling from culture

ware and shearing.

3. RESULTS AND DISCUSSION

3.1. Crystal Structure. Single-crystal X-ray diﬀraction

analysis revealed that compound 1 crystallized in a monoclinic

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Figure 1. Crystal structure of compound 1: (a) coordination environment of Zn²⁺in compound 1; (b) coordination nodes of the OBA²⁻ligand;

(c) 2D layer of compound 1 and L₁ligand; (d) 3D structure viewed along the a axis; (e) space-ﬁlling diagram of compound 1 viewed along the a

axis. Color code: C, light gray; O, red; N, blue; Zn, turquoise.

crystal system with space group P2₁/c. As presented in Figure

S2, the asymmetric unit of compound 1 consists of two

crystallographically independent Zn²⁺, two OBA²⁻ligands, and

one independent L₁ligand. Zn1 is in a [ZnO₄N₂] distorted

octahedral geometry deﬁned by four oxygen atoms (O1, O2,

O7A, and O8A) from two OBA²⁻ligands with Zn1−O bond

lengths in the range of 1.938(6)−2.702(6) Å and two nitrogen

atoms from two L₁ligands with Zn1−N bond distances falling

in 2.033(6)−2.107(6) Å (Figure 1a). The 6-connected Zn2 is

coordinated by ﬁve carboxyl oxygen atoms (O3, O5, O4D,

O5D, and O6D) from four diﬀerent OBA²⁻ligands and one

nitrogen atom (N2C) from one L₁ligand, resulting in an

irregular octahedral coordination geometry. The Zn2−O and

Zn2−N distances fall into the range of 1.779(14)−2.773(6) Å

and 2.009(6) Å, respectively. All Zn−O and Zn−N bond

distances of Zn1 and Zn2 match well with those reported

previously.^44,45It is worth noting that one independent Zn1

atom forms a mononuclear [Zn(COO)₂N₂] node and two

adjacent Zn2 atoms construct a binuclear [Zn₂(COO)₄N₂]

node (Figure 1a). The [Zn(COO)₂N₂] and [Zn₂(COO)₄N₂]

nodes are connected by OBA²⁻ligands adopting μ₃:η¹:η¹:η¹:η²

and μ₃:η¹:η¹:η¹:η¹coordination nodes (Figure 1b), forming a

ﬁsh-scale 2D layer with a window size of 18.75 × 8.27 Å²

taking the van der Waals radius into account (Figure 1c). As

shown in Figure 1d, the novel 2D layers of compound 1 are

further linked by L₁ligands, ﬁnally constructing a 3D porous

framework. From the view of topology,⁴⁶the OBA²⁻ligand, L₁

ligand, mononuclear Zn unit, and binuclear Zn unit can be

considered to be 2-, 3-, 4-, and 6-connected nodes, respectively

(Figure S3). In particular, the OBA²⁻ligands exhibit two kinds

of 2-connected models. Thus, compound 1 can be described as

a 2,2,3,4,6-connected net with a short Schlaﬂi symbol of {4·8 ×

10}₂{4·8·12⁴}₂{4²·8²·10²·12⁷·14·16}{4}₂{8}₂(Figure S3). The

scalene 1D channel can be seen along the a axis (Figure 1e),

and the total solvent-accessible volume calculated by the

PLATON software is 31.7%.⁴⁷

3.2. Framework Stability. Prior to performance analysis,

the PXRD experiments were conducted for conﬁrming the

phase purity of compound 1. A comparison of the peaks shows

good consistency between the experimental and simulated

PXRD patterns (Figure S4), conﬁrming the excellent phase

purity of the polycrystalline sample. In order to further

investigate the chemical stability of compound 1, 25 mg of the

as-synthesized sample was immersed in various acid/base

aqueous solutions with pH values ranging of 2−12 and

diﬀerent boiling solvents (ethyl acetate, CH₃OH, CHCl₃,

CH₂Cl₂, n-hexane, isopropyl alcohol, N-methyl-2-pyrrolidone,

N,N-diethylformamide, acetone, CH₃CH₂OH, CH₃CN, DMA,

N,N-dimethylformamide, and H₂O) for 12 h, respectively. The

corresponding PXRD patterns of compound 1 after treated are

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in agreement with the peak positions of the as-synthesized

sample, indicating that the structure of compound 1 is still

intact in these solutions (Figure 2a,b). According to the hard−

free DMA molecules. Upon further heating, no weight loss can

be found up to 325 °C because there is a platform in the range

of 265−325 °C and the framework begins to collapse at higher

temperatures. Meanwhile, we heated the fresh compound 1 at

diﬀerent temperatures in air, and the PXRD patterns of

compound 1 heated to 300 °C match closely with that of the

fresh sample (Figure 2c).

The above results indicate that compound 1 possesses

excellent chemical and thermal stabilities because it can retain

its integrity in H₂O (pH = 2−12), in diﬀerent boiling solvents,

and at high temperature (300 °C) in air.

3.3. Gas Adsorption. The permanent porosity of activated

compound 1 was tested by N₂adsorption−desorption

isotherms at 77 K. As shown in Figure 3a, compound 1

Figure 3. (a) N₂adsorption isotherm of compound 1 at 77 K. (b)

Emission spectra of H₂OBA ligand (λ_ex= 346 nm), L₁ligand (λ_ex

290 nm), and compound 1 (λ_ex= 290 nm).

Figure 2. Stability exploration of compound 1: (a) PXRD patterns of

compound 1 after being immersed in diﬀerent boiling solvents for 24

h; (b) PXRD patterns of compound 1 after being soaked in H₂O with

diﬀerent pH values (2−12) for 24 h; (c) PXRD patterns of

compound 1 after being heated at diﬀerent temperatures in air.

exhibits type I adsorption isotherms, which is classic for

microporous materials. The adsorption amount of N₂at 1 atm

reached 282 cm³g⁻¹. The Brunauer−Emmett−Teller and

Langmuir surface areas are 573 and 882 m²g⁻¹, respectively.

Meanwhile, the pore size is 5.9 Å, calculated by using the

density functional theory (DFT) model, further conﬁrming its

microporosity. Furthermore, the H₂adsorption isotherms for

compound 1 were measured at 77 K (Figure S6). The H₂

uptake of compound 1 reaches a maximum of 97.3 cm³g⁻¹at 1

atm. This adsorption amount is higher than those of some

reported porous materials,⁵³⁻⁵⁵indicating its potential

application for H₂adsorption and storage.

3.4. Luminescence Sensing of Compound 1. Because

the aromatic ligands and d¹⁰metal centers can be used to

construct promising MOFs with potential luminescence, we

explored the luminescent properties of the free ligands and

compound 1. As shown in Figures 3b and S7, the emission

spectra of the H₂OBA and L₁ligands exhibit moderate

intensity with emission maxima at 563 and 372 nm (λ_ex= 346

soft acid−base theory, the Zn²⁺ions as soft acids tend to bind

with N-containing linkers as soft bases to construct stable

structures.²⁵Zn and N in compound 1 form a strong bonding

interaction, which can reduce the damage of coordination

bonds in H₂O. The hydrophobic benzene rings and saturated

coordination nodes in compound 1 can eﬀectively protect the

coordination bonds from the attack by H₂O and enhance the

framework stability in aqueous solution.⁴⁸As reported in the

previous literature, the mixed ligands combining mixed-donor

linkers (N and O donors) can also enhance the stability of

MOFs in H₂O.⁴⁹⁻⁵²To further explore the thermal stability of

compound 1, TGA measurement was carried out from room

temperature to 800 °C under a N₂atmosphere. As shown in

Figure S5, a weight loss of about 13.22% can be observed at

90−265 °C (calcd 15.3%), corresponding to the escape of two

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and 290 nm), respectively. These emissions may be assigned to

the intraligand π → π* and/or n → π* charge transitions.

Meanwhile, compound 1 exhibits an enhanced emission peak

at 360 nm under excitation at 290 nm, indicating that the

luminescence emission of this Zn-based MOF originates from

the L₁ligand.⁵⁶Compared to the L₁ligand, the emission of

compound 1 is blue-shifted by 12 nm and distinctly enhanced,

which may be caused by the constrained coordination of Zn²⁺

with the organic ligands in the stable framework. The emission

spectra of compound 1 under diﬀerent excitation wavelengths

were also measured, which do not show emission peaks similar

to the 563 nm peak of H₂OBA (Figure S8). Furthermore, the

wavelength of the maximum emission exhibits obvious shifts

when compound 1 is dispersed in various organic solvents such

as CH₃OH, CH₃CH₂OH, ethyl acetate, isopropyl alcohol, N-

methyl-2-pyrrolidone, CH₃CN (Figure S9). This phenomenon

conﬁrms that compound 1 could be employed to identify

diﬀerent organic solvents through the wavelength shifts.⁵⁷

Inspired by the excellent water stability, strong luminescence

intensity, and aromatic pyridine rings of compound 1, the

luminescence detection toward metal cations was investigated.

Thus, a suspension of compound 1 was fully mixed with

aqueous solutions of M(NO₃)_x(M = Zn²⁺, Co²⁺, Cd²⁺, Mg²⁺,

K⁺, Ni²⁺, Mn²⁺, Na⁺, Fe³⁺, Cr³⁺, Pb²⁺, Cu²⁺, and Al³⁺),

respectively. The emission spectra of suspensions incorporat-

ing metal ions with an ion concentration of 1 mM were

recorded (Figure S10a). As illustrated in Figure 4a, the

disturbing cations, indicating that compound 1 possesses

excellent selectivity for Fe³⁺(Figure S11).

Meanwhile, we also explored the selective recognition of

3−

compound 1 toward diﬀerent anions (NaX, where X = PO₄₂₋

CO₃²⁻, NO₃, ClO₄, Cl⁻, Br⁻, SO₄²⁻, NO₂, CrO₄

−

Cr₂O₇²⁻). Similar to the cation-sensing experiments, the

luminescence intensities of aqueous suspensions containing

compound 1 and various anions (1 mM) were measured

(Figure S10b). As shown in Figure 4b, most tremendous

2−

quenching eﬀects are observed in the presence of CrO₄and

Cr₂O₇²⁻, but other ions make compound 1 have negligible

quenching. The eﬀective quenching eﬃciencies of CrO₄²⁻and

2−

Cr₂O₇are 99.84% and 99.88%, respectively. Selective

quenching of compound 1 to chromate ions was explored by

interference tests. The other anions do not obstruct the

signiﬁcant quenching of compound 1 with the addition of

2−

CrO₄and Cr₂O₇(Figures S12 and S13), suggesting the

perfectly selective sensing and antiinterference capability of

compound 1 for chromate ions in aqueous solution.

Besides the satisfactory selectivity, the performance indexes

with short response time, excellent sensitivity, low LOD, and

well recyclability are also crucial to compound 1 as a feasible

chemosensor. To explore the response time of the three kinds

of ions, we charged separately 1 mL of a Fe³⁺, CrO₄²⁻, or

Cr₂O₇²⁻aqueous solution (10 mM) to glass sample bottle with

9 mL of an aqueous dispersion of compound 1 and quickly

shook to well mix. Subsequently, we recorded the correspond-

ing luminescence spectra. As shown in Figure 5a, the

luminescence of compound 1 quenches rapidly and the

quenching eﬃciency is almost 100% in 25 s after the addition

of the corresponding ions (Fe³⁺, 99.58%; CrO₄²⁻, 99.48%;

Cr₂O₇²⁻, 99.35%). This striking result indicates the fast

response rate and short response time of compound 1 for

Fe³⁺, CrO₄²⁻, and Cr₂O₇²⁻. Simultaneously, three kinds of ion

aqueous solutions (10 mM) were titrated gradually into a

suspension of compound 1 to study the sensitivity of this

chemosensor. In the antiinterference and titration experiments,

the maximum emission wavelength deviated in the blank

experiments, which may be due to insuﬃcient grinding of the

MOF material and the short ultrasonic treatment time,

resulting in the uneven particle size of compound 1 in H₂O.

Following an increase of the Fe³⁺concentration, the

luminescence intensity of compound 1 gradually weakened

(Figure S14a). Similar results were also observed in the related

2−

experiments of CrO₄and Cr₂O₇(Figure S14). To further

evaluate the sensitivity of compound 1, the Stern−Volmer (S−

V) equation (I₀/I = 1 + K_SV[M], where K_SVis the quenching

constant, [M] is the ion concentration, I₀and I are the

luminescence intensities of the suspension before and after the

addition of ion aqueous solutions) was employed to analyze

the luminescence quenching eﬃciency. As shown in Figure 5,

the S−V curves for Fe³⁺, CrO₄²⁻, and Cr₂O₇show a good

2−

Figure 4. Luminescence intensities of compound 1 dispersed in

aqueous solutions of diﬀerent (a) cations and (b) anions (1 mM)

under excitation at 290 nm.

linear correlation (R²= 0.99, 0.97, and 0.99) in the

concentration ranges of 0−0.1, 0−0.183, and 0−0.21 mM,

respectively. The calculated K_SVof Fe³⁺is 42210.27 M⁻¹,

which is much higher than those in some reported

works.^14,58,59Furthermore, the LOD of compound 1 toward

Fe³⁺is calculated by using the equation of LOD = 3σ/slope (σ

= standard deviation) with a low LOD of 1.06 μM. Meanwhile,

the K_SVand LOD of compound 1 toward CrO₄²⁻/Cr₂O₇²⁻are

11605.28 M⁻¹and 3.87 μM and 18972.72 M⁻¹and 2.37 μM,

respectively. The LOD values of this compound for Fe³⁺,

luminescence intensity of compound 1 is inﬂuenced by the

metal ions. In particular, the luminescence of compound 1 is

almost completely quenching toward Fe³⁺with a quenching

eﬃciency of 99.96%, while the quenching percentages of other

ions are negligible or moderate. This reveals that compound 1

can selectively detect Fe³⁺over other explored cations. More

importantly, Fe³⁺leads to the dominant luminescence

quenching of compound 1 even in the presence of other

2−

CrO₄²⁻, and Cr₂O₇are comparable to those reported

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2−

Figure 5. (a) Quenching eﬃciencies of compound 1 for Fe³⁺, CrO₄²⁻, and Cr₂O₇in 25 s. S−V plots of I₀/I versus concentrations of Fe³⁺(b),

2−

CrO₄(c), and Cr₂O₇(d), respectively.

previously, further conﬁrming the potential application of

compound 1 as a chemosensor toward these ions (Tables S3 −

S5).^14,58−68

In order to further explore the practicability of this

chemosensor, the recycling experiments were carried out as

follows: compound 1 after grinding was placed in a H₂O

solution of Fe³⁺, CrO₄²⁻, and Cr₂O₇²⁻, respectively, and the

mixture was sonicated for several minutes. Then, we tested the

corresponding luminescence spectra, and the sample in

suspension solutions was washed with deionized water and

centrifuged several times. Subsequently, the recovered sample

was conﬁrmed by luminescence spectra (Figure S15). We

repeated the above experiments several times. As shown in

Figure 6, compound 1 can be simply and quickly regenerated

and reused to detect Fe³⁺, CrO₄²⁻, and Cr₂O₇six times,

2−

suggesting excellent recyclability. It is noted that the

luminescence intensity of compound 1 was reduced by half

after six cycles, indicating the strong interaction between Fe³⁺

and the framework of compound 1. After six cycles, the Fe³⁺

content of compound 1 is 0.64 wt %, which was measured by

ICP analysis.

The above experiments demonstrate that luminescent

compound 1 has excellent selectivity, short response time,

high sensitivity, low LOD, and well recyclability toward Fe³⁺,

CrO₄²⁻, and Cr₂O₇²⁻. Therefore, the mechanism investigation

is very necessary to further design a useful luminescent

chemosensor. First, the PXRD patterns of the samples were

recorded after immersion into a 1 mM aqueous solution of

Fe³⁺, CrO₄²⁻, and Cr₂O₇²⁻for 24 h (Figure S16). There is no

change in the peaks compared to the as-synthesized compound

1, conﬁrming that the structure is maintained even during the

sensing process and the luminescence quenching should not be

due to the structural collapse of this compound. To further

conﬁrm the possible quenching mechanism of compound 1

toward these ions, UV−vis absorption spectra of the aqueous

Figure 6. Six cycle experiments of compound 1 for sensing Fe³⁺(a),

CrO₄(b), and Cr₂O₇(c).

2−

solutions of anions and cations (1 mM) were recorded and

analyzed. It is apparent that Fe³⁺shows an absorption band

from 200 to 450 nm, which covers the excitation spectrum of

compound 1 (270−330 nm). However, no overlap can be

observed between the UV−vis absorption bands of other

cations and the excitation spectrum of this luminescence

sensor (Figure S17a).^59,63Therefore, it is very likely that Fe³⁺

can compete with this MOF for absorbing the excitation light.

Therefore, the competitive absorption between Fe³⁺and

compound 1 cut down the excitation eﬃciency, resulting in

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Inorganic Chemistry

Article

emission quenching. Meanwhile, the broad absorption bands

the MMM cannot emit ﬂuorescence in a dark room (Figure

7c) but shows strong blue emission under irradiation of a

laboratory UV lamp (254 nm). Under the same conditions, the

pure membrane PMMA only exhibits weak emission (Figure

S18), conﬁrming that the emission source is compound 1. The

uniform brightness also conﬁrms the homogeneous distribu-

tion of compound 1 in this MMM (Figure 7d). Besides, the

MMMs show good toughness because it can be bent (Figure

7e). The above properties indicate the great potential of this

test device for luminescence sensing application. When MMM

2−

of CrO₄and Cr₂O₇from 320 to 480 nm nearly cover the

emission spectrum of compound 1 (320−500 nm), which does

not overlap with the UV−vis absorption peaks of other anions

(Figure S17b). Therefore, resonance energy transfer from

2−

compound 1 (donor) to the CrO₄and Cr₂O₇ions

(acceptors) might occur and lead to luminescence quench-

ing.^58,65In addition, compound 1 can encapsulate Al³⁺, Cu²⁺,

and Pb²⁺because it has microporous channels. As shown in

Table S6, the results of ICP-ACS also conﬁrmed the successful

introduction of these ions into compound 1. The introduction

of Al³⁺, Cu²⁺, and Pb²⁺may consume energy through the

collision interaction between ions and the framework and

further decrease the energy-transfer eﬃciency within the

framework, leading to the reduction of luminescence of

compound 1.^69,70The size and electron conﬁguration of the

three ions are diﬀerent, so they have diﬀerent eﬀects on

ﬂuorescence attenuation.^71,72

2−

was gradually immersed in the Fe³⁺, CrO₄²⁻, and Cr₂O₇

aqueous solution, respectively, the strong blue luminescence of

MMM promptly disappeared with irradiation of a laboratory

UV lamp at 254 nm. Meanwhile, the ﬂuorescence emission of

MMM has no obvious change in other ionic aqueous solutions

(Figures S19 and S20). Remarkably, blue luminescence can be

immediately regenerated by simply removing MMM from the

aqueous solution of Fe³⁺, CrO₄²⁻, and Cr₂O₇(Figure 8).

2−

3.5. Luminescent MMMs of Compound 1. Inspired by

the excellent stability and sensing performance in a H₂O

solution, the luminescent MMMs were designed and prepared

to improve the convenience of operation and expand the

practical application of compound 1. As shown in Figure 7a,

the uniform membrane was prepared through the complete

mixing of compound 1 and PMMA in CH₂Cl₂. The PXRD

spectrum of this MMM contains the peaks of compound 1,

indicating that this compound retains its structural integrity in

the process of membrane preparation (Figure 7b). Meanwhile,

Figure 8. Photographs exhibiting the change of luminescence

emission of luminescent MMM before, during, and after sensing of

2−

Fe³⁺(a−c), CrO₄(d−f), and Cr₂O₇(g−i) under UV-lamp

irradiation (λ_ex= 254 nm).

These dramatic results suggest that this MMM can quickly

detect these ions and then be easily recovered, making this

material have good practicability. To further explore the

sensitivity of MMM for these three kinds of ions, we quantiﬁed

the quenching eﬃciency including K_SVand LOD by the

titration experiments (Figure S21). As illustrated in Figure S22,

the S−V plots satisfy a linear relationship between the

luminescence intensity and concentration of Fe³⁺, CrO₄

2−

and Cr₂O₇in the range of 0−120 μM, with calculated K_SV

values being 10330, 13000, and 12600 M⁻¹, respectively.

Moreover, the LODs are estimated to be 4.35 μM for Fe³⁺,

3.46 μM for CrO₄²⁻, and 3.57 μM for Cr₂O₇²⁻, revealing a

quenching eﬀect comparable to that of the reported

luminescent MMMs. The structure of compound 1 in MMM

is intact after sensing (Figure S23). These novel results suggest

that this MMM can selectively detect Fe³⁺, CrO₄²⁻, and

Cr₂O₇²⁻with short detection time and low LOD and meet the

Figure 7. (a) Schematic illustration for the preparation process of

MMM incorporating compound 1 (the powder is compound 1; the

transparent blocks are PMMA). (b) PXRD patterns of PMMA,

MMM, and the as-synthesized compound 1. Photographs of

luminescent MMM incorporating compound 1 in a dark room (c)

and upon UV-light irradiation of 254 nm (d). (e) Photographs

demonstrating that large-area MMM is resilient to mechanical stress

and can be easily handled.

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Inorganic Chemistry

https://pubs.acs.org/10.1021/acs.inorgchem.0c03214

Article

Authors

high requirements of practical detection and fast recovery for

these targeted ions.

Bowen Qin − Advanced Energy Materials Research Center,

Faculty of Chemistry, Northeast Normal University,

Changchun 130024, P. R. China

Jingjing Qiu − Advanced Energy Materials Research Center,

Faculty of Chemistry, Northeast Normal University,

Changchun 130024, P. R. China

4. CONCLUSIONS

In summary, a water-robust Zn-based MOF (1) with mixed

nodes has been designed and synthesized by using H₂OBA

ligand, L₁ligand, and Zn(CH₃COO)₂·2H₂O under a

solvothermal method. This porous compound 1 with a pore

size of 5.9 Å is highly stable in diﬀerent boiling organic solvents

and H₂O solution with diﬀerent pH values (2−12) and has

strong luminescence emission. Meanwhile, compound 1 can

Godefroid Gahungu − Department of Chemistry, University

of Burundi, Bujumbura, Burundi

Complete contact information is available at:

selectively and sensitively detect Fe³⁺, CrO₄²⁻, and Cr₂O₇

2−

Notes

with a short response time even in the presence of other

interfering ions. Obviously, compound 1 can be reused six

times for the detection of these ions and has a low LOD of

1.06 μM toward Fe³⁺, which is much lower than those of the

The authors declare no competing ﬁnancial interest.

ACKNOWLEDGMENTS

■

2−

reported works. The LODs for CrO₄and Cr₂O₇are 3.87

and 2.37 μM, which are comparable to those of the reported

works. Interestingly, the luminescent MMMs, which are

designed and prepared by integrating the advantages of porous

MOFs and polymers, show outstanding performance with

quick response, good selectivity, and excellent sensitivity. This

work exhibiting an accessible luminescent MOF and a useful

MOF-based MMM with excellent sensing capability to Fe³⁺,

This work was supported by the National Natural Science

Foundation of China (Grants 21873018 and 21573036),

Foundation of the Education Department of Jilin Province

(Grant 111099108), Fundamental Research Funds for the

Central Universities (Grant 2412019QD006), Open Funds of

the State Key Laboratory of Rare Earth Resource Utilization

(Grant RERU2019020), and Jilin Provincial Research Center

of Advanced Energy Materials (Northeast Normal University).

2−

CrO₄²⁻, and Cr₂O₇in H₂O provides valuable guidance for

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AUTHOR INFORMATION

■

Corresponding Authors

Xiaoying Zhang − Advanced Energy Materials Research

Center, Faculty of Chemistry, Northeast Normal University,

Changchun 130024, P. R. China; Email: zhangxy218@

nenu.edu.cn

Haiyan Yuan − Advanced Energy Materials Research Center,

Faculty of Chemistry, Northeast Normal University,

Changchun 130024, P. R. China; Email: yuanhy034@

nenu.edu.cn

Jingping Zhang − Advanced Energy Materials Research

Center, Faculty of Chemistry, Northeast Normal University,

Changchun 130024, P. R. China; orcid.org/0000-0001-

8004-3673; Email: zhangjp162@nenu.edu.cn

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