Turn-on and Ratiometric Luminescent Sensing of Hydrogen Sulfide

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Turn-on and Ratiometric Luminescent Sensing of Hydrogen Sulfide based on Metal-Organic Frameworks Xin Zhang, Quan Hu, Tifeng Xia, Jun Zhang, Yu Yang, Yuanjing Cui, Banglin Chen, and Guodong Qian ACS Appl. Mater. Interfaces, Just Accepted Manuscript • DOI: 10.1021/acsami.6b12118 • Publication Date (Web): 09 Nov 2016 Downloaded from http://pubs.acs.org on November 14, 2016

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ACS Applied Materials & Interfaces

Turn-on and Ratiometric Luminescent Sensing of Hydrogen Sulfide based on Metal-Organic Frameworks †

‡

†

†

†

Xin Zhang, Quan Hu, Tifeng Xia, Jun Zhang, Yu Yang,*, Yuanjing Cui,*, †§

Banglin Chen, , †

and Guodong Qian*,

†

†

State Key Laboratory of Silicon Materials, Cyrus Tang Center for Sensor Materials

and Applications, School of Materials Science and Engineering, Zhejiang University, Hangzhou 310027, China ‡

Department of Pharmacy, School of Medicine, Hangzhou Normal University,

Hangzhou 310036, China §

Department of Chemistry, University of Texas at San Antonio, Texas 78249-0698,

United States

KEYWORDS: metal-organic frameworks, turn-on, fluorescent probes, hydrogen sulfide, ratiometric sensing

ABSTRACT: The sensing of hydrogen sulfide (H2S) has become a long-time challenging task. In this work, we developed a general strategy for sensing of H2S 1

ACS Paragon Plus Environment


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utilizing postsynthetic modification of a nano metal-organic frameworks (MOF) UiO-66-(COOH)2

with

Eu3+

and

Cu2+

ions.

The

nano

MOF

Eu3+/Cu2+@UiO-66-(COOH)2 displays the characteristic Eu3+ sharp emissions and the broad ligand-centered (LC) emission simultaneously. Because H2S can strongly increase the fluorescence of Eu3+ and quench the broad LC emission through its superior affinity for Cu2+ ions, the MOF Eu3+/Cu2+@UiO-66-(COOH)2 exhibits highly sensitive turn-on sensing of H2S over other environmentally and biologically relevant species under physiological conditions. Furthermore, this approach for fluorescent turn-on sensing of H2S is expected to extend to other water stable MOFs containing uncoordinated -COOH.

1. INTRODUCTION

Hydrogen sulfide (H2S), as an intriguing endogenous gasotransmitter in addition to nitric oxide (NO) and carbon monoxide (CO), is associated with various physiological and pathological processes.1-2 Fluorescence-based H2S sensing methods have been developed and have many advantages, such as good sensitivity, excellent selectivity, rapid response, and non-invasive sensing.3-4 However, the accuracy of the fluorescence intensity-based fluorescent probes is limited, because of numerous man-made environmental influence factors during the fluorescence intensity measurement, such as probe concentration, probe environment, and excitation intensity. In contrast, ratiometric fluorescent probe is able to rule out these drawbacks

2


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by self-calibration of two emission bands.5-9 In particular, fluorescent turn-on ratiometric probes are preferred to avoid false response and improved signal to noise ratio as the detection occurs relative to dark background.10 Recently, a few turn-on type fluorescent probes for detection of H2S have been reported, and the design strategies mainly rely on the typical reactive characteristics of H2S, including H2S-mediated nitro/azide reduction, copper/silver sulfide precipitation, and dual nucleophilic addition.11-20 Unfortunately, these probes reported up to now can hardly satisfy all the criteria such as turn-on, ratiometric sensing, highly selective and real-time detection. Furthermore, some molecule probes do not dissolve in aqueous solution, hampering their application in tackle biological issues. Therefore, the development of a probe with all these properties is still a challenging task and an active area of current research. In the past few decades, metal-organic frameworks (MOFs) are emerging as a novel class of promising microporous hybrid crystalline materials.21-23 In particular, the luminescent MOFs have gathered considerable attention owing to their potential applications in chemical sensors and light-emitting devices.24-28

Among them,

lanthanide MOFs (Ln-MOFs) are regarded as particularly significant due to their intense, longlived, sharp emission in the visible region. However, because of the higher coordination numbers and more variable nature of the Ln3+ coordination sphere, the rational design and preparation of desired Ln-based MOFs still remain a challenge. Recently, an increasing number of Ln-doped MOFs constructed by 3



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postsynthetic modification (PSM) have been prepared.29-31 The luminescence of lanthanides can be tuned by the introduction of transition metal ions (d block) into the lanthanide (f block) organic frameworks.32-34 In this contribution, we intended to the development of a novel turn-on and ratiometric fluorescent probe that utilize Cu2+ to tune the luminescence of Eu3+@UiO-66-(COOH)2 for H2S sensing. Designing ideas and strategies are shown in Scheme 1. As a UiO-66 analogue, UiO-66-(COOH)2, which contains two extra free carboxylic acid function groups per ligand with interesting porosity, was selected as a parent framework.35 The reactive nature of the uncoordinated carbonyl groups, as well as the high thermal and chemical stabilities of UiO-66-(COOH)2 turn it into a good candidate to bind with metal cations. As a result, a new class of lanthanide luminescent MOFs was generated by encapsulating Eu3+ into UiO-66-(COOH)2 crystals. The key point of the probe design was the way to introduce the reactive site for H2S into the framework. Considering the flexibility of functionalization of MOFs, the design could be done by the introduction of active metal center (Cu2+) into the framework as the H2S-responding site. Cu2+ ions in aqueous solution have been found to be able to quench the fluorescence of Eu3+ ions.33-34 This inspired us to incorporate Cu2+ ions into Eu3+@UiO-66-(COOH)2 to form a luminescence-quenched Eu3+/Cu2+@UiO-66-(COOH)2. H2S has a strong affinity for Cu2+ ions, which as a result, weaken the quenching effect of Cu2+ ions on Eu3+ ions. Therefore, in the

4


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presence of NaHS (commercially available H2S donor), the changes on the fluorescence intensity of Eu3+/Cu2+@UiO-66-(COOH)2 would be observed.

2. EXPERIMENTAL SECTION Synthesis of UiO-66-(COOH)2. In a round-bottom flask equipped with reflux condenser and magnetic stirrer, 1,2,4,5-benzenetetracarboxylic acid (H4btec) (2.54 g, 10 mmol) and zirconium tetrachloride (ZrCl4) (2.43 g, 10.4 mmol) were dispersed in distilled water (60 mL) and acetic acid (40 mL) at room temperature under stirring and then heated at 100 oC for 24 h to yield a powder product. The product was soaked in anhydrous methanol for 3 days at room temperature, during which time the extract was decanted and fresh methanol was added every day. Then the sample was treated with acetone similarly for another 5 days. This process was carried out to wash out residual reagents in the pores. After removal of acetone by decanting, the sample was dried under a dynamic vacuum at 70 oC to yield the final product.

Preparation

of

Eu3+@UiO-66-(COOH)2.

Eu3+@UiO-66-(COOH)2

was

prepared by heating the mixture of compound UiO-66-(COOH)2 (0.1 g) and Eu(NO3)3·6H2O (0.446 g, 1 mmol) in distilled water (10 mL) at 60 oC. The solid was isolated by centrifugation for 10 min at 8000 rpm, extensively 3 times washed with distilled water followed by exchanging it with acetone over 5 days. During this period, acetone was freshly exchanged 3 times per day. Then, the volatile acetone was removed under vacuum at 70 oC. 5



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Preparation of Eu3+/Cu2+@UiO-66-(COOH)2. Eu3+/Cu2+@UiO-66-(COOH)2 was prepared by heating the mixture of compound UiO-66-(COOH)2 (0.1 g), Eu(NO3)3·6H2O (0.446 g, 1 mmol), and Cu(NO3)2·2.5H2O (0.465 g, 1 mmol) in distilled water (10 mL) at 60 oC. The solid was isolated by centrifugation for 10 min at 8000 rpm, extensively 3 times washed with distilled water followed by exchanging it with acetone over 5 days. During this period, acetone was freshly exchanged 3 times per day. Then, the volatile acetone was removed under vacuum at 70 oC.

Luminescent Sensing Experiments. For the experiments of sensing various biologically relevant species, powder samples of MOFs (2 mg) were introduced into HEPES buffer (2 mL) (10 mM, pH 7.4) of NaF, NaCl, NaBr, NaI, HCOONa, CH3COONa, NaNO2, NaNO3, Na2HPO4, Na3PO4, (NaPO3)3, Na2S2O3, Na2SO4, NaHCO3, NaN3, Na2SiO3, Angeli’s salt (AS, the HNO source), DEA/NONOate (the NO source), glutathione (GSH), homocysteine (Hcy) and cysteine (Cys). The mixtures were then shaken to form suspensions for luminescent measurements. The concentration of MOFs was 1mg/mL. All the fluorescence spectra were measured in HEPES buffer (10 mM, pH 7.4) at an excitation of 305 nm light.

3. RESULTS AND DISCUSSION

3.1. Crystal Structure and Characterization

In

virtue

of

the

Zr-based

MOF

UiO-66-(COOH)2

or

Zr6O4(OH)4(O2C-C6H2-CO2-(CO2H)2)6·xH2O (x ≈ 16) high chemical and thermal 6


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stability, it has attracted intense research interests during the past few years.35-36 This porous solid frame material is constructed of Zr6-octahedra [Zr6O4(OH)4] but bounded to

benzene-1,2,4,5-tetracarboxylic

acid

(H4btec)

ligand

to

shape

cubic

three-dimensional (3D) microporous material involving tetrahedral and octahedral cages linked through triangular windows (Figure 1a).37 Field-emission scanning electron microscope (FE-SEM) and transmission electron microscopy (TEM) images (Figure S1, Supporting Information) were taken to reveal the morphology of the synthesized UiO-66-(COOH)2. In this work, the reflux reactions were employed to obtain UiO-66-(COOH)2 single crystals with particle sizes of 80-100 nm. It is also worth mentioning here that a smaller particle size is conducive to guest species transfer within UiO-66-(COOH)2 crystals. As can be seen from Figure 1b, the simulated power X-ray diffraction (PXRD) pattern of primary UiO-66 is featured by two peaks at 7.4° and 8.5° respectively, representing the crystal plane (111) and (200).38 In order to determine the phase purity of synthesized MOFs, PXRD experiments have been conducted at ambient conditions (Figure 1b). The PXRD pattern of synthesized UiO-66-(COOH)2 was in a good match with that simulated from UiO-66 single crystal structure data, indicating an isostructural UiO-66 framework topology (Figure 1b). Notably, only two carboxylate arms of the H4btec act as linkers yet the rest of the two is uncoordinated in their protonated form of -COOH. The uncoordinated -COOH groups point to the pores to constitute an open form configuration. The presence of the free -COOH groups on the linkers were 7



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further evidenced by characterizing the UiO-66-(COOH)2 under study with the FT-IR spectrum (Figure S2, Supporting Information). A strong band is observed at 1716 cm-1, which can be attributed to the C=O stretching vibration of uncoordinated – COOH function groups in UiO-66-(COOH)2. The permanent porosity of the prepared UiO-66-(COOH)2 was determined by N2 sorption isotherm after guest removal exhibiting Brunauer-Emmett-Teller (BET) surface areas of 620.09 m2g-1 (Figure S3, Supporting Information). This value, which is higher than the literature reported 415 m2g-1 of UiO-66-(COOH)2, probably owing to a better sample activation process and crystallinity,35 is quite lower than the parent framework UiO-66 of 1110 m2g-1.39 The reduction of the BET surface area could be ascribed to the steric hindrance effect of uncoordinated –COOH groups within the pores, which lessened the access of N2 molecules. The reactive nature of the uncoordinated –COOH groups and the permanent porosity of UiO-66-(COOH)2 are potentially available for conducting various modifications. UiO-66-(COOH)2 was metalated by the reaction of metal cations with the uncoordinated –COOH groups in water at 60 oC. The ICP-MS analysis, FT-IR spectra, N2 sorption isotherm, and PXRD patterns analysis certificated that the metal cations were successfully coordinated to the free –COOH in the

pores

of

UiO-66-(COOH)2.

The

metal

cations

loading

level

in

Eu3+@UiO-66-(COOH)2 and Eu3+/Cu2+@UiO-66-(COOH)2 were quantified by ICP-MS measurement (Table S1, Supporting Information), which show the molar ratio of Zr:Eu and Zr:Eu:Cu are 2.28:1 and 7.14:3.13:1, respectively. As shown in 8


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Figure S2 (Supporting Information), the band of uncoordinated –COOH groups almost entirely disappeared at 1716 cm-1, illustrating the coordinated interactions between the uncoordinated –COOH and the metal cations. After incorporating metal cations, Eu3+/Cu2+@UiO-66-(COOH)2 displayed analogous N2 sorption behavior to UiO-66-(COOH)2 (Figure S3, Supporting Information), yet the BET surface area of Eu3+/Cu2+@UiO-66-(COOH)2, as expected, showed a decreased value of 378.48 m2g-1, which is owing to the steric hindrance effect of the metal cations within the pores.

The

PXRD

patterns

for

Eu3+@UiO-66-(COOH)2

and

Eu3+/Cu2+@UiO-66-(COOH)2 gained under the same conditions were identical with that for the as-synthesized sample of UiO-66-(COOH)2, indicating that metalation does not destroy the stable framework. The successful Eu3+ and Cu2+ loading did not affect the morphology of UiO-66-(COOH)2, as demonstrated by SEM and TEM in Figure

S1

(Supporting

Information).

After

addition

of

NaHS

into

Eu3+/Cu2+@UiO-66-(COOH)2, the materials retained its crystallinity well, as evidenced by the PXRD pattern (Figure S4, Supporting Information).

3.2. Photoluminescence and Logic Gate Properties

The

successful

constitution

of

Eu3+@UiO-66-(COOH)2

and

Eu3+/Cu2+@UiO-66-(COOH)2 were also verified by studying the spectra in HEPES buffer (pH=7.4) (Figure 2 and Figure S5, Supporting Information). In the inclusion of Cu2+, the fluorescence intensity of Eu3+ in Eu3+/Cu2+@UiO-66-(COOH)2 was remarkably suppressed and the ligand-centered (LC) emission was obviously 9



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enhanced

(Figure

2a).

The

fluorescence

emission

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spectrum

of

the

Eu3+/Cu2+@UiO-66-(COOH)2 suspension allows the observation of two types of luminescence: one is the sharp characteristic emissions of Eu3+ and the other is the broad LC emission at 393 nm. The presence of the characteristic emissions of Eu3+ in the fluorescence emission spectra validate an energy transfer (ET) from the H4btec ligands to Eu3+, yet the strong LC emission in the fluorescence emission spectra suggests that the ET efficiency is low (the intensity ratio I615/I393 of the 5D0→7F2 line at 615 nm to that of LC emission at 393 nm is 0.45). With the unsaturated electronic state (3d9), Cu2+ tends to gain electrons. This electronic state may promote the interaction of Cu2+ ions with the Lewis basic carboxylic oxygen sites within Eu3+/Cu2+@UiO-66-(COOH)2 through the cooperative effect as the form of O-Cu-O.40-41 As shown in Scheme 2, the bonding interaction decreased the antenna efficiency of the H4btec ligands to Eu3+, inducing the luminescence quenching of Eu3+.42 According to Pearson’s hard-soft acid-base theory,43 Cu2+ is a soft ion (soft acid) and interacts priorly with sulfide (a soft base). Therefore, the Cu2+ ions in Eu3+/Cu2+@UiO-66-(COOH)2 constructed highly stable species with the targeting sulfide ion (Ksp of CuS = 1.27 × 10-36).44 In comparison, in the presence of sulfide, the enhanced

Eu3+-luminescence

and

the

decreased

LC

emission

of

Eu3+/Cu2+@UiO-66-(COOH)2 demonstrated the more effective ET (I615/I393 = 4.16). Moreover, with the two inputs Cu2+ (Input1) and NaHS (Input2), the emission properties

of

Eu3+@UiO-66-(COOH)2

were

successfully

explained

as

the 10


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IMPLICATION type logic gate. In this system, the presence and absence of the two chemical inputs Cu2+ and NaHS can be defined as “1” and “0” states, and the fluorescence intensity ratio of I615/I393 as the output for the logic gate. Figure 2a and b show the luminescence features of the Eu3+@UiO-66-(COOH)2 in the presence of the four possible input combinations (Input1, Input2), which were (0, 0), (1, 0), (0, 1), and (1, 1). The fluorescence intensity ratio of I615/I393 was greatly reduced only in the presence of Cu2+ (1, 0) due to the ET process. However, the fluorescence intensity ratio of I615/I393 above the threshold level (1, I615/I393) was observed in the absence (0, 0) and presence of both the inputs (1, 1) and also NaHS alone (0, 1). Therefore, monitoring the fluorescence intensity ratio of I615/I393 and with the two inputs as Cu2+ and NaHS, an IMPLICATION type logic gate can be constructed. Equivalent combinatorial logic circuit and truth table for the IMPLICATION logic gate are shown in Figure 2c and d.

3.3. Sensing Properties Fascinating fluorescence and logic gate properties of Eu3+/Cu2+@UiO-66-(COOH)2 made it particularly attractive for potential applications like fluorescent sensors. As an excellent chemosensor, rapid response, high selectivity and sensitivity are quite crucial.

As

shown

in

Figure

S6

(Supporting

Information),

probe

Eu3+/Cu2+@UiO-66-(COOH)2 could detect sulfide more quickly (within 30 s) than the majority of the reported H2S probes (typically 2 min - 2 h) Supporting

Information).

In

order

to

verify


11-13, 45-52

the

(Table S2,

selectivity

of 11


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Eu3+/Cu2+@UiO-66-(COOH)2, we tested the fluorescence response of this probe to the environmentally and biologically relevant species in HEPES buffer (pH=7.4). As shown in Figure 3 and Figure S7 (Supporting Information), the fluorescent probe demonstrates virtually negligible fluorescence in response to 5 mM of the environmentally and biologically relevant species (F-, Cl-, Br-, I-, HCOO-, CH3COO-, NO2-, SiO32-, NO3-, HPO42-, PO43-, P3O93-, S2O32-, SO42-, HCO3-, N3-, HNO, NO, GSH, Hcy and Cys). The LC emission in Eu3+/Cu2+@UiO-66-(COOH)2 indicated an evidently selective fluorescence enhancing behaviour merely with SiO32- ions (I615/I393 = 0.1034) and had no bearing on sulfide sensing. HNO and the thiol-containing amino acids (GSH, Hcy and Cys) showed a certain degree influence on the fluorescence intensity of Eu3+/Cu2+@UiO-66-(COOH)2 and also had no bearing on sulfide sensing. However, the addition of NaHS (5 mM) induced the large and instant increase of fluorescence

intensity

of

Eu3+

and

quenching

the

LC

emission

of

Eu3+/Cu2+@UiO-66-(COOH)2. The probe Eu3+/Cu2+@UiO-66-(COOH)2 could be useful for selectively sensing sulfide, even with the involvement of potential competition analytes (Figure S8, Supporting Information). The fluorescence responses of Eu3+/Cu2+@UiO-66-(COOH)2 were further investigated with diﬀerent concentrations of NaHS (Figure 4 and Figure S9, Supporting Information). As shown in Figure 4a, the good relationship (R2 = 0.97947) of the fluorescence intensity ratio (I615/I393) with sulfide concentration can be established as a function of I615/I393 = -3.95·exp(-0.002·CNaHS)+4.22 in the range of 12


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0-5 mM, suggesting that sulfide also can be detected quantitatively using Eu3+/Cu2+@UiO-66-(COOH)2. As illustrated in Figure 4b, the plots of fluorescence intensity ratio I615/I393 vs the concentration of NaHS well follow a good linear relationship (R2 = 0.98931) within the concentration range of 0-625 µM and it can be curve-fitted into I615/I393 = 0.005·CNaHS + 0.333. The limit of detection (LOD=3δ/S, where 3 is the factor at the 99% confidence level; δ is the standard deviation for twenty replicating fluorescence measurements of blank solutions; and S is the slope of the calibration curve) value of Eu3+/Cu2+@UiO-66-(COOH)2 towards sulfide was calculated to be 5.45 µM, which is comparable with or even better than some other previously reported small-molecule and MOF-based fluorescent probes for H2S (Table S2, Supporting Information).11-13,

47-52

Taking into consideration the

concentration of H2S that triggers physiological responses ranges of 10-600 µM

53-55

the proposed probe is expected to have practical potential for sensing of H2S in important biological samples.

4. CONCLUSION In summary, we illustrated a general designing strategy of luminescent IMPLICATION logic gate for fluorescent turn-on and ratiometric sensing of H2S based on functionalized nano MOF, Eu3+/Cu2+@UiO-66-(COOH)2. The MOF Eu3+/Cu2+@UiO-66-(COOH)2 simultaneously displays the characteristic Eu3+ sharp emissions and the broad LC emission. Because of the strong enhancement on the fluorescence of Eu3+ and quenching the broad LC emission by H2S through its high 13



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Page 14 of 29

affinity for Cu2+ ions, this probe exhibits rapid response, and high sensitive in-situ fluorescent turn-on sensing of H2S over other environmentally and biologically relevant species under physiological conditions. It is suggested that this work may arouse research interests in the field of MOF-based sensors for H2S to gain a deeper understanding of H2S production, distribution, and action.

ASSOCIATED CONTENT

Supporting Information

General experimental details, SEM and TEM images, and other luminescence data. This material is available free of charge via the Internet at http://pubs.acs.org.

AUTHOR INFORMATION Corresponding Author *

E-mail:

[email protected]

(Y.

Yang),

[email protected](Y.

Cui),

[email protected] (G. Qian)

Notes The authors declare no competing financial interest.

ACKNOWLEDGMENT The authors gratefully acknowledge the financial support for this work from the National Natural Science Foundation of China (Nos. 51372221, 51472067, 51472217, 51432001 and 51632008), Zhejiang Provincial Natural Science Foundation of China 14


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(Nos. LR13E020001 and LZ15E020001), and Fund amental Research Funds for the Central Universities (Nos. 2015QNA4009, 2015FZA4008, and 2014XZZX005).

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SCHEMES

Scheme 1. Synthetic scheme and representative crystalline structure: (a) the tetrahedral cage of UiO-66-(COOH)2, (b) the tetrahedral cage of

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Eu3+/Cu2+@UiO-66-(COOH)2, Eu3+@UiO-66-(COOH)2.

and

(c)

the

tetrahedral

Page 24 of 29

cage

of

FIGURES

24


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Figure 1. Crystal structure and characterization of MOFs. (a) 3D cubic framework structure of UiO-66-(COOH)2 in ball-and-stick representation. The large yellow spheres represent the void regions inside the cages. For clarity, the hydrogen atoms have been removed from the structural plots. (b) PXRD patterns of the simulated UiO-66,

as-synthesized

UiO-66-(COOH)2,

Eu3+@UiO-66-(COOH)2,

and

Eu3+/Cu2+@UiO-66-(COOH)2.

25



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Figure 2. Operations of the IMPLICATION logic gate. (a) Fluorescence spectra of Eu3+@UiO-66-(COOH)2 in the presence of four input modes: (Magenta) no input; (Yellow) 5 mM NaHS; (Dark yellow) 1 mM Cu2+; (Navy) 5 mM NaHS and 1 mM Cu2+. (b) Fluorescence intensity ratio (I615/I393) in the form of a bar representation, with a threshold of I615/I393 = 1 for output 1 or 0. (c) IMPLICATION logic gate represented using a conventional gate notation; an inactive output signal is obtained when Cu2+ = 1 and NaHS = 0. (d) Truth table for the IMPLICATION logic gate; Cu2+ and NaHS are inputs to the system; fluorescence intensity ratio I615/I393 is the output signal of Eu3+@UiO-66-(COOH)2.

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Scheme 2. Schematic illustration of the fluorescence detection mechanism.

Figure 3. Ratios of fluorescence intensity (I615/I393) of Eu3+/Cu2+@UiO-66-(COOH)2 towards various analytes (5 mM) after 30 s of analyte addition.

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Figure 4. Concentration dependence of the fluorescence intensity ratio (I615/I393). The concentration (a) 0-5 mM and (b) 0-625 µM of NaHS.

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Table of Contents Graphic and Synopsis

Turn-on and Ratiometric Luminescent Sensing of Hydrogen Sulfide based on Metal-Organic Frameworks †

‡

†

†

†

Xin Zhang, Quan Hu, Tifeng Xia, Jun Zhang, Yu Yang,*, Yuanjing Cui,*, †§

Banglin Chen, ,

and Guodong Qian*,

†

†

KEYWORDS: metal-organic frameworks, turn-on, fluorescent probes, hydrogen sulfide, ratiometric sensing A general strategy to achieve turn-on and ratiometric H2S sensing by encapsulating Eu3+ and Cu2+ into a metal-organic framework (MOF) UiO-66-(COOH)2 is developed. Because H2S can strongly increase the characteristic Eu3+ emissions and quench the broad ligand-centered (LC) emission through its superior affinity for Cu2+ ions, Eu3+/Cu2+@UiO-66-(COOH)2 exhibits highly selective and sensitive turn-on and ratiometric sensing of H2S.

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Turn-on and Ratiometric Luminescent Sensing of Hydrogen Sulfide

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