Fluorescence Quenching Investigation of Methyl Red Adsorption on

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Fluorescence Quenching Investigation of Methyl Red Adsorption on Aluminum-Based Metal–Organic Frameworks Jun-Kai Chen, Shan-Min Yang, Bing-Han Li, Chia-Her Lin, and Szetsen Lee Langmuir, Just Accepted Manuscript • DOI: 10.1021/acs.langmuir.7b04240 • Publication Date (Web): 06 Jan 2018 Downloaded from http://pubs.acs.org on January 6, 2018

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Fluorescence Quenching Investigation of Methyl Red Adsorption on Aluminum-Based Metal–Organic Frameworks

Jun-Kai Chen, Shan-Min Yang, Bing-Han Li, Chia-Her Lin,* and Szetsen Lee* Department of Chemistry, Chung Yuan Christian University, Jhongli District, Taoyuan City, 32023, Taiwan

Abstract The adsorption of methyl red (MR) isomers (ortho, meta, para) on metal–organic frameworks (MOFs) was investigated by using fluorescence quenching technique. All three MR isomers were found to quench the fluorescence of MOFs effectively. Non-linear fluorescence quenching trends were observed in Stern-Volmer plots. A modified non-linear Stern-Volmer equation with the concepts of multiple adsorption sites, adsorption strength, and quencher accessibility was successfully adopted to fit the fluorescence quenching data. The fitted parameters were correlated with the structural properties of MRs and MOFs. The order of quenching efficiency was found to be m-MR > p-MR > o-MR for all MOFs. It indicates that MR molecules not only adsorb via carboxylate-metal bonding, but also adsorb through π–π interactions between the aromatic rings of MR and linker molecules in MOFs. The position of the carboxylate group in MRs and the structure of the linkers in MOFs are the key factors affecting the fluorescence quenching efficiency.

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*Corresponding authors. E-mail address: [email protected] (Szetsen Lee), [email protected] (Chia-Her Lin)


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1.

Introduction Metal–organic frameworks (MOFs) are a novel type of microporous hybrid

organic–inorganic crystalline materials, which exhibit an ample range of optical properties resulting from their versatile structures. Many articles have been published in the studies of the optical properties of MOFs, in both fundamental and application aspects. Investigations in band gap tuning,1 luminescence enhancement,2–4 surface enhanced Raman scattering,5–7 second-harmonic generation,8,9 and photoluminescence10 have been reported. In MOFs, the inorganic and the organic components as well as metal-to-ligand or ligand-to-metal charge transfer can generate luminescence.2–4 In addition, some guest molecules within MOFs can also emit and induce or quench luminescence of MOFs (guest-dependent luminescence). The variation of MOF structures can be used to alter emissive properties for molecular sensing on the changes in local coordination environment.10 Usually MOFs containing metal ions emit luminescence from the organic (linker) component rather than on the inorganic (metal) component. Emission from paramagnetic metal complexes is usually weak because ligand-field (d–d) transitions may lead to strong reabsorption or quenching of fluorescence generated from the organic linkers. On the other hand, MOFs with metal ions without unpaired electrons, especially those with filled orbital, e.g. d10 configurations, can have strong fluorescence based on linkers.3,11 This explains why Zn- and Cd-based MOFs are strongly luminescent, but those MOFs with paramagnetic metal


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ions are not.3 MOFs containing Al also fit into the strong luminescence category.12,13 The studies of the adsorption of dyes14–16 and volatile organic compounds17–20 on MOFs have been conducted to a great extent. The understanding of the fundamental mechanisms of the removal of environmental pollutants using MOFs is of primary concern. Up to date, typical methods used in the study of adsorption in MOFs are gravimetric,17,18 volumetric,21 and calorimetric22 measurements. Alternatively, optical methods such as FTIR15,23 and UV-visible absorption15,24 have been used to characterize the dye adsorption behavior of MOFs. Fluorescence quenching in MOFs has also been successfully adopted in chemical sensing for aromatic molecules,15,25 explosives,26,27 metal ions,12,13,28 and studying charge-transfer interactions29 and guest molecule diffusivity.30 In this work, we probed the adsorption behavior of methyl red (MR) isomers (ortho, meta, para) by observing the fluorescence quenching of Al-based MOFs. The fluorescence intensities of the Al-containing MOFs used in this study are relatively weak as compared to those of Zn-based MOFs, but enough for performing quenching experiments. The MOF fluorescence intensities were found to be inversely proportional to the amount of adsorbed MR molecules. The observed non-linear fluorescence quenching trends suggest the simultaneous presence of dynamic quenching and static quenching.31–37 The goal of this work is to find a general non-linear Stern-Volmer (SV) model to describe fluorescence quenching with non-linear trends. Starting from the traditional form of the SV model, by adding the


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concepts of multiple adsorption sites, adsorption strength, and quencher accessibility, a “Goldilocks form” of SV model is proposed. Traditional methods of studying dynamic quenching require the use of time-resolved techniques for measuring fluorescence lifetimes and quenching rate constants.34, 36–38 However, in this work, by fitting the fluorescence quenching data with the general non-linear SV equation, the dynamic quenching constant can be extracted with the static approach. The fitted parameters are correlated with the structural properties of MOFs and MR isomers.

2.

Experimental Section The MOFs used in this study include MIL–100 (Al),39 MIL–69(Al),40,41 DUT–4(Al),41

and CYCU-4(Al).42 MIL–100 (Al) [Al3F(H2O)2O(BTC)2·nH2O] is an aluminum trimesate-based MOF with an organic linker 1,3,5–benzenetricarboxylate (BTC). MIL-69 [Al(OH)(NDC)] and DUT-4 [Al(OH)(NDC)] are 2,6-naphthalenedicarboxylate (NDC)-based frameworks. CYCU-4(Al) is an infinite number of rod-shaped aluminum carboxylate secondary building units [Al(OH)(SDC)] with an organic linker 4,4-stilbenedicarboxylate (SDC) to build up the frameworks. All MOFs were synthesized and activated according to published procedures.39–44 An X-ray diffractometry model (Bruker D8 ADVANCE Eco) was used for powder X-ray diffraction (PXRD) pattern recording. Please refer to Supporting Information for the PXRD parameters used in this work and the PXRD data of synthesized


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MOFs. MR dyes (o-, m-, and p-) were used as received without further purification. Ethanol solutions of MRs (0 ~ 0.1 mM), and MOFs (5×10-3 mmol) were mixed and ultrasonicated for 15 min to reach good dispersion before fluorescence quenching experiments. Wavelength at 350 nm was chosen for excitation using a fluorescence spectrometer (FluoroSENS, Gilden Photonics Ltd.). Fluorescence emission of MOFs between 360 and 440 nm was monitored to avoid the interference from the fluorescence emission of MRs or background. The fluorescence intensities of MRs are of several orders of magnitudes weaker than that of the MOFs used in this study (Supporting Information, Figure S5).

3.

Results and discussion The structures of MR isomers are shown in Figure 1. The trends of the fluorescence

quenching intensity ratios I0/IC of MOF-MR solutions are plotted in Figure 2, where I0 and IC are the fluorescence intensities for MOFs without quencher (MR) (i.e. pure MOFs) and MOFs with various MR concentrations (C = 10-4 ~ 10-7 M), respectively. For the fluorescence spectra of all the MOF-MR pairs with various concentrations of MRs added, please refer to Figure S2 in Supporting Information. Apparently, all MOFs have non-linear trends of the fluorescence quenching intensity ratios I0/IC with different MR concentrations. Therefore, the simple linear SV equation (eq. 1) does not apply.


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I0 = 1 + K SV C IC

(1)

In eq. 1, KSV is the quenching constant. It can be either dynamic (KD) or static (KS), if there is only one single type of quenching (static or dynamic) process.36 The non-linear trend in SV plots have been interpreted by several studies.31,36,37,45,46 Usually, the reason for the SV curve with “upward” (positive deviation) non-linear trend along the x-axis (quencher concentration) can be ascribed to the participation of static quenching mechanism at higher quencher concentrations.31,45,46 The simultaneous presence of dynamic (collisional) and static quenching is one of the origins of non-linear behavior in SV plots (eq. 2). I0 = (1 + K S C )(1 + K D C ) IC

(2)

Static quenching involves the formation of quencher-fluorophore pairs, which are associated with an equilibrium constant KS. If using the Langmuir adsorption isotherm model47–49 (eq. 3) to interpret the phenomenon of static-only quenching, the formation of the quencher-fluorophore pair is equivalent to the formation of the adsorbate-adsorbent pair. Therefore KS represents both the static quenching constant in the SV equation (eq. 2) and the equilibrium adsorption constant in the Langmuir isotherm equation (eq. 3),

θ=

KSC 1+ KSC

(3)

where θ is the surface fractional coverage. The quadratic form of SV equation in eq. 2 appears to be perfect for fitting the non-linear fluorescence quenching data in this work. However, it is symmetric, i.e. the fitted


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parameters KD and KS cannot be distinguished. Swapping these two parameters does not change eq. 2. We have tried to fit our fluorescence quenching data with eq. 2, appreciable deviation was observed. Apparently the quadratic SV equation in eq. 2 could not provide satisfactory interpretation to our MOF-MR fluorescence quenching data trends. Studies on protein fluorescence quenching often find that proteins in the native (folded) state can have a fraction of the emission that is not accessible to quenchers, and that denaturation (unfolded state) of the proteins usually displays the accessibility of all the fluorophores, such as tryptophan residues, to quenchers.31–34 A similar model has also been used to account for the efficient energy transfer inside MOFs and electron transfer quenching at the MOF/solution interface.36,37 Since MOFs have porous three-dimensional structures, the fractional accessibility of MOF fluorophores to quenchers is possible. By considering the concept of fractional accessibility,31,34 eq. 2 is modified to eq. 4,

I 0 (1 + K S C )(1 + K D C ) = 1 + (1 − f ) K S C IC

(4)

where the f factor represents the fraction of the accessible fluorophores in MOFs. Please refer to Supporting Information for the comprehensive derivation of eq. 4. When f = 1, eq. 4 reduces back to eq. 2. Nonetheless, appreciable deviation was still there by fitting the MOF-MR fluorescence quenching data with eq. 4. We further consider the MOF surface as a non-uniform surface, i.e. there are several possible sites for MR molecules to adsorb on the MOF surface (or trapped inside the pores).


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The possible ways of adsorption are via open and unsaturated metal sites coordination (with carboxylate groups), ligand ring π-π interaction, and other types of (weak) physisorption or due to adsorption at different crystal faces of the MOFs. 17 The Sips equation35,47–49 (eq. 5) is well known and suited for describing the multi-site adsorption phenomenon,

θ=

KSC n 1+ KSCn

(5)

where the exponent n is called the adsorption strength, the effective adsorption sites47–49 or the index of affinity heterogeneity.35 n does not have to be an integer. It is an averaged value. Different sites have different affinity for binding. The Sips exponent n with values greater than unity implies a result of surface heterogeneity and positive cooperativity between MRs and MOFs.50 Weak binding affinity shows n values less than 1. In the study of surface adsorption, it is well-known that Sips isotherm (eq. 5) is a combined expression of Langmuir and Freundlich models for describing the heterogeneous adsorption systems.47 At low adsorbate concentrations, it becomes the Freundlich isotherm; while at high concentrations, it follows a monolayer adsorption capacity characteristic of the Langmuir isotherm. The Freundlich model describes the non-ideal and reversible adsorption, not restricted to monolayer adsorption. This model can be applied to multilayer adsorption with non-uniform distribution of adsorption sites. The MOF surface can be considered as a non-uniform surface with inequivalent adsorption sites. Therefore, the non-linear SV equation in eq. 4 is further modified into eq. 6.


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n I 0 (1 + K S C ) (1 + K D C ) = IC 1 + (1 − f ) K S C n

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(6)

Table 1 summarizes the fitted parameters KS, KD, n, and f using eq. 6. The first thing draws our attention is that, except for o-MR adsorbed on CYCU-4, the f values are all nearly equal to 1 and the n values are all larger than 1. The near unity f values indicate that the quenching of MOF fluorescence by MRs is quite efficient. Also, high KS values are accompanied with high n values, especially for DUT-4 and MIL-69. It suggests that energy transfer efficiency or the quenching efficiency in DUT-4 and MIL-69 is high with m- and p-MR as quenchers. DUT-4 and MIL-69 have the same kind of linkers, 2,6-NDC, which has a fused ring structure. The interconversion between these two structures does not occur.51 Such a “rigid” structure makes the conjugated frameworks in the entire MOF to be less twisted. Studies have shown that conjugated aromatic molecules are efficient fluorescence quenchers due to energy transfer by non-covalent interaction.52–56 In contrast, for o-MR adsorbed on MIL-69 and DUT-4, KS and n are relatively small. It can be attributed to the position of the carboxylate group in o-MR, which cannot coordinate on the Al sites of MOFs effectively. The azo bond in o-MR is perturbed by hydrogen bonding with the carboxylate group and thus loses its π character. Therefore the resonance structure of the MR molecule is interrupted. The π-π interaction with the linker molecules in MOFs is reduced. Hence the quenching efficiency is affected. MIL-100, with BTC as linkers, can be somewhat flexible by the change in the inorganic


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node or the dihedral angle between the BTC and the secondary building unit (SBU). The flexibility of CYUC-4 is also expected because SDC is an inherent flexible linker,57 not as rigid as BTC and 2,6-NDC. Such a flexible structure allows twisting heterogeneity and significant structural disorder. This disorder disrupts the framework conjugation and interrupts the electronic delocalization. Hence the quenching energy transfer efficiencies of MIL-100 and CYCU-4 are not as high as those of DUT-4 and MIL-69. Therefore KS and n of MIL-100 are relatively smaller than those of MIL-69 and DUT-4. Despite the flexibility of CYCU-4, the higher KS and n values of CYCU-4 with m- and p-MR as compared with MIL-100 are due to that SDC has two aromatic rings. Strong π-π interaction is expected. The f factor of o-MR on CYCU-4 is 0.4781, which indicates that nearly half of the fluorophores in CYCU-4 cannot be quenched effectively by o-MRs. This is correlated with small (and less than 1) n value of o-MR on CYCU-4. As mentioned before, the resonance structure of o-MR is interrupted. The π-π interaction with the linker molecules in CYCU-4 is reduced. Hence the quenching efficiency is affected. In general, m-MR shows the largest KS and n values for almost all types of MOFs. In contrast, o-MR has the smallest KS and n values. It can be ascribed to the position of the carboxylate group. Apparently the adsorption between m-MR and MOF surface is the most versatile and effective. m-MR cannot form internal hydrogen bonds and is in twist form.58–60 Such a structure allows m-MR to dock on MOF surface simultaneously via the carboxylate


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group, azo bond, and the ring which reflects the large n values between 1.4 and 2.0. For p-MR, which is also in twist form, can only contact on the MOF surface either by the carboxylate group or by the ring/azo group. Simultaneous contact on the MOF surface with all functional groups is difficult. Therefore, the order of n values is m-MR > p-MR > o-MR. The fitted KD constants are of the order of 104 for all MOFs. No apparent trends are related to the structures of MOFs (pore size and surface area, Table 2) and MRs (carboxylate group position). Typical fluorescence lifetimes of MOFs are known to be between 3 ns and 30 ns.10,61,62 This means that the bimolecular collision rate constants kq of MOFs and MRs are between 3 × 1011 M−1 s−1 and 3 × 1013 M−1 s−1. It is beyond the diffusion-controlled limit (1010 M−1 s−1).36 Since the solutions of MOFs and MRs were treated with ultrasonic agitation, we believe that this might explain the large values of kq estimated in this work.

4.

Conclusions The fluorescence quenching effect of MRs adsorbed in MIL–100, CYCU-4, DUT-4, and

MIL–69 was investigated. Non-linear fluorescence quenching trends were observed in SV plots. A modified non-linear SV equation with the concepts of multiple adsorption sites, adsorption strength, and quencher accessibility was successfully adopted to fit the fluorescence quenching data. The fitted parameters in the non-linear SV equation, KS, KD, n, and f, were interpreted with the interactions and energy transfer efficiency between MOFs


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and MRs. m-MR shows the largest KS and n values among all types of MOFs. In contrast, o-MR has the smallest KS and n values. It can be ascribed to the position of the carboxylate group in MRs. Besides the open and unsaturated metal sites coordination with carboxylate groups, there are other ways of adsorption are via ligand ring π-π interaction and weak physisorption or due to adsorption at different crystal faces of the MOFs. The static quenching constant (KS) and the adsorption strength (n) are the most sensitive parameters in the quenching efficiency between MOFs and MRs. The dynamic quenching constants (KD) are of the same order for all MOFs. No apparent trends of KD are related to the pore size and surface area of MOFs and carboxylate group position in MRs.

Supporting Information for Publication Synthesis and characterization data for starting materials and all MOFs, including PXRD data, fluorescence spectra of MOF-MR pairs, and the derivation of non-linear SV equations are available free of charge via the Internet at http://pubs.acs.org.

Acknowledgments We dedicate this paper in the memory of the late Professor Hsi-Ya Huang (Department of Chemistry, Chung Yuan Christian University), whose enthusiasm and encouragement supported this work. We acknowledge financial support from Chung Yuan Christian


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University and the Ministry of Science and Technology of Taiwan (MOST 103-2632-M-033-001-MY3, MOST 105-2119-M-033-004, and MOST 106-2113-M-033-010).


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Figure Caption: Figure 1. Structures of MR isomers. Figure 2. Stern-Volmer (SV) plots of MOFs with various concentrations of MRs added in ethanol at 350 nm excitation. The curves are fitted with the modified non-linear SV equation (eq. 6).


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o-MR

m-MR

p-MR

Figure 1. Structures of MR isomers.


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30

70

MIL-69(Al)+o-MR MIL-69(Al)+m-MR MIL-69(Al)+p-MR

25

DUT-4(Al)+o-MR DUT-4(Al)+m-MR DUT-4(Al)+p-MR

60

50

I0/IC (Arbi. Unit)

I0/IC (Arbi. Unit)

20

15

10

40

30

20

5 10 0 0 0.0

-5

2.0x10

-5

4.0x10

-5

6.0x10

-5

8.0x10

-4

1.0x10

-4

0.0

1.2x10

-5

2.0x10

MR concentration (M)

-5

4.0x10

-5

6.0x10

-5

1.0x10

-5

1.0x10

8.0x10

-4

1.2x10

-4

-4

1.2x10


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40


20

CYCU-4(Al)+o-MR CYCU-4(Al)+m-MR CYCU-4(Al)+p-MR

35 30 25

15

I0/IC (Arbi. Unit)

I0/IC (Arbi. Unit)

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Langmuir

10

20 15 10

5 5 0

0 0.0

-5

2.0x10

-5

4.0x10

-5

6.0x10

-5

8.0x10

-4

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-4

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0.0

-5

2.0x10


-5

4.0x10

-5

6.0x10

8.0x10

-4


Figure 2. Stern-Volmer (SV) plots of MOFs with various concentrations of MRs added in ethanol at 350 nm excitation. The curves are fitted with the modified non-linear SV equation (eq. 6).


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Table 1. Fitted MOF parameters using the modified non-linear SV equation (eq. 6). Unit: KS: M-1; KD: M-1.

Al-MOF

MIL-69

DUT-4

MIL-100

CYCU-4

dye

KS

KD

n

f

R2

o-MR

6.14×104

1.21×104

1.1670

0.9999

0.9988

m-MR

6.64×108

2.64×104

2.0031

0.9999

0.9978

p-MR

7.10×107

1.68×104

1.8264

0.9999

0.9994

o-MR

3.20×106

4.03×104

1.5935

0.9999

0.9958

m-MR

2.56×108

5.89×104

1.8690

0.9999

0.9975

p-MR

1.95×107

5.03×104

1.6475

0.9999

0.9989

o-MR

1.40×104

1.34×104

1.0545

0.9999

0.9989

m-MR

2.99×105

3.16×104

1.1978

0.9983

0.9991

p-MR

5.70×104

1.36×104

1.0487

0.9999

0.9927

o-MR

8.59×102

4.19×104

0.6073

0.4781

0.9971

m-MR

5.75×106

2.79×104

1.4467

0.9968

0.9936

p-MR

1.09×106

4.11×104

1.3315

0.9999

0.9962


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Table 2. MOF structural properties in this work. Al-MOFs

Pore size (nm)

Window size

Specific surface

(nm)

area (m2/g),

Ligand

BET/Langmuir MIL-100

0.65, 2.5, 3

0.52, 0.88

2152/2919

BTC

MIL-69

0.3, 0.77

0.38

10/15

2,6-NDC

DUT-4

0.85

1.29

1308/1996

2,6-NDC

CYCU-4

1.2, 1.6, 2.1

1.2

1910/2671

SDC

References: MIL-100(Al),39 MIL-69(Al),40,41 DUT-4(Al),41 CYCU-4(Al).42


Langmuir

Table of Content Graphic

25


20

I0/IC (Arbi. Unit)

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

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o-MR

m-MR

p-MR

Figure 1. Structures of MR isomers.


Langmuir

30

70


25

DUT-4(Al)+o-MR DUT-4(Al)+m-MR DUT-4(Al)+p-MR

60

50

I0/IC (Arbi. Unit)

I0/IC (Arbi. Unit)

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15

10

40

30

20

5 10 0 0 0.0

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0.0

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6.0x10

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8.0x10

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1.2x10


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CYCU-4(Al)+o-MR CYCU-4(Al)+m-MR CYCU-4(Al)+p-MR

35 30 25

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I0/IC (Arbi. Unit)

I0/IC (Arbi. Unit)

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

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10

20 15 10

5 5 0

0 0.0

-5

2.0x10

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4.0x10

-5

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-5

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0.0

-5

2.0x10


-5

4.0x10

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6.0x10

8.0x10

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Figure 2. I0/IC Stern-Volmer plots of MOFs with various concentrations of MRs added in ethanol at 350 nm excitation. The curves are fitted with the modified non-linear SV equation (eq. 6).


Fluorescence Quenching Investigation of Methyl Red Adsorption on

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