A luminescent metal-organic framework with pre-designed functionalized ligands as an efficient fluorescence sensing for Fe3+ ions

Article abstract of DOI:10.1016/j.jssc.2018.12.005

Metal-organic frameworks are a class of attractive materials for fluorescent sensing. Here, we report the exploration of fluorescent Zn-based amine/azine-functionalized MOF, TMU-17-NH₂, ([Zn(NH₂-BDC)(4-bpdb)].2DMF; NH₂-BDC = amino-1,4-benzenedicarboxylic acid, 4-bpdb = 1,4-bis(4-pyridyl)-2,3-diaza-2,3-butadiene) for highly selective and sensitive detection of Fe³⁺ in DMF solution. TMU-17-NH₂ shows fast recognition of Fe³⁺ ion with a response time of< 1 min and detection limit of 0.7 μM (40 ppb), and the luminescence is completely quenched in 10^?3 M DMF solution of Fe³⁺. Furthermore, the static quenching constant is calculated to be upper than 41,000 M^?1 by the fluorescence titration experiment in low concentration of Fe³⁺. No interferences from 250 μM As³⁺, Cd²⁺, Zn²⁺, Co²⁺, Ni²⁺, Cu²⁺, Pb²⁺, Mn²⁺ and Al³⁺ were found for the detection of Fe³⁺. The efficient fluorescent quenching effect is attributed to the photoinduced electron transfer between Fe³⁺ ions and the amino-functionalized ligand in this MOF. Moreover, the introduced azine N donors in the 4-bpdb ligand of TMU-17-NH₂ additionally donate their lone-pair electrons to the Fe³⁺ ions, leading to significantly enhanced detection ability. Furthermore, the regenerated TMU-17-NH₂ still has high selectivity for Fe³⁺ ions, which suggests that the functionalized TMU-17-NH₂ is a promising luminescent probe for selectively sensing of Fe³⁺ ions.

Full text of DOI:10.1016/j.jssc.2018.12.005

Author’s Accepted Manuscript
A Luminescent Metal-Organic Framework with
Pre-Designed Functionalized Ligands as an
3+
Efficient Fluorescence Sensing for Fe Ions
Yeganeh Davoudabadi Farahani, Vahid Safarifard
www.elsevier.com/locate/yjssc
PII:
S0022-4596(18)30546-2
https://doi.org/10.1016/j.jssc.2018.12.005
YJSSC20506
DOI:
Reference:
To appear in: Journal of Solid State Chemistry
Received date: 24 October 2018
Revised date: 1 December 2018
Accepted date: 4 December 2018
Cite this article as: Yeganeh Davoudabadi Farahani and Vahid Safarifard, A
Luminescent Metal-Organic Framework with Pre-Designed Functionalized
3+
Ligands as an Efficient Fluorescence Sensing for Fe Ions, Journal of Solid
State Chemistry, https://doi.org/10.1016/j.jssc.2018.12.005
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A Luminescent Metal-Organic Framework with Pre-Designed
Functionalized Ligands as an Efficient Fluorescence Sensing for
3+
Fe Ions
*1
Yeganeh Davoudabadi Farahani, Vahid Safarifard
Department of Chemistry, Iran University of Science and Technology, Tehran 16846-13114, Iran.
Email: vsafarifard@iust.ac.ir
Abstract:
Metal-organic frameworks are a class of attractive materials for fluorescent sensing. Here, we
report the exploration of fluorescent Zn-based amine/azine-functionalized MOF, TMU-17-NH2,
(
1
[Zn(NH -BDC)(4-bpdb)].2DMF; NH -BDC = amino-1,4-benzenedicarboxylic acid, 4-bpdb =
2
2
3
+
,4-bis(4-pyridyl)-2,3-diaza-2,3-butadiene) for highly selective and sensitive detection of Fe in
3
+
DMF solution. TMU-17-NH shows fast recognition of Fe ion with a response time of <1 min
and detection limit of 0.7 µM (40 ppb), and the luminescence is completely quenched in 10 M
DMF solution of Fe . Furthermore, the static quenching constant is calculated to be upper than
4
interferences from 250 μM As , Cd , Zn , Co , Ni , Cu , Pb , Mn and Al were found
for the detection of Fe . The efficient fluorescent quenching effect is attributed to the
photoinduced electron transfer between Fe ions and the amino-functionalized ligand in this
2
-3
3
+
-1
3+
1000 M by the fluorescence titration experiment in low concentration of Fe . No
3
+
2+
2+
3+
2+
2+
2+
2+
3+
3
+
3
+
MOF. Moreover, the introduced azine N donors in the 4-bpdb ligand of TMU-17-NH
2
3
+
additionally donate their lone-pair electrons to the Fe ions, leading to significantly enhanced
3
+
detection ability. Furthermore, the regenerated TMU-17-NH still has high selectivity for Fe
2
ions, which suggests that the functionalized TMU-17-NH is a promising luminescent probe for
2
3
+
selectively sensing of Fe ions.
Graphical abstract
Fluorescent Zn-based amine/azine-functionalized TMU-17-NH MOF for highly selective and
2
3
+
sensitive detection of Fe .
1
Tel: +98 21 77240540, Fax: +98 21 73021415
1

3
+
Keywords: Metal-Organic Frameworks; Fluorescence; Functionalization; Sensing; Fe
1
. Introduction
Metal ions play key roles in life and environment, and the detection of these metal ions is quite
3
+
important. Iron is an important metal in industry [1]. However, its cationic form, Fe , have
3
+
different influences on the human body and other biological tissues. Fe ion is one of the most
essential elements for either humans or other living organisms on account of their significance in
many biochemical processes and biological systems. Iron deficiency or excess both will lead to
various severe function condition disorders, such as sleep loss, skin diseases, iron deficiency
3
+
anemia and decreased immunity [2]. In addition, Fe is a common inorganic pollutant in water.
3
+
The presence of excess Fe in drinking water could result in a number of problems related to
3
+
human health. The recommended guideline level of Fe in water is 0.3 ppm [3]. Therefore, the
3
+
selective detection of Fe is a very important subject in biological research as well as water
treatment industry.
Various analytical techniques such as spectrophotometry, voltammetry and atomic absorption
spectroscopy have been developed for determination of iron [4]. However, these analytical
methods are often restricted by complicated sample preparation, sophisticated instruments and
high cost. Therefore, it is important to develop efficient and economical approaches to sense and
3
+
distinguish Fe ions. Recently, fluorescent sensors have been considered as some of the
2

promising and compelling devices for sensing metal ions or other small molecules because of
their high sensitivity, good reliability, and real-time detection [5].
Metal-organic frameworks (MOFs), formed by the connection of metal centers or clusters and
organic linkers through coordination bonding, have gathered immense attention due to not only
their intriguing structures of diverse pore topologies and accessible cages and tunnels, unusual
properties of permanent nanoscale porosity, high surface area, and the availability of in-pore
functionality and outer-surface modification, but also their application potential in gas storage,
separation, catalysis, proton conduction, sensing and so on [6-9]. In recent years, luminescent
sensing with MOF materials has been considered as one of the most promising technologies for
chemical and biological detection applications due to their easily induced luminescence, diverse
advantages in structural and functional components, and various detecting mechanisms [10-12].
So far, an increasing number of luminescence MOFs have been reported and they have shown
great potential as effective luminescence sensors important targets such as cations, anions, small
molecules, gas, and vapors [13]. By the incorporation of guest molecules and ions into their
pores, MOF fluorescence sensors can respond with different degrees of fluorescence
enhancement or quenching behaviors [14].
3
+
Till now, several MOFs have shown good performance for the detection of Fe ions [15-22].
3
+
However, the detection processes of these MOFs towards Fe ions take relative long time
because of the detection mechanisms of them are based on the ion exchange between the
3
+
framework metal ions and Fe ions [23-27]. Thus, there is an urgent need to develop new
3
+
fluorescent MOF sensors for the selective and fast detection of Fe ion [28, 29]. Amine and
azine groups in organic luminescent materials are an electron-donating groups [30, 31], while
metal ions often need to accept electrons. Therefore, these two functional groups in a
luminescent MOF may be used as binding sites to bind with metal ions [32]. Encouraged by the
above aspects, the work reported herein demonstrates that the highly luminescent MOF,
3
+
[
Zn(NH -BDC)(4-bpdb)].2DMF, (TMU-17-NH ) reveals excellent potential for sensing Fe
2 2
ions. Mixed-linker microporous TMU-17-NH consists of both amino-1,4-benzenedicarboxylic
2
acid (NH -BDC) and the N,N΄-bipyridine-type rigid linkers (4-bpdb = 1,4-bis(4-pyridyl)-2,3-
2
diaza-2,3-butadiene) with ZnN O secondary-building units (SBUs). TMU-17-NH has been
2
4
2
previously synthesized and reported for the selective synthesis of tetrahydro-chromenes, which
the amine group of NH -BDC ligand has been found to be a base catalyst active site [33].
2
3
+
Very recently, a lot of studies on luminescent MOFs for sensing heavy metal ions such as Fe
2
+
and Cu ions have been developed on the basis of interactions between metal ions and Lewis
base sites within the ligands and energy or electron transfer from the ligand to metal ions [34-
3
+
3
6]. Although excellent studies on a highly selective fluorescent probe for the detection of Fe
were reported by Zhou’s group [37], Sun’s group [20], and Wang’s group [38], the advantage of
3
+
TMU-17-NH is that the overall quenching constant for Fe in the concentration range of 50-
2
-1
2
50 μM is quantified to be ~41000 M and detection limit of 0.7 µM, which is comparable to
that of other reports (Table 1). It is very exciting that TMU-17-NH , as a Fe -probe, was not
3
+
2
3

3
+
2+
2+
3+
2+
2+
2+
influenced by the presence of other metal ions such as As , Cd , Zn , Co , Ni , Cu , Pb ,
2
+
3+
3+
Mn and Al . More importantly, the MOF could achieve recognition of Fe ion in DMF
solution with a response time of <1 min and 94% fluorescence quenching efficiency.
EXPERIMENTAL SECTION
Chemicals, Reagents and Apparatus.
Starting reagents for the synthesis were purchased and used without further purification from
commercial suppliers (Sigma-Aldrich, Merck and others). Zn(NO ) ·6H O and amino-1,4-
3
3
2
benzenedicarboxylic acid (NH -BDC) were used to synthesize TMU-17-NH . N,N-
2
2
Dimethylformamide (DMF) was used as the solvent to purify TMU-17-NH . Aqueous solutions
2
2
+
2+
2+
2+
3+
2+
3+
2+
2+
3+
of Cd , Zn , Pb , Co , As , Mn , Al , Cu , Ni and Fe were prepared from
CdCl ·2.5H O, Zn(NO ) ·6H O, Pb(NO ) , Co(NO ) ·6H O, NaAsO , Mn(OAc) ·4H O,
2
2
3 3
2
3 2
3 2
2
2
2
2
Al(NO ) ·9H O, Cu(NO ) ·3H O, NiCl ·6H O, and Fe(NO ) ·9H O, respectively. The infrared
3
3
2
3 2
2
2
2
3 3
2
spectra were recorded on a Nicolet Fourier Transform IR, Nicolet 100 spectrometer in the range
-1
5
00-4000 cm using the KBr disk technique. X-ray powder diffraction (XRD) measurements
were performed using a Philips X’pert diffractometer with monochromated Cu-k radiation
α
(λ=1.54056Å). The simulated XRD powder pattern based on single crystal data was prepared
using Mercury software [39]. The fluorescence experiments were performed at room temperature
on a Shimadzu RF-6000 fluorescence spectrometer (kyoto, Japan) with a photomultiplier voltage
−
1
of 700 V, scan speed of 60,000 nm min , excitation slit width of 900 nm, emission slit width of
00-800 nm, and a 380 nm optical filter. The fluorescent emission spectra were recorded in the
2
wavelength range of 300-800 nm upon excitation at 270 nm.
Preparation of 1,4-bis(4-pyridyl)-2,3-diaza-2,3-butadiene (4-bpdb).
1
mL (11 mmol) of hydrazine was added dropwise to a solution of pyridine-4-carboxaldehyde
(2.2 mL, 22 mmol) dissolved in ethanol (15 mL). Two drops of formic acid were added and the
mixture was stirred at room temperature for 24 h. The yellow solid that formed was filtered and
washed several times with ethanol/ether (1:1). Yield, 79%.
Preparation of [Zn(NH -BDC)(4-bpdb)].2DMF (TMU-17-NH ).
2
2
The TMU-17-NH , [Zn(NH -BDC)(4-bpdb)].2DMF was prepared by following the published
2
2
protocol [33]. Briefly, Zn(NO ) ·6H O (0.297 g, 1 mmol), 4-bpdb (0.210 g, 1 mmol), NH -BDC
3
2
2
2
(0.181 g, 1 mmol) were dissolved in 15 ml DMF. The mixture was placed in a Teflon reactor and
heated at 80 °C for 3 days, and then it was gradually cooled to room temperature during 24h. The
-1
crystals were obtained in a 33% yield. FT-IR (cm ): 3462.05 (s), 3364.90 (s), 1676.84 (vs),
1
8
609.95 (vs), 1574.38 (vs), 1425.94 (vs), 1380.69 (vs), 1253.26 (s), 1094.03 (m), 1014.26 (m),
30.72 (s), 772.04 (s), 688.79 (s), 519.39 (s). Before luminescence process, the crystals were
4

washed with 3 × 5 mL of DMF followed by 3 × 5 mL of CH Cl . The crystals were then
2
2
soaked in 5 mL of CH Cl for 3 days with fresh CH Cl added every 24 h. After 3 days the
2
2
2
2
CH Cl solution was decanted, and the crystals were dried at 80 °C under vacuum for at least 24
2
2
h.
Luminescent Experiments.
TMU-17-NH suspensions for fluorescence experiments were prepared by dispersing 1 mg of
2
TMU-17-NH powder in 4 mL of DMF under ultrasonication (80 W) for 10 min. To a 1 cm × 1
2
-1
cm quartz cell, a TMU-17-NH suspension (250 mg L , 4 mL) and certain amounts (20 μL) of
Fe solution were sequentially added. The mixtures were then used for fluorescence
measurements. The fluorescence data were collected after 5 min.
2
3
+
RESULTS AND DISCUSSION
Characterization of TMU-17-NH2.
The MOF TMU-17-NH has been previously synthesized and reported in the literature [33]. A
2
guest-filled phase of TMU-17-NH , was synthesized by the solvothermal reaction of NH -BDC,
2
2
4
-bpdb, and Zn(NO ) ·6H O in DMF at 80 °C for 72 h (Fig. 1a). TMU-17-NH is a two-fold
3 2 2 2
interpenetrated 3D framework bridged by dicarboxylates and 4-bpdb linkers. The 2D Zn-
dicarboxylate layers are extended along the a-axis to form distorted square grids which are
further pillared by 4-bpdb linkers along the b axis to form a 3D pillared-bilayer interpenetrated
framework with a topology that can be described as a primitive cubic lattice (pcu) (Fig. 1b). As a
results, the pore surface of TMU-17-NH is decorated with pendant amine (-NH ) and azine (=N-
2
2
N=) functional groups. TMU-17-NH has 1D channels of approximately 3 Å in cross section
2
(
including van der Waals radii) that are not large enough to be accessible for N (kinetic
2
diameters for N : 3.75 Å) adsorption. The experimental XRD pattern of the synthesized TMU-
2
1
7-NH was in good agreement with the simulated one [33], showing the successful preparation
2
of TMU-17-NH2 (Fig. 1c). To evaluate the thermal stability for further utilization,
thermogravimetric analysis (TGA) of TMU-17-NH was carried out under flowing N
2
2
atmosphere with the temperature range from 30 to 600 ℃ (Fig. 2a). The TG curve of the TMU-
7-NH presents two events. The first one is the elimination of the trapped solvents in the pores
1
2
over the temperature range of ∼25-270 to form the guest-free phases, [Zn(NH -BDC)(4-bpdb)].
2
TMU-17-NH material is thermally stable up to 300 ℃, above which the total decomposition of
2
the organic ligand occurs, implying the good thermal stability of this MOF. The chemical
stability of TMU-17-NH was examined by suspending samples of the MOF in ethanol,
2
acetonitrile, methanol, tetrahydrofuran, water and dichloromethane (Fig. 2b). TMU-17-NH2
samples were immersed in the desired solvent overnight at ambient temperature. The MOF was
found to be soluble in water, however, at the same time the PXRD patterns reveal that TMU-17-
NH can be resistant to the other solvents molecules.
2
Fluorescent Properties of TMU-17-NH2.
5

The most significant structural feature of TMU-17-NH is the presence of free amine and azine
2
sites within the pores, highlighting the potential for its recognition of metal ions and thus for
sensing functions. When TMU-17-NH was dispersed in DMF solution, it has an emission peak
2
at 440 nm with a weak luminescent intensity, as shown in Fig. 3a, which is approximately four
times lower than the ligand NH -BDC dispersed in DMF. This interesting phenomenon can be
2
attributed to the ligand-to-metal charge transfer (LMCT) effect upon the ligand NH -BDC
2
coordination with Zn-O clusters to form the TMU-17-NH framework. Furthermore, the solvent
2
induced luminescence quenching effect may also play an important role in weakening the
luminescent intensity of TMU-17-NH in DMF solution, which was mainly dependent on the
2
polarity of solvent molecules and their coupling with the Zn-O nodes as reported [40].
First of all, the luminescence properties of TMU-17-NH in different solvent emulsions were
2
investigated. Compound TMU-17-NH (1 mg) was dispersed in different commonly used organic
2
solvents (4 ml) like ethanol (EtOH), methanol (MeOH), N,N′-dimethylformamide (DMF),
tetrahydrofuran (THF), dichloromethane (CH Cl ) and acetonitrile (MeCN). As shown in Figure
2
2
3
b, the PL intensity is largely dependent on the solvent molecules, particularly in the case of
DMF, which exhibited the most significant enhancing effects. Shifts in emission are observed
upon changing the solvent, and the wavelength corresponding to the maximum peak decreases in
the order DMF > CH CN > CH Cl > EtOH > THF > MeOH (Fig. 3c).
3
2
2
To examine the potential of TMU-17-NH for sensing metal ions, the activated TMU-17-NH (1
2
2
2
+
2+
2+
2+
mg) was immersed in 4 mL DMF solutions containing M(NO ) (M = Zn , Pb , Co , Cu ,
3
x
2
+
3+
3+
Cd , Al , or Fe ), NaAsO , Mn(OAc) and NiCl to form the metal ion incorporated MOF
2
2
2
suspension for luminescence studies. The suspension-state luminescent measurements illustrated
that various metal ions display markedly different effects on the luminescence intensity of TMU-
1
7-NH . As shown in Figure 4a-b, the luminescence intensity of TMU-17-NH shows the
2 2
quenching effect for all metal ions, but the quenching degree is heavily dependent on the species
3
+
2+
2+
of metal ions. Among the metal ions studied, the quenching effects of Fe , Ni and Cu are
3
+
very pronounced, especially for Fe ions. Moreover, the color changes seen by the naked eye
3
+
have also been recorded (Fig. 4b, inset). After Fe is incorporated, the luminescence intensity of
the TMU-17-NH suspension reduces to 0.04 from 1. The luminescence intensity of TMU-17-
2
3
+
2+
NH (metal-ion free) was quenched by about 96, 83 and 62% when incorporated with Fe , Ni
and Cu , respectively. Obvious color change from bright yellow to brownish red of the TMU-
7-NH solid samples was also observed after the Fe incorporation (Fig. 4a, inset). Clearly,
2
TMU-17-NH can highly efficiently sense Fe ions through the fluorescent quenching, which
2
2
+
3
+
1
3
+
2
can also be easily recognized by the naked-eye.
The chemically closely related metal ions can frequently interfere with ion binding. So, to
3
+
2+
2+
examine the selective detection of TMU-17-NH on Fe , the quenching effects of Ni and Cu
2
on the luminescence intensity of MOF must be considered. We studied the optical properties of
n+
3+
2+
2+
the M @TMU-17-NH (M = Fe , Ni and Cu ) when immersing in different concentrations
2
of metal cations (Fig. 5). It is obvious that the luminescence intensity at 440 nm is gradually
6

decreased as the concentration of the metal cations increase. As demonstrated in Figure 4a, when
3
+
the Fe concentration increased from 0 to 1000 μM, the emission intensity of TMU-17-NH was
2
gradually decreased and a good linear relationship (correlation coefficient R = 0.997) between
3
+
the emission intensity of TMU-17-NH and the concentration of Fe was observed in the range
2
3
+
3+
of 0-250 μM of Fe (Fig. 5a; inset). The luminescence intensity of the Fe -incorporated TMU-
7-NH from a 50 μM DMF solution of Fe(NO ) is about half that of the metal-ion free MOF,
1
2
3 3
3
+
indicating the potential of TMU-17-NH for the sensing of Fe .
2
The fluorescence quenching follows the Stern–Völmer (SV) equation: I /I=1 + K [M], where I
0
0
SV
and I correspond to the luminescence intensity for TMU-17-NH in absence and presence of
2
metal cations, respectively, [M] is the metal concentration, and KSV is the Stern–Völmer
constant. It is thus expected that the reduction in luminescence intensity is proportional to the
3
+
concentration of the metal ion. It was found that the SV plot of TMU-17-NH towards Fe ions
2
is nearly linear at low concentration range, but subsequently deviates from linearity and bends
upwards at higher concentrations (Fig. 5a; inset). Moreover, the K
is calculated via
SV
-1
3+
luminescent data. An exceptionally high K value of 41565 M was obtained for Fe , while
SV
2
+
2+
-1
KSV for Ni and Cu were 16180 and 5766 M , respectively. For metal ion-incorporated TMU-
3
+
2+
3+
2+
1
7-NH , K [Fe ]/K [Ni ] and K [Fe ]/K [Cu ] are 2.6 and 7.2, respectively, in which
2
n+
SV
SV
SV
n+
SV
K [M ] represent the K of M @TMU-17-NH . Obviously, TMU-17-NH shows highly
selective sensing for Fe ions (see Fig. 5d). The sensitivity of TMU-17-NH is three to four
SV
SV
2
2
3
+
2
orders of magnitude higher than that for previously reported MOFs with K values in the range
SV
-
1
of tens to hundreds M (Table 1). The high sensitivity allows us to easily identify the existence
3
+
of a small amount of Fe ions.
To further corroborate the high sensitivity of TMU-17-NH compared to other reported systems,
2
3
+
we determined the detection limit for Fe on the basis of the 3δ IUPAC criterion [41, 42]. Based
on the slope of the calibration curve (K) values and the standard deviations (S ) from ten
b
repeated fluorescent measurements of blank solutions, the detection limits (3S /K) of TMU-17-
b
3
+
NH towards Fe ion in DMF were calculated to be 0.7 μM (corresponding to 40 ppb). This is
also comparable to or better than some previously reported Fe fluorescent sensors (Table 1)
2
3
+
Additionally, to investigate the time-response characteristic of the TMU-17-NH sensor toward
2
3
+
Fe , the fluorescence intensity of TMU-17-NH (λ = 270 nm) was measured as a function of
immersion time in DMF solution of 100 µM Fe (Fig. 6a). With increasing time, the PL
intensity decreases for the Fe -loaded sample. Fe induced fluorescence quenching reaction is
2
ex
3
+
3
+
3+
very fast. The emission intensity of TMU-17-NH at 440 nm has been decreased to more than 6.7
2
times in less than 1 min and a constant value in 8 min (Fig. 6a). This response is much faster than
that in the some previous reports (about 24 h or 72 h). This rapid response is quite appealing and
3
+
may be attribute to the fact that Fe ions can rapidly diffuse into the channels of TMU-17-NH
and interact with luminophores on the pore surface.
2
7

It should be pointed out that usually many metal ions coexist in practical biological and
environmental systems. Further experiments for the effect of coexisting metal ions on the
3
+
quenched fluorescence intensity of TMU-17-NH by Fe were performed to show the selectivity
2
3
+
of TMU-17-NH for detecting Fe (Fig. 6b). It is very encouraging that TMU-17-NH shows
2
2
high anti-interference from other coexisting metal ions. The tolerable concentrations of the
3
+
2+
2+
2+
2+
interfering metal ion with the detection of 150 μM Fe were 250 μM for Co , Cu , Mn , Ni ,
2
+
3+
3+
3+
Cd , Al or As . It is quite cheerful that the quenching effect of the Fe ion on TMU-17-NH -
2
luminescence is not influenced by the addition of other metal ions, further underlining the high
3
+
selectivity of this MOF for detecting Fe and the great potential of TMU-17-NH for real sample
2
application.
According to the above studies, we attempted to explore the possible sensing mechanism of
3
+
fluorescence quenching for Fe . Generally, the reasons for the fluorescence quenching caused
by MOFs can be attributed to the following reasons: the ionic exchange, the collapse or change
of the structure, and the competitive photon absorption between the adsorbed ions and MOFs
[
43]. The PXRD measurements were carried out to study the structures of the original and metal
3
+
ion incorporated MOF samples of Fe @TMU-17-NH . As shown in Fig. 2b, the PXRD patterns
of the Fe @TMU-17-NH is similar to that of TMU-17-NH ruling out the collapse of the MOF
2
3
+
2
2
framework. We infer that the energy transfer may be responsible for the quenching phenomenon.
According to the reported literature, porous MOFs with Lewis basic sites, such as pyridyl, amide,
anionic sulfonate sites, can have significant interactions with guest metal ions [44, 45]. We
3
+
therefore suggest that the quenching might be related to the interaction between the Fe ions and
3
+
pendant amine and azine functional groups in TMU-17-NH . The interaction between the Fe
2
ions and the NH -BDC ligands minimizes the energy transfer efficiency from NH -BDC to the
2
2
2
+
Zn ions within TMU-17-NH , thus decreasing the luminescence intensity.
2
As a sensor, regeneration is another important issue. The reusability of the fluorescence sensing
3
+
performances of the MOF probe towards Fe ions was examined up to four cycles. For checking
its recyclability, the MOF compound was filtered off after each fluorescence titration
experiment. The filtered materials were washed with DMF repeatedly and then dried in an oven.
TMU-17-NH showed outstanding recovery of its initial fluorescence intensity, even after four
2
consecutive cycles of fluorescence sensing experiments, demonstrating its good recyclability and
stability for the detection applications (Fig. 6c).
CONCLUSIONS
Lewis basic sites within porous MOFs are expected to play very important roles for their
recognition of small Lewis acidic molecules and metal ions, and thus to find functionalities in
chemical transformation and sensing. Here we used the amine-functionalized dicarboxylate
ligand as well as the ligand contain a bridging azine group in the middle to functionalize the
3
+
TMU-17-NH luminescent MOF as a stable fluorescent sensor for detecting Fe . This
2
3
+
luminescent probe shows high selectivity for sensing Fe in DMF solution with excellent
8

-1
sensitivity (K ~ 41000 M ) and a detection limit of 0.7 µM, and fast response speed (<1 min).
sv
The present results may provide a facile route to design and synthesize functional MOFs with
applications in fluorescent sensors.
ACKNOWLEDGMENTS
Support of this investigation by Iran University of Science and Technology and Iran’s National
Elites Foundation is gratefully acknowledged.
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5
1
1

Figure 1. (a) Chemical Structure of NH -BDC and 4-bpdb. (b) Structure of the cuboidal block of
2
TMU-17-NH , showing Zn units linked by eight NH -BDC and eight 4-bpdb units; (c)
2
2
Representations of the structure of the open phase TMU-17-NH which contains 1D channels.
2
The two interpenetrating frameworks are shown in blue and green. The disordered guest
molecules are omitted for clarity.
1
2

Figure 2. (a) TGA of as-synthesized (red) and activated TMU-17-NH (blue). (b) PXRD of
2
TMU-17-NH : as-synthesized (red), simulated (black); immersed in EtOH (blue), CH CN (pink),
2
3
3
+
MeOH (green), THF (dark blue), CH Cl (purple) and TMU-17-NH @Fe (dark purple).
2
2
2
1
3

Figure 3. (a) The PL spectra of TMU-17-NH (blue), NH -BDC (red) and 4-bpdb (yellow) suspensions in
2
2
DMF. (b) The PL spectra of TMU-17-NH -solvent emulsions at room temperature, where the red, blue,
2
yellow, purple, green and gray curves denote the intensities of TMU-17-NH -DMF, -CH CN, -CH Cl , -
2
3
2
2
EtOH, -THF and -MeOH, respectively. (c) Comparisons of the luminescence intensity of TMU-17-NH2-
solvent emulsions at room temperature.
1
4

Figure 4. (a) Emission spectra of TMU-17-NH (1 mg) in DMF (4 ml) with various metal ions (250 μM).
2
3
+
(
Inset: photographs of Fe @TMU-17-NH sample as solid). (b) Relative luminescence intensities of
2
n+
n+
M @TMU-17-NH in DMF. (Inset: photographs of M @TMU-17-NH sample in DMF).
2
2
1
5

3
+
2+
Figure 5. The luminescence spectra of TMU-17-NH with different concentration of (a) Fe , (b) Ni and
2
2
+
(
c) Cu ions. Inset of (a): Stern-Völmer (SV) plots of the fluorescence emissions of TMU-17-NH
2
quenched by Fe in DMF with 270 nm excitation (emission decays were monitored at 440 nm). (d) SV
3+
plots of the fluorescence emissions of TMU-17-NH quenched by different metal ions in DMF with 270
2
nm excitation (λ = 440 nm).
em
1
6

3
+
Figure 6. (a) Fluorescence quenching of TMU-17-NH by 250 μM Fe in DMF as a function of time
(
2
3
+
λ = 270 nm). (b) Comparison of the luminescence intensity of TMU-17-NH with 150 μM Fe in DMF
ex 2
in presence of 250 μM of other metal ions, monitored at 270 nm. (c) Quenching and recovery tests of
3+
TMU-17-NH for 100 μM Fe in DMF.
2
1
7

3
+
Table 1. TMU-17-NH sensing parameter comparison with other MOF based Fe sensors.
2
-
1
MOF
KSV (M )
Detection limit (μM)
Line range
0-0.5 mM
20-700 μM
20-700 μM
0.0025-0.15 mM
0.0025-0.15 mM
0-2 mM
Ref
[27]
3
+
3
Eu @MIL-53-COOH (Al)
5.12 × 10
0.5
0.8
4
[46]
BUT-15
1.66 × 10
3
[46]
BUT-14
2.17 × 10
3.8
4
[47]
[
Cd(L)(BPDC)]·2H O
3.63×10
2.21
7.14
1.77
3.76
_
2
4
[47]
[
Cd(L)(SDBA)(H O)]·0.5H O
3.59×10
2
2
3
[48]
[
Cd(ATA)(L)]·2H O
3.838 × 10
2
3
0-2 mM
[48]
[
Zn(ATA)(L)]·H O
0.557×10
1.519 × 10
4.479 × 10
2
3
3
0-5 mM
[49]
[
[
Eu(HL)(DMF)(H O) ]·3H O
2 2 2
0-5 mM
[49]
Tb(HL)(DMF)(H O) ]·3H O
50
2
2
2
4
-
[50]
[
Eu(L)(BPDC) (NO )]·H O
5.16 × 10
4.30 × 10
1.78 × 10
0.5
1
/2
3
3
4
-
[50]
[
Tb(L)(BPDC) (NO )]·H O
0.5
1
/2
3
3
4
0.01875-0.225 mM
[51]
[
Cd(5-asba)(bimb)]
Eu K (dcppa) (H O) ].5H O
0.0061
−6
−2
4
4
10 -10
M
[52]
[
4.3 × 10
4.2 × 10
1
2
2
2
2
6
2
TMU-17-NH2
0.7
50-250 µM
This works
1
8