Selective detection and removal of mercury ions by dual-functionalized metal-organic frameworks: design-for-purpose

Article abstract of DOI:10.1039/c9nj03951a

In this study, through introducing a new functional group into the structure, the performance and efficiency of MOFs as a sensor for heavy metal cations have been improved. It was observed that the N1,N3-di(pyridine-4-yl) malonamide ligand (-NH-CO-CH

Full text of DOI:10.1039/c9nj03951a

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Selective detection and removal of mercury
ions by dual-functionalized metal–organic
Cite this: New J. Chem., 2019,
frameworks: design-for-purpose†
43, 18079
Leili Esrafili, ‡ Maniya Gharib‡ and Ali Morsali
*
In this study, through introducing a new functional group into the structure, the performance and
efficiency of MOFs as a sensor for heavy metal cations have been improved. It was observed that the
2
N1,N3-di(pyridine-4-yl) malonamide ligand (–NH–CO–CH –CO–NH–)(S), one of the pillar linkers, has
not directly entered into the structure of the synthesized MOFs. To solve this issue, three new structures
based on copper metal–organic frameworks and amide-functionalized pillar ligands (–NH–CO–),
TMU-46, 47 and 48 have been synthesized under hydrothermal conditions. An exciting aspect of the
2+
acylamide pillar ligands is their efficient detection ability of Hg (mercury ion) in the presence of other
2+
2+
3+
heavy metal cations such as Cd (cadmium), Cu (copper), and Cr (chromium). Due to their chelating
effect on heavy metal cations, we hypothesized that decoration of the MOF wall with both malonamide
and acylamide struts would promote their Lewis basic properties, and improve the removal capacity of
2+
heavy metal ions. A new linker, containing suitable functional group malonamide (S) to enhance Hg
cation sensing, was successfully exchanged and the produced material was labeled TMU-46S, TMU-47S
and TMU-48S. Designing dual-functionalized MOFs is our ‘‘design-for-purpose’’ approach for the decora-
tion of MOF walls by suitable functional groups resulting in high removal capacity and sensitivity of heavy
metal ions. To the best of our knowledge, this is the first report of a mixed amide-malonamide based MOF
2+
ꢀ1
Received 30th July 2019,
which provides a proper coordination site to coordinate strongly to Hg ions, along with 714 mg g
Accepted 16th October 2019
ꢀ1
2+
maximum adsorption capacity and 186087 M
Ksv. Generally, we attributed the impressive Hg sensing of
TMU-48S to synergistic effects of both hydrophilic chelating malonamide and acylamide functional groups.
Hence, the results represent an effective strategy in designing and developing multi-functional MOF-based
materials and their application in removal processes and environmental protection efforts.
DOI: 10.1039/c9nj03951a
rsc.li/njc
enhancement or quenching. Special features of MOFs including
pore size and functional group of the linkers can be controlled
Introduction
A new class of crystalline porous materials, Metal–Organic by changing the sizes and type of ligands, which enhances the
Frameworks (MOFs), provide a chance for combining inorganic detection sensitivity by concentrating analytes to high levels, and
building units and organic linkers to make great fluorescent using different functional pillar ligands and acids to construct
materials, because of their high selectivity, quick-response, MOFs with various and specific functional groups makes specific
1
–6
operability, and reversibility. In contrast to ordinary porous recognition sites with unique selectivity through host–guest
7–13
sensors such as zeolites, activated carbons and mesoporous interactions.
silicas, the LMOFs (Luminescence MOFs) are promising mate- Due to unique advantages, various structures of MOFs have
rials due to the effective interactions between the guest and been employed as an ideal luminescent material for sensing
2
+
2+
3+
3+
structure backbone resulting in various degrees of luminescent different kinds of metal cations such as Hg , Cu , Fe , Cr
1
4–16
etc.
This reflects the growing trend of application of MOFs
for the detection and sensing of a wide range of pollutants such
as metallic ions, biomolecules, aromatic compounds, volatile
organic compounds and nitro aromatic materials. At times,
It is not easy to synthesize single crystals of MOFs via particular
functionalized ligands. Consequently, we have employed
Department of Chemistry, Faculty of Sciences, TarbiatModares University, P. O. Box
1
+
†
4115-175, Tehran, Iran. E-mail: Morsali_a@modares.ac.ir; Tel: +98-21-82884416,
98-9122420568
Electronic supplementary information (ESI) available: Experimental details,
PXRD patterns, TGA, IR spectroscopy and sensing graphs and details (PDF),
and crystallographic data (CIF). CCDC 1849865. For ESI and crystallographic data
in CIF or other electronic format see DOI: 10.1039/c9nj03951a
1
7–19
a strategy to swap the linkers.
Solvent-Assisted Ligand
‡
L. E. and M. G. contributed equally to this work.
Exchange (SALE) is a well-known post-synthesis pathway that
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provides a route to incorporate desired ligands that is un-
2
0–25
obtainable via traditional synthetic pathways.
In addition,
a SALE method enhances MOF performances in gas uptake,
2
6–30
catalysis, proton conductivity, and sensing applications.
However, a large number of sensor-based MOFs
3
1–35
have been
examined for the sensing and removal of heavy metal cations
3
6–40
owning to security, environmental and humanitarian issues,
but only a few of them have exhibited selective sensing and
4
1–43
detection behavior towards heavy metal cations.
Because of
2+
the toxicity and water stability of mercury (Hg ), it causes serious
problems for environmental remediation and human health.
In particular, the accumulation of Hg in the human body,
especially the central nervous system, even at ultra-trace concen-
4
4–48
2
+
0
0
Fig. 1 N,N -(1,4-Phenylene)diisonicotinamide (bpfb), N,N -bis-(4-pyridyl-
49,50
tration, can cause a variety of diseases
like digestive, kidney formamide)-1,5-naphthalenediamine (bpfn), N,N0-bis(4-pyridinyl)-terephthal-
and neurological sickness. Thus, it is necessary to design and amide (bpta), and N1,N3-di(pyridine-4-yl) malonamide S linkers.
synthesize a convenient, low cost and highly sensitive adsorbent
2
+
for Hg analytes. MOFs with particular desired functions and
properties can be employed as fluorescent sensor probes. So far,
some functional luminescent MOFs have been manufactured
2
+
acylamide multifunctional luminescent MOFs to identify Hg
through a fluorescence method.
5
8–62
5
1–54
2+
In this discussion, we have applied the acylamide and
malonamide functional groups to design and synthesize six
isoreticular Cu-MOF based luminescent probes. The studied
pillar likers are N,N -(1,4-phenylene)diisonicotinamide (bpfb),
N,N -bis(4-pyridinyl)-terephthalamide (bpta), N,N -bis-(4-pyridyl-
formamide)-1,5-naphthalenediamine (bpfn) and N1,N3-di(pyridine-
for harmful ion sensing.
Hg detection by MOFs has
encountered challenges like long response times, the poor dis-
persion of MOFs in water, framework collapse, and noticeable
effects of other cations on the PL spectra. These problems lead to
lower sensitivity, precision, and accuracy of detection. Using
multi functionalized paddlewheel MOFs is a promising strategy
due to their higher stability and presence of multiple fluorophore
sites immobilized in the solid support in designing and developing
MOF-based probes.
0
0
0
4-yl) malonamide linkers used in SALE (see Fig. 1).
Therefore, after incorporating S linker which has a malon-
amide functional group with the chelate site into TMU-46, 47
and 48, the properties of the new compounds significantly
changed, especially in TMU-48S. The malonamide function
2
+
Most reported MOF based luminescence sensors for Hg
ions can be classified into two categories: (1) nitrogen based
2
+
(–NH–CO–CH –CO–NH–) of the daughter MOFs includes
2
probes, which can bind strongly to Hg including amino, urea,
imine, amide, and CQN functional groups. Nitrogen based
functional groups have been known to demonstrate superior
two adjacent amide groups with localized free non-bonding
lone-pairs which can be considered as a Lewis basic moiety.
The removal ability of these new functionalized TMUs was
2
+
selectivity and sensitivity for Hg ions and are often applied as
a sorbent. (2) Sulfur-containing sensors, due to the inherent
2
+
further elucidated by the effective separation of Hg from
2
+
2
+
water. Due to the orientation changes of Hg connected into
the acylamide-pillar ligands by deviation to the naphthyl rings
and also the existence of a malonamide moiety with a chelating
site, TMU-48S has shown sensing property enhancement.
ability of Hg ions to form soft–soft interactions with S (Hgꢁ ꢁ ꢁS
interaction) to produce a luminescence signal, are always
considered as an effective sorbent for mercury ions. However,
these probes have encountered challenges like: (1) both amine
and sulfide based MOFs can easily suffer from aerial oxidation,
(
2) Hgꢁ ꢁ ꢁS interaction is not specific; sulfur based functions can
Experimental section
Synthesis of parent MOFs
also adsorb many other heavy metal ions in reasonable quan-
tities such as Cd and Pb , (3) sulfur-based MOFs do not show
2
+
2+
thermal and chemical stability and they have expensive and A solid mixture of Cu (NO ) ꢁ6H O (0.29 g, 1 mmol), H oba
3
2
2
2
unstable ligands. As it is clear, the functional group plays a key (0.258 g, 1 mmol) and 0.318 g of bpfb (1 mmol) for TMU-46,
role in the process of absorbing and sensing heavy metal ions. 0.318 g of bpta (1 mmol) for TMU-47 and 0.368 g of bpfn
In comparison to sulfur-based probes, hard atom-containing (1 mmol) for TMU-48 were dissolved in DMF (15 mL). The
functional groups like amide as Lewis basic sites, are also reaction solution was sonicated and then placed in an isothermal
promising materials for selective mercury binding. Then oven at 120 1C for 2 days resulting in green crystals. The mixture
amide-decorated MOFs are potentially suitable for interaction was then isolated by washing with DMF and dried at room
2
+
with Hg due to their ability to form covalent mercury-amide temperature. Yield: 65%, 35% and 58% for TMU-46, TMU-47
5
5–57
ꢀ1
linkages under ordinary conditions.
However, there are and TMU-48 respectively. Data for (TMU-46), FT-IR (cm ): 1692
numerous reports on the role of hydroxyl, acylamide, amino (m), 1598 (vs), 1507 (s), 1396 (vs), 1332 (m), 1297 (m), 1240 (vs),
and other hard functional groups in polymer based sensors 1161 (vs), 1200 (m), 778 (m), 660 (m), 525 (m). EA on solvent free
in heavy metal ion sensing and removal. To the best of our sample: (%) C, 57.28; H, 3.69 Cu, 13.27; N, 5.58; O, 19.68. Data for
ꢀ
1
knowledge, there is no report based on malonamide and (TMU-47), FT-IR (cm ): 1674 (vs), 1598 (vs), 1541 (m), 1513 (m),
1
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Fig. 2 SALE process in TMU-46 and resulting TMU-46S.
1
400 (vs), 1310 (m), 1218 (vs), 1160 (s), 1089 (m), 1068 (m), 1015 diffraction analysis shows that TMU-46 crystallizes in centro-
/n of the monoclinic crystal system.
7.19; H, 3.58 Cu, 13.5; N, 5.62; O, 19.67. Data for (TMU-48). FT-IR The PXRD pattern of the simulated TMU-46 compound is
(m), 800 (m), 659 (m), 523 (m). EA on solvent free sample: (%) C, symmetric space group P2
1
5
ꢀ
1
(
cm ): 1668 (vs), 1596 (vs), 1572 (m), 1503 (s), 1386 (vs), 1232 (s), similar to the synthesized pattern (Fig. S1, ESI†). Besides, single
161 (vs), 1086 (m), 1063 (m), 1017 (m), 876 (m), 655 (m), 519 (m). crystal X-ray diffraction analysis shows that TMU-46 has a
EA on solvent free sample: (%) C, 59.52; H, 3.7 Cu, 12.31; N, 5.58; pillared-paddlewheel two interpenetrated structure manufac-
1
O, 18.5.
tured from a binuclear Cu unit (Cu1 and Cu2). The Cu (COO)4
2
clusters were pillared by bpfb spacer ligands. In the asymmetric
unit, there is one bpfb as a nitrogen donor pillar and two oba as
oxygen donor ligands. Cu1 is surrounded by four O atoms (O1A,
O1B, O4A, O4B) from four carboxylate oba ligands, one N atom
Synthesis of TMU-46S, 47S and 48S via the SALE pathway to
create daughter MOFs
1.2 g of TMU-46, 47 MOFs and 1 g of TMU-48 MOF was placed
(N4) from the pyridine bpfb ligand and another Cu2 atom
into a 15 mL solution of DMF in a capped vial solution
containing 120 mg (2 mmol) of S ligand, respectively. The vial
was placed in an oven at 80 1C for 3 days and the ligand
composing a distorted octahedral geometry. Each Cu2 is also
six-coordinated by four O atoms (O2A, O2B, O5A and O5B) from
four carboxylate oba ligands, one N atom from the pyridine
spacer and Cu1 (Fig. 3a). The Cu1 and Cu2 bond distance is
1
solution was refreshed every day. For H NMR analysis, 1–3 mg
of paddlewheel Cu-MOF crystals (activated with acetonitrile)
was placed into a 1.5-dram vial containing D SO and DMSO
2 4
2 4
2.637 Å. 2D Cu (COO) sheets of TMU-46 are connected through
the N donor pillar spacers (bpfb), extending the structure to 3D
frameworks (Fig. 3b). The pore size of TMU-46 is measured to
be 13.677 Å (taking the van der Waals radius of each atom into
account), and there are different potential Lewis basic moieties
inside the internal surface of these pores which are: (1) amide
groups of the bpfb ligands, (2) etheric oxygen atom of carb-
oxylate oba ligands and (3) p-conjugated aromatic phenyl rings
mixture and then the crystals were sonicated to complete the
digestion process. After 5 days, H NMR demonstrated that
1
77% of bpta, 74% of bpfb and 50% of bpfn was replaced by S
ligands (Fig. 2). Crystalline powders of TMU-46S, 47S and 48S
after activation were characterized by analytical CHN elemental
analysis (Table 1).
(Fig. 3d). Also, the analysis of network topology of TMU-46
Results and discussion
Synthesis and characterization
demonstrates mab topology. The structure consists of a 3D
framework with V5TiSc2O2N2C2. From the topological point of
view, the net is uninodal and 6-connected with a point symbol
Three new MOFs; TMU-46, TMU-47 and TMU-48 were synthe-
sized under solvothermal reaction using copper nitrate, H oba
as an oxygen donor ligand and bpfb, bpta and bpfn as nitrogen
donor linkers, respectively (Fig. 1). The structure of TMU-46 was
characterized by X-ray crystallography. The single crystal X-ray
{
4^4.6^10.8} (Fig. 3b)
2
Thermogravimetric analysis (TGA) of TMU-46, TMU-47 and
TMU-48 shows an initial weight loss at 400 1C, 377 1C and
50 1C respectively, attributed to release of trapped DMF
3
molecules within the structure. Finally the decomposition of
the guest-free frameworks was observed at 420 1C for TMU-46
and TMU-47 and 350 1C for TMU-48 and the final residual
weight for TMU-46, 47 and 48 corresponds to that of CuO
Table 1 Elemental analysis of daughter MOFs structures
TMU-46S
TMU-47S
TMU-48S
(Fig. 4). Moreover after activation and evacuation of the cavities
C
52.84
3.85
18.66
8.22
C
52.08
3.77
19.02
8.38
C
53.82 the thermal stability of daughter compounds TMU-46S, 47S,
H
Cu
N
H
Cu
N
H
Cu
N
3.50
and 48S was evaluated by TGA. Thermal stability analysis of the
compounds indicated two steps which are attributed to the
18.39
8.10
O
16.43
O
16.75
O
16.20 removal of ligands. According to TGA analysis of the parent
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Fig. 3 Coordination environment around Cu (a), representation of the simplified three-dimensional structure (b), three-dimensional structure and
two-fold interpenetration structure of TMU-46 along the b axis (c), and the orientation of amide groups inside the pores along the b axis (d).
Fig. 4 Thermal gravimetric analysis of TMU-46, 47 and 48.
Fig. 5 Thermal gravimetric analysis of TMU-46S, 47S and 48S after
activation.
compounds the first step showed elimination of S linkers. The
decomposition temperatures ranged from 400 to 550 1C. The (Fig. 7). To explore the effect of SALE on the BET surface
first weight loss at 350 1C, 300 1C, and 250 1C for TMU-46S, area of the daughter compounds, N2 adsorption/desorption
TMU47S and TMU-48S respectively, was attributed to the measurement was also performed on the samples. The results
removal of the S linker from the structure backbone (Fig. 5).
indicate that all the daughter materials have lower BET surface
Fig. 6 shows the comparison of PXRD patterns of TMU-46 areas than the parents. The order of the BET surface areas for
2
ꢀ1
single crystal and the as-synthesized samples prepared by a the daughter MOFs is as follows: TMU-46S (510 m
g
) 4
2
ꢀ1
2
ꢀ1
solvothermal method (TMU-47,48 and all dughter MOFs). The TMU-47S (520 m
g ) 4 TMU-48S (408 m g ) (Fig. 7).
H NMR shows 73%, 74%, and 50% replacement of the parent
bpta, bpfb and bpfn pillars by S linkers after 5 days (Fig. S3 and
1
good agreement between these PXRD patterns indicates that all
parent and daughter samples are structurally identical.
To investigate the porosity of the TMU-46, 47 and 48 Table S2, ESI†). Fig. 6 shows the powder X-ray diffraction
structures, N adsorption/desorption was carried out at 77 K (PXRD) patterns of TMU-46, TMU-47, and TMU-48 as well as
2
and 1 bar on the samples. The Brunauer–Emmett–Teller (BET) the daughters TMU-46S, 47S and 48S. Clearly, all the functio-
specific surface area for TMU-46, 47, and 48 was obtained nalized daughter MOFs are isostructural to their parents.
2
ꢀ1
2
ꢀ1
2
ꢀ1
as 550 m
g
, 580 m
g
and 533 m
g
respectively In addition, successful incorporation of S can be evidenced
1
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Fig. 6 PXRD of the as-synthesized TMU-46 (black), TMU-47 (red) and TMU-48 (blue). The PXRD of the as-synthesized TMU-46S (pink), TMU-47S
green) and TMU-48S (dark blue).
(
Fig. 7 Nitrogen adsorption–desorption isotherms at 77 K for (a) TMU-46, (b) TMU-47, and (c) TMU-48 and the daughter MOFs.
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by their optical changes. Compared to the parent crystals, the emission ones are at 400 nm, respectively (Fig. S7–S12, ESI†).
color of the daughter samples was gradually changed from Moreover, excitation at 300 nm can be assigned to the intra-
6
4
green into brown represented the light brown color of the S ligand p–p* transition of the aromatic rings. All the fluores-
linkers but the size and morphology of the daughter crystals are cence emission experiments were carried out with a stable
similar to those of their parents (Fig. 8 and Fig. S29, ESI†).
As revealed from FTIR spectroscopy, the first characteristic luminescent treatments of daughter compounds were studied at
suspension of reported crystals (3 mg) in water (3 mL). The
ꢀ1
peak of TMU-46, 47 and 48 at 3327 cm corresponded to room temperature. In TMU-48S, the excitation and emission
amide NH stretching absorption, and also amide NH bending wavelengths are at 330 and 480 nm in water, respectively. Also,
ꢀ
1
absorption appears at 1540 cm . In all MOFs, two strong TMU-46S and TMU-47S show 320 and 475 nm for the excitation
ꢀ
1
bands at 1640 and 1690 cm corresponded to the asymme- and emission wavelengths, respectively. From the results of the
trical and symmetrical vibrations of the carboxylate group observations, we have found that the luminescence behavior of
(Fig. S4, ESI†). It can be remarked that the carbonyl stretching the parent compounds is different from their daughter samples.
band in malonamide with two resonance forms (enol and keto The excitation and emission wavelengths are distinct and also
ꢀ
1
form) at 1642 cm is broader than the amide group, which have increased suggesting that the spacer S entered into the
may be attributed to the presence of the S ligand in the MOF MOF combinations and changed the luminescent properties.
structure (Fig. S3, ESI†).
In order to study the sensing ability of Cu-MOFs for heavy
Elemental analysis from EDS data demonstrate the elemental metal ions, a series of suspension steady state luminescent
2+
constituents of the parent and daughter MOF samples. In addition, experiments were carried out. Remarkably, when the Hg solution
2
+
EDS spectra shows the appearance of Hg cations after the concentration increased, the fluorescence intensity of the
adsorption process by TMU-48S (Fig. S5, ESI†).
suspension showed a significant quenching (Fig. S13–S15,
2
+
ESI†). The concentration range of the Hg solution for TMU-
Luminescence properties and metallic ion sensing of Cu-MOFs
ꢀ1
48 is 50–350 ppm, which shows the value of Ksv = 14 076 M
.
The luminescence behavior of MOFs can originate from Comparison of the Ksv values for TMU-46, 47 and 48 toward
p-conjugated aromatic ligands, different functional groups, different metal cations is shown in Fig. S16–S18 (ESI†). On the
metal ions, and host–guest interaction of the frameworks with other hand, between Cu-MOFs (TMU-46, 47 and 48), TMU-48
2
+
specific guests, and thus luminescent MOFs have been con- shows high specificity in distinguishing the target Hg from
sidered as promising candidates for applications as chemical other metal ions, which is due to the linker-based lumines-
6
3
sensors. Since our reported crystals have p-electron conju- cence behavior with p-extended conjugation of TMU-48. This
gated functional groups, they were hypothesized to be potential trait arises from the naphthyl ring, in which the metallic ions
fluorophores with remarkable photoluminescence (PL) properties. tend to deviate and move toward them and moreover develop
Fig. S13–S15 (ESI†) show that the TMU-48 excitation and emission cation-pi interactions. In addition, the other site of interaction
wavelengths are at 315 and 425 nm in water, respectively. Also, is the amide’s oxygen, which displayed the electronic structure
TMU-46 and TMU-47 show 310 excitation wavelengths and the of the mercury ion and the strong electrostatic interaction
Fig. 8 From left to right: the color of the parent crystals TMU-46, 47 and 48 changed from green into brown in the daughter samples TMU-46S, 47S and
4
8S after SALE via S (ꢂ100).
1
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2+
Fig. 9 Fluorescence emission spectra of (a) TMU-46, (b) TMU-47 and (c) TMU-48 dispersed in water solution at different concentrations of Hg
excited at 315 nm. (d) Comparison graph between the parent compounds and daughter structure in Hg , Ag and Pb adsorption.
,
2+
+
2+
2
+
2+
between the Hg ion and the Lewis basic sites of acylamide selective photoluminescence sensor for the detection of Hg
6
5,66
oxygen in the Cu-MOF.
By comparing the compounds’ Ksv ions was investigated in the presence of similar amounts of
3
+
2+
2+
values, one can conclude that the changes were as follows: some heavy metal ions such as Cr , Cu and Cd . Different
TMU-48 4 TMU-47 4 TMU-46. cation solutions were added into the detection system of the
After examining the parent compounds, we studied the MOFs and different Ksv values of luminescence were aroused
effect of the S ligand in the daughter samples on increasing (Fig. 10). Among the parent and daughter samples, the most
2
+
the absorption of metal cations. Fig. S17–S25 (ESI†) shows that effective MOF in adsorbing Hg was TMU-48S and the order
the S spacer has a significant result on increasing the fluores- of the Ksv values was TMU-48S 4 TMU-47S 4 TMU-46S 4
ꢀ
1
2+
cence efficiency for cation absorption. This can be attributed to TMU-48. The Ksv value = 186 087 M for TMU-48S towards Hg
the existence of malonamide sites, which are able to form a in the presence of different metal cations shows a highly
2
+
chelate with metal cations. In addition, the presence of aro- selective response to Hg ions (Fig. S25–S28, ESI†). The
matic rings that make cation–p interactions has been effective. quenching constants (Ksv) for the daughter MOFs were also
Fig. 9 shows the comparison graph between the parent studied by monitoring the fluorescence intensity response of
compounds and daughter structures in adsorption of Hg , these MOFs upon different concentrations of heavy metal ions.
Ag and Pb . The most notable K values among the daughter For TMU-46S, TMU-47S and TMU-48S, the Stern–Volmer plots
2
+
+
2+
sv
compounds in the concentration range of 0.1–25 ppm were predict linear dependence on analyte concentration, with Ksv of
ꢀ1
ꢀ1
ꢀ1
2+
observed for TMU-48S due to its S linkers with chelate sites and 78 297 M , 113 148 M , and 186 087 M for Hg cations,
also two naphthyl rings, which are suitable sites for the develop- respectively. In addition, Fig. 11 demonstrates the comparison
ment of cation–p interaction. The ability of TMU-48S to act as a of the fluorescence response of TMU-46S, 47S and 48S to
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paddlewheel frameworks, inductively coupled plasma (ICP)
analysis and Atomic Absorption Spectroscopy (AAS) were also
used to investigate the framework stability. ICP and AAS
analyses significantly confirmed that more than 99% of the
Cu metal center had not leached into the solution.
Decoration of the pore-walls and surface of the porous
materials with a specific functional group is a very practical
strategy for achieving high removal capacity for small targets
like heavy metal ions. Comparison of the maximum limit of
detection (LOD) of TMU-48S with MOFs exhibiting different
2
+
active sites and porosity, for Hg metal cation removal is
provided in Table 2. As it is observed, the efficiency of the
proposed MOFs as sorbents is higher or at least comparable
with other sorbents reported for the sensing and removal of
2
+
toxic heavy-metal ions such as Hg . Since TMU-48S shows
moderate porosity, we attributed the impressive adsorption
capacity of Hg ions to synergistic effects of both the chelating
amide and malonamide functional groups. These investiga-
tions explain that due to the presence of naphthyl rings the
designed structure of the MOF has the ability to make strong
Fig. 10 Comparison of the selective function graph between the parent
compounds and daughter structures in the presence of heavy metal
cations.
2
+
10 ppm of different metal ions in water. Efficiency evaluation of cation–p interactions (Table 3). Moreover, these TMU daughter
the daughter MOFs presented in this study was compared with compounds were reused for three cycles by centrifuging the
2
+
some previously reported MOFs. Fig. 10 illustrates the high dispersed solution after Hg sensing and frequently washing
2
+
selectivity of TMU-48S toward Hg among other heavy metal the dispersed solution with water after use (Fig. 11).
cations. Furthermore, after activation and the sensing process,
Evaluation of the adsorption capacity of TMU-48S
the PXRD patterns of TMU-46, TMU-47, TMU-48 and their
daughter samples TMU-46S, 47S, and 48S show high structural To investigate the maximum adsorption capacities, 3 mg of
stability and confirmed that the structures’ crystallites were TMU-48S after pH adjustment to 6, was soaked in 10 mL
2
+
not changed (Fig. S30, ESI†). To check the stability of these distilled water spiked with varying concentrations of Hg in
Fig. 11 (a) Relative fluorescence response of TMU-46S, 47S and 48S to 10 ppm of different metal ions in water. (b) Regeneration and reuse of TMU-46S,
2+
4
7S and TMU-48S for Hg sensing.
2+
Table 2 Comparison of maximum limits of detection (LOD) of several sorbents for Hg metal ion sensing
Compound name
Zn(tpbpc)
Ligand
Reported LOD in Hg(II) sensing
Ref.
ꢀ
[
2
]
tpbpc
0.32 mM
0.1 ppm
17.6 nM
10.9 nM
25.0 ppb
67
TMU-48S (our synthesis compound)
UiO-66-NH and a T-rich FAM-labeled ssDNA
S, bpfn
This work
2
NH
2
-BDC DNA
68
69
70
48
UiO-66@Butyne
COF-LZU8
Butyne
Thioether-based hydrazonelinked
DPT (3,6-di(pyridin-4-yl)-1,2,4,5-tetrazine)
ꢀ
6
[
Zn(OBA)-(DPT)0.5]ꢁDMF
1.8 ꢂ 10
M
1
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2+
Table 3 Comparison of maximum adsorption capacities (q
m
) of some sorbents for the removal of Hg metal ions
ꢀ1
Compound name
In @MIL-101
Ligand
Reported q
m
(mg g
)
Ref.
2
S
3
A mixture of terephthalic acid and In
Hydroxyisophthalic acid, N4,N4 -di(pyridineyl)biphenyl-4,4 -dicarboxamide
A mixture of terephthalic acid and Fe
Bpfn and S
2
S
3
q
m
q
m
q
m
q
m
= 518
= 333
= 264
= 714
71
72
73
0
0
Zn(hip)(L)ꢁ(DMF)(H
Fe @SiO @HKUST-1
TMU-48S
2
O)
3
O
4
2
3
O
4
@SiO
2
This work
Fig. 12 Interaction energies for amide-containing ligand bpta (a), bpfb (b), bpfn (c) and S (d) in the presence of metal cations. (e) Schematic
representation of the framework interaction with a metal ion.
ꢀ
1
the range of 20–200 mg L . Equilibrium of the composite is should be considered simultaneously. TMU-48S contains malon-
reached within 15 minutes. The sample was separated centri- amide and acylamide functional groups in which malonamide
fugally to remove any solids and the concentration of remaining groups act as an excellent guest-interactive site in the removal
target ions was determined by ICP analysis. The q
lated by the Langmuir equation are 714, 454 and 204 mg g
m
values calcu- process and can trap this metal ion inside the pores through
ꢀ1
double (CQO)ꢁꢁꢁmetal ion interaction. Also there are other
2
+
2+
+
2+
for Hg , Pb and Ag , which follow the same sequence of Hg
4
different potential basic sites inside this TMU structure, which
2+
+
Pb 4 Ag . The Elemental analysis from EDS data showed are (I) the etheric oxygen atom of oba ligand, (II) the p-electron
the appearance of Hg cations after the adsorption process by density of phenyl rings and (III) carboxylate oxygen atoms of oba
2+
2ꢀ
TMU-48S (Fig. S31–S33, ESI†).
coordinated to the Cu metal. To correlate the experimental results
and the probable interaction between cations with different
ligands, MEP plots were calculated for the isolated ligands,
according to the literature methodology. Fig. S34 (ESI†) displays
the MEP isosurfaces of the studied ligands, where positive and
Theoretical calculations
To investigate the possible mechanism for Hg(II) removal by the negative phases are represented in red (for electrophilic site) and
TMU-48S structure, all potential sites of the TMU-48S framework blue (for nucleophilic site) colors, respectively. The MEP may be
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considered as a valuable electrostatic property in order to under- frameworks (TMU-46, TMU-47 and TMU-48). TMU-48S exhibited
2+
stand the relative polarity and relation between structure and an excellent selectivity for Hg sensing in the presence of the
activity. As shown in Fig. S34 (ESI†), for all compounds, the regions other heavy metal cations with the 0.1 ppm limit of detection.
around the nitrogen or oxygen atoms of the amide and malon- The calculated detection limit of TMU-48S in comparison with
amide groups represent the most electron rich region (light blue TMU-48 (the parent sample) demonstrates significant increased
2+
color), which demonstrates the potential of these sites in an amount. Meanwhile, the efficient removal of Hg from water of
electrophilic attack. In fact, the presence of a strong electrophilic these new functionalized MOFs offered the possibility for practical
site in the diketon of the malonamide groups can allow them to applications.
interact with heavy metal ions, while the same sites in TMU-46, 47
and 48 are mainly delocalized on the surface of aromatic rings and
are probably weaker to interact with positive ions. As shown in
Conflicts of interest
Fig. 12 the optimized geometry of the metal cation and functional
group of the ligands is specified. In the bpta and bpfb ligands the
There are no conflicts to declare.
interaction is between the carbonyl functional group and metal
cation and in bpfn the interaction is between the carbonyl and the Acknowledgements
naphthyl rings. Also for S ligands, the interaction is among the
This work was funded by Tarbiat Modares University.
malonamide functional group and metal cation in a chelate form.
The interaction energies for amide-containing ligands and
malonamide ligand bpta, bpfb, bpfn and S in the presence of
metal cations were studied. Table S3 (ESI†) shows the interaction
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8088 | New J. Chem., 2019, 43, 18079--18091 This journal is ©The Royal Society of Chemistry and the Centre National de la Recherche Scientifique 2019

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