An excellent example illustrating the fluorescence sensing property of cobalt-organic frameworks

Article abstract of DOI:10.1039/C9DT00005D

A novel luminescent cobalt-organic framework was assembled and employed as a chemosensor for the detection of metal ions via fluorescence enhancement and quenching. The sensing mechanism was based on the exposed pyridyl and carboxyl functional sites in the MOF structure, which interacted with metal ions via Lewis acid-base interactions and electrostatic interactions.

Full text of DOI:10.1039/C9DT00005D

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An excellent example illustrates the fluorescence sensing
property of cobalt–organic framework
Received 00th January 20xx,
Accepted 00th January 20xx
Chuan-Lei Zhang,a,b Zi-Teng Liu, Heng Xu, He-Gen Zheng* , Jing Ma* , Jing Zhao*
c
b
a
c
a
DOI: 10.1039/x0xx00000x
www.rsc.org/
A novel luminescent cobalt–organic framework is assembled and expensive chemicals, stability, and lack of molecular
employed as a chemosensor for the detection metal ions through organization. MOFs (Metal−organic frameworks) are relatively
fluorescence enhancement and quenching, respectively. The a new class of hybrid crystalline materials with diverse
sensing mechanism is based on exposed pyridyl and carboxyl structural characteristics constructed from metal ions or metal
functional sites in the MOF structure, which interact with metal clusters and organic ligands. Over the past few years,
ions for Lewis acid-base interactions and electrostatic interactions. significant progress has been made in the uses of the
8
luminescence property of MOFs for sensing of metal ions.
Aluminum and Iron ions are very important to biological and
environmental systems. The trivalent cation of aluminum
Generally, the diverse functional sites and pore topologies
make MOFs as suitable sensory materials through the
molecular level interactions between the frameworks and the
1
3
+
(
Al ) existing in natural water and most plants can enter the
2
human body through foods and drinking water. The trivalent
cation of Iron (Fe ) is known to participate in oxygen uptake,
oxygen metabolism and electron transfer. Abnormal
concentrations of Al and Fe3+ may result in numerous
diseases such as bone softening and encephalopathy.
Notably, the iron binding protein can carry Al ion to the brain
to damage the central nervous system. Thus, it is of
considerable importance to detect Al and Fe3+ with high
selectivity and sensitivity, both in the environment and in
organisms.
9
analytes. An efficient strategy to prepare MOFs for the use of
3
+
metal ion sensing is to use aromatic organic molecules with
nonbonded functional ligand sites, in which the aromatic part
provides luminescence, and the nonbonded functional sites
3
3+
4
10
interact with the metal ions. However, immobilization of
3
+
functional sites within the MOFs has been still challenging due
to their high reactivity during the synthesis of MOFs.
5
3+
Among the MOFs sensors, most research focused on the
3+
3+
high-cost lanthanide metals, heavy metal ions (Ir , In , etc)
10
2+
and d electron configuration transition metal ions (Zn ,
Among the sensing methods, luminescence-based methods
have gained much attention in recent years due to their
advantageous features such as easy of manipulation, high
sensitivity, and real-time monitoring with rapid response
2+
11
Cd )-based MOFs. The highly economic and abundant cobalt
ion with magnetic performance has been studied very few on
MOF sensor. This is mainly because non-d electron
configuration usually has low emission performance due to d-d
transition in the visible region. On the basis of the above
consideration, we assembled a 2D luminescent Co-MOF,
12
10
6
time. Recently, some luminescent sensors with detection of
3
+
3+
Al or Fe ions have been reported, and even fewer sensors
3
+
3+
have been reported for sensing Al
simultaneously in pure water. Nevertheless, their widespread
use is limited owing to multistep processing, requirement of
and Fe
ions
[
2 2 2 4
Co(H L)(TPY)(H O)]·H O {H L = [1,1':4',1''-terphenyl]-2',4,4'',5'-
7
tetracarboxylic acid and TPY = 4,3':5',4''-terpyridine}, 1, with
free Lewis acid and base functional groups under
3
+
hydrothermal reaction. It senses Al ion through 5.3 fold
3
+
luminescence enhancing and Fe ion through complete
quenching in aqueous medium. As a sensor, 1 shows response
in the second timescale and its sensitivity is well maintained
even in the presence of interfering ions. The very high
selective and sensitive luminescence behavior indicates that it
is strong molecular level interaction between the MOF and
metal ions. The sensing mechanism can be considered as
photo-induced electron transfer (PET). At the same time, we
give further experimental proof and theoretical calculation to
a. State Key Laboratory of Coordination Chemistry, School of Chemistry and
Chemical Engineering, Collaborative Innovation Center of Advanced
Microstructures, Nanjing University, Nanjing 210023, P. R. China
b.
Anhui Provincial Laboratory of Optoelectronic and Magnetism Functional
Materials, School of Chemistry and Engineering, Anqing Normal University,
Anqing 246011, China.
c.
Theoretical and Computational Chemistry Institute, School of Chemistry and
Chemical Engineering, Nanjing University, Nanjing 210093, China.
d. † Electronic supplementary information (ESI) available. CCDC 1559997. For ESI
and
e.
crystallographic data in CIF or other electronic format see DOI:
1
0.1039/x0xx00000x
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DOI: 10.1039/C9DT00005D
Fig.2 Changes of luminescence intensity with respect of emission of 1 with different
metal ions.
measurement for the solid powdered sample of 1 was carried
out in the temperature range 1.8-300 K under an applied field
of 1000 Oe.
The χ
temperature, the χ
greater higher than that expected for two high-spin Co ions
through the spin only formula (2 × 1.875 = 3.75 cm mol
M
T versus T plot is shown in Fig. S6, at room
3
−1
M
T value is 5.54 cm mol K, which is
II
3
−1
K
with g = 2.0). It could be due to the occurrence of an
4
unquenched orbital contribution typical of the
T
1g ground
T product
remains relatively constant from 300 to 100 K. Below 100K, the
II
13
state for octahedral high-spin Co complex. The χ
M
3
−1
Fig. 1 (a) Coordination model of binuclear cobalt. (b) Coordination environment of 1
χ
M
T rapidly drops to a minimum value of 4.93 cm mol K at
0 K, which is mainly due to the spin-orbital coupling of the
Co(II) ions. Upon further cooling, the χ T value increases
with 30% ellipsoid probability. (c) The 1D chain containing “swinging cantilever”. (d)
2
2 2 2 4
(μ-OH ) L ligands. (e) The AA···
}4+ and H
The 2D network constructed by dinuclear {Co
stacked 2D structure. (f) Topological representation of the parallel stacking 2D layers of
M
3
−1
sharply to a value of 5.28 cm mol K at 5 K, and then falls
rapidly to 4.62 at 1.8 K. The increase below 20 K is caused by
the ferromagnetic coupling between the Co(II) ions (Co···Co
1
.
illustrate this phenomenon.
The structure of 1 was determined by single-crystal X-ray
diffraction study. As shown in Fig. 1, each H L ligand is
4
coordinated with three Co(II) ions via its two lateral
carboxylate groups, and its two terminal carboxyl groups exist
in the structure through strong hydrogen bonds interaction
M
distance is 3.061 Å), whereas the final decrease of χ T below 5
K indicates the presentence of magnetic anisotropy in 1. The
magnetic susceptibilities can be fitted well by the PHI
1
4
program though the spin Hamiltonian given in eq 1 (ESI),
-
1
-1
with J = 1.48 cm ，g
=
1
= g
2 1 2 1 2
= 1.71，D = D =43.6 cm ，E = E
-1
6.75 cm . The positive exchange parameter J confirms the
(Fig. 1b, Tab. S3). The TPY ligand is coordinated with cobalt ion
ferromagnetic intra-complex magnetic interaction in this
molecular, which is in agreement with other reported
only through one pyridine group, and the other two pyridine
groups also exist in the form of hydrogen bonds. Two
octahedral cobalt(II) centers are bridged by two μ-1,3
carboxylate ligands and two water molecules. The two bridging
carboxylate groups are disposed trans to each other across the
II
15
polynuclear Co complexes. The frontier molecular orbitals
and spin density distributions on this cobalt center clearly
show the character of unpaired electron, as seen in Fig. S9-S10,
II
which is also indicated the high-spin Co ions in 1.
4
+
{
2 2 2
Co (μ-OH ) } plane, and two TPY ligands disposed anti across
The luminescence spectra of 1 dispersed in water are shown
in Fig. S12. 1 exhibits luminescence at λmax = 400 nm, upon
excitation at 318 nm. The emission observed is due to
intraligand transitions (π* → π and π* → n transitions) of two
aromatic ligands bonded with the metal ions. To explore the
the Co-Co vector completes the pseudo-octahedral
coordination sphere of each cobalt(II) center. Thus, the
4
+
dinuclear {Co
through crossing each other to generate a 2D network (Fig.
d), which was modified by TPY ligands to give rise to a 2D
2
(μ-OH
2 2
) }
4
were connected by H L ligands
1
ability of
1 to sense a trace amount of metal ions,
structure containing “swinging cantilever” (Fig. 1c). The
exposed pyridyl and carboxyl functional sites have significant
role for the selective sensing of Al and Fe ions in aqueous
medium.
For the presence of centrosymmetric dinuclear CoII in
complex 1, we focused on its magnetic behavior. Variable-
temperature direct-current (dc) magnetic susceptibility
luminescence-based titrations were performed with the
incremental addition of analytes (metal ions) to 1 dispersed in
water (Fig. S13). Fig. 2 shows that most metal ions have
varying degrees of quenching effects on the luminescence
3
+
3+
-3
intensity (up to 50 μL, 5 × 10 M), such as the luminescence
2
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PET). Simultaneously, the aromatic system of the pyridine ring
(
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3+
039/C9DT00005D
was perturbed. For example, when trivaDleOnIt: 1F0e.1 ions, a strong
Lewis acid, coordinated to the open nitrogen atoms, under
ultraviolet light excitation, the excited state electrons from
3
+
nitrogen atoms transferred to Fe ions, and the perturbation
of the aromatic system led to deformation of the distribution
1
8
of π-electron density in the pyridine ring. This behavior was
consistent with the N 1s XPS spectra, N 1s peak from free
pyridyl nitrogen atoms of 1 at 396.98 eV is shifted to 397.22 eV
3
+
after 1 immersed in Fe ions (Fig. S15-16).
Fig.3 Differential types of interactions of Al3+ ions and all other metal ions (M2+ / M3+)
with the ligand unit of 1. The Lewis acid−based interactions are represented using the
arrows, and the electrostatic interactions (only for Al3+ ions) are represented using
Additionally, for the quenching effect, the liquid UV−vis
absorption spectra were recorded. As seen in Fig. S17, the
maximum absorption peak of 1 appears at 283 nm, while Fe3+
dotted line.
ions have a wide absorption band from 250 to 400 nm. This
intensity at 400 nm is about 64.5 % of 1 when Cu ions are band covered not only the whole range of 1 in absorption
2
+
3
+
involved, and Fe ions can almost quench the luminescence of spectrum, but also nearly half range in emission spectrum. For
1
. However, when Al3+ ions are present, the luminescence other metal ions, apparent absorption bands were not
2
+
intensity is enhanced compared to the 1, which is nearly 5.3 observed (Cu has less absorption). Upon light excitation,
times of the initial luminescence intensity along with the blue there is a competition of absorbing light source energy
3
+
shifting of ∼5 nm of the emission peak. The results indicate between Fe and 1, and part of the emitted light from excited
3
+
luminescence behavior of 1 can be significantly different only state of 1 will also be absorbed by Fe ions. In consequence, 1
3
+
3+
3+
in the presence of Al and Fe ions. The luminescence presented the best quenching effect for Fe ions.
3
+
enhancement behavior in the presence of Al ions in aqueous
To prove the above mechanism of the luminescence
3
+
medium can be clearly detected at as low as 1.67 μM enhancement behavior for Al ions, we performed the
concentration, which is equivalent to 45 ppb of Al3 ions in titrations in acidic aqueous medium where the possibility of
water (Fig. S14). The observed detection limit is significantly the formation of anionic carboxylate is expected to reduce. For
lower than the higher limit of U.S. Environmental Protection this purpose, 50 μL of 5 × 10- M aqueous solution of HCl
+
3
3
+
Agency (EPA) recommendation of Al ion for drinking water solution was added to the aqueous mixture of the MOF prior
3+
(
200 ppb).16
to the luminescence titration using Al ions. The result shows
To explain the observed luminescence titration behaviors in 1.21 times enhancement in luminescence intensity upon the
the presence of these metal ions, we considered two addition of 50 μL (5 × 10- M) Al ions, which is nearly 4.38
3
3+
important factors: (i) Differential types of interactions of the times less than the value obtained in original pure aqueous
free carboxyl groups of H L ligand with the metal ions and (ii) medium (Fig. S18). Similarly, we also performed the titrations
4
Lewis acid−base interactions between the metal ions and the in alkaline aqueous medium, where the formation of
free pyridyl groups of the TPY of the MOF. Among these ions, carboxylate will be accelerated. This result shows 2.70 times
3
+
charge-to-size ratio of Al ions is so high that it is expected to enhancement in luminescence intensity, which is about half of
cause rupture of the O−H bonds of the carboxyl groups to form the value obtained in original pure aqueous medium. The
+
17
carboxylates and hydronium ions (H
3
O ) in aqueous medium,
lesser amount of enhancement behavior supports the
3
+
whereas the free pyridyl groups of TPY act as simple electron interaction between the Al ions and MOF through the
donor to Al3+ ions through Lewis acid−base type interactions. It carboxyl groups of H L ligand.
4
is likely that the possibility of strong electrostatic interactions
between the Al3+ ions and carboxylates (anionic) is responsible
To further prove that the fluorescence quenching behavior is caused by Lewis
acid−base interactions, detailed studies on
for the blue shifting of the emission peak with the incremental
addition of the Al3+ ions. Additionally, the formation of
4
-
carboxylate increases the overall electron density on L , which
resulted in enhanced luminescence behavior. However, for
other metal ions, the carboxyl groups acting as Lewis acid
cannot react with them. It is likely the reduced electron
density on the pyridyl ring through Lewis acid−base
interactions is responsible for luminescence quenching. The
3
+
schematic of the above interactions of the Al ions and the
other metal ions with the ligands are shown in Fig. 3.
Lewis acid−base interactions between the metal ions and
the free pyridyl of the TPY, actually, are the nitrogen atoms of
pyridyl groups binding to metal ions in a subtle and weak
fashion. Thus, the excited state electrons can be transferred to
the metal ions, achieving a photo-induced electron transfer
Fig.4 Emission spectra of 1 dispersed in water upon incremental addition of 5 × 10-3
Eu3+ solution.
M
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the luminescence properties of 1 were carried out. Lanthanide
metal ions have large charge-to-radius ratio, belong to hard
acid. They are easy to coordinate with uncoordinated carboxyl
Conflicts of interest
There are no conflicts to declare.
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DOI: 10.1039/C9DT00005D
3
+
groups in 1 to enhance the Lewis acid-base interaction. So Eu
and Tb3+ ions are used to instead of Fe for titration
3+
Notes and references
experiments. As shown in Fig. 4 and Fig. S19, with the increase
of Eu3+ / Tb3+ ions concentration, the intensity of the emission
1
(a) J. J. Leddy, Electrochemistry, Past and Present, 1989, ch.
3
3, vol. 390, p. 478; (b) P. G. Nelson, J. Chem. Educ., 1991,
8, 732; (c) M. Zhao, Z. F. Deng, J. Tang, X. F. Zhou, Z. Chen,
peak (λmax = 400 nm) of the original solution gradually
6
3
+
decreases until nearly disappears, just as in the case of Fe
X. T Li, L. Yang and L. J. Ma, Analyst, 2016, 141, 2308.
3
+
3+
ions. However, the characteristic peaks of Eu / Tb ions
2
(a) W. H. Ding, W. Cao, X. J. Zheng, D. C. Fang, W. T. Wong,
and L. P. Jin, Inorg. Chem., 2013, 52, 7320; (b) M. I. Yousef,
A. M. A. EI-Morsy and M. S. Hassan, Toxicology, 2005, 215,
appear and gradually increase (λmax = 594, 617, 654, 698 nm
3
+
3+
for Eu ; λmax = 491, 545, 592, 617 nm for Tb ), indicating that
9
7.
3
+
3+
there are stronger interactions between 1 and Eu / Tb ions.
It shows that both nitrogen atoms from pyridyl groups and
oxygen atoms from carboxyl groups in 1 have interactions with
metal ions. This was also consistent with the XPS spectra, N 1s
peak from free pyridyl nitrogen atoms of 1 at 396.98 eV is
3
4
(a) M. Y. Deng, S. Wang, C. S. Liang, H. X. Shang and S. M.
Jiang, RSC Adv., 2016, 6, 26936; (b) R. Patidar, B. Rebary, G.
R. Bhadu and P. Paul, J. Lumin., 2016, 173, 243.
(a) A. S. Pithadia and M. H. Lim, Curr. Opin. Chem. Biol., 2012,
1
6, 67; (b) E. Gauthier, I. Fortier, F. Courchesne, P. Pepin, J.
Mortimer and D. Gauvreau, Environ. Res., 2000, 84, 234; (c)
T. P. Flaten, Brain Res., 2001, 55, 187; (d) C. H. Fan, X. M.
Huang, L. H. Han, Z. L. Lu, Z. Wang and Y. P. Yi, Sens.
Actuators, B, 2016, 224, 592.
3
+
shifted to 397.45 eV after 1 immersed in Eu ions, and the O
s peak is also shifted from 529.62 to 530.18 eV. This result
1
strongly supports the second hypothesis about fluorescence
quenching.
5
6
G. D. Fasman, Coord. Chem. Rev., 1996, 149, 125.
(a) J. R. Lakowicz, Principles of Fluorescence Spectroscopy,
Motivated by very high luminescence enhancement in the
3
rd ed.; Kluwer Academic: New York, 2006; (b) N. B.
3
+
presence of Al ions, we thought to check the selectivity for
Shustova, A. F. Cozzolino, S. Reineke, M. Baldo and M. Dincă,
J. Am. Chem. Soc., 2013, 135, 13326.
(a) V. D. Suryawanshi, A. H. Gore, P. R. Dongare, P. V.
Anbhule, S. R. Patil and G. B. Kolekar, Spectrochim. Acta, Part
A, 2013, 114, 681; (b) S. Vallejos, A. Muñoz, S. Ibeas, F.
Serna, F. C. García and J. M. García, J. Hazard. Mater., 2014,
Al ions in the presence of Fe3+ ions (the highest quenchable
3+
metal ions). In a specially designed experimental protocol, the
7
8
luminescence spectrum of
1 in aqueous medium was
-3
recorded, and to this 1 μL of 5 × 10 M aqueous solution of
3
+
-3
Fe ions was added sequentially followed by 1 μL of 5 × 10
M
2
76, 52.
3
+
aqueous solution of Al ions in similar way; the corresponding
(a) L. N. Li, S. S. Shen, R. Y. Lin, Y. Bai and H. W. Liu, Chem.
Commun., 2017, 53, 9986; (b) X. D. Sun, S. Yao, C. Y. Yu, G.
H. Li, C. M. Liu, Q. S. Huo and Y. L. Liu, J. Mater. Chem. A,
2018, 6, 6363.
3
+
emissions were monitored. With the addition of initial Fe
ions, obvious fluorescence quenching occurs. Then, the
3
+
addition of aqueous solution of Al ions gave greater
luminescence enhancement. With the further addition
sequences, this intermittent switching behavior continues to
appear, just like a flashlight, demonstrates high selectivity of
9
1
Z. H. Xiang, C. Q. Fang, S. H. Leng and D. P. Cao, J. Mater.
Chem. A, 2014, 2, 7662.
0 J. Rocha, L. D. Carlos, F. A. A. Paz and D. Ananias, Chem. Soc.
Rev., 2011, 40, 926.
the 1 for Al3+ ions in the presence of Fe3+ ions. The results can
11 H. H. Li, Y. B. Han, Z. C. Shao, N. Li, C. Huang and H. W.
Hou, Dalton Trans., 2017, 46, 12201.
be easily visualized by plotting the percentage of luminescence
intensity versus volume of analyte added (Fig. S20).
In conclusion, a cobalt–organic framework with exposed
1
2 W. M. Chen, X. L. Meng, G. L. Zhuang, Z. Wang, M. Kurmoo,
Q. Q. Zhao, X. P. Wang, B. Shan, C. H. Tung and D. Sun, J.
Mater. Chem. A, 2017, 5, 13079.
pyridyl and carboxyl functional sites has been synthesized and 13 (a) B. N. Figgis, M. Gerloch, J. Lewis, F. E. Mabbs and G. A.
Webb, J. Chem. Soc. A, 1968, 2086; (b) M. Gerloch and P. N.
Quested, J. Chem. Soc. A, 1971, 3729.
4 F. C. Nicholas, P. A. Russell, D. T. Lincoln, S. Alessandro and S.
M. Keith, J. Comput. Chem., 2013, 34, 1164.
5 (a) V. Mishra, F. Lloret and R. Mukherjee, Inorg. Chim. Acta,
2006, 359, 4053; (b) F. Lloret, M. Julve, J. Cano, R. Ruiz-
Garcia and E. Pardo, Inorg. Chim. Acta, 2008, 361, 3432.
6 National Secondary Drinking Water Regulations of the US
EPA: Maximum Contaminant Levels, http://water.epa.gov
characterized. The compound demonstrates highly selective
3
+
3+
and sensitive fluorescent sensor for Al and Fe through
fluorescence enhancement and quenching in aqueous
medium, respectively. The fluorescence sensing mechanism
based on Lewis acid-base interaction and electrostatic
interaction has been verified by acid-base titration and
lanthanide ion titration, which is also theoretically confirmed.
It is important that the observed detection limit of 45 ppb,
which is considerably lower than the higher limit of U.S. EPA
1
1
1
/
drink/contaminants/secondarystandards.cfm, accessed 24
March 2013.
3
+
recommendation of Al ion for drinking water (200 ppb), 17 J. Huheey, E. A. Keiter and R. L. Keiter, Inorganic Chemistry:
3
+
Principles of Structure and Reactivity, 4th ed.; Pearson
Education: New York, 2000.
8 S. Cogal, M. Kiristi, K. Ocakoglu, L. Oksuz and A. U. Oksuz,
Mater. Sci. Semicond. Process, 2015, 31, 551.
make this MOF a useful Al sensor for real life application.
This work was supported by grants from the National
Natural Science Foundation of China (No. 21771101,
1
2
1371092), the Natural Science Research Key Project of Anhui
Provincial Department of Education (No. KJ2017A346), and
China Postdoctoral Science Foundation (No. 2017M621717).
4
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For Table of Contents Use Only
An excellent example illustrates the fluorescence sensing
property of cobalt–organic framework
Chuan-Lei Zhang,a,b Zi-Teng Liu, Heng Xu, He-Gen Zheng* , Jing Ma* , Jing Zhao*
c
b
a
c
a
A cobalt–organic framework senses metal ions through fluorescence enhancement
and quenching based on exposed pyrimidyl and carboxyl functional sites.