Thioether-Based Fluorescent Covalent Organic Framework for Selective Detection and Facile Removal of Mercury(II)

Article abstract of DOI:10.1021/jacs.5b10754

Heavy metal ions are highly toxic and widely spread as environmental pollutants. New strategies are being developed to simultaneously detect and remove these toxic ions. Herein, we take the intrinsic advantage of covalent organic frameworks (COFs) and dev

Full text of DOI:10.1021/jacs.5b10754

Subscriber access provided by KUNGL TEKNISKA HOGSKOLAN
Article
A Thioether-Based Fluorescent Covalent Organic Framework
for Selective Detection and Facile Removal of Mercury(II)
San-Yuan Ding, Ming Dong, Ya-Wen Wang, Yan-Tao
Chen, Huai-Zhen Wang, Cheng-Yong Su, and Wei Wang
J. Am. Chem. Soc., Just Accepted Manuscript • DOI: 10.1021/jacs.5b10754 • Publication Date (Web): 15 Feb 2016
Downloaded from http://pubs.acs.org on February 15, 2016
Just Accepted
“Just Accepted” manuscripts have been peer-reviewed and accepted for publication. They are posted
online prior to technical editing, formatting for publication and author proofing. The American Chemical
Society provides “Just Accepted” as a free service to the research community to expedite the
dissemination of scientific material as soon as possible after acceptance. “Just Accepted” manuscripts
appear in full in PDF format accompanied by an HTML abstract. “Just Accepted” manuscripts have been
fully peer reviewed, but should not be considered the official version of record. They are accessible to all
readers and citable by the Digital Object Identifier (DOI®). “Just Accepted” is an optional service offered
to authors. Therefore, the “Just Accepted” Web site may not include all articles that will be published
in the journal. After a manuscript is technically edited and formatted, it will be removed from the “Just
Accepted” Web site and published as an ASAP article. Note that technical editing may introduce minor
changes to the manuscript text and/or graphics which could affect content, and all legal disclaimers
and ethical guidelines that apply to the journal pertain. ACS cannot be held responsible for errors
or consequences arising from the use of information contained in these “Just Accepted” manuscripts.
Journal of the American Chemical Society is published by the American Chemical
Society. 1155 Sixteenth Street N.W., Washington, DC 20036
Published by American Chemical Society. Copyright © American Chemical Society.
However, no copyright claim is made to original U.S. Government works, or works
produced by employees of any Commonwealth realm Crown government in the course
of their duties.

Page 1 of 9
Journal of the American Chemical Society
1
2
3
4
5
6
7
8
9
A Thioether-Based Fluorescent Covalent Organic Framework
for Selective Detection and Facile Removal of Mercury(II)
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
San-Yuan Ding,† Ming Dong,† Ya-Wen Wang,† Yan-Tao Chen,† Huai-Zhen Wang,† Cheng-Yong
Su,‡ and Wei Wang*,†,§
†State Key Laboratory of Applied Organic Chemistry, College of Chemistry and Chemical Engineering, Lanzhou
University, Lanzhou, Gansu 730000, China
‡School of Chemistry and Chemical Engineering, Sun Yat-Sen University, Guangzhou 510275, China
§Collaborative Innovation Center of Chemical Science and Engineering, Tianjin 300071, China
ABSTRACT: Heavy metal ions are highly toxic and widely spread as environmental pollutants. New strategies are being
developed to simultaneously detect and remove these toxic ions. Herein, we take the intrinsic advantage of covalent or-
ganic frameworks (COFs) and develop the fluorescent COFs for sensing applications. As a proof-of-concept, a thioether-
functionalized COF material, COF-LZU8, was “bottom-up” integrated with multi-functionality for the selective detection
and facile removal of mercury(II): the π-conjugated framework as the signal transducer, the evenly-and-densely distribut-
ed thioether groups as the Hg2+ receptor, the regular pores to facilitate the real-time detection and mass transfer, together
with the robust COF structure for the recycle use. The excellent sensing performance of COF-LZU8 was achieved in terms
of high sensitivity, excellent selectivity, easy visibility, and real-time response. Meanwhile, the efficient removal of Hg2+
from water and the recycle use of COF-LZU8 offered the possibility for practical applications. In addition, XPS and solid-
state NMR investigations verified the strong and selective interaction between Hg2+ and the thioether groups of COF-
LZU8. This research not only demonstrates the utilization of fluorescent COFs for both sensing and removal of metal ions,
but also highlights the facile construction of functionalized COFs for environmental applications.
amplifying13 the fluorescence change. In this regard, the
INTRODUCTION
rigid COF framework serves not only for the structural
preservation but also for the signaling function. Secondly,
both the “bottom-up” and “post-synthesis”14 strategies can
be applied to attach the specific ionophoric moieties to
the fluorescent COF framework. Thus, the selective sens-
ing toward diverse applications may be achieved. Thirdly,
the porous crystalline structure offers an excellent scaffold
for hosting the metal ions for detection. Therefore, the
easy access to the ionophoric sites within the regular
channels is guaranteed for the real-time response. Finally,
the enrichment of toxic metal ions inside the robust po-
rous framework can be fulfilled as the prerequisite for
further removal.
Herein, we report the first application of fluorescent
COFs for detecting and removing the metal ions. As a
proof-of-concept, a thioether-functionalized hydrazone-
linked COF, COF-LZU8, was designed and applied for the
selective sensing and effective removal of the toxic Hg2+
(Figure 1). The convenient “bottom-up” construction of
COF-LZU8 was successfully achieved from the monomers
1 and 2 under solvothermal conditions. In significant
difference from 1 and 2, COF-LZU8 showed strong
fluorescence both in solid state and upon the dispersion
in solution. Importantly, the real-time fluorescence
response did show the efficient quenching of COF-LZU8
upon the addition of Hg2+, together with its color change
under a portable UV lamp. The excellent sensitivity and
Because of the high toxicity and bioaccumulation, heavy
metal ions released into the environment can lead to a
wide range of severe diseases.1 Considerable efforts2 are
accordingly being made to develop new methodologies
for effective detection and removal of these toxic ions. For
example, many types of small molecules have been de-
signed as the fluorescent sensors for selective and sensi-
tive detection of the metal ions.3 On the other hand, vari-
ous absorbents, such as porous silicas,4 hydrogels,4b, 5 na-
noparticles6 and metal-organic frameworks (MOFs),7 are
being tested for the possible removal of the toxic ions.
However, most of the developed methods can only per-
form either the detection or the removal task separately,
which limits their practical applications.
Covalent organic frameworks (COFs) are a new type of
crystalline porous materials purely constructed with
organic building blocks.8 Characteristic in tunable
functionality, regular pore structure and high surface area,
many COFs have recently been constructed with different
covalent linkages.9 However, the potential applications
(such as for gas storage,10 optoelectricity,11 catalysis,12 and
sensing9g, 9l) are still at the very early stage of fundamental
research. We envisioned that the functional COFs could
work as the new candidates for both detection and
removal of metal ions. Firstly, the extended π-conjugation
framework can act as an excellent signal transducer by
ACS Paragon Plus Environment

Journal of the American Chemical Society
Page 2 of 9
1
2
selectivity toward the Hg2+ detection were verified in the
experiments. In addition, X-ray photoelectron and solid-
presence of other competitive cations. The removal ability
of COF-LZU8 was further elucidated by the effective
separation of Hg2+ from water. Both detection and
removal of Hg2+ could be facilely realized in the recycling
state NMR spectroscopic analysis revealed the strong and
selective interaction between Hg2+ and the sulfur atoms of
COF-LZU8.
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
Figure 1. Synthesis of COF-LZU8 via the co-condensation of 1 and 2 under solvothermal conditions (see SI for details). For clari-
ty, the extended COF-LZU8 structure is not shown. With the π-conjugated framework as the fluorophore and the thioether
groups as the Hg2+ receptor, the synthesized COF-LZU8 was applied for both detection and removal of Hg2+ (see the main text).
COF-LZU8 itself exhibited strong fluorescence upon excitation at 390 nm. Upon the addition of Hg2+, the fluorescence of COF-
LZU8 was effectively quenched. Photographs of COF-LZU8 under a UV lamp (λex = 365 nm) visualizes the significant change in
the fluorescence emission before (left) and after (right) the adsorption of Hg2+.
and-densely distributed thioether side-chains as the iono-
RESULTS AND DISCUSSION
phore (cation receptor). These unique characteristics to-
gether with the regular porous structure are expected to
be beneficial for the performance in selective detection
and facile removal of Hg2+. In addition, with the total yield
of 74%, the key thioether monomer 1 was facilely synthe-
sized in three steps from the cheap and commercially
available compound 4 (Scheme 1). As a result, COF-LZU8
can be readily prepared on a large scale for the potential
practical use.15
Designed Synthesis and Structural Characterization.
As shown in Figure 1, the thioether-based hydrazone-
linked COF material, COF-LZU8, was “bottom-up”
constructed from the simple monomers 1 and 2 under
solvothermal conditions. The synthetic conditions are
quite mild and COF-LZU8 could be reproducibly prepared
(see SI for details). Our approach not only built the rigid
and extended π-conjugation framework as the fluoro-
phore (signal transducer), but also embedded the evenly-
Scheme 1. Facile three-step synthesis of the thioether monomer 1 from the cheap and commercially available compound 4. The
total yield from 4 to 1 is 74%. See SI for the synthetic details.
The crystalline structure of COF-LZU8 was determined
by powder X-ray diffraction (PXRD) analysis with Cu Kα
radiation. The experimental PXRD pattern (Figure 2,
black) shows the diffraction peaks with the d spacings of
25.30, 12.62, 9.60, 7.23, and 3.32 Å, corresponding to the
100, 200, 210, 220, and 001 reflections, respectively. The
lattice modeling and Pawley refinement (Materials Studio,
version 6.0) gave the optimized parameters of a = b =
29.030 (± 0.446) Å and c = 3.667 (± 0.057) Å for the unit
cell with the space group of P3. After the geometrical en-
ergy minimization, the refined PXRD pattern (Figure 2,
orange) matches well with the experimental profile. Com-
parison of the observed and the simulated PXRD patterns
(Figures S12 and S13) suggested the preferable structure as
the eclipsed arrangement (Figure 2). Similar to the 2D
cyclotricatechylene16 and cyclotriveratrylene17 COFs, COF-
LZU8 possesses a contorted structure (Figure 2, side view)
with the interlayer distance of ~3.7 Å.
ACS Paragon Plus Environment
2

Page 3 of 9
Journal of the American Chemical Society
1
2
3
4
5
6
7
8
9
10
11
12
13
14
15
16
17
Figure 3. N2 adsorption and desorption isotherms of COF-
LZU8. Inset: the pore size distribution (with an average pore
width of ~13 Å) of COF-LZU8 calculated by NLDFT.
Figure 2. Experimental (black), Pawley refined (orange) and
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
predicted (blue) PXRD patterns of COF-LZU8. The difference
plot between the observed and the refined PXRD patterns is
presented in brown. Inset: the eclipsed structure proposed
for COF-LZU8. C: sky blue, N: green, O: purple, and S: red. H
atoms are omitted for clarity.
Identification of the eclipsed structure for COF-LZU8
was further supported by the pore-size-distribution analy-
sis. Nitrogen adsorption−desorption experiment at 77 K
(Figure 3) showed a type-I N2 adsorption isotherm, which
is characteristic for a microporous structure. A slight off-
set may exist between the adjacent layers of COF-LZU8
which results in pore constrictions.11h The hysteresis in the
desorption curve should be attributed to N2 trapped in
these pore constrictions. The Brunauer−Emmett− Teller
(BET) surface area of COF-LZU8 was calculated as 454 m2
g-1; and the total pore volume was 0.36 cm3 g-1 (P/P0 =
0.99), which matches with that (0.39 cm3 g-1) calculated by
PLATON18. The nonlocal density functional theory
(NLDFT) gave rise to a narrow pore-size-distribution with
an average pore width of ~13 Å (Figure 3, inset), which
agrees well with the calculated value of 12.3 Å based on
the eclipsed-structure (Figure 2).
With the robust hydrazone linkage in its structure,
COF-LZU8 is insoluble and stable in common organic
solvents, such as N,N-dimethylformamide (DMF),
tetrahydrofuran (THF), dimethyl sulfoxide (DMSO),
acetone, and trichloromethane. Importantly, COF-LZU8 is
also insoluble and very stable in water; its crystalline
structure can be preserved in aqueous solution with pH =
3−13 (Figure S16). Thermogravimetric analysis (TGA)
indicates that COF-LZU8 is thermally stable up to 310 ºC
(Figure S21). The scanning electron microscopy (SEM)
images showed that COF-LZU8 possessed the uniform
sphere-like morphology with the average particle size of
~250 nm (Figure S22). A characteristic vibrational band
appeared at 1621 cm-1 in the FT-IR spectrum of COF-LZU8
(Figure S1), indicating the successful condensation of 1
and 2 via the formation of -C=N- bonds.
Figure 4. Fluorescence spectra of 1 (gray), 2 (purple), the
model compound 3 (green), COF-LZU8 (blue), and Hg/COF-
LZU8 (red) in the solid state. Meanwhile, insoluble COF-
LZU8 can well be dispersed in solvents, and the fluorescence
spectrum of COF-LZU8 dispersed in acetonitrile was shown
(black).
Fluorescence Property. The luminescence properties of
COF-LZU8 were studied by UV/Vis and fluorescence spec-
troscopy. COF-LZU8 displayed a UV absorption band at
~390 nm in the solid state (Figure S3). Due to the extend-
ed π-conjugation in the COF-LZU8 framework, its absorp-
tion band is bathochromic (Figure S3) from those of the
monomers (1 and 2) and the model compound 3 (as a
small-molecule counterpart of COF-LZU8, structure
shown in Figure 4). The fluorescence spectrum of COF-
LZU8 in the solid state (Figure 4, blue) exhibits a maxi-
mum emission band at 460 nm with an absolute quantum
yield (ФF) of 3.5 % upon excitation at 390 nm. On the con-
trary, the corresponding monomers 1 and 2 display essen-
tially no fluorescence in the solid state upon excitation at
390 or 366 nm (Figure 4, gray and purple). Meanwhile, the
model compound 3 only shows a weak emission band at
415 nm (Figure 4, green) and a low quantum yield (ФF =
0.8 %) upon excitation at 366 nm. The significant differ-
ence in the luminescence properties between COF-LZU8
ACS Paragon Plus Environment
3

Journal of the American Chemical Society
and its structural monomers (1, 2, or 3) should be attribut-
Page 4 of 9
1
2
3
4
5
6
7
8
ed to the extended π-conjugation framework in COF-
LZU8. This difference highlights the uniqueness of using
π-conjugated COF materials for fluorescent applications.
Moreover, COF-LZU8 is readily dispersed in acetonitrile
and other organic solvents (such as THF, DMF, and etha-
nol), exhibiting a strong emission peak at ~450 nm upon
excitation at 390 nm (Figure 4, black, and Figure S6).
Therefore, COF-LZU8 keeps its high fluorescence in sol-
vents as well, which is the prerequisite for the real-time
detection of Hg2+ in solution.
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
It is worth to mention that the 2D imine-linked COF-
LZU112a shows negligible fluorescence in the solid-state
(Figure S4). The fully planar structure of COF-LZU1 may
lead to the “aggregation-caused quenching (ACQ)”, which
normally turns off the light emission.19 As shown in Figure
2, the contorted structure could efficiently avoid the ACQ
effect20 and, therefore, renders COF-LZU8 highly fluores-
cent. Meanwhile, the strong fluorescence of COF-LZU8
might be contributed partially from the thioether groups,
the similar case has been found for the thioether-
functionalized MOF.21
Detection and Removal of Hg2+. The finding of strong
fluorescence for COF-LZU8 intrigued us to further explore
its sensing applications. Mercury has been recognized as
one of the most toxic heavy metals, leading to the
dysfunction of organs and central nervous system.22 New
probes are highly desired to monitor and remove Hg2+ in
23
environmental and biological systems.22a,
Due to the
Figure 6. Selectivity of COF-LZU8 toward the Hg2+ detection:
distinct π-donor character and specific affinity, sulfur
groups are the privileged ionophoric receptor for Hg2+.
Aiming to reach the high sensitivity, unique selectivity,
and simple handling, fluorescent organosulfur compounds
have been tested as the Hg2+ chemosensor24. Accordingly,
we applied the thioether-based fluorescent COF-LZU8
firstly for the possible detection and removal of Hg2+.
(a) the fluorescence response of COF-LZU8 in the presence
of different metal ions (33.3 μM) in acetonitrile (λex = 390 nm),
and (b) the competition experiments. The green bars in (b)
represent the emission intensity of COF-LZU8 in the pres-
ence of the competition ions (2.0 equiv.), while the purple
bars represent the emission intensity upon the addition of
Hg2+ (1.0 equiv) to the above solutions. The error bars repre-
sent the standard deviation of three independent measure-
ments.
The sensitivity of COF-LZU8 toward the Hg2+ detection
was evaluated via the real-time fluorescence response. For
this purpose, the stock solution of Hg(ClO4)2 was
gradually added to the suspension of COF-LZU8 in
acetonitrile and the corresponding fluorescence spectra
were measured immediately. As shown in Figure 5, upon
the addition of Hg2+, the quenching of the fluorescence
emission of COF-LZU8, accompanied by its color change
under a portable UV lamp (with λex = 365 nm), was clearly
observed. 83% of the total fluorescence intensity of COF-
LZU8 could be quenched upon the addition of only 33.3
μM of Hg2+. In contrast, another hydrazone-linked COF-
42 with the ethoxy groups as the side chains9i, exhibited
an extremely weak response to Hg2+ (Figures S8). This
discrepancy suggested that the quenched fluorescence in
Figure 5 should be originated from the interaction
between the thioether groups of COF-LZU8 and Hg2+. The
detection limit for Hg2+ was determined as 25.0 ppb
(Figure S11). This value represents an excellent
sensitivity22a toward the Hg2+ detection, which is superior
Figure 5. Fluorescence titration of COF-LZU8 dispersed in
acetonitrile upon the gradual addition of Hg2+ (λex = 390 nm).
Inset: Photographs of fluorescence emission change (under a
UV lamp with λex = 365 nm) of COF-LZU8 in acetonitrile
upon the addition of Hg2+.
to many thioether-functionalized chemosensors24g, 24l
We further measured the Hg2+ contents in the COF-
.
ACS Paragon Plus Environment
4

Page 5 of 9
Journal of the American Chemical Society
LZU8 samples by inductively coupled plasma (ICP)
analysis. The ICP results indicate that COF-LZU8 with
1
2
3
4
5
6
7
8
11.6% of the Hg2+ content (corresponding to 1.6 Hg atoms
per unit cell), resulted in 80% of the fluorescence
quenching (Figures S9 and S10). Given that one unit cell
contains 6 thioether groups, unsaturated coordination of
these thioether groups with Hg2+ can already lead to the
efficient fluorescence-quenching. This result indicates
clearly the amplified fluorescence response for COF-LZU8
in the detection of Hg2+.
9
The selectivity of COF-LZU8 toward the Hg2+ detection
was then investigated by testing the fluorescence change
in the presence of other metal ions. As shown in Figure 6a,
among all the metal ions examined, only Hg2+ caused the
significant fluorescence-quenching of COF-LZU8. To veri-
fy the selectivity of COF-LZU8 for the practical detection
of Hg2+, the competition experiments were further con-
ducted by addition of Hg2+ (1.0 equiv.) to the acetonitrile
solution of the competitive metal ions (2.0 equiv.). As
shown in Figure 6b, neither of the competitive metal ions
showed an appreciable influence on the Hg2+ detection.
These results further identified that COF-LZU8 exhibits a
satisfactory selectivity toward the Hg2+ detection.
The sensing methods developed so far can hardly
perform the double duties of both detection and removal
of metal ions, especially in aqueous solution. We found
that COF-LZU8 worked well in aqueous solution. Similar
to the cases in organic solvents, COF-LZU8 showed high
fluorescence in water and efficient fluorescence-
quenching upon the addition of Hg2+ (Figure S6). In order
to assess the Hg2+ uptake capacity in water for potential
practical applications, we treated 20 mg of COF-LZU8
with an aqueous solution of Hg(ClO4)2 (0.1 M, 2.2 mL)
followed by dryness. Similar to those found in acetonitrile
(Figure 5) and in water (Figure S6), the obtained Hg/COF-
LZU8 powders displayed essentially no fluorescence upon
excitation at 390 nm (Figure 4, red). The ICP analysis
identified the high Hg content of 23.6% wt, corresponding
to ~2.5 Hg atoms per unit cell. To further illustrate the
effective removal of Hg2+ from water, COF-LZU8 (5 mg)
was suspended in a dilute aqueous solution of Hg(ClO4)2
(10 ppm in 5.0 mL). After the suspension was stirred for 3
h, the concentration of the residual Hg2+ in water was less
than 0.2 ppm, indicating that over 98% of Hg2+ could be
effectively removed even in extremely dilute solutions. In
comparison with the thioether-functionalized MOF,21
COF-LZU8 shows even better capability for the Hg2+ re-
moval. This may be attributed to the 2D eclipsed structure
of COF-LZU8: the straight channel with the diameter of
~13 Å facilitates the access between the Hg2+ and the S
atoms of COF-LZU8.
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
Figure 7. Recycle use of COF-LZU8 for selective detection
and facile removal of the toxic Hg2+. Upon the treatment in
aqueous Na2S solution, COF-LZU8 was facilely recovered and
could be repeatedly used.
On the basis of the excellent Hg2+ uptake capacity, we
further explored the recycle use of COF-LZU8. Upon the
simple treatment with 10 equiv. of aqueous Na2S solution
to exchange the adsorbed Hg2+ out,25 the fluorescence of
COF-LZU8 could be easily recovered (see SI for details).
As shown in Figure 7, this Hg2+ adsorption-desorption
cycle could be repeated at least for three times without
significant loss in the sensitivity and responsiveness of
COF-LZU8. Meanwhile, the crystalline structure of COF-
LZU8 was well preserved after the Hg2+ exchange (Figure
S15).
Figure 8. XPS spectra of COF-LZU8 (a) and Hg/COF-
LZU8 (b). The experimental spectra are shown in solid
curves, while the fitting data are shown in dotted curves.
Interaction between Thioether and Hg2+. To gain
further insights into the structure-property-performance
relationship for COF-LZU8 in the selective detection and
effective removal of Hg2+, we applied X-ray photoelectron
spectroscopy (XPS) to evaluate the interaction between
Hg2+ and the thioether groups. As shown in Figure 8a, the
binding energy (BE) of S 2p3/2 in COF-LZU8 was measured
as 163.1 eV, which is in agreement with that generally
found26 for organosulfur compounds. On the contrary,
upon the adsorption of Hg2+, the BE of S 2p3/2 in Hg/COF-
LZU8 was mostly shifted to 164.6 eV (Figure 8b). The
positive shift by 1.5 eV evidenced the interaction between
Unlike in organic solvents, the uniform dispersion of
COF-LZU8 in water is, however, not easy to maintain.
This phenomenon prevented us from obtaining the quan-
titative data for the systematic assessment of its sensing
performance in water. Nevertheless, this would not hinder
the potential use of COF-LZU8 at all for both detection
(Figure S6) and removal of Hg2+ in water, especially when
the advantages of COF-LZU8 (such as easy preparation,
low cost, excellent stability, and insensitivity to pH values)
are taken into account together with its excellent capabil-
ity for removing the Hg2+ (vide infra).
ACS Paragon Plus Environment
5

Journal of the American Chemical Society
Page 6 of 9
S and Hg2+ in Hg/COF-LZU8: electron was further
sensors because Hg2+ possesses intrinsically a chelation-
donated to Hg2+, which makes the S atoms electron-
deficient.26
enhanced quenching effect.27 In the case of COF-LZU8, it
is very likely that the Hg2+ firstly bound to the S atoms
and the electrons were then transferred from the π-
conjugation framework to the unoccupied orbitals of Hg2+,
resulting in the significant fluorescence quenching. Other
metal ions, such as Cd2+, could not bind with the S atoms
effectively and, therefore, their effects on the fluorescence
are negligible.
1
2
3
4
5
6
7
8
Solid-state 13C cross-polarization magic-angle spinning
(CP/MAS) NMR spectroscopy further provided the
overwhelming evidence for the strong and selective
interaction between Hg2+ and the S atoms. The 13C
CP/MAS NMR spectrum of COF-LZU8 was shown in Fig-
ure 9a and the signal assignments are depicted on the top.
Upon the adsorption of Hg2+, the 13C NMR signal at 28
ppm, which is ascribed to the methylene carbons adjacent
to the S atoms in COF-LZU8, was down-field4b shifted to
35 ppm (Figure 9b); while other signals kept unchanged.
This result identified the strong interaction between S and
Hg2+ in Hg/COF-LZU8. Meanwhile, after the treatment of
Hg/COF-LZU8 with Na2S, the 13C CP/MAS NMR spectrum
recorded thereafter (Figure 9c) closely resembles to that
of fresh COF-LZU8. This result verified the recycle use of
COF-LZU8 for facile removal of Hg2+. Moreover, in order
to further confirm the selective interaction with Hg2+, the
fresh COF-LZU8 was treated with Cd(ClO4)2. The
obtained Cd/COF-LZU8 sample (Figure 9d) gave almost
the identical 13C CP/MAS NMR spectrum as COF-LZU8,
indicating that no interaction existed between Cd2+ and
COF-LZU8.
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
CONCLUSION
In conclusion, we demonstrate in this contribution that
fluorescent COFs can be designed and synthesized for
efficient detection and removal of metal ions. Possessing
the unique nature of the extended π-conjugation
framework, tunable functionality, and regular pore
structure, COF materials are new and ideal candidates
with intrinsic “all-in-one” sensing functionalities. As an
example towards the potential practical application, a
thioether-based hydrazone-linked COF-LZU8 was ration-
ally designed and facilely constructed for highly sensitive
detection and effective removal of Hg2+. Beneficial from
the high stability of the hydrazone linkages, together with
the dense distribution of the thioether groups and the
straight channels in COF-LZU8, the recycle use of COF-
LZU8 were additionally achieved towards the simultane-
ous detection and removal of the toxic Hg2+.
This research not only explored the construction
strategy of fluorescent COFs, but also expanded the COF
applications from gas storage/separation to cation
sensing/adsorption/removal. In addition, by realizing the
new application of hydrazone-linked COF, we paved the
way to construct and utilize the structurally-robust hydra-
zone COFs. The synthetic route in Scheme 1 should be
applicable to synthesize a diverse of monomers for con-
structing functional hydrazone COFs with tunable pore
size: the existence of alkenyl (or alkynyl) side chains in the
key intermediate 5 makes many modifications possible.
We expect that our effort will benefit the future research
on employing fluorescent and hydrazone COFs for
environmental and industrial applications.
ASSOCIATED CONTENT
Supporting Information. Experimental procedures; FT-IR,
UV/Vis, and fluorescence spectra; SEM images, PXRD pat-
terns, modeling details and atomic coordinates; N2 adsorp-
tion and TGA traces; liquid NMR data. This material is avail-
able free of charge via the Internet at http://pubs.acs.org.
Figure 9. 13C CP/MAS NMR spectra of COF-LZU8 (a),
Hg/COF-LZU8 (b), and Hg/COF-LZU8 after the treatment
with Na2S (c). The signal at 28 ppm of COF-LZU8 (a) was
down-field shifted to 35 ppm in Hg/COF-LZU8 (b), which
verified the strong interaction between Hg2+ and S. On the
contrary, the 13C CP/MAS NMR spectrum of Cd/COF-LZU8
(d) is almost identical to that of COF-LZU8 (a), indicating
that COF-LZU8 can not bind with Cd2+ strongly.
AUTHOR INFORMATION
Corresponding Author
* E-mail: wang_wei@lzu.edu.cn
Notes
The authors declare no competing financial interest.
ACKNOWLEDGMENT
Therefore, the selective detection and effective removal
of Hg2+ for COF-LZU8 stems indeed from the strong and
selective interaction between Hg2+ and the S atoms. It is
well known that most of the Hg2+ sensors are "turn off"
This work was supported by the National Natural Science
Foundation for Distinguished Young Scholars of China (No.
21425206), the 973 Program of China (2012CB821701), and the
ACS Paragon Plus Environment
6

Page 7 of 9
Journal of the American Chemical Society
(10) (a) Furukawa, H.; Yaghi, O. M., J. Am. Chem. Soc. 2009,
Specialized Research Fund for the Doctoral Program of High-
1
2
3
4
5
6
7
8
131, 8875; (b) Doonan, C. J.; Tranchemontagne, D. J.; Glover, T. G.;
Hunt, J. R.; Yaghi, O. M., Nature Chem. 2010, 2, 235.
er Education (20120211130004). We thank Prof. Qi-Yu Zheng
(Institute of Chemistry, The Chinese Academy of Sciences)
and Prof. Wei Yu (Lanzhou University) for helpful discussion.
(11) For selected examples, see: (a) Wan, S.; Guo, J.; Kim, J.;
Ihee, H.; Jiang, D., Angew. Chem., Int. Ed. 2008, 47, 8826; (b)
Wan, S.; Guo, J.; Kim, J.; Ihee, H.; Jiang, D., Angew. Chem., Int. Ed.
2009, 48, 5439; (c) Spitler, E. L.; Dichtel, W. R., Nat. Chem. 2010,
2, 672; (d) Wan, S.; Gandara, F.; Asano, A.; Furukawa, H.; Saeki,
A.; Dey, S. K.; Liao, L.; Ambrogio, M. W.; Botros, Y. Y.; Duan, X.;
Seki, S.; Stoddart, J. F.; Yaghi, O. M., Chem. Mater. 2011, 23, 4094;
(e) DeBlase, C. R.; Silberstein, K. E.; Truong, T.-T.; Abruña, H. D.;
Dichtel, W. R., J. Am. Chem. Soc. 2013, 135, 16821; (f) Dogru, M.;
Handloser, M.; Auras, F.; Kunz, T.; Medina, D.; Hartschuh, A.;
Knochel, P.; Bein, T., Angew. Chem., Int. Ed. 2013, 52, 2920; (g)
Bertrand, G. H. V.; Michaelis, V. K.; Ong, T.-C.; Griffin, R. G.;
Dincă, M., Proc. Natl. Acad. Sci. U.S.A. 2013, 110, 4923; (h) Spitler,
E. L.; Koo, B. T.; Novotney, J. L.; Colson, J. W.; Uribe-Romo, F. J.;
Gutierrez, G. D.; Clancy, P.; Dichtel, W. R., J. Am. Chem. Soc. 2011,
133, 19416.
(12) (a) Ding, S.-Y.; Gao, J.; Wang, Q.; Zhang, Y.; Song, W.-G.;
Su, C.-Y.; Wang, W., J. Am. Chem. Soc. 2011, 133, 19816. (b)
Stegbauer, L.; Schwinghammer, K.; Lotsch, B. V., Chem. Sci. 2014,
5, 2789. (c) Fang, Q.; Gu, S.; Zheng, J.; Zhuang, Z.; Qiu, S.; Yan, Y.,
Angew. Chem., Int. Ed. 2014, 53, 2878.
(13) (a) Thomas, S. W.; Joly, G. D.; Swager, T. M., Chem. Rev.
2007, 107, 1339; (b) Toal, S. J.; Trogler, W. C., J. Mater. Chem.
2006, 16, 2871.
(14) (a) Nagai, A.; Guo, Z.; Feng, X.; Jin, S.; Chen, X.; Ding, X.;
Jiang, D., Nature Commun. 2011, 2, 536; (b) Bunck, D. N.; Dichtel,
W. R., Chem. Commun. 2013, 49, 2457.
(15) Pending patent, Chinese Patent Application (No.
201310693080.9).
(16) Yu, J.-T.; Chen, Z.; Sun, J.; Huang, Z.-T.; Zheng, Q.-Y., J.
Mater. Chem. 2012, 22, 5369.
(17) Song, J.-R.; Sun, J.; Liu, J.; Huang, Z.-T.; Zheng, Q.-Y.,
Chem. Commun. 2014, 50, 788.
(18) Spek, A. L., J. Appl. Crystallogr. 2003, 36, 7.
(19) Birks, J. B., Photophysics of Aromatic Molecules, Wiley
London, 1970.
(20) Hong, Y.; Lam, J. W. Y.; Tang, B. Z., Chem. Soc. Rev. 2011,
40, 5361.
(21) He, J.; Yee, K.-K.; Xu, Z.; Zeller, M.; Hunter, A. D.; Chui, S.
S.-Y.; Che, C.-M., Chem. Mater. 2011, 23, 2940.
(22) (a) Nolan, E. M.; Lippard, S. J., Chem. Rev. 2008, 108, 3443;
(b) Quang, D. T.; Kim, J. S., Chem. Rev. 2010, 110, 6280.
(23) For selected examples, see: (a) Climent, E.; Marcos, M.
D.; Martínez-Máñez, R.; Sancenón, F.; Soto, J.; Rurack, K.;
Amorós, P., Angew. Chem., Int. Ed. 2009, 48, 8519; (b) Ye, B.-C.;
Yin, B.-C., Angew. Chem., Int. Ed. 2008, 47, 8386; (c) Ros-Lis, J. V.;
Casasús, R.; Comes, M.; Coll, C.; Marcos, M. D.; Martínez-Máñez,
R.; Sancenón, F.; Soto, J.; Amorós, P.; Haskouri, J. E.; Garró, N.;
Rurack, K., Chem.-Eur. J. 2008, 14, 8267; (d) Li, J.; Wu, Y.; Song, F.;
Wei, G.; Cheng, Y.; Zhu, C., J. Mater. Chem. 2012, 22, 478.
(24) For selected examples, see: (a) Nolan, E. M.; Lippard, S. J.,
J. Am. Chem. Soc. 2003, 125, 14270; (b) Yoon, S.; Albers, A. E.;
Wong, A. P.; Chang, C. J., J. Am. Chem. Soc. 2005, 127, 16030; (c)
Coronado, E.; Galán-Mascarós, J. R.; Martí-Gastaldo, C.;
Palomares, E.; Durrant, J. R.; Vilar, R.; Gratzel, M.; Nazeeruddin,
M. K., J. Am. Chem. Soc. 2005, 127, 12351; (d) Zhao, Y.; Zhong, Z., J.
Am. Chem. Soc. 2006, 128, 9988; (e) Huang, J.; Xu, Y.; Qian, X., J.
Org. Chem. 2009, 74, 2167; (f) Yang, Y.-K.; Yook, K.-J.; Tae, J., J.
Am. Chem. Soc. 2005, 127, 16760; (g) Descalzo, A. B.; Martínez-
Máñez, R.; Radeglia, R.; Rurack, K.; Soto, J., J. Am. Chem. Soc.
2003, 125, 3418; (h) Rurack, K.; Kollmannsberger, M.; Resch-
Genger, U.; Daub, J., J. Am. Chem. Soc. 2000, 122, 968; (i) Chen,
X.; Nam, S.-W.; Jou, M. J.; Kim, Y.; Kim, S.-J.; Park, S.; Yoon, J.,
Org. Lett. 2008, 10, 5235; (j) Dong, M.; Wang, Y.-W.; Peng, Y.,
Org. Lett. 2010, 12, 5310; (k) Wu, J.-S.; Hwang, I.-C.; Kim, K. S.;
REFERENCES
(1) Schwarzenbach, R. P.; Escher, B. I.; Fenner, K.; Hofstetter,
T. B.; Johnson, C. A.; von Gunten, U.; Wehrli, B., Science 2006,
313, 1072.
(2) (a) You, L.; Zha, D.; Anslyn, E. V., Chem. Rev. 2015, 115,
7840; (b) Vendrell, M.; Zhai, D.; Er, J. C.; Chang, Y.-T., Chem. Rev.
2012, 112, 4391; (c) Kreno, L. E.; Leong, K.; Farha, O. K.; Allendorf,
M.; Van Duyne, R. P.; Hupp, J. T., Chem. Rev. 2012, 112, 1105; (d)
Jung, J. H.; Lee, J. H.; Shinkai, S., Chem. Soc. Rev. 2011, 40, 4464;
(e) Dave, N.; Chan, M. Y.; Huang, P.-J. J.; Smith, B. D.; Liu, J., J.
Am. Chem. Soc. 2010, 132, 12668.
(3) (a) Carter, K. P.; Young, A. M.; Palmer, A. E., Chem. Rev.
2014, 114, 4564; (b) Li, X.; Gao, X.; Shi, W.; Ma, H., Chem. Rev.
2014, 114, 590; (c) Kim, H. N.; Ren, W. X.; Kim, J. S.; Yoon, J.,
Chem. Soc. Rev. 2012, 41, 3210; (d) Du, J.; Hu, M.; Fan, J.; Peng, X.,
Chem. Soc. Rev. 2012, 41, 4511; (e) Quang, D. T.; Kim, J. S., Chem.
Rev. 2010, 110, 6280.
(4) (a) Wu, Z.; Zhao, D., Chem. Commun. 2011, 47, 3332; (b)
Feng, X.; Fryxell, G. E.; Wang, L.-Q.; Kim, A. Y.; Liu, J.; Kemner,
K. M., Science 1997, 276, 923.
(5) (a) Esser-Kahn, A. P.; Iavarone, A. T.; Francis, M. B., J. Am.
Chem. Soc. 2008, 130, 15820; (b) Oh, Y.; Morris, C. D.; Kanatzidis,
M. G., J. Am. Chem. Soc. 2012, 134, 14604.
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
(6) Khajeh, M.; Laurent, S.; Dastafkan, K., Chem. Rev. 2013,
113, 7728.
(7) (a) Yee, K.-K.; Reimer, N.; Liu, J.; Cheng, S.-Y.; Yiu, S.-M.;
Weber, J.; Stock, N.; Xu, Z., J. Am. Chem. Soc. 2013, 135, 7795; (b)
Abney, C. W.; Gilhula, J. C.; Lu, K.; Lin, W., Adv. Mater. 2014, 26,
7993; (c) Fang, Q.-R.; Yuan, D.-Q.; Sculley, J.; Li, J.-R.; Han, Z.-B.;
Zhou, H.-C., Inorg. Chem. 2010, 49, 11637.
(8) (a) Côté, A. P.; Benin, A. I.; Ockwig, N. W.; O'Keeffe, M.;
Matzger, A. J.; Yaghi, O. M., Science 2005, 310, 1166; (b) Feng, X.;
Ding, X.; Jiang, D., Chem. Soc. Rev. 2012, 41, 6010; (c) Ding, S.-Y.;
Wang, W., Chem. Soc. Rev. 2013, 42, 548; (d) Colson, J. W.;
Dichtel, W. R., Nature Chem. 2013, 5, 453.
(9) For selected examples, see: (a) El-Kaderi, H. M.; Hunt, J.
R.; Mendoza-Cortes, J. L.; Côté, A. P.; Taylor, R. E.; O'Keeffe, M.;
Yaghi, O. M., Science 2007, 316, 268; (b) Tilford, R. W.; Gemmill,
W. R.; zur Loye, H.-C.; Lavigne, J. J., Chem. Mater. 2006, 18, 5296;
(c) Colson, J. W.; Woll, A. R.; Mukherjee, A.; Levendorf, M. P.;
Spitler, E. L.; Shields, V. B.; Spencer, M. G.; Park, J.; Dichtel, W.
R., Science 2011, 332, 228; (d) Uribe-Romo, F. J.; Hunt, J. R.;
Furukawa, H.; Klock, C.; O'Keeffe, M.; Yaghi, O. M., J. Am. Chem.
Soc. 2009, 131, 4570; (e) Zhang, Y.-B.; Su, J.; Furukawa, H.; Yun, Y.;
Gándara, F.; Duong, A.; Zou, X.; Yaghi, O. M., J. Am. Chem. Soc.
2013, 135, 16336; (f) Kandambeth, S.; Mallick, A.; Lukose, B.; Mane,
M. V.; Heine, T.; Banerjee, R., J. Am. Chem. Soc. 2012, 134, 19524;
(g) Das, G.; Biswal, B. P.; Kandambeth, S.; Venkatesh, V.; Kaur,
G.; Addicoat, M.; Heine, T.; Verma, S.; Banerjee, R., Chem. Sci.
2015, 6, 3931; (h) Kuhn, P.; Antonietti, M.; Thomas, A., Angew.
Chem., Int. Ed. 2008, 47, 3450; (i) Uribe-Romo, F. J.; Doonan, C. J.;
Furukawa, H.; Oisaki, K.; Yaghi, O. M., J. Am. Chem. Soc. 2011, 133,
11478; (j) Jackson, K. T.; Reich, T. E.; El-Kaderi, H. M., Chem.
Commun. 2012, 48, 8823; (k) Nagai, A.; Chen, X.; Feng, X.; Ding,
X.; Guo, Z.; Jiang, D., Angew. Chem., Int. Ed. 2013, 52, 3770; (l)
Dalapati, S.; Jin, S.; Gao, J.; Xu, Y.; Nagai, A.; Jiang, D., J. Am.
Chem. Soc. 2013, 135, 17310; (m) Beaudoin, D.; Maris, T.; Wuest, J.
D., Nat. Chem. 2013, 5, 830; (n) Zhou, T.-Y.; Xu, S.-Q.; Wen, Q.;
Pang, Z.-F.; Zhao, X., J. Am. Chem. Soc. 2014, 136, 15885; (o) Zeng,
Y.; Zou, R.; Luo, Z.; Zhang, H.; Yao, X.; Ma, X.; Zou, R.; Zhao, Y., J.
Am. Chem. Soc. 2015, 137, 1020.
ACS Paragon Plus Environment
7

Journal of the American Chemical Society
Page 8 of 9
Kim, J. S., Org. Lett. 2007, 9, 907; (l) Kim, S. H.; Song, K. C.; Ahn,
1
2
3
4
5
6
7
8
S.; Kang, Y. S.; Chang, S.-K., Tetrahedron Lett. 2006, 47, 497; (m)
Lee, M. H.; Lee, S. W.; Kim, S. H.; Kang, C.; Kim, J. S., Org. Lett.
2009, 11, 2101; (n) Li, M.-J.; Chu, B. W.-K.; Zhu, N.; Yam, V. W.-W.,
Inorg. Chem. 2007, 46, 720.
(25) (a) Zhou, Y.; Zhu, C.-Y.; Gao, X.-S.; You, X.-Y.; Yao, C.,
Org. Lett. 2010, 12, 2566; (b) Wu, D.; Huang, W.; Lin, Z.; Duan, C.;
He, C.; Wu, S.; Wang, D., Inorg. Chem. 2008, 47, 7190; (c) Findlay,
D. M.; McLean, R. A. N., Environ. Sci. Technol. 1981, 15, 1388; (d)
Armstrong, R. D.; Porter, D. F.; Thirsk, H. R. J. Phys. Chem. 1968,
72, 2300.
9
(26) Wagner C. D.; Riggs W. M.; Davis L. E.; Moulder J. F.; E.,
M. G., Handbook of X-ray Photoelectron Spectroscopy. Perkin
Elmer: Eden Prairie, MN, 1979.
(27) (a) Silva, A. P. d.; Gunaratne, H. Q. N.; Gunnlaugsson, T.;
Huxley, A. J. M.; McCoy, C. P.; Rademacher, J. T.; Rice, T. E.,
Chem. Rev. 1997, 97, 1515; (b) Lee, H.; Lee, H.-S.; Reibenspies, J.
H.; Hancock, R. D., Inorg. Chem. 2012, 51, 10904.
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
ACS Paragon Plus Environment
8

Page 9 of 9
Journal of the American Chemical Society
1
2
TOC
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
ACS Paragon Plus Environment
9