Luminescent covalent organic framework as a recyclable turn-off fluorescent sensor for cations and anions in aqueous solution

Article abstract of DOI:10.1039/c9tc03265g

Covalent organic frameworks (COFs) have shown great potential for use in ion sensing; however, applications of existing COFs are limited to sensing either cations or anions. In this study, a three-dimensional COF, COF-TT, is constructed by reacting the bis(tetraoxacalix[2]arene[2]triazine) core with tetra(p-aminophenyl)methane to provide a luminescent sensor. COF-TT exhibits ultrahigh thermal stability and exceptional chemical stability in aqueous solutions over a broad pH range from 2 to 14, which signifies immense practical potential for sensing applications. Excellent selectivity and high sensitivity of COF-TT toward Fe³⁺ cations and CrO₄^2-, Cr₂O₇^2-, and MnO₄^- anions are evident via luminescence quenching. COF-TT also exhibits excellent recyclability in terms of washing and re-exposure cycles. Both experimental data and theoretical calculations are employed to unveil the mechanisms of the quenching effect and sensing properties of COF-TT.

Full text of DOI:10.1039/c9tc03265g

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ISSN 2050-7526
PAPER
Nosang V. Myung, Yong-Ho Choa et al.
A
noble gas sensor platform: linear dense assemblies of
a multi-layered
single-walled carbon nanotubes (LACNTs) in
ceramic/metal electrode system (MLES)
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DOI: 10.1039/C9TC03265G
ARTICLE
Luminescent Covalent Organic Framework as a Recyclable Turn-
Off Fluorescent Sensor for Cations and Anions in Aqueous
Solution†
Ming Li,a Zhonghua Cui,*b Shirui Pang,a Lingkun Meng,a Dingxuan Ma,c Yi Li,a Zhan Shi*a and Shouhua
Feng a
Received 00th January 20xx,
Accepted 00th January 20xx
Covalent organic frameworks (COFs) have shown great potential for use in ion sensing; however, applications of
existing COFs are limited to sensing either cations or anions. In this study, a three-dimensional COF, COF-TT, is
constructed by reacting the bis(tetraoxacalix[2]arene[2]triazine) core with tetra(p-aminophenyl)methane to provide a
luminescent sensor. COF-TT exhibits ultrahigh thermal stability and exceptional chemical stability in aqueous solutions
over a broad pH range from 2 to 14, which signifies immense practical potential for sensing applications. Excellent
DOI: 10.1039/x0xx00000x
-
selectivity and high sensitivity of COF-TT toward Fe3+ cations and CrO42-, Cr2O72-, and MnO4 anions are evident via
luminescence quenching. COF-TT also exhibits excellent recyclability in terms of washing and re-exposure cycles. Both
experimental data and theoretical calculations are employed to unveil the mechanisms of the quenching effect and
sensing properties of COF-TT.
laboratory and in industry.11 However, excess Fe3+ cations in
biological systems can catalyze the production of reactive
oxygen species (ROS) and harm nucleic acids and proteins.
Uptake of Cr6+ in the human body can lead to serious
Introduction
Over the past decade,
a large number of covalent organic
frameworks (COFs)1 have been synthesized due to their highly
ordered structures, permanent porosities, and physicochemical
stabilities against moisture. The structural features of COFs impart
these new materials with immense potential for advanced
applications. These COFs are structurally diverse and can be utilized
as platforms in practical applications, such as gas storage2 or
separation,3 catalysis,4 drug delivery,5 fluorescence recognition,6
batteries,7 energy storage,8 and various other potential
applications.9 Significant advancements have been made via
cytotoxicity and carcinogenicity. KMnO4 is a strong oxidizing
agent, and is toxic and corrosive. Therefore, the design and
manufacture of sensing materials with high selectivity and
desired sensitivity with respect to Fe3+, CrO42-, Cr2O72-, and
-
MnO4 ions are significant for biological and environmental
safety.
COFs with luminescence properties were proposed by Liu and
co-workers as materials to detect such toxic ions.6e However,
existing luminescent COFs probe either cations or anions, which
greatly limits their commercial development. In this regard, the
exploitation of luminescent COFs that can efficiently sense both
attempts to use luminescent COFs as sensing materials in cationic
and anionic solutions6b,
6e
as well as for medical diagnosis and
monitoring (e.g., environmental and industrial manufacturing).
It is well recognized that Fe3+, CrO42-, Cr2O72-, and MnO4- play
important roles in the field of life sciences. Fe3+ ions are
essential for living organisms because they play a vital role in
various biological processes, such as oxygen metabolism,
-
cations (Fe3+) and anions (CrO42-, Cr2O72-, and MnO4 ) is essential
for mass production. Furthermore, the majority of reported
6e
luminescent materials detect ions in organic solvents,6d,
because development of sensors that operate in aqueous
solutions remains challenging. Therefore, sensors that function
in aqueous solution are desirable.
Herein, we report a highly porous and chemically stable
luminescent covalent organic framework, denoted as COF-TT,
oxygen absorption, and electron transfer, and they also find
wide spread applications in industry.6b, 10 In addition, CrO4
,
2-
Cr2O72-, and MnO4 ions are ubiquitous oxidants used both in the
-
derived
bis(tetraoxacalix[2]arene[2]triazine) and the rigid tetra(p-
aminophenyl)methane combined through solvothermal
from
a
flexible
core
consisting
of
a. State Key Laboratory of Inorganic Synthesis and Preparative Chemistry, College of
Chemistry, Jilin University, Changchun 130012, China
a
b. Institute of Atomic and Molecular Physics, Jilin University, Changchun 130012,
China
preparation. The three-dimensional COF exhibited excellent
-
sensitivity to detect Fe3+ cations and CrO42-, Cr2O72-, and MnO4
c. College of Chemistry and Molecular Engineering, Qingdao University of Science
and Technology, Qingdao 266042, China
† Electronic supplementary information (ESI) available: Synthetic procedures,
anions based on a turn-off sensing mechanism. Moreover, the
luminescence quenching mechanisms were investigated using
characterization details, reusability, and additionalanalytical results
.
See
DOI: 10.1039/x0xx00000x
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both experimental studies and theoretical calculations. The
luminescence properties of cations can be ascribed to the
interactions of guest molecules with the COF skeleton, while the
luminescent characteristics of anions are attributed to
competitive adsorption of anions to the COF skeleton.
View Article Online
DOI: 10.1039/C9TC03265G
Cl
Cl
N
N
N
N
N
N
O
O
O
O
O
N
O
N
N
Cl
DMAc, K2CO3
+
,
℃
120
48h
NH2
H2
N
NH2
Figure 2. FT-IR spectra of (a) the bis(tetraoxacalix[2]arene[2]triazine) core, (b) tetra(p-
aminophenyl)methane, and (c) COF-TT.
NH2
Figure 1. Schematic illustration of the synthesis of COF-TT.
Experimental
Synthesis of COF-TT
Tetra(p-aminophenyl)methane (152.1 mg, 0.3 mmol), extra dry
DMAc (15 mL), and K2CO3 (166.0 mg, 1.2 mmol) were mixed well
under
nitrogen
at
0
°C
(Figure
1).
Bis(tetraoxacalix[2]arene[2]triazine) core (234.8 mg, 0.4 mmol)
was dissolved in 5 mL DMAc and slowly added to the former
mixture, then vigorously stirred at 120 °C for 2 days. The crude
product was collected by filtration and subsequently washed
with tetrahydrofuran (THF) and water, followed by Soxhlet
extraction with THF (48 h). The final activated brown powder of
COF-TT was obtained after vacuum drying overnight at 120 °C
(317.6 mg, 82.1% yield).
Figure 3. Solid-state 13C NMR spectrum of COF-TT.
According to the experimental structural characterization of COF-
TT, we established a sound theoretical model based on Focite
force-field calculations (Forcite Geometry Optimization-
Convergence) using Accelrys’ Materials Studio (MS) v. 8.0
software (Accelrys Software Inc., San Diego, CA, USA).5 The
simulations offered an optimized structure of I-43D space group
and a unit cell with a = b = c = 40.75 Å and α = β = γ = 90° (Fig. S1).
The powder X-ray diffraction (PXRD) pattern displays three broad
diffraction signals at 2θ  7°, 13°, and 19°, thus revealing a low
degree of crystalline order for COF-TT powders (Fig. S2). The
simulated and experimental PXRD patterns of the framework
were in agreement. The morphology of COF-TT was observed by
using scanning electron microscopy (SEM) and transmission
electron microscopy (TEM). As shown in Fig. S3, the COF-TT solid
consists of particles and nanoparticles. Thermogravimetric
analysis (TGA) plots revealed that COF-TT was stable up to 400 °C
(Fig. S4).
Results and discussion
Structural characterization
The structural integrity of COF-TT was verified by Fourier
transform infrared (FT-IR) spectroscopy (Figure 2). The
disappearance of the tensile vibrations characteristic of C-Cl at
875 cm-1 indicated that all the Cl atoms were replaced. The peak
at 1565 cm-1 was attributed to a C=N stretching vibration, which
indicated that the triazine ring moiety was grafted in the
framework. Furthermore, the peaks at 3396 and 1621 cm-1,
attributed to the N-H stretching vibration and primary amines,
respectively, disappeared, confirming substitution of the amine
groups in tetra-(4-anilyl)-methane.3e, 6b Structural features of
COF-TT were also characterized using solid-state 13C NMR
spectroscopy (Figure 3). The signal at 62.5 ppm corresponded
to the sp3 carbons of COF-TT. Seven distinct peaks ranging from
100 to 200 ppm were assigned to the sp2 carbons associated
Porosity of COF-TT
To investigate the porosity, gas sorption of COF-TT was probed
using CO2 (3.30 Å), Ar (3.54 Å), and N2 (3.64 Å) (The value in
parentheses stand for the size of the molecule.). CO2 adsorption
of COF-TT at 195 K was highly consistent with typical type I gas
adsorption isotherms, confirming that micropores exist in COF-TT
(Figure 4). The Langmuir and Brunauer-Emmett-Teller (BET)
surface areas of COF-TT were 736 and 528 m2·g-1, respectively.
with
the
cage-like
structure
of
bis(tetraoxacalix[2]arene[2]triazine) and benzene rings.
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Furthermore, the aperture size distribution was modeled using
density functional theory (DFT), which revealed that the main
aperture size of COF-TT is 5.2 Å. N2 sorption isotherms at 77 K and
Ar sorption isotherms at 87 K of COF-TT were obtained (Fig. S6);
the BET surface areas calculated from these two isotherms were
446 m2·g-1 and 451 m2·g-1, respectively, different from those
reported in a previous paper.3b
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DOI: 10.1039/C9TC03265G
Figure 5. Relative fluorescence intensities (excitation at 490 nm) of COF-TT dispersed in
acidic aqueous solutions containing different cations (0.01 mol·L-1).
To further investigate the sensitivity and quantitative sensitization of
the quenching behavior, a concentration gradient experiment was
carried out by changing the Fe3+ concentration. The titration of Fe3+
illustrated that the luminescence intensity of COF-TT gradually
decreases with increasing Fe3+ concentration (Figure 6a), showing
that the quenching process is diffusion-controlled (Figure 6b). When
the Fe3+ concentration reached 0.9 mM, luminescence of the
emulsion was hardly detected, demonstrating its obvious sensitivity
for sensing Fe3+. The curve representing the relationship between
the relative luminescence intensity and the Fe3+ concentration can
be fitted using the formula: I0/I = 1.135 exp([M]/0.265) + 0.223
(where I0 is the luminescence intensity of the H2O@COF-TT
suspension, I is the luminescence intensity of the Mn+@COF-TT
suspension, and [M] (mM) is the concentration of the Fe3+ ion). For
Fe3+, using the Stern–Volmer (SV) equation I0/I = 1 + KSV[M], the
relationship between I0/I and [M] can be fitted well. In the low-
concentration range, the SV curve is almost linear (Figure 6b, inset).
The KSV values are approximately 1.3 × 104 M-1, which are comparable
or even better than those for previously reported compounds for
monitoring Fe3+.10b-10e The detection limit (3σ/ksv) of COF-TT was
calculated from the Ksv value and standard deviation of ten repeated
luminescence measurements of the blank solution (Table S1, ESI†).
For COF-TT, Fe3+ cations are the most effective quenchers with a
detection limit of 3.69 × 10-4 M, similar to those of other reported
compounds.10a-10c The low detection limit indicates that COF-TT has
extremely high detection sensitivity for Fe3+ ions.
Figure 4. CO2 sorption isotherms of COF-TT at 195 K. Inset: Pore size distribution for COF-
TT calculated using DFT.
Fluorescence sensing of cations
The cation recognition properties of COF-TT were measured and the
luminescence response corresponding to the interaction of COF-TT with
different cations in aqueous suspension was investigated using a series
of spectroscopic methods. For comparison, an equal amount of solid
COF-TT was ground and dispersed in separate aqueous solutions
containing equal volumes of MClx (M = Mn2+, Ba2+, Na+, Ca2+, Pb2+, Li+,
Mg2+, K+, Cd2+, Zn2+, Ni2+, Co2+, Al3+, Fe2+, Cr3+, Cu2+ and Fe3+; [M] = 10–2
mol·L-1; x = 1–3, depending on the corresponding charge). The resultant
mixtures were then immersed for 1 h in an ultrasonic bath to form
uniform dispersion suspensions, then at r.t. to form Mn+@COF-TT
(positive ion incorporated COF-TT) solutions for fluorescence studies.
The fluorescence intensities of the Mn+@COF-TT solutions were
recorded and compared (Figure 5). The detailed fluorescence
quenching efficiencies of different metal ions for COF-TT are shown in
Fig. S11. Interestingly, Mn+@COF-TT suspensions show obviously
different fluorescence intensities. The luminescence intensity of COF-TT
at 490 nm (Figure 5) exhibits negligible enhancement or quenching
upon addition of Mn2+, Ba2+, Na+, Ca2+, Pb2+, Li+, Mg2+, K+, Cd2+, Zn2+, Ni2+,
Co2+, Al3+ or Fe2+, whereas marked fluorescence quenching was
observed for Cr3+and Cu2+. In sharp contrast, Fe3+ produced pronounced
luminescence quenching, reducing the fluorescence intensity by 98.4%
compared with that of the initial COF-TT in aqueous solution. More
importantly, COF-TT immersed in an aqueous solution containing Fe3+
ions retained its characteristic luminescence after rinsing with H2O (Fig.
S12), enabling highly selective and repeated sensing of Fe3+.
Figure 6. (a) Luminescence spectra of the Fe3+@COF-TT suspensions upon addition
of Fe3+
(0–1.2 mM) in acidic aqueous solution (Ex at 490 nm). (b) The plot of relative
luminescence intensity vs. Fe3+ concentration. Inset: The linear correlation for the plot
of I0/I as a function of Fe3+ concentration.
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Mechanism for the luminescence quenching of cations
between Fe3+ ions and Lewis bases in the COF-TT framework, which is
View Article Online
DOI: 10.1039/C9TC03265G
stronger than that for other metal ions. Furthermore, the interaction
The luminescence quenching of cations for COFs is probably caused by
three simultaneous behaviors: (i) collapse of the framework, (ii)
competition between cations and COFs for absorption, and (iii) strong
interactions between cations and COFs. Experimentally, interactions
between Fe3+ and the COF-TT framework lead to luminescence
quenching, further supported by the results of theoretical calculations.
The PXRD spectra (Fig. S8) show that the framework of COF-TT
remains unchanged after treatment with Fe3+ aqueous solution.
The UV-visible absorption spectrum Fe3+ ions in aqueous
solution has almost no overlap with the excitation spectrum of
COF-TT (Fig. S13). Therefore, there is no competitive adsorption
between Fe3+ and COF-TT. To elucidate the possible mechanism
of the strong luminescence quenching from Fe3+, O 1s and N 1s
X-ray photoelectron spectroscopy (XPS) and theoretical
calculations were performed on COF-TT and some metal ions
containing Mn+@COF-TT. The O 1s peak for metal ion-free COF-
TT changed from 513.65 to 534.42 eV upon Fe3+ addition (Figure
7a), and the bonding energy of N 1s in Fe3+@COF-TT increased
by 0.76 eV compared to that of free COF-TT, which shifted to
399.94 from 399.18 eV (Figure 7b), representing weak
interactions between Fe3+ and oxygen or nitrogen atoms. Strong
interactions between oxygen or nitrogen atoms and Fe3+ ions
led to changes in the electronic energy level of the ligand,
resulting in inefficient energy transfer in COF-TT. As shown in
Figure 7, the interactions of Mg2+, Mn2+, Ni2+, Fe2+, and Co2+ with
nitrogen or oxygen atoms are negligible; therefore, the O 1s and N 1s
peaks were not shifted in the XPS spectra. On the other hand, Cr3+ and
Cu2+ ions shifted the bond energy of N 1s to 399.52 eV and 399.61 eV,
respectively, while the O 1s peak remained unchanged probably due to
weak interactions between Cr3+ or Cu2+ with COF-TT. Compared with
other Mn+@COF-TT, the XPS profiles of oxygen atoms showed the only
obvious shift and the XPS spectra of nitrogen atoms have the most
obvious shift in Fe3+@COF-TT. Therefore, the mechanism underlying the
selective luminescent detection of Fe3+ involves the interaction
between Fe3+ ions and oxygen atoms in the COF-TT framework is strong,
while there is almost no interaction between other metal ions and COF.
Optimized ground-state structures using the CAM-B3LYP methods
combined with the def2SVP basis sets are displayed in Figure 8. In the
case of calculations including solvent effects, the water was used in the
polarizable continuum model (PCM).12 The initial structures for the
optimization process were taken from the crystal structures, and six
oxygen and three nitrogen atoms were fixed during the optimization
process for better geometrical prediction based on the crystal
structures. All the calculations were carried out using the GAUSSIAN09
program
bis tetraoxacalix[2]arene[2]triazine
Fe···N distance in the triazine unit
distance in the amine groups 2.309 Å) (Figure 8c
packages.
The
Fe···O
distance
2.118 Å) (Figure 8a
in
the
, the
(
)
core
(
)
(
2.143 Å) (Figure 8b), and the Fe···N
(
) were used to
determine the corresponding bond energies. The interaction energies
of Fe3+@COF-TT in water corresponding to Figure 8a, b, c were −10.1,
−21.8, −2.0 kcal·mol-1, respectively, while the corresponding energies in
gas phase were −24.0, −36.2, and 14.7 kcal·mol-1. (Table S1) The
interaction of Fe3+ with oxygen atoms in COF-TT was not a feature
observed in other ions that consistent with the results shown in Figure
7a. The results of XPS analysis and the data in Figure 8 and Table S1
reveal that the ratio of the interaction of Fe···O to fluorescence
quenching is larger than that to Fe···N in Fe3+@COF-TT. Although the O
site is 10 kcal/mol lower than the energy required for adsorption at the
N site, there is Cl···H adsorption at the N-position so that its energy is
lower. In addition, the fact that COF-TT has many O layouts and almost
no steric hindrance is also an important factor. The calculations further
revealed the interaction of Fe3+ ions with nitrogen atoms of COF-TT.
Both XPS experimental data and theoretical calculations show that
quenching is mainly due to the exchange of electrons in the
bis
through Lewis acid-base interactions.
(tetraoxacalix[2]arene[2]triazine)
core after bonding with Fe3+ ions
Figure 7. (a) O 1s and (b) N 1s XPS spectra of the original COF-TT and Mn+@COF-TT.
4 | J. Name., 2012, 00, 1-3
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(a)
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(b)
(c)
Figure 8. Preferential Fe3+ ion bonding sites and the corresponding bonding energies, obtained by theoretical calculations (C, gray; N, blue; O, red; H, white; Fe, orange; Cl, green).
Fluorescence sensing of anions
demonstrating that the quenching process is diffusion-controlled
(Figure 10b–12b). Luminescence of the suspensions could not be
detected when 0.8, 0.8, or 0.7 mM of CrO42-, Cr2O72-, or MnO4- anions
were present, respectively, indicating good sensitivity of COF-TT to
detect K2CrO4, K2Cr2O7, and KMnO4 as a turn-off sensor. The variation in
relative fluorescence intensity as a function of anion concentration (i.e.,
Due to the prominent luminescence features of COF-TT, we also
became interested in exploring the turn-off fluorescence sensing of
anions. To investigate this potential application of COF-TT, equal masses
of ground COF-TT materials were immersed in isometric KnX aqueous
solutions (X = ClO3 , BrO3 , CO32-, C2O42-, OAc-, SCN-, IO3 , Br-, ClO4 , OH-,
-
-
-
-
-
CrO42-, Cr2O72-, and MnO4 ) is shown in Figures 10-12. The relative
NO3 , SO42-, I-, Cl-, PO43-, F−, CrO42-, Cr2O72-, and MnO4 ; [X] = 10–2 mol·L-1;
n = 1–3, depending on the corresponding charge) for the luminescence
study. The fluorescence intensities of the solutions were recorded and
compared (Figure 9). Detailed fluorescence quenching efficiencies of
COF-TT for different anions are shown in Fig. S14. The luminescence
intensity at 490 nm increased negligibly after the addition of KClO3,
KBrO3, K2CO3, K2C2O4, KOAc, KF, KCl, KBr, KI, KNO3, KOH, KSCN, K2SO4,
K3PO4, KIO3, and KClO4. However, addition of K2CrO4, K2Cr2O7, and
KMnO4 showed significant luminescence quenching toward COF-TT.
Furthermore, multiple cycles of experiments on COF-TT for detecting
-
-
fluorescence intensity can be calculated as follows: I0/I = 1.281
exp([X]/0.253) + 0.185, I0/I = 1.621 exp([X]/0.253) − 0.553, and I0/I =
1.870 exp([X]/0.241) − 1.019, respectively (I0 is the fluorescence
intensity of H2O@COF-TT and I is the fluorescence intensity of
anion@COF-TT emulsions; [X] (mM) is the anion concentration). For
CrO42-, Cr2O72- and MnO4-, the curves of I0/I vs. [X] can be readily applied
to the Stern–Volmer equation I0/I = 1 + KSV[X], with the SV curve being
nearly linear in the low-concentration range (Figure 10b–12b, inset).
-
The KSV values for CrO42-, Cr2O72-, and MnO4 were calculated to be
approximately 1.4 × 104 M-1, 1.4 × 104 M-1 and 1.5 × 104 M-1,
respectively, comparable or superior to the reported as-synthesized
materials for detecting CrO42-, Cr2O72-, and MnO4.11 The detection limits
(3σ/KSV) of COF-TT were also determined based on the Ksv values and
the standard deviations for ten repeated fluorescence measurements
of H2O@COF-TT solutions (Table S1). CrO42-, Cr2O72-, and MnO4- are the
most efficient quenchers of COF-TT, with detection limits of 3.43 × 10-4
M, 3.43 × 10-4 M, and 3.20 × 10-4 M, respectively, comparable to the
-
CrO42-, Cr2O72-, and MnO4 anions indicated that COF-TT can regain its
fluorescence intensity after washing with H2O (Fig. S15–S17). These
results indicated that COF-TT can be applied as a turn-off fluorescence
sensor for sensing CrO42-, Cr2O72-, and MnO4- ions with high sensitivity
and excellent recyclability.
values for other reported quenching materials.11b The low detection
2-
limits suggest that COF-TT has very high detection sensitivity for CrO4
,
-
Cr2O72-, and MnO4 .
Figure 9. Relative fluorescence intensities (upon excitation at 490 nm) of COF-TT
dispersed in acidic aqueous solutions containing different anions (0.01 mol·L-1).
Figure 10. (a) Luminescence spectra of the CrO42-@COF-TT suspensions upon addition of
CrO4 (0–1.2 mM) in acidic aqueous solution (Ex at 490 nm). (b) The plot of relative
luminescence intensity as a function of CrO42- concentration. Inset: The linear correlation
for the plot of I0/I as a function of CrO42- concentration.
Sensitivity and quantitative sensitization of the quenching behavior are
important criteria for a fluorescent sensor. Therefore, concentration
gradient experiments were performed by varying the concentrations of
CrO42-, Cr2O72-, and MnO4- ions. Titrations illustrated that the
fluorescence intensity of COF-TT decreases with increasing
2-
-
concentration of CrO42-, Cr2O72-, or MnO4 anions (Figure 10a–12a),
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Conclusions
View Article Online
DOI: 10.1039/C9TC03265G
In conclusion, COF-TT, a porous multifunctional luminescent
COF sensor with high chemical and thermal stabilities, was
prepared for the first time via a simple route. COF-TT showed
2-
high sensitivity and selectivity to Fe3+ cations, and CrO42-, Cr2O7
-
, and MnO4 anions via luminescence quenching, indicating its
immense potential for the qualitative and quantitative
detection of these ions. Furthermore, the luminescent sensing
mechanism was confirmed using both experimental and
theoretical calculations. Our findings advance COFs as a
functional platform for luminescent sensors, and expand the
scope of materials designed to produce new COFs with unique
luminescent sensing functions.
Figure 11. (a) Luminescence spectra of the Cr2O72-@COF-TT suspensions upon addition
of Cr2O72-(0–1.2 mM) in acidic aqueous solution (Ex at 490 nm). (b) The plot of relative
2-
luminescence intensity as
a
function of Cr2O7 concentration. Inset: The linear
correlation for the plot of I0/I as a function of Cr2O72- concentration.
Acknowledgements
This work was supported by the Foundation of the Natural Science
Foundation of China (no. 21771077, 21771084 and 21621001), the
National Key Research and Development Program of China (no.
2016YFB0701100), the 111 Project (no. B17020)
-
Figure 12. (a) Luminescence spectra of the MnO4 @COF-TT suspensions upon addition of
-
different contents of MnO4 (0–1.2 mM) in acidic aqueous solution (Ex at 490 nm). (b) The
Conflicts of interest
The authors declare no competing conflicts of interest.
plot of relative luminescence intensity as a function of MnO4- concentration. Inset: The
linear correlation for the plot of I0/I as a function of MnO4- concentration.
Notes and references
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Mechanism for the luminescence quenching byanions
Luminescence quenching by anions occurs due to structural collapse,
interactions of anions with the COF skeleton, or competition of the
excitation wavelength of the skeleton with the UV absorption by the
anions. Consequently, the mechanisms of fluorescence quenching by
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CrO42-, Cr2O72-, and MnO4 anions were further explored. First, PXRD
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0.055%, 0.053%, and 0.045% in CrO42-, Cr2O72-, and MnO4 @COF-TT,
-
-
respectively, verifying that CrO42-, Cr2O72-, and MnO4 are not fixed in the
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3
4
-
wide absorption bands ranging from 230 to 550 nm for MnO4 were
observed, which almost overlapped with the adsorption band of COF-
TT in the range 210–310 nm, while other anions showed no apparent
absorption in this wavelength range. This observation testified that the
strong competitive adsorption of excitation energy between COF-TT
-
and CrO42-, Cr2O72-, or MnO4 is the primary cause of luminescence
quenching in the presence of these anions, which can suppress
excitation energy transfer to an organic ligand of COF-TT by absorbing
most of the excitation energy, resulting in a sharp decrease in the
emission intensity or even quenching.
6 | J. Name., 2012, 00, 1-3
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