Covalent organic frameworks as pH responsive signaling scaffolds

Article abstract of DOI:10.1039/c6cc05748a

A β-ketoenamine based covalent organic framework, COF-JLU4, was synthesized by condensation of 2,5-dimethoxyterephthalohydrazide with triformylphloroglucinol under solvothermal conditions. This COF has strong crystallinity, good porosity, photoluminescence properties and wettability for water. It can serve as the first COF-based fluorescent pH sensor in aqueous solutions.

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A β-ketoenamine based covalent organic framework possesses highly crystalline,
good porosity, stability and luminescent properties. It can serve as first COF-based
fluorescent pH sensor in aqueous solutions.

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Covalent organic frameworks as pH responsive signaling scaffolds
Yuwei Zhanga,d, Xiaochen Shena, Xiao Fengb, Hong Xiac, Ying Mua* and Xiaoming Liua*
Received 00th January 20xx,
Accepted 00th January 20xx
DOI: 10.1039/x0xx00000x
www.rsc.org/
the best of our knowledge, this study is the first example of
applying COF materials as fluorescent pH sensors.
A β-ketoenamine based covalent organic framework, COF-JLU4,
was
synthesized
by
condensation
of
2,5-
dimethoxyterephthalohydrazide with triformylphloroglucinol
under solvothermal conditions. This COF has strong crystallinity,
good porosity, photoluminescent properties and as well as
wettability for water. It can serve as first COF-based fluorescent
pH sensor in aqueous solutions.
COF-JLU4 was synthesized by condensation of 2,5-
dimethoxyterephthalohydrazide with triformylphloroglucinol under
solvothermal conditions (Scheme 1A). The resulting COF was
unambiguously characterized by Fourier transform IR spectroscopy,
13C cross-polarization/magic-angle-spinning (CP/MAS) NMR
spectroscopy, and elemental analysis. The FT-IR spectra of the COF-
JLU4 does not show the characteristic stretching vibration peaks of
imine and hydroxyl groups (Fig. S1, ESI†). Compared with its
monomer (1639 cm-1), the carbonyl C=O peak of COF-JLU4 shifts to
a lower energy characteristic of β-ketoenamine carbonyl stretch
(1619 cm-1), changes simultaneously broad and merges with the
C=C stretching vibration (1585 cm-1), which attributed to the strong
intramolecular hydrogen bonding in the keto forming of the 2D
conjugated sheet (Fig. S1, ESI†).8 The atomic precision construction
of COF-JLU4 was further verified by the 13C CP/MAS NMR (Fig. S2,
ESI†). The peaks at 182 and 161 ppm correspond to the carbon
atoms of the carbonyl groups. The high-field peak at ~55 ppm can
be assigned to the methoxy groups of the aromatic rings. Elemental
analysis of the COF-JLU4 revealed that the C, H and N contents were
51.36, 4.08, and 13.77%, respectively, which were close to the
53.63, 3.94, and 15.64% expected for an infinite 2D sheet. Field-
emission scanning electron microscopy (FE-SEM) revealed regular
belt shape (Fig. S3A, ESI†). High resolution tunneling electron
microscopy (HR-TEM) exhibited porous texture (Fig. S3B, ESI†),
which were close to the lattice resolved by X-ray diffraction (XRD).
The XRD measurement was performed on samples of COF-JLU4 to
determine its crystallinity. The experimental XRD pattern displayed
a strong peak at 3.31° and relatively weak signals at 5.91, 6.76,
9.26°, which were assigned to 100, 110, 200, and 210 facets,
respectively. The slight broad peak at higher 2θ (~26.9°) is mainly
due to π-π stacking between the COF layers and corresponds to the
001 plane (Fig. 1A, red curve). The simulation of the XRD pattern of
the AA stacking mode can reproduce the experimental data in the
peak position and intensity (yellow curve), while the staggered AB
mode produced a profile (green curve) that deviates from the
experimental pattern. The Pawley refinement showed an XRD
As a novel class of porous crystalline materials, covalent organic
frameworks (COFs) are assembled from molecular building blocks
using reticular chemistry via covalent bond formation, and exhibit
periodic architectures, low densities and predesignable pore
parameters.1 The rational construction in their skeleton structure
promises these porous materials tailored to specific functional
applications, including gas storage and separation,2 catalysis,3
optoelectronics,4 energy storage,5 and proton conduction.6
However, these materials have only rarely been explored as
chemosensors,7 in particular, pH dependent fluorescent properties
in COFs is still unexplored. The weak stability and wettability of COF
skeletons in aqueous solution is an obvious bottleneck, which
restricted their potential for pH sensing.
In this contribution, a new two-dimensional (2D) COF with high
crystalline and large surface area, termed COF-JLU4 was designed
and constructed through continuous reversible and irreversible
reactions. Interestingly, the COF-JLU4 skeleton features keto-
enamine form, and it shows an outstanding hydrolytic stability and
wettability. Furthermore, COF-JLU4 exhibits pH dependent
fluorescence response in aqueous solutions with wide pH range. To
a.
State Key Laboratory for Supramolecular Structure and Materials, College of
Chemistry, Jilin University, Changchun, 130012, P. R. China.
E-mail: xm_liu@jlu.edu.cn
b.
School of Chemistry, Beijing Institute of Technology, Beijing, 100081, P. R. China.
c. State Key Laboratory on Integrated Optoelectronics, College of Electronic Science
and Technology, Jilin University, Changchun, 130012, P. R. China.
d.College of Resoureces and Environmnet, Jilin Agricultural University, Changchun,
130118, P. R. China.
†Electronic Supplementary Informaꢀon (ESI) available: Synthesis of monomer,
model compound and COF-JLU4, FT-IR, PXRD, SEM and TEM images for COF-JLU4,
carbon dioxide adsorption isotherms and the isosteric heat of adsorption of COF-
JLU4. See DOI: 10.1039/x0xx00000x
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Scheme 1. (A) Schematic Representation of the Synthesis of COF-JLU4; (B)
Top and Side Views of the AA Stacking Structure of COF-JLU4.
Fig. 1 (A) XRD profiles of COF-JLU4 experimentally observed (red), simulated
by using AA-stacking (orange) and AB-stacking (green) models. (B) XRD
profiles of COF-JLU4 experimentally observed (red), Pawley refinement
(blue), and their difference (black). Crystal structures of (C) the AA-stacking
and (D) the AB-stacking models.
pattern (Fig. 1B, blue curve) that is in well agreement with the
experimentally observed pattern, as evidenced by their negligible
difference (black curve). The P3 space group with parameters of a =
b = 31.154 Å, c = 3.4 Å, and α = β = 90°, γ = 120° was simulated.
The porous structure was investigated by nitrogen sorption
measurements at 77 K. The COF-JLU4 exhibited typical type-IV
isotherm, which indicated that the new COF was a mesoporous
material (Fig. 2A). The Brunauer-Emmett-Teller (BET) surface area
and pore volume were estimated to be 923 m2 g–1 and 0.573 cm3 g–
1, respectively. The SABET value of COF-JLU4 is higher than the
majority of reported β-ketoenamine-linked 2D-COFs,8 and
comparable to the highest value of 1120 m2 g–1 for DAAQ-TFP
COF.5a The pore size distribution evaluating by nonlocal density
functional theory was centered at 2.2 nm (Fig. 2B), which is
reasonably close to the predicted pore size (2.3 nm). Carbon dioxide
(CO2) adsorption isotherms for COF-JLU4 were collected, showing
that this material can store up to 128 mg g–1 of CO2 at 273 K and 1.0
bar, with the isosteric heat (Qst) value of 33 kJ mol–1 at low coverage
(Fig. S4, ESI†).
Fig. 2 (A) Nitrogen adsorption (●) and desorption (○) isotherm curves of
COF-JLU4 measured at 77 K. (B) Pore size distribution profile of COF-JLU4.
and fluorescence spectra of the COF-JLU4 in water and the solid
Thermogravimetric analysis (TGA) was done for the COF-JLU4 state were showed in Fig. 3A. COF-JLU4 displayed an absorption
under a nitrogen atmosphere, which revealed that it was stable up band at 380 nm in the solid state with a shoulder peak at 488 nm
to 305 oC (Fig. S5, ESI†). To evaluate the chemical stability of COF- (red line in Fig. 3A). Compared with monomers and model
JLU4, we dispersed the COF samples in different organic solvents compound, the absorption band of COF-JLU4 is occurred an obvious
such as tetrahydrofuran (THF), hexane, toluene, CHCl3 and the red-shift, which implied an extended π-conjugation over a layer of
aqueous solution with different pH values at room temperature for network (Fig. S7, ESI†). The fluorescence spectrum of COF-JLU4 in
2 h. And then the samples were collected by filtration, washed with
the solid state displayed orange luminescence and an emission
acetone, and dried under vacuum at 100 oC for 10 h. Pleasurably, all band observed at 568 nm with a quantum yield of 2.9% upon
the samples display strong diffraction peaks in XRD patterns and excitation at 380 nm (green line, Fig. 3A). However, the
same network connection from FT-IR profiles (Fig. S6a and S6b, corresponding monomers exhibited almost no emission in the solid
ESI†), implying that the good crystllinity is retained in the COF state upon excitation at 322 and 376 nm, respectively (Fig. S8, ESI†).
samples. The good chemical stability may attribute to the hydrogen The obvious difference in the fluorescence properties between
band interactions on the skeleton of COF-JLU4.8
COF-JLU4 and its monomers can be attributed to the extended π-
conjugation skeleton of COF and restriction of intramolecular
rotation in the framework. Furthermore, this COF can be easily
The luminescence properties of COF-JLU4 were investigated by
UV/vis and fluorescence spectroscopy. The electronic absorption
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dispersed in water, owing to abundant heteroatoms in the skeleton. enhancement and blue-shift in acidic solution are assumed to arise
And a blue-shifted emission band with a peak at 475 nm was from protonation of nitrogens.12 Furthermore, we investigated the
obtained upon excitation at 418 nm (blue line, Fig.3A).
fluorescent response of the model compound to pH value, and
found that the fluorescence intensity of the model compound also
decreases gradually with pH value of solutions increasing (Fig. S9,
ESI†).
Fig. 4 The reversible optical responses of COF-JLU4 (A) between the neutral
(pH = 7.0) and basic condition (pH = 13.0), (B) between the neutral (pH = 7.0)
and acidic condition (pH = 1.0). Insets: luminescence images of COF-JLU4 in
different pH solutions under a UV lamp with λex = 365 nm.
Fig. 3 (A) Electronic absorption (in water: black line, in the solid state: red
line) and fluorescence spectra (in water: blue line, in the solid state: green
line) of COF-JLU4. (B) pH dependent fluorescence of COF-JLU4 in the
aqueous solutions with pH ranging from 0.9-13.0 measured under excitation
We further explored the recyclability and reusability of COF-JLU4
as a fluorescent pH sensor. For example, when 13.0 of pH value for
solution, COF-JLU4 exhibited the same degree of fluorescence
of 334 nm. (C) Photoluminescent intensity of COF-JLU4 at 428 nm in the quenching after each cycle, and the fluorescence intensity
aqueous solutions with different pH values. (D) Deprotonation and
protonation processes of COF-JLU4 framework in acidic and basic media.
recovered to a similar level when the pH value of solution was
changed to 7.0 (Fig. 4a). Moreover, COF-JLU4 exhibited an
enhancement in fluorescence upon acidic solution (pH = 1.0), even
after reuse (Fig. 4b). Pleasantly, the crystallinity of COF-JLU4
framework is still well maintained after multiple use (Fig. S10, ESI†).
Compared with electrodes, fluorescent pH responsive sensors
possess numerous advantage including facile pH detection, ease of
device fabrication and flexibility of design and exhibit greatly
important in environmental and life sciences.9 Currently, some
nanoscale fluorescent pH sensors such as metal-organic
frameworks (MOFs) have been constructed successfully.10
Unfortunately, however, the investigation of luminescent COFs as
fluorescent pH sensors has not been explored.1-8 The COF-JLU4 has
good luminescence and wettability in aqueous solution, which
In conclusion,
a β-ketoenamine based 2D covalent organic
framework (COF-JLU4) was successfully designed and synthesized
under solvothermal conditions. The new COF has excellent porosity
with a high BET surface area of over 900 m2 g-1, strong crystallinity,
good stability and photoluminescent properties. Furthermore, we
also highlight that the crystalline framework exhibits interesting
fluorescence response for pH value in aqueous solution, showing
for the first time the utility of the crystalline COFs for developing
pH-sensing systems. This research expands the scope of COFs and
constitutes an important step toward porous crystalline organic
architecture with designed functions.
provides
a suitable platform and inspires us to explore its
application as a nanoscale pH responsive sensor. Fig. 3B shows the
emission spectra of COF-JLU4 in aqueous solution at various pH
values. Interestingly, the fluorescence intensity and wavelength of
COF are strongly correlated with pH value of the dispersed solution
in our experimental pH range (0.9-13.0). The most acidic solution
displayed strongest fluorescence, while weakest intensity of
fluorescence was obtained with pH = 13.0 solution. The intensity of
emission peak at 428 nm almost increases with pH decreasing in
experimental range (Fig. 3c). When the pH changed from 9.0 to 13.0,
the fluorescence intensity of COF-JLU4 decreased gradually, which
is attributed to a deprotonation process of the nitrogen in the
framework (Fig. 3d).11 In comparison, when the pH is below 4.5,
COF-JLU4 presents that the luminescent emission of the 428 nm
peak increases dramatically with decreasing pH. The fluorescence
This work was supported by the National Natural Science
Foundation of China (Nos. 51473064, 61435005, 21274050 and
U1462111).
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